Advanced Energy Materials

The image depicts a series of electronic-type sorted single-walled carbon nanotubes suspended in solution. The rays of light emanating from behind the samples is suggestive of the solar energy that these materials help capture in organic photovoltaic cells. On p. 785 Mark Hersam and co-workers demonstrate that the green metallic solutions yield transparent electrodes with improved stability compared to their red semiconducting counterparts.

Efficient charge collection is important for achieving high conversion efficiency in solar cells. On p. 829 Hyun Suk Jung, Jin Young Kim, and coworkers show that three-dimensional transparent conducting oxide (TCO) substrates based on aligned In-Sn-O (ITO) nanowire arrays provide a universal strategy for improving charge collection properties of solar cells by significantly reducing the charge travel distance in the active material.

Nanostucturing is considered to be a promising strategy for thermoelectric materials with high efficiency (Z·T). Novel nanostructures such as multilayers, nanowires and nanomeshes based on traditional materials and silicon are of great interest for fundamental research. Nanograined bulk-materials, based on scalable synthesis processes, are thought to be worth implementing in outperforming standard devices for room temperature and high temperature applications.

A new chemical route to Cu₂ZnSn(S_xSe_1-x)₄ based thin film solar cells has been developed using spin coating of commercially available molecular precursors from an environmentally friendly non-toxic solvent. 4.1% efficiency solar cells were achieved after selenization of Cu₂ZnSnS₄. This technique could provide simple, facile, and reproducible fabrication for efficient and large area solar cells.

A hierarchically nanostructured composite of MnO₂/conjugated polymer/graphene is designed and fabricated for lithium ion batteries. The composite can produce a reversible capacity more than ten times that of plain MnO₂-based devices. The described approach can be used to create desired hierarchically nanostructured composite electrodes for broad applications in energy conversion/storage systems.

A ZnO–ZnS solid solution nanowire array photoanode is developed based on an alternative sensitization of a ZnO–ZnS solid solution nanowire array for solar hydrogen generation with considerably enhanced photocurrent – more than 195% greater compared to pristine ZnO nanowires. This solid solution structure demonstrates a better photoactivity enhancement effect than traditional quantum dot sensitization, as well as allowing hydrogen generation.

An integrated computational and experimental study of FeS₂ pyrite reveals that phase coexistence is an important factor limiting performance as a thin-film solar absorber. This phase coexistence is suppressed with the ternary materials Fe₂SiS₄ and Fe₂GeS₄, which also exhibit higher band gaps than FeS₂. Thus, the ternaries provide a new entry point for development of thin-film absorbers and high-efficiency photovoltaics.

Nanoconfined MgH₂ is destabilized compared to its bulk counterpart because of an interface energy effect. The hydrogen equilibrium pressure increases by an order of magnitude when decreasing the Mg layer thickness from 10 to 2 nm. This relates to an interface energy change of 0.3 J m⁻².

Graphite/silicon composite electrodes are prepared with PANa polymer as a binder. Morphological characters and electrode performance are compared with those of PVdF. The PANa layer behaves like SEI at the interface with ionic liquid, resulting in the highly reversible electrode performance.

The use of size-controlled Ag nanoparticles (NPs) synthesized by a polyol process in the BHJ shows improved values of J_sc, FF, and the EQE. These results are due to an enhanced light absorption caused by the plasmonic field effect of Ag cluster from NPs and improved charge transport in the active layer. Ag clusters reduce the series resistance and induce an efficient charge transport system in the BHJ active layer.

Conjugated small molecules can show excellent performance in bulk heterojunction solar cells. Using a simple solution spinning process, high power conversion efficiencies have been achieved by employing these molecules. As an example, DCAO7T/PC₆₁BM based solar cells exhibit power conversion efficiency values of up to 5.08%, with a short- circuit current density of 10.74 mA/cm², an open-circuit voltage of 0.86 V, and a fill factor of 55.0%.

Organic bulk heterojunction (OBHJ) photovoltaics, incorporating open-cage fullerenes with orifice-size of 8, 12, 13, 16 and 20-membered-rings as n-type materials, display power conversion efficiency (PCE) up to 2.9% by the 8-membered-ring fullerene under AM 1.5G irradiation; their PCEs decrease as the orifice-size of open-cage fullerene increases, primarily due to electron-mobility descends as the cage is ruptured to larger holes.

PEDOT nanotube arrays present a better catalytic activity with I⁻/I₃⁻ redox than standard platinized counter electrodes used for dye sensitized solar cells exhibiting thereby a photoconversion efficiency as high as 8.3%. The PEDOT layer introduces an additional series resistance which is compensated by its excellent catalytic performance yielding counter electrodes as good as platinized ones, or even better.

