Patent Application
20110220920
The present invention relates to light emitting devices and, more particularly, to methods of forming light emitting devices that have high color rendering index (“CRI”) values and to light emitting devices that have such high CRI values.
Light emitting diodes (“LEDs”) are well known solid state lighting devices that are capable of generating light. LEDs generally include a plurality of semiconductor layers that may be epitaxially grown on a semiconductor or non-semiconductor substrate such as, for example, sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide substrates. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. As the electrons and holes flow toward each other, some of the electrons will “collide” with a hole and recombine. Each time this occurs, a photon of light is emitted, which is how LEDs generate light. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device (i.e., the area where the electrons and holes recombine).
LEDs typically have a narrow wavelength distribution that is tightly centered about a “peak” wavelength (i.e., the single wavelength where the radiometric emission spectrum of the LED reaches its maximum as detected by a photo-detector). For example, the spectral power distributions of a typical LED may have a full width of, for example, about 10-30 nm, where the width is measured at half the maximum illumination (referred to as the full width half maximum or “FWHM” width). Accordingly, LEDs are often identified by their “peak” wavelength or, alternatively, by their “dominant” wavelength. The dominant wavelength of an LED is the wavelength of monochromatic light that has the same apparent color as the light emitted by the LED as perceived by the human eye. Thus, the dominant wavelength differs from the peak wavelength in that the dominant wavelength takes into account the sensitivity of the human eye to different wavelengths of light.
As most LEDs are almost monochromatic light sources that appear to emit light having a single color, LED lamps that include multiple LEDs that emit light of different colors have been used in order to provide solid state light emitting devices that generate white light. In these devices, the different colors of light emitted by the individual LED chips combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue light emitting LEDs, the resulting combined light may appear white, or nearly white, depending on, for example, the relative intensities, peak wavelength and spectral power distributions of the source red, green and blue LEDs.
White light may also be produced by surrounding a single-color LED with a luminescent material that converts some of the light emitted by the LED to light of other colors. The combination of the light emitted by the single-color LED that passes through the luminescent material along with the light of different colors that is emitted by the luminescent material may produce a white or near-white light. For example, a single blue-emitting LED chip (e.g., made of indium gallium nitride and/or gallium nitride) may be used in combination with a yellow phosphor, polymer or dye such as for example, cerium-doped yttrium aluminum garnet (which has the chemical formula Y3Al5O12:Ce, and is commonly referred to as YAG:Ce), that “down-converts” the wavelength of some of the blue light emitted by the LED, changing its color to yellow. Blue LEDs made from indium gallium nitride exhibit high efficiency (e.g., external quantum efficiency as high as 60%). In a blue LED/yellow phosphor lamp, the blue LED chip produces an emission with a dominant wavelength of about 450-460 nanometers, and the phosphor produces yellow fluorescence with a peak wavelength of about 550 nanometers in response to the blue emission. Some of the blue light passes through the phosphor (and/or between the phosphor particles) without being down-converted, while a substantial portion of the light is absorbed by the phosphor, which becomes excited and emits yellow light (i.e., the blue light is down-converted to yellow light). The combination of blue light and yellow light may appear white to an observer. Such light is typically perceived as being cool white in color. In another approach, light from a violet or ultraviolet emitting LED may be converted to white light by surrounding the LED with multicolor phosphors or dyes. In either case, red-emitting phosphor particles (e.g., a CaAlSiN3 (“CASN”) based phosphor) may also be added to improve the color rendering properties of the light, i.e., to make the light appear more “warm,” particularly when the single color LED emits blue or ultraviolet light.
As noted above, phosphors are one known class of luminescent materials. A phosphor may refer to any material that absorbs light at one wavelength and re-emits light at a different wavelength in the visible spectrum, regardless of the delay between absorption and re-emission and regardless of the wavelengths involved. Accordingly, the term “phosphor” may be used herein to refer to materials that are sometimes called fluorescent and/or phosphorescent. In general, phosphors may absorb light having first wavelengths and re-emit light having second wavelengths that are different from the first wavelengths. For example, “down-conversion” phosphors may absorb light having shorter wavelengths and re-emit light having longer wavelengths.
LEDs have been combined with phosphor layers by dispensing phosphor-containing encapsulant material (e.g., epoxy resin or silicone) over the LED to cover the LED. However, it can be difficult to control the geometry and/or thickness of the phosphor layer. As a result, light emitted from the LED at different angles can pass through different amounts of luminescent material, which can result in an LED with non-uniform color temperature as a function of viewing angle. Because the geometry and thickness is hard to control, it can also be difficult to consistently reproduce LEDs with the same or similar emission characteristics.
Another conventional method for coating a phosphor layer onto an LED is by stencil printing. In a stencil printing approach, multiple LEDs are arranged on a substrate with a desired distance between adjacent LEDs. A stencil is provided having openings that align with the LEDs, with the holes being slightly larger than the LEDs and the stencil being thicker than the LEDs. The stencil is positioned on the substrate with each of the LEDs located within a respective one of the openings in the stencil. A composition is then deposited in the stencil openings, covering the LEDs, with a typical composition being a phosphor in a silicone polymer that can be cured by heat or light. After the holes are filled, the stencil is removed from the substrate and the stenciling composition is cured to a solid state.
Like the volumetric dispense method described above, the stenciling method may also present difficulties in controlling the geometry and/or layer thickness of the phosphor containing polymer. The stenciling composition may not fully fill the stencil opening, resulting in non-uniform layers. The phosphor-containing composition can also stick to the stencil opening, which may reduce the amount of composition remaining on the LED. These problems can result in LEDs having non-uniform color temperature and LEDs that are difficult to consistently reproduce with the same or similar emission characteristics.
Another conventional method for coating LEDs with a phosphor utilizes electrophoretic deposition (EPD). The phosphor particles are suspended in an electrolyte based solution. A plurality of LEDs are immersed in the electrolyte solution. One electrode from a power source is coupled to the LEDs, and the other electrode is arranged in the electrolyte solution. The bias from the power source is applied across the electrodes, which causes current to pass through the solution to the LEDs. This creates an electric field that causes the phosphor particles to be drawn to the LEDs, covering the LEDs with the conversion material.
After the LEDs are covered by the phosphor particles, they are removed from the electrolyte solution so that the LEDs and their phosphor particles can be covered by a protective resin. This adds an additional step to the process and the phosphor particles can be disturbed prior to the application of the resin. During the deposition process, the electric field in the electrolyte solution can also vary such that different concentrations of phosphor particles can be deposited across the LEDs. Additionally, the electric field in the electrolyte solution may act preferentially according to particle size thereby increasing the difficulty of depositing mixed phosphors of different particle sizes. The phosphor particles can also settle in the solution, which can also result in different phosphor particle concentrations across the LEDs. The electrolyte solution can be stirred to prevent settling, but this presents the danger of disturbing the particles already on the LEDs.
Still another coating method for LEDs utilizes droplet deposition using systems similar to those in an ink jet printing apparatus. Droplets of a liquid phosphor-containing material are sprayed from a print head. The phosphor-containing droplets are ejected from a nozzle on the print head in response to pressure generated in the print head by a thermal bubble and/or by piezoelectric crystal vibrations. However, in order to control the flow of the phosphor-containing composition from the ink-jet print head, it may be necessary for the print head nozzle to be relatively small. In fact, it may be desirable to engineer the size and/or shape of the phosphor particles to prevent them from catching in the nozzle and clogging the print head.
LEDs are used in a host of applications including, for example, backlighting for liquid crystal displays, indicator lights, automotive headlights, flashlights, specialty lighting applications and even as replacements for conventional incandescent and/or fluorescent lighting in general lighting and illumination applications. In many of these applications, it may be desirable to use phosphors or other luminophoric mediums to provide a lighting source that generates light having specific properties. It may also be desirable to apply the phosphors or other luminophoric mediums to such lighting sources so that they have good conformity and/or uniformity.
