Flexible Flat Panel Displays E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Wiley Series in Display Technology
• Sprache: Englisch

Seitenzahl: 818

Veröffentlichungsjahr: 2023

Beschreibung

Flexible Flat Panel Displays

A complete treatment of the entire lifecycle of flexible flat panel displays, from raw material selection to commercialization

In the newly revised Second Edition of Flexible Flat Panel Displays, a distinguished team of researchers delivers a completely restructured and comprehensive treatment of the field of flexible flat panel displays. With material covering the end-to-end process that includes commercial and technical aspects of the technology, the editors have included contributions that introduce the business, marketing, entrepreneurship, and intellectual property content relevant to flexible flat panel displays.

This edited volume contains a brand-new section on case studies using the Harvard Business School format that discusses current and emerging markets in flexible displays, such as an examination of the use of electronic ink and QD Vision in commercial devices.

From raw material selection to device prototyping, manufacturing, and commercialization, each stage of the flexible display business is discussed in this insightful new edition. The book also includes:

• Thorough introductions to engineered films for display technology and liquid crystal optical coatings for flexible displays
• Comprehensive explorations of organic TFT foils, metallic nanowires, adhesives, and self-healing polymer substrates
• Practical discussions of flexible glass, AMOLEDs, cholesteric displays, and electronic paper
• In-depth examinations of the encapsulation of flexible displays, flexible batteries, flexible flat panel photodetectors, and flexible touch screens

Perfect for professionals working in the field of display technology with backgrounds in science and engineering, Flexible Flat Panel Displays is also an indispensable resource for professionals with marketing, sales, and technology backgrounds, as well as senior undergraduates and graduate students in engineering and materials science.

Leseprobe

Wiley– SID Series in Display Technology

Series Editor: Dr. Ian Sage

Advisory Board: Paul Drzaic, Ioannis (John) Kymissis, Ray Ma, Ian Underwood, Michael Wittek, Qun (Frank) Yan

Flexible Flat Panel Displays, Second Edition

Darran R. Cairns, Dirk J. Broer, and Gregory P. Crawford

Liquid Crystal Displays - Addressing Schemes and Electro-Optical Effects, Third Edition

Ernst Lueder, Peter Knoll, and Seung Hee Lee

Amorphous Oxide Semiconductors: IGZO and Related Materials for Display and Memory

Hideo Hosono, Hideya Kumomi

Introduction to Flat Panel Displays, Second Edition

Jiun-Haw Lee, I-Chun Cheng, Hong Hua, and Shin-Tson Wu

Flat Panel Display Manufacturing

Jun Souk, Shinji Morozumi, Fang-Chen Luo, and Ion Bita

Physics and Technology of Crystalline Oxide Semiconductor CAAC-IGZO: Application to Displays

Shunpei Yamazaki, Tetsuo Tsutsui

OLED Displays: Fundamentals and Applications, Second Edition

Takatoshi Tsujimura

Physics and Technology of Crystalline Oxide Semiconductor CAAC-IGZO: Fundamentals

Noboru Kimizuka, Shunpei Yamazaki

Physics and Technology of Crystalline Oxide Semiconductor CAAC-IGZO: Application to LSI

Shunpei Yamazaki, Masahiro Fujita

Interactive Displays: Natural Human-Interface Techniques

Achintya K. Bhowmik

Addressing Techniques of Liquid Crystal Displays

Temkar N. Ruckmongathan

Modeling and Optimization of LCD Optical Performance

Dmitry A. Yakovlev, Vladimir G. Chigrinov, and Hoi-Sing Kwok

Fundamentals of Liquid Crystal Devices, Second Edition

Deng-Ke Yang and Shin-Tson Wu

3D Displays

Ernst Lueder

Illumination, Color and Imaging: Evaluation and Optimization of Visual Displays

P. Bodrogi, T. Q. Khan

Liquid Crystal Displays: Fundamental Physics and Technology

Robert H. Chen

Transffective Liquid Crystal Displays

Zhibing Ge and Shin-Tson Wu

LCD Backlights

Shunsuke Kobayashi, Shigeo Mikoshiba, and Sungkyoo Lim (Eds.)

Mobile Displays: Technology and Applications

Achintya K. Bhowmik, Zili Li, and Philip Bos (Eds.)

Photoalignment of Liquid Crystalline Materials: Physics and Applications

Vladimir G. Chigrinov, Vladimir M. Kozenkov, and Hoi-Sing Kwok

Projection Displays, Second Edition

Mathew S. Brennesholtz and Edward H. Stupp

Introduction to Microdisplays

David Armitage, Ian Underwood, and Shin-Tson Wu

Polarization Engineering for LCD Projection

Michael G. Robinson, Jianmin Chen, and Gary D. Sharp

Digital Image Display: Algorithms and Implementation

Gheorghe Berbecel

Colour Engineering: Achieving Device Independent Colour

Phil Green and Lindsay MacDonald (Eds.)

Display Interfaces: Fundamentals and Standards

Robert L. Myers

Reffective Liquid Crystal Displays

Shin-Tson Wu and Deng-Ke Yang

Display Systems: Design and Applications

Lindsay W. MacDonald and Anthony C. Lowe (Eds.)

Flexible Flat Panel Displays

Edited by

Darran R. Cairns University of Missouri, Kansas City, USA

Dirk J. Broer Eindhoven Technical University, Netherlands

Gregory P. Crawford Miami University, Florida, USA

Second Edition

This edition first published 2023

© 2023 John Wiley & Sons Ltd

Edition History

© 1e, 2005 John Wiley & Sons Ltd

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The right of Darran R. Cairns, Dirk J. Broer, and Gregory P. Crawford to be identified as the authors of the editorial material in this work has been asserted in accordance with law.

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Library of Congress Cataloging-in-Publication Data

Names: Cairns, Darran R., editor. | Broer, Dirk J., editor. | Crawford, Gregory P., editor.

Title: Flexible flat panel displays / edited by Darran R. Cairns, Dirk J. Broer, Gregory P. Crawford.

Description: Second edition. | Hoboken, NJ : John Wiley & Sons, 2023. | Includes bibliographical references and index.

Identifiers: LCCN 2021052707 (print) | LCCN 2021052708 (ebook) | ISBN 9781118751114 (hardback) | ISBN 9781118751060 (pdf) | ISBN 9781118750889 (epub) | ISBN 9781118751077 (ebook)

Subjects: LCSH: Information display systems. | Liquid crystal displays. | Electroluminescent display systems.

