Quantum Dot Display Science and Technology
Description
COMPREHENSIVE REFERENCE PRESENTING ALL ASPECTS OF QUANTUM DOT-BASED DISPLAY TECHNOLOGIES IN FOUR PARTS, SUPPORTED WITH PEDAGOGICAL FEATURES
Quantum Dot Display Science and Technology presents all aspects of quantum dot (QD) based display technologies, divided into four general topic areas: the basic science of quantum dots, QD photoluminescent technologies, QD electroluminescent technologies, and other display related QD technologies. Composed of 14 chapters, this book includes a list of pedagogical features such as tables, illustrations, process flow charts, and more to provide active learning for the reader. This book also includes information on future quantum dot displays and the major milestones in the field.
Quantum Dot Display Science and Technology discusses topics including:
- The basic physics and photophysics of QD, explaining why QD can offer better color and higher brightness
- QD material systems and compositional families as well as principles and practices of QD synthesis
- Quantum dot enhancement film and quantum dot color conversion for LCDs, OLEDs, and µLEDs
- Quantum dot electroluminescent displays and QD-LED panel processes based on ink-jet printing and lithography
- QD for lighting and photodetector applications
- Future outlook for QD displays
Published in partnership with the Society for Information Display (SID), Quantum Dot Display Science and Technology is the perfect resource for updated information on quantum dots and their applications for professionals working in displays, consumer electronics, and product design and development.
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Persons
PAUL ALIVISATOS is the 14th President of the University of Chicago, USA, where he also holds a faculty appointment as the John D. MacArthur Distinguished Service Professor in the Department of Chemistry, the Pritzker School of Molecular Engineering, and the College.
EUNJOO JANG is a Professor of Sungkyunkwan University, South Korea. She received her Ph.D. in 1998 from the Chemical Engineering Department at Pohang University of Science and Technology (POSTECH). She joined Samsung in 2000 and has been developing various QD materials and optoelectronic devices since 2023.
RUIQING MA is a Fellow of Society for Information Display (SID). He received his Ph.D. in Chemical Physics in 2000 from the Liquid Crystal Institute at Kent State University, USA. Before joining Meta in 2022, he was the Senior Director of R&D at Nanosys.
Content
Series Editor's Foreword xv
About the Editors xvii
Preface xix
Acknowledgments xxi
1 Physics and Photophysics of Quantum Dots for Display Applications 1
Einav Scharf, Uri Banin
1.1 Introduction 1
1.2 Quantum Confinement and Band Structure 1
1.3 Absorption Spectrum 4
1.4 Charge Carrier Dynamics 6
1.5 Surface Passivation and Heterostructure Band Alignment 8
1.6 Emission Intermittency (Blinking) and Stability 9
1.7 Emission Linewidth 12
1.8 Dimensionality Effects 15
1.9 Collective Emission 16
1.10 Summary and Outlook 18
References 18
2 Quantum Dot Material Systems, Compositional Families 23
Sudarsan Tamang, Karl David Wegner, Peter Reiss
2.1 Introduction 23
2.2 II-VI Semiconductor QDs 25
2.2.1 Cadmium Chalcogenide QDs 25
2.2.2 Zn Chalcogenide QDs 27
2.3 III-V Semiconductor QDs: Overview and Properties 35
2.3.1 Introduction 35
2.3.2 Indium Phosphide Quantum Dots 37
2.3.3 Indium Arsenide Quantum Dots 47
