`Baretz et al.
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`I lllll llllllll Ill lllll lllll lllll lllll lllll 111111111111111111111111111111111
`US006600175Bl
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`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,600,175 Bl
`*Jul. 29, 2003
`
`(54) SOLID STATE WHITE LIGHT EMITTER
`AND DISPLAY USING SAME
`
`(75)
`
`Inventors: Bruce Baretz, West Milford, NJ (US);
`Michael A. Tischler, Danbury, CT (US)
`
`(73) Assignee: Advanced Technology Materials, Inc.,
`Danbury, CT (US)
`
`( *) Notice:
`
`This patent issued on a continued pros(cid:173)
`ecution application filed under 37 CFR
`1.53( d), and is subject to the twenty year
`patent term provisions of 35 U.S.C.
`154(a)(2).
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 1212 days.
`
`(21) Appl. No.: 08/621,937
`
`(22) Filed:
`
`Mar. 26, 1996
`
`Int. Cl.7 ................................................ HOlL 33/00
`(51)
`(52) U.S. Cl. ........................... 257/100; 257/88; 257/98;
`257/99
`(58) Field of Search .............................. 257/88, 98, 99,
`257/100
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`3,593,055 A * 7/1971 Geusic ........................ 257/98
`3,763,405 A * 10/1973 Mitsuhata .................... 257/98
`3,932,881 A * 1/1976 Mita ........................... 257/98
`4,992,704 A
`2/1991 Stinson
`...................... 315/312
`5,126,214 A * 6/1992 Tokailin ...................... 257/99
`5,208,462 A * 5/1993 O'Conner .................... 257/98
`5,405,709 A
`4/1995 Littman et al.
`............. 428/690
`5,583,349 A * 12/1996 Norman ....................... 257/88
`5,660,461 A * 8/1997 Ignatius ....................... 257/88
`
`FOREIGN PATENT DOCUMENTS
`
`JP
`JP
`
`60170194
`04289691
`
`9/1985
`10/1992
`
`JP
`
`5-152609
`
`* 6/1993
`
`257/98
`
`OTHER PUBLICATIONS
`
`Sato, Y. et al, "Full-Color Fluorescent Display Devices
`Using a Near-UV Light Emitting Diode", Jpn. J. Appl.
`Phys. vol. 35 (1996) pp. L 838-L839.
`J.I. Pankove and E.R. Levin, "Scanning Electron Micros(cid:173)
`copy Studies of GaN" J. Appl. Phys. vol. 46, (1975), pp.
`1647-1652.
`I. Akasaki, et al., "Photoluminescence of Mg-doped p-type
`GaN and Electro luminescence of GaN p-n Junction LED" J.
`Lumion., vol. 48-49, (1991) pp. 666-670.
`H. Amano et al., UV and Blue Electroluminescence from
`Al.GaN:Mg/GaN LED treated eith Low-Energy Electron
`Beam Irradiation (LEEBI), Inst. Phys. Conf Ser. vol. 106,
`(1990), pp. 725-730.
`Munch et al, "Silicon Carbide Light-Emitting Diodes with
`Epitaxial Junctions" Solid State Electronics, vol. 19, (1976)
`p. 871.
`Zhang Jin Chao et al., White Light Emitting Glasses, Journal
`of Solid State Chemistry, 93, 17-29 (1991), pp. 17-29.
`
`(List continued on next page.)
`
`Primary Examiner-Jerome Jackson
`(74) Attorney, Agent, or Firm-Steven J. Hultquist;
`Margaret Chappuis
`
`(57)
`
`ABSTRACT
`
`A light emitting assembly comprising a solid state device
`coupleable with a power supply constructed and arranged to
`power the solid state device to emit from the solid state
`device a first, relatively shorter wavelength radiation, and a
`down-converting luminophoric medium arranged in receiv(cid:173)
`ing relationship to said first, relatively shorter wavelength
`radiation, and which in exposure to said first, relatively
`shorter wavelength radiation, is excited to responsively emit
`second, relatively longer wavelength radiation. In a specific
`embodiment, monochromatic blue or UV light output from
`a light-emitting diode is down-converted to white light by
`packaging the diode with fluorescent organic and/or inor(cid:173)
`ganic fiuorescers and phosphors in a polymeric matrix.
