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

Published on July 5th, 2018. The purpose of this work was to explore the potential of CuGa(S,Se)2 thin films as wide-EG 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-EG 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-EG transmittance is likely hampered by a defect band 100–400 meV below EC. 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.