2.4 More Recent Families of QDs 50
2.4.1 I-III-VI Chalcopyrite-type QDs 50
2.4.2 Metal Halide Perovskite NCs 54
2.5 Summary and Outlook 60
References 62
3 Principles and Practices for Quantum Dots Synthesis 81
Derrick Allan Taylor, Justice Agbeshie Teku, Jong-Soo Lee
3.1 Introduction 81
3.2 Principles of Colloidal Quantum Dot Synthesis 84
3.2.1 Basic Chemistry of Quantum Dot Synthesis 84
3.2.2 Innovatory Experimental Techniques for Monitoring Evolving Nanocrystals 93
3.2.3 Colloidal Quantum Dots (II-VI and III-V) 94
3.3 Practices of Colloidal Quantum Dot Synthesis 95
3.3.1 Practices 98
3.3.2 Post-synthetic Methods 104
3.4 Summary and Outlook 112
References 114
4 Quantum Dot Enhancement Film 131
Zhong Sheng Luo, Jeff Yurek
4.1 Introduction 131
4.2 Understanding Color for Displays 132
4.2.1 Measuring Display Color Performance: Chromaticity Gamut 134
4.2.2 NTSC 1953 in Practice 135
4.2.3 LCDs and Display Color in the 1990s and 2000s 136
4.3 Color in the Modern Era - Defining the Ultimate Visual Experience 138
4.3.1 Color Volume 139
4.3.2 High Dynamic Range 141
4.3.3 Clarity 142
4.4 Quantum Dots for QDEF Applications 143
4.4.1 Quantum Dot Wavelength Tunability 144
4.4.2 Narrower Spectrum for Better Color 145
4.5 Quantum Dot Enhancement Film 146
4.5.1 Origins of the QDEF Concept 146
4.5.2 Design Requirements 149
4.5.3 Resin System 150
4.5.4 Barrier Film 150
4.5.5 QD Coating 152
4.5.6 QDEF Fabrication Process 152
4.5.7 QDEF in a Display 154
4.5.8 Heavy Metals and Environmental Regulation 155
4.6 Barrierless Quantum Dot Enhancement Film 156
4.6.1 QD Requirements for Barrierless QDEF 157
4.6.2 Construction and Manufacturing 158
4.6.3 Application 158
4.7 Quantum Dot Diffuser Plate 159
4.7.1 Quantum Dot Requirement 159
4.7.2 Construction and Manufacturing 160
4.7.3 Application 161
4.8 Summary and Outlook 161
References 162
5 Quantum Dot Color Conversion for Liquid Crystal Display 167
Zhifu li, Ji li, Yanan Wang, Hanming li
5.1 Introduction 167
5.2 Thin-film Transistor Liquid Crystal Display 168
5.2.1 Color Perception of Human Eyes 168
5.2.2 Basic Structure and Principle of Liquid Crystal Display 169
5.2.3 Advantages of Quantum Dot Liquid Crystal Display 172
5.3 Quantum Dot Color Conversion for Liquid Crystal Display 173
5.3.1 Quantum Dot Backlight 173
5.3.2 Quantum Dot Color Filter 178
5.4 Summary and Prospects 191
References 193
6 Quantum Dot (QD) Color Conversion for QD-Organic Light-Emitting Diode 197
Keunchan Oh, Hyeokjin Lee, Gakseok Lee, Taehyung Hwang
6.1 Introduction to Quantum Dot-Organic Light-emitting Diode 197
6.2 Color Conversion Materials 199
6.2.1 Quantum Dots in QD-OLED 200
6.2.2 Optical Scattering Particle 204
6.2.3 Surface Ligand Modification 207
6.2.4 Photo Enhancement and Degradation 210
6.3 Color Conversion Architecture 212
6.3.1 Bank 212
6.3.2 Color Filter 214
6.3.3 Optical Recycling Layer 215
6.3.4 Reflection 217
6.4 Inkjet Printing of CCM 218
6.4.1 Inkjet Equipment and Inspection 219
6.4.2 Rheological Properties of Colloidal QD Ink 220
6.4.3 Large Area Uniformity 224
6.5 Conclusion and Future Work 225
References 226
7 Quantum Dots for Augmented Reality 231
Jason Hartlove
7.1 Why Quantum Dots for Augmented Reality? 231
7.2 Augmented Reality Glasses: The Need for High-efficiency Small Emitters 232
7.2.1 ARG Requirements 232
7.2.2 Display Engine Approaches 235
7.3 QD Color Conversion Performance and Reliability Requirements 247
7.3.1 Quantum Dot PLQY 247
7.3.2 Quantum Dot Absorption 248
7.3.3 Flux Stability 249