Classification: LCC TK7882.I6 F55 2023 (print) | LCC TK7882.I6 (ebook) | DDC 621.3815/422--dc23/eng/20211116

LC record available at https://lccn.loc.gov/2021052707

LC ebook record available at https://lccn.loc.gov/2021052708

Cover Image: © metamorworks/Shutterstock

Cover Design: Wiley

Set in 9.5/12.5pt STIXTwoText by Integra Software Services Pvt. Ltd, Pondicherry, India

Contents

Cover

Serious page

Title page

Copyright

Series Editor’s Foreword

List of Contributors

1 Introduction

1.1 Toward Flexible Mobile Devices

1.2 Flexible Display Layers

1.3 Other Flexible Displays and Manufacturing

2 Engineered Films for Display Technology

2.1 Introduction

2.2 Factors Influencing Film Choice

2.2.1 Application Area

2.2.2 Physical Form/Manufacturing Process

2.2.3 Film Property Set

2.2.3.1 Polymer Type

2.2.3.2 Optical Clarity

2.2.3.3 Birefringence

2.2.3.4 The Effect of Thermal Stress on Dimensional Reproducibility

2.2.3.5 Low-bloom Films

2.2.3.6 Solvent and Moisture Resistance

2.2.3.7 The Effect of Mechanical Stress on Dimensional Reproducibility

2.2.3.8 Surface Quality

2.3 Summary of Key Properties of Base Substrates

2.4 Planarizing Coatings

2.5 Examples of Film in Use

2.6 Concluding Remarks

Acknowledgments

3 Liquid Crystal Optical Coatings for Flexible Displays

3.1 Introduction

3.2 LCN Technology

3.3 Thin-film Polarizers

3.3.1 Smectic Polarizers

3.3.2 Cholesteric Polarizers

3.4 Thin-film Retarders

3.4.1 Reactive Mesogen Retarders

3.4.2 Chromonic Liquid Crystal-based Retarders

3.4.3 Liquid Crystal Alignment and Patterned Retarders

3.5 Color Filters

3.6 Conclusion

4 Large Area Flexible Organic Field-effect Transistor Fabrication

4.1 Introduction

4.2 Substrates

4.3 Photolithography

4.4 Printing for Roll-to-roll Fabrication

4.4.1 Inkjet Printing

4.4.2 Gravure and Flexographic Printing

4.4.3 Screen Printing

4.4.4 Aerosol Jet Printing

4.4.5 Contact Printing

4.4.6 Meniscus Dragging

4.5 Conclusions

5 Metallic Nanowires, Promising Building Nanoblocks for Flexible Transparent Electrodes

5.1 Introduction

5.2 TEs Based on Metallic Nanowires

5.2.1 Metallic Nanowires, New Building Nanoblocks

5.2.2 Random Network Fabrication

5.2.3 Optical Characterization

5.2.4 Electrical Characterization

5.2.5 Mechanical Aspect

5.3 Application to Flexible Displays

5.3.1 Touch Screens

5.3.2 Light-emitting Diodes Displays

5.3.3 Electrochromic Flexible Displays

5.3.4 Other Displays

5.4 Conclusions

6 Optically Clear Adhesives for Display Assembly

6.1 Introduction

6.2 OCA Definition and General Performance Specifications

6.3 Application Examples and Challenges

6.3.1 Outgassing Tolerant Adhesives

6.3.2 Anti-whitening Adhesives

6.3.3 Non-corrosive OCAs

6.3.4 Compliant OCAs for High Ink-step Coverage and Mura-free Assembly of LCD Panels

6.3.5 Reworkable OCAs

6.3.6 Barrier Adhesives

6.4 Summary and Remaining Challenges

7 Self-healing Polymer Substrates

7.1 Introduction

7.2 General Classes of Self-healing Polymers

7.2.1 Types of Dynamic Bonds in Self-healing Polymers

7.2.2 Supramolecularly Crosslinked Self-healing Polymers

7.2.2.1 Hydrogen Bonding

7.2.2.2 π–π Stacking

7.2.2.3 Ionic Interactions

7.2.3 Dynamic-covalently Crosslinked Self-healing Polymers

7.2.3.1 Cycloaddition Reactions

7.2.3.2 Disulfides-based Reversible Reactions

7.2.3.3 Acylhydrazones

7.2.3.4 Boronate Esters

7.3 Special Considerations for Flexible Self-healing Polymers

7.4 Incorporation of Electrically Conductive Components

7.4.1 Metallic Conductors

7.4.2 Conductive Polymers

7.4.3 Carbon Materials

7.4.4 Polymerized Ionic Liquids

7.5 Additional Possibilities Enabled by Three-dimensional Printing

7.6 Concluding Remarks

8 Flexible Glass Substrates

8.1 Introduction

8.2 Display Glass Properties

8.2.1 Overview of Display Glass Types

8.2.2 Glass Properties

8.2.2.1 Optical Properties

8.2.2.2 Chemical Properties

8.2.2.3 Thermal Properties

8.2.2.4 Surface Properties

8.2.2.5 Permeability

8.3 Manufacturing of Thin “Flexible” Glass

8.3.1 Float and Downdraw Technology for Special Glass

8.3.2 Limits

8.3.2.1 Thickness Limits for Production

8.3.2.2 Surface Quality Limits for Production

8.4 Mechanical Properties

8.4.1 Thin Glass and Glass/Plastic Substrates

8.4.2 Mechanical Test Methods for Flexible Glasses

8.5 Improvement in Mechanical Properties of Glass

8.5.1 Reinforcement of Glass Substrates

8.5.1.1 Principal Methods of Reinforcement

8.5.1.2 Materials for Reinforcement Coatings

8.6 Processing of Flexible Glass

8.6.1 Cleaning

8.6.2 Separation

8.7 Current Thin Glass Substrate Applications and Trends

8.7.1 Displays

8.7.2 Touch Panels

8.7.3 Sensors

8.7.4 Wafer-level Chip Size Packaging

9 Toward a Foldable Organic Light-emitting Diode Display

9.1 Panel Stack-up Comparison: Glass-based and Plastic-based Organic Light-emitting Diode

9.1.1 Technology for Improving Contrast Ratio of OLED Display

9.2 CF–OLED for Achieving Foldable OLED Display

9.2.1 Mechanism of the AR coating in CF–OLED

9.2.2 Optical Performance of CF–OLED

9.3 Mechanical Performance of CF–OLED

9.3.1 Bi-directional Folding Performance and Minimum Folding Radius of SPS CF–OLED

9.4 Touch Panel Technology of CF–OLED

9.5 Foldable Application

9.5.1 Foldable Technology Summary

9.5.1.1 Polymer Substrates and Related Debonding Technology

9.5.1.2 Alternative TFT Types to LTPS

9.5.1.3 Encapsulation Systems to Protect Devices against Moisture

9.5.2 Novel and Next-generation Display Technologies

10 Flexible Reflective Display Based on Cholesteric Liquid Crystals

10.1 Introduction to Cholesteric Liquid Crystal

10.2 Reflection of CLC

10.3 Bistable CLC Reflective Display

10.4 Color Design of Reflective Bistable CLC Display

10.4.1 Mono-color Display

10.4.2 Full-color Display

10.5 Transitions between Cholesteric States

10.5.1 Transition from Planar State to Focal Conic State

10.5.2 Transition from Focal Conic State to Homeotropic State

10.5.3 Transition from Homotropic State to Focal Conic State

10.5.4 Transition from Homeotropic State to Transient Planar State

10.5.5 Transition from Transient Planar State to Planar State

10.6 Driving Schemes

10.6.1 Response to Voltage Pulse

10.6.2 Conventional Driving Scheme

10.6.3 Dynamic Driving Scheme

10.6.4 Thermal Driving Scheme

10.6.5 Flow Driving Scheme

10.7 Flexible Bistable CLC Reflective Display

10.8 Bistable Encapsulated CLC Reflective Display

10.9 Production of Flexible CLC Reflective Displays

