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Abstract :
[en] Improving the charge carrier mobility in organic-based devices such as LEDs, solar cells and FETs is of crucial importance in order to achieve standardized performances. To reach this objective, a detailed understanding at the molecular scale of the influence of organization of within the materials on the charge carrier mobility is a key issue. In disordered materials, the hopping regime is a reliable model to depict charge transport. Indeed, due to positional and energetic disorders, charge carriers spend enough time on a (sub-)molecular units in order for polaron relaxation to take place. In this framework, charge transport has often been described using the semi-classical Marcus theory, with the main parameters, (transfer integral and reorganization energy) calculated with quantum chemistry [1]. In this contribution, we will highlight by means of molecular modeling the subtle interplay between the molecular organization and the charge transport properties of crystalline as well as disordered organic semiconductors. We will first report on the impact of chemical substitution on the molecular packing as well as magnitude of the charge carrier mobility in single crystals of tetrathiafulvalene derivatives [2], ending up with rules of design. We will also address charge delocalization in solution-processed pentacene derivatives by comparing experimental and theoretical polaron absorption [3]. Moving to disordered systems, we will describe the use of a combination of molecular dynamics (MD) simulations and quantum chemistry in order to determine both molecular morphology and charge transport properties. We will illustrate it in the case study of discotic molecules [4] and show how this powerful tool helps us to reveal the dynamic nature of charge transport in liquid crystalline materials. Finally, we will present a molecular picture of the different microscopic processes (ionization, charge recombination and dissociation) induced by molecular doping in organic semiconductors, which lead to the creation of mobile charge carriers [5].
