藉由多超薄載子捕獲層及電洞傳輸層增強高穩定度白光有機發光二極體之研究

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(1)國立高雄大學應用物理研究所碩士論文. 藉由多超薄載子捕獲層及電洞傳輸層增強高穩定度白光有機發光二極體之研究. Investigation of high stability white organic light-emitting diode enhancement by using multiple ultra-thin carrier trapping layers and the hole transport layer 研究生：林豐益指導教授：黃建榮博士. 中華民國一百零二年七月. I.

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(3) 研究成果 (A) 國際期刊： 1. Kan-Lin Chen, Chien-Jung Huang, Dei-Wei Chou, Fong-Yi Lin and Chih-Chieh Kang, ” High stability white organic light-emitting diode (WOLED) using double ultra-thin carrier trapping layers”, Revised on Journal of Luminescence, Aug. 12, 2012. (SCI, Impact Factor: 2.1). 2. Kan-Lin Chen, Chien-Jung Huang, Fong-Yi Lin and Chih-Chieh Kang ”Formation of charge-transfer (CT) complex enhancement performances in white organic light-emitting diode (WOLED)” , submitted on Organic electronics, Aug. 21, 2013. (SCI, Impact Factor: 3.8). (B) 研討會論文： 1. Kan-Lin Chen, Chien-Jung Huang, Fong-Yi Lin, Dei-Wei Chou, Teen-Hang Meen, Wen-Ray Chen, Chern- Hwa Chen, ” Effect of multi-trapping layers on high color stability of white organic light-emitting diode (WOLED) ”, 2013 Second International Conference on Innovation, Communication and Engineering (ICICE 2013), Accepted on June 15, 2013.. III.

(4) Investigation of high stability white organic light-emitting diode enhancement by using multiple ultra-thin carrier trapping layers and the hole transport layer. Abstract Recently, organic light-emitting diodes (OLED) has attracted a great deal of attention as the new full-color displays and solid-state lighting technology. OLED superior characteristics such as high brightness, fast response time, wide viewing angle and low operating voltage make it with great potential for commercialization. Furthermore, the white organic light-emitting diode (WOLED) lighting technology has been extensively studied. And then, there are several methods to obtain WOLEDs, For example, using multi-layer stack of three primary colors (red, green and blue), two complementary colors (blue and yellow) and two or three colors of the dye doped into single host material. The structure of indium tin oxide (ITO) (100nm)/ Molybdenum trioxide (MoO 3 ) (15nm)/ 4,4’-Bis(2,2-diphenylvinyl)-1,1’-biphenyl (DPVBi) (10nm)/ 5,6,11,12-tetraphenylnaphthacene (Rubrene). (0.2nm)/. DPVBi. (24nm)/. Rubrene. (0.2nm)/. DPVBi. (6nm)/. 4,7-Diphenyl-1,10-phenanthroline (BPhen) : Cesium carbonate (Cs 2 Co 3 ) (10nm) /Al (120nm) with high color purity and stability white organic light-emitting diode (WOLED) was fabricated. The function of the multiple-ultra-thin layer (MUTL), such as Rubrene, is as the yellow light-emitting layer and trapping layer. The results show that the MUTL has excellent carrier IV.

(5) capture effect, resulting in high color stability of the device at different applied voltages. This study elucidates the optoelectronic properties of high efficiency white organic light-emitting diodes (WOLED) with molybdenum oxide (MoO 3 ) doping into N, N0-di (naphthalene-1-yl)–N, N0-diphenyl-benzidine (NPB) as a p-doping hole-transport layer (p-HTL). The device with a MoO 3 - doping NPB layer shows a turn-on voltage of 2.01 V and the maximum power efficiency 4.6 lm/W. The X-ray photoelectron spectroscopy (XPS) and UV-vis-NIR absorption spectra of MoO 3 -doping NPB layer revealed that the MoO 3 -doping NPB p-HTL had an improvement on holes injection. The improvement is caused by the formation of the charge transfer (CT) complex (NPB+-MoO 3 －) that is generated by doping MoO 3 into NPB, markedly increasing the number of holes carrier, improving the balance of the electrons and holes in recombination zone. The pure white light emission with Commissions Internationale De L’Eclairage (CIE) coordinates of (0.335, 0.321) was achieved at the operating voltage of 6 V. This device shows the maximum luminance of 12230 cd/cm2 and the maximum luminous efficiency of 7.01 cd/A at an operating voltage of 7 V.. V.

(6) 藉由多超薄載子捕獲層以及電洞傳輸層增強高穩定度白光有機發光二極體之研究摘要白光有機發光二極體可應用作為液晶顯示幕背光源與全固化照明光源，因此是一極具商品化潛力之有機發光二極體，白光有機發光二極體可以由光之三原色混合而成，亦可由兩種互補色形成。至今已有許多研究專門研究如何提高白光有機發光二極體之色純度、穩定度以及亮度，本論文將對此領域的現況進展做一較完整介紹，特別是元件結構調變及高效率白光有機發光二極體。在本實驗中具有高色純度和穩定度的結構indium tin oxide (ITO) (100nm)/ Molybdenum trioxide. (MoO 3 ). (15nm)/. 4,4’-Bis(2,2-diphenylvinyl)-1,1’-biphenyl. (DPVBi). (10nm)/. 5,6,11,12-tetraphenylnaphthacene (Rubrene) (0.2nm)/ DPVBi (24nm)/ Rubrene (0.2nm)/ DPVBi (6nm)/ 4,7-Diphenyl-1,10-phenanthroline (BPhen) : Cesium carbonate (Cs 2 Co 3 ) (10nm) /Al (120nm) 已經被製作了。多超薄層的功能，如Rubrene，作為黃色發光層和捕獲層。在實驗中，我們發現MUTL具有優異的載子捕獲效果。因此，可以使正負載子在整個發光層中充分複合，導致黃光和藍光產生激子的數量有相對增加，因而達到黃光和藍光的最佳互補產生高穩定性的白光。實驗結果表明，元件具有高色穩定度不會因為外加電壓的不同，而造成CIE座標會有太大的位移。另一方面，我們將MoO 3 摻雜到NPB當中作為電洞傳輸層，使其增加電洞注入以及導電性。結果發現，元件效率獲得進一步改善，元件最起始作電壓可以在 2.01V被獲得，亮度可以提升至 12230 cd/cm2。最後，我們使用有系統的物理和電性 VI.

(7) 分析於MoO 3 -doped NPB薄膜，在這當中發現MoO 3 -doped NPB薄膜會有電荷轉移的現象，而這現象可以確定是造成WOLED特性提升的重要原因。. VII.

(8) 誌謝回憶過往二年來的研究所生活中，首先要感謝我的指導教授－黃建榮教授諄諄教誨，感謝老師在我求學過程中耐心地指導與教誨，讓我學習正確的研究態度與做事的方法，對於處理各種事情考慮會更加嚴謹、周詳，進而學習待人處世的人生哲學。此外，感謝老師對於論文寫作的指導與建議，得以讓本論文順利如期完成，在此感謝他的指導與協助。還要感謝兩位指導老師周德威老師、陳甘霖老師指正和協助，對於論文內容提供許多寶貴意見與匡正之處，使得本論文得以更臻完善，在此一併致上謝意。感謝在實驗室一起為學術研究努力的學長高家元、柯中喬以及學弟妹們陳育豪、黃保勛、葉涵、巫俐瑩，於課業上的互相砥礪及討論，以及在實驗上的種種幫助使我得以在專業技能、知識上有所進步。在本論文完成的同時，我也同時將這份喜悅和我的家人以及關愛我的朋友們一起分享。. VIII.

(9) Contents 第一章. 緒論-------------------------------------------------------1. Chapter 1 Introduction----------------------------------------------3 1-1 General introduction------------------------------------- -----3 1-2 The history of organic light-emitting diode-----------------4 1-3 The benefits of organic light-emitting diode----------------5 1-3-1 The difficulty of organic light-emitting diode------------6 1-4 OLED Market opportunity-------------------------------------7 1-5 The motion and purpose of this deliberation----------------8 第二章. 有機發光二極體的背景理論-------------------------10. Chapter 2 OLED Background theory ----------------------------11 2-1 Theory of organic light-emitting diode (OLED) -----------11 2-2 Emission mechanism-------------------------------------------12 2-2-1 Fluorescent theory--------------------------------------------13 2-3 Factors for the performance of OLEDs----------------------14 2-3-1 The OLED composite----------------------------------------15 2-4 The two forms of organic light-emitting displays-passive matrix and active matrix displays-----------------------------22 2-5 The full-color technology of OLED--------------------------24 IX.

