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

Abstract

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 SnS0.45Se0.55-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.