Ultrafast carrier dynamics in BiVO$_{\mathrm{4}}$ thin film photoanode material: time-resolved THz spectroscopic study.

Recent demonstrations of 3{\%} solar conversion efficiency in thin film BiVO$_{\mathrm{4}}$ make it a promising photoanode material for photoelectrochemical water oxidation [1]. With a bandgap of 2.4 eV, it strongly absorbs UV and visible light up to 520 nm. However, its efficiency is limited by extremely poor carrier mobility, with values from 0.01 to 1 cm$^{\mathrm{2}}$ /Vs reported in the literature, and often attributed to formation of small polarons [2]. The precise nature of conductivity in BiVO$_{\mathrm{4}}$ is, however, not well-established. We use time-resolved terahertz (THz) spectroscopy as a non-contact probe of microscopic photoconductivity of a 100 nm-thick BiVO$_{\mathrm{4}}$ film. THz spectroscopy allows probing the dynamics of photo-injected carriers over nanometer length scales, and thus provides insight about transport of carriers inside the 100-200 nm grains. We find that intra-grain mobility may be as much as several orders of magnitude higher than macroscopic mobility that is affected by the grain boundaries. References [1] P. M. Rao et al., Nano Lett. 14, 1099 (2014) [2] A.J.E. Rettie et al., Appl. Phys. Lett. 106, 022106 (2015).