10.9.1 Color e-Book with Single-layered Structure

10.9.2 Roll-to Roll E-paper and Applications

10.10 Conclusion

11 Electronic Paper

11.1 Introduction

11.2 Electrophoretic Display

11.2.1 Development History and Working Principle

11.2.2 Materials

11.2.2.1 Colored Particles/Pigments

11.2.2.2 Capsule Shell Materials

11.2.2.3 Suspending Medium (Mobile Phase)

11.2.2.4 Charge Control Agents

11.2.2.5 Stabilizers

11.2.3 Device Fabrication

11.2.4 Flexible EPD

11.3 Electrowetting Displays

11.3.1 Development History and Working Principle

11.3.2 Materials

11.3.2.1 Absorbing (Dyed) Hydrophobic Liquid

11.3.3 Device Fabrication

11.3.4 Flexible EWD

11.4 Other E-paper Display Technologies and Feasibility of Flexibility

11.4.1 PCD

11.4.2 LPD

11.5 Cholesteric (Chiral Nematic) LCDs

11.6 Electrochromic Displays

11.7 MEMS Displays

12 Encapsulation of Flexible Displays: Background, Status, and Perspective

12.1 Introduction

12.2 Background

12.3 Multilayer TFE Technology

12.3.1 Multilayer Approach

12.3.2 Inorganic Layer Deposition Techniques

12.3.3 Organic Layer Deposition Techniques

12.4 Current Technology Implementation

12.5 Future Developments

12.6 Conclusions

Acknowledgments

13 Flexible Battery Fundamentals

13.1 Introduction

13.2 Structural and Materials Aspects

13.2.1 Shape

13.2.2 One-dimensional Batteries

13.2.3 Two-dimensional Planar Batteries

13.2.4 Solid versus Liquid Electrolyte

13.2.5 Carbon Additives

13.3 Examples of Flexible Batteries

13.4 Future Perspectives

14 Flexible and Large-area X-ray Detectors

14.1 Introduction

14.2 Direct and Indirect Detectors

14.3 Thin-film Photodiode Sensors for Indirect-conversion Detectors

14.3.1 Performance Parameters

14.3.2 Photodiode Materials on Plastic Substrates

14.3.2.1 Amorphous Silicon

14.3.2.2 Organic Semiconductor Materials

14.4 TFT Array

14.4.1 Pixel Architecture and Transistor Requirements

14.4.2 Flexible Transistor Arrays

14.5 Medical-grade Detector

14.6 Summary and Outlook

15 Interacting with Flexible Displays

15.1 Introduction

15.2 Touch Technologies in Non-Flexible Displays

15.2.1 Resistive Touch Sensors

15.2.2 4-Wire Resistive

15.2.3 5-Wire Resistive

15.2.4 Capacitive Sensing

15.2.5 Surface Capacitive

15.2.6 Projected Capacitive

15.2.7 Infrared Sensing

15.2.8 Surface Acoustic Wave

15.2.9 Bending Wave Technologies

15.3 Touch Technologies in Flexible Displays

15.4 Summary

16 Mechanical Durability of Inorganic Films on Flexible Substrates

16.1 Introduction

16.2 Flexible Display Materials

16.2.1 Property Contrast between Coating and Substrate Materials

16.2.2 Determination of Mechanical Properties of Inorganic Coatings

16.3 Stress and Strain Analyses

16.3.1 Intrinsic, Thermal, and Hygroscopic Stresses and Strains

16.3.2 Strain Analysis of Multilayer Films under Bending

16.3.3 Critical Radius of Curvature

16.4 Failure Mechanics of Brittle Films

16.4.1 Damage Phenomenology under Tensile and Compressive Loading

16.4.2 Experimental Methods

16.4.3 Fracture Mechanics Analysis

16.4.4 Role of Internal Stresses

16.4.5 Influence of Film Thickness on Critical Strain

16.5 Durability Influences

16.5.1 Influence of Temperature

16.5.2 Fatigue

16.5.3 Corrosion

16.6 Toward Robust Layers

16.7 Final Remarks

Acknowledgments

Nomenclature

17 Roll-to-roll Production Challenges for Large-area Printed Electronics

17.1 Introduction

17.2 Infrastructure

17.3 Equipment

17.4 Materials

17.5 Processing

17.6 Summary

18 Direct Ink Writing of Touch Sensors and Displays: Current Developments and Future Perspectives

18.1 Introduction

18.2 DIW and Ink Development

18.3 Applications of DIW for Displays and Touch Sensors

18.4 Future Challenges and Opportunities

19 Flexible Displays for Medical Applications

19.1 Introduction

19.1.1 Flexible Displays in Medicine

19.1.2 A Brief Historical Perspective

19.1.3 Application of Flexible Displays for Biochemical Analysis

19.1.4 OLEDs and Organic Photodiodes as Optical Excitation Sources and Detectors

19.1.5 Device Integration

19.1.6 Fluorescence, Photoluminescence Intensity, and Decay-time Sensing

19.2 Flexible OLEDs for Oxygen Sensors

19.3 Glucose Sensing Using Flexible Display Technology

19.4 POC Disease Diagnosis and Pathogen Detection Using Flexible Display Optoelectronics

19.5 Flexible Display Technology for Multi-analyte Sensor Array Platforms

19.5.1 Integrated LOC and Flexible Display Devices

19.5.2 Multiplexed Sensor Platforms

19.6 Medical Diagnostic Displays

19.7 Wearable Health Monitoring Devices Based on Flexible Displays

19.7.1 Monitoring Vital Signs Using Flexible Display Technology

19.7.2 Flexible Display Technology for Phototherapy

19.7.3 Smart Clothing Using Flexible Display Technology

19.8 Competing Technologies, Challenges, and Future Trends

19.9 Conclusion

Acknowledgment

Conflicts of Interest

Index

End User License Agreement

List of Tables

CHAPTER 02

Table 2.1 Table of solvent...

Table 2.2 Comparative rigidity of...

Table 2.3 Comparison of the...

CHAPTER 03

Table 3.1 Specification of the...

CHAPTER 07

Table 7.1 Common polymers and...

CHAPTER 08

Table 8.1 Relation of production...

CHAPTER 09

Table 9.1 Reflective luminance and...

Table 9.2 Optimal device thickness...

Table 9.3 Criteria and specification...

CHAPTER 10

Table 10.1 Common commercially...

CHAPTER 14

Table 14.1 Specifications of x...

CHAPTER 16

Table 16.1 Substrates foils and...

List of Illustrations

CHAPTER 02

Figure 2.1 Illustration of the...

Figure 2.2 Chemical structures of...

Figure 2.3 Glass transitions of...

Figure 2.4 SEM of high...

Figure 2.5 Spectra of...

Figure 2.6 Shrinkage of heat...

Figure 2.7 Growth of cyclic...

Figure 2.8 SEM of surface...

Figure 2.9 Spectra of reflectance...

Figure 2.10 Moisture pickup versus...

Figure 2.11 Effect of RH...

Figure 2.12 Moisture loss of...

Figure 2.13 Change in storage...

Figure 2.14 Distortion through a...

Figure 2.15 Frequency distribution of...

Figure 2.16 Summary of PET...

Figure 2.17 Effectiveness of...

Figure 2.18 Shows that there...

Figure 2.19 Thermal mechanical...

CHAPTER 03

Figure 3.1 (a) Photopolymerization...

Figure 3.2 Polarized UV/V...

Figure 3.3 Examples of smectic...

Figure 3.4 Examples of smectic...

Figure 3.5 Intensity gradient...

Figure 3.6 Reflection of right...

Figure 3.7 Schematic set-up...

Figure 3.8 The slow axis...

Figure 3.9 RM polarization...

Figure 3.10 (a) RM stabilized...

Figure 3.11 Photo-aligning RMs...

