Enhanced solar cell performance of Cu₂ZnSn(S,Se)₄ thin films through structural control by using multi-metallic stacked nanolayers and fast ramping process for sulfo-selenization

In this paper, Cu₂ZnSn(S,Se)₄ (CZTSSe) thin-films were prepared by sulfo-selenization of metal precursors in H₂S environment instead of using metal selenides/sulfides as precursors. High quality CZTSSe thin films were obtained using multi-stacking metallic nanolayer precursors undergoing a fast ramping process. For the preparation of metallic stacked nanolayer precursors, we have developed a 9-layer sequential deposition of Sn/Zn/Cu metal stack onto Mo-coated soda lime glass substrate by RF-sputtering. Due to inevitable metal inter-diffusion during the sulfo-selenization, we further studied the effect of the Sn/Zn/Cu metal stacking number (therefore, the layer thickness) on the quality of thin film with respect to its device performance. In the device prepared with conventional 3-layer stack, due to insufficient inter-diffusion of precursors, excessive Cu-rich secondary phase was formed at the back contact region and resulted in poor performance of devices. By using the modified 9-layer stacked precursor and fast ramping heating process the device efficiency can be improved from 4.9% to 7.7% and open circuit voltage from 0.44 to 0.5 V. This improvement can be ascribed to a compact, smooth microstructure, presence of bronze formation and the suppression of Cu-rich bi-layer formation in the 9-layer approach.