Thiol–ene photo-curable hybrid silicone resin for LED encapsulation: enhancement of light extraction efficiency by facile self-keeping hemisphere coating

Abstract

Encapsulation with polymer materials can not only protect LED chips, but also enhance light extraction efficiency (LEE), and thus it has become necessary for the fabrication of LED devices. We developed a new concept of LED encapsulation with photo-curable hybrid silicone resin, which allows facile encapsulation at room temperature. The photo-curable silicone liquid resin based on thiol–ene photopolymerization is comprised of styrene-modified mercaptopropyl-polysilsesquioxane (St-POSS-SH) with a ladder-like polysilsesquioxane containing acrylic and phenethyl sulfide groups (LPSQ). Even under open air condition, the resulting liquid silicone resin can be photo-cured in 2 minutes at room temperature, and can keep a hemisphere shape during encapsulation without a mold. The optical, mechanical and thermal properties of the resulting cured silicone resin were evaluated in detail. The silicone resin is transparent and possesses a high refractive index of 1.545(@450 nm). The encapsulation of blue GaN LED with this self-keeping hemisphere-shaped photo-cured silicone resin achieved a LEE enhancement of 25.6%, compared to flat encapsulation induced by heat. Contrary to thermal encapsulation, the encapsulation of an LED chip with the photo-curable silicone resin does not involve the processes of mold and demold, which simplifies the LED encapsulation procedure, and obviously decreases the operation cost. We believe that this novel photo-curable silicone resin based on thiol–ene photopolymerization will greatly improve the encapsulation of LED chips.