Flexible and Stretchable Electronics
Materials, Designs, and Devices
edited by

Run-Wei Li | Gang Liu

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Flexible and Stretchable Electronics

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Flexible and Stretchable Electronics
Materials, Designs, and Devices

edited by

Run-Wei Li
Gang Liu

www.TechnicalPDF.com

Published by
Jenny Stanford Publishing Pte. Ltd.
Level 34, Centennial Tower
3 Temasek Avenue
Singapore 039190

Email:
Web: www.jennystanford.com
British Library Cataloguing-in-Publication Data
A catalogue record for this book is available from the British Library.

Flexible and Stretchable Electronics:
Materials, Designs, and Devices
Copyright © 2020 by Jenny Stanford Publishing Pte. Ltd.
All rights reserved. This book, or parts thereof, may not be reproduced in any form
or by any means, electronic or mechanical, including photocopying, recording
or any information storage and retrieval system now known or to be invented,
without written permission from the publisher.

For photocopying of material in this volume, please pay a copying fee through
the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923,
USA. In this case permission to photocopy is not required from the publisher.

ISBN 978-981-4800-46-4 (Hardcover)
ISBN 978-0-429-05890-5 (eBook)

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Contents


Preface

xiii

1. Organic Field-Effect Transistors for Flexible Electronics
Application1

Jung-Yao Chen and Cheng-Liang Liu

1.1Introduction
2

1.2
Device Structures and Operation Principle
3

1.3
Important Device Parameters
5

1.3.1 Field-Effect Mobility
6

1.3.2 Current ON/OFF Ratio
6

1.3.3 Threshold Voltage
7


1.3.4 Subthreshold Swing
7

1.4
Materials
7

1.4.1 Organic Semiconductors
8

1.4.1.1 p-Type
8

1.4.1.2 n-Type10

1.4.2 Gate Dielectric Materials
12

1.4.3Electrode Materials
12

1.4.4 Substrate Materials
13

1.5
Overview of Processing Techniques
13

1.5.1 Vacuum Deposition
14


1.5.2 Solution-Processed Deposition
14

1.6
Flexible Organic Transistor Device
15

1.7
Flexible Organic Phototransistor
18

1.7.1 Introduction
18

1.7.2 Important Device Parameters of
Organic Phototransistor
19

1.7.2.1 Photoconductive gain (G)
19

1.7.2.2 Photocurrent/dark current
ratio (P)
20

1.7.2.3 Photosensitivity (R)
20

1.7.2.4 Quantum efficiency (η)

20

1.7.2.5 Photodetectivity (D*)
20

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2.

















1.7.3

1.8

Examples of Flexible Organic
Phototransistors21
1.7.3.1 Donor–acceptor system
21
1.7.3.2 Photochromism
23
1.7.3.3 Photopolymerization
24
Conclusion
26

Flexible and Organic Solar Cells
Bing Cao
2.1Introduction
2.2
Basic Solar Cell Concepts
2.2.1 Structure of Organic Solar Cells
2.2.2 Operation Principle of Organic Solar
Cells
2.2.3 Photovoltaic Parameters
2.3

Donor Materials Development
2.3.1 Conjugated Polymers
2.3.2 Conjugated Small Molecules
2.4
Acceptor Materials Development
2.4.1 Fullerene Derivatives
2.4.2 Non-fullerene Small Molecules
2.5
Interfacial Materials and Device Engineering
2.6
Flexible and Organic Solar Cells