(10) 2-5-1 The introduction of full color technology----------------25 2-5-2 The comparison with full color technology--------------27 2-6 Material technologies------------------------------------------28 2-7 OLED Structures-----------------------------------------------29 2-8 The measurement of characteristics for OLEDs------------31 第三章. 實驗步驟-------------------------------------------------33. Chapter 3 Experiment procedure----------------------------------34 第四章. 結果與討論----------------------------------------------36. Chapter 4 Result and discussion-----------------------------------37 4-1 The characteristics of various thicknesses of Rubrene layer for WOLEDs-----------------------------------------------------37 4-2 The influence of different locations of Rubrene layer for WOLEDs ---------------------------------------------------------39 4-3 The characteristics of multiple-ultra-thin layer (MUTL) for WOLEDs ---------------------------------------------------------43 4-4 The characteristics of MoO 3 -doped NPB layers for WOLEDs ---------------------------------------------------------45 4-5 The dependence of luminous efficiency and power efficiency on the voltage of WOLEDs ------------------------47 X.

(11) 4-6 UV-vis-NIR absorption spectrophotometer analysis -------47 4-7 X-ray photoelectron spectroscopes (XPS) analysis --------48 第五章. 結論與未來工作-----------------------------------------51. Chapter 5 Conclusion and future work----------------------------53 5-1 Conclusion--------------------------------------------------------53 5-2 Future work-------------------------------------------------------54 References-------------------------------------------------------------55. XI.

(12) 第一章緒論在過去的幾年以來，由於反應速率、亮度、視角、壽命等諸多優點及較低的製造成本，加上技術的成熟，陰極射線管已主宰顯示器及電視機的市場幾十年，不管是在電腦的螢幕或家庭視聽娛樂器材上，依然具有競爭優。但是重量過重、體積龐大的缺點已成為它的致命傷。為因應視覺享受的大面積化及可攜帶性的輕便化要求，短短十年間，新的平面顯示器技術陸續被開發出來，例如液晶顯示器、電漿顯示器、場發射顯示器、真空螢光顯示器、發光二極體、電激發光等。目前在平面顯示器的技術當中，又以有機發光二極體（ Organic/Polymer Light-Emitting Diode, O/PLED）最被看好。因為相較於目前市場上平面顯示器的主流薄膜電晶體 - 液晶顯示器（ Thin Films Transistor – Liquid Crystal Display,TFT-LCD），有機發光二極體擁有自發光、不需背光源、更輕薄短小以及更寬廣的視角等優點，所以已被認為是這個世代平面顯示器的主流技術。自從 1987 年，美國柯達公司的 Tang 等人利用熱蒸鍍法將小分子有機材料製成雙層元件後， OLED 的發展有了進一步的突破，也引起了研究者廣泛的興趣及持續的研究。在 1990 英國劍橋大學 J. H. Burroughes 等人提出以高分子 PPV 材料作為發光層的 OLED 元件，有機高分子材料在 OLED 元件亦佔有一席之地。為了讓WOLED達到照明應用的需求，如何製作高穩定度、高效率、高色純度的 WOLED，便成為本實驗研究發展的重要課題。而長波長的黃光材料Rubrene，在兩色互補的白光技術中扮演重要角色。除此之外，Rubrene的高量子效率也有助於元件效率提升，因此本論文藉由調變發光層中Rubrene的位置，研究製作高色純度及高穩定度的WOLED。此外，藉由適當MoO 3 的摻雜來提升NPB的載子傳輸的能力，進而達到改善元件的效果。接續研究探討MoO 3 -doped NPB的摻雜作為電洞傳輸層，對元件所造成的影響，並利用電性和物性的分析，討論MoO 3 -doped NPB的機制。本論文共分為五章節，第一章為緒論；第二章為 OLED 的基本原理，包含有機發光二極體的理論及 OLED 製作白光的方法；第三章包含實驗的步驟，製作方法，和量測儀器； 1.

(13) 第四章則是不同參數下 WOLED 的光電特性的實驗結果與討論；第五章是本論文的結論及後續研究的建議。. 2.

(14) Chapter 1 Introduction 1-1 General introduction Since Tang and Van Slyke reported the efficient double-layer organic light-emitting devices (OLEDs) in 1987 [1], and the research on OLEDs has been attractive and promoted considerably. Much study has been focused on improving the performance of OLEDs [2–5]. The series of efforts contain the growth of new efficient materials and optimum device fabrications. In recent years, OLEDs are considered to be one of the flat-panel displays of the next generation [6]. An organic light-emitting diode (OLED) is light-emitting diode (LED) whose electroluminescent emission layer is composed of film of organic compounds. The layer usually contains a polymer substance that allows to be deposited. They are deposited in rows and columns onto a flat substrate by a simple "printing" process. The resulting matrix of pixels can emit light of different colors. Such systems can be used in television screens, and computer displays. OLEDs can also be used in light sources for general space illumination, and large-area light-emitting elements. OLEDs typically emit less light per area than inorganic solid-state based LEDs which are usually designed for use as point-light sources. A significant benefit of OLED displays beyond traditional liquid crystal displays (LCDs) is that OLEDs do not require a backlight as functioning. Thus they consume even less power. Because there is no need for the backlight, an OLED display can be far thinner than a LCD panel. 3.

(15) OLED display devices also can be more effectively fabricated than LCD.. 1-2 The history of organic light emitting diode At first Bernanose and co-workers created electroluminescence with organic materials in the early 1950s by applying a high-voltage alternating current (AC) field to crystalline thin films of acridine orange and quinacrine [7-10]. In 1960, researchers at Dow Chemical developed AC-driven electroluminescent cells using doped anthracene [11]. The materials with inferior electrical conductivity limited light output until more conductive organic materials became obtainable, particularly the polypyrrole, and polyacetylene. Later in a 1977 paper, Hideki Shirakawa et al. reported high conductivity in similarly oxidized and iodine-doped polyacetylene [12]. Alan J. Heeger, Alan G. MacDiarmid & Hideki Shirakawa received the 2000 Nobel Prize in Chemistry for "The discovery and development of conductive organic polymers". The first diode device was developed at Eastman Kodak by Dr. Ching Tang and Steven Van Slyke in the 1980's. This diode, causing the name "OLED", used a novel two-layer structure with parting hole transporting and electron transporting layers such that recombination and light emission took place in the middle or the organic layer. That led to the fact that the operating voltage decrease and efficiency increase, promoting OLED research and development. In addition, the idea was applied to polymers in the Burroughs et al. 1990 paper in the “Journal Nature” demonstrating a very-high-efficiency green light emitting polymer [13].. 4.

(16) 1-3 The benefits of organic light emitting diode There are so many advantages and excellences for OLEDs compared with liquid crystal display (LCD), plasma display panel (PDP) and cathode ray tube (CRT) display, attracting much attention. For instance, high contrast ratio, fast response time, wide viewing angle (up to 170 degrees), high brightness, low-operating voltage(3-10V), low power consumption, the ease of fabrication, extensive operating temperature range(- 40℃~85℃) and low cost [14, 15]. OLEDs self emit light, so there is no need for color filter and backlight as compared with LCD. Therefore, OLED pixels show colors correctly and un-shifted even if the viewing angle approaches 90 degrees from normal. In addition, OLEDs possess superior range of colors, higher brightness, high contrast ratio, and fast response time. OLEDs can emit various colors by using the extensive choices of organic fluorescent dyes. For LCD, the origin of light is backlight and there is no true and optimal black. It exists light leak phenomenon for LCD even if at the black state because liquid crystal molecules can not work ideally and completely. However, OLED does not produce any light emission in a black state and consumes no power at the same time. It exist not only color filters but also polarizers in LCD, wasting more than half of the light emitted from backlight with polarizer, and color filters filter out two-thirds of the light which transmitted from liquid crystal layer in Figure 1.1 . OLEDs also have a faster response time (1 microsecond) than standard LCD screens (2-8 milliseconds). OLEDs have attracted much interest due to their potential for flat-panel displays. Besides, 5.

