Organic Light Emitting Diodes

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Abstract An OLED is an electronic device made by placing a series of organic thin films between two conductors. When electrical current is applied, a bright light is emitted. OLEDs are lightweight, durable, power efficient and ideal for portable applications. OLEDs have fewer process steps and also use both fewer and lower-cost materials than LCD displays. More and more people believe that OLEDs can replace the current technology in many applications. First part of this seminar is a physical overview of semiconducting properties of organic polymers and takes a deeper insight in their electron structure. Second part of seminar deals with structural OLED properties and phosphorescence material doping phenomena as improvement of efficiency. Then it offers some technical facts, the question of stability of OLEDs and so present and future aspects of use in actual products. Contents 1 Introduction 3 2 The physics behind OLEDs 4 2.1 Organic semiconductors . . . . . . . . . . . . . . . . . . . . . . 4 2.2 SSH model of electronic structure . . . . . . . . . . . . . . . . . 5 2.3 Doping, charge transport . . . . . . . . . . . . . . . . . . . . . . 7 2.4 Light emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3 OLED device structure 10 3.1 Basic PLED - Charge injection . . . . . . . . . . . . . . . . . . . 10 3.2 Basic SMOLED. . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.3 Phosphorescent doping (Singlet, triplet excitons) . . . . . . . . . 12 4 Technical facts, Stability 13 5 Matrix types of OLEDs, Products 14 6 Conclusion 16 7 References 17 2
1 Introduction The basic idea of organic LED (OLED) operation is fairly simple and similar to that of inorganic LED's. Electrons and holes are injected into an "active" region. There, they recombine resulting in photon emission. Electrons are injected in the conduction band of the appropriate organic material and holes are injected into the valence band. These carriers then move by diffusion until they meet and recombine to form excitons. When these excitons decay to their ground state, photon emission occurs. This is the same process as in fluorescence, but here caused by induced electricity and thus called electroluminiscence. In the 1960s the first LEDs on the basis of semi-conducting metals were developed and rapidly found their way to applications. At the same time, the phenomenon of electroluminescence was also observed for organic materials (anthracene single crystals). But it was not until 1987 the research group at Eastman Kodak Research Laboratories reported an organic electroluminescent diode with some remarkable characteristics [1]. By choosing organic instead of inorganic semiconductors in a novel device resembling a conventional p-n diode, they produced intensive electroluminescence while using a low-drive voltage. This efficiency indicated great potential for display applications, and intense research and development efforts on OLED ensued in numerous industrial laboratories, particularly in Japan. A little more than a decade later, the first OLED displays have been commercialized, and the technology is poised to challenge the dominance of liquid-crystal displays (LCDs) in many applications. OLEDs do not have to be manufactured in semiconductor factories (like LEDs). Nor are they limited to relatively small sizes. Organic LEDs can potentially be made with a low-cost printing line, much like you print a newspaper, so very high resolution of displays which consume little power is achievable. OLED devices can be divided into two classes: depending on the type of organic layer [8]: • Small molecule devices (SMOLED) • Organic polymer devices (PLED or LEP). Small-molecule devices are fabricated using vacuum evaporation techniques, whereas polymer structures can be applied using spin-casting or even ink-jet printing techniques. Originating at Eastman Kodak Co. (Rochester, NY), the small-molecule technology has achieved commercialization first. There has been a big advance recently also on PLEDs first discovered by researchers at Cambridge University in 1989. OLEDs based on “small molecules” have tris (8-hydroxyquinolinato) aluminum (Alq3) as the prototype (Figure 1), and only a few derivatives have been proposed so far. Organic materials used for two different types of OLED : Figure 1 : SMOLED prototype – Alq3 PLED prototype - PPV OLEDs based on polymers have poly-paraphenylene-vinylene (PPV) as the prototype (Figure 1), but over the years a number of derivatives of PPV or other polymers have been proposed and used. 3
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Organic Light Emitting Diodes

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