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Comprehensively covering inorganic flexible optoelectronics and their applications
This highly application-oriented book provides an overview of the vibrant research field of inorganic flexible optoelectronics ? from materials to applications ? covering bulk materials as well as nanowires, thin films, nanomembranes for application in light emitting diodes, photodetectors, phototransistors, and solar cells.
Edited and written by world-leading experts in the field, Inorganic Flexible Optoelectronics: Materials and Applications begins by covering flexible inorganic light emitting diodes enabled by new materials and designs, and provides examples of their use in neuroscience research. It then looks at flexible light-emitting diodes based on inorganic semiconductor nanostructures ? from thin films to nanowires. Next, the book examines flexible photodetectors with nanomembranes and nanowires; 2-D material based photodetectors on flexible substrates; and IV group materials based solar cells and their flexible photovoltaic technologies. Following that, it presents readers with a section on thin-film III-V single junction and multijunction solar cells and demonstrates their integration onto heterogeneous substrates. Finally, the book finishes with in-depth coverage of novel materials based flexible solar cells.
-A must-have book that provides an unprecedented overview of the state of the art in flexible optoelectronics
-Supplies in-depth information for new and already active researchers in the field of optoelectronics
-Lays down the undiluted knowledge on inorganic flexible optoelectronics ? from materials to devices
-Focuses on materials and devices for high-performance applications such as light-emitting diodes, solar cells, and photodetectors
Inorganic Flexible Optoelectronics: Materials and Applications appeals to materials scientists, electronics engineers, electrical engineers, inorganic chemists, and solid state physicists.
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Veröffentlichungsjahr: 2019
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Table of Contents
Cover
Preface
1 Flexible Inorganic Light Emitting Diodes Enabled by New Materials and Designs, With Examples of Their Use in Neuroscience Research
1.1 Introduction
1.2 Flexible Micro‐Inorganic LEDs (μ‐ILEDs)
1.3 Flexible Quantum Dot LEDs (QLEDs)
1.4 Flexible Perovskite LEDs (PeLEDs)
1.5 Flexible 2D Materials‐Based LEDs
1.6 Opportunities for Flexible Optoelectronic Systems in Neuroscience Research
1.7 Conclusion
References
2 Flexible Light‐Emitting Diodes Based on Inorganic Semiconductor Nanostructures: From Thin Films to Nanowires
2.1 Introduction
2.2 Flexible LEDs Based on Thin‐Film Transfer
2.3 Nanowire LEDs and Their Potential Advantages
2.4 Flexible LEDs Based on Inorganic Bottom‐Up Nanowires
2.5 Conclusions
References
3 Flexible Photodetectors with Nanomembranes and Nanowires
3.1 Introduction
3.2 Flexible Photodetectors
3.3 Performance Parameters
3.4 Fabrication of Donor Substrates for Transferrable NMs
3.5 Transfer Printing of Single Crystalline Semiconductor NMs
3.6 Semiconductor NM‐Based Flexible Photodetectors
3.7 Fabrication of NW‐Based Flexible Detectors
3.8 Fabrication of Flexible Photodetectors Based on AgNWs/CdS NWs
3.9 Results and Discussion
3.10 Conclusions and Outlook
References
4 2‐D Material‐Based Photodetectors on Flexible Substrates
4.1 Introduction
4.2 Performance Metrics of Photodetectors
4.3 Working Mechanisms of 2D Photodetectors
4.4 2D Photodetectors on Flexible Substrates
4.5 Outlook and Perspectives
References
5 IV Group Materials‐Based Solar Cells and Their Flexible Photovoltaic Technologies
5.1 Introduction
5.2 IV Group Materials‐Based Solar Cells
5.3 Flexible Solar Cells Technology with Group IV Materials
5.4 Mechanics Analysis
5.5 Applications
5.6 Conclusions
References
6 Thin‐Film III–V Single Junction and Multijunction Solar Cells and Their Integration onto Heterogeneous Substrates
6.1 Introduction
6.2 III–V Solar cells
6.3 Thin‐Film III–V Solar Cells on Flexible Substrates
6.4 Applications
6.5 Future Generations
6.6 Conclusion
References
7 Novel Materials‐Based Flexible Solar Cells
7.1 Flexible Perovskites Solar Cells
7.2 Flexible CdTe/CdS Solar Cells
7.3 Infrared Colloidal Quantum Dots Solar Cell
References
Index
End User License Agreement
List of Tables
Chapter 3
Table 3.1 Obtained length and diameter of different NWs.
Table 3.2 Different sets of rise and decay time obtained with different ...
Table 3.3 Comparison of critical parameters for various NW photodetector...
Chapter 5
Table 5.1 Failure modes and their critical applied strain for Si ribbons...
Chapter 7
Table 7.1 Summary of the performance of non‐indium oxide flexible PSCs. ...
Table 7.2 Summary of the performance of indium oxide flexible PSCs.
List of Illustrations
Chapter 1
Figure 1.1
Flexible μ‐ILEDs
. (a) (Top) Schematic illustrati...
Figure 1.2
Flexible μ‐ILEDs: Examples of integrated devices in biomedi
...
Figure 1.3
Flexible QLEDs: Materials designs, fabrication techniques,
...
Figure 1.4
Flexible QLEDs: State‐of‐the‐art devices and their applicat
...
Figure 1.5
Light responsive, dual‐functional DHNR LEDs
. (a) Ener...
Figure 1.6
Flexible PeLEDs
. (a) Emission‐wavelength tunability of CH
3
N...
Figure 1.7
Flexible 2D material based LEDs
. (a) A broad library of 2D ...
Figure 1.8 Schematic illustration of key stimulus and recording capabili...
Figure 1.9
Miniaturized optoelectronic elements for implantable wirele
...
Figure 1.10
Miniaturized and wireless tools for optogenetic stimulatio
...
Figure 1.11
Miniaturized, wireless tools for use in the peripheral ner
...
Chapter 2
Figure 2.1 Epitaxial lift‐off of conventional thin‐film LEDs. (a) Schema...
Figure 2.2 Waterproof flexible GaAs‐based micro‐LED array by micro‐trans...
Figure 2.3 (a) Schematic illustration of the exfoliation process of a re...
Figure 2.4 Photos of InGaN LEDs grown on h‐BN‐on‐sapphire substrates tra...
Figure 2.5 Maximum external quantum efficiency of different commercial n...
Figure 2.6 Schematic illustrations of (a) radial and (b) axial nanowire ...
Figure 2.7 InGaN nanowire LEDs produced by selective area growth with a ...
Figure 2.8 Selective area growth MOVPE InGaN core–shell nanowire LEDs. (...
Figure 2.9 (a) Schematic illustration of a ZnO nanowire LED directly gro...
Figure 2.10 (a) Electroluminescence spectra of an AlGaN/GaN nanowire arr...
Figure 2.11 (a) Tilted view and (b) cross‐sectional view SEM images of a...
Figure 2.12 (a) Schematic illustration of a flexible self‐emitting displ...
Figure 2.13 Flexible nitride nanowire LED. (a) ZnO/nitride LEDs under op...
Figure 2.14 (a–c) Schematic illustration of the fabrication procedures a...
Figure 2.15 Photographs of operating flexible (a) blue, (b) green, and (...
Figure 2.16 Tilted SEM image of InGaN/GaN core/shell nanowires grown on ...