(17) OLEDs can fabricate onto flexible substrate like plastic substrate and they enable new applications such as roll-up displays. OLEDs can be also printed onto any suitable substrate using an inkjet printer or even screen printing technologies [16]. OLEDs have no need for backlight and also printed onto any suitable substrates, so they can also be much thinner than LCDs and rightly have a significantly lower cost than LCD or PDP. OLEDs quite possibly could be produced like newspapers, i.e., electronic paper. OLEDs can be also more effectively manufactured than LCDs and PDPs. The thickness of OLED panel is thinner than 2 millimeters and the cost for OLEDs is cheaper than LCDs (about 20%). Besides, OLEDs generally operate at 2 to 10 Volts. OLED pixels only consume power as they are lit, and can be more efficient than LCDs. No matter at full black or full write state, the backlights for LCDs function all the time that consume much power.. 1-3-1 The difficulty of organic light emitting diode Although there are so many advantages and excellences for OLEDs, but OLEDs still exist some disadvantages and problems which are needed to overcome and improve. For instance, degradation of OLED materials has limited their use [17]. Above all, LCDs constantly improved manufacturing process step by step to cost down and search for new ways to increase the brightness and board the viewing angles. It is the challenge for OLEDs due to manufacturing technique of LCD improves incessantly. The hugest technical problem for OLEDs is the lifetime confine of the organic materials. In 6.

(18) particular, blue OLEDs historically have had a lifetime of around 14,000 hours (5 years at 8 hours a day) when used for flat-panel displays, which is lower than characteristic lifetime of LCD, LED or PDP technology – each currently achieve about 60,000 hours, depending on manufacturer and model. But in 2007, experimental PLEDs were created which can sustain 400 cd/m² of luminance for over 198,000 hours for green OLEDs and 62,000 hours for blue OLEDs [18]. By the way, the invasion of water into displays can damage or destroy the organic materials. Therefore, improved sealing processes are important for practical manufacture.. 1-4 OLED market opportunity OLED technology is used in commercial applications like small screens for mobile phones and conveyable digital audio players, and car radios, digital cameras. Due to the high light output of OLEDs, it facilitates readability in sunlight for such portable applications. Portable displays are sometimes used, so the lower lifespan of OLEDs is unimportant. OLEDs have been used in most Motorola and Samsung color cell phones, as well as some Sony Ericsson phones, notably the Z610i, and some models of the Sony Walkman [19]. It is also found in the Creative Zen V/V Plus series of MP3 players. Nokia has also introduced recently some OLED products, including the 7900 Prism and Nokia 8800 Arte. On the October 1st, 2007, Sony announces that an OLED television produce, which was released in Japan in December 2007 [20]. Newer OLED applications include signs and space illumination. The second-generation flash-based Clix mp3 player, released in April 2007 by 7.

(19) iriver, displays video on a 320x240 2.2" AMOLED screen of 262K colors. Samsung unveiled a 31-inch OLED TV at the January 2008 CES in Las Vegas and is promising much larger screens to generate. Use of OLEDs may be subject to patents held by Eastman Kodak and others. Kodak has licensed its patents to other firms such as LG for commercialization [21]. There is speculation that the next generation 3G version of the Apple iPhone may use OLED technology [22].. 1-5 The motion and purpose of this deliberation Recently, organic light-emitting diodes (OLED) has attracted a great deal of attention as the new full-color displays and solid-state lighting technology. OLED superior characteristics such as high brightness, fast response time, wide viewing angle and low operating voltage make it with great potential for commercialization [23]. Furthermore, the white organic light-emitting diode (WOLED) lighting technology has been extensively studied. And then, there are several methods to obtain WOLEDs, For example, using multi-layer stack of three primary colors (red, green and blue), two complementary colors (blue and yellow) and two or three colors of the dye doped into single host material In the doping co-deposition process, to accurately control the evaporation rate and the concentration of the two or more materials is very difficult, resulting in its deposition process and reproducibility being poor. However, non-doping technique can accurately control to avoid the above problems in the fabrication process. Currently, Tsuji et al. and Xie et al. have reported 8.

(20) non-doped-type WOLEDs with the single-ultra-thin layer (SUTL) structure, which is a simple device structure and good reproducibility, making them very suitable for low-cost lighting applications and conducive to commercialization [24-26]. In this work, we employ non-doped method to fabricate WOLEDs with a multiple-ultra-thin layer (MUTL) structure. The material of 5, 6, 11, 12-tetraphenylnaphthacene (Rubrene) was used as yellow light sources in MUTL structure. Rubrene is the most common material, which has good light-emitting efficiency, high color saturation, good stability and good capture efficiency of charge carriers. In addition, the photoluminescence (PL) quantum efficiency of the Rubrene can be close to 100% [27-29]. It is usually deemed host emitting layer (EML) due to the higher efficiency of blue light. Moreover, DPVBi is not only emitting but also transfer the incomplete energy from DPVBi to the Rubrene [30, 31]. So far the study of fluorescence WOLED (FWOLED) based on the non-doped multiple-ultra-thin layer (MUTL) is rarely reported. Therefore, we introduced a simple process for the non-doped FWOLEDs with a MUTL structure. Meanwhile, the influential factors for the improvement of FWOLEDs performance were investigated in detail. However, the mechanism of the influences of MUTL structure on the electroluminescence (EL) and the color stability of FWOLEDs is also presented.. 9.

(21) 第二章有機發光二極體的背景理論本章節主要是說明有機發光二極體的原理，包含發光機制及螢光理論。對於有機發光二極體有機材料和電極金屬的選擇，元件結構的組成及特性有一連串的說明。藉此可歸納出影響有機發光二極體光電特性的原因，和提升元件效率及色純度的方向。此外，主動及被動 OLED 的應用和驅動方式、OLED 全彩化的方法及優缺點，亦是本章節探討的範疇。當 OLED 受到外加電場作用，電子及電洞分別由陽極及陰極注入有機層，並在有機材料再結合放出特定波長的光。在 OLED 結構中，各個有機層，都有各自的功能及必須具有的特性，包含能階的匹配、材料的穩定性，發光的效率都是必須考量的項目。此外，陽極或陰極金屬材料(功函數, 可見光波段的透光性)的選擇，在 OLED 中亦扮演重要的角色。如何降低載子的注入能障，使電子電洞注入及傳輸更平衡、提升電子電洞在發光層再結合的比率，是 OLED 具有絕佳光電特性不可或缺的課題。 OLED製作白光的主要方法包含side-by-side method、color conversion method、 color filter method 、two-color complementary method。此四種方法都有各自的優勢和缺點及改進的方向。藍光和黃光在兩色互補法(two-color complementary method)中扮演重要角色，而本實驗主要是改變發光層中Rubrene的位置加上MoO 3 -doped NPB作為電洞傳輸層改善白光 OLED的色純度、穩定度及提升元件效率。. 10.

(22) Chapter 2 OLED background theory 2-1 Theory of organic light emitting diode (OLED) The anode is positive as compare with the cathode as applying a voltage. That generates a current of electrons to flow through the device from cathode to anode. Thus, the cathode contributes electrons to the emissive layer and the anode retires electrons from the conductive layer; in other words, the anode offers holes to the conductive layer. At once, electric field give rise to the fact that the electrons and the holes towards each other and then they recombine. Actually, the recombination zones are in emissive layer which is closer to the cathode, because in organic semiconductors holes own great mobility than electrons (unlike in inorganic semiconductors). The recombination causes a fall in the energy levels of electrons, attending an emission of radiation that emission frequency is in the visible region. To realize charge carriers distribution and generated excitons information are important for obtained optimal OLED. Besides, choosing proper thickness of organic layer is crucial to balance of charge carriers. Towards this goal, a series of data are generated for particular purposes in our experiment, including current density-voltage (J-V) characteristics with changing thicknesses of organic layer, and the variations for the balance of charge carriers with various thicknesses of organic layer.. 11.

(23) 2-2 Emission mechanism There are two emission mechanisms in OLEDs, one is fluorescence emission mechanism and the other is phosphorescence. It generates a current of electrons to flow through the device from cathode to anode as applying a voltage. The electrons inject from a low work function cathode into the LUMO (the lowest unoccupied molecular orbital) of the organic material and holes inject from a high work anode into the HOMO (the highest occupied molecular orbital) of the organic material. Then, electric field give rise to the fact that the electrons and the holes towards each other and then they recombine, and excited from ground state to excited state. In OLED, excited states are separated the singlet state from the triplet state. For singlet state, the electrons in the same energy orbital have anti-parallel spins. If transition for the excited state with reversal of the spin to the electrons, the electrons in the same energy orbital have parallel spins, and the excited state is called triplet state. Figure 2.1 shows the situation of charge carriers recombination and the process for emission transmission. In OLEDs, fluorescence material generates fluorescence that maximum efficiency is 25 %. The other 75 % excitons in triplet energy level released to the ground state with phosphorescence (in Figure 2.2) [32]. For host-guest system, host material owns high gap value than guest owns. The gap value is the value difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for organic materials. The host-guest system generates emission spectrum of the guest material by energy transfer from 12.

