Compact bis-adduct fullerenes and additive-assisted morphological optimization for efficient organic photovoltaics

Bis-adduct fullerenes surrounded by two insulating addends sterically attenuate intermolecular interaction and cause inferior electron transportation. In this research, we have designed and synthesized a new class of bisadduct fullerene materials, methylphenylmethano-C₆₀ bis-adduct (MPC₆₀BA), methylthienylmethano-C₆₀ bis-adduct (MTC₆₀BA), methylphenylmethano-C₇₀ bis-adduct (MPC₇₀BA), and methylthienylmethano-C₇₀ bis-adduct (MTC₇₀BA), functionalized with two compact phenylmethylmethano and thienylmethylmethano addends via cyclopropyl linkages. These materials with much higher-lying lowest unoccupied molecular orbital (LUMO) energy levels successfully enhanced the V_oc values of the P3HT-based solar cell devices. The compact phenylmethylmethano and thienylmethylmethano addends to promote fullerene intermolecular interactions result in aggregation-induced phase separation as observed by the atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of the poly(3-hexylthiophene-2,5-diyl) (P3HT)/bis-adduct fullerene thin films. The device based on the P3HT/MTC₆₀BA blend yielded a V_oc of 0.72 V, a J_sc of 5.87 mA/cm², and a fill factor (FF) of 65.3%, resulting in a power conversion efficiency (PCE) of 2.76%. The unfavorable morphologies can be optimized by introducing a solvent additive to fine-tune the intermolecular interactions. 1-Chloronaphthalene (CN) having better ability to dissolve the bis-adduct fullerenes can homogeneously disperse the fullerene materials into the P3HT matrix. Consequently, the aggregated fullerene domains can be alleviated to reach a favorable morphology. With the assistance of CN additive, the P3HT/MTC₆₀BA-based device exhibited enhanced characteristics (a V_oc of 0.78 V, a J_sc of 9.04 mA/cm², and an FF of 69.8%), yielding a much higher PCE of 4.92%. More importantly, the additive-assisted morphological optimization is consistently effective to all four compact bis-adduct fullerenes regardless of the methylphenylmethano or methylthienylmethano scaffolds as well as C₆₀ or C₇₀ core structures. Through the extrinsic additive treatment, these bis-adduct fullerene materials with compact architectures show promise for high-performance polymer solar cells. (Chemical Equation Presented).