Pursuant to some embodiments of the present invention, methods of forming light emitting devices are provided in which a solid state lighting source is heated. A luminescent solution is then applied to the heated solid state lighting source to form the light emitting device. The luminescent solution includes a first material that down-converts the radiation emitted by the solid state lighting source to radiation that has a peak wavelength in the green color range and that has a full width half maximum emission bandwidth that extends into the cyan color range, and at least one additional material that down-converts the radiation emitted by the solid state lighting source to radiation having a peak wavelength in another color range
In some embodiments, the luminescent solution may be applied to the solid state lighting source while the solid state lighting source is at a temperature of at least about 90 degrees Celsius. In some embodiments, the method further involves curing the luminescent solution to provide a recipient luminophoric medium on the solid state lighting source. The luminescent solution may include a binder material and/or a solvent. The curing of the luminescent solution may evaporate at least some of the solvent.
In some embodiments, the first material may be a first phosphor that down-converts the radiation emitted by the solid state lighting source to a radiation having a peak wavelength between 525 nanometers and 550 nanometers, and that has a full width half maximum emission bandwidth that extends below 500 nanometers. The at least one additional material may be a second phosphor and a third phosphor which together down-convert the radiation emitted by the solid state lighting source to a radiation having at least one peak wavelength greater than 580 nanometers. In some embodiments, the second phosphor may down-convert the radiation emitted by the solid state lighting source to a radiation having a peak wavelength between 530 nanometers and 585 nanometers, and the third phosphor may down-convert the radiation emitted by the solid state lighting source to a radiation having a peak wavelength between 600 nanometers and 660 nanometers.
In some embodiments, the solid state lighting source may be a light emitting diode that emits light having a dominant wavelength in the blue color range, and the light emitting device may emit a warm white light having a correlated color temperature between about 2500K and 4500K and has a CRI value of at least 90. The luminescent solution may be wavelength conversion particles that are suspended in a solution that includes a volatile solvent and a binder material. The volatile solvent may be evaporated via thermal energy in the heated solid state lighting source. In other embodiments, the luminescent solution may be wavelength conversion particles suspended in a solution including a nonvolatile solvent and a binder material. In such embodiments, the nonvolatile solvent and/or binder may be cured via thermal energy in the heated solid state lighting source.
In some embodiments, the solid state lighting source may be a singulated light emitting diode having a top surface and a wirebond pad on the top surface. A wire may be bonded to the wirebond pad before heating the singulated light emitting diode. In other embodiments, the solid state lighting source may be a light emitting diode wafer. This wafer may be singulated into a plurality of light emitting diode chips after the luminescent solution is applied thereto. In some embodiments, the luminescent solution may be applied to the heated solid state lighting source by spraying a layer of a first atomized luminescent solution onto the heated solid state lighting source, then curing the layer of the first atomized luminescent solution, and then spraying a layer of a second atomized luminescent solution onto the cured layer of the first atomized luminescent solution.
In some embodiments, the first phosphor may be a first aluminum garnet-based phosphor, the second phosphor may be a second aluminum garnet-based phosphor, and the third phosphor may be a nitride- or oxynitride-based phosphor. In some embodiments, the first phosphor may be a cerium activated phosphor, the second phosphor may be a cerium activated phosphor, and the third phosphor may be a europium activated phosphor.
Pursuant to further embodiments of the present invention, light emitting devices are provided that include a light emitting diode (“LED”) that emits light having a dominant wavelength in the blue color range and a conformal recipient luminophoric medium that is configured to down-convert at least some of the light emitted by the LED. The recipient luminophoric medium includes at least a first phosphor that down-converts the radiation emitted by the LED to radiation having a peak wavelength in the green color range, a second phosphor that down-converts the radiation emitted by the LED to radiation having a peak wavelength in a second color range having wavelengths above the green color range, a third phosphor that down-converts the radiation emitted by the LED to radiation having a peak wavelength in a third color range having wavelengths above the green color range, and a binder material that is cured by heat energy in the LED at the time the conformal recipient luminophoric medium layer is applied to the LED.
In some embodiments, the first phosphor has a full width half maximum emission bandwidth that extends into the cyan color range. The second phosphor may down-convert the radiation emitted by the LED to radiation having a peak wavelength in the yellow color range, and the third phosphor may down-convert the radiation emitted by the LED to radiation having a peak wavelength in the red color range. The phosphors included in the recipient luminophoric medium and the LED may be configured to together emit warm white light having a correlated color temperature between about 2500K and 3300K and having a CRI of at least 90. The first phosphor may be a LuAG:Ce phosphor, and the dominant wavelength of the LED is between about 460 nanometers and 470 nanometers. The conformal recipient luminophoric medium may include a first conformal recipient luminophoric medium layer that is directly on the LED and a second conformal recipient luminophoric medium layer that is on the first conformal recipient luminophoric medium layer, where at least one of the first conformal recipient luminophoric medium layer and the second conformal recipient luminophoric medium layer includes light diffuser particles. The light emitting devices according to various embodiments of the present invention may be designed to provide a warm white light having a high color rendering index.
Pursuant to still further embodiments of the present invention, packaged light emitting devices are provided that include a submount, an LED mounted on the submount, and a recipient luminophoric medium conformally coated on the LED and on the submount. This recipient luminophoric medium includes at least a first material that down-converts the radiation emitted by the LED to radiation having a first peak wavelength, a second material that down-converts the radiation emitted by the LED to radiation having a second peak wavelength, and a third material that down-converts the radiation emitted by the LED to radiation having a third peak wavelength.
In some embodiments, the LED emits light having a dominant wavelength in the blue color range, and the first material down-converts the radiation emitted by the LED to radiation having a peak wavelength in the green color range that has a full width half maximum emission bandwidth that extends into the cyan color range. The second material may down-convert the radiation emitted by the LED to radiation having a peak wavelength in the yellow color range. The third material may down-convert the radiation emitted by the LED to radiation having a peak wavelength in the red color range. In some embodiments, the packaged light emitting device further includes comprising an additional LED that is mounted on the submount. The additional LED may emit light having a dominant wavelength in, for example, the red color range or the blue color range.
Pursuant to still further embodiments of the present invention, packaged light emitting devices are provided that include a submount, an LED mounted on the submount, and a recipient luminophoric medium conformally coated on the LED and on the submount. The recipient luminophoric medium may include a first material that down-converts the radiation emitted by the LED to radiation having a peak wavelength in the green color range that has a full width half maximum emission bandwidth that extends into the cyan color range and a second material that down-converts the radiation emitted by the LED to radiation having a second peak wavelength in a color range having wavelengths above the green color range.
Pursuant to yet additional embodiments of the present invention, packaged LEDs are provided that include a submount, at least two LEDs that emit light having a dominant wavelength in the blue color range mounted on the submount, and a conformal recipient luminophoric medium that is mounted to receive light emitted by the at least two blue LEDs. The conformal recipient luminophoric medium may include a first phosphor that down-converts the radiation emitted by the at least two blue LEDs to radiation having a peak wavelength in the green color range that has a full width half maximum emission bandwidth that extends into the cyan color range and a second phosphor that down-converts the radiation emitted by the at least two blue LEDs to radiation having a peak wavelength in a second color range having wavelengths above the green color range. In some embodiments, the second phosphor may down-convert the radiation emitted by the at least two blue LEDs to radiation having a peak wavelength in the yellow color range, and the conformal recipient luminophoric medium may further include a third phosphor that down-converts the radiation emitted by the at least two blue LEDs to radiation having a peak wavelength in the red color range.
Pursuant to additional embodiments of the present invention, methods of forming a packaged LED are provided in which an LED is mounted on a submount. A luminescent solution is prepared that includes at least a first material that down-converts the radiation emitted by the LED to radiation having a first peak wavelength, a second material that down-converts the radiation emitted by the LED to radiation having a second peak wavelength, and a third material that down-converts the radiation emitted by the LED to radiation having a third peak wavelength. This luminescent solution is then conformally coated onto the LED and on the submount.
In some embodiments, the luminescent solution is sprayed onto the LED and on the submount. In some embodiments, the LED is heated such that the LED is at a temperature of at least about 90 degrees Celsius when the luminescent solution is sprayed onto the LED. The heat from the heated LED may cure the luminescent solution to form a conformal recipient luminophoric medium on the LED.
The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that, when used in this specification, the terms “comprises” and/or “including” and derivatives thereof, specify the presence of stated features, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, operations, elements, components, and/or groups thereof.
It will be understood that when an element such as a layer, region or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.
It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions and/or layers, these elements, components, regions and/or layers should not be limited by these terms. These terms are only used to distinguish one element, component, region or layer from another element, component, region or layer. Thus, a first element, component, region or layer discussed below could be termed a second element, component, region or layer without departing from the teachings of the present invention.
Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element's relationship to another element as illustrated in the figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower”, can therefore, encompasses both an orientation of “lower” and “upper,” depending on the particular orientation of the figure.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
Embodiments of the invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention. The thickness of layers and regions in the drawings may be exaggerated for clarity. Additionally, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing.
As used herein, the term “solid state light emitting device” may include a light emitting diode, laser diode and/or other semiconductor device which includes one or more semiconductor layers, which may include silicon, silicon carbide, gallium nitride and/or other semiconductor materials, an optional substrate which may include sapphire, silicon, silicon carbide and/or other microelectronic substrates, and one or more contact layers which may include metal and/or other conductive materials. The design and fabrication of solid state light emitting devices are well known to those skilled in the art. The expression “light emitting device,” as used herein, is not limited, except that it be a device that is capable of emitting light.
Solid state light emitting devices according to embodiments of the invention may include III-V nitride (e.g., gallium nitride) based LEDs or lasers fabricated on a silicon carbide, sapphire or gallium nitride substrates such as those devices manufactured and/or sold by Cree, Inc. of Durham, N.C. Such LEDs and/or lasers may (or may not) be configured to operate such that light emission occurs through the substrate in a so-called “flip chip” orientation. Solid state light emitting devices according to embodiments of the present invention include both vertical devices with a cathode contact on one side of the chip, and an anode contact on an opposite side of the chip and devices in which both contacts are on the same side of the device. Some embodiments of the present invention may use solid state light emitting devices, device packages, fixtures, luminescent materials/elements, power supplies, control elements, and/or methods such as described in U.S. Pat. Nos. 7,564,180; 7,456,499; 7,213,940; 7,095,056; 6,958,497; 6,853,010; 6,791,119; 6,600,175, 6,201,262; 6,187,606; 6,120,600; 5,912,477; 5,739,554; 5,631,190; 5,604,135; 5,523,589; 5,416,342; 5,393,993; 5,359,345; 5,338,944; 5,210,051; 5,027,168; 5,027,168; 4,966,862, and/or 4,918,497, and U.S. Patent Application Publication Nos. 2009/0184616; 2009/0080185; 2009/0050908; 2009/0050907; 2008/0308825; 2008/0198112; 2008/0179611, 2008/0173884, 2008/0121921; 2008/0012036; 2007/0253209; 2007/0223219; 2007/0170447; 2007/0158668; 2007/0139923, and/or 2006/0221272.
Visible light may include light having many different wavelengths. The apparent color of visible light can be illustrated with reference to a two-dimensional chromaticity diagram, such as the 1931 CIE Chromaticity Diagram illustrated in
As shown in
Light that generally appears green or includes a substantial green component is plotted in the regions 11, 12 and 13 that are above the white region 10, while light below the white region 10 generally appears pink, purple or magenta. For example, light plotted in regions 14 and 15 of
It is further known that a binary combination of light from two different light sources may appear to have a different color than either of the two constituent colors. The color of the combined light may depend on the wavelengths and relative intensities of the two light sources. For example, light emitted by a combination of a blue source and a red source may appear purple or magenta to an observer. Similarly, light emitted by a combination of a blue source and a yellow source may appear white to an observer.
Each point in the graph of
As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish, as the wavelength associated with the peak radiation of the black-body radiator becomes progressively shorter with increased temperature. This occurs because the wavelength associated with the peak radiation of the black-body radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants that produce light which is on or near the black-body locus 16 can thus be described in terms of their correlated color temperature (CCT). As used herein, the term “white light” refers to light that is perceived as white, is within 7 MacAdam ellipses of the black-body locus on a 1931 CIE chromaticity diagram, and has a CCT ranging from 2000K to 10,000K. White light with a CCT of 4000K may appear yellowish in color, while white light with a CCT of 8000K or more may appear more bluish in color, and may be referred to as “cool” white light. “Warm” white light may be used to describe white light with a CCT of between about 2500K and 4500K, which is more reddish or yellowish in color. Warm white light is generally a pleasing color to a human observer. Warm white light with a CCT of 2500K to 3300K may be preferred for certain applications.
The ability of a light source to accurately reproduce color in illuminated objects is typically characterized using the color rendering index (“CRI”). The CRI of a light source is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference black-body radiator when illuminating eight reference colors. Thus, the CRI is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the black-body radiator. Daylight generally has a CRI of nearly 100, incandescent bulbs have a CRI of about 95, fluorescent lighting typically has a CRI of about 70 to 85, while monochromatic light sources have a CRI of essentially zero. Light sources for general illumination applications with a CRI of less than 50 are generally considered very poor and are typically only used in applications where economic issues preclude other alternatives. Light sources with a CRI value between 70 and 80 have application for general illumination where the colors of objects are not important. For some general interior illumination, a CRI value of greater than 80 is acceptable. A light source with color coordinates within 4 MacAdam step ellipses of the planckian locus 16 and a CRI value that exceeds 85 is more suitable for general illumination purposes. Light sources with CRI values of more than 90 provide greater color quality.
For backlight, general illumination and various other applications, it is often desirable to provide a lighting source that generates white light having a relatively high CRI, so that objects illuminated by the lighting source may appear to have more natural coloring to the human eye. Accordingly, such lighting sources may typically include an array of solid state lighting devices including red, green and blue light emitting devices. When red, green and blue light emitting devices are energized simultaneously, the resulting combined light may appear white, or nearly white, depending on the relative intensities of the red, green and blue sources. However, even light that is a combination of red, green and blue emitters may have a low CRI, particularly if the emitters generate saturated light, because such light may lack contributions from many visible wavelengths.
According to some embodiments of the present invention, LEDs and other solid state light emitting devices are provided that emit warm white light having high CRI values. Solid state lighting devices according to embodiments of the present invention may emit light having CRI values that exceed 90 and may have a color point that is within 7 MacAdam ellipses of the black-body locus on a 1931 CIE chromaticity diagram and that has a correlated color temperature of between about 2500K and about 4500K. In other embodiments, the solid state lighting devices may emit light having CRI values that exceed 90 and may have a color point that is between 0.385 and 0.485 ccx and 0.380 and 0.435 ccy on the 1931 CIE chromaticity diagram and may have a correlated color temperature of between about 2500K and about 4500K. In some embodiments, the correlated color temperature is between about 2500K and about 3300K. The LEDs according to embodiments of the present invention may achieve these high CRI values and warm white light output with a relatively high luminous flux as compared to conventional single-die LEDs that use a comparable LED and have a comparable color point, as such conventional LEDs typically have either lower CRI values or reduced luminous flux.
The LEDs according to some embodiments of the present invention use luminophoric mediums to provide warm white light having the high CRI value. Herein, the term “luminophoric medium” refers to a medium which includes one or more luminescent materials such as, for example, phosphors. A wide variety of luminescent materials are known, with exemplary materials being disclosed in, for example, U.S. Pat. No. 6,600,175 and U.S. Patent Application Publication No. 2009/0184616. In addition to phosphors, other luminescent materials include scintillators, day glow tapes, nanophosphors, quantum dots, and inks that glow in the visible spectrum upon illumination with (e.g., ultraviolet) light. Exemplary luminophoric mediums include layers that include luminescent materials that are coated on solid state light emitting devices and clear encapsulents (e.g., epoxy-based or silicone-based curable resin) that include luminescent materials that are arranged to partially or fully cover one or more solid state light emitting devices.
Currently, single-die solid state lighting devices are available that comprise an LED that emits radiation having a dominant wavelength in the blue color range (e.g., radiation with a dominant wavelength of 465 nanometers). A luminophoric medium is coated or placed on, above and/or around the die, or otherwise arranged to receive the blue light emitted by the LED (herein a luminophoric medium that is arranged to receive the light emitted by a solid state lighting source such as an LED is referred to as a “recipient luminophoric medium”). The recipient luminophoric medium comprises an encapsulant material such as, for example, silicone that has YAG:Ce phosphor particles and (Ca1-xSrx)SiAlN3:Eu2+ phosphor particles suspended therein. The YAG:Ce phosphor particles down-convert blue light received from the LED to yellow light, and the (Ca1-xSrx)SiAlN3:Eu2+ phosphor particles down-convert the blue light received from the LED to red light.
