Advanced Materials for Printed Flexible Electronics (Springer Series in Materials Science, 317) [1st ed. 2022] 3030798038, 9783030798031

This book provides a comprehensive introduction to printed flexible electronics and their applications, including the ba

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Table of contents :
Preface
Contents
About the Author
Chapter 1: Fundamentals and Design Guides for Printed Flexible Electronics
1.1 Historical Perspectives on Printed Flexible Electronics
1.2 Printing Requirements for Printable Materials
1.2.1 Ink Formulation
1.2.2 Inks for Flexible Devices
1.2.3 Inks for Stretchable Devices
1.2.4 Inks for Self-Healing Devices
1.2.5 Polymer Substrate Formulation
1.3 Design Guidelines for Printed Flexible Electronics
1.3.1 3D Modeling and Printing Process Control
1.3.2 Design Guideline for 3D Printing
1.3.3 Materials Design for Flexible and Stretchable Electronics
1.4 Fabrication Technology for 3D Printed Flexible Electronics
1.4.1 Nozzle-Based 3D Printing Technologies
1.4.2 Light-Based 3D Writing Technologies
1.4.2.1 Two-Photon Lithography
1.4.2.2 Projection Micro-stereolithography
1.4.2.3 Continuous Liquid Interface Production
1.4.3 Representative Multi-material and Hybrid 3D Printing Processes
1.4.4 Stress-Controlled Folding of 3D Systems
1.4.4.1 4D Printing
1.4.4.2 Micro- and Nanoscale Origami
1.4.5 Mechanically Guided Assembly
References
Chapter 2: Process and Material Characterization in Printed Flexible Electronics
2.1 Fluid Characterization
2.1.1 Rheology and Wetting Behavior
2.1.1.1 Viscosity
2.1.1.2 Surface Energies and Surface Tensions
2.1.1.3 Viscoelasticity
2.1.1.4 Direct Imaging
2.1.1.5 Dynamic Measurements
2.1.2 Jet Breakup and Drop Formation
2.1.3 Characteristics of Jet Fluids with Solid Fillers
2.1.3.1 Rheology of Particle Suspensions
2.1.3.2 Shear Thinning Fluids
2.1.3.3 Phase-Changing Inks and Three-Dimensional Printing
2.1.4 Ink Drop Impact and Reaction with Substrate
2.1.4.1 Drop Impact on Powder and Three-Dimensional Printed Structures
2.1.4.2 Drop Impact on Textile Surfaces
2.1.5 Solidification
2.1.6 Curing and Sintering
2.1.6.1 Thermal Sintering
2.1.6.2 Electrical Sintering
2.1.6.3 Photonic Sintering
2.1.6.4 Microwave Sintering
2.2 Solid Feedstock Materials Characterization Techniques
2.2.1 Filament for Fused Deposition
2.2.1.1 Filament Diameter Consistency
2.2.1.2 Density
2.2.1.3 Porosity
2.2.1.4 Moisture Content
2.2.1.5 Thermal Properties
2.2.1.6 Microstructure Analysis of Composite Filament
2.2.2 Powder for Additive Manufacturing Processes
2.2.2.1 Powder Morphology
2.2.2.1.1 Sieve Analysis
2.2.2.1.2 Microcopy Analysis
2.2.2.1.3 Laser Light Diffraction
2.2.2.1.4 Influence of Particle Size and Size Distribution on Part Properties
2.2.2.1.5 Effect of Particle Shape and Surface Roughness
2.2.2.2 Powder Chemistry
2.2.2.2.1 X-Ray Photoelectron Spectroscopy
2.2.2.2.2 Auger Electron Spectroscopy
2.2.2.2.3 Energy Dispersive X-Ray Spectroscopy
2.2.2.2.4 Inductively Coupled Plasma Optical Emission Spectroscopy
2.2.2.2.5 Inert Gas Fusion
2.2.2.2.6 Effect of Powder Chemistry
2.2.2.3 Powder Microstructure
2.2.2.3.1 Metallography
2.2.2.3.2 X-Ray Diffraction
2.2.2.3.3 Thermal Analysis Methods
2.3 Aerosol Jet Printing Process Characterization
2.3.1 Working Principle of Aerosol Jet Printing
2.3.1.1 Atomization Approach
2.3.1.2 Materials Transport, Focusing, and Deposition
2.3.2 Aerosol Jet Printing Parameters
2.3.2.1 Sheath and Atomizer Gas Flow
2.3.2.2 Tool Path and Design Rules
2.3.3 Future Aerosol Jet Printing Process Modification and Application
2.4 Printed Thin-Film Characterization
2.4.1 Optical Characterization
2.4.1.1 Optical Microscopy
2.4.1.2 UV-Vis Spectroscopy
2.4.2 Additional Surface Topography
2.4.2.1 Stylus Profilometry
2.4.2.2 Confocal and White-Light Microscopy
2.4.2.3 Atomic Force Microscopy
2.4.3 Electrical Conductivity Measurement
2.5 Mechanical Characterization of Printed Flexible Electronics
2.5.1 Determining Materials Constants
2.5.2 Bending Deformation
2.5.3 Stretching Deformation
2.5.4 Shear and Twisting Deformation
2.5.5 Adhesion, Cohesion, and Scratch Testing
2.5.6 Impact Resistance
2.6 Durability of Flexible Electronics
2.6.1 Engineering Stress Distribution Across Layers
2.6.2 Nanoribbons and Nanomembranes
2.6.3 Separation of Brittle Components
2.6.4 Future Perspectives
References
Chapter 3: Conductive Materials for Printed Flexible Electronics
3.1 Introduction
3.2 Advanced Metal-Based Materials for Micro/Nanoscale 3D Printing
3.2.1 Metal Nanoparticles
3.2.1.1 Synthesis of Metal Nanoparticles
3.2.1.2 Stabilization of Dispersed Metal Nanoparticles Against Aggregation
3.2.1.3 Stabilization of Metal Nanoparticles Against Oxidation
3.2.1.4 Formulation of Metal-Based Conductive Inks
3.2.1.5 Metal-Based Conductive Inks for Printing 3D Structures
3.2.2 Metal Nanowires
3.2.3 Liquid Metal Inks
3.2.4 Reactive Metal Inks
3.3 Carbon-Based Materials
3.3.1 Graphene-Based Inks
3.3.2 Carbon Nanotube-Based Inks
3.4 Transparent Oxide Conductors
3.5 Conductive Polymer Inks
3.6 Perspectives and Future Development Trends of Conductive Inks
3.6.1 Traditional Polymer Thick Film Inks
3.6.2 Printing Inks for In-Mold Electronics
3.6.3 Stretchable Conductive Inks
3.6.3.1 Sputtering/Etching or Laser-Cutting Conductive Films on Stretchable Substrates
3.6.3.2 Embedding Stretchable Conductive Materials in Stretchable Substrates
3.6.3.3 Thinning or Developing Meandering Patterns
3.6.3.3.1 Pre-strained Substrate Approach
3.6.3.3.2 Localized Node Bonding Approach
3.6.3.3.3 Helix Structure Approach
3.6.4 Enabling Limited Stretchability by Printing Conductive Ink on Stretchable Substrates
References
Chapter 4: Semiconducting Materials for Printed Flexible Electronics
4.1 Introduction
4.2 Flexible Inorganic Semiconducting Materials
4.2.1 Thin Films of Silicon
4.2.2 Films of Transparent Oxides
4.2.2.1 ZnO Films Deposited from the Gas Phase
4.2.2.2 ZnO Films Spin-Cast from Colloidal Solutions
4.2.2.3 Films of ZnO-Based Binary and Ternary Oxides
4.2.3 Films of Chalcogenides
4.2.3.1 Films of Chalcogenide Nanocrystals
4.2.3.2 Films of Chalcogenides Derived from Liquid Precursors
4.2.4 Nanoscale Inorganic Semiconductors Formed with Bottom-Up Approaches
4.2.5 Nanoscale Inorganic Semiconductors Formed with Top-Down Approaches
4.3 Organic Semiconductors for Flexible Electronics
4.3.1 Historical Perspective
4.3.2 Material Types
4.3.3 Basic Properties of Organic Semiconductors
4.3.3.1 Physical Properties
4.3.3.2 Optical Properties
4.3.3.3 Charge Carrier Transport
4.3.4 Architectures and Properties of Organic Semiconductor Devices
4.3.5 Organic Semiconductor Structural Design in Printed Electronics
4.4 Printable Organic Small Molecular Semiconductors
4.4.1 p-Type Small Molecular Semiconductors
4.4.2 n-Type Small Molecular Semiconductors
4.5 Printable Polymeric Semiconductors
4.5.1 p-Type Conjugated Polymer Semiconductors
4.5.2 n-Type Conjugated Polymers
4.5.3 Perspectives of Solution-Processed Polymer Semiconductors
4.6 Composite Organic Semiconductors
4.6.1 Polymer-Fullerene Bulk Heterojunctions
4.6.2 Polymer-Polymer Semiconductor Composites
4.6.3 Organic-Inorganic Composites of Semiconductor Nanocrystals
4.6.4 Nanoconfinement of Polymer Semiconductors with Improved Stretchability
References
Chapter 5: Substrate and Encapsulation Materials for Printed Flexible Electronics
5.1 Substrate Materials
5.1.1 General Requirements for Flexible Substrates
5.1.2 Types of Substrate Materials
5.1.2.1 Polymer Substrate Materials
5.1.2.2 Inorganic Substrate Materials
5.1.2.3 Fibrous Substrate Materials
5.2 Dielectric Materials
5.2.1 Inorganic Dielectrics
5.2.2 Polymer Dielectrics
5.2.2.1 Poly(vinyl alcohol)
5.2.2.2 Cyanoethyl Polymers
5.2.2.3 Poly(vinylidene fluoride) and Its Copolymers
5.2.3 Electrolyte Dielectrics
5.2.3.1 Polymer Electrolytes
5.2.3.2 Polyelectrolytes
5.2.3.3 Ionic Liquids
5.2.3.4 Ion-Gels
5.2.4 Hybrid Dielectrics
5.2.4.1 Self-Assembled Nano-dielectrics
5.2.4.2 Inorganic/Polymer Blends
5.3 Encapsulation
5.3.1 Encapsulation Evaluation Methods
5.3.2 Traditional Encapsulation Approaches
5.3.3 Chemical Vapor Deposition Technology for Encapsulation
5.3.4 Atomic Layer Deposition for Encapsulation
5.3.5 Thin Film Encapsulation for Flexible Devices
References
Chapter 6: Printed Flexible Thin-Film Transistors
6.1 Types of Transistors
6.1.1 Bipolar Junction Transistors
6.1.1.1 NPN Transistor
6.1.1.2 PNP Transistor
6.1.2 Field-Effect Transistors
6.1.2.1 Junction-Field-Effect Transistor
6.1.2.2 Metal-Oxide-Semiconductor Field-Effect-Transistor
6.1.3 Other Emerging Transistors
6.2 Structure and Operation of Thin-Film Transistors
6.3 Printing Techniques and Printed Components of Thin-Film Transistors
6.3.1 Printing Techniques
6.3.2 Printed TFTs on Rigid Substrate
6.3.2.1 Printed Semiconductor Layer
6.3.2.1.1 Organic Semiconductor
6.3.2.1.2 Carbon-Based Semiconductor
6.3.2.2 Printed Dielectric Layer
6.3.2.3 Printed Electrodes
6.3.2.4 Fully Printed TFTs
6.3.3 Printed TFTs on Flexible Substrate
6.3.3.1 Polymer Substrates
6.3.3.1.1 Partly Printed TFTs on Flexible Substrate
6.3.3.1.2 Fully Printed TFTs on Flexible Substrate
6.3.3.2 Paper Substrate
6.4 Printed Organic Thin-Film Transistors
6.4.1 Materials for OTFTs
6.4.1.1 Organic Semiconductors
6.4.1.2 Gate Dielectrics in OTFTs
6.4.1.3 Other Materials Used in OTFTs
6.4.2 Device Structures Used for OTFTs
6.4.3 Manufacturing Process and Integration of OTFTs
6.4.3.1 Processes Compatible with Established Industry Facilities
6.4.3.2 Full Printing Processes for OTFTs
6.4.3.3 Challenges and Outlook for OTFT Technologies
6.5 Printed Inorganic Thin-Film Transistors
6.5.1 Printed Oxide Transistors
6.5.1.1 Vacuum Deposition-Based Metal Oxide TFTs
6.5.1.2 Solution-Processed n-Type Metal-Oxide-Semiconductors
6.5.1.2.1 Basics of Sol–Gel Oxide Chemistry
6.5.1.2.2 Low-Temperature Route for Solution-Processed n-Type Oxide Semiconductors
Novel Precursor Approaches
Novel Posttreatment Methods
6.5.1.2.3 Current Challenges in Solution-Processed n-Type Oxide Semiconductors
6.5.1.3 Solution-Processed p-Type Metal-Oxide-Semiconductors
6.5.1.3.1 Basics of p-Type Oxide Semiconductors
6.5.1.3.2 Copper Oxide
6.5.1.3.3 Tin Monoxide
6.5.1.3.4 Nickel Oxide
6.5.1.3.5 Current Challenges in Solution-Processed p-Type Oxide Semiconductors
6.5.2 Carbon Nanotubes for Thin-Film Transistors
6.5.2.1 SWCNT-TFT Fabrication
6.5.2.1.1 CNT Fabrication
6.5.2.1.2 Separation of Metallic and Semiconducting CNTs
6.5.2.1.3 CNT Film Fabrication Process
6.5.2.1.4 SWCNT-TFT Structure and Fabrication Process
6.5.2.2 Electrical, Optical, and Mechanical Properties of SWCNT-TFTs
6.5.2.2.1 Electrical Properties
6.5.2.2.2 Optical Properties
6.5.2.2.3 Mechanical Properties
6.5.2.3 Outlook on Carbon Nanotubes-Based Thin-Film Transistors
