Understanding Fundamental Processes in Poly(9,9-Dioctylfluorene) Light-Emitting Diodes via Ultrafast Electric-Field-Assisted Pump-Probe Spectroscopy

Femtosecond electric-field-assisted pump-probe measurements are a new approach to the study of fundamental processes within organic optoelectronic devices. Here we report a detailed study of organic light-emitting diodes based on poly(9,9-dioctylfluorene), using both temporal and spectral information to investigate polaron generation, due to field-induced singlet dissociation, and their subsequent recombination. The fundamental event in electroluminescence is time resolved: we find that initially free polarons coalesce into intermediate pairs of both singlets and triplets multiplicity which subsequently decay into the neutral state. Our results indicate that the efficiency of singlet formation, $\beta \approx 0.7$, is much higher than expected from simple state degeneracy arguments ($\beta \approx 0.25$).