Fabrication of a CuInS2 photoelectrode using a single-step electrodeposition with controlled calcination atmosphere

Abstract

n-type CuInS₂ thin films were fabricated by a one-step electrodeposition of Cu/In/S precursors on a transparent fluorine-doped tin oxide (FTO) substrate followed by calcination in a mixture of N₂ and H₂ at 500 °C without the need of toxic H₂S gas for sulfurization. The analyses of structural and optical properties revealed that tetragonal chalcopyrite CuInS₂ thin films were obtained when heat treated in a mixture of 8% H₂ and 92% N₂; whereas calcination in pure N₂ resulted in the formation of CuInS₂ with a considerable amount of In₂O₃ and Cu_xS impurities. The synthesized n-CuInS₂ thin film has an optical bandgap of 1.3 eV with the conduction band at ca. −1.2 V vs. Ag/AgCl at pH 6.5. The heat treatment in a mixture of H₂ and N₂ provided a reducing atmosphere to suppress the oxide formation of the electrodeposited Cu/In/S elements. It also increased the donor density of CuInS₂ sixfold by increasing the density of the sulfur vacancies (known to be electron donors) at the grain surface. As a result of the improved purity and increased donor density, the hydrogen treated CuInS₂ thin film yielded a photocurrent density of ∼8 mA cm⁻² at 0.5 V vs. Ag/AgCl in 0.25 M Na₂S and 0.35 M Na₂SO₃ under visible light illumination, which is a fourfold enhancement as compared to that obtained from the CuInS₂ film calcined in pure N₂.