1. Field of the Invention
Embodiments of the present invention are directed in general to novel silicate-based yellow and/or green phosphors (herein referred to as yellow-green phosphors) for use in a white light illumination system such as a white light emitting diodes (LED). In particular, the yellow-green phosphors of the present invention comprise a silicate-based compound having at least one divalent alkaline earth element and at least one anion dopant, wherein the optical performance of the novel phosphors is equal to or exceeds that of either known YAG:Ce compounds or known silicate-based compounds that do not take advantage of the benefits of including an anion dopant.
2. State of the Art
White LED's are known in the art, and they are relatively recent innovations. It was not until LED's emitting in the blue/ultraviolet region of the electromagnetic spectrum were developed that it became possible to fabricate a white light illumination source based on an LED. Economically, white LED's have the potential to replace incandescent light sources (light bulbs), particularly as production costs fall and the technology develops further. In particular, the potential of a white light LED is believed to be superior to that of an incandescent bulbs in lifetime, robustness, and efficiency. For example, white light illumination sources based on LED's are expected to meet industry standards for operation lifetimes of 100,000 hours, and efficiencies of 80 to 90 percent. High brightness LED's have already made a substantial impact on such areas of society as traffic light signals, replacing incandescent bulbs, and so it is not surprising that they will soon provide generalized lighting requirements in homes and businesses, as well as other everyday applications.
There are several general approaches to making a white light illumination system based on light emitting phosphors. To date, most white LED commercial products are fabricated based on the approach shown in FIG. 1, where light from a radiation source does affect the color output of the white light illumination. Referring to the system 10 of FIG. 1, a radiation source 11 (which may be an LED) emits light 12, 15 in the visible portion of the electromagnetic spectrum. Light 12 and 15 is the same light, but is shown as two separate beams for illustrative purposes. A portion of the light emitted from radiation source 11, light 12, excites a phosphor 13, which is a photoluminescent material capable of emitting light 14 after absorbing energy from the source 11. The light 14 can be a a substantially monochromatic color in the yellow region of the spectrum, or it can be a combination of green and red, green and yellow, or yellow and red, etc. Radiation source 11 also emits blue light in the visible that is not absorbed by the phosphor 13; this is the visible blue light 15 shown in FIG. 1. The visible blue light 15 mixes with the yellow light 14 to provide the desired white illumination 16 shown in the figure.
A known yellow phosphor that has been used in the art according to the scheme illustrated in FIG. 1 is a YAG-based phosphor having a main emission peak wavelength that varies in the range of about 530 to 590 nm depending on the composition, especially the amount of gadolinium (Gd) atoms substituting yttrium (Y) atoms constituting the YAG-based phosphor. Another factor that influences the main emission peak wavelength is the amound of the Ce3+ added as a luminescent center. It is known that the peak emission wavelength shifts to longer wavelengths as either the substitution amount of Gd or the amount of Ce3+ is increased. Color control of the white light may be accomplished by changing the output ratio between the blue light emitted by the blue LED and the yellow light emitted by the YAG-based phosphor.
U.S. Pat. No. 5,998,925 to Shimizu et al. discloses the use of a 450 nm blue LED to excite a yellow phosphor comprising a yttrium-aluminum-garnet (YAG) fluorescent material. In this approach a InGaN chip functions as a visible, blue-light emitting LED, and a cerium doped yttrium aluminum garnet (referred to as “YAG:Ce”) serves as a single phosphor in the system. The phosphor typically has the following stoichiometric formula: Y3Al5O12:Ce3+. The blue light emitted by the blue LED excites the phosphor, causing it to emit yellow light, but not all the blue light emitted by the blue LED is absorbed by the phosphor; a portion is transmitted through the phosphor, which then mixes with the yellow light emitted by the phosphor to provide radiation that is perceived by the viewer as white light.
The YAG:Ce phosphors of the prior art have known disadvantages. One disadvantage is that when used in an illumination system it may contribute to production of white light with color temperatures ranging from 6,000 to 8,000 K, which is comparable to sunlight, and a typical color rendering index (CRI) of about 70 to 75. These specifications are viewed as a disadvantage because in some instances white light illumination systems with a lower color temperature are preferred, such as between about 3000 and 4100 K, and in other cases a higher CRI is desired, such as above 90. Although the color temperature of this type of prior art system can be reduced by increasing the thickness of the phosphor, the overall efficiency of the system decreases with such an approach.
Another yellow phosphor that has been used in the art according to the scheme illustrated in FIG. 2 is a silicate-based phosphor described by T. Maeda et al. in U.S. Patent Application Publication 2004/0104391 A1, published Jun. 3, 2004. In this publication, T. Maeda et al. describe a silicate-based phosphor according to the formula (Sr1-a1-b1-xBaa1Cab1Eux)2SiO4, where 0≦a1≦0.3; 0≦b1≦0.8; and 0<x<1. This yellow-yellowish phosphor emits a fluorescence having a main emission peak in the wavelength range from 550 to 600 nm, inclusive, with a wavelength range 560 to 590 nm being preferred. Still more preferable was a phosphor emitting a fluroescence having a main emission peak in the wavelength range 565 to 585 nm, both inclusive.
That the YAG-based phosphors exemplified by U.S. Pat. No. 5,998,925 to Shimizu et al., or the silicate-based phosphors of T. Maeda et al. in U.S. Patent Application Publication 2004/0104391 A1, can produce white light according to the visible excitation source scheme of FIG. 1, may be understood in part by studying an excitation spectra shown in FIG. 2, taken from the Maeda et al. patent application. FIG. 2 is a graph showing the excitation and emission spectra of Maeda et al.'s silicate phosphor and a YAG-based phosphor. Maeda et al.'s silicate based phosphor is a yellow (or yellow/yellowish, as they describe it) phosphor which has an excitation peak around 250 to 300 nm, and absorbs light in a wavelength range of 100 to 500 nm to emit a yellow/yellowish fluorescence having an emission peak in the 550 to 600 nm range; i.e., from yellow-green to yellow to orange. Accordingly, if light from the yellow/yellowish phosphor of Maeda et al. is combined with the blue light from a blue-light-emitting device, the resulting light is substantially white in nature.
Maeda et al. concede in FIG. 2 that their silicate phosphor has a low luminous efficacy, the luminous efficacy of their silicate-based phosphor being only half of that of a YAG-based phoshpor under 470 nm excitation when the silicate-based phosphor is excited by blue light in the wavelength range greater than 430 nm and less than or equal to 500 nm. This necessitates the use of a larger amount of Maeda et al.'s phosphor relative to a YAG-based phosphor in order to obtain the same color of light according to the scheme of FIG. 2. In a description provided by Maeda et al., the luminescent layer is “relatively thick” compared to that which would have been used had the luminescent been a YAG-based phosphor. In this case the blue light intensity used as a part of white illumination will be significantly reduced by relative thick layer of Maeda et al's yellow phosphors.
What is needed is an improvement over the silicate-based, yellow phosphors of the prior art where the improvement is manifestated at least in part by an equal or greater conversion efficiency from blue to yellow. The enhanced yellow phosphor with low gravity density and low cost may be used in conjunction with a blue LED to generate light whose color output is stable, and whose color mixing results in the desired uniform, color temperature and color rendering index.