HP researchers show prototype of printed, plastic, color display

HP Laboratories Bristol has developed a prototype of a display that is bistable, color, plastic and is made by imprinting and lamination processes that eliminate the expensive vacuum deposition and photolithography used to make today’s flat panels.

The 3cm x 4cm, 128 x 96 x RGB prototype liquid crystal display (click to see prototype) does not require an active matrix and can display 125 colors. The technologies used to create the prototype are at an early stage, but are designed to scale to paper-like resolutions over large areas so that future products can affordably deliver full-color, print quality from a low-cost printed display.

The development is targeted at applications such as electronic books and magazines and digital posters and photographs, rather than video displays such as TVs and computer monitors.

Many more pixels for ‘printed’ material

Current display technologies have price and performance points that suit them to high-value products such as notebook computers, televisions and mobile products including PDAs, mobile phones and digital cameras.

The primary focus for these products has been to create displays that are video-enabled, and which usually have fewer than 2 megapixels. In contrast, current printers support fine detail and crisp edges using 20 to 40 times the number of pixels per unit area compared with the average computer monitor and can deliver this resolution over large areas.

The prototype HP Labs display is a bistable passive matrix, which means that displays with as many pixels as desired can be built. Most of today’s LCDs have an active matrix -- a TFT (Thin Film Transistor) embedded in each pixel to keep it turned on between periodic updates. In the HP Labs prototype the pixel stays on or off without this help, remembering its state for as long as required.

Paper-like display technologies are beginning to emerge, but so far many developments still need active matrices and have focused on reflective, black-and-white displays.

“More and more content is produced in color. New display technologies cannot afford to ignore that if they are ever to compete with books, art, magazines and posters,” said Adrian Geisow, manager of Displays Research, HP Laboratories, Bristol.

Low cost, large area manufacturing

Current flat screen displays are made on glass by processes very similar to those used for making silicon chips. As displays get larger, the factories to make them have become extremely expensive.

The new HP Labs prototype has been created out of plastic, which is easier to handle than large sheets of glass, and which reduces the weight and thickness of the display. However, it is more difficult to make good TFTs on plastic, so eliminating them avoids this problem and the costs associated with the complex manufacturing processes involved.

In the HP Labs prototype, the ability of the pixel to remember its state is produced by tiny posts less than a thousandth of a millimeter wide, which are imprinted on to the plastic. These posts hold the liquid crystal in one of two orientations, corresponding to ‘on’ and ‘off’.

The display also has electrodes that are integrated with the printed color filters, further simplifying the device. The electrodes and color filters are made by imprinting shapes on to the plastic, and then using the shapes as templates for the color filter and electrode materials. This gives very precise control of features – such as metal lines five microns wide.

“All of the patterning in the prototype has been carried out by printing-like processes,” said Geisow. “The details of the processes are still being developed, and we expect it will take a few more years of further applied research to properly develop and assess their commercial potential."

Given HP’s position in the printing and imaging business, and the volume of display products that the company sells, "any development that can deliver thin, light, and attractive displays at low cost is of great interest," he said. "This is particularly so if it can display images that would otherwise need to be printed. We believe that this is an important advance.”