Microenvironment Created by SnSe₂ Vapor and Pre‐Selenization to Stabilize the Surface and Back Contact in Kesterite Solar Cells

Abstract The ambient air‐processed preparation of kesterite Cu₂ZnSn(S,Se)₄ (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)₂ 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 SnSe₂ 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)₂ layers near the back contact can be significantly reduced with increasing SnSe₂ 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 SnSe₂ 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.