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MOSCOW, June 7. /TASS/. Scientists from the Center for Photonics and Quantum Materials, Skolkovo Institute of Science and Technology (Skoltech, Russia) and their colleagues from the UK and Taiwan have designed a novel photonic white light-emitting diode (LED) structure with record-breaking efficiency parameters. This new technology opens the route towards ultra-thin and cheap LEDs.
Schematic representation of a novel LED crystalThe optical society of America
"Compared to other modern emitters, it would be much easier to integrate the newly designed structures into typical LED screens, lowering the overall costs. Moreover, the new approach allows for more control on the directionality of the light emission, which is energetically more effective. All there features make our development very appealing to the lighting industry," said Dr. Mael Brossard of the Skoltech Center for Photonics and Quantum Materials.
The standard white LEDs are based on a semiconductor gallium nitrate GaN, which emits the light in blue and ultraviolet regions when activated with the electric current. Then, the emitted photons are absorbed by other substances followed by reemission of the light in a more broad wave range producing a white spectrum. Such substances capable of converting absorbed energy to the visible light call luminophores. Recently, quantum dots have been suggested for applications as luminophores as the QDs irradiation parameters (e.g. the color) could be easily tuned.
Top scanning electron microscope view of a novel LED crystalThe optical society of America
However, in the first prototypical white LEDs, the luminophore has been deposited as a separate layer above the blue LED consisted of GaN, which has strongly limited the efficiency of the whole device. The major part of GaN irradiation has reflected on the layer interface and as a result has not reached the QDs. In this work, the authors have suggested the new hybrid structure where the QDs are encapsulated in the blue LED layer in a periodic way. Such an approach increases significantly the quantum yield, i.e. the ratio between the number of emitted and absorbed photons, as well as other characteristics of the device.
"The excellent coupling between the blue emission from the GaN LED and the QD emitters provides unprecedented color conversion and effective quantum yield, a key metric for white light LEDs. This allows us to completely avoid the encapsulation in the epoxy layer that typically contains the white phosphors to obtain ultra-thin LED devices," Brossard said.
The effective quantum yield of the new device reaches the breaking value of 110%. The development made by scientists from Skoltech, University of Southampton (UK), Luxtaltek Corporation (Taiwan), and National Chiao Tung University (Taiwan) is presented in the recent in Optica, an open-source journal of The Optical Society of America.