Download Module 5: Electroluminescent Displays

Module 5: Electroluminescent Displays
5.1. Introduction
Electroluminescent (EL) refers to the non-thermal generation of light resulting from
the application of an electric field to a substance. EL display devices can be classified
into four groups:
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ac thin film EL
ac powder EL
dc thin film EL
dc powder EL
Two of these have reached the commercial market: ac power ELdevices predominantly used for backlights in LCD applications, and ac thin film devices - used
in stand alone displays.
EL displays deal with light emission generated by impact excitation of a light emitting
center referred to as an activator or a luminescent center, by high energy electrons.
The electrons acquire their high energy from an electric field (100 V/mm). Initial
efforts to utilize EL phenomena focused on powder EL devices, but with the
advances in thin film technology, the majority of recent developments center around
thin film technology.
Thin film EL display devices based on ZnS:Mn materials have gained acceptance in
demanding markets where high brightness and large viewing angle displays are
necessary. The strongest selling point of a thin film EL display is high legibility
because light is emitted from a sub-micrometer thick device. The other positive
attributes of a EL display include high contrast (> 7:1 at 500 lux), wide viewing angle
(> 160 degrees), fast dynamic response (~ms), and high resolution. The fast
response enables video rate addressing capability to display true video images.
however, the biggest challenge of EL display devices is producing a full color image.
Initially, insufficient luminance of the primary colors prevented full color display. This
was overcome in the 1990's with the development of better color phosphors.
5.2. History
The first demonstration of the EL phenomena dates back to 1936 when Destriau
observed light emission from a ZnS phosphor powder dispersed in an insulating
medium, sandwiched between two electrodes. Until the development of SnO 2
transparent conductive substrates, there were no attempts to use EL technology in
practical display devices. The usefulness of SnO2 transparent conductive films
pushed the development of practical EL devices forward.
During the 1950's and 1960's, development focused on illumination sources, such as
nightlights. It was determined that the powder EL technology had serious multiplexing
limitations, so displays were restricted to fixed legend displays, mostly in instrument
panels. The powder materials in multiplexing applications had limited luminance, high
operating voltage, poor contrast, and severe luminance degradation (over 500 an
hour).
The first thin film EL devices were also fabricated during the 1950's and 1960's. This
was a promising device because the luminance-voltage curve was much steeper
than powder devices, which gave hope to passively address these devices. Passive
addressing, up to this point, was not possible because powder based devices had
poor luminance-voltage characteristics. During this time frame, researchers still
battled with short lifetimes.
In the 1960's, thin-film processing became feasible with improved manufacturing
techniques. A double-insulating layer type ac thin-film structure was introduced in
1967. However, the reliability issue was still the prominent hurdle for these devices to
break into commercial applications.
In 1974, researchers made a breakthrough on the materials requirements, achieving
a luminance of 1000 fL (3400 cd/m2) for more than 1000 hours. The device was a
ZnS:Mn thin-film ac double insulating structure layer. This demonstration rejuvenated
the interest in EL technology that eventually brought EL technology to the
commercial market. In 1983, volume production of 6" diagonal monochrome display
commenced.
The main target in the 1990's was to produce multicolor thin film displays from EL
technology. Many materials studies were directed at efficient color EL phosphors to
find suitable combinations of new host materials. The progress in full-color EL has
been realized through parallel developmental efforts of materials, thin-film processing
methods and EL device structures. A summary of the historical milestones is
presented below.