6.5.2.3.1 Alignment
6.5.2.3.2 Metal Contact
6.5.2.3.3 Semiconducting CNT Purity
6.5.2.3.4 N-Type Device
6.5.2.3.5 Integration
6.5.3 Thin-Film Transistors Based on Graphene and Graphene/Semiconductor Heterojunctions
6.5.3.1 Graphene Acting as Channel Material in Thin-Film Transistors
6.5.3.2 Graphene Acting as Electrode Material in Thin-Film Transistors
6.5.3.2.1 Preparation of Graphene/Semiconductor Heterojunctions
Mechanical Stacking Method
Direct CVD Growth of 2D Nanomaterials Heterostructures
6.5.3.2.2 Graphene/Inorganic Semiconductor Heterojunction TFTs
6.5.3.2.3 Graphene/Organic Semiconductor Heterojunction TFTs
6.5.3.3 Outlook on Graphene-Based Thin-Film Transistors
6.5.4 High-Mobility Thin-Film Transistors Based on Multilayer 2D Materials
6.5.4.1 Rationale
6.5.4.2 Common 2D Materials for TFTs
6.5.4.3 Applications of 2D TMDs TFTs
6.5.4.3.1 Flexible Devices
6.5.4.3.2 Transparent Devices
6.5.4.3.3 Optoelectronic Devices: Sensitive Photodetectors
6.5.4.4 Outlook on High-Mobility Thin-Film Transistors
References
Chapter 7: Printed Flexible Organic Light-Emitting Diodes
7.1 Introduction
7.2 Working Principle of Organic Light-Emitting Diodes
7.2.1 Basic Light Phenomena
7.2.1.1 Incandescence
7.2.1.2 Luminescence
7.2.1.2.1 Photoluminescence
7.2.1.2.2 Electroluminescence
7.2.2 OLED Device Structure
7.2.3 OLED Working
7.2.4 OLED Classification
7.2.4.1 Passive-Matrix OLED (PMOLED)
7.2.4.2 Active-Matrix OLED (AMOLED)
7.2.4.3 Transparent OLEDs
7.2.4.4 Top- and Bottom-Emitting OLED
7.2.4.5 White OLEDS
7.2.4.6 Flexible OLED
7.2.4.7 Phosphorescent OLED
7.2.5 OLED Characterization
7.2.5.1 Internal Quantum Efficiency
7.2.5.2 External Quantum Efficiency
7.2.5.3 Outcoupling Efficiency
7.2.5.4 Efficacy
7.2.5.5 Lifetime Issues
7.2.5.6 Routine Testing for Performance Evaluation of OLED Device
7.2.6 OLED Fabrication Techniques
7.2.6.1 Physical Vapor Deposition
7.2.6.2 Screen Printing
7.2.6.3 Inkjet Printing
7.2.6.4 In-line Fabrication
7.2.6.5 Roll-to-Roll Process
7.3 General Materials and Components of OLEDs
7.3.1 Substrate
7.3.1.1 Geometric Properties: Hermiticity and Surface Roughness
7.3.1.2 Substrate Material Requirements
7.3.2 Anode
7.3.3 Cathode
7.3.4 Organic Emissive Materials
7.3.5 Amorphous Molecular Materials for Hole- and Electron-Transporting
7.3.5.1 Hole Transporting Amorphous Molecular Materials
7.3.5.2 Electron-Transporting Amorphous Molecular Materials
7.3.6 Solution-Processable OLED Materials
7.3.7 Encapsulation for OLEDs
7.4 White Lighting OLEDs
7.4.1 White Light Emission Mechanism
7.4.1.1 White Light Emission from Small-Molecule-Doped Polymer Films
7.4.1.1.1 Fluorescence-Emitting Dopants
7.4.1.1.2 Phosphorescent Emitters
7.4.1.1.3 Hybrid Fluorescent Blue/Phosphorescent Green and Red Systems
7.4.1.2 White Emission from Multiple Light-Emitting Polymers
7.4.1.2.1 Blended Polymeric Systems
7.4.1.2.2 White Light from Polymer Heterolayers
7.4.1.3 Single-Component Polymer Systems
7.4.1.3.1 Conjugated Copolymers Comprising Main Chain Chromophores
7.4.1.3.2 Copolymers with Side-Chain Chromophores
7.4.1.4 Outlook on the Development of Polymer White OLEDs
7.4.2 White OLEDs Based on Small Molecules
7.4.3 Light Outcoupling Improvement and Efficiency Limitation of White OLEDs
7.5 Flexible Quantum Dot Light-Emitting Diodes
7.5.1 Material Design for Efficient QLEDs
7.5.2 Device Structures and Operation Principles of QLEDs
7.5.3 Patterning Technology of QDs for Full-Color Displays
7.5.4 Flexible White QLEDs
7.5.5 Flexible Transparent QLEDs
7.5.6 Potential Applications of Flexible QLEDs
7.5.7 Outlook on Flexible and Wearable QLEDs
References
Chapter 8: Printable Solar Cells from Solution Processable Materials
8.1 Operating Principles of Printable Solar Cells
8.1.1 Fundamentals of Solar Cells
8.1.2 Device Structure
8.1.3 Operating Principles
8.1.4 Performance Characteristics
8.1.4.1 Fill Factor
8.1.4.2 Open Circuit Voltage
8.1.4.3 Short Circuit Current Density
8.1.4.4 Absorption Coefficient
8.1.4.5 Recombination and Diffusion Length
8.1.4.6 Photovoltaic Cell Efficiency Limit
8.2 Solution-Processed Organic Polymeric Solar Cells
8.2.1 Historical Perspective
8.2.2 Tandem Solar Cells
8.2.2.1 Interconnecting Layer Materials
8.2.2.2 Processing Multijnction Stacks and Light Management
8.2.2.3 Active Layer Materials
8.2.2.4 Upscaling
8.3 Solution-Processed Inorganic CIGS/CZTS Thin-Film Solar Cells
8.4 Organic–Inorganic Hybrid Perovskite Solar Cells
8.5 Outlook and Future Perspective
References
Chapter 9: Printed Flexible Electrochemical Energy Storage Devices
9.1 Perspectives on Electrochemical Energy Storage
9.1.1 Classification of Electrochemical Energy Storage
9.1.1.1 Basic Battery Operation
9.1.1.2 Basic Operation of Capacitor and Supercapacitor
9.1.2 Miniaturization of Electrochemical Energy Storage Devices for Flexible/Wearable Electronics
9.2 3D Printing for Electrochemical Energy Storage Applications
9.2.1 Printing Technologies for Electrochemical Energy Storage Device Fabrication
9.2.1.1 Basic 3D Printing Systems and Processes
9.2.1.2 Materials Considerations
9.2.2 Performance Optimization Strategies
9.2.2.1 Performance Metrics
9.2.2.2 Optimization Strategies
9.2.2.2.1 Utilization of Nanomaterials
9.2.2.2.2 Electrical Transport Optimization
9.2.2.2.3 Ionic Transport Optimization
9.2.2.2.4 Mechanically Robust Design
9.2.2.2.5 Scaling Up Performance Optimization
9.2.3 Advances in 3D-Printed Electrochemical Energy Storage Devices
9.2.3.1 Sandwich-Type Configurations
9.2.3.1.1 Sandwich-Type Batteries
9.2.3.1.2 Sandwich-Type Electrochemical Capacitors
9.2.3.2 In-Plane Configurations
9.2.3.2.1 In-Plane Batteries
9.2.3.2.2 In-Plane Electrochemical Capacitors
9.2.4 Outlook on Printed Electrochemical Energy Storage Devices
9.3 Printed Battery Architectures
9.3.1 Printing Technique Adoption
9.3.2 Preparation of Battery Component Inks
9.3.2.1 Printed Electrodes
9.3.2.2 Printed Electrolytes and Separator Membranes
9.3.3 Electrochemical Performances of Printed Batteries
9.3.4 Advances in Printed Battery Systems and Their Applications
9.3.4.1 Zn-Based Batteries
9.3.4.2 Li-Ion Batteries
9.3.5 Perspectives and Future Development Directions
9.4 Printed Flexible Supercapacitors
9.4.1 Device Structures of Printed Supercapacitors
9.4.2 Printable Materials for Supercapacitors
9.4.2.1 Electrode Materials
9.4.2.1.1 Carbon-Based Electrode Materials
9.4.2.1.2 Metal-Based Electrode Materials
9.4.2.1.3 Conducting Polymers
9.4.2.1.4 2D Nanomaterials Beyond Graphene
9.4.2.1.5 Metal-Organic Frameworks
9.4.2.2 Electrolytes
9.4.2.2.1 Aqueous Gel Polymer Electrolytes
9.4.2.2.2 Organic Gel Polymer Electrolytes
9.4.2.2.3 Ionic Liquid-Based Gel Polymer Electrolytes
9.4.2.2.4 Redox-Active Gel Electrolytes
9.4.2.3 Current Collectors
9.4.2.3.1 Metal Current Collectors
9.4.2.3.2 Carbon-Based Current Collectors
9.4.2.4 Substrates
9.4.2.4.1 Metal Foils
9.4.2.4.2 Polymer-Based Plastic Substrates
9.4.2.4.3 Paper Substrates
9.4.2.4.4 Textiles
9.4.3 Advances of Printed Supercapacitors
9.4.3.1 Inkjet Printing
9.4.3.2 Screen Printing
9.4.3.3 Three-Dimensional (3D) Printing
9.4.3.4 Transfer Printing
9.4.3.5 Pen-Based Direct Ink Writing
9.4.3.6 Roll-to-Roll (R2R) Printing
9.4.3.7 Patterned Coating Methods
9.4.3.8 Outlook on Printed Supercapacitors
9.4.4 Applications of Printed Supercapacitors
9.4.4.1 Multifunctional Supercapacitors
9.4.4.2 Supercapacitors Working as Power Units for Sensors
9.4.4.3 Supercapacitors Working as Energy Storage Units for Ambient Energy Sources
9.4.5 Challenges and Future Perspectives
References
Chapter 10: Printed Flexible Sensors and Sensing Systems
10.1 Introduction
10.2 Working Principle of Sensors
10.3 Printable Materials and Component Integration
10.3.1 Substrates for Flexible Sensors
10.3.2 Conducting Materials
10.3.2.1 Metals
10.3.2.2 Amorphous Oxide Conductors
10.3.2.3 Carbon Conductors
10.3.2.4 Organic Conductors
10.3.3 Semiconductors
10.3.3.1 Metal Oxide Semiconductors
10.3.3.2 Organic Semiconductors
10.3.3.3 Flexible Silicon
10.3.3.4 Transition Metal Dichalcogenides
10.3.3.5 Black Phosphorus
10.3.3.6 Perovskites
10.3.4 Dielectric Materials
10.4 Printed Flexible Sensors
10.4.1 Printable Pressure Sensors
10.4.1.1 Piezoresistive Sensors
10.4.1.2 Piezoelectric Sensors
10.4.1.3 Piezocapacitive Sensors
10.4.1.4 Triboelectric Sensors
10.4.2 Printable Strain Sensors
10.4.3 Temperature Sensors
10.4.4 Humidity Sensors
10.4.5 Magnetic Sensors
10.4.6 Chemical Sensors
10.4.7 Electromagnetic Radiation Sensors
10.4.8 Multimodal Sensors
10.4.9 Electropotential Sensors
10.4.10 Ultrasonic Sensors
10.5 Integration of Printed Sensors into Systems
10.6 Future Perspectives
References
Chapter 11: Printed Flexible Hybrid Electronics
11.1 State-of-the-Art Development
11.1.1 The Roles of Printed Electronics and Standard Silicon Integrated Circuits
11.1.2 The Merit of Flexible Hybrid Electronics
11.2 Core Components of the Flexible Hybrid Electronics
11.2.1 Substrate
11.2.2 Inks and Printing Techniques
11.2.3 Printed Sensors and Circuits
11.3 Thinned Silicon ICs and Assembly Process in FHE
11.3.1 Thinning Silicon ICs and Connecting to FHE
11.3.2 Conductive and Nonconductive Adhesives
11.3.3 Assembly Process for Rigid Components in FHE
11.4 Printed Antennas for Wireless Power and Communications
11.4.1 Printed Antennas for Communication Purposes
11.4.2 Printed Coils for Wireless Power Transfer
11.5 Printed Power Sources: Batteries, Solar Cells, and Energy Harvesters
11.5.1 Printed Energy-Storage Modules
11.5.2 Printed Energy-Harvesting Modules
11.6 Quality Assurance
11.6.1 High-Resolution Patterning
11.6.2 Uniformity
11.6.3 Flexibility/Stretchability
11.6.4 Durability
11.7 Reliability Evaluation
11.8 Application
11.8.1 Wearable Health Monitoring with FHE
11.8.2 Industrial, Environmental, and Agricultural Monitoring with FHE
11.8.3 Structural Health Monitoring with FHE
11.9 Challenges and Future Trends
References
Chapter 12: Current Trends and Prospects in Advanced Manufacturing for Printed Electronics
12.1 Introduction
12.2 Electronic Materials and Components
12.3 Techniques and Processes in Printed Electronics
12.3.1 Techniques in Printed Electronics
12.3.1.1 2D-Printing Technologies
12.3.1.2 3D-Printing Technologies
12.3.1.3 4D-Printing Technologies
12.3.2 Processes in 3D-Printing Electronics
12.4 Current Trends in 3D-Printed Electronics
12.4.1 Research and Development
12.4.1.1 Common Devices
12.4.1.2 Antennas
12.4.1.3 Flexible Electronics
12.4.1.4 Batteries
12.4.2 Integrated 3D-Printing Systems for Mass Production
References
Abbreviations
Index
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Advanced Materials for Printed Flexible Electronics (Springer Series in Materials Science, 317) [1st ed. 2022]
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Springer Series in Materials Science 317

