OLED

An organic light emitting diode (English organic light emitting diode, OLED) is a shining thin film component of organic semiconducting materials, the difference to inorganic light emitting diodes ( LED) differs in that the electric current density and luminance are low and no single crystal materials are required. Compared to conventional ( inorganic ) LEDs, organic light-emitting diodes can therefore be produced at lower cost thin-film technology, but their durability is currently less than the conventional LEDs.

The OLED technology for displays ( for the time being in smartphones and tablet computers, and later in larger area, TVs, PCs, monitors ) suitable and displays. Another field of application is the large-scale lighting. Due to the material properties of a possible use of OLEDs as a flexible screen and an electronic paper is interesting.

Design and operation

OLEDs are composed of multiple organic layers. Here (HTL engl. hole transport layer, ) is usually applied to the anode consisting of indium -tin - oxide ( ITO), which is located on a glass plate, a hole transport layer is applied. Is between ITO and HTL - often a layer of PEDOT / PSS, which serves to lower the injection barrier of the holes and preventing the diffusion of indium into the transition - depending on the production method. On the HTL, a layer is applied, which contains either the dye ( 5-10 %) or - more often - completely consists of the dye, such as aluminum tris (8- hydroxyquinoline), Alq3. This layer is called the emitter layer (English emitter layer, EL ). In this optional nor electron conduction layer ( engl. electron transport layer ETL) is applied. At the end, a cathode consisting of a metal or an alloy with a low electron work function such as calcium, aluminum, barium, ruthenium, magnesium -silver alloy, applied by vapor deposition in high vacuum. As a protective layer and to reduce the injection barrier for the electrons is mostly deposited a very thin layer of lithium fluoride, cesium fluoride, or silver between the cathode and E (T) L.

Electron ( = negative charge) will now be injected from the cathode, while the anode provides holes ( = positive charge ). Hole and electron drift toward each other and meet in the ideal case in the EL, which is why this layer is also called recombination. Electrons and holes form a bound state, which is referred to as an exciton. Depending on the mechanism provides the exciton already the excited state of the dye molecule is, or the decay of the exciton, the energy for the excitation of the dye molecule is available. This dye has different excited states. The excited state can transition to the ground state and emit a photon ( light particle ). The color of the emitted light depends on the energy gap between the excited and ground state and can be selectively altered by varying the dye molecules. A problem is posed by non-radiative triplet states dar. These can be redissolved by the addition of so-called " Exzitoren ".

The use and choice of organic materials

For the polymers made ​​from organic LEDs, the abbreviation PLED (English polymer light emitting diode) has prevailed. As SOLED or SMOLED be called OLEDs made ​​from the rare "small molecules" ( small molecules). Derivatives of poly (p -phenylene vinylene) (PPV ) may be used as dyes in PLEDs frequently. In recent times, dye molecules are used, which can be expected to a four-fold higher efficiency than with the above-described fluorescent molecules. In these more efficient OLEDs organometallic complexes are used in which the light emission from triplet states is performed ( phosphorescence).

These molecules are also referred to as triplet emitters; the dye can be excited by the ambient light, which can lead to luminescence. The goal, however, is to produce self-luminous screens which use the organic electroluminescence.

Benefits

One advantage of OLED screens over the conventional liquid crystal displays ( LCDs) is the very high contrast because they do not require backlighting: black pixels do not emit light. While LCDs act only as colored filters and still shines some light in the dark state, OLEDs emit colored light only when driven, what is also very good color representation promises. This method is much more efficient, making OLEDs, especially when displaying dark images require less energy. For this reason, OLED devices was less warm lowered as appropriate devices with LCD screens, though by switching from CCFL to LED for LCD backlight, the energy expenditure for liquid crystal displays. Due to the low energy demand OLEDs can be well used in small, portable devices such as laptops, mobile phones and MP3 players. Because of unneeded backlight, it is possible to make very thin OLEDs. One on the "Display 2008" imagined model from Sony has a depth of only 0.3 millimeters.

The reaction time ( engl. response time ) of an OLED screen is a few units under 0,001 milliseconds ( 1 microsecond) and is thus about 1000 times faster than the currently fastest LCD with a millisecond.