Single-die solid state lighting devices are also currently available that comprise an LED that emits radiation having a dominant wavelength in the blue color range that have a recipient luminophoric silicone encapsulent material that includes phosphor particles that down-convert the blue light received from the LED to green light, and phosphor particles that down-convert the blue light received from the LED to red light. These devices produce a warm white light having a significantly higher CRI value such as, for example, a CRI between 84-94. Herein these devices are referred to as green/red phosphor devices. However, because phosphors that emit light having a peak wavelength in the green color range may be less efficient than various other phosphors, the green/red phosphor devices may have luminous flux values that are significantly lower (e.g., 25-30% or more) than the luminous flux values of a comparable yellow/red phosphor devices.
In order to increase the CRI value of the conventional yellow/red phosphor devices, the dominant wavelength of the blue LED that is included in the devices was increased. In particular,
In
For each of Curves A-C, the y-axis represents the normalized luminous flux of the device, where the luminous flux at each wavelength is graphed as a percentage of the luminous flux at the peak emission wavelength for that particular curve. Accordingly,
As shown in Curves A and B of
As shown in Curve C of
While the device of Curve C of
In some embodiments, these solid state light emitting devices comprise a blue LED that includes a recipient luminophoric medium that has first, second and third phosphors included therein. The first phosphor may down-convert light received from the blue LED to light having a peak wavelength in the green color range. This phosphor may have a sufficiently broad FWHM bandwidth such that its FWHM emission spectra falls into at least part of the cyan color range. In some embodiments, this first phosphor may comprise a LuAG:Ce phosphor (i.e., cerium doped LuAG). LuAG:Ce phosphors may have a peak emission wavelength of between 535 and 545 nanometers, and a FWHM bandwidth of between about 110-115 nanometers. As such, the FWHM bandwidth of LuAG:Ce phosphors may extend throughout the entire cyan color range. The second phosphor may down-convert light received from the blue LED to light having a peak wavelength in the yellow color range. In some embodiments, this second phosphor may comprise a YAG:Ce phosphor. The third phosphor may down-convert light received from the blue LED to light having a peak wavelength in the red color range. In some embodiments, this third phosphor may comprise a (Ca1-xSrx)SiAlN3:Eu2+ phosphor.
In some embodiments, the LuAG:Ce phosphor and the YAG:Ce could be grown together in a single cerium-doped structure that includes lutetium, yttrium, aluminum and oxygen. For example, the LuAG:Ce phosphor and the YAG:Ce could be implemented together as a Lu1-xYxAl5O12:Ce material. Such a material would act as both a first phosphor that emits light like a LuAG:Ce phosphor and a second phosphor that emits light like a YAG:Ce phosphor (which would provide a combined spectra having a peak between the peak wavelength of the LuAG:Ce phosphor and the peak wavelength of the YAG:Ce phosphor). Thus, it will be appreciated that the first and second phosphors may comprise two separate phosphors, two separate phosphors that are mixed together, and/or a composition in which both phosphors are grown together in the same structure.
It will also be appreciated that pursuant to some embodiments of the present invention, the first phosphor may comprise a first aluminum garnet-based phosphor, the second phosphor may comprise a second aluminum garnet-based phosphor, and the third phosphor comprises a nitride- or oxynitride-based phosphor. In some of these embodiments, the first phosphor may emit light in the green color range and the second phosphor may emit light in the yellow color range. In some embodiments, the first phosphor may comprise a LuAG:Ce phosphor or may comprise any other aluminum garnet-based phosphor, and the second phosphor may comprise a YAG:Ce or may comprise any other aluminum garnet-based phosphor. The third phosphor may comprise any suitable nitride- or oxynitride-based phosphor, and is not limited to calcium and strontium based nitride- or oxynitride-based phosphors. It will be understood that the use of first and second aluminum garnet-based phosphors may have certain advantages in particular implementations, as the two aluminum garnet-based phosphors may have good compatability when they are, for example, mixed together in a binder, and as aluminum garnet-based phosphors may exhibit good stability and have other desirable features for use in luminophoric mediums.
As shown in Curve D, the yellow/red phosphor device exhibited a CRI value of 86.6. This device was similar to the device used to generate Curve B of
In contrast, the devices of Curves E and F both achieved CRI values of greater than 90. In particular, the green/yellow/red phosphor device that was used to generate Curve E achieved a CRI value of 90.6, and the green/red phosphor device that was used to generate Curve F achieved a CRI value of 91.4. In each case, these devices had an increased percentage of their luminous flux in the cyan color range, which is attributable to the inclusion of the green phosphor that had significant emission in the cyan color range. Curve F also shows that the omission of any yellow phosphor in the device used to generate Curve F shifts the major peak in the spectra (i.e., the peak between 600 and 700 nanometers) to the right (i.e., to higher wavelengths). In the devices used to generate Curves D-F, the ratio of the yellow phosphor to the red phosphor was again varied so that each of the devices would produce a light having a color that was at approximately the same location on the 1931 CIE Chromaticity Diagram illustrated in
As can be seen from
As shown in
In particular, Curve I in
Turning now to
As noted above, in some embodiments, the recipient luminophoric medium may include a LuAG:Ce phosphor that emits light having a peak frequency between 535 nanometers and 545 nanometers and a FWHM of 110-115 nanometers, which is primarily in the green color range, aYAG:Ce phosphor that emits light having a peak frequency between 545 nanometers and 565 nanometers and a FWHM of 115-120 nanometers, which is primarily in the yellow color range, and a (Ca1-xSrx)SiAlN3:Eu2+ phosphor that emits light having a peak frequency between 630 nanometers and 650 nanometers and a FWHM of 85-95 nanometers, which is primarily in the red color range. In some embodiments, the ratio of LuAG:Ce phosphor to YAG:Ce phosphor may be between about 3-to-1 and 1-to-3 by weight. In a more specific embodiment, the ratio of LuAG:Ce phosphor to YAG:Ce phosphor may be between about 1.5-to-1 and 1-to-1.5 by weight. In some embodiments, the ratio of the combination of the LuAG:Ce phosphor and the YAG:Ce phosphor to the red phosphor may be between about 1-to-1 and 9-to-1 by weight.
In some embodiments of the present invention, light emitting devices are provided that include a solid state lighting source such as, for example, a blue or ultraviolet LED, and a recipient luminophoric medium for down-converting at least some of the radiation emitted by the solid state lighting source. The luminophoric medium may include a material such as a first phosphor that down-converts the radiation emitted by the solid state lighting source to radiation having a peak wavelength in the green color range that has a FWHM bandwidth that extends into the cyan color range, and one or more additional materials that down-convert the radiation emitted by the solid state lighting source to radiation having a peak wavelength in another color range.
In some embodiments, the first phosphor may down-convert the radiation emitted by the solid state lighting source to a radiation having a peak wavelength between 525 nanometers and 550 nanometers, and has a FWHM emission bandwidth that extends below 500 nanometers. In some embodiments, the one or more additional materials may be second and third phosphors that together down-convert the radiation emitted by the solid state lighting source to radiation having a peak wavelength greater than 580 nanometers. In some embodiments, the second phosphor may down-convert the radiation emitted by the solid state lighting source to radiation having a peak wavelength in the yellow color range, and the third phosphor may down-convert the radiation emitted by the solid state lighting source to radiation having a peak wavelength in the red color range. The first phosphor may be a LuAG:Ce phosphor, the second phosphor may be a YAG:Ce phosphor, and the third phosphor may be a (Ca1-xSrx)SiAlN3:Eu2+ phosphor. Such a light emitting device may emit a warm white light having a correlated color temperature between about 2500K and 4500K, a CRI value of at least 90, and a color point that is within 7 MacAdam ellipses of the black-body locus on a 1931 CIE chromaticity diagram. In some embodiments, the correlated color temperature may be between about 2500K and 3300K.
In further embodiments of the present invention, light emitting devices are provided that include an LED that emits light having a dominant wavelength in the blue color range, and a recipient luminophoric medium that is configured to down-convert at least some of the light emitted by the LED. The recipient luminophoric medium may include at least a first phosphor that down-converts the radiation emitted by the LED to radiation having a peak wavelength in the green color range, a second phosphor that down-converts the radiation emitted by the LED to radiation having a peak wavelength in a second color range having wavelengths above the green color range, and a third phosphor that down-converts the radiation emitted by the LED to radiation having a peak wavelength in a third color range having wavelengths above the green color range. The first phosphor may have a FWHM emission bandwidth that extends into the cyan color range.
