Unified picture of the photoexcitations in phenylene-based conjugated polymers: Universal spectral and dynamical features in subpicosecond transient absorption

Using subpicosecond transient absorption spectroscopy, we investigate the p rimary photoexcitations in thin films and solutions of several phenylene-ba sed conjugated polymers and an oligomer. We identify several features in th e transient absorption spectra and dynamics that are common to all of the m aterials which we studied from this family. The first spectral feature is a photoinduced absorption (PA) band peaking near 1 eV that has intensity-dep endent dynamics that match the stimulated emission dynamics exactly over tw o orders of magnitude in excitation density. This band is associated with s inglet intrachain excitons. The second spectral feature (observed only in t hin films and aggregated solutions) is a PA band peaking near 1.8 eV, that is longer Lived than the 1 eV exciton PA band, and that has dynamics that a re independent (or weakly dependent) on excitation density. This feature is attributed to polarons, generated through a mechanism that is sample depen dent. In pristine samples, polarons are generated via a mechanism that is q uadratic in exciton density, whereas in photodegraded samples or samples do ped with electron accepters, the generation mechanism becomes Linear in exc iton density.