OLED Display Fundamentals and Applications
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Content
About the Author xi
Preface xiii
Series Editor's Foreword to the Second Edition xv
1 Introduction 1
References 5
2 OLED Devices 7
2.1 OLED Definition 7
2.1.1 History of OLED Research and Development 7
2.1.2 Luminescent Effects in Nature 8
2.1.3 Difference Between OLED, LED, and Inorganic ELs 11
2.1.3.1 Inorganic EL 11
2.1.3.2 LED 11
2.2 Basic Device Structure 12
2.3 Basic Light Emission Mechanism 14
2.3.1 Potential Energy of Molecules 14
2.3.2 Highest Occupied and Lowest Unoccupied Molecular Orbitals (HOMO and LUMO) 15
2.3.3 Configuration of Two Electrons 17
2.3.4 Spin Function 20
2.3.5 Singlet and Triplet Excitons 20
2.3.6 Charge Injection from Electrodes 24
2.3.6.1 Charge Injection by Schottky Thermionic Emission 25
2.3.6.2 Tunneling Injection 28
2.3.6.3 Vacuum-Level Shift 28
2.3.7 Charge Transfer and Recombination 29
2.3.7.1 Charge Transfer Behavior 29
2.3.7.2 Space-Charge-Limited Current 29
2.3.7.3 Poole-Frenkel conduction 32
2.3.7.4 Recombination and Generation of Excitons 33
2.4 Emission Efficiency 36
2.4.1 Internal/External Quantum Efficiency 36
2.4.2 Energy Conversion and Quenching 37
2.4.2.1 Internal Conversion 37
2.4.2.2 Intersystem Crossing 37
2.4.2.3 Doping 38
2.4.2.4 Quenching 40
2.4.3 Outcoupling Efficiency of OLED Display 42
2.4.3.1 Light Output Distribution 42
2.4.3.2 Snell's Law and Critical Angle 43
2.4.3.3 Loss Due to Light Extraction 44
2.4.3.4 Performance Enhancement by Molecular Alignment 45
2.5 Lifetime and Image Burning 46
2.5.1 Lifetime Definitions 46
2.5.2 Degradation Analysis and Design Optimization 47
2.5.3 Degradation Measurement and Mechanisms 50
2.5.3.1 Acceleration Factor and Temperature Contribution 50
2.5.3.2 Degradation Mechanism Variation 50
2.6 Technologies to Enhance the Device Performance 51
2.6.1 Thermally Activated Delayed Fluorescence 51
2.6.2 Other Types of Excited States 53
2.6.2.1 Excimer and Exciplex 53
2.6.2.2 Charge-Transfer Complex 53
2.6.3 Charge Generation Layer 54
References 56
3 OLED Manufacturing Process 61
3.1 Material Preparation 61
3.1.1 Basic Material Properties 61
3.1.1.1 Hole Injection Material 61
3.1.1.2 Hole Transportation Material 62
3.1.1.3 Emission Layer Material 62
3.1.1.4 Electron Transportation Material and Charge Blocking Material 63
3.1.2 Purification Process 67
3.2 Evaporation Process 68
3.2.1 Principle 68
3.2.2 Evaporation Sources 72
3.2.2.1 Resistive Heating Method 72
3.2.2.2 Electron Beam Evaporation 75
3.2.2.3 Monitoring Thickness Using a Quartz Oscillator 76
3.3 Encapsulation 79
3.3.1 Dark Spot and Edge Growth Defects 79
3.3.2 Light Emission from the Bottom and Top of the OLED Device 80
3.3.3 Bottom Emission and perimeter sealing 81
3.3.4 Top Emission 82
3.3.5 Encapsulation Technologies and Measurement 83
3.3.5.1 Thin-Film Encapsulation 84
3.3.5.2 Face Sealing Encapsulation 87
3.3.5.3 Frit Encapsulation 88
3.3.5.4 WVTR Measurement 88
3.4 Problem Analysis 91
3.4.1 Ionization Potential Measurement 91
3.4.2 Electron Affinity Measurement 92
3.4.3 HPLC Analysis 93
3.4.4 Cyclic Voltammetry 94
References 96
4 OLED Display Module 99
4.1 Comparison Between OLED and LCD Modules 99
4.2 Basic Display Design and Related Characteristics 101
4.2.1 Luminous Intensity, Luminance, and Illuminance 101
4.2.1.1 Luminous Intensity 101
4.2.1.2 Luminance 102
4.2.1.3 Illuminance 103
4.2.1.4 Metrics Summary 104
4.2.1.5 Helmholtz-Kohlrausch Effect 106
4.2.2 OLED Current Efficiencies and Power Efficacies 106
4.2.3 Color Reproduction 109
4.2.4 Uniform Color Space 115
4.2.5 White Point Determination 116
