Fine-Tuning of Molecular Self-Assembly Morphology via Synergistic Ternary Copolymerization and Side Chain Optimization of Low-Cost Polymer Donors Toward Efficient Organic Solar Cells.

Self-assembly morphology optimization of organic photovoltaic materials is crucial to improve the performance of organic solar cells (OSCs). Herein, three low-cost PTQ derivative donors, PTQ17, PTQ18, and PTQ19 are developed by synergistic ternary copolymerization and side chain optimization of utilizing different benzothiadiazole (BT) units, to fine-tune molecular self-assembly morphology. PTQ17, containing difluorinated BT, shows the tightest π-π packing and strongest molecular crystallinity, leading to excessive molecular aggregation and phase separation morphology in active layer. In contrast, PTQ19, containing dialkoxy-substituted BT, has the weakest molecular crystallinity, resulting in the worst long-range ordered molecular packing and the smallest phase domains in active layer. Remarkably, PTQ18, containing monofluorinated and monoalkoxy-substituted BT, has moderate molecular crystallinity and the best compatibility with acceptor, resulting in the most ideal microscopic morphology of active layer with desirable domain size and phase separation features. In result, the PTQ18-based binary OSC achieves an outstanding efficiency of 19.68%; and further optimized energy level alignment leads to an enhanced PCE of 20.06% in the PTQ18-based ternary device. This work demonstrates the importance of self-assembly morphology modification of organic photovoltaic molecules in improving performance of OSCs, and it has guiding role in design of high-performance organic photovoltaic materials.