Iron pyrite (FeS₂) is considered a promising earth-abundant semiconductor for solar energy conversion with the potential to achieve terawatt-scale deployment. However, despite extensive efforts and progress in the past two decades the solar conversion efficiency of iron pyrite remains below 3%, primarily due to a low open circuit voltage (V_OC). Here I report a comprehensive investigation on the limitations imposed by intrinsic bulk and surface defect states on the semiconducting properties of n-type single crystals and nanostructures of iron pyrite to understand its puzzling low V_OC. We utilized electrical transport, optical, photoelectrochemical (aqueous and acetronitrile electrolytes), and surface sensitive measurements, to enable a detailed characterization of the bulk and surface defect states and the construction of a detailed band energy diagram for iron pyrite. A holistic evaluation of the impact of both bulk and surface states revealed that despite a high density of surface states, full ionization of the bulk deep states creates a degenerate n-type surface space charge region that reduces the usable barrier height and satisfactorily explains the limited photovoltage and poor photoconversion efficiency of pyrite single crystals. I will build on these findings to discuss how nanocrystalline and polycrystalline films are limited by both bulk and surface defect states, but their successful solar applications appear to be ultimately hindered by a strong Fermi level pinning. In contrast to the significant challenges as solar absorbers, we found the pyrite-phase transition metal disulfides (MS₂, M= Fe, Co, Ni and their alloys), especially CoS₂, are highly efficient electrocatalysts for hydrogen evolution reaction (HER) and polysulfide and triiodide reduction reactions important for photoelectrochemical solar conversion. Furthermore, microwires or nanowires of CoS₂ can be controllably synthesized with high surface areas to substantially boost their catalytic performance.

Hydrogen, a clean, storable, and high-energy density energy carrier, is a promising sustainable alternative for meeting the global energy demand and achieving an environmentally friendly fuel economy. Utilizing an integrated photoelectrochemical (PEC) system to directly convert solar to hydrogen via water splitting is one of the most promising approaches. Platinum and other noble metals remain the best catalysts for solar-driven hydrogen evolution reaction (HER), but the high cost and scarcity greatly limit their large scale deployment. In this work, we develop low-cost, yet highly efficient and robust photocathodes based on heterostructures of silicon photocathodes and new amorphous group 6 metal chalcogenides (MoS₂, WS₂, MoSe₂) electrocatalysts for solar-driven HER. A facile chemical vapor deposition (CVD) method was used to directly grow amorphous group 6 metal chalcogenides on planar p-Si or nanostructured n⁺p Si, forming a high-quality interface between the catalyst and light absorber. The structure and chemical compositions were characterized using Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, etc. Excellent PEC performance was achieved at simulated 1 sun irradiation as revealed by the large onset potential of photocurrent and high current density. Superior stability was also demonstrated over repeated scans and long-term operation, suggesting these heterostructures are promising earth-abundant alternatives to photocathodes based on noble metal catalysts for solar-driven hydrogen production.

The cadmium chloride annealing treatment is an essential step in the manufacture of efficient thin film CdTe solar cells. In previous work we have shown that the primary effect of the treatment is to remove high densities of stacking faults from the as-deposited material. Use of density functional theory has shown that some of the higher energy stacking faults are hole traps. Removal of these defects dramatically improves cell efficiency.
In this study we focus on the effect of the activation treatment on the underlying n-type cadmium sulphide layer. A range of techniques has been used to observe the changes to the microstructure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the micro-structural properties of the cells have also been undertaken. Techniques used include High Resolution Transmission Electron Microscopy (HRTEM) for sub-grain analysis, EDX for chemical analysis and XPS and SIMS for composition-depth profiling.
By studying the effect of increasing the treatment time and temperature, we will show that the cadmium sulphide layer depletes to the point of complete dissolution into the absorber layer. We will also show that chlorine penetrates and decorates the grain boundaries in the cadmium sulphide. In addition we will show that chlorine builds up at the heterojunction and concentrates in voids at the cadmium telluride/cadmium sulphide interface. A combination of these effects damages the electrical performance of the solar cell.

An organic-inorganic hybrid bulk heterojunction artificial leaf fabricated by depositing the hybrid film composed by CdSe:P3HT (10:1 (w/w)) with 1,2-ethanedithiol (EDT) treatment on Au and using Pt as co-catalysts is demonstrated in this work. This photocathode shows stable water reduction and hydrogen bubble generation with a photocurrent of -0.36 mA/cm² at 0 V vs reversible hydrogen electrode (RHE) under illumination of AM1.5G (100 mW/cm²) in a mild electrolyte. The record EQE of 45% at wavelengths between 400-480 nm and unprecedented V_oc of 0.8 V_RHE are observed. A flexible hybrid electrode is also fabricated, showing an EQE of 65% at 400 nm wavelength. Time-resolved photoluminescence (TRPL) is performed to clarify the working mechanism of the hybrid. The long lifetime of the CdSe (45; 667 ps) is reduced to 3; 25 ps in the blend, indicating the fast charge separation upon illumination. The large mismatch between the integrated photocurrent from EQE spectra and J-V curves indicates charge transport problems. By transient photocurrent measurements and comparing the internal quantum efficiency (IQE) measured illuminating the electrode both from the front-side and back-side indication of electron accumulation in the liquid/solid interface is found.

The earth abundant, solution phase grown light absorber Cu₂ZnSn(S_xS_1-x)₄- (CZTS,Se) exhibits complex surface compositional and electronic properties that require detailed understanding if optimal heterojunction characteristics and device performance are to be achieved. In order to study the surface and, by analogy, grain boundary properties of polycrystalline CZTS,Se, an array of surface and bulk characterization techniques including X-ray (XPS) and femtosecond ultraviolet photoelectron spectroscopy (fs-UPS), secondary ion mass spectrometry (SIMS), photoluminescence imaging (PLI) and Kelvin probe force microscopy (KPFM) were employed. Air annealing of CZTS,Se prior to device fabrication substantially increases device efficiency and absorber PL intensity but produces an oxide covered surface. Upon removal of the oxide the resulting surface is Cu poor and Zn rich relative to the bulk and SIMS reveals substantial diffusion of O and Na into the absorber interior, primarily along grain boundaries. Various processing conditions are observed to change the Cu content of the surface and will be described. XPS studies of the oxide before removal suggests the formation of Se-O-Na complexes that are likely present along grain boundaries. Using fs-UPS to measure the electronic structure we find that the oxide-free CZTS,Se surface exhibits upward band bending indicating the presence of negative charge trapped in the surface region. In addition we find the Fermi level located near the middle of the band gap. Deposition of sub-nanometer layers of Cu followed by a mild anneal removes the upward band bending indicating that Cu vacancies are the source of negative charge. Scanning force microscopy and KPFM were employed to study the structural and electronic properties of the grain surfaces and their boundaries. The role of Cu, the presence of point defects and the impact of these findings on grain boundary properties will be described.

Efficient Copper Zinc Tin Sulphide Selenide (CZTSSe) thin film photovoltaic devices were fabricated with a new, fast, simple and environmentally friendly preparation method. Our process is based upon a versatile and instantaneous synthesis of a Cu-Zn-Sn-S colloid. Dispersing this colloid in a mixture of water (90%) and ethanol (10%), spraying it, and annealing sequentially the samples in two different atmospheres allow us to grow large grain crystalline layers. We measured cell efficiencies up to 9% under simulated AM1.5G (cell area 0.25 cm²). To the best of our knowledge, this achievement represents the highest performances reached to date with CZTSSe deposited by spray, notwithstanding the unprecedented environmental friendliness of our process.
Detailed characterizations of both the active layer and the final devices were performed in order to identify the weaknesses of our solar cells. X-Ray Fluorescence (XRF) mappings indicate a spatially homogeneous composition, slightly Cu poor and Zn rich. Time resolved photoluminescence measurements (TRPL) reveal a minority carrier lifetime of about 5 nanoseconds, while E-Beam Induced Current (EBIC) indicate a charge carrier collection length of about 200 nm. Furthermore, temperature dependent I-V curves and admittance studies indicate thermally activated phenomena with activation energies of 100 and 260 meV respectively. We attribute the former energy to a barrier at the Mo/CZTSSe interface and the latter energy to the Cu_Sn anti-site defect. Finally, temperature dependent photoluminescence allow us to conclude on the presence of a donor state at 130 meV from the conduction band.

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 point defects on the Zn and Cu sublattices may lead to the observed low Voc and to a significant reduction in solar cell efficiency. The tunable energy x-rays available at 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 Cu_Zn, Zn_Cu, V_Cu, and V_Zn point defects present in thin films of polycrystalline CZTS in order to shed light on the relative success of different growth conditions. We find a significant amount of Cu and Zn site swapping present ubiquitously in thin films. We will discuss the implications of these defects on the low Voc in CZTS and to the film growth conditions.

The high potential in terawatt-scale energy conversion increased the attention and excitement on earth abundant Cu, Zn, Sn, S, and Se, CZT(S_xSe_1-x)₄ based solar cells, and lead to rapid development of photo conversion efficiencies (PCE) to ~ 12.6 %. Nevertheless, the low open circuit voltages (< 0.5 Volts) inhibits approaching to the Shockley-Queisser limit, single-junction detailed balance, (~31%). Solution based depositions, such as colloidal nanocrystal or molecular based inks, provide potentially lower-cost and higher-throughput alternative routes to conventional vacuum based processes while resulting in high optoelectronic quality CZT(S_xSe_1-x)₄ films. Herein, we focus on the effect of utilizing volatile species SnSe, SnS, and S during selenium incorporating sintering process on the optoelectronic quality of CZT(S_xSe_1-x)₄ films that are deposited through a molecular solution route using dimethylsulfoxide, a safer and environmentally friendly alternative to the current state-of-the-art hydrazine route. The improvement in optoelectronic quality of the absorber layers is observed via absolute photoluminescence intensity, where the quasi-Fermi-level-splitting (QFLS) is shown to increase from 0.34 to 0.46. This effect is also seen in the open circuit voltage (V_OC) in the completed devices; the V_OC is observed to increase from 0.400 V to above 0.480 V with a constant band gap of ~1.11 eV. The improved V_OC is attributed to reduction of the potential fluctuations in the band gap due to impurity related donors and acceptors. PCE from solar cells completed with these absorber films reached 10.8% (without metal top grid area or anti-reflection coating), which is would be approximately 11.6% total area efficiency with antireflective coating deposited on the completed solar cells. In addition, the V_OC of the solar cells increased with the thickness of the CZT(S_xSe_1-x)₄ absorber layer. However, formation of large CZT(S_xSe_1-x)₄ grains in films thicker than 1 mm with molecular ink route is challenging. The regular selenium incorporating sintering leads to bimodal distributions in the microstructure of the CZT(S_xSe_1-x)₄ films thicker than 1 mu;m: the upper layer of the absorber layer results in large micrometer-scale grains and the lower layer results in smaller nanometer-scale grains. The utilization of the SnSe, SnS, and S with Se results in an increase in the grain size; CZT(S_xSe_1-x)₄ grains larger than 1 mm are achieved while decreasing film surface roughness and improving the compactness of the grains. The change in the microstructure via the utilization of these volatile species is attributed to the formation of sulfide species with the alkali-metals (e.g. Na₂S) coming from the soda-lime glass which assists the grain growth as previously observed in CIGS based absorber materials.

We present an one-dimensional device model for the hydrazine processed kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cell with a world record efficiency of ~12.6%. Our simulation results performed using wxAMPS software show that the presented model fits almost perfectly to numerous experimental observations. Not only the V_OC, J_SC, FF and efficiency under normal operating conditions are matched, but also all the features in temperature vs. V_OC, sun intensity vs. V_OC and the quantum efficiency measurements are quantitatively reproduced. We also emphasize that the material properties incorporated into the device model are consistent with the values derived from various characterization techniques. Interface defects are deliberately omitted from the model in order to show that all the experimentally observed features can be accounted for by the bulk properties of CZTSSe. Notably the features in the temperature vs. V_OC and sun intensity vs. V_OC are usually interpreted as evidence for interface dominated recombination. In contrast to this common interpretation, our model shows that bulk properties of CZTSSe can completely explain these same features. An electrical gap (i.e., a mobility gap) that is smaller than the optical gap is essential to fit the V_OC data. This is an analogous feature to that observed in amorphous materials and further emphasizes the significance of disorder and tail states in CZTSSe solar cells.

We report on the fabrication of Cu₂ZnSnS₄ (CZTS) thin films via electrochemically depositing precursor stacks on Mo-coated glass in a variety of orders: Cu/Sn/Cu/Zn, Cu/Zn/Cu/Sn, Zn/Cu/Sn/Cu, and Sn/Cu/Zn/Cu. Raman spectroscopy and X-ray diffraction alanysis showed the formation of a single CZTS phase with good crystallinity and strong (1 1 2) orientation. The quality of the annealed CZTS thin films was found to depend on the stacking order. The CZTS films made of the stack with a Zn top layer showed a larger grain size than the stacks using Sn and Cu as top layers. The optical absorption coefficient of the annealed CZTS thin films was > 10⁴ cm^minus;1 in the visible region, indicating the direct band gap nature of the fabricated films. The hole mobility, hole concentration and resistivity of the annealed CZTS films were found to depend strongly on the stacking order. Also, the efficiency of the fabricated solar cell devices, ranging from 2% to 4%, depends on the stacking order. The Cu₂ZnSnS₄ thin film solar cell showed a conversion efficiency of 4.08% measured under the air mass 1.5 global (AM1.5G) sunlight with a reasonably high fill factor of 0.77.

In this study, we investigate the intrinsic point defects in solar energy conversion materials Cu₂ZnSnSe₄ (CZTSe) and CuInSe₂ (CISe) using density functional theory within the Generalized Gradient Approximation (GGA). This study gives a close analysis of the donor-type defects in these materials since donor-type defects have higher capture cross sections for minority carriers (electrons) and thus dominate Shockley-Read-Hall (SRH) recombination in these solar cells. A comparison between the defect transition levels for CZTSe and CISe reveals that in CZTSe, the Sn-on-Cu and Sn-on-Zn antisites are recombination centers with defect states close to midgap, while in CISe the In-on-Cu antisite has a shallow defect level. The concomitant higher SRH recombination rate in CZTSe solar cells reduces the concentration of minority carriers and hence the separation of quasi Fermi levels under illumination. This might explain the origin of the low open circuit voltage (V_oc) values for CZTSe solar cells compared to CISe solar cells.

Thin-film solar cells using kesterite Cu₂ZnSn(S_xSe_1-x)₄ (CZTSSe) absorbing layers may prove to be a viable technology for cost-effective manufacturing. However, CZTSSe devices have been observed to suffer from a host of significant loss mechanisms, which may include interface recombination, potential fluctuations, poor carrier collection, a blocking back contact, and multistep recombination via deep trap states. CZTS devices fabricated at our labs with efficiencies higher than 6% are characterized using J-V, C-V, and QE analysis. With knowledge of depletion width and minority carrier diffusion length, it is shown that the voltage-dependency of photocurrent collection can be estimated and subsequently used to correct light J-V data, providing for more accurate extraction of diode model parameters. Various 1-dimentional heterostructure simulations are conducted with SCAPS software. Comparisons between experimental data and simulation results are made, giving insight into physical processes that may be occurring in the devices.

Integrating plasmon resonant nanostructures with photocatalytic semiconductors shows great promise for high efficiency photocatalytic water splitting. However, the electrochemical instability of most III-V semiconductors severely limits their applicability in photocatlaysis. In this work, we passivate p-type GaP with a thin layer of n-type TiO₂ using atomic layer deposition. The TiO₂ passivation layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. In addition, the TiO₂ passivation layer provides an enhancement in photoconversion efficiency through the formation of a charge separating pn-region. Plasmonic Au nanoparticles deposited on top of the TiO₂-passivated GaP further increases the photoconversion efficiency through local field enhancement. These two enhancement mechanisms are separated by systematically varying the thickness of the TiO₂ layer. Because of the tradeoff between the quickly decaying plasmonic fields and the formation of the pn-charge separation region, an optimum performance is achieved for a TiO₂ thickness of 0.5nm. Finite difference time domain (FDTD) simulations of the electric field profiles in this photocatalytic heterostructure corroborate these results. The effects of plasmonic enhancement are distinguished from the natural catalytic properties of Au by evaluating similar photocatalytic TiO₂/GaP structures with catalytic, non-plasmonic metals (i.e., Pt) instead of Au. This general approach of passivating narrower band gap semiconductors enables a wider range of materials to be considered for plasmon-enhanced photocatalysis for high efficiency water splitting.

We are currently exploring semiconductor nanowire heterojunctions for the direct solar-to-fuel conversion.We are in the process of developing a fully-integrated and highly-sensitive nanowire probe platform for cancer cells. Developing of such flexible nanowire probes would enable us to monitor in-vivo biological processes within living cells and will greatly improve our fundamental understanding of cell functions, intracellular physiological processes, and cellular signal pathway. There are several key features associated with these cell nanowire probes: minimal invasiveness, high flexibility, photoeletrochemical sensing principle with highly localized excitation and detection scheme and nonlinear optical conversion capability. We also design nanoscale electrical and optical systems that can greatly expand our capability in probing, imaging, monitoring and manipulating biological processes with unprecedented resolution, sensitivity and precision. Push the limits of ultrasensitive nanowire nanoelectronic sensors. And we establish the IrO₂minus;heminminus;TiO₂ nanowire arrays for label-free, real-time and sensitive photoelectrochemical detection of glutathione. The sensitivity achieved is sim;10 nM in bu#64256;er. The cell extracts containing glutathione are robustly detected, with sim;8000 cells/mL for HeLa cells and sim;5000 cells/mL for human embryonic kidney 293T cells. We report a nitrogen-doped carbon nanodot (N-Cdot)/TiO₂ nanowire photoanode for solar-driven, real-time and sensitive photoelectrochemical probing of the cellular generation of H₂S. This low-cost, sensitive and stable N-Cdot-NWs/living cell interface can open up new avenues to the Cdot-based, as well as other semiconductor-based NWs/living cell interface.We demonstrate the GOx-functionalized TiO₂ (TiO₂-GOx) nanowires exhibit a high sensitivity of ~0.9 nM in detection of glucose in buffer. The capability of detecting glucose in mouse serum is also demonstrated.We also synthesize Cdot-Folate-Dox for Drug Delivery and animal imaging.

Polycrystalline Copper-zinc-tin-sulfide/selenide (CZTS,Se) compounds have received wide research interest due to their potential as inexpensive absorber materials composed of earth-abundant elements. Photovoltaic devices fabricated on CZTS,Se have reached the highest (record) conversion efficiency of the 12.6 %. One of the key parameters to further boost the conversion efficiency is to control the concentration of recombination sites at the surface, in the grain boundaries, and in the bulk. Surface states formed on the sample surface as a result of carbon and oxygen contamination can act as non-radiative recombination sites which limit the ultimate cell efficiency. Therefore, a surface-cleaning method which can effectively reduce the amount of surface oxygen and carbon is necessary for CZTS,Se processing. In this work, 2 mu;m thick CZTS,Se films were prepared by spin coating hydrazine-based precursor solutions onto Mo-coated soda lime glass substrates in a nitrogen-filled glove box. To clean the CZTS,Se surfaces, three different wet cleaning recipes were used: 1) NH₄OH only; 2) HCl followed by NH₄OH; 3) H₂O₂ followed by NH₄OH. The effect of the wet cleaning on the surface composition including carbon and oxygen content has been studied via X-ray photoelectron spectroscopy (XPS) and femtosecond ultraviolet photoelectron spectroscopy (fs-UPS). Spatial variation of work function over the surface upon surface cleaning was measured via Kelvin Probe Force Microscopy (KPFM). The stability of the clean surface against re-oxidation in ambient was modeled by density functional theory (DFT). The H₂O₂/NH₄OH recipe showed the best result reducing the amount of surface O and C down to 5% and 20%, respectively. This is due to the oxidizing effect of H₂O₂ which converted the carbonaceous surfaces contaminants into oxides which were later removed by NH₄OH. DFT calculations are consistent with a group VI surface being stable against oxidation by ambient moisture. KPFM measurements showed strongly non-homogeneous surfaces after both NH₄OH-only and H₂O₂/NH4OH clean. Areas with work functions different from CZTS,Se could result from binary chalcogenides formed during the growth that were subsequently covered by the native oxide. NH₄OH etch successfully removed the covering oxide and made those phases visible to KPFM.

Iron Pyrite has been identified as an excellent material for solar cell application due to its large absorption coefficient and the suitable bandgap. The sulfurization of hematite nanoparticles is a promising method to obtain pyrite nanoparticles of high electronic quality. We combine iron pyrite nanoparticles, obtained by sulfurization of iron oxide NPs, as absorber material with the well-established thin film silicon technology by embedding them into a pin type device. In this type of device defects are generated in the thin film matrix due to silicon growth on surfaces covered by nanoparticles. We studied the effect of these defects on the device performance by including inert silicon dioxide nanoparticles in solar cells.
We compared the spectral response of solar cells containing nanoparticles to that of reference devices. The solar cells were fabricated by interrupting the deposition sequence and spin coating of nanoparticles onto the devices under N2 atmosphere. Devices containing hematite and silicon dioxide nanoparticles and reference solar cells without particles were prepared in the same deposition run. The particles were positioned either at the p/i interface, in the middle of the intrinsic layer or at the i/n interface of p-i-n type amorphous silicon solar cells. An increase of spectral response below the optical bandgap of amorphous silicon was observed for the devices containing nanoparticles. We carefully analyzed the spectral response of the devices with particles at various positions of the layer stack. We separated the contribution of current generated in the nanoparticles from charge carriers that are excited in additional defects in the matrix surrounding the particles.
By including an additional sulfurization step after spincoating hematite nanoparticles onto the device we realized solar cells containing pyrite nanoparticles as absorber. The sulfurization was carried out by a plasma process in H₂S atmosphere. Sulfurization parameters were optimized to achieve improved electronic properties of the nanoparticles while additionally minimizing the influence of the sulfurization process on the underlying silicon layers.

Thin films based on Sn-S compounds are currently of great interest because of their potential applications in photovoltaic and optoelectronic devices. S and Sn are abundant in nature, inexpensive and much less toxic than most of the materials used in the industry for manufacturing solar cells. In addition to their photovoltaic properties, the chalcogenide materials such as SnS, SnS₂, Sn₂S₃, Sn₃S₄ and Bi₂S₃ are of great interest due to its applications in the fabrication of optoelectronic and thermoelectric devices, and as a holographic recording medium. In this work SnS and SnS:Bi thin films, with suitable properties for their use in manufacturing solar cells, were grown by a new technique of sulfurization of the metallic precursors. The deposited films were studied through transient photoconductivity measurements under illumination and decay, and photocurrent as a function of light intensity; their micro-structure was also characterized through Scanning Electron Microscopy (SEM) technique. The studies allowed establishing the effect of adding Bi on the recombination processes of free charge carriers which affect the photoconductivity of the deposited films, and to identify the types of recombination occurring in them.

We report the development of a multifunctional, solar-powered photoelectrochemical (PEC)-pseudocapacitive-sensing material system for simultaneous solar energy conversion, electrochemical energy storage, and chemical detection. The TiO2 nanowire/NiO nanoflakes and the Si nanowire/Pt nanoparticle composites are used as photoanodes and photocathodes, respectively. A stable open-circuit voltage of 0.45 V and a high pseudocapacitance of up to 455 F g-1 are obtained, which also exhibit a repeating charging-discharging capability. The PEC-pseudocapacitive device is fully solar powered, without the need of any external power supply. Moreover, this TiO2 nanowire/NiO nanoflake composite photoanode exhibits excellent glucose sensitivity and selectivity. Under the sun light illumination, the PEC photocurrent shows a sensitive increase upon different glucose additions. Meanwhile in the dark, the open-circuit voltage of the charged pseudocapacitor also exhibits a corresponding signal over glucose analyte, thus serving as a full solar-powered energy conversion-storage-utilization system.

Current H₂ production is based around steam methane reforming or coal gasification, energy intensive routes with a large carbon footprint. This limits the widespread use of H₂ fuel cells for automotive and stationary power applications. The development of simple, affordable and environmentally benign methods for H₂ production, distribution and storage are amongst the great challenges facing mankind over the next 50 years. Semiconductor photocatalysis offers vast potential for the generation of H₂ from water or biomass resources. In response, we have systematically explored the Au/TiO₂ system for solar H₂ production from water and biofuels.¹ A series of Au/P25 TiO₂ photocatalysts (Au = 0.5-10 wt.%) were fabricated and subjected to detailed physico-chemical characterisation and photocatalytic testing. At optimal Au loadings (0.5-1 wt.%) Au/P25 TiO₂ photocatalysts exhibited remarkable activities for H₂ production (asymp;2 L/kgCatal.min, QY = 23%) under UV irradiation of comparable intensity to that provided by the sun. For comparison, a 1 kW PEM fuel cell requires 15 L of H₂ per min, indicating the viability of Au/TiO₂ catalysts for H₂ production for fuel cell applications. H₂ production rates increased by a further 25% under a combined UV/Vis flux due to visible light stimulation of the Au nanoparticles (NP&’s) surface plasmon resonance. To deconvolute the key role of the P25 TiO₂ supports mixed phase nature (~80% anatase, 20% rutile) in promoting H₂ production, anatase and rutile were isolated and functionalised with Au NP&’s (3 wt.% loading). H₂ production rates of Au/anatase (22 mmol g^-1 h^-1) and Au/rutile (10 mmol g^-1 h^-1) were substantially lower than in 3 wt.% Au/P25 (32 mmol g^-1 h^-1). EPR studies reveal transfer of photoexcited electrons from rutile&’s conduction band to anatase lattice electron traps under UV light, increasing the number of charge carriers available for photoreactions at the TiO₂ surface. Our data provides strong evidence that synergistic electron transfer at interfacial sights of TiO₂ polymorphs and supported Au NP&’s (catalytic ‘hot spots&’) leads to the high H₂ production rates observed in Au/P25 TiO₂ systems and is useful for the development of new and improved photocatalysts.
1. V. Jovic, W.-T. Chen, D. Sun-Waterhouse, M.G. Blackford, H. Idriss, G.I.N. Waterhouse, J. Catal., 305 (2013) 307-317.

There is considerable interest in heterojunction (a-Si:H/c-Si) based solar cells due to several advantages over a conventional c-Si homojunction including: cost-effective processes, better temperature coefficient and lower silicon consumption [1-2]. The performance of this kind of solar cell critically depend on emitter thickness and doping [3-4]. In this work, we investigate a new HIT cell design with a highly doped junction that increases the electric field which further enhances the Impact Ionization and Tunneling rates that could result in increase of the J_sc. Moreover the effect of emitter doping and thickness on the performance of a-Si(n+)/c-Si(p+) is experimentally studied. Two different emitter designs solar cell structures are fabricated, Sample A is a structure having 80 nm ITO stack over thinner and highly doped a-Si emitter layer (22 nm thick and 2.8x10²¹cm^-3 phosphorous doping concentration) on p⁺ boron doped p-type Si wafer whereas for Sample B the solar cell structure is same with thicker and less doped emitter layer (60 nm thick and phosphorous doping concentration of 4x10²⁰ cm^-3) The current density-voltage (J-V) characteristics were measured using solar simulator at 1 sun with AM1.5G. The results shows that the V_oc increases by 80 mV while the J_sc increases from 11.8mA/cm² to 4.9mA/cm² for Sample A (higher doping and lower thickness of a-Si emitter layer) compared to Sample B. Also the fill factor and efficiency for Sample A is 56.9 % and 4.02 % respectively whereas Sample B has low efficiency of 1.9 % and small fill factor of 46.2%. The results also show an improved spectral response for thinner and high doping a-Si emitter layer. The peak IQE is 55% and peak EQE is 44 % for sample A while the Sample B has 24% and 18% peak IQE and EQE respectively. In summary, emitter design for a-Si (n+)/c-Si (p+) HIT cell is carried out. The results show an increase in solar cell parameters like J_sc, V_oc, efficiency, fill factor and spectral response with thinner (22nm) and higher doping emitter(2.8x10²¹ cm^-3) a-Si layer. Finally, the results highlight the importance of emitter design for future HIT cell technologies. References: [1] Batzner,D.et al. Characterisation of over 21% efficient Silicon Heterojunction cells developed at Roth & Rau Switzerland. In Proceedings of the 26th EU Photovoltaic Specialists Conference 2011. [2] Mishima, T., et al. Development status of high-shy;#8208;efficiency HIT solar cells. Solar Energy Materials and Solar Cells, 2011. 95(1): p.18-21. Design criteria for amorphous/crystalline silicon heterojunction solar cells, - a simulation study R. Stangl *, A. Froitzheim, M. Schmidt, W. Fuhs. [3] Effect of emitter layer doping concentration on the performance of a silicon thin film heterojunction solar cell. Chin. Phys. B Vol. 22, No. 1 (2013) 016803. [4] Effect of emitter layer doping concentration on the performance of a silicon thin film heterojunction solar cell. Chin. Phys. B Vol. 22, No. 1 (2013) 016803

Water splitting using solar energy holds unique promise for renewable and sustainable energy. However, implementation requires increased efficiency, which can be realized by enlarging the solar absorption bandwidth and improving the separation of the charge carriers and reaction products. To this end, we have developed nano-photochemical cells composed of open-ended carbon nanotube arrays with photocatalysts on the outside and metal co-catalysts on the inside of each nanotube. The carbon nanotubes were synthesized within titania nanotubes, improving the visible light response of the photocatalyst and promoting fast charge separation. The oxidation sites are on the outside of the tubes, where the titania is located, and the reduction sites are on the inside of the tube, which was decorated with Pt nanoparticles.
We will present results on the enhanced photocatalytic performance, determined from the degradation of methylene blue, and the photoelectrochemical performance, determined from photocurrent density under visible light. The separation of the oxidation and reduction sites is verified by the photodeposition of metal and metal oxide under visible light. Ag nanoparticles were deposited on the inside of the tubes, indicating that this is the location of the reduction sites, while MnO₂ was deposited on the outside of the tubes, implying that this is the location of the oxidation sites. This demonstrates the spatial separation of oxidation and reduction sites within a single nanotube with enhanced photocatalytic and photoelectrochemical activity under visible light. This work has been partially supported by the NSF through Grants DMR 0645596 and CMMI 1335417.

We present here a method to assemble CdS nanorods and Co3O4 nanoparticles by electrostatic interactions into photocatalytic systems that simultaneously oxidize water and mediate electron transfer. The different nano-elements were characterized by TEM, absorbance and electrochemical measurements. Layered films of CdS nanorods and Co3O4 nanoparticles were first assembled onto ITO electrodes to show the efficient generation of high photocurrents as opposed to CdS or Co3O4 alone. The CdS/Co3O4 electrodes showed 100mV lower over-potentials than the dark reaction which was caused by better electron-hole transfer. Moreover, we showed that photoillumination produced high amounts of oxygen from the CdS/Co₃O₄ electrode. We also show in this work that dispersed clusters of CdS nanorods and Co3O4 nanoparticles held together by acid-base interactions can efficiently oxidize water and reduce methylene blue in solution. Furthermore, we demonstrate that this self-assembly approach can be used to couple well-defined BiVO₄ nanostructures to CdS nanorods or Co-Pi catalysts with the goal of producing fuel in the absence of using any sacrificial components or biasing the electrodes.

Solar-driven water splitting via photoelectrochemical process in presence of water and semiconductor, such as iron oxide (hematite), offers sustainable and unlimited manner to generate renewable energy free from carbon emission. Decades of investigation and hematite still remains as great photoanode candidate to realize at low-cost and high efficiency half reaction of the overall water splitting. Indeed, many improvements were achieved in this current year making the use of hematite believable and not so far from the real device. This work describes the influence of temperature of thermal treatment and use of different metal oxides (WO₃ and SnO₂) modifying the hematite films surface. The formation of nanostructured layer distributed along of conductor glass substrate was observed by top-view of scanning electron microscopy images. Hematite films modified with SnO₂ and WO₃ were analyzed by X-ray diffraction and its crystallographic arrangement was identified using PDF catalog. Besides, using the diffraction data was found that the nanostructure growth preferentially oriented at the highly conductive (001) basal plane of hematite, perpendicular to the substrate. Photoeletrochemical performance enhanced when low amount of metal oxide (WO₃ and SnO₂) was used to modify hematite modified. Additionally, the hematite electrodes modified with WO₃ layer and annealed at high temperature (at 750shy; ^oC) showed an expected reduction of water oxidation efficiency. While hematite electrode surface with SnO₂ layer showed an improvement from 1.4 to 2.6 mA.cm^-2 of water oxidation reaction efficiency. Understanding the reason of this distinct behavior is the subject of ongoing research in our laboratory.
Acknowledgements
We gratefully acknowledge #64257;nancial support from the Brazilian agencies of FAPESP (Grants 2011/19924-2, 2012/19926-8 and 2013/07296-2), CAPES and CNPq (Grants no. 473669/2012-9).

Earth abundant Cu₂ZnSn(SeS)₄ (CZTS) thin films have great potential for photovoltaic because the cost of raw material is much lower than that of other existing thin film PV technologies. Interest in CZTSS has increased exponentially because the device structure and physics are very similar to the well-studied high efficiency CuInGaSe₂ (CIGS) solar cells. Several kinds of vacuum and non-vacuum techniques have been reported for the preparation of CZTS solar cells. The demonstration of 12%-efficient solar cells makes CZTS a promising alternative material to current thin-film absorbers, such as CIGS and CdTe. For the sputter process, most CZTS layers were deposited by using several metallic and/or compound targets, followed by annealing in a Se/S-contained vapor. However, the control of element composition of CZT from several cathodes makes the process quite complex. According to our previous results, we reported 14.6%-efficient CIGS solar cells by employing a single CIG-ternary target. In this study, the CZTS absorbers were also formed by depositing a CZT thin film and then treated first in H₂Se and subsequently in H₂S.
Thin CdS buffer layers are mostly used as an n-type semiconductor material for forming p-n junction with p-type CZTS absorber layer. However, the applications could be limited in the future due to the prohibition of toxic cadmium in electronic devices in some countries. A replacement of CdS buffer layer by using other non-toxic materials has been of considerable interest for the development of CZTS solar cells. Zinc sulfide (ZnS) based buffer layer is one of the important substitutes for Cd-free devices. However, these efficiencies are still lower than those of the cells with CdS buffer layer. This study investigates ZnS and CdS buffer layers on CZTS solar cells. As the Cd-free buffer layer is used, the light soaking is workable to slightly improve cell performance. After the device fabrication, the Cd-containing and Cd-free CZTS solar cells have achieved the efficiency of 6 % and 1.8 %, respectively.

