Printed Electronics
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Persons
Prof Zheng Cui, Chinese Academy of Sciences, China
Zheng Cui obtained a PhD in electronic engineering in 1988 and became a Visiting Fellow at the Microelectronics Research Center, Cambridge University, UK, in 1989. Cui joined the Rutherford Appleton Laboratory, UK, in 1993 and subsequently became a Principal Scientist and group leader there in 1999. In October 2009, after working in the UK for 20 years, Cui returned to China and founded the Printable Electronics Research Center at the Suzhou Institute of Nanotech, which was the first research center dedicated to printed electronics R&D in China.
CONTRIBUTORS
Dr Song Qiu, Chinese Academy of Sciences, China
Song Qiu received a B.S. Polymer Materials and Engineering in 2000, followed by a PhD in Polymer Chemistry and Physics?from Jilin University in 2005.
Dr Jian Lin, Chinese Academy of Sciences, China
Jian Lin received his PhD degree in Polymer Chemistry and Physics from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2008.
Dr. Jianwen Zhao, Chinese Academy of Sciences, China
Jianwen Zhao received his PhD degree in Technical Institute of Physics and Chemistry, Chinese Academic of Science, in 2008.
Dr. Chang-Qi Ma, Chinese Academy of Sciences, China
Chang-Qi Ma received his bachelor's degree in Chemistry from Beijing Normal University in 1998. In 2003 he obtained his PhD degree at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences in Beijing with Professor B.-W. Zhang.
Dr. Zheng Chen, Chinese Academy of Sciences, China
Zheng Chen received the B.S. degree in materials physics and PhD degree in condensed matter physics from the University of Science and Technology of China, in 2002 and 2007, respectively.
Dr. Wenming Su, Chinese Academy of Sciences, China
Wenming Su is anAssociate professor of Printable Electronics Research Center, Suzhou Institute of Nanotech, Chinese Academy of Sciences.
Content
Preface xii
1 Introduction 1
Zheng Cui
1.1 What is Printed Electronics? 1
1.2 The Importance of Developing Printed Electronics 11
1.3 Multidisciplinary Nature of Printed Electronics 15
1.4 Structure and Content of the Book 17
References 19
2 Organic Printable Electronic Materials 21
Song Qiu and Chunshan Zhou
2.1 Introduction 21
2.2 Organic Conductive Materials 22
2.2.1 Characteristics of Organic Conductive Materials 22
2.2.2 History of Organic Conductive Materials 23
2.2.3 Conductive Polymer 23
2.2.3.1 Structural Conductive Polymer 23
2.2.3.2 Composite Conductive Polymer 25
2.2.4 PEDOT 25
2.3 Printable Organic Small Molecular Semiconductors 27
2.3.1 Fused Aromatic Compounds 28
2.3.2 Heterocyclic Sulfur Compounds and Oligothiophenes 30
2.3.3 Other Materials with High Mobility 33
2.4 Printable Polymeric Semiconductor 34
2.4.1 P-type Polymer Semiconductors 35
2.4.1.1 Sulfur-containing Heterocyclic Polymeric Semiconductors 35
2.4.1.2 Phenyl-containing Polymeric Semiconductors 37
2.4.1.3 Other p-type Polymeric Semiconductors 39
2.4.2 N-type Polymer Semiconductors 39
2.4.3 Ambipolar Transistor and Related Polymer Materials 41
2.4.4 Outlook 43
2.5 Other Printable Organic Electronic Materials 44
2.5.1 Organic Insulating Materials 44
2.5.2 Organic Materials for Sensors 47
2.6 Summary 49
References 49
3 Inorganic Printable Electronic Materials 54
Zheng Chen
3.1 Introduction 54
3.2 Metallic Materials 56
3.2.1 Metallic Ink 56
3.2.2 Post-printing Process 63
3.2.3 Metal Nanowire 64
3.3 Transparent Oxide 66
3.3.1 Transparent Oxide Semiconductor and Conductor 66
3.3.2 Low Temperature Solution Processing 68
3.3.3 Doped Transparent Oxide Nanoparticles 71
3.4 Single-wall Carbon Nanotube 72
3.4.1 Preparation and Selective Chemistry of SWNT 72
3.4.2 Purification of SWNT 76
3.4.3 Metallic SWNT Thin Film 77
3.4.4 Semiconducting SWNT Thin Film 79
3.5 Graphene 83
3.6 Silicon and Germanium 86
3.7 Metal Chalcogenides Semiconductor and Quantum Dots 90
