A History of Electroluminescent Displays
By
Jeffrey A. Hart
Indiana University
Stefanie Ann Lenway
University of Minnesota
and
Thomas Murtha
University of Minnesota
Second Draft
September 1999
We are grateful for the
comments provided by Christopher King, Sey Shing Sun, Richard Tuenge, T. Peter
Brody, and Runar Tornqvist. Research
assistance was provided by Craig Ortsey.
This research was supported by a grant from the Alfred P. Sloan
Foundation. Please do not cite or quote
without the written permission of the authors.
Introduction
Electroluminescent
displays (ELDs) have their origins in scientific discoveries in the first
decade of the twentieth century, but they did not become commercially viable
products until the1980s. ELDs are
particularly useful in applications where full color is not required but where
ruggedness, speed, brightness, high contrast, and a wide angle of vision is
needed. Color ELD technology has
advanced significantly in recent years, especially for microdisplays. The two main firms that have developed and
commercialized ELDs are Sharp in Japan and Planar Systems in the United States.
What Is
Electroluminescence?
There
are two main ways of producing light: incandescence and luminescence. In incandescence, electric current is passed
through a conductor (filament) whose resistance to the passage of current
produces heat. The greater the heat of
the filament, the more light it produces.
Luminescence, in contrast, is the name given to "all forms of
visible radiant energy due to causes other than temperature."[1]
There
are a number of different types of luminescence, including (among others):
electroluminescence, chemiluminescence, cathodoluminescence, triboluminescence,
and photoluminescence. Most "glow
in the dark" toys take advantage of photoluminescence: light that is
produced after exposing a photoluminescent material to intense light. Chemiluminescence is the name given to
light that is produced as a result of chemical reactions, such as those that
occur in the body of a firefly.
Cathodoluminescence is the light given off by a material being bombarded
by electrons (as in the phosphors on the faceplate of a cathode ray tube). Electroluminescence is the production of
visible light by a substance exposed to an electric field without thermal
energy generation.[2]
An
electroluminescent (EL) device is similar to a laser in that photons are
produced by the return of an excited substance to its ground state, but unlike
lasers EL devices require much less energy to operate and do not produce
coherent light. EL devices include light emitting diodes, which are
discrete devices that produce light when a current is applied to a doped p-n
junction of a semiconductor, as well as EL displays (ELDs) which are
matrix-addressed devices that can be used to display text, graphics, and other
computer images. EL is also used in
lamps and backlights.
There
are four steps necessary to produce electroluminescence in ELDs:
1.
Electrons
tunnel from electronic states at the insulator/phosphor interface;
2.
Electrons
are accelerated to ballistic energies by high fields in the phosphor;
3.
The
energetic electrons impact-ionize the luminescent center or create
electron-hole pairs that lead to the activation of the luminescent center; and
4.
The
luminescent center relaxes toward the ground state and emits a photon.
All
ELDs have the same basic structure.
There are at least six layers to the device. The first layer is a baseplate (usually a rigid insulator like
glass), the second is a conductor, the third is an insulator, the fourth is a
layer of phosphors, and the fifth is an insulator, and the sixth is another
conductor.
Figure 1. Structure of an Electroluminescent Display |
|
|
|
Source: Planar Systems. |
ELDs
are quite similar to capacitors except for the phosphor layer. You can think of an ELD as a "lossy
capacitor" in that it becomes electrically charged and then loses its
energy in the form of light.[3] The insulator layers are necessary to
prevent arcing between the two conductive layers.
An
alternating current (AC) is generally used to drive an ELD because the light
generated by the current decays when a constant voltage is applied. There are, however, EL devices that are DC
driven (see below).
Scientific Origins
Electroluminescence
was first observed in silicon carbide (SiC) by Captain Henry Joseph Round in
1907.[4] Round reported that a yellow light was
produced when a current was passed through a silicon carbide detector.[5] Round was an employee of the Marconi Company
and a personal assistant to Guglielmo Marconi.
