BiVO₄-Liquid Junction Photovoltaic Cell with 0.2% Solar Energy Conversion Efficiency

BiVO₄ is an important photoanode material for water oxidation, but its photoelectrochemistry regarding the triiodide/iodide redox couple is not well understood. Here, we use a combination of open circuit potential measurements, photoelectrochemical scans, and liquid surface photovoltage spectroscopy (SPS) to confirm that BiVO₄/triiodide/iodide electrolyte contacts produce up to 0.55 V photovoltage under 23 mW/cm^-2 illumination from a 470 nm LED. Inspired by these results, we construct FTO/BiVO₄/KI(I₂)_aq/Pt sandwich photoelectrochemical cells from electrochemically grown 0.5 × 0.5 cm² BiVO₄ and Mo-doped BiVO₄ films. Under AM 1.5 illumination, the devices have up to 0.22% energy conversion efficiency, 0.32 V photovoltage, and 1.8 mA cm^-2 photocurrent. Based on SPS, hole transfer to iodide is sufficiently fast to prevent the competing water oxidation reaction. Mo doping increases the incident photon-to-current efficiency to up to 55% (at 425 nm under front illumination) by improving the BiVO₄ conductivity, but this comes at the expense of a lower photovoltage resulting from recombination at the Mo defects and a detrimental Schottky junction at the interface with FTO. Additional photovoltage losses are caused by the offset between the BiVO₄ valence band edge and the triiodide/iodide electrochemical potential and by electron back transfer to iodide at the FTO back contact (shunting). Overall, this work provides the first example of a BiVO₄-liquid photovoltaic cell and an analysis of its limitations. Even though the larger band gaps of metal oxides constrain their solar energy conversion efficiency, their transparency to visible light and deep valence bands makes them suitable for tandem photovoltaic devices.