OLEDs and Polymer Displays

OLEDs have three main advantages over its silicon counterpart: high contrast, high brightness, and low power consumption. And in addition, the OLED has fewer restrictions in terms of size. Organic light-emitting diodes were invented by Tang and Van Slyke [390], who found that p-type and n-type organic semiconductors could be combined to form diodes. This breakthrough initiated great efforts in the development of new molecular materials and device structures. In 1990, the researchers at Cambridge University, under the direction of Richard Friend, reported a similar light-emitting effect in a semiconducting organic polymer film consisting of PPV [391].

In recent years, not only polymer transistors and ICs obtain fast progress, but also polymer OLEDs and displays have impressive development. Major electronic firms such as Philips, Pioneer, Lucent Technologies, IBM, Dupont, Xerox, Mitsubishi, and Hitachi, and smaller companies such as Cambridge Display Technology (CDT), Universal Display, the Franco-German group Aventis, and Uniax, are currently working on the OLEDs and organic full-color displays. CDT Company, which was founded when luminous polymers were discovered at the University of Cambridge, owns the patent to the first luminous polymer, PPV Cambridge Display Technology is working on flexible displays based on poly LED and has shown a first prototype of a polymer display developed in cooperation with Seiko-Epson in 1998. In 1999, the color polymer display by ink-jet printing of emitting layers such as PPV and poly-fluorene (F8) was reported [251]. Here, control of the thickness and shape of the droplet, which eventually sets into a high-resolution pixel, remains an as-yet-unsolved problem. As the light-emitting polymer films are very thin, it is quite difficult to use the conventional photolithography technique to pattern them. In 2001, the University of Cambridge has made an agreement with Seiko-Epson and a regional Cambridge operation called Plastic Logic to manufacture full-color display from sophisticated polymers using ink-jet printing [392].

Since the late 1990s, OLEDs have entered the stage of commercialization. In November 1997, Pioneer Company in Japan commercialized a monochrome 256 x 64 dot matrix OLED display for automotive applications [393]. After that, Pioneer Electronics has launched production of a car-stereo system that has an organic multicolor graphic display, while Philips is currently setting up a production line for polymer displays [394]. Organic EL was originally released by Pioneer with three models in 1999 and is now taken to the next level with the ability to download images directly to the face of the radio. Its OEL displays working at very low voltage requirements (<5 volts) have three models, including DEH-P9300, DEH-P6300, and DEH-P7300 [395]. Multi-color organic EL provides a very wide viewing angle, eliminates the need for backlighting, without loss of visibility from sunlight or a bright environment.

In September 2001, Lucent Bell Laboratories and E-Ink, a Cambridge, MA, Company, have developed a flexible electronic display prototype that is the world's first electronic paper [396, 397]. This prototype electronic paper display, holds its image when its power is shut off, so it only requires electric current when the screen changes. The display is made up of a flexible electronic circuit printed on a sheet of mylar and electronic ink made up of tiny capsules that hold white particles suspended in black dye. An electrical current moves the particles to the top or bottom of the capsules, changing the display's image. The display represents a significant scientific advance over previous works in plastic electronics and paper-like displays. The prototypes consist of a 25-square-inch display area made up of several hundred pixels. The transistors in these circuits are made of plastic materials and are fabricated with a low-cost printing process that uses high-resolution rubber stamps.

Recent advances in boosting the efficiency of OLED have also been made. By using guest-host organic material systems, where the radiative guest fluorescent or phosphorescent dye molecule is doped at a low concentration between 0.5 and 5% into the conducting host molecule, the efficiency can be increased to 10% or higher for phosphorescence or up to about 3% for fluorescence. Currently, the best efficiency of OLED has exceeded that of incandescent light bulbs. Efficiencies of 20 lumens per watt have been reported for yellow-green emitting polymer devices, and 40 lm/W attained for phosphorescent molecular OLEDs, compared to less than 20 lm/W for a typical incandescent lightbulb [390]. According to the report by Yu et al., the luminence efficiency of 3.3 lumens per watt has been reached for the red-light emitting of double-layer organic EL devices by doping the emitting layer with the N,N'-bis[4-(N,N-dimethylamino)-benzylidene]diaminomaleonitrile (BAM) [398].

Vladimir Bulovic and co-workers at Princeton University demonstrated transparent OLEDs (or TOLEDs), which could be used in either transparent head-up or high-contrast displays [391]. But, developing reliable organic devices remains a challenge. Charge conduction requires very high electrical fields (1-5 MV/cm), so it is only the extreme thinness of OLEDs that enables them to operate at relatively low voltages. That thinness can contribute to rapid device degradation through the generation and subsequent growth of dark spots. The problem is especially acute when the devices are exposed to the atmosphere. Organic displays work reliable only when their constituent organic materials are stable. One use means to extend the lifetime of the devices by using organic phosphors. The key point of this approach is to reduce the amount of time a light-emitting molecule remains in the excited state. Through use phosphor emitters, OLEDs have the potential to achieve operational lifetimes of many hundreds of thousands of hours, easily meeting the demands on display performance. The ultimate goal of using high-efficiency, phosphorescent, flexible OLED displays in the cell phones, personal digital assistants, laptop computers, and even for home video applications may be realized no more than in a few years. However, there are still some challenges to be met, such as achieving higher efficiencies, lower power consumption, and longer device lifetime.

Princeton University developed a micro-patterning technique of organic electronic devices by the process of cold-welding of metal cathodes followed by lift-off from the organic substrate [399]. They fabricated a 17 x 17 passive matrix display, with a pixel size of 440 /m by 320 /m.

There have been tremendous advances in polymer microelectronics and optoelectronics during the last decade. Such achievement will have huge impacts on the future of polymer microelectronics in various applications like information technology. Some kinds of polymer-integrated circuits and field-effect transistors with high mobilities have already been fabricated. The performance can even compete with the amorphous silicon device in some cases. Polymer semiconductors, such as pentacene, deposited by vacuum sublimation, have the best performances because of their very well-ordered structures. However, great improvements made in solution-processable organic semiconductors, with their mobilities only one order of magnitude lower than those of vapor-deposited TFTs, result in even lower process technologies, such as the stamping, ink-jet printing, and self-assembling without the lithography and vacuum facilities. Using the organic LEDs with high efficiencies, full color displays may eventually replace LCDs for use in laptops, PDAs, and even home video displays. Although the chemistry and physics behind the polymer or plastics are not well understood, science is close to creating a next-generation chip that can help the digital revolution mature. More and more polymer microelectronic and optoelectronic devices may enter the stage of commercialization. However, there is still much to be done for the commercialization of polymer or organic electronic devices in the market. Higher operation frequency, a higher integrated level, higher device performance, a lower power consumption, a longer device lifetime, and performance stability are all challenges to us.

Nano self-assembly and other nanofabrication techniques could be approaches in resolving the above problem and can lead to the commercialization of polymer microelectronics/optoelectronics.

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