Polymers in Organic Electronics: Polymer Selection for Electronic, Mechatronic, and Optoelectronic Systems 1927885671, 9781927885673

Table of contents :
Cover
Polymers in Organic Electronics:
Polymer Selection for Electronic,
Mechatronic & Optoelectronic Systems
Copyright
Dedication
Table of Contents
Acknowledgments
1 Introduction to Polymers for
Electronic Engineers
2 Electronics
for Polymer Engineers
3 Optimized Electronic
Polymers, Small Molecules,
Complexes, and Elastomers for
Organic Electronic Systems
4 Optimization of Electrical,
Electronic and Optical
Properties of Organic
Electronic Structures
5 Optimization of Polymeric
Structures of Organic Printed
Circuit Boards
6 Optimized Polymeric
Structures of Organic Active
Electronic Components
7 Polymeric Structures
Optimized for Organic Passive
Electronic Components
8 Optimizing Polymeric
Structures in
Organic Optoelectronics
9 Optimizing Polymeric
Structures of Organic
Electronic Packages
Index
Back Cover

Citation preview

Polymers in Organic Electronics Polymer Selection for Electronic, Mechatronic & Optoelectronic Systems Sulaiman Khalifeh

Toronto 2020

Published by ChemTec Publishing 38 Earswick Drive, Toronto, Ontario M1E 1C6, Canada © ChemTec Publishing, 2020 ISBN 978-1-927885-67-3 (hard cover); 978-1-927885-68-0 (E-PUB)

Cover design: Anita Wypych

All rights reserved. No part of this publication may be reproduced, stored or transmitted in any form or by any means without written permission of copyright owner. No responsibility is assumed by the Author and the Publisher for any injury or/and damage to persons or properties as a matter of products liability, negligence, use, or operation of any methods, product ideas, or instructions published or suggested in this book.

Library and Archives Canada Cataloguing in Publication Title: Polymers in organic electronics: polymer selection for electronic, mechatronic & optoelectronic systems / Sulaiman Khalifeh. Names: Khalifeh, Sulaiman, 1959- author. Description: Includes bibliographical references and index. Identifiers: Canadiana (print) 20190238720 | Canadiana (ebook) 20190238739 | ISBN 9781927885673 (hardcover) | ISBN 9781927885680 (PDF) Subjects: LCSH: Organic electronics. | LCSH: Electronic polymers. | LCSH: Electronics-Materials. Classification: LCC TK787.K53 2020 | DDC 621.382-dc23

Printed in Australia, United States, and United Kingdom

To my family Wife. Magdolen, Daughters. Shaza and Arije Son. Zain Alabdien

v

Table of Contents 1 1.1 1.2 1.3 1.3.1 1.3.2 1.4 1. 4.1 1. 4.2 1. 4.3 1.4.4 1.5 1.5.1 1.5.2 1.5.3 1.6 1.7 1.8 1.9 1.9.1 1.9.2 1.9.3 1.9.4 1.9.5

INTRODUCTION TO POLYMERS FOR ELECTRONIC ENGINEERS Overview Synthetic electronic polymers Chemistry of electronic polymers Electronic resins Hydrocarbons (nature and electronic applications) Concepts of electronic polymers Bond type of polymer Chain geometry of polymers Characteristics and properties of polymers Polymer morphology Classification of polymer families and types Electronic thermoplastic polymers Electronic thermosetting polymers Electronic elastomers Micro- and nano-electronic polymers Electronic copolymers and copolymerization Electronic oligomers Electronic polymer-based compounds Electronic inorganic polymers Electronic organometallic polymers Electronic complex polymers Electronic small molecules Electronic nanocomposites

1 1 2 5 8 9 10 12 12 13 14 15 21 23 24 24 25 25 26 27 27 27 27 28

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11

ELECTRONICS FOR POLYMER ENGINEERS Electrical conductivity of electronic polymers Electronic polymers “electrical conductivity” theory Electronic polymers “charge transport and charge transfer” theory Electronic polymers “molecular orbital” theory Electronic polymers “valence bond and Lewis structure” theory Electronic polymers “electroluminescent” theory Electronic polymers “piezoelectricity” theory Electronic polymers “electroactivity” theory Fundamentals of microelectronics for polymers Fundamentals of nanoelectronics for polymers Fundamentals of optoelectronics for polymers

33 33 35 37 37 39 40 41 42 43 43 44

vi

3 3.1 3.2 3.2.1 3.2.2 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.4 3.4.1 3.4.2 3.5 3.5.1 3.5.2 3.5.3 3.6 3.6.1 3.6.2 3.6.3 3.7 3.7.1 3.7.2 3.7.3 3.7.4 3.8 3.8.1 3.8.2 3.8.3 3.9 3.9.1 3.9.2 3.10 3.10.1 3.10.2 3.10.3 3.10.4 3.10.5 3.10.6

Table of Contents

OPTIMIZED ELECTRONIC POLYMERS, SMALL MOLECULES, COMPLEXES, AND ELASTOMERS FOR ORGANIC ELECTRONIC SYSTEMS Electronic polymers Electroactive polymers Electronic-electroactive polymers Ionic-electroactive polymers Non-electroactive polymers Chemically-activated polymers Shape memory polymers Electronic inflatable structure polymers Electronic light-activated polymers Magnetically-activated polymers Electronic thermally-activated gels Electronic conductive (conjugated and doped) polymers Electronic extrinsically conductive polymers Electronic intrinsically (inherently) conductive polymers Electronic piezoelectric and pyroelectric polymers Electronic bulk piezoelectric polymers Electronic piezoelectric/polymeric composites Electronic voided charged piezoelectric polymers Microelectronic polymers Microelectronic three-dimensional conjugated macromolecules Microelectronic low-k polymers in microelectronics Organic/inorganic hybrid nanocomposites for microelectronics Nanoelectronic polymers (nanopolymers) Electroactive nanostructured polymers Self-assembled nanostructured polymers Non-self-assembled nanostructured polymers Numbered nanoscale dimension polymers Optoelectronic polymers Optoelectronic light-emitting polymers Optoelectronic light transporting polymers Optoelectronic light receiving (absorbing) polymers Actuation polymers Stretchable electronic polymers Robotic polymers Electronic small molecules Electronic small molecules based on polycyclic aromatics Solution-processable electronic small molecules Electronic small molecule dyes Donor-π-acceptor structure electronic small molecules Optoelectronic small molecules Organic π-conjugated electronic small molecules

49 49 50 51 61 64 65 66 68 69 70 70 71 72 73 83 85 87 88 88 88 94 97 97 98 99 100 101 101 101 133 134 136 136 140 142 142 143 144 145 146 148

vii

3.11 3.11.1 3.11.2 3.11.3 3.12 3.12.1 3.12.2 3.12.3 3.12.4 3.12.5 3.12.6 3.12.7 3.12.8

Organic electronic complexes Polymeric metal complexes Small molecule complexes Heavy-metal complexes Electronic elastomers Electronic liquid crystalline elastomers Ferroelectric elastomers Electrostrictive grafted elastomers Optoelectronic elastomers Electrostatic elastomers Electroviscoelastic elastomers Electromagnetic-interference-shielding elastomers Electronic stretchable elastomers

152 153 154 155 161 162 163 163 164 166 166 167 167

4

OPTIMIZATION OF ELECTRICAL, ELECTRONIC AND OPTICAL PROPERTIES OF ORGANIC ELECTRONIC STRUCTURES Overview Electrical properties Electronic properties HOMO-LUMO energy (band) gaps Electronic excitation energy Absorption wavelength Optical properties Transparency and colorlessness Refractive index Optical absorption Birefringence Optical transmission Polarizability Haze Photoconductivity Optical emission Luminescence

185 185 188 192 192 193 193 194 194 195 196 196 196 197 197 197 198 199

OPTIMIZATION OF POLYMERIC STRUCTURES OF ORGANIC PRINTED CIRCUIT BOARDS Overview Polymers for conventional printed circuit boards Dielectric substrate-based polymeric printed circuit boards Prepreg polymeric printed circuit boards Polymeric single-sided printed circuit boards Polymeric structures of double-sided printed circuit boards Polymeric structures of multilayered printed circuit boards

203 203 204 206 209 213 216 216

4.1 4.2 4.3 4.3.1 4.3.2 4.3.3 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.4.6 4.4.7 4.4.8 4.4.9 4.4.10 5 5.1 5.2 5.2.1 5.2.2 5.2.3 5.2.4 5.2.5

viii

5.3 5.3.1 5.3.2 5.3.3 5.3.4 5.3.5 5.3.6 5.4 5.5 5.5.1 5.5.2 5.5.3 5.6 5.6.1 5.6.2 5.6.3 5.6.4 5.6.5 5.7 6 6.1 6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.3 6.4 6.5 6.5.1 6.5.2 6.5.3 6.5.4 7 7.1 7.2 7.2.1

Table of Contents

Polymeric structures of flexible printed circuit boards Polymeric structures of single-sided flexible printed circuit boards Polymeric structures of double-sided flexible printed circuit boards Polymeric structures of multilayer flexible printed circuit boards Polymeric structures of rigid-flexible printed circuit boards Polymeric structures of dual access (back-bared) flexible printed circuit boards Polymeric structures of polymer thick-film flexible printed circuit boards Polymeric structures of ultra-multilayer printed circuit boards Polymeric structure of three-dimensional printed circuit boards Polymers in molded interconnected devices Combination of molded interconnected device polymers Manufacturing methods of molded interconnected devices Functions of advanced printed circuit boards optimized Printed circuit boards embedded in a polymeric substrate Polymeric microelectronic printed circuit boards Polymeric nanoelectronic printed circuit boards Polymeric optoelectronic printed circuit boards Polymeric structures of smart-textile printed circuit boards Polymeric structures of rapid printed circuit boards (state of the art)

218 220 222 222 223

224 225 226 227 229 231 235 235 236 237 238 241 242

OPTIMIZED POLYMERIC STRUCTURES OF ORGANIC ACTIVE ELECTRONIC COMPONENTS Overview Polymeric structures of organic semiconductors Polymeric structures of organic integrated circuits Polymeric structures of organic transistors Polymeric structures of organic diodes Polymeric structures of organic optoelectronic systems Polymeric structures of organic display technologies Polymeric structures of organic discharge devices Polymeric structures of organic power sources Polymeric structures of organic batteries Polymeric structures of organic fuel cells Polymeric structures of organic thermoelectric generators Polymeric structures for organic piezoelectric pressure

251 251 252 254 257 263 290 314 316 317 317 320 323 326

POLYMERIC STRUCTURES OPTIMIZED FOR ORGANIC PASSIVE ELECTRONIC COMPONENTS Overview Organic film resistors Thin film resistors

341 341 341 341

224

ix

7.2.2 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.4 7.4.1 7.4.2 7.5 7.6 7.6.1 7.6.2 7.6.3 7.7 7.7.1 7.7.2 7.7.3 7.7.4 7.8 7.9 7.9.1 7.9.2 7.9.3 7.9.4 7.9.5 7.9.6 7.9.7

Thick film resistors Organic capacitors Organic film capacitors Aluminum polymer capacitors Tantalum polymer capacitors Functional polymer capacitor Organic magnetic systems Magnetic polymers Organic/polymeric magnets Organic networks Organic transducers Piezoelectric polymer transducers Ionic polymer transducers Elastomeric transducers Organic sensors Organic gas sensors Organic optical sensors Organic fiber optic-sensors Organic, flexible sensors Organic antennas Organic actuators All-organic/polymeric actuators Conducting polymer actuators Ionomeric polymer-metal composite actuators Piezoelectric polymer actuators Flexible elastomeric actuators Conjugated polymer actuators Polymeric microactuators

342 342 343 345 345 346 346 347 348 349 352 352 353 354 354 355 356 358 359 360 362 363 364 365 366 367 370 370

8 8.1 8.2 8.2.1 8.2.2 8.2.3 8.2.4 8.2.5 8.3 8.4 8.5 8.5.1 8.5.2 8.5.3

OPTIMIZING POLYMERIC STRUCTURES IN ORGANIC OPTOELECTRONICS Overview Optical polymers Optical electroactive conjugated polymers Transparent (photonic) polymers Optical organic photovoltaic polymers Electroluminescent polymers Electro-phosphorescent polymers Properties of optical polymers Physical properties of optical polymers Organic optoelectronic systems Optical polymers for forming organic optoelectronic emitters Optical polymers for organic electroluminescent systems Organic photonics

393 393 393 394 396 399 402 420 437 441 444 446 454 456

x

Table of Contents

8.5.4 8.5.5 8.5.6 8.5.7

Organic optical amplifiers Organic optical detectors and receivers Organic optoelectronic thin-films Organic electro-optic modulators

9

OPTIMIZING POLYMERIC STRUCTURES OF ORGANIC ELECTRONIC PACKAGES Overview Polymers in organic electronic packaging Polymeric structures of packaging systems Polymeric dual in-line package Polymeric single in-line package Polymeric zig-zag in-line package Structures of organic microelectronic packaging Practical concept of organic microelectronic packaging Organic microelectronic packages Electrically and thermally conductive polymer adhesives Organic microelectromechanical packaging Polymeric thin-film multilayer packaging Microelectromechanical packaging Vacuum and air cavity packaged organic microelectromechanical systems Organic encapsulation gels Organic near-hermetic (quasi-hermetic) materials Organic nanoelectronic packaging Polymeric system-on a-chip (or nanochip) Polymeric nanoscaled systems Nanoelectronic circuit packaging (nanopackaging) Organic nanoelectromechanical packaging Organic optoelectronic packaging Polymeric optoelectronic waveguides Organic optocoupler (optoisolator) packaging Organic microoptoelectromechanical systems packaging Polymeric packages Polymeric adhesive packages

9.1 9.2 9.3 9.3.1 9.3.2 9.3.3 9.4 9.4.1 9.4.2 9.5 9.6 9.6.1 9.6.2 9.6.3 9.6.4 9.6.5 9.7 9.7.1 9.7.2 9.7.3 9.7.4 9.8 9.8.1 9.8.2 9.8.3 9.9 9.10

INDEX

461 462 465 467

485 485 486 490 490 491 492 492 492 493 495 496 497 499 499 503 504 506 506 507 513 514 514 516 518 519 525 528 543

Acknowledgments My thanks go to my family, whose encouragements made me to do always the best. Special thanks go to the staff of the “Manufacturing systems engineering department” at the University of Warwick in UK, especially the supervisors of Automation and Robotic, Manufacturing Strategy, Polymers, Material Science, and CAD-CAM classes.

1

Introduction to Polymers for Electronic Engineers 1.1 OVERVIEW Conventional materials such as metals and ceramics have been replaced by polymers (homopolymers, copolymers, composites, complexes, blends of small molecules, and alloys), in electronic, microelectronic, and nanoelectronic systems because of their complex molecular characteristics and attractive electrical, electronic, mechanical, physical, chemical, and optical properties.1 Polymers help to achieve low cost, short lead time, reduced weight, and they can be molded into very complex shapes, having properties that can easily satisfy the requirements. The above features are crucial in microelectronics and nanoelectronics based on three-dimensional printed circuit boards 3D-PCBs; molded integrated devices MID; flexible (bendable and stretchable) polymeric substrates; integrated electronic systems (such as integrated circuits ICs); and electronic components working at high temperatures. The further examples are the developed generations of microelectromechanical systems MEMS and nanoelectromechanical systems NEMS which started with utilization of silicone Q polymers in integrated circuits.2 Note: Silicone polymer (abbreviated as Q) should not be confused with the silicon chemical element (abbreviated as Si). Generally, polymers are compounds of organic nature consisted of different combinations of carbon, oxygen, hydrogen, nitrogen, in addition to other elements. Polymers are available in the form of solid, liquid, filament, or powder states. According to their chemical nature, polymers can be grouped into families, subfamilies, and members. Polymers can be formed (molded) through the application of both heat and pressure. Electric conductivity (σ) of some polymers represents the key feature of structuring electronic, microelectronic, nanoelectronic, and optoelectronic systems. Organic electronics (also called organic electronic components or polymeric electronic components) term was the starting point of forming electronic polymers. The electric conductivity of both inorganic and organic compounds can be measured in Siemens/cm (or S/cm). σ < 10-7 for insulators, 10-7 < σ 102 for metals, and σ >> 1020 for superconductors).46 Electronic polymers (also called organic electronics such as electrically active polymers EAPs having inherent electrical conductivity) cover a considerable field of material science that focuses on the development of electrically conducting polymers, small molecules, and complexes designed for development of novel electronic, microelectronic, and nanoelectronic systems (including displays, organic light-emitting diodes OLEDs, organic

2

Introduction to Polymers for Electronic Engineers

electroluminescent devices, organic thin-film transistors OTFTs, nanorobots, organic solar cells/photovoltaics, sensors, and actuators).47 Examples of electronic polymers include polyacetylene PAC having electric conductivity σ = 104-105 S/cm; polyaniline PANI (σ = 102-103 S/cm); poly(p-phenylene vinylene) PPV (σ = 103-104 S/cm); and polythiophene PT (σ = 103-104 S/cm).3 Generally, the electrical conductivity of such polymers results from a process called “doping” (oxidation or reduction).46 The main classes of electronic polymers include electroactive polymers EAPs (also called intelligent polymers), shape-memory polymers/alloys, ferroelectric polymers, and piezoelectric polymers/materials. They are characterized by the capability of change in dimension or shape upon external stimuli such as electric field or light.3,6 For example, the ferroelectric polymer, called poly(vinylidene fluoride) PVDF, is a member of electronic polymers applied widely in actuators and sensors of robots (included microrobots and nanorobots). Micro and nanomachining technologies such as microinjection molding are the micro- and nano-tools used for structuring polymeric micro and nanoelectronic systems.4 For example, some of the new microelectronics being molded recently include hearing aids, sensors, biomedical components, and fiber-optic components. Some companies can make micro- or nano-parts weighing 0.0002 grams with tolerances of 0.004 mm and tighter. So, what are the electronic polymers?

1.2 SYNTHETIC ELECTRONIC POLYMERS Polymers (commercially called plastics or plastic materials) are compounds of organic nature (often produced from crude oil) with high molecular weight, which can be shaped by the application of heat and pressure, such as the applied heat and pressure of an injection molding process. Chemically, they are built up of monomers as repeating units of shorter carbon-containing compounds having attractive properties such as low density, low electrical conductivity, and toughness. According to their chemical nature, polymers can be included in one of the three important groups (families):3,5 • Thermoplastic polymers TPs (also called thermoplastics or thermoplastic families), which can be repeatedly melted by heating and solidified by cooling, such as polyethylene PE, polypropylene PP, polystyrene PS, acrylonitrile-butadienestyrene ABS, etc. • Thermosetting polymers TSs (also called thermosets or thermosetting families) having chemical structures changed during molding processes (heat and pressure) to become permanently solidified, such as silicones, polyurethanes PUR, epoxy EP, and phenol-formaldehyde PF. • Elastomers ELs (rubbers families) which can be considered as thermoplastics but with thermosetting properties. They are available in the form of thermoplastic elastomers and thermosetting elastomers. The chemical names of some polymers can be expressed by their abbreviations or trade names; such as polyamide polymer abbreviated as PA and commercially known as Nylon®,93 or polymethylmethacrylate abbreviated as PMMA and commercially known (often) as Plexiglas®.94 As illustrated in Figure 1.1,6 crude oil is the main source of syn-

1.2 Synthetic electronic polymers

3

thetic polymers (plastic materials). The polymerization methods represent the chemical processes by which polymers are synthesized. For example, polystyrene indicates polystyrene polymer, unsaturated polyester UPR, poly(vinyl chloride) PVC, polyethylene PE, and poly(ethylene terephthalate) PET.

Figure 1.1. A representation of some polymers derived from raw materials such as crude oil. [Adapted, by permission, from Anthony L. Andrady, Plastics and the Environment, © 2003 John Wiley & Sons, Inc.]

Polyethylene can be given as an example of polymer synthesis (it is the most common thermoplastic polymer). It can be synthesized by polymerization process called “Ziegler-Natta” polymerization of ethylene (derived from crude oil). This process can be achieved by introducing ethylene gas, specific solvents, and the “Ziegler-Natta” catalyst (under pressure) into a reaction vessel. After several chemical processes, the product comes in the form of polyethylene powder. Most polymers can be blended with other organic or inorganic compounds called additives or modifiers (in molten, liquid, or powder states) in order to improve their properties such as moldability.7 For example, pure polyethylene polymers cannot be molded unless compounded with modifiers or additives (20%-50%), especially for certain electronic applications, for enhancing their features such as mechanical, thermal, optical, and physical properties. Examples of commercial additives are listed in Table 1.1.6,70,71 Note, how important is the function of such modifiers. For example, compounding some types of polymers with chalks reduces their costs, and prevents the probability of color change; antioxidants minimize oxidation rate of a polymer at service temperature. The properties of polymers can be optimized by the use of additives.6,71

4

Introduction to Polymers for Electronic Engineers

Table 1.1. Effect of additives on polymer properties.6,70,71 Additive

Effect on performance

Plasticizer (e.g., phthalates, adipates, Softening polymers and enhancing their molding prosebacates, etc.) cesses, especially in plasticized (soft) poly(vinyl chloride) applications. Heat stabilizers (e.g., Zn stabilizer)

Low fogging in pure and clarity applications such as polymethylmethacrylate displays; also odor-free applications.

Antioxidants (e.g., peroxide decom- Reduce rates of oxidation, improve service temperatures, posers) slow degradation in outdoor applications. Antistatics (e.g., carbon black, steel Reduce retention of electrostatic charges and accidental fibers, etc.) fires and explosions Fillers (e.g., calcium carbonate)

Reduce costs and prevent color changes

Impact modifiers (e.g., rubbers)

Improved impact resistance, especially for heavy-duty applications.

Curing agents (e.g., isocyanates, and Crosslink polyurethanes, give rubbery properties to silimany other) cones and many other thermosetting resins Antiblocking additives (e.g., silica or Reduce adhesion between films and other materials in conerucamide) tact.

A simple process of compounding polymers is illustrated in Figure 1.2.14,73,77,97,99,100 This process consists of the following four stages8,45,49 1. Extrusion stage; in which polymers are mixed with additives, modifiers, and fillers and melted together in the twin-screw of a plastic injection molding machine; and extruded through a tool called an extruder die in the form of soft strands. 2. Cooling stage in which the soft strands are quenched continuously in a water bath to solid state. 3. Pelletizing stage in which the solidified strands are cut into a form of small cylindrical pellets (granulates). 4. Drying and packaging of pellets to prepare raw material for molding plastic products. Recent techniques of synthesis and compounding of polymers were focused on the development of electrically conductive polymers to overcome the problems of conventional materials used for the production of electronic systems, such as high costs, long lead time, and difficulties in processing and assembly. The electrical insulating polymers can be converted into conductive materials by compounding them with conductive fillers such as for example, silver. In other developments, the intrinsically conducting polymers were introduced as semi-conductive materials acting as polymeric or organic semiconductors used in polymeric/organic electronic systems, because their chemical structures can conduct electricity without the need for silver powders. These polymers can be called as electrical or electronic polymers. Important examples of developed electronic systems based on electronic polymers include flexible polymeric/plastic screens of computers, televisions, transducers, wearable electronics, and mobile phones.50 According to their process of synthesis (polymerization process), electronic polymers can be classified as ionically conductive polymers such as poly(ethylene oxide) con-

1.3 Chemistry of electronic polymers

5

taining lithium perchlorate used for forming the solid-state electrolyte of organic/ polymeric batteries, and electronically conductive polymers such as polyacetylenes used in polymer light-emitting diodes.

Figure 1.2. The principles of polymer compounding.14,73,77,97,99,100

1.3 CHEMISTRY OF ELECTRONIC POLYMERS Based on their chemical composition, polymers can be divided into two types: 1. Carbon chain-based polymers containing only aliphatic (linear) carbon atoms in their backbone chain, such as polypropylene PP known as commodity plastics. 2. Hetero chain-based polymers containing different atoms (such as oxygen, nitrogen, or sulfur) in their backbone chains, in addition to carbon such as polycarbonate PC classified as engineering plastics. Generally, the values of both density and tensile strength of carbon chain-based polymer are lower than those of hetero chain-based polymers, while both of them are useful in electronic systems. Examples of carbon chain-based polymers and hetero chain-base polymers are listed in tables (Tables 1.2 and 1.3).12,74,75,76 Note: Electronic carbon chain-based polymers such as acrylonitrile-butadiene-styrene ABS, rigid poly(vinyl chloride) R-PVC, polymethylmethacrylate PMMA, and polytetrafluoroethylene PTFE; can be used as photoresist polymers Ph-P, fused deposition modeling FDM (prototyping) wires, laser cutting (prototyping) polymers Ls-RPT, laser direct structuring LDS, laminated object manufacturing (prototyping) polymers LOM, novolac polymers (photoresist polymer of integrated circuits) NOV, and computerized numerical control CNC prototyping polymers Ls-RPT. They can be used for the following electronic applications: molded integrated devices MID, electrostatic discharge devices EDS, ultrasonic welding USW, laser cutting (proto-

6

Introduction to Polymers for Electronic Engineers

typing) LSW, printed circuit boards PCBs, integrated circuits ICs, ultra-multilayer printed circuit boards Um-PCBs. On the other hand, electronic hetero chain-based polymers such as poly(ethylene terephthalate) PET, polycarbonate PC, polyoxymethylene (acetal) POM, polyetheretherketone PEEK, poly(phenylene sulfide) PPS, cellulose acetate CA, polyamide-6 PA6, polyesters (thermoset) PESt, epoxy EP, phenol-formaldehyde PF, urea-formaldehyde UF, melamine-formaldehyde MF, and polyurethane (thermoset) PUR can be used for the same applications as electronic carbon chain-based polymers in addition to single-sided flexible printed circuit boards SSF-PCB, multilayered flexible printed circuit boards FPC, and reaction injection molding RIM.12,74,75,76 Table 1.2. Examples of electronics applications of selected carbon-chain based polymers.12,74,75,76 Polymers Polyacetylene PAC Crosslinked methacrylate)

Electronics and optical applications Applied as the important conducting polymer used in organic electronic systems such as light-emitting diodes.

poly(2-hydroxyethyl Used in optical systems such as contact lenses.

Acrylonitrile-butadiene-styrene ABS Can be used as a photoresist Ph-P polymer or laser direct structuring LDS polymer. It is essential for the production of molded integrated devices MID. Rigid poly(vinyl chloride) R-PVC

Can be used as a photoresist polymer Ph-P.

Polymethylmethacrylate PMMA of Applied where toughness, stability, and purity are required, glass transition temperature 3-35oC such as the core of optical liners and the second layer of integrated circuit packaging for stress relief. Polytetrafluoroethylene PTFE of Applied when chemical resistance is required in addition to glass transition temperature -97 to high-temperature stability. High styrene content type is 126oC used for abrasion-resistant applications Ethylene-propylene diene terpoly- As an elastomer, it is applied for structuring organic elecmer EPDM tronic systems where outstanding oxygen, ultraviolet (UV), weather, fatigue, and moisture resistance, and good electrical properties are required. Styrene-butadiene rubber SBR

Used where ductility and impact resistance are required.

Table 1.3. Examples of electronics applications of selected hetero-chain polymers.12,74,75,76 Polymer

Electronics and optical applications

Poly(ethylene terephthalate) PET of Applied in electronic systems and circuits where thin glass transition temperature 69oC films, transparent layers, and flexible substrates are needed. The commercial types of this polymer, such as Mylar®76 films (Mylar®92 EL/C) can be used for manufacturing the mask of lithography process of integrated circuits. Polycarbonate PC of glass transition Applied where flame and heat resistant and tough and transparent properties are needed for optical storage temperature 150oC devices such as CDs, DVDs, and HD-DVDs. It can be used as a photoresist polymer Ph-P.

1.3 Chemistry of electronic polymers

7

Table 1.3. Examples of electronics applications of selected hetero-chain polymers.12,74,75,76 Polymer Silicone PDMS

Q

Electronics and optical applications

polydimethylsiloxane Applied where heat resistance and flexible electrical insulating and packaging properties are required.

Polyoxymethylene (acetal) POM of Can be considered as an actuation polymer used in organic glass transition temperature (-85oC) robotic systems and fabricating gear transmission modules. Polyetheretherketone PEEK of high Considered as a high-performance polymer, especially service temperature (melting tem- when high-temperature resistance is needed. perature 334oC) Poly(phenylene sulfide) PPS of glass Applied as laser-direct structuring LDS polymer. It is used transition temperature 85oC widely for fabricating fused deposition-modeling FDM wires. Known as an electrically conductive polymer and can be used as a transistors packaging polymer. Cellulose acetate CAC

Applied in the form of fibers when high resistance to shrinkage, stretching, and wrinkling properties is important. It can be used in vibration and ultrasonic welding applications.

Polyamide-6 PA6 of high service Applied when high-temperature stability and high strength temperature (melting temperature are required. 215oC) Polyesters (thermoset) PESt

Applied when flammability is expected.

Epoxy EP

The most essential polymer used in the form of laminates for printed circuit boards such as ultra-multilayered printed circuit boards.

Polyurethane (thermoset) PUR

Applied as the second layer of integrated circuits packaging for stress relief purposes.

The chemical structure of a polymer can be expressed by its chemical formula that represents the proportions of the combined atoms. Nomenclatures of most polymers and organic compounds listed in this book are based on the systematic method of the International Union of Pure and Applied Chemistry IUPAC. The function of IUPAC rules is to help chemical scientists to name single-strand organic polymers S-SOP in a systematic manner based on polymer structure.9 Examples of polymers chemical formulas, include (C2H4)n for polyethylene PE [IUPAC name: poly(methylene)] and (C2F4)n for polytetrafluoroethylene PTFE [IUPAC name: poly(1,1,2,2-tetrafluoroethylene)]. An example of writing the IUPAC name of chemical compound or polymer is the IUPAC name of the aliphatic polymer called [IUPAC: 2-methyl-1-propanol]. The nomenclature of a compound should be built from four parts: prefix, word root, primary suffix, and secondary suffix as shown in Figure 1.3.72 (for IUPAC name- 2-methyl-1-propanol). In reference to Figure 1.3, a “prefix” denotes the substituent group, such as (−Cl). The “root” term indicates the number of carbon atoms in the longest possible chain such as (1) as a number of carbon atoms, and (meth-) as a word root. The “primary suffix” term denotes the nature of carbon in the polymer/organic compound, such as (“Ane”, -bond).

8

Introduction to Polymers for Electronic Engineers

Figure 1.3. An example of writing IUPAC name of an aliphatic compound. [Data from KEA, Organic chemistry: Nomenclature of organic compounds, (Bridge course) 2012, www.kea.kar.nic.in.]

“Secondary suffix” term represents the functional group in an organic molecule, and it is attached to the primary suffix (such as the secondary suffix -”ol” that represents the functional group “−OH” of alcohol compound. The chemical formula of a polymer should not be confused with either empirical formula or molecular formula; where, a chemical formula specifies a structure of the simplest of molecules and chemical substances. An example of the chemical formula is “butane” (also called butylene); with the empirical formula (C2H5), the molecular formula (C4H10), and the condensed (or semi-structural) formula (CH3CH2CH2CH3).10,72 The properties of electronic polymers depend mainly on their chemical structure, so that their electrical properties can be achieved by conjugation process by which the molecules of polymers gain alternating double and single bonds, which in turn provides the pathway for free-electron charge carriers. The resulting polymers can be classified as conjugated polymers such as polyacetylene, polythiophene, polyaniline, and polypyrrole. Creating the electrical properties of conjugated polymers depends on a chemical process called “doping” by which a polymer is treated either with oxidizing agents (removal of electrons) or reduction agents (addition of electrons). The chemical structure of conjugated polymers or electronic polymers can be optimized, so that, several grades of polymers can be derived (such as ring and nitrogen substituted derivatives of polyaniline).51 1.3.1 ELECTRONIC RESINS The “resin” term can be considered as an alternative term of plastic or polymer, but specifically, it denotes the state of natural/synthesized organic compounds formed from noncrystalline or viscous liquid substances. Mostly, this term relates to polymers used in the form of liquid such as epoxy and silicone resins used for packaging and encapsulating electronic systems. Such resins can be optimized by adding the so-called “additives” such as promoters, plasticizers, curing agents, etc. These additives are used to enhance the functionality of these liquid polymers to improve their resistance to water absorption, UV resistance, and to avoid the negative influence of cracking stresses.11 The main physical characterizations of resins are solubility in alcohol, fusibility and flammability, transparency (translucent), and yellowish to brown color. Most of the natural resins have been replaced by synthetic resins (thermoplastics, thermosets, and elastomers) produced by

1.3 Chemistry of electronic polymers

9

several types of polymerization processes from molecules of organic nature named “monomer.” Where, a monomer can be converted into “dimer,” “trimer,” “tetramer,” “polymer,” etc.9 An example of natural resins is GR (also called rosin or gum rosin) derived from turpentine of pine trees;8,9 and an example of converting a monomer to a polymer, is the conversion of cyclopentadiene CPD monomer into either polycyclopentadiene PCPD9,10 polymer by addition polymerization or to polypentenamer PPTM [IUPAC: poly(pent-1ene-1,5-diyl)] by ring-opening metathesis polymerization.9,11 Both of these polymers are used in optoelectronic applications. According to their nature (natural or synthesized), polymers can be classified as homopolymers (formed from just one type of a repeat unit) and copolymers (formed from two or more different types of monomers); as illustrated in Figure 1.4.48,78 Note: Monomers of type (A) can be combined together to form homopolymers of either straight chains or branched chains; while, some monomers of type (B) can be combined together to form copolymers with either branched chains or straight chains as well. Such combinations result in different properties of the final polymers (depending on the arrangement of their monomers either straight polymers or branched polymers).48

Figure 1.4. The simple principle of homopolymers and copolymers formation.48,78

An example of homopolymers is polystyrene [IUPAC name: poly(1-phenylethylene)] with the formula (C8H8)n because it combines only styrene monomer; and an example of the copolymer is ethylene-vinyl acetate EVA with the formula [(C2H4)n(C4H6O2)m], because it combines different monomers. 1.3.2 HYDROCARBONS (NATURE AND ELECTRONIC APPLICATIONS) Chemically, hydrocarbons are compounds of organic nature formed as networks of only carbon C atoms (joined together) and hydrogen H atoms attached to them in several types of configurations such as the tetrahedral network of methane CH4 [IUPAC name]. In relation to their sources and properties, hydrocarbons can be divided into two types: aliphatic hydrocarbons (also called unsaturated aliphatic hydrocarbons UAHC), and aromatic (also

10

Introduction to Polymers for Electronic Engineers

called aromatic hydrocarbons ARHC). Aliphatic hydrocarbons are hydrocarbons of petroleum origin without benzene rings, and available in the form of three groups (derived according to the types of bonds), such as alkanes, alkenes, and alkynes. Alkane (also called paraffin) with the general formula (CnH2n+2) is a saturated hydrocarbon with just one bond, such as in [IUPAC name: methane] with the formula (CH4). An alkene (also called olefin) is an aliphatic hydrocarbon with a carbon-carbon double bond, such as ethylene [IUPAC name: ethene] with the formula (C2H4).9,12,13 An alkyne (also called acetylene) with the general formula (CnH2n-2) is the unsaturated aliphatic hydrocarbon with a carbon-carbon triple bond, such as ethyne [the IUPAC name of acetylene] with the formula (C2H2).12,13 Thousands of electronic polymers are of hydrocarbon nature such as polynorbornene rubber PNR known as purely hydrocarbon polymer with high thermal stability and soluble in common organic solvents. Triphenylene T-Ph polymer is another example of an electronic polymer of hydrocarbon origin. It is classified as a flat polycyclic aromatic hydrocarbon and used in optical and electronic applications due to its capability of emitting bluish-purple fluorescence under UV irradiation. Dibenzo{[f,f'']-4,4',7,7'-tetraphenyl}diindeno[1,2,3-cd:1',2',3'-lm]perylene} DBfP is the most important electronic polymer of hydrocarbon nature (as a fluorescence small molecule of polyaromatic hydrocarbon polymer) and used for structuring electrochemistry and electrogenerated chemiluminescence due its capability of exhibiting absorption at 333 nm (in tetrahydrofuran solvent), photoluminescence at 610 nm (in tetrahydrofuran solvent), and orange-red light with efficiency of 2%. The highly conjugated small molecule pentacene PNC is considered as an organic semiconductor because it has linearly-fused benzene rings. In the form of thin films, it is used for structuring low-voltage organic thinfilm transistors, organic light-emitting diodes, and pentacene field-effect transistors.52

1.4 CONCEPTS OF ELECTRONIC POLYMERS Polymer term [IUPAC name: macromolecule] consisted of two words “poly” (meaning: many) and “mer” (meaning: part), is a large molecule consisted of one or more repeating units called “mers” linked together by the covalent chemical bonds. For example, a monomer that forms polyethylene PE polymer can be represented by equation [1.1]9 nCH2=CH2 (under temperature and pressure) → (CH2-CH2)n

[1.1]

where: CH2=CH2 (CH2-CH2)n n

represents monomer (ethylene gas). represents polyethylene polymer. number of monomers reacting >> 1.

Trivial system can be considered as the simple standard of naming polymers; where, polymers can be named by introducing the name of a specific monomer onto the prefix “poly” without a space or hyphen.9 Polymer can be a very large molecule “macromolecule”53 with repeated units of the same type; its structure looks like a chain made of several links, such as that illustrated in Figure 1.5.101-103 Note: Each link of this chain is considered as a basic unit (mer) of C, H, O, and/or Si. Many basic units (mers: can be called monomers) should be repeated in a regular pattern and hooked together (polymer-

1.4 Concepts of electronic polymers

11

ized) to make a long chain (polymer). As a result, a monomer consists of one mer; while, a polymer consists of more than 1000 mers along its chain.16,54 In this case, the “degree of polymerization term” means how many monomeric units are linked to form the polymer chain. Molecular weight is a term associated with polymerization processes for describing the mass of a molecule. Polymeric compounds are of synthetic types formed by a series of repeated polymerization steps.15,17 Alloy and blend terms are used to indicate polymeric compounds formed by the simple mixing of two or more polymers to get a new compound of improved properties.

Figure 1.5. Representation of polymer chain.101-103

Examples of alloys (blends) include poly(phenylene oxide)/high-impact polystyrene, polycarbonate/acrylonitrile-butadiene-styrene, and acrylonitrile-butadiene-styrene/ poly(vinyl chloride); and examples of commonly used synthetic polymers include polyethylene and polypropylene which can be chemically optimized by introducing ethylene and propylene monomers one by one to the growing chain. The commonly used polymers are of organic nature and called electronic polymers or organic polymers because their backbone chains are built of carbon atoms; there are polymers called inorganic polymers because their structures have no carbon atoms. Electronic polymers containing inorganic and organic components are named electronic hybrid polymers.18 According to their mechanisms of polymerization, polymers can be classified as addition electronic polymers structured of a chain reaction (addition polymerization or free-radical polymerization) such as polyethylene and polystyrene polymers; and condensation polymers (backbones include non-carbon atoms) structured chemically by a condensation reaction such as alkyd, phenol-formaldehyde, and phenol resins.9,13 “Polymerization” indicates the chemical reaction by which monomer molecules (such as, for example, ethylene groups) are reacted together to form a polymer chain or three-dimensional network. The main two types of polymerization processes are addition polymerization (also called vinyl polymerization or chain-growth polymerization) in which unsaturated monomers are reacted by a chain mechanism involving active sites on the growing chain; and condensation polymerization by which two or more molecules are combined with the separation of water or other simple substance. Three types of polymerization processes are related to addition polymerization: 1. Free-radical polymerization by which the propagating species is a long-chain free radical initiated by the introducing free radicals; 2. Ziegler-Natta polymerization by which polymers of high linearity structures are produced under low pressure using catalysts (Ziegler catalysts); 3. Metallocene catalysis polymerization by which olefins are polymerized at high activities.9,13

12

Introduction to Polymers for Electronic Engineers

An example of macromolecule is the ethylene group consisted of two hydrogen atoms H2 and one carbon atom C. Such a molecule may consist of 2,500 methylene groups. Examples of addition polymers can be produced by addition (vinyl) polymerization include polyethylene, polypropylene, poly(vinyl chloride), polystyrene, polymethylmethacrylate, polytetrafluoroethylene, polyacrylonitrile, poly(vinyl acetate), and poly(vinyl alcohol). Examples of polymers formed by free radical polymerization include polystyrene, polymethylmethacrylate, poly(vinyl acetate), and branched polyethylene (low-density polyethylene). Examples of polymers formed by condensation polymerization are polyamide, alkyd resins, and polyurethanes.13 Several types of chemical substances are added (either during polymerization or by physical mixing) to enhance their structures or improve their properties (such as catalysts, accelerators, promoters, inhibitors, plasticizers, etc.). Catalysts are used for changing the rate of a chemical reaction. Accelerators are used for increasing reaction rates. Promoters are used to accelerate the reactions of catalysis and improve their performance. Inhibitors are used to reduce the efficiency of a catalyst. Plasticizers are used for making polymers more flexible and to lower viscosity of reacting mass. Examples of inhibitors include quinones [IUPAC name: p-benzoquinone] with the formula (C6H4O2); hydroquinones [IUPAC name: benzene-1,4-diol] with the formula (C6H6O2); aromatic nitro compounds, aromatic amines. Some inhibitors such as tetrachlorobenzoquinone (also called 2,3,5,6-tetrachloro-1,4-benzenediole) act as inhibitors just by addition to the growing radical chain. Polymers formed by conventional polymerization processes (such as condensation, free radical, addition polymerization processes), are insulators (of electrical conductivity >1020 S/cm). Electronic polymers require doping (oxidation or reduction) for conductivity purposes such as polyacetylene of σ = 104-105 S/cm, polyaniline of σ = 102-103 S/cm, poly(p-phenylene vinylene) of σ = 103-104 S/cm, and polythiophene of σ = 103-104 S/cm. Dopant materials used for doping processes include (I2, Br2, Li, Na, AsF5, HCl, FeCl3, BF3, and R-SO3H).55,68 1. 4.1 BOND TYPE OF POLYMER The polymer bonds represent the attraction between its molecules, such as intermolecular bonds (also called van der Waals bonds or secondary forces), which represent the attractions between just one molecule and a neighboring molecule; and intramolecular bonds (also called primary forces or covalent bonds) which represent the forces of attraction that hold an individual molecule together. 1. 4.2 CHAIN GEOMETRY OF POLYMERS Polymers have three types of chain geometries: linear; branched; and crosslinked (also called 3-D network); as illustrated by Figure 1.6.9,34,104-105 Shapes or geometries of the molecules (chains) play a key role in determining the properties of polymers. Polymers of linear geometries (linear polymer such as linear polyamide 6/6) have simple chains. Polymers of branched geometries (branched polymers) have many side chains. Polymers of

1.4 Concepts of electronic polymers

13

crosslinked geometries (crosslinked or 3-D network polymers) have chains connected by bonding. The function of such groups is to joint two polymer chains or two parts of the same chain. The classification of polymers into thermoplastics, thermosets, and elastomers; depends on chain geometry of polymers. For example, linear polymers with minor branched structures can be classified as thermoplastics due to flexibility of their chains (such as polyethylene); while crosslinked polymers can be classified as thermosets because the crosslinking state makes polymers stronger and not recyclable such as phenolformaldehyde.9,13,19 Examples of linear polymers include poly(vinyl chloride), polyethylene, and polyamide because they have repeat units joined end to end in a single chain; while, the reaction of phenol with formaldehyde [IUPAC name: methanol or AKA formaldehyde] with the formula CH2O, represents the best example of forming a network (from a tri-functional molecules).19

Figure 1.6. The main geometries of molecules (chains).9,34,104-105

1. 4.3 CHARACTERISTICS AND PROPERTIES OF POLYMERS As illustrated in Figure 1.7,9,20,34 the physical classification of polymers into thermoplastic, thermosets, and elastomers depends mainly on the relationship between stress and strain properties. On the other hand, such classification can be described by another property such as temperature. For example, polymers become more brittle at low temperatures (but they can be optimized by blending with elastomers). Figure 1.7, line (A) describes thermosets or brittle polymers of brittle fracture. Line (B) describes thermoplastics or plastic polymers of ductile fracture. Line (C) describes the behavior of elastomeric polymers. For optimizing the structures of electronic, optoelectronic, microelectronic, and nanoelectronic systems, electric/electronic properties should be strongly considered, such as dielectric constant,79,80 dissipation factors, static electrification, electrical conductivity, electric discharge, dielectric breakdown, and arc resistance. Examples of polymer properties include:20 1. Physical properties (such as specific gravity and water absorption). 2. Mechanical properties (such as tensile strength, flexural strength, tensile modulus, flexural modulus, compressive strength, impact strength, hardness, and coefficient of static friction). 3. Thermal properties (such as thermal conductivity, coefficient of thermal expansion, specific heat, and maximum continuous temperature). 4. Electrical properties (such as volume resistivity, dielectric strength,90 dielectric constant,80 dissipation factor,80 and arc resistance).

14

Introduction to Polymers for Electronic Engineers

5.

Chemical/physical properties (such as solubility, permeability, environmental stress cracking and crazing, chemical attack, photooxidation, aging and weathering, ignition, and combustion). 6. Optical properties (such as refractive index, spectral transmission, optical dispersion, optical loss, and birefringence). 7. Processing properties (such as injection pressure, mold temperature, melt flow, injection cylinder temperatures, and molding shrinkage).21 Just for more information, examples of additional polymer characteristics include the presence of chain molecules, the chemical nature of the monomeric units, and the arrangement of the polymeric macromolecules.

Figure 1.7. Classification of polymers depending on physical properties.9,20,34

1.4.4 POLYMER MORPHOLOGY Polymer morphology is a physical phenomenon that focuses on studying the structures and relationships of polymers. Importance of this phenomenon is the capability of describing the arrangement of molecules on a large scale. Such an arrangement can be classified either amorphous, crystalline, or semi-crystalline.21,57 Often, polymers have a semi-crystalline arrangement. This arrangement consists of small crystalline portions (domains) (crystallites) surrounded by domains of amorphous phase as shown in Figure 1.8.9,20,34,104 Examples of crystalline polymers include acetal, poly(ethylene terephthalate), polyamide, polytetrafluoroethylene, polyethylene, and polypropylene; while, examples of amorphous polymers include acrylonitrile-butadiene-styrene, polymethylmethacrylate, polycarbonate, poly(phenylene oxide), polystyrene, and poly(vinyl chloride).

Figure 1.8. A representation of both amorphous and crystalline domains in polymers.9,20,34,104

1.5 Classification of polymer families and types

15

1.5 CLASSIFICATION OF POLYMER FAMILIES AND TYPES According to ASTM- D4000 (ISO 1043), polymers can be classified by “Modulus-based classification, properties-based classification, performance-based classification, and families-based classifications (widely adopted).” Modulus-based classification means Young's modulus, elastic modulus, or tensile modulus-based classification; where a polymeric sample is deformed when a force is applied (both stressed and strained). The function of stress S [N/m2] property is to describe the “intensity at point in a product of the forces that act on a given plane through the point”; while, the function of strain T [mm/mm] property is to describe the change in the length of the polymeric sample dL (elongation or compression) over its original length L [m]. Hooke's law describes the relationship between stress S and strain T.22,23 Stress S [N/m2] is expressed by the load F [N] applied per unit area A [m2]m as described by equations [1.2-1.4].22,56,107 S = F/A

[1.2]

Strain T [mm/mm] is expressed by the change in the length of the polymeric sample dL over its original length L [m] T = dL/L

[1.3]

In Hooke's law, the strain is proportional to stress, and the modulus of elasticity E or Young's modulus is the constant of proportionality. E = S/T

[1.4]

Rigid, semi-rigid, and non-rigid polymers represent the three important groups in modulus-based classification. Rigid polymers having Young's modulus greater than 3.0 GPa are used for optimizing printed circuit boards PCBs substrates and antenna dishes. Semi-rigid polymers having Young's modulus of 0.35 GPa (at 23oC and 50% RH) are used to optimize engineering products for automotive and biomedical applications. Non-rigid (flexible) polymers having Young's modulus lower than 0.1 MPa are used in gaskets for electronic devices.23,24 Modulus properties of selected polymers used for different electronic systems are listed in Table 1.4.20,25,81 Table 1.4. Modulus properties of selected polymers used for optimizing the structures of electronic systems.20,25,81 Modulus-based classification Rigid (brittle) polymer

Semi-rigid (ductile) polymer Flexible polymers

Polymers Polystyrene PS Polymethylmethacrylate PMMA Polyamide PA93 Rigid polyvinylchloride R-PVC Polypropylene PP High-density polyethylene HDPE Polytetrafluoroethylene PTFE Low-density polyethylene LDPE Rubbers (elastomers)

Young’s modulus [GPa]

Tensile strength [MPa]

3-3.3 3.3 2-3.5 2.4-3 1.2-1.7 0.55-1 0.35 0.15-0.24 0.002-0.1

35-65 80-90 60-110 80-90 50-70 20-37 17-28 7-17 400 MV/m), electrostrictive strain (>7%), and relatively high modulus (>0.3 GPa). Examples of cyano-polymer CynP include14,17-18 polyacrylonitrile PAN, poly(vinylidene cyanide) P(VDCN) families, and polymers with cyano-groups in the side chains. As a high-grade chemical with virtually no interfering impurities, polyacrylonitrile and its copolymers can be commercially fabricated in the form of fibers containing vinyl monomer, methyl acrylate, methyl methacrylate MMA, or vinyl acetate VAC. Optimal properties of optoelectronic systems obtained from polyaniline can be achieved by radical polymerization resulting in poly(acrylonitrile-allyl-cyanide) P(AN-ALCN). The optimized properties include high crystallinity (para-crystallinity), high optical transparency, and high dielectric relaxation strength in the glass transition region. Vinylidene cyanide VDCN is a highly reactive monomer that undergoes rapid ionic polymerization in the presence of almost any weak base to form a hydrolytically unstable homopolymer. The optimal use of vinylidene cyanide in piezoelectric applications is in the form of a homopolymer due to its large piezoelectric constant (the polarization of its repeat unit is 4.5 (dimensional:D) in trans-conformation). Similar to ferroelectric transition, poly(vinylidene cyanide-co-vinyl acetate) exhibits dielectric peaks near its Tg. It has a dipole moment (µ) of 10-30 cm. Ferroelectric polyurethanes used for structuring organic optoelectronic systems are polyurethane PUR polymers with ferroelectric properties such as polarization (50-60 mC/m2). Examples of ferroelectric polyurethanes include poly(trimethylene-co-heptamethylene dicarbamate)-3,7-polyurethane (abbreviated as PMHHCPU or 3,7-PUR), and poly(pentamethylene-co-hexamethylene dicarbamate)-5.6polyurethane (abbreviated as PPHCPU or 5.6-PUR). Both of them have Tg of 31oC. 3,7-

3.2 Electroactive polymers

53

PUR has Tm of 142oC, while 5.6-PUR of 160oC. Both of them have a “remnant polarization” of 50-60 mC/m2. This value is stable for 3,7-PUR over Tg up to 115oC.14 Examples of poly(vinylidene cyanide) derivatives include poly(vinylidene cyanideco-vinyl acetate) P(VDCN-VAc), poly(vinylidene cyanide-co-vinyl benzoate) P(VDCNVBz), poly(vinylidene cyanide-co-vinyl propionate) P(VDCN-VPr), poly(vinylidene cyanide-co-vinyl pivalate) P(VDCN-VPiv), poly(vinylidene cyanide-co-methylmethacrylate) P(VDCN-MMA), and poly(vinylidene cyanide-co-isobutylene) P(VDCN-IB).8,14,17 The electronic properties of these polymers are listed in Table 3.2.8 Table 3.2. Electronic properties of some poly(vinylidene cyanide) cyano-polymers. [Data from Anthony L. Andrady, Plastics and the Environment, © 2003 John Wiley & Sons, Inc.] Glass transition temperature Tg [oC]

Dielectric constant80 ε

Dielectric relaxation strength Dε

Piezoelectric constant d31 [pC/N]

Pyroelectric constant p [µC/Km2]

Remnant polarization Pr [mC/m2]

Acetate-based vinylidene cyanides such as poly(vinylidene cyanide-co-vinyl acetate) 178

5.6

120

7.0

10

35

Benzoate-based vinylidene cyanides such as poly(vinylidene cyanide-co-vinyl benzoate) 184

5.6

115

5.2

10

21

Propionate-based vinylidene cyanides such as poly(vinylidene cyanide-co-vinyl propionate) 176

5.8

85

9.8

30

28

Pivalate-based vinylidene cyanides such as poly(vinylidene cyanide-co-vinyl pivalate) 172

5.8

100

7.0

12

33

Acrylate-based vinylidene cyanides such as poly(vinylidene cyanide-co-methylmethacrylate) 146

5.4

30

2.2

7

12

Butylene-based vinylidene cyanides such as poly(vinylidene cyanide-co-isobutylene) 75

5.0

16

1.0

3.5

6

Ferroelectric liquid crystal polymers can be termed as “mesogens” Mgs (that indicates substance capable of inducing liquid crystalline states (also known as the liquid crystal phase)). They are available in the form of lyotropic liquid crystals, which can be reformed by variation of the concentration of amphiphilic molecules in a suitable solvent and in the form of thermotropic liquid crystals, which can be observed using temperature variation (their components are determined or changed by temperature). They are used in contemporary electronic displays. Liquid crystals exhibit ferroelectric properties, and for this reason, they are called ferroelectric liquid crystals. To optimize the structures of these electronic displays (and other optoelectronic systems), the ferroelectric polymers such as ferroelectric liquid crystal polymers must show the phases of permanent polarization without the need for the electric field. Moreover, they must have excellent electro-optic properties, especially for structuring reflective displays, heat-repelling sheets, optical shutters, and dynamic holography.9,18-20

54

Optimized Electronic Polymers, Small Molecules, Complexes, and Elastomers for Organic

