Fundamentals of Organic Luminescence

Transcription

1 Fundamentals of Organic Luminescence S.M. Sawde Applied Physics dept., Priyadarshini Institute of Engineering and Technology, Nagpur, India R.R.Patil, Institute of Forensic Science, R.T.Road Civil Lines, Nagpur, India S.V Moharil Department of Physics, R.T.M Nagpur University, Nagpur, India Abstract Luminescence is observed in inorganic as well as organic materials. In inorganic materials activators plays important role in imparting luminescent properties. The activators which are used are mostly rare earth ions or the 3d elements. Several inorganic phosphors have been developed and are routinely used in day to day life. However inorganic phosphors have certain disadvantages. They are not cost effective. Large displays cannot be made easily as it is very difficult to make large size films. The electroluminescent display requires more power. With the miniaturization of electronic devices it is desired to have displays which are cost effective, requires less power to operate, efficient, colorful. For such applications phosphors based on Organic materials are suitable. Keywords Organic Luminescence, OLEDs I. LUMINESCENCE Luminescence is emission of light by a substance not resulting from heat; it is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light emitted by a substance as a result of heating. II. TYPES OF LUMINESCENCE A. Fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. The most striking example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, while the emitted light is in the visible region, which gives the fluorescent substance a distinct color that can only be seen when exposed to UV light. Fluorescence has many practical applications, including mineralogy, gemology, chemical sensors (fluorescence spectroscopy),fluorescent labelling, dyes, biological detectors, cosmic-ray detection, and, most commonly, fluorescent lamps. B. Phosphorescence Phosphorescence is a specific type of photoluminescence related to fluorescence. In simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for "glow-in-the-dark" materials which are "charged" by exposure to light.commonly seen examples of phosphorescent materials are the glow-in-the-dark toys, paint, and clock dials that glow for some time after being charged with a bright light such as in any normal reading or room light. Organic luminescence Luminescence is the phenomenon in which a substance absorbs energy in some form or the other and re-emits a fraction of it as visible or near-visible radiation. Evidently it is a two step process- the excitation of the electronic system of the substance and the subsequent emission of the photons; these steps may not be separated by intermediate processes. However it is a broad term; several categories of luminescence are possible in crystals, depending upon the means employed to excite the electrons. These categories include: Luminescence is observed in inorganic as well as organic materials. In inorganic materials activators plays an important role in imparting luminescent properties. The activators which are used are mostly rare earth ions or the 3d elements. Several inorganic phosphors have been developed and are routinely used in day to day life. However inorganic phosphors have certain disadvantages. They are not cost effective. Large displays cannot be made easily as it is very difficult to make large size films. The electroluminescent display requires more power. With the miniaturization of electronic devices it is desired to have displays which are cost effective, requires less power to operate, efficient, colorful. For such

2 applications phosphors based on Organic materials are suitable. Luminescence in organic molecules originates from the excitation of Pi electrons. The organic materials are held together by Vander Walls forces between molecules and are therefore molecular solids. The consequence of this molecular nature is that the luminescence processes in organic materials are associated with the excited states of molecules. Those hydrocarbons which contain double or triple bonds between the carbon atoms, that is the unsaturated hydrocarbons, commonly give rise to strong luminescence emission. It is the excited states of electrons systems which are of interest for organic luminescence and in particular double bonded molecules such as aromatic hydrocarbons. These electrons are less tightly bound to their parent carbon nuclear than the localized 6 electrons and those require less energy to excite them. It is very easy to excite the pi electrons when the host forms complex with metal ions resulting the efficient luminescence from the formed complexes. The metal ions could be alkali - alkaline earth ion, transition metal ion or lanthanide ion. III. INTRODUCTION OF OLED OLED is a series of organic films that, when activated with electricity, will emit light. 1. An organic light-emitting diode (OLED) is a lightemitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. 2. Passive matrix OLED (PMOLED)- PMOLED display uses a fairly simple control scheme in which you control one row or line sequentially. PMOLED displays don t make use of a storage capacitor so the pixels are actually off for a majority of the time. The lack of a storage capacitor requires that more voltage be used to make the display brighter. Higher voltage causes for a shorter lifetime of the OLED. The more lines you add the more voltage you have to use. PMOLED displays are restricted in resolution and size so as OLED display technology advances PMOLED will unlikely be used to create large displays. Generally PMOLED displays there are a restriction on resolution and size. Usually up to 3 displays are used to display small character data or icons. 3. Active matrix OLED (AMOLED)- displays require a thin film transistor backplane to switch each pixel on or off. This scheme allows for larger display sizes and higher resolution.currently low temperature polycrystalline silicon TFT backplanes are used. A. Structure Bottom or top distinction refers not to orientation of the OLED display, but to the direction that emitted light exits the device. OLED devices are classified as bottom emission devices if light emitted passes through the transparent or semi-transparent bottom electrode and substrate on which the panel was manufactured. Top emission devices are classified based on whether or not the light emitted from the OLED device exits through the lid that is added following fabrication of the device. Top-emitting OLEDs are better suited for active-matrix applications as they can be more easily integrated with a non-transparent transistor backplane. The TFT array attached to the bottom substrate on which AMOLEDs are manufactured are typically non-transparent, resulting in considerable blockage of transmitted light if the device followed a bottom emitting scheme [1]. 2. 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 contrast, making it much easier to view displays in bright sunlight [2]. This technology can be used in Head-up displays, smart windows or augmented reality applications. 3. Graded Heterojunction Graded heterojunction OLEDs gradually decrease the ratio of electron holes to electron transporting chemicals [3]. This results in almost double the quantum efficiency of existing OLEDs. 4. Stacked OLEDs 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. 5. Inverted OLED In contrast to a conventional OLED, in which the anode is placed on the substrate, an Inverted OLED uses a bottom cathode that can be connected to the drain end of an n-channel TFT especially for the low cost amorphous silicon TFT backplane useful in the manufacturing of AMOLED displays. 1. Bottom or top emission

