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An organic light-emitting diode OLED or organic LED , also known as organic electroluminescent organic EL diode , [1] [2] is a light-emitting diode LED in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. This organic layer is situated between two electrodes ; typically, at least one of these electrodes is transparent.
OLEDs are used to create digital displays in devices such as television screens, computer monitors , and portable systems such as smartphones and handheld game consoles. A major area of research is the development of white OLED devices for use in solid-state lighting applications. There are two main families of OLED: those based on small molecules and those employing polymers. In the PMOLED scheme, each row and line in the display is controlled sequentially, one by one, [6] whereas AMOLED control uses a thin-film transistor TFT backplane to directly access and switch each individual pixel on or off, allowing for higher resolution and larger display sizes.
In LEDs doping is used to create p- and n- regions by changing the conductivity of the host semiconductor. OLEDs do not employ a p-n structure. Doping of OLEDs is used to increase radiative efficiency by direct modification of the quantum-mechanical optical recombination rate.
Doping is additionally used to determine the wavelength of photon emission. An OLED display works without a backlight because it emits visible light. Thus, it can display deep black levels and can be thinner and lighter than a liquid crystal display LCD.
OLED displays are made in the same way as LCDs, but after TFT for active matrix displays , addressable grid for passive matrix displays or indium-tin oxide ITO segment for segment displays formation, the display is coated with hole injection, transport and blocking layers, as well with electroluminescent material after the first 2 layers, after which ITO or metal may be applied again as a cathode and later the entire stack of materials is encapsulated.
They applied high alternating voltages in air to materials such as acridine orange dye, either deposited on or dissolved in cellulose or cellophane thin films. The proposed mechanism was either direct excitation of the dye molecules or excitation of electrons. In , Martin Pope and some of his co-workers at New York University developed ohmic dark-injecting electrode contacts to organic crystals.
These contacts are the basis of charge injection in all modern OLED devices. Pope's group also first observed direct current DC electroluminescence under vacuum on a single pure crystal of anthracene and on anthracene crystals doped with tetracene in [17] using a small area silver electrode at volts. The proposed mechanism was field-accelerated electron excitation of molecular fluorescence.
Pope's group reported in [18] that in the absence of an external electric field, the electroluminescence in anthracene crystals is caused by the recombination of a thermalized electron and hole, and that the conducting level of anthracene is higher in energy than the exciton energy level. Also in , Wolfgang Helfrich and W. Schneider of the National Research Council in Canada produced double injection recombination electroluminescence for the first time in an anthracene single crystal using hole and electron injecting electrodes, [19] the forerunner of modern double-injection devices.
In the same year, Dow Chemical researchers patented a method of preparing electroluminescent cells using high-voltage — V AC-driven — Hz electrically insulated one millimetre thin layers of a melted phosphor consisting of ground anthracene powder, tetracene, and graphite powder.
It used a film of poly N-vinylcarbazole up to 2. The light generated was readily visible in normal lighting conditions though the polymer used had 2 limitations; low conductivity and the difficulty of injecting electrons.
His contribution has often been overlooked due to the secrecy NPL imposed on the project. When it was patented in [22] it was given a deliberately obscure "catch all" name while the government's Department for Industry tried and failed to find industrial collaborators to fund further development. Research into polymer electroluminescence culminated in , with J.
Burroughes et al. Kido et al. They announced the world's first 2. 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 part or all of the molecule. These materials have conductivity levels ranging from insulators to conductors, and are therefore considered organic semiconductors.
The highest occupied and lowest unoccupied molecular orbitals 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 synthesised by J. 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, [43] or block a charge from reaching the opposite electrode and being wasted. 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. Anodes are picked based upon the quality of their optical transparency, electrical conductivity, and chemical stability. 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 electron-transport layer part of 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.
As electrons and holes are fermions with half integer spin , an exciton may either be in a singlet state or a triplet state depending on how the spins of the electron and hole have been combined. Statistically three triplet excitons will be formed for each singlet exciton. Decay from triplet states phosphorescence is spin forbidden, increasing the timescale of the transition and limiting the internal efficiency of fluorescent devices.
Phosphorescent organic light-emitting diodes make use of spin—orbit interactions to facilitate intersystem crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and improving the internal efficiency. Indium tin oxide ITO is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the HOMO level of the organic layer.
Metals such as barium and calcium are often used for the cathode as they have low work functions which promote injection of electrons into the LUMO of the organic layer. Two secondary benefits of the aluminum capping layer include robustness to electrical contacts and the back reflection of emitted light out to the transparent ITO layer. Imperfections in the surface of the anode decrease anode-organic film interface adhesion, increase electrical resistance, and allow for more frequent formation of non-emissive dark spots in the OLED material adversely affecting lifetime.
