Kyushu University Claims OLED Breakthrough

Jun 1, 2014
Tetsuo Nozawa, Nikkei Electronics
The newly-developed assistant dopant and the mechanism of light emission. Each color is produced by a single dopant.
The newly-developed assistant dopant and the mechanism of light emission. Each color is produced by a single dopant.
[Click to enlarge image]
The fluorescent dopant materials used for devices and the spectra at the time of light emission
The fluorescent dopant materials used for devices and the spectra at the time of light emission
[Click to enlarge image]

The Center for Organic Photonics and Electronics Research (OPERA) of Kyushu University developed an OLED device that emits light by using a fluorescent material with an internal quantum efficiency of 100%.

The device was realized by using the "thermally activated delayed fluorescence (TADF)," which OPERA has been developing, as an assistant dopant and dispersing it in the light-emitting layer of a conventional fluorescent OLED device. Compared with the previous TADF, the new technology enables to make a material and device with a more versatile and easier method and improves the durability of a device (See related article).

"Outside engineers call it an ultimate OLED technology," said Chihaya Adachi, project leader of OPERA.

In general, the light-emitting layer of an OLED device is made by using a host material that generates excitors from electric current and a dopant material that is directly involved in light emission.

The host material used for the light-emitting layer of the latest OLED device is a "versatile material that has been used for conventional OLEDs," said Hajime Nakanotani, associate professor at Kyushu University and the author of the thesis announced this time.

The fluorescent materials used as dopants (light-emitting materials) are blue light-emitting TBPe, green light-emitting TTPA, orange light-emitting TBRb, red light-emitting DBP, etc, which are versatile materials. With them, the external quantum efficiency is usually up to 3-4%.

This time, a TADF material was added as an assistant dopant to the light-emitting layer using each of those materials. As a result, the external quantum efficiency was improved to 13.4% with blue light, 15.8% with green light, 18.0% with orange light and 17.5% with red light.

The latest technology will bring about the following two effects. First, because a device made of a fluorescent material, which can be designed with a high degree of freedom and for which a large amount of knowhow has already been accumulated, is used as a basic structure, it becomes easier to develop a high-luminous efficiency OLED device.

Second, the technology is expected to drastically improve the emission lifetime of high-luminous efficiency OLED devices. It is because an assistant dopant helps the energy transfer between host and dopant materials. Because a fluorescent material, whose electrochemical stability is high, continues to be used as a dopant directly involved in light emission, the durability of the device drastically improves, Kyushu University said.