As LED usage becomes increasingly prevalent, the industry seeks to improve efficiency through the use of higher index encapsulants engineered from modified nano-zirconia composites, which promise to deliver enhanced light extraction, better color conversion control, and improved overall performance.
A crucial factor to optimal luminous efficacy is maximizing the refractive index of LED encapsulants. A higher index directly correlates to reduced total internal reflection (TIR) between LED chip and encapsulant. Closely matching the refractive index of the LED chip to the index of the encapsulant can result in near-perfect extraction, however the differential between LED semiconductor compounds with high indices and optically clear organic encapsulants with low indices makes this a challenge. Other characteristics of these materials, such as low optical absorption, photothermal stability, and mechanical properties can also reduce the range of appropriate constituents and generally limit these encapsulant types to refractive index values below 1.6.
An encapsulants’ refractive index can be increased through the addition of transparent high-index inorganic nanoparticles, provided nanoparticle agglomeration can be prevented, good optical clarity can be achieved. This occurs when nanoparticles clump together to create larger, light dispersing particles.
High-index nano particle/silicone composites have been developed using nano-zirconium oxide (nano-ZrO2) resulting in both satisfactory photothermal stability and high optical clarity. By incorporating organic dyes directly into this nanoparticle structure, highly stable color-conversion can also be accomplished.
The “polymer brush” technique employs matrix-compatible polymer ligands to coat the nanoparticle surface and impede agglomeration. The utilization of this approach with bimodal-brush-modified nano-zirconia filled silicone displayed a 7% improvement in light extraction when compared to pure silicone.
Graded-index lens structures can also enhance LED extraction efficiency and far-field emission control by varying the refractive index of filler loadings, with low-index layers surrounding high index materials closest to the LED chip.
Initial tests to determine reliability involved evaluating sets of red, green, and blue LEDs manually encapsulated with both unmodified and nanoparticle enhanced silicone at varying levels of usage. Green and blue LEDs retained over 85% of optical output at 1000 hours, and though it had been postulated that the high photonic energy of the blue LEDs would be more likely to induce photochemical degradation, it was the red LEDs in which the highest output degradation was experienced.
The incorporation of organic dye to the nano-ZrO2 at a molecular level results in drastic reduction in post-processing degradation with no discernable impact to the crystalline structure. Also being explored are more effective synthetic methods of organic reagent extraction to reduce blue light absorption due to the slight yellow coloration from organic impurities. In addition to developments in organic color conversion and index engineering, these advances can also lead to new techniques in exotic optical systems fabrication, due to the elimination of the use of matrix by the usage of the surface-bound polymer brush and the maximization of filler loading.
Testing will continue to assess of the capabilities of nano-ZrO2-filled silicones and to analyze their potential for commercial applications. Though evaluations are still in early stages, the variety of plausible uses for this type of technology may lead to some very interesting developments in LED encapsulant industry standards.