Two-dimensional (2D) Dion-Jacobson (D-J)-type cesium lead iodide CsPbI ₃ 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 CsPbI ₃ with fine-tuning functionalities. Specifically, for the first time we embedded fluorinated aromatic cations in 2D D-J CsPbI ₃, 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– 1}Pb _nI _{3 n+ 1}. The results show that (F-PDA)Cs _{n– 1}Pb _nI _{3 n+ 1} demonstrates more significant charge transfer between organic and inorganic layers than (PDA)Cs _{n– 1}Pb _nI _{3 n+ 1}, and it is confirmed in the femtosecond transient absorption experiment. Moreover, the interactions of the fluorinated spacer with the [PbI ₆] ^{4 –} plane effectively manipulate the crystallization quality, and thus the ion migration and defect formation of target 2D CsPbI ₃ are inhibited. As a result, we obtained a record power conversion efficiency (PCE) beyond 15% for 2D D-J (F-PDA)Cs ₃Pb ₄I ₁₃ (n = 4) PSCs with significantly improved environmental stability compared with the three-dimensional (3D) counterparts.

Iodine vacancies and uncoordinated iodide ions of CsPbI₃ films are mainly responsible for nonradiative recombination. Here, we report a composition-engineering passivation method that through guanidium (GA⁺) and I⁻ forms strong hydrogen bonds to passivate iodine vacancies and reduce defects. Both experimental and theoretical results confirmed strong chemical interactions between GA⁺ and uncoordinated I⁻ in the GA_xCs_1−xPbI₃ bulk or at the grain boundary. Moreover, GA⁺ doping could slow down the crystallization speed of perovskite films during the deposition process. As a result, we observed GA⁺ modified films with much lower defect density, larger grain size, and better carrier extraction and transportation. Upon GA⁺ passivation, the power conversion efficiency (PCE) is boosted from 18.01% to 19.05%, with open-circuit voltage (V_OC) enhancement from 1.08 V to 1.14 V.

Two-dimensional (2D) Ruddlesden–Popper (RP) CsPbI₃ perovskite possesses superior phase stability by introducing steric hindrance. However, due to the quantum and dielectric confinement effect, 2D structures usually exhibit large exciton binding energy, and the charge tunneling barrier across the organic interlayer is difficult to eliminate, resulting in poor charge transport and performance. Here, a multiple-ring aromatic ammonium, 1-naphthylamine (1-NA) spacer is developed for 2D RP CsPbI₃ perovskite solar cell (PSC). Theoretical simulations and experimental characterizations demonstrate that the 2D RP CsPbI₃ perovskite using 1-NA spacer with extended π-conjugation lengths reduces the exciton binding energy and facilitates the efficient separation of excitons. In addition, its cations have a significant contribution to the conduction band, which can reduce the bandgap, promote electronic coupling between organic and inorganic layers, and improve interlayer charge transport. Importantly, the strong π–π conjugation of 1-NA spacer can enhance intermolecular interactions and hydrogen bonding, and prepare high-quality films with preferred vertical orientation, resulting in lower defect density, and directional charge transport. As a result, the (1-NA)₂(Cs)₃Pb₄I₁₃ PSC exhibits a record 16.62% performance with enhanced stability. This work provides an efficient approach to improve charge transport and device performance by developing multiple-ring aromatic spacers.