In some embodiments, the phosphors included in the recipient luminophoric medium and the LED are configured to together emit warm white light having a correlated color temperature between about 2500K and 4500K (or even between about 2500K and 3300K) and/or having a CRI of at least 90. In some embodiments, the second phosphor down-converts the radiation emitted by the LED to radiation having a peak wavelength in the yellow color range, and the third phosphor down-converts the radiation emitted by the LED to radiation having a peak wavelength in the red color range. The first phosphor may comprise a LuAG:Ce phosphor, the second phosphor comprises a YAG:Ce phosphor, and the third phosphor comprises a (Ca1-xSrx)SiAlN3:Eu2+ phosphor.
In still further embodiments, light emitting devices are provided that include a LED that emits light having a dominant wavelength in the blue color range and a recipient luminophoric medium that is configured to down-convert at least some of the light emitted by the LED. The recipient luminophoric medium includes a first material that down-converts the radiation emitted by the LED to radiation having a peak wavelength in the green color range and a second material that down-converts the radiation emitted by the LED to radiation having a peak wavelength in a second color range having wavelengths above the green color range. The spectra emitted by the light emitting device has two distinct peaks in the visible spectrum, including a first peak in the blue color range, a second peak at wavelengths longer than the wavelengths associated with the green color range.
In some embodiments, the first material may be a first phosphor such as, for example, a LuAG:Ce phosphor, that down-converts the radiation emitted by the LED to radiation having a peak wavelength in the green color range and that as a FWHM emission bandwidth that extends into the cyan color range, and the second material may be a second phosphor such as, for example, aYAG:Ce phosphor, that down-converts the radiation emitted by the LED to a radiation having a peak wavelength in the yellow color range and a third phosphor such as, for example, (Ca1-xSrx)SiAlN3:Eu2+ phosphor, which down-converts the radiation emitted by the LED to a radiation having a peak wavelength in the red color range. The LED may emit a warm white light having a correlated color temperature between about 2500K and 4500K (or even between about 2500K and 3300K) and a CRI value of at least 90.
In some embodiments, the LED (or other solid state light emitting device) may emit light having a dominant frequency between about 430 nanometers to 470 nanometers. The first phosphor may emit light having a peak frequency between about 525 nanometers to 545 nanometers. The second phosphor may emit light having a peak frequency between about 545 nanometers to 585 nanometers. The third phosphor may emit light having a peak frequency between about 600 nanometers to 650 nanometers.
The phosphor particles for the green, yellow and red phosphors may range in diameter (the term diameter is used loosely, as the particles will not necessarily have a spherical shape) may range from about 1 micron to about 30 microns, with about half of the particles being from between about 4 microns to about 20 microns in diameter. In some embodiments, at least half of the particles of the green, yellow and red phosphors may have a size (diameter) in the range between 2 microns and 20 microns.
A solid state light emitting device 30 will now be descried that includes a luminophoric medium according to embodiments of the present invention with reference to
As shown in
It is also understood that additional layers and elements can also be included in the LED 34, including but not limited to buffer, nucleation, contact and current spreading layers as well as light extraction layers and elements. It is also understood that the oppositely doped layers can comprise multiple layers and sub-layers, as well as super lattice structures and interlayers. The active region can comprise, for example, a single quantum well (SQW), multiple quantum well (MQW), double heterostructure and/or super lattice structure. The active region and doped layers may be fabricated from different material systems, including, for example, Group-III nitride based material systems such as GaN, aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and/or aluminum indium gallium nitride (AlInGaN). In some embodiments, the doped layers are GaN and/or AlGaN layers, and the active region is an InGaN layer.
The LED 34 may be an ultraviolet, violet or blue LED that emits radiation with a dominant wavelength in a range of about 380 nm to about 475 nm.
The LED 34 may include a conductive current spreading structure 36 on its top surface, as well as one or more contacts 38 that are accessible at its top surface for wire bonding. The spreading structure 36 and contacts 38 can both be made of a conductive material such as Au, Cu, Ni, In, Al, Ag or combinations thereof, conducting oxides and transparent conducting oxides. The current spreading structure 36 may comprise conductive fingers 37 that are arranged in a pattern on the LED 34 with the fingers spaced to enhance current spreading from the contacts 38 into the top surface of the LED 34. In operation, an electrical signal is applied to the contacts 38 through a wire bond as described below, and the electrical signal spreads through the fingers 37 of the current spreading structure 36 into the LED 34. Current spreading structures are often used in LEDs where the top surface is p-type, but can also be used for n-type materials.
The LED 34 may be coated with a luminophoric medium 39 according to embodiments of the present invention. As discussed above, this recipient luminophoric medium 39 may include multiple phosphors (or other luminescent materials) that absorb at least some of the LED light and emit a different wavelength of light such that the LED emits a combination of light from the LED and the phosphors. In some embodiments, the recipient luminophoric medium 39 includes particles of a green phosphor, particles of a yellow phosphor and particles of a red phosphor mixed therein (together and/or in separate layers). It will be understood that the recipient luminophoric medium 39 may comprise any of the recipient luminophoric mediums discussed in the present disclosure.
The recipient luminophoric medium 39 may be coated on the LED 34 using many different methods, with suitable methods being described in U.S. patent application Ser. Nos. 11/656,759 and 11/899,790, both entitled Wafer Level Phosphor Coating Method and Devices Fabricated Utilizing Method. Alternatively the recipient luminophoric medium 39 may be coated on the LED 34 using other methods such an electrophoretic deposition (EPD), with a suitable EPD method described in U.S. patent application Ser. No. 11/473,089 entitled Close Loop Electrophoretic Deposition of Semiconductor Devices. Several exemplary methods of coating or otherwise applying the recipient luminophoric medium 39 onto the LED 34 are described below.
An optical element or lens 70 (see
The solid state light emitting device 30 may comprise an LED package having different sizes or footprints. In some embodiments, the surface area of the LED chip 34 may cover more than 10% or even 15% of the surface area of the submount 32. In some embodiments, the ratio of the width W of the LED chip 34 to the diameter D (or width D, for square lens) of the lens 70 may be greater than 0.5. For example, in some embodiments, the solid state light emitting device 30 may comprise an LED package having a submount 32 that is approximately 3.45 mm square and a hemispherical lens having a maximum diameter of approximately 2.55 mm. The LED package may be arranged to hold an LED chip that is approximately 1.4 mm square. In this embodiment, the surface area of the LED chip 34 covers more than 16% of the surface area of the submount 32.
The top surface 40 of the submount 32 may have patterned conductive features that can include a die attach pad 42 with an integral first contact pad 44. A second contact pad 46 is also included on the top surface 40 of the submount 32 with the LED 34 mounted approximately at the center of the attach pad 42. The attach pad 42 and first and second contact pads 44, 46 may comprise metals or other conductive materials such as, for example, copper. The copper pads 42, 44, 46 may be plated onto a copper seed layer that is, in turn, formed on a titanium adhesion layer. The pads 42, 44, 46 may be patterned using standard lithographic processes. These patterned conductive features provide conductive paths for electrical connection to the LED 34 using known contacting methods. The LED 34 can be mounted to the attach pad 42 using known methods and materials.
A gap 48 (see
Referring to
The pads 42, 44, 46 provide extending thermally conductive paths to conduct heat away from the LED 34. The attach pad 42 covers more of the surface of the submount 32 than the LED 34, with the attach pad extending from the edges of the LED 34 toward the edges of the submount 32. The contact pads 44, 46 also cover the surface of the submount 32 between the vias 56 and the edges of the submount 32. By extending the pads 42, 44, 46, the heat spreading from the LED 34 may be improved, which may improve the operating life of the LED and/or allow for higher operating power.