3.7.1 Metal Chalcogenides Semiconductor 90
3.7.2 Quantum Dots 90
3.8 Nanoparticle/Polymer Dielectric Composites 92
3.9 Summary 95
References 96
4 Printing Processes and Equipments 106
Jian Lin
4.1 Introduction 106
4.2 Jet Printing 108
4.2.1 Inkjet Printing 108
4.2.1.1 Working Principles 108
4.2.1.2 Pattern Preparation 108
4.2.1.3 Application in Printed Electronics 110
4.2.2 Aerosol Jet Printing 111
4.2.2.1 Working Principle 112
4.2.2.2 Pattern Preparation 112
4.2.2.3 Advantages and Challenges 113
4.2.3 Electrohydrodynamic Jet Printing 114
4.2.4 Advantages and Disadvantages 114
4.3 Direct Replicate Printing 115
4.3.1 Screen Printing 116
4.3.1.1 Working Principle 116
4.3.1.2 Screen Mask 117
4.3.1.3 Advantages and Disadvantages 118
4.3.1.4 Applications 118
4.3.2 Gravure Printing 118
4.3.2.1 Principle and System 118
4.3.2.2 Gravure Plate 120
4.3.2.3 Advantages and Disadvantages 120
4.3.2.4 Applications in Printed Electronics 121
4.3.3 Flexographic Printing 122
4.3.3.1 Principle and System 122
4.3.3.2 Printing Plate 123
4.3.3.3 Advantages and Disadvantages 123
4.3.3.4 Applications in Printed Electronics 125
4.4 Indirect Replicate Printing 125
4.4.1 Offset Printing 125
4.4.2 Gravure Offset Printing 126
4.4.3 Pad Printing 128
4.5 Pre-printing Processes 129
4.5.1 Pattern Design 129
4.5.2 Modification of Surface Energy 130
4.5.3 Surface Coating 131
4.5.4 Embossing and Nanoimprinting 131
4.6 Post-printing Processes 134
4.6.1 Sintering 134
4.6.2 UV Curing 135
4.6.3 Annealing 135
4.7 Summary 136
References 137
5 Printed Thin Film Transistors 145
Jianwen Zhao
5.1 Introduction 145
5.2 Types of Transistors 146
5.3 Working Principles of Transistors 147
5.3.1 Basic Mechanism of MOSFETs 147
5.3.2 Charge Carriers and Carrier Mobility 149
5.3.3 Basic Parameters of TFT 149
5.3.3.1 Effective Mobility 149
5.3.3.2 Operating Voltage 151
5.3.3.3 Device Capacitance 151
5.3.3.4 Threshold Voltage (Vt) 153
5.3.3.5 Subthreshold Swing (SS) 155
5.3.3.6 On/off Current Ratio (Ion /Ioff) 155
5.3.3.7 Hysteresis 156
5.3.3.8 Transconductance (gm) 157
5.3.3.9 Stability 157
5.4 Structures and Fabrication of TFTs 157
5.4.1 Structures of TFTs 157
5.4.2 Characteristics of TFTs 159
5.4.3 Fabrication of TFTs 160
5.4.3.1 Fabrication of Electrodes 160
5.4.3.2 Fabrication of Active Layer 163
5.4.3.3 Fabrication of Dielectric Layers 167
5.5 Fully Printed TFTs 172
5.5.1 Printability of Electronic Materials 172
5.5.2 Influence of Surface Morphology 173
5.5.3 Interface Effect of Printed TFTs 173
5.5.3.1 Effect of Semiconductor/Dielectric Interface 175
5.5.3.2 Effect of Semiconductor/Semiconductor Interface 176
5.5.3.3 Effect of Semiconductor/Electrode Interface 177
5.5.4 Effect of Channel Length 178
5.5.5 Summary of Issues in Printing TFTs 179
5.5.5.1 Printable Inks and Printing Processes 179
5.5.5.2 Printed Electrodes 180
5.5.5.3 Printed Dielectric Thin Films 180
5.6 Advances in Printed TFTs 180
5.6.1 Printed Inorganic TFTs 181
5.6.1.1 SWCNT TFTs 181
5.6.1.2 Metal Oxide TFTs 182
5.6.1.3 Metal Dichalcogenide and Graphene TFTs 184
5.6.2 Printed Organic TFTs 187
5.7 Basics of Printed Logic Circuits 189
5.7.1 NAND and NOR Gates 190
5.7.2 Inverter 190
5.7.3 Ring Oscillator 190
5.7.4 Flip-flop 193
5.7.5 Backplane Driving Circuits for Display 194
5.8 Summary 196
References 197
6 Printed Organic Thin Film Solar Cells 201
Changqi Ma
6.1 Introduction 201
6.1.1 Solar Energy and its Utilization 201
6.1.2 Classification of Solar Cells 202
6.1.3 A Brief History of Organic Thin-Film Solar Cells 203
6.2 Working Principles and Characterization of Organic Solar Cells 205
6.2.1 Physical Processes 205
6.2.2 Basic Structure 206
6.2.3 Characterization 208
6.2.3.1 I-V Characteristics 208
6.2.3.2 Spectrum Response 209
6.2.4 The Main Factors Influencing Device Performance 209
6.2.4.1 Photon Absorption Ability of Organic Semiconductors 210
6.2.4.2 Energy Level Arrangement of Donor and Acceptor 210
