Nanoparticle-loaded encapsulation materials for light-emitting diode applications
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Nanoparticle-loaded encapsulation materials for light-emitting diode applications F. W. Mont1, H. Luo2, M. F. Schubert1, J. K. Kim1, E. F. Schubert1,2, and R. W. Siegel3 1 Future Chips Constellation, Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY, 12180 2 Future Chips Constellation, Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY, 12180 3 Department of Materials Science and Engineering and Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, NY, 12180
ABSTRACT Nanoparticle-loaded encapsulants provide unique optical and material properties for the enhancement of light-extraction efficiency in light-emitting diodes (LEDs). We report on the uniform dispersion of TiO2 nanoparticles with average diameter of 40 nm in epoxy, and the demonstration of a refractive index (n) of 1.68 at 400 nm wavelength, higher than that of pure epoxy (n = 1.53). It is found that proper chemical surfactants and nanoparticle preparation are critical to eliminate agglomeration of nanoparticles. Theoretical analysis of optical scattering in nanoparticle-loaded encapsulation materials reveals that although the size and loading factor of nanoparticles greatly influence scattering, specular transparency of the encapsulant film occurs if the thicknesses of the films are kept below the optical scattering length. Furthermore, the encapsulants benefit from an optimized scattering coefficient as demonstrated by threedimensional ray-tracing simulations showing light-extraction efficiency enhancements greater than 50%.
INTRODUCTION High light-extraction efficiency in light-emitting diodes (LEDs) is primarily focused on designs such as surface roughening1-3, mirrors4, and photonic crystals5. An encapsulant is placed between the semiconductor and air to enhance light-extraction efficiency, but light-extraction is limited due to the low refractive index values of the encapsulant. Typical refractive indices lie in the range of 1.4 to 1.6, far below the semiconductor refractive index of 2.5 to 3.5. Due to this refractive index contrast between the semiconductor and the encapsulant, the escape cone of the LED is narrow. Photons incident upon the semiconductor-encapsulant boundary that are beyond the critical angle (escape cone) will be totally internally reflected in the semiconductor. Encapsulants with high refractive indices are therefore needed to achieve high light-extraction efficiency. High-n nanoparticle-loaded encapsulants with optimized scattering properties will overcome these fundamental limitations.
EXPERIMENTAL DETAILS Titanium dioxide (TiO2) nanoparticles of average size 40 nm in its rutile form are dried in a nitrogen oven at 125°C for 24 hours to remove moisture and organic compounds present in the
as-received nanoparticles. Immediately after drying, the nanoparticles are mixed in a solvent while continuously stirred. For surface treatment of the nanoparticles, either Surfactant A or Surfactant B is th
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