[UPDATE] Interviews with Engineers OLED "Super Top Emission"

"XEL-1" --- the world's first* OLED panel TV. OLED panels are noted for being extremely slim (thanks to self-emitting luminescence), and for boasting high contrast and rapid response times. However, Sony's task was to develop a technology that would add the key feature of stunningly advanced picture quality. "Super Top Emission" proved to be that technology. How did this technology come about? We took the opportunity to talk to its developer.
* As of October 1, 2007, based on Sony research.

The development of OLED was a series of challenges and discoveries. When problems emerged, there were no precedents, and everything depended on the efforts of the development team. Ultimately, the beauty and advanced potential of OLED was what led me to strive for commercialization of this technology.

The main merits of OLED are:

The panel is self luminescent, offering a wide viewing angle and high contrast.No backlight is necessary---making it possible to create panels far slimmer than LCD-supported panels.Rapid response time means no motion blur.

Top emission and bottom emission are the two main light emission methods for OLED and each is based on a different structure. In the bottom emission method, the structure causes light, generated by the organic material, to travel downward toward the TFT backplane substrate. However, the presence of opaque pixel driver circuits on the backplane partially block the light. By contrast, with the top emission method (the method used by Sony), the structure causes light to travel to the "top" side with no interference from the TFT circuits. When Sony launched full-scale OLED development in 1999, bottom emission was the dominant method due to its comparatively easier manufacturing process. Manufacturing for top emission had proved difficult, and research had not advanced.

Furthermore, the dominant type of OLED display at the time was the passive matrix type. Here, the electrodes which sandwich the film of organic material are strips, with the anode strips arranged perpendicular to the cathode strips. In the passive matrix type, voltage is applied to each strip, causing the organic material to react by emitting light. Thus, this type of OLED display was not suited to achieving the kind of high resolution exhibited by today's TVs. To realize a viable OLED TV, it was necessary to develop active matrix types, where each pixel is supported by an individual TFT circuit for emitting light.

From the outset, Sony was intent on developing OLED panels to be used in TVs. If these TVs were to carry the Sony logo, they would have to be of higher resolution and more advanced in terms of picture quality compared to conventional displays. The decision was therefore made to pursue development of the more difficult active matrix type via the top emission method.

Our first challenges centered on selecting raw materials for the cathode and anode electrodes, and establishing a production process. This was a tale of trial and error, but we finally confirmed the best materials and production process.

The next step was developing technology to enhance color generation. OLED panels at the time showed changes in color if viewed from an angle. Green turned to orange when viewed from one angle and to yellow when viewed from another. This was due to the "microcavity structure." Resonance occurred within the light-emitting part of the OLED, and the wavelength of the selected light would change. This in turn led to a change in color perceived by the human eye.

With the microcavity structure, colors easily change based one's angle of vision. For this reason, the microcavity structure was developed for projector use rather than "direct-view" TVs. To determine whether the microcavity structure could be used for OLED TVs, I made my own simulation software program and continued testing. Ultimately, we found an optimal device structure where color did not change even if the angle of vision was altered.

Many challenges remained. Our struggles to achieve high contrast were being thwarted by ambient light reflected by highly reflective film on the bottom of the OLED panel. Reflection of ambient light turned a once beautifully deep black to a much lighter hue. At the time, a circular polarizer was used to prevent the reflection of ambient light. Although this was effective, it also reduced the amount of light emitted from the OLED by about 40%. I found that a combination of the microcavity structure and color filters proved highly effective in reducing ambient light. The microcavity structure reduced the reflective nature of the generated color, while the color filters suppressed other forms of reflectivity. However, initially many engineers were doubtful that color filters would be as effective in OLED as they had been in LCDs in terms of color reproduction. While LCDs utilize backlights, OLED panels are self-luminescent which raised questions as to whether filters could be effectively utilized. Nevertheless, when we switched on a panel fitted with built-in color filters, we saw a dramatic improvement in contrast. In addition, the panel showed a two-fold increase in brightness compared to panels using circular polarizer. This was the start of Sony's unique Super Top Emission method.

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