Otis Singleton
Electroluminescence (EL) is a phenomenon where light is generated by an electric field. It is the generation of light through the radiative recombination of holes and electrons injected into the material from electrodes. EL is distinct from other forms of luminescence, such as fluorescence, chemiluminescence, and mechanoluminescence, as it is driven solely by electrical energy. This phenomenon is applied in display devices, such as LEDs and OLEDs, and has applications in lighting, displays, and advertising.
| Characteristics | Values |
|---|---|
| Definition | Electroluminescence (EL) is the generation of light through the radiative recombination of holes and electrons injected into the material from electrodes. |
| Mechanism | There are two distinct mechanisms that can produce electroluminescence in crystals: pure or intrinsic and charge injection. |
| Materials | EL materials include powdered zinc sulfide, thin-film zinc sulfide, naturally blue diamond, semiconductors containing Group III and Group V elements, and certain organic semiconductors. |
| Devices | Common EL devices include light-emitting capacitors (LECs), flat electroluminescent panels, and automotive instrument panel backlighting. |
| Applications | EL is used in lighting, display technologies, advertising, and sensors and indicators. |
| Power Consumption | EL technologies have low power consumption compared to competing lighting technologies such as neon or fluorescent lamps. |
| Light Characteristics | EL light is not directional and appears the same from all angles of view. The emitted light colour depends on the energy difference between the electron's excited and ground states. |
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What You'll Learn
EL devices are composed of powder or thin films
Electroluminescence (EL) is an optical and electrical phenomenon in which a material emits light in response to the passage of an electric current or a strong electric field. EL devices are fabricated using either organic or inorganic electroluminescent materials. The active materials are semiconductors of wide enough bandwidth to allow the exit of light. The most common EL devices are composed of either powder or thin films. Powdered zinc sulfide doped with copper or silver is primarily used in lighting applications, emitting a greenish or bright blue light respectively.
Thin-film zinc sulfide doped with manganese produces an orange-red colour. The colour emitted by the device can be controlled by the dopant material used. For example, the most typical inorganic thin-film EL (TFEL) is ZnS:Mn with yellow-orange emission. The dopant materials in EL devices enhance efficiency and control the colour of the emitted light.
The performance of an EL device depends on the quality of the layers of material. The properties of the deposited thin films, such as thickness, porosity, and composition, can be controlled using solution-based techniques. However, achieving uniformity in the deposited films can be difficult due to the uneven distribution of solvents and solutes.
EL films are Lambertian radiators, meaning the brightness of the surface appears the same from all angles of view. The light emitted from the surface is perfectly homogeneous and is well-perceived by the eye. This property, along with the low power consumption of EL devices, has made EL technology valuable to the advertising industry. Manufacturers can control which areas of an EL sheet illuminate and when, allowing advertisers to create more dynamic advertising that is still compatible with traditional advertising spaces.
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EL displays use the principle of electron–hole pair recombination
Electroluminescence (EL) is an optical and electrical phenomenon where a material emits light in response to an electric current or a strong electric field. EL displays use the principle of electron-hole pair recombination, where electrons and holes are injected into the recombination region from the cathode and anode. This process is distinct from photoluminescence, where electrons are excited by incident photon absorption.
In EL displays, the electron-hole pair recombination occurs in the following manner: electrons and holes are injected into the emissive material from the cathode and anode contacts. These contacts are oriented opposite each other. When an electron and a hole recombine, the electron relaxes into the hole, resulting in the emission of a photon with energy characteristic of the optical transition. This charge recombination is fundamental to the process of electroluminescence in light-emitting diodes (LEDs) and organic LEDs (OLEDs).
The electron-hole pair is the fundamental unit of generation and recombination in inorganic semiconductors. The process of carrier generation and recombination is essential to the operation of optoelectronic semiconductor devices such as photodiodes, LEDs, and laser diodes. In semiconductors, there is a continuous transition of electrons between the conduction band and the valence band, with an energy gap of about 1.1 eV in silicon. When an electron falls from the conduction band into the valence band, a recombination process occurs, and an electron-hole pair disappears. This recombination process results in the emission of a photon of light.
The ABC model is often used to describe electron-hole recombination in LEDs, assuming the equal injection of carriers into the active region. However, this model has limitations in explaining experimental results, especially when the current is shown on a logarithmic scale. The ABC model also does not account for the shape of the EQE versus current curve. To address these limitations, phase-space filling has been incorporated into the model, adding a carrier dependence to each coefficient.
