High refractive index coating of phosphor-in-glass for enhanced light extraction efficiency of white LEDs

The coating of phosphor-in-glass with high refractive index was successfully fabricated for enhanced light extraction efficiency of white LEDs through a multilayer screen-printing and low-temperature sintering procedure. The effects of sintering temperature on the B2O3–SiO2–ZnO glass coating and YAG:Ce phosphor concentration on the phosphor-in-glass coating were investigated. The B2O3–SiO2–ZnO glass matrix possesses low glass transition temperature and appropriate thermal expansion coefficient. With the addition of La2O3 and WO3, high refractive index of 1.82 at 460 nm is acquired. The amorphous glass coating, sintered at 600 °C, yields an optimized transmittance of 50%. Furthermore, negligible thermal degradation is observed in the phosphor-in-glass coating, and the emission peak under the 460-nm excitation is at ~540 nm. When increasing the concentration of phosphor, luminous efficacy and correlated color temperature are both improved, but color rendering index is deteriorated. As the refractive index increases, the light extraction efficiency of white LEDs is enhanced due to the reason that the match of refractive index between the glass matrix and the phosphor decreases the total internal reflection at the interface.