3 (HOMO and LUMO) of organic semiconductors are analogous to the valence and conduction bands of inorganic semiconductors [5]. Positive charged electron holes and negative charged electrons injected from either side to the organic material. Electron holes and electrons meet and recombine in the emissive layer. The molecules in the organic emissive layer enter an excited high-energy condition (because of point 2). The energy level of the organic emissive layer relaxes and the energy is emitted in the form of light. B. Working Principle Figure.2 Schematic of a bilayer OLED: 1. Cathode ( ), 2. Emissive Layer, 3. Emission of radiation, 4. Conductive Layer, 5. Anode (+) 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 the entire 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 C. HOMO/LUMO PAIRS HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. The energy difference between the HOMO and LUMO is termed the HOMO LUMO gap. HOMO and LUMO are sometimes referred to as frontier orbitals [6]. The difference in energy between these two frontier orbitals can be used to predict the strength and stability of transition metal complexes, as well as the colors they produce in solution [7]. Roughly, the HOMO level is to organic semiconductors what the valence band maximum is to inorganic semiconductors and quantum. The same analogy exists between the LUMO level and the conduction band minimum. All π electrons system contains an even number of π electrons. Since there are two electrons per π bond the ground state of π electrons is singlet state.if the π electrons are excited without change of spin the resultant excited states S 1,S 2,S 3 etc. are also single states. However if the excited π electron suffers a spin reversal between the ground and excited state is a triplet state. Electronic transitions between a singlet and triplet state are formal forbidden by quantum mechanical selection rules. The absorption process therefore occurs, principally between the ground state and the singlet states S 1,S 2,S 3, etc. Excitation to levels higher than S 3 usually results in excitation of 6 electrons and thus the excited state of 6 electrons complete with π electron states for the excitation energy. It should be noted that each tripled state lies below the corresponding singlet state- a situation dictated by Hunds rule.

4 Luminescence may be produced by absorption into any of the excited singlet states S 1S 2S 3 etc. However the primary fluorescence emission occurs from the lowest excited singlet state S 1 irrespective of the initial state recited. The radiative transitions from higher excited states S 2,S 3 etc. are very weak due to the rapid and efficient non-radiative process of internal conversion between S 2,S 3 etc. and the lowest excited state S 1. S1 S0 Fluorescence S1 S0 Internal conversion S1 T1 Intersystem crossing II. array simultaneously. This may not sound like a big difference, but does offer a wide range of benefits including: Lower power consumption - An OLED display doesn't need any of the electronics and circuitry used to drive the LED back light and LCD shutter from a LED display, which makes oleds more efficient. LED screens produce black simply by fully closing the pixel shutter the back light is still shining (it never actually turns off) but the light itself is being blocked. T1 S0 Phosphorescence T1 S0 Intersystem crossing Figure.3 Possible physical process following absorption of a photon by a molecule. OLED VS LED I. The leds in today's LED televisions are actually used only to provide a white back light, which then shines through a rapidlyrefreshing LCD shutter array which tints the emanating light. Oleds, on the other hand, operate as both light source and color III. Better durability and lighter weight - Ditching the back light and shutter arrays also means manufacturers can replace the heavier, shatter-prone glass substrates often used in LED displays with lighter, stronger plastic substrates. And with the advent of injet-based printable oleds, these light producing compounds can be applied to more exotic and malleable surfaces. Additionally, the OLED films themselves are quite durable and can withstand a wider operating temperature range than regular leds without failing [19]. I. ADVANTAGES OF OLED I. Lower cost in the future Oleds can be printed onto any suitable substrate by an inkjet printer or even by screen printing [8] theoretically

