Improving the Efficiency of Solid State Light Sources

Abstract

This proposal addresses the national need to develop a high efficiency light source for general illumination applications. The goal is to perform research that would lead to the fabrication of a unique solid state, white-emitting light source. This source is based on an InGaN/GaN UV-emitting chip that activates a luminescent material (phosphor) to produce white light.White-light LEDs are commercially available which use UV from a GaN chip to excite a phosphor suspended in epoxy around the chip. Currently, these devices are relatively inefficient. This research will target one technical barrier that presently limits the efficiency of GaN based devices. Improvements in efficiencies will be achieved by improving the internal conversion efficiency of the LED die, by improving the coupling between the die and phosphor(s) to reduce losses at the surfaces, and by selecting phosphors to maximize the emissions from the LEDs in conversion to white light. The UCSD research team proposes for this project to develop new phosphors that have high quantum efficiencies that can be activated by the UV-blue (360-410 nm) light emitted by the GaN device. SCIENTIFIC ACHIEVEMENTS: DEVELOPMENT OF NEW PHOSPHORS Objective:The main goal for the UCSD team was to develop new phosphor materials with a very specific property: phosphors that could be excited at long . The photoluminescence of these new phosphors must be activated with photons emitted from GaN based dies. The GaN diodes can be designed to emit UV-light in the same range (λ=350-410 nm). A second objective, which is also very important, is to search for alternate methods to fabricate these phosphors with special emphasis in saving energy and time and reduce pollution. Achievements:The UCSD group has pioneered a method, combustion synthesis, to rapidly produce multicomponent oxides. The combustion synthesis method is based in the use of metal nitrates (oxidizers) and a fuel (urea, carbohydrazide or hydrazine) that initiates a highly exothermic reaction, as described in more detail in [1,2]. As an example of this process, EuAlO 3 :Eu 3+ is synthesized by reacting two precursors, Eu(NO 3 ) 3 and 3 Al(NO 3 ) 3 using hydrazine as a fuel in a controlled atmosphere which is heated below <200ºC to produce the combustion reaction. The powder obtained is red-emitting (λ=610 nm) with strong absorption in the UV (λ= 365 nm). The product is then exposed to a heated ammonia flow which yields EuAlO 3 :Eu 2+ green luminescent powders. Thus, a new phosphor material that absorbs efficiently photons in the range λ=250-400 nm and with a broad emission peak centered at λ=525 nm is synthesized.In this work we also found that combustion synthesis is a powerful technique to develop With the same method is possible to produce Y 2 SiO 5 :Ce 3+ a blue-emitting oxide phosphor that is efficiently excited at λ=365 nm. Figure 1 shows these three phosphors excited by UV at a wavelength of 365 nm and the corresponding luminescence spectrum of the blend of these three colors producing a white-emitting phosphor.An...