Fabrication and characterization of ultraviolet light stable sol-gel siloxane Hybrid materials for UV LED encapsulant

Abstract

We studied UV transparent and resistant sol-gel siloxane hybrid materials with superior long-term sta-bility as applicable to UV-LED encapsulant. An organic groups in siloxane encapsulants can be easily degrad-ed by UV light and heat, and thereby showing transmittance decreases and surface cracks. The sol-gel de-rived organic-inorganic siloxane hybrid materials have many advantages due to their thermal and light stabil-ity, high transparency, low modulus, low moisture permeability, and tunable refractive index. For reliability of UV-LED during UV irradiation, siloxane hybrid materials based encapsulants have advantages due to the high bonding energy and cross-linking density of siloxane network. First, we prepared vinyl-methyl oligosiloxane and hydrogen-methyl oligosiloxane based methyl-siloxane hybrid materials. The oligosiloxane resins were synthesized by a hydrolytic sol-gel condensation re-action of organo-alkoxysilane precursors. Then, the methyl hybrid materials were fabricated by thermal hy-drosilylation reaction of the oligosiloxane resins under platinum catalyst. The vinyl-methyl oligosiloxane resin was used for high thermal stability and densely cross-linked network, while the linear vinyl-methyl siloxane resin was applied for high UV transparency due to their large free volume and minimized Pt catalyst content. We demonstrated high UV transparency (>82 %, at 300 nm), which is comparable with that of PDMS (poly-dimethylsiloxane). The methyl-siloxane hybrid material showed remarkable UV resistance without transmit-tance decrease due to the densely cross-lined siloxane network (stable for more than 1000 hours under UV-B (280 ~ 320 nm)). To improve deep UV stability, the fluoro-siloxane hybrid materials are fabricated by a hydrosilyla-tion reaction vinyl-fluoro oligosiloxane and hydrogen-methyl oligosiloxane resins. The vinyl-fluoro oli-gosiloxane resin shows high condensation degree, vinyl functionality for providing a high cross-linking density and thermal decomposition temperature. Also, a substantial vinyl groups are source of radical formation, which improves UV stability through a stabilization of radicals. The superior deep UV resistance (UV-C) was achieved by minimizing undesired organic residues and the presence of fluorine groups with high bonding energy. Due to the high siloxane bonds content and cross-links content, the fluoro hybrimer exhibited excel-lent thermal stability $(> 500^\circ C)$ and long-term UV stability (250 nm, 2000 hours) compared with convention-al organo-silicone based LED encapsulant. The methyl and fluoro hybrimers will be a promising candidate to be used as a deep UV-LED for sterilization and water purification.