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- Sprache: Englisch
Interface Engineering in Organic Field-Effect Transistors
Systematic summary of advances in developing effective methodologies of interface engineering in organic field-effect transistors, from models to experimental techniques
Interface Engineering in Organic Field-Effect Transistors covers the state of the art in organic field-effect transistors and reviews charge transport at the interfaces, device design concepts, and device fabrication processes, and gives an outlook on the development of future optoelectronic devices.
This book starts with an overview of the commonly adopted methods to obtain various semiconductor/semiconductor interfaces and charge transport mechanisms at these heterogeneous interfaces. Then, it covers the modification at the semiconductor/electrode interfaces, through which to tune the work function of electrodes as well as reveal charge injection mechanisms at the interfaces.
Charge transport physics at the semiconductor/dielectric interface is discussed in detail. The book describes the remarkable effect of SAM modification on the semiconductor film morphology and thus the electrical performance. In particular, valuable analyses of charge trapping/detrapping engineering at the interface to realize new functions are summarized.
Finally, the sensing mechanisms that occur at the semiconductor/environment interfaces of OFETs and the unique detection methods capable of interfacing organic electronics with biology are discussed.
Specific sample topics covered in Interface Engineering in Organic Field-Effect Transistors include:
- Noncovalent modification methods, charge insertion layer at the electrode surface, dielectric surface passivation methods, and covalent modification methods
- Charge transport mechanism in bulk semiconductors, influence of additives on materials’ nucleation and morphology, solvent additives, and nucleation agents
- Nanoconfinement effect, enhancing the performance through semiconductor heterojunctions, planar bilayer heterostructure, ambipolar charge-transfer complex, and supramolecular arrangement of heterojunctions
- Dielectric effect in OFETs, dielectric modification to tune semiconductor morphology, surface energy control, microstructure design, solution shearing, eliminating interfacial traps, and SAM/SiO2 dielectrics
A timely resource providing the latest developments in the field and emphasizing new insights for building reliable organic electronic devices, Interface Engineering in Organic Field-Effect Transistors is essential for researchers, scientists, and other interface-related professionals in the fields of organic electronics, nanoelectronics, surface science, solar cells, and sensors.
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Seitenzahl: 492
Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Title Page
Copyright
Preface
Author Biographies
List of Acronyms and Abbreviations
1 Introduction
1.1 Different Interfaces in OFETs
1.2 Brief Historic Overview of Interface Engineering in OFETs
1.3 Scope of the Book
2 Interfacial Modification Methods
2.1 Noncovalent Modification Methods
2.2 Covalent Modification Methods
2.3 Efforts in Developing New Methods
3 Semiconductor/Semiconductor Interface
3.1 Influence of Additives on a Material's Nucleation and Morphology
3.2 Enhancing the Performance Through Semiconductor Heterojunctions
3.3 Integrating Molecular Functionalities into Electrical Circuits
4 Semiconductor/Electrode Interface
4.1 Work Function Tuning for Better Contact
4.2 Installing Switching Effects at Semiconductor/Electrode Interface
5 Semiconductor/Dielectric Interface
5.1 Dielectric Modification to Tune Semiconductor Morphology
5.2 Eliminating Interfacial Traps
5.3 Integrating New Functionalities
6 Semiconductor/Environment Interface
6.1 Device Optimization to Improve Sensing Performance
6.2 OECT‐Based and EGOFET‐Based Sensors
7 Interfacing Organic Electronics with Biology
7.1 Integration of OFETs/OECTs with Nonelectrogenic Cells
7.2 Integration of Flexible Bioelectronics with Electrogenic Cells
7.3 Light/Cell/Device Interfaces
8 Concluding Remarks and Outlook
8.1 New Challenges in Molecular Design
8.2 High‐Quality OSC Films: Self‐Assembly Control
8.3 High‐Performance Scalable Flexible Optoelectronics
8.4 Exploration of Novel Structures: Organic/2D Heterostructures and Vertical Structures
8.5 Instability: Stability in Aqueous Media and Thermal Stability in Hygienic Applications
8.6 Multifunctional Sensor Systems
References
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 The device performance of the devices with SAM‐modified SiO
2
diele...
Table 2.2 The device performance of the devices with SAM‐modified high‐
k
die...
Table 2.3 Summary of the device performance of SAMFETs.
Chapter 3
Table 3.1 Common nucleating agents.
