专业英语-王宁(P1505085208)-材料科学与工程学院.docx

研究生课程论文 学院：材料科学与工程学院年级：研究生一年级姓名学号：课程论文成绩评语：任课老师签字： 年 月 日White Organic Light-Emitting DiodesFirst grade graduate, optical engineering, WangNing(P1505085208)Abstract: A simplified white organic light-emitting diodes (WOLEDS) employing a p-type interlayer is designed, simultaneously realizing high efficiency, high color rendering index (CRI), low efficiency roll-off, low voltage and stable color. As flat-panel light sources they are intrinsically glare-free and generate light over a large area. But these improvements require joint research efforts in chemistry and the materials sciences to design better materials as well as in physics and engineering to invent new device concepts and design suitable fabrication schemes. This article reviews its current developments in the field of WOLEDs and puts a special focus on the future trends.Keywords: White Organic Light-Emitting Diodes; color-rendering index；external quantum efficiency.1 Introduction Device efficiency is the single most important parameter describing the performance of any light source and is typically measured in lumens per watt (lmW1), i.e., the integral light output per consumed electrical power. This measure for the efficiency is a photometric unit, meaning that it is scaled to the sensitivity of the human eye. It should therefore more precisely be referred to as luminous efficacy, although the term efficiency is widely used today. Another frequently used figure-of-merit is the external quantum efficiency (EQE), i.e., the average number of photons emitted per unit charge passed through the device. In order for light to be perceived as white by the human eye, the emission spectrum should closely match the spectrum emitted by an incandescent blackbody with a temperature between 2500 and 6500 K. 1 For instance, sunlight has a color temperature of 5800 K and the color temperature of light emitted by incandescent light bulbs ranges from 2000 to 3000 K. The emission color can also be described by the CIE (Commision Internationale de lclairage) coordinates. The CIE coordinates describe how the human eye perceives the emission color of any light source (with an arbitrary emission spectrum) using a pair of two numbers (x, y). Figure 3shows the CIE coordinates of several reference spectra and a few exemplary WOLED emission spectra in a so-called CIE diagram. Also shown are the CIE coordinates of an incandescent black-body over a range of temperatures (Planckian locus). The CIE coordinates of the WOLEDs with particularly high luminousefficacy tend to be located on the greenish side of the Planckianlocus because the human eye perceives green light as particularly bright. However, since the emission from such devices is characterized by a disturbing green hue, care has to be taken when comparing the efficacy of WOLEDs with substantially different CIE coordinates.Figure 3 A typical CIE diagram that is used to compare the emission color of different OLEDs.The overall efficiency of an OLED is influenced by a variety of factors. When expressed as the external quantum efficiency (EQE), it is given by the product of the charge recombination efficiency (), the ratio of generated singlet and triplet excited states (s/t), the internal electroluminescence quantum efficiency (q), and the fraction of photons extracted from the structure (out). While it is widely assumed that the recombination efficiency approaches unity in todays most efficient device structures, the other factors often introduce significant loss. Forinstance, the outcoupling effi- ciency, out, in conventional devices is only around 20% 2 ,unless special light-extraction schemes are applied. 3-6 The electroluminescence quantum efficiency depends on the active material, but also on its local surroundings and might thus be well-below unity, even for materials with high photoluminescence quantum efficiency. 7 Similar to conventional LEDs, WOLEDs are typically driven by a constant DC current and require a low bias voltage (27V). While this means that they are not directly compatible with the AC power grid, the required voltage conversion can be integrated into the housing of the device at very moderate cost and is much less challenging than the up-conversion required for starting most gas discharge sources, including fluorescent lamps. 2 History DevelopmentThe history of organic light-emitting diodes (OLEDs) that emit white light started some 15 years ago when Kido and co-workers reported that they had succeeded in fabricating OLEDs generating light that contained wavelengths across the entire visible part of the spectrum. 8 Today, researchers throughout the world in both industry and academia are developing these white OLEDs (WOLEDs) for the next generation of solid-state light sources and very recently the first products have become commercially available. Besides their many other interesting properties, WOLEDs are conceptually different from most light sources that are currently on the market in that the light is generated and emitted over a sizeable area, ranging from sub-square centimeter in lab samples to over several square decimeters in current prototypes and potentially up to square-meter dimensions in future products. Therefore, these organic light sources are intrinsically glare-free and can provide very homogenous illumination, even in tight spaces where this is very challenging to achieve using conventional, point-shaped light sources. This feature is likely to allow WOLEDs to enter the lighting market as a complementing technology in the very near future. Initially, their unique properties are likely to find use in exclusive markets where the quality of the light source dominates over other factors such as production cost. Examples where this is currently envisioned include indoor lighting in the automotive and the aviation sector.It seems, however, that complementing the lighting market will only be a first step. Governments around the world have launched serious research initiatives to reduce illumination-related energy consumption which currently constitutes 810% of the electric power consumption in Europe and the United States. 