Designing Multifunctional Donor-Acceptor-Type Molecules to Passivate Surface Defects Efficiently and Enhance Charge Transfer of CsPbI2Br Perovskite for High Power Conversion Efficiency.

High-density and multitype surface defects of CsPbI2Br perovskite induce charge recombination and accumulation, hindering its device efficiency and stability. However, the surface defect types of CsPbI2Br perovskite are still unclear, and conventional organic molecules only passivate one specific defect and cannot achieve good overall passivation. Here, density functional theory is used to explore surface defect types and properties of CsPbI2Br with calculating the defect formation energy and electronic structure. Results show that the dominant deep-level defects are cationic defects (PbBr) under Br-poor conditions and anionic defects (Ii and Bri) under moderate and Br-rich conditions, originating from Pb-Pb bonding and I-I bonding. Multifunctional organic molecules containing donor and acceptor groups are used to passivate both cationic and anionic defects simultaneously. It turns out that the deep-level defects are effectively decreased by forming strong interaction of N-Pb, O-Pb, and halide-Pb bonds. Moreover, the electron and hole transfers from perovskite to molecules increase dramatically to -9.06 × 1012 and 2.60 × 1012 e/cm2 and maybe improve the efficiency of power conversion. Our findings not only reveal the surface defect properties of CsPbI2Br, but also offer an approach for designing new multifunctional passivators for perovskite solar cells with high conversion efficiency.