THIN-FILM inorganic electroluminescent(EL) displays are shattering longstandingtechnological barriers in cost, scalability, andhigh-luminance full color. Recent advancesmight finally answer the continuing questionof whether the technology will always be lim-ited to high-performance niche markets.
The general advance in flat-panel-display(FPD) technology has accelerated to the pointthat the bar for initial mass deployment hasbeen raised to a daunting level for any novelor rapidly evolving display technology. Furthermore, the impressive breakthroughs ininorganic EL technology have been largelyovershadowed by technologies that are not as
ready for commercialization, but are moreimpressive to investors because they providenon-traditional attributes such as physicalflexibility. Inorganic EL technology is clearlya dark horse in the display race, but it is in therace because of its increasingly rapid progressand undeniable advantages in image qualityand manufacturing simplicity.
There are many good reviews on the richhistory of inorganic EL technology spanningthe 1960s and 1970s (development of thin-film EL technology), 1970s and 1980s (com-mercialization of ac thin-film EL displays),and 1990s (development of active-matrix EL and early thick-dielectric EL displays).Despite these productive R&D efforts, accom-panied by various examples of commercial-ization, it is the early years of the currentdecade that are likely to establish the long-term prospects for inorganic EL technology.
Of course, we have already seen the devel-opment of previous generations of inorganicEL displays with performance levels whichhave justified a significant expansion in theapplicability of EL technology. For example,high-performance full-color active-matrix ELmicrodisplays were nearly commercialized byPlanar Systems in the 1990s. Regardless ofsuch past efforts, inorganic EL technology hasnot fundamentally changed since its inceptionin terms of the commercial markets served bysmall-to-medium monochrome displays forthe medical, industrial, and military sectors.
Some analysts argue that, barring a newparadigm that shifts the development of inor-ganic EL technology onto a significantly dif-
ferent path, the technology will be confined toits current market niche of high-contrastrugged monochrome displays. However,inorganic EL technology is now generatingfull-color pixel formats that rival the perfor-mance of some of the best mainstream flat-panel technologies. If inorganic EL technol-ogy is to expand, the time is now.
Inorganic EL TechnologyThe benefits and challenges of state-of-the-artinorganic EL technology stem directly fromEL-device structure and operation. A com-mon feature of all commercially viable ELdevices is their double electrical-insulator(dielectric) structure. The basic thin-film EL(TFEL) panel structure consists of a self-heal-ing metal row electrode (non-propagatingbreakdown at dielectric defects), twodielectrics sandwiching a light-emitting phos-phor, a light-emitting phosphor that emitslight via hot-electron impact excitation ofluminescent dopants, and a transparent indiumtin oxide (ITO) column electrode [Fig. 1(a)].
The dielectrics capacitively couple an acvoltage to the phosphor, allowing for thereversible electrical breakdown of the phos-phor layer, commonly ZnS:Mn, which emitsamber light. The contrast of TFEL displayscan be enhanced using one of several clevertechniques that nearly eliminate reflectionfrom the rear thin dielectric and metal rowelectrode.
In the early 1990s, Dr. Xingwei Wu of iFireTechnology was able to replace the difficult-to-implement thin-film dielectrics (which are
Inorganic EL Displays at the Crossroads
A proven flat-panel technology for two decades, inorganic EL technology must now achieve multiple-provider commercialization of low-cost full-color displays – or resign itself to stagnant niche markets.
by Jason C. Heikenfeld and Andrew J. Steckl
Jason C. Heikenfeld is a display scientistleading the development of BDEL displays atExtreme Photonix LLC, 3130 Highland Ave.,3rd Floor, Suite 3225, Cincinnati, OH 45219-2374; telephone 513/475-6615, fax 513/221-1891, e-mail: [email protected]. Andrew J. Steckl foundedExtreme Photonix Corp. in 2001 and is nowits president, as well as a professor at theUniversity of Cincinnati; telephone 513/556-4777, e-mail: [email protected]. The authorsthank R. Tuenge, J. Laney, and M. Bowen(Planar); P. Beatty (ViewPoint); X. Wu (iFireTechnology); J. Zavada (Army ResearchOffice); D. Morton and E. Forsythe (ArmyResearch Laboratory); J. Wager and J. Bender (Oregon State University); A. Krasnov; and P. Rack (University of Tennessee) for support, encouragement, andmany useful discussions.
