Microenvironment Created by SnSe₂ Vapor and Pre-Selenization to Stabilize the Surface and Back Contact in Kesterite...

The ambient air-processed preparation of kesterite Cu₂ ZnSn(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.