Exciton dissociation mechanisms in the polymeric semiconductors poly(9,9-dioctylfluorene) and poly(9, 9-dioctylfluorene-co-benzothiadiazole)

We present femtosecond transient absorption measurements on the semiconductor conjugated polymers poly(9,9-dioctylfluorene) (F8) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). Detailed photophysical modeling reveals that, in F8, sequential excitation, first to the lowest singlet excited state, and then to a higher-energy state resonant with the pump photon energy, is predominantly responsible for the rapid (<150 fs) dissociation of photoinduced excitons. Resonant sequential excitation accesses high-energy states that can promptly evolve to charged or tripler states. In F8BT, however, we find that sequential excitation plays a lesser role in fast polaron-pair generation, and that exciton bimolecular annihilation can explain the charge population. We suggest that the electrophilic benzothiadiazole groups in F8BT facilitate charge formation by dissociation of the excited state formed by exciton-exciton annihilation. Modeling also reveals that exciton bimolecular annihilation can occur via two separate and competing processes. We find that in F8, the dominant mechanism involves exciton diffusion and collision. 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.