Figure 2.17 Selective area epitaxy of GaN nanowires on CVD‐graphene nano...
Figure 2.18 (a) Photograph of the flexible LED wrapped around a paper cl...
Chapter 3
Figure 3.1 SEM image of magnified view of ZnO NW of area outlined in whi...
Figure 3.2
SEM image of synthesized hierarchical CdS Nano wires under
...
Figure 3.3 Photograph of hybrid photodetectors on printing paper.
Figure 3.4
A schematic process flow of the fabrication of GeOI wafer
: ...
Figure 3.5
Generic process for Si NM release from SOI and transfer
. (a...
Figure 3.6 (a–c) Optical, cross‐sectional schematic, and microscopic ima...
Figure 3.7
The schematic illustration of the device fabrication proces
...
Figure 3.8 (a) Drain current–gate voltage characteristics (
I
DS
–
V
GS
) ...
Figure 3.9 (a) Microscopic image of transferred Si NM grid on PET substr...
Figure 3.10 (a)
I–V
characteristics of the diode in dark and under ...
Figure 3.11 (a) Process flow of Ge‐on‐polyimide structure. (b) A photogr...
Figure 3.12 (a) Representative image of an array of single crystal 250 n...
Figure 3.13 (a) A schematic process flow for the Ge NM‐based MSM photode...
Figure 3.14
A flow chart for FAMT process
: (a) Formation of release ho...
Figure 3.15 (a) Cross‐sectional and (b) three‐dimensional views of flexi...
Figure 3.16
Measured flexible InP p–i–n photodetector characteristics
...
Figure 3.17 SEM image of single crystal Si NW and catalytic
chemical vap
...
Figure 3.18 Assembly of NWs.
Figure 3.19 Schematic illustration of the flexible CdS‐NWs photodetector...
Figure 3.20 (a) SEM cross‐sectional view of the Ag/CdS NWs overlapping a...
Figure 3.21
I
–
V
characteristics of the hybrid photodetector as a functi...
Figure 3.22 Dark current and photocurrent for the free standing films un...
Figure 3.23 Time‐dependent on/off switching of the device based on free‐...
Figure 3.24
I
–
T
curve of flexible photodetector bend with different ben...
Figure 3.25 Plot showing the current responses to dynamic loading and un...
Figure 3.26 Enlarged view of one cycle in photoresponse characteristics ...
Figure 3.27 Single modulation cycle of photodetector exposed to the blue...
Figure 3.28
I
–
V
characterstics under dark and light intensity for CdSe ...
Figure 3.29 Relative capacitance variation pressure curves of capacitive...
Figure 3.30 UV–VIS absorbtion spectra of P3HT film, CdSe nano wire film ...
Figure 3.31 Room temperature photocurrent spectra in logarithmic scale u...
Chapter 4
Figure 4.1
Schematic representation of the four photocurrent generatio
...
Figure 4.2
Schematic illustration of the photogating effect by a graph
...
Figure 4.3
Thin films of solution‐processed GO
. Photographs of G...
Figure 4.4 (a) Photograph of a graphene‐coated PET substrate, and schema...
Figure 4.5 (a) Schematic representation of the energy level alignment (t...
Figure 4.6 (a) Responsivity of the photoconductor based on PbS QDs and g...
Figure 4.7 (a) Schematic cross‐sectional and top views of the top‐gate g...
Figure 4.8 (a) The picture of the flexible substrate with the patterned ...
Chapter 5
Figure 5.1 Schematic diagram of a PERC cell (passivated emitter and rear...
Figure 5.2 Structure of crystalline silicon heterojunction solar cell wi...
Figure 5.3
Schematic image and IV curve of the HJ‐IBC cell
. (a) ...
Figure 5.4 (a) Optical, (b) top view SEM, and (c) cross‐sectional SEM im...
Figure 5.5
SEM images
of (a) close‐packed PS monolayer template on the ...
Figure 5.6
Scheme of two possible diffusion models during metal‐assist
...
Figure 5.7
Topological images of nc‐SiO
x
:H/μc‐Si:H film st
...
Figure 5.8 Process flow for fabricating exfoliated homojunction germaniu...
Figure 5.9
Development of C/Si heterojunction solar cells
. The timelin...
Figure 5.10 Process sequence for Si solar cell fabrication by layer tran...
Figure 5.11
Atomic force microscopy
(
AFM
) images with 20 m sq
...
Figure 5.12 Process flow for
single heterojunction structure
(
SHJ
) solar ...
Figure 5.13
Flexible ultrathin Si films and Si films with various shap
...
Figure 5.14 The bend strength of PERC solar cell with thickness varying ...
Figure 5.15 (a–c) Schematic illustration of steps for transferring Si ri...
Figure 5.16 (a) Beam model of post‐buckling analysis; (b) stress analysi...
Figure 5.17 The stress to strength ratio versus the Si ribbon thickness....
Figure 5.18 The critical radius of curvature versus the thickness ratio ...
Figure 5.19 Photovoltaic textile as a tent (http://cssproperty.com/new‐g...
Figure 5.20
Photo‐electrochemical nitrogen reduction
. (a) Schema...
Chapter 6
Figure 6.1 Theoretical Shockley–Queisser (SQ) detailed balance efficienc...
Figure 6.2 (a) Schematic illustration of the electron–hole pairs generat...
Figure 6.3 (a) Schematic illustration of the controlled spalling process...
Figure 6.4 (a) Schematic illustration of epitaxial lift‐off (ELO) proces...
Figure 6.5 (a)
Scanning electron microscopy
(
SEM
) image and schematic ill...
Figure 6.6 (a) Optical images (b) and SEM cross‐sectional images of the ...
Figure 6.7 (a) Schematic illustration of the luminescent concentrators (...
Figure 6.8 (a) Vanguard I satellite (https://nssdc.gsfc.nasa.gov/nmc/spa...
Figure 6.9 (a) Schematic illustrations (b) and SEM images of the 3J InGa...
Figure 6.10 (a) Schematic illustrations of spectrum splitting in multiju...
Figure 6.11 (a) Schematic illustrations of photon dynamics in multijunct...
Chapter 7
Figure 7.1 Structure of Perovskite (ABX
3
).
Figure 7.2 Processing scheme for perovskite thin film using spin‐coating...
Figure 7.3 (a) Schematic illustration of slot‐die coating with a gas‐que...
Figure 7.4 Schematic of perovskite film formation through vapor‐Assisted...
Figure 7.5 Schematics of the perovskite film fabrication using MAI and P...
Figure 7.6 (a) Images of fiber‐shaped flex‐PSC undergoing tying (top) an...
Figure 7.7 Schematic of (a) “partial” and (b) “complete” encapsulation a...
Figure 7.8 Optical transmission photographs showing the loss of Ca film ...
Figure 7.9 Degradation scheme of CH
3
NH
3
PbI
3
perovskite solar cells during...
Figure 7.10 Structure of CdTe solar cells. Cross‐sectional scanning elec...
Figure 7.11 Electron beam‐induced current (EBIC) measurements of CdTe so...
Figure 7.12 Electrical characteristics of CdTe solar cells. (a) Current ...
Figure 7.13 Concept of roll‐to‐roll coating for production of CdTe solar...
Figure 7.14 Transmittance of polyimide (PI) films. Transmittance (
T
) (so...