The LED package 30 further comprises a metalized area 66 on the back surface 54 of the submount 32, between the first and second mounting pads 50, 52. The metalized area 66 may be made of a heat conductive material and may be in at least partial vertical alignment with the LED 34. In some embodiments, the metalized area 66 is not in electrical contact with the elements on top surface of the submount 32 or the first and second mounting pads 50, 52 on the back surface of the submount 32. Although heat from the LED is spread over the top surface 40 of the submount 32 by the attach pad 42 and the pads 44, 46, more heat will pass into the submount 32 directly below and around the LED 34. The metalized area 66 can assist with this dissipation by allowing this heat to spread into the metalized area 66 where it can dissipate more readily. The heat can also conduct from the top surface 40 of the submount 32, through the vias 56, where the heat can spread into the first and second mounting pads 50, 52 where it can also dissipate.
As shown in
In some embodiments, each of the LEDs 104 may be coated with a recipient luminophoric medium according to embodiments of the present invention (not shown). As discussed above, this recipient luminophoric medium may include multiple phosphors (or other luminescent materials) that absorb at least some of the LED light and emit a different wavelength of light such that the LED emits a combination of light from the LED and the phosphors. In some embodiments, the recipient luminophoric medium includes particles of a green phosphor, particles of a yellow phosphor and particles of a red phosphor mixed therein (together and/or in separate layers). It will be understood that the recipient luminophoric medium may comprise any of the recipient luminophoric mediums discussed in the present disclosure.
The recipient luminophoric medium may be applied to the LEDs 104 in any suitable manner including, for example, by coating the wafer(s) from which the LEDs 104 grown using the coating methods described in the above-referenced U.S. patent application Ser. Nos. 11/656,759, 11/899,790, or 11/473,089, or by any of the exemplary methods of coating or otherwise applying a recipient luminophoric medium onto an LED at either the wafer level or the die level that are described below. As described below, the recipient luminophoric medium may alternatively be deposited onto and/or built into a lens 110 of packaged light emitting device 100, or may be provided between the lens 110 and the LEDs 104. Exemplary techniques for providing the recipient luminophoric medium on or next to a lens such as lens 110 will be discussed below.
Each LED 104 in the packaged light emitting device 100 may include a conductive current spreading structure 106 on its top surface such as the current spreading structures 36 discussed above with respect to light emitting device 30, as well as one or more contacts 108 that are accessible at its top surface for wire bonding. An optical element or lens 110 is formed on a the top surface of the submount 102, over the four LEDs 104, to provide environmental and/or mechanical protection. The lens 110 may be any suitable lens, including any of the lenses 70 discussed above with respect to the light emitting device 30 of
While
It will also be appreciated that additional wire bonds or other contacting structures may be provided that may be used to electrically connect the LEDs 104 in series, in parallel, or in a combination thereof. Two such wirebonds 109 are shown in
The top surface of the submount 102 may have patterned conductive features that can include die attach pads 112 which the LEDs 104 may be mounted on. Electrical contact may be made to each of the LEDs 104 using known wire bonding and contact methods such as, for example, those discussed above with respect to the light emitting device 30 of
The bottom side of the packaged light emitting device 100 (
As is apparent from the figures, the packaged light emitting device 120 of
The recipient luminophoric medium may be applied to the LEDs 124 or the lens 130, or mounted therebetween, in any suitable manner including, for example, each of the methods discussed above with respect to packaged light emitting device 100 and/or the various additional methods discussed below.
The packaged light emitting device 140 of
It will be appreciated that while
As noted above, in some embodiments, the recipient luminophoric mediums according to embodiments of the present invention may be directly coated onto a surface of a semiconductor wafer before the wafer is singulated into, for example, individual LED chips. One such process for applying the recipient luminophoric medium will now be discussed with respect to
Referring to
As shown in
As shown in
Different factors determine the amount of LED light that will be absorbed by the luminophoric medium coating 232 in the final LED chips 210, including but not limited to the size of the phosphor particles, the percentage of phosphor loading, the type of binder material, the efficiency of the match between the type of phosphor and wavelength of emitted light, and the thickness of the luminophoric medium coating 232. Many different phosphors can be used in the recipient luminophoric medium coating 232 according to the present invention. As discussed above, in some embodiments, the recipient luminophoric medium coating 232 may include a green phosphor, a yellow phosphor and a red phosphor. The green phosphor may have a FWHM bandwidth that may extend into, or even all the way across, the cyan color range. The green phosphor may comprise, for example, LuAG:Ce. Other phosphors that emit at or near the green color range include, but are not limited to, Sr6P5BO20:Eu; MSi2O2N2:Eu2+; and Zinc Sulfide:Ag with (Zn,Cd)S:Cu:Al, or other combinations. The yellow phosphor may comprise, for example, YAG:Ce. Other suitable yellow phosphors include Tb3-xRExO12:Ce(TAG) where RE=Y, Gd, La, Lu; and Sr2-x-yBaxCaySiO4:Eu. The red phosphor may comprise, for example, (Ca1-xSrx)SiAlN3:Eu2+. Other red or orange phosphors that could be used in certain embodiments include Lu2O3:Eu3+; (Sr2-xLax)(Ce1-xEux)O4; Sr2Ce1-xEuxO4; Sr2-xEuxCeO4; SrTiO3:Pr3+, Ga3+; CaAlSiN3:Eu2+; and/or Sr2Si5N8:Eu2+. It will be understood that many other phosphors can used alone or in combination to achieve the desired combined spectral output.
Different sized phosphor particles can be used including, but not limited to, 10-100 nanometer (nm)-sized particles to 20-30 μm sized particles, or larger. Smaller particle sizes typically scatter and mix colors better than larger sized particles to provide a more uniform light. Larger particles are typically more efficient at converting light compared to smaller particles, but emit a less uniform light. In some embodiments, the phosphor particles for the green, yellow and red phosphors may range in size from about 1 micron to about 30 microns, with about half of the particles being from between about 4 microns to about 20 microns. In some embodiments, at least half of the particles of the green, yellow and red phosphors may have a size (diameter) in the range between 2 microns and 20 microns. Different sized phosphors can be included in the recipient luminophoric medium coating 232 as desired before it is applied such that the end coating 232 can have the desired combination of smaller sizes to effectively scatter and mix the light, and larger sizes to efficiently convert the light.
The coating 232 can also have different concentrations or loading of phosphor materials in the binder, with a typical concentration being in range of 30-70% by weight. In one embodiment, the phosphor concentration is approximately 65% by weight, and is may be generally uniformly dispersed throughout the binder. In other embodiments the coating 232 can comprise multiple layers of different concentrations or types of phosphors, and the multiple layers can comprise different binder materials. One or more of the layers can be provided without phosphors. For example, a first coat of clear silicone can be deposited followed by phosphor loaded layers. As another example, the coating may comprise, for example, a three layer coating that includes a first layer having a first phosphor with a peak wavelength in the green color range that is coated directly on the LED chips 210, a second layer having a second phosphor with a peak wavelength in the yellow color range that is coated directly on the first layer, and a third layer having a third phosphor with a peak wavelength in the red color range that is coated directly on the second phosphor. Numerous other layer structures are possible, including multi-layers that include multiple phosphors in the same layer. Intervening layers or elements could also be provided between layers and/or between the coating and the underlying LED chips 210.
After the initial coating of the LED chips 210 with the recipient luminophoric medium coating 232, further processing is needed to expose the pedestals 228. Referring now the
As shown in
Following singulation, the LED chips 210 can be mounted in a package, or to a submount or printed circuit board (PCB) without the need for further processing to add phosphor. In one embodiment the package/submount/PCB can have conventional package leads with the pedestals 228 electrically connected to the leads. A conventional encapsulation can then surround the LED chip 210 and electrical connections.
Pursuant to further embodiments of the present invention, recipient luminophoric mediums may be applied to a light emitting device while the light emitting device is exposed to a curing agent. The curing agent may be, for example, heat, radiation, a material that is present on or in the light emitting device, or other agents that speed up the curing of the recipient luminophoric medium.
Reference is now made to
A recipient luminophoric medium is applied to the heated light emitting device (block 252). In some embodiments, the recipient luminophoric medium may be applied in the form of a luminescent solution that may be atomized using a flow of pressurized gas. While typically the luminescent solution will comprise a liquid mixture, the term “solution” is used broadly herein to cover any mixture of substances, whether or not such mix is homogeneous and regardless of the form of the substances. The atomized luminescent solution may be sprayed or otherwise deposited onto the heated light emitting device using the flow of pressurized gas. By way of example, the atomized luminescent solution may be sprayed onto the heated light emitting device using an air pressurized spray system. It will be appreciated that the atomized luminescent solution may be applied at different and/or multiple angles, directions and/or orientations.
