United States Patent [191
`King et al.
`
`[11] Patent Number:
`{45] Date of Patent:
`
`4,963,788
`Oct. 16, 1990
`
`54 THIN FILM ELECI‘ROLUMINESCENT
`[
`1 DISPLAY WITH IMPROVED CONTRAST
`[75] Inventors: Christopher N. King Richard E.
`Convert’ both of pogtland 0mg
`’
`_
`[73] Asslgnw Planar Systems, 1119-, Beaverton,
`Oreg'
`[21] Appl. No.: 218,848
`.
`Jul‘ 14’ 1988
`[22] Flled:
`[51] Int. Cl. ................... .. H05B 33/04, I-Ii?)55BB3333//O268,
`[52] U S Cl
`313/503. 313/505
`313/509; 313/512
`[58] Field of Search .............. .. 313/512, 503, 509, 505
`[56]
`References Cited
`U_S_ PATENT DOCUMENTS
`
`.
`
`.
`
`'
`
`5
`
`giggling? et a1‘ """" "
`
`OTHER PUBLICATIONS
`Ketchpel et al., “Development of an Effective Black
`Layer for Electroluminescent (EL) Displays,” SPIE,
`vol. 457, Advances in Display Technology IV, (1984).
`Abe et 8.1., “AC Thin-Film EL Display with PrMnO3
`Black Dielectric Material,” Society for Information
`Display 85 Digest, pp- 215, 217
`Schimizu et al., “High Contrast EL With New Light
`Absorbing Material GeNx,” Japan Display ’86.
`‘
`Primary Examiner_palmer C. DeMeo
`Attorney, Agent, or Firm—Chernoff, Vilhauer, McClung
`8‘ stem‘
`ABSTRACT
`[57]
`A TFEL device having improved contrast includes a
`laminate having a phosphor layer sandwiched between
`front and rear insulating layers placed upon a substrate
`supporting a set of front transparent electrodes. The
`
`' ' ' '
`
`rear set of electrodes are transparent or semitransparent
`' ' ' ' " 313/503 X
`4’143’297 3/1979 Fischer
`so as not to re?ect ambient light toward the viewer. 'The
`____ __ 313/509
`4:237:449 9/1931 Takeda et ah
`TFEL laminate is contained within a cavity created by
`4,356,429 10/1982 Tang ..................... .. 313/503
`an enclosure secured to the substrate by an adhesive.
`4,357,557 11/1982 Inohara et a1.
`.... .. 313/509
`4,417,174 11/1983 Kamllo et al‘
`313/503 X Darkly dyed ?ller material is injected into the cavity
`"""
`Iss5]?
`whose rear inside wall may have a dark coating. The
`.... “ 313/505
`4’670’69o 6/1987 Ketchpé'l":
`semitransparent electrodes may be made of gold or may
`4:672:264 6/1987 Higton .......................... .. 313/505 X
`be made of transparent indium tin Oxide having narrow
`aluminum bus bars for improved conductivity.
`
`FOREIGN PATENT DOCUMENTS
`
`2074787 10/1981 United Kingdom .............. .. 313/509
`
`13 Claims, 1 Drawing Sheet
`
`VALEO EX. 1014_001
`
`

`

`I
`US. Patent
`Us, Patent 4
`
`Oct. 16, 1990
`0ct.16,1990
`
`’
`’
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`4,963,788:
`4,963,788;'
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`19
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`11
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`VALEO EX. 1014_002
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`VALEO EX. 1014_002
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`

