Seiko-Epson and CDT Develop LEP Display

Seiko-Epson and Cambridge Display Technology (CDT) have developed a full colour display using CDT’s light emitting polymer (LEP) technology.

The prototype colour display has been made using CDT’s red, green and blue polymer materials and an industry first ink-jet printing process developed for the project.

The colour display achieves colour quality equal to current liquid crystal display (LCD) technology and is comparable to displays found in many portable computer products.

This latest development follows an initial announcement in 1998 when CDT and Seiko-Epson demonstrated a monochrome plastic television display based on LEP technology and announced their intention to develop a colour LEP display.

The prototype colour display measures 2.5 inch square, has a resolution of 200 by 150 pixels, with 16 grey scaling at system level and will be targeted at initial market entry points for LEP displays products such as mobile phones and personal digital assistants (PDAs). Beyond this, CDT and Seiko-Epson expect this technology eventually to penetrate all other display markets.

Based on an innovative manufacturing technique which uses ink-jet printing to deposit individual pixels made up of the red, green and blue LEP materials directly onto the substrate, potential display size is limited only by the size of the available wafer with no impact on the overall throughput when deployed in existing manufacturing lines.

“The pre-production colour light emitting polymer display being shown by CDT and Seiko-Epson has a colour density similar to current LCDs. The techniques being jointly developed by the two companies means that the manufacturing cost of an LEP display will be significantly less than the cost of producing conventional LCD or cathode ray tube displays,” said Dr. Shimoda, general manager of basic research, Seiko-Epson.

“CDT is very excited about the market opportunity for a full colour light emitting polymer display. The market needs a video-capable display for emerging mobile applications that is light-emissive, colour and low-power. CDT and Seiko-Epson are working towards the commercial availability of a product with precisely these characteristics. Over time we expect this technology eventually to penetrate all other display markets,” said Dr Daniel McCaughan, president and chief operating officer (COO), CDT.

Manufacturing Process

The ink-jet technique allows the LEP material to be printed from a liquid solution and brings advantages when compared to existing and emerging display manufacturing processes by eliminating the need for backlights, colour filters and polarisers used in LCD displays and complex multi-shadowing techniques for depositing small molecules.

The structure of the display consists of two polymer layers, a conducting polymer layer covering all the pixels and a light emitting colour polymer layer where each pixel consists of a third of each of the red, green and blue colour LEP materials.

Based on a poly-silicon active matrix driver the colour display uses a digital drive scheme based on time and area variables addressing each pixel separately. This is in contrast to the Thin Film Transistor (TFT) analogue drive system of the previous monochrome display, which was more susceptible to the variance of the TFT threshold voltage.