Hole-Confining Concept for Blue Organic Light Emitting Diode

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2 Hole-Confining Concept for Blue Organic Light Emitting Diode A thesis submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirements for the degree of Master of Science (M.S.) in the department of Electrical and Computer Engineering of the College of Engineering June 2011 By Balasaheb Darade B.E (Electronics and Tele-Communication Engineering) Jawaharlal Nehru Engineering College, Aurangabad, India, 2005 Thesis Advisor and Committee Chair: Dr. Marc M. Cahay

3 Abstract Organic Light Emitting Diodes (OLEDs) have received wide attention in recent years because of their advantages of light-weight, wide viewing angle, self-emission and promise of low cost. There is continuous increase in demand for OLEDs applications particularly in solid state lighting and color displays. With such huge potential there is also an urgent need for more research in improving efficiency and lifetime. Blue OLEDs are the bottleneck in both of these important performance parameters among three primary colors (green, red and blue). The main objective of this thesis was to implement blue OLEDs and improve their efficiencies and lifetimes by using a novel hole-confining structure. Generally, in organic semiconductors, the hole mobility is higher than electron mobility by few orders of magnitude. This mismatch of electron and hole mobilities is detrimental because it leads to a lower recombination rate and leads to a degradation in device performance. In this thesis, we used the concept of a hole confining structure to overcome this challenge. This helps enhancing the recombination rate and the overall device performance. In chapter 2, a step by step description of the various OLED fabrication procedures is presented. Also different OLED devices made in the Spintronics and Nanoelectronics lab at the University of Cincinnati so far are discussed. In chapter 3, we implemented a blue OLED with a ITO/TPD/BCP/LiF/Al structure first, and then improved the OLED performance, by using a ITO/PEDOT:PSS/TPD/BCP/Alq 3 /LiF/Al structure resulting in a brighter blue OLED. This work was the first successful fabrication and characterization of blue OLEDs in Dr. Cahay s lab. iii

4 Finally, a comparison of all devices fabricated is presented, and suggestions for future work are offered. iv

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6 Acknowledgements I would like to express my sincere gratitude to my advisor Dr. Marc Cahay for his invaluable guidance, support and encouragement during good as well as bad times. I would like to thank Dr. Boolchand and Dr. Ferendeci for taking the time to read my thesis and for serving on my committee. I would like to thank all my friends in Cincinnati and US who made my stay enjoyable here. There are so many people who made a difference in my life in Cincinnati. I express my sincere gratitude to them for their constant support. vi

7 Table of Contents Chapter 1: Introduction Outline Background Advantages, Disadvantages and Applications of OLEDs Advantages of OLEDs Disadvantages of OLEDs Applications of OLEDs Working of OLEDs Carrier Injection Carrier Transport Carrier Recombination and Emission Performance Parameters External Quantum Efficiency Luminance Efficiency Thesis Outline Chapter 2: Fabrication of OLED Outline Device Structure and material selection Material Selection.16 vii

8 2.3 OLED Fabrication Procedure Substrate preparation ITO substrate patterning Spin coating of PEDOT:PSS Thermal evaporation of organics Thermal evaporation of cathode bi-layer..29 Chapter 3: Blue OLED using a hole blocking concept Outline Concept Structure and properties of the materials used PDOT:PSS TPD and NPB Bathocuproine (BCP) tris(8-hydroxyquinoline) aluminum (Alq 3 ) Electron and hole carrier transport/mobilities Hole blocking material Proposed structure Experimental Results & Discussion Chapter 4: Conclusions Conclusions viii

9 4.2 Suggestions for future work: References ix

10 List of Figures 1.1 First commercial display for car stereo by Pioneer/Toyota (1998) The LG 31 inch 3D OLED TV Source: a). Cross-section of bi-layer OLED, b). A flat energy band diagram of bi-layer OLED under no-bias, c). A flat energy band diagram of bi-layer OLED under bias Schottky-type thermal carrier injection [6] Fowler-Nordheim tunneling carrier injection [6] ITO substrate with a Kapton tape for selective patterning of the ITO substrate for anode formation Hot plate for ITO etching Patterned substrate after ITO etching Chemat technology spin-coater Photograph of thermal evaporation Revap-3000 setup Schematic diagram of vacuum chamber and Revap-3000 used in OLED fabrication [2, 12] Picture of Revap-3000 resistive heating mechanism showing the boat containing the organic and the copper current leads Various boats used in thermal deposition of OLED materials Schematic and actual picture of the molybdenum strip acting as shadow mask Cathode mask, original (top) and schematic representation (bottom) Schematic of final device after cathode deposition...31 x

11 3.1 Chemical structure of PEDOT:PSS ( Alex/) Molecular structure of TPD Molecular structure of NPB Molecular structure of BCP Molecular structure of Alq Schematic diagram of proposed ITO/PEDOT:PSS/TPD/BCP/Alq 3 /LiF/Al structure to form blue OLED Flat band energy diagram for proposed blue OLED using BCP as hole blocking/electron transporting material Flat band energy diagram of Device B: ITO/PEDOT:PSS/TPD/Alq 3 /BCP/LiF/Al using the concept of hole confinement [28] Flat band energy diagram of Device C: ITO/PEDOT:PSS/TPD/BCP/ Alq 3 /BCP/LiF/Al [28] Luminance as a function of current density for Devices B and C [28] IV curve comparison of Device B and Device C [28] IV curve for Device F IV curve for Device F IV curve for Device F IV curve negative voltage for Device F IV curve for Device F Illustration of blue emission during testing of device F 49 xi