33

33
34
34
35
36
38
39
45
49
49
50
54
56

3. Flexible Parylene-C Material and Its Applications in
MOSFETs, RRAMs, and Sensors

81

Yimao Cai, Min Lin, and Qingyu Chen

3.1
An Introduction to Parylene
82

3.1.1 Types and Growth of Parylene Thin
Films
82

3.1.2 Properties of Parylene-C Thin Films
83

3.2
Application of Parylene-C in MOSFETs
84

3.2.1 Gate Dielectric
84

3.2.2 Substrate
87

3.2.3 Encapsulation Gate Dielectric
89

3.3
Application of Parylene-C in RRAM91


3.4
Application of Parylene-C in Sensors
96

3.4.1 Flow Sensors
96

3.4.2 pH Sensors
98

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3.5

3.4.3 Force Sensors
3.4.4 Pressure Sensors
Conclusion

100
101
104


4. Resistive Switching Phenomenon for Flexible and
Stretchable Memories
113

Xiaohui Yi, Shuang Gao, Jie Shang, Bin Chen,
Gang Liu, and Run-Wei Li

4.1Introduction
114

4.2
Design Principle of Flexible Resistive
Switching Memory
117

4.3
Flexible Resistive Switching Storage Media
Materials119

4.3.1 Inorganic Materials
119

4.3.2 Organic Materials
122

4.3.2.1 Organic resistive
switching memory with
small molecules
124


4.3.2.2 Blends or mixtures of
memory polymer materials 127

4.3.2.3 Polymer matrices for
electroactive components
129

4.3.2.4 Single-component polymer
active materials
134

4.3.3 Inorganic–Organic Hybrid Materials
141

4.3.3.1 Metal-organic frameworks
141

4.3.3.2Perovskite
144

4.4
Conclusion and Outlook
146
5. Two-Dimensional Materials for Flexible In-Plane
Micro-Supercapacitors157

Kaiyue Jiang, Chongqing Yang, and Xiaodong Zhuang

5.1
Introduction

157

5.2
In-Plane Micro-Supercapacitors
158

5.3
Graphene
160

5.3.1 Reduced Graphene Oxide
160

5.3.2 Electrochemically Exfoliated
Graphene
162

5.3.3 Laser-Scribed Graphene
165

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Contents












5.4
5.5
5.6
5.7

5.3.4 Graphene Composites
MXenes
Two-Dimensional Metal Oxides
5.5.1 Layered Double Hydroxides
5.5.2 V2O5/MWNT
Two-Dimensional Soft Materials
5.6.1 Two-Dimensional Coordination
Polymer Framework
5.6.2 Two-Dimensional Thiophene
Summary and Outlook

6. Flexible On-Chip Interdigital Micro-Supercapacitors:
Efficient Power Units for Wearable Electronics

Guozhen Shen, Kai Jiang, and Di Chen


6.1
Introduction

6.2
Fabrication Methods

6.2.1 Conventional Photolithography
Method

6.2.2 Laser-Scribing Method

6.2.3 Printing Method

6.3
Stretchable On-Chip MSCs

6.4
Integrated Systems

6.5
Conclusion
7.











168
173
174
175
177
178
180
180
181
191
192
195
196
199
203
205
209
213

Flexible and Stretchable Sensors
221
Tie Li, Yudong Cao, Chunyan Qu, and Ting Zhang
7.1
Introduction
222
7.2
Classes of Architectural Strategies for
Flexible and Stretchable Sensors

223
7.2.1 One-Dimensional Fibrous
Configuration224
7.2.2 Two-Dimensional Planar
Configuration
228
7.2.3 Three-Dimensional Blocks
Configuration230
7.2.4 Nature-Inspired Structure for
Flexibility and Stretchability
233
7.3
Classes of Functional Materials for Flexible
and Stretchable Sensors
234

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7.4
7.5


7.3.1 One-Dimensional Nanowire Materials
7.3.2 Two-Dimensional Planar Materials
7.3.3 Semiconductors
7.3.4 Other Special Functional Materials
Flexible and Stretchable Sensors for Human
Information Detection
Conclusion

