| Organic light emitting devices (OLEDs) create great potential for developing lightweight emissive flat panel displays. Among the several potential strategies for achieving full-color OLED displays, vertical stacking of OLEDs emitting in the three primary colors provides for high fill-factor, highest possible resolution, and moderate fabrication complexity. This thesis discusses the principles, fabrication, and potential applications of stacked OLEDs (SOLEDs).Transparent OLEDS (TOLEDs) are necessary components in the SOLED architecture. We have demonstrated both semitransparent, metal-containing and high-transparency, metal-free TOLEDs, based on the fact that the absorption of many organic electroluminescent solids are substantially blue shifted relative to their emission due to Franck-Condon relaxation.When indium tin oxide (ITO) is deposited to form the intermediate electrodes in a SOLED, high-temperature processes can not be used due to the instability of the organic materials at elevated temperatures. Room-temperature sputtered ITO films with optimized conductance and transparency do not efficiently inject holes into overlying hole transporters, resulting in high operating voltage for the upper stacked elements. This has been overcome by using an oxygen enhanced process.The layered structure of a SOLED gives rise to microcavity resonances that distort the emission colors if not properly controlled. A quantitative model of microcavities in OLEDs has been used in SOLED design to minimize the impact of such effects on emission colors.Based on these principles and techniques, we demonstrated both metal-containing and metal-free SOLEDs having adequate performance for many display applications.The design of OLED displays is systematically studied. Strategies for achieving full color are compared. The SOLED scheme, with moderate fabrication complexity and moderate or minimal extra loss, provides for a three-fold increase in fill-factor and can achieve the highest possible resolution.Finally, the prospects for OLED displays are considered. A realistic scenario is that OLEDs will first fill some market niches where the technology has considerable advantages. Only after critical issues such as manufacturability, uniformity and yield are resolved, will OLED technology prove itself profitable in some niche market. In that case, it will undoubtedly attract sufficient investment for R&D and infrastructure construction to compete with other well established, ever improving display technologies. |
