Performance Enhancement of Mixed‐Halide all‐Inorganic Perovskite Solar Cells via Ionic Liquid Additive

All‐inorganic perovskites hold great promise for applications in tandem and indoor photovoltaics. However, their device performance is constrained by poor film crystallinity and high density of bulk and interfacial defects. Herein, an ionic liquid 1‐ethyl‐3‐methylimidazolium trifluoroacetate (EMIMTFA) is introduced into the CsPbI 1.5 Br 1.5 perovskite precursor. It modulates the colloidal dispersion, retards crystallization, and improves film quality. The EMIM + passivates Pb 2+ ‐related defects, while the TFA − anion plays multiple roles at the buried interface. It forms Pb–O coordination bonds that raise the formation energies of both iodine and lead vacancies, suppressing the generation of interfacial defects. Through its carbonyl and fluorine groups, TFA − anchors on the buried TiO 2 surface, passivates oxygen vacancy defects, and constructs an interfacial dipole layer. EMIMTFA suppresses carrier recombination and promotes the separation and transport of photogenerated charges. As a result, the power conversion efficiency (PCE) of carbon‐based, hole‐transport‐layer‐free CsPbI 1.5 Br 1.5 devices increases from 11.22% to 13.42%. Encapsulated devices maintain 89% of their initial PCE after 100 h of continuous maximum power point tracking under illumination. EMIMTFA is also effective in CsPbI 2 Br and CsPbIBr 2 systems, where devices with the same architecture achieve PCEs of 14.90% and 10.72%, respectively. This work provides experimental support for all‐inorganic perovskite solar cells.