Development of White LEDs Using Nanophosphor-InP Blends
Investigating Organization
Sandia National Laboratories
Principal Investigator(s)
Lauren Shea Rohwer
Subcontractor
None
Funding Source
Building Technologies Program/NETL
Award
DOE Share: $600,000
Contract Period
10/01/06 - 04/30/08
The wavelength conversion approach to solid-state lighting (SSL) utilizes LEDs that emit near-UV or blue light in combination with phosphors that absorb the excitation light and convert it to visible light. Although good color rendering can be achieved with this approach, the strong scattering of the micron-size phosphor particles disperses the output light, leading to poor beam quality and low lighting efficiency. The scattering of a particle is proportional to the square of its mass or volume, so at constant particle loading the scattering scales as the volume of a particle. Reducing the particle size to a few nanometers would essentially eliminate collimation losses due to scattering.
This project proposes to develop blends of oxide nanophosphors and semiconductor quantum dots (QDs) in encapsulants to produce high conversion efficiency white-emitting blends with a variety of correlated color temperatures and good color rendering index. The ultimate goal of this research is to produce white LEDs containing encapsulated nanophosphor-QD blends that are superior to LEDs made with QDs or traditional phosphors alone.
Our approach is to select from the best available phosphors those that we believe can be synthesized in nanoscale form. We targeted several oxide phosphors with high quantum yield (QY) and strong absorption in the near-UV/blue spectral region. The main challenge is to synthesize these phosphors as nanoparticles, and maintain their high QY at the nanoscale.
Using pure solution methods, we have recently synthesized our first red-emitting oxide nanophosphor with particle sizes of 2-30 nm. The particles are crystalline and single-phase. The photoluminescence emission and excitation spectra were measured and are both comparable to the bulk phosphor. With this material, we have another option for the red component in blends. We will also use non-toxic InP QDs which absorb in the blue spectral region.
The development of the blends will be guided by a mathematical model that we will develop for the computation of the absorption, emission, and re-emission in blends of multiple luminescent species. Computation of the emission cascade from blends of nanophosphors, quantum dots, and mixtures thereof will be performed to determine the optimal blend for the desired color properties. The computer predictions of the conversion efficiency, CRI and CCT for specific blends will be experimentally verified. If this work is successful, we hope to achieve a white LED with much improved optical properties, significantly better collimation, high conversion efficiency, and excellent stability.