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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
Polymers for Light-Emitting Devices and Displays provides an in-depth overview of fabrication methods and unique properties of polymeric semiconductors, and their potential applications for LEDs including organic electronics, displays, and optoelectronics. Some of the chapter subjects include: * The newest polymeric materials and processes beyond the classical structure of PLED * Conjugated polymers and their application in the light-emitting diodes (OLEDs & PLEDs) as optoelectronic devices. * The novel work carried out on electrospun nanofibers used for LEDs. * The roles of diversified architectures, layers, components, and their structural modifications in determining efficiencies and parameters of PLEDs as high-performance devices. * Polymer liquid crystal devices (PLCs), their synthesis, and applications in various liquid crystal devices (LCs) and displays. * Reviews the state-of-art of materials and technologies to manufacture hybrid white light-emitting diodes based on inorganic light sources and organic wavelength converters.
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Table of Contents
Cover
Preface
1 Applications of Polymer Light-Emitting Devices and Displays
1.1 Introduction
1.2 Background
1.3 The Mechanism of Light Emission
1.4 Widely Used Polymers in PLED Applications
1.5 Parameters to be Considered for Display Applications
1.6 Applications in Large and Small Area Devices
1.7 Conclusion
References
2 Polymer Light-Emitting Devices by Solution Processing
2.1 Introduction
2.2 Materials for Fabrication of PLEDs and Their Performance at Solution Processing
2.3 Specific Phenomena at PLED—Energy Transfers, Traps, Excitons Formation, and Color Tuning
2.4 Conclusions
References
3 DFT Computational Modeling and Design of New Cyclopentadithiophene (CPDT) Derivatives for Highly Efficient Blue Emitters in OLEDs
3.1 Introduction
3.2 Computational Methods
3.3 Molecular Geometry
3.4 Frontier Molecular Orbitals
3.5 Molecular Electrostatic Potential Maps
3.6 Optical Absorption and Emission Properties
3.7 ICT Properties
3.8 OLEDs Modulation
3.9 Conclusion
References
4 Conjugated Polymer Light-Emitting Diodes
4.1 Introduction
4.2 History, Classification, and Characteristics of Polymer OLED Material
4.3 Polymer OLED Device Construction and Working
4.4 Blue Light-Emitting Diodes
4.5 Green Light-Emitting Diodes
4.6 Red Light-Emitting Diodes
4.7 Multicolor Light-Emitting Diodes
4.8 Advantages of OLEDs over Other Liquid Crystal Display
4.9 Applications of OLEDs
4.10 Challenges and Future Possibilities
4.11 Conclusion
References
5 Application of Electrospun Materials in LEDs
5.1 Introduction
5.2 Electrospun Nanofibers Technology
5.3 Electrospun Materials for LEDs
5.4 Conclusions
References
6 Luminescent Polymer Light-Emitting Devices and Displays
Abbreviation
6.1 Introduction
6.2 Chronological Development
6.3 Basic Principles Behind Luminescence of Polymers
6.4 Classification of Polymer Light-Emitting Diode
6.5 Dependence of Various Performance Parameters on Structural Factors
6.6 Life Time and Stability
6.7 Recent Developments, Challenges, and Constraints
6.8 Conclusions
References
7 Polymer Liquid Crystal Devices and Displays
7.1 Introduction
7.2 History and Progress
7.3 Polymer Liquid Crystal: An Overview
7.4 Applications of PLCs
7.5 Conclusions
References
8 Hybrid Inorganic-Organic White Light Emitting Diodes
8.1 Introduction
8.2 Hybrid Devices and Other Ambiguities
8.3 Necessity of a Host Matrix
8.4 Materials for Hybrid LEDs
8.5 Color Tuning and Rendering
8.6 Stability
8.7 Conclusions
References
Index
Also of Interest
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Main parameters of the all-solution processed inverted PLED with ...
Table 2.2 Main properties of the commercially available 100 μm polymer subs...
Chapter 3
Table 3.1 Calculated total energies and dipole moments at DFT// B3LYP/6-311...
Table 3.2 Calculated energetic parameters at DFT//B3LYP/6-311g (d,p) level.
Table 3.3 The vertical transition energies (nm) and their oscillator streng...
Table 3.4 The vertical transition energies (nm) and their oscillator streng...
Table 3.5 Reorganization energies
λ
(eV) for hole (
λ
hole
) and elect...
Chapter 4
Table 4.1 Comparison of small molecule LED and polymer LED
Chapter 6
Table 6.1 Chronological development and performances of luminescent polymer...
Chapter 8
Table 8.1 Overview of the MOF-based HWLEDs reported in literature.
List of Illustrations
Chapter 1
Figure 1.1 The basic structure of a PLED.
Chapter 2
Figure 2.1 (a) Typical structure of conventional organic based light-emitt...
Figure 2.2 Physical processes revealed in the organic-based LED, related t...
Figure 2.3 Contact points between functional layers in PLED.
Figure 2.4 (a–c) PLEDs with various electron-transporting host materials. ...
Figure 2.5 (a) Energy bands diagram of the solution produced light-emittin...
Figure 2.6 (a) Energy level alignment of the device; (b) current density—l...
Figure 2.7 (a) J-V-L characteristics of PLED with quantum dots with single...
Figure 2.8 (a) Current density-voltage-luminance (J-V-L) characteristics o...
Figure 2.9 (a) Solution-processable fluorescent compounds; (b) Correspondi...
Figure 2.10 AFM 2D and 3D topography of the emission layer prepared by syn...
Figure 2.11 (a) Band diagram, (b) normalized EL spectra, (c) the current-vol...
Figure 2.12 (a) Normalized EL spectrum of the SPB-02T/MEH-PPV/PFO structur...
Figure 2.13 (a) Sheet resistance variation at different bending cycles for...
Figure 2.14 (a) Photo of all-solution processed yellow PLED of electrochem...
Figure 2.15 (a) Blue PLEC bent to 2.5-mm-radius of curvature; (b) Efficien...
Figure 2.16 (a) Device structure: ITO/HAT-CN/VB-FNPD/PVK5Firpic/PFN5CsF/ m...