He was an inventor in his own right with 117 patents to his name by the
end of his life.[6]
The
second reported observation of electroluminescence did not occur until 1923,
when O.V. Lossev of the Nijni-Novgorod Radio Laboratory in Russia again
reported electroluminescence in silicon carbide crystals.[7]
B.
Gudden and R.W. Pohl conducted experiments in Germany in the late 1920s with
phosphors made from zinc sulfide doped with copper (ZnS:Cu). Gudden and Pohl were solid state physicists
at the Physikalisches Institut at the University of G`ttingen.[8] They reported that the application of an
electrical field to the phosphors changed the rate of photoluminescent decay.[9]
The
next recorded observation of electroluminescence was by Georges Destriau in
1936, who published a report on the emission of light from zinc sulfide (ZnS)
powders after applying an electrical current.[10] Destriau worked in the laboratories of
Madame Marie Curie in Paris. The Curies
had been early pioneers in the field of luminescence because of their research
on radium. According to Gooch, Destriau
first coined the word "electroluminescence" to refer to the
phenomenon he observed.
Gooch
also argues that one should keep in mind the differences between the
"Lossev effect" and the "Destriau effect:"
The Lossev effect should be distinguished from the
Destriau effect. Destriau's work
involved zinc sulphide phosphors, and he observed that those phosphors could
emit light when excited by an electric field…[The Lossev effect, in contrast,
involves electroluminescence] in p-n junctions.[11]
During
World War II, a considerable amount of research was done on phosphors in
connection with work on radar displays (which was later to benefit the
television industry in the form of better cathode ray tubes). Wartime research also included work on the
deposition of transparent conductive films for deicing the windshields of
airplanes. That work was later to make
possible a whole generation of new electronic devices.
In
the 1950s, GTE Sylvania fired various coatings, including EL phosphors onto
heavy steel plates to create ceramic EL lamps.[12] During this period, most research focused on
powder EL phosphors to get bright lamps requiring minimal power and with a
potentially long lifetime. Research funding
was cut back when it was determined that product lifetimes were too short
(approximately 500 hours).[13]
The
first thin-film EL structures were fabricated in the late 1950s by Vlasenko and
Popkov.[14] These two scientists observed that luminance
increased markedly in EL devices when they used a thin film of Zinc Sulfide
doped with Manganese (ZnS:Mn).
Luminance was much higher in thin film EL (TFEL) devices than in those
using powdered substances. Such devices
however were still too unreliable for commercial use.
Russ
and Kennedy introduced the idea of depositing insulating layers under and above
the phosphor layer on a TFEL device.[15] The implications for reliability of TFEL
devices was not appreciated at the time, however.
Soxman
and Ketchpel conducted research between1964 and 1970 that demonstrated the
possibility of matrix addressing a TFEL display with high luminance, but again
unreliability of the devices remained a problem.[16]
In
the mid-1960s, there was a revival of EL research in the United States focused
on display applications. Sigmatron
Corporation first demonstrated a thin-film EL (TFEL) dot-matrix display in
1965. Unfortunately, Sigmatron was
unable to successfully commercialize these displays and it folded in 1973.[17]
In
1968, Aron Vecht first demonstrated a direct current (DC) powered EL panel
using powdered phosphors.[18] Research on powdered phosphor DC-EL devices
continued, especially for use in watch dials, nightlights and backlights, but
most subsequent research on ELDs focused on thin-film AC driven devices. An early example was the work of Peter Brody and his associates at
Westinghouse Research Laboratories on EL and AM-EL devices between 1968 and
1974.[19]
In
1974, Toshio Inoguchi and his colleagues at Sharp Corporation introduced an
alternating current (AC) TFEL approach to ELDs at the annual meeting of the
Society for Information Display (SID).