9, 10 The further development of WOLED technology is an integral part of most of these programs since organic lighting is generally considered to offer a range of advantages over competing technologies.3 Situation Analysis Traditionally, the OLED research community has been divided into two groups, each favoring a specific fabrication method and class of materials resulting from that choice. Following the original report on OLEDs, 11 .the gas-phase small-molecule community promotes the deposition of low molecular weight molecules by thermal evaporation under high vacuum. More recently, other gas phase deposition processes, including chemical vapor deposition (CVD) have also received attention because they might allow reduction of the manufacturing cost. 12 .The solution-based-polymer community,which as originally concerned mostly with relatively high-molecular-mass conjugated polymers, 13 is now concerned with all types of organic semi-conductors (including small molecules, oligomers, polymers, and nanoparticles) that can be deposited by solution-based methods, such as spin-coating or, ideally, printing. In recent years, the clear distinction has started to blur and hybrid device concepts combining solution-processable and vapor-deposited materials are increasingly considered as a promising route to cost-efficient, large-scale manufacturing of highly efficient WOLEDs. Related requirement is that the emission spectrum must be continuous so that the color of any object illuminated by the source is sampled properly. The so-called color-rendering index (CRI), which is represented by a number between 0 and 100, usually serves as a measure for this property. A CRI 80 is required for indoor-lighting applications. The broad acceptance of fluorescent tubes and their light bulb analogues, the energy saving lamp, has been hindered by the fact that their emission spectrum is not continuous resulting in unnatural and often irritating color tint. Likewise, the inorganic counter-parts of WOLEDs, the white LEDs, often suffer from a poor CRI since their emission spectrum is formed by the super position of phosphors, typically with relatively narrow emission bands. Since the emission color of WOLEDs can be readily tuned by chemical modification of the electroluminescent organic materials forming the device, WOLEDs have a much better chance of generating emission spectra that are perceived as natural white light by the human eye and current WOLEDs achieve CRI values up to around 90. 14 However, it has been found that maintaining emission color and a CRI that is constant over a wide range of brightness levels is relatively challenging. 15, 16The acceptable manufacturing cost depends on the expected lifetime, the number of devices required to achieve a certain brightness level, and on the specific application. In general, WOLEDs are predicted to be a cost-efficient light source once the technology is fully developed. The amorphous morphology of the involved organic materials renders them compatible with low-cost material processing techniques, such as spin-coating or printing, 17 and with inexpensive substrates, such as glass or even plastic foil. 18 However, todays WOLED prototypes are often fabricated by thermal evaporation of small molecules under high vacuum. Although the required processes are rap-idly improved, this remains technologically challenging and is a relatively expensive and slow process, in particular when used with large substrates. A quicker mass-market entry of WOLED lighting is therefore anticipated if the full low-cost potential of the technology can been harnessed.The environmental burden of any product must be assessed in terms of manufacturing, operation, and disposal. WOLEDs promise relatively low energy consumption during manufacturing as few or no high-temperature processes are involved. The toxic load is also low compared to the processes typically involved in processing inorganic semiconductors. However, purification of the widely used OLED electrode material indium tin oxide (ITO) is not a “green” process and an intense search for alternative electrode materials is currently under way. 19 Researchers also look at reducing the amount of organic sol-vents required during synthesis and processing of the organic semiconductors forming the OLED structure. 20 During its operational lifetime, the device efficiency is the main factor determining the environmental burden. In terms of device disposal, WOLEDs, and all organic semiconductor based devices, are beneficial because they can be combusted, leaving mainly CO2and H2O. Unlike inorganic LEDs and especially fluorescent tubes, OLEDs contain no or much smaller amounts of toxic components such as arsenic or mercury. Incandescent light bulbs are characterized by a poorly reproducible lifetime, which is on average well below 3000h. Their short lifetime is partly compensated by the low manufacturing cost and low environmental burden upon disposal. However, the replacement process itself can be expensive and there seems to be wide agreement that future light sources need to exceed the lifetime of incandescent light bulbs by one to two orders of magnitude. Although achieving high operational lifetimes has been very challenging for OLEDs, dramatic improvements were realized over the past few years by further purification of the materials and by careful optimization of the device structure and fabrication processes. Today, OLEDs achieve lifetimes in excess of 30 000 h at 5000 cd m 2, which is a brightness level that is more than sufficient for indoor illumination. 4 Future TendencyIn the future, attention should be paid to the quality of the white light emitted by WOLEDs. Although many devices feature high CRIs, which in many cases exceed those achieved by fluorescent tubes, the emission is often relatively far-off the Planckian locus. This results in unacceptable (often greenish) hues of the emitted light. Matching commonly accepted white-points, such as the D65 point, while maintaining a good CRI, might prove particularly challenging for all-phosphorescent devices, which will require stable deep-blue-emitting phosphors to meet this specification. It is now widely accepted that efficient WOLEDs require harvesting of the triplet states by phosphorescent emitters. However, it is still unclear whether all-phosphorescent architecturesor triplet-recycling devices with blue singlet emitters will ultimately reach the best balance of quantum efficiency, operating voltage, and long-term stability. Some attention should be paid to this field.5 ConclusionsWOLEDs have seen rapidly increasing levels of attention in industry and academia over the past years and their performance, particularly in terms of device efficiency, has greatly improved. Todays most efficient devices outperform both incandescent light bulbs and most conventional fluorescent tubes, indicating that organic lighting is a viable concept for general lighting. WOLEDs offer unique form factors that currently cannot be provided by any other efficient light source, which will help to increase customer demand once competitively priced devices reach the market.Material and device fabrication cost remains a technical challenge to the commercial availability of organic lighting. We expect future research to put a particular focus on alternative electrode materials replacing the expensive ITO. Further development of economic deposition methods for organic compounds will also play a major role. In order for these high-throughput fabrication techniques to be successful, future research will have to focus on adapting the materials used in WOLEDs to these processes and vice versa.Reference1.M. S. Rea , L.H., New York 2000.2.N. C. Greenham , R.H.F., D. D. C. Bradley, Adv. Mater, 1994. 6: p. 491.3.A. Khnen , M.C.G., N. Riegel , P. Zacharias , K. Meerholz Appl. Phys, Lett. 2007: p. 91.4.M. Thomschke , R.N., M. Furno , K. Leo