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OLED stands for Organic Light Emitting Diode. The "organic" in OLED refers to organic material. Carbon is the basis of all organic matter. Examples of carbon-based substances include sugar, wood and the majority of plastics. The "LED" stands for "Light Emitting Diode" and describes the process of converting electric energy into light. There are two types of OLEDs small molecule OLED and polymer OLED. Sony uses the small molecule type because it has a longer lifespan.
I.Â Â INTRODUCTION
An organic light emitting diode (OLED), also light emitting polymer (LEP) and organic electro luminescence (OEL), is a light-emitting diode (LED) whose emissive electroluminescent layer is composed of a film of organic compounds. The layer usually contains a polymer substance that allows suitable organic compounds to be deposited. They are deposited in rows and columns onto a flat carrier by a simple "printing" process. The resulting matrix of pixels can emit light of different colors.
Such systems can be used in television screens, computer monitors, small, portable system screens such as cell phones and PDAs, advertising, information and indication. OLEDs can also be used in light sources for general space illumination, and large-area light-emitting elements. OLEDs typically emit less light per area than inorganic solid-state based LEDs which are usually designed for use as point-light sources.
A significant advantage of OLED displays over traditional liquid crystal displays (LCDs) is that OLEDs do not require a backlight to function. Thus, they can display deep black levels, draw far less power, and can be much thinner and lighter than an LCD panel. OLED displays also naturally achieve much higher contrast ratio than LCD screens using either CCFL or LED Backlights.
Like an LED, an OLED is a solid-state semiconductor device that is 100 to 500 nanometers thick or about 200 times smaller than a human hair. OLEDs can have either two layers or three layers of organic material; in the latter design, the third layer helps transport electrons from the cathode to the emissive layer. In this article, we'll be focusing on the two-layer design.
II. OLED COMPONENTS
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.
III.Â Â MAKING OLED
biggest part of manufacturing OLEDs is applying the organic layers to the substrate. This can be done in three ways:
- Vacuum deposition or vacuum thermal evaporation (VTE) - In a vacuum chamber, the organic molecules are gently heated (evaporated) and allowed to condense as thin films onto cooled substrates. This process is expensive and inefficient.
- Organic vapor phase deposition (OVPD) - In a low-pressure, hot-walled reactor chamber, a carrier gas transports evaporated organic molecules onto cooled substrates, where they condense into thin films. Using a carrier gas increases the efficiency and reduces the cost of making OLEDs.
- Inkjet printing - With inkjet technology, OLEDs are sprayed onto substrates just like inks are sprayed onto paper during printing. Inkjet technology greatly reduces the cost of OLED manufacturing and allows OLEDs to be printed onto very large films for large displays like 80-inch TV screens or electronic billboards.
IV. HOW DO OLED'S EMIT LIGHT?
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.)
V.Â TYPES OF OLED
There are several types of OLEDs:
- Passive-matrix OLED
- Active-matrix OLED
- Transparent OLED
- Top-emitting OLED
- Foldable OLED
- White OLED
Each type has different uses. In the following sections, we'll discuss each type of OLED. Let's start with passive-matrix and active-matrix OLEDs.
A. Passive-matrix OLED (PMOLED)
PMOLEDs have strips of cathode, organic layers and strips of anode. The anode strips are arranged perpendicular to the cathode strips. The intersections of the cathode and anode make up the pixels where light is emitted. External circuitry applies current to selected strips of anode and cathode, determining which pixels get turned on and which pixels remain off. Again, the brightness of each pixel is proportional to the amount of applied current.
PMOLEDs are easy to make, but they consume more power than other types of OLED, mainly due to the power needed for the external circuitry. PMOLEDs are most efficient for text and icons and are best suited for small screens (2- to 3-inch diagonal) such as those you find in cell phones, PDAs and MP3 players. Even with the external circuitry, passive-matrix OLEDs consume less battery power than the LCDs that currently power these devices.
B. Active-matrix OLED (AMOLED)
AMOLEDs have full layers 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.
AMOLEDs consume less power than PMOLEDs because the TFT array requires less power than external circuitry, so they are efficient for large displays. AMOLEDs also have faster refresh rates suitable for video. The best uses for AMOLEDs are computer monitors, large-screen TVs and electronic signs or billboards.
C. Transparent OLED
Transparent OLEDs have only transparent components (substrate, cathode and anode) and, when turned off, are up to 85 percent as transparent as their substrate. When a transparent OLED display is turned on, it allows light to pass in both directions. A transparent OLED display can be either active- or passive-matrix. This technology can be used for heads-up displays.
D. Top-emitting OLED
Top-emitting OLEDs have a substrate that is either opaque or reflective. They are best suited to active-matrix design. Manufacturers may use top-emitting OLED displays in smart cards.
