Copper tin sulfide (CTS) absorber thin films obtained by co-evaporation: Influence of the ratio Cu/Sn

Highlights: • Copper tin sulfide (CTS) thin films were grown by co-evaporation at different Cu/Sn atomic ratios. • Smooth Cu₂SnS₃ layers with large grains are obtained at Cu/Sn ⩾ 1.5 and T ⩾ 350 °C. • At 450 °C, the cubic Cu₂SnS₃ phase changes to tetragonal phase. • Cu₂SnS₃ presents suitable optical and electrical properties for use as photovoltaic absorbers. - Abstract: Copper tin sulfide thin films have been grown on soda-lime glass substrates from the elemental constituents by co-evaporation. The synthesis was performed at substrate temperatures of 350 °C and 450 °C and different Cu/Sn ratios, adjusting the deposition time in order to obtain thicknesses above 1000 nm. The evolution of the morphological, structural, chemical, optical and electrical properties has been analyzed as a function of the substrate temperature and the Cu/Sn ratio. For the samples with Cu/Sn ⩽ 1, Cu₂Sn₃S₇ and Cu₂SnS₃ have been observed by XRD. Increasing the Cu/Sn to 1.5, the Cu₂SnS₃ phase was the majority, being the formation completed at Cu/Sn ratio around 2. The increment of the substrate temperature leads to a change of cubic structure to tetragonal of the Cu₂SnS₃ phase. The chemical treatment with KCN was effective to eliminate CuS excess detected in the samples with Cu/Sn > 2.2. The samples with Cu₂SnS₃ structure show a band gap energy increasing from 0.9 to 1.25 eV and an electrical resistivity decreasing from 7 ∗ 10⁻² Ω cm to 3 ∗ 10⁻³ Ω cm when the Cu/Sn atomic ratio increases from 1.5 to 2.2