View source for Organic light-emitting diodes

== Introduction ==

Organic light-emitting diodes (OLEDs) are a category of light-emitting diodes (LEDs) in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. This organic layer is situated between two electrodes; typically, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, and portable systems such as smartphones and handheld gaming consoles. A major area of research is the development of white OLED devices for use in solid-state lighting applications.

== History and Development ==

The concept of electroluminescence in organic materials was first observed in the early 1950s by André Bernanose and co-workers at the Nancy-Université in France. They discovered that certain organic compounds emitted light when subjected to alternating current in a high-voltage field. However, it wasn't until the 1980s that significant advancements were made. In 1987, Ching W. Tang and Steven Van Slyke at Eastman Kodak developed the first practical OLED device, which was a breakthrough in the field.

== Structure and Working Principle ==

The basic structure of an OLED consists of several layers:

1. **Substrate**: This is usually made of glass or plastic and provides the foundation for the OLED.
2. **Anode**: The anode is typically made of indium tin oxide (ITO) and is transparent.
3. **Organic Layers**: These include the hole transport layer (HTL), emissive layer (EML), and electron transport layer (ETL). The organic materials used are typically small-molecule compounds or polymers.
4. **Cathode**: The cathode is usually made of metals such as aluminum or calcium.

When a voltage is applied across the OLED, electrons are injected from the cathode into the ETL, and holes are injected from the anode into the HTL. These electrons and holes migrate towards the EML, where they recombine to form excitons. The decay of these excitons results in the emission of light.

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== Types of OLEDs ==

There are several types of OLEDs, each with unique properties and applications:

1. **Passive-Matrix OLED (PMOLED)**: These are simpler and cheaper to manufacture but are less efficient and have shorter lifespans compared to other types.
2. **Active-Matrix OLED (AMOLED)**: These are more complex and expensive but offer higher resolution and better power efficiency.
3. **Transparent OLED (TOLED)**: These can be made to be transparent, allowing for applications like heads-up displays.
4. **Top-Emitting OLED (TEOLED)**: These emit light from the top and are used in applications where the substrate is opaque.
5. **Foldable OLED**: These are designed to be flexible and can be used in foldable devices.
6. **White OLED (WOLED)**: These emit white light and are used in lighting applications.

== Materials ==

The materials used in OLEDs can be broadly classified into small molecules and polymers.

1. **Small Molecules**: These are often used in high-performance OLEDs. They are deposited using vacuum thermal evaporation (VTE).
2. **Polymers**: These can be processed using solution-based techniques like spin coating and inkjet printing, making them suitable for large-area applications.

Common materials include:

- **Hole Transport Materials (HTMs)**: N,N'-di(naphthalen-1-yl)-N,N'-diphenyl-benzidine (NPB)
- **Emissive Materials**: Tris(8-hydroxyquinolinato)aluminium (Alq3)
- **Electron Transport Materials (ETMs)**: Bathophenanthroline (BPhen)

== Advantages and Disadvantages ==

=== Advantages ===

1. **High Contrast Ratio**: OLEDs can achieve true blacks, as they can turn off individual pixels.
2. **Wide Viewing Angles**: OLED displays maintain color accuracy and brightness over a wide range of viewing angles.
3. **Fast Response Time**: OLEDs have faster response times compared to LCDs, making them ideal for video playback and gaming.
4. **Flexibility**: OLEDs can be made on flexible substrates, allowing for innovative form factors.

=== Disadvantages ===

1. **Lifespan**: Blue OLEDs have a shorter lifespan compared to red and green OLEDs.
2. **Burn-In**: Prolonged display of static images can lead to burn-in, where ghost images remain on the screen.
3. **Cost**: The manufacturing process for OLEDs is more expensive compared to traditional LCDs.

== Applications ==

OLED technology has a wide range of applications:

1. **Displays**: Used in smartphones, televisions, computer monitors, and wearable devices.
2. **Lighting**: White OLEDs are being developed for use in general lighting applications.
3. **Automotive**: OLEDs are used in dashboard displays and taillights.
4. **Medical Devices**: OLEDs are used in various medical imaging devices.

== Future Prospects ==

The future of OLED technology looks promising with ongoing research aimed at overcoming current limitations. Key areas of focus include:

1. **Improving Lifespan**: Developing more stable blue emissive materials to extend the lifespan of OLED displays.
2. **Reducing Costs**: Finding cost-effective manufacturing techniques to make OLEDs more affordable.
3. **Enhancing Efficiency**: Increasing the power efficiency of OLEDs to reduce energy consumption.
4. **Expanding Applications**: Exploring new applications in areas such as virtual reality (VR) and augmented reality (AR).

== See Also ==

* [[Electroluminescence]]
* [[Indium Tin Oxide]]
* [[Flexible Electronics]]
* [[Solid-State Lighting]]
* [[Virtual Reality]]

== References ==

[[Category:Display Technology]]
[[Category:Organic Electronics]]
[[Category:Lighting Technology]]