`
`26 Claims, 3 Drawing Sheets
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`16
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`LOWES 1005, Page 1
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`US 6,600,175 Bl
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`OIBER PUBLICATIONS
`
`Shosaku Tanaka, et al., Bright White-Light Electrolumines(cid:173)
`cence Based on Nonradiative Energy Transfer in Ce- and
`Eu-doped SrS Thin Films, Appl. Phys. Lett. 51 (21), Nov.
`23, 1987, pp. 1661-1663.
`M. Berggren et al., White Light From an Electroluminescent
`Diode Made From poly[3( 4-octylphenyl)-2,2'-bithiophene]
`and an Oxadiazole Derivatives, J. Appl. Phys. 76 (11), Dec.
`1, 1994, pp. 7530--7534.
`
`J. Kida et al., White Light-Emitting Organic Electrolumi(cid:173)
`nescent Devices Using the poly(N-vinylcarbazole) Emitter
`Layer Doped with Three Fluorescent Dyes, Appl. Phys.
`Lett., 64 (7) Feb. 14, 1994, pp. 815-817.
`N. El Jauhar et al., White Light Generation Using Fluores(cid:173)
`cent Glasses Activated by Ce 3
`+, Tb 3
`+ and Mn2
`+ Ions, J. De
`Physique IV, Colloque C2, supplement au j. de Physique III,
`vol. 2, Oct. 1992, pp. 257-260.
`* cited by examiner
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`LOWES 1005, Page 2
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`FIGUREl
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`FIGURE2
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`Jul. 29, 2003
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`FIGURE 3
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`14
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`17
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`FIGURE4
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`11
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`-G- -G- -G- -
`G- -G- -G- -G- -G- -
`-
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`30
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`31
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`controller
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`FIGURES
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`63
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`43
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`40
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`I
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`1
`SOLID STATE WHITE LIGHT EMITTER
`AND DISPLAY USING SAME
`
`FIELD OF THE INVENTION
`
`This invention relates to solid state light emitting devices
`such as light emitting diodes and more particularly to such
`devices which produce white light.
`
`15
`
`2
`whose breakage makes the lamp useless, and by extension,
`the message on the sign board illegible. Due to severe
`environmental conditions on roadways, glass breakage of
`incandescent and fluorescent lamps is an all too frequent
`5 mishap. The solid state LED lamp has no filaments to break
`and is housed within a durable plastic casing, as the primary
`device envelope or package (typically being of considerable
`thickness), thereby exhibiting a high level of imperviousness
`to extreme outdoor environmental stresses. With respect to
`10 outdoor signage applications, displays can contain up to 1
`million or more pixels or lamps. Thus the maintenance costs
`related to replacement of non-operational incandescent
`lamps or miniature fluorescent (or neon) lamps are high and
`unfortunately, continual.
`Hence, an emerging trend in the manufacturing and
`marketing of informational displays or signage, especially
`for outdoor usage, is to utilize solid state LED lamps as
`replacement for more conventional incandescent bulbs. The
`major end user benefits are the lower power consumption
`20 costs and the longer operational lifetime (hence, reducing
`maintenance costs). A further benefit is the rapid relaxation
`times of a solid state device affording an opportunity to
`display rapidly changing information messages incorporat-
`ing video or lifelike animation.
`Given the desirability of white light displays (e.g., com-
`mercial bank "time and temperature" message boards, sta(cid:173)
`dium scoreboards), considerable effort has been expended to
`produce white light LEDs. Although the recent availability
`of the blue LED makes a full color, and by extension a white
`30 light display realizable, conventionally it has been consid(cid:173)
`ered that such a display would require multiple LEDs. The
`multiple LEDs would be then incorporated into complicated
`and expensive LED modules to obtain the required broad
`band illumination necessary to provide white light. Even if
`35 a discrete LED lamp were constructed that provides white
`illumination (as opposed to the utilization of a multitude of
`single die, single color discrete LED lamps in a module or
`sub-assembly), the current state of the art requires the
`utilization of multiple LED dies and typically at least four
`40 electrical leads to power these dies. U.S. Pat. No. 4,992,704
`issued to Stinson teaches a variable color light emitting
`diode having a unitary housing of clear molded solid epoxy
`supporting three LED dies characterized as producing color
`hues of red, green and blue, respectively. There have been
`45 some recent introductions of commercial "full-color" LED
`lamps, that are essentially discrete lamps which afford a
`means of producing white light. All currently available
`examples of such lamps contain a minimum of three LED
`dies (or chips)--0ne red, one green and one blue, encapsu-
`50 lated in a single epoxy package. The chips are powered via
`at least 4 electrical leads. These complicated multiple die,
`variable color devices provide an expensive and complicated
`method of offering white light illumination. Furthermore,
`these multiple die white lamps are rather inefficient in the
`55 present state of the art, offering luminosity far below that
`realized by existing monochromatic light emitting diode
`lamps, even when a very large quantity of dies are func(cid:173)
`tionally incorporated into the discrete lamp assembly.
