The Key Role of Subtle Substitution for a High-Performance Ester-Modified Oligothiophene-Based Polymer Used in Photovoltaic Cells

In organic solar cells (OSCs), developing high-performing easily synthesized photoactive materials is essential for pursuing cost- effective balance. Herein, we have designed and synthesized a pair of wide-band-gap polymers (PBDE4T-0F and PBDE4T-2F), using the low synthesis cost dicarboxylic ester-substituted quaterthiophene as the building block. Despite the minor change of molecular structure for polymer PBDE4T-xF, the fluorine substituent in polymer PBDE4T-2F greatly enhances its interchain aggregation. The higher aggregation tendency of ester-modified polymer in solution is beneficial for reducing both the aggregate size and π-π stacking distance of blend film, which contribute to the highly efficient exciton dissociation and symmetric charge transport. An impressive power-conversion efficiency (PCE) of 16.1% is achieved for the PBDE4T-2F:BTP-eC9-based device, while its counterpart only delivers a PCE of 5.8% with distinctly lower short-circuit current density (J_sc) and fill factor. Notably, the aggregation effect of donor polymer has also been found to be associated with the energy level shifts, and thus the variation of charge transfer energy and voltage losses for blend system. The results suggest that simultaneously reduced voltage loss and increased J_sc can be expected by further finely tuning the aggregation behavior of the ester-modified oligothiophene-based donor polymer.