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.

Automated summary

The study demonstrates that using the more water-resistant Li-PFSI instead of Li-TFSI can significantly improve the efficiency and stability of PSCs. Furthermore, higher fluorine substitution can enhance charge-carrier transfer and photovoltaic performance in PSCs.