4.2.6 Color Boost 119
4.2.7 Viewing Condition 120
4.3 Passive-Matrix OLED Display 121
4.3.1 Structure 121
4.3.2 Pixel Driving 122
4.4 Active-Matrix OLED Display 125
4.4.1 OLED Module Components 125
4.4.2 Two-Transistor One-Capacitor (2T1C) Driving Circuit 127
4.4.3 Ambient Performance 136
4.4.3.1 Living Room Contrast Ratio 136
4.4.3.2 Chroma Reduction Due to Ambient Light 137
4.4.4 Subpixel Rendering 138
References 139
5 OLED Color Patterning Technologies 143
5.1 Color-Patterning Technologies 143
5.1.1 Shadow Mask Patterning 143
5.1.1.1 Shadow Mask Process 143
5.1.1.2 Blue Common Layer 146
5.1.1.3 Polychromatic Pixel 147
5.1.2 White+Color Filter Patterning 148
5.1.3 Color Conversion Medium (CCM) Patterning 149
5.1.4 Laser-Induced Thermal Imaging (LITI) Method 149
5.1.5 Radiation-Induced Sublimation Transfer (RIST) Method 151
5.1.6 Dual-Plate OLED Display (DOD) Method 152
5.1.7 Other Methods 153
5.2 Solution-Processed Materials and Technologies 153
5.3 Next-Generation OLED Manufacturing Tools 158
5.3.1 Vapor Injection Source Technology (VIST) Deposition 158
5.3.2 Hot-Wall Method 163
5.3.3 Organic Vapor-Phase Deposition (OVPD) Method 164
References 165
6 TFT and Driving for Active-Matrix Display 167
6.1 TFT Structure 167
6.2 TFT Process 169
6.2.1 Low-Temperature Polysilicon Process Overview 169
6.2.2 Thin-Film Formation 172
6.2.3 Patterning Technique 173
6.2.4 Excimer Laser Crystallization 177
6.3 MOSFET Basics 180
6.4 LTPS-TFT-Driven OLED Display Design 183
6.4.1 OFF Current 183
6.4.2 Driver TFT Size Restriction 184
6.4.3 Restriction Due to Voltage Drop 185
6.4.4 LTPS-TFT Pixel Compensation Circuit 190
6.4.4.1 Voltage Programming 190
6.4.4.2 Current Programming 192
6.4.4.3 External Compensation Method 193
6.4.4.4 Digital Driving 194
6.4.5 Circuit Integration by LTPS-TFT 197
6.5 TFT Technologies for OLED Displays 200
6.5.1 Selective Annealing Method 200
6.5.1.1 Sequential Lateral Solidification (SLS) Method 200
6.5.1.2 Selective Annealing by Microlens Array 200
6.5.2 Microcrystalline and Superamorphous Silicon 202
6.5.3 Solid-Phase Crystallization 205
6.5.3.1 MIC and MILC Methods 205
6.5.3.2 AMFC Method 205
6.5.4 Oxide Semiconductors 207
References 210
7 OLED Television Applications 215
7.1 Performance Target 215
7.2 Scalability Concept 217
7.2.1 Relationship between Defect Density and Production Yield 217
7.2.1.1 Purpose of Yield Simulation 217
7.2.1.2 Defective Pixel Number Estimation Using the Poisson Equation 217
7.2.2 Scalable Technology 217
7.2.2.1 Scalability 218
7.3 Murdoch's Algorithm to Achieve Low Power and Wide Color Gamut 219
7.3.1 A Method for Achieving Both Low Power and Wide Color Gamut 219
7.3.2 RGBW Driving Algorithm 221
7.4 An Approach to Achieve 100% NTSC Color Gamut With Low Power Consumption Using White + Color Filter 224
7.4.1 Consideration of Performance Difference between W-RGB and W-RGBW Method 224
7.4.1.1 Issues of White+Color Filter Method for Large Displays 224
7.4.1.2 Analysis of W-RGBW Approach to Circumvent Its Trade-off Situation 224
7.4.1.3 Design of a Prototype to Demonstrate That Low Power Consumption Can Be Achieved with Large Color Gamut 229
7.4.1.4 Product-Level Performance Demonstration by the Combination of Scalable Technologies 230
References 233
8 New OLED Applications 235
8.1 Flexible Display/Wearable Displays 235
8.1.1 Flexible Display Applications 235
8.1.2 Flexible Display Substrates 235
8.1.3 Laser Liftoff Process 236
8.1.4 Barrier Technology for Flexible Displays 240
8.1.5 Organic TFTs for Flexible Displays 241
8.1.5.1 Organic Semiconductor Materials 242
8.1.5.2 Organic TFT Device Structure and Processing 243
