Organic Thin Film Transistor Integration E-Book

Flora Li

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 432

Veröffentlichungsjahr: 2011

Beschreibung

Research on organic electronics (or plastic electronics) is driven by the need to create systems that are lightweight, unbreakable, and mechanically flexible. With the remarkable improvement in the performance of organic semiconductor materials during the past few decades, organic electronics appeal to innovative, practical, and broad-impact applications requiring large-area coverage, mechanical flexibility, low-temperature processing, and low cost. Thus, organic electronics appeal to a broad range of electronic devices and products including transistors, diodes, sensors, solar cells, lighting, displays, and electronic identification and tracking devices A number of commercial opportunities have been identified for organic thin film transistors (OTFTs), ranging from flexible displays, electronic paper, radio-frequency identification (RFID) tags, smart cards, to low-cost disposable electronic products, and more are continually being invented as the technology matures. The potential applications for "plastic electronics" are huge but several technological hurdles must be overcome. In many of these applications, transistor serves as a fundamental building block to implement the necessary electronic functionality. Hence, research in organic thin film transistors (OTFTs) or organic field effect transistors (OFETs) is eminently pertinent to the development and realization of organic electronics. This book presents a comprehensive investigation of the production and application of a variety of polymer based transistor devices and circuits. It begins with a detailed overview of Organic Thin Film Transistors (OTFTs) and discusses the various possible fabrication methods reported so far. This is followed by two major sections on the choice, optimization and implementation of the gate dielectric material to be used. Details of the effects of processing on the efficiency of the contacts are then provided. The book concludes with a chapter on the integration of such devices to produce a variety of OTFT based circuits and systems. The key objective is to examine strategies to exploit existing materials and techniques to advance OTFT technology in device performance, device manufacture, and device integration. Finally, the collective knowledge from these investigations facilitates the integration of OTFTs into organic circuits, which is expected to contribute to the development of new generation of all-organic displays for communication devices and other pertinent applications. Overall, a major outcome of this work is that it provides an economical means for organic transistor and circuit integration, by enabling the use of a well-established PECVD infrastructure, while not compromising the performance of electronics. The techniques established here are not limited to use in OTFTs only; the organic semiconductor and SiNx combination can be used in other device structures (e.g., sensors, diodes, photovoltaics). Furthermore, the approach and strategy used for interface optimization can be extended to the development of other materials systems.

Leseprobe

Table of Contents

About the Authors

Title Page

Copyright

Dedication

Preface

Glossary

Abbreviations

Mathematic Symbols

Definitions

Chapter 1: Introduction

1.1 Organic Electronics: History and Market Opportunities

References

Chapter 2: Organic Thin Film Transistor (OTFT) Overview

2.1 Organic Semiconductor Overview

2.2 OTFT Operation and Characteristics

2.3 OTFT Device Architecture

2.4 OTFT Device Material Selection

2.5 Summary

References

Chapter 3: OTFT Integration Strategies

3.1 Technological Challenge in OTFT Integration

3.2 Overview of Processing and Fabrication Techniques

3.3 OTFT Fabrication Schemes

3.4 Summary and Contributions

References

Chapter 4: Gate Dielectrics by Plasma Enhanced Chemical Vapor Deposition (PECVD)

