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Organic EL Panel: A Great Leap Forward


1 March 2005


The largest organic EL panel in mass production has been developed by Sony. Using super top emission technology, together with the microcavity structure and colour filters, these panels create high-quality images with lower levels of power consumption than conventional panels.


"The Sony EL panel achieves a gamut that is 147% of the CRT gamut, roughly twice that of conventional panels."

While organic EL panels, due to their self-luminescence and fast response times, provide superlative moving characteristics, this panel also features excellent colour reproducibility and high efficiency. Thus the organic EL technology provides more than adequate performance to support future high-quality moving pictures for mobile applications and to also be deployed in larger TV sets.

The panel achieves high brightness (150cd/m²), high contrast (1000:1) and high resolution (151ppi). Furthermore, its colour reproducibility greatly exceeds that of conventional organic EL panels. While the colour gamut of conventional panels had an area only approximately 70% of that of standard CRT monitors, this panel achieves a gamut that is 147% of the CRT gamut, roughly twice that of conventional panels. Taken together, these picture-quality characteristics ensure that the panel delivers stunning images.

At the same time, this panel holds the per unit area of screen power consumption to levels lower than those of conventional panels. This was achieved by using unique Sony-developed device structures.

LIGHT EXTRACTION METHODS

The means by which the light generated by EL is extracted from the TFT substrate side is called bottom emission. Since the EL drive pixel circuits are present on the TFT substrate, the area from which light can be extracted is limited and the efficiency with which the light generated by EL is used is reduced. By way of contrast, in top emission light is extracted from the sealing substrate side and the light generated by EL can be extracted efficiently.

NO CIRCULAR POLARISER REQUIRED

"The panel represents a significant technological upgrade over conventional panels and shows the way for the future of organic EL technology."

In conventional panels, a circular polariser (retardation film and polariser) was installed on the panel surface. While this was provided to prevent the reflection of ambient light, it also reduced the amount of EL light emitted. The panel does not use a circular polariser, but instead uses the microcavity structure and colour filters. At the same time as preventing the reflection of ambient light, the microcavity structure and colour filters achieve an increase in colour purity.

The ability to use colour filters relatively easily is a feature of top emission. In bottom emission panels, the colour filters (OCCF or On-Chip Colour Filter) are formed on the TFT circuit, while the organic film is formed on top of the filters, thus increasing the technical difficulty of manufacturing the panels. It is the combination of top emission, the microcavity structure and the colour filters that allows the panel to achieve its superb picture quality with low power consumption.

FULLY FIXED SEALING

Previously CAN sealing, in which an inert gas is enclosed, was used. More recently, glass substrates that have been carved out from the inside and the edge sections retained have been used for sealing. These are sealing methods that minimise damage to the organic films caused by the sealing process.

In this panel, the problem of damage to the organic films caused by the sealing process has been resolved and a fully fixed structure that can better withstand external mechanical shocks has been created. This has allowed Sony to reduce the thickness of the sealing glass and make the panel even thinner. Furthermore, it paves the way for the creation of ultra-thin panels using plastic film sealing.

Clearly, the panel represents a significant technological upgrade over conventional panels and shows the way for the future of organic EL technology. It is Sony's unique super top emission technology that makes this leap possible.

MICROCAVITY STRUCTURE

"Sony has developed the unique super top emission technology that takes maximum advantage of organic EL technology."

One of the features of this panel is that the thicknesses of the organic films differ for the RGB colours. This is because the film thickness is selected to match the optical path length between the cathode and anode electrodes to the EL spectral peak wavelength for each colour (microcavity structure).

Another feature is that the cathode electrode is a semi-transparent film. The light, whose wavelength matches the optical path length between the electrodes, repeatedly reflects and interferes (multiple reflectance interference) between the cathode semi-transparent film and the (reflective film) anode electrode. As a result, the spectrum of the extracted light is sharpened and the colour purity increased.

THE MICROCAVITY STRUCTURE

The effect of reducing ambient light reflection is provided by the microcavity structure and the colour filters. When the organic film optical path length is matched to the wavelength of the green EL light, the green component of the ambient reflected light is cut.

At the same time, the ambient reflected light with colours other than green is cut by the green filter. In other words, when a colour filter is combined with the microcavity structure, essentially all ambient reflected light can be cut. This allows high contrast to be achieved without the use of a circular polariser, and the same effect applies for green, red and blue operate in the same manner.

COLOUR FILTER COLOUR SELECTION WHITE EL TECHNIQUE

Sony has also developed a microcavity structure colour filter colour selection white EL technique. With this technique, the whole panel is manufactured using an EL film that generates white light emission, and colour filters are used for RGB colour selection. Sony exhibited a 12.5-type prototype panel using this technique at SID 2004.

FUTURE DEVELOPMENTS

Sony has developed the unique super top emission technology that takes maximum advantage of organic EL technology and supports mass production. The company is committed to providing the highest performance organic EL panels. It also aims to maintain a commanding lead in EL panel technology over other manufacturers in this area.