Effect of ether chain length in the perylenediimide-based dimeric acceptors with asymmetric 6-(thien-2-yl)benzo[b]thiophene linker

Abstract

Asymmetric acceptors have gained the increasing attention because of the stronger intermolecular binding energy, the larger natural dipole moment, and the antiparallel packing. However, asymmetric perylene diimide (PDI) dimeric acceptors were not fully investigated. In this contribution, two asymmetric 6-(thien-2-yl)benzo[b]thiophene (T-BTh) core linked dimeric PDI acceptors, T-BTh-(PDI-OMe)₂ and T-BTh-(PDI-EG)₂, with the outside-bay methoxy (OMe) and 2-methxoyethanoxyl (EG) modified PDI-OMe and PDI-EG aromatic moieties as flanked wings, were developed to elucidate the effect of ether chain length. Extending the flexible ether chain from OMe to EG acquired a decreased thermo-stability, a weakened absorption response between 450 nm and 650 nm, a close solution state aggregation, a strengthened aggregation in both pristine and blend films, and a deepened E_LUMO. Thus, the relatively shorter OMe ether chain modified PTB7-Th:T-BTh-(PDI-OMe)₂-based device delivered a V_OC of 0.84 V, a J_SC of 6.62 mA cm⁻² and an FF of 41.81 %, contributing to a winning PCE of 2.33 % and better storage device stability. In the similar testing condition, longer EG ether chain modified PTB7-Th:T-BTh-(PDI-EG)₂-based device got the decreased PCE as low as 0.99 %. The decline in PCE could be primarily attributed to the deepened the E_LUMO, the worse absorption, the reduced electron mobility resulting from poor miscibility, and the rougher surface morphology. When the electron donor PM6 was applied, T-BTh-(PDI-OMe)₂-based device afforded the higher V_OC of 0.96 V but reduced PCE of 1.61 %. These preliminary results implied that it should be cautious to extend the bay-position flexible ether chain, which would affect the molecular geometry, optoelectronic property, and further reduce device efficiency of the asymmetric core linked PDI-based dimers.