Manipulate energy transport via fluorinated spacers towards record-efficiency 2D Dion-Jacobson CsPbI₃ solar cells

Two-dimensional (2D) Dion-Jacobson (D-J)-type cesium lead iodide CsPbI₃ perform remarkably in terms of stability. However, the complex interactions between spacer and inorganic layers limit its excellent progress in perovskite solar cells (PSCs). Herein, starting from the considerable structural diversity of organic spacers, we engineer 2D CsPbI₃ with fine-tuning functionalities. Specifically, for the first time we embedded fluorinated aromatic cations in 2D D-J CsPbI₃, and successfully applied it into construction of high-performance PSCs. Compared with constitutive 1,4-diaminobenzene (PDA), the fluorinated 2-fluorobenzene-1,4-diamine (F-PDA) component greatly expands the dipole moment from 0.59D to 3.47D, which reduces the exciton binding energy of the system. A theoretical study shows that the spacer layer and inorganic plane are more enriched with charge accumulation in (F-PDA)Cs_n-1Pb_nI_3n+1. The results show that (F-PDA)Cs_n-1Pb_nI_3n+1 demonstrates more significant charge transfer between organic and inorganic layers than (PDA)Cs_n-1Pb_nI_3n+1, and it is confirmed in the femtosecond transient absorption experiment. Moreover, the interactions of the fluorinated spacer with the [PbI₆]^4- plane effectively manipulate the crystallization quality, and thus the ion migration and defect formation of target 2D CsPbI₃ are inhibited. As a result, we obtained a record power conversion efficiency (PCE) beyond 15% for 2D D-J (F-PDA)Cs₃Pb₄I₁₃ (n = 4) PSCs with significantly improved environmental stability compared with the three-dimensional (3D) counterparts.