Flexible Electronics (eBook)

William Wong received his B.S. from the University of California, Los Angeles, in 1990, his M.S. from the University of California, San Diego, in 1995 and his Ph.D. from the University of California, Berkeley, in 1999.  He was an associated research engineer for Siemens Solar Industries in Camarillo, CA, form 1990-1992. Since 2000, he has been a senior member of the research staff at the Palo Alto Research Center. Alberto Salleo received his physics degree in 1994 from Ecole Polytechnique in France; his M.S. from the University of California, Berkeley in 1998; and his Ph.D. from the University of California, Berkeley, in 2001.  He has held several positions such as visiting scholar and graduate student researcher and currently is a researcher in the Electronic  Materials Laboratory at the Palo Alto Research Center. In January, 2006, he will become an Assistant Professor for the Department of Materials Science and Engineering at Stanford University.

Flexible Electronics: Materials and Applications 2
Preface 6
Contents 9
Contributors 16
to 1 Overview of Flexible Electronics Technology 18
1.1 History of Flexible Electronics 18
1.2 Materials for Flexible Electronics 19
1.2.1 Degrees of Flexibility 20
1.2.2 Substrates 22
1.2.2.1 Thin Glass 23
1.2.2.2 Plastic Film 24
1.2.2.3 Metal Foil 24
1.2.3 Backplane Electronics 25
1.2.3.1 Silicon Thin-Film Transistors 26
1.2.3.2 Organic Thin-Film Transistors 27
1.2.3.3 Transparent Thin-Film Transistors 27
1.2.3.4 Materials for Interconnects and Contacts 28
1.2.4 Frontplane Technologies 29
1.2.4.1 Liquid Crystal Displays 29
1.2.4.2 Electrophoretic Displays 30
1.2.4.3 Organic Light-Emitting Displays 31
1.2.4.4 Sensors 32
1.2.4.5 Actuators 33
1.2.4.6 Electronic Textiles 33
1.2.5 Encapsulation 33
1.3 Fabrication Technology for Flexible Electronics 35
1.3.1 Fabrication on Sheets by Batch Processing 35
1.3.2 Fabrication on Web by Roll-to-Roll Processing 36
1.3.3 Additive Printing 37
1.4 Outlook 37
to 2 Mechanical Theory of the Film-on-Substrate-Foil Structure: Curvature and Overlay Alignment in Amorphous Silicon Thin-Film Devices Fabricated on Free-Standing Foil Substrates 46
2.1 Introduction 46
2.2 Theory 49
2.2.1 The Built-in Strain 0 bi 52
2.3 Applications 53
2.3.1 Strain in the Substrate, 0 s (T d ), and the Film, 0 f (T d ), at the Deposition Temperature T d 53
2.3.2 Strain in the Substrate, 0 s (T r ), and the Film, 0 f (T r ), at Room Temperature T r 55
2.3.3 Radius of Curvature R of the Workpiece 59
2.3.4 Strain of the Substrate and the Curvature of the Workpiece for a Three-Layer Structure 63
2.3.5 Experimental Results for a-Si:H TFTs Fabricated on Kapton 64
2.4 Conclusions 67
to 3 Low-temperature Amorphous and Nanocrystalline Silicon Materials and Thin-film Transistors 69
3.1 Introduction 69
3.2 Low-temperature Amorphous and Nanocrystalline Silicon Materials 71
3.2.1 Fundamental Issues for Low-temperature Processing 71
3.2.2 Low-temperature Amorphous Silicon 72
3.2.3 Low-temperature Nanocrystalline Silicon 72
3.3 Low-temperature Dielectrics 73
3.3.1 Characteristics of Low-temperature Dielectric Thin-film Deposition 73
3.3.2 Low-temperature Silicon Nitride Characteristics 73
3.3.3 Low-temperature Silicon Oxide Characteristics 74
3.4 Low-temperature Thin-film Transistor Devices 75
3.4.1 Device Structures and Materials Processing 76
3.4.2 Low-Temperature a-Si:H Thin-Film Transistor Device Performance 77
3.4.3 Contacts to a-Si:H Thin-film Transistors 78
3.4.4 Low-temperature Doped nc-Si Contacts 80
