Nanowires, with their unique capability to bridge the nanoscopic and macroscopic worlds, have already been demonstrated as important materials for different energy conversion. One emerging and exciting direction is their application for solar to fuel conversion. The generation of fuels by the direct conversion of solar energy in a fully integrated system is an attractive goal, but no such system has been demonstrated that shows the required efficiency, is sufficiently durable, or can be manufactured at reasonable cost. One of the most critical issues in solar water splitting is the development of suitable photoelectrodes with high efficiency and long-term durability in an aqueous environment. Semiconductor nanowires represent an important class of nanostructure building block for direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. Nanowires can be readily designed and synthesized to deterministically incorporate heterojunctions with improved light absorption, charge separation and vectorial transport. Meanwhile, it is also possible to selectively decorate different oxidation or reduction catalysts onto specific segments of the nanowires to mimic the compartmentalized reactions in natural photosynthesis.
Recently, We have developed a fully integrated system of nanoscale photoelectrodes assembled from inorganic nanowires for direct solar water splitting. Similar to the photosynthetic system in a chloroplast, the artificial photosynthetic system comprises two semiconductor light absorbers with large surface area, an interfacial layer for charge transport, and spatially separated cocatalysts to facilitate the water reduction and oxidation. Under simulated sunlight, a 0.12% solar-to-fuel conversion efficiency is achieved, which is comparable to that of natural photosynthesis. The result demonstrates the possibility of integrating material components into a functional system that mimics the nanoscopic integration in chloroplasts. It also provides a conceptual blueprint of modular design that allows incorporation of newly discovered components for improved performance.

The push for the development of inexpensive, earth-abundant catalysts for photoelectrochemical water-splitting has demonstrated significant advancements. For example, Nocera&’s cobalt-phosphate and nickel borate catalysts have been successfully used for the oxidation of water with moderate overpotentials. However, these catalysts require near-neutral and weakly basic pH electrolytes in order to maintain their performance and usability. In order to successfully collect the generated fuels in photoelectrochemical cells (PECs), the use of electrolytic membranes and porous separators is necessity. In this study, we focus on identifying and measuring the series resistance losses that could arise in aqueous buffered-membrane electrolytes in PECs. Potentiometric and pH measurements were used to study the contribution of solution resistance, membrane resistance and pH gradient formation at 25 mA cm^-2. A comparison between different combinations of membranes and buffered electrolytes was done, in which we identified membrane pH gradient formation as the greatest contributor to voltage losses on the PECs.

CIGS (CuIn_xGa_1-xSe₂) is a well-known solar cell material, and should thus be an interesting material for solar water splitting applications. Despite this almost no work has been directed towards utilizing CIGS for renewable hydrogen production.
Here we demonstrate that CISG is a highly interesting material for solar hydrogen applications, with the potential of deliver photocurrents of technological importance. CIGS in itself has a suitable conduction band position for the hydrogen producing half-reaction, and photocurrents of 6 mA/cm² for the photo reduction are demonstrated. The stability in water under illumination is however a problem as for most other efficient materials. We demonstrate how the problem can be solved by spatially separating the charge carrier generation process from the catalysis step by increasing the distance of charge transport.
Finally we report results from a monolithic CIGS-device with over 8% solar-to-hydrogen efficiency for the total reaction.

The long term goal of our research is the direct conversion of carbon dioxide and water with visible light to a liquid fuel in a nanoscale assembly made of robust, Earth abundant materials. Focusing on inorganic heterobinuclear light absorbers and metal oxide catalysts, nanoscale assemblies are explored that afford the coupling of light absorbers and catalysts across a nanometer thin proton transmitting silica layer under separation of the water oxidation catalysis from all other photosynthetic processes. Effort ranges from establishing efficient visible light driven water oxidation catalysts and units for carbon dioxide photoreduction, to methods for the efficient charge transport across product separating silica membranes.
A photosynthetic unit consisting of a heterobinuclear ZrOCo(II) site anchored on a silica surface has been shown to reduce CO2 to CO, with the electrons provided by an Ir oxide nanocluster catalyst for water oxidation. Transient optical spectroscopy of such all-inorganic heterobinuclear groups excited to the metal-to-metal charge-transfer states revealed long lifetimes consistent with the photocatalytic activity of these light absorber-catalyst units. Very active Co3O4 nanoparticles for visible light sensitized water oxidation allowed us to detect surface reaction intermediates of oxygen evolution on a metal oxide catalyst in a time resolved manner for the first time. The technique used was rapid-scan FT-IR spectroscopy of an aqueous suspension in the attenuated total reflection mode. The temporal behavior and isotopic composition of intermediates allowed us to establish their kinetic relevancy and mechanistic role. In parallel, we have developed Co3O4(4 nm)/SiO2(2 nm) core/shell constructs with embedded molecular wires (oligo paraphenylenevinylene) for controlled charge transport from the visible light chromophore on the outside across the silica layer to the Co oxide catalyst core on the inside. Efficient hole transfer was demonstrated by transient absorption spectroscopy using spherical core-shell particles. Implementation in the form of Co3O4/SiO2 core shell nanotubes and nanotube arrays for closing of the photosynthetic cycle under product separation is in progress.

REFERENCES
[1] H.S. Soo, A. Agiral, A. Bachmeier, and H. Frei, J. Am. Chem. Soc. 134, 17104 (2012).
[2] A. Agiral, H.S. Soo, and H. Frei, Chem. Mater. 25, 2264 (2013).
[3] M. Zhang, M. De Respinis, and H. Frei, submitted.

Photoelectrochemical (PEC) water splitting is one of the ideal alternatives for hydrogen production from clean and abundant solar energy. La₅Ti₂CuS₅O₇ (LTC) was reported as a visible light responded oxysulfide semiconductor whose wavelength of absorption edge was at 650 nm equal with a band gap value of 1.9 eV [1-3]. Furthermore, the abundance of Ti, Cu and La element in the earth&’s crustal rocks are 6320, 68 and 35 ppm respectively, which is much higher than the widely studied PEC materials such as Ga (19 ppm) and In (0.24 ppm) [4]. We had reported that LTC exhibit photocatalytic activity for both water reduction and oxidation under visible light irradiation in the presence of sacrificial reagents [2,3]. These results indicate that LTC has the potential for overall water splitting to utilize a large portion of sunlight (lambda;< 650 nm) as a photocatalyst. Then, the LTC thin-film photoelectrode was fabricated by a novel particle transfer (PT) technology [5]. The PEC water splitting reaction was carried in a commonly used three-electrode system under visible light irradiation. The current-voltage curve for LTC thin film electrode under intermittent visible-light irradiation showed a clear photocathodic current, which implied a p-type semiconductor character. The correlation between the amount of H₂ evolution and photocatodic current showed a nearly 100% faradic efficiency on the LTC photoelectrode. Then, the zero-bias PEC water splitting was carried out by combining the LTC photocathode with other photoanode prepared by the PT method, which finally accomplished stoichiometric H₂ and O₂ production from water under visible light irradiation.
References
1. V. Meignen, L. Cario, A. Lafond, Y. Moelo, C. Guillot-Deudon, and A. Meerschaut, J. Solid State Chem., 177 (2004) 2810.
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3. T. Suzuki, T. Hisatomi, K. Teramura, Y. Shimodaira, H. Kobayashi and Kazunari Domen, Phys. Chem. Chem. Phys., 14 (2012) 15475.
4. N. N. Greenwood and A. earnshaw, Chemistry of the Elements, 2nd Edition, 1997.
5. T. Minegishi, N. Nishimura, J.kubota and Kazunari Domen, Chem. Sci., 4 (2013) 1120.

The development of sustainable energy forms is one of the most important problems of our time because of the ever increasing energy consumption together with the CO2 related climate problems. The conversion of solar light into electrons and holes which are used to split water into hydrogen and oxygen is one of the possible ways to address the world's pressing energy supply and storage problem. The properties determining the usefulness of a material to be used as light harvester in a photoelectrochemical cell include (i) a band gap in the visible range with band edges well positioned with respect to the redox levels of water, (ii) good mobility allowing electrons and holes to reach the surface before recombining, (iii) chemical/structural stability under irradiation, and (iv) low cost and nontoxicity. In previous works, we performed a computational screening for new materials with focus on one- and two-photon water splitting [1,2]. We found 20 and 12 promising materials for visible light harvesting in the one- and two-photon schemes in the space of 20000 cubic perovskites. Now, we move from the perovskite crystal structure and extend the screening to a more general collection of materials that are all known to exist in nature (as present in the Materials Project database [3]). The descriptors are the heat of formation, the bandgap, and the band edge positions. The heat of formation is calculated with respect to solid and dissolved phases using Pourbaix
diagrams [4]. The bandgaps are evaluated using the GLLB-SC functional recently implemented in the GPAW electronic structure code. The band edges are calculated using an empirical formula based on the electronegativities of the constituent atoms. Based on the screening, we suggest a handful of materials for the one- and two-photon water splitting devices for further experimental investigation.
References
[1] I.E. Castelli, T. Olsen, S. Datta, D.D. Landis, S. Dahl, K.S. Thygesen, and K.W. Jacobsen, Energy Environ. Sci., 5, 5814 (2012).
[2] I.E. Castelli, D.D. Landis, K.S. Thygesen, S. Dahl, I. Chorkendorff, T.F. Jaramillo, and K.W. Jacobsen, Energy Environ. Sci., 5, 9034 (2012).
[3] https://www.materialsproject.org/.
[4] I.E. Castelli, K.S. Thygesen, and K.W. Jacobsen, Accepted in Topics in Catalysis (2013).

Carbon-based catalysts have been attracting attention in renewable energy technologies due to the low cost and high stability, but their insufficient activity is still a challenging issue. Here, we demonstrate that graphene (Gr) can catalyze the hydrogen evolution reaction (HER); thus, Gr can enhance the performance of silicon (Si) photocathodes through a significant decrease in the overpotential.
To evaluate the photocathodic behavior of Gr loaded on a p-type Si (Gr-Si) electrode, a current density was measured as the potential was swept from 0.4 V to -1.0 V vs. Reversible Hydrogen Electrode (RHE). The potential of Gr-Si at -1 mA/cm² (the onset potential, V_OS) is 0.01 V vs. RHE, and this V_OS is a positive shift by 0.18 V compared to that of bare Si (-0.17 V vs. RHE). Dark current density was also measured using heavily arsenic doped n+ type Si electrode. In the dark condition, the positive shift in 0.14 V of V_OS (-0.49 V vs. RHE for Gr-Si) also shows higher activity for HER compared to that of the bare Si (-0.63 V vs. RHE), which results in the photovoltage of 0.50 V by depositing Gr on bare Si. This result indicates that Gr acts as an effective catalyst for HERs on the Si photocathode.
To investigate the electrocatalytic activity of Gr, cyclic voltammetry was measured with a rotating disk electrode using Gr-loaded glassy carbon (Gr-GC) electrode. V_OS for Gr-GC was -0.18 V vs. RHE; this V_OS is shifted positive by 60 mV compared to that for the bare GC. This result means that the Gr has electrocatalytic activity for HER. To quantitatively gain more insight into the catalytic activity of Gr, the potential-current density curves were converted into a plot of the potential as a function of the logarithm of current density; this plot is called a Tafel plot. If the electrochemical desorption step (H_ads + H₃O⁺ + e^- = H₂ + H₂O, the Heyrovsky reaction) is the rate-determining step for HER, a Tafel slope of 40 ~ 118 mV/decade is measured and is dependent of the value of the adsorbed hydrogen coverage (theta;_H = 0 ~ 1). The observed Tafel slope of 74 mV/decade in the current work indicates that the kinetics of the HER on Gr-GC electrodes is determined by the Heyrovsky reaction because theta;_H has an intermediate value (0 ~ 1). Moreover, the Gr-GC electrode also showed an enhanced exchange current density (J₀) of 2.73e-6 A/cm², which is higher than the J₀ value for bare GC (1.63e-6 A/cm²). The higher J₀ indicates that electron transfer or the adsorption/desorption of protons at the electrode/electrolyte can occur more easily with a lower kinetic barrier. From the Tafel analysis, the HER catalytic activity of the Gr catalyst is identified from the increase in J₀.
In summary, we have presented Gr catalyst that enhanced the photoelectrochemical performance of the Si-photocathode. Our approach in this study exploits a strategy to develop metal-free carbon-based catalysts with high efficiency for solar-driven hydrogen fuel production.

Recently, transition metal sulfides, such as MoS2 and NiS, have attracted considerable attentions as noble-metal-free co-catalysts for efficient photocatalytic H2 production. Herein, we developed a transition metal sulfide, NiS2, as a new noble-metal-free co-catalyst for photocatalytic H2 production. NiS2 co-catalyst was deposited onto the surface of graphitic carbon nitride (g-C3N4) via hydrothermal method. Results show that NiS2 co-catalyst exhibits an excellent H2 evolution performance which is even better than that of the commonly used noble metal cocatalyst, Pt. The 2wt% NiS2 deposited g-C3N4 photocatalyst displays a highest photocatalytic H2 production rate of 406.3 umol/h-1/g-1 from triethanolamine (TEOA) aqueous solution under visible light irradiation., which is 3 times higher than that of 1 wt% Pt loaded g-C3N4. Further studies reveal that the enhancement in photocatalytic H2 production by NiS2 deposition is due to efficient suppression of photoexcited electrons and holes recombination. This research also demonstrates the great potential of transition metal sulfides as noble-metal free co-catalysts for photocatalytic H2 production and provides a new insight on developing active metal sulfide co-catalysts.

Silicon is cheap, earth abundant material which is suitable for mass producible, solar energy conversion for water splitting. Although a p-type Si (p-Si) photocathode shows a negative potential appropriate for the hydrogen evolution reaction (HER) at the conduction band-edge, a large amount of applied overpotential is required to drive water splitting because the position of a conduction band is not negative enough to result in a kinetically reasonable HER. Adding metal-catalyzed nanoparticles, [1] metal-oxide thin film, [2] or an interface passivation layer [3] has recently been reported to overcome the kinetic limitation of Si photocathodes.
Here, we focus on how the schottky barrier height (SBH) has been increased by inserting an atomic-layer-deposited Al2O3 interlayer (ALD-Al2O3) into an interface between electrolyte and a Si substrate. This attempt provides a solution to mitigate the requirement of overpotential without additional optical losses. 5~20cycles of ALD-Al2O3 were applied on a lightly boron-doped (1~10Omega;cm) p-Si wafer. HER currents were measured in 0.5M sulfuric acid used for electrolyte. A significant amount of positive fixed charges observed in ALD-Al2O3 films electrically induced electrons to segregate at the Si surface such that an accumulation layer has been formed at the Si band-edge to enhance the SBH. As a result, a charge transfer resistance (Rct) of the photocathode was greatly reduced, and the overpotential of ~150mV was also decreased at a current density of 20mA/cm2. The remarkable increase in photocurrent was observed by a factor of 5.84 (from 1.80 to 10.52mA/cm2) at -0.4 V versus reversible hydrogen electrode (RHE). Tafel-plots and Mott-Schottky analyses reveal that a significant improvement in charge carrier transfer with a SBH enhancement is attributed to a thin interlayer of ALD-Al2O3 formed at the Si/electrolyte interface.
[1] I. Oh, J. Kye, and S. Hwang, Nano. Lett. 12(1), 292 (2012)
[2] K. Sen, N. Park, Z. Sun, J. Zhou, J. Wang, X. Pang, S. Shen, S. Y. Noh, Y. Jing, S. Jin, P. K. L. Yu, and D. Wang, Energy Environ. Sci. 5, 7872 (2012)
[3] F. L. Formal, N. Tétreault, M. Corniz, T. Moehl, M. Grätzel, and K. Sivula, Chem. Sci. 2, 737 (2011)

Earth abundant photovoltaic materials attracted significant attention about 10 years ago as the world focused on alternate energy generation to supplement and eventually replace fossil fuels as sources. Future projections at that time, of substantial long term market penetration by solar to achieve its full potential, drove the realization that the envisioned Terawatts capacity needed to satisfy the world demand could only be addressed by abundant photovoltaic materials for the anticipated capacity. At the beginning of this earth abundant PV development activity, the benchmark incumbent technology consisted of primarily both multi-crystalline and single crystal silicon, providing efficiencies of 11 - 18%, panel prices of $3 to $5/watt and global manufacturing capacity of single digit GW. Estimations of efficiency, cost and capacity projected 10 years forward to current (2013) at that time for both types of silicon seemed to be achievable by earth abundant PV solutions. Significant technological progress has been made since then by both the incumbent technology, silicon, as well as several earth abundant PV solutions. The economic viability of commercializing an earth abundant PV technological solution includes not only the requirement for abundance but the commercial solar solution also must integrate the overall material cost of the active device structure and supporting materials, the commercial scale manufacturing costs of the PV solution, the product requirements and system performance value to the customer relative to the incumbent offering. We will share our insights as to whether the earth abundant PV community is now closer to a viable commercial offering then 10 years ago.

Terawatt scale deployment of solar energy will demands the exploration of new high performance earth abundant optoelectronic materials options for photovoltaics and solar fuels. That implies a need for materials that are both abundant and sustainably harvestable from the earth&’s crust, and which also can be designed as high efficiency heterostructures. In this context, we outline the critical role of heterostructure interfaces in optoelectronic performance with case examples in cuprous oxide, zinc phosphide photovoltaic, and Zn-IV nitride semiconductors. We examine the dramatic dependence of open circuit voltage on interface stoichiometry at zinc oxide/cuprous oxide interfaces, and the effect of heterostructure design on current transport interfaces between zinc phosphide and ZnS, ZnSe, ZnO and CdS. Design of electrochemically stable photoanodes and photocathodes for solar fuels applications poses an additional constraint of anode-electolyte robustness against corrosion under widely varying pH conditions. We compare solar fuel photoanode designs based on stable oxides such as bismuth vanadium oxide with approaches based on interface passivation and protection of less-stable III-V compound semiconductor interfaces for solar fuels devices.

The II-IV-N₂ compounds are closely related to the wurtzite-structure III-N semiconductors, but have a mixed A-site composition. The choice of different group II and group IV elements provides chemical diversity that can be exploited to tune the structural and electronic properties of the II-IV- N₂ compounds. Specifically, ZnSn_xGe_1-xN₂ alloys with optical band gaps ranging from 2-3.1eV can be tuned to span a large portion of the solar spectrum, and can therefore be a viable earth-abundant light absorber and replacement for InGaN in nitride optoelectronic devices. They exhibit local order as demonstrated via X-ray absorption fine structure spectroscopy (EXAFS) and a linear relationship between the (002) peak position and composition in X-ray diffraction studies, indicating continuous access to the entire range of band gap values without phase separation or the need for more complicated growth strategies. The bowing parameter is 0.29 eV for the measured band gaps of ZnSn_xGe_1-xN₂, and 0.67 eV for the calculated band gaps. Although they are different, both values are significantly smaller than that of In_1-xGa_xN, indicating that the ZnSn_xGe_1-xN₂ alloy band gaps can be tuned almost linearly by controlling the Sn/Ge composition.
In this presentation we will describe theoretical studies of the optoelectronic behavior and defect physics of the ZnSnN₂ alloy series, as well as experimental investigations via X-ray absorption and emission spectroscopy of the alloys that probe the conduction and valence band partial density of states, which are in excellent agreement with first principles theory. Resonant inelastic scattering observations to provide understanding of the role of Ge when incorporated into the ZnSnN₂ lattice will be presented along with carrier dynamics of ZnSn_xGe_1-xN₂ alloys as elucidated from photoluminescence (room and low temperature) and pump-probe spectroscopy.

Zinc phosphide (Zn₃P₂) is a promising candidate for scalable photovoltaics, with a reported direct band gap of 1.5 eV and a long minority-carrier diffusion length (>5µm). However understanding the interface electronic properties of Zn₃P₂ is important to the performance of heterojunction devices incorporating Zn₃P₂ as a thin-film absorber. In this work, we have determined the energy-band alignment of epitaxial heterojunctions between Zn₃P₂ and n-type II-VI materials: ZnS, ZnSe, ZnO, and CdS as well as III-V materials: GaAs and GaP. The valence-band discontinuities were determined using high-resolution X-ray photoelectron spectroscopy measurements via the Kraut method. Amongst the band alignments measured, the ZnSe/Zn₃P₂ heterojunction demonstrated a large valence-band offset of -1.21 ± 0.11 eV and a negligible conduction-band offset of -0.03 ± 0.11 eV, indicating a nearly ideal alignment for a photovoltaic device. High-resolution transmission electron micrographs of the ZnSe/Zn₃P₂ interface showed that the morphological properties of the ZnSe epilayer were dominated by a thin (~1.5 nm) amorphous layer at the crystalline Zn₃P₂ surface. Various growth conditions were investigated in an attempt to remove this amorphous interfacial layer and improve the ZnSe crystallinity and the extent to which the films could be doped. Finally, the device properties of the ZnSe/Zn₃P₂ heterojunctions, including substrate and superstrate configurations grown epitaxially on GaAs, were characterized using current-voltage measurements performed under dark and simulated Air Mass (AM) 1.5, 1-Sun illumination. These results represent significant progress towards the realization of efficient, earth abundant Zn₃P₂ solar cells.

Semiconductors intended for use in photovoltaics (PV) are often limited by the unintentional incorporation of chemical impurities. Transition metals have long been viewed as one of the most problematic contaminants in PV absorbers; much effort has been invested into developing techniques to avoid or remove contamination that adversely affects device performance. One of the most important unanswered questions in this field involves the actual spatial distribution of contaminant atoms in processed material, which requires compositional characterization on the sub-100 nm scale. Recent advances have enabled Atom Probe Tomography to measure the atomic-scale spatial distribution of impurities in semiconductors with parts-per-million chemical sensitivity and sub-nanometer resolution, representing an entirely new, and very powerful, form of metrology. Atom Probe is not limited to crystalline samples, and has been shown to yield complete, detailed three-dimensional atomic reconstructions of compound semiconductors as well as oxides. Using silicon as a well-understood model system, we examine by Atom Probe Tomography the incorporation and segregation of transition metal impurities. Challenges involving the evaporation, ionization, and detection of these impurities are addressed, and prospects for rapidly localizing and identifying contaminants in other, novel Earth-abundant materials are discussed.

Pyrite (FeS2) is an earth abundant semiconductor that has experienced increased attention in the past few years. Much time has been invested on studying how to creating thin films of pyrite material for optoelectric devices utilizing spincoating/dipcoating solution-based nanocrystals and also chemical vapor deposition growth. Here we present a novel Iron Sulfide (FeS) nanowire ink precursor which can be used to create very uniform films, which can then be sulfurized to convert to the final pyrite material. Solar devices will be presented utilizing the pyrite thin-film which features a unique ZnO nanowire electron blocking layer which is shown to help improve device performance. This study opens up new, easily scalable strategies for pyrite thin film creation coupled with distinctive device fabrication.