E. Foldable OLED
Foldable OLEDs have substrates made of very flexible metallic foils or plastics. Foldable OLEDs are very lightweight and durable. Their use in devices such as cell phones and PDAs can reduce breakage, a major cause for return or repair. Potentially, foldable OLED displays can be attached to fabrics to create "smart" clothing, such as outdoor survival clothing with an integrated computer chip, cell phone, GPS receiver and OLED display sewn into it.
F. White OLED
White OLEDs emit white light that is brighter, more uniform and more energy efficient than that emitted by fluorescent lights. White OLEDs also have the true-color qualities of incandescent lighting. Because OLEDs can be made in large sheets, they can replace fluorescent lights that are currently used in homes and buildings. Their use could potentially reduce energy costs for lighting.
VI. OLED ADVANTAGES AND DISADVANTAGES
The LCD is currently the display of choice in small devices and is also popular in large-screen TVs. Regular LEDs often form the digits on digital clocks and other electronic devices. OLEDs offer many advantages over both LCDs and LEDs:
- The plastic, organic layers of an OLED are thinner, lighter and more flexible than the crystalline layers in an LED or LCD.
- Because the light-emitting layers of an OLED are lighter, the substrate of an OLED can be flexible instead of rigid. OLED substrates can be plastic rather than the glass used for LEDs and LCDs.
- OLEDs are brighter than LEDs. Because the organic layers of an OLED are much thinner than the corresponding inorganic crystal layers of an LED, the conductive and emissive layers of an OLED can be multi-layered. Also, LEDs and LCDs require glass for support, and glass absorbs some light. OLEDs do not require glass.
- OLEDs do not require backlighting like LCDs . LCDs work by selectively blocking areas of the backlight to make the images that you see, while OLEDs generate light themselves. Because 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. It is much more difficult to grow and lay down so many liquid crystals.
- 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.
B. Problems with OLED
OLED seems to be the perfect technology for all types of displays, but it also has some problems:
- Lifetime - While red and green OLED films have longer lifetimes (46,000 to 230,000 hours), blue organics currently have much shorter lifetimes (up to around 14,000 hours.
- Manufacturing - Manufacturing processes are expensive right now.
- Water - Water can easily damage OLEDs.
VII. USEFUL APPLICATION
It is used for aviation lights and military purposes also.
A. PAPI LIGHTS
The PAPI is an instrument helping to carry out a correct approach (in the vertical plane) on an aerodrome or an airport. It is generally located approximately 300 meters beyond the landing threshold of the runway.
The Precision Approach Path Indicator (PAPI) is a light system positioned beside the runway that consists of two, three, or four boxes of lights that provide a visual indication of an aircraft's position on the glidepath for the associated runway.
The PAPI is usually located on the left or right side of the runway(at 90Ëš to the runway centre line which are typically spaced at 9 metres apart.) Units are identical and can be seen up to five miles during the day and twenty miles at night. It has two or four lights installed in a single row instead of far and near bars that would be characteristic of Visual Approach Slope Indicator (VASI).
Each box of lights is equipped with an optical apparatus that splits light output into two segments, red and white. Depending on the angle of approach, the lights will appear either red or white to the pilot. Ideally the total of lights will change from white to half red, moving in succession from the runway side to the outer side. The pilot will have reached the normal glidepath (usually 3 degrees) when there is an even split in red and white lights. If an aircraft is beneath the glidepath, red lights will outnumber white; if an aircraft is above the glidepath, more white lights are visible.
During aircraft descent, this system, along with other airport lights, may be activated by the pilot by keying the aircraft microphone with the aircraft's communication radio.
The Precision approach path indicator is based on the Fresnel Lens principle.
B. ITS COMPONENTS
CCR(CONSTANT CURRENT REGULATOR)
It is a device used to keep the current constant at varying voltages or load .It consist of thyrister TRAK .
Suppose when any of the bulb/OLED damages, the load decreases, due to that current will also decrease as there is no load or less load across it. Then CCR increases the voltages so that the current remains constant.
NOTE: CCR doesn't works in open circuit because if it is open circuit , sometimes the voltages become very high and in open circuit it will damage CCR .
CCR and CONTROL UNIT isÂ so designed thatthey always remain in closed circuit even when all the lights are cut off.
There is a transformer, relay, circuit board (PCB).Two wires from CCR are connected transformer. Wire 1 is connected to secondary of transformer then to relay normally close. Other end of relay is connected with wire 2. Primary of the transformer is connected to PCB. A connection is taken just after the secondary of transformer and before relay. Another connection is taken from other end of relay. These both connections are taken to PAPI lights.