Cyclic Structured Ionic Molecules Formulating Multi‐Dimensional Perovskite Structures of Highly Stable and Efficient Perovskite Solar Cells

Abstract Perovskite solar cells (PSCs) achieve power conversion efficiency (PCE) above 26% but remain reliant on inert‐atmosphere processing and toxic antisolvents, hampering scale‐up. Here, ambient‐condition (RH <50%) fabrication of PSCs is reported using an ethyl acetate antisolvent modified with ring‐size‐tunable N‐heterocyclic ammonium ionic liquids (ILs): 1‐(2‐ethoxyethyl)‐1‐methylpyrrolidinium (PYR + ) and 1‐(2‐ethoxyethyl)‐1‐methylpiperidinium (PIP + ). Comparative analysis reveals that the PYR + cation with the smaller ring size not only fosters an ultrathin interfacial low‐dimensional perovskite layer, enhances (100) α‐phase orientation, and suppresses trap states, but also induces beneficial in‐plane compressive lattice strain, collectively yielding high‐quality films with superior crystalline order. In situ GIWAXS under one‐sun illumination and electrical bias demonstrates that IL treatment prevents α→δ phase transitions and PbI 2 formation, directly correlating with stable photocurrent output. PSCs incorporating PYR + achieve a champion PCE of 24.7% and retain over 90% of their initial efficiency after 1000 h of damp‐heat (85 °C/85% RH) stress. These findings establish ring‐size‐tunable N‐heterocyclic ammonium ionic liquids as scalable, multifunctional additives for ambient‐processed, high‐performance, and durable PSCs.