Pyrite FeS2 is undergoing a tremendous resurgence of interest as a candidate thin-film solar absorber based on abundant, low-cost, and non-toxic elements. However, FeS2-based solar cells have suffered from low open circuit voltages (~0.1 V), and low efficiency, although the origins are not clear. Conduction mechanisms and doping are similarly poorly understood in FeS2, a simple example being the commonly observed p-doping in thin films, in contrast with the n-doping in typical bulk crystals. Understanding these issues could contribute significantly to improvements in FeS2-based solar cells. It is in this context that we have performed a comprehensive study of conduction mechanisms in FeS2 films synthesized via ex situ sulfidizing Al2O3(0001)/Fe films at temperatures in the range 100 C le; TS le; 700 C. In our initial work, we detected a crossover in transport mechanism around 450 C, from intergranular hopping to conventional charge transport [1]. Detailed analysis identified the origin as residual nanoscopic Fe clusters embedded in an FeS2 matrix, eventually extinguished at high TS. Significantly, the crossover was also accompanied by a three order of magnitude increase in Hall coefficient and a reversal of its sign. The apparent p-type conduction in the hopping regime was argued to be an artifact of hopping [1], challenging the viewpoint of predominantly p-type behavior in FeS2 films.
More recently, we have performed a detailed study of the response of these high TS transport properties to vacuum and S annealing, to probe possible Fe:S stoichiometry effects. As a function of vacuum annealing (up to 550 C) we observe a rapid crossover from close to diffusive to intergranular hopping transport. Structural characterization identifies formation of pyrrhotite (a FeS1±x phase), which we confirm to be highly conductive, again implying hopping due to nanoscopic secondary phase formation. Complementary annealing experiments in S were plagued by porosity problems. As a whole the annealing results show little promise for precise manipulation of the Fe:S ratio to control doping, at least when starting from these relatively heavily unintentionally doped (1020-1021 cm-3) low mobility (0.1-0.01 cm2/Vs) films. To address this high doping/low mobility, and to determine whether the n-type behavior we observe is general, we also synthesized FeS2 on multiple substrates. This is important given the possibility of potential dopant out-diffusion from the substrate. A variety of substrates were employed, including glasses, and other single crystal and amorphous substrates. Hall measurements revealed n-type behavior in all cases, but interestingly, with quite different electron density and mobility. Trends with respect to chemical constituents in the substrate will be discussed in detail, and have important implications for dopants in FeS2 films.
Work supported by UMN-IREE grant (RL-0004-11).
[1] Zhang, Manno, Baruth, Johnson, Aydil and Leighton, ACS Nano 7, 2781 (2013).

Numerical modeling of Hall effect data is used to demonstrate the existence of a strong hole inversion layer at the surface of high-quality n-type single crystals of iron pyrite (cubic FeS2) grown by a flux method. The presence of the hole inversion layer is corroborated by photoemission spectroscopy and electrochemical impedance measurements. The inversion layer can explain both the low photovoltage of pyrite solar cells and the universal heavy p-type conductivity of polycrystalline pyrite thin films that have together perplexed researchers for the past thirty years. We find that the thickness and hole concentration of the inversion layer can be modified by mechanical and chemical treatments of the pyrite surface, suggesting that it may be possible to eliminate this hole-rich layer by passivating surface and near-surface defects. Furthermore, an analysis of high-temperature electrical conductivity and optical transmission data firmly establishes that the electronic and optical band gap is in fact ~0.90 eV at room temperature, confirming that photovoltages in excess of 500 mV should be attainable from pyrite under solar illumination.

Iron pyrite (FeS₂), is an earth-abundant semiconductor that has generated recent interest due to its promising properties for solar energy conversion (band gap of 0.95 eV and high absorption coefficient α ~ 10⁵ cm^-1). Despite intensive efforts in synthesizing pyrite thin films there has not been any report of solar conversion efficiency. The only reported conversion efficiency has been for photoelectrochemical cells of pyrite single crystals which displays low photovoltage and low efficiency (< 3%). We hypothesize that the lack of photovoltage and the low performance in pyrite materials originates from intrinsic bulk and surface defects. In order to demonstrate our hypothesis, we report electrochemical and photoelectrochemical studies on pyrite single crystals grown by CVT, in contrast with device transport studies on single crystal pyrite nanorods and nanoribbons synthesized via sulfidation. Herein, we report the J-V characteristics, electrochemical impedance spectroscopy, and Mott-Schottky analysis of pyrite single crystals and the gating of field-effect transistors based on pyrite nanostructures, and discuss how the observed behaviors are a result of bulk defects and a Fermi level pinning which causes a surface inversion. We provide a comprehensive explanation into how a degenerately doped p-type surface inversion layer of pyrite that originates from a high density of intrinsic surface states is the main reason for the low solar performance in pyrite single crystals and the observed heavy p-type like transport in pyrite nanostructures. We will further discuss our efforts in passivating the surface defects in both pyrite single crystals and nanostructures.

Iron pyrite (cubic β-FeS2), an earth abundant and nontoxic semiconductor, has attracted resurgent attention as a promising candidate for solar energy conversion thanks to its suitable band gap (0.95 eV indirect, 1.03 eV direct), high absorption coefficient (~6×10^5 cm^-1), excellent resistance to photocorrosion for photoelectrochemical applications. However, the application of pyrite for solar cells has been hindered by its low open circuit voltage (< 200 mV) and thus low efficiency (~3%), which is likely the results of phase impurities, and rich bulk and surface defects. Therefore, it is pivotal to understand and reveal the mystery of pyrite electrical transport mechanism arising from bulk and surface defects. Single crystalline phase-pure pyrite nanoplates offer a versatile platform to study their physical properties. Here, we employ electrolyte field-effect gating and Hall effect measurements of pyrite nanoplates to investigate and confirm the n-type bulk pyrite with heavily p-doped surface inversion layer. Furthermore, by combining the multilayer Hall effect model and Poisson equation, we obtain key physical parameters including the bulk electron and surface hole carrier concentrations, mobilities, the thicknesses of surface inversion layer and depletion layer. This fundamental study confirms that the surface Fermi level pinning is one of the main obstacles that prevent pyrite to become a high performance solar material and will allow us to improve the solar performance of pyrite nanostructures and thin films.

Iron pyrite (cubic FeS2) with its exceptional optical absorption and suitable band gap is a promising candidate for earth-abundant thin-film solar cells. Using ambient pressure synchrotron x-ray spectroscopies, we report the electronic properties and phase transformation of pyrite thin films under in situ heating in ultrahigh-vacuum environment. The low-temperature Fe and S L-edge absorption spectra indicate an increasing density of states (DOS) above the valence band, and the nondestructive photoemission depth profiles also suggest an increasing DOS below the conduction band. Together they reveal a band gap narrowing related to surface states of defects and sulfur vacancies created by heating. Above 430 °C, x-ray photoelectron spectroscopy and in situ x-ray diffraction study indicate a phase transformation from pyrite to pyrrhotite, where a significant change of the S/Fe ratio from 2:1 to 1:1 occurs. The results provide crucial information on surface states and phase transition of pyrite based thin-film solar cells.

Kesterite Cu2ZnSn(S,Se)4 (CZTS) thin films are attracting a lot of interest as an alternative system to Cu(In,Ga)Se2 (CIGS) thin films, owing to their majority carrier type (p-type), proper band gap energy (1.0-1.5 eV), and high optical absorption coefficient (> 10^4 cm-1). More promisingly, the CZTSSe is composed of earth-abundant (cf. In in CIGS), environmentally-friendly (cf. Cd in CdTe), and relatively cheap elements. Here, we fabricated metallic Cu-Zn-Sn (CZT) precursor thin films via electrochemical deposition from aqueous metal salt solution on Mo-coated soda-lime glass substrates, and the influence of the subsequent sulfurization/selenization condition on the structural, electrical, and photovoltaic properties of the CZTSSe thin films was investigated. The as-deposited films are composed of binary metallic alloys, which can be converted to the highly crystalline CZTS phase after sulfurization at temperatures above 500 oC. The composition of the CZT film barely changes during the sulfurization, and small amount of CuS-based secondary phases exists even at 550 oC. However, a quick post-annealing KCN treatment effectively and selectively removes the secondary phase, evidenced by the Raman spectroscopy. The formation of the CuS-based secondary phase can be suppressed by slowing down the hearting rate during the sulfurization process, leading to an increased conversion efficiency of ~ 4%. The selenization process has been found to accelerate the crystallization process to CZTSe and the grain growth compared to the sulfurization process, and thus, to enhance the photovoltaic properties, exhibiting a high conversion efficiency of ~ 8%.

Given the terawatt-scale of future energy needs, the most promising future photovoltaic materials should be Earth abundant with their primary mineral resources distributed across several geographic regions and their supply chains robust to reduce concerns of price volatility. In addition, the process of forming the solar cell should be scalable, low-cost, and not utilize dangerous or toxic materials. The strongest initial candidate appears to be kesterite structures of Cu2ZnSn(S,Se)4 (CZTSSe) and similar alloy materials.
Conventionally, thin film chalcopyrite and kesterite solar cells have been synthesized by evaporating or sputtering metals followed by sulfurization or selenization. More recently, two potentially low-cost high-throughput approaches have been demonstrated that form the quaternary or pentenary chalcogenide directly from solution-phase processes. One is based on first synthesizing multinary sulfide nanocrystals and then sintering them to form a dense layer. The other approach utilizes molecular precursors dissolved in hydrazine. Both approaches reach their highest device efficiencies by incorporating Se to form Cu2ZnSn(Sx,Se1-x)4 devices, and each has yielded higher efficiency devices than the best vacuum deposited absorbers. The hydrazine route has yielded the most efficient CZTS-based devices thus far.
The presentation will focus on our development of the nanocrystal-ink based routes to materials and devices and a new molecular-ink route that utilizes benign solvents (avoiding the use of hydrazine). For both systems we will show the results of composition spread experiments coupled with spatially resolved photoluminescence and Raman scattering to reveal the effects of native point defects and doping on material quality and device performance. Finally, the current state-of-the art and performance limitations for the material will be review.

We examine photoluminescence spectra (PL) of Cu2ZnSnSe4/CdS/ZnO solar cells via temperature-dependent and illumination power-dependent measurements. Our cells are fabricated by H2Se selenization of sputtered Cu, Zn, Sn multilayers and show a total area efficiency of 9.2 % with an open circuit voltage of 416 mV and a short circuit current density of 38 mA/cm2. PL measurements offer an opportunity to study the defect states in the absorber layer.
The experimental results lead us to propose a recombination model for our cells that is able to explain both temperature dependent PL as well as power dependent PL results. At low temperatures and moderate excitation power, the quasi-donor-acceptor recombination (Q-DAP) between electrons localized at distorted donor states and holes at distorted acceptor states dominates. At higher temperatures and/or higher excitation power, the recombination involves electrons in the conduction band and distorted acceptor levels and the transition changes from Q-DAP to a quasi-free to bound transition (Q-FB). We can link the low Voc values generally observed in CZTSe solar cells to the presence of strong potential fluctuations in the absorber layer, as these potential fluctuations will give rise to tunneling enhanced recombination, thereby increasing the recombination currents as compared to the case without potential fluctuations.

Copper zinc tin sulfide (Cu₂ZnSnS₄ or CZTS) solar cells with the highest power conversion efficiencies are fabricated on Mo-coated soda lime glass (SLG), a carryover from Cu(In_xGa_1-x)Se₂ (CIGS) solar cells. In CIGS solar cells, Na diffusion from the SLG into the CIGS film has been shown to enhance the power conversion efficiency. Na diffusion is also expected when CZTS is deposited on Mo-coated SLG. In fact, SLG hosts a variety of other impurities such as K, Ca, Mg, and Al that may also diffuse into CZTS. However, a systematic investigation of whether these impurities diffuse into CZTS and how they affect the film properties has not yet been conducted. To this end, we have investigated the effects of the substrate and the intentional addition of individual impurities on the microstructure and electronic properties of CZTS films. Thin CZTS films were synthesized via ex situ sulfidation of Cu-Zn-Sn films co-sputtered on a variety of substrates, including, crystalline quartz, amorphous quartz, sapphire, SLG and Pyrex. These Cu-Zn-Sn precursor films were then loaded into quartz ampoules with 1 mg of S, evacuated to 10^-6 Torr, sealed and sulfidized at 600 ^oC for 8 hours. The sulfidized films were then characterized using a suite of techniques including X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Concentration depth profiles were examined using time-of-flight secondary ion mass spectrometry (TOF-SIMS). CZTS films synthesized on SLG were found to have significantly larger grains than films grown on any of the other substrates. Furthermore, we found that by simply including a bare additional piece of SLG in the sulfidation vessel, the grain size of films grown on impurity-free quartz increases from 100's of nm to greater than 1 mu;m. This conclusively demonstrates that the impurity atoms found in SLG are volatilized in a sulfur atmosphere and transported via the vapor phase to neighboring films. TOF-SIMS experiments implicated Na, K and Ca as the impurities responsible for this enhanced grain growth. To investigate the effects of these impurities individually, we introduced very small and controllable amounts of either Na, K, or Ca into the sulfidation ampoule. By including impurities at levels as low as 10^-6 moles of Na, or 10^-7 moles of K, in the 8 cm³ sulfidation ampoule, the grain size of CZTS on quartz substrates was increased dramatically from 100's of nm to greater than 1 mu;m, while Ca loading had little effect. The electronic properties of CZTS films synthesized with different amounts of impurities in the sulfidation tube were also studied using temperature dependent conductivity and Hall effect. The effects of alkali metal impurities on the microstructure and electrical properties of CZTS films will be discussed in the context of their applications to solar cells.
Work supported by National Science Foundation through CBET-0931145 and in part by the UMN Initiative for Renewable Energy and the Environment (IREE).

Tin monosulfide (SnS) is a candidate Earth-abundant solar cell absorber material. It is attractive due to its high absorption coefficient (α > 10⁴ cm^-1)¹, suitable band structure (1.1 eV indirect and 1.3 eV direct) and potentially high carrier mobility (Hall mobility reported above 100 cm²/Vs).² SnS has sufficient elemental abundance to reach terawatt levels of photovoltaic module production, which cannot be reached by CdTe and CIGS because of the limited availability of Te and In.
Industrial production of both CdTe and CIGS involve crucial efficiency-boosting annealing steps that increase grain size, improve electrical properties and reduce interface defects. The analogous process for tin monosulfide has yet to be explored and optimized.
It was previously shown that RF sputtered SnS films annealed in H₂S ambient at 400 °C show promising grain growth from near amorphous, ~10 nm grains to a 40-200 nm range. The Sn/S ratio shifted from sulfur-rich (Sn/S < 1) to a stoichiometric ratio of ~1.0 after annealing. Additional thermodynamic simulations suggest a phase evolution during annealing³, indicating a natural tendency of this material toward SnS “phase purification.” This is a significant result, because minority phase formation can be difficult to control in films grown rapidly by industrial methods. The effects of annealing in a sulfur-containing, or H₂S gas, ambient are suggested to promote grain growth and control of the Sn/S ratio.
We hypothesize that annealing in a sulfur-containing atmosphere may fill sulfur vacancies, which have been calculated to lie near mid-gap for SnS.¹ Filling these sulfur vacancies should improve solar cell performance by reducing Shockley-Reed-Hall carrier recombination. The use of a mixed H₂S + H₂ gas environment allows fine control over the S₂ gas partial pressure, offering the possibility of point defect control in SnS films.

We will report the effects of annealing thermally evaporated SnS thin films in 4% H₂S + 4% H₂ (N₂ balance). We characterize changes in grain structure, optical properties, and electronic properties using SEM, XRD, 4-point-probe resistivity measurements, Hall effect measurements, and spectrophotometry. Current methods for producing thermally evaporated SnS solar cells reach an efficiency of approximately 2%. We will present the results of H₂S + H₂ annealing on solar cell performance, using an established device stack: glass/Mo/SnS/ZnO_xS_y/ZnO/ITO/Ag.⁴

[1] J. Vidal, S. Lany, M. d&’Avezac, A. Zunger, A. Zakutayev, J. Francis, J. Tate, Appl. Phys. Lett. 100 (2012) 032104.
[2] K.T. R. Reddy, N. K. Reddy, and R.W. Miles, Sol. Energy Mat. Solar Cells 90 (2006) 3041.
[3] V. Piacente, S. Foglia, P. Scardala, J. Alloys Compd. 177 (1991) 17.
[4] P. Sinsermsuksakul, K. Hartman, S. Bok Kim, J. Heo, L. Sun, H. Park, R. Chakraborty, T. Buonassisi, R. G. Gordon, Appl. Phys. Lett. 102 (2013) 053901.

In order for photovoltaic (PV) technology to contribute significantly to society&’s energy supply, device components must be abundant, cheap and environmentally benign. One candidate that satisfies these criteria as well as exhibiting almost ideal electronic properties is the photo-absorber tin sulfide. SnS is reported to have a higher optical absorption coefficient and a more suitable band gap for light absorption at peak intensity than current commercially available materials. However, the record device efficiency for SnS PV cells is only 2.0 % to date,[1] far below those obtained for similar absorbers.

We employ a combination of first-principles calculations and single crystal growth to study the multiphasic tin sulfide system with the goal of identifying the limiting attributes for PV technology. Our methods provide insight into thermodynamic stabilities, reaction pathways and electronic configurations, which allow us to ultimately comment on the photovoltaic applicability of a given structure. We are also able to predict the characteristic signatures of intrinsic chemical and physical phenomena and suggest measurements in order to identify them.
Our key results include the prediction that a recently reported structure of SnS, zinc-blende, has been mis-assigned. This phase is unstable, with large negative phonon modes and spontaneous distortions upon introduction of moderate conditions (e.g. 300K) in simulation.[2] We have developed a clear synthetic route for obtaining phase pure materials, which have been characterised using X-ray diffraction, Raman spectroscopy and time-resolved microwave conductivity measurements. Finally, our electronic calculations reveal a band mismatch between SnS and common PV device components; i.e. the molybdenum metal contact. Beyond this, we are able to suggest optimal contacts that would allow for the full photovoltaic potential of tin sulfide to be achieved.[3]
1) P. Sinsermsuksakul, K. Hartman, S. B. Kim, J. Heo, L. Sun, H. H. Park, R. Chakraborty, T. Buonassisi, R. G. Gordon; Appl. Phys. Lett., 102, 053901 (2013). http://dx.doi.org/10.1063/1.4789855
2) L. A. Burton and A. Walsh; J. Phys. Chem. C 116, 45, 24262 (2012). http://dx.doi.org/10.1021/jp309154s
3) L. A. Burton and A. Walsh; Appl. Phys. Lett. 102, 132111 (2013). http://dx.doi.org/10.1063/1.4801313

Bulk photovoltaic systems can provide a solution for decentralized and grid-compatible electricity generation. Tin sulfide (SnS), a layered metal chalcogenide, showing high optical absorption, has been identified as an attractive material for photovoltaic absorbers [1]. We used density functional theory methods to study trends in the electronic and optical properties of model single-layer, double-layer and bulk structures of SnS.
We find that the optoelectronic properties of the material can vary significantly with respect to the number of layers and the separation between them. For instance, the calculated band gap is wider for fewer layers and increases with tensile strain along the layer stacking direction [2]. We conclude from these results that either a restriction of the number of layers or the application of uniaxial strain could be used to obtain improvements in the performance of SnS as absorber material.
[1] H. Noguchi, A. Setiyadi, H. Tanamura, T. Nagatomo, and O. Omoto, Solar Energy Materials and Solar Cells 35, 325-331 (1994)
[2] G. A. Tritsaris, B. D. Malone, E. Kaxiras, Journal of Applied Physics 113, 233507 (2013)

Materials with physical properties that are insensitive to the presence of defects in crystal structure are rare but very desirable in solar energy conversion applications. For example Cu(In,Ga)Se2 (CIGS) can have enormous deviations from nominal compositions yet still result in good performance. Unfortunately, Cu(In,Ga)Se2 contains elements that may constrain its large scale fabrication in the future, so development of the next generation of Earth-abundant absorbers with similar properties is highly desirable. Here we demonstrate defect tolerance in copper nitrides, a new class of next-generation Earth-abundant solar absorber materials, using a binary copper nitride Cu3N. This prototypical copper nitride material is known to have optical absorption and electrical transport properties that are reasonably suitable for solar absorber applications and achievable at low substrate temperature synthesis conditions: 1.5 eV absorption onset, 0.9 eV band gap and 10^15 - 10^17 p-type doping at ~100 C synthesis temperature.
Defect tolerance of Cu3N is experimentally manifested by insensitivity of its electrical conductivity to the presence of point defects as well as low-angle and high-angle grain boundaries identified by the means of diffraction, microscopy and spectroscopy measurements. In the pure semiconducting Cu3N electrical conductivity is 10^-3 -10^-2 S/cm, regardless of these of crystallographic imperfections. Defect tolerance of electrical conductivity in Cu3N is theoretically supported by first principles calculations of point defects in this material. According to the theoretical results, Cu3N has no deep bulk point defects that can act as scattering centers for majority charge carriers in the experimental conductivity measurements or as recombination traps for minority charge carriers in solar energy conversion applications.
We propose that the observed defect tolerance of electrical properties in Cu3N originates from its anti-bonding character of the valence band maximum. In semiconductors with such electronic structure, crystallographic defects states are likely to fall in the energy bands, not in the energy band gap. This in turn leads to effective-mass-like shallow defect states close to the band edges that cause minimal scattering to both majority and minority electric charge carrier transport. This explanation of defect tolerance in Cu3N invokes only the character of the involved atoms and their electronic energy states. Therefore, it is likely that other copper nitrides, in particular ternaries, will also have similar defect tolerant properties. This leads to a conclusion that copper nitrides are a new class of defect-tolerant next-generation Earth-abundant solar absorber materials.
This research is supported by the U.S. Department of Energy, office of Energy Efficiency and Renewable Energy, as a part of a Next Generation PV II project “Ternary copper nitride absorbers” within the SunShot initiative.

A series of copper transition metal nitrides have been synthesized by using different methods, including ion-exchange and high pressure reactions. A combination of theoretical calculations and experimental studies were applied for the analyses of electronic structures and optical properties of these materials. Our results indicate that ternary copper nitrides may have considerable potential as absorbers in earth abundant solar cells. For example, layered CuTaN2 was synthesized by an ion exchange reaction of CuI and NaTaN2 as previously reported. Based on the results of EDX analysis, the Cu:Ta ratio of the CuTaN2 sample was1:1 within the overall errors when examining powders of ±10 % and no Na was detected. The crystal structure and thermal stability of CuTaN2 was accurately determined by Rietveld analysis of the powder X-ray Diffraction profile and by TGA analysis, respectively. CuTaN2 crystallizes in a rhombohedral structure with space group R-3mH as shown in [Figure 1]. CuTaN2 possesses a band gap of 1.53(x) eV, which is in reasonable agreement with density functional theory calculations of Cu containing nitrides. Similar materials may be even better suited for solar cell application.

