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Keywords:

  • high band-gap;
  • CIGS;
  • CuInxGa1 − xS2;
  • solution process;
  • solar cells

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES
  9. Supporting Information

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.

INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES
  9. Supporting Information

CuInxGa1 − xSySe2 − y (CIGSSe) thin film as an absorber layer is the most important component in CIGSSe thin film solar cells, which have been considered the most promising alternative to crystalline silicon solar cells [1]. CIGSSe is a direct band-gap material that offers high absorptivity, thus enabling highly efficient solar cell devices (~20%) with only ~2 µm thick absorber film [2]. In addition, CIGSSe also has a very attractive property whereby the band-gap of the material can be easily tuned by controlling the composition, which is potentially useful for tandem solar cell architecture. For example, by substituting In with Ga and/or Se with S, the band-gap of CIGSSe can gradually increase in the range of 1.0 (CuInSe2) and 2.4 eV (CuGaS2) [3].

In general, CIGSSe thin films have been fabricated by vacuum-based processes such as co-evaporation and sputtering techniques. However, to fabricate CIGSSe thin film more cost effectively, solution-based printing methods have also been intensively developed in recent years [4, 5]. Even though there are some differences depending on the methods, solution-based CIGSSe thin film preparation is generally conducted by (1) ink (or paste) preparation, (2) coating, and (3) heat treatment. The ink can be prepared with CI(G)S(Se) or CuInGa oxide nanoparticles that are synthesized beforehand or by precursors of metal nitrates, chlorides, acetates, or binary compounds (e.g. Cu2S, In2Se3, and Ga2Se3) directly dissolved in organic solvent [6-11]. Some organic binders and/or additives are frequently added to allow for suitable viscosity in the nanoparticle or precursor solution [12, 13]. The ink is then coated on conducting substrates (e.g. Mo-coated glass) via drop-casting, spin coating, doctor-blading, screen printing, and so forth [5]. To achieve proper thickness, the coating process can be repeated with a drying process at moderate temperature in the intervals [14]. After being coated on a conducting substrate, the films were annealed under inert gas, sulfur, or selenium environments to form polycrystalline CIG(S)Se alloy [4].

To date, most efforts for fabrication of solution-based CIGSSe thin film solar cells have been devoted to selenized compounds (e.g. CuInSe2 (CISe) or CuInGaSe2 (CIGSe)), which have relatively low band-gaps (<1.2 eV). When considering a tandem configuration of CIGSSe solar cells, however, the preparation of higher band-gap CIGSSe thin film is also required. In general, double junction CIGSSe tandem cells require CIGSSe films with band-gaps of 1.6–1.7 and 1.1–1.2 eV for top and bottom cells, respectively [15]. In addition, the higher voltage that can be achieved with the high band-gap materials is desirable in solar cell modules because energy loss due to series resistances can be reduced [3]. More importantly, the band-gap of around 1.5 eV is closer to the optimum value for theoretical maximum solar cell efficiency [16].

In this study, we suggest a precursor solution-based method for the preparation of high band-gap CuInGaS2 (CIGS) thin film. To achieve such a film, a two-step heating process was involved after coating of the precursor solution paste on an Mo-coated glass substrate: the first process was for the formation of the mixed oxide layer of Cu, In, and Ga with the elimination of carbon residue by air annealing; the second was for the formation of the CIGS alloy by sulfurization (Figure 1). In solution-based method, removal of residual carbon impurities is of importance because they may induce efficiency loss by creating series resistance through the formation of a thick carbon layer between absorber layer and metal electrode [17]. Open circuit voltage (Voc) of the solar cell device can be also diminished by facilitating the charge recombination process because of the presence of carbon impurities in the bulk of the absorber film. In addition to the residual carbon impurity problem, densely packed film morphology is also very crucial issue in the solution-based CIGSSe solar cells because heat treatment process (e.g. sulfurization and/or selenization) often resulted in absorber film with high degree of porosity [18]. The solar cell devices with porous absorber film often suffered from short-circuiting or low Voc related with leak current by direct contact of the buffer (e.g. CdS), window (e.g. ZnO), and metal electrode material (e.g. Al) with the Mo layer of the back electrode.

image

Figure 1. Schematic illustration of the CuInGaS2 (CIGS) thin film fabrication process.

