Electroluminescence

Electroluminescence (EL) is an optical and electrical phenomenon in which a material emits light in response to an applied electric current or a strong electric field. Unlike incandescence, which relies on heat to produce radiation, or chemiluminescence and mechanoluminescence, which originate from chemical or mechanical processes, electroluminescence results directly from electronic excitation within a material. This makes it a cornerstone of modern display and lighting technologies, particularly where thin, uniform and energy-efficient illumination is required.

Mechanism

Electroluminescence arises from radiative recombination between electrons and electron holes within a semiconductor. When an electric field or current provides energy, electrons transition to higher energy levels and subsequently fall back to lower states, releasing this excess energy as photons. In pn-junction devices, such as light-emitting diodes, the recombination zone forms at the junction of differently doped semiconductor regions.
In contrast, in electroluminescent displays, high-energy electrons accelerated by a strong alternating electric field excite phosphor materials, producing light. The emitted spectrum typically has a broad bandwidth of about 85 nm. Notably, improvements in solar cell efficiency have been shown to correlate with improved electroluminescent efficiency due to enhanced open-circuit voltage.

Characteristics

Electroluminescent devices offer several distinctive operational advantages:

• Low power consumption, significantly lower than fluorescent or neon technologies.
• Thin form factor, enabling flexible design applications in advertising, instrumentation and portable devices.
• Lambertian reflectance, meaning the emitted brightness appears uniform from all viewing angles.
• Monochromatic emission, often centred in a narrow spectral band perceived clearly by the human eye.
• Colour versatility, with the commonly used greenish glow providing optimal visibility for minimal power.

Because EL lamps are not negative-resistance devices, they do not require the same stabilising circuitry as neon or fluorescent lamps. Developments in multispectral phosphors have further enabled colour-shifting effects through variation of drive frequency.

Electroluminescent Materials

Both organic and inorganic semiconductors can serve as EL materials, provided they possess a bandgap allowing photon emission:

• Inorganic electroluminescent materials:
  • Powdered zinc sulfide doped with copper (green light) or silver (blue light)
  • Thin-film ZnS:Mn producing orange-red emission
  • Boron-doped natural blue diamond
  • Group III–V semiconductors such as indium phosphide, gallium arsenide and gallium nitride
• Organic electroluminescent materials:
  • Organometallic complexes such as Ru(bpy)₃²⁺(PF₆)₂ emitting yellow-green light
  • Organic materials used in organic light-emitting diodes (OLEDs)

These materials are manufactured either as powders for lighting applications or thin films for flat-panel displays.

Practical Implementations

A variety of EL devices and systems have emerged across industries:

• Light-emitting capacitors (LECs)Electroluminescent panels operate as capacitors with a phosphor-based dielectric. When charged, the phosphor emits photons. Transparent electrodes, often made of indium tin oxide, permit emission across the device surface. LECs date to early General Electric developments of the late 1930s and remain prevalent in night lights and control-panel illumination.
• Automotive and aerospace instrumentationChrysler introduced electroluminescent instrument panels in 1960 under the name Panelescent Lighting, with each pointer individually lit. EL technology was similarly employed in NASA’s Apollo spacecraft instrumentation, including the Apollo Guidance Computer’s DSKY interface.
• Consumer electronics backlightingElectroluminescent panels are widely used as backlights in devices such as wristwatches, pagers and thermostats. Their uniform glow provides gentle illumination at low power consumption. Battery-operated applications typically require a high-voltage boost converter, which may produce a faint audible whine when active.
• Thin-film EL displaysCommercialised in the 1980s by companies such as Sharp, Finlux and Planar Systems, thin-film electroluminescent (TFEL) displays offered ruggedness, wide viewing angles and long operational life. They were especially useful before liquid crystal technologies became mature. Later innovations included Timex’s 1992 Indiglo EL display and the development of full-colour thin-film EL materials.
• Active Matrix Electroluminescent (AMEL) displaysAMEL technology integrates control circuitry on the substrate, allowing high resolution and precise pixel addressing. Demonstrated resolutions exceed 1000 lines per inch and can be fabricated directly on silicon.
• Thick-film Dielectric Electroluminescent (TDEL) technologyDeveloped by iFire Technology Corp., TDEL blends thin-film and thick-film processes to produce durable flat-panel displays. These employ a thick dielectric layer combined with a thin phosphor layer between electrode grids.

Examples of Applications

Electroluminescence has been adopted in many fields due to its efficient and uniform illumination:

• Advertising and signage – dynamic electroluminescent billboards and panels
• Night lights – long-lasting EL units used in domestic settings
• Backlit displays – in aviation, computing and portable electronics
• Industrial and scientific instrumentation – where readability and ruggedness are essential