Accurate Determination of the Charge Transfer Efficiency of Photoanodes for Solar Water Splitting

TL;DR

This paper compares methods for accurately measuring charge transfer efficiency in photoanodes for solar water splitting, emphasizing the advantages of intensity modulated photocurrent spectroscopy (IMPS) over traditional techniques.

Contribution

It demonstrates that IMPS provides more reliable xt measurements and offers guidelines for optimal experimental conditions to reduce errors.

Findings

IMPS yields more accurate xt values than voltammetry or chopped light methods.

Different analysis techniques and light conditions can cause significant variability in xt results.

Optimal measurement conditions involve using bias light intensity close to the standard solar spectrum.

Abstract

The oxygen evolution reaction (OER) at the surface of semiconductor photoanodes involves photo-generated holes that oxidize water. A certain fraction of the holes that reach the surface recombine with electrons from the conduction band, giving rise to the surface recombination loss. The charge transfer efficiency, xt, defined as the ratio between the flux of holes that contribute to the water oxidation reaction and the total flux of holes that reach the surface, is an important parameter that helps to distinguish between bulk and surface recombination losses. However, accurate determination of xt by conventional voltammetry measurements is complicated because only the total current is measured and it is difficult to discern between different contributions to the current. Chopped light measurement and hole scavenger measurement techniques are widely employed to determine xt, but they often lead to errors. Intensity modulated photocurrent spectroscopy (IMPS) is better suited for accurate determination of xt because it provides direct information on both the total photocurrent and the surface recombination current. Careful analysis of IMPS measurements at different light intensities is required to account for nonlinear effects. We compare the xt values obtained by these methods using heteroepitaxial hematite photoanodes. A wide spread of xt values is obtained by different analysis methods and different light sources and light intensities. Statistical analysis of the results show good correlation between different methods for measurements carried out with the same light source, light intensity and potential. However, there is a considerable spread in the results obtained by different methods. For accurate determination of xt, we recommend IMPS measurements with a bias light intensity such that the irradiance is as close as possible to the standard solar spectrum.