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Abstract 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 (SnF 2 ) has been widely used as a basis along with various strategies to improve the optoelectronic properties of low‐bandgap SnPb perovskites and efficient cells. However, fully understanding the roles of SnF 2 in both films and devices is still lacking and fundamentally desired. Here, the functions of SnF 2 in both low‐bandgap (FASnI 3 ) 0.6 (MAPbI 3 ) 0.4 perovskite films and efficient devices are unveiled. SnF 2 regulates the growth mode of low‐bandgap SnPb perovskite films, leading to highly oriented topological growth and improved crystallinity. Meanwhile, SnF 2 prevents the oxidation of Sn 2+ to Sn 4+ 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 SnF 2 content, leading to more defects. This work provides in‐depth insights into the roles of SnF 2 additives in low‐bandgap SnPb films and devices, assisting in further investigations into multiple additives and approaches to obtain efficient low‐bandgap PSCs.
Chen et al. (Wed,) studied this question.