Pyramid pixels promise sharp picturesBy Kimberly Patch, Technology Research News
Pyramids may be the key to sharper, cheaper electronic displays.
The color flatscreens used in electronic devices like laptops, cellphones and miniature television screens are made up of many tiny red, green and blue light emitting diodes (LED's) that produce the tiny dots, or pixels of light that make up the picture. One focus of flatscreen research has been cramming more pixels on the screen, because this makes for a higher resolution picture.
Researchers from the University of California at Los Angeles have come up with a different angle on the problem. They have devised a way to coax light from three colored LED's through a single, tiny plastic pyramid. Effectively, the three types of pixels are stacked into one space, tripling resolution in one fell swoop.
"We built the red, green and blue [LED's] in a vertical structure" said Yang Yang, an associate professor at UCLA. "They mix the light to give you any color that you want, [and] they do not take the real estate" of separate LED's, he said.
Because the pyramids mix light at the pixel level, a screen made with this technology will continue to produce a range of colors close up. In contrast, taking a magnifying glass to a conventional screen will reveal the separate red, blue, and green dots that give the illusion of many colors.
The pyramid pixel method may also prove cheaper than traditional flatscreens because it does not require shadow masking. Today's LED displays are manufactured using sheets of metal containing many tiny holes to guide the separate dots of red, green and blue organic materials as they are deposited on the screen. "The holes are so small it requires [a] very thin metal sheet for the shadow mask. It's not easy to fabricate a large [shadow mask and] it's not easy to maintain," said Yang.
In practice, the pyramid shape acts like its own shadow mask, shielding the different color LED's from each other.
"Permanent shadow masks have been used ... but not in the way Yang has been using them here," said Mark Thompson, a chemistry professor at the University of Southern California. "I don't know that anybody else has looked at building structures and using those as sort of in situ shadow masks -- using the shadowing of the pyramid," Thompson said. "It's an interesting approach that could have a lot of interesting applications," he added.
Some of the pyramid pixel's potential advantages are also shared by a pixel stacking scheme under development by Universal Display Corp. Thompson contributed to the basic research behind that scheme, which literally stacks red, blue, and green elements like pancakes into one pixel using a standard manufacturing process that includes shadow masking. The stacked pixels emit mixed light that changes color as the the ratio of currents in the three pixels is varied.
Like the pyramids, the stacked approach produces true color pixels that are effectively higher resolution and can be looked at closely without breaking up. The tricky part of the pancake pixel scheme was working out how to connect all the pixel elements, something Yang has not yet reported on, said Thompson.
In theory, the pyramid pixel displays could cost 30 percent less to manufacture then screens that use sheets of metal for shadow masking, Yang said. The manufacturing process for depositing the pyramid pixels has yet to be worked out, but it will be similar to a process used by a type of 3M film, Yang said.
Yang has implemented his scheme in a prototype pyramid about ten times the size needed. The next step is to shrink the prototype down to about 100 microns, he said.
According to Yang, the technology could be ready for practical use in about two years.
The pyramid pixel research was funded by UCLA and by a corporate partner who did not want to be named. Yang Yang's research partner was Shun-Chi Chang, also from UCLA. They published a technical paper on their research in the August 14, 2000 issue of Applied Physics Letters.
The research behind Universal Display's stacked pixel scheme was published in Science June 27, 1997 and Applied Physics Letters, November 11, 1996.
Timeline: 2 years
Funding: Corporate, University
TRN Categories: Semiconductors and Materials
Story Type: News
Related Elements: Photo 1, Photo 2; Technical paper, "Pyramid-Shaped Pixels for Full-Color Organic Emissive Displays," Applied Physics Letters, August 14, 2000; Technical paper "Three Color Tunable Organic Light Emitting Devices," Science, June 27, 1997; Technical paper "Color-tunable organic light-emitting devices," Applied Physics Letters November 11, 1996.
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