Table 3.2 Summaries of the performance by using OSC/Insulating polymer blend...
Chapter 5
Table 5.1 Summaries of OFET‐based memory cells by using various charge‐trapp...
Chapter 6
Table 6.1 Summary of OFET‐based sensors for selective sensing.
List of Illustrations
Chapter 1
Figure 1.1 Schematic illustration of a typical OFET architecture.
Figure 1.2 Interface engineering of interfaces in OFETs. (a) Interface withi...
Figure 1.3 Timeline showing key developments in interface‐engineered functio...
Chapter 2
Figure 2.1 Chemical structures of the charge insertion layer (CIL). (a) Meta...
Figure 2.2 Modification of polymer dielectrics. (a) Schematic representation...
Figure 2.3 Chemical structures of SAMs used to modify metal electrodes.
Figure 2.4 Molecular structures of SAMs used to modify the SiO
2
surface. Sur...
Figure 2.5 Chemical structures of SAMs used to modify high‐
k
inorganic diele...
Figure 2.6 Organic semiconducting molecular design for SAMFEFs. (a) Schemati...
Figure 2.7 Molecular structures of OSCs for SAMFETs.
Chapter 3
Figure 3.1 Interfaces within the semiconductor layers. (a) On the macroscale...
Figure 3.2 Molecular packing arrangements of TIPS–pentacene prepared under d...
Figure 3.3 Microstructures of conjugated polymer films. (a) A semicrystallin...
Figure 3.4 High‐boiling‐point solvent additives and nonsolvent additives. (a...
Figure 3.5 Additive‐assisted crystallization. (a–d) Schematic illustrations ...
Figure 3.6 Template‐mediated crystallization. (a) Schematic representation s...
Figure 3.7 Phase separation of multicomponent systems. (a) Benefits of phase...
Figure 3.8 Molecular structures of (a) polymer/polymer blends and (b) small ...
Figure 3.9 Three representative methods to blend small‐molecule OSC material...
Figure 3.10 Blending with insulating polymer elastomer. (a) Schematic illust...
Figure 3.11 Nanoconfinement effect enhances the stretchability of a polymer ...
Figure 3.12 Semiconductor heterojunctions. (a) Schematic energy‐level diagra...
Figure 3.13 Planar bilayer structures. (a) Chemical structures of bilayer he...
Figure 3.14 Long‐range ordered single‐crystal film. (a) Schematic illustrati...
Figure 3.15 Molecular‐level heterojunctions. (a) Schematic illustration show...
Figure 3.16 Supramolecular arrangements of different heterojunctions. (a) Sc...
Figure 3.17 Supramolecular arrangements of different heterojunctions. (a) Cr...
Figure 3.18 Charge‐trapping‐induced memory effect. (a) Charge‐transfer dopin...
Figure 3.19 Interfacial energy diagram of photochromic blends. (a) Left: The...
Figure 3.20 Photochromism‐induced switching and memory effect. (a) Schematic...
Chapter 4
Figure 4.1 Schematic representation of electron injection from a metal elect...
Figure 4.2 Modified work function by the SAM methods. Materials are classifi...
Figure 4.3 SAM modification of metal electrodes in OFETs. (a) Schematic devi...
Figure 4.4 CIL modification mechanism of metal electrodes in OFETs. (a) Sche...
Figure 4.5 Chemical structures of polymer‐based electrodes.
Figure 4.6 CNT as electrodes for OFETs. (a) Schematic drawing of a cut SWCNT...
Figure 4.7 Graphene‐based carbon electrodes for OFETs. (a) Schematic and (b)...
Figure 4.8 Carbon‐based molecular junctions. (a) Schematic of using SWCNTs a...
Figure 4.9 Three switching mechanisms. (a, b) Switching behaviors induced by...
Chapter 5
Figure 5.1 Charge‐transport physics at the semiconductor/dielectric interfac...
Figure 5.2 Energy gap and static dielectric constant of representative diele...
Figure 5.3 Semiconductor morphology control by SAMs at the semiconductor/die...
Figure 5.4 Molecular structure effect of SAMs in surface energy. (a) Chemica...
Figure 5.5 Semiconductor morphology control by SAMs at the semiconductor/die...
Figure 5.6 Roughness effects. (a) Schematic illustration showing the differe...