(24) host material to guest material. In other words, if the emission spectrum of host material overlaps more and more the absorption spectrum of guest material, the effect of energy transfer gets better and better. In addition, the guest material can trap charge carriers and generate emission with the wavelength it owns. This is another emission mechanism for guest material that it is no need to consider the mechanism of energy transfer [33].. 2-2-1 Fluorescent theory In our study, a series of data and characteristic must be analyzed and discussed. For the proposal, it is important to realize that the fluorescent theory and the main optical process that generate in organic molecule. The process shows in Figure 2.3. In Figure 2.4, the most stable molecule have even electrons, all the electrons assemble together in pair and reversed spin in the ground state, and it is called singlet ground state (S0). As the molecule absorbs external radiation, the electrons in ground state will be excited to higher energy level. It is called that the excited singlet state (S1) or excited triplet state (T1). The ground and excited states all have many vibration and rotation degrees of freedom. When molecules absorb light or ultraviolet ray, the electrons in the ground state will jump to the excited state. The electrons in the excited state are called as excitons, and they will decay by moving to the ground state in different ways (light or heat). First, in the case of releasing energy by light emission, electrons are excited to a higher singlet state. Soon, the excited electrons release to the lowest singlet state and emit fluorescence. The organic materials absorb energy and the electrons jump to excited state, and then rapidly 13.

(25) return to the ground state by emitting fluorescence. The light emission can also be generated by electrons releasing from the triplet state, which are called phosphorescence. The emission of phosphorescence provides longer life as compared with the emission of fluorescence. In order to generate fluorescence, electrons must absorb energy and jump to the excited state with higher energy from the ground state with the lowest energy. However, the lowest vibration energy level of singlet state overlap higher vibration energy level of triplet state, electrons will spin inversely into the energy level of excited triplet state. The phenomenon is called intersystem crossing, and the electrons can return to any vibration energy level of ground state by releasing the emission of phosphorescence.. 2-3 Factors for the performance of OLEDs The major factors that influence the performance of OLEDs are discussed in the section. It is well known that major factors include the situation of charge carrier injection, the characteristic of charge carrier transporting, and the balance of electrons and holes. The situation of charge carrier injection is related to the difference of energy levels between electrodes and organic layer or neighboring organic layers. In organic semiconductors, holes own great mobility than electrons (unlike in inorganic semiconductors). In addition, a material is usually proper for particular charge carrier. Above all, it is necessary to chose suitable materials with unique characteristics and fabricate proper demonstrate for assembling organic materials to generate higher performance of OLEDs. 14.

(26) 2-3-1 The OLED composite A typical OLED is composed of an emissive layer, a conductive layer, a substrate, and anode and cathode terminals. The layers are fabricated with particular organic molecules that conduct electricity. Their levels of conductivity range from those of insulators to those of conductors, and so they are called organic semiconductors. Most basic OLEDs comprised a single organic layer, for instance the first light-emitting polymer device synthesized by Burroughs et al. involved a single layer of poly (p-phenylene vinylene). Multilayer OLEDs can have more than two layers to improve device efficiency. For good conductive properties, layers may be chosen to help charge injection at electrodes by providing a more proper energy level, or block charge carriers from reaching the opposite electrode and being wasted. It was recognized that the quantum efficiency of OLEDs depends on the status of carrier injection, the mobility of charge carriers, and the balance of the holes and electrons [34, 35]. Above all, light emission is the consequence of the recombination of holes and electrons injected from the electrodes to the organic emissive layer. Such carrier recombination generates excited molecules, which eventually emit light. Thus, the device efficiency is highly dependent on both carrier recombination efficiency of the emissive material. It is widely recognized that unbalanced charge carriers due to higher hole mobility in the hole transport layer (HTL) and slower electron mobility in the electron-transport layer (ETL) leads to reduced efficiency of OLEDs. Thus, it is important to balance the injected charges to improve device performance. 15.

(27) The organic films for OLED can separate from several layers with particular functions. The general device can contain substrate, anode, hole injection layer (HIL), buffer layer, hole transporting layer (HTL), emission layer (EML), hole blocking layer (HBL), electron transporting layer (ETL), electron injection layer (EIL), and cathode. The detail of function and characteristic for each layer are discussed as follows: The organic films must be fabricated on a substrate. The common substrates are glass and plastics. The plastics substrate can be applied to flexible OLED. As a good anode material, several characteristics must be considered. The characteristics contain high conductivity, high work function, high transparency in visible light, and good morphological stability. Indium tin oxide (ITO) is commonly used as the anode material [36]. It is transparent to visible light (better than 90％) and has a high work function (4.5 ~ 4.8 eV) [37] which promotes injection of holes into the polymer layer. The methods of ITO thin film depositions are sputter [38, 39], chemical vapor deposition (CVD) [40], and spray pyrolysis [41]. However, ITO thin film is grown on general glass substrate with high temperature (great than 215 ℃). The high temperature is not suitable for plastic substrate, which result in buckle and deformation for plastic substrate. The difficult is so important for flexible OLED to overcome. In order to achieve good injection efficiency for hole, the treatment in ITO surface attracts much attention. It is believed that the work function of ITO without treatment is about 4.5~ 4.8 eV. In addition, the contaminant in the ITO surface also decreases the wok function [42]. The work 16.

(28) function of ITO increases to above 5.0 eV by O2 plasma and UV ozone [43-45] treatments. The treatments also improve the characteristics of interface with organic layer to increase hole injection, decrease operating voltage, and increase stability of OLED. Organic materials are deposited on the ITO-coated glass substrate. Because the organic thin films contact with ITO directly, ITO surface characteristics deeply influence the performance of OLED. The O2 plasma treatment increase the wok function, removing its surface contaminant, and enhance the hole injection. However, there are several handicaps in the O 2 plasma treatment at low pressure, such as vacuum system is expensive and the size of ITO substrate is limited by the size of vacuum chamber [46]. Another problem for ITO anode is the diffusion of indium into the organic layer as device functions. The diffusion result in the decay of device performance [47]. Besides, the spikes in the ITO surface cause low uniformity of ITO surface, which procures current leakage. Even if the work function of ITO anode increases after O2 plasma treatment or UV ozone treatment, the wok function of ITO is still lower than the highest occupied molecular orbital for common hole transporting layer (about 0.4 eV). Thus, it is beneficial to improve hole injection between anode and hole transporting layer as inserting hole injection layer (HIL) in the interface between node and hole transporting layer. The ideal HIL material should have the characteristics as following: matching ITO work function, morphological stability, good thermal stability, and adhesion promotion. Proper HIL can not only improve efficiency but also increase the longevity for OLEDs. Organic hole injection layer materials usually have ability of hole transporting. The 17.

(29) familiar HIL materials contain copper phthalocyanine (CuPc) [48], polyaniline [49] in small molecules, and poly ethylenedioxy thiophene (PEDOT) in polymer material [50]. PEDOT owns many advantages such as smoothing the surface of ITO, decreasing threshold voltage, and extending the longevity for OLEDs [51]. Most of hole transporting materials are tri-arylamine that applied to xerography former. They all have high hole mobility, which are about 10-3 ~10-4 cm/Vs. For hole transporting layer (HTL) in OLEDs, they need characteristics such as easy to inject hole, efficient hole mobility, good thermal stability, and easy synthesis. Ideal materials of HTL must be deposited as thin films without pinholes. If the HTL material with high glass transition temperature enables to form stable and amorphous morphology, they will unchangeably generate pinholes in thin film. The large use of HTL is NPB. NPB has advantages such as easy synthesis and simply purify. However, its glass transition temperature (about 98℃) is low. The new HTL materials are emphasized that the characteristics of the high glass transition temperature and stable thin film morphology. Besides, it is also important to search the optimum control of hole injection and hole transporting. The major function of electron injection layer (EIL) is promotion of electron injection from cathode to the electron transporting layer (ETL). The common use of HIL is LiF. With LiF as HIL, the applying voltages of OLED decrease, and the phenomenon is attributed to reason that thin LiF film avoid directly contact between Al and Alq 3 , which effectively decrease interface 18.

