Ultrafast electron dynamics in excited states of conjugated thiophene-fluorene organic polymer (pF8T2) thin films

The electronic states of poly(9,9-dioctylfluorenyl- alt -bithiophene) pF8T2 on H/Si(100) substrates, prototypical for organic photovoltaics, were investigated by ultrafast photoelectron spectroscopy and by time-resolved fluorescence studies. Occupied and unoccupied electronic states were analysed by ultraviolet photoelectron spectroscopy (UPS), static and dynamic femtosecond two-photon photoemission (2PPE), and time-correlated single photon counting (TCSPC). Time-resolved measurements allow assessment of population lifetimes of intermediate states. The combination of time-resolved photoelectron spectroscopy and fluorescence excitation allows following the electronic dynamics in excited states from the femtosecond to the nanosecond time scale. For this prototypical material the electron kinetic energy resolved lifetimes range from about a few tens of femtoseconds up to hundreds of picoseconds. After annealing these types of organic thin films the efficiency of organic solar cells usually increases. We show that annealing does not influence the initial ultrafast charge generation processes, but the long-lived states. However, the nanosecond scale fluorescence lifetimes measured by TCSPC are prolonged after annealing, which therefore is identified as the cause of a greater exciton diffusion range and thus is beneficial for charge carrier extraction. Three distinct ultrafast lifetimes are observed by tr-2PPE in pF8T2, and three longer ones by TCSPC. Annealing extends the long lifetimes and thus facilitates a better charge separation, but has no effect on the carrier generation.