Aromatic Imidazole Diammonium‐based 2D Dion–Jacobson Perovskites with Reduced Exciton Binding Energy

2D Dion–Jacobson (2D DJ) perovskites are considered as promising photovoltaic materials due to their structural stability and spacer designability. Here, a spacer cation with an aromatic imidazole ring, 2‐(1H‐imidazol‐2‐yl)ethylammonium (HE), is successfully applied to construct 2D DJ perovskite. It's found that the high polarity of the HE spacer strengthens the interaction between organic and inorganic layers and reduces the exciton binding energy to 67.8 meV, resulting in promoted charge dissociation, compared with the aliphatic 1,4‐butanediammonium (BDA) spacer with a similar length. The HE spacer enlarges the micelle size in precursor solution and suppresses the formation of low‐n value phases. In consequence, the HE‐based perovskite film exhibits better quality than the BDA‐based one, with lower defect density and longer carrier lifetime. The optimized device based on (HE)(MA 0.75 FA 0.25 ) 4 Pb 5 I 16 film achieves a champion power conversion efficiency up to 18.40%, much higher than that of the BDA‐based device (15.03%). Besides, the unencapsulated device based HE exhibits improved moisture and thermal stability.