(30) barrier [52]. But, up to now, the mechanisms of LiF layer in OLED is indefinite. The exact mechanisms of LiF layer need to demonstrate and discuss. The conditions for good electron transporting layer (ETL) contain efficient electron mobility, good thermal stability, easy synthesis, formation of thin film with amorphous morphology, ability to block holes, and easy to inject electrons. The ideal HTL material must have proper values of HOMO and LUMO. The suitable value of LUMO (low LUMO) with ETL can effectively inject electrons from cathode into ETL. The suitable value of HOMO (high HOMO) with ETL can effectively block holes in emission layer and improve the probability of charge carrier recombination. It is perfect for uniform mobility of holes and electrons. However, the mobility of electrons is far smaller than the mobility of holes actually in organic materials. Thus, it is crucial to ETL with high electron mobility, and it makes the recombination zone far away the cathode to increase excitons generation. The ETL with high glass transition temperature and good thermal stability is important to avoid producing heat accumulation in high current density. Besides, the thin film of ETL must be uniform and there are not pinholes which formed by thermal evaporation or spin coating. In 1987, Tang and Van Slyke utilize Alq 3 to emit high efficiency electroluminescence. Besides, Alq 3 owns some advantages such as good thermal stability and easy to deposit thin film without pinholes [53]. Thus, Alq 3 is generally used as emission layer or HTL in OLEDs. Proper work functions of cathode and anode are important for effective injection of holes 19.

(31) and electrons into organic layers [54-56]. Due to most organic materials have LUMO value (2.5~3.5 eV) and HOMO value (5~6 eV). Cathode must be a low work function metal and anode must be a high work function metal, so it is possible to produce the lowest injection barriers. The good cathode has characteristics like Ohmic electron contact, adhesion to ETL, and low work function. For effective electrons injection, low work function metals, like calcium (Ca) [57], magnesium (Mg) [58], are used as cathodes. In addition, some stable metals such as aluminum (Al) and silver (Ag) are common use of cathodes in OLEDs. Some composite cathodes such as Mg:Ag and Li:Al also become cathodes in OLEDs. To add silver into magnesium is not only improving the stability of cathode but also increasing the ability of adhesion with Alq 3 [59]. It is believed that the mobility of holes in HTL is faster than the mobility of electrons in ETL. The fact of unbalanced mobility for charge carriers results in bad recombination condition and reduces efficiency of organic light emitting devices. The difficulty can be solved in two ways. One way is the improvement of electrons injection from cathode to organic layers, and increasing probability of recombination for charge carriers. This section contains proper cathode with low work function to increase electrons injection and good ETL materials with high mobility of electrons to transport electrons fast. The other way is inserting a buffer layer between anode and HTL to improve the hole injection and decrease the apply voltages. Choosing suitable buffer layer and depositing thin film of ideal thicknesses for buffer layer are beneficial to increase efficiency of OLEDs. The common buffer layers contain TiO 2 , SiO 2 [60], and LiF [61]. CuPc is 20.

(32) inserting as buffer layer between anode and HTL to slow down the transport of holes and balancing the transport for charge carriers [62, 63]. Thus, the bad injection of hole with CuPc layer improves the recombination of charge carriers and promotes the efficiency of OLEDs. Besides, the hole blocking layers (HBL) have characteristic that it can limit holes in the emission layer to increase the probability of recombination for charge carriers. The general HBL materials are such as BCP, and BPhen. The materials as emission layer (EML) in OLEDs require the characteristics that contain whose light emission must be in the range of visible light and high photoluminescence (PL) quantum efficiency and good thermal stability. Besides, the combination of host material with excellent transporting and emission characteristics and the guest material whose emission characteristic is good can effectively produce all kinds of colors emission. High internal quantum efficiency is composed of good carrier injection (proper work function for electrode; suitable HOMO and LUMO for organic materials), charge carriers balance (better carrier transporting ability; ideal recombination zone in particular organic layer), and emission material with high PL quantum efficiency and nice color saturation. For improvement of carrier injection, for instance, it contains ITO pretreatment and adjunction of hole injection layer and electron injection layer. For improvement of carrier balance, it includes multilayer structure and the optimizing thickness of organic layers.. 21.

(33) 2-4 The two forms of organic light emitting displays-passive matrix and active matrix displays Just like passive-matrix LCD versus active-matrix LCD, OLEDs can be categorized into passive-matrix and active-matrix displays. Active-matrix OLEDs (AMOLED) require a thin film transistor backplane to switch the individual pixel on or off, and can make higher resolution and larger size displays possible.. Passive matrix OLED The development and commercialization activities of OLED have been booming over the past few years. The initial market entry point for OLED displays is expected to be in portable imaging products such as cellular phones, MP3 and car audio systems, applications that are currently served by traditional LCD and Vacuum Fluorescent Displays (VFD). Passive matrix driven OLED (PMOLED) actually fits the rising demand of color display in cell phones because it has a lower cost structure (PMOLED requires 4 photo steps while C-STN LCD requires 6 photo steps), and a thin profile compared to LCD module with backlight. Although the main display and sub-display panels nowadays mainly use STN-LCD or TFT-LCD, there is a fast increasing trend for replacing the LCD into OLED in the coming years. In a PMOLED display (Figure 2.5), a matrix of electrically-conducting rows and columns forms a two-dimensional array of picture elements called pixels. Sandwiched between the orthogonal column and row lines, thin films of organic material are activated to emit light by applying electrical signals to designated row and column lines. The more current that is applied, the brighter the pixel 22.

(34) becomes. For a full image, each row of the display must be charged for 1/N of the frame time needed to scan the entire display, where N is the number of rows in the display. For example, to achieve a 100-row display image with brightness of 100 nits, the pixels must be driven to the equivalent of an instantaneous brightness of 10,000 nits for 1/100 of the entire frame time. While PMOLEDs are fairly simple structures to design and fabricate, they demand relatively expensive, current-sourced drive electronics to operate effectively. In addition, their power consumption is significantly higher than that required by a continuous charge mode in an active-matrix OLED. When PMOLEDs are pulsed with very high drive currents over a short duty cycle, they do not typically operate at their intrinsic peak efficiency. These inefficiencies come from the characteristics of the diode itself, as well as power losses in the row lines. Power analyses have shown that PMOLED displays are most practical in sizes smaller than 2” to 3” in diagonal, or having less than approximately 100 row lines. PMOLEDs make great sense for many such display applications, including cell phones, MP3 players and portable games [64].. Active matrix OLED Active-matrix OLED displays provide the same beautiful video-rate performance as their passive-matrix OLED counterparts, but they consume significantly less power. This advantage makes active-matrix OLEDs especially well suited for portable electronics where battery power consumption is critical and for displays that are larger than 2” to 3” in diagonal. An active-matrix OLED (AMOLED) display (Figure 2.6) consists of OLED pixels that have been deposited or 23.

(35) integrated onto a thin film transistor (TFT) array to form a matrix of pixels that illuminate light upon electrical activation. In contrast to a PMOLED display, where electricity is distributed row by row, the active-matrix TFT backplane acts as an array of switches that control the amount of current flowing through each OLED pixel. The TFT array continuously controls the current that flows to the pixels, signaling to each pixel how brightly to shine. Typically, this continuous current flow is controlled by at least two TFTs at each pixel, one to start and stop the charging of a storage capacitor and the second to provide a voltage source at the level needed to create a constant current to the pixel. As a result, the AMOLED operates at all times (i.e., for the entire frame scan), avoiding the need for the very high currents required for passive matrix operation [65]. For a high resolution display like a TV, a TFT backplane is necessary to drive the pixels correctly. Currently LTPS-TFT (low temperature poly silicon) is used for commercial AMOLED displays. LTPS-TFT has variation of the performance in a display, so various compensation circuits have been reported [66]. Due to the size limitation of the excimer laser used for LTPS, the AMOLED size was limited. To cope with the hurdle related to the panel size, amorphous-silicon/microcrystalline-silicon backplanes have been reported with large display prototype demonstrations [67, 68].. 2-5 The full-color technology of OLED The full color is the most important technology in practical application for flat panel display. The method for the full color displays are introduced as following, there are Side-by-Side, Color 24.