In some embodiments, the luminescent solution comprises wavelength conversion particles such as phosphor particles that are suspended in a solution that includes a volatile liquid solvent and a binder material. In such embodiments, the volatile liquid solvent may be at least partially evaporated via thermal energy in the heated light emitting device to provide a conformal layer of wavelength conversion particles on the light emitting device. In some embodiments, the luminescent solution may alternatively or additionally include a nonvolatile liquid. In such embodiments, the nonvolatile liquid may be cured via the thermal energy in the heated light emitting device.
In some embodiments, the light emitting device may be a singulated light emitting device that may have a top surface with one or more wirebond pads thereon and/or a bottom surface with one or more wirebond pads thereon.
Respective wires may be bonded to these wirebond pad(s) before heating the light emitting device and before spraying the luminescent solution onto the light emitting device. In other embodiments, the light emitting device may comprise a semiconductor wafer that receives the luminescent solution, and this wafer may thereafter be singulated into a plurality of individual light emitting devices after the recipient luminophoric medium layer is formed thereon.
In some embodiments, multiple layers may be applied to the light emitting device to form the recipient luminophoric medium. These layers may or may not be the same. For example, a first layer may include a first set of at least one luminescent material and a second layer may include a second set of at least one luminescent material, where the first and second sets are different. Since each layer may rapidly cure once it is deposited on the heated light emitting device, subsequent layers may be applied directly thereafter. However, some embodiments provide that the light emitting device may be allowed to cool between layers and then heated again for subsequently applied layers.
As shown in
As shown in
As shown in
The reflector cup 362 is optional. For example, the LED chip 370 could be mounted on a substrate 360, printed circuit board or other support member without any reflector around the LED chip 370. Moreover, the reflector cup 362 and the substrate 360 could be merged together as a unitary structure. The substrate 360 could also include a leadframe, and a package body may be formed on the leadframe surrounding the LED chip 370 to define the optical cavity 364. Likewise, the LED chip 370 could be mounted in any of the exemplary packages discussed above with reference to
Still referring to
As shown in
The volatile solvent liquid may be evaporated by the thermal energy of the heated substrate 360 and LED chip 370, leaving the phosphor particles (and perhaps other elements such as diffuser particles that may be in the luminescent solution 354) in the binder material to provide the conformal recipient luminophoric medium 380. However, in some cases, a non-volatile liquid, such as silicone and/or epoxy resin, may be used as a carrier liquid for the phosphor/diffuser particles, in which case the non-volatile liquid may be cured by the thermal energy of the heated substrate 360 and LED chip 370 to provide a conformal recipient luminophoric medium 380 on the LED chip 370.
Referring to
As shown in
As shown in
It will be appreciated that the recipient luminophoric medium 380 may be applied to the lens 394 before, during and/or after assembly with the LED chips 370A-D. For example, some embodiments provide that an array of multiple lenses may be heated and then the luminescent solution 354 is applied thereto. In still other embodiments, the luminescent solution 354 could be sprayed or otherwise deposited in a micro mold to form the recipient luminescent medium 380 which may then be removed from the mold and placed on the light emitting device (e.g., an LED chip 370).
It will also be appreciated that in the various embodiments described herein the recipient luminophoric medium 380 may be formed as a plurality of layers. In such embodiments, the various layers of the recipient luminophoric medium 380 can have the same and/or different luminescent materials (and/or other materials) therein. For example,
In some embodiments, three layers 380A, 380B, 380C may be provided. The first layer 380A may include phosphor particles configured to convert incident light to wavelengths centered about a first peak wavelength (e.g., a peak wavelength in the yellow color range), the second layer 380B may include phosphor particles configured to convert incident light to wavelengths centered about a second peak wavelength that is in a different color range than the first peak wavelength (e.g., a peak wavelength in the green color range), and the third layer 380C may include phosphor particles configured to convert incident light to wavelengths centered about a third peak wavelength that is in a different color range than both the first and second peak wavelengths (e.g., a peak wavelength in the red color range). Accordingly, light output by the packaged LED chip 370 can be a mixture of primary light emitted by the LED chip 370 and secondary light emitted by the different phosphors or other luminescent materials included in the layers 380A, 380B and 380C. Such light can have improved color rendering properties compared to light generated using only one kind of phosphor.
As shown in
The light emitting device can then be energized, for example, by applying a voltage across anode and cathode terminals of an emitting portion, and one or more optical characteristics of the resulting device, such as output power, color point, and/or correlated color temperature can be measured (block 408). For example, the light output by the light emitting device can be measured by an optical sensor 335, and the results can be provided to a controller 320 (see
If it is determined that the device can be reworked, the light output corresponding to the light emitting device can be tuned by determining the amount and type of additional phosphor needed to correct the color point/CCT of the structure (block 414). A second conformal layer 380B can be applied (block 404). In some embodiments, the test may be performed while the light emitting device is still heated. The light emitting device may also be heated during the application of the second conformal layer 380B. The second conformal layer 380B may include the same and/or different type from the phosphor used in the first conformal layer 380A. The operations of blocks 404-414 can be repeated multiple times to achieve desired optical characteristics. However, if too much phosphor is applied, the light emission characteristics may deteriorate due to reabsorption and/or excessive absorption of light from the optical element, at which point the light emitting device may fail the test at block 410.
The system 300 may be used to spray a luminescent solution 354 onto the light emitting device 310. However, it will be appreciated that in other embodiments the luminescent solution 354 may be applied using other application techniques such as pouring, dipping, rolling, and/or brushing, among others. A heating device 337 applies heat (thermal energy) 339 to the light emitting device 310 to increase the temperature of the light emitting device 310 prior to spraying the luminescent solution 354 thereon. As shown in
In some embodiments, the liquid in the supply line 336 may include a binder that includes organic and/or organic-inorganic hybrid materials. The binder material may be, for example, a liquid silicone and/or liquid epoxy, and the volatile or nonvolatile solvent material may be, for example, alcohol, water, acetone, methanol, ethanol, ketone, isopropynol, hydrocarbon solvents, hexane, ethylene glycol, methyl ethyl ketone, xylene, toluene, and combinations thereof. In some embodiments, the binder may have an index of refraction of greater than about 1.25. Some embodiments provide that the index of refraction of a binder material may be greater than about 1.5. It may be desirable to have high light transmission across the visible spectrum. In some embodiments, the binder may have a transmission of light including about 90% or greater in a wavelength range of at least about 440 nm to about 470 nm. In some embodiments, the binder may have a transmission of light including about 95% or greater in a wavelength range of at least about 440 nm to about 470 nm In some embodiments, the binder may have a transmission of light including about 98% or greater in a wavelength range of at least about 440 nm to about 470 nm. In some embodiments, the binder may have a transmission of light of at least about 90% or greater, about 95% or greater, and/or about 98% or greater for other wavelengths in the visible spectrum, such as green, yellow and/or red. In general, a volatile solvent may dry or evaporate off shortly after being deposited. A volatile or nonvolatile solvent material can include particles therein that are to be deposited onto the LED structure, such as particles of a luminescent material (e.g., a phosphor) and/or particles of a light-scattering material, such as titanium dioxide, among others. The liquid in the supply line 336 may be provided from one of a plurality of fluid reservoirs 330A-D, which are attached to the supply line 336 through respective input lines 332A-D. The flow of liquid through the input lines 332A-D can be controlled by electronically-controlled mass flow controllers 334A-D, respectively.
As shown in
In some embodiments, the reservoirs 330A-D can also include a diffuser reservoir 330D that contains a liquid solvent in which a concentration of diffuser particles is suspended. In some embodiments, the diffuser reservoir 330D can include diffuser particles at a concentration that is greater than a concentration at which the diffuser particles will be applied onto the optical element 310.
One or more of the reservoirs 330A-D can be pressurized, so that flow from the reservoirs 330A-D can be obtained by positive pressure into the supply line 363. In particular, the solvent reservoir 330A and the binder reservoir 330B can be pressurized. In some embodiments, the phosphor reservoir 330C and/or the diffuser reservoir 330D may not be pressurized, so that flow from the phosphor reservoir 330C and/or the diffuser reservoir 330D may be induced into the supply line 336 by negative pressure caused by flow through the supply line 336. In some embodiments, the pressure in the liquid supply line 336 need not be high, since the force for spraying the liquid onto the optical element 310 may be provided by a high-pressure gas line 344.