about 0.2 µm thick) in a TFEL display with aneasily manufactured screen-printed thick-film-dielectric layer which is about 20 µm thickand has an εr ~ 1000’s to 10,000’s [Fig. 1(b)].This thick-dielectric EL (TDEL) display pos-sesses higher capacitance and strong diffuse-emission outcoupling, which substantiallyboost panel luminance and efficiency. Thesecharacteristics, along with parasitic-energy-recovery techniques, justify a projected powerconsumption of about 200 W for iFire’s 34-in.HDTV product that the company plans to pro-duce in 2005. Recognizing the benefit ofcombining the low reflectivity of TFEL displays with the high luminance of TDELdisplays, Extreme Photonix LLC has recentlydeveloped the black thick-dielectric EL(BDEL) display structure [Fig. 1(c)].
In commercial EL-display panels, the phos-phor is excited by a bipolar pulse with a dura-tion in the tens of microseconds and thenemits radiatively within milliseconds, allow-ing for passive-matrix addressing and smear-free video images.
Typical inorganic EL displays are biased ata refresh rate between 60 and 240 Hz. The
EL emission normally exhibits a thresholdvoltage within the range of 120–180 V and anefficient modulation voltage in the 0–40-Vrange, or as high as 80 V, above threshold.Although not discussed here, it is worth not-
ing that several other high-performance inor-ganic EL-display variations have been exten-sively investigated, including edge-emittingTFEL, transparent TFEL, and active-matrixTFEL-on-silicon displays.
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Fig. 1: These EL-display structures are suitable for passive-matrix displays: (a) thin-film EL (TFEL), (b) thick-dielectric EL (TDEL), and (c) black-dielectric EL (BDEL).
Fig. 2: iFire Technology’s current 17-in.TDEL prototype exhibits higher luminance,resistance to differential aging, and greatermanufacturing simplicity, thanks to the com-pany’s Color-by-Blue approach. iFire Technology
Instability and Rapid ProgressIf a technology can seem to be heading in twodirections at the same time, the 2002–2003period seemed to suggest just that for inor-ganic EL technology. Several announcementsdisturbed the general perception of inorganicEL technology. It is a common mistake toperceive TFEL technology as the indicator forthe health and future of all forms of inorganicEL technology, but it is understandablebecause only TFEL-based displays are soldcommercially at present.
• Beaverton, Oregon, August 2002.Planar Systems, Inc., the establishedleader in EL displays, announced consol-idation of TFEL-display production to itsEspoo, Finland, facility.
• Tokyo, Japan, September 2002. TDKCorp. announced a color TDEL displaywith more than 200 cd/m2 that is basedon technology licensed from iFire Technology.
• Novato, California, November 2002.Global-Tech Appliances announced
plans to sell the TFEL-display businesssector of LiteArray, followed by a man-agement buyout of that business to formViewPoint Displays.
• Toronto, Canada, April 2003. iFireTechnology announced the Color-by-Blue approach for EL technology, whichled to the fabrication of a 600-cd/m2 pro-totype and could reduce capital require-ments for manufacturing by 15%.
• Baltimore, Maryland, May 2003.Extreme Photonix LLC demonstrated thefirst monochrome BDEL-display proto-type with commercially viable luminanceand pixel format and a reflectivity of lessthan 3%.
Planar Systems described their move as partof a company-wide push to higher profitabil-ity by closing its U.S. manufacturing facilities– which produced significant numbers of cus-tom TFEL products – and focusing on its lineof more-profitable standard TFEL panels pro-duced in Finland using an atomic-layer-epi-taxy (ALE) process. During this same period,TDK Corp. resurfaced, after a fairly quietR&D period, with a nearly product-ready full-color QVGA panel for automotive displays.
The most impressive achievement of theseturbulent 2 years is iFire’s 17-in. prototypeTDEL panel, which now exhibits a maximumfull-color luminance greater than 600 cd/m2
(Fig. 2). Such a luminance level was practi-cally unimaginable in the mid-1990s, whenmuch smaller full-color TFEL and TDEL displays could realize only 50 cd/m2 or so.
