We theoretically investigate light trapping with disordered 1D photonic structures in thin-film crystalline silicon solar cells. The disorder is modelled in a finite-size supercell, which allows the use of rigorous coupled-wave analysis to calculate the optical properties of the devices and the short-circuit current density J_sc. The role of the Fourier transform of the photonic pattern in the light trapping is investigated, and the optimal correlation between size and position disorder is found. This result is used to optimize the disorder in a more effective way, using a single parameter. We find that a Gaussian disorder always enhances the device performance with respect to the best ordered configuration. To properly quantify this improvement, we calculate the Lambertian limit to the absorption enhancement for 1D photonic structures in crystalline silicon, following the previous work for the 2D case [M.A. Green, Progr. Photovolt: Res. Appl. 2002; 10(4), pp. 235–241]. We find that disorder optimization can give a relevant contribution to approach this limit. Finally, we propose an optimal disordered 2D configuration and estimate the maximum short-circuit current that can be achieved, potentially leading to efficiencies that are comparable with the values of other thin-film solar cell technologies. Copyright © 2013 John Wiley & Sons, Ltd.

We theoretically investigate light trapping in thin-film crystalline silicon solar cells with photonic structures combining order and disorder. We find that the richer Fourier spectrum of disordered patterns is responsible for increased coupling of incident sunlight inside the solar cell. We derive guidelines for disorder optimization, and we prove that partially disordered photonic structures represent a valid way to approach the Lambertian limit to light trapping.

In recent years, zinc oxide has been investigated as a front electrode material in hydrogenated amorphous silicon/hydrogenated microcrystalline silicon (a-Si:H/µc-Si:H) tandem solar cells. Such as for other transparent conducting oxide materials and applications, a proper balancing of transparency and conductivity is necessary. The latter is directly related to the density and the mobility of charge carriers. A high density of charge carriers increases conductivity but leads to a higher absorption of light in the near-infrared part of the spectrum due to increased free-carrier absorption. Hence, the only way to achieve high conductivity while keeping the transparency as high as possible relies on an increase of carrier mobility. The carrier density and the mobility of sputtered Al-doped zinc oxide (ZnO:Al) can be tailored by a sequence of different annealing steps. In this work, we implemented such annealed ZnO:Al films as a front electrode in a-Si:H/µc-Si:H tandem solar cells and compared the results with those of reference cells grown on as-deposited ZnO:Al. We observed an improvement of short-circuit current density as well as open-circuit voltage and fill factor. The gain in current density could be attributed to a reduction of both sub-band-gap absorption and free-carrier absorption in the ZnO:Al. The higher open-circuit voltage and fill factor are indicators of a better device quality of the silicon for cells grown on annealed ZnO:Al. Altogether, the annealing led to an improved initial conversion efficiency of 12.1%, which was a gain of +0.7% in absolute terms. Copyright © 2013 John Wiley & Sons, Ltd.

A post-deposition annealing procedure was used to tailor the opto-electrical properties of large-area sputtered Al-doped zinc oxide (ZnO:Al) films on glass, which were subsequently used as substrates for a-Si:H/µc-Si:H tandem solar cells. The improved properties of the ZnO:Al led to a gain of +0.7% in conversion efficiency. Using this technique, we achieved a conversion efficiency of 12.4% in the initial state and 11.1% stabilized after 500 h of light soaking.

We exploited alternated electrodeposition of Cu, Sn and S to obtain Cu_xSn_yS_z thin films. These materials are kesterite-type chalcogenides that have attracted a relevant interest from worldwide researchers as low cost and high conversion efficiency solar cell devices. Films were grown on silver substrate, controlling the growth of the electrodeposited structures at the nanometric level. The obtained films were characterized by diffuse reflectance spectroscopy, voltammetric stripping and atomic force microscopy. Experimental bandgap energies resulted linearly modulated by changes of chemical composition and thickness. On the basis of these results, we candidate electrodeposition as a room temperature method to obtain thin films for solar cell technology with low energy investment and negligible environmental impact. Copyright © 2013 John Wiley & Sons, Ltd.

In this study, a new approach to synthesize thin films of ternary copper and tin sulfides, technologically relevant for photovoltaic semiconductors, characterized by low cost of raw materials and absence of toxicity, is proposed. This approach is established within the electrodeposition ECALE technique, which enables the opportunely change of the sequence of deposition and of the number of cycles to obtain Cu_xSn_yS_z thin films with different composition and thickness, precisely adjusting the bandgap depending on the target device.

The degradation observed on a 7-kWp Si-x photovoltaic array after 17 years of exposure on the roof of the Solar Energy Institute of the Polytechnic University of Madrid is presented. The mean peak power degradation has been 9% over this time, or an equivalent to 0.53% per year, whereas peak power standard deviation has remained constant. The main visual defects are backsheet delamination at the polyester/polyvinyl fluoride outer interface and cracks in the terminal boxes and at the joint between the frame and the laminate. Insulation resistance complies well with the requirements of the International Electrotechnical Commission 61215 tests. Copyright © 2013 John Wiley & Sons, Ltd.

The degradation observed on a 7 kWp Si-x PV array after 17 years of exposure at the roof of the Solar Energy Institute of the Polytechnic University of Madrid is presented. Mean peak power degradation has been 9% during this time, or an equivalent to 0.56% per year, while peak power standard deviation has remained constant. Although modules have shown some visual defects, they have passed the isolation tests considered in the IEC-61215.

Many thin film photovoltaic (PV) technologies can be sensitive to corrosion induced by the presence of water vapor in the packaging materials. Typically impermeable front and backsheets are used in conjunction with an edge-seal around the perimeter to prevent water vapor ingress. These edge-seal materials are often made of a polyisobutylene resin filled with desiccant, which dramatically increases the time for moisture to reach sensitive module components. While edge-seals can prevent moisture ingress, even the lowest diffusivity transparent encapsulant materials are insufficient for the lifetime of a module. To evaluate the performance of edge-seal and encapsulant materials in a manner that simulates their function in a PV module, an optical method was devised where ingress is detected by reaction of a Ca film with water. Using this method, we have exposed test samples to heat and humidity allowing quantitative comparison of different edge-seal and encapsulant materials. Next, we use measurements of polymer diffusivity and solubility to evaluate the ability to model this moisture ingress. Here, we find good agreement between these two methods highlighting the much greater ability of polyisobutylene materials to keep moisture out as compared with typical encapsulant materials used in the PV industry. Copyright © 2013 John Wiley & Sons, Ltd.

In this work, we present a technology for a high precision nanostructure replication process based on ultraviolet nanoimprint lithography for the application in the field of thin-film photovoltaics. The potential of the technology is demonstrated by the fabrication of microcrystalline silicon thin-film prototype solar cells. The high accuracy replication of random microstructures made from sputtered and etched ZnO:Al, used to scatter the incident light in thin solar cells, is shown by local topography investigations of the same 7.5 × 7.5 µm² area on the master and the replica. Different types of imprint resists and imprint moulds were investigated to find the optimal, high precision replication technology. Two types of thin-film silicon solar cells, in p-i-n and n-i-p configuration, were fabricated to study the potential of the imprint technology for different applications. It is shown that solar cells deposited on an imprinted glass hold similar performances compared with reference solar cells fabricated with a standard process on textured ZnO:Al. Thus, it is demonstrated that the replication of light scattering structures by using an imprint process is an attractive method to decouple the scattering properties from the layer forming the electrical front contact. Because a simple and cheap high throughput process is used, this study additionally proves the relevance for the industrial mass production in the field of photovoltaics. Copyright © 2013 John Wiley & Sons, Ltd.

The high accuracy replication of random microstructures made from sputtered and etched ZnO:Al is shown by local topography investigations of the same area on the master and the replica. Different types of imprint resists and imprint moulds were investigated to find the optimal, high precision replication technology. It is demonstrated that the replication of light scattering structures by using an imprint process is an attractive method to decouple the scattering properties from the layer forming the electrical front contact.

InAs quantum dots (QDs) have been incorporated to bandgap engineer the (In)GaAs junction of (In)GaAs/Ge double-junction solar cells and InGaP/(In)GaAs/Ge triple-junction solar cells on 4-in. wafers. One sun AM0 current–voltage measurement shows consistent performance across the wafer. Quantum efficiency analysis shows similar aforementioned bandgap performance of baseline and QD solar cells, whereas integrated sub-band gap current of 10 InAs QD layers shows a gain of 0.20 mA/cm². Comparing QD double-junction solar cells and QD triple-junction solar cells to baseline structures shows that the (In)GaAs junction has a V_oc loss of 50 mV and the InGaP 70 mV. Transmission electron microscopy imaging does not reveal defective material and shows a buried QD density of 10¹¹ cm⁻², which is consistent with the density of QDs measured on the surface of a test structure. Although slightly lower in efficiency, the QD solar cells have uniform performance across 4-in. wafers. Copyright © 2013 John Wiley & Sons, Ltd.

InAs quantum dots (QDs) have been incorporated to bandgap engineer the (In)GaAs junction of (In)GaAs/Ge double-junction solar cells and InGaP/(In)GaAs/Ge triple-junction solar cells on 4-in. wafers. The QD solar cells show uniform performance across 4-in. diameter wafers under AM0 illumination. Current gains of 0.020 mA/cm²/QD layer are measured for all samples.

This article reviews all the experimental tests carried out to analyze the performance of a FluidReflex photovoltaic concentrator. This novel concentrator concept consists of a single reflective stage immersed in an optical fluid. The presence of the fluid entails significant advantages. It not only allows a high system optical efficiency and increases the attainable concentration but also enhances the heat dissipation from the cell. In addition, the electrical insulation is improved, and the problem of water vapor condensation inside the module is avoided. A complete characterization is addressed in this paper. Among the experimental results, a measured optical efficiency of 83.5% for a concentration of 1035× stands out. Copyright © 2013 John Wiley & Sons, Ltd.

This article presents a novel concentrator concept, called FluidReflex, which consists of a single reflective stage immersed in an optical fluid. The presence of the fluid entails significant advantages. It not only allows a high system optical efficiency and increases the attainable concentration but also enhances the heat dissipation from the cell. In addition, the electrical insulation is improved, and the problem of water vapor condensation inside the module is avoided.

The electrical properties of Cu(In,Ga)Se₂/Mo junctions were characterized with respect of MoSe₂ orientation and Na doping level using an inverse transmission line method, in which the Cu(In,Ga)Se₂ (CIGS)/Mo contact resistance could be measured separately from the CIGS film sheet resistance. The MoSe₂ orientation was controlled by varying the Mo surface density, with the c-axis parallel and normal orientations favored on Mo surfaces of lower and higher density, respectively. The effect of Na doping was compared by using samples with and without a SiO_x film on sodalime glass. The conversion of the MoSe₂ orientation from c-axis normal to parallel produced a twofold reduction in CIGS/Mo contact resistance. Measurements of the contact resistances as a function of temperature showed that the difference in CIGS/Mo contact resistance between the samples with different MoSe₂ orientations was due to different barrier heights at the back contact. Comparison between Na-doped and Na-reduced samples revealed that the contact resistance for the Na-reduced system was four times of that of the doped sample, which showed more pronounced Schottky-junction behavior at lower temperature, indicating that Na doping effectively reduced the barrier height at the back contact. Copyright © 2013 John Wiley & Sons, Ltd.

The electrical properties of Cu(In,Ga)Se2/Mo junctions were successfully characterized in terms of contact resistance using modified (inverse) transmission line method, in which the Cu(In,Ga)Se2/Mo contact resistance could be measured separately from the CIGS film sheet resistance. The measurement showed that the substantial Na doping as well as the switching of MoSe2 structure toward c-axis parallel orientation could reduce the barrier height at back contact, favoring higher photovoltaic conversion efficiency.

In this paper, we investigate the morphology, optical, and electrical properties of silicon nanowire (SiNW) arrays and their applications on inorganic/organic hybrid solar cells. The SiNW arrays are obtained by two kinds of metal-assisted chemical etching (MacEtch) processes. One is depositing assisted metal, for example, Ag, by an electron gun evaporator before etching (BE). The other is depositing assisted Ag during the etching (DE) process. The results reveal that BE method MacEtch can produce more uniform and denser SiNW arrays, but the coverage of organic materials to SiNWs is less. In terms of optical properties, the BE method SiNW arrays have higher reflectance than the DE method ones, except the wire length less than 1 µm in the wavelength range less than 500 nm. Regarding the electrical property, the minority-carrier lifetime is higher for the DE method SiNW arrays because of less surface area of SiNWs. Therefore, the best cell performance happens on the DE method SiNW/organic hybrid solar cell with wire length less than 1 µm. The short-circuit current density (J_sc) is 28.55 mA/cm², open-circuit voltage (V_oc) is 0.524 V, and power conversion efficiency (PCE) is 9.56%. Copyright © 2013 John Wiley & Sons, Ltd.

SiNW/PEDOT:PSS hybrid solar cells with wire arrays formed by depositing assisted metal during the etching process (DE method) have a better cell performance of PCE of 9.56%, J_sc of 28.55 mA/cm², and V_oc of 0.524 V. However, cells with wire arrays formed by depositing assisted metal before the etching process (BE method) have a worse cell performance of PCE of 8.66%, J_sc of 26.38 mA/cm², and V_oc of 0.517 V.

The advanced semiconductor finger solar cell is a silicon wafer cell designed for industrial implementation. The concept incorporates a selective emitter with a front side grid metallisation that combines the advantages of both screen printing and plating techniques. Screen-printed metal forms the busbars and a few thick but widely spaced fingers. The current is carried to these fingers via narrow, closely spaced, thinly plated, laser-doped lines formed perpendicular to the fingers. This paper explains the evolution from the original semiconductor finger cell design to this advanced version and outlines the issues encountered in the process development thus far. An encouraging result of 18.5% efficiency has been achieved on a large area cell with fully industrial equipment, and several opportunities for significant improvement are identified. Copyright © 2013 John Wiley & Sons, Ltd.

The Advanced Semiconductor Finger cell concept incorporates a selective emitter with a front side metallisation that combines both screen-printing and plating techniques. Screen-printed metal forms the busbars and widely-spaced fingers. The current is carried to these fingers via narrow, closely-spaced, thinly-plated, laser-doped lines formed perpendicular to the fingers. This paper reports the evolution of this cell design and its process development. An encouraging 18.5% efficiency was demonstrated on large area cells using fully industrial equipment.

Tracking systems can increase the amount of electricity generated by photovoltaic (PV) modules, by actively orienting each module to intercept more solar energy. We find that horizontal one-axis tracking systems can increase PV generation by 12–25% relative to south-facing fixed mount PV systems with 25° tilts in the contiguous USA, and two-axis tracking systems can increase PV generation by 30–45% relative to fixed mount systems. Tracking systems increase PV generation more significantly in arid regions such as the southwest USA than in humid regions with persistent cloud cover such as the Pacific Northwest and coastal Atlantic states. We find that fixed and tracking PV systems have similar interannual variability in their generation profiles, and this variability is primarily driven by project location. Tracking PV projects cost more than fixed tilt systems, per unit capacity, and we explore how much more tracking projects could cost while generating similar levelized costs of energy as fixed tilt systems. We define this as the breakeven additional cost of tracking and find that it is primarily driven by three factors: (i) regional tracking performance, (ii) fixed tilt system costs that tracking projects compete against, and (iii) additional tracking operation and maintenance costs. Using this framework, we explore the relative competitiveness of tracking systems for a range of fixed and tracking PV prices and evaluate how tracking competitiveness varies by region. Copyright © 2013 John Wiley & Sons, Ltd.

Tracking systems actively orient photovoltaic (PV) modules to intercept more direct solar radiation, enabling tracking modules to generate more electricity than modules in fixed orientations. The increase in tracking PV system output, relative to fixed tilt systems, is highest in arid US regions than in regions with frequent and persistent cloud cover. We characterize the relationships between fixed and tracking PV costs that enable tracking systems to reduce the levelized cost of energy for PV systems in different US locations and explore how these cost relationships are likely to evolve as PV prices continue to decline.

Quasi-monocrystalline silicon wafers have appeared as a critical innovation in the PV industry, joining the most favorable characteristics of the conventional substrates: the higher solar cell efficiencies of monocrystalline Czochralski-Si (Cz-Si) wafers and the lower cost and the full square-shape of the multicrystalline ones. However, the quasi-monocrystalline ingot growth can lead to a different defect structure than the typical Cz-Si process. Thus, the properties of the brand new quasi-monocrystalline wafers, based on low and high crystal defect densities, have been for the first time studied from a mechanical point of view, comparing their strength with that of both Cz-Si monocrystalline and typical multicrystalline materials. The study has been carried out employing the four line bending test and simulating them by means of FE models. For the analysis, failure stresses were fitted to a three-parameter Weibull distribution. High mechanical strength was found in all the cases. However, the quasi-monocrystalline wafers characterized by large density of bulk defects, due to the noticeable density of extended defects, showed lower fracture tensions. Copyright © 2013 John Wiley & Sons, Ltd.

The mechanical strength comparison of the new quasi-monocrystalline silicon substrates with the conventional monocrystalline Czochralski and multicrystalline, shows that etched wafers have similar values of the characteristic fracture stress (σ_θ) for a reference area ΔA=0.004mm².

The existence of a minimum threshold stress (λ) for single crystal substrates is indicative of a robust structure preventing the failure at low stresses.

The major scattering or uncertainty of the quasi-monocrystalline sets (lower value of parameter β) can be improved controlling the defect density.

This paper describes the measurement of photovoltaic module performance over a range of temperatures and irradiances according to the international standard IEC 61853 Part 1. The purpose of this work is to assess the reproducibility of power matrix measurements obtained using two methods specified in the standard: under natural sunlight with a tracker, and with a solar simulator. A comparison of results using the third principal method (under natural sunlight without tracker) is also summarised for completeness. The same measurement techniques have been employed to measure four modules of different technologies, namely mono and poly crystalline Si, CdTe and CIS. The method used to vary the irradiance in the natural sunlight with tracker and solar simulator approaches is based on un-calibrated mesh filters. The uniformity and effect on spectrum of the mesh filters have been studied, and the impact of these on the measurements estimated. Measurements from all methods are compared over as much of the ranges as possible. The results show that for all modules, the reproducibility is within the estimated measurement uncertainty. The suitability of the different methods is discussed in light of the results and the limitations of the various methods as applied to different modules technologies. On the basis of the results, parts of IEC 61853 Part 1 will be introduced into the ISO 17025 laboratory accreditation at the European Solar Test Installation (ESTI). Copyright © 2013 John Wiley & Sons, Ltd.

This paper describes the measurement of photovoltaic module performance over a range of temperatures and irradiances according to the international standard IEC 61853 Part 1 to provide an assessment of the reproducibility of power matrix measurements obtained using different methods. The results show that for three different module technologies, the reproducibility is within the estimated measurement uncertainty. The suitability of the different methods is discussed including their limitations as applied to different modules technologies.

In this work, we investigate the effect of ageing Mo-coated substrates in a dry and N₂ flooded cabinet. The influence was studied by preparing Cu(In,Ga)Se₂ solar cells and by comparing the electrical performance with devices where the Mo layer was not aged. The measurements used for this study were current–voltage (J-V), external quantum efficiency (EQE), secondary ion mass spectroscopy (SIMS) and capacitance–voltage (C-V). It was concluded that devices prepared with the aged Mo layer have, in average, an increase of 0.8% in efficiency compared with devices that had a fresh Mo layer. Devices with aged Mo exhibited a nominal increase of 12.5 mV of open circuit voltage, a decrease of 1.1 mA/cm⁻² of short circuit current and a fill factor increase of 2.4%. Heat treatment of fresh Mo layers in oxygen atmosphere was also studied as an alternative to ageing and was shown to provide a similar effect to the aged device's performance. Copyright © 2013 John Wiley & Sons, Ltd.

The ageing of the Mo back contact shows small but positive changes in the electrical performance of Cu(In,Ga)Se2 thin-film solar cells, namely an increase of the open circuit voltage, fill factor and efficiency. By ageing the Mo back contact, a slightly higher net acceptor concentration of Na contents similar to the ones of the reference sample is found. Heat treatment of fresh Mo layers in an oxygen atmosphere showed similar effects to ageing.

The successful transition of dye-sensitised solar cell (DSC) manufacture from laboratory to factory requires new thinking in terms of lowering cost and removing time consuming manufacturing process. Platinisation of the fluorine doped tin oxide (FTO) glass counter electrode is essential for the operation of a conventional DSC and is usually carried out by thermal decomposition of chloroplatinic acid at 385 °C for 30 min. Here, near infrared radiation is used to directly heat the FTO layer resulting in full platinisation in 12.5 s. These platinised electrodes behave identically to those produced via conventional static thermal treatment in assembled DSC devices. Copyright © 2013 John Wiley & Sons, Ltd.

Platinisation of the fluorine doped tin oxide (FTO) glass counter electrode is essential for the operation of a conventional dye-sensitised solar cells (DSC) and is usually carried out by the thermal decomposition of chloroplatinic acid at 385 °C for 30 min. In this work, we have used near infrared radiation to directly heat the FTO layer resulting in full platinisation in 12.5 s. These platinised electrodes behave identically to those produced via conventional static thermal treatment in assembled DSC devices.

We investigate the angular behavior of the upper bound of absorption provided by the guided modes in thin film solar cells. We show that the 4n² limit can be potentially exceeded in a wide angular and wavelength range using two-dimensional periodic thin film structures. Two models are used to estimate the absorption enhancement; in the first one, we apply the periodicity condition along the thickness of the thin film structure, but in the second one, we consider imperfect confinement of the wave to the device. To extract the guided modes, we use an automatized procedure that is established in this work. Through examples, we show that from the optical point of view, thin film structures have a high potential to be improved by changing their shape. Also, we discuss the nature of different optical resonances that can be potentially used to enhance light trapping in the solar cell. We investigate the two different polarization directions for one-dimensional gratings, and we show that the transverse magnetic polarization can provide higher values of absorption enhancement. We also propose a way to reduce the angular dependence of the solar cell efficiency by the appropriate choice of periodic pattern. Finally, to obtain more practical values for the absorption enhancement, we consider the effect of parasitic loss that can significantly reduce the enhancement factor. Copyright © 2013 John Wiley & Sons, Ltd.

Variations of versus k_∥ (cm⁻¹) and energy (eV) in S polarization for a complete solar cell stack.

Key findings of the paper:

• We find the upper bounds of absorption enhancement in thin film solar cells on the basis of gratings by considering their modal properties.
• The 4n² limit can be potentially exceeded in a wide angular and wavelength range using two-dimensional periodic thin film structures.
• Parasitic absorption can significantly reduce the enhancement factor.

The impacts of air annealing, light soaking (LS), and heat–light soaking (HLS) on cell performances were investigated for ZnS(O,OH)/Cu(In,Ga)Se₂ (CIGS) thin-film solar cells. It was found that the HLS post-treatment, a combination of LS and air annealing at 130 °C, is the most effective process for improving the cell performances of ZnS(O,OH)/CIGS devices. The best solar cell yielded a total area efficiency of 18.4% after the HLS post-treatment. X-ray photoelectron spectroscopy showed that the improved cell performance was attributable to the decreased S/(S + O) atomic ratio, not only in the surface region but also the interface region between the ZnS(O,OH) and CIGS layers, implying the shift to an adequate conduction-band offset at the ZnS(O,OH)/CIGS interface. Copyright © 2013 John Wiley & Sons, Ltd.

The J–V characteristics of MOCVD-ZnO:B/ZnS(O,OH)/CIGS solar cells before and after one-sun light soaking (LS) for 10 min and subsequent heat-light soaking (HLS) for 40 and 120 min. The total-area efficiency was improved from 8.5% to 11.3%, 16.7%, and 17.5% after 10-min LS, 40-min HLS, and 120-min HLS post-treatments, respectively. The aim of this work is to investigate the effects of post-treatments such as post-annealing, LS and HLS before and after the completion of cell fabrication on the cell performances and film properties.

Time-resolved photoluminescence (TRPL) measurements are one of the key metrics available to determine the minority-carrier lifetime in the absorber layer of direct band gap photovoltaic devices. Direct measurement of the minority-carrier lifetime is essential to understanding the impact of changes in deposition and processing on material quality. Unfortunately, the TRPL signal is determined by a complex convolution of multiple physical factors including bulk carrier lifetime, interface recombination velocity, electric field, doping density, photo-excited carrier density, and carrier mobility. To gain clarity, we have used numerical simulations to analyze the carrier dynamics after a CdTe device is illuminated with a short light pulse. After the light pulse, the photo-generated carriers undergo complex dynamics including drift, diffusion, interface, and bulk recombination. In this work, we develop a new formalism that enables much greater insight into which factors dominate the TRPL decay dynamics. By breaking down the carrier dynamics into drift, diffusion, and recombination terms, we have developed six-factor, four-factor, and two-factor analyses that provide clear understanding of which physical factors dominate the decay dynamics under various conditions and at different times during the decay. We show that in a typical CdTe device under the typical experimental conditions used in our laboratories, the faster part of the decay is dominated by charge separation, whereas the slower part is dominated by carrier recombination. Therefore, under the conditions investigated in this study, the slower part of the decay is a better parameter to explain the defect density in the CdTe layer. Copyright © 2013 John Wiley & Sons, Ltd.

We develop a new formalism that enables insight into the carrier dynamics during the TRPL decay. By breaking down the carrier dynamics into drift, diffusion, and recombination terms, this analysis provides clear understanding of which physical factors dominate the decay dynamics under various conditions and at different times during the decay. We show that in a typical CdTe device, the faster part of the decay is dominated by charge separation, whereas the slower part is dominated by carrier recombination.

Photovoltaic performance of cross-linkable Ru(2,2′-bipyridine-4,4′-bicarboxylic acid)(4,4′-bis((4-vinyl benzyloxy)methyl)-2,2′-bipyridine)(NCS)₂ (denoted as RuS dye) adsorbing on TiO₂ mesoporous film was enhanced by polymerizing with either ionic liquid monomer, 1-(2-acryloyloxy-ethyl)-3-methyl-imidazol-1-ium iodide (AMImI), to form RuS-cross-AMImI or di-functional acrylic monomer with ether linkage, triethyleneglycodimethacrylate (TGDMA), to form RuS-cross-TGDMA. Their cross-linking properties were investigated by UV–vis spectroscopy by rinsing with 0.1 N NaOH aqueous solution. The power conversion efficiencies (PCEs) of dye-sensitized solar cells (DSSCs) with RuS-cross-AMImI and RuS-cross-TGDMA both reached over 8% under standard global air mass 1.5 full sunlight. The increased PCE for DSSCs with RuS-cross-AMImI comparing with cross-linked RuS was attributed to the I⁻ counterion of AMImI increasing the charge regeneration rate of RuS dye, whereas that with RuS-cross-TGDMA was attributed to the Li⁺ coordination property of TGDMA. The photovoltaic performance of RuS-cross-TGDMA was also slightly better than that of RuS-cross-AMImI because of higher open-circuit photovoltage (V_oc) and short-circuit photocurrent (J_sc). Its higher V_oc was supported by the Bode plot of impedance under illumination and Nyquist plots at dark, whereas higher J_sc was supported by the incident monochromatic photon-to-current conversion efficiency spectra and charge extraction experiments. Copyright © 2013 John Wiley & Sons, Ltd.

RuS dye cross-linking with TGDMA on the TiO₂ mesoporous layer can retain up to 93% after being rinsed with 0.1 N NaOH aqueous solution.

A photovoltaic (PV) hybrid system combines PV with other forms of electricity generation, usually a diesel generator. The system presented in this paper uses concentration photovoltaic (CPV) as the main generator in combination with a storage system and the grid, configured as the backup power supply. The load of the system consists of an air conditioning system of an office building. This paper presents the results obtained from the first months of operation of the CPV hybrid system installed at Instituto de Sistemas Fotovoltaicos de Concentración facilities together with exhaustive simulations in order to model the system behaviour and be able to improve the self-consumption ratio. This system represents a first approach to the use of a CPV in office buildings complemented by an existing AC-coupled hybrid system. The contribution of this paper to the analysis of this new system and the existing tools available for its simulation, at least a part of it, can be considered as a starting point for the development of these kinds of systems. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents the results obtained from the first months of operation of the concentration photovoltaic hybrid system installed at Instituto de Sistemas Fotovoltaicos de Concentración's facilities, together with exhaustive simulations. It is a first approach to the use of a concentration photovoltaic in office buildings. The contribution of this paper to the analysis of this new system and the existing tools available for its simulation, at least a part of it, can be considered as a starting point for the development of these kinds of systems.