OLEDs can be industrially produce not only expensive vacuum and clean room conditions. Another advantage is based on the alternative, OLEDs in mass and large areas cheaply as they can be implemented at pressure by technical means, which is seldom the case of electronic components and systems and is not in the classic LEDs on the case. The cost advantage results from the fact that the electrically conducting coloring layers can be then coated in a modified ink jet printing method, or more recently applied in the offset printing, and also without a vacuum - vapor deposition. Leaders in this area of ​​soluble OLED material systems are DuPont and Merck. The first OLEDs were printed under laboratory conditions in 1987. Leading exhibition and congress for printed electronics is the LOPEC fair in Munich each year. At Drupa 2012 trade fair for the printing industry, including printed OLEDs were identified as market worth billions.

Disadvantages

The greatest technical problem is the relatively short lifetime of some organic materials consisting of elements In dar. (O) LEDs is referred to as the average operating life time after which the luminance is decreased to one half. For white light sources and monitors the blue component is limiting the overall useful life. 2011 were reported for white light sources 5000 hours ( at 1000 cd / m²) and 12,000 hours (at 100 cd / m²).

However, several important aspects must at the official information on the life of OLED materials are observed: the ( maximum possible or in relation thereto reduced ) initial brightness at which the lifetime measurement begins, the time to drop the brightness to 50 percent of this initial value and the different temperatures, wherein the OLEDs are driven ( to ). A well-cooled OLED ( regardless of color ) with low initial brightness so always has a much longer life than an OLED, which is operated without cooling from the beginning with the maximum luminosity. In addition, the lifetime is usually extrapolated theoretically based on the shortest value: Since it is impractical to test tens or even hundreds of thousands of hours at medium or low luminosity an OLED material, use the lifetime at maximum luminosity and expects them to the lower lighting levels to. The fact that the boom in OLED monitors failed until now, has mainly to do with this life and quality differences for OLED colors and materials.

Just as water and oxygen can destroy the organic material. Therefore, it is important to encapsulate the component and to protect it from external influences. The required rigid, inorganic encapsulation reducing flexibility. Meanwhile, however, the organic materials are more resistant to water and oxygen than previous versions. Corrosion especially the highly reactive injection layer of calcium and barium is at risk. Typical failure symptoms are circular, growing unlit areas, so-called "Dark Spots ". Cause is often a particle load during vapor deposition of metal layers. The microscopic edges of the multilayer structure to be undermined by corrosion, which leads to the decrease of effective pixels illuminated surface with screen applications.

Commercial OLEDs on flexible substrate are still in the implementation phase, as all flexible plastic substrates have a high permeability to oxygen and humidity. Thin glass (glass with a thickness of at most about 0.2 mm) is difficult to handle in processing, also the indium tin oxide anode material is a hard material and therefore brittle. Repeated rolling in and out by a small radius leads to breaking and rapid failure ( increase in resistance ) of the anode.

History

In the 1950s, the electroluminescence in organic materials was discovered by A. Bernanose at the University of Nancy in France. Substances such as acridine orange were deposited in thin films from cellulose or cellophane or dissolved and subjected to an AC field. This mechanism is based on the direct excitation of the dye molecules or electrons.

Martin Pope and co-workers at New York University developed ohmic electrode contacts 1960 for the injection of charge carriers in organic crystals in the unilluminated state. In addition, she described the necessary energetic requirements ( work functions ) for electrode contacts, the electrons and holes ( electron holes ) can be injected into an organic semiconductor. Such contacts are the basis for the charge injection in all modern OLED devices.

In 1963, also Pope group detected for the first DC voltage (DC) luminescence under vacuum to a pure anthracene single crystal and tetracene doped anthracene crystals with a small silver electrode at 400 V. This mechanism is based on the field of accelerated electrons excite the molecular fluorescence. Pope group reported in 1965 for an electroluminescence anthracene crystals caused by the recombination of electrons and holes thermalized without an external electric field, and on the other that, when the guide anthracene energy level is higher than the exciton energy level.

Also in 1965 produced Wolfgang Helfrich and WG Schneider of the National Research Council of Canada electroluminescence by double recombination of injected for the first time in an anthracene single crystal by the use of hole and electron -injecting electrodes, the precursors of modern double -injecting devices.

In the same year 1965 from Dow Chemical researchers patented a process for the production of electroluminescent cells from an electrically insulating, 1 mm thin film of the molten phosphorus with incorporated Anthracenpulver, tetracene, and graphite powder, with an alternating voltage ( 100 to 3000 Hz, 500 to 1500 V ) was operated. This mechanism is based on electronic excitation of graphite and anthracene molecules at the contacts.

