Organic Light Emitting Diodes (OLED)
Function in Today’s Displays
Eng 314, 1:25 pm MWF
October, 4 2010 (keep this date in mind, it might be outdated for some things)
The use of Organic Light Emitting Diodes (OLED) in displays is a new developing field which is described and analyzed for its advantages and disadvantages over previous technologies.Introduction:
Organic Light Emitting Diodes, also known as Organic LED or OLED, uses organic polymers that when excited by electricity will produce light. LED’s, the predecessor of OLED’s, work in the same way except they use a large amount of advanced semiconductor material which takes up a lot of space and is quite expensive. You can see these LED’s in stores being sold as an alternative light source over the incandescent light bulbs, costing ten to fifteen times as much. Because of the low pixel density, due to their large size, and high manufacturing cost LED’s were never integrated into a displays.
With the advancements in organic polymers they can now replace that large expensive semiconductor with organic polymers. These polymers are cheaply made in mass quantities and can be easily printed on a fifty inch surface in less than two minutes. (Katherine) Since OLED’s can be cheaply made in mass quantities and have a good pixel density from their small size they are poised to take over the screen market. Currently the majority of screens use Liquid Cristal Displays (LCD) which has over nine main components vs OLED’s four. LCD’s complex design makes the manufacturing process quite expensive when compared to OLED and with the technology behind LCD’s already at the pinnacle of its development it still cannot compete with the potential performance that OLED’s have. (4dsystems)How OLEDs Work:
Fig: 1 (Craig) Fig: 2 (Craig)
OLEDs are comprised of 4 main parts, the Cathode, Anode, Organic layers, and the Substrate. The Cathode and Anode are both transparent conductors that the power supply connects to which are aligned in rows perpendicular to each other whose intersection determines what section, or pixel, of the OLED is turned on, or emits light. Then there is the Substrate, this is what the OLED is built on top of and is usually reflective so that light will only be emitted from one side, making the OLED seem brighter, it could also be transparent for some special applications. Last is the Organic Layers which is composed of two parts, the Conductive and Emissive Layers, this is where the light is made. This process starts when the power supply applies current, the flow of electrons, to a selected pixel or section of the Organic Layers. This current is then forced to flow through the Organic Layers producing light, for a more detailed explanation read the figure 2.Applications:
Since these OLED’s are small you can build up a large array of them to produce a display that could be used for many different applications, such as a cell phone, TV, or a computer monitor. The fundamental principle by which any screen today can display an image is by putting three different colors, red, green, and blue, closely together side by side, the distance between them can vary significantly depending on the application. This will trick the human eye into seeing those three different colors as one solid color when they look at it from a distance. Then by varying the intensity of each of those three colors you can reproduce the entire color spectrum, these three colors together are called a pixel. You can do this same thing with OLED’s but since OLED’s are thin and can be made transparent you could stack all three colors on top of each other allowing for an even higher pixel density and resolution for pictures to be displayed for an even more realistic look.
“OLEDs on the market today rely on an expensive, small-scale technique called shadow-mask evaporation to pattern the light-emitting organic molecules that make up the pixels in these displays.” (Katherine) These displays where initially small, two to three inches in diameter with resolutions under 176x220 pixels in 2006, but over the past 4 years they were able to scale up the shadow-masking process for LG’s 15” monitor with 1366x768 pixels in 2010 (OLED-Info). This display will retail for $2,725, which is unable to compete with the cheap LCD’s on the market at the time. In May 2010 DuPont introduced a new process that could print, just link an inkjet printer, a 50” substrate in less than two minutes by using active molecules inks. This process retains the reliability and performance that shadow-masking offers while also allowing for the possibility of cheap large scale mass production. (Katherine)
Once you are able to cheaply manufacture OLED devices LCDs will no longer be the main choice for a display. OLED’s have many areas in which they outperform LCD’s, such as lower complexity, thinner design, faster refresh rate, lower power consumption, higher brightness, higher contrast ratio, and wider viewing angle. Since OLED’s only have four main parts in there screen while LCD has over nine main parts their displays are significantly thinner, from 0.3mm to 5mm vs LCD’s 7mm and up. OLED’s can also refresh the image more than ten times as fast as LCD’s can, meaning an image with a lot of movement will look sharper on an OLED while the LCD would look blurred. Also with this extremely high refresh of about 600Hz it can easily support the new 3D imaging technique that needs a 120Hz or faster display. (4dsystems)
Power consumption on an OLED display is a catch twenty two. OLED power consumption is directly related to how white the image it is displaying, the whiter the image the higher the power consumption. On a solid black screen OLED’s will consume less than 10% that of an LCD, on an average picture it is about 65% less, but on solid white it can consume over three times as much. To capitalize on this feature of OLED displays the software developers change the graphical user interface to use more black backgrounds with lighter text and lowered the brightness of white to a lighter shade of gray. With these simple software design changes the OLED display will consume on average 50% of an LCD, when displaying the same image. (4dsystems)
Fig: 3 (4dsystems)
Since in an OLED display each pixel emits its own light, unlike in an LCD where the light is emitted from backlight over the entire display which makes the blacks look grey, the contrast ratio of the image is significantly higher, contrast ratio defined as the brightness of white over the brightness of black. When looking at the picture to the left this is should be easily noticed. Along with a high contrast ratio OLED also have an extremely large viewing angle when compared to LCD’s.
