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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: 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. 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 The double insulating layers that sandwiched the phosphor protects it from moisture and imperfections that typically result in catastrophic failure. Direct electron flows from electrodes to phosphor are blocked by insulators, resulting in a high breakdown field. 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.