Examples of ferroelectric liquid crystal polymers include9,8,21-23 1. p-decyloxybenzylidene-p'-amino-2-methylbutylcinnamate DOBAMBC 2. cholesteric liquid crystals such as cholesteryl benzoate molecule (also called 5-cholesten-3-yl benzoate) 3. N-(4-methoxybenzylidene)-4-butylaniline molecule (also called benzenamine-4butyl-N-[(4-methoxyphenyl)methylene]) 4. polysiloxane 5. polyacrylates 6. polyethers 7. poly(vinyl ethers). To get the optimal optical properties of liquid crystal displays structured from pdecyloxybenzylidene-p'-amino-2-methylbutylcinnamate DOBAMBC polymer, this ferroelectric liquid crystal polymer should be a low molecular weight grade. It is advisable to use the ferroelectronic liqid polymer (having formula C34H50O2) that is chemically polymerized from an ester of cholesterol and benzoic acid with helical structure because cholesteryl benzoate softens at 145°C, yielding a cloudy fluid, which can be changed to the originally expected clear liquid at 178.5°C. N-(4-methoxybenzylidene)-4-butylaniline with the formula (C18H21NO) is attractive for structuring optoelectronic systems because it has an initial phase transition temperature (liquid, nematic) of 160oC and temperature of final phase transition (liquid) of 145oC. Polysiloxane of ferroelectric liquid crystal grade is widely used for structuring optoelectronic systems as well.1,3,21-23 The ferroelectric polyamides (also called “odd-numbered nylons”)215,317 are formed by recurring amide (−CO−NH−) linkages in the hydrocarbon units of their chain for optimizing the structures of electronic systems. The term “odd-numbered nylons”316,317 denotes the number of carbon atoms between amide groups. Generally, polyamides exhibit interesting dielectric properties (polyamide-11 has a dielectric constant ε of 3 at 25oC), which significantly changes with temperature and frequency. Ferroelectric polyamide grades are used for optimizing the structures of optoelectronics due to their capability of exhibiting ferroelectric current densities J versus electric fields E, and considerable values of remnant polarization. For example, the values of remnant polarization of a traditional polyamide (55 mC/m2) can be highly improved up to 180 mC/m2 with the application of ferroelectric-polyamide of polyamide-3 grade.8,14,24,26-27 Examples of ferroelectric polyamides include polyamide-11, polyamide-9, polyamide-5, polyamide-3, polyamide-6, and polyamide-7. Both polyamide-11 and polyamide-6 have dipoles and hydrogen bonds in their chemical structures. Polyamide-9 shows remnant polarization of 135 mC/m2, while polyamide-3 of 180 mC/m2. Polyamide-5 exhibits the highest melting point of any of the ferroelectric polyamides and stable piezoelectric response up to a temperature of 250°C. This means that the piezoelectric properties of ferroelectric polyamides are temperature dependent. For example, the polarization response of ferroelectric polyamides is low at room temperature. The ferroelectric polyamide-11 of the formula (−(−NH−CO−(CH2)10−]n−) exhibits a strong induced anisotropy.8,24,26-27 Ferroelectric cellular polymers (also called ferroelectric foams) are piezoelectric polymer foams of extreme softness compared to other polymers. The main important feature of these cellular polymers is the capability of exhibiting piezoelectric and pyroelectric

3.2 Electroactive polymers

55

properties after electric charging. Among electroactive polymers, ferroelectric cellular polymers can be considered as the optimal group for structuring optoelectronic systems due to their low dielectric constant, large piezoelectric coefficient (d33 = 200 pC N-1), large electro-optic coefficient (g33 = 30 VmN-1), and Young's modulus of 0.002 GPa. Ferroelectric cellular polymers include ferroelectric polyurea, polythiourea (such as pyromellitic dianhydride PMDA). As illustrated in Figure 3.2,12,327 ferroelectric foam, such as cellular polypropylene, has structure filled with air gaps so that it is considered as an internally charged voided polymer. Ferroelectric foams (including cellular polypropylene) as polymer-air composites are soft due to their high air content as well as due to the size and shape of the polymer walls.14-15

Figure 3.2. The structure of cellular polypropylene as an example of ferroelectric cellular polymers.12,327

3.2.1.2 Dielectric elastomer-electroactive polymers Dielectric elastomer-electroactive polymers (also called electrostatically stricted polymers or dielectric elastomers) acting as actuators are the elastomers in which actuation is generated by electrostatic forces between two electrodes, which activate the polymer. They exhibit low elastic stiffness and high dielectric constants under the electrostatic field, inducing large actuation strain. That is why they are called electrostatically stricted polymer actuators. The majority of available elastomers for structuring dielectric elastomerelectroactive actuators are those based on acrylic elastomers ACR-Es, silicone elastomers QEs, and polyurethane elastomers PUR-Es. To optimize the efficiency of organic actuators, sensors, generators, microrobots, and artificial muscles, which have been structured from electroactive elastomers, such as acrylic, silicone, and polyurethane elastomers, the electroactive elastomers should reduce actuation through polarization, produce large actuation strain on application of an electric field due to “Maxwell stress effect.” They are highly deformable dielectric media, light-weight, flexible, low cost, and have good processability. Table 3.34,10,321-322 contains several types of elastomers that can be used as dielectric elastomer-electroactive polymers (such as artificial muscles and robots) with their dielectric and mechanical properties. These include isoprene thermoset elastomer, chlorinated thermoset elastomer, butadiene rubber, nitrile thermoset elastomer (30% acrylonitrile contents), styrene-butadiene thermoset elastomer (25% styrene contents), isobutylene-isoprene rubber, chlorosulfonated polyethylene thermoset elastomer, ethylenepropylene rubber, urethane, and silicone. Important to remember is that optimal types of dielectric elastomer-electroactive polymers include silicone elastomers QEs, acrylic elastomers ACR-Es, and polyurethane elastomers PUR-Es. Acrylic elastomers are considered

56

Optimized Electronic Polymers, Small Molecules, Complexes, and Elastomers for Organic

as super electroactive polymer EAP due to their highest areal actuation strain (~160%), highest elastic energy density (~3.4 MJ/m3), and highest pressure (~7 MPa). Silicone elastomers include Dow Corning® HS3324,366 and Nusil® CF 19-2186324,366 as commercial grades. Acrylic elastomers include VHB® 49104 grade. Polyurethane elastomers include TPU® 58 888.7 VHB® 4910324,366 has the highest actuation strain among these examples. The silicone HS3® has a maximum areal strain of 94-163%.4,10,28-29,32-33 Table 3.3. Characteristics of the selected dielectric and mechanical properties of elastomers that can be used in dielectric elastomer-electroactive polymer applications.4,10,321,322 Dielectric constant [at 1kHz]

Dielectric loss factor [at 1kHz]

Young's modulus [x106 Pa]

Eng. stress [MPa]

Break stress [MPa]

Ultimate strain [%]

Isoprene-based electroactive polymers such as isoprene thermoset elastomer 2.68

0.002-0.04

1.3

15.4

30.7

470

Chlorine-based electroactive polymers such as chlorinated thermoset elastomer 6.5-8.1

0.03-0.86

1.6

20.3

22.9

350

Butadiene-based electroactive polymers such as butadiene thermoset rubber -

-

1.3

8.4

18.6

610

Nitrile-based electroactive polymers such as nitrile thermoset elastomer 5.5 (at 106 Hz] 35 [at 106 Hz]

-

16.2

22.1

440

20-55

-

2-10

80-500

Urethane-based electroactive polymers 5-8

0.015-0.09

3.0-3.5

0.001-0.10

-

-

Silicone-based electroactive polymers -

-

3.2.1.3 Electrostrictive graft elastomers Electrostrictive graft elastomers are elastomeric polymers consisting of two components: (1) flexible macromolecule backbone (2) crystallizable side chains attached to the backbone (called grafts, as illustrated in Figure (a) 3.34,34), where, (b) represents the grafts on the backbone. These grafts crystallize, forming physical crosslinking sites. These sites are important for a three-dimensional elastomer network, which generates an electric field in response to polar crystal domains. The actuation mechanism of electrostrictive graft elastomers is based on the development of electrostriction and the Maxwell contribution phenomenon. The electrostriction term denotes any change in the shape of a system due to the rearrangement of its molecules in the presence of an external electric field. The physical principle of electrostrictive graft elastomers is related to the polar crystal domains. These crystal domains are primary contributors to electrostrictive-mechanical functionality. The efficiency of organic actuators from electrostrictive graft elastomers depends on (1) high electric field-induced strain (~4%), (2) high modulus (~550 MPa), (3) good processability, excellent electrical and mechanical toughness, (4) high piezoelectric strain associated with electric-field induced strain responses, (5) the ability to incorporat-

3.2 Electroactive polymers

57

ing nanofillers (such as carbon nanotubes/electrostrictive graft elastomers to enhance strains versus electric fields). These characteristics make electrostrictive graft elastomers more attractive for application in electronic systems having light-weight, flexibility, low cost of processing, and the ability to be molded into any desirable shape. Actuators represent the best application of electrostrictive graft elastomers.4,7,34

Figure 3.3. Representation of (a) molecular structure and (b) morphology of electrostrictive graft elastomers.4,34

Examples of electrostrictive graft elastomers used to optimize the efficiency of organic actuators, microactuators, and robots include4,7,36 1. electron irradiated poly(vinylidene fluoride-co-trifluoroethylene) PVDF-TrFE. The electron irradiation (also called electron beam processing) is a process in which βradiation of high energy is used to treat an object for a variety of purposes. For this purpose, poly(vinylidene fluoride-co-trifluoroethylene) can be considered as an optimal electrostrictive graft elastomer 2. poly(vinylidene fluoride-co-trifluoroethylene-co-hexafluoropropene) P(VDF-TrFEHFP), in which bulky (comonomer) hexafluoropropene HFP reduces the degree of crystallinity of poly(vinylidene fluoride-co-trifluoroethylene) 3. polyurethane PUR elastomer (especially the commercial grades having significant electrical-field-induced strains, high specific energy, and small response, such as Estane® 58888-NAT021323 − polyurethane electrostrictive graft elastomer grade). According to “Devonshire theory,” the electrically induced strain response in the electrostrictive graft elastomer (poly(vinylidene fluoride-co-trifluoroethylene) results from the electric field-induced phase transition between non-polar and polar phases in the crystalline area. The “Devonshire theory” describes many cubic perovskite ferroelectrics, such as barium titanate, by a sixth-order expansion of the free energy in the polar order parameter. In conclusion, the poly(vinylidene fluoride-co-trifluoroethylene) exhibits high electrostrictive strain levels when irradiated with high-energy electron radiation. The chemical structure of Estane® 58888-NAT021323 involves 4,4'-methylene-bis(phenyl isocyanate) and 1,4-butanediol as hard segments HSs while poly(tetramethylene oxide) as a soft segment. This polymer has the alternating soft and hard segments, which give unique possibilities to regulate polymer properties by varying the length of soft and hard blocks and the density of hard blocks. The selection of either polyether or polyester (as a soft segment) affects the flexibility of the polymer. Estane® 58888-NAT021323 has a molecular weight of 1000 g/mol, the density of 1130 kg/m3, breaking stress of 38 MPa, and breaking elongation of 640%.7,36

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Electrostrictive papers (such as silver laminated paper) are electronic polymers capable of behaving as actuators. Such electronic polymers involve a multitude of discrete particles − fibrous in nature − forming a network structure. An example of such paper is formed by two silver laminated papers (bonded with a suitable polymeric adhesive) with electrodes formed from silver and placed on the outside surfaces. Upon applying an electric voltage to the electrodes, a bending displacement is immediately observed. These types of actuators have light-weight and are easy to fabricate. They are used as active sound absorbers, flexible speakers, and smart shape control systems. Their performance depends on the excitation voltage, the host paper, and the type of polymeric adhesive used to bond the papers.The polymeric structure of active sound absorbers, flexible speakers, and smart shape control systems can be optimized by (1) increasing the capability of producing high displacement levels with small forces under an electrical excitation, (2) having electroactive properties to depend on polymeric adhesives, (3) operation according to the electrostriction effect associated with a combination of the electrostatic force of electrodes and the intermolecular interaction of the adhesives.4,7,11 Cellophane having an amorphous structure is another example of electrostrictive paper. Compared with the crystalline structure, this cellophane shows a better response due to its amorphous cellulose with a low degree of polymerization. The responsible for the strain of cellophane paper is a combination of the piezoelectric effect and the ionic migration effect both associated with the dipole moment of its constituents. The properties of an organic electronic system structured from electrostrictive cellulose papers can be achieved by preparing these papers in the form of strips having large bending displacement with low force.4,38 3.2.1.4 Electrostatic term The electrostatic term describes the behavior and properties of either stationary electric charges or the slow-motion of the charges. Electrostatic properties characterize buildup of charges on the surface of objects due to their contact with other surfaces. Electrostatic discharge is an important term related to the unexpected flow of electricity between two contacted electrically charged objects. It should be avoided in electronic systems such as organic integrated circuits because it may cause damage. To optimize the structures of organic integrated circuit (or organic microactuators) formed from electrostatic elastomers, silicone rubber is used due to its prevention of electrostatic discharge.119,121-122 3.2.1.5 Electroviscolastic elastomers Electroviscolastic elastomers are the solid forms of electrorheological fluids (before crosslinking). This solid form can be described as a suspension of polymers (non-conducting polymers, but electrically active polymers) as a polar phase in a low dielectric-constant liquid (such as silicone elastomers). The viscosity of solution can be changed by an electric field (100%) than those induced in electroactive polyelectrolyte gels.4,10,12 Examples of these very important ionic polymer gels include4,7,30,45,48-49 1. polyacrylonitrile is known as a pH-activated polymer and used for structuring actuators. The function of pH is to cause the deformation of gels. 2. poly(acrylic acid) gel that is changing shapes and sizes when placed between electrodes surrounded by an aqueous solution. Its function is to shrink near the anode when it touches the electrode. This gel is not touching an electrode; it swells near the anode 3. poly(vinyl alcohol) gel is a non-ionic polymer gel, but it is used for structuring actuators, which bend more than 90o when subjected to an electrical field because it involves dimethylsulfoxide as a dielectric solvent 4. poly(vinyl chloride) polymer that becomes poly(vinyl chloride) gel in the presence of dioctyl phthalate (as a plasticizer). Organic exchange membranes are an important class of organic electronic systems acting as actuators, which can be optimized by ionomeric polymer-metal composites (matrices) because they can make changes in the ion concentration upon the application of an external electric field. Such changes attract water and cause deflection towards one of the metal electrodes. Often, they consist of the hydrophobic polymer backbone and hydrophilic anionic side-chains. Ion exchange membranes from these polymers exhibit swelling (e.g., 200-300 µm film) on one side and shrinkage on the other side due to the non-uniform distribution of water in the polymer electrolyte network. Another important application of these polymers is in transducers exhibiting a measurable charge across the effective electrodes when subjected to an imposed bending stress. They can be used in organic bending soft actuators because they can produce large bending motion under a low applied electric field (~10 kV/m) across the metalized or conductive surface. Ionomeric polymermetal composites can be driven with a low voltage of 10 Hz) in water, have stable mechanical and chemical properties, are able to work in water (or under wet conditions), and their bending deformation decreases with the increase of frequency.4,7,10 Figure 3.5308,320-321 shows the working principle of ionomeric polymer-metal composites. Note: (a) represents the natural or initial state (prior actuation or prior to the application of an electric field). At this stage, the aqueous ionomeric polymer-metal composites have a flat shape. (b) represents the actuation state during which the application of an electrical potential across the ionomeric polymer-metal composite induced the cations to be spatially redistributed as they diffuse towards the cathode. This state results in forming a cation-rich layer along the cathode side and depleting cations from the anode side.52,308

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Figure 3.5: A representation of ionomeric polymer-metal composites.308,320-321

Ionomeric polymer-metal composites used for structuring organic ion exchange membranes include4,6,7,10,50,53 1. perfluorinated polymers or compounds such as perfluorosulfonate and perfluorocarboxylate. Both of them are perfluorinated ionomers with anionic-terminated side groups, such as Aciplex®330, Nafion®304,305, and Flemion®329 grades 2. polystyrene ionomers with anionic-substituted phenyl rings. Aciplex®330, Nafion®304,305, and Flemion®329 grades form an optimized group of commercial ionomeric polymer-metal composites for organic electronic biomimetic systems because they require low voltages to stimulate a bending response (1-10 V) with low frequencies below 1 Hz. Moreover, they can be used as collagen fibers, which are composed of charged ionic natural polymers. Nafion®304,305 grades, such as Nafion® 117, are attractive for structuring polyelectrolyte membrane-electrodes. As a perfluorinated copolymer of good flexibility, Nafion®304,305 is more suitable for membranes that can be chemically coated with platinum electrodes. In this case, it deforms and bends when a low voltage (~1-5 V) is applied across the electrodes in an aqueous solution. Flemion®329 is considered the best selection for fuel cells exposed to hydrolysis.4,6-7,10,50,53 Ion exchange polymers299,306 (also called ion exchange resins) represent the optimized class of ionic-electroactive polymers because they combine crosslinked porous polymeric (organic) substances (0.5-1 mm diameter) having functional (ionogenic) groups with mobile ions. They can be classified as either cation or anion exchange polymers. The class of cation exchanger polymers exchanges positively charged ions (cations), while the anion exchange polymers exchange negatively charged ions (anions). Both types of ion exchange polymers can be fabricated as polydispersed spherical beads (0.25-1.25 mm size) of stable mechanical and physical properties for structuring nanoelectronic systems.31,54,114 Styrene-divinylbenzene SDVB copolymer is an example of ion-exchange polymers.299,306 Examples of strong acid cation polymers are those of hydrogen form: (RSO3H) or sodium form: (R-SO3Na). Sulfonated polystyrene S-PS is a strong acid cation polymer of hydrogen form: (R-SO3H). An example of weak acid cation polymers is the hydrogen form (R-COOH). An example of strong base anion polymers is the hydroxide

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form (R-NH3OH). Quaternary ammonium Q-Am is based on a strong base anion polymer of hydroxide form (R-NH3OH). Note: the polymer consists of polystyrene with amine groups to form a cation exchanger.31,54,114 Nanocomposites302 used as electroactive polymers for nanoelectronic systems contain carbon nanotubes CNTs (also called buckytubes) of 10 nm diameter, which can consist of one or several graphene sheets terminated by hemispherical half-fullerenes (fullerene-60 C60, fullerene-70 C70, fullerene-80 C80). As actuators, multiwall carbon nanotubes MWNTs are less effective than single-wall carbon nanotubes SWNTs due to lower solvent-accessible area. According to the relationship between graphene sheets and nanotubes, nanotubes can be classified as (1) zigzag tubes (in which hexagonal chains form closed loops around the circumference), (2) armchair tubes AcTs (of hexagons oriented in the direction parallel to the tube axis), (3) chiral tubes (not falling into either of those two classes). Characteristics of carbon nanotubes include the acceptability of low driving voltages (1-5 V), charged surfaces-depended actuation, ease of blending with common polymeric materials (resulting in high reinforced, super-tough and/or conducting composites), and good electrical and mechanical properties (high electric conductivity, high thermal conductivity, mechanical strength, thermal resistivity/stability, large aspect ratios,307 and superior field emission properties).4,7,10,55,59 Examples of carbon nanotubes include16 1. ferroelectric field-effect transistors-based single-wall carbon nanotubes (abbreviated as FeFETs-SWCNTs) films 2. polyaniline-based multiwall carbon nanotubes (abbreviated as PANI-MWCNTs) composites. According to its chemical structure, the non-redox doping process in polyaniline results in the transformation from its insulating state into a conducting state in the form of emeraldine-salt/multiwall carbon nanotube composite (abbreviated as EDN-salt/MWCNT) of green color. As nanocomposites, field-effect transistor-based single-wall carbon nanotubes are used in ferroelectric memories because they exhibit memory hysteresis loop, large memory window, and very low power consumption. With polyaniline-based multiwall carbon nanotubes, hydrochloric acid HCl acts as a solution for aniline monomers (of polyaniline) in which multiwall carbon nanotubes are suspended. Ammonium hydroxide acts as de-dopant aqueous.

3.3 NON-ELECTROACTIVE POLYMERS The non-electroactive polymers (also called non-electrical deformation polymers) are the most suitable polymers for organic nanoelectronic systems. They exhibit volume or shape change in response to specific applied action. The main difference between electroactive polymers and non-electroactive polymers is that mechanical stresses can cause degradation of the electroactive polymers EAPs, while the effect of mechanical stresses is not observed in the case of non-electroactive polymers. Non-electroactive polymers include chemically activated polymers ChAP, shape memory polymers SMPs, inflatable structure polymers, light-activated polymers, magnetically-activated polymers, and thermally-activated gels.4,12,61

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3.3.1 CHEMICALLY-ACTIVATED POLYMERS The chemically-activated polymers are the first class of non-electroactive polymers showing changes in dimensions (sizes or shapes) depending on interaction with chemical materials. This can be observed in the case of doping a rubber piece in oil container, where it slowly swells upon the interaction with the solvent. Properties of such polymers depend on the interaction with ionic species contacted with the gel (including the pH environment or contact with solvents).4,12,61 Examples of chemically-activated polymers include56,61,63-64,66 1. pH-actuated polymeric gel such as a poly(acrylic acid) gel. Poly(vinyl alcohol)poly(acrylic acid) is also a part of this family 2. temperature-responsive matrix gels such as poly(acrylic acid)/poly(N-isopropylacrylamide), abbreviated as PAA/PNIPAM 3. self-oscillating polymer gels such as poly(N-isopropylacrylamide-co-ruthenium-4,4' -bipyrimidine) PNIPAM-co-Ru(bpy)3 gel (also called poly(N-isopropylacrylamideco- tris(2,2'-bipyridyl)ruthenium(III)) 4. self-assembling block and di-block copolymer gels such as poly(ethylene glycol)poly(DL-lactic acid), poly(ethylene glycol)-poly(DL-lactide-co-glycolide, and poly(ethylene glycol)-polycaprolactone 5. polyacrylonitrile gel fiber that exhibits a big change in length in a few seconds upon moving from acid to base environment. At the same time, it shows a very big change in volume. Poly(acrylic acid) PAA behaves as an electrolyte due to the presence of many carboxylic groups along with the polymer molecules. It can be produced as a gel-type electrolyte by a γ-ray polymerization of a concentrated aqueous solution of acrylic acid. Smart or intelligent gels (hydrogels) are randomly grafted polymers/copolymers such as the grafted copolymer PAA/PLO. PAA represents poly(acrylic acid), while PLO represents pluronic (also called poloxamer) used in block copolymers of ethylene oxide EOX and propylene oxide POX. Among known active polymer gels, poly(vinyl alcohol)-poly(acrylic acid) can be considered as optimal for organic electronic-based gels system because it expands and contracts in response to specific environmental stimuli (such as pH change). It is mechanically stable, easy, and safe to both fabricate and actuate, expands upon moving from acidic to basic solutions, and contracts upon moving from basic to acidic solutions. Poly(acrylic acid)/poly(N-isopropylacrylamide) PAA/PNIPAM (as a temperature-responsive matrix gel) should be opaque in order to optimize its functions. This can be achieved by polymerizing it on clear polyacrylamide gels, which can be formed by cooling and finally become opaque. The optimal results of poly(N-isopropylacrylamide) PNIPAM applications can be observed when polymerizing it in the form of temperature-responsive hydrogel for structuring chemically activated organic electronic systems. Similarly, the function of poly(N-isopropylacrylamide-co-ruthenium-4,4'-bipyrimidine) gel PNIPAM/ Ru(bpy)3 can be optimized for such structures by copolymerizing poly(N-isopropylacrylamide) and a vinyl-substituted derivative of ruthenium(III)-2,2'-bipyridine Ru(bpy)3, with N,N'-methylene-bis-acrylamide (also called N-N'-methylenebis(acrylamide) MBA) as the crosslinker. Poly(ethylene glycol)-poly(DL-lactic acid), poly(ethylene glycol)-poly(DLlactide-co-glycolide, and poly(ethylene glycol)-polycaprolactone are injectable biodegradable polymers with reverse gelation properties.56,63-64,66

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3.3.2 SHAPE MEMORY POLYMERS The shape memory polymers SMPs311 (also called smart polymers or functional polymers) are non-electroactive polymers because they act as a stimuli-responsive polymeric class, changing shape because of the application of external stimuli such as thermal stimulus. For example, thermal shape memory polymers can change their shapes when heated above a predetermined temperature, and they can be formed into two shapes (permanent shape and recovery phase). These polymers can be affected by transition Ttrans, melting Tm, and glass transition temperature Tg, stress σ, strain ε, and modulus E. Ttrans is the temperature at which a polymer changes from one state to another. Melting temperature is the highest temperature at which a semi-crystalline phase of a polymer melts into an amorphous state. Glass transition temperature is the temperature at which a polymer moves from a hard, glass-like state, to a rubber-like state. Stress is the force exerted on the polymer per area. The strain is the deformation per unit length due to stress. Modulus is Young's modulus or elastic modulus. To optimize the mechanism of activating shape memory polymers, the following three phases should be squinted: Transition between permanent shape → temporary shape → permanent shape as illustrated in Figure 3.6.309-310 These change in shape can be divided into the following four steps: (1) loading, (2) cooling, (3) unloading, (4) recovery. During these steps, the temperature and stress-strain response are considered. Note: The permanent shape of the tested polymeric sample is transformed into a temporary shape through a programming process. The permanent shape is recovered when the tested polymeric sample is heated above the switching temperature. Group (b) represents the cyclic thermo-mechanical test used as quantitative analysis for measuring the performance of shape memory polymers. During the first step, the shape memory polymer with the permanent strain εp(N-1) at deformation temperature Td should be equilibrated and followed by subjecting the shape memory polymer to predefined stress at deformation temperature. Deformation (such as in length) at deformation temperature is defined as εld (N). Where “p” means permanent, “l” indicates loading, “d” indicates deformation temperature, and “u” means unloading. At the second step, the temporary deformation (such as length εld (N)) should be fixed by cooling the shape memory polymer to Tf under loading stress. Where “εl(N)” indicates the fixed strain at Tf under loading. The third step is achieved by unloading the stress to zero or a specific lower constrain stress σc. Where “εu(N)” represents the resulting strain after unloading. It should be noted that the shape of the tested polymeric sample is recovered under either zero stress, or a specific constrain stress σc at Tr. Where “εp(N)” represents the final recovered strain. According to the result of the above test, the shape memory polymers can be classified corresponding to the number of shapes, such as one-shape, dual-shape, triple-shape, and multi-shape memory effect polymer. One-shape memory effect-shape memory polymers OSME-SMPs can be considered as conventional shape memory polymers because they are designed to memorize only one permanent shape. Dual-shape memory effect polymers are those of glass transition temperature ranging from 55 to 130oC. Triple-shape memory effect-shape memory polymers TSME-SMPs can be considered as polymeric crosslinked networks, which combine two incorporated polymeric chains with discrete transition temperatures. Multi-shape memory effect-shape memory polymers MSME-SMPs are the polymeric networks con-

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sisting of more than two macroscopic layers. To optimize their applications, shape memory polymers should be evaluated for ability of (1) producing actuation by phase change, (2) exhibit large deformation strains (several hundred percentages), (3) showing a radical change ranging from a normal rigid polymer to a very stretchy elastic and back on compound, (4) exhibiting two types of memory (current (or temporary) form and stored (or permanent) form) (5) deforming in the rubbery state, (6) accepting the utilization of carbon nanotubes to improve the rubbery state elastic modulus, (7) depending their performance on both molecular structure and the mode of deformation, (8) showing high level of flexibility.4,71-74

Figure 3.6. A simple example showing the principle of shape memory polymers SMPs.309-310

Optimal types of shape memory polymers used for structuring organic electronic systems, especially organic actuators and artificial muscles, include4,71-72.76 1. polyene-based shape memory polymers such as shape-memory polynorbornene, shape memory poly(trans-isoprene), shape-memory poly(styrene-butadiene), and shape memory poly(methylene-1,3-cyclopentene) 2. shape memory polyurethanes SMPU312 available in the form of linear phase-segregated multi-block copolymers (such as polyurethane/polybenzoxazine-based shape memory polymers PUR/PBa-SMP) 3. high-stress recovery shape memory polymers such as poly(vinyl alcohol) shape memory polymers PVA-SMP 4. fully recoverable high-strain shape memory polymers such as adjusted acrylate network 5. sharp transition temperatures shape memory polymers such as biodegradable shape memory polyurethane prepared by crosslinking, 8-arm star-branched poly(ε-caprolactone) macromers 6. multiple shape memory polymers such as polyol oligosilsesquioxane.

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The polyene-based shape memory polymers such as polynorbornene must contain carbon-carbon double bonds to optimize their functions for such applications because the interaction of such bonds represents the conjugation states of polynorbornene. The commercial-grade of polynorbornene shape memory polymer, such as Norsorex®331 grade, has an effective shape memory effect based on the formation of the physically crosslinked network as a result of entanglements of the high molecular weight linear chains. Poly(methylene-1,3-cyclopentene) is a member of optimal shape memory polymers because it has a strong shape memory effect with a glass transmission temperature of 7.6oC and the small endothermic melting peak of 68oC. Polyurethane is a special grade of conventional polyurethane family, but it can be prepared as a member of optimal shape memory polymers with a high degree of flexibility and a wide range of mechanical properties associated with glass transition temperatures. Polyurethane/polybenzoxazine-based shape memory polymer (abbreviated as PUR/PBa-SMP) is a derivative of shape memory polyurethane SMPU,312 which can be formed by incorporation of polybenzoxazine PB-a (as a thermosetting polymer) with polyurethane (as a thermoplastic polymer). The shape memory polyurethane polymers SMPUs312 (including polyurethane/polybenzoxazine-based shape memory polymer) show shape memory effect due to micro-phase segregation and high shape recovery performance (recovery stress of 13 MPa and shape recovery ratio of 93%).71,76,79 High recovery stress shape memory polymers represent the special type of optimal shape memory polymers for structuring organic actuators because this type has improved rubbery state elastic modulus resulting from the incorporation of fillers such as carbon nanotubes within the shape memory networks. An example of such types is the shape memory polymer called poly(vinyl alcohol) PVA-SMP, which has a wide range of glass transition temperatures (50-200oC) as compared with poly(vinyl alcohol)-filled with carbon nanotubes of glass transition temperature ~80oC. The fibers of poly(vinyl alcohol)filled with carbon nanotubes (abbreviated as CNT-PVA) have much-improved storage modulus both at the glassy state and at the rubbery state. For example, when deformed at 70 or 90°C, the poly(vinyl alcohol)-filled with carbon nanotubes exhibits maximal stress of ~150 MPa, one to two orders of magnitude greater than the stress generated by conventional shape memory polymers. Sharp transition temperatures of shape memory polymers depend on their structure, which is modulated as an amorphous switching form with a narrow glass transmission temperature (10°C). Multiple functional shape memory polymers are systems based on the semi-crystalline polycaprolactone segment, such as the polyol oligosilsesquioxane/polycaprolactone networks. Their function can be optimized for structuring organic actuators by formulating them to exhibit two-way shape memory properties under an appropriate constant tensile load. Chemically crosslinked polycaprolactone possesses the best two-way shape memory properties because (1) maximum strain increment is 44%, (2) a recovery ratio of 85% under a stress of 0.4 MPa.71-72,74 3.3.3 ELECTRONIC INFLATABLE STRUCTURE POLYMERS As illustrated in Figure 3.7,4,159,291,314-315 pneumatic artificial muscles (often called “Braided” or “Mckibben” muscles) represent the simple inflatable structure, where polymers used for structuring them are called inflatable structure polymers as an important

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type of non-electroactive polymers. Pneumatic artificial muscles can be defined as contractible linear motion gas-pressured engines because their structure consists of core elements able of acting as inlet and outlet elements. These elements formed as flexible reinforced closed membrane attached at both the ends to the fittings. The function of membranes structured of electronic inflatable structure polymers can be optimized by inflating gas pressures, where it bulges outward radially, resulting in axial contraction of the shell. Such contraction results in a pulling force on its load. The actuation provides unidirectional linear force and motion. The mechanical power should be transferred to the load through the fittings. An optimum application can be gained from inflatable structures in the case of the robotic hands illustrated in Figure 3.7.4,57,80,313

Figure 3.7. A representation of different contraction levels of the artificial muscles, robotic Sheffield hand, and the chemical structure of poly-p-phenylenebenzobisoxazole PBBO as an inflatable structure polymer.4,159,291,314,315

Optimal electronic (organic) inflatable structure polymers as a group of non-electroactive polymers used for structuring artificial muscles and robotic hands include4 1. poly-p-phenylenebenzobisoxazole PBBO fiber cored sleeves due to its high flexibility associated with ultra-high-strength properties 2. dielectric elastomers such as butadiene rubber, styrene-butadiene thermoset elastomer, nitrile thermoset elastomer, and isobutylene-isoprene rubber due to their ultrahigh flexibility associated with dielectric properties. Poly-p-phenylenebenzobisoxazole has good mechanical properties (tensile strength of 76 MPa) and thermal stability. Both these groups of electronic (organic) inflatable structure polymers are used for structuring artificial muscles due to their lightweight, flexibility, low cost, easy fabrication, large strain, high response rate, and high output power at low strain. 3.3.4 ELECTRONIC LIGHT-ACTIVATED POLYMERS The light-activated polymers represent the class of electroactive polymers (such as polyelectrolyte gels) capable of responding mechanically under the radiation of a specific wavelength. In fact, polyelectrolyte gels can be considered as the optimal grades of electronic light-activated polymers because their mechanical response comes in the form of dimen-

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sional change caused by the chemically-induced ionization. Such a mechanism is called the mechano-photo-chemical effect. Examples of light-activated polymers include 1. polymeric gels made of poly(N,N-dimethylglutamanilide) (also called poly(p-N,Ndimethylamine)-N-gamma-D-glutamanilide)). Importance of this polymeric gel is its capability of producing dilation of 35% in each dimension when exposed to light (UV illumination) 2. light-activated poly(methyl acrylate acid) gels based on cis-trans photoisomerizable (p-phenylazophenyl) trimethylammonium iodide dye that can produce 10% elongation when irradiated with a laser beam for 10 minutes.4,7-8,64 3.3.5 MAGNETICALLY-ACTIVATED POLYMERS The magnetically-activated polymers (also called magnetoelastic, magnetostrictive polymers, or ferrogels) can be considered as the optimized sensitive polymeric materials due to their ability to show strain caused by changes in the magnetic field. Ferrogels have a chemically crosslinked network; that is why they become swollen by ferrofluids. Magnetic particles of these gels should be attached to the polymer chains by strong adhesive forces. The applied magnetic field gradient acts as a driving force. The movements are elongation, contraction, bending, and rotation.4 The optimal types of magnetically-activated polymers are those that can be formed by incorporation of magnetic particles and polymeric gels, such as4,84,86 1. magnetic particles: Fe2O3, magnetic γ-Fe2O3, samarium-cobalt SmCo, samarium iron nitride SmFeN, etc. 2. polymeric gels such as poly(N-isopropyl acrylamide), poly(vinyl alcohol), poly(vinyl methyl ether), and polymer network matrix containing crystallizable poly(ethylene glycol) side chains and poly(ε-caprolactone) crosslinks named segmented caprolactone-ether methacrylate. As a type of magnetically activated polymers, the magneto-responsive gel poly(Nisopropyl acrylamide) is considered as a thermosensitive polymer loaded with γ-Fe2O3 (16%) as a ferromagnetic powder. As with poly(vinyl methyl ether), the function of such powder is to increase the heat transfer rate in the gel with a magnetic strength of 830 Oersted, 2.08 KHz). 3.3.6 ELECTRONIC THERMALLY-ACTIVATED GELS The thermally-activated gels are polymeric gels having the ability to undergo thermal phase transitions (volume changes). This phase transition takes place at temperatures of 20-40oC, exhibiting a contraction force that can reach 100 KPa with a response time of 2090 s. To optimize the efficiency of an electronic thermally-activated gel for organic artificial muscles and thermoresponsive soft actuators, it should be polymerized so that it shows volume phase transition, induced by temperature change, and the ability to use both hot and cold water for actuation.4 Examples of thermally activated gels include4 1. N-substituted polyacrylamide derivatives and polypeptides 2. di-functional benzoxazine monomers 3. thermally activated gels that can be considered as the most widely used thermallyactivated gels. Poly(vinyl methyl ether) PVEM is completely soluble in water at temperatures below the phase-transition temperature. The optimized poly(vinyl methyl ether) should have the ability to undergo phase transition at 38oC so that its transition produces volume change and can be crosslinked into a hydrogel by γ-ray irradiation.

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In addition to that, the swelling ratio of poly(vinyl methyl ether) gels produced in the form of fibers should be decreased with temperature increasing. As a result, such a reaction increases as the temperature nears the transition point.

3.4 ELECTRONIC CONDUCTIVE (CONJUGATED AND DOPED) POLYMERS The conductive polymers (also called conjugated polymers CGPs) are polymers of organic nature capable of conducting electricity. The chemical structure of a conductive polymer should be optimized so that it exhibits an actuation upon the electronic change of oxidation state. “Doping” is the main chemical/physical (reduction/oxidation) process by which conducting polymers are created. A conductive polymer with an appropriate dopant, such as hydrogen chloride HCl or sulfuric acid H2SO4, can exhibit chemically- and electrochemically-controllable electronic conductivities. Electrical conduction state is a result of either electrons (n-type) or holes (p-type) (σ ~ 10-1 S cm-1). The function of the doping process is getting ions that can flow across the covalently bonded chains of conjugated polymers. This function can be optimized by the utilization of dopants capable of increasing the conductivity depending on their ability to create states in the energy gap observed close to either the conduction or the valence band.4,7,87,100 As a result, conductive polymers have 1. the capability of reducing actuation through ion/mass transportation 2. can be formulated as a matrix for enzymes in biosensors 3. need low actuation voltage ~2 V (that make them attractive actuator materials) 4. their conductivity values decrease at low temperatures.4,7,87,100 The right understanding of the electrical concept of conductive polymers starts from the structure of a polymeric atom consisting of a nucleus (made up of protons with a positive charge), neutrons (electrically neutral), and electrons (organized outside the nucleus in shells or levels of energy). Electrons are arranged on three levels, the 1st level (also called the first shell) can hold a maximum of 2 electrons, the 2nd level (also called the second shell) can hold a maximum of 8 electrons, and the 3rd level (also called the third shell) can hold a maximum of 18 electrons. If more electrons are added, they occupy additional shells. Generally, an outer shell is known as the “valence shell” because it determines the electrical conduction or insulation characteristics of the associated atom. Metallic atoms are known as atoms with free electrons because they easily give up their electrons (that is why the are described as electrically conductive), whereas the atoms of non-metallic materials have the ability to accept electrons that is why they are called “insulators.” Ordinary polymers are total insulators, but conductive polymers are those gaining long chain having current flowing properties.88,374 The optimal way for the creation of conductive polymers is doping them with dopants of negative or positive charges (oxidizing or reducing agents: reduction/oxidation ReDox). Importance of such a doping process is to allow current to travel down the chain. Thus, dopant materials enable conductive polymers to exhibit p-type or n-type conductivity. As illustrated in Figure 3.8,261,319 the conductive polymers can be classified as “extrinsically conductive polymers” or “intrinsically (inherently) conductive polymers.” Extrinsically conductive polymers include “conductive element filled polymers” and

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“blended conducting polymers,” while, intrinsically (inherently) conductive polymers include “π-conjugated polymers” and “doped conducting polymers.” Some scientists consider π-conjugated polymers as the same as the conductive polymers. Others consider that there is a difference between them. For more precision, we assume that every intrinsically (inherently) conductive polymer is a π-conjugated polymer, but π-conjugated polymers are not conductive without doping.5,88-90,261

Figure 3.8. Classification of the atomic structure of conducting polymers.261,319

3.4.1 ELECTRONIC EXTRINSICALLY CONDUCTIVE POLYMERS The extrinsically conductive polymers are members of optimized conjugated polymers because of their ability to be filled with external conducting ingredients such as carbon black or blended with conducting polymers. An example of extrinsically conductive polymers is the polyacetylene matrix containing a specific amount of electrically conducting carbon black (powdered graphite). The electrical conductivity of such a matrix can be highly optimized by causing that the particles of incorporated carbon material contact each other. Carbon black is the most frequently used filler, having a very high surface area (1000 m2/g), considerable porosity, and improved filamentous properties. The optimal electrically conducting metallic powders used as fillers in the conjugated polymers as electronic extrinsically conductive polymers include Ni, Cu, Ag, Al, and Fe. Silver-loaded epoxy adhesives can be considered as the optimal member of a conductive element filled polymers related to electronic extrinsically conductive polymers used in organic electronic resistors, antistatic components, self-regulating heaters, etc. An optimizing the efficiency of blended conducting polymers related to electronic extrinsically conductive polymers for structuring organic electronic systems can be achieved by blending the original polymer with a conducting polymer. The function of such blending is to get polymers of improved physical, chemical, electrical, and mechanical properties. Examples of blended conducting polymers include the polymeric composites consisting of soluble conjugated polymers such as polyaniline. Important to know is that poly(3-octylthiophene) with certain non-conducting polymers such as polymethylmethacrylate, poly(p-phenylene terephthalamide), and polyethylene intrinsically (inherently) conductive polymers are electronically conducting organic polymers not because of compounding them with metallic conductivity materials, but due to their content of conjugated π-electron in their back-

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bones which increased their conductivity to a large extent (such as polypyrrole).5,8789,100,261

Optimized intrinsically (inherently) conductive polymers for structuring organic electronic systems include 1. polyacetylene and its derivatives due to their considerable conductivity and bandgap values (σ = 103-1.7x105 S/cm, 1.5 eV respectively) 2. polyaniline and its derivatives having σ = 30-200 S/cm and bandgap =3.2 eV 3. polypyrrole and its derivatives having σ = 102-7.5x103 S/cm and bandgap = 3.1 eV 4. polythiophene and its derivatives having σ = 10-103 S/cm bandgap = 2.0 eV 5. poly(p-phenylene) and its derivatives having σ = 102-103 S/cm and bandgap = 3.0 eV 6. poly(phenylene vinylene) and its derivatives having σ = 3-5x103 S/cm and bandgap = 2.5 eV. The above conducting polymers belong to the polyaromatic polyene family. The mother family is called polyenes (polyaromatics) and includes the following sub-families 1. thiophenes (polythiophene, poly(ethylene dioxythiophene), and poly(3-n-alkyl thiophene)) 2. acetylenes (polyacetylene), anilines (polyaniline), pyrroles (polypyrrole, polycarbazole, polyindole, polyazepine, poly(vinyl pyrrolidone), and diketopyrrolopyrrole) 3. phenylenes (poly(p-phenylene), poly(phenylene vinylene), and poly(phenylene oxide)) 4. fluorines (polyfluorine, polyperene, polyazolene, and polynaphthalene), derivatives of polyacetylene including polymethylacetylene, polyphenylacetylene, polydiphenylacetylene, poly(1-alkyl-2-phenylacetylene), poly(1-chloro-2-phenylacetylene), and poly(1-phenyl-2-p-(triphenylsilyl)phenylacetylene). These derivatives are available in the form of soluble conducting polymers and have been synthesized to show electroluminescent properties. Derivatives of poly(phenylene vinylene) include (in the form of soluble conducting polymers) poly(2-methoxy-5-(2'-ethyl-hexyloxy)-pphenylenevinylene) MEH-PPV, poly(2-butyl-5(2'-ethylhexyl)-1,4-phenylene vinylene) BuEH-PPV (also called poly(2-butyl-5-(2-ethylhexyl)-p-phenylenevinylene)), and poly(2,5-dimethoxy-p-phenylene vinylene) PDMeOPV. These derivatives have been synthesized for the applications in electroluminescent and light-emitting devices. Derivatives of polythiophene include (in the form of soluble conducting polymers) poly(3-alkyl thiophene) P3AT used for structuring solar cells and transistors.5,87-89,100,261 3.4.2 ELECTRONIC INTRINSICALLY (INHERENTLY) CONDUCTIVE POLYMERS Intrinsically (inherently) conductive polymers represent the optimal class of electronic conductive (conjugated and doped) polymers because their electrical conductivity is inherent that is why this class is used in electromagnetic-interference EMI shielding, conductive layers (for organic light-emitting diodes OLEDs), organic field-effect transistors OFETs, opto-active layers for OLEDs, and anti-corrosion coatings (for iron and steel). They can be divided into two types, including π-conjugated polymers and doped conducting polymers. 3.4.2.1 Electronic π-conjugated polymers The electronic π-conjugated polymers can be considered as an optimal class of electronic intrinsically (inherently) conductive polymers due to the presence of π-electrons in their chain's backbones (that permit electrons movement). The function of π-electrons is to

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increase the conductivity of these conductive polymers to a large extent. Because of the presence of double bonds and lone pairs of electrons, the conduction of electricity takes place. Originally, the term “π-electrons” refers to an electron which resides in the πbond(s) of a double bond or a triple bond or in a conjugated π-orbital. For example, allyl carbanion has four π-electrons. As a result: (1) both of the two π-electrons and “resonance contributor” can be assigned to the π-bond portion of the carbon-carbon double bond (2) the other π-electron pair is a lone pair in a conjugated π-orbital.5,28,49,88-89 The efficiency of organic electronic systems (especially those which require electrical conductivity to a large extent) can be optimized by the right selection of electronic πconjugated polymers such as5,88-89,92-93 1. conductive polymers which include polypyrrole, polythiophene, polyaniline, and polyacetylene 2. conjugated molecules which include butadiene, benzene, furan, N-N'-dimethylformamide, and allylic carbocations 3. cyclic, partially conjugated polymers or completely conjugated polymers 4. conjugated pigment systems (also called conjugated systems in pigments) 5. homo-conjugated polymers such as 1,4-pentadiene 6. crosslinkable conjugated polymers such as oxetane-functionalized conjugated family 7. heterocyclic or polyheterocyclic conducting or conjugated polymers HC-CPs which offer unique electronic and physical properties 8. nanoparticle-based conjugated polymers and oligomers for applications in optoelectronics. Annulene family can be considered as the optimal of completely conjugated polymers used for structuring organic electronic systems that need electrical conductivity to a large extent. Conjugated pigments (such as phthalocyanine compounds, porphyrin compounds, and chromophores) have been introduced for structuring such electronic systems due to their ability to form charge-transfer complexes. Homo-conjugated polymers have been selected as optimal members of electronic π-conjugated polymers for the same applications due to their capability of overlapping two π-systems separated by a non-conjugating group, such as CH2. Polypyrrole can be selected as an optimal member of inherently conducting polymers for organic/polymeric electromagnetic shielding systems due to its chain doping of electrons and its capability of exhibiting a high impact strength. Optimization of the function of organic optical systems that need electrical conductivity to a large extent depends on the utilization of the electronic π-conjugated polymer polythiophene due to its optical properties resulting from its conjugated backbone.88 It is known that thiophenes are among the optimal electrically conductive polymeric families used in organic electronic and optical systems. The most important derivatives of this family include polythiophene PT, poly(ethylene dioxythiophene) PEDOT, poly(3-nalkyl thiophene) P3AT, poly(3-octylthiophene) P3OT, poly(3-hexyl thiophene) P3HT, poly(3-butylthiophene) P3BT, poly(3-undecyl-2,2'-bithiophene) P3UBT, poly(2-(3-thienyl)-ethoxy-4-butylsulfonate) PTEBS, poly(3-(4'-octylphenyl)thiophene) POPT, and poly(3-dodecyl thiophene) P3DT.95 The thermally stable conjugated polyanilines are also among the optimal electrically conjugated polymeric families due to their unique thermal stability needed for environmental and aerospace organic electronic and optical systems and their ability to have three

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stable oxidation states (leuco-emeraldine base, emeraldine salt, and per-nigraniline base. For example, the doped emeraldine salt form of polyaniline has very high conductivity and thermal stability, so that it is used widely for forming the hole-injection layers of polymeric light-emitting diodes PLEDs and organic photovoltaics (especially for aerospace applications). Poly(phenylene vinylene) is an optimized solvent-based conjugated polymer of phenylene family because it belongs to low-bandgap electronic polymers. It can be used as a donor in solar cells (especially due to its high absorption coefficient and high electron affinity).312 Poly(phenylene vinylene) PPV family (containing the most important electronic conjugated polymers) can be used as donors in polymeric solar cells. Derivatives of this family include58,94-95 cyano-polyphenylene, cyano-para-phenylenevinylene CN-PPV, poly(2methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene MEH-PPV, poly(2-methoxy-5-(2'ethyl-hexyloxy)-1,4-phenylene(1-cyano)vinylene) MEH-CN-PPV, poly((2-methoxy-5(3',7'-dimethyloctyloxy)-1,4-phenylene-vinylene) MDMO-PPV, poly(2,5-bis((2'-ethylhexyl)oxy)-1,4-phenylenevinylene) BEH-PPV, poly(2,5-dioctyloxy-p-phenylenevinylene) DOO-PPV, and poly(2-butyl-5-(2'-ethylhexyl)-1,4-phenylenevinylene) BuEH-PPV. For example, both poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene MEH-PPV and poly(2-butyl-5-(2'-ethylhexyl)-1,4-phenylenevinylene) BuEH-PPV are the first members of optimized poly(phenylene vinylene) PPV family used as donors in soluble organic solar cells because they are chemically polymerized in the form of soluble electrically conjugated polymer. Poly((2-methoxy-5-(3',7'-dimethyloctyloxy))-1,4-phenylene-vinylene)] MDMO-PPV is the optimized type of poly(phenylene vinylene) used in high performance organic photovoltaics. For optimizing the efficiency of very highly stable organic electronic systems under the effects of irradiation, the π-conjugated polymer polyacetylene can be considered as the optimal choice of a very stable conducting polymer upon irradiation with laser light in thin layers. As the p-type semiconductor, polyacetylene is soluble in halogenated hydrocarbons and aromatic solvents. Poly(1,2-bis(benzylthio)acetylene) is an example of polyacetylene derivative. Regarding the chemical structure of polyacetylene, conjugation term can be referred to as the regular alternation of single and double bonds between atoms of a molecule. That is why π-conjugated polymers can be described as a π-conjugated material. Additional examples of π-conjugated polymers include butadiene, benzene, furan resins, dimethylformamide, and allyl carbocations.58,94-96,98 Polybutadiene is the πconjugated polymer that belongs to unsaturated aliphatic hydrocarbons having general formula (C4H6)n. Its double bonds are separated by sigma bonds. Benzene is an aromatic hydrocarbon combining a ring of six carbon atoms. These atoms are bound by alternating single and double bonds. Furan is the π-conjugated aromatic compound having the fivemembered ring with two alternating double bonds and oxygen. Dimethylformamide is the π-conjugated polar aprotic solvent that can be added to polymeric conducting films to increase their conductivity. Allyl carbocations are electronic π-conjugated carbocations CCiN with a vinyl group as a substituent (next to a double bond). Annulenes ANN are completely conjugated polymers of monocyclic hydrocarbon nature.

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According to IUPAC, the electronic conjugated polymeric family of annulenes, are named as n-numbered annulenes ([n]ANN), where [n] indicates the number of carbon atoms in their ring. For example, [4]ANN is named cyclobutadiene, [6]ANN is named benzene, [8]ANN is named cyclooctatetraene, [14]ANN is named cyclotetradecaheptaene, [18]ANN is named cyclooctadecanonaene, and [22]ANN is named cyclodocosahendecaene. Selecting n-numbered annulenes as an optimal polymeric family for structuring organic electronic systems structured for absorbing visible light due to their high energy. This family includes cyclobutadiene ANN (IUPAC name: cyclobuta-1,3-diene), benzene [6]ANN (IUPAC name: benzene), cyclooctatetraene [8]ANN (IUPAC name: cycloocta1,3,5,7-tetraene), cyclotetradecaheptaene [14]ANN ([IUPAC name: (1E,3Z,5E,7Z,9E,11E,13Z)-cyclotetradeca-1,3,5,7,9,11,13-heptaene), and cyclooctadecanonaene [18]ANN. Pigment conjugated systems represent the optimal class of conjugated electron systems for organic electronic systems fabricated for absorbing visible light due to their ability to give rise to strong colors. The function of pigment conjugated systems depends on the principle “when an electron in the system absorbs a photon of light of the right wavelength; it can be promoted to a higher energy level.” The lowest possible absorption energy corresponds to the energy difference between the highest occupied molecular orbital HOMO and the lowest unoccupied molecular orbital LUMO. Examples of HOMO-LUMO absorption wavelengths for the conjugated polymers are 217 for 1,3butadiene, 252 for hexatriene, and 304 nm for octatriene.99-101 Phthalocyanine is the first compound of π-conjugated polymers used as conjugated pigment system. The chromophore is the conjugated polymer consisting of a series of conjugated bonds. 1,4-pentadiene is a type of homo-conjugated polymer. Oxetane-functionalized conjugated polymers represent the family of crosslinkable conjugated polymers related to electroactive polymers. All these electronic π-conjugated polymers and pigments can be considered as members of optimal π-conjugated polymers for structuring thin films of organic electronic systems due to their high solubility, good film-forming properties in the non-crosslinked form. They can achieve complete insolubility upon crosslinking.375 Oxetane-functionalized conjugated polymers represent the optimal family of electronic crosslinkable conjugated polymers for the thin films of organic field-effect transistors due to their considerable hole mobilities µ+. This family includes the following polymers and copolymers375 1. polymers: poly(spirobifluorene)s PSBF, derivatives of polyfluorine copolymers, carbazole, triphenylamine derivatives, and oxetane-functionalized poly(3-alkylthiophene) Ox-P3AT 2. copolymers: polyfluorine copolymers (with the following derivatives, fluorenephenylene copolymers, fluorene-alt-bithiophene copolymers, and hyperbranched polyfluorenes). Important abbreviations: Ox: oxirane. PSBF: poly(spirobifluorene). H-Ox: hexyloxy-oxetane. F8T2: poly(9,9'-dioctyl fluorene-cobithiophene). F8T2Ox1: oxetane-functionalized derivative of poly(9,9-dioctylfluorene-alt-bithiophene): with one oxetane group. F8T2Ox2: oxetane-functionalized derivative of poly(9,9-dioctylfluorene-alt-bithiophene): with two oxetane groups. Cz-Ox: oxetane-functionalized compounds containing carbazole moieties. ICz: indolo[3,2b]carbazole-based materials. TPA: triphenyl amine. DBr: dibromide. Ar: aryl group.