Figure 2.17 (a) Current-voltage and brightness-voltage characteristics of ...
Figure 2.18 (a) Display structure consisting of multiple PLEDs; (b) Micros...
Figure 2.19 (a) Modification of the surface wettability of the BS; (b) Pat...
Figure 2.20 (a) Microscopic images of the patterned emissive structures 5 ...
Figure 2.21 Calculated spatial distribution of HOMO and LUMO of PCzTPP and...
Figure 2.22 (a) Efficiency-current density curves of improved devices base...
Figure 2.23 (a) Device architecture of the three color-tunable PLED; (b) C...
Figure 2.24 (a) Current-brightness-voltage characteristics of one of the t...
Figure 2.25 Time-resolved EL responses of the PLED: the reverse voltages (...
Figure 2.26 Chemical structures of the monomers and star-shaped white-emis...
Figure 2.27 Device structure and measurement: (a) Energy levels of the mat...
Chapter 3
Scheme 3.1 Molecular structures of the studied CPDT dimers.
Scheme 3.2 Optimized molecular structures of CPDT dimers.
Figure 3.1 Mulliken atomic charge distribution at ground and excited state...
Figure 3.2 Bond lengths at optimized ground (GS) and excited (ES) states o...
Figure 3.3 Frontier molecular orbitals (FMOs) and energy levels at ground ...
Figure 3.4 Density of state (DOS) plots of D1 and D2
.
Figure 3.5 Electronic different density (EDD) between ground and excited s...
Figure 3.6 Molecular electrostatic potential surface (MEPs) of D1 and D2
.
...
Figure 3.7 Simulated UV-Vis-NIR optical absorption spectra with oscillator...
Figure 3.8 Compared UV-Vis-NIR optical absorption spectra of investigated ...
Figure 3.9 Emission spectra with oscillator strength (vertical lines) of t...
Figure 3.10 The simulated emission spectra at TD-DFT//B3LYP/6-311g(d,p) of...
Figure 3.11 Fitting Gaussian and Lorentz peaks of emission spectra of CPDT...
Figure 3.12 Schematic plot of reorganization energy.
Figure 3.13 The representative CIE color coordinates of D1 and D2.
Figure 3.14 Typical OLED architecture including D1 or D2 as emitting layer...
Figure 3.15 Current-voltage (I-V) characteristics of OLED devices based on...
Chapter 4
Figure 4.1 Classification of polymers according to emissive materials.
Figure 4.2 The basic OLED device architecture.
Figure 4.3 Functionalized conjugated polymer having donor (5 mol.% triphen...
Figure 4.4 Applications of OLED. Source: https://www.elprocus.com/oled-dis...
Chapter 5
Figure 5.1 Light-emitting smart textile
Figure 5.2 Illustration of electrospinning instrument
Figure 5.3 Various applications of electrospun nanofibers
Figure 5.4 (a) Schematic of the electrospinning. (b) Composite-film prepar...
Figure 5.5 (a) PL spectrum, and (b) UV-Vis absorption of CCAL membranes wi...
Figure 5.6 (a) Diagram (left) and photographs (right) of the configuration...
Figure 5.7 (a) Photograph, (b) SEM image, (c, d) Fluorescence microscopy i...
Figure 5.8 Integrated emission vs. water soaking time for NCs embedded in ...
Figure 5.9 CIE diagram of emission spectra of x% Dy
3+
-doped ZnO nanofibers...
Figure 5.10 (a) Schematic representation of the electrospun setup for core...
Chapter 6
Figure 6.1 Copolymerization of (I) poly-p phenylene vinylene and (II) poly...
Figure 6.2 Structure of polyfluorine-based green light-emitting diode comp...
Figure 6.3 Structure of poly(9,9-dihexylfluorene-2,7-diyl).
Figure 6.4 Structure of BDOH-PF [10].
Figure 6.5 Molecular structures of (a) Bphen and (b) PBI-H.
Figure 6.6 Two different mechanisms of luminescence.
Figure 6.7 Architecture of conventional (a) bottom-emitting and (b) top-em...
Figure 6.8 Architecture of inverted (a) bottom-emitting and (b) top-emitti...
Figure 6.9 Loss of out-coupling efficiency due to diversion of EML at the ...
Figure 6.10 Copolymers of F8 and TBT (a) F8TBT-out and (b) F8TBT-in having...
Figure 6.11 Red colored emission by reduction of HOMO-LUMO gap.
Figure 6.12 Electrons and holes are transported through the suitable phase...
Figure 6.13 In the MEH-PPV/PMMA blends carrying lesser MEH-PPV envisage di...
Figure 6.14 Multi-layered PLED structure containing polar solvent at the F...
Figure 6.15 Morphological changes, shape of the aggregates, i.e., (a) nodu...
Figure 6.16 AFM images of doped EML coats envisaging changes in shape of a...
Chapter 7
Figure 7.1 Different classifications of liquid crystals [6].
Figure 7.2 Various phases of liquid crystals [28].
Figure 7.3 Fundamentals of LCs materials. (a) The nematic LCs phase has a ...
Scheme 7.1 Synthesis of the chemically disordered liquid crystal polymer [80...
Scheme 7.2 Synthesis of the chemically ordered liquid crystal polymer [80]....
Scheme 7.3 Synthetic route of polymer from monomer unit [81].
Figure 7.4 LCs lenses. As shown schematically (a) and experimentally, look...
Figure 7.5 Liquid-crystal biosensors. (a, b) A well-aligned liquid-crystal...
Figure 7.6 Patterned liquid-crystal-polymer based actuators can undergo ra...
Chapter 8
Figure 8.1 Creating white light with LEDs. Reprinted from LED Light Spectr...
Figure 8.2 Structures of (a) OLEDs, (b) Hybrid OLEDs, (c) Inorganic phosph...
Figure 8.3 Position of the standard white illuminants A, B, C, D
65
, and E, and...
Figure 8.4 Sketch of FRET between a Cyan Fluorescent Molecule (CFM) and a ...
Figure 8.5 Pictures of the blue LED emitting at 450 nm (a) before and afte...
Figure 8.6 Sketch of an HWLED prepared as a stack of individual fibers. Re...
