Light-Emitting Electrochemical Cells (eBook)

Dr. Rubén D. Costa holds an independent junior group leader position at the Department of Physical Chemistry I, University of Erlangen-Nuremberg, Germany. His research group focuses on sustainable hybrid lighting technologies based on low-cost and environmentally friendly approaches to fulfill the “Green Photonics” requirements.

Foreword 5
Light-Emitting Electrochemical Cells: organic semiconductor devices augmented by ions 5
Preface 8
Contents 10
Introduction to the Light-Emitting Electrochemical Cell Technology 12
1 Light-Emitting Electrochemical Cells: Mechanisms and Formal Description 13
Abstract 13
1.1 Purpose and Aims 14
1.2 Overview 14
1.2.1 Background 14
1.2.2 Figures of Merit and Device Architectures 17
1.2.3 Suggested Operational Mechanisms for LECs 20
1.2.3.1 Electrochemical Doping Model (ECDM) 20
1.2.3.2 Electrodynamic Model (EDM) 20
1.2.3.3 Preferential Electrochemical Doping Model (PECDM) 21
1.2.4 Current Understanding of Operational Mechanism of LECs 22
1.2.5 Basic Equations to Describe LEC Operation 22
1.2.5.1 Drift and Diffusion for Ionic and Electronic Charges 23
1.2.5.2 Poisson’s Equation 23
1.2.5.3 Binding Energy for Anion/Cation and Ion/Electronic Charge Pairs 23
1.2.5.4 Electron–Hole Recombination 24
1.2.5.5 Continuity Equations 25
1.2.5.6 Boundary Conditions 25
1.3 Transient Phenomena 26
1.3.1 Turn-on and the Role of Ion Motion 26
1.3.1.1 Studies in Planar LECs 27
1.3.1.2 Studies in Stacked LECs 31
1.3.2 Polarization Reversal and Hysteresis 32
1.3.3 Degradation, Side Reactions, and Electrochemical Stability 34
1.4 Steady-State Phenomena 38
1.4.1 Potential and Ion Distribution 38
1.4.1.1 EDM 38
1.4.1.2 ECDM 39
1.4.1.3 PECDM 41
1.4.2 Position and Width of the Recombination Zone 42
1.4.2.1 Studies in Planar LECs 42
1.4.2.2 Electrical Impedance Spectroscopy 43
1.4.2.3 Studies in Stacked LECs 43
1.4.3 Current-Voltage Characteristic 45
1.4.4 Luminescence Quenching and Reabsorption 47
1.4.5 Color Tuning and Cavity Effects 49
1.4.6 Efficiency: Values and Limits 50
1.5 Conclusion and Outlook 51
References 51
Definition and Role of the Ionic Additives 56
2 Optical-Beam-Induced-Current Imaging of Planar Polymer Light-Emitting Electrochemical Cells 57
Abstract 57
2.1 Polymer Light-Emitting Electrochemical Cells 57
2.1.1 Background 57
2.1.2 Frozen-Junction LECs 59
2.1.3 Extremely Large Planar LECs 62
2.2 Scanning Optical Imaging of Planar LECs 65
2.2.1 The Optical-Beam-Induced Current (OBIC) Technique 65
2.2.2 OBIC Scanning of Planar LECs with a Micromanipulated Cryogenic Probe Station 69
2.2.3 Concerted OBIC and Scanning PL Imaging of Planar LECs with a Fluorescence Microscope 70
2.3 OBIC and Scanning PL Probing of a Frozen Planar p-i-n Junction 72
2.3.1 Introduction 72
2.3.2 Experimental Details 72
2.3.3 Resolving the Depletion Width of a Planar p-i-n Junction 73
2.4 High-Resolution OBIC and Scanning PL Imaging of a Frozen Planar Polymer p-n Junction 75
2.4.1 Introduction 75
2.4.2 Experimental Details 75
2.4.3 Results and Discussion 76
2.5 Conclusion and Outlook 80
Acknowledgements 80
References 81
3 Optical Engineering of Light-Emitting Electrochemical Cells Including Microcavity Effect and Outcoupling Extraction Technologies 84
Abstract 84
3.1 Introduction 85
3.1.1 Microcavity Effect in Organic Thin-Film Devices 85
3.1.2 Optical Modes in Organic Thin-Film Devices 85
3.1.3 Organization of This Chapter 86
3.2 Tailoring Output EL Spectrum of LECs by Employing Microcavity Effect 86
3.2.1 Suppression of Blue-Green Emission to Achieve Purer White EL 86
3.2.2 Non-doped White LECs Based on a Single Emissive Material 88
3.2.3 Non-doped Near-Infrared LECs Based on Interferometric Spectral Tailoring 90
3.3 Outcoupling Extraction Technologies to Enhance Device Efficiencies of LECs 92
3.3.1 Enhancing Light Extraction by Employing Microlens Array 92
3.3.2 Recycling the Trapped EL by Employing Red Color Conversion Layers 93
