有機發(fā)光半導體(OLED).docx

有機發(fā)光半導體有機發(fā)光二極管（英文：Organic Light-Emitting Diode，縮寫：OLED）又稱有機電激發(fā)光顯示（英文：Organic Electroluminesence Display，縮寫：OLED）與薄膜晶體管液晶顯示器為不同類型的產(chǎn)品，前者具有自發(fā)光性、廣視角、高對比、低耗電、高反應速率、全彩化及制程簡單等優(yōu)點，有機發(fā)光二極管顯示器可分單色、多彩及全彩等種類，而其中以全彩制作技術最為困難，有機發(fā)光二極管顯示器依驅動方式的不同又可分為被動式（Passive Matrix，PMOLED）與主動式。有機發(fā)光二極管可簡單分為有機發(fā)光二極管和聚合物發(fā)光二極管（polymer light-emitting diodes, PLED）兩種類型，目前均已開發(fā)出成熟產(chǎn)品。聚合物發(fā)光二極管主要優(yōu)勢相對于有機發(fā)光二極管是其柔性大面積顯示。但由于產(chǎn)品壽命問題，目前市面上的產(chǎn)品仍以有機發(fā)光二極管為主要應用。歷史有機發(fā)光二極管技術的研究，起源于鄧青云博士，他出生于香港，于英屬哥倫比亞大學得到化學理學士學位，于1975年在康奈爾大學獲得物理化學博士學位。鄧青云自1975年開始加入柯達公司Rochester實驗室從事有機發(fā)光二極管的研究工作，在意外中發(fā)現(xiàn)有機發(fā)光二極管。1979年的一天晚上，他在回家的路上忽然想起有東西忘記在實驗室，回到實驗室后，他發(fā)現(xiàn)在黑暗中的一塊做實驗用的有機蓄電池在閃閃發(fā)光從而開始了對有機發(fā)光二極管的研究。到了1987年，鄧青云和同事 Steven 成功地使用類似半導體 PN結的雙層有機結構第一次作出了低電壓、高效率的光發(fā)射器。為柯達公司生產(chǎn)有機發(fā)光二極管顯示器奠定了基礎。由此被譽為OLED之父。OLED英文名為Organic Light-Emitting Diode，縮寫：OLED），中文名(有機發(fā)光二極管)更是鄧青云命名的。 到了1990年，英國劍橋的實驗室也成功研制出高分子有機發(fā)光原件。1992年劍橋成立的顯示技術公司CDT（Cambridge Display Technology），這項發(fā)現(xiàn)使得有機發(fā)光二極管的研究走向了一條與柯達完全不同的研發(fā)之路。 OLED最大的優(yōu)勢是無需背光源，可以自發(fā)光可做得很薄，可視角度更大、色彩更富、節(jié)能顯著、可柔性彎曲等等?？蓮V泛利用在各個領域，目前OLED更多使用AMOLED技術，在2013年的柏林國際電子消費品展（IFA）上，更有曲面OLED電視機種出現(xiàn)并引起注意。結構OLED基本結構：1. 陰極 ()；2. 發(fā)光層（Emissive Layer, EL）；3. 陽極空穴與陰極電子在發(fā)光層中結合，產(chǎn)生光子；4. 導電層（Conductive Layer）；5. 陽極 (+)有機發(fā)光二極管基本結構是由一薄而透明具半導體特性之銦錫氧化物（ITO），與電力之正極相連，再加上另一個金屬陰極，包成如三明治的結構。整個結構層中包括了：電洞傳輸層（HTL）、發(fā)光層（EL）與電子傳輸層（ETL）。當電力供應至適當電壓時，正極電洞與陰極電子便會在發(fā)光層中結合，產(chǎn)生光子，依其材料特性不同，產(chǎn)生紅、綠和藍三原色，構成基本色彩。OLED的特性是自發(fā)光，不像薄膜晶體管液晶顯示器需要背光，因此可視度和亮度均高，且無視角問題，其次是驅動電壓低且省電效率高，加上反應快、重量輕、厚度薄，構造簡單，成本低等，被視為 21世紀最具前途的產(chǎn)品之一。驅動方式不過，有機發(fā)光二極管也與 LCD 一樣其驅動方式也分為主動和被動式兩種。被動式下依照定位發(fā)光點亮，類似郵差寄信；主動式則和薄膜晶體管液晶顯示器相同，在每一個有機發(fā)光二極管單元背增加一個薄膜晶體管，發(fā)光單元依照晶體管接到的指令點亮。簡言之，主動被動矩陣分法，主要指的是在顯示器內(nèi)打開或關閉像素的電子開關型式典型的有機發(fā)光二極管由陰極、電子傳輸層、發(fā)光層、電洞輸運層和陽極組成。電子從陰極注入到電子輸運層，同樣，電洞由陽極注入進空穴輸運層，它們在發(fā)光層重新結合而發(fā)出光子。與無機半導體不同，有機半導體（小分子和聚合物）沒有能帶，因此電荷載流子輸運沒有廣延態(tài)。受激分子的能態(tài)是不連續(xù)的，電荷主要通過載流子在分子間的躍遷來輸運。因此，在有機半導體中，載流子的移動能力比在硅、砷化鎵、甚至無定型硅的無機半導體中要低幾個數(shù)量級。 在實際的OLED中，有機半導體典型的載流子移動能力為10-310-6cm2/VS。因為載流子移動能力太差，OLED器件需要較高的工作電壓。如一個發(fā)光強度為1000cd/m2的OLED，其工作電壓約為78V。因為同樣的原因，OLED受空間電荷限制，其注入的電流密度較高。通過一厚度為的薄膜的電流密度由下式定義：J=(9 / 8)e M (V2/d3)式中是電荷常數(shù)、是載流子遷移率、為薄膜兩端的電壓。在一般的機發(fā)光二極管中，全部有機膜的厚度約為1000囝 。實際上，有機發(fā)光二極管的發(fā)光功率與電流有JVm的關系，其中m 2。Burrows和Forrest制得的TPD/Alq器件的高達，他們認為，值大是因為“阱”（或稱極化子）的緣故。最近，他們又證實具有很強的溫度依賴性，并且電荷是通過“阱”來輸運的。 在發(fā)光層中，摻雜客體螢光染料能極大地提高OLED的性能和特性。例如，只要摻雜1%的紅色螢光染料DCM、Alq式機發(fā)光二極管的最大發(fā)射峰即可從520nm遷移到600nm；摻雜少量的MQA（一種綠色染料）將使機發(fā)光二極管的效率提高2至3倍，在同樣的亮度下工作壽命可提高10倍。有機發(fā)光二極管所用的物料是有機分子或高分子材料。將來可望應用于制造平價可彎曲顯示幕、照明設備、發(fā)光衣或裝飾墻壁。2004年開始，有機發(fā)光二極管已廣泛應用于隨身MP3播放器。器件效率Schema einer有機發(fā)光二極管迄今為止，發(fā)綠光的有機發(fā)光二極管是最有效的器件，這是因為人眼對綠光最為敏感。Tang曾報道，用香豆素摻雜Alq的器件具有56l的效率。據(jù)文獻報道，效率最大的發(fā)綠光的有機發(fā)光半導體是由Sano制成的，用Bebq作為HTM，其效率為15l。