Organometal Halide Perovskite Solar Cells with Improved Thermal Stability via Grain Boundary Passivation Using a Molecular Additive

Organometal halide perovskite solar cells (PeSCs) are regarded as promising photovoltaics due to their outstanding power conversion efficiencies (PCEs). However, even though their PCEs are achieved over 20%, their intrinsically poor stability is a big bottleneck for their practical uses. Here, a simple method is reported using phenyl‐C61‐butyric acid methyl ester as a molecular additive to improve thermal stability of organometal halide perovskite crystals, which also improves the PCEs of the associated PeSCs. Moreover, by varying the grain size of perovskite crystals up to ≈150 µm, it is demonstrated that grain boundary plays a significant role in their thermal stability. Cells with smaller grain interface area (i.e., larger grain size) have higher thermal stability. The additive is located at grain boundaries and found to induce electron transfer reactions with halogens in the perovskite. The reaction products chemically passivate perovskite crystals and strongly bind halogen atoms at grain boundaries to their crystal lattice, preventing them from exiting from the crystal lattice, which improves thermal stability of perovskite crystals. This study offers a simple method for improving thermal stability of perovskite without any loss and opens up the possibility of the use of various molecular additives to achieve highly stable PeSCs. Chemical passivation of organometal halide perovskite crystals with phenyl‐C61‐butyric acid methyl ester (PCBM) is investigated. PCBM located at perovskite grain boundaries induces electron transport reactions with halogens in perovskite and chemically passivates the grain boundaries. It prevents halogens at the grain boundaries from exiting the crystal lattice and thereby results in improved thermal stability of organometal halide perovskite solar cells.