30 Jan 2008
High quality light has been produced by a white OLED using a novel three emission layer structure.
A white OLED (WOLED) boasting a higher quantum efficiency, power efficiency and colour rendering index compared with previous devices has been unveiled. The US team used a three emission layer (EML) structure to obtain a peak power efficiency of 64±3 lm/W at a luminance of 1000 cd/m2 and CRI of 81. Typical CRI values for WOLEDs are between 75-80. (Applied Physics Letters 91 263503)
"The power efficiency we obtained is one of the highest values among WOLEDs reported so far," Yiru Sun, a researcher at Princeton University, told optics.org. "What's more, we have made no compromise in CRI for high power efficiency as we have also achieved a good quality of white colour balance."
The CRI value indicates the ability of a light source to reproduce the colours of an object it illuminates. "It is important to develop light sources with high CRI values particularly for indoor lighting applications," explained Sun. "A CRI value larger than 70 is considered to have a good quality of white light, similar to sunlight."
Until now, electrophosphorescent WOLEDs have been fabricated with only one or two emission layers, in which excitons are formed. The crucial hurdle facing scientists is triplet-triplet annihilation, which can occur at the emission layer from a pileup of excitons. During the process one triplet exciton is destroyed, reducing the efficiency of the device.
Sun found that introducing a 3-EML structure lowered the local density of excitons and reduced the exciton annihilation effect. What's more, she found that combining the 3-EML structure with carefully selected phosphorescent dopants allowed optimum efficiency and colour rendition to be achieved.
"The three sections of the EML are for red, blue and green (RGB) emission separately. Excitons are generated in multiple locations across the full width of the EML which reduces triplet-triplet annihilation," explained Sun. "We chose each of the three RGB dopants so as to have favourable energy alignment with the three hosts. By achieving optimum efficiency for all three colours we enhanced the efficiency of the whole device."
The 3-EML structure offers the additional advantage of allowing the colour intensity to be adjusted. "We can produce a balanced white colour by changing the thickness and doping concentration of the RGB emission region," added Sun.
Although lifetimes of the device were not measured, the team recognizes that this is one of the major issues facing OLEDs and plans to address this in the future. "We will investigate more advanced dopant/ host materials that can be incorporated into this structure to achieve higher efficiency and longer lifetime," concluded Sun.
This research was carried out under the supervision of lead researcher Stephen Forrest from the University of Michigan, US.