`The utility of solid state lamps that offer white light
`60 illumination is clear. However, at present there is a very
`limited number of such solid state lamps available. In
`signage applications where a small pixel of light is fre(cid:173)
`quently required to offer the highest possible resolution of
`the message or video image, the most practical solid state
`65 lamps for display applications are the LED lamps. The LED
`lamp can have very narrow angles of irradiance and are very
`small in size when compared with other means of providing
`
`BACKGROUND OF THE INVENTION
`Solid state light emitting devices, including solid state
`lamps including LEDs are extremely useful because they
`potentially offer lower fabrication costs and long term
`durability benefits over conventional incandescent and fluo-
`rescent lamps. Due to their long operation (burn) time and
`low power consumption, solid state light emitting devices
`frequently provide a functional cost benefit, even when their
`initial cost is greater than that of conventional lamps.
`However, because large scale semiconductor manufacturing
`techniques can be used, many solid state lamps can be
`produced at extremely low cost. One such device is the solid
`state light emitting diode (LED) which has low fabrication
`costs, long operational lifetimes and low maintenance costs.
`Light emitting diodes (LEDs), and similarly constructed 25
`super luminescent diodes and semiconductor diode lasers,
`are commercially available and a wide variety of designs
`and manufacturing techniques have been developed. In
`addition to applications such as indicator lights on home and
`consumer appliances, audio visual equipment, telecommu(cid:173)
`nication devices and automotive instrument markings, such
`LEDs have found considerable application in indoor and
`outdoor informational displays. But until recently, LEDs
`have produced light only in the red, green or amber ranges
`and have not been generally suitable for replacing, for
`example, incandescent bulbs, with normally a white
`luminescence, in a wide variety of display applications. The
`recent introduction of a bright blue LED, however, allows
`white light LED systems to be realized and thus has the
`potential to open the display market to LEDs by providing
`a practical means to achieve both full color and white light
`illumination.
`The practical advantages of LED displays over those
`using incandescent bulbs are many. The operational lifetime
`(in this case, defined as continual illumination) of a LED is
`on the order of ten years or over 50,000 hours, whereas
`incandescent bulbs often burn out in the order of 2000 hours,
`thus leaving an empty pixel in the display message. Such
`recurrent failures make a display unreadable and, therefore,
`not useful. These conditions (i.e., broken or missing pixels)
`require constant repair leading to a significant maintenance
`problem for providers of display signs based on incandes(cid:173)
`cent illumination devices. With the long operational lifetime
`of a LED-based sign board, the pixels rarely burn out and the
`illuminated message remains legible over long operational
`periods.
`Similarly, LED lamps are considerably more robust.
`When exposed to stress, mechanical shocks, or temperature
`variations often encountered in an outdoor environment they
`are less likely to fail than incandescent lamps. This attribute
`is especially important when the signage is utilized in an
`environment such as vehicular traffic, e.g., roadway signage
`to mark highway construction sites, bridges, tunnels, or
`traffic control markings, in which perishable filaments used
`in the incandescent lamps frequently break due to constant
`vibrational motion. Further, incandescent and fluorescent
`lamps are constructed with fragile glass exterior casings
`
`LOWES 1005, Page 6
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`US 6,600,175 Bl
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`3
`a radiant surface. However, the methods of fabricating white
`LED lamps are limited. A conventional approach is to
`fabricate a large cluster of red, green and blue LED discrete
`lamps, housed in multiple lamp (up to 30) subassemblies or
`modules. By providing multiple power sources to control all
`of the discrete lamps, these large modules can appear, from
`a distance, to provide white light by the spatial mixing of
`blue, green and red sub-pixels of light given off by the
`individual discrete LED lamps that comprise the module.
`While the lamps that make up the modules may be indi- 10
`vidually addressable, and hence, offer the opportunity to,
`selectively and individually, provide red, green and blue
`light (or combinations thereof), such modular systems are
`complex and costly means of providing white light for a
`solid state display. Further, as these modules are rather large, 15
`the ultimate resolution of the display will always be lower
`than that of a conventional single lamp pixel display.
