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2D/3D Perovskites: Introduction of Hydrophobic Ammonium Salts with Halogen Functional Groups for High-Efficiency and Stable 2D/3D Perovskite Solar Cells (Adv. Funct. In article number 1807565, Xu Pan, Songyuan Dai, and co-workers design novel type 2D/3D perovskites by firstly introducing hydrophobic ammonium salt cations with halogen functional groups into 3D perovskite. Owing to the notable hydrophobicity of alkylammonium cations with halogen functional groups and the formed 2D/3D perovskite structure, the highly efficienct 2D/3D perovskite solar cells exhibit superior moisture resistance and thermal stability. [Extracted from the article]

2D perovskites have attracted extensive attention due to their excellent stability compared with 3D perovskites. However, the intrinsic hydrophilicity of introduced alkylammonium salts effects the humidity stability of 2D/3D perovskites. Devices based on longer chain alkylammonium salts show improvement in hydrophobicity but lower efficiency due to the poorer charge transport among various layers. To solve this issue, two hydrophobic short‐chain alkylammonium salts with halogen functional groups (2‐chloroethylamine, CEA+ and 2‐bromoethylamine, BEA+) are introduced into (Cs0.1FA0.9)Pb(I0.9Br0.1)3 3D perovskites to form 2D/3D perovskite structure, which achieve high‐quality perovskite films with better crystallization and morphology. The optimal 2D/3D perovskite solar cells (PSCs) with 5% CEA+ display a power conversion efficiency (PCE) as high as 20.08% under 1 sun irradiation. Because of the notable hydrophobicity of alkylammonium cations with halogen functional groups and the formed 2D/3D perovskite structure, the optimal PSCs exhibit superior moisture resistance and retain 92% initial PCE after aging at 50 ± 5% relative humidity for 2400 h. This work opens up a new direction for the design of new‐type 2D/3D PSCs with improved performance by employing proper alkylammonium salts with different functional groups. [ABSTRACT FROM AUTHOR]