Figure 8.7 Schematic diagram for the formation of fluorescein-silica nanop...
Figure 8.8 (a) Optical microscope images of LEDs coated with the color conve...
Figure 8.9 CIE chromaticity diagram illustrating the development of coordi...
Figure 8.10 Solutions and suspensions fluorescence images of (upper panel)...
Figure 8.11 Illustration of fluorescence turn-on process of AIEgens by RIR...
Figure 8.12 Emission spectra of coumarin in CHCl
3
(excitation wavelength a...
Figure 8.13 Pictures of raw curcuma, curcuma powder, and curcuminoids. Rep...
Figure 8.14 Soxhlet apparatus setup. Reprinted from M. Al Shafouri, N. M. ...
Figure 8.15 (a) Representation of a bio-HLED with a cascade coating based ...
Figure 8.16 (a) The spectrum and photograph (in the inset) of fluorescent ...
Figure 8.17 Structure of MOFs: examples of arrangements of inorganic and o...
Figure 8.18 TEM images of CDs dispersed in a PMMA matrix of molecular weig...
Figure 8.19 EL spectrum of the white LED based on CDs operated at 350 mA, ...
Figure 8.20 (a) CIE color gamut for the NTSC system, the Rec. 2020 standar...
Figure 8.21 Fluorescence spectra of PFA after progressive heat treatment o...
Figure 8.22 Chromatic coordinates in CIE diagram of a Lumogen
®
F Yell...
Guide
Cover
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Scrivener Publishing
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Publishers at Scrivener
Martin Scrivener ([email protected])
Phillip Carmical ([email protected])
Polymers for Light-Emitting Devices and Displays
Edited by
Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri
This edition first published 2020 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2020 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-65460-5
Cover image: Pixabay.comCover design by Russell Richardson
Preface
Polymer light-emitting diodes (PLEDs) or organic light-emitting diodes (OLEDs) are organic semiconductor light sources that emit light in response to an electric current. These PLEDs are promising devices with the aforementioned features to convert electrical energy to light energy, which is the necessary component of any display technology. OLEDs have received increasing attention since they were first developed in 1989. The development of OLEDs has attracted considerable interest in innovations for our daily life and future. They have promising applications in flat panel displays, electronic products, automotive, flexible displays, industrial products, and future wearables due to unique electrical and optical properties including low-cost, easy processing, low-operating voltage, energy-saving, eco-friendliness, thinner and smaller in size, lightweight, flexibility, and cost-effective fabrication process.
Polymers for Light-Emitting Devices and Displays provides an in-depth overview of fabrication methods and unique properties of polymeric semiconductors, and its potential applications for LEDs, organic electronics, displays, optoelectronics, and so on. Engineers, chemists, material science, and research scholars, students, and faculty members working in the area of organic electronics will benefit by understanding the materials used in optoelectronics. Based on thematic topics, the book edition contains the following eight chapters:
Chapter 1 is a detailed summary of the working principles of PLEDs. Different polymers used in PLEDs and limitations of PLEDs in the illumination system where the intensity of light is very high compared with displays like televisions and laptops are discussed.
Chapter 2 presents an overview of the newest polymeric materials and processes beyond the classical structure of PLED, leading to the low-cost and all-solution processed devices with enhanced parameters, which are closer to commercial production line requirements and custom needs.
Chapter 3 seeks to obtain a better understanding of the fluorescence quenching behavior and intramolecular charge transfer (ICT) character of two kinds of cyclopentadithiophene (CPDT) derivatives. A comparative study based on the optoelectronic properties of CPDT dimers for their highly efficient blue emitters in OLEDs is developed using the density functional theory (DFT) approach.
Chapter 4 deals with conjugated polymers and their application in the light-emitting diodes (OLEDs and PLEDs) as optoelectronic devices. It provides basic information on the classification of polymers and their modification via functionalization, copolymerization, doping, etc., for device fabrication, function, and use of conjugated polymers in blue, red, green, and multicolored light-emitting diodes along with challenges and their future perspectives. Additionally, the chapter focuses on the advantages/ disadvantages and application of OLED technology in various fields.
Chapter 5 discusses the novel and noteworthy work carried out on electrospun nanofibers used for LEDs. It mainly focuses on the fabrication technology for producing electrospun nanofibers and how metal oxide semiconducting, perovskite, rare earth ion-doped, and coordination polymeric electrospun nanofibers are useful in designing smart clothes and LEDs.
Chapter 6 summarizes the roles of diversified architectures, layers, components, and their structural modifications in determining efficiencies and parameters of PLEDs as high-performance devices. Additionally, some recently developed materials and concepts, including white PLEDs, quantum dots, thermally activated delayed fluorescence, and transparent PLED, are discussed in detail.
Chapter 7 gives a general idea of polymer liquid crystal devices (PLCs), their synthesis, and applications in various liquid crystal devices (LCs) and displays.
Chapter 8 reviews the state-of-art of materials and technologies to manufacture hybrid white light-emitting diodes based on inorganic light sources and organic wavelength converters. It takes stock of the benefits—but also the weak spots—of the hybrid technology to envisage its future impact among the well-established inorganic lighting technologies.
Editors
Inamuddin
Rajender Boddula
Mohd Imran Ahamed
Abdullah M. Asiri
1Applications of Polymer Light-Emitting Devices and Displays
D. Prakash Babu1, S. Naresh Kumar1, N. Suresh Kumar2, K. Chandra Babu Naidu3* and D. Baba Basha4
1School of Applied Sciences, REVA University, Bangalore, India
2Department of Physics, JNTUA, Anantapuramu, India
3Department of Physics, GITAM Deemed to be University, Bangalore, India
4Department of Physics, College of Computer and Information Sciences, Majmaah University, Al’Majmaah, Saudi Arabia
AbstractThis chapter gives information of polymer light-emitting diodes (PLEDs) and their applications. Besides, background, types, and the development of PLEDs also discussed. Further, the behavior of different PLEDs has been discussed with respect to various parameters, brightness, color purity, light conversion efficiency, and color stability are discussed.