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Poly(9,9'-dioctyl fluorene-co-bithiophene) F8T2 (also called poly(9,9-dioctyl fluorene-alt-bithiophene) is the optimal type of fluorene-alt-bithiophene conjugated copolymers used for structuring organic field-effect transistors films OFETFs due to its hole mobility µ+ up to 10-2 cm2/Vs. Both poly(9,9-dioctylfluorene-alt-bithiophene) F8T2Oxx1 (single oxetane group) and poly(9,9-dioctylfluorene-alt-bithiophene) F8T2xOx2 (two oxetane groups) are examples of oxetane-functionalized derivatives of F8T2-based oxitane family. Indolo[3,2-b]carbazole as a type of oxetane-functionalized compound containing carbazole moieties is soluble in common organic solvents and yields insoluble films upon crosslinking in the presence of photo-acids. It shows high performance in hole transport layers HTL.376 Heterocyclic conjugated polymers represent the optimal electronic conjugated polymer-based on heterocyclic groups used in heavy duty organic electronic systems because they have the best stability among electronic conjugated polymers (in addition to the highly enhanced mechanical and thermal properties, such as strength, modulus, and glass transition temperature Tg). This family includes phthalazinone, quinazolinone, benzopyrenequinone, and acridone groups. The incorporation of the heterocyclic group into polymer chain results in enhanced properties such as strength, modulus, and glass transition temperature. They are used in organic field-effect transistors, organic light-emitting diodes, and flexible organic photovoltaics. To optimize the functions of p-conjugated polymers containing nitrogen (including heterocyclic polymers such as pyridine, pyrimidine, quinoline, and quinoxaline) for structuring organic electronic systems needing high levels of electrical conductivity, they have to be doped with specific doping material to achieve high levels of electrical conductivity.377-378 A cyclic compound (also called ring compound) is a compound in which one or more series of atoms in the compound are connected to form a ring. On the other hand, carbocyclic compounds (in addition to natural cyclic compounds) are available in the form of complex cyclic compounds. A heterocyclic compound is a cyclic compound in which the ring(s) of its atoms combine at least two different elements. In fact, heterocyclic systems can be considered as the optimal conjugated compounds for building-blocks of electrically conductive systems, especially those for biological applications.92,101,379 The optimal grades of “5-membered heterocyclics 5-MH”92,101,379 used for structuring organic electronic having thermal stability include pyrrole PYRL, furan Fr, thiophene TH, imidazole IMs, oxazole Oxz, thiazole TZ, pyrazole PyZl, isoxazole IXZ, isothiazole ITZ, and indole ID. The optimal grades of “6-membered heterocyclics 6-MH”97,161 include pyridine Pyr, quinoline QNl, pyrylium PYLM, pyridazine PyDz, pyrimidine PyMd, pyrazine PYZ, and isoquinoline IQL.379 Pyrrole, furan, and thiophene are the optimized grades of 5-membered heterocyclics because the structures of their π-electrons have planar, aromatic, isoelectronic with cyclopentadienyl anion form. Pyridine Pyr is a member of optimized 6-membered heterocyclics 6-MH. It has ability to resist oxidation at ring carbon atoms. Moreover, it has the ability to undergo side-chain oxidation as a preference for oxidation of the ring. Both quinoline QNl and isoquinoline IQl are also optimized members of 6-membered heterocyclics 6-MH

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because their electrophilic reactions can be easily achieved by ammonium salts in the presence of strong acidic conditions.92,101 Among groups of heterocyclic conjugated polymers, three of them can be selected as the most suitable groups for organic electronic systems of high thermal stability. They are as follows58,92,94,101,104-106 • 1st group − poly(thiazole)s PThZ, poly(thiadiazole)s, poly(benzothiazoles)s, and poly(thioimidine)s • 2nd group − poly(selenophene)s PSph, poly(pyridine-2,5-diyl) PPY, poly(pyridazine-3,6-diyl) PPDD, poly(quinolinediyl-5,8-diyl) PQND, poly(isoquinoline1,4-diyl) PIQL, poly(isoquinoxaline-5,8-diyl) PIQX, poly(1,5-naphthyridine-2,6diyl) PNRD, polyquinoxaline PQX, and poly(quinoxaline-5,8-diyl) 5,8PQX • 3rd group − polyquinolines, polyanthrazolines, poly(phenylquinolone), poly(phenylquinoxaline)s, polytriazines, polypyrrolones, polyimide, polyamideimide, polybenzimidazole, polysiloxane, and poly(benzoxazole)s. Among sulfur-containing heterocyclic conjugated polymers, poly(thiazole)s PThZ, poly(thiadiazole)s, poly(benzothiazoles)s, and poly(thioimidine)s are the most interesting members of this family because of their high thermal stability, good solubility, and processability. Thiazole (or 1,3-thiazole)-based heterocyclic conjugated polymers are those containing the chemical elements of sulfur and nitrogen. Both conjugated polymers have large π-electron delocalization and great aromaticity. As a type of five-membered ring compounds, thiadiazole is a heterocyclic organic compound combining one sulfur and two nitrogen atoms. Among “5-membered 1,3-thiazole ring fused to a benzene ring” electrically conjugated compounds benzothiazoles-based conjugated polymers represent the optimal group for organic light-emitting devices. For example, poly(selenophene)s, poly(pyridine-2,5-diyl), poly(pyridazine-3,6-diyl), poly(quinolinediyl-5,8-diyl), poly(isoquinoline-1,4-diyl), poly(isoquinoxaline-5,8-diyl), poly(1,5-naphthyridine-2,6-diyl), polyquinoxaline, and poly(quinoxaline-5,8-diyl) are among the most applied heterocyclic conjugated polymers in organic light-emitting devices due to their heterocyclic structures, which provide conjugated polymers with tunable properties such enhanced stability. The azo-substituted poly(p-phenylene)s poly(pyridine-2,5-diyl) is the best grade characterized by stability in the air (without oxidation) with a sufficiently large bandgap to generate electroluminescence in the blue wavelength region. Pyridazine-3,6-diyl moiety of poly(pyridazine-3,6-diyl) can be used as an acceptor (with arylamines as the donor) for forming “two-photon absorption compounds” of dipolar and quadrupolar types. Poly(1,5naphthyridine-2,6-diyl) is a linearly structured electrically conjugated polymer that can be synthesized by the electrochemical de-halogenation polycondensation of 2,6-dichloro-1,5naphthyridine using nickel complexes. Polyquinolines, polyanthrazolines, polyphenylquinolone, poly(phenylquinoxaline)s, polytriazines, polypyrrolones, polyimide, polyamide-imide, polybenzimidazole, polysiloxane, and poly(benzoxazole)s are the representatives of the optimal group of electrically heterocyclic conjugated polymers for structuring the thin films of organic optoelectronic systems of low bandgap properties. For example, polyquinolines and polyanthrazolines are originally organic optoelectronic polymers of novel applications, such as forming thin films for optical devices with bandgap of

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2.0-3.1 eV. In addition to that, they can be classified among the optimal n-type semiconducting polymers due to their reversible reduction with formal potentials of -1.57 to -2.08 V. Among conjugated polymers containing 9,9-diphenylfluorene moieties, polyamideimide represents the optimal member because it is completely conjugated polymer containing amide units in its main chain. This optimized polymer is soluble in aprotic polar solvents. Importance of such polymer is its ability to form optically transparent films by solution casting. The aromatic ring structured polyimide is the high performance conducting polymer characterized by unique thermal, chemical, radiation, and thermo-oxidative stabilities with outstanding mechanical and electrical properties. Polybenzimidazole is the optimal aromatic heterocyclic conjugated polymer for structuring organic electronic systems, fibers, and films if fire-resistance is required. Nanoparticles-based conjugated polymers represent the optimal types of conjugated polymers and oligomers (especially those based on fluorine polymers and oligomers) used for structuring organic optoelectronic systems such as organic light-emitting diodes, organic photovoltaics, organic imaging and sensing systems, and organic field-effect transistors due to their tunable and exceptional fluorescent properties and their high absorption cross-section, excellent fluorescence brightness, and high chromophore density.92-93,101,104,110 Nanoparticle-based conjugated fluorine oligomers and polymers include93,111 1. fluorene-acetylene polymer FAP, polyfluorenes PFs, poly(9,9-dioctylfluorene) P9FO, poly(ethylhexyl fluorine) PF2/6, poly(trimethyl dodecyl fluorine) F1112, azide-base polyfluorenes PFAz, poly(9,9-dioctylfluorene)-base poly(p-phenylene vinylene) POPPV, and fluorene-fluorenone copolymers (such as 2,7-poly(9,9-dialkylfluorene-co-fluorenone) PFFO) 2. oligoflurene O-Fs, (4-(dicanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4Hpyran DCM, naphthalene-based fluorene FNF, quinoxaline-based fluorene FQF, thienopyrazine-based fluorene FTF, and benzothiadiazole-based fluorene FBF. The function of nanoparticle-based conjugated polymers using polymers and fluorene oligomers can be optimized by utilizing fluorene moieties as building blocks for πconjugated systems due to their high charge carrier mobility, good solubility in organic solvents, and high tunable fluorescence. Fluorene-acetylene polymer is considered as an optimized member of polymers (not based on oligomers) due to its variable emission wavelength from the blue for the pure fluorene polymer to red for 2% incorporated perylene diimide dye in the copolymer due to (partial) energy transfer. The most important feature of such nanoparticles is their ability to shift the emission wavelength of the polyfluorene nanoparticles PFs-NPs (in comparison with the polymer in chloroform) from blue to green and the quantum yield (which is normally seen for π-conjugated polymer films). The efficiency of poly(9,9-dioctylfluorene)-base poly(p-phenylene vinylene) for structuring organic optoelectronic systems such as organic light-emitting diodes can be optimized by color tuning of the electroluminescence from blue to green, which can be achieved by the energy transfer from the poly(9,9-dioctylfluorene) energy donor to the poly(9,9-dioctylfluorene)-base poly(p-phenylene vinylene) being the energy acceptor. Poly(9,9-dioctylfluorene)s can be used as blue-emitting polymer. Among nanoparticlebased conjugated polymers/oligomers, both oligoflurene and (4-(dicanomethylene)-2methyl-6-(4-dimethylaminostyryl)-4H-pyran are optimal “light-emitting oligomers” (especially oligoflurene due to its ability to be self-assembled to stable nanoparticles upon

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injection of tetrahydrofuran THF solution into water, causing a blue emission). Among the electrically conjugated supra-molecule fluorene family, naphthalene-based fluorine, quinoxaline-based fluorene, benzothiadiazole-based fluorene, and thienopyrazine-based fluorene can be considered as the optimal members due to their ability to act as light-sensing and imaging oligomers. In conclusion, typical characteristics of π-conjugated polymers include • they are of organic molecules • can be doped to increase their conductivity • used for structuring organic light-emitting diodes and photovoltaic cells • have low energy of excitations in the visible spectral region • can be considered as semiconducting due to their framework of alternating single and double carbon-carbon bonds • σ-bonds can be found in all conjugated polymers • most conjugated polymers have a bandgap between highest occupied molecular orbital and lowest unoccupied molecular orbital ranging from 1.5 to 3 eV and a high absorption coefficient of ~105/cm • most conjugated polymers only absorb light in the blue and green; they only absorb light having wavelengths of less than 650 nm • and they have a bandgap larger than 1.9 eV.92-93,95,106 3.4.2.2 Electronic doped conducting polymers The electronic doped conducting polymers represent the group of conventional polymers with the ability to become electrically conductive polymers by “doping process” with specific doping materials, which cause the removal of electrons (also called oxidation or abbreviated as p-doping), or injecting electrons (also called reduction or abbreviated as ndoping). During this process, electrons in the π-bonds jump around the polymer chain, so that as soon as they moved along the molecule, the electric current can be generated. For example, an oxidation process with chemical doping material called iodine I2, motivates electrons to be jerked out of the polymer, leaving “holes” in the form of positive charges that can move along the chain. By doping, the conductivity of the polymer can be increased from 10-3 to 3000 Sm-1. Dopants (also called doping materials or charge-transfer agents) are chemical materials able to act as electron acceptors or electron donors. For the applications of π-conjugated polymers, the nature of dopants is either “ion” (Cl- or small molecules such as polydimethylsiloxane) or “polymer” (such as polystyrene sulfonate). In conclusion, the main two types of doping that result in doped conductive polymers are pdoping (oxidative doping) and n-doping (reductive doping). According to equation [3.1],88,96 p-doping (also called oxidative doping) can be achieved by the oxidation process (removal of (e-) from the polymer π-backbone). This formation is known as “polaron.” The second oxidation of polaron formation results in the recombination of radicals that yields two positive charge carriers on each chain mobile nature. Figure 3.996,258 shows the p-doping mechanism of polyacetylene. p-doping: Polymer (such as polyacetylene) + A → (Polymer)n+ An-

[3.1]

3.4 Electronic conductive (conjugated and doped) polymers

81

where: polymer (polymer)n+ AnA

denotes polyacetylene, polypyrrole, polythiophene, etc. denotes the p-doped polymer such as “p-doped polyacetylene” denotes dopant.

Dopants (also called doping materials or oxidative dopants for p-doping process) represent “Lewis acids” (including iodine I2 vapor, iron(III) chloride FeCl3, iodine/carbon tetrachloride I2/CCl4), bromine, arsenic pentafluoride, aluminum chloride, molybdenum(III) chloride, molybdenum(V) chloride, etc.87,100

Figure 3.9: A representation of p-doping mechanism of polyacetylene (as an electronic doped conducting polymer). [Data from references 96,258]

According to some scientists, “soliton” indicates the conjugational defect in polyacetylene as an example. A soliton can be considered as a pseudo-particle, while some scientists consider it as being responsible for the high electric conductivity. In fact, some solitons always exist on a polyacetylene chain as a consequence of synthesis. Solitons can be created in the form of pairs (soliton/antisoliton pairs: meaning the conservation of particle number). According to Figure 3.9, the doping mechanism of polyacetylene can be represented by the following four steps • Case (1) which represents the addition (removal) of an electron at the bottom of the conduction band (from the top of the valence band) of polyacetylene • Case (2) that shows the partial filling of the conduction valence band, where the radical anion (cation) is named a polaron • Case (3) represents the formation of “bipolaron” upon the dimerization of two polarons as a result of the addition (removal) of the second electron on a chain (of a negative/positive polarons). This step can lower the total energy. • Case (4) shows the capability of bipolarons to lower the total energy.62,92,96,113 The resultant electrical conductivities (Siemens/cm or S/cm) of polyacetylene doped with several types of oxidation are listed in Table 3.4.96,356

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Table 3.4. Examples of electrical conductivity [σ] values resulting from doping polyacetylene with several types of (oxidative dopants).96,356 Oxidative dopants

Electrical conductivity, σ [S/cm]

Iodine I2

360 (vapor)

Iodine monobromide

360 (vapor)

Hydrogen bromide

7x10-4 (vapor)

Arsenic pentafluoride

560 (vapor)

Selenium hexafluoride

180 (vapor) 180 (CH3NO2) 9.0 (toluene)

Molybdenum(V) chloride

563 (anisole) 563 (toluene)

Tungsten hexachloride

365 (toluene) 8.48 (anisole)

Note: According to equation [3.2],88,96 n-doping (reductive red-doping) can be achieved by a reduction process (represented by the addition of an e- to the polymer). According to this state, both polaron and bipolaron can be formed in two steps. Such formation should be followed by the recombination of radicals forming two negative charge carriers on each chain of polyacetylene (responsible for the required electric conduction). Figure 3.1096,258 shows the mechanism of the n-doping of polyacetylene. n-doping: Polymer (such as polyacetylene) + B → (Polymer)n- Bn+

[3.2]

where: polymer B (polymer)n- Bn+

represents polyacetylene, polypyrrole, polythiophene. etc. represents dopant material (also called reductive dopants Re for n-doping process) such as Lewis base, sodium Na, potassium K, lithium naphthalide, etc. represents the n-doped polymer, such as n-doped polyacetylene.

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83

Figure 3.10. A representation of n-doping mechanism of polyacetylene. [Data from references 96,258.]

3.5 ELECTRONIC PIEZOELECTRIC AND PYROELECTRIC POLYMERS “Piezoelectricity” denotes electricity generated by applied pressure, and the “piezoelectric effect” is the relation of linear electromechanical interaction between the electrical state and the mechanical action on crystalline materials. It can be expressed by the “piezoelectric strain coefficient” (abbreviated as d31 [pC/N]). Other applied properties include “piezoelectric stress coefficients” (abbreviated as g31, g33 ((V/m)/(N/m), (Vm/N), or (m/ m)/(C/m2)) and “electromechanical coupling factor” (abbreviated as kt, k31).380-381 “Electronic piezoelectric polymers” are the optimal group of amorphous and semicrystalline polymers for use in organic actuators, robots, and artificial muscles due to their piezoelectric poling properties. “Poling” term denotes the process by which piezoelectric materials could be induced to be piezoelectric. Piezoelectric polymers can be classified as “piezoelectric semi-crystalline polymers” and “piezoelectric amorphous polymers.” Piezoelectric polymers include poly(p-xylylene), polysulfone, poly(vinyl fluoride), poly(bis-chloromethyloxetane), aromatic polyamides, synthetic polypeptide, and synthetic cyanoethyl cellulose.381-382 “Pyroelectricity” term describes the ability of certain materials/polymers to generate a temporary voltage upon exposure to heating or cooling conditions. “Pyroelectric effect” phenomenon denotes the part between electrical and thermal corners of a triangle crystal consisted of kinetic, electric, and thermal energies or angles). Pyroelectric coefficient (also called “remnant polarization”) Pr [mC/m2] is an important property related to pyroelectricity, meaning the change in the spontaneous polarization vector with temperature. Remnant polarization represents the polarization during poling minus the atomic and electronic polarizations.116 Pyroelectric materials can be classified as “single crystals,” “ceramics,” “polymers,” and “thin films.” “Electronic pyroelectric polymers” represent the optimal group of these pyroelectric materials due to their capability of satisfying the general

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requirements of all pyroelectric materials (at the same time) such as high pyroelectric coefficient, low relative permittivity, physical and chemical stability, low piezoelectric response, low cost, high quality, ease of processing, and stability against de-poling.117 Examples of such electronic pyroelectric polymers include383 1. poly(vinylidene fluoride) and its copolymers such as vinylidene fluoride copolymers 2. poly(vinylidene fluoride) tetrafluoroethylene copolymer56,116-117 3. poly(vinylidene fluoride) trifluoroethylene copolymer. Important to remember that both “piezoelectric polymers” and “pyroelectric polymers” (in addition to “ferroelectric polymers”) are the main three types of electroactive polymers.383 The simple method to distinguish ferroelectricity, piezoelectricity, and pyroelectricity is the nature of crystal materials as shown in Figure 3.11.4

Figure 3.11. A simple method to distinguish ferroelectricity, piezoelectricity, and pyroelectricity by the nature of crystalline materials. [Adapted, by permission, from Kwang J. Kim and Satoshi Tadokoro, Electroactive Polymers for Robotic Applications: Artificial Muscles and Sensors, © Springer-Verlag London Limited, 2007.]

According to Figure 3.12,117 the electronic piezoelectric polymers can be classified as 1. 2. 3.

“electronic bulk piezoelectric polymers” related to the piezoelectric semi-crystalline polymers and amorphous dipolar polymers or piezoelectric amorphous polymers “electronic piezoelectric/polymer composites” available in the form of piezoelectric particles embedded in polymer and piezoelectric pillars inside the polymer “electronic voided charged piezoelectric polymers” available in the form of polymers with air voids with charged surfaces in polar form.

Figure 3.12. A representation of classification of electronic piezoelectric polymers. [Data from reference 117.]

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3.5.1 ELECTRONIC BULK PIEZOELECTRIC POLYMERS The electronic bulk piezoelectric polymers represent the optimal group of electronic piezoelectric semi-crystalline polymers and electronic piezoelectric amorphous polymers for structuring high-performance inorganic actuators due to their inherent piezoelectric effect resulting from the molecular structure of the base polymer and its orientation. Electronic bulk piezoelectric polymers are characterized by the molecular structures that are inherently containing molecular dipoles. The dipoles are able to be reoriented within the bulk material and kept in their preferred orientation state.117 Electronic piezoelectric semicrystalline polymers can be considered as semicrystalline polymer-based piezoelectric actuation. The semicrystalline polymers (such as poly(vinylidene fluoride) PVDF, polyimide, liquid crystal polymers, and Parylene-C) are not completely crystalline because they have amorphous phases (partially crystalline or semicrystalline). To optimize their piezoelectricity, they must have polar crystalline phases. The electronic piezoelectric amorphous polymers also include amorphous polymer-based piezoelectric actuation. In fact, piezoelectricity in amorphous polymers differs from that in semicrystalline polymers and inorganic crystals. For example, the orientation polarization of molecular dipoles in amorphous polymers is responsible for their piezoelectricity. The most important piezoelectric properties of electronic piezoelectric amorphous polymers are their glass transition temperature Tg because it defines the poling process conditions.56,116-117 Examples of electronic piezoelectric semicrystalline polymers include4,56,116-117 1. poly(vinylidene fluoride) PVDF, poly(vinylidene fluoride) trifluoroethylene copolymer PVDF-TrFE, polyamide-11 PA11, polyurea-9 P-UR9 having properties listed in Table 3.5.116,229,234,248 2. polyimide, polyamide-5, Parylene-C, and biopolymers such as synthetic polypeptide (which combines polymethylglutamate and polybenzyl-L-glutamate) 3. bilaminates (also called Nylon 11318-poly(vinylidene fluoride) bilaminates) 4. the thermosetting piezoelectric polyureas such as polyrurea-9 P-UR9 and polyurea11 P-UR11. Among this group of polymers, bilaminates can be considered as the optimal ones for structuring organic electromechanical systems due to its considerable value of piezoelectric strain coefficients d31 that ranges from 53 to 62 [pC/N]. Table 3.5. Comparison of piezoelectric properties of some piezoelectric semicrystalline polymers.116,229,234,248 Properties Piezoelectric semicrystalline polymers

Glass transition temp. [oC]

Melting temp. [oC]

Max. use temp. [oC]

Piezoelectric strain coefficients d31 [pC/N]

Piezoelectric strain coefficients d33 [pC/N]

Poly(vinylidene fluoride) PVDF

-35

175

80

20-28

-37.7

Poly(vinylidene fluoride) trifluoroethylene copolymer PVDF-TrFE

32

150

90-100

12

-31.4

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Optimized Electronic Polymers, Small Molecules, Complexes, and Elastomers for Organic

Table 3.5. Comparison of piezoelectric properties of some piezoelectric semicrystalline polymers.116,229,234,248 Properties Piezoelectric semicrystalline polymers

Glass transition temp. [oC]

Melting temp. [oC]

Max. use temp. [oC]

Piezoelectric strain coefficients d31 [pC/N]

Piezoelectric strain coefficients d33 [pC/N]

Bilaminates (nylon 11poly(vinylidene fluoride) bilaminates)

68

195

185

3 at 25oC (4 at 107oC)

−

Polyrurea-9 P-UR9

50

180

−

−

−

Poly(vinylidene fluoride) PVDF (polymerization of H2C=CF2 monomers) is the most used polymer for organic electromechanical systems, but it cannot be considered the optimal because its piezoelectric strain coefficient d31 (20-28 pC/N) is lower than that of bilaminates (53-62 pC/N). On the other hand, it can be optimized by forming conductive films because of their ability to create defects in their connecting chains, which can be observed by connecting two fluorene groups or CH2 groups. For films fabricated from this polymer, the increase of defects in the polymeric chain causes an increase of polarity, which increases the piezoelectric response. The secret behind the excellent properties of poly(vinylidene fluoride) and its copolymers such as poly(vinylidene fluoride-co-trifluoroethylene) (as an electronic piezoelectric polymer) is in their crystalline structure that is responsible for creating the piezoelectric effect. Films fabricated from poly(vinylidene fluoride-co-trifluoroethylene) exhibit (at room temperature) a piezoelectric coefficient d33 of (-38 pm/V) and a coupling factor k33 of 0.33. The odd-numbered polyamides316-317 (such as polyamide-5, polyamide-7, and polyamide-11) are interesting electronic piezoelectric polymers, but they have lower piezoelectric constants than poly(vinylidene fluoride) (at room temperature). For example, the odd-numbered polyamide316-317 grades, such as polyamide-7 PA-7 and polyamide-11 PA-11 have piezoelectric strain coefficients d31 of 17 [pC/N] and 14 [pC/N] with coupling factors k31 of 0.054 and 0.049, respectively. The thermosetting piezoelectric polyureas, such as polyurea-9 PUR-9 and polyurea-11 PUR11 are available in the form of insoluble powders of highly crosslinked resins having various aliphatic or aromatic groups. They cannot be considered as optimal grades for structuring organic electromechanical systems because their piezoelectric strain coefficients d31 15 (pC/N) are lower than that of bilaminate (53-62 pC/N). They can be useful in specific organic electromechanical systems, where high pyroelectric coefficient (not piezoelectric coefficient), high thermal stability, and low dielectric loss are required. The piezoelectric strain coefficients d31 of these two types of polymers can be optimized in organic electromechanical systems, in which high resistance to temperature is required because they have the ability to increase their piezoelectric strain coefficients as the temperature increases.4,56,116-117 The group of electronic piezoelectric amorphous polymers used for organic sensors and electronic systems, where large dielectric relaxation strength is required, includes116,119

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87

1.

nitrile substituted polymers such as polyacrylonitrile PAN, poly(vinylidenecyanideco-vinyl acetate) PVDCN-VAC, poly(vinyl chloride) PVC, poly(vinyl acetate) PVAc, polyphenylethernitrile PPEN, and poly(1-bicyclobutanecarbonitrile) 2. nitrile containing polyimide (abbreviated as (β-CN)APB-ODPA, where, APB is the abbreviation of 1,3-aminophenoxy benzene (a type of diamine), (β-CN) APB is the abbreviation of 2,6-bis(3-aminophenoxy)benzonitrile (also a type of diamine), and ODPA is the abbreviation of 4,4'-oxydiphthalic anhydride. Properties of some piezoelectric amorphous polymers are listed in Table 3.6.116,234,248 For example, vinylidene cyanide VDCN-based copolymers such as poly(vinylidenecyanideco- vinyl acetate)s are considered optimized polymers for such applications because they exhibit large dielectric relaxation strengths and strong piezoelectricity. Similarly, poly(vinylidenecyanide-co-vinyl acetate) has been selected for such applications because it exhibited large relaxation strength, the largest value of pyroelectric coefficient (remnant polarization) Pr of 55 [mC/m2]. Poly(vinyl chloride) and poly(vinyl acetate) are known as polymers having weak piezoelectric activity. The carbon-chlorine dipole in poly(vinyl chloride) can be oriented to produce a low level of d31 of 0.5 to 1.3 [pC/N]. Among the above piezoelectric polymers, nitrile containing polyimide (β-CN)APB-ODPA can be selected as an optimal choice for high temperature piezoelectric organic sensors because it has the highest glass transition temperature (220oC) among piezoelectric amorphous polymers. Its chemical structure involves polar functional groups, and it has a piezoelectric strain coefficient d31 of 5 [pC/N] at 150oC and a pyroelectric coefficient (remnant polarization) Pr of 20 [mC/m2]. Table 3.6. Comparison of piezoelectric properties of some piezoelectric amorphous polymers.116,234,248 Properties Piezoelectric amorphous polymers

Glass transition temp. [oC]

Remnant polarization Pr [mC/m2]

Piezoelectric coefficient d31 [pC/N]

Poly(vinyl chloride) PVC

80

16

5

Polyacrylonitrile PAN

90

25

2

Poly(vinyl acetate) PVAc

30

5

−

Poly(vinylidenecyanide-co-vinylacetate) PVDCN-VAC

170

50

10

Polyphenylethernitrile PPEN

145

12

−

Nitrile-containing polyimide (β-CN)APB-ODPA

220

20

5 at 150oC

3.5.2 ELECTRONIC PIEZOELECTRIC/POLYMERIC COMPOSITES The electronic piezoelectric/polymeric composites (also called electronic piezocomposites) are polymers (not electromechanically active such as poly(vinylidene fluoride)) with embedded inorganic piezoelectric material such as ceramic. An impediment process makes piezocomposites to become an optimized member because this process combines the advantages of both filler and polymers, such as higher coupling factor and dielectric constant of ceramics and the mechanical flexibility of polymers. These electronic piezocomposites can be classified as “electronic piezoelectric particles embedded in polymers”

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Optimized Electronic Polymers, Small Molecules, Complexes, and Elastomers for Organic

or “electronic piezoelectric pillars inside polymers.” The electronic piezoelectric particles embedded in polymers are available in the form of rods or particles embedded in a polymer. They are prepared for electromechanical devices of a millimeter to micrometer scales. The electronic piezoelectric pillars dispersed in polymers are in the form of inorganic cylindrical or square pillars embedded in the polymer matrix. The gap between every two pillars (pitch) should be in the range of 130-1000 µm.56,117 3.5.3 ELECTRONIC VOIDED CHARGED PIEZOELECTRIC POLYMERS The electronic voided charged piezoelectric polymers (also called electronic cellular polymers, such as the cellular polydimethylsiloxane PDMS) can be considered as an optimized member of electronic piezoelectric polymers due to containing internal gas voids in their structures. Such structures exhibit a high piezoelectric coefficient d33 of 20x103 [pC/N]. Note: As soon as the polymer surfaces surrounding the voids are charged, the voided charged polymer behaves as an electronic piezoelectric material considering electrical and mechanical energy. This concept can be called as “ferroelectrets” or “piezoelectrets.” The electronic voided charged piezoelectric polymers have the ability to reduce actuation through polarization, exhibit attractive advantages such as high strength, high impact resistance, low dielectric constant, and elastic stiffness, and have very low density with higher voltage resistivity, low acoustic and mechanical impedance, high dielectric breakdown and operating field strength, good sensor characteristics, and the ability to pattern electrodes on the film surface.4,116,119

3.6 MICROELECTRONIC POLYMERS The microelectronic polymers form a group of electronic polymers used in organic microelectronic systems due to their low capacitance constants. The family of microelectronic polymers includes the following important members 1. microelectronic three-dimensional conjugated macromolecules used as light-emitting materials (such as hyperbranched polymers) 2. microelectronic low-k polymers of low-k dielectrics such as poly(m-diethynyl benzene) of low dielectric constant (k=2.7) 3. organic/inorganic hybrid nanocomposites for general-purpose organic microelectronic systems.57,69,123,125-126 3.6.1 MICROELECTRONIC THREE-DIMENSIONAL CONJUGATED MACROMOLECULES Among microelectronic polymers, the microelectronic three-dimensional conjugated macromolecules can be considered as the optimal member for structuring high performance organic microsized light-emitting diodes because of their ability to decrease aggregation of materials, resulting in the improvement of the light-emitting efficiency. As illustrated in Figure 3.13,96,106,127,254 microelectronic three-dimensional conjugated macromolecules can be classified as hyperbranched polymers, dendrimers (related to dendritic polymers), dendronized (dendrigrafted) polymers, star-shaped polymers, spirobifluorene-based polymers, and spiro-bridged ladder-type oligomers and polymers.126-127

3.6 Microelectronic polymers

89

Figure 3.13. A representation of classification of microelectronic polymers associated with some structural examples.96,106,127,254

3.6.1.1 Microelectronic electronic hyperbranched polymers The microelectronic electronic hyperbranched polymers are the high-branched microelectronic 3-dimensional conjugated macromolecules. The microelectronic hyperbranched polymers have been selected as the optimal class for structuring organic microelectronic systems where chemical and physical working conditions are too-sensitive due to their very large molecules. For example, they are widely applied in electronic devices due to their unique chemical and physical properties such as the highly branched and compact three-dimensional structures, low intrinsic viscosities, and a large number of terminal functional groups. The “degree (fraction) of branching” term with a percentage ranging from 0 to 100% is the property (in addition to molecular weight distribution) that strongly influences the physical and chemical properties of polymeric materials. An optimization of microelectronic hyperbranched polymers begins during their chemical preparation by means of single monomer methodology or double monomer methodology. The resultant conjugated hyperbranched polymers have high-efficiency light emission, light absorption, and active electroluminescent layers of microscaled organic optoelectronic systems. For exceptional microelectronic applications, this class can be chemically prepared in the form of “ion-conducting hyperbranched elastomers” such as electrolytes for organic microelectronics applications, or “nanohyperbranched polymers” for use in microelectronics hostguest encapsulation.125-128 Among the family of conjugated hyperbranched polymers, the following polymers can be considered as the optimal members for microscaled organic optoelectronic systems such as micro-scaled organic light-emitting diodes because they can exhibit exceptional non-linear optical properties and excellent optical limiting properties. These members include125,127 1. microelectronic hyperbranched poly(phenylene vinylene) HB-PPV, microelectronic hyperbranched poly(3,5-bis-vinyl benzene) HB-PBB, microelectronic hyperbranched poly(β,β-dibromo-4-ethynylstyrene) HB-PDE, microelectronic hyper-

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Optimized Electronic Polymers, Small Molecules, Complexes, and Elastomers for Organic

branched poly(arylene/1,1-vinylene) HB-PAV, microelectronic hyperbranched polyarylene HB-PARY, microelectronic hyperbranched poly(9-tetradecanyl-3,6-dibutadienyl-carbazole) HB-PTDDBC, and microelectronic hyperbranched poly(4-(2cyano-2-methoxy-carbonylvinyl)aniline) HB-PCMA 2. microelectronic hyperbranched polycoumarines-(2H-1-benzopyran-2-ones) HBPCBO (with acetate side group), microelectronic hyperbranched polythiophene HBPT, microelectronic hyperbranched polyaniline HB-PANI, and microelectronic hyperbranched polytriphenylamine HB-PTPA 3. ion-conducting hyperbranched elastomers which include126,127 microelectronic hyperbranched polyetherester HB-PEE, microelectronic hyperbranched poly(bis(diethyleneglycol)benzoate) caped with acetyl groups HB-PDBA, and microelectronic hyperbranched poly(bis(diethyleneglycol)benzoate) caped with 3,5bis((3',6',9'-trioxodecyl)oxy)benzoyl groups HB-PDBT. Both microelectronic hyperbranched poly(phenylene vinylene) and hyperbranched poly(3,5-bis-vinyl benzene) are phenylene-vinylene-based polymers with a relatively low yield and low molecular weight. They are optimized members of microelectronic hyperbranched polymers used in optical and magnetic-based microscaled organic electronic systems due to their unique optical, electrical, and magnetic properties (in addition to excellent and desirable processability). The microelectronic hyperbranched poly(β,βdibromo-4-ethynylstyrene) has been selected as an optimized sub-family because it includes both fully conjugated hyperbranched polymer and oligomer. Microelectronic hyperbranched poly(arylene/1,1-vinylene) has alternating arylene and 1,1-vinylene units. Microelectronic hyperbranched polyarylene was the first optimal member of hyperbranched polymers for structuring microscaled organic optoelectronic systems due to excellent optical limiting properties, good thermal stability, and desirable processability. Microelectronic hyperbranched poly(9-tetradecanyl-3,6-dibutadiynyl-carbazole) is used as a hole transport layer in organic light-emitting diodes. Microelectronic hyperbranched poly(4-(2-cyano-2-methoxy-carbonylvinyl)aniline) has non-linear optical properties and can be used as a compound possessing donor and acceptor chromophores. Microelectronic hyperbranched poly(coumarine-(2H-1-benzopyran-2-ones)) is the hyperbranched coumarin-containing polymer that can be used as organic light-emitting material because it contains a higher content of coumarin units. Microelectronic hyperbranched polythiophene is a member of good light-absorbing materials based on conjugating the thiophene group with an improving-solubility alkyl group. Both microelectronic hyperbranched polyaniline and hyperbranched polytriphenylamine have good magnetic properties and transition absorption. Microelectronic hyperbranched polyetheresters are amorphous polymers with high solvating power for appropriate ions. They have good ion transport and electrochemical stability. Microelectronic hyperbranched poly(bis(diethyleneglycol)benzoate) caped with 3,5-bis((3',6',9'-trioxodecyl)oxy)benzoyl groups exhibits higher ionic conductivity than microelectronic hyperbranched poly(bis(diethyleneglycol)benzoate) caped with acetyl groups, but electronic hyperbranched poly(bis(diethyleneglycol)benzoate) caped with 3,5-bis((3',6',9'-trioxodecyl)oxy)benzoyl groups shows better electrochemical stability.125,127 As a class of microelectronic hyperbranched polymers related to the family of microelectronic polymers, nanohyperbranched polymers are light-emitting polymers for struc-

3.6 Microelectronic polymers

91

turing both micro and nanoscaled organic light-emitting diodes. This class includes the following members126-127 1. compositions of amphiphilic hyperbranched polyglycerols HB-PGs with palmitoyl chloride in the presence of pyridine 2. compositions of hydroxy-terminated hyperbranched polyester HB-PESt, and hyperbranched-organometallic poly(1,1'-ferrocenylene(n-alkyl)silyne) HB-PFS. Lightemitting hyperbranched polymers include126-127 hyperbranched carbazole-based polymers HB-CZ, hyperbranched triphenylamine-based polymers HB-TPA, and hyperbranched porphyrin polymers HB-PRO. Compositions of amphiphilic hyperbranched polyglycerols, hydroxy-terminated hyperbranched polyester, and hyperbranched-organometallic poly(1,1'-ferrocenylene(n-alkyl)silyne) are optimal polymers for structuring organic dye-based microelectronics because the composition of amphiphilic hyperbranched polyglycerols is available in the form of dyes suitable for the utilization in irreversible encapsulation of polymer. Compositions of hydroxyterminated hyperbranched polyester are suitable for such applications due to their high molecular weight. Hyperbranched-organometallic poly(1,1'-ferrocenylene(nalkyl)silyne) is desirable for magnetic-based organic microelectronic systems because it has high magnetizability and a negligibly small hysteresis loss. The hyperbranched carbazole-based polymers are suitable for structuring organic microelectronic systems of high working temperatures because they have high blue light quantum efficiencies in solution and solid-state and excellent thermal stability to annealing under air. Hyperbranched triphenylamine-based polymers are used as hole-transporting materials in light-emitting diodes. They have an average molecular weight of 9800. As a high molecular weight polymer with good solubility in common organic solvents, hyperbranched porphyrin polymers are used in organic red light-emitting diodes due to their intense red emission peak at around 655 nm and a shoulder at 713 nm. Characteristics of hyperbranched polymers include the ability to be formed from traditional small molecular monomers or emerging macro-monomers via a one-pot polymerization process. They are widely applied in the form of light-emitting polymers, nanoscience and technology polymers, supra-molecular compounds, hybrid composites, coatings, adhesives, and modifiers.125-126 3.6.1.2 Microelectronic dendrimers The microelectronic dendrimer term indicates the class of three-dimensional conjugated macromolecules of highly branched dendritic polymers (arborols and cascade molecules) such as polyamidoamines and dendrimeric fragments. They can be produced by divergent polymerization such as polyamidoamines, while some of them can be produced by convergent polymerization such as dendrimeric fragments. Microelectronic dendrimers are a sub-family of microelectronic three-dimensional conjugated macromolecules related to the family of microelectronic polymers due to their exceptional structure based on the number of generations (the number of layers or shells of repeat units) and have three major elements of structure, such as a core, an inner shell, and an outer shell. Their chemical structures are shown in Figure 3.14132,357 as an example. They include chemical structures of three types of dendrimers, such as microelectronic polyamidoamine PAMAM, microelectronic cyano-star dendrimer CyS-DRM, and microelectronic aliphatic polyester dendrimer AP-DRM. The optimal known dendrimer is polyamidoamine of diamine core (such as ethylenediamine), which reacts with methyl acrylate, and then another ethylene-

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diamine to make the generation-0 ((G0)PAMAM: means the generation (0) of polyamidoamine). These generations are numbered starting from 0-generation (abbreviated as G0) up to 10-generation (G10) as illustrated in Figure 3.15.240,300-301 “G0” has a molecular weight of 517, while “G10” has molecular weight of 934,720.70,75,106,127,131 Note: The structures are adopted just as examples for star hyperbranched polymers and dendrimers like star hyperbranched polymers. The surface group acts as attachment of various monomers to form branches, and functional core represents the starting point of building generation.

Figure 3.14. Examples of dendrimers.132,357

Microelectronic cyano-star dendrimer is a five-sided macro-cycle of pentagonal cyano-star macro-cycle with cyanostilbene CH donors, binds anions and forms microelectronic dialkylphosphate (3)-rotaxanes. Microelectronic aliphatic polyester dendrimer represents the fifth generation of aliphatic polyester dendrimer bearing 128 terminal hydroxyl groups in five steps.70,75,131

Figure 3.15. A representation of generations of dendrimers.240,300-301

3.6 Microelectronic polymers

93

3.6.1.3 Microelectronic dendronized polymers The microelectronic dendronized polymers are microelectronic dendrimers with a core of linear polymer and a backbone of a conjugated polymer. The microelectronic dendronized polymers represent the optimal class of electronic hyperbranched polymers for organic nanowires and light-emitting polymers because they are obtained by direct polymerization of dendritic macro-monomers, or by attaching dendrons to a linear polymeric core.72,127,132 Conjugated polymers used as backbones of microelectronic dendronized polymers include72,126,132 microelectronic poly(p-phenylene), microelectronic poly(p-phenylenevinylene)s, microelectronic poly(p-phenylene-ethynylene)s, microelectronic poly(triacetylene)s, microelectronic polyacetylene, microelectronic polythiophene, microelectronic binaphthyl-based poly(arylene)s, microelectronic dendronized polyfluorenes DD-PFs, microelectronic dendronized poly(4-hydroxystyrene) DD-PHxS, and microelectronic dendronized porphyrin polymers DD-PRO. The microelectronic polymers, such as poly(pphenylene), poly(p-phenylene-vinylene)s, poly(triacetylene)s, polyacetylene, polythiophene, and binaphthyl-based poly(arylene)s (as some types of microelectronic dendronized polymers) are more attractive for forming small molecules as light-emitting polymers than conjugated polymers. Microelectronic dendronized polyfluorenes have bulky poly(p-phenylene) dendrimer substituent. Microelectronic fluorene-based dendronized polymers have good absorption, emission spectra, and molecular modeling properties. Microelectronic dendronized poly(4-hydroxystyrene) is a microelectronic dendronized linear polymer of high molecular weight and suitable for forming cores of dendronized polymers. Microelectronic dendronized porphyrin polymer is a member of the red-light-emitting family. It can be synthesized by the coupling of dendritic macromonomers and porphyrin monomers. It exhibits good solubility and emission peak at about 605-611 nm.72,126,132 3.6.1.4 Microelectronic star polymers The microelectronic star polymers or “microelectronic star-shaped polymers S-SP” are a class of microelectronic three-dimensional conjugated macromolecules having star-like structures. They can be classified as red and blue light-emitting polymer. They are conjugated, hyperbranched polymers structured of a core and a number of linear chains emanating from it.126 This class includes the following polymers as examples126 microelectronic star-shaped porphyrins SS-PRO, microelectronic fluorene, microelectronic triphenylamine-based star-shaped polymers SS-(FN-TPA), and microelectronic carbazole-oligofluorene)-based star-shaped polymer SS-(CZ-OFN). Microelectronic star-shaped porphyrins have four oligofluorene arms at their meso positions, exhibiting a red emission peak at 658 nm and a shoulder at 715 nm. Microelectronic fluorene and microelectronic triphenylamine-based star-shaped polymers having bromo end-groups exhibit good solubility in common organic solvents, good film-forming ability, and pure saturated red light emission. In addition, they can show a deep red emission at about 662 nm and a shoulder at 726 nm. The microelectronic carbazole-oligo-fluorene-based star-shaped polymer is an optimal member of microelectronic star-shaped polymers available in the form of oligomer for forming films of flexible organic displays due to its chemical structure based on four mon-

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odisperse starburst oligomers bearing a 4,4’,4’’-tris(carbazol-9-yl)-triphenylamine core and oligofluorene six arms.126 3.6.1.5 Microelectronic spirobifluorene-based polymers The microelectronic spirobifluorene-based polymers represent the fifth class of microelectronic spirobifluorene-based polymers related to the family of microelectronic polymers. The microelectronic spirobifluorene-based polymers are building blocks for stable, blue light-emitting polymers due to their high rigid three-dimensional structures, and for microscaled organic optoelectronic systems such as microscaled organic photovoltaics due to their optimized photophysical properties.126 Microelectronic spirobifluorene and fluorene alternating copolymers SPFNC126 are examples of microelectronic spirobifluorene-based polymers. Compared with conventional types such as alkyl substituted polyfluorenes, these complexes can be used in the form of blue light-emitting copolymer because of more stable and narrower blue emission peak (associated with a smaller tail at longer wavelengths). 3.6.1.6 Microelectronic spiro-bridged ladder-type oligomers and polymers The microelectronic spiro-bridged ladder-type oligomers and polymers represent the sixth class of microelectronic spirobifluorene-based polymers of the family of microelectronic polymers. This class can be synthesized by cross-coupling, oxidation, and boron trifluoride-ether-catalyzed cyclization reactions, which result in unique properties (such as the unusual three-dimensional conformation associated with intense emission with a very small Stokes shift) making microelectronic spiro-bridged ladder-type oligomers and polymers the optimal class of electronic three-dimensional conjugated macromolecules. Importance of such unique microelectronic three-dimensional confirmation is that this conformation has very good color stability and prevents planar ladder polymer backbone from aggregation in solid films.126 Boron trifluoride-based oligomer is an example126 of spiro-bridged ladder-type oligomers and polymers because its rigid coplanar structure can enhance its conjugation, carrier mobility, and luminescence intensity. They can be synthesized from a ladder-type oligo-p-phenylene to show good photo-physical properties and to be used in microscaled organic photovoltaics. The “ladder-type oligo-p-phenylene” exhibits blue emission with high fluorescence quantum efficiency. 3.6.2 MICROELECTRONIC LOW-K POLYMERS IN MICROELECTRONICS The microelectronic low-k polymers225 have low dielectric constant80 polymers, and they represent the third important group of microelectronic polymers. The “low-k-dielectric” term describes the relative permittivity εr319 represented by the ratio of the permittivity of a substance to the permittivity of free space. An example of a microelectronic low dielectric constant polymer is poly(m-diethynyl benzene) with k=2.7. Microelectronic low-K dielectric polymers are insulating polymers, but they are classified as microelectronic polymers because of their polarization when subjected to an externally applied electric field. Generally, “k” indicates the physical measure of the electric polarizability of a polymer, while “electric polarizability” indicates the ability of a polymer to induce electric dipoles (separated positive and negative charges) under the externally applied electric

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field. “Polarization P” term is related to the electric field “E” and the displacement “D” as given by the following equations [3.3]-[3.6]225,364 D = ε0 E + P

[3.3]

where: P

related to “E” through “χe” the electric susceptibility of the dielectric, thus

P = ε0χeE

[3.4]

According to equations [3.3] and [3.4]: D = ε0(1 + χe)E = ε0kE where:

ε0 P

[3.5]

permittivity of the free space also the density of atomic electric dipole per unit volume

P = Σp/V = Np

[3.6]

where: P N

dipole moment density of dipoles.

To optimize the function of microelectronic low-k polymers in microelectronics for structuring microscaled organic electronic systems, they should be chemically prepared, so that they can exhibit considerable electrical, thermal, chemical, and mechanical properties. That is why the choice may have a significant effect on a device's performance and lifetime. Most of the microelectronic low-k polymers are suitable for applications in microelectronic devices. Such applications depend on the stability of their chemical structure, thermal and mechanical stability, high glass transition temperature (up to 400oC). Examples of microelectronic low-k polymers include microelectronic linear structured low-k polymers, microelectronic branch-structured low-k polymers, and microelectronic network-structured low-k polymers.1-2,43,125,136 3.6.2.1 Microelectronic linear dielectric polymers The microelectronic linear dielectric polymers are optimal types because of their relative relationship between the electric moment induced by the particles during polarization state and the intensity of the electric field applied to these particles. The polarizability of a dielectric material describes the electric dipole moment per unit volume divided by the electric field and permittivity of space. Generally, the dielectric polymers of low polarizabilities are desirable for designing microelectronic low-K polymers. Also, polymers having single C−C and C−F bonds exhibit low electronic polarizability so that they make fluorinated and non-fluorinated aliphatic macromolecules more attractive for the applications of low-k associated systems.139 The optimal types of microelectronic linear structured low-k polymers used for structuring microsized organic low-k electronic systems include139-140 microelectronic polyimide-based polymers (linear polyimide LPI) such as fluorinated polyimides, polyarylether PAE, polynorbornene rubber PNR, polytetrafluoroethylene, and polysilsesquioxane PSQO. Microelectronic polyimide-based polymers have k values larger than 3.0, but they are more suitable for such applications due to their excel-

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lent mechanical strength, thermal stability, and high chemical resistance. Polyarylether is considered as an optimized member due to its excellent thermo-mechanical properties. Moreover, it can be classified as an isotropic polymer with the glass transition temperature of 175oC and k of 2.8-2.93. The pure hydrocarbon polymer polynorbornene rubber is also a member of the group because of its highest thermal stability among organic polymers and solubility in common organic solvents, with a glass transition temperature of 365oC and k of 2.2. Polysilsesquioxane is also the member of this group due to good thermomechanical properties with k of 2.7.139,142 3.6.2.2 Microelectronic branching structured low-k polymers The microelectronic branching structured low-k polymers represent the optimized subfamily of microelectronic linear structured low-k polymers used for organic microelectronics of low-k values. This sub-family includes the following members144,384 1. microelectronic grafted-based polymers/copolymers GP 2. microelectronic hyperbranched-based polymers. Conjugated polymers can be considered as particular types of electronic branched copolymers in which the side chains are structurally distinct from the main chain. Conjugated polymers are available in the form of electronic di-block, electronic tri-block, electronic symmetric single graft, electronic π-architecture, etc. The optimal type of such electronic polymers for structuring microelectronics is microelectronic polyimide grafted polyhedral oligomeric silsesquioxane POSS-g-PI for structuring microsized organic electronic systems where low-k is required due to its low-k value. On the other hand, microelectronic branching structured low-k polymers include144 (1) conjugated polymers, such as polyimide-grafted polyhedral oligomeric silsequioxane POSS-g-PI and grafted polyimide with methyl methacrylate to polyhedral oligomeric silsequioxane, and (2) hyper-branched-based polymers, such as hyper-branched polycarbosiloxane HB-PCSO. Polyhedral oligomeric silsequioxane POSS has k of 3.2, while polyimide-grafted polyhedral oligomeric silsequioxane of 2.32. The glass transition temperature of polyimide-grafted polyhedral oligomeric silsequioxane is slightly lower than that of polyhedral oligomeric silsequioxane, while the coefficient of thermal expansion CTE is higher. Polyimide-grafted polyhedral oligomeric silsequioxane is used for forming nanoporous crystalline films for structuring microelectronic devices. Hyper-branched polycarbosiloxane used for structuring thin-films of microelectronic devices has high molecular weight with a dielectric constant of 2.6-3.1.144 3.6.2.3 Microelectronic network structured low-k polymers The microelectronic network structured low-k polymers are the third optimized type of microelectronic low-k polymers used for structuring microelectronics of low-k values. They are available in the form of organic and inorganic networks with excellent thermal, mechanical, and chemical properties. Microelectronic organic networks are based on elemental carbon such as diamond-like carbon, while microelectronic inorganic networks such as ceramic and amorphous silica are widely applicable due to their low polarizability and superior thermal and mechanical properties.43,136 The members of choice of microelectronic network structured low-k polymers are43,136

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97

1.

microelectronic organic networks (including diamond-like carbon and benzocyclobutene). Diamond-like carbon is considered as an amorphous organic network structured polymer with a dielectric constant of 2.7 and can be deposited in the form of films with compressive stress spanning values from 200-800 MPa. The thermosetting benzocyclobutene, which is prepared from 1,3-divinyl-1,1,3,3-teramethyldisiloxane-bis-benzocyclobutene monomer, is a silicon-containing derivative for microelectronics applications with a dielectric constant of 2.65 2. microelectronic inorganic networks, including polyhedral oligomeric silsesquioxane, polyhydrogen-silsesquioxane, and polymethyl silsesquioxane. Microelectronic polyhedral oligomeric silsesquioxane includes microelectronic polymethylsilsesquioxane and microelectronic polyhydridosilsesquioxane. Both microelectronic polymers have a dielectric constant of 2.6. 3.6.3 ORGANIC/INORGANIC HYBRID NANOCOMPOSITES FOR MICROELECTRONICS Hybrid nanocomposites302 are the materials composed of two or more components, in which the characteristic dimension of one constituent is 1-100 nm. They represent the third optimal class of the family of microelectronic polymers because they are the most suitable materials/polymers for spin-on, and ultra-low dielectric constant composites (k 21%, glass transition temperature >320oC, and absorption at 303 nm (in CH2Cl2 solvent). Poly(vinyl carbazole) having backbone formed from vinyl group of N-vinylcarbazole monomer by cationic or free radical polymerization is not a conjugated polymer, but it can be used as an optoelectronic light-emitting polymer for structuring white organic light-emitting diodes due to its highest occupied molecular orbital/unoccupied molecular orbital HOMO/LUMO energy = 5.6 eV/2.0 eV. The optoelectronic light-emitting polymer N,N'-bis(3-methylphenyl)-N,N'bis(phenyl)-9,9-dioctylfluorene can be considered as an optimized member of organic diodes-based optoelectronic light-emitting polymers for structuring white organic lightemitting diodes due to its absorption at 376 nm (in tetrahydrofuran) and photoluminescence at 401 nm (also in tetrahydrofuran), while the optoelectronic light-emitting polymer N4,N4'-bis(dibenzo[b]thiophen-4-yl)-N4,N4'-diphenyl-4,4'-diamine due its glass transition temperature >430oC, absorption at 279/338 nm (in CH2Cl2) and photoluminescence of 407 nm (in CH2Cl2). The optoelectronic light-emitting polymer N,N'-bis(phenanthren9-yl)-N,N'-bis(phenyl)-benzidine has a glass transition temperature of 410oC, absorption at 336 nm (in tetrahydrofuran), and photoluminescence at 454 nm (in tetrahydrofuran). The function of “electron transport layer” ETL (also can be called complementary transport layer CTL or the complementary to hole transparent layer HTL) in structuring white organic light-emitting diodes is to assists the electron transport from cathode to the associated emissive layer EML due to the level of lowest unoccupied molecular orbital LUMO of the electron transport material in respect to the associated metallic cathode. The metallic cathode (Mn, Ca, and Al, or their alloys with Ag) is responsible for the electron injection to the electron transport layer. The anode is made out of indium tin oxide ITO. To facilitate electron transport, the highest occupied molecular orbital HOMO of the electron transport layer should be lower than the highest occupied molecular layer of the emissive layer. So, transferring the holes to electron transport layer can be prevented.17,169,171-174,166

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Optimal optoelectronic light-emitting polymers that can be used for forming the electron transport layers of white organic light-emitting diodes include17,166,169,173,175 optoelectronic ((2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazol) purified by sublimation PBDO-pur, optoelectronic aluminum quinolinolate Alq3, optoelectronic 2,9naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline NBphen, optoelectronic 1,3-bis(2-(2 2'-bipyridine-6-yl)-1,3,4-oxadiazo-5-yl)benzene Bpy-OXD, optoelectronic 8-hydroxyquinolinolato-lithium Liq, optoelectronic 4-(naphthalen-1-yl)-3,5-diphenyl-4H-1,2,4-triazole NTAZ, and optoelectronic methyl-2-(4-(naphthalen-2-yl)phenyl)1H-1-methyl-2-(4(naphthalen-2-phenyl)-1H-imidazo[4,5f][1,10]phenanthroline 2-NPIP. For example, the photoelectronic light-emitting polymer (2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazol) purified by sublimation was selected as an optimized optoelectronic light-emitting polymer for fabricating the electron transport layers of white organic light-emitting diodes of green-emitting light due to its electroluminescence. An organic light-emitting diodes structured of this optoelectronic light-emitting polymer in addition to poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate acid) PEDOT:PSS, poly(vinyl carbazole) PVK, tris(2-phenylpyridine) iridium(III) Ir(ppy)3, and tetra-butyl phenyl-5biphenyl-1,3,4-oxadiazole PBD can result in emission of green light and maximum luminance of 10,000 cd/m2. Originally, this optoelectronic light-emitting polymer has a melting temperature of 210oC, absorption at 305 nm (in tetrahydrofuran), exciting light (or emission spectrum) λexc 305 nm, maximum emission peak (or maximum emission wavelength) λemmax of 364 nm (in ethanol). Similarly, aluminum quinolinolate was selected as an optimized optoelectronic light-emitting polymer for fabricating the electron transport layers of white organic light-emitting diodes of green-emitting light due to its electroluminescent nature. An organic light-emitting diodes structured of this optoelectronic lightemitting polymer in addition to molybdenum trioxide MoO3, N,N'-bis(1-naphthyl)-N,N'diphenyl-1,1'-biphenyl-4,4'-diamine NPD, (bathophenanthroline)2 BPhen can result in emission of green light with maximum luminance of 20,000 cd/m2, external quantum efficiency ΦELex of 1.2%, and turn-on voltage of 2.8 V. Optoelectronic 2,9-bis(naphthalen-2yl)-4,7-diphenyl-1,10-phenanthroline NBphen should be mixed with n-type dopant dicesium molybdate Cs2MoO4 containing carbon disulfide of alkaline metal as an electron donor to be used for the electron transport layers of organic light-emitting diodes. In addition to this mixture, the structured organic light-emitting diodes of copper-phthalocyanine CuPc, lithium-quinolate complex Alq3, and N,N'-di(naphthalene-1-yl)-N,N'-diphenylbenzidine NPB may exhibit a melting point >340oC, absorption at 264/349 nm (in tetrahydrofuran), and photoluminescence at 412 nm (in tetrahydrofuran). Optoelectronic 8hydroxyquinolinolato-lithium Liq is available in the form of lithium-quinolate complex used for fabricating both electron transport and electron injection layers of organic lightemitting diodes of the maximum cut-off frequency of 12.1 MHz, melting point Tm >310oC, absorption at 261 nm (in tetrahydrofuran), and photoluminescence at 331 nm (in tetrahydrofuran). The optoelectronic light-emitting polymer 4-(naphthalen-1-yl)-3,5diphenyl-4H-1,2,4-triazole NTAZ has a melting point >260oC, absorption at 264 nm (in CH2Cl2), and photoluminescence at 367 nm (in CH2Cl2). The optoelectronic light-emitting polymer methyl-2-(4-(naphthalen-2-yl)phenyl)-1-H1-methyl-2-(4-(naphthalen-2-phe-

