Theoretical Studies of Structural and Electronic Properties of Donor-Acceptor Polymers

The development of new electronic devices requires the design of novel materials since the existing technologies are not suitable for all applications. In recent years, semiconducting polymers (SCPs) have evolved as fundamental components for the next generation of costumer electronics. They provide interesting features, especially flexibility, light weight, optical transparency and low-cost processability from solution. The research presented in this thesis was devoted to theoretical studies of donor-acceptor (DA) copolymers formed by electron-deficient 3,6-(dithiophene-2-yl)-diketopyrrolo[3,4-c]pyrrole (TDPP) and different electron-rich thiophene compounds. This novel type of SCPs has received a lot of attention due to experimental reports on very good electronic properties which yielded record values for organic field-effect transistor applications. In order to get a deeper understanding of the structural and electronic properties, the main objective of this work was to study this material type on the atomic scale by means of electronic structure methods. For this, density functional theory (DFT) methods were used as they are efficient tools to consider the complex molecular structure. This work comprises three main parts: a comparative study of the structural and the electronic properties of TDPP based DA polymers obtained by means of different theory levels, the calculation of the intermolecular charge transfer between pi-pi stacked DA polymer chains based on the Marcus transfer theory and investigations of molecular p-doping of TDPP based DA polymers. For the first, DFT using different functionals was compared to the density functional based tight binding (DFTB) method, which is computationally very efficient. Although differences in structural properties were observed, the DFTB method was found to be the best choice to study DA polymers in the crystalline phase. For the second, correlations between the molecular structure and the reorganization energy are found. Moreover, the dependency of the electronic coupling element on the spatial shape of the frontier orbitals is shown. Furthermore, a Boltzmann-type statistical approach is introduced in order to enable a qualitative comparison of different isomers and chemical structures. For the last part, the p-doping properties of small, multi-polar dopant molecules with local dipole provided by cyano groups were investigated theoretically and compared with experimental observations. The one with the strongest p-doping properties was studied in this work for the first time on a theoretical basis. Comparing these different p-dopants, rich evidence was found supporting the experimentally observed doping strength