Influence of Fullerene Multiadducts on the Morphology and Charge Photogeneration of Their Photovoltaic Blends with Poly(3-hexylthiophene)

A series of benzene octyl ether adducted fullerene derivatives (PCBOEs) of higher LUMO levels with reference to PCBM were synthesized and were blended with P3HT to assemble polymer solar cells (PSCs). Upon increasing the adduct order, the PCBOE-based PSCs exhibited higher open-circuit voltage (<i>V</i>_OC) but increasingly lower short-circuit current density (<i>J</i>_SC) with respect to the P3HT:PCBM device. Morphological analyses of the P3HT:fullerene blends reveal different extents of fullerene aggregation, which are shown to impact intimately on the charge photogeneration dynamics. An increase of adduct order is found to accelerate geminate charge recombination and to induce nanosecond nongeminate charge recombination that is absent in the cases of monoadducted PCBOE and PCBM. The severer charge recombination found for higher order fullerene adducts is ascribed to the deterioration of electron mobility owing to the discontinuity of the fullerene phase. The results suggest that, upon increasing the adduct order, the electron and hole transport in photoactive layers turn from ambipolarly balanced to space-charge limited states, a crucial issue for the application of multiadducted fullerenes as the electron accepting materials of PCSs