Organic LEDs Shine Invisibly

Long-lasting near-infrared LEDs could be used to make cheap, flexible night-vision displays and sensors.

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).

Universal Display Corporation’s phosphorescent organic LED display can be built on a flexible plastic substrate. The company, working with researchers at the University of Southern California and Princeton University, has now made near-infrared emitting LEDs and plans to make a near-infrared version of the flexible display. The display would be invisible to the naked eye but visible through night-vision goggles for covert military operations. Credit: Julie Brown, Universal Display Corporation

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.