Figure 5.7 Effect of the nanostructured dielectric surfaces on the device pe...
Figure 5.8 Self‐structured dielectrics. (a) Schematic illustration showing t...
Figure 5.9 Polymer encapsulation of dielectrics dielectrics. (a) Schematic d...
Figure 5.10 OFETS with air‐gap dielectrics. (a) Device structure and (b) ele...
Figure 5.11 Effects of the dielectric interface on the device performance. (...
Figure 5.12 SAM/high‐
k
hybrid dielectrics. (a) Schematic of SAM/metal–oxide ...
Figure 5.13 SAM/high‐
k
hybrid dielectrics for flexible electronics. (a) Comp...
Figure 5.14 Organic semiconducting molecular design for SAMFEFs. (a) Schemat...
Figure 5.15 Morphology control of SAMFETs. (a–d) Effect of mixed SAMs on mor...
Figure 5.16 Schematic description of current trends in developing different ...
Figure 5.17 Photoresponsive semiconductor/dielectric interfaces. (a) Schemat...
Figure 5.18 Other functional dielectrics. (a) Layout of pressure‐sensing org...
Figure 5.19 Conventional floating gates. (a) Schematic of a floating gate st...
Figure 5.20 Charge storage layers. (a) Schematic architecture of an OFET str...
Chapter 6
Figure 6.1 Device engineering. (a) Gas dielectric FETs based on CuPc nanowir...
Figure 6.2 Bilayer heterojunction sensor. (a) p–n junctions sensor based on ...
Figure 6.3 Remote floating gate device for a DNA sensor with two
organic cha
...
Chapter 7
Figure 7.1 Electrochemical control of cell culture on substrates. (a) Neutra...
Figure 7.2 Cell sensors. (a) Schematic illustration of a cell‐based OECT bio...
Figure 7.3 Organic bioelectronics. (a) Schematic illustration of bioelectron...
Figure 7.4 OECT‐based electrophysiological measurements. (a) Cartoon showing...
Figure 7.5 Device/brain interfaces. (a) Left: picture of the probe conformin...
Figure 7.6 Photoresponsive cells/device interfaces. (a) Schematic showing th...
Figure 7.7 Integrated OFET‐OECTs. (a) Structure of the OFET‐OECT integrated ...
Figure 7.8 Skin‐inspired electronics. (a) Schematic of a skin‐inspired organ...
Chapter 8
Figure 8.1 Multifunctional sensor systems and future applications. (a) Devel...
Guide
Cover Page
Table of Contents
Title Page
Copyright
Preface
Author Biographies
List of Acronyms and Abbreviations
Begin Reading
References
Index
Wiley End User License Agreement
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Interface Engineering in Organic Field‐Effect Transistors
Xuefeng GuoHongliang Chen
Authors
Prof. Xuefeng Guo
Peking University
202 Chengfu Rd
Haidian District
100871 Beijing
China
Prof. Hongliang Chen
Zhejiang University
No. 866 Yu Hang Tang Rd
Xihu District
310027 Hangzhou
China
Cover Image: © blackdovfx/Getty Images
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© 2023 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany
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Print ISBN: 978‐3‐527‐35145‐9
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Preface
The development of microelectronics into modern integrated circuits over the past 75 years was the result of continuous efforts of huge teams of scientists and engineers. The device performance of microelectronic devices is mainly determined by the properties of the materials used and their corresponding interfaces, just as Nobel laureate Herbert Kroemer coined, “the interface is the device.” Understanding and governing the charge transfer phenomena occurring at the interfaces could revolutionize a number of related fields, including batteries, catalysis, data storage, energy harvesting, imaging displays, sensing devices, and even solar cells. For these reasons, interface engineering of these heterojunctions has captured the attention of a broad cross section of scientific and industrial communities.
In comparison with traditional complementary metal oxide semiconductor transistors, the advantages of organic field‐effect transistors (OFETs) originate from how OFETs can be manufactured, including the considerable variety in molecular design, low cost, lightweight, mechanical flexibility, solution processability, and large‐area fabrication. The key concept of this book is to use interface engineering strategies to improve the inherent properties of OFETs as well as to build novel functional devices based on OFETs. Both are of great importance for laying the foundation for the OFET research. For the first point, rapid progress has been made on each isolated component of OFETs to improve the performance, with major emphasis on organic semiconducting material design and relevant film processing methods. Although the improved electronic properties of these materials could offer device performance advantages, interfacial barriers between the semiconductors, dielectric layers, and electrodes are often treated as secondary concerns in comparison with the generation and preservation of carrier population in the semiconductor itself. In pursuit of high performance, OFETs will inevitably require an optimized carrier transport pathway that leverages all the device layers and their corresponding interfaces. This should be as important as the material design and relevant film processing methods.