8.1.5.3 Organic TFT Characteristics 245
8.2 Transparent Displays 245
8.3 Tiled Display 247
8.3.1 Passive-Matrix Tiling 247
8.3.2 Active-Matrix Tiling 248
References 252
9 OLED Lighting 255
9.1 Performance Improvement of OLED Lighting 255
9.2 Color Rendering Index 257
9.3 OLED Lighting Requirement 259
9.3.1 Correlated Color Temperature (CCT) 260
9.3.2 Other Requirements 262
9.4 Light Extraction Enhancement of OLED Lighting 262
9.4.1 Various Light Absorption Mechanisms 262
9.4.2 Microlens Array Structure 266
9.4.3 Diffusion Structure 266
9.4.4 Diffraction Structure 268
9.4.5 Reduction of Plasmon Absorption 268
9.4.5.1 Plasmonic Loss Mechanism 268
9.5 Color Tunable OLED Lighting 269
9.6 OLED Lighting Design 272
9.6.1 Resistance Reduction 272
9.6.2 Current Reduction 272
9.7 Roll-to-Roll OLED Lighting Manufacturing 273
References 275
Appendix 277
Index 281
Chapter 2
OLED Devices
2.1 OLED Definition
2.1.1 History of OLED Research and Development
Before any in-depth discussion of OLED display structure, let us consider the initial origins of OLED technology, which are based on early observations of electroluminescence (EL). In the early 1950s, a group of investigators at Nancy University in France applied high-voltage alternating-polarity fields in air to thin films of cellulose or cellophane containing deposited or dissolved acridine orange and quinacrine, and observed light emission [1]. One mechanism identified in these reaction processes involved excitation of electrons. Then in 1960, a team of investigators at New York University (NYU) made ohmic (a nonrectifying charge injection, which shows linear current-voltage relationship) dark-injecting electrode contacts to organic crystals and described the necessary workfunctions (energy requirements) for hole and electron-injecting electrode contacts [2]. These contacts are the source of charge injection in all present-day OLED devices. The same NYU group also studied direct-current (DC) EL in vacuo on a single pure anthracene crystal and tetracene-doped anthracene crystals in the presence of a small-area silver electrode at 400 V [3]. The proposed mechanism for this reaction was termed field-accelerated electron excitation of molecular fluorescence. The NYU group later observed that in the absence of an external electric field, the EL in anthracene crystals results from recombination of electron and hole and that the conducting-level energy of anthracene is higher than the exciton energy level [4].
Because of the association between EL and later OLED development on the basis of these and other early EL studies, the term organic EL gradually emerged and is still used today. EL includes two basic phenomena:
- 1. Light emission due to the presence of excited molecules caused by accelerated electrons (i.e., electrons that are accelerated to higher energy levels)
- 2. Light emission due to electron-hole recombination, as in all light-emitting diodes (LEDs).
Phenomenon 1 is the narrower definition. Current OLED devices, after Tang and Van Slyke's "first OLED paper," utilize exclusively LED-like emission mechanisms, that is, phenomenon 2.
Table 2.1 lists the differences between a liquid crystal display (LCD) and an OLED display. The OLED has a very short response time and is capable of using "punching" (an imaging technique for enhancing the local luminance to emphasize the highlighted region of an image). The punching technique is used in cathode ray tubes (CRTs), which can have much higher luminance of a dot than the screen luminance. An OLED can use a similar operation, while a normal LCD display cannot.