Keywords: Polymer, light-emitting diodes, efficiency, color purity
1.1 Introduction
In the past one decade, the display technology has undergone several technological advancements and industries and household are looking for low cost, flexible, power efficient, and durable displays. Polymer light-emitting diodes (PLEDs), which convert electric energy into light, are promising devices with aforementioned features to convert electrical energy to light energy, which is the necessary component of any display technology. High temperature resistance, short response time, smooth brightness, and a large viewing angle are the additional advantages with PLEDs [1]. These special characteristics of PLEDs give the scope to use them in the applications where a large array of displays is required [2]. At present, inorganic light-emitting diodes are widely used. The advancement of technology demands advancement of display also, some times, the display device needs to be flexible, this flexibility can be easily provided by PLEDs. In this chapter, the basic structure of PLED, the mechanism of light emission, and different applications are discussed.
1.2 Background
In 1990, an article was first published in Nature on “Light-emitting polymers” by J. H. Burroughes, Richard Friend, and others [3].
Basic structure of PLED
The basic structure of a PLED is illustrated in Figure 1.1. It consists of thin layers of light-emitting polymers film sandwiched between a transparent electrode which is anode and a non-transparent electrode which is cathode. Indium tin oxide (ITO) layer coated on glass substrate is most commonly used as the transparent anode. The glass provides the mechanical support for the PLED. ITO being transparent to light allows the light photon created inside the diode to escape from the device. There are two polymer layers in a typical PLED structure; among them is the hole transporting layer and the other is the light-emitting layer. Generally, the metal cathode is deposited over of the polymers by means of thermal evaporation.
Figure 1.1 The basic structure of a PLED.
1.3 The Mechanism of Light Emission
Electron-hole recombination causes emission of a photon in visible region. Electrons are injected from the cathode to the LUMO (lowest unoccupied molecular orbit) and the holes are injected from the anode to the HOMO (highest occupied molecular orbit) of a conducting polymer. The reliability and the efficiency of the diode are strongly influenced by the materials which form the cathode, anode, and the emissive layers. A typical PLED may either be a single- layer device or a multilayer device. One example of PLED is the one fabricated from conjugated polymers including polyacetylene, polythiophene polypyrrole (PPy), poly (para-phenylene vinylene), and polyaniline (PANI) [4]. Another example of the active element used in PLED is the poly (p-phenylene vinylene) (PPV).
PLEDs like polymer light-emitting diodes and polymer light-emitting electrochemical cells gain huge interests owing to their high capabilities to serve as next generation illuminants and displays. Contrasted to inorganic light-emitting materials, conducting polymers possess very good film-forming behavior enabling the deposition uniformly by solution-based techniques, for example, screen printing and spin-coating that are competent of upscaling to industrial scale manufacturing. Polyfluorene, poly(p-phenylene vinylene), polycarbazole, and poly(p-phenylene) are widely researched; their solubility, morphology, stability, doping, etc., are proved to increase the device performance.
For example, poly(p-phenylene) films are widely synthesized by precursor methods because of its insolubility in commonly used organic solvents. Its solubility can be increased by the preparation of conducting polymers (ladder type) which leads to improved co-planarity [5]. Furthermore, a complete color display may be achieved through adjusting the structure of the molecule to regulate the energy gap of the HOMO-LUMO. Also, small amount of molecule doping proved to give desired luminous properties. Other than light emitting, color changes (electrochromic devices) and strain (electromechanical actuator) can also be stimulated by applying electric energy. Further, the electromechanical actuators directly convert the electrical energy to mechanical energy. These materials find applications in fabricating robotics, artificial muscles, etc. Electrochromic devices produce revocable color variation in reaction to the applied electric field, which makes it suitable for electronic skins and smart windows.
Owing to the tunable redox states under electricity, the conducting polymers are the fascinating materials for high performance electrochromic devices and electromechanical actuators. Further enhancement of the electrochromic or actuating ability of the polymer and the response speeds is improved by incorporating the graphene and other nanocarbon materials. For example, the multiwalled carbon nanotubes incorporated with polyaniline through an electrochemical deposition technique to aid as composite electrodes which can exhibit large conductivity ranges from 100 to 1,000 S/cm−1 that enables reversible and rapid electrochromic developments within short time [6]. PLEDs are not loaded with only merits. They do have a main disadvantage of weathering of the polymers with time. The disadvantage of the PLED technology is the sensitivity of the organic light-emitting materials to the atmospheric oxygen and water vapor. Hence, to protect PLEDs, a weather proof transparent polymer which is chemically and physically stable must be used for encapsulation.
1.4 Widely Used Polymers in PLED Applications
Among wide choices of polymers, some particular polymers gained special attention owing to their processability and functionality advantages that are discussed below. The advancement of important polymer material groups has been discussed here. This report gives the group of materials which can exhibit utmost potential till date to be espoused as the emissive materials in PLED applications, for example, the poly(fluorene)s, the poly(phenylenevinylene)s, etc. Polyfluorene homo- and co-polymers are purposefully emphasized because they are not well re-viewed, much progress has been made only recently, and this group of polymers are rapidly developed as a most promising viable LED polymeric material of widespread commercial interest.
1.4.1 Polyfluorene-Based Luminescent Polymers
Fukuda et al. reported the first fluorene-based polymers, by ferric chloride oxidative polymerization of 9-alkyl-fluorene and 9,9-dialkylfluorene [7, 8]. Their molecular weight was relatively low, with some-extent of separating and non-conjugated connections across locations except 2 and 7 [7, 8]. By using the transition-metal-catalyzed reactions of monomeric 2,7-dihalogenatedfluorenes, researchers introduced the homo-polymers for minimization of branching, improving regiospecificity. Further, Suzuki and co-workers discovered the palladium-catalyzed synthesis of mixed biphenyls from aryl bromide and phenylboronic acid [9, 10].
1.4.2 Polyfluorene Homo-Polymers
In general, polyfluorenes with substituents C6 or C9 are solvable in traditional organic diluters like aromatic hydro-carbons, chlorinated-hydrocarbons, etc. [11]. The polymers with large molecular weight does not consist separate glass transition. The polymers with straight alkyl substituents exhibit liquid crystallinity and tend to be semicrystalline. For example, F8, exhibits constant liquid crystallinity up to the temperature 270oC [12]; however, the polymer-materials having diverged alkyl-substituents exhibit non-crystallinity. Further, entire polymers while excited with UV emits a strong blue light, either in solution or in their solid state. They have a wide and drab absorption spectrum, whereas the photoluminescence spectrum shows distinct vibrionic-structures [13]. In general, the Stoke’s shift lower than 50 MeV indicates a prolonged conformation.
