German Researchers Claim First Transparent OLED Pixels

[Ryan Ruck]

German Researchers Claim First Transparent OLED Pixels

If you’ve seen the movie “Minority Report” and marveled at the transparent computer screens used by Tom Cruise, you’ll appreciate what German researchers have concocted in their labs: entirely transparent OLED (organic light emitting diode) pixels.

The researchers, located at the Technical University of Braunschweig, are claiming the development to be a world’s first.

Their approach is to use transparent TFTs (thin-film transistors) made of a 100-nanometer-thick layer of zinc-tin-oxide, which transmits more than 90 percent of visible light. Such transistors are more often made of silicon, which is used for LCDs (liquid crystal displays) but is highly absorptive in the visible part of the spectrum.

In the transparent displays, the TFTs and the OLED pixels are positioned next to each other. The OLED pixel can be placed on top of the TFT driver circuit without interference.

In addition, because 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 200 degrees Celsius, cheap, flexible plastic substrates can be used.

In the devices developed by the researchers, 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 the Technical University of Braunschweig expects the first prototype transparent OLED displays to be available in two years.

Transparent displays could have numerous applications, including screens that supply surgeons with additional information in their field of view or car windshields that allow drivers to view instructions while driving, according to Riedl.

[Ryan Ruck]

High Efficiency Flat Light Source Could Be The End For The Light Bulb

The end of the light bulb is nigh! Scientists studying organic light-emitting devices (OLEDs) have made a critical leap from single-color displays to a highly efficient and long-lived natural light source. The invention is the latest fruit of a 13-year OLED research program led by Mark Thompson, professor of chemistry at USC and Stephen Forrest of the University of Michigan. If the device can be mass-manufactured cheaply - a realistic expectation, according to Thompson - interior lighting could look vastly different in the future. Almost any surface in a home, whether flat or curved, could become a light source: walls, curtains, ceilings, cabinets or tables. Since OLEDs are transparent when turned off, the devices could even be installed as windows or skylights to mimic the feel of natural light after dark - or to serve as the ultimate inconspicuous flat-panel television.

"This process will enable us to get 100 percent efficiency out of a single, broad spectrum light source," Thompson said.

Thompson and Forrest previously invented efficient single- color displays now ready to enter the market in next-generation cell phones. But subsequent attempts by several groups to create white-light OLEDs fell short. The biggest issue was the fast burnout time of the blue component, since blue is one of the primary colors needed to make white.

The Nature paper presents a quantum mechanical trick that solves this problem. First, the researchers followed their standard recipe for making an OLED: placing four ultra-thin organic layers on glass or transparent plastic. Three of the layers serve as highways for charges to reach a central "emissive" layer.

When the oppositely charged molecules meet in the emissive layer, electrons jump from the negatively charged molecules to the positive ones, and ultimately relax to their starting energy. In the process, light is emitted, which can be tuned to cover a broad range of wavelengths.

Previous OLEDs used phosphorescent blue, green and red dyes to generate light with greater energy efficiency than all-fluorescence based devices (phosphorescence and fluorescence, both expressions of energy that is released as excited electrons fall back into their regular orbit, differ mainly in the speed of their response).

Thompson and Forrest found that they could substitute a fluorescent dye for blue without sacrificing the superior properties of OLEDs.

In fact, the researchers reported, the fluorescent dye should prolong the lifetime of the blue component and also uses 20 percent less energy. "We're hoping this will lead to significantly longer device lifetimes in addition to higher efficiency," Thompson said.

According to Forrest, the device eventually could achieve three times the efficiency of standard incandescent light bulbs.

"With a future emphasis on manufacturing technology, this structure may provide an important, low-cost and efficient means that will replace incandescent lighting in many different applications," Forrest wrote.

The tallest remaining hurdle to production of these devices may have nothing to do with the OLED itself, Thompson said, but with the plastic layer to be used as a backing in economical large-area devices. All mass-produced plastics allow some humidity to pass through to the OLED, eventually degrading it.

"There's no plastic that's hermetic enough to make devices that will last a long period," Thompson said, while predicting that this problem can be solved. Already, Universal Display Corp. has developed the group's research into a commercially feasible process for making cell phone screens.

Funding for the research came from the Department of Energy and Universal Display Corp., which holds exclusive licensing rights to the group's OLED inventions.