Organic LEDs Shine Invisibly

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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."