FEMTOSECOND PROBING OF EXCITON RELAXATION AND TRANSPORT DYNAMICS IN POLYBITHIOPHENE

The relaxation and transport dynamics of singlet excitons in 100 nm thick, electrochemically prepared polybithiophene films were investigated by monitoring the time evolution of photoinduced bleaching of the S0–S1 absorption and photoinduced absorption (S1–Sn) with femtosecond resolved transient absorption spectroscopy. The decay dynamics of both photoinduced bleaching and photoinduced absorption in the spectral range from 500 to 800 nm are observed to be independent of the pump pulse wavelength and can be fitted by a double exponential, the time constants of which are τ1=120±20 fs and τ2=2±0.3 ps. The fast decay process with ∼τ1=120 fs was ascribed to the transport dynamics of the initially generated free excitons migrating over parallel aligned polymer segments to structural defects acting as traps. The competing process, occurring predominantly in structurally disordered regions, is structural relaxation of the initially generated free excitons to self-trapped excitons decaying with a lifetime of about...