EL displays offer advantages such as low power consumption and thin, flexible materials. Manufacturers can precisely control the illuminated areas of an EL sheet, making it valuable for advertising applications like billboards and signs. EL displays also provide a homogeneous light emission that appears the same from all angles of view, enhancing their versatility in various lighting and display applications.
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EL technologies have low power consumption
Electroluminescence (EL) is an optical and electrical phenomenon where a material emits light in response to an electric current or a strong electric field. This occurs through the radiative recombination of electrons and holes in a semiconductor material, typically injected from cathode and anode contacts. The excited electrons release their energy as photons, emitting light.
The low power consumption of EL technologies also leads to reduced heat production, allowing processors to run cooler and be packed into systems more tightly, increasing their longevity. Additionally, the low heat emission of EL displays makes them well-perceived by the human eye, making them valuable for advertising applications such as billboards and signs.
EL displays can be made to consume even less power through techniques such as power gating, where sleep transistors are used to disable entire blocks when not in use. This is particularly useful for systems that remain dormant for long periods and only "wake up" to perform periodic activities, which are often battery-powered, making power consumption a key design consideration.
The low power consumption of EL technologies, coupled with their thinness and ability to control the illuminated areas precisely, has made them a popular choice in the advertising industry.
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EL materials emit light in response to an electric current
Electroluminescence (EL) is an optical and electrical phenomenon where a material emits light in response to an electric current or a strong electric field. This phenomenon is distinct from black body light emission resulting from heat (incandescence), chemical reactions (chemiluminescence), reactions in a liquid (electrochemiluminescence), sound (sonoluminescence), or other mechanical action (mechanoluminescence), or organic electroluminescence.
EL materials emit light through the radiative recombination of holes and electrons injected into the material from electrodes. This process is fundamental to the operation of light-emitting diodes (LEDs) and organic LEDs (OLEDs). LEDs are typically made from III-V inorganic semiconductors, such as gallium arsenide (GaAs), while OLEDs are made from organic compounds that function as semiconductors.
In the case of LEDs, the recombination of electrons and electron holes in a semiconductor produces light. The wavelength of the light depends on the energy band gap of the semiconductors used, and the colour of the light can be altered by changing the composition of the alloys used in the semiconducting element.
OLEDs, on the other hand, are composed of organic materials such as small organic molecules or polymers. These materials are electrically conductive due to the delocalization of pi electrons, and they offer advantages such as thin, low-cost displays with a wide viewing angle and high contrast.
EL technologies have gained popularity in the advertising industry due to their low power consumption and the thinness of the material. EL manufacturers can control precisely which areas of an electroluminescent sheet illuminate, enabling advertisers to create dynamic advertisements that still fit within traditional advertising spaces.
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EL is distinct from other types of luminescence
Electroluminescence (EL) is distinct from other types of luminescence. Luminescence is the spontaneous emission of radiation from an electronically excited species (or from a vibrationally excited species) not in thermal equilibrium with its environment. It is characterised by electrons undergoing transitions from excited quantum states. EL is an optical and electrical phenomenon, in which a material emits light in response to the passage of an electric current or a strong electric field.
EL is distinct from black body light emission resulting from heat (incandescence). Incandescence is the emission of light from a hot material, where the atoms of the material are in a high state of agitation. EL is also different from chemiluminescence, the emission of light as a result of a chemical reaction, and electrochemiluminescence, which is caused by an electrochemical reaction.
EL differs from sonoluminescence, which is the emission of light caused by sound, and mechanoluminescence, which is the result of mechanical action on a solid. It is also distinct from triboluminescence, which occurs when bonds in a material are broken when that material is scratched, crushed, or rubbed.
EL is also different from photoluminescence (PL), where electrons are excited by incident photon absorption. In EL, the electric field applied for LED operation can cause a shift in the emission, known as the Stark effect.
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Frequently asked questions
Electroluminescence (EL) is the generation of light through the radiative recombination of holes and electrons injected into the material from electrodes.
Electroluminescence involves the conversion of electrical energy into non-thermal emitted light. It is driven solely by the electrical energy itself.
The most common electroluminescent devices are composed of either powder or thin films. Light-emitting capacitors, or LECs, are a term used to describe electroluminescent panels.
Electroluminescence is distinct from photoluminescence (PL) in which electrons are excited by incident photon absorption. Photoluminescence relies on chemical reactions or light absorption, unlike electroluminescence.