`Whereas multiple discrete LED dies housed within a
`single polymeric matrix (as taught by Stinson) may provide
`a discrete LED lamp such that the illumination could appear 20
`white to an observer, the individual LED dies would still
`need to be individually powered and the lamp would require
`multiple leads in order to effect the simultaneous emission of
`multiple wavelength light. Thus, this multiple die LED lamp
`would be rather expensive to fabricate, and would require 25
`expensive and complicated circuitry to power and control in
`an outdoor display. Despite these problems, both methods
`point to the utility of generating white illuminance.
`It would thus be highly desirable to develop a simple solid
`state LED lamp, with a minimum of power leads, (i.e., 2) 30
`exactly as practiced in single color LED lamps, such that
`three domains of red, green and blue light are generated and
`yet the white light emission is apparent to an observer, all
`while offering significantly reduced die costs (one versus
`three) and low fabrication costs in the design of correspond- 35
`ing displays and signage, high medium resolution (small
`pixel or lamp size), rapid switching to the on and off states
`(to enhance live video imaging), and with a high luminous
`efficiency.
`It is well known that so-called fluorescent lamps provide 40
`white light illumination. In a fluorescent lamp, the Hg vapor
`in the vacuum tube is excited by an electrical discharge. The
`excited Hg atoms emit light, primarily in the ultraviolet
`region (e.g., 254 nm, 313 nm, 354 nm), which is absorbed
`by the inorganic phosphors coating the inside walls of the 45
`tube. The phosphors then emit light. These inorganic phos(cid:173)
`phors are designed as such to offer white light emission by
`"down-converting" (i.e., transforming a higher frequency,
`shorter wavelength form of energy to a lower frequency,
`longer wavelength form of energy) the ultraviolet emissions 50
`of the excited states of atomic Hg into a broad spectrum of
`emitted light which appears as white to the observer.
`However, these light emitting devices are not solid-state, and
`miniaturization of these fluorescent bulbs to provide suitable
`pixel resolution for display applications has never been 55
`practically accomplished. In fact, the primary application of
`miniature fluorescent lamps (with long operational lifetimes,
`but unfortunately high power consumption when compared
`with solid state LED lamps) in displays is to provide back
`lighting to liquid crystals that are individually addressed at 60
`the pixel level. Furthermore, these miniature fluorescent
`lamps remain fragile light emitting devices by virtue of their
`glass housings and are unsuitable for use in display appli(cid:173)
`cations in which the lamps are exposed to extreme environ(cid:173)
`mental stresses. Such stresses can not only break the glass 65
`housing, but effect delamination of the powder coatings
`from the interior wall of the glass housing. It would be
`
`4
`desirable to generate white light by radiative energy transfer,
`where the luminescent centers are an integral part of the
`assembly such that a thick, difficult-to-fracture housing
`structure (plate or bulb) could provide white illumination
`from the interior thickness of such housing structure, and not
`from a semi-permanent powder coating placed on one side
`of a housing surface.
`In a further example of generating white light, in the
`absence of phosphor coatings, it was disclosed in Chao, et
`al., "White Light Emitting Glasses," Journal of Solid State
`Chemistry 93, 17-29 (1991) (see also El Jouhari, N., et
`al.,"White light generation using fluorescent glasses acti(cid:173)
`vated by Ce 3
`+, Tb3
`+ and Mn2
`+ ions," Journal de Physique IV,
`Colloque C2, supplement au Journal de Physique III, Vol(cid:173)
`ume 2, October 1992, C2-257 to C2-260), that vitreous
`materials are capable of generating white light by simulta(cid:173)
`neous emission of blue, green and red emitting fluorescent
`centers in B2 0 3 -based glass that simultaneously contain
`Ce 3 +, Tb3 +, and Mn2 + as activators. These glasses provide
`white illumination by offering the blue emission of Ce3
`+ as
`well as by the transfer of excited state energy from the Ce 3
`+
`to Te 3 + and Mn2 +, whose luminescence occurs respectively
`in the green and red parts of the visible light spectrum.
`Mixed rare earth borates can be used to provide white
`light illumination, via down conversion, with excitation of
`the borate powders with a primary (ultraviolet) radiation
`between 250 nm and 300 nm. Similarly, for cathode ray
`applications, white light-emitting mixed fluorescent materi-
`als can be made by careful formulation of green fluorescent
`materials ( 48 to 53% w/w), red fluorescent materials (37 to
`40% w/w) and blue fluorescent materials (10 to 13% w/w).
