Organic light-emitting diodes (OLEDs) are set to revolutionize lighting technology,
ushering in an era of thin, flexible, and ultra-bright devices. At the heart
of recent OLED devices are phosphorescent metal complexes that, when stimulated
by an electric voltage, produce a sustained emission of light with higher efficiency
than other sources. Furthermore, because OLEDs create their own light, they
eliminate the need for backlights used in liquid crystal displays, and therefore
consume low amounts of power.
 | | A phosphorescent iridium–amidinate complex (top left) serves as an excellent emitter (bottom), enabling the first successful fabrication of highly efficient non-doped phosphorescent OLEDs (top right). |
Although the advantages of OLEDs are impressive, manufacturing these devices
remains a challenging and expensive process. Phosphorescent OLEDs are normally
fabricated by via a process known as 'doping' where metal complexes
are added into a host matrix under strict concentration requirements. If the
metal concentration is too high, the complexes interact and quench each other's
phosphorescent abilities.
Now, a team of scientists led by Zhaomin Hou from the RIKEN
Advanced Science Institute in Wako has developed a way to eliminate precise
doping limits from the OLED manufacturing process. By using a metal dopant containing
molecular groups that block the self-quenching interactions, the scientists
have, for the first time, fabricated high-efficiency OLEDs with a wide range
of doping concentrations.
Hou and colleagues modified a phosphorescent iridium metal complex with a class
of molecules known as amidinates. These molecules bind to iridium through a
nitrogen atom that localizes electrons near the center of the metal complex.
Bulky carbon groups on the edges of the complex are inert and prevent the materials
from attaching and self-quenching their phosphorescence.
Prototype OLED devices made with the iridium–amidinate complex exhibited
a bright yellow-green emission using very low driving voltages. The scientists
found that a wide range of doping concentrations—from 7% to 100%—could
be used to produce the OLED devices.
“One of the research projects in my group led to an efficient synthesis
of various amidinates,” says Hou. “We envisioned that a geometrically
hindered amidinate group might overcome the problems encountered previously
in phosphorescent metal complexes.”
According to Hou, the iridium complex itself possesses charge-transport ability,
removing the need for a host matrix. Moreover, because of the excellent performance
and the ease of synthesis, the iridium–amidinate phosphorescent complexes
should have high potential in practical applications such as flat-panel displays
and organic lighting.
“We are now applying the amidinate molecules to phosphorescent metal
complexes that emit light at different wavelengths,” says Hou. “This
will allow us to produce new high performance OLED devices with different colors.”
- Liu, Y., Ye, K., Fan, Y., Song, W., Wang, Y. & Hou, Z. Amidinate-ligated
iridium(III) bis(2-pyridyl)phenyl complex as an excellent phosphorescent material
for electroluminescence devices. Chemical Communications published online
2009 (doi: 10.1039/b902807b).
Posted June 4th, 2009
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