與發(fā)綠光的OLED比較，對發(fā)紅光和藍光的OLED的研究工作少得多。目前已知的，效率最好的發(fā)藍光的OLED是由Idemitsu的Hosokawa等人研制的，其發(fā)光效率為5.0l，對應的表面量子效率為2.4%。據(jù)Tang等人報道，將DCM染料攙入Alq制成了發(fā)紅光的OLE器件，其發(fā)光效率為2.5l。 需要說明的是，上述文獻所報道的發(fā)光效率，都是在發(fā)光強度約為100cd/m2或更小的條件下測得的。而實際應用的有機發(fā)光半導體是由多路驅動的，最大的發(fā)光強度要高一些。因此，顯示象素會被驅動到很高的發(fā)光強度，導致發(fā)光效率下降。也就是說，隨著發(fā)光亮度增加，發(fā)光效率將因驅動電壓的增加而降低。發(fā)綠光的有機發(fā)光半導體，在發(fā)光亮度為10,000cdm2時，其發(fā)光效率降為2lm/W，只有低亮度下的30%。發(fā)紅光和藍光的有機發(fā)光半導體，其發(fā)光效率隨著發(fā)光亮度的增加降低得更多。因此，有機發(fā)光半導體技術可能更適用于不需要有源矩陣驅動的小尺寸、低顯示容量的顯示器件。 器件的壽命和衰變在過去的幾年中，對有機發(fā)光半導體器件的壽命有過一些報道。但由于每個實驗室測量器件壽命的方法不同，無法對這些數(shù)據(jù)進行有意義的比較。在報道中，應用最多的測量器件壽命的方法，是在器件維持一恒定電流的條件下，測量從初始亮度下降至一半亮度的時間。據(jù)柯達公司的VanSlyke報道，亮度在2000cd/2時，器件的工作壽命達到了1000小時。Sano也報道了，在TPD中摻雜紅熒烯得到的器件，其初始亮度為500cd/m2、半亮度壽命為3000小時。對壽命進行比較的最佳量值是亮度和半亮度壽命的乘積。據(jù)報道，該量值對使用壽命最長的器件是：綠光為7,000,000cd/m2-hr；藍光為300,000cd/m2-hr；紅橙色為1,600,000cd/m2-hr。一個雙倍密封的有機發(fā)光半導體器件的儲存壽命約為年。特色與關鍵技過去的市場上有機發(fā)光半導體一直沒辦法普及，主要的問題在于早先技術發(fā)展的有機發(fā)光半導體樣品大多是單色居多，即使采用多色的設計，其發(fā)色材料和生產(chǎn)技術往往還是限制了有機發(fā)光半導體發(fā)色的多樣性。實際上有機發(fā)光半導體的影像產(chǎn)生方法和CRT顯示一樣，皆是借由三色RGB畫素拼成一個彩色畫素；因為有機發(fā)光半導體的材料對電流接近線性反應，所以能夠在不同的驅動電流下顯示不同的色彩與灰階。OLED的特色在于其核心可以做得很薄，厚度為目前液晶的1/3，加上有機發(fā)光半導體為全固態(tài)組件，抗震性好，能適應惡劣環(huán)境。有機發(fā)光半導體主要是自體發(fā)光的，讓其幾乎沒有視角問題；與LCD技術相比，即使在大的角度觀看，顯示畫面依然清晰可見。有機發(fā)光半導體的元件為自發(fā)光且是依靠電壓來調整，反應速度要比液芯片件來得快許多，比較適合當作高畫質電視使用。2007年底SONY推出的11吋O有機發(fā)光半導體電視XEL-1，反應速度就比LCD快了1000倍。有機發(fā)光半導體的另一項特性是對低溫的適應能力，舊有的液晶技術在零下75度時，即會破裂故障，有機發(fā)光半導體只要電路未受損仍能正常顯示。此外，有機發(fā)光半導體的效率高，耗能較液晶略低還可以在不同材質的基板上制造，甚至能成制作成可彎曲的顯示器，應用范圍日漸增廣。有機發(fā)光半導體與LCD比較之下較占優(yōu)勢，數(shù)年前OLED的使用壽命仍然難以達到消費性產(chǎn)品（如PDA、移動電話及數(shù)碼相機等）應用的要求，但近年來已有大幅的突破，許多移動電話的屏幕已采用OLED，然而在價格上仍然較LCD貴許多，這也是未來量產(chǎn)技術等待突破的。潛在應用有機發(fā)光半導體技術的主要優(yōu)點是主動發(fā)光?，F(xiàn)在，發(fā)紅、綠、藍光的有機發(fā)光半導體都可以得到。在過去的幾年中，研究者們一直致力于開發(fā)有機發(fā)光半導體在從背光源、低容量顯示器到高容量顯示器領域的應用。下面，將對OLED的潛在應用進行討論，并將其與其它顯示技術進行對比。有機發(fā)光半導體在1999年首度商業(yè)化，技術仍然非常新?，F(xiàn)在用在一些黑白簡單色彩的汽車收音機、移動電話、掌上型電動游樂器等。都屬于高階機種。目前全世界約有100多家廠商從事OLED的商業(yè)開發(fā)，有機發(fā)光半導體目前的技術發(fā)展方向分成兩大類：日、韓和臺灣傾向柯達公司的低分子有機發(fā)光半導體技術，歐洲廠商則以PLED為主。兩大集團中除了柯達聯(lián)盟之外，另一個以高分子聚合物為主的飛利浦公司現(xiàn)在也聯(lián)合了EPSON、DuPont、東芝等公司全力開發(fā)自己的產(chǎn)品。2007年第二季全球有機發(fā)光半導體市場的產(chǎn)值已達到1億2340萬美元。在中國企業(yè)方面，早在2005年，清華大學和維信諾公司決定開始OLED大規(guī)模生產(chǎn)線建設，并最終在昆山建設了OLED大規(guī)模生產(chǎn)線；廣東省也積極上馬有機發(fā)光半導體專案，截至2009年12月，廣東已建、在建和籌建的有機發(fā)光半導體生產(chǎn)線項目有5個，分別是汕尾信利小尺寸有機發(fā)光半導體生產(chǎn)線、佛山中顯科技的低溫多晶硅TFT（薄膜場效應晶體管）AMOLED生產(chǎn)線專案、東莞宏威的有機發(fā)光半導體顯示幕示范生產(chǎn)線項目、惠州茂勤光電的AMOLED光電項目、彩虹在佛山建設的有機發(fā)光半導體生產(chǎn)線項目。在有機發(fā)光半導體微型顯示器方面，云南北方奧雷德光電科技股份有限公司是世界第二家、中國第一家具備批量生產(chǎn)能力的AMOLED微型顯示器的生產(chǎn)廠商，微型顯示器多與光學組件配合，進行便攜的近眼式應用，可應用于紅外系統(tǒng)、工業(yè)檢測、醫(yī)療器械、消費電子等多個領域。根據(jù)調研公司DisplaySearch的報告，全球有機發(fā)光半導體產(chǎn)業(yè)2009年的產(chǎn)值為8.26億美元，比2008年增長35%。中國成為全球有機發(fā)光半導體應用最大的市場，中國的手機、移動顯示設備及其他消費電子產(chǎn)品的產(chǎn)量都超過全球產(chǎn)量的一半。