8. Liquid Metal–Enabled Functional Flexible and
Stretchable Electronics

Xuelin Wang and Jing Liu

8.1
Introduction

8.2
Materials and Properties of Gallium-Based
RTLMs

8.2.1 Compositions of Gallium-Based
RTLM Alloys

8.2.2 Basic Properties of LMs in Flexible
Electronics

8.3
Design and Fabrication of LM Flexible
Electronics


8.3.1 Planar Electronics Printing

8.3.2 3D Printing

8.4
Applications: LM Soft Devices

8.4.1 LM Sensor

8.4.2 LM Coil

8.4.3 LM e-Skin and Wearable
Bioelectronics

8.4.4 LM-Conformable Electronics

8.4.5 Other Applications

8.5
Discussion and Conclusion

9. Printing Technology for Fabrication of Flexible and
Stretchable Electronics

Wei Yuan and Zheng Cui

9.1
Introduction


9.2
Printing Process

9.2.1 Jet Printing (Non-contact Printing)

9.2.1.1 Inkjet printing

9.2.1.2 Aerosol-jet printing

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238
240
244
246
254
267
268
269
269
271
274
274
277
278
279
281
282
283

283
284
295
296
298
299
300
300

ix


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Contents




























10.












9.2.1.3

9.3

9.4


9.5

9.6

Electrohydrodynamic-jet
printing301
9.2.2 Replicate Printing (Impact Printing)
302
9.2.2.1 Screen printing
302
9.2.2.2 Gravure printing
303
9.2.2.3 Flexographic printing
304
9.2.2.4 Offset printing
304
9.2.2.5 Roll-to-roll printing
305
Printable Inks
305
9.3.1 Metal Materials
305
9.3.2 Transparent Conducting Oxide Inks
308
9.3.3 Carbon Nanomaterials
309
9.3.4 Semiconductor Nanomaterials
311
9.3.5 Reactive Inks

312
9.3.6 Stretchable Inks
313
Post-printing Process
314
9.4.1 Thermal Sintering
315
9.4.2 Photonic Sintering
316
9.4.3 Plasma, Microwave, and Electrical
Sintering
318
Applications
320
9.5.1 Transparent Conductive Films
320
9.5.2 Printed TFTs
323
9.5.3 Printed Solar Cells
324
9.5.4 Printed OLEDs
326
9.5.5 Printed Stretchable Circuits
327
Summary
329

Mechanics and Control of Smart Flexible Structures
Guoyong Mao and Shaoxing Qu
10.1 Introduction

10.2 Wavy Designs
10.2.1 Small Deformations of Wavy Ribbons
10.2.2 Large Deformations of Wavy Ribbons
10.2.3 Partially Boned Wavy Ribbons
10.2.4 Wavy Membranes
10.3 Island–Bridge Designs
10.3.1 Straight Interconnects
10.3.2 Serpentine Interconnects

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346
347
348
349
350
352
355
355
359


Contents






10.4
10.5

Index

10.3.3 Fractal Interconnects
Origami/Kirigami Designs
Conclusion



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366
374
383

xi


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Preface

The last decade has witnessed evolutional development of flexible
electronics and they are reshaping the way we live, work, and
communicate with the world. Setting themselves apart from
the traditional silicon-based semiconductor devices, the unique

capability of bending and stretching, together with the merits of
light-weight and low-cost fabrications, endows flexible electronics
imaginative versatility for soft circuits in portable energy and
industrial equipments, wearable consumer digital gadgets,
implantable health-monitoring electronic skins, medicine robots and
prosthetics, and even the fictional human (brain)-machine docking
that appears only in sci-fi movies. As an important technological
advance and one of the most promising information technologies,
flexible electronics have received broad and intensive attention from
both the academic and industrial community ever since their birth
twenty years ago.
The fabrication of flexible electronics requires multiscale
manufacture on soft substrates, spanning from the nanofeatures,
microstructures, to large-area macro-integration. What determines
their performance characteristics and, in principle, enables a host of
potential applications is the accurate control of multiple functional
materials at their interfaces, such as organics, metals, and ceramics,
which show completely different mechanical and electrical properties,
and multidisciplinary investigations in the area of chemistry, physics,
materials science, biotechnology, and electronic engineering. In this
book, we provide a timely and systematic overview of the operating
mechanisms, materials selection, device structure and circuit
design, fabrication methods, and range of applications of flexible
electronics, with the aim to deliver a comprehensive summary and
present the prospects of the specific field. The materials used for
flexible electronics range from those that carry intrinsic molecular
and structural flexibility, namely, organic semiconductors, organic–
inorganic hybrids, two-dimensional graphene derivatives and
analogs, and layered metal oxides, to room-temperature liquid
metals that demonstrate the fluidity of common liquid and electrical


