Highly Efficient OLEDs
Materials Based on Thermally Activated Delayed Fluorescence

The essential resource that offers a comprehensive understanding of OLED optimizations

Highly Efficient OLEDs. Materials Based on Thermally Activated Delayed Fluorescence (TADF) offers substantial information on the working principle of OLEDs and on new types of emitting materials (organic and inorganic). As the authors explain, OLEDs that use the Singlet-Harvesting mechanism based on the molecular property of TADF work according to a new exciton harvesting principle. Thus, low-cost emitter materials, such as Cu(I) or Ag(I) complexes as well as metal-free organic molecules, have the potential to replace high-cost rare metal complexes being currently applied in OLED technology.

With contributions from an international panel of experts on the topic, the text shows how the application of new TADF materials allow for the development of efficient OLED displays and lighting systems. This new mechanism is the gateway to the third-generation of luminescent materials. This important resource:

• Offers a state-of-the-art compilation of the latest results in the dynamically developing field of OLED materials
• Is edited by a pioneer in the field of OLED material technology
• Contains a detailed application-oriented guide to new low-cost materials for displays and lighting
• Puts the focus on the emerging fields of OLED technology

Written for materials scientists, solid state chemists, solid state physicists, and electronics engineers, Highly Efficient OLEDs. Materials Based on Thermally Activated Delayed Fluorescence offers a comprehensive resource to the latest advances of OLEDs based on new TADF materials.

Preface xv

1 TADF Material Design: Photophysical Background and Case Studies Focusing on Cu(I) and Ag(I) Complexes 1
Hartmut Yersin, Rafał Czerwieniec, Marsel Z. Shafikov, and Alfiya F. Suleymanova

1.1 Introduction 1

1.2 TADF,Molecular Parameters, and Diversity of Materials 4

1.2.1 TADF and Phosphorescence 6

1.2.2 Minimizing ΔE(S₁–T₁) 7

1.2.3 Importance of k_r(S₁–S₀) 7

1.3 Case Study: TADF of a Cu(I) Complex with Large ΔE(S₁–T₁) 15

1.3.1 DFT and TD-DFT Calculations 16

1.3.2 Flattening Distortions and Nonradiative Decay 16

1.3.3 TADF Properties 18

1.3.4 Radiative S₁→S₀ Rate, Absorption, and Strickler–Berg Relation 20

1.4 Case Study: TADF of a Cu(I) Complex with Small ΔE(S₁–T₁) 22

1.4.1 DFT and TD-DFT Calculations 22

1.4.2 Emission Spectra and Quantum Yields 23

1.4.3 The Triplet State T1 and Spin–Orbit Coupling 23

1.4.4 Temperature Dependence of the Emission Decay Time and TADF 28

1.5 Energy Separation ΔE(S₁–T₁) and S₁→S₀ Fluorescence Rate 30

1.5.1 Experimental Correlation Between ΔE(S₁–T₁) and kr(S₁→S₀) for Cu(I) Compounds 31

1.5.2 Quantum Mechanical Considerations 32

1.6 Design Strategies for Highly Efficient Ag(I)-Based TADF Compounds 34

1.6.1 Ag(phen)(P2-nCB): A First Step to Achieve TADF 34

1.6.2 Emission Quenching in Ag(phen)(P2-nCB) 36

1.6.3 Sterical Hindrance. Tuning of the Emission Quantum Yield up to 100% 38

1.6.4 Detailed Characterization of Ag(dbp)(P2-nCB) 40

1.7 Conclusion and Future Perspectives 45

Acknowledgments 46

References 46

2 Highly Emissive d10 Metal Complexes as TADF Emitters with Versatile Structures and Photophysical Properties 61
Koichi Nozaki and Munetaka Iwamura

