Unraveling the Impact of a Cyclized Phenylethylamine‐Derived Spacer Cation on the Structural, Electrical, and Photovoltaic Performance of Quasi‐2D Ruddlesden‐Popper Perovskites

Abstract The discovery of new ligand molecules is crucial for advancing the performance and stability of 2D perovskites in optoelectronic devices. In this study, dihydroindole (IDN) cation, a novel organic spacer derived from the cyclization of phenylethylamine (PEA), is employed to fabricate stable and efficient quasi‐2D Ruddlesden‐Popper (RP) perovskite solar cells (PSCs). The IDN‐based perovskite, (IDN) 2 PbI 4 , exhibits an average Pb─I─Pb bond angle exceeding 170°, with minimal distortion in the inorganic layer. Furthermore, the IDN molecules possess a larger dipole moment, reducing exciton binding energy to 79.86 meV. The IDN‐based perovskite films demonstrate exceptional quality, with significantly enlarged grain sizes. This is attributed to the interaction between IDN molecules and [PbI 6 ] 4− octahedra, which enhances crystallinity, decreases trap density, extends carrier diffusion length, and increases carrier lifetime. The optimized device achieves an efficiency of 17.60%, markedly surpassing that of PEA‐based devices (11.46%). Unencapsulated IDN‐based quasi‐2D RP PSCs exhibit superior thermal and humidity stability, making them promising for practical applications. These findings offer an effective strategy for the development of novel spacer cations, paving the way for high‐performance 2D RP PSCs.