`

`1
`
`THIN FILM ELECTROLUMINESCENT DISPLAY
`WITH IIVIPROVED CONTRAST
`
`The following invention relates to a high ef?ciency
`TFEL device for providing an optical display having
`improved contrast without substantially attenuating the
`luminance of the panel.
`
`10
`
`4,963,788
`2
`approach is shown in a device described in U.S. Pat.
`No. 4,547,702 in which a dark ?eld layer consisting of
`6-10% of a noble metal, such as gold, dispersed within
`a ceramic, such as magnesium oxide, is used between
`the phosphor and rear insulator or is used as the rear
`insulator. In either case, the resulting luminance versus
`voltage characteristic is not steep enough for good
`matrix display operation, and a higher-than-10% gold
`content causes excess conductivity resulting in break
`down of the phosphor layer as well as undesirable lat
`eral conduction between electrodes.
`In yet another type of proposed device, GeNx is
`sandwiched as an embedded dark layer within the rear
`insulator. As with other structures that employ a black
`layer added between the phosphor layer and the rear
`electrode, this layer affects the dielectric properties of
`the insulator, and, hence the reliability of the panel with
`regard to dielectric breakdown.
`
`BACKGROUND OF THE INVENTION
`Thin ?lm electroluminescent (TFEL) display panels
`are constructed using a set of transparent front elec
`trodes, typically made of indium tin oxide (ITO), and a
`transparent phosphor layer sandwiched between trans
`parent dielectric layers situated behind the front elec
`trodes. A rear electrode set is disposed behind the rear
`insulating layer and is usually constructed of aluminum
`which provides good electrical conductivity and has a
`self-healing failure feature because it acts as a localized
`fuse at breakdown points. Aluminum also enhances the
`luminance of the display by re?ecting back toward the
`viewer most of the light that would otherwise be lost to
`the rear of the display. While this re?ected light nearly
`doubles the light of the displayed image, the aluminum
`electrode also re?ects superimposed ambient light that
`interferes with the display information and reduces the
`contrast of the display.
`To minimize the reflection of ambient light, an antire
`?ection coating is typically used on the front glass.
`Also, dark backgrounds behind the display are com
`monly provided. The TFEL laminar stack is situated
`within an enclosure sealed against the substrate, and the
`rear wall of this enclosure is usually blackened to block
`light from extraneous light sources behind the display,
`and to absorb ambient light passing through the display
`from the front. Another method of improving the con
`trast and attenuating the amount of light reflected from
`the rear aluminum electrodes is to use an external circu
`larly polarized contrast enhancement ?lter in front of
`the display. However, such ?lters can be expensive and
`40
`typically attenuate the display luminance by 60% or
`
`25
`
`30
`
`35
`
`more.
`
`‘ -
`
`Another approach that has been tried in the past has
`been to use ITO transparent electrodes for the rear
`electrode set. This reduces re?ectance and allows ambi
`ent light to pass on through to the back of the display
`where it can be absorbed. However, ITO is more resis
`ti‘ve than any metallic electrodes such as those made of
`aluminum, and must be made much thicker to achieve
`adequate electrical conductivity. Thick layers of ITO
`do not exhibit the self-healing characteristics of alumi
`num rear electrodes. This leads to an unacceptable loss
`in device reliability due to dielectric breakdown.
`In yet another approach, shown in Steel et al., U.S.
`Pat. No. 3,560,784, a light absorbing layer is incorpo
`rated into the thin ?lm laminate structure. This refer
`ence suggests that if a conventional metallic rear elec
`trode is used, then a light absorbing layer may be added
`as an insulating layer or as a conductive layer to achieve
`a black layer display. Insertion of a dark layer immedi
`ately behind the phosphor layer, however, can interfere
`with the phosphor/insulator interface leading to infe
`rior display performance. The light pulse for one polar—
`ity may be reduced which can give rise to a ?icker
`effect as well as to a loss in overall brightness.
`Another approach has been to utilize a black opti
`cally absorbing layer behind the rear insulating layer
`and in front of the rear aluminum electrode. A similar
`
`45
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`55
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`65
`
`SUMMARY OF THE PRESENT INVENTION
`The present invention provides an improved contrast
`display for a TFEL panel which includes a substrate
`supporting a laminar thin ?lm structure including a set
`of transparent front electrodes, a phosphor layer sand
`wiched between front and rear insulating layers, and a
`semitransparent set of rear electrodes that exhibits good
`self-healing characteristics deposited on the rear insulat
`ing layer, all contained within an enclosure sealed
`against the substrate. The cavity thus formed includes
`within it an optically absorbent material such as a dark
`?uid for absorbing ambient light to improve the con
`trast of the display.
`The thin transparent rear electrodes may be made of
`gold and the optically absorbent material may include a
`black dye dissolved in silicone oil or a solid ?ller mate
`rial injected into the cavity. Additionally the optically
`absorbent material may include a black coating which is
`deposited on the rear wall of the enclosure inside the
`cavity.
`As an alternative embodiment, the rear electrodes
`may be totally transparent. Totally transparent elec
`trodes such as those made from indium tin oxide (ITO)
`however, have poor conductivity if made thin enough
`to exhibit self-healing characteristics. Thus, a narrow
`bus bar made of aluminum or some other highly con
`ductive and self-healing material may be provided
`which extends colinearly, and in contact with, each
`electrode. The bus bars are narrow, having a width of
`between 5% and 25% of each respective ITO electrode.
`To provide good electrical contact and adhesion, a thin
`chromium strip may be interposed between each bus bar
`and its corresponding electrode.
`In either case the electrodes will appear to be trans
`parent or nearly transparent and will not reflect ambient
`light back toward the viewer as conventional rear elec
`trodes do. This will allow the ambient light to be ab
`sorbed by the dark ?ller material in the cavity behind
`the rear electrodes.
`It is a principal object of this invention to provide an
`AC TFEL display device having improved contrast
`while at the same time maintaining high ef?ciency with
`out substantially attenuating the luminance of the dis
`play.
`A further object of this invention is to provide a
`TFEL panel having improved contrast utilizing trans
`parent or semitransparent rear electrodes with an opti
`cally absorbent material interposed behind the elec
`trodes.
`
`VALEO EX. 1014_003
`
`

