Luminescent Pt (II) complexes and their application in white OLEDs Introduction Display technology is increasingly required toadapt and evolve in order to meet the demands of today’s society.
One of themost promising display technologies, in development and in use currently, isOLED display technology. The desire for efficient OLED displays iswarranted as they offer a range of advantages when compared to othertechnologies. For example, OLED displays can be produced on flexible plastic substrateswhich enables the manufacturing of flexible OLED displays which gives host to awide gamut of potential applications.
J Non-flat OLED displays havealready seen use consumer technology in the production of curved OLED TV’s andsmartphones in Samsung’s “Edge” range of devices. Fig 1: Samsung’s flexible OLED display technologyK Another advantage of OLED displays is thatthey offer better picture quality via greater contrast ratios and viewingangles which can be attributed to the direct light that OLEDs emit. BecauseOLED displays do not employ a backlight, they do not suffer from some of thedrawbacks of LCD displays such as not being able to display true blackscorrectly and generally being thicker than their OLED counterparts. This isbecause OLEDs, when inactive, do not consume power or emit any light whichmeans they are able to deliver true blacks.N OLED displays arealso lighter than traditional LCD which can again be attributed to the lack ofa backlight or refraction panel. OLED displays also have significantly fasterresponse times than LCD displays. LCD displays can facilitate a refresh of downto 1ms and a refresh rate of 240Hz, however LG have claimed that OLED displayscould potentially reach a stage where they have a response time that is 1,000times faster than conventional LCD displays (0.
001ms). M OLEDs are not without their drawbacks. Recently, theefficiency of OLEDs have been under scrutiny in an attempt to reduce the energyusage of OLED devices like lighting systems and displays.
O Whilstfluorescent OLED displays have reached the stage where they are reliable forpractical uses, however, because of the nature of fluorescence, they can onlyhave a maximum quantum efficiency of 25% which is the calculated as the amountof photons created per injected carrier. This is because, of all theexcited-state populations, only the singlet spinstates are fluorescent and only make up a minor portion (around 25%).QAn area of research that is currently of major interest is the usephosphorescent complexes in OLED devices. These devices are known as PHOLEDs(Phosphorescent Organic Light-Emitting Diode) and offer significant advantagesover current OLED devices which are already seen as a major step forward indisplay technology when compared to consumer LCD devices. With phosphorescent molecule containing heavy metals and TADF(Thermally Activated Delayed Fluorescence) materials, a quantum efficiency of100% is achievable. VW Fluorescence vs PhosphorescenceFluorescenceis the core principal by which current consumer OLED devices operate.Fluorescence can described as the absorption of photons by a molecule in thesinglet ground state which are then promoted to a singlet excited state. As themolecule relaxes to the ground state, it release a photon of a lowerwavelength, and therefore lower energy.
ADPhosphorescenceoffers a variation of this principle. A phosphorescent material gradually emitsthe photons it absorbs over a longer period of time than fluorescence which istypically around 10 nS. This can be attributed to electrons undergoingintersystem crossing into an excited triplet state from which the emission oflight via phosphorescence occurs.
Fig 2: Jablonski diagram depicting fluorescence and phosporescenceAD How OLEDswork An OLED (organic light-emitting diode) is anLED that utilizes an organic material as the electroluminescent layer thatproduces light as a response to an electric current. This layer sits betweentwo electrodes where one of the electrodes is typically transparent. OLEDs canbe used as a light source in many devices such as computer monitors, televisionscreens, mobile phones and smart watches, among many other devices. Researchinto the development of white OLEDs for use in solid-state lighting is aparticular area of research which is of major interest. ABC Two main types of OLEDs exist; OLEDs that usesmall males and OLEDs that utilize polymers. Mobile ions can be added to OLEDsto create LECs (light-emitting electrochemical cell) which have a differentmechanism of operation. There are two primary schemes that can be used tocontrol OLED displays and, depending on which one is used, result in eitheractive-matrix OLEDs (AMOLED) or passive-matrix OLEDs (PMOLED) being manufactured.
With active-control, a thin-film transistor backplane is used which allowsdirect access to each OLED in the display which means they can be switched onand off independently. A passive-matrix control scheme controls each row andline of the display sequentially. AMOLED offers more advantages than PMOLED asit allows for facilitates larger display sizes at higher resolutions.G Conventional OLEDs consist of an organic layerplaced in between two electrodes which is situated on a substrate. As aconsequence of the delocalization of pi electrons, the organic molecules areable to conduct electricity.
The materials used in the OLED are regarded asorganic semiconductors as they have various levels of conductivity, fromconductors to insulators. AB Fig 5: Thestructure of an OLEDH Oneof the most simple polymer OLED systems only contained one organic layer. Thiswas created by J. H.
