Investigation of the photocorrosion of n-GaP photoanodes in acid with in situ UV-Vis spectroscopy†

Semiconductors of the III–V class are among the most promising materials for high efficiency solar fuels applications, particularly in tandem devices. Unfortunately, these materials are known to undergo oxidative corrosion as photoanodes in strongly acidic or alkaline electrolyte. In striving to overcome this challenge, researchers need to understand the fundamental degradation behavior during photoelectrochemical operation. Studying the in situ photocorrosion process has largely been limited thus far to expensive and labor-intensive techniques that are not widely available in most labs. Herein, the corrosion of n-GaP, a promising III–V material for tandem top subcells, was investigated in strongly acidic electrolyte using an in situ UV-Vis spectroscopy technique to monitor dissolved Ga and P species as a function of applied bias and time. The changing faradaic efficiency of the electrochemical GaP oxidation reaction was calculated from this data and used to interpret the corrosion process in conjunction with SEM and XPS characterization. Most notably, p⁺-GaP and n-GaP displayed strikingly different corrosion behavior, with p⁺-GaP dissolving uniformly across the active area and the corrosion faradaic efficiency increasing to a steady value. Illuminated n-GaP, in contrast, showed initially high corrosion faradaic efficiency which decreased during operation and was attributed to the phenomena of anisotropic surface etching and micropore formation during the beginning stages of photoanodic operation. In addition, corrosion measurements were made with thin conformal coatings of TiO₂ as a protective barrier layer on the GaP surface. Although the protective coating slowed the rate of GaP dissolution, the TiO₂ layers produced herein contributed significant charge-transfer resistance and still showed similar trends in the corrosion faradaic efficiency vs. time as the bare n-GaP.