Exciton dissociation mechanisms in the polymeric semiconductors poly(9,9-dioctylfluorene) and poly(9, 9-dioctylfluorene-co-benzothiadiazole) - art. no. 165213

We present femtosecond transient absorption measurements on the semiconduct or conjugated polymers poly(9,9-dioctylfluorene) (F8) and poly(9,9-dioctylf luorene-co-benzothiadiazole) (F8BT). Detailed photophysical modeling reveal s that, in F8, sequential excitation, first to the lowest singlet excited s tate, and then to a higher-energy state resonant with the pump photon energ y, is predominantly responsible for the rapid (<150 fs) dissociation of pho toinduced excitons. Resonant sequential excitation accesses high-energy sta tes that can promptly evolve to charged or tripler states. In F8BT, however , we find that sequential excitation plays a lesser role in fast polaron-pa ir generation, and that exciton bimolecular annihilation can explain the ch arge population. We suggest that the electrophilic benzothiadiazole groups in F8BT facilitate charge formation by dissociation of the excited state fo rmed by exciton-exciton annihilation. Modeling also reveals that exciton bi molecular annihilation can occur via two separate and competing processes. We find that in F8, the dominant mechanism involves exciton diffusion and c ollision. In F8BT, however, additional annihilation of spatially separated excitons occurs when they interact through the Forster transfer mechanism, where the critical distance for annihilation in F8BT is 4 nm.