Triplet energy transfer in conjugated polymers. III. An experimental assessment regarding the influence of disorder on polaronic transport

It is a general notion that, in organic semiconductors, the transport of electronic excitations, such as neutral excitons of singlet or triplet type and charge carriers, is controlled by both, polaron and disorder effects. For compounds with low energetic disorder triplet exciton diffusion can be described in the framework of Marcus-theory [see Sudha Devi et al.,Phys. Rev. B 78, 045210 (2008)], and a theoretical model for diffusion in more disordered compounds has been developed [see Fishchuk et al., Phys. Rev. B 78, 045211 (2008)]. Here we experimentally demonstrate that such a modified Marcus-type model is suitable to describe triplet exciton transport in commonly used poly($p$-phenylene)-type polymers and oligomers. In particular, we provide a quantitative spectroscopic assessment of the polaronic and the disorder contribution to triplet exciton transport as a function of conjugation length. Franck-Condon analyses of the phosphorescence spectra and temperature-dependent triplet diffusion combined with analytic transport theory demonstrate that, in contrast to charge carriers, Marcus-type jump rates with dominantly polaronic activation energies control the motion of triplet excitons above a transition temperature.