The performance of photovoltaic (PV) modules is generally rated under standard test conditions (STC). However, the performance of thin-film photovoltaic modules is not unique even under STC, because of the “metastability”. The effects of the light soaking and thermal annealing shall be incorporated into an appropriate energy rating standard. In this study, the change in I–V characteristics of thin-film PV modules caused by the metastability was examined by repeated indoor measurements in addition to round-robin outdoor measurements. The investigated thin-film modules were copper indium gallium (di)selenide (CIGS), a-Si : H, and a-Si : H/µc-Si : H (tandem) modules. The increase in the performance of the CIGS module between the initial and final indoor measurements was approximately 8%. Because of light-induced degradation, the indoor performance of the a-Si : H and a-Si : H/µc-Si : H modules decreased by approximately 35% and 20%, respectively. The performance was improved by about 4–6% under high temperature conditions after the initial degradation. The results suggest that the performance of thin-film silicon modules can seasonally vary by approximately 4–6% only due to thermal annealing and light soaking effects. The effect of solar spectrum enhanced the outdoor performance of the a-Si : H module by about 10% under low air mass conditions, although that of the a-Si : H/µc-Si : H modules showed a little increase. The currents of these a-Si : H/µc-Si : H modules may be limited by the bottom cells. Therefore, it is required to optimize the effect of solar spectrum in addition to the effects of light soaking and thermal annealing, in order to achieve the best performance for thin-film silicon tandem modules. Copyright © 2013 John Wiley & Sons, Ltd.

Changes in I-V characteristics of thin-film PV modules caused by the metastability were examined by repeated indoor measurements in addition to outdoor measurements. The increase in the performance of a CIGS module between the initial and final indoor measurements was approximately 8%. Due to light-induced initial degradation, the indoor performance of a-Si:H and a-Si:H/µc-Si:H modules decreased by approximately 35% and 20%, respectively, and improved by 4% to 6% under high temperature conditions after the initial degradation.

A method to report photovoltaic (PV) system degradation rates without using irradiance data is demonstrated. First, a set of relative degradation rates are determined by comparing daily AC final yields from a group of PV systems relative to the average final yield of all the PV systems. Then, the difference between relative and absolute degradation rates is estimated using a Bayesian statistical analysis. This approach is verified by comparing to methods that utilize irradiance data. This approach is significant because PV systems are often deployed without irradiance sensors, so the analysis method described here may enable measurements of degradation using data that were previously thought to be unsuitable for degradation studies. Copyright © 2013 John Wiley & Sons, Ltd.

Degradation rates for photovoltaic systems are studied by a new method that does not require irradiance data. First, a set of relative degradation rates are determined by comparing daily AC final yields from a group of photovoltaic systems. Then, the difference between relative and absolute degradation rates is estimated using Bayesian statistical analysis.

Porous silicon plays an important role in the concept of wafer-equivalent epitaxial thin-film solar cells. Although porous silicon is beneficial in terms of long-wavelength optical confinement and gettering of metals, it could adversely affect the quality of the epitaxial silicon layer grown on top of it by introducing additional crystal defects such as stacking faults and dislocations. Furthermore, the epitaxial layer/porous silicon interface is highly recombinative because it has a large internal surface area that is not accessible for passivation. In this work, photoluminescence is used to extract the bulk lifetime of boron-doped (10¹⁶/cm³) epitaxial layers grown on reorganised porous silicon as well as on pristine mono-crystalline, Czochralski, p⁺ silicon. Surprisingly, the bulk lifetime of epitaxial layers on top of reorganised porous silicon is found to be higher (~100–115 µs) than that of layers on top of bare p⁺ substrate (32–50 µs). It is believed that proper surface closure prior to epitaxial growth and metal gettering effects of porous silicon play a role in ensuring a higher lifetime. Furthermore, the epitaxial layer/porous silicon interface was found to be ~250 times more recombinative than an epitaxial layer/p⁺ substrate interface (S ≅ 10³ cm/s). However, the inclusion of an epitaxially grown back surface field on top of the porous silicon effectively shields minority carriers from this highly recombinative interface. Copyright © 2013 John Wiley & Sons, Ltd.

Photoluminescence measurements combined with modelling are used to extract bulk lifetimes of epilayers and the effective recombination velocity at the epilayer-substrate interface. It is shown that lifetime can be higher in epilayers grown on reorganised porous silicon compared with that on pristine silicon because of the gettering property of porous silicon. Furthermore, although the interface recombination is excessive at the interface with exposed porous silicon, a back surface field can act as a very effective shield for minority carriers.

The routine availability of key component materials has been highlighted as a potential constraint to both extensive deployment and reduction in production costs of thin-film photovoltaic (PV) technologies. This paper examines the effect of material availability on the maximum potential growth of thin-film PV by 2050 using the case of tellurium (Te) in cadmium telluride (CdTe) PV, currently the dominating thin-film technology with the lowest manufacturing cost. The use of system dynamics (SD) modelling allows for a dynamic treatment of key Te supply features and prospects for reductions in PV demand via material efficiency improvements, as well as greater transparency and a better understanding of future recycling potential. The model's projections for maximum Te-constrained CdTe PV growth by 2050 are shown to be higher than a number of previous studies using static assumptions—suggesting that a dynamic treatment of the resource constraints for CdTe inherently improves the outlook for future deployment of this technology. In addition, the sensitivity analysis highlights certain complex correlations between the maximum potential CdTe growth by 2050 and the rated lifetime of PV modules as well as the reported size of global Te resources. The highest observed sensitivities are to the recovery rate of Te from copper anode slimes, the active layer thickness, the module efficiency and the utilisation rate of Te during manufacturing, all of which are highlighted as topics for further research. Copyright © 2013 John Wiley & Sons, Ltd.

The availability of tellurium (Te) has been highlighted as a significant constraint to the growth of CdTe PV. In this study, the use of system dynamics modelling allows for a transparent and dynamic treatment of future Te supply to and demand from CdTe PV. The impact of several techno-economic parameters on maximum potential CdTe growth by 2050 is evaluated with the conclusion that further reductions in material intensity, and increased recovery rates of Te from copper anode slimes are needed to improve Te availability for CdTe PV.

This paper describes the results of an intercomparison of spectroradiometers for global and direct normal incidence irradiance in the visible and near-infrared spectral regions together with an assessment of the impact these results may have on the estimation of the short-circuit current (I_SC) calibration of photovoltaic devices and on the spectral mismatch calculation. The intercomparison was conducted in the framework of the European project Apollon with the additional participation of external partners from the Italian project for the long-term monitoring of solar radiation for photovoltaics. Six institutions and six spectroradiometer systems, representing different technologies and manufacturers, were involved. Prior to the intercomparison, all participating partners calibrated their own instrument(s) according to their usual procedures in order to verify the entire measuring and traceability chain. The difference in measured spectra shape and amplitude showed to have an impact on I_SC calculation of less than 3% and less than 6% for single-junction and multi-junction devices, respectively. When only the shape of the spectra is considered, the spectral mismatch ranges from 1.7% to 4.7% depending on the spectral response of the device. Copyright © 2013 John Wiley & Sons, Ltd.

An intercomparison of spectroradiometers for outdoor solar spectrum measurements is described, and results are presented. Systematic differences in measurement results are analyzed to assess the impact on photovoltaic device calibration. Short-circuit current calculation using spectra data set acquired by different instruments as well as spectra mismatch calculations are performed for single-junction, double-junction, and triple-junction devices.

The photovoltaic (PV) market is experiencing vigorous growth, whereas prices are dropping rapidly. This growth has in large part been possible through public support, deserved for its promise to produce electricity at a low cost to the environment. It is therefore important to monitor and minimize environmental impacts associated with PV technologies. In this work, we forecast the environmental performance of crystalline silicon technologies in 2020, the year in which electricity from PV is anticipated to be competitive with wholesale electricity costs all across Europe. Our forecasts are based on technological scenario development and a prospective life cycle assessment with a thorough uncertainty and sensitivity analysis. We estimate that the energy payback time at an in-plane irradiation of 1700 kWh/(m² year) of crystalline silicon modules can be reduced to below 0.5 years by 2020, which is less than half of the current energy payback time. Copyright © 2013 John Wiley & Sons, Ltd.

On the basis of technological scenario development and a prospective life cycle assessment, we forecast the environmental performance of crystalline silicon technologies in 2020, the year in which PV is anticipated to be competitive with wholesale electricity costs all across Europe. We estimate that the energy payback time at an in-plane irradiation of 1700 kWh/(m²*year) of crystalline silicon modules can be reduced to below 0.5 years by 2020, which is less than half of the current energy payback time.

Cu₂ZnSnSe₄ (CZTSe) thin film solar cells have been produced via co-evaporation followed by a high-temperature annealing. In order to reduce the decomposition of the CZTSe, a SnSe₂ capping layer has been evaporated onto the absorber prior to the high-temperature treatment. This eliminates the Sn losses due to SnSe evaporation. A solar cell efficiency of 5.1% could be achieved with this method. Moreover, the device does not suffer from high series resistance, and the dominant recombination pathway is situated in the absorber bulk. Finally, different illumination conditions (white light, red light, and yellow light) reveal a strong loss in fill factor if no carriers are generated in the CdS buffer layer. This effect, known as red-kink effect, has also been observed in the closely related Cu(In,Ga)Se₂ thin film solar cells. Copyright © 2013 John Wiley & Sons, Ltd.

CZTSe thin film solar cells have been produced via co-evaporation followed by a high-temperature annealing including a SnSe2 capping layer. A solar cell efficiency of 5.1% has been achieved. The device exhibits low series resistance down to 120K, dominant bulk recombination, and the device is studied under different illumination conditions where a red kink effect is observed.

We investigated the behavior of carrier populations generated at the interface of an n-Si wafer to an Si quantum dot (QD) array embedded in SiO₂ with a photon flux ranging from 1.24 to 2.48 eV (1000 to 500 nm). The optically assisted IV method was used with the Si wafer as hot carrier (HC) absorber and the Si QD array as energy selective contact (ESC). Charge carriers obtain excess energy from photons with energies significantly exceeding the band gap, resulting in an HC population. This reduces the bias field required to provide kinetic energy by field emission. The ESC can collect HCs at lower bias voltages. Tunneling resonances show the energy selective behavior under illumination at 80 K and room temperature (295 K). The data at 80 K can arguably be interpreted as an HC population in Si within the range of the ballistic mean free path from the QD array. We discovered a correlation between the energetic shift of the average hot hole temperature near the valence band edge and the energy of the photons impinging on the mesa structures. The optically assisted IV technique delivers a proof of principle for operation of an Si QD array as an ESC at room temperature, and furthermore for an HC solar cell with one ESC at 80 K. Copyright © 2013 John Wiley & Sons, Ltd.

Steady-state carrier populations at the interface of an n-Si wafer to an Si quantum dot array embedded in SiO₂ were characterized by optically assisted IV with photon energies from 1.24 to 2.48 eV (1000 to 500 nm). At 80 K, hot carrier populations in Si exist within the range of the ballistic mean free path from the quantum dot array, with a clear correlation between the average hot hole temperature and the photon energy. We deliver proof of principle for an energy selective contact at room temperature and a hot carrier solar cell with one energy selective contact at 80 K.

Cu(In,Ga)Se₂ (CIGS) thin films co-evaporated by 1-stage, 2-stage, and 3-stage processes have been studied by spectroscopic ellipsometry (SE). The disappearance of a Cu_2-xSe optical signature, detected by real time SE during multistage CIGS, has enabled precise endpoint control. Band gap energies determined by SE as depth averages show little process variation for fixed [Ga]/([In] + [Ga]) atomic ratio, whereas their broadening parameters decrease with increasing number of stages, identifying successive grain size enhancements. Refined SE analysis has revealed band gap profiling only for 3-stage CIGS. Solar cells incorporating these absorbers have yielded increased efficiencies in correlation with phase control, grain size, and band gap profiling. Copyright © 2013 John Wiley & Sons, Ltd.

Cu(In,Ga)Se₂ thin films co-evaporated by 1-stage, 2-stage, and 3-stage processes have been studied by spectroscopic ellipsometry (SE). The disappearance of a Cu_2-xSe optical signature, detected by real time SE during multistage CIGS, has enabled precise endpoint control. Band gap energies determined by SE as depth averages show little variation with process, whereas broadening parameters decrease with increasing number of stages. Refined SE analysis has revealed band gap profiling only for 3-stage CIGS. These results were correlated with solar cells parameters.

Thin-film silicon solar cells often rely on a metal back reflector separated from the silicon layers by a thin rear dielectric as a back reflector (BR) design. In this work, we aim to obtain a better insight into the influence of the rear-dielectric/Ag BR design on the optical performance of hydrogenated microcrystalline silicon (µc-Si:H) solar cells. To allow the application of a large variety of rear dielectrics combined with Ag BRs of diverse topographies, the solar cell is equipped with a local electrical contact scheme that enables the use of non-conductive rear dielectrics such as air or transparent liquids of various refractive indices n. With this approach, detached Ag BRs having the desire surface texture can be placed behind the same solar cell, yielding a direct and precise evaluation of their impact on the optical cell performance. The experiments show that both the external quantum efficiency and the device absorptance are improved with decreasing n and increasing roughness of the BR. Calculations of the angular intensity distribution of the scattered light in the µc-Si:H are presented. They allow for establishing a consistent picture of the light trapping in the solar cell. Copyright © 2013 John Wiley & Sons, Ltd.

The microcrystalline silicon (µc-Si:H) solar cell is equipped with a local electrical contact scheme that enables the use of non-conductive rear dielectrics, such as air or transparent liquids of various refractive indices n, combined with detached BRs with the desired surface topography. The quantum efficiency is improved by decreasing n and increasing roughness of the BR. Calculations of the angular intensity distribution of the scattered light in µc-Si:H allow us to establish a consistent picture of the light trapping.

Intermediate band solar cell provides novel alternative to multi-junction solar cell, but its efficiency is significantly degraded when spectral overlap exists between different absorption bands. Here, a scheme using non-uniform sub-bandgap state filling together with intermediate band transport is proposed to resolve the spectral overlap issue. On the basis of detailed balance calculation, spectrally decoupled devices using low–high state filling is shown to achieve 52.8% conversion efficiency when 4 eV spectral overlap is present between absorption coefficients of different bands, compared with baseline efficiency equal to 35.1% for conventional half-filled intermediate band devices. If a base material without intermediate band is added to the two section low–high state filling devices, the efficiency is further increased to 61.5%, which approaches efficiency of 63.2% for intermediate band devices with no spectral overlap and 63.8% for unconstrained triple-junction tandem cells. The junction thermalization loss associated with proposed new structures is shown to be equal to conventional half-filled intermediate band devices. Copyright © 2013 John Wiley & Sons, Ltd.

Spectral overlap between different absorption bands limits the practical efficiency achievable by intermediate band solar cells. By using non-uniform state filling, the spectral overlap can be effectively decoupled. This work shows that theoretical efficiency achieved by intermediate band solar cells with non-uniform state filling is close to ideal non-overlap efficiency limit.

Three novel heteroleptic amphiphilic polypyridyl Ru-complexes, coded MH08–10, with hetero-aromatic electron-donor ancillary ligands containing N-benzylcarbazole (MH08), dibenzofurane (MH09) and benzothiophene moieties (MH10) were synthesized to study the influence of different heterocyclic electron donors on the interrelationship of photophysical and electrochemical properties, and device performances for dye-sensitized solar cells (DSSCs). MH08 showed a remarkably high molar extinction coefficient of 27,650 M⁻¹cm⁻¹. MH08–TBA was synthesized from MH08 by converted one COOH group into −COO⁻⁺N(C₄H₉)₄ to investigate the effect of deprotonating one carboxylic group on the Fermi level and electron injection. When compared under the same experimental device conditions using 0.3M t-butylpyridine (TBP), the short-circuit photocurrent density (J_SC) and total conversion efficiency (%η) of MH08–10 were MH08>MH09>MH10. The differences in %η and J_SC of MH08–10 were ascribed to the conjugation length coupled with the electron donation and hole-transport strength of the ancillary ligands, which were in the following order N-benzylcarbazole>dibenzofurane>benzothiophene. Moreover, MH08–TBA showed J_SC of 19.56 mAcm⁻² and %η of 9.76% compared to 17.16 mAcm⁻² and 9.12% of the benchmark dye N719. The superior performance of MH08–TBA was attributed to its better light harvesting and enhanced incident-photon-to-current efficiency (IPCE) conversion. DFT/TD-DFT calculations utilizing the energy functional B3LYP and the full-electron basis set DGDZVP were performed to calculate HOMO and LUMO energies, vertical electronic excitations, lowest singlet-singlet electronic transitions (E_0-0), and excited state oxidation potentials. Excellent agreement was found between the experimental results and calculated data. Copyright © 2013 John Wiley & Sons, Ltd.

Three novel heteroleptic polypyridyl Ru(II)-complexes, coded MH08-10 were synthesized for DSSCs. When compared under the same experimental device conditions, the J_SC and %η of MH08-10 were MH08>MH09>MH10. The differences in %η and J_SC of MH08-10 were attributed to the conjugation length coupled with the electron donation and hole-transport strength of the ancillary ligands, which were in the following order MH08>MH09>MH10, and dye MH08-TBA outperformed the champion dye N719. Excellent agreement between the experimental photophysics results and TD-DFT were found.

There is a large gap between photovoltaic (PV) demand and PV supply in China. For a long time, more than 90% of the PV cells were exported to other countries, mainly to European and US region. But in 2012, US government decided to impose high tariffs on solar modules from China, then European Commission will announce their tariffs in 2013. In this context, we discussed the question on how the supply in China will be driven by demand in the following years by analyzing the driving forces for domestic demand in past 10 years, the structure and status of PV supply chain in China, and the future domestic demand to 2020. The point made by this study is that about half of current capacity in China will survive to next wave of strong demand. But the premise is that Chinese government should make more efforts to clear technical and nontechnical barriers for grid connection. Copyright © 2013 John Wiley & Sons, Ltd.

It is very serious in 2013 for Chinese manufacturers. About half of current capacity will disappear. As grid parity is achieved in other countries in Asia and Africa, the supply will grow gradually, but the ratio of supply to domestic demand would not exceed 2.

The principal objective of this study is to analyze the performance of sun-trackers devices compared with fixed flat plate systems using data from a widely used irradiation prediction software (PVGIS). We analyze typical parameters as daily and monthly sum of global irradiation (Hd and Hm) or average daily and monthly estimated electricity production (Eh and Em) and also their associated costs and land requirements (usually described by the ground cover ratio, GCR). It was observed that the influence of these two last parameters is quite important to the calculation of the payback time of the whole installation. As a final result, it is concluded that although the dual-axis tracker allows the maximum energetic performance, a 38% higher than a fixed system on average, if we take into account the GCR value and calculate the surface performance ratio, the most efficient configuration is the horizontal-axis solar tracker. Copyright © 2013 John Wiley & Sons, Ltd.

The performance of sun-trackers devices has been compared with fixed flat plate systems using data from Photovoltaic Geographical Information System. If we take into account the associated costs and land requirements (ground cover ratio), then the most efficient configuration is the horizontal-axis solar tracker.

Silicon nanowires (SiNWs) combined with a conducting polymer are studied to constitute a hybrid organic/inorganic solar cell. This type of cell shows a particularly high interfacial area between SiNWs and the polymer so that interfacial control and interface optimization are required. For that purpose, we terminated the SiNW surfaces with well selected functional groups (molecules) such as native oxide (hereinafter SiO₂-SiNW), hydrogen (hereinafter H-SiNW) and methyl (hereinafter CH₃-SiNW). A radial hetero-junction solar cell is formed, and the cell parameters with and without interface control by functionalization with molecules are compared. Electronically, the three surfaces were close to flat-band conditions. The CH₃-SiNW, H-SiNW and SiO₂-SiNW produced a surface dipole of −0.12, +0.07 and 0.2 eV and band bending of 50, 100 and 170 meV, respectively. The surface properties of functionalized SiNWs are investigated by photoelectron yield (PY) and photoemission spectroscopy. PY studies on functionalized SiNWs are presented for the first time, and our results show that this type of measurement is an excellent option to carry out interface optimization of NWs for envisaged nano-electronic and photonic applications. The solar cell efficiency is increased dramatically after terminating the surface with CH₃ molecules due to the decrease of the defect emission. The differently functionalized SiNW surfaces showed identical absorbance, reflectance and transmission so that a change in PY can be attributed to the Si–C bonds at the surface. This finding permits the design of new solar cell concepts. Copyright © 2013 John Wiley & Sons, Ltd.

Silicon nanowires (SiNWs) combined with a conducting polymer were studied to constitute a hybrid organic/inorganic solar cell. SiNW surfaces were terminated by functional groups such as native oxide, hydrogen and methyl and investigated by photoelectron yield and photoemission spectroscopy. A radial hetero-junction solar cell was formed, and cell parameters with and without interface functionalization with molecules were compared. The solar cell efficiency was increased after terminating the surface with CH₃ molecules due to a decrease in defect emission.

A multi-objective optimization was performed to allocate 2 MW of photovoltaic (PV) among four candidate sites on the island of Lanai, Hawaii, such that energy was maximized and variability in the form of ramp rates was minimized. This resulted in the Pareto-optimal set, an optimal solution set that provides a range of geographic allotment alternatives for fixed PV capacity. Within the Pareto-optimal set, a trade-off was found between energy produced and variability experienced, whereby a decrease in variability always necessitates a simultaneous decrease in energy. With this development, system designers have a method to select the preferred combination of energy generation and variability within the set of optimal alternatives to meet their needs. A design point within the optimal set was selected for study that decreased extreme ramp rates by more than 50% while decreasing annual energy generation by only 3% above the maximum generation allocation. To quantify the allotment mix selected, a new metric called the “ramp ratio” was developed. It compares ramping magnitude when all capacity is allotted to a single location to the aggregate ramping magnitude in a distributed scenario. The ramp ratio quantifies simultaneously how much more smoothing a distributed scenario would experience than single-site allotment and how much a single site is being underutilized for its ability to reduce aggregate variability. This paper creates a framework for use by cities and municipal utilities to reduce variability impacts while planning for high penetration of PV on the distribution grid, thereby maximizing the value of investments. Copyright © 2013 John Wiley & Sons, Ltd.

A multi-objective optimization was performed to allocate a fixed capacity of photovoltaics among candidate sites such that energy was maximized and variability in the form of ramp rates was minimized, resulting in an optimal solution set of geographic allotment alternatives. A trade off_ was found between energy produced and variability experienced, whereby a decrease in variability necessitates a simultaneous decrease in energy. A method is presented for system designers to select the preferred combination of energy generation and variability to meet their needs.

Earth abundant kesterite solar cells have achieved 7–10% cell efficiency mostly by processes that separate the film deposition and the annealing into two sequential steps. In contrast, co-evaporation onto a high-temperature substrate, demonstrating previous success in chalcopyrite (Cu(In,Ga)Se₂) solar cells, allows real-time composition control. Chalcopyrite research widely supports the model that Cu-rich growth conditions assist grain growth, and subsequently, the endpoint composition can be adjusted back to Cu-poor via monitoring the surface emissivity of the film. On the basis of the same intentions, the recent development of co-evaporated kesterite (Cu₂ZnSnSe₄) adapts the concept and achieves 9.2% efficiency. To understand the effect of growth strategies, this study examines the phase evolution, grain morphology, and device performance in Cu-rich growth and other strategies (Zn-rich and close-to-stoichiometric). By characterizing films obtained from interrupted depositions and also interpreting the variation in surface emission during growths, this study found a subtle hindrance in the reaction of Cu_xSe_y and ZnSe possibly caused by the volatile nature of SnSe_x. The hindrance explains why, distinctive from chalcopyrite, little difference in grain size is observed between kesterite films made by Cu-rich versus Zn-rich growth at these deposition rates. At last, a Zn-rich growth 9.1% device, certified by the National Renewable Energy Laboratory, is presented, which equals the performance of the previously-reported Cu-rich growth device. At the present stage, we believe the Cu-rich and Zn-rich growth share equal promise for the optimization of kesterite solar cells. Copyright © 2012 John Wiley & Sons, Ltd.

High-temperature co-evaporation is attractive for pursuing high copper zinc tin chalcogenide cell efficiency, as it affords the real-time manipulation of elemental deposition rates and their simultaneous reaction to form the final phase during film formation. Such deposition conditions of simultaneous high temperature and high vacuum encountered in the single-step process result in different growth mechanisms, bring up different issues, and also open new possibilities to fabricate absorbers for efficient solar cells. This study explores depositions starting from different initial compositions and examines the phase evolution, grain morphology, and device performance of resulting copper zinc tin selenide films.

Technically effective and economically efficient voltage control is a major issue in distribution systems with high amounts of installed capacity from dispersed generators, such as photovoltaic. In this paper, the results of an encompassing cost–benefit analysis for different voltage control strategies are presented. The investigated voltage control strategies comprise two different reactive power control methods and one combined reactive power/active power control method, each applied by inverters of utility scale photovoltaic systems. The results are gained by performing 12-month root-mean-square simulations with a 1 min resolution, using the model of a real distribution grid as well as complex generation and load models. The simulations show that local reactive power provision methods as well as temporal active power output curtailment methods are capable of reducing the necessity of a voltage-driven grid reinforcement. However, the economic benefit of those voltage control strategies highly depends on the parameterization of the respective control algorithm. Copyright © 2013 John Wiley & Sons, Ltd.

The cost–benefit analysis shows that voltage-dependent active and reactive power control strategies of photovoltaic inverters can be used to increase the hosting capacity of a medium voltage grid on short notice and hence to save grid reinforcement costs for the distribution grid operator. The additional costs for the plant operators are highly dependent on the voltage control strategy applied.

For crystalline silicon solar cells, the efficient collection of light at wavelengths in the infrared is a challenge because of long absorption lengths. Especially for thinner wafers, an efficient light-trapping scheme, such as the patch texture, is required for high short-circuit current densities. We have measured the light-trapping properties of patch textures produced by laser assisted texturing (LAST) on polished ⟨100⟩silicon wafers, and compared them with ray-tracing simulations. Single-sided random pyramid textures are created for comparison. Excellent agreement between simulations and measurements is achieved by employing diffuse scattering with a narrow angular distribution in the simulations, confirming the successful implementation of the process. We use our optical measurements of the textures for simulations of textures with rear reflectors, where we also investigate the influence on light-trapping properties when varying geometry and reflectance properties. The results from the optical simulations are imported into the solar cell simulation program PC1D. For a 50  μm-thick solar cell, we simulate an improvement in J_sc of up to 0.4  mA/cm² when going from single-sided random pyramid textures to patch textures, even when the performance of the texture is limited by process inaccuracies. Removing the physical inaccuracies of the laser system, the potential gain in J_sc on a 50  μm-thick cell with a patch texture covering the complete wafer surface is 0.8  mA/cm². We therefore conclude that the LAST method for creating patch textures is suitable to achieve an improved J_sc in thin monocrystalline silicon solar cells. Copyright © 2013 John Wiley & Sons, Ltd.

Light-trapping structures for monocrystalline silicon created by etching through laser openings in etch barrier are presented. Light-trapping potential exceeds that of random pyramid textures.

To decrease the cost of ownership of photovoltaic systems, less costly and more reliable photovoltaic inverters must be developed. Insulated gate bipolar transistors are a significant cause of inverter failures and system inefficiencies, so a thorough understanding of their strengths and weaknesses with regards to inverters is necessary. This paper summarizes the current state of experimentation surrounding the use of IGBTs in photovoltaic inverters and discusses their construction, use, lifetime, and reliability of IGBTs regularly used in photovoltaic inverters. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.

To decrease cost of ownership of photovoltaic systems, less costly and more reliable photovoltaic inverters must be developed. Insulated gate bipolar transistors (IGBTs) are a significant cause of inverter failures and system inefficiencies, so a thorough understanding of their strengths and weaknesses with regards to inverters is necessary. This paper summarizes the current state of experimentation surrounding the use of IGBTs in photovoltaic inverters and discusses the construction, use, lifetime, and reliability of IGBTs regularly used in photovoltaic inverters.

This paper presents the impact of non-homogeneous deposits of dust on the performance of a PV array. The observations have been made in a 2-MW PV park in the southeast region of Spain. The results are that inhomogeneous dust leads to more significant consequences than the mere short-circuit current reduction resulting from transmittance losses. In particular, when the affected PV modules are part of a string together with other cleaned (or less dusty) ones, operation voltage losses arise. These voltage losses can be several times larger than the short-circuit ones, leading to power losses that can be much larger than what measurements suggest when the PV modules are considered separately. Significant hot-spot phenomena can also arise leading to cells exhibiting temperature differences of more than 20 degrees and thus representing a threat to the PV modules' lifetime. Copyright © 2013 John Wiley & Sons, Ltd.

The impact of non-homogeneous deposits of dust on the performance of a PV array is presented. The results are that inhomogeneous dust leads to larger power losses when the affected PV modules are part of a string together with less dusty ones than when considered separately. Voltage losses can arise and be several times larger than the ones resulting from the transmittance decrease. Hot-spot phenomena with temperature differences of more than 20 degrees can also appear.