Figure 7.15 Electrical characteristics of a monolithically integrated fl...
Figure 7.16 Structures of CdTe/CdS solar nanopillar (SNOP) cells. (a) Ba...
Figure 7.17 SNOP cell fabrication stages. (a) SEM image of highly period...
Figure 7.18 Flexible SNOP cells. (a) Structure of SNOP cell embedded in ...
Figure 7.19 Flexible CdTe solar cells on thin‐film glass. (a) Transmitta...
Figure 7.20 Bending tests of flexible CdTe solar cells. Electrical perfo...
Figure 7.21 Flexible CdTe solar cells with 16.42% efficiency. NREL‐certi...
Figure 7.22 Performance enhancement of CdTe solar cells. (a) Enhanced QE...
Figure 7.23 The Solar power spectrum at the surface of Earth and the cor...
Figure 7.24 Dark current and photocurrent versus applied bias at the ITO...
Figure 7.25 Photocurrent spectral responses and absorption spectra. Main...
Figure 7.26 Organic and atomic ligand passivation strategies. The molecu...
Figure 7.27 Surface engineering of CQD solids for air stability. To real...
Figure 7.28 Solar cell efficiency chart from NREL. The progress of CQD s...
Guide
Cover
Table of Contents
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Inorganic Flexible Optoelectronics
Materials and Applications
Edited by Zhenqiang Ma and Dong Liu
Copyright
Editors
Prof. Zhenqiang Ma
University of Wisconsin‐Madison
Department of Electrical and Computer Engineering
1415 Engineering Drive
Madison, WI 53706
United States
Dr. Dong Liu
University of Wisconsin‐Madison
Department of Electrical and Computer Engineering
1415 Engineering Drive
Madison, WI 53706
United States
Cover Image: © Creative Commons,
© Oleksandra Korobova/Getty Images
The image has previously been published
in Zhang et al. Nature Communications
volume 8, Article number: 1782 (2017).
All books published by Wiley‐VCHare carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.:applied for
British Library Cataloguing‐in‐Publication DataA catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN:978‐3‐527‐34395‐9
ePDF ISBN:978‐3‐527‐81296‐7
ePub ISBN:978‐3‐527‐81298‐1
oBook ISBN:978‐3‐527‐81300‐1
Preface
In conventional optoelectronics, inorganic semiconductors are employed as functional materials primarily due to their long‐term stability under mechanical, electrical, and environmental stress as well as processing compatibility with existing infrastructures used for deposition, crystallization, doping, etc. However, the rigid form of the optoelectronic devices renders them difficult to be employed in many application situations. In contrast, mechanically flexible optoelectronic devices and systems can enable a much broader range of applications than what their rigid counterparts can do. Examples include rollable displays, deformable or implantable light sources for biological study and health care, skin sensors for a humanoid robot, energy harvesters embedded in clothing, flexible solar cells, etc. Organic materials can easily fulfill the mechanical flexibility requirement of flexible optoelectronic devices along with the additional advantages of low cost, compatibility with various flexible substrates, and large‐scale manufacturing capability. However, organic materials suffer from oxidation, recrystallization, and temperature‐induced degradation, which can jeopardize device performance in terms of electrical conductivity, interfaces quality, etc.
This book focuses on flexible optoelectronics based on inorganic materials. The frontier developments and applications of flexible optoelectronics are comprehensively captured and elucidated within seven chapters, which cover all types of optoelectronic devices and all inorganic materials that have been used to develop the devices to date. The inorganic material forms include 3D (“bulk” and micros‐sized “bulk” materials), quasi 2D (crystalline nanomembranes), 2D, 1D (nanowires), and 0D (quantum dots), ranging from Group IV to Group III–V, to II–VI, and to perovskites. The optoelectronic devices covered in this book include light emitting diodes (LED), photodetectors (PD), and photovoltaic devices. Some novel applications of flexible optoelectronic devices beyond their traditional ones (e.g. lighting, energy harvesting, and light detection), such as neuroscience research, are also included in the book.
The book is intended as a comprehensive reference for advanced‐level students and researchers with backgrounds in semiconductor materials and electronic devices, in particular, for those who are interested in flexible electronics and optoelectronics. Chapter 1, coauthored by Hao Zhang, Philipp Gutruf, and John A. Rogers, presents a comprehensive view of flexible LEDs made of various methods, including flexible micro‐inorganic LEDs (μ‐ILEDs), flexible quantum dot LEDs (QLEDs), flexible perovskite LEDs (PeLEDs), and flexible 2D materials‐based LEDs. The emerging application research opportunities of flexible optoelectronic systems in neuroscience are also presented. Chapter 2, coauthored by Nan Guan and Maria Tchernycheva, is dedicated to nanowire‐based flexible LEDs. Chapter 3, coauthored by Munho Kim, Jeongpil Park, Weidong Zhou, and Zhenqiang Ma, presents an overview of flexible photodetectors based on transferrable single crystalline semiconductor nanomembranes and nanowires, followed by Chapter 4, coauthored by Qin Lu, Wei Liu, and Xiaomu Wang, which is dedicated to 2D photodetectors. Chapter 5, coauthored by Ying Chen, Ye Jiang, Yin Huang, and Xue Feng, overviews the flexible photovoltaics employing Column IV materials. Flexible solar cells based on thin‐film III–V materials are reviewed by He Ding and Xing Sheng in Chapter 6, where both single junction and multijunction solar cells are included. In Chapter 7, flexible solar cells employing perovskite materials and CdTe/CdS are reviewed by Dong Liu, Kwangeun Kim, Jisoo Kim, Jiarui Gong, Tzu‐Hsuan Chang, and Zhenqiang Ma.
We would like to thank all the contributingauthors for their time and effort in preparing the contents of this book. Their pioneering and extensive work in the related fields has enabled the rapid developmental progress in the field witnessed in the last decade or so. We hope that this book can provide a handy reference to researchers during their continued explorations and further expansion of applications of this exciting field. We also hope that the book can be an inspiring introduction to those who are interested in entering this field, bringing with them new ideas.