During the last two decades there has been a proliferation of solution-phase batch colloidal nanocrystal synthesis methods. Solution-phase processes offer excellent control over the size and composition of nanocrystals but there is a perspective gaining momentum that the ligands attached to the nanocrystal surfaces and the presence of residual solvent degrades performance when these nanocrystals are incorporated into thin films and optoelectronic devices. Using gas-phase synthesis and gas-phase deposition of nanocrystals, it is possible to form films with nearly the ideal random close packed density without exposure to solvents or need for ligands. Our group&’s work to date has focused on nonthermal plasma synthesis of silicon and germanium nanocrystals followed by dense film formation via inertial impaction. However, very little attention has been paid to compound semiconductors. In particular, the plasma synthesis of technologically important metal-sulfide nanocrystals remain completely unexplored.
We have developed a new generalizable approach for making metal-sulfide nanocrystals in the gas phase using a nonthermal sulfur-argon plasma. A metalorganic precursor and cyclic sulfur molecules are dissociated by electron impact reactions to form metal sulfide nanoparticles with controllable composition and controllable size. For example, zincblende (cubic) ZnS nanocrystals, were formed using diethyl zinc (DEZ) as the Zn precursor. Scherrer analysis of the X-ray diffraction (XRD) peak broadening and transmission electron microscopy (TEM) agreed and indicated nanoscrystals with diameters less than 10 nm. The metal to sulfur ratio could be controlled through the DEZ feed rate. Surprisingly, the elemental sulfur feed rate, in the range we explored, had little effect on the sulfur content of the particles. Sulfur rich products (low DEZ feed rate) exhibited visible light absorption while stoichiometric ZnS (high DEZ feed rate) showed negligible absorption in the visible range of the spectrum and a clear absorption onset near the bulk bandgap of ZnS (3.7 eV). Crystalline copper sulfides were made using hexafluoroacetylacetone Cu(+1) vinyltrimethylsilane (HFAC)Cu(VTMS) as the copper precursor. Depending on the feed rate of the (HFAC)Cu(VTMS) relative to sulfur, various phases were observed by XRD, from Cu metal, to Cu2S, to CuS. This particular copper precursor exhibits a rich plasma chemistry. Interestingly, if the present results on ZnS and CuxS are compared to previous experiments on Si and Ge, both the CuxS and ZnS nanocrystals are much smaller than expected from the precursor partial pressures and the residence time. This suggests that the metal sulfide growth is dominated by nucleation, in contrast to the Si and Ge which exhibit characteristics of surface growth. Finally, the synthesis of tin sulfides and iron sulfides will be discussed.

Thin film solar cells consisting of earth-abundant and non-toxic materials were made from pulsed chemical vapor deposition (pulsed-CVD) of SnS as the p-type absorber layer and atomic layer deposition (ALD) of Zn(O,S) as the n-type buffer layer. Solar cells with a structure of glass/Mo/SnS/Zn(O,S)/ZnO/ITO/Ag are studied by varying treatments of the SnS and Zn(O,S) layers. Annealing SnS in pure hydrogen sulfide increased the mobility by more than one order of magnitude, and improved the open-circuit voltage up to 273 mV, fill factor up to 52%, and power conversion efficiency up to 2.2%, which are higher than our current champion cell [2]. A short-circuit current density of 16 mA/cm² was achieved. Solar cell performance will be further optimized by adjusting the oxygen to sulfur ratio of Zn(O,S) [1,2] and by in situ nitrogen doping using ammonium hydroxide to serve as both an oxygen and nitrogen source. [1] H. H. Park, R. Heasley, and R. G. Gordon, Appl. Phys. Lett.102, 132110 (2013). [2] P. Sinsermsuksakul, K. Hartman, S. Kim, J. Heo, L. Sun, H. H. Park, R. Chakraborty, T. Buonassisi, and R. G. Gordon, Appl. Phys. Lett.102, 053901 (2013).

The solid solution between ZnO and GaN is the best single system capable of utilizing visible light to stably drive solar water splitting (to renewably produce H₂ fuel) developed to date. The lowest band gaps observed for this system are found for Zn-rich samples, suggesting that samples which are rich in the earth-abundant cation may be the most desirable for water splitting applications. We find that defects are common in samples with high Zn contents, and describe methods for identifying and quantifying defects in bulk samples. Our results provide a framework for exploring the influence of synthesis and post-processing conditions on defect formation, and for testing the impact of defects on photoelectrochemical activity.

Cu₂ZnSn(S,Se)₄ (CZTSSe) currently offers the highest performance, measured in terms of power conversion efficiency (PCE), among thin-film PV devices based on “earth-abundant” metals, with PCE now reaching above 11%. Despite rapid increase in demonstrated performance, the current levels are not yet sufficient for commercialization when compared to other more established thin-film materials, such as Cu(In,Ga)(S,Se)₂ (CIGS) and CdTe, which have efficiencies of as high as 20%. Among the three device characteristics that determine PCE—J_sc, FF and V_oc—modest improvements in efficiency can be gained by addressing J_sc and FF. However, the most important deficiency in the current generation of devices (relative to either analogous CIGS or CdTe device performance or the Shockley-Queisser limit) can be found in V_oc. In this talk we will examine progress in understanding and mitigating limitations that are holding back device performance in the current generation of CZTSSe devices. Key issues include the nature of bulk/surface defects and phase purity of the CZTSSe absorbers.

The heterojunctions formed between solution phase grown Cu2ZnSn(SxSe1- x)4(CZTS,Se) and a number of important buffer materials including CdS, ZnS, ZnO, and In2S3, were studied using femtosecond ultraviolet photoelectron spectroscopy (fs-UPS) and photovoltage spectroscopy. Time synchronized pump (1.55eV/photon) and probe (20-40 eV/photon) light pulses from an amplified 50 fs Ti:sapphire laser were employed for these experiments. Under static conditions (no pump pulse) standard UPS spectra were collected. With the pump pulse present, a dense electron-hole plasma screens the depletion field and flattens the CZTS,Se bands. With this approach we extract the magnitude and direction of the CZTS,Se band bending, locate the Fermi level within the band gaps and measure the band offsets between the absorber and buffer under flatband conditions. This approach is particularly useful when studying chemical bath deposited buffer layers. We find spike conduction band offsets (CBO) for the CdS/CZTS,Se heterostructure over all ranges of S and Se content. We find a somewhat smaller spike CBO for the In2S3/CZTSSe heterostructure and have achieved the highest reported device efficiency (7.6%) for that system. For ZnS/CZTS,Se, the large CBO (1.1 eV) results in a device with no short circuit current (Jsc) flow while for ZnO, the ~0 eV CBO produces good Jsc but low open circuit voltage (Voc). More detailed correlations between heterostructure electronic properties and device performance will be discussed. We will also discuss two-color pump/probe experiments in which the band bending in the buffer layer can be independently determined. Finally, studies of the bare CZTS,Se surface will be discussed including our observation of mid-gap Fermi level pinning and its relation to Voc limitations and bulk defects.

Having a high absorption coefficient and an optimal bandgap similar with that of CIGS solar cells, CZTS solar cell is intrinsically even attractive owing to a large reserve in the earth crust and non-toxic feature.[1] Since CZTS is a complex material system and a single phase production is not yet controllable, the thermal co-evaporation approach is now holding the champion efficiency 8.4% for CZTS solar cells,[2] in contrast with over 20%[3] for its CIGS counterpart. CZTS at this stage mainly imitates the relevant device structure of CIGS as a shortcut for development, which may not be the optimal due to the difference between CZTS and CIGS. In fact, parasitic secondary phases are un-avoidable yet and detrimental to device performance in CZTS materials, which form either carrier transport barrier like ZnS and SnS2 or shunting path such as Cu2-xS and CuxSnSy.[4] One of the major challenges for phase control is the defects inducing Mo/CZTS interface due to the lower Gibbs free energy of MoS2 compared with that of CZTS or Cu2SnS3.[5] Voids, detrimental secondary phases, thick MoS2 (in the order of hundreds of nm) are generally observed for untreated Mo/CZTS interface due to the reaction between Mo and CZTS (a decomposition reaction for CZTS).[5, 6] To conquer such similar problem both TiN[7] and ZnO[8] barrier layers have been proved to be effective in reducing the thickness of MoSe2 and the amount of defects at Mo/Cu2ZnSnSe4 interface, and therefore enhancing the cell efficiency; To suppress or even avoid the decomposition of CZTS at the back contact, four potential solutions have been proposed and investigated as follows. (1) Ultrathin ZnS overcoating has been introduced as it is relatively stable in comparison with MoS2; (2) Rapid thermal annealing (RTA) of the Mo layer was also explored as this increases the grain size of Mo and crystallinity of CZTS grown on such layer; (3) RTA treated Mo with ultrathin Ag overcoating is attempted aiming to block the reaction between Mo and CZTS. Ultrathin ZnO overcoating is adopted in this investigation for comparison purpose. The results indicate that these solutions reduce the amount of voids, secondary phases at the interfaces and the thickness of MoS2 substantially, and improve the device performance consequently. Notice that the solutions should not be limited to a specific CZTS preparation procedure and in this investigation the photoactive layer is prepared by sputtering metal stack precursor followed by sulfurisation.
References:
1 Ji, S., Ye,C., Rev. Adv. Sci. Eng.1, 42 (2012).
2 Shin, B., et al., Prog. Photovolt., Res. Appl. 21, 72 (2013).
3 Jackson, P., et al., Prog. Photovolt., Res. Appl. 19, 894 (2011).
4 Platzer-Björkman, C., et al., Sol. Energy Mater. .Sol. C. 98,110 (2012).
5 Scragg, J.J., et al., J. Am. Chem. Soc. 134,19330 (2012).
6 Wätjen, J.T., et al., Thin Solid Films 535, 31 (2013).
7 Shin, B., et al., Appl Phys Lett 101,053903-4 (2012).
8 Lopez-Marino, S., et al., J Mater Chem A, 2013.

The earth abundant kesterite Cu₂ZnSnS₄ material with a high absorption coefficient, direct band gap, and good long-term stability has become an attractive candidate for use in solar cell absorber layers, as compared to the traditional CdTe and Cu(In,Ga)(S,Se)₂ (CIGS) thin-film absorber layers. However, the narrow compositional window for obtaining a stable single phase in the phase diagram leads to difficulties in the manufacturing process of Cu₂ZnSn(S,Se)₄. Additionally, the existing substrate-type device configuration for these solar cells uses a molybdenum (Mo) back contact, which becomes a critical bottleneck for further increasing the power conversion efficiency because of serious disadvantages suffered by the (i) presence of a Schottky barrier at the Mo/Cu₂ZnSn(S,Se)₄ interface and (ii) decomposition of Cu₂ZnSn(S,Se)₄ at the Mo interface. Therefore, we here introduce a low-cost and Mo free superstrate-type device configuration of Au/Cu₂ZnSn(S,Se)₄/CdS/TiO₂/ITO/glass, which includes a bifunctional interlayer of CdS to circumvent the formation of Mo(S,Se)₂ and avoid the occurrence of potential decomposition pathways. We also demonstrate the tunable properties of Cu₂ZnSn(S_xSe_1-x)₄ nanocrystals followed by a relatively mild sulfurization process, as compared to the harsher selenization reaction. Using a facile hot-injection approach for synthesizing Cu₂ZnSn(S_xSe_1-x)₄ nanocrystals with varied Se to (S+Se) ratio, not only the role of Se in Cu₂ZnSn(S_xSe_1-x)₄ nanocrystals but also the evolution of Cu₂ZnSn(S_xSe_1-x)₄ nanocrystals to Cu₂ZnSn(S_xSe_1-x)₄ film during the sulfurization step are systematically investigated. It is found that minimizing the possibility for the loss of Sn during the heat treatment and producing a compact film with large grain size are beneficial for the device performance. As a proof-of-concept, our superstrate-type architecture without using any binder has exhibited the conversion efficiency of 1% with V_oc of 353 mV, J_sc of 7.75 mAcm^-2, and FF of 36.66%. It indicates that a low-cost, Mo(S,Se)₂-free superstrate-type architecture is potential for earth abundant Cu₂ZnSn(S,Se)₄ solar cell applications.

As is common in other thin film photovoltaic technologies, the most common form of Cu2ZnSnS4 (CZTS) in solar cell applications is polycrystalline, i.e., with grain boundaries of various crystallographic orientations. To date, there is still insufficient evidence to determine the role of grain boundaries in CZTS. Investigating grain boundary-free CZTS (grown epitaxially on a low resistivity substrate) would unambiguously answer this question. Fortunately, CZTS and Si have a nearly perfect lattice match; the reported a-axis lattice constant, a of CZTS ranges from 0.5426 - 0.5435 nm (c-axis lattice constant, c is twice of a in CZTS) while that of Si is 0.5431 nm. We have demonstrated epitaxially-grown CZTS films on Si(001) for the first time using a thermal co-evaporation technique with a valved cracker as a sulfur source. A substrate temperature as high as 370 °C and proper substrate cleaning (HF-dip followed by thermal desorption of surface hydrogen) are found to be necessary for the epitaxial growth. Theta-2Theta X-ray diffraction measurements reveal that the overall orientation of the CZTS films follows the underlying Si(001) substrate. Direct evidence of the epitaxial relationship between the CZTS and the Si is provided by transmission electron microscopy measurements. Formation of structural defects such as twinning and the effect of the growth temperature on the defects are also discussed.

The rapid rise in power conversion efficiencies of thin-film solar cells based on the p-type semiconductors Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe) attests to their potential as low-cost, earth-abundant alternatives to CdTe and CuInGaSe₂, the two leading commercial solar absorber materials. To date, many CZTS and CZTSe thin film formation methods have been developed, and solar cells with efficiencies exceeding 11 % have been demonstrated. However, increasing solar cell efficiencies relies heavily upon trial-and-error optimization of multiple film properties during the film&’s synthesis. One of these properties is the film&’s microstructure. Ideally, a monolayer of single crystal grains with sizes on the order of the film thickness is required to reduce charge recombination at grain boundaries. This talk will focus on elucidating the factors that affect the microstructure of thin CZTS films by comparing two synthesis methods: (i) annealing, in a sulfur containing atmosphere, of thin films deposited from colloidal CZTS nanocrystal dispersions, and (ii) high-temperature sulfidation of copper-zinc-tin precursor films deposited by co-sputtering. In the first method, 20 to 30 nm diameter CZTS nanocrystals are synthesized in hot (150-300 ^oC) oleylamine from copper, zinc, and tin diethyldithiocarbamates. Following this, thin coatings (2-3 mu;m) are drop cast from colloidal dispersions of these nanocrystals, which are then annealed in a well-controlled closed system to form large grain microcrystalline films. In this approach, CZTS already forms prior to annealing and the large surface energy of the nanocrystals drives the microstructure development. Abnormal and normal grain growth compete with one another, leading to bimodal grain size distributions. The factors that affect the abnormal and normal grain growth rates include, the annealing temperature, the substrate, and the sulfur and tin sulfide partial pressures. In the second method, CZTS formation and microstructure development takes place simultaneously during the sulfidation of the copper-zinc-tin precursor films. In this approach, abnormal grain growth is not observed. In both methods, however, the film microstructure is a strong function of the underlying substrate. Specifically, substrates containing alkali metal impurities enable the formation of significantly larger grains at lower processing temperatures.

Kesterite thin film solar cells made from Cu, Zn, Sn, Se and S (CZTS/Se) are promising for their earth abundant materials and solution processability. To date, these devices have achieved efficiencies of around ~11%, underperforming compared to other thin film technologies such as CIGS or CdTe. A major limiting factor is the low open circuit voltage resulting from recombination losses. Interestingly, polycrystalline devices have shown higher efficiencies than the single crystal counterparts. The nature of these recombination pathways and the role of grain boundaries are not fully understood. Additionally, the beneficial role that grain boundaries provide may be important for improving device performance. We seek to characterize the defect physics of these devices using scanning Kelvin probe microscopy (SKPM). SKPM is a non-contact technique that provides information about surface potential and topography. We correlate local electrical properties and structure on the nanoscale using this instrument. We analyze cross sections of CZTS devices to probe both the grain boundaries and interfaces in working devices processed under different conditions.

The sustainable PV material, Cu2ZnSnS4 (CZTS), can form in two different tetragonal polymorphs, known as kesterite and stannite respectively. These differ in the ordering of Cu and Zn cations within the (004) planes. The energy difference between the two phases is only ~3 meV/atom, meaning that the material is prone to Cu, Zn cation disorder, as evidenced by neutron diffraction experiments. The disordering can potentially take place at the unit cell level in the form of point defects (vacancies and/or anti-site atoms) or on a much larger scale where the material can be viewed as consisting of domains of kesterite and stannite. The latter scenario is not expected to have a significant effect on device performance, since theoretical calculations have shown the optical properties for pure kesterite and pure stannite to be similar. However, point defects are important because they can dope the material as well as act as recombination centres. Luminescence measurements on CZTS have indicated that the material contains a high density of charged point defects which result in a fluctuating electrostatic potential.
In this contribution direct evidence for Cu, Zn disordering in CZTS is obtained by measuring the chemical composition of individual atom columns using aberration-corrected scanning transmission electron microscopy. Many of the measurements indicated that Cu and Zn were inter-mixed; this may be a genuine feature of the sample or an experimental artefact due to spreading of the electron probe as it propagates through the specimen, resulting in a measurement that represents the average composition over several neighbouring atom columns. However, evidence for atom columns consisting entirely of ZnCu anti-site atoms was also found. The composition inhomogeneity results in local degenerate doping and causes strong electrostatic potential fluctuations due to uncompensated ZnCu donors. This is the first time such composition inhomogeneities have been observed directly. The role composition fluctuations have on device performance will also be discussed. For example, uncompensated donors are expected to be stronger recombination centres compared to uncompensated acceptors in p-type CZTS material.

The interest in Cu₂ZnSn(S,Se)₄ (CZTS) for photovoltaic (PV) applications is motivated by the similarities to the promising material Cu(In,Ga)Se₂ (CIGS) while being comprised of non-toxic and earth abundant elements. Competition between the kesterite (necessary for PV applications) and the stannite phase of CZTS, as well as a number of binary and ternary competing phases affects the power conversion efficiencies of CZTS devices. However, the structural similarities of a number of these phases make their identification through standard x-ray diffraction challenging. Furthermore, the strong possibility of site-swapping of the Zn and Cu between their respective sublattices may also lead to a high level of defects which reduce solar cell efficiency. The tunable energy x-rays available at modern synchrotron sources provide a site and element specific probe to investigate such disorder. We have used resonant x-ray diffraction techniques to quantitatively determine the crystallographic phases and level of site-swapping disorder present in thin films of polycrystalline CZTS in order to shed light on the relative success of different growth conditions. Our goal is to understand and characterize the structural differences and defect levels of films grown under different conditions. By comparing with device efficiencies for these films, we can identify those structural features with the greatest effect on PV performance and the growth conditions to effectively control them.

Grain boundaries are ubiquitous in inorganic thin-film PV and, unless passivated, reduce the device efficiency by lowering the open circuit voltage through Shockley-Read-Hall recombination within the space charge region. In the sustainable PV material, Cu2ZnSnS4 (CZTS), secondary phases are also commonly observed. Typical secondary phases include sulphides of copper and tin, Cu2SnS3 and ZnS which has a high exothermic heat of formation. Secondary phases can affect device efficiency through a number of different mechanisms (e.g. variation in band gap and hence absorption, effect on series and shunt parasitic resistances), but one of the factors that needs to be considered is the recombination rate at the heterointerface between CZTS and the secondary phase. This is similar to the role of grain boundaries on device performance. The recombination velocity measures the ability of an interface to act as a minority carrier sink; interfaces with a larger recombination velocity are more deleterious to device performance. An interface typically has a large recombination velocity if it contains deep electronic states within the band gap, the origin of these defect states being due to lattice mismatch and/or segregation at the interface.
Cathodoluminescence (CL) is a technique that measures the light emitted by a solid when illuminated by an electron probe, such as that in a scanning electron microscope (SEM). The CL intensity is lower in the vicinity of an interface due to recombination. By analysing the variation of CL intensity as a function of distance from the interface the recombination velocity can be extracted. This technique has been applied to heterointerfaces for a number of different secondary phases in CZTS. SnS was found to have a high recombination rate, but heterointerfaces for ZnS and Cu2SnS3 were electrically passive, i.e. the recombination velocity was lower than the bulk carrier diffusion velocity. To examine the cause for the low recombination velocity the atomic structure of the heterointerface was examined using a transmission electron microscope. The interface between CZTS and ZnS for example, showed registry of the c-planes of tetragonal CZTS with the cubic planes of ZnS. The lattice registry is facilitated by the similarity of crystal structure and lattice parameters between CZTS and ZnS, Cu2SnS3 secondary phases, which are based on tetrahedral bonding. Due to the relatively small lattice mismatch secondary phases such as ZnS and Cu2SnS3 do not lower device efficiency through recombination, although they may have an effect through other processes, such as light absorption.

As a semiconductor oxide, hematite (alpha iron oxide) possesses many attractive properties for photoelectrochemical (PEC) water splitting, including the suitable bandgap (2-2.2 eV) for efficiencies as high as 16% and stability against corrosion over a wide range of pH. In addition, its abundance makes it possible to utilize the material in large scale. The utilization of hematite as a PEC electrode material also faces great challenges. For example, the short hole diffusion distance within hematite limits the effectiveness in charge separation; the relatively positive positions of the band edges renders complete water splitting unachieveable. To combat these problems, we proposed and tested a number of strategies, most of which are based on forming homo- or hetero-junctions. The idea is to keep the desirable properties of hematite and compensate its shortcomings using additional material components. Here we present our recent results toward this direction. To improve charge separation, we introduced a highly conductive nanostructure that we call nanonets, which help collect photogenerated electrons, thereby minimizing electron-hole recombinations. To reduce the need for externally applied potentials, we coated a p-type layer on top of n-type hematite, which produces a buried junction. The photovoltage created by the junction assists water splitting. To absorb photons with energies lower than the band gap of hematite, we combined Si nanowires with hematite and showed that the two absorbers can work in concert to greatly reduce the applied potential and still enable PEC water splitting. These proof-of-concept demonstrations show that with good synthesis controls, the goal of high-performance water splitting by hematite is within reach. The concepts demonstrated here should also be applicable to other earth abundant, oxide-based semiconductors for water splitting purposes. They will likely contribute significantly to our efforts of building a renewable energy powered future.

In this study, a facile solution-based method was developed to fabricate hematite nanorods coated with ultrathin overlayer of TiO₂ or Ag_xFe_2-xO₃. The core/shell nanorod structures of α-Fe₂O₃/TiO₂ and α-Fe₂O₃/AgxFe_2-xO₃ were obtained by annealing solution-fabricated β-FeOOH nanorod arrays which were first ultrasonicated in TiO₂ sol and Ag+ aqueous solution, respectively. The formed overlayer of TiO₂ or AgxFe_2-xO₃ was estimated to be ~ 3 nm in HRTEM images, and evenly coated on the surface of α-Fe₂O₃ nanorod core. In the α-Fe₂O₃/TiO₂ nanorod structure, TiO₂ overlayer could extract photogenerated holes from α-Fe₂O₃ core via the quantum-mechanical tunneling process, resulting in promoted charge carrier separation. With compared to the pristine α-Fe₂O₃ nanorod film, the α-Fe₂O₃/TiO₂ nanorod film shows greatly improved photoelectrochemical performance for water splitting, with IPCE increased by a factor of 4.5 from ~2.0% to 9.0% at 400 nm. In the α-Fe₂O₃/AgxFe_2-xO₃ nanorod structure, the surface doping of Ag⁺ ions gave rise to increased electron donor density as estimated from the Mott-Schottky results, which also led to the enhancement in photoelectrochemical performance with IPCE at 400 nm increased from ~2.0% to ~7.5%.

Growing energy consumption and its impact on the environment increases the demand for sustainable global development. This arouses interests in utilizing earth-abundant materials for development of solar energy harvesting devices. Hematite is a promising candidate material for the photoanode in a photoelectrochemical cell (PEC). It has an appealing band gap (1.9-2.2 eV) and is stable, non-toxic, inexpensive and earth-abundant. However, its light penetration depth is relatively short (~120 nm). The diffusion length of minority charge carrier is very short (2-4 nm); and the charge mobility in hematite is very low. In order to circumvent the drawbacks of a hematite photoanode, a vertically aligned hematite nanorod array has been incorporated into a plasmonic nanostructure. The plasmonic metal-hematite heterostructure has exhibited much larger photocurrent than the monolithic hematite counterpart under the full-spectrum solar radiation. The finite difference time domain (FDTD) simulation shows that the improved photoelectrochemical performance is originated from both the photonic enhancement and the plasmon-induced resonant energy transfer (PIRET) process.