A high band-gap (~1.55 eV) chalcopyrite compound film (CuInGaS₂) was synthesized by a precursor solution-based coating method with an oxidation and a sulfurization heat treatment process. The film revealed two distinct morphologies: a densely packed bulk layer and a rough surface layer. We found that the rough surface is attributed to the formation of Ga deficient CuInGaS₂ crystallites. Because of the high band-gap optical property of the CuInGaS₂ absorber film, a solar cell device with this film showed a relatively high open circuit voltage (~787 mV) with a power conversion efficiency of 8.28% under standard irradiation conditions. Copyright © 2013 John Wiley & Sons, Ltd.

A high band-gap (~1.55 eV) CuInGaS₂ film is synthesized by a precursor solution-based coating method with an oxidation and a sulfurization. Because of the high band-gap optical property of the CuInGaS₂ absorber film, a solar cell device with this film shows a relatively high open circuit voltage (787 mV) with a power conversion efficiency of 8.28% under standard irradiation conditions.

In this paper, concentrating photovoltaic (CPV) systems coupled with various inverter configurations are modeled, compared, and tested. Because CPV systems use optics to concentrate sunlight onto highly efficient PV cells, the systems are affected not only by mismatches in the I–V characteristics among individual PV cells but also by the electro-optical mismatches of each concentrator. The best way to minimize power losses by these mismatches is having higher quality controls in aligning at the time of manufacturing and installation. To mitigate the power losses when mismatches are present, electrical components can be considered at the expense of additional cost. The developed models for central, string, and micro-inverters allow an accurate estimation of power losses in CPV systems and can be used to find an optimum solution for various power conversion schemes on the basis of the given mismatch conditions. Simulation results show that a CPV system with micro-inverters outperforms a CPV system with conventional inverters. Experimental test results under normal operation validate that power losses in a CPV system can be reduced by more than 5% by using the micro-inverter scheme. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents concentrating photovoltaic systems coupled with various inverter configurations including optical mismatch among concentrating units. The developed models for central, string, and micro-inverters allow an accurate estimation of power losses. Experimental test results under normal conditions validate the proposed power loss models.

High performance Cu₂ZnSnSe₄ (CZTSe) photovoltaic materials were synthesized by electrodeposition of metal stack precursors followed by selenization. A champion solar cell with 7.0% efficiency is demonstrated. This is the highest efficiency among all of the CZTSe solar cells prepared from electrodeposited metallic precursors reported to-date. Device parameters are discussed from the perspective of material microstructure and composition in order to improve performance. In addition, a high performance electrodeposited CZTS (S only) solar cell was demonstrated and its device characteristics were compared against the CZTSe (Se only) cell. Using secondary ion mass spectrometry for the analysis of the chemical composition of the absorber layer, a higher concentration of oxygen in the electrodeposited absorber is thought to be the root cause of the lower performance of the electrodeposited CZTS or CZTSe solar cells with respect to a solar cell fabricated by evaporation. The grain boundary areas of Sn-rich composition are thought to be responsible for the lower shunt resistance commonly observed in CZTSe devices. We measured the longest minority carrier lifetime of 18 ns among all reported kesterite devices. This work builds a good baseline for obtaining higher efficiency earth-abundant solar cells, while it highlights electrodepositon as a low cost and feasible method for earth-abundant thin film solar cell fabrication. Copyright © 2012 John Wiley & Sons, Ltd.

High performance Cu₂ZnSnSe₄ (CZTSe) photovoltaic materials were synthesized by electrodeposition of metal stack precursors followed by selenization. A champion CZTSe solar cell with 7.0% efficiency is demonstrated. This is the highest efficiency among all of the CZTSe solar cells prepared from electrodeposited metallic precursors reported to-date. In addition, a high performance electrodeposited CZTS (S only) solar cell was demonstrated and its device characteristics were compared against the CZTSe (Se only) cell.

Authenticity of conventional circuit model, to interpret the characteristics of polymer solar cells (PSCs) is examined. Conventional circuit model is found to be quite limited, and various assumptions used there are not valid for PSCs. By understanding the nature of photovoltaic characteristics, through detailed investigations, we developed an improved circuit model, which explains correctly the behavior of PSCs under different environmental conditions. Investigations are carried out on the solar cells, made of the blend of regioregular poly(3-hexylethiophene) (P3HT) and phenyl [6,6] C₆₁ butyric acid methyl ester (PCBM). The model is developed by treating both the dark and illuminated characteristics separately, even the characteristics were dealt with separately in reverse and forward biases. The formulated equivalent circuit model helps us in explaining many other important features, observed in the characteristics of PSCs. Copyright © 2013 John Wiley & Sons, Ltd.

The conventional circuit model is found to be quite limited, and various assumptions used there are not valid for polymer solar cells (PSCs). By understanding the nature of photovoltaic characteristics, through detailed investigations, we developed an improved circuit model, which explains correctly the behavior of PSCs under different test conditions. The formulated equivalent circuit model helps us in explaining many important features observed in the characteristics of PSCs.

Various measurements and experiments are performed to establish the mechanism of passivation on emitter and base of conventionally manufactured solar cell with p-type base. The surface coatings on the emitter are removed. The bare surface is then coated with silicon (Si) nanoparticles (NPs) with oxygen termination. It shows an increase in the cell efficiency up to 14% over bare surface of solar cell. The NPs show enhancement in light scattering from the surface, but shows an increase in the recombination lifetime indicating an improved passivation. When back contact is partially removed, the coating on bare back side ( p-type) of the solar cell also improves the cell efficiency. This is also attributable to the increased recombination lifetime from the measurements. Same NPs are seen to degrade the surface of n and p-type Si wafers. This apparently contradictory behaviour is explained by studying and comparing the emitter (n-type) surface of the solar cell with that of n-type Si wafer and the back surface ( p-type) with that of p-type Si wafer. The emitter surface is distinctly different from the n-type wafer because of the shallow p–n junction causing the surface depletion. Back surface has aluminium (Al) metal trace, which plays an important role in forming complexes with the oxygen-terminated Si NPs (Si–O NPs). With these studies, it is observed that increase in the efficiency can potentially reduce the thermal budget in solar cell preparation. Copyright © 2013 John Wiley & Sons, Ltd.

The silicon (Si) nanoparticles (NPs) with oxygen termination are shown to passivate the n-type emitter surface of Si solar cell. The same NPs do not passivate surface of n-type Si wafer. The figures show the density of state as obtained from scanning tunnelling spectroscopy measurements on emitter and wafer surfaces. The absence of gap states for emitter surface compared with n-type wafer surface is evident. The passivation is a result of proximity of p-n junction for the emitter surface.

This paper presents three key factors that cause system mismatches and power losses in high-concentration photovoltaic (HCPV) systems. The first factor is the I–V mismatch within a module, similar to the manufacturing mismatches in conventional photovoltaic modules. The second factor is the misalignments amongst modules, and the third factor is the tracking control. Unlike in the conventional photovoltaic systems, the second and the third factors in HCPV systems introduce larger electro-optical mismatches due to narrow acceptance angles. We have developed a model to address these three factors. It allows an accurate estimation of power losses in HCPV systems, which enabled us to propose configurations to reduce power losses without adding additional electrical components to the system. Simulation results show that the power harvest can be increased as much as 8.5% for a system using open-loop controls by simply increasing the number of strings at the time of calibration. Experimental test results are presented for validation. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents mismatches and power losses in high concentration photovoltaic systems. We developed a model based on three factors: mismatch with in a module, mismatch among modules, and tracking control. The proposed open-loop control increases power up to 8.5% without extract electrical components.

Photovoltaic (PV) systems incorporated with sun-tracking technology have been proposed and verified to effectively increase the power harvest. However, the actual power generated from a PV module has not been investigated and compared with that analyzed from theoretical models of the PV material. This study proposes a novel method for estimating the power benefit harvested by a two-axis sun-tracking type (STT) PV system. The method is based on semiconductor theory and the dynamic characteristics, including maximum power point tracking of PV modules that can be integrated with the database of annual solar incidences to predict the power harvested by any STT PV system. The increment of annual energy provided by an STT PV system installed at any arbitrary latitude, compared with that by a fixed-type system, can be accurately estimated using the proposed method. To verify the feasibility and precision performance of this method, a fixed-type and a two-axis STT PV system were installed at 24.92° north latitude in northern Taiwan and tested through long-term experiments. The experimental results show that the energy increments estimated by the theoretical model and actual measurement are 19.39% and 16.74%, respectively. The results demonstrate that the proposed method is capable of predicting the power benefit harvested by an STT PV system with high accuracy. Using our method, a PV system installer can evaluate beforehand the economic benefits of different types of PV systems while taking different construction locations into consideration, thereby obtaining a better installation strategy for PV systems. Copyright © 2012 John Wiley & Sons, Ltd.

This study proposes a novel method for estimating the power benefit harvested by the sun-tracking type (STT) photovoltaic (PV) system. The method is based on the semiconductor theory, and the dynamic characteristics of PV modules can be integrated with the solar incidence database to predict the energy harvested by an STT PV system. The annual energy increment provided by an STT PV system compared with that by a fixed-type PV system can be accurately estimated by the proposed method.

Back contacts for Si solar cells made by Al evaporation and screen printing Al paste were studied by transmission electron microscopy. Si was found to diffuse into the Al during heating. Si diffusion formed vacancies in the Si wafer and Al could then penetrate the Si wafer in spiked formations. The Al spikes retracted during cooling, leaving a doped back surface field region. Copyright © 2012 John Wiley & Sons, Ltd.

Back contacts made by Al evaporation and screen printing Al paste for Si solar cells were studied by TEM. Si was found to diffuse into Al during heating. Si diffusion made vacancies in the Si wafer. Al could then penetrate the Si wafer in spiked formations. Al spikes retreated during cooling, leaving a doped region (BSF).

The era of the seed-cast grown monocrystalline-based silicon ingots is coming. Mono-like, pseudomono or quasimono wafers are product labels that can be nowadays found in the market, as a critical innovation for the photovoltaic industry. They integrate some of the most favorable features of the conventional silicon substrates for solar cells, so far, such as the high solar cell efficiency offered by the monocrystalline Czochralski-Si (Cz-Si) wafers and the lower cost, high productivity and full square-shape that characterize the well-known multicrystalline casting growth method. Nevertheless, this innovative crystal growth approach still faces a number of mass scale problems that need to be resolved, in order to gain a deep, 100% reliable and worldwide market: (i) extended defects formation during the growth process; (ii) optimization of the seed recycling; and (iii) parts of the ingots giving low solar cells performance, which directly affect the production costs and yield of this approach. Therefore, this paper presents a series of casting crystal growth experiments and characterization studies from ingots, wafers and cells manufactured in an industrial approach, showing the main sources of crystal defect formation, impurity enrichment and potential consequences at solar cell level. The previously mentioned technological drawbacks are directly addressed, proposing industrial actions to pave the way of this new wafer technology to high efficiency solar cells. Copyright © 2012 John Wiley & Sons, Ltd.

The innovative seed-cast growth process can entail a turning point for the affected and fluctuating photovoltaic market. However, this approach still needs to be reviewed before moving to mass scale activities in a stable manner. In this paper, sources of defect generation, low performance ingot regions and the reutilization of the original seed (cost reduction) are discussed from the results of several industrial scale experiments using casting furnace stations. Some strategies to improve the process quality are also proposed.

We present a new method to characterize bifacial solar cells under standard test conditions (STC). The method considers the bifacial operation of the cell and provides the characteristics for simultaneous front and rear side illumination rather than providing the front and the rear side characteristics separately. The method involves measurements of front side electrical parameters (efficiency, open-circuit voltage, short-circuit current and fill factor) and rear side short-circuit current under STC. Two new parameters are introduced, namely bifacial 1.x efficiency (effective efficiency) and gain-efficiency product, which are calculated from the measured STC parameters. The former provides information related to the cell design considering the bifacial operation, whereas the latter provides the end-use benefits from the modules with bifacial cells for a particular installation. To calculate the bifacial 1.x efficiency and the gain-efficiency product, a one-diode solar cell equivalent circuit is used. Characteristic plots are shown for the newly introduced parameters as a function of rear-side illumination for various example solar cells. A sensitivity analysis is performed to understand the influence of each single-sided STC solar cell parameter on the newly introduced parameters. This sensitivity analysis shows that the fill factor and the rear-to-front current ratio are the most critical parameters for bifacial solar cells. Copyright © 2012 John Wiley & Sons, Ltd.

The method characterizes the bifacial solar cells considering the bifacial operation, i.e. simultaneous front and rear side illumination. The method involves measurements of front side electrical parameters and rear side short-circuit current under STC conditions. The new parameters, bifacial 1.x efficiency and gain-efficiency product provides information related to the cell design and the end-use benefits from the bifacial modules. The method shows that the fill factor and the rear-to-front current ratio are the most critical parameters for bifacial solar cells.

Single layers and combined layer systems with Cu(In,Ga)(S,Se)₂, ZnS-nanodot (nd) and In₂S₃ layers were investigated by surface photovoltage spectroscopy in the Kelvin-probe arrangement and compared with the open-circuit voltage (V_OC) of solar cells. The In₂S₃ and ZnS-nd layers were prepared by the spray ion layer gas reaction (ILGAR) technique from Indium chloride (InCl₃), Indium acetylacetonate (In(acac)₃) and Zinc acetylacetonate, respectively. The surface photovoltage signals of Cu(In,Ga)(S,Se)₂ were larger for the Cu(In,Ga)(S,Se)₂/ZnS-nd/In₂S₃ than for the Cu(In,Ga)(S,Se)₂/In₂S₃ layer system showing that a ZnS-nd layer additionally passivated the Cu(In,Ga)(S,Se)₂ surface. ILGAR In₂S₃ deposition from InCl₃ precursor solution led to a modification of surface defects of ZnS-nd and to generation of defect states below the band gap of Cu(In,Ga)(S,Se)₂, which has not been observed for deposition from Indium acetylacetonate precursor. Defect generation during ILGAR In₂S₃ deposition with InCl₃ precursor resulted in a lower V_OC of Cu(In,Ga)(S,Se)₂/ZnS-nd/In₂S₃/ZnO : Al solar cells. Copyright © 2012 John Wiley & Sons, Ltd.

ZnS nanodots deposited on Cu(In,Ga)(S,Se)₂ absorber and surface photovoltage (SPV) spectra of Mo / ZnS-nd / In₂S₃ for In₂S₃ deposited by spray ILGAR from chloride and acetylacetonate solutions demonstrating the decreased (increased) SPV signal related to defects below (absorption above) the bandgap of ZnS-nd / In²S³ (acac).

According to uncertainty calculations, the values recorded by means of commercial monitoring systems are expected to be less accurate than those recorded by a system optimized for the measurement of electrical parameters—the so-called dedicated system (DS). This study aims to verify if a larger expected uncertainty for commercial system (CS) actually turns into a larger spread of the measurements around the true value. In the Airport Bolzano Dolomiti plant, CS and DS are installed on the same photovoltaic arrays. The comparison performed considers the detailed uncertainty budget for the two systems using three performance indicators—energy, yield and performance ratio. Results show that the uncertainty level of the CS is much larger; for example, on performance ratio, it is about four times larger with respect to the optimized one (respectively ±16% and ±4%). Three sources mainly contribute to the uncertainty: measurements of irradiance, current and voltage. The measured values of the electrical parameter are compared in order to verify if the results of the budget calculations turn into a real difference. Results show that the CS is accurate in measuring current and voltage, respectively, ~2% and ~5% of difference from the DS, but not for irradiance—here, the difference is higher than 10%. In particular, the irradiance measured by the CS is systematically smaller; therefore, the performance ratio calculated through the CS is always overestimated and often larger than 100%. Copyright © 2012 John Wiley & Sons, Ltd.

A detailed uncertainty budget calculation was performed on commercial and dedicated monitoring systems for photovoltaic plant. The respective uncertainties on performance ratio are ±4% and ±16%. The values measured by the systems on crystalline silicon and amorphous silicon module arrays were compared over a 4-month period. The commercial system is accurate regarding current (<5%) and voltage (<2%), but it systematically underestimates irradiance (>10%), leading to an overestimation of performance ratio: +5% and +15%, respectively, for crystalline silicon and amorphous silicon arrays.

Methodical and intensive surface plasmon (SP) excitation trials were carried out on various dielectric-metal interfaces to optimize plasmonic photocurrent enhancements in organic P3HT-PCBM photovoltaic thin films. The SPs were optically excited via the diffraction grating method using single, crossed, and parallel grating schemes, with trials yielding optimal grating and film thickness parameters. Photocurrent enhancements up to 355% were demonstrated with TM-polarized incident light on single and parallel grating structures, while both TM and TE-polarized incident light enhancements were present on crossed grating structures. When compared with the photocurrent enhancements seen on single gratings, those seen on parallel gratings were comparable in magnitude but were shown over a broader optical band. This broadening of the optical band was due to the simultaneous SP excitations by the two superimposed gratings in the parallel scheme. Copyright © 2012 John Wiley & Sons, Ltd.

Surface plasmon excitation trials were carried out on various dielectric-metal interfaces in order to optimize plasmonic photocurrent enhancements in organic P3HT-PCBM photovoltaic thin films. The SPs were optically excited via the diffraction grating method using single, crossed and parallel grating schemes. Photocurrent enhancements up to 355% were demonstrated. When comparing the photocurrent enhancements seen on single and crossed gratings, those seen on parallel gratings were comparable in magnitude, but shown over a broader optical band.

Extremely low upper-limit effective surface recombination velocities (S_eff.max) of 5.6 and 7.4 cm/s, respectively, are obtained on ~1.5 Ω cm n-type and p-type silicon wafers, using silicon nitride (SiN_x) films dynamically deposited in an industrial inline plasma-enhanced chemical vapour deposition (PECVD) reactor. SiN_x films with optimised antireflective properties in air provide an excellent S_eff.max of 9.5 cm/s after high-temperature (>800 °C) industrial firing. Such low S_eff.max values were previously only attainable for SiN_x films deposited statically in laboratory reactors or after optimised annealing; however, in our case, the SiN_x films were dynamically deposited onto large-area c-Si wafers using a fully industrial reactor and provide excellent surface passivation results both in the as-deposited condition and after industrial-firing, which is a widely used process in the photovoltaic industry. Contactless corona-voltage measurements reveal that these SiN_x films contain a relatively high positive charge of (4–8) × 10¹² cm⁻² combined with a relatively low interface defect density of ~5 × 10¹¹ eV⁻¹ cm⁻². Copyright © 2012 John Wiley & Sons, Ltd.

Extremely low S_eff.max of 5.6 and 7.4 cm/s, respectively, are obtained on ~1.5 Ω cm n and p-type silicon wafers, using dynamically-deposited plasma silicon nitride (SiN_x) films. Such low S_eff.max values were previously only attainable for SiN_x films deposited statically in laboratory reactors or after optimised annealing; however, in our case the SiN_x films were dynamically deposited onto large-area c-Si wafers using a fully industrial reactor and provide excellent surface passivation results both in the as-deposited condition and after industrial-firing, which is a widely used process in the photovoltaic industry.

Analytical modeling of p-i-n solar cells constitutes a practical tool to extract material and device parameters from fits to experimental data, and to establish optimization criteria. This paper proposes a model for p-i-n solar cells based on a new approximation, which estimates the electric field taking into account interface potential drops at the intrinsic-to-doped interfaces. This leads to a closed-form current/voltage equation that shows very good agreement with device simulations, revealing that the inclusion of the interface potential drops constitutes a major correction to the classical uniform-field approach. Furthermore, the model is able to fit experimental current/voltage curves of efficient nanocrystalline Si and microcrystalline Si p-i-n solar cells under illumination and in the dark, obtaining material parameters such as mobility-lifetime product, built-in voltage, or surface recombination velocity. Copyright © 2012 John Wiley & Sons, Ltd.

A new current(J)-voltage(V) equation for p-i-n solar cells is obtained, containing drift-diffusion transport and interface potential drops at the limits of the intrinsic layer. It turns out that accounting for interface potential drops constitutes a major correction to the classical uniform-field approach. Furthermore, the model fits experimental current/voltage curves of efficient nc-Si and μc-Si p-i-n solar cells under illumination and in the dark, yielding material parameters such as mobility-lifetime product, built-in voltage or effective surface recombination velocity.

A dye-sensitized solar cell (DSC) made of nanoporous ZnO film on aluminum-doped zinc oxide (ZnO/AZO) transparent substrate has higher solar-to-electric energy conversion efficiency than a DSC consisting of nanoporous ZnO film deposited on conventional fluorine-doped tin oxide (ZnO/FTO) transparent substrate. The ZnO/AZO DSC gave an overall conversion efficiency of 7.2% whereas the ZnO/FTO yielded a conversion efficiency of 4.5%. The film-substrate orientation and higher light harvesting of the nanoporous ZnO film on the AZO after heating in air are mainly attributed to the higher energy conversion efficiency of the ZnO/AZO DSC. Copyright © 2012 John Wiley & Sons, Ltd.

In this work, AZO films used in ZnO-dye-sensitized solar cells (DSC) as transparent electrode have higher solar-to-electric energy conversion efficiency than conventional fluorine-doped tin oxide (FTO) films. The porous ZnO on both substrates after heating resulted in major changes in the crystallographic properties of the resultant ZnO films. The changes in the crystallographic properties in the ZnO films significantly affected the photovoltaic properties of the two DSCs. The ZnO/AZO DSC showed superior cell performance than ZnO/FTO DSC.

An accelerated irradiance and temperature cycle test (AITCT) has been developed as a method to evaluate the long-term performance stability of amorphous silicon (a-Si) photovoltaic (PV) devices. The AITCT simulates the daily light–dark cycle in 6 min (0.1 h). It also simulates the annual temperature cycle while controlling the temperature at 45 °C above the average monthly outdoor ambient temperature. This allows the influence of the day–night cycle and seasonal variation to be included in the acceleration factor for single-junction a-Si PV devices. The initial degradation and seasonal variation of performance of a-Si PV devices simulated by the AITCT agreed well with experimental results of 4-year outdoor exposure. Subsequent tests with the AITCT equivalent of 30-year outdoor exposure revealed that rapid degradation in the efficiency of a-Si PV devices would not occur by repeating the cyclic changes corresponding to seasonal variations following the initial degradation. The AITCT is able to accelerate further recovery in addition to light-induced degradation. Furthermore, the AITCT is applicable to other PV devices with light-intensity dependencies related to light-induced degradation as well as thermal recovery dependencies, such as multi-junction PV devices consisting of a-Si layers and other materials. This point will be discussed. Copyright © 2012 John Wiley & Sons, Ltd.

We have developed an accelerated irradiance and temperature cycle test to evaluate the long-term performance stability of amorphous silicon photovoltaic devices. The results of the accelerated irradiance and temperature cycle test agreed well with the experimental results of outdoor exposure, and subsequent simulation of 30-year outdoor exposure revealed that the I–V parameter of cells would not undergo rapid degradation. Data obtained by the accelerated irradiance and temperature cycle test will be useful in improving the estimation accuracy of electrical energy generated from thin-film photovoltaic power systems.

This paper reports temperature influence on radiation degradation of hydrogenated amorphous silicon (a-Si : H) solar cells. Degradation behaviors of a-Si : H solar cells irradiated with protons at 331 K are compared with that at 298 K (room temperature). Variations with time in the post-irradiation electrical properties are also investigated. It is found that the radiation degradation of the electrical properties at 331 K is significantly smaller than that at room temperature. Also, all the electrical properties (short-circuit current, open-circuit voltage, output maximum, and fill factor) recover with time after irradiation even at room temperature. The characteristic time of thermal annealing of short-circuit current is larger as the temperature is higher. These results indicate that temperature during irradiation and elapsed time from irradiation to measurement is an important parameter for radiation degradation of a-Si : H solar cells. Therefore, these parameters should be controlled in conducting the ground radiation tests. Copyright © 2012 John Wiley & Sons, Ltd.

Temperature influence on radiation degradation of a-Si:H solar cell is clarified using in-situ I-V measurement techniques. The radiation degradation is strongly affected by irradiation temperature and the thermal recovery of radiation degradation always occurs even during irradiation. Both the irradiation rate and the temperature should be controlled in conducting irradiation tests of a-Si:H solar cells for space use.

Research efforts have been directed toward photovoltaic technologies using “abundant” base metals such as copper and zinc (e.g., CZTS or more recently CZTSSe) to overcome the material constraint challenges posed by tellurium, indium, germanium and gallium in current generation technologies (e.g., CdTe, CIGS, a-Si/thin-film Si). These materials are limited in supply because they are minor byproducts of copper, zinc, lead and aluminum production and their economic production is inherently linked to that of the base metals. On the other hand, although the base metals currently are abundant, their reserves are not inexhaustible. In addition to resource availability, the main sustainability metrics for large scales of photovoltaics growth are low cost and minimum environmental impact. As photovoltaics installations grow to greatly displace traditional power generation infrastructures, recycling will play an increasingly important role in managing their end-of-life and relieving pressure on the prices of critical materials. Identifying potential issues of current technologies can help implement take-back or recycling program ahead of time. This work explores the material recycling potential of various commercial and under development photovoltaic technologies. It sheds light on a dimension of sustainability that has not been investigated before. On the basis of entropy analyses, documented by the experience of recycling electronic products, we show that recycling of some types of photovoltaic modules that are based on “abundant” materials could be burdened by complexity and lack of value, creating, therefore, concerns associated with both end-of life environmental impacts and resource availability. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.

This work explores the material recycling potential of various commercial and under development photovoltaic technologies. It sheds light on a dimension of sustainability that has not been investigated before. Based on entropy analyses, documented by the experience of recycling electronic products, we show that recycling of some types of PV modules that are based on “abundant” materials could be burdened by complexity and lack of value, creating, therefore, concerns associated with both end-of life environmental impacts and resource availability.

Semprius has a novel micro-cell based approach that addresses the cost, performance and reliability requirements of high concentration photovoltaic systems. A design that has a geometric concentration ratio of >1100 suns and three-junction 0.36 mm² micro-transfer printed cells is now complete. A module efficiency of 33.9% at a direct normal irradiance of 850 W/m² and cell temperature of 25°C has been independently validated for this design by the Instituto de Energia Solar at the Universidad Politecnica de Madrid. This is the highest measured module efficiency for any PV module, designed for commercial use. Several research, development and demonstration (RD&D) systems have been installed with these modules to collect early on-sun data and validate the technology. This paper presents module characterization and on-sun system results from a 3.5-kWp RD&D system installed at Instituto de Sistemas Fotovoltaicos de Concentracion, Puertollano, Spain by Semprius and Siemens. In addition, results from cleaning experiments and thermal performance of the system are presented from another RD&D system in Tucson. Comparisons of the performance of the Tucson RD&D system with co-located one-axis Si and fixed-tilt Si systems are also presented. Copyright © 2012 John Wiley & Sons, Ltd.

Semprius' novel microcell based high-concentration photovoltaic design has a concentration of >1100X and an externally validated efficiency of 33.9% at standard test condition. The design approach enables substrate reuse, short optical path, unique low-cost two-stage refractive optics, zero-cost thermal management, improved reliability and a highly scalable, massively parallel manufacturing process. Module characterization and on-sun results from a 3.5 kW research, development and demonstration system at the Instituto de Sistemas Fotovoltaicos de Concentracion, Spain are presented.

The surface microstructures of molybdenum (Mo) back contacts were shown to play a crucial role in the preferred orientations of Cu(In,Ga)Se₂ (CIGS) films. The lower surface density of Mo tends to drive the growth of CIGS films toward favoring a (220)/(204) orientation, attributed to the higher likelihood of a MoSe₂ reaction. This work showed that the presence of a very thin layer on a Mo bilayer facilitated the tuning of the CIGS grain orientations from strongly favoring (112) to strongly favoring (220)/(204) without sacrificing the electrode conductivity. The efficiency of Na-doped CIGS cells was increased toward decreasing Mo surface density, that is, increasing (220)/(204) CIGS orientation. Although slight changes in Na doping found between different Mo surface properties could contribute in part, the comparison with Na-reduced CIGS cells showed that it was more likely due to the (220)/(204) orientation-related enhancement of CdS/CIGS junction characteristics, which were possibly attributed to a favorable CdS reaction and a reduction in the defect metastabilities. Copyright © 2012 John Wiley & Sons, Ltd.

The preferred orientations of Cu(In,Ga)Se₂ (CIGS) films were strongly influenced by the surface density of Mo back contacts. Trilayer Mo back contact, consisting of very thin surface layer and a conventional bilayer, can provide the better controllability for the grain orientations of CIGS overlayer as well as its own good electrical and mechanical properties. The strong enhancement of CIGS grains toward (220)/(204) preferred orientation was demonstrated to considerably improve its photovoltaic conversion efficiency.

