Singapore and Ann Arbor, MI--A pure-blue organic LED (OLED) created by researchers at the Institute of Materials Research and Engineering (IMRE) and the University of Michigan has a maximum external quantum efficiency (EQE) that is about twice that of the best previous device.1 Using the new emitter in OLED displays will (of course) make them more efficient; also, lifetimes can be increased because the electrical current can be lowered. The color of the OLED is also purer blue than that of previous devices.
Four major improvements
The emitter's higher EQE is a result of multiple improvements. First, electron-withdrawing groups were added to the oligofluorene molecular structure of the OLED, balancing the hole and electron injection and transport. Second, the thickness of the emitting layer was optimized. Third, the OLED was thermally annealed. And fourth, the emitter was doped in a host material called 4,4'-bis(carbazol-9-yl)biphenyl (CBP). The result is an EQE of 9.4% and a blue color with CIE coordinates of (0.147, 0.139).
The new material is solution-processable, making it cost-effective and straightforward to manufacture in very large sizes.
Current OLED displays contain either phosphorescent or fluorescent emitting materials. Though phosphorescent OLEDs have efficiencies that reach 11%, they are less stable, have shorter lifetimes, and produce poor color quality, having a light-blue color output. Blue light has a wider band gap that requires higher energy for effective blue-light emission, and inherently has lower efficiency and a shorter lifetime than other colors. The new blue OLED is fluorescent; this type of OLED material is more stable compared to phosphorescent materials and has a longer lifetime (not to mention that pure blue color).
While the results of the research were published early this year, the researchers have since recorded efficiencies that have gone beyond the published data.
1. Chang-Gua Zhen et al., Advanced Functional Materials, article first published online 18 January 2011; DOI: 10.1002/adfm.201002165.
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