Theoretical, cross-scale investigations of structure-property relationships of molecular materials in organic electronics

Organic semiconductors offer many advantages over conventional semiconductors such as silicon or gallium arsenide. These include, for example, the structural diversity of the molecules, which allows application-specific optimization of the material properties, the resource-saving manufacturing processes of electronic components or the mechanical flexibility of corresponding films. While organic light-emitting diodes are already used in commercial products and allow the development of high-resolution displays, deficits in performance prevent the widespread use of technologies such as organic field-effect transistors or solar cells. Optimizing organic devices requires a detailed understanding of underlying effects at the molecular level. In this way, optimized materials can be developed in a targeted manner and changes can be made to the architecture of the devices. The aim of this dissertation thesis is to gain knowledge of low-mass organic semiconductors based on hierarchical material simulations. This means that the properties of this class of materials are studied across scales and related to the molecular structure. The parallel model development for the simulation of the materials allows the systematic study of molecular and film structures. This is a necessary prerequisite to define design strategies for improved materials. Single molecule studies based on quantum chemical methods allow a criteria-based preselection of suitable molecules. Thus, they reduce the effort for synthesis and experimental characterization. Exemplarily, an analysis of the material class of BODIPYs as near-infrared absorbing donor materials for organic solar cells as well as for radical molecules is performed. In both cases, the manipulation of the frontier orbitals or the wave functions of the unpaired electrons by means of benzanellation, push-pull strategies as well as the control of the molecular sterics, among others, proves to be a target-oriented design strategy. Further development of these structure-property relationships is based on quantum chemical analysis of molecular clusters. In this way, intermolecular effects can be systematically analyzed on length scales down to 5 nm. Single-molecule studies support the interpretation of the results and reduce the required computational resources. For example, the simulation of low-energy charge transfer absorption at the donor-acceptor interface allows the analysis of disorder processes and elemental properties of organic materials. Contrary to the widely used Marcus theory, it is shown that high-energy molecular vibrational modes do not behave classically at room temperature. This leads to the definition of the temperature-dependent reduced relaxation energy, which only considers contributions of classical low-energy modes, or to the use of alternative models for the description of charge transport. Apart from these single-molecule contributions, the influence of environment-dependent molecular deformations can be observed by combining molecular dynamics simulations and time-dependent density functional theory calculations. In active layers with low donor concentration, these contribute dominantly to the broadening of low-energy charge transfer absorption in addition to the contributions of intramolecular reduced relaxation energies. Thus, they lead to voltage loss in organic solar cells and can be used to broaden the spectroscopic window in organic near-infrared detectors. Based on the results, improved design rules for donor materials can be derived. For example, stiffening the molecular backbone or attaching small functional groups can reduce these contributions. Although single molecule and molecular cluster simulations help to understand the behaviour of organic materials in electronic devices and their impact on performance, the extracted trends apply to largely homogeneous structural environments. This is often not guaranteed in organic films, motivating the simulation of representative film sections. Theoretical studies have so far largely described the properties of crystalline or amorphous structures. To characterize polycrystalline film structures, the combination of a statistical growth algorithm with force-field-based molecular dynamics simulations is suitable. This approach is applicable to diverse molecules and requires a comparatively small set of input information with crystal structure data and force field parameters. Structures constructed in this way provide a basis for systematic, order-dependent analysis of charge carrier mobility, as demonstrated for C${}_{60}$. Contrary to the assumption that charge transport usually becomes worse with lower structural order within the film, it is shown for C${}_{60}$ that energetic homogeneity can be more important than structural order. In polycrystalline C${}_{60}$ structures, there is an energetic gradient between molecules at grain boundaries and molecules within the crystallites, leading partially to smaller electron mobilities than for amorphous structures. With its sequence of single-molecule studies, molecular cluster analyses, and film simulations, the dissertation thesis provides in-depth insights into the structure-property relationships of organic semiconductors and, at the same time, methodologically provides new impulses for further theoretical investigation of them. Going beyond these individual advances, it demonstrates that an understanding of single molecules is necessary to model and understand phenomena in molecular clusters. Moreover, both single molecule and molecular cluster studies are necessary to describe effects in film sections. This further motivates the performance of hierarchical material studies or multiscale simulations.