1.4.3 Polyfluorene Alternating Copolymers
Tertiary aromatic amines are very good hole-transport materials, viable for photoconductors and LEDs. Preparation of large molecular weight, varying co-polymers containing different aromatic amines and 9,9-dialkylfluorene is possible through the Pd-catalyzed polymerization process. These alternating polymers are all soluble in conventional organic solvents, excellent film formers, and are good blue emitters. These polymer films exhibit discrete and adjustable oxidation capacities through cyclic-voltammetry that could be cycled exclusive of any significant alteration. The mobilities of positive charge carriers of the above said polymers are relatively large (3 × 10−4 to 1 × 10−3 cm2/Vs) [14–16]. Due to these large mobilities of holes, these polymeric materials recommended for the applications in photoconductors also in LEDs for transportation of holes. Attempting to create polymers with distinctive properties, the alternating copolymer approach has been extended to other conjugated monomers, such as triarylamine, thiophene, etc. [11]. The co-polymers consisting large molecular weight exhibit high photoluminescence and emission spectra of the co-polymers may be associated with degree of co-monomers delocalization, e.g., the copolymers of bithiophene produce the spectra in yellow region, cyano-stilbene produces the spectra in green region, and thiophene in bluish green region [11].
1.4.4 Derivatives of PPV
The emission of yellow-green light by PPV under electrical stimulation was discovered in past decade, since then, several researchers focused on optimization of PPV and to make this as a potential material [17, 18, 19]. Most importantly, some of the advancements have taken place in preparation, regulating the balance in charge carriers, improving the efficiency in power, and enhancing the life-time, also in adjusting the emission of wavelength. Owing to the vinylene linkages, photo-oxidatively, the PPV chemical structure is unstable, also there is some restriction in the improving the saturated blue rich and red rich emitters. Even though these problems continuing to encounter the initiation of PPV into the display devices commercially, noteworthy development has been made towards the controlling and optimization of the PPV materials to make these are potential aspirants for the applications in PLED devices [11].
1.4.5 Soluble Precursors of PPV
For poly (arylene vinylene) series, the parent structure is PPV owing to absence of functional groups to improve solubility, rigid structure, and propensity to develop crystalline morphology, these materials are stubborn and directly not processable form the solution. Meanwhile for polymeric emission systems solvent process ability is a necessary characteristic; further, the soluble precursors to the PPV which can be molded as films then transformed to PPV through heating have been established.
1.4.6 Derivatives of PPV for Solution-Processing
PPVs are generally difficult to process but possess properties that are capable of good PLED candidates. To overcome the processing difficulty, plenty of research is devoted towards the advancement of soluble PPVs. Making of thin films is easy with soluble PPVs, exclusive of successive thermal-conversion. In 1991, Heeger et al. reported the applications of PPVs particularly 2,5-dialkoxy functional PPVs; according to them, the alkoxy groups having at least one bulky or long polymer groups are soluble in diverse organic solvents which include xylene, chloroform, etc. The functionality of the bulkier materials has been described to interrupt the propensity of PPV in order to increase the efficiency of EL.
1.4.7 Polyphenylenes
In the area of polymer light-emitting devices, there exist another class of conjugated polymer group called PPP (poly(1,4-phenylene)); these PPP materials consist large bandgap and permit blue light emission. Subsequently, the design of blue emitters which are having high efficiency and long life time endures a major task in advancement of the polymers. Therefore, activities of research in focused on PPP to emphasize the methods towards PPP thin-films through solvable precursor polymer materials which are thermally converted, in addition to the improvement of soluble PPPs. These class polymers have high molecular weights that are sufficient to mold films including excellent integrity that have been accomplished besides diodes with blue-emission that have been built and reported with considerable efficiencies [20].
1.5 Parameters to be Considered for Display Applications
The following parameters are considered for making different display technologies:
Color purity and brightness,
Light conversion efficiency, and
Durability.
1.5.1 Color Purity and Brightness
Entire spectrum of colors (and infrared) is possible with different PLEDs. For orange and green emitting PLEDs, life cycles of over 10,000 hours have been reported. However, till now the data is not available, blue devices with high cyclic life. This causes the loss of blue component of light emitted with time and the PLED will eventually lose the entire blue light, and hence, the visible light emitted cannot maintain color purity with time. So, a blue emitting polymer with lifetime equal to orange and green ones is barely needed in realizing a display device to maintain the emission color through the lifetime of the device.
High luminance values may be achieved at small voltages. For orange PLEDs, the observed starting value of emission is of around 1.79 V which is above the bandgap. A brightness of 100 cd/m2 is reported around 2.5 V for the same PLED. It might be associated with a distinct brightness of 60 cd/m2 for computer and laptop displays. Even a 50-nm thin film polymers are reported to emit light. If the layer thickness increases, for instance, 100 nm, the voltage rises approximately by 1V. The low-voltage process enables device operation possible in ordinary less-expensive integrated chips. Up to 10,000 cdm−2 brightness can be achieved at as low as 6 V. In pulsed open, even 100,000 cdm−2 brightness is possible to achieve. Even some groups proved laser action is possible in polymer devices with intensity more than 1,000,000 cdm−2.
1.5.2 Light Conversion Efficiency
The efficiency of a PLED depends on the external efficiency which is calculated in forward direction. However, the value of external efficiency observed in forward direction is large compared to the values observed using integrated spheres. In addition, sometimes, the samples behave like an optical fiber, in which total internal reflection takes place when the angle of incidence is greater than the critical angle, due to this in PLEDs considerable quantity of light which is produced in the emissive-layer escapes through the sideways of the sample. Ching Tang et al. [21] proposed a simple solution to determine the whole value in terms of candela per ampere. Green PLEDs exhibit highest efficiency of 75 cd/A. Further enhancements can be achieved through rising in the efficiency of photoluminescence and also by enhanced electron-injection. In general, PLEDs’ efficiencies are far superior than normal bulbs and LEDs. In current display technologies, PLED is far superior and simple to fabricate.
