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’sflexible OLED display technology K  Another advantage of OLED displays is thatthey offer better picture quality via greater contrast ratios and viewingangles which can be attributed to direct light that OLEDs emit. Because OLEDdisplays do not employ a backlight, they do not suffer from some of thedrawbacks of LCD displays such as not able to display true blacks correctly andgenerally being thicker than their OLED counterparts. OLEDs, when inactive, donot consume power or emit any light which means they are able to deliver trueblacks.N OLED displays are also lighter than traditional LCD whichcan again be attributed to the lack of a backlight.

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OLED displays also havesignificantly faster response times than LCD displays. LCD displays canfacilitate a refresh of down to 1ms and a refresh rate of 240Hz, however LGhave claimed that OLED displays could potentially reach a stage where they havea response time that is 1,000 times faster than conventional LCD displays (0.001ms). M  However, OLEDs are notwithout their drawbacks. Recently, the efficiency of OLEDs have been underscrutiny in an attempt to reduce the energy usage of OLED devices like lightingsystems and displays. O Fluorescent OLED displays have reached thestage where they are reliable for practical uses, however, because of thenature of electrofluorescene, they can only have a maximum quantum efficiencyof 25% which is the calculated as the amount of photons created per injectedcarrier. This is because, of all the excited-state populations, only the singlet spin states are fluorescent and only make up a minor portion(around 25%).

Q With phosphorescent molecule containing heavy metalsand TADF (Thermally Activated Delayed Fluorescence) materials, a quantumefficiency of 100% is achievable. VW  It is necessary to understand the workingprinciple of OLED displays to properly assess the advantages and disadvantagesof PHOLED (Phosphorescent Oragnic Light-Emitting Diodes)  displays.  History 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 hole injectionin today’s OLED devices. In 1963, they also managed to observe DC (directcurrent) electroluminescence on a solitary crystal of anthracene and on tetracene-dopedanthracene 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, electroluminescencein anthracene can be attributed to the conducting energy level being higher thanexcitation level and to the recombination of thermalized 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 injecting electrodesmade 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 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 either active-matrixOLEDs (AMOLED) or passive-matrix OLEDs (PMOLED) being produced. Withactive-control, a thin-film transistor backplane is used which allows directaccess to each OLED in the display which means they can be switched on and offindependently. A passive-matrix control scheme controls each row and line ofthe display sequentially. AMOLED offers more advantages than PMOLED as itallows for facilitates larger display sizes at higher resolutions.G Conventional OLEDs consist of an organic layerplaced in between two electrodes, all of this structure is situated on asubstrate. As a consequence of the delocalization of pi electrons, the organicmolecules are able to conduct electricity.

The materials used in the OLED areregarded as organic semiconductors as they have various levels of conductivity,from conductors to insulators. P Fig 5: Thestructure of an OLEDH Fig 6: Adiagram depicting how OLEDs emit lightH One of the most simple polymer OLED systemsonly contained one organic layer. This was created by J. H. Burroughes and his colleagues in 1990 and utilized a solitary layer of poly(p-phenylenevinylene).

Z Fig 6: Monomerof poly(p-phenylenevinylene)However, the manufacturing OLEDs that employ multiple layers ispossible which generally leads to better efficiency. As well as conductiveproperties, different materials may be chosen to aid charge injection atelectrodes by providing a more gradual electronic profile,26 orblock a charge from reaching the opposite electrode and being wasted.27 Manymodern OLEDs incorporate a simple bilayer structure, consisting of a conductivelayer and an emissive layer. More recent developments in OLED architectureimproves quantum efficiency (up to 19%) by using a graded heterojunction.

28 In the gradedheterojunction architecture, the composition of hole and electron-transportmaterials varies continuously within the emissive layer with a dopant emitter.The graded heterojunction architecture combines the benefits of bothconventional architectures by improving charge injection while simultaneouslybalancing charge transport within the emissive region.29During operation, a voltage is applied across the OLED such thatthe anode is positive with respect to the cathode. Anodes are picked based uponthe quality of their optical transparency, electrical conductivity, andchemical stability.30 A current of electrons flows through the device fromcathode to anode, as electrons are injected into the LUMO of the organic layerat the cathode and withdrawn from the HOMO at the anode. This latter processmay also be described as the injection of electron holes into the HOMO.Electrostatic forces bring the electrons and the holes towards each other andthey recombine forming an exciton, a bound state ofthe electron and hole.

