Di(4-methylphenyl)methano-C(60) Bis-Adduct for Efficient and Stable Organic Photovoltaics with Enhanced Open-Circuit Voltage

Abstract

A new class of fullerene bis-adducts di(4-methyl-phenyl)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 degrees C over 20 h.