4.1 Overview of Gate Dielectrics

4.2 Experimental Details and Characterization Methods

4.3 Material Characterization of PECVD SiNx Films

4.4 Electrical Characterization of OTFTs with PECVD Gate Dielectric

4.5 Summary and Contributions

References

Chapter 5: Dielectric Interface Engineering

5.1 Background

5.2 Experimental Details

5.3 Impact of Dielectric Surface Treatments

5.4 Impact of Oxygen Plasma Exposure Conditions

5.5 Summary and Contributions

References

Chapter 6: Contact Interface Engineering

6.1 Background

6.2 Experimental Details

6.3 Impact of Contact Surface Treatment by Thiol SAM

6.4 Impact of Execution Sequence of Surface Treatment

6.5 Summary and Contributions

References

Chapter 7: OTFT Circuits and Systems

7.1 OTFT Requirements for Circuit Applications

7.2 Applications

7.3 Circuit Demonstration

7.4 Summary, Contributions, and Outlook

References

Chapter 8: Outlook and Future Challenges

8.1 Device Performance

8.2 Device Manufacture

8.3 Device Integration

References

Index

About the Authors

Flora M. Li is currently a Senior Scientist at Polymer Vision, Eindhoven, The Netherlands. Prior to this, she was a Research Associate/NSERC Postdoctoral Fellow at the Centre of Advanced Photonics and Electronics (CAPE) in the Electrical Engineering Division of the University of Cambridge, UK. She received her Ph.D. degree in Electrical and Computer Engineering from the University of Waterloo, Canada, in 2008. She was a Visiting Scientist at Xerox Research Centre of Canada (XRCC) from 2005– 2008. Her research interests are in the field of nano- and thin-film technology for applications in large area and flexible electronics, including displays, sensors, photovoltaics, circuits and systems. Dr. Li has co-authored a book entitled CCD Image Sensors in Deep-Ultraviolet (2005), and has published articles in various scientific journals.

Arokia Nathan holds the Sumitomo/STS Chair of Nanotechnology at the London Centre for Nanotechnology, University College, London, and is a recipient of the Royal Society Wolfson Research Merit Award. He is also the CTO of Ignis Innovation Inc., Waterloo, Canada, a company he founded to commercialize technology on thin film silicon backplanes on rigid and flexible substrates for large area electronics. He has held Visiting Professor appointments at the Physical Electronics Laboratory, ETH Zürich and at the Engineering Department, University of Cambridge, UK. He received his Ph.D. in Electrical Engineering from the University of Alberta. He held the DALSA/NSERC Industrial Research Chair in sensor technology, was a recipient of the 2001 Natural Sciences and Engineering Research Council E.W.R. Steacie Fellowship, and was awarded the Canada Research Chair in nano-scale flexible circuits. Professor Nathan has published extensively in the field of sensor technology and CAD, and thin film transistor electronics, and has over 40 patents filed/awarded. He is the co-author of two books, Microtransducer CAD and CCD Image Sensors in Deep-Ultraviolet, and serves on technical committees and editorial boards in various capacities.

Yiliang Wu received his Ph.D. degree in polymer science from the Tokyo Institute of Technology, Japan, in 1999. After two years of postdoctoral studies at Queen's University, Kingston, Canada, he joined the Xerox Research Centre of Canada in 2001 as a research scientist working on organic transistor materials design and process. Currently, Dr. Wu is a Principal Scientist at the centre, leading Xerox's Printable Electronic Materials project. He is the holder of over 85 US patents and has authored/co-authored 78 peer-reviewed papers and two book chapters.

Beng Ong is Program Director of Polymer & Molecular Electronics and Devices for the Science & Engineering Research Council (SERC), Agency for Science, Technology and Research (A*Star), and Director at the Institute of Materials Research & Engineering (IMRE), Singapore. He currently also holds positions as a Professor at the Nanyang Technological University, Singapore, as Adjunct Professor at McMaster University and the University of Waterloo, Canada, as Fellow/Visiting Professor at Shanghai JiaoTong University and as an Honorary Professor at the East China University of Science and Technology in Shanghai. Prior to his relocation to Singapore in 2007, Professor Ong was a Senior Fellow of the Xerox Corporation, and a manager of Advanced Materials and Printed Electronics at the Xerox Research Centre of Canada. Professor Ong currently has a patent portfolio of 187 US patents and about 100 refereed papers on enabling materials, processes, and integration technologies. Many of the technology programs he managed at Xerox have won awards including the Nano-50 Award for Materials Innovation (2007) and for Nanotechnology Commercialization (2007), the ACS Innovation Award (2006), the Connecticut Quality Improvement Gold Award (2006), and the Wall Street Journal Technology Innovation Runner-up Award (2005).

The Editors

Dr. Flora M. Li

University of Cambridge

Electrical Engineering

9 JJ Thomson Avenue

Cambridge CB3 0FA

United Kingdom

Currently at

Polymer Vision – Wistron

Kastanjelaan 1000, building SFH

5616 LZ, Eindhoven

The Netherlands

Prof. Arokia Nathan

University College London

London Centre of Nanotechnology

17-19 Gordon Street

London WC1H 0AH

United Kingdom

Dr. Yiliang Wu

Xerox Research Centre Canada

2660 Speakman Drive

Mississauga, Ontario L5K 2L1

Canada

Prof. Beng S. Ong

Institute of Materials Research and Engineering (IMRE)