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xiv

Preface

conductivity of metals. Fundamental circuit elements of resistors,
capacitors, and transistors are then produced with these materials
to construct sensors, memory and energy storage devices, and
circuits.
This book is a result of the great efforts of our contributing authors,
who have covered major areas of flexible electronics. In Chapter
1, Drs. Jung-Yao Chen and Cheng-Liang Liu discuss the materials,
charge-transport model, deposition methods, and integration of
organic field-effect transistor (OFET) for photo detecting. In Chapter
2, Dr. Bing Cao mainly presents an overview on the device structure,
operating principle, and the latest advancement of new materials
for high-performance organic solar cells (OSCs). Dr. Yimao Cai and
coworkers have compiled Chapter 3, which focuses on the use of
CMOS-compatible poly(para-xylylene) material in metal–oxide
semiconductor field-effect transistors (MOSFETs), resistive random
access memories (RRAMs), and sensor applications. In Chapter
4, Dr. Xiaohui Yi and coauthors offer a deep understanding of the
designing strategy for flexible RRAM devices, followed by particular
emphasis on organic and inorganic–organic hybrid resistive
switching materials and devices. Chapters 5 and 6, contributed by
Drs. Zhuang’s and Shen’s research groups, respectively, review the
device structure, working principle, materials selection, fabrication

approach, and integration of in-plane microcapacitor as on-chip
power supply in stretchable electronic skin systems. Drs. Tie Li and
Ting Zhang and their coworkers identify the special characters and
requirements for flexible and stretchable sensors and summarize the
strategies of structural design and functional materials for detecting
human information in Chapter 7. Beyond the electroactive materials
employed in the above-mentioned device applications, Drs. Xuelin
Wang and Jing Liu present the use of liquid metal as soft conductor
and electrode in flexible and stretchable electronics in Chapter 8.
After an overview of the performance benchmarks for the different
materials and devices of flexible electronics in Chapter 9, Drs. Wei
Yuan and Zheng Cui introduce the manufacturing processes, inks,
post-treatments, and important applications of the high-throughput,
cost-effective, and eco-friendly printing technology for producing
wearable electronics, portable energy harvesting and storage
devices, flexible display panels, and human-skin sensors. Finally,
the mechanical design concept, theory, simulation, and applications
of smart flexible structures for optimizing the overall performance

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Preface

of stretchable electronics are discussed by Drs. Guoyong Mao and
Shaoxing Qu in Chapter 10
As we enter the 21st century Big Data and Artificial Intelligence
era, information technology has changed rapidly over the past few
years and is mainly being driven by the new practical requirements and application scenarios that we had never expected before.
Looking back into the 1980s, none of the prophets could have imagined that a palm-sized telephone will accomplish complex tasks

that were being done separately by clumpy desktop computers,
brick-like walkie-talkies, mechanical film cameras, etc. Thanks to
the fourth technological revolution, the “mission impossible” is becoming “impossible is nothing” and flexible electronics show great
promise for the future as they will closely interconnect every human
being, every object, and every circumstance. As this is a newborn
area, the theoretical framework of flexible electronics is still incomplete and global efforts are needed to facilitate their transition from
research and laboratory prototypes to commercial products. This
book examines the research progress achieved in this field around
the world, and through it we hope to inspire interests to advance
flexible electronics technology, especially colleagues and university
students.

