OLED stands for organic light emitting diodes. Organic light emitting diodes (OLEDs) have received a lot of attention over the world as a new type of display technology. OLEDs have more advantages over the conventional display panel. It uses flat light emitting technology which is made possible by installing organic thin films between two conductors. The organic panels emit light when electricity is applied through the setup. They are very flexible when subjected to fitting as they are thinner, much simpler to make and can even be folded or rolled or even stretched and can be made transparent also. OLEDs are mainly of two types Active-matrix OLED (AMOLED) and Passive-Active OLED (PMOLED). As per current development OLED’s provide the best colour viewing experience, finest contrast, quick receptivity and wide viewing angles. OLEDs are current-driven devices. Both Active matrixes TFT’s and Passive matrix Technologies are used for display and addressing purposes for high speed display of moving pictures and faster response.
Components of OLEDs:
OLEDs consists of the following components:
- Substrate: In an OLED, substrate can be plastic, glass, foil. It supports the OLED.
- Anode: Indium tin oxide (ITO) is the anode components. It removes the electron i.e. add holes when current flows through the device.
- Cathode: Depending upon the type of OLED, the cathode component can be barium, calcium, and aluminium. It injects electrons when current flows through the device.
- Conducting layer: This layer is made up of plastic and it transports holes from the anode. Example of one conducting layer that is used in the device is polyaniline.
- Emissive layer: It is made up of organic plastic molecules and it transfers electron from the cathode; this is where light is made. One polymer used in emissive layer is polyfluorene.
Construction of OLEDs:
OLED is a solid-state semiconductor device that is 100 to 500 nanometres thick or about 200 times smaller than a human hair. A simple OLED is made up of 5 or 6 different layers. The top layer is called Seal and the bottom layer is called substrate, they both are made up of glass or plastic. In between these layers, there is a negative terminal called cathode and positive terminal called anode and between the anode and cathode are two layers made from organic layers called emissive layer (where the light is produced and it is next to the cathode) and the conductive layer (it is next to the anode).
Working of OLEDs:
OLEDs emit light through a process call electrophospherence.
The process is as follows:
When a voltage is applied the OLED, electrons start flowing from cathode to the anode through the organic layers. The cathode gives electron to the emissive layer of organic molecules and anode removes the electron from the conducting layer of organic molecules. Added electrons make emissive layer negatively charged which is similar to the n-type in junction diode and conductive layer becomes positively charged which is similar to the p-type. Positive holes are much more mobile than negative electrons so they jump across the boundary from the conductive layer to the emissive layer. When a hole (a lack of electron) meets an electron, the two things cancel out and release a brief burst of energy in the form of a particle of light—a photon, in other words. This process is called recombination, and because it’s happening many times a second the OLED produces continuous light for as long as the current keeps flowing.
The colour of the light depends upon the type of organic molecule in emissive layer. The intensity or brightness of the light depends on the amount of electrical current applied: the more current, the brighter the light.
Types of OLEDs:
Mainly there are two types of OLEDs:
1) Active-Matrix OLED (AMOLED) 2) Passive-Matrix OLED (PMOLED)
AMOLEDs are full layer of cathode, organic molecules and anode, but the anode layer overlays a thin film transistor (TFT) array that forms a matrix. The TFT array itself is the circuitry that determines which pixels get turned on to form an image. It consumes less power, they are efficient for large display panels, and it has best refresh rates suitable for videos. They are used as a display panels in monitors, large screen TVs and billboards.
PMOLEDs basically consist of organic layers and anode strips in perpendicular order to the cathode strips. Intersection of strips makes pixels where light is emitted. The brighter the pixel the more amount of current is applied. External circuit provides current to some strips to turn on or off the pixels. The brightness of each pixel depends upon the intensity of the applied current. It consumes more power than the other OLEDS but less power compared to the other LCD displays.
They are easy to make and are cheaper compared to AMOLED. They are used in music players and cell phones.
Advantages of OLED:
- The plastic, organic layers of an OLED are thinner, lighter and more flexible than the crystalline layers in an LED or LCD.
- OLEDs are brighter than LEDs and also help to keep a proper contrast ratio.
- OLEDs do not require backlighting, they consume much less power than LCDs (most of the LCD power goes to the backlighting). This is especially important for battery-operated devices such as cell phones.
- OLEDs are easier to produce and can be made to larger sizes. Because OLEDs are essentially plastics, they can be made into large, thin sheets.
- OLEDs have large fields of view, about 170 degrees. Because LCDs work by blocking light, they have an inherent viewing obstacle from certain angles. OLEDs produce their own light, so they have a much wider viewing range.
Thanks to the very simple construction of the device, the panels can get extremely thin and makes it easy to install it in smartphones. As compared to LCD displays which need a backlight, these modern equipment do not have any such requirement. The absence of backlight helps these devices to reciprocate very deep blacks which in turn helps in keeping a very good contrast ratio, this even helps save energy. Research and development in OLEDs have resulted in future applications like dashboards and in flexible displays. Video images seem more realistic and updated.