3.4.5 Low-temperature nc-Si TFTs 82
3.5 Device Stability 83
3.6 Conclusions and Future Prospective 85
to 4 Amorphous Silicon: Flexible Backplane and Display Application 90
4.1 Introduction 90
4.2 Enabling Technologies for Flexible Backplanes and Displays 91
4.2.1 Flexible Substrate Technologies 91
4.2.1.1 Flexible Stainless Steel Substrates 91
4.2.1.2 Flexible Plastic Substrates 92
4.2.1.3 Flexible PEN Plastic Substrates 94
4.2.2 TFT Technologies for Flexible Backplanes 97
4.2.2.1 Low-temperature a-Si TFT 102
4.2.3 Display Media for Flexible Displays (LCD, Reflective-EP, OLED) 104
4.2.3.1 LCD Media 104
4.2.3.2 Electrophoretic Display Media 104
4.2.3.3 OLED Display Media 105
4.2.4 Barrier Layers 105
4.3 Flexible Active Matrix Backplane Requirements for OLED Displays 106
4.3.1 Active Matrix Addressing 107
4.3.1.1 Voltage Programming 107
4.3.1.2 Current Programming 109
4.4 Flexible AMOLED Displays Using a-Si TFT Backplanes 110
4.4.1 Backplane Fabrication Using PEN Plastic Substrates 110
4.4.2 Flexible OLED Display Fabrication 113
4.4.3 Flexible AMOLED Display Fabrication with Thin-film Encapsulation 115
4.5 Flexible Electrophoretic Displays Fabricated using a-Si TFT Backplanes 117
4.6 Outlook for Low-Temperature a-Si TFT for Flexible Electronics Manufacturing 117
to 5 Flexible Transition Metal Oxide Electronics and Imprint Lithography 122
5.1 Introduction 122
5.2 Previous Work 123
5.3 Properties of Transistor Materials 128
5.3.1 Semiconductors 128
5.3.2 Dielectrics 130
5.3.3 Contact Materials 131
5.4 Device Structures 131
5.5 Fabrication on Flexible Substrates 134
5.5.1 Imprint Lithography 135
5.5.2 Self-Aligned Imprint Lithography 137
5.5.3 SAIL Transistor Results 141
5.5.4 Summary of Imprint Lithography 142
5.6 Flexible TMO Device Results 142
5.7 Future Problems and Areas of Research 147
5.7.1 Carrier Density Control 149
5.7.2 Low-Temperature Dielectrics 150
5.7.3 Etching of TMO Materials 150
5.7.4 P-type TMO 151
5.7.5 Stability 151
5.7.6 Flexure and Adhesion of TMO 152
5.7.7 Flexible Fabrication Method Yields 152
5.8 Summary 153
to 6 Materials and Novel Patterning Methods for Flexible Electronics 158
6.1 Introduction 158
6.2 Materials Considerations for Flexible Electronics 159
6.2.1 Overview 160
6.2.2 Inorganic Semiconductors and Dielectrics 160
6.2.3 Organic Semiconductors and Dielectrics 161
6.2.4 Conductors 164
6.3 Print-Processing Options for Device Fabrication 165
6.3.1 Overview 165
6.3.2 Control of Feature Sizes of Jet-Printed Liquids 166
6.3.3 Jet-Printing for Etch-Mask Patterning 168
6.3.4 Methods for Minimizing Feature Size 169
6.3.5 Printing Active Materials 171
6.4 Performance and Characterization of Electronic Devices 172
6.4.1 Overview 172
6.4.2 Bias Stress in Organic Thin-Film Transistors 173
6.4.2.1 Continuous Biasing 173
6.4.2.2 Pulsed Biasing 175
6.4.2.3 Long-Term Stress Effects 177
6.4.3 Nonideal Scaling of Short-Channel Organic TFTs 178
6.4.4 Low-Temperature a-Si:H TFT Device Stability 180
6.4.5 Low-temperature a-Si:H p--i--n Devices 182
6.5 Printed Flexible Electronics 185
6.5.1 Overview 185
6.5.2 Digital Lithography for Flexible Image Sensor Arrays 185
6.5.3 Printed Organic Backplanes 187
6.5.3.1 Hybrid Fabrication 187
6.5.3.2 All-Printed Electronics 189
6.6 Conclusions and Future Prospects 190
to 7 Sheet-Type Sensors and Actuators 197
7.1 Introduction 197
7.2 Sheet-type Image Scanners 198
7.2.1 Imaging Methods 199
7.2.2 Device Structure and Manufacturing Process 200
7.2.3 Electronic Performance of Organic Photodiodes 204