(36) filter (CF) method, and Color conversion method (CCM). OLED is divided into monochromatic colors, colorful (Area color) and full-color (Full color), each of the display area is still monochrome and full color is from R, G, B duplication of pixel RGB (pixel) component, the more fine resolution, the higher the resolution of the display.. 2-5-1 The introduction of full color technology Side-by-Side Pixelation (Figure 2.7) Side-by-Side Pixelation is the common use to industry for full-color technology, the method is that the Red, Green, and Blue organic materials are respectively fabricated onto the substrate by thermal deposition with metal shadow mask. The fine pitch shadow mask need to consider the issue that includes resolution, undercut angle, surface roughness, holding mechanism, and thermal stability [69]. The color of each pixel is composed of three primary colors (R, G, B) with unlike intensity by different driving voltages. The technology is focused on the color purity and efficiency for materials. The advantage for this method is that RGB materials emit independently to achieve the best luminous efficiency, but the deficiencies are that R, G, B materials require different driving voltage and cause poor color balance. In addition, there is the metal shadow mask in the process of manufacture, so precise degree of system must be improved. To Side-by-Side Pixelation method in PM-OLED, the problems are the purity, efficiency, and life of the red material. The manufacturers for full-color display with Side-by-Side Pixelation method contain Tohoku Pioneer, Sanyo, Kodak, NEC, Toshiba, RitDisplay and Teco, and so on. 25.

(37) Color conversion method (CCM) (Figure 2.8) Color conversion method is the use of blue OLED by the Blue emission, through the color conversion array converted to Red, Green, and Blue color. For the color conversion efficiency recently, the ratio is about 50 percent that it converted the Blue emission to the green emission, and the ratio is about 30 percent that it converted the Blue emission to the red emission. In theory, it can be completely converted the blue emission to red or green color. But the materials of conversion easily absorb the adjacent elements of light, and emit once again, resulting in the problem of contrast. Thus, manufacturers need to search for the organic materials with high-conversion efficiency. Besides, it is important to consider the design of structure in the device. Proper design of structure in the device can prevent the phenomenon of the interference with different colors. The development of technology for color conversion method can improve the difficulty in that three primary colors need different driving voltages. It causes the problem of the design for driving IC. It is essential to produce the blue emission with high efficiency and excellent color saturation. The high efficiency blue emission can avoid producing the bad current efficiency after energy transfer. Besides, the efficiency of Color conversion method is low than that of Side-by-Side Pixelation method because of the existence for the middle layer of Mesosphere material. Because it is easier to design the drive circuit in color conversion method and there are better characteristics in active-matrix mode.. 26.

(38) Color filter (CF) method (Figure 2.9) Color filter method is the way that is composed of white light and color filters to achieve the demand of full color display. The white light emission produced from the combination of three primary colors (Red, Green, Blue) or complementary colors. The biggest advantages of this full-color technology is that it can be directly utilized the color filter technology of liquid crystal display. However, the method results in decay of brightness due to the emission pass through the color filters. The color filters filter out two-thirds of the light, and decrease two-thirds of the efficiency for light emission. In addition, the costs increase with color filters in manufacture process and the white light emission is hard to generate. So many problems need to solve, which are cost down and better transmission of color filters.. 2-5-2 The comparison with full color technology The major advantage of Side-by-Side Pixelation method is that luminous efficiency and contrast ratio can achieve optimization. But the precise degree of the metal shadow mask and poor resolution need to improve. The biggest advantages of this full-color technology is that it can be directly utilized the color filter technology of liquid crystal display. There are few problems of resolution for Side-by-Side Pixelation method and Color conversion method in large-scale panels. For Side-by-Side Pixelation method, it uses ITO substrate and three independent guns for three primary colors(R, G, B). For Color conversion method and Color filter method, it is necessary for color conversion array or color filters collocate with the ITO 27.

(39) substrate respectively, and cost up. With the same yield, the methods of color filters and color conversion have low cost in materials and equipment as compared with the method of Side-by-Side Pixelation. For Side-by-Side Pixelation method, it needs different driving voltages for three primary colors, and decadent time is also unlike. That causes imbalance colors and poor color saturation. In the quality of panel display, there is no need for color filters or color conversion arrays in Side-by-Side Pixelation method, and it increases luminous efficiency and the contrast ratio. But the resolution is bad than that whose color filters method and color conversion method own.. 2-6 Material technologies Small molecules OLED technology was first developed at Eastman Kodak Company by Dr. Ching W. Tang using small molecules. The production of small-molecule displays often involves vacuum deposition, which makes the production process more expensive than other processing techniques. Since this is typically carried out on glass substrates, these displays are also not flexible.. Polymer light-emitting diodes Polymer light-emitting diodes (PLED), also light-emitting polymers (LEP), involve an electroluminescent conductive polymer that emits light as applying driving voltage. They are used as a thin film for full-spectrum color displays and require a relatively small amount of 28.

(40) power for the light produced. No vacuum is required, and the emissive materials can be applied on the substrate by a technique derived from commercial inkjet printing [70, 71]. The substrate used can be flexible, such as PET [72]. Thus flexible PLED displays, also called Flexible OLED (FOLED), may be produced inexpensively.. 2-7 OLED structures Bottom emission (Figure 2.10) Bottom emission uses a transparent or semi-transparent bottom electrode to get the light through a transparent substrate. Generally, the light emission in OLEDs emit from the ITO coated onto glass substrate. In active-matrix OLED, the OLED is controlled by thin film transistor (TFT). If OLED is fabricated in the way of bottom emission, the actual emission is limit to small area. The light emission is restricted to the thin film transistor (TFT) and metal circuits which fabricated on the glass substrate. It decreases the ratio of actual emitting area which is called the aperture ratio. The problem will get worse as the number of thin film transistor (TFT) raises to improve the difference of pixels.. Top emission (Figure 2.11) Top emission [73, 74] uses a transparent or semi-transparent top electrode to get the light through the counter substrate. The anode with high reflection reflects the light emission to pass through the transparent cathode. For top emission OLEDs, there is not the problem of the aperture ratio. As the number of thin film transistor (TFT) raises, the light emission is 29.

(41) unchangeably due to the light is through the counter substrate. The aperture ratio for top emission OLED is a lot greater than that for bottom emission OLED. If bottom emission OLED with low aperture ratio intent to achieve the same aperture ratio as that whose top emission OLED is, it is necessary to increase the current density of each pixel. The organic materials and panel display will accelerate to decay and produce reductive longevity.. Transparent OLED (Figure 2.12) Transparent organic light-emitting device (TOLED) uses a proprietary transparent contact to create displays that can be made to be top-only emitting, bottom-only emitting, or both top and bottom emitting (transparent). Transparent organic light-emitting device can greatly improve contrast, making it much easier to view displays in bright sunlight. It is important to the transparency of cathode for transparent and top emission OLEDs due to all the light emission pass through the metal cathode. It makes the ratio of output light emission by controlling the transparency of the cathode. In order to increase the transparency of cathode, the thicknesses of cathode metal must be thin enough. However, thinner metal results in worse conductivity and reduce the operating stability of the devices. In addition, the metal absorbs the light emission and wastes the light emission. It is difficulty to produce the cathode with good transparency and conductivity at the same time. Even if the cathode is ITO with good transparency and conductivity, it also involves in the problem of manufacturing process. It is hard to sputter the ITO cathode onto the organic layers without harming the organic layers. Thus, the development 30.

(42) of all kinds of protecting layers and the improvement of the damage caused from sputtering the ITO cathode onto the organic layers are points for top emission or transparent OLEDs.. Inverted OLED (Figure 2.13) In contrast to a conventional OLED, in which the anode is placed on the substrate, an Inverted OLED (IOLED) uses a bottom cathode that can be connected to the drain end of an n-channel TFT especially for the low cost amorphous silicon TFT backplane useful in the manufacturing of AMOLED displays[75]. In inverted OLED, the protecting layers such as CuPc, PTCDA [76], Pentacene [77], and PEDOT [78] are used to avoid the harm with sputtering ITO anode. Due to cathode must deposit onto glass and need to etch the proper figure, the metals such as Li, Ca, and Mg with low work function are not suitable for the use.. 2-8 The measurement of characteristics for OLEDs The efficiency of OLEDs can be defined as power efficiency (lm/W), and current efficiency or luminance efficiency (cd/A). Besides, the quantum efficiency of OLED has two parts: internal quantum efficiency and external quantum efficiency. Above descriptions are explained as follows:. Quantum efficiency Because the electroluminescence is belonged to current driving (holes and electrons recombine after injecting), the quantum efficiency is more proper to describe the characteristics of light emission for OLEDs. The quantum efficiency is defined as the ratio of the number of 31.

