Sub-Bandgap Photon-to-Current Conversion in Bismuth Vanadate Photoanodes and Its Impact on the Maximum Photocurrent Density Achievable for Water Splitting

Abstract

The physical properties of bismuth vanadate (BiVO4) make it an appealing semiconductor photoanode for water oxidation in photoelectrochemical cells that aim to produce hydrogen or other solar fuels. However, it has been estimated that its relatively wide bandgap limits achievable photocurrent densities to approximately 7.5 mA/cm2under 1 sun AM1.5G illumination. Here, we perform high-sensitivity external quantum efficiency measurements and demonstrate that sub-bandgap states within BiVO4also contribute to photocurrent generation, regardless of the fabrication method or mesoscopic structure. Based on these results and considering Lambertian scattering at the electrolyte/BiVO4interface, we show that the maximum theoretical current density from BiVO4can be as high as 12.2 mA/cm2, when assuming complete absorption and conversion of sunlight photons extending to the lowest photon energy for which we experimentally measured photocurrent generation promoted by sub-bandgap states. This finding opens new avenues for design of BiVO4photoanodes with efficiencies that are much greater than were previously assumed to be possible.