`

`15
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`25
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`4,963,788
`3
`4
`ity of gold. To compensate for its poor conductivity,
`Yet a further object of this invention is to provide an
`improved contrast TFEL panel having adequate lumi
`the ITO electrodes are each provided with bus bars 21
`nance, high electrical reliability and high efficiency
`made of aluminum which extend colinearly with each
`electrode and in contact with it. Each bus bar 21 typi
`utilizing a transparent or semitransparent rear electrode
`structure having good self-healing characteristics.
`cally has a width ranging from 5% to 25% of the width
`The foregoing and other objectives, features, and
`of the ITO electrode 20. To improve adhesion a thin
`chromium strip 23 interposed between the bus bar and
`advantages of the invention will be more readily under
`stood upon consideration of the following detailed de
`the ITO electrode may be used. For example, the bus
`scription of the invention, taken in conjunction with the
`bar may have a thickness of 900 Aoand the chromium
`accompanying drawings.
`strip may have a thickness of 100 A. The bus bars 21
`enable the ITO electrodes 20 to be made thin enough so
`BRIEF DESCRIPTION OF THE DRAWINGS
`that they exhibit the same self-healing properties as
`aluminum or gold while compensating for the loss in
`FIG. 1 is a partial cutaway view of a TFEL device
`conductivity. For greater conductivity thin gold may
`constructed according to the invention employing semi
`also be used in place of ITO with the aluminum bus bars
`transparent rear electrodes.
`21.
`FIG. 2 is a partial cutaway view of a TFEL device
`constructed according to the present invention and
`As with the emobodiment of FIG. 1, a filler 17’ which
`including transparent rear electrodes having auxiliary
`may be black-dyed silicone oil is inserted into a cavity
`bus bars.
`formed by enclosure 19' secured to the substrate 10'
`with adhesive 11’. A black coating 18’ is placed on the
`FIG. 3 is a partial cutaway view of a TFEL device
`showing a further re?nement of the invention as shown
`rear inner wall of the enclosure 19'.
`in FIG. 2 employing light absorbing stripes to attenuate
`A further improvement in the alternative form of the
`reflectance from the rear bus bars with which they are
`invention (Refer to FIG. 3) is to include an additional
`optically aligned.
`patterned light absorbing ?lm 22 directly in front of the
`reflective bus bars 24 backing transparent conductors 34
`DETAILED DESCRIPTION OF THE
`to reduce or eliminate the reflection of ambient light
`INVENTION
`from the bus bars. This ?lm can be located at any level
`Referring to FIG. 1, a TFEL device includes a glass
`in the thin ?lm stack, but the recommended location is
`substrate 10 supporting a laminar stack comprising the
`to deposit it as the ?rst ?lm on the substrate 26. To
`maximize the optical transmission of the overall display,
`TFEL display elements. The stack includes a set of
`the ?lm 22 need only be in front of each bus bar 24, and
`transparent front electrodes 12 and a sandwich struc
`ture including a phosphor layer 14 sandwiched between
`therefore can be patterned so that the light absorbing
`front and rear insulating layers 13 and 15, respectively.
`?lm 22 is removed between the bus bar locations. If
`desired, a buffer layer 28 of transparent insulating mate
`semitransparent rear electrodes 16 are deposited on the
`rear insulator 15 and extend in a direction perpendicular
`rial, such as aluminum oxide or silicon nitride, may be
`deposited over the patterned light absorbing ?lm 22, to
