Microenvironment Created by SnSe Vapor and Pre-Selenization to Stabilize the Surface and Back Contact in Kesterite Solar Cells.

The ambient air-processed preparation of kesterite Cu ZnSn(S,Se) (CZTSSe) thin film is highly promising for the fabrication of low-cost and eco-friendly solar cells. However, the Sn volatilization loss and formation of a thick Mo(S,Se) interfacial layer during the traditional selenization process pose challenges for fabricating high-efficiency CZTSSe solar cells. Here, CZTS precursors prepared by a sol-gel process in ambient air are selenized and assisted with SnSe vapor via one- and two-step selenization to prepare a CZTSSe absorber on a Mo film and, subsequently, solar cells. For one-step selenization, the thickness of the fine grain and Mo(S,Se) layers near the back contact can be significantly reduced with increasing SnSe vapor partial pressure in the mixed selenization atmosphere, while the device efficiency is only 7.97% due to the severe interface recombination. For two-step selenization, the desired morphology and stoichiometry of the absorber can be achieved through the assistance of Sn-poor precursors selenized with high SnSe vapor partial pressure to regulate the Sn content in CZTSSe, yielding the highest efficiency of 10.85%. This study improves the understanding of the key role of the microenvironment during film growth towards the production of high-efficiency thin film solar cells and other photoelectronic devices.