Figure 3.12 Cinnamate containing...

Figure 3.13 Fabrication of a...

Figure 3.14 Photoisomerizable...

Figure 3.15 Light-responsive...

Figure 3.16 Photoisomerizable...

CHAPTER 04

Figure 4.1 (a) Process flows...

Figure 4.2 (a) Process procedure...

Figure 4.3 The variety of...

Figure 4.4 (a) Contact angle...

Figure 4.5 (a) Steps in...

Figure 4.6 Examples of the...

Figure 4.7 (a) Three-dimensional...

Figure 4.8 Common printing techniques...

Figure 4.9 A schematic of...

Figure 4.10 (a) Process flow...

Figure 4.11 Stretchable electrodes...

Figure 4.12 A combination printed...

Figure 4.13 Process flow for...

Figure 4.14 SEM image of...

Figure 4.15 (a) Process flow...

Figure 4.16 Left: Thermal nanoimprint...

Figure 4.17 A schematic drawing...

Figure 4.18 Schematics of knife...

CHAPTER 05

Figure 5.1 (a)(b)(c...

Figure 5.2 (a) AgNWs on...

Figure 5.3 A1–A3...

Figure 5.4 (A1−A6...

Figure 5.5 Demonstration of a...

Figure 5.6 Left: schematic...

Figure 5.7 (a) A bent...

Figure 5.8 (a) Schematic illustration...

CHAPTER 06

Figure 6.1 Refractive index of...

Figure 6.2 Effect of display...

Figure 6.3 Relative capacitances in...

Figure 6.4 Typical cross section...

Figure 6.5 Morphology control in...

Figure 6.6 Whitening phenomena in...

Figure 6.7 Cross section of...

Figure 6.8 Test configuration for...

Figure 6.9 Resistance of the...

Figure 6.10 Principal stress...

Figure 6.11 Shear storage modulus...

Figure 6.12 Tensile load testing...

Figure 6.13 OCA UV curing...

Figure 6.14 Time-lapse images...

Figure 6.15 Thixotropic, printable LOCA...

Figure 6.16 Shear-thinning behavior...

Figure 6.17 LOCA adhesive patch...

Figure 6.18 Automated wire-cutting...

CHAPTER 07

Figure 7.1 (A) Autonomic healing...

Figure 7.2 (A) Sequential and...

Figure 7.3 (A) sol-gel...

Figure 7.4 Applications of self...

Figure 7.5 Demonstrates a variety...

Figure 7.6 CT scans of...

CHAPTER 08

Figure 8.1 Transmission of D263...

Figure 8.2 Chemical durability of...

Figure 8.3 Length change of...

Figure 8.4 Contribution of single...

Figure 8.5 Basic process steps...

Figure 8.6 Large thin glass...

Figure 8.7 Thickness data for...

Figure 8.8 Detailed diagram of...

Figure 8.9 Four-point bending...

Figure 8.10 Edge impact test...

Figure 8.11 Set-up for...

Figure 8.12 Influence of edge...

Figure 8.13 Edge strength...

Figure 8.14 Improved surface...

Figure 8.15 Edge quality of...

Figure 8.16 Edge quality of...

Figure 8.17 Principal structure of...

CHAPTER 09

Figure 9.1 (a) Structure of...

Figure 9.2 Example of inorganic...

Figure 9.3 Appearance of OLED...

Figure 9.4 (a) Basic component...

Figure 9.5 Schematic of individual...

Figure 9.6 Major drawbacks of...

Figure 9.7 Process flow of...

Figure 9.8 Reflectance of cavity...

Figure 9.9 Panel specification of...

Figure 9.10 Schematic of a...

Figure 9.11 Strain distribution of...

Figure 9.12 AMOLED panel with...

Figure 9.13 Luminance ratio of...

Figure 9.14 Luminance (L) ratio...

Figure 9.15 Comparison of the...

Figure 9.16 Schematics of the...

Figure 9.17 Touch signal and...

Figure 9.18 The side-wall...

Figure 9.19 The stretchable AMOLED...

CHAPTER 10

Figure 10. 1 Schematic diagram of...

Figure 10.2 Chemical structures of...

Figure 10. 3 Reflection spectrum of...

Figure 10.4 Peak reflectance as...

Figure 10.5 Side view of...

Figure 10.6 Schematic diagram of...

Figure 10.7 Schematic diagram of...

Figure 10.8 Schematic diagram of...

Figure 10.9 LC director configurations...

Figure 10.10 Side view of...

Figure 10.11 Top view of...

Figure 10.12 Side view of...

Figure 10.13 Side view of...

Figure 10.14 Reflection spectrum of...

Figure 10.15 Reflection of bistable...

Figure 10.16 Schematic diagram of...

Figure 10.17 Schematic diagram of...

Figure 10.18 Reflection versus continuously...

Figure 10.19 (a) Schematic diagram...

Figure. 10.20 Schematic diagram...

Figure. 10.21 The trends of research...

Figure. 10.22 The structure of the...

Figure. 10.23 The color single-layer...

Figure 10.24 (a) the good...

Figure 10.25 (a) Traditional sub...

Figure 10.26 (a) The 10...

Figure 10.27 Schematic diagram of...

Figure 10.28 The blue/white...

Figure 10.29 (a) The roll...

Figure 10.30 (a) Results of...

Figure 10.31 (a) Structure of...

Figure 10.32 Wireless controlled thermal...

Figure 10.33 (a) Soft flexible...

Figure 10.34 The new process...

Figure 10.35 The image quality...

Figure 10.36 The applications of...

Figure 10.37 (a) The characteristics...

Figure 10.38 (A) and (B...

CHAPTER 11

Figure 11.1 2008–2018...

Figure 11.2 Demos of color...

Figure 11.3 Demos of flexible...

Figure 11.4 Schematic of EPD...

Figure 11.5 Zeta potential. Source...

Figure 11.6 Exploded view of...

Figure 11.7 Flexible EPD demonstrated...

Figure 11.8 Schematic drawing of...

Figure 11.9 An example of...

Figure 11.10 Process flow for...

Figure 11.11 Raster filling using...

Figure 11.12 EWD operation on...

Figure 11.13 Schematic of PCD...

Figure 11.14 QR-LPD working...

Figure 11.15 ECD principle...

Figure 11.17 Example of a...

Figure 11.18 Image of a...

Figure 11.19 Schematic of IMOD...

CHAPTER 12

Figure 12.1 Comparison of LCD...

Figure 12.2 Defect density versus...

Figure 12.3 Pictorial description of...

Figure 12.4 TFE evolution history...

Figure 12.5 Comparison between structure...

Figure 12.6 Example of particle...

Figure 12.7 Example of PDL...

Figure 12.8 Effective diffusivity versus...

Figure 12.9 Swelling of each...

Figure 12.10 Comparison of display...

Figure 12.11 Mask-less deposition...

Figure 12.12 Royole’s...

Figure 12.13 TEM cross section...

Figure 12.14 Novel experimental linear...

Figure 12.15 Samsung stretchable backplane...

Figure 12.16 LGD Rollable TV...

Figure 12.17 LGD Rollable transparent...

Figure 12.18 Fabrication process flow...

CHAPTER 13

Figure 13.1 Rudimentary illustration of...

Figure 13.2 (a) Depiction of...

Figure 13.3 Demonstration of functional...

Figure 13.4 Comparison of the...

Figure 13.5 Comparison of elemental...

