Di(4-methylphenyl)methano-C60 Bis-Adduct for Efficient and Stable Organic Photovoltaics with Enhanced Open-Circuit Voltage

A new class of fullerene bis-adducts—di(4-methylphenyl)methano-C 60 bis-adduct (DMPCBA), di(4-fluorophenyl)methano-C 60 bis-adduct (DFPCBA), and diphenylmethano-C 60 bis-adduct ( DPCBA )—were rationally designed and easily synthesized. Compared to the lowest unoccupied molecular orbital (LUMO) energy level of PC 61 BM (−3.95 eV), the double functionalization effectively raises the LUMO energy levels of these fullerene materials to ca. −3.85 eV, regardless of the substituent groups (CH 3 –, F–, and H−) at the para -position of the phenyl rings. This phenomenon suggests that the plane of the phenyl groups is preferentially parallel to the fullerene surface, leading to poor orbital interactions with C 60 and negligible electronic effect. Importantly, such geometry sterically protects and shields the core C 60 structure from severe intermolecular aggregation, rendering it intrinsically soluble, morphologically amorphous, and thermally stable. The device based on the P3HT: DMPCBA blend exhibited an open-circuit voltage ( V oc ) of 0.87 V, a short-circuit current density ( J sc ) of 9.05 mA/cm 2, and a fill factor (FF) of 65.5%, leading to a high power conversion efficiency (PCE) of 5.2%, which is superior to that of the P3HT: PC 61 BM -based device. Most significantly, the amorphous nature of DMPCBA effectively suppresses the thermal-driven aggregation and thus stabilizes the morphology of the P3HT: DMPCBA blend. Consequently, the device retained 80% of its original PCE value against thermal heating at 160 °C over 20 h.