Nature and light dependence of bulk recombination in Co-Pi-catalyzed BiVO ₄ photoanodes

BiVO ₄ is considered to be a promising photoanode material for solar water splitting applications. Its performance is limited by two main factors: slow water oxidation kinetics and poor charge separation. We confirm recent reports that cobalt phosphate (Co-Pi) is an efficient water oxidation catalyst for BiVO ₄ and report an AM1.5 photocurrent of 1.7 mA/cm ² at 1.23 V vs RHE for 100 nm spray-deposited, compact, and undoped BiVO ₄ films with an optimized Co-Pi film thickness of 30 nm. The charge separation of these films depends strongly on light intensity, ranging from 90% at low light intensities to less than 20% at intensities corresponding to 1 sun. These observations indicate that the charge separation efficiency in BiVO ₄ is limited by poor electron transport and not by the presence of bulk defect states, interface traps, or the presence of a Schottky junction at the back-contact. © 2012 American Chemical Society.