Burroughes andhis colleagues in 1990 and utilized a solitary layer of poly(p-phenylenevinylene).ZFig 6: Monomerof poly(p-phenylenevinylene) Inan OLED, an electric current flows from the cathode to the anode, injecting electronsin the LUMO of cathode which are then withdrawn from the HOMO of the anodewhich is also referred to hole injection. The hole and the electron are broughttogether via electrostatic forces and combine to form an exciton. Because holesmove more freely in organic semiconductors than electrons, this process occurs moreclosely to the emissive layer. When the exciton relaxes it releases radiation inthe visible spectrum producing light which is where OLEDs function as a lightemitting device originates. The difference in HOMOand LOMO energy levels determines the frequency of the light emitted. Fig 7: Adiagram depicting how OLEDs emit lightH Anodesare selectively chosen based on a few key criteria including their chemicalstability optical transparency and their electrical conductivity.
Apopular material that is used for this is indium tin oxide it’s high workfunction encourages the injection of holes in the HOMO of the organic layer andit is transparent AE. Barium and calcium are common cathode materials because of thelow work functions they possess as they encourage the injection of electrons inthe organic layer AF. Manufacturing of OLEDs that employ multiple layers is possible whichgenerally leads to better efficiency.
A range of materials can be used to influenceconductive properties or to potentially improve charge injection at theelectrodes by offering a more contoured electronic profile.AA MostOLED devices in production today use abilayer structure which constituted of a conductive and emissive layer asdepicted in Fig 5. There are a few different OLED architectures which offer different advantages. Aninteresting development in OLED technology that has been shown to improveinternal quantum efficiency, is the implementation of a graded heterojunctionarchitecture. A graded heterojunction acts an interface between the conductiveand emissive layers of an OLED.P This architecture varies the configuration of electron/hole transportmaterials within the emissive layer utilizing a dopant emitter.Thisapproach to device architecture is particularly advantageous as it improvescharge injection and balances charge transport in the emissive region.
Thisapproach to device architecture could potentially yield an internal quantumyield double that of conventional OLED systems.ACAn interesting area of research is the use ofelectrophosphorescent Pt(II) complexes as a substitute to traditionalfluorescent compounds which are common today. Early history of OLED technology Electroluminescence in organic materials wasfirst observed by André Bernanose and his colleagues at the French universityNancy-Université in 1953.
High alternating voltages in air were applied tocompounds like alcidine orange. The compounds were either dissolved in ordeposited on thin cellophane films or cellulose. The initial observations madeattributed the electroluminescence to excitation of electrons or directexcitation of the dye molecules. DEF Martin Pope and his colleagues at New YorkUniversity developed ohmic dark-injecting electrode contacts toorganic crystals in 1960. They also defined the required energetic requirementsfor electrode contacts and electron and hole injection. RST Theelectrode contacts are utilized as the foundation of electron and holeinjection in today’s OLED devices. In 1963, they also managed to observe DC(direct current) electroluminescence on a solitary crystal of anthracene and ontetracene-doped anthracene crystals using a silver electrode at 400 volts. U Fig3: Antrhacene Fig 4: Tetracene Popes group’s research continued and in1965 they observed that when an external electric field is not supplied,electroluminescence in anthracene can be attributed to the conducting energylevel being higher than excitation level and to the recombination ofthermalized hole and electron.
X The first reported observation ofelectroluminescence in polymers was reported by Roger Partridge at the NationalPhysical Laboratory and the paper was published in 1983. A 2.2 µM thick poly(N-vinylcarbazole) film between two charge injectingelectrodes made up the device.
Y The first practical OLED wasmade in 1987 by Steven Van Slyke and Ching W. Tang for the Eastman Kodakcompany and utilized conventional fluorescent materials.O PhosphorescentOLEDsRather like OLEDs, PHOLEDs produce light via electroluminescence ofan organic semiconductor layerin an electric current. Electrons and holes are injected into the organic layerat the electrodes and form excitons, a bound state ofthe electron and hole.However, phosphorescent OLEDs generate light from both tripletand singlet excitons, allowing the internal quantum efficiency of such devicesto reach nearly 100%.5This is commonly achieved by doping a host molecule withan organometallic complex.
These contain a heavy metal atom at the centre of the molecule, for exampleplatinum6 or iridium, of which thegreen emitting complex Ir(mppy)3 isjust one of many examples.1 The large spin-orbitinteraction experienced by the molecule due to this heavy metalatom facilitates intersystem crossing,a process which mixes the singlet and triplet character of excited states. Thisreduces the lifetime of the triplet state,78 therefore phosphorescenceis readily observed.Typically, a polymer such as poly(N-vinylcarbazole) is used as ahost material to which an organometallic complex is addedas a dopant. Iridium complexes54 such asIr(mppy)352 arecurrently the focus of research, although complexes based on other heavy metalssuch as platinum53 have alsobeen used.
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