Interplay between carrier dynamics and structure of poly(alkylthiophene) and poly(dialkylfluorene) films

Abstract

Carrier dynamics properties of poly(alkylthiophene) are investigated via their radiative and nonradiative recombination rates, depending on the length of the alkyl side chain. The recombination rates are determined by luminescence quantum efficiency and photoluminescence decay time measurements in the picosecond range. A large decrease in radiative rate, going along with an increase in nonradiative rate, was observed in the solid state when compared to dilute solution, and was associated with the well known π stacking of the thiophene rings. As already reported from field effect mobility measurements, the study of poly(hexylthiophene) and poly(octylthiophene) in the solid state showed an increase of the carrier transport for polythiophene with the shortest alkyl chain length. We then discuss the differences between poly(dinonylfluorene) and poly(alkylthiophene), both prone to aggregation. In the solid state, the origin of the emission band in the yellow part of the emission spectrum, which appears upon oxidative aging or annealing, is still controversial. Nevertheless, we explain here that this redshift results from fluorenone excimer/aggregate emission. However, the amount of aggregated species is too small to allow good carrier transport.