We have revealed that a new type of solar cells, intermediate-band-assisted hot-carrier solar cells using indirect-bandgap absorbers, can solve serious issues involved in intermediate-band solar cells (IB-SCs) and hot-carrier solar cells (HC-SCs). We have analyzed energy dissipation processes in these cells using the detailed balance models. In an HC-SC, energy dissipation related to entropy generation associated with hot-carrier extraction is a major issue when the thermalization rate of hot carriers is too rapid, although energy dissipation caused by the thermalization is sufficiently reduced. An IB-SC requires “photon selectivity,” where the spectral absorption ranges for the different interband transitions have no overlap with each other; otherwise, additional energy dissipation due to thermalization occurs. When hot carriers are extracted from an IB absorber, the feature that the current density is decreased because of the two-step excitation via the IB significantly reduces the entropy generation and consequently relaxes the requisite for the thermalization time. Even though the photon selectivity is broken, the increasing carrier energy in excess of each bandgap is efficiently converted to electricity by extracting hot carriers. The current density is further decreased using an indirect-bandgap absorber, leading to even higher efficiency. The limiting conversion efficiency of this new type with no photon selectivity has been calculated to be higher than those of IB-SCs with perfect selectivity by around 10%, assuming the thermalization time being 1 ns. This improvement is contrasting to the fact that the efficiency of HC-SCs is only slightly higher than the Shockley–Quisser limit at 1 sun. Copyright © 2012 John Wiley & Sons, Ltd.

We have revealed that a new type of solar cells, intermediate-band-assisted hot-carrier solar cells using indirect-bandgap absorbers, can solve serious issues involved in intermediate-band solar cells and hot-carrier solar cells. This type no longer requires “photon selectivity”, where the spectral absorption ranges for the different interband transitions have no overlap with each other and relaxes the requisite for the thermalization time of hot carriers.

The flattened light-scattering substrate (FLiSS) is formed by a combination of two materials with a high refractive index mismatch, and it has a flat surface. A specific realization of this concept is a flattened two-dimensional grating. When applied as a substrate for thin-film silicon solar cells in the nip configuration, it is capable to reflect light with a high fraction of diffused component. Furthermore, the FLiSS is an ideal substrate for growing high-quality microcrystalline silicon (µc-Si:H), used as bottom cell absorber layer in most of multijunction solar cell architectures. FLiSS is a three-dimensional structure; therefore, a full-wave analysis of the electromagnetic field is necessary for its optimal implementation. Using finite element method, different shapes, materials, and geometrical parameters were investigated to obtain an optimized FLiSS. The application of the optimized FLiSS in µc-Si:H single junction nip cell (1-µm-thick i-layer) resulted in a 27.4-mA/cm² implied photocurrent density. The absorptance of µc-Si:H absorber exceeded the theoretical Yablonovitch limit for wavelengths larger than 750 nm. Double and triple junction nip solar cells on optimal FLiSS and with thin absorber layers were simulated. Results were in line with state-of-the-art optical performance typical of solar cells with rough interfaces. After the optical optimization, a study of electrical performance was carried out by simulating current–voltage characteristics of nip solar cells on optimized FLiSS. Potential conversion efficiencies of 11.6%, 14.2%, and 16.0% for single, double, and triple junction solar cells with flat interfaces, respectively, were achieved. Copyright © 2012 John Wiley & Sons, Ltd.

Accurate optical modeling using finite element method of µc-Si:H-based solar cell on a FLiSS substrate has been achieved. The subsequent optimization of FLiSS configuration together with the usage of available and less absorptive materials has led to highly efficient µc-Si:H-based single junction thin-film silicon solar cell, characterized by a potential 11.6% conversion efficiency and photocurrent density higher than 27 mA/cm² in a 1-µm-thick absorber layer. Current-matched ultra-thin double and triple junction solar cells on the optimized FLiSS structure have been optoelectrically designed, showing potential 14% and 16% conversion efficiencies, respectively.

Tunnel junctions (TJ) made of p-Al_0.1 Ga_0.9As/n-GaAs are used because of their high peak current and low series resistance, but are not fully transparent. The influence of reducing the thickness of these tunnel junctions on the characteristics of InGaP/GaAs tandem cells was investigated. It was found that ultra-thin TJs with excellent performance can be realized. Even for a 7.5/6-nm thick TJ, which is the thinnest possible in our growth reactor, the peak current density is at least 600 A/cm². The series resistance of the TJs was found to be at a constant level of 0.6 ± 0.2 mΩ cm² for all total thicknesses of the TJ in the 13.5–40 nm range. Because of a lower absorption in the TJ, a tandem cell with a 7.5/6-nm thick TJ, compared with a cell with a 20/20-nm thick TJ, gained 0.53 ± 0.05 mA/cm² in short circuit current to a value of 14.8 mA/cm². Copyright © 2012 John Wiley & Sons, Ltd.

Photon absorption in the tunnel junction (TJ) of a III–V multijunction solar cell reduces the short circuit current. Therefore, in this paper, we have investigated the performance of ultra-thin Al_0.1 Ga_0.9As/GaAs TJs. An excellent peak current density of more than 600 A/cm² and a low series resistance of 0.6 mΩ cm² has been realized for a tandem cell with a thickness as low as 13.5-nm, which compared to a standard 40-nm thick TJ raises the short circuit current by 0.5 to 14.8 mA/cm².

In order to demonstrate that three-dimensional carbon nanotube-based photovoltaic devices show an increase in power output over similar planar cells, we have produced cells with texturing through the use of vertically aligned carbon nanotube arrays and cells without this texturing. The output power of these cells at varying incident angles of light was measured. The textured cells show an increase in the normalized power output compared with similar planar cells when the solar flux is at off-normal angles. The power output versus incident angle curve takes an inverted C-type curve as predicted by the theory developed previously, with very good agreement between experimental results and theoretical predictions. Copyright © 2012 John Wiley & Sons, Ltd.

In order to demonstrate that three-dimensional carbon nanotube-based photovoltaic devices show an increase in power output over similar planar cells, we have produced cells with texturing through the use of vertically aligned carbon nanotube arrays and cells without this texturing. The textured cells show an increase in the normalized power output compared with similar planar cells when the solar flux is at off-normal angles. The power output shows very good agreement between experimental results and theoretical predictions.

So-called “air mass functions” of photovoltaic modules are used to approximate the effects of spectral responsivity and to correct short-circuit current to or from a reference condition. These empirical functions are determined from outdoor measurements with test modules mounted on two-axis solar trackers and then calculated from plots of normalized calibration value (short-circuit current divided by total irradiance) versus optical air mass. Because they are incorporated into a number of photovoltaic system modeling and sizing software programs, the accuracy of the functions has direct implications for system costs. We discuss the assumptions associated with these functions that are generally not considered or ignored, and study their variability with respect to atmospheric constituents. The variability study included a 6-month outdoor measurement on a crystalline-Si module and a software simulation of the same module using a solar spectral irradiance model. We conclude that air mass functions depend on the measurement location and time, and therefore are not unique to a particular device. Also, using these functions introduces two distinct errors, the magnitudes of which are unknown without knowledge of spectral irradiance conditions. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.

Empirical polynomial expressions of module short-circuit current over total irradiance versus absolute optical air mass are known as “air mass functions”, and are used to approximate the effects of spectral responsivity in several photovoltaic system performance models. From 6 months of outdoor data and a spectral sensitivity model, this paper demonstrates that such functions are not fundamental properties of modules because they vary with time and location. Thus, these functions can actually increase error in system performance modeling.

ZnO nanowire (NW) grown on triple-junction (TJ) solar cells via the hydrothermal growth method to enhance efficiency is investigated. In this paper, experimental results indicate that TJ solar cells with ZnO NW as an antireflection (AR) coating have the lowest reflectance in the short wavelength spectrum, as compared with those of bare TJ solar cells (without AR coating) and solar cells with SiN_x and TiO₂/Al₂O₃ AR coatings. ZnO NW has the lowest light reflection among all experimental samples, especially in the range of ultraviolet to green light (300–500 nm). It was found that ZnO NW could enhance the conversion efficiency by 6.92%, as compared with the conventional TJ solar cell. In contrast, SiN_x and TiO₂/Al₂O₃ AR coatings could only enhance the conversion efficiency by 3.72% and 6.46% increase, respectively. The encapsulated results also suggested that the cell with ZnO NW coating could provide the best solar cell performances. Furthermore, all samples are measured at tilt angles of 0°–90° and results show that the solar cells with ZnO NW have the highest efficiency at all tilt angles. Furthermore, a small NW diameter increases light absorption. Copyright © 2012 John Wiley & Sons, Ltd.

In this research, it was found that ZnO nanowire (NW) enhances the conversion efficiency by 6.92%, as compared with the conventional triple-junction (TJ) solar cell. In contrast, SiN_x and TiO₂/Al₂O₃ antireflection coatings only enhance the conversion efficiency by 3.72% and 6.46% increase, respectively. Compared with the unencapsulated ZnO NW TJ cell, it was found that J_sc of the encapsulated TJ cell with ZnO NW increased by 0.41 mA/cm². The encapsulated results suggested that cell with ZnO NW could provide the best electrical performances.

The quality control of photovoltaic modules in terms of the output power to satisfy the technical specification is of great importance for producers as well as consumers and also represents a major issue of certification procedures. Previous work focused on one-sided specification limits to reject underperforming samples (lots) of photovoltaic modules or solar cells. In the present paper, we generalize the classic acceptance sampling methodology and derive sampling plans on the basis of two-sided specification limits. Those sampling plans can be constructed for arbitrary output power distributions by making use of flash data tables. For the out-of-spec setting, the sampling plans are solutions of rather involved nonlinear equations. Explicit formulas, which resemble known sampling plans, can only be obtained under symmetry assumptions. Further, the solution depends on the ratio of overperforming modules to underperforming modules. We investigate by numerical studies to which extent the required sample size depends on that ratio and the shape of the underlying output power distribution. The application to real examples indicates that in practice, the new approach often results in substantially smaller control samples than classic approaches. Copyright © 2012 John Wiley & Sons, Ltd.

The evaluation of solar cells and PV modules has to rely on random samples and their statistical analysis. In this article, the classic acceptance sampling methodology is generalized to the case of two-sided specification limits. Valid sampling plans are constructed for arbitrary output power distributions by making use of flash data tables. It turns out that the associated operating characteristic curve is parameterized by the ratio of underperforming and overperforming modules. We investigate by numerical simulation studies to which extent the required sample size depends on that ratio and on the shape of the power output distribution. A real data example indicates that the new approach often results in substantially smaller control samples than classic approaches.

There are a wide variety of solar thermal, photovoltaic, and hybrid photovoltaic–thermal concentrator systems and concepts that use liquids for optical adaptation and heat transfer. Depending on the functional purpose of the fluid: heat transfer, optical adaptation, spectral filtering, or a combination of these applications, different fluid properties are required. This review studies actively cooled solar concentrators and their associated requirements for liquids. The most suitable candidate fluids available in the market are assessed according to their properties and applications, with a special emphasis on fluid toxicity and long-term performance. Copyright © 2012 John Wiley & Sons, Ltd.

There exist a wide variety of solar thermal, photovoltaic, and hybrid photovoltaic-thermal concentrator systems and concepts that use liquids for optical adaptation and heat transfer. Depending on the functional purpose of the fluid, which can be used for heat transfer, optical adaptation, spectral filtering, or a combination of these applications, different fluid property requirements need to be met. This review studies these generic, actively cooled solar concentrators and their associated requirements for liquids. The most suitable fluid candidates available in the market are assessed according to their properties and applications, with a special emphasis on fluid toxicity and long-term performance.

Variability of solar power is a key driver in increasing the cost of integrating solar power into the electric grid because additional system resources are required to maintain the grid's reliability. In this study, we characterize the variability in power output of six photovoltaic plants in the USA and Canada with a total installed capacity of 195 MW (AC); it is based on minute-averaged data from each plant and the output from 390 inverters. We use a simple metric, “daily aggregate ramp rate” to quantify, categorize, and compare daily variability across these multiple sites. With this metric, the effect of geographic dispersion is observed, while controlling for climatic differences across the plants. Additionally, we characterized variability due to geographical dispersion by simulating a step by step increase of the plant size at the same location. We observed maximum ramp rates for 5, 21, 48, and 80 MW_AC plants, respectively, as 0.7, 0.58, 0.53, and 0.43 times the plant's capacity. Copyright © 2012 John Wiley & Sons, Ltd.

The results from this study show that relative power output fluctuations are reduced as a function of solar PV plants size.

An algorithm to calculate the current in the two-diode equivalent circuit of a solar cell is described and characterized in detail. It enables fitting measured current–voltage characteristics with hundreds of voltage points and six fit parameters at practically instantaneous speeds and can handle thousands of voltage points within a few seconds, without simplifications of the two-diode model. This performance enables routine two-diode model parameter extraction at in-line speeds, which may help to enhance cell characterization for module integration. The source code is publicly available. Copyright © 2012 John Wiley & Sons, Ltd.

An open-source algorithm to calculate the current in the two-diode equivalent circuit of a solar cell is described and characterized in detail. It enables fitting measured current-voltage characteristics with hundreds of voltage points and six fit parameters at practically instantaneous speeds and can handle thousands of voltage points within a few seconds, without simplifications of the two-diode model. This performance enables routine two-diode model parameter extraction at in-line speeds, which may help to enhance cell characterization for module integration.

Large-area multicrystalline silicon solar cells fabrication by laser doping is studied in this paper. The liquid dopant solution is sprayed onto the SiN_x:H film to act as dopant source. Laser doping is performed to locally melt silicon substrates, and phosphorus dopant atoms are incorporated into the liquid silicon by liquid-phase diffusion to form a selective emitter. Light-induced plating is carried out for front side metallization. The influences of laser energy density and pulse overlap on electrical performance of large-area multicrystalline silicon solar cells are obtained. The laser energy density and pulse overlap are optimized in consideration of sufficient built-in voltage and small-scale laser-induced damage. The typical spectral response for large-area multicrystalline silicon solar cells by laser doping is presented. The typical efficiency distribution for 1-day production of the 10 MW production line shows the overall average efficiency above 18% on large-area commercial-grade multicrystalline silicon substrates for the 4 months of operation, confirming the potential for transferring high-efficiency selective emitter silicon solar cells by laser doping into a production line. Copyright © 2012 John Wiley & Sons, Ltd.

Large-area multicrystalline silicon solar cells fabrication by laser doping is studied in this paper. The liquid dopant solution is sprayed onto the SiN_x:H film to act as dopant source. Laser doping is performed to locally melt silicon substrates, and phosphorus dopant atoms are incorporated into the liquid silicon by liquid-phase diffusion to form a selective emitter. Light-induced plating is carried out for front side metallization. The influences of laser energy density and pulse overlap on electrical performance of large-area multicrystalline silicon solar cells are obtained. The laser energy density and pulse overlap are optimized in consideration of sufficient built-in voltage and small-scale laser-induced damage. The typical spectral response for large-area multicrystalline silicon solar cells by laser doping is presented. The typical efficiency distribution for 1-day production of the 10 MW production line shows the overall average efficiency above 18% on large-area commercial-grade multicrystalline silicon substrates for the 4 months of operation, confirming the potential for transferring high-efficiency selective emitter silicon solar cells by laser doping into a production line.

We present the integrated development of back-contacted solar cells and an adequate module interconnection for a high-efficiency photovoltaic module. We report on a large area (125 × 125) mm² back-junction back-contact n-type solar cell metallized with aluminum having a total area conversion efficiency of 20.7%. To transfer the high conversion efficiency to the module, we use the laser welding process named aluminum-based mechanical and electrical laser interconnection for module integration of these back-contacted solar cells. We further analyze the impact of the busbars of our back-junction back-contact cells on the cell performance and report on optimization of the cell/module interface. The new cells have an adapted rear-side geometry by omitting the emitter busbar. A proof-of-concept module consisting of these cells is presented. Copyright © 2012 John Wiley & Sons, Ltd.

We present the integrated development of back-contacted solar cells and an adequate module interconnection for a high-efficiency photovoltaic module. The impact of the busbars of our back-junction back-contact cells on the cell performance is analyzed, and we report on optimization of the cell/module interface. As a first step towards modules with busbar-free cells, we fabricated cells with only one busbar and interconnect them in series.

In this study, we present a new light absorption enhancement method for p-i-n thin film silicon solar cells using pyramidal surface structures, larger than the wavelength of visible light. Calculations show a maximum possible current enhancement of 45% compared with cells on a flat substrate. We deposited amorphous silicon (a-Si) thin film solar cells directly onto periodically pyramidal-structured polycarbonate (PC) substrates, which show a significant increase (30%) in short-circuit current over reference cells deposited on flat glass substrates. The current of the cells on our pyramidal structures on PC is only slightly lower than that of cells on Asahi U-type TCO glass (Asahi Glass Co., Tokyo, Japan), but suffer from a somewhat lower open circuit voltage and fill factor. Because the used substrates have a locally flat surface area due to the fabrication process, we believe that the current enhancement in the cells on structured PC can be increased using larger or more closely spaced pyramids, which can have a smaller flat surface area. Copyright © 2012 John Wiley & Sons, Ltd.

A new light absorption enhancement method for p-i-n thin film silicon solar cells using micro-pyramidal structures, much larger than the wavelength of light, shows a significant increase (30%) in short-circuit current over reference cells deposited on flat glass substrates. Calculations show a maximum possible current enhancement of 45%, in spite of 30% locally flat area on the substrate. The cell on micro-structured PC shows an efficiency of 6.4%, which is only around 1% absolute lower than the cell on Asahi U-type glass substrate

Multistep hydrothermal (MSH) process is employed for growth of TiO₂ nanocorals onto the conducting fluorine-doped tin oxide-coated glass substrates. The surface morphological features and physical properties of TiO₂ films were investigated by field emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Fourier transform Raman spectroscopy, room temperature photoluminescence spectroscopy and X-ray photoelectron spectroscopy. The surface morphology revealed the formation of TiO₂ corals having nanosized (30–40 nm) polyps. The photoelectrochemical properties of the TiO₂ nanocoral electrodes were investigated in 0.1 M NaOH electrolyte under ultraviolet illumination. The results presented in this study highlight two major findings: (i) tuning the photoelectrochemical response and photoconversion efficiency via controlled thickness of TiO₂ nanocorals by MSH route and (ii) the substantial increase in short-circuit photocurrent (J_sc) because of the improved charge transport through TiO₂ nanocorals prepared via MSH process. Copyright © 2012 John Wiley & Sons, Ltd.

Multistep hydrothermal (MSH) processed nanocoral TiO₂ for efficient dye sensitized solar cells.

Hydrogenated amorphous silicon (a-Si : H) films are prepared by plasma-enhanced chemical vapor deposition (PECVD) with a triode electrode configuration in which a SiH₄–H₂ glow-discharge plasma is confined spatially away from the substrate. Although the deposition rate (0.1–0.5 Å/s) is lower than that of the conventional diode PECVD process (2.5 Å/s), the light-induced degradation in conversion efficiency (Δη/η_ini) of a single-junction solar cell is substantially reduced (e.g., Δη/η_ini ~ 10% at an absorber thickness of t_i = 250 nm), and efficiencies after light soaking (LS) maintain >9% for t_i = 180–390 nm. By applying the improved a-Si : H layers as top cell absorbers in a-Si : H/hydrogenated microcrystalline silicon (µc-Si : H) tandem solar cells, the light-induced degradation can be reduced further (e.g., Δη/η_ini ~ 5% at t_i = 250 nm). As a result, we obtain confirmed stabilized efficiencies of 9.6% (LS condition: 100 mW/cm², 50 °C, 1000 h) and 11.9% (LS condition: 125 mW/cm², 48 °C, 310 h) for a-Si : H single-junction and a-Si : H/µc-Si : H tandem solar cells, respectively. Copyright © 2012 John Wiley & Sons, Ltd.

Hydrogenated amorphous silicon (a-Si : H) films are grown by plasma-enhanced chemical vapor deposition with a triode electrode configuration in which a SiH₄ H₂ glow-discharge plasma is confined spatially away from the substrate. By this method, the light-induced degradation of solar cell efficiency is reduced substantially. The stabilized efficiencies of 9.6% and 11.9% are obtained for a-Si : H single-junction and micromorph tandem solar cells, respectively.

The temperature dependence of the I–V parameters of different III–V multi-junction concentrator cells at several concentration levels was investigated. Moreover, the influence of spectral changes on the temperature coefficients of multi-junction solar cells was examined. Complete sets of temperature coefficients of a metamorphic Ga_0.35In_0.65P/Ga_0.83In_0.17As dual-junction cell, a metamorphic Ga_0.35In_0.65P/Ga_0.83In_0.17As/Ge triple-junction cell and a lattice-matched Ga_0.50In_0.50P/Ga_0.99In_0.01As/Ge triple-junction cell determined under well-controlled laboratory conditions are reported. Copyright © 2012 John Wiley & Sons, Ltd.

The temperature dependence of the I–V parameters of different III–V multi-junction concentrator cells at several concentration levels was investigated. Complete sets of temperature coefficients of a metamorphic dual- and triple-junction cell as well as of a lattice matched triple-junction cell determined under well-controlled laboratory conditions are reported.

Dielectric scattering structures are a promising way of trapping light in solar cells. Titanium dioxide is a particularly attractive candidate material because of its high refractive index and ability to be deposited on a finished solar cell. Here, we present an experimental demonstration of photocurrent enhancement in thin film recrystallised silicon solar cells using TiO₂ pillar arrays fabricated on the rear of the cells using nanoimprint lithography. A short circuit current enhancement of 19% is measured experimentally, and excellent agreement with numerical simulations is obtained. We show numerically that by replacing the Ag capping present on the cells with a detached rear Ag back reflector, the enhancement could reach 37%. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, we present an experimental demonstration of photocurrent enhancement in thin film recrystallised silicon solar cells using TiO₂ pillar arrays fabricated on the rear of the cells using nanoimprint lithography. A short circuit current enhancement of 19% is measured experimentally, and excellent agreement with numerical simulations is obtained. We show numerically that by replacing the Ag capping present on the cells with a detached rear Ag back reflector, the enhancement could reach 37%.

We show the benefits of spatially resolved pseudo fill factor analysis on multicrystalline silicon solar cells. Hereby, we present a method based on quasi-steady-state photoluminescence-calibrated photoluminescence images at varying generation rate. We verify the method by a comparison with global and local Suns-V_oc measurements and apply Suns-PLI to multicrystalline heterojunction samples with and without conductive top layer, the latter being not accessible by Suns-V_oc. Thereby, we obtain detailed insight into the influence of injection-dependent local recombination on fill factor and of losses only due to recombination-driven lateral balancing currents. The conclusions are supported by Spice network simulations. Copyright © 2012 John Wiley & Sons, Ltd.

Calibrated injection-dependent photoluminescence imaging for spatially resolved pseudo fill factor analysis; Good agreement with global and local Suns-Voc measurements; Fundamental fill factor limitations due to recombination-driven lateral current flow could be investigated.

In this paper, we present a temperature-dependent network model of a concentrator photovoltaic (CPV) module. The ability of this network model to calculate different interconnection schemes within CPV modules is validated, and there is good agreement between the measured and calculated data. The model is used to quantify the influence of an inhomogeneous current and of a temperature distribution between the solar cells on the power output of a module. The different interconnection schemes that combine parallel and series connections are compared. The optimal interconnection scheme strongly depends on the variations in the short current densities and temperature differences between the solar cells as well as on the risk of ‘sudden death’ of individual solar cells. Optimal interconnection schemes for several scenarios are developed. A combination of parallel and series interconnections is found to be the most robust interconnection. Copyright © 2012 John Wiley & Sons, Ltd.

A validated temperature-dependent network model of a concentrator photovoltaic module with good agreement to measured data is presented. The model is used to quantify the influence of an inhomogeneous current and temperature distribution between the solar cells on module power output. Optimal interconnection schemes of concentrator photovoltaic modules strongly depend on variations of short current densities and temperature differences between the solar cells and on the risk of ‘sudden death’ of individual solar cells. Optimal interconnection schemes for several scenarios are developed.

Starting from the end of 2020, all new buildings will have to be Nearly Zero Energy Buildings (Nearly ZEBs—ED 2010/31/EU recast). This new ‘energy paradigm’ might be a revolution for architecture and for Photovoltaics (PV) too, but there are both cultural and technical obstacles to overcome. There is a need to re-think the way buildings are designed (integrating renewables for being ZE). There is a need to re-think the way PV is designed in buildings. PV will be gaining an increasing relevance in the ZEBs design, thanks to its features and potentialities (suitability for any kind of energy demand of the building, easiness of building integration, cost). In a ZEB scenario, PV is very suitable for generating energy, ‘on site’ and ‘at site’; this enlarges the perspective of use of PV from the architectural scale to a wider scale, including the space close to the building or even to the urban and landscape scale. In such a new context, the existing research on the relationships between PV and architecture, focusing mainly on the way the PV components are used in relation to the envelope (Building-integrated PV/Building-added (Attached) PV), is no longer sufficient. The authors envision possible formal results, opportunities and challenges, for the use of PV in ZEBs, as well as new research issues for the future relationships between PV and ZEBs from the architecture and landscape design point of view. Copyright © 2012 John Wiley & Sons, Ltd.

There is a need to rethink the way PV is used in relation to ZEBs and other buildings. With tall buildings, or in dense urban areas, wall and roof integration may not be sufficient to achieve a Net Zero Energy Balance. The PV system will, in numerous cases, have to extend beyond the building's footprint. The design challenge hence moves from the architectural to the landscape or even to the urban scale. This encourages new research in order to take up this challenge. Image A. Scognamiglio.

In this study, the optimum material parameters capable of providing high efficiencies close to the detailed balance limit are determined for intermediate band solar cells. A diffusion model, including the overlap effect between absorption coefficients, is used during the calculations for the first time. It is obtained that to achieve high efficiencies close to the detailed balance limit; the effective density of state value, N_CV, should be higher than 10¹⁷ cm⁻³ and the carrier mobility should be larger than 200 cm²/Vs, where the light concentration should not be higher than nearly 1000 sun. Besides, it is found out that the optimum intermediate band level and the base width depend on the mobility and effective density of state values. So they need to be optimized according to the material parameters. The effect of overlap between absorption coefficients on the performance of intermediate band solar cells is also investigated. Copyright © 2012 John Wiley & Sons, Ltd.

The optimum material parameters capable of providing high efficiencies close to the detailed balance limit are determined for intermediate band solar cells using a diffusion model, including the overlap effect between absorption coefficients. It is obtained that to achieve high efficiencies, the effective density of state value, N_CV, should be higher than 10¹⁷ cm⁻³ and the carrier mobility should be larger than 200 cm²/Vs, where the light concentration should not be higher than nearly 1000 sun.

Nanostructured crystalline silicon is promising for thin-silicon photovoltaic devices because of reduced material usage and wafer quality constraint. This paper presents the optical and photovoltaic characteristics of silicon nanohole (SiNH) arrays fabricated using polystyrene nanosphere lithography and reactive-ion etching (RIE) techniques for large-area processes. A post-RIE damage removal etching is subsequently introduced to mitigate the surface recombination issues and also suppress the surface reflection due to modifications in the nanohole sidewall profile, resulting in a 19% increase in the power conversion efficiency. We show that the damage removal etching treatment can effectively recover the carrier lifetime and dark current–voltage characteristics of SiNH solar cells to resemble the planar counterpart without RIE damages. Furthermore, the reflectance spectra exhibit broadband and omnidirectional anti-reflective properties, where an AM1.5 G spectrum-weighted reflectance achieves 4.7% for SiNH arrays. Finally, a three-dimensional optical modeling has also been established to investigate the dimension and wafer thickness dependence of light absorption. We conclude that the SiNH arrays reveal great potential for efficient light harvesting in thin-silicon photovoltaics with a 95% material reduction compared to a typical cell thickness of 200 µm. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents the optical and photovoltaic characteristics of silicon nanohole arrays fabricated using polystyrene nanosphere lithography and reactive-ion etching techniques for large-area processes. A damage removal etching is subsequently introduced to mitigate the surface recombination issues and also suppress the surface reflection due to modifications in the nanohole sidewall profile, resulting in a 19% increase in the power conversion efficiency. Silicon nanohole arrays reveal great potential for efficient light harvesting in thin-silicon photovoltaics.

A squaraine dye is tested for novel application in a near-infrared-active organic photovoltaic cell that is subsequently optimized to obtain a power conversion efficiency of 2.4 ± 0.3%. The optimization utilizes an Alq3 buffer layer and macroscopic structure control through the addition of co-solvents in the spin-casting process. Co-solvent addition increases the amount of aggregates present as measured through linear absorption spectroscopy, and there is a concurrent increase in both efficiency and short-circuit current. An interpretation of the greatly increased current density is presented that describes how increased J-aggregation likely increases hole mobility and, as a result, charge separation of the photogenerated excited state. Copyright © 2012 John Wiley & Sons, Ltd.

