LEDs, or light-emitting diodes, are semiconductor devices that emit light when an electric current passes through them. When a voltage is applied across the LED, electrons from the n-type semiconductor layer are pushed across the junction towards the p-type semiconductor, while holes (positive charge carriers) from the p-type move in the opposite direction. As electrons and holes recombine at the p-n junction, they release energy in the form of photons (light particles). This process generates light.

LEDs can be classified into Organic LEDs (OLEDs) and Inorganic LEDs based on the materials used in their construction. OLEDs use organic semiconductor materials for the generation of light, commonly used in display technologies like TVs, smartphones, and monitors. Inorganic LEDs find applications in lighting, large outdoor displays, digital billboards, electronic displays, etc. While these classifications are fundamental, there are various subcategories and specialized types of LEDs within each main type.

Inorganic LEDs

Inorganic LEDs are semiconductor devices that emit light when an electric current passes through them. It specifically refers to LEDs that use inorganic semiconductor materials like gallium nitride (GaN), gallium arsenide (GaAs), or similar compounds in their construction. These inorganic compounds offer durability and efficiency, making inorganic LEDs widely usable in diverse applications.

Inorganic LEDs emit light across various wavelengths based on the semiconductor materials used.

When electrons recombine with holes within the device, energy is released in the form of photons, producing visible light.

An inorganic LED consists of two types of semiconductors: an N-type semiconductor (excess electrons) and a P-type semiconductor (electron deficiencies or "holes"). The interface between these two types of semiconductors is called a P-N junction.

When a voltage is applied across the P-N junction, electrons from the N-type semiconductor are pushed toward the P-type semiconductor, and the holes from the P-type semiconductor move toward the N-type semiconductor. This movement of electrons and holes is initiated by the electric field created by the applied voltage.

As electrons from the N-type semiconductor recombine with the holes in the P-type semiconductor at the P-N junction, energy is released in the form of photons (light particles). The energy released during this recombination process corresponds to a specific wavelength of light that determines the color of the emitted light.

The released photons escape the semiconductor material, creating visible light (380 nm - 700 nm). The color of the light emitted by the LED depends on the energy band gap of the semiconductor material. Different materials have different band gaps, resulting in LEDs emitting various colors such as red, green, blue, infrared or even ultraviolet, depending on the specific semiconductor used.

Inorganic LEDs encompass various subtypes based on their construction materials and intended applications:

• Gallium Arsenide (GaAs) LEDs: Often used in infrared applications.
• Aluminum Gallium Arsenide (AlGaAs) LEDs: Commonly employed in red and infrared LEDs.
• Gallium Phosphide (GaP) LEDs: Utilized in red, yellow, and green LEDs.
• Indium Gallium Nitride (InGaN) LEDs: Known for blue, green, and violet/UV LEDs.

Organic LEDs (OLED)

OLED (Organic Light Emitting Diode) is a type of LED that emit light when an electric current passes through them, eliminating the need for a backlight. They offer vibrant colors, deep blacks, fast response times, wide viewing angles, and energy efficiency due to their ability to control individual pixels, enhancing image quality and enabling innovative form factors in various devices such as TVs, smartphones, and wearable screens.

They are inherently flexible, a characteristic that stands as a prominent advantage within this display technology. Unlike traditional LED displays, which are rigid and often rely on glass substrates, OLEDs can be manufactured on flexible substrates like plastic or metal foil. This property allows OLED displays to be thin, lightweight, and bendable.

• Curved Displays: OLEDs can be curved to fit the shape of a curved surface, enabling the creation of curved or even foldable displays.
• Flexible Screens: OLEDs can be used in devices with flexible screens, such as smartphones, tablets, and wearable devices. This flexibility enhances the design possibilities for these devices.
• Rollable Displays: Some OLED displays can be rolled up like a sheet of paper, making them suitable for applications where space-saving and portability are crucial.
• Transparent Displays: OLEDs can also be made transparent when turned off, allowing for applications where transparency is desirable, such as in car windshields or augmented reality devices.

