Sony first became involved in OLED research around 1994. A growing number of organizations had established OLED R&D projects after the publication of a paper in 1987 describing a thin-film OLED device fabricated using vapor deposition. In this sense, Sony was a latecomer to this field. At the time, Trinitron was still Sony’s core technology for display devices. Of course, the Company was also working on the development of next-generation flat-panel display devices and had established parallel projects focusing on various types of devices, including the Plasmatron (plasma addressed liquid crystal) and field emission display (FED) systems.
“Various systems were being tried at that time. It was as if they were in competition with each other. There was extensive debate on which technology would be the winner.”
Not everyone thought that OLED was likely to become a major future display technology, and the development of display devices based on OLED technology did not begin in earnest until 1998. Tetsuo Urabe was a member of the OLED display development team established that year.
Technology had already been developed to create light using OLED. However, Sony wanted to develop an OLED display for TV use. This achievement would necessitate the creation of a screen made up of large numbers of picture elements. Sony decided to use an active matrix system based on thin-film transistor (TFT) technology, which is also used in LCD panels. The consensus view at the time was that it would be very difficult to apply this technology to the development of an OLED display. However, Urabe and his colleagues began to develop an active matrix driver for an OLED-based system.
“There was growing interest in the concept of an OLED system with an active matrix driver. It was seen as a technology for the future. Sony was a latecomer to OLED R&D, but we were among the first to start developing the technology for use as a television display device.”
Successful Development of 13-inch OLED Display in 2001
The first problem in using an active matrix system to drive an OLED display was variation in pixel brightness. This variation results from differences in the characteristics of the TFTs positioned in each pixel.
“In an OLED display, the TFTs drive the luminescence themselves. This means that any variation in TFT characteristics end up as variations in the brightness of individual pixels.”
Since creating TFTs with identical characteristics is virtually impossible, Urabe’s team decided to focus instead on the development of a method to compensate for this. After studying several possible solutions, they decided to use current mirror circuits.
Current mirror circuits consist of two circuits that are mirror images of each other. When a current flows in one of the circuits, the same exact current will flow through the other one. These circuits were attached to neighboring pixels. Provided both pixels in each pair have the same TFT characteristics, there will be no variation in pixel brightness between them. Using this concept, Urabe’s team was able to overcome the brightness variation problem by arranging large numbers of pixels symmetrically. In 2001, they succeeded in developing the world’s first 13-inch active matrix OLED display. At the time, it was the largest in the world.
Challenges in Establishing Mass-production Technology
Sony had developed a 13-inch OLED display, but it was still only a prototype. The first challenge on the path to commercialization would be to extend the life of the product. When first developed, the display was completely useless as a commercial product since its brightness declined dramatically in just two or three days. There were countless additional challenges, including the choice of organic materials and drive system and the method used to stack thin organic layers. The development team also had to consider the structure of the organic layers, and the method used to isolate the materials from the external environment. Urabe and his team solved each of these problems in turn by conducting a massive program of testing and evaluation. The work was so intense that team members sometimes fought over access to larger pieces of testing equipment.
The next challenge was the establishment of production technology. Before OLED products could be launched commercially, Sony needed a production technology able to mass-produce panels without any loss of quality. One of the most difficult tasks was reducing the number of defective pixels. The organic film in an OLED panel is only a few hundred nanometers thick. This extremely thin layer is sandwiched between electrodes, and the presence of even a minute particle of dust can prevent the current from flowing to the organic film, resulting in a dead pixel. To prevent dead pixels, it’s necessary to eliminate dust, so the team began to remove all possible sources of dust from the production line. They also sought to minimize the effects of dust by increasing the thickness of the film as much as possible without compromising its characteristics. Another solution involved the use of lasers to repair any dead pixels discovered after production.
This process culminated in 2004 with the launch of the Courier PEG-VZ90, the first PDA with an OLED panel.
Enhancing Japan’s Competitiveness with OLED
The XEL-1, the world’s first OLED television, was launched on schedule in December 2007. Urabe recalls the supreme happiness he felt at the shipment ceremony at Sony EMCS Corporation’s Inazawa TEC, where the XEL-1 is manufactured.
“People from various departments were involved in the XEL-1 project. They were all at the facility for the shipment ceremony. That was our greatest moment, because all of us felt the satisfaction of having created a new product that was a world’s first.”
This is the first time in 12 years that Sony has won an Okochi Memorial Award. According to Urabe, the real significance of the award will only become apparent when OLED technology matures into a key product category for Sony and a driving force for Japan’s competitiveness.
Sony is still developing OLED technology. Current goals are to create large-screen OLED televisions at a commercially viable cost.
Super Top Emission
Microcavity Structure
In addition to the unique OLED panel structure described above, Sony’s Super Top Emission technology (Fig. 4) utilizes a microcavity structure and color filters to simultaneously enhance color purity, attain higher contrast and achieve lower power consumption.
The microcavity structure utilizes light resonance effects between the two electrodes. Red, Green and Blue all have different light wavelengths. Therefore the thickness of the organic film corresponding to each color is adjusted to produce the spectral peak wavelength (the optimum light) for each color. Only light that possesses the same wavelength as the distance between the “cathode electrode semitransparent film” and the “anode electrode reflective film” resonates. Light wavelengths that do not match are weakened. As a result, the spectrum of the extracted light is sharpened while brightness and color purity are enhanced. This ensures the strongest light from each color.
n conventional panels, a circular polarizer (retardation film and polarizer) is installed on the panel surface to prevent the reflection of ambient light. However this structure also reduces the amount of electroluminescent light emitted by less than half. Sony rejected the circular polarizer and instead created a microcavity structure combined with color filters. This both prevents the reflection of ambient light and enhances color purity. The results are lower power consumption with longer life and advanced picture quality.
Figure 6 shows the effects of reducing ambient light achieved by the microcavity structure and the color filters. When the organic layer optical path length is matched to the wavelength of green electroluminescent light, the internally generated green light is strengthened, while the green component of the ambient reflected light is cut. At the same time, the color filter removes non-green colors from the ambient reflected light. High contrast can therefore be achieved without using a circular polarizer, and power consumption is reduced by half.
Thanks to Sony.net for this informations!
