Effect of charge selective contacts on the quasi Fermi level splitting of CuGa3Se5 thin film photocathodes for hydrogen evolution and methylviologen reduction

Abstract

The copper chalcopyrite CuGaSe₂ and the defect-related phase CuGa₃Se₅ are promising photocathode materials for solar hydrogen generation. Existing devices exhibit photocurrents nearing 68% (CuGa₃Se₅)–86% (CuGaSe₂) of their theoretical limit but they are plagued by photovoltage losses that reduce their energy conversion efficiency. To evaluate the reasons, we determine the light intensity dependent quasi-Fermi level splitting (QFLS) in p-CuGa₃Se₅/liquid junctions for the first time, using the vibrating Kelvin probe surface photovoltage technique. The QFLS, or internal photovoltage, corresponds to the maximum electrochemical work a photoelectrode can perform under a given illumination condition. In the presence of water or methylviologen (2+/+) as electron acceptors, the QFLS of Mo/p-CuGa₃Se₅/liquid junctions reaches 0.22 to 0.29 V, respectively, under 49 mW cm⁻² 400 nm illumination, while the QFLS of a FTO/p-CuGa₃Se₅ photoelectrode is only 0.15–0.16 V. The lower voltage of the latter is attributed to a Schottky junction at the back contact, which limits majority charge carrier (hole) extraction from the semiconductor. Photovoltage losses also result from Fermi level pinning of the minority carriers at surface states 0.5 eV above the CGSe valence band. This problem can be overcome by chemical bath deposition of a CdS overlayer, which functions as a selective contact for electron extraction from CuGa₃Se₅ and which raises the QFLS to 0.44 V at 49 mW cm⁻². No significant QFLS enhancement occurs upon adsorption of Cd²⁺ ions to the CuGa₃Se₅ electrode surface, suggesting that Cd²⁺ adsorption alone does not remove the Fermi level pinning effect. Overall, these results provide a better understanding of the effect of surface treatments and charge selective contacts on the photovoltage of CuGa₃Se₅ photoelectrodes and indicate pathways to improve its solar fuel conversion efficiency.