Efficient and Stable Mesoporous CsSnI3 Perovskite Solar Cells via Imidazolium‐Based Ionic Liquid Additive

Inorganic tin halide perovskite compound with its eco‐friendly property has attracted tremendous attention of researchers in the field of lead‐free perovskite solar cells. However, the trap‐assisted nonradiative recombination caused by deep‐level defects originating from surface undercoordinated Sn 2+ cations significantly deteriorates the CsSnI 3 device's performance. Herein, adding low concentrations of an ionic liquid 1‐ethyl‐3‐methylimidazolium acetate (EMIMAc) shows promise in controlling deep‐level defects in CsSnI 3 perovskites. Both experimental observation and theoretical simulation reveal that EMIMAc can have strong electrostatic attraction and coordination interaction with the surface undercoordinated Sn 2+ through the lone electron pairs of carboxyl functional groups and the donated π electrons from electron‐rich imidazole moieties, leading to a reduced deep‐level defect density and a restrained nonradiative recombination. Consequently, the processed CsSnI 3 perovskite solar cells based on a printable fluorine‐doped tin oxide/compact‐TiO 2 /mesoporous‐TiO 2 /Al 2 O 3 /NiO/carbon framework achieve a power conversion efficiency as high as 8.54%, which is the champion efficiency among all the reported CsSnI 3 mesoporous perovskite solar cells up to now. In addition, the unencapsulated devices have shown an impressive long‐term stability with only ≈6% efficiency degradation after over 2000 h aging under nitrogen atmosphere.