Novel Active Layer Nanostructures for White Light Emitting Diodes (Phase I)
Investigating Organization
Dot Metrics Technologies
Principal Investigator(s)
Mr. Mike Ahrens
Subcontractor
None
Funding Source
Small Business Innovation R&D, Phase 1
Award
DOE Share: $100,000
Contract Period
7/13/04 - 4/13/05
The most efficient solid state white lights developed to date typically use a bright blue light emitting diode, as a blue source, and, simultaneously, to optically excite an inorganic downconverter, converting a fraction of the blue light to yellow. The yellow light is mixed with the leftover blue to be perceived by the human eye as white. The energy efficiency of such a "white light emitting diode" is limited because the photonic downconversion process suffers from a fundamental energy loss ("Stokes shift") as higher energy blue photons are converted to yellow. Also, the color uniformity in the illuminated region is not ideal, because the geometry of the blue source (a chip) is different than the source geometry of the yellow source (a layer atop the chip). Recent results on InGaN LEDs have highlighted the positive effect of nanostructure on LED efficiency (O'Donnell, Martin et al. 1999). Dot Metrics Technologies and UNC Charlotte are working to incorporate multicolor nanostructured active layers into light emitting devices, to achieve the same advantages with more color flexibility. In Phase I, we are formulating mixtures of various sizes of semiconductor quantum dots and integrating them into quantum dot composite structures. The color of the peak luminescence of a semiconductor quantum dot is dictated by the quantum size effect when the particle size is small compared to the Bohr-exciton radius (Brus 1984). Deposited quantum dot samples are analyzed with fluorescence microoscopy and scanning probe microscopy. A preliminary LED design has been developed and LED devices are currently being fabricated in a designed experiment to determine optimum conditions for high-efficiency white light emission.
References:
Brus, L. E. (1984). "Electron–electron and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state." Journal of Chemical Physics 80(9): 4403-4409.
O'Donnell, K. P., R. W. Martin, et al. (1999). "Origin of luminescence from InGaN diodes." Physical Review Letters 82: 237-240."