7.4 Summary and Outlook 250
References 251
8 CdSe-based Quantum Dot Light-emitting Diodes 253
Yiran Yan, Longjia Wu, Weiran Cao, Xiaolin Yan
8.1 Overview of Quantum Dot Light-emitting Diode Development 253
8.2 Functional Layers 255
8.2.1 QD-emitting Layer 255
8.2.2 Hole Transport Layer 260
8.2.3 Electron Transport Layer 262
8.3 Aging Mechanism 264
8.3.1 Degradation Mechanism 264
8.3.2 Positive Aging Mechanism 272
8.4 Summary and Outlook 277
References 277
9 Quantum Dot Light-emitting Device Materials, Device Physics, and Fabrication: Cadmium-free 283
Igor Coropceanu, Heeyoung Jung, Christian Ippen
9.1 Introduction 283
9.1.1 Benefits of Quantum Dot Light-emitting Devices 283
9.1.2 Why Cd-free QD-LED? 284
9.2 Survey of Materials 285
9.2.1 General Considerations 285
9.2.2 Indium Phosphide 286
9.2.3 Zinc Telluride Selenide 290
9.2.4 I-iii-vi 293
9.3 Surface Chemistry 293
9.3.1 General Introduction to NC - Organic Interface 293
9.3.2 Inorganic Termination 293
9.3.3 Anchoring Group 294
9.3.4 Ligand Body 294
9.3.5 Organic Ligand Exchange for Improved Charge Transport 295
9.3.6 Inorganic and Mixed Organic/Inorganic Surface Treatments 296
9.4 Device Physics and Fabrication 298
9.4.1 Device Architectures 298
9.4.2 Evaluation Metrics 300
9.4.3 HTL Optimizations 301
9.4.4 ETL Optimizations 302
9.4.5 Positive Aging 302
9.4.6 Degradation Mechanisms 303
9.5 Patterning for Display Fabrication 305
9.5.1 General Considerations 305
9.5.2 Optical Methods 306
9.5.3 Inkjet Printing 308
9.6 Summary and Outlook 309
9.6.1 Performance Development of Cd-free vs. Cd-based QD-LEDs 309
9.6.2 What is Still Missing for Cd-free QD-LEDs? 311
References 311
10 Quantum Dot Light-emitting Diode Panel Process: Inkjet Printing 323
Dong Jin Kang, Changhee Lee
10.1 Inkjet Printing Technology for QD Patterning in Full-color Displays 323
10.2 Ink Formulation for Inkjet-Printed QD-LED Displays 325
10.2.1 Quantum Dot Inks 325
10.2.2 Organic Charge-transport Material Ink 328
10.2.3 Inorganic Charge-transport Material Inks 331
10.3 Inkjet Printing Processes and Device Performance of QD-LED Display Panels 331
10.3.1 Device Structure and Operation Mechanism of QD-LEDs 331
10.3.2 Device Characteristics of QD-LEDs 333
10.3.3 Inkjet Printing Processes for Fabricating QD-LED Display Panels 335
10.3.4 Drying and Thermal Baking Processes for QD-LED Panels 339
10.3.5 Device Performance of Inkjet-printed QD-LED Display Panels 342
10.4 Current Challenges in Inkjet Printing for QD-LED Display and Future Outlook 347
10.5 Summary and Outlook 348
References 349
11 Photolithographic Patterning Techniques for Quantum Dot Light-emitting Diodes 355
Yanzhao Li, Shaoyong Lu, Zhuo Chen, Zhuo Li, Xiangbing Fan, Peng Bai, Haoyu Yang, Dong li
11.1 Introduction 355
11.2 Photolithography Technology 357
11.2.1 Basics of Photolithography 357
11.2.2 Photolithographic Patterning of Quantum Dots 359
11.3 Indirect Photoresist-assisted Photolithographic Patterning of Quantum Dots 360
11.3.1 Protective Photoresists 360
11.3.2 Sacrificial Photoresists 363
11.4 Direct Photoresist-free Photolithographic Patterning of Quantum Dots 366
11.4.1 Patterning Using Native Ligands 367
11.4.2 Patterning Through Ligand Exchange 374
11.4.3 Photolithographic Patterning for Maintaining Photophysical Properties of Quantum Dots 377
11.5 Industrial Progress 381
11.6 Summary and Outlook 382
References 383
12 Quantum Dots in Light-emitting Diodes for General Lighting 387
Benjamin Mangum, Juanita Kurtin
