Wavelength Conversion Materials
Inorganic phosphors doped with rare-earth metals are used for a variety of applications. A wide array of these phosphors has been developed for use with fluorescent lamps. The first SSL white LEDs have used an yttrium aluminum garnet doped with trivalent cerium (YAG:Ce+3) to convert output from a blue LED into very broad-band yellow light. The sum of the two emissions appears white. For efficient white lighting with good color rendering, quantum efficiencies >85% (for phosphors absorbing in the near-UV) at operating temperatures >155 oC (for phosphors in intimate contact with the LED) will be needed.
A red-emitting phosphor centered near 610 nm with narrow band emission (required because of the steep drop-off in human eye sensitivity at longer wavelength) and absorption in the near-UV or blue region has been difficult to achieve. Broader-band emission is acceptable in the green region because of the eye’s sensitivity to a wider range of wavelengths in this region. Phosphors based on divalent europium (Eu+2) are available in the green and even into the blue emission region.
Photon Recycling Semiconductors
This approach is a photon down-conversion scheme similar to using yellow phosphors, in which an AlGaInP photoluminescent quantum well (QW) or active layer is laminated to a GaN LED. Some of the blue emission from the GaN chip is absorbed in the phosphide layer, which emits complementary yellow light. The combined blue and yellow emission produces white light.
White light can be produced using semiconductor nanoparticles. For example, the band-gap of CdS nanoparticles can be tuned over the entire visible spectrum by the changing their size (because of quantum confinement effects) and surface characteristics (e.g., coating the nanoparticles with ZnS or changing the nature of chemical groups bonded to the nanoparticle surface).