If you have seen the movie "Minority Report" and marveled at the see through computer screens used by Tom Cruise in the Department of Pre-crime, you should find this update on Transparent OLED (TOLED) technology of interest.

Art Berman
Insight Media Consultant

TOLEDs have only transparent components (substrate, cathode and anode) and, when turned off, are up to 85% as transparent as their substrate. When a TOLED display is turned on, light is emitted and passed in both the up and down directions. That means it can create a display or light-emitting surface that is viewable from both sides. A TOLED display can be either active or passive matrix.

Several companies and universities are developing TOLED technology. Here is a summary of activities in a few of the more interesting programs

Osram Opto Semiconductors has developed a white TOLED that achieves a luminous efficacy of over 20 lm/W at a brightness of 1000 cd/m2. The prototype is transparent when either on or off and currently exhibits a transparency of 55%. The OLED has white color coordinates of x = 0.396, y = 0.404 and an active illumination area of 90 cm2.

Marion Reichl, Public Relations Manager at Osram, offered the company’s concept for usage of the technology: "They could be used as light partitions that are almost invisible by day and then provide a pleasant diffused light at night. They could also be used as mood lighting, canopies of light or integrated into bathroom and kitchen designs."

Osram could not comment on when the first products would be commercially available, but expects the market launch to be predominantly in the luxury lighting design sector. "The initial volume applications will probably be in the functional design segment in which the principal criteria will be style and quality of light."

At the Technical University of Braunschweig, scientists have developed a technique for embedding OLED pixels on layers of transparent TFTs, creating see-through displays. The goal here is to manufacture devices inexpensively by using flexible plastic substrates that are capable of withstanding high temperatures.

To accomplish this, the Braunschweig approach uses transparent TFTs made of a 100-nanometer-thick layer of zinc-tin-oxide. Such a TFT is more than 90% transmissive in the visible. This is in contrast to the more common silicon-based TFT, the type used in LCDs. Silicon TFTs cannot be used in TOLEDs because they are highly absorptive in the visible. Since the TFT layers are thin, they can be deposited on large areas with conventional techniques. And, because these techniques can be performed at temperatures below 200oC, inexpensive, flexible plastic substrates can be used. The OLED pixels can be positioned next to or even placed on top of the TFT driver circuit

In the prototype displays developed at Braunschweig, the brightness of the OLED pixels varied from 0 to 700 candelas per square meter by changing the voltage of the driving TFTs. By comparison, typical computer screens today reach a brightness of approximately 300 candelas per square meter.

Thomas Riedl, head of the organic and inorganic lasers team of the High-Frequency Institute at Braunschweig, expects the first prototype transparent OLED displays to be available in two years.

Supported, in part, by the U.S. Department of Energy under a Small Business Innovation Research grant, researchers at Universal Display Corporation have developed a prototype high-resolution, active-matrix, TOLED display. It is based on the company’s phosphorescent OLED (which claims up to a 4:1 power advantage over existing fluorescent OLED technology) along with amorphous silicon backplane technology developed by Professor Jin Jang at Kyung Hee University in Seoul, Korea. The Universal Display prototype has a resolution of 120 x 160 (QQVGA) at 200 dots-per-inch and is monochrome. A power efficiency of 45 lumens/watt has been demonstrated.

Universal Display envisions TOLEDs built into vision-area applications, such as architectural windows that provide lighting when daylight is insufficient and automotive windshields for navigation and warning systems. TOLEDs may also enable the development of novel helmet-mounted or heads-up systems for virtual reality, industrial and medical applications.

Researchers at the Fraunhofer Institute in Germany envision using OLED overlays on top of LCDs. These could display specific information without interfering with the main picture.

As a preview of coming attractions, it is possible to stack red, green and blue TOLEDs to make a transparent, full color display.

With all the scientific horsepower and funding currently directed at the development of TOLED technology, it seems likely that consumer products will follow. Insight Media will report on developments as they arise.

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