The Sharp device used zinc sulfide doped with manganese (ZnS:Mn) as the
phosphor layer and yttrium oxide (Y2O3) for the
sandwiching insulators. This was the
first high-brightness long-lifetime ELD ever made. Sharp introduced a monochrome ELD television in 1978. The paper Inoguchi published on his group's
research helped to reinvigorate EL research in the rest of the world, including
at Tektronix, a U.S. electronics firm based in Portland, Oregon.[20]
Tektronix'
research on EL began in 1976. The
management at Tektronix were familiar with the work reported by Inoguchi's
team. They decided to start a new
program on ELDs at Tektronix Applied Research Laboratories. The work begun there was continued when the
Tektronix researcher left to create a spinoff firm called Planar Systems. Several other large U.S. companies also were
conducting research on ELDs in the 1970s, including: IBM, GTE, Westinghouse,
Aerojet General, and Rockwell. All
these companies realized that ELDs had potential advantages over existing LCD
technology in the following areas:
1.
Contrast
2.
Multiplexing,
and
3. Viewing angle.
The
most important problem that had to be solved before mass production of ELDs
could begin was increasing the reliability of the EL thin film stack. Since the devices operated at very high
field levels -- about 1.5 MV/cm -- there was a high probability that they would
break down, especially if there was insufficient uniformity in the stack. Sharp, Tektronix, and Lohja Corporation in Finland were able to solve
this problem between 1976 and 1983 using slightly different approaches.
The
second major problem was to get access to high-voltage drivers for the displays. Sharp ended up developing their own; Tom
Engibous developed drivers for EL displays at Texas Instruments by modifying
the design his group had done for plasma displays.[21] Planar used the TI drivers in its products
until it could find additional suppliers.
The
introduction to the market in 1985 of Grid and Data General laptops with EL
displays from Sharp and Planar respectively helped to build the foundations for
the nascent laptop computer industry at a time when LCDs did not have
sufficient brightness or contrast to be used in commercial products. Both Planar and Sharp monochrome ELDs used a
phosphor layer made from zinc sulfide doped with manganese (ZnS:Mn). These displays gave off an amber
(orange-yellow) color that was bright but also pleasing to the eye.
|
Figure
2. A Monochrome (Amber) EL Notebook Display |
|
|
|
Source:
Planar Systems. |
Research on Color ELDs
One
of the key disadvantages of ELDs relative to liquid crystal displays (LCDs) was
that until 1981 ELDs were not capable of displaying more than one color. Even after 1981, color ELDs were limited to
a limited range of colors (red, green, and yellow) until 1993 when a blue
phosphor was discovered.
In
1981, Okamoto reported that a rare-earth doped ZnS could be used in the
phosphor layer of a TFEL device.[22]
In
1984, William Barrow of Planar and his colleagues announced that they were able
to get blue-green emissions from strontium sulfide doped with cerium (SrS:Ce).
In
1985, Shosaku Tanaka at Tottori University and his colleagues reported that
they had duplicated the work done at Planar on SrS:Ce phosphors but added that
they had gotten calcium sulfide (CaS) to emit a deep red color. In 1988, Tanaka's group announced that they
had gotten white light from a TFEL display using a combination of SrS:Ce and
SrS doped with Europium (SrS:Eu). The
idea here was to use the white light in connection with a color filter to
produce a full color display analogously to the way that it is done in liquid
crystal displays. The advantage of
doing this with ELDs was that such a display would not require a
backlight. The main disadvantage was
the added cost and difficulty of introducing a color filter.
In
1994, Soininen and coworkers at Planar International in Finland announced that
a SrS:Ce/ZnS:Mn white phosphor deposited by atomic layer epitaxy achieves
sufficient luminance and stability for use in color EL display products.[23]
Further
work on blue phosphors was done by Reiner Mach and his colleagues at the
Heinrich Hertz Institute in Berlin.
Additional work on SrS:Ce blue phosphors was done at Westaim
Corporation.