Single-walled carbon nanotubes (SWNTs) sorted by electronic type are employed as organic photovoltaic device anodes. Metal-enriched SWNT films yield device efficiencies that are fifty times greater than their semiconducting counterparts. Through sheet resistance, UV-vis-NIR optical absorbance, and X-ray photoelectron spectroscopy measurements, the OPV charge blocking layer PEDOT:PSS is found to reverse the original chemical doping of the SWNT films. The relative insensitivity of metallic SWNTs to chemical doping thus explains the improved performance of metal-enriched SWNT films as OPV anodes.

Spontaneous formation of dipoles at the P3HT:PCBM interface after annealing is proposed based on UV photoelectron spectroscopy data, the integer charge transfer model and photoinduced absorption data. The interfacial dipoles create an extra barrier for (non-)geminate recombination at the interfaces and thus can explain the reduced recombination and high efficiency observed in annealed regioregular-P3HT:PCBM solar cells.

Interconnected hollow carbon nanospheres (HCNSs) were prepared by pressure-assisted reduction and graphitization of sucrose in autoclaves without template. The obtained HCNSs with a large surface area, very thin graphitic shells, and an interconnected structure exhibit excellent performances as the electrode material for lithium ion batteries.

Conjugated polymer (P3HT)/fullerene (PCBM) blends sensitized by near-infrared quantum dots (PbS QDs) are investigated by optical spectroscopy. Pristine, binary and ternary blends of the materials with a systematic variation of the QDs to P3HT/PCBM ratio are studied by absorption, photoluminescence and pump-probe absorption. The study discusses charge transfer processes in ternary blends of organic semiconductors with near-infrared QDs for solution-processed photodetectors and photovoltaics.

Enhanced efficiency and stability of a high performance organic solar cell is demonstrated when the hole collecting indium-tin oxide contact is modified with solution-processed nickel oxide and the high work-function contact is matched with the donor ionization potential. We use photoemission spectroscopy to characterize the band edge positions in the contact layer, a multilayer matrix optical model and a diode model to explain the improvements in contact quality in comparison to the hole-transport layer, polyethylene dioxythiophene:polystyrene sulfonate.

A high-energy Li[Ni_0.54Co_0.12Mn_0.34]O₂ cathode material with two heterostructures, consisting of a layered core phase and a spinel shell phase, was developed using a PVP-functionalized coating of a Mn precursor on Li[Ni_0.7Co_0.15Mn_0.15]O₂. This material demonstrates a reversible capacity of 200 mA h g⁻¹ and an excellent cycling and rate capability at 60 °C. Most importantly, this material with two heterostructures demonstrates substantially reduced heat generation compared to Li[Ni_0.5Co_0.2Mn_0.3]O₂.

Core-shell nanowire array photoelectrodes composed of an ITO core and a TiO₂ shell are prepared using a vapor transport method and TiCl₄ treatments. Dye-sensitized solar cells fabricated using the core-shell nanowires exhibit extremely fast charge collection. The charge-collection time does not increase with increasing electrode thickness, implying that the thickness of solar cells can be increased without losing charge-collection efficiency.

Reversible degradation of V_OC and FF of inverted organic solar cells is demonstrated in experiments with concentrated sunlight and by forward biasing in the dark. The degradation could be prevented by simple electrical treatment and even fresh cells can be improved by the same process. The cell degradation and restoration are attributed to generation and degeneration of shunts in ZnO layer.

David Mitzi and co-workers have studied defects in Cu(In,Ga)Se₂ (CIGS), intentionally induced with controlled processing, with both material and device characterization techniques on p. 845. Structural defects indentified are directly correlated with defect levels extracted from admittance spectroscopy and device behaviors. Reduction of such defects leads to significant improvement of the device photovoltaic performance.

Defects in Cu(In,Ga)Se₂ (CIGS), intentionally induced with controlled processing, are studied with both material and device characterization techniques. Structural defects indentified are directly correlated with defect levels extracted from admittance spectroscopy and device behaviors. Reduction of such defects leads to significant improvement of device photovoltaic performance.

New copolymer semiconductors, based on thiazolothiazole and dithienosilole moieties (PSOTT, PSEHTT and PSOxTT), were synthesized and used to demonstrate effects of side chains on the performance of solar cells. Solar cells under one sun illumination showed an average power conversion efficiency of 2.1% (PSOxTT), 4.1% (PSOTT) and 5.0% (PSEHTT), demonstrating the dramatic effects of the type and topology of side chains.

The concept of dye sensitization of fullerenes is presented for organic photovoltaic devices. Enhanced light harvesting, current generation and power conversion efficiency are shown for sensitized solar cells using different donor materials, namely a discotic small molecule hexa-peri-hexabenzocoronene (HBC) and a near-IR absorbing donor polymer poly-cyclopenta-dithiophene-benzothiodiazole (PCPDTBT).