(43) photons output to the number of injecting electrons.. External quantum efficiency (EQE, ηext ) The external quantum efficiency is defined as the ratio of the number of. photons output. from the device to the number of injecting electrons.. Internal quantum efficiency (IQE, η int ) Because OLED is the multilayer structure, the light emission generating from the emission layer will lose because of the effect of waveguide and the effect of the resorption. The internal quantum efficiency is defined as the actual efficiency that excluded the above effects.. Light-coupling efficiency (η c ) The light coupling efficiency is defined as the ratio of the external quantum efficiency to the internal quantum efficiency.. Luminance efficiency (η L ) The luminance efficiency is defined as the ratio of the candela output to the input current (cd/A). It focuses on the characteristics of emission materials.. Power efficiency (η P ) The power efficiency is defined as the ratio of the lumen output to the input electrical watts (lm/W). It considers the characteristics of energy consumption and the design of energy system.. 32.

(44) 第三章實驗步驟關於 ITO 基板的處理包含以下步驟: 1. 使用丙酮清洗 5 分鐘，去除 ITO 表面的雜質污染。 2. 使用甲醇清洗 5 分鐘，去除 ITO 表面的殘留丙酮。 3.使用去離子水在超音波振盪器中清洗 10 分鐘。 4. 使用氮氣吹乾 ITO 基板。 5. 利用 spin coating 的方式在 ITO 表面塗上正型光阻。 6. 在烘箱中軟烤 10 分鐘。 7. 將 ITO 基板曝光，形成所需的電極形狀。 8. 在烘箱中硬烤 10 分鐘。 9. 利用鹽酸蝕刻出所需的圖案。 10. 重覆上述 1~3 步驟。熱蒸鍍包含以下步驟: -6. 1. 在熱蒸鍍腔體放入處理過的ITO基板，抽真空至 10 torr。 2. 控制材料加熱溫度及鍍率，依序鍍上所需薄膜厚度。 3.破真空，準備量測。元件的量測: 利用 PR655 搭配 Keithley 2400 在大氣下量測得到 OLED 的光電特性。 33.

(45) Chapter 3 Experiment procedure Indium tin oxide (ITO) coated glass with a sheet resistance of approximately 15Ω/ were consecutively cleaned in ultrasonic bath containing detergent water, acetone, ethanol and de-ionized (DI) water for 20 min each, then dried with an nitrogen (N 2 ) flow. All organic layers were deposited by high-vacuum (10-6 Torr) thermal evaporation. Thermal deposition rates for organic materials, inorganic materials and Al were about 1 Å/sec, 1 Å/sec and 10Å/sec, respectively. The evaporation rate and thickness of the thin films were monitored using a quartz crystal oscillator system (Sigma, SID-142). The structures of devices are as follows: A.. ITO/. Molybdenum. trioxide. (MoO 3 ;. N,N0-bis-(1-naphthyl)-N,N0-biphenyl-1,10-biphenyl-4,40-diamine. 15nm)/ (NPB;. 40nm)/. DPVBi (10nm)/ Rubrene (0.2nm)/ DPVBi (30nm)/ 4,7-Diphenyl-1,10-phenanthroline: cesium carbonate (BPhen: Cs 2 Co 3 =4:1; 10nm)/ Aluminum (Al; 120nm) B.. ITO/ MoO 3 (15nm)/ NPB (40nm)/ DPVBi (34nm)/ Rubrene (0.2nm)/ DPVBi (6nm)/ BPhen : Cs 2 Co 3 =4:1 (10nm)/Al (120nm). C.. ITO/ MoO 3 (15nm)/ NPB (40nm)/ DPVBi (10nm)/ Rubrene (0.2nm)/ DPVBi (24nm)/ Rubren (0.2nm)/ DPVBi (6nm)/ BPhen: Cs 2 Co 3 =4: 1 (10nm)/ Al (120nm). In these devices, MoO 3 and NPB are used as hole-injecting layer and hole-transport layer. The DPVBi acts as blue-emitting layer. The ultra-thin Rubrene (UTR) was selected as yellow 34.

(46) light-emitting layer. The BPhen: Cs 2 Co 3 is used as electron transporting layer and hole blocking layer. The chemical structures of the organic materials and the structures of the device are shown in Fig. 1. The active area of the device was 0.6 cm2. To measure the properties of the device, a voltage was applied by using a Keithley 2400 programmable voltage-current source (Keithley SourceMeter 2400; USA). EL spectra and CIE coordination of the devices were measured by PR655 spectra scan spectrometer (Kollmorgen Instrument PR655; USA). All measurements were carried out at room temperature in air without encapsulating the devices. The absorption spectra of the NPB, MoO 3 and MoO 3 -doped NPB films were recorded by a spectrophotometer (U-3900 Hitachi Japan). X-ray photoelectron spectroscopy (XPS) measurements were made in an ultrahigh-vacuum system. The XPS device, which comprised a high-power Mg Kα (1253.6 eV) line X-ray source and an angle-resolved electron energy analyzer, had an energy resolution of 0.2eV.. 35.

(47) 第四章結果與討論在本實驗中，學生首先固定DPVBi的厚度，並改變Rubrene的厚度。根據實驗結果，當元件的Rubrene厚度為 0.2 nm時有最佳的光電特性，接著改變Rubrene的位置。根據實驗結果，當Rubrene接近陰極時元件有較大的效率。最後增加黃光發光層Rubrene，使其提升載子捕獲效率。根據實驗結果，當兩層Rubrene在適當的位置時元件有最佳的亮度、色純度以及色穩定度。此外發現當MoO 3 -doped NPB作為電洞傳輸層元件效率有進一步的提升，由於NPB具有良好的熱穩定性以及成膜性，所以被廣泛的作為電洞傳輸層在ITO及有機層之間，而MoO 3 為過渡金屬氧化物為傳統電致變色的材料，具有高功函數以及強的電子接受能可以容易產生電子轉移在摻雜層中，因此在本實驗中，選用MoO 3 氧化物作為摻雜材料，以NPB當作電洞傳輸層主體。本論文最後得證MoO 3 氧化物摻雜到NPB中對元件效率有極大影響。. 36.

(48) Chapter 4 Result and discussion In order to fabricate WOLED, we first attempted to obtain the optimizing thicknesses of Rubrene. Because yellow light is required as one of the two-color complementary to obtain white light. In this section, the electrical and optical properties of the WOLEDs will be discussed by various process parameters, such as the location of Rubrene and the ratio of MoO 3 -doped. In our experiment, the energy band diagrams of the multilayer of WOLED shows in Fig 4.1.. 4-1 The characteristics of various thicknesses of Rubrene layer for WOLEDs We change the thicknesses of Rubrene layer from 0.1 ~ 0.3 nm and fix other organic layers. The devices with different thicknesses of Rubrene layer are designed as following: Device:. ITO/MoO 3 (15nm)/NPB (40 nm)/ DPVBi (20 nm)/Rubrene (0.1 nm)/ DPVBi (20 nm)/. BPhen: Cs 2 Co 3 (4: 1) (10nm)/Al (100 nm). Device:. ITO/MoO 3 (15nm)/NPB (40 nm)/ DPVBi (20 nm)/Rubrene (0.2 nm)/ DPVBi (20 nm)/. BPhen: Cs 2 Co 3 (4: 1) (10nm)/Al (100 nm). Device:. ITO/MoO 3 (15nm)/NPB (40 nm)/ DPVBi (20 nm)/Rubrene (0.3 nm)/ DPVBi (20 nm)/. BPhen: Cs 2 Co 3 (4: 1) (10nm)/Al (100 nm). The BPhen: Cs 2 Co 3 is used as electron transporting layer. We hope that Rubrene layer have good capture efficiency of charge carriers in the experiment due to we want to produce white light 37.

(49) emission. In order to generate good performance of WOLED, the balances of blue and yellow emission intensity are so important to be considered. In our experiment, the device with proper thickness of Rubrene layer produces high luminance. Figure 4.2 shows the current density-voltage (J-V) characteristics of the devices with various thicknesses of Rubrene layer. The current density at 14V of the devices, whose thicknesses of Rubrene layer are 0.1~0.3 nm are 59.6, 65.1, and 58.5 mA/cm2, respectively. We can observe that the device whose thickness of Rubrene layer is 0.2 nm has the best J-V characteristics as compared with other devices.. Figure 4.3 shows the luminance-voltage curves of the devices with various thicknesses of Rubrene layer are 0.1~0.3 nm respectively. The maximum luminance of devices is 1476, 1806, and 1566 cd/m2, respectively. The maximum luminance is 1806 cd/m2 at 14 V and the CIE coordinate is (0.270, 0.286) when Rubrene layer is 0.2 nm. Besides, the device with thicknesses of Rubrene layer is 0.2 nm the luminance high than other devices at the same voltage. The phenomenon tells us that proper thickness of Rubrene layer results in better luminance for WOLED. The device with 0.2 nm Rubrene layer is ideal because of the best luminance in other devices at the same applying voltage. Figure 4.4 shows normalized electroluminescence spectra of the devices at 14V with various thicknesses of Rubrene layer at 0.1, 0.2 and 0.3 nm respectively. According to the earlier report, the peak wavelengths of the DPVBi layer and the Rubrene layer were 436nm and 556nm, 38.

