Ultrafast photogeneration of inter-chain charge pairs in polythiophene films

Photoexcitation dynamics in films of polythiophenes with different side groups were studied by transient absorption spectroscopy using 80 fs pulses. We show that inter-chain charge pairs (CP) are generated with ∼20% efficiency in a polymer with short inter-chain distance (∼4 Å) within 100 fs after photoexcitation. We discuss two mechanisms for inter-chain charge separation: (i) electron or hole transfer from an initially excited intra-chain singlet state or (ii) optical excitation of mixed exciton–charge transfer states, which quickly evolve to inter-chain CPs.