12 3.18 Luminance vs wavelength for Device F...50 xii

13 List of Tables 3.1 Review and comparison of various OLED devices...41 xiii

14 Chapter 1 Introduction 1.1 Outline There is an increasing demand for organic light emitting diode (OLED) based displays for applications in TVs as well as mobile phones, handheld electronic devices and digital camera among others. OLEDs are the light emitting devices (LEDs) where the active region is composed of organic materials. Hereafter, we give a brief background and history about the OLED technology development and discuss its advantages. Next, we describe the working of a typical bi-layer OLED and discuss its performance parameters. 1.2 Background Electroluminescence (EL) is an electrical and optical non-thermal emission process in which a material emits light in response to the passage of an electric current or an applied electric field. In molecular films composed of small molecules with unsaturated bonds the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) are used to describe their electronics properties [1]. The interest in organic EL was revived in 1987 when efficient, low-voltage EL was reported by Tang and VanSlyke at Eastman Kodak Company, for an organic thin-film device with a novel heterojunction structure [2]. The reported device consisted of a junction formed by vacuum deposition of two thin layers ( Å) of small molecular weight hole transporting 1

15 diamine and light-emitting/electron transporting tris (8-hydroxyquinoline) aluminum (Alq 3 ) materials between an Indium Tin Oxide (ITO) anode and a Mg-Ag cathode. The report of this novel green-light-emitting organic device with unique characteristics of high EL efficiency, fast response, low drive voltage and simple fabrication procedure initiated worldwide interest in the development of OLEDs. Another significant work was published in 1990 by Burroughes et al. where EL was reported from a single-layer OLED using a polymer material [3]. The device consisted of highly fluorescent poly (para-phenylene vinylene) (PPV) deposited using simple spin-coating process as the active material sandwiched between two electrodes [4]. These discoveries have led to intense worldwide interest in new materials for incorporation into OLEDs for practical applications, primarily in full-color flat-panel displays and solid state lighting. 1.3 Advantages, Disadvantages and Applications of OLEDs Color displays and solid state lighting are already used in commercial applications and have some advantages over existing liquid- crystal displays (LCDs) Advantages of OLEDs: Self-luminous, i.e., with OLED we do not need to use backlight. This leads to better efficiency compared to other display technologies, High brightness, even at low temperature, Reduced ecological footprint on our environment, Lower monitory expenditure on energy, Flexible substrate, OLED can be fabricated on plastic surfaces, and therefore can be used in flexible electronics applications, 2

16 Low cost and easy fabrication: For instance inkjet printing and screen printing are mass production technologies which are used for OLED fabrication, Wide range of color selectivity is available to choose from, Thin, compact and lightweight devices. They seems very user friendly and good input/output (I/O) design, Wide viewing angle approaching 170 degrees, OLEDs require a low operating voltage, possess a fast response time and demonstrate potential for high resolution applications, OLEDs can be refreshed almost 1,000 times faster than liquid crystal displays (LCDs), and OLED displays have greater contrast ratio compared to LCDs Disadvantages of OLEDs: Lifetime: OLED performance degrades rapidly when in contact with water and moisture. Therefore, proper encapsulation is required, Low mobility in amorphous organic materials, White, Red, Green OLED typically have lifetime of 46,000 to 230,000 hours that is about 5-25 years. The lifetime of blue OLEDs is about 14,000 hours, i.e., about 1.6 year. So there is lot more work needed in that area in the design of efficient and durable blue OLEDs, and OLED processing is still pretty expensive and also OLED industry has to compete against the existing multi-billion dollar LCD market. 3

17 1.3.3 Applications: Figure 1.1: First commercial display for car stereo by Pioneer/Toyota (1998). The first commercial application of OLEDs was introduced by Pioneer Electronics as the front panel of a car stereo in 1997 using a passive OLED display (see Fig. 1.1). OLEDs have been used in a wide gamut of commercial applications such as low-power, bright, colorful, small screens for mobile phones, portable digital audio/mp3 players, car radios and digital cameras [5]. Companies such as LG, Samsung, and Motorola have manufactured and commercialized various cell phone displays using passive-matrix OLED (or PMOLED) and active-matrix OLED (or AMOLED) displays. In 2007, Sony became the first company to announce an OLED television for commercial sale when they introduced the XEL-1 - an 11-inch, 3 mm thick AMOLED Digital Television with high resolution, contrast ratio and low power consumption. The LG 31 inch 3D OLED TV (see Fig 1.2), Samsung droid charge, Android OS and a 8 mp camera, 4G support are few more examples. 4

18 Figure 1.2: The LG 31 inch 3D OLED TV. Source: Working of OLEDs The working mechanism of an OLED can be described in three different stages as follows: 1. Carrier Injection, 2. carrier Transport, and 3. carrier recombination and emission. Before going into further details we will see the diagrammatic explanation of flat energy band diagram without applied bias and after applied bias. 5

19 Figure 1.3: a). Cross-section of bi-layer OLED, b). A flat energy band diagram of bi-layer OLED under no-bias, c). A flat energy band diagram of bi-layer OLED under bias. 6