The second point is that how to “use” OFETs is another important issue in OFET research. Of course, huge progress has been made to develop a variety of functional devices based on OFETs, such as chemo/biosensors, photodetectors, integrated circuits, and flexible electronics. They can be realized by using various strategies, for example, designing functional semiconducting materials, developing semiconductor processing methods, and tailoring functional interlayers. In this book, however, we will specifically survey the most recent and important works employing interface engineering strategies to develop functional OFET devices. We believe that interfacial modification is a much simpler, more efficient way to integrate functionalities.
This book will focus on interface engineering of OFETs and aim to provide fundamental understandings of the interplay between the molecular structure, assembly, and emergent functions at the molecular level and consequently offer novel insights into designing a new generation of multifunctional integrated circuits and sensors toward practical applications. Actually, the interface engineering strategy can be leveraged to improve the efficiency and stability of perovskite solar cells. We believe that this book will draw widespread interest from researchers working on organic electronics, chemo‐ and biosensors, solar cells, flexible electronics, and other device‐related fields.
We would like to thank Wiley for offering the opportunity to prepare this book, especially Ms. Alice Qian. We would also like to express our deep gratitude to Ms. Katrina Maceda and Ms. Monica Chandra Sekar who have contributed their time and outstanding expertise to make this book possible. Also, thanks to Dr. Weining Zhang for the helpful discussions during the preparation of the book. Finally, we hope that the readers of this book will find it both useful and delightful.
Beijing, ChinaDecember 2022
Xuefeng Guo and Hongliang Chen
Author Biographies
Xuefeng Guo received his PhD in 2004 from the Institute of Chemistry, Chinese Academy of Sciences, Beijing. From 2004 to 2007, he was a postdoctoral research scientist at the Columbia University Nanocenter. He joined the faculty as a professor under the “Peking 100‐Talent” Program at Peking University in 2008. In 2012, he won the National Science Funds for Distinguished Young Scholars of China. His current research is focused on functional nanometer/molecular devices. Professor Xuefeng Guo has authored over 210 scientific publications and has received numerous scientific awards, including the First prize of Ministry of Education Natural Science Award.
Hongliang Chen received his PhD in 2016 from the College of Chemistry and Molecular Engineering, Peking University, under the guidance of Professor Xuefeng Guo. From 2016 to 2018, he worked as a research scientist at the Core R&D department in Dow Chemical Company. Then he moved to Northwestern University in United States and worked as a postdoctoral research fellow in Professor Sir Fraser Stoddart's group from 2018 to 2021. He joined Zhejiang University as an assistant professor under “ZJU 100‐Talent” Program in June 2021. His research interests are focused on organic functional devices and molecular electronics.
