Fullerene Derivative Layer as a Charge Transfer Bridge for Efficient and Stable Perovskite Solar Cells

Abstract To address the suboptimal charge transfer and recombination losses at perovskite/ Phenyl‐C61‐butyricacidmethylester (PCBM) interface in inverted perovskite solar cells (PSCs), a fullerene derivative interlayer is introduced, which can passivate the defects at the perovskite surface via interactions with the phosphonic acid group, while the fullerene part of the molecule interacts with PCBM and ensures efficient charge transfer. The use of 4‐(1′,5′‐Dihydro‐1′‐methyl‐2′H‐[5,6]fullereno‐C60‐Ih‐[1,9‐c]pyrrol‐2′‐yl)phenylphosphonic acid (CPPA) interlayer results in significant shortening of charge carrier lifetime indicating improved charge extraction, which leads to as significant enhancement of power conversion efficiency (PCE) for both CsFA (from 22.8% to 24.6%) and CsFAMA (from 22.1% to 25.1%) PSCs. The observed improvement can be attributed to the synergistic effects of the phosphonic acid and fullerene in CPPA molecule, as benzylphosphonic acid interfacial layers yields significantly smaller changes in charge carrier lifetime and device performance. The CPPA interlayer also results in enhanced stability, with CPPA‐containing devices retaining 90% of the initial PCE after 2000 h in the dark in ambient without encapsulation, while encapsulated devices retain 89% of the initial value after 1000 h of MPP testing under 1 Sun illumination, as well as exhibit stable performance outdoors under ambient sunlight for 112 days.