Looking into a Brighter Future

What led to OLED?

In the past couple years, liquid crystal displays (LCDs) have become more prominent in the computer display market. The main advantage of LCDs over the older traditional computer monitors (cathode ray tube displays, CRTs for short) is their form factor and power consumption. CRTs can hog your whole desk and leave you with nothing but 3 inches to work, while LCDs sit nicely in the back, almost blending into the wall. Before LCDs were available in full color, they were originally used in digital watches, calculators, and small desk clocks. As the technology progressed, it became possible to create displays made up of many small dots, and control each dot individually to form a large picture. Today, we see LCDs in many products, including laptop, digital camera, and cell phone displays. However, if you own a digital camera, especially an older one, you are probably familiar with how short the battery lasts when you use the LCD screen instead of the viewfinder. LCDs require a lot of power to operate. This is because the screen itself doesn’t actually emit light, it only changes the color of the light passing through it. In order to see the screen, there needs to be a white light shining through it, as a backlight. It is the backlight that requires so much power to operate. The amount of power is still much less than that required to operate a CRT computer display, but it is a lot for a little battery powered device to handle. Also familiar to many people with digital cameras or laptops is the narrow angle at which you can view the screen and still see accurate colors. This has been one of the major drawbacks to getting a flat panel display for your computer as opposed to sticking with your CRT. Could there be a new display technology that would consume less power and be visible from all angles? Enter OLED.

OLED stands for organic light emitting diode. Displays made of OLEDs have a number of advantages over LCDs and the like. OLEDs emit their own light directly from each dot of the screen, meaning no backlight is required. This takes care of the problem of the viewing angle and means these screens require far less power to operate. Kodak was the first company to research OLED technology, and received the first patent in 1987. OLEDs have made slow progress since then, but are expected to advance much faster in the years to come. At present, Kodak is working with Sanyo as a joint venture called SK Display to develop and manufacture these OLED screens.

How OLEDs Work

Each cell of an organic LED display functions in a similar way to a regular LED. It is made up of a layer of emitting material sandwiched between a p-type and an n-type layer. The two sandwiching layers are very similar to the p- and n-type layers in transistors and diodes. The n-type layer is doped with a material that has an excess of electrons, while p-type layer is doped with a material with extra “holes” in the energy levels. When power is applied to the diode, the extra electrons in the n-type layer are repelled away from the electrons coming in from the negative side of the battery. They flow through the layer of emitting material where they combine with a hole from the p-type side. The electron drops to a lower energy state, giving off a photon. By making the emission layer with different compounds, the frequency of the emitted photon can be controlled, changing the color of the light. Three OLEDs, red, green and blue, are combined to make one dot on the screen. By controlling the brightness of each color, any color in the spectrum can be produced.

Fig. 1 One cell of an OLED display

Because the OLED itself emits light, it only uses power when it is lit. This makes it much more efficient than LCD displays which have a backlight that is always on.

An OLED differs from a regular LED and other semiconductors in that it does not have to be made with compounds in a crystalline structure. Instead, it is made of carbon-based molecules. This means it is much easier and cheaper to manufacture, one of its greatest advantages over the LCD screens.

Displays

In order to use these OLEDs to create a picture, they must be arranged in a grid, spaced very close together. There are a number of ways of doing this. The simplest screen uses only one color of OLED. There is one cell per pixel on the screen. This type of display can be used for car radios, where full color is not needed. In order to get a full color display, capable of showing photos and videos, you can place red, green and blue cells very close next to each other. All three combined create one pixel of the image. By varying the brightness of each individual color, it is possible to create any color. When you want to show an image on the screen, you start with the top row. The top row is “activated” by applying a voltage, and then you can input which pixels you want to turn on by applying voltages to the appropriate columns. The top row is deactivated, and the second row goes next. Eventually, you reach the bottom row and start again at the top.

Fig. 2 Addressing a passive-matrix OLED display

The reason you don’t see the screen drawing itself in rows is because it all happens so quickly. The entire screen is scanned approximately 60 times per second. This type of display is called a passive-matrix display. On an active-matrix display on the other hand, each pixel can be addressed individually at all times. This means during one screen refresh, each pixel is on the whole time. This produces super sharp, full-motion video. An even more space-efficient method is to stack the red, green and blue cells on top of each other. Since they can be made transparent, this is not a problem. When the colors are stacked instead of next to each other, this means each pixel can be placed much closer together, allowing for much higher resolution displays than our current LCD panels.

