A new molecule developed by researchers at the University of Michigan and the University of Southern California shines a deep blue that is close to meeting the stringent brightness requirements of the National Television Systems Committee.
“Bright, deep blue, phosphorescent emitters have been very elusive. Our work has resulted in deep, display quality blue at very high efficiency and extremely high brightness,” said Stephen Forrest, the Peter A. Franken Distinguished University Professor of Engineering and Paul G. Goebel Professor of Engineering.
Each pixel in an organic LED (OLED) display is typically made up of red, green and blue OLEDs that shine with different brightnesses to achieve the full palette of color. Phosphorescent organic LEDs (PHOLEDs) have the potential to use a quarter of the electricity required for organic LEDs, saving battery life and electric bills, but blue PHOLEDs haven’t yet achieved the necessary brightness and richness of color. While green and red PHOLEDs are already used in Samsung and LG displays, the blue still comes from regular OLEDs.
The new molecule, an iridium complex, produces the deep blue color through phosphorescence. The exceptional brightness is thanks in part to the way that the team used the molecule in the device. It does triple duty, serving as the light emitter while also conducting positive charges into the light emitting layer and keeping the negatively charged electrons from leaving it, preventing wasted electricity.
LEDs work by running an electrical current through a semiconductor. Electrons come in from one side while positively charged “holes”, which are spaces in molecules that can be filled by electrons, come in from the other side. When the electrons and holes come together in the light emitting section, they release their energy in the form of light. In the new PHOLED, this recombination takes place in the iridium complexes and produces deep blue light.
The PHOLED built by Jaesang Lee, first author on the paper in Nature Materials and a graduate student in electrical engineering and computer science, is layered. The central layer emits the light. It is a mix of a host organic semiconductor and the new iridium complex.
The light-emitting layer is sandwiched between a hole-blocking layer and the electron-blocking layer (the new iridium complex). This keeps the electrons and holes from escaping the light-emitting portion, which helps the PHOLED shine brightly at high electrical currents.
“Since the holes travel through the electron-blocking layer, which is made of the same iridium complex as the light emitters, there is no energy loss when the holes enter the light-emitting layer,” said Lee. “As a result, the device is more efficient.”
The team’s device also takes advantage of a life-extending design that they discovered last year. When too many energetic electrons and holes are concentrated in one place, usually near the electron-conducting layer, they can break apart the molecules responsible for emitting the light.
To prevent this problem, Lee built the light-emitting portion so that there were fewer iridium complexes near the electron-conducting layer, drawing electrons deeper into the light-emitting section.
“This exciting result gets us very close to device and materials designs that will result in unprecedented efficiencies and lifetimes for displays and even ultrahigh efficiency OLED lighting,” said Forrest.
A paper on this work appears in Nature Materials, titled “Deep blue phosphorescent organic light-emitting diodes with very high brightness and efficiency.”
This work was supported by the Air Force Office of Scientific Research and Universal Display Corporation.
The university has applied for patent protection and has licensed this concept to Universal Display Corporation.
Forrest is also a professor of electrical engineering and computer science, material science and engineering, and physics.