Direct Time-Resolved Observation of Carrier Trapping and Polaron Conductivity in BiVO4

BiVO4 is a promising photoanode candidate for water splitting applications, but its microscopic charge carrier transport properties are not yet fully understood. We investigated the photoinduced carrier mobility for undoped and 1% tungsten-doped BiVO4 thin films in an early time window from 1 ps to 1 ns using THz spectroscopy. The combined electron hole effective mobility gradually decreases with time by 1 order of magnitude starting at an upper limit of similar to 0.4 cm(2) V-1 s(-1). The loss is attributed to carrier localization. We provide for the first time direct time-resolved evidence of hole polaron formation accompanied by the temporal buildup of a polaron population in parallel to initial carrier trapping. A mobility of 0.02 cm(2) V-1 s(-1) is found for the self trapped carriers, which leads to a thermal hopping activation energy of similar to 90 meV.