Twisted-graphene-like perylene diimide with dangling functional chromophores as tunable small-molecule acceptors in binary-blend active layers of organic photovoltaics

This study presents the synthesis of small-molecule acceptors having the structure A-D-A′-D-A - where A, A′, and D represent the end group, the core and π-bridge unit, respectively - that form the active layers with the polymer PM6 for organic photovoltaics. Increasing the number of core perylenetetracarboxylic diimide (PDI) units and conjugating them with thienothiophene (TT) or dithiophenepyrrole (DTP) π-bridge units enhanced the intramolecular charge transfer (ICT) and also increased effective conjugation, thereby, improving the light absorption and molecular packing. The absorption coefficient of hPDI-DTP-IC2F (two PDI with DTP) has the highest value (8 × 104 cm-1) because it featured the greatest degree of ICT, being much larger than that of PDI-TT-IC2F (one PDI with TT), hPDI-TT-IC2F (two PDI with TT) and PDI-DTP-IC2F (one PDI with DTP) (1.64 × 104 cm-1). The PM6:hPDI-DTP-IC2F device provided the highest power conversion efficiency (PCE) of 11.6%; this value was more than twice that of the PM6:PDI-DTP-IC2F (4.8%) device. This large increase in the PCE of the devices from the one-PDI core to two-PDI core case can be attributed to the two-PDI core case having (i) a stronger ICT, (ii) proper molecular packing that provided higher and more balanced carrier mobilities and (iii) a smaller energy loss than for the one-PDI case. Therefore, increasing the number of PDI units that were conjugated with suitable chromophores for stronger ICT in small molecule acceptors can be an effective way for enhancing the efficiency of organic photovoltaics.