Organic light emitting diode (OLED) display technology has been grabbing headlines in recent years. Now one form of OLED displays, LIGHT EMITTING POLYMER (LEP) technology is rapidly emerging as a serious candidate for next generation flat panel displays. LEP technology promises thin, light weight emissive displays with low drive voltage, low power consumption, high contrast, wide viewing angle, and fast switching times.One of the main attractions of this technology is the compatibility of this technology with plastic-substrates and with a number of printer based fabrication techniques, which offer the possibility of roll-to-roll processing for cost-effective manufacturing.
LEPs are inexpensive and consume much less power than any other flat panel display. Their thin form and flexibility allows devices to be made in any shape. One interesting application of these displays is electronic paper that can be rolled up like newspaper Cambridge Display Technology, the UK, is betting that its light weight, ultra thin light emitting polymer displays have the right stuff to finally replace the bulky, space consuming and power-hungry cathode ray tubes (CRTs) used in television screens and computer monitors and become the ubiquitous display medium of the 21st century.
Light emitting polymers or polymer based light emitting diodes discovered by Friend et al in 1990 has been found superior than other displays like, liquid crystal displays (LCDs) vacuum fluorescence displays and electro luminescence displays. Though not commercialised yet, these have proved to be a mile stone in the filed of flat panel displays. Research in LEP is underway in Cambridge Display Technology Ltd (CDT), the UK. In the last decade, several other display contenders such as plasma and field emission displays were hailed as the solution to the pervasive display. Like LCD they suited certain niche applications, but failed to meet broad demands of the computer industry.Today the trend is towards the non_crt flat panel displays. As LEDs are inexpensive devices these can be extremely handy in constructing flat panel displays. The idea was to combine the characteristics of a CRT with the performance of an LCD and added design benefits of formability and low power. Cambridge Display Technology Ltd is developing a display medium with exactly these characteristics. The technology uses a light-emitting polymer (LEP) that costs much less to manufacture and run than CRTs because the active material used is plastic.
In active matrix architecture, a thin film polysilicon transistor on the substrate address each pixel individually. Active matrix displays are not limited by current consideration. Seiko-Epson, Tosibha (Tokyo,Japan), and Samsung (Seoul, Korea) have now demonstrated full colour active matrix displays. One exciting possibility is that polymer transistors, which can be manufactured by techniques similar to those used for LEP patterning, could be used to drive an LEP display. Such an approach would potentially lend itself to roll-to-roll processing on flexible substrates.
In order to manufacture the polymer two techniques are used.
Spin coating process
This technique involves spinning a disk, that is glass substrate at a fixed angular velocity and letting a small amount of polymer solution to drop on the top of the disk. It is shown in the figure. Spin coating machine used has a few thousands rotations per minute.
The robot pours the plastic over the rotating plate, which in turn, evenly spreads the polymer on the plate. This results in an extremely fine layer of the polymer having a thickness of 100 nanometers. Once the polymer is evenly spread, it is a\baked in an oven to evaporate any remnant liquid.
Printer based technique
LEPs can be patterned using a wide variety of printing techniques. The most advanced is ink-jet printing (figure). Resolution as high as 360 dpi have been demonstrated, and the approach are scalble to large-screen displays. Printing promises much lower manufacturing cost.
TYPES OF LEPs
The types of LEPs available in the market include flexible, stacked and transparent.
Flexible organic LEPs
They are built on flexible substrates instead of glass substrates. These materials provide the ability to conform, bend or roll a display into any shape. So these find application on helmet face shields, military uniforms, shirtsleeves and automotive windshields.
Stacked organic LEPs
They use pixel architecture and offers high-definition display resolution and true-colour quality for the next generations display applications. With this type, each pixel emits the desired colour and thus is perceived correctly, no matter what size it is and from where it is viewed.
Transparent organic LEPs
The employ an innovative transparent contact to achieve an enhanced display. They can be top, bottom or both top and bottom emitting (transparent). Bi-directional LEPs will provide two independent displays emitting from opposite faces of the display. With portable products shrinking and desired information content expanding, transparent LEPs are a great way to double the display area for the same display size.
LEPs are promising, low cost solutions for today’s flat panel displays. Although not commercialised yet, these replace bulky and heavy CRT displays in the near future. However research is underway to improve the efficiency and lifetime of the polymer displays.
A panel of industry leaders predicted that LEP technology would storm the market in the near few years and we will find LEP in every sphere of life about ten years from now.
LEP technology is now set to change the products we use to view the world.