20 1.4.1 Carrier Injection This first stage of carrier injection involves the injection of carriers from the electrodes into the organic layers in the presence of an applied electric field [6]. When a forward bias is applied, holes are injected from the positively biased anode into the hole transport layer (HTL) and electrons are injected from the negatively biased metal cathode into the electron transport layer (ETL). This injection of holes and electrons from the electrodes into the organic layers is generally described using two theories: Schottky thermal injection and Fowler-Nordheim (FN) tunneling injection. Both mechanism are discussed briefly next. (i) Schottky thermal injection The Schottky-type carrier injection is obtained via impurity or structural disordered levels with thermal assistance. This mechanism of Schottky thermal injection is described by the following equation [1], as shown in Fig. 1.4 [7]. [1] where m is the effective mass of the carrier (electron or hole), k is Boltzmann s constant, h is Planck s constant, T is the temperature, q is the elementary charge, Φ Bn is the barrier height for carrier injection, and V is the applied voltage between the anode and the cathode. 7

21 Figure 1.4: Schottky-type thermal carrier injection [6]. (ii) Fowler-Nordheim tunneling injection This mechanism involves the tunneling injection of carriers from the electrode into the organic layer through the triangular barrier created by the band bending due to the high applied electric field. Carrier injection by Fowler-Nordheim tunneling mechanism can be described by the equation [2] and is illustrated in Fig. 1.5 [6]. [2] For efficient hole injection from the anode into the HTL, the difference between the HOMO of the HTL and the Fermi energy level E f of the anode should be as low as possible. The work function of ITO can be increased by oxygen plasma treatment which helps in increasing the work function of typical ITO to match the HOMO levels of the commonly used organic hole transport materials [8]. Similarly, for efficient injection of electrons from the cathode into the ETL, various methods such as using low work function cathodes or n-type doping of ETL are implemented to improve electron injection and consequently lower the operating voltage [9]. 8

22 Figure 1.5: Fowler-Nordheim tunneling carrier injection [6] Carrier Transport The transport of carriers across the thin organic layers in an OLED is through hopping process of carriers under the influence of applied bias [6].Taking the traps within the organics into consideration, charge transport through the organic layers can be divided into four different regimes: ohmic, space-charge limited, trap-charge limited and trap-filled space-charge limited [10]. (i) Ohmic transport regime: When a very low electric field is applied, the initial current flows or charge transport is limited only to the contact interface between the electrodes and the organic layers and not the bulk of the organics. Thus, the holes and electron currents typically exhibit ohmic behavior. (ii) Space-charge limited transport regime: Following the initial ohmic current flow, the increased carrier injection due to increased applied field creates a space charge at the interface 9

23 between the electrode and the organic semiconductor [6]. When the applied voltage increases, the resulting strong carrier injection into the low-mobility materials leads to charge accumulation in the organic film. This charge build-up screens out the applied field partially and results in its redistribution. The resulting behavior in the I-V characteristics is considered space-charge limited regime. (iii) Trap-charge limited transport regime: Under high applied electric field, relatively large number of carriers are injected into the organics and fill up the traps that may be present within the organic layers. This is classified as the trap-charge limited regime [10]. (iv) Trap filled space-charge limited transport regime: Once all the traps sites in the organic layers are filled up by the injected carriers at a very high voltage, the additional injected carriers are free to move in the presence of space-charge effects only without any influence of chargetrapping [10]. This regime is called the trap-filled space-charge limited regime Carrier Recombination and Emission As the holes and electrons are transported across the organic layers towards opposite biased electrodes, they eventually interact via coulomb interaction to form an exciton, resulting singlet and triplet excitons [11]. Parallel spin pairs recombine to form triplet excitons and antiparallel spin pairs recombine to form singlet excitons. Statistically, it is understood that carrier recombination creates singlet and triplet excitons at a ratio of 1:3 [6]. In fluorescent OLEDs, singlet excitons decay radiatively and in phosphorescent OLEDs, both singlets and triplets 10

24 contribute to photon emission. The HOMO and LUMO levels of the organic materials used in the device help determine in which layer the carriers recombine and photon emission occurs. 1.5 Performance Parameters There are various parameters used to characterize the performance of OLEDs: external quantum efficiency (EQE), radiance efficiency, radiance, luminance efficiency, luminance and lifetime External Quantum Efficiency The external quantum efficiency or EQE (denoted as η EQE ) is defined as the number of photons emitted out of the front of the device divided by the number of charge carriers injected into the device [6]. The expression for EQE is given by: η EQE = ξ γ r ST η PL where, ξ is the outcoupling efficiency, γ is charge balance factor, r ST is the ratio of singlet excitons to triplet excitons and η PL is the photoluminescence quantum yield. A more detailed description of these quantities is given next. ξ - The outcoupling efficiency is defined as the ratio of the number of photons emitted out of the front surface of the device to the number of photons generated inside the device. Ideally this ratio should be equal to unity, but it is always less than unity due to internal reflection and absorption 11

25 loss in the substrates and organics as the light travels through the adjacent layers on its way out of the device. γ - The charge balance factor is a measure of the balance between the holes and electrons injected into the device and the probability of their recombination with each other. Ideally, it should be unity, such that every electron and hole injected would recombine to form an exciton and subsequently decay radiatively. It is expected that this factor can be optimized by varying the thicknesses and compositions of the HTLs and ETLs and monitoring the I-V and current efficiencies and EQEs of the devices. Taking this idea into consideration, the devices in this thesis were designed to improve the charge balance factor by reducing the mobility of holes by confining them in the emitting layers. r ST Is the ratio of singlet excitons to the triplet excitons formed from the recombination of the injected charge carriers. As explained in section 1.5, the ratio of single excitons to triplet excitons is 1:3 according to spin statistics. η PL - Photoluminescence quantum yield is defined as the radiative yield of the singlet excitons formed Luminance Efficiency The luminance efficiency/luminance current efficiency (cd/a) is the ratio of the luminance (L, cd/m 2 ) of the light emitted to the input current density (J, A/m 2 ) [10]. The 12

