Electronic excitations, phonons, and electron-phonon coupling in acenes
In this dissertation we present first principles electronic structure calculations of acene solids, achieving better understanding of their electronic and vibrational properties relevant to their optoelectronic function. The family of acene molecular crystals serves as a testbed for broader classes of organic crystalline semiconductors, which have come under increasing focus for their many favorable optoelectronic properties. Among them are relative ease of processing, strong and tunable absorption, and charge carrier mobilities sufficiently high for applications. Despite numerous computational and experimental efforts, the details of the underlying mechanisms of these optoelectronic processes are still actively disputed, especially concerning the role of electron-phonon coupling and its impact on the acene electric structure at finite temperatures. To further improve the efficiency of these systems, and to develop new materials that can overcome existing challenges, better understanding of the underlying principles and structure-function relationships that determine acene properties is thus needed.
Here, we calculate the charged and neutral electronic excitations of the acene crystal series within many-body perturbation theory (MBPT), based on van der Waals (vdW)–corrected density functional theory (DFT). We compare the performance of various functionals and vdW corrections in predicting the experimental lattice parameters and investigate the sensitivity of excited states to these structural parameters. Generally, low-lying charged and neutral excitations are well described by the MBPT methods used here, provided that optimized geometries close to experiment are used. The inclusion of vdW interactions to account for the weak intermolecular interactions in molecular crystals is thus found to be a prerequisite for the predictive and accurate calculation of excited state energies in these systems.
To investigate the effect of vibrational coupling at zero and finite temperatures in organic crystals, we calculate the phonon band structure and electron-phonon contributions to the electron self-energy for the case of the naphthalene crystal. We first provide a comprehensive analysis of the computed phonon band structure, comparing to neutron diffraction data. Again, vdW corrections are necessary to obtain phonon frequencies from DFT calculations that are in good agreement with experiment. Based on these results, we compute the electron-phonon self-energy in naphthalene using vdW-corrected DFT and MBPT to lowest order. This self-energy provides the contribution of phonons to the renormalization of band structure energies and to the scattering lifetimes of electronic states. The resulting renormalized band gap at room temperature, and the temperature-dependent mobilities of electron and hole charge carriers are in good agreement with experimental values. Finally, we explore an eigenvalue–self-consistent computational scheme for the electron-phonon self-energy that leads to the prediction of strong satellite bands in the quasiparticle band structure.
The methods presented in this dissertation are general and our conclusions are applicable to other molecular crystals, thus providing a template for future predictive calculations of optoelectronic properties of acenes and related systems, in which both structures and excited states are calculated entirely from first principles.