藉由三層電洞阻擋層及摻雜客體材料製備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. 藉由三層電洞阻擋層及摻雜客體材料製備高穩定度白光有機發光二極體之研究 Investigation of high stable white organic light-emitting diodes by using triple hole-blocking layer and doping guest material. 研究生：李育蓁指導教授：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). 期刊論文(Journal). 1. Kan-Lin Chen, Chien-Jung Huang, Dei-Wei Chou, Yu-Chen Lee and Chih-Chieh Kang “The stable color purity of blue organic light-emitting diodes by using triple hole-blocking layer”, Submitted to Journal of Physics and Chemistry of Solids, Aug. 2013. 2. Kan-Lin Chen, Chien-Jung Huang, Yu-Chen Lee and Chih-Chieh Kang, “Enhancing color purity and stable efficiency of white organic light diodes by using hole-blocking layer”, Submitted to Solid-State Electronics, Aug. 2013. (B). 研討會論文(Conference). 1. Kan-Lin Chen, Yu-Chen Lee, Dei-Wei Chou, Chien-Jung Huang, Wen-Ray Chen and Teen-Hang Meen, “The Color Purity Stability of Pure Blue Oganic Light-Emitting Diodes by Using Double Hole Blocking Layer”, 2012 International Electron Devices and Materials Symposium (IEDMS), Kaohsiung, Taiwan, Nov. 29-30, 2012, AP-02.. III.

(4) 誌謝轉眼間兩年的碩士生活即將告一段落了。回想剛進入實驗室時，老師讓學生選擇未來的研究方向，到課餘時間的學習，了解OLED的基本觀念、原理，讓求學過程中更加充實。首先要感謝指導教授黃建榮教授，在這兩年的時間，老師用心的指導在課業及研究上，不斷的在實驗過程中提點，並且在討論數據過程中激發出各種想法，也不厭其煩地指導論文寫作及協助；於生活上，老師也常與我們分享生活中的經驗以及待人處事的道理，讓學生一生受用無窮。在這邊誠摯的感謝黃建榮老師，謝謝老師的用心指導讓這兩年的研究生活顯得更臻充實。感謝周德威教授、陳甘霖教授特別撥冗前來指導實驗及研究，讓學生可以順利完成論文，在此一併致上誠摯感謝。另外，特別感謝口試委員張守進教授、閔庭輝教授感謝他們在百忙之中特地前來參加我的口試，並且指導學生論文修改事宜。接著感謝家元學長及中喬學長，總是在陷入瓶頸時拉我一把，在他們的幫助下，才能順利完成論文；另外感謝同學豐益、學弟保勛和育豪、學妹葉涵和俐瑩，在這段期間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。感謝學長蔣宗勳、吳佳霖、蘇修賢，沒有你們的幫忙和鼓勵及互相吐槽也不會有今天這本論文；感謝同學蔡沛修、吳宗勳和吳佳仲，沒有你們的支持，我想我會一直停滯不前。謝謝朋友溫乘焜和何效儒給予的英文幫助，沒有你們的幫助這本論文也不會順利完成。最後，感謝我的家人，在我的求學過程給予我各種鼓勵和支持，衷心的感謝你們。特別感謝老哥、老妹和譯鴻的鼓勵和打氣，願將這本論文與你們分享，謝謝！. IV.

(5) Investigation of high stable white organic light-emitting diodes by using triple hole-blocking layer and doping guest material Advisor(s): Dr. Chien-Jung Huang Institute of Applied Physics National University of Kaohsiung Student: Yu-Chen Li Institute of Applied Physics National University of Kaohsiung. ABSTRACT This research includes two parts as mentioned: (I) high color stable blue organic emitting diodes and (II) high efficiency white organic emitting diodes by using triple hole blocking layer. In part (I), blue organic light-emitting diodes (OLEDs) with triple hole-blocking layer (THBL). structure,. consist. of. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. (BCP),. 4,4'-bis(2,2'diphenylvinil)-1,1'-biphenyl (DPVBi) and (4,4’-N,N’-dicarbazole)biphenyl (CBP), have been fabricated. Regardless of applied voltage variation, the luminance efficiency of the OLEDs with THBL structure was increased by 41% as compared with the dual hole-blocking layer (DHBL) structure. The CIE coordinates of (0.157, 0.111) of device with THBL structure is close to pure blue emission than that of other devices of DHBL. There is a coordinate with the slight shift of ±△ x, y = (0.001, 0.008) for the device with THBL structure during the applied voltage of 6-9V. The results indicate that the excitons can be effectively confined in the emitting layer of device, leading to an enhancement of luminance efficiency and more stable coordinate. In part (II), the white organic light-emitting diodes with THBL formation sandwich V.