Figure 13.6 Manufacturing process of...

Figure 13.7 (a) Paper battery...

Figure 13.8 (a) Voltage-time...

Figure 13.9 (a) Voltage-time...

CHAPTER 14

Figure 14.1 Schematic of the...

Figure 14.2 Direct-conversion detectors...

Figure 14.3 Example of bulk...

Figure 14.4 (A) Photomicrograph of...

Figure 14.5 Optical images captured...

Figure 14.6 (A) Photograph of...

CHAPTER 15

Figure 15.1 Schematic diagram showing...

Figure 15.2 Schematic diagram showing...

Figure 15.3 Schematic diagram showing...

Figure 15.4 Schematic diagram showing...

Figure 15.5 Schematic diagram showing...

Figure 15.6 Schematic diagram showing...

Figure 15.7 Schematic diagram showing...

Figure 15.8 Schematic diagram showing...

Figure 15.9 Schematic diagram showing...

Figure 15.10 Process flow of...

Figure 15.11 Ultra-low reflectance...

Figure 15.12 Hybrid-sensing scheme...

Figure 15.13 A 6- x...

Figure 15.14 Two FabriTouch pads...

Figure 15.15 Curved OLED display...

Figure 15.16 5-inch single...

Figure 15.17 A micrograph of...

CHAPTER 16

Figure 16.1 Young’s...

Figure 16.2 Effective modulus versus...

Figure 16.3 Sketches of internal...

Figure 16.4 Residual stress dynamics...

Figure 16.5 Internal stress in...

Figure 16.6 Normalized strain in...

Figure 16.7 Optical micrographs of...

Figure 16.8 Buckling morphologies in...

Figure 16.9 Fragmentation process of...

Figure 16.10 Predicted εcrit...

Figure 16.11 Electron micrographs of...

Figure 16.12 Fatigue damage map...

Figure 16.13 Critical number of...

CHAPTER 17

Figure 17.1 Schemat of ideal...

Figure 17.2 Major R2R large...

Figure 17.3 Substrate and coating...

Figure 17.4 Operational principles of...

Figure 17.5 Pyramid diagram showing...

Figure 17.6 Schematic representation of...

Figure 17.7 Representation of printing...

Figure 17.8 Increase of electrical...

Figure 17.9 Market forecast for...

Figure 17.10 Urban installation of...

CHAPTER 18

Figure 18.1 (a) DIW apparatus...

Figure 18.2 Optical images of...

Figure 18.3 (a) Normalized change...

Figure 18.4 (A) Optical and...

Figure 18.5 (a1–3...

Figure 18.6 (a) Transparent conductor...

Figure 18.7 3D printed 2...

Figure 18.8 Printed ACEL 1D...

Figure 18.9 Schematic illustration of...

Figure 18.10 Images of (a...

Figure 18.11 The capacitive touch...

CHAPTER 19

Figure 19.1 Normalized emission spectra...

Figure 19.2 (A) Evolution of...

Figure 19.3 The effect of...

Figure 19.4 (A) Schematic illustration...

Figure 19.5 (A) Flexible bottom...

Figure 19.6 (A) Schematic diagram...

Figure 19.7 (A) LOC flexible...

Figure 19.8 (A) Flexible displays...

Figure 19.9 (A) Concept for...

Figure 19.10 (A) Integrated solid...

Guide

Cover

Serious page

Title page

Copyright

Table of Contents

Series Editor’s Foreword

List of Contributors

Begin Reading

Index

End User License Agreement

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Series Editor’s Foreword

The first edition of Professor Crawford’s Flexible Flat Panel Displays was one of the first volumes in the Wiley-SID series of technical books to be published. Since its appearance it has achieved the distinction of becoming the single most successful work in the series, based on cumulative sales. These facts illustrate the continuing importance and the technical challenge of mass producing high-quality, reliable devices in a flexible format, as well as the comprehensive analysis of the technical issues involved, which the editor and authors brought to the topic. In 2005 when the first edition was produced, the whole field of flexible displays was immature, and the volume necessarily focused mainly on the enabling technologies and technical challenges faced by those seeking to develop flexible devices and routes to their mass manufacture.

Today, the status of these devices has been transformed, and this new and completely revised edition of the book reflects that. Foldable organic light-emitting diode (OLED) displays are widely available on mobile phones and tablet devices, albeit restricted mainly to premium devices, while flexible liquid-crystal displays (LCDs) and especially electrophoretic displays have achieved maturity. The potential of large-area conformable displays to open new fields of application and product design is rapidly expanding in areas such as automotive interiors and industrial systems, while curved computer monitors are firmly established as mainstream devices. Meanwhile, the aspiration of product engineers to exploit displays with the free flexibility of paper or fabric remains problematic. The technical challenges which flexible devices face are in many cases the same as those that could be addressed in the first edition; bending any electronic device can lead to undesirable consequences ranging from a reversible shift in semiconductor characteristics through fatigue failure of different materials to catastrophic failure of conductor tracks, encapsulation, or the thin-film transistors (TFTs) themselves. However, state-of-the-art solutions to these difficulties have in many cases advanced both in technical approach and in performance in ways which could hardly be dreamed of in 2005. Encapsulation materials, conductive layers, and advanced semiconductors are examples where the technology has been transformed.

In this new edition of Flexible Flat Panel Displays the editors Professor Cairns, Professor Crawford, and Professor Broer have assembled a comprehensive and thoroughly updated overview of the field, key challenges that remain to be overcome, and approaches to materials, processes, and operating modes to further improve the availability, quality, and durability of flexible display screens. The advances in the technology that have been achieved since the publication of the first edition have also impacted the application space for flexible devices and forward-looking views of flexible sensor systems are also included. The work will be a valuable resource and reference, not only for scientists and engineers concerned with flexible display and electronic devices but for all interested in current developments in display technology, in integrating displays in new products, and in such diverse areas as Internet of Things and wearable devices.

January 2023

Ian Sage

Great Malvern

List of Contributors

Karen S. Anderson

Virginia G. Piper Center for Personalized Diagnostics, The Biodesign Institute at Arizona State University, USA

Dirk J. Broer

Eindhoven University of Technology, Department of Chemical Engineering & Chemistry, Laboratory of Functional Organic Materials & Devices, Eindhoven, The Netherlands

Darran R. Cairns

West Virginia University, Statler College of Engineering

University of Missouri – Kansas City, School of Science and Engineering, USA

Marco Roberto Cavallari

Departamento de Engenharia de Sistemas Eletrônicos, Escola Politécnica da Universidade de São Paulo, São Paulo, Brazil

Department of Renewable Energies. UNILA, Federal University of Latin American Integration, Foz do Iguącu, PR, Brazil

Progyateg Chakma

Department of Chemistry and Biochemistry, Miami University, USA

Chi-Shun Chan

AUO Display Plus Corp., Taiwan

Yi-Hong Chen

AUO Display Plus Corp., Taiwan

Janglin Che

Industrial Technology Research Institute, Hsinchu, Taiwan

Jennifer M. Blain Christen

School of Electrical, Computer, and Energy Engineering, Goldwater Center #208, 650 E. Tyler Mall, ASU Tempe Campus, Arizona, USA

