Titre de la dissertation: “Theoretical modeling of charge transport parameters in Organic Semiconductors”

Promoteurs: Monsieur Jérôme Cornil et Monsieur David Beljonne

Résumé de la dissertation: Organic semiconductors encompass promising materials for low cost, portable electronics but their use in everyday life is hindered by poor performance. In modern numerical technology, the semiconductor is the physical support for binary coding when encased in a device called a transistor. In essence, organics electronics relates to the study and fabrication of hardware based on organic chemistry. The rate at which a transistor can switch from zero to one (and hence the speed at which one can perform computations) is strongly dependent on the mobility (µ) of the charge carrier across the device.Within the material, the electronic mobility is strongly dependent on the properties of the charge transport states of the crystal, which are themselves dependent on a small number of parameters: the site energies, the reorganization energy, the electronic coupling between molecules and their fluctuation due to temperature. In turn these fundamental parameters depend on the shape of the molecule and its crystal packing. We are not capable of reliably predicting the crystal structure of a new molecule because of the soft interactions holding the structure together. But, provided with a structure, a theoretician may compute these parameters, thus enriching the empirical knowledge that has been built in the past forty years quest for high performance organic semiconductors.This thesis provides design rules of hand for organic semiconductors in the introduction. In particular, the roles of the transfer integrals and their distributions are highlighted with toy models. The first chapter details a four-year long collaboration with the university of Rennes on the structure-property relationship of carbazole nanohoop molecules. A special focus is set the dimensionality of the transport network based on hopping theory, which quite often rationalizes the performances of the devices. The second chapter presents an approach to dissect the standard distribution of transfer integral distributions () in a molecule called DN4T. Normal modes of vibration are computed both at the DFT and classical force field level, at gamma point. This was done in collaboration with other colleagues from the CMN group and Pr. Claudio Melis from Cagliari university. These data sets are then compared and their influence on the transfer integral is computed and compared. We find that hole transport in DN4T likely suffers from a specific shearing mode that is also very detrimental to charge transport in other materials. We also find that anharmonicity plays an extremely important role to estimate how low frequency modes affect charge transport. The last chapter explores the role of phonon dispersion on EPCs and the effect of phonon-phonon interactions. We take these latter effects into account by running long molecular dynamics simulations in big super cells. We then compute the frequency dependence of the transfer integral fluctuations and compare the results with frozen-phonon EPCs computed from the same cell size. We find that the phonon dispersion doesn’t contribute significantly to the transfer integral standard deviation. On the other hand, we observe a shift in frequency and a smoothing effect that could find its source in mode-mixing.