Dot Matrix Displays
A full-color display requires the patterning of individual red, green, and blue pixels. This is usually a technologically complicated process, but because LEPs can be deposited from solution, patterning using a wide variety of printing techniques is possible. The most advanced technique is ink-jet printing (figure 1). CDT have demonstrated resolutions as high as 360 dpi, and it is likely that much higher resolutions will be obtained in the near future. The inkjet approach is easily scaled to large-screen displays. Printing promises the potential of a much lower manufacturing cost than equivalent LCD or small-molecule OLED technologies. Commercial inkjet printing systems for LEP manufacturing are being developed by CDT subsidiary Litrex Corp. (Pleasanton, CA) and by Seiko-Epson (Suwa, Japan).
Litrex work closely with CDT and have developed an inkjet system that can create droplets as small as 14pL. It is shown in Figure 2.
Pixel displays can either by active or passive matrix. Figure 3 shows the differences between the two matrix types, active displays have transistors so that when a particular pixel is turned on it remains on until it is turned off.
The matrix pixels are accessed sequentially. As a result passive displays are prone to flickering since each pixel only emits light for such a small length of time. Active displays are preferred, however it is technically challenging to incorporate so many transistors into such small a compact area.
Nonetheless, Seiko-Epson, Toshiba (Tokyo, Japan), and Samsung (Seoul, Korea) have succesfully created full-colour active-matrix LEP displays. Research is also being conducted into creating plastic transistors using semi-conducting polymers and applying them with the same inkjet process as the LEPs (visit Plastic Logic). If this is achievable LEP screens would be extremely cheap; need a display? Just print it off!
eMagin, a US display manufacturer have released a kit that contains an 852x600 pixel colour OLED display. This uses rival LEP-type technology developed by Eastman Kodak, but it is fundamentally very similiar to the LEP technology developed by CDT. The exciting thing is that full-colour LEP displays are now availiable, you can read more about it here. LEP technology is widely believed to be more marketable than the small molecule OLED technology developed by Kodak, but as things stand now, OLEDs have longer lifetimes.. according to eMagin.
So, What of the Future?
Here's a recent quote from OEMagazine from an article by CDT employee, Mark Leadbeater:
LEP technology combines simple solid-state construction, low drive voltage, and high-efficiency OLEDs with large-area patternability and the possibility of flexible substrates. This combination of characteristics provides a powerful base for building high-information-content displays ranging from microemissive displays to large-area screens. We expect that LEP technology will initially be used to extend the performance of current displays in mobile communications, computers, and consumer electronics but will quickly lead to new applications. The necessary manufacturing infrastructure and supply chain are under development for this emerging industry, including commercial material systems, production techniques, and capital equipment such as ink-jet printers. Building from the fundamental advantages of polymer chemistry, LEP display technology is now set to significantly change the products we use to view the world.
And how long must you wait until you can get a Sony 64" LEP widescreen television? Best estimates seem to be 2004 for the first commercial products (eMagin only released an industry kit), but I expect that 2004 is a little overenthusiastic. If you want regular new information on this exciting field, I will update the frontpage News section when I can.
last updated: 19-Jun-2002