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1981 Color thin-film EL devices based on rare earth doped ZnS
1984 Blue emission from SrS:Ce thin-film EL device
1985 Multicolor emission from CaS and SrS based thin-film EL
1986 Luminance improvement of SrS:Ce
1986 Prototype multicolor thin-film EL display
1987 Luminance improvement of ZnS:Tb,F by sputtering
1988 White emission from SrS:Ce,Eu
1988 Prototype full-color thin-film EL display
1991 Multicolor thin-film EL display using an inverted, patterned-color-filter
device structure based on ZnS:Mn
1992 Red/Green/Yellow multicolor EL display in the inverted, patterned-colorfilter device structure with ZnS:Mn phosphor
1993 First commercial multicolor (Red/Green/Yellow) thin-film EL display
panel in EGA format
1993 Bright blue emission from CaGa2S4:Ce and prototype full-color thin-film
EL display
1994 First commercial full-color thin-film EL display monitor
5.3. EL Device Structures
The four basic types of EL devices will be discussed here.
5.3.1. ac Thin-Film EL Technology
The basic structure of ac thin-film EL technology is the double-insulating structure
shown below.
The central layer in the insulator sandwich is the phosphor layer wich emits light
when a large electric field is applied (100V/mm). Because of this large electric field,
any imperfection (e.g. dust) in the phosphor layer will result in catastrophic failure.
Therefore, when thin film insulating layers are used, a current limiting layer is placed
on both sides of the phosphor layer making it a more reliable device. The insulating
layers limit the maximum current ot the capacitative charging and discharging
displacement current level. The electodes on top and bottom of the insulatorphosphor-insulating layers complete a basic capacitance structure. The top-set of
these electrodes should be transparent to permit emitted light to pass to the viewer.
The transparent electode is usually fabricated from indium-tin-oxide (ITO) and the
bottom electrode is typically aluminum (Al). The approximate layer thicknesses of the
insultating layers range from 0.3 - 0.5 mm and the phosphor layer tends to be
between 0.5 - 1.0 mm. The visual appearance of the device in the undriven state is
diffusely scattering of ambient light - largely a result of the phosphor layer. The
scattering from the phosphor and the reflections from the Al electodes can be
managed (supressed) by neutral density filters in a circular polarization filter.
5.3.1.1. Advantages of Double-Insulating EL Device
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The double insulating layers that sandwiched the phosphor protects it from
moisture and imperfections that typically result in catastrophic failure.
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Direct electron flows from electrodes to phosphor are blocked by insulators,
resulting
in
a
high
breakdown
field.
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Trapped charges at the phosphor layer/insulating layer interface create an
internal polarization which increases the effective field under ac drive
conditions.
Emission colors are controlled by adding
various luminescent centers.
CaS and
based
SrS:Ce
CaS:Ce
CaS:Eu
SrS
blue-green
green
red
The energy band diagram for ac EL thin-film is shown below.
Zn based
ZnS:Tm,F blue
ZnS:Tb,F green
ZnS:Sm,F red-orange
The following is the five step process for Zn-based EL devices. For CaS or SrS
based EL technologies, there is one additional mechanism characterized by a fieldinduced ionization of luminescent centers and subsequent trapping of low energy
electrons resulting in EL emission. Our discussion here for the above diagram is
restricted to ZnS:Mn devices.
1. Above the threshold voltage, electrons are injected from the interface states
between the phosphor layer and insulating layer by high field assisted
tunneling.
2. The injected electrons are accelerated and gain kinetic energy to the extent
that they excite luminescent centers on the host lattice.
3. Hot electrons are accelerated (i.e. high energy electrons) and directly excite
the luminescent centers through impact excitation mechanisms. When these
electrons in the excited states of the luminescent centers make radiative
transitions to the ground state, light emission is realized.
4. Hot electrons proceed through the phosphor layer until they are trapped by the
insulating layer interface on the anode side, resulting in polarization.
5. When the polarity of the applied voltage waveform is reversed, the same
process takes place in the opposite direction in the phosphor layer.
This tunneling injection mechanism described above is insensitive to temperature, so
they can operate between -100 to 100 degrees Celcius.
The typical luminance verses voltage (L-V) and luminous efficiency (n - V) voltages is
shown below. This is an example plot for ZnS:Mn devices.
