Understanding the Effects of Fluorine Substitution in Lithium Salt on Photovoltaic Properties and Stability of Perovskite Solar Cells

Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) additive is widely employed to improve the hole mobility of the hole-transporting layer (HTL) in perovskite solar cells (PSCs). However, the hygroscopic nature of Li-TFSI is not beneficial to the long-term stability of PSCs. Herein, a new more water-resistant Li-PFSI is used to replace Li-TFSI. As a result, the best power conversion efficiency (PCE) of 22.14% is achieved for Li-PFSI-treated PSCs, exceeding that of the control cell with Li-TFSI (20.25%). Importantly, the Li-PFSI-based cell shows impressive environmental and thermal stability. Moreover, we first comparatively investigate the effect of the amount of fluorine substitution in lithium salt (2F for Li-FSI, 6F for Li-TFSI, and 10F for Li-PFSI) on the HTL's physical properties and their photovoltaic performance in PSCs. We found that more fluorine substitution can improve the HTL charge-carrier transfer and photovoltaic performance in PSCs. Our findings provide key missing information for designing new additives toward efficient and stable PSCs.