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The advent of organic light-emitting diodes (OLEDs) has led to a host of new applications for these light-weight and energy-efficient devices. Until recently, however, OLED research was focused on visible emission. Now, US scientists have developed a near infra-red OLED, a NIROLED, you might say. Such a device could be used in night-vision devices and potentially in small-scale, portable NIR spectrometers or lab-on-a-chip systems. Mark Thompson of the University of Southern California and colleagues at Princeton University, Steve Forrest's group at the University of Michigan, and Julie Brown's team at Universal Display Corporation have used a phosphorescent platinum-porphyrin complex as a doping agent to create the new class of NIR OLED. OLEDs sandwich a dye-containing emission layer between two electrodes, which responds to an applied voltage by emitting light as electrons and holes pumped to and from the electrodes to this layer trigger excitation and subsequent relaxation of the dye molecules. The emission layer is usually doped with a fluorescent dye, which provide for suitable emission at visible wavelengths. However, OLED researchers anticipate that phosphorescent dopants will make for even more efficient OLEDs. Phosphorescent dyes emit light for a longer period of time, because they are "trapped" in their excited state and cannot return to their ground state as easily. The emission wavelength of any OLED depends on the energy gap between the relaxed and the excited state, which depends almost wholly on the structure of the dye molecule. Thompson and his team turned to a platinum-porphyrin complex as a potential phosphorescent doping agent. Tweaking the precise structure allowed them to tweak the band gap so that the saddle-shaped porphyrin derivative (tetraphenyltetrabenzoporphyrin) with its central platinum atoms emits in the infrared; very efficiently. The team says their NIR OLEDs have lifetimes of greater than 1000 hours at 90% efficiency, which makes them suitable for many night-vision display and sensing applications. They suggest that achieving even longer-wavelength emission should be possible by extending the conjugation length of the ligand porphyrin structure. "I am not sure if these NIR devices will be used in spectrometers or not," Thomson told us, "To be honest, we decided to push in this direction for the challenge of getting there, combined with the idea that such devices would be good to have to displays, signage and other emissive applications that are readily visible with night vision goggles." However, he adds that they are now thinking of many other applications for these devices. "What we have here are highly efficient, extremely long-lived devices," he says, "The next challenge is to shift to longer wavelengths, which is in the works." Related links:
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