Vertical phase segregation suppression for efficient FA-based quasi-2D perovskite solar cells via HCl additive

Ruddlesden–Popper (RP) layered perovskites (with general formula of (RNH₃)₂A_n−1M_nX_3n+1) have drawn tremendous attention due to their excellent ambient stability while retaining promising device performance. However, quasi-2D perovskites with mixed-cation often suffer from severe phase segregation and vertically aligned multiple perovskite phases (with small n phases concentrated on the bottom and large n phases on the top) are usually formed. Here, we report a strategy to suppress vertical phase segregation by using hydrochloric acid (HCl) additive. As a result, homogenous quasi-2D perovskite films with uniform phase distribution and enhanced film quality are successfully obtained via this additive strategy, which is confirmed by PL measurements. With uniform distribution of different n value phases and high-quality film, FA-based quasi-2D perovskite solar cells with an efficiency of 13.16% and excellent stability was fabricated. This strategy paves a way to develop stable and efficient quasi-2D perovskite solar cells by suppressing phase segregation.