Tuning 2D Perovskite Passivation: Impact of Electronic and Steric Effects on the Performance of 3D/2D Perovskite Solar Cells

Abstract Surface passivation with 2D perovskites is a powerful strategy to achieve improved stability and performance in perovskite solar cells (PSCs). Various large organic cations have been successfully implemented, led by phenylethylammonium ( PEA ⁺ ) and its derivatives. However, systematic studies on large sets of cations to understand the effect of substituent position on 2D perovskite passivation and device performance are lacking. Herein, a collection of halogenated PEA ⁺ iodide salts ( x ‐XPEAI where x : ortho ( o ), meta ( m ) , para ( p ), X : F, Cl, Br) are synthesized by a facile method and deposited on top of 3D perovskite. The 2D perovskite layer formation is confirmed by X‐ray diffraction (XRD) and grazing‐incidence wide‐angle X‐ray scattering analyses for all cations, regardless of the nature and position of the halogen. Density functional theory analysis reveals that lower formation energies and higher interfacial dipoles achieved by m ‐substituted cations are responsible for enhanced performance compared to their o ‐ and p ‐ counterparts. While the m ‐BrPEAI ‐treated device shows a champion efficiency of 23.42%, ( V _OC =1.13 V, FF=81.2%), considering average efficiencies, stability, and reproducibility, the treatment with m ‐ClPEAI salt yields the best overall performance. This comprehensive study provides guidelines for understanding the influence of large cation modification on performance and stability of 3D/2D PSCs.