α-SnWO 4 is an n-type metal oxide semiconductor that has recently attracted attention as a top absorber material in a D4-tandem device for highly efficient solar water splitting due to the combination of an ideal bandgap (∼1.9 eV) and a relatively negative photocurrent onset potential (∼0 V vs RHE). However, up to now, α-SnWO 4 photoanodes have not shown high photoconversion efficiencies for reasons that have not yet been fully elucidated. In this work, phasepure α-SnWO 4 films are successfully prepared by pulsed laser deposition. The favorable band alignment is confirmed, and key carrier transport properties, such as charge carrier mobility, lifetime, and diffusion length are reported for the first time. In addition, a hole-conducting NiO x layer is introduced to protect the surface of the α-SnWO 4 films from oxidation. The NiO x layer is found to increase the photocurrent for sulfite oxidation by a factor of ∼100, setting a new benchmark for the photocurrent and quantum efficiency of α-SnWO 4 . These results provide important insights into the photoelectrochemical properties and limitations of α-SnWO 4 and point toward new strategies to further improve the performance of this promising material.

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The earth-abundant ternary compound BaZrS3, which crystallizes in the perovskite-type structure, has come into view as a promising candidate for photovoltaic applications. We present the synthesis and characterization of polycrystalline perovskite-type BaZrS3 thin films. BaZrO3 precursor layers were deposited by pulsed laser deposition and sulfurized at various temperatures in an argon-diluted H2S atmosphere. We observe increasing incorporation of sulfur for higher annealing temperatures, accompanied by a red shift of the absorption edge, with a bandgap of E g = 1.99 eV and a large absorption strength >105 cm–1 obtained for sulfurization temperatures of 1000 °C. X-ray diffraction analysis and SEM indicate enhanced crystallization at the higher annealing temperatures, but no evidence for a crystalline solid solution between the BaZrO3 and BaZrS3 phases is found. The charge carrier sum mobility estimated from optical-pump–terahertz-probe spectroscopy indicates increasing mobilities with increasing sulfurization temperature, reaching maximum values of up to ∼2 cm2 V–1 s–1.

Photoelectrochemical water splitting is a promising route to produce hydrogen from solar energy. However, corrosion of photoelectrodes remains a fundamental challenge for their implementation. Here, we reveal different dissolution behaviors of BiVO 4 photoanode in pH-buffered borate, phosphate, and citrate (hole-scavenger) electrolytes, studied in operando employing an illuminated scanning flow cell. We demonstrate that decrease in photocurrents alone does not reflect the degradation of photoelectrodes. Changes in dissolution rates correlate to the evolution of surface chemistry and morphology. The correlative measurements on both sides of the liquid–semiconductor junction provide quantitative comparison and mechanistic insights into the degradation processes.

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.