OLEDAn OLED (organic light-emitting diode) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound which emits light in response to an electric current. This layer of organic semiconductor is situated between two electrodes; typically, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, portable systems such as mobile phones, handheld game consoles and PDAs. A major area of research is the development of white OLED devices for use in solid-state lighting applications.123There are two main families of OLED: those based on small molecules and those employing polymers. Adding mobile ions to an OLED creates a light-emitting electrochemical cell (LEC) which has a slightly different mode of operation. OLED displays can use either passive-matrix (PMOLED) or active-matrix addressing schemes. Active-matrix OLEDs (AMOLED) require a thin-film transistor backplane to switch each individual pixel on or off, but allow for higher resolution and larger display sizes.An OLED display works without a backlight; thus, it can display deep black levels and can be thinner and lighter than a liquid crystal display (LCD). In low ambient light conditions (such as a dark room), an OLED screen can achieve a higher contrast ratio than an LCD, regardless of whether the LCD uses cold cathode fluorescent lamps or an LED backlight.ContentsHistoryThe first observations of electroluminescence in organic materials were in the early 1950s by Andr Bernanose and co-workers at the Nancy-Universit in France. They applied high alternating voltages in air to materials such as acridine orange, either deposited on or dissolved in cellulose or cellophane thin films. The proposed mechanism was either direct excitation of the dye molecules or excitation of electrons.4567In 1960, Martin Pope and some of his co-workers at New York University developed ohmic dark-injecting electrode contacts to organic crystals.8910 They further described the necessary energetic requirements (work functions) for hole and electron injecting electrode contacts. These contacts are the basis of charge injection in all modern OLED devices. Popes group also first observed direct current (DC) electroluminescence under vacuum on a single pure crystal of anthracene and on anthracene crystals doped with tetracene in 196311 using a small area silver electrode at 400 volts. The proposed mechanism was field-accelerated electron excitation of molecular fluorescence.Popes group reported in 196512 that in the absence of an external electric field, the electroluminescence in anthracene crystals is caused by the recombination of a thermalized electron and hole, and that the conducting level of anthracene is higher in energy than the exciton energy level. Also in 1965, W. Helfrich and W. G. Schneider of the National Research Council in Canada produced double injection recombination