Review Of Organic Light Emitting Diode Production And Uses

1. Introduction The production and use of organic light emitting diodes (OLEDs) has, in recent years, become an extremely important area of research in both physics and chemistry. This is due to the fact that efficiency and image quality in monitors and other forms of screens now plays an essential part in both business and the entertainment industries. They are also very versatile in their applications and can be incorporated into most materials. The idea of OLEDs first arose from the organic electroluminescence of crystals and the efficient emission of multilayer materials[1]. Initially most research was aimed at accurately tuning the wavelength of OLEDs and maximising the emission intensity, however new research tends to look at improving the mediation of charge carriers (in terms of injection and blocking), making more thermally stable OLEDs and also increasing their operational lifetime[1]. The most common use of OLEDs is in the production of screens for televisions, mobile phones, tablet computers and laptop monitors. However more recent research has looked into making transparent screens, that is, screens which display information on what would appear to be a thin layer of glass. An example of this is the transparent screen Microsoft demonstrated in February 2012[2]. This is a good example of the more obscure uses of OLEDs; it has been designed such that the user can `put their hands into the screen` and use motion detection to control the 3D applications running on the device. It is products like these that have fuelled the research in this field as much of the revenue required to fund the research will come from companies building these products; companies want to develop these products as in the modern world new technology is one of the more lucrative markets.

2. Theory of Organic Light Emitting Diodes 2.1. How OLEDs Work The basic structure of an OLED is multilayered, a potential is passed through a series of layers and this leads to the organic electroluminescence. From the negative end of the cell, a current passes into a low workfunction cathode, then into the electron transport/emissive layer, through the hole transport layer and finally into the transparent anode before returning back to the cell[1]. When a forward bias is used, holes are injected from the anode into the hole transport layer causing it to become oxidized, and electrons are injected into the electron transport layer causing it to become reduced. The charge carriers move in the applied electric field and recombine, creating bound excited electron hole pairs in the emissive layer. These pairs settle back to the ground state by radiative decay. A basic energy band diagram for these processes can be seen in figure 1.

2.2. Production of OLEDs. The material used to make an OLED can vary both the intensity and wavelength of the emitted photons, therefore it is essential to use the correct compounds when designing an OLED. With reference to the emission spectrum, most OLEDs aim to have a broad emission spectrum so that they can be used to make full colour displays or for backlighting for other types of screen[6]. This is achieved by using a combination of different materials. An example of this is aluminium tris(8-hydroxyquinoline) (Alq) which has an emission spectrum spanning 450-650 nm, combined with a thin layer of 2-napthyl-4-,-bis(4-methoxyphenyl)-1,3-oxazole (NAPOXA), shifts the overall emission spectra towards the blue end of the spectrum. The thickness of the thin layer is normally 20 nm as seen in figure 2. The dichloromethane (DCM) doped Alq is added to boost the red component of the emission and thus attempt to generate white light[3].

2.3. Implementation of OLEDs into products On a large scale a screen of OLEDs is made by using a large sheet of conductive glass, a sheet of cathode, and the multilayered OLED mixture injected between the two, this is then covered with a sheet of non-conducting glass for safety and then placed into a case. In some cases a transparent cathode can be used such that the screen is transparent. Foldable OLEDs can also be made using flexible foils and plastics, this means that not only can they be used for screens but can also be incorporated into fabrics for fashion, used for round displays and several other applications. White OLEDs are beginning to be used for lighting and are more efficient than fluorescent lights. They also have more balanced spectra of colours and can therefore produce truly incandescent lighting.

4. Major Results into the Research of Organic Light Emitting Diodes 4.1. White OLEDs Ko and Tao in 2001 produced a very bright white OLED (WOLED) by controlling the layer thicknesses and using the correct ratio of dopant to Alq. They found that their WOLED had luminance of 24,700 cd m-2 and efficiency of 1.93 lm/W at 6.5V.[8] In 2009 research results were revealed into the efficiency of WOLEDs. The investigation looked into the power efficiency in lm W-1 of four different devices as a function of the luminance in cd m-2. It was found that in comparison with a fluorescent tube, WOLEDs using an index-matched half-sphere to couple the light into the air were up to twice as efficient, this can be seen in figure 4[5].

