Silicon Chip Online - OLED Displays: Better Than Plasma Or LCD

In 2002, OLED displays began to appear in small consumer appliances like cameras and mobile phones. The superiority of this new technology will ensure that it replaces LCDs in many more applications within the next few years. And that might just be the beginning!

What is an OLED?

Scientists have long known about the electrolumin-escence of organic crystals. Early attempts at generating light with organic electroluminescent (EL) devices were not developed past the experimental stage, as they required high excitation voltages (upwards of 100V) and were very power inefficient.

An important step in the evolutionary process began with the use of thin-film organic layers. The first EL thin-film device used a single organic layer sandwiched between two injecting electrodes (Fig.1).

Operation of these single-layer devices is relatively straightforward. When a voltage is applied across the electrodes, holes are injected from the anode and electrons from the cathode. These carriers migrate through the organic layer until they meet and recombine to form an exciton. Relaxation from the excited to ground states then occurs, causing emission of light.

Single-layer EL devices are impractical because of the extremely accurate matching required between the electrodes and the organic material. Essentially, mismatching results in carriers crossing the structure without combining with an opposite sign, thus wasting energy.

Technology breakthrough

Kodak scientists Ching Tang and Steve Van Slyke demonstrated an efficient, low-voltage OLED for the first time in 1987. Their device used two layers of organic thin-film material.

In the two-layer EL device, one layer is optimised for hole injection and transport while the other is optimised for electron injection & transport. In this way, each sign of charge is blocked at the interface between layers, in effect "waiting" until a partner is found.

Tang and Van Slyke also improved on the composition and construction of the EL cell, resulting in a bright, efficient device that operates on less than 10V.

Due to the monopolar nature of the organic layers, EL devices conduct current in one direction only; in other words, they behave like diodes, hence the common name "OLEDs".

In one and two-layer devices, the organic compounds must perform two major functions. They must be luminescent as well as hole/electron transporters. By incorporating a third organic layer chosen specifically for its luminescent qualities, researchers have been able to further improve efficiencies by optimising each layer for a specific function.

OLED structure

Fig.2 shows the physical structure of an RGB OLED cell. A conductive, transparent anode material such at indium-tin-oxide (ITO) is first deposited on a transparent substrate. Next, the organic layers are added. Lastly, a reflective metal cathode of magnesium-silver alloy or lithium-aluminium completes the structure. Incredibly, the thickness of the structure, minus the substrate, is only about 300nm. This means that most of the total weight and thickness is due to the substrate itself.

OLED types

To date, OLEDs can be divided into two groups, depending on the processes used to apply the thin-film organic layers during manufacture. Small Molecular OLEDs (SMOLEDs) use organic material with very small molecular structures. This allows the layers to be built using sophisticated vacuum vapour deposition.

On the other hand, Polymer OLEDs (Poly-OLEDs) utilise organic polymers, which consist of much larger molecular structures. These are commonly applied with simpler solution processing (spin coating) methods.

Recent advances in chemical-resistant polymers have also enabled traditional photolithography technichques to be brought to bear. Inkjet printing methods have also proved popular due to their high resolution and "on-the-fly" design versatility.

OLEDs in colour

Using fluorescent dopants in the luminescent layers, manufacturers have been able to produce OLEDs in many colours, including the three primaries (red, green & blue).

White OLEDs are realised with the use of dual emitting layers of complementary colours. By individual control of the drive level to each layer, hue can be adjusted from pale yellow to light blue.

OLED displays versus LCDs

Because of their small size and relatively high efficiency, OLEDs are ideally suited for use in flat-panel displays. Liquid crystal display (LCD) technology is the current leader in this area. So how do OLEDs stack up?

As you've probably guessed, OLED displays offer significant advantages over LCDs. Being self-luminous, they require no backlighting. By contrast, LCDs require either an external light source (reflective type) or a fluorescent or LED backlight (transflective type). No backlighting means OLED displays are smaller in size, use less power, weigh less and cost less.

Their self-luminous nature is also responsible for two other important advantages. First, they have a virtually unlimited viewing angle (165°). LCDs, on the other hand, are limited by the "aperture" effect. In addition, they have very high brightness and contrast (>100:1). This is something that LCDs can't hope to match. A backlit LCD typically looks "washed out" under bright light.

Equally importantly, OLED displays have almost instantaneous update speed. The response time of LCDs has always been a problem, particularly when displaying real-time video. The microsecond switching speeds of the OLED has entirely eliminated this issue!

In summary, OLED displays have:

High brightness and contrast
Ultra-wide viewing angle
No backlight required
Thin, compact form factor
Fast response time
Low power consumption

Display types

In common with their LCD counterparts, OLED displays are currently being manufactured in both active and passive types.

Passive-matrix display panels are typically created by depositing the electrode material in a matrix of rows and columns (Fig.3). An OLED is formed at the intersection of each row and column line. Display electronics can illuminate any OLED (pixel) in the array by driving the appropriate row line and column line. A video image is created by sequentially scanning through all rows and columns, briefly switching on the pixels needed to display a particular image. An entire display screen is scanned ("refreshed") in about 1/60 second.

Active-matrix displays use TFT (thin-film transistor) technology. Every OLED cell is controlled by at least two transistors. All transistors in the array are individually addressable in a row/column format. However, unlike the passive-matrix display, the transistor circuits retain the state (on/off) and level (intensity) information programmed by the display electronics. Therefore, the light output of every pixel is controlled continuously, rather than being "pulsed" with high currents just once per refresh cycle.

Active-matrix displays are considerably more expensive than passive displays, but they boast brighter, sharper images and use less power.

Monochrome (single colour) displays are generally of the passive type. Full-colour displays may be either active or passive. Similarly to other display technologies, the full colour spectrum is generated by modulating individual red, blue and green OLED cells positioned side-by-side in a "triad" arrangement.

Universal Display Corp. has recently announced a different architecture for full-colour display. In their Stacked OLED (SOLED), they stack red, green and blue sub-pixels on top of one another instead of next to one another. This provides a three-fold increase in display resolution and enhances picture quality.

Availability

Researchers still have a lot of work to do before OLED displays are ready for the majority of mainstream applications. Of particular concern is the longevity and intensity of the light-emitting layers. In addition, manufacturing methods need to be improved in order to produce high yields at low costs.

Small passive-matrix OLED displays can already be found in many consumer items, such as mobile phones, hand-held games, music systems and in-car instrumentation.

Kodak and Sanyo Electric Co., Ltd., produced the first full-colour 2.4" active-matrix OLED display in 1999. Less than a year later, they produced a larger, 5.5" model, and in 2002 demonstrated a 15" display. Since then, at least one manufacturer has demonstrated a 19" full-colour display.

The first commercially available active-matrix display is to be found in Kodak's new EasyShare LS633 zoom digital camera, available in Australia this year (see photos).

Where to next?

According to some sources, more than 80 companies and universities around the world are involved in OLED research. Clearly, there is a great deal of interest and much potential in this new technology.

For example, several companies have recently demonstrated highly flexible display panels fabricated on plastic substrates. Apart from making panels much more robust, this breakthrough could also allow very cheap mass production, where displays are produced in a roll-to-roll, printed medium style.

Yet another discovery involves the use of non-metallic transparent anodes. Manufacturers will soon be able to make OLED panels that are over 85% transparent (when not active). The applications are mind-boggling!