Colin Tong

Advanced Materials for Printed Flexible Electronics

Springer Series in Materials Science Volume 317 Series Editors Robert Hull, Center for Materials, Devices, and Integrated Systems, Rensselaer Polytechnic Institute, Troy, NY, USA Chennupati Jagadish, Research School of Physics and Engineering, Australian National University, Canberra, ACT, Australia Yoshiyuki Kawazoe, Center for Computational Materials, Tohoku University, Sendai, Japan Jamie Kruzic, School of Mechanical & Manufacturing Engineering, UNSW Sydney, Sydney, NSW, Australia Richard M. Osgood, Department of Electrical Engineering, Columbia University, New York, USA Jürgen Parisi, Universität Oldenburg, Oldenburg, Germany Udo W. Pohl, Institute of Solid State Physics, Technical University of Berlin, Berlin, Germany Tae-Yeon Seong, Department of Materials Science & Engineering, Korea University, Seoul, Korea (Republic of) Shin-ichi Uchida, Electronics and Manufacturing, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan Zhiming M. Wang, Institute of Fundamental and Frontier Sciences - Electronic, University of Electronic Science and Technology of China, Chengdu, China

The Springer Series in Materials Science covers the complete spectrum of materials research and technology, including fundamental principles, physical properties, materials theory and design. Recognizing the increasing importance of materials science in future device technologies, the book titles in this series reflect the state-­ of-­ the-art in understanding and controlling the structure and properties of all important classes of materials. More information about this series at http://www.springer.com/series/856

Colin Tong

Advanced Materials for Printed Flexible Electronics

Colin Tong Bolingbrook, IL, USA

ISSN 0933-033X     ISSN 2196-2812 (electronic) Springer Series in Materials Science ISBN 978-3-030-79803-1    ISBN 978-3-030-79804-8 (eBook) https://doi.org/10.1007/978-3-030-79804-8 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

This book is dedicated to my wife Dali and to our parents, and our family.

Preface

Printed electronics is attracting a great deal of attention in both research and commercialization as it enables fabrication of large-scale, low-cost electronic devices on a variety of substrates. Printed flexible electronics has been explored for manufacturing of widespread flexible, stretchable, wearable, and conformal electronic devices with conventional, 3D, and hybrid printing technologies. This has enabled a wide variety of applications, such as transparent conductive films, thin-film transistors, printable solar cells, flexible energy harvesting and storage devices, electroluminescent devices, and wearable sensors. At the intersection between integrated 3D printing technologies and electronics, advanced materials are helping to bridge the gap between the creation of structure and function. Advanced materials in 3D printing include a range of advanced polymers, nanoparticles, ceramics, graphene, etc. These materials are enabling traditional and advanced electronic applications while taking advantage of the design and fabrication flexibility of additive manufacturing systems. Additive processes allow new mechanical, optical, and electronic products to be fabricated with nearly any geometry, reducing traditional manufacturing constraints. Innovative materials are a key enabling component of many printed flexible electronic devices and range from organic semiconductors to quantum dots and from carbon nanomaterials to conductive adhesives. Functional materials are a fundamental part of the value chain for printed flexible electronics since the materials need to combine electronic/semiconducting functionality by being flexible, stretchable, wearable, and/or solution processable as well as being stable and straightforward to manufacture. This can be a significant technical challenge, leading to widespread innovation in materials research and development. As a systematic reference source or textbook, Advanced Materials for Printed Flexible Electronics is aimed at enabling today’s students, researchers, and engineers to understand current status and future trends in printed flexible electronics, and acquire skills for using materials and additive manufacturing processes in the design of printed flexible electronics and develop a cross-functional approach that brings together materials, 3D printing, electronics, and processes in the context of technologies used in mass production of printed flexible electronics that takes vii

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economic and regulatory aspects into account. This book provides a comprehensive introduction to basic principles of modern printing technologies, formulations of printable inks, performance characterization methods, device design and fabrication processes, and a wide range of materials used for printed flexible electronics, as well as their most suitable applications. Integrated coverage ranges from the application of scientific and engineering principles for materials to enable different types of printed flexible electronics; properties, performance, modeling, fabrication, characterization, and application of innovative and functional materials used in each electronic system; the complex relationships between materials selection, optimizing design, and device operating conditions in each electronic system; to research and development challenges of novel emerging materials for future printed hybrid electronic systems. Finally, the current trends and prospects in advanced manufacturing for printed flexible electronics are addressed. It is a great pleasure to acknowledge the help and support I have received from my colleagues and friends. I would like to express my sincere gratitude to Dr. Sam Harrison and all other editing staff who have done a fantastic job on the publication of this book. Chicago, IL

Colin Tong

Contents

1 Fundamentals and Design Guides for Printed Flexible Electronics������������������������������������������������������������������������������������������������    1 1.1 Historical Perspectives on Printed Flexible Electronics ������������������    2 1.2 Printing Requirements for Printable Materials ��������������������������������    6 1.2.1 Ink Formulation��������������������������������������������������������������������    7 1.2.2 Inks for Flexible Devices������������������������������������������������������   11 1.2.3 Inks for Stretchable Devices ������������������������������������������������   14 1.2.4 Inks for Self-Healing Devices����������������������������������������������   18 1.2.5 Polymer Substrate Formulation��������������������������������������������   19 1.3 Design Guidelines for Printed Flexible Electronics��������������������������   22 1.3.1 3D Modeling and Printing Process Control��������������������������   23 1.3.2 Design Guideline for 3D Printing ����������������������������������������   25 1.3.3 Materials Design for Flexible and Stretchable Electronics����������������������������������������������������������������������������   27 1.4 Fabrication Technology for 3D Printed Flexible Electronics ����������   31 1.4.1 Nozzle-Based 3D Printing Technologies������������������������������   32 1.4.2 Light-Based 3D Writing Technologies ��������������������������������   34 1.4.3 Representative Multi-material and Hybrid 3D Printing Processes ������������������������������������������������������������������������������   37 1.4.4 Stress-Controlled Folding of 3D Systems����������������������������   38 1.4.5 Mechanically Guided Assembly ������������������������������������������   44 References��������������������������������������������������������������������������������������������������   48 2 Process and Material Characterization in Printed Flexible Electronics������������������������������������������������������������������������������������������������   53 2.1 Fluid Characterization����������������������������������������������������������������������   53 2.1.1 Rheology and Wetting Behavior ������������������������������������������   54 2.1.2 Jet Breakup and Drop Formation������������������������������������������   60 2.1.3 Characteristics of Jet Fluids with Solid Fillers ��������������������   62 2.1.4 Ink Drop Impact and Reaction with Substrate����������������������   65

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2.1.5 Solidification������������������������������������������������������������������������   67 2.1.6 Curing and Sintering ������������������������������������������������������������   68 2.2 Solid Feedstock Materials Characterization Techniques������������������   70 2.2.1 Filament for Fused Deposition���������������������������������������������   70 2.2.2 Powder for Additive Manufacturing Processes��������������������   74 2.3 Aerosol Jet Printing Process Characterization����������������������������������   84 2.3.1 Working Principle of Aerosol Jet Printing����������������������������   84 2.3.2 Aerosol Jet Printing Parameters��������������������������������������������   87 2.3.3 Future Aerosol Jet Printing Process Modification and Application ��������������������������������������������������������������������   93 2.4 Printed Thin-Film Characterization��������������������������������������������������   95 2.4.1 Optical Characterization ������������������������������������������������������   95 2.4.2 Additional Surface Topography��������������������������������������������   95 2.4.3 Electrical Conductivity Measurement����������������������������������   97 2.5 Mechanical Characterization of Printed Flexible Electronics����������   98 2.5.1 Determining Materials Constants ����������������������������������������   98 2.5.2 Bending Deformation�����������������������������������������������������������  100 2.5.3 Stretching Deformation��������������������������������������������������������  101 2.5.4 Shear and Twisting Deformation������������������������������������������  104 2.5.5 Adhesion, Cohesion, and Scratch Testing����������������������������  105 2.5.6 Impact Resistance ����������������������������������������������������������������  107 2.6 Durability of Flexible Electronics����������������������������������������������������  107 2.6.1 Engineering Stress Distribution Across Layers��������������������  107 2.6.2 Nanoribbons and Nanomembranes ��������������������������������������  109 2.6.3 Separation of Brittle Components����������������������������������������  111 2.6.4 Future Perspectives ��������������������������������������������������������������  111 References��������������������������������������������������������������������������������������������������  113 3 Conductive Materials for Printed Flexible Electronics������������������������  119 3.1 Introduction��������������������������������������������������������������������������������������  119 3.2 Advanced Metal-Based Materials for Micro/Nanoscale 3D Printing����������������������������������������������������������������������������������������  121 3.2.1 Metal Nanoparticles��������������������������������������������������������������  121 3.2.2 Metal Nanowires ������������������������������������������������������������������  131 3.2.3 Liquid Metal Inks������������������������������������������������������������������  132 3.2.4 Reactive Metal Inks��������������������������������������������������������������  135 3.3 Carbon-Based Materials��������������������������������������������������������������������  136 3.3.1 Graphene-Based Inks������������������������������������������������������������  136 3.3.2 Carbon Nanotube-Based Inks ����������������������������������������������  138 3.4 Transparent Oxide Conductors ��������������������������������������������������������  140 3.5 Conductive Polymer Inks������������������������������������������������������������������  143 3.6 Perspectives and Future Development Trends of Conductive Inks����������������������������������������������������������������������������  145 3.6.1 Traditional Polymer Thick Film Inks������������������������������������  146 3.6.2 Printing Inks for In-Mold Electronics����������������������������������  148

