有機(jī)發(fā)光半導(dǎo)體(OLED).docx

有機(jī)發(fā)光半導(dǎo)體有機(jī)發(fā)光二極管（英文：Organic Light-Emitting Diode，縮寫：OLED）又稱有機(jī)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(yīng)速率、全彩化及制程簡單等優(yōu)點(diǎn)，有機(jī)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(shù)最為困難，有機(jī)發(fā)光二極管顯示器依驅(qū)動(dòng)方式的不同又可分為被動(dòng)式（Passive Matrix，PMOLED）與主動(dòng)式。有機(jī)發(fā)光二極管可簡單分為有機(jī)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(jī)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(jī)發(fā)光二極管為主要應(yīng)用。歷史有機(jī)發(fā)光二極管技術(shù)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(xué)得到化學(xué)理學(xué)士學(xué)位，于1975年在康奈爾大學(xué)獲得物理化學(xué)博士學(xué)位。鄧青云自1975年開始加入柯達(dá)公司Rochester實(shí)驗(yàn)室從事有機(jī)發(fā)光二極管的研究工作，在意外中發(fā)現(xiàn)有機(jī)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(shí)驗(yàn)室，回到實(shí)驗(yàn)室后，他發(fā)現(xiàn)在黑暗中的一塊做實(shí)驗(yàn)用的有機(jī)蓄電池在閃閃發(fā)光從而開始了對有機(jī)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(dǎo)體 PN結(jié)的雙層有機(jī)結(jié)構(gòu)第一次作出了低電壓、高效率的光發(fā)射器。為柯達(dá)公司生產(chǎn)有機(jī)發(fā)光二極管顯示器奠定了基礎(chǔ)。由此被譽(yù)為OLED之父。OLED英文名為Organic Light-Emitting Diode，縮寫：OLED），中文名(有機(jī)發(fā)光二極管)更是鄧青云命名的。 到了1990年，英國劍橋的實(shí)驗(yàn)室也成功研制出高分子有機(jī)發(fā)光原件。1992年劍橋成立的顯示技術(shù)公司CDT（Cambridge Display Technology），這項(xiàng)發(fā)現(xiàn)使得有機(jī)發(fā)光二極管的研究走向了一條與柯達(dá)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(gè)領(lǐng)域，目前OLED更多使用AMOLED技術(shù)，在2013年的柏林國際電子消費(fèi)品展（IFA）上，更有曲面OLED電視機(jī)種出現(xiàn)并引起注意。結(jié)構(gòu)OLED基本結(jié)構(gòu)：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(jié)合，產(chǎn)生光子；4. 導(dǎo)電層（Conductive Layer）；5. 陽極 (+)有機(jī)發(fā)光二極管基本結(jié)構(gòu)是由一薄而透明具半導(dǎo)體特性之銦錫氧化物（ITO），與電力之正極相連，再加上另一個(gè)金屬陰極，包成如三明治的結(jié)構(gòu)。整個(gè)結(jié)構(gòu)層中包括了：電洞傳輸層（HTL）、發(fā)光層（EL）與電子傳輸層（ETL）。當(dāng)電力供應(yīng)至適當(dāng)電壓時(shí)，正極電洞與陰極電子便會在發(fā)光層中結(jié)合，產(chǎn)生光子，依其材料特性不同，產(chǎn)生紅、綠和藍(lán)三原色，構(gòu)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(qū)動(dòng)電壓低且省電效率高，加上反應(yīng)快、重量輕、厚度薄，構(gòu)造簡單，成本低等，被視為 21世紀(jì)最具前途的產(chǎn)品之一。驅(qū)動(dòng)方式不過，有機(jī)發(fā)光二極管也與 LCD 一樣其驅(qū)動(dòng)方式也分為主動(dòng)和被動(dòng)式兩種。被動(dòng)式下依照定位發(fā)光點(diǎn)亮，類似郵差寄信；主動(dòng)式則和薄膜晶體管液晶顯示器相同，在每一個(gè)有機(jī)發(fā)光二極管單元背增加一個(gè)薄膜晶體管，發(fā)光單元依照晶體管接到的指令點(diǎn)亮。簡言之，主動(dòng)被動(dòng)矩陣分法，主要指的是在顯示器內(nèi)打開或關(guān)閉像素的電子開關(guān)型式典型的有機(jī)發(fā)光二極管由陰極、電子傳輸層、發(fā)光層、電洞輸運(yùn)層和陽極組成。電子從陰極注入到電子輸運(yùn)層，同樣，電洞由陽極注入進(jìn)空穴輸運(yùn)層，它們在發(fā)光層重新結(jié)合而發(fā)出光子。