(6) structure which generate white emission were fabricated. The 5,6,11,12-tetraphenylnapthacene (Rubrene), CBP and DPVBi were used as emitting materials in the device. The function of CBP layer is not only an emitting layer but also a hole-blocking layer (HBL), and the Rubrene was doped into the CBP. The optimal configuration structure was indium tin oxide (ITO)/ Molybdenum trioxide (MoO 3 ) (5nm)/ [4,4-bis[N-(1-naphthyl)-N- phenylamino]biphenyl (NPB) (35nm)/ CBP (HBL1) (5nm)/ DPVBi (I) (10nm)/ CBP (HBL2): Rubrene (4:1) (3nm)/ DPVBi (II) (30nm)/ CBP (HBL3) (2nm)/ 4,7-diphenyl-1,10-phenanthroline (BPhen) (10nm)/ Lithium fluoride (LiF) / aluminum (Al). The result showed that the device with Rubrene doped in CBP (HBL2) exhibited a stable white emission with the color coordinates of (0.322, 0.368), and coordinate with the slight shift of ±△ x, y = (0.001, 0.011) for applied voltage of 8-12V was observed.. VI.

(7) 藉由三層電洞阻擋層及摻雜客體材料製備高穩定度白光有機發光二極體之研究指導教授：黃建榮博士國立高雄大學應用物理系學生：李育蓁國立高雄大學應用物理系. 摘要本研究可分為二個部分，分別是第一部分的高顏色穩定度藍色有機發光元件和第二部分的高效率白光有機發光元件藉由使用三層電洞阻擋層(THBL)。首先，在第一部分藍色有機發光元件製作是使用 2,9-dimethyl-4,7-diphenyl-1,10 -phenanthroline (BCP) 和 (4,4’-N,N’-dicarbazole)biphenyl (CBP) 組成三層電洞阻擋層， 4,4'-bis(2,2'diphenylvinil)-1,1'-biphenyl (DPVBi)做為藍光發光層。不論施加的外加電壓增加，元件的發光效率利用THBL結構可以有效提升 41%相比於雙層電洞阻擋層結構。利用 THBL結構製作的元件CIE座標是(0.157, 0.111)接近純藍光的發光，而座標位移僅有±△ x, y = (0.001, 0.008)在外加電壓 6-9V。這個結果指出激子可以被有效限制在元件的發光層，使得亮度效率的提升和更穩定的座標。在第二部分的白色有機發光元件製作是利用THBL形成三明治結構並將黃光客體材料Rubrene分別摻雜進入電洞阻擋層內。我們得到一個最佳的結構是Indium tin oxide (ITO)/ Molybdenum trioxide (MoO 3 ) (5nm)/ [4,4-bis[N-(1-naphthyl)-N- phenylamino]biphenyl (NPB) (35nm)/ CBP (HBL1) (5nm)/ DPVBi (I) (10nm)/ CBP (HBL2): Rubrene (4:1) (3nm)/ DPVBi (II) VII.

(8) (30nm)/ CBP (HBL3) (2nm)/ 4,7-diphenyl-1,10-phenanthroline (BPhen) (10nm)/ Lithium fluoride (LiF) / aluminum (Al)。結果顯示將Rubrene摻雜到HBL2 展示一個穩定的白色發光與 (0.322, 0.368)的顏色座標，和座標僅有輕微的位移±△ x, y = (0.001, 0.011)在外加電壓 8-12V。. VIII.

(9) Contents 第一章緒論------------------------------------------------------------------------------------------------1 Chapter 1 Introduction--------------------------------------------------------------------------------2 1-1. General introduction-----------------------------------------------------------------------------------2. 1-2. The history of organic light-emitting diode--------------------------------------------------------3. 1-3. The benefits of organic light-emitting diode-------------------------------------------------------4 1-3-1 The difficulty of organic light-emitting diode-------------------------------------------5. 1-4. OLED Market opportunity-------------------------------------------------------------------------6. 1-5. The motion and purpose of this deliberation-----------------------------------------------------7. 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)--------------------------------------------------12 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 display--------------------------------------------------------------------------------------------------22 2-5 The full-color technology of OLED----------------------------------------------------------------25 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--------------------------------------------------32 IX.