A SrS:Cu blue phosphor showing improved blue color and efficiency was reported by Sey-Shing Sun of Planar in 1997. Planar demonstrated true white color EL prototype displays using this blue phosphor in a SrS:Cu/ZnS:Mn multi-layer structure. The SrS:Cu phosphor will enable color EL displays to be produced with a wider color gamut.[24]
Barrow
and his team at Planar announced a prototype of a multi-color EL display using
ZnS:Mn and ZnS:Tb phosphor layers in 1986.
By 1988, they had a prototype full-color display using a patterned
phosphor structure. Commercial
production of multicolor ELDs did not occur until 1993 at Planar, however, and
full color ELDs have been produced only in the form of microdisplays (see
section below on AMEL microdisplays).[25] These color AMEL microdisplays used the ALE
SrS:Ce/ZnS:Mn white phosphor with either sequential or spatial color filtering.
Planar Systems
Planar
Systems, Inc., was formed in 1983 as a spinoff from Tektronix. It was founded by three senior managers from
Tektronix' Solid State Research and Development Group: John Laney, James Hurd, and Christopher
King.[26] Hurd became the President and CEO, Laney
worked on manufacturing issues, and King became the firm's chief technical
officer. Tektronix gave Planar its
rights to certain technologies in exchange for an equity stake (in 1994 its
share was still 7.5 percent).[27] Planar remained privately held until it went
public in 1993.
In
1984, Planar opened its first manufacturing facility in Beaverton, Oregon. It shipped its first bulk order in 1985 to
Nippon Data General for an early laptop computer with a CGA (640x200) EL panel.
Once
volume manufacturing of ELDs began, a number of additional problems had to be
solved in order to improve prospects for sales in the competitive markets for
flat panel displays:
1.
Luminous
efficiency had to be increased;
2.
Better
driving methods were needed; and
3.
Gray
scale capability of ELDs had to be enhanced.
The
initial ELD prototypes had brightness levels of only about 20 foot lamberts
(fLs). Commercial products in the 1990s
were to have brightness levels of 100 fLs.
The
initial drive scheme for ELDs at Planar was to apply a single polarity voltage
pulse to each line of the display and then an opposite polarity pulse to the
entire panel. This was called "the
refresh method." In 1984/85, it
was determined that this drive method led to "burn in" -- some pixels
would become unusable over time. A new
drive scheme invented by Tim Flegal called symmetric drive replaced the refresh
method. In symmetric drive, pulses of
alternate polarities were applied to each line so that a net zero dc voltage
was developed. This prevented
"burn in."
Tim
Flegal was also responsible for pioneering a variety of gray scale driving
methods, including pulse width, analog voltage, and frame rate modulation. High performance analog drivers at
reasonable prices were difficult to obtain, and Planar had difficulty getting
Texas Instruments to supply them because of the relatively low volumes involved
(from TI's perspective), but eventually Planar found a new supplier for these
circuits: Supertex.[28]
One
of Planar's key markets after the decline in demand for monochrome displays for
laptop computers was military displays.
Planar provided EL displays to defense contractors like Norden Systems
and Computing Devices Canada, Ltd. (CDC).
These displays were monochrome with limited gray scaling. Planar diversified its sales out of military
applications toward industrial and medical equipment. By the mid 1990s, over a
third of Planar's sales were to medical equipment firms.
|
Figure
3. Sales of Planar Displays by Type
of Application, c. 1998. |
|
|
|
Source:
Planar Systems at http://www.planar.com
on July 28, 1999. |
Because
of Planar's willingness to work with customers to adapt products for specific
applications, it was able to command a price premium over the products of its
main competitor, Sharp. By the late
1980s, Planar controlled over 90 percent of the world market for ELDs.
