Evolution of excitons and polarons in polythiophene from femtoseconds to milliseconds

We have studied the photoexcitation dynamics in electrochemically polymerized polythiophene thin films using transient photomodulation spectroscopy measured from 100 fs to 20 ms in the spectral range from 0.25 to 2.2 eV, and by the novel technique of absorption-detected magnetic resonance (ADMR). Our results show that photogenerated exciton polarons are formed within 300 fs; they subsequently decay in the nanosecond time domain by ID diffusion toward recombination centers. The dominant excitations after about 250 ns are charged polarons and bipolarons, which are identified by optical transitions correlated with a spin- j ADMR signal at g ≈ 2.003.