3 Research Link

Singapore 117602

Singapore

Cover

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ISBN: 978-3-527-40959-4

ePDF: 978-3-527-63446-0

ePub: 978-3-527-63445-3

Mobi: 978-3-527-63447-7

Dedication

Flora Li dedicates this book to her extraordinarily amazing family, for their unconditional love and unwavering support: David, Adda, Christina, Ben, and

Preface

Organic semiconductors offer great promise for large area, low-end, lightweight, and flexible electronics applications. Their technological edge lies not only in their ease of processability but in their ability to flex mechanically. This makes them highly favorable for implementation on robust substrates with non-conventional form factor. Since its proof of concept in the early 1980s, progress in organic electronics has been impressive with performance attributes that are competitive with the inorganic counterparts. In particular, organic electronics is attractive from the standpoint of complementing conventional silicon technology, thriving in a different market domain that targets lower resolution, cost-effective mass production items such as identification tags, smart cards, smart labels, and pixel drivers for display and sensor technology.

While the material properties and processing technology for organic semiconductors continue to advance and mature, progress in organic thin film transistor (OTFT) integration and its scalability to large areas has not enjoyed the same pace. A major driving force behind this technology lies in the ability to manufacture low-end, and disposable electronic devices. This in turn demands a fabrication process that allows high volume production at low cost. The process should be able to produce stand-alone devices, device arrays, and integrated circuits of acceptable operating speed, functionality, reliability, and lifetime. However, this comes with its fair share of challenges, which we have attempted to address in this book. It is intended as a text and/or reference for graduate students in Electrical Engineering, Materials Science, Chemistry, and Physics, and engineers in the electronics industry.

Most of the results presented here stem from research conducted at the Giga-to-Nano Labs, University of Waterloo, and the Xerox Research Centre of Canada (XRCC), which granted access to its high quality, high performance, stable organic semiconductor materials. We acknowledge the contributions of several colleagues in these laboratories whose expertise ranged from materials processing and TFT integration to circuit and system design. We especially thank Prof. A. Sazonov (University of Waterloo), Dr Yuri Vygranenko (Instituto Superior de Engenharia de Lisboa), Dr D. Striakhilev (Ignis Innovation Inc.), Prof. P. Servati (University of British Columbia), Dr S. Koul (General Electric), Dr M.R.E. Rad (T-Ray Science), Dr C.-H. Lee (Samsung Electronics), Dr G. Chaji (Ignis Innovation Inc.), Dr K. Sakariya (Apple Computers), Dr S. Sambandan (PARC), Dr H.-J. Lee (DALSA Inc.), Dr K. Wong (University of Waterloo), R. Barber (University of Waterloo), Dr G.-Y. Moon (LG Chemicals), Dr I.W. Chan (ETRI).

We would also like to acknowledge the support of other colleagues: Prof. W.I. Milne, Dr. P. Beecher, and Dr C.-W. Hsieh of University of Cambridge, A. Ahnood and J. Stott of University College London, and Prof. G. Jabbour and Dr H. Haverinen of Arizona State University and Oulu University.

The text has evolved from a series of courses offered to graduate students in Electrical Engineering as well as doctoral dissertations covering different aspects of large area electronics. The scope of this book is to advance OTFT integration from an engineering perspective, and not material development, which is the strength of chemical physicists. By assimilating existing materials, techniques and resources, the book explores a number of approaches to deliver higher performance devices and demonstrate the feasibility of organic circuits for practical applications. Much of the material in the book can be presented in about 30 hours of lecture time. The text begins with an assessment of organic electronics and market opportunities for OTFT technology. The latter is further described in Chapter 2.1, examining device architectures and material selection. Strategies to enable circuit integration are presented in Chapter 3.1, while Chapter 4.1 explores optimization of gate dielectric composition and structure. Interface engineering methodologies for OTFTs to enhance the dielectric/semiconductor and contact/semiconductor interfaces are described in Chapters 5.1 and 6.1. Chapter 7.1 presents examples of functional circuits for active-matrix display and other applications. Chapter 8.1 concludes with a glimpse of future challenges related to OTFT integration.

This book would not have been possible without the support of various institutions and funding agencies: University of Waterloo, Xerox Research Centre of Canada, University College London, University of Cambridge, Nanyang Technological University, Natural Sciences and Engineering Research Council of Canada, Ontario Centres of Excellence, and The Royal Society.