Despite intensive studies of transition metal based electrocatalysts for water splitting, the precise knowledge about the roles of each transition metal in a catalyst remains far from complete. In this work, FeNiCo LDH ultrathin nanosheets (< 2 nm) with variable Co incorporations were synthesized, providing a model ternary system for tuning the OER electrocatalytic activity. A low overpotential down to 0.21 V has been established. Moreover, by annealing the LDH precursor, we obtained FeNiCo oxide nanosheets, which showed even higher electrocatalytic activity with much faster OER kinetics. In general, the Tafel slope decreases with increasing Co content for both nanosheets. The Co incorporation increases the specific surface area, modulates the electronic structure and reduces the charge transfer resistance of the LDH nanosheets, thereby boosting the electrocatalytic activities. Higher oxidation state metal ions are detected in the oxide nanosheets stemming from the annealing treatment and are held responsible for the further increase of the OER activity.

In this work we present promising photocathode for solar hydrogen production based on nanostructured multiferriocs namely, bismuth iron oxide, BFO, doped with 20% Ca, Ba and Cu. Hydrogen generation through photoelectrochemical (PEC) water splitting using solar energy is proven to be one of the ideal solutions to energy and environmental problems. Nano materials were synthesized using a facile sol gel method and photocathode were fabricated using doctor blading technique on ITO glass substrate. Both structural and morphological characterization were carried out using XRD, Mossbauer spectroscopy and TEM to study the doping effect on BFO. Photoelectrochemical performance was determined using linear sweep voltammetry. In all the three dopants, photocurrent was enhanced compared to pure BFO case. The highest photocurrent recorded was for the Ca doped BFO film with value of -0.4 mA cm^-2 at -0.5 V versus Ag/AgCl in 1 M NaOH electrolyte. Photocurrent density in case of Cu and Ba was found to be -0.05 and -0.19 mA cm^-2 respectively. Enhancement of photocurrent with doping was related, to some extent, to the decrease in particles size achieved by doping compared to the particle size of pure BFO. Also, doping at such high percentage, above solubility limit, was found to cause phase separation and the appearance of other magnetic phases like a and or g Fe2O3 which are n-type semiconductors as recorded from XRD and Mossbauer. This could form nano PN junction that could increase the electron hole life time minimizing recombination possibility and enhance the photocurrent.

Efficient utilization of the sun as a renewable and clean energy source is one of the major goals of this century due to increasing energy demand and environmental impact. Solar thermal fuels are materials that capture and store the sun&’s energy in the form of chemical bonds, which can then be released as heat on demand and charged again. Previous work on solar thermal fuels faced challenges related to the cyclability of the fuel over time, as well as the need for higher energy densities. Recently, it was shown that by templating photoswitches onto carbon nanostructures, both high energy density as well as high stability can be achieved.
In this work, we explore alternative molecules to azobenzene in such a nano-templated system. We employ the highly accurate quantum Monte Carlo (QMC) method to predict the energy storage potential for each molecule. Our calculations show that in many cases the level of accuracy provided by density functional theory (DFT) is sufficient. However, in some cases, such as dihydroazulene, the drastic change in conjugation upon light absorption causes the DFT predictions to be inconsistent and incorrect. For this case, we compare our QMC results for the geometric structure, band gap, reaction enthalpy, charge density and degree of conjugation with different DFT functionals. Additionally, we also investigate ways of tailoring band gap in such conjugated complex molecules.

Cu₂ZnSnS₄ (CZTS) consisting of only earth-abundant, non-toxic elements draws great interest presently as a promising photovoltaic absorbing material.¹ However, due to the complex quaternary nature of this material, composition and phase control still remains a tough issue to be solved. In this work, instead of the conventional bulk thin film structure, we develop an extremely thin absorber (ETA) solar cell type construction by coating nanoscale Cu₂ZnSnS₄ absorber layer on TiO₂ mesoscopic film (_mesoTiO₂) deposited on FTO glass. Using a three-step aqueous solution-based successive ionic layer adsorption and reaction (SILAR) approach, a precursor layer comprising Cu, Zn, Sn and S was well coated on TiO₂ mesoscopic film with controlled atomic ratio composition. Then we subject the composite _mesoTiO₂@CZTS film to a fast and controlled annealing method to promote the crystallization of the kesterite CZTS phase. Currently, Raman spectroscopy is the most feasible method for Cu₂ZnSnS₄ phase identification. As part of the annealing process development, we have employed in-situ Raman monitoring for the first time to investigate the phase formation dynamics of Cu₂ZnSnS₄ and we determined the optimum crystallization temperature range. By performing multi-wavelength excitation Raman scattering with four lasers we were able to confirm the formation of crystalline Cu₂ZnSnS₄ coating while eliminating formation of other possible secondary phases. The in-situ Raman monitored annealing procedure constitutes an invaluable tool towards building kesterite phase pure and impurity phase-free Cu₂ZnSnS₄ coated TiO₂ mesoscopic films for developing new generation low-cost, non-toxic PV devices.
1. C. M. Fella, Y. E. Romanyuk and A. N. Tiwari, Solar Energy Materials and Solar Cells, 2013, 119, 276-277.

Cu₂ZnSnS₄ (CZTS) thin film possesses suitable optical bandgap (E_g = 1.5 eV), high optical absorption coefficient (> 10^-4 cm^-1) and high theoretical conversion efficiency (~ 30%) when implanting in the thin-film solar cells. Since all elements are abundant in the earth and the preparation method is non-toxic, CZTS becomes the promising alternative to replace conventional Cu(In,Ga)Se₂ (CIGS) as the absorber layer of thin-film solar cells.
In this work, CuS, ZnS and SnS₂ powders are adopted as the starting materials. The powders were mixed at the molar ratio of Cu:Zn:Sn:S = 2:1:1:4, poured into a mold and pressured into 2-inch disc form, and subjected to an appropriate sintering treatment to yield a single-phase CZTS sputtering target. Afterward, the CZTS target was transferred to a sputtering system to deposit the CZTS thin films on Mo-glass substrate at various working pressures (1, 5, 10 and 30 mtorr). The CZTS layers were then annealed at 560°C in Se vapor ambient using forming gas as the carrying gas to form the Cu₂ZnSnSe₄ (CZTSe) absorption layers. Subsequently, CZTS thin-film solar cells with the structure of Mo/CZTSe/CdS/i-ZnO/IZO/Al were prepared and their performances were evaluated.
In microstructure characterizations of CZTSe layers, Raman spectroscopy revealed the Cu₂Se phase in CZTSe layer prepared at working pressure of 1 mtorr although X-ray diffraction (XRD) analysis indicated the CZTSe layers are all single-phase regardless of the working pressure. Energy dispersive spectroscopy (EDS) analysis found that the Cu content of CZTSe layer decreases with the increase of working pressure. Since EDS detected the stoichiometry of CZTSe layer prepared at working pressure of 1 mtorr is Cu:Zn:Sn:Se = 27.1:13.4:11.0:48.5, Cu-rich feature might thus result in the secondary phase in such a layer. EDS analysis found that the CZTSe layer prepared at 5-mtorr working pressure exhibits the stoichiometric ratio of Cu:Zn:Sn:Se = 24:14.5:12.4:49.1 which is the closest to the ideal CZTSe stoichiometric ratio of 2:1:1:4. UV-Visible spectroscopy indicated the E_g of such a CZTSe layer is about 1.1 ~ 1.2 eV with the best transport properties of p-type carrier concentration = 3.1×10¹⁸ cm^-3 and mobility = 7.829 cm²×V^-1×sec^-1 as revealed by the Hall measurement.
In solar cell performance evaluated under the AM1.5 illumination condition, the best CZTSe thin-film solar cell sample exhibited the open-circuit voltage of 0.354 V, short-circuit current density of 25 mA/cm², fill factor of 57.4 % and conversion efficiency of 5.08 %. The preparation of single-phase CZTS sputtering target and the fabrication of CZTSe thin-film solar cell with satisfactory conversion efficiency are successfully demonstrated in this work.

Photovoltaic cells that transmit the visible part of the solar spectrum and only absorb photons from the UV part of the solar spectrum for photocurrent generation are a promising way to harvest energy on large area window areas of high-rise buildings or on glass roofs. While maintaining the function of these structures, such devices allow for conversion of UV photon energy that would otherwise simply be dissipated by the window glass.
We present PLD-grown NiO/ZnO pn-heterojunctions with average transmittance of around 50% in the visible range that work as UV-active solar cells. Open-circuit voltages of more than 500 mV and comparably high short-circuit current densities of around 1.5 mA/cm² under illumination with a solar simulator have been measured, resulting in power conversion efficiencies approaching 1%. Measurements of the external quantum efficiency revealed a photocurrent gain coefficient of around 3 at short-circuit, and we attribute this to a carrier sweep-out effect that comes about by strongly differing transit times for photogenerated electrons and holes. This finding is supported by results of temperature-dependent current-voltage analysis of these devices.

One of the main challenges within the field of renewable energy is overcoming the intermittency of energy harvesting. As our largest source of available energy is the sun, one promising approach for meeting this challenge is to convert solar energy into storable fuels¹, which can be transported and used when required. The simplest fuel solution is hydrogen produced by water splitting, with the only waste product being oxygen. This approach has been demonstrated with photovoltaic (PV) and electrolyser technologies², but since both elements require large areas (and large quantities of material), an integration of the two technologies is desirable. This can be performed simply by functionalizing the surface of the PV cell and bringing it into direct contact with water³. The challenge, however, is to identify materials, with band gap energy and band positions suitable for water splitting, and good chemical stability.
Here, direct bandgap gallium phosphide (GaP) nanowires, grown with wurtzite crystal structure, were studied for the purpose of PEC water splitting. Vertically aligned p-doped nanowire arrays have exhibited 2.9% applied bias solar to hydrogen efficiency. To maximize the photoelectrochemical current, and therefore the molecular hydrogen production, several factors were studied. The growth parameters were changed in order to study the effect of nanowire geometry on resistance and absorption. The surface of the nanowires was also modified after growth by atomic layer deposition of an insulating oxide layer, and electrochemical deposition of both earth abundant and precious metal catalysts, to boost efficiency.
These factors were investigated using cyclic voltammetry, photocurrents of >10mA/cm² have been recorded under AM1.5 100mW/cm² illumination, as well as high open circuit potentials of > 0.7V (vs. RHE). Gas chromatography was also used to caculate a faradaic efficiency for hydrogen production of >97%.
References
¹ O. Khaselev, et al., Science 280, 425 (1998)
² F. Dimroth, et al., 2006 IEEE 4th World Conf. Photovolt. Energy Conf., 640 (2006)
³ F. F. Abdi, et al., Nat. Commun4, 2195, (2013)

For achieving high efficiency and a sustained performance of photoelectrochemical hydrogen generation processes strategic nanoengineering of the microstructure, electronic structure and chemical composition is most crucial. In the context of water splitting, metal oxides has been the main workhorse photocatalysts for study, due to their better resistance to aqueous, electrochemical and photo-corrosion. However in terms of the optical, electronic and catalytic properties many sulfide, nitride and phosphide -based photocatalysts are more favorable, although their degradation needs to be appropriately taken care of.
Herein we present a metal chalcogenide-Nickel Sulfide (NiS₂) as a potential photoanode for photoelectrochemical water splitting application. NiS₂ has been demonstrated earlier only as a co-catalyst for this application, where due to its favorable valance band potential it enhanced the water oxidation efficiency of the main catalyst. In our work, we have used NiS₂ as the active photocatalyst and shave howed that it is highly capable of driving the oxidation/reductions efficiently. Obtaining phase pure NiS₂ with uniform morphology is a challenge. In our work, we have synthesized single-phase NiS₂nanosheets from adefect-induced (non-stoitiometric) NiO_x, by calcining it under sulphur atmosphere. With a bandgap of ~1.6 eV, NiS₂ absorbs the entire visible spectrum of solar energy. We demonstrate a remarkable current density of 2.3mA/cm²at 0.6 V with respect to Ag/Agclwith NiS₂as photoanode under near-neutral pH. Further for a sustained photoelectrochemical performance, we have deposited ZnS as a thin passivation layer. The demonstrated solar-to-hydrogen efficiency is better than many metal oxide based photocatalystsand present a strong potential to study such Sulfide-based nanosystemsfurther.

To make photoelectrochemical (PEC) water splitting an economically competitive technology, we need photoelectrodes made of earth-abundant elements that can deliver high efficiencies. With a suitable band gap (2.0 eV) to yield theoretical efficiencies as high as 16%, hematite has attracted significant research attention in the past decades. The light-to-charge conversion of a PEC cell is enabled by the development of a built-in #64257;eld within the semiconductor. It is believed that surface states of energies within the band gap often act to reduce the built-in #64257;eld. It has been recognized as a potential source of problems in terms of photovoltage generation. In our study, we found that hematite prepared by atomic layer deposition (ALD) produces photocurrents when illuminated by near-infrared light (lambda; = 830 nm), whose energy is smaller than the band gap of hematite. The phenomenon was inferred to be a result of valence band to surface state transition. Thermodynamics of the hematite/water interface was studied under open-circuit conditions and it was discovered that the equilibrium potential of the hematite surface in electrolyte was more negative than water oxidation potential by at least 0.4 V. With a layer of amorphous NiFeO_x coating, the equilibrium potential of hematite was restored to water oxidation potential (1.23 eV), and the photo response under 830 nm illumination was annihilated. More interestingly, the hypothesis was further supported by the observation that surface states of similar energy were present on TiO₂ electrodes. A facile NiFeO_x deposition has proven e#64256;ective in diminishing the observed changes in photocurrent responses and open-circuit potentials. These results reveal new insights into surface states of hematite and metal oxide electrodes. Our results establish that much can be harvested from hematite as an earth abundant photoanode material, despite its many challenges. We continue to view it as a favorable photoanode choice for sustainable solar water splitting applications.

Water splitting is a thermodynamically uphill reaction. It has the potential to store harvested solar energy for redistribution and re-utilization and has recently attracted significant research attention. Among existing methods to carry out the reaction, semiconductor-based photoelectrochemical route is appealing because this method promises low cost and long lifetime. However, sluggish kinetics at the surface of the photoelectrodes (both photoanodes and photocathodes) demands the use of catalyst to help facilitate the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. There are currently a large number of different materials that have been studied as HER or OER catalysts, but a persisting concern with each of these materials is the long term instability resulting in catalytic deactivation. Here using HER catalyst as a platform, we study a method to stabilize the catalysts on the surface of photoelectrodes. Our method employs a facile electrodeposition of a mesoporous silica film atop of the catalyst acting as a protection layer. Our initial tests use platinum as a HER catalyst for a proof of concept system, which can later be extended to more practical catalysts. The diffusion and kinetics of the HER reaction using Pt are unaffected by the addition of this protection layer, and initial stability tests show promises for an increase in long term stability. The deposition of a protection layer allows for a promising solution to stabilize HER and OER catalyst by physically restraining the catalysts to the surface. This facile surface treatment will contribute significantly to the construction of high-performance, long-stability photoelectrodes for solar fuel production.

A method was developed to accurately map out doping profiles in silicon. The method was demonstrated on ion implanted p-type silicon (boron doped) with a junction depth of approx. 450 nm (also measured using Secondary Ion Mass Spectroscopy, SIMS). Uniform well controlled layers of silicon oxide (50 nm - 100 nm) were grown using anodization, an electrochemical room temperature process without changing the doping profile. The silicon oxide was subsequently etched using a buffered oxide etch. Finally the free carrier density in the remaining sample was measured rapidly and accurately using terahertz time domain spectroscopy (THz-TDS). By repeating this process 5 times until the junction was etched the doping profile could be reconstructed accurately. The underlying physics and potential of the method will be discussed.

Tungsten trioxide (WO₃) materials have a narrow band gap than titanium dioxide and have been studied due to their high visible light absorption ability, good stability under acid environments for solar cell applications. Also, block-copolymer (BCP) membranes are versatile templates for the fabrication of controllable porous nanostructures. In this study, we introduce a method to increase a porosity of an interconnected WO₃ nanostructure by blending polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP). Firstly, we fabricated the nanoporous templates based on cylindrical micro-domain via selective swelling treatment. We used ethanol as a selective solvent of P4VP. It was possible to increase the porosity of WO₃ nanostructure by controlling volume fraction of P4VP. Secondly, we filled the pores of BCP templates with ammonium metatungstate (AMT) as a tungsten trioxide precursor. We fabricated an interconnected WO₃ nanostructure film with a high surface area and used this film as a photoanode to improve light harvesting in dye-sensitized solar cells. The films were characterized using X-ray diffraction, scanning electron microscopy, absorbance spectroscopy, current-voltage characteristics, impedance spectroscopy and incident photon-electron conversion efficiency analysis.

High reflection and low build-in electrical field hindered the efficiency of planar n-Si/organic heterojunction hybrid solar cells. Here, we demonstrate a MoO₃ layer can simultaneous solve the two big issues, resulting a record power conversion efficiency (PCE) of 13.8%. Optical analysis reveals that a proper thickness of MoO₃ film on top of Si/organic surface can act as antireflection coating thus increasing the short-circuit current density (J_sc). The high work function MoO₃ on the polymer induces an inversion layer in Si underneath which improves charge separation and leads to less surface recombination, resulting in an optimized open-circuit voltage (V_oc) of ~650 mV. This shows a new promising route towards low cost, high efficiency photovoltaics by improving optical and electrical properties.

Since the first report of photo-induced water splitting by TiO₂ electrodes was published [1], the potential of this reaction to convert photon energy into H₂ energy has been extensively investigated. Due to its simplicity, water splitting using a powdered photocatalyst is currently a subject of interest, with most research focusing on the visible light sensitization of catalysts in order to effectively utilize incoming solar energy. Numerous photocatalysts have been identified that can generate H₂ in the presence of a proper sacrificial agent irradiated with visible light (half water splitting). Among them, the longest wavelength of visible light that can produce H₂ is ca. 700 nm, as reported by Kudo using sulfide solid-solution photocatalysts [2]. β-FeSi₂, composed of only earth-abundant elements, is a semiconductor, known to possess a narrow band-gap of 0.85 eV and high stability [3,4]. We employed β-FeSi₂ as a H₂ production photocatalyst. Due to its band-gap, we can expect that it can utilize near-infrared light as well as whole visible light. In fact, β-FeSi₂ could evolve H₂ in the presence of S₂O₆^2- as a sacrificial agent, under light irradiation whose wavelength was larger than 1300 nm. This system can serve as both H₂ production and environmental preservation with solar energy because S₂O₆^2- in known as one of the water pollutants.
References: [1] A. Fujishima, K. Honda, Nature, 238, 37 (1972). [2] A. Kudo, Int. J. Hydro. Ener., 32, 2673 (2007). [3] I. Nishida, Phys. Rev. B, 7, 2710 (1973). [4] D. Leong, M. Harry, K. J. Reeson, K. P. Homewood, Nature, 387, 686 (1997).

Stable 2-Dimensional molybdenum disulfide (MoS₂) nanostructures are an active subject of research due to their unique properties. Recently, nanostructured MoS₂ became a strong candidate for cheap and, yet efficient, catalysts of hydrogen evolution reaction (HER) to replace Pt. The HER activity for MoS₂ is strongly correlated with the number of exposed edge sites that have quite different characteristics from the catalytically inert (0001) basal planes. Controlling the exposed edge sites of nanostructured MoS₂ is the key to success for HER. We report on, for the first time, the atomic layer deposition (ALD) of MoS₂ nanostructures. The resulting MoS₂ was flake-like and crystalline nanostructures, allowing for the direct formation of nanoporous films on many different kinds of substrate, e.g., SiO₂/Si, Au and FTO (fluorine doped tin oxide). Non-ideal ALD growth on planar surfaces due to the layered nature of MoS₂ structures makes it possible to control over the fraction of active edge sites that is exposed. Exchange current densities for HER of 0.3 - 7.9 mA/cm² at -0.3V versus reversible hydrogen electrode (RHE) were obtained with the function of the number of ALD cycles, corresponding to the thicknesses of the MoS₂ flakes. The corresponding on-set potentials were systematically varied with the relative area of edge sites of our MoS₂ which is controlled by the number of ALD cycle. Remarkably, the maximum current density of 7.9 mA/cm²@ - 0.3V vs.RHE, which is comparable to planar Pt, was obtained from the optimized thickness of the ALD-grown MoS₂ flakes. Our preliminary results provide us insight into formation of layered materials by ALD and generalization into other materials systems.

TiO₂/hematite (α-Fe₂O₃) heterojunction nanostructured photoanodes were constructed on fluorine-doped tinoxide (FTO) substrates for use in the photoelectrochemical (PEC) water splitting cells. An N⁺-n hereojunction of TiO₂ and hematite is proposed to build an electric field in the space charge region, which facilitates the charge separation in the hematite layer. The rutile TiO₂ nanostructures were grown on the FTO substrates by hydrothermal method and the hematite thin layers were subsequently deposited on the surfaces of TiO₂ nanostructures using chemical vapor deposition (CVD). Compared to the thin-film hematite photoanode, an eightfold enhanced photocurrent density was measured in the TiO₂/hematite heteronanostructure PEC cell at 1.23 V versus reversible hydrogen electrode (RHE) under AM 1.5 at 100 mW cm^-2. The overpotential for water oxidation was decreased by the surface modification of the TiO₂/hematite photoanode with cobalt catalysts. The dynamics of charge separation and charge transfer in the TiO₂/hematite nanostructured photoelectrode were investigated using hydrogen peroxide as a hole scavenger. Significant enhancements of both charge separation and charge injection in the hematite photoanode were achieved through morphology and interfacial energetics controls via constructing the TiO₂/hematite heteronanostructures. The details will be reported in the presentation.

Among all sources of renewable energy, solar energy is the only one that can be utilized on a scale comparable to our growing energy needs. Its utilization, however, must address challenges associated with its diurnal and intermittent nature. In principle, the challenge can be addressed by carrying out water splitting reactions by methods such as photoelectrochemistry, which harvests and stores solar energy in chemical bonds, in a way similar to natural photosynthesis. For this reaction, we need highly performing photoelectrode for both the oxidation (anode) and reduction (cathode) reactions. From a cost and stability perspective, metal oxides are attractive candidates. While significant research efforts have been devoted to studying metal oxides as photoanodes, comparably little has been done on using metal oxides as photocathodes. This is because conventional wisdom considers photocathode should be a narrow band gap material. Recent advances in using narrow band gap semiconductors such as Si as photoanode challenges this view and opens up doors to study wide band gap metal oxides (e.g. Cu₂O) as photocathode. As non-toxic, low-cost p-type semiconductor, Cu₂O exhibits a band gap of 2 eV. Nevertheless, due to the energy mismatch of Cu₂O Fermi level with water reduction potential, researchers have been unable to obtain a high photovoltage (typically <0.6 V). Here we present a strategy that has the potential to solve the problem. Our strategy centers around using another earth abundant material, ZnS, which is an n-type semiconductor with a more negative Fermi level than water reduction potential to decorate the surface of Cu₂O, so as to increase the degree of band bending within Cu₂O. Under illumination, an enhanced built-in field is expected, so is the measurable photovoltage. To produce pure phase Cu₂O, we prepared the material by thermal oxidation of polished Cu foil. Then a thin film of ZnS was deposited on the surface of Cu₂O, followed by the growth of a TiO₂ protection layer. The fabrication was concluded by the decoration of earth-abundant hydrogen evolution catalysts (NiMo/CoMo). Under illumination, the heterojunction exhibits an increase of the photovoltage by as much as 20%. Open circuit potential (OCP) measurements confirmed that the change was due to the shift of surface thermodynamics. A new door is opened to construct complete water splitting systems using only earth abundant elements.

Cu₂SnS₃ (CTS) thin films were synthesized by sulfurization of electrodeposited Cu/Sn metallic precursors in the sulfur atmosphere. Two different sulfurization processes were produced and compared: (a) one-step sulfurization and (b) two-step sulfurization (TSS). The one-step prepared samples were performed by conventional sulfurization method in a tubular furnace. Sulfurization of Cu/Sn precursors occurred when the temperature was higher than the melting point of sulfur (~120 ^oC). In the TSS approach, Cu/Sn precursors were alloyed in the vacuum first before the sulfurization. The property of the prepared samples was determined by structural, morphological, compositional and optical analysis. The study of crystal structure was carried out using x-ray diffraction. The morphology and the composition of the CTS thin films were characterized by scanning electron microscopy and energy dispersive spectroscopy. Raman and photoluminescence analysis were performed at room-temperature using a 532 nm He-Cd laser. Time-of-flight secondary ion mass spectrometer provides the detailed elemental distribution after the alloy process and sulfurization. The TSS process improved the CTS thin films, yielding uniform, large and dense grains without the cracks and the secondary phases included CuS, Cu_2-xS and SnS. Furthermore, it also demonstrated that TSS improved the CZTS solar cells with high power conversion efficiency.
In this work, electroplating was used for depositing Cu/Sn metallic precursors. The process is of particular interest due to the advantages of the low-cost, large-area, and no toxic. The metallic Cu/Sn precursors were electroplated in a conventional three-electrode electrochemical cell. The Mo-coated soda lime glass substrate was used as the working electrode, a platinum electrode as a counter electrode, and saturated calomel electrode as a reference electrode. The Mo substrates were cleaned ultrasonically in acetone, isopropanol, distilled water and finally dried under flowing nitrogen.
Cu and Sn layers were sequentially electrodeposited in the order of Mo/Cu/Sn at room temperature. Cu layer was firstly plated onto the Mo-coated soda lime glass substrate in an aqueous solution containing copper sulfate (0.02 M) and trisodium citrate (0.085 M). Then, Sn layer was deposited onto Cu using acidic bath of tin sulfate (0.022 M) and trisodium citrate (0.12 M). Trisodium citrate was added as the complexing agent. The sulfuric acid was used to adjust the pH value between in 4.5 to 5.0. After the deposition, the precursors were rinsed in de-ionized water and dried under flowing nitrogen, followed by the heat treatment.

Photoelectrochemical (PEC) reduction of CO₂ into chemical fuels such as CH₄ and CH₃OH or other chemical feedstock is one of the promising ways to utilize the solar energy and to solve the problem of energy crisis. In most of these researches, CO₂ bubbling into the electrolytes is used and regarded as the active species. However, since the CO₂ could dissolve in and react with the water to form HCO₃^- and CO₃^2-, these anions could also be the active species. Both CO₂ and HCO₃^- have been reported as the active species in the electrochemical reduction of CO₂ [1, 2]. However, the problem is still not clear. Therefore, the active species in the electrochemical reduction of CO₂ in KHCO₃ solution was studied on a Cu electrode at elevated temperatures without the CO₂ bubbling in this research. The elevated temperature was used to help the decomposition of the HCO₃^- to generate CO₂ gas.
Results showed that hydrogen was the major product (over 96% faradaic efficiency) in the electrochemical reduction of 3.0 mol L^-1 KHCO₃ without the CO₂ bubbling on a Cu working electrode at 40 and 60^oC and a bias of -1.6 V (vs. Ag/AgCl) for 1 hour. Products of CO, C₂H₄ and CH₄ were also detected even without the CO₂ bubbling. However, the total faradaic efficiency of these products was less than 1%. As the temperature increased from 40^oC to 60^oC, the faradaic efficiency of the CO and CH₄ decreased while that of the C₂H₄ increased. When the concentration of the KHCO₃ decreased from 3.0 to 0.1 mol L^-1, there were no products related to the CO₂ reduction found at 60^oC, only hydrogen generation. Experiments for measuring the CO₂ gas generation from the 0.1 and 3.0 mol L^-1 KHCO₃ at 60^oC were carried out to investigate the spontaneous decomposition of the HCO₃^- into the CO₂ gas. More than 35 mL of CO₂ gas was collected after 2 hours in 3.0 mol L^-1 KHCO₃ while only 2 mL of CO₂ gas was collected in 0.1 mol L^-1 KHCO₃ at 60^oC. This suggests that higher concentration of KHCO₃ is favorable for the decomposition of the HCO₃^- into CO₂ gas at elevated temperature. Therefore, higher concentration of 3.0 mol L^-1 KHCO₃ could generate much more CO₂ gas than 0.1 mol L^-1 KHCO₃ from the decomposition of the HCO₃^- at elevated temperature, which then, promoted the reduction of CO₂. As a result, the active species of the electrochemical reduction of CO₂ at elevated temperatures should be the CO₂ gas dissociated from the HCO₃^-.
References
[1] B. Innocent, et al., Appl. Catal. B-Environ., 2010, 94, 219-224.
[2] Y. Hori, et al., J. Chem. Soc. Faraday Trans. 1, 1989, 85, 2309-2326.

We present electrical, optical, composition and structural properties of InGaN thin films grown by rf magnetron sputtering. We also present efficiency, fill factor, open circuit voltage and short circuit current values of our successful solar cells made using rf sputtering. For the solar cells, V_oc = 400 mV and low short circuit current values were measured both under sunlight and laboratory conditions. As a first step, several samples of InGaN thin films were deposited on cover glass, high temperature aluminosilicate glass and silicon (111) substrates. Some films were grown at different pressures ( 4 mTorr to 14 mTorr ) by varying Argon and Nitrogen flow rate. Samples at different substrate temperatures ( 100 ^oC, 200 ^oC, 300 ^oC, 400 ^oC and 600 ^oC) keeping all other parameters constant were grown. Films of varying In/Ga ratio were grown. Bandgap values of 1.4 eV, 1.6 eV, 1.75 eV, 2.0 eV and 2.5 eV were measured using UV Spectrophotometer and Tauc plots. Resistivity of 6.3E-3 Omega;.cm, mobility of 2.462E+0 cm²/Vs, conductivity of 1.58E+2 /Omega;.cm and bulk carrier concentration of -1.66E+21 /cm³ values for InGaN thin films grown at room temperature were measured using HMS-3000 Hall effect measurement system. Amorphous/Crystalline nature was verified using X-Ray Diffraction ( XRD) measurements and SEM images. Atomic composition of In_xGa_1-xN , 0< x < 0.8 was calculated from Energy-dispersive X-ray spectroscopy (EDXS) measurements. Sputtering conditions for thin films with the best resistivity, mobility and carrier concentration values were recorded and used to deposit n-type InGaN layers while making heterojunction with intrinsic thin layer ( HIT ) solar cells. Silicon with bulk carrier concentration of 2.74E+19 /cm³ and mobility of 2.99E+2 cm²/Vs as p-type substrate, n-type InGaN active layer, aluminum back contact and ITO front contact solar cells were made with different i-layers in between. It has been found that successful InGaN solar cells with improved V_oc and I_sc values compared to that of test cells can be made using cost effective rf magnetron sputtering method.

Early stage development of new earth-abundant non-toxic absorber materials is hampered by the complex number of loss mechanisms and limiting factors in the device. Identifying whether recombination-active bulk defects, sub-optimal band alignment at the heterojunction, interface recombination, back contact recombination, or light management problems are contributing to the losses requires simultaneous interpretation of multiple characterization measurements. The standard methods for interpreting such measurements rely on fitting to fairly simple models, where only one or two recombination mechanisms are taken into account at a time. Since many material and device parameters in new systems are undetermined, there is little guarantee that a fit between a single experiment and model is unique or physically meaningful.
To avoid these problems we present a method for synthesizing temperature- and illumination-dependent current-voltage, capacitance voltage, quantum efficiency, and bulk material measurements to create a well-constrained description of an under-developed photovoltaic device. This methodology uses a combination of numerical device modeling (using SCAPS1D) and analytic solutions for limiting cases. The numerical simulations insure that we can include many loss mechanisms at the same time. To constrain these models we perform systematic fitting procedure between the numerical simulation and experimental characterization results. Additionally, the analytic descriptions give insight into mechanism and allow us to determine whether the fit is robust, and therefore more likely unique.
We perform this analysis on tin sulfide devices as a test case. We find that for our standard devices stack, giving an efficiency of 3.88%, a non-optimal band alignment is limiting the Voc. We also predict bulk lifetime efficiencies on the order of 100 ps. These fitting predictions are compared with further experimental results. We perform our analysis on devices that take into account these learnings, and make suggestions for future improvements.

Solar cells have drawn much attention as one of the most promising candidates for renewable clean energy. Solar cells can be roughly divided into bulk and thin film devices in terms of their structures. For bulk devices, silicon-based materials, including single and polycrystalline structures, which are the leading materials, have been extensively studied. However, drawbacks such as indirect band-gap and poor light absorption could result in poor efficiency for Si-based solar cells. Among all solar materials, Cu(In, Ga)Se₂ (CIGS) is the most promising material owing to its excellent light trapping ability, broadband light absorption, and environment-friendly manufacturing processes. Laboratory-scale Cu(In, Ga)Se₂ (CIGS) solar cells have now reached over 20% conversion efficiency. In the mid-1990s, it was observed that CIGS grown on soda-lime glass (SLG) would perform better than CIGS grown on sodium-free substrates, and the reason for this improvement was that the CIGS contained Na. The influence of Na during the growth of CIGS has since then been studied by several research groups; see, for instance, Although the influence of Na is still a debated topic in the CIGS community, the main Na studies point toward the following conclusions: Na increases the free carrier density by at least one order of magnitude and this is associated with a lower number of compensating donors. Na influences the CIGS growth, resulting, in some cases, in smaller grains and, in some cases, in larger grains and increased texturing of thin films. At the diffusion temperatures (~ 600 °C), Na from the glass diffuses through the molybdenum back contact and into the forming CIGS film. This makes the solar cell efficiency very dependent on the properties of the molybdenum back contact.
In this study, Mo-Na alloy (90:10) target was used for incorporating Na in CIGS layer grown on stainless-steel substrate. Bi-layer Mo back contact composed of adhesion and electrode layer was deposited by DC magnetron sputtering system. Grain size of Mo-Na layer can be controlled with Ar gas pressure and glow discharge power. SIMS depth profile shows that Na concentration diffused from Mo-Na layers of different grain size was varied although the thickness of Mo-Na layers deposited was identical. We can adjust Na concentration diffused in CIGS to variation of grain size. It demonstrates that Na is diffused along the grain boundaries. Also, efficiency of CIGS solar cell was improved by Na incorporation.

Today, demands for sustainable energy such as solar, wind and biofuel have been increased due to global warming and energy crisis. Among them, solar energy has been expected to provide sufficient capacity for global energy consumption. Therefore, lots of efforts have been dedicated for increasing of the solar energy conversion efficiency. The converted solar energy can be stored in photoelectrochemical cell for water splitting or CO₂ decomposition.
Hematite (α-Fe₂O₃) is promising material for solar water splitting, which is stable in ambient condition, non-toxic, earth-abundant and moderate band gap (1.9-2.2thinsp;eV). Nevertheless, several drawbacks such as short hole diffuse length (2-4 nm), short lifetime of charge carrier, poor mobility and insufficient energy band position for absorbing visible wavelength are inadequate for using photoelectrode. Therefore, hematite can be engineered into a nanostructure to overcome those limitations.
Moreover, integrating metal nanoparticle with nanostructured hematite provides an effective way to extend absorption spectrum region of solar wavelength. To achieve the integrated structure for metal oxide nanostructure and metal, several routes such as atomic layer deposition, chemical vapor deposition and other vacuum-based technique were established. However, all of them requires high-cost and time-consumption method.
In this study, facial synthesis method for integrated metal oxide nanostructure and metal nanoparticle was introduced. In detail, α-Fe₂O₃/Ag hybrid nanostructure was fabricated by using hydrothermal and UV irradiation, respectively. To achieve rapid reaction time, high-power UV lamp was adopted. The fabricated nanostructure was evaluated by various analytic tools to determine the plasmon effect on the integrated semiconductor/metal nanostructure. Consequently, photoeclectrochemical cell was demonstrated for the water splitting application.