26 luminance current efficiency is useful for gauging the influence of current on the device performance. 1.6 Thesis Outline In this thesis, we are reporting the first working blue OLED fabricated in the Spintronics and Nanoelectronics lab at University of Cincinnati. OLED research was started in our lab by K. Garre as a part of his Masters thesis [12]. First he had reproduced double-layer OLED structures similar to the one demonstrated Tang and VanSlyke. He successfully demonstrated green emitting OLED with a ITO/TPD/Alq 3 /Al structure. Then Garre further replaced Al with LaS cathode deposited by pulsed laser deposition and fabricated inverted LaS cathode OLED. Further, S. Mohan introduced a novel hole confining concept for efficient green OLEDs [13]. He realized it with a ITO/PEDOT/TPD/BCP/Alq 3 /BCP/LiF/Al structure. Next, N. Bhandari optimized the luminescence efficiency of green OLED using hole and electron blocking layers [28]. In this thesis, we are reporting the first high efficient blue OLED created in our lab using the hole confining concept. This thesis is mainly divided into 4 chapters: In chapter 1, there is brief discussion about background of OLEDs, basic operation, applications and device performance. In chapter 2, there are two sections. First, we study the general device structure and material selection. Then we discuss the OLED fabrication procedures used in the Spintronics and Nanoelectronics lab at the University of Cincinnati. 13

27 In chapter 3, the actual device structures, experimental results and their analysis are described. First we describe the molecular structure and properties of the various organic materials used. Next, the hole blocking nature of the BCP layer is used to fabricate blue OLEDs. We have documented the various types of OLEDs developed in our lab over the years. They all are reviewed in a tabular form including the blue OLED with BCP as hole blocking layer. Chapter 4 contains our conclusions and suggestions for future work. 14

28 Chapter 2 Fabrication of OLED 2.1 Outline In this chapter, we describe the OLED fabrication. First, we go through an overview of general device structure and material selection. Then we discuss the OLED fabrication procedures used in the Spintronics and Nanoelectronics lab at the University of Cincinnati. 2.2 Device Structure and material selection Tang and VanSlyke were the first to demonstrate a double layer OLED [2]. We take their device structure as a starting point of our discussion below. That device has the following basic configuration: Anode/Hole transport layer (HTL)/Electron transport later (ETL)/Cathode [ITO/TPD/Alq 3 /Al]. This device was first reproduced in our lab by K. Garre for his Masters thesis [12]. Subsequently, there have been further developments in structure and fabrication method over the years in our lab. One important milestone was using the concept of hole confinement. As the mobility of holes in TPD is much higher than that of electrons in Alq 3, it causes charge leakage and a decrease of efficiency. By adding the thin layer of BCP before the Alq 3 and another thick layer of BCP after Alq 3, holes were confined in the emitting layer leading to an increase in emission and efficiency. All the above devices had green emission in Alq 3. The purpose of this work was to fabricate a blue OLED to achieve confined emission to the TPD 15

29 layer rather than Alq 3. We restricted holes to the hole transport layer (TPD) by adding a thick layer of BCP after TPD. The important factors in device structure design are as follows: charge balance factor, location of recombination, and the more the number of layers, the more the complexity in designing and fabricating the device. It will also add up to the driving voltage of the device Material Selection: The important factors in selecting the device layers are as follows: Suitable selection of anode and cathode materials such that their work function values closely match the HOMO and LUMO levels, respectively. Matching energy level of organic-inorganic interface is useful for better efficiency, and ease of availability (can be reproduced easily). Anode: Indium tin oxide (ITO) over glass has been the favorite choice of substrate in OLED fabrication. Hole injection layer: ITO is most common choice for the anode, but it too has some downsides which includes: 16

30 High resistivity, and moderate surface roughness. (ITO is deposited on glass surface usually using sputtering from ITO source). A rough surface can damage the organic layers degrading device performance. PEDOT:PSS or Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) is used as the hole injection layer on top of the ITO surface. It has been shown to lead, to a reduced threshold voltage by more than 50% and an increase in maximum luminescence of OLED devices by three orders of magnitude [16]. Hole transportation layer: This layer must have good hole mobility. N, N'-diphenyl-N, N'-(3- methylphenyl)-[1,1'-biphenyl]-4,4'- diamine (TPD), N, N '-bis(1-naphthyl)- N, N '- diphenyl- 1,1'-biphenyl-4,4'-diamine (NPB) and copper phthalocyanine (CuPc) are commonly used. Also TPD has been a popular choice because of its better hole mobility (1-2x10-3 cm 2 /V.s) [18]. Also More importantly, has blue emission at its peak. Hole blocking layer: BCP was selected for the hole blocking layer (Section 3.4). It has a deep HOMO level (good for hole blocking) and LUMO level comparable to that of ETL (it is also a good electron transporting layer). 17

31 Electron transport layer: Alq 3 (8-hydroxyquinoline aluminum salt) is very good ETL material. Also it has superior electron transporting ability. The electron mobility in Alq 3 is 1.4x10-5 cm 2 V.s [19]. Electron injection layer: By adding LiF (Lithium Fluoride) at the ETL - cathode, the interface the effective cathode work function to 3.5 ev can be reduced by pulling down the vacuum level. Cathode: The cathode is usually selected with a low work function. Al, Mg, Ca, Li are some of the choices for the cathode. Al has a work function of 4.3 ev and it is easily available and cheap. Also it has been shown that Al-based composite cathodes reduce the driving voltage while increasing luminous and device efficiency [20]. 2.3 OLED Fabrication Procedure The fabrication of blue OLED, has evolved from the basic configuration ITO/TPD/Alq 3 /Al to a device containing a hole confinement structure. The details of the fabrication steps are outlined below. Blue OLED: ITO/PEDOT:PSS spin coating/organic layers deposition/ Bi-layer (LiF/Al) cathode. The different OLED fabrication steps are: 1. Substrate preparation, 2. ITO substrate patterning, 18

