News and Reviews....

    Waiting For OLEDs    June 2004

    The new buzz in displays is not LCD or Plasma but OLEDS (See Flexible Displays July 2002). OLEDS stands for Organic Light Emitting Diode. It is a display technique that comes from the large digital displays seen at venues and Times Square made up of LEDs. What makes a Light Emitting Diode (LED) Organic is that conventional LEDs, used in everything from toys to tail lights are made from semiconducting elements and dopants on the periodic table such as silicon, indium aluminum, and phosphorous, but OLEDS are built from organic materials such as hydrogen, oxygen, carbon, and nitrogen that are formed into monomers and polymers. OLEDs roughly simulate the semiconducting effect of their more popular inorganic counterparts but use an entirely different charge-transfer mechanism.

    What is interesting is that forecasters predicted by now that OLEDs would have pushed the Liquid Crystal Displays (LCD) into obscurity. That was the prediction in late 1990 when LCDs were hampered by high cost, not so great performance, and OLEDs were touted as ready to take over. What really happened was that the hype and actual lab results met and did not coincide. OLEDs were not ready for market and so in the next three to five years LCD corrected their problems and manufacturing geared up to production. 

    Compared to LCDs, OLEDs produce richer colors, have a faster response time and wider viewing angle. Where LCDs sandwich a liquid between two plates and require a complex system of polarizers and color filters, OLEDs are entirely solid state, thinner and lighter. They need fewer parts and process steps to manufacture, making them potentially less costly (25% to 50% less) to manufacture. In addition, there is the prospect of being able to deposit OLED material on flexible plastic rather then glass, raising the possibility of having displays that are curved or flexible, or that can even be rolled up.

    Researchers continue to make good progress, although there are still barriers to scale. Finding the right materials to achieve all the necessary qualities - efficiency, long life, high brightness, and color purity and melding them into a high yield manufacturing technique. Researchers have cut two main paths to the fabrication of OLEDs. One uses so-called small molecule materials, the other uses polymer materials. Of these, the small molecule approach starts as a powder and is typically deposited on a glass or silicon substrate using a vacuum deposition process. Polymer materials, in contrast, start as a liquid and are deposited on a glass substrate by liquid processes such as spin-coating.          

    Right now Eastman Kodak has managed to make a blue-emitting material worthy of commercial production. Dow began marketing its Lumation Green, Red, Blue, and White light-emitting polymers of which Green has been designed into a palm-size MP3 player from Go Dot Technologies. But the lifetime for Blue from Dow is well below what is needed for mainstream displays. OLEDs have appeared in other products to date, what is needed is for researchers to find more materials that emit high efficiencies of red, green and blue.

    Even with these high-efficiency materials manufactures must employ an array of active-matrix thin film transistors to drive the pixels in a large display, as is done for LCDs. But much of the thin film manufacturing equipment is owned by, and their technology patented by, the LCD makers, many of whom, as mentioned are more interested in selling LCDs then pursuing an upstart technology.

    Another factor is longevity. Although opinions and requirements vary, for TV and computer displays, a 25,000 to 50,000 hour lifetime is widely expected. Cell phone displays are expected to last about 10,000 hours. Today, while LCDs easily last 30,000 to 50,000 hours, which is when the backlight burns out, most OLEDs are good for 5,000 to around 10,000 hours (researchers at Covion have reported lifetimes of more then 20,000 hours using extrapolated data), depending on brightness.

    OLEDs will be the TV of the future, just when it will be available to the general public, is not predictable at this time. Polymer OLEDs are seen as holding the greatest hope for ink-jet printing onto plastic substrate for producing flexible displays.