Planar
purchased the Finlux Display Electronics unit of Lohja Oy (Finland) in December
1990. Finlux was renamed Planar
International, Ltd. Its headquarters
remained in Espoo, Finland. The main reason for the purchase of Finlux was to
obtain a marketing and production base in Europe but an important secondary
reason was to get access to Finlux's atomic layer epitaxy (ALE) technology (see
the section on Finlux below).[29]
EL
displays were not well suited to military applications by the early 1990s. By that time, the military wanted color
displays that were bright enough to be seen in airplane cockpits and tanks
under a variety of environmental lighting conditions. In August 1994, Planar purchased the avionics display operations
of Tektronix and formed a wholly owned subsidiary called Planar Advance to
manage this business.[30] Planar Advance initially invested about $10
million in CRT-based displays for cockpits, but was blindsided by the DoD's
policy of switching to ruggedized TFT LCDs.
In response to this shift, Planar Advance purchased TFT LCD glass from
dpiX and assembled them into "mil spec" units for the DoD. This move permitted Planar to diversify its
display offerings out of ELDs but it also necessitated a redefinition of the
core competence of the firm.
In
1992, Planar helped to organized a consortium to develop color ELDs called the
American Display Consortium. This
consortium was funded by the Department of Commerce under the Advanced Technology
Program (ATP) created by the Clinton administration. The total funding for the consortium was to be $30 million; half
funded by the government and half by the consortium's private firms. The National Institute for Standards and
Technology (NIST) supervised the consortium on behalf of the Department of
Commerce. Other members of this
consortium were: Candescent Technologies, dpiX, Electro Plasma, FED
Corporation, Kent Display Systems, Lucent Technologies, OIS, Photonics Imaging,
SI Diamond, Standish Industries, Three-Five Systems, and Versatile Information
Products.
In
Spring 1995 Planar organized a consortium to develop the next generation of
High Resolution and Color TFEL Displays. This consortium was funded by the
Department of Defense under the DARPA managed Technology Reinvestment Program
(TRP). The total funding for the consortium was to be
$30 million; half funded by the government and half by the consortium’s private
firms. Other members of the consortium
were: AlliedSignal Aerospace, Computing Devices of Canada, Ltd., Advanced
Technology Materials, Boeing, CVC Products, Georgia Tech Research Institute,
Hewlett Packard, Honeywell, Lawrence Livermore National Laboratory, Los Alamos
National Laboratory, Oregon State University, Positive Technologies and the
University of Florida.[31]
In
1989, the Defense Advanced Research Projects Agency (DARPA) began to fund work
on advanced displays as part of its High Definition Systems program. DARPA issued a Broad Area Announcement in
that year and in subsequent years asking for proposals.
Planar
won one of the first grants from DARPA in 1990 and used the funds to set up a
laboratory to develop color ELDs.
Planar
participated in a variety of DARPA programs, but perhaps the most significant
was its work with Kopin and the David Sarnoff Research Center on active matrix
EL (AMEL) microdisplays beginning in 1993.
The AMEL device is processed on a silicon wafer
substrate using the inverted EL structure with a transparent ITO top
electrode. The lower EL electrode is
the top metallization layer of the silicon IC.
ALE
was used to make the device because of its excellent "conformal
coating" characteristics. ALE
resulted in very few pinhole defects, a key requirement for reliable EL devices
with top electrodes.
The
pixel size of the first generation of AMEL displays was 24 microns. The second generation of displays used
pixels of 12 microns. Smaller pixels
meant higher resolution, lower power consumption and lower cost of production
for a given display format.[32]
In
October 1995, Planar announced an arrangement to supply AMEL displays to
Virtual I-O, a Seattle-based manufacturer of consumer head mounted displays for
virtual reality entertainment systems.[33] Unfortunately, Virtual I-O went bankrupt in
1997 before any of these displays could be sold to the public.
|
Figure
4. Picture of a Color AMEL
Microdisplay |
|
|
|
Source:
Planar Systems. |
In
March 1996, Planar was a awarded a DoD contract to supply an AMEL-based head
mounted display (HMD) for the military's Land Warrior Program.[34] On May 16, 1996, Planar announced that it
had developed an AMEL microdisplay that was one-inch square, 3mm thick, and
weighed only 4 grams. In 1997 Planar announced that it had developed a 0.75
inch diagonal full-color VGA AMEL microdisplay using an LC sequential color
shutter.[35](ref: R.
