Blue phosphorescent OLED lifetime increased by 10X, reaching more than 600 hours
Researchers at the University of Michigan (UM; Ann Arbor, MI) have extended the lifetime of blue organic LEDs (OLEDs) by a factor of ten.1
These days, OLEDs are big business, having found their way into smartphone displays, as well as into TVs of ever-increasing screen size. They allow ultrathin and even curved displays; in addition, OLEDs have other advantages over conventional liquid-crystal displays, such as a naturally very wide viewing angle and the highest contrast available (blacks are truly black). In these red-green-blue (RGB) displays, each pixel contains red, green, and blue modules.
Phosphorescent OLEDs, also known as PHOLEDs, produce light through a mechanism that is four times more efficient than fluorescent OLEDs. Green and red PHOLEDs are already used in OLED-based TVs, as well as in Samsung and LG smartphones; however, the blue OLEDs remain the lower-efficiency fluorescent version.
"Having a blue phosphorescent pixel is an important challenge, but they haven't lived long enough," says Stephen Forrest, an engineering professor at UM. He and his colleagues demonstrated the first PHOLED in 1998 and the first blue PHOLED in 2001.
Stacked OLED
The researchers achieved their new results by broadening the blue OLED's emissive layer and giving it a graded dopant concentration profile; a two-unit stacked device they fabricated has a lifetime of 616±10 hours, which is the time for the device to degrade to 80% of its initial 1000 cd/m2 luminance.
If the light-emitting PHOLED molecules are evenly distributed, the energetic electron-hole pairs tend to accumulate near the layer that conducts electrons, causing damaging energy transfers, so instead, the researchers arranged the molecules so that they were concentrated near the device's hole-conducting layer and were sparser toward the electron conductor. This drew electrons further into the material, spreading out the energy so that it caused a lower degradation rate.
This new distribution alone extended the lifetime of the blue PHOLED by three times. Then, the researchers split the design into two layers, halving the concentration of light-emitting molecules in each layer. This configuration is the one that lasted ten times longer.
This research was supported and is licensed for commercialization by Universal Display (Ewing, NJ).
Source: http://www.eurekalert.org/pub_releases/2014-09/uom-lla092314.php
REFERENCE:
1. Yifan Zhang et al., Nature Communications (2014); doi: 10.1038/ncomms6008