7.2.4 Organic Transistors 205
7.2.5 Photosensor Cells 207
7.2.6 Issues Related to Device Processes: Pixel Stability and Resolution 209
7.2.7 A Hierarchal Approach for Slow Organic Circuits 210
7.2.8 The Double-Wordline and Double-Bitline Structure 210
7.2.9 A New Dynamic Second-Wordline Decoder 213
7.2.10 Higher Speed Operation with Lower Power Consumption 213
7.2.11 New Applications and Future Prospects 214
7.3 Sheet-Type Braille Displays 215
7.3.1 Manufacturing Process 215
7.3.2 Electronic Performance of Braille Cells 218
7.3.3 Organic Transistor-based SRAM 224
7.3.4 Reading Tests 225
7.3.5 Future Prospects 226
7.4 Summary 226
to 8 Organic and Polymeric TFTs for Flexible Displays and Circuits 229
8.1 Introduction 229
8.2 Important Organic TFT Parameters for Electronic Systems 230
8.2.1 Field-Effect Mobility 230
8.2.2 Threshold Voltage 233
8.2.3 Subthreshold Swing 234
8.2.4 Leakage Currents 236
8.2.5 Contact Resistance 236
8.2.6 Capacitances and Frequency Response 237
8.2.7 TFT Nonuniformity 239
8.2.8 Bias-Stress Instability and Hysteresis 239
8.3 Active Matrix Displays 241
8.3.1 Introduction 241
8.3.2 Liquid Crystal and Electrophoretic Displays 242
8.3.2.1 Introduction 242
8.3.2.2 Electro-Optic Response of Liquid Crystal Materials 245
8.3.2.3 Electro-Optic Response of Electrophoretic Materials 247
8.3.2.4 Liquid Crystal and Electrophoretic Display Architecture 248
8.4 Active Matrix OLED Displays 250
8.4.1 Introduction 250
8.4.1.1 Electro-Optic Response of Organic Light-Emitting Diodes 251
8.4.1.2 OLED Display Architectures 252
8.4.1.3 Nonideal Behavior in AMOLED Pixels 255
8.5 Using Organic TFTs for Electronic Circuits 256
8.5.1 Thin-Film Transistor Circuits 256
8.5.1.1 Comparison with Silicon CMOS 256
8.5.1.2 Digital OTFT Design 258
8.5.2 Frequency Limitations of OTFTs 260
8.5.3 Integrated Display Drivers 261
8.5.4 Radio Frequency Identification Tags 262
8.5.4.1 Introduction 262
8.5.4.2 Using OTFT Technology for RFID Tags 264
8.6 Conclusion 270
to 9 Semiconducting Polythiophenes for Field-Effect Transistor Devices in Flexible Electronics: Synthesis and Structure Property Relationships 275
9.1 Introduction 275
9.2 Polymerization of Thiophene Monomers 277
9.2.1 General Considerations 278
9.2.2 Synthetic Routes for the Preparation of Thiophene Polymers 278
9.2.2.1 Chemical Oxidation Routes 279
9.2.2.2 Electrochemical Oxidation Routes 281
9.2.2.3 Transition Metal Catalyzed Cross-coupling Methodologies 281
9.2.2.4 Dehalogenative Polymerization 287
9.3 Poly(3-Alkylthiophenes) 287
9.3.1 Electrical Properties 289
9.3.2 Thin-film Device Processing and Morphology 290
9.3.3 Doping and Oxidative Stability 291
9.4 Polythiophene Structural Analogues 293
9.5 Thienothiophene Polymers 300
9.5.1 Poly(Thieno(2,3-b)Thiophenes) 300
9.5.2 Poly(Thieno(3,2-b)Thiophenes) 302
9.6 Summary 306
to 10 Solution Cast Films of Carbon Nanotubes for Transparent Conductors and Thin Film Transistors 311
10.1 Introduction: Nanoscale Carbon for Electronics, the Value Proposition 311
10.2 Carbon NT Film Properties 312
10.2.1 Carbon Nanotubes: The Building Blocks 312
10.2.2 Carbon Nanotube Network as an Electronic Material 312
10.2.3 Electrical and Optical Properties of NT Films 314
10.2.3.1 Concentration Dependent Conductivity 315
10.2.3.2 Temperature Dependent Conductivity 316
10.2.3.3 Frequency Dependence and Optical Conductivity 316
10.2.3.4 Geometric Factors 317
10.2.4 Doping and Chemical Functionalization 318
10.3 Fabrication Technologies 319
10.3.1 Solubilization 320
