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FOR IMMEDIATE RELEASE
Date: March 7, 1996
Contact: Jacquelyn Savani (609) 258-5729

New Device May Transform Television-Computer Display Technology

Princeton, N.J. -- A March 7 Nature article announces that a team of five scientists at Princeton and the University of Southern California (USC) has discovered how to make a transparent organic light emitting device (TOLED). According to the article, ``This is the crucial first step toward realizing high definition, full-color and head-up displays using organic materials.''

Current display technology for televisions and computers includes two main types: (1) the bulky cathode ray tube (CRT) bulging from the back of conventional televisions and desktop computers and (2) the flat panel of laptop computers with liquid crystal display (LCD).

This new device is light-emissive like CRTS, but made out of thin organic films and therefore especially good for making flat displays. The promise is that this device is the key technology for making high definition televisions whose flat displays can be hung on the wall like a painting or propped, perhaps, on an easily movable easel structure. or imagine laptops with low energy consuming displays that are light emissive like CRT displays. Then imagine desktop computers with flat displays. Another potential use besides television and computer displays is head-up displays such as maps embedded in automobile windshields.

The device is grown on a conducting substrate (basically glass), precoated with a transparent indium tin oxide (ITO) thin film. On that by evaporation in a vacuum is deposited a thicker layer of TPD (short for N,N'-diphenyl-N,N'-bis[3-methylphenyl]1-1'biphenyl-4,4'diamine). TPD is a ``hole'' conducting compound in which the absence of an electron acts as a carrier of a positive charge. The TPD layer is followed by a thick layer of the electron conducting and highly electroluminescent organic compound tris (8-hydroxyquinoline) aluminum (Alq3).

The action that creates color takes place in the electron conducting layer (Alq3 in this case emits green light). An electron injected from the contact onto the molecules in the layer is attracted by the positive hole and gives off a photon with the frequency of either green, red or blue light. What determines the various colors is the molecular structure of the light emitting material -- the greater the separation of electron and hole in energy, the more energetic the photon released and the bluer the light.

On top of the Alq layer goes the top contact, the most innovative component of the device built by the Princeton-USC scientists because it is transparent. This electron-
injecting contact is made by depositing through a shadow mask a thin layer of magnesium-silver alloy (Mg-Ag), onto which is sputter-deposited a thick ITO layer.

So the device consists of five films laid down on glass. Sandwiched between top and bottom ITO films are three films: the hole injecting TPD layer and the electron injecting Mg-Ag layer and between them the Alq layer -- where holes and electrons meet and give off light.

Stephen Forrest, director of Princeton's Advanced Technology Center for Photonics and optoelectronic materials (POEM), explains: ``The full color display of cathode ray tubes is created by mixing three primary colors. At each pixel there is a triad of colored phosphors, each red, green or blue, turned on separately with electron guns. The intensity with which each one is turned on determines the ultimate color the eye perceives.

``What we can now do with these organic devices is to place the three primary color emitters in a single, very small stacked structure. By having intervening transparent contacts, we can energize to different extent the red, green and blue devices all in a single stack. The light penetrates through all transparent contacts and other organic layers and out comes a mixture of colors. So this device makes it possible to fabricate an `RGB' pixel, which is the main element in a full-color flat-panel display.,,

The making of a single pixel with three colors could have profound implications for manufacturing because only one type of pixel need be made, instead of three -- thereby reducing three separate fabrication steps to one, points out Forrest.

The transparency of the device depends on a unique property of organic materials -- they are extremely transparent to their own radiation. A material, for instance, emitting in' the green, absorbs in the near ultra-violet, so for lack of absorption green shines forth.

In addition to Forrest, the authors of the Nature paper include two Princeton graduate students in electrical engineering, Vladimir Bulovic and Gong Gu, research scholar Paul Burrows, and Mark Thompson, a chemistry professor at USC.

Funding for this research came from Universal Display Corp. and ARPA. The Air Force Office of Scientific Research (AFOSR) has supported fundamental aspects of Forrest's work under which the basic technology for organic materials was developed. Transfer of the TOLED technology to industry is expedited by the POEM Center, which receives support from the New Jersey Commission on Science and Technology.




Caption: A transparent organic light emitting device (TOLED) consists of five films laid down on glass. Sandwiched between top and bottom ITO films are three films: the hole injecting TPD layer and the electron injecting Mg-Ag layer and between them the Alq layer -- where holes and electrons meet and give off light.

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