List of Acronyms and Abbreviations
1D
one‐dimensional
2D
two‐dimensional
3D
three‐dimensional
AFM
atomic force microscopy
AZOs
azobenzenes
BCB
divinyltetramethyldisiloxane‐bis(benzocyclobutene)
BTBT
[1]benzothieno[3,2‐
b
][1]‐benzothiophene
C
8
‐BTBT
2,7‐dioctyl[1]‐benzothieno[3,2‐
b
][1]benzothiophene
CIL
charge insertion layer
CNTs
carbon nanotubes
CT
charge‐transfer
D–A
donor–acceptor
DAE
diarylethene
DNTT
dinaphtho[2,3‐
b
:2′,3′‐
f
] thieno[3,2‐
b
]thiophene
DOS
densities of state
E
A
energy gap of OSC
E
F
Fermi level of electrodes
E
VAC
vacuum energy level
FLG
few‐layer graphene
GOs
graphene oxides
Gr
graphene
HMDS
hexamethyldisilazane
HOMO
highest occupied molecular orbital
k
relative permittivity or dielectric constant
LB
Langmuir–Blodgett
LUMO
lowest unoccupied molecular orbitals
MWCNTs
multiwalled carbon nanotubes
NDI
naphthalenediimide
NHCs
N‐heterocyclic carbenes
OFET
organic field‐effect transistor
OLET
organic light‐emitting transistor
OSCs
organic semiconductors
P3HT
poly(3‐hexylthiophene)
PA
Phosphonic acid
PCBM
[6,6]‐phenyl‐C
61
‐butyric acid methyl ester
PCMs
photochromic molecules
PDI
perylenediimide
PDMS
polydimethylsiloxane
PEN
polyethylene naphthalate
PHBD
photo hybrid bilayer dielectric
PLA
polylactide
PMMA
poly(methyl methacrylate)
POM
polyoxometalate
PS
polystyrene
PSA
poly(stearyl acrylate)
PSS
poly(sodium‐
p
‐styrenesulfonate)
PVA
poly(vinyl alcohol)
P(VDF–TrFE)
poly(vinylidene fluoride‐trifluoroethylene)
PVP
poly(vinyl phenyl)
PVPyr
poly(4‐vinylpyridine)
PαMS
poly(α‐methylstyrene)
RGO
reduced graphene oxide
SAM
self‐assembled monolayer
SAMFETs
self‐assembled monolayer field‐effect transistors
SEBS
poly(styrene‐
b
‐(ethylene‐
co
‐butylene)‐
b
‐styrene)
SEM
scanning electron microscopy
SLG
single‐layer graphene
SPs
spiropyrans
SWCNTs
single‐walled carbon nanotubes
TCNB
1,2,4,5‐tetraacyanobenzene
TCNQ
tetracyanoquinodimethane
TEM
transmission electron microscopy
TIPS–pentacene
6,13‐bis(triisopropylsilylethynyl) pentacene
TTF
tetrathiafulvalene
V
D
source–drain voltage
V
G
gate voltage
V
th
threshold voltage
WEG
weak epitaxy growth
W
F
work function
γ
tot
total surface energy
ε
0
vacuum permittivity
μ
DT
aligned dipole moment of the SAM
μ
M–S
M—S bond dipole interface
ϕ
m
electrode work function
1Introduction
1.1 Different Interfaces in OFETs
When Nobel laureate Herbert Kroemer coined the famous phrase that “the interface is the device” [1], he pointed out the essential importance of the interfaces between different materials in any device action, especially for semiconducting thin‐film devices, which have shown astonishing successes in photonic and electronic applications. From the point of view of a typical organic field‐effect transistor (OFET), the device performance is mainly determined by the properties of the materials used and their corresponding interfaces. In general, an OFET [2–5] consists of three electrodes (source, drain, and gate), a dielectric layer, and an organic semiconductor (OSC) layer (Figure 1.1). To improve the device performance and even build novel functionalities, interface engineering of these heterojunctions is currently a research focus because of its promising potential for further applications in broad areas ranging from integrated circuits and energy conversion to catalysis and chemical/biological sensors [6–14]. In fact, scientists in a variety of disciplines have been devoting great efforts to this concept, which started with simple improvement of the device performance and has branched out in different directions, indicating the interdisciplinary of these efforts [15–18]. In comparison with traditional complementary metal oxide semiconductor transistors, the advantages of OFETs originate from how they can be manufactured, including considerable variety in molecular design, low cost, light weight, mechanical flexibility, solution processability, and large‐area fabrication.
Figure 1.1 Schematic illustration of a typical OFET architecture.
There are four major contact interfaces in OFETs, namely, the interface within the semiconductor layers, semiconductor/electrode interface, semiconductor/dielectric interface, and semiconductor/environment interface (Figure 1.2). In general, the interface within the semiconductor layers [19, 20] contains traps, grain boundaries, and other defects, which provide scattering sites for carriers and are detrimental to charge transport (Figure 1.2a). Recent studies [21, 22], however, proved that semiconductor doping and blends had a positive influence on the crystallinity and morphology of the semiconductor films. Several strategies have been developed to achieve high‐quality thin films, as well as to form good heterojunctions, thus affording ambipolar OFETs that are useful for building future OFET‐based integrated devices, accelerating charge separation, and improving the device performance. By designing mixed or segregated crystal heterojunctions made of OSCs, functional devices such as photodetectors, memory devices, and ambipolar transistors can be realized.
