Cross‐Linkable Fullerene Electron Transport Layer with Internal Encapsulation Capability for Efficient and Stable Inverted Perovskite Solar Cells

A stable and compact fullerene electron transport layer (ETL) is crucial for high‐performance inverted perovskite solar cells (PSCs). However, traditional fullerene‐based ETLs like C60 and PCBM are prone to aggregate under operational conditions, a challenge recently recognized by academic and industrial researchers. Here, we designed and synthesized a novel cross‐linkable fullerene molecule, bis((3‐methyloxetan‐3‐yl)methyl) malonate‐C60 monoadduct (BCM), for use as an ETL in PSCs. Upon a low‐temperature annealing at 100 °C, BCM undergoes in‐situ cross‐linking to form a robust cross‐linked BCM (CBCM) film, which demonstrates excellent electron mobility and a suitable band structure for efficient PSCs. Our results show that PSCs incorporating CBCM‐based ETL achieve an impressive efficiency of 25.89% (certified: 25.36%), significantly surpassing the 23.25% efficiency of PCBM‐based devices. The intramolecular covalent interactions within CBCM films effectively prevent aggregation and enhance film compactness, creating an internal encapsulation layer that mitigates the decomposition and ion migration of perovskite components. Consequently, CBCM‐based PSCs show exceptional stability, maintaining 97.8% of their initial efficiency after 1000 hours of maximum power point tracking, compared to only 78.6% retention in PCBM‐based devices after less than 820 hours.