01 October 2002
LECs' future looks flat
University Park, Pa. - When it comes to computers, it's all about gaining more power while engineering in quality components that don't drain a battery. That is where the polymer light-emitting electrochemical cell (LEC) comes in, said an international team of electrical engineers.
"The color-variable LEC can provide a solution to simple, low-cost color displays," said Cheng Huang, a graduate student in electrical engineering at Penn State.
Huang and his team investigated color-tunable, light-emitting devices and the attributes necessary for any organic or polymer electroluminescent device used to provide full-color displays.
Devices for flat-panel, full-color displays must have high luminance intensity and efficiency, full-color capability, fast response time, and the ability to avoid cross talk.
Also important for these high-performance content displays is the quality of the image on the display, which means high contrast ratio, wide color gamut, and long-term stability.
"Achievement of color tunability in light-emitting devices is important for multicolor or full color displays, and various approaches for LED development have been tried," said Huang. "The dual or multicolored polymer LEC is a new direction for light-emitting devices fabricated from semiconducting electroluminescent polymers."
LECs are a blend of polymers, including semiconducting luminescent polymers and a polyelectrolyte, as well as two metallic electrodes. When the proper voltage applies to the electrodes, a p-n junction forms in place, and the luminescent polymer emits light.
Polymer LECs have many advantages for flat-panel, full-color displays. The researchers developed a voltage-controlled, two-color bipolar, fast response LEC based on ionic conductive poly(phenylene vinylene) derivatives. They used a bilayer structure-different luminescent polymers sandwiched between two electrodes.
The change in bias voltage moved the p-n junction from one polymer layer to the other, causing the LEC to emit either red/orange light or yellow/green light. The response time for light emission was well within the requirements of a flat-panel display.
Full-color representation relies on the combination of three light colors: red, green, and blue. To obtain true full color, the pure red, green, and blue must be the exact required wavelengths, and efficient, stable electroluminescent polymer materials must develop so that when combined, they create all possible colors.
While the researchers have not created the proper red or green and do not yet have a blue LEC, they have created one cell that can produce a yellow and a red.
They have also created an LEC that responds in less time than the human eye can notice, satisfying the response time requirement.
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