Physical properties of chalcogenide Sn-Bi-S graded thin films annealed in argon

The development of cost-effective and nontoxic thin film materials is vital for fabrication of solar cells. We are presenting a combinatorial synthesis approach (CSA) for the deposition of chalcogenide Sn-BiS graded thin films by thermal evaporation. Post-deposition thermal annealing in the temperature range of 200-500 degrees C in an argon atmosphere has been carried out for the Sn-BiS thin films. The effect of annealing treatment and initial composition on the structural properties of the Sn-Bi-S graded thin films was studied by using energy-dispersive X-ray spectroscopy, X-ray diffraction (XRD), and Raman spectroscopy. XRD measurements showed that the thin films were grown in polycrystalline structure. Different microstructural parameters such as crystallite size, dislocation density, and microstrain were estimated after post-deposition thermal treatment and found annealing temperature dependent. From the transmission spectra the estimated optical band gap energy values were found in the range 1.27-1.43 eV for the (Sn/Bi) molar ratio of 2.18-0.67 in a typical sample annealed at 400 degrees C. Photoconductivity response was determined for incident light of wavelength 300-1100 nm and was observed to be annealing temperature and Sn/Bi molar ratio dependent. Photoconductivity was also noted to depend upon the Sn/Bi molar ratio with Sn-rich samples giving the strongest response. Sn-rich compositions also showed p-type conductivity over the temperature range of 350-400 degrees C. These findings show that the CSA has potential for the screening of high-quality Sn-Bi-S thin films.