Strategic improvement of Cu₂MnSnS₄ films by two distinct post-annealing processes for constructing thin film solar cells

The synthesis approach using a non-toxic solution-based method accompanied with special post-annealing processes demonstrates the great potential to realize industrial manufacture of earth-abundant materials for sustainable photovoltaics. Exploration of an appropriate post-annealing process is significant to gain insight into the crystallization of solar materials. The earth-abundant Cu₂MnSnS₄ (CMTS) thin films were fabricated by post-sulfuring as-prepared precursors and the results have been compared with that of directed-annealed samples. It was found that the sulfurization procedure not only can support to enhance the crystallinity but also help to obtain a nearly stoichiometric CMTS thin film which also free of SnS secondary phase. This reaction also contributes to sustainable incorporation of S into CMTS to a considerable extent and formation of notable large grains; whereas, it leads to the formation of a typical bi-layer microstructure containing large grains (500-800 nm) on the top and fine grains at the bottom. Through the sulfurization approach, the device performance is improved with the power conversion efficiency shows a 170% increase which mainly via the boost of open circuit and fill factor compared with that of direct-annealed processed. These results offers a novel research direction for preparing pure-sulfide CMTS thin film for the potential application in low-cost solar cells.