(12) United States Patent
`Baretz et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,600,175 B1
`*Jul. 29, 2003
`
`US006600175B1
`
`(54) SOLID STATE WHITE LIGHT EMITTER
`AND DISPLAY USING SAME
`
`(75) IIlVeIltOfSI Bryce BaI‘etZ: West Milford, NJ (Us);
`Mlchael A- TlSChleI‘, Danbury, CT (Us)
`_
`_
`(73) Asslgnee? gdvgneed Technology Materlals, Inc»
`an ury> CT (Us)
`
`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 ER. Levin, “Scanning Electron Micros
`Sltélgizies of GaN” J. Appl. Phys. vol. 46, (1975), pp.
`
`(
`
`*
`
`-
`
`~
`
`) Nonce'
`
`-
`
`-
`
`-
`
`Th1? patent llsslied 015111 eticonémued pros‘
`ecu 10H app {Ca lot}
`6 un er 37 CFR
`1.53(d), and 1s subJect to the tWenty year
`ligfatxztirm provlslons of 35 USC'
`'
`
`_
`
`'
`
`I. Akasaki, et al., “Photoluminescence of Mg—doped p—type
`GaN and Electroluminescence 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
`
`t
`
`t '
`
`t
`
`Subject to any disclaimer, the term of this
`$1 Se nc 1; Signs edl glrzagiussted under 35
`'
`'
`'
`y
`y '
`
`'
`
`'
`(21) Appl- No-- 08/621,937
`(22) Filed:
`Man 26’ 1996
`
`’
`
`'
`
`_. .
`
`'
`
`.
`
`.
`
`231e9a9I3)IrIr);d1;t21§n7(3I6EEBI)’ Inst’ PhyS' Conf Sen V01’ 106’
`Munch et al, “Silicon Carbide L1ght—Em1ttmg 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.
`
`.
`
`.
`
`(51) Int. c1.7 .............................................. .. H01L 33/00
`(52) US. Cl. ......................... .. 257/100; 257/88; 257/98;
`257/99
`(58) Field of Search ............................ .. 257/88, 98, 99,
`257/100
`
`(56)
`
`References Cited
`Us PATENT DOCUMENTS
`*
`'
`2 * 13/
`5191151; ---------------------- -- 257/98
`3,932,881 A *
`1; 1976 M5211 ata ~~~~~~~~~~~~~~~~~~ ~~
`4’992’704 A
`2/1991 stinsoil' ' ' ' ' ' ' '
`' ' ' "315/312
`5:126j214 A * 6/1992 Toka?in
`257/99
`5,208,462 A * 5/1993 UCOnner "
`257/98
`5,405,709 A
`4/1995 Limnan et a1_
`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
`
`(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 ?rst, relatively shorter Wavelength radiation, and a
`down-converting luminophoric medium arranged in receiv
`mg'relationship to said ?rst, relatively shorter Wavelength
`radiation, and Which in'exposure'to said ?rst, relatively
`shorter Wavelength radiation, is excited to responsively emit
`second, relatively longer Wavelength radiation. In a speci?c
`embodiment, monochromatic blue or UV light output from
`a light-emitting diode is down-converted to White light by
`packaging the diode With ?uorescent organic and/or inor
`ganic ?uorescers and phosphors in a polymeric matrix.
`
`26 Claims, 3 Drawing Sheets
`
`1 1
`10
`//////// /
`/ 5x55513552
`/ /////// ////
`// /////// /////
`
`20
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`/1;2
`/
`/ // ///
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`///// //// // ////
`///// ///
`/////
`//// )X/
`/
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`//// / / ///////
`////
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`////
`/ ///////
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`///
`/// -
`/
`//// /// ///////
`/ // /// ///////
`/ // /// ///////
`/// /// ///////
`/// /// ///////
`
`14
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`17
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`7
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`TCL 1005, Page 1
`
`

`

`US 6,600,175 B1
`Page 2
`
`OTHER PUBLICATIONS
`
`Shosaku Tanaka, et a1., Bright White—Light Electrolumines
`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 a1., White Light From an Electroluminescent
`Diode Made From po1y[3(4—octylpheny1)—2,2‘—bithiophene]
`and an OXadiaZole Derivatives, J. Appl. Phys. 76 (11), Dec.
`1, 1994, pp. 7530—7534.
`
`J. Kido et a1., White Light—Emitting Organic Electrolumi
`nescent Devices Using the po1y(N—vinylcarbaZo1e) Emitter
`Layer Doped With Three Fluorescent Dyes, Appl. Phys.
`Lett., 64 (7) Feb. 14, 1994, pp. 815—817.
`N. E1 Jouhar et a1., White Light Generation Using Fluores
`cent Glasses Activated by Ce3+, Tb3+ and Mn2+ Ions, J. De
`Physique IV, Colloque C2, supplement au j. de Physique 111,
`vol. 2, Oct. 1992, pp. 257—260.
`
`* cited by examiner
`
`TCL 1005, Page 2
`
`