1.5.3 Color Stability
Stability of polymers and their properties are main concerns regarding this polymer technology. In the present scenario, PLEDs’ displays and backlights for LCDs can meet customer specification only in orange light-emitting materials. Quick evolution is happening to grow blue and green emitting polymers to similar extent of constancy. Some researchers proved that it’s possible to fabricate device on elastic substrates as an alternative of rigid glass-substrates. But these devices are reported to have short lifetime around 1 day. These devices are ruined by diffusion of water through the plastic film because of the sensitivity of the device for O2 and H2O, it may lead to rapid corrosion of the positive electrode of the device. Recently, advancement in the production of elastic films consisting good water barrier properties, flexible light-emitting films, and displays was made possible.
1.6 Applications in Large and Small Area Devices
1.6.1 Displays
PLED technology can be employed to fabricate small and simple unicolor segmented displays to complicated and large full-colored displays. The application ranges are typically classified based on the size of the display.
1.6.1.1 Matrix and Small Segmented Displays, ≤25 cm2
The appliances are where the information displayed is limited, for example, in car dash boards, professional-equipment, etc. Utmost of the display area is typically monochrome. Comparing PLEDs with other technologies, reflective LCD is advantageous as a function of power consumption, but it has a poor contrast, it has a dull visual aspect, it has low viewing angle, and it is not readable in the dark. PLED exhibits a discrete advantages in slimness and an improved power consumption factor in the range of 10 to 100, combining with a backlight and LCD. Some of the advantages of PLED devices are thin, response time is fast, graphics resolution is very high, display brightness is high, and contrast is also high.
1.6.2 Thin and Flat Light Sources
In present scenario, application of PLEDs as sources of light, apart from some special cases, is questionable. When compared with power efficiency of 19.99 Lm/W for florescent sources and 60 Lm/W for incandescent tubes and the PLED power efficiency is as low as of 4 to 10 Lm/W. Also, the lifetime of polymer-based LED devices is limited by the high intensity of light needed for illumination. Nevertheless, PLED devices might be utilized in all types of applications in signaling such as brake lights for cars, decorative light sources, potentially rear lights, etc. LCD backlighting is one explicit application where the source of the light is really lightweight, thin, and flat. The usage PLEDs in comparatively less in large-area applications, less than 100 cm2, can be distinguished based on the purpose for which they are used. Also, the backlight not only used to increase the contrast of the display in daylight but also used to illuminate the display in dark for example car stereo, radio sets, etc.
1.6.3 Cloth-Type PLEDs
The formation top emission OLEDs on a substrate of fabric is an easy way in advancement of OLEDs. The substrates of fabrics are categorized thru spatial voids and through these voids water can easily pass. Besides the assembly of constituent fibers forms the uneven surface; unfortunately, this is not compatible with the OLEDs. Hence, to prepare OLED-compatible fabric substrates, it is necessary to introduce the methods to eliminate the spatial voids then reformation of surface to be even [22–24]. There exist two stages to form a glass-like surface which can be used as fabric substrate to embed OLEDs [24]. Foremost, for partial planarization, i.e., fill the valleys, low viscous ductile polyurethane (PU) is spin coated on self-assembled fabric substrate. Next, to reduce the roughness of the surface, a high viscous polyurethane is deposited on the fabric before clean guide substrate is transported to the subsequent fabric substrate via lamination at room temperature. Subsequently, the distinguished methods of top-emission OLED production and multibarrier encapsulation, extremely robust wearable OLEDs were attained on the modified fabric substrate [24]. Even after this promising achievement, remaining practical problems should be overcome to use in realistic wearable displays. The predicted planarization procedure for fabric substrate well-suited with OLEDs weakens the nature of the fabric like softness, breathability, etc.
Furthermore, accomplishing consistent wash ability of the device might be another critical obstacle in the perspective of wearable electronic clothes. In view of the ability to concede actual light-emitting fabrics, the fiber-shaped OLEDs are much closer to cloth-type display concept, due to tiny and curved substrate, it is difficult to prepare OLEDs on the fiber substrate. B. O’Conner et al. reported a technique that the deposition of layers of OLEDs on the fiber is retain the rotation of the fiber in vacuum deposition [25]. Another reported technique is dip-coating method which is cost-effective and simple and can be employed for solution-based PLED-fibers [26]. Whereas both techniques surely proved the possibility of OLEDs on fibers, advanced studies on electrical addressing and reliable encapsulation schemes for the accumulated fibers are required for working as a display device [27].
1.6.4 PLEDs in Wearable Electronics
S. Choi et al. [28] have fabricated a light-emitting fabric, which is efficient and flexible, making it suitable for displays which can be worn like clothes. The PEN fibers are used for weaving the fabric, and the thermal lamination is used to form planarization layer onto this fabric. The surface roughness of Rq =2.073nm, these fabrics make them appear very smooth. Organic light-emitting diodes were placed thru thermal evaporation and the additive protective layers by transparent-flexible encapsulation effectively block the saturation of water and oxygen. Besides, the prepared device exhibits the luminance of around 35,844 Cd/m2 and maximum current efficiency of about 70.43 Cd/A. In addition, the device on the material was found to operate stably after harsh bending, even at a bending radius of 2 mm for 3,000 cycles and a bending radius of 1cm after 30,000 cycles. However, more bending of these fabrics results in leakage current within the device and cracks on the fabric. These fabrics can find various electronic textile industrial applications like curtain manufacturing, serves as functional and table clothes in and healthcare, fashion, as well as in the automobile industries.
1.7 Conclusion
The construction and working principles of PLEDs is discussed in detail. Different polymers used in PLEDs are discussed. The important parameters that are the key factors to be considered while adopting the polymer electroluminescent materials for PLEDs like brightness, color purity, light conversion efficiency, and color stability are discussed. Finally, the application of the PLEDs to small, midsize, and large size displays are discussed. Also, their application in flexible displays and cloth type wearable displays are discussed. These PLEDs are undergoing a rapid development and may soon be available in all forms of displays. The present limitations of PLEDs in illumination system where the intensity of light is very high compared with displays like television and laptops will soon be rectified with improved polymers.