In the last decade, several semiconductors oxides such as Fe2O3, WO3 and SrTiO3 have been considering as promising material for photoanode in a photoelectrochemical cell (PEC). These oxides semiconductors showed high photocurrent and in general, it performance is directly associated to the morphological control at the nanoscale. In this work, we demonstrate an alternative and promising way to produce metal oxide photoanodes with high performance for water oxidation. In this approach, we processed Fe2O3, WO3 and SrTiO3 thin films using a colloidal dispersion of oxides nanoparticles as the precursor and these nanoparticles were synthesized by a non-aqueous method. This method combines multi-step deposition procedure with phase transformation or crystallization during the sintering process. The first step in this approach consists of the preparation of a stable colloidal dispersion of nanoparticles (with controlled stoichiometry) with an organic surface layer in a suitable solvent. In the second step, the colloidal suspension is deposited by dip coating, forming a homogeneous and continuous thin film on the transparent conductor oxide (TCO) substrate. In the final step, the layer is transformed into the suitable semiconducting oxide film through an adequate combination of multi-step deposition and sintering under an oxygen atmospheric flow. The sintering and phase transformation process has been analyzed in detail, using in-situ TEM characterization. The use of colloidal dispersion for the preparation of photoanode thin film is a facile method and has been used in our research group to process hematite and WO3 photoanodes with very good activity. For instance, hematite photoanode reached 1.7 mA.cm-2 at 1.23 VRHE and a photocurrent of 1.96 mA.cm-2 at 1.23 VRHE was reported for WO3 photoanode.

In the search for earth abundant photovoltaic absorbers, chalcostibite (CuSbS2) is emerging as a material of primary interest. Theoretical calculations in this study and others have placed the absorption onset within the optimal range for an absorber (1.5eV), and have indicated a steep absorption profile nearing a maximum absorption coefficient value of 10^5 cm^-1 within +0.2eV of the onset. However, experimental verification of these properties has been less forthcoming due to the large variations in sample quality found in the literature. The goal of this work is to produce high quality thin films of chalcostibite using the well-understood RF magnetron sputter deposition technique. The resulting films were then subjected to in-depth electrical, optical, and microstructural characterization in order to assess chalcostibite&’s potential for PV applications, as well as to validate theoretical predictions.
The first challenge was to identify deposition parameters that yield phase pure chalcostibite in this complex ternary system. This was accomplished by generating combinatorial libraries through variable co-sputtering of Cu2S and Sb2S3 targets in the presence of an imposed substrate temperature gradient, with and without the addition of H2S. The resulting libraries were characterized using high throughput, spatially resolved measurements of film structure (XRD), composition (XRF), and optical properties. The libraries contained tetrahedrite, skinnerite, and chalcostibite ternary phases, and the transitions among the individual phases were in agreement with previously published phase diagrams.
It was observed that the sticking coefficient of Sb2S3 is dramatically lower than Cu2S at moderately high temperatures (400 - 500 C). This finding suggested a pathway to grow stoichiometric CuSbS2 by providing an over-flux of Sb2S3. Under these conditions it was found that excess Sb2S3 did not stick to the substrate, leaving only the phase pure CuSbS2. This has been identified as a strategy for synthesis of high quality, phase pure, polycrystalline thin films of CuSbS2. Uniform films of chalcostibite grown in this manner were then characterized for optical absorption spectra, mobility, carrier concentration, work function, grain size and morphology. Initial results are promising, confirming the theoretical optical absorption onset as well as the high absorption coefficient value just above the onset. Conductivity is within the acceptable range for an absorber (0.01-0.1 S/cm). SEM measurements of grain size are on the micron scale.
The project “Rapid Development of Earth-abundant Thin Film Solar Cells” is supported as a part of the SunShot initiative by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC36-08GO28308 to NREL

Highly porous WO3/TiO2 heterostructures were fabricated by combinatorial magnetron sputtering. Under varying sputter pressures diverse WO3 microstructures were obtained by depositing the films in form of wedges, while layers of TiO2 nanocolumns were grown thereon by oblique angle deposition method. This combinatorial fabrication approach allows screening of a large quantity of compositions having different film thicknesses, microstructures and porosities with the aim of improving photoelectrochemical (PEC) solar water splitting characteristics. The high-throughput PEC characterizations were performed using a specially designed scanning droplet cell (SDC). The reference films with individual layers of WO3 and TiO2 were fabricated and assessed for comparing the PEC characteristics of layered heterostructures. The PEC properties of individual layers displayed morphology-based differences. Due to the short carrier diffusion length in WO3 films, constrained photocurrent densities are observed for dense films. On the contrary, porous WO3 nanostructures demonstrated relatively increased photocurrents. TiO2 nanocolumns showed length-dependent characteristics, where the photocurrent increased with increasing length of the TiO2 nanocolumns. Heterostructures with the combination of WO3-wedge/TiO2-nanocolumnar layers exhibited strikingly improved PEC properties in contrast to the individual TiO2 and WO3 layers. The maximum photocurrent density of 0.11 mA/cm2 attained on highly porous combinatorial WO3/TiO2 materials libraries is about an order of magnitude higher than that of individual WO3 films and two-fold as that of individual TiO2 layer. Evidently, the charge separation effect and an efficient charge carrier transfer process on WO3/TiO2 heterostructures enhanced the photoconversion efficiency.

Optical splitting of sunlight into spectral bands guided to materials with different bandgaps has the potential to dramatically increase the efficiency of photovoltaic modules without the challenges of stacked multi-junction devices. This flexibility could enable the accelerated incorporation of novel Earth-abundant materials into high-efficiency device architectures. It also allows for more module designs including simple 4-terminal tandem structures. Many approaches to spectral splitting have been tried and are under development today.
We hypothesize that general design constraints exist for any optical system to split the solar spectrum. Specifically, 2D solar cell device simulations have identified two parameters of merit for these systems: one describing the accuracy with which photons are guided to absorbers of appropriate bandgap, and one describing the spatial variation of illumination intensity across the device. Furthermore, these simulations indicate that there are threshold values for each parameter, above which they have little impact and below which, they have a very strong impact on performance.
Through further simulation using the TCAD Synopsys Sentaurus Device package, these threshold values and the precise nature of these relationships will be determined, providing design guidelines for spectral splitting optics. Through simulation and experiment, we will evaluate varying multi-color approaches combining silicon with Earth-abundant, non-toxic absorbers being developed in our laboratory, such as Cu₂O, whose bandgap of 1.9 eV is almost ideal for a silicon tandem structure.

Multijunction nanostructures which combine metallic and semiconductor moieties were synthesized by wet chemical approach and their photocatalytic properties were studied in the reaction of hydrogen generation under UV and visible light. Highest photoctatlytic efficiency under UV light was observed for CNT nanotubes decorated with TiO2, while optimal efficiency under Visible light was achieved in the CdSe nanowires decorated with CdS quantum dots and Pt nanoparticles. Increased photoefficiency was explained by improved charge separation (studied by transient absorption spectroscopy and photoluminescence lifetime) and better dispersion and surface stability in the above mentioned composites. Charge separation was also studied in Cu2S and CuInS2 single-crystaline nanowires prepared by microwave assisted solvothermal method.

Luminescent solar concentrators (LSC&’s) are slab waveguides doped in order to absorb sunlight and produce luminescence, which is then delivered to photovoltaics (PV&’s) placed at the edges. This device concept that obviates the need for tracking is attracting renewed interest by exploiting new optical materials and photophysics to improve the performance. We present here a new LSC form-factor: flexible and robust polymeric fiber LSC (or FLSC). In this embodiment we harness the inherently scalable process of thermal fiber drawing to produce continuous, extended lengths of uniform polymer fiber with precisely controlled internal transverse geometry and external morphology drawn from a ‘preform&’ - a macroscopic scaled-up model of the target fiber structure. We utilize the telecommunications-fiber fabrication methodology, which provides the potential for mass-production. Additionally, we exploit flexible and robust polymer fibers in lieu of brittle glass fibers. This feature may enable the incorporation of these FLSC&’s in fabrics for mobile energy applications. We multiplex several functionalities monolithically in these polymer FLSC&’s - with no post-drawing processing - that are realized with a non-conventional cross-section designed to optimize solar concentration: a square with a half-circle cap. The curved top surface acts as an axially extended cylindrical microlens that provides primary concentration achieved by focusing incident sunlight into the core along the fiber length. Sunlight is absorbed by one or more of a variety of potential luminescent dopants located in the fiber with controllable concentration. Finally, the fiber provides optical guidance to the captured portion of the emitted luminescence confined within it by total internal reflection. Crucially, the transverse optical confinement reduces the amount of necessary PV&’s by half compared to a LSC having the same solar-area coverage. The measured FLSC conversion efficiencies in the proof-of-principle demonstration reported here approach those of traditional slab LSC&’s while accruing additional advantages in enhanced mechanical flexibility and reduced weight and cost. A tandem FLSC structure achieves optical conversion efficiency of ~ 4% for 10-cm-long fibers. These findings indicate the potential for optical fiber fabrication having an impact on solar energy harvesting, which may have wide implications for mobile energy applications.

Iron pyrite is a promising solar absorber material since it is inexpensive and has excellent properties, such as a band gap of 0.95 eV and a high absorption coefficient and high mobility in single crystals. However, past studies have only demonstrated low photovoltages and solar conversion efficiencies of less than three percent. Our recent research led us to hypothesize that the low photovoltage is due to a surface inversion layer caused by surface Fermi level pinning. In this work, we will present our surface studies of pyrite single crystals or nanostructures using Kelvin probe force microscopy and field effect gating measurements. We investigate surface Fermi level pinning and further examine the influence of passivation and modification treatments on the surface properties. In combination with extensive Hall effect measurements on pyrite nanoplates and electrochemical measurements on single crystals, these experiments will demonstrate the importance of pyrite&’s surface on its electrical properties and allow us to screen strategies to effectively control pyrite&’s surface properties to enable it as a high performance solar material.

The large scale utilization of semiconductor nanomaterials for solar energy conversion applications requires the use of earth-abundant (i.e. cheap) and non-toxic materials. This has sparked interest in the nanocrystalline form of materials such as pyrite. [1] The use of pyrite nanocrystals in solar cells however has been hampered by surface defects of the nanocrystals of which the effect is amplified in nanoscale materials with large surface to volume ratios. Here we report on the role of surface ligands on the electronic and optical properties of pyrite nanocrystal thin films. [2] We show that the ligand anchor group modifies the absorption of the thin films leading to shifts of the absorption onset of up to ~100 meV. The conductivity and photoconductivity on the other hand is determined by combined effects of the anchor and bridging group of the ligands. For the latter, both physical length as well as electronic states are important.
[1] W. Li et al. J. Mater. Chem. 21, 17946-17952 (2011).
[2] W. Li et al. submitted.

In this work, for the first time we report the fabrication of quaternary copper, zinc, tin, sulphur (CZTS) microfibers using an efficient method of electrospinning following the sequence sulphurization. The precursor fibers are electrospun from the solvent composed of Cu(CH3COO)2, Zn(CH3COO)2 and SnCl4 as the solutes, ethylene glycol methyl ether as the solvent, and polyvinylpyrrolidone (PVP) as the plasticizer. CZTS fibers are obtained after annealing the precursor fibers in sulphur ambient at 550 centigrade degree for 1h. Scanning electron microscopy (SEM) characterization presents a detailed morphologies of CZTS microfibers. X-ray diffraction (XRD) studies showed that CZTS microfibers have good chalcopyrite crystallization with strongly orientational growth in (112) crystal direction. The chalcopyrite crystallization phase formation is also confirmed by the Raman spectra. The chemical composition determined by energy-dispersive spectroscopy (EDS) testifies that the three elements of Cu, Zn, Sn are in an elemental ratio of about 2:1:1, while the element S is much more excessive than its stoichiometric ratio in CZTS compound. The excessive S content may be caused by the strong absorbance via nano-scale pinholes formed by crossed micro fibers. Possible mechanism contributing to the orientational growth of CZTS microfibers is discussed. This study shall be beneficial to expose the inherent growth mechanism of high-quality CZTS compound and thus high-performed CZTS-based solar cells.

Photoelectrochemical water splitting has been highlighted as elegant mechanism to storage solar energy. Several preparation methods are being widely studied to produce photoelectrode with high efficiency on the solar light conversion. Among the n-type metal oxide semiconductors studied as photoactive electrodes, WO3 is a promising material to produce oxygen photoelectrochemically because it has a band gap of 2.6 eV which facilitates a maximum theoretical efficiency of 8% for solar photon conversion. This work describes a synthetic method to produce a WO3 nanowire as well as to prepare photoanodes by colloidal nanowire deposition. We also studied the influence of a nanowire phase on the photoanode performance for water splitting. Among the nanowires synthesized by using nonhydrolytic media, the orthorhombic WO3.H2O nanowire produced a photoanode with excellent performance (a photocurrent of 1.96 mAcm-2 at 1.23 VRHE) and good photocurrent stability during long-term analysis (chronoamperometry test). In situ TEM analysis also was used to understand the morphology transformation during the sintering process occurs in two steps: the fragmentation of nanowires into very small nanoparticles occurs at 500-600 oC followed by grain and pore growth processes, which is controlled by coalescence and grain boundary migration. This phenomenon was observed for the orthorhombic WO3.H2O nanowires. The structural and photoelectrochemical characterization showed the importance of nanostructural features such as exposed (200), (020), and (002) facets and porosity in the WO3 photoanode performance.

Fuel cells are not limited by the Carnot cycle like heat engines, and thus can achieve better fuel efficiencies. Photoactive earth abundant materials would minimize costs of fuel cells, however improved material structures are needed to increase surface area and maximize efficiency. We report a novel optical fuel cell device that utilizes Si-microwire electrodes, coated in tungsten and titanium metal oxides and nanoparticle catalysts, for the photooxidation of methanol fuel. We characterized the anode through a series of photoelectrochemistry experiments under AM 1.5 illumination, while monitoring the decrease of methanol and formation and depletion formaldehyde, one of the products formed before complete oxidation to carbon dioxide. Cyclic voltammograms (CV) revealed that current densities increased by 10 to 20 µA/cm² upon the addition of methanol while using a metal oxide, Si-microwire photoanode, and increased up to 60 µA/cm² with the addition of the nanoparticle catalysts. The formaldehyde concentration and current densities, regardless of nanoparticle presence, peaked around 10 minutes of sequential CV scans. Addition of Pt nanoparticle catalysts resulted in a 2.5 increase in formaldehyde production, corresponding to our maximum current density achieved. Further optimization of the anode structure through varying doping type and concentration and nanoparticle loading should improve current densities and lead to better power output for use of an earth abundant photoanode in a fuel cell.

Zn3P2 is considered an ideal candidate for scalable photovoltaics, with a reported direct band gap of 1.5 eV and a long minority-carrier diffusion length (>5µm). The fabrication of Zn3P2 devices requires a method for the reproducible deposition of high quality Zn3P2 thin films. Polycrystalline tetragonal (P42/nmc) zinc phosphide (Zn3P2) thin films were deposited on glass substrates by r.f. reactive sputtering of zinc (Zn) in a mixture of phosphine (PH3) and Ar. It was found that the structural and optoelectronic properties of the films are strongly dependent on various growth parameters including deposition temperature (Tdep), gun power, gas flow rates, and total pressure (PTot). Under optimal growth conditions (Tdep ~ 200-230 deg. C, PH3 flow < 10 % and PTot: 1.2E-02 mbar), the films are polycrystalline in nature, with crystallite sizes up to 100-130 nm, which are oriented along the (220) direction. Lowering the Tdep or increasing the PH3 content resulted in amorphous films. The films were found to be stoichiometric as confirmed by EDS, while cross-sectional SEM/EDS mapping indicated compositional homogeneity across the depth. The average surface roughness under optimal growth conditions is less than 10 nm. As grown films were p-type with high carrier concentration (~10E16 cm-3) but low mobilities (< 1 cm2/V.s). Electrical properties of the films can be improved by post-growth annealing in an inert atmosphere. Optical measurements showed the presence of strong absorption in the visible region with an absorption coefficient on the order of 10E4 cm-1. Preliminary results for Mg-Schottky diodes and heterojunctions will also be discussed.

Photovoltaics may represent a general and efficient approach to solve the global energy crisis, and have attracted extensive attention from both academia and industry for many years. However, the development of inorganic photovoltaics has long been hindered by complex fabrication and high costs. As a result, increasing interest has been recently paid to dye-sensitized solar cells which can be fabricated by an easy solution process with low cost. In the development of dye-sensitized solar cells, it is critically important but remains challenging to discover new electrode materials to replace the conventional indium and platinum which have obvious disadvantages including high cost, complex fabrication, and chemical instability during the use. To this end, carbon nanotube (CNT) and graphene may represent two of the most promising candidates due to the combined advantages including high surface area and stability, excellent electrical and electrocatalytic properties, and abundant supply.
Therefore, we designed several CNT film electrodes with special structure for rapid electrons transportation, for example, multi-walled CNTs were partially unzipped to produce nanoribbons which bridged nanotubes to form a network structure for rapid charge transfer. The best energy conversion efficiency have been up to 9.05%, which is better than the traditional platinum based DSCs. These CNT electrodes show great potential to replace the platinum electrode in practical application.
In addition, graphene/platinum composite fibers have been prepared, which exhibits excellent flexibility, mechanical strength and electrical conductivity. The composite fibers were applied as counter electrodes in the photovoltaic wires, where a titanium wire impregnated with perpendicularly aligned titania nanotubes functioned as the working electrode. The resulting photovoltaic wires exhibits a certified maximum energy conversion efficiency of 8.45%, which is much higher than that of other wire-shaped photovoltaic devices.
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8 Huang, S., dagger; Yang, Z., dagger; Zhang, L., et al. J. Mater. Chem., 2012, 22, 16833. (dagger; joint first author)
9 Yang, Z., Li, L., Lin, H., et al. Chem. Phys. Lett., 2012, 549, 82.

Nonvolatile unipolar resistive switching has been observed in multifunctional pure BiFeO₃ (BFO) and Sm doped BiFeO₃ (BFSO) thin films in stack geometry. The photo-current responses have been studied in Pt/BFO/SRO and Pt/BFSO/SRO structures both in low and high resistance states. SrRuO₃ (SRO), grown by pulse laser deposition (PLD), was used as a bottom electrode whereas the pure and doped BFO films were grown by RF sputtering. The initial forming voltage required for reliable resistance switching was found to decrease by doping Sm into BFO compared to pure BFO films of similar thickness. After the initial forming process repeatable switching of the resistance of both the BFO and BFSO was observed between two stages of low and high resistances (LRS and HRS). The ratio of resistance of high and low resistance states was found to enhance by two order of magnitude from ~ 10³ in case of pure BFO to ~ 10⁵ in case of Sm doped BFO. The non overlapping switching voltages were in the range of 0.8-1.5 V and 5-7.5 V in case of pure BFO and 0.7-1 V and 4-6 V in case of Sm doped BFO. The temperature dependent measurements of the resistance of the device indicated metallic and semiconducting conduction behavior in low and high resistance states in both BFO and BFSO based structures. The current conduction mechanism of both structures in the low resistance states was found to be dominated by the Ohmic behavior whereas in case of high resistance state and at higher voltages it followed Poole-Frenkel emission. The pure Pt/BFO/SRO and Pt/BFSO/SRO structures showed efficient photo-response with significant increase in photocurrent in low resistance state compare to high resistance state when illuminated with solar spectrum using an incident light having power density of ~ 1kW/m². Further works are underway. These and BFO with other dopings will be discussed at the meeting.

Photoelectrochemical cells in regenerative and artificial photosynthetic schemes may hold the key to global energy sustainability and have been a major area of research, particularly in recent years. Two key features of good working electrodes for these cells are strong light absorption and efficient charge transport. In order to develop materials that enhance light absorption, thereby reducing the amount of active light absorbing material required for the cell, we have investigated light concentration using surface plasmon resonance. Our initial proof of concept that uses hierarchical multi-shelled nanoparticles demonstrates that the best way to do this is by using coupled plasmonic systems. These possess broadband surface plasmon resonance throughout the visible light region of the solar spectrum, enabling plasmonic light concentration over a wide frequency range. For implementation as photoanodes in artificial photosynthetic cells, efficient water oxidation catalysis is also required. Toward that end, we are investigating different types of water oxidation catalysts that can be easily interfaced with nanostructured electrodes.

Tin sulfide, SnS, is a promising candidate for thin film solar cell devices, with a theoretical efficiency of 32%. There is a need to characterize the microstructure of SnS films to identify efficiency-limiting defects. Focused-Ion-Beam (FIB) Tomography is a powerful three dimensional characterization technique for analyzing the microstructure of SnS films. A volume of the sample is cross-sectioned, then progressively ion milled. A Scanning Electron Micrograph is taken after each milling step. After processing and aligning them, these micrographs are used to generate a three-dimensional reconstruction of the sample.
We grow SnS by thermal evaporation and use FIB Tomography to evaluate the effects of different substrates and growth temperatures on the film microstructure. For instance, porosity is a defect that reduces optical absorption, and therefore device performance, and can be uniquely quantified by reconstructing the pores within the film using FIB Tomography. Statistical information such as pore size and spatial distribution is extracted as a function of growth condition. We observe a significant difference in porosity between SnS films grown on molybdenum, gold, and glass.
This technique is material-independent, in that it can be applied to any earth-abundant thin film. In addition, 3-dimensional reconstruction derived from Atom Probe Tomography can help probe the microstructure on the atomic scale and identify atomic-scale impurities and defects, complementing the microscale capabilities of FIB tomography to yield a high degree of defect characterization with only two techniques.

The introduction of a Nb2O5 thin layer between transparent conducting oxide and TiO2 electrodes has been considered to enhance the power conversion efficiency of dye-sensitized solar cells (DSSCs). Firstly, we deposited Nb thin films on F-doped SnO2/glass substrates by using RF magnetron sputtering and then TiO2 electrodes were screen-printed on their top and post-annealing treatment. The Nb thin films were oxidized during the annealing process and the oxidized layer played a key role of improving the performance of our DSSCs. Importantly, the incorporation of the Nb2O5 layer resulted in the enhancement of the solar-cell efficiency from 4.91% (TiO2 only) to 5.6% and the photo-current density from 10.2 mA/cm2 (TiO2 only) to 12.8 mA/cm2.

