First-Principles Theory for Understanding Excitons in Stacked Organic Assemblies*

Organic semiconductors are tunable light absorbers with promise as solar energy conversion materials, with their efficiency highly dependent on the nature and energy of electron-hole pairs or excitons formed upon light absorption. Excitons in these materials are controlled by the interplay between inter- and intra-molecular electronic as well as vibrational interactions, which is not yet well-controlled in devices. Here, we utilize first-principles theory to investigate the optoelectronic and vibrational properties of stacks of functionalized PTCDI DNA base surrogates as a model system to study inter- and intra-molecular interactions. We apply time-dependent density functional theory, along with a Franck-Condon analysis of vibronic effects, to finite stacks of molecules that have been recently synthesized. We determine that the intra- and inter-molecular interactions result in distinct vibrational, electronic, and optical properties. For a periodic assembly of PTCDI, many-body perturbation theory predicts a bandstructure with significant bandwidth (~0.8 eV), consistent with strong inter-molecular electronic interactions, and several spatially delocalizated low-energy optically excited-states. By incorporating electron-phonon interactions, we find that at T = 300K, the optical absorption is altered from T = 0 K due to allowed indirect transitions, while exciton delocalization and binding energy, a measure of intermolecular electronic interactions, remains constant. Overall, this work demonstrates that excitonic properties can be modified via inter-molecular electronic and vibrational interactions.