2.1 Introduction 61

2.2 Phosphorescence and TADF Mechanisms 62

2.3 Structure-Dependent Photophysical Properties of Four-Coordinate [Cu(N^N)₂] Complexes 64

2.4 Flattening Distortion Dynamics of the MLCT Excited State 76

2.5 Green and Blue Emitters: [Cu(N^N)(P^P)] and [Cu(N^N)(P^X)] 77

2.6 Three-Coordinate Cu(I) Complexes 79

2.7 Dinuclear Cu(I) Complexes 80

2.8 Ag(I), Au(I), Pt(0), and Pd(0) Complexes 84

2.9 Summary 85

References 86

3 Luminescent Dinuclear Copper(I) Complexes with Short Intramolecular Cu–Cu Distances 93
Akira Tsuboyama

3.1 Introduction 93

3.2 Overview of Luminescent Dinuclear Copper(I) Complexes 94

3.2.1 Structure 94

3.2.2 Luminescence Properties 99

3.3 Structural and Photophysical Studies of the Dinuclear Copper(I) Complexes: [Cu(μ-C∧N)]₂ (C∧N=2-(bis(trimethylsilyl)methyl) pyridine Derivatives) 100

3.3.1 Outline 100

3.3.2 X-ray Crystallographic Study 101

3.3.3 Photophysical Properties 102

3.3.3.1 Absorption Spectrum 102

3.3.3.2 DFT Calculation 103

3.3.3.3 Emission Properties 104

3.3.3.4 Emission Decay Kinetic Analysis 105

3.3.4 OLED Device 110

3.3.5 Experimental 111

3.3.5.1 Synthesis 111

3.3.5.2 Measurement, Calculation, and Device 111

3.3.5.3 X-ray Structure Analysis 112

3.3.5.4 DFT Calculation 112

3.3.5.5 OLED Device 112

3.4 Conclusion 112

Acknowledgment 113

References 114

4 Molecular Design and Synthesis of Metal Complexes as Emitters for TADF-Type OLEDs 119
Masahisa Osawa and Mikio Hoshino

4.1 Introduction 119

4.2 Cu(I) Complexes for OLEDs 122

4.2.1 Energy Levels of Molecular Orbitals in Tetrahedral Geometries 122

4.2.2 Ligand Variation 123

4.3 Mononuclear Cu(I) Complexes for OLEDs 126

4.3.1 Bis(diimine) Type 131

4.3.2 [Cu(NN)(PP)]⁺ Complexes with phen or bipy Derivatives as Ligands 131

4.3.3 [Cu(NN)(PP)]⁺ Complexes with NN Ligands OtherThan phen or bipy Derivatives 134

4.3.4 Tetrahedral Cu(I) Complexes with the LUMO on the PP Ligand 142

4.3.5 Charge-NeutralThree-Coordinate Cu(I) Complexes 146

4.4 Dinuclear Cu(I) Complexes for OLEDs 155

4.4.1 Dinuclear Cu(I) Complexes Possessing {Cu2(𝜇-X)2} Cores 155

4.4.2 Other Dinuclear Cu(I) Complexes 157

4.5 Another Group of Metal Complexes Exhibiting TADF 157

4.6 Conclusion 160

Acknowledgments 160

Appendix 161

4.A.1 Schematic Structures of 1–86 161

4.A.2 Abbreviations and Molecular Structures of Materials for OLEDs 168

References 171

5 Ionic [Cu(NN)(PP)]+ TAD9727 F Complexes with Pyridine-based Diimine Chelating Ligands and Their Use in OLEDs 177
Rongmin Yu and Can-Zhong Lu

5.1 Introduction 177

5.2 The Influence of Molecular and Electronic Structure on Emissive Properties of Cu(I) Complexes 178

5.3 Heteroleptic Diimine/Diphosphine [Cu(NN)(PP)]+ Complexes with Pyridine-Based Ligand 181

5.3.1 [Cu(NN)(PP)]⁺ Complexes with 2,2′-bipyridyl-based Ligands 181

5.3.1.1 [Cu(NN)(PP)]⁺ Complexes with 2-(2′-pyridyl)benzimidazole and 2-(2′-pyridyl)imidazole-based Ligands 182

5.3.2 [Cu(NN)(PP)]⁺ Complexes with 5-(2-pyridyl)tetrazole-based Ligands 185

5.3.3 [Cu(NN)(PP)]⁺ Complexes with 3-(2′-pyridyl)-1,2,4-triazole-based Ligands 187

5.3.4 [Cu(NN)(PP)] Complexes with 2-(2-pyridyl)-pyrrolide-based Ligands 188

5.3.5 [Cu(NN)(PP)]⁺ Complexes with 1-(2-pyridyl)-pyrazole-based Ligands 189

5.3.6 [Cu(NN)(PP)]⁺ Complexes with Carbazolyl-modified 1-(2-pyridyl)-pyrazole-based Ligands 191