University of Wisconsin–Madison
Madison, WI 53706, USA
February 08, 2019
Zhenqiang (Jack) Ma
Dong Liu
1Flexible Inorganic Light Emitting Diodes Enabled by New Materials and Designs, With Examples of Their Use in Neuroscience Research
Hao Zhang1, Philipp Gutruf2, and John A. Rogers3
1Northwestern University, Department of Materials Science and Engineering, 2145 Sheridan Road, Evanston, IL, 60208, USA
2University of Arizona Tucson, Department of Biomedical Engineering, 1230 N Cherry Ave., Tucson, AZ 85719, USA
3Northwestern University, Simpson Querrey Institute for Nano/Biotechnology, Center for Bio‐integrated Electronics, Departments of Materials Science and Engineering, Biomedical Engineering, Chemistry, Mechanical Engineering, Electrical Engineering and Computer Science, Civil and Environmental Engineering, 2145 Sheridan Road, Evanston, IL, 60208, USA
1.1 Introduction
Light emitting diodes (LEDs) representessential components in nearly all solid‐state lighting systems. Although conventional LEDs formed from epitaxial materials grown on rigid, brittle, and planar substrates are the most dominant technology, flexible LEDs continue to be of great interest, originating primarily from concepts in flexible, paper‐like displays from the 1990s [1]. Flexible LEDs, as defined by their ability to be bent, twisted, and deformed in other ways, also serve as the basis for advanced optoelectronic technologies, ranging from next‐generation displays in large or portable formats to wearable/implantable devices capable of intimate contact with soft, curvilinear bio‐interfaces. Organic materials, including polymers and small molecules, are natural choices for flexible LEDs due to their favorable mechanical properties, their ability to provide multicolor light emission in ultrathin, lightweight films, and their low‐temperature processability and associated compatibility with plastic substrates [1]. Challenges in performance degradation from photooxidation and other subtle effects and their limited color purity remain as key hurdles for organic light emitting diodes (OLEDs). By contrast, LEDs that exploit inorganic semiconductor materials as emissive layers outperform their organic counterparts in terms of brightness, lifetime, efficiency, and color purity. Recent progress in materials designs, fabrication concepts, and assembly approaches now enable high‐performance, flexible classes of inorganic light emitting diodes (ILEDs). Integrating these ultrathin components with flexible electronics establishes the basis for system‐level, advanced systems for deformable, high‐brightness displays and for biomedical tools that provide diagnostic/therapeutic capabilities. The two main approaches toward flexible ILEDs use (i) microscale ILEDs fabricated from high‐quality epitaxial materials grown on source wafers, subsequently released and assembledon flexible target substrates using the techniques of transfer printing, and bridged by structurally optimized interconnects (Section 1.2); and (ii) ILEDs formed with emissive layers composed of solution‐processed semiconductors and/or low‐dimensional nanomaterials. The former approach mainly relies on processing of well‐established, high‐performance III–V semiconductors with a novel set of techniques, while the latter deploys diverse classes of new materials, including colloidal semiconductor nanocrystals (or quantum dots [QDs], see Section 1.3), metal halide perovskites (Section 1.4), and two‐dimensional (2D) materials (Section 1.5).
This chaptersummarizes the most recent advances and key remaining challenges associated with flexible ILEDs from both the materials and device perspectives. The focus is on their unique properties as candidates in flexible ILEDs and state‐of‐the‐art devices design and performance. In addition, recent progress in integrating flexible ILEDs into system‐level optoelectronic platforms for various applications highlights the current state of the field. The use of miniaturized, flexible ILEDs to optogenetically modulate neural activity (described in Section 1.6) represents one of the most recent cases.
1.2 Flexible Micro‐Inorganic LEDs (μ‐ILEDs)
A combination of properties such as brightness, efficiency, color purity, and lifetime makes III–V semiconductor‐based LEDs the most attractive candidates for solid‐state lighting applications compared to almost all other options, including OLEDs [2,3]. Existing techniques to incorporate commercial ILEDs into systems such as billboard‐scale displays involve robotic, pick‐and‐place assembly of ILEDs diced from a wafer source, followed by device‐by‐device, (sub)millimeter‐scale packaging, and interconnecting of these components with a collection of bulk wires and heat sinks [4]. These conceptually old techniques are ineffective for assembling ultrasmall (<200 μm × 200 μm, microscale), ultrathin (<50 μm) μ‐ILEDs into dense, highly pixelated arrays, particularly on flexible substrates. A set of unconventional processes, starting with the rational design of the μ‐ILEDs in released configurations but still tethered to the underlying growth wafer, followed by transfer printing to a target substrate, circumvents the abovementioned restrictions [5–9]. This sort of deterministic assembly approach enables the use of μ‐ILEDs in a wide range of applications, from high‐resolution flexible/deformable displays to cellular‐scale biocompatible lighting sources for sensing, therapy, and neuroscience research [410–21]. This section focuses on recent developments in μ‐ILEDs, including the materials design and fabrication concepts, as well as their unconventional implementation at circuit and system levels.
The first demonstrations of flexible assembly of μ‐ILEDs involved red‐emitting AlInGaP epitaxial structures. [4]The active layers include emissive quantum wells (6 nm thick In0.56Ga0.44P wells, with 6 nm thick barriers of Al0.25Ga0.25In0.5P on top and at the bottom), cladding films, spreaders, and contacts, all grown on GaAs source wafers via metal organic chemical vapor deposition techniques with high levels of control over thedoping profiles (layer configurationshown in Figure 1.1a). Multiple photolithography and selective etching steps define the lateral dimension (as small as 25 μm × 25 μm) and yield isolated arrays of ILEDs. After selective removal of the underlying sacrificial layer (AlAs) by immersion in hydrofluoric acid (HF), the isolated μ‐ILEDs remain tethered to the GaAs source substrate via “breakaway” photoresist anchors [4,18]. Next, a soft elastomeric stamp with engineered features of relief selectively retrieves a set of μ‐ILEDs (or solid “inks”) from the source wafer (“donor” substrate) and delivers them to a target “receiver” substrate in a desired pattern and in a parallel manner (Figure 1.1a, bottom panel), fully automated using computer‐controlled printing systems. This process, referred to as “transfer printing,” exploits nonspecific van der Waals interactions at the interface between the solid inks and the relief features on the surfaces of the soft stamps. Detailed descriptions and demonstrations of transfer printing techniques in heterogeneous assembly of highly dissimilar materials can be found in recent reviews [5,7]. Transfer printing, together with related strategies (e.g. undercut of sacrificial layers, photoresist anchors to tether inks, and chemical design of adhesives [17,23,24] to improve the yield of delivery), enables the deterministic manipulation of a large number of μ‐ILEDs, each with dimensions that can extend in the range of a few microns in lateral sizes and tens of nanometers in thickness, at room temperature and onto nearly any substrate of interest. Compared to the conventional pick‐and‐place methods, the soft stamps used in transfer printing enable assembly of material structures and devices that can be smaller, by orders of magnitude, and far more fragile; the parallel operation affords throughput speeds that are many orders of magnitude higher, depending on the layouts. The ultrathin (∼2.5 μm in initial demonstrations, and far thinner in more recent demonstrations) form factors of μ‐ILEDs allow for the use of standard, planar processing methods to define the conductive interconnects (metals [4,12,18] or graphene [25]) in direct or matrix addressable configurations, without the need for wire bonding. Mechanically designed arc‐shaped bridges can accommodate strains of ∼22% by changing their shapes, thereby allowing for highly stretchable red‐emitting μ‐ILED displays on polydimethylsiloxane(PDMS) substrates (Figure 1.1b) [4]. Demonstrations of μ‐ILED displays of this type show no noticeable changes in color or device performance (e.g. current–voltage characteristics) after up to 500 stretching cycles to strains of >20% (Figure 1.1c) [4].