(10) 第三章實驗步驟----------------------------------------------------------------------------------------34 Chapter 3 Experiment procedure----------------------------------------------------------------36 第四章結果與討論------------------------------------------------------------------------------------37 Chapter 4 Result and discussion------------------------------------------------------------------38 4-1 The characteristics of various thicknesses of DPVBi layer for Blue OLEDs-----------------39 4-2 The characteristics of various materials of double hole blocking layers layer for Blue OLEDs-------------------------------------------------------------------------------------------------41 4-3 The characteristics of various thicknesses of triple hole blocking layers for blue OLEDs-------------------------------------------------------------------------------------------------43 4-4 The characteristics of different material of electron transporting layer for blue OLEDs ---46 4-5 The characteristics of devices with insert MoO 3 layer for blue OLED------------------------49 4-6 The characteristics of devices with change triple hole blocking layer of material for blue OLEDs-------------------------------------------------------------------------------------------------50 4-7 The characteristics of Rubrene doped CBP of triple hole blocking layer for white OLEDs-------------------------------------------------------------------------------------------------51. 第五章結論與未來工作-----------------------------------------------------------------------------57 Chapter 5 Conclusion and future work--------------------------------------------------------59 5-1 Conclusion-------------------------------------------------------------------------------------------59 5-2 Future work------------------------------------------------------------------------------------------60. References-------------------------------------------------------------------------------------------------61. X.

(11) 第一章緒論有機發光二極體(Organic light-emitting Diode, OLED)具有自發光(不需背光源)，厚度薄，重量輕，操作電壓低，操作溫度範圍大，高亮度，低耗能，應答速度快製程簡單，以及廣視角等優點。相較於現今市面上廣範使用的薄膜電晶體-液晶顯示器(Thin Films Transistor-Liquid Crystal Display, TFT-LCD)，有機發光二極體擁有許多光電特性及製程上的優勢。除此之外，OLED 還具有可撓的特性，在攜帶方便及輕薄短小的考量下，有機發光二極體具備相當大的發展潛力。. 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 OLED 達到全彩化的需求，如何製作高亮度、高效率、高色純度的 RGB 三原色，便成為 OLED 研究發展的重要課題。而短波長的藍光 OLED 元件，在色轉換法(CCM) 的全彩技術中扮演重要角色。除此之外，在白光 OLED 製作方面不論是利用三原色法或互補色法，高效率的藍光都是不可或缺的。因此本論文藉由調變發光層的厚度，研究製作高色純度的白光 OLED；此外加入電洞阻擋層及額外的黃光摻雜層，對元件作進一步的研究及改善。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 全彩化的方法；第三章包含實驗的步驟，製作方法，和量測儀器；第四章則是不同參數下 OLED 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。. 1.

(12) 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 and the stable efficiency 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 this 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. T the resulting matrix of pixels can emit light of different colors, i.e., red, green and blue. Such systems can be used in television screens, mobile phone display 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 2.

(13) 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. 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 3.

(14) Nature” demonstrating a very-high-efficiency green light emitting polymer [13].. 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, attracted 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 unshifted 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). 4.

(15) OLEDs have attracted much interest due to their potential for flat-panel displays. Besides, 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. 5.

(16) The hugest technical problem for OLEDs is the lifetime confine of the organic materials. In 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 lifetime of OLEDs is unimportant. OLEDs have been used in most Samsung, HTC and Apple smart-phone, notably the Samsung galaxy SIII, and some models of the Nikon camera [19]. It is also found in the Sony PlayStation series of Handheld game console. Asus has also introduced recently some OLED products, including the Asus Padfone. 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 6.

(17) 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]. In June 2013 by LG and Sumsang were make public peddle 55-inch flexible and unflexible by AMOLED television [22].. 1-5 The motion and purpose of this deliberation The most important goal for OLEDs is its application in the full color on the plane display. Due to the purpose, the high efficiency and better color purity of three primary colors red, green, and blue (R、G、B)is crucial to consider. One of methods for full color display is the color conversion method (CCM), which blue OLED (BOLED) emit blue emission and the fluorescent dyes absorb the blue emission. After absorbing the blue emission, the fluorescent dyes emit red, green, and blue emission, respectively. Thus, the stable characteristic of blue emission is so important for full color displays with CCM. In addition, ideal blue emission is beneficial to fabricate high efficiency white organic light emitting diodes (WOLED) by three primary colors method or complementary colors method. Another method to manufacture full color plane display is composed of white emission and color filters [23]. The white emission must own high efficiency and purity to achieve pure and high luminance three primary colors(R、G、B) with particular color filter, respectively. Above all, ideal blue emission is not only crucial for full color 7.

