Organic light-emitting diode (OLED)
History :
It was first developed in France in early 1950s. After that in 1960s AC-driven electroluminescent cells using doped anthracene was developed. In 1987 OLED diode technology was introduced by researchers at the Eastman Kodak company. Chemists, Ching W Tang and Steven Van Slyke were the principal inventors. In June of 2001, Van Slyke and Tang got an Industrial Innovation Award from the American Chemical Society for their work with organic light-emitting diodes(OLED). They observed that a bright luminance was obtained in an OLED device with two thin organic layers sandwiched between anode and cathode. And in 1990 electroluminescence in polymers was discovered.
Architecture of OLED :
(Figure 1)
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Thickness of an OLED is about 100 to 500 nanometers. OLED is made up of organic electroluminescent material. This material is consists of small molecules or polymers. It contains two layers : a conducting layer and an emissive layer. This electrodes are sandwiched between two electrodes. The conducting layer is made of organic plastic molecules. The emissive layer is made of a film of organic compound. Out of these one of the electrodes should be transparent. The conducting layer allow holes to pass from anode. And the emissive layer allows electrons to pass from cathode. One example of material used in making this material is ; polyaniline is used to make conducting layer and polyfluorene is used to make emissive layer. Three layer OLEDs are also possible .In the three layers based OLED; the conductive layer is replaced by two more effective layers. OLED also contains substrate and anode and cathode terminals. Due to delocalization of π electrons in organic layer, we get current. The interface between a conducting and emissive layer is an efficient place for the recombination of holes and electrons and hence electroluminescence. So this OLEDS are charge injection devices which require a simultaneous supply of holes and electrons to the interface between two electrodes ( Refer to Figure 1). The substrate is used to support the OLED. Substrate is made up of foil, plastic or it can also be glass. The anode is usually transparent. And it is made up of indium tin oxide. This material allows visible light to pass through it . It also has a suitable work function which is helpful in injecting holes into different layers. The type of cathode depends on type of OLED required. We can also use even a transparent cathode. Metals like calcium and aluminium are used because they contain less work function than anode, which is required to injecting electrons into different layers.
How the emissive layers and conducting layers are added to the substrate :
To add emissive layer and conducting layer to the substrate there are mainly three methods is being used.
1. Inkjet Printing Technique – This technique is mostly used. The method is similar to the paper printing mechanism in which organic layers are sprinkled onto the substrates. This method is so efficient and it can be used for printing very large displays. TV screen can also be manufactured using this method.
2. Organic Vapour Phase Deposition (OVPD) – This technique is efficient in the sense that it requires low cost. Organic molecules hit a cooled substrate, which was evaporated in low pressure and high temperature chamber. The gas is carried onto the substrate with the help of a carrier gas.
3. Vacuum Thermal Evaporation (VTE) – This method is also known as vacuum deposition method. In this method we provide heat to organic molecules, so that this organic molecules evaporate and subside on the substrate . The heating method is complex and the strictness parameters would be highly accurate, this method is low cost as well.
4. Transfer-printing – In this method we create alignment marks on device substrates by metal deposition, etching and photolithography . Thin polymer layers are also added for increasing resistance to surface and particles defects. To form bottom electrode anode layer is placed at backside. Then OLED layers are applied to anode by vapor deposition processes, and covered with conductive metal electrode layer. This method can be used for printing substrates of size approx 500 nm x 400 nm
Working :
The substrate is used to support the OLED. The anode injects more holes when there is a path of current. Holes pass through conducting layer. Electrons are produced by cathode. The desired light is produced in emissive layer. First of all we set anode positive with respect to the cathode. So electron flows from the cathode to the anode. So holes flow in conductive layer .
Electrons recombine with holes in organic layer. As this process goes on the conductive layer is now positively charged and emissive layer is negatively charged. A recombination of electrons and holes occur because of electrostatic force. Holes are more mobile than electrons, the recombination happens in emissive layer. As a result of this process light is produced. The frequency of this emitted light is visible region . If we want to obtain a colour display , many organic layers are used. OLED emits light by process of electrophosphorescence.. This recombination causes photon emission(i.e. light). The color of this light depends on type of organic layer used. And intensity of emitted light depends on amount of current.
