Electroluminescence (EL) refers to the phenomenon where light is emitted as a direct result of an applied electric field. Unlike other luminescence mechanisms that rely on chemical reactions or light absorption, electroluminescence is driven solely by the electrical energy itself.
Key features of electroluminescence
- Energy source: An electric field directly interacts with electrons, exciting them to higher energy levels.
- Electron transitions: Excited electrons return to their ground state, releasing excess energy as light (determining the colour).
- Materials: Certain materials like semiconductors and phosphors are susceptible to electroluminescence under electric fields.
Applications of electroluminescence
- Lighting technologies: LEDs, OLEDs, and EL displays use this principle for efficient lighting.
- Sensors and indicators: Some sensors and indicator lights rely on electroluminescence for visual signalling.
- Back-lighting: Electroluminescent panels can be used for back-lighting in devices like LCD screens.
Mechanisms involved in electroluminescence
- Injection electroluminescence: An electric field injects electrons or holes into a material, leading to recombination and light emission (common in LEDs and OLEDs).
- Impact excitation: High-energy electrons collide with atoms or molecules, exciting them and causing light emission (used in some EL displays).
About Recombination
In the context of electroluminescence, “recombination” refers to the process where an excited electron and a “hole” (absence of an electron) reunite within a material, releasing energy as light.
Components
- Excited electron: An electron within the material absorbs energy (often from an electric field) and occupies a higher-energy state.
- Hole: Not a physical particle, but the absence of an electron in a specific location within an atom, acting like a positive charge seeking an electron.
The Process
- Injection: An electric field introduces electrons or holes into the material.
- Movement: These injected particles move through the material.
- Encounter: An excited electron and a hole meet within the material.
- Recombination: They “recombine,” meaning the electron fills the hole, returning to its ground state.
- Energy release: During recombination, the excess energy is released as a photon of light, causing electroluminescence.
Key Points
- Emitted light colour depends on the energy difference between the electron’s excited and ground states.
- Recombination is crucial for converting electrical energy into light in LEDs, OLEDs, and other electroluminescent devices.
| Light sources | Emission mechanism | Description | Examples |
|---|---|---|---|
| LIGHT-EMITTING PROCESS | |||
| Luminescence | Light emission due to the excitation of electrons in a material. | Electrons within a material gain energy and then release light as they return to a lower energy state. | Bioelectroluminescence Electroluminescence Photoluminescence - Fluorescence - Phosphorescence Sonoluminescence Thermoluminescence |
| Blackbody radiation (Type of thermal radiation) | Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero. | Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero. | All objects above temperature of absolute zero. |
| Chemiluminescence | Light from natural and artificial chemical reactions. | Light from natural and artificial chemical reactions. | Bioluminescence Chemiluminescent reactions: - Luminol reactions - Ruthenium chemiluminescence |
| Nuclear reaction | Light emission as a byproduct of nuclear reactions (fusion or fission). | Light emitted as a byproduct of nuclear reactions. | Nuclear reactors Stars undergoing fusion |
| Thermal radiation | Light emission due to the thermal excitation of atoms and molecules at high temperatures. | Light emission due to the thermal excitation of atoms and molecules. | Sun Stars Incandescent light bulbs |
| Triboluminescence | Light emission due to mechanical stress applied to a material. | Light emission due to the mechanical stress applied to a material, causing the movement of electric charges and subsequent light emission. | Sugar crystals cracking Adhesive tape peeling Quartz crystals fracturing. |
| Natural light source | |||
| Fireflies Deep-sea creatures Glowing mushrooms | Bioluminescence | Light emission from biological organisms. | Involves the luciferase enzyme. |
| Sun Stars | Nuclear Fusion | Light emission as a byproduct of nuclear fusion reactions in stars. | Electromagnetic spectrum (visible light, infrared, ultraviolet). |
| Fire Candles | Thermal radiation | Light emission due to the thermal excitation of atoms and molecules during the combustion of a fuel source. | Burning of a fuel source, releasing heat and light. |
| Artificial light source | |||
| Fluorescent lights Highlighters Safety vests | Chemiluminescence | Light emission from chemical reactions. | Fluorescence (absorption and re-emission of light). |
| Glow sticks Emergency signs | Chemiluminescence | Light emission due to phosphorescence - a type of chemiluminescence. | A type of chemiluminescence where light emission is delayed after the initial excitation. |
| Glow sticks Light sticks | Chemiluminescence | Chemiluminescence | Light emission from a chemical reaction that does not involve combustion. |
| Tungsten light bulbs Toasters | Thermal radiation | Heated filament radiates light and heat. | Light emission from a hot filament. |
| Fluorescent lamps LED lights | Electroluminescence | Excitation of atoms by electric current. | Light emission when electric current excites atoms in a material. |
| Neon signs | Electrical Discharge | Discharge of electricity through gas. | Light emission when electricity flows through a gas. |
| Sugar crystals cracking Pressure-sensitive adhesives | Triboluminescence | Light emission from friction or pressure. | Light emission due to mechanical forces. |
| Fluorescent paint Highlighters Safety vests | Photoluminescence | Absorption and subsequent re-emission of light at a lower energy. | Absorption and re-emission of light. |
Light Sources: Mechanism, examples, and everyday applications
Footnote: Cerenkov radiation and Synchrotron radiation are not included in the table because they are not conventionally classified as light sources.
- Electroluminescence (EL) refers to the phenomenon where light is emitted as a direct result of an applied electric field. Unlike other luminescence mechanisms that rely on chemical reactions or light absorption, electroluminescence is driven solely by the electrical energy itself.
Key features of electroluminescence
- Energy source: An electric field directly interacts with electrons, exciting them to higher energy levels.
- Electron transitions: Excited electrons return to their ground state, releasing excess energy as light (determining the colour).
- Materials: Certain materials like semiconductors and phosphors are susceptible to electroluminescence under electric fields.