Today i was chatting with my friend about the various options available in market for mobile phones.The conversation curved its way to the latest T.V (led t.v) and finally halted upon AMOLED screen of GALAXY S2.It halted as we both had no stuff to discuss on it.Hope reading this article somewhat makes you understand OLED (AMOLED)
OLED
INTRODUCTION
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
HISTORY
The discovery of the electroluminescence property in organic materials in 1950s is considered to be the stepping stone of OLED.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.
COMPONENTSThe 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.
An OLED consists of the following parts:
- Substrate (clear plastic, glass, foil) - The substrate supports the OLED.
- Anode (transparent) - The anode removes electrons (adds electron "holes") when a current flows through the device.
- Organic layers - These layers are made of organic molecules or polymers.
- Conducting layer - This layer is made of organic plastic molecules that transport "holes" from the anode. One conducting polymer used in OLEDs is polyaniline.
- Emissive layer - This layer is made of organic plastic molecules (different ones from the conducting layer) that transport electrons from the cathode; this is where light is made. One polymer used in the emissive layer is polyfluorene.
- Cathode (may or may not be transparent depending on the type of OLED) - The cathode injects electrons when a current flows through the device.
WORKING
OLEDs emit light in a similar manner to LEDs, through a process called electrophosphorescence.
The process is as follows:
- The battery or power supply of the device containing the OLED applies a voltage across the OLED.
- An electrical current flows from the cathode to the anode through the organic layers (an electrical current is a flow of electrons).
- The cathode gives electrons to the emissive layer of organic molecules.
- The anode removes electrons from the conductive layer of organic molecules. (This is the equivalent to giving electron holes to the conductive layer.)
- At the boundary between the emissive and the conductive layers, electrons find electron holes.
- When an electron finds an electron hole, the electron fills the hole (it falls into an energy level of the atom that's missing an electron).
- When this happens, the electron gives up energy in the form of a photon of light
- The OLED emits light.
- The color of the light depends on the type of organic molecule in the emissive layer. Manufacturers place several types of organic films on the same OLED to make color displays.
- The intensity or brightness of the light depends on the amount of electrical current applied: the more current, the brighter the light.
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.
Hope you found it interesting!!
Again have prepared a presentation have a look at it. Presentation
References
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