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In this study, we show that a high band-gap (~1.55 eV) CIGS film can be fabricated by the simple precursor solution paste coating method, which resulted in minimum carbon residue and low degree of porosity. Because of these benign properties of the CIGS absorber film, the solar cell device with this film showed high power efficiency (8.28%) with high Voc (~787 mV). More importantly, in this method, most procedures, including paste preparation and film deposition, were conducted in air conditions, not in restricted conditions (e.g. in a glove box), which will yield more potential for commercialization.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES
  9. Supporting Information

A precursor mixture solution was prepared by dissolving appropriate amounts of Cu(NO3)2·xH2O (99.999%, Alfa Aesar, 1.0 g), In(NO3)3·xH2O (99.99%, Alfa Aesar, 1.12 g), and Ga(NO3)3·xH2O (99.999%, Alfa Aesar, 0.41 g) in methanol (7.0 mL), followed by the adding of a methanol solution (7.0 mL) with polyvinyl acetate (PVA) (Aldrich, 1.0 g). After the mixture solution was stirred with a magnetic bar for 30 min, a paste suitable for a spin coating was prepared. In this process, the composition of Cu, In, and Ga can be easily adjustable by controlling the amount of precursor.

The paste was spin-casted on the glass substrate, and the film was dried on a hotplate at 150°C for 3 min and subsequently at 250°C for 7 min. To obtain the desired thickness of the film (~1.2 µm), the aforementioned process was repeated about six times (generally ~200 nm thick film was obtained for each deposition). After coating and drying, the first annealing process, air annealing, was performed at 300°C for 30 min under ambient conditions. This annealing temperature was chosen to remove the binder material (PVA) on the basis of our thermal gravimetric analysis (TGA2960) (Figure S1) as well as to minimize possible oxidation of Mo layer of glass substrate. The second annealing process, sulfurization, was carried out at 500°C for 30 min under H2S(1%)/N2 gas environment.

A solar cell device was fabricated according to the conventional Mo/CIGS/CdS/i-ZnO/n-ZnO/Ni/Al structure. A 60nm thick CdS buffer layer was prepared on CIGS film by chemical bath deposition, and i-ZnO (50 nm)/Al-doped n-ZnO (500 nm) were deposited by the radiofrequency magnetron sputtering method. An Ni (50 nm) and Al (500 nm) grid was prepared as a current collector by thermal evaporation. The active area of the completed cell was 0.449 cm2.

Structural characterization of the films was performed using a scanning electron microscope (SEM, FEI inc., Hilsboro, OR, USA) with a 10kV acceleration voltage and an X-ray diffraction (XRD, Shimadzu, Tokyo, Japan) with Cu-Kα radiation (λ = 0.15406 nm). Optical properties were measured by ultraviolet-visible spectroscopy (UV-Vis, Varian, Palo Alto, CA, USA). Composition analysis was carried out with an electron probe microanalyzer (EPMA, JEOL Ltd., Tokyo, Japan) and energy-dispersive X-ray spectroscopy (EDX, E2V Tech. Inc., Elmsford, NY, USA). The depth profiling was conducted by Auger electron spectroscopy (AES, Ulvac-Phi, Kanagawa, Japan). The film thickness was measured with a surface profiler (Veeco, Plainview, NY, USA). Device performances were characterized using a class AAA solar simulator (Wacom, Saitama, Japan) and an incident photon conversion efficiency (IPCE) measurement unit (Soma Optics, Tokyo, Japan).

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES
  9. Supporting Information

The procedure of the CIGS thin film fabrication is illustrated in Figure 1. A paste with a composition ratio of 1:0.7:0.3 (Cu/In/Ga) was spin-casted onto an Mo-coated glass substrate. This process was repeated six times with drying in the intervals to achieve ~1.2 µm thickness. The film was then annealed at 300°C under ambient conditions to remove carbon residues from the organic solvent and/or the binder material. Notably, the amount of carbon residue was measured to be around the accuracy limit (~3 at.%) of our EPMA analysis. The formation of only amorphous states of the mixed oxide was confirmed by XRD where no apparent peaks appeared. The film also showed a densely packed film morphology with almost no pores as seen in SEM data (Figure S2).

To synthesize the CIGS alloy film, a mixed oxide film of Cu, In, and Ga was reacted with dilute H2S gas (H2S(1%)/N2) at elevated temperature (500°C). As can be seen in Figure 2(a), the XRD pattern shows a peak at 28.0° 2θ, with weak peaks at 32.5°, 46.6°, and 55.3° 2θ. The most intense peak, at 28.0° 2θ, indicates the polycrystalline CIGS alloy with a (112) orientation. The other prominent peaks correspond to the (204)/(220) and (116)/(312) phases. The presence of these peaks clearly indicates the polycrystalline chalcopyrite structure of CIGS, which is in good agreement with a Joint Committee on Powder Diffraction Standards reference (JCPDS #27-0159) as well as other reported values [19].

image

Figure 2. X-ray diffraction patterns (a) and absorbance (b), and typical cross-sectional (c) and top view (d) scanning electron microscope images of the CuInGaS2 (CIGS) thin film obtained after sulfurization. The inset of (b) is a plot of (αhv)2 versus hv for evaluating the band-gap. The boxes (marked by 1, 2, and 3) in (c) and (d) indicate the areas where energy-dispersive X-ray spectroscopy analyses were carried out, and the inset in (d) is an enlarged scanning electron microscope image corresponding to area 2.