5.3.7 [Cu(NN)(PP)]⁺ Complexes with 1-phenyl-3-(2-pyridyl)pyrazole-based Ligands 192

5.3.8 [Cu(NN)(PP)]⁺ Complexes with 3-phenyl-5-(2-pyridyl)-1H-1,2,4- triazole-based Ligands 193

5.4 Conclusion and Perspective 194

References 195

6 Efficiency Enhancement of Organic Light-Emitting Diodes Exhibiting Delayed Fluorescence and Nonisotropic Emitter Orientation 199
Tobias D. Schmidt andWolfgang Brütting

6.1 Introduction 199

6.2 OLED Basics 200

6.2.1 Working Principle 200

6.2.2 Electroluminescence Quantum Efficiency 202

6.2.3 Delayed Fluorescence 203

6.2.4 Nonisotropic Emitter Orientation 204

6.2.5 Optical Modeling 205

6.3 Comprehensive Efficiency Analysis of OLEDs 206

6.4 Case Studies 209

6.4.1 Treating the OLED as a Black Box 209

6.4.2 Highly EfficientThermally Activated Delayed Fluorescence Device 214

6.4.3 Low Efficiency Roll-Off Triplet–Triplet Annihilation Device 218

6.5 Conclusion 222

Acknowledgments 223

References 223

7 TADF Kinetics and Data Analysis in Photoluminescence and in Electroluminescence 229
Tiago Palmeira and Mário N. Berberan-Santos

7.1 TADF Kinetics 229

7.1.1 Introduction 229

7.1.2 Excitation Types 231

7.1.3 Photoexcitation 232

7.1.3.1 Rate Equations 232

7.1.3.2 Fluorescence and Phosphorescence Decays 232

7.1.3.3 Steady-state Fluorescence and Phosphorescence Intensities 233

7.1.3.4 Excited-state Cycles 235

7.1.3.5 TADF Onset Temperature 238

7.1.3.6 Conditions for Efficient TADF 239

7.1.4 Electrical Excitation 240

7.1.4.1 Steady State 240

7.1.4.2 Conditions for Efficient Electroluminescence 241

7.1.5 More Complex Schemes 244

7.2 TADF Data Analysis 245

7.2.1 Introduction 245

7.2.2 Steady-state Data 245

7.2.2.1 Delayed Fluorescence and Phosphorescence Intensities as a Function of Temperature: Rosenberg–Parker Method 245

7.2.2.2 Prompt and Delayed Fluorescence Intensities as a Function of Temperature 245

7.2.2.3 Delayed Fluorescence Intensity as a Function of Temperature 249

7.2.3 Decay Data 249

7.2.4 Combined Steady-state and Decay Data 250

7.2.4.1 Linear Relation Between Delayed Fluorescence Lifetime and Intensity Ratio 250

7.2.4.2 Linearized Relation for the Determination of ΔEST 250

7.3 Conclusion 252

Acknowledgment 252

References 252

8 Intersystem Crossing Processes in TADF Emitters 257
Christel M. Marian, Jelena Föller, Martin Kleinschmidt, andMihajlo Etinski

