Semiconducting Polymer Composites

Edited by Xiaoniu Yang

Principles, Morphologies, Properties and Applications

www.wiley-vch.de

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The first part of Semiconducting Polymer Composites describes the prin-ciples and concepts of semiconducting polymer composites in general, addressing electrical conductivity, energy alignment at interfaces, mor-phology, energy transfer, percolation theory and processing techniques. In later chapters, different types of polymer composites are discussed: mixtures of semiconducting and insulating or semiconducting and semiconducting components, respectively. These composites are sui-table for a variety of applications that are presented in detail, including transistors and solar cells, sensors and detectors, diodes and lasers as well as anti-corrosive and anti-static surface coatings.

From the contents:

● Solubility, Miscibility, and the Impact on Solid-State Morphology● Nanoscale Morphological Characterization for Semiconductive Polymer Blends● Energy Level Alignment at Semiconductive Polymer Interfaces: Correlating Electronic Energy Levels and Electrical Conductivity● Energy and Charge Transfer● Percolation Theory and its Application in Electrically Conducting Materials● Processing Technologies of Semiconducting Polymer Composite Thin Films for Photovoltaic Cell Applications● Thin-Film Transistors Based on Polythiophene/Insulating Polymer- Composites with Enhanced Charge Transport● Semiconducting Organic Molecule/Polymer Composites for Thin- Film Transistors● Enhanced Electrical Conductivity of Polythiophene/Insulating Polymer Composite and its Morphological Requirement● Intrinsically Conducting Polymers and Their Composites for Anticorrosion and Antistatic Applications● Conjugated–Insulating Block Copolymers: Synthesis, Morphology, and Electronic Properties● Fullerene/Conjugated Polymer Composites for the State-of-the-Art Polymer Solar Cells● Semiconducting Nanocrystal/Conjugated Polymer Composites for Applications in Hybrid Polymer Solar Cells● Conjugated Polymer Blends: Toward All-Polymer Solar Cells● Conjugated Polymer Composites and Copolymers for Light-Emitting Diodes and Laser● Semiconducting Polymer Composite Based Bipolar Transistors● Nanostructured Conducting Polymers for Sensor Development

ISBN 978-3-527-33030-0

Xiaoniu Yang is presently an assistant director of the Chang-chun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences (CAS). He is also the director of the Polymer Composites Engineer-ing Laboratory of CAS. Having obtained his acade-mic degrees from CIAC, he spent a few years in Germany and the Netherlands before taking up his present appoint-ment at CIAC. Professor Yang has authored more than 80 scientific publications, filed more than 20 patents and received the National Science Foundation Award for Distin-guished Young Scholars of China in 2009. He is also a committee member of the Applied Chemistry Division of the Chinese Chemical Society.

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Edited by

Xiaoniu Yang

Semiconducting PolymerComposites

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Edited by Xiaoniu Yang

Semiconducting Polymer Composites

Principles, Morphologies, Properties and Applications

The Editor

Prof. Xiaoniu YangChinese Academy of SciencesPolymer Composite EngineeringChangchung 130022China

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Contents

List of Contributors XVPreface XXI

1 Solubility, Miscibility, and the Impact on Solid-State Morphology 1Florian Machui and Christoph J. Brabec

1.1 Introduction 11.2 General Aspects 21.2.1 Solubility 31.2.2 Miscibility–Thermodynamic Relationships 51.3 Solubility, Solvents, and Solution Formulations 61.3.1 Solubility 61.3.2 Solvents 91.3.2.1 Impact of Different Solvents on the Solid-State

Morphology 101.3.2.2 Non-Halogenic Solvents 141.3.2.3 Solvent Blends 151.3.2.4 Addition of Poor Solvents 161.3.2.5 Processing Additives 181.3.2.6 Solution Concentration 211.3.3 Conclusive Outlook 211.4 Miscibility 221.4.1 Methods 221.4.1.1 Glass Transition 231.4.1.2 Surface Energy 231.4.1.3 Photoluminescence Quenching 241.4.2 Polymer–Polymer Miscibility 261.4.3 Polymer–Fullerene Miscibility 281.4.4 Phase Diagrams 301.5 Conclusions 32

References 34

jV

2 Nanoscale Morphological Characterization for SemiconductivePolymer Blends 39Joachim Loos

2.1 Introduction 392.2 The Importance of Morphology Control 402.3 The Classic Blend: MDMO-PPV/PCBM as a Model for an

Amorphous Donor System 422.4 Intermezzo: Morphology Imaging with Scanning Transmission

Electron Microscopy 482.5 Volume Characterization of the Photoactive Layer: Electron

Tomography 502.6 Measuring Nanoscale Electrical Properties: Conductive AFM 562.7 Current Progress and Outlook 60

References 62

3 Energy Level Alignment at Semiconductive Polymer Interfaces:Correlating Electronic Energy Levels and Electrical Conductivity 65Nobuo Ueno

3.1 Introduction 653.2 General View of Electronic Structure of Organic Solids 653.2.1 Introduction to Correlating Electronic Structure and Electrical