Rising production of large area thin-film solar cell (TFSC) technology has increased the need for earth-abundant transparent conductive oxides (TCOs). TCOs are commonly used as electrodes both above and below the solar absorbing region in TFSCs. Due to its low cost and relative abundance, F- or Sb-doped SnO2 is the most widely deposited TCO for many energy generation and electronic applications. However, high quality SnO2 has limited applications in copper zinc tin sulfide (CZTS) and amorphous silicon (a-Si) TFSCs due to a processing temperature of ~450C. High performance TFSCs also require additional optical coatings above the TCO. It would be highly advantageous to lower the processing temperature of SnO2 and simultaneously enable light trapping and antireflection (AR).
This paper demonstrates a multifunctional nanostructured Sn-rich SnO TCO thin film with a significantly lower processing temperature along with light trapping, light scattering, and AR properties. The films were obtained by co-sputtering Sn and SnO2 on Si and were amorphous as deposited. Crystallization occurred during a tube furnace annealing in an N2 environment above 225C. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) showed randomly oriented crystalline SnO nanostructure and segregated Sn nanocrystals. Sn-rich SnO films had an electrical conductivity of ~10^4S/cm, better than the best F- or Sb-doped SnO2 TCOs obtained at 450C (1.6x10^3S/cm). Interference microscopy on etched films with an Sn:SnO2 molar ratio of ~1:1 indicated a thickness of ~155nm; ellipsometry at 30°, 50° and 70° each confirmed an index of refraction of 3.0.
Identical films were prepared on quartz for optical characterization. Spectrophotometry with an integrating sphere showed <20% diffuse reflectance between 520nm and 900nm without thickness optimization suggesting potential AR properties. Diffuse transmission in this range was between 10% and 16%. SnO band gap is between 2.5eV and 3.0eV indicating it is optically transparent in this wavelength range—oscillations in the reflectivity measurements support that light is internally reflected with little absorption. Low diffuse transmission can be explained by light trapping within the film: SnO textured surface-light interaction and Sn nanocrystal plasmonic scattering likely results in confined large-angle scattering. On high index TFSCs such as Si, this will result in strong optical confinement within the solar absorbing layer.
Due to its low processing temperature, high electrical conductivity, light-trapping properties and antireflective potential, nanostructured Sn-rich SnO is a very promising earth-abundant TCO candidate. This multifunctional film can serve as TCO, light trapping, and AR layers simultaneously and can be used as both top and bottom electrodes on solar cells. Nanostructured Sn-rich SnO has the potential to both reduce the cost and improve the performance of earth-abundant TFSCs.

Atomic layer deposition (ALD) offers the ability to uniformly deposit conformal films over high aspect ratio nanostructure scaffolds. This makes ALD particularly well-suited for photoelectrochemical (PEC) applications, where thin photoactive film coatings can maximize the effective absorption length and also minimize the charge carriers&’ path length to the interface with the electrolyte. Although the deposition of TiO2 films over such scaffolds has been shown to vastly improve PEC performance, the large bandgap intrinsic to TiO2 still prevents such devices from harvesting more of the solar spectrum. To overcome this problem, substitutional N-doping into TiO2&’s oxygen vacancy sites by NH3 treatments has shown promise in narrowing the bandgap and promoting visible light absorption. However, the overall effect of such doping on TiO2 PEC performance is still not well understood, especially for thin ALD films. We report on a systematic study of the effects of NH3 calcination treatment conditions on the performance of TiO2 films less than 50 nm thick for the purpose of water oxidation. The amount and type of nitrogen incorporation are characterized as a function of depth by x-ray photoelectron spectroscopy (XPS) and the chemical compositions of the as-deposited surfaces of select films are characterized by in-vacuo XPS. The surface versus bulk charge transfer effects, flatband voltages, and donor concentrations are analyzed by electrochemical impedance spectroscopy (EIS). The combination of these tools provides insight into the consequences of nitridation and enables the development of optimal thickness and treatment conditions for N:TiO2 films. Specifically, this presentation summarizes how NH3 treatments that are successful in substitutionally incorporating nitrogen into the TiO2 lattice can also introduce detrimental surface defects that are more pronounced as the film thickness decreases. In comparison, the introduction of N atoms into the bulk of the film does not have such a strong influence on PEC performance. The fundamental understanding of these N doping effects enables engineering of semiconductor films that can be integrated onto nanostructured scaffold for superior PEC performance.

Renewable sources of energy are increasingly needed and solar production of hydrogen fuel from water offers significant potential to contribute to these needs. Bismuth vanadate (BiVO4) has received much interest as a promising visible-light-active photocatalyst for water splitting and pollutant decomposition. BiVO4 has been found to exhibit phase-dependent photocatalytic activity. For instance, the monoclinic phase (m-BiVO4) exhibits much higher activity than the tetragonal phase (t-BiVO4), although their main difference is a slight structural distortion in m-BiVO4. Besides the underlying reason for the superior performance of m-BiVO4 over t-BiVO4, mechanisms underlying the enhanced photocatalytic activity of m-BiVO4 through incorporation of impurities such as phosphorous still remain poorly understood. For instance, doping of W, Mo or P into BiVO4 has been experimentally found to lead to a significant increase in the photooxidation current of water. In this talk, we will present theoretical evidence for phase dependence in the localization and transport of excess charge carriers, based on hybrid density functional theory calculations; this may shed some insight into why m-BiVO4 and t-BiVO4 exhibit a considerable difference in photocatalytic performance. In addition, the effect of impurity doping will be discussed with our theoretical explanation and prediction. The improved understanding may offer important guidance for the rational design of BiVO4-based materials for high efficiency solar-powered hydrogen generation.

A novel synthesis technique for the production of copper zinc tin sulfide (CZTS) nanocrystals has been developed using aerosol spray pyrolysis. CZTS is a quaternary semiconducting material that shows promise as a replacement to common semiconductors such as CdTe and CIGS for use in photovoltaic devices. CIGS is currently being commercialized in the photovoltaic industry, but rare and expensive indium and gallium components threaten its long term viability. CZTS looks to be one of the best alternatives to CIGS with all earth abundant and non-toxic materials and a band gap of 1.5 eV [1]. A number of synthesis techniques have been thoroughly studied and detailed previously. In our novel approach, we synthesis single phase 15 nm nanocrystals, starting with zinc, copper, and tin diethyldithiocarbamate precursors in a toluene solvent. The precursor solution is aerosolized using an ultrasonic nebulizer wherein the droplets are vacuumed through a tube furnace and nucleation occurs. We reproducibly synthesize kesterite, Cu2ZnSnS4, nanocrystals. This technique continuously converts the chemical precursor into high-purity nanopowder with a production rate of ~50 mg/hour for an un-optimized, lab-scale reactor. Using the same precursor chemistry, we have also been able to deposit high-quality CZTS thin films directly onto arbitrary substrates using a spray pyrolysis technique. A discussion of process parameters on the stoichiometry of either the nanoparticles or the thin films will be presented. Results from extensive characterization via Raman spectroscopy, EDS, XRD, TEM and XPS will be presented. We are currently in the process of producing a printable ink to coat CZTS as the absorbing layer for use in photovoltaic devices.
[1] H. Wang. “Progress in Thin Film Solar Cells Based on Cu2ZnSnS4,” International Journal of Photoenergy, 2011.

Orthorhombic SnS and GeS are layered materials made of earth-abundant elements which have the potential to play a useful role in future photovoltaic devices. We report on first principles calculations of the quasiparticle spectra of these two materials, predicting the type and magnitude of the fundamental band gap, a quantity which shows a strong degree of scatter in the experimental literature. Additionally, in order to evaluate the possible role of GeS as an electron-blocking layer in a SnS-based photovoltaic device, we investigate the band offsets of the interfaces between these materials along the three principle crystallographic directions. We find that while the valence-band offsets are similar along the three principle directions, the conduction-band offsets display a substantial amount of anisotropy.

The Zn-IV-N₂ family represents an earth abundant element alternative to current III-N devices due to their similar range of band gaps. There are a significant number of reports on the fabrication of the wide band gap ZnSiN₂ and ZnGeN₂, but little is known regarding the other family members. ZnSnN₂ is predicted to exhibit a direct band gap of 2 eV, assuming an ordered orthorhombic structure. We have synthesized a series of single crystal ZnSnN₂ thin films using plasma assisted molecular beam epitaxy on (111)-oriented yttria-stabilized zirconia (YSZ) and (001)-oriented LiGaO₂ substrates. Films exhibit a strong quality dependence on the metal flux ratio, with the best films obtained with a Zn:Sn flux in excess of 20:1. DFT predictions indicate that ZnSnN₂ should crystalize in the orthorhombic structure due to the periodic ordering of Zn and Sn. However, it is also expected that disorder in the cation sub-lattice will result in a monoclinic distortion due to changes in the periodicity. The fully disordered structure is monoclinic with a γ value near 120°, and bears a close resemblance to wurtzite InN. Similar behavior has been seen in the related materials ZnGeN₂ and ZnSnP₂ where variations in the order parameter have been observed. As with those materials, fully disordered ZnSnN₂ is also expected to possess a narrower band gap than its ordered counterpart. In fact, DFT calculations performed using the special quasi-random structure give a band gap narrowing of 0.97 eV compared to the orthorhombic structure.
Characterization of samples by x-ray diffraction yields lattice parameters for a monoclinic structure consistent with a disordered lattice. All films grown to date exhibit the monoclinic structure, regardless of the growth conditions, substrate, or crystal quality. Hall effect measurements indicate that all samples are degenerate, with a carrier concentration above 10²⁰ cm^-2. Films exhibit optical absorption edges in the range of 2 - 2.4 eV, depending on growth conditions. Care must be employed when interpreting optical transitions due to competing effects that can alter the apparent band gap, as has been seen with other materials. Despite what would be expected from a pure Burstein-Moss shift, a higher energy absorption edge is observed for films with lower carrier concentrations. We believe this to be due to increased cation disorder in the high carrier concentration film resulting in a narrower fundamental band gap. Far from being detrimental, variations in band gap with respect to degree of ordering present an opportunity for band gap engineering without need of alloying.
This material is based upon work supported by the National Science Foundation under Grant Nos. DMR-1244887 (UB), DMR-0605734 (FAMU), EPSRC Grant Nos. EP/G004447/2 (UL) and EP/F067496/1 (UCL). D.O.S. acknowledges the Ramsay Memorial Trust.

Iron pyrite has seen a renewed interest in the past few years as a promising earth-abundant, non-toxic semiconductor material for solar energy conversion. Pyrite has many appealing properties such as direct/indirect bandgaps of 1.03 & 0.95 eV, large absorptivity of 6×10⁵ cm^-1, and high carrier mobility (for single crystals). However, recent attempts to make solar cells using pyrite nanostructures or thin films have been hampered by the presence of impurity phases (such as the marcasite polymorph) and low photovoltage. To date, the only means of synthesizing phase-pure pyrite thin films requires a harsh sulfidation step at high temperatures to eliminate the other impurity phases.
Here we report a direct chemical vapor deposition (CVD) synthesis of phase-pure pyrite thin films with micron-sized crystalline domains using FeCl₃ and di-tert butyl disulfide (TBDS) precursors on a conducting cattierite (CoS₂) substrate. Growth of a phase-pure thin film is attributed to the epitaxy between isostructural pyrite and cattierite (with only ~2% lattice mismatch), which promotes the growth of pyrite and suppresses the marcasite impurity phase. The phase-purity was confirmed using X-Ray Diffraction, Raman Spectroscopy, and Energy Dispersive X-Ray Spectroscopy. The transport properties and electrochemical properties of the pyrite thin films will be reported. The ability to grow high quality, phase-pure pyrite thin films represents an important first step in the eventual realization of practical solar cell devices based on pyrite.

Zinc tin nitride (ZnSnN2) is a novel earth-abundant semiconductor. It is a member of the II-IV-Nitride family and is analogous to the well-characterized III-Nitrides that are currently employed in light-emitting diodes and sensors. Promising results from hybrid DFT simulations of ZnSnN2 predicted a direct band gap of 1.4 eV, which lies in the permissible regime for solar energy conversion. This result motivated the search for reasonable fabrication methods to ZnSnN2. Similarly to the III-Nitrides that posses tunable band gap energies, ZnSnxGe1-xN2 alloys should also be tunable. Recently, we demonstrated that they are tunable and we are developing ZnSnxGe1-xN2 alloys for their potential application in photovoltaics, light-emitting diodes, and sensors. In order to advance this work and fabricate devices their electronic characterization is important and we report on the relative band alignments for ZnSnN2.
Zinc tin nitride thin films were fabricated by RF reactive sputtering on sapphire and GaN templates on sapphire. The thin films were shown by x-ray diffraction studies to be polycrystalline with grain sizes ranging from 30-80 nm. The resistivities were determined to be in the 10-2 Ohm-cm range and exhibited high n-type doping.
Using photoelectron spectroscopy the work functions of treated and untreated surfaces and their extrapolated ionization potentials were determined. To further locate the relative valence band positioning of ZnSnN2, band-offset measurements using the Kraut band alignment method were used. This measurement involves the deposition of metal overlayers in-situ to the ZnSnN2 film. X-ray photoelectron spectroscopy was used to detect core-level peak shifts and valence band onsets in order to define the valence band offset with the respective metal work function. The metals studied were platinum, gold, and magnesium. Using effective band gaps from ellipsometry and the compiled data the band bending energy diagrams and barrier heights for the Schottky junctions were determined. Devices constructed with the selected metal contacts, however, all displayed ohmic behavior. However, the ohmic junctions were shown to be photoconductive. The observed photoresponse provides a positive outlook for the continued development of ZnSnN2 for solar cells and photon sensors.

Kesterite materials such as Cu2ZnSnS4 (CZTS) use abundant elements to form effective thin-film inorganic photovoltaic absorbers.[1] Following five years of intensive research, the bulk and defect properties of these materials are becoming well understood.[2] However, what is the best way to synthesise them?
A wide range of formation reactions has been explored in the experimental literature; an ideal industrial-scale reaction would be solution-based, operating at moderate temperatures and pressures and controllable by easily-manipulated parameters. The CZTS crystal structure consists of a relatively simple close-packed framework (shown to the right); however, the phase equilibrium between the various binary precursors and intermediates is relatively complex. An ab initio thermodynamic screening approach is used to explore the operating envelopes of some of these reactions, with a view to identifying the most industrially-promising methods.
Reaction energies are modelled using Density Functional Theory with the PBEsol generalised gradient approximation (GGA), and temperature-dependent properties are modelled using lattice dynamics within the harmonic approximation. FHI-aims is a modern quantum chemistry code which scales well for large systems, and vibrational calculations are carried out using the supercell approach with the Phonopy package. The result is the free energy, and associated thermodynamic potentials, of CZTS and its component states, which can be used to model reactions as a function of temperature and pressure.
1. “Kesterite Thin-Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4” A. Walsh, S. Chen, S.-H. Wei and X. G. Gong, Advanced Energy Materials 2, 400 (2012).
2. “Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Earth-Abundant Solar Cell Absorbers” S. Chen, A. Walsh, X. G. Gong and S.-H. Wei, Advanced Materials 25, 1522 (2013).

In this presentation, we report a low temperature, facile solution reaction route for the fabrication of QDSSCs made with Ag2S quantum dot/ZnO nanowires, simultaneously ensuring a low manufacturing cost and environmental safety. Simple hydrothermal reaction was adopted to synthesize ZnO nanowire array, while successive ionic layer absorption reaction (SILAR) was used to produce Ag2S quantum dots on the ZnO nanowire surface.
For the comparison, the most widely used sensitizer in quantum dot solar cell field, CdS, was also synthesized on the ZnO nanowire. Both heterostructure, Ag2S/ZnO and CdS/ZnO, were analyzed to estimate their prospect in photovoltaic application.
UPS analysis shows that the band structures of both photoanodes have type-II band structures, which facilitate electron transfer and collection. Then, both electrodes were used as photoanodes in quantum-dot sensitized solar cells (QSSC). In the solar cell performance test under the illumination of 1 sun condition, Ag2S QDSSC exhibited a considerably higher photocurent with a strongly enhanced light harvesting efficiency compared with the CdS QDSSC. The Ag2S QDSSC showed lower open circuit voltage, resulting in almost the same power conversion efficiency of 1.2%.
Reasons for the different properties were also investigated through the open-circuit voltage decay (OCVD) measurement, eletrochemical impedance spectroscopy (EIS) analysis, and incident photon-to-current efficiency (IPCE) test. Due to relatively narrow band gap energy of Ag2S, Ag2S Cell exhibited enhanced light harvesting and detoriated recombination rate at the same time. The fast recombination caused low open circuit voltage. Finally both cell showed same photovoltaic efficiency of 1.2%.
Although both the cell exhibited the same efficiency, it is meaningful that Ag2S could replace CdS in QDSSC, ensuring environmental non-toxicity. Also, it showed greatly improved light harvesting ability, which makes it a good sensitizer candidate if futher research on Voc improvement follows.

Low cost, stable, earth-abundant materials for photovoltaic applications are increasingly attracting people&’s interest as viable, ready to commercialize alternatives for solar cells components. One promising approach is the Metal-Insulator-Semiconductor (MIS) concept, where a thin insulator layer is playing the role of light absorber and charge separator. We are reporting on the production of hybrid MIS structures involving ordered titanium oxide nanotube arrays integrated with insulating iron oxide or copper oxide layers and metallic copper. The synthesis process involved only inexpensive, easy to scale up steps such as electrochemical anodization and deposition. The intrinsic nanotubular architecture is ensuring a very large active surface area, several orders of magnitude larger than the flat thin films. Structural and morphological properties of the new structures were studied by means of SEM and TEM imaging, energy-dispersive X-ray spectroscopy (EDS) and elemental mapping, and X-ray diffraction (XRD). A solar simulator (1.5AM) was employed for characterizing the lab scale MIS (TiO₂ | Fe₂O₃/Cu₂O | Cu) solar cells by means of IV curves. Open circuit values over 0.5V, in line with the current commercial standards, have been already demonstrated, and the optimization of the synthesis process and structural design of the cells is currently investigated.

In order to make use of the waste heat caused by the unabsorbed light based on photovoltaic(PV) effect, a novel hybrid dye-sensitized solar cells (DSSC) with the synergies of PV and thermoelectric(TE) effect has been proposed in present work. The main idea is to prepare a composite hybrid DSSC photoanode which can simultaneously achieve PV and TE conversion by incorporating the excellent TE Bi-Te alloys into TiO2 nanomaterial. In this paper, Bi2Te3 nanoplates with different size were doped in the TiO2 nanoparticle photoanode and the effect of the Bi2Te3 size on the properties of DSSCs was analyzed. It is found that with the decrease of the size of the Bi2Te3 nanoplates, the TE performance became better and the dye absorption and the conversion efficiency of DSSCs were improved. Preliminary results show that the efficiency of DSSC with Bi2Te3 increased at least 15.3% compared to the un-doped. By further optimizing the parameters, the performance of DSSC is estimated to have a much more enhancement. Therefore, the new idea of combination PV and TE provides an alternative way to improve the performance of DSSC.

In order to avoid to use the expensive ITO as the transparent electrodes of dye-sensitized solar cells(DSSC), Ga-doped ZnO(GZO) thin films were employed in present work, meanwhile ZnO nano composite structure (nanoparticles (NPs)+ nanowires (NWs) )were applied as photoelectrode for DSSC. GZO thin films were prepared by RF magnetron sputtering. It shows that GZO films were polycrystalline with a preferred 002 orientation, their electrical and optical properties were compared to those of commercial ITO and fluorine-doped tin oxide (FTO). Then a compact ZnO film was deposited on GZO film by RF magnetron sputtering and ZnO NPs or (NPs+ NWs) was coated on this compact layer to adsorb dye. The DSSCs with GZO transparent electrodes showed better photo-electric conversion performance than those with FTO, which can be attributed to the ZnO compact layer can prevent the oxidation of GZO film at high temperature and reduce the charge recombination at the interface between transparent conductive oxide and electrolyte. Meanwhile, it is found the compound nanostructure DSSC exhibits a significant enhancement of the short-circuit current density, fill factor and overall conversion efficiency, which can ascribed to NPs enlarging dye adsorption area by filling in the gaps between NWs and the NWs affording fast electron transport channels.

Over the past few years, a variety of strategies have been employed to improve the photocatalytic performance of semiconductor photocatalysts. In this work another successful strategy has been developed. This work demonstrates the efficient harvesting of solar energy using organic dye with TiO2 and reduced graphene oxide (rGO) for solar water splitting without using noble metals. Herein, a novel organic photo sensitizer, the derivative of aryl bis-amide (ABA) is designed and developed which has good stability and reusability. Also, a series of environmentally benign novel sensitizers were prepared and applied for the present system. The graphene oxide was prepared from natural graphite by using a modified Hummers method. The experimental results illustrate the potential of rGO as co-catalyst for the steady evolution of H2. In a typical synthetic procedure, the highly crystalline mesoporous nanostructure TiO2 was obtained leading to a broad absorption spectrum towards the red shift in DRS band. In the first instance, after slow initiation, the rate of H2 production accelerated over time which was corroborated by the formation of rGO. The resultant ABA-TiO2-rGO exhibited significantly higher photocatalytic performance because of a rapid separation of photogenerated electrons and holes by the rGO cocatalyst. In the present work, we have undertaken a systematic study on the use of various SEDs in order to probe the efficacy of hydrogen production over these photocatalysts.
In the experimental setup, ABA-TiO2-rGO of 0.010 g in 20 mL aqueous methanol solution (15%V) under Xe arc lamp irradiation (450 W) gave H2 of 18 mmol gminus;1h-1. The photocatalyst is cost effective and eco-friendly and also presents advantages such as suppression of charge recombination, improvement of interfacial charge transfer and increase of active sites. Salient results obtained by further studies will be discussed.

Cuprous oxide is an earth abundant, low cost and non-toxic p-type semiconductor with a direct band gap of 2.1 eV that has been employed as an absorber layer in solar cells. Although the Schokley Queisser (SQ) efficiency limit for Cu₂O based solar cells is 20%, the efficiency obtained to date is rather low. The poor efficiency in Cu₂O based solar cells can be traced back to fundamental electronic band structure and absorption properties. Cu₂O has a direct forbidden band gap and a high photon absorption threshold (~2.6 eV). The electronic band structure of copper oxide reveals the forbidden lowest energy transitions as the prime cause for higher absorption threshold. Thus, by tuning the electronic band structure to allow the lowest energy transitions, the photon absorption threshold (in energy) can be decreased. In this study, we alloyed Cu₂O with ZnO to modify the electronic band structure and found that the absorption threshold is significantly reduced.
ZnO alloyed copper oxide thin films were synthesized via combinatorial rf magnetron co-sputtering from Cu₂O and ZnO targets onto a 2” x 2” glass substrate. The Cu₂O and ZnO targets were set at 15° with respect to the substrate and a composition gradient was obtained across the library. Single-phase polycrystalline copper zinc oxide alloys were synthesized at RT and 3 mTorr Ar pressure. From x-ray fluorescent measurements, the concentration of Zn present in these alloys was found to be 6 - 13 at%. The optical reflection and transmission was measured in the energy range from near infrared to visible and the absorption coefficient was calculated. The absorption coefficient increased with increasing Zn content in the energy range 1.6 - 2.4 eV and a maximum of up to 3*10⁴ cm^-1 was observed at 2 eV. Thus the absorption threshold was significantly reduced. Zn alloyed copper oxides were also grown at higher temperatures and different pO₂ and their optical properties will be discussed.
This research is supported by the U.S. Department of Energy, office of Energy Efficiency and Renewable Energy, as a part of a Next Generation PV II project within the SunShot initiative.