Cambridge, London, Toronto, Singapore 2010

Flora M. Li, Arokia Nathan,

Yiliang Wu, and Beng S. Ong

Glossary

Abbreviations

AC alternating current

AFM atomic force microscopy

Ag silver

Al aluminum

Al2O3 or AlOx aluminum oxide

ALD atomic layer deposition

AMLCD active-matrix liquid crystal display

AMOLED active-matrix organic light emitting diode

a-Si:H or a-Si amorphous silicon

Au gold

BCB benzocyclobutene

C60 fullerene

CMOS complementary metal oxide semiconductor

CNT carbon nanotube

CT charge transfer

CTC charge transfer complex

Cu copper

C–V capacitance–voltage characteristics

CVD chemical vapor deposition

D6HT dihexyl-sexithiophene

DC direct current

DFH-4T diperflurorohexylquarter-thiophene

DIP dual in-line package

DOS density of states

Dpi dots per inch

EDM electro-discharge machining

E-Paper electronic paper

ERDA elastic recoil detection analyses

F16CuPc hexadecafluoro-phthalocyanine

F8T2 poly(9,9′-dioctyl-fluorene-co-bithiophene)

FTIR fourier transform infrared spectroscopy

GIXRD grazing-incidence X-ray diffraction

HF hydrofluoric acid

HMDS hexamethyldisilazane

HOMO highest occupied molecular orbital

IC integrated circuit

ICP inductively coupled plasma

IEEE Institute of Electrical and Electronics Engineers

IJP inkjet printing

IP ionization potential

I–V current–voltage characteristics

LCD liquid crystal display

LUMO lowest unoccupied molecular orbital

MIS metal-insulator-semiconductor

MOS metal-oxide-semiconductor

MNB 2-mercapto-5-nitro-benzimidazole

Mo molybdenum

MOSFET metal oxide semiconductor field effect transistor

MTR multiple trapping and release model

N2 nitrogen

NH3 ammonia

NMOS n-channel or n-type metal oxide semiconductor

NW nanowire

O2 plasma oxygen plasma

ODTS octadecyltrichlorosilane

OFET organic field effect transistor

OLED organic light emitting diode

OTFT organic thin film transistor

OTS or OTS-8 octyltrichlorosilane

P3HT poly(3-hexylthiophene)

PA polyacetylene

PANI polyaniline

PBTTT poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene)

PCBM phenyl-C61-butyric acid methyl ester

PECVD plasma enhanced chemical vapor deposition

PEDOT:PSS poly(3,4-ethylene dioxythiophene) doped with polystyrene sulfonic acid

PEN poly(ethylene naphthalate)

PET poly(ethylene terephthalate)

Ph.D. doctor of philosophy

PI polyimide

PMMA poly(methyl methacrylate)

PPV poly(p-phenylene vinylene) or polyphenylene vinylene

PQT poly(3,3′′′-dialkylquaterthiophene)

Pt platinum

PT polythiophene

PTV poly(thienylene vinylene)

PVA polyvinyl acetate or polyvinyl alcohol

R&D research and development

RCA clean a standard set of wafer cleaning steps

RCA Radio Corporation of America

RF radio frequency

RFID radio frequency identification

RIE reactive ion etching

SAM self-assembled monolayer

SiH4 silane

SiNx silicon nitride

SiO2 silicon dioxide

SiOx silicon oxide

SnO2 tin oxide

TFT thin film transistor

TiO2 titanium oxide

UV ultraviolet

UW University of Waterloo

XPS X-ray photoelectron spectroscopy

XRCC Xerox Research Centre of Canada

ZnO zinc oxide

Mathematic Symbols

φB injection barrier

ΦM work function of the electrode (metal)

[N]/[Si] nitrogen to silicon ratio, to describe stoichiometry or composition of SiNx

μFET field effect mobility

Ci gate capacitance per unit area

CS storage capacitor

EG band-gap energy

fmax maximum switching frequency

gm transconductance

ID drain current

IG gate current

Ileak leakage current

IOFF off current

ION on current

ION/IOFF on/off current ratio

IS source current

IPS ionization potential of the semiconductor

L channel length

RCONTACT contact resistance

S inverse subthreshold slope (V dec–1)

τ transit time

VBG bottom-gate voltage

VDD positive supply voltage

VDS drain-source voltage

VGS gate-source voltage

VON,VSO onset voltage or switch-on voltage

VSS negative supply voltage

VT threshold voltage

VTG top-gate voltage

W channel width

Definitions

Definitions of selected terms cited from Wikipedia webpage. http://en.wikipedia.org/wiki/Main_Page.

