Unveiling Roles of Tin Fluoride Additives in High-Efficiency Low-Bandgap Mixed Tin-Lead Perovskite Solar Cells

Low-bandgap mixed tin-lead perovskite solar cells (PSCs) have been attracting increasing interest due to their appropriate bandgaps and promising application to build efficient all-perovskite tandem cells, an effective way to break the Shockley-Queisser limit of single-junction cells. Tin fluoride (SnF2) has been widely used as a basis along with various strategies to improve the optoelectronic properties of low-bandgap Sn-Pb perovskites and efficient cells. However, fully understanding the roles of SnF2 in both films and devices is still lacking and fundamentally desired. Here, the functions of SnF2 in both low-bandgap (FASnI(3))(0.6)(MAPbI(3))(0.4) perovskite films and efficient devices are unveiled. SnF2 regulates the growth mode of low-bandgap Sn-Pb perovskite films, leading to highly oriented topological growth and improved crystallinity. Meanwhile, SnF2 prevents the oxidation of Sn2+ to Sn4+ and reduces Sn vacancies, leading to reduced background hole density and defects, and improved carrier lifetime, thus largely decreasing nonradiative recombination. Additionally, the F- ion preferentially accumulates at hole transport layer/perovskite interface with high SnF2 content, leading to more defects. This work provides in-depth insights into the roles of SnF2 additives in low-bandgap Sn-Pb films and devices, assisting in further investigations into multiple additives and approaches to obtain efficient low-bandgap PSCs.