Ultrafast photogeneration mechanisms of triplet states in para-hexaphenyl

We present femtosecond pump-probe measurements, both conventional and elect ric held-assisted, on organic light-emitting devices based on para-hexaphen yl. The dominant triplet excition generation mechanism is assigned to nonge minate bimolecular recombination of photogenerated, spin-1/2 polarons, This process is active within a few hundred femtoseconds after photoexcitation and involves about 20% of the initially excited states. At higher photoexci tation densities, we observe an additional triplet generation mechanism, wh ich occurs in the 10-ps time domain, due to fusion of singlet excitons and subsequent fission into correlated triplet pairs. The latter decay on the 1 0(2)-ps time scale by geminate recombination.