The development of earth-abundant electronic nanomaterials compatible with low cost and low curing temperatures is one of the main challenges for printed plastic electronics and photovoltaics. Chemical syntheses of inorganic nanocrystals (NCs) is a powerful tool to provide a wide range of conducting and semiconducting nanomaterials dispersed in colloidal solutions that are solution processable to create solid-state devices such as field-effect transistors or solar cells. Doping of metal oxide NCs allow tailoring the bandgap, optical properties, carrier concentration thus conductivity [1]. Regarding their electric properties, high quality aluminum-doped ZnO (AZO) colloïdal solutions have been published showing high conductivities in combination with decent transparency, but high curing temperature were needed making them incompatible with printing onto plastic substrates.
Here we present two different approaches to synthesis AZO nanoparticles: first synthetic route that allow controlling both the aluminum doping level and the NCs morphologies (spheres, triangles or hexagons) using alkyl chain bearing ligands during synthesis. This made additional ligand exchange procedure necessary to replace the existing by shorter ligands promoting high conductivity between NC. In a second approach, we therefore developed the synthesis of ligand free AZO nanoparticles that are highly soluble in polar solvents. The solution processing of these AZO nanoparticle into thin films allowed us to generate highly transparent conductive films without any further temperature treatment. Electron tomography was used to study the 3D shape of the AZO in detail, while impedance spectroscopy, photoemission spectroscopy and conductivity measurements were applied to characterize the electronic properties of the AZO NCs.
[1] Yizheng Jin et al. Journal of Nanomaterials, 2012, 985326

The importance of sunlight driven fuel generation lies in the potential for a substantial impact on replacing fossil fuel by renewable solar fuel sources. Many persistent challenges of developing an artificial photosynthetic system for demanding chemical transformations in the solar fuel generation processes exist. One of the essential challenges is to achieve effective separation of water oxidation and fuel generation (carbon dioxide reduction) catalysis sites by an ion conducting, gas impermeable membrane at the nanoscale. Several chemical intermediates are formed in the multi-electron transfer steps in the photosynthetic cycle, especially for the fuel generation reactions. Incorporating a membrane between the oxidation catalysts and the reduction sites can potentially lead to minimized crossover reactions and reduced charge recombination. Ideally, the separation membrane should also control the charges and ions transfer processes to minimize efficiency degrading processes and possess enough electronic and ionic conductivity. Integrating such a separation membrane with nanoscale thickness into an artificial photosynthetic system, as in the case of natural photosynthesis, will mark a major step toward closing the artificial photosynthetic cycle at the nanoscale. To meet aforementioned challenge, our group proposed an artificial photosynthetic unit composed of asymmetrically functionalized core-shell cobalt oxide/silica nanotube arrays featuring organic molecular wires (p-oligo(phenylenevinylene), PV3) embedded in the ultrathin silica shell membrane. Herein, our latest progress toward synthesizing the designed heteronanostructure using atomic layer deposition and surface chemical modification will be discussed. The photogenerated hole transfer and proton transport property across the oxide membrane are quantitatively characterized using photoelectrochemical method, transient optical spectroscopy and electrochemical impedance spectroscopy. The knowledge learned from these studies will be invaluable to optimize artificial photosynthetic system performance and may open doors to a wide range of applications which require robust separation membranes with specific electronic and ionic transport property.

We report here the structure and properties of ZnSn_xGe_1-xN₂ thin films. These earth-abundant semiconductors have an electronic structure and tunable band gap similar to In_xGa_1-xN. Calculations predict that replacing the group III elements (In and Ga) with a combination of group II (Zn) and group IV (Sn, Ge) elements yields ZnSn_xGe_1-xN₂ with a band gap that is tunable over a range from 1.4 eV to 2.9 eV, which covers a large portion of the solar spectrum. It has also been shown that there is a smaller lattice mismatch between ZnSnN₂ and ZnGeN₂, compared to In_xGa_1-xN over the same energy gap range.
Thin films of ZnSn_xGe_1-xN₂ were deposited on c-sapphire and c-GaN template substrates using reactive RF magnetron sputtering from a Zn_0.75Sn_0.25 pressed powder target and a single-crystal Ge elemental target in a nitrogen-rich atmosphere at ~250 °C. The film composition, measured by energy dispersive X-ray spectroscopy, was varied by keeping the power constant on the Zn_0.75Sn_0.25 target and changing the power on the Ge target. The combined target is zinc-rich because the high vapor pressure of Zn limits its incorporation at deposition temperatures above ~200 °C.
On c-GaN substrates, ZnSn_xGe_1-xN₂ films are predominantly oriented with <001> normal to the interface. X-ray diffractograms reveal a linearly increasing (002) peak position in 2theta; as x decreases, indicating a continuous change in the c lattice parameter and an absence of any observable phase separation between ZnSnN₂ and ZnGeN₂. The optical band gap is estimated from absorption obtained using spectroscopic ellipsometry, and increases from ~2.2 eV to ~3.1 eV as x decreases. Films with x = 1 have a larger band gap than expected, attributed to the Burstein-Moss effect, which is further evidenced by a large donor carrier concentration from Hall measurements. The resistivity of the films strongly increases with decreasing x, suggesting that the Burstein-Moss effect diminishes as the Sn content decreases.
Thin films of ZnSn_xGe_1-xN₂ have been synthesized over a wide range of x values with no observable phase separation. In addition, it has been shown that the band gap is tunable from ~2.2 eV to ~3.1 eV. We suggest that ZnSn_xGe_1-xN₂ is a promising low-cost and earth-abundant alternative to In_xGa_1-xN for photovoltaic applications.

Solar cell devices have nearly become a mainstay in terms of use for private energy production. Despite this advancement, the current solar cell materials still face many limitations such as encroachment on maximum solar conversion efficiency values, rare or toxic nature of the constituent materials, or expensive and complicated fabrication techniques. In response, a new class of sustainable materials is required to meet these challenges. In the past, copper sulfide was a candidate for solar cells because of the attractive band gap of the material, however because of issues with the material stability, the research was ultimately abandoned. Recent advancements however provide opportunities for improving the characteristics for the basic copper sulfide material by incorporating an additional element to the chalcogenide structure. For example, we have utilized a wet chemical synthetic method to incorporate various metals such as zinc or iron into copper sulfide nanoparticles. These resulting copper metal sulfide particles display unique and tailorable semiconducting properties as a function of the particle composition, size and structure. In addition, the nanoparticles lend themselves to easy device preparation through spray deposition of the nanoparticle ink. The presentation will focus on our recent results in the synthesis and characterization of a new class of copper metal sulfide nanoparticle with applications in solar cell devices. The results will be discussed using characterization techniques such as XRD, XPS, TEM, STEM-HAADF, EDS Elemental Mapping and others.

Cost effective solar cells with high efficiency based on abundant non-toxic materials is the long term target of present research and development. Multijunction, material saving thin film solar cells, including nanostructures or nanoparticles is considered as an important option for future solar cell technologies. Copper(I) oxide (Cu2O) as an abundant, non-toxic material with a suitable band gap of 2.1 eV is an appropriate candidate for an absorber in top cells of thin film tandem solar cells.
Commercial available copper(II) oxide (CuO) nanoparticles were transformed in Cu2O via oven annealing at 1000°C in nitrogen atmosphere. The Cu2O particles are investigated with respect to application as active absorber material in solar cells. Microstructure investigations were carried out by XRD, TEM and Raman spectroscopy. Absorption characteristics were determined by photo thermal deflection spectroscopy (PDS). The electronic properties were investigated by photoluminescence (PL) spectroscopy at room temperature and down to 130 K in the visible to NIR energy range with a spatial resolution of 1 µm.
According to TEM results the CuO particles are agglomerated with a particle size of 30 nm to 100 nm. After the 1000°C temperature treatment in nitrogen, the material is further agglomerated and transformed into Cu2O as can be deduced from XRD. Also the Raman spectra are dominated by the Cu2O Raman modes. PDS reveals an absorption edge at about 2 eV which is in agreement with the Cu2O band gap. PL at room temperature exhibits strong excitonic band edge luminescence of Cu2O at 1.967 eV as well as defect luminescence at about 1.6 eV probably caused by copper and oxygen vacancies. A weak signal at the position of the CuO emission is tentatively attributed to remaining CuO. At room temperature the excitonic band edge emission is about two orders of magnitude more intense than the defect luminescence, indicating a very high quality material. Furthermore, the excitonic emission reveals a fine structure which is due to scattering at LO phonons and shifts to higher energies when cooling the sample to 130 K. Concomitantly, the slope of the PL emission becomes steeper at the high energy side of the spectrum which can be explained by a change in the occupation of the excitonic states. At 130 K the 1s exciton can be located at 2.019 eV with the respective LO replicas at 2.017 eV and 2.041 eV. Additionally, excitonic transition at 2.139 eV and 2.156 eV can be resolved and attributed to the n=2 and n=3 transitions, respectively. The temperature dependence of the 1 s exciton reveals a maximum at 190 K, the reason is not yet clear. All results point to a very high quality of the particles which are therefore considered to be ready for implementation into solar cells.

Cu2SnS3 (CTS) thin film is a promising material for low-cost thin film solar cells, because it is composed of abundantly available materials. CTS films were grown by thermal evaporation in a vacuum system using evaporation sources for Cu, Sn, and S. All elements were co-deposited onto soda lime glass (SLG) substrates and Mo coated SLG substrates. The deposition was performed at different substrate temperatures. From the results of x-ray fluorescence measurements, it was confirmed that the nearly stoichiometric CTS thin films were obtained in all substrate temperature. The influence of substrate temperature on the structural and electrical properties of CTS thin films grown by co-deposition was investigated.

Solar thermal fuels are a potential all-in-one solution for capturing, converting, transporting and delivering solar energy sustainably and cleanly by using sunlight to photochemically generate highly metastable isomers that later release the stored energy as heat. We have developed a new approach for achieving meaningfully high energy densities by templating photoswitchable molecules on nanoscale templates to access highly strained molecular conformations and engineer specific interactions between neighboring chromophores. We have experimentally demonstrated a bulk energy density of 56 Wh/kg with ~14% external quantum efficiency in a highly cyclable and robust material consisting of simple azobenzenes covalently bound to single-walled carbon nanotubes. This system validates our approach and indicates that the predicted energy densities of ~200 Wh/kg are achievable with other, similarly template azobeznene variants. This talk discusses these findings, the effects of templating azobenzenes on their thermal and photoisomerization kinetics, and the role of inter-template interactions in increasing energy densities and storage lifetimes.

Over the last decades, photocatalyts have aroused a growing interest because of their ability to harvest light and subsequently induce chemical reactions on the interface of the material surface and the surrounding homogeneous phase. Given the nature of a photocatalytic reaction, the catalyst surface properties become crucial to improve the overall catalytic features. On the other hand, it is equally important to improve the material morphology and electronic structure for both effective photoinduced charge separation and transport. Inasmuch as the use of nanosized materials has captured all the attention in this field, this latter strategy has undergone lesser relevance since nanoparticles behave very differently from their macroscopic analogues. However, the increasing capability of researchers to create higher quality nanosized materials with excellent crystallographic properties has awaken again the significance to govern and control materials properties such as charge carrier density, space charge layer width, and Fermi level position, among others.
From the diverse applications of the photocatalytic materials, photoelectrochemical water splitting has recently undergone special awareness by virtue of the actual and future energetic juncture. Titanium dioxide is considered the material par excellence in water splitting applications despite its light absorption limitation caused by its large band gap (3.0 eV). Many attempts have been tried to circumvent such limitation and improve its overall photocatalytic efficiency. Among them, tailoring the interface through the control of the electrode nanostructure has shown promising results. In this regard, single crystalline TiO2 nanorods have been successfully grown on FTO substrates through a simple hydrothermal route. Since then, the number of publications based on this material that attempt to improve its photocurrent has increased, reaching values very next to the theoretical efficiency values for TiO2. However, the grounds for the improvement of the photocatalytic activity of TiO2 nanorods have not been fully elucidated yet. In this work, we present an ammonia-induced reduction treatment of titanium dioxide rutile nanorods that triggered a synergistic surface modification of titania electrodes. A model, based on fundamental physical parameters (e.g. donor density, Fermi level position, depletion layer width and surface states) is proposed to disclose the relevance of each interfacial property on the observed photoefficiency modification.

Dye-sensitized solar cells (DSCs) are electrochemical solar cells, in which platinum (Pt) plays a crucial role as counter electrode and catalyzing the triiodide/iodide transitions. Despite the high catalytic activity of Pt, extensive studies aimed at discovery of cheaper substitutes for this material has been pursued. Among these efforts, earth-abundant elements with their dichalcogenide formations are of high interest due to their low-cost and high catalytical activity. In this work, the potential applications of molybdenum disulfide (MoS2) atomic layers counter electrodes, synthesized by chemical vapor solid-state reaction and fabricated on fluorine-doped tin oxide (FTO) substrates, are examined. This counter electrode has high light transmission in the visible range (70-80%), and the DSCs made from them show comparable photon-to-electron conversion efficiencies (4.4%) to those made of Pt (5.6%). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis reveals that the catalytical activity of the 2-dimensional MoS2 is lower than that of Pt. Open-circuit voltages (Voc) are found to positively shifted (30mV) for all the MoS2 cells. We propose a model to explain the mechanism of MoS2 Voc tuning and utilize a Mott-Schottky analysis to signify this theory. Despite the limited specific surface area, the 2-dimensional MoS2 exhibits reasonable catalytical activity. Hence, we propose that better DSCs&’ performance could be expected as the morphology of MoS2 atomic layers is modified and sufficient active edges are exposed.

We present a complete, easily fabricated, high-performance top contact system for use with photovoltaic and thermophotovoltaic cells. The system incorporates a transparent conductor and a two-stage antireflective treatment. To create an economically sustainable design, choice of materials is restricted to those that do not require large amounts of rare-earth elements. The proposed system will use niobium-doped titanium dioxide (TNO) as the conductor combined with a continuously graded antireflective coating composed of titanium dioxide and silicon dioxide. To further suppress reflection of incident light, the silicon dioxide surface will be etched to form a nano-scale “moth&’s eye” pattern.
TNO was chosen as the conductor for the proposed top contact system for several reasons: First, it displays electrical and optical performance on par with ITO. Second, it minimizes the amount of rare earth material used. Third, the exceptionally large window of transparency, extending well into the infrared, makes TNO ideal for use with wide-spectrum, multi-junction solar cells or thermophotovoltaic cells operating in the near IR. Lastly, the crystalline structure of titanium dioxide shows excellent compatibility with silicon dioxide, allowing the two materials to be blended into a stress-free crystalline composite. This compatibility with silicon dioxide is exploited to form the graded antireflective layer on top of the conductive layer.
To improve the antireflective behavior, a regular pattern of sub-wavelength scale (200nm) cones is etched into the SiO2 surface to create a gradual optical transition between air and glass. Shadow nanosphere lithography (SNL) is used to form the etch mask to pattern the antireflective treatment. This technique, employing a close-packed monolayer of polystyrene nanospheres, provides a fast, low-cost method to create nano-scale patterns over large areas.
By combining new materials and recent advances in nano-scale fabrication, we are able to produce a useful and versatile top contact for optoelectronic devices. Several variants of the system were fabricated and tested to determine the best design and the procedure was optimized to create a reliable fabrication process.

We report the demonstration of single-layered and multi-layered tantalum oxide (Ta2O5) nanorod arrays, first fabricated by an anodization process, and then converted into tantalum oxynitride (TaON) nanorod arrays through nitridation in an ammonia atmosphere. The morphology and crystal structure of the TaON nanorods are found to depend on the anodization and annealing conditions, respectively. Dense carpets of TaON arrays show an enhanced optical absorption activity in the visible part of the light spectrum. DFT calculations showed covalent character of Ta-O and Ta-N bonds that results in a large dispersion in the valence band maximum raising it up, which can be held responsible for the observed bandgap narrowing and visible light response of TaON. When used to photoelectrochemically split water (1 M KOH, AM 1.5G illumination, and 0.6 V applied DC bias), the multilayer nanorod films show visible light incident photon conversion efficiencies (IPCE) as high as 7.5%. Comparison to flat TaON films indicates that the enhanced visible-light photochemical activity is related to the ordered one-dimensional morphology of fabricated TaON nanorods.

In the decades to come, the accomplishment of ‘grid&’ and ‘fuel&’ parity visualise massive deployment of PV in power production. The existing PV technologies only fulfil the production requirement tentatively due to their one or more limitation such as Ag in c-Si; Te in CdTe; In, Ga in CIGS; and Ru in DSSC. The sustainability of PV production in future calls for a new PV technology based on low cost, abundant, environment friendly and necessarily compatible with the existing manufacturing technology. Earth-abundant Cu-Zn-Sn chalcogen kesterites [Cu2ZnSn(S, Se)4] (CZTS) emerge as potential alternative to existing major thin-film PV technologies. The bandgap of the CZTS absorber can be tailored using incorporation of Se or partial replacement of Sn with Ge thereby efficiency enhancement through utilization of more available solar spectrum. It is still attractive to develop new methods for bandgap tuning and investigation of important properties of CZTS thin film absorbers as Se and Ge not worthy in terms of cost and rare availability.
The present study investigates the growth of Fe-rich CZTS thin-films at different deposition conditions. The effect of the compositional variation of constituent Zn and Sn was also explored. The deficiency of S was controlled using post deposition annealing in H2S ambient. The optimum growth conditions were realized with good control of Zn and Sn composition at elevated temperature. The electrical behaviour of Fe-rich CZTS semiconductor with metals like Cu, Ag and Pd was also studied. This study provides an alternative compositional approach to obtain the optimum bandgap for solar energy harvesting using Cu-Zn-Sn chalcogen kesterites thin-film solar cell.

As a promising strategy to address challenges caused by increasing need of renewable energy, semiconductor (SC) based photoelectrochemical (PEC) solar water splitting has received growing attention. Among compounds that have been studied, hematite (α-Fe₂O₃) stands out for its earth abundance and, hence, low cost. An important issue in using hematite for solar water splitting is the relatively positive band edge positions. We sought to address the issue by combining hematite with another earth abundant element of Si. The resulting material not only uses the solar spectrum more efficiently but also changes the band edge positions of hematite to better match water oxidation and reduction potentials. The design was realized by growing high-quality hematite thin film by atomic layer deposition (ALD) on Si nanowires in a conformal fashion, enabling a high current density and an onset potential of 0.6 V vs.RHE which represents one of the lowest reported for hematite photoanode devices. The cathodic shift on the onset potential compared to the planar sample was attributed to the gained photovoltage by the introducing of hematite-Si buried solid junction. A systematic study concerning potential pitfalls of using PEC characterization to study water splitting will be presented. For instance, we show that side reactions other than water oxidation (oxygen evolution) can contribute to photocurrents, in which case the PEC data need to be treated with great caution. Direct measurement of photovotlages generated by hematite and Si, respectively, was carried out to unambiguously confirm the function mechanism of the dual absorber system. Similar studies also allowed us to examine the failing mechanisms that prevented us from measuring performance improvement when hematite was combined with a high-performance oxygen evolution catalyst of MnO_x.

Photoelectrochemical (PEC) reduction of CO₂ into useful organics is regarded to be one of the most promising way to utilize the solar energy because it converts the unstable solar energy into stable chemical energies. However, the photoelectrodes used in the PEC reduction of CO₂ need to be carefully and subtly designed to satisfy both requirements of high energy conversion efficiency and high product selectivity which are very difficult to achieve simultaneously. Therefore, separate the PEC process into photovoltaic (PV) and electrochemical (EC) processes would be another promising way to solve such problems because the high energy conversion efficiency could be achieved from the PV part while the high product selectivity could be obtained from the EC part. Meanwhile, in most reactions of PEC or EC reduction of CO₂, gaseous CO₂ bubbling was used as the carbon source. However, during this process, most of the CO₂ is exhausted directly into the air without any reaction, which not only causes a waste of the carbon source but also makes the separation of the gas products and the unreacted CO₂ inevitable. Therefore, in this research, the carbonate (CO₃^2-) and bicarbonate (HCO₃^-) solution were used as the carbon source and their effect were studied and compared with the CO₂ bubbling using an electrochemical method on a Cu electrode.
In this research, A copper wire (phi; 0.5 mm, 99.999%, Nilaco) was used as the working electrode. An Ag/AgCl electrode saturated with NaCl was selected as the reference electrode along with a Pt wire as the counter electrode. The CO₂ (99.995%, Taiyo Nippon Sanso) was bubbling for 10 min before and during the experiments when the CO₂ was used as the carbon source.
Results from the cyclic-voltammetric measurement showed that the CO₂ bubbling didn&’t affect the reaction too much when the KHCO₃ was used as the electrolyte. This is probably because the dissolved CO₂ changed into HCO₃^- in this pH region. Hori et al. reported that the production of CH₄ and C₂H₄ increased steeply while the H₂ generation dropped quickly with the cathodic potential from -1.2 to -1.4 V vs. NHE in the electrochemical reduction of CO₂ in KHCO₃ electrolyte with CO₂ bubbling [1]. The Voltammogram obtained in the present research was very similar to the results. However, When the K₂CO₃ or KCl was used as the electrolyte, not only the onset potential shifted positively, but also the current changed obviously with the CO₂ bubbling. According to the pH value, in the K₂CO₃ solution, most dissolved CO₂ became CO₃^2-, while in the KCl solution, the dissolved CO₂ changed into H₂CO₃. Therefore, the HCO₃^- has an important role in the CO₂ reduction.
Reference
[1] Y. Hori, et al., J. Chem. Soc., Faraday Trans. 1, 1989, 85, 2309.

Cu₂ZnSnS₄(CZTS) is one of the most promising absorber materials for low-cost thin film solar cells because of the suitable optical band gap of about 1.5 eV and the large absorption coefficient over 104 cm-1. The band gap energy of CZTS is optimal for a middle cell in a 3-cell tandem and close to the optimal top cell in 2-cell stacks with a Si bottom cell. The kesterite structure of CZTS is analogous to that of cubic Si [1]. The low lattice mismatch [2] demonstrates that single crystal cubic silicon would be suitable as a substrate material for the epitaxial growth of a tetragonal kesterite CZTS thin film.
In addition to obtain a more detailed understanding of the grain boundaries and interfaces and elucidation of flaws in the absorber layers which are critical to optimise the parameters to get the best quality of CZTS epitaxial layers on Si, further exploration of the experimental design for device optimisation is investigated. Epitaxial layers of quaternary compounds CZTS are grown on Si substrates via Molecular Beam Epitaxy (MBE). A CZTS/ Si heterojunction is made as the first step to obtain the band offsets between CZTS and single crystal Si wafers. The morphological and structural properties of the CZTS layers are investigated with scanning electron microscopy (SEM), transmission electron microscopy (TEM) atomic force microscopy (AFM), electron backscatter diffraction (EBSD), Raman spectroscopy and X-Ray diffraction (XRD). The composition of the films is determined by X-ray fluorescence (XRF). The optical property of CZTS films is investigated by photoluminescence (PL). Ultraviolet photoelectron spectroscopy (UPS) measurement is employed to study the band offsets between the CZTS epilayer and Si.
References
1. Schorr, S., The crystal structure of kesterite type compounds: A neutron and X-ray diffraction study. Solar Energy Materials and Solar Cells, 2011. 95(6): p. 1482-1488.
2. Oishi, K., et al., Growth of Cu2ZnSnS4 thin films on Si (100) substrates by multisource evaporation. Thin Solid Films, 2008. 517(4): p. 1449-1452.

Photoelectrochemical (PEC) cells offer the ability to convert solar energy to stored chemical energy through the splitting of water into molecular oxygen and hydrogen. Hematite (α-Fe₂O₃) has recently emerged as a promising photoanode material for the generation of oxygen from water splitting due it favorable optical band gap (E g -2.2 eV), excellent chemical stability in aqueous environments, ample abundance and low cost. This work describes a simple and inexpensive method to make pure and doped-hematite photoactive films with a controlled nanostructure. Hematite photoanodes was synthesized by a hydrothermal technique using FeCl₃ and urea at 100 °C in different times. As-prepared hematite photoanode was doped with addition of a 70 µL of aqueous solution of tin (IV) chloride before heat treatment at 750 °C for 30 min. The hematite phase was confirmed by X-ray diffraction data after the thermal treatment and no additional peak related to SnO₂ was observed. FE-SEM images illustrate the hematite films composed by rods vertically aligned on the conductor glass substrate. TEM images exhibit the rods covered like a shell by a thin layer of SnO₂. The photoeletrochemical performance was carried out in absence and presence of light using a solar simulator coupled with AM 1.5 global filters. The highest value of photoresponse for pure hematite films was found around 1.42 mA.cm^-2 (samples synthesized during 2 hours) at 1.23 V_RHE and for doped samples approximately 1.28 mA.cm^-2 (samples synthesized during 24 hours) at 1.23 V_RHE. Finally, a simple and cost-effective preparation of photoactive hematite photoanodes was described exhibiting a significant enhancement in photoeletrochemical performance for pure and modified hematite films with addition of SnO₂ layer covering the rods like a shell.

