Fine-Tuning Alkyl Chains on Quinoxaline Nonfullerene Acceptors Enables High-Efficiency Ternary Organic Solar Cells with Optimizing Molecular Stacking and Reducing Energy Loss

Material design of guest acceptor is always a big challenge for improving the efficiency of ternary organic solar cells (OSCs). Here, a pair of isomeric nonfullerene acceptors based on quinoxaline core, Qx-p-C7H8O and Qx-m-C7H8O, is designed and synthesized. By moving the alkoxy chain attached on side phenyl from meta-position to para-position, both & pi;-& pi; stacking distance and crystallinity are enhanced simultaneously. They obtain the uplifted lowest unoccupied molecular orbital level. Compared to Qx-m-C7H8O, Qx-p-C7H8O exhibits wider absorption spectrum and higher extinction coefficient. Using D18-Cl:N3 as host materials, the addition of guest acceptor Qx-p-C7H8O significantly improves the power conversion efficiency (PCE) from 17.61% to 18.49% because of higher open-circuit voltage (0.875 V) and short-circuit current density (27.85 mA cm(-2)). This can be attributed to the faster exciton dissociation, more balanced carrier mobility, fine fiber morphology, and lower energy loss in the ternary devices. However, Qx-m-C7H8O-based ternary device achieves relatively low PCE of 17.17% because this device shows extremely low electron mobility. The results indicate that molecular stacking, film morphology, etc., can be effectively modulated by fine-tuning the side chains of guest materials, which may be an effective design rule for further improving the PCE of OSCs.

Automated summary

In this study, a pair of isomeric nonfullerene acceptors based on quinoxaline core were designed and synthesized. The positioning of the alkoxy chain was adjusted to enhance the π-π stacking distance and crystallinity, resulting in improved performance of the organic solar cells. The results demonstrate that fine-tuning the side chains can effectively improve the efficiency of organic solar cells.