Formation of SnS phase obtained by thermal vacuum annealing of SnS2 thin films and its application in solar cells

Abstract

We demonstrate a simple approach to fabricate single-phase SnS thin films by thermal vacuum annealing of SnS2 layers obtained by the close-spaced vacuum sublimation method. It was found that the initial non-annealed SnS2 films exhibit typical chemical composition for SnS2 with ratio of Sn/S = 0.49. The structural quality of the SnS2 phase was studied by X-ray diffraction and Raman spectroscopy measurements. In particular, it was established that films have a hexagonal 2H-SnS2 crystal structure. The field emission scanning electron microscope analysis of the surface and cross-section shows plate-like crystallites with an average size of 2 µm. Annealing of the SnS2 samples was carried out at 300, 400, and 500 °C for 30, 60, and 90 min for each temperature, and at 600 °C for 30 min. It was shown that concentration of S gradually decreases with increasing annealing temperature and time. The samples annealed at 500 °C for 30, 60, and 90 min demonstrate a typical SnS composition ratio of Sn/S = 0.96. Further, by using X-ray diffraction, Raman, and energy dispersive X-ray analysis, we found that annealing of the samples at 500 °C for 30, 60, and 90 min provides a phase transition from hexagonal SnS2 to orthorhombic SnS. The shorter annealing time and temperature leads to the mixed SnS, Sn2S3, and SnS2 phase composition. The shape and size of plate-like crystallites remains the same after annealing. However, randomly distributed nano-pores were observed. Transmittance and reflectance measurements of the SnS2 and SnS films show both direct and indirect optical band gaps in the materials. For SnS films a large absorption coefficient of 104–105 cm−1 above the band gap was found. The current-voltage characteristic of the ITO/SnS2/Sn structure shows small rectification current, while the current-voltage curve of ITO/SnS/Sn is linear. The dark resistivity was found to be 1.01 × 105 Ω cm and 1.18 × 103 Ω cm for SnS2 and SnS films, respectively. The heterojunction structure was obtained by annealing of the SnS2 deposited on the ITO/CdS structure to obtain the SnS phase. The p-SnS/n-CdS heterojunction shows weak photovoltaic response under illumination at AM 1.5 conditions, namely, open circuit voltage (Voc), short circuit current density (Jsc), and fill factor (FF) of 0.35 V, 34.08 µA/cm2, and 0.42, respectively.