Two-step field-induced singlet dissociation in a fluorene trimer

We time resolve the event of singlet separation into free carriers in an oligofluorene using femtosecond field assisted pump-probe technique. Steady state photoinduced absorption (PA) measurements have also been employed to identify triplet absorption (T-1-T-n) in the spectral region of 1.8 eV. As a result of an applied electric field of 2.2 MV/cm singlets are observed to undergo a dissociation process consisting of first singlet splitting into bound polaron pairs and second further dissociation of polaron pairs into free carriers. Subsequent free carrier recombination yields a new population of singlets and triplets. Our results suggest that nongeminate recombination leads to the formation of 35% singlets and 65% triplets.