Run-Wei Li
Gang Liu
Autumn 2019

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Chapter 1

Organic Field-Effect Transistors for
Flexible Electronics Application

Jung-Yao Chena and Cheng-Liang Liub

aDepartment

of Chemical Engineering, National Chung Cheng University,
Chiayi 62102, Taiwan
bDepartment of Chemical and Materials Engineering, National Central University,
Taoyuan 32001, Taiwan
;

During the past decades, with the advent of novel organic
semiconductors (such as conjugated polymers and small molecules)
and low-cost process requirement, significant interest and progress
in the field of organic electronics have been reported. In particular,
organic field-effect transistors (OFETs) have become the backbone
of the recent developments in the large-area flexible electronics and
found usage in numerous applications such as the digital and analog
circuits, memories, photovoltaic, and sensors. This chapter gives
the present advancements in OFET devices based on an overview of
the individual layers of OFETs, charge transport model, deposition
methods, and their integration of flexible substrate. Besides,
their applications in organic phototransistors based on the OFET
architecture are briefly addressed at the end of the chapter.
Flexible and Stretchable Electronics: Materials, Designs, and Devices
Edited by Run-Wei Li and Gang Liu
Copyright © 2020 Jenny Stanford Publishing Pte. Ltd.
ISBN 978-981-4800-46-4 (Hardcover), 978-0-429-05890-5 (eBook)
www.jennystanford.com

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2

Organic Field-Effect Transistors for Flexible Electronics Application

1.1 Introduction
Organic electronics have been a field of broad academic and industrial
interest for the past few decades [1–3]. A wide range of organic
semiconductors are currently being explored steadily for organic
electronics primarily due to the growing need for substituting
Si-based technology with some organic-based materials offering
relatively simple and commercial products and applications. Organic
devices demonstrate numerous advantages such as light weight, low
cost, mechanical and molecular flexibility, and low-temperature
processing allows cost-effective production, which eventually leads
to huge benefits on various fronts, such as fabrication with flexible
substrates.
The transistor is a fundamental element for all modern
electronics. Here, transistors based on organic semiconductors as
active layer are referred to as OFETs. The performance of OFETs is
comparable to but still lower than that of conventional inorganic
transistors; however, the production cost and flexibility of Sibased devices are obvious constraints. Consistent advancements in
fabrication techniques provide the ideal solution to the realization
of flexible and large-area electronic circuits. The charge transport
properties can be undoubtedly enhanced with the optimization of
fabrication methodology and the synthesis of novel organic-based
materials.
Excellent review articles are available in the recent years, which
introduce the overall picture of OFETs or deal with each layer in detail
[4–14]. This chapter aims to give the readers a brief overview of the
building blocks in the chemical structures of organic semiconductors

and fabrication and characterization in OFET devices and to provide
their flexible electronic applications. This chapter is organized into
eight sections, including the current introduction section. Section
1.2 covers the study of the major developments in OFET structures
and operation principles. The characteristic parameters of OFETs
for analyzing device performance are discussed in Section 1.3.
Several first-generation p-type and n-type molecular and polymeric
semiconductors used in the active layer are reported in Section
1.4. Major processing techniques involved in the fabrication are
illustrated in Section 1.5. Furthermore, Section 1.6 enumerates
several representative studies on flexible OFETs. Section 1.7

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Device Structures and Operation Principle

demonstrates the application of OFETs on phototransistors. Finally,
concluding remarks related to OFETs are summarized in Section 1.8.

