Modelling Charge Carrier Dynamics in Organic Semiconductors

Electronic devices made of organic molecules are starting to show their transfomative power in various fields of application today. However, as with most technologies, progress is eventually bounded by how well the inner workings of the components are understood. For electronic devices, as the name suggests, this mostly concerns the behavior of electrons or, more generally, electric charge carriers. To understand and predict device properties, knowledge of the mechanisms that govern the fate of charge carriers is indispensable. In an organic material, those mechanisms are closely related to material properties on a molecular level. Thus, the micro- and macroscale are linked in a complex manner and many questions about these links are still open. This work aims to advance the understanding of three important aspects of the field: the time-evolution of charge carrier states, the mechanism of molecular doping and the efficiency of organic solar cells and photodetectors. All three are strongly affected by a common property of organic materials: disorder. Specifcally, we extend the theoretical framework of describing the time-dependence of charge carrier motion in disordered semiconductors and use it to predict the time-dependence of recombination in organic solar cells. We find that, just as transport, recombination slows down with time, and establish a quantitative method of extracting material characteristics from the measured time-dependence of recombination. To analyze the influence of molecular doping on charge transport, we develop a computational method based on percolation theory. We show that for organic semiconductors, the popular transport energy model can not be used to predict the thermoelectric properties. The latter are important since they are often used to measure the amount of free charges introduced by doping. We are able to accurately model the activation energy of conductivity and study the important length scales and the influence of molecular parameters. Finally, we investigate the consequences of disorder on the performance of solar cells and photodetectors by studying the timescale and efficiency of the separation of photo-generated positive and negative charges. We find that, depending on the conditions, separation can in fact be either enhanced or hindered by disorder effects