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

• Authors: Zihao Xu, Emory University; Bingya Hou, University of Southern California; Fengyi Zhao, Emory University; Sa Suo, Emory University; Yawei Liu, Emory University; Haotian Shi, University of Southern California; Zhi Cai, University of Southern California; Craig Hill, Emory University; Djamaladdin Musaev, Emory University; Matthew Mecklenburg, University of Southern California; Stephen B Cronin, University of Southern California; Tianquan Lian, Emory University
• Journal Title: JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
• Volume: Volume 145, Number 5
• Publisher: AMER CHEMICAL SOC | 2023-01-30, Pages 2860-2869
• Type of Work: Article | Final Publisher PDF
• Publisher DOI: http://doi.org/10.1021/jacs.2c10578
• Subjects: Engineering, Electronics and Electrical
• Keywords: Science & Technology; Physical Sciences; Chemistry, Multidisciplinary; Chemistry; multiple elementary steps; including diffusion; drift; charge; FRANZ-KELDYSH OSCILLATIONS; SEMICONDUCTOR ELECTRODES; 2ND-HARMONIC GENERATION; WATER; TIO2; GAAS; EVOLUTION; STATE; FIELD; PHOTOANODES

Abstract

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 (ϕCS) and provides support for a transient photocurrent model that divides IPCE into the product of quantum efficiencies of light absorption (ϕabs), charge separation (ϕCS), and photoreduction (ϕred), i.e., IPCE = ϕabsϕCSϕred. Our study shows that there are two general key loss pathways: recombination within the bulk GaP that reduces ϕCS and interfacial recombination at the junction that decreases ϕred. 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.