Macroscopic control of solution conditions prior to spin-coating allows for effective nanoscale control of donor morphology. This morphology has been optimized to give efficiency and short-circuit gains in a bulk heterojunction solar cell.

We present a measurement technique that we call ‘edge-light’, which allows a very sensitive detection of cracks in solar wafers and cells. For polycrystalline material, we found to be able to detect much smaller cracks than possible with conventional equipment. The technique is based on dark-field illumination with the illuminating light entering the wafer (or the cell) from the edges and being kept inside the wafer by total internal reflection. As detector, an infrared line-scan camera is used, giving high resolution images. In this configuration, even small cracks appear with high contrast, whereas the grain structure is not visible. In the same setup, photoluminescence or electroluminescence can be recorded in the same run in order to gain information on electronic properties. For the case of photoluminescence, only the scan line needs to be irradiated by the optical excitation source. Consequently, the necessary optical power can be lower and/or exposure times can be shorter. The combination of several measurement channels (crack detection, luminescence) with different light/excitation sources and a single camera is achieved by a special application software for an field programmable gate array (FPGA) frame grabber. The integration of the two measurements principles in a single setup allows simultaneous determination of mechanical stability and electronic quality properties. The setup was proven to work under inline conditions. Copyright © 2012 John Wiley & Sons, Ltd.

Edge-light is a combination of a very sensitive crack detection principle with luminescence measurements, usable for silicon wafers and cells. It is realized in a single setup with only one camera. All images are generated in a single pass in continuous motion.

A double-layer film, consisting of an upper layer of ZnO nanosheets and a lower layer of ZnO nanoparticles (designated as ZnONS/NP), was synthesized for the photoanode of a dye-sensitized solar cell (DSSC) by a one-step potentiostatic electrodeposition on a conducting fluorine-doped tin oxide substrate at 70 °C in a solution containing zinc nitrate and sodium acetate, followed by the pyrolysis of the film at 300 °C. The growth mechanism of the double-layer nanostructure was studied by monitoring the morphological changes at various periods of electrodeposition. The effects of the concentration of acetate anion on the morphology of the double-layer structure were also studied. The double-layer film of ZnONS/NP showed a better self-established light scattering property, compared with that of a thin film of ZnO nanoparticles, prepared without acetate anion. The concentration of an acetate anion in the electrolyte for the electrodeposition of the double-layer film, the electrodeposition period, and the period for dye adsorption were optimized for obtaining the best performance for a DSSC with a photoanode consisting of the double layer. A metal-free dye, coded as D149, was used in this research. A conversion efficiency of 4.65% was achieved for a DSSC (0.2376 cm²) with the photoanode, consisting of the double-layer film, under 100 mW/cm² illumination in the wavelength range of 400–800 nm. X-ray diffraction patterns, thermo gravimetric curves, elemental analysis, scanning electron microscopic images, transmission electron microscopic image, transmission spectra, and electrochemical impedance spectra were used to explain observations. Copyright © 2012 John Wiley & Sons, Ltd.

A novel double-layer thin film of ZnO with an underlayer of porous nanoparticles and an overlayer of nanosheets was synthesized on a conducting substrate by a simple one-step potentiostatic electrodeposition, followed by a pyrolysis treatment. After the growth mechanism of the double layer was confirmed, the film was applied for high-efficiency dye-sensitized solar cells to achieve an efficiency of 4.65%.

The crystal growth shape (CGS) and equilibrium crystal shape (ECS) of silicon in Si melt are observed using in situ observation. Fully faceted silicon CGSs, which are dominated by the {111} facets, are observed from the {112} and {110} orientation. Silicon CGS in three-dimensional in Si melt is octahedral in shape, bounded by {111} facets. Silicon ECSs in the melt are obtained by the relaxation from the CGSs and exhibit the {111} facets separated by curved interface. Copyright © 2012 John Wiley & Sons, Ltd.

The crystal growth shape (CGS) and equilibrium crystal shape (ECS) of silicon in Si melt are observed using in situ observation. Fully faceted silicon CGSs, which are dominated by the {111} facets, are observed from the {112} and {110} orientation. Silicon ECSs in the melt are obtained by the relaxation from the CGSs, exhibit the {111} facets separated by curved interface.

Polycrystalline silicon films of 10 μm thickness are formed on glass in a single-step continuous wave diode laser crystallisation process, creating large crystal grains up to 1 mm wide and 10 mm long. Solar cells are formed on the layers by employing a rear point contacting scheme. Intermediate layers between the glass and the silicon are shown to heavily influence the cell characteristics. A stack of silicon oxide/silicon nitride/silicon oxide has produced the best cell efficiency so far of 8.4 % with open-circuit voltage of 557 mV. With simple optimisation of the contacting scheme, 10 % efficient cells are expected in the near future. Copyright © 2012 John Wiley & Sons, Ltd.

Polycrystalline silicon films are formed on glass via diode laser crystallisation, creating large crystal grains up to 1 mm wide and 10 mm long. Solar cells are formed employing a rear point contacting scheme achieving cell efficiency of 8.4% with open-circuit voltage of 557 mV. Simple optimisation of the contacting scheme is expected to enable over 10% efficient cells in the near future.

An energy balance model for concentrating photovoltaic and thermal (CPVT) systems is presented. In the model, the CPVT system and its environment are represented using a set of input parameters. The main outputs of the model are the system's electrical and thermal efficiencies. The model accounts for optical losses. Thermal losses are derived from a thermal network model of the hybrid receiver. The solar cell performance is modeled as a function of the temperature and the irradiance. The robustness of the model is demonstrated by a sensitivity analysis of all input parameters.

The influence of the operating temperature on the electrical and thermal performances and the overall efficiency of the CPVT system are discussed. The limiting cases of maximum electrical and thermal power outputs are presented. Further, the influence of the concentration ratio on the electrical and thermal performance and on the partitioning of these two power outputs is analyzed in detail. It is shown that high concentration reduces the thermal losses considerably and increases the electrical efficiency. At concentration ratios above 300, the system operates with an overall efficiency of 75% at temperatures up to 160 °C. Copyright © 2012 John Wiley & Sons, Ltd.

An energy balance model for concentrating photovoltaic and thermal systems is presented and applied to investigate the influence of operating temperature and concentration ratio on the hybrid performance, that is, the electrical and thermal efficiencies. It is shown that high concentration offers several advantages to a hybrid photovoltaic and thermal system. As the concentration increases, the electrical efficiency also increases and, at the same time, thermal losses are reduced significantly, enabling overall conversion efficiencies of 75% at operating temperatures up to 160 °C.

Plasma treatment (PT) of the buffer layer for highly H₂-diluted hydrogenated amorphous silicon (a-Si:H) absorption layers is proposed as a technique to improve efficiency and mitigate light-induced degradation (LID) in a-Si:H thin film solar modules. The method was verified for a-Si:H single-junction and a-Si:H/microcrystalline silicon (µc-Si:H) tandem modules with a size of 200 × 200 mm² (aperture area of 382.5 cm²) under long-term light exposure. H₂ PT at the p/i interface was found to eliminate non-radiative recombination centers in the buffer layer, and plasma-enhanced chemical vapor deposition at low radio-frequency power was found to suppress the generation of defects during the growth of a-Si:H absorption layers on the treated buffer layers. With optimized H₂ PT of the a-Si:H single-junction module, the stabilized short circuit current and fill factor increased, and the stabilized open circuit voltage moves beyond its initial value. The results demonstrate 7.7% stabilized efficiency and 10.5% LID for the a-Si:H single-junction module and 10.82% stabilized efficiency and 7.76% LID for the a-Si:H/µc-Si:H tandem module. Thus, the growth of an a-Si:H absorption layer on a H₂ PT buffer layer can be considered as a practical method for producing high-performance Si thin film modules. Copyright © 2012 John Wiley & Sons, Ltd.

Plasma treatment of hydrogenated amorphous silicon (a-Si:H) absorption layers is proposed to improve efficiency and mitigate light-induced degradation in a-Si:H thin film solar modules. H₂ plasma treatment at the p/i interface eliminated non-radiative recombination centers in the buffer layer. Plasma-enhanced chemical vapor deposition at low radio frequency power suppressed the generation of new defects during the growth of a-Si:H absorption layers on the treated buffer layers. The proposed method is practical for producing high-performance Si thin film modules.

Most photovoltaic (PV)-modules have polymeric materials as encapsulants and back sheets. Usually these materials are not water vapour-tight and air-tight, but allow permeation of gases from the ambient atmosphere, especially oxygen and water vapour, which could contribute to degradation of the PV-modules.This work gives a short overview on how the permeation process through polymers takes place and how it is influenced by the material properties and ambient conditions. Results of permeation and diffusion measurements for water vapour as well as oxygen are shown for various encapsulation and back sheet materials with special focus on temperature dependence. The results demonstrate differences in permeability and temperature dependence for the investigated materials, but also show similarities for material groups. All investigated materials feature a Fickian behaviour within the measurement conditions even when crossing glass temperature or melting temperature. With these results, an Arrhenius-model was fitted to extrapolate permeation and diffusion values to a temperature range the PV-modules can be exposed to during lifetime. Copyright © 2012 John Wiley & Sons, Ltd.

This work gives a short overview how the permeation process through polymers takes place and how it is influenced by the material properties and ambient conditions. Results of permeation and diffusion measurements for water vapor as well as oxygen are shown for various encapsulation and back sheet materials with special focus on temperature dependency. The results not only demonstrate differences in permeability and temperature dependency for the investigated materials but also show similarities for material groups.

We model the electron photoemission from metal nanoparticles into a semiconductor in a Schottky diode with a conductive oxide electrode hosting the nanoparticles. We show that plasmonic effects in the nanoparticles lead to a substantial enhancement in photoemission compared with devices with continuous metal films. Optimally designed metal nanoparticles can provide an effective mechanism for the photon absorption in the infrared range below the semiconductor bandgap, resulting in the generation of a photocurrent in addition to the photocurrent from band-to-band absorption in a semiconductor. Such structure can form the dais of the development of plasmonic photoemission enhanced solar cells. Copyright © 2012 John Wiley & Sons, Ltd.

We model the electron photoemission from metal nanoparticles into in a Schottky diode. We show that plasmonic effects in the nanoparticles lead to a substantial enhancement in photoemission compared with devices with continuous metal films. Such structure can provide an effective mechanism for the generation of a photocurrentt in the infrared range below the semiconductor bandgap, resulting in the development of plasmonic photoemission enhanced solar cells.

Photovoltaic (PV) module efficiency and reliability are two factors that have an important impact on the final cost of the PV electricity production. It is widely accepted that a good adhesion between the encapsulant and the different substrates of a PV module is needed to ensure long-term reliability. Several testing procedures exist that use a metric derived from the force at interface failure to characterize the adhesion. It has, however, not been demonstrated that those metrics relate directly to the interfacial adhesion (defined as the surface energy density needed to break interfacial bonds), and the obtained results usually relate to an apparent adhesion strength. In this work, we describe a new design for compressive-shear testing of polymer layers bonded to rigid substrates. We use it to characterize real interfacial adhesion of ethylene-vinyl acetate (EVA) and polyvinyl-butyral (PVB) to a glass substrate before and after degradation in damp-heat. Our results show that a peak-force based metric is unable to capture the evolution of adhesion through degradation, and a new metric based on the elastic strain energy of the encapsulant is proposed. Moreover, we show that PVB adhesion to glass is much more affected by damp-heat exposure where polymer saturation takes place, in comparison with the adhesion of EVA to glass. The presented characterization protocol is a powerful tool that can help in assessing the reliability of an encapsulant facing specific degradation conditions. Copyright © 2012 John Wiley & Sons, Ltd.

A new compressive-shear approach to measure adhesion of photovoltaic encapsulants to glass is presented. Evolution of ethylene-vinyl acetate and polyvinyl-butyral adhesion to glass through damp-heat degradation is characterized. It is shown that an energy based metric is necessary to describe the evolution of adhesion, which is positive in the case of ethylene-vinyl acetate and negative in the case of polyvinyl-butyral.

Chemical analysis of individual atom columns was carried out to determine the crystal structure and local point defect chemistry of Cu₂ZnSnS₄. Direct evidence for a nanoscale composition inhomogeneity, in the form of Zn enrichment and Cu depletion, was obtained. The lateral size of the composition inhomogeneity was estimated to be between ~1.5 and 5 nm. Photoluminescence confirmed the presence of a broad donor–acceptor transition consistent with the observed cation disorder. Areas of relatively high concentration of Zn_Cu⁺ antisite atom donors locally increases the electrostatic potential and gives rise to band bending. Troughs in the conduction band and peaks in the valence band are ‘potential wells’ for electrons and holes, respectively. For a solar cell, these prevent minority carrier electrons from diffusing towards the edge of the space charge region, thereby reducing the carrier separation efficiency as well as reducing the carrier collection efficiency of majority carrier holes. Furthermore, electrons and holes ‘trapped’ within potential wells in close proximity have a high probability of recombining, so that the carrier lifetime is also reduced. High quality Cu₂ZnSnS₄ crystals free from composition inhomogeneities are therefore required for achieving high efficiency solar cell devices. Copyright © 2012 John Wiley & Sons, Ltd.

Aberration corrected scanning transmission electron microscopy is used to investigate Cu, Zn cation disorder in Cu₂ZnSnS₄ by measuring the composition of individual atom columns. A nanoscale composition inhomogeneity is observed with a high concentration of Zn_Cu⁺ donors. The spatially fluctuating electrostatic potential gives rise to potential wells in the electronic band structure. Deep potential wells reduce the carrier separation and collection efficiencies of solar cells. Chemically homogeneous Cu₂ZnSnS₄ material is therefore required for producing high-efficiency devices.

Antireflective light trapping glass nanostructures fabricated by a non-lithographic process are investigated for their angle dependent properties to improve the omnidirectional performance of solar modules. Optical transmission and solar cell module I-V measurements are used to understand the dependence of angular performance of nanostructures in the packaging glass. Nanostructures 100–400 nm in height demonstrate an increase in solar light transmission both for normal as well as oblique incidence and measurements show that a ~200-400 nm nanostructure height is optimum for solar modules, providing an absolute increase of 1% in the power conversion efficiency at normal incidence and a gain in short circuit current density over a 120° angular cone of solar incidence. This shows that packaging glass texturing can be an important and often-overlooked method to yield substantial gain in solar module efficiency. Copyright © 2012 John Wiley & Sons, Ltd.

The omnidirectional properties of nanostructured glasses have been investigated. It was found that glasses containing nanostructures of height 200–400 nm exhibited an absolute increase of ~ 3-4% in transmission as compared to a planar glass sample. A corresponding increase was also observed for the short circuit current density for solar modules with nanostructured glass samples as their packaging cover. The improvement in the performance of the nanostructured glass samples was observed over a 120° cone of solar reception.

In this paper, a study on the mismatch effect due to the use of different photovoltaic (PV) modules classes in large-scale solar parks is presented. For this purpose, a new model for simulating current–voltage and power–voltage characteristics is introduced. The model is then applied for calculating mismatch losses in a number of case studies for a PV plant built in Bari, southern Italy. First, in order to test the effectiveness of the model, this is applied to homogeneous strings and field showing that the mismatch losses are zero. Subsequently, the use of inhomogeneous strings (i.e. made of modules belonging to different power classes) is investigated. Finally, the behaviour of 1 MW_p homogeneous and inhomogeneous PV fields is investigated, again with a focus on the mismatch effect. The operational conditions have been introduced starting from the definition of European efficiency. The use of standard test conditions can in fact lead to gross approximations because mismatch losses depend, as well as, on PV module characteristics, electrical connections and electrical architecture, also on the location of the PV system. The results presented in this work can be used both by PV system designers for carrying out yield calculations, and by operation and maintenance personnel for substituting modules during operation without compromising the productivity of the plant. Copyright © 2012 John Wiley & Sons, Ltd.

A new model for simulating current–voltage and power–voltage characteristics is introduced for the study on the mismatch effect due to the use of different photovoltaic module classes in large-scale solar parks. The results presented in this work can be used both by photovoltaicPV system designers for carrying out yield calculations, and by operation and maintenance personnel for substituting modules during operation without compromising the productivity of the plant.

To improve CdS/CdTe cell/module efficiencies, CdS window layer thinning is commonly applied despite the risk of increased pin-hole defects and shunting. An alternative approach is to widen the band gap of the window layer (2.42 eV for CdS) via alloying, for example, by forming compositions of Cd_1−xZn_xS. In this study, the performance of Cd_1−xZn_xS/CdTe thin-film solar cells has been studied as a function of x (from x = 0 to 0.9), widening the window layer band gap up to and over 3.4 eV. Optimum Cd_1−xZn_xS compositions were clearly identified to be around x = 0.7, and limitations to the achievable photocurrent and conversion efficiencies have been addressed. Copyright © 2012 John Wiley & Sons, Ltd.

CdS window layer thinning is commonly applied to increase photocurrent in chalcogenide solar cells, despite the risk of increased shunting. An alternative approach, by forming compositions of Cd_1−xZn_xS, is shown to improve the blue response of CdTe solar cells. An optimum Zn content, found to be x ≈ 0.7, boosts the average conversion efficiency by 37%.

Concentrator photovoltaic (CPV) systems are one of the most promising technologies for future energy supply. Several studies reported the interest of using a Fresnel lens coupled with a secondary optical element in such a system. For high concentration factor, the optimization of the optical configuration plays a key role regarding electrical performances. On the other hand, the thermal management of the solar cell is also critical to ensure a better module efficiency. This paper presents a study of a ×1024 CPV system performances and a methodology for estimating the optical chain efficiency, the cell temperature impact and the alignment requirements. Module efficiencies were then measured as a function of the cell temperature and correlated to optical performances through current-tension characterizations under real solar illumination conditions and the estimation of the power density received by the solar cell. The system yield was up to 27% for a cell temperature around 30 °C, confirming that high concentration ratio should be of great interest in the near future. A 1D model was also developed in order to quantify the possible improvements of this CPV system. Using a solar cell with an efficiency of 36.7% at ×600, we then demonstrated that the ×1024 CPV system could reach up to 30% in standard test conditions. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents a study of ×1024 concentrator photovoltaic system performances and a methodology for estimating the optical chain efficiency, the cell temperature impact and the alignment requirements. Module efficiencies were measured as a function of the cell temperature and correlated to optical performances through current-tension characterizations under real solar illumination conditions. The system yield was up to 27% for a cell temperature around 30°C, confirming that high concentration ratio should be of great interest in the near future.

This contribution investigates the effect of seeding the growth of thin film microcrystalline silicon (µc-Si : H) deposited by radio frequency plasma-enhanced chemical vapor deposition on the material properties of µc-Si : H film and the device performance of p-i-n and n-i-p µc-Si : H solar cells. By means of Raman measurement, x-ray diffraction (XRD) and transmission electron microscopy (TEM), we investigate the structure of seeded µc-Si : H. In particular, the effect of seed layers on the crystallinity development is investigated. Measurements of the depth profile of the crystalline mass fraction using Raman spectroscopy show that seed layers lead to a more rapid and uniform crystallinity development in growth direction. The amorphous incubation layer is suppressed and crystallization begins directly from onset of film growth without evolving through the intermediate growth phases. From TEM analyses, we observe that crystal sizes are not affected by seed layers. Horizontal cracks are however observed to dominate the early growth of µc-Si : H in p-i-n solar cell and this is reduced upon seeding. For the n-i-p cells, these cracks are not affected by seeding. XRD results also indicate that the use of seed layers does not affect the crystal sizes but affects the direction of preferential orientation. Solar cell external parameters show that seeding of p-i-n solar cells leads mainly to increase in short-circuit current density, J_sc with a slight drop in open-circuit voltage, V_oc. For the n-i-p cells, a reverse effect is observed. In this case, the V_oc increases and the J_sc decreases. Copyright © 2012 John Wiley & Sons, Ltd.

The use of seed layer for the radio frequency plasma-enhanced chemical vapor deposition growth of thin film microcrystalline silicon facilitates crystal nucleation and eliminates the adverse effect of amorphous silicon incubation layer. This way, it is possible to grow microcrystalline silicon of uniform crystalline volume fraction directly from a crystalline growth phase without the growth evolving through the intermediate phases.

We have analysed and optimised a laser process for the sintering of the TiO₂ layers in dye solar cells (DSCs). Through a thermographic characterisation of the process, we show that it is possible to scale and process large areas uniformly (16 cm²). We fabricated DSCs with nanocrystalline (nc)-TiO₂ films sintered by using pulsed ultraviolet laser with an average output power P varying from 1 W to 7 W whilst mainting a constant power conversion efficiency η. The highest efficiency reached for a laser sintered DSC was 7%. The time required to sinter 1 m² of nc-TiO₂ film was found to decrease hyperbolically with P, which is important for determining process takt times. We quantified the embodied energy (EE) required to sinter 1 m² of the active TiO₂ layer for a variety of different processes, and found that the EE for the laser sintering process with a system wall plug efficiency of 3.5% to be competitive with the more conventional oven and belt furnace treatments. We outline the main features required from a laser system to carry out an efficient, energetically favourable and industrially applicable automated process with competitive throughput. Copyright © 2012 John Wiley & Sons, Ltd.

We have analysed and optimised laser sintering of nc-TiO2 layers for dye solar cells (best cell efficiency 7%) showing that large areas can be processed uniformly. The time required to sinter 1m2 of nc-TiO2 was found to decrease hyperbolically with laser output power. The embodied energy of the sintering process via efficient lasers was found to be competitive with conventional oven/furnaces. We outline the features required from a laser system to carry out an industrially applicable process with competitive throughput.

This paper focusses on the physical conditions for a degradation mechanism of photovoltaic modules, known as potential-induced degradation. The analysis was made on several levels. At first, the influence of humidity and temperature on the potential-induced leakage current has been investigated, the second step consists of an accelerated test scheme in a climatic chamber and the third one is outdoor exposure with high voltage stress in two different climate regions. The humidity has a huge impact on the leakage current. Therefore, a test in the climate chamber accelerates the stress found in the field of some orders of magnitude. Copyright © 2012 John Wiley & Sons, Ltd.

The impact of temperature, humidity, bias voltage, and contact to the ground on the leakage current was investigated. The temperature dependence obeyed an Arrhenius relation with an activation energy of 75 kJ/mol. The correlation with the bias voltage showed an ohmic behavior. The leakage current differed by orders of magnitude on the ground contact at the module glazing. First, results for the dependence on the ambient climate could be reported. A surprisingly high leakage current was found at rainy days.

Multi-junction solar cells offer extremely high power conversion efficiency with minimal semiconductor material usage, and hence are promising for large-scale electricity generation. However, suppressing optical reflection in the UV regime is particularly challenging due to the lack of adequate dielectric materials. In this work, bio-inspired antireflective structures are demonstrated on a monolithically grown Ga_0.5In_0.5P/In_0.01Ga_0.99As/Ge triple-junction solar cell, which overcome the limited optical response of reference devices. The fabricated device also exhibits omni-directional enhancement of photocurrent and power conversion efficiency, offering a viable solution to concentrated illumination with large angles of incidence. A comprehensive design scheme is further developed to tailor the reflectance spectrum for maximum photocurrent output of tandem cells. Copyright © 2012 John Wiley & Sons, Ltd.

Biologically inspired antireflective structures are incorporated into a monolithically grown In_0.5Ga_0.5P/In_0.01Ga_0.99As/Ge triple-junction solar cell using scalable polystyrene nanosphere lithography. The subwavelength structures exhibit remarkable antireflection in the UV, which is hardly attainable with common thin-film coatings. Consequently, the nanostructured device shows omni-directional enhancement of photocurrent and power conversion efficiency because of alleviated current matching. A comprehensive design scheme is also developed to tailor the reflectance spectrum for maximum photocurrent output of tandem cells.

Achieving the maximum power output from photovoltaic (PV) modules is indispensable for the operation of grid-connected PV power systems under varied atmospheric conditions. In recent years, the study of PV energy for different applications has attracted more and more attention because solar energy is clean and renewable. We propose an efficient direct-prediction method to enhance the utilization efficiency of thin film PV modules by tackling the problem of tracking time and overcoming the difficulty of calculation. The proposed method is based on the p–n junction recombination mechanism and can be applied to all kinds of PV modules. Its performance is not influenced by weather conditions such as illumination or temperature. The experimental results show that the proposed method provides high-accuracy estimation of the maximum power point (MPP) for thin film PV modules with an average error of 1.68% and 1.65% under various irradiation intensities and temperatures, respectively. The experimental results confirm that the proposed method can simply and accurately estimate the MPP for thin film PV modules under various irradiation intensities and temperatures. In future, the proposed method will be used to shed light on the optimization of the MPP tracking control model in PV systems. Copyright © 2012 John Wiley & Sons, Ltd.

The proposed method is based on the p–n junction recombination mechanism so that it can be applied to all kinds of photovoltaic modules and its performance is not influenced by weather conditions. The results indicate that the method can be used to simply and accurately estimate the maximum power point for thin film photovoltaic modules under various irradiation intensities and temperatures. Using the method will shed light on the optimization of the maximum power point tracking control model in photovoltaic system.

Hydrogenated amorphous silicon (a-Si:H) is conventionally deposited using static plasma-enhanced chemical vapor deposition (PECVD) processes. In this work, a very high frequency (VHF) dynamic deposition technique is presented, on the basis of linear plasma sources. This configuration deploys a simple reactor design and enables continuous deposition processes, leading to a high throughput. Hence, this technique may facilitate the use of flexible substrates. As a result, the production costs of thin-film silicon solar cells could be reduced significantly.

We found a suitable regime for the homogeneous deposition of a-Si:H layers for growth rates from 0.35–1.1 nm/s. The single layer properties as well as the performance of corresponding a-Si:H solar cells are investigated and compared with a state-of-the-art radio frequency (RF) PECVD regime. By analyzing the Fourier transform infrared spectroscopy spectra of single layers, we found an increasing hydrogen concentration with deposition rate for both techniques, which is in agreement with earlier findings. At a given growth rate, the hydrogen concentration was at the same level for intrinsic layers deposited by RF-PECVD and VHF-PECVD.

The initial efficiency of the corresponding p–i–n solar cells ranged from 9.6% at a deposition rate of 0.2 nm/s (RF regime) to 8.9% at 1.1 nm/s (VHF regime). After degradation, the solar cell efficiency stabilized between 7.8% and 5.9%, respectively. The solar cells incorporating intrinsic layers grown dynamically using the linear plasma sources and very high frequencies showed a higher stabilized efficiency and lower degradation loss than solar cells with intrinsic layers grown statically by RF-PECVD at the same deposition rate. Copyright © 2012 John Wiley & Sons, Ltd.

A dynamic very high frequency-plasma-enhanced chemical vapor deposition (PECVD) technique is presented that targets high deposition rates and good uniformity for the growth of hydrogenated amorphous and microcrystalline silicon absorber layers. In this work, we investigate the influence of the deposition rate of the a-Si:H absorber layer on the degradation behavior of the solar cells and compare the results with a state-of-the-art radio frequency-PECVD technique. It is found that solar cells with dynamically deposited intrinsic layers perform as well as solar cells with conventionally deposited intrinsic layers.

The presented study gives an integrated overview on the prospects of glow discharge (GD) methods in the chemical analysis of photovoltaic materials. With a focus on recent research and important photovoltaic (PV) materials, the GD coupled analytical methods, high resolution mass spectrometry (MS), time-of-flight-mass spectrometry (TOF-MS) and optical emission spectrometry (OES) are discussed. Each exemplary study carried out will point out the most suitable GD technique for the problem at hand, at the same time showing ways to increase analytical accuracy and to overcome typical instrumental restrictions. Challenging GD-MS analyses of thin and ultra thin films (down to 20 nm) as well as GD-MS and GD-OES studies of ready-to-use modules were carried out, showing the reader the application potential of GD methods in a PV development or production process. For the first time, novel cell concepts based on crystalline silicon on glass and silicon nanowires are analyzed by GD-OES, revealing precise chemical information on the devices. Copyright © 2012 John Wiley & Sons, Ltd.

With a focus on important photovoltaic (PV) materials, the glow discharge (GD) coupled analytical methods high resolution mass spectrometry (HR-MS), time-of-flight-mass spectrometry (TOF-MS) and optical emission spectrometry (OES) are discussed. Challenging HR-GD-MS analyses, ultra thin films as well as HR-GD-MS and GD-OES, studies of ready-to-use modules were carried out, showing the application potential of GD methods in a PV development process. For the first time, novel cell concepts based on crystalline silicon on glass and silicon nanowires are analyzed by GD-OES.