Alkanes (also Alkyl)

Chemical compounds that consist only of the elements carbon (C) and hydrogen (H) (i.e., hydrocarbons), wherein these atoms are linked together exclusively by single bonds (i.e., they are saturated compounds) without any cyclic structure (i.e., loops). An alkyl group is a functional group or side-chain that, like an alkane, consists solely of singly-bonded carbon and hydrogen atoms.

Charge transfer complex (CT complex)

An electron donor–electron acceptor complex, characterized by electronic transition(s) to an excited state. In this excited state, there is a partial transfer of elementary charge from the donor to the acceptor. A CT complex composed of the tetrathiafulvalene (TTF, a donor) and tetracyanoquinodimethane (TCNQ, an acceptor) was discovered in 1973. This was the first organic conductor to show almost metallic conductance.

Conductive polymer (also conducting polymer)

Polymer that is made conducting, or “doped,” by reacting the conjugated semiconducting polymer with an oxidizing agent, a reducing agent, or a protonic acid, resulting in highly delocalized polycations or polyanions. The conductivity of these materials can be tuned by chemical manipulation of the polymer backbone, by the nature of the dopant, by the degree of doping, and by blending with other polymers. Conductive polymer is an organic polymer semiconductor, or an organic semiconductor.

Conjugated polymer

A system of atoms covalently bonded with alternating single and double carbon–carbon (sometimes carbon–nitrogen) bonds in a molecule of an organic compound. This system results in a general delocalization of the electrons across all of the adjacent parallel aligned p-orbitals of the atoms, which increases stability and thereby lowers the overall energy of the molecule.

Dielectric (also insulator)

A non-conducting substance, that is, an insulator. Although “dielectric” and “insulator” are generally considered synonymous, the term “dielectric” is more often used when considering the effect of alternating electric fields on the substance while “insulator” is more often used when the material is being used to withstand a high electric field. Dielectric encompasses the broad expanse of nonmetals (including gases, liquids, and solids) considered from the standpoint of their interaction with electric, magnetic, of electromagnetic fields. In this book, the terms “dielectric” and “insulator” are used interchangeably.

Electrode (also contact)

An electrical conductor (e.g., metallization) used to make contact with a nonmetallic part of a circuit (e.g., a semiconductor). The gate/source/drain metal layer of the TFT is referred to as an electrode. The connection between the source/drain metal layer and the semiconductor layer (i.e., when we speak of the interface) is referred to as the “contact.” In this book, the terms “electrode” and “contact” are used almost interchangeably.

Insulator (also dielectric)

A material that resists the flow of electric current. It is an object intended to support or separate electrical conductors without passing current through itself. An insulation material has atoms with tightly bonded valence electrons. The term electrical insulation often has the same meaning as the term dielectric.

Mobility (also carrier mobility, field-effect mobility, effective mobility)

The state of being in motion. Carrier mobility is a quantity relating the drift velocity of electrons or holes to the applied electric field across a material; this is a material property. Field-effect mobility or effective mobility describes the mobility of carriers under the influence of the device structure in field-effect transistors. Field-effect mobility is device-specific, not material-specific, and includes effects such as contact resistances, surface effects, and so on.

Organic compounds

Chemical compounds containing carbon-hydrogen (C–H) bonds of covalent character.

Organic electronics (also plastic electronics)

A branch of electronics that deals with conductive polymers, plastics, or small molecules. It is called “organic” electronics because the polymers and small molecules are carbon-based, like the molecules of living things. This is as opposed to traditional electronics which relies on inorganic conductors such as copper or silicon.

Organic semiconductor (also polymer semiconductor)

Any organic material that has semiconductor properties. Both short chain (oligomers) and long chain (polymers) organic semiconductors are known. There are two major classes of organic semiconductors, which overlap significantly: organic charge-transfer complexes, and various “linear backbone” polymers derived from polyacetylene. This book focuses on the investigation of polymer organic semiconductors; thus, in most cases, the term “organic semiconductor” and “polymer semiconductor” are used interchangeably.

OTFT (also OFET)

An organic thin film transistor (OTFT) or organic field effect transistor (OFET) is a field effect transistor using an organic semiconductor in its channel.

Plastic

A general term for a wide range of synthetic or semi-synthetic polymerization products. Plastics are polymers, that is, long chains of atoms bonded to one another.

Polymer

A substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds.