Tuenge, et al., SID 97 Digest (1997), p.862 ).
Planar now has a brighter full-color microdisplay capable of displaying
32k colors that does not require the LC shutter.
Planar
has experienced a steady growth in revenues (see Figure 5 below).
|
Figure
5. Annual Revenues for Planar Systems
in $Millions, 1984-98 |
|
|
|
Source:
Annual Report (1998) accessed at http://www.planar.com on July 15, 1999. |
|
|
Its
profits also steadily increased in both absolute terms and per share but with a
decline in 1998. Planar went public
with an IPO in 1993.
A Brief History of Sharp's
EL Operations
The
head of research at Sharp, Sanai Mita, was convinced that ELDs could be used
eventually to make flat TVs. Mito was
formerly a professor at Osaka Municipal University. He and his team mounted a major effort in the mid 1970s to
develop TFELs.[36]
The
key research at Sharp was done by Toshio Inoguchi and his colleagues. The successful demonstration of a working
TFEL display in September 1978 at the Consumer Electronics Show in Chicago was the "finest
hour" of Inoguchi's group. This
display was only a few inches in diagonal, but it was also only 3 cm
thick.
Sharp
began mass production of ELDs in 1983.
One of its earliest displays was used in the U.S. Space Shuttle's obital
navigation system in that same year.[37] Another early application of a Sharp ELD
was in a Grid laptop computer. This
display provided resolution equivalent to a quarter VGA (320x240).
1983
was also the year that Shinji Morozumi at Seiko announced that his group was
able to build a TFT LCD television.
That announcement took Sharp by surprise and they redirected their
efforts toward catching up with Seiko in LCDs.
By 1987, Sharp was able to market their own TFT LCD television.[38] They were able to capitalize on their lead
in mass production of STN LCDs for calculators to quickly develop production
technologies for high-volume TFT manufacturing. After 1987, TFT LCD production was far more important to Sharp's
corporate strategy than EL production.
Nevertheless, the firm remained active in both research and production
of ELDs, providing strong competition to Planar and Lohja. Sharp continues to market EL displays for
niche markets.
A Brief History of the
Finlux Display Division of Lohja Oy[39]
In
1975, a research group headed by Dr. Tuomo Suntola recognized that thin film
electroluminescence would be an ideal flat panel display technology provided
that luminance stability and reliability problems could be overcome. To solve
these problems a new thin-film deposition method called atomic layer epitaxy
(ALE) was developed (see Figure 6). The basic idea was to build thin films
layer by layer using surface-controlled chemical exchange reactions. The result
is a dense, pinhole film with very good step coverage properties. This research activity started in a small
company called Intrumentarium that was acquired in 1977 by Lohja Oy, a Finnish
conglomerate which was primarily a manufacturer of construction material. Lohja was the second largest Finnish
electronics company after Nokia, and the new ELD technology was considered a
good fit for its strategy of diversification into electronics.
Figure 6. ALE sequences for a binary compound (courtesy of Tuomo Suntola)
|
|
B.
When all bonding sites are filled the surface reaction is saturated. Bonding
sites for the second precursor have been created. |
C.
Second precursor reacts with the surface created in steps A and B. Chemisorption
occurs as long as bonding sites are available, until saturation … |
D.
and the formation of bonding sites for the first precursor begins
again. The cycle of sequences A to D are repeated the necessary number
of times for the desired layer thickness. |
Excellent
ELD results based on its proprietary ALE technology were for the first time
presented at the annual meeting of the Society for Information Display (SID) in
1980, where they received a lot of attention.
In 1983, three large information boards were delivered to the Helsinki
Vantaa airport. Each of these was comprised of more than 700 character modules.