In the aftermath of these developments, twoconclusions are emerging: (1) monochromeTFEL technology remains viable and prof-itable and (2) the quest for significant expan-
sion of inorganic EL technology now dependson advances in variations of TDEL displays.Support for this argument could be found atthe SID 2003 International Symposium held inBaltimore. For the first time in many years,not a single TFEL-technology paper was pre-sented, nor a single product displayed; all ofthe papers and demonstrations on inorganic ELtechnology concerned the thick-dielectric type.
The effect of thick dielectrics on the com-mercialization of EL technology has proven tobe much more significant than the recent tech-nical developments in other secondary displaytechnologies, such as carbon nanotubes forfield-emission displays (FEDs). Althoughefforts by ViewPoint Displays and other com-panies to increase the market share of TFELdisplays through high-volume low-cost manu-facturing are under way and may yet bearfruit, the fact remains that TFEL-technologyR&D has diminished rapidly in just a fewyears, while TDEL approaches are attractingthe majority of corporate R&D.
Meeting the ChallengesIn order to be successful in penetrating large-screen TV, automotive, or other lucrativehigh-volume markets, there are several chal-lenges EL technology must overcome if it isto expand beyond the traditional niche mar-kets served by monochrome TFEL panels(Table 1). The qualitative assessments in thetable represent the full potential of TFEL,TDEL, and BDEL technologies based on pre-sent understanding.
Sunlight-readable TFEL displays having amonochrome luminance of 50–300 cd/m2 anda low reflectivity of about 1% have long beencommercialized. The luminous efficiency of
22 Information Display 12/03
EL displays
Table 1: A Comparison of theCharacteristics of Inorganic EL
Key: + adequate; ++ good; +++ high performance; ++++ stateof the art.
Extreme Photonix LLC
Fig. 3: Extreme Photonix’s BDEL small graphics prototype demonstrates the easy legibility ofinorganic EL displays due to their high contrast.
1–2 lm/W that has been achieved for ZnS:Mnphosphor used in TFEL panels is partly due tothe high sensitivity of the eye to this phos-phor’s yellow emission. The relative lumi-nance penalty associated with red and bluephosphors, with efficiencies of less than 0.5 lm/W in TFEL displays, has preventedpractical full-color products.
Another challenge is that the TFEL dielec-tric capacitance – and, consequently, the powerinput coupled to the phosphor layer – is gener-ally limited to less than 40 nF/cm2 because ofthe thickness requirement of thin-filmdielectrics for high-voltage stability. Also, inTFEL devices, the dielectric/phosphor/dielec-tric stack must be free of high-field points,which in turn causes highly specular reflec-tions and strong wave-guiding of the phosphor-generated light. This limits the light outcou-pling of high-contrast panels to just about 5%.
In TDEL devices, on the other hand, thehigher dielectric capacitance (up to hundredsof nF/cm2) and diffuse light-outcoupling (inthe tens of percent) permit achievement ofluminous efficiency approaching 10 lm/W for ZnS:Mn. The power consumption of inorganic EL panels, which is proportional toCV2, is dominated by parasitic capacitances
and only slightly increases with increasingdielectric capacitance. This and several otherfactors boost efficiency in TDEL and BDELpanels, including the ability to increase thethickness of the phosphor layer without anincrease in operating voltage.
TFEL and BDEL displays can be designedfor superior dark contrast and an extremelysteep luminance–voltage (L–V) slope. This isdesirable for easy legibility (Fig. 3) andreduces much of the parasitic panel powerconsumption associated with modulation volt-age. The very steep L–V slope necessary toachieve a modulation voltage less than 40 Vrequires that thin films be formed directly onsmooth glass for a uniform electric field overthe entire pixel area, which is necessary forsimultaneous electrical breakdown of thephosphor layer. However, a more gradualL–V slope is required for ease of operation ingray-scale mode.
The simplest way to achieve a gradual L–Vslope is to use high- and low-field points toachieve varied electric-field distribution overthe pixel area. However, a thick dielectric isgenerally required for this approach becausehigh-field points cause “punch-through” ofthin dielectrics, resulting in electrical shorts.
A gradual L–V slope is inherent in the TDELdevice structure because of its non-planarthick/thin-film morphology, which can easilybe implemented in a BDEL structure as well.
