Panasonic Electric Works Co Ltd announced that it realized an OLED device with a luminous efficiency of 128lm/W for lighting purposes at the 72th Meeting of the Japan Society of Applied Physics, which runs from Aug 29 to Sept 2, 2011.
The device is being developed in a project of Japan's New Energy and Industrial Technology Development Organization (NEDO). The efficiency of 128lm/W is higher than the 102lm/W efficiency that Universal Display Corp (UDC) announced in June 2008. Though Panasonic Electric Works disclosed some information about the OLED device at SID 2011, an academic conference that took place in May 2011, it disclosed more details this time.
This time, Panasonic Electric Works drastically improved its light extraction techniques for white organic light-emitting diodes (WOLEDs), Takuya Komoda of the company said. Specifically, the company inserted a light extraction board composed of a highly-refractive material between a light-emitting layer and a glass substrate and used a high-reflectivity material for the metal on the back side.
Also, Panasonic Electric Works minutely calculated the optimal position of each light-emitting layer and designed the OLED device based on the calculation results. With those techniques, the company drastically reduced the total reflection inside the device and doubled light extraction efficiency to about 40%.
An OLED device made by using those techniques and using phosphorescent materials for red- and green-light emissions and a fluorescent material for blue-light emission has an area of 25cm2, a luminous efficiency of 56lm/W (with a brightness of 1,000cd/m2), a color rendering index (Ra) of 91 and an estimated brightness half-life of 150,000 hours or longer.
Furthermore, Panasonic Electric Works made another OLED device by using only phosphorescent materials for red-, green- and blue-light emissions. With the same light extraction layer mentioned above, it has a luminous efficiency of 80lm/W, an Ra of 83 and a brightness half-life of 10,000 hours.
When a semispherical highly-refractive lens similar to the lens that UDC used to realize the efficiency of 102lm/W was used for light extraction, a 2 x 2mm OLED device had a luminous efficiency of 128lm/W. This highly-refractive lens is thick and not suited for use in large-area light-emitting devices. But, if light extraction techniques are improved, it will become possible to realize an efficiency of 130lm/W, Komoda said.
The 130lm/W efficiency is the goal of NEDO's project for the development of OLED lighting apparatuses. It is said that a luminous efficiency of 200lm/W or higher can be achieved with white LEDs. However, due to the use of a diffuser panel, etc, the effective luminous efficiency of white LEDs is expected to be about 130lm/W.
In general, OLED lighting apparatuses do not use instruments that block light. Therefore, the luminous efficiency of an OLED device is equal to the luminous efficiency of a lighting apparatus using the OLED device.
"In the case of OLED lighting, a lamp is equal to an apparatus," Komoda said. "If we take advantage of this fact, it is possible that OLED lamps will rapidly become popular in about 2020. In the future, lighting apparatuses will possibly disappear."