與無機(jī)半導(dǎo)體不同，有機(jī)半導(dǎo)體（小分子和聚合物）沒有能帶，因此電荷載流子輸運(yùn)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(yùn)。因此，在有機(jī)半導(dǎo)體中，載流子的移動(dòng)能力比在硅、砷化鎵、甚至無定型硅的無機(jī)半導(dǎo)體中要低幾個(gè)數(shù)量級。 在實(shí)際的OLED中，有機(jī)半導(dǎo)體典型的載流子移動(dòng)能力為10-310-6cm2/VS。因?yàn)檩d流子移動(dòng)能力太差，OLED器件需要較高的工作電壓。如一個(gè)發(fā)光強(qiáng)度為1000cd/m2的OLED，其工作電壓約為78V。因?yàn)橥瑯拥脑?，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(jī)發(fā)光二極管中，全部有機(jī)膜的厚度約為1000囝 。實(shí)際上，有機(jī)發(fā)光二極管的發(fā)光功率與電流有JVm的關(guān)系，其中m 2。Burrows和Forrest制得的TPD/Alq器件的高達(dá)，他們認(rèn)為，值大是因?yàn)椤摆濉保ɑ蚍Q極化子）的緣故。最近，他們又證實(shí)具有很強(qiáng)的溫度依賴性，并且電荷是通過“阱”來輸運(yùn)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(jī)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(jī)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(jī)發(fā)光二極管所用的物料是有機(jī)分子或高分子材料。將來可望應(yīng)用于制造平價(jià)可彎曲顯示幕、照明設(shè)備、發(fā)光衣或裝飾墻壁。2004年開始，有機(jī)發(fā)光二極管已廣泛應(yīng)用于隨身MP3播放器。器件效率Schema einer有機(jī)發(fā)光二極管迄今為止，發(fā)綠光的有機(jī)發(fā)光二極管是最有效的器件，這是因?yàn)槿搜蹖G光最為敏感。Tang曾報(bào)道，用香豆素?fù)诫sAlq的器件具有56l的效率。據(jù)文獻(xiàn)報(bào)道，效率最大的發(fā)綠光的有機(jī)發(fā)光半導(dǎo)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(lán)光的OLED的研究工作少得多。目前已知的，效率最好的發(fā)藍(lán)光的OLED是由Idemitsu的Hosokawa等人研制的，其發(fā)光效率為5.0l，對應(yīng)的表面量子效率為2.4%。據(jù)Tang等人報(bào)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(xiàn)所報(bào)道的發(fā)光效率，都是在發(fā)光強(qiáng)度約為100cd/m2或更小的條件下測得的。而實(shí)際應(yīng)用的有機(jī)發(fā)光半導(dǎo)體是由多路驅(qū)動(dòng)的，最大的發(fā)光強(qiáng)度要高一些。因此，顯示象素會被驅(qū)動(dòng)到很高的發(fā)光強(qiáng)度，導(dǎo)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(qū)動(dòng)電壓的增加而降低。發(fā)綠光的有機(jī)發(fā)光半導(dǎo)體，在發(fā)光亮度為10,000cdm2時(shí)，其發(fā)光效率降為2lm/W，只有低亮度下的30%。發(fā)紅光和藍(lán)光的有機(jī)發(fā)光半導(dǎo)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(jī)發(fā)光半導(dǎo)體技術(shù)可能更適用于不需要有源矩陣驅(qū)動(dòng)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(jī)發(fā)光半導(dǎo)體器件的壽命有過一些報(bào)道。但由于每個(gè)實(shí)驗(yàn)室測量器件壽命的方法不同，無法對這些數(shù)據(jù)進(jìn)行有意義的比較。在報(bào)道中，應(yīng)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(shí)間。據(jù)柯達(dá)公司的VanSlyke報(bào)道，亮度在2000cd/2時(shí)，器件的工作壽命達(dá)到了1000小時(shí)。Sano也報(bào)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(shí)。對壽命進(jìn)行比較的最佳量值是亮度和半亮度壽命的乘積。據(jù)報(bào)道，該量值對使用壽命最長的器件是：綠光為7,000,000cd/m2-hr；藍(lán)光為300,000cd/m2-hr；紅橙色為1,600,000cd/m2-hr。一個(gè)雙倍密封的有機(jī)發(fā)光半導(dǎo)體器件的儲存壽命約為年。特色與關(guān)鍵技過去的市場上有機(jī)發(fā)光半導(dǎo)體一直沒辦法普及，主要的問題在于早先技術(shù)發(fā)展的有機(jī)發(fā)光半導(dǎo)體樣品大多是單色居多，即使采用多色的設(shè)計(jì)，其發(fā)色材料和生產(chǎn)技術(shù)往往還是限制了有機(jī)發(fā)光半導(dǎo)體發(fā)色的多樣性。