Conductivity 653.2.2 Evolution of Electronic Structure from Single Molecule

to Molecular Solid 673.2.3 Evolution of Electronic Structure from Single Atom to

Polymer Chain 703.2.4 Polaron 723.2.5 Energy Level Alignment at the Interface 733.3 Experimental Methods 753.3.1 Ultraviolet Photoelectron Spectroscopy 753.3.2 Penning Ionization Electron Spectroscopy 783.4 Valence Electronic Structure of Organic Semiconductors:

Small Molecules 793.4.1 Energy Band Dispersion and Band Transport Mobility 793.4.2 Electron–Phonon Coupling and Hopping Mobility 843.4.2.1 Fundamental Aspects on Charge Hopping 843.4.2.2 Reorganization Energy and Small Polaron Binding

Energy 863.5 Valence Electronic Structure of Polymers 903.5.1 Quasi-One-Dimensional Band Dispersion Along

Polymer Chains 903.5.1.1 s-Bond Polymer 903.5.1.2 p-Conjugated Polymer Chain 913.5.2 Pendant Group Polymers: Is the Surface of Solution-Cast Film

Clean on Molecular/Atomic Scale? 92

VIj Contents

3.5.3 P3HT: Electronic Structure and Control of p-Electron DensityDistribution at the Surface for Realizing a Functional Interface 93

3.6 Role of the Interface Dipole Layer: Its Impact on the EnergyLevel Alignment 101

3.7 Future Prospects 103References 103

4 Energy and Charge Transfer 107Ralf Mauer, Ian A. Howard, and Fr�ed�eric Laquai

4.1 Introduction 1074.2 Energy Transfer 1084.2.1 Electronic Structure and Excited States of Conjugated

Polymers 1084.2.1.1 Excitons: The Nature of Excited States in Conjugated Polymers 1084.2.2 Excited State Dynamics in Conjugated Polymers 1134.2.2.1 Role of Disorder in Energy Transfer 1134.2.2.2 Singlet Exciton Energy Transfer 1144.2.2.3 Triplet Exciton Dynamics 1154.2.3 Energy Transfer: Relevance to Device Performance 1234.3 Charge Transfer in Polymer/Fullerene Composites 1254.3.1 Theoretical Background 1254.3.1.1 Theory of Charge Transfer 1254.3.1.2 Theory of Field and Temperature Dependence of Charge

Separation 1294.3.2 The Role of Charge Transfer States for Charge Separation 1314.3.2.1 Parameters Influencing the Separation of Charge Transfer

States 1334.3.3 Charge Transfer: Relevance to Device Performance 139

References 140

5 Percolation Theory and Its Application in ElectricallyConducting Materials 145Isaac Balberg

5.1 Introduction 1455.2 Lattice Percolation 1465.3 Continuum Percolation 1525.4 Percolation Behavior When the Interparticle Conduction Is by

Tunneling 1545.5 The Structure of Composite Materials 1565.6 The Observations and Interpretations of the s(x) Dependence in

Composite Materials 1595.6.1 The Percolation Threshold 1595.6.2 The Critical Behavior of s(x) 1615.7 Summary and Conclusions 165

References 168

Contents jVII

6 Processing Technologies of Semiconducting Polymer Composite ThinFilms for Photovoltaic Cell Applications 171Hui Joon Park and L. Jay Guo

6.1 Introduction 1716.2 Optimization of Bulk Heterojunction Composite

Nanostructures 1736.3 Fabrication of Sub-20 nm Scale Semiconducting Polymer

Nanostructure 1826.3.1 Nanoimprint Mold Fabrication 1836.4 Conclusions 186

References 187

7 Thin-Film Transistors Based on Polythiophene/Insulating PolymerComposites with Enhanced Charge Transport 191Longzhen Qiu, Xiaohong Wang, and Kilwon Cho

7.1 Introduction 1917.2 Fundamental Principle and Operating Mode of OTFTs 1937.3 Strategies for Preparing High-Performance OTFTs Based on

Semiconducting/Insulating Blends 1947.4 Blend Films with Vertical Stratified Structure 1947.4.1 Phase Behavior of Polymer Blends 1947.4.2 One-Step Formation of Semiconducting and Insulating

Layers in OTFTs 1987.4.3 Improved Environmental Stability 2017.4.4 Patterned Domains of Polymer Blends 2017.4.5 Improved Charge Carrier Mobility 2047.4.6 Crystallization-Induced Vertical Phase Segregation 2067.5 Blend Films with Embedded P3HT Nanowires 2077.5.1 P3AT Nanowires 2087.5.2 Polymer Blends with Embedded P3HT Nanowires 2097.5.3 Nanowires from Conjugated Block Copolymers 2127.5.4 Electrospun Nanowires from Conjugated Polymer

Blends 2127.6 Conclusions and Outlook 214

References 214

8 Semiconducting Organic Molecule/Polymer Compositesfor Thin-Film Transistors 219Jeremy N. Smith, John G. Labram, and Thomas D. Anthopoulos

8.1 Introduction 2198.1.1 OFET Device Operation 2208.1.2 Small-Molecule/Polymer Film Morphology 2228.2 Unipolar Films for OFETs 2248.2.1 Oligothiophene/Polymer Blends 2248.2.2 Acene/Polymer Blends 227