12.1 Benefits of Quantum Dots for Illumination 387
12.2 Illumination Landscape: The Need for Narrow Emitters 387
12.2.1 Background: Blackbody Emitters vs. LEDs 387
12.2.2 Making White LEDs: Spectral Engineering 390
12.2.3 Background: Color Metrics 392
12.2.4 The Ideal Spectrum and Theoretical Maximums 395
12.3 SSL Devices and Solution Development 399
12.3.1 Power Classes 399
12.3.2 Quantum Dots for Illumination 400
12.3.3 Form Factor 401
12.3.4 Pairing QDs with Other Phosphors 403
12.4 QD Performance and Reliability Requirements 405
12.4.1 QD Performance Requirements: PLQY 406
12.4.2 QD Performance Requirements: FWHM 406
12.4.3 QD Performance Requirements: Flux Droop 407
12.4.4 Performance Requirements: Thermal Droop 408
12.4.5 Reliability Testing: LM80 testing 408
12.4.6 Reliability Testing: Color Point Shift 409
12.4.7 Reliability Testing: Lumen Maintenance 410
12.5 Summary and Outlook 411
References 412
13 Quantum Dot Photodetector Technology 415
Pawel Malinowski, Itai Lieberman, Jonathan S. Steckel, Andras Pattantyus-Abraham
13.1 Introduction to Sensing with Quantum Dots 415
13.1.1 Photoconductive Devices 416
13.1.2 Photodiodes 416
13.1.3 Phototransistors 417
13.1.4 Other Light Sensing Techniques 418
13.2 Figures of Merit for QD Sensors 418
13.2.1 QD Films and Stacks 418
13.2.2 Photodetector Performance Metrics 419
13.2.3 Image Sensors Performance Metrics 423
13.2.4 Reliability 425
13.3 QD Photodetector Materials and Devices 426
13.3.1 QD Core and Photodetectors 426
13.3.2 QDPD Comparison 431
13.3.3 Evolution of QD Image Sensors 431
13.4 Conclusion and Outlook 434
13.4.1 Use Cases and Applications 434
13.4.2 Outlook 438
References 439
14 Future of Quantum Dots in Displays and Beyond 445
Peter Palomaki
14.1 Introduction 445
14.2 Implementation of QDs Past, Present, and Future 446
14.2.1 Past Technologies 446
14.2.2 Present Technologies 447
14.2.3 Future Technologies 447
14.3 QD Materials 452
14.3.1 CdSe and InP 452
14.3.2 Perovskite 453
14.3.3 I-III-VI QDs 455
14.3.4 Nitrides 456
14.3.5 Material Usage 456
14.3.6 Anisotropic QD Systems 458
14.3.7 Stability 460
14.4 Optical Properties 462
14.4.1 Linewidth 462
14.4.2 Light Absorption 465
14.4.3 Spectral Engineering and Re-absorption 465
14.4.4 QDs and Phosphors 466
14.4.5 Four or More Primaries 467
14.5 Regulatory 468
14.6 Non-display Applications 470
14.6.1 Solar Spectrum Engineering 470
14.6.2 QD Solar Cells 471
14.7 Summary 472
References 473
Index 477
Chapter 1
Physics and Photophysics of Quantum Dots for Display Applications
Einav Scharf, Uri Banin
Institute of Chemistry and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem, Israel
1.1 Introduction
Semiconductor quantum dots (QDs) are nanocrystals composed of hundreds to thousands of atoms, forming a lattice of nanometric size. They exhibit highly bright and stable emission, with color tunable via size, composition, and shape, and can achieve fluorescence quantum efficiency approaching unity [1]. These qualities along with their narrow emission linewidth make them prominent building blocks for display applications. The unique characteristics of QDs already enhance the properties of existing display technologies, and with further developments, they are bound to penetrate display device technologies even to greater extent. In this chapter, we will review briefly the fundamental principles governing the optoelectronic characteristics of such QDs, serving as a basis for their utility in displays.