8.1 Introduction 257

8.1.1 Electroluminescent Emitters 257

8.1.2 Thermally Activated Delayed Fluorescence 258

8.2 Intersystem Crossing Rate Constants 259

8.2.1 Condon Approximation 260

8.2.1.1 Electronic Spin–Orbit Coupling Matrix Elements 261

8.2.1.2 Overlap of VibrationalWave Functions 262

8.2.2 Beyond the Condon Approximation 263

8.2.3 Computation of ISC and rISC Rate Constants 264

8.2.3.1 Classical Approach 265

8.2.3.2 Statical Approaches 265

8.2.3.3 Dynamical Approaches 265

8.3 Excitation Energies and Radiative Rate Constants 266

8.3.1 Time-Dependent Density FunctionalTheory 266

8.3.2 DFT-Based Multireference Configuration Interaction 267

8.3.3 Fluorescence and Phosphorescence Rates 268

8.4 Case Studies 269

8.4.1 Copper(I) Complexes 269

8.4.1.1 Three-Coordinated Cu(I)–NHC–Phenanthroline Complex 270

8.4.1.2 Four-coordinated Cu(I)–bis-Phenanthroline Complexes 275

8.4.2 Metal-Free TADF Emitters 277

8.4.2.1 1,2,3,5-Tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) 279

8.4.2.2 Mechanism of the Triplet-to-Singlet Upconversion in the Assistant Dopants ACRXTN and ACRSA 282

8.5 Outlook and Concluding Remarks 285

References 286

9 The Role of Vibronic Coupling for Intersystem Crossing and Reverse Intersystem Crossing Rates in TADF Molecules 297
Thomas J. Penfold and Jamie Gibson

9.1 Introduction 297

9.1.1 Background to Delayed Fluorescence 300

9.1.2 The Mechanism of rISC 302

9.2 Beyond a Static Description 303

9.2.1 Obtaining the Potential Energy Surfaces 304

9.2.1.1 Vibronic Coupling Model Hamiltonian 306

9.2.2 Solving for the Motion of the Nuclei 309

9.2.2.1 Multiconfigurational Time-Dependent Hartree Approach 310

9.2.2.2 Density Matrix Formalism of MCTDH: 𝜌MCTDH 311

9.3 Case Studies 312

9.3.1 Ultrafast Dynamics of a Cu(I)–phenanthroline Complex 313

9.3.2 The Contribution of Vibronic Coupling to the rISC of PTZ-DBTO2 316

9.4 Conclusions and Outlook 322

References 323

10 Exciplex: Its Nature and Application to OLEDs 331
Hwang-Beom Kim, Dongwook Kim, and Jang-Joo Kim

10.1 Introduction 331

10.2 Formation and Electronic Structures of Exciplexes 332

10.3 Optical Properties of Exciplexes 336

10.3.1 Photoluminescence of Exciplexes 336

10.3.2 Absorption Spectra of Exciplexes 338

10.4 Decay Processes of the Exciplex in Solution 339

10.4.1 Fluorescence Rate Constant for the Exciplex State 340

10.4.2 Contact Radical Ion Pair (CRIP) Versus Solvent-separated Radical Ion Pair (SSRIP) 342

10.4.3 Charge Separation Versus Charge Recombination 343

10.4.4 Intersystem Crossing (ISC) in the Exciplex 345

10.5 Exciplexes in Organic Solid Films 346

10.5.1 Prompt Versus Delayed Fluorescence 347

10.5.2 Spectral Shift as a Function of Time 350

10.6 OLEDs Using Exciplexes 353

10.6.1 Exciplexes as Emitters 353

10.6.2 Exciplexes as Sensitizers 356

10.7 Summary and Outlook 360

Appendix 360

10.A.1 Small Molecular Pairs of Donors and Acceptors Forming Exciplexes 360

10.A.2 Small Molecules with Electron-donatingMoieties Forming Exciplexes 360

10.A.3 Small Molecules with Electron-accepting Moieties Forming Exciplexes 365

10.A.4 Small Molecules with Electron-donating and Electron-accepting Moieties Forming Exciplexes 368

References 370

11 Thermally Activated Delayed FluorescenceMaterials Based on Donor–AcceptorMolecular Systems 377
Ye Tao, Runfeng Chen, Huanhuan Li, Chao Zheng, andWei Huang

