Ionic Liquid Top Surface Engineering: In Situ GIWAXS Insights Into 1D/3D Heterojunction Perovskite Formation

Abstract Low‐dimensional perovskites have been demonstrated repeatedly to improve the performance of perovskite photovoltaic devices in both light‐to‐electricity conversion efficiency and device durability. In this work, the ionic liquid (IL) 1‐ethyl‐3‐methylimidazolium hydrogen sulfate (EMIMHSO 4 ) is innovatively introduced as a capping layer, which interacts with the residual PbI 2 on the 3D perovskite top surface to generate the 1D perovskite, EMIMPbI 3 . By adjusting the concentration of the IL, 1D perovskite formations with distinct morphologies is achieved. Time‐resolved GIWAXS studies allows to track the formation process of 1D perovskite on the 3D perovskite top surface. Moreover, GIWAXS at varying angles provide insights into the IL's role in the management of PbI 2 distribution. Further analyses, including in situ PL and ICP‐MS, demonstrates that the 1D perovskite layer effectively enhances the stability of the final 3D perovskite. This stabilization, combines with defect passivation of the 1D perovskite and its effective management of residual PbI 2 , enables the 1D/3D device to achieve a PCE of 25.01%, a marked improvement over the pristine 3D device. Additionally, the 1D/3D device displays superior long‐term durability, retaining 94% of its initial PCE after 2000 h in a nitrogen atmosphere, while the 3D device maintains only 66% of its initial PCE.