Organic Light Emitting Diode popularly known as OLED is a solid-state device that consists of thin films of organic molecules that generate a bright light on the application of electric current. They are made by a series of organic thin films placed between two conductors.
Research in the field of electroluminescence in organic materials started in the early 1950s by Andre Bernanose and his co-workers at the Nancy University of France. The first practical OLED was built by chemists Ching Wan Tang and Steven Van Slyke of Eastman Kodak in 1987. Since then OLEDs have become popular in research and for practical use.
Components of an OLED
OLEDs can be made up of either two or three layers of organic material. A two-layer OLED consists of the following parts:
- Substrate– OLED is supported by either a thin translucent glass or foil material known as substrate. It acts as the base for OLED.
- Anode– When an electric current is passed, the anode loses an electron or adds holes. It is transparent. It is also known as an emitter.
- Organic Layer– They are made up of organic molecules or polymers such as hydrogen or carbon which are placed on top of the anode.
- Conductive Layer– It consists of organic plastic molecules such as polyaniline that are used to transport holes from the anode.
- Emissive Layer – This is the layer where light is made. It is made up of organic plastics molecules, but different ones from the conducting layer that transmits electrons from the cathode. One such polymer is polyfluorene.
- Cathode– When an electric current is applied the cathode injects electrons. It is made up of metals such as aluminum and calcium. It may be transparent or not. The cathode is also known as a conductor.
How Does an OLED Emits Light?
OLEDs work on the phenomenon of electroluminescence. The process of emission of light through an OLED is as follows:
- A voltage is applied across the OLED with the help of a battery or a power supply present in the device.
- After this generation of electric current takes place which flows from cathode to anode through the organic layers. The emissive layer receives the electrons from the cathode and then passes them to the conductive layer. The anode then removes the electron from the conductive layer which is equivalent to the generation of electron holes in the conductive layer.
- Electron finds the electron holes present between the emissive and conductive layers. To fill the hole electron releases energy. This energy is released in the form of photons of light and hence light is emitted.
- The generation of different colors of light depends on the type of organic materials used in the emissive layer. So different types of organic films are placed on the same OLED for color displays.
- The intensity of the brightness of the light can be increased by increasing the amount of current.
Types of OLEDs
Based on their use different types of OLED available in the market. Some commonly used OLEDs are as follows:
- Passive-matrix OLED (PMOLED) – It consists of strips of the cathode, organic layer, and strips of the anode. In PMOLED anode strips are placed perpendicular to the cathode strips. Light is emitted at the pixels, formed by the intersection of cathode and anode. The external circuit is used for the application of current to the selected strips of cathode and anode, which determines the turning on and off of pixels. They are easy to construct but consumes more power than other types of OLED due to an external circuit. They are used for text and icons and are best suited for the small screen of size 2-3 inch diagonal. They are used in devices used for metering, as they require a selective display. It is also used in Personal Digital Assistant(PDAs), cell phones, and MP3 players.
- Active-matrix OLED (AMOLED) – It consists of full layers of the cathode, organic molecules, and anode. It also consists of a thin film transistor overlaid by the anode forming a matrix. TFT acts as a circuit for determining the turning on and off of the pixels to produce an image. They are suitable for large display as they consume less power than PMOLED due to the presence of TFT. They have a faster refresh rate which makes them suitable for videos. AMOLEDs are used in large-screen TVs, computer monitors, and electronic signs and billboards.
- Transparent OLED – In transparent OLED the entire cathode, anode, and substrate are transparent in nature. In the off state, they are up to 85 percent transparent as to their substrate. When turned on they emit light in both directions which makes them suitable for creating displays that are top as well as bottom emitting. They can be either active- or passive-matrix. This type of OLED is used in head-up displays, smart windows, and mobile phones.
- top-emitting OLED – This type of OLED consists of a substrate that is either opaque or reflective. Active-matrix design is best suited for this type of OLED. It is used in smart cards.
- Foldable OLED – Their substrate is made up of very flexible metallic foils or plastics which make them very lightweight and durable. As they are durable it reduces breakage and therefore suited for cell phones and PDAs. It is also used in computer chips and GPS devices. They can also be attached to fabrics to create smart clothing.
- White OLED – They emit white light which is brighter, energy-efficient, and more uniform than fluorescent lights. They are made in large sheets which can replace fluorescent light, currently used in homes and buildings. This will lead to a reduction in energy costs for lighting.
- Inverted OLED – In this type of OLED cathode is situated at the bottom. This method reduces the cost of OLED but they have very little application.
- Stacked OLED – In this type of OLED pixel gap is reduced by using composite colors as subpixels on top of each other. Color depth is also increased, so they are now used in some television displays.
- Organic layers present in an OLED are better than the crystalline layers in LED or LCD as they are flexible, lighter, and thinner than crystalline layers.
- Due to the presence of organic layers, OLEDs are brighter than LEDs.
- OLEDs consume less power than LCD and LEDs.
- They have faster response time and better picture quality and viewing angle than LEDs.
- Currently, the biggest drawback of an OLED is its lifetime. The Red and Green OLED films have a lifetime ranging from 46, 000 to 230, 000 hours while Blue organics have a shorter lifetime of around 14, 000 hours.
- The process of manufacturing an OLED is expensive right now.
- They are easily damaged by water.
OLEDs are currently used in smartphones, television, digital cameras, computer monitors, portable gaming consoles, etc.
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