Organic Light Emitting Diode Essay

Published: 2020-04-22 15:06:56
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Abstract:

Organic Light Emitting Diode is a scalable nano level emerging technology in Flat Panel Displays and as a White Light Source with efficient features. This paper focuses on OLED structure, principle aspects, fabrication methodology and different techniques to replace current white light sources like Incandescent bulbs, Fluorescent tubes, and even display techniques like Liquid Crystal Displays, Plasma technologies. OLEDs can be fabricated using Polymers or by small molecules. OLED matrix displays offer high contrast, wide viewing angle and a broad temperature range at low power consumption. These are Cheaper, Sharper, Thinner, and Flexible. An OLED is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current.

This layer of organic semiconductor material is situated between two electrodes. Generally, at least one of these electrodes is transparent. There are two main families of OLED s: those based on small molecules and those employing polymers. Adding mobile ions to an OLED creates a Light-emitting Electrochemical Cell or LEC, which has a slightly different mode of operation. OLED displays can use either passive-matrix (PMOLED) or active-matrix addressing schemes. Active-matrix OLED s (AMOLED) require a thin-film transistor backplane to switch each individual pixel on or off, but allow for higher resolution and larger display sizes. An OLED display works without a backlight.

Thus, it can display deep black levels and can be thinner and lighter than a liquid crystal display (LCD). In low ambient light conditions such as a dark room an OLED screen can achieve a higher contrast ratio than an LCD, whether the LCD uses cold cathode fluorescent lamps or the more recently developed LED backlight. Due to its low thermal conductivity, an OLED typically emits less light per area than an inorganic LED. OLEDs are used in television screens, computer monitors, small, portable system screens such as mobile phones and PDA s, watches, advertising, information, and indication.

OLEDs are also used in large-area light-emitting elements for general illumination. OLED s have a potential of being white-light sources that are ¢Bright, power-efficient and long lived, by emitting pleasing white light

¢Ultra-thin, lightweight, rugged, and conformable

¢Inexpensive, portable

Introduction:

OLEDs are energy conversion devices (electricity-to-light) based on Electroluminescence. Electro-luminescence is light emission from a solid through which an electric current is passed. OLEDs are more energy-efficient than incandescent lamps. The luminous efficiency of light bulbs is about 13 20 lm/W but the latest experimental green emitting OLEDs already have luminous efficiency of 76 lm/W, though at low luminance. The development is on track for OLEDs to effectively compete even with fluorescent lamps, which have the luminous efficiency of 50 100 lm/W. One big advantage of OLEDs is the ability to tune the light emission to any desired color, and any shade of color or intensity, including white.

Achieving the high Color Rendition Index (CRI) near 100 (the ability to simulate the most pleasing white color, sunlight), is already within the reach of OLEDs. Another advantage of OLEDs is that they are current-driven devices; where brightness can be varied over a very wide dynamic range and they operate CRT is still continuing as top technology in displays to produce economically best displays. The first best look of it is its Cost. But the main problems with it are its bulkiness, Difficulties in Extending to Large area displays as per construction. Even though Liquid Crystal Displays have solved one of problem i.e. size, but it is not economical.

So in this present scenario the need for a new technology with both these features combined leaded to invention of OLED.OLED which is a thin, flexible, Bright LED with self luminance which can be used as a display device. The main drawback of LCD display is its Less viewing angle and highly temperature depending which moves us towards a new technology. Thus OLED promises for faithful replacement of current technology with added flavors like Less Power Consumption and Self Luminance .Both Active matrix TFTs and Passive matrix Technologies are used for display and addressing purposes for high speed display of moving pictures and faster response. Already some of the companies released Cell Phones and PDAs with bright OLED technology for color full displays.

One of the new lighting technology which emerged within the past two decades and has the potential of becoming more energy-efficient then the existing light sources is the Solid State Lighting technology of Organic Light Emitting Diodes (OLEDs). The available data about OLEDs and technical projections indicate that the amount of energy needed to generate the same amount of light can be eventually reduced by up to 50%.If the consumption of electric energy used for lighting is reduced by the desired 50%; the savings to the society would amount to approximately $25B per year (1). In addition to the savings, less consumed energy would amount to less produced energy and, consequently, less pollution of water and air.

According to the latest estimates, the use of electricity may be reduced by 50% by the year 2020, sparing the atmosphere some 45 million tons of carbon emissions annually. The potential savings also depends on how quickly and to what extend these developments occur (2). This study also indicates that it is primarily the price breakthrough that will facilitate the market penetration of the new sources of light. In other words, even though the technological advances may lead to significant reduction of energy, the market will not accept SSL unless the cost is reduced as well. If SSL achieves a price breakthrough, far more energy will be saved. Today, incandescent light bulbs dominate the residential and light industrial lighting market where the initial cost and aesthetics are the key drivers. Fluorescent lamps are used in the commercial sector where the combined cost of the lighting fixtures and the consumed energy is the principal driver.

OLEDs are unconventional, large area thin film, nearly two-dimensional devices. They are distributed (diffused) light sources, distinctly different from point sources such as light bulbs. Also, OLEDs will operate at very low voltages, of the order of 3 5 V. Therefore, the introduction of OLEDs as sources of light for general lighting applications will cause a major paradigm shift in the lighting industry. Not only a new lighting infrastructure will be required, but also many new jobs will be created. While significant research is still needed, OLEDs will soon achieve the efficiency to compete directly with incandescent sources (light bulbs).

