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Self-interaction and charge transfer in organic semiconductors
(2009)
- The fascinating properties of organic molecular semiconductors paved the way for a new class of electronic devices such as organic light-emitting diodes, transistors, or solar cells. Despite an inferior efficiency as compared to commonly used silicon-based technologies, organic semiconductors promise the advent of fully flexible devices for large-area displays and solar cells, printed transistors as low-cost radio frequency identification (RFID) tags, displays for electronic books, and disposable measuring instruments for medical diagnosis. Hence, the investigation of organic molecular semiconductors has emerged as a vibrant field of development both in industry and in academia, spanning a wide range of subjects from physics, chemistry, and materials science to engineering and technology. Theoretical physicists can contribute to this progress by developing methods that allow to determine the electronic properties of organic semiconductors from first principles and thus deepen our knowledge of the underlying electronic processes in organic electronic devices. The calculation of the electronic properties of molecular semiconductors issues a serious challenge to theoretical physicists and chemists. Typically, organic semiconductor molecules employ several hundreds of electrons. For systems of that size, approaches that work with model Hamiltonians are typically not accurate enough in predicting many important electronic properties. However, solving the many-particle Schrödinger-equation by employing highly accurate perturbation theory approaches is often numerically too expensive to be considered as a convenient alternative. Hence, density functional theory (DFT) naturally arises as the method of choice. However, although in theory DFT yields an exact formulation of quantum mechanics, the quality of the results obtained from DFT calculations in practice strongly depends on the used approximations to the so-called exchange-correlation functional. This work concentrates on the problem of self-interaction, which is one of the most serious problems of commonly used approximative density functionals. As a major result of this work, it is demonstrated that self-interaction plays a decisive role for the performance of different approximative functionals in predicting accurate electronic properties of organic molecular semiconductors. This is particularly true for the calculation of ionization potentials, photoelectron spectra, dissociation, and charge-transfer processes. In search for a solution to the self-interaction problem, a new concept for correcting commonly used density functionals for self-interaction is introduced and applied to a variety of systems, spanning small molecules, extended molecular chains, and organic molecular semiconductors. It is further shown that the performance of functionals that are not free from self-interaction can vary strongly for different systems and observables of interest, thus entailing the danger of misinterpretation of the results obtained from those functionals. The underlying reasons for the varying performance of commonly used density functionals are discussed thoroughly in this work. Finally, this thesis provides strategies that allow to analyze the reliability of commonly used approximations to the exchange-correlation functional for particular systems of interest. This cumulative dissertation is divided into three parts. Part I gives a short introduction into DFT and its time-dependent extension (TDDFT). Part II provides further insights into the self-interaction problem, presents a newly developed concept for the correction of self-interaction, gives an introduction into the publications, and discusses their basic results. Finally, the four publications on self-interaction and charge-transfer in extended molecular systems and organic molecular semiconductors are collected in Part III.
