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Monday, January 24, 2000
4 p.m. at UCSD Faculty Club

Professor S.S. Penner
Department of Mechanical and Aerospace Engineering
University of California, San Diego

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Organic light emitting diode (OLED) has long been recognized for its potential applications in flat-panel displays. As an emissive device, OLED offers many unique advantages including high luminance efficiency, low drive voltage, and RGB colors. As a display technology, OLED is considered as an alternative to the industry-standard liquid crystal display (LCD) with several distinctive features: high contrast at any viewing angles, fast response, and low power consumption (no back-light). This presentation will trace the development of the OLED technology, from the perspective of an industrial researcher, beginning with the invention of the novel device structure by Tang and Van Slyke [APL 51, 913 (1987)]. Along the development path in the last decade were steady improvements in organic luminescent and transport materials and in device configurations, resulting in significant increase in OLED performance and reliability. Analytical studies using UPS and XPS tools have probed in considerable detail the energetic states of the organic/metal and the organic/organic junctions, yielding useful information for further improving the device structure. For the fabrication of OLED display panels, novel but simple methods were successfully devised for patterning the cathode and the RGB color pixels, providing a path for low-cost manufacturing and the demonstration of passive-matrix color QVGA displays. For high-resolution OLED displays, the active-matrix panel architecture is ideally compatible with the current-driven OLED, and polysilicon thin-film-transistor (TFT) technology well developed for LCD has recently been adapted to produce both monochrome and color active-matrix OLED displays with an excellent viewing quality suitable for video applications.

* Dr. Ching Tang is a senior member of the technical staff of Kodak Research Laboratories, Rochester, NY. He leads the research group of the Display Technology Laboratory responsible for the development of organic light emitting diodes for flat-panel display applications.

Polymers can be used to modify the electro-optic response of liquid crystals and to create entirely new electro-optic effects. In this talk I will review my work in polymer liquid crystal composites, beginning with the development of polymer dispersed liquid crystals in the mid-1980's and continuing with the development of bistable, relective cholesteric materials and devices. I will end with a report on the formation of polymer walls in the inter-pixel region of liquid crystal displays. The walls are formed by the patterned electric field produced by the row and column electrodes in a multi-plexed display. The polymer walls will be particularly useful for displays using flexible plastic substrates, making roll to roll processing possible.

* John West is Director of the Liquid Crystal Institute at Kent State University and of the NSF Science and Technology Center for Advance Liquid Crystalline Optical Materials. He received his Ph.D. in Chemistry in 1980 from Carnegie Mellon University. Since joining the Liquid Crystal Institute in 1984 as a Senior Research Fellow, he has conducted research on combining polymers and liquid crystals. He concentrates his research on the development of PDLC and cholesteric materials for use in electro-optic devices and on photoalignment of liquid crystals.

In order to use organic light emitting diodes (OLEDs) in display applications it is important to be able to accurately tune the color of emission. Doping of OLEDs with fluorescent dyes has been known for many years as a useful means to control the color of OLEDs. Unfortunately, the use of a fluorescent dye leads to an upper limit of 25% on the internal quantum efficiency, due to the small fraction of singlet excitons created on hole-electron recombination. The use of phosphorescent dopants, however, allows the efficient utilization of both singlet and triplet excitons, removing the 25% upper limit on the internal efficiency. We have fabricated saturated red, orange, yellow and green OLEDs, utilizing phosphorescent dopants. The quantum efficiencies of these devices are quite good, with measured external efficiencies as high as 10% (internal eff. » 40%). The phosphorescent dopants in these devices are heavy metal containing molecules (i.e. Pt, and Ir), prepared as both metalloporphyrins and organometallic complexes. The high level of spin orbit coupling in these metal complexes gives efficient emission from triplet states. In addition to emission from the heavy metal dopant, it is possible to transfer the exciton energy to a fluorescent dye, by Förster energy transfer. The heavy metal dopant in this case acts as a sensitizer, utilizing both singlet and triplet excitons to efficiently pump a fluorescent dye. I will discuss the important parameters in designing electrophosphorescent OLEDs as well as their strengths and limitations.

I will also discuss some of our recent work with new electron transporters and anode materials. The most common electron transporter in OLEDs is aluminum tris (8-hydroxyquinolate), while this material is very useful, its optical gap in the green part of the spectrum complicates its use in blue OLEDs. We have recently found that octasubstituted cyclooctatetraenes make excellent electron transporting layers in OLEDs. Their optical gaps are in the UV, making their use in blue OLEDs very straightforward. If time permits I will also discuss our work in novel anodes for OLEDs. Nearly all of the OLED work thus far has centered in use of indiumtinoxide (ITO) as a transparent anode. We have found that thin film metal nitrides will also act as good semitransparent anodes. For example, a 100 Å film of TiN is > 60% transparent and can be used as a replacement for ITO. The resulting OLED has a better quantum efficiency that the ITO based device and may have a longer lifetime due the refractory nature of TiN. TiN and other metal nitrides are readily prepared by standard CVD methods.

* Professor Mark Thompson graduated from the University of California at Berkeley in 1980, and from the California Institute of Technology with a Ph.D. degree in chemistry in 1985. He then took a S.E.R.C. postdoctoral fellowship with Prof. Malcolm Green in the Inorganic Chemistry Laboratory at Oxford, working on projects involving layered organometallic materials and the study of new materials for nonlinear optics. In 1987 Thompson joined the faculty of the Chemistry Department at Princeton as an Assistant Professor and in June of 1995 took an Associate Professor's position in the Department of Chemistry at the University of Southern California. His research is aimed at understanding the optical, electronic and catalytic properties of novel organic and metal-organic materials.

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