Energy efficiency and flexible lighting applications have long been the promise of organic light emitting diodes (OLEDs). The technology hasn’t lived up to its promise, however, because in typical OLEDs, only 20 percent of the light generated is released from the device. That means that most light is trapped inside the bulb, making it highly inefficient.
Beam me up: A new OLED design could help the devices emit far more light. Electron microscope images show the top of the OLED with organic and aluminum layers (top) and an organic grid before depositing the organic and aluminum layers (middle). The bottom image shows polymer micro lenses on the surface of the glass substrate.
Researchers at the University of Michigan and Princeton University believe that they’re on to a way to break the OLED-efficiency logjam. The scientists have designed an OLED that boosts illumination by 60 percent using a combination of an organic grid working in tandem with small micro lenses that guide the trapped light out of the device.
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Stephen Forrest, a professor of electrical engineering and physics at Michigan, and Yuri Sun, from Princeton University, described the work in the August issue of Nature Photonics.
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In OLEDs, white light is generated by using electricity to send an electron into nanometer-thick layers of organic materials that behave like semiconductor materials. Typically, the light in the substrate is internally reflected and runs parallel and not perpendicular. That’s the crux of the problem because the light can’t escape in the vertical direction without some coaxing. In Forrest’s devices, the grids refract the trapped light, sending it to the five micrometers dome-shaped micro lenses. The light is sent off in a vertical orientation that helps release the trapped rays.
Forrest and his coworkers report that the technology emits about 70 lumens from a watt of power. In comparison, incandescent lightbulbs emit 15 lumens per watt. Fluorescent lights put out roughly 90 lumens of light per watt but have liabilities: they produce harsh light, lack longevity, and use environment-damaging substances like mercury.
Forrest says that the next step in the research is to use OLEDs that are more efficient than those the team used in the current project. Looking beyond the research lab work on these OLEDs, he is cautiously optimistic that it should be possible to scale up the manufacturing of the devices, and that production costs for manufacturing the new OLEDs will be competitive.
Today, an estimated 22 percent of the electricity produced goes to lighting buildings. A highly efficient form of OLED lighting could significantly reduce the electricity demand and boost savings. Another factor influencing broad adoption of LEDs is the fact that they outlast incandescent bulbs. Over the next 20 years, the rapid adoption of LED lighting in the United States could reduce electricity demands by 62 percent and thus eliminate 258 million metric tons of carbon emissions, according to the Department of Energy.
It will take several years to replace current lighting in office buildings and homes with OLEDs. But the continued progress in increasing the efficiencies of the devices is encouraging to researchers. “Luckily, OLEDs are the light that just keeps giving,” says Forrest, who has spent much of his professional research career focused on OLEDs. “There is so much to be done and so much that’s been done, but this is nonetheless a quite exciting advancement.”
