Wide Band Gap CuGa(S,Se)₂ Thin Films on Transparent Conductive Fluorinated Tin Oxide Substrates as Photocathode Candidates for Tandem Water Splitting Devices

The purpose of this work was to explore the potential of CuGa­(S,Se)₂ thin films as wide-E_G top cell absorbers for photoelectrochemical (PEC) water splitting. A synthesis was developed on fluorinated tin oxide (FTO) photocathodes by converting copper-rich co-evaporated CuGaSe₂ into CuGa­(S,Se)₂ via a post-deposition annealing. We found it necessary to first anneal CuGaSe₂ at low-temperature in sulfur then at high-temperature in nitrogen to preserve the transparency and conductivity of the FTO. Using this two-step synthesis, we fabricated a 1.72 eV CuGa­(S,Se)₂ photocathode with a saturation current density and photocurrent onset potential of 10 mA/cm² and −0.20 V versus reversible hydrogen electrode, respectively. However we found that the PEC performance and sub-E_G transmittance, worsened with increasing copper content. Using flatband potential measurements and the Gerischer model, we show that divergences in PEC performance of CuGa­(S,Se)₂ photocathodes can be explained by differences in conduction band minimums and Fermi levels. We also explain that sub-E_G transmittance is likely hampered by a defect band 100–400 meV below E_C. Additional external quantum efficiency measurements of a high-efficiency 1.1 eV Cu­(In,Ga)­Se₂ photovoltaic driver, while shaded by the CuGa­(S,Se)₂ photocathode, yielded a short-circuit current density of 4.14 mA/cm² revealing that CuGa­(S,Se)₂ shows promise as a top cell for PEC water splitting.