1.2 Quantum Confinement and Band Structure
The electronic structure of semiconductor QDs manifests a manifold of fully occupied valence band (VB) states and a manifold of empty conduction band (CB) states, separated by the bandgap. Excitation of the QD, for example, by the absorption of a photon, promotes an electron to the CB, leaving an electron vacancy, a hole, in the VB. This electron-hole pair, termed as an exciton, is bound by Coulomb attraction, granting a typical exciton binding energy and an exciton Bohr radius [2]. The Bohr radius is analogous to the Bohr radius of an electron orbiting the nucleus of a hydrogen atom. However, in the case of the QD, it is influenced by the dielectric environment and the effective mass of the semiconductor electron and hole charge carriers, resulting in an exciton Bohr radius in the nanometric scale. QDs that are smaller than the exciton Bohr radius of the corresponding semiconductor exhibit strong quantum confinement, which leads to discretization of the energy levels and size-dependent electronic and optical properties (Figure 1.1a) [3]. By varying the size of the QD, for suitable semiconductors such as InP or CdSe, its emission color can thus be tuned from blue for the smallest QDs with a diameter smaller than 2 nm, through the entire visible spectrum and to the near-infrared for large QDs with a radius of 10 nm (Figure 1.1b).
Figure 1.1 (a) Quantum confinement effect in quantum dots (QDs). The electronic structure varies with the size of QD. A bulk semiconductor (in gray) presents a fundamental bandgap between the valence band (VB) and conduction band (CB). Upon formation of QDs, the discrete states arise due to the quantum confinement effect and the bandgap increases from large to small QDs. (b) Emission from a series of CdSe QDs with sizes ranging from smaller than 2-6 nm with colors covering the visible spectrum, from blue to red, respectively, demonstrating the quantum confinement effect. (c) Electronic structure of the first two energy levels in the VB and CB.
The QD behavior can be derived by solving the particle-in-a-spherical-box problem, describing the behavior of an electron and a hole in the QD [4]. This involves solving the Schrödinger equation, in its central potential form (the potential depends only on radius):
(1.1)where the Hamiltonian has separable radial and angular components. The first two terms are of the kinetic energy. is the square of the angular momentum operator, is the potential energy, and is the wave function. This problem resembles the problem of the hydrogen atom considering it being a central potential that depends on radius, and accordingly the solution for the angular part of the wave functions is similar as well. However, the main difference between the derivation of the hydrogen atom and of the QD is in the actual form of the potential energy. In the hydrogen atom, the electron is attracted to the nucleus by the Coulomb potential, whereas in the case of an electron in a QD, the potential inside the spherical box is zero and, for simplification as a first approximation, is infinite outside the box. In both the hydrogen atom and the QD, the potential depends solely on the radial distance and is independent of the angle. Therefore, in both problems, the angular solution is described by spherical harmonics. In QDs, the spherical solution can be described by spherical Bessel functions. Since the solution of the angular equation is the spherical harmonics, the energy levels are defined by four quantum numbers: the principal quantum number n, the angular momentum quantum number , the angular momentum projection quantum number , and the spin quantum number . In contrast to the hydrogen atom, the condition on the relation between the principal and angular momentum quantum numbers is canceled. Accordingly, the energy levels are denoted as for the electron and for the hole states, with denoted in numbers, and using common notation of for , for , for , etc. The first energy levels in the QD are thus , , for the electron (hole) in the CB (VB; Figure 1.1c) [5]. As in the hydrogen atom, the value of is in the range of to , resulting in a degeneracy of . The energy levels under the strong confinement approximation, for QDs that are smaller than the exciton Bohr radius, are described by:
(1.2)where is the effective mass of the electron or the hole , is the radius of the QD, and is the allowed solutions arising from demanding that the wave function is 0 on the surface of the QD. For the band edge optical transition (), . The strong confinement approximation allows to treat the electron and hole as uncorrelated, neglecting in the first step the Coulomb interaction [6, 7]. Then, the Coulomb term is reintroduced using perturbation theory. This redshifts the bandgap by adding the weak attractive Coulomb interaction term [8], as approximated by:
(1.3)where is the bulk bandgap energy, and are the electron and hole effective masses, respectively, is the electron charge, and is the dielectric constant.
