Revealing the Performance-Limiting Factors in α-SnWO₄ Photoanodes for Solar Water Splitting

α-SnWO₄ 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-SnWO₄ photoanodes have not shown high photoconversion efficiencies for reasons that have not yet been fully elucidated. In this work, phase-pure α-SnWO₄ 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-SnWO₄ 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-SnWO₄. These results provide important insights into the photoelectrochemical properties and limitations of α-SnWO₄ and point toward new strategies to further improve the performance of this promising material. Copyright © 2018 American Chemical Society.