VIIIj Contents

8.3 Polymer/Fullerene Ambipolar OFETs 2328.3.1 Polymer:Fullerene Blend Morphology 2338.3.1.1 Solvent and Polymer Molecular Weight 2358.3.1.2 Blend Composition 2368.3.1.3 Temperature- and Time-Dependent Annealing 2388.3.1.4 Effect of Fullerene Molecular Weight 2408.3.2 Polymer:Fullerene Bilayer Diffusion 2418.3.2.1 Modeling Fullerene Diffusion 2438.4 Conclusions 245

References 246

9 Enhanced Electrical Conductivity of Polythiophene/Insulating PolymerComposite and Its Morphological Requirement 251Guanghao Lu and Xiaoniu Yang

9.1 Introduction 2519.2 Phase Evolution and Morphology 2539.3 Enhanced Conductivity of Conjugated Polymer/Insulating

Polymer Composites at Low Doping Level: InterpenetratedThree-Dimensional Interfaces 258

9.4 Conductivity of Semiconducting Polymer/Insulating PolymerComposites Doped by Molecular Dopant 260

9.5 Mechanisms for the Enhanced Conductivity/Mobility 2619.5.1 Improved Crystallinity and Molecular Ordering 2619.5.2 “Self-Encapsulation” Effect 2629.5.3 Bulk 3-D Interface and Reduced Polarization of Matrix at

Interface 2639.5.4 “Zone Refinement” Effect 2649.5.5 Reduced Polaron–Dopant Interaction 2659.6 Perspective 267

References 268

10 Intrinsically Conducting Polymers and Their Compositesfor Anticorrosion and Antistatic Applications 269Yingping Li and Xianhong Wang

10.1 ICPs and Their Composites for Anticorrosion Application 26910.1.1 Introduction 26910.1.2 Protection Mechanism 27010.1.2.1 Anodic Protection Mechanism 27010.1.2.2 Inhibitory Protection Mechanism 27410.1.2.3 Cathodic Protection Mechanism 27410.1.2.4 Comprehensive Understanding on Protection

Mechanism of ICPs 27710.1.3 Matrix Resin of Conducting Composite Coating 27810.1.4 Processing Methods 27910.1.5 Conclusions and Perspectives 280

Contents jIX

10.2 Antistatic Coating 28210.2.1 Introduction 28210.2.2 Synthesis of Processable ICPs 28310.2.3 Processing of ICPs for Antistatic Application 28410.2.4 Water-Based Polyaniline and Its Complex 28610.3 Summary 288

References 289

11 Conjugated–Insulating Block Copolymers: Synthesis, Morphology,and Electronic Properties 299Dahlia Haynes, Mihaela C. Stefan, and Richard D. McCullough

11.1 Introduction 29911.2 Oligo- and Polythiophene Rod–Coil Block Copolymers 30011.3 Poly(p-phenylene vinylene) Block Copolymers 30811.4 Polyfluorenes 31311.5 Other Semiconducting Rod–Coil Systems 31911.6 Conjugated–Insulating Rod–Rod Block Copolymers 32011.7 Conclusions and Outlook 322

References 322

12 Fullerene/Conjugated Polymer Composite for theState-of-the-Art Polymer Solar Cells 331Wanli Ma

12.1 Introduction 33112.2 Working Mechanism 33212.2.1 Unique Properties of Organic Solar Cells 33212.2.2 Understanding the Bulk Heterojunction Structures 33212.2.3 Device Parameters and Theoretical Efficiency 33412.2.3.1 Short-Circuit Current Density 33512.2.3.2 Open-Circuit Voltage 33612.2.3.3 Fill Factor 33712.2.3.4 Theoretical Efficiency 33812.3 Optimization of Fullerene/Polymer Solar

Cells 33812.3.1 Design of New Materials 33912.3.1.1 Absorption Enhancement 34012.3.1.2 Fine-Tuning of HOMO and LUMO Energy

Levels 34412.3.1.3 Mobility and Solubility Improvement 34512.3.1.4 New Fullerene Derivative 34512.3.2 Optimization of Polymer Solar Cell Devices 34612.3.2.1 Morphology Control 34612.3.2.2 Device Architectures 35112.4 Outlook 354

References 355

Xj Contents

13 Semiconducting Nanocrystal/Conjugated Polymer Composites forApplications in Hybrid Polymer Solar Cells 361Michael Krueger, Michael Eck, Yunfei Zhou, and Frank-Stefan Riehle

13.1 Introduction 36113.2 Composite Materials 36113.2.1 Colloidal Semiconductor Nanocrystals 36113.2.2 Conjugated Polymers 36613.3 Device Structure 36713.3.1 Photoactive Layer 36713.3.2 Device Principle 37113.3.3 Band Alignment and Choice of Donor/Acceptor Pairs 37313.4 State of the Art of Hybrid Solar Cells 37413.5 Novel Approaches in Hybrid Solar Cell Development 38113.5.1 Utilization of Less Toxic Semiconductor NCs 38113.5.2 In Situ Synthesis of Ligand-Free Semiconductors in

Conjugated Polymers 38113.5.3 Utilization of One-Dimensional Structured Donor–Acceptor