Water splitting to generate hydrogen by using solar energy is a candidate process to solve the global energy problem. There are two proposed major methods to split water with light energy. One is the photoelectrochemical way and the other is the electrochemical water splitting with the electric power made by solar cells. Especially for the oxygen generation, the oxygen evolving overpotential is a major problem to realize the system. Even precious metal oxides such as IrO2 or RuO2 are used as the catalysts, the overpotential is much higher than hydrogen evolving overpotential. Therefore, it is required that new oxygen evolving catalyst, which reduces overpotential much and consist of earth-abundant materials. In this sense, CaMn2O4*nH2O is one of the candidates of oxygen evolving catalysts for water-splitting[1]. This material has the close structure to oxygen evolution center of photo synthesis II reaction of a plant. However, the properties are still obscure because there are a few reports about CaMn2O4*nH2O. Even the synthesis condition of CaMn2O4*nH2O has not been clarified, thus, we investigated the condition of CaMn2O4*nH2O and evaluated the amount of coordinated water in this report.
We synthesized CaMn2O4*nH2O by using hydrothermal method as follows [1]. Firstly, Ca(NO3)2*4H2O (2.0 mmol, 477 mg) and MnCl2*H2O (2.8 mmol, 560 mg) were dissolved in 10 mL of water. Secondly, the mixture was stirred for about 15 min. at room temperature. Slow addition of aqueous solution of KMnO4 (1.2 mmol, 191 mg) and KOH (150mmol, 9.79 mg) in 10 mL of water resulted in a dark precipitate. Thirdly, the precipitate was collected by centrifugation, re-suspended in 10mL of water, transferred to a teflon-lined autoclave, and placed in an oven. The hydrothermal process was carried out for 72 h at 210°C and 180°C, respectively. Finally, the obtained suspension were collected by centrifugation and washed using distilled water before being allowed to dry for one day at 60°C in air.
Rod-like structure with the diameter of about 2 um was observed as-grown samples by scanning electron microscopy (SEM). In order to evaluate the coordinated water, we performed a calcination process 900°C for 1 h. The sample weight decreases 4.6% and 15.0 % after the autoclave temperature of 210°C and 180°C, respectively. The main sample component was CaMn2O4 analyzed from the powder X-ray diffraction. Considering from the weight decreasing and the component after the calcination process, the as-grown product with autoclave temperature of 210°C is CaMn2O4*H2O and that of 180°C is CaMn2O4*2H2O. From the catalyst activation point of view, the coordinated water affects the catalyst ability [1]. The catalyst ability for the electrochemical oxygen evolution of water-splitting will be evaluated in the future.
[1] M. M. Najafphour, et al., Angew. Chem. Int. Ed., 49, 2233, 2010

Hematite has emerged as a good photocatalyst for efficient solar water splitting. Various methods have been used to prepare high efficiency hematite nanostructures. However, a facile and cheap synthesis method of hematite nanostructures with high efficiency is critical for future practical application. Here we report a facile synthesis of hematite nanostructures for solar water splitting via a simple pyrolysis of ferrocene under ambient pressure. The photocurrent of the carbon-coated hematite nanostructures can be 2.1 mA cm-2 at 1.23 V vs. RHE, compared to a value of 0.5 mA cm-2 for hematite without the carbon layer. The carbon layer is a few nm thick covering the surface of hematite nanostructures. X-Ray photoelectron spectroscopy and X-ray absorption spectroscopy revealed that the electronic structure of hematite was significantly modified with the existence of oxygen vacancy, which was responsible for the remarkable photo- current. The carbon layer plays an important role for the appearance of oxygen vacancy. The simple and cheap method could be scaled up easily which may pave the way for the practical application for efficient solar water splitting.

The beneficial effect of TiO₂ 2D nanosheet morphology on the photocatalytic activity and electronic coupling of TiO₂-based hybrid materials is studied by the comparative investigation of 2D TiO₂-Ag₃PO₄ nanohybrid with P25-Ag₃PO₄ (i.e. 0D TiO₂-Ag₃PO₄) nanohybrid. In comparison with the hybridization of P25 TiO₂ 0D nanocrystals with Ag₃PO₄, that of TiO₂ 2D nanosheets with Ag₃PO₄ gives rise not only to a stronger modification of electronic structure but also to a more prominent depression of electron-hole recombination. This underscores that the electronic coupling between the hybridized components is much more prominent for TiO₂ 2D nanosheet than for TiO₂ 0D nanoparticle, as evidenced by X-ray photoelectron spectroscopy. This is mainly attributable to the subnanometer-level thickness and highly anisotropic 2D morphology of the exfoliated nanosheet. The 2D TiO₂-Ag₃PO₄ nanohybrid shows much higher photocatalytic activity under visible light irradiation (lambda; > 420 nm) than do the P25 0D TiO₂-Ag₃PO₄ nanohybrid and the highly efficient Ag₃PO₄ photocatalyst, highlighting the excellent photocatalytic activity of this 2D nanohybrid material. The present study clearly demonstrates that the exfoliated 2D nanosheet of titanium oxide can be used as an effective platform for exploring highly efficient hybrid-type photocatalysts.

Earth-abundant p-type Cu2ZnSnSe4 (CZTSe)/n-type CdS hetero-junction has recently shown high potential for photovoltaic application. However, these devices usually exhibit a low Voc which could be explained -among others- by a high concentration of electrical defects in the vicinity of the CZTSe/CdS interface. The present contribution aims at determining the impact of the physical properties of the hetero-junction on the electrical performance of the corresponding devices. CZTSe thin film absorbers with different chemical compositions were prepared by DC sputtering of CuSn, Zn and Cu metallic layers on glass/Mo substrates, followed by a subsequent annealing under H2Se. After the standard KCN etch, CdS buffer layers were deposited on these absorbers by chemical bath using different deposition times. All solar cells were completed using a i-ZnO/ZnO:Al bilayer deposited by RF-sputtering. The resulting devices were characterized using SEM-EDX and advanced electrical characterization techniques such as I(V,T), EQE, CV and DLCP. Both the deposition time of the buffer layer and the composition of the absorber are found to affect the Voc of the cells and the concentration of defects in the devices. SCAPS simulations will be used in order to discuss these results.

Cu-M-N, where M = Earth-abundant metal ion, are a promising class of materials for application as solar absorbers. This class of materials exhibits suitable band gaps for absorption in the solar spectrum, and is predicted to be defect-tolerant, making them ideal for synthesis in thin-film form using scalable techniques such as sputtering. In much the same way that CIGS materials progressed from binary Cu2S to CIS and finally to CIGS, the Cu-M-N class of materials can be studied by starting with binary Cu3N. In the present study, the ability to dope Cu3N either n-type or p-type depending on deposition conditions is investigated, as is the possibility of degenerate doping.
Thin films of Cu3N were deposited from a metallic copper target using reactive RF-magnetron sputtering that utilized an atomic nitrogen source. Target power density was varied from 0.4 Wcm-3 to 1.6 Wcm-3 in one set of experiments, and in a second set of experiments the ratio of nitrogen plasma exposure time to copper plasma exposure time was varied from unity (plasma exposure time equal for both) up to 2 (copper plasma shut off for half the time). All depositions were performed at temperatures below 100°C. Hall and Seebeck effect measurements were used to characterize carrier type and concentration with respect to deposition conditions. Microstructural characterization was performed via X-ray diffraction and field emission scanning electron microscopy.
It was found that Cu3N grown under copper-rich conditions (greater than 0.6 Wcm-3) exhibited n-type conductivity with carrier concentrations of approximately 1019 cm-3 and Seebeck coefficients on the order of -10 mu;VK-1, while films grown under copper-poor conditions (0.4 to 0.6 Wcm-3) exhibited p-type conductivity with carrier concentrations in the range of 1015 - 1016 cm-3 and Seebeck coefficients on the order of 300 mu;VK-1. For films grown under conditions in which the ratio of nitrogen plasma exposure time to copper plasma exposure time was greater than unity, it was found that p-type carrier concentration was increased to approximately 1017 cm-3 and the Seebeck coefficient concurrently decreased to approximately 120 mu;VK-1. These data suggest the possibility to manipulate Cu3N carrier type to create a pn-homojunction device. Furthermore, the ability to obtain carrier concentrations as high as 1017 holes cm-3 via simple changes to the deposition conditions indicates that binary Cu3N and the larger class of Cu-M-N materials can be manipulated to meet the challenges of device integration.
This research is supported by the U.S. Department of Energy, office of Energy Efficiency and Renewable Energy, as a part of a Next Generation PV II project “Ternary copper nitride absorbers” within the SunShot initiative.

Photoelectrochemical (PEC) cells offer a more elegant, clean and sustainable way to store solar energy as chemical energy through the splitting of water into its primitive form (H2 and O2). Among many metal oxides pointed as candidates for this application, the fundamental characteristics of hematite (α-Fe2O3), such as abundance, excellent chemical stability in an aqueous environment and favorable optical band gap, emerged as a promising photoanode. Although attractive, the poor optoelectronic properties necessitate a large application of over potential for split water assisted by solar irradiation, limiting the high performance of this material. Since the electrode was built using materials in nanoscale, significant advances were achieved. In the last years, our research group has developed and applied hematite vertical nanorod to split water into molecular hydrogen and oxygen. This work highlights the influence of temperature of thermal treatment and use of SnO2 and ZnO doping hematite films. In addition, recent progress in the use of a hydrothermal process to build hematite electrodes for improving photocatalytic activity was discussed. X-ray diffraction and X-ray absorption near edge structure spectra were used to investigate the phase evolution from iron oxyhydroxide (FeOOH) to pure hematite phase. The formation of nanorods distributed along of substrate was observed by top-view SEM images and the rod growth preferentially oriented at the highly conductive (001) basal plane of hematite, perpendicular to the substrate. The good photoeletrochemical performance in pure and doped hematite photoanode was attributed to the structural, morphological and catalytic properties working in great harmony.

Cuprous oxide (Cu₂O) is a promising alternative to traditional photovoltaic materials because of its low cost, the abundance of its component elements in the earth&’s crust, and its uniquely straightforward processing. Crystalline wafers of Cu₂O made by thermal oxidation of Cu foils can have minority carrier diffusion lengths of up to 10 µm and hole mobilities of up to 100 cm²V^-1s^-1. Cu₂O is a native p-type semiconductor with relatively high absorbance in the visible region above the gap. Furthermore, Cu₂O has an optical band gap we have measured to be 1.9 eV which gives it a detailed balance efficiency for a homojunction of ~25% under AM 1.5G illumination.
Despite a favorable band gap and minority carrier properties, the highest energy conversion efficiency achieved in a photovoltaic device with a Cu₂O absorber layer is 5.38%. There are several challenges to making a high efficiency Cu₂O device, including the difficulties of doping the material, its relatively low chemical stability compared to other oxides, and a lack of good heterojunction partners. In this work we studied both the effects of the low chemical stability and the lack of good heterojunction partners for Cu₂O by X-ray photoelectron spectroscopy (XPS) of thin hetereostructures. We report on interfacial reactions and the energy band alignment for Cu₂O/ZnO, Cu₂O/ZnSe, and Cu₂O/ZnS heterojunctions.
Cu₂O is an especially reactive oxide due to its low enthalpy of formation, which is -168.6 kJ/mol. Furthermore, Cu₂O may be both either oxidized or reduced easily, as it is in the copper (I) oxidation state. We have used XPS to characterize chemical reactions at the heterojunction interface by analyzing shifts in the core level and auger peak positions of Cu₂O covered with 0.5-1.5 nm of ZnO. For example, by altering the partial pressure of O₂ in the atmosphere during ZnO sputtering, we can produce a Cu₂O/ZnO interface that is Cu-rich, stoichiometric, or O-rich. Furthermore, photovoltaic device performance is strongly correlated with interface species. We have found the Cu₂O/ZnO heterojunctions made with stoichiometric interfaces have V_OC = 530.4 ±4 mV. Devices with O-rich interfaces have V_OC = 102.3 ±11 mV. Devices with Cu-rich interfaces have V_OC = 347.2 ±30 mV.
Using XPS we were also able to determine the valence band offset between Cu₂O/ZnO. The valence band offset can be combined with the band gap to yield the more useful value of conduction band offset. For Cu₂O/ZnO we obtained a value of ΔE_C = -1.3, which indicates that the conduction band offset will be a limiting factor in Cu₂O/ZnO photovoltaic device efficiency. We will also report interfacial reactions and valence band offsets in ZnSe/Cu₂O and ZnS/Cu₂O heterojunctions.

Due to its high absorption coefficient, non-toxicity and the abundance of its composing elements, cuprous oxide (Cu₂O) is a promising absorber material for photovoltaic applications, even despite of the relatively large band gap (2.17 eV). With increasing success, more attention has recently been paid to cuprous oxide heterojunctions, employing different transparent conducting oxides as window layers. In order to demonstrate the influence and potential improvement of a more favorable band alignment on the photovoltaic conversion efficiency, we chose several compositions of Al_xGa_1-xN grown on sapphire via metal organic vapor phase epitaxy in conjunction with cuprous oxide thin films, prepared by radio frequency magnetron sputtering. Interface properties were investigated via X-ray photoelectron spectroscopy. For device characterization, J-V characteristics, capacitance spectroscopy, and external quantum efficiency were measured.

Oxide semiconductors receive an increased interest as solar absorber materials for photovoltaics or photoelectrochemical water splitting. Such applications have, however, stringent requirements for the band-structure, optical, electrical, and transport properties, which are usually not fulfilled simultaneously. Therefore, we employ a computational design process to identify the compositions and structures that have favorable properties.
Example 1: Cu₂O is considered as a potential earth abundant photovoltaic absorber, but it suffers from a large band gap of 2.1 eV. This problem is aggravated by the dipole-forbidden nature of the optical transition, and the effective absorption threshold of a thin film around 2.5 eV leaves too little overlap with the solar spectrum. A second problem is that the p-type doping due to native defects in the 10¹⁵ cm^-3 range is insufficient for an efficient p-n junction device design. Thus, we are attempting to optimize the optical and electrical properties by considering complex semiconductor alloys, where aliovalent (II) and isovalent (VI) elements are mixed into the cuprite structure. Our theoretical predictions are based on supercell defect calculations, GW band-structure calculations, and thermodynamic modeling, employing a model that extends the usual dilute defect model to moderate alloy concentrations.
Example 2: From a fundamental point of view, the electronic structure of d5 oxides like Fe₂O₃ and MnO has interesting features suggesting that advantageous semiconducting properties may be found in this materials class [1]. However, the proximity of the states of the half-filled d-shell to the conduction or valence band edge leads to self-trapping of electrons and holes in Fe₂O₃ and MnO, respectively. Not only does the resulting small polaron transport mechanism impede carrier transport, but the self-trapped electron or hole states also cause deep gap levels that can act as recombination centers for minority carriers. A key finding of the study in Ref. [1] was that the undesirable self-trapping of holes due to a Mn^+II/Mn^+III transition can be avoided in a tetrahedral coordination of Mn. Exploiting both structure-property and composition-structure relationships, we theoretically predict that Mn_1-xZn_xO alloys undergo a transition from the octahedral rocksalt to the tetrahedral wurtzite structure at x = 0.38. Further alloy electronic structure calculations for such alloys predict band-gaps, optical properties, and a band-lineup (relative to the vacuum level) suggesting that these wurtzite-phase MnZnO alloys could be interesting materials for water splitting.
Supported by the US Department of Energy, Office of Energy efficiency and Renewable Energy within the SunShot initiative (Example 1) and Office of Basic Energy Sciences as part of an Energy Frontier Research Center (Example 2).
[1] H. Peng and S. Lany, Phys. Rev. B 85, 201202(R) (2012).

Cu₂O is an earth abundant semiconductor that has been identified as a promising photovoltaic material due to its high absorption and long minority carrier diffusion length. It is composed of non-toxic elements and can be easily manufactured by thermal oxidation of Cu foils. While the bulk properties of Cu₂O are attractive for photovoltaic applications, surface stability has been one of the most pressing issues that needs to be resolved to achieve high efficiency devices. Cu₂O is intrinsically p-type and thus requires a heterojunction partner. However, heterojunction formation introduces the possibility of deleterious interface reactions and increased interface defects that impede high solar cell performance. Although the detailed balance limit has been calculated to exceed 20%, the highest reported photovoltaic efficiency for a Cu₂O absorber device is 5.38%. The major reasons for low efficiency are phase instability of the Cu₂O surface and high resistivity of thermally oxidized wafers.
It has been shown that the technique used to deposit the heterojunction partner has a significant effect on device performance and cleanest interfaces produce highest efficiency devices. One way to achieve a cleaner heterojunction interface is to deposit the interface in situ by plasma-assisted Molecular Beam Epitaxy (MBE), which allows precise interface control in an ultra-high vacuum (UHV) environment. Phase-pure Cu₂O thin films have been grown on single crystalline MgO substrates. In situ Reflection High-Energy Electron Diffraction (RHEED) as well as X-Ray Diffraction (XRD) indicate that the hetero-epitaxial orientation relationship is (100) and (110) Cu₂O on (110) MgO and (110) Cu₂O on (100) MgO. Film resistivities measured by four-point probe technique vary between 1000-3000 Omega;-cm, based on partial pressure of oxygen during growth. Thin homo-epitaxial films of Cu₂O are then grown on bulk thermally oxidized Cu₂O wafers, which are polycrystalline with grain size of approximately 1mm. The heterojunction partner, ZnO (0001), is then deposited immediately without breaking vacuum. Phase purity of the interface is confirmed by X-ray Photoelectron Spectroscopy (XPS). Hall mobilities as well as device performance will be reported.

Cuprous oxide (Cu₂O) is an earth-abundant p-type semiconductor promising for scalable photovoltaic applications compatible with terawatts-level deployment. While its theoretical maximum power conversion efficiency is ~20%, device efficiencies with this material have remained low due to non-ideal interfaces and unfavorable band alignments. A deep valence band edge position and a low carrier density of Cu₂O layer often result in a highly resistive Schottky barrier at Cu₂O-metal interface. To mitigate the absorber-electrode interface problem and create an electron-reflecting back surface field to promote photo-generated carrier collection, a heavily-doped hole-transporting layer with a proper band-position needs to be introduced at the interface.
In this contribution, we demonstrate the potential of nitrogen-doped cuprous oxide (Cu₂O:N) film as a p-type hole-transporting layer for engineering metal-semiconductor interface in photovoltaic devices. In-situ doping of nitrogen during Cu₂O film growth results in a carrier (hole) density over 10¹⁹ cm^-3 and a reduced electrical resistivity down to 0.18 Omega;-cm. By controlling the nitrogen content in the film, its electrical and optical properties are properly tuned to form a tunnel junction with low contact-resistance, and to minimize optical absorption by the layer. We fabricate Cu₂O-based heterojunction thin-film solar cells and insert a 20-nm-thick Cu₂O:N hole-transporting layer between a silver back-contact and a Cu₂O light-absorbing layer. The insertion of a Cu₂O:N layer results in a sizeable enhancement of fill-factor and power conversion efficiency of the solar cells. The Cu₂O:N thin-film may also be useful in various photovoltaic materials systems, improving their back-contact properties as well as widening the range of possible back-contact materials.
[1] Y. S. Lee et al., Energy Environ. Sci., DOI:10.1039/c3ee24461j (2013)
[2] Y. S. Lee et al., under review (2013)

One dimensional (1D) inorganic materials are gaining increasing attention because of their unique structural features and interesting functional properties. Given the structural stability, they show promising application potential in vacuum as well as in oxidizing atmospheres, which provides them a competitive edge over their carbon-based counterparts. A number of synthetic procedures have been developed and demonstrated for 1D nanostructures that have led to intriguing morphological variations (wires, tubes, belts, rods, etc.), however the control over radial and axial dimensions remains a continuing challenge. In addition, the choice of material is rather limited. We have developed a generic approach for the size-selective and site-specific growth of nanowires by combining vapor-liquid-solid (VLS) approach with the chemical influence of molecular precursors.
The device potential of nanowires and heterostructures both as individual device elements and as ensemble was evaluated in photo- and gas sensors and battery applications as self-supporting electrode materials. For instance, illuminating tin oxide NWs with UV photons triggers interesting photo-conductance, which can be modulated by tuning the wire diameters. The stable photo-response over several on-off cycles demonstrated their potential for applications in UV detectors or optical switches, where the NWs can act as resistive elements whose conductance changes by charge-transfer processes. In addition, tin oxide nanowires grown on copper foils were used for electrochemical energy storage and photoelectrochemical water splitting (PEC) applications. This talk will address the generic feature of our approach for the synthesis of oxide nanowires of various compositions and present the results obtained on potential scaling-up through electrospinning techniques.

Hybrid nanowire heterostructures of multiple components and controlled dimensions, capable of being tailored and assembled during the synthesis, are of substantial research interest, as they allow for the realization of devices with synergetic and unconventional functions in solar-driven energy conversion. Here, we will present our recent works in designing and fabrication of multi-component hybrid nanostructures for photoelectrochemical (PEC) energy conversion, including the following several scenarios/materials. (1) In-situ elemental doping: By an in-situ hydrothermal synthesis with both the Ti and Sn precursors, the controlled Sn doping in high-density TiO2 nanowire arrays is achieved, which shows increased donor density and enhanced PEC conversion efficiency. (2) Post-growth elemental doping: By a simultaneous etching and post-growth doping, W atoms are selectively doped into pre-grown TiO2 nanowire arrays, and the resultant nanowires exhibit both enhanced surface area and charge carrier density. (3) Surface graphene coating: By an in-situ photo-reduction method that directly deposits graphene layer on WO3 nanostructures, a substantial increase of the charge transfer efficiency at the semiconductor/electrolyte interface is achieved, leading to an enhanced PEC activity. All these hybrid oxide nanowire photoanodes are highly stable in PEC conversion and can serve as potential candidates for pure TiO2 materials in a variety of solar energy driven applications.

The growing demand for energy and increasing concerns for the effect of the abuse of fossil fuels on the environment force the scientific world to search for alternative clean and safe energy sources. Solar energy is one appealing option as it is free, abundant and of course renewable nevertheless only a small fraction of the energy by alternative and renewable sources is currently obtained by sunlight. In the frame of this issue we explored the properties of a novel photoactive organic-inorganic fluid hybrid material based on titanium oxide hydrates and commodity materials such as polyalcohols for the production of low cost green energy from solar light. This “solar” fluel, in fact, absorbs sun light and converts it and stores it into chemical energy that can be released when requested by allowing the fluid to interact with oxygen through an electrochemical cell. More importantly once discharged, the fluid can be re-activated by exposing it again to sunlight. The mechanism of the photoactivation and discharge of the hybrid have been studied via UV-vis spectroscopy, electronic paramagnetic resonance (EPR) and potentiometric analyses, and the results are reported here along with considerations to further improve the overall energy storage and conversion set up.

