Facile Synthesis of o-Xylenyl Fullerene Multiadducts for High Open Circuit Voltage and Efficient Polymer Solar Cells

The ability to control the lowest unoccupied molecular orbital (LUMO) level of an electron-accepting material is a critical parameter for producing highly efficient polymer solar cells (PSCs). Soluble bis-adducts of C-60 have great potential for improving the V-OC in PSCs because of their high LUMO level. In this work, we have developed a novel o-xylenyl. C-60 bis-adduct (OXCBA) via a [4 + 2] cycloaddition between C-60 and an irreversible diene intermediate from dibromo-o-xylene. OXCBA was successfully applied as the electron acceptor with poly(3-hexylthiophene) (P3HT) in a PSC, showing a high efficiency of 5.31% with V-OC of 0.83 V. This composite showed a nearly 50% enhancement in efficiency compared to the P3HT:PCBM control device (3.68% with V-OC of 0.59 V). Furthermore, tuning the molar ratio between C-60 and the a,alpha,alpha'-dibromo-o-xylene group from 1:1 to 1:3 in the reaction scheme enables facile control over the number of o-xylenyl solubilizing groups ultimately tethered to the fullerene, thus producing o-xylenyl C-60 mono-, bis-, and tris-adducts (OXCMA, OXCBA, and OXCTA) with different LUMO levels. As the number of solubilizing groups increased, V-OC values of the P3HT-based BHJ solar cells increased from 0.63 V (OXCMA) to 0.83 V (OXCBA) to 0.98 V (OXCTA). This series of o-xylenyl C-60 multiadducts provides a model system for investigating the molecular structure-device function relationship, especially with respect to changes in the number of solubilizing groups on the electron acceptor.