Multifunctional silicone nanocomposites for advanced LED encapsulation
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Multifunctional silicone nanocomposites for advanced LED encapsulation Ying Li, Peng Tao, Richard W. Siegel, and Linda S. Schadler Department of Materials Science and Engineering and Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, NY 12180, U. S. A. ABSTRACT The addition of high refractive index (RI) inorganic nanoparticles (NPs) to LED encapsulation materials can lead to higher light extraction efficiency. In addition, the NPs can be carriers for additional functionality such as color conversion. Using a simple “grafting-to” approach, bimodal polydimethylsiloxane (PDMS) brushes were grafted onto high-RI ZrO2 NPs. Subsequently, an organic phosphor, 6-[fluorescein-5(6)-carboxamido]hexanoic acid (FCHA), was attached onto the PDMS-grafted ZrO2 NPs via a facile ligand exchange process. The bimodal polymer brush design enables homogenous dispersion of the surface functionalized NPs within the silicone matrix. The functionalized NPs with ~53 wt% ZrO2 core have a ~0.08 higher RI than neat silicone, and the NP-filled silicone nanocomposites exhibit a transparency of ~ 90% in the 550-800 nm wavelength range. In addition, the nanocomposites could be excited at a wavelength around 455 nm by a blue LED and undergo secondary yellow emission at around 571 nm. It is expected that the prepared nanocomposites can be used as high-efficiency, nonscattering, color-tuned materials for advanced LED encapsulation. INTRODUCTION Compared to epoxy-based LED encapsulants, which tend to yellow over time with exposure to high operating temperatures and/or absorption of UV-blue light, silicone resins have higher photochemical and thermal stability, high transparency in the UV-visible region, low water permeability, and tunable hardness.[1] Silicone encapsulants would open up exciting new luminaire designs and allow for penetration of phosphor-converted LED lamps into the solidstate-lighting market if they had a higher RI and better color-conversion properties.[2, 3] Increasing the RI increases the angle of the light-escape cone, thereby enhancing the light extraction efficiency.[4] Adding non-scattering color conversion properties to the encapsulant opens up new luminaire geometries. Currently, the most commonly used phosphors are composed of an inorganic host substance, such as yttrium aluminum garnet (YAG), doped with rare-earth elements.[5] With increasing concerns over the resource depletion of rare-earth elements, organic fluorescent materials have attracted attention because of their low cost, ease of fabrication, color tuning via modifying ʌ–ʌ* transitions through molecular/structure design, solubility in organic solvents enabling molecular-level doping into polymers, and generally good compatibility with polymer matrices.[3] In this work, we have demonstrated the preparation of multifunctional silicone nanocomposites with combined high RI and color conversion functionality by uniformly dispersing organic-phosphor functionalized high-RI ZrO2 NPs within a silicone matrix. In order to achieve higher NP loading, and
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