Anti-dimerization 56π-electron fullerene adduct bearing bulky functional groups for inverted perovskite solar cells with enhanced interfacial stability

Solution-processible fullerene derivatives have been extensively used as electron transport layers (ETLs) of inverted perovskite solar cells (PSCs); however, the commonly used 58π-electron [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) often tends to dimerize, especially under continuous illumination, severely compromising long-term stability. Herein, we develop a 1,4-unsymmetrical addition strategy and synthesize two novel 56π-electron fullerene derivatives bearing multiple bulky functional groups such as tert-butyl, indole and azaindole, designated as C₆₀-TFB and C₆₀-TFP, which not only play the role of anti-dimerization but also leverage a combined passivating effect of various heteroatom-containing functional groups. These groups establish robust interfacial bonding, enhancing interfacial stability by inhibiting the migration of iodide ions (I⁻) and silver (Ag). Consequently, C₆₀-TFB and C₆₀-TFP exhibit excellent optoelectronic properties, enabling favorable energy level alignment with perovskites. PSC devices based on C₆₀-TFB and C₆₀-TFP ETLs achieve a significantly enhanced power conversion efficiency (PCE) of 25.55% and 25.93%, respectively, relative to a 24.08% PCE for the PCBM-based control devices. After over 1000 h of continuous illumination at 55°C, the optimized C₆₀-TFP–based devices demonstrate excellent stability, retaining 81.9% of their initial efficiency, whereas only 62.9% retention is achieved for the PCBM-based control device, indicating a dramatic enhancement of operational stability. © The Author(s) 2025. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.