1 research outputs found
Direct <i>In Situ</i> Measurement of Quantum Efficiencies of Charge Separation and Proton Reduction at TiO<sub>2</sub>‑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 (Ï•CS) and provides support for a transient photocurrent model that divides
IPCE into the product of quantum efficiencies of light absorption
(Ï•abs), charge separation (Ï•CS),
and photoreduction (Ï•red), i.e.,
IPCE = ϕabsϕCSϕred. Our study shows that there are two general key loss pathways: recombination
within the bulk GaP that reduces Ï•CS and interfacial
recombination at the junction that decreases Ï•red. 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