Efficient hydrogen evolution from water over a thin film photocathode composed of solid solutions with a composition gradient of ZnTe and CdTe

Abstract

Photoelectrodes utilising cadmium telluride (CdTe) have shown promising properties as photocathodes for solar hydrogen evolution from water, such as a large cathodic photocurrent, owing to their long optical absorption edge of about 840 nm, and a large driving force for the reaction according to a flat band potential of close to 0.9 V_RHE. However, their performance is limited, especially in a relatively high potential region. For the fabrication of a practical photoelectrochemical device, photocathodes with sufficiently high onset potential and high photocurrent at around 0.6 V_RHE are desired. In this study, a thin film CdTe based photocathode composed of a bilayer structure with zinc telluride (ZnTe) as the back contact was successfully fabricated by sequential deposition of ZnTe and CdTe using the close space sublimation method. The addition of a small amount of Cu is essential to obtain an efficient photocathode and, interestingly, a composition gradient is introduced through the formation of a solid solution at the ZnTe–CdTe interface. Post-deposition rapid thermal annealing further enhanced the cathodic photocurrent resulting in 4.0 mA cm⁻² at 0.5 V_RHE and a half-cell solar-to-hydrogen conversion efficiency of 3.6% at 0.36 V_RHE with incident photon-to-current conversion efficiencies of >70% at 480–640 nm.