Direct In Situ Measurement of Quantum Efficiencies of Charge Separation and Proton Reduction at TiO2-Protected GaP Photocathodes

Photoelectrochemical solar fuel generation at the semiconductor/liquid interface consists of multiple elementary steps, including charge separation, recombination, and catalytic reactions. While the overall incident light-to-current conversion efficiency (IPCE) can be readily measured, identifying the microscopic efficiency loss processes remains difficult. Here, we report simultaneous in situ transient photocurrent and transient reflectance spectroscopy (TRS) measurements of titanium dioxide-protected gallium phosphide photocathodes for water reduction in photoelectrochemical cells. Transient reflectance spectroscopy enables the direct probe of the separated charge carriers responsible for water reduction to follow their kinetics. Comparison with transient photocurrent measurement allows the direct probe of the initial charge separation quantum efficiency (Phi(CS)) and provides support for a transient photocurrent model that divides IPCE into the product of quantum efficiencies of light absorption (Phi(abs)), charge separation (Phi(CS)), and photoreduction (O-red), i.e., IPCE = Phi(abs)Phi(CS)Phi(red). Our study shows that there are two general key loss pathways: recombination within the bulk GaP that reduces Phi(CS) and interfacial recombination at the junction that decreases Ored. Although both loss pathways can be reduced at a more negative applied bias, for GaP/TiO2, the initial charge separation loss is the key efficiency limiting factor. Our combined transient reflectance and photocurrent study provides a time-resolved view of microscopic steps involved in the overall light-to-current conversion process and provides detailed insights into the main loss pathways of the photoelectrochemical system.

Automated summary

Photoelectrochemical solar fuel generation involves multiple elementary steps, and understanding the individual efficiency loss processes has been challenging. In this study, simultaneous measurements of transient photocurrent and transient reflectance spectroscopy were conducted to investigate the microscopic efficiency loss processes in water reduction on titanium dioxide-protected gallium phosphide photocathodes. The results provided insights into the key loss pathways and the factors limiting the overall efficiency of the photoelectrochemical system.