Workng of Organic LED (OLED)

Have you ever thought of having a high-definition TV that is bigger than your usual LCD display, and that consumes only half the power of an ordinary one? What if you could easily roll the TV and pack it up so that you can use it for a trip? Yes, this is possible with the organic Light Emitting Diode (OLED) technology. With such a component you can easily make PC monitors, high clarity cell-phone screens, watches and also indicators. They can also be used for illuminating a large area and is sure to replace the Liquid Crystal Display Technology (LCD) in the near future.

These types of displays can be built on even your clothing and are flexible enough to be portable. As it is in the early stage of development, they are not being marketed at a high rate.

TAKE A LOOK : WORKING OF LIQUID CRYSTAL DISPLAY

TAKE A LOOK : WORKING OF LIGHT EMITTING DIODE

What is an OLED?

OLED’s are simple solid-state devices (more of an LED) comprised of very thin films of organic compounds in the electro-luminescent layer. These organic compounds have a special property of creating light when electricity is applied to it. The organic compounds are designed to be in between two electrodes. Out of these one of the electrodes should be transparent. The result is a very bright and crispy display with power consumption lesser than the usual LCD and LED.

OLED – Comparison with LCD and LED

Like a LCD, the OLED does not require a backlight for its normal working. This makes them more advantageous in saving space and also weight. It also helps them in displaying deeper black levels than LCD’s. OLED is also capable of making a high contrast ratio when it is displayed in a dark room than LCD as well as LED.

Introduction of OLED

The discovery of the electroluminescence property in organic materials in 1950s is considered to be the stepping stone of OLED.

Later in 1960, a scientist called Martin Pope discovered an ohmic, dark injecting electrode contact to organic nature of crystals. With this he was also able to explain the work functions for both the holes and electrons while injecting electrode contacts. These dark injecting holes and electrons formed to be the base for an OLED device. The technique was further experimented with DC electroluminescence under different conditions. Later it was found that electroluminescent materials can also act as doped insulators. Thus came the discovery of a double injection induced OLED device.

OLED’s, as can be seen here are simple solid-state devices (more of an LED) comprised of very thin films of organic compounds in the electro-luminescent layer.The first proper OLED was manufactured in 1980 by Dr. Ching W Tang and Steven Van Slyke. The OLED had a double layer structure. When the holes and electrons were transported separately and when combined together produced a light in the organic layer centre. This light was produced at a very low operating voltage with high efficiency. Now more research is being done with the application of OLED on polymer so as to obtain a higher efficiency OLED.

Components in an OLED

The components in an OLED differ according to the number of layers of the organic material. There is a basic single layer OLED, two layer and also three layer OLED’s. As the number of layers increase the efficiency of the device also increases. The increase in layers also helps in injecting charges at the electrodes and thus helps in blocking a charge from being dumped after reaching the opposite electrode. Any type of OLED consists of the following components.

1. An emissive layer
2. A conducting layer
3. A substrate
4. Anode and cathode terminals.

As the emissive layer and the conducting layer is made up of organic molecules (both being different), OLED is considered to be an organic semi-conductor, and hence its name. The organic molecules have the property of conducting electricity and their conducting levels can be varied form that of an insulator to a conductor.

The emissive layer used in an OLED is made up of organic plastic molecules, out of which the most commonly used is polyfluorene.

The conducting layer is also an organic molecule, and the commonly used component is polyaniline.

The substrate most commonly used may be a plastic, foil or even glass.

The anode component should be transparent. Usually indium tin oxide is used. This material is transparent to visible light. It also has a great work function which helps in injecting holes into the different layers.

The cathode component depends on the type of OLED required. Even a transparent cathode can be used. Usually metals like calcium and aluminium used because they have lesser work functions than anodes which helps in injecting electrons into the different layers.

Working of an OLED

Before going on to the detailed explanation of its working, it is important to know how the emissive layers and conducting layers are added to the substrate. There are mainly three basic methods for this operation. They are:-

1. Inkjet Printing Technique – This is the cheapest and most commonly used technique. The method is same as the paper printing mechanism where the organic layers are sprayed onto the substrates. This method is also highly efficient and they can be used for printing very large displays like billboards and also big TV screens.

2. Organic Vapour Phase Deposition (OVPD) – This is also an efficient technique which can be carried out at a low cost. A cooled substrate is being hit by the organic molecules, which was evaporated in a low pressure, 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 commonly known as vacuum deposition method. This operation is carried out by gently heating the organic molecules so that they evaporate and subside on the substrates. As the heating method is complicated and the strictness of parameters should be highly accurate, this method is economical as well.

After the organic material has been applied to the substrate the real working of the OLED begins.

The substrate is used to support the OLED. The anode is used to inject more holes when there is a path of current. The conducting layer is used to carry the holes from the anode. The cathode is used to produce electrons when current flows through its path. The emissive layer is the section where the light is produced. This layer is used to carry the electrons form the cathode.