In order to improve the performance of solar cells made from earth-abundant materials, we propose to apply the electrochemical impedance spectroscopy (EIS) technique for readily determining pn-interface properties such as uniformity, defect characteristics, and carrier-recombination time.
Many earth-abundant-material solar cells are promising for solar energy applications. However, it is difficult to obtain high-efficiency solar cells because of poor crystal and/or interface quality compared with commercialized thin-film or crystalline solar materials. In fact, the electrical properties of earth-abundant-material solar cells, especially concerning the pn-interface, have yet to be comprehensively clarified because these solar cells exhibit a complex structure comprising stacked layers of several materials and interfaces.
Recently, deep level transient spectroscopy (DLTS) and admittance spectroscopy (AS) have become very popular techniques for determining the energy levels of the defects and/or the majority carrier-trap properties of semiconductors. In contrast, EIS is a nondestructive method used by chemical or material researchers for investigating defect/degradation properties. We have proposed this simple and convenient technique for investigations of the physical properties of a pn-interface with the goal of improving device properties [1,2].
In this presentation, we propose that the carrier-transfer time-constant (quasi-lifetime) is revealed through the pn-interface carrier recombination time in a simple EIS measurement. For improving earth-abundant-material solar cell performance, an EIS measurement can quickly and simply determine pn-interface properties such as uniformity, defect properties, and carrier recombination time.
We applied the EIS technique to thin-film solar cells such as CIGS-, tin monosulfuide (SnS)-, nickel oxide (NiO)-, and Cu₂SnS₄ (CTS)-related solar cells and obtained the quasi-lifetimes by fitting the data. The quasi-lifetime reflects the carrier transfer or captor properties around the pn-interface. We also describe the relationship between the quasi-lifetime and the recombination process in the solar cell. This is the first step toward the practical application of EIS as a new method for improving the efficiency of earth-abundant-material solar cells.
[1] Our group, Thin Solid Films, 535(2013)287. [2] Our group, Electrochimica Acta, 131(2014) 236.

As an earth abundant material, silicon stands out as one of the most promising semiconductors with its outstanding good performance on light absorption, incident photon to electron conversion efficiency, charge transfer property and rich knowledge for solar energy driven devices. Photoelectrochemical (PEC) water splitting by silicon based photoelectrodes has potential to generate hydrogen as clean energy source. However, one of the drawbacks of silicon for PEC water splitting is the stability issue in aqueous solutions. In order to address this problem, the surface protection/passivation of silicon by TiO₂ oxide heterojunctions were investigated.
In this study, we passivated the surface of silicon by employing the atomic layer deposition (ALD) to synthesize TiO₂. As photocathode, the electrons generated by solar irradiation in silicon were easily flow from its conduction band into the TiO₂ passivation layer, and then injected into the electrolyte for reduction reactions. By carefully control the passivation layer condition, the water splitting efficiency of silicon was shown to be improved. Fundamental researches of investigating the passivation layer thickness effect, the annealing effect and the redox potentials influences in this paper can help to understand the energetic structure of Si/metal oxide heterostructures and the original of the water splitting efficiency improvement.

Photocatalytic reactions are a comparatively new and promising approach for converting solar energy to chemical energy and the removal of pollutants from environment. Titanium dioxide is regarded as one of the best photocatalysts in several ways: it is relatively inexpensive, chemically stable, and has a wide band gap so that acts as a powerful oxidant.
The main challenge of photocatalysis with TiO₂ is the low efficiency due to electron/hole recombination and the lack of absorption of visible light. In this research we present a new synthetic method to integrate copper metal and TiO₂ by a redox-hydrolysis synergistic microwave hydrothermal reaction. Copper metal works as an electron sink to help the electron-hole separation. Moreover, the localized surface Plasmon resonance of metallic copper may make it possible for the TiO₂ layer to use visible light. The copper nanowire has a relatively large surface area and thin (about 10 to 20 nm) TiO₂ surface coating in order to assist the charge transfer between carriers approaching the wire surface and solution species.
The TiO₂ layer is crystalline so that the recombination centers from amorphous structure is limited. Because copper oxides are typical p-type semiconductors, this could be also helpful for a formation of a PN junction with TiO₂, a comparison between the combination of Cu-TiO₂ and CuO_x-TiO₂ has been carried out, to demonstrate the possible future development of photocatalyst based on resonance enhanced Cu doped titania. This catalyst shows a good hydrogen evolution rate under white light in the presence of a molecular electron donor. These results suggest the nanostructure combination between copper metal and titania is promising for the future development of high-efficiency solar spectrum titania based photocatalyst.

Production of hydrogen through splitting of water using solar energy in a photoelectrochemical (PEC) cell is one of the most attractive and environment friendly method. To design and develop a low-cost, efficient and durable PEC system, selection of semiconductor photoelectrode has become a vital task. Amongst the various semiconductors hematite has been proven to be a promising photo-electrode due to its abundance, chemical stability in aqueous environments and favourable band gap of 2.0-2.2 eV for solar energy absorption. However, its performance stands backside in PEC cell due to poor charge transport and band edge misalignment relative to redox level of water. Introduction of carbon nanotubes (CNTs) as a conducting scaffold can improve charge transport property of poorly conducting semiconductors like α-Fe₂O₃ by providing an expressway for electron transport and thus is expected to enhance the PEC response of α-Fe₂O₃ thin film.
The main focus in this paper is to modify the properties of hematite thin films with single wall carbon nanotube (SWCNT) in the direction of improving its PEC response. The fascinating and exceptional properties of CNT are not much exposed in such type of practical applications. To achieve the goal, SWCNT/ Fe₂O₃ thin films were obtained by coating hematite layer onto pre-deposited SWCNT thin film over conducting glass substrate. Suspension of SWCNT (purchased from Sigma Aldrich) in ethanol was spin coated onto ITO at 1000 rpm to obtain 2 and 4 layers of SWCNT thin films and afterward sintered at suitable temperature. Hematite thin films were obtained by using sol-gel spin coating method. 0.3 M solution of Fe(NO₃)₃.9H₂O was prepared using 2 Methoxy Ethanol (2-MET). The obtained solution was magnetically stirred for 2 hrs at and maintained at 80°C. Afterwards a reddish brown colloidal gel was obtained. The gel was then spiun coated over the predeopsited SWCNT films for 2 layers. Finally, the samples were sintered in the furnace at 500°C. These films were indexed as 2H (2 layers of Hematite), 2S-2H (2 layers of Hematite over 2 layers of SWCNT), 4S-2H (2 layers of Hematite over 4 layers of SWCNT). Thin films so prepared were implemented as photoanode in photoelectrochemical (PEC) cell for hydrogen generation. XRD, SEM, RAMAN, UV-visible spectroscopy and IPCE measurements were used to characterize the thin films. Increasing the layers of SWCNT was observed to increase absorption in visible region, thus clearly highlighting the synergistic effect of SWCNTs in the layered films. Also IPCE was found to increase 1.8 times for the best sample (4S-2H) as compared to the pristine sample at 550 nm. Sample 4S-2H exhibited 8 fold increase in photocurrent density as compared to the pristine film. Highest photocurrent density of 0.5 mA/cm² at 0.75 V/SCE was observed for 4S-2H sample. In virtue of their superior photoelectrochemical performance, SWCNTs/Fe₂O₃ thin films found substantial potential in PEC water splitting reactions.

Novel solar cells containing a p-type NiO layer were fabricated by a scalable reactive RF sputtering method using an inexpensive metallic Ni target. We eventually observed a detectable photovoltaic effect from this visible-light-transparent solar cells. Thin film NiO-based solar cell is attractive because their optical transparency permits greater flexibility in terms of installation locations.
Wide bandgap oxide semiconductors exclusively exhibit n-type conductivity and this fact gives technical limitations on their practical use. For instance, both p- and n-type layers having bandgap energies higher than 3.3 eV are essential to form a transparent p-n junction used for the visible light to pass through. Earth-abundant NiO shows only p-type conductivity, and is composed of inexpensive and less toxic elements. In addition, NiO films can be easily obtained by oxidizing a Ni film. The authors have proposed “NiO-based invisible solar cells [1]” that can supply a small electrical power for an “invisible sensor and/or transistor”. These solar cells can be installed on a wall or a ceiling without a wired connection. Moreover, by using an energy harvesting device, user will not be aware of these invisible devices and will not experience the stress associated with being sensed or monitored. Therefore, there is a great opportunity for various environments, and the application possibilities for invisible devices based on NiO-based solar cells are limited only by imagination.
In this presentation, we will introduce some fundamental features of NiO-based solar cells while focusing on their band diagram, transport properties, and optical properties. In addition, the interface properties of p-NiO / n-ZnO deposited by a conventional RF sputtering method will be shown: polycrystalline NiO and ZnO thin films with a thickness of approximately 200-400 nm were deposited by sputtering a metallic Ni and ZnO ceramics, respectively, and a mixture of Ar and O2 gases was used as the sputtering gas. The fabricated NiO-related solar cells exhibited a photovoltaic effect under illumination. Moreover, optical transmittance of greater than 70% was obtained in the wavelength range of 400-800 nm.
This work was supported in part by The Kurata Memorial Hitachi Science and Technology Foundation, and The Sasakawa Scientific Research Grant from The Japan Science Society.
[1] M. Warasawa, Y. Watanabe, J. Ishida, Y. Murata, S. F. Chichibu, and M. Sugiyama, Jpn. J. Appl. Phys. 52 (2013) 021102

Iron pyrite is one among the most promising earth abundant material candidates for photovoltaic applications having high absorption coefficient around 5 X 10⁵ cm^-1 at hv>1.4 eV and band gap of 0.95 eV. However, achieving thin films of pure pyrite phase (and minimizing other detrimental phases such as marcasite, troilite and pyrrhotite) and maintaining the S:Fe ratio at 2:1 through the entire film thickness, has been challenging and contributed as main limiting factors for achieving descent iron pyrite based solar cell efficiencies. In this work, we report synthesis of pyrite thin films using either Di-tert-butyl disulfide (TBDS) or Hydrogen sulfide (H₂S) in one-step sulfurization of iron oxides films on bare and Mo-coated rigid and flexible glass substrates. Evolution of pyrite films with sulfurization time was also studied. Synthesized films were characterized for their surface morphology, phase identification and chemistry using SEM, XRD, Raman Spectroscopy, XPS and TEM. Electrical and optical properties of these films were also studied. Surface along with bulk chemistry of the synthesized films was investigated in detail using XPS. The S:Fe atomic ratio as well as their chemical bonding states were monitored to obtain and maintain 2:1 ratio through the entire film thickness as a function of the sulfurization time by performing XPS depth profile using Ar sputtering. Also, the effect of TBDS and H₂S on sulfur diffusion within the film is presented. Atomic resolution images of the bulk film were obtained with TEM. The potential of the iron pyrite thin films that were grown on flexible glass was also discussed for flexible solar cell applications.

Ternary and quaternary copper-based chalcogenides are garnering increased attention for use as photocathodes for photoelectrochemical (PEC) solar-to-hydrogen conversion given their easily tunable electronic and optical properties together with outstanding photocurrent values and decent onsets for hydrogen evolution. To date, high-performance photocathodes have been prepared by costly and unscalable techniques such as vacuum co-evaporation methods, whereas cost-effective solution processing routes have yet to show comparable photoelectrochemical properties. Indeed, while colloidal nanocrystals inks of such chalcogenides have already proven to be promising building blocks for preparing active layers for photovoltaic application, their application in PEC devices has been disregarded to date.
In this work, colloidal inks of ternary and quaternary chalcogenides (e.g. Cu(In,Ga)S₂ and Cu₂ZnSnS₄) were synthesized and used to prepare highly crystalline photocathodes. We employ a bottom-up approach where the introduction of different dopants (antimony or bismuth) act as flux agents, promoting the grain growth of the chalcogenide nanocrystals and reducing the temperature required for annealing. In such a way, the as-deposited nanocrystals with an average size of 20 nm turn into well-defined crystallites of micrometer size. Interestingly, the grain growth induces an enhancement in the PEC response of more than one order of magnitude reaching values of ge;1 mAcm^-2 (at 0 V vs. RHE), presumably due to improved charge transport caused by the reduction of grain boundaries. Additionally, surface modification with different overlayers (CdS, CdSe, ZnSe, etc.) demonstrated further enhancement of the PEC performance, boosting the photocurrent values (ge; 3mAmiddot;cm^-2, at 0 V vs. RHE) and shifting the corresponding onset up to 0.4 V (vs. RHE) in the absence of any co-catalyst. Surprisingly, both bare and CdS-coated photocathodes show long-term stability without ubiquitous protective layers of TiO₂ or AZO:TiO₂ indicating efficient charge separation and high surface catalytic activity. Overall, these results pave the way for the development of a new generation of solution-processable and high-performance copper-based chalcogenide photocathodes.

Efficient and low-cost solar energy conversion devices require large active surfaces of thin films with controlled properties. For large area deposition the properties are more difficult to maintain, and therefore a careful choice of the deposition techniques are required to deposit individual layers. Chemical techniques of electrodeposition and spray pyrolysis are very versatile and economic for development of solar absorbers using earth abundant inorganic materials. In the present work we report on the surfactant-free synthesis of SnS nanoparticles as sensitizers on ZnO nanowire arrays leading to the fabrication of extremely thin absorbing layers prepared using continuous spray pyrolysis (CoSP) and electrodeposition, respectively. The high-surface-area nanowire arrangement was precisely controlled by electrodeposition parameters and these nanowires were over coated by SnS using CoSP technique. Both the deposition technique were optimized to produce cost effective nanostructures on large areas possessing a high degree of crystallinity of SnS nanocrystals and ZnO hexagonal prismatic nanorods. This method allows a facile and rapid deposition of SnS nanostructures on ZnO nanowire arrays without the need for post thermal treatment. The morphology and crystalline phase of the ZnO/SnS nanostructures were studied by scanning electron microscopy and X-ray diffraction. Conducting AFM was conducted to the see the junction formation. Coverage of ZnO nanowires with SnS nanoparticles creates p-n junctions for photovoltaic effect. This paper will report on fabrication details and analysis of extremely thin absorber n-ZnO/p-SnS structure on ITO-coated glass substrates.

Semiconductor photocatalysis has emerged as an interesting area of research since the discovery of Honda-Fujishima effect.^[1] The practical realization of a hydrogen-based energy economy calls for the design and development of efficient and earth abundant photocatalytic materials. In this study, TiO₂/MoO₂/graphene composites have been prepared by a solar radiation-assisted co-reduction method, wherein ammonium tetrathiomolybdate salt and graphite oxide are reduced to MoO₂ and graphene respectively along with TiO₂. The method involved the utilization of focused pulses of natural sunlight using a simple convex lens,^[2] thereby eliminating the need for harmful reducing agents. The compound was characterized by XRD and SEM for phase identification and morphology. The TiO₂/MoO₂/graphene composite exhibits superior photocatalytic water splitting activity without using any co-catalyst. In addition, we demonstrate the electrocatalytic hydrogen production using this earth abundant catalyst, which shows high current density (60 mA/cm²) and low Tafel slope (47 mV/dec). The hydrogen evolved during photo-electro catalysis was detected by gas chromatography.
References
[1] Fujishima A, Honda K., Nature, 283, 37 (1972)
[2] S. Ramaprabhu et al., US Patent Publication, US20130056346 A1 (2013).

#8203;ZnSnN₂, a Zn-IV-N₂ compound semiconductor, is a candidate “earth-abundant element” alternative to traditional materials for photovoltaic (PV) applications. In addition to having low toxicity, both zinc and tin have well established domestic recycling infrastructures already in place, ensuring future availability. While conventional alloying with other members of the Zn-IV-N₂ family offers a range of band gaps stretching from the infrared to the ultraviolet, controlled introduction of cation sublattice disorder offers the opportunity to directly engineer the band gap to match the terrestrial solar spectrum without the introduction of either Ge or Si. Here we report experimental evidence for this band gap tuning using a combination of optical absorption, Hall effect, and x-ray photoelectron spectroscopy, which are compared to density functional theory (DFT) calculations. All single-crystal thin films were grown by plasma-assisted molecular beam epitaxy using yttrium-stabilized zirconia (111) substrates.
DFT calculations of the bandstructure for both fully ordered and fully disordered cation sublattices predict a reduction of almost 1 eV from the 2.03 eV band gap of the fully ordered structure. To explore this experimentally, a range of thin films were grown under different conditions to obtain a range of disorder. When optical absorption measurements are corrected for Burstein-Moss shift using measured electron concentration and extraction of the effective mass from DFT computed bandstructure, a clear trend emerges with films having higher quality and less disorder corresponding to larger band gaps. Further support for this conclusion was provided by comparing valence band and shallow core level spectra obtained using high-energy x-ray photoelectron spectroscopy (HAXPES). Distinct differences in the calculated density of states as well as Sn 4d to Zn 3d separations are readily apparent in the experimental data, with the film of poorer quality much closer to what is predicted for the fully disordered structure, and a film of much higher quality exhibiting features close to what is expected for the fully ordered structure.
This work was supported in part by NSF grants DMR-1244887 DMR-1410915, and EPSRC grant no. EP/G004447/2.

Complex metal oxides such as perovskites and spinels represent a rich family of materials featuring a wide range of optical and electronic properties. Research in the area of solar energy conversion has been mostly dominated by a relatively narrow range of oxides including TiO₂, ZnO, Fe₂O₃ and Cu₂O. Although the latter may also exhibit p-type conductivity, oxide materials are commonly used as photoanodes, particularly in the field of solar fuels. In this contribution, we shall describe the preparation and properties of nanostructured photocathode materials based on LaFeO₃ and Cu_xZn_1-xBi₂O₄.
The preparation of these materials followed a novel solution processable route based on ionic liquid/cellulose mixtures. Dehydrated precursors are generated via a simple evaporation route to ensure complete sequestration of metal ions into the ionic liquid phase, with the incorporation of cellulose as weakly-coordinating chelating agent. Upon calcination of the ionic liquid precursors, mixed oxides with a high degree of phase purity and high yields can be obtained. These oxides are obtained as particles with sizes in range of 50 nm, which can be later deposited at electrodes employing conventional techniques such as screen-printing.
The LaFeO₃ nanostructures synthesized by this new method is characterized by a direct band gap of 2.56±0.07 eV. Photoelectrochemical studies in Ar-saturated solutions at pH 12 provides clear evidence of direct hydrogen photogeneration at potentials up to 1 V more positive than the formal hydrogen potential. Our results also show that the potential associated with the valance band edge overlaps with the reversible potential for the oxygen evolution. The properties of the materials as photoelectrodes for direct H₂ generation will discussed based on the photocurrent dependence on photon flux and electrode potential. In the case of Cu_xZn_1-xBi₂O₄, the H₂ generation efficiency is clearly affected by the Zn content. The potential impact of these materials not only in solar fuels but also the field of all-oxide photovoltaic devices will be briefly discussed.

Metal oxides are interesting materials for developing all-oxide thin film photovoltaics. They are cheap, abundant, mostly non-toxic and suitable for multi-junction cells. The optical material properties, such as band gap and adsorption coefficient, are compatible for converting sunlight to electricity. However most metal oxides have limited electrical properties with short charge carrier lifetime and low mobility. Those limitations need to be addressed in order to make efficient all-oxide photovoltaics.
Improved performance of oxide thin films on amorphous/poly-crystalline glass substrates could be realized by increasing the grain size and crystallinity. This can be achieved by using nanosheet engineering where highly crystalline nanosheets are positioned on the glass substrate or FTO/ITO electrode layer before growth of the oxide thin films. Such 1 nm thin nanosheets can consist of titanates or calciumniobates and have individual dimensions of several square micron.
Thin films of TiO₂ and ZnO are commonly used as electron transport layers in combination with cuprous oxide absorber layers for all-oxide photovoltaics. However, current photovoltaics with cuprous oxide absorber layers exhibit low efficiencies, far below the Shockley-Queisser limit, and improvement of the thin film quality is critical. We have deposited these oxide multilayers were deposited on glass/ITO substrates by pulsed laser deposition. Nanosheet engineering was applied to determine the effect on the structural and electrical properties. In-situ XPS characterization and ex-situ XRD analysis show in detail the enhanced crystalline ordering, while the photovoltaic performance was studied as well. Enhancement of thin film properties on cheap, low quality substrates by nanosheet engineering could contribute significantly to thin film all-oxide photovoltaics as future solar cells.

The importance of oxygen evolution reaction (OER) has been well recognized for electrolysis, photoelectrochemical water splitting and solar fuel productions, yet significant challenges still remain to find low cost catalyst materials that are both efficient and stable. Towards the search for OER catalysts based on earth-abundant materials, there has been a growing interest to investigate lithium-containing oxides, previously known as cathode materials of lithium ion batteries. For example, LiCoO₂ and LiCoPO₄ were studied thoroughly for OER catalysis in both alkaline and neutral electrolytes.¹ Structural and electrochemical analysis of LiCoO₂ and LiCoPO₄ revealed the influence of surface composition and structure on the catalysts&’ steady state activity. Systematically examination of LiMn_1.5Ni_0.5O₄ with different surface planes/facets showed its OER activity increased in the order of truncated octahedral < cubic < spherical < octahedral.² More recently, a new catalyst based on lithium manganese pyrophosphate was studied for water oxidation at neutral pH, from which the role of Mn valency in OER catalysis was elucidated via controlled delithiation (Li_2-xMnP₂O₇, x=0, 0.3, 0.5, 1).³ In this study, we report for the first time hydrothermal synthesis of lithium cobalt germanate (Li₂CoGeO₄). Elemental composition, morphology and crystal structure of this compound were characterized by various analytical techniques including SEM, TEM, ICP, and XRD analyses. Structure refinement and DFT calculation suggests the resulting Li₂CoGeO₄ from hydrothermal synthesis has a crystal structure isostructural with Li₂ZnGeO₄, significantly different from previous reports. Electrochemical studies confirmed Li₂CoGeO₄ as an active catalyst for oxygen evolution reaction (OER). In alkaline electrolyte (0.1 M NaOH), rotating disk electrodes made with Li₂CoGeO₄ as catalyst have a Tafel slope of c.a. 67 mV/dec and an overpotential of 330 mV at 50 mu;A/cm²_cat, about 90 mV less than electrodes containing another known OER catalyst, Co₃O₄. X-ray photoelectron spectroscopy analyses revealed surface cobalt was oxidized from 2+ to 3+ during the reaction, in agreement with observation from cyclic voltammetry studies.
(1) S. W. Lee et al., JACS, 2012, 134, 16959-16962.
(2) T. Maiyalagan, K. R. Chemelewski and A. Manthiram, ACS Catalysis, 2013, 4, 421-425.
(3) J. Park et al., JACS, 2014, 136, 4201-4211.

Zinc oxide doped with Ga or Al forming n+ conducting high band gap (3.34 eV) layer combined with low 2.4 eV band gap n-CdS buffer layer constitutes an integral part of the semiconductor window layer in thin film heterojunction solar cells. Replacing toxic heavy metal Cd is highly desirable from ecological considerations both in solar cell fabrication and deployment. With current perspectives on solar cells based on earth abundant non-toxic photo-absorbers, Cd free alternative to CdS buffer layer is an important milestone in technology of these solar cells. In this context, ZnS and Zn(S,OH), ZnS(O,OH) or Zn(S,O,OH) as buffer layers are investigated. An important aspect not adequately addressed in thin film solar cell buffer layers is mismatch in band alignment with p-type absorber layers of low 1.1-1.5 eV band gap. This work investigates bilayer n-ZnO_1-xS_x/n-ZnO structures as a possible Cd-free buffer layer in thin film solar cells and show band gap can be engineered by varying S (x=0-0.4) due to raised valence band energy following bowing effect of alloying S anion of differenrt electronegativity at O-sites.
The n-ZnO_1-xS_x/n-ZnO bilayer structures were synthesized with controlled anion (S) doping by substituting O-atoms over surface region of n-ZnO film. This is achieved by chemical vapor sulfurization of sputter deposited 35-75 nm thick ZnO film at 400°C under 100 sccm flow of di-tert-butyl disulfide (TBDS) vapor preheated to 100°C. S-radicals formed by pyrolysis of TBDS by diffusion in ZnO substitute at O-sites to produce S-alloyed layer ZnO_1-xS_x at the surface. The flow rate and time was used to control S-alloying with x=0.1-0.9. X-ray diffraction study over various sulfurization times show systematic diminishing of the ZnO (002) and evolution of the ZnS(111) diffraction peaks confirming S-diffusive-alloying of ZnO. X-ray photoelectron spectroscopy depth profile study of ZnO_1-x S_x (x=0.9) film was modeled to elucidate S-diffusion kinetics. Distinctive changes in Raman spectra of ZnO_1-xS_x films were interpreted for S- substitution at O-sites by comparing ZnO and ZnS films spectra. Band gap of n-ZnO_1-xS_x/n-ZnO bilayer structures was studied by optical absorption spectroscopy. The absorption coefficient values were fitted in the Tauc relation for direct band gap and optical band gap was determined. The decrease in band gap from ZnO value 3.21 eV is consistent with the time evolution of the S-alloying process converting ZnO surface layer to ZnO_1-xS_x with varying composition x. The optical band gap lowering to 2.4 eV was realized which is close to the band-gap alignment typical for CdS buffer layer used in thin film solar cells. Thin film heterojunction device in the superstrate structure, glass/ITO/n-ZnO_1-xS_x/n-ZnO/p-CuSbS₂/Ag was fabricated and photovoltaic performance of the bilayer buffer was evaluated. This paper will report the results of these investigations.

Solar thermal fuels (STF) store the energy of sunlight, which can then be released later in the form of heat, offering an emission-free and renewable solution for both solar energy conversion and storage. Recently, we have predicted a new class of functional materials based on templated azobenzene derivatives that have the potential to address these challenges [Kolpak and Grossman, 2011]. However, to release the energy stored in STF efficiently, it is highly desirable to design a catalyst that can trigger the energy-releasing backward reaction at room temperature. Using ab initio simulations, we discover that the adsorption energy of trans and cis azobenzene can be tuned through straining of metal surfaces such as copper and gold. We show further from simulations that this phenomenon can be used to tune the catalytic property of metal surfaces for the energy-releasing backward reaction of azobenzene. With charge density and band structure analysis, we also discuss the physical mechanism underlying the change of adsorption properties of azobenzene on strained metal surfaces.

Understanding semiconductor surface states is critical for their applications, but fully characterizing surface electrical properties is challenging. Such challenge is especially crippling for earth-abundant semiconductor iron pyrite (FeS₂), whose promise for solar energy conversion has been suggested to be held back by rich surface states. Here, by taking advantage of the high surface-to-bulk ratio in nanostructures and effective electrolyte gating, we develop a general method to fully characterize both the surface inversion and bulk electrical transport properties through electrolyte-gated Hall measurements of pyrite nanoplate devices. Our study directly confirms that pyrite is n-type in the bulk and p-type near the surface due to strong inversion and yields the concentrations and mobilities of both bulk electrons and surface holes. Further, solutions of the Poisson equation reveal a high-density of surface holes accumulated in a 1.3 nm thick strong inversion layer and an upward band bending of 0.9-1.0 eV. This work suggests that the high-density of surface states and the thin inversion layer are key factors contributing to the low solar performance of pyrite, especially helping to explain the universal p-type conductivity and lack of photovoltage in polycrystalline and nanocrystalline pyrite. Moreover, we found that in pyrite single crystals, in addition to surface states, high-density deep donor states are found as intrinsic bulk defects to severely limit the open circuit voltage of pyrite. Both high-density bulk and surface states are responsible for low solar performance of semiconducting iron pyrite.

Different strategies have been used to develop photocatalysts that function under visible-light irradiation to efficiently utilize solar energy to produce hydrogen from water (i.e. water splitting), reduce CO₂, or decompose toxic organic chemicals. The intimate combination of two semiconductors, one active in visible light and the other in ultraviolet, is a synthesis strategy that has been shown to lead to formation of composites with enhanced visible light photocatalytic activity. ^{1, 2} We have modified the surface of UV light active photocatalyst Tantalum oxide nanoparticles by in-situ synthesis of ultrafine visible light active bismuth oxide using benzyl alcohol. The Bi₂O₃/Ta₂O₅ composite obtained was characterized by powder X- ray diffraction (PXRD), thermogravimetric analysis (TGA), Brunaer-Emmett-Teller (BET) surface area measurement, microprobe techniques (SEM and TEM), diffuse reflectance UV-Vis spectroscopy. Photocatalytic activity was evaluated by studying the generation of hydrogen from water under visible light irradiation (l sup3; 400 nm). The results show that the as-prepared composite extends the light absorption range and restrict photogenerated charge-carrier recombination, resulting in enhanced photocatalytic activity for hydrogen evolution.
References
(1) Liu, H.; Shon, H. K.; Sun, X.; Vigneswaran, S.; Nan, H. Appl. Surf. Sci.2011, 257, 5813-5819.
(2) Parida, K. M.; Nashim, A.; Mahanta, S. K. Dalton Trans2011, 40, 12839-12845.

It has been show that a tandem/multi-junction solar cell consisting of a single crystal silicon cell paired with a cell of band gap near 1.9 eV can yield efficiencies in the range of 35%. The most common partner that has been studied to date is Cu₂O. However, this material degrades to CuO resulting in a substantial decrease in efficiency. Another excellent candidate for an earth abundant absorber material is WS₂ which can be directly grown as a p-type semiconductor with a band gap near 1.9 eV. In this work we present the structural, optical, and electrical properties of thin film WS₂ grown via the sulphurization of sputter deposited tungsten films. We will show that highly textured films with an optical band gap in range of 1.9 eV can be reproducibly grown. In addition we will present the optical constants, and absorption coefficient as well as Hall mobilities and carrier density as a function of process conditions. Preliminary results on doping with halogens to make n-type material will also be discussed. We will employ these results to numerically simulate WS₂ solar cells as well as tandem/multi-junction structures with silicon.

Hydrogenated amorphous silicon carbide (a-SiC:H) exhibits a great potential as a semiconducting photocathode material for photoelectrochemical (PEC) water splitting due to its narrow band gap, its high stability and its composition made from earth abundant elements. Several limitations for a-SiC:H include the large onset potential which is due to the interfacial energetics at the semiconductor-liquid junction, and the slow reaction kinetics on the phototocathode surface. To address the former, we deposited an n-type thin titanium dioxide (TiO₂) layer onto the p-type/intrinsic a-SiC:H, forming a p-i-n junction. The p-i-n structure created an internal electric field that increased the built-in potential, which subsequently improved the drift mechanism of separated charge carriers across the intrinsic layer. In addition, we investigated several doping strategies to alter the compositional structures of a-SiC:H photocathodes to control the interfacial energetics, which showed a variation of photocurrent onset potentials. Finally we have investigated various noble metal-free hydrogen evolution catalysts deposited on a-SiC:H photocathodes, showing enhanced surface reaction kinetics at significantly lower bias voltages. Electronic band diagrams have been developed to illustrate the interfacial energetics of the a-SiC:H photocathodes.
This study identified and addressed the possible performance limitations for a-SiC:H photocathodes, and achieved a significant advance in constructing a highly efficient solar water splitting device.

The need for sustainable energy sources is always current and photovoltaics can be one of the main solutions. Of the major thin film alternatives, Cu(In,Ga)Se₂ (CIGS) is the highest performing, but due to the high price and limited availability of the metal indium, interest has been focused on a search for replacements for this material. One of the most promising alternatives is Cu₂ZnSnS₄ (CZTS), which is a p-type semiconductor with a band gap of 1.4-1.5 eV, close to the optimum value for solar energy conversion [1].
In this work, we investigate the synthesis of CZTS thin films using thermal evaporation from Cu, Zn and Sn pellets in a metal precursor stack on Mo-coated soda-lime glass substrates and post-annealing of the stack in a sulfur atmosphere. X-ray diffraction and Raman scattering results show that the films are kesterite phase CZTS. Grain sizes of 1 um or above are obtained from SEM images. A solar cell built from CZTS thin film exhibited an open-circuit voltage of 410 mV, a short-circuit current of 8.59 mA/cm², a fill factor of 0.41 and a conversion efficiency of 1.46%.
Exploring the limiting factors of poor performing CZTS solar cells, we varied the Cu and Zn layer thicknesses in precursors to obtain CZTS thin films with various compositions. De-ionized (DI) water soaking has also been tested. Through capacitance-voltage measurements, we have determined the carrier concentration profile of CZTS thin films. It was observed that Cu deficiency leads to CZTS thin films with lower carrier concentration. This might be due to a reduction in Cu_Zn antisite defect formation which may cause the higher carrier concentration in near stoichiometric films. This is consistent with the reported optimal composition of CZTS absorber for making solar cells (Cu/(Sn+Zn)=0.8-0.95, Zn/Sn=1.1-1.25) [2]. Meanwhile, DI water soaking is found to reduce the carrier concentration near the CZTS surface. The reason for this effect, which has been reported elsewhere [3], is not clear. It may be related to the removal of interfacial defects and/or impurities after soaking. It was also found that the CZTS/CdS interfacial carrier concentration peaks at some depth in some of our devices. Such a carrier concentration profile may influence the device performance by bending the energy bands outside the space charge region in a way that reduces the carrier collection efficiency. Further study on detailed carrier concentration depth profile with various film compositions and selective etching techniques are needed.
References
[1] H. Flammersberger, Experimental study of Cu₂ZnSnS₄ thin films for solar cells, Uppsala Universitet, 2010.
[2] A. Fairbrother, Development of a Selective Chemical Etch To Improve the Conversion Efficiency of Zn-Rich Cu₂ZnSnS₄ Solar Cells, J. Am. Chem. Soc. 2012, 134, 8018minus;8021.
[3] H. Katagiri, K. Jimbo, W. Maw, K. Oishi, M. Yamazaki, H. Araki, A. Takeuchi, Development of CZTS-based thin film solar cells, Thin Solid Films 517 (2009) 2455-2460.