`While the devices in the above examples vary in concept
`and construction, they demonstrate the utilization of red,
`green and blue fluorescent materials, all inorganic in
`composition, which when excited by photons or electron
`beams, can release multiple wavelengths of secondary light
`emission (luminescence of either fluorescent or phosphores(cid:173)
`cent character) to exhibit white light to the observer. This is
`generally true, even if microscopic domains of discrete
`colored light emission can be observed on the Lambertian
`surface of the light emitting device.
`Tanaka, S., et al., "Bright white-light electroluminescence
`based on nonradiative energy transfer in Ce- and Eu-doped
`SrS films," App. Phys. Lett. 51 (21), Nov. 23, 1987,
`1662-1663, describes the generation of a white-light emit(cid:173)
`ting thin-film electroluminescent (EL) device using Ce- and
`Eu-doped strontium sulfide (SrS) inorganic phosphors. In
`the EL excitation of the SrS:Ce,Eu device, nonradiative
`energy transfer from the Ce 3
`+ luminescent center to the Eu2
`+
`luminescent center plays an important role in generating
`broad EL emission extending from the blue to the red,
`thereby generating white light.
`Similarly, some recent discussions of AlGaN electrolu(cid:173)
`minescent systems with Zn and Si dopants have indicated
`that some white light can be generated. While it is useful for
`a single device to be constructed in which dopants offer a
`multitude of luminescent wavelengths, dopants invariably
`alter the electrical and lattice structures of semiconductors
`and as such, the performance of these devices are consid(cid:173)
`erably poorer than for corresponding semiconductors free of
`dopant that emit monochromatic irradiation, as a result of
`being dopant-free.
`Until recently, most light emitting diodes have been
`semiconductor-based and most electroluminescent devices
`have been inorganic based. While organic materials have
`been utilized to prepare certain thin-film electroluminescent
`
`LOWES 1005, Page 7
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`devices, no organic based LEDs are commercially available.
`Further, organic-based LEDs are at present plagued by
`extremely short operational lifetimes due to degradation of
`the organic charge-transfer materials. In all of these systems,
`the organic materials, used in thin films on conducting
`inorganic substrates such as ITO, are actively participating
`in the electron-hole recombination necessary to generate an
`excited state, and, by subsequent radiative decay, light.
`Recently, the literature has discussed approaches directed
`to fabricating organic LED or electroluminescent devices
`and in certain cases, white light emission has been observed
`from these experimental designs. As an example, white light
`from an electroluminescent diode made from poly[3( 4-
`octylphenyl)-2,2'-bithiophene] and an oxadiazole derivative
`have been reported. Spectroscopic analysis indicates that the 15
`apparent white light is composed of blue ( 410 nm), green
`(530 nm),and red-orange (520 nm) luminescent centers.
`Electroluminescent devices incorporating the red fluoresc(cid:173)
`ing material Rhodamine onto an inorganic substrate have
`been effective in yielding some white light as well.
`White light emission from thin film organic electrolumi(cid:173)
`nescent cells based on poly(vinylcarbazole PVK) thin films
`on ITO-coated glass has also been recently reported. The cell
`has the construction of Mg:Ag:Alq:TAZ:doped PVK:ITO(cid:173)
`:Glass where the conducting ITO layer injects holes into the 25
`organic based PVK thin film layer which has high hole drift
`mobilities. Simultaneously, electrons are injected by the
`tris(8-quinolato) aluminum (III) complex layer Alq, into the
`hole blocking electron transporting layer composed of the
`organic molecule 3-( 4'tert-butylphenyl)-4-phenyl-5-( 4'- 30
`biphenyl)-1,2,4-triazole, TAZ. At the interface of the organic
`poly(vinlycarbazole) layer with the TAZ layer, recombina(cid:173)
`tion of holes and electrons take place which excites the
`organic, aromatic, carbazole pendant moiety that comprises
`the polymer. It is well known that the excited carbazole 35
`moiety within the polymer aggregates in the excited state
`leads to blue excimer emission, in the absence of quenchers
`or dopants. In the example of the organic
`Mg:Ag:Alq:TAZ:doped PVK:ITO:Glass electroluminescent
`device, the quenchers of excimeric emission, are the dopants 40
`blue emitting 1,1,4,4-tetraphenylbuta-l,3-diene (TPB),
`green emitting 7-diethylamino-3-(2'benzothiazoyl)coumarin
`(Coumarin-6), and red emitting dicyanomethylene-2-
`methyl-6-p-dimethylaminostyryl-4H-pyran (DCM-1 ).