32 3. spin coating of PEDOT:PSS, 4. thermal evaporation of organics, 5. and thermal evaporation of cathode bi-layer Substrate preparation The first step in the fabrication is selecting the anode substrate. The first step is to select a glass or plastic substrate coated with ITO. This is common practice in display industry. We used ITO coated over glass substrate from Delta Technologies Inc., in Minnesota. Their specifications are as follows: Glass thickness: 1.1 mm ITO thickness: 1200 angstroms Sheet resistance: Ω/sq Size: 3 inch by 2 inch Commercially obtained ITO coated glass substrates are usually characterized by material roughness, square resistance and layer transparency [21]. 3 inch by 2 inch ITO coated glass is cut into four 1 inch by 1.5 inch substrate. The ITO substrate is put side facing down on a bed of clean wipes and then cut with a diamond cutter. This procedure is to avoid any roughness, distortion or scratches on the ITO side which may lead to shorts or uneven organic layer thickness during deposition leading to device degradation. A diamond scribe is used to cut the ITO substrate in three even pieces. Then each substrate is properly washed in de-ionized (DI) water and blown dry. 19

33 2.3.2 ITO substrate patterning To define the anode area of OLED, we used a wet etching method. About 1 cm wide Kapton tape is used to protect the area to be used as an anode (see Fig 2.1). The Kapton tape is highly resistant to acid bath. The etching solution is made up of 20% HCl and 5% HNO3 in a beaker. This beaker is kept on a hot plate covered with aluminum foil and heated to around 55 degree C. Etching is carried for about 6 minutes (see Fig 2.2). Figure 2.1: ITO substrate using Kapton tape for patterning of anode. 20

34 Figure 2.2: Hot plate for ITO etching. Figure 2.3: Patterned substrate after ITO etching. 21

35 Then the substrate is washed with DI water for about 2 minutes. An example of an etched substrate is shown in Fig 2.3. Next, the Kapton tape is carefully removed from the substrate by scrubbing it off and then washed with detergent and Q-tips. The substrate is then rinsed with DI water again to remove any remaining detergent solution. The etched substrate needs to be thoroughly cleaned and baked before thermal evaporation. 20 % by weight solution of ethanolamine in DI water is heated to 80% in an ultrasonic bath. Magnetic stir bars are used to agitate the solution to achieve homogeneity. ITO substrates are then dipped in the solution carefully keeping ITO side up for about 15 minutes. The ITO substrates are then removed and rinsed in DI water, dry blown in pure nitrogen gas and baked at 80 degree C for 15 minutes. Baking drives out any moisture and hence improves the device performance. ITO substrates are stored it in a nitrogen glow box if not used immediately. Appropriate ITO substrate preparation is important for fabrication of efficient devices. Any residual film can result in imperfections within the emissive area on the device [11] Spin coating of PEDOT:PSS Next, we deposit the hole transport layer by spin coating. We have selected baytron aqueous dispersion (PEDOT:PSS) from H.C. Starck Co. for the HTL layer. Spin coater used was from Chemat Technology, as shown in Fig

36 Figure 2.4: Chemat technology spin-coater. The substrate is placed on a chuck in the spin coater with the ITO facing up. A small vacuum is applied through a hole in the chuck to keep the substrate in place. 5 ml of PEDOT: PSS is then spread over the substrate and then the following two spin cycles are applied rpm for 9 seconds, followed by rpm for 45 seconds. As a result, we obtain a spin coated PEDOT:PSS layer with thickness of about 30 nm. This layer is then subsequently baked at 80 degree C for about 15 minutes. 23

37 2.3.4 Thermal evaporation of organics Thermal evaporation was used for all other organic layers after the PEDOT deposition. First, organics were deposited at low temperature and then a cathode bi-layer at high temperature using different masks. Revap-3000 resistive heating unit from MDC Vacuum Products Inc, was used for thermal evaporation. A picture of the Revap-3000 is shown in Fig 2.5. Figure 2.5: Photograph of thermal evaporation Revap-3000 setup. 24

38 Figure 2.6: Schematic diagram of vacuum chamber and Revap-3000 used in OLED fabrication [2, 12]. 25

39 A schematic diagram of the vacuum chamber is shown in Fig 2.6. A photograph of the boat containing the organics and current leads of the Revap-3000 system are shown in Fig Figure 2.7: Picture of Revap-3000 resistive heating mechanism showing the boat containing the organic and the copper current leads. The vacuum chamber is pumped by mechanical and turbo pumps. In the resistive heating unit, a current is applied through the boats leading to material deposition on the target substrate. The substrate holder is placed right above the material holder (called boats) about cm away. A quartz crystal monitor is used to measure the film thickness deposited on the target substrate. Care needs to be taken that the substrate does not blocks the crystal thickness monitor. 26