3.3.3 Recycling the Trapped EL by Employing Waveguide Coupling 95
3.4 Conclusion and Outlook 97
Acknowledgements 98
References 98
4 The Use of Additives in Ionic Transition Metal Complex Light-Emitting Electrochemical Cells 100
Abstract 100
4.1 Polymer Additives to Decrease Self-quenching for Improved Efficiency 100
4.1.1 Layer-by-Layer Techniques 101
4.1.2 Blended Inert Polymers 102
4.2 Host/Guest LECs to Control Color and Improve Efficiency 104
4.3 Ionic Additives for Improved Ion Redistribution and LEC Performance 108
4.3.1 Electric Double Layer Formation and Charge Injection 108
4.3.2 Electrolyte—Salt Combinations 108
4.3.3 Ionic Liquids 109
4.3.3.1 Ionic Liquids Shown to Decrease Turn-on Time 109
4.3.3.2 High Ionic Conductivity Ionic Liquids Yield High Peak Luminance 110
4.3.3.3 Ionic Liquids in LECs Under AC and Pulsed Operation 112
4.3.4 Lithium Salt Additives 112
4.3.4.1 Lithium Additives Improve ITMC-Based LECs 114
4.3.4.2 Scanning Probe Study of Lithium Salt Additives 117
4.3.4.3 Optimal Lithium Salt Concentration 119
4.3.4.4 Electrochemical Impedance Spectroscopy of LECs with Lithium Salt Additive 121
4.3.4.5 Counterion Dependence of Lithium Salts 123
4.4 Outlook 123
Acknowledgements 124
References 124
5 Improving Charge Carrier Balance by Incorporating Additives in the Active Layer 127
Abstract 127
5.1 Introduction 127
5.1.1 Characteristics of Light-Emitting Electrochemical Cells (LECs) 127
5.1.2 Charge Carrier Balance in LECs 130
5.1.3 Organization of this Chapter 131
5.2 Optical Technique to Probe Charge Carrier Balance in LECs 131
5.3 Incorporating Carrier Trappers in LECs 133
5.4 Incorporating Salts in LECs 137
5.5 Incorporating Carrier Transport Materials in LECs 139
5.6 Conclusion and Outlook 141
Acknowledgements 142
References 142
6 Morphology Engineering and Industrial Relevant Device Processing of Light-Emitting Electrochemical Cells 144
Abstract 144
6.1 Introduction 144
6.2 Film Morphology and Polymer Solid Electrolytes 147
6.2.1 Introduction 147
6.2.2 LEC Active Layer Morphology: PSE Phase Separation 148
6.2.3 Effect of the PSE Molecular Weight on the Film Morphology 148
6.2.4 Effect of the PSE Monomer Ratio on the Film Morphology 153
6.3 LEC Fabrication by Scalable Methods 156
6.3.1 Introduction 156
6.3.2 Gravure Printing 157
6.3.2.1 Gravure-Printed Polymer-Based LECs 158
6.3.2.2 Gravure-Printed Small Molecule-Based LECs 160
6.3.3 Inkjet Printing 161
6.3.4 Slot-Die Coating 161
6.3.5 Spray Coating 164
6.4 Conclusion 166
Acknowledgements 166
References 166
Traditional and New Electroluminescent Materials 169
7 Development of Cyclometallated Iridium(III) Complexes for Light-Emitting Electrochemical Cells 170
Abstract 170
7.1 Introduction 170
7.2 Synthetic Approaches to [Ir(C^N)2(N^N)]+ Complexes 173
7.2.1 Use of [Ir2(C^N)4(?-Cl)2] Dimers 173
7.2.2 Solvento Complexes 175
7.3 Development of Ligand Types in [Ir(C^N)2(N^N)]+ Emitters 176
7.3.1 Archetype [Ir(ppy)2(bpy)]+ and [Ir(ppy)2(phen)]+ Complexes 176
7.3.2 Functionalizing N^N Ligands in [Ir(ppy)2(bpy)]+ and [Ir(ppy)2(phen)]+ with Bulky Substituents 178
7.3.3 Cyclometallating Ligands with Nitrogen-Rich Heterocycles 180
7.3.4 N^N Ligands with Nitrogen-Rich Heterocycles 182
7.3.5 Designing N^N Ligands for Red-Emitting Iridium(III) Complexes 184
7.4 Increasing Stability Through Intramolecular ?-Stacking in [Ir(C^N)2(N^N)]+ Luminophores 187
7.5 Effects on LEC Stability of Introducing Fluoro-Substituents into Cyclometallating Ligands 192
7.6 Fluorine-Free, Blue-Shifted Emitters 195
7.7 Replacing the N^N Ligand in [Ir(C^N)2(N^N)]+ Emitters by N-Heterocyclic Carbenes 196
7.8 Effects of Incorporating Peripheral, Charged Domains in Ir-iTMCs and the Design of Anionic Ir-iTMCs 199
7.9 Conclusions 202
Acknowledgements 202
References 202
8 Recent Advances on Blue-Emitting Iridium(III) Complexes for Light-Emitting Electrochemical Cells 206
Abstract 206
