Investigating strategies for high-efficiency flexible kesterite solar cells from solution-based synthesis

High-efficiency thin film solar cells play a key role in the production of Building Integrated Photovoltaic (BIPV) or Product Integrated Photovoltaic (PIPV). Among the most interesting emerging photovoltaic (PV) materials, kesterite absorbers such as Cu2ZnSn(SxSe1-x)4 (CZTSSe) have attracted great attention in recent years. The ambient air-processed deposition of CZTSSe films is suitable for the production of low-cost and environmentally friendly flexible devices. In this work, we report on the optimization and functionalization of the back contact to design kesterite solar cells grown by molecular ink method on Mo foil, which, thanks to its high-temperature stability, high conductivity and high purity, is one of the most interesting flexible substrates. In addition, to improve the crystal morphology and promote the grain growth of the kesterite absorber, thus achieving good device efficiencies, intrinsic and extrinsic doping of CZTSSe were also investigated and the incorporation of Ag was proved to be an effective strategy to achieve high-quality flexible solar cells. The Li and Na introduction directly into the precursor solution was also studied to overcome the lack of alkali metal elements (typically needed to promote the kesterite crystallization) in the flexible substrate. Finally, thanks to these approaches, kesterite flexible solar cells with an efficiency of over 11% have been obtained, offering promising prospects for further progress in producing highly efficient flexible kesterite solar cells.