`U.S. Pat. No. 5,045,709 issued Apr. 11, 1995 to J. E. 45
`Littman et al.discloses a white light emitting internal junc(cid:173)
`tion organic electroluminescent device comprising an anode,
`an organic electroluminescent medium and a cathode. The
`organic electroluminescent medium further comprises a hole
`injecting and transporting zone contiguous with the anode, 50
`and an electron injecting and transporting zone contiguous
`with the cathode. The electron injecting and transporting
`zone further comprises an electron injecting layer in contact
`with the cathode. The portion of the organic electrolumi(cid:173)
`nescent medium between the the electron injecting layer and 55
`the hole injecting and transporting zone emits white light in
`response to the hole-electron recombination, and comprises
`a fluorescent material and a mixed ligand aluminum chelate.
`Japanese Patent Publication 04289691 of Mitsubishi
`Cable Industries, Ltd., published Oct. 14, 1992, discloses an 60
`electroluminescent device comprising a fluorescent dye(cid:173)
`fixed silica layer coated with a transparent electrode layer, a
`luminescing (light-emitting) layer containing a phosphor, a
`backside electrode layer, a water-sorbing layer, an encapsu(cid:173)
`lating film, and an insulating layer.
`In the Mitsubishi patent publication, the silica layer may
`be formed by a sol gel process using metal alkoxides in a
`
`6
`solvent such as ethanol, isopropanol, or dimethyl ether. A
`Rhodamine 6G-doped silica layer is described to exhibit
`white luminescence. The luminescing layer may be for
`example on the order of 15 microns in thickness, and is
`formed by a sol gel technique yielding ZnS or ZnCdS doped
`with a dopant such as copper, aluminum, manganese,
`chlorine, boron, yttrium, or rare earth dopant. The luminesc(cid:173)
`ing layer may also contain scattered phosphor material. The
`average grain size of grains in the luminescing layer is
`generally greater than 10 microns, and preferably is in the
`range of from 15 to 40 microns. The luminescing layer may
`for example contain from 30 to 80% phosphor. A disclosed
`advantage of the foregoing structure is that one can change
`the phosphor in the luminescing layer, and thereby change
`the color of the whole material.
`Japanese Patent Publication 60170194 of Sony
`Corporation, published Sep. 3, 1985, discloses a white
`light-emitting electroluminescent device with a luminescent
`layer containing a mixture of a blue-green-emitting phos(cid:173)
`phor and Rhodamine S. Since Rhodamine S strongly fluo-
`20 resces orange by excitation with a bluish-green light, a white
`light of high luminosity may be obtained even at low
`voltage. This reference discloses a phosphor emitting blue(cid:173)
`green light, in which ZnS is doped with Cu and Cl, as well
`as a phosphor emitting yellow light, in which ZnS is doped
`with Cu and Mn. ZnS may also be doped with Cu and Br to
`produce green light.
`The Sony patent publication discloses a multilayer elec-
`troluminescent article, including sealing layers of protective
`film of a material such as Aclar polymer, a polyester layer,
`a transparent electrode formed of indium tin oxide (ITO), a
`light-emitting layer, and a backside electrode. The light-
`emitting layer may comprise 50--95% by weight of ZnS
`doped with the aforementioned dopant species (e.g., 0.045%
`wt. Cu, and 0.020% wt. Cl) and 5-50% wt. Rhodamine S.
`Not withstanding the progress made in using organic
`fluorescers as luminescent sites within either electron(cid:173)
`transport or hole-transport layers and affording thin-film
`interfacial hole-electron recombination, the current state of
`the art finds it difficult to generate organic based LED dies
`with reasonable operational lifetimes. By their very nature,
`these donor-acceptor complexes are prone to reaction with
`the surrounding medium. As a result, many of these organic
`molecules degrade under constant excitation to the excited
`state and consequently the organic-based LEDs fail. Those
`fluorescers with extremely high quantum yields of
`fluorescence, which by definition necessitate short excited
`state lifetimes and are unlikely to be quenched or degraded
`by oxygen or other reactants, do not have sufficient electron
`or hole transport properties to allow for device-wide local(cid:173)
`ized hole-electron recombination in the ground state.
