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 & Aring;, 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 degrees C-475 degrees 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 degrees C, the thin film formed on a SnS-CUB film or glass substrate at 450 degrees C is SnS-CUB. At a substrate temperatures of 200 degrees C-350 degrees 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 (sigma) of SnS-CUB and SnS-ORT films are 10(-7) and 0.01 Omega(-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; sigma, 0.02 Omega(-1) cm(-1)) thin films by vacuum thermal evaporation offer new outlook for their applications.