1.2  Device Structures and Operation Principle

Typical OFETs consist of four main components: active organic
semiconducting layer, gate dielectric (insulator), three terminals
(gate, source, and drain contact), and supporting substrate. In
principle, three-terminal organic electronic devices include intrinsic
π-conjugated organic molecules and polymers as active channel
materials; inorganic oxide and organic insulator as dielectric;
conducting polymers, metals, or carbon materials as electrodes;
and Si, flexible transparent polymer, and stretchable polymers

as substrate. The active organic semiconductor layer is generally
located in the channel between the source and drains, isolated from
the electrode by the dielectric. OFETs can act as a modified capacitor,
where one plate is from the gate electrode and the second from the
source/semiconductor/drain circuit. The application of voltage
can modify the resistance of the organic semiconductor; therefore,
a typical application of a transistor is acting as an amplifier of the
electrical signal or switch.
According to the gate electrode and relative position of organic
semiconductors and source/drain electrode, the representative
arrangement may vary into top-gate bottom-contact (TGBC), topgate top-contact (TGTC), bottom-gate bottom-contact (BGBC),
and bottom-gate top-contact (BGTC) configuration, as shown in
Fig. 1.1. There are also some special novel structures such as dualgate, side-gate, and vertical structure. The performance of OFETs
strongly depends on their structures, which is mainly due to
different charge injection paths and areas between source and drain.
For TC OFETs, the source and drain electrodes are placed on the top
of semiconductors. On the other hand, organic semiconductors are
deposited onto prefabricated source and drain electrodes for BC
OFETs. Considering BG devices as an example, the application of
a gate voltage induces a current channel at the interface between
semiconductor and dielectric layer. A BGTC exhibits a higher
current due to the relatively large injection area for charge carriers,
resulting in a lower contact resistance. In contrast, the BGBC

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Organic Field-Effect Transistors for Flexible Electronics Application

structure demonstrates an inferior performance due to high metal/
semiconductor contact resistance, which is an effect of small contact
area as well as a non-uniform deposition of the semiconductors
with distinct morphologies around the prefabricated source and
drain, even in the case of identical materials, process dimensions,
and processing skills. Although the device performance is low,
the BGBC OFETs allow simper and cost-effective processing/
patterning schemes over BGTC OFETs since deposition of organic
semiconductors constitutes the last fabrication step with greater
flexibility.

Bottom-Gate
Bottom-Contact
(BGBC)

Bottom-Gate
Top-Contact
(BGTC)

Top-Gate
Bottom-Contact
(TGBC)

Top-Gate
Top-Contact
(TGTC)
Contact electrodes

Semiconductor
Dielectric
Gate/substrate

Figure 1.1  Structures and materials of four OFET architectures.

The operation principle of OFETs is similar to the traditional
metal-oxide-semiconductor field-effect transistor (MOSFET) used
in silicon integrated circuits. OFETs operate as voltage-controlled
current source on applying the gate bias (Vg), and mobile charge
carriers are accumulated near the semiconductor/dielectric interface
that allows the current channel through the active semiconductor
layer on applying drain voltage (Vd). It should be noted that the

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Important Device Parameters

source serves as the grounded electrode. Hole and electron in the
semiconductor act as charge carriers mostly depending on the
relative energy level relationship between the semiconductor and
source/drain contact, which can be divided into three operation
modes, including p-type, n-type, and ambipolar channel. Current–
voltage (I–V) characteristics of typical OFETs can be expressed from
the linear (Eq. 1.1) and saturation region (Eq. 1.2).





Ê Vg - Vth - Vd ˆ
ÊW ˆ
( Id )lin = Á ˜ mCi Á
˜ Vd
Ë L¯
2
Ë
¯
ÊW ˆ
( Id )sat = Á ˜ mCi (Vg - Vd )2
Ë 2L ¯

(1.1)
(1.2)

where Id is the drain current, W and L are the width and length of the
channel, μ is the field-effect mobility, Ci is the capacitance per unit
area of the gate dielectric, and Vth is the threshold voltage. At low
Vd, Id builds up linearly due to the presence of carriers all along the
channel, following Ohm’s law. As Vd further increases and approaches
Vg, pinch-off channel occurs and Id becomes independent of Vd. Thus,
any further increase in Vd does not contribute more toward the
increase in Id. The region is called the saturation operation.