Figure 1.1Flexible μ‐ILEDs. (a) (Top) Schematic illustration of an AlGaInP μ‐ILED with integrated Ohmic contacts on flexible polyurethane substrate. (Bottom) Optical micrographs of an array of ILEDs (Left: 25 μm × 25 μm, square geometries; Right: characters “LED”) in their on state without illumination. (b) Photographs (main and inset) of a passive matrix, stretchable ILED display that uses a noncoplanar mesh configuration, on a rubber substrate. (c) Voltage (V) needed to generate a current of 20 μA measured after stretching cycles to 500 times at an applied strain of 22%. The inset shows the I–Vbehavior after these cycling tests. (d) Optical image of a 6 × 6 array of μ‐ILEDs (100 μm × 100 μm, and 2.5 μm thick, in an interconnected array with a pitch of ∼830 μm) with noncoplanar serpentine bridges on a thin (∼400 μm) PDMS substrate (Left‐hand frame). Schematic illustration (Right) and corresponding photograph (Inset) of a representative device, with encapsulation. (e) (Left) Optical images of an array of μ‐ILEDs (3 × 8) with serpentine interconnects on a band of PDMS twisted to different angles (0° (flat), 360° and 720°). (Right) Optical image of an array of μ‐ILEDs (6 × 6), tightly stretched on the sharp tip of a pencil, collected with external illumination. The white arrows indicate the direction of stretching. (f) (Top) Optical image of a fully interconnected array of InGaN μ‐ILEDs on PET. Inset shows the scanning electron microscopy(SEM) image of a dense array of InGaN μ‐ILEDs after anisotropic etching of the near‐interfacial region of a Si (111) wafer. (Bottom) Arrays of GaN μ‐ILEDs (12 devices) on a 4 mm × 15 mm strip of PET, tied into a knot to illustrate its deformability. (g) Illustrations showing the fabrication steps of an array of red‐emitting AlGaInP flexible vertical light emitting diodes (f‐VLEDs) with multi‐quantum well layers. (h) Photographs of a 3 × 3 array of f‐VLEDs in a bent state on a glass rod (radius of curvature = 5 mm) and in a flat state (inset).
Source: Reproduced with permission from Park et al. [4]. Copyright 2009, The American Association for Advancement of Science.
Source: Reproduced with permission from Kim et al. [12]. Copyright 2010, Nature Publishing Group.
Source: Reproduced with permission from Kim et al. [15]. Copyright 2011, The National Academy of Sciences.
Source: Reproduced with permission from Kim et al. [17]. Copyright 2012, John Wiley & Sons.
Source: Reproduced with permission from Jeong et al. [22]. Copyright 2014, The Royal Society of Chemistry.
Further advanced mechanical designs in the interconnect geometries enable arrays of μ‐ILEDs to remain operational even under extreme modes of deformation [8,26]. Figure 1.1d shows an array of μ‐ILEDs bridged by serpentine‐shaped interconnects. Sandwiching the metal traces between two photodefined layers of epoxy places them near the neutral mechanical plane(NMP), thereby significantly reducing the strains induced by bending [12]. A second cycle of transfer printing can deliver these interconnected μ‐ILEDs to prestrained PDMS substrates through selective bonding at specific locations. Releasing the strain leads to a noncoplanar configuration of the serpentine interconnects via processes of mechanical buckling, to further improve the stretchability. The wide range of choices of mechanical design allows stable and robust device operation at repeated, extreme deformational changes (e.g. 100 000 cycles of stretching along the horizontal direction at 75% strain). Besides simple uniaxial stretching, such μ‐ILED arrays remain operational under biaxial, shear, twisting, and other mixed distortional modes (Figure 1.1e) [12]. Electrical measurements reveal no measurable changes in current–voltagecharacteristics after up to 1000 cycles. These optimized mechanical designs enable intimate integration of μ‐ILEDs with soft, curvilinear surfaces of human skin/internal organs and on nonplanar surgical tools (e.g. balloon catheters) that involve significant changes in shape during operation, as described later in this section [12,13].
Similar material designs and fabrication concepts can be extended to blue‐emitting μ‐ILEDs based on InGaN [15–17]. The high internal quantum efficiencyand external quantum efficiency(IQEand EQEgreater than 70% and 60%, respectively), long lifetime (>50 000 hours), and luminous efficiency (>200 lm/W) make InGaN LEDs one of the most widespread options in solid‐state lighting applications [27]. Depending on the growth substrates, anisotropic wet etching [15]or laser lift‐off [16,17] can be used to separate lithographically defined, isolated μ‐ILEDs from their growth wafers. Silicon is of particular interest as a growth substrate due to the availability of large, low‐cost wafers and simple schemes for release based on anisotropic etching techniques. Specifically, the large difference (over 100 times) in etching rates of Si(110) compared to Si(111) in a hot potassium hydroxide bath [28,29] allows for freely suspended, isolated μ‐ILEDs where lithographically defined segments of InGaN serve as anchors (Figure 1.1f, inset in top panel) [15]. These approaches bypass the need for conventional laser lift‐off techniques, thereby enabling high‐throughput, parallel production of millions of devices in forms configured for transfer printing with micron‐scale position accuracy. Additionally, the large bandgap of GaN enables a remarkably convenient means for metallization of interconnects, where backside exposure through the transparent active layers yields self‐aligned traces without the need for photomasking. The ease in deterministic assembly of ultrathin, ultrasmall (100 μm × 100 μm) blue‐emitting μ‐ILEDs on target substrates and the straightforward methods for registration of electrical interconnects result in simple routes to large‐area, flexible μ‐ILED arrays (Figure 1.1f) [15]. Laminating a thin layer of a down‐converting phosphor embedded in PDMS and adding a diffuser film on top yield a uniform, white color emission over areas >100 times larger than that of a traditional LED die with the same amount of InGaN.
As an alternate substrate, sapphire is widely used to grow high‐quality, state‐of‐the‐art GaN epitaxial layers [17]. Here, separation of patterned μ‐ILEDs from the sapphire requires a laser lift‐off process. Via a dual transfer printing procedure, μ‐ILEDs with dimensionsas small as 25 μm × 25 μm can be made with the radiant efficiency up to ∼10% in this way. Transfer printing such μ‐ILEDs on flexible substrates of polyethylene terephthalate(PET) yields systems with stable operation when strongly bent and twisted (Figure 1.1f, bottom panel) [17]. Moreover, introducing wireless powering components (e.g. a rectangular spiral inductor metal coil) leads to an integrated, implantable blue‐emitting μ‐ILED array with the ability to operate in a continuous or periodic mode, in a purely wireless manner [16]. This type of system served as the foundation for recent advances in implantable, wireless optogenetic tools for neuroscience, as described in Section 1.6 .
In the above cases, the assembly of μ‐ILEDs relies on their release from a source wafer via selective etching or laser lift‐off, followed by deterministic assembly via transfer printing. An alternative approach (Figure 1.1g) [22,30] requires no sacrificial layers or transfer printing steps. Here, red‐emitting AlGaInP epitaxial layers grown on GaAswafers bond to a thin polyimidelayer (PI, 25 μm, preprinted with bottom electrodes) via an anisotropic conductive film(ACF). Subsequent wet etching removes the entire GaAs wafer and exposes the epitaxial structures for further epoxy passivation and top electrode deposition. The resulting red‐emitting, flexible vertical μ‐ILEDs on PI substrate remain operational in the bent form (bending radius, 5 mm) (Figure 1.1h) [22]. A disadvantage of this approach is that it consumes the growth wafer, thereby preventing its reuse for additional cycles of growth.
