Selenium-containing two-dimensional conjugated fused-ring electron acceptors for enhanced crystal packing, charge transport, and photovoltaic performance

Although two-dimensional fused-ring electron acceptors (FREAs) have recently attracted increasing attention, designing highly efficient two-dimensional FREAs with a new heterocycle-based two-dimensional central core for further development is still a great challenge. Herein, two chlorinated two-dimensional fused-ring electron acceptors (namelyC8T-BDT4ClandC8T-BDSe4Cl) were synthesized for polymer solar cells, featuring with the two-dimensional conjugated central donor units by alkylthienyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) and benzo[1,2-b:4,5-b′]diselenophene (BDSe), respectively. Compared withC8T-BDT4Cl,C8T-BDSe4Clexhibits a red-shifted absorption spectrum and a narrower optical bandgap. Moreover, we first studied the single crystals of such two-dimensional conjugated FREAs and the crystallographic analysis indicates that both acceptors presented 3D network packing with three independent molecular conformations, butC8T-BDSe4Clexhibits more enhanced multiple intermolecular interactions with slightly shorter π-π interaction distances. AC8T-BDSe4Cl:PM6-based blend film exhibited a more favorable morphology with better face-on intermolecular packing with a longer coherent length and more suitable phase separation, resulting in more efficient charge separation, transport, and collection. Consequently, benefiting from the above-mentioned merits, polymer solar cells (PSCs) based onC8T-BDSe4Cl:PM6presented a much higher power conversion efficiency (PCE) of 13.5% with higherJ_scthan that of sulfur-containing analogue-basedC8T-BDT4Cl:PM6(12.2%). These results indicate a unique application prospect of the selenium-substituted FREAs and demonstrate that combination of the two-dimensional conjugated central core and selenium-substituted in the central electron-rich position strategy is an effective approach to improve theJ_scvalue and PCE of two-dimensional FREA-based PSCs.