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109

nyl)-1H-imidazo[4,5f][1,10]phenanthroline 2-NPIP has melting temperature >360oC, absorption at 275 nm (in CH2Cl2), and photoluminescence at 412 nm (in CH2Cl2). Optoelectronic bipolar polymers represent the optimal class of electrically bipolar conjugated polymers used for structuring optoelectronic systems because its members have intrinsic electron and hole transport properties, unique electrochemical and photophysical properties, and the ability to be used as good electron acceptors (due to the vacant boron pz orbital). Bipolar organic polymers are able to transport both positive and negative charge carriers. They have improved electron and hole transport and enhanced electroluminescence properties, that is why they are used for organic light-emitting diodes.169,173,177 The optimal members of optoelectronic bipolar polymers used for forming both hole and electron transport layers for high performance organic light-emitting diodes include169,174 optoelectronic poly(N-(2'-ethylhexyl)-carbazole-3,6-diyl-1'',3'',4''-oxadiazole-2'',5''-diyl-2’’’,5’’’-dioctyloxy-1’’’,4’’’-phenylene-1,3,4-oxadiazole-2,5-diyl) PCOPO, optoelectronic diphenyl phosphine oxide based hosts PhAMB-1T, and optoelectronic 2-4-(bis(9,9-dimethylfluorenyl)amino)phenyl)-5-(dimesityl boryl)thiophene FIAMB-1T. For example, optoelectronic poly(N-(2'-ethylhexyl)-carbazole-3,6-diyl1'',3'',4''-oxadiazole-2'',5''-diyl-2’’’,5’’’-dioctyloxy-1’’’,4’’’-phenylene-1,3,4-oxadiazole2,5-diyl) is the first optimized member of optoelectronic bipolar polymers because (1) its carbazole-based chemical structure contains both an electron-deficient aromatic oxadiazole unit and an electron-rich carbazole moiety, (2) it can emit greenish-blue light (475 nm), (3) it has a bandgap energy of 2.82 eV with highest occupied molecular orbital and lower molecular orbital energies of 5.60 and 2.66 eV, respectively. On the other hand, optoelectronic 2-(4-(bis(9,9-dimethylfluorenyl)amino)phenyl)-5-(dimesityl boryl)thiophene can be considered as an optimized member of optoelectronic bipolar polymers used for the same structures because its organoborane-based chemical structure can exhibit optical bandgaps of 0.16 and 0.3 eV, especially when used for structuring phosphorescent organic light-emitting diodes. Despite the optoelectronic light-emitting polymer 2-(4(bis(9,9-dimethylfluorenyl)amino)phenyl)-5-(dimesityl boryl)thiophene is used just for forming the hole transport layers of organic light-emitting diodes, but it is still considered as an optimized member of optoelectronic bipolar polymers because its bipolar fluorescent-based chemical structure results in hole transport layer of green light-emitting color without the need of doping process. The function of emissive (emission) layer can be optimized for structuring white organic light-emitting diodes by locating it where electronhole recombination can be achieved by electroluminescence, which results in arrangement of hole and electron injection from adjoining layers because of suitable positions of both highest occupied molecular orbital and lowest unoccupied molecular orbital of the associated emissive layer. As a result, both electron transport and emissive layers must facilitate the migration of electrons into the emissive layer and block holes transporting from the emissive layer to the electron transport layer. The lowest unoccupied molecular orbital of the emissive layer should be matched with that of the electron transport layer so that electrons can readily enter the emissive layer.166,169,174 The optimal grades of optoelectronic light-emitting polymers used for fabricating the emissive layers of organic light-emitting diodes are the bipolar polymeric mixtures such

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Optimized Electronic Polymers, Small Molecules, Complexes, and Elastomers for Organic

as17,163 4,4',4''-tris(carbazol-9-yl)triphenylamine/bis[2-(2-hydroxyphenyl)-pyridine]beryllium (tcta:Bepp2), 2,8-di(t-butyl)-5,11-di(4-(t-butyl)phenyl)-6,12-diphenylnaphthacene TBRb, and p-bis(p-N,N-diphenylaminostyryl)benzene DSA-Ph. The optoelectronic compound called 4,4',4''-tris(carbazol-9-yl)triphenylamine/bis(2-(2-hydroxyphenyl)-pyridine)beryllium is the first optimal type of bipolar polymeric mixture used for structuring “hybrid white light-emitting diodes” due to its capability resulting in a high-efficiency structure, stable spectral emission, and low-efficiency roll-off at high luminance. This mixture is used as the host of phosphorescent emissive layers and spacer, but for the developed applications, it is more suitable for fabricating red phosphorescent emissive layer/green phosphorescent emissive layer/spacer/blue fluorescent emissive layer. Separately 4,4',4''-tris(carbazol-9-yl)triphenylamine tcta can act as hole electron layer and electron blocking layer and can be used as a host of red and green phosphorescence emissive layer. Bis(2-(2-hydroxyphenyl)-pyridine)beryllium Bepp2 acts as electron transport layer and hole blocking layer, but it can also act as a good electron transport layer with high electron mobility of about 10-4 cm2/Vs and wide bandgap with high triplet energy of (2.6 eV), which is high enough for emitting red and green phosphors. The optoelectronic lightemitting polymer p-bis(p-N,N-diphenylaminostyryl)benzene can be considered as an optimized grade of bipolar polymeric mixtures for fabricating the emissive layers of hybrid white light-emitting diodes due to its ability to act as a “fluorescent dopant” with bis(2-(2hydroxyphenyl)-pyridine)beryllium as a host for blue. Generally, such hybrid white organic light-emitting diodes can exhibit a current efficiency of 30.2 cd/A, a power efficiency of 32.0 lm/W and an external quantum efficiency ΦELex of 13.4% at a luminance of 100 cd/m2, keeps a current efficiency of 30.8 cd/A, a power efficiency of 27.1 lm/W and ΦELex of 13.7% at a 1000 cd/m2.17,163 Both optoelectronic light-emitting polymers 2,8-di(t-butyl)-5,11-di(4-(t-butyl)phenyl)-6,12-diphenylnaphthacene and p-bis(p-N,N-diphenylaminostyryl)benzene in the form of a bipolar polymeric mixture can act as emissive layers (yellow and blue, respectively) for structuring multilayered white organic light-emitting diodes. But individually, 2,8-di(t-butyl)-5,11-di(4-(t-butyl)phenyl)-6,12-diphenylnaphthacene can be considered as a “fluorescent emitter” for structuring thermally activated delayed fluorescence-organic light-emitting diodes and fluorescent organic light-emitting diode types (in which the electroluminescent efficiency and operational stability were enhanced by the rapid conversion of triplet excitons). The 2,8-di(t-butyl)-5,11-di(4-(t-butyl)phenyl)-6,12-diphenylnaphthacene can be used with (2s,4r,6s)-2,4,5,6-tetrakis(3,6-dimethyl-9H-carbazol-9-yl) isophthalonitrile 4CzIPN-Me as an assistant dopant. For such structures, the combination of 2,8di(t-butyl)-5,11-di(4-(t-butyl)phenyl)-6,12-diphenylnaphthacene and (2s,4r,6s)-2,4,5,6tetrakis(3,6-dimethyl-9H-carbazol-9-yl) isophthalonitrile shows a large spectral overlap between the absorption spectrum of 2,8-di(t-butyl)-5,11-di(4-(t-butyl)phenyl)-6,12diphenylnaphthacene and the photoluminescent spectrum of (2s,4r,6s)-2,4,5,6tetrakis(3,6-dimethyl-9H-carbazol-9-yl) isophthalonitrile. Considering the general structure of organic light-emitting diodes, the function of the cathode is to inject electrons into the associated electron injection layer. The main function of the “electron injection layer” EIL (and electron transport layer ETL explained above) is to inject electrons as soon as the

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111

electric field is applied. Holes are injected using hole injection and transport layers.163,166,178 The optimal optoelectronic light-emitting polymers that can be used for fabricating electron injection layer (also called the complementary to hole transport layer) include17,173,166 optoelectronic tris(phenylpyrazole)iridium Ir(ppz)3, optoelectronic poly(9,9-bis-(6'-(diethanolamino)hexyl)fluorine) PFN-OH, optoelectronic cesium carbonate Cs2CO3, and optoelectronic molybdenum trioxide MoO3. For example, the optoelectronic tris(phenylpyrazole)iridium can be considered as the first optimal optoelectronic light-emitting polymer for fabricating the electron injection layers of organic light-emitting diodes due to its high-lying lowest unoccupied molecular orbital level, high melting temperature >270oC), absorption at 242, 320 nm (in tetrahydrofuran), and photoluminescence at 423 nm (in tetrahydrofuran). Optoelectronic poly(9,9-bis-(6'-(diethanolamino)hexyl)fluorine) represents the second optimized optoelectronic light-emitting polymer for fabricating electron injection layers of organic light-emitting diodes in the form of a water-soluble compound due to its ability to introduce air-stable high-workfunction cathode and enhancing the efficiency of the associated structure at the same time. Thus, the operating voltage can be reduced as a result of forming an interface dipole between the electron injection layer and the associated cathode. As a third optimized optoelectronic polymer used for fabricating the electron layers of organic light-emitting layers, the optoelectronic cesium carbonate can be added on the side of the lithium fluoride LiF cathode to improve the electron transport and extraction of the structured organic lightemitting and organic solar cell systems. On the other hand, the function of the anode is to inject holes into the associated hole injection layer. The function of the “hole transport layer” HIL is to act as a buffer between the associated indium tin oxide ITO anode and adjacent hole transport layer, which results in facilitating the hole injection state. The optimal optoelectronic polymer used for forming this hole injection layer is the highly conductive polymer called poly(3,4-ethylenedioxythiophene) PEDOT doped with polystyrenesulfonate PSS (abbreviated as PEDOT:PSS) due to its ability to facilitate the required hole injection state.166,173-174,179 Several types of electronic polymers are available for fabricating hole injection layers of organic light-emitting diodes, such as17,173 optoelectronic titanium oxide phthalocyanine TiOPC, optoelectronic pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile PPDN, optoelectronic N,N'-diphenyl-N,N'-di-(4-(N,N-diphenylamino)phenyl)benzidine NPNPB, optoelectronic N,N,N',N'-tetrakis(4-methoxyphenyl)benzidine MeO-TPD,179 optoelectronic N,N'-diphenyl-N,N'-di-(4-(N,N-ditolylamino)phenyl)benzidine NTNPB, and optoelectronic N4,N4'-(biphenyl-4,4'-diyl)bis(N4,N4',N4'-triphenylbiphenyl-4,4'diamine) TPT1. For example, optoelectronic titanium oxide phthalocyanine that is chemically available in the form of a complex of phthalocyanine of metal oxide in the complex center is used for fabricating hole injection layers of organic light-emitting diodes due to its attractive optical and electronic properties. The thin films formed from this optoelectronic light-emitting polymer can be highly improved by doping with tris-(8-hydroxyquinolinato) aluminum compound at different concentrations. Thus, the absorption peak of both tris-(8-hydroxyquinolinato) aluminum and titanium oxide phthalocyanine can be

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increased up to 387 nm and 707 nm, respectively, while the photoluminescence intensity peak to 517 nm. Similarly, the optoelectronic pyrazino[2,3-f][1,10]phenanthroline-2,3dicarbonitrile is used for the same structures, but due to its high melting temperature (>270oC), its absorption at 307 nm (in CH2Cl2), and its fluorescence at 847nm (in CH2Cl2). The optoelectronic N,N'-diphenyl-N,N'-di-(4-(N,N-diphenylamino)phenyl)benzidine can be considered as an optimized member for fabricating hole injection layers of “multilayered organic electronic light-emitting diodes” of fluorescent type due to low activation voltage, large external quantum efficiency, current efficiency, power efficiency of 3.21 lm/W, and maximum brightness of up to 42,325 cd/m2. The optoelectronic N,N,N',N'-tetrakis(4-methoxyphenyl)benzidine can be used for the same applications due to the increased brightness (14,300 cd/m2 at 13 V). It can be doped with the p-type doping compound called 2,2-(perfluoronaphthalene-2,6-diylidene)dimalononitrile to act as an optical spacer. This optoelectronic light-emitting polymer has a high melting temperature (>300oC), absorption at 302/351 nm (in tetrahydrofuran), and fluorescence at 429 nm (in tetrahydrofuran). The optoelectronic N,N'-diphenyl-N,N'-di-(4-(N,N-ditolylamino)phenyl)benzidine can also be used in the same application due to its high melting temperature (>350oC), absorption at 327 nm (in tetrahydrofuran), and fluorescence at 458 nm (in tetrahydrofuran). Optoelectronic N4,N4'-(biphenyl-4,4'-diyl)bis(N4,N4',N4'-triphenylbiphenyl-4,4'-diamine) has melting temperature >370oC and absorption at 394 nm (in CH2Cl2). The function of the “charge blocking layer” CBL is to act as a barrier for holes or electrons. The high level of highest occupied molecular orbital or the low level of lowest unoccupied molecular orbital facilitates the injection of holes or electrons into these layers. Important to know that the hole transport layer which facilitates hole injection from the associated anode to the emissive layer acts as an exciton blocking layer EBL and helps to confine excited states to the related emissive layer EML.1-2,173-174 The optimal electronic polymers used for fabricating both “hole blocking layer” HBL297 and “exciton blocking layer” EBL include1-2,107,166,173 optoelectronic bathocuproine BCP, optoelectronic 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole TAZ, optoelectronic 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) TPBi, and optoelectronic bis(2-methyl-8-quinolinolate)-4-(phenylphenolato)aluminum Balq. For example, the optoelectronic bathocuproine is used for fabricating hole blocking layers of both structuring blue and white organic multilayered electroluminescent systems and organic light-emitting diodes due to its ability to block holes depending on its thickness. On the other hand, holes can tunnel through the thin layer of bathocuproine (15 nm thickness). The optoelectronic light-emitting polymer bathocuproine has a melting temperature (>240oC), absorption at 277 nm (in tetrahydrofuran), and fluorescence at 386 nm (in tetrahydrofuran). Optoelectronic 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole is also used for the same application because it can be added in the form of thin layer between the layer of luminescent material and the electron transport layer or the metallic electrode to balance the charge injection and transport rates. The optoelectronic 2,2',2"(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) is considered as an optimized electronic polymer for fabricating hole blocking layers of organic light-emitting diodes due to

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ability of fabricating the exciton blocking layers of these diodes in addition to the ability of forming hole transport layers of “phosphorescent organic light-emitting diodes” and “fluorescent organic light-emitting diodes.” This polymer has a melting temperature (>350oC), absorption at 305 nm (in tetrahydrofuran), and fluorescence at 359/370 nm (in tetrahydrofuran).118,173,181 The optoelectronic bis(2-methyl-8-quinolinolate)-4-(phenylphenolato)aluminum can also be used for the same application due to its ability to sandwich it in the form of a thin layer between the associated emissive and electron transport layers of organic light-emitting diodes, phosphorescent organic light-emitting diodes, and small-molecule organic light-emitting diodes. Such applications result in a variety of structures with improved efficiencies. This optoelectronic light-emitting polymer has a melting temperature (>230oC), absorption at 259 nm (in tetrahydrofuran), and fluorescence at 334/477 nm (in tetrahydrofuran). Phosphorescent organic light-emitting diodes belong to the class of organic light-emitting diodes which combine heavy metals in their structures to facilitate the intersystem crossing (due to the spin-orbit coupling consequently resulting in the improved internal quantum efficiency of up to 100%). “Phosphorescent emitters” term (also called phosphorescent polymers) represent the optimized class of optoelectronic light-emitting polymers used in organic light-emitting diodes due to their ability to take the advantage of spin statistics. These phosphorescent emitters (as guests), often dispersed in a host material, act as good conductors, which transfer energy to the associated phosphorescent emitters.120,173-174,182 The optimized class of phosphorescent polymers (phosphorescent emitters) used as guests for structuring phosphorescent organic light-emitting diodes include the following members173-174,183-184 iridium complexes such as iridium(III)bis(4',6'-difluorophenylpyridinato)tetrakis(1-pyrazoyl)borate FIr6, iridium(III)bis((4,6-difluorophenyl)-pyridinatoN,C2')picolinate FIrpic, iridium(III)-2-phenylpyridine Ir(ppy)3, and bis(2-(2'-benzothienyl)-pyridinato-N,C3')iridium(III) (acetylacetonate) Btp2Ir(acac). For example, the phosphorescent emitter iridium(III)bis(4',6'-difluorophenylpyridinato)tetrakis(1-pyrazoyl) borate represents the optimized member of iridium complexes used for phosphorescent organic light-emitting diodes because it is originally known as a blue-emitting phosphorescent metal-organic complex (blue dopant) capable of deep-blue emission. Moreover, it has higher triplet energy of 2.72 eV compared to 2.65 eV for phosphorescent emitter iridium(III)bis((4,6-difluorophenyl)-pyridinato-N,C2')picolinate of the same optimized class. Both of these two members are chemically related to “heteroleptic complexes” of fluorosubstituted phenylpyridine ligands and anionic 2-picolinic acid (or poly(pyrazolyl)borate) as an auxiliary ligand. Important to know is that “heteroleptic” term describes an organometallic compound of two or more types of the ligand. The iridium(III)bis(4',6'-difluorophenylpyridinato)tetrakis(1-pyrazoyl)borate has a melting temperature >280oC, absorption at 387 nm (in tetrahydrofuran), and fluorescence at 461/490 nm (also in tetrahydrofuran solvent).1-2,173-174,183-184 The phosphorescent emitter iridium(III)bis((4,6-difluorophenyl)-pyridinatoN,C2')picolinate represents the second optimized member of the above class related to optoelectronic light-emitting polymers for structuring blue phosphorescent organic light-

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emitting diodes due to its functionality as a blue-emitting organic phosphor (blue dopant) suitable for doping into hosts such as tetra-aryl silanes and short conjugation length of carbazole derivative. Such a process may result in good quantum efficiency, but with a relatively high operating voltage required because of poor charge transport in the associated host materials. Organic light-emitting diode structured with the phosphorescent emitter iridium(III)bis((4,6-difluorophenyl)-pyridinato-N,C2')picolinate has a maximum quantum efficiency of 6.1% and a luminous power efficiency ηP of 7.7 lm/W, and a peak luminance of 6,400 cd/m2. It has melting temperature >270oC, absorption at 258 nm (in tetrahydrofuran), and fluorescence at 472 nm (in tetrahydrofuran).1-2,165,173,185 The optoelectronic light-emitting polymer iridium(III)-2-phenylpyridine is the optimal grade of phosphorescent emitter used for structuring white polymer light-emitting diodes because of it is a green-emitting organic phosphor (green dopant) with a maximum external quantum efficiency ΦELex of 12% and color rendering index CRI of 80. It is an organo-transition metal compound representing triplet emitters. This means it can exhibit an emission (also called “phosphorescence”) from the lowest excited electronic triplet state to the electronic singlet ground state. It has a melting temperature >300oC, absorption at 282/377 nm (in tetrahydrofuran), and fluorescence at 513 nm (in tetrahydrofuran). The optoelectronic light-emitting polymer bis(2-(2'-benzothienyl)-pyridinato-N,C3')iridium(III) (acetylacetonate) is the optimal phosphorescent emitter used for structuring fluorescent organic light-emitting diodes and organic light-emitting diodes due to its ability of acting as a red-emitting organic phosphor (red dopant) and the ability to be used as an organometallic guest with polyfluorene as a host to improve the efficiency of the organic light-emitting diodes. It gives red electroluminescence with ΦELex of 12.3%, 38 lm/W, and >50 cd/A. It has melting temperature >270oC, absorption at 271 nm (in tetrahydrofuran), and fluorescence at 563 nm (in tetrahydrofuran).1-2,165,173,185 Additional examples of polymers acting as phosphorescent emitters for phosphorescent organic light-emitting diodes include166,186 triazine-based polymers, tetraphenylsilane derivatives, diphenylphosphineoxide based hosts, and carbazole derivatives. Triazinebased polymers used for “phosphorescent organic light-emitting diodes” PhOLEDs are a sub-family containing the following members 2-phenoxy-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)-1,3,5-triazine POBICT, 4,6-bis(12-phenylindolo[2,3-a]carbazole-11-yl)1,3,5-triazine BICT, 2-benzenecyano-4,6-bis(12-phenylindolo[2,3-a]carbazole-11-yl)1,3,5-triazine BBICT, 2,4-bis-cyanophenyl-6-(12-phenylindole[2,3-a]carbazole-11-yl)1,3,5-triazine BCPICT, and 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a]carbazole-11-yl)1,3,5-triazine PBICT. The members of this sub-family are doped phosphorescent systems (chemically prepared through aromatic nucleophilic substitution reactions) with a molecular backbone of 4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)-1,3,5-triazine whose πconjugation is distorted by steric hindrance introduced through bulky substituents. As a result, these members can show an absorption peak at 258, 289, and 325 nm. Their emission peaks range from deep blue to green. Each one of these members can be compounded with 4,4’-bis(diphenylphosphine oxide) biphenyl PO1 so that all resulting compounds can be used for fabricating the emissive layers of phosphorescent organic light-emitting diodes. Another compound can be applied for the fabrication of hole transport layers, such

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as N,N'-di(naphthalene-1-yl)N,N'-diphenylbenzidine NPB, electron transport layers such as (bathophenanthroline)2 Bphen, exciton blocking layers such as 4,4',4''-tris(carbazol-9yl)triphenylamine tcta, hole injection layers such as 1,4,5,8,9,11-hexaazatriphenylene HATCN, and electron injection layer such as lithium fluoride LiF. 2,4-Bis-cyanophenyl-6-(12-phenylindole[2,3-a]carbazole-11-yl)-1,3,5-triazine BCPICT can be selected as the optimal member of sub-family of triazine-based polymers for structuring red emission phosphorescent organic light-emitting diodes due to its ability to act as novel host polymer with low dopant concentration and low roll-off efficiency. Moreover, it can be doped with a phosphorescence emitting complex from iridium Ir, 2(3,5-dimethylphenyl)-4-methylquinoline (mphmq), and 2,2,6,6-tetramethylheptane-3,5dionate (tmd) (abbreviated as Ir(mphmq)2(tmd)) to act as an emitting layer having different dopant concentrations. On the other hand, the second member of this sub-family can be used for the same applications because it has been previously prepared as a host from the connection of 1,3,5-triazine with cyanophenyl unit so that the resulting structure can exhibit the lowest small energy gap ΔEST and the narrowest bandgap, resulting from the strong electron-withdrawing ability of cyanophenyl. Several types of optoelectronic lightemitting polymers can be derived from the phosphorescent emitter tetraphenylsilane for structuring phosphorescent organic light-emitting diodes. These derivatives include the following members1-2,173-174,186 1. ultra-high energy-gap hosts UGHs, such as bis(2-methylphenyl)diphenylsilane UGH-1, p-bis(triphenylsilyl)benzene UGH2, and 1,3-bis(triphenylsilyl)benzene UGH-3 2. triphenyl(4-(9-phenyl-9H-fluoren-9-yl)phenyl)silane TPSi-F 3. 4,4'-di(triphenylsilyl)-biphenyl BSB-Cz. These three derivatives of ultra-high energy-gap hosts, having very low glass transition temperatures (26-50°C), are used for structuring blue phosphorescent organic lightemitting diodes due to their desirable optoelectronic properties. For example, the first member bis(2-methylphenyl)diphenylsilane has absorption at 265 nm (in CH2Cl2) and fluorescence at 298 nm (in CH2Cl2). The second member bis(triphenylsilyl)benzene has a melting temperature >270oC, absorption at 265 nm (in CH2Cl2), and fluorescence at 296 nm (in CH2Cl2). The third member 1,3-bis(triphenylsilyl)benzene has a melting temperature >270oC, absorption at 265 nm (in CH2Cl2), and fluorescence at 301/418 nm (in CH2Cl2). The third member has the ability to be used with derivatives of carbazole such as mbis(N-carbazolyl)benzene-co-host system to form the emitting layer of structured blue phosphorescent organic light-emitting diodes to increase the maximum external quantum efficiency up to 20%.173-174 As an attractive sub-family of phosphorescent emitters related to optoelectronic light-emitting polymers, diphenylphosphine oxide-based host is one of the optimized subfamilies related to phosphorescent emitters for structuring blue phosphorescent organic light-emitting diodes PhOLEDs due to their high triple energy ET, low operating voltage, enhanced electron transport, and their ability of lowering both highest occupied molecular orbital and lowest unoccupied molecular orbital, which makes them suitable compounds for electron transport and hole blocking. The optimal members of this sub-family are as follows173-174 4,4’-bis(diphenylphosphine oxide) biphenyl PO1, 2,7-bis(diphenylphos-

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phine oxide)-9,9-dimethylfluorene PO6, 3,6-bis(diphenylphosphoryl)-9-phenylcarbazole PO9, dibenzo[b,d]thiophene-2,8-diylbis(diphenylphosphine oxide) PO15222, dibenzofuran DBF, N-(4-diphenylphosphoryl phenyl)carbazole MPO12, and 2,6,14-tris(diphenylphosphine oxide)triptycene TPOTP. The three optimal types of this group for structuring blue phosphorescent organic light-emitting diodes are 4,4’-bis(diphenylphosphine oxide) biphenyl, 2,7-bis(diphenylphosphine oxide)-9,9-dimethylfluorene, and dibenzo[b,d]thiophene-2,8-diylbis(diphenylphosphine oxide) due to their high triplet energies, which are able to host blue phosphorescent emitters. The 4,4’-bis(diphenylphosphine oxide) biphenyl-based bi-layered organic light-emitting diodes may result in electroluminescence at 338 nm, but doping 4,4’-bis(diphenylphosphine oxide) biphenyl with Ir(III)bis((4,6-difluorophenyl)-pyridinato-N, C2’)picolinate may result in an electrophosphorescence of external quantum efficiency ΦELex of 7.8% at 0.09 mA/cm2 and 5.9% at 13 mA/cm2. The 2,7-bis(diphenylphosphine oxide)-9,9-dimethylfluorene is a charge transporting host polymer, which can be used for structuring short-wavelength phosphor-doped organic lightemitting diodes. The 2,7-bis(diphenylphosphine oxide)-9,9-dimethylfluorene has the ability to be doped with Ir(III)bis((4,6-difluorophenyl)-pyridinato-N, C2’)picolinate. It has sky-blue emission with peak ΦELex of 8.1% and a luminous power efficiency ηP of 25.1 lm/W. Among the sub-family of diphenylphosphineoxide based hosts related to phosphorescent emitters of optoelectronic light-emitting polymers, both 3,6-bis(diphenylphosphoryl)-9-phenylcarbazole and dibenzo[b,d]thiophene-2,8-diylbis(diphenylphosphine oxide) are optimized members used for blue phosphor-doped organic light-emitting diodes as host polymers due to the inductive electron-withdrawing effect of diphenylphosphoryl group, lowering both highest occupied molecular orbital and lowest unoccupied molecular orbital. On the other hand, doping both 3,6-bis(diphenylphosphoryl)-9-phenylcarbazole and N-(4-diphenylphosphoryl phenyl) carbazole with iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2']picolinate may result in ΦELex of 6-8% at operating voltage < 7 V.112,181,206 For more developed applications, 3,6-bis(diphenylphosphoryl)-9-phenylcarbazole can be used for generation of white organic light-emitting diodes as a blue emitter doped with mer-tris(N-dibenzofuranyl-N-methylimidazole) Ir(III) Ir(dbfmi) which may result in ηP of 59.9 lm/W, luminous efficiency of 49.9 cd/A, and external quantum efficiency of 16.7% at 5,000 cd/m2. The optimal application of dibenzo[b,d]thiophene-2,8-diylbis(diphenylphosphine oxide) can be achieved by fabrication of electron transport layers of blue phosphorescent organic light-emitting diodes so that the resulting structure can have the following characteristics: glass transition temperature of 106oC, highest occupied molecular orbital/lowest unoccupied molecular orbital 6.6/2.9 eV, and triple energy of 3.07 eV.172,187 Carbazole derivatives, such as a sub-family of phosphorescent emitters, can be optimized for structuring phosphorescent organic light-emitting diodes by improving their triple energy, tuning their glass transition temperature, and enhancing their fluorescence levels. In this case, the following derivatives can be considered for the applications173-174 4,4'-N,N'-dicarbazolyl-biphenyl CBP, 4,4'-N,N'-dicarbazole-biphenyl mCP, m-bis(N-car-

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bazolyl)benzene CBBz, 4,4’-bis(carbazol-9-yl)-2,2’-dimethylbiphenyl CDBP, 3,6-bis(carbazol-9-yl)-9-(2-ethyl-hexyl)-9H-carbazole TCz1, 9-(4-tert-butylphenyl)-3,6bis(triphenylsilyl)-9H-carbazole CzSi, 9-phenyl-3,6-bis(9-phenylfluoren-9-yl)carbazole CBZ1-F2, carbazole bridged fluorene CBF, and 9,9'-(5-(triphenylsilyl)-1,3-phenylene)bis(9H-carbazole) SimCP. For example, 4,4'-N,N'-dicarbazolyl-biphenyl represents the first optimized member of carbazole family with phosphorescent emitters for structuring blue-emitting phosphorescent organic light-emitting diodes due to its triple energy of 2.56 eV and glass transition temperature of 120oC. This carbazole-based host has a melting temperature >320oC, absorption at 292/318 nm (in tetrahydrofuran), and fluorescence at 369 nm (in tetrahydrofuran). Compared with 4,4'-N,N'-dicarbazolyl-biphenyl, the second member of this sub-family 4,4'-N,N'-dicarbazole-biphenyl is desirable for such structures due to its higher triple energy (2.9 eV), but it should not be used as a host because of its low glass transition temperature (65oC). The 4,4'-N,N'-dicarbazole-biphenyl has a melting temperature >250oC, absorption of 292/338 nm (in tetrahydrofuran), and fluorescence at 345/360 nm (in tetrahydrofuran). The m-bis(N-carbazolyl)benzene, 4,4’-bis(carbazol-9-yl)-2,2’-dimethylbiphenyl has melting temperature >280oC, absorption at 292/ 340 nm (in tetrahydrofuran), and fluorescence at 349/364 nm (in tetrahydrofuran). 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)carbazol is the third optimized member of carbazole sub-family of phosphorescent emitters for structuring the new generation of organic light-emitting diodes called “complex organic light-emitting diodes” because of their high triplet energy, and efficient carrier injection and transport. To achieve the efficiency of the structure of this new generation, it is desirable to use additional compound for such structure such as (4-(4-(5-phenyl-5,10-dihydrophenazine)phenyl)-3,5-diphenyl1,2,4-triazole) PPZ-4TPT325,367 to act as a donor unit (in addition to its unique photophysical properties and low energy level indicating blue-shift of the emission band), and bis(2(diphenylphosphino)phenyl)ether oxide DPEPO of lowest triplet excited state of 3.3 eV to act as a host due to its high triplet energy. The 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)carbazol has a melting temperature >320oC, absorption at 275/301 nm (in CH2Cl2), and fluorescence at 354 nm (in CH2Cl2). Similarly, the next member called 9-phenyl-3,6bis(9-phenylfluoren-9-yl)carbazole related to carbazole/fluorene hybrids is used for structuring organic light-emitting diodes as hosts, due to its thermal and morphological stability. Moreover, it can be used as hosts with iridium(III)bis[(4,6-difluorophenyl)-pyridinatoN,C2']picolinate as a guest for structuring electroluminescent efficient organic light-emitting diodes.174,186,188-189 The 4,4’-bis(carbazol-9-yl)-2,2’-dimethylbiphenyl has a melting temperature >280oC, absorption at 292/340 nm (in tetrahydrofuran), and fluorescence at 349/364 nm (in tetrahydrofuran). 3,6-bis(carbazol-9-yl)-9-(2-ethyl-hexyl)-9H-carbazole has absorption at 293 nm (in tetrahydrofuran) and fluorescence at 396 nm (in tetrahydrofuran). The last member of carbazole sub-family related to phosphorescent emitters 9,9'-(5-(triphenylsilyl)-1,3-phenylene)bis(9H-carbazole) is an optimized member for structuring both blue phosphorescent organic light-emitting diodes and white organic light-emitting diodes due to its ability to be used as a host polymer, and its comparable energy levels of the highest occupied molecular orbital and the lowest unoccupied molecular orbital levels. Additional

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polymers useful with this polymer are 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) acting as an electron transport layer, iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2']picolinate acting as a blue phosphorescence dopant, bis[2-(2-pyridinylN)phenyl-C](2,4-pentanedionato-O2,O4)iridium(III) acting as a green phosphorescence dopant, and iridium(III)-bis(2-methyldibenzo-[f,h]chinoxalin) (acetylacetonate) Ir(MDQ)2(acac), which is a red phosphorescence dopant. The 9,9'-(5-(triphenylsilyl)-1,3phenylene)bis(9H-carbazole) has absorption at 293/312/345 nm (in tetrahydrofuran) and fluorescence at 446 nm (in tetrahydrofuran).190 3.8.1.2 Organic solar cells-based light-emitting polymers Organic solar cells-based light-emitting polymers (also called “organic photovoltaic cellsbased light-emitting polymers” or “organic solar photovoltaic cells”) are electrical systems-based polymers capable of converting the energy of light into electricity (depending on the phenomenon of “photovoltaic effect”). Organic solar cells-based light-emitting polymers represent the second class of optoelectronic light-emitting polymers, depending on the classification of their final applications. The photovoltaic effect is the physical/ electrical phenomenon focusing on the creation of electric current in a polymer upon exposure to light. As a result, the organic solar cells-based optoelectronic light-emitting polymers can be considered as the second optimized group of optoelectronic light-emitting polymers related to electronic polymers used for structuring organic solar cells-based light-emitting polymers due to their capability of converting sunlight into electricity via absorption of photons of light with a high level of efficiency.385-386 This group includes the following important types129,386-387 1. organic dye-sensitized solar cell 2. hybrid solar cell 3. multi-junction (tandem) solar cell 4. plasmonic (organic) solar cell 5. organic (plastic) solar cell (also called organic phototransistors) 6. polymeric solar cell 7. organic quantum dot solar cell (also called organic quantum dot sensitized solar cells) 8. organic thin-film solar cell (also called organic thin-film photovoltaics).124,191,221,368 The optimal organic photovoltaic cells-based light-emitting polymers used for structuring organic/polymeric solar cells (Figure 3.18)227,284 include light-emitting poly(phenylene vinylene), light-emitting poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylene vinylene) MEH-PPV, light-emitting poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4phenylene(1-cyano)vinylene) MEH-CN-PPV, light-emitting poly(3-(4'-octylphenyl)thiophene) POPT, light-emitting poly(3-(4'-1"-oxooctylphenyl)thiophene) POOPT, light-emitting poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene-vinylene) MDMO-PPV, light-emitting thiophene-isothianaphthene copolymer PDTI, light-emitting poly(3-hexyl thiophene), or (also called poly(3-n-hexyl-2,5-thienylene)) P3HT, light-emitting cyanopolyphenylene CN-PPV, light-emitting polyacetylene, light-emitting phthalocyanines, and light-emitting benzothiadiazole pyrrole copolymer PTPTB. These polymers are optimal for organic/polymeric solar cells due to their ability to form flexible thin-film photovoltaics, which convert light into electricity with the assistance of plasmons or producing electricity from sunlight based on the photovoltaic effect phenomenon.

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Figure 3.18. Examples of light-emitting polymers used for structuring organic solar cells.227,284

“Organic dye-sensitized organic solar cells” are thin-film photovoltaics based on semiconductors sandwiched between a photosensitized anode and an electrolyte. To optimize the structures of this type of organic solar cells, they should be fabricated from dyebased polymers having extreme light- and temperature-stability such as 1-ethyl-3 methylimidazolium tetracyanoborate and high conversion efficiency such as copper-diselenium.207 “Hybrid solar cell” is an organic/inorganic semiconductor-based solar cell. The optimization of structures of such solar cells depends on the right selection of both organic and inorganic semiconductors. Organic semiconductors must consist of conjugated polymers such as light-emitting poly(3-hexyl thiophene) P3HT, light-emitting poly(2-methoxy,5(2’-ethyl-hexyloxy)-p-phenylenevinylene)), and light-emitting poly(2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b,3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)) due to their ability of absorbing light as the donor and transport holes, while inorganic semiconductors are preferred, such as based on oxides (e.g., zinc oxide), which can act as acceptor and electron transporter. “Organic tandem solar cells” (also called “multi-junction solar cells”) are organic solar cells structured with multiple p-n junctions which can be formed from various semiconducting materials. The p-n junction can produce an electric current in response to the wavelength of light. Organic quantum dot sensitized solar cells are solar cells based on the utilization of quantum dots as absorbing photovoltaic materials. Structures of this type can be optimized by photovoltaic deposition of one or more thin layers or thin films of photovoltaics on glass or polymeric substrate. In conclusion, optimizing the structures of all types of organic solar cells based on optoelectronic light-emitting polymers can be achieved by proper selection of the light-emitting polymers which can improve the following characteristics 1. short-circuit current density Jsc [mA/cm2] 2. open-circuit voltage Voc [V]

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fill factor FF (or ff) maximum power conversion efficiency ηmp [%] quantum efficiency for charge carrier generation Φcc.17,124,129,150,191 The function of these characteristics is to help the designers of organic solar cells to select proper light-emitting polymers (optoelectronic light-emitting polymers) for each type of these solar cells. For example, selecting the polymeric blend consisting of poly(3hexyl thiophene) P3HT with methanofullerene PCBM (P3HT/PCBM (1:1)) results in a structure of 10.6 mA/cm2 short-circuit current density, 0.61 V open-circuit voltage, 0.67 fill factor, and 4.4% maximum power conversion efficiency. The selection of light-emitting poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene-vinylene)) MDMO-PPV with methanofullerene (MDMO-PPV/PCBM) results in a structure of 5.25 mA/cm2 shortcircuit current density, 0.82 V open-circuit voltage, 0.61 fill factor, 2.5% maximum power conversion efficiency, and 0.5 quantum efficiency for charge carrier generation (at 470 nm). Note: The nominal solar efficiency is greater than 10%. The majority of electrically conducting/conjugated polymers are widely used for organic/polymeric solar cells, but just some of them have sufficient efficiencies, such as those having low bandgaps, high molar absorption coefficients, and multiple binding sites to the oxide of conjugated polymers, including the following classes of light-emitting polymers 1. conjugated 2. carboxylic acid-based 3. fluorene-containing 4. small bandgaps 5. carbazole-based 6. poly(ethylene dioxythiophene) PEDOT-types. For example, light-emitting conjugated polymers have been chosen as the first optimized class of light-emitting polymers used for structuring organic/polymeric solar cells due to their ability of using them as electron donor and electron acceptor materials rather than utilizing them as semiconductors p-n junctions because the molecules forming the electron donor region of solar cells are of conjugated polymers types. In fact, each type of light-emitting conjugated polymers used for structuring solar cells has its own specific properties to achieve the required performance. For example, the light-emitting conjugated polymers, such as light-emitting poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene, poly(3-hexyl thiophene), and poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-pphenylenevinylene) contain side chains that make them soluble in common organic solvents. On the other hand, copper phthalocyanine must generally be deposited by evaporation onto a substrate to be fabricated in the form of film.17,124,129,133,150,191,194-195 In conclusion, the optimal members of light-emitting conjugated polymers/copolymers that can be used for structuring organic/polymeric solar cells (see Figure 3.18) include17,124,129,195,236 light-emitting poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4phenylene-vinylene); poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene; poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene(1-cyano)vinylene); cyano-polyphenylene, poly(3-hexyl thiophene); poly(3-(4'-octylphenyl)thiophene); light-emitting poly(3-(4'-(1'',4'',7''-trioxaoctyl)-phenyl)thiophene); thiophene-isothianaphthene copolymer; benzothiadiazole/pyrrole copolymer; and thiophene(poly(N-dodecyl-2,5,-bis(2'-thienyl)pyrrole,2,1,3-benzothiadiazole) copolymer.

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It is better to blend each one of the following light-emitting π-conjugated polymers/ copolymers poly((2-methoxy-5-(3',7'-dimethyloctyloxy))-1,4-phenylene-vinylene)), poly(3-hexyl thiophene), and benzothiadiazole pyrrole copolymer, with methanofullerene PCBM so that each one of them acts as donor polymer for structuring organic/polymeric solar cells, while methanofullerene (optimized acceptor) forms segregated phase with such donors and has good electron affinity. Blending the light-emitting poly((2-methoxy5-(3',7'-dimethyloctyloxy)-1,4-phenylene-vinylene with methanofullerene) can result in improved characteristics such as an short-circuit current density of 5.25 mA/cm2, the open-circuit voltage of 0.82 V, fill factor of 0.61, maximum power conversion efficiency of 2.5%, and quantum efficiency for charge carrier generation of 0.5. Blending poly(3hexyl thiophene) with methanofullerene can result in the following improved characteristics, including short-circuit current density of 9.5 mA/cm2, the open-circuit voltage of 0.63 V, fill factor of 0.68, the maximum power conversion efficiency of 5.1%. Also, blending the light-emitting benzothiadiazole pyrrole copolymer with methanofullerene can result in 3.1 mA/cm2 short-circuit current density, 0.72 V open-circuit voltage, 0.38 fill factor, and 1% maximum power conversion efficiency.17,124,129,195,236 Both light-emitting polymers cyano-polyphenylene and poly(2-methoxy,5-(2'-ethylhexyloxy)-p-phenylenevinylene are natural “bulk heterojunction polymers,” which can act as acceptors and donors. They can increase the interfacial surface area between blend components, which results in enhanced interfacial charge separation. But, it should be noted that the utilization of both light-emitting polymers poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene as a donor with poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4phenylene(1-cyano)vinylene) as an acceptor does not results in the required improved characteristics without utilization of the high charge mobilities of poly(phenylene vinylene) and the knowledge on effects of the cyano-substitution on lowering the lowest unoccupied molecular orbital with a little effect on the highest occupied molecular orbital, which lowers the optical bandgap. Optimizing the characterization of organic/polymeric solar cells structured of the light-emitting poly(3-(4'-1"-oxooctylphenyl)thiophene) or the light-emitting poly(3-(4'-octylphenyl)thiophene)s including poly(3-(4'-octylphenyl)thiophene), poly(3-(4'-(1'',4'',7''-trioxaoctyl)-phenyl)thiophene), and poly(3-(4'-1"-oxooctylphenyl)thiophene) as donors can be achieved by blending with poly(3-hexylthiophene)/ methanofullerene(6,6-phenyl C61 butyric acid methyl ester) PCBM61 as acceptor. The poly(3-(4'-octylphenyl)thiophene)s have bandgaps of 1.7 eV.124,129,191,237 As the second class of light-emitting conjugated polymers, the light-emitting carboxylic acid-based polymers can be used for structuring organic/polymeric solar cells, but just some of them can be considered as the optimal members such as133-134,194,196 polythiophene acetic acid PTAA (also called poly(3-thiophene acetic acid), poly(2-(3-thienyl) ethanol hydroxyl carbonyl-methylurethane) H-PURET, polythiophene-carboxylic groupbased polymer PT-CO2H, and poly(phenylene ether) resins-carboxylic group based polymer PPE-CO2H. For example, the utilization of both light-emitting polymers poly(thiophene acetic acid) and poly(2-(3-thienyl) ethanol hydroxyl carbonyl-methylurethane) as two members of carboxylic acid-based polymers results in an organic/polymeric solar cell structures of maximum power conversion efficiency (1.5%). Such structures can be opti-

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mized by increasing this percentage through the addition of an electron-donating ionic liquid to the liquid electrolyte. Both light-emitting carboxylic acid-based polymers, such as polythiophene-carboxylic group based polymer and poly(phenylene ether)-carboxylic group based polymer have anionic conjugated polyelectrolyte nature and are used for structuring “titanium oxide film-solar cells.” Fluorene-containing polymers-based solar cells depend on the utilization of two materials/polymers as a donor (p-type) and acceptor (n-type) materials/polymers. Many conjugated polymers in their undoped state (semiconducting state) are electron donors on photoexcitation (electrons promoted to the antibonding π* band). Substituted fullerenes, such as poly(3-hexylthiophene)/ methanofullerene(6,6-phenyl C61 butyric acid methyl ester) PCBM61 are widely used as acceptors. Some structures of organic solar cells depend on the solubility of polythiophenes or derivatives of poly(phenylene vinylene) as donor polymers. Importance of fullerenes is that their “9-carbon” is ready for the cooperation with alkyl groups to increase the solubility of the polymer without causing an additional twist in the main chain.133,194,197 The optimal “light-emitting fluorene-based polymers” (also copolymers and blends) used for structuring organic solar cells include129,198 1. light-emitting blend of 2-ethylhexyl-substituted polyfluorene PF2/6 with violanthrone VTh (abbreviated PF2/6:VTh as a blend). Violanthrone is an organic compound − a vat dye and a precursor to other vat dyes 2. light-emitting polyfluorene copolymers such as poly-2,7-(9,9'-dioctyl-9H-fluorene)alt-4,7-benzo[c][1,2,5]-thiadiazole F8BT 3. light-emitting fluorene-containing triarylamine units such as poly(2,7-dioctyl-9Hfluorenyl-alt-4,4'-(N-4-butylphenyl)-diphenylamine) TFB, poly(2,7-dioctyl-9H-fluorenyl-alt-4,4'-(N1,N4-bis(4-butylphenyl)-diphenylbenzene-1,4-diamine) PFB, poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methylphenyl)imino)-1,4phenylene-((4-methylphenyl)imino)-1,4-phenylene)) PFM, and poly(2,7- (9,9-di-noctylfluorene)-(1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene-((4methoxyphenyl) imino)-1,4-phenylene)) PFMO. Table 3.7198 includes the main properties of some light-emitting fluorene-containing triarylamine as emission layer. 2-Ethylhexyl-substituted polyfluorene with violanthrone blend (1:4) is the first optimal member of light-emitting fluorene-based polymers for forming the emissive layers of organic solar cells due to external quantum efficiency ΦELex up to 2.5% measured at 610 nm or efficiency up to 3.0% measured at 410 nm after thermal annealing. The light-emitting poly-2,7-(9,9'-dioctyl-9H-fluorene)-alt-4,7-benzo[c][1,2,5]-thiadiazole is a low bandgap copolymer structured of benzothiadiazole with dioctylfluorene used for solar cells, but, practically, it can be considered as the first optimal member of light-emitting fluorene-based polymers for structuring “bulk heterojunction solar cells” because the dioctylfluorene-based polymer called poly(2,7-(9,9-dioctylfluorene) has a non-dispersive hole mobility μh (hole transport mobility) of 4x10-4 cm2/Vs, which is more suitable for such structures in addition to structuring “photoactive systems.” Poly-2,7-(9,9'-dioctyl-9H-fluorene)-alt-4,7-benzo[c][1,2,5]-thiadiazole is the third optimal member of light-emitting fluorene-based polymers due to its ability of acting as electron transport layer of organic solar cells, where the introduction of electron-deficient benzothiadiazole co-monomers

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123

into fluorene-based copolymers increases the ability to reduce the polymer, making it a better electron acceptor. Such an introduction results in reducing the bandgap of the associated polymer to give a better overlap of the solar spectrum. The electron and hole transport properties of poly-2,7-(9,9'-dioctyl-9H-fluorene)-alt-4,7-benzo[c][1,2,5]-thiadiazole have electron mobility values of 2x10-3 cm2/Vs and µh of 1x10-3 cm2/Vs. Among lightemitting fluorene-containing triarylamine units, the following three polymers light-emitting fluorene-containing triarylamine units, poly(2,7-dioctyl-9H-fluorenyl-alt-4,4'(N1,N4-bis(4-butylpheny))-diphenylbenzene-1,4-diamine), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene-((4-methylphenyl)imino)1,4-phenylene)), and poly(2,7- (9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene-((4-methoxyphenyl) imino)-1,4-phenylene)) can be considered as the optimal “bulk heterojunction polymers” due to their ability of acting as hole transport layers for structuring heterojunction solar cells. The poly(2,7-dioctyl-9H-fluorenylalt-4,4'-(N-4-butylphenyl)-diphenylamine) has been shown to be non-dispersive with a hole mobility of 1x10-2 cm2/Vs, while poly(2,7-dioctyl-9H-fluorenyl-alt-4,4'-(N1,N4bis(4-butylpheny))-diphenylbenzene-1,4-diamine) has been shown to yield the best solar cell device efficiencies in polymer blends with poly(2,7-(9,9-di-n- octylfluorene)-3,6-benzothiadiazole).129,198 Table 3.7. The properties of some fluorene-containing triarylamine acting as the emissive layer of organic/polymeric solar cells. [Data from reference 198] Photoluminescence ηPL

Highest occupied molecular orbital gap [eV]

Lowest unoccupied molecular orbital gap [eV]

Optical gap [eV]

poly(dialkyl fluorene) APFO

80

5.8

2.8

3.0

light-emitting poly(2,7-dioctyl-9H-fluorenyl-alt-4,4'-(N-4-butylphenyl)-diphenylamine) TFP

40

5.3

2.3

3.0

light-emitting poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methylphenyl) imino)-1,4-phenylene-((4-methylphenyl) imino)-1,4-phenylene)) PFM

20

5.0

2.1

3.0

light-emitting poly(2,7- (9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methoxyphenyl) imino)-1,4-phenylene-((4-methoxyphenyl) imino)-1,4-phenylene)) PFMO

40

5.0

2.0

5.8

Polymers of blue emission color

The main feature of a conducting/conjugated polymer is the relationship between its flexible and the energy difference between the highest occupied molecular orbital HOMO and lowest unoccupied molecular orbital LUMO of the associated polymer. The lowest values of bandgap mean the higher electrical conductivity of a polymer. Advantages of a small bandgap of a polymer (conductive polymer) include the increase in electrical conductivity, enhancement of the nonlinear optical properties, and enabling the polymer to absorb light up to a longer wavelength that is very much desirable for structuring organic/ polymeric solar cells. As a result, the so-called “low (small)-bandgap polymers” are con-

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jugated polymers of bandgap less than 2 eV, and desirable for structuring highly efficient organic/polymeric solar cells with suitable electron acceptors. Two classes of low-bandgap polymers are used in organic/polymeric solar cells “traditional low-bandgap polymers” and “optimized low-bandgap polymers.” The traditional low-bandgap polymers include polyarylenene, poly(arylene vinylene), and the derivatives of poly(arylene ethynylene) created by linkage of aryl-aryl, aryl-vinyl, and aryl-ethynyl-aryl. An objective of incorporating arylenes into the chains of such polymer is to improve its stability, provide anchoring points for side chains, and enhance control of their properties such as solubility. Combining two types of traditional low-bandgap polymers such as poly(3-hexyl thiophene) as a hole transport layer and methanofullerene as an electron transport layer of an organic solar cell structure results in improved parameters such as 5% efficiency, lowest unoccupied molecular orbital level of 0.3-0.5 eV, highest occupied molecular orbital of -4.3 eV, and bandgap of 1.5-1.8 eV. On the other hand, optimized low-bandgap polymers can act as the hole transparent layer and have their own energy levels which make them attractive as the electron transport layers (despite their reduced mobility).129 Such optimized low-bandgap polymers/copolymers for structuring organic/polymeric solar cells include129,135,199-200 1. low-bandgap polymers containing 4,7-di-2-thienyl-2,1,3-benzothiadiazole LBGDBT 2. low-bandgap polymers containing 4,9-di-2-thienyl-1,2,5-thiadiazolo-3,4-g]quinoxalines LBG-DTQx 3. low-bandgap polymers containing thieno[3,4-b]pyrazines LBG-TPs 4. arylene vinylene-based low-bandgap polymers LBG-AV 5. low-bandgap polymers containing 4H-cyclopenta[2,1-b,3,4-b']dithiophene LBGCPDTh. Low-bandgap polymers containing 4,7-di-2-thienyl-2,1,3-benzothiadiazole LBG-DBT include the following derivatives129,135,199,200 (1) poly(4,7-bis(4-octyl-2thienyl)-2,1,3-benzothiadiazole) PB4BT, (2) poly(4,7-bis(3-octyl-2-thienyl)-2,1,3benzothiadiazole) PB3BT, (3) poly(4-(3-octyl-2-thienyl)-7-(4-octyl-2-thienyl)-2,1,3benzothiadiazole) PB34BT, (4) poly(4,7-bis(4-decanyl-2-thienyl)-20,10,30-benzothiadiazole-thiophene-2,5-) PDDBT. Let us have the derivatives of low-bandgap polymers containing 4,7-di-2-thienyl2,1,3-benzothiadiazole as an example. Comparing with poly(4,7-bis(3-octyl-2-thienyl)2,1,3-benzothiadiazole), poly(4,7-bis(4-octyl-2-thienyl)-2,1,3-benzothiadiazole), and poly(4-(3-octyl-2-thienyl)-7-(4-octyl-2-thienyl)-2,1,3-benzothiadiazole) of bandgaps of 2.10, 2.10, and 1.65 eV respectively, poly(4,7-bis(4-decanyl-2-thienyl)-20,10,30-benzothiadiazole-thiophene-2,5-) has the lowest (optimized) bandgap of 1.38 eV. This decreased bandgap is likely due to an increase in solubility as a result of longer side chains and increase in molecular weight. As a result, a solar cell such as bulk heterojunction solar cells type structured with poly(4,7-bis(4-decanyl-2-thienyl)-20,10,30-benzothiadiazolethiophene-2,5-), especially combined with poly(3-hexylthiophene)/methanofullerene(6,6phenyl C61 butyric acid methyl ester) PCBM61, results in a structure of maximum power conversion efficiency of 13% and a photocurrent response up to 880 nm.129 The sub-family of low-bandgap polymers containing 4,9-di-2-thienyl[1,2,5]thiadiazolo[3,4-g]quinoxalines, is the second sub-family related to the main family called “optimized low-bandgap polymers” used for structuring organic/polymeric solar cells. This sub-family includes the following members129,135,199-200

3.8 Optoelectronic polymers

1.