References
1. Liang, J., Lu, L., Xiaofan, N., Zhibin, Y., Qibing, P., Elastomeric polymer light-emitting devices and displays.
Nat. Photonics
, 7, 817–824, 2013.
2. Hameed, S., Predeep, P., Baiju, M., Polymer light emitting diodes - A review on Materials and techniques.
Rev. Adv. Mater. Sci.
, 26, 30–42, 2010.
3. Burroughes, J.H., Bradley, D.D.C., Brown, A.R., Marks, R.N., Mackay, K., Friend, R.H., Burns, P.L., Holmes, A.B., Light-emitting diodes based on conjugated polymers.
Nature
, 347, 539–541, 1990.
4. Belgacem, M.N. and Gandini, A.,
Monomers, Polymers and Composites from Renewable Resources
, University of Aveiro, CICECO, Chemistry Department, Portugal, Amsterdam, 2008.
5. Tasch, S., Niko, A., Leising, G., Scherf, U., Efficient white light-emitting diodes realized with new processable blends of conjugated polymers.
Appl. Phys. Lett.
, 68, 1090, 1996.
6. Chen, X., Lin, H., Chen, P., Guan, G., Deng, J., Peng, H., Smart, stretchable supercapacitors.
Adv. Mater.
, 26, 4444–4449, 2014.
7. Fukuda, M., Sawaka, K., Yoshino, K., Electronic Characterization of New Bright-Blue-Light-Emitting Poly(9,9-dioctylfluorenyl-2,7-diyl)-End Capped With Polyhedral Oligomeric Silsesquioxanes.
Jpn. J. Appl. Phys.
, 28, 1433, 1989.
8. Fukuda, M., Sawaka, K., Yoshino, K., Synthesis of fusible and soluble conducting polyfluorene derivatives and their characteristics.
J. Polym. Sci., Polym. Chem. Ed.
, 31, 2465, 1993.
9. Miyaura, N., Yanagi, T., Suzuki, A., The Palladium-Catalyzed Cross-Coupling Reaction of Phenylboronic Acid with Haloarenes in the Presence of Bases.
J. Synth. Commun.
, 11, 513–519, 1981.
10. Friend, R., Gymer, R., Holmes, A., Burroughes, J., Marks, R., Taliani, C., Bradley, D., Dos Santos, D., Bredas, J., Logdlund, M., Salaneck, W., Electroluminescence in conjugated polymers.
Nature
, 397, 121–128, 1999.
11. Bernius, T., Inbasekaran, M., O’Brien, J., Wu, W., Progress with Light-Emitting Polymers.
Adv. Mater.
, 12, 1737–1750, 2000.
12. Grell, M., Bradley, D., Inbasekaran, M., Woo, E., A glass-forming conjugated main-chain liquid crystal polymer for polarized electroluminescence applications.
Adv. Mater.
, 9, 798–802, 1997.
13. Bernius, M., Inbasekaran, M., Woo, E., Wu, W., Wujkowski, L., Polyfluorenes.
J. Mater. Sci.: Mater. Electron.
, 11, 111, 2000.
14. Redecker, M., Bradley, D., Inbasekaran, M., Wu, W., Woo, E., High Mobility Hole Transport Fluorene-Triarylamine Copolymers.
Adv. Mater.
, 11, 241– 246, 1999.
15. Redecker, M., Bradley, D., Inbasekaran, M., Woo, E., Mobility enhancement through homogeneous nematic alignment of a liquid-crystalline polyfluorene.
Appl. Phys. Lett.
, 74, 1400, 1999.
16. Redecker, M., Bradley, D., Baldwin, K., Smith, D., Inbasekaran, M., Wu, W., Woo, E., An investigation of the emission solvatochromism of a fluorene-triarylamine copolymer studied by time resolved spectroscopy.
J. Mater. Chem.
, 9, 2151–2154, 1999.
17. R. Wessling and R. Zimmerman, US Patent 3 401 152, 1968.
18. R. Wessling and R. Zimmerman, US Patent 3 706 677, 1972.
19. Braun, D., Heeger, A., Kroemer, H., Improved efficiency in semiconducting polymer light-emitting diodes.
J. Electron. Mater.
, 20, 945–948, 1991.
20. Yang, Y., Pei, Q., Heeger, A., Efficient blue polymer light-emitting diodes from a series of soluble poly(paraphenylene)s.
J. Appl. Phys.
, 79, 934, 1996.
21. Tang, C.W., Direct interaction with large-scale display systems using infrared laser tracking devices.
Sot. Inf. Display Conf. Proc.
, p. 181, 1996.
22. Kim, W., Kwon, S., Lee, S.M., Kim, J.Y., Han, Y., Kim, E., Choi, K.C., Park, S., Park, B.C., facilitated embedding of silver nanowires into conformally-coated iCVD polymer films deposited on cloth for robust wearable electronics.
Org. Electron.
, 14, 3007–3013, 2013.
23. Kim, H., Kwon, S., Choi, S., Choi, K.C., Solution-processed bottom-emitting polymer light-emitting diodes on a textile substrate towards a wearable display.
J. Inf. Disp.
, 16, 179–184, 2015.
24. Kim, W., Kwon, S., Han, Y.C., Kim, E., Choi, K.C., Kang, S.H., Park, B.C., Wearable Electronics: Reliable Actual Fabric-Based Organic Light-Emitting Diodes: Toward a Wearable Display.
Adv. Electron. Mater.
, 2, 1600220, 2016.
25. O’Connor, B., An, K.H., Zhao, Y., Pipe, K.P., Shtein, M., Fiber shaped light emitting device.
Adv. Mater.
, 19, 3897–3900, 2007.
26. Kwon, S., Kim, W., Kim, H., Choi, S., Park, B.C., Kang, S.H., Choi, K.C., High Luminance Fiber-Based Polymer Light-Emitting Devices by a Dip-Coating Method.
Adv. Electron. Mater.
, 1, 1500103, 2015.
