Highly efficient perovskite solar cells by building 2D/3D perovskite heterojuction in situ for interfacial passivation and energy level adjustment

Passivating the interfacial defects and reducing the interfacial non-radiative recombination losses are the keys to improving the photovoltaic performance of three-dimensional (3D) perovskite solar cells (PVSCs). Stacking two dimensional (2D) perovskites on 3D perovskite is a promising method for interfacial treatment that improves the stability and efficiency of PVSCs. Herein, we developed conjugated fluorinated benzimidazolium cation (FBIm+) which can be inserted between 3D perovskite and hole-transporting layer (HTL) to form 2D perovskite in situ. The 2D single crystal structures of (FBIm)2PbI4 and (FBIm)2PbBr4 were achieved and confirmed by single-crystal X-ray diffraction (XRD), while few single crystals of 2D perovskite based on imidazolium or benzimidazolium anchors have been reported. The 2D perovskite can passivate the interfacial defects, induce better crystallinity and orientation, conduct lower trap density and extend carrier lifetime. Furthermore, the energy level arrangement can be regulated by changing the counterion from iodide to bromide, which can efficiently improve the hole extraction and device performances. As a consequence, the best efficiency of 23.00% for FBImBr-incorporated devices was achieved, while only 20.72% for the control device. Meanwhile, the PVSCs modified by FBImBr displayed excellent environmental stability due to the constructed hydrophobic 2D perovskite layer which can effectively block moisture permeation. This work develops a new path to design novel conjugated organic passivants to form 2D/3D perovskite structures.