Effects of Polymer Packing Structure on Photoinduced Triplet Generation and Dynamics

The study of photoexcitations dynamics as a function of thin film microstructures for solution-processed organic semiconductors is essential to provide physical insights needed to further developments in the field of organic electronics as a whole. Here, the effects of polymer packing structure on triplet generation and dynamics in poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) thin films of different molecular weights (M-n = 9-255 kg/mol), both in the pristine and annealed states, were studied by photoinduced absorption spectroscopy and quantum chemical calculations. For pristine films, the lowest molecular weight gives rise to the strongest triplet absorption signal resulting from the enhanced generation through an efficient intersystem crossing process. Upon annealing, an increase in the triplet lifetime is measured. These changes are associated with a restructuring of F8BT molecules packing dictated by the strong dipole on the BT unit, which subsequently affects the interchain exciton migration.