2

Organic Thin Film Transistor (OTFT) Overview

By definition, organic materials describe a large class of chemical compounds whose molecules contain carbon. The dividing line between organic and inorganic is somewhat controversial and historically arbitrary, but, generally speaking, organic compounds have carbon–hydrogen bonds, and inorganic compounds do not [1]. Until about 40 years ago, all carbon-based organic compounds and polymers1 were regarded as insulators. Organic polymer materials (or plastics) were used as inactive packaging and insulating materials. This narrow perspective rapidly changed as a new class of polymer known as a conductive polymer was discovered in the 1960–1970s [2–4]. Today, there is a tremendous research effort focused on using conductive polymers for electronic fabrication. Depending on the resistivity levels, conductive polymers can behave as semiconductors (referred to as “organic semiconductors”), or they can be highly doped to behave as conductors or metals.

An organic semiconductor is loosely defined as any organic material that has semiconductor properties. Both short chain (oligomers) and long chain (polymers) organic semiconductors are known. There are two major classes of organic semiconductors, with considerable overlap: organic charge-transfer complexes, and various “linear backbone” polymers derived from polyacetylene. Charge transfer (CT) complexes are obtained by pairing an electron donor molecule with an electron acceptor molecule, and are characterized by electronic transition(s) to an excited state. One example of a CT complex is a tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) crystal, where TTF serves as a donor and TCNQ serves as an acceptor. The TTF-TCNQ crystal was the first organic conductor found to show almost metallic conductivity (in 1972). Organic semiconductors based on linear-backbone polymers are obtained from doped conjugated polymers. In this book, discussion of “organic semiconductors” pertains to this class of polymer-based organic semiconductors.

Transistors based on organic semiconductors as the active layer to control current flow are commonly referred to as organic thin film transistors (OTFTs), or sometimes organic field effect transistors (OFETs). The simplest OTFT configuration is shown in Figure 2.1. Generally speaking, a thin film transistor (TFT) is composed of three main parts: a thin semiconductor layer, a dielectric (or insulator), and three electrodes (gate, source, and drain). The source (S) and drain (D) electrodes directly contact the semiconductor, whereas the gate (G) electrode is separated from the semiconductor by a dielectric layer. The gate turns the device on and off with an applied voltage, and thus controls the current flow (IDS) in the semiconductor between the source and drain electrodes.

Figure 2.1 Cross-section of a basic OTFT structure, in bottom-gate and top-contact configuration.

This chapter presents an overview of OTFT technology. The organic semiconductor, which is the heart or foundation of the OTFT, is introduced in Section 2.1. The fundamental properties of organic semiconductors and recent progress in material development are reviewed. The basic operation and characteristics of the OTFT are discussed in Section 2.2, along with the parameter extraction techniques used for characterizing OTFT performance. The device architectures and material systems, specific to the context of this book, are examined in Sections 2.3 and 2.4, respectively. Please note that the terms “organic” and “polymer” are used interchangeably in this book. Generally speaking, “organic” covers a wider scope of materials, where “polymer” is a subset of organic. The book focuses primarily on polymer semiconductors.

2.1 Organic Semiconductor Overview

The operation and performance of OTFTs are largely dictated by the characteristics of the active organic semiconductor layer. In particular, the mobility of the device is related to the efficiency of charge transport through the semiconductor channel. This section begins with a review of the unique conjugated chemical structure that gives rise to electrical conduction in organic semiconductor materials. Typical charge transport mechanisms in organic materials are presented. These models pertain primarily to non-crystalline (or disordered) organic semiconductors (e.g., polymers); the transport mechanism in well-organized organic molecular crystals is different, and is outside the scope of this book. The recent progress in material development and classifications of organic semiconductors are considered.

2.1.1 Basic Properties

Polyacetylene was one of the first polymers reported to be capable of conducting electricity [5], and it was discovered that oxidative doping with iodine causes the conductivity to increase by 12 orders of magnitude [6]. The “doped” form of polyacetylene had a conductivity of 105 S m−1. As a comparison, an insulator such as teflon has a conductivity of 10−16 S m−1, and a metal such as silver or copper has a conductivity of 108 S m−1 [4]. However, polyacetylene reacts rapidly and irreversibly with oxygen, is insoluble in organic solvents, and is difficult to process. Progress was made with the discovery of polythiophene (PT) and polyphenylenevinylene (PPV), which exhibited better characteristics than polyacetylene [7]. Figure 2.2 shows the chemical structure of these three conductive polymers.

