OLED Displays
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Organic Light Emitting Diode (OLED) technology was invented in the 1980's. There were two types that emerged. One was Eastman Kodak's molecular type (or thin film) and the other was the polymer type (or thick film). The thick film technology was mostly developed at the University of Cambridge which spawned Cambridge Display Technologies (CDT).
The key point to understand about OLEDs is that just like an LCD, they must be used in conjunction with an active matrix array except for the very simplest character type displays. To get sufficient current to drive an OLED array, one must use a low temperature polysilicon process. OLEDs are not very efficient, so when multiplexed, they have to be driven with relatively high currents to get an adequate amount of light.
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Positive OLED Characteristics:
- Since it is a light emitter, it creates light that is Lambertian so it can be seen uniformly at all angles and gives a very pleasing effect.
- The biggest strength of OLEDs is that they do not require a backlight and can be made thinner than any other technology used today. A 2 mm thick OLED is a reality today where the thinnest LCD is 3 mm.
Negative OLED Characteristics:
- Dynamic display efficiency. While you can write a few lines of static text with great efficiency, video requires more power than an LCD. OLEDs are more efficient for small graphics or text because they only consume power in the area where they are addressed.
- To date, the reliability has not come up to the levels of LCDs.
- It is particularly difficult to drive the blue colors where the luminance efficiency is very low. As a consequence, the lifetime is reduced, and burn-in is also an issue.
To make a graphics OLED, one must use a low temperature polysilicon process - so that it becomes more expensive than LCDs at large sizes. Small size OLEDs have been manufactured by Sanyo partnering with Kodak to make very simple consumer devices. Although larger (15") XGA panels have been demonstrated, the manufacturing processes using CMOS process Low Temperature Polysilicon (LTPS) has been demonstrated and some products are appearing.
OLEDs are an interesting technology and one to be watched carefully, but it has limitations in terms of market. They work best at low temperatures and are prone to failure at higher temperatures. Since you have to control burn-in effects, one must to be careful about the driving levels. At the light levels required in high ambient light, there is considerable risk of burn-in. In general, they are not used in highly mobile applications, and it is unlikely that we will see these devices in automobiles and PDAs and other applications that are better matched to transflective LCDs, because of the expectations of operability in all lighting ambients.
Occasionally, you will see articles talking about flexible OLEDs. For OLEDs that are not highly multiplexed such as simple character modules, it is possible to make a display that does not require a polysilicon or CG-silicon backplane. To date, graphics OLEDs await plastic LTPS or CG-Silicon to enable flexibility, just as is the case for LCDs. It then becomes possible (especially for thick film devices) to build an LCD directly on a plastic material.
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