`to the transparent front electrodes 12 so that pixel points
`avoid any reaction with the next deposited transparent
`of light are created when electrodes in both sets are
`energized simultaneously. The semitransparent rear
`conductor layer 30, which is typically indium tin oxide.
`With this con?guration for the light absorbing ?lm 22,
`electrodes 16 may be fabricated from gold, and as such,
`provide high conductivity but do-not re?ect ambient
`it is isolated electrically from the subsequently depos
`ited conductors 30, and therefore does not compromise
`light back toward the viewer to the same degree that
`the electrical characteristics of the light emitting stack
`aluminum electrodes would. The gold electrodes ex
`comprising insulators 31 and 33 sandwiching phosphor
`hibit the self-healing characteristics of aluminum and
`are highly conductive, thus providing good electrical
`layer 32. The light absorbing layer therefore, does not
`reliability and high efficiency without high reflectance
`need to have any particular electrical requirements.
`The light absorbing stripes 22 may be optically
`from the rear electrode layer.
`The TFEL components are sealed against the sub
`opaque or may constitute a partially transmissive ?lter,
`with either neutral density or wavelength-selective
`strate 10 by an enclosure 19 which may be af?xed to the
`?ltering. For a multicolor display, the light absorbing
`substrate 10 by any suitable adhesive 11. An optically
`absorbent material may be injected into the cavity de
`transmission characteristics can be matched to the emit
`?ned by the enclosure 19 to further absorb ambient
`ted light, i.e., a red transmitting ?lter may be used in
`light. This may take the form of a silicone oil 17 which
`front of a red emitting area bus bar, etc., to substantially
`preserve the emitted light while substantially blocking
`is conventionally used as a ?ller material or a solid ?ller
`of the type disclosed in Ser. No. 104,166 entitled “Seal
`the ambient light reflected from the bus bar. Even in the
`Method and Construction for TFEL Panels Employing
`case of a neutral density ?lter with transmission T, the
`Solid Filler” and assigned to the same assignee. This
`display contrast can be improved because the emitted
`light is reduced by the factor T, whereas the ambient
`silicone oil 17 may include a black dye to make it opti
`cally absorbent. Optical absorption is also enhanced by
`light fraction R, reflected from the bus bar, is reduced
`providing a black coating 18 on the rear inside cavity
`by T2 due to absorption on both the inward and out
`ward passage of the re?ected light path.
`wall of the enclosure 19.
`.
`An alternative embodiment is shown in FIG. 2 which
`The light absorbing stripes 22 can be deposited on the
`includes all the components of FIG. 1 with the excep
`surface of the substrate 26. If the stripes 22 are thick,
`they can be tapered at the edges for better step coverage
`tion that the rear electrodes are transparent. Phosphor
`layer 14' is sandwiched between insulators 13’ and 15'
`of subsequent layers. In the alternative the substrate 26
`and are supported by electrode layer 12’ on glass sub
`may be prepared with recesses or channels to receive
`strate 10'. Transparent rear electrodes 20 may be fabri
`the stripes 22. This may be necessary if the stripes are
`cated from indium tin oxide (ITO). The conductivity of
`very thick where it may be dif?cult to provide tapered
`ITO, however, is signi?cantly less than the conductiv
`edges.
`.
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`VALEO EX. 1014_004
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`

