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

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journal contribution
posted on 2018-06-03, 00:00 authored by Alexander D. DeAngelis, Kimberly Horsley, Nicolas Gaillard
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.