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The morphologies of the CIGS films obtained after sulfurization were also investigated by SEM (Figure 2(c) and (d)). A densely packed film with very low degree of porosity was observed in the cross-sectional SEM image (Figure 2(c)). Interestingly, however, large crystallites were also formed on the upper part of the film, which induces rough surface morphology, as seen in the top view of the SEM image (Figure 2(d)). To elucidate the nature of these large crystallites, various composition analyses were performed. First, the composition profile with respect to the film depth was obtained by Auger depth profiling (Figure 3(a)). No apparent variation of Cu and S concentration was observed from the top to the bottom of the film. However, significantly lower Ga content was found concomitantly with higher In content at the upper part of the film up to the film depth of ~500 nm, implying that the crystallites on the upper part of the film can possibly be attributed to the formation of a Ga poor (or In rich) phase. The thickness of the rough layer was estimated to be 250–500 nm on the basis of the cross-sectional SEM images; this estimation also confirms the presence of Ga poor (or In rich) crystallites on the upper part of the film. Second, EDX analysis was carried out on several restricted areas as marked by rectangular boxes in Figure 2(c) and (d). Areas 1 and 2 reflect the composition of the dense layer region, whereas area 3 reflects that of the rough surface layer. Results showed that In was in abundance in the large crystallites, relatively (Table 1). Finally, we also performed EPMA analysis for the particular area where a large crystallite exists, as can be seen in Figure 3(b). Consistently, the Ga poor and In rich composition feature was clearly seen on the isolated crystallite (Figure 3(b)). Notably, these crystallites are not binary compound such as CuS2 because they remain even after the potassium cyanide (KCN) leaching treatment.

image

Figure 3. (a) The composition profile with respect to the CuInGaS2 film depth obtained by Auger depth profiling and (b) electron probe microanalyzer mapping of the surface area where a large crystalline exists on the CuInGaS2 film.

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Table 1. Composition ratio of the local area of the CuInGaS2 film obtained by energy-dispersive X-ray spectroscopy (EDX) and electron probe micro analyzer (EPMA).
Analytical methodAreaAtomic ratio
CuInGaS
  • *

    The atomic ratios were obtained by normalizing each atomic concentration with respect to the of Cu.

EDX11.000.75 ± 0.030.31 ± 0.052.17 ± 0.20
21.000.75 ± 0.040.29 ± 0.032.07 ± 0.10
31.000.99 ± 0.050.07 ± 0.022.13 ± 0.10
EPMA 1.000.78 ± 0.050.18 ± 0.021.91 ± 0.05

The optical property of the CIGS film was also investigated by ultraviolet-visible (UV-Vis) absorption spectroscopy. As can be seen in Figure 2(b), absorption showed below 900 nm. The estimated direct optical band-gap of the film is found to be 1.57 ± 0.02 eV (see Supporting Information). Because of the replacement of Se with S this value is much bigger than that of general CIGSe films (~1.1 eV) having a Ga/(Ga + In) composition ratio (~0.3) similar to that of our CIGS film [20].

Finally, solar cell devices were constructed using the CIGS films. The conventional configuration (Mo/CIGS/CdS/i-ZnO/n-ZnO/Ni/Al) and recipes were applied to fabricate devices in which a CdS buffer layer and a ZnO window layer were prepared by chemical bath deposition and sputtering deposition methods, respectively. Notably, antireflection coating was not applied for our solar cell device. As can be seen in Figure 4(a), the current density–voltage (J–V) characteristics of the devices revealed a highest efficiency of 8.28% with an open circuit voltage (Voc), short circuit current density (Jsc), and fill factor of 787 mV, 17.0 mA/cm2, and 61.9%, respectively. The IPCE data (Figure 4(b)) showed that the photocurrents were generated below 840 nm wavelength, which was well matched with the absorbance spectra of the CIGS film shown in Figure 2(b), implying that the photovoltaic response arises from the CIGS film. The band-gap can be also calculated from the IPCE data (see Supporting Information). As can be seen in the inset of Figure 4(b), a band-gap of 1.53 ± 0.01 eV was obtained; this is also consistent with the results from the absorbance data (1.57 ± 0.02 eV) as well as the photoluminescence data (1.55 eV, Figure S3).

image

Figure 4. (a) The current density–voltage characteristics and (b) incident photon conversion efficiency (IPCE) spectrum of the CuInGaS2 solar cell device. The inset in (b) is a plot of (αhv)2 versus hv for evaluating the band-gap using IPCE data.