Nanostructures for Hybrid Film Formation 38213.5.4 Toward Nanostructured Donor–Acceptor Phases 38413.6 Outlook and Perspectives 39013.6.1 Hybrid Solar Cells Versus Pure OPVs 39013.6.2 Hybrid PV and OPV Versus Other PV Technologies 391

References 393

14 Conjugated Polymer Blends: Toward All-Polymer Solar Cells 399Christopher R. McNeill

14.1 Introduction 39914.2 Review of Polymer Photophysics and Device Operation 40014.3 Material Considerations 40114.4 Device Achievements to Date 40314.5 Key Issues Affecting All-Polymer Solar Cells 40714.5.1 Interfacial Charge Separation 40714.5.2 Morphology 41114.5.3 Charge Transport 41814.6 Summary and Outlook 421

References 421

15 Conjugated Polymer Composites and Copolymersfor Light-Emitting Diodes and Laser 427Thien Phap Nguyen and Pascale Jolinat

15.1 Introduction 42715.2 Properties of Organic Semiconductors 42815.3 Polymer-Based Composites 42915.4 Use of Polymer Composites in Photonic Applications 43015.4.1 Organic Light-Emitting Diodes 430

Contents jXI

15.4.1.1 Efficiency of OLEDs 43215.4.1.2 Color Emission 43315.4.1.3 Stability 44015.4.2 Organic Semiconductor Lasers 44115.4.2.1 Working Principle 44115.4.2.2 Materials 44315.4.2.3 Types of Resonators 44615.4.2.4 Applications and Developments 44815.5 Conclusions 451

References 452

16 Semiconducting Polymer Composite Based Bipolar Transistors 457Claudia Piliego, Krisztina Szendrei, and Maria Antonietta Loi

16.1 Introduction 45716.2 Basics of Organic Field-Effect Transistors 45816.2.1 Operation Principles of FETs 45816.2.1.1 Unipolar FETs 45916.2.1.2 Bipolar FETs 46016.2.2 Current–Voltage Characteristics 46116.2.2.1 Unipolar FET 46116.2.2.2 Bipolar FETs 46216.2.3 Device Configurations 46216.2.4 Role of the Injecting Electrodes 46316.2.5 Applications: Inverters and Light-Emitting Transistors 46416.3 Bipolar Field-Effect Transistors 46516.3.1 Single-Component Bipolar FETs 46516.3.2 Bilayer Bipolar FETs 46916.3.3 Bulk Heterojunction Bipolar FETs 47416.3.3.1 Coevaporated Blends 47516.3.3.2 Polymer–Small Molecule Blends 47616.3.3.3 Hybrid Blends 47916.3.3.4 Polymer–Polymer Blends 48016.4 Perspectives 485

References 486

17 Nanostructured Conducting Polymers for Sensor Development 489Yen Wei, Meixiang Wan, Ten-Chin Wen, Tang-Kuei Chang, Gaoquan Shi,Hongxu Qi, Lei Tao, Ester Segal, and Moshe Narkis

17.1 Introduction 48917.2 Conducting Polymers and Their Nanostructures 49017.3 Synthetical Methods for Conducting Polymer Nanostructures 49317.3.1 Hard-Template Method 49417.3.2 Soft-Template Method 49617.3.3 Electrospinning Technology 49617.4 Typical Conducting Polymer Nanostructures 497

XIIj Contents

17.4.1 Polyaniline (PANI) 49717.4.2 Poly(3,4-ethylenedioxythiophene) (PEDOT) 50117.4.3 Polypyrrole (PPy) 50217.5 Multifunctionality of Conducting Polymer Nanostructures 50317.6 Conducting Polymer-Based Sensors 50517.6.1 Gas Sensors 50617.6.2 pH Sensors 50917.6.3 Biosensors 51017.6.4 Artificial Sensors 51117.7 Summary and Outlook 512

References 513

Index 523

Contents jXIII

List of Contributors

Thomas D. AnthopoulosImperial College LondonDepartment of PhysicsSouth Kensington CampusLondon SW7 2AZUK

Isaac BalbergThe Hebrew UniversityThe Racah Institute of PhysicsJerusalem 91904Israel

Christoph J. BrabecFriedrich-Alexander UniversityDepartment of Material Scienceand EngineeringInstitute of Materials for Electronicsand Energy TechnologyMartensstrasse 791058 ErlangenGermany

and

Bavarian Center for Applied EnergyResearch (ZAE Bayern)AmWeichselgarten 791058 ErlangenGermany

Tang-Kuei ChangNational Cheng Kung UniversityDepartment of Chemical EngineeringTaiwan 70101Taiwan

Kilwon ChoPohang University of Scienceand TechnologyDepartment of Chemical EngineeringSan 31 Hyojia-dong, NamguPohang 790-784Korea

Michael EckFMF - FreiburgerMaterialforschungszentrumInstitute for Microsystems TechnologyStefan-Meier-Str. 2179104 FreiburgGermany

