Exciton temperature dependence dictated by localization in organic semiconductors

The response of organic semiconductors in optoelectronic devices is dominated by room temperature excitons. However, exciton calculations of organic crystals at finite temperature can be challenging, due to the need to combine the effects of exciton-phonon coupling and thermal expansion. Here we overcome this challenge by employing a combination of Green's function methods for the electronic structure, and non-perturbative finite difference methods for the interaction with the lattice. We apply our methodology to the acene series of molecular crystals and find that different localization ranges of excitons lead to different responses to exciton-phonon coupling and thermal expansion. As a result, the energies of singlet and triplet excitons have different temperature dependencies, as do the singlet energies between the different materials. We expect that the combination of exciton-phonon coupling and thermal expansion, together with their interplay with the exciton localization range, provides a general mechanism for understanding the temperature dependence of exciton energies in molecular crystals.