Uncovering chemical structure-dependency of ionic liquids as additives for efficient and durable perovskite photovoltaics

Ionic liquids (ILs) have garnered significant attention in the field of perovskite solar cells (PSCs) due to their versatile capabilities in manipulating the crystallization kinetics, optimizing the morphology, and effectively passivating defects, leading to high-efficiency and long-term stability. While previous studies have mostly focused on individual ILs or different anions and their application in PSCs, the distinctive distribution of cations and anions within ILs underscores the significance of optimizing cationic structures and elucidates the particular role of cations in ILs on device performance. In this work, the implications of juxtaposing ILs incorporating carboxyl-functionalized imidazolium cations against conventional imidazolium-based counterparts on the crystallization kinetics, perovskite film quality, and the photovoltaic performance of perovskite devices. This strategy involving carboxyl-functionalized cations in ILs with a higher affinity to coordinate Pb2+, supported by chemical spectroscopy testing and a quantitative measure of the bond strength via Crystal orbital Hamiltonian group (COHP) calculations. The modified ILs more prominently results in enhancing the degree of PbI2 conversion into perovskite, slowing down crystallization rates and inducting formation of higher-quality perovskite films. Benefitting from the strong chemical interaction provided carboxyl-functionalized cations and widespread distribution of carboxyl-functionalized cations throughout the perovskite films, accomplishing comprehensive synergistic passivation of defects in perovskite films. This led to the suppression of non-radiative recombination and enhanced carrier transport properties. The incorporation of carboxyl modified ILs with suitable chemical structures resulted in a remarkable enhancement of the champion power conversion efficiency (PCE) to 24.21% in the ILs-treated device, coupled with excellent long-term stability.