Ultrafast spectroscopy of photoexcitations in α-sexithienyl films: evidence for excitons and polaron-pairs

We have used transient and steady state photomodulation techniques for studing the photoexcitations in thin polycrystalline films of α-sexithienyl. We show that singlet Frenkel excitons and charge transfer excitons are formed within 1 ps. The initial Frenkel exciton population decays via a bimolecular annihilation process followed by a slower, trap-mediated recombination. The charge transfer state is long lived and gives rise to separated charges in the nanosecond time domain.