| Low operating voltages, a wide range of emission wavelengths, and solution processing make polymer light-emitting diodes attractive for high-growth markets including flexible displays, large-area displays, and solid-state lighting. However, the external efficiencies of these devices must be improved in order to compete with existing technologies. Currently, the majority of the light generated inside polymer LEDs remains trapped within the device by total internal reflection. Extracting this trapped light can significantly increase the external efficiency. In this thesis, I use both theoretical tools and experimental results to study light extraction from polymer LEDs. First, I examine the optical properties of the light-emitting polymer. The properties of this layer have important implications for light extraction and need to be measured carefully. I have developed a method to accurately measure the optical properties of a light-emitting polymer by using grating outcoupling. The results show that the polymer layers are anisotropic and dispersive. Using numerical modeling techniques, I predict the emission into air, substrate, polymer/indium tin oxide (ITO) and surface plasmon modes of a polymer light-emitting diode. The results give good insight into the possible efficiency increases that can be expected for various light extraction techniques. In addition, the effects of various optical properties and layer thicknesses on the optical performance of the device are reported. I show how modification of the substrate can be used to focus light into mode types that can be easily extracted. I then report my experimental results for two very different light extraction techniques. First, I demonstrate how Bragg gratings can be used to extract light from waveguide modes in the polymer/indium tin oxide (ITO) layers. With an optimized Bragg grating, I have increased the external power efficiency by 25% at high brightness levels. In addition, I have used substrate-shaping techniques to extract light from waveguide modes in the glass substrate. In this case, I was able to increase the efficiency by over 60%. Finally, I compare my numerical and experimental results to summarize the possible benefits of various light extraction strategies. |