(50) respectively [79, 80]. It is believed the fact that light emission of three devices is composed of yellow light emission and blue light emission. In addition, we can find that the devices whose thickness of Rubrene layer are 0.1 and 0.3nm all have an EL peak without the balances of blue and yellow emission intensity. The device with 0.2 nm Rubrene layer has CIE coordinate (0.270, 0.286) which lies in about the blue color zone. However, the device with 0.2 nm Rubrene layer at 14V owns better luminance of 1806 cd/m2.. 4-2 The influence of different locations of Rubrene layer for WOLEDs In this study, the WOLED which consists of the blue emission layer and the yellow emission layer was fabricated as the energy band diagrams of the multilayer of WOLED shown in Fig 4.5. The Rubtene layer was inserted in the light-emitting layer of DPVBi to form the structure of DPVBi (10nm EML1)/ Rubrene (0.2nm)/DPVBi (30nm EML2) for the device A. The yellow emission of the device A was caused by the Rubrene layer. Figure 4.6 (a) shows the EL spectra of the device A at the applied voltage of 3~7V. The peak wavelengths of the DPVBi layer and the Rubrene layer were 436nm and 556nm, respectively. The intensity of blue emission was higher than that of the yellow emission, and the location of EL spectra for the blue and the yellow emission did not shift as the voltage increased. In addition, the CIE coordinates of the device A at the applied voltage of 3~7V were shown in Fig 4.6 (b). It was found that the CIE coordinates for the device A changed from (0.269, 0.299) at 39.

(51) 3 V to (0.249, 0.259) at 7 V. By comparing for the CIE coordinates (0.330, 0.330) of standard white light, the error value of the CIE coordinates were about (- 0.081, - 0.071) at the 7V, i. e., the shift of the CIE coordinates is about (-7.4%, -13.4%) during the applied voltage of 3~7V. That is to say, the CIE coordinates of the device A were unstable. As for the EL phenomenon in the applied voltage of 3~5V, some of the holes which were injected from the anode via Rubrene layer into the EML2 can be directly trapped by the Rubtene layer, which may be due to the effective hole trapping of rubrene molecules. However, the Rubtene layer has excellent charge carriers trapping property, but it is unable to trap lots of the injected holes. On the other hand, the electron mobility in BPhen: Cs 2 Co 3 of 3.9×10-4 cm2/Vs was less than the hole mobility of 5.5×10-4 cm2/Vs in NPB, so lots of electrons from electrode inject just into EML2, i. e., the electrons recombine with holes in the EML2 close cathode. Therefore, the electrons were difficult to be directly trapped by the Rubtene layer at 3~5V. As a result, the great majority electrons and holes would meet in EML2. Similarly, for the voltage of 6~7V, more charge carriers were injected into Rubrene layer and EML2, so the amount of recombination for electrons and holes were also relatively increased in Rubrene layer and EML2, resulting in an enhancement in the intensity of peaks, as shown in Fig. 4.6 (a). However, the Rubtene layer in the device A cannot trap enough electrons and holes to generate exciton. Therefore, optimum complementary color of the blue and the yellow emission intensity was not achieved. According to the results obtained above, the best recombination zone was in the EML2 of the 40.

(52) device A. For the device B, the Rubrene layer inserted in and closed to the cathode to form the structure of DPVBi (34nm EML1)/Rubrene (0.2nm)/DPVBi (6nm EML2) was fabricated as the energy band diagrams of the multilayer of WOLED shown in Fig 4.7. However, by comparing 4.6 (a) and 4.8 (a), it is found that the yellow emission intensity of the device B was stronger than that of device A because of its position of the Rubrene layer being changed. Although the Rubrene layer was inserted into the best recombination zone and has excellent charge carrier trapping effect, the blue emission and the yellow emission did not reach the best complementary color. As mentioned above, the optimal location is in EML2 of device A. Thus, the Rubrene layer was inserted into better recombination zone near the EML2/BPhen: Cs 2 Co 3 interface of device B. The difference between LUMO energy of Rubrene (-3.2 eV) and that of DPVBi (-2.8 eV) is 0.4 eV. Similarly, the difference between HOMO value of Rubrene (-5.4 eV) and that of DPVBi (-5.9 eV) is 0.5 eV [81, 82]. It is expected the electrons and holes can be trapped in the Rubrene layer as well as accumulated in the Rubrene/EML2 interface, leading to an enhancement in the intensity of yellow emission. Figure 4.8 shows the EL spectra and CIE coordinates of the device B. With the applied voltage increasing, most of the electrons and holes can be directly trapped and recombined to form exciton in the Rubrene layer. Moreover, the relative intensity of yellow is higher than that of the blue emission, i. e., the exciton number of Rubrene layer is more than that of DPVBi layer. In Fig. 4.8 (a), the intensity of both yellow emission and blue emission 41.

(53) become large, but the ratio of the enhancement in yellow emission is larger than that of blue emission. Thus, the CIE coordinate gradually shifts toward an orientation of white color. For example, when the applied voltage is 3V and 4V, the location of CIE coordinates shows at (0.462, 0.481) and (0.425, 0.437), respectively. By comparing for the CIE coordinates of standard white light, the error value of the CIE coordinates were gradually reduced. It is confirmed that the better recombination zone of device B is in the Rubrene /EML2 interface. With the applied voltage increasing to 5V, the recombination zone of electrons and holes gradually shifts toward the Rubrene /EML2 interface; the corresponding CIE coordinates is (0.391, 0.405). We thus conjecture that few excitons are generated in the EML2, resulting in an enhancement in the blue emission. On the other hand, more electrons and holes were trapped in the Rubrene, i. e., more excitons are generated in the Rubrene, leading to an enhanced yellow emission. At the voltage of 6~7V, CIE coordinates shift again from (0.368, 0.385) to (0.348, 0.365), and gradually shifts toward the CIE coordinates of standard white light. When a high voltage is applied, the concentration of electrons and holes increase which significantly influences the zone of the exciton generation. The movement of the electrons and holes are similar toward opposite electrode. Thus, the zone of the exciton generation in device B becomes broad as the EML1/ Rubrene and Rubrene /EML2 interfaces. This implies that the location of Rubrene not only improve color shift of CIE coordinates but also enhance carries recombination rate. By comparing for the CIE coordinates of standard white light, the error value of the CIE 42.

(54) coordinates were about (0.018, 0.035) at the 7V, i. e., the shift of the CIE coordinates is about (-24.7%, -24.1%) during the applied voltage of 3~7V. With the results obtained above, the CIE coordinates of device B is near the CIE coordinates of standard white light by the ultra-thin Rubrene layer inserted into EML2, but the CIE coordinates of device B is still unstable during the applied voltage of 3~7V.. 4-3 The characteristics of multiple-ultra-thin layer (MUTL) for WOLEDs Figure 4.9 shows the energy band diagram of device C, ITO/ MoO 3 / NPB/ DPVBi/ Rubrene/ DPVBi/ Rubren/ DPVBi/ BPhen: Cs 2 Co 3 / Al, and EL spectra and CIE coordinates during the applied voltage of 3~7V as shown in Fig 4.10 (a) and (b). It is found that there is a balance or complementary color in blue emission and yellow emission, as shown in Fig. 4.10 (a). Besides, at the applied voltage of 5V, a pure white emission with CIE coordinates of (0.331, 0.332) is observed. When the voltage continued to increase to 6 and 7V, the CIE coordinates were respectively (0.341, 0332) and (0.346, 0.339), and showed a little change. This is due to the fact that the ratio of blue and yellow emission intensity from EL spectra of Fig. 4.10 (a) is almost the same and about unit. Furthermore, this improvement in chromaticity can be attributed to the MUTL structure in the emission layer, resulting in a balance in the relative intensity of blue and yellow emission. Thus, by introducing a MUTL structure in the emission layer, the WOLED has more stable spectra characteristics than that of device A and B with the increase of bias voltage. 43.