Restricting the liquid–liquid phase separation of PTB7-Th:PF12TBT:PC71BM by enhanced PTB7-Th solution aggregation to optimize the interpenetrating network

The current understanding of the active layer morphology in ternary organic solar cells (OSCs) is superficial owing to more variables and complexity compared to that of binary OSCs. The PTB7-Th:PF12TBT:PC71BM ternary system with complementary polymer absorption spectra and efficient energy transfer from PF12TBT to PTB7-Th was anticipated to have an outstanding performance. However, only a limited improvement in the power conversion efficiency (PCE) was achieved when the ternary devices were processed from CB/DIO. This is because large PC71BM domains formed with the addition of amorphous PF12TBT and the PF12TBT molecules embedded in the large PC71BM domains served as trap sites. Constraining the formation of large PC71BM domains and avoiding PF12TBT molecules being fully embedded in PC71BM domains are needed for a better performance. Polymer pre-aggregation before liquid–liquid phase separation is beneficial to construct bicontinuous interpenetrating networks with proper phase-separated domains. Therefore, para-xylene was introduced into CB/DIO to cause a weak polymer–solvent interaction for enhanced PTB7-Th solution-phase aggregation. The enhanced PTB7-Th aggregation in the CB/PX/DIO solution restricted the extent of the liquid–liquid phase separation, and a well-developed polymer network formed with improved PTB7-Th crystallinity, which prevented large PC71BM domains from forming via fluid-phase Ostwald ripening in the liquid–solid phase separation stage. The appearance of PF12TBT emission and increased PTB7-Th emission in the ternary film processed from CB/PX/DIO suggested that fewer PF12TBT molecules were embedded in the PC71BM domains, and the energy transfer from PF12TBT to PTB7-Th was more efficient. Thus, PCE increased from 8.09% for binary blends processed from CB/DIO to 9.28% for ternary blends processed from CB/PX/DIO.