Halide exchange in the passivation of perovskite solar cells with functionalized ionic liquids

Applying additives to perovskite precursor solutions is an effective way to passivate perovskite films for the construction of perovskite solar cells (PSCs) with increased power-conversion efficiencies (PCEs) and enhanced stabilities. Ionic liquids (ILs) with halide anions have been applied as additives. Here, we report the application of imidazolium-based ILs, containing the nitrile functionality in the cation and chloride anion, as additives for triple-cation (Cs+, formamidinium [FA+], methylammonium [MA+]) PSCs. We found that a Cl-I exchange takes place between the I− anion of the triple-cation precursors and the Cl− anion of the IL. The fabricated PSCs containing perovskite films with the ILs show high PCEs (in the best case 22.86%) and high stabilities, with the unencapsulated devices retaining about 95% of their original efficiencies after 1,000 h. The mechanistic basis for the passivation effect of the IL additives in the perovskite films is described.