`However, their proximity to the holes, as dopants in a hole
`transporting layer, as an example, may make the excited
`states of the luminophors more easily oxidized than would
`normally be the case. This would be especially true for
`excited state species, even if the ground state of the lumi(cid:173)
`nophors are stable to the holes in the hole-transporting layer.
`Similarly arguments regarding excited state reduction would
`be applicable for dopants sequestered within an electron(cid:173)
`transport layer.
`It would be most desirable, then, if a white light emitting
`LED device could be fabricated that took advantage of the
`simultaneous emission of red, green and blue luminescent
`centers, using both inorganic and organic fluorescers or
`phosphors without requiring theses species to be in proxi-
`65 mate contact with the transporting layers.
`It is the purpose of the present invention to provide while
`light solid state luminescent devices using a single die,
`
`LOWES 1005, Page 8
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`US 6,600,175 Bl
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`7
`which initially provide monochromatic radiation and
`wherein the monochromatic radiation is converted to poly(cid:173)
`chromatic white light, thus providing a solid state illumina(cid:173)
`tion device with white illuminance, without the need for
`multiple power leads or for more than one discrete LED 5
`lamp.
`
`SUMMARY OF THE INVENTION
`The present invention relates broadly to a light emitting
`assembly comprising a solid state device which is suitably
`joined by circuit-forming means to a power supply, con- 10
`structed and arranged to power the solid state device and
`induce the emission from the solid state device of a first,
`relatively shorter wavelength radiation. The solid state
`device is structurally associated with a recipient down(cid:173)
`converting luminophoric medium which when impinged by 15
`the first, relatively shorter wavelength radiation is excited to
`responsively emit a radiation in the visible white light
`spectrum.
`In accordance with a specific embodiment of the present
`invention, an LED operative to emit, for example, mono- 20
`chromatic blue or ultraviolet (UV) radiation is packaged
`along with fluorescent organic and/or inorganic fluorescers
`and phosphors in an insulating polymeric matrix. The mono(cid:173)
`chromatic blue or UV radiation output of the LED is
`absorbed and then down converted by the fluorphore or 25
`phosphor to yield longer wavelengths to include a broad
`spectrum of frequencies which appear as white light.
`This use of fluorescers and/or phosphors to effect down
`conversion of light from an LED in a solid state light
`emitting device using a packing dye material is a significant 30
`departure from prior art teaching. In addition to allowing for
`the generation of white light from a blue or ultraviolet
`emitting LED die with a typical p-n junction construction,
`devices in accordance with the invention can be variously
`constructed to provide an essentially infinite series of col- 35
`ored (visible) light emissions, of either narrow or broad
`spectral distribution, from one single p-n junction construc(cid:173)
`tion. The concept can be extended to any solid-state light
`emitting device, including super luminescent diodes, diode
`layers, electroluminescent cells, electroluminescent 40
`displays, organic and polymer based light emitting diodes
`and/or devices, even those not requiring semiconductor p-n
`junctions, providing an insulating matrix or housing can be
`attached to or incorporated within the device.
`As used herein, the term "solid state device," used in 45
`reference to the device for generating the primary radiation
`which subsequently is down-converted to a longer wave(cid:173)
`length radiation in such visible white light spectrum, means
`a device which is selected from the group consisting of
`semiconductor light emitting diodes, semiconductor lasers, 50
`thin film electroluminescent cells, electroluminescent dis(cid:173)
`play panels, organic based light-emitting diodes, polymeric(cid:173)
`based light-emitting diodes, and internal junction organic
`electroluminescent devices.
`As used herein, the term "luminophoric medium" refers to
`a material which in response to radiation emitted by the solid
`state device emits light in the white visible light spectrum by
`fluorescence and/or phosphorescence.
`Other aspects, features and embodiments of the invention
`will be more fully apparent from the ensuing description and
`claims.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a schematic elevational cross-sectional view of
`a down-converting solid state device assembly for producing
`white light according to one embodiment of the present
`invention.
`
`8
`FIG. 2 is a schematic elevational cross-sectional view of
`a another white light generating assembly according to
`another embodiment of the invention.
`FIG. 3 is a schematic elevational cross-sectional view, in
`enlarged scale, of a portion of the device of FIG. 1.
`FIG. 4 is a schematic representation of a display which
`may usefull