125

low-bandgap poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',9'-di-2-thienyl-6',7'-diphenyl-[1',2',5']thiadiazolo-[3',4'-g]quinoxaline)) APFO-Green1 2. low-bandgap poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',9'-di-2'-thienyl-6',7'-diethylheptyl-hexyl[1',2',5']thiadiazolo-[3',4'-g]quinoxaline)) APFO-Green3 3. low-bandgap poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',9'-di-2-thienyl-6',7'-4-(2ethylhexyloxy)phenyl-[1',2',5']thiadiazolo-[3',4'-g]quinoxaline)) APFO-Green4. All these three members either separate or in the form of blends are optimized for structuring organic/polymeric solar cells due to their desirable values of their maximum conversion efficiencies, highest occupied molecular orbital level, and lowest unoccupied molecular orbital level. For example, the low-bandgap poly(2,7-(9,9-dioctyl-fluorene)-alt5,5-(4',9'-di-2-thienyl-6',7'-diphenyl-[1',2',5']thiadiazolo-[3',4'-g]quinoxaline)) has bandgap of 1.3 eV, the highest occupied molecular orbital level of -5.3 eV, and the lowest unoccupied molecular orbital level of -4.0 eV. Blending this polymer with the derivatives of methanofullerene such as 3'-(3,5-bis-trifluoromethylphenyl)-1'-(4-nitrophenyl)pyrazolino[60]fullerene BRPF60 or 3'-(3,5-bis-trifluoromethylphenyl)-1'-(4-nitrophenyl)pyrazolino[70]fullerene BRPF70 can result in a structure of improved maximum power conversion efficiency (up to 0.3%), while blending low-bandgap poly(2,7-(9,9-dioctylfluorene)-alt-5,5-(4',9'-di-2-thienyl-6',7'-4-(2-ethylhexyloxy)phenyl-[1',2',5']thiadiazolo[3',4'-g]quinoxaline)) with 3'-(3,5-bis-trifluoromethylphenyl)-1'-(4-nitrophenyl)pyrazolino[70]fullerene can result in a structure of improved maximum power conversion efficiency (up to 0.7%).129,199 The sub-family low-bandgap polymers containing thieno[3,4-b]pyrazines is the third sub-family related to the main family called “low-bandgap polymers”95 used for organic solar cells. This sub-family includes the following members129,135,199-200 poly(5,7-bis-(3octylthiophen-2-yl)thienopyrazine) PB3OTP, poly(5,7-bis(3-dodecylthiophen-2yl)thieno[3,4-b]pyrazine) copolymer BTTP, poly(5,7-bis(3-dodecylthiophen-2yl)thieno[3,4-b]pyrazine-alt-2,5-thiophene) copolymer BTTP-T, poly(5,7-bis(3-dodecylthiophen-2-yl)thieno[3,4-b]pyrazine-alt-9,9-dioctyl-2,7-fluorene) copolymers BTTP-F, poly(5,7-bis(3-dodecylthiophen-2-yl)thieno[3,4-b]pyrazine-alt-1,4-bis(decyloxy)phenylene) copolymer BTTP-P, and poly((thiophene-2,5-diyl)-alt-(2,3-diheptylquinoxaline-5,8diyl)) copolymer PTHQx. All these members are optimal low-bandgap polymers for structuring organic/polymeric solar cells because they are available in the form of conjugated copolymers with attractive maximum absorption at 667-810 nm, highest occupied molecular orbital level ranging from 4.6 to 5.04 eV, and small desirable optical low-bandgaps ranging from 1.1 to 1.6 eV. The last one poly((thiophene-2,5-diyl)-alt-(2,3-diheptylquinoxaline-5,8-diyl)) copolymer which was already compounded with 5,8-dibromo-2,3diheptyl-quinoxaline and 2,5-thiophenediylbis(trimethylstannane) can act as a donoracceptor conjugated copolymer with hole mobility μ+ of 3.6x10-3 cm2/Vs.129 The sub-family of low-bandgap polymers, which are arylene vinylene-based is the fourth sub-family related to the main family called “low-bandgap polymers” used for structuring organic/polymeric solar cells. This sub-family includes the following members129,135,199-200 poly(9,9-dioctylfluorene-2,7-diyl-alt-2,5-bis(2-thienyl-1-cyanovinyl)-1(2'-ethylhexyloxy)-4-methoxy-benzene-5',5'-diyl) FR3-S, and poly(9,9-dioctylfluorene2,7-diyl-alt-2,5-bis(2-thienyl-2-cyanovinyl)-1-(2'-ethylhexyloxy)-4-methoxy-benzene-

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5'',5'-diyl) PFR4-S. Both members can be considered as optimal low-bandgap polymers for structuring organic/polymeric solar cells because they are also available in the form of conjugated copolymers, with attractive maximum absorption at 460 and 537 nm, respectively (at solid state), and can be blended with methanofullerene so that the resulting structure has improved parameters such as open-circuit voltages of 0.85 and 0.81 V and maximum power conversion efficiency of 0.98 and 1.02%, respectively. The sub-family of low-bandgap polymers containing 4H-cyclopenta[2,1-b,3,4b']dithiophene is the fifth sub-family related to the main family called “low-bandgap polymers” used for structuring organic/polymeric solar cells. This sub-family includes the following members poly(cyclopenta[2,1-b:3,4-b']dithiophene-4-one) PDOCPTCK, poly(4,4dialkylcyclopentadithiophene-co-pyridine) PDOCPTPy, and poly(4,4-dialkylcyclopentadithiophene-co-phenanthroline) PDOCPTPh. They have low bandgap values of 2.5, 2.0, and 2.5 eV, respectively. These three members are based on the homopolymer called poly(4,4-dialkylcyclopentadithiophene) homopolymer PDOCPT of the optical low-bandgap of 1.8 eV.129,135,200 Carbazole-based polymers have been already included as an optimized family for structuring organic light-emitting diodes, but the members of this family can be modified so that they can be used for structuring organic/polymeric solar cells. This family includes the following members polyvinylcarbazole PVK, poly(carbazolene vinylene) PCV (also called poly(N-(4-octyloxyphenyl)-2,7-carbazolene vinylene)), and poly(N-(4-octyloxyphenyle)-2,7-carbazolenevinylene-alt-3",4"-dihexyl-2,2',5',2",5",2"',5"',2""-quinquethiophenevinylene-1",1"-dioxyde) PCPTDO. Polyvinylcarbazole have a high bandgap that is not suitable for structuring solar cells. To overcome this problem, it can be used as a hole transport layer (p-type polymer), which may result in a structure of improved maximum power conversion efficiency up to 2.4%. Poly(carbazolene vinylene) is a copolymer prepared from 2,7-carbazolene vinylene with thiophene-containing units to get a low-bandgap of 2.78 eV. Blending the derivatives of carbazole with methanofullerene results in improving the required maximum power conversion efficiency of up to 0.4%. The performance of PCPTDO copolymer can be improved by increasing thiophene Th units. This may result in increasing the absorption from 500 to 725 nm, bandgap BG of 2.0 eV, and maximum power conversion efficiency of 0.8%.129,135,200 Additional sub-families of conducting polymers can be used for structuring organic/ polymeric solar cells, such as129 1. the sub-family called poly-3,4-ethylenedioxythiophene-type polymers PEDOT-TP 2. the sub-family called conductive polyethylene dioxythiophene. The first sub-family is related to polymers containing 3,4-dialkoxythiophene family. This sub-family includes the following members for structuring organic/polymeric solar cells poly(3,4-ethylenedithiathiophene) PEDTT, poly(3,4-ethylenediselena)thiophene PEDST, poly(3,4-propylenedioxythiophene) PProDOT, 2,2',2'',3,3',3''-hexahydro-5,5':7',5'-terthieno[3,4-b]-1,4-dioxin-6',6'-dioxide EDT-EDO-EDT, and 5,5'-(1,1-dioxido-2,5-thiophenediyl)bis[3,4-b]-1,4-dioxin EDT-O-EDT. Both poly(3,4-ethylenedithiathiophene) and poly(3,4-ethylenediselena)thiophene have optical low-bandgaps of 2.15 and 1.79 eV respectively, while, both 2,2',2'',3,3',3''-hexahydro-5,5':7',5'-terthieno[3,4-b]-1,4-dioxin6',6'-dioxide and 5,5'-(1,1-dioxido-2,5-thiophenediyl)bis[3,4-b]-1,4-dioxin, which are

3.8 Optoelectronic polymers

127

chemically based on S,S-dioxide, have low-bandgaps of 1.7 eV, and 1.95 eV respectively. Poly(3,4-propylenedioxythiophene) of flexible conjugated backbone structure has optimal properties because of its low oxidation potential, relatively high chemical and thermal stability, and high conductivity (σ=2x10-6 S/cm). Structures resulting from the above polymers have a maximum power conversion efficiency of 0.8%.129,194 The second sub-family has high stability, highly conductive oxidized (doped) state, and low-bandgap ranging from 1.6 to 1.7 eV. The function of conductive poly(ethylene dioxythiophene) can be optimized for structuring organic solar cells by utilizing it with “waterborne poly(3,4-ethylene dioxythiophene)” doped with “poly(styrene sulfonate) acid PEDOT:PSS” which acts as a buffer layer between the anode typically formed from a transparent conductive oxide and the photoactive charge generating layer. Such applications result in a poly(ethylene dioxythiophene) of 300-500 S/cm conductivity, 0.2 absorption constant K1 at 550 nm, and 4.2 eV work function.129,135,200 3.8.1.3 Organic laser polymer-based optoelectronic light-emitting polymers The organic laser polymer-based optoelectronic light-emitting polymers represent the third class of optoelectronic light-emitting polymers, depending on the classification of their final applications. Their function depends mainly on the utilization of “laser,” which originally focuses on emitting monochromatic, spatially coherent, and highly polarized light. The active materials used for structuring some laser systems are of two types 1. host/guest systems consisting of a host material (such as 4,4'-N,N'-dicarbazolylbiphenyl, 4,4'-N,N'-dicarbazole-biphenyl, and 2-tetra-butyl phenyl-5-biphenyl-1,3,4oxadiazole) doped with organic dye molecules (such as rhodamine, coumarin, and pyrromethenes) 2. systems consisting of conjugated polymers. “Organic laser” that uses an organic material (semiconductors or conjugated polymers) as the gain medium consists of (1) an active material such as conjugated polymer having light of narrow linewidth with high quantum yield emission (under high excitation density conditions) (2) a resonator whose function is enabling the build-up of certain resonant modes, which essentially determine the lasing characteristics. The main lasing parameters related to laser polymers include threshold intensity for lasing Ithreshold [µ/cm2] and excitation conditions represented by the generated laser pulses (measured by τ in femtoseconds).17,201,162,203 The optimal laser polymers are light-emitting polymers used for structuring optoelectronic light-emitting systems such as17,162,328 poly(2,5-dioctyloxy-pphenylenevinylene) DOO-PPV, poly(2,5-bis((2'-ethylhexyl)oxy)-1,4-phenylenevinylene) BEH-PPV, poly(2-butyl-5-(2'-ethylhexyl)-1,4-phenylenevinylene) BuEH-PPV, methylsubstituted ladder-type poly(p-phenylene) m-LPPP, and poly[3-(2,5-dioctyl phenyl)thiophene] PDOPT. Poly(2,5-dioctyloxy-p-phenylenevinylene) and poly(2,5-bis((2'-ethylhexyl)oxy)-1,4-phenylenevinylene) are conjugated polymer suitable for structuring organic lasing systems with desirable properties as shown in Table 3.8.203

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Optimized Electronic Polymers, Small Molecules, Complexes, and Elastomers for Organic

Table 3.8. Lasing parameters of some laser polymers. [Adapted by permission, from Theo Hoyer, Wilfried Tuszynski, Christoph Lienau, Ultrafast photo-dimerization dynamics in a-cyano-4hydroxycinnamic and sinapinic acid crystals, Chem. Phys. Lett., 443, 107-112, 2007.] Type of resonator RST

Excitation conditions

Ithreshold [µ/cm2]

Poly(2,5-dioctyloxy-p-phenylenevinylene) DOO-PPV

Microring

λ=512 nm τ=100 ps

0.1

Poly(2,5-bis((2'-ethylhexyl)oxy)-1,4phenylenevinylene) BEH-PPV

Microring

λ=555 nm, τ=100 fs

25

Microcavity

λ =515 nm, τ=10 ns

4.5

Flexible distributed feedback

λ=400 nm, τ=150 ps

1.7

Microcavity

λ=510 nm, τ=90 fs

0.12

Laser polymers

Poly(2-butyl-5-(2'-ethylhexyl)-1,4phenylenevinylene) BuEH-PPV Methyl-substituted ladder-type poly(p-phenylene) m-LPPP Poly[3-(2,5-dioctyl phenyl)thiophene] PDOPT

3.8.1.4 Organic thin-film transistors-based light-emitting polymers The organic thin-film transistors-based light-emitting polymers represent the fourth class of optoelectronic light-emitting polymers, depending on their final applications. They are light-emitting polymers used for structuring thin-film transistors (also called light-emitting transistors) or for fabricating the active layers of organic chip and flexible electronic systems, such as radio frequency identification tags and flexible displays. Important to remember is that the performance of the organic field-effect transistor can be highly improved depending on202 1. the stability and the charge carrier mobility of conjugated p-type polymers (of conductivity σ = 2 cm2/Vs) 2. n-type polymers (of σ = 1 cm2/Vs) 3. small molecules used widely for structuring bipolar field-effect transistors. In fact, such an efficiency structure can be optimized by blending p-type and n-type conjugated polymers to form “p-n-type heterojunction polymers” (“heterojunction” obtained by co-evaporation or solution process). An objective of such blending is optimizing the final properties of the resulting compound by gaining advantageous properties of the two compounded components (p-n-types) in a single structure. The most important feature of “p-n-type heterojunction polymers” is that the bipolar202 charge transport in these polymers has electron and hole mobilities of 0.01-0.1 cm2/Vs.204 The light-emitting polymers related to the family of organic thin-film transistors-based optoelectronic lightemitting polymers and used as thin-film transistors-based light-emitting polymers (Figure 3.19)202,204 include poly((4,4’-bis(2-octyl)dithieno[3,2-b:’’,3’-d]silole)-2,6-diyl-alt-(2,5bis(3-octylthiophen)-2yl)thiazolo[5,4-d]thiazole)) PSOTT, poly(benzobisthiazolealt-3octylquarterthiophene) PBTOT, poly(3-hexyl thiophene) P3HT, poly(benzobisimidazobenzophenanthroline) BBL, 6,6’-(2,2’-octyloxy-1,1’-binaphthalene) binaphthyl BN-PFO, and poly(9,9-dioctylfluorene-co-9,9-di(4-methoxy)phenylfluorene) F8DP.171,202,204,388

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Poly((4,4’-bis(2-octyl)dithieno[3,2-b:’’,3’-d]silole)-2,6-diyl-alt-(2,5-bis(3-octylthiophen)-2yl)thiazolo[5,4-d]thiazole)) PSOTT, poly(benzobisthiazolealt-3-octylquarterthiophene) PBTOT, poly(3-hexyl thiophene) P3HT are considered as the optimal members of this family because they are conjugated polymers ready for forming thin-film transistors with p-type layers which are highly suitable for structuring “bipolar field-effect transistors”. Poly((4,4’-bis(2-octyl)dithieno[3,2-b:’’,3’-d]silole)-2,6-diyl-alt-(2,5-bis(3-octylthiophen)-2yl)thiazolo[5,4-d]thiazole)) is a poly(thiazolothiazole)-based polymer of high crystallinity similar to poly(3-hexyl thiophene). Poly((4,4’-bis(2-octyl)dithieno[3,2b:’’,3’-d]silole)-2,6-diyl-alt-(2,5-bis(3-octylthiophen)-2yl)thiazolo[5,4-d]thiazole))-based transistors have a maximum mobility of 0.01 cm2/Vs and average threshold voltages Vt of 13.4 V. Poly(benzobisimidazo-benzophenanthroline) acts as n-type layer. 6,6’-(2,2’-octyloxy-1,1’-binaphthalene) binaphthyl is a blue-emitting organic laser polymer (related to statistical copolymers) consisting of 2,7-(9,9-dioctylfluorene) and 6,6'-(2,2´-octyloxy1,1´-binaphthyl). It is known as a lasing polymer having an absorption peak at 382 nm and photoluminescence spectrum of two vibronic peaks at 428 and 449 nm. Poly(9,9-dioctylfluorene-co-9,9-di(4-methoxy)phenylfluorene) is also another blue-emitting copolymer, but consists of 2,7-(9,9-dioctylfluorene) with 9,9-di(4-methoxy)phenylfluorene conjugated repeat units. Its absorption band has a single peak at 390 nm with an extinction coefficient of 3x105 cm-1, while its photoluminescence spectrum shows a structured emission that exhibits vibronic peaks at 425, 450, and 480 nm.171,204-205

Figure 3.19. Examples of organic thin-film transistors-based optoelectronic light-emitting polymers.202,204

3.8.1.5 Organic metallocene polymer-based optoelectronic light-emitting polymers The organic metallocene polymer-based optoelectronic light-emitting polymers represent the fifth class of optoelectronic light-emitting polymers, depending on the classification of their final applications. These polymers have been optimized for structuring optoelectronic-based metallocene systems, because “metallocene” (as a compound) consists of two

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cyclopentadienyls bound to a metal center (M) in the oxidation state II, acting as a lightemitting conjugated polymer. Examples of metallocene derivatives include organotitanium compound titanocene dichloride and the organometallic complex vanadocene dichloride. The “metallocene” term is related to “ferrocene” known as bis(η5-cyclopentadienyl)iron(II). On the other hand, the “metallocene containing polymer” term (also called metallocene-based polymer or ferrocene (ferrocenyl)-based polymer), represents the optimized class of light-emitting conjugated polymers for structuring light-emitting systems due to high levels of quantum efficiency, brilliant solid-state blue luminescence that is suitable for structuring organic luminescent systems, highly improved thermal stability that is suitable of environment applications, and excellent solubility. Metallocene-containing polymers can be synthesized from conjugated polymers with ferrocene units as pendants or as end-groups.206 The optimal members of metallocene polymers/copolymersbased light-emitting polymers related to optoelectronic light-emitting polymers include206 polyferrocenylacetylene PFcA, polyferrocenylacetylene/polyphenylacetylene copolymer PFcA-PPA, and polyferrocenylacetylene/polynorbornene copolymer PFcA-PNBE. Polyferrocenylacetylene has been selected because of acting as an “electroactive polymer” containing fluorene unit in its backbone. These ferrocene units play an important role in tuning the conductivity of this polymer. Moreover, it exhibits high thermal stability with a maximum weight loss at 440oC. Both polyferrocenylacetylene/polyphenylacetylene copolymer and polyferrocenylacetylene/polynorbornene copolymer are copolymerized by adding phenylacetylene or norbornene monomer to the red-brown solution of polyferrocenylacetylene. Polyferrocenylacetylene/polynorbornene copolymer is an optimized member of the above structures due to its decomposition maximums at 220oC and 490oC, indicating the presence of two blocks in the copolymer structure. The polyferrocenylacetylene/polyphenylacetylene copolymer is selected because of decomposition at 400oC indicates that both blocks undergo decomposition at the same temperature.114,206 3.8.1.6 Organic photosensors-based optoelectronic light-emitting polymers The organic photosensors-based optoelectronic light-emitting polymers represent the sixth class of optoelectronic light-emitting polymers, depending on the classification of their final applications. The light-emitting polymers that have been used for structuring “organic photosensors” are called “photosensor polymers” or “organic photosensor polymers.” Photosensors structured of polymers are called “organic photosensors” or “polymeric photosensors” (also can be called “organic polymer thin-film photosensors).” The photosensor polymers are conjugated polymers, such as polythiophene derivatives, polyarylethers, poly(phenylene ether)s, etc. The main feature of these conjugated polymers is the ability to act as “photosensing materials” with a high level of quantum efficiency. Important to know is that large area photosensors fabricated in the form of the metal-polymer-metal structure show high photosensitivity (0.1-0.5 A/W in the visible and near UV).208 As the sixth optimized class of optoelectronic light-emitting polymers, organic photosensors based-light-emitting polymers includes the following members58,94 poly(9,9'dioctyl fluorene-co-bithiophene) F8T2, 3,4,9,10-perylene-tetracarboxylic diimide PTCDI, poly((2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl)-

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alt-((2,2':5',2''-terthiophene)-5,5''-diyl)) PDPP3T, and the nanohybrids of benzo[ghi]perylene monoimide with cadmium-doped zinc oxide (abbreviated as BPyM/ CZO). 3,4,9,10-Perylene-tetracarboxylic diimide is considered as an optimized member of the above class of optoelectronic light-emitting polymers because it is electrically known as an important “electroluminescent polymer” of enhanced photosensitivity. The “photosensitivity” makes it very much suitable for structuring organic photosensors as n-type active layer on a transparent substrate formed from a transparent polymer such as polyethersulfone. The conjugated polymer poly((2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6dioxopyrrolo[3,4-c]pyrrole-1,4-diyl)-alt-((2,2':5',2''-terthiophene)-5,5''-diyl)) is the next optimized member of the above class for the same application because it has attractive low bandgap (1.3 eV) and high hole mobility of 0.04 cm2/Vs. These two important properties enable it to absorb the near-infrared parts of the spectrum. The last member called nanohybrid of benzo[ghi]perylene monoimide with cadmium-doped zinc oxide is utilized because it is a lamellar hybrid inorganic/organic nanostructure consisting of two alternating layers: benzo[ghi]perylene monoimide and cadmium-doped zinc oxide. Such a nanohybrid structure has high photosensitivity and visible orange photoluminescence emission.209 3.8.1.7 Organic light-emitting electrochemical cells-based optoelectronic light-emitting polymers The organic light-emitting electrochemical cells-based optoelectronic light-emitting polymers, which represent the seventh class of optoelectronic light-emitting polymers depending on the classification of their final applications was not classified as an optimized subfamily because it was related to the attractive family of “optoelectronic light-emitting polymers,” but because “organic light-emitting electrochemical cell” family represented alternative of organic light-emitting diodes as light-emitting source. Moreover, this alternative family is known because it exhibits unique electrical/physical properties such as 1. light-emission with a low threshold voltage 2. improved electroluminescence efficiency 3. high quantum efficiency 4. balanced electron and hole injection 5. high power conversion efficiency 6. long operating lifetime. The structures of light-emitting electrochemical cells involve active layers of polymeric blends (conjugated polymers and electrolytes), which contain both electronic and ionic conductors sandwiched between two electrodes. With such structure, the formed positivetype (p-type) and negative-type (n-type) doped layers are stabilized by counter ions available in the ion-conducting phase.210 Organic light-emitting electrochemical cell-based light-emitting polymers (copolymers, oligomers, or complexes) include211-212 luminescent, ionic, and cationic types, (1) luminescent because light-emitting electrochemical cells exhibit improved electroluminescence efficiencies compared to organic light-emitting diodes, (2) ionic and cationic because light-emitting electrochemical cells consist of an active polymer layer, which originally contains both electronic and ionic conductors sandwiched between two electrodes.

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Luminescent complexes include211-212 1. iridium(III)-2-phenylpyridine Ir(ppy)3 2. iridium/2-phenylpyridine/2,2'-bipyridine-based complex [Ir(ppy)2(bpy)]+ 3. ruthenium(III)-2,2'-bipyridine [Ru(bpy)3]2+ 4. ruthenium (II) complexes with terdentate polypyridyl ligands [Ru(tpy)2]2+. These four members represent the optimal complexes used for structuring light-emitting electrochemical cells because they are highly luminescent, have emission energies ranging from blue to red regions (λmax = 456 to 667 nm), with quantum yields between 0.2 and 0.9 (in degassed dichloromethane at room temperature). Organic light-emitting electrochemical cells based on ionic transition metal complexes include211,389-392 1. (4,4’-bis(1,1-dimethylethyl)-2,2’-bipyridine-N1,N1’)bis(3,5-difluoro-2-(5-(trifluoromethyl)-2-pyridinyl-N)phenyl-C)iridium(III) hexafluorophosphate (Ir(df(CF3)ppy)2(dtb-bpy))(PF6), 2. (4,4’-bis(1,1-dimethylethyl)-2,2’-bipyridine-N1,N1’)bis(2-(2-pyridinyl-N)phenylC)iridium(III) hexafluorophosphate (Ir(ppy)2(dtb-bpy))(PF6) 3. iridium complex of 1-phenylpyrazolyl 4. 2,2'-biquinoline (Ir(ppz)2(biq)) (PF6) (PF6) indicates an anion hexafluorophosphate of the formula (or PF6-). These complexes can also be called as “cyclometalated complexes” related to iridium(III) of “ionic transition metal complexes” class (known as “cyclometalated complexes of ionic transition metal complexes” Ir-iTMCs). Both “ionic transition metal complexes-based light-emitting electrochemical cells” and “charge-transport materials-based light-emitting electrochemical cells,” have superior luminescence, photochemical, and electrochemical properties, simple device structure, low operation voltage, and compatibility with air-stable electrodes. The “positively charged complex ion” is called a “cationic complex” (denoting cationic transition metal complexes) in which a “cation” is a positively charged ion. On the other hand, a “negatively charged complex ion” is called an “anionic (ionic) complex” (denoting ionic transition metal complexes) in which an “anion” is a negatively charged ion. For example, tetraamminecopper(II) sulfate (Cu(NH3)4)2+ is a complex cation (cationic), dithiosulfatosilver(I) (Ag(S2O3)2)3- is a complex anion.193-194 (4,4’-bis(1,1-dimethylethyl)-2,2’-bipyridine-N1,N1’)bis(3,5-difluoro-2-(5-(trifluoromethyl)-2-pyridinylN)phenyl-C)iridium(III) hexafluorophosphate (Ir(df(CF3)ppy)2(dtb-bpy))(PF6), (4,4’bis(1,1-dimethylethyl)-2,2’-bipyridine-N1,N1’)bis(2-(2-pyridinyl-N)phenyl-C)iridium (III) hexafluorophosphate (Ir(ppy)2(dtb-bpy))(PF6), and iridium complex of 1-phenylpyrazolyl and 2,2'-biquinoline (Ir(ppz)2(biq))(PF6) can be considered as the optimal members of the ionic transition metal complexes used for structuring organic light-emitting electrochemical cells, because211,390 1. (Ir(df(CF3)ppy)2(dtb-bpy))(PF6) (as a cyclometalated complex of ionic transition metal complexes equipped with two multi-fluorinated cyclometalating monoanion of phenyl-pyridine-based ligands) has light-emitting electrochemical cells with bluegreen emission, and can show a slight blue-shift in the electroluminescent peak from λmax = 520 to 500 nm in going from positive to negative voltages 2. (Ir(ppy)2(dtb-bpy))(PF6) is suitable for structuring single-layered light-emitting electrochemical cells emitting yellow light, it has photoluminescence quantum occurring in oxygen-free acetonitrile solution of 23.5%, and shows emission lifetime of 0.56 µs, and under certain conditions, it may exhibit blue-shifted electroluminescence

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spectrum of λmax at 560 nm at 3 V, luminance level of 200 cd/m2, and external quantum efficiency of 0.2% 3. (Ir(ppz)2(biq))(PF6) is more desirable for structuring green emission light-emitting electrochemical cells of electroluminescent spectrum λmax = 542 nm.211-212 Light-emitting electrochemical cells based on cationic transition metal complexes include211-212 1. iridium complex of 1-(2,4-difluorophenyl)pyrazole and (4,4'-di(tert-butyl)-2,2'bipyridine) (abbreviated as (Ir(dfppz)2(dtb-bpy))((PF6)) 2. iridium complex of 2-phenylpyridine and 2-2'-biquinoline (abbreviated as (Ir(ppy)2(biq))(PF6) 3. iridium complex of monoanion of phenyl-pyridine and 4,5-diaza-9,9'-spirobifluorene (abbreviated as (Ir(ppy)2(dasb))(PF6)). These complexes can also be called as cyclometalated complexes of iridium(III) of cationic transition metal complexes class and considered as optimized members of cyclometalated complexes of cationic transition metal complexes Ir-cTMCs) because (1) in addition to their original function and of ionic transition metal complexes depending on the host-guest concept, by dispersing them in the same cationic derivatives, they can form cationic terfluorene derivatives (cationic transition metal complexes), (2) the blue-emission light-emitting electrochemical cells structured from (Ir(dfppz)2(dtb-bpy))(PF6) complex, exhibit electroluminescence at wavelength of maximum optical absorption of λmax = 492 nm and considerable external quantum efficiency of 4.6% at low voltage. This blueemission complex can be doped with the red-emission complex (Ir(ppy)2(biq))(PF6) and the orange-emission complex (Ir(ppy)2(dasb))(PF6) to achieve the best performance white-emission light-emitting electrochemical cells, with external quantum efficiency of 7.4% and a luminous power efficiency ηP = 15 lm/W.211-212 3.8.2 OPTOELECTRONIC LIGHT TRANSPORTING POLYMERS Just as the optoelectronic light-emitting polymers were considered the first optimized family of optoelectronic polymers (for structuring organic light-emitting systems), optoelectronic light transporting polymers are the second optimized family for structuring organic light transporting systems. It is considered as an optimized family for structuring organic light transporting systems because polymers used for such structures (called “optoelectronic light transporting polymers”) have attractive properties required for such applications. These properties include transparency and the ease of transmitting visible light. The “transparency” of utilized polymers is the optical property required for structuring optoelectronic systems with flexible polymeric transparent substrates (instead of glass substrates). The most efficient light transporting polymers have amorphous nature (transparency is related to crystalline nature such as in polyethylene or semicrystalline nature such as in low-density polyethylene). The light scattering of a transparent light transporting polymer is achieved when its crystallite size is increased. Important to remember is that (1) ”transmittance” depends on the refractive index n, (2) some fillers may make a polymer translucent instead of transparent (3) “gloss” can be determined by light reflection, where, most polymers have smooth surfaces, and high gloss (4) “birefringence” (double reflection) state can be observed mainly in oriented polymers. In conclusion, the optical properties of common light transporting polymers include refractive index n, Abbe value, light transmission, haze, absorption coefficient, and yellowness index. The “light

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transmission” (transmittance or light transmittance in %) denotes the fraction of incident electromagnetic power that is transmitted through a polymeric sample. The refractive index n (index of refraction) describes how light propagates through a medium. The “Abbe value” Vd (also called Abbe number or V-number) is used for measuring the dispersion of a polymer (variation of refractive index versus wavelength). The “haze” term describes the non-clarity of a transparent polymer resulting from exposure to dust, smoke, and dry particles. The “absorption coefficient” (α) is the physical coefficient that determines the ability of radiation (of a particular wavelength) to penetrate a polymer before it is absorbed. As a spectrophotometric data-based number, the function of “yellowness index” YI is to show the change in color of a polymer from clear or white to yellow.213-215 Optoelectronic light transporting polymers of amorphous nature include213-214 1. commercial polymethylmethacrylate PMMA such as acrylate or Perspex®337 grade, polycarbonate PC, polystyrene PS, poly(vinyl chloride) PVC (with a range of 8892% in apparent light transmittance), and cyclic olefin copolymer COC. These polymers are classified as thermoplastics. The related polymers classified as thermosets include thermoset polyesters UPR and allyl diglycol carbonate ADCt 2. some transparent, crystalline polymers such as poly(ethylene terephthalate) PET can transmit the visible light because its crystallites are smaller than the wavelength of radiation 3. sulfone-based polymers called sulfone polymers such as poly(phenylene sulfone) (Radel®193 and Veradel®338 grades), polyethersulfones, and polysulfones (Udel®339 trade name). Among transparent thermoplastic polymers, the above types represent an optimized group of optoelectronic transporting systems because they have light transmission values ranging from 88 to 92%. Optical properties of some light transporting polymers are listed in Table 3.9.340-341 Note: Poly(phenylene sulfone)s, polyethersulfones, and polysulfones classified as sulfone polymers have a combination of transparency, high clarity, low haze, high heat resistance, toughness, hydrolytic stability, and chemical resistance. They exhibit over 74-87% light transmission, 1.5-3.1% haze (1.78 mm thick.), 7.0-19.0 YI (91.78 mm thick), 1.634-1.675 refractive index, and 18.7-23.3 Abbe value. Table 3.9. Examples of optoelectronic properties of some light transporting polymers.340-341 Properties Refractive index n

Polymethyl methacrylate PMMA

Polycarbonate PC

1.489-1.497 (1.49)

1.580-1.593 (1.586)

Polystyrene PS

Cyclic olefin copolymer COC

Allyl diglycol carbonate ADCt

1.585-1.604 (1.59)

1.527-1.537

1.5

Abbe value Vd

57.2

34

30.8

55.8

59.0

Light [%]

92.95

85.91

87.92

90.92

92.0

Haze [%]

1-2

1-3

2-3

1-2

1014 Ω/sq, fluorinated ethylene-propylene copolymer = 1015 Ω/sq, polyamide-6 = 5x1010 Ω/sq, polyamide6/6 = 1011 Ω/sq, polyamide-imide = 8-50x1017 Ω/sq, polycarbonate= 1015 Ω/sq, polytetrafluoroethylene = 1015 Ω/sq, polyetherimide = 4x1015 Ω/sq, high-density polyethylene, low-density polyethylene, ultra-high molecular weight polyethylene = 1013 Ω/sq, poly(ethylene terephthalate) = 1013 Ω/sq, polyimide = 1016 Ω/sq, polymethylmethacrylate = 1014 Ω/sq, polyoxymethylene = 1015 Ω/sq, and poly(phenylene oxide) = 2x1016 Ω/sq. According to ASTM-D495, the arc resistance can be defined as the time (in seconds) that a polymer resists the formation of a surface-conducting path when subjected to an intermittently occurring arc of high-voltage and low-current characteristics. Electrical insulating polymers are divided into high and moderate insulating polymers. High electrically insulating polymers are those characterized by high electric resistivity, low dielectric constant, and very low power factor (such as polytetrafluoroethylene of volume resistivity >1020 Ω.cm, dielectric strength 180 kV/cm, the dielectric constant of 2.1 at 60 Hz, and

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power factor of 1015 Ω.cm, dielectric strength 145 kV/cm, dielectric constant of 4.0 at 60 Hz, and power factor of 90% at above 415 nm wavelength), superior thermal stability, and a desirable glass transition temperature (250oC).4,18,49-51 Poly(vinyl chloride) has desirable transmitted light intensity (82.3%) with incident light entering the light trap of 99.4%. Poly(ethylene terephthalate) has transmitted light intensity of 88.8% with incident light entering the light trap of 99.4%. Polycarbonate is the fifth optimized member due to its transmitted light intensity of 88.3% with incident light entering the light trap of 99.4%.51 4.4.2 REFRACTIVE INDEX The “reflection” term denotes the portion of light that bounces off the surface of a polymeric sample. Two types of reflection of electronic polymers are observed “diffused reflection” that represents the light reflected in many directions from an uneven polymeric surface, and “direct reflection” that represents the light reflected at an identical angle to the incident beam (it is often referred to as “specular reflection).” Generally, the values of “refractive indices” of electronic polymers are ranging from 1.30 to 1.70. The value of a refractive index plays an important role in optimizing refractive index-dependent organic electronic structures such as “waveguide fibers” of both single- and multi-modes with polymeric cores of refractive indices higher than their polymeric cladding. To optimize the structures of such fibers (either a single- or multi-mode waveguide), it is important to select the suitable index difference between core and cladding. This difference depends strongly on the dimensions of waveguide and the waveguide of the light source.23,49,52 On the other hand, it should be noted, that the “dense packing” or large “polarizability” optimizes (increases) the refractive index. Both high molar refractivity and low molar volumes optimize (increase) the related refractive index of the electronic polymer, as well. The following electronic polymers are considered as optimized members of refractive index-dependent organic electronic structures such as organic waveguide fibers due to their high refractive indices poly(2-chlorostyrene) of 1.610 refractive index, poly(1-naphthyl methacrylate) 1.640, poly(pentabromobenzyl acrylate) 1.670, poly(pentabromobenzyl methacrylate) 1.710, poly(vinyl phenyl sulfide) 1.657, poly(N-vinylphthalimide) 1.620, and poly(2-vinylthiophene) 1.638 refractive index.53 Examples of refractive indices of some other selected electronic polymers include76 poly(hexafluoropropylene oxide) refractive index = 1.497, fluorinated ethylene-propylene copolymer = 1.3380, poly(pentadecafluorooctyl acrylate) = 1.3390, poly(tetrafluoro-3(heptafluoropropoxy)propyl acrylate) = 1.3460, polytetrafluoroethylene = 1.3500, poly(trifluorovinyl acetate) = 1.3750, poly(methyl hydrosiloxane) = 1.3970, polydimethylsiloxane = 1.4035, poly(vinylidene fluoride) = 1.4200, poly(ethylene oxide) = 1.4539, poly(vinyl ethyl ether) = 1.4540, poly(phenylene oxide) = 1.4570, poly(ethylene glycol) = 1.4590, poly(vinyl acetate) = 1.4665, poly(vinyl methyl ether) = 1.4670, cellulose acetate butyrate = 1.4750, cellulose acetate = 1.4750, ethyl cellulose = 1.4790, poly(ethyl methacrylate) = 1.4850, poly(vinyl alcohol) = 1.5000, polyethylene = 1.5100, polyacrylonitrile =

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1.5187, poly(acrylic acid) = 1.5270, high-density polyethylene = 1.5400, polyamide-6/6 = 1.5650, polycarbonate = 1.5860, polystyrene = 1.5894, poly(vinylidene cyanide) = 1.6000, polysulfone = 1.6330, and poly(vinyl carbazole) = 1.6830. Note: high refractive index polymers have a refractive index >1.50. These polymers are required for anti-reflective coatings and photonic devices such as light-emitting diodes and image sensors. 4.4.3 OPTICAL ABSORPTION The “absorption” term related to optoelectronic polymers indicates the light is captured and dissipated as heat within a polymer as it passes through. Such absorption can be classified as high-level absorption, exponential region, and weak absorption. High-level absorption is the main optical transition. The optical absorption coefficient determines how far the optoelectronic polymer light of a particular wavelength can penetrate before it is absorbed. According to this concept, the optical absorption-dependent organic electronic structures such as organic solar cells can be optimized by selecting polymers having high absorption coefficient due to their absorption of photons, which excite electrons into the conduction band.16,46,49,55 In conclusion, the optimized optoelectronic polymers for such structures are those having maximum optical absorption coefficient values such as55 polymethylmethacrylate ~105 [cm-1] at 170 nm wavelength, poly(vinyl alcohol) ~106 [cm-1] at 170 nm wavelength, polyimide >105 [cm-1] at 170 nm wavelength, polyethylene, polypropylene, and polytetrafluoroethylene ~103 [cm-1] at 170 nm wavelength. 4.4.4 BIREFRINGENCE The double refraction of light in a transparent electronic polymer is termed as “birefringence,” which may be caused by the presence of orientation-dependent differences in refractive indices. Birefringence can be considered as a scale for measuring orientation in optically anisotropic polymers. For isotropic electronic polymers, the birefringence term is related to the stress build-up within the electronic polymer due to processing or thermal treatment. To optimize the birefringence-dependent organic electronic structures, the selected electronic polymers should be in the range from 10-5 to 10-6. Note: aromatic compounds such as aromatic polyamides exhibit a very large birefringence (up to 0.24).23,52,58 4.4.5 OPTICAL TRANSMISSION Light on the surface of an electronic polymeric sample is partially transmitted, reflected, and some absorbed. Transmittance is an important parameter used for describing the optical transmission by polymers, given as the ratio of the light passing through to the incident light on the polymeric samples, and the reflectance the ratio of the light reflected to the incident light. A gloss of polymeric film is another important parameter related to transmission. It shows the function of the reflectance and the surface finish of a polymer. To optimize the optical transmission-dependent organic electronic structures, the electronic polymers must have transmission percentages ranging from 87 to 92%, such as polymethylmethacrylate = 92%, polystyrene = 87-90%, polycarbonate = 85-91%, cyclic olefin copolymer = 92%, and 1,3-adamantane dicarbonyl dichloride = 89-91%.4,29,30,49,59

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4.4.6 POLARIZABILITY The light polarization represents the direction in which light oscillates (light is an electromagnetic wave). According to the application of electronic polymers, light is polarized when it is reflected from a transparent polymeric sample. The “degree of polarization” depends on the type of polymer and on the angle at which the light is reflected. Three types of polarizability are contributed to the total polarization of polymers, including electronic, atomic, and dipole orientation. To optimize the polarizability-dependent organic electronic structures, it is advisable to use aromatic polymers such as aromatic polyimide rather than aliphatic polymer such as aliphatic polyester, due to its better packing and electronic polarizability.23,31,52,60 4.4.7 HAZE The haze is a physical parameter used to measure scattering as the ratio of diffuse to total transmission. Haze is caused by imperfections on the surface of the polymeric sample, especially low-density polyethylene thin films. Haze term describes polymers having a cloudy or milky appearance.4,49 4.4.8 PHOTOCONDUCTIVITY The photoconductivity is the optoelectronic phenomenon focused on the electrical conductivity of electronic polymers due to the absorption of electromagnetic radiation such as visible light, UV, infrared, or gamma radiation. Photoconductive polymers are very good insulators in the dark, but they become conductive when exposed to light. To optimize photoconductivity-dependent organic electronic structures, the photoconducting polymers have to be selected, which are able to absorb light to permit photoexcitation of electrons from the ground state, and allow migration of photo-excited electrons, holes, or both through the photoconductive polymer in the electric field towards electrodes. In addition to charge-transport polymers, photoconductive polymers can be classified as p-type or ntype (depending on the majority carrier). Most photoconductive polymers are of p-type because of their function of transporting only holes.61 Optimized photoconducting polymers used for photoconductivity-dependent organic electronic structures include48,57,61 polypyrrole, polythiophene, poly(vinyl carbazole), poly(phenylene vinylene), polymethylphenylsilane, poly(2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b,3,4-b’]dithiophene)alt-4,7-(2,1,3-benzothiadiazole)), poly(N-dodecyl-2,5,-bis(2'-thienyl)pyrrole,2,1,3-benzothiadiazole, poly(9,9-dioctyl-fluorene)-4,7-di(2'-thienyl)-2,1,3-benzothiadiazole, and poly(3-hexyl thiophene). Poly(vinyl carbazole) was the first optimized photoconductive polymer (with acceptable electron acceptors) used for photoconductivity-dependent organic electronic structures such as electron-photography systems due to its high level of photoconductivity. It can be fabricated in the form of layers (13 µm thickness) with 2,4,7-trinitrofluorenone used as a single-layer photoconductor. Poly(phenylene vinylene) and polymethylphenylsilane have been selected as the second and third optimized members because they exhibit extremely high carrier mobilities. Poly(2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1b,3,4-b’]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)) has been classified as the fourth optimized member because it is naturally a low bandgap conjugated polymer with an opti-

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Optimization of Electrical, Electronic and Optical Properties of Organic Electronic Structures

cal bandgap of ~1.5 eV. It can act as an electron donor with methanofullerene (electron acceptor) in photoelectronic systems.57 4.4.9 OPTICAL EMISSION The “emission” term describes the conversion of the higher energy quantum mechanical state of a particle to a lower state with the emission of a photon, resulting in the production of radiation. According to this concept, the doped conjugated polymers of long-range pelectron delocalization behave similar to metals, while the naturally (undoped) conjugated polymers behave similarly to the semiconducting materials. Optimizing the optical emission-dependent organic electronic structures can be achieved by switching their optical/ physical properties between doped and undoped states. These changes include polymer volume, absorption color, and reversible photoluminescence quenching.33,49,62 After such switching, the optimized photoconductive polymers (also called the optimized light-emitting polymers) become useful for optical emission-dependent organic electronic structures such as polymeric light-emitting diodes. These polymers include33,54,62 1. poly(vinyl carbazole) 2. poly(p-phenylene) and its derivatives 3. poly(2-decyloxy-1,4-phenylene) 4. poly(1,4- phenylene-1,2-diphenylvinylene) 5. poly(phenylene ether) 6. poly(9,9-dioctylfluorene) 7. poly(2-methoxy-5-(3',7'-dimethyloctyloxy))-1,4-phenylene-vinylene) 8. poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene 9. poly(2,5-bis(3’,7’-dimethyloctyloxy)-1,4-phenylenevinylene) 10. poly(9,9-(2'-ethylhexyl)fluorene). Poly(vinyl carbazole) was selected as the first optimized light-emitting polymer for the optical emission-dependent organic electronic structures due to its attractive levels of orbital energy, such as the highest occupied molecular orbital HOMO of 5.8 eV and lowest unoccupied molecular orbital LUMO of 2.2 eV. Poly(p-phenylene) and its derivatives have been classified as the second optimized class for the same structures because they can emit deep blue light with a significant portion of the emission in the UV region. Poly(2-decyloxy-1,4-phenylene) exhibits high photoluminescence and electroluminescent quantum yields. It has emission peak at about 410 nm. Poly(1,4- phenylene-1,2-diphenylvinylene) is naturally a green emitter with high photoluminescence efficiency and stability. Polyphenylene ether has optical properties, which are approximately similar to those of the well-know poly(phenylene vinylene). Poly(9,9-dioctylfluorene) has photoluminescence quantum yield >70%. Poly((2-methoxy-5-(3',7'-dimethyloctyloxy))-1,4-phenylenevinylene)) has highest occupied molecular orbital of 5.4 eV and lowest unoccupied molecular orbital of 3.2 eV (fluorescence exciting light (also called emission spectra) λex of 485 nm and maximum emission peak (also called or maximum emission band or maximum emission wavelength) λem of 555 nm in toluene). Poly(2-methoxy,5-(2'-ethyl-hexyloxy)p-phenylenevinylene has the highest occupied molecular orbital of 5.3 eV and the lowest unoccupied molecular orbital of 3.0 eV (fluorescence exciting light of 493 nm and maximum emission peak λem of 554 nm in toluene). Poly(2,5-bis(3’,7’-dimethyloctyloxy)-1,4phenylenevinylene) has fluorescence exciting light of 480 nm and maximum emission

4.4 Optical properties

199

peak of 548 nm in toluene. Poly(9,9-(2'-ethylhexyl)fluorene) has fluorescence exciting radiation of 362 nm and a maximum emission peak of 409 nm in chloroform.33,54,62 4.4.10 LUMINESCENCE The electrically driven emission of radiation from non-crystalline organic materials/polymers represents the concept of organic electroluminescence. This phenomenon is applied for the luminescence-dependent organic electronic structures such as two-layered organic light-emitting, where one organic layer is applied to transport holes while the other is to transport electrons. The optimization of luminescence-dependent organic electronic structures starts from two-layered structures because the interface between them provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence.54,57,61-62 The optimized electro-luminescent polymers related to optoelectronic polymers used for luminescence-dependent organic electronic structures include 1-aminoanthracene 1AANC, 2-aminoanthracene 2AANC, 3-aminobiphenyl ABP, acetyl acetonate ACAC (or acac), 9-acetyl-3,6-diiodo-carbazole ADIC, 2-aminofluorene 2AF, 2-amino-9-fluorenone A9F, anthracene ANC, 1,1-bis(4-aminophenyl)-cyclohexane APhCh, 1-aminopyrene APRN, 4-amino-p-terphenyl APTPh, 9,9-bis(4-aminophenyl)-fluorene BAPF, 9,9-bis(4-aminophenyl)-fluorene purified by sublimation BAPF-pur, bis(4biphenylyl)amine BBPYA, bathocuproine BCP, purified bathocuproine BCP-pur, bis(4formylphenyl)-phenylamine BFPPA, 9,9-bis(4-hydroxyphenyl)fluorine BHPF, 4,4'-bis(5methyl-2-benzoxazolyl)stilbene BMBXS, purified 4,4'-bis(5-methyl-2-benzoxazolyl)stilbene BMBXS-pur, 3-(2-benzimidazolyl)-7-(diethylamino)coumarin BMDEAC, 2,5-bis(1naphthyl)-1,3,4-oxadiazole BND, bathophenanthroline Bphen, (bathophenanthroline)2 (bphen)2, purified bathophenanthroline Bphen-pur, 2-bromoanthracene 2BrANc, 9-bromoanthracene 9BrANc, 2-bromobiphenyl 2BrBP, 3-bromobiphenyl 3BrBP, 4-bromobiphenyl 4BrBP, 2-bromodibenzothiophene BrDBT, 2-bromofluorene 2BrF, 9bromofluorene 9BrF, bis(4-bromophenylamine) BrPA, tris(4-bromophenyl)amine BrPhA, 9-(4-bromophenyl)-carbazole BrPhC, 3-bromo-1,10-phenanthroline BrPhen, 9,9-bis(4bromophenyl)-fluorene BrPhF, 1-bromopyrene BrPn, 1-(4-bromophenyl)-1,2,2-triphenylethylene BrPPE, 2-bromo-5-phenylthiophene BrPTh, and 4-bromotriphenylamine BrTPA.77-78 Depending on the way of spinning the electron, “luminescence” can be classified as “fluorescence” or “phosphorescence.” The energy of phosphorescent emission is lower than that of fluorescent emission. Phosphorescence represents the emission of photons from triplet states. It is, thus, emission of light from an excited state (also called “triplet excited state”) to a ground state (singlet “ground state”), but with different multiplicity. This process involves both an electronic state change and a spin-flip. Phosphorescent polymers (also called “phosphorescent emitters”) are complexes exhibiting a phosphorescence of very high efficiency (100%).17,19,42,48,63 Compared with fluorescent, the phosphorescent emitters can be selected as optimized for luminescence-dependent organic electronic structures due to their ability to exhibit long excited state lifetime. The optimized complexes used for phosphorescent-dependent organic electronic structures such as organic light-emitting diodes and photo-phosphorescent polymeric light-emitting diodes include tris(1-phenylisoquinoline)iridium Ir(piq)3, fac-tris(2-phenylpyridine) iridium(III)

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Optimization of Electrical, Electronic and Optical Properties of Organic Electronic Structures

Ir(ppy)3, iridium(III)bis((4,6-difluorophenyl)-pyridinato-N,C2')picolinate FIrpic, iridium(III)-bis(4',6'-difluorophenyl pyridinato)tetrakis(1-pyrazolyl)borate FIr6, tris(phenbis(2-(2-pyridinyl-N)phenyl-C)(2,4-pentanedionatoylpyrazole)iridium Ir(ppz)3, O2,O4)iridium(III) Ir(ppy)2(acac), bis(2-(2'-benzothienyl)-pyridinato-N,C3')iridium(III) (acetylacetonate) Ir(btp)2(acac), and platinumoctaethyl-porphyrin PtOEP.17,18,63 Fluorescence represents the emission of radiation from an excited state (triplet excited state) to a ground state (singlet ground state), but with the same multiplicity. Compared with the exciting radiation, fluorescence-dependent organic electronic structures are optimized due to their fluorescence emission at lower frequencies because the emissive transition takes place after the energy is partially dispersed to the environment as vibrational energy. On the other hand, when compared with phosphorescence emission, fluorescence-dependent organic electronic structures are also optimized because the fluorescence emission has higher energy.42 Fluorescent polymers (also called fluorescent emitters), which can be optimized for the above structures, especially due to their ability to absorb light in the UV and blue region, in addition to emitting light in the visible region, include anthracene ANC, arylmethine ArM, cyanine CYNN, fluorescein isothiocyanate FITC, phydroxybenzylidene-imidazolidone FPh, bis(4-(N-(1-naphthyl)phenylamino) phenyl)fumaronitrile NPAFN, 2,5,8,11-tetra-tert-butylperylene TBPe, fluorescent polymethylmethacrylate Fsc-PMMA, naphthalene NA, oxadiazoles OXD, oxazine OXZ, benzo[def]phenanthrene PRN, rose bengal RBg, (2,4-bis(4-(N,N-diisobutyl amino)-2,6dihydroxy phenyl)squaraine) SQN, tetrapyrrole TPYR, and xanthen Xth.18

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Tony Blythe and David Bloor, Electrical Properties of Polymers, Second Edition, © University of Durham, UK, 2005. Jan Kalinowski, Organic Light-Emitting Diodes: Principles, Characteristics & Processes, © CRC Press, 2004. Polymers, chemical structure, and Symbols L. H. Sperling, Introduction to Physical Polymer Science, Fourth Edition, © 2006, John Wiley & Sons, Inc. J. A. Brydson, Plastics Materials, 7th Edition, © J. A. Brydson, 1999. Alastair Buckley, Organic Light-Emitting Diodes (OLEDs): Materials, Devices and Applications, © Woodhead Publishing, 2013. Jiri George Drobny, Polymers for Electricity and Electronics: Materials, Properties, and Applications, © John Wiley & Sons, 2012. Patrick Schmidt-Winkel, Synthesis Design of Polar Polymers and Nanostructured Porous Silica, © University of California, Santa Barbara, 1999. Charles Harper, Handbook of Plastics, Elastomers & Composites, 4th Edition, McGraw-Hill Professional, 2002. Gooch, Jan, Encyclopedia Dictionary of Polymers, © Springer-Verlag New York, 2011. ASTM D924 - 15, Standard Test Method for Dissipation Factor (or Power Factor) and Relative Permittivity (Dielectric Constant) of Electrical Insulating Liquids, Book of Standards Vol. 10.03. Thomas Roy Crompton, Polymer Reference Book, © iSmithers Rapra Publishing, 2006. Karasz, Frank, Dielectric Properties of Polymers, © Holder Plenum Press, New York, 1972. R.W. Munn, Andrzej Miniewicz, and Bogdan Kuchta, Electrical and Related Properties of Organic Solids, © Springer Science & Business Media, 2012. Natti S, Basic Polymer Engineering Data, © Hanser Publications. Wallace C.H. Choy, Organic Solar Cells: Materials and Device Physics, © Springer-Verlag London, 2013. M. Aldissi, Intrinsically Conducting Polymers: An Emerging Technology, © Springer Science & Business Media, 1993.

