AMOLED Active Matrix Organic Light Emitting Diode

- details about the AMOLED, active matrix organic light emitting diode, its technology, operation and use.

The AMOLED or Active Matrix Organic Light Emitting Diode, has many similarities to the PMOLD.

However the AMOLED incorporates more elements of the drive circuitry, making it easier to address individual elements as well as providing more flexibility and a greater level of overall performance and capability.

As a result the AMOLED display is being used more widely and in more exacting applications where higher levels of performance are required.

AMOLED basics

The AMOLED provides a considerably greater degree of flexibility and control when compared to its passive relation.

The AMOLED display consists of a matrix of OLED pixels, each having an anode, cathode and a layer of organic material between them. These pixels are activated by a thin film transistor array which controls the current to each pixel, enabling it to be activated and when current flows through it, light is generated.

Typically two transistors are used for each pixel - one to turn the charge to the pixel on and off, and a second to provide the constant current. This eliminates the need for the very high currents required for passive matrix OLED operation

TFT backplane technology is an essential element for AMOLED display fabrication. Two primary TFT backplane technologies, namely polycrystalline silicon (poly-Si) and amorphous silicon (a-Si), are used today in AMOLEDs. These technologies offer the potential for fabricating the active matrix backplanes at low temperatures (below 150C) directly onto flexible plastic substrates for producing flexible AMOLED displays.

The TFT array uses a very small amount of energy, yet unlike an LCD, it is able to refresh very fast. This means that the AMOLED display is very suited to television and other displays where moving graphics are to be seen.

AMOLED life expectancy

One of the chief issues with AMOLED displays is the lifetime. Although much has been achieved to improve the displays, the first ones to be introduced commercially only had lifetimes of around 15 000 hours.

Figures of this order are quite acceptable for mobile phones which are replaced fairly regularly and the display may only be on for relatively short periods, but for televisions or computer monitors that have much higher usage rates, the half life becomes an issue.

AMOLED applications

The AMOLED has a number of advantages over its passive relation. This means that AMOLED displays can be used in many more areas.

The first commercially available AMOLED display was produced as early as 2003. It was manufactured by SK Display and was used in a number of products including camera and personal media players.

Since then AMOLED displays have been used in a number of televisions. Currently sizes are not as large as those available with LCD or Plasma displays, but in view of the anticipated cost advantages that are likely to be gained from the use of AMOLED displays, much investment is being directed towards the development of AMOLEDs.

As such the major application for AMOLEDs is likely to be within televisions and computers, although the life of the display is currently an issue.

Comparison of Screen technologies

There are a number of contenders for screen technology within the television, computer monitor and other related areas. Although cathode ray tubes are now well out of the picture, developers of equipment have a choice of technologies of which the AMOLED is one.

  • Potentially the lowest cost
  • Consumes lowest power (when backlight of others included)
  • Self emissive
  • Displays wider colour range than LCD
  • No screen burn potential
  • Shorter overall life (red and green half life ~ 10 - 40 k hours, blue ~ 1-k hours
  • Medium cost
  • Lower power consumption than plasma
  • Requires backlight
  • Colour range not good
  • No screen burn potential
  • Backlight bulb typically requires replace at around 60 k hours
  • Highest cost
  • Highest power consumption
  • Requires backlight
  • Displays a very deep black
  • Screen burn potential
  • Half life ~ 60 k hours

By Ian Poole

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