(18) plane display but also for white space illumination. There are so many researches focused on blue light emitting devices and an extensive variety of organic compounds have been used for this purpose recently [24-26]. Blue light with short wavelength enable to make other colors via color filters or color conversion. If we can fabricate blue light, which possess high luminance and high efficiency, the full color panel displays are generated effectively. Saito demonstrated the devices with multiplayer structure [27-29]. It is widely known that hole mobility in hole-transporting layer (HTL) is higher than electron mobility in electron-transporting layer (ETL). Unbalanced charge carriers result in worse efficiency of organic light-emitting devices (OLEDs). Thus, it is important to balance charge carriers [30, 31]. In this study, a triple hole blocking layer to improve the color purity of blue OLED by modulation thicknesses and features of triple hole blocking layer and by confining exciton in BCP/DPVBi/BCP HOMO and LUMO energy levels as double the quantum-well structure, achieving the best color purity of blue is presented. In order to improve the luminance and fabricate white light, we were improved by the insertion 5,6,11,12-tetraphenylnapthacene (Rubrene) is used as a guest doping material layer into the CBP hole blocking layer. The CBP layer doped with Rubrene can effectively control the carrier recombination in the EML that will enable to reach the white light emission. It is due to the fact that the highest occupied molecular orbital (HOMO) of CBP is higher than that of Rubrene. However, we used the THBL structure so 8.

(19) that the carrier can be effectively confined and controlled at the interface of CBP/DPVBi, resulting that the CIE color coordinates of devices can be close to the standard CIE of white light emission. Simultaneously, the mechanism of devices with different Rubrene doped CBP layers is presented. It has excellent electrons transport and holes blocking ability. The CBP layer doped with Rubrene can effectively control the carrier recombination in the EML that will enable to reach the white light emission. It is due to the fact that the highest occupied molecular orbital (HOMO) of CBP is higher than that of Rubrene. However, we used the THBL structure so that the carrier can be effectively confined and controlled at the interface of CBP/DPVBi, which are close to the white coordinates of the national television system committee (NTSC) standard (0.33, 0.33).. 9.

(20) 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 全彩化的主要方法包含 side-by-side method、color conversion method、 color filter method 。此三種方法都有各自的優勢和缺點及改進的方向。藍光在色轉換法(color conversion method)中扮演重要角色，而本實驗主要是利用發光層厚度的改變加上三層電洞阻擋層及摻雜黃光發光層改善白光 OLED 的色純度。. 10.

(21) 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 emission 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.

(22) 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 LUMO for organic materials. The host-guest system generates emission spectrum of the guest material by energy transfer from host material to guest material. In other 12.

(23) 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 (S 0 ). 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 (S 1 ) or excited triplet state (T 1 ). 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.

(24) 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.

(25) 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.

(26) 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.

(27) function of ITO increases to above 5.0 eV by O 2 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 O 2 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 O 2 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.

(28) 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.

(29) 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.

(30) 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.

(31) 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.

(32) 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.

(33) 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.

(34) 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].. 24.

(35) 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 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 Side-by-Side Pixelation (Figure 2.7) 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 25.

(36) 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.. Color conversion method (CCM) Color conversion method (Figure 2.8) 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 26.

(37) 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.. Color filter (CF) method Color filter method (Figure 2.9) 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 27.

(38) 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 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. 28.

(39) 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 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 Bottom emission (Figure 2.10) 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.. 29.

(40) Top emission Top emission (Figure 2.11) [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 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 Transparent organic light-emitting device (TOLED) (Figure 2.12) 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 30.

(41) 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 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 In contrast to a conventional OLED, in which the anode is placed on the substrate, an Inverted OLED (IOLED) (Figure 2.13) 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.. 31.