Gregory P. Crawford

President, Miami University, USA

Zachary A. Digby

Department of Chemistry and Biochemistry, Miami University, USA

Albert I. Everaerts

3M Company St. Paul, USA

Gerwin Gelinck

Holst Centre, TNO, Eindhoven, The Netherlands

Eindhoven University of Technology, Eindhoven, The Netherlands

Andreas Habeck

Schott AG, Germany

Alex Henzen

Electronic Paper Display Institute, South China Normal University, China

Norbert Hildebrand

Schott North America Inc., NY, USA

Annie Tzuyu Huang

AUO Display Plus Corp., Taiwan

Silke Knoche

Schott AG, Germany

Dominik Konkolewicz

Department of Chemistry and Biochemistry, Miami University, USA

Anke Kruse

Schott AG, Germany

Ioannis Kymissis

Professor, Electrical Engineering Columbia University, New York, USA

Principal Engineer, Lumiode, New York, USA

Zachary A. Lamport

Electrical Engineering Columbia University, New York, USA

Meng-Ting Lee

AUO Display Plus Corp., Taiwan

Yves Leterrier

Laboratoire de Technologie des Composites et Polymères (LTC), Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

Chun-Yu Lin

AUO Display Plus Corp., Taiwan

Johan Lub

Eindhoven University of Technology, Department of Chemical Engineering & Chemistry, Laboratory of Functional Organic Materials & Devices, Eindhoven, The Netherlands

W.A. MacDonald

DuPont Teijin Films (UK) Limited

Lorenza Moro

Vice President CTO Group, Palo Alto, USA

Uwadiae Obahiagbon

School of Electrical, Computer and Energy Engineering, Arizona, USA

Owain Parri

Merck Chemicals Ltd., Southampton, UK

Armin Plichta

Schott AG, Germany

Dr. Grzegorz Andrzej Potoczny

OPVIUS GmbH, Nuremberg, Germany

J.W. Shiu

Industrial Technology Research Institute, Hsinchu, Taiwan

Konstantinos A. Sierros

West Virginia University, Statler College of Engineering

West Virginia University, Department of Mechanical & Aerospace Engineering, USA

Jean-Pierre Simonato

Director of Research, Simonato, CEA

Jonathan HT Tao

AUO Display Plus Corp., Taiwan

Robert Jan Visser

Applied Materials, Santa Clara California, USA

Andreas Weber

Schott AG, Germany

Anthony S. Weiss

University of Missouri – Kansas City in the School of Science and Engineering, USA

Nicholas Winch

West Virginia University, Statler College of Engineering

Chih-Hung Wu

AUO Display Plus Corp., Taiwan

Deng-Ke Yang

Advanced Materials and Liquid Crystal Institute, Chemical Physics Interdisciplinary Program and Department of Physics, Kent State University, USA

M. H. Yang

Industrial Technology Research Institute, Hsinchu, Taiwan

Dong Yuan

Electronic Paper Display Institute, South China Normal University, China

Guofu Zhou

Electronic Paper Display Institute, South China Normal University, China

1 Introduction

Darran R. Cairns1, Gregory P. Crawford2, and Dirk J. Broer3

1 West Virginia University, Statler College of Engineering and University of Missouri – Kansas City, School of Science and Engineering2 President, Miami University, USA3 Eindhoven University of Technology, Department of Chemical Engineering & Chemistry, Laboratory of Functional Organic Materials & Devices, Eindhoven, The Netherlands

1.1 Toward Flexible Mobile Devices

Displays and how we use them have gone through some major changes already in the twenty-first century. Mobile displays have developed from displaying text and some rudimentary graphics to highly interactive, high-resolution devices capable of streaming high-definition video. In addition to the advances in the display technologies, mobile devices also have high-resolution cameras, multiple internal sensors, powerful computer processers, multiple communication chips, and large area rechargeable batteries. Against this backdrop the requirements for flexible displays to be used in many mobile device applications far exceed those near the turn of the century. However, despite these challenging requirements, there are now beginning to be commercial products with amazing capabilities. An example of the type of approach that can be used to develop commercial foldable displays is described by Meng-Ting Lee et al. in Chapter 9. The significant improvements in organic light-emitting diodes (OLEDs) have opened possibilities in the development of foldable displays.

While improvements in OLEDs have been critical to recent developments in flexible displays there are a range of other critical components that would also need to be flexible for the development of truly mobile devices. One particularly important development has been in the field of transparent conductive coatings where metallic nanowires are becoming a commercial reality with important flexible properties. This is described in detail by Jean-Pierre Simonato in Chapter 5. For some applications an outer surface of glass would be very useful as an oxygen and moisture barrier or to protect underlying layers. Some examples of flexible glass are discussed in Chapter 8 by Armin Plitchta et al. For truly flexible devices large flexible power sources such as batteries will be needed, and flexible batteries are discussed in Chapter 13 by Nicholas Winch et al.

These are certainly exciting times for the development of flexible mobile devices. We have highlighted some of the key developments we discuss in this book that could be incorporated into such a device, but we will also need to understand how flexibility impacts functionality such as the rich touch input we expect. Some important aspects of integrating touch in mobile devices is described by Darran Cairns and Anthony Weiss in Chapter 15.

1.2 Flexible Display Layers

It is likely that polymer films will be used widely in flexible displays, and this raises myriad challenges, not least of these being durability. For applications where a polymer film is part of the outermost surface of the device, cuts and abrasions of the outermost polymer layer can reduce display performance. One approach to mitigate for this is to use self-healing polymers, which is discussed by Progyateg Chakma et al. in Chapter 7. One vitally important issue is damage to inorganic layers in a flexible display which can lead to cracking this is discussed by Yves Letterier in Chapter 16.

Mechanical damage is not the only thing to be considered in flexible display components. It is also important to tune the optics of polymer for the application and to mitigate for the underlying properties of polymer substrates. This can be achieved through the design of engineered polymer films as described by Bill McDonald in Chapter 2 and through the design of optical coatings as described by Owain Parri et al. in Chapter 3. The ability to tune properties in multiple ways opens a range of ways to design display components.

Two additional layers that play critical roles in the development of flexible displays are optically clear adhesives, discussed by Albert Everaerts in Chapter 6, and the thin film encapsulation layer used to protect OLEDs, discussed by Robert Jan Visser and Lorenza Moro in Chapter 12. Optically clear adhesives allow components to be laminated with minimal optical losses and enable complex stacks to be engineered and assembled. We discussed earlier how OLEDs are enabling advances in flexible devices but for OLEDs to have reasonable lifetimes they must be encapsulated—and it is this encapsulation that had enabled OLEDs to become a display of choice in flexible applications. One additional component that is required for flexible devices is a flexible backplane and Zachary A. Lamport et al. describe flexible backplanes using organic transistors in Chapter 4.

1.3 Other Flexible Displays and Manufacturing

We discussed earlier how our expectations of mobile devices has changed with expectations for high-fidelity video and computing power necessitating flexible batteries and touch sensors with a high-resolution display. We have also highlighted how OLEDs have become widely used in large part because of these expectations. However, not all devices need to play high-resolution video or require significant computing power. For applications such as e-readers the requirements are very different with low-power consumption and high-contrast ratio being more important than speed. Two important technologies that have found important niches are cholesteric liquid crystal displays, described by Deng-Ke Yang in Chapter 10, and electronic paper, described by Guofu Zhou in Chapter 11. There are currently some commercial products that can be manufactured on flexible substrates even if they are not ultimately used in a flexible form factor.

