Grain Boundaries Limit the Charge Carrier Transport in Pulsed Laser Deposited α-SnWO₄ Thin Film Photoabsorbers

Recently, α-SnWO₄ 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-SnWO₄ thin films are elucidated to optimize their charge carrier transport properties. A high-temperature treatment is found to enhance the photoconductivity of α-SnWO₄ 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 cm² V^-1 s^-1 in α-SnWO₄. This is comparable to values found for BiVO₄, 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-SnWO₄ photoanodes. Copyright © 2020 American Chemical Society.