(42) 2-8 The measurement of efficiency for OLEDs The efficiency of OLEDs can be defined as power efficiency (lm/W), and current efficiency or luminous efficiency (cd/A). Besides, the quantum efficiency of OLED has two parts: internal and external quantum efficiency. Above descriptions are explained as follows:. External quantum efficiency (EQE, ηext ) η ext or EQE, is the number of photons released from the device per number of injection hole - electron pairs.. Internal quantum efficiency (IQE, η int ) η int or IQE, is the number of photons generated inside the device per number of injection hole - electron pairs. A large fraction of generated photons stays trapped and absorbed inside the device.. Power efficiency (η P ) η P is the ratio of the lumen output to the input electrical watts (lm/W).. Luminous efficiency (η L ) The above paragraphs describe the ways of improving the internal quantum efficiency (IQE) and external quantum efficiency (EQE) and deal with maximizing the light output from recombination of the charge carrier pairs, holes and electrons, that are available in the device. To improve the luminous efficiency from the current levels - that is to maximize the lumen output per unit power (lm/W), more charge carriers should be supplied per unit electrical power ( or less 32.

(43) power should be used to keep the lumen output). For example, the same current densities (same light output) should be achieved at lower operating voltages. In other words, the organic light emitting diode circuit should provide less resistance.[ 79]. 33.

(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(CMOS Grade)清洗 5 分鐘，去除 ITO 表面的雜質污染。 2. 使用甲醇(CMOS Grade)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( >50%)吹乾 ITO 基板。 5. 利用 spin coating 的方式在 ITO 表面塗上正型光阻(MICROPOSIT S1805)。 6. 在烘箱中軟烤 30 分鐘。 7. 將 ITO 基板曝光(Optical Associates Inc. MODEL 500, Mask Aligner)，形成所需的電極形狀。 8. 在烘箱中硬烤 30 分鐘。 9. 利用鹽酸(36.5-38.0%)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≦4.8×10 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: 34.

(45) 利用 PR-655(Kollmorgen Instrument; USA) 搭配電源供應器(Keithley SourceMeter 2400; USA)在大氣下量測得到 OLED 的光電特性。. 35.

(46) Chapter 3 Experiment procedure The structure consists of the ITO as the anode, the NPB as hole transport layer, CBP as hole blocking layer, Rubrene as a yellow guest doping material, DPVBi as blue-emitting layer, BPhen as an electron-transport layer, LiF as electron buffer layer and Al as a cathode. The OLEDs were formed with an area of 0.06 cm2 at the intersections between the ITO anode and the cathode stripes. The sheet resistivity of the indium-tin-oxide (ITO) thin films on glass substrates used in this study was 10 Ω/sq. The ITO substrates were cleaned by using acetone(CMOS Grade) and methanol(CMOS Grade) at 25℃ for 5 min and were rinsed in de-ionized water thoroughly. The chemically cleaned ITO substrates were then dried by using N 2 gas( >50%). After the ITO substrate was loaded into an evaporation system, organic layers were sequentially deposited at a rate of 0.5 ~ 1.5 Å/s onto substrates at room temperature by thermal evaporation from resistively -6. heated tantalum boats at a base pressure of ≦4.8×10 Torr. The deposition rates were controlled by using a quartz crystal monitor. The characteristic current (I)-voltage (V)-luminance (B) curves of the OLEDs were measured by using both the power source (Keithley SourceMeter 2400; USA) and the photospectrometer (Kollmorgen Instrument PR655; USA), controlled using computer software. All the measurements were carried out with the devices in air immediately after fabrication without encapsulation.. 36.

(47) 第四章結果與討論在有機材料中，電洞相較於電子具有較大的移動率。為了使電子電洞有更佳的注入效果以及增加載子在發光層再結合的機率，選擇合適的有機材料、使結構能階更匹配；調變有機層的厚度、讓載子傳輸更平衡是 OLED 製作的重要課題。除此之外，加入三層電洞阻擋層，並搭配適當的摻雜層位置，可以提升元件的色純度。在本實驗中，學生首先改變DPVBi的厚度，並固定其它有機材料的厚度。根據實驗結果，當元件的DPVBi厚度為 40 nm時有最佳的光電特性。接著利用不同電洞的阻擋層材料，固定雙層電洞阻擋層之位置。根據實驗結果，當DPVBi的厚度是 40 nm時元件有較大的亮度。最後製作出三層電洞阻擋層，根據實驗結果，當三層電洞阻擋層的厚度分別是 6 nm、 2 nm、2 nm時元件有最佳的亮度及發光效率。此外為了增加三層電洞阻擋層的電洞注入能力，所以選擇具有高電洞注入的無機材料MoO 3 做為電洞注入層，根據實驗結果，發現元件加入MoO 3 層可以使亮度提升，進而產生更佳的效率。為了製作出純白光元件，將 Rubrene 摻雜進入 CBP 層中，隨著位置的變換可以得到色純度接近標準的白光座標，但是仍然有改善的空間。因此將發光層的厚度做調整，目的是改善元件的色純度與穩定性。實驗結果發現此結構的確可以改善白光 OLED 的色純度與穩定性。. 37.