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3.6.3 Stretchable Conductive Inks ������������������������������������������������  148 3.6.4 Enabling Limited Stretchability by Printing Conductive Ink on Stretchable Substrates������������������������������������������������  153 References��������������������������������������������������������������������������������������������������  155 4 Semiconducting Materials for Printed Flexible Electronics����������������  159 4.1 Introduction��������������������������������������������������������������������������������������  159 4.2 Flexible Inorganic Semiconducting Materials����������������������������������  160 4.2.1 Thin Films of Silicon������������������������������������������������������������  161 4.2.2 Films of Transparent Oxides������������������������������������������������  163 4.2.3 Films of Chalcogenides��������������������������������������������������������  165 4.2.4 Nanoscale Inorganic Semiconductors Formed with Bottom-Up Approaches������������������������������������������������  167 4.2.5 Nanoscale Inorganic Semiconductors Formed with Top-Down Approaches ������������������������������������������������  169 4.3 Organic Semiconductors for Flexible Electronics����������������������������  172 4.3.1 Historical Perspective ����������������������������������������������������������  175 4.3.2 Material Types����������������������������������������������������������������������  176 4.3.3 Basic Properties of Organic Semiconductors ����������������������  178 4.3.4 Architectures and Properties of Organic Semiconductor Devices����������������������������������������������������������������������������������  184 4.3.5 Organic Semiconductor Structural Design in Printed Electronics����������������������������������������������������������������������������  187 4.4 Printable Organic Small Molecular Semiconductors������������������������  190 4.4.1 p-Type Small Molecular Semiconductors����������������������������  190 4.4.2 n-Type Small Molecular Semiconductors����������������������������  193 4.5 Printable Polymeric Semiconductors������������������������������������������������  195 4.5.1 p-Type Conjugated Polymer Semiconductors����������������������  196 4.5.2 n-Type Conjugated Polymers������������������������������������������������  198 4.5.3 Perspectives of Solution-Processed Polymer Semiconductors��������������������������������������������������������������������  199 4.6 Composite Organic Semiconductors������������������������������������������������  200 4.6.1 Polymer-Fullerene Bulk Heterojunctions ����������������������������  203 4.6.2 Polymer-Polymer Semiconductor Composites ��������������������  207 4.6.3 Organic-Inorganic Composites of Semiconductor Nanocrystals��������������������������������������������������������������������������  208 4.6.4 Nanoconfinement of Polymer Semiconductors with Improved Stretchability������������������������������������������������  212 References��������������������������������������������������������������������������������������������������  217 5 Substrate and Encapsulation Materials for Printed Flexible Electronics������������������������������������������������������������������������������������������������  221 5.1 Substrate Materials����������������������������������������������������������������������������  221 5.1.1 General Requirements for Flexible Substrates ��������������������  222 5.1.2 Types of Substrate Materials������������������������������������������������  223 5.2 Dielectric Materials��������������������������������������������������������������������������  226

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5.2.1 Inorganic Dielectrics������������������������������������������������������������  227 5.2.2 Polymer Dielectrics��������������������������������������������������������������  229 5.2.3 Electrolyte Dielectrics����������������������������������������������������������  236 5.2.4 Hybrid Dielectrics����������������������������������������������������������������  239 5.3 Encapsulation������������������������������������������������������������������������������������  242 5.3.1 Encapsulation Evaluation Methods��������������������������������������  244 5.3.2 Traditional Encapsulation Approaches ��������������������������������  245 5.3.3 Chemical Vapor Deposition Technology for Encapsulation������������������������������������������������������������������  247 5.3.4 Atomic Layer Deposition for Encapsulation������������������������  247 5.3.5 Thin Film Encapsulation for Flexible Devices ��������������������  250 References��������������������������������������������������������������������������������������������������  253 6 Printed Flexible Thin-Film Transistors�������������������������������������������������  257 6.1 Types of Transistors��������������������������������������������������������������������������  257 6.1.1 Bipolar Junction Transistors ������������������������������������������������  258 6.1.2 Field-Effect Transistors��������������������������������������������������������  259 6.1.3 Other Emerging Transistors��������������������������������������������������  262 6.2 Structure and Operation of Thin-Film Transistors����������������������������  263 6.3 Printing Techniques and Printed Components of Thin-Film Transistors����������������������������������������������������������������������������������������  269 6.3.1 Printing Techniques��������������������������������������������������������������  269 6.3.2 Printed TFTs on Rigid Substrate������������������������������������������  272 6.3.3 Printed TFTs on Flexible Substrate��������������������������������������  282 6.4 Printed Organic Thin-Film Transistors ��������������������������������������������  290 6.4.1 Materials for OTFTs ������������������������������������������������������������  290 6.4.2 Device Structures Used for OTFTs��������������������������������������  295 6.4.3 Manufacturing Process and Integration of OTFTs����������������  296 6.5 Printed Inorganic Thin-Film Transistors������������������������������������������  303 6.5.1 Printed Oxide Transistors������������������������������������������������������  303 6.5.2 Carbon Nanotubes for Thin-Film Transistors ����������������������  316 6.5.3 Thin-Film Transistors Based on Graphene and Graphene/Semiconductor Heterojunctions��������������������  325 6.5.4 High-Mobility Thin-Film Transistors Based on Multilayer 2D Materials��������������������������������������������������  333 References��������������������������������������������������������������������������������������������������  340 7 Printed Flexible Organic Light-Emitting Diodes����������������������������������  347 7.1 Introduction��������������������������������������������������������������������������������������  347 7.2 Working Principle of Organic Light-Emitting Diodes����������������������  348 7.2.1 Basic Light Phenomena��������������������������������������������������������  348 7.2.2 OLED Device Structure��������������������������������������������������������  352 7.2.3 OLED Working ��������������������������������������������������������������������  354 7.2.4 OLED Classification ������������������������������������������������������������  355 7.2.5 OLED Characterization��������������������������������������������������������  359 7.2.6 OLED Fabrication Techniques����������������������������������������������  362

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7.3 General Materials and Components of OLEDs��������������������������������  365 7.3.1 Substrate��������������������������������������������������������������������������������  365 7.3.2 Anode������������������������������������������������������������������������������������  366 7.3.3 Cathode ��������������������������������������������������������������������������������  367 7.3.4 Organic Emissive Materials��������������������������������������������������  367 7.3.5 Amorphous Molecular Materials for Hole- and Electron-­Transporting ����������������������������������������������������������  370 7.3.6 Solution-Processable OLED Materials ��������������������������������  371 7.3.7 Encapsulation for OLEDs ����������������������������������������������������  372 7.4 White Lighting OLEDs��������������������������������������������������������������������  372 7.4.1 White Light Emission Mechanism���������������������������������������  373 7.4.2 White OLEDs Based on Small Molecules����������������������������  379 7.4.3 Light Outcoupling Improvement and Efficiency Limitation of White OLEDs ������������������������������������������������  380 7.5 Flexible Quantum Dot Light-Emitting Diodes ��������������������������������  383 7.5.1 Material Design for Efficient QLEDs ����������������������������������  384 7.5.2 Device Structures and Operation Principles of QLEDs��������  386 7.5.3 Patterning Technology of QDs for Full-Color Displays ������  388 7.5.4 Flexible White QLEDs����������������������������������������������������������  389 7.5.5 Flexible Transparent QLEDs������������������������������������������������  392 7.5.6 Potential Applications of Flexible QLEDs ��������������������������  394 7.5.7 Outlook on Flexible and Wearable QLEDs��������������������������  396 References��������������������������������������������������������������������������������������������������  397 8 Printable Solar Cells from Solution Processable Materials ����������������  401 8.1 Operating Principles of Printable Solar Cells ����������������������������������  401 8.1.1 Fundamentals of Solar Cells ������������������������������������������������  402 8.1.2 Device Structure��������������������������������������������������������������������  404 8.1.3 Operating Principles�������������������������������������������������������������  405 8.1.4 Performance Characteristics ������������������������������������������������  407 8.2 Solution-Processed Organic Polymeric Solar Cells��������������������������  412 8.2.1 Historical Perspective ����������������������������������������������������������  412 8.2.2 Tandem Solar Cells ��������������������������������������������������������������  414 8.3 Solution-Processed Inorganic CIGS/CZTS Thin-Film Solar Cells����������������������������������������������������������������������������������������  424 8.4 Organic–Inorganic Hybrid Perovskite Solar Cells����������������������������  426 8.5 Outlook and Future Perspective��������������������������������������������������������  429 References��������������������������������������������������������������������������������������������������  431 9 Printed Flexible Electrochemical Energy Storage Devices������������������  433 9.1 Perspectives on Electrochemical Energy Storage ����������������������������  433 9.1.1 Classification of Electrochemical Energy Storage����������������  434 9.1.2 Miniaturization of Electrochemical Energy Storage Devices for Flexible/Wearable Electronics��������������������������  440

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9.2 3D Printing for Electrochemical Energy Storage Applications��������  446 9.2.1 Printing Technologies for Electrochemical Energy Storage Device Fabrication����������������������������������������������������������������  447 9.2.2 Performance Optimization Strategies ����������������������������������  449 9.2.3 Advances in 3D-Printed Electrochemical Energy Storage Devices����������������������������������������������������������������������������������  453 9.2.4 Outlook on Printed Electrochemical Energy Storage Devices����������������������������������������������������������������������������������  457 9.3 Printed Battery Architectures������������������������������������������������������������  458 9.3.1 Printing Technique Adoption������������������������������������������������  459 9.3.2 Preparation of Battery Component Inks ������������������������������  461 9.3.3 Electrochemical Performances of Printed Batteries ������������  467 9.3.4 Advances in Printed Battery Systems and Their Applications��������������������������������������������������������������������������  468 9.3.5 Perspectives and Future Development Directions����������������  475 9.4 Printed Flexible Supercapacitors������������������������������������������������������  478 9.4.1 Device Structures of Printed Supercapacitors����������������������  480 9.4.2 Printable Materials for Supercapacitors��������������������������������  482 9.4.3 Advances of Printed Supercapacitors ����������������������������������  491 9.4.4 Applications of Printed Supercapacitors������������������������������  506 9.4.5 Challenges and Future Perspectives��������������������������������������  512 References��������������������������������������������������������������������������������������������������  517 10 Printed Flexible Sensors and Sensing Systems��������������������������������������  523 10.1 Introduction������������������������������������������������������������������������������������  523 10.2 Working Principle of Sensors ��������������������������������������������������������  526 10.3 Printable Materials and Component Integration ����������������������������  531 10.3.1 Substrates for Flexible Sensors������������������������������������������  532 10.3.2 Conducting Materials����������������������������������������������������������  534 10.3.3 Semiconductors������������������������������������������������������������������  541 10.3.4 Dielectric Materials������������������������������������������������������������  545 10.4 Printed Flexible Sensors ����������������������������������������������������������������  546 10.4.1 Printable Pressure Sensors��������������������������������������������������  546 10.4.2 Printable Strain Sensors������������������������������������������������������  551 10.4.3 Temperature Sensors����������������������������������������������������������  553 10.4.4 Humidity Sensors����������������������������������������������������������������  553 10.4.5 Magnetic Sensors����������������������������������������������������������������  554 10.4.6 Chemical Sensors����������������������������������������������������������������  556 10.4.7 Electromagnetic Radiation Sensors������������������������������������  557 10.4.8 Multimodal Sensors������������������������������������������������������������  558 10.4.9 Electropotential Sensors�����������������������������������������������������  558 10.4.10 Ultrasonic Sensors��������������������������������������������������������������  559 10.5 Integration of Printed Sensors into Systems ����������������������������������  559

