Effect of ambient gas on grain growth of CZTS layer: study on device efficiency

Higher grain size is one of the important requirements for any absorber material to achieve a high efficiency solar cell. Among various other parameters, thermal conductivity of the ambient gas used during annealing is also expected to change the grain size of the thin film material. In this work, a similar to 1 micron thick CZTS layer was prepared on Mo coated soda lime glass substrate using co-sputtering of precursor followed by sulfurization (annealing in sulphur vapour and Argon/Nitrogen gas). Annealing environment was changed by varying the gas (N-2/Ar) during the sulfurization process to study the effect of thermal conductivity of gas on CZTS layer and cell performance. Cells prepared using nitrogen gas sulfurization is found to be more efficient as compared to argon gas sulfurization. This has been discussed as a function of higher thermal conductivity of sulfurizing gas which led to the larger grain size. Reduction in grain boundaries due to larger grain size help in more charge collection and hence improving device properties.