High-efficiency perovskite solar cells prepared by using a sandwich structure MAI-PbI₂-MAI precursor film

Two-step deposition has been widely used in the perovskite layer preparation for perovskite solar cells due to its attractive morphology controllability. However, the limited diffusivity of CH₃NH₃I (MAI) might cause some PbI₂ to remain in the perovskite film. The residual PbI₂ in the perovskite film would lead to inferior performance of devices, such as, low power conversion efficiency (PCE), poor reproducibility and weak air stability. In this work, we developed a sandwich structure MAI-PbI₂-MAI precursor film to prepare a PbI₂-free CH₃NH₃PbI₃ perovskite film. In comparison to the two-step approach, the MAI-PbI₂-MAI precursor film with a typical sandwich structure formed a uniform and pinhole-free perovskite film without any PbI₂ residue, which could significantly improve the performance of the devices. Moreover, the bottom MAI layer of the MAI-PbI₂-MAI precursor film could improve the interfacial contact of the porous TiO₂ layer, leading to the promotion of the charge transfer and reduction of the recombination rate. Therefore, the devices fabricated from the sandwich structure MAI-PbI₂-MAI precursor films showed dramatic improvements of open-circuit voltage (V_oc), short-circuit current density (J_sc), fill factor (FF) and PCE. As a result, a promising PCE of 17.8% with good long-term air stability was achieved for the MAI-PbI₂-MAI precursor film based PSC, which is better than that prepared by a two-step approach.