electroluminescence for the first time in an anthracene single crystal using hole and electron injecting electrodes,13 the forerunner of modern double injection devices. In the same year, Dow Chemical researchers patented a method of preparing electroluminescent cells using high voltage (5001500 V) AC-driven (1003000Hz) electrically insulated one millimetre thin layers of a melted phosphor consisting of ground anthracene powder, tetracene, and graphite powder.14 Their proposed mechanism involved electronic excitation at the contacts between the graphite particles and the anthracene molecules.Electroluminescence from polymer films was first observed by Roger Partridge at the National Physical Laboratory in the United Kingdom. The device consisted of a film of poly(n-vinylcarbazole) up to 2.2 micrometres thick located between two charge injecting electrodes. The results of the project were patented in 197515 and published in 1983.16171819The first diode device was reported at Eastman Kodak by Ching W. Tang and Steven Van Slyke in 1987.20 This device used a novel two-layer structure with separate hole transporting and electron transporting layers such that recombination and light emission occurred in the middle of the organic layer; this resulted in a reduction in operating voltage and improvements in efficiency that led to the current era of OLED research and device production.Research into polymer electroluminescence culminated in 1990 with J. H. Burroughes et al. at the Cavendish Laboratory in Cambridge reporting a high efficiency green light-emitting polymer based device using 100nm thick films of poly(p-phenylene vinylene).21Universal Display Corporation holds the majority of patents concerning the commercialization of OLEDs.Working principleSchematic 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 all of the 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 (HOMO and LUMO) of organic semiconductors are analogous to the valence and conduction bands of inorganic semiconductors.Originally, the most basic polymer OLEDs consisted of a single organic layer. One example was the first light-emitting device synthesised by J. H. Burroughes et al., which involved a single layer of poly(p-phenylene vinylene). However multilayer OLEDs can be fabricated with two or more layers in order to improve device efficiency. As well as conductive properties, different materials may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile,22 or block a charge from reaching the opposite electrode and being wasted.23 Many modern OLEDs incorporate a simple bilayer structure, consisting of a conductive layer and an emissive layer. More recent developments in OLED architecture improves quantum efficiency (up to 19%) by using a graded heterojunction.24 In the graded heterojunction architecture, the composition of hole