A study on the correlation between the chemical structures of organic conjugated materials and performances in organic photovoltaics

Abstract

Organic photovoltaics (OPVs) have been attracted as a promising alternative for clean and sustainable energy due to their solution-processable, low-cost, lightweight and flexible sources. Although many electron donors and acceptors as active materials for OPVs have intensively studied, additional improvement of PCE must be required to commercialize OPVs. Therefore, the fundamental study on the correlation between the chemical structures of active materials and PCE in OPVs is urgently needed. Herein, we developed a model system for a model system for investigating the molecular structure-device function relationship and may contribute to advancing the commercialization of OPVs. The ability to control the lowest unoccupied molecular orbital (LUMO) level of an electron-accepting material is a critical parameter for producing highly efficient OPVs. Soluble bis-adducts of $C_{60}$ have great potential for improving the $V_{OC}$ in OPVs due to their high LUMO level. 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 α,α``-dibromo-o-xylene. OXCBA was successfully applied as the electron acceptor with poly(3-hexylthiophene) (P3HT) in a OPV, 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 α,α``-dibromo-o-xylene 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). Also, structural control of s...