Bicomponent Random Approach for the Synthesis of Donor Polymers for Efficient All-Polymer Solar Cells Processed from A Green Solvent

All-polymer solar cells (all-PSCs) can offer unique merits of high morphological stability to thermal and mechanical stress. To realize their full potential as flexible or wearable devices, it is highly desirable that the all-PSCs can be fabricated from a green solvent with simple post-treatment to avoid thermal annealing on the flexible substrate. This posed a severe challenge on material design to tune their properties with suitable solubility, aggregation, and morphology. To address this challenge, here, a simple bicomponent random approach on a D–A-type polymer donor was developed by just varying the D–A molar ratio. Under this approach, a series of new random polymers PBDTa–TPDb with different molar ratios of the D component of 2D-benzo[1,2-b:4,5-b′]dithiophene (BDT) and A component of thieno[3,4-c]pyrrole-4,6-dione (TPD) were designed and synthesized. The energy levels, light absorption, solubility, and packing structure of random donors PBDTa–TPDb were found to vary substantially with the various D–A molar ratios. The devices based on PBDTa–TPDb/P(NDI2HD-T) were fabricated to explore the synergistic effects of the processing solvent and composition of D–A-type random polymers. The results show that nanoscale morphology, balanced miscibility/crystallinity of blend, and photovoltaic properties could be rationally optimized by tuning the composition of random donors. As a result, as-cast all-PSC-based optimal donor PBDT5–TPD4 achieves the best power conversion efficiency (PCE) of 8.20% processed from a green solvent, which performs better than that the reference polymer (PCE: 6.41%). This efficiency is the highest value for all-PSCs from BDT–TPD-based donors. Moreover, the optimized devices were relatively insensitive to the thickness of the active layer and exhibited good stability.