The performance was limited by the poor electrical conductivity of the former organic materials. This limitation has been improved through the discovery and development of highly conductive polymers. So watching Roger Partridge from the British National Physical Laboratory in 1975 for the first time the electroluminescence of polymer films. The later patented in 1983 and published in a professional journal setup consisted of a up to 2.2 micron thin film of poly (N- vinylcarbazole ) between two charge injecting electrodes.

Ching W. Tang and Van Slyke of Steven Eastman Kodak Company have reported for the first time in 1987 from a diode structure. In this case, a novel two-layer structure with a separate hole and electron - transporting layer is used, so that recombination and light emission in the middle of the organic layer occurred. This led to a lower operating voltage and higher efficiency and set the transition to today's OLED research and production dar.

JH Burroughes et al in 1990, developed by the University of Cambridge efficient, green light emitting device with the use of 100 nm thin film of poly (p -phenylene vinylene). In 1996, the first device with a luminous polymer of Cambridge Display Technology ( CDT) has been presented. In November 2006, created a scientist at Pacific Northwest National Laboratory ( PNNL ), a blue OLED with a quantum yield of 11 % at 800 cd / m².

State of the art

OLEDs could be used in many applications to replace the commonly used today LCDs and plasma screens. The life span are still some problems to, because the red, green and blue phosphor dots age at different rates. Through this irregular aging of the individual colors it comes in the overall picture over time to color shifts by a limited - ideally automatic - readjustment can be compensated (especially on strengthening the blue emission ).

The basic patents for OLED structures date from the 1980s. The Kodak company has been a leader. Since 1980, on the subject of about 6,600 patents known. Research interests are in Japan, South Korea and the United States. Most patents are registered in Japan, followed by the USA and Europe. Germany is about 4.5 % to third place behind the U.S. with about 22%.

Since OLEDs are more expensive than LCDs, they come so far only in special applications. Because of the smaller size, they offer greater design freedom for the device manufacturer. Even the power consumption of OLEDs is often lower because no backlight is required. The main applications of OLED screens are currently in small displays for mobile phones and other, especially small portable devices.

Large screens are not yet available at competitive prices. Problems are mainly the encapsulation of the components and the elaborate control of the pixel dar. In LCDs the control with less power is because LCD pixels are reversed as electrical capacity by an applied voltage only, the light energy is generated by the backlight. In contrast, OLEDs must itself be subjected to the energy required for the light output to produce electroluminescence. They are current-controlled, so the previously used, mature technology of the LCD area can not be directly transmitted.

For small OLED screens, the control over a so-called passive matrix can be made: A given pixel is activated by applying a voltage to a row and column, for which two lines are necessary. For large screens, this method is not sufficient. The main reason why a passive matrix for large screens is inappropriate, is that the bulk resistances have greatly increased, so that the driving force is no longer sufficient to drive the respective pixels. To control the screen active matrix has to be used here, in which each pixel is addressed individually via its own transistor, making four lines necessary; Derived from active-matrix OLED ( active matrix organic german light emitting diode) Samsung sells this technique under the name AMOLED or Super AMOLED development. The provision of switching (voltage signals ) as well as supply current is (as with plasma screens ) consuming and therefore very expensive and one of the main reasons for the high cost of large screens.

The technique is regarded as the latest Super AMOLED , also sold by Samsung; here the PenTile matrix was removed, so now each pixel has all three primary colors. Thus, more will be " merged " with no PenTile matrix several pixels to mix all the colors. Due to this change, the resolution of such displays affects significantly higher, and there stand out individual pixels. Further improvements are better black levels, increased contrast, more colors to be displayed, lower power consumption and reduced thickness of the display. However, the PenTile effect at very high pixel densities is well in excess of 300 ppi ( pixels per inch ) is not or hardly noticeable upon close inspection. This is one of the reasons why Samsung can use with newer products to full HD still SuperAMOLED screens, without fear of a reduced quality. Likewise, this kind of screens are more energy efficient, since approximately one third of the sub-pixel is missing, that need not be energized.

The main supplier of OLED technology are the company Osram, Philips, Sony, LG, Samsung SDI, RiTdisplay, Univision, Pioneer and TDK. Philips and Osram increased in 2004 and 2007 out of the display business and they only produce OLED bulbs.

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