Organic semiconductors are highly 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. Here, we utilize first-principles density functional theory (DFT) and many-body perturbation theory (MBPT) to investigate the optoelectronic and vibrational properties of stacks of functionalized perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) DNA base surrogates as a model system to study inter- and intra-molecular interactions. We apply time-dependent DFT (TDDFT), along with a Franck-Condon analysis of vibronic effects, to finite stacks of molecules that have been recently synthesized. By systematically increasing the number of stacked molecules from one to four, and comparing to measured optical spectra, we establish the accuracy of our approach. We determine that the intra- and inter-molecular interactions result in distinct vibrational, electronic, and optical properties, with inter-molecular coupling playing a more signficiant role as the number of molecules in the chain increases. Additionally, for a periodic assembly of PTCDI, MBPT 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. However, by incorporating the influence of electron-phonon interactions, we demonstrate that at finite temperature, the bandwidth is reduced and exciton binding increased, leading to localization of excited-states. Overall, this work demonstrates that excitonic properties can be modified via inter-molecular electronic and vibrational interactions.

First-principles theory for understanding excitons in stacked organic assemblies

2019 Spring National Meeting

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