The influence of a retro-reflective texture cover on light in-coupling and light-trapping in thin film silicon solar cells is investigated. The texture cover is applied to the front glass of the cell and leads to a reflectance as low as r ≈ 3% by reducing the reflection at the air/glass interface and indirectly also reducing the reflections from the internal interfaces. For weakly absorbed light in the long wavelength range, the texture also enhances the light-trapping in the solar cell. We demonstrate an increase of the short circuit current density of exemplary investigated thin film silicon tandem solar cells by up to 0.95 mA cm⁻² and of the conversion efficiency by up to 0.74% (absolute). For a planar microcrystalline solar cell, the enhancement of light-trapping was determined from the reduced reflection in the long wavelength range to be up to 17%, leading to an increase of the external quantum efficiency of up to 12%. Copyright © 2012 John Wiley & Sons, Ltd.

A texture foil attached to the substrate glass of solar cells surpasses the maximum efficiency gains achievable by anti-reflective coatings. The photograph shows the substructure of the scattering foil on top of a reflecting silicon wafer. Besides the anti-reflex effect, the texture enhances the internal light-trapping.

The design of effective future promotion polices for photovoltaics (PV) needs the lessons learned from past experiences. The major objective of this study is to analyse the major PV markets over time to identify the effects caused by the two main promotion schemes: the achievement of economic profitability by means of Feed-in Tariffs (FiTs) and the use of the Willingness-to-Pay (WTP) in investment subsidies. For this purpose, indicators have been defined that characterise the promotion policies for grid-connected PV since the middle of the 1990s: (i) dissemination effectiveness, (ii) costs for the public, (iii) development of system prices over time, (iv) consumer's WTP and (v) profitability for the consumer.

The following are the major conclusions of this analysis. (i) If financial incentive programmes are implemented over a reasonable time frame, they work with respect to both significant price decreases as well as increases in quantities. (ii) FiT schemes and also investment subsidies and combined concepts are able to increase the market penetration and the diffusion of PV systems. They are especially relevant in the context of optimising the own use of PV electricity generated. (iii) Regarding the design of promotion systems, it is important that on the one hand, they consider customers WTP, and on the other hand, they include a well-defined dynamic component, which considers the effects of Technological Learning. In this context, capacity corridors, as were introduced in Germany, are essential. This tool allows predictable legislations and the correction of incentive payments without generating boom and bust cycles. Copyright © 2012 John Wiley & Sons, Ltd.

The major objective of this study is to analyze the major PV markets over time in order to identify the effects caused by the two main promotion schemes: the achievement of economic profitability by means of Feed-in Tariffs or the use of the Willingness-to-Pay in investment subsidies. With looming grid parity, a major challenge will be to link incentives for the effective own use of PV with market-based prices for feeding PV electricity into the grid.

To date, the majority of quality controls performed at PV plants are based on the measurement of a small sample of individual modules. Consequently, there is very little representative data on the real Standard Test Conditions (STC) power output values for PV generators. This paper presents the power output values for more than 1300 PV generators having a total installed power capacity of almost 15.3 MW. The values were obtained by the INGEPER-UPNA group, in collaboration with the IES-UPM, through a study to monitor the power output of a number of PV plants from 2006 to 2009. This work has made it possible to determine, amongst other things, the power dispersion that can be expected amongst generators made by different manufacturers, amongst generators made by the same manufacturer but comprising modules of different nameplate ratings and also amongst generators formed by modules with the same characteristics. The work also analyses the STC power output evolution over time in the course of this 4-year study. The values presented here could be considered to be representative of generators with fault-free modules. Copyright © 2012 John Wiley & Sons, Ltd.

To date, there is very little representative data on the real Standard Test Conditions (STC) power output values for PV generators. This paper presents the power output values for more than 1300 PV generators (15.3 MW in total). The study shows the power dispersion that can be expected amongst generators of the same manufacturer and amongst generators of different manufacturers. The work also analyses the STC power output evolution over time in the course of this 4-year study.

Cu₂ZnSnS₄ (CZTS) is a promising thin-film absorber material that presents some interesting challenges in fabrication when compared with Cu(In,Ga)Se₂. We introduce a two-step process for fabrication of CZTS films, involving reactive sputtering of a Cu-Zn-Sn-S precursor followed by rapid annealing. X-ray diffraction and Raman measurements of the sputtered precursor suggest that it is in a disordered, metastable CZTS phase, similar to the high-temperature cubic modification reported for CZTS. A few minutes of annealing at 550 °C are sufficient to produce crystalline CZTS films with grain sizes in the micrometer range. The first reported device using this approach has an AM1.5 efficiency of 4.6%, with J_sc and V_oc both appearing to be limited by interface recombination. Copyright © 2012 John Wiley & Sons, Ltd.

Cu₂ZnSnS₄ is a promising thin-film absorber material that presents some interesting challenges in fabrication when compared with Cu(In,Ga)Se₂. Here, we demonstrate the development of large Cu₂ZnSnS₄ grains during rapid annealing of reactively sputtered Cu-Zn-Sn-S precursor films. A metastable precursor phase is proposed to explain structural properties and the high rate of grain growth.

Several works report on seasonal fluctuations of power production of amorphous silicon (a-Si). These oscillations are due to two overlapping phenomena (i) spectral and (ii) the Staebler–Wronski effects. It is hence difficult to assess—for a given location and climatic conditions—which one has the largest impact. By means of a straightforward approach based on two sets of single-junction a-Si photovoltaic modules (stored indoors/exposed outdoors) and on two different I–V measurement set-ups (indoor and outdoor), we were able to separate the different contributions to this phenomenon. For the test-site of Lugano, seasonal oscillations account for performance variations of a-Si of ~10% (±5% around an annual average value with a minimum around the mid of January and a maximum around mid-July). The time-phase of the overall effect lies in between that of the two distinguished phenomena. (i) Spectral variations seem to have the highest impact on the outdoor performance of a-Si with an amplitude corresponding to 10.5% (± ~5.2%). Moreover, the influence of spectral variations on the outdoor performance of a-Si (and for comparison of c-Si) was modeled, and the experimental data were found to be in excellent agreement with the theoretical simulation; (ii) the Staebler–Wronski effect has a slightly lower influence with an amplitude of ~8% (±4% with a minimum at the middle of February and a maximum around mid-August). Because of the position (46°N) and average climatic conditions (southern Alpine climate) of Lugano, these observations are possibly representative of a large part of continental Europe. Copyright © 2012 John Wiley & Sons, Ltd.

Several works report on seasonal fluctuations of power production of amorphous silicon (a-Si). These oscillations are due to two overlapping phenomena (i) spectral and (ii) the Staebler–Wronski effects. In this work, we separate between the different contributions to this phenomenon. For the test-site of Lugano (46°N), seasonal oscillations account for performance variations of a-Si of ~10% (±5% around an annual average value with a minimum around the mid of January and a maximum around mid-July). The time-phase of the overall effect lies in between that of the two distinguished phenomena. (i) Spectral variations seem to have the highest impact on the outdoor performance of a-Si with an amplitude corresponding to 10.5% (± ~5.2%); (ii) the Staebler–Wronski effect has a slightly lower influence with an amplitude of ~8% (±4% with a minimum at the middle of February and a maximum around mid-August).

The influence of the thickness of atomic layer deposited Zn_1−xSn_xO_y buffer layers and the presence of an intrinsic ZnO layer on the performance of Cu(In,Ga)Se₂ solar cells are investigated. The amorphous Zn_1−xSn_xO_y layer, with a [Sn]/([Sn] + [Zn]) composition of approximately 0.18, forms a conformal and in-depth uniform layer with an optical band gap of 3.3 eV. The short circuit current for cells with a Zn_1−xSn_xO_y layer are found to be higher than the short circuit current for CdS buffer reference cells and thickness independent. On the contrary, both the open circuit voltage and the fill factor values obtained are lower than the references and are thickness dependent. A high conversion efficiency of 18.0%, which is comparable with CdS references, is attained for a cell with a Zn_1−xSn_xO_y layer thickness of approximately 13 nm and with an i-ZnO layer. Copyright © 2012 John Wiley & Sons, Ltd.

The influence of the thickness of atomic layer deposited Zn1-xSnxOy buffer layers and the presence of an intrinsic ZnO layer on the performance of Cu(In,Ga)Se₂ solar cells are investigated. Both the open circuit voltage and the fill factor values are found to be thickness dependent, where a highest conversion efficiency of 18.0 % is obtained for a cell with a Zn1-xSnxOy layer thickness of approximately 13 nm.

Because of recent advances in the production and installation of photovoltaic (PV) systems, the international conformity of PV module performance measurement has become increasingly important. The increase in PV production sites is particularly significant in the Asian region. The present paper summarizes and discusses the results of a round-robin intercomparison of crystalline silicon modules among national laboratories and certified testing laboratories in the Asian region conducted from 2009 to 2011. Most of the values of P_max measured at the different laboratories were within a ±2% range, although some P_max results showed differences of up to about 3%. This result is comparable to that obtained in the recent intercomparison among international laboratories. Possible sources of difference in the measured values of I_sc, V_oc, FF, and P_max are discussed, for further improvement of international conformity in PV measurement technologies. Copyright © 2012 John Wiley & Sons, Ltd.

Results of a round-robin intercomparison of crystalline silicon modules among national laboratories and certified testing laboratories in the Asian region are discussed. Most of the values of P_max measured at the different laboratories were within a ±2% range, although some P_max results showed differences of up to about 3%. Possible sources of difference in the measured values of I_sc, V_oc, FF, and P_max are discussed, for further improvement of international conformity in photovoltaic measurement technologies.

In this paper, the main causes that are able to limit the efficiency of Distributed Maximum Power Point Tracking (DMPPT) are analyzed in detail. It will be shown that, to get full profit from DMPPT, it is necessary that the bulk inverter voltage belongs to an optimal range whose position and amplitude are functions of the following factors: the number of PV modules and dedicated DC/DC converters in a string, the atmospheric operating conditions characterizing each PV module (irradiance and temperature values), the voltage and current ratings of the physical devices the DC/DC converters are made of, and the adopted DC/DC converter topology. Moreover, it will be given proof of the necessity to couple the DMPPT function with a suitable centralized MPPT function carried out by the inverter through the proper control of its own DC input voltage. Copyright © 2012 John Wiley & Sons, Ltd.

In order to get full profit from DMPPT, it is necessary that the input inverter voltage belongs to an optimal range whose position and amplitude are functions of the number of PV modules and dedicated DC/DC converters in a string, the atmospheric operating conditions characterizing each PV module, the voltage and current ratings of the physical devices the DC/DC converters are made of and the adopted DC/DC converter topology.

Thin-film module technologies are known for their metastability, and a study of this behaviour for different types of thin-film modules is presented. The modules investigated through a series of controlled light-soaking procedures are copper–indium sulfide (CIS), copper–indium–gallium diselenide (CIGS), cadmium telluride (CdTe), triple-junction amorphous silicon (a-Si), micromorph silicon (a-Si/μ-Si) and thin-film crystalline silicon (CSG). The objective of the paper is to investigate whether after the stabilization point, as defined in the international qualification standard IEC 61646, there is any further significant change in the maximum power of the module. It was found that all CIS and CIGS modules investigated in this study stabilize according to IEC 61646, and no further significant change in maximum power is observed. The same result was obtained also for the CSG module. To the contrary, CdTe, triple-junction a-Si and a-Si/μ-Si modules continued to show further change in maximum power even after they stabilize according to IEC 61646. For the purposes of module qualification, given the need to stay ‘within reasonable constraints of cost and time’, the stability procedure of IEC 61646 could be considered as satisfactory. However, in order to perform sufficient preconditioning of thin-film modules prior to precision calibration, a new more complete standard procedure is needed, tailored to the specific technology. For example, tighter stability limits lower than the current 2%, which would have the effect of increasing the number of light-soaking periods required. Copyright © 2012 John Wiley & Sons, Ltd.

A study of the metastability of different types of thin-film photovoltaic modules is presented. The power stabilization point, as defined in the international qualification standard IEC 61646, is evaluated using controlled light soaking. Some of the tested modules are found to show further change in maximum power even after they stabilize according to IEC 61646.

Wafer quality is extremely important in determining yield and efficiency of solar cells. Ideally, this wafer quality should be determined for incoming wafers before solar cell fabrication based on the electronic quality of the wafers. Recent papers have discussed methodologies for doing this by using lifetime measurement and pattern recognition of photoluminescence (PL) images. This paper compares results from quasi-steady-state photoconductance (QSSPC) lifetime measurements with PL imaging pattern recognition of dislocations. By using a more complete analysis of the lifetime and the PL data than performed in some recent publications, a more detailed physical picture is presented here, which reconciles contradictions between previous results. In particular, the differences between PL and QSSPC lifetime measurements on as-cut wafers are discussed. The trends in voltage prediction based on measured lifetime, doping, and PL-determined dislocation densities are shown. Copyright © 2012 John Wiley & Sons, Ltd.

Wafer quality is extremely important in determining yield and efficiency of solar cells. Ideally, this wafer quality should be determined for incoming wafers before solar cell fabrication based on the electronic quality of the wafers. This paper compares results from quasi-steady-state photoconductance lifetime measurements with photoluminescence (PL) imaging pattern recognition. A detailed physical picture is presented here that reconciles contradictions between previous results. The trends in voltage prediction based on measured lifetime, doping, and PL-determined dislocation densities are shown.

Organic photovoltaics (OPVs) are expected to be a low cost, environmentally friendly energy solution with advantageous properties such as flexibility and light weight that enable their use in new applications. Considerable progress in power conversion efficiencies has brought OPV technology closer to commercialization. However, little consideration has been given to potential environmental impact associated with their production. Although environmental life cycle studies of OPV exist, their scope is narrow or too reliant on outdated technologies. Some of the most significant recent improvements are the result of new semiconductors materials, which have not yet been assessed from a life cycle perspective. Therefore, this study calculates life cycle embodied energy for 15 new materials encompassing a variety of donor, acceptor, and interface compounds showing the most promise in organic electronics. With the use of new inventory data, life cycle energy impact associated with production of both single junction and multi-junction architectures has been calculated including bulk heterojunction polymer, planar small molecule, and planar-mixed small molecule devices. The cumulative energy demand (CED) required to fabricate small molecule and polymer photovoltaics were found to be similar from 2.9 to 5.7 MJ/Wp. This CED is on average of 50% less than for conventional inorganic photovoltaics, motivating the continued development of both technologies. The use of fullerenes was shown to have a dramatic impact on polymer solar cells, comprising 18–30% of the CED, despite only being present in small quantities. Increases in device efficiency are shown to marginally reduce CED for both small molecule and polymer designs. Copyright © 2012 John Wiley & Sons, Ltd.

The embodied energy has been calculated for 15 new materials used in organic electronics, encompassing a variety of donor, acceptor, and interface compounds. With the use of this new inventory data, the life cycle energy impact of 26 different organic photovoltaic devices comprising single junction and multi-junction architectures has been calculated including bulk heterojunction polymer, planar small molecule, and planar-mixed small molecule. The cumulative energy demand is on average of 50% less than for conventional inorganic photovoltaics, motivating the continued development of both technologies.

The effective minority carrier lifetimes on epitaxial silicon thin-film material have been measured successfully using two independent microwave-detected photoconductivity decay setups. Both measurement setups are found to be equally suited to determine the minority carrier lifetime of crystalline silicon thin-film (cSiTF) material. The different measurement conditions to which the sample under investigation is exposed are critically analyzed by both simulations and measurements on a large number of lifetime samples. No systematic deviation between the lifetime results from different measurement setups could be observed, underlining the accuracy of the determined lifetime value. Subsequently, a method to separate the epitaxial bulk lifetime and the total recombination velocity, consisting of front surface and interface recombination between the epitaxial layer and the substrate, is presented. The method, based on different thicknesses of the epitaxial layer, is applied to all batches of this investigation. Each batch consists of samples with the same material quality but different epitaxial layer thicknesses whereas different batches differ in their material quality. In addition, the same method is also successfully applied on individual cSiTF samples. From the results, it can be concluded that the limiting factor of the effective minority carrier lifetime for the investigated solar-grade cSiTF material is the elevated recombination velocity at the interface between epitaxial layer and the substrate compared with microelectronic-grade material. In contrast, the samples cannot be classified into different material qualities by their epitaxial bulk lifetimes. Even on multicrystalline substrate, solar-grade material can exhibit high epitaxial bulk lifetimes comparable to microelectronic-grade material. Copyright © 2012 John Wiley & Sons, Ltd.

The effective carrier lifetime of crystalline silicon thin-film material has been measured on a wide range of different material qualities using two independent microwave-detected photoconductivity decay setups. No systematic deviation between the two measurement setups could be observed. Through a variation of the epitaxial layer thickness, the recombination in the interface region between the layer and the underlying substrate has been identified to be the limiting factor of the effective lifetimes.

In testing maximum power point tracking (MPPT) algorithms running on electronic power converters for photovoltaic (PV) applications, either a PV energy source (PV module or PV array) or a PV emulator is required. With a PV emulator, it is possible to control the testing conditions with accuracy so that it is the preferred option. The PV source is modeled as a current source; thus, the emulator has to work as a current source dependent on its output voltage. The proposed emulator is a buck converter with an average current mode control loop, which allows testing the static and dynamic performance of PV facilities up to 3 kW. To validate the concept, the emulator is used to evaluate the MPPT algorithm of a 230-W experimental microinverter working from a single PV module. Copyright © 2012 John Wiley & Sons, Ltd.

When researching about maximum power point tracking methods, it is mandatory to have a reliable photovoltaic (PV) emulator that assures the repeatability of the results, capable of simulating all the possible climatic conditions (variations of irradiance and temperature). The PV emulator proposed in this paper can simulate the behavior of PV power source with an open-circuit voltage of 500 V (maximum voltage) and a short-circuit current of 9 A (maximum current), with a power limitation of 3000 W.

A highly efficient ZnO photoanode for dye-sensitized solar cells was successfully grown by a simple, low cost, and scalable method. A nanostructured coral-shaped Zn layer was deposited by sputtering onto fluorine-doped tin oxide/glass slices at room temperature and then thermally oxidized in ambient atmosphere. Stoichiometry, crystalline phase, quality, and morphology of the film were investigated, evidencing the formation of a highly porous branched nanostructure, with a pure wurtzite crystalline structure. ZnO-based dye-sensitized solar cells were fabricated with customized microfluidic architecture. Dye loading on the oxide surface was analyzed with ultraviolet-visible spectroscopy, and the dependence of the cell efficiency on sensitizer incubation time and film thickness was studied by current-voltage electrical characterization, incident photon-to-electron conversion efficiency, and impedance spectroscopy measurements, showing the promising properties of this material for the fabrication of dye-sensitized solar cell photoanodes with a solar conversion efficiency up to 4.58%. Copyright © 2012 John Wiley & Sons, Ltd.

A highly efficient ZnO photoanode for dye-sensitized solar cells (DSSCs) was successfully grown by a simple, low cost, and scalable method. A nanostructured coral-shaped Zn layer was deposited at room temperature sputtering technique and then thermally oxidized. ZnO-based DSSCs were fabricated with customized microfluidic architecture, and the dependence of the cell efficiency on sensitizer incubation time and film thickness was studied showing the promising properties of this material for the fabrication of photoanodes, with solar conversion efficiency up to 4.58%.

In this paper, a detailed exergetic analysis based on the variation of meteorological parameters was performed for a solar power generation system. All wind and solar energy and exergy characteristics were examined in order to identify the variables that affect the power output of the photovoltaic (PV) system. Atmospheric variables such as air temperature, humidity and wind speed and their effects, shadow effects, tracking losses, and low radiation losses of the PV power output were investigated aiming at identifying the real and net solar energy output. It was shown that some usually disregarded atmospheric variables in planning new PV plants, in fact, do play a significant role on the plant's overall exergetic efficiency as wind chill temperature. The solar potential around a windy coastal hilly area was studied and presented on the basis of field measurements and simulations. Understanding atmospheric parameters variation appears to be of great importance for estimating correctly and trustworthy the energy yield, and special focus was given to that in this paper. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, a detailed exergetic analysis based on the variation of meteorological parameters was performed for a solar power generation system. The air psychrometric analysis, under this case study, showed that air density varies from site to site and monthly as well (1.152–1.228 kg/m³), affecting the energy productivity. It was found that for the proposed PV park, production is 3.2% higher compared with what would happen if this parameter was neglected.

A process to fabricate a parallel connection dye-sensitized solar cell (DSSC) module has been developed using commercially available materials and screen printed silver grid. The process is not only simple but also easy to manipulate and therefore facilitates researchers in evaluating new materials in a module platform. By changing the design of the silver grid pattern, it was found that the performance of DSSC modules can be controlled. With the silver grid, DSSC modules have shown that a 7% conversion efficiency can be reached. Modules fabricated by this process, but with a non-volatile electrolyte system, passed a 60 °C, 1000 h thermal aging test. Copyright © 2012 John Wiley & Sons, Ltd.

A process to fabricate a parallel connection dye-sensitized solar cell (DSSC) module has been developed using commercially available materials and screen printed silver grid. The process is not only simple but also easy to manipulate and therefore facilitates researchers in evaluating new materials in a module platform. By changing the design of the silver grid pattern, it was found that the performance of DSSC modules can be controlled, and a 7% conversion efficiency can be reached.

The impact of laterally non-uniform carrier lifetime on the determination of the lifetime from photoconductance-based measurements, based on the self-consistent method proposed by Trupke and Bardos, is investigated using a simple model. It is shown that the method can result in an overestimation of the mean lifetime, with the magnitude of the error mainly dependent on the distribution of the effective lifetime across the area sensed by the photoconductance coil. Although in many cases the error introduced will be relatively small (in the order of 5% or less), much larger errors can result in some cases, such as for samples that feature small areas with a significantly higher than average lifetime. The error can be eliminated through independent measurement of the sample optical properties. Experimental measurements confirm the model predictions. Copyright © 2012 John Wiley & Sons, Ltd.

The impact of lateral nonuniform carrier lifetime on the determination of the lifetime from photoconductance-based measurements, based on the self-consistent method, is investigated. It is shown that the method can result in an overestimation of the mean lifetime, with the magnitude of the error mainly dependent on the distribution of the effective lifetime across the area sensed by the photoconductance coil. The error can be eliminated through independent measurement of the sample optical properties. Experimental measurements confirm the model predictions.

III–V concentrator photovoltaic systems attain high efficiency through the use of series connected multi-junction solar cells. As these solar cells absorb over distinct bands over the solar spectrum, they have a more complex response to real illumination conditions than conventional silicon solar cells. Estimates for annual energy yield made assuming fixed reference spectra can vary by up to 15% depending on the assumptions made. Using a detailed computer simulation, the behaviour of a 20-cell InGaP/In_0.01GaAs/Ge multi-junction concentrator system was simulated in 5-min intervals over an entire year, accounting for changes in direct normal irradiance, humidity, temperature and aerosol optical depth. The simulation was compared with concentrator system monitoring data taken over the same period and excellent agreement (within 2%) in the annual energy yield was obtained. Air mass, aerosol optical depth and precipitable water have been identified as atmospheric parameters with the largest impact on system efficiency. Copyright © 2012 John Wiley & Sons, Ltd.

The behaviour of a 20-cell InGaP/In_0.01GaAs/Ge multi-junction concentrator system was simulated in 5-min intervals over an entire year with agreement within 2% between measured and modelled annual energy yields. Air mass, aerosol optical depth and precipitable water have been identified as the atmospheric parameters with the largest impact on system efficiency.

We have designed and fabricated dye solar cell (DSC) modules with optimized geometries and processes. Integrated interconnections were made following the “Z” architecture for series connections. Several modules were prepared varying the materials, multilayer combination of the TiO₂ active layers, and the fabrication processes. With the best combination of TiO₂ multilayers, titanium tetrachloride (TiCl₄) treatment, a back reflector/diffusor, and optimized layout of cells via simulations, we fabricated a DSC module with a conversion efficiency of 6.9% on 43 cm² aperture area and 9.4% on active area. This result confirms that an effective scale-up of high performance Z-series-connected DSC modules can be achieved comparable with other thin film technology. Note that the materials used to produce the devices of this work are all commercially available: an important result for a technology that is being developed for industrial application. Copyright © 2012 John Wiley & Sons, Ltd.

We have designed and fabricated dye solar cell (DSC) Z-type modules with optimized geometries and materials. Through optimization of the layout of the modules using PSPICE simulations, the multilayer combination and treatments of the TiO₂ active layers, and the fabrication processes, a conversion efficiency of 6.9% on the 43-cm² aperture area and 9.4% on the active area was achieved. This result confirms that an effective scale-up of opaque high-performance Z-series-connected DSC modules, all realized with commercially available materials, can be accomplished.

Back-contacted, ultrathin (<10 µm), and submillimeter-sized solar cells made with microsystem tools are a new type of cell that has not been optimized for performance. The literature reports efficiencies up to 15% using thicknesses of 14 µm and cell sizes of 250 µm. In this paper, we present the design, conditions, and fabrication parameters necessary to optimize these devices. The optimization was performed using commercial simulation tools from the microsystems arena. A systematic variation of the different parameters that influence the performance of the cell was accomplished. The researched parameters were resistance, Shockley–Read–Hall (SRH) lifetime, contact separation, implant characteristics (size, dosage, energy, and ratio between the species), contact size, substrate thickness, surface recombination, and light concentration. The performance of the cell was measured with efficiency, open-circuit voltage, and short-circuit current. Among all the parameters investigated, surface recombination and SRH lifetime proved to be the most important. Through completing the simulations, an optimized concept solar cell design was introduced for two scenarios: high and low quality materials/passivation. Simulated efficiencies up to 23.4% (1 sun) and 26.7% (100 suns) were attained for 20-µm-thick devices. Copyright © 2012 John Wiley & Sons, Ltd.

Back-contacted, ultrathin (<10 µm), and submillimeter-sized solar cells made with microsystem tools are a new type of cell that has not been optimized for performance. In this paper, we present the design conditions and fabrication parameters necessary to optimize these devices via simulations. Through completing the simulations, an optimized concept solar cell design was introduced for two scenarios: high and low quality materials/passivation. Simulated efficiencies up to 23.4% (1 sun) and 26.7% (100 suns) were attained for 20-µm-thick devices.

Even within the simplest real solar cell model, the exact value of the fill factor (FF) is only computable by numerical calculations. Here, we perform approximations to the power–voltage curve given by the one-diode model with series and shunt resistance losses, obtaining explicit expressions for the voltage and current at the maximum power point, and thus an explicit approach for the FF. Over a broad range of possible solar cell parameters, including cells where the impact of shunt losses on the fill factor is not negligible, the approximate equations yield relative errors typically around 1%. The equations are applied to explore the dependence of FF on alternative buffer material thickness of organic solar cells, and to investigate the incidence of shunt and series resistance losses on the FF of Cu(In,Ga)Se₂ solar cells under indoor illumination conditions. Copyright © 2012 John Wiley & Sons, Ltd.

We obtain accurate equations for the fill factor of solar cells suffering from both series and shunt resistance losses. The equations are applied on two examples: visualization of optimum alternative buffer layer thickness of organic solar cells, and investigation of shunt and series resistance limitations to the fill factor of Cu(In,Ga)Se₂ solar cells under indoor illumination conditions. The figure shows how our model follows the fill factor of organic solar cell data where series and shunt resistance limit the fill factor.

Electric charge transport simulations of symmetrically doped radial pn junction silicon nanorod solar cells were performed using the Technology Computer-aided Design software suite by Silvaco. Two schemes of electric contacting were applied, the first one consisting of a cathode wrapped around the cladding of the rod and the second one in a cathode located only on the top rod surface. In both cases, the anode was implemented just below the bottom end of the p-type rod core. P-type cores and n-type shells of the rods were assumed, with dopant densities of 10¹⁸ cm^− 3 in both regions. The location of the pn junction was chosen such that well-formed space charge regions could be established with the outer end of the n-type depletion region being adjacent to the cylindric surface of the nanorod. Rod radii and rod lengths were varied and optimized in a three-step process for both types of contacting schemes. It was found that inhomogeneous carrier generation profiles diminish the open-circuit voltage in case of a wrapped cathode configuration. Most realistic is the usage of a top contact configuration with rod radii of 2 µm and lengths of around 100 µm, leading to a cell efficiency of about 15%. Further enhancement of performance is expected, if light trapping of the nanorod layer is taken into account and photonic light harvesting is applied. Copyright © 2012 John Wiley & Sons, Ltd.

Computer-based simulations were performed on radial pn junction silicon nanorod and microrod solar cells to find optimized geometric dimensions and explore the effects of different types of contact configurations. A top contact configuration rendered superior to a wrapped contact configuration within the chosen set of material parameters. Maximization of cell efficiency assuming Lambert–Beerian absorption led to optimized rod radii in the 1-μm range and to optimized rod lengths in the 100-μm range for top contact configuration.

