This thesis deals with theoretical studies of the electronic structure of molecules used in the context of molecular electronics. Both studies with model Hamiltonians and first principle calculations have been performed. The materials studied include molecular crystals of pentacene and DNA, which are used as active material in field-effect transistors and as tentative molecular wires, respectively. The molecular magnet compound TCNE and surface modification by means of chemisorption of TDAE on gold are also studied.

Molecular crystals of pentacene are reported to have the highest field-effect mobility values for organic thin film field-effect transistors. The conduction process in field-effect transistors applications occurs in a single layer of the molecular crystal. Hence, in studies of transport properties molecular crystals of pentacene can be considered as a two dimensional system. An open question of these system is if the charge transport is bandlike or if as a result of disorder is a hopping process. We address this question in two of the included papers, paper I and paper II.

The conducting properties of DNA are of interest for a broad scientific community. Biologist for understanding of oxidatively damaged DNA and physicist and the electronics community for use as a molecular wire. Some reports on the subject classifies DNA as a conductor while other report insulating behavior. The outcome of the investigations are heavily dependent on the type of DNA being studied, clearly there is a big difference between the natural and more or less random sequence in, e.g., λ-DNA and the highly ordered syntethic poly(G)-poly(C) DNA. It has been suggested that long-range correlation would yield delocalized states, i.e., bandlike transport, in natural DNA, especially in the human chromosome 22. In paper III we show that this is not the case. In general our results show that DNA containing an approximately equal amount of the four basis is an insulator in a static picture.

An emerging research field is spintronics. In spintronic devices the spin of the charge carrier is as important as the charge. One can envision a device where spin alone is the carrier of information. In realizing spintronic devices, materials that are both magnetic and semiconducting are needed. Systems that exhibit both these properties are organic-based magnets. In paper IV the electronic structure of the molecular magnet compound TCNE is studied, both experimentally and theoretically.

The injection of carriers from metal contacts to organic semiconductors is central to the performance of organic based devices. The interface between the metal contact and the organic material has been pointed out to be one of the device parameters that most significantly influences the device performance. This relates to the process of injection of charge carriers in to the organic material. In some contact and organic material combinations the energy barrier for charge injection can be very high. The barrier can be reduced by modify the interface dipole, this is achieved by a monolayer of adsorbed molecules at the interface. The molecule TDAE chemisorbed on gold is studied in paper V.

The electronic structure of materials for use in organic electronics is studied in this thesis. The first part includes applied research in the form of studies of polymers for use in polymer light emitting devices. The second part is more directed toward organic based spintronics and contains research regarding a room temperature organic ferrimagnetic material. Common for the studies, apart from that all regard organic material, are the use of electron spectroscopy techniques. The studies give new spectroscopic evidence of how the energy level alignment occurs between electrical conductors and spin coated semiconducting polymers, i.e. alignment at the anode side of polymer light emitting devices. The studies prove theoretical predictions regarding spontaneous charge injection forming positive polaronic species in the semiconducting polymer, pinned to the Fermi level of the substrate. The first part also includes studies of novel conducting polymers, based on PEDOT and polyaniline, with work functions spanning from 4.2 eV to 6.4 eV. In the case of organic magnets, our design and construction of the purpose built vacuum deposition system allowed for the first time oxygen free films of the extremely reactive organic ferrimagnet, vanadium tetracyanoethylene (V(TCNE)x), to be studied by means of several different electron spectroscopies, proving or disproving several previous results and assumptions.

The research included in this thesis is regarding materials physics in organic electronics. The thesis consists of two projects: one applied and one basic science in nature. The applied project concerns modification of charge injection properties in organic electronics. The choice of the materials used as anodes and cathodes in polymer light emitting devices, PLEDs, plays a crucial role in device performance. The electrodes, often multi-component/layer systems, need to be chosen in such a way that that they fit the properties of the active material. Here, the electronic, chemical and physical properties of materials for so-called hole transporting layers (HTL) were studied and used to explain and improve the performance of polymer-based light emitting diodes (PLED). The study focused on the properties most important for devices, e.g. film morphology, work function and chemical composition, and the results were compared to PLED 1-V characteristics and luminescence efficiency. The second project aimed to provide basic understanding of issues concerning charge confinement (and charge injection) in small molecules. The n-doping of an component of an organic-based magnetic semiconductor was studied as a first step towards understanding the electronic and magnetic properties of the actual magnetic material.