The CuSbS₂ belonging to simple ternary Cu-Sb-S system is a potential material among the earth abundant photo-absorbers. It effectively blocks Cu migration instability in erstwhile Cu_2-xS/CdS solar cells, can replace expansive In and Ga in popular Cu(InGa)(SSe)₂ (CIGS) solar cells and in composition more tolerant than current favorite Cu₂ZnSnS₄ (CZTS) solar cells. CuSbS₂ has a direct band gap 1.38-1.56 eV and high absorption coefficient, 1-5x10⁴ cm^-1 both relevant for photovoltaic solar cells. Even with extensive research in CuSbS₂ thin films, there is no unanimity on opto-electronic properties, band-gap and p-type doping, due to sensitivity of the structure and composition to thin film preparatory methods. Inclusion of secondary phases, CuS, Sb₂S₃ and Cu₃SbS₄ in CuSbS₂ synthesized by direct methods, chemical spray or vacuum deposition and indirect methods, sulfurization of Cu and Sb multilayers in S or H₂S vapor are the constrains hampering solar cell development.
This research provides a new perspective on single step synthesis of photosensitive monophase CuSbS₂ films by electrodeposition in ionic liquid electrolyte based on choline chloride and urea (ChCl:U) eutectic mixture. We show that CuSbS₂ thin film form by co-deposition of constituents Cu, Sb and S in a self-regulating electrochemical environment. Chemical stability of ChCl:U over wide potential range and at high temperatures enables better composition control by unrestricted choice of deposition potentials and highly crystalline phase. Biodegradable and nontoxic ChCl:U enables multiple depositions for low cost solar cell fabrication using green methods.
In this work, Cu and Sb deposition potentials using CuCl₂ and SbCl₃ precursors in ChCl:U are established as -0.54V and -0.48V vs. Pt, respectively by cyclic voltammetry. Corresponding, binary Cu_xS and Sb_xS_y electrodeposition was realized with added Na₂S₂O₃ as sulfur source. Based on this data,CuSbS₂ films deposition was optimized for potentials and bath composition. X-ray diffraction studies show monophase CuSbS₂ films in the chalcostibite orthorhombic crystal structure deposit at -0.65V vs. Pt. in ChCl:U with 1:1 Cu/Sb precursor ratio. Deviant Cu/Sb ratio study revealed Cu₃SbS₃ and Sb₂S₃ inclusion at 1:0.071 and 1:1.4, respectively. Electrochemical impedance studies show film growth kinetics by diffusive ionic transport. Direct band gap of 1.64 eV for single phase CuSbS₂ film and 1.74 eV with inclusive secondary phases was established by optical absorption. Electrical conduction study shows formation of p-type CuSbS₂ films. Heterojunction device in Glass/TCO/nZnO/pCuSbS₂/Ag structure with rectifying I-V curves also confirms it. This paper will present the ionic liquid electrodeposition, growth mechanism, structure and optoelectronic properties of CuSbS₂ films and the photovoltaic characteristics of heterojunction solar cells based on electrodeposited p-CuSbS₂ thin film photo-absorber with n-ZnO window layer

Oxidation of water as an abundant and common oxidant is essentially required for all solar-fuel devices, because water-oxidation components provide the protons and electrons needed for water reduction and/or CO₂ reduction at the photo-cathode side. To date, all existing non-oxide photovoltaic materials are not stable for water oxidation. A recently developed protective coating strategy that utilizes atomic layer deposited (ALD) TiO₂ show promises for stabilizing efficient non-oxide semiconductor light absorbers, like Si and GaAs, against photo-corrosion or passivation. Successful extension of this strategy could largely leverage the materials development and scalable manufacturing in the photovoltaic industry, thus significantly advancing the field of solar fuel production.
Here, we will show that the ALD-TiO₂ protection strategy can be extended to ternary semiconductor compounds like GaAsP with direct band gaps of 1.7-1.9 eV and II-VI semiconductors like CdTe with 1.4 eV band gap. Because ALD is a conformal coating process, we have also stabilized Si and GaAs wires for efficient, quantitative and stable oxidation. For example, ALD-TiO₂ coated Si microwires show 400 mV photovoltage, 5 mA cm^-2 photocurrent density, with ~100% Faradaic efficiency and >50 hours stability for water oxidation. GaAs nanowire arrays with 4% filling fraction enhance their optical absorption at broad wavelengths and incident angles, thus reducing the materials usage without sacrificing their performance. In addition, the photocurrent per electrochemical active surface area of wires as compared to planar photoanodes is dramatically reduced. We will also discuss further study of TiO₂/light absorber interfaces in order to improve the performance of protected III-V and II-VI photoanodes.

Minority carrier lifetime (tau;) is an all-important figure of merit for any photovoltaic (PV) absorber, and much ongoing research in PV can be fairly described as attempts to improve the lifetime. Therefore, reliable lifetime measurements are essential to enable rapid and rational engineering of PV technologies. For wafer-based PV using legacy materials such as Si and GaAs, measurements of tau; are relatively straightforward. The bulk lifetimes tend to be fairly long (over 1 mu;s), and passivation techniques to reduce the surface recombination velocity are widely known. Unfortunately the same cannot be said for most thin film PV materials. The bulk tau; is typically on the order of 1 ns, even for high performing materials such as polycrystalline CdTe [1]. Furthermore, surface passivation is generally not well understood and therefore it is difficult to experimentally distinguish the time scales for non-radiative recombination in the quasi-neutral, space charge, and interface regions.
Here we report minority carrier lifetime measurements on SnS thin films and semi-fabricated solar cells. SnS is an attractive material for thin film PV. It has strong manufacturing advantages and has seen recent efficiency gains, including a current record 4.36% certified, 0.24 cm² area device [2]. Numerical modeling suggests that bulk tau; is on the order of 100 ps in the record devices, and efficiencies of 10% or greater could be readily achieved with bulk tau; on the order of 1 ns [3].
We use an optical pump and a THz probe to measure time-dependent free carrier absorption (FCA), which is a direct measure of the excess carrier concentration. We model our data with diffusive charge dynamics, and we distinguish surface from bulk recombination by simultaneously fitting the model to multiple data sets acquired with different pump colors. We use these first-ever lifetime measurements on SnS to corroborate and improve our numerical device modeling. We show that THz lifetime measurements can be used to provide rapid feedback to bulk and interface processing such as controlled annealing and native oxidation. Finally, we briefly review bulk and interface processes that improve the lifetime and enable further PV efficiency gains.
1. T. A. Gessert, et al., 37^th IEEE Photovoltaic Specialist Conference (2011).
2. P. Sinsermsuksakul, et al., Adv. Energy Mater. (accepted, 2014).
3. N. Mangan, et al., 40^th IEEE Photovoltaic Specialist Conference (2014).

A simple and inexpensive electrodeposition method is introduced to fabricate a highly efficient n-Si/SiO_x/Co/CoOOH photoanode for the photoelectrochemical oxidation of water. A thin layer of cobalt is electrodeposited onto n-silicon and the surface layer of cobalt is photo-oxidized to cobalt oxyhydroxide. The photoanode generates a 0.47 V photovoltage, 35 mA cm^-2 short-circuit photocurrent density, 0.62 fill factor, and 10.2% conversion efficiency with 100 mW cm^-2 simulated sunlight. The interfacial energetics are quantitatively analyzed. The photoanode has a barrier height of 0.91 eV, compared with the 0.71 eV barrier height that is expected for a solid-state Schottky barrier. This leads to a 260 mV enhancement of the photovoltage, which is attributed to an adventitious 0.5 nm thick SiO_x tunnel junction that minimizes surface states. The n-Si/SiO_x/Co/CoOOH photoanode functions as an efficient solid-state metal-insulator-semiconductor photovoltaic cell in series with a low overpotential water-splitting electrochemical cell.

Semiconductor hybrid nanostructures have tremendous potential in energy technology. The use of nanostructures reduce material usage and improve efficiency in photovoltaic devices. ZnO-CuO hybrid nanostructures have a perfect band alignment which favor its application in photovoltaic cells. Introduction of these unified electronic materials can form tandem solar nano devices for energy applications. Here, we report structural and optical characteristics of ZnO-CuO hybrid nanostructures fabricated by hydrothermal process. Hybrid nanostructures were synthesized onto indium tin oxide (ITO/ZnO) thin film substrates by energy efficient hydrothermal process by changing the pH of the precursor solution from 10 to 13 and temperature from 50 C to 200 C. X-ray diffraction, Raman spectroscopy, photoluminescence spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy were employed to characterize these exclusive nanopowder. Rietveld analysis of XRD data confirm the phase mixture nanocomposite of ZnO-CuO with different ratios of ZnO/CuO. From SEM it is proved that the nanocomposite are nanorods type structure. The length of the nanorods vary from 50 nm to 800 nm. EDX data showed that the nanorods are the composite of ZnO-CuO- Cu₂O. Raman spectra of nanocomposite consists of optical vibrational mode of ZnO, CuO, and Cu₂O. The photoluminescence spectra has peaks at 390 nm, 540 nm, 620 nm and 775 nm which confirm that the nanocomposite absorb in the solar visible region. Current-voltage behavior showed the diode characteristics of the nanopowder based devices with a cut off voltage of 0.5 V. This synthesis process can be beneficial for fabricating nanoscale devices used for optoelectronic applications.

Copper Indium Gallium Diselenide (Culn_xGa_1-xSe₂, CIGS) is a I-III-VI₂ semiconductor material of chalcopyrite structure which is used as an absorber layer to produce high efficiency with low cost solar cells. Chemically synthesized CIGS nanoparticles have drawn much attention due to their size dependent unique optical and electrical properties. In this present investigation, CIGS nanoparticles were synthesized by dissolving 1:0.7:0.3:2 molar ratio of copper (I) chloride, indium (III) chloride, gallium (III) chloride and elemental selenium in oleylamine (OLA). The solution was heated to 260°C with constant stirring under nitrogen atmosphere by varying time from 2h to 8h. When the reaction was terminated, the solution was cooled down to room temperature. CIGS nanoparticles purified with ethanol and chloroform using centrifugation process. Synthesized CIGS nanoparticles were characterized using X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), energy dispersive x-ray analysis (EDAX) and UV-VIS-NIR (Ultraviolet-Visible-Near infrared) spectroscopy. From XRD results, we observed the phase transformation from ternary (CIS) to quaternary phase (CIGS) as time increases. In Raman spectra, the characteristic peak of CIGS is obtained around 177cm^-1 after 4h. FESEM micrograph depicts CIGS nanoparticles size which varies from 20 to 80nm (2-8h). EDAX analysis confirms the composition of CIGS which is close to the desired stoichiometry. The bandgap of CIGS nanoparticles is found to be 1.2eV using UV-VIS-NIR absorption spectrum. To sum up, the reaction time plays a crucial role to get single phase of CIGS. These CIGS nanoparticles can be deposited as a thin film for solar cell applications.

Cu/ZnO systems are well known and used commercially for high temperature thermal synthesis of methanol from syngas. We propose a photoelectrochemical system for this process, that is carbon dioxide reduction, under milder, room temperature reaction conditions via the fabrication of sputtered Cu catalysts supported on Si photocathodes coated with Atomic Layer Deposition layers of ZnO (ZnO_ALD). The effect of ALD layer thickness, addition of dopants, and pre-treatment such as annealing of the ALD layer and/or catalyst will be evaluated. The electrode properties, such as morphology, crystal structure of the layers, and optical response, will be probed via SEM, XRD, UV-Vis spectroscopy and ellipsometry. Product selectivity and faradaic efficiency for CO₂ reduction over the novel catalyst will be probed in a 3-electrode photoelectrochemical cell with online mass spectroscopy (MS) and liquid and gas phase gas chromatography-mass spectroscopy (GC-MS). The product distribution as a function of potential in the dark over Cu and Cu/ZnO_ALD/n⁺-Si will be compared to illuminated Cu/ZnO_ALD/p-Si.

Nanoparticle suspensions, or nanofluids, have promising applications in solar-thermal energy conversion, due to their ability to efficiently absorb sunlight and even concentrate solar irradiance using plasmonic effects. Several investigative works have focused on the formation of local steam nanobubbles around the individual nanoparticles, using high intensity lasers (asymp;10¹⁰-10¹² W/m²) for excitation. More recently, a series of studies were published proposing a mechanism for solar steam generation via localized heating around the nanoparticles. However, the mechanism behind the nanobubbles formation is still not completely understood and is a subject of ongoing research debate. Here, we will report on the results of our experimental and computational studies that suggest steam generation in nanofluids is caused by non-localized heating of the fluid surrounding the nanoparticle. We measure the temperature (up to 100°C) and vapor generation rates (76±4%) of graphitized carbon black, carbon black, and graphene flake water-based nanofluids under broadband solar illumination at low concentration (10 suns, 10⁴ W/m²). A transient macroscale heat transfer model is developed based on heating of the bulk nanofluid, and the results match closely with the experiments. Further numerical models are developed to elucidate more clearly the global heating effects of the nanoparticles, which arise due to the particle concentration dependent overlapping of the thermal boundary layers. Lastly, the vapor generation rates of the nanofluids are used to explain the results of previous studies in an attempt to reconcile the discrepancies between the experiments in the literature. This work aims to theoretically and experimentally describe the solar-assisted generation of steam using carbon particle based nanofluids, and has implications for a wide range of applications such as renewable steam generation, desalination, distillation, and power generation. This work was supported by the Cooperative Agreement between the Masdar Institute of Science and Technology, UAE and the Massachusetts Institute of Technology (MIT), USA.

Tin (II) Monosulfide (SnS) has theoretical promise as a new material for thin film photovoltaics (PV). Despite a full decade of rigorous research to develop SnS-based devices, improvement beyond single-digit percent efficiencies seems unattainable. Engineering this material into a usable device is crucial for future development. Our group has been investigating the optical and structural properties of magnetron sputtered SnS_x thin films from a SnS₂ target [1,2,3,4]. This work will investigate the properties that govern open-circuit voltage, including band gap, series resistance, carrier concentration and built-in potential. Some of these parameters are directly related to the junction material paired with p-SnS. Several partner materials will be presented with p-SnS including, but not limited to highly doped n-TiO₂ and n-SnS. Current work is underway to produce n-type SnS as well with potential to produce a homojunction. The optoelectronic properties of SnS make it a suitable material for PV. Its high absorption coefficient, greater than 10⁴ cm^-1, and band gap near 1.3 eV are well matched with the solar spectrum. SnS also has a carrier concentration greater than 10¹⁵ cm^-3 and potential to be both n-type and p-type. Our group is able to produce dense SnS thin films with optimal electronic properties. Sputtering the material gives great control over the material properties and recent work optimizing post-deposition heat treatment has shown great promise for improving the material. Tin sulfide thin films were sputtered on glass and oxidized silicon substrates at varying substrate-to-target distances, substrate temperature, target power, and chamber pressure. The sputter target was a 3” SnS₂ with 99.999% purity (LTS Research Laboratories, Inc.). These sulfur-rich samples were then annealed under medium vacuum (<2x10^-6 Torr) in the deposition chamber at 400°C to produce a uniform α-SnS, which is most likely to be p-type. Producing n-type SnS is possible via annealing of the films in a methanol/SnCl₄ solution. Production of homojunction SnS-based thin film devices is not found in the literature. Our work aims to produce these devices for the first time and compare them to a well-known partner material such as ZnO.
[1] R. E. Banai, et al., in Proceedings of 2012 IEEE 38th Photovoltaics Specialists Conference, Austin, 2012, pp. 164-169.
[2] R. E. Banai, et al., IEEE Journal of Photovoltaics, vol. 3, no. 3, pp. 1084-1089, 2013.
[3] R. E. Banai, et al., in Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th, Tampa, 2013, pp. 2562 - 2566.
[4] R. E. Banai, et al., in Photovoltaic Specialists Conference (PVSC), 2014 IEEE 40th, Denver, 2014.

Metal oxide nanostructures with hetero-contacts and phase boundaries offer unique platform for designing materials architectures for sensing applications. Besides the size and surface effects, the modulation of electronic behaviour due to junction properties leads to modified surface states that promote selective detection of analytes. The growing possibilities of engineering nanostructures in various compositions (pure, doped, composites, heterostructures) and forms (particles, tubes, wires, films) has intensified the research on the integration of different functional material units in a single architecture to obtain new sensing materials. In addition, new concepts of enhancing charge transduction by surface functionalization and use of pre-concentrator systems are promising strategies to promote specific chemical interactions, however the challenge related to reproducible synthesis and device integration of nanomaterials persist.
This talk will present how chemically grown and designed nanoparticles, nanowires and nanocomposites of different metals and metal oxides lead to new fucntional properties, which can be transformed into advanced materials technologies.

Due to their well-defined pinhole free structure, ALD grown thin films offer the possibility to investigate the material&’s properties. In this study, we have focused on the pristine material&’s water splitting efficiencies depending on the deposition parameters. Concerning the low absorption coefficient of TiO₂, the photoactivity of the ALD grown films was very poor, which was improved by doping TiO₂ thin films for water splitting with a series of different dopant natures: hydrogen plasma and nitrogen plasma as electron dopants and vanadium as hole dopant. While shorter plasma treatment times have led to an increase in photocatalytic activity up to 0,7 to 1 mA/cm², the vanadium doping has emerged as an obstacle for charge carrier separation due to phase separation. Film thicknesses in the range of 100 nm allowed us to prove the existence of Ti³⁺-ions in hydrogen-plasma treated thin films via XPS analysis and therefore correlate the enhancement in photoactivity with this ion.

WO₃/TiO₂ heterostructures in different weight percentages of WO₃ were successfully synthesized through one step hydrothermal environmentally-friendly process. The formation of heterostructures were verified by the presence of both orthorhombic WO₃ and anatase TiO₂ crystalline phases in XRD measurements and by the interface identification in HRTEM images. It is well known that the heterostructure formation by one step synthesis is a difficult procedure to control as different oxides have unique synthesis conditions, (pH, temperature, etc.). In this work we report a facile and green process in order to obtain heterostructures by crystallizing WO₃ in the presence of TiO₂ pre-formed particles to enhance the contact between two oxides. According to SEM images the heterostructures consists of plate-like WO₃ covered with TiO₂ nanoparticles. However, the surface area of these particles was significantly reduced after the synthesis that can be attributed to the micro meter size WO₃. The electronic properties of these structures were also investigated and the bandgap values were estimated from DRS data by applying in Tauc equation. The materials showed a characteristic absorption in UV region. The photocatalytic activity toward rhodamine-B dye degradation was evaluated by UV-vis technic and also the hydroxyl radicals, the active species for the dye oxidation, were quantitatively investigated by measuring the fluorescence derived by the reaction with terephthalic acid. The schematic mechanism was investigated to understand the photocatalytic process upon UV and visible light illumination, based on the band structure of semiconductors. A low photocatalytic activity was observed under visible light, as expected due to the bandgap energy and the band positions determined by the electronic analysis of the materials. It is well known that the heterogeneous photocatalysis under UV light could be influenced by two types of possible mechanisms, the direct and indirect photocatalysis. We observed that both mechanisms are acting in the heterostructured systems and WO₃ particles in the dye adsorption mechanism verification and hydroxyl radical formation experiment. However, for TiO₂ particles only the indirect photocatalysis mechanism was observed. In indirect photocatalysis, the photogenerated charges (electron-hole) are formed and migrated until the semiconductor surface. The redox reactions occur at the surface by the adsorbed molecules like H₂O and OH, thus the hydroxyl radical formed in the semiconductor surface will govern the dye oxidation mechanism, as observed for the photocatalytical tests.

Numerous new materials and device structures have been widely explored in order to cut the cost of photovoltaic (PV) manufacture. In our group, we have developed various nanostructured semiconductors for high-performance and low cost solar cell. Here, nanostructured silicon acts as acceptor. The donor organic materials can be either small molecules or conjugated polymers. Here, we demonstrate that hybrid PVs based on organic conjugated molecular and silicon nanowire (SiNWs) arrays can achieve a high PCE (~13%) by controlling the phase separation as well as surface passiviation.
An advantage of hybrid devices presents the excellent light harvest capability of SiNWs as well as simple fabrication process. The antireflection property of SiNW arrays fabricated by chemical etching method is significantly enhanced over wide spectrum range. In addition, we can passiviate the surface defect by methyl group termination, which suppress the surface recombination velocity dramatically. Furthermore, we can control the phase separation by tuning the density of SiNWs array and the shell thickness of polymer. Conjugated organic materials exhibit low-cost solution processability. PVs employing organic and SiNWs hybrid materials as photoactive layer exhibit the potency to benefit from the advantages of both organic and Si components. The utilization of the organic molecular allows hybrid cells to be superior over conventional silicon ones in terms of their cost and scalable solution processing. Hybrid solar cells attract wide research interests in the photovoltaic community. Especially, hybrid composites of conjugated organic materials and nanostructured inorganic materials are potential candidates for cost-effective and efficient solar-energy-harvesting devices.

Yong Zhou¹, Wenjun Luo¹ and Zhigang Zou^1*
1Ecomaterials and Renewable Energy Research Center (ERERC), Nanjing University, 22 Hankou Road, Nanjing 210093, China,
The concept of using solar energy to solve the global energy and environmental problems are has been intensified from the standpoints to a technological assessment, since the energy and environmental issues in a global level are important themes tackled in the 21st century. The mass consumption of fossil fuels after 20th century has produced negative properties in future such as the exhaustion of petroleum resources and the contamination of environment. In order to continue the global human life, it is very important to exploit new clean energy resources instead of fossil fuels without heavy burden to energy and environment. Exactly the solar energy conversion satisfies above conditions. In this talk, we will introduce advance and development of the solar energy conversion research in our group and the relative research project.
Keywords: Photocatalys, solar fuel, solar energy conversion
Email:[email protected]

Synthesizing fuels or organic substances from sunlight, CO₂ and H₂O is an ideal reaction to help alleviate global warming and fossil fuel shortages. We have proposed the novel concept of selective CO₂ reduction using a hybrid photocatalyst comprising of semiconductor (SC) photosensitizer coated with a metal complex electrocatalyst (MCE). The SC/MCE hybrid photocatalyst combining zinc-doped indium phosphide (InP) with a ruthenium complex polymer (RuCP) electrocatalyst can reduce CO₂ to formate with very high selectively under visible light irradiation in water.^[1,2] Hence, by wiring the InP/[RuCP] photocathode for CO₂ reduction with a TiO₂ photoanode for H₂O oxidation in two compartment cell separated with proton exchange membrane, solar formate production by the Z-scheme reaction utilizing only sunlight, CO₂ and H₂O was successfully realized without an external bias application in previous study.^[3] However, the conversion efficiency for solar energy to chemical energy (formic acid) is 0.04% over the TiO₂//InP/[RuCP] system. Therefore, the efficiency needs to be improved in order to be able to discuss the feasibility of an artificial photosynthetic system.
In the present system, the TiO₂ photoanode was replaced with reduced SrTiO₃(SrTiO_3-x) which has more negative potential of the conduction band minimum to facilitate electron transfer from the conduction band of the photoanode to the valence band of the photocathode. The SrTiO_3-x //InP/[RuCP] wiring system generates formate by reducing CO₂ using water as the electron donor and the proton source. The conversion efficiency from solar energy to chemical energy reached 0.14%^[4], which is approximately half that of plants in nature and 4 times higher than that of the previous TiO₂//InP/[RuCP] system. Our present result shows that the design of band-configuration is crucial for the enhancement of solar formate production from CO₂ and H₂O by the Z-scheme reaction.
In this presentation, we will also introduce the success in further improvement of the solar conversion efficiency.
Refrence
[1] S. Sato, et al. Angew. Chem. Int. Ed. 2010,49, 5101.
[2] T. Arai, et al. Chem. Commun.2010,46, 6944.
[3] S. Sato, et al. J. Am. Chem. Soc. 2011,133, 15240.
[4] T. Arai, et al. Energy. Environ. Sci.2013,6, 1274.

Exploring suitable materials for photocatalytic water splitting has attracted tremendous efforts in recent decades. The observation of Sr_1-xNbO₃ as effective photocatalyst has opened up a novel window to apply metallic oxides in photocatalytic energy conversion. However the experimental band structure and how the photogenerated electron-hole pairs are separated without significant internal field in Sr_1-xNbO₃ type of metallic oxides remain unexplained. Here we report that the long lifetime of the photogenerated excitations in these materials may be responsible for their photocatalytic properties. We find a mid-gap-band forming in their bandgap and the electrons excited from the mid-gap-band into the conduction band are used for splitting water. The formation of the mid-gap-band strongly depends on the preparation oxygen pressure, which also influences the conductivity and the optical transmission of the epitaxial film.

Photoelectrochemical (PEC) water splitting is the simplest and cleanest route that directly converts sun light to hydrogen and potentially it will enable a low cost production of hydrogen. One of the biggest challenges for the realization of the PEC water splitting is to develop an efficient photoanode having a good light absorption, fast charge transport and transfer properties simultaneously. Typically metal oxides are considered to be good candidates because of their excellent photochemical stability and low cost. However, their poor material quality, such as large amount of defects, low surface area, low charge carrier&’s mobility/conductivity, which largely originated from the preparation method, limits the charge transport and transfer properties.
In this talk, I will present two novel flame processing techniques, i.e., flame reduction and doping, for metal-oxide photoanodes that allows to greatly improve the charge transport and transfer properties, hence improving the PEC water-splitting performance. First, we developed a rapid flame reduction method to generate controllable amount of oxygen vacancies in TiO₂ nanowires (NWs) that leads to nearly three times improvement in the PEC water-splitting performance. The flame reduction method has unique advantages of a high temperature (>1000 ^oC), ultra-fast heating rate, tunable reduction environment, and open-atmosphere operation, so it enables rapid formation of oxygen vacancies (<1min) near the surface region without damaging the nanowire morphology and crystallinity, and even applicable to various metal oxides. Second, we designed an ex-situ novel doping method which combines versatile solution phase chemistry and rapid flame annealing process (i.e., Sol-Flame) to dope TiO₂ NWs with cobalt (Co). The sol-flame doping method not only preserves the morphology and crystallinity of the TiO₂ NWs, but also allows fine control over the Co dopant profile by varying the concentration of Co precursor solution. Finally, we extended the sol-flame doping method to codope TiO₂ NWs with tungsten and carbon (W, C) by sequentially annealing W-precursor coated TiO₂ nanowires in flame and CO gas. This is the first experimental demonstration that codoped TiO₂:(W, C) nanowires outperform monodoped TiO₂:W and TiO₂:C and double the saturation photocurrent of undoped TiO₂ for PEC water-splitting.

Uranium oxides or uranium doped metal oxides are promising materials as catalysts for visible light driven photo-oxidation of water. Suitable precursors for the material synthesis of these compounds were scarce limiting the use of gas phase or liquid phase material processing from metal organic molecular compounds. The current work describes the synthesis of new volatile uranium and uranyl complexes suitable for gas phase deposition of the respective oxides as pure material or as dopant into different oxide matrices. These complexes show significantly enhanced properties in terms of volatility and stability, which improves their application in CVD processes. The complexes have been used as single source precursors for the chemical vapour phase deposition of uranium oxide thin films using a low pressure, cold wall MOCVD and plasma-enhanced CVD. The resulting nanostructured films are tested as electrode materials in photoelectrochemical cells.

Global climate warming and environment pollution have spurred scientists to develop new high-efficient and environmental-friendly energy technologies. Hydrogen is an ideal fuel for fuel cell applications. Hydrogen has to be produced from renewable and carbon-free resources using nature energies such as sunlight if one thinks of clean energy and environmental issues. In this regard, a photoelectrochemical (PEC) cell consisting of semiconductor photoelectrodes that can harvest light and use this energy directly for splitting water is a more promising way for hydrogen generation. Abundant and inexpensive oxide semiconductor such as ZnO has been recognized as a promising photoelectrode, but the photoconversion efficiency is substantially limited by its large band gap and rapid charge recombination. Recently, doping with 4d transition metal, such as Mo, has been carried out to remarkably enhance the PEC performance of many photoanodes including TiO₂, BiVO₄, and Fe₂O₃. Nevertheless, 1-D Mo-doped ZnO nanostructures have not been reported for PEC water splitting. We report the first demonstration of cobalt phosphate (Co-Pi) assisted Zn_1-xMo_xO nanorods (NRs) as visible-light-sensitive photofunctional electrodes to fundamentally improve the performance of ZnO NRs for PEC water splitting. The maximum photoconversion efficiency could be successfully achieved as high as 1.05%, with the significant photocurrent density of 1.4 mA cm^-2. More importantly, in addition to achieve the maximum incident photon to current conversion efficiency (IPCE) value of 86%, it could be noted that the IPCE of Zn_1-xMo_xO photoanodes at the monochromatic wavelength of 450 nm is up to 12%. Our PEC performances are comparable to those of many oxide-based photoanodes in recent reports. The improvement in photoactivity of PEC water splitting may be attributed to the enhanced visible-light absorption, increased charge-carrier densities, and improved interfacial charge-transfer kinetics due to the synergistic effects of Mo incorporation and Co-Pi modification, thus contributing to photocatalysis. The new design of constructing highly photoactive Co-Pi assisted Zn_1-xMo_xO photoanodes enriches the doping community and sheds light on developing high efficiency photoelectrodes for solar-hydrogen field.
Reference:
[1] Y.G. Lin, Y.K. Hsu, Y.C. Chen, S.B. Wang, J.T. Miller, L.C. Chen, and K.H. Chen, Energy Environ. Sci., 5, 8917 (2012).
[2] Y.G. Lin, Y.K. Hsu, Y.C. Chen, L.C. Chen, S.Y. Chen, and K.H. Chen, Nanoscale, 4, 6515 (2012).
[3] Y.K. Hsu, Y.C. Chen, Y.G. Lin, L.C. Chen, and K.H. Chen, J. Mater. Chem., 22, 2733 (2012).
[4] Y.G. Lin, Y.K. Hsu, A.M. Basilio, Y.T. Chen, K.H. Chen, and L.C. Chen, Optics Express, 22, A21 (2014)
[5] Y.K. Hsu, S.Y. Fu, M.H. Chen, Y.C. Chen, and Y.G. Lin, Electrochimica Acta, 120, 1 (2014).
[6] Y.G. Lin, Y.C. Chen, J.T. Miller, L.C. Chen, K.H. Chen, and Y.K. Hsu, ChemCatChem, doi: 10.1002/cctc.201400012 (2014)

High performance and inexpensive photocathodes for solar water reduction are urgently needed to advance photoelectrochemical (PEC) devices toward commercial application. P-type delafossite CuFeO₂ is a promising candidate given its known stability, CB-edge energy position, and band-gap of 1.5 eV, making it possible to reach STH conversion efficiencies higher than 10% in a tandem device. Moreover its composition from earth abundant atoms accords with solar energy use on a global scale. However, the difficulty in preparing thin-film electrodes using the typical solid-state reactions known to produce delafossites has limited PEC investigations of this material. Herein, we report significant advances in CuFeO₂ photocathodes based on the development of a solution-based, sol-gel method to afford tunable and reproducible p-CuFeO₂ thin-films.
While initial photocurrents obtained with the bare material in a planar geometry only exceeded 1 mA.cm^-2 in a sacrificial O₂-purged alkaline electrolyte under AM 1.5G illumination, a thickness optimization and extensive physical characterization gave deep insight into the limitations and advantages of the material. Our electrodes were found to harvest visible light up to 830 nm, and exhibit a remarkable photocurrent onset at +0.8 V vs RHE, demonstrating its suitability in a D4 tandem cell. Importantly, the electrode was found to be stable under operation for several days.
Subsequently we demonstrate that the limitation of charge recombination in the active layer can be overcome using optimized semiconductor doping, employing nanostructured scaffolds of p-CuAlO₂, and adding charge extraction overlayers resulting in photocurrents exceeding 2 mA.cm^-2. Moreover, we demonstrate that the slow charge transfer kinetics to the hydrogen evolution reaction can be improved by adding cocatalysts to the surface of CuFeO₂ giving high Faradic efficiency for solar hydrogen production. Finally we demonstrate that our optimized CuFeO₂ photocathodes can operate in tandem with an n-type oxide photoanode to split water without an external bias.

This presentation will provide an overview of recent progress in the development of oxide-based photocatalytic approach for solar-driven production of hydrogen from water. The emphasis will be on fundamental aspects of disorder-engineered titanium dioxide nanoparticles, starting with an introduction of the basic electronic band structure resulted from disorder incorporation, followed by an analysis of the origin of favorable performance exhibited by disorder-engineered titanium dioxide nanoparticles on photocatalysis.

Photoelectrochemical water splitting offers a robust and scalable method to produce hydrogen in a renewable, sustainable, and cost effective way. The more demanding half reaction involved in water splitting is the oxygen evolution reaction, which requires the transfer of 4 holes/electrons. Thus, much focus has centered on finding suitable n-type semiconductor materials with suitable band gap energies to absorb a substantial amount of visible light, while also having appropriate valence band edge positions to drive oxygen evolution. BiVO4 has gained recent attention as a promising photoanode material due to its modest band gap energy (2.4 eV), stability in aqueous environments, and simple preparation methods. However, this material still requires elemental doping and surface catalyst incorporation in order to achieve respectable photocurrent densities at the water splitting potential (1.23 V vs. RHE) on the order of 4~5 mA/cm2. In this work, we have investigated a new method to improve the performance of BiVO4 photoanodes deposited by spray pyrolysis. By holding the photoanodes under open circuit conditions in an aqueous electrolyte and shining light on them for several hours, the photoactive performance improves drastically, and approaches the behavior of both Co-Pi catalyzed BiVO4 and BiVO4 when using a hole scavenger such as H2O2. The possible cause of this enhancement can be the ‘cleaning&’ of the semiconductor surface, which reduces surface state defects, and thus reduces surface recombination of photogenerated holes and electrons. This technique is simple and should be applicable to other photoanodes, particularly metal oxides, who have shown similar surface energy states.

Fabricating and engineering photoelectrochemically-active low-cost and non-toxic metal oxides is essential for the development and implementation of solar hydrogen in our societies as a renewable energy carrier. More over, the use of seawater as unique electrolyte will unfold the full potential of solar water splitting as a clean, cost effective and sustainable source of hydrogen. Our approach combines a well controlled large scale, low temperature and surfactant-free aqueous synthesis of metal oxide quantum rods/dots-based arrays where component size, shape, crystal structure and electronic properties are tailored and synchrotron-based in-depth characterization techniques of their bulk and interfacial electronic structure. Low cost visible light-active Iron oxide-based heteronanostructured photoelectrodes have been developed showing several mA.cm^-2 at pH 7 in simulated seawater at 1 Sun under solar simulator. Size effects on the surface chemistry, carrier dynamics, electronic structure (bandgap, band edges, orbital character and symmetry), and photoelectrochemical properties (photocurrent and onset potential) as well as electrical conductivity of such compounds will be demonstrated and discussed.