and electron-transport materials varies continuously within the emissive layer with a dopant emitter. The graded heterojunction architecture combines the benefits of both conventional architectures by improving charge injection while simultaneously balancing charge transport within the emissive region.25During operation, a voltage is applied across the OLED such that the anode is positive with respect to the cathode. Anodes are picked based upon the quality of their optical transparency, electrical conductivity, and chemical stability.26 A current of electrons flows through the device from cathode to anode, as electrons are injected into the LUMO of the organic layer at the cathode and withdrawn from the HOMO at the anode. This latter process may also be described as the injection of electron holes into the HOMO. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole. This happens closer to the emissive layer, because in organic semiconductors holes are generally more mobile than electrons. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of this radiation depends on the band gap of the material, 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 spins of the electron and hole have been combined. Statistically three triplet excitons will be formed for each singlet exciton. Decay from triplet states (phosphorescence) is spin forbidden, increasing the timescale of the transition and limiting the internal efficiency of fluorescent devices. Phosphorescent organic light-emitting diodes make use of spinorbit interactions to facilitate intersystem crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and improving the internal efficiency.Indium tin oxide (ITO) is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the HOMO level of the organic layer. A typical conductive layer may consist of PEDOT:PSS27 as the HOMO level of this material generally lies between the workfunction of ITO and the HOMO of other commonly used polymers, reducing the energy barriers for hole injection. Metals such as barium and calcium are often used for the cathode as they have low work functions which promote injection of electrons into the LUMO of the organic layer.28 Such metals are reactive, so they require a capping layer of aluminium to avoid degradation.Experimental research has proven that the properties of the anode, specifically the anode/hole transport layer (HTL) interface topography plays a major role in the efficiency, performance, and lifetime of organic light emitting diodes. Imperfections in the surface of the anode decrease anode-organic film interface adhesion, increase electrical resistance, and allow for more frequent formation of non-emissive dark spots in the OLED material adversely affecting lifetime. Mechanisms to decrease anode roughness for ITO/glass substrates include the use of thin films and