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10.6 Future Perspectives ������������������������������������������������������������������������  561 References��������������������������������������������������������������������������������������������������  565 11 Printed Flexible Hybrid Electronics������������������������������������������������������  567 11.1 State-of-the-Art Development��������������������������������������������������������  567 11.1.1 The Roles of Printed Electronics and Standard Silicon Integrated Circuits��������������������������������������������������������������  568 11.1.2 The Merit of Flexible Hybrid Electronics��������������������������  568 11.2 Core Components of the Flexible Hybrid Electronics��������������������  570 11.2.1 Substrate������������������������������������������������������������������������������  570 11.2.2 Inks and Printing Techniques����������������������������������������������  572 11.2.3 Printed Sensors and Circuits ����������������������������������������������  575 11.3 Thinned Silicon ICs and Assembly Process in FHE����������������������  575 11.3.1 Thinning Silicon ICs and Connecting to FHE��������������������  575 11.3.2 Conductive and Nonconductive Adhesives������������������������  577 11.3.3 Assembly Process for Rigid Components in FHE��������������  578 11.4 Printed Antennas for Wireless Power and Communications����������  579 11.4.1 Printed Antennas for Communication Purposes ����������������  579 11.4.2 Printed Coils for Wireless Power Transfer��������������������������  581 11.5 Printed Power Sources: Batteries, Solar Cells, and Energy Harvesters ��������������������������������������������������������������������������������������  581 11.5.1 Printed Energy-Storage Modules����������������������������������������  582 11.5.2 Printed Energy-Harvesting Modules����������������������������������  582 11.6 Quality Assurance ��������������������������������������������������������������������������  583 11.6.1 High-Resolution Patterning������������������������������������������������  583 11.6.2 Uniformity��������������������������������������������������������������������������  584 11.6.3 Flexibility/Stretchability ����������������������������������������������������  585 11.6.4 Durability����������������������������������������������������������������������������  585 11.7 Reliability Evaluation ��������������������������������������������������������������������  586 11.8 Application��������������������������������������������������������������������������������������  587 11.8.1 Wearable Health Monitoring with FHE������������������������������  587 11.8.2 Industrial, Environmental, and Agricultural Monitoring with FHE����������������������������������������������������������������������������  588 11.8.3 Structural Health Monitoring with FHE ����������������������������  589 11.9 Challenges and Future Trends��������������������������������������������������������  593 References��������������������������������������������������������������������������������������������������  595 12 Current Trends and Prospects in Advanced Manufacturing for Printed Electronics ����������������������������������������������������������������������������  597 12.1 Introduction������������������������������������������������������������������������������������  597 12.2 Electronic Materials and Components��������������������������������������������  599 12.3 Techniques and Processes in Printed Electronics ��������������������������  600 12.3.1 Techniques in Printed Electronics��������������������������������������  600 12.3.2 Processes in 3D-Printing Electronics����������������������������������  604

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12.4 Current Trends in 3D-Printed Electronics��������������������������������������  604 12.4.1 Research and Development������������������������������������������������  604 12.4.2 Integrated 3D-Printing Systems for Mass Production��������  609 References��������������������������������������������������������������������������������������������������  612 Abbreviations ��������������������������������������������������������������������������������������������������  615 Index������������������������������������������������������������������������������������������������������������������  623

About the Author

Colin Tong  is a materials expert with considerable professional experience in the past two decades. His research and development activities and industrial practices cover a broad range of different fields with a special focus on materials testing and characterization, component design and processing of advanced composite materials, metallurgy, thermal management of electronic packaging, electromagnetic interference shielding, integrated optical waveguides, functional metamaterials and metadevices, energy materials, as well as flexible and printed electronics. He holds a Ph.D. degree in materials science and engineering and a master’s as well as a bachelor’s degree in materials and mechanical engineering. Dr. Tong has published 5 books and over 30 peer-reviewed papers, and he holds 9 patents. He is a senior member of Institute of Electrical and Electronics Engineers (IEEE). He received the Henry Marion Howe Medal from ASM International for his contribution to research and development on advanced aluminum composite materials in 1999.

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Chapter 1

Fundamentals and Design Guides for Printed Flexible Electronics

Abstract Three-dimensional (3D) printing, known as additive manufacturing, includes a family of technologies consisting of novel ink materials, flexible substrates, and unique processing methods that can be integrated to create flexible, stretchable, and wearable electronics. These technologies can be used to fabricate components and full systems mainly in a layer-by-layer manner and offer various options regarding cost, feature details, and organic and inorganic materials. The most popular materials are printable organic, inorganic, and hybrid semiconductors with various functional structures (i.e., 1D, 2D and 3D, even 4D), including polymers, metals, composites, ceramics, and nanomaterials. 3D printing enables the creation of complex geometric shapes and merging of selected functional components into any configuration thus supplying an innovative approach for the fabrication of multifunctional end-use devices that can potentially combine mechanical, optical, chemical, electronic, electromagnetic, fluidic, thermal, and acoustic features. On the other hand, rapid advances in modern electronics place ever-accelerating demands on innovation towards more robust and versatile functional components. In the flexible electronics domain, novel material solutions often involve creative uses of common materials to reduce cost, while maintaining uncompromised performance. Moreover, mechanically durable and highly stretchable materials are fundamentally important to the development of flexible and stretchable devices. Therefore, there has been enormous progress in the materials, designs, and associated assembly techniques as well as manufacturing processes for flexible/stretchable electronic systems and subcomponents, such as transistors, amplifiers, sensors, actuators, light-emitting diodes, photodetector arrays, photovoltaics, energy generation and storage devices, and bare die integrated circuits. This chapter will highlight the fundamentals and design guides for 3D-printed flexible electronics, including historical perspectives, printing requirements for printable materials, design strategies, and advanced fabrication technologies for printed flexible electronics.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 C. Tong, Advanced Materials for Printed Flexible Electronics, Springer Series in Materials Science 317, https://doi.org/10.1007/978-3-030-79804-8_1

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1.1  Historical Perspectives on Printed Flexible Electronics Printed electronics combines a set of printing technologies used to create electrical devices on various substrates such as semiconductor devices manufactured by various printing processes on flexible or stretchable substrates. Today, with increasing interest in both printing technology and electronic devices, new innovations combining these two great technologies have been designed to produce novel electronics through low-cost and large-area printing processes. The printed electronics is often related to organic electronics or plastic electronics, in which one or more inks are composed of carbon-based compounds. These ink materials can be deposited by solution-based, vacuum-based, or other processes. Printed electronics, in contrast, specifies the process, and, subject to the specific requirements of the printing process selected, can utilize any solution-based materials and other printable materials, such as organic semiconductors, inorganic semiconductors, metallic conductors, and nanomaterials. Printed electronics should at least include the following key components: (1) a flexible (circuit) board to be used as the substrate, (2) the use of inks (semiconductors) and other printable materials, and (3) a continuous printing processing technique or hybrid printing process to combine different approaches together. Historically, the first use of the term “printed electronics” dates back to the early twentieth century. The technology was initially an attempt to fabricate flexible conductors, which could be applied to various materials while simplifying the interconnections within the complex electronic circuits. By 1950, the printed circuit board (PCB) was developed, which involves copper sheet lamination. The contemporary concept of printed electronics, which includes conductors, semiconductors, and insulators using inks, was initially proposed in the early 1990s. At that time, the goal was to deposit conducting wires using conductive ink. Since then, printing technology has encompassed a variety of processing techniques, such as spray-coating, stamping, and inkjet printing, all of which enable roll-to-roll processing. Furthermore, developments in transfer printing have enabled the transfer of traditional silicon-based devices onto flexible circuit boards or other desirable types of flexible substrates. As a result, printable technology has found use in various applications, including healthcare, displays, memory, and sensors, showing great promise for future technology. This platform enables information to be shared through attaching or implanting the technology in clothes, clocks, and human skin (Coombs 1996; Rim et al. 2016). The need and interest for the development of flexible electronics can be traced back to the 1960s. The need to capture solar energy in space application and the energy crisis of the late 1960s were the motivating factors for research on flexible electronics. The shift from rigid to flexible solar panels was driven by the benefits of flexible panels such as a larger active area, lighter weight, and more resistance to thermal and vibration shocks. More applications such as flexible ribbons/wires in computers led to further growth in the use of flexible electronics. Unlike conventional silicon electronics that are limited to rigid wafers, flexible electronic devices have been demonstrated on plastics, paper, fibers, and even

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biological tissues. To make flexible electronics that are compatible with delicate surfaces, low-temperature processing is required. This need has led to the development of materials such as organic conductors and semiconductors as well as advanced solution-based techniques that enable low-temperature processing. Thus, flexible electronics promote the use of alternative manufacturing technologies, such as 3D and roll-to-roll printing for electronics. Moreover, the flexible devices enable a wide range of applications, in fields ranging from energy sustainability to smart sensor networks to bioelectronics (Loo and Ng 2013). Furthermore, applications such as robotics and soft robotics, prosthetics (E-skin, for example), implantable electrodes, and wearable systems have led to significant growth in the field of flexible electronics. By incorporating a degree of flexibility, the performance of multifunctional electronic systems could be extended for various applications. However, applications where large deformation is experienced, it is imperative to have stretchability. As an example, large deformation is experienced at complex surfaces such as the knees and elbows of a humanoid robot where the need for tactile skin is needed. Therefore, the development of flexible and stretchable devices has enabled new pathways and interaction mechanisms for applications such as wearable electronics, consumer electronics, structural electronics, stretchable and conformable electronics (Dang et al. 2017). In addition, the development of wearables has ushered in a new era of personalized Internet-of-Things (IoT) wherein the devices are intimately mated with the human body for various healthcare, entertainment, and sports applications. In order to realize feasible wearable technology, significant progress in printing technology is required. At this point, three-dimensional (3D) printing technology is one of the best candidates. If a specific system can be developed that performs a particular task based on information at home by consolidating materials and ideas, it would be a major advancement for future printing technology (Rim et al. 2016). 3D printing technology can be employed for the fabrication of flexible and stretchable devices with complicated geometries through layer-by-layer assembly based on the digital 3D model. Progress in the field of engineering has resulted in complex 3D printing processes that involve advanced dispensing technologies, for instance, including pneumatic, piezoelectric, aerosol, electrohydrodynamic, and thermal processes to print intricate and high-resolution architectures. These 3D printed flexible electronic devices not only possess complicated geometries with precisely prescribed microarchitectures and excellent mechanical property for satisfying all kinds of individual requirements but also can be produced anywhere on-­demand by integrating various 3D printing materials and ideas at home or in the workplace. These advantages render 3D printed flexible electronic devices as the future trend and provide great opportunities for the development of electronic devices in various fields ranging from biomimetic devices to soft robots (Yang et al. 2018). Much of the success of printed flexible systems can be attributed to innovations in materials engineering that have led to novel inks and other printable materials comprising of new nanomaterials, polymers, and composites. New generation of printed devices include soft, stretchable, and anatomically compliant devices that enable efficient bio-integration and withstand high tensile stress associated with on-­body

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1  Fundamentals and Design Guides for Printed Flexible Electronics

Fig. 1.1  Multifunctional, flexible electronic systems based on engineered nanostructured materials. (Modified from Takei et al. 2010 and Takahashi et al. 2011 with permission from American Chemical Society and Springer Nature)

applications, as shown in Fig. 1.1 (Ko et al. 2012). Progress in materials science has also led to the development of self-healing printed electrochemical systems for wearable applications. In fact, development and research on printed electronics has led to a vast number of physical devices, covering temperature, pressure, strain, light and mechanical sensors, and chemical devices, covering biosensors, pH, ions, gas, or vapor sensors. Furthermore, the trillion sensors movement was founded to accelerate and coordinate the development to achieve the upcoming sensor demand such as capacitive, piezoresistive, piezoelectric, photodetectors, digital X-ray, temperature, bio, and gas sensors. The driving fields of interest hereto span, for instance, the Internet-of-Things, wearables for daily life applications such as personal health care, or industry-relevant applications such as local energy harvesting, supervision of grocery transportation, or telecommunications. In many cases, the unique properties of printed electronics such as flexibility, conformability, or transparency can generate new markets, since conventional inorganic devices usually lack these properties. For example, organic solar cells could be fabricated comprising a certain semi-­ transparency and conformability, which could be beneficial for implementation into building facades and windows. In addition to that, the use of environmentally friendly solvents, biocompatible materials, and low energy consumption during the fabrication process and therefore lower fabrication costs could promote the demand for printed devices (Eckstein 2016; Kim et al. 2017).