(48) Chapter 4 Result and discussion 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. In our experiment, the energy band diagrams of the multilayer of blue OLED shows in Fig 4.1. The performance of devices was investigated with various thicknesses of organic layer such as NPB, Alq 3 , and DPVBi layer. The result shows that the device with proper thickness of organic layers generating better electron injection enhances efficiency and luminance for BOLED. In addition, proper thickness of triple CBP layers produces purer blue emission.. 38.

(49) 4-1 The characteristics of various thicknesses of DPVBi layer for blue OLEDs The emission layer plays an important role in OLED. In order to find the optimum characteristics for Blue OLED, we must find ideal recombination zone in organic layers. Thus, we change the thicknesses of DPVBi layer are from 20 nm ~ 50 nm and fix other organic layers. The devices with different thicknesses of DPVBi layer are designed as following: Device A:. ITO/NPB (40 nm)/ DPVBi (20 nm)/Alq 3 (20 nm)/LiF (0.5 nm)/Al (100 nm). Device B:. ITO/NPB (40 nm)/ DPVBi (30 nm)/Alq 3 (20 nm)/LiF (0.5 nm)/Al (100 nm). Device C:. ITO/NPB (40 nm)/ DPVBi (40 nm)/Alq 3 (20 nm)/LiF (0.5 nm)/Al (100 nm). Device D:. ITO/NPB (40 nm)/ DPVBi (50 nm)/Alq 3 (20 nm)/LiF (0.5 nm)/Al (100 nm). Figure 4.2 shows the current density-voltage (J-V) characteristics of the devices A-D with various thicknesses of DPVBi layer. The current density at maximum voltage of the devices, whose thicknesses of DPVBi layer are 20, 30, 40, and 50 nm, are 233, 176, 156 and 148 mA/cm2, respectively. We can observe that the device whose thickness of DPVBi layer is 20 nm has the best J-V characteristics as compared with other devices. In the same applying voltage, the device with thinner thickness of DPVBi layer owns better J-V characteristics. This suggests that DPVBi layer has an effect of blocking the charge carriers. Haskal obtained that the charge mobility in Alq3 layer is 3.4 ± 0.2 times than in DPVBi layer [80]. So DPVBi layer plays an important role in electrons transporting as compared with 39.

(50) Alq3 layer. Thus, the major recombination zone lie in DPVBi layer with blue light as the thickness of DPVBi layer is thick enough to restrict the holes in DPVBi layer. Figure 4.3 shows the luminance-voltage of the devices A-D with various thicknesses of DPVBi layer about 20, 30, 40 and 50 nm respectively. The maximum luminance of devices is 2340, 2430, 3060 and 2780 cd/m2, respectively. The maximum blue luminance is 3060 cd/m2 at 12 V. Besides, the device A whose thickness of DPVBi layer is 20 nm, the luminance is lower than other devices at the maximum voltage. The phenomenon tells us that proper thickness of DPVBi layer results in better luminance for OLED. Figure 4.4 shows normalized electroluminescence spectra and corresponding CIE coordinates of the devices A-D at 12V with various thicknesses of DPVBi layer are 20, 30, 40, and 50 nm respectively. When the DPVBi thickness is increased to 20 nm, the EL peak of the spectra was exhibit the dual peaks. Further increase in the DPVBi thickness (up to 50 nm) results in the collapse if the dual peaks into a single peak. In table 4.1, the device C, the EL peak wavelengths are 436 nm and CIE coordinate are (0.212, 0.271). The device C with at 102V owns high luminance 3060 cd/m2 and better luminous efficiency 1.96 cd/A with close to blue light emission. Base on experiment data, we consider the device with 40 nm DPVBi layer is optimum because of better luminance and luminous efficiency as compared with other devices. However the light emission of this device is composed of blue and green emission, the phenomenon needs 40.

