Chalcopyrite CuIn(S_1–<i>x</i>Se_<i>x</i>)₂ for Photoelectrocatalytic H₂ Evolution: Unraveling the Energetics and Complex Kinetics of Photogenerated Charge Transfer in the Semiconductor Bulk

Abstract

Cu(In,Ga)(S,Se)2 (CIGS) chalcopyrites and their derivatives are potential candidates for use in photoelectrochemical (PEC) H2 generation because of their well-matched solar absorption properties and agreeable band positions relative to the water reduction redox potential. In this work, we present the photoelectrochemical characterization and H2 evolution performance of the highly tunable anion CIGS derivative series CuIn(S1–xSex)2. The photocathodes were subjected to Mott–Schottky analysis and chopped-light irradiation to determine their flat band potential. With excellent agreement between the two techniques, the flat band potentials were determined to increase successively with increasing x in CuIn(S1–xSex)2, with all band positions possessing appropriate energetics for the thermodynamic requirement of reducing H2O to H2. The photocathodes were then subjected to identical bulk H2 evolution conditions for 10–12 h, maintaining ∼90% faradaic efficiency. The rates of photoelectrocatalysis were found to vary significantly within the CuIn(S1–xSex)2 series, ranging over 2 orders of magnitude using a constant light intensity of 100 mW/cm2. Carrier transport behavior beyond the space charge region where charge diffusion dominates is suggested as a limiting factor for the H2 evolution rate of these p-type materials through the indirect influence of the majority carrier concentration.