lengths, low exciton binding energy, and ease of solution processability, which promise the perovskite-based photovoltaic devices both high efficiency and low manufacture cost. The intense effort to improve power conversion efficiencies (PCEs) has resulted in PCEs of perovskite solar cells (PSCs) increasing from 3.8% in 2009 [5] to over 23% in 9 years. [6,7] Changes in material composition (e.g., using mixed cations and halides together into a 3D lead−halide perovskites, such as [FA/MA]Pb[I/Br] 3 , [8] [FA/Cs]Pb[I/Br] 3 , [9,10] or [Cs/FA/MA]Pb[I/Br] 3[11] ) have played essential role to improve the performances and structural stability. However, the intrinsic instability of the 3D lead−halide perovskites with regard to moisture, heat, light, and oxygen remains to be entirely circumvented. [12,13] Two main measures have been taken to increase the stability of PSCs: providing sufficient protection to the perovskite material [14] and increasing the intrinsic stability of the perovskite material. [15] More importantly, techniques that can improve device stability without sacrificing the efficiency are critical for the future of PSCs. In this regard, strategies called multidimensional perovskite (MDP) using Ruddlesden-Popper type perovskite (Class I) or low dimensional polymorphs passivated 3D perovskites (Class II) were brought into being increasing the intrinsic stability of the perovskite material. [16] Ruddlesden-Popper phase layered perovskites ((RNH 3 ) 2 (MA) n−1 Pb n X 3n+1 , n = 1, 2, 3, 4, 5, …) [17][18][19][20][21] which was introduced by Smith et al. showed albeit low PCE the superior ambient stability, [18] where RNH 3 are large aliphatic or aromatic ammonium cations represented by n-butylammonium (BA) and 2-phenylethylammonium (PEA). (BA) 2 (MA) 2 Pb 3 I 10 as a light absorber retained its performance after exposure to a highhumidity environment for 60 d. [20] So far the highest reported PCE of Ruddlesden-Popper type perovskite-based solar cell is 15.42%. [22] Very recently, Zhang et al. utilized synchrotron source based in situ measurement to disclose the phase transition kinetics during the crystallization of Ruddlesden-Popper type perovskite. This study provided insight into the relationship between phase purity, quantum well orientation, and photovoltaic performance. [23] On the other hand, the approaches that involve low dimensional polymorphs passivated 3D perovskites, or 3D-2D perovskite stacked structures attracted extensive interest in recent years. This approach features not only the Supported by the density functional theory (DFT) calculations, for the first time, a fluorinated aromatic cation, 2-(4-fluorophenyl)ethyl ammonium iodide (FPEAI), is introduced to grow in situ a low dimensional perovskite layer atop 3D perovskite film with excess PbI 2 . The resulted (p-FC 6 H 4 C 2 H 4 NH 3 ) 2 [PbI 4 ] perovskite functions as a protective capping layer to protect the 3D perovskite from moisture. In the meantime, the thin layer facilitates charge transfer at the interfaces, thereby reducing the...