For several years roll-to-roll manufacturing has been advanced as a justification for flexible electronics because of the ability to fabricate devices in volume. There are a number of challenges with roll-to-roll fabrication and some of these are highlighted by Greg Potoczny in Chapter 17. More recently robotic deposition and direct writing is opening new approaches to manufacturing allowing for precise deposition of coatings and circuitry. This manufacturing approach is discussed by Kostas Sierros and Darran Cairns in Chapter 18.

Finally, we also include two chapters related to applications. Uwadiae Obahiagbon et al. detail some applications of flexible displays in medical applications in Chapter 19. We expect this to be an exciting area moving forward. In Chapter 14, Gerwin Gelinck describes his work on large area flexible x-ray detectors, which we believe will be useful in incorporating additional sensing in flexible devices and displays.

2 Engineered Films for Display Technology

W.A. MacDonald

DuPont Teijin Films

2.1 Introduction

Since the early 2000s there has been an explosion in interest in printed electronics and flexible displays in terms of both exploring the potential to develop new business opportunities and developing the technology base. The opportunity to exploit flexible substrates in roll-to-roll (R2R) production has excited the interest of the plastic films and associated processing and coating industries.

To replace a rigid substrate such as glass, however, a plastic substrate needs to be able to offer some or all of the properties of glass i.e. clarity, dimensional stability, thermal stability, barrier, solvent resistance, and low coefficient of linear thermal expansion (CLTE) coupled with a smooth surface. In addition, a further functionality such as a conductive layer might be required. No plastic film offers all these properties so any plastic-based substrate replacing glass will almost certainly be a multilayer composite structure [1–3]. However, not all applications require such a demanding property set and over the past decade plastic films have found application in areas broader than the flexible organic light-emitting diode (OLED) displays initially envisaged in the early days of the technology development. These include applications such as electrophoretic displays driven by thin-film transistor (TFT) arrays printed on plastic film, printed memory, and sensors. In addition there has been a smearing of the boundary between flexible devices and the use of printed electronics and/or flexible substrates in rigid devices – an example being the use of conductive films in touchscreens incorporated into smartphones and tablets.

This chapter will review the progress made in developing plastic substrates for flexible displays over the past decade and polyester films will be used as the main examples.

2.2 Factors Influencing Film Choice

2.2.1 Application Area

The requirements for the different applications areas envisaged for printable electronics are very different and will require substrates with different property sets.

This is summarized in Figure 2.1 and this classification is divided into “simple” organic circuitry, e.g. radio frequency identification (RFID), organic-based active-matrix backplanes, inorganic TFTs, and OLED displays. As one moves up the list the substrate requirements in terms of properties such as dimensional stability, surface smoothness, low coefficient of thermal stability (CLTE), conductive layers, and barrier become increasingly complex and more demanding and this will be reflected in the cost of the substrate. Substrates with low temperature stability including coated papers and oriented polypropylene film with good surface quality are adequate for some applications such as RFID and simple printed electronic devices, but a film with high temperature stability and dimensional reproducibility coupled with excellent surface quality will be required for more complex applications associated with displays such as active-matrix backplanes and OLED technology. Obviously a substrate with a property set and price commensurate with the application should be chosen and this section will focus on substrates appropriate for display application.

Figure 2.1 Illustration of the changing property requirements of different applications.

2.2.2 Physical Form/Manufacturing Process

The physical form of the display and whether it is

flat but exploiting light weight and ruggedness,

conformable, one time fit to non-flat surface,

flexible and handleable, e.g. electronic newspaper, or

rollable

will also influence film choice. Initially in the early 2000s, the vision was of flexible OLED displays and rollable displays in particular. Over the past decade, however, with the emergence of smartphones and tablets, the general public have become used to the portability and user interface of such devices and the interest in a rollable or foldable display device has waned. At the time of writing this chapter, however, serious interest in foldable and flexible displays has again emerged as the major electronic companies envisage flexibility as key to providing new innovation in the next generation of smartphones and tablets. In addition to this, flexible electronics offers robustness over glass-based rigid devices, and a further general trend on film substrates is to go thinner to reduce the bulkiness of a device.

Whether the film is manufactured by batch or R2R can influence film selection. Although R2R processing can be used for specific stages of device manufacture, for example barrier or conductive coatings, and for less complex printed electronic applications, for the most part more complex device manufacture such as active-matrix backplanes is carried out by a batch-based process on a rigid carrier. This fits with existing semiconductor manufacturing tooling equipment and the rigid carrier can also be exploited to control the dimensional stability. However, batch processing introduces new challenges such as bowing of the rigid carrier due to a mismatch in coefficient of thermal expansion and shrinkage behavior between the film and the rigid carrier. The major factors influencing this are the rigidity of the carrier (more rigid gives less bow) and the thickness of the film (less bow with thinner film). A second issue is release of the film from the carrier without damaging the printed circuitry and this remains an active area of research.

2.2.3 Film Property Set

2.2.3.1 Polymer Type

Polyethylene terephthalate (PET), e.g. DuPont Teijin (DTF) Films Melinex® polyester film, and polyethylene naphthalate (PEN), e.g. DuPont Teijin Films Teonex® polyester film, are biaxially oriented semicrystalline films [4, 5]. The difference in chemical structure between PET and PEN is shown in Figure 2.2.

Figure 2.2 Chemical structures of PET and PEN films.

The substitution of the phenyl ring of PET by the naphthalene double ring of PEN has very little effect on the melting point (Tm), which increases by only a few degrees. There is, however, a significant effect on the glass transition temperature (Tg), the temperature at which a polymer changes from a glassy state to a rubbery state, which increases from 78oC for PET to 120 oC for PEN [2]. PET and PEN films are prepared by a process whereby the amorphous cast is drawn in both the machine direction and transverse direction. The biaxially oriented film is then heat set to crystallize the film [4, 5].

The success of polyester film in general application comes from the properties derived from the basic polymer coupled with the manufacturing process of biaxial orientation and heat setting described earlier. These properties include high mechanical strength, good resistance to a wide range of chemicals and solvents, low water absorption, excellent dielectric properties, good dimensional stability, and good thermal resistance in terms of shrinkage and degradation of the polymer chains. Fillers can be incorporated into the polymer to change the surface topography and opacity of the film. The film surface can also be altered by the use of pretreatments to give a further range of properties, including enhanced adhesion to a wide range of inks, lacquers, and adhesives. These basic properties have resulted in PET films being used in a wide range of applications, from magnetic media and photographic applications, where optical properties and excellent cleanliness are of paramount importance to electronics applications such as flexible circuitry, and touch switches, where thermal stability is key. More demanding polyester film markets, which exploit the higher performance and benefits of PEN, include magnetic media for high-density data storage and electronic circuitry for hydrolysis-resistant automotive wiring [6]. This property set provides the basis on which one can now build to meet the demands of the printed electronic market.

It is interesting to contrast these films with the other films that are currently being considered for flexible electronics applications. The main candidates are shown in Figure 2.3, which lists the substrates in terms of increasing glass transition (Tg)

Figure 2.3 Glass transitions of film substrates of interest for flexible electronics applications.

