Ultrafast photogeneration mechanisms of triplet states inpara-hexaphenyl

We present femtosecond pump-probe measurements, both conventional and electric field-assisted, on organic light-emitting devices based on para-hexaphenyl. The dominant triplet exciton generation mechanism is assigned to nongeminate bimolecular recombination of photogenerated, spin-$\frac{1}{2}$ polarons. This process is active within a few hundred femtoseconds after photoexcitation and involves about 20% of the initially excited states. At higher photoexcitation densities, we observe an additional triplet generation mechanism, which 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 ${10}^2-\mathrm{ps}$ time scale by geminate recombination.