Hematite has emerged as a good photocatalyst for efficient solar water splitting due to its favorable optical band gap (2.1-2.2 eV), extraordinary chemical stability in oxidative environment, abundance, and low cost. According to theoretical prediction, the solar-to-hydrogen efficiency of hematite can be 16.8% and the water splitting photocurrent can be 12.6 mA cm^-2. However, the practical performance of hematite for solar water splitting is far from the ideal case which has been limited by several factors such as poor conductivity, short lifetime of the excited-state carrier (10^-12s), poor oxygen evolution reaction (OER) kinetics, short hole diffusion length (2-4 nm), and improper band position for unassisted water splitting. In our recent work, enormous efforts have been focused on improving the performance of hematite nanostructure photoelectrode. Different methods such as morphology control, elemental doping, and improvement of the charge transport of hematite have been developed to improve the performance of hematite photoelectrode in solar water splitting. We present the preparation of Ti-doped and H₂-treated hematite nanostructures. The H₂-treated hematite photoelectrode showed a high photocurrent of 2.28 mA/cm² at 1.23 V vs. RHE, which was over 2.5 times than that for pristine hematite. Moreover, when compared to hematite photoelectrode with oxygen vacancies but treated by controlling the oxygen content in the sintering process, a cathodic shift of the onset potential was observed by about 120 mV (from 0.99 to 0.87 V vs. RHE). The cathodic shift of the onset potential was attributed to the surface effect of H₂-treatment. The Ti-doped hematite nanostructures with optimized oxygen vacancies achieved a remarkable maximum photocurrent value of of 4.56 mA/cm² at 1.6 V vs. RHE. Moreover, Ti-doping can expand the applicative partial oxygen pressure to a wide range compared to that for undoped hematite. The expansion of partial oxygen pressure range might be useful for the practical application. The coupling of extrinsic Ti-doping and intrinsic oxygen vacancies stands for an effective strategy to design oxide-based photoanodes for efficient solar water oxidation.
Reference:
M. Li, J. J. Deng, A. W. Pu, P. P. Zhang, H. Zhang, J. Gao, Y. Y. Hao, J. Zhong and X. H. Sun, Journal of Materials Chemistry A, 2, 6727 (2014)
A.W. Pu, J.J. Deng, M. Li, J. Gao, H. Zhang, Y.Y. Hao, J. Zhong and X.H. Sun, Journal of Materials Chemistry A, 2, 2491(2013)
J.J. Deng, X.X. Lv, J. Gao, A.W. Pu, M. Li, X.H. Sun and J. Zhong, Energy & Environmental Science, 6, 1965 (2013)
J.J. Deng, J. Zhong, A.W. Pu, D. Zhang, M. Li, X.H. Sun, S.T. Lee, Journal of Applied Physics, 112, 084312 (2012)

Photoelectrochemical (PEC) hydrogen production using semiconductor electrodes is one of the promising methods of hydrogen production from water using solar energy. The first report on the PEC hydrogen production was a system with a n-type semiconductor, TiO₂-Pt system. Some recent studies have focused on high-efficiency n-type semiconductor photoanodes and photocathodes. However, semiconductor-metal electrodes systems such as a TiO₂-Pt system generally require an external voltage for the hydrogen generation process to take place. The development of a PEC hydrogen generation system which does not require any external voltage is crucial for the efficient use of solar energy. Such a system is ultimately an artificial photosynthesis system that can generate hydrogen and oxygen separately. So far, various electrode systems have been reported as systems without any external voltage. Our research has been focusing on low-cost and/or toxic chemical-free oxide materials containing of iron and calcium such as Fe₂O₃ and CaFe₂O₄. Here we show the electrical conductivity of CaFe₂O₄ with a p-type semiconducting behavior and the possibility of CaFe₂O₄-Fe₂O₃ electrodes system for the application as PEC hydrogen production without an external applied voltage was examined. The open circuit voltage of photo cell where p-type CaFe₂O₄ (10 cm²) and n-type Fe₂O₃ (2cm²) electrodes were connected in 0.1 M NaOH aqueous solution was 0.48 V, and the short-circuit current (V = 0) was 440 mA. Although the photo current for the CaFe₂O₄-Fe₂O₃ electrodes system decreased immediately after the start of illumination, the CaFe₂O₄-Fe₂O₃ electrodes system showed an activity for hydrogen production from water without applying an external voltage.

Hematite, with a 2.1 eV bandgap, can theoretically achieve over 10% solar-to-fuel efficiency. While it exhibits excellent stability under alkaline conditions and is truly earth-abundant, the major obstacle to its development as a photoanode material is the short hole diffusion length. Many efforts have been aimed at improving the performance of Fe₂O₃ photoanodes: by fundamentally improving the material crystal structure by annealing or its conductivity by doping, or by nanoengineering the photoanode through nanostructuring or surface catalyst deposition.
Here, we create tailored, hierarchically-nanostructured hematite photoanodes to improve photoresponse. An inverse-opal framework is used as the foundation of the experimental effort, with several additional modifications. Illuminated photoelectrochemical water oxidation measurements of the inverse-opal hematite films reveal an early turn-on potential, suggesting a significant photovoltage is achieved in the nanostructured films. shy;
Mesopores are then inserted into the inverse-opal framework, improving absorption as measured by UV-Visible Spectroscopy and slightly improving water oxidation photocurrent. Absorption is further increased by photodeposition of Au nanoparticles onto the inverse-opal framework, promoting subbandgap light absorption in the structure. The underlying mechanisms of the gold nanoparticle contribution will be discussed.
The results are compared with contrasting efforts using ultrathin (1-25 nm) single crystal lanthanum iron oxide for water splitting to gain insight into the carrier collection lengths of the materials and their water splitting potential.

In the past, the electrochemical instability of III-V semiconductors has severely limited their applicability in photocatlaysis. As a result, a vast majority of the research on photocatalysis has been done on TiO₂, which is chemically robust over a wide range of pH. However, TiO₂ has a wide band gap (3.2eV) and can only absorb ~4% of the solar spectrum, and thus will never provide efficient solar energy conversion/storage on its own. Here, we report photocatalytic CO₂ reduction with water to produce methanol using TiO₂-passivated GaP photocathodes under 532nm wavelength illumination. The TiO₂ layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. Here, the GaP surface is passivated using a thin film of TiO₂ deposited by atomic layer deposition (ALD), which provides a viable, stable photocatalyst without sacrificing photocatalytic efficiency. In addition to providing a stable photocatalytic surface, the TiO₂-passivation provides substantial enhancement in the photoconversion efficiency through passivation of surface states, which cause non-radiative carrier recombination. In addition to passivation effects, the TiO₂ deposited by ALD is n-type due to oxygen vacancies, and forms a pn-junction with the underlying p-type GaP photocathode. This creates a built-in field that assists in the separation of photogenerated electron-hole pairs, further reducing recombination. This reduction in the surface recombination velocity (SRV) corresponds to a shift in the overpotential of almost 0.5V. No enhancement is observed for TiO₂ thicknesses above 10nm, due to the insulating nature of the TiO₂, which eventually outweighs the benefits of passivation.

Major increases in conversion efficiencies and reductions in solar electricity costs would result by combining direct photovoltaic with indirect thermal conversion in concentrating solar plants. A straightforward way of accomplishing this would be to mount solar cells in good thermal contact with the heat receiving elements of parabolic mirror, Fresnel lens or power tower solar plants thus combining a photovoltaic with a thermal cycle in a single facility.
The reason this approach to improved conversion efficiencies has not been implemented is the fact that solar cells operating efficiently at temperatures of 400 Celsius and above are not currently available. Efforts to develop such cells have been only partially successful up to now for a variety of reasons that will be elucidated.
Graphene shell/aligned nanowire wide bandgap semiconductor materials such as graphene/nano Ti02 structures offer a new approach to the creation of high temperature solar cells. They are expected to display efficient photoinduced charge separation at the interface (1) together with very low dark currents due to the 3.2 eV bandgap of Ti02. Their excellent photocatalytic properties, accompanied by sizeable photocurrents when irradiated with sunlight, have been extensively documented. Work has begun to determine their photovoltaic properties using a novel cell design that takes full advantage of light harvesting nanophotonics.
A primary requirement that must be fulfilled by solar cells operating at high temperatures is that the interface between the two materials making up the cell remains stable. Interdiffusion as well as defect concentrations and composition must not change significantly at least up to operating temperatures. The graphene/Ti02 composite is a prime candidate for meeting these conditions based on the well explored high temperature properties of its constituents and their behavior as revealed for example by the ternary carbon/titanium/oxygen phase diagram.
Special features arising from the combination of the unique photophysical properties of graphene with the well known properties of Ti02 as a solar cell material will be described. The earth abundant and environmentally benign nature of the constituents is an additional attractive feature of these cells. The aim of the presentation is to evaluate the current status of a particular approach to the development of high temperature solar cells that can function so as to enable hybrid concentrating solar systems.
1. R. Long, N. J. English, and O. V. Prezhdo, J. Am. Chem. Soc. 2012, 134, 14238-14248

TiO₂ has been investigated as one of the most promising semiconductor materials for photocatalytic water splitting, pollutants degradation and dye-sensitized solar cells. However, the wide band gap of TiO₂ (3.2 eV for anatase) poses a critical challenge in the improvement of solar conversion efficiency, which could be well mitigated by coupling with narrow band gap semiconductors to form a heterostructure. In the present study, a novel graphitic carbon nitride quantum dots (g-CNQD)/TiO₂ nanotube arrays (NTA) photoanode was successfully synthesized by a simple two-step method which includes an electrochemical anodization technique followed by a facile organic molecular linkage. Mercaptopropionic acid was employed as bifunctional molecule linker to connect colloidal g-CNQD to TiO₂ NTA. SEM, HRTEM, FTIR and XPS analysis confirmed the presence of g-CNQD in the prepared electrode. The successful modification of TiO₂ by g-CNQD was found to improve the photoelectrochemical activity significantly because of enhanced separation and transfer of photo-generated electron-hole pairs. The optimized g-CNQD/TiO₂ NTA showed nearly 3.6-fold photocurrent of that from TiO₂ NTA alone. In addition, the prepared g-CNQD/TiO₂ NTA demonstrated superior photoelectrocatalytic activity and stability in the degradation of Rhodamine B (RhB), with kinetic constant nearly 3 times of that from TiO₂ NTA.

TiO₂ has been extensively investigated as a photoanode for photoelectrochemical water splitting due to its excellent photochemical stability, suitable band edge positions, low cost and environmental friendliness. However, TiO₂ typically show little photo-activity in the visible range because of its large band-gap and poor optical absorption in the visible range. Nitrogen (N) doped TiO₂ has been shown to exhibit improved visible light absorption yet typically with limited photocatalytic activity in the visible range. Here we discuss our recent effort in exploring an ion implantation approach to introduce N-dopants into single crystalline TiO₂ nanowires, and probe the fundamental role of N-doping in TiO₂ for improved visible photo-activity. Single crystalline rutile TiO₂ nanowire arrays were synthesized and implanted with nitrogen by using variable implantation dosage and acceleration voltage. The resulted doped nanowire arrays were then used as photoanodes in photoelectrochemical cells (PEC) for water splitting reaction. TEM studies show that the single crystalline structure of the TiO₂ nanowires is well maintained after N-implantation/thermal annealing processes. Optical diffusive reflectance studies show that the N-implanted TiO₂ nanowire arrays exhibit significant absorption in the visible range. X-ray photoelectron studies show that the as-implanted nitrogen atoms mainly occupy the interstitial sites and a proper anneal can restore the crystallinity to convert the interstitial nitrogen atoms into substitutional ones. PEC studies show that the as-implanted TiO₂ show no obvious improvement while the annealed sample exhibits a substantial improvement of the photoresponse in the visible range, with an incident photon to current conversion efficiency (IPCE) up to ~23% at 420 nm and ~18% at 450 nm. With few defects and trapping states, the single crystalline N-TiO₂ nanowires provide an excellent system for probing the fundamental role of N-doping. To this end, we have further determined the charge separation efficiency, charge injection efficiency, and well as the electrochemical impedance properties of the nanowires before and after N-implantation. These studies suggest that the N-doping can improve the charge transport properties as well as charge separation and injection efficiency in TiO₂ nanowires. Therefore, the enhanced photoactivity of N-implanted TiO₂ can be attributed to the increased visible light absorption and suppression of charge recombination in the TiO₂ nanowires.

This paper focuses on the degradation of solar cell absorbance due to localized heat. A decrease in optical absorbance represents a huge problem because of long-term solar cell degradation, decrease in absorption coefficient and a reduction in solar cell conversion efficiency. This decreases the photo-generating current hence reduces the effective efficiency of the solar device. This paper investigates the reduction in a-Si:H module absorption and correlates this with hot spot formation. Infrared Thermography was used for mapping of the module temperature profile, while IR flying meter software was used to identify hot spot centre. Fourier Transformation Infrared Spectroscopy (FTIR) was used for absorption characterization. The study was undertaken during outdoor deployment of five PV modules. This method was chosen so as to deduce a practical effect of hot spot formation on the module absorption ability. The results show a direct correlation between localized heat and absorption degradation, the final paper will present the detailed results.

Si is an excellent absorber material choice for use in 2-photon photoelectrochemical hydrogen production [1-3]. To date nearly all studies of silicon photoelectrodes have employed frontal illumination despite the fact that in most 2-photon tandem water-splitting concepts the silicon is the “bottom” cell in the tandem stack and therefore illuminated from the back with respect to the electrolyte [4-6]. Here, by experiemt and by modelling, we explore how photocurrent depends on carrier diffusion length (Le) and surface recombination velocity and we quantify their relative importance. A bifacial light absorbing structure (p+pn+ Si) is tested under back-illumination conditions which mimic the actual working environment in a tandem water splitting device. The thickness of the absorbing Si layer is varied to assess the impact of the diffusion length/thickness ratio (Le/L) on performance. It is shown how the induced photocurrent of a back-illuminated sample increases as wafer thickness decreases. Compared to a 350 µm thick sample, a 50 µm sample shows a 2.7-fold increase in photocurrent, and consequently also a higher open circuit voltage. Photocurrent increases with the Le/L-ratio only up to a certain point, beyond which the surface recombination velocity becomes the dominant loss mechanism. The present study is perhaps the first experimental demonstration of a Si wafer based photocathode under back-illumination.
Refs.
1. S. Hu, C. Xiang, S. Haussener, A. D. Berger, N. S. Lewis, An analysis of the optimal band gaps of light absorbers in integrated tandem photoelectrochemical water-splitting systems, Energy Environ. Sci., 2013, 6, 2984-2993.
2. E. L. Warren, S. W. Boettcher, J. R. McKone, N. S. Lewis, Photoelectrochemical water splitting: silicon photocathodes for hydrogen evolution, Proc. of SPIE, 2010, 7770, 77701F-1.
3. R. H. Coridan, M. Shaner, C. Wiggenhorn, B. S. Brunschwig, N. S. Lewis, Electrical and photoelectrochemical properties of WO3/Si tandem photoelectrodes, Phys. Chem. C., 2013, 117, 6949-6957.
4. B. Seger, T. Pedersen, A. B. Laursen, P. C. K. Vesborg, O. Hansen, I. Chorkendorff, Using TiO2 as a conductive protective layer for photocathodic H2 evolution, J. Am. Chem. Soc., 2013, 135, 1057minus;1064.
5. B. Seger, A. B. Laursen, P. C. K. Vesborg, T. Pedersen, O. Hansen, S. Dahl, I. Chorkendorff, Hydrogen production using a molybdenum sulfide catalyst on a titanium-protected n+p-silicon photocathode, Angew. Chem. Int. Ed., 2012, 51, 9128-9131.
6. M. R. Shaner, K. T. Fountaine, S. Ardo, R. H. Coridan, H. A. Atwater, N. S. Lewis, Photoelectrochemistry of core-shell tandem junction n-p+-Si/WO3 microwire array photoelectrodes, Energ. Environ. Sci., 2014, 7, 779-790.

Thin-film photovoltaics utilizing Cu(In,Ga)Se₂ (CIGS) absorber layers have reached efficiencies approaching 16% in commercially-available large-area modules and are now produced on an industrial scale. One aspect critical to further improving working device efficiencies is developing an understanding of the various interfaces within the solar cell heterostructure. Of particular importance is the interface between the absorber and the buffer layer, a crucial layer involved in charge collection that most commonly consists of CdS in record-efficiency devices.
Here we use both theoretical and experimental methods to characterize the absorber-buffer interface of physical-vapor-deposited (PVD) CdS/CIGS heterojunctions prepared at the MiaSolé production line. We report on hybrid density functional theory calculations to supplement and explain detailed transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) measurements of the hetero-interface that identify the coexistence of CdS domains in both cubic and hexagonal phases. We calculate the activation energies for the diffusion of absorber-related point defects within the buffer, both for relaxed and strained conditions for each phase. Our results confirm that CIGS-related defects are more prone to interdiffuse within CdS buffer layers in the zincblende rather than wurtzite modification and that Cu clustering within the buffer may be exacerbated in wurtzite CdS epitaxially grown on CIGS, consistent with observed composition profiles.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Department of Energy office of Energy Efficiency & Renewable Energy (EERE) through the SunShot Bridging Research Interactions through collaborative Development Grants in Energy (BRIDGE) program.

Long persistence phosphors enable the storage and slow discharge of light. It is anticipated that incorporation of such a delayed light release source will improve light harvesting efficiency for photovoltaic applications. One promising long persistence phosphors candidates for photovoltaics is strontium aluminate co-doped with Eu²⁺ and Dy³⁺ ions, compounds of which are already widely commercialized in phosphorescent paints for variety of decorative and safety signage applications, due to their relatively high luminescence intensity. The incorporation of boron into the compounds has been found to extend the afterglow from minutes to longer than 8 hours, while the intensity remains sufficient for reading the periodic table in the dark for ca. 10 minutes, and decayed to 1/e after 125 minutes. Moreover, 97 mol% of these compounds consist of earth-abundant, inexpensive and stable elements, rendering them well-suited for development for sustainable solar technologies.
In this study we present the development of the modified solution-polymerization processing to achieve environmentally stable photoactive compounds of boron-containing SrObull;(Al₂O₃)₂and (SrO)₄bull;(Al₂O₃)₇ doped with 1 mol% Eu²⁺ and 1 mol% Dy³⁺(SA2EDB and S4A7EDB). We have found that the presence of B lowers the necessary calcination temperature for the formation of crystal structures of the SA phases, without additional phase formation. The sintered powder mixture of SA2ED with the 30 mol% addition of B resulted in the formation of SA2EDB phase containing 7.46 mol% of B incorporated into the parent crystal phase, which was determined by the inductively coupled plasma-optical emission spectroscopy (ICP-OES). Electron energy loss spectroscopy (EELS) showed that trigonally coordinated B is stable in both, the SA2EDB and S4A7EDB crystal phases. Optical characterization of the synthesized powders showed persistence peaking at 490 nm wavelength (from photoluminescence) and decaying over 14 hours (PL glow decay curves). Detailed compositional analysis, which was performed by the aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) combined with EELS revealed variations in the cation (Sr/Al ratio) stoichiometry at the nanometer scale.
In conclusion, our results showed that sustainable engineering of earth-abundant, low-cost and stable long-persistence strontium aluminate ceramic phosphors offer high potential for applications in solar energy conversion

Different forms of carbon prepared from diverse syntyetic routs are currently being used in various fields of research including energetical, environmental, electrical, chemical, and biomedical application. In general, carbon based materials like graphene, carbon nanotube, carbon nitride, diamond like carbon etc. are prepared from gaseous precursors such as CVD, PVD and ion-assisted sputtering techniques [1]. We believe that the large scale synthesis of nanostructured carbon should be free from using excess energy for firing, sintering, melting and expensive equipment. We, propose herewith “Submerged Liquid Plasma (SLP)” technique for direct formation of nanostructured carbon material and nitrogen polymers (NPs) at ambient conditions. The SLP process provides number of advantages which includes (a) simple reaction set up (b) reaction can be carried out at ambient conditions (c) periodic collection of samples gives clear information about the product (d) simple procedure and less operating cost. In the present study, we utilized SLP technique for the direct synthesis of NPs. Under SLP, organic compounds which have either unsaturated or high energy functional group (e.g. C#65309;C, C#65309;N and Cequiv;N) form stable free radical monomer and initiate polymerization reaction.[1] The most significant differences are that the polymers produced by plasma process do not contain regular or repeating units and are enriched with radicalized functional groups. We have succeeded to prepare Nitrogen functionalized Graphene Nano sheets by these SLP in acetonitrile liquids.[2,3] We could confirm the functionalized Graphenes are electrochemically active. Then we are challenging to prepare the Nano-particle/Graphene hybrids by this SLP methods. Low temperature and/or Soft Processing of nanostructured carbon and NPs by SLP process will open up new possibilities for the development of functionalized/hybrid nanostructured carbon materials for various applications including Photovoltaic and Solar Cell areas.
References
1) J. Senthilnathan, C. C. Weng, J. D. Liao, M. Yoshimura, Sci. Rep. 3, 2414; DOI:10.1038/srep02414 (2013)
2) J. Senthilnathan, M.Yoshimura et al., J. Mater Chem A,(2014) 2, 3332-3337, a Hot Paper 2014
3) J. Senthilnathan,K. SanjeevaRao, M. Yoshimura, et al ., Scientific Reports, 4(2014), 04237 & 04395

Solar water splitting is a clean and sustainable process for large-scale production of hydrogen which is an effective carrier to capture solar energy and to be used as a fuel directly or possibly by hydrocarbon synthesis. In the photoelectrochemical process, the oxidation of water to O₂ is an important half-reaction that accompanies with the hydrogen generation from water. Although TiO₂ has shown stable photoelectrocatalytic properties, its bandgap is large, thereby limiting the efficiency significantly. On the other hand, Si has an optimal band gap for efficient solar-energy conversion, while it is unstable in aqueous media. For efficient and sustainable solar water splitting, we investigate epitaxial anatase TiO₂ on Si that could form a quasi-2D electron gas (2DEG) or hole gas (2DHG) at the interface, thereby inducing intrinsic electrostatic fields to separate photo-induced charge carriers and increase surface reactivity. We use ab initio density functional theory calculations to examine the electronic structure and thermodynamic stability of various interface configurations between TiO₂ and Si. We find quasi-2D metallic monolayers induced from the epitaxial TiO₂ on Si, although the system is charge neutral overall (i.e., the system with a Si/SiO/TiO₂ interface). The offset of a Ti dxy state occurs due to epitaxial strain on a TiO₂ film, and the electronic band alignment from Si to TiO₂ shift the electronic structures of Si and TiO₂ at the interface up and down, respectively. Therefore, 3p state of Si and 4d_xy state of Ti are partially occupied at the interface, resulting in the 2DHG at Si interface and 2DEG at TiO₂ monolayers. ZrO₂ and HfO₂ films on Si are also examined briefly.

Using the density functional theory (DFT) and time dependent DFT, within the generalized gradient approximation (GGA), the electronic and optical properties of stoichiometric (ZnS)_n nanoparticles were calculated. The dependence of the gap on the size (n) of the nanoparticle will be presented. The effect of replacing S atoms with P, Se or Te atoms in the (ZnS)_n nanoparticles and its influence in the gap will be also shown. The results will be compared with recently published studies of (CdSe)_n nanoparticles (M. M. Sigalas, E. N. Koukaras, and A. D. Zdetsis, RSC Advances 4, 14613 (2014)). These types of nanoparticles can be used in quantum dot sensitized solar cells and the present computational study can guide further experimental studies to choose the right size and composition of the nanoparticles in order to get the maximum absorption of the solar spectrum.

With solution-process-ability, scale-able fabrication and purification, and earth-abundant input materials (carbon), semiconducting single-walled carbon nanotube (SWNT) networks represent promising materials for near-infrared solar cell (SC) applications, offering long-wavelength light harvesting devices. Recently, this potential has motivated aggressive experimental device investigation.^1-4 Despite this interest, the basic design questions remain: even to an order of magnitude, what thicknesses, nanotube orientations, densities, and chirality mixtures should we focus on? What is the impact of impurities, and nanotube length? There is to date no quantitative model of SWNT/nanorod network solar cell operation analogous to bulk semiconductor p-n junction photovoltaics, allowing a rigorous understanding of the physical tradeoffs driving experimental observations and informing what research will enable technological progress. To address this issue, we develop a deterministic model of SWNT PVs based on photon, exciton, and free carrier population balances derived directly from the underlying SWNT photophysics. The model accounts for arbitrary distributions of nanotube chiralities, lengths, orientations, defect types and concentration, bundle fraction and size, and density. The potent framework provides a novel approach for modelling nanostructured SCs.
We show that feasible devices can achieve external quantum efficiencies above 60%. We find that there is an optimal device thickness that is a function of nanotube density, orientation, and quenching site concentration. This thickness stems from a tradeoff between exciton generation and diffusion to the electrodes, and is at a minimum at the limit of close-packed nanotube density. We show that this minimum characterizes a given device design and scales with average nanotube length to exponent 0.4. The normalized difference between optimal thickness and this close-packed limit scales inversely with density to the 0.24 power. Practically, in-plane aligned nanotube configurations yield optimal thicknesses less than 10 nm, increasing to a range of 50 to 200 nm for vertical alignment. Due to weak inter-SWNT exciton transport relative to exceptional intra-SWNT diffusion, vertically-aligned films are unambiguously favored at densities above 3% of the close-packed limit; at lower densities however an optimum emerges at an intermediate angle to compensate for weaker light absorption of vertical nanotubes. Comparison to published experimental devices displays the model&’s utility for device design.
(1) Bindl, D. J.; Shea, M. J.; Arnold, M. S. Chemical Physics2013, 413, 29.
(2) Jain, R. M.; Howden, R.; Gleason, K. K.; Strano, M. S. et al.Adv Mater2012, 24, 4436.
(3) Shea, M. J.; Arnold, M. S. Applied Physics Letters2013, 102, 243101.
(4) Bernardi, M.; Lohrman, J.; Kumar, P. V.; Kirkeminde, A.; Ferralis, N.; Grossman, J. C.; Ren, S. Q. ACS Nano2012, 6, 8896.

Solar thermal fuels make use of molecules that undergo reversible photo-isomerization to store solar energy and convert it into thermal energy [Kucharski, T. J., et al. Energy Environ. Sci.4, 4449 (2011).]. Because solar thermal fuels produce no emissions and can store and convert energy within one material, they are attractive option for a renewable alternative energy source. Azobenzene, which undergoes trans- to cis- photo-isomerization, has drawn attention as a candidate material for solar thermal fuels. However, both isomers are photoactive in similar regions of the solar spectrum, and the metastable cis-isomer exhibits a significant absorption coefficient, leading to a photo-stationary state (dynamic equilibrium of the two directions of photoisomerization) with a significant amount of the lower energy trans isomer and a resulting low energy storage capacity. In MRS 2014 Spring meeting, using time-dependent density functional theory (TDDFT) methods to analyze the spectra of azobenzene, we showed that energy-charged-state molecule (cis-isomer) content at the photostationary state can be improved from 73 percent for pure azobenzene to 83 percent and 97 percent by functionalizing azobenzene and a designing different geometry for azobenzene, respectively. In this talk, we attempt to understand how such improvements in absorption spectra and photostationary state of azobenzene could be made by looking at the effects of substitution and geometry on the charge density and excited states.

This talk will review recent work employing first principles DFT+U and GW theories aimed at understanding and optimizing mechanisms of solar energy conversion to electricity by non-standard light-absorbers made from abundant elements: nickel-oxide (NiO) alloys, Copper Zinc Tin Sulfide/Selenide (CZTS), and lead halide perovskites (LHPs). NiO has been used as back electrode in tandem dye-sensitized solar cells but its wide band gap and low valence band edge hinders light absorption and hole injection. We will describe how band edge/gap engineering via alloying may overcome both problems. CZTS is a zincblende-like compound, with a tunable direct band gap (1.0 to 1.5 eV) that also contains only cheap, non-toxic elements. The record efficiency for CZTS is still too low (~11%), limited by fast non-radiative recombination of charge carriers at point defects, interfaces, secondary phases or grain boundaries. We will show that carefully exploitation of surface and interface properties may reduce formation of recombination centers and thereby improve efficiency. LHPs have taken the photovoltaic community by storm during the last couple of years; we contribute by offering insights into the special electronic structure character of the LHPs that lead to long lifetimes and efficient transport of charge carriers

Rare earth elements are amongst the most critical materials - non only is the world market supply limited but also they are crucial for a number of energy related technologies. However, due to their special photophysical propoerties rare earth materials are of great interest as photonic materials. It has been estabilished that the efficiency of semi-conductor solar cells can be improved substantially through special rare earth materials that can efficiently convert high energy UV photons into more than one photon of lower energy that matches the band gap of the SCSS material, so called quantum-cutting materials. Substituting most of the precious rare earth ions and simultaneously retaining the efficiency of the energy conversion phosphor at the nanoscale is a major and quite challenging goal. In that context alkaline earth fluorides appear to be the cheapest, benign and readily available materials. We succeeded in synthesizing highly efficient quantum cutting phosphors based on alkaline earth host lattices with low rare earth ion concentrations nanoparticles doped with trivalent lanthanide ions. As doping trivalent ions into a host with divalent cations requires charge compensation, this effect was thoroughly studied by powder X-ray and electron diffraction, luminescence spectroscopy and ²³Na, ¹³⁹La and ¹⁹F solid state NMR spectroscopy to gain a better understanding of these materials. In the end, the rare earth content (cation content) of nanoparticles with a size of less than 10 nm could be reduced by 94 % compared to typical GdF₃:Eu VUV quantum cutting material while simultaneously retaining the quantum cutting efficiency close to the theoretical limit (199 %). With such performance values, these materials appear quite promising for solar energy conversion.

WO₃ is a benchmark material for photoelectrochemical water splitting (PEC-WS) with demonstrated saturated photocurrent of up to 60% of the theoretical maximum. Moreover, it has been used in a wide range of heterostructure architectures coupled with a lower band gap semiconductor (e.g. with Si, Fe2O3, TiO2, BiVO4). Hence, the development of WO3 nanostructures with excellent transport properties, light trapping capability and engineered electronic band structure is of paramount importance in the field of PEC-WS. In this contribution, we present our recent work on quasi-1D hyperbranched WO₃ nanostructures exhibiting unmet photocurrent values at low bias voltage of 1.7 mA/cm² at 0.7 V vs RHE, while reaching state of the art saturated photocurrents at higher biases. The quasi-1D hyperbranched WO₃ nanostructures are fabricated by pulsed laser deposition (PLD) exploiting self assembly from the gas phase. Overall, the novel photoanode resembles a forest composed of individual, high aspect-ratio, tree-like structures, assembled from crystalline nanoparticles. These novel structures are stable upon annealing in air at 500 °C, exhibiting monoclinic crystalline phase. The hierarchical quasi-1D nature of each tree represents an innovative compromise between nanorods/nanotubes (better electron transport) and the conventional isotropic nanoparticle photoanode (high surface area). Moreover, the peculiar geometry tends to scatter and trap light, similarly to what shown with TiO2 hyperbranched nanostructures by some of the authors in a previous work. Optical characterization and IPCE measurements confirm enhanced absorption and photoaction at wavelengths as high as 500 nm. The origin of the excellent behaviour at low biases is attributed to the low density of defects and long lifetime of electrons in the hyperbranched nanostructure as suggested by electrochemical impedance spectroscopy (EIS). This ensemble of peculiar properties candidate these quasi-1D WO3 nanostructures as a promising material for PEC-WS per se, and as electron acceptor scaffold in heterostructure architectures. Preliminary results with oxygen evolution reaction (OER) electrocatalyst will be also presented.

The present study demonstrates the development and synthesis of a nano-scale catalyst for methanol steam reforming and its integration into a catalytic reactor inside a non-concentrating solar-thermal collector. The solar collector is used to capture thermal energy at sufficient temperature to drive hydrogen production by steam reforming of methanol.
In conventional fuel reforming systems, the thermal energy required to preheat water and fuel to the reaction temperature, evaporate liquid water and fuel, compensate heat losses, and overcome the reaction enthalpy of the catalytic steam reforming is generated by burning part of the initial fuel. This typically costs approximately half of the fuel. In the solar-powered system of this study, all fuel can be converted to hydrogen, since the heating requirement is fulfilled by solar power.
Catalyst particles fabricated by a novel flame spray pyrolysis (FSP) method resulting in a highly active catalyst with high surface-to-volume ratio were compared to a commercially produced catalyst (BASF F3-01). Both catalysts consisted of CuO/ZnO/Al₂O₃ of identical composition (CuO 40wt%, ZnO 40wt%, Al₂O₃ 20wt%). Reaction temperatures between 220 and 295°C, methanol-water inlet flow rates between 2 and 50 mu;l/min, and reactor masses between 25 and 100 mg were tested for their effect on methanol conversion and the production of undesired carbon monoxide. 100% methanol conversion can be easily achieved within the operational conditions mentioned for this flame-made catalyst - at reactor temperatures of 255°C more than 80% methanol conversion can be reached for methanol-water inlet flow rates as high as 10 mu;l/min. The FSP catalyst demonstrates similar catalytic abilities as the commercial counterpart, produces a consistent gas composition and produces lower overall CO production. Furthermore, the FSP catalyst demonstrates a better suitability to fuel cell use through its higher resistance to degradation and smaller production of carbon monoxide over long-term use.
The novel design of a non-concentrating solar collector reaches temperatures up to 270°C, which is sufficient for methanol steam reforming. By using optimized nano-structured Cu/ZnO/Al₂O₃ catalyst particles, methanol steam reforming was experimentally demonstrated under solar irradiance of 1 sun (1000 W/m²). It was possible to generate up to 7 liters of hydrogen per m² of irradiated area, which corresponds to approximately 1000 W/m² output of hydrogen (using lower heating value).
This study presents the synthesis of the nanocatalysts, the design and fabrication of the solar collector-reactor, its testing and optimization, and the integration and testing of catalytic nanoparticles inside the solar collector, resulting in an integrated solar collector-reactor entirely powered by solar irradiation.

Harvesting solar energy by photon absorption in metal nanostructures and collecting photo-generated hot electrons via the processes of thermionic emission and/or tunneling has been recently actively explored as a promising alternative approach to traditional photovoltaics as well as for catalysis and photo-detection. Traditionally, metals such as Au or graphene were considered as candidates for gapless photon absorbers that are potentially capable of full spectrum harvesting; however, to date, the demonstrated conversion efficiencies have been extremely low. We will discuss a way to increase the efficiency limits for full-spectrum harvesting of solar light and hot electron extraction via the use of thin graphite layers. The use of graphite instead of more conventional materials such as gold or graphene offers carbon-based broadband light harvesting as well as potentially higher conversion efficiency, lower cost and easier fabrication. We will demonstrate that graphite has high promise as a photoactive material as it simultaneously offers (1) high broadband photon absorption, (2) slow hot electron decay, and (3) electron DOS that yields a sizeable population of hot electrons with the energies high enough to be collected across the potential barrier.

Recent advances on tin monosulfide (SnS) based thin-film solar cells have resulted in certified efficiency records of 3.88% [1] and 4.36% [2] for thermally evaporated and pulsed chemical vapor deposited SnS, respectively. In principle, congruent evaporation of SnS can enable high throughput processing and industrial scale-up (similar to CdTe). However, despite rapid developments in the SnS device performance, efficiencies > 10% are desired to attract attention in the photovoltaic industry.
Here, we present our latest device progress on thermally evaporated SnS solar cells. We present a detailed current loss analysis on our champion device, speaking for a straight forward continuation of our work to reduce recombination losses in the SnS bulk and at the SnS / buffer interface as well as shading and reflection losses in the device stack. We have shown that post-deposition treatment of SnS films can improve the bulk and surface properties but it may also change the preferred grain orientation and the SnS band-edge energies [1, 3]. By systematically varying the stoichiometry of our Zn(O,S):N buffer layers, we show that the optimal buffer layer composition may depend on the SnS post-treatment. To reduce reflection losses, we present first results on applying an anti-reflection coating to our SnS devices. In all our optimization studies, we attempt to stick to a simple device stack to guarantee high device reproducibility and low batch-to-batch variation.
1. V. Steinmann, et. al. submitted (2014).
2. P. Sinsermsuksakul, et al., Adv. Energy Mater. accepted (2014).
3. V. Stevanovicacute;, et al., Appl. Phys. Lett.104, 211603 (2014).

