Charge Separation Enhancement Mechanism By p-n Junction in Water Reduction Electrode

Abstract

Solar water splitting overall efficiency largely depends on the photoanode/photocathode charge carrier separation efficiency. Understanding the competition between charge separation and charge recombination is crucial to improve the device efficiency. In this study first we discussed the challenges in studying such process with femtosecond transient spectroscopy. Then we demonstrated a feasible method by combining transient reflectance (TR) spectroscopy and in situ electrochemical measurement to provide critical information on charge carrier separation efficiency and IPCE on a water/CO2 reduction photoanode, p-type gallium phosphate with protective titanium dioxide (TiO2) layer. Enhanced charge separation at the interface of gallium phosphate (GaP, 100) single crystal and TiO2 layer is observed by TR. The separated charge carrier concentration obtained by reflectance change shows strong correlation to the IPCE measured in situ under same condition. Through this correlation we provided direct evidence that the IPCE is heavily determined by the transiently separated charge carriers. This study also provides an applicable and powerful method to study similar HER and OER semiconductor electrode photogenerated charge carrier dynamics at ultrafast time scale.