`

`5
`The stripes 22 are positioned on the substrate to lie in
`front of, that is along the optical line of sight, of a
`viewer viewing the panel from in front of the substrate
`26. The bus bars 24 are positioned toward respective
`edges of the electrodes 34 so that one stripe 22 may
`effectively lie in front of each two bus bars 24. This
`obviates the need for depositing a large plurality of very
`thin light absorbent stripes on the substrate.
`If desired, a circularly polarized ?lter (not shown)
`may be used with the structure of FIG. 1 to further
`reduce the re?ected light and to achieve acceptable
`contrast in high ambient light conditions. Circularly
`polarized ?lters, however, have the effect of attenuating
`the luminance of the panel by as much as 60%. Never
`theless, in high ambient light conditions, such a ?lter
`may be desirable.
`The contrast ratio of a display is de?ned as the ratio
`of the luminance of the display when it is “on” to its
`luminance when it is “off.” Any illumination adds to
`both conditions so that the contrast ratio is equal to the
`“on” luminance plus the background illumination times
`the re?ectance divided by the “off” luminance plus the
`background illumination times the re?ectance. A stan
`dard TFEL panel with no ?lter conventionally pro
`vides a luminance of 20 fL and has a diffuse reflectance
`of 10%, so that with a background luminance of 1000 fc,
`its contrast ratio is 1.2. By comparison, a panel employ
`ing transparent gold electrodes as disclosed herein pro
`vides a contrast ratio of 1.86 and a luminance of 14 fL.
`The structure of the invention therefore provides a
`signi?cant increase in contrast with only a moderate
`penalty in luminance.
`If a circularly polarized ?lter with 35% transmission
`is added to the standard display to improve its contrast,
`the result is a luminance of 7 fL and a contrast ratio of
`1.98. In comparison, the panel disclosed herein, without
`any ?lter, has nearly comparable contrast (1.86) but
`provides twice the luminance (l4 fL).
`Application of the circularly polarized flter to the
`panel disclosed herein reduces its luminance to 4.9 fL
`but raises the contrast ratio to 6.1. That is, when circular
`polarizer ?lters are used on both panels, the gold elec
`trode panel provides three times as much contrast and
`70% of the luminance of the standard panel. Therefore,
`depending upon the ?lter con?guration, the panel dis
`closed can provide either improved luminance or supe
`rior contrast to a standard panel.
`The terms and expressions which have been em
`ployed in the foregoing speci?cation are used therein as
`terms of description and not of limitation, and there is
`no intention, in the use of such terms and expressions, of
`excluding equivalents of the features shown and de
`scribed or portions thereof, it being recognized that the
`
`4,963,788
`6
`scope of the invention is de?ned and limited only by the
`claims which follow.
`What is claimed is:
`1. In a TFEL device for providing an optical display, -
`a substrate supporting a laminar thin ?lm re including a
`set of transparent front electrodes and a phosphor layer
`sandwiched between front and rear insulating layers,
`the improvement comprising:
`(a) a set of at least semitransparent rear electrodes
`deposited on said rear insulating layer;
`(b) a set of conductive bus bars arranged colinearly
`and in contact with each electrode in said set of
`transparent rear electrodes; and
`(c) enclosure means sealed against said substrate for
`de?ning a cavity enclosing said laminar thin-?lm
`structure, said cavity including within it an opti
`cally absorbent material disposed behind the rear
`electrode set for absorbing ambient light to im
`prove the contrast of the optical display.
`2. The TFEL device of claim 1 wherein said set of at
`least semitransparent rear electrodes is made of indium
`tin oxide.
`3. The TFEL device of claim 1 whereirLthe bus bars
`are made of aluminum.
`4. The TFEL device of claim 3 further including a
`chromium strip interposed between each aluminum bus
`bar and its corresponding electrode.
`5. The TFEL device of claim 4 wherein each said
`chromium strip has a thickness on the order of 100 A
`and each aluminum bus bar has a thickness on the order
`of 900 A.
`6. The TFEL device of claim 1 wherein the optically
`absorbent material comprises a black dye dissolved in
`?ller material occupying the cavity.
`7. The TFEL device of claim 6 wherein the tiller
`material is a silicone oil.
`8. The TFEL device of claim 7 wherein the optically
`absorbent material further comprises a black coating
`deposited on a rear wall of the enclosure means inside
`said cavity.
`,
`9. The TFEL device of claim 1 further comprises
`light absorbent stripes disposed on the substrate and
`optically aligned with each bus bar.
`10. The TFEL device of claim 9 wherein the bus bars
`are positioned toward edges of their respective elec
`trodes whereby one light absorbent stripe is optically
`aligned with a pair of bus bars.
`11. The TFEL device of claim 9 further comprising a
`thin ?lm buffer layer interposed between the transpar
`ent front electrodes and the light absorbent stripes.
`12. The TFEL device of claim 9 wherein the light
`absorbent stripes have tapered edges.
`13. The TFEL device of claim 1 wherein said set of
`rear electrodes is made of gold.
`* * *
`=0!
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`65
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`VALEO EX. 1014_005
`
`

`

`UNITED STATES PATENT AND TRADEMARK OFFICE
`CERTIFICATE ()F CGRRECTION
`43631788
`
`PATENT N0. :
`
`I
`DATED
`lNvEN-l-oms) :
`
`October 16, 1990
`Christopher N. King and Richard E. Coovert
`
`It is certified that error appears in the above~identi?ed patent and that said Letters Patent is hereby
`corrected as shown below:
`
`Column 1, lines 28-29: change "antire-flection" to -
`anti—reflection——.
`
`Column 6, line 5: change "re" to --structure—-.
`
`Signed and Sealed this
`
`Fourteenth Day of July, 1992
`
`Attest:
`
`Arresting Officer
`
`Acting Commissioner of Patents and Trademarks
`
`DOUGLAS B. COMER
`
`VALEO EX. 1014_006
`
`