Understanding the Role of Fluorine Groups in Passivating Defects for Perovskite Solar Cells ​.

Introducing fluorine (F) groups into a passivator plays an important role in enhancing the defect passivation effect for the perovskite film, which is usually attributed to the direct interaction of F and defect states. However, the interaction between electronegative F and electron-rich passivation groups in the same molecule, which may influence the passivation effect, is ignored. We herein report that such interactions can vary the electron cloud distribution around the passivation groups and thus changing their coordination with defect sites. By comparing two fluorinated molecules, heptafluorobutylamine (HFBM) and heptafluorobutyric acid (HFBA), we find that the F/-NH2 interaction in HFBM is stronger than the F/-COOH one in HFBA, inducing weaker passivation ability of HFBM than HFBA. Accordingly, HFBA-based perovskite solar cells (PSCs) provide an efficiency of 24.70% with excellent long-term stability. Moreover, the efficiency of a large-area perovskite module (14.0 cm2) based on HFBA reaches 21.13%. Our work offers an insight into understanding an unaware role of the F group in impacting the passivation effect for the perovskite film. ​.