11.1 Introduction 377

11.2 TADF OLEDs 380

11.2.1 Device Structures and Operation Mechanisms of TADF OLED 380

11.2.2 TADF Molecules as Emitters for OLEDs 382

11.2.3 TADF Molecules as Host Materials and Sensitizers for OLEDs 382

11.2.4 Host-free TADF OLEDs 383

11.3 Basic Considerations in Molecular Design of TADF Molecules 384

11.3.1 Design Principles of Donor–Acceptor Molecular Systems for TADF Emission 384

11.3.2 Control of Singlet–Triplet Energy Splitting (ΔEST) 386

11.3.3 Modulation of Luminescent Efficiency of TADF Emission 389

11.4 Typical Donor–Acceptor Molecular Systems with High TADF Performance 391

11.4.1 Cyano-based TADF Molecules 391

11.4.2 Nitrogen Heterocycle-based TADF Molecules 396

11.4.3 Diphenyl Sulfoxide-based TADF Molecules 405

11.4.4 X-bridged Diphenyl Sulfoxide-based TADF Molecules 407

11.4.5 Diphenyl Ketone-based TADF Molecules 408

11.4.6 X-bridged Diphenyl Ketone TADF Molecules 410

11.5 Organoboron-based TADF Molecules 411

11.6 TADF Polymers 412

11.7 Intermolecular D–A System for TADF Emission 413

11.8 Summary and Outlook 417

References 417

12 Photophysics of Thermally Activated Delayed Fluorescence 425
AndrewMonkman

12.1 Introduction 425

12.2 Comments on the Techniques Used in Our Studies 428

12.3 Basic Absorption and Emission Properties 428

12.4 Phosphorescence and Triplet State Measurements 438

12.5 Characteristics of the Delayed Fluorescence 440

12.5.1 Time-resolved Emission in Solution 440

12.5.2 Time-resolved Emission in Solid State 446

12.5.3 Kinetics of the 1CT Prompt State 449

12.6 UnderstandingWhich Excited States are Involved 450

12.7 Excited-state Properties 452

12.8 Dynamical Processes 455

12.9 Emitter–host Interactions 457

12.10 Energy Diagram for TADF 459

12.11 Final Comments 459

Acknowledgments 461

References 461

13 Thioxanthone (TX) Derivatives and Their Application in Organic Light-emitting Diodes 465
XiaofangWei, YingWang, and PengfeiWang

13.1 Organic Light-emitting Diodes 465

13.2 Pure Organic TADF Materials in OLEDs 467

13.3 TX Derivatives for OLED 468

13.3.1 High Efficient OLEDs Based on TX-based TADF Materials 468

13.3.1.1 Design and Characterization of TX-based TADF Emitters 468

13.3.1.2 Nondoped OLEDs Based on TADF Emitters with QuantumWell Structure 481

13.3.1.3 White OLEDs Based on Blue Fluorescent Emitter and Yellow TX-based TADF Emitters 486

13.3.2 TADF Host for Phosphorescent Emitters 490

13.4 Concluding Remarks and Outlook 495

Acknowledgments 496

References 496

14 Solution-Processed TADF Materials and Devices Based on Organic Emitters 501
Nidhi Sharma,Michael YinWong, Ifor D.W. Samuel, and Eli Zysman-Colman

14.1 Introduction 501

14.1.1 Solution-Processed Blue TADF Materials and Devices 504

14.1.2 Solution-Processed Green TADF Materials and Devices 512

14.1.3 Solution-Processed Yellow-to-Red TADF Materials and Devices 523

14.1.4 Comparison of State-of-the-Art Solution-Processed OLEDs to Vacuum-Deposited Counterparts 526

14.1.5 Solution-Processed TADF Polymers and Dendrimers 527

14.2 Summary and Outlook 537

References 538

15 Status and Next Steps of TADF Technology: An Industrial Perspective 543
Alhama Arjona-Esteban and Daniel Volz

15.1 What Does the MarketWant? 543

15.1.1 The Emitter Materials: Heart of the OLED 544

15.1.2 Processing Aspects 547

15.1.3 Sustainability Aspects 549

15.1.3.1 Availability Issues 549

15.1.3.2 Recycling Considerations 550

15.1.4 Realization of Efficient and Stable Blue OLEDs 550

15.1.4.1 The Blue Gap 550

15.1.4.2 Key Performance Indicators 551

15.2 Mastering Blue OLEDs with TADF Technology 552

15.2.1 Current Status of Blue TADF Technology: Academia 552

15.2.2 Current Status of Blue TADF Technology: Industry 554

15.3 An Alternative Approach: TADF Emitters as (Co) Hosts 559

15.3.1 General Remarks 559

15.3.2 First Attempts of Using TADF as Hosts 561

15.3.3 Discussion of Various Concepts 562

15.3.3.1 TADF as Host for Other TADF Emitters 562

15.3.3.2 TADF as Host for Fluorescent Materials 563

15.3.3.3 TADF as Host for Phosphorescent Emitters 564

15.4 Outlook:What to Expect from TADF Technology in the Future 566

References 567

Index 573