A thick dielectric [Fig. 4(c)] provides toler-ance to high-field points and other commonthin-film defects (pinholes, particulates,scratches), which, in turn, results in strongmanufacturing advantages over thin-filmhigh-field EL technologies such as organic EL (OEL) technology [Fig. 4(a)] and TFELtechnology [Fig. 4(b)]. This manufacturingadvantage reduces capital-equipment investment for production, increases panelthroughput and yield, and is one of the factors allowing inorganic EL technology tomove closer to the production of affordablemass-marketable displays.
Excellent dark-room contrast ratio is thestarting point for high image fidelity and iseasily achieved in all EL technologies. Moredifficult is maintaining that contrast in brightlighting conditions, which has long been aproblem in plasma-display panels (PDPs) andcathode-ray tubes (CRTs) because they usediffusely reflecting powder phosphors. Thethick dielectrics used in TDEL and BDELdevices are formed primarily from powder
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Fig. 4: Surface or film irregularities can cause electrical shorts in OEL and TFEL pixel structures and dark-spot formation on OEL structures.These problems are avoided when thick/thin-film hybrid EL structures are used.
formulations and produce a strong diffusereflectivity as deposited. In TDEL devices,this problem can be solved.
Extreme Photonix LLC has developed ablack- or color-pigmentation method thatturns a thick-film dielectric black (roughly2–3% reflectivity), making TDEL displayslegible at higher-ambient-light levels and insunlight. The black thick dielectric separatesBDEL displays from other thick-film displaysby eliminating – and outperforming – externalcontrast-enhancement techniques such as neutral-density filters.
Several promising variations of this black-or color-dielectric approach are discussed inthe next section. One final issue is that of dis-play reliability and lifetime. In general, inor-ganic EL technology is one of the most rugged technologies available, largely free from ther-mal sensitivity, shock, vibration, and otherharsh-environment issues. And, with mono-
chrome displays exhibiting field-proven life-times exceeding 100,000 hours and full-colorpanels now exceeding lifetimes of 30,000hours, EL technology has a clear lifetimeadvantage over other flat-panel emissive tech-nologies such as PDP and OEL technologies.
The Future of Full-Color EL Displays To penetrate major display markets, inorganicEL displays clearly need a full-color technol-ogy that is competitive in both performanceand cost of manufacturing. The conventionalapproach to efficient (minimum use of colorfilters) full-color integration is the sequentialdeposition and patterning of red-, green-, andblue-phosphor layers. An alternative approachis to use color-conversion media (CCM). Here,the output of a strong blue or violet/ultravioletlight source is absorbed in layers of fluorescentCCM materials, which then emit photons atlonger wavelengths of light.
This approach was first demonstrated witha luminance of a few cd/m2 by Y. Cho in1997, using inorganic CCM layers and a UV-emitting TFEL phosphor. Current full-color17-in. iFire Technology panels use what thecompany calls its Color-by-Blue approach, inwhich a single blue-phosphor layer, red andgreen CCM, and a light-blue color filter areused to create the RGB subpixels (Fig. 5).These panels have a luminance of about 600 cd/m2 without neutral-density contrastenhancement and 200–300 cd/m2 with con-trast enhancement.
The CCM approach greatly simplifies full-color EL panels because it eliminates the needfor patterning hydro-sensitive sulfide phos-phors, allows a single phosphor anneal (differ-ent phosphors often require different anneal-ing temperatures and durations), results in asingle L–V slope characteristic and therebysimplifies column drivers for modulation ofRGB subpixels, and alleviates differential-aging problems. The CCM approach wouldnot be practical for inorganic EL displays if itwere not for breakthroughs in the develop-ment of saturated blue EL phosphors in thelate 1990s, the leading example of which isBaMgAlS:Eu (developed by Noboru Miuraand his colleagues), which was recentlyreported to deliver a luminous efficiencyexceeding 1 lm/W.
The CCM approach is easily adaptable toinverted EL displays because organic CCM lay-ers can be printed at the end of the fabricationprocess, which follows the high-temperature
annealing (550–750°C) of the EL phosphor.With quantum efficiencies of over 90%, CCMlayers themselves can be very efficient, and canbe made from simple combinations of inorganicpowder phosphors or of organic fluorescentdyes in a PMMA (or other polymer) matrix.
