Recently, α-SnWO 4 attracted attention as a material to be used as a top absorber in a tandem device for photoelectrochemical water splitting due to its nearly optimum band gap of ∼1.9 eV and an early photocurrent onset potential of ∼0 V versus RHE. However, the mismatch between the charge carrier diffusion length and light penetration depth which is typical for metal oxide semiconductorscurrently hinders the realization of high photoconversion efficiencies. In this work, the pulsed laser deposition process and annealing treatment of α-SnWO 4 thin films are elucidated to optimize their charge carrier transport properties. A hightemperature treatment is found to enhance the photoconductivity of α-SnWO 4 by more than 1 order of magnitude, as measured with time-resolved microwave conductivity (TRMC). A complimentary analysis by time-resolved terahertz spectroscopy (TRTS) shows that this improvement can be assigned to an increase of the grain size in the heat-treated films. In addition, TRTS reveals electron−hole charge carrier mobilities of up to 0.13 cm 2 V −1 s −1 in α-SnWO 4 . This is comparable to values found for BiVO 4 , which is one of the best performing metal oxide photoanode materials to date. These findings show that there is a significant potential for further improving the properties of α-SnWO 4 photoanodes.

Mobilities and lifetimes of photogenerated charge carriers are core properties of photovoltaic materials and can both be characterized by contactless terahertz or microwave measurements. Here, the expertise from fifteen laboratories is combined to quantitatively model the current‐voltage characteristics of a solar cell from such measurements. To this end, the impact of measurement conditions, alternate interpretations, and experimental inter‐laboratory variations are discussed using a (Cs,FA,MA)Pb(I,Br)3 halide perovskite thin‐film as a case study. At 1 sun equivalent excitation, neither transport nor recombination is significantly affected by exciton formation or trapping. Terahertz, microwave, and photoluminescence transients for the neat material yield consistent effective lifetimes implying a resistance‐free JV‐curve with a potential power conversion efficiency of 24.6 %. For grainsizes above ≈20 nm, intra‐grain charge transport is characterized by terahertz sum mobilities of ≈32 cm2 V−1 s−1. Drift‐diffusion simulations indicate that these intra‐grain mobilities can slightly reduce the fill factor of perovskite solar cells to 0.82, in accordance with the best‐realized devices in the literature. Beyond perovskites, this work can guide a highly predictive characterization of any emerging semiconductor for photovoltaic or photoelectrochemical energy conversion. A best practice for the interpretation of terahertz and microwave measurements on photovoltaic materials is presented.

α‐SnWO4 is a promising metal oxide photoanode material for direct photoelectrochemical water splitting. With a band gap of 1.9 eV, it ideally matches the requirements as a top absorber in a tandem device theoretically capable of achieving solar‐to‐hydrogen (STH) efficiencies above 20%. It suffers from photoelectrochemical instability, but NiOx protection layers have been shown to help overcome this limitation. At the same time, however, such protection layers seem to reduce the photovoltage that can be generated at the solid/electrolyte junction. In this study, an extensive analysis of the α‐SnWO4/NiOx interface is performed by synchrotron‐based hard X‐ray photoelectron spectroscopy (HAXPES). NiOx deposition introduces a favorable upwards band bending, but also oxidizes Sn2+ to Sn4+ at the interface. By combining the HAXPES data with open circuit potential (OCP) analysis, density functional theory (DFT) calculations, and Monte Carlo‐based photoemission spectra simulation, the presence of a thin oxide layer at the α‐SnWO4/NiOx interface is suggested and shown to be responsible for the limited photovoltage. Based on this new‐found understanding, suitable mitigation strategies can be proposed. Overall, this study demonstrates the complex nature of solid‐state interfaces in multi‐layer photoelectrodes, which needs to be unraveled to design efficient heterostructured photoelectrodes for solar water splitting.

fuel can be used on-site and on-demand but can also be stored and transported for off-site use. [5] However, practical PEC applications put stringent demands on photoabsorber materials in terms of efficiency, cost, and stability. Significant trade-offs have to be made, and this has thus far impeded the commercialization of PEC technology. [3,4] High solar to hydrogen conversion efficiencies approaching 20% have been achieved with photoelectrodes based on high-quality III-V semiconductors, such as GaInP 2 and GaAs. [6] However, their cost is likely to be prohibitive, and many of them suffer from instability under PEC operating conditions. The primary materials criteria are suitable bandgap energy to absorb a large fraction of solar photons with sufficient energies to enable water splitting, good electrical conductivity to enable photogenerated charge carrier extraction, favorable energy-band positions to enable carrier injection, and long-term stability in an aqueous environment. [4] Additionally, the material should be abundant and inexpensive in order to make PEC technology competitive with the chemical production of hydrogen from coal or natural gas. Almost all possible elemental and binary semiconductors have been investigated as photoelectrodes for water splitting, but none fulfill all the requirements. Therefore, the search will have to be expanded to ternary or even more complex materials. Metal-oxides offer many unique advantages as photoabsorber materials for PEC water splitting. [7-9] They have a variety of The widespread application of solar-water-splitting for energy conversion depends on the progress of photoelectrodes that uphold stringent criteria from photoabsorber materials. After investigating almost all possible elemental and binary semiconductors, the search must be expanded to complex materials. Yet, high structural control of these materials will become more challenging with an increasing number of elements. Complex metal-oxides offer unique advantages as photoabsorbers. However, practical fabrication conditions when using glass-based transparent conductive-substrates with low thermal-stability impedes the use of common synthesis routes of high-quality metal-oxide thin-film photoelectrodes. Nevertheless, rapid thermal processing (RTP) enables heating at higher temperatures than the thermal stabilities of the substrates, circumventing this bottleneck. Reported here is an approach to overcome phasepurity challenges in complex metal-oxides, showing the importance of attaining a single-phase multinary compound by exploring large growth parameter spaces, achieved by employing a combinatorial approach to study CuBi 2 O 4 , a prime candidate photoabsorber. Pure CuBi 2 O 4 photoelectrodes are synthesized after studying the relationship between the crystal-structures, synthesis conditions, RTP, and properties over a range of thicknesses. Single-phase photoelectrodes exhibit higher fill-factors, photoconversion efficiencies, longer carrier lifetimes, and increased stability than nonpure photoelectrodes...