40 The crystal thickness monitor is calibrated to give the thickness depending on the material being deposited. Different boats are used depending on various factors such as evaporation temperature, rate of evaporation, and reactivity of the material to be deposited with the boat material. The different types of boats used in our lab for OLED fabrication are shown in Fig 2.8. All the organic materials used are in powder form. They were deposited using either molybdenum boat with cluster mask or an alumina oxide crucible. Boats with cluster mask give better uniform thin films and crucible gives better control over the deposition rate. Once the organic materials are loaded, the ITO substrate is mounted vertically above it. There is a molybdenum strip with two screws on either side on the holder. The molybdenum strip has dual function. One is to hold the substrate and the other is to act as a shadow mask for organic deposition. A schematic and actual picture of the molybdenum strip is shown in Fig 2.9. After the first organic layer is deposited, we need to move the copper lead manually to deposit another organic material in another boat/crucible. The deposition rate is controlled by the percentage of maximum power supplied by controlling the current through the leads. This procedure can be automatized through a control panel for the power and crystal monitor. Typically the required power for a particular material changes over time and is better controlled manually. 27

41 Figure 2.8: Various boats used in thermal deposition of OLED materials. 28

42 Figure 2.9: Schematic and actual picture of the molybdenum strip acting as shadow mask Thermal evaporation of cathode bi-layer Once the organics are deposited the vacuum chamber is brought back up to atmospheric pressure. Then to define the cathode area another shadow mask is mounted over the substrate. LiF is loaded in a boat [Fig 2.8.b] and a small piece of aluminum wire loaded in another boat [Fig 2.8.d]. The chamber is closed and brought down to base pressure. First the chamber is pumped to the pressure of around 100 mtorr using a mechanical pump and then down to 2x10-5 Torr using a turbo pump. Once the cathode evaporation is done, the device is ready for testing. It is preferred to do the testing on the same day to reduce exposure to humidity. If not tested immediately, the device should be transferred to a nitrogen glow box. 29

43 Figure 2.10: Cathode mask, original (top) and schematic representation (bottom). 30

44 Figure 2.11: Schematic of final device after cathode deposition. In the final device (as shown in Fig. 2.11), the grey color represents the area where the cathode is deposited. The green region is the final emitting area. 31

45 Chapter 3 Blue OLED using a hole blocking concept 3.1 Outline In this chapter, we discuss actual device structure, the device characterization and experimental results and their analysis. We describe the molecular structure and properties of the various organic materials used. Then we discuss charge transport properties of electron and hole in each material layer. This is very important factor in determining the EQE of OLED as well as defining its characteristics and luminous efficiency. The hole blocking nature of the BCP lead us to the fabrication of blue OLEDs. We are comparing these devices to various OLEDs developed in our lab over the years. 3.2 Concept Structure and properties of the materials used We describe the molecular structure and properties of the organic materials used in OLED fabrication. This provides more understanding into device physics and working of OLED PDOT:PSS The conducting polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT PSS) is a polymer mixture of two ionomers [33]. It is electrically conducting, transparent, printable, and flexible. So it is a good candidate to be used for flexible, printable 32

46 electronics. It has good transmission and low absorption in the visible range. It has been used in photo-film industry for a long time. It is a conductive polymer which can lower the drive voltage as it eases the hole injection into the device. It can be easily spin coated and hence has potential to be used as an alternative to ITO. By adding solvents like PEDOT: PSS, its conductivity can be increased significantly. Figure 3.1: Chemical structure of PEDOT:PSS ( Alex/). As mentioned in chapter 2, PEDOT:PSS is spin coated over ITO prior to depositing other organic layers. It resulted in increased luminescence of OLED and reduces the anode ionization potential leading to a decrease in threshold voltage. 33

47 3.2.2 TPD: N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-l,l'biphenyl-4,4'diamine and NPB(N,N -bis(1-naphthyl)-n,n -diphenyl-1,1 biphenyl 4,4 -dimaine) TPD and NPB are commonly used as hole transport layers (HTL s) in organic devices. And their molecular structures are shown in the figures below. Figure 3.2: Molecular structure of TPD Figure 3.3: Molecular structure of NPB TPD and NPB films have similar hole mobility [22] and only slightly different HUMO/LOMO levels. Therefore, devices fabricated with either TPD or NPB have comparable 34

48 characteristics. The main requirement for a good HTL layer are good hole mobility, low resistance/barrier for holes injection from the anode and a high glass temperature (Tg). TPD is reported to have hole mobility of 10-2 to 10-3 cm 2 /(V.s) [32]. TPD has a peak in its emission spectrum in the blue Bathocuproine (BCP) 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) also known as Bathocuproine is a commonly used electron transporting and hole blocking material for OLEDs. It has a deep HOMO level of around 6.7 ev and a LUMO level at 3.2 ev which provides a high barrier for the flow of holes and allows electron flow. Figure 3.4: Molecular structure of BCP tris(8-hydroxyquinoline) aluminum (Alq 3 ) Alq 3 is commonly used as an electron transporting material as well as emitting material. It has a LUMO level of 3.1 ev which makes it a good electron transporting material. The peak in 35

49 the emission spectrum of Alq 3 lies in the green region around 545nm. It can degrade rapidly when forced to transport holes [23]. Figure 3.5: Molecular structure of Alq Electron and hole carrier transport/mobilities The working of an OLED can be described in three stages- carrier injection, carrier transport, and carrier recombination and emission. Carrier injection involves the introduction of carriers from the electrodes into the organic layers in the presence of an applied electric field. Transport of carriers across the thin organic layers in an OLED is through the hopping of carriers under the influence of applied bias [29]. As the holes and electrons are transported across the 36