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Although 3D printing technologies are well-established, the field of flexible electronics mandates the development of a new class of printable inks with properties that enable the devices to perform flawlessly in conditions commonly experienced by the human body and other application areas. For example, the printed wearable devices must be small, thin, light, soft, and should adhere well to the complex three-­ dimensional curvature of the human body without causing any irritation. Such atomically compliant printed devices should maintain conductive pathways under severe mechanical deformations and survive in a wide range of ambient conditions (e.g., temperature, humidity), and must be made of innocuous, biocompatible materials. Meeting these conditions is indeed challenging and requires innovations in materials science through the development of novel ink formulations. Significant progress in printing technology is thus required for realizing high-performance flexible and wearable devices (Kim et al. 2017).

IN-DEPTH: Additive Manufacturing and Its Benefits Additive manufacturing (AM) is defined as the process of joining materials to make parts from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing and formative manufacturing methodologies. It has been categorized into seven basic processes: • Vat photopolymerization—additive manufacturing process in which liquid photopolymer in a vat is selectively cured by light-activated polymerization. • Material jetting—additive manufacturing process in which droplets of build material are selectively deposited. • Binder jetting—additive manufacturing process in which a liquid bonding agent is selectively deposited to join powder materials. • Powder bed fusion—additive manufacturing process in which thermal energy selectively fuses regions of a powder bed. • Material extrusion—additive manufacturing process in which material is selectively dispensed through a nozzle or orifice. • Directed energy deposition—additive manufacturing process in which focused thermal energy is used to fuse materials by melting as they are being deposited. • Sheet lamination—additive manufacturing process in which sheets of material are bonded to form an object. Out of these seven categories, Vat photopolymerization has, so far, not been used for metals. But, as it is currently being used for ceramics (with photopolymer resins that are impregnated with ceramic powders, and then transformed into pure ceramics in a secondary furnace sintering process), it too, has the potential to, one day, be used for metal. Some of the main benefits of AM include less waste, new design possibilities, increased functionality of the products, and flexible production. Examples include complex parts that are expensive, or impossible, to manufacture by

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1  Fundamentals and Design Guides for Printed Flexible Electronics

other methods, parts consisting of many consolidated separate pieces into a single component that can be manufactured directly, tools with curved cooling channels for optimized cooling, and lightweight topology optimized parts. Great examples are presented, from all around the world, with very high weight reduction, greatly reduced lead times, costs, and waste compared to conventional manufacturing. A side-effect of this global hype is that this can lead to too high a general expectation of the technology, and the challenges in actually implementing them are not always foreseen (Ålgårdh et al. 2017).

1.2  Printing Requirements for Printable Materials Printed electronics evolved into a fusion of microelectronics, chemistry, and printing/process engineering, comprising the formulation of functional inks, controlling the thermodynamic effects during and after the printing, and drying mechanisms. Conductive materials are equally important to the organic semiconducting compounds and are necessary for wiring, electrodes, or contacts. Nowadays, a large diversity of printable metals in the form of metal-organic decomposition (MOD) inks, nanoparticle, or nanowire dispersions are available, including precious metals like gold, silver, platinum, or the cost-efficient metal copper. Beyond that, carbon-­ based compounds, such as graphene or carbon nanotubes (CNTs), found applications in transparent electrode systems and show outstanding performances. Immense progress has been achieved, where the sinter conditions (e.g., temperature), of especially silver inks, could be decreased from initially 200 to 300 °C to almost room temperature. Also, highly conductive and transparent polymer electrodes have been discovered and studied extensively and depict the only solution-processable transparent and homogeneously conductive electrode material. Nevertheless, vast quantities of conductive and semiconducting materials and resistive inks have been developed to further push the realization of fully printed electronic devices on cost-­ efficient and flexible substrates, such as PET or paper. Novel sinter techniques, including photonic, low-pressure argon plasma, microwave, or DC current sintering additionally enhance the degrees of freedom in solution-processable metal inks (Au, Cu, Pt, etc.) on low-temperature substrates. However, a crucial, but may be initially almost subordinated printing step turned out to be by far more complex than suggested. Nearly every printing technique and source material requires a tailored formulation, where the boiling point, the viscoelastic properties, as well as thermodynamic effects during printing and drying, have to be considered and in many cases adjusted (Eckstein 2016).

1.2  Printing Requirements for Printable Materials

7

Fig. 1.2  Key components (conductive fillers, additives, solvents, and binders) of general printable inks for electronic devices. (Adapted from Bocchetta et al. 2020 with open access (MDPI))

1.2.1  Ink Formulation A typical printing ink comprises of fillers, binders, additives, and solvents, as shown in Fig.  1.2. The selection of these components ultimately depends on the type of printing methodology to be followed. The fillers are the active component of the ink that provides it with the characteristic features required for specific applications. Depending on the applications, the fillers could be metallic, ceramic, organic, or a combination of thereof. Combining the attributes of nano-science with printing technology has resulted in synthesized inks involving tailor-made nanomaterials, such as nanosheets, nanowires, nanoparticles, or their composites. The other important component of an ink is the binder—a polymeric material that helps in the homogeneous dispersion of the fillers into the ink. Upon printing, the binders play the role of holding the ink components together upon solvent evaporation and also help bind the printed trace onto the substrate. A rich variety of binders with acrylic, silicone, styrene, fluoroelastomers, or urethane backbones have been developed as ink binders for printing flexible, stretchable, and self-healing devices. The choice of the binder ultimately depends on the properties of the fillers. By identifying the surface chemistry of the filler particles, a suitable binder can be selected that allows their homogeneous dispersion within the ink. The choice of the binder also depends on the application of the printed devices. For example, water-soluble binders should be avoided for fabricating devices that will be exposed to aqueous media. Various types

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1  Fundamentals and Design Guides for Printed Flexible Electronics

of UV and heat curing binders have been developed to meet the needs of a wide range of applications. The other crucial component of the ink is the solvent, the vehicle that allows the ink to flow. The ink solvent should provide good solubility to the polymeric binder and impart favorable viscosity, surface tension, and homogeneity. The ideal solvents for a particular set of binders and fillers can be selected based on Hillenbrand and Hansen solubility parameters that estimate the cohesive energy between a solvent molecule and other components of the ink. This is critical for obtaining homogeneous ink formulations and optimal printability. The solvent selection becomes especially challenging while printing multilayer devices. When a fresh layer of ink is printed onto a printed film, the solvent present in the ink can potentially dissolve and damage the underlying printed film. This problem can be addressed by controlling combination of ink, such as binder, filler, and solvent. In case each layer is printed with a different set of solvent and binder so that each binder is soluble only in its corresponding solvent. Due to the difference in the solubility parameters, the inks will have minimal effect on each other while fabricating multilayer devices. Apart from filler, binder, and solvent, additives are also included to impart desired rheological, wetting, healing, or stretching properties to the inks. Additives in the form of surfactants, adhesion improvers, humectants, penetration promoters, and stabilizers have thus been used to tailor the ink properties for specific applications (Kim et al. 2017). In addition, the different printing methods require specific ink properties for precise resolution and performance. Table 1.1 shows printing parameters for some printing methods. For instance, the major advantage of screen printing is its ability to reproducibly print high-aspect-ratio structures. This is achieved due to the high viscosity of the inks realized by incorporating higher binder loadings, compared to other printing techniques. The viscosity ideally resides in the range of 1000–10,000 cps for thin films (25–100μm) but can go as high as 50,000 cps for much thicker prints (>300μm). Unlike screen printing, gravure or flexography are template-based printing techniques that rely on less viscous inks for fabricating thin films (300,000 1–100 1–100 60 mN/m), which makes inkjet printing quite challenging (Brett Walker and Lewis 2012). Therefore, more and more reactive silver inks are under development to achieve better performance and working at low temperatures even at room temperature. Reactive copper and nickel inks have been developed with a modified electroless plating process. The typical composition of an electroless copper-plating bath consists of a copper salt, a complexing agent, a reducing agent, and a pH adjustor. On contact with the plating solution, metal nanoparticles on the substrate catalyze the reduction of the copper ions to metal. In contrast, CuSO4 and sodium citrate powders were dissolved in distilled water to form the copper ink of concentrations up to 1.25 M. Citrate is a copper chelator that increases the solubility of the copper and the stability of the solutions. For the reducing ink, NaBH4 was dissolved in 0.5 M NaOH solution (pH 14). The high pH stabilizes the NaBH4. For nickel line printing, 2.5 M aqueous NiSO4 solution was prepared as the nickel ink and 1.25 M NaBH4 was used as reducing ink (Li et al. 2009).

3.3  Carbon-Based Materials As an important type of conductive materials, carbon-based materials such as carbon black, graphene, carbon nanotubes (CNTs), and carbon nanofiber have been added into polymeric materials to form conductive inks due to their excellent conductive and sensing abilities. For instance, carbon black particle inks were used to fabricate flexible strain sensors and piezoresistive sensors by material extrusion, which could flexible strain sensors and piezoresistive sensors by material extrusion, which could embed functional sensors and electronics in a single and low-cost process (Chang et al. 2018). This section will focus on graphene and carbon nanotubes.

3.3.1  Graphene-Based Inks Two-dimensional single-layer graphene flake has a thickness of 0.34  nm corresponding to interlayer spacing of graphite. Graphene sheet is characterized by a very high Fermi velocity (106 m/s) and a high intrinsic inplane conductivity. The sheet resistance of graphene varies with the number of layers, N, as Rs ∼ 62.4/N [Ω/ square] for highly doped graphene. Graphene and its derivatives are produced by several methods: from graphite by mechanical and liquid phase, exfoliation, chemical vapor deposition (CVD), solvothermal synthesis from organic compounds, by chemical cross-linking of polycyclic aromatic hydrocarbons, and by thermal decomposition of SiC.  However, compared to pristine graphene (PG), graphene oxide (GO) is more often used as a precursor for the formulation of conductive inks because of its dispersibility in water and high electrical conductivity after

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post-­printing reduction, which can be performed by chemical or thermal treatment. GO is usually obtained by oxidation of graphite powder in the presence of strong acids and oxidants (Kamyshny and Magdassi 2014). Dispersions of PG are usually prepared by ultrasonication of graphite in water or organic solvents such as terpineol, N-methylpyrrolidone, γ-butyrolactone, 1,3-dimethyl-2-imidazolidinone, o-dichlorobenzene, and ethanol. To obtain stable dispersions suitable for inkjet ink formulations, various stabilizing agents are used: polycyclic aromatic hydrocarbons; surfactants such as sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (NaDDBS), sodium deoxycholate, sodium cholate, and CTAB; polymers such as PVP and PVA; and ethyl cellulose. One of the major disadvantages of such dispersions is the low graphene loading, which is usually in the range of 0.002–0.1 wt% (Wajid et al. 2012). GO contains hydroxyl, epoxy, carbonyl, and carboxylic groups that make it easily dispersible in water and polar organic solvents such as N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), and ethylene glycol. Preparation of stable GO dispersions does not require addition of stabilizing agents that is obvious advantage of GO-based inks compared to PG-inks. The concentrations of GO in inks are usually in the range of 0.1–1.0 wt% (Dai et al. 2011). Because of low graphene concentration, to achieve low resistivity, multiple printing of graphene-based inks is usually required. In addition, electrical resistivity can be significantly decreased by post-deposition annealing in air at 300–400 °C or at higher temperatures in an inert atmosphere in order to remove the stabilizing organic material. Figure 3.8 shows the morphology of the film obtained by multiple printing the graphene layers onto PI substrate (Lee et al. 2013). To make the GO-printed films conductive, they should be reduced. Chemical (e.g., hydrazine vapor, hydrides) and thermal reduction (in an inert atmosphere in the presence of a reducing gas, H2), or combination of chemical reduction and heating are usually used. Reduction of GO to graphene affects only the surface layer of the deposited film, and the underlying material remains an insulator. Thermal

Fig. 3.8  Field-emission SEM images of films obtained by printing 50 graphene layers onto polyimide substrate (left image—top view, right image—cross-sectional view). (Adapted from Lee et al. 2013 with permission from Elsevier)

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reduction is more effective than chemical reduction but requires high temperatures (>550 °C) that are not suitable for heat-sensitive plastic substrates. In addition, reactive inkjet printing has been developed to obtain conductive films of graphene (sheet resistance >100 kΩ/square). This approach is based on sequential printing of GO ink and ink containing a reducing agent, ascorbic acid mixed with FeCl2, on a substrate heated to 60 °C (Kim et al. 2014).