In photoanodes for O2 evolution such as ZnO and WO3, an overlayer of Co3O4 improves photo-excited current densities and photo-onset potentials. Yet, the efficiency of photoinduced hole injection into Co3O4 and its role in improving hole transfer at the Co3O4/electrolyte interface have been difficult to ascertain, due primarily to the lack of a spectroscopic signature of photo-generated holes in Co3O4 and partly to porosity in the Co3O4 overlayer. We first report on a transient optical study of Co3O4 that finds spectral signatures of prominent hole absorptions. The hole absorptions are thought to come from holes generated at the valence band edge states prior to recombination with electrons trapped in mid-gap d-states. We then report on in-situ transient optical spectroscopy of an atomic layer deposited heterojunction with minimal porosity where holes are photo-injected into the valence band edge of Co3O4 from the underlying photoanode. Kinetics for hole transfer at the photoanode/Co3O4 junction and the Co3O4/electrolyte interface are monitored as a function of electrochemical potential through surface-sensitive reflectivity measurements.

Semiconducting metal oxides nanostructures are being extensively studied for the solar energy applications, such as DSSC, QDSSC, photocatalysts and solar fuel cells. A tailored structural complexity and appropriate heterojunction formation will provide opportunities for performance enhancement. In this talk I present mainly our research on TiO2 nanostructures (both 1D nanorods and 3D inverse opals) in photoelectrochemical (PEC) sacrificial water splitting. Quantum dots sensitizers are fabricated based on both conventional SILAR and recent ALDIER (atomic layer deposition and ion exchange reaction) methods. The sensitization properties to TiO2 will be systemically discussed in terms of bandgap and interface band alignments. Furthermore, a smart combination of 3D inverse opal with 1D nanorods allows enhanced light harvesting and higher sensitizer loading, and subsequently higher photocurrent levels.
Extension to other earth-abundant materials, Cu2O and Nb2O5, for PEC application will also be covered in this talk. In particular, the cuprous oxides are obtained by a facile electrodeposition using 3D template. Interesting effect of surface modification by ALD will be discussed.
This work is contributed from Jingshan Luo, Chuanwei Cheng, Siva Krishna Karuturi, Ignacio Minguez Bacho, and Alfred Iing Yoong Tok.
References
1. Li, H. X.; Cheng, C. W.; Li, X. L.; Liu, J. P.; Guan, C.; Tay, Y. Y.; Fan, H. J., Composition-Graded ZnxCd1-xSe@ZnO Core-Shell Nanowire Array Electrodes for Photoelectrochemical Hydrogen Generation. J Phys Chem C 2012, 116, 3802-3807.
2. Cheng, C. W.; Karuturi, S. K.; Liu, L. J.; Liu, J. P.; Li, H. X.; Su, L. T.; Tok, A. I. Y.; Fan, H. J., Quantum-Dot-Sensitized TiO2 Inverse Opals for Photoelectrochemical Hydrogen Generation. Small 2012, 8, 37-42.
3. Luo, J. S.; Ma, L.; He, T. C.; Ng, C. F.; Wang, S. J.; Sun, H. D.; Fan, H. J., TiO2/(CdS, CdSe, CdSeS) Nanorod Heterostructures and Photoelectrochemical Properties. J Phys Chem C 2012, 116, 11956-11963.
4. Karuturi, S. K.; Luo, J. S.; Cheng, C. W.; Liu, L. J.; Su, L. T.; Tok, A. I. Y.; Fan, H. J., A Novel Photoanode with Three-Dimensionally, Hierarchically Ordered Nanobushes for Highly Efficient Photoelectrochemical Cells. Adv Mater 2012, 24, 4157-4162.
5. Luo, J. S.; Karuturi, S. K.; Liu, L.; Su, L. T.; Tok, A. I. Y.; Fan, H. J., Homogeneous Photosensitization of Complex TiO2 Nanostructures for Efficient Solar Energy Conversion. Sci Rep 2012, 2, Artn 451.

The efficient use of solar energy is now one of the great challenges in science and technology. In these days, variety materials have been investigated for use as photo-anodes for water-splitting by sunlight.
Among these materials, ferrite oxide such as Fe2O3 Fe3O4 are regarded as a promising system because of their probabilities of bandgap engineering, which lie well within the visible-IR spectrum, as well as their low costs, electrochemical stabilities, and environmental compatibilities. Therefore, a considerable number of studies have been performed on the photoelectrochemical (PEC) properties ofα-Fe2O3.[1,2]
We have demonstrated that enhanced photocurrent in Rh-substituted α-Fe2O3 thin films grown by a pulsed laser deposition. The Rh-substituted α-Fe2O3 (Fe2-xRhxO3; 0.0le;xle;2.0) thin films were grown on α-Al2O3(110) substrates with a Ta-doped SnO2 electrode layer by pulsed laser deposition.[3,4] Highly oriented epitaxial films with pure corundum structures were successfully fabricated over the entire compositional range. The optical absorption spectra of the films indicate narrowing of the bandgap with increasing Rh content. Consequently, the photoelectrochemical performance was improved in the Rh-substituted films. We found that the optimum Rh content lies at around x=0.2, where the photocurrent is significantly enhanced over a wavelength range of 340-850 nm. The bandgap of the films decreased with increasing Rh content. The PEC efficiency was significantly enhanced in the films with lower Rh contents, in the visible and NIR regions. The findings of this research are expected to be useful in the development of the solar fuel conversion systems based on alpha-Fe2O3.
References
1. M.Seki, H.Yamahara and H.Tabata, Appl. Phys. Express 5 (2012) 115801
2. M.Seki, H.Tabata, H. Ohta, K. Inaba, and S. Kobayashi, Appl. Phys. Lett. 99 (2011) 242504
3. W. Badalawa, H. Matsui, A. Ikehata, and H. Tabata, Appl. Phys. Lett. 99 (2011) 011913
4. W. Badalawa, H. Matsui and H. Tabata, J.Appl. Phys. 109 (2011) 053502

Photoelectrochemical (PEC) water splitting is a promising method to convert the immense energy from the sun into storable chemical fuels. One of the key challenges in advancing the efficiency and performance of solar water splitting devices is to improve the water oxidation reaction carried out over semiconductor photoanodes. Copper tungstate (CuWO4) is a an attractive candidate for PEC water splitting due to its ideal band gap energy (2.25 eV), n-type conductivity, and stability in aqueous solutions. However, there have only been limited studies on this material for PEC applications, and thus the overall performance for solar to hydrogen conversion is still very low. Therefore, there is a need to identify the performance limiting factors in CuWO4 in order to determine the fundamental materials deficiencies so that they can be overcome, allowing this material to achieve high water splitting efficiencies.
Here, we report the use a spray pyrolysis technique to deposit nanostructured thin films of CuWO4. The structural, optical, and photoelectrochemical properties of these films were investigated by XRD, UV-Vis, EIS, XPS, SEM.
In addition to fabricating highly active pure CuWO4 films, we have also successfully doped CuWO4, which has shown drastic improvements in the incident photon conversion efficiency (IPCE) and significantly higher photocurrent values at the water splitting potential of 1.23 V vs. RHE. These improvements have been explained by the increase in charge carrier density due to the substitutional doping of higher vacancy atoms as well as improved charge carrier mobility. In addition, we have also applied various oxygen evolution catalysts (OECs) to the CuWO4 surface, and shown improvements in the catalytic efficiency of the overall photoanode. The results have shown promising activity for solar water splitting over photoanodes made from earth abundant materials.

Sb₂S₃ is widely considered as an attractive photovoltaic material, based on abundant, non-toxic elements. However, the maximum efficiency reported for solar cells based on this semiconductor does not exceed 6.5%. In order to understand the main loss mechanisms responsible of these quite low performances, we have measured light intensity dependent J-V curves, Transient Microwave Photo-Conductivity, Steady State Photocurrent Grating, Modulated Photocurrent, and Photoconductivity on Sb₂S₃ based devices. All techniques converge toward the same observation: The main recombination route controlling the density of charge carriers in the absorber is of secondary order and appears to stem from an exponentially decaying density of tail states within the conduction band of the material. This conclusion has direct and drastic implications upon the performances of Sb₂S₃ based solar cells.
Another interesting metal sulphide that is currently largely studied consists in a quaternary compound comprising Copper, Zinc, Tin and Sulfur (CZTS). We have developed a new, simple synthesis route to produce a liquid, non-toxic CZTS source that we spray on Molybdenum substrate. The resulting devices have been characterized with many different techniques revealing the strength and weaknesses of our active layers. The loss mechanisms identified and ways forward will be discussed.

Thousands of inorganic materials have band gaps between 1 and 2 eV, a necessary precondition for consideration as a absorber materials for thin-film PV. This band-gap range covers the popular Schokley-Quisser (SQ) criterion for identifying candidate absorbers, but the SQ approach is not a sufficient initial filter. In this talk, I will present our recent work in searching for novel highly absorbing inorganic materials according to the new metric “spectroscopic-limited maximum efficiency” (SLME)[PRL, 108, 068701(2012)]. We focus on main group Cu-based ternary chalcogenides, i.e., CupMqVIr, where M = I, II, III, IV, V (main group) and p,q,r are stoichiometric ratio numbers. Thin-film solar cells hold the promise of reducing the cost of sunlight-to-electricity compared to conventional crystalline silicon. In a solar cell with an ultra-thin film absorber, a strong intrinsic electric field can drive photo-generated carrier extraction, mitigating the constraints on the mobility and lifetime of minority carriers and thus being more defect-tolerant. The realization of such cell is largely predicated on the availability of materials that exhibit both a strong absorption across the solar spectrum and an abrupt onset at the band gap. Based on first-principles quasiparticle theory, we have calculated SLME, band gap, and absorption spectra for all CupMqVIr materials that have been reported in the inorganic crystal structure database. Tens of materials (e.g., CuSbS2 and Cu3SbS4) exhibit higher absorption strength than CuInSe2. By analyzing the fundamental physical factors that control absorption strength in these compound semiconductors, new insights into “absorption design principles” have been developed. Theory- driven experimental work on some of those candidate materials will also be presented. (This work is supported by the DOE Energy Frontier Research Center "Center for Inverse Design.")

The development of earth-abundant photovoltaic materials is critical if photovoltaics are to generate a significant fraction of the world&’s energy in the future. Ternary Cu-Se absorbers have demonstrated both high efficiency and long-term stability; we believe these effects can also be achieved in the ternary Cu-S system. In this work, we determine optimized synthesis parameters for Cu₂SnS₃ through combinatorial sputtering, and explore device integration of this material.
Theory indicates that several compounds within the Cu-Sn-S phase space may be suitable for absorber applications, including Cu₂SnS₃, Cu₄SnS₄ and Cu₄Sn₇S₁₆. For example, the Cu₂SnS₃ phase has a calculated 0.8 eV band gap and an absorption coefficient of up to 10⁵ cm^-1; the other two Cu-Sn-S phases are calculated to have similar properties. However, in contrast to the other two phases, Cu₂SnS₃ has been calculated to have no defects that can lead to Fermi level pinning. Therefore, we have focused our experimental efforts on the Cu₂SnS₃ phase.
There has been limited prior work on the synthesis and characterization of Cu₂SnS₃. The measured optical gaps of Cu₂SnS₃ span a wide range from 0.83-1.6 eV, with absorption coefficients of 10⁴-10⁵ cm^-1. The current record efficiency for a cell with a Cu₂SnS₃ absorber layer is 2%; a 6% cell was recently reported using an alloyed Cu₂Sn_0.83Ge_0.17S₃ absorber.
We have synthesized thin films of Cu₂SnS₃ using combinatorial RF sputtering from Cu₂S and SnS₂ targets with spatial gradients in temperature and composition. When coupled with spatially resolved characterization techniques, this has allowed us to quickly explore the optimal growth conditions for the Cu₂SnS₃ phase. Using this understanding, we have synthesized Cu₂SnS₃ films under both Cu-rich and Sn-rich conditions. The properties of the films grown under Sn-rich conditions were more advantageous for absorber applications, especially when the high sub-gap absorption of the Cu-rich films is considered.
The films grown under Sn-rich conditions were p-type with carrier concentrations of about 1*10¹⁸ cm^-3. The films had an absorption edge at 0.8 eV, with an absorption coefficient up to 10⁵ cm^-1. The work function was measured to be about 5 eV. Conductivity measurements under dark and illuminated conditions showed the films to be photoactive. Temperature dependent Hall effect measurements indicated a 150 meV shallow acceptor activation energy. The films had low values of mobility (1-2 cm²/Vs), which we believe to be due to the small grain size (~100 nm). Current research is focused on increasing the grain size and integrating the optimized Cu₂SnS₃ absorber layers into PV device prototypes; the results of these efforts will also be reported.
The project “Rapid Development of Earth-Abundant Thin Film Solar Cells” is supported as a part of the SunShot initiative by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC36-08GO28308 to NREL.

Today's solar cell technologies based on crystalline silicon wafers and thin film CdTe and CIGS are limited for deployment at the grid parity level (TW production scale). These technologies are restricted by expensive refinement to solar grade or raw materials supply, confining production levels to few GW per year. Hence, new solar absorber materials need to be recognized to solve the growing energy demands with an environmentally conscious, renewable and rapidly scalable technology, using earth-abundant elements found in concentrated deposits. Moreover, materials with rapid onset to high absorption allow the development of thin film PV devices of superior efficiency, with drift field aided extraction of photogenerated carriers.
Utilizing a collaborative experimental and computational approach using optical band gap and absorption as design metrics (SLME), the Cu3-V-VI4 material system was identified as photovoltaic absorbers that satisfy the above criteria. We demonstrated Cu3SbS4 (Eg=0.88 eV) polycrystalline thin films exhibiting exceptionally rapid onset to high absorption (α>10^5 1/cm at Eg+0.6 eV), as well as high hole carrier mobility mu;=15 cm^2/Vs. In this contribution we explore the formation energies of intrinsic defects in Cu3-V-VI4 materials using DFT methods and provide experimental support to the findings.

Cu2ZnSn(SxSe1-x)4 (CZTSSe) solid solution has attracted considerable excitement in photovoltaic (PV) community since it is earth abundant and possesses a reasonably high cell efficiency (> 10%). However, routes to synthesis of these PV materials are limited by very little fundamental knowledge of thermochemistry and reaction pathways. In the present work, phase stability of CZTSSe due to partial disordered Cu and Zn atoms as well as S and Se atoms has been studied in terms of first-principles calculations within the density functional theory. The temperature and pressure dependences of binodal and spinodal decompositions of the pseudo-binary CZTS-CZTSe system have been studied based on the cluster expansion and the partition function approaches with input from first-principles. It is found that the low energy structures due to partial disordered Cu and Zn are the ones with space groups I-4 (#82) and P-42c (#112).

The U.S. Department of Energy (DOE) aggressively supports the development of low-cost, high-efficiency photovoltaic (PV) technologies through the SunShot Initiative, which seeks to make solar electricity cost-competitive with fossil fuel based sources of electricity, without subsidies, by 2020. For solar energy to become competitive with other energy resources, the installed cost for utility-scale photovoltaic (PV) solar systems must reach $1 per watt or, equivalently, 5-6cent; per kilowatt hour (kWh). The SunShot Initiative currently supports research and development of PV materials and devices with earth-abundant constituents to enable terawatt scale deployment. Current projects span materials such as CZTS; pyrites (FeS2); Cu-M-N; (Zn, Mg)Cu- oxysulfides; etc. A brief overview of projects addressing challenges of earth-abundant materials and devices will be presented. Current and forthcoming programs to support materials science and research for development of PV devices and systems will also be described.

We demonstrate that one important factor limiting hydrazine processed kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with efficiencies reaching above 11% is the existence of band-edge tail states due to electrostatic potential fluctuations. Detailed comparison of internal quantum efficiency (IQE), photoluminescence (PL) and temperature-dependent time-resolved photoluminescence (TRPL) data indicates that the electrostatic potential fluctuations for CZTSSe devices are roughly twice as severe as in higher-performing Cu(In,Ga)(S,Se)2 devices. The formation of compensating charged defects, such as [Cu_Zn + Zn_Cu], and the lower dielectric constant of CZTSSe are suggested as the main factors determining the amplitude of these fluctuations.

CZTS (Cu2ZnSnS4) and related alloys like CZTSSe (Cu2ZnSn(SxSe(1-x))4) have potential as attractive options for low-cost, high-efficiency thin-film solar cells. With optimal band-gaps (1.1 eV to 1.5 eV) and high absorption coefficients (>10^4 cm^-1), these quaternary direct-gap semiconductors can be produced with earth-abundant elements. However, the record laboratory efficiency for a CZTS or CZTSSe cell is about 11 %, significantly trailing that of CIGS (Cu(In,Ga)Se2) and CdTe thin-film solar cells. Further improvement of the cell performance can benefit from a deeper understanding of the photovoltaic properties of the CZTS materials at the level of their microstructures. Here, we characterize the local carrier collection properties of a CZTSSe solar cell prepared from solution precursors. The p-type CZTSSe absorber layer was synthesized employing a colloidal ink comprised of nanoparticle metal sulfides in an organic vehicle. The ink was coated onto a Mo coated glass substrate and annealed in the presence of elemental selenium as described previously. The device construction was completed by deposition of CdS emitter, i-ZnO buffer, ITO transparent conductor and silver grid lines by standard methods. Electron beam-induced current (EBIC) measurements were performed to map the local carrier collection efficiencies with a spatial resolution as high as asymp; 20 nm on the top surface and in cross-section of the devices. The spatial maps of current are compared for low- and high-efficiency CZTSSe devices. The cross-sectional EBIC images reveal that the large-grain top layer (asymp; 700 nm) is electrically active, while the particulate-like bottom CZTSSe layer (asymp; 500 nm; carbon-rich) remains inactive. We will discuss optimal designs of the device and interface engineering at the p-n junctions.

Copper zinc tin sulfide (Cu₂ZnSnS₄, or CZTS), copper zinc tin selenide (Cu₂ZnSnSe₄, or CZTSe), and their alloys are being considered for light absorbing materials for thin film solar cells because they are comprised of abundant, nontoxic and inexpensive elements. We synthesize colloidal dispersions of CZTS nanocrystals (inks) using rapid thermal decomposition of copper, zinc, and tin diethyldithiocarbamates in presence of hot (150-300 °C) oleylamine and then use these inks to drop cast nanocrystal coatings on various substrates which are then annealed to form large grain microcrystalline films. Specifically, the substrates with the nanocrystal coatings are placed in quartz tubes with Sulfur, Selenium, or a mixture of the two, evacuated to 10^-6 Torr, sealed, and then heated for 1-8 hours at temperatures from 500 °C to 800 °C. Isothermal annealing in a closed system provides good control of the annealing temperature and known Sulfur and/or Selenium pressures over the film. Moreover, any decomposition products such as tin sulfide cannot leave the system, minimizing tin loss from the film. After annealing, the microstructure, the elemental composition and the phase composition of the microcrystalline films are examined using a battery of characterization methods including X-ray diffraction (XRD), Raman spectroscopy, various electron microscopies and energy dispersive X-ray spectroscopy (EDS). Within the detection limits of these techniques, both the starting nanocrystals and the annealed films are CZTS or CZTSSe, if annealed under Selenium vapor. We have studied the extent to which Sulfur and Selenium vapor pressures, substrate, carbon content in the nanocrystal coating, annealing time, and annealing temperature affect the mechanisms by which the microcrystalline films form and how their microstructure evolves. Depending on the annealing conditions, the CZTS nanocrystals sinter and grow to sizes ranging from a hundred nanometers to a few microns. In addition to sintering, we observe abnormal grain growth, which can lead to formation of single-crystal CZTS grains up to 10 microns in size. The surface energy difference between the nanocrystals and the large grains is the driving force for abnormal grain growth, which appears to be enhanced at high temperatures but reduced significantly at high Sulfur pressures and on soda lime glass. Annealing under selenium vapor and sulfur vapor yield different microstructures. In particular, annealing in Selenium leads to carbon segregation either as a continuous several hundred-nanometer thick film under several micrometer thick large CZTSSe crystals or as thin fibrous coatings weaving in between smaller CZTSSe grains. In contrast, annealing with S does not show distinct carbon-rich layers or fibrous coatings after annealing.

We report on the low-temperature (4K) photoluminescence (PL) spectra of Cu2ZnSnS4 (CZTS) as a function of excitation intensity. The radiative recombination is characteristic of heavily-compensated material with a high quasi donor-acceptor pair density, as determined by the changes in peak position and peak height with excitation intensity. This is further supported by measurements of carrier lifetimes at different wavelengths. The blue-shift in the defect peak position can be used to estimate the average donor-acceptor pair spacing. Probing a large number of samples with varying compositions indicates a relationship between sample composition and donor-acceptor pair density. Additionally, we will show how the PL spectra of completed devices are correlated with their photovoltaic performance.

Presently commercialized thin-film solar PV technologies based on CIGS and CdTe have attracted considerable attention due to high power conversion efficiencies. Nevertheless, the limitation in scalability due to the use of rare and expensive elements is a barrier to achieving terawatt-scale fabrication. Tin monosulfide (SnS) is a promising earth-abundant alternative because of its suitable bandgap (1.1 eV indirect, 1.3 eV direct) [1,2], strong optical absorption (α > 10⁴ cm^-1) [2], good transport properties (reported Hall mobilities of > 100 cm²/V#9679;s) [3], and ease of film deposition.
We demonstrate congruent evaporation of SnS in a CdTe-like manufacturing process. SnS can be thermally evaporated at rather low temperatures (> 1 Å/s at le; 600 °C). The precursor can be phase-purified by distillation in the growth chamber. These properties allow for potentially low-cost and readily scalable fabrication. We anneal commercially available Sn_xS_y powder to obtain phase-pure SnS feedstock (confirmed by XRD) prior to thermal evaporation. Congruent evaporation of SnS is confirmed by comparing deposition rates to the equilibrium vapor pressures of SnS. We obtain phase-pure SnS thin-films on varied substrates including glass, metals (e.g., Mo and Au), and metal oxides (e.g., ITO). This enables a variety of PV device architectures. We characterize the grain morphology, texture, optical and electronic properties of the as-deposited films by SEM, XRD, spectrophotometry, and Hall effect measurements and compare for the different underlying metal and metal oxide substrates. The thin-film properties are further evaluated as a function of different processing parameters (e.g., deposition rate, substrate temperature and film thickness).
We present a proof-of-concept thin-film solar cell based on thermally evaporated SnS, following the previously developed substrate device stack (glass/metal/SnS/ZnO_xS_y/ZnO/ITO/Ag) [4]. We also demonstrate SnS-based superstrate devices (glass/metal oxide/buffer/SnS/metal), and we compare the two stack architectures. Upon device optimization, our best performing SnS-based devices yield power conversion efficiencies near 2%, similar to the performance of the current champion device [4]. We will present J-V and quantum efficiency measurements, demonstrating considerably high open-circuit voltages up to 300 mV and short-circuit current densities approaching 20 mA/cm². In closing, we present state-of-the-art SnS devices fabricated using a manufacturing process with proven scalability, harnessing the underlying physical property of congruent evaporation intrinsic to only a handful of binary PV candidate compounds.
1. Vidal, J. et al., Appl. Phys. Lett. 100, 032104 (2012).
2. Sinsermsuksakul, P. et al., Adv. Energy Mater. 1, 1116-1125 (2011).
3. Ramakrishna Reddy, K., et al., Sol. Energy Mater. Sol. Cells 90, 3041-3046 (2006).
4. Sinsermsuksakul, P. et al., Appl. Phys. Lett. 102, 053901 (2013).