Insights to the production of SnS-cubic thin films by vacuum thermal evaporation for photovoltaics

Thin films of SnS-CUB with a lattice constant of 11.6 Å, 32 units of SnS per cell and an optical bandgap (E _g) of 1.7 eV (direct), are mostly produced by chemical techniques. This cubic polymorph is distinct from its orthorhombic polymorph (SnS-ORT) with an E _g of 1.1 eV. This work is on the deposition of SnS-CUB thin films of 100–300 nm in thickness by thermal evaporation at substrate temperatures of 400 °C–475 °C on glass or on a chemically deposited SnS-CUB thin film (100 nm). Under a slow deposition rate (3 nm min⁻¹) from a SnS powder source at 900 °C, the thin film formed on a SnS-CUB film or glass substrate at 450 °C is SnS-CUB. At a substrate temperatures of 200 °C–350 °C, the thin film is of SnS-ORT. A low atomic flux and a higher substrate temperature favor the growth of SnS-CUB thin film. The E _g of the SnS-CUB film is nearly 1.7 eV (direct gap), and that of the SnS-CUB film is 1.2 eV (indirect gap). The electrical conductivity (σ) of SnS-CUB and SnS-ORT films are 10^–7 and 0.01 Ω^–1 cm⁻¹, respectively. A proof-of-concept solar cell of the SnS-CUB thin film showed an open circuit voltage of 0.478 V, compared with 0.283 V for the SnS-ORT solar cell. The insights to the deposition of SnS-CUB and SnS_0.45Se_0.55-CUB (E _g, 1.57 eV; σ, 0.02 Ω⁻¹ cm⁻¹) thin films by vacuum thermal evaporation offer new outlook for their applications.