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Seizo Miyata, Organic Electroluminescent Materials and Devices, © CRC Press, 1 Edition (July 16, 1997. Melissa Mushrush, Antonio Facchetti, Michael Lefenfeld, Howard E. Katz, Tobin J. Marks, Easily Processable Phenylene-Thiophene-Based Organic Field-Effect Transistors and Solution-Fabricated Nonvolatile Transistor Memory Elements, J. Am. Chem. Soc. 2003, 125. John A. Emerson and John M. Torkelson, Optical and Electrical Properties of Polymers, Volume 214, © Cambridge University Press, 1991. Anish Khan, Mohammad Jawaid, Aftab Aslam Parwaz Khan, and Abdullah M. Asiri, Electrically Conductive Polymers and Polymer Composites: From Synthesis to Biomedical Applications, © Wiley-VCH, 2017. Larry Rupprecht, Conductive Polymers and Plastics, © William Andrew, 1999. Faiz Mohammad, Specialty Polymers: Materials and Applications, © I. K. International Pvt Ltd, 2007. Julie P. Harmon and Gerry K. Noren, Optical Polymers: Fibers and Waveguides, © American Chemical Society, 2001. Abay Gadisa, Studies of Charge Transport and Energy Level in Solar Cells Based on Polymer/Fullerene Bulk Hetero-junction, Linköping University, © Linköping 2006. Safa Kasap and Peter Capper, Springer Handbook of Electronic and Photonic Materials, © Springer US, 2007. Sergei Baranovski, Charge Transport in Disordered Solids with Applications in Electronics, © John Wiley & Sons, 2006. Lioz Etgar, Hole Conductor Free Perovskite-based Solar Cells, © Springer International Publishing, 2016. William R. Salaneck, Kazuhiko Seki, Antoine Kahn, and Jean-Jacques Pireaux, Conjugated Polymer and Molecular Interfaces: Science and Technology for Photonic and Optoelectronic Application, © CRC Press, 2001. Donald L. Wise, Gary E. Wnek, Debra J. Trantolo, Thomas M. Cooper, and Joseph D. Gresser, Photonic Polymer Systems: fundamentals, Methods, and Applications, © 1998 by Marcel Dekker, Inc. C. P. Wong, Polymers for Electronic & Photonic Application, © Elsevier, 2013. Dennis H. Goldstein, Polarized Light, 3rd Edition, © CRC Press, 2010. Eugenio Cantatore, Applications of Organic and Printed Electronics, © Springer Science+Business Media New York 2013. Zhigang Li, Zhigang Rick Li, and Hong Meng, Organic Light-Emitting Materials and Devices, © CRC Press, 2006. Carlo Jacoboni, Theory of Electron Transport in Semiconductors: A Pathway from Elementary Physics to Nonequilibrium Green Functions, © Springer Science & Business Media, 2010. Wolfgang Brütting and Chihaya Adachi, Physics of Organic Semiconductors, © John Wiley & Sons, 2012. Rungtiwa Chidthong, Theoretical Investigation on Structural and Electronic Properties of FluorenePyridine Copolymer, Kasetsart University, 2007. Jianhui Hou, Mi-Hyae Park, Shaoqing Zhang, Yan Yao, Li-Min Chen, Juo-Hao Li, and Yang Yang, Bandgap and Molecular Energy Level Control of Conjugated Polymer Photovoltaic Materials Based on Benzo1,2-b:4,5-b’)dithiophene, Macromolecules 2008, 41l, © 2008 American Chemical Society. Y. Li, Organic Optoelectronic Materials, © Springer International Publishing Switzerland 2015. Eric Daniel G. Owacki, Niyazi Serdar Sariciftci, Ching W. Tang, Organic Solar Cells, © Springer Science+Business Media New York 2013. Hari Singh Nalwa, Handbook of Organic Electronics and Photonics, © American Scientific Publishers, 2008. He Seung Lee, Albert. S. Lee, Kyung-Youl Baek and Seung Sang Hwang, Low Dielectric Materials for Microelectronics, © 2012 Hwang et al., Pintech. Pierpaolo Brulatti, New luminescent iridium (III) complexes containing NCN cyclometallated ligands: synthesis, photophysical properties and emission tuning, Durham University, 2010. 2007 Wiley-VCH Verlag GmbH & Co. T. Akasaka et al, Chemical Science of π-Electron Systems, © Springer Japan 2015. Stroble, G, R, The Physics of Polymers: Concepts for understanding their Structure and behavior, © Springer, 2007. Donald J. Dahm, Interpreting Diffuse Reflectance and Transmittance: A Theoretical Introduction to Absorption Spectroscopy of Scattering Materials, © IM Publications LLP, 2007. J. A. Chilton, M. Goosey, Special Polymers for Electronics and Optoelectronics, © Springer

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Optimization of Electrical, Electronic and Optical Properties of Organic Electronic Structures Netherlands, 1995. David I. Bower, An Introduction to Polymer Physics, © D. I. Bower 2002, Cambridge University. Cesare Soci, In-Wook Hwang, Daniel Moses, Zhengguo Zhu, David Waller, Russel Gaudiana, Christoph J. Brabec, and Alan J. Heeger, Photoconductivity of a Low-Bandgap Conjugated Polymer, Adv. Funct. Mater. 2007, 17, © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Thomas A. Germer, Joanne C. Zwinkels, and Benjamin K. Tsai, Spectrophotometry, Volume 46: Accurate Measurement of Optical Properties of Materials (Experimental Methods in the Physical Sciences), © Academic Press, 2014. Jeffrey J. McDowell, Cross-linkable Light-Emitting Conjugate and Metallocene Polymers: Synthesis, Properties and Application, University of Toronto 2013, © by Jeffrey. J. McDowell 2013. Kai Tao, Video IV Monitor, WO2014187432A1. WO Application patent, 2017. Xingcun Colin Tong, Advanced Materials for Integrated Optical Waveguides, © Springer Nature, Germany (2013). Properties and Behavior of Polymers, 2 Volume Set, © John Wiley & Sons, 2012. W. Schnabel, Polymers and Light: Fundamentals and Technical Applications, © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2007. David I. Bower, An Introduction to Polymer Physics, © Cambridge University Press, 2002. Mylar® Polyester Film, Electrical properties Hari Singh Nalwa, Advanced Functional Molecules and Polymers: Electronic and photonic properties, Volume 3, © CRC Press, 2001. Cambridge Polymer Group, The Theory of Birefringence, © Cambridge Polymer Group, 2004. K. S. Lee, Polymers for Photonic Applications, © Springer Science & Business Media, 2002. Serge Huard, Polarization of Light, © Wiley, 1997. L. S. Hung, C. H. Chen, Recent Progress of Molecular Organic Electroluminescent Materials and Devices, Material Science and Engineering R39, 2002, © 2002 Elsevier Science B.V, PII:S0927-796X(02)00093-1. Klaus Müllen, Ullrich Scherf, Organic Light-emitting Devices: Synthesis, Properties and Applications 1st Edition, © Wiley-VCH, 2006. WS Hampshire Inc, Teflon, Teflon® is a registered trademark of DuPont, www.catalog.wshampshire.com. Polymer Database; Introduction to Polymers: A Property Database Online Preface; Polymers 2011 © 2013 Taylor & Francis Group. The Universal Selection Source: Plastics & Elastomers; Dielectric Constant; © SpecialChem 2017. Polymer Properties Database; Conducting Polymers; © 2015 polymerdatabase.com. Omnexus; Arc resistance; © SpecialChem 2017. Roel S. Sánchez-Carrera; Theoretical Characterization of Charge Transport in Organic Molecular Crystals; Georgia Institute of Technology, 2008. Sara Righi; Charge Transport Properties of Organic Conjugated Polymers for Photovoltaic Applications; Università di Bologna; 2014. Manisha Bajpai, Ritu Srivastava, Ravindra Dhar, and R. S. Tiwari; Review on Optical and Electrical Properties of Conducting Polymers; Indian Journal of Materials Science Volume 2016 (2016), Article ID 5842763; DOI: 10.1155/2016/5842763. Sateesh Prathapani; How is the hole transported through the metal in an application to a solar cell? Indian Institute of Technology Bombay; © 2008-2017 researchgate.net. van Mensfoort, S.L.M.; Billen, J.G.J.E.; Vulto, S.I.E.; Janssen, R.A.J.; and Coehoorn, R; Electron transport in polyfluorene-based sandwich-type devices: Quantitative analysis of the effects of disorder and electron traps; Physical Review B, 80, 033202, 2009; DOI: 10.1103/PhysRevB.80.033202. Xingang Zhaoab and Xiaowei Zhan; Electron transporting semiconducting polymers in organic electronics; Chem. Soc. Rev., 40, 3728-3743, 2011,40; DOI: 10.1039/C0CS00194E. Yixing Yang, Efficient Near-Infrared and Ultraviolet Organic Light Emitting Devices, University of Florida, 2011 Dow Corning; Optical Properties, Definitions, and Measurements; Form N0. 11-2-02-01; © 2012 Dow corning corporation. Scientific Polymer; Refractive Index of Polymers by Index; © 2013 Scientific Polymer, Inc. Electronic Materials Index, Well-Being Center, Seoul, Korea; 2010. TCI, Organic Electroluminescence Materials, 2012, www.tcichemicals.com.

5

Optimization of Polymeric Structures of Organic Printed Circuit Boards 5.1 OVERVIEW The polymeric printed circuit boards (also called “organic printed circuit boards”) PCB are the simplest self-assembled structures of interconnected microelectronic/nanoelectronic components on a thin film substrate formed from polymer-based composites such as epoxy. As illustrated in Figure 5.1,30,107 such structure has several advantages, including very low coast, good electromechanical properties, easy to be manufactured in any shape, and applicable for multilayer configurations. These printed circuit boards can be electrically connected to organic/polymeric circuits by either “through-hole technology” THT (older process) or “surface mount technology” SMT (newer process). More recent solution includes “integrated circuits” IC (also called “microchip”) by which the polymeric printed circuit boards can be built with more circuits and components that are electrochemically grown in place on the surface of a very small chip. The other method is called a “hybrid circuit” that contains some components, which can be grown onto the surface of the substrate without the need to be mounted on the surface and soldered.1-2,8

Figure 5.1. Structure of organic/polymeric printed circuit boards.30,107

Three-dimensional polymeric printed circuit boards 3D-PCPs are the most developed solutions, where both single-sided or multilayered polymeric printed circuit boards can be fabricated in the form of 3D-prototypes depending on the “miniature technologies” such as “molded interconnect device” MID. With molded interconnect device technology, the copper traces act as an electrical circuit on the polymeric substrate that can be fabricated by “two-component injection molding,” “hot stamping, photolithography,” “in-mold

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Optimization of Polymeric Structures of Organic Printed Circuit Boards

circuit film,” or “laser direct structuring” LDS which can be considered as the most flexible technology. Such a process can be called “three-dimensional molding of interconnected devices” 3D-MID, while the resultant product can be called “three-dimensional printed circuit board” 3D-PCB as illustrated in Figure 5.2.129 The direct laser structuring can be achieved immediately after applying single-component injection molding of the carrier. Examples of polymers used for fabricating the polymeric substrates by injection molding process include: liquid crystal polymers LCP, poly(butylene terephthalate) PBT, Trogamid® (amorphous polyamide) (known as: PA 6-3-T), and poly(ethylene terephthalate)/poly(butylene terephthalate) blend PET/PBT.2-5,8

Figure 5.2. Three-dimensional molded integrated device 3D-MID.[Data from Laser Micronics GmbH, 3-Dimensional Circuitry, Laser Direct Structuring Technology (LPKF-LDSTM) for Molded Interconnect Devices, 2012.]

The concept of “optoelectronic printed circuit boards” has been developed to satisfy the increasing demand for high data rates along with progressive miniaturization of devices and components. Optoelectronic printed circuit board represents the integration of optics in a polymeric printed circuit board that is the need to utilize optical fibers, the generation of waveguides by UV lithography, embossing, or direct laser writing. An example of commercial polymers used for structuring optoelectronic printed circuit boards is Ormocer®195, which is a type of inorganic/organic hybrid polymer.3,129

5.2 POLYMERS FOR CONVENTIONAL PRINTED CIRCUIT BOARDS The main function of printed wiring is to support circuit components and interconnect these components electrically. Several types of flexible and rigid printed wiring have been used, including traditionally printed circuit boards (single-sided, double-sided, and multilayered), ultra-multilayered printed circuit boards, and three-dimensional printed circuit

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boards. These types are based on variable dielectric materials (polymers and composites, conductors types, number of conductor planes, rigidity, flexibility, etc.) Rigid printed circuit boards of substrates formed from thermosetting polymers, such as epoxy composites, and flexible printed circuit boards of substrates formed from thermoplastic polymers (such as polyethylene terephthalate PET) are the main two types. Thermosetting composites can be used in the form of filled or glass fiber reinforced epoxy resin EP-GFR, while paper reinforced phenolic (phenol-formaldehyde PF) resin with a bonded copper foil or silicone Q substrates is used for fabricating very small chips (microchips) and stretchable circuits.2,7-8,13 The first optimized type of glass fiber reinforced epoxy resin used for structuring conventional printed circuit boards is available in the form of epoxy glass, fire-retardant grade-4 (abbreviated as FR-4). It has very high thermal stability, good mechanical and electrical properties, flame retardance, excellent bonding to copper foil, electroless copper, and glass fibers. It is approved by National Electrical Manufacturers Association NEMA for synthetic resin bonded papers. FR-4 is a polymeric composite material made of woven glass impregnated with plasticized epoxy resin. The second optimized resin is phenolic glass, fire-retardant grade-2 (abbreviated as FR-2), which is a composite material made of paper impregnated with plasticized phenol-formaldehyde resin for the same application. The third optimized type is silicone, which has been used as a flexible substrate for fabricating hybrid stretchable circuits (Figure 5.3114,125,144). It consists of a millimeter thick polydimethylsiloxane PDMS (IUPAC name: polydimethylsiloxane) having the formula ((C2H6OSi)n) within which two concentric discs of polyimide PI foil (50 μm thick) are embedded. It is a derivative of polyorganosiloxane used as packaging polymer for electronic devices, as well. Polydimethylsiloxane is the first optimized derivative of silicone family used for structuring polymeric double-disk stretchable substrate due to its high strain property. Note: polymeric materials used for fabricating printed circuit board substrates are made of differential dielectrics. This means that they can be selected to provide different insulating values depending on the requirements of the circuit. For this reason, the two types of substrate that can be selected for traditional printed circuit boards are dielectric printed circuit boards and prepreg printed circuit boards.7,13

Figure 5.3. A representation of the polymeric double-disk stretchable substrate.114,125,144

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5.2.1 DIELECTRIC SUBSTRATE-BASED POLYMERIC PRINTED CIRCUIT BOARDS The dielectric materials (including most polymers) are insulators (non-conducting electricity) resisting the flow of an electrical current. Dielectric polymer represents the optimized class of dielectric materials used for dielectric substrate-based polymeric printed circuit boards, including polytetrafluoroethylene PTFE, epoxy glass, fire-retardant grade4 (FR-4), epoxy glass, fire-retardant grade-1 (FR-1), composites of epoxy with cotton paper (CEM-1),107 and composed of nonwoven glass core combined with epoxy resins (CEM-3). The laminates of thermoplastic polymer polytetrafluoroethylene represent the first member of dielectric polymers used for antennas and base stations because of their high melt viscosity. The laminates are fabricated under the lamination pressure of 3.1-3.27 MPa.206 An example of commercial polytetrafluoroethylene grade used as a polymer for structuring the substrate of double-sided printed circuits is Teflon®146. Figure 5.47,148 is an example of multilayer laminate containing polytetrafluoroethylene used for printed circuit boards. Polytetrafluoroethylene (Teflon®146) is bonded with chloro-trifluoroethylene CTFE (IUPAC name: 1-chloro-1,2,2-trifluoroethene) having formula C2ClF3 or fluorinated ethylene-propylene copolymer FEP produced by free-radical polymerization of mixture of hexafluoropropylene and tetrafluoroethylene.8-10,12,30,150

Figure 5.4. Example of multilayer laminate containing polytetrafluoroethylene for printed circuit boards.7,148

Epoxy glass, fire-retardant grade-4 FR-4 is the second optimized grade for structuring dielectric substrate-based polymeric printed circuit boards due to its very good mechanical and electrical properties, ability to resist flame, and excellent bonding to copper foil, electroless copper, and glass fibers. Epoxy is available as “difunctional-epoxy” DfEP blend formed by reacting epichlorohydrin and bisphenol-A with flame-retardant additive, “tetrafunctional epoxy” TfEP blend, and “multifunctional epoxy” MfEP blend, listed in Table 5.1.7,110-111 Importance of epoxy polymer is in acting as self-extinguishing binder (self-extinguishing term is used to describe the ability of a material/polymer to cease burning upon removal of the source of flame). Epoxy glass, fire-retardant grade-4 available in the form of laminate is resistant to high temperature and water absorption\, has good electrical insulation, and good machinability. To optimize both physical and electrical properties of the laminate, its glass fibers should be made perpendicular to one

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another. Some of these properties are listed in Table 5.2.7,110-111 An example of epoxy glass, fire-retardant grade-4 (FR-4) two-layer structure is illustrated in Figure 5.5.107,151 Such structure consists of a woven glass fiber mesh soaked in the organic polymer (epoxy as resin matrix) with copper layers laminated (sometimes filled with specific materials).7,30,106,108 Table 5.1. Optimized epoxy types for dielectric substrate-based polymeric printed circuit boards.7,110-111 Properties

Units

(condition) o

Difunctional epoxy DfEP blend

Tetrafunctional epoxy TfEP blend

Multifunctional epoxy MfEP blend

C

130

130-140

160-190

Dielectric constant

at 1MHz

4.5

4.6

4.4

Dissipation factor

at 1MHz

0.025

0.025

0.025

%

0.70

0.06-0.013

0.60-0.013

Glass transition temperature

Moisture absorption

Table 5.2. The general properties of epoxy glass, fire-retardant grade-4 (FR-4) and epoxy glass, fire-retardant grade-1 (FR-1).7,110-111 Properties

Units

FR-4

FR-1

g/cm3

1.85

1.335

%

0.10-0.14

>1.2

Physical properties Density Water absorption (3.175 mm thick)

Mechanical properties Rockwell hardness (M-scale)

−

110

−

Bond strength

kg

>1000

−

Peel strength

N/mm

−

>1.2

Flexural strength

MPa

345-440

~100

Tensile strength

MPa

>310

41.1

Izod impact strength

J/m

44-54

−

Young's modulus

GPa

21-24

−

Electrical properties Dielectric breakdown

kV

>50

−

Dielectric strength80

MV/m

20

43

Relative permittivity

−

4.8

−

Dissipation factor

−

0.017

1x1010

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Optimization of Polymeric Structures of Organic Printed Circuit Boards

Table 5.2. The general properties of epoxy glass, fire-retardant grade-4 (FR-4) and epoxy glass, fire-retardant grade-1 (FR-1).7,110-111 Properties Volume resistivity Insulation resistance

Units

FR-4

FR-1

Ω-cm

−

>5x1011

Ω

−

>1x1011

Thermal properties Glass transition temperature Tg Flammability UL94

o

C

120-125

−

−

−

FV0

According to Figure 5.5, and Table 5.1, the polymeric matrix of epoxy glass, fireretardant grade-4 FR-4 laminates consists of bi-, tetra- or multi-functional epoxy groups. Note: epoxy EP is chemically derived from the reaction of bisphenol-A epoxy BPAE with epichlorohydrin ECO which creates diglycidyl ether of bisphenol A (DGEBA) (also referred to as oxirane OXr). OXr (DGEBA) reacts in subsequent resin polymerization, curing the polymeric matrix. Higher crosslinking in the cured system can be achieved by the use of epoxy monomers with more than two epoxy functional groups per molecule.7

Figure 5.5. Example of two-layer structure of epoxy glass, fire-retardant grade-4 (FR-4).107,151

Epoxy glass, fire-retardant grade-1 FR-1 is the second optimized epoxy glass, fireretardant grade used in dielectric substrate-based polymeric printed circuit boards. It is a thermoset polymeric composite formed from the paper base with plasticized epoxy resin. Properties of this composite were listed in Table 5.2. Composite of epoxy with cotton paper (abbreviated as CEM) represents the third optimized class of polymeric composites used for structuring dielectric substrate-based polymeric printed circuit boards. These composites are commercially available in the form of five numbered grades, including CEM-1, CEM-2, CEM-3, CEM-4, and CEM-5. Where numbers from 1 to 5 are related to the type of the base. For example, number 1 means cotton (or cellulose) paper and epoxy.7,13,16

5.2 Polymers for conventional printed circuit boards

209

CEM-1 is a polymeric composite consisting of woven glass fabric surfaces and paper core combined with epoxy resin. It is easy to punch, and has excellent mechanical and electrical properties, and higher flexural strength than paper-based grades. It is used in radio receivers, smoke detectors, and single-sided printed circuit boards. Its properties are listed in Table 5.3.7,110-111 CEM-3 is a composite of nonwoven glass core combined with epoxy resin. Similarly to CEM-1, it consists of epoxy resin with woven glass cloth surfaces, but its core is nonwoven matte fiberglass. The nonwoven mate fiberglass optimizes through-hole plating. It has better fine-line capability than FR-4. It has a very smooth surface and milky white color. It can be used as an alternative to FR-4, known as a flame retardant epoxy copper-clad plate glass material.7,13,16 Table 5.3. The general properties of composites of epoxy with cotton paper CEM-1 and composites of non-woven glass core combined with epoxy resin CEM-3.7,110-111 Properties

Units (Condition)

CEM-1

CEM-3

−

60

−

Peel strength

N/mm

>1.2

>6.0

Flexural strength

kg/m2

1-76-2.11x107

1-90-2.32x107

Pulling strength

Electrical properties 80

Dielectric constant

Dielectric breakdown Arc resistance

at 1MHz

>4.4

>4.6

kV

−

>40

s

>118

>115

at 1 MHz

0.022

0.0016

Surface resistance

MΩ

>30000

>104

Volume resistivity

MΩm

>5000

>105

Dissipation factor

−

0.045

−

flame retardant

flame retardant

110

132

Loss tangent

Thermal properties Flammability UL94 Glass transition temperature Tg

− o

C

5.2.2 PREPREG POLYMERIC PRINTED CIRCUIT BOARDS The prepreg term (derived from pre-impregnated) is related to reinforced polymer matrix consisting of reinforcements combined with a thermosetting liquid resins (typically epoxy), where the role of liquid epoxy resin is to provide good environmental resistance, high toughness, and easy processing, while the role of reinforcement (such as woven and nonwoven cloth, filaments, roving, etc.) is to provide excellent mechanical properties. The

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Optimization of Polymeric Structures of Organic Printed Circuit Boards

main forms of prepregs are undirectional (one direction of reinforcements) and fabric form (woven, NCF, NC2®152), as shown in Figure 5.6.109,153-154 NCF is the abbreviation of “non-crimp fabric,” while CN2® is the trade name of “multi-matrix fabric.”19 Reinforcements, advantages, and applications of these types are listed in Table 5.4.109,153-154

Figure 5.6. Prepregs for polymeric printed circuit boards.109,153-154

Table 5.4. Reinforcements, advantages, and applications of prepregs.109,153-154 Properties

Commercial grades

Examples of applications

Undirectional prepreg

High strength and stiffness. Available in low fiber weights (~100 g/m2) and high fiber weights (~3000 g/m2) for glass

-

PCBs and space applications

NCF

Low time investment, cost-effective technology. High strength and stiffness, and capability of optimizing weight distribution within a fabric with non-crimp

-

PCBs

NC2®

Non-gap construction allows great flexibility Hexcel's of fiber orientation. Provides strength and HexForce®stiffness. Products of homogeneous filament NC2®154 distribution in the matrix with improved mechanical properties.

Reinforcements

PCBs and automobiles

Two groups of polymeric composites are optimized for structuring prepreg substratebased polymeric printed circuit boards, including 1. epoxy glass, fire-retardant grade FR-n 2. composite of epoxy with cotton papers CEMn The first optimized group FR-n includes the following six members epoxy glass, fire-retardant grade-2 (FR-2), epoxy glass, fire-retardant grade-3 (FR-3), epoxy glass, fireretardant grade-4 (FR-4), epoxy glass, fire-retardant grade-5 (FR-5), epoxy glass, fireretardant grade-6 (FR-6), and glass epoxy laminate number 10 (G-10). The second opti-

5.2 Polymers for conventional printed circuit boards

211

mized group CEMn includes the following five composites of epoxy with cotton paper (CEM-1), composite with cellulose (cotton) paper core and woven glass fabric surface (CEM-2), composites of nonwoven glass core combined with epoxy resin (CEM-3), composite made of woven glass and epoxy resin (CEM-4), and composite made of woven glass and polyesters (CEM-5).7,13 Epoxy glass, fire-retardant grade-2 FR-2 that is available in the form of polymeric composite sheets consisting of cotton paper impregnated with plasticized thermosetting phenolic resin (known as phenol-formaldehyde) represents the first optimized member of the first group FR-n for structuring low-end consumer organic electronic systems due to its good electrical and mechanical properties and good machinability (drilling, sawing, milling, and punching). Sometimes, it is used for simple structural shapes and electrical insulation. Its electrical properties include dielectric constant 4.5 at 1 MHz, dissipation factor 0.024-0.26 at 1 MHz, and dielectric strength of 29134 V/mm.19,21 Epoxy glass, fireretardant grade-3 FR-3 that is also available in the form of polymeric composite sheets consisting of cotton (cellulose) paper, but impregnated with plasticized epoxy resin (not phenolic resin), represents the second optimized member of the first group FR-n for structuring prepreg substrate-based polymeric printed circuit boards due to its flexural strength of 78.6 MPa, tensile strength of 5.1 MPa, notched Izod impact strength of 130.8 J/m, and flammability of UL94-V1. Epoxy glass, fire-retardant grade-4 FR-4 has the same structure and applications as FR-2, but represents the third optimized member of the first group FRn because it is cheaper than FR-2. Epoxy glass, fire-retardant grade-5 FR-5 has a similar composition to FR-4, but with higher thermal stability. It represents the fourth optimized member of the first group FR-n for structuring prepreg substrate-based polymeric printed circuit boards, especially if higher thermal stability is required. Other attractive properties include glass transition temperature Tg of 175oC, excellent dimensional stability, UL certification number E176891 with UL flammability class 94 V-0, high luminance contrast between epoxy and copper for laser type A.O.I, volume resistivity of 1x107 MΩ.cm, dielectric breakdown >60 kV, moisture absorption 165 N/m.23 Epoxy glass, fire-retardant grade-6 FR-6 that is available in the form of opaque white polymeric laminates consisting of matte glass with plasticized polyester resins represents the fifth optimized member of the first group FR-n for structuring organic electronic systems of low-capacitance or high-impact, due to its flexural strength of 12 MPa, operating temperature of 110oC, relative water absorption of 0.40%, dielectric constant of 4.5, and coefficient of thermal expansion of 12 μm/moC for X-axis (15012 μm/moC for Y-axis). Glass epoxy laminates number-10 G-10 that is available in the form of polymeric composite sheets consisting of woven glass, impregnated with plasticized epoxy resin represents the sixth optimized member of the first group FR-n for structuring prepreg substrate-based polymeric printed circuit boards needed for computers and telecommunication due to its tensile strength of 310-240 MPa, flexural modulus of 18-16 GPa, dielectric constant of 5 at 1 MHz, water absorption of 0.1%, and 94 HB flammability rating.8,24-25

212

Optimization of Polymeric Structures of Organic Printed Circuit Boards

Composite with cellulosic cotton paper core and woven glass fabric surface CEM-2 that is available in the form of fabric impregnated with epoxy represents the first optimized member of the second group CEM-n for structuring prepreg substrate-based polymeric printed circuit boards, especially, when good electrical properties are required due to its volume resistivity of 1014 Ω.cm, surface resistivity of 1012 Ω, dielectric constant of 5.2, dissipation factor of 0.02, and electric strength of 50 kV, flexural strength of 379 MPa, peel strength of 19 N/m, water absorption of 0.1%. Composite made of woven glass and epoxy resin CEM-4 is quite similar to CEM-3 but not flame retardant. Composite made of woven glass and polyesters CEM-5 represents the second optimized member of the second group CEM-n for structuring organic ball grid array and naked die technology due to its good dimensional stability resulting from its content of glass fibers.8,27 The general description and applications (of the above) laminates for structuring printed circuit boards are summarized in Table 5.5.7,110-111 Table 5.5. Descriptions and applications of laminates.7,110-111 Grade (color)

Resin base

Features and applications

FR-2 (opaque brown)

phenolic

Punchability. Low cost. Good electrical and punching qualities. Used where tight dimensional stability is not required (such as radios and calculators applications)

FR-3 (opaque cream)

epoxy

High insulation resistance. Higher electrical and physical properties than the FR-2. Used in consumer products (such as computers and communication applications)

FR-4 (translucent)

epoxy

Structured epoxy glass with a self-extinguishing resin system. Good electrical, physical, and thermal properties. Used where high technology applications are needed such as aerospace, communication, and computers

FR-5 (translucent)

epoxy

The glass transition temperature of 150-160°C. Used where retaining strength and electrical performance at elevated temperatures is required

FR-6 (opaque white)

polyester

Structured to satisfy the requirements of low-capacitance or high-impact applications

G-10 (translucent)

epoxy

General purpose

CEM-1 (opaque tan)

epoxy

Structured of epoxy resin paper core with glass on the laminate surface, composite mechanical characteristics of glass. Compared with FR-2 and FR-3, its punchability is similar, but electrical and physical properties are better. Used for the applications of smoke detecting

CEM-3 (translucent)

epoxy

Punchability. Its properties are similar to FR-4. Its structure is similar to CEM-1 but more expensive. Used in applications such as home computers, car electronics, and home entertainment products

Laminates of organic printed circuit board substrates can be fabricated by curing layers of fiberglass, cloth, or paper (as reinforcement) with thermosetting resins, under predetermined temperature and pressure, in the form of integrated uniform thickness. Cloths

5.2 Polymers for conventional printed circuit boards

213

and glass fibers are available as fiber-containing sheets, woven fabric, and mats. Often, an electronic grade E-glass (E grade glass for reinforcing polymers) is the most used type.7,27 Examples of reinforcement (reinforcing materials) mixed with polymers are glass fibers, carbon fibers, and Kevlar®156. Glass fibers are commonly used for reinforcing printed circuit boards, while carbon fibers are used where very light-weight is needed. Kevlar®156 is used when high tensile strength is required.11,28 Fiberglass is used widely with thermosetting polymers (such as epoxy, vinyl esters, and polyesters) as binders to be molded in the form of final products for heavy-duty applications such as aircraft, automobiles, organic printed circuit board substrates, and other electronic, mechatronics applications. Characteristics of fiberglass include lightweight, very high tensile strength (but lower than that of carbon fibers), good electrical properties, good dimensional stability, good chemical resistance, low water absorption, and good heat resistance. It is brittle and a little bit expensive.8,11,28-29 Examples of standard glass fibers types include 1. A-glass (alkali-lime glass with little or no boron oxide) 2. E-CR-glass (electrical/chemical resistance, alumino-lime silicate with less than 1 wt% alkali oxides and high acid resistance) 3. C-glass (alkali-lime glass with high boron oxide content, used for glass staple fibers and insulation) 4. D-glass (borosilicate glass, named for its low dielectric constant) 5. R-glass (aluminosilicate glass without magnesium oxide and calcium oxide with high mechanical requirements as reinforcement) 6. S-glass (aluminosilicate glass without calcium oxide but with high magnesium oxide content and high tensile strength). Its diameter is 9 µm. The most common types of glass fiber used in fiber reinforced polymers is E-glass (of 5-25 µm diameter). Factors to be considered when selecting reinforcement for the manufacturing of organic printed circuit board laminates include stiffness, weight, abrasion resistance, and damage tolerance. Resin selection depends on fabric compatibility, service conditions, and desired characteristics of the finished part. There are three common types of thermosetting resin to choose from, including epoxy, polyester, and vinyl esters. Epoxy has the highest performance, and it is used in weight critical, higher strength, and higher dimensional stability products. Polyester is less expensive, offers corrosion resistance, and is more forgiving than epoxies. For this reason, they are the most widely used. Vinyl ester is often described as a cross between epoxy and polyester.11,28 5.2.3 POLYMERIC SINGLE-SIDED PRINTED CIRCUIT BOARDS The polymeric single-sided printed circuit boards (Figure 5.730,128) have connectors assembled on one surface of the polymeric substrate. They are used for simple circuitry in power supplies and loudspeaker switches. Optimization of polymeric structures of singlesided printed circuit boards lowers cost, increases durability, and eases production. These improvements can be achieved using epoxy glass, fire-retardant grade-4 (FR-4) in 1.6 mm glass fiber epoxy resin laminate composite. A thin copper layer is protected by a polymeric coating called “photoresist.” FR-4 improves glass transition temperature Tg (up to 300oC) and permits the use of high melting point solders. Table 5.631,110-111 includes properties of some of these laminates. According to the table, the two key properties of a lami-

214

Optimization of Polymeric Structures of Organic Printed Circuit Boards

nate needed more improvement, such as relative permittivity (dielectric constant80) and dissipation factor.30-31

Figure 5.7. The polymeric structure of a traditional single-sided PCB.30,128

Table 5.6. Properties of some high-performance laminates compared to epoxy glass, fire-retardant grade-4 (FR-4).31,110-111 Tg, oC

Dielectric constant80 (10 GHz)

Dissipation factor (10 GHz)

Standard epoxy-based laminates of high glass transition temperature

130-150

4.5

0.022

Epoxy-based laminates of high glass transition temperature

170-180

4.4

0.02

Poly(phenylene ether)-based laminates

174

3.4

0.009

Epoxy/poly(phenylene oxide) blend-based laminates

180

3.9

0.013

210-230

3.6-4.1

0.01-0.014

Laminates based on polyimide

280

4.2-4.3

0.02

Hybrid laminates of hydrocarbon resins with ceramic

>280

3.5

0.004

Laminates of liquid crystal polymers

280

2.8

0.002

Laminate materials (polymers)

Laminates based on cyanate ester resins and laminated epoxy with cyanate ester resins

Examples of organic substrates are polymer ball grid array packages. Types of organic substrates include paper, woven glass, composite, epoxy resin, polyester resin (or heat-resistant plastic substrates), flexible, and multilayer wiring substrates. An optimization of organic (polymeric) substrates begins with definition of their characteristics, such as low electrical permittivity, improved effectiveness by implementing cost-effective bare printed circuit board innovation, simple fabrication process, improved quality and performance of electronic products in a global manufacturing environment, and good coefficient of thermal expansion that matches printed circuit board. Figure 5.8157 shows a simple structure of an organic (polymeric) substrate in which several types of organic resins (polymers) can be used. These resins include epoxy, epoxy-novolac, phenol-formaldehyde, bismaleimide-triazine BT resins, polyimide, cyanate ester or polycyanate esters, poly(phenylene oxide), and polyetherimide.8,13-14,32-36

5.2 Polymers for conventional printed circuit boards

215

Figure 5.8. Structure of polymeric substrate. [Data from Sarang Shidore, Package Substrates, December 2005, www.coolingzone.com]

“Polymeric photoresist” is a polymer/oligomer containing light-sensitive (photoactive) compounds and alkaline-soluble resin. The choice of photoactive compounds influences photolithography and photoengraving processes. The main components needed for the photoresist include non-photosensitive polymeric substrates such as cyclized poly(cisisoprene) (support component), negative photosensitive polymers such as 1-methoxy-2propanol acetate or positive such as polymethylmethacrylate acting as binder, photosensitive crosslinking agents (chromophores) such as 2,6-bis(4-azidobenzal)-4-methylcyclohexanone compound, coating solvents such as the mixture of n-butyl acetate, n-hexyl acetate, and 2-butanol to dissolve resins, and photoactive compounds acting as inhibitors. Polymeric photoresists are of two types “negative tone resists” (also called “negative resist” or “negative photoresist”) and “positive tone resist” (also called “positive resist” or “positive photoresist”) as shown in Figure 5.9.110,113 To optimize the design and printing of small chips for organic electronic systems such as organic microelectronic systems, the negative tone resist should become insoluble upon exposure to radiation.37,56-58,60,62-63,6667,100,102-103 Optimal photoresist polymers include16,36-39,68-69,104 1. polymethylmethacrylate 2. poly(methyl glutarimide) PMGI 3. diazonaphthoquinone/novolac DNQ/NOV 4. epoxy-based or 1-methoxy-2-propanol acetate-based photoresist polymer SU8 5. poly(vinyl cinnamate) PVCM 6. cyclized isoprene rubber-based photopolymer.101,112 Polymethylmethacrylate acts as a positive photoresist, a negative photoresist, UV light absorber, and liquid resist type. Poly(methyl glutarimide) acts as a positive tone resist polymer. Diazonaphthoquinone/novolac acts as a positive tone resist polymer. Epoxybased or 1-methoxy-2-propanol acetate-based photoresist polymer SU8 provides a highresolution mask for the fabrication of semiconductor systems. Poly(vinyl cinnamate) is a liquid resist polymer. Cyclized isoprene is negative photoresist polymer.16,36-39,68-69,104

216

Optimization of Polymeric Structures of Organic Printed Circuit Boards

Figure 5.9 Comparison of positive and negative tone resist.110,113

5.2.4 POLYMERIC STRUCTURES OF DOUBLE-SIDED PRINTED CIRCUIT BOARDS The polymeric double-sided printed circuit boards (also called “two-side printed circuit boards,” “two-layered printed circuit boards,” “bilayered printed circuit boards,” “bi-sided printed circuit boards,” or “double-sided plated through holes printed circuit boards”) are epoxy glass, fire-retardant grade-4 (FR-4)-based printed circuit boards coated with conducting material on both sides,40 as illustrated in Figure 5.10.7,30,149 To optimize the polymeric structures of double-sided printed circuit boards, the procedure should be conducted as follows 1. the core plates should be formed from fully crosslinked thermosetting polymers called C-stage resins because they are fully cured (crosslinked) resin 2. the required prepregs should also be formed from crosslinked thermosetting polymers called B-stage resins because they are partially cured (crosslinked) resins.

Figure 5.10. Structure of polymeric double-sided printed circuit boards.7,30,149

5.2.5 POLYMERIC STRUCTURES OF MULTILAYERED PRINTED CIRCUIT BOARDS The polymeric multilayered printed circuit boards consist of more than three conducting layers separated by polymeric insulating layers, such as polymer-based prepregs. The inner conductive layers can be formed before the bonding lamination. The main advantage of multilayered printed circuit boards is the reduced size of miniaturized microelectronics

5.2 Polymers for conventional printed circuit boards

217

parts. They accommodate the increasing complexity and density of circuitry used in applications, such as high-speed computers and signal processors. Optimization of multilayered printed circuit boards, such as those consisting 32 layers, depends mainly on right selection of polymers especially those commercially available grades such as polytetrafluoroethylene PTFE known as Teflon®146 grade, epoxy glass, fire-retardant grade-2 FR-2, epoxy glass, fire-retardant grade-3 FR-3, epoxy glass, fire-retardant grade-4 FR-4, epoxy glass, fire-retardant grade-5 FR-5, composites of epoxy with cotton paper CEM-1, composites of non-woven glass core combined with epoxy resin CEM-3, phenol-formaldehyde, polyimide, and polyamide. Importance of such polymers/composites is their ability to be fabricated with thicknesses ranging from 0.1 mm to 3.2 mm (sometimes even 6.5 mm), drilled to 0.2 mm diameter, tracked width of 100 µm with track spaces of 100 µm.111,207 Both epoxy glass, fire-retardant grade-2 FR-2 and epoxy glass, fire-retardant grade-4 FR-4 have been selected as optimized polymeric composites for structuring polymeric structures of multilayered printed circuit boards due to their attractive properties such as high-speed circuiting, especially when the dielectric thickness becomes crucial.8,44 Examples of polymeric plates used for polymeric multilayered printed circuit boards are illustrated in Figure 5.11,158 containing the following groups of materials9

Figure 5.11. Example of polymers used for structuring polymeric multilayered PCBs. [Data from Multilayer PCBs with special dielectric constant, © Ellwest KG 2003-2012.]

1.

core plates are structured from polytetrafluoroethylene of dielectric constant 2.1-3.0 or FR-4 (dielectric constant 4.1) 2. prepregs plates structured from commercial FR-4 such as Norplex®159 grade of high dimensional stability, high mechanical strength, machinability, and static dissipation 3. conducting polymers such as pyridine Ppy polymers are considered as the optimized generation of organic materials due to their attractive electrical and optical properties (similar to those of metals and inorganic semiconductors). Conducting polymers exhibit high chemical and physical stability and low toxicity of the monomer and easy chemical or electrochemical synthesis. The main feature of pyridine is that its conductivity can be increased up to 102 S/cm by inserting doping anions such as p-sulfonic acid PTS, as shown in Figure 5.12.47,116,117 Because of the above advantages and features, pyridine is used as a conductive pre-coat for the metalization of multilayered printed circuit boards holes. This is done by etching of FR-4 by in situ deposition of a thin film of pyridine, resulting in an FR-4/Ppy electrode. PTS is an organic compound used as dopant material.46-48

218

Optimization of Polymeric Structures of Organic Printed Circuit Boards

Figure 5.12. Example of metalization of organic substrate-printed circuit board holes.47,116-117

5.3 POLYMERIC STRUCTURES OF FLEXIBLE PRINTED CIRCUIT BOARDS The polymeric flexible printed circuit boards have electrically conducting components mounted on a polymeric flexible (bendable) thin films. Polymer-based adhesives are used for connecting their layers. Optimization of polymeric structures of flexible printed circuit boards, such as those known as thin-film bendable printed circuit boards, depends mainly on the selection of polymers, especially commercially available grades such as poly(ethylene terephthalate) PET (Mylar®120), polyetheretherketone, polyimide PI films (Kapton,®118,119 Aramid,®155 and Nomex®155), and glass fiber reinforced epoxy resin EP-GFR, due to their dielectric characteristics. Dielectric polymers such as poly(ethylene terephthalate), poly(ethylene naphthalate), polyetherimide, and fluorocarbons such as fluorinated ethylene-propylene copolymer can be used, as well. Table 5.748-49,110,121 shows the most important properties of some of these dielectric polymers.51,79 These optimized dielectric polymers are associated with the optimized polymerbased adhesives such as polyester, polyimide, acrylate, and modified epoxy due to their ability to provide secure joints between substrate and electrically conducting components to join circuits together if multilayer or rigid-flex constructions are required and to provide a protective cover layover to exposed conductors once they are formed. The selection of such polymer-based adhesives depends mainly on their dielectric constant. General, properties of flexible printed circuit boards adhesives are listed in Table 5.8.48-49,110,121 In conclusion, polymeric flexible printed circuit boards have high dimensional stability, good thermal resistance, tear resistance, good electrical properties, high flexibility, low moisture absorption, chemical resistance, low cost, and availability. Types of polymeric flexible (bendable thin films) printed circuit boards include polymeric single-sided flexible printed circuit boards, polymeric dual access (or back-bared) flexible printed circuit boards, sculptured polymeric flexible printed circuit boards, polymeric double-sided flexible printed circuit boards, polymeric multilayer flexible printed circuit boards, polymeric

5.3 Polymeric structures of flexible printed circuit boards

219

rigid-flexible printed circuit boards, and polymer thick film flexible printed circuit boards.48-51,78,121 Table 5.7. Examples of properties of dielectric polymers used for structuring polymeric flexible (bendable) PCBs.48-49,110,121 Property

Polyesters

Polyimide

Polyamide

Glass fiber reinforced epoxy

75-689 E

Mechanical properties Tensile strength, MPa

151-193 E

172-207 E

11 H

Elongation at break, %

60-165

60-80

7-10

3-5

Tear strength initiation, kg/cm

177-265

177

N/A

N/A

2125-4429

1417-1770

8858-15945

N/A

Tear strength propagation, kg/cm

Physical properties Flexibility Dimensional stability

E

E

F/G

E

F/G

G

E

F/G

144 VG

20 F

9.6 F

Electrical properties 80

Dielectric strength, kV/mm 80

135 VG

Dielectric constant (at 1 kHz)

3.1

3.0

2.0

4.2-5.3

Volume resistivity, Ω-cm

1018

1018

1016

1015

P

E

E

E

-60/+105

-200/+300

-75/+200

UP TO 180

27 L

20 L

22 L

10-12 VL

90-110

220-260

−

90-165

Solderability

Thermal properties Operating temperature Coefficient of thermal expansion Glass transition temperature

Chemical properties Chemical resistance

G

G

F

VG

Moisture absorption

VL

H

L

VH

H

M

M

Relative costs Cost

L

E= Excellent, VG= Very good, G= Good, F= Fair, M= Moderate, H= High, L= Low, VL=Very low, N/A not applicable

220

Optimization of Polymeric Structures of Organic Printed Circuit Boards

Table 5.8. Examples of properties of polymer-based adhesives used in polymeric flexible (bendable) PCBs.48-49,110,121 Property

Polyimide

Polyester

Acrylate

Modified epoxy

Mechanical properties Peel strength, kg/cm

0.36-0.98

0.53-0.89

0.25-0.21

0.89-1.25

Adhesive flow, mm

1014 Ohm.cm, Izod impact strength = 13-35 J/m, tensile strength

308

Optimized Polymeric Structures of Organic Active Electronic Components

= 80 GPa for biaxial film, density = 1.3-1.4 g/cm3, and upper working temperature = 115170oC. The addition of graphene is reducing resistance by half. The optimized polymers have improved conductivity, flexibility, and become attractive photoactive donors. These optimized polymers/blends (even oligomers) include 3,7-bis(5-(4-n-hexylphenyl)-2-thienyl)dibenzothiophene-5,5-dioxide 37HPTDBTSO, poly-(3-hexylthiophene):[6,6]-phenylC61 butyric acid methyl ester P3HT:PCBM blend, poly(3,4-ethylenedioxy thiophene) doped with poly(styrene sulfonate) acid PEDOT:PSS, and 3,7-bis(5-(4-n-hexylphenyl)-2thienyl)dibenzothiophene-5,5-dioxide. They are phenylene-thiophene type oligomers, acting as attractive photoactive donors compatible with flexible substrates of poly(ethylene terephthalate). The 7-bis(5-(4-n-hexylphenyl)-2-thienyl)dibenzothiophene-5,5-dioxide can be used as a blend solution with a weight ratio of 1:2 to phenyl-C61-butyric acid methyl ester PCBM61 to exhibit attractive properties, such as high open-circuit voltage for the fabrication of organic flexible thin-film solar cells.218,220 Optimization of the polymeric structure of “organic ultrathin thin-film solar cells” of less than 2 µm in thickness (Figure 6.30) can be achieved by building them on polymeric substrates of poly(ethylene terephthalate) to withstand extreme mechanical deformation and have unprecedented solar cell-specific weight. Poly(ethylene terephthalate) can act as a capacitor dielectric film. Such cells consist (generally) of cadmium/silver Ca/Ag electrode and poly(3,4-ethylenedioxy thiophene) doped with poly(styrene sulfonate) acid layer fabricated on poly-(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (both of them are bulk heterojunction polymers). Poly(3,4-ethylenedioxy thiophene) doped with poly(styrene sulfonate) acid layer acts as a transparent electrode, while poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester layer acts as an active layer. The power conversion efficiency can be as high as 4.2%. This structure can be rebuilt so that poly(ethylene terephthalate) layer can be replaced by graphene spin-coated poly(vinylidene fluoride-trifluoroethylene) copolymer P(VDF-TrFE) ferroelectric polymer. In this case, the poly(vinylidene fluoride-trifluoroethylene) copolymer film is used for its crystallinity, which enhances the electrostatic doping effect of graphene. It can be utilized as a substrate or/and a doping layer simultaneously with ultrathin-film thickness obtained by a solution process.214-215

Figure 6.30. Organic ultrathin-film solar cells.214-215,234

Figure 6.3162,283 represents the structure of an “organic stretchable thin-film solar cell,” which depends on the strain. The stretchable electronics is available in the form of organic artificial electronic muscles, organic electronic skin, organic electronic bio-inter-

6.2 Polymeric structures of organic semiconductors

309

faces, organic textile electronics, organic solar cells, etc. For example, the organic electronic super skin is used widely in robotic applications so that it results in an ability to stretch and return to its normal shape. It permits applications that deform according to the human body and can be used for robotic elbows and other movements. It is important to know that stretchable solar cells enable the concept of conformal photovoltaics on curved or wrinkled surfaces, such as textiles and fabrics.124,221

Figure 6.31. Organic stretchable thin-film solar cells.62,283

The main constituents of “organic stretchable thin-film solar cells” are top electrode and stretchable and transparent electrodes. Their function is to improve the reliability of systems subjected to strains and stretchable substrates. Liquid eutectic of gallium and indium (which converts into a solid film in contact with air) can be used as the top electrode. The associated stretchable, transparent, and conducting electrodes are used in the form of films fabricated from highly conductive and transparent poly-(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) PEDOT:PSS polymers. To optimize the functions of these films, they should be treated with fluoro-surfactant additive. Such treatment results in improved sheet resistance Rs by 35% (where Rs of 4-layers poly-(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) = 46 Ohm). The stretchable substrates of organic stretchable thin-film solar cells are fabricated in the form of multilayer films from highly conductive and stretchable polydimethylsiloxane PDMS. Polydimethylsiloxane is polymeric organosilicone compound, which can be found in the form of an electro-active elastomer having ductility or elastic (Young's) modulus of 0.003 MPa. It can be treated with ultraviolet/ozone UV/O3 and can be coated with poly-(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid), which can then be treated by fluoro-surfactant, such as Zonyl®288 to produce a mechanically resilient film with improved conductivity. Such organic solar cells can also be based on bulk-heterojunction films of [6,6]-phenyl-C61 butyric acid methyl ester-based poly(3-hexyl thiophene), diketopyrrolopyrrole-thieno(3,2b) thiophene copolymer DPPT-TT, or poly(3-octylthiophene) P3OT.62,85,222,225,227 To optimize the polymeric structure of organic nanostructured solar cells, the following steps should be taken 1. form a highly flexible thin or ultrathin film (10

−

3V

−

Carbon nanotubes

2-4

>10

1.2 V

−

The above polymers used for structuring actuators are inherently conductive polymer types (exhibit the electrical properties of metals or semiconductors in addition to their good mechanical properties). The actuators are called polymer actuators. Polymer actuators can be classified as all-organic/polymeric (also called electroactive polymer), conducting polymer, ionomeric polymer-metal composite, piezoelectric polymer, flexible elastomeric, conjugated polymer actuators, and polymeric microactuators.112 7.9.1 ALL-ORGANIC/POLYMERIC ACTUATORS The “all-organic” term is related to electroactive polymers EAPs. They can also be called “electroactive polymer actuators” with the ability to change their shape in response to electrical stimulation. The optimized class of electroactive polymers used for structuring all-organic/polymeric actuators, such as “ionic electroactive polymers/copolymers” (also can be called “electroactive polymer/copolymers containing ionic liquids”) includes poly(styrene sulfonate-b-methyl butylene) PSS-b-PMB block copolymers, which can be actuated by the displacement of ions in polymer. The main feature of these electroactive polymers/copolymers is their ability to be controlled by external electric fields. Because of its high ionic conductivity and large electrochemical stability, poly(styrene sulfonate-bmethyl butylene) is selected as an optimized polymer for structuring all-organic polymer actuators.101,108,113-115 Advantages of electroactive polymers (especially electroactive nanostructured polymers) include light weight, easy to be fabricated, ability to be stimulated by low electric fields, excellent displacement (>200% areal strain) under applied electric field, and attractive properties for designing organic actuators for advanced engineering, biomimetic, and biomedical applications. Applications of these actuators include robotics, microsensors, and artificial muscles. The Maxwell stress (as an electrostatic pressure) represents the mechanical response to electrical stimulation, and it is used for medical and underwater applications. Optimizing this structure depends on the utilization of electrodes formed as films from electroactive polymers, such as polyimide with single-walled carbon nanotubes

364

Polymeric Structures Optimized for Organic Passive Electronic Components

SWCNT/PI (as a matrix). This matrix can be synthesized using diamine, such as 2,6-bis(3aminophenoxy) benzonitrile (b-CN)APB (also abbreviated as (β-CN)APB), and a dianhydride, such as 4,4-oxydiphthalic anhydride ODPA. D-EAPs, an organic electronic braille actuator is an example of an all-organic polymer actuator able to provide sufficient stimulus to be read by a blind person.80,83,101,108,113-117 7.9.2 CONDUCTING POLYMER ACTUATORS The conducting polymer actuators can be described as long-chain organic molecules. Successive carbon atoms are bound along the axis of the chains of these molecules alternately by one shared electron pair (single, or σ-bond) and by two shared electron pairs (double, or π-bond). The optimized polymer selected for structuring conducting polymer actuators is polypyrrole PPYR, which can be doped and have a very high molecular weight. This polymer is attractive for structuring time-dependent change actuators (in which the intercalated/deintercalated species can complicate the control of actuation). Poly(3-octylthiophene), thiophene derivatives, and polyaniline are also widely used.73,85,90-91,106,118-119 Other optimized conducting/insulating polymers can also be selected for structuring conducting polymer actuators including94,101,114,120 1. 1-methoxy-2-propanol acetate SU-8 patterned conductive silicone Q polymer 2. spin-coat Teflon AF®256 films 3. poly(perfluorobutenyl vinyl ether) PFEVE (including Cytop®68 series such as CTL809M®257 grade) films 4. polyimide PI (including Pyralin®73) 5. parylene PAY 6. polypyrrole PPYR doped with bis-trifluoromethane-sulfonimide TFSI (abbreviated as PPYR:TFSI) 7. poly(3-hexyl thiophene) P3HT and poly(thienylene vinylene) PDDT 8. poly(3-methylthiophene) P-3MT, 9. poly((E)-4,4'',-didecoxy-3'-styryl[2.2':5,2'']terthiophene) P-E-DST. 1-Methoxy-2-propanol acetate SU-8 patterned conductive silicone polymer has been optimized for structuring conducting polymer actuators. It was previously used for associated beams of organic microfluidic, electrostatic fluids, accelerometers, and microelectromechanical system MEMS applications. Spin-coated Teflon AF®256 films have hydrophobic properties. CTL-809M®257 film has a high dielectric constant similar to bulk polytetrafluoroethylene PTFE. Polyimide PI (Pyralin®73) has flexibility, temperature stability, and toughness. Parylene PAY provides a conformal protective coating. Highly conductive polypyrrole doped with bis-trifluoromethane-sulfonimide PPYR:TFSI is used instead of polypyrrole doped with hexafluorophosphate (PPYR:PF6), polypyrrole doped with tetrafluoroborate (PPYR:BF4), polypyrrole doped with perchlorate ClO4 (PPYR:ClO4), or polypyrrole doped with phenolsulfonate PP-S (PPYR:PPS) due to its higher strain at equivalent conditions. On the other hand, poly(3-hexyl thiophene) P3HT and poly(3-dodecyl thiophene) PDDT can be used for structuring polythiophene PTH actuators of 2% electromechanical strain. Poly(3-methylthiophene) P-3MT is used for structuring polythiophene actuators of 2% electromechanical strain when used in an organic electrolyte and 0.5% strain when used ionic liquid electrolyte. Poly((E)-4,4''didecoxy-3'-styryl[2.2':5,2'']terthiophene) P-E-DST is used for structuring polythiophene

7.9 Organic actuators

365

actuators of 11.5% strain.94,101,114,120 The actuation properties and performance of doped polypyrrole actuators (as conducting polymer actuators) are given in Table 9.5.208 Table 7.5. Actuation properties and performance of doped polypyrrole actuators (as conducting polymer actuators).208 Work density [kJ/m3]

Strain [%]

Strain rate [%/s]

Working stress [MPa]

PPYR:TFSI

−

11.2-12.4

0.7-0.8

10.6-22

PPYR:PF6

40

2

−

0.5

PPYR:BF4

−

9.5

−

1.3

PPYR:PPS

−

1.2

−

5

PPYR:DBS

100

1.4

−

5

PPYRs

7.9.3 IONOMERIC POLYMER-METAL COMPOSITE ACTUATORS The ionomeric polymer-metal composite actuators are made of electroactive polymers EAPs capable of exhibiting large deformations under a low applied voltage. They consist of ion exchange membranes and metal electrodes (formed by electroless chemical plating of platinum Pt on both surfaces of the membranes). Conventionally, metal electrodes can be formed by impregnating polymeric membranes (modified polytetrafluoroethylene, such as Teflon®213) in metal precursor solution followed by reduction to the particulate metal electrode. To optimize polymeric structures of these ionomeric polymer-metal composite actuators, the metal electrodes should be replaced by highly conductive polymers such as polyaniline (e.g., Nafion® 11168-169) due to its high electrical conductivity (203428 S/cm), and because it is perfluorinated ion-exchange membrane that has a wide variety of commercial uses having tensile modulus of 249 MPa, tensile strength of 43 MPa, and elongation of 225%.70,73,106,108 Due to their mechanical and electrical properties that make them highly attractive for actuators, carbon nanotubes CNTs can be considered as an optimized base for structuring ionomeric polymer-metal composite actuators. The main feature of carbon nanotubes for such an application is that they deform elastically by several percents, thus storing very large amounts of energy thanks to their crystalline nature and morphology. Figure 7.14126,142,151,200,210 shows carbon nanotubes, which were compounded with poly(vinylidene fluoride) PVDF and an ionic liquid such as 1-butyl-3-methylimidazolium tetrafluoroborate BMIM-BF4 for structuring high-performance biomorph actuator. Carbon nanotubes act as electrodes. Multi-walled carbon nanotubes form a physical gel (called bucky-gel) upon grounding with imidazolium-based ionic polymer liquids, due to a specific interaction between the imidazolium ion component and the π-electronic nanotube surface.95,113-114,123,125-126

366

Polymeric Structures Optimized for Organic Passive Electronic Components

Figure 7.14. Multi-walled carbon nanotube-based ionomeric polymer-metal composite actuators.126,142,151,200,210

7.9.4 PIEZOELECTRIC POLYMER ACTUATORS The piezoelectric polymer actuators are polymer-based products in which actuation provides large forces (few mN), especially in the case of thick polymeric piezoelectric films. The piezoelectric properties include piezoelectric constant dij [10-12 C/N], relative permittivity [εr], density [g/cm3], Young's modulus [GPa], and acoustic impedance of [106 kg/ m2.s]. The function of a piezoelectric polymer actuator is to convert an electrical signal to precisely controlled physical displacement (stroke). The optimized piezoelectric polymers, which can satisfy requirements for structuring piezoelectric polymer actuators include poly(vinylidene fluoride) PVDF, poly(vinylidene fluoride-co-trifluoroethylene) PVDF-TrFE, and poly(L-lactic acid) PLLA.69,129-130 Poly(vinylidene fluoride-co-trifluoroethylene) (an important derivative of poly(vinylidene fluoride)) is the most suited for this purpose because it can be used by inkjet printing process for structuring high-performance piezoelectric polymer actuators on a flexible/polymeric substrates such as those formed from poly(ethylene terephthalate) or polymethylmethacrylate (microfluidic applications). Poly(L-lactic acid) has been considered as an optimized polymer for the same actuators for biomimetic applications. Polymers (as organic materials) are widely applied as piezoelectrics due to their unique properties and advantages over other piezoelectrics, even though the piezoelectric response of polymers is less pronounced than that of single crystal inorganic materials. These advantages are listed in Table 7.6101,132-133,156 (piezoelectric polymers vs. another piezoelectrics, such as piezoceramic and shape memory alloys).101,132-133,156 Table 7.6. Comparison of smart materials used for structuring actuators in general.101,132-133,156 Property

Unit

Piezoelectric polymers

Piezoceramic

Shape memory alloys

Actuation strain

%

2-5

0.1-0.3

10oC easily exceeds that generated by electroactive polymers at several kV. Furthermore, the actuation energy density per squared Kelvin is more than four times higher than that of aluminum, which is known for its high coefficient of thermal expansion among metals.5,163 Thermal properties (including coefficient of thermal expansion at 300K) of some materials used for the manufacturing of thermal expansion polymer microactuators are listed in Table 7.13.144,184

Figure 7.25. The structure of thermoelectrical polymer microactuators based on polymethylmethacrylate/gold PMMA/Au. [Adapted, by permission, from V. Vidyaa, G. Arumaikkannu, Hybrid Design of a Polymeric Electrothermal Actuator for Microgripper, Int. J. Mech. Ind. Eng. (IJMIE), 1, 2, 31-5, 2011.]