50 organic layers towards opposite biased electrodes, they interact via Coulomb interaction to form an exciton [29]. Excitons decay radiatively leading to photon emission. The carrier mobility of organic materials is a very important factor in understanding of the operating mechanism in OLED devices. Charge transport (charge balance factor) plays a major role in determining the efficient (high EQE) of an OLED device. Fabricating blue OLED with optimized charge balance factor (close to one) is one of the important goal for this thesis. The main motivation was to obtain blue emission with available resources and without using any dopants. In conventional OLED structure- ITO/TPD[HTL]/Alq 3 [ETL]/Al, the mobility of holes in TDP the (HTL) is larger than mobility of electrons in Alq 3 (ETL) at least by a factor of two. Also the hole barrier at the ITO (Anode)/TDP interface is lesser than at the Alq 3 /Al cathode which results in electron/hole imbalance. This cause holes leakage to the cathode and also emission toward the cathode. To obtain blue emission we need to confine the holes in HTL. In this thesis we have proposed to add a layer of effective hole blocking layer (HBL) in between HTL and ETL. For the hole blocking material following requirements must be satisfied [30]: The material must have a wide energy band-gap, The material must have a deep HOMO level (i.e., a high ionization energy) so that it can effectively restrict the flow of holes, The LUMO level should be closely aligned with that of the ETL so it allows free flow of electrons towards the anode, and 37

51 Furthermore, the hole blocking material should be readily sublimable, forming uniform amorphous films, since these OLEDs consist of several organic layers that cannot be readily prepared by solution deposition methods. 3.4 Hole blocking material BCP is the commonly used material which meets all the above criteria. BCP has a deep HOMO level of 6.7 ev. BCP has been used in OLEDs either as [31]: hole blocking layer, electron transporting and buffer layer, hole-trapping layer, and exciton-blocking layer. In this work, we have used BCP as a hole blocking as well as electron transporting layer to obtain efficient blue emission in TPD. Thicker layer of BCP improves electron transport, and also block holes whereas thin layer slows down the hole transport but do not block them completely. In his Masters thesis, S. Mohan used a concept of hole confinement for green emission in Alq 3 using two layers of BCP (thin and thick) [13]. This helped confining holes in the emitting layer improving charge balance factor and leading to better EQE. Also, further BCP- Alq 3 -BCP structures were used by Mohan, Bhandari and Rakurthi in Spintronics and 38

52 Nanoelectronics lab at the University of Cincinnati, leading to improved EQE performance for respective OLEDs. 3.5 Proposed structure In this work, we decided to use BCP both as hole blocking as well as ETL in HTL (here TPD) and ETL (here Alq 3 ). The TDP/BCP interface results in barrier of ~ 1.3 ev (HOMO) for holes from HTL to travel towards the cathode, and hence confines holes in TPD. Also comparable LUMO levels make BCP/Alq 3 to easily transport electrons. In this work, we are proposing the structure ITO/PEDOT:PSS/TPD/BCP/Alq 3 /LiF/Al to form a blue OLED (shown in Fig 3.6). We study how to optimize the thickness of each layer in the next section. Figure 3.6: Schematic diagram of proposed ITO/PEDOT:PSS/TPD/BCP/Alq 3 /LiF/Al structure to form blue OLED. 39

53 Figure 3.7: Flat band energy diagram for proposed blue OLED using BCP as hole blocking/electron transporting material. 3.6 Experimental Results & Discussion Table 3.1 gives a comparison of the various OLEDs fabricated in the Spintronics and Nanoelectronics laboratory at University of Cincinnati over the years. This gives an overview and understanding of device operation for comparison and the results of their analysis. 40

54 Table 3.1 Review and comparison of various OLED devices Device A B C D E F G Layers ITO Yes Yes Yes Yes Yes Yes Yes PDOT No Yes, Yes, No No Yes, No ~ 30 nm ~ 30 nm ~ 30 nm TPD (Å) BCP (Å) Alq 3 (Å) BCP (Å) LiF (Å) Al (Å) Emission Green Green Green Green Blue Blue Blue 41

55 Device A: ITO/TPD/Alq 3 /Al Green OLED This was the first demonstration of a double-layer green OLED similar to the one first described in the original work of Tang and Van Slyke by K. Garre in his Masters thesis [2, 12]. This OLED consists of an ITO anode, TPD as HTL, Alq 3 as ETL and emitting layer and Al as a cathode. The separation of HOMO level in HTL and LUMO level in ETL is 2.3 ev. This leads to electron and hole recombination at the HTL/ETL interface and exciton formation. Electron hole recombination eventually leads to green light emission. The current-voltage (IV) curve of this device was recorded for forward and reverse bias. A diode like behavior was observed as expected. There is a Fowler-Nordheim emission of electrons and holes across the triangular barriers formed at the ETL/cathode and ITO/HTL, respectively. As the forward bias is increased, these triangular barriers become narrower leading to an exponential increase in the current [12]. Device B and C: Masters thesis of Nikhil Bhandari [28]. These structures were studied by Nikhil Bhandari in his Masters thesis using the concept of hole confinement to improve the efficiency and overall performance of green OLEDs. In Device B, BCP was used as a hole barrier and Alq 3 as a emitting layer whereas in Device C there were two layers of BCP. As suggested by K. Garre, in this structure, one thin layer of BCP was added before the emitting layer as a hole impeding layer (HiL) to slow down the holes. 42

56 Figure 3.8: Flat band energy diagram of Device B: ITO/PEDOT:PSS/TPD/Alq 3 /BCP/LiF/Al [28]. Figure 3.9: Flat band energy diagram of Device C: ITO/PEDOT:PSS/TPD/BCP/ Alq 3 /BCP/LiF/Al [28] 43