The optical transitions, which are typically seen in absorption and emission, are dictated by selection rules [6]. The transition probability is proportional to:
(1.4)where is the wave function of the electron or the hole , and is the transition dipole moment operator.
Under the envelope function approximation, the electron and hole wave functions are separated into the periodic Bloch part and the envelope part (see Eq. 1.5) [9]. Integration of the Bloch part is related to the bulk crystal lattice, approximated to be unaffected by the QD size ( in Eq. 1.5). Integration of the envelope part yields the overlap term for the electron-hole wave functions, and as the eigenstates of a particle-in-a-sphere are orthonormal, we obtain:
(1.5)where is the Bloch term of the bulk semiconductor and is the envelope function ( or for electron or hole, respectively). is the oscillator strength in the bulk semiconductor and is the Kronecker delta function. Accordingly, the allowed optical transitions are those that conserve the principal and angular momentum quantum numbers of the electron-hole envelope functions of a particle-in-a-sphere (, ) [9, 10].
According to the similarity to degeneracy of the hydrogen atom energy levels, QDs can be referred to as artificial atoms [11]. This property can be probed by scanning tunneling microscopy, where a voltage is applied on a tip hovering above the QD at a distance of ~1 nm, and the tunneling current is measured. Figure 1.2a shows the tunneling I-V curve of an InAs QD [12]. Plotting the tunneling conductance spectrum reveals the density of states (Figure 1.2b). In the positive bias, the CB energy levels are probed, revealing a doublet of the two electrons in the energy level, separated by a charging energy. After a larger separation, a sextet is resolved to be assigned to the six electrons occupying the energy level. In negative bias, the VB states are slightly more convoluted, as the spacing between the energy levels is smaller. The difference between the VB and CB apparent density of states arises from the typically heavier hole, confining the energy levels closer to the band edge, and from the different orbitals constructing the bands. The CB is typically essentially constructed from the empty atomic s orbitals of the corresponding cationic element composing the semiconductor (i.e. In3+ in the case of InAs), whereas the VB is typically constructed from the atomic p orbitals related to the corresponding anionic element (As3- in the case of InAs). At , the bulk VB has a twofold degeneracy of the heavy hole and light hole p3/2 bands and below them the split-off hole p1/2 band (Figure 1.2c) [13]. This leads to a rich and dense level structure in the VB of such QDs.
Figure 1.2 (a) Tunneling I-V curve of an InAs quantum dot (QD). The QD is linked to a gold substrate and the scanning tunneling microscopy (STM) tip scans it from the top (right inset). The left inset presents a 10 × 10 nm2 STM topographic image of the QD. (b) Tunneling conductance spectrum presenting the density of states in the CB (VB) in positive (negative) bias. is the charging energy.
Source: (a, b) Reproduced from [12]/with permission of Springer...
