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University of Michigan Claims Replacing ITO in OLED Stack with Ag:Cu Increases EQE by 20%
Research funded by Zenithnano Technology, a company co-founded L. Jay Guo, of the University of Michigan, to commercialize his lab’s inventions of transparent, flexible metal electrodes, produced an electrode that improves the light output of an OLED by 20%. The University of Michigan has filed for patent protection.
In today’s OLED devices, ~80% of the light is trapped inside the device, due the trapping of light inside the device, associated with waveguiding that prevents light from exiting at an angle close to perpendicular, which causes the light to get reflected back and guided sideways through the device. A good portion of the lost light is simply trapped between the anode and cathode on either side of the light-emitter. The anode, which is made of ITO causes most of the loss.
Research funded by Zenithnano Technology, a company co-founded L. Jay Guo, of the University of Michigan, to commercialize his lab’s inventions of transparent, flexible metal electrodes, produced an electrode that improves the light output of an OLED by 20%. The University of Michigan has filed for patent protection.
In today’s OLED devices, ~80% of the light is trapped inside the device, due the trapping of light inside the device, associated with waveguiding that prevents light from exiting at an angle close to perpendicular, which causes the light to get reflected back and guided sideways through the device. A good portion of the lost light is simply trapped between the anode and cathode on either side of the light-emitter. The anode, which is made of ITO causes most of the loss.
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In a lab device, light that is trapped can be seen shooting out the sides rather than traveling through to the viewer. “Untreated, it is the strongest waveguiding layer in the OLED,” Guo said. “We want to address the root cause of the problem.” By using a layer of silver just five nanometers thick, deposited on a seed layer of copper rather than ITO, Guo’s team maintained the electrode function while eliminating the waveguiding problem in the OLED layers altogether. “Industry may be able to liberate more than 40% of the light, in part by replacing the conventional indium tin oxide electrodes for our nanoscale layer of transparent silver,” said Changyeong Jeong, first author and a Ph.D. candidate in electrical and computer engineering. Even though light is no longer guided in the OLED stack, that freed-up light can still be reflected from the glass. The research team did not address the transparency of the new anode, nor its ability to deliver the same number of holes supplied by ITO. |
In order to prove that they had eliminated the waveguiding in the light-emitter, Guo’s team had to stop the light trapping by the glass, too. They did this with an experimental set-up using a liquid that had the same index of refraction as glass, so-called index-matching fluid—an oil in this case. That “index-matching” prevents the reflection that happens at the boundary between high-index glass and low-index air.
Once they’d done this, they could look at their experimental set-up from the side and see whether any light was coming sideways. They found that the edge of the light-emitting layer was almost completely dark. In turn, the light coming through the glass was about 20% brighter. The finding is described in the journal Science Advances, in a paper titled, “Tackling light trapping in organic light-emitting diodes by complete elimination of waveguide modes.”
The University of Michigan has a long history with OLEDs stemming from Dr. Stephen’s Forest’s work on phosphorescent emitters, which was licensed by UDC and is today used overwhelmingly by panel makers for red and green emitters. But it is surprising to learn that swapping out ITO for a combination of silver and copper, would not have been tried by the many R&D groups working on OLEDs both within and outside of the panel makers, given the improvement is EQE.
Once they’d done this, they could look at their experimental set-up from the side and see whether any light was coming sideways. They found that the edge of the light-emitting layer was almost completely dark. In turn, the light coming through the glass was about 20% brighter. The finding is described in the journal Science Advances, in a paper titled, “Tackling light trapping in organic light-emitting diodes by complete elimination of waveguide modes.”
The University of Michigan has a long history with OLEDs stemming from Dr. Stephen’s Forest’s work on phosphorescent emitters, which was licensed by UDC and is today used overwhelmingly by panel makers for red and green emitters. But it is surprising to learn that swapping out ITO for a combination of silver and copper, would not have been tried by the many R&D groups working on OLEDs both within and outside of the panel makers, given the improvement is EQE.
