Significantly enhanced photocurrent for water oxidation in monolithic Mo:BiVO4/SnO2/Si by thermally increasing the minority carrier diffusion length

Abstract

Transition-metal-oxide semiconductors are promising photoanodes for solar water splitting due to their excellent chemical stability and appropriate bandgaps. However, in absorbers such as BiVO₄, TiO₂, α-Fe₂O₃, and WO₃, charge carriers localize as small polarons or become trapped, leading to low minority carrier mobilities. This limits the minority carrier collection efficiency in the quasi-neutral region of the light absorber, and lowers the overall photoactivity. In this work, we demonstrate that modestly elevating the temperature activates minority carrier hopping in monoclinic BiVO₄, significantly enhancing the saturation photocurrent without a substantial anodic shift of the onset potential, and is an attractive alternative to employing complex passivation layers and nanostructured templates towards achieving the theoretical photocurrent density. Specifically, using a Mo:BiVO₄/SnO₂/Si tandem photoanode/photovoltaic, increasing the absolute temperature by 11% from 10 to 42 °C elevates the saturation photocurrent from 1.8 to 4.0 mA cm⁻². This strong temperature enhancement, 3.8% K⁻¹, is 5 times greater than that in α-Fe₂O₃. Concurrently, the onset potential shifts slightly from 0.02 V to 0.08 V versus the reversible hydrogen electrode (or equivalently, from −1.22 V to −1.13 V versus the equilibrium potential of oxygen evolution). Our observation contrasts with the prevailing understanding that the energy conversion efficiency generally decreases with temperature as a result of reduced photovoltage. Thermally-activating minority carrier transport represents a general pathway towards enhancing the photoactivity of light absorbers where hopping conduction limits the minority carrier collection in the quasi-neutral region.