L. Jay GuoThe University of MichiganMacromolecular Scienceand Engineering

and

Department of ElectricalEngineeringand Computer ScienceAnn Arbor, MI 48109USA

jXV

Dahlia HaynesCarnegie Mellon UniversityThe McCullough Group5000 Forbes AvenueWarner Hall 608Pittsburgh, PA 15213USA

Ian A. HowardMax Planck Institute for PolymerResearchAckermannweg 1055128 MainzGermany

Pascale JolinatUniversit�e de ToulouseLaboratoire Plasmaet Conversion d’Energie118, Route de Narbonne31062 Toulouse cedex 9France

Michael KruegerFMF - FreiburgerMaterialforschungszentrumInstitute for Microsystems TechnologyStefan-Meier-Str. 2179104 FreiburgGermany

John G. LabramImperial College LondonDepartment of PhysicsLondon SW7 2AZUK

Fr�ed�eric LaquaiMax Planck Institutefor Polymer ResearchAckermannweg 1055128 MainzGermany

Yingping LiGraduate School of ChineseAcademy of SciencesBeijing 100039China

Maria Antonietta LoiUniversity of GroningenZernike Institutefor Advanced MaterialsNijenborgh 4Groningen 9747 AGThe Netherlands

Joachim LoosUniversity of GlasgowKelvin Nanocharacterisation Centre(KNC)Scottish University Physics Alliance(SUPA)

and

School of Physics and AstronomyGlasgow G12 8QQScotlandUK

Guanghao LuChinese Academy of SciencesChangchun Instituteof Applied ChemistryChangchun 130022China

Wanli MaSoochow UniversityInstitute of Functional Nano and SoftMaterialsNo. 1, Shizi StreetSuzhou, 215123China

XVIj List of Contributors

Florian MachuiFriedrich-Alexander UniversityDepartment of Material Scienceand EngineeringInstitute of Materials for Electronicsand Energy TechnologyMartensstrasse 791058 ErlangenGermany

Ralf MauerMax Planck Institute for PolymerResearchAckermannweg 1055128 MainzGermany

Richard D. McCulloughCarnegie Mellon UniversityThe McCullough Group5000 Forbes Avenue, Warner Hall608Pittsburgh, PA 15213USA

Christopher R. McNeillMonash UniversityDepartment of MaterialsEngineeringClayton, Victoria 3800Australia

Moshe NarkisTechnion-Israel Institute ofTechnologyDepartment of ChemicalEngineeringHaifa 32000Israel

Thien Phap NguyenUniversit�e de NantesInstitut des Mat�eriaux Jean RouxelCNRS2, rue de la Houssinière44322 Nantes Cedex 3France

Hui Joon ParkThe University of MichiganMacromolecular Science andEngineeringAnn Arbor, MI 48109USA

Claudia PiliegoUniversity of GroningenZernike Institute forAdvanced MaterialsNijenborgh 4Groningen 9747 AGThe Netherlands

Hongxu QiTsinghua UniversityDepartment of ChemistryBeijing 100084China

Longzhen QiuHefei University of TechnologyAcademyofOpto-ElectronicTechnologyKey Lab of Special Display TechnologyMinistry of EducationNational Engineering Lab of SpecialDisplay TechnologyNational Key Lab of AdvancedDisplay Technology193 Tunxi RoadHefei 230009China

List of Contributors jXVII

Frank-Stefan RiehleFMF - FreiburgerMaterialforschungszentrumInstitute for Microsystems TechnologyStefan-Meier-Str. 2179104 FreiburgGermany

Ester SegalTechnion-Israel Instituteof TechnologyDepartment of Chemical EngineeringHaifa 32000Israel

Gaoquan ShiTsinghua UniversityDepartment of ChemistryBeijing 100084China

Jeremy N. SmithImperial College LondonDepartment of PhysicsLondon SW7 2AZUK

Mihaela C. StefanCarnegie Mellon UniversityThe McCullough Group5000 Forbes AvenueWarner Hall 608Pittsburgh, PA 15213USA

Krisztina SzendreiUniversity of GroningenZernike Institute for AdvancedMaterialsNijenborgh 4Groningen 9747 AGThe Netherlands

Lei TaoTsinghua UniversityDepartment of ChemistryBeijing 100084China

Nobuo UenoChiba UniversityGraduate School of AdvancedIntegration ScienceInage-kuChiba 263-8522Japan

Meixiang WanChinese Academy of SciencesInstitute of ChemistryCenter for Molecular ScienceOrganic Solid LaboratoryBeijing 100080China

Xianhong WangChinese Academy of SciencesChangchun Institute of AppliedChemistryKey Laboratory of PolymerEcomaterialsRenmin Street 5625Changchun 130022China

Xiaohong WangHefei University of TechnologyAcademyofOpto-ElectronicTechnologyKey Lab of Special Display TechnologyMinistry of EducationNational Engineering Lab of SpecialDisplay TechnologyNational Key Lab of AdvancedDisplay Technology193 Tunxi RoadHefei 230009China

XVIIIj List of Contributors

Yen WeiTsinghua UniversityDepartment of ChemistryBeijing 100084China

and

National Cheng Kung UniversityDepartment of ChemicalEngineeringTainan 70101Taiwan

Ten-Chin WenNational Cheng Kung UniversityDepartment of ChemicalEngineeringTaiwan 70101Taiwan