`

`U.S. Patent
`
`Jul. 29, 2003
`
`Sheet 1 of3
`
`US 6,600,175 B1
`
`FIGURE 1
`
`20
`
`///x,2//xx
`/xrV////
`/////z
`
`xx ///x
`
`16
`
`FIGURE 2
`
`11
`
`18
`
`12
`
`TCL 1005, Page 3
`
`

`

`U.S. Patent
`
`Jul. 29, 2003
`
`Sheet 2 of3
`
`US 6,600,175 B1
`
`FIGURE 3
`
`12
`
`16
`
`18
`
`13
`
`22
`
`\ 14
`17
`
`FIGURE 4
`
`controller
`
`TCL 1005, Page 4
`
`

`

`U.S. Patent
`
`Jul. 29, 2003
`
`Sheet 3 of3
`
`US 6,600,175 B1
`
`FIGURE 5
`
`FIGURE 6
`
`82
`
`80
`/
`
`a4
`
`\
`
`_ _ _
`
`88
`
`J
`
`86
`
`Wm“,
`‘ X;
`M7,; m
`\
`
`92
`
`90
`
`TCL 1005, Page 5
`
`

`

`US 6,600,175 B1
`
`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.
`
`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 bene?ts over conventional incandescent and ?uo
`rescent lamps. Due to their long operation (burn) time and
`loW poWer consumption, solid state light emitting devices
`frequently provide a functional cost bene?t, 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
`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
`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, de?ned 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 signi?cant maintenance
`problem for providers of display signs based on incandes
`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
`traf?c control markings, in Which perishable ?laments used
`in the incandescent lamps frequently break due to constant
`vibrational motion. Further, incandescent and ?uorescent
`lamps are constructed With fragile glass exterior casings
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`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 ?uorescent lamps is an all too frequent
`mishap. The solid state LED lamp has no ?laments 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
`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 ?uorescent (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 bene?ts are the loWer poWer consumption
`costs and the longer operational lifetime (hence, reducing
`maintenance costs). A further bene?t 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
`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
`light display realiZable, conventionally it has been consid
`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
`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
`electrical leads to poWer these dies. US. 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
`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)—one red, one green and one blue, encapsu
`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 inef?cient in the
`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
`tionally incorporated into the discrete lamp assembly.
`The utility of solid state lamps that offer White light
`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
`quently required to offer the highest possible resolution of
`the message or video image, the most practical solid state
`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
`
`TCL 1005, Page 6
`
`

`

`US 6,600,175 B1
`
`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
`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,
`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
`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
`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)
`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 signi?cantly reduced die costs (one versus
`three) and loW fabrication costs in the design of correspond
`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
`ef?ciency.
`It is Well knoWn that so-called ?uorescent lamps provide
`White light illumination. In a ?uorescent 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
`tube. The phosphors then emit light. These inorganic phos
`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
`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 ?uorescent bulbs to provide suitable
`pixel resolution for display applications has never been
`practically accomplished. In fact, the primary application of
`miniature ?uorescent 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
`the pixel level. Furthermore, these miniature ?uorescent
`lamps remain fragile light emitting devices by virtue of their
`glass housings and are unsuitable for use in display appli
`cations in Which the lamps are exposed to extreme environ
`mental stresses. Such stresses can not only break the glass
`housing, but effect delamination of the poWder coatings
`from the interior Wall of the glass housing. It Would be
`
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`desirable to generate White light by radiative energy transfer,
`Where the luminescent centers are an integral part of the
`assembly such that a thick, dif?cult-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 ?uorescent glasses acti
`vated by Ce3+, Tb3+ and Mn2+ ions,” Journal de Physique IV,
`Colloque C2, supplement au Journal de Physique III, Vol
`ume 2, October 1992, C2-257 to C2-260), that vitreous
`materials are capable of generating White light by simulta
`neous emission of blue, green and red emitting ?uorescent
`centers in B2O3-based glass that simultaneously contain
`Ce3+, Tb“, 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 Ce3+
`to Te3+ 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 ?uorescent materi
`als can be made by careful formulation of green ?uorescent
`materials (48 to 53% W/W), red ?uorescent materials (37 to
`40% W/W) and blue ?uorescent 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 ?uorescent materials, all inorganic in
`composition, Which When excited by photons or electron
`beams, can release multiple Wavelengths of secondary light
`emission (luminescence of either ?uorescent or phosphores
`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 ?lms,” App. Phys. Lett. 51 (21), Nov. 23, 1987,
`1662—1663, describes the generation of a White-light emit
`ting thin-?lm electroluminescent (EL) device using Ce- and
`Eu-doped strontium sul?de (SrS) inorganic phosphors. In
`the EL excitation of the SrS:Ce,Eu device, nonradiative
`energy transfer from the Ce3+ 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
`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
`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-?lm electroluminescent
`
`TCL 1005, Page 7
`
`

