Activation Energies of Diffusion for Relaxed Singlet and Triplet Excitons over Conjugated Polymer Chains

Exciton diffusion in nonadiabatic regime can be modeled using an activated expression for relaxed singlet and triplet excitons formed on conjugated chains. A displaced but undistorted harmonic oscillator model has been used to predict the energy required for an exciton to hop to the nearest site on a chain. The activation energies of diffusion for excitons are estimated for some well-known conjugated polymers used in organic electronics. On average, the activation energies for triplet exciton migration have been found to be almost three times larger than those for singlet exciton migration. The activation energies for singlet excitons in a perfectly planar conjugated chain without torsional disorder have been found to be usually lower than the thermal energy available at room temperature. Inclusion of torsional energetic barriers can significantly increase the total activation energy of exciton transport over conjugated chains. The results provide an explanation for long-range exciton transport observed in ordered phases of conjugated polymer films. The theoretical findings also suggest that the activation barriers for exciton diffusion are not very dependent on the electronic structure of the conjugated units but mostly affected by the presence of torsional disorder, which has to be dealt with carefully in computational studies.