External Quantum Efficiency Spectra of BiVO4 Thin Film Photoanodes under Bias Illumination

Abstract

External quantum efficiency (EQE) of bismuth vanadate thin film photoanodes, measured in a pH 7 potassium phosphate buffer solution with sodium sulfite hole scavenger, was observed to substantially decrease when measured under white light bias (LB). While the EQE exhibited a fast initial decrease across its full spectral range, a ∼3.5 eV (350 nm) feature under front illumination conditions became disproportionally suppressed after being under LB (strongest when it is also incident on the front side of the sample) for several tens of minutes, in spite of this wavelength being outside the spectral range encompassed by the LB source. Applied potential does not have a strong effect on the qualitative behavior. From its different decay time, the wavelength-specific decrease of the 3.5 eV feature, and its responsible mechanism, is distinct from the initial, spectrally uniform decrease of EQE, which happens at a faster timescale and is similar for all illumination conditions. To more closely examine the suppression of the 3.5 eV feature, we compare calculated depth-dependent optical generation profiles and behaviors under different illumination conditions, which imply the involvement of in-gap states and long-lived states deeper into the conduction (or alternatively, valence) band. Possible mechanisms are discussed.