Experimental OLEDs are already more energy-efficient than incandescent lamps The luminous efficiency of light bulbs is about 13 -20 lm/W but the latest experimental green emitting OLEDs already have luminous efficiency of 76 lm/W, albeit at low luminance. The development is on track for OLEDs to effectively compete even with fluorescent lamps, which have the luminous efficiency of 50 100 lm/W. One big advantage of OLEDs is the ability to tune the light emission to any desired color, and any shade of color or intensity, including white .Achieving the high color rendition index (CRI) near 100 (the ability to simulate the most pleasing white color, sunlight), is already within the reach of OLEDs. Another advantage of OLEDs is that they are current-driven devices, where brightness can be varied over a very wide dynamic range and they operate uniformly, without flicker.

All this has created a great deal of optimism that OLEDs will be accepted and welcome by the general public as long as they are inexpensive. Yet another advantage of OLEDs is that they could be deposited on any substrate: glass, ceramics, metal, thin plastic sheets, fabrics, flexible and conformable substrates, etc., and therefore, could be fabricated in any shape and design. This will open new architectural and design possibilities. Freedom to produce sources of any shape or color will create radically new illumination culture. In a nutshell, OLEDs have a potential of being large area, white-light sources that are * Bright, power-efficient and long lived, emitting pleasing white light * Ultra-thin, light weight, rugged, and conformable

* Inexpensive

This qualitative comparison is based on the assumption that the development of OLEDs will be successful. Monumental challenges, however, still exist to reach the goal. Over the next 5 years, the lighting market will grow to about $40B/y. Based on the novel features; OLEDs may soon capture 10% of that market. As the efficiency and cost approach the targets fluorescent lamps, 50% of the market may be captured in 10-12 years.

1.4 White Light from OLEDs

OLEDs are uniquely suitable as sources of white light. The structure of light emitting Fluorescence or phosphorescence additives can be tailored to emit any desired color (see section 5.1). Mixing light from two or more sources (dopants or layers) gives light whose color is determined by the weighted average of the CIE coordinates of these sources. Given the enormous variety of known and yet-to-be synthesized dopants, both fluorescent and phosphorescent, with broad emission spectra of choice, practically any shade of white or any temperature of white light can be generated in OLEDs. Many devices have already been made in the laboratory scale and tested and some of them almost perfectly simulate the sunlight. The methods of generating white light are described in Sections 5.1.4. And 5.1.5.

2. OLED Components

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, well be focusing on the two-layer design.

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. The 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.

3. Working Principle of Oled

OLEDs emit light in a similar manner to LEDs, through a process called electro phosphorescence.

The process is as follows:

1. The battery or power supply of the device containing the OLED applies a voltage across the OLED. 2. 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.) 3. 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 thats missing an electron). When this happens, the electron gives up energy in the form of a photon of light (see How Light Works). 4. The OLED emits light.

5. 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. [pic]

Schematic of a bilayer OLED:

1. Cathode (−), 2. Emissive Layer, 3. Emission of radiation, 4. Conductive Layer, 5. Anode (+)

A typical OLED is composed of a layer of organic materials situated between two electrodes, the anode and cathode, all deposited on a substrate. The organic molecules are electrically conductive as a result of delocalization of pi electrons caused by conjugation over all or part of the molecule. These materials have conductivity levels ranging from insulators to conductors, and therefore are considered organic semiconductors. The highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) of organic semiconductors are analogous to the valence and conduction bands of inorganic semiconductors. Originally, the most basic polymer OLEDs consisted of a single organic layer. One example was the first light-emitting device synthesized by J. H. Burroughs et al., which involved a single layer of poly (p-phenylene vinylene). However multilayer OLEDs can be fabricated with two or more layers in order to improve device efficiency.

As well as conductive properties, different materials may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile, or block a charge from reaching the opposite electrode and being wasted. Many modern OLEDs incorporate a simple bilayer structure, consisting of a conductive layer and an emissive layer. More recent developments in OLED architecture improves quantum efficiency (up to 19%) by using a graded heterojunction. In the graded heterojunction architecture, the composition of hole and electron-transport materials varies continuously within the emissive layer with a dopant emitter. The graded heterojunction architecture combines the benefits of both conventional architectures by improving charge injection while simultaneously balancing charge transport within the emissive region. During operation, a voltage is applied across the OLED such that the anode is positive with respect to the cathode.

A current of electrons flows through the device from cathode to anode, as electrons are injected into the LUMO of the organic layer at the cathode and withdrawn from the HOMO at the anode. This latter process may also be described as the injection of electron holes into the HOMO. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole. This happens closer to the emissive layer, because in organic semiconductors holes are generally more mobile than electrons.

The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of this radiation depends on the band gap of the material, in this case the difference in energy between the HOMO and LUMO. OLEDs are solid-state devices composed of thin films of organic molecules that create light with the application of electricity. OLEDs can provide brighter, crisper displays on electronic devices and use less power than conventional light-emitting diodes (LEDs) or liquid crystal displays (LCDs) used today.

4. Types of OLEDs: Passive and Active Matrix

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, well discuss each type of OLED. Lets start with passive-matrix and active-matrix OLEDs.

1. Passive-matrix OLED (PMOLED)

PMOLEDs has 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.

2. 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. 3. 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.

4. 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.

5. 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. 6.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.

Reference:
http://impnerd.com/the-history-and-future-of-oled
http://en.wikipedia.org/wiki/Organic_light-emitting_diode
http://www.oled-research.com/oleds/oleds-history.html
http://www.voidspace.org.uk/technology/top_ten_phone_techs.shtml#keep-your-eye-on-flexible-displays-coming-soon http://www.pocket-lint.com/news/news.phtml/23150/24174/samsung-say-oled-not-ready.phtml http://www.cepro.com/article/study_future_bright_for_oled_lighting_market/

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