Dynamics of photoexcitations in a poly[3-dodecylthiophene] thin film studied by femtosecond visible–near-infrared absorption spectroscopy

Femtosecond transient absorption of a poly(3-dodecylthiophene) film is measured in a wide spectral range from visible to near-infrared. Our results show that the transient absorption consists of four decay components. The two faster components are assigned to self-trapping and the subsequent relaxation process of photogenerated excitons. The relaxation time constant of the self-trapped exciton is evaluated as 680±50 fs. The excited species corresponding to the third slow component is identified as a pair of intrachain, oppositely-charged polarons on the basis of its power-law decay and its spectral peak around 1.4 eV. A long-lived transient absorption due to interchain polarons is also observed.