Hetero Bis-Addition of Spiro-Acetalized or Cyclohexanone Ring to 58π Fullerene Impacts Solubility and Mobility Balance in Polymer Solar Cells

Fullerene bis-adducts are increasingly being studied to gain a high open circuit voltage (V_oc) in bulk heterojunction organic photovoltaics (OPVs). We designed and synthesized homo and hetero bis-adduct [60]­fullerenes by combining fused cyclohexanone or a five-membered spiro-acetalized unit (SAF₅) with 1,2-dihydromethano (CH₂), indene, or [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). These new eight 56π fullerenes showed a rational rise of the lowest unoccupied molecular orbital (LUMO). We perform a systematic study on the electrochemical property, solubility, morphology, and space-charge-limited current (SCLC) mobility. The best power conversion efficiency (PCE) of 4.43% (average, 4.36%) with the V_oc of 0.80 V was obtained for poly­(3-hexylthiophene) (P3HT) blended with SAF₅/indene hetero bis-adduct, which is a marked advancement in PCE compared to the 0.9% of SAF₅ monoadduct. More importantly, we elucidate an important role of mobility balance between hole and electron that correlates with the device PCEs. Besides, an empirical equation to extrapolate the solubilities of hetero bis-adducts is proposed on the basis of those of counter monoadducts. Our work offers a guide to mitigate barriers for exploring a large number of hetero bis-adduct fullerenes for efficient OPVs.