Singlet Fission in Luminescent and Nonluminescent II-conjugated Polymers

We have applied the action spectrum technique to the photoinduced absorption (PA) band of triplet excitons (TE) in luminescent polymers such as PPV and mLPPP, and nonluminescent polymers such as polydiacetylene (PDA), to obtain the TE photogeneration quantum effiency excitation spectra. In all three cases the TE photogeneration spectrum shows an onset at the optical gap, and in addition they also show a second photogeneration onset at excitation energies higher than the optical gap. This second onset is explained by singlet exciton fission into triplet pairs. We introduce a model to fit the spectra and obtain the TE energy as a fitting parameter.