They proved that ALE technology could meet reliability requirements necessary
for commercial use. That technology was licensed to Sintra Alcatel in France in
1983. However, the driver costs of the
ELD character modules were too high to make them commercially viable, and as a
result Finlux began development of a 9-inch 512x256 matrix display for computer
and industrial applications. A large
manufacturing plant was constructed in a new science park set up in Espoo close
to Helsinki. Core manufacturing technologies,
including ALE deposition equipment, were developed in-house, which delayed the
start of mass production until 1986.
Half-page ELD matrix displays with resolutions of 640x200, 650x350 and
640x400 were subsequently manufactured at this plant.
The
investments and development costs for ELDs were essentially funded internally
by Lohja Oy because little public or customer-paid funding was available. This
situation changed when color ELD development was started in 1988 as part of an
EU-supported international consortium. The first color EL display based on an
innovative device structure was brought to market in 1993.
Lohja
Corporation was never able to make the Finlux Display Division profitable
because of a lack of experience in managing microelectronics businesses. The Finnish economy benefited from rapid
economic growth from the late 1970s until the late 1980s. But when the Soviet Union broke apart in
1991, the Finnish economy suffered because of its dependence upon the Soviet
Union as a customer for exports. In
1991, the Finlux Display Division was sold to Planar Systems and was renamed
Planar International.
The
two ELD operations were of approximately the same size at the time of the
merger. The merger permitted savings in
marketing costs and materials purchases.
Planar Systems succeeded in making Planar International profitable in
just a few years by using more experienced management, but without changing
manufacturing technology and with only minor changes in staffing. The ALE
manufacturing technology still forms the basis for the production of high
volume ELDs at both Planar Systems and Planar International. Much of the color development results that
were achieved in Finland were also of direct benefit to the work on color ELDs
at Planar Systems in the United States, and in particular the AMEL
microdisplays discussed above.
In
addition, in 1996, Planar Systems began to market a new generation of
monochrome ELDs called ICEBrite displays.
The ICEBrites combined ALE grown phosphors and insulators with high
contrast layers developed by Eric Dickey in the late 1980s.[40]
|
Figure
7. Picture of an ICEBrite Display |
|
|
|
Source:
Planar Systems. |
Organic Light Emitting
Diodes (OLEDs)
In
the late 1990s, several research laboratories announced that they had made
breakthroughs in getting thin films of organic materials to emit light
analogously to EL devices. Because
organic materials offered a number of process advantages over inorganic
phosphors, these announcements were taken very seriously by potential
investors. This is not the place to go
into the details of these developments.
Suffice it to say that the emergence of OLEDs led to a relative decline
in interest in further work on color ELDs.
Planar Systems set up its own OLED program in collaboration with ___ as
did several other display manufacturers.
It is possible that inability to solve the technological problems that
have to be solved in order to manufacture OLEDs in high volumes will result in
a return to research on color ELDs and other alternatives to TFT LCDs. For the moment, however, the momentum is
with the OLED research groups.
Conclusions
Electroluminescent
displays (ELDs) have a venerable history starting with the experiments of
Captain Henry J. Round in 1907, O.V. Lossev in the Soviet Union, and Georges
Destriau in France. Electroluminescence
was mostly a scientific curiosity until the invention of thin film deposition
techniques and the discovery that a sandwich of conductors, insulators and
phosphors could result in a very efficent
and long-lasting form of emissive display. ELDs were very important in the early days of the laptop computer
industry and remained important in niche markets for military, medical and
industrial equipment where high brightness, speed, contrast, and ruggedness are
necessary. The rise of the color TFT
LCD display forced the ELD producers to engage in research on color ELDs with
the result that there are now multicolor ELDs on the market and full-color
AMELs in development for microdisplays.
The ELD industry is currently limited to two major players: Planar and
Sharp. Planar acquired its only
European competitor, the Finlux Display Division of Lohja Oy, in 1990. Sharp remains committed to competing in ELDs
but its main focus is on liquid crystal displays. Most of the important research on ELDs remains within the
corporate laboratories of Planar and Sharp, but several publicly funded
research laboratories and consortia have also made important contributions to ELD
technology.
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