Vacuum annealed tin sulfide (SnS) thin films for solar cell applications

Thin films of tin sulfide (SnS) were grown on a glass substrate at an optimum temperature of 300 degrees C by thermal evaporation technique. Following the deposition, films were vacuum annealed at different temperatures in the range of 100 to 300 degrees C for 2 h. The effect of annealing temperature (T-a) on the composition, surface morphology, microstructure, optical and electrical properties was investigated. Elemental analysis showed sulfur deficiency of annealed films and the Sn to S atomic percent ratio increased from 1.0 to 1.1. XRD analysis confirmed the orthorhombic crystal structure of the films with (111) preferred orientation and phase purity. Degree of preferred orientation decreased with increase in T-a and the diffraction peaks corresponding to other planes intensified. Increasing the T-a to 300 degrees C led to an increase in crystallite size to 129 nm. Results indicated presence of several crystallites in the grains of as-deposited films. AFM studies revealed the fragmentation of larger grain and the average grain size reduced form 265 nm for as-deposited films to 132.8 nm for the film annealed at 300 degrees C. An apparent shift in absorption edge towards longer wavelengths is observed for films annealed at T-a > 200 degrees C. The optical constant such as optical band gap, extinction coefficient (k), absorption coefficient (alpha) and refractive index (n) have been evaluated. The optical band gap of SnS thin films varied marginally with the annealing temperature and remained in between 1.33-1.29 eV. The extinction coefficient of the film annealed at 300 degrees C was enhanced and is found to be 0.85 at 700 nm. At the annealing temperature of 300 degrees C, the SnS films had enhanced electrical properties: the electrical resistivity was 7.8 Omega cm, the p-type carrier concentration was up to 2.17 x 10(16) cm(-3), and the mobility was about 36.9 cm(2)V(-1)s(-1). The variation of physical parameters with T-a has been explained by taking into account the crystallite size and the presented values are discussed with relevance to solar cells.