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New materials and device structure for organic light-emitting diodes

Posted on:2011-12-07Degree:Ph.DType:Dissertation
University:University of Southern CaliforniaCandidate:Wu, ChaoFull Text:PDF
GTID:1448390002960463Subject:Engineering
Abstract/Summary:
Organic light-emitting diode (OLED) refers to any light-emitting diode (LED) using exclusively molecular and polymeric materials. Since the first demonstration in late 80s, OLED has been the topic of intense interest to both scientific and industrial community. With the progresses made on improving performance, the most advanced OLEDs have demonstrated near unity quantum efficiencies and over tens of thousands of hour-long lifetime. They have been developed to the point that that they are commercially available in small, hand-held, full color displays and showed great promise in lighting applications as well. All the aforementioned achievements were driven by a series of new materials, device structures and fabrication techniques, which will continue to be the force moving OLED technology towards wider success. This dissertation follows the mainstream of the OLED research, trying to incorporate materials with attractive properties into both conventional and newly-designed device structures, in the mean time demonstrating the advantages in simplicity and performance.;Chapter 1 briefly introduces the basics of OLED and the breakthroughs in the history of OLED research, including the first heterojuction device, doping concept and the application of phosphorescence materials.;Chapter 2 focuses on understanding how the molecular structure affects a property of profound significance to OLED, triplet exciton diffusion length. The study uses a simple and effective device structure to probe the energy transfer and triplet exciton diffusion in this paper. The deliberate mismatch of the HOMO between hosts and dopants and the good hole mobility of host materials are key to the success. We are able to correlate the device performance to the triplet exciton diffusion length and explain the huge discrepancy, based on the data obtained from experiments and theoretical calculations.;The versatility of the device structure discussed in Chapter 2 is proven in Chapter 3, with both two-component and three-component White OLEDs (WOLED) being fabricated using this design. Thanks to the new broad-band yellow emitter, two-component WOLED demonstrated excellent quantum efficiency and superior electroluminescence stability. The study shows the promise of this simple structure in yielding comparable performance to its complicated conventional counterpart.;Chapter 4 unveils a new type of materials called "soft slats". In this dissertation, "soft salt" are composed of an organometallic cation and an organometallic anion, both Ir-based. The attractive features include solution-processing ready, amipolar and flexible in property tuning. Energy transfer between the ions in solution is observed, and found to take place at diffusion controlled rates. OLEDs prepared using a simple structure yielded external quantum efficiencies close to theoretical maximum, if the front-orbital energies between two ions are aligned properly.;In the end, a thermally cross-linkable electron-transporting polymer is studied with the emphasis on the application in bottom-emitting inverted OLED in Chapter 5. The polymer layer can be easily prepared by baking the spin-casted monomer film under mild conditions. And it is shown that without the aid of interfacial electron injection layer, the polymer lowers the electron injection barrier, resulting in the encouraging device performance.
Keywords/Search Tags:Device, Materials, OLED, Light-emitting, Polymer, New, Triplet exciton diffusion, Performance
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