self-assembled monolayers. Also, alternative substrates and anode materials are being considered to increase OLED performance and lifetime. Possible examples include single crystal sapphire substrates treated with gold (Au) film anodes yielding lower work functions, operating voltages, electrical resistance values, and increasing lifetime of OLEDs.29Single carrier devices are typically used to study the kinetics and charge transport mechanisms of an organic material and can be useful when trying to study energy transfer processes. As current through the device is composed of only one type of charge carrier, either electrons or holes, recombination does not occur and no light is emitted. For example, electron only devices can be obtained by replacing ITO with a lower work function metal which increases the energy barrier of hole injection. Similarly, hole only devices can be made by using a cathode made solely of aluminium, resulting in an energy barrier too large for efficient electron injection.303132Material technologieseditSmall moleculeseditAlq3,20 commonly used in small molecule OLEDsEfficient OLEDs using small molecules were first developed by Dr. Ching W. Tang et al.20 at Eastman Kodak. The term OLED traditionally refers specifically to this type of device, though the term SM-OLED is also in use.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.33 Fluorescent dyes can be chosen to obtain light emission at different wavelengths, and compounds such as perylene, rubrene and quinacridone derivatives are often used.34 Alq3 has been used as a green emitter, electron transport material and as a host for yellow and red emitting dyes.The production of small molecule devices and displays usually involves thermal evaporation in a vacuum. This makes the production process more expensive and of limited use for large-area devices than other processing techniques. However, contrary to polymer-based devices, the vacuum deposition process enables the formation of well controlled, homogeneous films, and the construction of very complex multi-layer structures. This high flexibility in layer design, enabling distinct charge transport and charge blocking layers to be formed, is the main reason for the high efficiencies of the small molecule OLEDs.Coherent emission from a laser dye-doped tandem SM-OLED device, excited in the pulsed regime, has been demonstrated.35 The emission is nearly diffraction limited with a spectral width similar to that of broadband dye lasers.36Researchers report luminescence from a single polymer molecule, representing the smallest possible organic light-emitting diode (OLED) device.37 Scientists will be able to optimize substances to produce more powerful light emissions. Finally, this work is a first step towards making molecule-sized components that combine electronic and optical p