Fig: 4 (4dsystems)
Figure 4 charts the contrast ratio, luminance, and color gamut, how well defined the colors are from each other, of an OLED display against an LCD with the same image. From looking at the data presented one should notice that throughout the 60° swing the OLED’s numbers dropped very little where as the LCD’s dropped substantially and quite rapidly over just 60°. This advantage in viewing angle makes OLED a prime candidate for billboards and banners, where people can view them from almost any angle and still manage to make out what is on the display. (4dsystems)
Currently, OLEDs are being made on a glass substrate and are then encapsulated with metal or glass lids to seal the device from the elements. By replacing this glass substrate with a plastic substrate it becomes thinner and lighter and by capitalizing on the flexible nature of the plastic substrate you now have a flexible display. (Sugimoto) These flexible displays are very thin, only 0.3mm to 0.6mm thick, allowing them to be very durable and reliable in devices that see a lot of vibrations or flexing, like a laptop monitor. These flexible devices however are not as reliable as the nonflexible OLEDs because of the plastics poor sealing properties. The organic polymers inside the OLED will start to decay reducing the life span of the device If it is not properly sealed. (Sugimoto)Current Limitations:
OLEDs although having many advantages over other previous technologies they are not perfect and do have some pitfalls of their own. The major and most pressing of these limitations is its high sensitivity to oxygen and water, which will work together to decompose the organic elements making the OLED device useless. To compensate for this the developers make sure to seal the OLEDs between two impermeable layers, usually glass or metal. Plastic substrates that are used in flexible displays are not impermeable and will allow moister and oxygen to get into the organic layers deteriorating it over time. There are some preventive measures that can be taken to help prevent this, adding a moister barrier film and then a surface smoothing film. The barrier film is added to the plastic substrate as a shield to protect from moister and oxygen penetration. This surface however is not perfect and has lots of small pits in it. These pits allow moister to settle in them allowing the moister to etch its way through the protective film. To prevent this a second layer is added that makes the surface smooth, without little pits, thus preventing moister collection. With the combination of these two protective films you can increase the lifespan of the flexible OLED to about that of an OLED built off a glass substrate. (Sugimoto)
Another large problem is the lifespan of the blue light emitting diode, which is significant shorter than the red and green colors. This large difference in color lifespan means that as the display gets older the blue OLED will fading out faster than the other two colors, making your image have a noticeable color shift, missing blue. There are currently workarounds for this such as making the blue OLED larger than the other two colors and program in an expected decay rate while slowly increase the intensity of blue over time. This quick fix helps masks the issue making it less noticeable but it does not address the real problem. Researchers are currently working on new polymers and techniques that could improve the lifespan on the blue OLED to that of the red and green but are currently unable to do so. (OLED-Info)
The last pressing issue with OLEDs is its lack luster performance when exposed to direct sun light. Since OLEDs emit their own light they can be bright indoors but when viewed under direct sunlight they become difficult to read. The direct sun light drowns out the light that is being emitted from the OLED as the sun light enters the OLED and gets reflected back to the user from the reflective backside. That reflective film used on the backside of the OLED to help improve brightness is now hurting it. To prevent this you could put an anti-reflective coating or foil on the back of the OLED, but in doing so you will cut the brightness of the OLED by 40% in areas that do not have direct sun light. You could compensate for this by increasing the brightness but most electronic devices that see direct sun light are mobile devices that run off batteries. There has yet to be any development in fixing this issue but, as always, companies are researching into ways to improve this weakness. (OLED-Info)Conclusion:
OLEDs though they do have some pitfalls when it comes to use in outdoor applications they still have high potential in indoor applications, such as TV’s and monitors. These device have too many advantages over today’s LCD technology for it not to replace them once the techniques behind manufacturing are perfected and some kind of solution is worked out for blue OLED issue. Given a few more years of research and field testing we should see OLED displays become reasonably priced for the average consumer in many shapes, forms, and sizes.Works Cited:
Sugimoto, Akira et al. “Flexible OLED Displays Using Plastic Substrates.” Quantum Electronics
IEEE. Web. 25 Sep. 2010.
Craig, Freudenrich. “How OLEDs Work.” howstuffworks.com. 2005. 2-3. Web. 25 Sep. 2010.
“Introduction to OLED Displays Design Guide for Active Matrix OLED (AMOLED) Displays.” 4dsystems.com.au. n.d. n.p. Web. 20 Sep. 2010.
Katherine, Bourzac. “New Inks Could Mean Cheaper OLED Screens.” technologyreview.com. 17 May 2010. n.p. Web. 20 Sep. 2010.
“OLED TV.” Oled-info.com. n.d. n.p. Web. 20 Sep. 2010.