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Weaker Microcavities Enable Brighter, Whiter Organic LEDs

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Hank Hogan

Consumer display companies have declared white organic LEDs the wave of the future. Combined with a color filter to create red, green or blue and a transistor to switch pixels on and off, top-emitting organic LEDs promise full-color displays that are bright and stable.


The researchers investigated three structures of organic LEDs (left to right): conventional top-emitting, transparent and electrode-modified. Theelectrode-modified design avoids the microcavity effect that causes spectralnarrowing of the output.

But building the emitters upon which the scheme depends has been a challenge. Researchers at National Chiao Tung and National Tsing Hua universities, both in Hsinchu, Taiwan, have demonstrated a fabrication technique that solves this problem.

Top-emitting organic LEDs suffer from a microcavity effect in which the reflective end caps of the devices and their length together allow only certain wavelengths to emerge. As a result, it is difficult to generate broadband white light, explained National Chiao Tung graduate student Shih-Feng Hsu. Hsu worked on the project under the direction of Chin H. Chen, a professor at the university’s Display Institute.

The investigators have modified a conventional top-emitting organic LED structure comprising a white-light organic LED material sandwiched between various thin metal films and sitting atop a substrate. Typically, silver is in both end caps. The film at the anode is thick enough to be reflective, and the one at the cathode is thin enough — 15 nm or so —  to be semitransparent. The two form a microcavity, and the light from the organic LED bounces between them, escaping only at certain wavelengths.

In the new device’s geometry, the researchers use a silver anode coated with a polymerized fluorocarbon film and cap the silver on the cathode with an index-matching film of tin oxide. This keeps the anode highly reflective, which is important to luminance efficiency, while diminishing the microcavity effect. Hsu noted that the absence of an indium-tin-oxide layer also shortens the optical length of the device, which further alleviates the microcavity effect.

The top-emitting white organic LEDs built with the new scheme display an efficiency of 9.6 lm/W, several times that of emitters built using the conventional approach. However, Hsu cautioned that direct comparisons are difficult because the output from a standard device has an electroluminescent full width half maximum of 20 nm, while the corresponding figure for the new devices is 136 nm.

The group is working to create a balanced white-light source by adding red emission to the device’s output.

Applied Physics Letters, June 20, 2005, 253508.

Photonics Spectra
Sep 2005
An electronic device consisting of a semiconductor material, generally germanium or silicon, and used for rectification, amplification and switching. Its mode of operation utilizes transmission across the junction of the donor electrons and holes.
color filterConsumerFeatureslight sourcesorganic LEDstransistorLEDs

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