10.3.2 Deposition 320
10.3.2.1 Spraying 321
10.3.2.2 Slot Coating 322
10.3.2.3 Spin Coating 322
10.3.2.4 Filtration/Stamping 322
10.4 Carbon NT Films as Conducting and Optically Transparent Material 323
10.4.1 Network Properties: Sheet Conductance and Optical Transparency 323
10.4.2 Applications: ITO Replacement 326
10.4.3 Challenges and the Path Forward 326
10.5 TFTs with Carbon Nanotube Conducting Channels 327
10.5.1 Device Characteristics 328
10.5.2 Device Parameters 330
10.5.2.1 Mobility 331
10.5.2.2 ON/OFF Ratio 332
10.5.2.3 Device Capacitance 333
10.5.2.4 Operating Voltage 334
10.5.2.5 Threshold Voltage 335
10.5.2.6 Subthreshold Swing 335
10.5.2.7 Hysteresis 335
10.5.2.8 Device Stability 336
10.5.2.9 Doping and Logic Elements 337
10.5.3 Challenges and the Path Forward 337
10.6 Conclusions 338
to 11 Physics and Materials Issues of Organic Photovoltaics 343
11.1 Introduction 343
11.2 Basic Operation 343
11.2.1 Photocurrent 345
11.2.2 Dark Current 345
11.3 Organic and Hybrid Solar Cell Architectures 346
11.4 Materials 347
11.5 Light Absorption 348
11.6 Exciton Harvesting 352
11.6.1 Effects of Disorder 354
11.6.2 Extrinsic Defects 358
11.6.3 Measuring Exciton Harvesting 358
11.6.4 Approaches to Overcome Small Diffusion Lengths 361
11.7 Exciton Dissociation 363
11.8 Dissociating Geminate Pairs 364
11.9 Heterojunction Energy Offsets 369
11.10 Charge Transport and Recombination 371
11.10.1 Diffusion-Limited Recombination 373
11.10.2 Interface-Limited (Back Transfer Limited) Recombination 374
11.10.3 Measurements Relevant for Extracting Charge 377
11.11 Nanostructures 378
11.12 Efficiency Limits and Outlook 381
to 12 Bulk Heterojunction Solar Cells for Large-Area PV Fabrication on Flexible Substrates 386
12.1 Introduction and Motivation 386
12.1.1 Photovoltaics 386
12.1.2 Technology Overview 387
12.1.3 Motivation for Large-Area, Solution-Processable Photovoltaics 388
12.2 The Concept of Bulk Heterojunction Solar Cells 389
12.2.1 Basics of Organic Solar Cell Materials 390
12.2.2 Fundamentals of Photovoltaics 391
12.2.2.1 Solar Radiation 391
12.2.2.2 Band Considerations of a Two-Level System 392
12.2.2.3 Transport Phenomena 394
12.2.2.4 Current--Voltage Characteristic 396
12.2.2.5 The V oc 397
12.2.3 Understanding and Optimization of BHJ Composites 398
12.2.3.1 The One-Diode Model for Organic Solar Cell 399
12.2.3.2 Consequences of the One-Diode Model 405
12.2.3.3 Application to Large-Area Solution-Processed Solar Cells 409
12.3 Challenges for Large-Area Processing 414
12.3.1 Production Scheme 414
12.3.2 Encapsulation of Flexible Solar Cells 417
12.4 Conclusions 421
to 13 Substrates and Thin-Film Barrier Technology for Flexible Electronics 426
13.1 Introduction 426
13.2 Barrier Requirements 427
13.2.1 Generic Requirements 429
13.2.1.1 Barrier Level 429
13.2.1.2 Optical Properties 429
13.2.1.3 Mechanical Flexibility 429
13.2.1.4 Compatibility of the Barrier Coating 430
13.2.2 Substrate-Specific Requirements 430
13.2.2.1 Substrate Options 430
13.2.2.2 Chemical Resistance 431
13.2.2.3 High-Temperature Stability 432
13.2.2.4 Surface Quality 432
13.3 Thin-Film Barrier Technology 432
13.3.1 Historical Background 432
13.3.2 Permeation Measurement Techniques 433
13.3.3 Permeation Through Thin-Film Barriers 439
13.3.3.1 Simple Single Layer Barrier Films 439
13.3.3.2 Advanced Barrier Films 442
13.4 BarrierDevice Integration 449
13.4.1 Substrate and Barrier Compatibility with OLEDs 450
13.4.2 Thin-Film Encapsulation 453
13.5 Concluding Remarks 455
Index 463