3.3.2  Carbon Nanotube-Based Inks Carbon nanotubes (CNTs) are cylindrical hollow nanostructures with walls composed of one-atom-thick sheets of carbon with graphene structure. Depending on the chirality along the graphene sheet, CNTs possess semiconducting or metallic properties. They exist both as single-walled (SWCNTs) and as multi-walled (MWCNTs), and their length is from less than a micron up to tens of microns. The diameter of SWCNTs is typically in the range of 0.4–4 nm, while the outer diameter of MWCNTs is in the range of a few to tens of nanometers. The intrinsic electrical resistivity of individual CNTs was found to be as low as 10−6 Ω cm for SWCNT and 3 × 10−5 Ω cm for MWCNT. However, in most cases, due to the presence of various defects or impurities formed during the CNT synthesis, the resistivities of individual CNTs, as well as the resistivities of their assemblies, were much higher. Introducing acceptor dopants into CNT conjugated systems by oxidation in air or HNO3, resulted in increased conductivity of CNT fibers. Because of the superior mechanical properties of individual CNTs, thin films made of randomly distributed CNTs were shown to possess unique mechanical properties, such as flexibility and stretchability, which are essentially important for the fabrication of flexible electronic devices (Kamyshny and Magdassi 2014). CNTs are produced by three major methods: electric arc discharge (a high temperature method—above 1700 °C), laser ablation, and chemical vapor deposition (CVD). The most often used is the DC discharge between two graphite water-cooled electrodes with diameters of 6–12  mm, in a chamber filled with helium at sub-­ atmospheric pressure, although hydrogen and methane gases are also used. Usually, MWCNTs are produced by this method, if no catalyst is used. SWCNTs are usually produced when a transition metal catalyst (Fe, Ni, Co, Mo, Y) is used. Laser evaporation/ablation (Nd:YAG or CO2 lasers) of pure graphite targets is one of the superior methods to grow high-quality and high-purity SWCNTs. Catalytic CVD, thermal or plasma enhanced, which is the catalytic decomposition of hydrocarbons or carbon monoxide, is now the standard method for the production of high purity CNTs. The most frequently used catalysts are Fe, Co, or Ni (Prasek et al. 2011). The major challenge in formulating CNT inks is to obtain a stable dispersion of non-aggregated nanotubes in a proper liquid vehicle, at a low viscosity. Since the CNTs are very hydrophobic, three routes for obtaining such dispersions are presented (Kamyshny and Magdassi 2014): (a) dispersing the CNTs in organic solvents without dispersing agents, (b) dispersing the CNTs in an aqueous media using

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dispersants such as surfactants (anionic, cationic, and nonionic) or polymers, and (c) chemical modification of the CNT with functional groups, which favor the interactions of CNTs with the dispersing medium (for example, to obtain water-dispersible CNTs, nitric acid is usually used to produce oxygen-containing carboxyl, carbonyl, and hydroxyl groups on the surface of a nanotube). The major problem with organic solvents without dispersing agents is the inability to obtain dispersions with high CNTs concentration, more than 0.1 g/L. To obtain aqueous dispersions of CNTs, the surfactants Triton X-100 and SDS are the most widely used. Other surfactants include Tween 20 and Tween 80, sodium cholate, CTAB, dodecyl trimethylammonium bromide (DTAB), and NaDDBS. Stabilization is achieved due to adsorption of the hydrophobic tails of the surfactant molecules on the nanotube surface, while the hydrophilic head groups are directed toward the aqueous phase. Polymeric stabilizers are rarely used. For all solubilization schemes, energy must be applied to the system in order to break the CNT aggregates. The most widely used approach to prepare homogeneous dispersions of CNTs is ultrasonication for a prolonged time. An alternative method for de-agglomerating CNT bundles is based on high-pressure homogenization. Superacids can also enable dispersion of the CNTs without applying high shear forces. For example, chlorosulfonic acid was identified as a true solvent for CNTs because it protonates the side walls, dissolves the CNTs as individuals, and promotes the formation of liquid crystals from a wide range of nanotube sources, including multi-walled nanotubes and long carpet-grown CNTs (Davis et al. 2009). CNT inks for conductive printing can be formulated by combining the above dispersions with suitable additives, according to the required printing method, and in order to improve printing performance. Liquid vehicles are water, organic solvents such as DMF, N-methyl-2-pyrrolidone, and γ-butyrolactone, as well as solvent mixtures. Typically, the content of CNTs in ink formulations is in a wide range of concentrations, 0.01–10  g/l. In fact, the actual concentration of CNT is not known, since the dispersion is either filtered or let to sediment prior to printing in order to separate the large CNT bundles. Concentration limitation is important depending on the printing methods used. Inkjet and aerosol jet require low viscosity ink, for example, while less critical to screen printing. Some commercial CNT-­ based inks, such as Nink-100 (MWCNT ink) and Nink-1100 (SWCNT ink) of NanoLab Inc. (USA), contain carboxyl (COOH) functionalized carbon nanotubes in an aqueous suspension, with the minimum concentration of additives to impart long-term stability and printability to the ink (Kamyshny and Magdassi 2014). Flexible transparent CNT films can be fabricated by rod-coating, spin coating, spray-coating, dip-coating, drop casting, vacuum filtration followed by transferring onto a proper substrate, electrophoretic deposition followed by hot pressing transfer, inkjet printing, and aerosol jet printing. As in the case of metal NWs and graphene sheets, both sheet resistance and transmittance are thickness-dependent, and CNT films with a thickness of 10–100 nm, possess high electrical conductivity and optical transmittance. Usually, SWCNTs are used for the fabrication of conductive transparent films, and electrical conductivity and transparency of such films on glass and plastic substrates are typically in the range of 60–870  Ω/square and

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70–90% respectively, depending on CNT dispersion, film thickness and method of preparation. Because of the superior mechanical properties of individual CNTs and their strong interactions with hydrophobic substrates, thin films made of randomly distributed CNTs show excellent mechanical performance, such as flexibility, stretchability, and foldability, which are important for flexible electronic devices and functional textile (Hu et al. 2010).

3.4  Transparent Oxide Conductors Transparent conducting oxides (TCOs) are known as materials that show both transparent and conducting properties, and their films are utilized in flat panel displays, electroluminescent devices as transparent electrodes, as well as modern devices including touch screens, portable electronics, flexible electronics, optics, biochemical/environmental sensors, transparent heaters, multifunctional windows, and solar cells. Generally, TCOs are metal oxides with high optical transmittance and high electrical conductivity. They are also referred to as wide-bandgap oxide semiconductors (band gap >3.2 eV). These materials have high optical transmission at visible wavelengths (400–700 nm) and electrical conductivity close to that of metals, which is often induced by doping with other elements. They also reflect the near infrared and infrared wavelengths. Since the bandgaps of these materials lie in the ultraviolet wavelength region they hardly absorb visible light, so they appear to be transparent to the human eye. These unique properties make TCOs widely applicable in current electronics which requires optical access behind electrical circuitry. In order to be considered as a TCO substrate, the film needs to possess a low electrical resistivity (~10−3 to 10−4 Ω cm) as well as high optical transparency toward visible light (>80% transmittance), due to their wide band gap (Ginley et  al. 2011; Jayathilake and Nirmal Peiris 2018). As shown in Table 3.1, more than 20 different doped binary TCOs have been developed of which ITO is preferred, while AZO and GZO come close to it in their electrical and optical performance. Doping In2O3 with Sn to form ITO substantially increased conductivity. It is believed that substituting Sn4+ for In3+ provides carrier Table 3.1  TCO compounds and dopants (Afre et al. 2018) TCO SnO2 ZnO In2O3 CdO GaInO3 CdSb2O3

Dopant Sb, F, As, Nb, Ta Al, Ga, B, In, Y, Sc, F, V, Si, Ge, Ti, Zr, Hf, Mg, As, H Sn, Mo, Ta, W, Zr, F, Ge, Nb, Hf, Mg In, Sn Sn, Ge Y

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141

electrons, as Sn4+ is supposed to act as a one-electron donor. Similarly, aluminum is often used for intentional n-type doping of ZnO, but other group III impurities, such as Ga and In, and group IV, such as Sn and Ge, also work. Doping by Al produced the relatively high conductivity AZO. Doping with nonmetallic elements is also common, e.g., ZnO:Ge (GZO), SnO2:F (FTO), and SnO2:Sb (ATO) (Afre et al. 2018). TCO films can be prepared by physical vapor depositions (PVD), including pulsed laser deposition, sputtering, and evaporation) and chemical routes also called solution-based fabrication methods (e.g., sol-gel process and dip-coating, etc.). PVD techniques can produce high-quality ITO films, but the limited usage of the target materials causes chemical wastes. Wet chemical routes have a problem in controlling the uniformity and thickness of the films. Solution-based fabrication methods can greatly reduce the cost and broaden the applications of transparent conducting oxides (TCO) films, such as Indium tin oxide (ITO) films. Several solution-based methods for making ITO films have been developed. The first method is mainly based on synthesizing ITO nanoparticles into a dispersion. Different size, shape, and composition of ITO nanoparticles can be synthesized and then dispersed in different solvents. ITO films can be made by these ITO nanoparticle dispersions using solution deposition methods. However, this type of method always uses long-chain surfactants on the nanoparticle surface to prevent agglomeration, which are difficult to remove or replace. This may have affected the electrical properties of resultant ITO films because long-chain surfactants generally serve as insulating layers between nanoparticles. Thermal annealing can remove the polymers but may also leave organic remnants and/or porosity between nanoparticles. The second method is based on making nanopowder dispersions using commercial ITO nanopowders. For example, ITO films have been made by dip-coating methods using a commercial ITO nanopowder-ethanol dispersion. The ball milling process helped disperse agglomerated ITO nanoparticles well, which made ITO films with excellent transparency of 95% and good resistivity of 5.10× 10−3 Ω cm after annealing treatment. But it was difficult to disperse all of the agglomerates so the surface morphology was not as good as those for synthesized ITO nanoparticle made films. The third method is using sol-gel ITO solution. First approach, the sol-­ gel ITO method was prepared using separate [In3+] and [Sn4+] solutions. The two solutions were only mixed before coating onto the substrate. The ITO films had about 90% transmittance and 10−4 Ω cm resistivity. Another approach is using a sol-­ gel solution fabrication method which mixed all of the precursors from the start. The optoelectronic properties were both good, which produced the highest figure of merit (1.19 × 10−2/Ω) among all the solution processed ITO films. In this sol-gel process, acetylacetone was not only used as the solvent to dissolve the indium salts and tin salts, but also served as chelating ligands to stabilize [In3+] and [Sn4+] ions from hydrolysis (Maruyama and Kojima 1988; Xia and Gerhardt 2016). Among the solution-based fabrication methods, inkjet and aerosol jet printing technology are promising in fabrication of ITO films with different patterns. The inks are prepared from dispersing ITO nanoparticles in solvent with suitable surfactant. To obtain good conductivity, it is necessary to anneal the printed films at high