A key feature of these types of flexible μ‐ILEDs is that they can conformally laminate onto curvilinear surfaces and find use in several unconventional applications, especially those in biomedical sensing, physiological monitoring, and clinical therapy. For example, a stretchable μ‐ILED array integrated with molded plasmonic crystals and an external photodetector offers capabilities in the quantitative monitoring of changes in refractive indices of fluids that pass through tubing, which is of relevance for use in intravenous delivery systems for continuous monitoring of nutrient dosage (Figure 1.2a) [12]. In another example, a flexible device with both μ‐ILEDs and micro‐inorganic photodetectors (μ‐IPDs) mounted on the fingertip of a glove serves as an optical proximity senor (Figure 1.2b) [12]. Encapsulation with biocompatible, PDMS layers renders these flexible μ‐ILEDs waterproof and operational even when completely immersed in bio‐fluids, allowing their use in implantable systems [12]. Flexible μ‐ILEDs and a collections of other interconnected devices can be mounted on commercial balloon catheters, for the sensing of a variety of physiological parameters as well as electrically and/or thermally stimulating tissues at the bio‐interface (Figure 1.2c) 13.
Figure 1.2Flexible μ‐ILEDs: Examples of integrated devices in biomedical applications. (a) Measurement results from a representative refractive index microsensor. (Inset) The sensor integrates a μ‐ILED array and molded plasmonic crystals and laminates directly on a flexible plastic tube with a sequence of glucose solutions passing through. Scale bar = 1 mm in inset. (b) Optical image of a proximity sensor with arrays of μ‐ILEDs (4 × 6) and μ‐IPDs, transfer printed on the fingertip region of a vinyl glove. The inset shows a plot of photocurrent as a function of distance between the sensor and an object (white filter paper) for different reverse biases and different voltages. (c) Optical image of a multifunctional balloon catheter in deflated and inflated states. The image shows arrays of temperature sensors (anterior), μ‐ILEDs (posterior), and tactile sensors (facing downward).
Source: Reproduced with permission from Kim et al. [12]. Copyright 2010, Nature Publishing Group.
Source: Reproduced with permission from Kim et al. [13]. Copyright 2011, Nature Publishing Group.
1.3 Flexible Quantum Dot LEDs (QLEDs)
QDs are semiconductor nanocrystals with sizes in the quantum‐confined regime (usually in the range of 2–20 nm) [31,32]. The discrete, quantized energy levels lead to well‐known size‐dependent optical properties (Figure 1.3a) [33]. The most enabling features of QDs as LED emitters are their broad spectral tunability (from UV to near infrared[NIR]) and narrow emission peaks (full width at half maximum[FWHM] smaller than 30 nm). Additionally, as with organic materials, QDs in colloidal form can be well dispersed in solvents and are amenable to a series of low‐cost, solution‐based techniques to assemble into large‐scale solid‐state materials [34,41]. These attributes, especially their exceptional color purity and extended color gamut, motivate research into electroluminescent(EL) quantum dot‐based light emitting diodes (QLEDs) as alternatives to OLEDs, with initial work published more than two decades ago [42,43]. On the other hand, applications that harness optically induced emission (or photoluminescence[PL]) of QDs in backlighting for liquid crystal displays or as downconverters in solid‐state lighting sources have evolved into mass‐produced consumer products, including the Samsung Quantum Dot TV and Amazon Kindle Fire HDX 7 Tablet [44]. Progress in QLEDs can be found in several recent reviews [34,35,45,46]. This section begins with a brief introductionto the unique optical properties of QDs and focuses on the most recentadvances of flexible QLEDs and their applications in high‐resolution displays as well as wearable, skin‐mounted devices.
Figure 1.3Flexible QLEDs: Materials designs, fabrication techniques, and bendable QLEDs. (a) (Top) Schematic diagram of bandgap and (bottom) emission color as a function of size of CdSe QDs. (b) Photoluminescence (PL) spectra of CdSe/ZnS and PbS/CdS core/shell colloidal QDs, demonstrating the size‐ and composition‐dependent tunability of QD emission color. (c) Representative RGB color spaces (solid lines) and chromaticity points of RGB QLEDs (squares) and cutting‐edge OLED products (triangles) relative to the CIE 1931 chromaticity diagram. While the state‐of‐the‐art OLEDs can only cover sRGB or Adobe RGB color space with the help of optical engineering, QLEDs easily meet the current standards and satisfy Rec. 2020. (d) Band engineering of QDs by forming heterostructures. Representative transmission electron microscopy(TEM) images and band structures of (Left) core/shell (continuously graded CdSe/CdxZn1−xSe/ZnSe0.5S0.5) QDs. Scale bar in the TEM image: 10 nm. (Right) Representative TEM image and band structures of DHNRs. Scale bars in the TEM image and the inset: 50 and 5 nm. (e) Intaglio transfer printing for high‐resolution RGB QLEDs. (Left) The PL image showing aligned RGB pixels (2460 ppi with the pixel size of 6 μm). (Right) The PL image of the RGB QD patterns via multiple aligned transfer printings. (f) Composite fluorescence images of electrohydrodynamic jet (E‐jet) printed dual‐color QD patterns. Inset shows an optical microscope image of a metal‐coated glass nozzle (5 μm inner diameter at the tip) and a target substrate during the E‐jet printing. (g) A typical device structure and energy band diagram of flexible QLED using inorganic/organic hybrid charge transporting layers. (h) Flexible full color QLED with RGB pixels (inset) patterned by transfer printing onto polyethylene naphthalate substrate. Inset: optical image of simultaneous electroluminescence emission of RGB patterned QDs. (i) Optical image of the flexible white QLEDs made by Intaglio transfer printing under the bias. Bending radius is 1 cm.
Source: Reproduced with permission from Goesmann and Feldmann [33]. Copyright 2010, John Wiley & Sons.
Source: Reproduced with permission from Shirasaki et al. [34]. Copyright 2013, Nature Publishing Group.
Source: Reproduced with permission from Pietryga et al. [35]. Copyright 2016, American Chemical Society.
Source: Reproduced with permission from Lim et al. [36]. Copyright 2018, Nature Publishing Group.
Source: Reproduced with permission from Oh et al. [37]. Copyright 2014, Nature Publishing Group.
Source: https://creativecommons.org/licenses/by/4.0/, [38].
Source: Reproduced with permission from Kim et al. [39]. Copyright 2015, American Chemical Society.
Source: Reproduced with permission from Kim et al. [40]. Copyright 2011, Nature Publishing Group.
Source: https://creativecommons.org/licenses/by/4.0/, [38].
A wealth of well‐established synthetic protocols (primarily wet chemistries) can yield QDs with tunable emission properties spanning the entire visible spectrum and NIR. A combination of QD sizes, compositions, heterostructures, as well as surface chemistry offers superior control of QD emission characteristics:
(i)
Spectral tunability
. Cadmium selenide (CdSe) QDs with different sizes (2.5–6.3 nm, smaller than the Bohr radius of bulk CdSe) exhibit different colors as a result of size‐dependent bandgaps (Figure
1.3
a) [
33
,
47
]. The spectral tunability also comes from changes in compositions/stoichiometries, as exemplified by CdSe‐ and PbS‐based QDs (Figure
1.3
b)
[34]
. Efficient emission in the NIR regime afforded by IV–VI [
48
,
49
], III–V [
50
,
51
], and other QDs provides a distinct advantage over organic‐based fluorophores.
(ii)
Color purity
. High‐quality, monodisperse QDs (size distribution within 5%) exhibit narrow emission (FWHM < 30 nm compared to 40–60 nm in OLEDs) and thus a wider color gamut, meeting the Rec. 2020 standard for ultrahigh definition TV (Figure
1.3
c) [
34
,
35
,
45
].