An OLED is a thin film optoelectronic device utilizing organic materials, such as small molecules, dendrimers, or polymeric substances, positioned between conductive electrodes termed the anode and cathode. These components are deposited onto a substrate, with the organic films ranging in thickness from tens to hundreds of nanometers.

OLEDs operate based on the phenomenon of electroluminescence in organic materials. Using an organic material or polymer as an active layer or emitter, an OLED transforms electric energy into light. The first OLED device was developed by Eastman Kodak in 1987.

Structure of OLED

The basic structure of an OLED cell involves layers of organic materials positioned between a conductive anode and a conductive cathode. OLED devices are constructed with several essential components:

• Substrate: This foundational layer, which can be made of plastic, glass, or metal foil, provides structural support to the OLED.
• Cathode: The cathode positioned at the top within a typical OLED structure carries a negative charge. It's typically composed of metals such as aluminum or calcium with low work functions, enabling the entry of electrons into the organic layers. It may or may not be transparent depending on the OLED type.
• Organic Layer: 
  • Emissive Layer: Situated between the cathode and the anode, this layer is responsible for generating light. It consists of organic molecules (emitters) such as Polyfluorene that emit photons when charge carriers recombine. This layer comprises a color-defining emitter doped into a host material that determines the color of the emitted light. Different types of color-defining emitters are used to produce various colors (red, green, blue, etc.) in OLED displays. Each emitter is designed to emit light at a particular wavelength corresponding to the desired color.
  • Conductive Layer: The conductive layer positioned above the anode is made of organic materials such as Polyaniline, known for its high hole mobility. They transport holes from the anode and facilitates their deeper penetration into the emissive layer for efficient light generation.
• Anode: The anode is positioned opposite the cathode at the bottom, it carries a positive charge and facilitates the movement of positive charge carriers (holes) into the organic layers. It is a transparent layer made of conductive materials like indium tin oxide (ITO), allowing emitted light to pass through.

When an electric current is applied to the cathode, charge carriers (holes and electrons) move from the electrodes into the organic layers and recombine in the emissive zone, creating excitons. Excitons are bound pairs of an electron and a positively charged "hole" in a material. They form when an electron gets excited to a higher energy level and leaves behind an empty space (the hole) in its original position. This electron-hole pair can attract each other due to their opposite charges, forming an exciton. 

These excitons can decay and release energy in the form of photons, producing light (electroluminescence), as they decays to a lower energy state. Excitons have specific properties, including stability and recombination behavior, which play a significant role in determining the efficiency and color characteristics of OLED displays.

The emission wavelength of an OLED can vary depending on the specific materials used in its construction. Different organic semiconductor materials are used in OLEDs to generate specific colors by emitting light within distinct wavelength ranges. The colors emitted correspond to the specific semiconductor materials utilized in the OLED structure.

The output power range can vary significantly based on the specific OLED device, its size, and its intended application.

Subtypes of OLEDs

• Passive Matrix OLEDs (PMOLEDs): These are simpler and more cost-effective OLEDs suitable for smaller displays like MP3 players or wearable devices.
• Active Matrix OLEDs (AMOLEDs): Offering better performance and suitable for larger displays like smartphones, tablets, and TVs, they use a matrix of thin-film transistors for individual pixel control.
• Transparent OLEDs: These OLEDs allow light to pass through when turned off, enabling applications like heads-up displays or transparent screens.
• Top-Emitting OLEDs: Designed to emit light from the top side, offering better efficiency and flexibility in manufacturing, commonly used in larger OLED displays.
• Foldable and Rollable OLEDs: Developed to create flexible and bendable displays, enabling devices like foldable smartphones or rollable TVs.
• White OLEDs (WOLEDs): These produce white light by combining multiple colored OLED subpixels, used in lighting and some display applications.

OLEDs are known for their flexibility, thinness, and lightweight nature, making them ideal for applications like flexible displays and lighting panels. It is used in displays, mobile phones, keyboards, light sources, etc.

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