7.9 Organic actuators

383

Table 7.12. Comparison of thermally expandable materials used for structuring thermoelectrical polymer microactuators.144 Properties Young's modulus, E [GPa] Poisson's ratio, v

Aluminum Al

Silicon Si

Epoxy SU-8

50-59% SU-8/Si

69-70

130

3.2

8.89-9.5

0.35-0.36

0.28

0.33

−

23.1

2.6

150-150.7

146.2-149

Linear CTE α [10-6/K] Thermal conductivity k [W/m/K] Maximum operating temperature [oC]

237

148

0.2

0.4-4.1

660 Tm

1414 Tm

238 Tg

238 Tg

Actuation stress Eα [MPa/K]

1.59

0.35

0.48

1.3

Energy density (1/2 Eα)2 [J/m3/K2]

18.41

0.48

36.32

95.04

Table 7.13. Thermal properties of some materials/polymers used for structuring thermoelectrical polymer microactuators of high coefficient of thermal expansion.144,184 Materials/ Polymers Silicon Si

Density [kg/m3]

Heat capacity [J/kg K]

Conductivity [W/mK]

Thermal expansion coefficient [10-6K-1]

2.330

710

156

2.3

Aluminum Al

2.700

920

230

23

Copper Cu

8.900

390

390

17

Chromium Cr

6.900

440

95

6.6

Polyimide Pl

21.500

133

70

9

Pyrene-N

1.110

837.4

0.12

69

Pyrene-C

1.290

711.8

0.082

35

Pyrene-D

1.4108

−

−

30-80

References 1 2 3 4 5 6 7 8 9

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Optimizing Polymeric Structures in Organic Optoelectronics 8.1 OVERVIEW The organic optoelectronics denotes systems structured from organic materials (polymers) such as organic light-emitting diodes OLEDs and organic prototype lamps. Such systems are characterized by ease and low cost of fabrication, have specific properties of active light emitters, flexibility, transparency, and scalability. System optimization mainly relies on the selection of polymers, which can satisfy the above characteristics to achieve functional objectives such as electrical and thermal conductivity, organic packaging and encapsulation, and interconnections. Let us consider polymeric structure of a flexible organic blue light-emitting diode (important class of organic optoelectronic systems) shown in Figure 8.1.187,219,243 It consists of titanium oxide layer to permit outflow of light, a transparent substrate formed from a composite of glass/polymer substrate acting as conducting anode based on poly(3,4-ethylene dioxythiophene) PEDOT doped with poly(styrene sulfonate acid) PSS (PEDOT:PSS).1-4,48

Figure 8.1. Organic optoelectronic device.187,219,243

8.2 OPTICAL POLYMERS The optical polymers (also called “optopolymers”) are transparent polymers with specific optical and physical properties used in optoelectronic systems such as anti-reflective films, liquid crystal displays LCD, lenses, adhesives, optical fibers, etc. Optical polymers are thermoplastics and thermosets. Thermosets are used for casting or transfer replication

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over thermoplastics. The optimized optoelectronic systems used for structuring organic/ polymeric optoelectronics, are listed in Tables 8.1 and 8.2 with their physical properties. Thermoplastic optical electronics can be produced by injection molding (e.g., injection of organic lenses) with very good surface uniformity and appearance. All optimized polymers (thermosets and thermoplastics) should satisfy the required optical, conducting, physical, and packaging properties. The application of product influences requirements. For example, when high purity is required, the optoelectronic polymer of unique optical/ physical properties such as refractive index, transmission, dispersion, and thermo-optic coefficients has to be used. Generally, twenty types of optical polymers can be included according to either their application, properties or both of them.5-6,8,35, 37,40,47 Classification of optical polymers begins with conventional transparent polymers as principal optical polymers (glass-like purity), such as polymethylmethacrylate, polycarbonate, styrene-acrylonitrile copolymer, polymethylpentene (e.g., TPX®269), and methacrylate-styrene copolymers. Classification based on both properties and applications includes conjugated polymers, electroluminescent polymers, fluorescent polymers, photonic polymers, etc. Poly(phenylene vinylene) and poly(p-phenylene) have chemical nature and properties of conjugated polymers that satisfy the optical requirements of electroluminescent applications. The optical polymers can be divided into 1. optical electroactive conjugated polymers 2. transparent (photonic) polymers 3. optical organic photovoltaic polymers 4. electroluminescent polymers 5. electrophosphorescent polymers.9 8.2.1 OPTICAL ELECTROACTIVE CONJUGATED POLYMERS The optical electroactive conjugated polymers have semiconducting and optical properties. For example, crosslinkable conjugated polymers containing oxitane OXn have oxitane units acting as the crosslinking moieties of organic field-effect transistors and organic light-emitting diodes. The main advantage of crosslinkable conjugated polymers is their high solubility and formation of films with attractive properties in non-crosslinked form. These polymers consist of electroactive or conjugated cores, pendant side chains carrying crosslinkable moieties, and solubilizing side chains. Solubilizing side chains include alkyl or alkoxy groups. The function of oxitane group is to act as crosslinkable moiety (similar to acrylate and styrene groups used for the same purpose). Oxitane group provides high crosslinking conversion with short conversion time and low shrinkage (less than 5%).10-12 The optimized groups of oxitane-functionalized conjugated polymers used for structuring optoelectronic systems include10-12 1. oxitane-functionalized polyspirobifluorenes PSBF, and N4,N4'-bis(4-(6-((3-ethyloxetan-3-yl)methoxy)hexyl)phenyl)-N4,N4'-bis(4-methoxyphenyl) biphenyl-4,4'diamine QUPD 2. oxitane-functionalized polyfluorene copolymers PFOs 3. oxitane-functionalized poly(3-alkylthiophene). The polyspirobifluorenes include the series of commercial-grades from PSBF1®10 to PSBF16®10 (depending on used aryl monomer). Oxitane-functionalized polyfluorene copolymers are related to poly(dialkyl fluorene) PF8 family which combines fluorene-phenylene copolymer FL-Ph, and fluorine-alt-bithiophene copolymers PFBTh (such as poly(9,9'-dioctylfluorene-2,7-

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diyl-co-benzothiadiazole) F8BT and poly(9,9-dioctylfluorene-alt-bithiophene) F8T2). Oxitane-functionalized poly(3-alkylthiophene) is crosslinkable conductive polymer widely used in the field-effect transistors due to its high hole mobility. Oxitane-functionalized polyspirobifluorenes belong to oxitane-functionalized conjugated polymers used for structuring optoelectronic systems. They can be prepared by reaction of spirobifluorine-diboronic ester with two or more aromatic dibromides in the presence of tetrakis(triphenylphosphine)palladium as a catalyst and tri-potassium phosphate TPPh as a base. The derivatives of this optimized group emit in the blue, green, or red part of the spectrum. For organic full-color displays, derivatives of oxitane-functionalized polyspirobifluorenes, such as the commercial grades FSBF2®10, PSBF10®10, or PSBF14®10 are useful. These derivatives can be prepared in a solution containing photoacid (4-(2-hydroxytetradecyl)-oxyl)-phenol)-phenyliodonium hexafluorantimonate, spincoated onto a substrate made out of film fabricated from indium tin oxide and poly(3,4ethylene dioxythiophene) doped with poly(styrene sulfonate). The system based on the commercial-grade PSBF2®10 shows the best performance with maximum efficiency ηmax = 1 cd/A, while systems based on commercial-grades PSBF10®10 and PSBF14®10 show maximum efficiency values of 2.9 and 3.0 cd/A respectively. N4,N4'-bis(4-(6-((3-ethyloxbiphenyl-4,4'-diamine etan-3-yl)methoxy)hexyl)phenyl)-N4,N4'-bis(4-methoxyphenyl) QUPD can be classified as a crosslinked oxitane-functionalized polyspirobifluorene homopolymer acting as a photo-crosslinkable hole transparent layer for structuring optoelectronic systems. N4,N4'-bis(4-(6-((3-ethyloxetan-3-yl)methoxy)hexyl)phenyl)-N4,N4'bis(4-methoxyphenyl) biphenyl-4,4'-diamine a derivative of N,N'-bis(4-butylphenyl)N,N'-bis(phenyl)benzidine TPd is known as photocrosslinkable hole transport material with two acid-sensitive oxetane groups. It can form films of 50-150 nm thickness with excellent optical transparency (>90% above 415 nm), hole mobilities of 10-4-10-3 cm2/Vs and superior thermal stability with glass transition temperature Tg = 250°C.10-11,14 Oxitane-functionalized polyfluorene copolymers (including poly(9,9-dioctylfluorene-alt-bithiophene)) are used as p-type channels for structuring organic field-effect transistors (as an important class of organic optoelectronic polymers), which have a hole mobility µh = 10-2 cm2/Vs and high stability at room temperature. Poly(9,9'-dioctylfluorene-2,7-diyl-co-benzothiadiazole) has maximum efficiency of ηmax = 1 cd/A at 80 nm. Optical electroactive conjugated polymers can act as charge-transport materials for forming optoelectronic thin-film systems because this conjugated polymer can transport charges and act as conductor or semiconductor in organic light-emitting diodes, polymer field-effect transistors, solar cells, etc.10 The bipolar p-n-diblock polymers satisfy the requirements of these systems, such as improved electron and hole transport and electroluminescent properties.10-11,14 The bipolar polymers include16 4’’’-phenylene-1,3,4-oxadiazole-2,5-diyl PCOPO, 2-(4-(bis(9,9dimethylfluorenyl)amino)phenyl)-5-(dimesityl boryl)thiophene FIAMB-1T, poly(N-(2’ethylhexyl)-carbazole-3,6-diyl-1’’,3’’,4’’-oxadiazole-2’’,5’’-diyl-2’’’ and 5’’’-dioctyloxy1’’’. PCOPO is copolymer containing an electron-rich carbazole moiety and an electrondeficient aromatic oxadiazole unit, while FIAMB-1T is a bipolar fluorescent polymer used for structuring white organic light-emitting diodes WOLEDs with non-doped structure. Note: PCOPO can be synthesized by polycondensation.82 Its films emit blue-green light

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(λmax = 475), with bandgap energy of 2.82 eV, higher occupied molecular orbital/lower unoccupied molecular orbital energies of 5.60 and 2.66 eV respectively.148 FIAMB-1T can act as both hole and electron transporting and the green light-emitting layers. It is used to insert two ultrathin layers and a thin layer between two layers of 2-(4-(bis(9,9-dimethylfluorenyl)amino)phenyl)-5-(dimesityl boryl)thiophene.17 8.2.2 TRANSPARENT (PHOTONIC) POLYMERS The transparent polymers also called “photonic polymers” have been optimized for structuring organic optoelectronic systems due to their the very high transparency and clarity similar to acrylics ACR, polycarbonate PC, cycloolefin polymers COP, cyclic olefin copolymers COC, optical poly(ethylene terephthalate) O-PET, polystyrene PS, allyl diglycol carbonate ADCt, and polysulfone PSU. The first group of photonic acrylics (including polymethylmethacrylate and alicyclic acrylate) can be optimized for structuring organic optoelectronic system because it has refractive index n of 1.49, high transparency of 92.5%, low birefringence, outstanding hardness, light deterioration resistance, high Abbe number Vd, surface resistivity of 1016 Ω, volume resistivity of 1013 Ωm, dielectric breakdown of 20 MV/m, dielectric constant DK of 3.7, and dielectric loss of 0.05 (at 60 Hz). Alicyclic acrylate ACAT has refractive index n of 1.5053, low birefringence (40 nm), low water absorption, Vd of 57, and high heat resistance.6,18-19,40,47 Examples of commercial grades of acrylics include Acrypet®279 (based on polymethylmethacrylate) and Optrez®118 (based on alicyclic acrylate). Examples of commercialgrades of polycarbonate include Panlite®280, a low impurities grade, and ST-3000®282, a low-birefringence. Zeonex®284 is the commercial-grade of cycloolefin polymer. Apel®283, Topas®285, and Arton®287 are norbornenes. The commercial grades of optical poly(ethylene terephthalate) include OKP-4®286 and TI-160®286. Odel-P1700®288, having hightemperature resistance, is an example of commercial-grade polysulfone.18 The highly transparent polymethylmethacrylates (including the commercial series of Acrypet®279, such as VH, MD, MF, V, VH5, Ir, and VR) have been optimized for structuring photonic polymer-based electronic systems because they have surface resistivity values >1016 Ω, volume resistivity values >1013 Ωm, dielectric constant around 3.7, water absorption percentage (24 h) around 0.3%, tensile stress levels from 62 to 78 MPa, and elongation at break from 5 to 8%. The optical alicyclic acrylates (including the commercial series Optrez®118 and its grades OZ-1000281, OZ-1100281, and OZ1310281) have been optimized for similar applications because they have refractive indices ranging from 1.5053 to 1.5121, Abbe numbers ranging from 54 to 57, transmittance ranging from 93 to 94%, and water absorption values (24h) of ~1.2%. The photonic polymer polycarbonate PC polymerized from bisphenol-A with carbonyl chloride or diphenyl ether has low levels of impurities, high transmittance of 89%, the high refractive index of 1.585, Abbe number of 30, high scattering, heat resistance, moldability, and high intensity. The low-birefringence polycarbonate grade is produced to solve problems of large birefringence of conventional polycarbonate. Its birefringence is lower than that of normal polycarbonate, lower water absorption than normal polycarbonate, and high rigidity. It is widely used for forming optical films that require a low birefringence.18

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Important to know is that the commercial series of Panlite®280 of the photonic polycarbonate (including grades AD-5503282 and L-1225Y282) have water absorption (24h) of 0.2%, light transmittance ranging from 88 to 89%, refractive index n of 1.585, Abbe number Vd of 30, and tensile modulus ranging from 2450 to 3400 MPa. The commercial ST3000®282 is a low-birefringence polycarbonate. Cycloolefin polymers based on acrylics and produced from cycloolefin monomers have been optimized for structuring high quality organic optoelectronic systems because they have high transparency (no absorption in visible region), light transmittance of 92%, low water absorption 1016 Ω/cm, the dielectric constant of 2.3 (at 1 MHz), dielectric tangent of 0.0002 (at 1 MHz), heat resistance, and glass transition temperature Tg of 123138oC.18 The photonic cyclic olefin copolymers, produced by the copolymerization of norbornenes and alpha-olefins, have low specific gravities, high transparency, low water absorption, low scattering, low birefringence, heat resistance, and high processability. Norbornene functional cycloolefin polymer with ester groups is suitable for surface coating and adhesion applications. It has low birefringence 1016 Ωcm, dielectric constant of 3 at 1MHz, dielectric loss of 0.02 at 1 MHz, high fluidity, water absorption of 0,4%, high heat resistance, and easy surface treatment and adhesion. The optical norbornenes copolymerized from tetracyclododecene and alpha-olefin (Apel®283) have water permeability levels of 0.09% g.mm/m2/d, the light transmittance of 91%, and n of 1.54. Topas®285 (other commercial-grade of norbornene copolymerized with alpha-olefin) has water absorption 1.6, low birefringence, water absorption of 0.15%, the light transmittance of 90%, and Abbe number of 27. The other commercial-grade TI-160® is available in the form of the olefin-maleimide copolymer, having low birefringence, the light transmittance of 89%, Vd of 50, water absorption of 0.5%, and outstanding rigidity and hardness. The optical polymer polysulfone is an excellent engineering thermoplastic with high heat resistance. It has been optimized for organic optoelectronic systems, especially those exposed to high working temperatures because it has heat resistance, heat deflection temperature of 174oC, high refractive index >1.6, water absorption of 0.3%, Abbe number of 23, light transmittance >84%, dielectric strength of 17K V/mm, dielectric constant of 3,3 at 60Hz, dissipation factor of 0.0007 at 60Hz, volume resistivity of 3x1016 Ωcm, and high intensity.18 Polystyrene and allyl diglycol carbonate ADC have been optimized for structuring organic optoelectronic systems because of their excellent light transmission and optical properties better than that of optical glass. For example, the commercial-grade of allyl diglycol carbonate, such as CR-39TM270 exhibits a refractive index of 1.498, Abbe number

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of 59.3, the transmission of 89-91%, transparency in the visible spectrum, and it is almost completely opaque in the ultraviolet range. Allyl diglycol carbonate can be polymerized from diethylene glycol-bis-allyl carbonate in the presence of diisopropyl peroxydicarbonate initiator. Polystyrene is known to have clearness, hardness, brittleness, the refractive index of 1.58-1.59, the transmittance of 88-90%, the dielectric strength of 20 MV/m, the dielectric constant of 2.5 at 1MHz, the volume resistivity of 1016 Ωcm, Abbe number of 31, and it is very inexpensive resin per unit of weight. For more developments, polymethylmethacrylate, polycarbonate, and polystyrene (as important classes of transparent or photonic polymers) can be used as host polymers with desirable optical properties for preparation of single-walled carbon nanotube-polymer. These transparent polymers were selected as optimized parts of composites with single-walled carbon nanotubes used in telecommunication wavelengths, due to their significant optical loss >0.6 dB cm-1. The “host” and “guest” terms are related to supramolecular chemistry used for describing complexes formed from two or more molecules or ions held together in unique structural relationships by forces other than those of covalent bonds. Individually, a host can be defined as an organic molecule or ion with binding sites able to converge in the complex, while a guest represents any molecule or ion with binding sites able to diverge in the complex.6,19,40,47,64,66 The polymer optical waveguides POWG used for structuring organically integrated optics depend on the use of transparent (photonic) polymers as core materials. Optical waveguide consists of a waveguide core surrounded by a waveguide cladding. The lightguiding state can be observed by the total internal reflection so that the refractive index of the core polymer should be higher than that of the cladding polymer. Polymers that can be optimized for structuring optical waveguides include polycarbonate ACR, halogenated acrylate H-ACR, fluorinated acrylate, polysiloxanes PSX, perfluorocyclobutane PFCB, benzocyclobutane BCB, and deuterated polysiloxanes DPSX. Both polycarbonate and halogenated acrylate have been selected as the first and the second optimized polymers used as core polymers for optical waveguide because they have an optical loss of 0.02 at 840 nm and 0.3 dB/cm at 1300 nm, respectively. For such applications, acrylics ACR represent the conjugate bases, salts, and esters of acrylic acid and its derivatives. Halogenated polymers are more hydrophobic.21,24-28 Fluorinated acrylate has been selected as the third optimized polymer because it has an optical loss ranging from 0.3-0.5 dB/cm at 1550 nm wavelength. Polysiloxane PSX (including commercial-grade Gemfire®289) has been selected as the fourth optimized polymer because it has an optical loss of 1.0 dB/cm at 1550 nm. Perfluorocyclobutane has been selected as the fifth optimized polymer for the same applications because it has an optical loss of 0.25 dB/cm at 1550 nm. Benzocyclobutane (CycloteneTM183) has been selected as the sixth optimized polymer because it has an optical loss of 0.8 dB/cm at 1300 nm wavelength. Deuterated polysiloxanes have been selected as the seventh optimized polymer because it has an optical loss of 0.17 dB/cm at 1310 nm. For applications in integrated optical telecommunication, poly(methyl methacrylimide) PMMAI is used for structuring optical planner waveguides. To optimize functions of the optical waveguide layer of poly(methyl methacrylimide), it should be sandwiched between silicon oxide SiO2 and

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polymethylmethacrylate PMMA layers because silicon oxide acts as a silica buffer layer, while polymethylmethacrylate acts as an optical cover layer (protection layer). The refractive indices of these layers are reduced ( 2.7 eV). The mterphenylene is the main building block in phenylene-based macro-cycles and foldamers, such as oxacalix[2]m-terphenylene[2]triazine and 5'-tert-butyl-[1,1',:3',1'']-terphenyl-4,4''diol. The foldamers are discrete chain molecules or oligomers which fold into conformationally ordered states in solutions. In other words, foldamers are oligomers, which have a tendency to form well-defined secondary structures.43 8.2.4.5 Optical silicon-containing polymers The optical silicon-containing polymers such as polysilanes PSNs have been optimized as the fifth group of electroluminescent polymers used for structuring organic optoelectronic systems such as organic electroluminescent systems and polymeric light-emitting diodes because members of this group participate in extensive improvement of efficiency of structured systems, leading to successful fabrication of high-efficiency electroluminescent devices in visible region made from sublimed molecular films and π-conjugated polymers. The optimized electroluminescent derivatives of polysilanes PSNs form important family of optical silicon Q-containing polymers for structuring electroluminescent systems, including diaryl-polysilanes (such as poly(bis(p-n-butylphenyl)silane PBPS), monoalkylaryl polysilanes MAA-PSNs (such as poly(methylphenylsilane) PMPS), dialkylpolysilanes (such as poly(di-n-butylsilane) PDBS), poly(di-n-pentylsilane) PDPS, and poly(di-nhexylsilane) PDHS. The members of the polysilane family act either as a hole-transporting materials or emissive materials Ems. The polysilane electroluminescent has two important features 1. it can provide novel types of near UV (NUV) or UV-visible sources, which are difficult to achieve with π-conjugated polymers and small dye molecules 2. electroluminescence produced high energy excitons and solid films of polysilanes, within which no electron has been detected using conventional techniques.33,44,75-76 Poly(bis(p-n-butylphenyl)silane has been selected as the first member of an optimized group of optical silicon-containing polymers used for structuring organic optoelectronic systems, such as electroluminescent systems due to its lowest work function among polysilane systems, while its bandwidth is less than 15 nm. Moreover, poly(bis(p-n-butyl-

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phenyl)silane thin films doped with a variety of electron acceptors (such as fluorene) exhibit high product of photocarrier generation and high charge carrier mobility. Polymethylphenylsilane is the first used polymer as a hole-transporting polymer in multilayer configurations. It exhibits non-dispersive hole transport with relatively high effective mobility 5 S/cm is deposited on top of rough ITO of conductivity 0.2 cm-1.126,156-157,162,164

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Figure 8.8 Ordering scheme for triplet emitters based on zero-filed splitting ZFS values of the emissive triplet state.46,58,156-157

Figure 8.9. A representation of the principle of physical complexity.61,316

Triplet emitters can be classified as trivalent iridium complexes, divalent ruthenium complexes, and divalent osmium complexes. Trivalent iridium complexes are triplet emitting compounds such as small molecule emitter fac-tris(2-phenylpyridine) iridium(III) Ir(ppy)3. The 'trivalent' term indicates an element of chemical structure having a valence of three as shown in Figure 8.9.61,316 This principle also represents divalent, tetravalent, in addition to tetravalent and pentavalent complex orders. Trivalent iridium complexes have been optimized for these structures because they can act as guests doped in different hosts materials, such as N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)-benzine TPD(3M) (triphenyl amine dimer) as a host material for structuring phosphorescent organic light-emitting

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diodes of the following layers, including indium tin oxide, thienopyrrole-dione TOD, factris(2-phenylpyridine) iridium(III), tris(8-hydroxyquinoline) aluminum Alq3, and aluminum.148,150-151,166 Platinum octaethyl porphine called platinumoctaethyl-porphyrin PtOEp can be used for such structures, as well. If phosphorescent organic light-emitting diode system is structured from N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)-benzine, fac-tris(2-phenylpyridine)iridium(III), and tris(8-hydroxyquinoline) aluminum layers, it would show a maximum intensity peak at 520 nm (5-15 V operating voltage), while it shows a maximum intensity peak at 520 nm if the structure is of thieno(3,4-c)pyrrole-4,6-dione, fac-tris(2phenylpyridine) iridium(III), and platinumoctaethyl-porphyrin layers. Generally, the decay of emission is at 520 nm upon excitation at 337 nm with different concentrations of thieno(3,4-c)pyrrole-4,6-dione in the blend with fac-tris(2-phenylpyridine) iridium(III), and in an inert polymeric binder.126,166,169,332 Platinum porphyrin derivatives can be considered as an optimized group of triplet emitter complexes such as platinumoctaethyl-porphyrin for structuring phosphorescent labels and sensors for determination of oxygen because the derivatives of this group can be formulated directly using platinum complexes of deuteroporphyrin IX, 2,7,12,18tetramethyl-3(8)-formyl-13,17-bis(2-methoxycarbonylethyl)porphyrin TFM-POR and copper and platinum complexes of 5,10,15,20,tetrakis(p-methoxycarbonylphenyl)porphyrin TMP-POR (known as a type of metal-complexes of substituted tetraphenylporphyrin).126,168-171 Porphyrins POR are super-molecules of the future because they can contain many different metal atoms in their centers. They are very stable due to their conjugation (alternating single and double bonds), and they have unique chemical properties. For example, they can be used as metal binders (ligands), in organic solar cells, as oxygen transport medium (hemoglobin), electron transfer medium (conductive polymers), etc. Platinum belongs to group VIII of transition metal. Their function is to form isostructural sub-molecular porphyrin assemblies. It should be noted that substantial change in the properties of platinum porphyrin complexes occurs on platinum(II) to platinum(IV) oxidation. For example, porphyrin platinum(II) Pt(II)POR complexes (as organometallic compounds or irregular metalloporphyrins) have efficient red phosphorescent emission useful in organic light-emitting diodes. That is because their strong π−π intermolecular interactions derived from their rigid planar coordination geometry result in the occurrence of self-quenching and affect their performance.137,171,173 Platinum(II) tetraphenyltetrabenzoporphyrin PtTPTBP is the first optimized derivative of porphyrin platinum(II). It is platinum π-extended porphyrin polymer (near-infrared range phosphor) for structuring near-infrared light-emitting diodes. Platinum(II) tetraphenyltetrabenzoporphyrin of the chemical structure shown in Figure 8.10176,273 acts as a guest polymer with tris(8-hydroxyquinoline) aluminum which acts as host polymer in organic light-emitting diodes. Such structures have external quantum efficiency EQE of 8.5%, the peak wavelength of 770 nm, and high efficient emission at 800 nm. The structure of near-infrared light-emitting diodes consists of indium tin oxide, N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine, and platinum(II) tetraphenyltetrabenzoporphyrin/tris(8-hydroxyquinoline) aluminum/BCP layers. Plati-

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num(II) tetraphenyltetrabenzoporphyrin is doped with tris(8-hydroxyquinoline) aluminum to optimize the efficiency of the structure and act as an emissive layer. N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine acts as hole transport layer, while bathocuproine BCP acts as electron transport layer. Platinumoctaethyl-porphyrin PtOEp is the second optimized derivative of platinum porphyrin for structuring near-infrared lightemitting diodes because with another compound called platinum tetrakis(pentafluorophenyl)porphyrin PtTFP it is suitable for forming films of optical oxygen-sensing systems if they are quenched in polystyrene PS. The /0/100 is a sensitivity of the sensing film, where /0 and /100 represent the detected phosphorescence intensities from a film exposed to 100% argon and 100% oxygen, respectively.48-49,54,115,145-146,176,179-181

Figure 8.10. A structural example of near-infrared light-emitting diodes from platinum porphyrin.176,273

Divalent ruthenium complexes have been considered as an optimized group of triplet emitter complexes based on tris-chelate complexes of divalent ruthenium and similar metal ions with polypyridine ligands such as 2,2'-bipyridine (bpy) (including Ru(II)tris(2,2’-bipyridine)). Ligands used for complexes of divalent ruthenium include 4,4'-bipyrimidine (bpm), and 2,2'-bipyridine (including ruthenium-4,4'-bipyrimidine and ruthenium-2,2'-bipyridine-4,4'-bipyrimidine). The “divalent” or “bivalent” terms indicate the element that has a valence of two. Divalent ruthenium coordinates with poly(4-vinylpyridine) or poly(vinyl alcohol).146,148,150-151,179-181 The divalent ruthenium polypyridine complexes represent the first optimized family of divalent ruthenium complexes. This family includes 1. homoleptic ruthenium(II) complexes such as ruthenium(III)-(5-chloro-1,10-phenanthroline) Ru(Cl-phen)3(PF6)2. Where (Cl-phen) is the abbreviation of 5-chloro-1,10phenanthroline, while PF6 is the abbreviation of hexafluorophosphate 2. heteroleptic ruthenium(II) complexes such as the mixed complex (heteroleptic complexes) consisting of ruthenium(III)-bis(5-chloro-1,1'-phenanthroline)-(2,2'-bipyridine) Ru(Cl-phen)2(bpy)(PF6)2 and ruthenium(III)-(5-chloro-1,1'-phenanthroline)bis(2,2'-bipyridine) Ru(Cl-phen)(bpy)2(PF6)2.182

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Ru(Cl-phene)32+, Ru(Cl-phen)2(bpy)2+, and Ru(Cl-phen)(bpy)22+ are cationic forms of the divalent ruthenium complexes, such as Ru(Cl-phen)3(PF6)2, Ru(Clphen)2(bpy)(PF6)2, and Ru(Cl-phen)(bpy)2(PF6)2, respectively. PF6 is the hexafluorophosphate salt that is used for forming the above complexes. The above divalent ruthenium complexes have been optimized as members of divalent ruthenium complex family because they have high purity, single emission band, and electronic emission spectra of these complexes exhibit strong emission bands at 587 nm for Ru(Cl-phen)32+, at 590 nm for Ru(Cl-phen)2(bpy)2+, and at 597 nm for Ru(Cl-phen)(bpy)22+.333 The divalent osmium complexes also represent an optimized family of triplet emitters. Osmium Os metal (similar to ruthenium Ru or iridium Ir metals) can be used to form complexes having the ability to emit light ranging in color from blue through red, making them ideal candidates for organic light-emitting diodes technology. Generally, the emissions of the various complexes range from yellow (560 nm) to yellow-green (550 nm) to green (520 nm). Divalent osmium complexes have the general form of osmium complexes, such as (OsCl(N-N)(L-L)(CO))+PF6. Where N represents a derivative of 1,10phenanthroline (phen) and L represents phosphine PHI type ligand, and PF6 represents hexafluorophosphate. The general form of divalent osmium complexes that feature strong red metal-to-ligand-charge-transfer MLCT phosphorescence and electro-phosphorescence is Os(II)-(N-N)2L-L. Where N-N is either a bipyridine (2,2'-bipyridine) (bpy) or a phenanthroline (1,10-phenanthroline) (phen), and L-L is either phosphine or an arsine. Note: polypyridyl ligands are the most important ligands used for synthesizing divalent osmium complexes.148,150-151 Examples of poly(pyridine-2,5-diyl) PPY ligands include151,266 1. bipyridene (bpy) ligands such as 4,4’-di(biphenyl)-2,2’-bipyridine (dbp-bpy) and 4,4’-di(diphenylether)-2,2’-bipyridine (ddpe-bpy) 2. phenanthroline (phen) derivatives such as 4,7-bis(p-methoxyphenyl)-1,10-phenanthroline pmp)2(phen), 4,7-bis(p-bromophenyl)-1,10-phenanthroline (pbp)2(phen), 4,7bis(4’-phenoxybiphen-4-yl)-1,10-phenanthroline (pbpy)2(phen), and 4,7-bis(4naphth-2-yl-phenyl)-1,10-phenanthroline (nyp)2(phen). Based on the above and other developed ligands, several types of divalent osmium complexes can be optimized as red phosphorescent and electrophosphorescent polymers such as those (first and second groups) listed in Tables 8.8 and 8.9.151,266 These complexes have been optimized as important members of divalent osmium complexes due to their ability to exhibit strong metal-to-ligand charge transfer absorption bands in the visible region and strong red phosphorescent emission ranging from 611 to 651 nm, with quantum efficiency Φ of up to 45% in ethanol solution at room temperature. Osmium can be doped into a blend of poly(N-vinylcarbazole) and 2-tert-butylphenyl-5-biphenyl-1,3,4oxadiazole for structuring double-layered organic light-emitting diodes. In this case, a brightness over 1400 cd/m2 can be obtained with a turn-on voltage of 8 V. The maximum external quantum efficiency of 0.64% can be achieved.148,150-151

8.2 Optical polymers

431

Table 8.8. Examples of divalent osmium complexes of strong red phosphorescence and electrophosphorescence.151,266 Divalent osmium complexes (phosphorescence and electrophosphorescence (red))

Emission

Quantum efficiency

1,2-bis(diphenylarseno)eth-

650

0.19

(Os(II)(4,4’-diphenyl-2,2’-bipyridine)2 cis-1,2-bis(diphenylphosphino)ethylene)2+(HFB)2. (Os(II)-(dp-bpy)(dpph)2e)

623

0.23

(Os(II)(4,4’-diphenyl-2,2’-bipyridine)2 cis-1,2-vinylene-bis(diphenylarsine))2+(Tf)2 Os(II)-(dp-bpy)2(dpa)2

640

0.25

(Os(II)(4,4’-bis(p-diphenylether)-2,2’-bipyridine)2 cis-1,2-vinylenebis(diphenylarsine))2+(Tf)2 Os(II)-(pdpe)2(bpy)v(dpa)2

645

0.27

(Os(II)(4,4’-bis(p-biphenyl)-2,2’-bipyridine)2 larseno)ethane)2+(HFB)2 Os(II)-(pbp)2(bpy)(dpa)2e

1,2-bis(dipheny-

651

0.22

(Os(II)(4,4’-bis(p-biphenyl)-2,2’-bipyridine)2 bis(diphenylarsine))2+(Tf)2 Os(II)-(pbp)2(bpy)v(dpa)2

cis-1,2-vinylene-

643

0.28

(Os(II)(bathophenanthroline)2 cis-1,2-bis(diphenylphosphino)ethylene)2+(Tf)2 Os(II)-(bphen)(dpph)2e

613

0.33

First group (Os(II)bis(4,4’-diphenyl-2,2’-bipyridine) ane)2+ Os(II)-(dp-bpy)2(dpa)2e

Table 8.9. Examples of divalent osmium complexes of strong red phosphorescence and electrophosphorescence.151,266 Divalent osmium complexes (phosphorescence and electrophosphorescence (red))

Emission

Quantum efficiency

cis-1,2-vinylene-bis(diphenylar-

623

0.38

(Os(II)(4,7-bis(p-methoxyphenyl)-1,10-phenanthroline)2 1,2-bis(diphenylarseno)ethane)2+(Ts)2 Os(II)-(pmp-phen)2(dpa)2e

635

0.27

(Os(II)(4,7-bis(p-methoxyphenyl)-1,10-phenanthroline)2 cis-1,2bis(diphenylphosphino)ethylene)2+(Ts)2 Os(II)-(pmp)2(phen)(dpph)2e

611

0.36

(Os(II)(4,7-bis(p-methoxyphenyl)-1,10-phenanthroline)2 cis-1,2-vinylene-bis(diphenylarsine))2+(Ts)2 Os(II)-(pmp)2(phen)v(dpa)2

629

0.45

(Os(II)(4,7-bis(p-bromophenyl)-1,10-phenanthroline)2 cis-1,2-vinylenebis(diphenylarsine))2+(Ts)2 Os(II)-(pbp)2(phen)v(dpa)2

635

0.39

Os(II)(4,7-bis(4’-phenoxybiphenyl-4-yl)-1,10-phenanthroline)2 cis-1,2vinylene-bis(diphenylarsine))2+(Ts)2 Os(II)-(pbpy)2(phen)v(dpa)2

637

0.40

cis-1,2(Os(II)(4,7-bis(4-naphth-2-ylphenyl)-1,10-phenanthroline)2 vinylene-bis(diphenylarsine))2+(Ts)2 Os(II)-(nyp)2(phen)v(dpa)2

637

0.41

Second group (Os(II)(bathophenanthroline)2 sine))2+(Ts)2 Os(II)-(bphen)v(dpa)2

Osmium Os complexes have been optimized for structuring “organic light-emitting diodes of true white color emission” due to their blue, green, and red emissions, and the ability to control amounts of each until the desired white color emission is achieved. As a

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Optimizing Polymeric Structures in Organic Optoelectronics

result, the “divalent osmium complexes” have been optimized for the same applications due to their strong green phosphorescence and electrophosphorescence, having bonding ligands of phosphine (arsine, antimony). The optimized members of divalent osmium complexes of strong green phosphorescence and electrophosphorescence include 1. chloro osmium carbonyl-cis-1,2-bis(diphenylphosphino)ethylene-3,4,7,8tetramethyl-1,10-phenanthroline hexafluorophosphate Os520 2. chloro osmium carbonyl-cis-1,2-bis(diphenylphosphino)ethylene-1,10-phenanthroline hexafluorophosphate Os550 3. chloro osmium carbonyl-1,2-diphenylphosphino ethane-1,10-phenanthroline hexafluorophosphate Os560. The number attached to the symbol Os (520, 550, or 560) refers to the emission wavelength.148,150-151 The above three divalent osmium complexes have been optimized for structuring organic light-emitting diodes of true white color emission because their polymeric bases exhibit strong (metal-to-ligand charge-transfer triplet state) absorption bands in the visible region and strong green phosphorescent emissions at 522, 553, and 561 nm, respectively, with quantum efficiencies of 0.75, 0.70, and 0.63, respectively. Other properties of these complexes include their metal-to-ligand charge-transfer triplet state entities that have very low to non-existent quantum yields, each of these complexes exhibits strong emission, and the quantum yields for these complexes are much stronger than those for red and orange emitting complexes. Important to note that the emission of osmium Os complexes is blue-shifted to the osmium emission of osmium(III)tris(2,2'-bipyridine) Os(bpy)3(PF6)2. Hexafluorophosphate PF6 is used as an important part of forming this complex. This complex is used as tris(2,2'-bipyridine) (bpy)3-based osmium(II). In addition to another popular complexes such as ruthenium(III)-tris(2,2'bipyridine)-perchlorate Ru(bpy)3(ClO4)2, osmium(III)-tris(2,2'-bipyridine) is optimized for emitting layer of organic solid-state light-emitting systems, which are alternatives to inorganic semiconductor light-emitting diodes and liquid crystal displays. Thus, they provide displays of flat shape, improved brightness, more flexible, and at a lower cost. Structuring organic light-emitting diodes based on osmium(III)-tris(2,2'-bipyridine) (of the chemical structure shown in Figure 8.1162) resulted in high brightness and low-voltage light-emitting devices. For example, a single-layer light-emitting diode cell can be structured of the following layers, including indium tin oxide layer as a substrate, thin-film of either osmium(III)-tris(2,2'-bipyridine) or ruthenium(III)-tris(2,2'-bipyridine)-perchlorate (spin-coated onto indium tin oxide substrate as an emitting layer), tris(8-hydroxyquinoline) aluminum layer for cathode contact, thieno[3,4-c]pyrrole-4,6-dione TPd layer as a hole transport layer, negative contact of gallium:tin (Ga:Sn) as a contact layer, and epoxy EP layer for encapsulation. As a result, the structure of indium tin oxide, osmium(III)tris(2,2'-bipyridine), and gallium/tin layers provides red emission of the maximum spectrum at ~700 nm.62,148,150-151,184

8.2 Optical polymers

433

Figure 8.11: A structural example of a single-layer light-emitting diode cell of osmium or ruthenium complexes. [Adapted, by permission, from Frank G. Gaoand, Allen J. Bard, High-Brightness and Low-Voltage Light-Emitting Devices Based on Tris-chelated Ruthenium(II) and Tris(2,2’-bipyridine) osmium(II) Emitter Layers and Low Melting Point Alloy Cathode Contacts, Chem. Mater., 14, 3465-3470, 2002.]

According to the structure shown in Figure 8.11 and depending on “chelating phosphine functionalized luminescent osmium(II) emitters,” the phosphorescent organic lightemitting diodes have been developed, as well. The optimized members of chelating phosphine functionalized luminescent osmium(II) emitters for structuring phosphorescent organic light-emitting diodes (Figure 8.12188) include osmium(II)-bis(3-(trifluoromethyl)5-(2-pyridyl)pyrazole)-cis-1,2-bis(diphenylphosphino)ethene Os(fppz)2(dppee), and osmium(II)-bis(2-pyridyl-3-trifluoromethyl-1,2,4-triazole)-cis-1,2-bis(diphenylphosphino)ethene Os(fptz)2(dppee). Optimization of these two emitters depends on their doping state so that they should be used as doped emitters for structuring orange/polymeric light-emitting diodes PLEDs to show maximum external quantum efficiency EQE of 13.3% (for osmium(II)-bis(2-pyridyl-3-trifluoromethyl-1,2,4-triazole)-cis-1,2-bis(diphenylphosphino)ethene). These two complexes are considered as osmium(II) pyridyl azolate complexes Os(II)pac for which either bis(diphenylphosphino)methane (dppm) or cis-1,2bis(diphenylphosphino)ethene (dppee) chelates were synthesized as ligands. These transition-metal complexes have strong phosphorescent emission at room temperature, remarkably high thermal stability, highly brightness luminescence, and good solubility in all organic solvents. Moreover, they are suitable for fabricating full-color displays and whiteemitting devices for general lighting applications.188 An organic light-emitting diode structured of osmium(II)-bis(3-(trifluoromethyl)-5(2-pyridyl)pyrazole)-cis-1,2-bis(diphenyl phosphino)ethene can be optimized by the utilization of this complex as dopant in the blend of poly(vinyl carbazole) PVK and 2-tertbutylphenyl-5-biphenyl-1,3,4-oxadiazole PBD because the resultant blend exhibits yellow emission with brightness of 7208 cd m-2, an external quantum efficiency of 10.4%, and luminous efficiency of 36.1 cd A-1 at current density of 20 mA cm-2. Utilizing osmium(II)-

434

Optimizing Polymeric Structures in Organic Optoelectronics

bis(3-(trifluoromethyl)-5-(2-pyridyl)pyrazole)-cis-1,2-bis(diphenylphosphino)ethene may give better brightness of 9212 cd m-2, external quantum efficiency of 12.5% and luminous efficiency of 46.1 cd A-1 at 20 mA cm-2.188

Figure 8.12. Polymer organic light-emitting diodes structured of chelating phosphine functionalized luminescent osmium(II) emitters. [Adapted, by permission, from Y.M. Cheng, G.H. Lee, P.T. Chou, L.S. Chen, Y. Chi, C.H. Yang, Y.H. Song, S.Y. Chang, P.I. Shih and C.F. Shu, Rational Design of Chelating Phosphine Functionalized Os(II) Emitters and Fabrication of Orange Polymer Light-Emitting Diodes Using Solution Process, Adv. Func. Mater., 18, 183-194, 2008.]

8.2.5.7 Optimized electrophosphorescent iridium(III) complexes The optimized electrophosphorescent iridium(III) complexes have been selected as the seventh type of optimized electrophosphorescence polymers related to organic optoelectronic polymers for structuring electrophosphorescence systems because platinum(II) and iridium(III)-based phosphorescent materials are the most suitable heavy-metal phosphorescent emitters due to their rationally tuning the emission wavelength over the entire visible range. Harvesting both singlet and triplet excitons and endow organic light-emitting diodes with the ability to achieve an internal efficiency of 100% represent the most important properties of phosphorescent materials. Iridium(III) complexes have been optimized for such applications using their high tunability in the emission color. Moreover, the strong spin-orbit coupling of these complexes results in singlet-triplet state mixing, giving highly efficient electrophosphorescence in organic light-emitting diodes. For example, organic light-emitting diodes based on iridium(III) complexes containing two heteroatoms in ligands, such as “thiazole,” “azine,” and “thiazine” have high efficiency and long luminance half-life. Azine ligands such as diazine are the most widely used for forming iridium(III) complexes. These are called iridium(III) diazine complexes.144 The optimized groups of iridium(III) diazine complexes used for structuring polymeric organic light-emitting diodes include11,144,186,188-191 1. blue phosphorescent emitting complexes such as iridium(III)-bis(2-(2,4-difluorophenyl)-pyrimidine) cyanide triphenylphosphine Ir(III)-(DFPPM)2(CN)(PPh3),and iridium(III)-bis(2-(2,4-difluorophenyl)-pyrimidine) chloro triphenylphosphine Ir(III)(DFPPM)2(Cl)(PPh3) 2. green phosphorescent emitting complexes such as iridium(III)-bis(2-phenylpyrimidine) (acetylacetonate) Ir(III)-(PPM)2(acac)

8.2 Optical polymers

3.