(iii)
Brightness
. As with conventional LEDs, the brightness or the radiant efficiency of QLEDs is closely related to EQE, which depends on the injection of charge carriers, light emission (quantified by quantum yield, defined as the ratio of radiative recombination rate to the sum of rates of radiative and nonradiative recombination), and light out‐coupling. Advanced synthetic procedures enable the formation of heterostructures, such as core–shell QDs [
36
,
52
,
53
],
double heterojunction nanorod
s (
DHNR
s) [
37
,
54
,
55
], and many others, in a precisely controlled manner at the nanometer scale (Figure
1.3
d). Core–shell QDs with near unity quantum efficiency are the most widely used materials in QLEDs [
56
,
57
]. More sophisticated, anisotropic DHNRs feature two larger bandgap semiconductors (CdS and ZnSe) with type II band offset surrounding and in contact with a smaller bandgap (CdSe) emitting center
[37]
. This material design allows independent control over the electron and hole processes, and more interestingly, increases the upper limit on light out‐coupling due to the anisotropic optical properties
[54]
. Details of the underlying mechanism that correlates heterostructure designs to enhanced EQEs can be found in a recent review
[35]
. The optimized compositional, structural, and surface control of QDs yields QLEDs with high efficiencies and brightness on par with the state‐of‐the‐art OLEDs (record EQE and brightness for QLEDs: red: 20.2%
[58]
, 106 000 cd/m
2
[59]
; green: 14.5%
[60]
, 218 800 cd/m
2
[61]
; blue: 10.7%
[60]
, 7600 cd/m
2
[62]
).
Fabrication of emissive QD layers in monochromic QLEDs typically exploits spin‐casting processes. These same techniques are not, however, amenable to the fabrication of RGB pixelated, full color displays due to cross‐contamination/redissolution that can arise during sequential steps in spin‐casting. Transfer printing methods, similar to those described for μ‐ILEDs, provide effective routes to pixelating QDs [38,5363–65]. Here, an elastomeric stamp (typically PDMS) delivers a uniform QD film (either spin‐cast on a stamp [65]or peeled off from a donor substrate [40,63]) to a target substrate in a deterministic, parallel manner. In one example, this solvent‐free method enables placement of RGB pixels (46 μm × 96 μm) across a 4‐in. display with 320 × 240 pixels (corresponding to 100 pixels per inch, or 100 ppi) [40]. The stamps can lead to discrepancies between the designed and printed QD patterns, especially for high‐resolution geometries and small pixel sizes (e.g. below 35 μm) [38]. An alternative form of transfer printing process addresses this limitation with the use of an intaglio trench that allows high‐resolution (2460 ppi), full color displays with uniform, ultrasmall RGB pixels (as small as 5 μm, Figure 1.3e) [38]. The high fidelity follows from the gentle contact between the QD thin film on the stamp and the intaglio trench, and subsequent slow delamination. During this process, cracks occur at the sharp edges of the trenches and only the noncontacting part of the QD layer (with sharp edges) remains on the stamp. The printing yields approach ∼100%, independent of pixel sizes. In addition to transfer printing of the QD layer, schemes now exist for transfer printing of multilayer assemblies (e.g. QD emitting layer/electron transport layer/cathode layer) from a donor substrate to a receiver substrate pre‐coated with hole transport layer and anodes, all enabled by the use of a sacrificial fluoropolymer coating [64]. The most enabling feature of this type of multilayer transfer printing is the ability to independently tailor band alignments between the charge transporting layers and QD layers that emit at different wavelengths. As an example, green QDs/TiO2and red QDs/ZnO pixels can be sequentially assembled on the same substrate for optimized device performance. Additionally, inkjet printing [39]or 3D printing [66]also provides useful routes to patterning QDs with elaborate designs. For example, inkjet printing enables sequential printing of QDs with different colors in programmable patterns with uniform thicknesses and ultrasmall pixel sizes (5 μm), in a fully automatic manner (Figure 1.3f) [39].
The high performance of QLEDs originates from rational materials design and advanced techniques to assemble ultrathin, high‐definition QD emitting pixels. The resulting capabilities serve as the basis for recent advances in flexible QLEDs as next‐generation displays as well as their integration with other flexible electronic components for skin‐mounted and bio‐interfaced applications. An optimized flexible QLED device structure (Figure 1.3g) typically includes a QD emitting layer sandwiched between two hybrid charge transport layers (an inorganic ZnO or TiO2electron transport layer and an organic hole transport layer such as poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐sec‐butylphenyl))diphenylamine)], or TFB) [38,40,6367–69]. This device structure utilizes solution‐based deposition of inorganic TiO2or ZnO sol–gel nanoparticles at temperatures that are compatible with flexible plastic substrates, in a way that leads to balanced electron/hole injection rates. The first demonstration of flexible QLEDs involved transfer printed RGB pixels on a polyethylene naphthalate(PEN) substrate (Figure 1.3h) [40], with no appreciable changes in luminous efficiency or current–voltage characteristics at a bending radius of 3 cm. Flexible white QLEDs with mixed QD active layers [70]or sequentially stacked RGB layers [63]require sophisticated control of the ratios of QDs of different colors and suffer from low efficiencies due to the inevitable energy transfer between different QDs. Pixelated white QLEDs enabled by intaglio transfer printing circumvent these issues to allow for excellent device performance (true white emission with a maximum EQE of ∼1.5%) and stable operation at different bending angles (up to 135°) (Figure 1.3i) [38].
In early examples, flexible QLEDs weretypically fabricated on plastic substrates with relatively large thickness (e.g. in the range of hundreds of micrometers for PET [71]), limiting their minimum bending radius to several tens of millimeters [45]. The use of thin tapes of polyimide (Kapton) allows for highly flexible and mechanically robust QLEDs, capable of mounting on and removal from the curved surfaces of many objects [72]. The high efficiency (EQE up to ∼4%) and brightness (over 20 000 cd/m2) largely remain (over 90% of the original brightness) after bending onto a 4 mm diameter rod for 300 cycles. In another example, a double layer composed of parylene and epoxy serves as an ultrathin (∼1.1 μm) substrate that is also biocompatible and waterproof [38]. The ultrathin form factor of QLEDs (Figure 1.4a, in total ∼2.6 μm) enables various deformations (bending, folding, or crumpling) and conformal integration on human skin as wearable tattoo‐like devices. The EQE and brightness (EQE ∼ 2.35% at 4.5 V and brightness ∼14 000 cd/m2at 7 V, Figure 1.4b) are among the highest of reported wearable LEDs and remain stable after 1000 cycles of uniaxial stretching to strains of 20% [38]. Device structure engineering and heterogeneous QD designs further improve the EL performance of flexible QLEDs. Introducing an interfacial layer of polyethylenimine ethoxylated(PEIE) between the green QD emitting layer and hole transport layer in an inverted architecture upshifts the valance band maximum of QDs and favors hole injection, leading to record‐high EQE (15.6%) and current efficiency (65.3 cd/A) on glass substrates (Figure 1.4c,d) [71]. Flexible QLEDs on PET using a similar device structure also show a maximum EQE of 8.4% and a current efficiency of 35.1 cd/A, both of which are the highest values reported for flexible QLEDs [71]. On the other hand, green‐emitting QDs with relatively thick shells (2 nm thicker compared to conventional core/shell QDs) show drastically suppressed nonradiative Auger recombination, leading to flexible QLEDs with the highest reported brightness (44 719 cd/m2at 9 V, Figure 1.4e) [69]. The exceptional electronic and mechanical properties of flexible QLEDs facilitate their integration with other emerging flexible electronic platforms into system‐level, skin‐mounted devices [68,69,73,74]. One representative example is a smart sensor system capable of monitoring and storing information related to pressure, temperature, and movements, and displaying them in QLED arrays (Figure 1.4f) [69]. Flexible red‐emitting QLEDs can also operate as light sources in wearable optical sensors for photoplethysmography(PPG) [73]. A single‐step process of transfer printing of the multilayers (Al/TiO2/QD/TFB/graphene/PEN) to a prestrained PDMS substrate, followed by buckling process, results in highly stretchable QLEDs. These devices show no degradation in performance when stretched at 70% strain or folded to a 35 μm bending radius of curvature. Together with an array of PbS QD‐based photodetectors, the integrated device can be wrapped around a fingertip to provide in situ monitoring of PPG pulses (Figure 1.4g) [73].