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Despite the rough surface morphology, the power conversion efficiency of solar cell devices with the CIGS film is relatively high compared with the previously reported result [21]. We believe that it is ascribed from an extremely low degree of porosity of the CIGS absorber film. To verify this, the porous CIGS film with smoother surface morphology (Figure S4) was prepared by almost same fabrication procedure except organic binder material used in the paste preparation (use ethyl cellulose instead of PVA). In fact, we do not know detailed incorporation mechanism of binders in thin film synthesis, but we could observe significantly different surface morphologies of the mixed oxide (Figure S2) as well as the surfurized films (Figure S4) from those of the films synthesized by the paste with PVA. The solar cell device with the porous CIGS film showed only less than 2% solar cell efficiency (Figure S4) with very low Voc (<400 mV), implying the presence of shunt paths originated from high degree of porosity of the film. On the basis of our observations, we were able to conclude that the porosity of the film is a more important factor than surface roughness in photovoltaic performance.

We also compared the solar cell performance of our CIGS thin film with other representative CIGSSe thin film solar cell records listed in Table 2. The highest solar cell efficiency (20.3%) with a Voc of 740 mV was attained using a CIGSe film that was made by the co-evaporation method; its band-gap was 1.14 eV [2]. Meanwhile, the CIGSSe thin film solar cells with a high band-gap (>1.5 eV) generally show much lower efficiency than those with low band-gap. For example, only sulfurized film (CIGS) exhibited the best efficiency of 12.9% even though vacuum-based deposition method (e.g. sputtering) was applied for CIGS film preparation [22]. Furthermore, on the basis of our literature survey, there are few examples of solution processed high band-gap (>1.5 eV) CIGS solar cells with efficiency comparable with that of low band-gap CIGSe solar cells, as can be seen in the data in Table 2. Our result is the first demonstration of a highly efficient and low cost solution-processed high band-gap (>1.5 eV) CIGS thin film solar cell.

Table 2. List of preparation method and band-gap of absorber layer, and Voc and efficiency of representative CuInxGa1 − xSySe2 − y (CIGSSe) thin film solar cells.
ProcessDeposition methodAbsorber layerBand-gap (eV)Voc (mV)Eff. (%)Ref.
  • *

    Result of this study.

  • CISe, CuInSe2; CIGSe, CuInGaSe2; CIGS, CuInGaS2.

Vacuum processCo-evaporationCIGSe21.1474020.3[2]
SputteringCIGS21.5383512.9[22]
EvaporationCIGS21.5377612.3[23]
Solution processNanoparticlesCIGSSe21.263012.0[7]
NanoparticlesCIGSe252013.6[9]
NanoparticlesCISe21.04408.2[24]
Precursor solutionCIGSSe21.1662315.2[8]
Precursor solutionCIGSe21.135258.01[25]
Precursor solutionCIGSe21.054197.3[26]
Precursor solutionCIS23202.15[21]
Precursor solution[*]CIGS21.557878.28

One of major problems of solution-processed CIGSSe solar cells is the low Voc. In general, Voc drop arises from the recombination processes in solar cells. As can be seen in Table 2, the gap of Voc between the solution-processed and the vacuum-processed CIGSSe thin film solar cells is very large (mostly over 200 mV) even though the same device configuration and deposition methods for each layer were applied except for the preparation of the CIGSSe absorber layer. Whereas, our device showed a Voc of 787 mV, which is even higher than the best one made by the evaporation method (776 mV) [23], and 48–92 mV lower than the world best record for a vacuum-based CIGS solar cell with a similar band-gap CIGS absorber (1.53 eV) [19, 22]. The relatively high Voc of our solar cell device may result from diminishing the recombination processes because of a very densely packed morphology (preventing short-circuiting; therefore, minimizing leak current) as well as a very low carbon contamination (minimizing recombination centers from impurities) of the CIGS thin film.

CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES
  9. Supporting Information

High band-gap CIGS alloy films were prepared using a precursor solution paste. Because of the air annealing process, we were able to obtain an almost carbon impurity free CIGS film. In addition, despite the rough surface morphology, the film was revealed to have a very densely packed morphology, which resulted in high power conversion efficiency (8.28%) with high Voc (787 mV) for the solar cell device.

ACKNOWLEDGEMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES
  9. Supporting Information

This work was supported by the program of Korea Institute of Science and Technology (KIST). Also, the authors would like to thank the program of Converging Research Center Program through a National Research Foundation of Korea Grants (2012-K001271) and (2012, University-Institute cooperation program), funded by the Ministry of Education, Science and Technology. We also thank Prof. Jae Kyu Song for the measurement of PL data.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES
  9. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGEMENTS
  8. REFERENCES
  9. Supporting Information
FilenameFormatSizeDescription
pip_2354_sm_tableandfigures.docxWord 2007 document1450KSupporting info item

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