Low-temperature synergistic effect of MA and Cl towards high-quality a-FAPbI3 films for humid-air-processed perovskite solar cells

Due to the hydrophilicity and black-phase instability of FA perovskites, ambient humidity is an unavoidable issue in the processing of perovskite solar cells (PSCs). MACl is among the most popular additives for improving perovskite films, but our experiments confirm that the direct addition of MACl into the precursor solution deteriorates the stability of the final alpha-FAPbI(3) films in humid air, which is attributed to the unwanted pinholes induced by MACl volatilization. To solve this problem, a novel confined-space annealing strategy (CSA) is intentionally developed to control the amount of MACl at a low level. Through retarding the volatilization of MACl and blocking moisture ingress, dense and delta-phase-free FAPbI(3) films with excellent crystallinity and stability are achieved at 100 degrees C under high humidity (RH: 60 +/- 10%). We further compare the same amounts of MAI and FACl additives with MACl, discovering that only when MA and Cl work together can pure alpha-FAPbI(3) films be obtained; therefore, a mechanism of MA-assisted nucleation and Cl-induced diffusion recrystallization is inferred. As a result, the PSCs employing optimal films yield a champion power conversion efficiency (PCE) of 17.27% and retain over 90% of the initial PCE after exposure to high humidity for 480 h. Our results offer deep insights into the thermodynamic and kinetic behaviors of MA and Cl in film growth and are beneficial for air-processed FA-based PSCs for commercial application.

Automated summary

In this study, a novel confined-space annealing strategy was developed to address the hydrophilicity and black-phase instability issues of FA perovskites. Through this strategy, stable and crystalline FAPbI(3) films were successfully fabricated under high humidity. The study also revealed that the combination of MA and Cl is essential for obtaining pure α-FAPbI(3) films, offering valuable insights for commercial applications of FA-based PSCs.