Nanocrystalline semiconductor LEDs with simple structure and high efficiency
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Nanocrystalline semiconductor LEDs with simple structure and high efficiency Dietrich Bertram, Volker Weiler, Dimitri Talapin1 and Horst Weller1, Nanomaterials and Devices, Philips Research Laboratories 52066 Aachen, Germany 1 Fachbereich Physikalische Chemie, University of Hamburg, 20045 Hamburg , Germany Abstract Nanocrystalline semiconductor particles exhibit a size dependent bandgap emission, due to size quantisation effects. These particles are derived from solution chemistry and can be made monodisperse under the right synthesis conditions. Compared to organic materials, the inorganic nanoparticles show much higher stability against oxidation and degradation, which makes them an interesting candidate for LEDs and displays. So far, LEDs based on semiconductor nanoparticles typically included low stability organic materials to provide charge injection. The talk will present a new class of nanoparticle LEDs, made without sensitive organic materials. These LEDs show high efficiencies, well defined color throughout the red to green part of the visible spectrum and improved stability under ambient conditions without excessive encapsulation. Using high quality monodisperse suspensions, high color purity is achieved for the emission which paves the road to cheap, high quality displays based on inorganic semiconductor nanoparticles. Introduction Electroluminescence (EL), which is the direct conversion of electrical current into light, has become a major research topic in recent years. Two different systems are at the heart of the activities: namely inorganic LEDs and organic LEDs. Whereas inorganic LEDs are highly advanced products with high brightness and all colors, they suffer from the drawback to be grown on a size limited crystalline substrate using expensive tools and difficult processes. Organic LEDs on the other hand are being deposited by many different techniques on virtually any substrate, especially in large sizes. However, the efficiencies and durability of organic LEDs is somewhat limited. Nanocrystalline semiconductor particles (Quantum Dots, QDs) could enable a third route, combining the best of both worlds: they can be processed by wet chemical means, making the deposition inherently cheap and substrate independent. Moreover, using direct bandgap semiconductor materials, the efficiencies and stability of these devices can become very high as well. The unique advantage of the QDs is that the emission wavelength shifts with size. Smaller size means increased quantum confinement and therefor increased bandgap. Thus the processing of the LEDs can be optimized for a specific material and the emission color can be varied by changing the size of the particles. Experimental details The present experiments were performed using QDs made by organometallic synthesis in solution. The details of the synthesis can be found e.g. in [1,2]. The material consists of a crystalline core of CdSe, surrounded by a thin shell of ZnS. This core/shell configuration has the advantage of increasing the luminescence effici
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