6.2.4.3 Morphology of Photoactive Layer 212
6.3 Materials for Organic Solar Cells 213
6.3.1 Transparent Substrate 214
6.3.2 Transparent Conductive Electrode 214
6.3.2.1 Metal Oxide Film 214
6.3.2.2 Conductive Polymer Film 215
6.3.2.3 Thin Metal Film and Metal Grid 215
6.3.2.4 Carbon-rich Materials 217
6.3.3 Organic Semiconductor Materials 218
6.3.3.1 p-Type Organic Semiconductors 218
6.3.3.2 n-Type Organic Semiconductors 223
6.3.4 Inorganic Semiconductors 227
6.3.5 Other Functional Materials 229
6.4 Inverted and Tandem Organic Solar Cells 229
6.4.1 Inverted Organic Solar Cells 229
6.4.2 Tandem Organic Solar Cells 231
6.4.3 Inverted Tandem Organic Solar Cells 231
6.5 Fabrication Methods 232
6.5.1 Spin Coating 233
6.5.2 Doctor Blading 235
6.5.3 Screen Printing 235
6.5.4 Inkjet Printing 237
6.5.5 Other Thin Film Deposition Techniques 237
6.6 Roll-to-roll Processing 237
6.7 Printable Perovskite Solar Cells 239
6.8 Summary and Outlook 239
References 240
7 Printed Organic Light Emission and Display 251
Wenming Su
7.1 Introduction 251
7.1.1 Overview of Lighting and Display 252
7.1.2 Overview of Organic Light Emitting Diodes (OLEDs) 253
7.2 Mechanism of Organic Light Emission 254
7.2.1 Charge Injection and Transport 255
7.2.2 Exciton Formation and Light Emission 256
7.2.3 Characterization of OLED Performance 256
7.2.3.1 Luminous Efficacy 256
7.2.3.2 Quantum Efficiency 257
7.2.3.3 Color 257
7.2.3.4 Three Primary Colors 258
7.3 Structures and Materials of OLED 259
7.3.1 Small Molecular OLED 259
7.3.1.1 Typical Structure 259
7.3.1.2 Electrode Materials 259
7.3.1.3 Fabrication Process 260
7.3.2 Polymer OLEDs 262
7.3.3 General OLED Materials 262
7.3.3.1 Charge Injection Materials 263
7.3.3.2 Charge Transport Materials 263
7.3.3.3 Emitter Materials 264
7.3.4 Soluble OLED Materials 265
7.3.4.1 Printable Polymer OLEDs 266
7.3.4.2 Printable Small Molecular OLEDs 266
7.3.4.3 Cross-linking Materials for Printable OLEDs 267
7.4 White Lighting OLEDs 267
7.4.1 White Light Emission Mechanism 267
7.4.2 Important Parameters 272
7.4.2.1 CRI 272
7.4.2.2 Efficiency and Light Extraction 273
7.4.2.3 Lifetime 275
7.4.3 Investment in OLED Lighting 275
7.5 Fabrication of OLED by Printing 277
7.5.1 Spin and Slot Die Coating 277
7.5.2 Inkjet Printing 278
7.5.3 Screen Printing 278
7.5.4 Roll-to-roll Printing 279
7.5.5 Current Status of the Printed OLED Industry 280
7.6 Summary 281
References 282
8 Encapsulation Technology for Organic Electronic Devices 287
Wenming Su
8.1 Introduction 287
8.2 Aging of Organic Electronic Devices 288
8.2.1 Characteristics and Mechanisms of Aging 288
8.2.2 Requirements for Organic Electronics Encapsulation 290
8.3 Principle of Encapsulation 291
8.3.1 Water/oxygen Penetration Mechanism through Thin Films 291
8.3.2 Organic/inorganic Multilayer Encapsulation 292
8.3.3 Measurement of Encapsulation Property 293
8.4 Thin-film Encapsulation Technology 296
8.4.1 History of Thin-film Encapsulation 297
8.4.2 Single Layer Thin-film Encapsulation 298
8.4.3 Multilayer Thin-film Encapsulation 298
8.4.4 BarixTM Thin-film Encapsulation 300
8.4.5 Thin Film Deposition Methods 301
8.4.5.1 PECVD 301
8.4.5.2 ALD 303
8.4.5.3 Parylene Deposition 303
8.4.6 Flexibility of Encapsulation Thin Film 304
8.4.7 Trends of Thin-film Encapsulation 306
8.5 Applications of Thin-film Encapsulation 307
8.5.1 Encapsulation of Flexible OLED 307
8.5.2 Encapsulation of Flexible OPV 309
8.6 Summary 313
References 314
9 Applications and Future Prospects of Printed Electronics 316
Zheng Cui
9.1 Introduction 316
9.2 Application Areas of Printed Electronics 317
9.2.1 Organic Photovoltaic 317
9.2.2 Flexible Display 321
9.2.3 Organic Lighting 324
9.2.4 Electronics and Components 326
9.2.5 Integrated Smart Systems 331
9.3 Challenges for Printed Electronics 333
9.3.1 Materials 333
9.3.2 Printing Process and Equipment 335
9.3.3 Encapsulation 335
9.3.4 Design Methodology and Standardization 336