This happens closer to the emissive layer, because inorganic semiconductors holes are generally more mobile thanelectrons. The decay of this excited state results in a relaxation of theenergy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of thisradiation depends on the band gap of thematerial, in this case the difference in energy between the HOMO and LUMO.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 spinsof the electron and hole have been combined. Statistically three tripletexcitons will be formed for each singlet exciton. Decay from triplet states (phosphorescence) is spin forbidden, increasingthe timescale of the transition and limiting the internal efficiency offluorescent devices. Phosphorescentorganic light-emitting diodes make use of spin–orbitinteractions to facilitate intersystem crossingbetweensinglet and triplet states, thus obtaining emission from both singlet andtriplet states and improving the internal efficiency.Indium tin oxide (ITO)is commonly used as the anode material.

It is transparent to visible light andhas a high work function whichpromotes injection of holes into the HOMO level of the organic layer. A typicalconductive layer may consist of PEDOT:PSS31 asthe HOMO level of this material generally lies between the work function of ITOand the HOMO of other commonly used polymers, reducing the energy barriers forhole injection. Metals such as barium and calcium are often used for the cathode asthey have low work functions whichpromote injection of electrons into the LUMO of the organic layer.32 Suchmetals are reactive, so they require a capping layer of aluminium to avoid degradation.Experimental research has proven that the properties of theanode, specifically the anode/hole transport layer (HTL) interface topographyplays a major role in the efficiency, performance, and lifetime of organiclight emitting diodes. Imperfections in the surface of the anode decreaseanode-organic film interface adhesion, increase electrical resistance, andallow for more frequent formation of non-emissive dark spots in the OLEDmaterial adversely affecting lifetime. Mechanisms to decrease anode roughnessfor ITO/glass substrates include the use of thin films and self-assembledmonolayers. Also, alternative substrates and anode materials are beingconsidered to increase OLED performance and lifetime.

Possible examples includesingle crystal sapphire substrates treated with gold (Au) film anodes yieldinglower work functions, operating voltages, electrical resistance values, andincreasing lifetime of OLEDs.33Single carrier devices are typically used to study the kinetics and charge transport mechanismsof an organic material and can be useful when trying to study energy transferprocesses. As current through the device is composed of only one type of chargecarrier, either electrons or holes, recombination does not occur and no lightis emitted. For example, electron only devices can be obtained by replacing ITOwith a lower work function metal which increases the energy barrier of holeinjection. Similarly, hole only devices can be made by using a cathode madesolely of aluminium, resulting in an energy barrier too large for efficientelectron injection.343536 An interesting area of research is the use ofelectrophosphorescent Pt(II) complexes as a substitute to traditionalfluorescent compounds which are common today. PhosphorescentOLEDsRather like OLEDs, PMOLEDs 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.

Electrons and holes are both fermions with half integer spin. An exciton is formed by the coulombic attraction betweenthe electron and the hole, and it may either be in a singlet state or a triplet state, depending on the spin states ofthese two bound species. Statistically, there is a 25% probability of forming asinglet state and 75% probability of forming a triplet state.23 Decay of the excitonsresults in the production of light through spontaneous emission.

In OLEDs using fluorescent organic molecules only, thedecay of triplet excitons is quantum mechanically forbidden by selection rules, meaning that the lifetime oftriplet excitons is long and phosphorescence is not readily observed. Hence itwould be expected that in fluorescent OLEDs only the formation of singletexcitons results in the emission of useful radiation, placing a theoreticallimit on the internal quantum efficiency (thepercentage of excitons formed that result in emission of a photon) of 25%.4However, 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. Platinum(II) complexes havebeen used as phosphorescent emitters in small-molecule OLEDs.4,12,20–25 Sincethe first phosphorescent OLED was reported with2,3,7,8,12,13,17,18octaethyl-21H,23H-porphine platinum(II) (PtOEP) as a redemissive dopant,4 platinum(II) complexes have been used to prepare OLEDs thatgive green, red, and even white EL with external quantumefficiencies as high as16.

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