Xiaoniu YangChinese Academy of SciencesChangchun Institute ofApplied ChemistryRenmin Street 5625Changchun 130022China

Yunfei ZhouFMF - FreiburgerMaterialforschungszentrumInstitute for Microsystems TechnologyStefan-Meier-Str. 2179104 FreiburgGermany

List of Contributors jXIX

Preface

The research on (semi-)conducting polymers has attracted dramatically increasedattention from both academic and industrial communities. The commercial pro-ducts based on these new materials, for example, polymer thin-film displays andpolymer solar cells, are already available on the market. Solution-based thin-filmdeposition technology makes it possible to carry out large-scale device fabricationwith very low cost, which has been regarded as the most attractive advantage ofsemiconducting polymers for applications in next-generation optoelectronicdevices. In most cases, a composite instead of only one polymer species is employedto realize the specific functionality of the device, which results in more scientificquestions that need to be answered, for example, with respect to morphological,interfacial, and mechanical properties as well as to charge transfer mechanismswithin the composite film. A book collecting the already existing knowledge on therespective topics is necessary for new researchers to become acquainted with thefield as well as for giving an overview and addressing the key questions within a shorttime. In addition, this book aims at giving a systematic and in-depth coverage ofsemiconducting polymer composites from their fundamental concepts to morphol-ogy control and their applications in real devices for researchers already working inthe field. Consequently, particular attention is given to the unique advantages ofsemiconducting polymer composites where polymers with specific functionalitiesare employed to form a multicomponent material with a desired morphology inorder to obtain required materials properties and high-performance devices.

This book contains three parts, where the first part describes the principles andconcepts of semiconducting polymer composites, including the mechanism ofmorphology formation, morphology characterization, energy level alignment atinterfaces, energy transfer between the components, percolation theory, and proces-sing techniques. These composites can be classified into two categories in terms offunctionality of the components, mainly the matrix polymer involved, which isdetailed in Parts II and III, respectively. Part II discusses the semiconducting/insulating polymer composites where a conjugated polymer or an organic semi-conductor is dispersed in an insulating polymer matrix, forming a composite withexceptional properties. Part III is concerned with semiconducting/semiconductingpolymer composites where conjugated polymers are used as the matrix. The

j XXI

applications of these composites in, for example, polymer solar cells, light-emittingdiodes, transistors, and biosensors are presented.I am greatly indebted to my colleagues who have been working in the respective

fields for years and have agreed to contribute their expertise to this book. Theirsupport made it possible to present the current state-of-the-art overview of semi-conducting polymer composites in terms of both its academic value and potentialapplications.I would also like to thank the people at Wiley-VCH who offered me this oppor-

tunity initially, helped me to overcome numerous difficulties, and made it becomereality eventually.

Changchun, China Xiaoniu YangApril 2012

XXII j Preface

1Solubility, Miscibility, and the Impact on Solid-State MorphologyFlorian Machui and Christoph J. Brabec

1.1Introduction

In recent years, organic semiconductors have been of increasing interest in aca-demic and industrial fields. Compared to their inorganic counterparts, they offervarious advantages such as ease of processing, mechanical flexibility, and potentialin low-cost fabrication of large areas [1]. Furthermore, modifications of the chemi-cal structure allow tailoring material properties and thus enhancing the applicabil-ity [2]. After the discovery of metallic conduction in polyacetylene in 1977 byHeeger, MacDiarmid, and Shirakawa, the path was paved for new material classesof electrical conductive polymers, possible due to chemical doping of conjugatedpolymers. This resulted in an increase of electrical conductivity by several orders ofmagnitude [3]. The main advantage of organic semiconductors is their processabil-ity from solution, which opens different applications such as flat panel displays andillumination, integrated circuits, and energy conversion [4–7]. Before widespreadcommercial application, further scientific investigations are necessary to achieveimproved device performance and environmental stability.The first organic solar cells were based on an active composite consisting of one

single material between two electrodes with different work functions. Light absorp-tion forms Coulomb-bound electron–hole pairs, so-called excitons, which have tobe separated for charge generation [8]. In single-material active layers, this is possi-ble by overcoming the exciton binding energy, either thermally or at the contacts[9]. Since both processes have rather low (

devices is the bulk heterojunction (BHJ), which consists of an interpenetrating net-work of a hole conductor and an electron acceptor, taking care of the low excitondiffusion length [12]. The main advantage of the BHJ concept is the increased inter-facial surface leading to very efficient exciton dissociation within the whole activelayer of the solar cell. The most commonly employed materials are conjugated poly-mers as donors and fullerene derivatives as acceptors [13–16]. By spontaneousphase separation, a specific nanostructure is formed that is decisive for thecharge transport, since charge separation takes place at the interface. In the field oforganic photovoltaics, several groups have realized devices with efficienciesover 6% [17–20]. Significant improvements have raised certified efficiencies upto 8.3% and novel concepts are under investigation to reach efficiencies beyond10% (Konarka, http://www.konarka.com; Heliatek, http://www.heliatek.com .) [21].For an efficient bulk heterojunction solar cell, good control of morphology is a