1.3  Important Device Parameters

The performance of OFETs is characterized in terms of the important
parameters, including the field-effect mobility, threshold voltage,
ON/OFF current ratio, and subthreshold swing. Basically all the
parameters can be evaluated by the output and transfer plot, where

the former provides a plot of Id versus Vd at constant Vg and the latter
provides a plot of Id versus Vg at constant Vd. The transition from
linear to saturation region is observed in the output curves. The
operation of performance of OFETs is generally governed by two key
mechanisms: (1) charge transport within the organic semiconductor
layer and (2) charge injection and extraction at the contact. Four
critical parameters—field-effect mobility (μ), current ON/OFF ratio
(ION/IOFF), threshold voltage (Vth), and subthreshold swing (SS)—are
measured.

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Organic Field-Effect Transistors for Flexible Electronics Application

Vg

(b)

Vd

ld1/2

log(ld)

(a)

ld


6

Vth

Vg

Figure 1.2 (a) Typical transfer and (b) output characteristics of OFET device
operation.

1.3.1  Field-Effect Mobility
Mobility is defined as the charge-carrier drift velocity per unit
electrical field, which is commonly used as a figure of merit for
reporting OFET performance of various organic semiconductor
materials by the research community. The field-effect mobility
can be extracted from the slope of the linear fit in the saturation
region on the plot of Id1/2 versus Vg. Important factors that strongly
affect mobility is the crystallinity and grain size of the active
semiconducting layer, which depends on how the layer is deposited.
Significant improvements in the mobility of organic semiconductors
are obtained by designing novel high-performance materials and
optimizing the fabrication methods, which may fulfill the possibility
of organic electronic devices for real applications.

1.3.2  Current ON/OFF Ratio

In addition to charge-carrier mobility, the other important device
parameter for OFETs is the current ON/OFF ratio, which is defined
as the ratio of the maximum Id (accumulation current) in the ON
state of the transistor to the minimum Id (depletion current) in the

OFF state of the transistor. During the OFF state, negligible Id flows
when Vg is small and independent of the bias applied between
the source and drain contacts (Vd). The OFETs turn ON when Vg is

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Materials

applied, which induces charge carriers in the semiconductors at the
interface with dielectric layer. This ratio is strongly influenced by the
channel conductance, mobility, thickness, and dielectric constant of
the dielectric layer. A high ON/OFF ratio is a desirable quality since
it is essentially the signal-to-noise ratio for a transistor.

1.3.3  Threshold Voltage

The threshold voltage Vth is the minimum Vg required for
accumulating charge carriers at the semiconductor/dielectric
interface forming the active channel between the source and drain.
By plotting Id1/2 versus Vg in the saturation region and performing
the linear curve fitting, Vth can be extracted from the intercept of
linearly fitted curve. Vth shows strong dependence on the dielectric
constant of insulator, channel length, and the thicknesses of active
and dielectric layer. Typically, a decrease in the thickness and an
increase in the dielectric constant of the dielectric layer result in a
significant reduction in Vth due to the high gate capacitance.

1.3.4  Subthreshold Swing


The subthreshold swing (SS) is the ratio of the change in the gate
biasing to the change in Id in the logarithmic scale, which can be
expressed as
SS =



∂VG

∂ log ID

(1.3)

The subthreshold operation of OFETs is closely related to the
mobility enhancement for carrier hopping, which determines an
efficient usage of the transistor as a switch. The active thin film
equality achieved during the fabrication process may affect this value.
A discontinuous semiconducting layer leads to the accumulation of
defects and higher trap density, which results in less steeper slope
(high SS).

1.4 Materials

Tremendous efforts have been put toward the synthesis of new
organic materials for enhancing the performance of OFETs. Material

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