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-C₆₀ bis-adduct (DFPCBA), and diphenylmethano-C₆₀ bis-adduct (DPCBA)-were rationally designed and easily synthesized. Compared to the lowest unoccupied molecular orbital (LUMO) energy level of PC₆₁BM (-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 (CH₃-, 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 C₆₀ and negligible electronic effect. Importantly, such geometry sterically protects and shields the core C₆₀ 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/cm², 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:PC₆₁BM-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.