key aspect, which is mainly influenced by the components’ solubility during proc-essing, the components’ miscibility, and the formation of the resulting film. Solu-bility describes to what extent a substance dissolves in a particular solvent. This isth e k ey ph en om en on wi th r eg ar d to th e de s ig n of in ks an d s ol ve nt sy s te ms wit hmutual multicomponent solubility regimes. The miscibility of several componentsin the film is mainly influenced by thermodynamic parameters. Film formation isadditionally influenced by the surface energy differences of the substrate to theprinting medium as well as by kinetic aspects.Upscaling from lab to mass production facilities is one of the major necessities

for cost optimization. In the case of organic solar cells, this is possible by large-arearoll-to-roll processing, allowing throughputs of 10 000m2 h�1. This is orders ofmagnitude higher compared to silicon processing capabilities [22]. Currently, themost employed deposition method for organic solar cells is spin coating, sinceinherent advantages such as high film uniformity and ease of production are suit-able for research activities. However, spin coating is very unfavorable for produc-tion due to its limitation in size. Doctor blading as an alternative coating techniqueis more suitable for larger area substrates and is easily transferred to roll-to-rollprocessing. For all of these techniques, it is necessary to know of the ink’s solubilityto adjust the formulation. Accordingly, the material parameters for ink definitionare viscosity, evaporation rate of the solvent systems, and the spreading behavior.These phenomena together define the quality and the functionality of an organicsemiconductor layer. Due to the high technical relevance for organic photovoltaicsand, more generally, for organic electronics, the impacts of these phenomena onthe performance and functionality of bulk heterojunction composite formation arethe major topics in this chapter.

1.2General Aspects

In general, chlorinated solvents are commonly used for processing in laboratories,which have restricted application in industrial operation due to safety risks and

2j 1 Solubility, Miscibility, and the Impact on Solid-State Morphology

http://www.konarka.comhttp://www.heliatek.com

processing costs. Environment-friendly inks are therefore one decisive criterionfor mass production that should provide full functionality. Since solubility is one ofthe determining factors for processing of the active layer in organic solar cells,several approaches are under investigation to predict solubility of the materialsin question.

1.2.1Solubility

Different approaches can be utilized to determine the solubility of a material. Whilesimulation of solubility is a helpful tool to predict material behavior, experimentalverification is of utmost importance. In order to reduce the expensive and time-consuming experimental efforts as well as frequent toxicity issues, simulations area welcome tool to accompany experiments. One possibility to predict the materialsolubility is the use of solubility parameters, which was first proposed by Hilde-brand and Scott and diversified by Hansen [23, 24]. In this approach, the energy ofmixing is related to the vaporization energies of pure components. For liquids aswell as for polymers, the solubility parameter d was defined as the square root ofthe cohesive energy density (CED) with DEv as energy of vaporization and Vm asaverage molar volume. Here the energy of mixing is related to the energies ofvaporization of the pure components according to Eqs. (1.1)–(1.3). The contribu-tions to DEv in Eq. (1.2) are the difference in enthalpy of evaporation DH, the abso-lute temperature T, and the global ideal gas constant R.

d ¼ CED1=2 ¼ ðDEv=VmÞ1=2; ð1:1Þ

DEv ¼ DH � RT : ð1:2ÞBlanks, Prausnitz, and Weimer assigned the separation of vaporization energy intoa nonpolar, dispersive part and a polar part [25, 26]. The polar part was furtherdivided into dipole–dipole contribution and hydrogen bonding contribution byHansen with dD as solubility parameter due to dispersion forces, dP as solubilityparameter due to polar dipole forces, and dH as solubility parameter due to hydro-gen bonding interactions according to Eq. (1.3) [27–29].

d2 ¼ d2D þ d2P þ d2H: ð1:3ÞHansen solubility properties are usually plotted in a three-dimensional coordi-nate system with the Hansen parameters as x, y, z axes. The coordinates of asolute can be determined by analyzing the solubility of a solute in a series ofsolvents with known Hansen parameters. By fitting a spheroid into the solubil-ity space, the solubility volume of this solute can be identified. The solubilityspace of a solute is defined by the origin of a spheroid, resulting from the threecoordinates, and the three radii in each dimension, with solvents inside thespheroid and nonsolvents outside. The radius of the sphere, R0, indicatesthe maximum difference for solubility. Generally, good solvents are within thesphere, and bad ones are outside of it. Furthermore, the solubility “distance”

1.2 General Aspects j3

parameter, Ra, between one solvent and one solute reflecting their respectivepartial solubility parameters can be defined with Eq. (1.4), with dD2 as disper-sive component for the solvent, dD1 as dispersive component of the solute, anda, b, and c as weighting factors. Setting of a¼ 4 and b¼ c¼ 1 was suggested byHansen based on empirical testing. To convert the Hansen spheroid into anellipsoid, different ratios of weighting factors are used. When the scale for thedispersion parameter is doubled, the spheroidal shaped volume is convertedinto a spherical body [24].