First, the anode is kept positive w.r.t the cathode. Thus there occurs an electron flow from the cathode to the anode. This electron flow is captured by the emissive layer causing the anode to withdraw electrons from the conductive layer. Thus, there occurs a flow of holes in the conductive layer. As the process continues, the conductive layer becomes positively charged and the emissive layer becomes negatively charged.

A combination of the holes and electrons occur due to electrostatic forces.  As the electrons are less mobile than the holes, the combination normally occurs very close to the emissive layer. This process produces light in the emissive region after there has been a drop in the energy levels of the electrons. The emissive layer got its name as the light produced in the emissive region has a frequency in the visible region. The colour of the light produced can be varied according to the type of organic molecule used for its process. To obtain colour displays, a number of organic layers are used. Another factor of the light produced is its intensity. If more current is applied to the OLED, the brighter the light appears. Take a look at the diagram given below.

Now consider the process when the anode is negative w.r.t the cathode. This will not make the device work as there will not be any combination of the holes and electrons. The holes will move towards the anode and the electrons to the cathode.

Different types of OLED’s

According to the type of manufacture and the nature of their use, OLED’s are mainly classified into 8 types. They are

1. Active Matrix OLED (AMOLED)

This type of OLED is suitable for high resolution and large size display. Though the manufacturing process is the same, the anode layers have a Thin-film transistor (TFT) plane in parallel to it so as to form a matrix. This helps in switching each pixel to it’s on or off state as desired, thus forming an image. This is the least power consuming type among others and also has quicker refresh rates which makes them suitable for video as well.

2. Passive Matrix OLED (PMOLED)

The design of this type of OLED makes them more suitable for small screen devices like cell phones, MP3 players and so on. Though this type is less power consuming than an LCD and LED (even if connected to other external circuitry’s), it is the most power consuming comparative to other OLED’s. This type is very easy to make as strips of anode and cathode are kept perpendicular to each other. When they are both intersected light is produced. As there are strips of anode and cathode, current is applied to the selected strips and is applied to them. This helps in determining the on or off pixels.

3. Inverted OLED

This type uses a bottom cathode, which is connected to the drain end of an n-channel TFT backplane. This method is usually used for producing low cost OLED with little applications.

4. Foldable OLED

This type is mainly used in devices which have more chance of breaking. As this material is strong it reduces breakage and therefore is used in cell phones, computer chips, GPS devices and PDA’s. They are also flexible, durable and lightweight. As its name explains, these OLED’s are foldable and can also be connected to clothes. They use different types of substrates like flexible metallic foils, plastics and so on.

5. Top Emitting OLED

This type of OLED is integrated with a transistor backplane that is not transparent. Such devices are suitable for matrix applications like smart cards. The substrate used for this device is of the opaque/reflective type. As a transparent substrate is used the electrode used is either semi-transparent or fully transparent. Otherwise the light will not pass through the transparent substrate.

6. Transparent OLED

This device has a good contrast even in bright sunlight so it is applicable in head-up displays, mobile phones, smart windows and so on. In this device, the entire anode, cathode and the substrate are transparent. When they are in the off position, they become almost completely transparent as their substrate. 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.

7. White OLED

This device creates the brightest light of all. They are manufactured in large sheets. Thus they can easily replace fluorescent lamps. They are also cost-effective and also consumes less power.

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 OLED’s

• The manufacture of OLEDD is highly economical and is more efficient than LCD and flat panel screens.
• It will be a great surprise to see displays on our clothing and fabrics. This technology will help in carrying huge displays in our hands.
• There is much difference in watching a high-definition TV to a OLED display. As the contrast ratio of OLED is very high (even in dark conditions), it can be watched from an angle of about 90 degrees without any difficulty.
• No backlight is produced by this device and the power consumption is also very less.
• OLED has a refresh rate of 100,000 Hz which is almost 9900 HZ greater than an LCD display.
• The response time is less than 0.01 ms. LCD needs a response time of 1 ms.

Disadvantages of OLED

• The power consumption of this device depends upon the colour that is displayed on the screen. Less than 50% power is only consumed when a black image is displayed, compared to an LCD. But the percentage increases to almost three times when a bright image such as a white colour is displayed. Thus, this device is disadvantageous for mobile applications.
• The OLED technology is only rising and due to this, the commercial availability of OLED products are very less. Though they can be easily made the fabrication process is considered expensive and thus the initial amount is expensive.
• As there is no reflective light technology used in such a device it has a very poor reading effect in bright light surroundings. Even if this is to be overcome additional power should be used.
• With time, the brightness of the OLED pixels will fade.
• The images displayed in this device are created by an artificial light source. So, the whole electricity has to be used to perform such an operation. LCD’s, on the other hand use some percentage of light from sunlight and also e-ink.
• The device is not at all water resistant.
• The lifetime of this device is much lesser when compared with an LCD or LED.

Applications of OLED

OLED’s are used as mobile phone screens, MP3 players, digital cameras, car radios, PDA’s and so on.