Are LECs the future of flat panel colour displays?

A team of electrical engineers has created a voltage-controlled, two-colour bipolar, fast response LEC that they believe will lead to low cost colour displays.

In the search for low-cost colour displays that do not drain a computer’s battery, the polymer light-emitting electrochemical cell (LEC) may be the next answer to the problem, according to an international team of electrical engineers.

‘The colour-variable LEC can provide a solution to simple, low cost colour displays,’ Cheng Huang, graduate student in electrical engineering at Penn State told attendees at the 224th American Chemical Society annual meeting in Boston.

Huang, working with Gang Huang, Suzhou University; J. Guo and Chang-Zheng Yang, Nanjing University, and Wei Huang and E.T. Kang, National University of Singapore, investigated colour tuneable light-emitting devices and the attributes necessary for any organic or polymer electroluminescent device used to provide full-colour displays.

Devices for flat-panel, full-colour displays must, according to the team, have high luminance intensity and efficiency, full-colour capability, fast response time and the ability to avoid crosstalk.

Also important for these high-performance content displays is the quality of the image on the display, which means high contrast ratio, wide colour gamut and long-term stability.

‘Achievement of colour tuneability in light-emitting devices is important for multicolour or full colour displays and various approaches for LED development have been tried,’ said Huang of Penn State. ‘The dual or multi coloured polymer LEC is a new direction for light-emitting devices fabricated from semiconducting electroluminescent polymers.

LECs are made of a blend of polymers including semiconducting luminescent polymers and a polyelectrolyte, as well as two metallic electrodes. When the proper voltage is applied to the electrodes, a p- n junction forms in place and the luminescent polymer emits light.

Polymer LECs are said to have many advantages for flat-panel, full-colour displays. The researchers developed a voltage-controlled, two-colour 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 said to be well within the requirements of a flat panel display.

Full-colour representation relies on the combination of three light colours: red, green and blue. To obtain true full colour, the pure red, green and blue must be the exact required wavelengths, and efficient, stable electroluminescent polymer materials must be developed, so that when combined in varying combination, they create all possible colours.

While the researchers have not created the proper red or green, and do not have a blue LEC as yet, they have created one cell that can produce both a yellow and a red. They also have created an LEC that responds in less time than the human eye can notice, satisfying the response time requirement.