5 making them cheaper to produce than LCD or plasma displays. However, fabrication of the OLED substrate is more costly than that of a TFT LCD, until mass production methods lower cost through scalability. Roll-to-roll vapour-deposition methods for organic devices do allow mass production of thousands of devices per minute for minimal cost, although this technique also induces problems in that devices with multiple layers can be challenging to make because of registration, lining up the different printed layers to the required degree of accuracy. II. Lightweight and flexible plastic substrates OLED displays can be fabricated on flexible plastic substrates leading to the possible fabrication of flexible organic light-emitting diodes for other new applications, such as roll-up displays embedded in fabrics or clothing. As the substrate used can be flexible such as polyethylene terephthalate (PET) [9] the displays may be produced inexpensively. Further, plastic substrates are shatter resistant, unlike glass displays used in LCD devices. IV. OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90 from normal. Better power efficiency and thickness Lcds filter the light emitted from a backlight, allowing a small fraction of light through. So, they cannot show true black. However, an inactive OLED element does not produce light or consume power, thus allowing true blacks. [64] Dismissing the backlight also makes oleds lighter because some substrates are not needed. This allows electronics potentially to be manufactured more cheaply, but, first, a larger production scale is needed, because oleds still somewhat are niche product [10.] When looking at top-emitting oleds, thickness also plays a role when talking about index match layers (imls). Emission intensity is enhanced when the IML thickness is nm. The refractive value and the matching of the optical imls property, including the device structure parameters, also enhance the emission intensity at these thicknesses[11]. III. Wider viewing angles and improved brightness Oleds can enable a greater artificial contrast ratio (both dynamic range and static, measured in purely dark conditions) and a wider viewing angle compared to lcds because OLED pixels emit light directly. V. Response time OLED response times are up to 1,000 times faster than LCD according to LG [12] putting conservative estimates at under 10 μs (0.01 ms), which in theory could accommodate refresh frequencies approaching 100 khz (100,000 Hz). Due to their extremely fast response time, OLED