Also, alternative substrates and anode materials are being considered to increase OLED performance and lifetime. Possible examples include single crystal sapphire substrates treated with gold Au film anodes yielding lower work functions, operating voltages, electrical resistance values, and increasing lifetime of OLEDs.
Single carrier devices are typically used to study the kinetics and charge transport mechanisms of an organic material and can be useful when trying to study energy transfer processes. As current through the device is composed of only one type of charge carrier, either electrons or holes, recombination does not occur and no light is emitted.
For example, electron only devices can be obtained by replacing ITO with a lower work function metal which increases the energy barrier of hole injection. Similarly, hole only devices can be made by using a cathode made solely of aluminium, resulting in an energy barrier too large for efficient electron injection. Balanced charge injection and transfer are required to get high internal efficiency, pure emission of luminance layer without contaminated emission from charge transporting layers, and high stability.
A common way to balance charge is optimizing the thickness of the charge transporting layers but is hard to control. Another way is using the exciplex. Exciplex formed between hole-transporting p-type and electron-transporting n-type side chains to localize electron-hole pairs. Energy is then transferred to luminophore and provide high efficiency. An example of using exciplex is grafting Oxadiazole and carbazole side units in red diketopyrrolopyrrole-doped Copolymer main chain shows improved external quantum efficiency and color purity in no optimized OLED.
Tang et al. Molecules commonly used in OLEDs include organometallic chelates for example Alq 3 , used in the organic light-emitting device reported by Tang et al. A number of materials are used for their charge transport properties, for example triphenylamine and derivatives are commonly used as materials for hole transport layers.
The 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. However, contrary to polymer-based devices, the vacuum deposition process enables the formation of well controlled, homogeneous films, and the construction of very complex multi-layer structures.
This high flexibility in layer design, enabling distinct charge transport and charge blocking layers to be formed, is the main reason for the high efficiencies of the small molecule OLEDs. Researchers report luminescence from a single polymer molecule, representing the smallest possible organic light-emitting diode OLED device.
Finally, this work is a first step towards making molecule-sized components that combine electronic and optical properties. Similar components could form the basis of a molecular computer. They are used as a thin film for full-spectrum colour displays. Polymer OLEDs are quite efficient and require a relatively small amount of power for the amount of light produced.
Vacuum deposition is not a suitable method for forming thin films of polymers. However, polymers can be processed in solution, and spin coating is a common method of depositing thin polymer films. This method is more suited to forming large-area films than thermal evaporation. No vacuum is required, and the emissive materials can also be applied on the substrate by a technique derived from commercial inkjet printing.
The metal cathode may still need to be deposited by thermal evaporation in vacuum. An alternative method to vacuum deposition is to deposit a Langmuir-Blodgett film.
Typical polymers used in PLED displays include derivatives of poly p -phenylene vinylene and polyfluorene. Substitution of side chains onto the polymer backbone may determine the colour of emitted light [63] or the stability and solubility of the polymer for performance and ease of processing.
Typically, a polymer such as poly N-vinylcarbazole is used as a host material to which an organometallic complex is added as a dopant. Iridium complexes [71] such as Ir mppy 3 [69] as of were a focus of research, although complexes based on other heavy metals such as platinum [70] have also been used. The heavy metal atom at the centre of these complexes exhibits strong spin-orbit coupling, facilitating intersystem crossing between singlet and triplet states.
By using these phosphorescent materials, both singlet and triplet excitons will be able to decay radiatively, hence improving the internal quantum efficiency of the device compared to a standard OLED where only the singlet states will contribute to emission of light. It had a transparent anode fabricated on a glass substrate, and a shiny reflective cathode.
Light is emitted from the transparent anode direction. To reflect all the light towards the anode direction, a relatively thick metal cathode such as aluminum is used. For the anode, high-transparency indium tin oxide ITO was a typical choice to emit as much light as possible. The downside of bottom emission structure is that the light has to travel through the pixel drive circuits such as the thin film transistor TFT substrate, and the area from which light can be extracted is limited and the light emission efficiency is reduced.
An alternative configuration is to switch the mode of emission. A reflective anode, and a transparent or more often semi-transparent cathode are used so that the light emits from the cathode side, and this configuration is called top-emission OLED TE-OLED. Unlike BEOLEDs where the anode is made of transparent conductive ITO, this time the cathode needs to be transparent, and the ITO material is not an ideal choice for the cathode because of a damage issue due to the sputtering process.
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