A detailed description of morphology is presented for blend films of a low-bandgap silole-containing conjugated polymer, poly[(4,4′bis(2-ethylhexyl)dithieno[3,2-b;2′ 3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl] (PSBTBT) with [6,6]phenyl-C61-butyric acid methyl ester (PCBM). Versatile techniques were used to characterize the morphology of the “As-Spun”, “Pre-Annealed”, and “Post-Annealed” thin films, mimicking the real solar cell devices, and to provide insight into the performance of these mixtures in actual devices.

Surface treatments are applied to photoanodes made of sub-micrometer-sized TiO₂ nanocrystallite aggregates of high surface area and roughness, for DSSC application. Pre-treatment of the fluorine-doped tin oxide (FTO) layer creates a thin underlayer of TiO₂, reducing recombination and improving contact with the FTO. Post-treatment creates a thin over-layer of amorphous TiO₂, which increases surface roughness for higher dye adsorption, enhances light-scattering and reduces recombination.

A simple, highly efficient, vacuum- processed small molecule solar cell based on merocyanine dyes – traditional colorants that can easily be mass-produced and purified – is presented. In the past, merocyanines have been successfully introduced in solution-processed as well as vacuum-processed devices, demonstrating efficiencies up to 4.9%. Here, further optimization of devices is achieved while keeping the same simple layer stack, ultimately leading to efficiencies beyond the 6% mark. In addition, physical properties such as the charge carrier transport and the cell performance under various light intensities are addressed.

A single-nanostructure electrochemical device framework is developed for testing the electrical properties of single Li battery nanostructures at different lithiation states and correlating these properties with structural changes via TEM. The electrical conductivity of single Si nanowires is found to vary by two to three orders of magnitude between lithiation and delithiation.

Porous Mn₃O₄/carbon aerogel composite electrodes prepared with a self-limiting anodic-electrochemical process, show outstanding capacitive properties and high energy and power densities because of the much-enhanced utilization of the functional Mn₃O₄ deposited as thin nanofibers along the backbone of the highly porous and conductive carbon aerogel matrix.

Efficient solid-state dye-sensitized solar cells are processed from porous TiO₂ photoanodes based on nanocrystals synthesized by laser pyrolysis. The strategy, which is compatible with production up-scale, allows a fine control of the nanocrystal physical properties. In comparison with commercial benchmark materials, the proposed photoanodes are found to greatly favor pore infiltration by the solid-state molecular glass, leading to improved device performance.

Graphene–cellulose paper (GCP) membrane materials fabricated by simple vacuum filtration are used as electrodes for flexible supercapacitors. The unique three-dimensional interwoven structure of graphene nanosheets and cellulose fibers equips the GCP with excellent mechanical flexibility, high rate capability and capacitance per geometric area of 81 mF cm⁻², and long cycling stability. GCP-based flexible polymer supercapacitors with various architectures are demonstrated.

Intra-molecular donor-acceptor interaction in photovoltaic polymers enhances inter-molecular donor-acceptor interaction in bulk-heterojunction solar cells, leading to an increase in charge dissociation, transport, and collection

The optical enhancement in conventional and inverted bulk heterojunction organic solar cells is systematically studied with respect to the anode and cathode buffer layers (BL). By removing parasitic absorption caused by low work function metals such as calcium, the photocurrent is increased by 25% leading to an improved efficiency of 3.9% for structures without calcium as a BL compared to reference cell performance of 3.2% using calcium as an electron accepting BL. Furthermore, the impact of the optical properties of transition metal oxides on device performance is assessed.

Binder-free flexible V₂O₅ nanowire (VNW)/carbon nanotube (CNT) composite paper electrodes are prepared for coin cell type (CR2032) supercapacitors. These composite paper electrodes show a highly porous VNW-CNT network that facilitates Li⁺ ion and electron diffusion, resulting in high power and energy performance. Additionally, Li⁺ ions are introduced to the VNWs during the synthesis, which further enhances the power (8.32 kW kg⁻¹) and energy (65.9 Wh Kg⁻¹) densities with improved cycling stability.

The power conversion efficiency of bulk heterojunction polymer solar cells based on a non-fullerene acceptor is enhanced 10-fold from 0.14 to 1.5% by using a processing additive in conjunction with an electron-blocking and a hole-blocking buffer layers. The morphology and device performance vary significantly with the concentration of the processing additive, reaching an optimum at 0.2 vol%, which is far lower than 2-3 vol% found in polymer/fullerene systems.

High efficiency bulk heterojunction solar cells based on a relatively recent polymer exhibit morphology and device physics that are very different from those of more commonly studied polymers. Though the polymer has achieved efficiencies exceeding 7%, its performance is limited by a distribution of hole-traps, which are related to the polymer morphology.

Bimolecular recombination in polymer solar cells was studied using different polymer: fullerene systems. Compared to regioregular poly(3-hexylthiophene) (P3HT), donor-acceptor polymer solar cells experience significant recombination loss at open circuit condition due to a higher degree of energetic disorder. Photo-induced carrier extraction with linear increasing voltage (Ph-CELIV) and transient photo-voltage (TPV) were carried out for the studies.