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High efficiency blue phosphorescent organic light emitting diodes

Posted on:2010-03-30Degree:Ph.DType:Dissertation
University:University of FloridaCandidate:Chopra, NeetuFull Text:PDF
GTID:1448390002475138Subject:Engineering
Abstract/Summary:
Organic light emitting diodes are touted as a promising candidate for solid state lighting. Keeping in mind the energy situation world is facing today, it is imperative to have a low cost, large area, low energy consumption lighting alternative and that's where organic light emitting diodes become very important. Also, OLEDs are very attractive from the standpoint of full color display application because of desirable properties such as wide viewing angle and easy fabrication. For both of these applications, white light is desirable which can be obtained from the combination of basic color components i.e. RGB. Phosphorescent organic materials are inherently four times more efficient compared to fluorescent materials. Using these phosphorescent materials, very high efficiencies have been achieved for red and green OLEDs. However, blue emitting phosphorescent devices were still lagging behind in terms of device efficiency until very recently.;The focus of this work has been to remove this weak link in development of high efficiency white OLEDs by studying the materials and device properties. For understanding factors impacting the device performance, effects of material properties such as triplet energy and mobility on the device performance of blue phosphorescent organic light emitting diodes (PHOLEDs) were investigated. The effect of triplet energy of different charge transport materials and the host materials were studied systematically. Also, the device performance was correlated with the mobility and transport properties for the materials used. Single carrier devices were fabricated to study and compare the transport of hole and electrons. It was found that these devices are largely hole dominant. Hence, it was expected that the recombination zone is located on the interface of the emitting layer and the electron transport layer which was verified experimentally by probing the recombination zone. The charge balance becomes even more significant in case of PHOLEDs as most of the conventionally used electron transport materials have lower triplet energy than that of blue phosphorescent dopants. Based on these findings, two major challenges were identified in these devices namely, (1) low triplet energy of the electron transport materials and (2) charge imbalance in the devices. Two approaches were used to get around these problems: (1) improving the electron transport in the device by use of doped transport layers and using high triplet energy high mobility electron transport material to confine the triplet excitons and tune the charge balance in the device and (2) using a mixed host architecture or ambipolar host materials to achieve charge balance in emitting layer. Based on these studies, very high efficiency devices were fabricated an efficiency of 50 lm/W. In the course of fabricating these high efficiency devices we discuss the device physics and the correlation of materials properties such as the energy level alignment of different layers on the device performance and characteristics of these blue phosphorescent devices.
Keywords/Search Tags:Organic light emitting, Light emitting diodes, Blue phosphorescent, Energy, High efficiency, Device, Electron transport, Materials
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