Figure 1.4Flexible QLEDs: State‐of‐the‐art devices and their applications in integrated, wearable systems. (a) Exploded view of the ultrathin, tattoo‐like wearable QLED. Inset shows a cross‐sectional SEM image in which the thicknesses of the encapsulation and active layers are shown. (b) (Left) The current density–voltage–luminance (J–V–L) characteristics of the ultrathin, wearable QLEDs shown in (a). (Right) Stable brightness in multiple stretching experiments (20%, 1000 times). The inset shows photographs of buckled and stretched ultrathin red QLEDs. (c) Cross‐sectional TEM and stacking sequence of a highly efficient, inverted QLED device with a polyethylenimine ethoxylated (PEIE) interlayer. Top right shows an operating QLED in highly bent state. (d) Current efficiency–EQE–luminance characteristics of inverted QLEDs without and with 15.5 nm thick PEIE interlayer. (e) (Left) Photographs of an ultrathin QLED display based on CdSe/ZnS core/shell QDs with thick shell, in rolled condition and mounted on skin. (Right) J–V–Lcharacteristics of the device. (f) Photographs of (Left) the touch sensor integrated with the ultrathin QLED display and (Right) the integrated wearable system subjected to external heat. (g) (Left) Skin‐mounted photoplethysmographic (PPG) sensor composed of QLEDs during LED operation at 8.4 V and QD photodetectors wrapped around the finger of a subject. (Right) Real‐time PPG signal pulse wave measured by a stretchable QD photodetector using the stretchable QLED or an indium tin oxide(ITO)‐based rigid QLED as a light source.
Source: https://creativecommons.org/licenses/by/4.0/, [38].
Source: Reproduced with permission from Yang et al. [72]. Copyright 2014, American Chemical Society.
Source: Reproduced with permission from Kim et al. [69]. Copyright 2017, John Wiley & Sons.
Source: Reproduced with permission from Kim et al. [73]. Copyright 2017, American Chemical Society.
In a broader context, a light‐responsive QLED represents an important, recent advance in this field [55]. The ability to combine both efficient photocurrent generation and high electroluminescence within a single system follows from the unique band diagrams in heterogeneous DHNRs (Figure 1.5a). DHNRs contain type‐I heterojunctions between the CdSe QDs and two surrounding materials (CdS and ZnSe), which also form type‐II offsets by themselves (Figure 1.5b). This band diagram (Figure 1.5a) allows separate control of injection of electron and holes and the more advanced, switchablility between light‐emitting and light‐detection modes by forward or reverse bias. Figure 1.5c,d demonstrates the temporal response of the dual‐functioning DHNR‐LEDs. Both EL characteristics (e.g. EQE of 8.0% at 1000 cd/m2under 2.5 V bias) and photoresponsivity (e.g. 200 mA/W) of this dual‐functional device compare favorably to state‐of‐the art QLEDs and commercial silicon photodetectors. A multilayered, 10 × 10 pixel device (Figure 1.5e) programmed by a circuit board demonstrates the “writing” action in response to laser excitation (Figure 1.5f). The switchability of the dual modes and the fast response enable their use in touchless displays with automatic brightness control (Figure 1.5g) as well as direct display‐to‐display data communication systems. Although the reported DHNR‐LEDs are fabricated on a glass substrate, the same material/device designs can be extended to flexible light‐responsive LEDs.
Figure 1.5Light responsive, dual‐functional DHNR LEDs. (a) Energy band diagram of DHNR‐LED along with directions of charge flow for light emissive (orange) and detection (blue) and a schematic of a DHNR. (b) A high magnificationscanning transmission electron microscopy(STEM) images of DHNRs. (c,d) Transient EL showing decay time and photocurrent in response to illumination by a blue LED source driven by 3 V, 50 μs square‐wave voltage pulses. (e) Schematic of a 10 × 10 DHNR‐LED array. (f) Photographs of a light‐responsive LED array with a laser pointer illuminating and turning on pixels along the path outlined by the orange arrows. (g) Automatic brightness control at the single‐pixel level in response to an approaching white LED bulb (Left) or a finger (Right) that blocks ambient light.
Source: Reproduced with permission from Oh et al. [55]. Copyright 2017, The American Association of Advancement of Science.
1.4 Flexible Perovskite LEDs (PeLEDs)
Metal halide perovskites (ABX3, where A is an alkali metal or organic cation, B is typically Pb or other group IV cations, X are halide anions or their mixtures) have attracted tremendous attention in the last several years, primarily due to their rapidly increasing photovoltaic(PV) powerconversion efficiencies, from <5% to certified values of 23.3% [75]. The exceptionally high PV performance also promises good light emitting properties according to the Shockley–Queisser detailed balance limit calculations [76,77]. Perovskite semiconductors feature easily tunable, direct bandgaps via compositional control of anions or cations. The emission spans over a broad spectral range (Figure 1.6a, 390–820 nm) [78]. Unlike QDs, the high color purity of perovskites (FWHM as small as 20 nm) stems from their intrinsic structural similarity to multi quantum wells (MQWs), irrespective of particle/grain size [84,85]. Additionally, the highly ionic nature of metal halide bonds allows for easy access to a broad range of perovskites from nanoscale materials (sub‐10 nm 0D QDs [86,87], 1D nanowires [88], and 2D nanoplatelets [89]), to polycrystalline thin films with variable grain sizes (sub‐100 nm to millimeters) [90,91], and millimeter‐scale, low defect density single crystals [92]. Most of these structures can be synthesized by simple, low‐temperature solution‐based methods, and in some cases, even in open beakers [86]. The unique optoelectronic properties and simple synthetic and fabrication approaches make perovskitesattractive, alternative light emitting materials for LEDs. The long carrier diffusion lengths (∼1 μm) and low exciton binding energies (tens of millielectronvolts) [93]areadvantages for the charge carrier separation in high‐performance solar cells, but they do not necessarily favor LED performance [79,84]. Strategies based on grain engineering [79,80] and chemical design [94–96] can reduce the rates of nonradiative recombination, leading to green and NIR PeLEDs with EQEs exceeding 10% on conventional, rigid substrates [94,96