Table 2.1 Differences between Liquid Crystal and OLED Displays
Parameter LCD OLED Response time Slow Fast Luminance boost Difficult Possible Viewing angle Narrower high contrast angle region Lambertian distributiona Number of components More Fewer Differential agingb Small Larger Susceptibility to water and O2 Largera Outgoing light distribution whose luminance is proportional to cos ?. To be discussed in Section 4.2.1.4.
b Luminance reduction in terms of use of a particular pixel and between colors. To be discussed in Section 2.5.
Table 2.2 outlines the chronological history of OLED technology development.
Table 2.2 Timeline for OLED Technology Development
Year Eventa Company/Institute 1960-mid-1970s D OLED crystal molecule, anthracene, etc.b NRC (Canada), RCA 1983 D First observation of electroluminescence from polymer film National Physical Laboratory 1987 P OLED diode structure paper in Appl. Phys. Lett.b Eastman Kodak 1988 P Double heterojunctionc Kyushu University 1990 P First PLED paper in Natureb Cambridge University 1994 P White OLED demonstrationc Yamagata University 1996 P first AMOLED demonstration (QVGA)b TDK 1998 D first phosphorescence OLEDb Princeton University 1999 D first passive OLED product Pioneer 1999 D Color OLED display by white + color filter methodc TDK 2001 D 0.72-in. headmount display by AMOLED on siliconb eMagin 2001 D 13-in. SVGA AMOLED prototypeb Sony 2001 D 2.1-in. 130-ppi AMOLED prototypeb Seiko Epson/CDT 2002 D 15-in. 1280×720 OLED prototypeb Eastman Kodak/Sanyo 2002 P Tandem OLED device demonstrationc Yamagata University 2003 D digital camera with 2.2-in. AMOLED displayb Eastman Kodak 2003 D Tiled 24-in. AMOLED prototype with by 12-in. displayb Sony 2003 D 20-in. phosphorescence AMOLED prototype by a-Si backplaneb ChiMei/IDT/IBM 2006 P White OLED with phosphorescent emitterc UDC 2007 D 11-in. OLED Television productc Sony 2007 P White OLED by all phosphorescent emittersc Konica Minolta 2008 P 12-in. Transparent OLED prototypec Samsung 2008 P 4-in. Flexible OLED prototypec Samsung 2008 P 100% NTSC low power OLED by white + color filter methodc Kodak 2009 D OLED lighting productc Philips 2009 P TADF OLED devicec Kyushu University 2010 P White OLED over 100 lm/W UDC 2011 D OLED lighting product by all phosphorescent emittersc Konica Minolta/Philips 2013 D 55-in. OLED Television product by white + color filter methodc LG display 2013 P 4KOLED Television prototypec Sony/Panasonic 2014 D Flexible OLED display productc LG display 2014 D Flexible OLED lighting product by roll-to-roll manufacturingc Konica Minoltaa Abbreviations in this column: a-si-amorphous silicon; AMOLED-active-matrix OLED; D-development of; P-publication or presentation/demonstration of; PLED-polymer (O)LED; ppi-pixels per inch; QVGA-quarter videographics array (320 × 240 pixels); SVGA-super videographics array (800 × 600 pixels).
b SID International Symposium (2003), 40 Years of SID Symposia-Nurturing Progress in EL/OLED Technology, Baltimore, MD. http://sid.org/Portals/sid/Files/DisplayHistory/EL-OLED_History.pdf.
c By Takatoshi Tsujimura, "Evolution and future of OLED lighting," OLED Forum Japan presentation, Kyushu University 11/12/2015.
The chronological sequence of development listed in Table 2.2 reflects the emergence of some general terms of classification of OLED technologies, including the following:
- Small-molecule OLED (SMOLED) and polymer OLED (PLED)
- Passive-matrix OLED (PMOLED) and active-matrix OLED (AMOLED) displays
- Fluorescent emission and phosphorescent emission.
The developments listed here and in Table 2.2 indicate that the rapid advances in OLED technologies resulted from extensive experimental trial and error. Each technology is discussed in further detail later in the book.
2.1.2 Luminescent Effects in Nature
There are several kinds of "luminescence" in nature, which can be explained by a mechanism similar to that of an OLED.
A molecule has multiple discrete energy levels, each able to hold two electrons. When electrons fill these levels completely, beginning from the lowest in energy, the system is stable. This is called the ground state.
If an electron is moved to an upper empty energy level, the resulting configuration is called an excited state. The excited state is normally unstable, so the electron tends to release the energy and return to the ground state. In such a...