`

`US 6,600,175 B1
`
`5
`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 ?lms 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
`apparent White light is composed of blue (410 nm), green
`(530 nm),and red-orange (520 nm) luminescent centers.
`Electroluminescent devices incorporating the red ?uoresc
`ing material Rhodamine onto an inorganic substrate have
`been effective in yielding some White light as Well.
`White light emission from thin ?lm organic electrolumi
`nescent cells based on poly(vinylcarbaZole PVK) thin ?lms
`on ITO-coated glass has also been recently reported. The cell
`has the construction of Mg:Ag:Alq:TAZ:doped PVKzITO
`:Glass Where the conducting ITO layer injects holes into the
`organic based PVK thin ?lm 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‘
`biphenyl)-1,2,4-triaZole, TAZ. At the interface of the organic
`poly(vinlycarbaZole) layer With the TAZ layer, recombina
`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
`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 PVKzITOzGlass electroluminescent
`device, the quenchers of excimeric emission, are the dopants
`blue emitting 1,1,4,4-tetraphenylbuta-1,3-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).
`US. Pat. No. 5,045,709 issued Apr. 11, 1995 to J. E.
`Littman et al.discloses a White light emitting internal junc
`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,
`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
`nescent medium betWeen the the electron injecting layer and
`the hole injecting and transporting Zone emits White light in
`response to the hole-electron recombination, and comprises
`a ?uorescent material and a mixed ligand aluminum chelate.
`Japanese Patent Publication 04289691 of Mitsubishi
`Cable Industries, Ltd., published Oct. 14, 1992, discloses an
`electroluminescent device comprising a ?uorescent dye
`?xed 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
`lating ?lm, 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
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`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
`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
`phor and Rhodamine S. Since Rhodamine S strongly ?uo
`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
`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
`?lm 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
`?uorescers as luminescent sites Within either electron
`transport or hole-transport layers and affording thin-?lm
`interfacial hole-electron recombination, the current state of
`the art ?nds it dif?cult 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
`?uorescers With extremely high quantum yields of
`?uorescence, Which by de?nition necessitate short excited
`state lifetimes and are unlikely to be quenched or degraded
`by oxygen or other reactants, do not have suf?cient electron
`or hole transport properties to alloW for device-Wide local
`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
`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
`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 ?uorescers or
`phosphors Without requiring theses species to be in proxi
`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,
`
`TCL 1005, Page 8
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`

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`US 6,600,175 B1
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`7
`Which initially provide monochromatic radiation and
`wherein the monochromatic radiation is converted to poly
`chromatic White light, thus providing a solid state illumina
`tion device With White illuminance, Without the need for
`multiple poWer leads or for more than one discrete LED
`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
`structed and arranged to poWer the solid state device and
`induce the emission from the solid state device of a ?rst,
`relatively shorter Wavelength radiation. The solid state
`device is structurally associated With a recipient doWn
`converting luminophoric medium Which When impinged by
`the ?rst, relatively shorter Wavelength radiation is excited to
`responsively emit a radiation in the visible White light
`spectrum.
`In accordance With a speci?c embodiment of the present
`invention, an LED operative to emit, for example, mono
`chromatic blue or ultraviolet (UV) radiation is packaged
`along With ?uorescent organic and/or inorganic ?uorescers
`and phosphors in an insulating polymeric matrix. The mono
`chromatic blue or UV radiation output of the LED is
`absorbed and then doWn converted by the ?uorphore or
`phosphor to yield longer Wavelengths to include a broad
`spectrum of frequencies Which appear as White light.
`This use of ?uorescers 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 signi?cant
`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 in?nite series of col
`ored (visible) light emissions, of either narroW or broad
`spectral distribution, from one single p-n junction construc
`tion. The concept can be extended to any solid-state light
`emitting device, including super luminescent diodes, diode
`layers, electroluminescent cells, electroluminescent
`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
`reference to the device for generating the primary radiation
`Which subsequently is doWn-converted to a longer Wave
`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,
`thin ?lm electroluminescent cells, electroluminescent dis
`play panels, organic based light-emitting diodes, polymeric
`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
`?uorescence and/or phosphorescence.
`Other aspects, features and embodiments of the invention
`Will be more fully apparent from the ensuing description and
`claims.
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`a
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`15
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`25
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`35
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`45
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`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.
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`65
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`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 usefully employ the device of FIGS. 1 and/or 2.
`FIG. 5 is a schematic elevational vieW of an electrolumi
`nescent cell device according to another embodiment of the
`invention.
`FIG. 6 is a schematic representation of the generaliZed
`light emitt