9.4 Summary and Outlook 336
References 337
Index 339
1
Introduction
Zheng Cui
1.1 What is Printed Electronics?
Printed electronics, as the name implies, is a type of electronics that are created by printing technology. To be more specific, it is an electronic science and technology based on conventional printing techniques as the means to manufacture electronics devices and systems. To most people, "printed electronics" is an unfamiliar phrase. Even experts in electronics may not have heard it. Many people may have it confused with conventional printing technology or mixed up with electronic printing. Conventional printing is for printing paper media, such as books, newspapers, and magazines. Even electronic printing is not printed electronics. Electronic printing is still conventional media printing but with more use of computers and electronic typesetting. A closer analogy to printed electronics would be electronics or integrated circuit (IC)-based electronics, rather than conventional printing. The aim of printed electronics is to make integrated electronic systems using printing technology instead of much more expensive and complex IC manufacturing technology.
Silicon-based IC technology has been in use for nearly 60 years. Modern silicon microelectronics and its manufacturing technology have evolved into an extremely complicated process. There are several hundreds of steps involved in producing a silicon IC chip, from the preparation of single crystal silicon substrates to making billions of transistors and getting these transistors interconnected, including repeated thin film deposition, lithography, etching, and packaging [1]. IC manufacturing has become so expensive that the latest deep UV photolithography system can cost tens of millions of dollars, whereas an extreme UV lithography system for making silicon chips at below 32 nm feature size has a price tag of more than $120 million [2]. The IC industry has become so investment intensive that only a handful companies in the world can afford to play in the field. On the other hand, printing is a very simple process compared to the IC manufacturing process, as illustrated in Figure 1.1 In order to turn a functional material into a functional structure or pattern on a silicon substrate, IC manufacturing has to go through thin film deposition, spin-coating photoresist layer, baking, photolithography, baking, developing, etching, and stripping of the photoresist masking layer. If printing is employed, the functional material can be directly printed as patterns onto the substrate. Only a subsequent annealing/sintering process is needed.
Figure 1.1 Comparison of IC manufacturing and printing processes. (a) Conventional IC manufacturing; and (b) printing process
Printing is an additive manufacturing process, similar to the deposition process in micro- and nanofabrication [3] but combined with patterning. In printed electronics, the components of an electronic device or a system can be made by printing in additive fashion. For example, for a field-effect transistor, the source, drain, and gate electrodes, as well as semiconductor and insulating layers, can all be printed in ink forms and layer by layer onto a substrate. It is very much like color printing in a conventional printing press, where each color ink is printed sequentially and several color layers are overlaid to form the final color print. Because of its similarity to the printing process, the machine to print electronics is not much different from a conventional media printer. Figure 1.2 compares a conventional roll-to-roll paper media printer and an electronics printer. They look almost the same. The only difference is the inks they use. The inks for printing electronics have conducting, semiconducting, or dielectric properties. They are electronic materials, not pigment, which is the key for printing to be used for printed electronics.
