A New Explanation of the Different Photocurrents of Bivo4 Photoanode Under Front and Back Illumination: Surface Hole Transfer Effected by Illumination Directions

Abstract

Illumination is the basic condition for the operation of the photoelectric chemical water separation device. In previous studies, it was found that different light directions of the BiVO4 photoanode produced different photocurrents. It is generally accepted that the carrier transport of BiVO4 is the reason that causes different photocurrents under different light directions. However, in addition to carrier transport, surface charge transfer is an important factor affecting photocurrent, but it has been ignored. In this work, the charge transfer efficiency of BiVO4 photoanode under front and back illumination was calculated, and the fractional occupy of surface state was analyzed by electrochemical impedance spectroscopy under different illumination directions. The results show that the hole transfer efficiency under front illumination is higher than that under back illumination. Under front illumination, the surface of the BiVO4 photoanode can directly absorb light, which lead to a decrease in fractional occupy of surface state, thereby reducing the probability of electron hole recombination and improving the efficiency of hole transfer. These findings provide a new explanation for the difference of BiVO4 photocurrents under different illumination directions, and demonstrate the usability of using electrochemical impedance spectroscopy under different illumination directions to analyze the surface states of photoelectrodes.

Keywords: Solar water splitting, BiVO4 photoanode, Surface hole transfer efficiency, Surface state