Dual function of formamidinium chloride additive improves the efficiency and stability of low-dimensional perovskite solar cells

Despite their excellent intrinsic stability, low-dimensional Ruddlesden-Popper (LDRP) perovskites face challenges with low power conversion efficiency (PCE), primarily due to the widen bandgap and limited charge transport caused by the bulky spacer cation. Herein, we introduce formamidinium chloride (FACl) as an additive into (4-FPEA)₂MA₄Pb₅I₁₆ perovskite. On the one hand, the addition of FACl narrows the bandgap through cation exchange between MA⁺ and FA⁺, thereby extending the light absorption range and enhancing photocurrent generation. On the other hand, this MA⁺/FA⁺ cation exchange decelerates the sublimation of methylammonium chloride and prolongs the crystallization of LDRP perovskite, leading to higher crystallinity and better film quality with a decreased trap-state density. Consequently, this approach led to a remarkable PCE of 20.46% for <n> = 5 LDRP perovskite solar cells (PSCs), ranking among the highest for MA/FA mixed low dimensional PSCs reported to date. Remarkably, our PSCs maintained 90% and 92% of the initial efficiency even after 1300 h at (60 ± 5)°C and (60 ± 5)% relative humidity, respectively. This work promotes the development of LDRP PSCs with excellent efficiency and environmental stability for potential commercial application.