The ability to grow efficient CdTe/CdS solar cells in substrate configuration would not only allow for the use of non-transparent and flexible substrates but also enable a better control of junction formation. Yet, the problems of barrier formation at the back contact as well as the formation of a p–n junction with reduced recombination losses have to be solved. In this work, CdTe/CdS solar cells in substrate configuration were developed, and the results on different combinations of back contact materials are presented. The Cu content in the electrical back contact was found to be a crucial parameter for the optimal CdCl₂-treatment procedure. For Cu-free cells, two activation treatments were applied, whereas Cu-containing cells were only treated once after the CdTe deposition. A recrystallization behavior of the CdTe layer upon its activation similar to superstrate configuration was found; however, no CdTe–CdS intermixing could be observed when the layers were treated consecutively. Remarkably high V_OC and fill factor of 768 mV and 68.6%, respectively, were achieved using a combination of MoO₃, Te, and Cu as back contact buffer layer resulting in 11.3% conversion efficiency. With a Cu-free MoO₃/Te buffer material, a V_OC of 733 mV, a fill factor of 62.3%, and an efficiency of 10.0% were obtained. Copyright © 2012 John Wiley & Sons, Ltd.

In this work, CdTe/CdS solar cells in substrate configuration were developed, and the results on different combinations of back contact materials are presented. A Cu-containing back contact buffer layer of MoO₃, Te, and Cu leads to remarkably high V_OC and fill factor of 768 mV and 68.6%, respectively, yielding 11.3% conversion efficiency. With a Cu-free MoO₃/Te buffer material, a V_OC of 733 mV, a fill factor of 62.3%, and an efficiency of 10.0% efficiency could be achieved.

Physical processes that lead to conversion of light into electrical energy inside photovoltaic devices happen at the nanoscale. Therefore, understanding of electrical properties of photovoltaic materials at this length scale is of paramount importance for improvement of device performance. In this paper, we describe and validate a new framework for high-resolution quantitative measurements of electrical and mechanical properties of compliant materials with sub-100-nm resolution. Previous approaches have generally suffered from uncertainty in the quantitative level of contact between the probe and the material being measured; the methodology presented here overcomes this obstacle. We use the broadly studied ITO/PEDOT:PSS/P3HT:PC₆₁BM system as an example to illustrate variability of chemical composition and electrical properties of the active layer at hundred-nanometers and micrometer length scales. Copyright © 2012 John Wiley & Sons, Ltd.

New technique for AFM-based quantitative electrical measurements is developed.Performance of the technique is validated on device-relevant OPV material. Electrical properties and chemical composition of the active layer found to be inhomogeneous at hundred-nanometers and micrometer length scales.

The effect of dust on photovoltaic modules is investigated with respect to concentration and spectral transmittance. Samples were collected in the form of raw dust as well as accumulated dust on exposed sheets of glass at different tilt angles. Spectral transmittance of the samples was determined. Transmittance variation between top, middle and bottom was identified for samples collected at different inclinations, where the worst case was seen at a tilt angle of 30^o with a non-uniformity of 4.4% in comparison with 0.2% for the 90° tilt. The measured data showed a decrease in transmittance at wavelengths <570 nm. Integrating this with measured spectral responses of different technologies demonstrates that wide band-gap thin-film technologies are affected more than, for example crystalline silicon technologies. The worst case is amorphous silicon, where a 33% reduction in photocurrent is predicted for a dust concentration of 4.25 mg/cm². Similarly, crystalline silicon and CIGS technologies are predicted to be less affected, with 28.6% and 28.5% reductions in photocurrent, respectively. The same procedure was repeated with varying Air Mass (AM), tilt angle and dust concentration values to produce a soiling ratio table for different technologies under different AM, tilt angle and dust concentration values. Copyright © 2012 John Wiley & Sons, Ltd.

The effect of dust on photovoltaic modules was investigated with respect to dust concentration and spectral transmittance. Total transmittance variation was identified for samples at different tilted positions that showed higher losses at flat orientation and higher transmittance variation at 30° tilt angle. The measured dust transmittance showed a spectral attenuation that affects wide band-gap thin-film technologies more than crystalline silicon technologies.

The colloids of YVO₄ nanoparticles on microtextured Si surface are demonstrated to have promising potential for efficient solar spectrum utilization in crystalline Si solar cells. The solar cells showed an enhancement of 4% in short-circuit current density and approximately 0.7% in power conversion efficiency when coated with YVO₄ nanoparticles. The properties of cells integrated with YVO₄ nanoparticles were characterized to identify the role of YVO₄ in improved light harvesting. The current experiments conclude that the colloids of YVO₄ nanoparticles not only act as luminescent down-shifting centers in the ultraviolet region but also serve as an antireflection coating for enhancing the light absorption in the measured spectral regime. Copyright © 2012 John Wiley & Sons, Ltd.

The colloids of YVO4 nanoparticles on microtextured Si surface are demonstrated to have promising potential for efficient solar spectrum utilization in crystalline Si solar cells. The current experiments conclude that the colloids of YVO4 nanoparticles not only act as luminescent down-shifting centers in the ultraviolet region but also serve as an antireflection coating for enhancing the light absorption in the measured spectral regime.

Metrics used in assessing irradiance model accuracy, such as root mean square error and mean absolute error, are precisely defined. Their relative (%) counterpart, however, can be subject to interpretation and may cover a wide range of values for a given set of data depending on reporting practice. This note evaluates different approaches for the reporting of relative metrics quantifying the dispersion accuracy of a model and formulates recommendations for the most appropriate approach. Copyright © 2012 John Wiley & Sons, Ltd.

This paper suggests that the mean absolute error (MAE) relative to the mean measured value may be the preferred metric to quantify the relative (percent) dispersion accuracy of irradiance models. This is because the metric is less sensitive to outliers and depends less upon the threshold applied to the validation data (e.g., whether nighttime values are included or not in the statistics).

An indoor method is presented for the quantification of the current-matching ratio of a multijunction cell within a concentrator under arbitrary spectral irradiance conditions. The cell current is measured across a very large spectral sweep to force the relevant subcells into a limiting condition. The light spectrum is monitored using component cells to avoid the need for a spectroradiometer and spectral response measurements. The method also provides an estimation of the current losses beyond the overall current mismatch, for example, losses produced in concentrators with chromatic aberration by the non-uniformity of the incident spectrum across the cell. The method has been applied to a pair of refractive point-focus concentrator systems; first, a 300X single-stage Fresnel lens over a lattice-matched GaInP/Ga(In)As/Ge triple-junction cell and second, a 1000X two-stage system with the same Fresnel lens over a homogenizing secondary lens that encapsulates a triple-junction cell of the same kind but smaller. The experiment demonstrates that the single-stage concentrator exhibits a higher sensitivity of the current mismatch to variations in the focal distance. Copyright © 2012 John Wiley & Sons, Ltd.

An indoor method is presented for the quantification of the current matching of a multijunction cell within a concentrator under arbitrary spectral irradiance conditions. The cell current is measured across a very large spectral sweep to force the relevant subcells into a limiting condition. The light spectrum is monitored using component cells. Results are presented for a pair of point-focus concentrators, showing a higher sensitivity to spectrum when no secondary lens is used.

Multicrystalline solar cells break down strongly at reverse voltages well below the theoretical limit. Previous explanations were based on assuming a constant depth of the junction below the surface. In this work, preferred phosphorous diffusion at special line defects in grain boundaries is shown to lead to spikes in the p–n junctions even below flat surfaces. The curvature radii of the spherical p–n junction bending are measured by electron beam-induced current to be in the range of 300–500 nm, leading to the observed type III avalanche breakdown voltages. Copyright © 2012 John Wiley & Sons, Ltd.

Avalanche pre-breakdown in multicrystalline solar cells is found to be due to submicron-sized nearly spherical dents in the p–n junction. These dents are assumed to be caused by enhanced diffusion of phosphorous during emitter formation at special crystallographic line defects. In spite of a flat solar cell surface, this phenomenon leads to pre-breakdown due to locally enhanced electric fields even in alkaline textured solar cells, which normally exhibit a flatter surface than acidic textured ones.

In this paper, a reliability analysis of a photovoltaic rural electrification (PVRE) programme is proposed considering the failures in the 13 000 installed Solar Home System (SHS) devices occurring over a long operating period of 5 years. A previous arrangement of the database and a brief explanation of the reliability concepts will serve to introduce the failure distribution of every component, from which the SHS lifetime operating features will be described. An application example will show the usefulness of the obtained results in the forecasting of spare parts during the maintenance period. The conclusions of this study may be useful in the scientific design of PVRE programme maintenance structures, with the goal of shedding some light on the technical management mechanisms in decentralised rural electrification. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents the results of a reliability study that was carried out with a real maintenance database from the 13 000-SHS PERG programme in 12 provinces in Morocco.

The failure distribution of every SHS component, registered for a period of 5 years, was evaluated to determine its reliability function R(t), failure rate λ(t) and mean time to failure.

The study opens the door to characterise the maintenance structure in a PVRE programme.

Partial shading is a commonly encountered mismatch problem in a photovoltaic system. In the drawing near perspective of their massive building integration, dye solar cell (DSC) modules may realistically receive different levels of irradiance, a situation similar to partial shading. In these conditions, the electrical characteristics of the DSC module significantly change. Here a general model for the description and the analysis of dye solar generators is proposed. A new equivalent circuit for DSCs has been developed that is characterized by the introduction of a second diode, capable to conveniently take into account the behavior of the reverse-biased cell/s. An experimental demonstration of the proposed two-diode model's validity is provided. A detailed description, based on numerical analysis, of the influence of partial shading on the photovoltaic performances of a DSC module made by four W-connected cells is given. We here demonstrate that the implementation of a two-diode model allows an excellent matching between the experimentally measured I–V characteristics of the partially shaded module and the simulated ones. Copyright © 2012 John Wiley & Sons, Ltd.

The I–V characteristics of a dye solar cell module working in several different partial shading conditions have been modeled through the implementation of a double diode-based equivalent circuit, that is the same equivalent circuit normally used to simulate the cell working in forward bias, now inclusive of a second additional diode that accounts for the reverse behavior of the shaded element/s. A perfect matching between the experimentally measured I–V characteristics and the simulation results has been revealed in every analyzed illumination condition.

Increasing silver prices and reducing silicon wafer thicknesses provide incentives for silicon solar cell manufacturing to develop new metallisation strategies that do not rely on screen printing and preferably reduce silver usage. Recently, metal plating has re-emerged as a metallisation process that may address these future requirements. This paper reports on the evolution of metal plating techniques, from their use in early silicon solar cells, to current light-induced plating processes. Unlike screen-printed metallisation, metal plating typically requires an initial patterning step to create openings in a masking layer for the subsequent self-aligned metallisation. Consequently, relevant recently-developed dielectric patterning methods are also reviewed because, in many cases, the plating process must be adapted to the properties of the patterning method used. The potential of new light-induced plating processes to form cost-effective copper metallisation is supported by the recent activity in the development of metal plating tools for commercial silicon solar cell manufacture. Copyright © 2012 John Wiley & Sons, Ltd.

Increasing silver prices and reducing silicon wafer thicknesses provide incentives for silicon solar cell manufacturing to develop new metallisation strategies that do not rely on screen printing and reduce silver usage. Recently, metal plating has re-emerged as a metallisation process that may address these future requirements. This paper reports on the evolution of metal plating techniques, from their use in early silicon solar cells to current light-induced plating processes.

The first prototype of the hybrid CPV-T ANU-Chromasun micro-concentrator has been installed at The Australian National University, Canberra, Australia. The results of electrical and thermal performance of the micro-concentrator system, including instantaneous and full-day monitoring, show that the combined efficiency of the system can exceed 70%. Over the span of a day, the average electrical efficiency was 8% and the average thermal efficiency was 50%. Copyright © 2012 John Wiley & Sons, Ltd.

The first prototype of the hybrid CPV-T ANU-Chromasun micro-concentrator (MCT) has been installed at The Australian National University (ANU, Canberra, Australia). The results of electrical and thermal performance of the MCT system, including instantaneous and full-day monitoring, show that combined efficiency of the system can exceed 70%. Over the span of a day, the average electrical efficiency was 8% and the average thermal efficiency was 50%.

In this study, we have investigated the Hall majority carrier mobility of p-type, compensated multicrystalline solar grade silicon (SoG-Si) wafers for solar cells in the temperature range 70–373 K. At low temperature (~70 K) the difference in the mobilities measured for the compensated and the uncompensated reference samples is the highest, and the measured mobility shows dependence on the compensation ratio. Mobilities decrease with increasing temperature, and towards room temperature, the mobilities of the different samples are in the same range. The measurements show that, for these samples, the contribution from lattice scattering is dominating over ionized impurity scattering at room temperature. In the range of interest for silicon solar cells (above room temperature), the trend in carrier mobility is similar for all the samples, and the measured value for the sample with low compensation ratio and low doping density is comparable to the uncompensated references. A comparison of resistivity and majority carrier density measured by the Hall setup at room temperature and by four-point probe and glow discharge mass spectroscopy, respectively, is reported as well. Copyright © 2012 John Wiley & Sons, Ltd.

Hall-effect measurements were carried out on compensated mc-Si samples, at temperatures between 70 K and 373 K. At low temperature, the samples scales with compensation ratio and show a decrease in majority carrier mobility compared to uncompensated references, due to increased ionized impurity scattering. At high temperatures, the compensated samples approach the uncompensated references, due to the dominant lattice scattering mechanism.

In this study, electroluminescence as a spatial characterisation technique is used to characterise a 6.9% efficient dye-sensitised solar cell. The obtained image is compared with a light beam-induced current scan image and a transmittance image. Results reveal the presence of inhomogeneities including those resulting from the topography of the cell and from defects, for example, presence of iodine crystals in the electrolyte, localised absence of dye in the active layer and poor adhesion of the active layer to the electrodes. The ability to identify such inhomogeneities within a relatively short acquisition time gives electroluminescence an advantage over the light beam-induced current technique. Copyright © 2012 John Wiley & Sons, Ltd.

Increasing silver prices and reducing silicon wafer thicknesses provide incentives for silicon solar cell manufacturing to develop new metallization strategies that do not rely on screen printing and reduce silver usage. Recently, metal plating has re-emerged as a metallization process that may address these future requirements. This paper reports on the evolution of metal-plating techniques, from their use in early silicon solar cells to current light-induced plating processes.

A novel methodology based on artificial neural networks is proposed as an alternative to algebraic and numerical procedures to determine the I-V curve of a module under different conditions. Although there are methods that use neural networks for approximating the I-V curve, this is the first time that the measurement of the spectrum is incorporated as an input. In addition, a suitable selection of the training samples used to build the model is fundamental in order to get an accurate approximation. This is why a training sample selection based on a Kohonen self-organizing map is performed in this paper instead of a random selection. With the use of this preliminary step, the performance of the network trained with spectral information improves over the one without spectral information. Copyright © 2012 John Wiley & Sons, Ltd.

A novel methodology based on artificial neural networks is proposed as an alternative to algebraic and numerical procedures to determine the I-V curve of a module under different conditions. It is the first time that the spectrum is incorporated as an input, and a selection of the training samples based on a Kohonen self-organizing map has been performed. With the use of this preliminary step, the performance of the network trained with spectral information improves over the previous models.

Evaluating the reliability, warranty period, and power degradation of high concentration solar cells is crucial to introducing this new technology to the market. The reliability of high concentration GaAs solar cells, as measured in temperature accelerated life tests, is described in this paper. GaAs cells were tested under high thermal accelerated conditions that emulated operation under 700 or 1050 suns over a period exceeding 10 000 h. Progressive power degradation was observed, although no catastrophic failures occurred. An Arrhenius activation energy of 1.02 eV was determined from these tests. The solar cell reliability [R(t)] under working conditions of 65°C was evaluated for different failure limits (1–10% power loss). From this reliability function, the mean time to failure and the warranty time were evaluated. Solar cell temperature appeared to be the primary determinant of reliability and warranty period, with concentration being the secondary determinant. A 30-year warranty for these 1 mm²-sized GaAs cells (manufactured according to a light emitting diode-like approach) may be offered for both cell concentrations (700 and 1050 suns) if the solar cell is operated at a working temperature of 65°C. Copyright © 2012 John Wiley & Sons, Ltd.

This work estimates the degradation and reliability of high concentration GaAs solar cells. Cells were tested under high thermal accelerated conditions and emulated operation under 700 or 1050 suns. Solar cell temperature working appeared to be the primary determinant of degradation [700X (left figure) and 1050X (right figure)], reliability and warranty period, with concentration being the secondary determinant. A 30-year warranty for these 1 mm²-sized GaAs cells may be offered for both cell concentrations if operated at a working temperature of 65 °C.

Most calculations of optimum photovoltaic (PV) performance focus on maximizing annual energy production. However, given the seasonally and daily time varying electricity demand and resulting variation in price, the PV orientation resulting in maximum energy yield may not yield the maximum economic benefit. With the use of historical solar irradiance and wholesale market prices for several locations in the USA, we evaluate the benefits of a variety of orientations for fixed and tracking PV arrays. We find that orienting fixed arrays slightly to the west of due south generally increases their economic value in the simulated systems because the reduced generation on an annual basis is more than offset by increased generation in high-value hours in late summer afternoons. However, this effect is small, typically providing an increase in value from 1% to 5%. The economic value of adjusting the orientation semi-annually (May 1st and September 1st) and monthly shows a modest increase in value from 3% to 5%. Several other implications of this analysis are also discussed. Copyright © 2012 John Wiley & Sons, Ltd.

Historical solar irradiance and wholesale market prices are evaluated to assess the benefits of a variety of orientations for fixed and tracking photovoltaic arrays. We find that orienting fixed arrays slightly to the west of due south generally increases their economic value in the simulated systems because of the time-varying value of electricity. This effect is small, typically providing an increase in value from 1% to 5%.

The basis for the temperature dependence of the principal performance parameters of single and multi-junction concentrator solar cells is examined, focusing on the impact of bandgap and irradiance. The analysis of cells in the radiative limit establishes fundamental bounds. A quasi-empirical model yields predictions consistent with available data. A simple method for estimating the temperature coefficients of key performance parameters is identified. The degree to which the efficiency penalty associated with cell heating can be mitigated by high irradiance is also evaluated. Copyright © 2012 John Wiley & Sons, Ltd.

The basis for the temperature dependence of the principal performance parameters of single and multi-junction concentrator solar cells is examined, focusing on the impact of bandgap and irradiance. The analysis of cells in the radiative limit establishes fundamental bounds. A quasi-empirical model yields predictions consistent with available data. The degree to which the efficiency penalty associated with cell heating can be mitigated by high irradiance is also evaluated.

We have studied experimentally the effect of different initial iron contamination levels on the electrical device properties of p-type Czochralski-silicon solar cells. By systematically varying phosphorus diffusion gettering (PDG) parameters, we demonstrate a strong correlation between the open-circuit voltage (V_oc) and the gettering efficiency. Similar correlation is also obtained for the short-circuit current (J_sc), but phosphorus dependency somewhat complicates the interpretation: the higher the phosphorus content not only the better the gettering efficiency but also the stronger the emitter recombination. With initial bulk iron concentration as high as 2 × 10¹⁴ cm⁻³, conversion efficiencies comparable with non-contaminated cells were obtained, which demonstrates the enormous potential of PDG. The results also clearly reveal the importance of well-designed PDG: to achieve best results, the gettering parameters used for high purity silicon should be chosen differently as compared with for a material with high impurity content. Finally we discuss the possibility of achieving efficient gettering without deteriorating the emitter performance by combining a selective emitter with a PDG treatment. Copyright © 2012 John Wiley & Sons, Ltd.

We have carried out a systematic study of the correlation between gettering efficiency and solar cell performance. The results reveal that the most efficient gettering treatment (=best bulk lifetime) is not always the best option for the solar cell operation. To achieve the best results, the gettering parameters should be chosen differently for materials with high and low impurity content.

A thin film-integrated device was constructed consisting of photovoltaic layers combined with additional layers to store charge in real time within the same device. In our design, a dye-sensitized solar cell and capacitor layers are integrated by a double-anodized titanium plate, which consists of TiO₂ nanotubes grown on either side by electrochemical anodization. The combination device can act either as an independent solar cell, a capacitor, or as a solar cell/capacitor device. The results presented here illustrate the capacitive behavior of high surface area nanotubular metal oxide films, with an achieved capacitance of 140 μF cm⁻². Copyright © 2012 John Wiley & Sons, Ltd.

This study shows the development of an integrated system, which can simultaneously generate and store electricity from a dye-sensitized solar cell. A capacitor is integrated along with the solar cell; this directly charges upon illumination and discharges whenever required. The present study shows a simple and inexpensive method of developing an integrated system, which could replace an independent solar cell and battery system for generation and storage.

We explore the potential of laser processing aluminium oxide (Al₂O₃)/amorphous silicon carbide (a-SiC_x:H) stacks to be used at the rear surface of p-type crystalline silicon (c-Si) solar cells. For this stack, excellent quality surface passivation is measured with effective surface recombination velocities as low as 2 cm/s. By means of an infrared laser, the dielectric film is locally opened. Simultaneously, part of the aluminium in the Al₂O₃ film is introduced into the c-Si, creating p+ regions that allow ohmic contacts with low-surface recombination velocities. At optimum pitch, high-efficiency solar cells are achievable for substrates of 0.5–2.5 Ω cm. Copyright © 2012 John Wiley & Sons, Ltd.

We explore the potential of laser processing aluminium oxide (Al₂O₃)/amorphous silicon carbide (a-SiC_x:H) stacks to be used at the rear surface of p-type crystalline silicon (c-Si) solar cells. For this stack, excellent quality surface passivation is measured with effective surface recombination velocities as low as 2 cm/s. By means of an infrared laser, part of the aluminium in the Al₂O₃ film is introduced into the c-Si, creating p+ regions that allow ohmic contacts with low-surface recombination velocities.

A reflective 3D crossed compound parabolic-based photovoltaic module (3D CCPC PV) was designed and its electrical and optical performance was analyzed for building integrated photovoltaic applications. A maximum power concentration of 3.0× was achieved compared to similar type of non-concentrating module. The reduction of the concentration factor from the geometrical concentration of 3.61× for the designed 3D CCPC were due to manufacturing errors, mismatch losses, series resistance losses, and thermal loses. The experimental output was validated by developing a MATLAB simulation code for its electrical performance. Good agreements were observed between experimental and electrical simulation with maximum electrical conversion efficiency of the concentrating system of 14%. The experimental characterization of the optical efficiency was found to show a deviation of 19.4% from the 3D ray tracing simulation efficiency of 94.6% for direct incidence. This deviation is mainly due to the fact that 3D ray tracing simulation does not take the non-uniform illumination distribution into account. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, a reflective 3D crossed compound parabolic concentrator based photovoltaic module was designed, fabricated and characterised for BIPV application. The measurements shows that the power increased by a factor of 3 compared to the similar solar cell area.

Solar spectral irradiance measurements on a routinely basis are relevant to study the influence of solar spectrum on the photovoltaic (PV) module performance, especially for thin film and third generation PV. Two spectroradiometers from EKO were added to the instrumentation available at the ESTER outdoor station of the University of Rome Tor Vergata. A detailed characterisation of the spectral irradiance at the site was carried on during more than 6 months of monitoring activity measuring spectral solar irradiance in the range 350–1700 nm with a time interval of 10 min on a horizontal plane. A wide variety of spectra were acquired in various weather conditions, and indications about the spectra behaviour on a daily and seasonal basis were obtained. Moreover, information about the effect of the weather conditions on the solar radiation spectral distribution were identified. The Average Photon Energy index was used as an indicator of the spectra characteristics. The same index was also used to evidence the solar spectrum influence on polycrystalline and double junction amorphous silicon PV modules. Copyright © 2012 John Wiley & Sons, Ltd.

Average Photon Energy (APE) index was used to characterise solar spectral irradiance and influence on double junction a-Si PV module. Detailed solar spectrum characterisation at the site evidenced an APE increment of 7% in summer with respect to winter. For overcast days an APE 10% higher than clear days was reported. Solar spectral dependence of double junction a-Si performance looks stronger than temperature dependence; no annealing effect was observed because the module temperature remained below 70°C.

The presented paper reports the results of the experimental work performed at the European Solar Test Installation, using an array of 70 polycrystalline silicon photovoltaic (PV) modules by the same manufacturer. After almost 20 years of continuous outdoor exposure, the modules were subjected to a comprehensive indoor test plan; in particular, electrical performance measurements were performed, together with a detailed visual analysis. It was also possible to perform a comparison between final and initial data (in particular IV characteristics): module average performance decay is 4.42% for the whole period. Degradation mechanisms, together with their effect on module lifetime, were also analyzed. Results of such a measurement exercise clearly show how PV device reliability over decades can guarantee safe investments, for the benefit of all PV users and stakeholders. The authors are currently installing the modules for further 20 years of outdoor exposure. Copyright © 2012 John Wiley & Sons, Ltd.

Photovoltaic modules degradation studies on arrays with a high number of samples from the same manufacturer are not common in scientific literature. We present the results of an extensive experimental campaign on 70 polycrystalline silicon modules after 20 years of outdoor exposure. A comparison between initial and current values of modules electrical parameters is also performed to analyze the effect of degradation mechanisms on module lifetime and module compliance with accelerated test procedures.

We systematically investigated the optical behaviors of an amorphous silicon solar cell with a core-shell nanograting structure. The horizontally propagating Bloch waves and Surface Plasmon Polariton waves lead to significant absorption enhancements and consequently short-circuit current enhancements of this structure, compared with the conventional planar one. The perpendicular carrier collection makes this structure optically thick and electronically thin. An optimal design is achieved through full-field numerical simulation, and a physical explanation is given. Our numerical results show that this configuration has ultra-broadband, omnidirectional, and polarization-insensitive responses and has a great potential in photovoltaics. Copyright © 2012 John Wiley & Sons, Ltd.

We systematically investigated the optical behaviors of an amorphous silicon solar cell with a core-shell nanograting structure. The propagating Bloch and Surface Plasmon Polariton waves lead to significant short-circuit current enhancements of this structure, compared with the conventional planar one. The optimal configuration has ultra-broadband, omnidirectional, and polarization-insensitive responses and has a great potential in photovoltaics.

A moth-eye anti-reflective structure was fabricated by hot-embossing and UV nanoimprint lithography on a solar cell protective film to suppress the reflection of incident light. Moreover, a superhydrophobic surface was developed by reducing the surface energy by forming a hydrophobic self-assembled monolayer coating on an anti-reflective structured resin surface. Therefore, the transmittance of incident light was increased by the anti-reflective structure. As a result, the solar cell efficiency was enhanced and the total accumulated electrical energy generated by a solar cell with a nano-patterned polymeric film was increased. The efficiency of each solar cell was evaluated by an analysis of its I-V characteristics using a solar simulator, and the external quantum efficiency according to the wavelength of incident light was analyzed by using an incident photon-to-current conversion efficiency system. Finally, the enhancement of the generated power was confirmed by a field test and a power charging experiment. Copyright © 2012 John Wiley & Sons, Ltd.

A moth-eye anti-reflective structure was fabricated by hot-embossing and UV nanoimprint lithography on a solar cell protective film. Moreover, a superhydrophobic surface was developed by forming a hydrophobic self-assembled monolayer on an anti-reflective structured resin surface. Because of the increase of transmittance of incident light, the solar cell efficiency was enhanced, and the total accumulated electrical energy generated by a solar cell with a nano-patterned polymeric film was increased.

In this paper, we present a theoretical model based on the detailed balance theory of solar thermophotovoltaic systems comprising multijunction photovoltaic cells, a sunlight concentrator and spectrally selective surfaces. The full system has been defined by means of 2n + 8 variables (being n the number of sub-cells of the multijunction cell). These variables are as follows: the sunlight concentration factor, the absorber cut-off energy, the emitter-to-absorber area ratio, the emitter cut-off energy, the band-gap energy(ies) and voltage(s) of the sub-cells, the reflectivity of the cells' back-side reflector, the emitter-to-cell and cell-to-cell view factors and the emitter-to-cell area ratio. We have used this model for carrying out a multi-variable system optimization by means of a multidimensional direct-search algorithm. This analysis allows to find the set of system variables whose combined effects results in the maximum overall system efficiency. From this analysis, we have seen that multijunction cells are excellent candidates to enhance the system efficiency and the electrical power density. Particularly, multijunction cells report great benefits for systems with a notable presence of optical losses, which are unavoidable in practical systems. Also, we have seen that the use of spectrally selective absorbers, rather than black-body absorbers, allows to achieve higher system efficiencies for both lower concentration and lower emitter-to-absorber area ratio. Finally, we have seen that sun-to-electricity conversion efficiencies above 30% and electrical power densities above 50 W/cm² are achievable for this kind of systems. Copyright © 2012 John Wiley & Sons, Ltd.

