Modification of the optoelectronic properties of sprayed In2S3 thin films by indium diffusion for application as buffer layer in CZTS based solar cell

Highlights: ► Resistivity of spray pyrolysed In2S3 reduced by five orders through indium diffusion. ► Crystallinity of sprayed In2S3 enhanced considerably by indium diffusion. ► Diffusion of metallic indium results in a buffer layer with conductivity gradient. ► CZTS/In2S3 solar cell was fabricated with a conversion efficiency of 1.85%. ► Possible band diagram for this type of CZTS/In2S3 heterojunction was proposed. -- Abstract: In this report the authors discuss the effect of diffusion of metallic indium on the optoelectronic properties of chemical spray deposited indium sulfide (In2S3) thin films which are prospective candidates for buffer layer application in thin film solar cells. Thin layers of metallic indium having different thicknesses were diffused into the films by evaporating different quantities of indium using vacuum evaporation technique followed by annealing. 'In' diffusion was done with an aim to reduce resistivity and improve the crystallinity to ensure better carrier collection. X-ray diffraction, X-ray photoelectron spectroscopy, optical absorption, photoluminescence and electrical studies were performed on the films. Analysis indicated that crystallinity attained a maximum for an optimum 'In' diffusion and then showed a retracing nature. Resistivity was found to decrease drastically from 2.3× 105 Ω cm [pristine] to 4.7 Ω cm [for the optimum indium diffused samples]. Using the optimized In2S3 layer and copper zinc tin sulfide (CZTS) deposited using chemical spray pyrolysis, a heterojunction device was successfully fabricated with a conversion efficiency of 1.85% and fill factor of 52%. The optimum quantity of indium to be diffused depends on the thickness of the In2S3 thin film