Influence of [Cu]/[Cu+Sn] molar ratios in p-type Cu–Sn–S photoelectrodes on their photoelectrochemical performances in water and salt–water solutions

Ternary p-type copper–tin–sulfide photoelectrodes are prepared on various substrates by using the sulfurization of co-sputtered Cu–Sn metal precursors. The influence of the [Cu]/[Cu+Sn] molar ratios in samples on structural and photoelectrochemical performances of samples in various electrolytes is examined. X-ray diffraction patterns of samples after the sulfurization process show that the samples change from the cubic Cu₂SnS₃ phase, the cubic Cu₂SnS₃/orthorhombic Cu₄SnS₄ mixing phases to the orthorhombic Cu₄SnS₄ phase with an increase in the [Cu]/[Cu+Sn] molar ratio in metal precursors. The phase transition from the cubic Cu₂SnS₃ to the orthorhombic Cu₄SnS₄ phase is observed at the [Cu]/[Cu+Sn] molar ratio in the range of 0.72–0.73 in metal alloys. Direct energy band gaps of the samples are in the range of 0.96–1.22 eV. All samples are the p-type semiconductors with the carrier concentration and mobility in the ranges of 8.79 × 10¹⁸–3.57 × 10²⁰ cm⁻³ and 2.22–0.27 cm²/V s, respectively, depending on the [Cu]/[Cu+Sn] molar ratio in samples. The maximum photoelectrochemical performances of the samples in 0.5 M K₂SO₄ and 1 M NaCl aqueous solutions are 0.55 and 0.34 mA/cm² at an external potential of −1.2 V versus an Ag/AgCl reference electrode under light illumination with the light intensity of 100 mW/cm², respectively.