Organic LEDs Shine Invisibly
Researchers at the University of Southern California have designed a phosphorescent dye molecule that emits near-infrared light and have used it to make long-lasting organic light-emitting diodes (OLEDs).
The diodes could be used to make a cheap and flexible near-infrared (NIR) display that would be unreadable to the naked eye but could be read with night-vision goggles. Such a display could be integrated into a soldier’s uniform or a device that could be stashed in a pocket, allowing soldiers to read communications at night without being spotted by enemy snipers.
These organic LEDs could also be converted into the infrared-detector diodes that make night vision possible. Infrared detectors are essentially the reverse of LEDs, converting light into an electric current. Warm objects emit infrared radiation, which has wavelengths longer than near-infrared radiation and is also invisible to the human eye. Just as the light detectors in cameras sense visible light, infrared sensors made of inorganic semiconductors detect infrared light in the night-vision goggles and cameras used by the military, police, border security agents, and firefighters. But detectors based on OLEDs would offer an important benefit: because the thin organic polymers that make up these diodes can be deposited on a variety of substrates, including bendable plastic, organic IR detectors could be flexible enough to incorporate into a helmet visor.
“Flexibility is very beneficial … next-generation displays are all going to be on flexible substrates,” says Mark Thompson, a chemistry professor at the University of Southern California, who led the research. Organic LEDs are the crucial technology for flexible displays, because they are easy and cheap to pattern on bendable substrates, he says. They are already being used in camera and cell-phone displays, and they hold tremendous promise for future large-area computer and television screens.
Research in organic LEDs has largely focused on visible-light applications; no one has previously made an organic LED that efficiently emits NIR light. Thompson and his colleagues at Princeton University and Universal Display Corporation, a company based in Ewing, NJ, described their organic LED online in Angewandte Chemie on January 9.
Organic LEDs that emit invisible NIR wavelengths could be used to make displays that you do not want everyone to see. “For covert military applications, night-vision displays will be very important, and these diodes would be key to that,” says Ghassan Jabbour, an optical-sciences professor at the University of Arizona in Tucson, who developed the first NIR-emitting organic molecules.
The secret to the new LED is a specially designed phosphorescent dye molecule that the researchers use in the emissive layer sandwiched between the device’s two electrodes. Typically, organic LEDs contain an emissive layer that is doped with fluorescent dyes. The electrodes inject negative electrons and positive “holes” into the layer, where the charged particles combine and excite the dye molecules. When the molecules return to their unexcited state, they emit photons. The new phosphorescent molecules emit very efficiently in the NIR region. They also emit light for a longer time than fluorescent dyes, increasing the lifetime of the device–a traditional weak point for organic materials.
The device emits at wavelengths close to 800 nanometers, which is right at the border of the visible and near-infrared spectrum, and boasts an efficiency of more than six percent, which is at least 60 times that of other NIR-emitting devices reported in the past. Right now, it runs for 1,000 hours at its maximum brightness. But at the lower brightness levels required in displays, “we’re talking at least a million hours,” Thompson says. By comparison, red or green organic LEDs have lifetimes of 100,000 hours, he says.
Gareth Redmond, who studies nanoscale organic photodetectors at the Tyndall National Institute in Cork, Ireland, calls the work a breakthrough toward NIR emission in organic material. Redmond says that the new organic LED shows “really good performance in terms of efficiency and lifetime which hasn’t been achieved before.”
Thompson and his colleagues plan to make other phosphorescent dye complexes that emit light at wavelengths longer than 800 nanometers, pushing deeper into the IR region. Then, Thompson says, it would be possible to “flip” the organic LED, converting it into an organic IR detector for a night-vision helmet visor. This would require modifying the device structure or tweaking the organic materials, but he says the conversion would be easy because LEDs and photodetectors are “cousins” with essentially the same diode structure but reverse functions–an LED converts electric current into light while a detector does the opposite.
But it is too early to say when such an organic IR detector would be available. It’s not just that the jury is still out, he says; “the jury hasn’t even been formed.”
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