4.2. Red OLEDs[7] In 2003 new iridium complexes were tested as red-orange emitters for OLEDs and were found to be highly efficient. The iridium dibenzo[f,h]quinoxaline acetylacetonate [Ir(DBQ)2(acac)] and iridium 2-methyldibenzo[f,h]quinoxaline acetylacetonate [Ir(MDQ)2(acac)] complexes were found to have strong luminescence at 618nm and 608nm wavelengths. The emission maximum for these devices is found to be 610-612nm, however this is dependent on the concentration of the iridium complex. The quantum efficiency of these devices ranged from 7.8% to 12.4% and the luminosities ranged from 45,440 cd m-2 at 13.6V, to 73,870 cd m-2 at 12.8V. The conclusion of this investigation was the synthesis of two ligands with very high brightness and power efficiencies.

4.3. Flexible Active-Matrix Organic Light Emitting Diode (AMOLED) Displays[9,10] Figures 5 and 6 show examples of the progression of flexible screens using OLEDs. It can be seen that the in the space of two years much progress had been made in the properties of these screens. In 2004 the TFT screen was made of inorganic materials with an organic electronic ink layered over the top, yet in 2006 the first organic TFT with OLED display was created. The lifetimes of these displays were as low as 21 hours yet paved the way for the development of flexible AMOLED displays. Flexible displays are useful as advertising tools but predominantly are used in mobile phone screens, this is because they are more durable than rigid screens and can endure far more physical stress[9,10].

4.4. Progression of the Field. The four papers referenced in the section demonstrate how the field began to grow and how it has branched out into different areas. None of the above uses have been perfected as of yet and because of the vast amount of OLEDs uses, many research groups are working towards making better screens, brighter WOLEDs and long living OLEDs. Each of the above papers has influenced the field in its own way; the initial stages of WOLEDs have led towards considering OLEDs as a more efficient form of lighting. The orange-red OLEDs have raised questions on how to improve the broadness of the emission spectra, and how to make each area of the spectrum more intense. The two papers on flexible screens have changed the mobile phone and computer industry, creating screens that are both highly durable, highly efficient and of high definition.

5. The Future of Organic Light Emitting Diodes In conclusion, what started off as a simple discovery of the organic electroluminescence of crystals has branched out and become a major area of science. The research that has gone into finding more efficient, brighter and wide spectra OLEDs has taken many years, whilst creating a perfect OLED will more than likely take many more years to come. The use of OLEDs has also spread through many technologies, from something as simple as a household light to the transparent screens mentioned earlier in this report. The field of OLEDs is far from petering out and in the future it is predicted OLEDs with much greater efficiencies, spectra and brightness will be discovered. It is also believed that more and more products will begin to use them as an alternative to other forms of projection or lighting and possibly for applications such as heads up displays on motor vehicles. With the global concern over energy consumption and production, OLEDs are definitely a step towards a brighter future.

6. References [1] Veinot J G C and Marks T J 2005 Toward the ideal light-emitting diode. The versatility and utility of interfacial tailoring by cross-linked siloxane interlayers Acc. Chem. Res. 38 632-43 [2] Wired Uk, 2012, Microsoft demos Kinect-powered transparent 3D desktop [online] [3] Dodabalapur A, Rothberg L J, Jordan R H, Miller T M, Slusher R R and Phillips J M 1996 Physics and applications of organic microcavity light emitting diodes J. App. Phys. 80 6954-64 [4] OLED-Info, 2012, OLED Companies [online] [5] Reineke S, Lindner F, Schwartz G, Seidler N, Walzer K, Lussem B and Leo K 2009 White organic light-emitting diodes with fluorescent tube efficiency Nat. 459 234-U116 [6] Chen C T 2004 Evolution of red organic light-emitting diodes: Materials and devices Chem. Mat. 16 4389-400 [7] Duan J P, Sun P P and Cheng C H 2003 New iridium complexes as highly efficient orange-red emitters in organic light-emitting diodes Adv. Mat. 15 224-28 [8] Ko C W and Tao Y T 2001 Bright white organic light-emitting diode App. Phys. Let. 79 4234-36 [9] Gelinck G H, Huitema H E A, Van Veenendaal E, et. al 2004 Flexible active-matrix displays and shift registers based on solution-processed organic transistors Nat. Mat. 3 106-10 [10] Zhou L S, Wanga A, Wu S C, Sun J, Park S and Jackson T N 2006 All-organic active matrix flexible display App. Phys. Let. 88 083502 [11] Ohmori Y, Kajii H, Sawatani T, Ueta H and Yoshino K 2001 Enhancement of electroluminescence utilizing confined energy transfer for red light emission Thin Solid Films 393 407-11