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OLED. Organic Light Emitting Diode

An organic light-emitting diode, also known as OLED (English acronym: Organic Light-Emitting Diode) is a diode that is based on an electroluminescent layer formed by an organic components film that reacts with a particular electrical stimulation, generating and emitting light themselves. There are many different OLED technologies, as many as the great diversity of structures (and materials) have been able to develop (and implement) to contain and maintain the electroluminescent layer and the type of organic components used. The main advantages of OLEDs are: lower cost, increased scalability, greater range of colors, more contrast and brightness, wide viewing angle, lower consumption and, in some technologies, flexibility. But degradation of OLED materials has limited its use at the moment. It is currently being investigated to solve the problems, the fact is that OLEDs will be a technology that can replace the current hegemony of LCD (TFT) and plasma display. Therefore, OLED can and will be used in all types of applications: television screens, computer screen, screens of handheld devices (mobile phones, PDAs, MP3 players ...), indicators and warning information, etc. with formats that under any design ranging from small dimensions (2 ") to huge sizes (equivalent to being achieved with LCD). By the OLEDs can also create large or small billboards and light sources to illuminate general areas. In addition, some OLED technologies have the ability to have a flexible structure, which has already led to development of folding screens, and perhaps in future displays on clothing and textiles etc.. Electroluminescence in organic materials was produced in the 50s by Bernanose and his staff. In a 1977 article, the Journal of the Chemical Society, Shirakawa et al. reported the discovery of high conductivity in iodine doped polyacetylene. [2] Heeger, MacDiarmid & Shirakawa received the Nobel Prize in Chemistry 2000 by the discovery and development of conductivity in organic polymers ". En un artículo de 1990 en la revista Nature, Burroughs et al. reportó el desarrollo de un polímero de emisión de luz verde con una alta eficiencia. Recently, in 2008, has appeared in Castilian a review and update of OLED technology. An OLED consists of two thin organic layers: emission layer and transmission layer , which in turn are between a thin film that makes the anode terminal and a cathode as it does. Generally, these layers are made of molecules or polymers that conduct electricity. Their electrical conductivity levels ranging from insulators to conductors, and therefore they are called organic semiconductors (see polymer semiconductor). The choice of organic materials and the structure of the layers determine the characteristics of operation: emitted color, lifetime and energy efficiency.

 

 

 

 

 

 

 

 

 

 

 

 

 

Working principle:

A voltage is applied across the OLED such that the anode is positive with respect to the cathode. This causes a current of electrons to flow through the device from cathode to anode. Thus, the cathode gives electrons to the emissive layer and the anode withdraws electrons from the conductive layer; in other words, the anode gives electron holes to the conductive layer. Soon, the emissive layer becomes negatively charged (excess electrons),while the conductive layer becomes rich in positively charged holes (for lack of electrons). Electrostatic forces bring the electrons and the holes towards each other and they recombine (in the inverse load sense there would not be recombination and the device would not work). This happens closer to the emissive layer, because in organic semiconductors holes are more mobile than electrons (not the case in inorganic semiconductors). Recombination is the phenomenon in which an atom captures an electron. That electron passes from one layer to a smaller one, releasing an energy that is equal to the difference between initial and final energies, in the form of photon. Recombination causes a radiation release at a frequency which is in the visible region, and you can observe a point of light in a specific color. The sum of many of these recombinations that happen simultaneously is what we call image.

 

 

 

 

 

 

 

Technologies:

* SM-OLED (Small-molecule OLED) SM-OLEDs are based on a technology developed by Eastman Kodak Company. production of small-molecule devices and displays usually involves thermal evaporation in a vacuum. This makes the production process more expensive, and of limited use for large-area devices, than other processing techniques (such as the following). Typically, glass substrates are used to create a vacuum, but this takes flexibility from screens although the molecules are themselves flexible.

* PLED (Polymer Light-Emitting Diodes) PLEDs or LEPs (Light-emitting polymers) have been developed by Cambridge Display Technology. LEPs involve an electroluminescent conductive polymer, that emits light when connected to an external voltage. They are used as a thin film for full-spectrum colour displays. Polymer OLED's are quite efficient and require a relatively small amount of power for the amount of light produced. No vacuum is required, unlike the SM-OLED is not necessary and polymers can be applied on the substrate using a a technique derived from commercial inkjet printing. The used substrate can be flexible as a PET plastic. With all this, PLEDs can be produced economically.

* TOLED (Transparent OLED) Transparent OLEDs use transparent or semi-transparent contacts on both sides of the device to create displays that can be made to be both top and bottom emitting (transparent). TOLEDs can greatly improve the contrast with the surroundings, which makes much easier to see the screens to sunlight.

