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.

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.

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.