6 VI. II. displays can also be easily designed to be strobed, creating an effect similar to CRT flicker in order to avoid the hold behavior used on both lcds and some OLED displays that creates the perception of motion blur [13]. Oleds can be printed onto any suitable substrate using an inkjet printer or even screen printing technologies, Use of flexible substrates could open the door to new applications such as roll-up displays and displays embedded in fabrics or clothing. DISADVANTAGES OF OLED 1) OUTDOOR PERFORMANCE. AS AN EMISSIVE DISPLAY TECHNOLOGY, OLEDS RELY COMPLETELY UPON CONVERTING ELECTRICITY TO LIGHT, UNLIKE MOST LCDS WHICH ARE TO SOME EXTENT REFLECTIVE; E-INK LEADS THE WAY IN EFFICIENCY WITH ~ 33% AMBIENT LIGHT REFLECTIVITY, ENABLING THE DISPLAY TO BE USED WITHOUT ANY INTERNAL LIGHT SOURCE. OLEDS TYPICALLY PRODUCE ONLY AROUND 200 NITS OF LIGHT LEADING TO POOR READABILITY IN BRIGHT AMBIENT LIGHT, SUCH AS OUTDOORS. DISPLAYS WITH SOME DEGREE OF REFLECTIVENESS INCREASE THEIR BRIGHTNESS AS AMBIENT LIGHT INCREASES, SO OVERCOMING UNWANTED SURFACE REFLECTIONS WITHOUT USING ANY ADDITIONAL POWER. 2) WATER DAMAGE. WATER CAN DAMAGE THE ORGANIC MATERIALS OF THE DISPLAYS. THEREFORE, IMPROVED SEALING PROCESSES ARE IMPORTANT FOR PRACTICAL MANUFACTURING. WATER DAMAGE MAY ESPECIALLY LIMIT THE LONGEVITY OF MORE FLEXIBLE DISPLAYS. 3) POWER CONSUMPTION. WHILE AN OLED WILL CONSUME AROUND 40% OF THE POWER OF AN LCD DISPLAYING AN IMAGE WHICH IS PRIMARILY BLACK, FOR THE MAJORITY OF IMAGES, IT WILL CONSUME 60 80% OF THE POWER OF AN LCD. HOWEVER IT CAN USE OVER THREE TIMES AS MUCH POWER TO DISPLAY AN IMAGE WITH A WHITE BACKGROUND SUCH AS A DOCUMENT OR WEBSITE. THIS CAN LEAD TO DISAPPOINTING REAL-WORLD BATTERY LIFE IN MOBILE DEVICES. 4) COLOR BALANCE ISSUES. ADDITIONALLY, AS THE OLED MATERIAL USED TO PRODUCE BLUE LIGHT DEGRADES SIGNIFICANTLY MORE RAPIDLY THAN THE MATERIALS THAT PRODUCE OTHER COLORS, BLUE LIGHT OUTPUT WILL DECREASE RELATIVE TO THE OTHER COLORS OF LIGHT. THIS DIFFERENTIAL COLOR OUTPUT CHANGE WILL CHANGE THE COLOR BALANCE OF THE DISPLAY AND IS MUCH MORE NOTICEABLE THAN A DECREASE IN OVERALL LUMINANCE. THIS CAN BE PARTIALLY AVOIDED BY ADJUSTING COLOUR BALANCE BUT THIS MAY REQUIRE ADVANCED CONTROL CIRCUITS AND INTERACTION WITH THE USER, WHICH IS UNACCEPTABLE FOR SOME USES. IN ORDER TO DELAY THE PROBLEM, MANUFACTURERS BIAS THE COLOUR BALANCE TOWARDS BLUE SO THAT THE DISPLAY INITIALLY HAS AN ARTIFICIALLY BLUE TINT, LEADING TO COMPLAINTS OF ARTIFICIAL-LOOKING, OVER- SATURATED COLORS. 5) SCREEN BURN-IN.

7 UNLIKE DISPLAYS WITH A COMMON LIGHT SOURCE, THE BRIGHTNESS OF EACH OLED PIXEL FADES DEPENDING ON THE CONTENT DISPLAYED. THE VARIED LIFESPAN OF THE ORGANIC DYES CAN CAUSE A DISCREPENCY BETWEEN RED, GREEN, AND BLUE INTENSITY. THIS LEADS TO IMAGE- PERSISTANCE, ALSO KNOWN AS BURN-IN. 6) LIFESPAN The biggest technical problem for oleds is the limited lifetime of the organic materials. In particular, blue oleds historically have had a lifetime of around 14,000 hours to half original brightness (five years at 8 hours a day) when used for flat-panel displays. This is lower than the typical lifetime of LCD, LED or PDP technology each currently rated for about 60,000 hours to half brightness, depending on manufacturer and model. However, some manufacturers displays aim to increase the lifespan of OLED displays, pushing their expected life past that of LCD displays by improving light out coupling. THUS achieving the same brightness at a lower drive current. I. APPLICATIONS Organic Light Emitting Diodes (OLED) is the next big thing in displays and lighting applications. As well as their familiar use in displays they can also be used as a very efficient and soft lighting source. They can be produced in every form, and it is possible to create transparent and flexible panels in different colours and sizes. The OLED is expected to be the next generation lighting and to revolutionalize the whole lighting and space concept. Oleds do not need light distribution elements. Furthermore, in contrast with LED lighting components they are not a point source technology. An OLED lighting panel has ultra thin layers of organic matter! Every layer has a function, but the thickness is less than 1/1000 of a human hair. The thin OLED layer is then sandwiched between positive and negative electrodes, and gives off light when a current is applied [19]. OLED technology is used in commercial applications such as displays for mobile phones and portable digital media players, car radios and digital cameras among others. Such portable applications favor the high light output of oleds for readability in sunlight and their low power drain. Portable displays are also used intermittently, so the lower lifespan of organic displays is less of an issue. Prototypes have been made of flexible and rollable displays which use oleds' unique characteristics. Applications in flexible signs and lighting are also being developed Editorial Policy. MATERIALS USED Molecules commonly used in oleds include organometallic chelates (for example Alq3, used in the organic light-emitting device reported by Tang et al.), fluorescent and phosphorescent dyes and conjugated dendrimers. 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[14].fluorescent dyes can be chosen to obtain light emission at different wavelengths, and compounds such as perylene, rubrene and quinacridone derivatives are often used. Alq3 has been used as a green emitter, electron transport material and as a host for yellow and red emitting dyes.

