In Situ Synthesized 2D Covalent Organic Framework Nanosheets Induce Growth of High‐Quality Perovskite Film for Efficient and Stable Solar Cells

Abstract Poor crystallinity of perovskite and extensive defects around grain boundaries are the acknowledged hindrances to obtaining high efficiency and long‐term stability for organic metal halide perovskite solar cells (PSCs). Here, a 2D covalent organic framework (2D COF) nanosheets, [(TPA)₁(TPhT)₁]_CN, is first in situ synthesized in a PbI₂ layer with a highly crystalline structure to precisely regulate the crystallization process of perovskite in the sequential deposition method. The existence of 2D COF nanosheets can decelerate intermolecular interdiffusion and induce perovskite crystals to grow along (110) planes with enlarged grain size. Meanwhile, 2D COF nanosheets distributed around the grain boundaries reduce the defect density and promote carriers transporting in the perovskite film. The superior properties of the perovskite film afford the champion PSC device with a power conversion efficiency of 22.04%, which is over 10% higher than the control device. Moreover, the target PSC also demonstrates outstanding long‐term stability. It can maintain over 90% of its initial value after 90 days storage in ambient conditions for unencapsulated devices. This work paves a new path for regulating the crystallization process of perovskites via 2D crystalline materials.