Understanding losses in OLEDs: optical device simulation and electrical characterization using impedance spectroscopy

Only about 5% of the invested electrical power is radiated as visible light from incandescent light bulbs, the other part of the power is lost, as it is emitted as heat. Therefore, using more efficient light sources appears to be a reasonable goal. Besides the common light bulb, there are several other types of illumination sources which are more efficient and therefore used in lighting applications, e.g., fluorescent tubes, compact fluorescent lamps (CFLs), and halogen incandescent lamps. Promising candidates for even more efficient, longer-lasting light sources derive from the area of 'solid-state lighting' (SSL), where light is generated by solid-state electroluminescence, i.e., radiative recombination of holes and electrons; in contrast to light generation by heat, as in incandescent light sources. SSL devices are 'light-emitting diodes' (LEDs, using inorganic semiconductors), or 'organic light-emitting diodes' (OLEDs, using organic materials). LEDs already have a vast distribution being used as indicators in everyday objects, e.g., in consumer electronics, traffic lights, cars, and others. They are also used as backlight in computer displays and flat-panel TVs, saving energy and providing almost constant luminance for very long times (half-luminance lifetimes of several 10,000 to 100,000 hours). For the same reasons, saving energy and extremely long-lasting, LEDs are more and more considered for lighting applications. There is another class of solid-state light sources besides inorganic LEDs: their organic counterparts using thin films of organic compounds -- the organic light-emitting diodes. One of their main advantages is that OLEDs are uniform surface light sources (and not point sources as LEDs, or, to some extent, incandescent lamps). This allows entirely new, fascinating lighting possibilities, such as luminescent ceilings or windows which are transparent at daytime and the source of room illumination at night. OLEDs can be fabricated on different substrates, usually on glass, but also on, e.g., thin metal or plastic foils. As a consequence, OLEDs can be flexible and can be adjusted to different surface forms. Also, cheap processing can be achieved using high-volume printing techniques, like roll-to-roll or inkjet-printing. To fully tap the potential of organic light-emitting diodes for lighting applications, they need further improvement concerning light-outcoupling, efficiency, and device lifetime. To achieve a fundamental understanding of the involved processes, these topics are addressed in this thesis: simulation based optical optimization, and electrical characterization and device degradation studies using impedance spectroscopy.Organische Leuchtdioden (organic light-emitting diodes, OLEDs) sind Flächenlichtquellen bestehend aus dünnen Schichten organischer Materialien. Sie bieten neuartige Möglichkeiten, Lichtquellen in die Architektur einzubinden. Um sie für einen breiten Markt attraktiv zu machen, müssen Lichtauskopplung, Effizienz und Bauteillebensdauer noch weiter verbessert werden. In der vorliegenden Dissertation werden daher Bauteiloptimierungen mithilfe von optischen Simulationen gezeigt. Des Weiteren werden OLEDs mit Impedanzspektroskopie elektrisch charakterisiert und ihr Alterungsverhalten untersucht