The features of the L-V curve reveal an extremely steep threshold effect. Below the
threshold voltage, Von, very little light is emitted. In the diagram, Von is defined as the
point where the luminance is 1cd/m2 and L30 is the point on the L-V curve defined as
the position of the L-V at 30 V above Vth. Practically, L30 is very important since drive
voltages are typically set 30 - 40 V above threshold. The luminous efficiency, h, is
also plotted on the same graph with units [lm/w]. Notice that h increases to above V th
where it reaches a maximum in the steep region of the luminous curve. After that, h
begins to weakly decrease.
The steep response of the L-V is highly desirable for electrically addressing
(multiplexing) a EL display. With such a non-linear response, a large number of pixels
can be addressed passively thereby avoiding the more complicated and expensive
active matrix .
5.3.2. ac Powder EL Technology
This technology is predominantly used for backlight technology rather than a stand
alone display. The device structure is shown below.
The phosphor layer is typically on the order of 50 -100 mm. The material is
composite-like, consisting primarily of ZnS powder dispersed in a dielectric binder.
The phosphor layer is sandwiched between two electrodes, one of which is a
transparent ITO layer and the other is typically aluminum. The dielectric material is a
large dielectric constant material, such as cyanoethylcellulose. In most cases, there
is an insulating layer between the Al electode and phosphor layer to avoid dielectric
breakdown. This insulating layer may not be necessary if ZnS grains are completely
isolated from each other.
To obtain EL emission, an electric field of 1 - 10 V/mm is required. Luminance
increases proportionally with frequency to 10 kHz, but lifetime also decreases in
roughly the same proportion. The luminance - luminancy efficiency curve is shown
below.
From the plot, one can expect 100 cd/m 2 at 400 V, which transforms into a luminous
efficiency of 1 lm/w.
Emission colors can be controlled by the addition of various kinds of luminescent
centers. A combination of copper (Cu) and chlorine (Cl) can provide either blue
emission peaks at 460 nm or green peaks at 510 nm, depending on the
concentration of Cl. The EL emission results from a recombination transition of
donor-acceptor pairs, where copper is the acceptor and Cl constitutes a donor.
The combination of copper (Cu) and aluminum (Al) (Zn:Cu,Al) yields green and the
additon of manganese (Mn) to Cu and Cl (ZnS:Cu,Cl,Mn) yields yellow-orange peaks
at 590 nm. Examples are shown below. Powder EL technology has a
characteristically broad emission peak (~100 nm) as shown below.
To understand the EL mechanism in powder EL, it is first worthwhile to describe the
observation of the emission. When the electric field is comparable to the EL
threshold, a pair of bright spots form. When the electric field is increased above
threshold, the bright spots turn into comet-shaped emission regions.
This phenomena is depicted above and can be explained in the following way. EL
powders at high temperatures, usually have hexagonal structure, but subsequent
cooling transforms them into a cubic structure. During this structural transformation,
copper exceeds the solubility limit precipitates on defects in ZnS particles. The result
is embedded CuxS conducting needles.
Between the CuxS precipitates and ZnS powder heterojunctions are formed. This is
depicted below.
These CuxS conducting needles act to concentrate or focus an applied electric field
at their tips. Therefore, an applied field of 1 - 10 V/mm can create local fields > 100
V/mm. The electric field is strong enough to induce tunneling of holes from one end
and electrons from the other. The holes are trapped on copper recombination centers
and upon reversal of fields, the emitted electrons recombine with the trapped holes to
produce light. The EL emission therefore occurs along the Cu xS precipitates. Larger
particle sizes lend to longer needles and greater field enhancement.
Since EL powder is typically used in backlight applications, lifetime is a key issue.
Long lifetime and high luminous efficiency are of course the tradeoff. The half-life of
current EL is approximately 2500 - 3000 hours at 200 V (400 Hz).