Figure 2.2 Chemical structure of three conjugated polymers: (a) polyacetylene, (b) polythiophene, and (c) polyphenylenevinylene [7].

All electrically conductive polymers share two principal properties. The first is the presence of “conjugated double bonds” along the backbone of the polymer. In conjugation, the polymer consists of alternate single and double bonds between the carbon atoms. This alternating structure can be observed in polyacetylene, displayed in Figure 2.2a. The second property is that the polymer must be “doped”, implying that electrons are removed (through oxidation) or introduced (through reduction). These extra holes or electrons can move along the molecule to contribute to electrical conductivity [4].

In the conjugated double bond structure, the single bond is a sigma (σ) bond, and the double bond consists of a σ-bond and a pi (π) bond [8]. σ-bonds, the strongest type of covalent bonds, require that both atoms give an electron from the s orbital. Thus, the electrons that form the σ-bond are attached to the two nuclei and are localized. π-bonds are a direct sharing of electrons between the p orbitals of two atoms. π-bonds are weaker than σ-bonds because their orbitals are further away from the positively charged nucleus. Normally, the electrons that form a π-bond are localized. However, in conductive polymers, π-orbitals of the neighboring double bonds overlap due to the conjugated structure, as illustrated in Figure 2.3a. This overlapping results in weakly localized (or “delocalized”) π-electrons that can move from one bond to another or move along the entire molecule. Therefore, delocalization, accomplished by the continuous overlapping π-orbitals of the conjugated backbone, makes the conduction of charge carriers along the polymer chain (i.e., intramolecular transport) possible [8].

Figure 2.3 (a) Conjugated and (b) non-conjugated structure of an organic molecule [8].

The system of alternating double and single bonds in the conjugated backbone gives rise to a separation of bonding and anti-bonding states, resulting in the formation of a forbidden energy gap and a spatially delocalized band-like electronic structure, as illustrated in Figure 2.4. The highest occupied molecular orbital (HOMO) consists of bonding states of the π-orbitals with filled electrons, and is analogous to the valence band in silicon. The lowest unoccupied molecular orbital (LUMO) consists of empty higher energy anti-bonding (π*) orbitals, and is analogous to the conduction band. The energy difference between the HOMO and LUMO defines the band-gap energy (EG). EG depends on the chemical structure of the repeating unit, and generally decreases with the number of repeat units in the chain [4]. The EG of conjugated polymers is typically in the energy range of 1–4 eV. This band-like structure, along with low electronic mobility, is responsible for the semiconducting properties observed in conjugated polymers. Due to the disordered nature of organic materials, conduction mainly takes place via phonon-assisted hopping and polaron-assisted tunneling between localized states; this is in contrast to crystalline semiconductors (e.g., silicon) where conduction occurs in energy bands through delocalized states.

Figure 2.4 Representative energy band diagram of an organic semiconductor.

Most conductive polymers in their neutral state are wide-band-gap semiconductors and exhibit very low conductivities [7]. To increase electrical conductivity, doping is required. The conductivity of a conjugated polymer can be modified by chemical doping or electrochemical doping, where oxidation and reduction reactions are used to achieve p-type (electron removal) and n-type (electron addition) doping, respectively [4]. These doping processes generate mobile charge carriers which move in an electric field, giving rise to electrical conductivity. In most cases, conductive polymers are doped by oxidative reactions; thus, p-type conductive polymer materials are more common, with holes as the majority transport carriers.

Highly conjugated organic materials can work as semiconductors because of their strong π-orbital overlap. When an electron is added or a hole is injected, the resultant charge becomes delocalized across the conjugated system. This injected charge acts as a carrier for current conducting through the molecule and through the organic semiconductor thin film. Transistors based on organic semiconductors as the active layer to control current flow are commonly referred to as organic thin film transistors.

2.1.2 Charge Transport

In conductive polymers, the charge transport is relatively easy along a conjugated molecular chain (i.e., intramolecular transport) because of the strong π-orbital overlap. However, due to the amorphous structure of polymer materials and the weak intermolecular interaction, charge transport between molecules (i.e., intermolecular transport) is more difficult. A polymer semiconductor often contains polycrystalline phases intermixed in disordered phases. These disorder segments tend to hinder charge transport. Typically, the charge transport between molecules is described as a thermally activated tunneling of charge carriers, commonly referred to as “hopping