The polymers can be further categorized into films that are semicrystalline (PET and PEN as mentioned earlier), amorphous and thermoplastic, and amorphous, but solvent cast. Polymers with a Tg higher than 150oC that are semicrystalline tend generally to have a Tm that is too high to allow the polymers to be melt processed without significant degradation – Victrex® PEEKfilm ACTIV® [7] is the highest performance semicrystalline material available in film form. The next category are polymers that are thermoplastic, but non-crystalline and these range from polycarbonate (PC), e.g. Teijin’s PURE-ACE® [8] and GE’s Lexan® [9], with a Tg of ~150oC to polyethersulphone (PES), e.g. Sumitomo Bakelite’s Sumilite® [10], with a Tg of ~220oC. Although thermoplastic, these polymers may also be solvent cast to give high optical clarity. The third category are high-Tg materials that cannot be melt processed and include substrates based on Akron Polymer Systems (APS) resins [11] and polyimide (PI), e.g. DuPont’s Kapton® [12].

PET and PEN by virtue of being semicrystalline, biaxially oriented, and heat stabilized (see Section 2.3.4) have a different property set to the amorphous polymers and for simplicity these two basic categories will be used when comparing and contrasting the properties of the film types and the importance to the property set required for flexible electronic application.

2.2.3.2 Optical Clarity

The clarity of the film is important for bottom emissive displays where one is viewing through the film and a total light transmission (TLT) of >85% over 400–800 nm coupled with a haze of less than 0.7% are typical of what is required for this application. The polymers listed earlier meet this requirement apart from polyimide, which is yellow.

Polyester films are used extensively in the light management within liquid crystal displays (LCDs). PET is the base film used in brightness enhancement films (BEF) where a prismatic coating on the surface of the film is used to recycle the scattered light and direct it through the LCD to increase the brightness.

In addition to BEF and diffuser films, highly reflective polyester films are used to reflect and recycle light from the light-guide plate in an LCD display. These films are polyester films with inorganic fillers where different levels of diffuse reflectance are achieved by generating voids during the manufacturing process in Figure 2.4. These are tailored in shape, size, and distribution by controlling filler type and film process technology to achieve different levels of performance. The total reflectance spectra of these specially designed films, e.g. Melinex® RFL1 and 2, are shown in Figure 2.5 compared to a standard white film.

Figure 2.4 SEM of high-reflectance films illustrating the different voiding effects.

Figure 2.5 Spectra of reflectance versus wavelength for highly reflective films.

2.2.3.3 Birefringence

Biaxially oriented films such as PET and PEN are birefringent. For LCDs that depend on light of known polarization this means that birefringent films, which would change the polarization state, are unlikely to be used as base substrates. That said, polyester films are used extensively to enhance LCD performance as discussed earlier. Films based on amorphous polymer are not birefringent and are more suitable for the base substrates for LCDs. Birefringence is not an issue with OLED, electrophoretic displays, and indeed some LCDs.

2.2.3.4 The Effect of Thermal Stress on Dimensional Reproducibility

PET and PEN films (Melinex® and Teonex®) are produced using a sequential biaxial stretching technology, which is widely used for semicrystalline thermoplastics [4, 5]. The process involves stretching film in machine and transverse directions (MD and TD) and heat setting at elevated temperature. As a consequence, a complex semicrystalline microstructure develops in the material, which exhibits remarkable strength, stiffness, and thermal stability. Various studies have been made of the biaxial structure of polyester film manufactured in this way and many descriptions have been written [13, 14]. The film comprises a mosaic of crystallites or aggregated crystallites accounting for nearly 50 wt% of its material and that tend to align along the directions of stretch. Adjacent crystallites may not, however, share similar orientations. Crystallites show only a small irreversible response to temperature, which may take the form of growth or perfection. The non-crystalline region also possesses some preferred molecular orientation, which is a consequence of its connectivity to the crystalline phase. Importantly, the molecular chains residing in the non-crystalline region are on average slightly extended and therefore do not exist in their equilibrium, Gaussian conformation.

Standard PET and PEN film will shrink 1–3% at temperatures above the Tg. PET and PEN films can be further exposed to a thermal relaxation process, in which film is transported under low tension through an additional heating zone at approximately 150oC for PET and 180–200oC for PEN. Some additional shrinkage is seen, which signifies a relaxation of the molecular orientation in the material [1–3, 14]. Fundamental measurements of fibers and films indicate that the relaxation occurs exclusively in the non-crystalline regions [15].

Figure 2.6 illustrates how the shrinkage of heat stabilized PET (Melinex®ST504/506) and PEN (Teonex® Q65) change with temperature. For applications requiring low shrinkage above 140oC, Teonex® Q65 is the preferred option and there is continuing work within DTF to optimize the heat stabilization process without having a detrimental effect on final film properties.

Figure 2.6 Shrinkage of heat stabilized PET and PEN film versus temperature.

In a batch-based device process it is also possible to anneal the film at temperatures around 200oC prior to processing on it. It has been shown that it is possible to achieve a level of shrinkage down to 25 ppm at 150oC with Teonex®Q65 [16].

The second factor that impacts on dimensional reproducibility is the natural expansion of the film as the temperature is cycled as measured by the CLTE. A low CLTE typically <20 ppm/oC is desirable to match the thermal expansion of the base film to the layers that are subsequently deposited. A mismatch in thermal expansion means that the deposited layers can become strained and cracked under thermal cycling. In the temperature range from room temperature up to the Tg the typical CLTE of PEN is 18–20 ppm and PET 20–25 ppm (but note that the Tg of PEN is 40oC higher than PET). Above the Tg, the natural expansion of PET and PEN films that have not been heat stabilized is dominated by the shrinkage the films undergo as the internal strains in the film relax as discussed earlier. However, the heat stabilized films discussed show only a very small increase in CLTE in the temperature range from the Tg to the temperature at which they were heat stabilized [1–3]. This contrasts favorably with the quoted coefficient of expansion of amorphous polymers, which is typically 50 ppm/°C [2, 3] below the Tg but can increase by a factor of three times above the Tg.

In addition to dimensional stability, another important factor to be considered is the upper processing temperature (Tmax) that a film can be used at. Although, as has been outlined, the Tg does not define Tmax with the semicrystalline polymers, it largely does with the amorphous polymers.

2.2.3.5 Low-bloom Films

Conductive coated films and their use in applications such as touch screens is outwith the scope of this chapter but an essential component of these films is that the base film in which the conductive coatings are deposited are dimensionally stable and do not “bloom” on heating through subsequent processing cycles. PET film contains 1.1–1.4 weight% cyclic oligomer, which can migrate to the surface of the film if held at elevated temperatures for tens of minutes [17, 18], as can occur for example in the manufacture of devices exploiting touchscreens. The presence of these oligomers on the surface gives rise to an increase in haze and this is commonly referred to as “bloom.” PEN with 0.3 weight% cyclic content has significantly lower cyclic oligomer content compared with PET and there is significantly less migration to the surface of the film (Figure 2.7).

Figure 2.7 Growth of cyclic oligomers on PET and PEN film with time at 100, 120, and 140°C.

One strategy to reduce this hazing effect is to exploit planarizing coatings (see Section 2.3.8) or hard coats to act as a barrier to cyclic oligomer migration.

Control of the polymerization process and film process to reduce the cyclic oligomer content offers a further strategy to yield a “low-bloom” film. Figure 2.8 shows the scanning electron microscope (SEM) image of the surface of films aged at 150oC for 60 minutes and the reduction of cyclic oligomer on the surface of the low-bloom film can be seen compared to a “normal” film. These low-bloom films, e.g. Melinex ® TCH, typically have a haze less than or equal to 1% after aging at 150oC for 60 minutes.

Figure 2.8 SEM of surface of normal and low-bloom film aged at 150°C for 60 minutes illustrating the reduction in cyclic oligomer on the surface.