* SOLED (Stacked OLED) Stacked OLEDs use a pixel architecture that stacks the red, green, and blue subpixels on top of one another instead of next to one another, leading to substantial increase in gamut and color depth, and greatly reducing pixel gap. Currently, other display technologies have the RGB (and RGBW) pixels mapped next to each other decreasing potential resolution.

 

 

 

 

 

 

 

Implementación en matrices:

In addition to the above technologies, OLED displays can be activated by a driving method of the current by matrix can have two different schemes and results in PMOL and AMOLED technologies.

* PMOLED (Passive-matrix OLED) PMOLEDs have tracks of cathodes, anodes tracks (perpendicular to the cathode) and organic layers in the intermediate. The intersections between cathodes and anodes compose the pixels where light is emitted. External circuitry applies current to the appropriate tracks, determining which pixels are switched on and which will be off. Again, the brightness of each pixel is proportional to the amount of current applied, which is distributed uniformly across all pixels (N pixels fed each with 1 / N of the applied current). PMOLEDs are easy to build, but consume more power than other types of OLEDs, mainly because the necessary power for external circuitry and the consumption that needs the variable illumination of the pixels. PMOLEDs are the most efficient to display text and icons, and acquire their better performance in smaller dimensions of 2 "or 3" diagonal, or within about 100 rows. PMOLEDs thus become the most suitable for using on small screens such as those found in mobile phones, PDAs and MP3 players. In addition, PMOLEDs consume less battery power than current LCDs that are used in these devices.

* AMOLED (Active-matrix OLED) AMOLEDs have complete layers of cathode, anode and organic molecules. On the anode layer it is overlayed an array of thin film transistor (Thin Film Transistor, TFT). The TFT array is the circuitry that determines which pixels turn on to form the image. AMOLEDs consume less power than TFT array PMOLEDs because it requires less power than external circuitry. Thus, AMOLEDs are more efficient and manage to have a faster refresh rates, ideal for video. The best applications which are located AMOLEDs are computer monitors, large TV screens and, if the price is permissive, large electronic signs.

 

Main advantages:

 

OLEDs offer many advantages over LCDs, LEDs and plasma displays.

Thinner and flexible. On the one hand, the organic layers of polymers or molecules in OLEDs are thin, light and more flexible than crystalline layers of an LED or LCD. Moreover, in some technologies the print media of OLEDs can be plastic, which provides flexibility for the rigidity of glass that supports the LCDs or plasma screens.

Cheaper in the future. In general, organic materials and plastic substrates will be much cheaper. Also, the manufacturing process of OLEDs can be used known inkjet technology (in English, known as inkjet), a fact that production costs decrease.

More brightness and contrast. The OLED pixels directly emit light. Therefore, compared to LCDs allow a greater range of colors, more brightness and contrast, and more viewing angle.

Less energy consumption. OLEDs do not require backlight technology, ie, a really off OLED element produces no light and no power, unlike LCDs can not show a true "black" and consuming power light up continuously. Thus, the OLED display images with less light output, and when fed from a battery can operate at length with the same load.

Greater scalability and new applications. Future ability to scale to large screens so far and collected by the LCDs and, above all, able to roll and fold the screen in some of OLED technologies that allow it opens the door to a whole new world of applications are coming.

 

Disadvantages and problems:

Short lifetimes. The red and green OLED layers have long lifetimes, however the blue layer is not as durable, now with a duration close to the 14,000 hours (8 hours daily for 5 years), this period of operation is much lower than the average of the LCD depending on model and manufacturer can reach 60,000 hours. Toshiba and Panasonic have found a way to solve this problem with a new technology that can double the life of the layer responsible for the blue color, placing life above average LCD screens. A metal membrane helps the light to pass from the polymer substrate through the glass surface more efficiently than current OLEDs. The result is the same image quality with half the brightness and double the expected lifetime. In 2007, experimental PLEDs could hold 400 cd / m² brightness for over 198,000 hours for green OLEDS and 62,000 for the Blues.

Expensive manufacturing process. Currently most of OLED technology are under investigation, and manufacturing processes (especially initially) are economically elevated, unless with a commitment to a design that is used in economies of scale.

Water. Water can easily damage OLEDs permanently.

Environmental impact. Organic compounds (molecules and polymers) has been that they are difficult to recycle (high cost, sophisticated techniques). This may cause an environmental impact very negatively on the future. Currently there is research to develop a new version of the organic LED that emits light only, but also collect solar energy to produce electricity. Currently there is no date for their marketing, but is already talking about how to do it for mass production. With this technology you could build all kinds of small electrical appliances through its own display could be self-sufficient energy.