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temperature to improve the links among the ITO particles. The high-temperature annealing process limits the substrates of the films, and cannot be used in some applications. Besides, even though under high-temperature annealing, the resistivity of the printed films is higher than the values of PVD films. Loose and porous structure of the printed films leads to the high resistivity. Another approach is using acetate solutions. The as-printed films are acetate precursors. To obtain target ITO films, it is necessary to decompose the acetate into oxides. Thus, the oxides could be formed “in situ” on the substrate after printing. This makes it possible to tune the structure and morphology of the ITO films during calcination (Fang 2012). An additional example is an aqueous precursor solution prepared for the deposition of Al doped ZnO thin film via ink jet printing. A Zn precursor solution and an Al precursor solution were prepared separately. The Al precursor was prepared as follows: a 0.01 mol of Al(NO3)3·9H2O (Sigma Aldrich, 98%) and 0.03 mol of malonic acid (C3H4O4, Sigma Aldrich, 99%) were dissolved in 10 ml water and stirred for 30 min. The pH was raised with ethanolamine (C2H7NO, Sigma Aldrich, 99%) till a clear, colorless solution was obtained at pH 9.5. Finally, the pH was adapted with formic acid (CHOOH, Acros Organics, 98%) till 7 and the precursor solution was diluted to a clear 0.5 mol/l Al precursor was obtained. The Zn precursor was prepared by dissolving 0.01 mol of Zn(CH3COO)2·2H2O (Sigma Aldrich, 99%) in 10 ml water and ethanolamine was added till a clear, colorless solution was obtained. The pH was adapted with formic acid till 7. At the end, a clear 0.5 mol/l Zn precursor was achieved by diluting the precursor solution. The final AZO precursor was obtained by adding a small amount of the Al precursor to the Zn precursor. The exact amount is determined by the final intended dopant concentration: dopant level (%) =100·Al/(Al + Zn). The printability of the ink was improved by the addition of 10 v% ethanol (Panreac, absolute). The amount of ethanol is added to the Zn precursor solution before diluted till 0.5 mol/l. The printed layers were dried at 120 °C on a hotplate for 10 min before they were thermally treated in a tube furnace till 500 °C in a wet 200 ppm O2/N2 atmosphere. The heating rate was kept at 10 °C/min with a dwell at 250 and 500 °C for 1 h. The wet atmosphere was established by bubbling the dry atmosphere through two glass bottles containing water at 20  °C.  Finally, samples were post-heated at 450 °C during 30 min in a dry Ar/5% H2 atmosphere. The optimal dopant concentration was set at 3 at% which resulted in thin films with a resistivity of 2.54 cΩ cm and an optical transmission larger than 90% over the visible range of the electromagnetic spectrum (Vernieuwe et al. 2016). Figure 3.9 shows a generic one-pot protocol for the synthesis of diverse nanocrystals (NCs) that can be used as inks, to prepare transparent conducting oxide films (such as ZnO, AZO, GZO, IZO, and ITO). The protocol uses inexpensive long-chain alkyl alcohols (e.g., 1-dodecanol) as reactants and oleylamine and oleic acid as surfactants. It is relatively simple and amenable to large-scale production compared to the conventional hot-injection method. The as-synthesized NCs can be easily dispersed in various organic solvents and, as such, be able to serve as inks for assembling thin and uniform films via solution-based processes. Large-scale centimeter-­wide films spin-coated by using the inks proved to be crack-free and with root-mean-square roughness values as low as 1.6  nm. The thickness and

3.5  Conductive Polymer Inks

143

a O

O R

O R

C O M1 (dopant)

H2O

R’ C O R”

C O M

OH M OH R” (DDL)

es

id

O e

r Pu

M

ox

M=M1=Zn, In, Ga

OD OH O R’ C O R”

M1

M1

op

OH

ed

ox

id

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M=Zn, M1=Al, In, Ga M=In, M1=Sn

b

Fig. 3.9  The one-pot strategy for the synthesis of transparent conducting oxide nanocrystal inks (modified from Song et al. 2015; Credit: https://pure.aston.ac.uk/ws/files/16273789/General_one_ pot_strategy_for_the_synthesis_of_high_performance_transparent_conducting_oxide_nanocrystal_inks.pdf): (a) Synthesis of both pure and doped TCO NCs. (b) Photographs of a series of TCO NC inks dispersed in toluene with stabilities of more than 1 year

electrical properties of the films can be effectively controlled and enhanced by UV treatment. ITO films spin-coated from NCs and then UV-treated under optimal conditions and annealed at 350 °C were 300-nm thick and showed an optimum resistivity of 112 Ω/square and optical transmittance as high as 87%. Such films prepared from NC inks have potential applications in various low-cost, large-area, and flexible optoelectronic devices fabricated via solution processes including inkjet and aerosol jet printing (Song et al. 2015).

3.5  Conductive Polymer Inks Conducting polymers have attracted much attention since their discovery in the late 1970s. Thanks to their outstanding characteristics such as light-weight and high flexibility, the use of polymer as active materials in electronic devices may allow replacement of the traditional metal-based devices and obtain fully plastic electronic devices. One of the technical challenges is due to the very low conductivity of some solution-processible organic compounds, which can lower the device performance (Vacca et al. 2015). Conductive polymers such as PEDOT, polypyrrole, and polyaniline have been used with inkjet and aerosol jet printing. A common feature of these materials is the presence of a conjugated pi-electron system present throughout the polymer, which gives them their conducting properties. These polymers have been widely used in sensing applications as their electricalor electrochemical properties can be sensitized to specific environmental or chemical factors by using an appropriate conducting polymer backbone or molecular dopant. These materials have also been used in

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applications such as electrochromic displays, batteries, fuel cells, and anti-static layers. These polymers typically have lower conductivities than metallic inks and may require the use of inert atmospheres due to their high susceptibility to ambient humidity and their reactivity with oxygen. Inkjet or aerosol jet printing is further complicated as these materials can exhibit non-Newtonian behavior. Under these conditions, a droplet of polymer solution remains attached to the nozzle by a long tail for several hundred microseconds. The tail will eventually detach from the nozzle when a pinch point is exceeded and will either collapse into the main droplet or disintegrate into a series of satellite droplets. The cause of the non-Newtonian behavior is not solely attributed to increased polymer concentration in the ink but is also due to elastic stresses caused by extensional flow in the nozzle. The conductivity of these materials is susceptible to substrate roughness. For example, photovoltaic devices with either spin coating or inkjet printing showed that the devices prepared with inkjet printed polymer films had better performance and higher efficiency than those produced with spin coating. This was attributed to the finer phase separation that occurred due to the rapid drying of inkjet printed droplets (Cummins and Desmulliez 2012). Among the conductive polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) presents very interesting properties including high electrical conductivity, environmental stability, and high transparency so that it has been proposed for different electronics applications, such as organic photovoltaic devices (OPVs) and organic light-emitting diodes (OLEDs). Moreover, PEDOT could be patterned by ink jet and aerosol jet printing techniques to fabricate flexible devices. However, PEDOT is not soluble in water so that aqueous inks are usually prepared using poly(styrene sulfonic acid) (PSS): the formation of the polyelectrolyte complex of PEDOT:PSS enhances the dispersibility of particles in water. The conductivity of the film depends on the ratio of PEDOT to PSS as well as on the size of the PEDOT:PSS particles dispersed in water; however, the pristine films present a very low conductivity ( 4) lead to greatly dispersed CBM, which in turn leads to high electron mobility if the carrier relaxation times are not significantly different between the constituent materials. As a result, amorphous oxide semiconductors can exhibit Hall-effect mobilities similar to those of their corresponding crystalline phases, even when grown at low temperatures (Presley et  al. 2006). Transparent TFTs made out of these materials are promising for a range of applications, such as backplanes for backlit liquid-crystal displays and for certain types of organic light-­ emitting diodes (OLEDs).

4.2.3  Films of Chalcogenides Chalcogenide compounds, including sulfides, selenides, and tellurides, are classes of semiconductors that have been used for transistors and other devices since the early days of solid-state electronics. In fact, the first TFT, demonstrated by Weimer (1962), was fabricated with a thin film of polycrystalline CdS in a structure similar to that of metal-semiconductor field-effect-transistors (MESFETs). Chalcogenide TFTs were successfully fabricated in the 1970s on not only rigid glass slides but also flexible plastics (e.g., Mylar films and Kapton strips) and certain types of paper. The emergence of MOSFETs and integrated circuits based on crystalline Si technology caused a decline in these development activities. The emergence of nanoparticle and nanowire versions of chalcogenides has led to a reexamination of these

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materials for transistor applications, particularly in fields such as large-area or flexible electronics, where silicon-wafer-based electronics cannot be used easily. Some TFTs have been fabricated with thin films of chalcogenide quantum dots (spin-cast from solutions) as well as crystalline layers derived from liquid precursors (Brody 1984; Sun and Rogers 2007). 4.2.3.1  Films of Chalcogenide Nanocrystals Chalcogenide nanocrystals have been used in TFTs. For instance, a pyridine solution of CdSe particles (average size

h

Donor FRET

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Electrical exitation (charge injection)

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Conjugated Polymer

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Exciton formation

Optical excitation

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Acceptor

Acceptor h

Fig. 4.22  Excitonic processes of conjugated polymer/nanocrystal composites (Adapted from Guzelturk et al. 2014 with permission from Royal Society of Chemistry): (a) exciton formation, (b) radiative and nonradiative recombination, (c) exciton diffusion, (d) exciton dissociation, and (e) excitation energy transfer (FRET and Dexter energy-transfer mechanisms). (HOMO: highest occupied molecular orbital level; LUMO: lowest occupied molecular orbital level, |X1> represents the exciton state, |g> is the ground state)

exciton diffusion/migration can happen either via exciton hopping within the delocalized excited-state landscape of CP or via Förster resonance energy transfer (FRET) between different chromophoric units of the CP. In the colloidal NCs, exciton diffusion dominantly takes place through long-range nonradiative energy transfer because excitons are confined to the NCs. The exciton diffusion length in the CPs and the NCs is typically on the order of 10 nm. For a solar cell, attaining a longer exciton diffusion length is desired for enhanced light-harvesting performance. In the case of a light-emitting diode (LED), exciton diffusion is undesired because it can cause trapping of the excitons at the trap/defect sites (Fig. 4.22c), causing nonradiative recombination (Collini and Scholes 2009; Xing et al. 2013). At the interfaces of materials having energetically staggered band alignment, excitons might dissociate through charge transfer. This process involves spatial overlap of the wave functions between different materials that are in close proximity (3.0 – – – 3.8

σRMS (nm) 0.4 3.3 0.3 ~4.0 0.2 0.3 0.1 0.3 0.1 3.6 0.1 0.99 0.3 0.2 0.6 0.2 0.2 2.8 1.6 1.8 (ITO) 6.0 3.0 (ITO) 1.6

Yieldb High Low Low Medium High High High High High Medium High High High High High High High Low Low Low Medium High High

230 5  Substrate and Encapsulation Materials for Printed Flexible Electronics

ZrO2

ZrTiOx

Sm2O3 Er2O3 Er2TiO5 TiO2

Nd2O3

LaAlOx LaLuO3 HfLaOx Pr2O3

Materials La2O3

Method ALD Spin coating + annealing ALD PLD Spin coating + annealing MOCVD Sputtering + RTA ALD E-beam Sputtering + RTA PLD Sputtering Sputtering Sputtering Spin coating + annealing Spin coating + annealing Spin coating + UV ALD Sputtering PLD Spin coating + annealing Spin coating + HPAc Spin coating + UV Spin coating + DUV Spray-pyrolysis + combustion Inkjet

Td (°C) 250 500 225 450 500 300 RT + 700 310 RT RT + 700 RT RT RT 200 250 200 RT 325 600 400 350 350 150 150 350 500

d (nm) 15 137 66 12 60 30 11 94 65 6.5 61.4 35 45 97 23 220 14 88 50 6.6 154 135 28 14 25 60

Ci (nF/cm2) 1239 75 134 2360 178 1000 1432 99 160 1634 223 217 297 373 560 110 705 226 530 3486 – – 478 410 370 325 k 21 11.6 10 32 22 26 17.8 10.5 11.7 12 15.5 8.6 15.1 41 27 27 18.9 22.5 30 26 – – 14.6 6.5 14.3 22

Jleak (A/cm2) at 2 MV/cma 1 × 10−8 2 × 10−6 @1.0 1 × 10−7 3 × 10−6 3 × 10−6 5 × 10−5 1 × 10−6 – 1 × 10−4 @1.0 1 × 10−7 1 × 10−6 @1.0 5 × 10−7