435

red phosphorescent emitting complexes such as iridium(III)-(2,4-diphenylquinoline) (acetylacetonate) Ir(III)-(DPQ)(acac) 4. yellow phosphorescent emitting complexes such as iridium(III)-bis(2-methyl-3phenylpyrazine) (acetylacetonate) Ir(III)-(MPPZ)2(acac). Both iridium(III)-bis(2-(2,4-difluorophenyl)-pyrimidine) cyanide triphenylphosphine and iridium(III)-bis(2-(2,4-difluorophenyl)-pyrimidine) chloro triphenylphosphine (related to the first optimized group of “blue phosphorescent emitting complexes”) use bis(2-(2,4-difluorophenyl)-pyrimidine) as cyclometalated ligand. They can be used as optimized phosphors for structuring organic light-emitting diodes of high-efficiency and long luminance half-life. When they are characterized by nuclear magnetic resonance NMR spectroscopy and mass spectrometry MS, they show emission peaks at 472 and 489 nm (for iridium(III)-bis(2-(2,4-difluorophenyl)-pyrimidine) chloro triphenylphosphine at 447 and 472 nm for iridium(III)-bis(2-(2,4-difluorophenyl)-pyrimidine) cyanide triphenylphosphine). Iridium(III)-bis(2-phenylpyrimidine) (acetylacetonate (related to the second optimized group of the green phosphorescent emitting complexes) uses ppm as ligand that has two symmetrical nitrogen atoms besides the phenyl ring. It is optimized as a member of green phosphorescent iridium(III) diazine complexes due to its absorption λabs at 261, 341, 406, and 640 nm, while the emission λem at 527 nm, and lower unoccupied molecular orbital energy of -2.93 eV.191 Iridium(III)-(2,4-diphenylquinoline) (acetylacetonate) of the third optimized group of red iridium(III) diazine complexes has been optimized as a member of iridium(III) diazine complexes due to its strong red photoluminescence at 660 nm. Moreover, it includes iridium(III)-2-(9,9-di-n-octyl-fluoren-2-yl)-4-phenylquinoline) (acetylacetonate) (Ir(III)(FPQ)(acac) that uses 2,4-diphenylquinoline DPQ ligand for structuring phosphorescent organic light-emitting diodes of the following layers poly(3,4-ethylene dioxythiophene) doped emitting layer and 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene TPBI. A blend of poly(vinyl carbazole) and 2-tetra-butyl phenyl-5-biphenyl-1,3,4-oxadiazole can be selected as a host. Poly(vinyl carbazole) acts as good hole transport layer, while 1,3,5tris(N-phenylbenzimidazol-2-yl)-benzene acts as electron-injection/transport layer. For such structures, iridium(III)-(2,4-diphenylquinoline) (acetylacetonate) shows absorption λabs at 275, 352, 441, 480, 522, and 564 nm, maximum photoluminescence λLC at 614 nm, and quantum yield of 0.14. It shows absorption λabs at 313, 382, 456, 500, 547, and 600 nm, maximum photoluminescence λLC at 625 nm, and quantum yield of 0.11.11,186,190,191 Iridium(III)-bis(2-methyl-3-phenylpyrazine) (acetylacetonate) has been optimized as a member of forth optimized group of “yellow phosphorescent emitting complexes” due to its ability to emit yellow light with a maximum peak at 575 nm. Importance of this member depends on its chemical structure that contains 2-methyl-3-phenylpyrazine mppz ligand that plays an important role in its cyclometalating procedure. It shows maximum absorption λabs at 258, 322, 410, and 496 nm, while the maximum emission λem at 575 nm, and lower unoccupied molecular orbital energy of -2.93 eV. The white organic light-emitting diodes WOLED can be fabricated using iridium(III) complexes. Such objective can be achieved by relatively broad orange-red excimer emission of iridium(II)-bis(5,6-dihydro-9,10-methylenedioxy-benzo[c]acridine) (acetylacetonate) Ir(III)-(dmba)2(acac) with efficient blue phosphorescent iridium(III)-bis((4,6-difluorophenyl)-pyridinato-N,C2)pico-

436

Optimizing Polymeric Structures in Organic Optoelectronics

linate. This combination results in optimized white organic light-emitting diodes due to their capability of exhibiting pure white emission with (commission international de l'eclairage CIE coordinates method to define colors) coordinates close to the ideal white emission. On the other hand, structures of triple-emitting-layered organic light-emitting diodes can be optimized by sandwiching blue-emitting layers between two orange-redemitting layers, so that, the resulting structures show stable white emission at different biases/brightness, maximum efficiencies of up to 12.2% and 27.0 cdA-1 for the forward viewing direction, corresponding to the total efficiencies of 19.7%, 45.9 cdA-1 and 32.1 lmW-1. Moreover, at high brightness of 1000 cdm-2, the electroluminescent efficiencies remained high at 8.8% and 19.2 cdA-1.182,189,192 The polymeric structures of top-emitting organic light-emitting diodes can also be optimized so that they can show white emission spectra by employing a three-color hybrid cavity structure with two highly efficient phosphorescent orange-red and green emitters. For example, iridium(III)-bis(2-methyldibenzo-[f,h]chinoxalin) (acetylacetonate) can be used as a three-color hybrid cavity structure, while fac-tris(2-phenylpyridine) iridium(III) and stable blue fluorescent emitter 2,5,8,11-tetra-tert-butylperylene can be used as two highly efficient phosphorescent orange-red and green emitters. Thus, these structures result in commission international de l'eclairage of (0.420, 0.407) at approximate luminance of 1000 cd/m2 and color rendering indices of up to 77. Electrophosphorescent iridium(III) complexes can be formed as blends, such as poly(3-methyl-4octylthiophene):bis(2-phenylbenzothiazole) iridium acetylacetonate (PMOT:BTIr) and poly(3-methyl-4-octylthiophene):platinum(II) 2,8,12,17-tetraethyl-3,7,13,18-tetramethyl porphyrin (PMOT:PtOX). Poly(3-methyl-4-octylthiophene):bis(2-phenylbenzothiazole) iridium acetylacetonate can be prepared from doping polythiophene-based light-emitting diodes with poly(3-methyl-4-octylthiophene) PMOT as a host and the phosphorescent compound bis(2-phenylbenzothiazole) iridium acetylacetonate BTIr as a guest, while, poly(3-methyl-4-octylthiophene)/platinum(II) 2,8,12,17-tetraethyl-3,7,13,18-tetramethyl porphyrin can be prepared from doping polythiophene-based light-emitting diodes with poly(3-methyl-4-octylthiophene) as a host and the phosphorescent compound platinum(II) 2,8,12,17-tetraethyl-3,7,13,18-tetramethyl porphyrin PtOX as a guest. The resulting structures of organic systems based on these two host-phosphorescent guest blends have optimized photoluminescent and electroluminescent properties due to their ability to show the existence of energy transfer from host to guest. Another type of such blends is tris(2,5-bis2’-(9’,9’-dihexylfluorene) pyridine) iridium(III) that can be doped into a blend of poly(vinyl carbazole) with 2-tetra-butyl phenyl-5-biphenyl-1,3,4-oxadiazole to get efficient and bright red electrophosphorescent light-emitting diodes. Characteristics of such structure are 50% ph/el external quantum efficiency, 7.2 cd/A luminous efficiency, and the maximum electroluminescent emission at 600 nm.138,147,178,193-195 The photophosphorescent iridium complex can be doped into a blend of poly(vinyl carbazole) with 2-tetra-butyl phenyl-5-biphenyl-1,3,4-oxadiazole to optimize the efficiency of green electrophosphorescent light-emitting diodes. A photophosphorescent iridium complex is represented by tris(9,9-dihexyl-2-(phenyl-4’-(pyridin-2’’yl))fluorene)iridium(III) which is used as a guest, while the blend of poly(vinyl carbazole)

8.3 Properties of optical polymers

437

with 2-tetra-butyl phenyl-5-biphenyl-1,3,4-oxadiazole as host. Characteristics of structuring organic light-emitting diodes of this complex include maximum electrophosphorescent emission at 550 nm, 8% external quantum efficiency of photons per electron, 29 cd A-1 luminous efficiency, and 3500 cd m-2 maximum brightness. The blend of poly(vinyl carbazole) and 2-tetra-butyl phenyl-5-biphenyl-1,3,4-oxadiazole can be replaced by another blend as host (such as the blend of poly(2,5-bis((5-tert-butylphenyl)-1,3,4-oxadiazole)styrene) P-Ct with thieno[3,4-c]pyrrole-4,6-dione TPD to increase the efficiency of structured electrophosphorescence devices. Ir(III)bis(5-methyl-2,3-diphenylpyrazine) (acetyl acetonate)) IrMDPP can be used as guest of this blend. Characteristics of electrophosphorescence device structured from blending poly(2,5-bis((5-tert-butylphenyl)-1,3,4oxadiazole)styrene), thieno[3,4-c]pyrrole-4,6-dione, and iridium(III)bis(5-methyl-2,3diphenylpyrazine) (acetyl acetonate)) P-Ct-TPD-IrMDPP include improved efficiency, 3702 cd/m2 maximum luminance efficiency, 0.83 cd/A external luminance efficiency, and the electroluminescent spectra color is shifted from green-yellow to yellow-orange as the iridium(III)bis(5-methyl-2,3-diphenylpyrazine) (acetyl acetonate) content is increased. Instead of blends, single polymers can be doped with iridium complexes for structuring electrophosphorescence devices. For example, poly(dialkyl fluorene) as a luminescent host polymer can be doped with platinumoctaethyl-porphyrin PtOEP as a red phosphorescent dye. The structured organic light-emitting diodes of this complex exhibit 3.5% maximum external quantum efficiency (4% platinumoctaethyl-porphyrin weight).147,194-195,197

8.3 PROPERTIES OF OPTICAL POLYMERS Optical properties required for structuring organic optoelectronic systems from optical polymers include a refractive index, spectral transmission, optical dispersion, optical losses, and birefringence. Refractive index n is the ratio of light's speed in a vacuum n1 to that in a second medium of greater density n2, where (refractive index of air) nair=1, (refractive index of water) nH2O=1.333, and (refractive index of oil) nOil=1.515. Generally, refractive indices range from 1.3 to 1.73, but for commercial grades of polymers, they range from 1.42 to 1.65. Polymers of refractive indices greater than 1.50 are considered having high-refractive-index HRIP, such as polystyrene PS (1.57260 at wavelength 1014 nm), polycarbonate PC (1.5672 at wavelength 1014 nm), and styrene-acrylonitrile copolymer SAN (1.5519 at wavelength 1014 nm). The highly transparent polymer polymethylmethacrylate has a minimum theoretical attenuation of 106 dB/km at 650 nm, due to the Rayleigh scattering and absorption of C−H bonds. Polycarbonate, which has n of 1.51, is used as a core of polymeric optical fibers (where polytetrafluoroethylene PTFE acts as a cladding layer). Polystyrene has attenuation of 114 dB/km at 670 nm.6,37,40,47 Optimized polymers used for structuring optoelectronic systems such as polymeric optical fibers should have refractive indices up to 1.80 (classified as high-refractive-index polymers). Optimizing the refractive index of a polymer means increasing the value of its index by introducing substituent with high molar refraction or by combining nanoparticles with polymer matrices (high-refractive-index polymer nanocomposites). These substituents include aromatic rings, sulfur-containing groups, halogens except fluorine, and

438

Optimizing Polymeric Structures in Organic Optoelectronics

organometallic moieties. It is useful to remember that the molar refraction R (equations 8.2274,304 and 8.3274,304) is a property of a dielectric defined by the equation:274 R = Vm[(n2 − 1)/(n2 + 2)]

[8.2]

where: n2 Vm

the index of refraction of the medium (at optical wavelengths) molar volume. It is related to the polarizability of the molecules that make up the medium by the Lorenz-Lorentz equation

R = NA/3e0

[8.3]

where: Avogadro's constant permittivity of a vacuum.200

NA e0

Polymers with refractive indices of approximately 1.3 to 1.4 are called low refractive index polymers (such as those used for anti-reflective coatings for optical lenses). It is important to note that the measurement of the refractive index of polymers requires contact liquids to create a planar contact between sample and prism. Selected optical polymers, refractive indices, and corresponding contact liquids are listed out in Table 8.10.275 Table 8.10. Selected optical polymers, refractive index n and corresponding contact liquids.275 Polymer Polycarbonate Polyetheretherketone Polystyrene Poly(ethylene terephthalate) Styrene-acrylonitrile copolymer Polyethylene Polymethylmethacrylate

Refractive index

Liquid

1.5860

methylene iodide

1.65-1.71

methylene iodide

1.5894 1.575 1.57-1.5704 1.51 1.4893-1.4899

mercury(II) iodide in saturated potassium solution

methylene iodide 1-bromonaphthalene 1-bromonaphthalene zinc chloride solution (ph400 nm, high refractive index in the range of 1.6512 to 1.6022 (at 589 nm), and Abbe-number in the range of 42.6 to 50.6. It can be polymerized as a composition of 2,5-bis(sulfanyl methyl)-1,4-dithiane, divinyl sulfone, and cyclohexane-1,4-dithiol CHDT.16 Poly(disulfanyl tricyclo[5.2.1.02,6]decane/divinyl sulfone) has been optimized as the third optimized member of the second optimized group due to its high refractive index (1.6052) and high Abbe-number (8.0) with a glass transition temperature of 74oC. It can be prepared by the reaction of poly(disulfanyl tricyclo[5.2.1.02,6]decane) DCDSH with divinyl sulfone. Poly(2,5-bis(sulfanylmethyl)-1,4-dithiane/bis(vinylsulfone)tricyclo[5.2.1.02,6]decane has been selected as the fourth optimized member of the second optimized group due to its high refractive index (1.6228) and high Abbe-number (45.8) with a glass transition temperature of 113oC. It can be prepared by the reaction of 2,5-bis(sulfanylmethyl)-1,4-dithiane with bis(vinylsulfone)tricyclo[5.2.1.02,6]decane. Both poly(disulfanyl tricyclo[5.2.1.02,6]decane/divinyl sulfone) and poly(2,5-bis(sulfanylmethyl)-1,4-dithiane/ bis(vinylsulfone)tricyclo[5.2.1.02,6]decane have good thermal stability, and they are used in optical lenses. Polyvinylsulfides family has been selected as the third optimized group due to its high refractive index in the range of 1.687-1.5460 and high Abbe-number in the range of 34.1-43.8 with glass transition temperature in the range of 41.9-124.0oC) They can be prepared by addition polymerization using 2,5-bis(2-thia-3-butenyl)-1,4-dithiane with a radical initiator.206-207 The optical losses of optical polymers used in communication applications are caused by absorption and scattering states. The optimized optical polymers used for structuring organic communication systems must have low values of optical losses (not more than 0.1 dB/cm in the communication spectral windows around 800, 1300, or 1500 nm), and must be characterized by transparency in the visible-wavelength region, and strongly absorbing in UV and throughout the far IR. For polymer optical fiber, optical losses are specific to geometry and handling of the fibers (but are not functions of the fiber polymers). Optical losses can be classified as bending, launching, and connector losses. Bending losses result from the distortion of fiber from the ideal straight-line configuration. Launching loss is related to an optical fiber not being able to propagate all the incoming light rays from an optical source. Connector losses are associated with the coupling of the output of one fiber with the input of another fiber, or couplings with detectors or other components.22-23,29 Birefringence term denotes the light's refraction in an anisotropic material (such as calcite) in two slightly different directions to form two rays. In plastic molding such as injection molding and manufacturing, birefringence is an important optical property of optical components observed during the related processes. That is why particular attention has to be paid to the edge configuration during the production of optical components by an injection molding process. The higher stress level at the edge of the optic causes birefringence and surface irregularities (this phenomenon is also called “edge effect”). Optical birefringence can be defined as the double refraction of light in a transparent, molecularly ordered material, which is manifested by the existence of orientation-dependent differ-

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ences in the refractive index. It is known that the refractive index of the crystalline lattice (such as amorphous polymers) is equal in all directions throughout it because many transparent solids are optically isotropic.18,213,335-336

8.4 PHYSICAL PROPERTIES OF OPTICAL POLYMERS Physical properties of optical polymers include hardness, rigidity, service temperature, outgassing, water absorption, and radiation resistance. Hardness denotes the ability of optical polymers to resist the static penetration (under a known load) of a sharp device made from a material of very high Young's modulus, such as glass or ceramic. Shore value (Shore A or Shore B scales) is used for measuring the hardness of soft polymers. Rockwell value (Rockwell E, I, and M scales) is used to measure rigid polymers. The international rubber hardness value is used for measuring the hardness of elastomers. The pencil test value is used to measure the hardness of surface coatings. Other types of tests can be used for measuring rigid polymers, such as Vickers and Brinell hardness. Microhardness is the technique by which the hardness and the mechanical properties of small-sized polymeric samples can be measured in the region of a micron. Such measurement is determined by the optical image of the residual width of penetration.104,216 The most developed testing methods are ultrasonic-hardness and nanohardness testers for thin films. The main physical properties of some optical polymers (included hardness property) are listed in Table 8.12.268 According to this table, several commercial types of optical polymers are selected, such as the commercial series of polymethylmethacrylate PMMA including Lucite®306, Plexiglass®253, and Polycast®307 grades. The commercial series of polycarbonate include Lexan®74 and Merlon®308 grades. The commercial-grades of polystyrene PS include Styron®309 and Lustrex®309. The commercial-grade of cyclic olefin copolymer COC includes Topas®285. The commercial-grades of cyclic olefin polymers COP include Zeonex®284 and Zeonor®284. The commercial-grade of polyetherimide PEI includes Ultem®300.104,216-217 Table 8.12. Physical properties of selected commercial grades of optical polymers (included hardness property).268 Physical properties Hardness (Rockwell) Specific gravity

Optical polymers PMMA

PC

PS

COC

COP

PEI

M97

M7

M90

M89

M89

M109

1.16-1.19 1.20-1.25 1.05-1.06 1.02-1.03 0.95-1.01

1.27

Coefficient of thermal expansion at 70oC

6.74

6.6-7.0

6.0-8.0

6.0-7.0

6.0-7.0

4.7-5.6

Max. service temperature, oC

60-70

124

82

130

130

170

PMMA − polymethylmethacrylate, PC − polycarbonate, PS − polystyrene, COC − cyclic olefin copolymer, COP − cyclic olefin polymer, PEI − polyetherimide

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Optimizing Polymeric Structures in Organic Optoelectronics

The rigidity is a physical property used to describe flexibility, stiffness, and non-pliability of optical polymers. Flexural modulus is related to rigidity as a function of the polymer's modulus of elasticity and shape. The rigidity of optical polymers depends on their chemical structures (for example, as the crystallinity increases, polymer rigidity increases). It also depends on the composition of a polymer such as additives, agents, radicals, and fillers. For example, the rigidity of the optical polymer polystyrene of α-methyl styrene radicals and styrene groups can be increased by increasing the number of styrene groups, while α-methyl styrene groups form less reactive radicals, and thus slow down the curing reactions. Regarding their chemical structures, the elastomeric polymers can exhibit elastomeric elasticity, while optical polymers used in the form of fibers (such as optical fibers), films, coatings, foams, or molded and fabricated components can exhibit a wide range of properties in terms of strength, rigidity, toughness, flexibility, elasticity, resilience, optical clarity, chemical and solvent resistance, etc. Reinforcements and fillers provide strength and rigidity, helping to support the structural load, but the clarity of optical polymers may be affected. This property (as mechanical property) creates good impact resistance of an optical polymeric component but generates torsion or compressive stress. That is why the profiles of optical surfaces should be maintained to sub-wavelength accuracy. As a result, the rigidity of a polymer increases as impact strength decreases. Examples of rigidity values of some optical polymers include acrylics of 3307 MPa, polycarbonate of 5512 MPa, polystyrene of 2136 MPa, poly(vinyl chloride) of 3314 MPa, commercial polyetherimide such as Ultem®300 of 3307 MPa, polyoxymethylene homopolymer of 2894 MPa, and high-density polyethylene of 1378 MPa.7,37,51,220-224

Figure 8.13. Examples of ranges of service temperature for some optical polymeric grades. [Data from Ensinger, Service Temperature, www.ensinger-online.com.]

The service temperature of the optical polymers (as a material characteristic) provides information about their thermal stability. Several terms are related to service temperature terms, such as “maximum service temperature,” “long-term service temperature,” “short-term service temperature,” “negative service temperature,” and “heat deflection temperature.” Maximum service temperature (Figure 8.13) denotes the highest temperature at which a polymeric material can be used without compromising its strength or other properties. The long-term service temperature indicates the maximum temperature at which a polymeric material has lost no more than 50% of its initial properties after 20,000 hours of storage in hot air. The short-term service temperature indicates the short-term

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peak temperature at which a polymeric material can tolerate over a short period (from minutes to occasionally hours) taking into account the stress level and duration, without sustaining damage. Heat deflection temperature denotes the temperature under which a deflection of 0.2% is achieved under a specific bending stress.310 Polyetherimide is a rigid polymeric material suitable for structuring heavy-duty optoelectronic systems due to its high rigidity and good creep resistance, as is polyoxymethylene, which has high rigidity and toughness. Polyethylene has toughness, lightweight, and very low water absorption. The toughness, electrical insulating properties, and abrasion resistance are the most important properties in the selection of acrylics (polymethylmethacrylate) for fabricating optical fibers and very clear optoelectronic components. Compared with high service temperature limits of glass (400-700oC), optical polymers have much lower service temperature limits (60-250oC).237 Water absorption of an optical polymer is a physical scale used to measure the content of H2O. This scale can be defined as the ratio of the weight of water absorbed by a polymeric material to its weight before absorption (dry state). Electrical (changes in electrical insulating properties), mechanical (reduction in strength), physical (dimensional stability), and optical (haze) properties of optical polymers are affected by water absorption. As a result, optimized optical polymers suitable for optoelectronic systems should have very low water absorption. The radiation resistance of an optical polymer denotes the energy (such as light, heat, or sound coming from a source and traveling through a polymeric material). Irradiation denotes exposure to penetrating radiation. Irradiation can be observed when all or part of the polymeric material is exposed to radiation from an unshielded source. In the field of antennas, resistance refers to radiation of electromagnetic waves from the antenna. In most circumstances, types of radiation include ionizing, x-ray, cosmic, and non-ionizing electromagnetic radiation. The ionizing radiation includes UV, alpha, beta, gamma, and neutron radiation. Non-ionizing electromagnetic radiation includes UV and visible light, infrared, microwave, radio waves, etc.4 Optical polymers are physically and chemically affected by radiation. For example, most of the optical polymers exhibit fluorescence upon irradiation by high-energy radiation. On the other hand, high-energy radiation of ultraviolet and ionizing sources, produce varying amounts of polymer crosslinking. Irradiation of polymers (especially optical polymers) is an important consideration in the assessment of altering properties of these polymers (such as electrical, optical, chemical, and mechanical properties). Poly(thioether sulfone) PES is an engineering thermoplastic polymer, which has excellent thermal stability (withstands temperature up to 180oC for a long time and retains many electrical and mechanical properties at 200oC). The interaction of radiation with this polymer destroys its initial structure and results in chain scission, chain aggregation, the formation of double bonds, and molecular emission. That is why this polymer should be modified before structuring optical systems such as light-emitting diodes, optical sensors, and anti-reflective coatings. Polymer optical fibers such as polyfluoroalkylmethacrylate used as a cladding layer for optical fibers of polymethylmethacrylate polymer core are affected by 60Co gamma-radiation. Studies indicated that even small doses of radiation cause some

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Optimizing Polymeric Structures in Organic Optoelectronics

decrease in the values of tensile modulus and elongation. Irradiated methacrylic polymers become yellow-brown.1,7,37,230

8.5 ORGANIC OPTOELECTRONIC SYSTEMS Optoelectronics represents branch of technology that combines both physics of light and electricity. This branch focuses on designing and manufacturing hardware of associated systems which convert electrical signals into photon signals and vice versa.247 It can be defined as the science of light. Its applications focus on both electrons and photons. A system is considered an optoelectronic system if it acts as an electrical-to-optical or opticalto-electrical transducer. Most optoelectronic systems make use of organic materials (polymers) called optical polymers. Such organic optoelectronic systems have the potential for cost and flexibility advantages over inorganic systems. The function of the polymeric structure shown in Figure 8.1477,95,109,303 (in relation to organic optoelectronic light-emitting diodes) is to act as a light emitter as soon as the external voltage is applied. The main two types of optoelectronic light-emitting diodes are 1. optoelectronic light-emitting diodes based on small organic molecules 2. optoelectronic light-emitting diodes based on organic polymers. Such devices have light-weight, are flexible and transparent, and have color tuning ability. These features make these systems to be an ideal modern light source (that is regarding their polymeric structure).234-235

Figure 8.14. A polymeric structural example of an optoelectronic light-emitting diode.77,95,109,303

The polymeric structure of optoelectronic light-emitting diodes of 100-500 nm, shown in Figure 8.14 consists of 1. two electrodes deposited on a flexible substrate (often polymeric substrate) 2. conducting layer that is made of organic/polymeric molecules to transport holes from the anode 3. emissive layer in the form of organic/polymeric film, which transports electrons from the cathode and emits light in response to an electric current. Optimizing the polymeric structure of such example depends mainly on making use of a polymeric family of poly(p-phenylene)s family because derivatives of this family are conjugated polymers. Conjugated polymers can produce electroluminescence in optoelec-

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tronic light-emitting diodes. Optimized polymers act as electron transport layers to produce electroluminescence in optoelectronic light-emitting diodes. They include 2-(4biphenyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole BTBO, tris(8-hydroxyquinoline) aluminum Alq3, 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene TPBi, and bathocuproine BCP.54,230 The optimized polymers can be selected to act as hole transport layers for producing electroluminescence in optoelectronic light-emitting diodes, and they include N,Ndiphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl-4,4-diamine TPD(4M) and 1,4-bis(1naphthylphenylamino)biphenyl (also called N,N'-di(naphthalene-1-yl)N,N'-diphenylbenzidine) NPB.54 The optimized electrochromic polymers that can be selected include poly(ethylene dioxythiophene) PEDOT, poly(3-methyl thiophene) P3MT, poly(3,3dimethyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine) PProDOT-(CH3)2, poly(3,4-ethylene dioxypyrrole) PEDOP, poly(3,4-(2,2-dimethyl propylene dioxy)-pyrrole) PProDOP(CH3)2, poly(3,6-bis(3,4-ethylenedioxy)thienyl)-N-methylcarbazole) PBEDOT-NMeCz, and poly(2-(3,4-ethylenedioxy)thienyl-(biphenyl)) PBEDOT-BP.97 Three developed techniques (electrochromic,100 electroluminescent, and lasing systems) are currently promising commercial applications. These systems are based on polymers, or more accurately electroactive polymers. Where a material (or a polymer) can be defined as an electroactive material/polymer if it responds to electrical stimulation with a reversible variation of one or more physical/chemical properties (Figure 8.1597). The electrochromism refers to a phenomenon displayed by some material/polymer, which reversibly changes color when a burst of charge is applied. Electrochromic polymers that are conjugated polymers able to change color in response to electronic signals are likely to spread into many applications. Electrochromic polymers such as polyaniline and polypyrrole represent optimized polymers for electrochromic applications because their colors can be controlled by electricity. For example, polypyrrole shows blue/violet color in the oxidized state, while the neutral state (unhoped) is yellow/green, but it presents a low cycle lifetime, discouraging its use for reliable devices at the moment. Polyaniline shows yellow (leuco-emeraldine), green (emeraldine salt), blue (emeraldine base), and black (pernigraniline) colors. Applications of electrochromic polymers include molecular electronics, electrical displays, optical computers, etc.52-53,71,97,104 According to their unique properties, nanoparticles based on π-conjugated polymers and oligomers are widely used in applications of optoelectronic systems. Nanoparticles (in the form of nanopowders, nanoclusters, or nanocrystals) are microscopic particles with at least one dimension less than 100 nm. Their unique properties include very small size, simple preparation method, and tunable and fluorescent properties. The applications include film fabrication for optoelectronic light-emitting diodes, bio-sensing, solar cells, and bio-imaging. Practically, nanoparticles based on π-conjugated polymers have been optimized for such applications because they can show excellent fluorescent brightness, high absorption cross-sections, and high effective chromophore density, which makes them attractive for imaging and sensing application.108,238

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Optimizing Polymeric Structures in Organic Optoelectronics

Figure 8.15. Seven-layer optoelectronic device. [Adapted, by permission, from F. Carpi, and D. De Rossi, Colours from Electro-active Polymers: Electro-chromic, Electroluminescent and Laser Devices Based on Organic Materials, Optics and Laser Technology 38, 292-305, 2006.]

Nanoparticles of organic nature that are used for structuring optoelectronic systems (e.g., polyfluorenes) are prepared by the so-called “mini-emulsion method” (50-500 nm size). The optimized fluorine-based organic nanoparticles include polyfluorene-acetylene PFAC polymer because it has blue emission, but it can be changed to red if perylene diimide dye is incorporated. Nanoparticles of polyfluorenes can be used to form optoelectronic films. The optimized derivatives of this family include poly(dialkyl fluorene) PFO, poly(ethylhexyl fluorine) PF2/6, poly(trimethyldodecyl fluorine) PF11112, and poly(2methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene) MEH-PPV due to their ability to be spin-coated onto glass substrates in the form of thin films. Annealing above the glass transition temperature, Tg may result in coalescence of nanoparticles and formation of multilayers. Optoelectronic devices can be classified as “organic optoelectronic emitters” (optoelectronic light-emitting diodes, laser, infrared, and displays), “electroluminescent,” “photonics,” “optical amplifier,” “optical detectors and receivers,” “optoelectronic thinfilms” (thin-film systems) OETF, and “electrooptic modulators.” Optoelectronic emitters include “optoelectronic light-emitting diodes” and “organic (polymeric lasers).” Photonics includes “ultrafast-photonics” and “photonic integrated circuits.”108,338 8.5.1 OPTICAL POLYMERS FOR FORMING ORGANIC OPTOELECTRONIC EMITTERS The light emission is physical radiation in the visible range caused by photons emitted by discrete semiconductor systems. A laser is a light emitter, providing an emission of 1015 Hz. Examples of emitters and lasers include 1. emitters (light sources, light-emitting diodes, lasers, displays, etc.) 2. lasers (early diode laser, vertical cavity surface-emitting laser, high power laser, quantum dot laser, quantum cascade laser, etc.)339-340

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A simple representation of light emission (based on the conjugated polymers for applications in optoelectronic systems) is shown in Figure 8.16.185,277 Note: electrical induction of optoelectronic systems results in the emission of light. The organic structure of such a system consists of a counter electrode such as silver, a substrate such as glass, a transparent electrode such as indium tin oxide/poly(3,4-ethylenedioxythiophene) ITO/ PEDOT, and the main component that is a thin layer of the conjugated polymer. The optimized conjugated polymers for these applications include poly(9,9'-dialkylfluorene) PDAF, polyspirobifluorene PSBF, poly(phenylene vinylene) PPV, poly(2-methoxy,5-(2'ethyl-hexyloxy)-p-phenylenevinylene MEH-PPV, and phenyl-substituted poly(phenylene vinylene) Ps-PPV.277 These polymers are arranged according to their visible spectra (Figure 8.16).

Figure 8.16. Representation of light emission.185,277

Conjugated polymers acting as laser materials and used for forming thin-film solidstate laser, and their systems can be called “semiconducting polymeric lasers.” It is useful to note that many conjugated polymers can be classified as “luminescent polymers” with a Stokes shift that separates emission sufficiently far from the absorption edge so that the self-absorption is minimal. Semiconducting luminescent polymers represent the optimized selection of laser materials with gain lengths in the micron regime.40,47,59,128,240-242 The optimized conjugated polymers used as luminescent polymers include40,47,128 poly(2butyl-5-(2'-ethylhexyl)-1,4-phenylenevinylene) BuEH-PPV, poly(2,5-bis(cholestanoxy)1,4-phenylenevinylene) BCHA-PPV, poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene) MEH-PPV, poly(2,5-bis((2'-ethylhexyl)oxy)-1,4-phenylenevinylene) BEHPPV, poly(2-methoxy-5-(3’-octyloxy)-1,4-phenylenevinylene) M3O-PPV, poly(9-hexyl9-(2’-ethylhexyl)fluorene-2,7-diyl) MEH-PF, poly(9,9-bis(3,6-dioxaoctyl)fluorene-2,7diyl) BDOO-PF, poly(2-((6’-cyano-6’-methylheptyl)oxy)-1,4-phenylene) CN-PPP, and

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Optimizing Polymeric Structures in Organic Optoelectronics

(4-(dicyanomethylene)-2-methylene)-2-methyl-6-(4-(dimethylamino)styryl)-4H-pyran DCM/PS.6,40,61-62 Thin films of poly(2,5-bis(cholestanoxy)-1,4-phenylenevinylene), poly(2methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene, and poly(2,5-bis((2'-ethylhexyl)oxy)-1,4-phenylenevinylene) have been selected as optimized conjugated polymers used as luminescent polymers because they show maximum absorption at 550 nm and maximum luminescence at 600 nm wavelength. Poly(2-butyl-5-(2'-ethylhexyl)-1,4phenylenevinylene) polymer and poly(2-butyl-5-(2'-ethylhexyl)-1,4-phenylenevinylene)/ poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene copolymer have been optimized for the same purpose because they can show maximum absorption at about 375 nm and maximum luminescence at 550 nm wavelength. Also, poly(9-hexyl-9-(2’-ethylhexyl)fluorene-2,7-diyl), poly(9,9-bis(3,6-dioxaoctyl)fluorene-2,7-diyl), and poly(2-((6’cyano-6’-methylheptyl)oxy)-1,4-phenylene) have been optimized because they can show maximum absorption at about 325 nm and maximum luminescence at 400 nm.341 8.5.1.1 Organic optoelectronic light-emitting diodes The organic optoelectronic light-emitting diodes are doped organic optoelectronic systems with a general polymeric structure shown in Figure 8.17.141,278 To optimize polymeric structures of organic optoelectronic light-emitting diodes (as light emitters), they must include at least one undoped organic layer between two electrodes. Three important layers can be utilized for this purpose: electron transport layers, hole transport layers, and emission layers. The optimized optical polymers can be used for structuring electron transport layers as n-type polymers, including tris(8-hydroxyquinoline) aluminum Alq3, 2-tetrabutyl phenyl-5-biphenyl-1,3,4-oxadiazole PBD, and m-methyl-tris(diphenylamine)triphenylamine MTDATA. The optimized optical polymers that can be used for structuring hole transport layers as p-type materials include N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1biphenyl-4,4-diamine TPd, and 1,4-bis(1-naphthylphenylamino)biphenyl NPB. The optimized emitting materials/polymers or emission layers can be structured from poly(dialkyl fluorene) or poly(phenylene vinylene) of hexyl, ethylhexyl, or trimethyldodecyl radicals. Poly(styrene sulfonate) doped polyaniline PANI:PSS can be used as an alternative for poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate acid) PEDOT:PSS to be deposited on a glass substrate as conducting polymer. The transparent electrode can be structured from indium tin oxide of the bandgap of 4.5-5.1 eV.63,141,244 To optimize functions of organic optoelectronic light-emitting diodes, the following factors should be considered sufficient brightness, efficiency, and low driving voltages. Efficiency has been discussed elsewhere in this book. For optoelectronic light-emitting diodes, low voltage of 10 V or less is sufficient, because the higher voltage will be a damaging level of current. In the SI system, the unit of brightness is candela/area cd/m2. The light-emitting polymers should have high brightness, efficiency, and compatibility. For this reasons, the structure of high-brightness optoelectronic light-emitting diodes must consider the following factors, including thermal conductive substrate, use of micro-cavity structure for improving performance of a device, adopting both charge carriers and excitons for avoiding the heat dissipation issue, reducing light-emitting area of a device

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for improving the heat dissipation, and adopting pulse voltage to be an alternative for realizing high-performance optoelectronic light-emitting diodes.51,56,247-,249

Figure 8.17. The optoelectronic light-emitting diode as a light emitter for optoelectronic devices.141,278

To optimize polymeric structures for high-brightness, high-performance, and longlived optoelectronic light-emitting diodes, the following light-emitting polymers can be selected as the optimized optical polymers/copolymers, including polyfluorenes PFs, poly(phenylene vinylene) PPV, and poly(p-phenylene) PPP. Polyfluorene-based lightemitting polymers such as 9,9-disubstituted 2,7-bis-1,3,2-dioxaborolanylfluorene have been selected as the first optimized family because it has high molecular weight, low polydispersity, and light emission of up to 10000 cd/m2 (at less than 6 V) with peak efficiency of 22 lm/W. Polyfluorene homopolymers such as polyfluorene substituted coumarin-6 (C6), coumarin-7 (C7), coumarin-8 (C8), and coumarin-9 (C9) can be used because they can act as strong blue emitters upon excitation with ultraviolet either in solution or in solid-state. Copolymers of polyfluorene of tertiary aromatic amines are suitable for the hole transport layer if they have high molecular weight. These copolymers include poly((2,7-(9,9-dialkylfluorene))-alt-(5,5-(4',7'-di-2'-thienyl-2,1,3-benzothiadiazole))) APFO and various aromatic amine conducting monomers (Ar).165,342 These copolymers are blue emitters, excellent for forming films with high hole mobility (3x10-4-1x10-3 cm2/Vs) (for these reasons, they are used in hole transport layer). Conjugated monomers optimized for such structures include thiophene due to its emission of bluish-green light, bithiophene due to its emission of yellow light, cyanostilbene due to its emission of green light, etc. Moreover, these monomers are highly photoluminescent. Poly(phenylene vinylene) and its derivatives have been selected as optimized optical polymers for improved and high efficiency optoelectronic light-emitting diodes due to their capability of emitting light (yellow-green) under electrical stimulation. The optimized

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Optimizing Polymeric Structures in Organic Optoelectronics

organic optoelectronic light-emitting diodes have a maximum brightness of 2200 cd/m2 and luminous efficacy of 11 cd/A.343 The optimized techniques can be selected to increase the brightness of optoelectronic light-emitting diodes of polymeric structures, including15,130,141,165,246 1. acid oxidized multiwall carbon nanotubes 2. structuring organic light-emitting diodes from elastomeric blends 3. white luminance organic light-emitting diodes, small molecules 4. hybrid organic-inorganic light emitters 5. organic phosphorescent light-emitting diodes 6. active-matrix organic light-emitting diodes 7. bipolar polymer organic light-emitting diodes. Acid oxidized multiwall carbon nanotubes act as a hole injection buffer layer. The addition of an acid oxidized multiwall carbon nanotubes buffer layer has enabled a further increase in the brightness of these diodes operating at 20,000 cd/m2 due to the enhanced hole injection by several orders of magnitude. It is useful to use triphenyldiamine polymer for forming transport layers due to its ability to act as a hole injector. Tris(8-hydroxyquinoline) aluminum polymer is used for relatively high electron mobility and its favorable energy level positions at the cathode. A carbon nanotube acts as a cathode buffer layer to achieve the maximum luminance of the reference device.250 Elastomeric blends as a class of optical compounds, such as ethoxylated trimethylolpropanetriacrylate, poly(ethylene oxide), and lithium trifluoromethane sulfonate of super yellow emitting light, have been selected for high brightness organic light-emitting diodes. In their application, it is advantageous to use indium tin oxide ITO as a transparent conductive electrode (spin-coated with a thin layer of poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) PEDOT:PSS, while the cathode is a composite of silver nanowire-polyurethane acrylate of 15 Ω/sq resistance. In such a structure, the system can be turned on at 6.8 V and reach its peak brightness of 2200 cd/m2 at 21 V. The achieved luminous efficacy is about 5.7 cd/A at the maximum brightness. The driving voltage at 10, 120, and 320 cd/m2 brightness is about 9, 14, and 16 V, respectively. Organic light-emitting diodes from matrix consisting poly(vinyl carbazole) PVK, 1,1,4,4tetraphenyl-1,3-butadiene TPBn, and 5,6,11,12-tetraphenyl anthracene TPLT (also called rubrene) exhibit white luminance of 4940 cd/m2 at 17 V.141 Several types of optical small molecules and polymers have been optimized as a technique related to optoelectronic light-emitting diodes due to their ability to improve required color stability and balanced charge transport properties, especially to reduce the global energy consumption of white organic light-emitting diodes. Achieving white lights from organic light-emitting diodes is a considerable challenge. The required development, in this case, is to use two or more emissive small conducting molecules to emit simultaneously colors that are together perceived as white in the form of multiple emission layers among the eight-layer stacked system. This system consists mainly of two emissive blue layers, a complementary yellow emissive layer to produce a two-color white, and multiple charge-transport/blocking layers that are capable of suppressing the change in electroluminescent light spectrum with drive current density (Figure 8.18130). The 3.9% external quantum efficiency (9.9 cd/A1, 2.7 lmW1) with commission international de l'Eclairage coordinates method to define colors, the coordinates of (0.31,0.41) can be achieved.15

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Figure 8.18. Emissive small conducting molecules used for structuring multiple white organic light-emitting diodes. [Data from Henry Tianhang Zheng, High Performance Organic thin-film Semiconductor Devices: Light Emission Properties and Resonant Tunneling Behaviors, PhD Thesis, University of Hong Kong 2009.]

The optimized optical polymers can be selected to act as emissive small conducting molecules (Figure 8.18), including yellow: 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)6,12-diphenyl tetracene TBRb, and blue p-bis(p-N,N-diphenyl-amino-styryl)benzene DAS-Ph. 2,8-Di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyl tetracene as a derivative of rubrenes RUN has been selected as the first optimized derivative of emissive small conducting molecules for structuring optical systems because it is originally considered as an important yellow fluorescent dye. Characteristics of rubrene include stable, desirable, more sensitive colors to human eyes and generally preferred by most users. It resists concentration quenching, improves the efficiency of organic light-emitting diodes, and enhances the device stability and lifetime. 2,8-Di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyl tetracene can be fabricated in the form of thick and thin films for photoluminescent devices due to its absorption spectrum and suitable excitation energy. It is used as a good dopant to achieve the Forster energy transfer and to assist light emission. The optical properties of 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyl tetracene are similar to those of rubrene. Lt-N732®311 and LT-E707®311 are examples of commercial rubrene.318,344 p-Bis(p-N,N-diphenyl-amino-styryl)benzene is optimized as the second derivative because it can act as a dopant polymer of 2-methyl-9,10-di(2-naphthyl) anthracene and can be used as the fluorescent polymer in white organic light-emitting diodes. Monochromatic red, green, and blue emission layers are used successfully for structuring highly efficient fluorescent white organic light-emitting diodes. The above red and blue emissive layers are based on 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7tetramethyljulolidin-4-yl-vinyl)-4H-pyran DCJTB doped with N,N'-di(naphthalene-1yl)N,N'-diphenylbenzidine and 2-methyl-9,10-di(2-naphthyl) anthracene, respectively. The green emissive layer is based on tris(8-hydroxyquinoline) aluminum doped with 10(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,1(H-(1)-benzopyropyrano[6,7-8-i,j] quinolizin-11)-one which is sandwiched between the red and the blue emis-

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Optimizing Polymeric Structures in Organic Optoelectronics

sive layers. For such structure, the maximum power efficiency, current efficiency, and quantum efficiency may come up to 15.9 lm/W, 20.8 cd/A, and 8.4%, respectively. The power efficiency at the brightness of 500 cd/m2 can be 7.9 lm/W, and the half-lifetime under the initial luminance of 500 cd/m2 is over 3500 h.34,218,345 Hybrid organic-inorganic light emitters have been considered as the fourth optimized technique used for increasing the brightness of optoelectronic light-emitting diodes because they are structured form micro-light-emitting diodes, from UV emitting microarrays, and specifically tailored quantum wells with conjugated polymers to access the entire visible spectrum (of transfer efficiency of 43% at 15 K with a 1/R2 dependence on distance between quantum well and polymer layer). Aluminum indium gallium nitride can be selected as inorganic material. The following polymers can be chosen as organic materials poly(9,9-dioctylfluorene-co-9,9-di(4-methoxy)phenylfluorene) (both blue and green emitter), and Sumitomo copolymer (red emitter). An objective of such a hybrid structure is to make use of a blend, which can support incomplete non-radiative resonant energy transfer, thus allowing all three colors to be emitted.246 Organic phosphorescent light-emitting diodes have been considered as the fifth optimized technique used for increasing the brightness of highly efficient organic phosphorescent (blue) light-emitting diodes structured from electrophosphorescent conjugated polymers as hosts. Such structures are based on cyclometalated iridium complex. The used electro-phosphorescent conjugated polymer hosts are of high triplet energy level, such as N,N'-dicarbazolyl-4,4'-biphenyl, N,N'-dicarbazolyl-3,5-benzene, and 3,5-bis(9carbazolyl) tetra-phenylsilane. Poly(vinyl carbazole) PVK has been used as a wide-gap host. Iridium(III)bis((4,6-difluorophenyl)-pyridinato-N,C2')picolinate can be doped into a poly(vinyl carbazole) host. For high efficiency, it is better to use conjugated polymers of wide-gap such as poly(9,9'-alkyl-3,6-silafluorene) PSiFC6C6 as host doped with a novel blue iridium complex called iridium(III) bis(2,4-difluorophenyl-2-pyridine) (2-(4H-1,2,4triazol-3-yl)pyridine (abbreviated as Ir(PPF)2(PZ). Light-emitting iridium complex with triazolate-based ancillary ligand (iridium bis(4,6-difluorophenylpyridinato)-3-(trifluoromethyl)-5-(pyridin-2-yl)-1,2,4-triazolate), and iridium bis(4,6-difluorophenylpyridinato)5-(pyridin-2-yl)-1H-tetrazolate are used as well. An advantage of the electroluminescent devices with Ir(PPF)2(PZ) doped into PSiFC6C6 is their ability to show efficient energy transfer from PSiFC6C6 host to the iridium complexes. The electroluminescence efficiency of these devices is much higher than those structured from poly(vinyl carbazole)s.142 Active matrices of organic light-emitting diodes have been considered as the sixth optimized technique used for increasing brightness of optoelectronic light-emitting diodes because their structures can act as panel display of organic light-emitting diodes controlled by transistor circuitry, and act as the small screen. Characteristics of such organic lightemitting diode-displays include lighter and wider viewing angles, exceptional color reduction, and outstanding contrast levels and higher brightness. The polymeric structures of active matrices organic light-emitting diode panel displays depend on small molecule organic semiconductors that have been used for thin-film electroluminescence devices such as tris(8-hydroxyquinoline) aluminum (as emission layer but also as the hole trans-

8.5 Organic optoelectronic systems

453

port layer). Thienopyrrole-dione can be used as hole transport layer, while 2-tetra-butylphenyl-5-biphenyl-1,3,4-oxadiazole as an emission layer. Active matrix organic lightemitting diodes depend on electroluminescent polymers, such as poly(p-phenylene vinylene) (green), the (orange-red) solution-processable conjugated polymers MEH-PPV, poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) OC1C10-PPV, and poly(3-hexyl thiophene). The reactive indices of polymeric layers of this structure are 1.83 at 460 nm, 1,76 at 5320 nm, and 1,72 at 625 nm. Luminance may reach up to 150 cd/ m2.51,346 The brightness of organic light-emitting diodes has been effectively improved by the use of bipolar polymers or conjugated polymers containing bipolar groups. The bipolar organic polymer is able to stabilize and transport both positive and negative charge carriers. In addition to poly(N-2-propynylpheno-thiazene) PNPPTZ, poly(N-propynyl)phenenothiazine PPPTZ is considered an optimized derivative of the conjugated polyacetylene family because it has two-charge transport centers (the conjugated main chain and the side chain chromophore). Important to know is that efficient bipolar charge transport has been revealed in aromatic polyimide, which can be prepared from 9,10-bis(m-aminophenylthio)-anthracene and 1,3-bis(3,4-dicarboxyphenoxy)benzene or 2,2-bis(4-(3,4-dicarboxyphenoxy)phenyl)propane dianhydrides using a method analogous to the chemical imidization method. The direct measurements of charge carrier mobilities in the aromatic polyimides at the ambient temperature and an electric field of 4x105 V.cm-1 gave practically the same values of 5x10-6 cm2V-1s-1 for electrons and holes. High performance organic light-emitting diodes (with Al electrodes) can be structured from organic bipolar polymer called poly(2-(4-(5-(4-(3,7-dimethyloctyloxy)phenyl)-1,3,4-oxadiazole-2yl)phenyloxy)-1,4-phenylenevinylene) Oxa-PPV, which can be prepared by introducing electron-deficient 1,3,4-oxadiazole OXD pendant units into poly(phenylene vinylene) PPV as a side chain (luminance efficiency of ~20 cd/A).16,236-237,239,347-348 8.5.1.2 Organic/polymeric lasers The laser is an abbreviation of Light Amplification by Stimulated Emission of Radiation. It can be defined as a long narrow beam of photons emitted from specially made diodes called laser diodes (by which electrical energy is converted into light energy). The organic/polymeric lasers include organic semiconductors influenced by the rapid development of organic light-emitting diodes. Organic lasers are based on π-conjugated molecules or polymers.233,236,241 The optimized members of such polymers include233,241,349-350 liquid dye lasers based on solutions of π-conjugated highly luminescent (dye) molecules, dye-doped lasers, solid-state organic lasers, thin-film based organic semiconductor lasers, diodes-based organic lasers, and optically-pumped organic lasers. The optimized optical polymers for forming organic/polymeric lasers include tris(8-hydroxyquinoline) aluminum Alq3, 2tetra-butylphenyl-5-biphenyl-1,3,4-oxadiazole PBD, 2-naphthyl-4,5-bis(4-methoxyphenyl)-1,3-oxazole NAPOXA, α-sexithiophene α-6T, (4-(dicanomethylene)-2-methyl-6-(4dimethylaminostyryl)-4H-pyran DCM, poly(phenylene vinylene) PPV, polyfluorenes PFs, poly-2,7-(9,9'-dioctyl-9H-fluorene)-alt-4,7-benzo[c][1,2,5]-thiadiazole F8BT, and poly(2methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene MEH-PPV.

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Optimizing Polymeric Structures in Organic Optoelectronics

Tris(8-hydroxyquinoline), 2-tetra-butylphenyl-5-biphenyl-1,3,4-oxadiazole, and 2naphthyl-4,5-bis(4-methoxyphenyl)-1,3-oxazole have been selected as the first three optimized optical polymers for organic/polymeric lasers because they can act as charge transporting hosts. 2-Naphthyl-4,5-bis(4-methoxyphenyl)-1,3-oxazole doped with (2-methyl6-(2-(2,3,6,7-tetrahydro-1H,5H-benjo[ij]quinolizine-9-yl)ethenyl)-4H-pyran-4-ylidenepropan-dinitrile DCMII is used as gain polymer for structuring ridge waveguide distributed Bragg laser. For this structure, 2-naphthyl-4,5-bis(4-methoxyphenyl)-1,3-oxazole host doped with (2-methyl-6-(2-(2,3,6,7-tetrahydro-1H,5H-benjo[ij]quinolizine-9-yl)ethenyl)-4H-pyran-4-ylidene-)propan-dinitrile is used as micro-disk laser. The emission spectrum of this micro-disk laser is of 3 µm diameter. This structure is based on distributed Bragg refractor lasers. In addition to that, the ultraviolet laser (λ ~ 392 nm) uses 2-tetrabutylphenyl-5-biphenyl-1,3,4-oxadiazole as the gain medium α-sexithiophene and (4(dicanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran have been selected as the fourth and fifth optimized members for the same applications because they can act as dopants (laser dyes).51 Electroactive polymers having attractive electroluminescent properties can be employed in the development of polymeric lasers because electroactive polymers are able to amplify light and couple to a resonator. In electroactive polymers, the light amplification is achieved through repeated stimulations (with luminescence) induced in the active polymer by a light beam, alternately reflected backward and forwards by the resonator. The optimized electroactive polymers used in structuring polymeric lasers systems include polythiophene doped with poly(ethylene dioxythiophene), poly(styrene sulfonate), poly(alkylene terephthalate), poly(3-hexyl thiophene), pentacene, polyaniline, polyfluorenes, polydialkylfluorene, and poly(dialkoxy-p-phenylene vinylene).51,97,236 8.5.2 OPTICAL POLYMERS FOR ORGANIC ELECTROLUMINESCENT SYSTEMS The electroluminescence can be considered as the base of light optoelectronics. Organic electroluminescent system-based organic light-emitting diodes are systems able to emit light in response to electric input. These organic light-emitting diodes are based on forming electron-hole-pairs within conjugated polymers such as poly(phenylene vinylene) PPV or its derivatives and commercial-grades such as Superyellow®312 to act as emission layers with semiconducting properties. As shown in Figure 8.19,231,297 optimizing the function of electroluminescence (or organic electroluminescence) requires placing conjugated polymers between the transparent anodes and metallic cathodes. The function of both transparent anodes and metallic cathodes is to inject electrons and holes, respectively, into the conjugated polymer placed between them during application of electric voltage. The charge carriers drift along the electric field towards each other and recombine via fluorescence. Thus, electroluminescence leads to highly efficient illuminants operated with low driving voltages.97,231-232 The electroluminescent polymers emit light upon the incidence of an electric field. Optimized polymers include poly(phenylene vinylene) as an active material for organic light-emitting diodes, and a conjugated copolymer of benzene and pyridine units such as copoly-(1,4-phenylenevinylene-2,6 pyridylenevinylene) used for micro-fabrication of organic light-emitting diodes, which are developed for flat electroluminescent dis-

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455

plays).351 The optimized electroluminescent polymers used in organic light-emitting diodes include the following groups77 1. substrates (polymethylmethacrylate, polycarbonate, and spatial silicate glasses) 2. electrodes (cathode and anode) (indium tin oxide ITO, aluminum Al, and polymers) 3. active layers (these layers include electron transport layers, hole transport layers, emission layers, dopants, and hole injection layers).

Figure 8.19. Electroluminescent device-based organic light-emitting diodes.231,297

The members of the first group have good transmission, high electric resistance, and are stable against H2O and O2. The members of the second group have good electric conductivity, good adhesion to the insulators, are stable up to 108 V/m, and have good transmission. Indium tin oxide is a very good electrical conductor and very transparent material (consists of 90% In2O3 and 10% SnO2).55,229 Three groups of polymers have been selected for application, including51,55,231-232 1. electron transport layers (tris(8-hydroxyquinoline) aluminum Alq3, the derivatives of polyoxadiazole PODA, such as 2-(biphenylyl)-5-phenyl-1,3,4-oxadiazole BpBO, 2tetra-butyl phenyl-5-biphenyl-1,3,4-oxadiazole PBD, and 2-(4-tert-butylphenyl)-5(4-biphenyl)-1,3,4-oxadiazole PBDO) 2. hole transport layers (thienopyrrole-dione TPD, and N,N'-bis(naphthalen-1-yl)-N,N'bis(phenyl)-2,2'-dimethylbenzidine α-NPD) 3. emission layers (tris(dibenzoylmethane)mono(1,10-phenanthroline)europium(lll) (Eu(DBM)3(Phen)), fac-tris(2-phenylpyridine) iridium(III) Ir(ppy)3, poly(phenylene vinylene) PPV, poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene MEHPPV, poly(vinyl carbazole) PVK, and poly(n-nonyl4'-(5-phenyl-1,3,4-oxadiazol-2yl)-(1,1'-biphenyl)-4-carboxylate acrylate) PVOXD). Thienopyrrole-dione has sharp emission peaks at 410-420 nm, purple, blue. Tris(8hydroxyquinoline) aluminum is a yellowish oily liquid luminescent dye related to aminoisoquinoline. Excellent luminescent materials and good electron transmission materials formed from quinoline derivatives are (tris(4-methyl-8-quinolinolato)aluminum(III)) Almq3. Tris(dibenzoylmethane)mono(1,10-phenanthroline)europium(lll) has 460 cd/m2

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Optimizing Polymeric Structures in Organic Optoelectronics

and 614 nm at 16 V. Poly(phenylene vinylene), poly(2-methoxy,5-(2'-ethyl-hexyloxy)-pphenylenevinylene, poly(vinyl carbazole), and poly(n-nonyl-4'-(5-phenyl-1,3,4-oxadiazol-2-yl)-(1,1'-biphenyl)-4-carboxylate acrylate) polymers can be used for forming emitting layers. The durable and heat resistant pigment copper-phthalocyanine CuPc and mmethyl-tris(diphenylamine)triphenyl amine) MTDATA can be used as a hole injection layer. Parylene and rubrene can be used as dopants.214 In addition to the above polymers/ materials groups, several types of polymeric complexes have the electroluminescent properties and can be used in organic light-emitting diodes (display technology), such as141,150,215,218 1. family of diamine ruthenium(II) complexes such as ruthenium(III)-2,2'-bipyridine Ru(bpy)3 2. family of phosphorescent iridium(III) complexes such as fac-tris(2-phenylpyridine)iridium(III) Ir(bpy)3 3. family of near-infrared emitting lanthanide complexes such as lanthanide mono-porphyrinate complex Ln(TPP)L. These families have been optimized for structuring electroluminescent device-based organic light-emitting diodes due to high emission and high efficiency of ~25%. Note: the mechanism of operation of solid-state electroluminescent devices structured from Ru(bpy)3 complex differs from that of traditional organic light-emitting diodes and is similar to that in electrochemical cells. Phosphorescent iridium(III) complexes are important for structuring organic lightemitting diodes, for example, organic light-emitting diodes structured from fac-tris(2phenylpyridine)iridium(III) use N,N'-diphenyl-N,N'-bis(m-tolyl)-1,1'-biphenyl-4,4'diamine and m-methyl-tris(diphenyl amine)triphenyl amine) as hole transport layer, while tris(8-hydroxyquinoline) aluminum use 2-tetra-butyl phenyl-5-biphenyl-1,3,4-oxadiazole as electron transport layer. These structures increase the efficiencies of the corresponding light-emitting devices. Near-infrared emitting lanthanide complex (Ln(TPP)L) has good photoluminescent and electroluminescent properties. This complex consists of 1. Lanthanide (such as erbium Er3+, neodymium Nd3+, paseodymium Pr3+, thulium Tm3+, and ytterbium Yb3+) coordinated with 5,10,15,20-tetraphenylporphyrin 1. capping ligands such as acetylacetonate, trispyrazoylborate, or (cyclopentadienyl) tris(diethoxyphosphito-P)cobaltate) anions.141,150,215,218 8.5.3 ORGANIC PHOTONICS Photonics is a field of science and technology interested in transport and manipulation of light. It includes generation, emission, transmission, modulation, signal processing, switching, amplification, and detection/sensing of light. Applications of photonics include Bragg mirrors, switches, filters, super-prisms, waveguides, and optical resonators. Polymers that can be optimized for structuring photonics are those of inexpensive grades and can be functionalized to achieve the required optical, electronic, or mechanical properties, in addition, to being compatible with various patterning methods. These polymers (called photonic polymers), characterized by electroluminescence, photoluminescence, or nonlinear optical properties, can act as matrices for optically active species (for dyes, liquid crystals, semiconductor quantum dots, or metal nanoparticles), and can coherently scatter light (because they possess topographic and/or compositional patterns). Conjugated polymers such as poly(phenylene vinylene), poly(p-phenylene), and polyfluorenes derivatives, rep-

8.5 Organic optoelectronic systems

457

resent the optimized photonic polymers. Poly(phenylene vinylene) derivatives include poly(2,5-bis((2'-ethylhexyl)oxy)-1,4-phenylenevinylene) BEH-PPV, poly(2,5-bis(cholestanoxy)-1,4-phenylenevinylene) BCHA-PPV, poly(2-methoxy,5-(2'-ethyl-hexyloxy)-pphenylenevinylene MEH-PPV, and poly(2-butyl-5-(2'-ethylhexyl)-1,4-phenylenevinylene) BuEH-PPV. Poly(p-phenylene) derivatives include poly(2-(6'-methylheptyloxy)1,4phenylene) CN-PPP. Polyfluorene derivatives include poly(9-hexyl-9-2'-ethylhexyl)fluorene-2,7-diyl) HEH-PF, and poly(9,9-bis(3,6-dioxaoctyl)fluorene-2,7-diyl) BDOO-PF. Copolymers of these derivatives include poly(2-butyl-5-(2'-ethylhexyl)-1,4-phenylenevinylene)/poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene copolymer BuEHPPV/MEH-PPV.6,40,47,56 More types of optical and photonic polymers can be optimized for structuring photonic systems such as those having excellent light transmission, being able to be molded into spherical, aspheric, and non-rotationally symmetric shapes, having high transparency, and good chemical resistance. These additional optimized polymers include polymethylmethacrylate PMMA, polystyrene PS, polycarbonate PC, cyclic olefin copolymer COC, and allyl diglycol carbonate ADCt. The optical properties (and other properties) of these polymers are listed in Tables (8.13-8.16).299 These polymers have light transmittance values >88% (380-1000 nm). Note: both polymethylmethacrylate and cyclic olefin copolymer have a transmittance of 92% (better than the previous group). The high refractive index of polycarbonate and polystyrene limits their transmittance.6,19,40,47 Table 8.13. Optical (and general) properties of some photonic polymers.299 Properties

Unit

Photonic polymers PMMA

PS

PC

COC

ADC

Optical properties Light transmission (3 mm thick. 400-700 nm)

%

92

87-90

85-91

92

89-91

Refractive index

−

1.491

1.589

1.586

1.533

1.50

Abbe number

−

57.4

31

30-30.3

58

58

Haze

%