The application of techniques capable of resolving ultrafast carrier recombination dynamics (<1 ns) is important for the development of earth abundant materials for harvesting solar energy [1]. By using time-resolved terahertz spectroscopy (TRTS), information on carrier mobility and lifetime can be decoupled and extracted all optically. In this work, we generate sub-picosecond and broadband terahertz pulses to detect the dynamics of free charge carriers in materials. We use optical-pump/terahertz-probe measurements to study chalcogen-hyperdoped silicon, a potential material for intermediate-band solar cells. Chalcogen-hyperdoped silicon (achieved via ion implantation followed by ns-laser melting) exhibits strong below-band-gap light absorption, and the optoelectronic properties are further controlled by dopant compensation [2]. We photoexcite the material with both above- and below-band-gap light to study the impact of ultrahigh dopant concentrations on carrier recombination. We find the recombination dynamics are described by two exponential decay time scales: a fast decay time scale ranges between 1 and 200 ps followed by a slow decay on the order of 1 ns. In contrast to prior theoretical predictions, we find that the carrier lifetime decreases with increasing dopant concentration up to and above the insulator-to-metal transition. Evaluating the material&’s figure of merit reveals an optimum doping concentration for maximizing solar cell performance. This study will impact our understanding of carrier transport processes in intermediate band solar cells, while also paving the way for the use of ultrafast terahertz spectroscopic techniques for direct evaluation of emerging materials&’ potential for both photovoltaic and photoelectrochemical devices.
[1] Baxter and Schmuttenmaer, J. Phys. Chem. B 110, 25229, (2006)
[2] Simmons et al, Adv. Funct. Mater 24, 2852, (2014)

Achieving low cost, high efficiency solar cells with earth-abundant materials will require a thorough understanding of the efficiency limitations and reliability issues. Because the defects that cause lower efficiency or device failure are often buried within the device, it is difficult or impossible to probe these defects with traditional characterization techniques. Developing new characterization tools that can non-destructively probe surfaces, interfaces and bulk material within photovoltaics in-situ will enable researchers to zero-in on limiting factors of their devices and provide insight into potential improvements.
Both surface and bulk defects play an important roll in device performance. To probe below the surface, we have developed a two-photon microscopy technique that allows for 3D imaging of solar cells. This microscopy technique operates on the principle that many semiconductors are generally transparent to infra-red (IR) light. Since only when sub-bandgap light is focused sharply does non-linear optical absorption occur, using a pulsed IR laser enables us to locally, sub-surface carriers. In particular, we are able to measure 3D maps of local carrier lifetimes, emission spectra, and induced photocurrent with this technique. We have now successfully imaged, in 3D, carrier dynamics within grains and at grain boundaries in polycrystalline PV materials. These grains and grain boundaries are below the surface of the material, and would be otherwise inaccessible without the penetrating power of the two-photon technique. By correlating our complementary near-field “surface-only” measurements with the multiphoton depth-resolved mapping, we gain penetrating insights into the complex relationships that determine efficiency bottlenecks in PV materials.

Optimization of the grain-boundary structures of multicrystalline silicon (mc-Si)-Si is a key issue to achieving high efficiency, because these regions act as recombination centers for carriers in solar cell materials. Multicrystalline Si with artificially-controlled grain orientations has been proposed as a means of reducing the number of electrically-active grain boundaries that lead to undesirable carrier recombination[1]. In the present study, we applied the spherical model including a grain boundary under a given misorientation, the right cluster is rotated about the <110> or the <112> orientation and the left cluster is rotated about the <110> or the <112> orientation. The <110> and <112> axes correspond to the preferred growth directions. These geometries correspond to a grain boundary in multi-crystalline silicon under dendrite growth conditions[2]. In this case, the prepared grain boundary is located at the center of the spherical model. To perform structural relaxation, we apply a Monte Carlo (MC) method based on the Tersoff potential [3] for the silicon system. Moreover, we used DFT method to understand relationship between sigma value and impurity atoms. First we study the dopant position and the nature of interaction between the grain boundary and transition metal, such as copper, iron, nickel and chromium. Finally we have studied the electronic changes that occurred up on doping the transition metal impurities in the grain boundary regions using Sigma grain boundaries. The segregation energy for the impurities in Sigma 3(111) follows the order of Fe greater than Cu, Ni, and Cr at the substitutional site and Cr greater than Cu, Fe, and Ni, at the interstitial site. The calculated values were positive, indicating that segregation is not favored in the Sigma 3(111) grain boundaries. When the metal impurity is placed at the substitutional site, a new state in the fundamental gap was observed in the density of states, the band gap is reduced, which may have an effect on the solar cell performance [4]. This research used computational resources of the HPCI system provided by Cyberscience Center, Tohoku University and Information Initiative Center, Hokkaido University through the HPCI System Research Project (Project ID: hp120014, hp140063). The authors would like to express their sincere thanks to the staff of the CCMS, IMR, Tohoku University for their continuous support of the SR16000-M1/320 supercomputing facility.
[1] N. Usami et al., Jpn. J. Appl. Phys. 45, 1734 (2006).
[2] I. Takahashi et al., J. Appl. Phys., 109, 033504 (2011).
[3] J. Tersoff, Phys. Rev. B39, 5566 (1989).
[4] A. Suvitha et al., Jpn. J. Appl. Phys. 49, 04DP02 (2010).

In recent years, self-assembly of nanomaterials into functional films has become a popular and effective strategy for nanofabrication. Since discovered in 2004, oil-water interfacial self-assembly of nanostructures has become a novel strategy for fabrication of nanofilms.[1] It is a powerful bottom-up approach for the film fabrication due to low cost and high efficiency, and is simply and universal for almost all low-dimensional nanostructures..
Herein, we present our present work on nanofilm and nanodevice fabrication from this novel and low-cost strategy. Layered rare-earth hydroxide nanoplatelets were densely trapped at an oil-water interface to from a high-quality nanofilm, and all the crystallites were densely aligned on the substrate with their [001] direction perpendicular to the substrate surface [2]. The rare-earth oxide nanofilm obtained from the annealing of the hydroxide precursor nanofilm showed excellent luminescence properties with very high emission intensity. Subsequently, a series of optoelectronic devices, such as NiCo₂O₄, SnO₂, ZnO monolayer and ZnO/ZnS composited nanofilm-based photodetectors, have successfully fabricated from this new method, which show high UV-light sensitivity, excellent stability and fast response time less than 0.3 s [3]. The as-constructed thin-film based optoelectronic devices are quite promising for applications such as optical communications, flame sensing, missile launch. Furthermore, we have fabricated a high-performance CoO nanofilm-based electrical resistive switching device from this self-assembly method [4]. A gigantic change in resistance induced by external electric field has been observed, and a very fast switching between the high resistance state (OFF) and low resistance state (ON) in ns level has also been realized for this device.
This novel and facile strategy significantly decreases the cost of fabrication procedure, and can also be extended to fabricate thin-film based supercapacitors, photovoltaic devices and field effect transistors (FETs) [5].
References
[1] H. W. Duan, D. Y. Wang, D. G. Kurthand, H. Möhwald. Angew. Chem. Int. Ed. 2004, 43, 5639.
[2] (a) L. F. Hu,, R. Ma, T. C. Ozawa, F. Geng, N. Iyi, T. Sasaki, Chem. Commun. 2008, 4897.
(b) L. F. Hu, R. Ma, T. C. Ozawa, T. Sasaki, Angew. Chem., Int. Ed. 2009, 48, 3846.
(c) J. Huan, L. F. Hu, X. S. Fang. ACS Appl. Mater. Interfaces 2014, 6, 1462.
[3] (a) L. F. Hu, L. M. Wu, M. Y. Liao,X. S. Fang. Adv. Mater. 2011, 23, 1988.
(b) L. F. Hu,L. M. Wu, M. Y. Liao, X. H. Hu, X. S. Fang. Adv. Funct. Mater. 2012, 22, 998.
(c) H. Chen, L. F. Hu, X. S. Fang, L. M. Wu. Adv. Funct. Mater. 2012, 22, 1229.
(d) L. F. Hu, M. Chen, W. Z. Shan, T.Zhan, M. Y. Liao, X. S. Fang, X. H. Hu, L. M. Wu. Adv. Mater. 2012, 24, 5872.
[4] R. Ma, M. Osada, L. F. Hu, T. Sasaki. Chem. Mater. 2010, 22, 6341.
[5] L. F. Hu, M. Chen, L. M. Wu, X. S. Fang. Chem. Soc. Rev. 2012, 41, 1350.

ZnSnN₂ as part of the Zn-IV-N₂ family of materials offers a potential earth abundant non-toxic element alternative for photovoltaic applications. ZnSnN₂ also offers the possibility to continuously tune the band gap by controlling the degree of order/disorder of the cation sublattice. The density functional theory (DFT) predicted structure for ZnSnN₂ is orthorhombic for the ordered lattice and is hexagonal for the disordered structure. We have grown a series of epitaxial single crystal thin films using the plasma-assisted molecular beam epitaxy technique to enable the determination of fundamental properties. Here we report the results of a detailed study of growth parameters as they affect compositional, structural and defect characteristics of the layers. Specifically, we have investigated a range of substrate temperatures and cation flux ratios.
Films grown with a near-unity metal flux ratio did not exhibit the desired stoichiometry, typically exhibiting Sn-rich composition ; unsurprisingly, reflection high energy electron diffraction (RHEED) patterns of these films indicated poor crystal quality. Increasing the zinc flux and the substrate temperature led to the growth of single crystal films with the desired composition, as confirmed by the RHEED patterns and Rutherford backscattering spectrometry (RBS). So far, hard x-ray synchrotron diffraction measurements have shown that most films have hexagonal lattice structures, however, there is evidence indicating variations from the hexagonal structure in at least one case.
All films grown to date are unintentionally degenerate n-type. The evolution of the RHEED patterns during growth indicate signs of strain in initial layers due to the lattice mismatch with the substrate, which may be producing defects within the films. Other types of defects and their dependence on the growth conditions are currently being investigated using positron annihilation, transmission electron microscopy and RBS/ion channeling measurements.
This work was supported in part by NSF grants DMR-1244887 DMR-1410915.

Efficient Cu₂ZnSnSe₄ (CZTSe) solar cells were fabricated with a simple, environmentally friendly, and scalable synthetic method for Cu₂ZnSnS₄ (CZTS) nanocrystals. CZTS nanoparticles were mechanochemically synthesized from elemental precursors on a relatively large scale (~20 g), during which no solvents or additives were used, thus alleviating the complex process of particle synthesis. An analysis of the time evolution of the crystalline phase and morphology of precursor powders revealed that the formation of the CZTS compound was completed in 0.5 h once initiated, suggesting that the mechanochemically-induced self-propagating reaction prevails. CZTS ink was prepared by dispersing the as-synthesized nanoparticles in an environmentally benign solvent (160 mg/mL in ethanol) without using any additives, after which it was cast onto Mo-coated glass substrates by a doctor-blade method. Subsequent reactive annealing at 560 °C under a Se-containing atmosphere resulted in substantial grain growth along with the nearly complete substitution of Se. The CZTSe solar cells therefrom exhibited power conversion efficiency levels as high as 6.1% (based on the active area, 0.44 cm²) with a relatively high open-circuit voltage (0.42 V) in comparison with the bandgap energy of 1.0 eV.

Large amount of energy is consumed for decomposition of toxic hydrogen sulfide (H₂S) gas emitted from various industrial process and also from natural sources. On the other hand, it is also true that H₂S has great potential for the source of hydrogen energy because of its low decomposition potential. For example, in recent years, various researchers reported the photocatalytic H₂ production from H₂S aqueous solution. In this case, only the solar energy is consumed during the decomposition of substance. Hence, the conversion of H₂S into H₂ using photocatalysts provide one of the solution for environment and energy problems. CdS is one of the most popular photocatalysts with the decomposition of H₂S under visible light irradiation, nevertheless noble metals (Pt, Pd and Au), which are expensive and not rich in nature, have been extensively used as H₂ production co-catalyst. Thus, in terms of commercial applications, economical co-catalysts with high activities are urgently needed for solar H₂ production. Recently, transition metal sulfides such as MoS₂ and NiS, have been discovered as noble-metal-free co-catalysts for efficient H₂ evolution when deposited onto the surface of TiO₂ [1]. However, its stability is relatively low (it cannot recycle if it is once dry up), and also stability of these co-catalysts under sulfurized condition. Therefore, relationship between the loaded condition (morphology, crystallinity, conjunction condition with photocatalysts) and properties (stabilities, activities) of transition metal (Ni, Mo, Fe, Co, Cu) sulfides on the surface of CdS photocatalysts was evaluated under sulfurized condition. CdS with capsule like morphology, called as stratified CdS photocatalysts, was used as base materials since it shows extremely high catalytic activities. Metal loaded s-CdS photocatalysts (MS_x/s-CdS, M= Ni, Mo, Fe, Co) was synthesized by utilizing in-situ photo-deposition method. For comparison, s-CdS and Pt/s-CdS (Pt: 0.3mM) was also synthesized. Photocatalytic activity was evaluated through the amount of H₂ production from the Na₂S solution under irradiation of Visible light. Synthesized materials were characterized by using XRD, STEM, and line analysis with EDX. Results of STEM and EDX show that NiS deposits on s-CdS surface. From result of photocatalytic activity, s-CdS showed very low H₂ production, while in-situ NiS photo-deposited s-CdS (NiS/s-CdS) showed high H₂ generation ratio (6.0ml/h), and its TOF (turn over frequency) was over 1. Photocatalytic activity of NiS/s-CdS has no deactivation during measurement. These results indicated that NiS can acts as co-catalyst under sulfurized condition and potential catalyst as economical co-catalyst. Other results will reported in our presentation.#12288;Part of this work has been supported by the Grant-in-Aid for Scientific Research (B) (No. 26281054). [1]INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 37 (2012) 17060-17067

#12288;#12288;Efficiency of CIS (CuInSe₂) solar cell, one of the attractive solar cell materials, has reached up to 20.9%, which is comparable with that of Si solar cells. However, it is also true that these cell are deposited in high vacuum condition, which lead the loss of natural resources. So, various researches were vigorously researched of chemical reduction method since it is ecological and also economical technique. Ye et al.^[1] synthesized Cu-In alloy particles by using chemical reduction method in organic solvent and the Cu-In alloy precursor films were transformed to CuInSe₂ films by selenization. The resulting cells demonstrate a maximum energy conversion efficiency of 3.92 %. However, total cost of this method is not low since organic solvent should be used. #12288;#12288;Therefore, in this study, CIS solar cell was tried to construct by using Cu-In alloy precursors prepared in aqueous phase by our method ^[2]. Cuminus;In alloy nanoparticles were synthesized by chemical reduction method under ambient conditions using metal chlorides as metal sources and sodium borohydride as a reducing agent. Synthesized Cu-In nanoparticles (0.4 g ) was dispersed in 4 mL of 3-methyl-3-methoxybutanol to make a sample dispersion. Cuminus;In alloy nanoparticles dispersion was coated onto Mo-sputtered glass substrates by spin coat to make Cuminus;In alloy precursor films. The desiccated thin films were selenized at temperatures ranging from 300 0C to 575 0C for 60min. #12288;#12288;By increasing the selenization temperature to 575 0C, the grain size of CuInSe₂ films were increased to around 1 mu;m. The CuInSe₂ thin film solar cells synthesized by this method exhibited a photovoltaic conversion efficiency of 0.5 % under AM 1.5 G illumination. However, many pinhole, which lead defective contact of electrode and causes a decrease in efficiency, was formed on the CuInSe₂ thin film surface at this treatment condition. Therefore, in order to form pinhole-free films by spin coat, particle size of Cu-In ally nanoparticles precursor was controlled to have narrow particle size distribution. Moreover, to control the dispersion stability of Cu-In alloy particles in aqueous solution, surface of Cu-In ally nanoparticles precursor was modified by using various surfactant, such as cetyltrimethylammonium bromide (CTAB), polyvinyl pyrrolidone (PVP, K25), and sodium dodecyl sulfate (SDS). As a result, diameters of Cu-In particles become 30-70 nm. Other results will be reported in our presentation. This work has been supported by the Grant-in-Aid for Scientific Research (B) (No. 26281054). [1] Ye Seul Lim, et.,al, J. Phys. Chem. C, 2013, 117 (23), pp 11930-11940, [2] H. Takahashi et.,al, Applied Catalysis A: General 392 (2011) 80-85.

Silicon based intermediate band semiconductors are candidates of high efficiency solar cells using earth-abundant materials. Emergence of broad and strong mid-infrared (MIR) optical absorption band has been reported for the silicon hyperdoped with sulfur and correlation between the intermediate band and this absorption band has been discussed. In spite of these efforts, origin of the MIR band has not been enough understood. In the present paper, correlation between crystallinity of hyperdoped layer and behavior of the MIR absorption band is discussed to clarify the origin of this absorption band.
We prepared silicon hyperdoped with sulfur by ion-implantation to dose of 1x10¹⁶ ions/cm² followed by pulsed YAG laser melting. The images of cross sectional transmission electron microscope indicate that the amorphous sulfur ion-implanted layers changed to monocrystal by following pulsed laser melting above the fluence of 1.0J/cm². The optical absorption spectra and photoconductivity of the samples prepared under different fluences were measured.
We found that the broad MIR absorption is not single band but is composed of at least two bands peaked at around 0.6 eV and 0.4 eV. The component below 0.4 eV might also exist. Furthermore, the band-tail and free-carrier absorptions overlap with these bands. The 0.4 eV band is observed when the laser fluence is less than 1.0 J/cm², which is not enough to form monocrystal layer. Since this band is also observed for the silicon wafer ion-implanted with silicon, the origin of this absorption does not directly correlate with sulfur atoms. The 0.4 eV band and band-tail absorption components are observed for as-sulfur-implanted wafer and their intensity attenuate with increasing the laser fluence. On the other hand, the 0.6 eV band is not observed for as-sulfur-implanted silicon wafer and it increases with the laser fluence. This ensures that presence of excess sulfur atoms and the laser melting process induce the 0.6 eV band. The 0.6 eV band and the free-carrier absorptions are predominant when prepared above 1.1 J/cm², which corresponds to the fluence to form monocrystal hyperdoped layer. These results indicate that the 0.6 eV absorption band is due to the meta-stable state induced during the hyperdoping of sulfur in monocrystal silicon lattice.
The decrement of the photocurrent observed above the band gap of pristine silicon wafer, which is due to the surface recombination, is reduced by the formation of sulfur hyperdoped surface layer. This indicates that the hyperdoped layer acts as surface passivation layer and excess sulfur atoms do not act as killer centers. However the photocurrents corresponding to the band-to-band and band-tail absorptions are observed, that corresponding to the 0.6 eV band was not observed. Correlation between carrier generation and optical absorption due to 0.6 eV band is discussed based on these results.

Silicon quantum dots (QDs) embedded in a high band gap Si-rich alloy are promising candidates for multi-junction solar cells of which the band gap can be tuned by the QD size. The mean size and size distribution are crucial parameters, but - due to their small dimensions - are difficult to measure. These parameters are often obtained using TEM, which is time-consuming and destructive, and Raman spectroscopy, which is inaccurate for samples with internal stress. Furthermore, these methods are limited: one can obtain the QD size and distribution, but not the absorption parameters that are of most interest.
In this contribution we show how we can obtain the QD absorption parameters directly using a fast, nondestructive measurement and simple analysis. These parameters can be extracted from absorption measurements using a simple routine in which sub-band gap and super-band gap absorption were fitted with cumulative normal distributions. For the embedded QDs we introduce a third cumulative normal distribution to account for their extra absorption. Its parameters reveal the QD absorption position and width. Earlier studies have shown that the position of QD absorption is determined by the QD size [2] and surface passivation [3]. Its width increases with an increase in QD size distribution [4].
For thermally annealed samples, Photothermal Deflection Spectroscopy (PDS) measurements show that an increase in annealing temperature leads to an increase in sub-band gap absorption and a redshift of the E₀₄ gap [1]. However, this shift cannot be solely attributed to QD absorption, as it is also affected by other changes in the material, like the broadening of tail states and increase in defect density caused by hydrogen effusion. We demonstrate our analysis on laser-annealed amorphous silicon oxide films. Laser annealing allows local annealing, and annealing of specific layers depending on their band gap and laser wavelength. For laser annealed a-Si_0.66O_0.34:H films we observe the crystallization onset at a fluence of 140 mJ/cm² from Raman measurements. With increasing laser fluences above this threshold, increasing crystalline fractions up to 0.8 are observed. In our PDS spectra we discern an absorption shoulder for samples containing QDs, which we attribute to absorption of embedded Si QDs. Our analysis shows a redshift of this shoulder from 2.5 to 2 eV for increasing laser fluences accompanied by a smaller absorption width, indicating larger mean QD sizes and more narrow size distribution. This energy range corresponds well with calculations performed using density functional theory [2,3]. Such a simple analysis has never been done before and provides a fast and nondestructive characterization of embedded QDs.
[1] Ding et al., 2013, Sol. Energ. Mat. Sol. Cells, 10.1016/j.solmat.2013.10.012
[2] Seino et al., 2009, Nanotechnology, 20, 135702
[3] Koponen et al., 2009, Phys. Rev. B, 79, 235332
[4] Ledoux et al., 2000, Phys. Rev. B, 62, 15942

Solar cells incorporating a tin sulphide (SnS) absorber have to date only achieved photovoltaic (PV) efficiencies of approximately 2% [1], whereas copper zinc tin sulphide/selenide (CZTS/Se) has reached cell efficiencies >11% [2]. However, secondary phase formation can readily occur in the quaternary structure leading to reduced photovoltaic performance of the solar cell. There may be a number of reasons for low PV cell efficiency for SnS-based solar cell devices, including small grain size leading to a high density of grain boundaries that act as recombination centres [3] and unsuitable chemical band offset [4] between the typical n-type CdS layer and p-type SnS. High growth temperatures have been shown to benefit grain size for CdTe-based solar cells [5]. The metal organic chemical vapour deposition (MOCVD) process offers a potential route for increasing SnS thin film grain size due to thermally stable high volatile Sn and S precursors allowing for relatively high deposition temperatures to be employed. An initial study was carried out using the MOCVD process with tetramethyltin as the tin source and diteriarybutylsulfide and diethyldisulfide as the sulfur source. A horizontal reactor configuration was used for initial experiments. High deposition temperatures (>500°C) were necessary for tin to nucleate on the substrate. Incorporation of sulfur was difficult using the mono-sulfide precursor, which was replaced with the di-sulfide precursor to enhance sulfur incorporation into the Sn-rich films. However, energy dispersive X-ray spectroscopy (EDX) showed that only ~10% of sulfur was in the upstream region of the SnS_x layer decreasing down to ~2% in the downstream region of the deposited film, indicating strong pre-reaction. In order to overcome the issue of low sulfur incorporation another MOCVD reactor was used with injectors&’ configured perpendicular to the substrate surface delivering both precursors over a shorter distance. The tin and sulfur precursors were premixed before injection to improve chemical reaction in the gas phase. This paper reports on the optical and structural properties of the consequent SnS_x films and their suitability for application in solar cell devices. References [1] P. Sinsermsuksakul, K. Hartman, S. B. Kim, J. Heo, L. Sun, H. H. Park, R. Chakraborty, T. Buonassisi, R. G. Gordon, Applied Physics Letters 102 (2013) 053901. [2] T. K. Todorov, J. Tang, S. Bag, O. Gunwan, T. Gokmen, Y. Zhu, D. B. Mitzi, Advanced Energy Materials 3 (2013) 34. [3] B. Ghosh, M. Das, P. Banerjee, S. Das, Solar Energy Materials and Solar Cells 92 (2008) 1099. [4] J. Xu, Y. Yang, Energy Conversion and Management 78 (2014) 260. [5] C. S. Ferekides, U. Balasubramanian, R. Mamazza, V. Viswanathan, H. Zhao and D. L. Morel, Solar Energy 77 (2004) 823.

Solar thermal fuels (STFs) are an unconventional paradigm for solar energy conversion and storage which is attracting renewed attention. A material absorbs sunlight and stores the energy chemically via an induced structural change, which can later be reversed to release the energy as heat. An example is the azobenzene molecule which has a cis-trans photoisomerization with these properties, and can be tuned by chemical substitution and attachment to templates such as carbon nanotubes [A. M. Kolpak et al., Nano Lett.11, 3156 (2011); T. Kucharski et al., Nat. Chem.6, 441 (2014)]. By analogy to the Shockley-Queisser limit for photovoltaics, we analyze thermodynamic constraints on STF efficiency, stored energy density, and storage lifetime. In particular, we derive limits to percent conversion in the photostationary state, the quantum yield for photoisomerization, and the structure of feasible potential-energy surfaces for STFs. We show optimal values of material parameters and compare to ab initio calculations and experimental measurements.

Luminescent solar concentrators (LSCs) have the potential to enhance the economic viability of solar energy. They concentrate both direct and diffuse incoming sunlight, and thus need no expensive tracking equipment. LSCs consist of a flat waveguide doped with fluorophores that absorb the incoming sunlight and re-emit it at a longer wavelength. Depending on the angle of re-emission, photons are trapped due to total internal reflection and guided towards the side surfaces of the waveguide. Solar cells fixed to the side surfaces will then convert photons into electricity. Since their inception in the late 70s, LSCs have been struggling with the following deficiencies: escape cone losses, re-absorption losses and limited spectral efficiency. Aligning the dye molecules with the help of a liquid crystal can lead to a higher trapping efficiency and thus lower escape cone losses. By aligning the polarizability axis of the dye molecule normal to the top surface, ie homeotropic alignment, the escape cone losses can be reduced from about 25% (random dye orientation) to 9% (perfectly aligned) for a waveguide with refractive index of 1.5. However, at a high degree of homeotropic alignment the absorption of incoming light is strongly reduced. To avoid the low absorption one can add a second dye, the absorption fluorophore, to the LSC that absorbs the incoming light more efficiently and transfers the energy to the homeotropically aligned dye, the emission fluorophore, through Förster resonance energy transfer (FRET). We have developed a Monte-Carlo ray tracing tool that incorporates FRET and dye alignment and can compute the efficiency of this LSC design. With this tool we can determine the optimal a) dye concentration, b) distance between the two types of dye molecules, c) absorption peaks separation between the absorption and emission dye and d) quality of homeotropic alignment. We show that the theoretical efficiency of a dye-doped LSC can be enhanced by up to 82.9% by utilizing FRET and dye alignment. The design becomes even more efficient by using a quantum dot as absorption fluorophore due to the quantum dot's wide absorption band. Additionally, deficiencies of quantum dots such as low quantum yield and high re-absorption are circumvented by the design. This is due to the energy transfer efficiency from the absorption to the emission fluorophore decreasing only marginally for lower quantum yields of the absorption fluorophore. Also, re-absorption by the absorption fluorophore is limited as the photon experiences a strong red-shift due to FRET. This knowledge gained from our modeling tool should greatly simplify fabrication of such advanced LSCs in the future. We aim to verify these promising results experimentally and further advance our ray tracer over the upcoming months and present our latest results at the conference.

A supercritical fluid-assisted electrochemical deposition has been developed to grow ZnO mesocrystals directly on conductive substrates for photoelectrochemical applications.^[1] The supercritical CO₂ and a nonionic surfactant were employed to form emulsified electrolyte, which significantly increased the supersaturation degree and promoted molecular diffusion to result in the growth of ZnO mesocrystals. The as-prepared ZnO mesocrystals exhibited fascinating optical properties at room temperature, including prominent near band-edge emission and substantially long exciton lifetime. The results showed that ZnO mesocrystals displayed remarkable photoactivity toward photoelectrochemical water oxidation, attributable to the facile charge transfer resulting from the highly oriented crystallinity as well as the lasting exciton survival enabling the further carrier utilization. The feasibility of the present electrochemical method for direct deposition of ZnO mesocrystals on conductive substrates shall push forward their advanced applications in a wide array of fields. The present sc-CO₂ emulsion-assisted electrochemical route can be readily extended to production of mesocrystals of other functional metal oxides, for example, TiO₂ and NiO.^[2,3]
References:
[1] W.-H. Lin, T.-F. M. Chang, Y.-H. Lu, T. Sato, M. Sone, K.-H. Wei, Y.-J. Hsu, J. Phys. Chem. C2013, 117, 25596minus;25603.
[2] T.-F. M. Chang, W.-H. Lin, Y.-J. Hsu, T. Sato, M. Sone, ECS Electrochem. Lett. , 2014, 3, D1-D2.
[3] T.-F. M. Chang, W.-H. Lin, Y.-J. Hsu, C.-Y. Chen, T. Sato, M. Sone, Electrochem. Commun.2013, 33, 68-71.

Iron pyrite (FeS₂) has attracted much attention for active layer of thin-film solar cells with non-toxic and earth abundant materials. FeS₂ has high absorption coefficient in visible to IR region due to the narrow band gap of 0.95 eV. Solution-derived FeS₂ has also been actively proposed, suggesting it has great possibility to realize large production of printed solar cells through cheaper process with quite low-cost material. The conversion efficiency of FeS₂ cells has recorded around 3 % as the highest value in photoelectrochemical cells. It has, however, a large potential for improvement by the controlling of impurity ions and fermi level. In this study, we developed FeS₂ film by electro-spray method, which is one of the spray techniques as a cheaper process, and estimated band gap of the solution-derived FeS₂ film by the changing of the precursor solution to understand the effect of impurity.
The electro-spraying generates droplets by applying a high voltage to a nozzle tip. It allows us to make smaller droplets than the other spray methods and obtain homogeneous and thin FeS₂ films on counter electrode. As precursor solutions, FeCl₃ or Fe-acetylacetonate solution in ethanol are prepared and sprayed to TCO substrate to form iron precursor film. Then, the iron precursor film is converted to FeS₂ film by annealing in sulfur vapor ambient, which is done with an individual temperature control system of substrate and sulfur at a lower and upper stream in tubular furnace, respectively. The material property of FeS₂ is evaluated after sulfuration process above 400^°C. The band gap of prepared film estimated by UV-Vis spectra exhibited 0.92 eV when using Fe-acetylacetonate precursor. On the other hands, the band gap with FeCl₃ precursor presented only 0.46 eV. We assume that the precursor impurity, such as chlorine deteriorates band structure of FeS₂.

Recently, considerable attention has been directed towards ZnO for thin film solar cell application [1-4]. One of the main advantages of ZnO is its wide direct band gap that enhances the blue light response and the open circuit voltage of the solar cell [2-3]. Moreover, ZnO has a dual functionality in which it can act as conductive oxide as well as an emitter layer due to its n-type nature [3,4]. ZnO can be deposited using different deposition methods such as evaporation, magnetron sputtering, chemical vapor deposition and Atomic Layer Deposition [1]. In this work, we investigate n-ZnO/i-aSi/p+ Si heterojunction solar cell. We compare the growing of ZnO by ALD and sputtering. In addition, the effect of the a-Si(i) layer is examined.
Due to the difference in the bandgap of ZnO and Si, the ZnO/Si hetrojunction exhibits a type II band alignment determined by Anderson model. The conduction band offset equals to ΔE_c=0.4eV which is much lower than the valance band offset is ΔE_v=2.55eV[5]. In the first part of the experiment we examined the conductivity of ZnO grown by ALD and sputtering. We deposited a 50nm n-ZnO on P⁺-Si Substrate to form the pn junction. Under 1 Sun at AM1.5G condition, ALD ZnO shows a better conductivity than sputtered ZnO. The measured V_oc and J_sc of ALD grown ZnO is 0.15V and 18 mA/cm² respectively. Whereas, sputtered ZnO has zero current and V_oc=0.12 V. The results indicate that ALD grown ZnO has better quality than sputtered ZnO.
In the second part, a 350nm intrinsic aSi was deposited in between the n-ZnO and P⁺-Si. Under 1 Sun at AM1.5G. Both ALD and sputtered n-ZnO shows a better open circuit equals to 0.52V and 0.56 V respectively. However, no significant change is observed in the short circuit current. The addition of the i-layer improves the performance of the cells and helps with carrier collections since is has a better lifetime. With these initial results, several experiment are done to investigate various ALD grown ZnO thicknesses (10, 30 and 50 nm). The optical and electrical charactarization of ALD and sputtered ZnO films is carried out.
[1] Kim, Joondong, et al. "Transparent and crystalline Al-doped ZnO film-embedded heterojunction Si solar cell." Materials Letters 75 (2012): 99-101.
[2] Shen, L., et al. "Studies on fabrication and characterization of a ZnO/p-Si-based solar cell." Superlattices and Microstructures 48.4 (2010): 426-433.
[3] Kozarsky, Eric, et al. "Thin film ZnO/Si heterojunction solar cells: Design and implementation." Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE. 2012.
[4] Mersich, Peter T., Tingfang Yen, and Wayne A. Anderson. "Hetero-interface modification in thin film ZnO/Si solar cells." Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE. 2009.
[5] Ye, J. D., et al. "Electroluminescent and transport mechanisms of n-ZnO#8725; p-Si heterojunctions." Applied physics letters 88.18 (2006): 182112.

Kesterite compounds with general compositions Cu₂(Zn,Fe)Sn(S,Se)₄ have attracted tremendous interests as photovoltaic and sensor materials. These materials are currently fabricated through combination of the elements via high temperature solid-state reaction, solvothermal reaction¹, high energy milling², and dip coating³. These synthesis routes typically generate the targeted compound along with minor secondary impurity phases. In addition, a high degree of disorder and anti-site defects are also present in kesterite phases synthesized from elements. Here we novel synthesis strategy for the fabrication of well ordered and stoichiometric Cu₂Mg_3-2xTi_xSe₄ (0 le; x le; 1.5) phases. Our strategy consists of a direct facile conversion from solid to solid of the CuSe₂ template precursor through high energy milling method in the presence of elemental Mg and Ti powders. For comparative purposes, direct reaction with elemental powders was also performed. X-ray powder diffraction of the synthesized materials suggested the formation of composites containing Cu₂Mg₃Se₄ and CuTi_0.75Se₂ in various ratios. Transmission electron microscopy revealed that both phases are intermixed in the atomic-scale suggesting their simultaneous nucleation from the CuSe₂ precursor. Optical absorption spectroscopy and the thermoelectric properties of various Cu₂Mg_3-2xTi_xSe₄ nanocomposites will be discussed and correlated to their nanometerscale internal structure.
(1) Guo, Q.; Ford, G. M.; Yang, W.-C.; Walker, B. C.; Stach, E. A.; Hillhouse, H. W.; Agrawal, R. Journal of the American Chemical Society2010, 132, 17384.
(2) Azanza Ricardo, C. L.; Su'ait, M. S.; Müller, M.; Scardi, P. Journal of Power Sources2013, 230, 70.
(3) Shinde, N. M.; Deshmukh, P. R.; Patil, S. V.; Lokhande, C. D. Sensors and Actuators A: Physical2013, 193, 79.