The flow of liquid through the supply line 336 can be controlled by an electronically controllable valve 340. When the valve 340 is open, liquid in the supply line 336 is supplied to the spray nozzle 350.
As further illustrated in
Referring still to
It will be appreciated that while
As illustrated, the heating device 337 applies heat 339 to the light emitting device 310 to increase the temperature of the light emitting device 310 prior to spraying the luminescent solution 354 thereon. Some embodiments provide that the heating device 337 may be electronically controlled by the controller 320 via electronic control line 329. In some embodiments, the heating device 337 may apply heat 339 to the light emitting device 310 during the spraying operation(s). In some embodiments, the heating device 337 may be used to heat the light emitting device 310 prior to the spraying operation(s) and/or may be operated independent of the controller 320.
Some embodiments provide that the heating device 337 includes a thermally conductive heating surface through which heat 339 is transferred to the light emitting device 310. In some embodiments, the heating device 337 may use a heat transfer media, such as, for example, heated air and/or gases, to transfer heat 339 to the light emitting device 310. Embodiments of the heating device may include electrically resistive and/or conductive and/or combustion related heat generating elements.
Some embodiments provide that the light emitting device 310 is heated to greater than 70 degrees Celsius. Some embodiments provide that the light emitting device 310 is heated to greater than 90 degrees Celsius. Some embodiments provide that the light emitting device 310 is heated to greater than 120 degrees Celsius. In some embodiments, the light emitting device 310 is heated to a temperature in a range of about 70 degrees Celsius to about 155 degrees Celsius. In some embodiments, the light emitting device 310 is heated to a temperature in a range of about 90 degrees Celsius to about 155 degrees Celsius. In some embodiments, the light emitting device 310 is heated to a temperature in a range of about 90 degrees Celsius to about 120 degrees Celsius. In some embodiments, the light emitting device 310 is heated to a temperature in a range of about 90 degrees Celsius to about 155 degrees Celsius. When the atomized luminescent solution 354 is deposited on the light emitting device 310, the thermal energy in the heated light emitting device 310 may rapidly cure and/or evaporate the solvent portion of the atomized luminescent solution 354. By rapidly curing and/or evaporating the solvent, settling and/or redistribution of the luminescent materials prior to curing may be reduced. In this regard, a more uniform concentration of the luminescent materials within the applied layer may be preserved, thus providing a substantially conformal layer on the light emitting device 310.
It will be further appreciated that a system 300 as illustrated in
A mixer 341 may be provided to mix supply line 336 constituents from various different ones of the reservoirs 330A-D. In some embodiments, the mixer 341 may include a static mixing element that causes the materials in the supply line 336 to mix by virtue of the flow therethrough. In other embodiments, an active mixing element may be provided that agitates the supply line 336 materials to keep particles in suspension and/or substantially uniformly distributed throughout the materials. Although not illustrated, pressure controllers may be provided for various ones of the components. For example, the reservoirs 330A-D and the nozzles 350 may include pressure controllers to provide control over the supply and/or delivery pressures, among others. Further, some embodiments may include static and/or active mixing elements in the reservoirs 330A-D. For example, the phosphor reservoir 330C and the diffuser reservoir 330D may use mixing elements to maintain the particles in suspension.
As is further shown in
Reference is now made to
A syringe 357 may be provided that includes a luminescent solution 354. The luminescent solution 354 may include, for example, one or more types of phosphor particles, one or more types of diffuser particles, a binder, and/or one or more solvents. The syringe 357 may be loaded with the luminescent solution 354 using, for example, a cartridge, shortly before the application operations to reduce settling and/or stratification of the components therein. In some embodiments, the syringe 357 may be coupled directly and/or closely to the nozzle 350 to reduce settling of suspended particles in the luminescent solution 354. Some embodiments provide that lateral fluid paths may be reduced and/or avoided as such paths may result in settling and/or stratification of the luminescent materials in the luminescent solution 354. In some embodiments, an active and/or static mixing element is provided with and/or within the syringe 357 to reduce settling.
A fluid pressurizer 356 may be provided to provide and/or control a fluid pressure within the syringe 357. Some embodiments provide that the fluid pressure may be substantially lower than the gas pressure provided by the gas pressurizer 342.
As further illustrated in
It will be appreciated that while
As illustrated, the heating device 337 applies heat 339 to the light emitting device 310 to increase the temperature thereof before the luminescent solution 354 is sprayed onto the light emitting device 310. Some embodiments provide that the heating device 337 may be electronically controlled by the controller 320 via electronic control line 329. In some embodiments, the heating device 337 may apply heat 339 to the light emitting device 310 during the spraying operation(s). In some embodiments, the heating device 337 may be used to heat the light emitting device 310 prior to the spraying operation(s) and/or may be operated independent of the controller 320.
Some embodiments provide that the heating device 337 includes a thermally conductive heating surface through which heat 339 is transferred to the light emitting device 310. In some embodiments, the heating device 337 may use a heat transfer media, such as, for example, heated air and/or gases, to transfer heat 339 to the light emitting device 310. Embodiments of the heating device 337 may include electrically resistive and/or conductive and/or combustion related heat generating elements.
It will be further appreciated that a system 500 as illustrated in
A sacrificial pattern 614 is formed on the electrical contacts 612. The sacrificial pattern 614 can include a material such as soluble polymer and/or glass, which can be applied and patterned using conventional photolithographic techniques. The sacrificial pattern 614 can be aligned with the underlying electrical contacts 612. Alternatively, the sacrificial pattern 614 may only cover portions of the electrical contacts 612, with some portions of the electrical contacts 612 being exposed. In some embodiments, the sacrificial pattern 614 can be wider than the electrical contacts 612, so that portions of the surface 610A of the LED wafer 610 adjacent the electrical contacts are also covered by the sacrificial patterns. Each of these configurations is illustrated in
Referring still to
After spray-coating the LED wafer 610, the sacrificial pattern 614 can be removed, for example, by exposure to a liquid solvent specific to the sacrificial pattern material, resulting in an LED wafer 610 as shown in
According to still further embodiments of the present invention, the recipient luminophoric medium 380 may be coated onto individual singulated light emitting devices 310. For example, pursuant to one such exemplary method, a light emitting device may be mounted on a reflective cup by means of a solder bond or conductive epoxy, and an encapsulant material such as, for example, silicone that has the phosphors suspended therein, may be used as the recipient luminophoric medium. This recipient luminophoric medium may be used, for example, to partially or completely fill the reflective cup.
While exemplary methods of applying a recipient luminophoric medium 380 and otherwise fabricating the light emitting devices according to embodiments of the present invention have been discussed above, it will be appreciated that that numerous other fabrication methods are available. For example, U.S. patent application Ser. No. 11/899,790, filed Sep. 7, 2007, the entire contents of which are incorporated herein by reference, discloses various additional methods of coating a recipient luminophoric medium onto a solid state light emitting device.
It is understood that although the present invention has been described with respect to LEDs having vertical geometries, it may also be applied to LEDs having other geometries such as, for example, to lateral LEDs that have both contacts on the same side of the LED chip.
Many different embodiments have been disclosed herein, in connection with the above description and the drawings. It will be understood that it would be unduly repetitious and obfuscating to literally describe and illustrate every combination and subcombination of these embodiments. Accordingly, the present specification, including the drawings, shall be construed to constitute a complete written description of all combinations and subcombinations of the embodiments described herein, and of the manner and process of making and using them, and shall support claims to any such combination or subcombination.
While embodiments of the present invention have primarily been discussed above with respect to solid state light emitting devices that include LEDs, it will be appreciated that according to further embodiments of the present invention, laser diodes and/or other solid state lighting devices may be provided that include the luminophoric mediums discussed above. Thus, it will be appreciated that embodiments of the present invention are not limited to LEDs, but may include other solid state lighting devices such as laser diodes.
In the drawings and specification, there have been disclosed embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.
This application claims priority under 35 U.S.C. §120 as a continuation-in-part of U.S. patent application Ser. No. 12/720,390, filed Mar. 9, 2010, the disclosure of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12720390 | Mar 2010 | US |
| Child | 13017983 | US |