R2a ¼ aðdD2 � dD1Þ2 þ bðdP2 � dP1Þ2 þ cðdH2 � dH1Þ2: ð1:4Þ

Further studies by Small revealed that solubility parameters of polymers couldalso be calculated by using group contribution methods, which was intensifiedby Hoy, van Krevelen, and Coleman et al. [30–33]. The properties of moleculesare investigated by separating them into smaller subgroups. The basic assump-tion is that the free energy of a molecule transfer between two phases is thesum of its individual contributions of groups, and that these group contribu-tions are independent of the rest of the molecule. There is an obvious trade-offin group contributions. It is possible to define several groups in different ways.The more the subgroups used, the more accurate the group contributionsbecome, but the less likely that there is sufficient statistical data to make predic-tions. More examples have been employed elsewhere [34–36]. For predictingsolubility parameters using the group contribution method, frequently the fol-lowing approach is used with Fi as molar attraction constant of a specific groupi and Vm as molar volume:

d ¼P

FiVm

��������: ð1:5Þ

Another method to predict solubility of solutes in different solvents is based onthe prediction of the activity coefficient using density functional theory [37]. Mol-ecules exhibit a rigid structure, but can possess different conformations, whosephysical and chemical properties depend on their ultimate three-dimensionalconfirmation. Jork et al. showed that different conformations have different influ-ence on the predictions of the activity coefficient [38]. Klamt et al. introduced aconductor-like screening model for real solvents (COSMO-RS), which allows apriori calculation of chemical potentials of one component within an arbitraryenvironment [39–42]. Here, modeling is realized by statistical thermodynamicswhere interacting molecules are substituted by corresponding pairwise interact-ing surface segments with densely packed contact areas. Since every segment hasa constant charge density s, the characterization of a molecule is possible byknowing the distribution function of the charge density P(s), the s-profile. Withthat the properties of the molecules are solely dependent on the number of seg-ments. The s-profile of a pure component results directly from the density func-tional theory calculation. Further methods are based on molecular dynamics orMonte Carlo simulations but discussions are beyond the topic of this chapter.

4j 1 Solubility, Miscibility, and the Impact on Solid-State Morphology

1.2.2Miscibility–Thermodynamic Relationships

A decisive criterion of organic semiconductor applications is their ability of mixing.In general, blends of two or more components can be categorized according to themiscibility of their phases in one-phase or multiphase systems. Miscibility is usu-ally defined by thermodynamic parameters. Here the Gibbs free mixing enthalpyDGm is decisive for compatibility of two phases. Figure 1.1 shows the Gibbs freeenergy as a function of compositions. If DGm is positive, the components are notmiscible (A). If DGm is negative and the second derivative is positive, both compo-nents are totally miscible (B). Independent of composition, a homogeneous blendis formed. If DGm is negative and the second derivative is negative as well, thecomponents are partially miscible (C). Phases with different composition areformed, which consist of both components [43, 44].

DGm ¼ DHm � TDSm: ð1:6ÞDGm can be determined according by changes in enthalpy (DHm) and entropy ofmixing the components (DSm). Compared to low molecular mass components, theentropy increase is low for mixing polymers. Mixing of two polymers results in asmaller increase of DSm as compared to a binary blend of two low molecular weightcomponents. Therefore, according to Eq. (1.6), the enthalpy change is the decisiveparameter for thermodynamic miscibility [43]. The relatively smaller increase ofentropy for polymers versus small molecules can be explained with Figure 1.2. Thetwo-dimensional grids in Figure 1.2 represent places for molecules or for polymersegments. The number of possible configurations W is significantly higher for the

Figure 1.1 Gibbs free energy of mixing as a function of composition. (According to Refs.[43, 44].)

1.2 General Aspects j5

arrangement with the small molecules. With S� kT ln(W), the lower entropyincrease for polymer blends becomes obvious.Since Gibbs free energy DGm cannot be determined directly, thermodynamic

models are used for the estimation. An often used model for polymer–polymer sys-tems is the Flory–Huggins theory [45]. The Flory–Huggins definition of the Gibbsfree energy and its implication on polymer blends are discussed in Eq. (1.7). Itdescribes the free energy of binary systems, with the first two parts of the equationrepresenting the entropic part and the third part describing the enthalpicphenomena. Here, wi is the volume fraction of component i, Vi is the molarvolume of component i, B12 is the interaction parameter, and R is the ideal gasconstant. In the case of polymer blends, the free energy is dominated by theenthalpic changes, which need to be negative for miscible systems. DHm is directlyproportional to the number of interactions between the two components, andbecomes negative for strong interactions such as ion, acid–base, hydrogen bonds,or dipole–dipole interactions.

DGm ¼ w1V1 ln w1 þw2V2

ln w2 þ w1w2B12� �

RTV : ð1:7Þ

1.3Solubility, Solvents, and Solution Formulations

1.3.1Solubility

In this chapter, the solubility of organic semiconductors, their influence on OPVdevices, and their correlation with Hansen solubility parameters (HSPs) are dis-cussed. Before that, experimental methods to determine the absolute solubility oforganic semiconductors are reviewed. High-performance liquid chromatography

Figure 1.2 Schematic depiction of blends with components of smaller molar mass (a) and highermolar mass (a). (According to Refs [43, 44].)

6j 1 Solubility, Miscibility, and the Impact on Solid-State Morphology