Cu₂O is an earth abundant semiconductor identified as a promising photovoltaic material due to its high absorption and good minority carrier diffusion length. While the bulk properties of Cu₂O are attractive for photovoltaic applications, surface instability is one of the most pressing issues that need to be resolved to achieve a high efficiency device. Cu₂O is intrinsically p-type and thus requires a heterojunction partner, which introduces the possibility of increased interface defects. The highest reported photovoltaic efficiency for a Cu₂O absorber device is 5.38%, while the detailed balance limit has been calculated to exceed 20%. One of the main reasons for low efficiency is phase instability of the Cu₂O surface. In addition, intrinsic thermally oxidized wafers of Cu₂O have shown to be too resistive for optimal device applications, and therefore thin film device development has an important role in increasing Cu₂O solar cell efficiency. It has been shown that the technique used to deposit the buffer layer and 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 (PA-MBE), which allows precise interface control in an ultra-high vacuum environment.
In this work Cu₂O thin films are deposited on various substrates relevant to device fabrication. In particular, we use MgO as a heteroepitaxial template for the growth of Cu₂O heterostructures. We discuss epitaxy of Cu₂O on suitable Ohmic contact layers including Pt grown on MgO, as well as promising buffer and emitter layer materials such as Ga₂O₃, also grown on MgO. We focus on MgO as a substrate because it is a close lattice match to Cu₂O and can be grown biaxially textured on an amorphous substrate, providing a path to dual junction cells. Epitaxial relationship and crystallinity are confirmed by reflection high energy electron diffraction (RHEED), and high resolution x-ray diffraction (HRXRD). Phase purity of the interface is confirmed by ex situ x-ray photoelectron spectroscopy (XPS). The interface structure and defects are further analyzed by cross-sectional transmission electron microscopy (TEM).

A summary of the NREL effort to grow epitaxial CZTS will be presented. The suitability of various lattice-matched substrates for epitaxial growth of CZTS was evaluated including silicon (various orientations), ZnS(110), Al₂O₃(0001), and gallium phosphide (GaP) (100). An overview of the wet-chemical techniques used to prepare a high-quality epi-ready surface for each substrate is covered, with special attention given to the limitations presented by CZTS MBE, namely the stability of the substrate surface with respect to sulfur vapor. The quality of the substrates prior to growth has been assessed by in-situ reflection high-energy electron diffraction (RHEED) and photoemission techniques. ZnS epitaxial growth was attempted on all substrates because ZnS is viewed as a simplified model system for CZTS and was used as a screening mechanism to evaluate the suitability of the substrate for CZTS growth. Epitaxial quality was assessed using RHEED during growth and via electron backscatter diffraction (EBSD) and transmission electron diffraction (TEM) post-growth. High-quality epitaxial growth of ZnS was achieved on both ZnS and GaP substrates utilizing a flux of ~1Ås^-1 of ZnS at a substrate temperature of 500 K as only one domain was observed via EBSD (matching the substrate orientation) and the TEM results showed an epitaxial interface between ZnS and the substrate. After the initial screening of substrates utilizing ZnS epitaxy, ZnS(110) and GaP(100) were determined to be most suitable for further study. It was subsequently discovered that GaP is likely unsuitable as a substrate for high-quality growth of epitaxial CZTS, as gallium readily diffuses through ZnS at temperatures greater than 500 K, forming a GaS phase at the surface of the specimen. ZnS remains the most promising substrate for high-quality epitaxial growth, and the results for the epitaxial growth of CZTS on ZnS (110) represent a best path forward for high-quality epitaxial CZTS growth. Results for the growth of epitaxial CZTS on ZnS(110) will be presented.

Photovoltaics based on chalcopyrite compounds, such as Cu(In,Ga)Se₂ (CIGS), show the highest performances of any device based on polycrystalline thin films. The active part of these devices consists of a thin film of CIGS coated with a compound such as CdS to form the photocurrent-collecting heterojunction. Understanding the nature of this heterojunction is critical to achieve high efficiency in the resulting device for two reasons. Firstly, recombination of excitons at the heterojunction can cause loss of current due to blue photons. Secondly, doping of the junction establishes the built in potential that, in part, determines the open circuit voltage. Therefore, it is important to know is the composition around this interface.
Previous studies have shown that clean surfaces of CIGS are Cu deficient. When the junction is formed with CdS, there is evidence that Cd dopes the surface of the CIGS resulting in a heavily n-type layer. These results were based on angle-resolved photoelectron spectroscopy, which is a valuable method for studying the near-surface chemistry of solids but is difficult to interpret because it averages over a significant depth, 1-2 nm for CIGS.
A unique method for studying the chemistry of only the top single monolayer of a film is low energy ion scattering (LEIS), also known as ion surface scattering. In addition to its extreme surface sensitivity, LEIS is generally free from the types of matrix effects which complicate quantitative analysis of data from Secondary Ion Mass Spectrometry (another commonly applied technique for surface analysis of photovoltaic materials). From this combination, LEIS is a very attractive technique for the analysis of the surfaces and interfaces of these thin film photovoltaic devices.
In this contribution, we will discuss some aspects of the application of LEIS to characterize the photovoltaic CdS - CIGS heterojunction, with particular attention to effects of sample preparation and cleaning. Different methods for the removal of the adventitious adsorbents from air exposure which obscure the surface signal. We also study the effect of removing the CdS coating by etching with dilute HCl, which reveals some evidence of elemental migration across the heterojunction.
These steps are necessary before this powerful technique can be routinely applied to characterise photovoltaic materials. However, these preliminary results are already providing insight into the composition around the heterojunction that ultimately determines device efficiency.

High-surface-area crystalline mesoporous d0 metal oxides such as Ta2O5, Nb2O5, and TiO2 were synthesized via a one-pot method using pluronic P-123 triblock copolymer as a structure directing agent. Mesostructure stability during metal oxide crystallization via high temperature calcination remains difficult to maintain in such synthesis. In this study, two heat treatments were proven to play key roles in achieving this goal. The first one is the intermediate heat treatment that preconsolidates the mesostructure and partially decomposes P-123 into carbon rich species. The second heat treatment relies on high temperature calcination under inert atmosphere to simultaneously form carbon wrapping on the metal oxides in situ and achieve crystallization of metal oxides. These two treatments successfully restricted the porous structure collapse and crystal size growth during high temperature crystallization. The as-synthesized crystalline mesoporous metal oxides show disordered mesoporous structures consisting of polycrystals. Notably, the commonly used P-123 surfactant without sp2-hybridized carbon forms amorphous carbon in situ, which effectively restricts mass transfer during crystallization and thus successfully prevents crystal size growth. The as-synthesized crystalline mesoporous Ta2O5, Nb2O5, and TiO2 displayed surface areas of 117.0, 125, and 76.2 m2/g, respectively, and pore sizes of 5.4, 8.1, and 13.9 nm, respectively. Crystal sizes amounted to 19, 28, and 34 nm for the mesoporous Ta2O5, Nb2O5, and TiO2, respectively. Porous and crystal structures of the as-synthesized samples are characterized using X-ray diffraction, thermogravimetric-differential thermal analysis, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and N2 sorption techniques. The photocatalytic performance of as-synthesized crystalline mesoporous Ta2O5 is also evaluated.Write you abstract here. The photocatalytic activity of the as-synthesized mesoporous Ta₂O₅ with the co-catalyst NiO_x for overall water splitting under ultraviolet (UV) light irradiation was systematically evaluated. The photocatalytic activity of crystalline mesoporous Ta₂O₅ showed about 3 times that of commercial Ta₂O₅ powder, 22 times that of amorphous mesoporous Ta₂O₅. Furthermore, the crystalline mesporous Ta3N5,which can effectively adsorb the visible light up to 600 nm, can be obtained by direct nitridation of mesoporous Ta2O5.

Photocatalytic water splitting has received considerable attentention because hydrogen is a clean, renewable, cheap, and viable alternative to fossil fuels. Most of the studied photocatalytic materials have been semiconductors as their electron-hole pairs are easily separated. Recently, a red metallic oxide, strontium deficient SrNbO₃ in the powder form, was reported for its photocatalytic properties¹. The water splitting efficiency was fully attributed to the high mobility which was responsible for electron-hole separation during the photocatalytic water splitting process. In our experiment, we grew epitaxial films of these niobates to measure the electrical properties accurately. We found that the mobility of single crystal SrNbO₃ film prepared by PLD was only 2.0 cm²/(Vmiddot;s), which is not high. To compare and understand the function of metallic oxides in the process of photocatalytic water splitting, we have prepared other two metallic oxides, CaNbO₃ and BaNbO₃. We found the photocatalytic activity of CaNbO₃ is higher than that of SrNbO₃, unlike BaNbO₃, which is lower than that of SrNbOshy;₃. However, we found no significant relation between the photocatalytic activity and the mobility. Interestingly, the lifetime of the photo-generated exaction in all these materials are very long (exceed 200ps), which may be responsible for their efficiency in the water splitting.
Reference
1 Xu, X., Randorn, C., Efstathiou, P. & Irvine, J. T. A red metallic oxide photocatalyst. Nature materials11, 595-598, doi:10.1038/nmat3312 (2012).

This study deposited tinsulphide (SnS) thin films using two electrodes electrochemical deposition technique from tinsulphate (SnSO₄), sodiumthiosulphate pentalhydrate (Na₂S₂O_3.5H₂O) and tetra-oxo-sulphate six acid (H₂SO₄) was used to adjust the pH of the bath. The thin films were characterised using Surface Profilometre, X-Ray Diffractometre (XRD), Uv-Visible spectrometer and four point probes. The Surface Profilometre reveals that the film is continuous but not uniformly distributed given a pinhole and crack free thin film. The thickness of the film was given to be 60nm. The XRD result shows that, the film has orthorhombic crystal structure. The grain size was estimated to be 0.61nm and inter-planar spacing to be 0.29nm. The Uv-visible spectrometer result reveals that, the film has good absorbance but poor reflectance and transmittance in the visible light region. A low direct optical energy band gap of 1.69 eV was found for the film. The four point probes give the sheet resistivity of the film to be 51.1 #8486;/#9633;, resistivity to be 5.12 x 10^-4 #8486;-cm and the conductivity was calculated to be 1.96 x 10³ #8486;^-1cm^-1. The I-V characteristic of ITO/SnS/Ag is linear indicating an ohmic contact between ITO and SnS as well as between SnS and Ag. Thus, the electrodeposited SnS thin film is a suitable candidate for absorber layer of thin film solar cells.
Key words: tinsulphide, thin films, electrodeposition, characterization and photovoltaic.

Tin monosulfide (SnS) is a candidate Earth-abundant solar cell absorber material due to its strong absorption in the visible wavelength range and potentially high carrier mobility (Hall mobility reported above 100 cm²/Vmiddot;s).¹ It has a 1.1 eV indirect band gap and 1.3 eV direct gap. The current certified record efficiencies are 4.36%² for a pulsed-CVD device and 3.88%³ for a SnS solar cell grown by thermal evaporation.
Current record devices include an annealing step, conducted in an Hshy;₂S ambient gas, which serves two purposes, to increase grain size and to control p-type carrier concentration: (i) During annealing, SnS grains grow and the density of grain boundaries is reduced, which is assumed to lead to reduced carrier recombination and improved efficiency; (ii) Carrier concentration is increased from the range of 1×10¹⁵ cm^-3 in as-deposited films to the range of 1×10¹⁶ cm^-3 in films after annealing, leading to improved transport in the absorber.
To decouple the effects of grain growth and increased carrier concentration, a two-step anneal is employed. A first anneal at high temperature followed by a slow cool down serves to grow grains and set a low carrier concentration in the thin film. A second anneal at various temperatures and sulfur partial pressures will reveal the effect of these two variables on carrier concentration, independent of grain size changes. A rapid cool down is employed at the end of the second anneal to freeze in the majority-carrier concentration and mobility of SnS thin films at high temperature.
Films will be analyzed by SEM, XRD and Hall effect measurements, while full device performance is evaluated by JV and QE measurements. Devices will be made using a stack similar to that in Reference 3: Si/SiO₂/Mo/SnS/ZnO_xS_y:N/ZnO/ITO/Ag.
Finally, annealing in high sulfur partial pressures is also hypothesized to impact the concentration of sulfur vacancies in the SnS film, which are predicted to lie mid-gap.⁴ Filling sulfur vacancies by annealing in high sulfur partial pressure may also lead to increased minority carrier lifetime through a reduction in deep recombination centers.
[1] K.T. R. Reddy, N. K. Reddy, and R.W. Miles, Sol. Energy Mat. Solar Cells 90 (2006) 3041.
[2] P. Sinsermsuksakul, L. Sun, S. W. Lee, H. H. Park, S. B. Kim, C. Yang, and R. G. Gordon, Advanced Energy Materials, accepted 2014.
[3] V. Steinmann, R. Jaramillo, K. Hartman, R. Chakraborty, R. E. Brandt, J. Poindexter, Y. S. Lee, L. Sun, A. Polizzotti, H. H. Park, R. G. Gordon, and T. Buonassisi, submitted 2014.
[4] J. Vidal, S. Lany, M. d&’Avezac, A. Zunger, A. Zakutayev, J. Francis, J. Tate, Appl. Phys. Lett. 100 (2012) 032104.

Prospective applications as a nontoxic and abundant absorber material in the design of novel solar cells raise the interest in the p-type semiconductor copper oxide and its related compounds. A profound knowledge of the electronic, optical, and structural properties of these materials is hence required. We present the results of our density functional theory (DFT) calculations of the electronic band structure for the three compounds Cu₂O, CuO, and Cu₄O₃ using first principles methods beyond the local density approximation. We compare the DFT+U approach to hybrid functional and quasiparticle calculations within the framework of the GW approximation. We assess the optical properties by calculating the dielectric function. Using the hybrid functional approach we further investigate the phase stability of the three copper oxide compounds in different temperature and pressure domains [1].
The ternary system Cu₂O_1-xS_x is of interest because of the possibility to tune the energy gap. For this alloy we compute the evolution of the lattice parameter by a substitutional supercell approach taking into account several configurations per concentration. The results are in very good agreement with experimental data and show that the increase of the lattice parameter can be described by Vegard&’s law only for low sulfur concentrations.
[1] M. Heinemann, B. Eifert, and C. Heiliger, Phys. Rev. B 87, 115111 (2013)

Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe) are emerging kesterite based absorbers for low cost and sustainable thin film solar cells. The world record kesterite based solar cells with 12.6% can be achieved by hydrazine-based liquid process, whereas there are safety and environmental issues to consider from manufacturing aspect. High-quality kesterite absorber could also be fabricated by widespread electroplating and post-chalcogenization process. However, to compete with other solar cell materials, improving the conversion efficiency and reducing material use are the key issues.
It has been reported that the nanorods solar cells can efficiently utilize the incident light with comparably small absorber volume. Here, we perform the FDTD simulation for structural optimization. Afterwards we use the electrodeposited Cu-Zn-Sn alloys as the precursors of CZTSe nanorods inside the anodic alumina templates. As a proof of concept, we fabricate the CZTSe nanorods solar cells with typical CIGS cell configuration suggesting a new way to achieve low-cost high-efficiency solar cells.

Mo back contact layers are usually deposited on soda-lime glass (SLG) substrates at room temperature (RT) for CIGS thin film solar cells application. The characterization of Mo thin films deposited on SLG at a high substrate temperature hasn&’t been reported until now. Significant changes in the Mo thin film characteristics and properties of the resulting CIGS/Mo interface were prospected. In the present work, we focused on Mo thin films deposited on SLG at high substrate temperatures. The characterizations focus on the crystal structure and morphology as well as the electrical properties of the Mo thin films deposited on SLG for various substrate temperatures. Our final goal is to investigate CIGS deposited on this novel high temperature grown Mo/SLG.
500nm-thick Mo thin films were deposited on SLG substrates by DC magnetron sputtering. The substrate temperature was varied from RT to 500°C. The sputter deposition was carried out in a pure Ar gas at a pressure of 0.5 Pa. CIGS thin film were grown on the Mo/SLG substrates by 3-stage co-evaporation process using a molecular-beam epitaxy (MBE) system at the maximum substrate temperature of 550°C. The crystal structure and morphology of the Mo thin film were measured by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrical properties on the Mo thin films were measured by four-point probe method. The optical properties of CIGS were characterized by room temperature time-resolved photoluminescence (TR-PL) measurements.
Mo grains were larger in size when increasing the substrate temperature, as seen on the plan-view and cross-sectional SEM images deposited on SLG. The XRD analysis revealed that the intensity of the (110) peak of the Mo thin film increased when increasing substrate temperature.
From the result of the electrical characterizations, the resistivity of the Mo thin films decreased by 1 order of magnitude with the increasing substrate temperature up to 500°C. This is related to the increased grain size from the high temperature that promotes the Mo atoms migration on the substrate. Thinner Mo layers could therefore be used, which can reduced manufacturing costs and promote the Na diffusion into CIGS thin film from the SLG substrate. The optical characterization of the CIGS thin films deposited on high temperature Mo films revealed that the PL intensity and PL life time increased with increasing the Mo deposition temperature. The SIMS data on the CIGS thin film and Mo/CIGS interface will also be presented.

Cu(InGa)S₂ (CIGS) compounds have drawn significant attention as a light absorber materials owing to their promising properties such as high absorption coefficients, direct and controllable bandgap and long-term stability. Recently, thin film solar cells based on nanostructured materials have been highlighted as one of the most efficient forms because ordered nanostructures increase light capture and enhance charge separation and transport via their unique optical and electrical properties. For CIGS-based thin film solar cell, nanostructured cell have been investigated through the adoption of other nanostructured materials, such as CdS nanowire, ZnO and ITO nanorod. However, direct fabrication and measurement of electrical and optical properties of CIGS nanostructure, such as CIGS nanowire, nanorod and nanotube-based devices have been rarely investigated. Although some synthetic methods for nanowire of Cu-based absorber materials have been reported including CIGS and Cu₂ZnSnS₄ (CZTS), most of the previously reported nanowires were too long (~50 mu;m) to be applied to practical 1-D devices.
Here, we demonstrate the fabrication of vertically-aligned CIGS nanorod arrays and photoelectrochemical performance of CIGS nanorod-based devices. The CIGS nanorod arrays were fabricated by using anodic aluminum oxide (AAO) membranes as templates for well-defined structures. Aqueous solution containing Cu, In, and Ga salts was infiltrated into AAO membranes and CIGS nanorods formed through the oxidation and sulfurization process. The structural, morphological and compositional characterization of the nanorods were carried out by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and energy-dispersed x-ray spectroscopy (EDS). The photoelectrochemical properties of CIGS nanorod arrays were characterized by three-electrode configuration using an Ag/AgCl reference electrode and a Pt wire counter electrode because the photoelectrochemical performance is usually used as a test-bed to demonstrate the feasibility of photovoltaic absorber materials. Under 1 sun illumination condition, the CIGS nanorod photoelectrode yielded a high photocurrent density as compared to the thin film counterpart. Our approach for the fabrication of CIGS nanorod arrays, reported here, will be the first step in realizing the high-performance nanostructured absorber materials-based solar cells.

Iron pyrite is a promising solar cell material due to its suitable bandgap (0.95eV), high absorption coefficient (ge;10⁵ cm^-1 for hvge;1.3eV), and in addition, it is nontoxic, cheap and abundant¹. However, the efficiency of only 2.8% reported in the 1980s² is limited by the high dark current, which results in small open circuit voltages of less than 0.2V. This low open voltage is attributed to trace impurities of marcasite or iron monosulfide, or to a large density of surface states. Previously we reported on the synthesis of phase pure FeS₂ nanocrystals (NCs) by the surfactant assisted hot injection method³. Here, we aim to reduce the surface states on FeS₂ NCs by ligand exchange during the NC film fabrication process (layer-by-layer dip-coating). Films are investigated by the time-resolved microwave conductivity technique and transient absorption spectroscopy to explore the charge carrier dynamics. Films with ligand exchange by 1,2-enthanedithiol (EDT) exhibit a 15 times higher photoconductance and a 10 times longer charge carrier lifetimes than films without ligand exchange. This indicates that the EDT linker molecule leads to a short interparticle distance and reduces the surface state density. Interestingly, the photoluminescence spectrum shows both indirect (around 900nm) and direct (around 1300nm) emission. A kinetic model explaining the charge carrier dynamics is proposed.
1. Wadia, C.; et al. Environ Sci Technol 2009, 43 (6), 2072-2077.
2. Ennaoui, A.; et al. Sol Energ Mat Sol C 1993, 29 (4), 289-370.
3. Bi, Y.; et al Nano Lett 2011, 11 (11), 4953-4957.

Transition metal doped anatase TiO₂ thin films are potential materials for next generation oxide devices due to their novel optical, electrical and magnetic properties. TiO₂ is also an attractive material as a photoactive material. it has been extensively used for photocatalytic and photoelectrochemical generation of solar hydrogen due to its its chemical stability, nontoxicity, and relatively low cost. It has several obstacle for the hydrogen production such as high band gap (3.2ev) Which allow only UV Spectrum. Recenly researcher s have discovered black TiO2( reduced tio2) which shows significant enhancement of photo absorption.
In our recent study we have grown black TiO₂ thin films by PLD without any hydrogen treatment which shows significant visible photo-abosorption. photocatalytic performance of the black TiO₂ has been studied. We found high concentration of Ti3+ defects such as Ti3+ interstitials and oxygen vacancies in the film which create the mid gap states in the TiO₂.

Defect states and impurity levels present in SnS films formed by sulfurization of tin precursors have been investigated by photoluminescence spectroscopy. The data is thoroughly analyzed in view of the application of SnS layers for solar cell development. A band diagram for the defect levels of SnS films is proposed and discussed.
Tin monosulfuide (SnS) is a promising candidate for the development of solar cells using earth-abundant-materials. This is because SnS has a direct bandgap of 1.3 eV and high light absorption coefficient (>10⁴ cm^-1). Although the recent results on SnS-related solar cells showed conversion efficiencies close to 4%, however, it is far below the theoretical efficiency. This is due to the fact that it is difficult to grow high purity and device quality SnS films with low defect density. Therefore an understanding of the complete defect structure of SnS is highly useful in the development of an efficient and environmentally benign solar photovoltaic cell. In general, the morphology, impurities and defects present in either thin films or at the interface of a heterojunction strongly affect the photoluminescence (PL) behavior. With regard to SnS films, the reported data on the PL properties is very meager compared to other solar cell materials such as CuInGaSe₂ and Cu₂ZnSnS₄.
Sulfurization of tin (Sn) layers is one of the most desirable processes for commercial production of SnS photo-absorbers. However, incorporation of sulfur (S) into tin is very crucial as S-rich growth results in the formation of additional SnS₂ or Sn₂S₃ phases while S-poor conditions causes Sn metal to remain in SnS layers. Therefore, the defect properties are highly sensitive to sulfurization conditions. In this study, the defect and impurity states present in SnS layers grown by sulfurization process are investigated using PL measurements. In addition, the defect levels existing at the interface of SnS-related heterojunction solar cells are also evaluated.
Sputtered Sn precursors were sulfurized at 200°C for 30 min and 350°C for 10 - 30 min to grow SnS films. The surface morphology and presence of additional phases in SnS films strongly depend on the sulfurization temperature profile and timing of each step during the process. Several donor and acceptor levels were identified using the PL spectra observed at various excitation intensities and temperatures. Using this data, a band diagram for the defect levels of SnS layers is proposed. These results might be the first step towards understanding the defect profile of SnS layer in the development of an efficient SnS-related solar cell.
[1] Our group, Jpn. J. Appl. Phys. 52 (2013) 021102.

Over the past decades, considerable amount of studies were focused on TiO₂ nanowire (NW) template-based hybrid solar cell structures such as hybrid, dye-sensitized¹ and organic solar cells² which can offer low cost, mature processing technology together with high efficiencies. Recently, in order to provide strong light absorption in the solar spectrum, various semiconductors such as CdS³, CdTe⁴, PbS⁵ have been used to sensitize metal oxide NW arrays in liquid- or solid-state quantum dot sensitized solar cells or semiconductor sensitized solar cells. However, the functionality of semiconductor interfaces plays a crucial role in all of these hybrid solar cells in which trapping or recombination of charge carriers can reduce photovoltaic (PV) efficiency. Controlling the impact of surface or interface-derived electronic states is, therefore, a prime goal in modern semiconductor processing.
We demonstrate that atomic layer deposition (ALD) coated ZnO embedded layer can efficiently passivate the NWs surface. The proposed solar cell structure consists of 5 main parts; 1) hydrothermally grown TiO₂ nanowires on FTO coated glass layer, 2) an ultrathin ALD deposited ZnO layer coated on TiO2 nanowires, 3) a thin layer of α-Si deposited by RF magnetron sputtering on TiO₂/ZnO HJ as absorbing layer, 4) redox electrolyte (I^-/I^3-) as a hole transfer mediator and finally 5) Pt-coated conducting glass as a counter electrode. The uniform TiO₂ nanowires are grown on FTO glass by hydrothermal technique. Afterwards nanowires are coated with ZnO by ALD reactor (Cambridge Nanotech Savannah S100). The substrate temperature is kept at 250 #730;C during the process. Various samples are prepared with different number of cycles (1, 2 and 3) of ZnO as an ultrathin layer on TiO₂ NWs. Densely-packed NWs with an average length of 1.1-1.6 mu;m have been obtained with diameters in the range of 80-150 nm.
Although the bare structure (no ZnO interfacial layer) shows poor efficiency, device performance is boosted remarkably by using the ZnO interfacial layer. It is also shown that thicker layers of such a high band material impede the electron injection noticeably and reduce the efficiency. It is demonstrated that an optimized ultrathin layer paves the way to efficient devices by reducing recombination at the interface without hampering electron injection capability.
¹ B.E. Hardin, H.J. Snaith, and M.D. McGehee, Nat. Photonics 6, 162 (2012).
² H. Hoppe and N.S. Sariciftci, J. Mater. Res. 19, 1924 (2011).
³ M. Seol, H. Kim, Y. Tak, and K. Yong, Chem. Commun. (Camb). 46, 5521 (2010).
⁴ G. Zhang, S. Jiang, Y. Lin, W. Ren, H. Cai, Y. Wu, Q. Zhang, N. Pan, Y. Luo, and X. Wang, J. Mater. Chem. A 2, 5675 (2014).
⁵ J. Jean, S. Chang, P.R. Brown, J.J. Cheng, P.H. Rekemeyer, M.G. Bawendi, S. Grade#269;ak, and V. Bulovicacute;, Adv. Mater. 25, 2790 (2013).

Vertically-oriented Ta-W-O nanotube array films were fabricated via the anodization of Ta-W alloy foils in HF-containing electrolytes. HF concentration is a key parameter in achieving well-adhered nanotube array structure. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance measurements confirm the staggered band-alignment between Ta₂O₅ and WO₃, which facilitates the separation of charge carriers. The nanotubes made of Ta-W films containing 10% W showed hundred-fold improvement in the measured photocurrent compared to pristine Ta₂O₅ upon their use to split water photoelectrochemically. This enhancement was related to the efficient charge transport as well as the red shift in absorption spectrum with increasing the W content, which was asserted by ultrafast transient absorption (TA) spectroscopy measurements. The TA measurements showed the elimination of trap states upon annealing Ta-W-O nanotubes and hence minimizing the charge carrier trapping, whereas the trap states remain in pristine Ta₂O₅ nanotubes even after annealing.

The development of earth abundant photovoltaic materials is critical if photovoltaics are to generate a significant fraction of the world&’s energy in the near future. The success of Cu₂ZnSn(S,Se)₄ as a thin film photovoltaic absorber have stimulated interest in a host of other earth abundant materials. One such promising material is Cu₂SnS₃, which has strong optical absorption and a favorable electronic structure for a photovoltaic absorber.
Despite theoretical work showing the promise of Cu₂SnS₃ as a photovoltaic absorber, device efficiencies remain low: the current device record, demonstrated in 2012, is 2.8%. One reason for this low device efficiency is that the carrier concentration in the Cu₂SnS₃ absorber layer is typically higher than desired for a high quality photovoltaic absorber, ranging from 10¹⁷-10¹⁹ holes/cm³. High carrier concentrations due to intrinsic defects can decrease the carrier diffusion length, leading to higher recombination losses and lower device efficiencies.
Our recent work has identified two approaches to gain control of the hole concentration in Cu₂SnS₃ thin films. High sulfur chemical potential during film deposition results in more Cu vacancies, leading to higher hole concentrations. High Cu/Sn ratios in the Cu₂SnS₃ films are believed to cause iso-structural alloying with a metallic Cu₃SnS₄ phase, also increasing the hole concentration. Thus, lowering the carrier concentration requires growth under both S-poor and Cu-poor conditions. However, even with careful control of these synthetic parameters, the carrier concentration remains around 10¹⁸ cm^-3, higher than is desired for device integration. In this talk, we explore how further reductions in carrier concentration can be achieved through the use of post-deposition anneals, which allow the film to reach thermal equilibrium.
Initial results have shown that annealing Cu₂SnS₃ films between 400-600°C under an SnS/Ar atmosphere produces a 1-2 orders of magnitude decrease in carrier concentration. The effects of different anneal atmospheres and times on the electronic properties, including Hall mobility and Seebeck coefficient, have been investigated. Changes in film morphology have also been assessed using SEM and AFM. We have coupled these experimental results with theory to develop a deeper understanding of the behavior of Cu₂SnS₃ under non-equilibrium and equilibrium conditions. The annealed films have been integrated into devices and characterized by JV and EQE measurements.
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.

CuSbS2 (chalcostibite) offers an earth abundant alternative to the successful Cu(In,Ga)Se2 (CIGS) photovoltaic absorber. Antimony is less expensive and more abundant than indium and gallium, yet it shares the same oxidation state and a similar ionic radius, resulting in similar electro-optical properties such as band gaps and band energy levels. However, the structure and bonding of the CuSbS2 is fundamentally different due to two additional, non-bonding (lone pair) electrons of Sb. It is unclear how the resulting layered structure of CuSbS2 will affect its solar energy conversion performance.
Here we present on our efforts to integrate CuSbS2 thin film absorbers into baseline solar cell device prototypes. High-quality CuSbS2 thin films are synthesized by self-regulated RF magnetron co-sputtering in excess of vapor of Sb2S3 at elevated substrate temperatures. The resulting stoichiometric (XRF) phase-pure (XRD) thin films have micron-scale grains (SEM), sharp absorption onset at 1.5 eV and tunable 10¹⁶ - 10¹⁷ cm^-3 doping level [1] The PV-grade CuSbS2 thin films are integrated into baseline photovoltaic devices with the substrate-type architecture (glass/metal/CuSbS2/heterojunction/TCO/grid) and are analyzed for Jsc, Voc, FF, efficiency (JV) and spectral response (EQE).
In order to improve the efficiency of the baseline devices, we use a number of experimental and theoretical methods to determine the scientific reasons that limit the performance. To separate bulk and surface recombination, we perform photoelectrochemical (PEC) experiments. To understand the bulk point defect chemistry, we probe defect levels within the gap via low temperature photoluminescence and compare the results to first-principles defect calculations. To address the surface recombination, we theoretically predict and experimentally implement alternative heterojunction partners.
[1] Welch, Adam et. al. Solar Energy Materials and Solar Cells (under review)

Cuprous oxide (Cu₂O), despite its band gap of 2.17 eV, is a promising material for photovoltaic applications, due to its high absorption coefficient, non-toxicity and the abundance of its composing elements. While recently more attention has been paid to heterojunctions, highest efficiencies were reached by employing copper sheets, which were oxidized and annealed at high temperatures. For technological applicability, a thin film deposition process with mass production capabilities, which provides decent film properties at low temperatures, has to be established. Such thin films however suffer from low carrier mobilities and lifetimes, due to their polycrystalline nature. It has been reported that post treatments with hydrogen can passivate grain boundaries. Copper oxide thin films were deposited at various substrate temperatures from a metallic copper target via reactive radio frequency sputtering, utilizing gaseous argon and oxygen under the addition of hydrogen. The films were characterized and the influence and incorporation of hydrogen was investigated via X-ray diffraction, scanning electron microscopy, secondary ion mass spectrometry, photoluminescence and Hall effect.

The development of Earth-abundant photovoltaic absorbers remains challenging due to insufficient information about the material properties of novel absorbers. Few measurements of the effective minority carrier lifetime exist for new materials, and where they do, there is still limited knowledge about the physical location and recombination activity of defects in the material. Without this knowledge, it is difficult to design treatments to mitigate or remove defects.
One strategy is to use the large quantity of information present in the open-circuit voltage (V_OC) of a completed cell, as this is an aggregate measure of recombination currents. The relative strength of various recombination pathways can be separated from one another using their functional dependence on injection, bias, and temperature.
In the present work, we apply temperature- and illumination-dependent measurements of the V_OC (JVTI measurements) to cells fabricated with several novel PV absorbers. These include cuprous oxide (Cu₂O) and tin monosulfide (SnS). We vary the interface defect density through chemical passivation of (or the intentional formation of) defect states, and quantify the role this plays in interface recombination. Similarly, we vary the electronic band alignment between the absorber and its contacts, to shift the maximum obtainable V_OC. We also study the influence of improved bulk material quality on V_OC through annealing of the absorber material. By modeling the individual recombination currents, we separate the relative strengths of interface, quasi-neutral, and space-charge region recombination in each of these cells.
We use this JVTI analysis to determine whether the low V_OC characteristic of these cells may be overcome by bulk and interface engineering, or whether the V_OC is intrinsically limited by a specific material or device property. This analysis may be generalized to other novel PV materials, to accelerate their learning curve and drive towards higher device efficiencies.

The earth-abundant and environmentally friendly photovoltaic materials, SnS, SnSe and Sn(S,Se) given their large optical absorption coefficients and long carrier lifetimes, These PV absorbers are relatively unexplored in comparison to the more mature but less abundant CdTe and Cu(In,Ga)Se₂ thin film absorbers, and more complex kesterites Cu₂ZnSn(S,Se)₄. The highest energy conversion efficiencies obtained for these materials, however, are still very low and development of alternative synthesis routes is required. Surprisingly, there is very little experimental thermochemical and phase equilibrium data on these two binary and ternary systems. The present work aims to develop a consistent thermodynamic description of the Sn-S-Se systems using the CALPHAD (CALculation of PHase Diagram) technique aided by first-principles calculations. The strength of the CALPHAD method is not only identifying inconsistent data for specified solution models but also for interpolation and, importantly, extrapolation outside the experimental temperature (T), composition (x), and pressure (P) ranges. Based on thermodynamic modeling of the Sn-S-Se system, the calculated T-x, P-x, and P-T phase diagrams are presented and their applications for synthesis of earth abundant photovoltaic absorber materials are discussed including the guidance they may provide when identifying possible growth conditions and when selecting appropriate annealing atmospheres.

