Efforts in the solar water splitting field for the past decade have established BiVO₄ as the highest performing metal oxide photoanode. Within this period, the AM1.5 photocurrents of BiVO₄ have been increased from hundreds of mA/cm² to ~7 mA/cm².^[1] However, high photocurrents (> 5 mA/cm²) are only achieved with nanostructuring, which poses additional complexities (e.g., light scattering) for the design and scalability of a tandem device for water splitting. Non-nanostructured BiVO₄ is limited by its poor carrier transport properties. In addition, with reported photocurrents already within 90% of the theoretical maximum, the solar-to-hydrogen (STH) efficiency of BiVO₄ is limited to < 9% due to its relatively wide bandgap of 2.4-2.5 eV.

Here, we demonstrate that these limiting factors can be alleviated by controlled anion and cation substitution in BiVO₄. To overcome the bandgap limitation, we incorporated sulfur into BiVO₄ by post-annealing in sulfur-rich atmosphere. As a result, the bandgap is reduced by up to ~0.3 eV, which increases the theoretical maximum STH efficiency to ~12%. We confirmed that sulfur substitutes oxygen in the lattice of BiVO₄ by a series of structural and chemical characterization (e.g., XRD, Raman, XPS). Moreover, hard X-ray photoelectron spectroscopy (HAXPES) reveals that the bandgap decrease is a result of an upward shift of the valence band maximum. Simultaneously, time-resolved microwave conductivity (TRMC) measurements reveal an improvement of charge carrier transport by the incorporation of sulfur; the mobility increases by a factor of ~5. Wavelength-dependent TRMC measurements confirm the photoactivity of the sulfur-incorporated BiVO₄ up to 560 nm, well beyond the bandgap of typical BiVO₄ films. In addition, we successfully introduced calcium into BiVO₄ thin films;^[2] calcium substitutes bismuth and acts as an acceptor-type dopant. Although it seems counterintuitive to introduce an acceptor dopant into n-type BiVO₄, this would allow the fabrication of p-type, and eventually p-n homojunctions based on BiVO₄. HAXPES measurements reveal that calcium out-diffuses towards the surface of the film, thereby creating a spontaneous p-n homojunction within the film. As a result of the internal electric field, the carrier separation efficiency was enhanced by a factor of ~2. Overall, this work underlines the importance of controlled ionic substitution in complex metal oxides, which may not only improve the performance but also enable new device architectures.

[1] Abdi and Berglund, J. Phys. D. Appl. Phys. (2017)

[2] Abdi et al. ChemPlusChem (2018)