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.