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.

Interface engineering is a simple and effective strategy for improving the photovoltaic performance and stability of perovskite solar cells (PSCs). Herein, an interface co-modification strategy is proposed, using [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 2-fluoro-1,4-phenylenediammonium iodide (2FPPD) to modify the electron transport layer (ETL)/perovskite (PVK) and the PVK/hole transport layer (HTL) interfaces, respectively. A series of characterizations demonstrate that the PCBM&2FPPD interface co-modification strategy effectively enhances the extraction and transport efficiency of carriers at the interface, passivates surface defects, inhibits the nonradiative recombination of carriers, and simultaneously inhibits ion migration. Moreover, this strategy improves the crystallinity and surface hydrophobicity of PVK and optimizes the energy level alignment of PSCs. As a result, all photovoltaic parameters are improved after optimization, where the power conversion efficiency (PCE) of PSCs has increased from 17.01% to 18.36%. Meanwhile, the optimized PSCs show excellent environmental stability, which can be stably stored in air (RH = 10-20%) for about 800 h.

Inorganic 2D layered CsPbI₃ is awaiting to overcome the phase instability of traditional 3D components. However, the most reported Ruddlesden–Popper (RP) phase 2D CsPbI₃ leads to larger interlayer distance and weaker interlayer coupling since the existence of the van der Waals gap, which deteriorates the performance of the device and makes the improvement of stability unsatisfactory. Herein, this work resorts ethylenediamine cations (EDA²⁺) to construct a series of Dion–Jacobson (DJ) phase 2D CsPbI₃ as (EDA)Cs_n−1Pb_nI_3n+1 with van der Waals gap eliminated. Combining simulation calculations and experiments, it is found that the (EDA)Cs_n−1Pb_nI_3n+1 has enhanced intermolecular forces to overcome the problem of insufficient crystallization power caused by large steric hindrance in the film assembly process compared to phenethylammonium-based RP phase analogues. In addition, profit from the reduced interlayer distance and stronger coupling, the rigidity of the structure is increased, and the annoying non-radiative recombination caused by structural fluctuations is alleviated. As a result, the 2D layered DJ phase CsPbI₃-based solar cells deliver eminent performance than RP phase analogues, especially the 2D (EDA)(Cs)₄Pb₅I₁₆ (n = 5) device exhibits a record PCE of 10.43% in this work, and significantly enhanced stability.

Two-dimensional (2D) Ruddlesden-Popper (RP) CsPbI3 exhibits enhanced phase stability compared with 3D CsPbI3. However, the issue of the uncontrollable crystallization process limits its photovoltaic performance. Here, the influence of a binary mixed solvent on the film quality and photovoltaic properties of (PEA)2Cs4Pb5I16 (n = 5) is studied in detail. It is demonstrated that the crystallization rate and crystal growth can be controlled by adjusting the amount of dimethyl sulfoxide (DMSO). Optimizing the solvent composition with adding 10% DMSO in pure dimethyl formamide (DMF) leads to perfect coverage, larger flaky 2D grains, reduced grain boundaries, and a better vertical orientation to the substrate due to the formation of a more stable intermediate phase. This can form good interface contact, which is beneficial to charge transport/extraction between TiO2 (electron transport layer, ETL) and perovskite, finally resulting in improved device performance. The enhancement of the power conversion efficiency of the optimized device based on DMF/DMSO (9:1) is 3.57% compared with the reference device based on pure DMF. This work illustrates the role of crystallization kinetics in the RP CsPbI3 film and offers a simple and effective method for high-performance 2D CsPbI3 solar cells.