(51) to be improved for pure blue light emission.. 4-2 The characteristics of various materials of double hole blocking layers for blue OLEDs Generally speaking, in conventional NPB/Alq3 OLEDs, the mobility of the holes in NPB (reported as about 10-4 cm-2 V-1 S-1) is much larger than the mobility of electrons in Alq 3 (reported as about 10-5 cm-2 V-1 S-1) [81]. Thus, it leads to the unbalance of holes and electrons in the luminescent region. In this study, a double hole blocking layer (HBL) to improve the color purity of blue OLED by modulation location and features of CBP and BCP layers and by confining excitons in HBL1/DPVBi/HBL2 highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels as the quantum-well structure, achieving the best color purity of blue are presented. The devices with different location and features of CBP and BCP layer are designed as following: Device E:ITO/NPB (40 nm)/CBP(8 nm)/DPVBi (40 nm)/CBP(2 nm)/Alq 3 (20 nm)/LiF (0.5nm) /Al (100 nm) Device F:ITO/NPB (40 nm)/CBP(8 nm)/DPVBi (40 nm)/BCP(2 nm)/Alq 3 (20 nm)/LiF (0.5nm) /Al (100 nm) Device G:ITO/NPB (40 nm)/BCP(8 nm)/DPVBi (40 nm)/CBP (2 nm)/Alq 3 (20 nm)/LiF (0.5nm) /Al (100 nm) 41.

(52) Device H:ITO/NPB (40 nm)/BCP(8 nm)/DPVBi (40 nm)/BCP(2 nm)/Alq 3 (20 nm)/LiF (0.5nm) /Al (100 nm). The schematic energy diagram of the fabricated OLEDs is shown in Figure 4.5. In this work, the total thickness of the CBP and BCP layers were kept constant at 10 nm. The lowest unoccupied molecular orbital (LUMO) levels of the NPB, the CBP, the BCP, the DPVBi and the Alq3 layers are -2.5, -3.2, -3.5, -2.8, -2.8 eV, and the corresponding highest occupied molecular orbital (HOMO) levels are -5.5, -6.3, -7, -5.9, -5.6eV, as obtained by using cyclic voltametry, respectively [82-85]. While the electrons are accumulated in the NPB/HBL1/DPVBi and DPVBi/HBL2/Alq 3 two wells due to the existence of the HBL in Figure 4.5, the holes are accumulated in the HBL1/DPVBi/HBL2 single well due to the existence of the DPVBi EML in Figure 4.5. Figure 4.6 shows the current density-voltage (J-V) characteristics of the devices E-H different location of CBP and BCP layer. The current density at 12V of the devices E-H are 191, 185, 153 and 117 mA/cm2, respectively. We can observe that the device whose both are CBP layer has the best J-V characteristics as compared with other devices. In the same applying voltage, the device with. both CBP layer owns better J-V characteristics.. Figure 4.7 shows the luminance-voltage curves of the devices E-H with different location of CBP and BCP layer. The maximum luminance of devices is 1340, 1370, 1130 and 965 cd/m2, respectively. The maximum blue luminance is 1370 cd/m2 at 12 V. Besides, the device D whose 42.

(53) both BCP layers, the luminance is lower than other devices at the same voltage. The phenomenon tells us that match energy level of HBL results in better luminance for OLED. Figure 4.8 shows normalized electroluminescence spectra and corresponding CIE coordinates of the devices E-H at 12V with different location of CBP and BCP layer. When the HBL is used BCP layer, the EL peak of the spectra were shown a little fall in the dual peaks. Further both of the BCP layers results in the collapse if the dual peaks into a single peak. In table 4.2, the device G, the EL peak wavelengths are 436 nm and CIE coordinate are (0.156,0.119). The CIE coordinates of device G is far away from the pure blue emission with CIE coordinates of (0.14, 0.08). It is attributed to the portion of carriers recombination at the Alq 3 layer, generating slight green emission. The difference in CIE coordinates of device A is ±△ x, y = (0.010, 0.005). The device G with at 12V owns high luminance 1130 cd/m2 and better luminous efficiency 0.738 cd/A with close to blue light emission.. 4-3 The characteristics of various thicknesses of triple hole blocking layers for blue OLEDs We improved the color purity of OLEDs by using structure of triple-hole blocking layer (THBL),. which. consisted. of. alternate. (4,4’-N,N’-dicarbazole)biphenyl. (CBP). and. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP). Both of them have excellent electron mobility and holes barrier ability. The BCP layer can effectively control the carrier in the EML 43.