Effect of Fullerene Tris-adducts on the Photovoltaic Performance of P3HT:Fullerene Ternary Blends

Fullerene tris-adducts have the potential of achieving high open-circuit voltages (V-OC) in bulk heterojunction (BHJ) polymer solar cells (PSCs), because their lowest unoccupied molecular orbital (LUMO) level is higher than those of fullerene mono- and bis-adducts. However, no successful examples of the use of fullerene tris-adducts as electron acceptors have been reported. Herein, we developed a ternary-blend approach for the use of fullerene tris-adducts to fully exploit the merit of their high LUMO level. The compound o-xylenyl C-60 tris-adduct (OXCTA) was used as a ternary acceptor in the model system of poly(3-hexylthiophene) (P3HT) as the electron donor and the two soluble fullerene acceptors of OXCTA and fullerene monoadduct (o-xylenyl C-60 monoadduct (OXCMA), phenyl C-61-butyric acid methyl ester (PCBM), or indene-C-60 monoadduct (ICMA)). To explore the effect of OXCTA in ternary-blend PSC devices, the photovoltaic behavior of the device was investigated in terms of the weight fraction of OXCTA (W-OXCTA). When W-OXCTA is small (<0.3), OXCTA can generate a synergistic bridging effect between P3HT and the fullerene monoadduct, leading to simultaneous enhancement in both V-OC and short-circuit current (J(SC)). For example, the ternary PSC devices of P3HT:(OXCMA:OXCTA) with W-OXCTA of 0.1 and 0.3 exhibited power-conversion efficiencies (PCEs) of 3.91% and 3.96%, respectively, which were significantly higher than the 3.61% provided by the P3HT:OXCMA device. Interestingly, for W-OXCTA > 0.7, both V-OC and PCE of the ternary-blend PSCs exhibited nonlinear compositional dependence on W-OXCTA. We noted that the nonlinear compositional trend of P3HT:(OXCMA:OXCTA) was significantly different from that of P3HT:(OXCMA:o-xylenyl C-60 bis-adduct (OXCBA)) ternary-blend PSC devices. The fundamental reasons for the differences between the photovoltaic trends of the two different ternary-blend systems were investigated systemically by comparing their optical, electrical, and morphological properties.