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 excitonic 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 (TDDFT), 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. Additionally, by combining TDDFT with a recently developed time-resolved non-adiabatic dynamics approach, we show that stacking increases the efficiency of non-radiative relaxation dynamics from a high excitonic state to the lowest energy exciton. 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.