Thermally evaporated SnS:Cu thin films for solar cells

Cu-doped tin sulphide (SnS) films with a thickness of about 300 nm have been grown on glass substrates by thermal evaporation technique. Different Cu-doped SnS films were obtained by controlling the Cu evaporation time to roughly alter Cu-doping concentration in SnS films (from 5.7 to 23 atom ). Then they were annealed at a temperature of 250 C and a pressure of 5.0 10^-3 Pa for 90 min. The structural, optical and electrical properties of the films were characterised by X-ray diffraction, atomic force microscopy, ultraviolet visible near infrared spectrometer and Hall-effect measurement system. All the films are polycrystalline SnS with orthorhombic structure, and the crystallites in the films are all exclusively oriented along (111) direction. Annealing can optimise the crystallinity of all the films. With the increase of Cu-doping concentration, the grain size of the films becomes larger and larger, but the roughness decreases. Meanwhile, the evaluated direct bandgap E_g of the SnS:Cu films initially decreases, reaches a minimum value of 1.38 eV with 15 atom Cu and then increases thereafter. The carrier concentration of the films increases sharply, while the resistivity of the films decreases straightly. All the films are of p-type conductivity. Using the optimised conditions, it is possible to prepare SnS:Cu thin films suitable for absorbers of thin film solar cells.