5.3.3. dc Thin Film EL Technology
The dc thin film EL technology has the simplest structure compared to other EL
techniques. However, these devices are mostly experimental and have not yet seen
the commercial market. The basic drawback with these dc devices is catastrophic
dielectric breakdown, so current limiting layers must always be considered.
A schematic diagram of a dc-type device structure is shown below.
One type of device that has been moderately successful is one fabricated from
ZnS:Mn, by inserting a thick resistive powder layer made from MnO 2 as a current
limiting layer between the phosphor layer and rear electrode. This is not a true dc thin
film device, but is better characterized as a thin film hybrid technology.
5.3.4. dc Powder EL Technology
Although this technology has been predominantly a research curiosity, there have
been demonstrations of panels with 640 X 200 pixel resolution. The device structure
is shown below, consisting of a phosphor layer (30 - 50 mm). This layer is made from
fine (0.5 - 1.0 mm) Mn-doped ZnS powder and a small amount of binder. The ZnS
powder is deliberately prepared with a CuxS coating. This has no insulating layer in
contrast to the ac powder EL technology.
To obtain stability and emission uniformity, a forming process is necessary. When a
voltage is first applied, a large current flows, the layer heats up and gradually a
narrow region ~1 mm thick, adjacent to the transparent anode begins to luminesce.
The luminance (luminance efficiency) versus voltage graph is shown below.
Under a pulse wave driven condition at 500 Hz, with a 1% duty cycle, almost the
same luminance level is realized as that of the dc drive case. Luminous efficiency is
not very high ~ 0.2 - 0.3 lm/W. For carefully prepared encapsulated devices, half-lifes
of 1000 hrs (dc) and 5000 hrs (dc pulse) are possible. EL emission spectrum are
shown below.
The emission of ZnS:Mn,Cu is yellow-orange (590 nm). In ZnS based EL devices
activated by rare-earth ions, terbium Tm3+ provides blue emission, Tb3+ and
Europium Eu3+ provides green emission, and Nd3+ and Sm3+ provides red output. In
CaS and SrS based devices, SrS:Ce,Cl gives blue-green emission, CaS:Ce,Cl
provides green emission and CaS:Eu,Cl gives red emission.
The figure below depicts the microscopic model of dc powder EL. Electromigration
drives all copper out of a region adjacent to the anode, creating a highly resistive
copper-free formed region near the anode. This resutls in a very large electric field in
the region (~100 V/mm).
Under a large electric field electrons, which have tunneled out of the Cu xS at the
edge of the formed region, are accelerated and excite Mn2+ luminescent centers by
impact. This is shown below.
When the excited electrons make transitions radiatively to the ground state, EL
emission is realized.
5.3.5. Organic Thin-Film EL Technology
Organic materials can also be used as EL diode devices. Two commonly used
molecules can be seen below
The substrate is an ITO coated glass and the first organic layer is on the order of >5
nm, which is a hole transport layer made of amorphous diamine film of molecular
structure. The second organic layer, on the order of 60 nm, is a luminescent layer as
well as an electron transport layer, made of 8-hydroxyquinoline aluminum (AlQ3). The
rear electrode is made of Mg or Ag in a 10:1 ratio. The figure below shows the
structure.
The emission color of undoped AlQ3 is greenish, and a luminance of more than 100
cd/m2 and a luminous efficiency of 1.5 lm/W is possible with a dc pulse drive voltage
of 10 V. In the device with an AlQ3 layer doped with fluorescent molecules, the EL
quantum efficiency is 2.5 x 10-2 photons/electron. The EL emission color can be
turned from blue-green to orange-red by a suitable choice of dopants, or by changing
the concentration of the dopants.
One can construct a three layer structure made of a hole-transport layer, a
luminescent layer or an electron-transport layer. By separating the luminescent
function from the hole or electron transport layer, it becomes possible to select
luminescent layer materials freely to obtain different emission colors. See figure
below.
By changing the luminescent material from anthracene to perylene, emission output
can range from blue to red.