Femtosecond Time-Resolved Fluorescence Study of P3HT/PCBM Blend Films

In order to understand the dependence of photoinduced initial processes on thermal annealing, the femtosecond time-resolved fluorescence dynamics of regioregular poly(3-hexylthiophene) (P3HT) in (thermally) annealed P3HT/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend films has been studied by using the fluorescence up-conversion technique. For comparison, a P3HT solution, pristine P3HT, and unannealed P3HT/PCBM blend films have been investigated as well. The fluorescence dynamics of the P3HT solution showed wavelength dependence. Excitation energy transfer between the segments and torsional relaxation possibly occurred in a time scale of several ps in the solution. Observed rise times at longer wavelength emission suggested the formation of these relatively lower emission states (at 650 and 700 nm). Charge transfer (or excitonic quenching) was the dominant process in the fs time scale with emission at 650 nm in the unannealed blend film. In the annealed blend film, the charge transfer (334 fs) and downhill relaxation (942 fs) of self-trapped (dynamic localized) excitons were competitive processes due to the well aligned nanodomains in the P3HT/PCBM blend films. There were different charge transfer rates at different excited states (650 and 700 nm) in the annealed film. The charge transfer process occurred faster at a lower excited state, and a stronger electronic and vibrational coupling in the annealed P3HT/PCBM films was revealed within these measurements as well. The ultrafast anisotropy decays suggested that a strong and ultrafast reorientation of the molecular dipole moments occurred at excited states. The anisotropy decay was mainly determined by the ultrafast process, whereas the energy could continuously migrate along or between P3HT chains in a time scale of similar to 100 ps. The ultrafast process suggested that there was an excitation delocalization associated with vibrational modes, as was consistent with the observation from steady-state measurements. On the basis of the understanding of the mechanisms above, the optimized cell performance has been established.