This study presents a new approach to solar energy conversion that combines photo- and thermionic electron emission. The process involves photo-induced electron emission across a narrow vacuum gap for a topping cycle, while heat generated by absorption of low energy photons can be transferred to a thermal heat engine such as a steam turbine. The combined efficiency of the two components could lead to an overall efficiency greater than 50%. The electron emission process is enabled by low work function doped diamond films deposited by chemical vapor deposition. Our group has deposited n-type diamond films on p-type Si substrates, which exhibit significant electron emissivity with photon illumination at elevated temperatures. The results are consistent with the photon-enhanced thermionic emission (PETE) mechanism, which have been described by several recent reports as a potential approach for high-efficiency concentrated solar cells. Results are presented for a diamond/Si structure and a model is proposes for a multi-layer emitter and collector structure for an isothermal energy converter. A key aspect of this structure is that the emitter and collector are maintained at the same temperature, which significantly reduces the complexity for device manufacture. We present a simulation of the performance of this isothermal topping device through a simplified physical model, and discuss its advantages.
This research is supported through the Office of Naval Research under grant number # N00014-10-1-0540.

Using an oriented array of 1D nanostructures in tallest possible height (i.e. greatest surface area) to maximize the solar-to-fuel conversion efficiency was proposed in a Science paper in 2007. This very idea has been disagreed by many other labs&’ papers, including one in Nature Materials in 2008 using ZnO nanowires 16-20 µm tall. This famous dispute in solar water splitting and solar cell fields has been ever since motivated us to explore the unknown photophysics around the 1D nanostructure.
Here w report our new discovery, in which densely packed ZnO nanowires were found to have an optimal height below 2 µm. while sparsely populated nanowires were found to be the taller the higher efficiency. These experimental data were confirmed by our optics simulation result, both of which were never reported in literature. On this basis, more publications have been put together, which will be briefly discussed here.

Dual junction tandem photovoltaic (PV) devices have extended coverage of solar spectrum compared to single junction solar cells. The state current state of art dual junction tandem solar cell achieved 32.6% efficiency under 1000 suns concentration. In this paper, we report on the computer based design of CdTe (1.5 eV) on InGaAs (0.74 eV) dual junction three terminal solar cell. The device was optimized for its active layers thicknesses and doping levels. An efficiency of 26.6% was predicted for the tandem PV device under standard testing condition (STC). We investigated the tandem PV device performance under different temperatures and we found that the device has a total efficiency temperature coefficient of about -0.15 %/ °C. We also investigated the device suitability for concentrated-photovoltaic (CPV) applications besides its original intended PV applications. The designed device proved to be a good candidate for CPV applications under 1000 suns.

Photocatalytic water splitting materials with the promise to generate cheap and clean hydrogen fuel to power future industry have been under intense investigation in recent years. Materials such as oxides, oxy-nitrides, compounds with different metal cations, and materials in various forms provide numerous opportunities; yet to study them all is a daunting task. So an effective high-throughput method is highly demanded. Combinatorial method, for its high- throughput feature can be power tool for the research. Here we report our efforts to develop a combinatorial method by reactive magnetron off-axis co-sputtering. Herein, different material systems and processes are studied by choosing proper element targets, controlling sputtering gas composition and substrate temperature. The composition of the as-deposited composition spreads changes continuously over the 3” wafer substrate and a resolution of 1 atom % is achieved. Because it deposits and processes all the compositions at once, this method rules out batch-to-bath variation and greatly improves the efficiency compared to traditional mix-and-sinter processes. The photocatalytic activity is characterized by an in-house made photoelectrochemical (PEC) system incorporating laser sources, an electrochemical setup for cyclic voltammetry (CV) scan and an automated positioning stage. This PEC setup enables us to locate the composition exhibiting the highest photocurrent on our combinatorial sample libraries in a buffered solution under different wavelength illuminations precisely and swiftly. Spatial distributions of sputtering rate for each of the targets with identical deposition condition were measured, so the composition on a specific point on our sample can be accurately estimated. Promising compositions located were further characterized using XRD for structures. Numbers of binary and ternary sample libraries have been synthesized and measured with the fast composition screening routine aforementioned. We discovered enhanced photocatalytic activity in Zn-substituted W-Zn-O thin-films prepared by reactive co-sputtering in Ar-O₂ (20 %), and post annealed in the air. A dramatic variation in photoactivity change as a function of Zn/W ratio was observed in a single composition spread. In a neutral electrolyte and under 405 nm visible light illumination, W_1-xZn_xO_3-2x films with 0.29 < x < 0.33 show a negative (improved) onset potential for water oxidation and a tenfold increase in photocurrent at a potential of 500 mV (vs. Ag/AgCl) compared to unsubstituted WO₃. X-ray diffraction confirms that the Zn-substituted phase is distinct from the known WO₃ and (sanmartinite) ZnWO₃ phases. Films annealed at 700 °C yield optimum performance. The negative onset potential allows this photoanode material to perform water oxidation without positive external potential required by most known W based oxide photocatalysts.

WO₃ absorbs the blue region of the solar spectrum so that near-infrared (NIR) photons are “lost” in solar energy conversion applications. Rare earth-doped nanoparticles (NPs), e.g. based on NaGdF₄:Er:Yb, are studied for their upconverting (UC) properties, i.e. the conversion of low-energy into high-energy photons [1]. Upon 980 nm illumination, NaGdF₄:Er:Yb UCNPs show two PL bands in the visible portion of the spectrum (located at about 550 and 650 nm) [2]. Here we report on the use of WO₃ sub-bandgap photons upconverted by NaGdF₄:Er:Yb to extend the spectral response of WO₃.
Blends (obtained from a WO₃ sol [3] and 1% weight/weight UCNPs in H₂O with WO₃: UCNPs 1:1 volume/volume ratio) were investigated in films drop-cast on SiO₂ substrates, treated at 550°C. UCNPs in various phases (cubic or hexagonal) and sizes (from 20 to 50 nm) were investigated. Interestingly, we observed that the ratio between the spectral intensity of the UCNPs PL bands located at 550 nm and 650 nm decreases from 1.2 to 0.8 for 45 nm hexagonal UCNPs blended with WO₃. For 30 nm cubic UCNPs, this ratio went from 0.6 to 0.45. These results suggest that a significant portion of the 550 nm emission is absorbed by WO₃, which would confirm the interest of the UCNP approach to extend the absorption properties of WO₃ towards NIR.
Electrical measurements in planar configuration carried out on Au electrode-patterned SiO₂ substrates comparing dark and 980 nm laser irradiation showed an encouraging current increase upon 980 nm irradiation. In order to disentangle optical and thermal contributions to the current, we investigated different power densities (1 to 10³ W/cm²) and exciting wavelengths. AFM, SEM, and XRD were used to investigate the effect of the power density on the film morphology and structure. Preliminary results suggest that using a power density of 1 W/cm² is sufficient to observe the UC process without inducing irreversible changes in our blends.
[1] Huang, X., Han, S., Huang, W., & Liu, X. (2013). Enhancing solar cell efficiency: the search for luminescent materials as spectral converters. Chemical Society Reviews, 42(1), 173-201.
[2] Vetrone, F., Naccache, R., Mahalingam, V., Morgan, C. G., & Capobianco, J. a. (2009). The Active-Core/Active-Shell Approach: A Strategy to Enhance the Upconversion Luminescence in Lanthanide-Doped Nanoparticles. Advanced Functional Materials, 19(18), 2924-2929.
[3] Santato, C., Odziemkowski, M., Ulmann, M., & Augustynski, J., “Crystallographically oriented mesoporous WO3 films: synthesis, characterization, and applications”, Journal of the American Chemical Society, 123(43), 2001, pp.10639-10649

With the relatively nontoxic nature and high chemical stability, ZnO is considered as a practically viable photocatalyst. However, the large band gap of ZnO prohibits it from harvesting light in the visible region, which hinders its applicability in the photoconversion process. Efforts to extend the light absorption range of ZnO and enhance the photocatalytic efficiency are thus being pursued. Metal ion doping has proven effective in extending the light absorption range of ZnO toward visible. Especially, the transition metal ion dopants (eg. Ni²⁺, Co²⁺, Fe³⁺) render ZnO with the room-temperature ferromagetism [1,2], which further enriches the functionality of ZnO.
In this work, a facile, green, ionic liquid based antisolvent method [3,4] was developed to prepare Ni²⁺-doped ZnO nanocrystals with controllable Ni²⁺ concentrations. A room-temperature ionic liquid, known as a deep eutectic solvent (DES), was used to dissolve the ZnO powers. When the ZnO-containing DES was injected into a bad solvent which shows no solvation to ZnO (e.g. water), ZnO nanocrystals were precipitated and grown due to the dramatic decrease of solubility. By adding Ni²⁺ ions in the bad solvent, the growth of ZnO in the antisolvent process was accompanied by Ni²⁺ doping, resulting in the formation of Ni²⁺-doped ZnO nanocrystals. The doped Ni²⁺ may invoke interband electronic transition within the bandgap of ZnO, giving rise to an additional absorption band at 400-800 nm. Due to the improved visible light harvesting, the Ni²⁺-doped ZnO showed significantly enhanced photocurrent generation in the photoelectrochemical water splitting. On the other hand, the introduction of Ni²⁺ induced pronounced room-temperature ferromagnetism for ZnO, which may further affect the photoactivity of Ni²⁺-ZnO in the photoelectrochemical cell, as a result of the magneto-optical-like effect.
[1] J. B. Cui, U. J. Gibson, Appl. Phys. Lett.2005, 87, 133108.
[2] D. Karmakar, S. K. Mandal, R. M. Kadam, P. L. Paulose, A. K. Rajarajan, T. K. Nath, A. K. Das, I. Dasgupta, G. P. Das, Phys. Rev. B2007, 75, 144404.
[3] J.-Y. Dong, Y.-J. Hsu, D. S.-H. Wong, S.-Y. Lu, J. Phys. Chem. C2010, 114, 8867.
[4] J.-Y. Dong, W.-H. Lin, Y.-J. Hsu, D. S.-H. Wong, S.-Y. Lu, CrystEngComm2011, 13, 6218.

ZnO plays an important role in novel solar cells as the n-type electron acceptor layer, and it is typically imagined to have sharp band edges. However, metal oxides typically have disorder, especially when fabricated by scalable, low temperature synthesis routes. Disorder leads to the creation of band-tails extending into the band gap. Here, we investigate the effect of ZnO conduction band-tails on the performance of colloidal quantum dot and cuprous oxide solar cells. We tuned the conduction band position of ZnO through Mg-doping using Atmospheric ALD for colloidal quantum dot solar cells (CQDSCs). Using photoluminescence, absorption, UPS, I - V and EQE measurements, we show that the ZnO conduction band-tail leads to V_OC losses due to electron thermalization. By reducing this loss mechanism, we increased the V_OC of ZnO - PbSe CQDSCs from 408 mV to 608 mV.^[1] We also synthesized cuprous oxide solar cells in the inverted architecture: glass/ITO/Cu₂O/Zn_1-xMg_xO/ITO, in which illumination is through the Zn_1-xMg_xO/ITO top contact. Impedance spectroscopy showed that this contact formed a Schottky barrier, but I - V measurements showed Ohmic charge transport. Using absorption measurements, we deduce that the band-tail states in the Zn_1-xMg_xO allow electrons to be injected to the ITO Fermi level through hopping, rather than overcoming the Schottky barrier. This finding provides greater flexibility in the selection of materials as the transparent top contact in inverted solar cells. Thus ZnO band-tails have a negative effect on performance at the p-n junction, but a positive effect at the electrode interface if Schottky barriers are present.
[1] R. L. Z. Hoye, et al. Adv. Energy Mater. 2014, Early View. DOI: 10.1002/aenm.201301544

This work identifies two key metal oxide properties that can be controlled to mitigate the negative influence of Schottky barriers on the performance of solar cells: metal oxide trap state density, and metal oxide carrier concentration for controlling the Schottky barrier width. These results are applicable to other solar cells reliant on transparent top contacts, such as CIGS and CZTS, in addition to the earth-abundant Cu₂O based photovoltaics studied here. We studied the role of the Schottky barrier between Mg-doped ZnO (Zn_1-xMg_xO) and indium-doped tin oxide (ITO) on the performance of electrochemically deposited inverted Cu₂O solar cells. The architecture of these is: glass/ITO/Cu₂O/Zn_1-xMg_xO/ITO, with illumination through the Zn_1-xMg_xO/ITO layer. Using a variety of electrical and optical characterization methods, we found that states in the Zn_1-xMg_xO band-tail enabled Ohmic electron flow through the Zn_1-xMg_xO/ITO Schottky barrier through hopping. Also, by controlling the Zn_1-xMg_xO thickness so that the Schottky barrier was sufficiently distant from the p-n junction to not influence the heterojunction built-in potential, high current densities were obtained. The efficiencies obtained from the optimized devices with ITO top contacts were double those obtained using Ohmic aluminium-doped zinc oxide transparent top contacts on these devices.

We have engineered the electronic structure at the interface between ZnO nanorod arrays and Cu₂O, through adjusting the membrane thickness and the carrier concentration of Cu₂O. The electrodeposition of Cu₂O at pH 11 acquired the highest carrier concentration, resulting in the largest interfacial electric field between ZnO and Cu₂O, which finally lead to the highest separation efficiency of photogenerated charge carriers. The optimized ZnO NRs array/Cu2O heterostructures exhibited enhanced PEC performance, such as elevated photocurrent and photoconversion efficiency, as well as excellent sensing performance for the detection of not only free glutathione (GSH) in PBS buffers but also GSH from cell extracts in serum even at applied bias of 0 V. Besides, the favorable selectivity, high reproducibility and extremely wide detection range, make such heterostructure promising candidate for PEC biosensing applications, maybe for the extended field of PEC water splitting or other solar photovoltaic beacons.

Nano-scale Bi₂Fe₄O₉ could be obtained by applying a modified sol-gel synthesis at low temperature, while EDTA was used as chelating agent. As a result of reaction between EDTA and Fe/Bi ions, the phase of Bi₂Fe₄O₉ could be controlled by adjusting the concentration of EDTA. Pure phase Bi₂Fe₄O₉ could be obtained when EDTA was 1:1 mol ratio with respect to the metal nitrates. SEM photos showed that morphology of the as-prepared powders were influenced by calcination temperature. The crystal growth and morphology evolution mechanism of Bi₂Fe₄O₉ in the process of sol-gel reactions was discussed. Additionally, the photocatalytic properties of the powders were explored and the crystallites with different morphologies have a direct impact to photocatalytic performance. The Bi₂Fe₄O₉ calcined at 650^oC showed the highest photocatalytic activity toward methyl orange degradation under visible irradiation, the degradation rate can be as high as 96%.

Successful integration of well-characterized p-WSe2 photocathodes with hydrogen evolution reaction (HER) electrocatalysts, viz. Pt was achieved. Preliminary results demonstrate considerable control over the morphology and the surface coverage of Pt catalyst particles photoelectrodeposited on WSe2 single crystals. Furthermore, catalyst particles seem to favor depositing on the edges sites versus the terraces of the crystal. This phenomenon is currently being investigated to achieve light-directed catalyst placement exclusively on the edge sites. Light-beam-induced current electrochemistry (LBIC-EC) experiments conducted in both the two and the three electrode electrochemical cell configurations showed a decrease in the photocurrent at the undecorated WSe2 edge sites indicating recombination of photo-generated carriers at these sites. On the other hand, platinized edge sites show an increase in photocurrent suggesting mitigation of recombination losses at the sites.

The silicon itself comprises a large proportion of the solar cell overall cost. Therefore, lowering its cost is directly related to the cost-reduction in the silicon solar cell manufacture. In the last few decades, there were a number of attempts to reduce the production cost of the silicon by reducing the thickness of the silicon wafer such as a sawing technique, a high-temperature induced spalling process and etc. Among these techniques, a novel method called Electrodeposit Assisted Stripping(EAS) process is one of the most effective and inexpensive kerf-free methods for acquiring a high quality thin-single crystalline silicon film. The mechanism of this process is a ‘spalling&’ where the maximum shear stress takes place not on the surface, but just below the surface, causing the spall off. During the EAS process, a metal stress layer of a certain thickness is electrochemically deposited onto the bulk silicon substrate inducing a thin silicon film to be lifted-off. The advantage of the EAS process is that this can be repeatedly carried out from the bulk silicon as long as it is worn out. In order to repeat this process, the initiation of the crack should be controlled to take place from the side of the bulk silicon. During spalling, however, the crack initiates from the inner surface on top of the bulk silicon leaving silicon residues at the edge of bulk silicon which should be removed to repeat the EAS process. To repeat the EAS process, these residues should be polished away, or the quality of a thin silicon film will be degraded as the steps of this process are carried out. In this work, pre-crack is created by a laser scribing at the side of the bulk silicon just below the top surface. This pre-crack is to directly initiate the crack propagation parallel to the top surface from the notch point. The effect of the laser scribing on silicon, such as resolidification of silicon due to the thermal energy created by the laser, is also investigated using different laser parameters - the pulse power, the pulse duration, the spot overlap and the pulse repetition cycle.

Conversion of Solar energy into the hydrogen via photoelectrochemical (PEC) splitting of water is expected to fulfill the increasing energy demand of the world in an efficient manner. Today, major impediments in the commercial viability of PEC splitting of water are the low efficiency of solar hydrogen production exhibited by existing semiconductor photoelectrodes and corrosion of semiconductors. In the search of suitable semiconductor, use of quantum dot modified metal oxide semiconductor needs to be explored for PEC generation of hydrogen. Semiconductor quantum dots are identified for their unique optical, electrical and chemical properties, significantly different from their bulk counterpart, which makes it applicable in various fields including solar energy applications also. To date, there are few reports on the use of small band gap QDs like CdSe, CdTe over wide band gap metal oxides like ZnO, TiO₂ in the field of photoelectrochemical cell. These QDs possess the large extinction coefficient, large intrinsic dipole moment that could provide the rapid charge separation. On the other hand, the less explored system is the combination of wide band gap QDs over small band gap metal oxide in this field. In this paper, small band gap hematite has been chosen as a main solar energy absorber, while wide band gap ZnO QDs decorated over it, as an efficient electron transport across the interface by reducing charge carrier recombination rate.
In the present study, doped hematite films have been synthesized over FTO substrate by electrodeposition method. These films were subjected to ZnO quantum dots sensitization for 24, 48, and 72 hours in a solution containing 0.5mg/ml ZnO QDs in ethanol. ZnO quantum dots were prepared by chemical route by mixing the 10mM Zn(CH₃COO)₂, 20mM NaOH in ethanol and left for 18 hours to obtained the desired size. After suitable annealing, these films have been characterized for optical, structural, morphological and photoelectrochemical properties. XRD confirmed the presence of hematite with rhombohedral structure and ZnO&’s with hexagonal structure, both in polycrystalline form. Size of ZnO QDs was ~15nm, as examined by HRTEM. As prepared hematite thin films were porous in nature and size of ZnO QDs are adequate to sit inside the pores of hematite. FE-SEM images confirmed that pores of hematite have been filled with ZnO QDs. These sensitized films, when used as photoelectrode in PEC cell, showed a significant increase in the photocurrent density as compared to unsensitized films. Red shifts in the absorption band edge and reduction in carrier recombination rate may be the major factors responsible for this increment in the photocurrent. The enhanced photo response has also been supported by increased negative value of flat band potential from -0.29V/SCE for unsensitized film to -0.8V/SCE for ZnO QDs sensitized hematite film, as examined by Mott-Schottky curve.

Solar hydrogen production through water splitting using photoelectrochemical (PEC) cells is considered as a promising approach for clean hydrogen fuel generation. Developing cost-effective photoelectrodes using earth-abundant materials with low-cost and scalable fabrication processes for efficient and durable solar water splitting is the key to make this technology practical at large scale. Moreover, having high PEC performance in neutral pH water is highly desirable because the natural water resources such as seawater are usually in a neutral condition and using a neutral solution preventing the use of strong acids and bases which can result in environmental issues.
Silicon is a promising material for the solar water splitting and hydrogen production due to its unique properties such as its proper bandgap, its suitable band position compared to the hydrogen evolution energy level, facile and scalable fabrication techniques to make high-surface-area silicon nanostructures, etc. On the other hand, there are two key factors limiting the application of silicon photoelectrodes for practical solar hydrogen generation including their large onset (turn-on) potential necessitating to use large external potential to drive hydrogen or oxygen evolution reaction (HER or OER), and their instability in the electrolyte. Herein, to improve the photocathodic performance and HER kinetics of 3D p-Si nanowire (NW) arrays fabricated by simple etching method and simultaneously to enhance the p-Si NWs stability, we grew uniform Fe₂O₃ nanocrystals (NCs) or nanorods (NRs) on the p-Si NW cores coated with a SnO₂ or TiO₂ layer (structure is called “core/shell/shell NWs (css-NWs)”) using a facile and scalable solution-processed synthesis method. The css-NW structures and their interfaces were characterized in detail using scanning, transmission, and scanning transmission electron microscopes as well as X-ray diffraction analysis. The css-NWs show significantly enhanced photocathodic performances in neutral pH water compared to the bare p-Si NWs or the corresponding core/shell NWs (cs-NWs) due to increased optical absorption, enhanced charge separation coming from the junctions, and improved HER kinetics. As a result, photocathodic reduction of water at 0 V versus reversible hydrogen electrode (RHE) was achieved in neutral pH water without using any metal catalyst. Moreover, the css-NWs show excellent photoelectrochemical stability of a couple of hours without any morphological changes. More significantly, the css-NWs with the gas-doped metal oxide show very low onset potential (about 0.49 V versus RHE) in the neutral pH water.
The achieved performances for the Si/metal-oxide NW photoelectrodes in the neutral pH water show promising potential for practical clean, efficient, cost-effective and durable solar hydrogen generation at large scales using earth-abundant materials with cost-effect fabrication processes.

Quantum dots (QDs)are one of the promising materials in the development of the third generation photovoltaics. QDs have the advantage of multiple exciton generation (MEG), high absorption coefficient and tunable bandgap, low cost and easy synthesis [1]. The QDs act as analog to dye molecules in QD sensitised solar cells (QDSSCs) when compared with traditional dye sensitized solar cells (DSSCs). The highest efficiency attained in QDSSCs till date is 8.55% [2] which is far below than the DSSCs (13%) [3]. Extending the absorption range of quantum dots is one of potential solutions for enhancing the photoconversion efficiencies.
Herein, we report the screen printing of multilayered TiO₂ films comprised of different particle sizes on FTO substrates. The sensitization of Pb_xCd_1-xS alloy quantum dots on these TiO₂ mesoporous layers is carried by successive ionic layer adsorption and reaction (SILAR) method. The advantage of SILAR method is high loading and wide coverage of quantum dots on the TiO₂ matrix. The bandgap of the Pb_xCd_1-xS alloy quantum dots has been tuned due to different sizes of nanoparticles in TiO₂ matrix. The optical studies reveal the extension of absorption peak from visible to near IR region due to tunable bandgap of quantum dots decorated on TiO₂ matrix. Device characteristics and morphological studies are under investigation.
References
1. Science, 334 (2010) 63-66
2. Nature Materials, (2014) doi:10.1038/nmat3984
3. Nature Chemistry, 6 (2014) 242-247

Cuprous oxide (Cu₂O) is a p-type semiconductor having a band gap of 2.0-2.6 eV, carrier concentration of 10¹⁴~10¹⁶ cm^-3 and mobility of 5 cm²/Vs at room temperature (293K). Due to these attractive properties, Cu₂O thin films have many applications such as gas sensors, resistive random access memory, photodiode, catalysist, anode in batteries, p-type thin film transistors. To prepare the deposited Cu₂O thin films, various techniques such as sputtering, chemical vapor deposition and electrodeposition have been reported. But with device scaling down and becoming complicated more, a more suitable deposition technique is needed for them. In this aspect, atomic layer deposition (ALD) is an attractive and versatile technique allowing the growth of highly conformal films at low temperatures. In this study, ALD-Cu₂O films were deposited on thermally grown SiO₂ and sapphire substrates at a deposition temperature of 140 °C using a sequential supply of bis(1-dimethylamino-2-methyl-2-butoxy)copper(II) and water vapor. The precursor used in this study has good properties, which are free of fluorine in the precursor and a vapor pressure of 1.2 torr at 80°C. With this precursor, excellent step coverage of ~ 100% was shown on trench structure has aspect ratio ~ 4.5 (top opening ~ 25nm). The ALD-Cu₂O film showed the electrical properties such as 2.4eV of band gap, 8.05 cm²/Vs of mobility, 10¹⁴ cm^-3 of carrier density and 10³ Omega;cm of resistivity. The developed ALD-Cu₂O process was applied to fabricate the nanowire photodiode. In order to fabricate ordered Si nanowire arrays, we used the nanosphere lithography to pattern metal catalysts for metal-assisted etching of silicon and obtained Si nanowires with 150 nm diameter (aspect ratio has 1:1.6). And then p-Cu₂O was deposited conformally by ALD to fabricate core-shell p-n junction diode and its electrical properties such as photo-current and dark-current were measured using probe station equipped with black light blue lamp (6W, 360nm). The rectification ratio (+5V/-5V) of nanowire structure was ~4 orders of magnitude in dark. With UV illumination, photo-response of p-Cu₂O/n-SiNW was ~1 order of magnitude higher than that of the planar p-n junction diode.

Semiconductor nanowires are among the most promising candidates for next generation high-efficiency photovoltaics. This is primarily due to their outstanding optical and electrical properties which provide large optical cross sections with high light concentration factors while simultaneously decoupling the photon absorption and charge carrier extraction length scales. These effects relax the requirements for both the minority carrier diffusion length and the amount of semiconductor necessary for the optimal utilization of the incident light power. Here we focus on metal-semiconductor core-shell nanowires, which have previously been predicted to show even better optical absorption than solid semiconductor nanowires. Here we fabricate and analyze for the first time such a structure using a single Au-Cu₂O core-shell nanowire solar cell. Spatially-resolved photocurrent maps using two different top contact geometries reveal that although the minority carrier diffusion length in the Cu₂O shell is less than 1 mm, the radial contact geometry still allows for facile photogenerated carrier collection along an entire nanowire. Wavelength-dependent photocurrent maps show a large decrease in photocurrent below the bulk band gap while current-voltage measurements yield a relatively high open-circuit voltage of 600 mV under laser illumination and an excellent dark diode turn-on voltage of 1V. The insight gained by this study can be used to extend the metal-semiconductor core-shell nanowire concept to low-cost, large-scale photovoltaic devices, utilizing for example metal nanowire electrode grids coated with epitaxialy grown semiconductor shells.

Quantum confined semiconductor nanocrystals have been widely investigated as light harvesting and charge separation components in photovoltaic and photocatalytic devices. The efficiency of these semiconductor nanocrystal-based devices depends on many processes, including light harvesting, carrier relaxation, charge separation and charge recombination. The competition between these processes determines the overall solar energy conversion (solar to electricity or fuel) efficiency. Compared with single component quantum dots (QDs), semiconductor nanoheterostructures, combining two or more materials, offer additional opportunities to control their charge separation properties by tailoring their compositions and dimensions through relative alignment of conduction and valence bands. Further integration of catalysts (heterogeneous or homogeneous) to these materials form multifunctional nanoheterostructures. Using CdSe/CdS/Pt, dot-in-rod nanorods(NRs) with Pt tips, as a model system, we are examining the mechanism of long-lived charge separation and H₂ generation in the presence of sacrificial electron donor. The rates of electron transfer, hole transfer and charge recombination are directly monitored by transient absorption and time-resolved fluorescence spectroscopy. In this talk, we will discuss the mechanism of exciton dissociation, the dependence of the rates of elementary charge transfer processes on the dimension (size and length) and band alignment in these materials, and how these rates affect the overall H2 generation efficiency.

Metal oxide (MO) semiconductors are very attractive materials for photovoltaic (PV) applications. MOs are typically chemically stable, non-toxic and, most important, earth abundant. Consequently, all-oxide-photovoltaics which fulfill the requirements for manufacturing methods at ambient conditions are expected to be durable, low cost systems. To bring all-oxide PV to a breakthrough, new materials have to be developed. The prospect of finding these unique materials lies in combinatorial material science which can produce novel MOs consisting of two, three, four or more elements. While most binary MOs are known, the number of unknown compositions is drastically increasing with the number of components.
Combinatorial MO synthesis allows producing a large number of material compositions on a single substrate. The resulting materials libraries are analyzed for their, structural, electrical and optical properties, while parallel combinatorial PV device libraries are investigated under PV operating conditions. Data analysis, storage and handling are organized using a dedicated database. Tools for data mining are applied to reveal complex correlations between the material properties and the device performance and to improve our understanding of all-oxide PV device operation. The methodology, new binary (A_kB_lO_n) and ternary (A_kB_lC_mO_n) MOs for all oxide photovoltaic cells will be reported.

Conventional flexible mono-crystalline silicon (Si) solar cells have a front-side contact. In this work we show innovative inter-digitated back contact for flexible mono-crystalline silicon solar cell. We also report the efficiency for different released thickness using a novel flex-Si process, which controls the thickness to reduce the required diffusion path for the generated minority carriers to reach the P-N junction. Additionally, nickel silicidation has been formed before the metal contact deposition.
The fabrication starts with a P-type Si < 100 > wafer (1-30 Omega;-cm, 500 mu; m thickness). First step was depositing SiO₂ (950 nm) on the whole wafer using Plasma Enhanced Chemical Vapor Deposition (PECVD). Respectively, n+ areas were defined through lithography pattering after etching the SiO₂ in Reactive Ion Etching (RIE). Source diffusion (Phosphorous TP-250) was used to create N+/P lateral junction. After the N+/P lateral junction formed, lithography patterning was used to leave the silicidation off metallurgical junction position and creates periodic holes (12-14 µm) through the whole contact areas. The silicidation formed by depositing Ni/TiN (40/25) nm using Magnetron Sputtering Deposition, followed by lifting off the deposited film from the defined pervious positions, respectively, 450 °C annealing for 300 sec was preformed in order to form nickel silicide. The unreacted metal was stripped in Piranha solution for 5 min. Structural characterization of NiSi was done using Scanning Electron Microscopy (SEM) and Grazing Incident X-ray Diffraction (GIXRD). Sheet Electrical Resistance has reduced after NiSi formation from 2.5 kOmega;/sq to 2.5 Omega;/sq. Additionally, the same lithography patterns which used for the silicidation was reused for the depositing contact metallization by electron beam evaporation.
The releasing process starts with deposition Al₂O₃ (45 nm) by Atomic Layer Deposition (ALD) to protect the solar cells. Next we create trenches within the holes after protecting the side walls with another layer of Al2O3 (45 nm) via ALD. Finally, we release this Si sheet with the devices on top. J-V measurements of solar cells were performed with a Keithley 2400 source meter and a Spectra-Physics 91160- 1000 solar simulator calibrated to 1 sun, AM1.5 G.
This work shows a novel processes that result in thin photoactive layers with high flexibility, periodic arrays of nano-structured junctions that dramatically amplify absorption while decreasing system parasitic resistance with inter-digitated back contact.

Non-toxic, cost-effective and abundant Group IV elements are potential materials for a broad range of electronic applications. However, silicon and germanium are indirect band gap semiconductors, which make their interactions with light much less efficient than those of direct-band gap semiconductors. This complicates their use in solar cells, escalating prices and holding utilization down. If one could, instead, convert these materials into direct-gap materials that could be handled and processed cheaply, the effect on solar cell development and deployment would be transformative. Theoretical studies suggest that the band structure of Si and Ge can be modified to reduce the energy difference between the first direct and indirect transitions, potentially to the point of “cross-over,” through the utilization of quantum confinement or by alloying with Sn. Evidence of partial direct behavior in small Si and Ge nanocrystals^1,2 is encouraging, but suggests confinement alone may not be enough to effect complete transition in practical materials. At the same time, alloying has been only cursorily investigated, as it is not easily achievable in bulk due to miniscule solubility of Sn in Ge and Si due to lattice strain. Here, we describe the combination of these approaches. Specifically, we have developed a facile microwave-assisted synthetic method for size- and shape-controlled production of colloidal Sn_xGe_1-x nanocrystals where x le; 0.3. The optical properties of the nanocrystals have been studied in conjunction with electronic structure calculations in order to estimate required tin concentrations for the indirect-to-direct transition. The details of the synthesis, structural and optical characterizations will be presented and discussed together with initial results on the synthesis of Sn_xSi_1-x nanocrystals.
¹ Sykora, M.; Mangolini, L.; Schaller, R. D.; Kortshagen, U.; Jurbergs, D.; Klimov, V. I. "Size-Dependent Intrinsic Radiative Decay Rates of Silicon Nanocrystals at Large Confinement Energies" Phys. Rev. Lett. , 2008, 100, 067401
² Lee, D. C. ; Pietryga, J. M.; Robel, I.; Werder, D. J.; Schaller, R. D. ; Klimov, V. I. "Colloidal Synthesis of Infrared-Emitting Germanium Nanocrystals" J. Am. Chem. Soc., 2009, 131(10), 3436

Sunlight is a renewable and carbon-neutral energy source on earth of abundant amount to replace fossil fuels for the rising worldwide energy demand. However, sunlight&’s intermittent nature is one of the major issues preventing its large-scale industrial use in a convenient way. Therefore, storage of sunlight energy in portable and transportable chemical fuels is of great interest. Photoelectrochemical (PEC) water splitting is one of the promising solutions to generate hydrogen fuel from water using sunlight irradiation. To date, tremendous research efforts have been devoted to realize high performance PEC water splitting devices. A major challenge is the development of high-performance photoanodes that can efficiently and stably oxidize water for the oxygen evolution.
Here, we report efficient and stable photoanodes made of particle-transferred BiVO₄ and LaTiO₂N photocatalysts in a sandwich structure with Nb and Ti metals sputtered on the backside realizing good electrical conductivity and a high-quality surface modification using dural co-catalysts enabling the enhanced charge separation and the improved oxygen evolving reactions. A high photocurrent density of ~3.5 mA/cm² is obtained with the BiVO₄ photoanode in 0.1 M potassium phosphate solution at pH 7 with an applied bias of 1.2 V_RHE under sunlight illumination. The applied-bias-solar-to-hydrogen conversion efficiency is up to 1.5%. Stoichiometric oxygen and hydrogen are stably generated on the BiVO₄ photoanode and the platinum electrode with Faraday efficiency of unity (100%) over 12 hours.

g-C₃N₄ is a promising material for the production of hydrogen from water via photo-catalysis, if we can tune its band gap to desirable levels. In this work, we present our experimental and ab initio results on the correlation between the band gap and the lattice constant of the g-C₃N₄ system. We first show that the system exhibits a correlation between stacking configuration and band gaps. Then, we show that a linear relation exists between the band gap and the lattice constant of the system. We explain this behavior by showing that the change in the lattice constant of the system produces a change in the overlap of wave functions of atoms in neighboring layers. In particular, the unoccupied p_z states experience a higher energy shift than any other occupied state (p_x or p_y). As a result, the band gap and the lattice constant of the g-C₃N₄ exhibit a direct relation between them. This result is of great importance, as it demonstrates the possibility to tune the band gap and the frequency at which g-C₃N₄ adsorbs light by changing the lattice constant of the system.