Despite the attractions of CCM, conven-tional color-integration approaches are stillbeing pursued. For example, the most recentTDK Corp. 4.25-in. 240 × 180-pixel TDELprototype uses a triple-patterned-phosphorprocess and produces bright displays at apower consumption of 10 W when displaying100% white. The display produces 200 cd/m2
and exhibits a 3:1 contrast ratio in an ambientof 20,000 lux.
There are several other possibilities for further increasing the luminance of full-colorinorganic EL displays. One such concept,under development at Extreme Photonix LLC,uses existing black- and color-dielectric tech-nology to boost the contrast-enhanced lumi-nance of Color-by-Blue EL panels. This con-cept is incorporated in the structure shown inFig. 5, and calls for the utilization of a high-contrast “blue” dielectric layer in a Color-by-Blue EL panel. Preliminary results show agood blue reflective chromaticity of approxi-mately x = 0.2 and y = 0.2 for high-capacitanceblue thick dielectrics based on lead magnesiumniobate and a blue-pigmentation dye compati-ble with the later patterning of CCM layers.
The blue dielectric absorbs incident red andgreen ambient light without reducing the out-coupling efficiency of the blue-phosphoremission. Of course, approximately 50% ofthe red and green CCM emission is absorbedby the blue dielectric, so subpixel areas mustbe adjusted accordingly. Nonetheless, the theoretical improvement in luminance andcontrast is significant. Assuming a panelwhite luminance of 1000 cd/m2 before con-trast enhancement, 80% diffuse reflectivityfrom the rear thick dielectric, and the additionof a 40% neutral-density filter, the result is 13% diffuse luminous reflectivity and 400-cd/m2 luminance. By using a blue thickdielectric, a 9% luminous reflectivity and aluminance of 600 cd/m2 can be achieved. Themost powerful embodiment would be a bluedielectric, with red and green color filtersabove the CCM, resulting in a reflectivity ofabout 2% and a luminance of 600 cd/m2 for afull-color contrast ratio of about 100:1 in a1000-lux ambient, and 3:1 in a 50,000-luxambient. This result would provide inorganic
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EL displays
Fig. 5: (a) Shown is a high-contrast full-color EL-display structure using a blue phos-phor, color-conversion media (CCM), and ablue filter. This sample structure of iFireTechnology's Color-by-Blue approachincludes an Extreme Photonix LLC bluedielectric for improved contrast. (b) Shown isthe full mechanism for contrast enhancementand emission outcoupling.
EL displays with an advantage in very-high-contrast performance over PDPs.
Color-EL-Display CommercializationThe prevalent model for monochrome TFELproduction and sales is not applicable to themass commercialization of inorganic color ELdisplays. Because of the extremely ruggedspecifications required by present TFEL-display markets, product qualification adds asignificant amount to the price tag for generalconsumers not requiring ruggedization. Reso-lution of this issue and several others isrequired for widespread acceptance of inor-ganic EL displays. Solutions for these prob-lems exist conceptually. iFire Technology’sproposed price model for its 34-in. HDTVproduct is less than $2500 compared to the present cost of about $500 for a 5-in. mono-chrome TFEL graphics panel.
Low manufacturing cost is essential, andthis generally requires the development ofcolor-EL-display manufacturing facilities inEast Asia. Market entry fueled merely bycosts lower than long-established technologies
is a tough sell, and EL technology will alsohave to convince investors and consumers thatit offers higher performance than existingtechnologies. An additional momentum boostwould be provided if difficulties in otheremerging technologies, such as OEL technol-ogy, prove to be insurmountable.
At the CrossroadsAll the development and all the speculationsupport one clear conclusion: inorganic ELtechnology is at a crossroads. In the authors’many discussions with those who know inorganic EL technology best – the longtimedevelopers and users of the technology – thepredictions of the future of EL technology canbe split into two groups. The first group saysthat because of the late emergence of themajority of inorganic EL-technology break-throughs it will be difficult for EL technologyto make the leap, since it must now do so as aninvasive technology. The second group saysthat inorganic EL technology is superior toexisting technology in so many ways that it canno longer be ignored, and it should be pursued
as a leading candidate technology by major display powers. Time will tell if the hard-wontechnical breakthroughs translate into massacceptance of inorganic EL displays. �
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