A new explanation of the different photocurrents of BiVO4 photoanode under front and back illumination: Surface hole transfer effected by illumination directions

Using solar energy for photoelectrochemical (PEC) water splitting is a popular way to store sustainable energy and reduce dependence on fossil fuels. Bismuth vanadate (BiVO 4 ) is a promising photoanode material for solar water splitting. In previous studies, it was found that different light directions of the BiVO 4 photoanode produced different photocurrents. It was generally accepted that the carrier transport of BiVO 4 was the reason that causes different photocurrents under different light directions. However, in addition to carrier transport, surface hole transfer may also be an important factor leading to different photocurrents under different light directions, but it has been ignored. In this work, the surface hole transfer efficiency of the BiVO 4 photoanode under front- and back-side illumination was calculated, and the fractional occupancy of surface states was analyzed by photoelectrochemical impedance spectroscopy. The results show that the hole transfer efficiency under front irradiation is about 1.56 times that under back irradiation at 0.1 sun illumination and about 1.2 times that under back irradiation at 1 sun illumination. Under front illumination, the surface of the BiVO 4 photoanode can directly absorb light, which leads to a decrease in the fractional occupancy of surface states, thereby reducing the probability of electron-hole recombination and improving the efficiency of hole transfer. These findings provide a new explanation for the different photocurrents of the BiVO 4 photoanode under different light directions and demonstrate the effectiveness of photoelectrochemical impedance spectroscopy under different light directions for analyzing the surface states of photoelectrodes.