The unique thing about OLED displays in contrast with LCD panels is that LCDs must be made of crystalline material, while OLEDs can be made with non-crystalline material, making OLEDs much cheaper and easier to produce. The production of LCDs is more like the production of semi-conductor chips. While this is relatively cheap and common, the manufacturing process for OLEDs is more like ink-jet printing, spraying ink down on a surface. This process is far less complicated, and is faster and cheaper than semiconductor manufacturing. OLEDs can be printed onto virtually any medium. This includes metal foil, paper, fabric, or even clear plastic. Because of this, the screens can be flexible.

Fig. 3 A flexible OLED display from Universal Display Corp.

The screens are also theoretically not limited to any particular size given their manufacturing process and addressing technique. This opens up the possibility to make large computer displays or even larger displays for events such as trade shows.

Today and Tomorrow

Currently, there are only a few products on the market that integrate OLED displays. Kodak and Sanyo are the leading developers of OLED technology. Kodak holds patents for many things pertaining to OLEDs, from the basic idea to manufacturing techniques. They are licensing the rights to several companies now, who are all actively pursuing this technology.

The first product on the retail market to use an OLED screen was Pioneer’s car radio. It used a one-color (blue) passive-matrix screen, with color modulators to get 2 more colors.

Fig. 4 Pioneer’s car radio that uses an OLED display

Soon after, Motorola followed with their Timeport mobile phone. The first active-matrix OLED display was the Kodak EasyShare LS633 digital camera. The display provides a 2.2 inch screen that is visible from virtually all angles in addition to being visible in the direct sunlight.

Fig. 5 Kodak’s EasyShare LS633 digital camera with a large 2.2-inch OLED display

There are many products currently under development by Kodak, Sanyo, Pioneer, Sony, Philips, DuPont, and Toshiba among others, that won’t be available to the public for a few more years. Some of the more commonplace ones are laptop display panels and handheld device screens. Also in development by Vodafone, is a “Visual Bracelet,” [6] part of their vision of the future. The bracelet is a display for a wearable computer, which uses a flexible OLED screen. The bracelet would use an active-matrix display, and be capable of displaying video. An application of OLED screens not too far from the future is a real-time bus schedule that can be wrapped around the poles at a bus stop. The display would be weatherproof, and contain live data about where each bus is, so you know exactly how long it will take to reach you. Taking advantage of the fact that these screens are flexible and can be transparent, it would be possible to make an electronic newspaper. It would be the size of a regular newspaper, and just as thin. You would be able to interact with the paper, touching links on news stories to video clips, all while drinking your morning cup of coffee at the table. After you had gone off to work, you’d have your pen-size eStick in your pocket. During a break you’d pull it out, unroll the screen, and read a chapter out of your favorite novel. Then you’d check your stock prices, and maybe check the live video feeds on your home security cameras. Back at home, you’d turn on the light, and instead of an energy-guzzling incandescent light bulb, or a buzzing fluorescent light reminiscent of the office, you’d have an OLED panel casting bright natural light in whatever color of light you wanted. Instead of wallpaper, you would have a giant OLED panel, so that you could change your “wallpaper” whenever you wanted. Or, you could mount video cameras on the outside of your house, and feed the video into the OLED wallpaper. If you wanted to watch TV, you could draw a TV screen on the wall, however big you wanted. You could move it around, change its size, and have as many TVs as you wanted. All thanks to the OLED technology. But we shouldn’t expect to see products like these for another good 20-30 years. At the moment, OLEDs are an emerging technology, but backed by several stable companies. We should, however, be seeing some more consumer products such as digital cameras, cell phones and radios come out in the next few years.

References

1. “Better Displays with Organic Films,” Webster E. Howard, Scientific American, Vol. 290, No. 2, pp. 76-81, Feb. 2004
2. “A Bright Future for Displays,” Bob Johnstone, Technology Review, Vol. 104, No. 3, pp. 80-85, April 2001
3. “The Silicon Web: The Physics Behind the Internet,” Michael G. Raymer, University of Oregon, 2002
4. “Display Products,” http://www.kodak.com/US/en/corp/display/
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6. “Organic Semiconductor World,” http://www.orgworld.de
7. “Kodak, Kyocera Digicams face off,” http://www.pcworld.com/news/article/0,aid,109682,00.asp
8. “Trio teams up for bendable screens,” http://www.zdnet.com.au/reviews/hardware/peripherals/0,39023417,20269493,00.htm
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13. “Kodak LS633 First with OLED Display,” http://www.dpreview.com/news/0303/03030216kodakls633.asp
14. “Vodafone Future Site,” http://www.vodafone.com/flash/futures/