Tech-on news website has reported that Japan Broadcasting Corp (NHK) has recently announced that it has dramatically extended the lifetime of a red OLED device for flexible displays and drastically reduced its power consumption.
NHK had previously exhibited a prototype full-colour display in 2015 that had been made on a plastic substrate. The display was also on exhibition at the recently held Open House event organised by NHK Science & Technology Research Laboratories (STRL). This allowed NHK to demonstrate that the plastic substrate-based display with almost no sealing had been continuously display video for more than a year after it was made.
The latest OLED device is based on the "inverted OLED (iOLED)" technology, which NHK STRL first announced way back in 2013. The key to this technology (structure) is the material used for the electron injection layer.
While many of OLED devices having a conventional structure use lithium fluoride (LiF), which is easily oxidized, NHK STRL used an organic material called "polyethyleneimine" for the iOLED that was prototyped in 2013.
Figure: NHK - iOLED
Although its electron injection layer was resistant to moisture and oxygen, it deteriorated by driving current. Further, its emission lifetime (the time it takes for the initial brightness of 1,000cd/m2 to be halved) was 1,000 hours or less, which is 1/10 that of commonly-used materials, even when it was sealed by glass.
By 2015 NHK STRL had developed an iOLED device on a plastic substrate with an improved emission lifetime. Unfortunately, the driving voltage increased from 3.9V in 2013 to 5.6V in 2015, lowering internal quantum efficiency from 100% to 70%.
The recent developments by NHK STRL used a new organic material and an unspecified additive for the electron injection layer of the red iOLED device. As a result, the driving voltage was lowered to 3.7V, the internal efficiency returned to 100%, and the emission lifetime was drastically increased to more than 10,000 hours.
For the future, NHK STRL plans to accelerate the development of green and blue OLED devices that have the same high energy-saving capability and long service life as the latest red OLED device.
A spokeperson for NHK said, "This is the world's first OLED device that has a long service life, high energy-saving capability (because of the low driving voltage) and resistance to oxygen and moisture."