Figure 1.2 Comparison of (a) conventional paper media printer; and (b) electronics printer.
(Courtesy of iPEN Co. Ltd.)
Printed electronics originated from organic electronics. In 1977, Alan Heeger, together with Alan G. MacDiarmid and Hideki Shirakawa, discovered that polymer could be conductive by doping certain molecules [4], which earned them the Nobel Prize in Chemistry in 2000. This discovery completely overthrew the conventional wisdom that organic polymer materials are always insulators. Following the discovery of conductive polymers, organic semiconductor materials were developed in 1983 [5] and organic field-effect transistors (OFETs) were first made in 1986 [6]. In the same period, Dr. C.W. Tang at Kodak developed organic photovoltaic (OPV) materials [7] and later invented the organic light-emitting diode (OLED) [8], from which organic electronics as a field of scientific interest started.
The reason the scientific community got interested in organic electronics was not only due to scientific curiosity but more importantly that they foresaw the prospect of printing electronic devices from organic polymers that could be naturally made into ink forms. Once they could be printed, electronic devices could be made on a massive scale at low cost, very much like printing newspapers. So from the early stage of development, people made attempts to process organic electronic materials in solution forms to make transistors [9]. In 1994, a research group led by Francis Garnier first reported OFETs made on plastic substrates [10]. Although only electrodes were printed and the organic semiconductors were deposited by vacuum evaporation, the significance of the work was that it proved transistors could be made on plastic substrates, opening the era of plastic electronics. Fully printed transistors were reported in 1997 when Dr. Zhenan Bao, working at Bell Labs, printed all layers including conductor, semiconductor, and dielectrics onto polyester (PET) film by a screen printing technique [11]. More recently, Professor Sirringhaus at Cambridge University made fully printed organic transistors by the inject printing technique [12].
It is apparent that organic electronics had its eye on low cost printing electronics from the beginning of its development. However, for a very long period, printing did not become the mainstream fabrication means for making organic electronic devices. The main reason lies in the fact that the charge mobility, which is a key property of semiconductor material, for the solution form of organic semiconductors is always lower than those small molecular organic semiconductors that cannot be made into solution form and have to be deposited by vacuum evaporation. In other words, transistors made by printable organic materials are not as good as those made by vacuum evaporated organic materials.
Charge mobility is the speed of electronic charge (electrons for n-type semiconductor or holes for p-type semiconductor) movement in semiconductor materials. It determines how fast a transistor switches at an applied external electric field. Table 1.1 lists the charge mobility of commonly used inorganic semiconductor materials, in comparison with organic semiconductor. It shows that the charge mobility of organic semiconductor materials is far lower than inorganic semiconductors.
Table 1.1 Charge mobility of organic and inorganic semiconductor materials
Semiconductor materials Charge mobility (cm2v-1s-1) GaAs 104 Single-crystal silicon 103 Poly silicon 10 Amorphous silicon 0.1-1 Organic semiconductor 10-4-1The research in organic electronics in its over 3 decades of development history has been mainly focused on how to improve the charge mobility, as it is obvious that only high mobility organic semiconductors have value in any practical applications. The last 25 years have indeed seen the steady improvement of charge mobility in organic semiconductors, as shown in Figure 1.3, which indicates the evolution of charge mobility from 1985 to 2010 for three different types of organic p-channel and n-channel semiconductor materials: vacuum-deposited small molecular organic materials, solution-processed small molecular organic materials, and solution-processed polymer materials [13].
Figure 1.3 Evolution of charge mobility for organic semiconductor materials.
(Adapted from [13] with permission from the Royal Society of Chemistry.)
The evolution curves reveal two things: first, there has been tremendous progress in improving the charge mobility of organic semiconductor materials. The mobility has increased 6 orders of magnitudes in the last 25 years. Second, the charge mobility of solution-processed polymeric organic semiconductor materials, though continuously improved, was always an order of magnitude lower than that of vacuum-deposited small molecular materials throughout the 25 years of development. Though the gap became smaller in the last few years due to the efforts in solution forms of organic semiconductors, small molecular organic semiconductor materials are still far better in terms of charge mobility [14]. As the ability to process organic semiconductor in solution form is the prerequisite of printing fabrication, low performance has prevented printing from becoming the preferred means of making organic...
