Bright infrared LEDs based on colloidal quantum-dots
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Bright infrared LEDs based on colloidal quantum-dots Liangfeng Sun1,4, *, Joshua J. Choi1,2, *, David Stachnik1, Adam C. Bartnik1, Byung-Ryool Hyun1, George G. Malliaras3, Tobias Hanrath2, Frank W. Wise1 1 School of Applied and Engineering Physics, 2School of Chemical and Biomolecular Engineering, 3Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA 4 Current affiliation: Department of Physics and Astronomy, Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, USA *These authors contributed equally to the work. ABSTRACT Record-brightness infrared LEDs based on colloidal quantum-dots have been achieved through control of the spacing between adjacent quantum-dots. By tuning the size of quantumdots, the emission wavelengths can be tuned between 900nm and 1650nm. INTRODUCTION Colloidal quantum dots (QDs) are promising materials for various optoelectronic device applications by virtue of their size- and shape-tunable optical and electronic properties. There is great interest in the development of low temperature solution processing of QD devices, for nextgeneration photovoltaics, photodetectors and light emitting diodes (LEDs). The central issue in the physics of optoelectronic devices based on nanostructured materials is the control of chargecarrier dynamics. However, manipulation of the excitons in these materials remains a significant challenge, and consequently device performance has been limited.
EXPERIMENT In our research, we demonstrated that the electronic coupling between the adjacent QDs can be dramatically varied by tuning the spacing between them [1]. A variation of a few angstroms in the spacing results in a few orders of magnitude change in exciton dynamics recombination and dissociation. The LEDs based on PbS QDs have reached record-brightness. The radiance is an order of magnitude higher than in previous QD LEDs. The maximum external quantum efficiency of the LEDs is about 2%. These solution processed LEDs can be fabricated to emit over the range 900 ˜ 1650 nm (Figure 1) and compete with the performance of state-ofthe-art infrared LEDs fabricated by planar epitaxial technology over the range 900 ˜ 1300 nm.
Figure 1. Emission spectra and an infrared image of LEDs. Normalized electroluminescence (solid) and PL (dashed) spectra of LEDs made of different size quantum dots. Inset: photograph of a device emitting at 1244 nm taken by an InGaAs camera [1]. Colloidal PbS quantum dots were synthesized using organometallic precursors. The poly(3,4ethylenedioxythiophene) poly(styrenesulphonate) (PEDOT:PSS), PbS quantum dots, and ZnO nanoparticle layers were successively spin-coated onto cleaned, pre-patterned indium tin oxide (ITO) substrates. The long-chain oleate ligands were displaced by bifunctional linker molecules of controlled length. Specifically, mercapto alkyl carboxylic acids of variable alkyl chain lengths, including 3-mercaptopropionic acid (MPA), 6-mercaptohexanoic acid (MHA), 8mercaptooctanoic acid (MOA), and 11-me
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