8.1 Introduction 207
8.2 Blue-Emitting Iridium(III) Complexes for LECs 207
8.2.1 Modification on [Ir(ppy)2(bpy)]+ 208
8.2.2 Using Ancillary Ligands Beyond the bpy Skeleton 212
8.2.3 Using Ancillary Ligands with Strong Ligand Field Strength 218
8.2.4 Using Cyclometalating Ligands Beyond the ppy Skeleton 222
8.3 Conclusion and Outlook 228
8.3.1 Current Status 228
8.3.2 Challenges and the Future 235
Acknowledgements 236
References 236
9 Thermally Activated Delayed Fluorescence Emitters in Light-Emitting Electrochemical Cells 239
Abstract 239
9.1 Introduction 239
9.2 Photoluminescence Mechanism: Fluorescence, Phosphorescence, and TADF 241
9.3 SM-Based LECs 242
9.3.1 State-of-the-Art of Blue-Emitting SM-Based LEC without TADF Mechanism 242
9.3.2 SM-Based LEC with TADF Mechanism 250
9.3.2.1 Designing TADF SMs 250
Application of TADF in Working Devices 250
9.3.2.2 Critical Outlook 254
9.4 Copper(I) Complexes Based LECs with TADF Mechanism 256
9.4.1 Designing TADF Copper(I) Complexes 256
9.4.1.1 LEC Performance 256
9.4.2 Critical Outlook 265
9.5 Conclusions 266
Acknowledgements 266
References 266
10 White Emission from Exciplex-Based Polymer Light-Emitting Electrochemical Cells 269
Abstract 269
10.1 Introduction 270
10.1.1 Energy, Electricity, and Lighting 270
10.1.2 Polymer White Light-Emitting Electrochemical Cells 271
10.1.2.1 Definition 271
10.1.2.2 Strategies for Obtaining White Emission from Polymer-Based Light-Emitting Electrochemical Cells 272
10.2 White Light-Emitting Electrochemical Cells Based on Exciplex Emission 275
10.2.1 Basic Concepts and Strategies 275
10.2.2 Optical Properties of Exciplexes Formed Between PFD and Amine Molecules 277
10.2.3 Characteristics of White-Emitting PLECs Based on Exciplex Emission 281
10.2.4 Strategy for Realizing Highly Efficient Exciplex-Based White-Emitting PLECs 282
10.3 Exciplex-Based LECs Made with Small-Molecule Compounds 284
10.4 Perspectives and Future Opportunities 286
References 287
11 Luminescent Cationic Copper(I) Complexes: Synthesis, Photophysical Properties and Application in Light-Emitting Electrochemical Cells 289
Abstract 289
11.1 Introduction 290
11.2 Complexes of General Formula [Cu(N^N)2][X] 291
11.3 Complexes of General Formula [Cu(P^P)(N^N)][X] 294
11.3.1 [Cu(P^P)(N^N)][X] Complexes Coordinated to 1,10-Phenanthroline 294
11.3.2 [Cu(P^P)(N^N)][X] Complexes Coordinated to 2,2-Bypiridine 298
11.3.3 [Cu(P^P)(N^N)][X] Complexes Coordinated to 2-Pyridyl-Aza-Heterocycles 304
11.3.4 LEC Devices Prepared with [Cu(P^P)(N^N)][X] Complexes 307
11.4 Complexes of General Formula [Cu(P^P)2][X] 311
11.5 Cationic Copper(I) Complexes Coordinated to Tripodal P^P^P or N^N^N Ligand 313
11.6 Polynuclear Cationic Copper(I) Complexes Based on Diphosphine and Dinitrogen Ligands 314
11.7 Cationic Copper(I) Complexes Coordinated to P^N Ligand 319
11.8 Cationic Copper(I) Complexes Coordinated to N-Heterocyclic Carbene Ligand 320
11.9 Conclusions 325
Acknowledgements 326
References 326
12 Small Molecule-Based Light-Emitting Electrochemical Cells 330
Abstract 330
12.1 Introduction 330
12.2 General Consideration of SM-LECs: Device Fabrication and Device Architecture 331
12.3 LEC Materials 332
12.3.1 Neutral SM-Based LECs 333
12.3.2 Ionic SM-Based LECs 337
12.3.3 Dyad-Endorsed LECs 342
12.3.4 Organic–Inorganic Host–Guest Systems Involving SMs in LECs 343
12.4 Conclusions and Outlook 347
Acknowledgements 348
References 348
13 Quantum Dot Based Light-Emitting Electrochemical Cells 351
Abstract 351
13.1 Introduction to Semiconductor Nanocrystals 352
13.2 Colloidal QDs 353
13.2.1 Type-I Core-Shell NCs 354
13.2.1.1 CdSe-Based QDs—QDs Used in LECs 355
13.3 Perovskites 355
13.3.1 Perovskite Nanostructures 357
13.3.2 Synthesis of Perovskite NCs 360
13.4 Applications of NCs into LECs 363
13.4.1 Core-Shell QDs Emitters for LECs 363
13.4.2 Perovskite Emitters for LECs 366
13.5 Conclusions and Outlook 369
Acknowledgements 369
References 369