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Organic electrode brightens OLEDs

22 Jan 2004

A new material for connecting organic light emitting diodes together could lead to brighter displays.

Brighter, more efficient OLED displays could be on the horizon thanks to a transparent organic electrode developed at Eastman Kodak’s Display Technology Laboratory. (Applied Physics Letters 84 167).

Stacking OLEDs on top of each other in a so-called tandem design is one way to achieve brighter, more stable displays. Unfortunately, the metal-based electrodes which usually connect multiple OLEDs are not very transparent.

Now, researchers at Kodak have come up with an attractive alternative to the traditional metal electrode. They discovered that a combination of n- and p-type doped organic layers gives a contact that has good electrical properties, is highly transparent, and easy to fabricate.

The use of a transparent contact in a tandem structure looks set to have a significant impact on display performance. As no light is lost between the separate OLEDs, the brightness of the display will scale linearly with the number of OLEDs that it contains. For example, a tandem device with three OLEDs is roughly three times as bright as a single OLED device.

The researchers have observed a luminous yield of ~130 cd/A for a green tandem device made up of three distinct OLEDs. According to the researchers, this means that a peak brightness in excess of 100,000 cd/m2 is possible for this device with a modest current density.

“In passive-matrix OLED displays, the ability to produce high peak brightness is critical as it directly impacts the display brightness, size, and resolution,” commented Liang Sheng Liao, a Kodak researcher. “By capitalizing on tandem OLED structure, it is possible to produce passive-matrix OLED displays that are both efficient and sunlight-readable for important applications such as in cellular phones.”

According to Liao, tandem OLED research is still in its infancy. “An issue is the increase in the number of layers in the tandem structure. A more simplified structure may be necessary to reduce the cost of fabrication,” he said. However, he added that “While we are not able to predict the time frame for commercialization at this point, we are confident that it will broaden the range of applications based on OLED technology.”

Siân Harris is features editor of Opto & Laser Europe magazine.

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