8 Ir(mppy)3 are currently the focus of research, although complexes based on other heavy metals such as platinum [18]have also been used. REFERENCES Alq3 [15] commonly used in small molecule oleds Polymer light-emitting diodes (PLED), also lightemitting polymers (LEP), involve an electroluminescent conductive polymer that emits light when connected to an external voltage. They are used as a thin film for fullspectrum colour displays. Polymer oleds are quite efficient and require a relatively small amount of power for the amount of light produced. Poly (p-phenylene vinylene), used in the first PLED Phosphorescent organic light emitting diodes use the principle of electro phosphorescence to convert electrical energy in an OLED into light in a highly efficient manner, with the internal quantum efficiencies of such devices approaching 100%.[16] Typically, a polymer such as poly(nvinylcarbazole) is used as a host material to which an organometallic complex is added as a dopant. Iridium complexes [17] such as 1. Kamtekar, K. T.; Monkman, A. P.; Bryce, M. R. (2010). "Recent Advances in White Organic Light-Emitting Materials and Devices (WOLEDs)". Advanced Materials 22 (5): doi: /adma PMID Bardsley, J. N. (2004). "International OLED Technology Roadmap". IEEE Journal of Selected Topics in Quantum Electronics 10: 3 4. doi: /jstqe US , Mark E. Thompson, Stephen R. Forrest, Paul Burrows, "High contrast transparent organic light emitting device display", published "Organic Light-Emitting Diodes Based on Graded Heterojunction Architecture Has Greater Quantum Efficiency". University of Minnesota. Retrieved 31 May Chu, Ta-Ya; Chen, Jenn-Fang; Chen, Szu-Yi; Chen, Chao-Jung; Chen, Chin H. (2006). "Highly efficient and stable inverted bottom-emission organic light emitting devices". Applied Physics Letters89 (5): Bibcode:2006ApPhL..89e3503C.doi: / ) Kho, Mu-Jeong, Javed, T., Mark, R., Maier, E., and David, C. (2008) Final Report: OLED Solid State Lighting - Kodak European Research, MOTI (Management of Technology and Innovation) Project, Judge Business School of the University of Cambridge and Kodak European Research, Final Report presented on 4 March 2008 at Kodak European Research at Cambridge Science Park, Cambridge, UK., pp ) Kamtekar, K. T.; Monkman, A. P.; Bryce, M. R. (2010). "Recent Advances in White Organic Light-Emitting Materials and Devices (WOLEDs)". Advanced Materials 22 (5): doi: /adma PMID ) D'Andrade, B. W.; Forrest, S. R. (2004). "White Organic Light-Emitting Devices for Solid-State Lighting". Advanced Materials 16(18): doi: /adma ) Pardo, Dino A.; Jabbour, G. E.; Peyghambarian, N. (2000). "Application of Screen Printing in the Fabrication of Organic Light-Emitting Devices". Advanced Materials 12 (17): doi: / (200009)12:17<1249::AID- ADMA1249>3.0.CO;2-Y. 9) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. (1992). "Flexible light-emitting diodes made from soluble conducting polymers". Nature 357 (6378): Bibcode:1992Natur G. doi: /357477a0. 10) Wong, William (16 December 2004). "Firefox: A Browser For Embedded Applications". Electronic Design 52 (28): ) Zhang, Mingxiao; Chen, Z.; Xiao, L.; Qu, B.; Gong, Q. (18 March 2013). "Optical design for improving optical properties of top-emitting organic light emitting diodes". Journal of Applied Physics113 (11): Bibcode:2013JAP...113k3105Z.doi: / )"LG 55EM9700" Retrieved ) "Why Do Some OLEDs Have Motion Blur?". Blur Busters Blog (based on Microsoft Research work) Retrieved ) Bellmann, E.; Shaheen, S. E.; Thayumanavan, S.; Barlow, S.; Grubbs, R. H.; Marder, S. R.; Kippelen, B.; Peyghambarian, N. (1998). "New Triarylamine- Containing Polymers as Hole Transport Materials in Organic Light-Emitting Diodes: Effect of Polymer Structure and Cross-Linking on Device Characteristics". Chemistry of Materials 10 (6): doi: /cm980030p.

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