Tailoring Ionic Liquid Chemical Structure for Enhanced Interfacial Engineering in Two‐Step Perovskite Photovoltaics

Abstract Ionic liquids (ILs) have emerged as versatile tools for interfacial engineering in perovskite photovoltaics. Their multifaceted application targets defect mitigation at SnO 2 ‐perovskite interfaces, finely tuning energy level alignment, and enhancing charge transport, meanwhile suppressing non‐radiative recombination. However, the diverse chemical structures of ILs present challenges in selecting suitable candidates for effective interfacial modification. This study adopted a systematic approach, manipulating IL chemical structures. Three ILs with distinct anions are introduced to modify perovskite/SnO 2 interfaces to elevate the photovoltaic capabilities of perovskite devices. Specifically, ILs with different anions exhibited varied chemical interactions, leading to notable passivation effects, as confirmed by Density Functional Theory (DFT) calculation. A detailed analysis is also conducted on the relationship between the ILs' structure and regulation of energy level arrangement, work function, perovskite crystallization, interface stress, charge transfer, and device performance. By optimizing IL chemical structures and exploiting their multifunctional interface modification properties, the champion device achieved a PCE of 24.52% with attentional long‐term stability. The study establishes a holistic link between IL structures and device performance, thereby promoting wider application of ILs in perovskite‐based technologies.