New Hybrid Nanoparticles Make for More Efficient LEDs

Achievement date: 
2015
Outcome/accomplishment: 

Researchers at the NSF-funded Lighting Enabled Systems & Applications ERC (LESA), headquartered at Rensselaer Polytechnic University, have demonstrated for the first time ever, the use of rare-earth-free, non-scattering hybrid nanoparticle-filled silicone as efficient phosphor-converted LED encapsulants. Taking advantage of a highly integrated material platform, organic fluorescent dyes have been successfully incorporated into high refractive-index nanoparticles, whose surfaces are carefully engineered with silicone polymer brushes. The organic dye concentration and intermolecular spacing are tuned to obtain different spectra and agglomeration of the dye is prevented. Meanwhile, the high refractive-index feature gives high light-extraction efficiency and suppresses light losses due to Rayleigh scattering. The surface-bound silicone brushes are further functionalized with cross-linkable groups to enable new LED package geometries and luminaire designs.

Impact/benefits: 

Currently, phosphor-converted LED encapsulants are largely limited by China's rare-earth export quotas, and are less energy-efficient due to the reduction in fluorescence intensity caused by dye aggregation, radical-sensitized photo-oxidation, and scattering losses.   Incorporating high conversion-efficiency organic dyes into high refractive-index nanoparticles offers a high degree of control over the dispersion and distribution of dye molecules. The wide band-gap crystalline nanoparticle provides oxidation barrier and eliminates photo-bleaching pathways. In addition to the nanoscale dimensions of the functionalized nanoparticles, scattering losses can be further minimized by refractive index matching between the LED die and the filled encapsulant. By introducing cross-linkable chemical components into the hybrid nanoparticle, the encapsulant materials can be molded in such a way as to provide brighter and smarter lighting systems.

Explanation/Background: 

High refractive index ZrO2 nanoparticles were synthesized using a non-aqueous surfactant-free approach, which offers appropriate nanoscale dimension, size distribution, and surface properties for surface ligand engineering. Using a simple “grafting-to” technique, multimodal PDMS polymer brushes were attached onto the particle surface to provide favorable enthalpic and entropic particle-particle interaction. Organic fluorescent molecules with suitable anchoring groups were grafted onto the particle surface or doped into particles for better reliability. Control over brush molecular weight, graft density, and nanoparticle loading provided several design degrees of freedom. With the coupling of vinyl groups at the end of PDMS brushes, the rare-earth-free color-converting encapsulant with high refractive index can be further molded into photonic crystals and graded index structures.