Types of OLEDs :
1. Passive-matrix OLED (PMOLED) :
(Figure 2)
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PMOLEDS is made up of strips of anode and cathode. This strips are perpendicular are placed perpendicular to each other. Intersection of this strips is pixel(refer to Figure 2). This pixel emits light . The brightness of pixels depends on current through them. We can control the current by external electronic circuit . We can apply current to selective strips. By this we can turn any pixel on or off. Advantage of PMOLED is that it is cheap and easy to fabricate. But PMOLED consumes more power. May be because of power required to run external circuitry. PMOLED is mostly used for small screens like music players, sub displays and sometimes mobile phones.
2. Active-matrix OLED (AMOLED) :
(Figure 3)
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Unlike PMOLED , AMOLED have continuous layers of anode, cathode and organic molecules. The anode layer contains layer made up of thin film transistor (TFT) plane in parallel to it so it forms a matrix. Through we can turn on and off desired pixel and hence generating output. This AMOLED consumes least power compared to other types of OLEDs. Also it has a quicker refresh rate which is suitable for video with high frames per seconds(fps). AMOLEDs are mostly used in computer monitors, TV with large displays, billboards and electronic signs.
3. Transparent OLED :
Transparent OLEDs (TOLEDs) have only transparent components: substrate, cathode and anode. When a TOLED display is turned on, it allows light to pass in both directions. This type of OLED can be included in both the active and passive matrix categories. As they have transparent parameters on both the sides, they can
create displays that are top as well as bottom emitting. This device has a good contrast even in bright sunlight so it is applicable in head-up displays, laptops (Figure 8b), mobile phones and smart windows.
Transparent OLEDs(TOLEDs) made up of only transparent components, substrate, cathode, and anode. TOLEDs emits light in both directions. Transparent OLEDs can be active type as well as passive type. Transparent OLEDs have very good contrast in even bright sunlight so it is useful in laptops(Figure 4), head up displays and mobile phones
(Figure 4)
Image source : ( https://www.youtube.com/watch?v=SaF9pdouvxw )
4. Foldable OLED :
Substrates of foldable OLEDs are created from very flexible metallic foils or plastics. It is durable and very lightweight .Foldable OLEDs are used in GPS devices, cell phones and large curved TVs. It can be also integrated with clothing item to make smart clothes. FOLEDs have very good resolution, a faster response and high contrast. For example of FOLEDs refer below figure.
(figure 5)
Image source : (http://www.redorbit.com/news/technology/1112968380/lg-begins-production-of-flexible-oled-smartphone-display-panel-100713/ )
5. Top-emitting OLED :
In this type of OLED, none-transparent transistor backplane is combined with OLED. Top-emitting OLED can be used for smart card. Here substrate is reflective type.
6. White OLED :
White OLED emits brighter light . It is also energy efficient. It can be manufactured in large sheets and manufacturing cost is also low. It is also suitable for car lighting. It can produce very dark black , so displays is crisp and and better contrast than LCD and LED. It is also very lightweight and thin. It uses less amount of power compared to other OLEDs. Despite consuming low power it has brighter screen with very wide view angle. White OLED can easily replace fluorescent lamps.
8. Stacked OLED :
This device uses the composite colours as sub pixels and also on top of each other. This causes the reduction in pixel gap and also an increase in colour depth. Thus they are being introduced as television displays.
Advantages of OLEDs :
Manufacturing cost is low and it is much more efficient than LCD or LED.
Response time of OLED is approximately 0.01 ms which ultra fast
It has much better contrast means black are truly black(pixel off), that implies better picture quality.
OLED displays have very wide viewing angle about 170 degrees.
OLED has a refresh rate of 100,000 Hz which is so high compared to LCD
There is no need of back light so it’s power consumption is low means environment friendly.
Disadvantages of OLED :
Lifespan is less compared to LCD or LED
There is no enough research and development happened, it is still in rising phase
With time, the brightness of the OLED pixels will fade.
It can be easily damaged by water
Limited market availability
Application :
OLEDs are used in mobile phone screens(Currently Samsung and Google flagship phones like iphone X and Pixe 2 xl have AMOLED display). AMOLED can also be used in MP3 players, digital cameras, car radios, computer display. As mentioned earlier due to AMOLED flexible displays are possible. Also now transparent displays are available which is AMOLED.
The Fraunhofer FEP institute, in collaboration with Nippon Steel & Sumikin Materials (NSMAT) and Nippon Steel & Sumitomo Metal Corporation (NSSMC), developed a new OLED lighting prototype that is made on a stainless steel substrate.