This paper investigates the optimum configuration of solar thermophotovoltaic systems comprising unconventional elements as multijunction cells and spectrally selective absorbers. The paper explains how using these elements, higher efficiencies can be achieved under less restrictive system design conditions.

We present a practical implementation of a solar thermophotovoltaic (TPV) system. The system presented in this paper comprises a sunlight concentrator system, a cylindrical cup-shaped absorber/emitter (made of tungsten coated with HfO₂), and an hexagonal-shaped water-cooled TPV generator comprising 24 germanium TPV cells, which is surrounding the cylindrical absorber/emitter. This paper focuses on the development of shingled TPV cell arrays, the characterization of the sunlight concentrator system, the estimation of the temperature achieved by the cylindrical emitters operated under concentrated sunlight, and the evaluation of the full system performance under real outdoor irradiance conditions. From the system characterization, we have measured short-circuit current densities up to 0.95 A/cm², electric power densities of 67 mW/cm², and a global conversion efficiency of about 0.8%. To our knowledge, this is the first overall solar-to-electricity efficiency reported for a complete solar thermophotovoltaic system. The very low efficiency is mainly due to the overheating of the cells (up to 120 °C) and to the high optical concentrator losses, which prevent the achievement of the optimum emitter temperature. The loss analysis shows that by improving both aspects, efficiencies above 5% could be achievable in the very short term and efficiencies above 10% could be achieved with further improvements. Copyright © 2012 John Wiley & Sons, Ltd.

A complete solar thermophotovoltaic system has been developed and tested outdoors. We have measured an overall conversion efficiency of about 0.8%, which to our knowledge, is the first solar-to-electricity efficiency reported for this kind of system. Details about the system development and characterization are given throughout the paper. Besides, we justify that an efficiency of 5% is attainable in the very short-term and that efficiencies above 10% are plausible in the mid-term with further improvements.

Traditional POCl₃ diffusion is performed in large diffusion furnaces heated to ~850 C and takes an hour long. This may be replaced by an implant and subsequent 90-s rapid thermal annealing step (in a firing furnace) for the fabrication of p-type passivated emitter rear contacted silicon solar cells. Implantation has long been deemed a technology too expensive for fabrication of silicon solar cells, but if coupled with innovative process flows as that which is mentioned in this paper, implantation has a fighting chance. An SiOx/SiN_y rear side passivated p-type wafer is implanted at the front with phosphorus. The implantation creates an inactive amorphous layer and a region of silicon full of interstitials and vacancies. The front side is then passivated using a plasma-enhanced chemical vapor deposited SiN_xH_y. The wafer is placed in a firing furnace to achieve dopant activation. The hydrogen-rich silicon nitride releases hydrogen that is diffused into the Si, the defect rich amorphous front side is immediately passivated by the readily available hydrogen; all the while, the amorphous silicon recrystallizes and dopants become electrically active. It is shown in this paper that the combination of this particular process flow leads to an efficient Si solar cell. Cell results on 160-µm thick, 148.25-cm² Cz Si wafers with the use of the proposed traditional diffusion-free process flow are up to 18.8% with a V_oc of 638 mV, J_sc of 38.5 mA/cm², and a fill factor of 76.6%. Copyright © 2012 John Wiley & Sons, Ltd.

This work presents a novel approach in utilizing implantation for silicon solar cells. First, phosphorus is implanted into textured silicon, then SiN_xH_y is deposited along with rear side passivation layers, then dopant activation is done using a metal-contaminated firing furnace. Cell results on 160-µm thick, 148.25-cm² Cz Si wafers with the use of the proposed traditional diffusion-free process flow are up to 18.8% with a V_oc of 638 mV, J_sc of 38.5 mA/cm², and a fill factor of 76.6%.

Investigations on the effect of direction of voltage sweeps, on the current density–voltage (J–V) characteristics in polymer bulk-heterojunction solar cells, based on the blend of poly(3-hexylthiophene) (P3HT) and phenyl [6,6] C₆₁ butyric acid methyl ester (PCBM), are reported with time. On the freshly prepared device, the direction of the voltage sweep did not have any effect; however, as the device started degrading, the change in direction of the voltage sweep resulted into different characteristics. Analysis beyond complete degradation, when all the photovoltaic parameters reduced to zero, revealed some interesting results. The J–V characteristics, measured with voltage sweep from −ve to +ve voltage, both in the dark and under illumination, were observed to pass through the second quadrant. On the other hand, with the change in the direction of voltage sweep, viz. from +ve to −ve voltage, the characteristics both in the dark and under illumination passed through the fourth quadrant. These results have been explained on the basis of polarization of the degraded active layer due to applied external voltage. This is an important effect and is observed to depend on the applied voltages during performance evaluation and becomes more prominent with time. This effect puts a question mark on the correctness of the method for calculation of the parameters of a degraded device. Studies on degradation of P3HT : PCBM solar cells showed that both the short circuit current density (J_sc) and the power conversion efficiency (η) decay exponentially, whereas the open circuit voltage (V_oc) decays almost linearly with time. Copyright © 2012 John Wiley & Sons, Ltd.

Degradation studies are presented on P3HT : PCBM solar cells with forward (+ve to −ve) and reverse (−ve to +ve) sweep of applied voltages. The direction of voltage sweep during performance evaluation is found to have crucial effect on the photovoltaic parameters. This effect is observed because of polarization of active layer due to degradation and depends on the applied voltage. The effect becomes more prominent with time. For seriously degraded cell, both the dark and illuminated density–voltage characteristics passed through the second quadrant for forward sweep and at the same time through fourth quadrant for reverse sweep.

The paper introduces a new type of series connection that considerably increases the active area and thus the efficiency of a thin film module by a superior arrangement of the patterning grooves. The new concept is mainly based on pointwise contacts instead of continuous, stripe-like contacts between adjacent cell stripes combined with a modified arrangement of the isolation grooves. We describe the functionality of the innovative series connection and present a method to calculate the optimal cell stripe geometry for cells based on the new series connection concept. Finally, the applicability of the new concept for a laser-patterned thin film silicon solar module is demonstrated. The new series connection leads to a relative efficiency increase of approximately 3% compared with the standard series connection for thin film modules. Copyright © 2012 John Wiley & Sons, Ltd.

The paper introduces a new type of series connection for thin film solar modules that is mainly based on pointwise contacts. We describe the functionality of the new concept and present a method to calculate optimal cell stripe geometries. Finally, the applicability of the new concept is demonstrated by means of an amorphous silicon thin film module. The new series connection leads to a relative efficiency increase of approximately 3% compared with the standard series connection.

For the first time, the sensitivity to impurities of the solar cell conversion efficiency is reported for a state-of-the-art (i.e., 18%) and advanced device architecture (i.e., 23%). The data are based on the experimental results obtained in the CrystalClear project for the state-of-the-art cell process and extrapolated to a device with excellent front and rear surface passivation. Both device structures are not assumed to work in low injection level as several studies assumed before, but real operating conditions are considered. This is a fundamental difference with the past and required for modeling future high efficiency devices. The impurity with highest impact is Ti, followed by Cu, Cr, Ni and Fe, which form together a group two order of magnitude less sensitive than the former. In high efficiency devices, a large reduction of the impurity impact is visible for impurities with large capture cross-section ratio like Fe, which reduces its relative difference in comparison with, for example, Cr, which has a small capture cross-section ratio. In general, advanced devices will be more sensitive to the impurity content than the state-of-the-art cell design. This effect is partly compensated by a reduction of the substrate thickness. The impurity sensitivity as function of the substrate thickness is reported. Copyright © 2012 John Wiley & Sons, Ltd.

For the first time, the sensitivity to impurities of the solar cell conversion efficiency is reported for a state-of-the-art (i.e., 18%) and advanced device architecture (i.e., 23%).

Both device structures are not assumed to work in low injection level. This is a fundamental difference with the past and required for modeling future high efficiency devices. In general, advanced devices will be more sensitive to the impurity content than the state-of-the-art cell design. The effect of the substrate thickness is also taken in consideration in the study.

A proof of concept study for a method of determining quantitative shunt values in silicon solar cells from photoluminescence images is presented. The method is based on interpretation of the luminescence intensity around a local shunt or recombination-active defect in terms of the extracted current. The theoretical relationship between the photoluminescence signal and the shunt current is derived. Experimental results on specifically prepared test structures show good agreement with known shunt resistance values. Experimental data on diffused wafers are presented. The effect of the front metallisation in complete cells on the appearance and interpretation of shunts in photoluminescence images is investigated experimentally. The limitations of the method are discussed. Copyright © 2012 John Wiley & Sons, Ltd.

A proof of concept study for determining quantitative shunt values from contactless photoluminescence images is presented. Experimental results calculated from specifically prepared test structures show good agreement with known shunt resistance values. The effect of the front metallisation on the appearance and interpretation of shunts in photoluminescence images is investigated experimentally.

In this paper, we demonstrate single-sided screen-printed emitters in thin monocrystalline Czochralski silicon (Cz-Si) wafers with an improved gettering of iron compared with conventional double-sided POCl₃ emitters. The phosphorus dopant pastes used have to be chosen carefully to provide a sufficiently low emitter sheet resistance and to avoid iron contamination. The iron concentration is determined in a non-destructive way from the minority carrier lifetime obtained by quasi-steady-state photoconductance measurements, down to levels not yet demonstrated for screen-printed emitters. In addition, the well-known metastable boron–oxygen complexes in Cz-Si have been transferred into a stable state by light-induced degradation prior to these measurements. Copyright © 2012 John Wiley & Sons, Ltd.

We demonstrate single-sided screen-printed emitters in thin Cz-Si wafers with an improved gettering of iron compared with conventional POCl₃ emitters. The phosphorus dopant pastes used have to be chosen carefully to provide a sufficiently low emitter shunt resistance and to avoid iron contamination. The [Fe] is determined in a non-destructive way from the minority carrier lifetime obtained by quasi-steady-state photoconductance measurements, down to levels not yet demonstrated for screen-printed emitters. Metastable B_SO_2i complexes have been transferred into a stable state by light-induced degradation prior to these measurements.

In this study, back-contacted back-junction n-type silicon solar cells featuring a large emitter coverage (point-like base contacts), a small emitter coverage (point-like base and emitter contacts), and interdigitated metal fingers have been fabricated and analyzed. For both solar cell designs, a significant reduction of electrical shading losses caused by an increased recombination in the non-collecting base area on the rear side was obtained. Because the solar cell designs are characterized by an overlap of the B-doped emitter and the P-doped base with metal fingers of the other polarity, insulating thin films with excellent electrical insulation properties are required to prevent shunting in these overlapping regions. Thus, with insulating thin films, the geometry of the minority charge carrier collecting emitter diffusion and the geometry of the interdigitated metal fingers can be decoupled. In this regard, plasma-enhanced chemical vapor deposited SiO₂ insulating thin films with various thicknesses and deposited at different temperatures have been investigated in more detail by metal-insulator-semiconductor structures. Furthermore, the influence of different metal layers on the insulation properties of the films has been analyzed. It has been found that by applying a SiO₂ insulating thin film with a thickness of more than 1000 nm and deposited at 350 °C to solar cells fabricated on 1 Ω cm and 10 Ω cm n-type float-zone grown silicon substrates, electrical shading losses could be reduced considerably, resulting in excellent short-circuit current densities of more than 41 mA/cm² and conversion efficiencies of up to 23.0%. Copyright © 2012 John Wiley & Sons, Ltd.

In this study, back-contacted back-junction n-type silicon solar cells featuring a large emitter coverage (point-like base contacts), a small emitter coverage (point-like base and emitter contacts), and interdigitated metal fingers have been fabricated. To prevent shunting, an insulating thin film is applied on the rear side. Hence, the charge carrier collection and the metallization geometry can be decoupled leading to a significant reduction of electrical shading losses and excellent short-circuit current densities of more than 41 mA/cm² and conversion efficiencies of up to 23.0%.

The effective doping concentration of the bulk of a silicon wafer is an important material parameter for photovoltaic applications. The techniques commonly used to measure the effective doping concentration are based on conductance or resistivity measurements and include both contacted methods, such as the four-point probe, and contactless approaches, such as eddy current measurements. Applying these techniques to diffused wafers is complicated by the fact that the total conductance is the sum of the bulk conductance and the diffused layer conductance. Without further information about the emitter properties, a clear separation of these two parameters is not possible. This paper demonstrates a contactless method for specifically measuring the effective doping concentration of the bulk without significant influence from diffused layers. Copyright © 2012 John Wiley & Sons, Ltd.

A new contactless method to detrmine the bulk doping concentration is presented. The method can be used throughout the solar cell fabrication process and on a wide variety of silicon substrates, without being affected by diffused or passivation layers. An excelent agreement was demonstrated between the post-diffusion doping concentration as obtained by the new method and the pre-diffusion doping concentration from dark conductance measurements.

The paper presents a simple approach to deriving I–V curves of photovoltaic panels and small arrays for arbitrary environmental conditions on the basis of three points of a single operating curve data and short current temperature coefficient only. The proposed method does not employ fitting of any type and is solely based on a numerical solution of a system of transcendental equations. The equations are expressed in a dimensionless form, simplifying both the solution and photovoltaic panel parameters' representation. The solution is used to find the values of normalized equivalent circuit elements for the available data and then perform an appropriate adjustment to obtain the operating curves for arbitrary conditions. The proposed method was applied to monocrystalline and polycrystalline commercial solar panels and was compared with both manufacturer-provided and experimentally measured operating curves to analyze the approach applicability and accuracy. Copyright © 2012 John Wiley & Sons, Ltd.

The paper presents a simple approach to deriving I–V curves of photovoltaic panels and small arrays for arbitrary environmental conditions on the basis of three points of a single operating curve data and short current temperature coefficient only.

A common misconception is that alkaline textured silicon solar cell surfaces are characterised by features that are pyramidal and bounded by {111} planes. In preference to the typical approach of observing scanning electron microscope images, we analyse reflection distributions from various pyramidal textures and find that {111} faceted pyramids are a poor approximation to the features on such surfaces. We conclude that features are hillocks, with an octagonal base. Furthermore, the characteristic base angle of the texture depends on the etchant and is closer to 50–52° than the commonly accepted value of 54.74°. Analyses of antireflection, light trapping, photogeneration and surface recombination properties of textured surfaces should take this feature morphology into account. The base angle has a strong influence on the hemispherical reflectance of the textured surface, with higher angles resulting in reduced reflectance. The influence of this reflection enhancement upon device performance is smallest when an optimised antireflection coating is applied; compared with an array of {111} faceted pyramids, a hillock morphology with 50° base angle results in a 0.2% reduction in photogenerated current in a typical cell. Additionally, as base angle is reduced, an encapsulant of increasingly higher refractive index is required to drive internal reflection at the air–glass interface of light initially reflected from the cell surface. The development of texturing processes resulting in higher base angles is encouraged. Copyright © 2012 John Wiley & Sons, Ltd.

A common misconception is that alkaline textured silicon solar cell surfaces are characterised by features that are pyramidal and bounded by {111} planes. We analyse reflection distributions from various pyramidal textures, and find that features are probably hillocks, with an octagonal base and with base angle closer to 50–52° than the commonly accepted 54.74°. Analyses of antireflection, light trapping, photogeneration and recombination properties of textured surfaces should be reconsidered. Development of texturing processes resulting in higher base angles is encouraged.

In this paper, a Distributed Maximum Power Point Tracking (D-MPPT) approach in photovoltaic (PV) applications is discussed. The proposed control method is suitable for the granular control of the PV generator at a module level or even at a sub-module level. D-MPPT is usually implemented by means of independent converters, each one of them running its own MPPT algorithm. Instead, the architecture proposed in this paper consists of only one digital controller, implementing a multivariable MPPT algorithm based on the Perturb and Observe approach, acting on a number of dc/dc converters, each one of them dedicated to a single PV module. The proposed control strategy reduces the number of current sensors with respect to the classical D-MPPT architecture and tracks the maximum power evaluated at the dc/dc converters' output. Planar solid immersion mirror simulations and experimental results confirm the validity of the approach and of the design guidelines proposed in the paper. Copyright © 2012 John Wiley & Sons, Ltd.

A novel approach to the Distributed Maximum Power Point Tracking (D-MPPT) in PhotoVoltaic (PV) applications is proposed for the granular control of the PV generator at a module level or even at sub-module level. The architecture consists of one central unit implementing a multivariable MPPT algorithm based on the Perturb and Observe approach. This solution reduces the number of current sensors with respect to the classical DMPPT architecture and tracks the maximum power evaluated at the dc/dc converters output. PSIM simulations and experimental results confirm the validity of the approach and of the design guidelines proposed in the paper.

This paper focused on the performance of photovoltaic-thermal (PVT) systems working in Bangkok for residential applications.

The PVT system is one which produces both electricity and low temperature heat at the same time. This paper investigated the performance of PVT systems that use different types of commercial solar PV panels. The characteristics of the PV panels were used as input parameters in the simulation. Each system comprises 2 m² of PVT collector area. Water draw patterns are those with a typical consumption of medium size houses in Bangkok, and the measured monthly average city water temperature of Bangkok has been used to estimate the energy output. The results show that the optimum water flow rate is 20 kg/h for all types of PVT collectors and the effect of water flow can improve the cell efficiency of PV cells. Moreover, the total energy output from the PVT collectors, which had glass covers is very significantly higher than those without one. The c-Si PVT panel gave the best performance with the highest rate of primary energy reduction. The payback time of each system is 6.4, 11.8, and 13.4 years for a-Si, mc-Si, and c-Si types of PVT system, respectively. This investigation concludes that from the viewpoint of system performance, c-Si PVT is the most promising type than whereas from the viewpoint of economy, a-Si PVT has the fastest payback time. Copyright © 2012 John Wiley & Sons, Ltd.

The optimum water flow rate is 20 kg/hr for all types of photovoltaic-thermal collectors, and the effect of water flow can improve the cell efficiency of photovoltaic cells.

This work describes a method developed for estimating the energy delivered by building integrated photovoltaics systems operating under non-standard conditions of irradiance and temperature. The method is based on calculation of the maximum power (P_Gmax) supplied by the modules array as a function of irradiance and ambient temperature, achieved by simulating its I–V and P–V curves using an algorithm which needs only the performance parameters supplied by the manufacturers. The energy generated by the PV system is estimated from monthly average values of P_Gmax calculated for using monthly average values of ambient temperature and irradiance obtained from data measured during 2 years. The method is applied to crystalline Si modules and tested by comparing the simulated I–V and P–V curves with those obtained by outdoor measurements as well as for comparing the energy produced during the years 2009 and 2010 with a 3.6 kWp building integrated photovoltaics system installed at the Universidad Nacional located in the city of Bogotá, Colombia, at 4°35′ latitude and 2.580 m altitude. The contrast of the simulated I–V and P–V curves for two different types of commercial Si-modules with those experimentally obtained under real conditions indicated that the simulation method is reliably. Copyright © 2012 John Wiley & Sons, Ltd.

This work describes a method developed for estimating the energy delivered by building integrated photovoltaics systems operating under non-standard conditions of irradiance and temperature. The method is based on calculation of the maximum power (P_Gmax) supplied by the modules array as a function of irradiance and ambient temperature, achieved by simulating its I-V and P-V curves using an algorithm that needs only the performance parameters supplied by the manufacturers.

We report on the performance of biomimicked antireflection coating applied to dilute nitride solar cell. The coating consists of nanostructures replicating the moth-eye geometry and has been fabricated by nanoimprint lithography directly within the window layer covering the dilute nitride absorbing junction. The mean reflectivity within the spectral range of 320–1800 nm remains under 5% for incident angles up to 45°. The effect of the coating on the cell performance was assessed by measuring the current–voltage characteristics under simulated solar illumination. A clear performance increase was identified when comparing a solar cell with the moth-eye coating with a solar cell having a standard SiN_x/SiO₂ coating. Copyright © 2012 John Wiley & Sons, Ltd.

We report on the performance of biomimicked antireflection coating applied to dilute nitride solar cell. The mean reflectivity within the spectral range of 320–1800 nm remains under 5% for incident angles up to 45°. The effect of the coating on the cell performance was assessed by measuring the current–voltage characteristics under simulated solar illumination.

This study focuses on the characterisation and the fabrication of solar cells using n-type multicrystalline silicon purified via the metallurgical route. Electrical and chemical analyses were performed on wafers taken from several positions along the crystallised ingot. The impact of the fabrication processing steps was investigated via effective carrier lifetime measurements. Solar cells were processed, and their efficiencies were found to be dependent on the position of the wafer along the ingot height, that is, the wafer's resistivity. A maximum conversion efficiency of 15.0% was obtained on cells from the bottom part of the ingot. In this study, the minimum resistivity value of 0.4 Ω cm resistivity is given in order to reach adequate cell efficiency. Light-soaking experiments were then performed on the fabricated cells. No significant variations of the cell performances were observed even after 110 h at 60 °C, meaning that the fabricated cells are stable under illumination. Copyright © 2012 John Wiley & Sons, Ltd.

In this study, solar cells were fabricated on n-type silicon purified via the mettalurgical route, and containing high amounts of boron and phosphorus. A maximum conversion efficiency of 15.0% is reported on cells from the bottom of the ingot with a resistivity of 0.4 ohm.cm. The fabricated solar cells are stable under illumination for the experimental light soaking conditions performed in this study.

Charge collection is one of the crucial processes to collect the induced current when a semiconductor sample is subjected to some external excitations such as the electron or photon beams. The charge collection probability is the basis in the study of this induced current particularly in the field of photonic devices, photovoltaic cells as well as in the characterization of semiconductor materials and devices. In this paper, the analytical expressions for the charge collection probability of the finite-dimension normal-collector configuration, with and without surface recombination at the free surfaces are presented. An excellent agreement has been found between the charge collection probability profiles computed using the presently derived analytical expressions and those obtained using a device simulator. The results have been used to study the effects of the various physical parameters on the charge collection probability. These analytical expressions are expected to enhance our understanding of the charge collection process. Copyright © 2012 John Wiley & Sons, Ltd.

The analytical expressions for the charge collection probability of the normal-collector configuration with finite sample width and sample thickness were derived using the Green's function method. The effects of the surface recombination velocities, the diffusion length, the size of the configuration, and the depth of the generation volume, on the charge collection probability, studied using these analytical expressions, are presented. The analytical results agree with those obtained using a device simulator, validating the correctness of these derived analytical expressions.

The variables presented in the current–voltage equation of a photovoltaic (PV) device are usually called PV parameters. There are several different methods for PV parameter extraction from measured data according to different models. However, many of these methods provide results that do not represent I–V curves of thin films devices correctly. This can occur because either the applied model or the PV parameter extraction methods are not suitable. It is also possible that the extracted parameters provide a good mathematical representation of the curves but without physical meaning (e.g. negative series resistance). This work presents a method for PV parameter extraction based on a modified double-diode model. In this model, the ideality factor related to the recombination of the charge carriers in the space-charge region is assumed as a variable. This method has been tested for different I–V curves of different PV module technologies providing very good results and parameters with physical meaning in all the cases. Copyright © 2012 John Wiley & Sons, Ltd.

This work presents a method for photovoltaic parameter extraction based on a modified double-diode model. In this model, the ideality factor related to the recombination of the charge carriers in the space-charge region is assumed as a variable. This method has been tested for different I-V curves of different photovoltaic module technologies providing very good results and parameters with physical meaning in all the cases.

We explore the potential utility of the II-IV-V semiconductor ZnGeAs₂ as the absorber material in solar cells. As-deposited ZnGeAs₂ films prepared by pulsed laser deposition are amorphous because of the limited substrate temperature that can be used without the rapid loss of volatile Zn and As. Thermal processing above 450 °C results in crystallization and improved electrical properties with hole mobilities as high as 58 cm²/V s. The annealed films were used to fabricate p-type ZnGeAs₂: n-type CdS cells on SnO₂-buffered borosilicate glass substrates in the so-called superstrate geometry. Light-induced currents of up to ~2 mA/cm² and open-circuit voltages of up to 470 mV were observed using backside illumination, indicating that these nascent devices hold potential for realizing high performance solar cells from earth-abundant elements. The performance of the devices fabricated to-date is degraded by conduction through shorts resulting from the presence of micron-sized pinholes in the absorber layer. Copyright © 2012 John Wiley & Sons, Ltd.

We described the first detailed fabrication and characterization of thin film solar cells based upon ZnGeAs2, a II-IV-V compound in the chalcopyrite family. We found these devices to have a small photoresponse and described physical limitations and improvement modes for device development

The electronic properties and the low environmental impact of Cu ₃ BiS ₃ make this compound a promising material for low-cost thin film solar cell technology. From the first principles, the electronic properties of the isoelectronic substitution of S by O in Cu ₃ BiS ₃ have been obtained using two different exchange–correlation potentials. This compound has an acceptor level below the conduction band, which modifies the opto-electronic properties with respect to the host semiconductor. In order to analyze a possible efficiency increment with respect to the host semiconductor, we have calculated the maximum efficiency of this photovoltaic absorber material. Copyright © 2012 John Wiley & Sons, Ltd.

From first principles, the electronic properties of the isoelectronic substitution of S by O in Cu₃BiS₃ have been obtained. The acceptor energy levels in the gap form a band in the alloying limit. Using the first-principles results, we have estimated the maximum efficiency of this photovoltaic absorber material.

Contactless photoconductance measurements are commonly used to extract the emitter saturation current density (J_oe) for crystalline silicon samples containing an emitter on the surface. We review the physics behind the analysis of J_oe and compare the commonly used approximations with more generalised solutions using two-dimensional device simulations. We quantify errors present in such approximations for different test conditions involving varying illumination conditions and surface properties in samples with the same emitter on both sides. The simulated J_oe obtained from the dark hole current from the emitter into the bulk is nearly the same as the simulated J_oe determined by photoconductance measurements of the rear diffusion. The simulated J_oe at the front emitter is equivalent to that at the rear emitter only when the sample is subject to a nearly constant and flat generation profile. For illumination conditions including visible light, the simulated J_oe at the front emitter is smaller than the simulated J_oe at the rear emitter. Both J_oe at the rear emitter and from the dark hole current in the emitter remain nearly constant over a wide range of base doping densities. The approximations used for the determination of J_oe from photoconductance measurements make J_oe dependent on the excess minority carrier density. Lifetime measurements demonstrate that, even in high-quality silicon, J_oe should be determined from the analytical solution as a function of excess minority carrier density including Shockley-Read-Hall recombination. Copyright © 2012 John Wiley & Sons, Ltd.

We review the physics behind the analysis of the emitter saturation current density (J_oe) and compare the commonly used approximations with more generalised solutions using two-dimensional device simulations. We show that the approximations used for the determination of J_oe from photoconductance measurements make J_oe dependent on the excess minority carrier density. Lifetime measurements demonstrate that, even in high-quality silicon, J_oe should be determined from the analytical solution as a function of excess minority carrier density including Shockley-Read-Hall recombination.

Rear sides of crystalline silicon solar cells are usually covered with aluminum on which it is difficult to solder. To ease soldering, we present a durability study for a Ni : V/Ag stack on evaporated Al as rear-side metallization. We adapt this cost-effective metallization stack from the microelectronic industry and investigate it as metallization for silicon solar cells. Here, a long-term stability of the metallization and of the solder joint must be guaranteed for 25 years and is therefore evaluated in detail by thermal aging experiments. During this experiment, the mechanical stability of the solder joints is measured. The chemical stability and the intermetallic compound (IMC) growth within the solder joints are examined by secondary electron microscopy, backscattered electron imaging, and energy dispersive X-ray analysis. Experiments with either a Sn–Ag-coated copper tab or pure Sn–Ag solder show two different sorts of IMCs at the Ni : V/Solder interface. With the copper tab, a Cu–Ni–Sn compound, presumably (Cu_1 - xNi_x)₆Sn₅, grows at the Ni/solder interface, whereas in case of a pure Sn–Ag solder, a Ni–Sn compound grows, which is likely to be Ni₃Sn₄. Analysis of the reaction kinetics leads to activation energies of 77 and 42 kJ/mol, respectively, for a diffusion-controlled IMC growth. By using temperature histograms of PV modules in the field, the necessary minimum Ni : V layer thickness is estimated: without a copper tab up to 1.6 µm Ni and with a copper tab less than 0.2 µm may be consumed by IMC formation during 25 years of lifetime. Copyright © 2012 John Wiley & Sons, Ltd.

A Ni : V/Ag stack on evaporated Al is investigated as rear-side metallization for silicon solar cells by a thermal aging of solder joints on this metallization. Depending on the presence of Cu on top of the solder, varying intermetallic compounds are formed at the Ni : V/solder interface. By analyzing the reaction kinetics and taking module temperature histograms into account, the necessary minimum Ni : V layer thickness for 25 years lifetime is estimated.