27. Lee, S.M., Kwon, J.H., Kwon, S., Choi, K.C., A review of flexible OLEDs towards highly durable unusual displays.
IEEE Trans. Electron Devices
, 64, 5, 2017.
28. Choi, S., Kwon, S., Kim, H., Kim, W., Kwon, J.H., Lim, M.S., Lee, H.S., Choi, K.C., Highly Flexible and Efficient Fabric-Based Organic Light-Emitting Devices for Clothing-Shaped Wearable Displays.
Sci. Rep.
, 7, 6424, 2017.
Note
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Corresponding author
:
2Polymer Light-Emitting Devices by Solution Processing
Mariya Aleksandrova
Technical University of Sofia, Department of Microelectronics, Sofia, Bulgaria
AbstractSolution processing of organic-based light-emitting devices (OLEDs) has been widely studied as a technology for low-cost, large area fabrication of optoelectronic devices. This chapter focuses on the usage of polymer films deposited by solution processing for the manufacturing of different single-colored or fill-colored electroluminescent displays. The survey focuses on the polymer molecules engineering and devices architecture for creating highly efficient polymer light-emitting devices (PLEDs) with parameters compatible to the commercially available OLED using small molecules. The relevance of the materials selection and deposition process modes are highlighted, in terms of energy bands alignment, smoothening of the films surfaces, and lack of intermixing at the layer interfaces. All these factors are crucial for achieving stable and efficient PLED. Some of the main achievements and challenges reported in the last few years are summarized and discussed in relation with the turn-on voltage, current density, maximum brightness, luminous efficiency, and possibility for commercialization of the devices. New light-emitting, hole and electron transporting materials, as well as electrodes and substrates materials are considered.
Keywords: All-solution processing, polymer light-emitting device, hole transporting layer, color tuning, polymer emissive layer, PEDOT:PSS, flexible substrates, energy level alignment
2.1 Introduction
The organic electronics is one of the most advanced branches of the engineering science. At present, it is possible to synthesize a great species of organic molecules that could be conductive, insulating, or semiconducting. Based on this classification, there are a huge number of organic materials, according to the band gap width and therefore to the electrical properties, which can be easily modified by replacing any of the major functional groups in the organic compound or by doping. By tailoring the molecular structure of the materials, it is possible to achieve great variety of properties and applications with easy tunable characteristics. Such flexibility in the capabilities of organic materials is very useful in the field of optoelectronics [1, 2].
Organic optoelectronics is a multidisciplinary field that includes physics of solid-state matter, synthetic chemistry, thin film deposition technologies, methods for structural characterization, and last, but not least, electronic engineering for processing the control signals to the device, or the useful signal generated from the device [3].
Organic semiconductors are divided into two major classes: low molecular weight compounds (organic crystals) and high molecular weight compounds (polymers). Recently, the research interest has been focused mainly on polymers. One of the major advantages of conductive polymers over the small molecules is their ability to be easily processed and applied as thin films from solution. This makes their manufacturing simple, fast, and cost-effective, because the solution processes are vacuum-free, not requiring complex and expensive equipment [4, 5]. Usually, printing and other solution-based processes are used for the polymeric films deposition. Since the most polymeric devices include several thin films, organic, as well as inorganic, the interface formation between them is also very important for the devices operation [6].
Nowadays, the consumer requirements to the display devices impose the need for development of increasingly attractive displays with qualities such as high brightness, a broad range of colors, color saturation, a wide viewing angle, small sizes, and low power consumption. Organic-based displays meet these requirements. Currently, the commercial organic light-emitting devices (OLEDs) could be found in the multi-color flat panel displays in TVs, cell phones, and digital cameras [7, 8]. Actually, all OLED displays on the market today use small molecules, and they are produced by expensive vacuum evaporation process [9]. Polymer light-emitting devices (PLEDs) technology also steps in the competition to make displays of commercial devices, like some of the latest MP3 players and audio systems, which are however low-volume series. Thus, the polymer-based products currently have only an advertising function to demonstrate trends in the development of the above-mentioned products of the modern electronics [10].
Because PLED displays have their own illumination and no backlighting is required, they tend to be even thinner and lighter than liquid-crystal displays (LCD). At the PLED matrix, it is possible control of the individual pixels; therefore, the consumption is significantly lower than at the LCD, where the entire panel should be continuously illuminated. PLEDs also provide higher contrast, more “true” colors and a wider viewing angle. These advantages are the same like for OLED, but additionally, PLED films can be deposited over a large area and at room temperature by spin-coating, inkjet printing, or spray deposition. The use of such techniques allows the application of the polymer layers to various types of substrates, including flexible foils, textile, and even paper.
2.1.1 Materials, Design, Main Parameters, and Characteristics of PLEDs
PLED is an electroluminescent (EL) device, at which the functional polymer layers are deposited between two electrodes. When applying a DC voltage, electrons and holes are injected from the electrodes into the light-emissive polymeric film. When the opposite charge carriers meet at this film, they recombine, resulting in light emission with wavelength depending on the band gap of the used polymer, which is semiconductor. At least one of the electrodes in the display structure must be transparent, in order to fluently emit the light generated inside the structure without optical losses, and at the same time, it must be conductive in order to have good injection properties (Figure 2.1a). Additionally, the electrodes’ work functions must be close in energy to one of the energy levels of the polymer—to the energy of the Highest Occupied Molecular Orbital (HOMO) for the anode and to the energy of the Lowest Unoccupied Molecular Orbital (LUMO) for the cathode (Figure 2.1b) [11]. In this term, the most suitable and commonly used material for anode is Indium Tin Oxide (ITO) and for cathode is aluminum. This represents the classical structure of a PLED. It involves vacuum deposition processes (sputtering and thermal evaporation) increasing the total price of the device and making more or less senseless the using of solution, low cost processes for the polymer films deposition only. That’s why, recently, the efforts have been focused on the finding of suitable materials and processes as alternative to the typical electrodes. An overview of these materials and technologies is made in this chapter.
Figure 2.1 (a) Typical structure of conventional organic based light-emitting device; (b) Band diagram, showing the energy levels alignment at the interface electrode/ polymer layer.