57 The IV curves of these devices showed a diode like behavior. The device with the BCP layer was found to have a slightly higher voltage for the same current thought it was brighter. The maximum luminance for Device B was found to be 2540 cd/m 2 and that for Device C was found to be 2870 cd/m 2. Device C showed the improvement of 11.5 % in luminance and 7.9% of EQE compared to what device B [28]. Figure 3.10: Luminance as a function of current density for Devices B and C [28]. 44

58 Figure 3.11: IV curve comparison of Device B and Device C [28]. Device D: ITO/TPD/BCP/Alq 3 /LiF/Al OLED This was our first attempt to obtain blue OLED emission. We used thick TDP and BCP layers where BCP is the hole blocking layer. However, we obtained green emission. This is probably because the Alq3 was too thick and the BCP layer too thin. Device E: ITO/ TPD/BCP/LiF/Al In the next device we increased the thickness of BCP and did not deposit Alq 3 at all. BCP has deep HOMO and LUMO levels comparable to Alq 3. This was the first time we obtained blue emission in the Spintronics and Nanoelectronics lab. It worked at low voltage for very short time. Slightly offset organic layers probably produced a leakage current. In this case, the holes reached the cathode too quickly and there was not enough electron-hole recombination for light 45

59 emission. Our next goal was to optimize the thickness of each layer for improving the brightness and efficiency of the next OLEDs. Device F: ITO/PEDOT:PSS/TPD/BCP/Alq 3 /LiF/Al This was the best blue device obtained so far in our lab. We fabricated the four devices in a single run in our vacuum chamber. Below are the IV curves for all the four devices. All of them show diode like characteristics and all of them showed blue emission. Figure 3.12: IV curve for Device F- 1 46

60 Figure 3.13: IV curve for Device F- 2 Figure 3.14: IV curve for Device F- 3 The third device included negative voltage readings to try and get a breakdown voltage as shown in Fig

61 Figure 3.15: IV curve negative voltage for Device F-3. The fourth device showed the smoothest IV curve. Figure 3.16: IV curve for Device F-4. 48

62 The successful blue emission is shown in the Fig The entire device area was blue emitting when tested. Figure 3.17: Illustration of blue emission during testing of device F. We further measured the luminous intensity using the ocean optics OOIIrrad software. As shown in Fig. 3.18, the emission peaked at 457 nm which is in the blue color range of the electromagnetic spectrum. 49

63 Figure 3.19: Luminance vs wavelength for Device F Device G: ITO//TPD/BCP/Alq 3 /LiF/Al This device started glowing at 12 V and was at its brightest at 25 V with a uniform glow. 50

64 Chapter 4 Conclusions 4.1 Conclusions A multi-layer, blue light emitting OLED has been successfully implemented using a ITO/PEDOT:PSS/TPD/BCP/Alq 3 /LiF/Al structure. The concept of hole confinement was used for achieving improved brightness and efficiency. Firstly, we have the various OLED devices fabricated in the Spintronics and Nanoelectronics laboratory at University of Cincinnati. The first green OLEDs were made by K. Garre using the device structure: ITO/TPD/Alq 3 /Al. The work was taken to the next level by N. Bhandari and S. Mohan in our lab. They used the hole impeding and hole blocking layer a well as improved charge injecting materials to lower the device operating voltage. The improved device structure they used was ITO/PEDOT:PSS/TPD/BCP/Alq 3 /BCP/LiF/Al. The first BCP layer was used as a hole impeding layer and the other as a hole blocking layer. BCP acts as a hole blocking layer because of its deep HOMO level. Devices fabricated by N. Bhandari implementing the hole confining structure exhibited significantly improvement by 11.5 % in luminance and 7.9% in EQE compared to a reference device without BCP as hole blocking layer. Our first attempt to implement a blue device at our lab (ITO/TPD/BCP/Alq 3 /LiF/Al) failed because the BCP and thick Alq 3 layers were too thin. The resulting device emitted in the green region of the visible spectrum. By increasing the BCP thickness and no Alq 3 deposition (ITO/TPD/BCP/LiF/Al), we implement the first blue emission in our lab. This was a short lived device. Most likely due to excess leakage current, holes reaching the cathode too quickly. 51

65 The ITO/PEDOT:PSS/TPD/BCP/Alq 3 /LiF/Al OLED was the best blue emitting OLED device structure implemented in our lab in which we used a thick BCP layer and a thin Alq 3. The hole confining structure was successful in reducing hole leakage to the electrode. It helped confining holes at the emitting interface (TPD/BCP) improving the charge balance factor and leading to better EQE. 4.2 Suggestions for future work: Device Encapsulation: We observed rapid degradation in the luminescence with time as devices that were made in our lab were not encapsulated. In fact, it is well-known fact that organic devices are very sensitive to moisture. The presence of moisture may damage the functionality of molecular device through hydrolysis. When organics are exposed to moisture, OLED performance degrades quickly [34]. So for better lifetime performance, nitrogen ambient should be used during encapsulation of the devices. Hole blocking layers: BCP devices with hole-confining structure demonstrated improved device efficiency. But they also exhibited poor device lifetimes which may be caused by the unstable nature of BCP material which crystallize after deposition. Apart from BCP, there have been reports of using TAZ (3-(4-Biphenylyl)-4-phenyl-5(4-tertbutylphenyl)-1,2,4-triazole),BPhen (4,7-diphyenyl-1,10-phenanthroline), PBD (2-(4'- tert-bytylphenyl)-5-(4'-diphenyl)-1,3,4-oxadiazole, and BAlq (Aluminum(III) bis(2-52