實(shí)際上有機(jī)發(fā)光半導(dǎo)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(gè)彩色畫素；因?yàn)橛袡C(jī)發(fā)光半導(dǎo)體的材料對電流接近線性反應(yīng)，所以能夠在不同的驅(qū)動(dòng)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(jī)發(fā)光半導(dǎo)體為全固態(tài)組件，抗震性好，能適應(yīng)惡劣環(huán)境。有機(jī)發(fā)光半導(dǎo)體主要是自體發(fā)光的，讓其幾乎沒有視角問題；與LCD技術(shù)相比，即使在大的角度觀看，顯示畫面依然清晰可見。有機(jī)發(fā)光半導(dǎo)體的元件為自發(fā)光且是依靠電壓來調(diào)整，反應(yīng)速度要比液芯片件來得快許多，比較適合當(dāng)作高畫質(zhì)電視使用。2007年底SONY推出的11吋O有機(jī)發(fā)光半導(dǎo)體電視XEL-1，反應(yīng)速度就比LCD快了1000倍。有機(jī)發(fā)光半導(dǎo)體的另一項(xiàng)特性是對低溫的適應(yīng)能力，舊有的液晶技術(shù)在零下75度時(shí)，即會破裂故障，有機(jī)發(fā)光半導(dǎo)體只要電路未受損仍能正常顯示。此外，有機(jī)發(fā)光半導(dǎo)體的效率高，耗能較液晶略低還可以在不同材質(zhì)的基板上制造，甚至能成制作成可彎曲的顯示器，應(yīng)用范圍日漸增廣。有機(jī)發(fā)光半導(dǎo)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(dá)到消費(fèi)性產(chǎn)品（如PDA、移動(dòng)電話及數(shù)碼相機(jī)等）應(yīng)用的要求，但近年來已有大幅的突破，許多移動(dòng)電話的屏幕已采用OLED，然而在價(jià)格上仍然較LCD貴許多，這也是未來量產(chǎn)技術(shù)等待突破的。潛在應(yīng)用有機(jī)發(fā)光半導(dǎo)體技術(shù)的主要優(yōu)點(diǎn)是主動(dòng)發(fā)光?，F(xiàn)在，發(fā)紅、綠、藍(lán)光的有機(jī)發(fā)光半導(dǎo)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(jī)發(fā)光半導(dǎo)體在從背光源、低容量顯示器到高容量顯示器領(lǐng)域的應(yīng)用。下面，將對OLED的潛在應(yīng)用進(jìn)行討論，并將其與其它顯示技術(shù)進(jìn)行對比。有機(jī)發(fā)光半導(dǎo)體在1999年首度商業(yè)化，技術(shù)仍然非常新?，F(xiàn)在用在一些黑白簡單色彩的汽車收音機(jī)、移動(dòng)電話、掌上型電動(dòng)游樂器等。都屬于高階機(jī)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(jī)發(fā)光半導(dǎo)體目前的技術(shù)發(fā)展方向分成兩大類：日、韓和臺灣傾向柯達(dá)公司的低分子有機(jī)發(fā)光半導(dǎo)體技術(shù)，歐洲廠商則以PLED為主。兩大集團(tuán)中除了柯達(dá)聯(lián)盟之外，另一個(gè)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(jī)發(fā)光半導(dǎo)體市場的產(chǎn)值已達(dá)到1億2340萬美元。在中國企業(yè)方面，早在2005年，清華大學(xué)和維信諾公司決定開始OLED大規(guī)模生產(chǎn)線建設(shè)，并最終在昆山建設(shè)了OLED大規(guī)模生產(chǎn)線；廣東省也積極上馬有機(jī)發(fā)光半導(dǎo)體專案，截至2009年12月，廣東已建、在建和籌建的有機(jī)發(fā)光半導(dǎo)體生產(chǎn)線項(xiàng)目有5個(gè)，分別是汕尾信利小尺寸有機(jī)發(fā)光半導(dǎo)體生產(chǎn)線、佛山中顯科技的低溫多晶硅TFT（薄膜場效應(yīng)晶體管）AMOLED生產(chǎn)線專案、東莞宏威的有機(jī)發(fā)光半導(dǎo)體顯示幕示范生產(chǎn)線項(xiàng)目、惠州茂勤光電的AMOLED光電項(xiàng)目、彩虹在佛山建設(shè)的有機(jī)發(fā)光半導(dǎo)體生產(chǎn)線項(xiàng)目。在有機(jī)發(fā)光半導(dǎo)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(xué)組件配合，進(jìn)行便攜的近眼式應(yīng)用，可應(yīng)用于紅外系統(tǒng)、工業(yè)檢測、醫(yī)療器械、消費(fèi)電子等多個(gè)領(lǐng)域。根據(jù)調(diào)研公司DisplaySearch的報(bào)告，全球有機(jī)發(fā)光半導(dǎo)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(jī)發(fā)光半導(dǎo)體應(yīng)用最大的市場，中國的手機(jī)、移動(dòng)顯示設(shè)備及其他消費(fèi)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