Figure 1.2 Interface engineering of interfaces in OFETs. (a) Interface within the semiconductor layers, (b) semiconductor/electrode interface, (c) semiconductor/dielectric interface, and (d) semiconductor/environment interface. SAM: self‐assembled monolayer; OSC: organic semiconductor; CIL: charge insertion layer.
Another important interface in OFETs is the semiconductor/electrode interface (Figure 1.2b) [9,23–25], which usually determines the efficiency of charge‐carrier injection and extraction. By carefully choosing the self‐assembled monolayer (SAM) or charge insertion layer (CIL) to modify metal electrodes, the charge‐injection barrier can be fine‐tuned at the interface, which could significantly reduce the contact resistance between semiconductors and electrodes, thus enhancing the device performance. More interestingly, by using functional layers, whose molecular conformation or electronic structure can be switched by external stimuli, the properties of OFETs can respond to external signals. This concept sets the foundation for building functional devices such as optical/electrical switches, memories, and photodetectors.
The semiconductor/dielectric interface is a vital interface [26, 27] that dominates carrier transport (Figure 1.2c) because (i) charge carriers are generated at the interface and (ii) the electrical current path is through at most the first few layers of molecules at the interface. In addition, the surface energy and microstructure of the dielectric interface have a significant impact on the local morphology of the semiconductor layer. Therefore, modification of this interface by SAMs or stimuli‐responsive layers offers an important and universal methodology to improve the device performance and to even integrate new molecular functionalities such as photo‐controllable memories, superconductors, and charge‐trap memories into organic electrical circuits.
The semiconductor/environment interface is useful for building functional OFETs for sensing applications [12, 13, 15, 17,28–30] (Figure 1.2d) by directly exposing the semiconductor layer to the environment being analyzed. This method offers some valuable advantages over other technologies, such as label‐free capability, real‐time detection, and compatibility with both the biological system and the electronics industry. Some intrinsic drawbacks, however, hamper the use of OFET‐based sensors for practical sensing applications. The first is the diffusion of the analyte into grain boundaries in the semiconductors, causing either trapping or doping of charge carriers. This could result in serious problems such as device instability and irreversibility because of the physically destructive interaction between semiconductors and analytes. Second, because of the signal amplification characteristics of OFETs, any nonspecific response or noise would cause interference with the intrinsic signal of the analytes, leading to a pseudo‐signal and poor selectivity. Therefore, rational control of interfacial charge transport is crucial to control the doping effects, increase the sensing selectivity/sensitivity, and even install new functionalities.
1.2 Brief Historic Overview of Interface Engineering in OFETs
We have summarized in Figure 1.3 the key developments to give a comprehensive big picture of the interface‐engineered functional OFETs. Starting from the first demonstration [31, 32] of OFETs in the 1980s to their very recent applications [21, 29, 30] in skin electronics, the performance and functionality of OFETs have evolved simultaneously and promoted each other, shedding light on their potential applications in healthcare, wearable electronics, and degradable electronics.
Figure 1.3 Timeline showing key developments in interface‐engineered functional OFETs [10, 21, 25, 28, 29, 31, 32, 37, 57, 71, 74, 77, 81, 114, 137, 143–145, 160, 168, 172, 180, 187, 205, 237–240, 298, 312, 315, 326, 331, 333, 348, 353, 359–361, 376, 393, 397, 417, 447, 459].
1.3 Scope of the Book
In this comprehensive book, we aim to provide a systematic survey of recent significant advances in developing effective methodologies of interface engineering in OFETs, from fundamental models to experimental techniques, for creating high‐performance and multifunctional optoelectronic devices. In Chapter 2, we will summarize interfacial modification methods in OFETs, which include physical and chemical modification methods. We will not go into details of them. Instead, summative tables and figures will be present to compare each method. Then, we focus on how to use interface engineering strategies to improve the inherent properties of OFETs at the semiconductor/semiconductor interface (Chapter 3), semiconductor/electrode interface (Chapter 4), semiconductor/dielectric interface (Chapter 5), semiconductor/environment interface (Chapter 6), as well as interfacing organic electronics with biology (Chapter 7). Both are of great importance to lie the foundation of the OFET research. We also provide a critical discussion of the limitations and main challenges that still exist for the development of practical applications in Chapter 8. Such a book will be invaluable for better understanding fundamental charge‐transport mechanisms at the interfaces and will offer novel insights into the device design concept, the device fabrication process, and the development of future multifunctional practical transistors.
2Interfacial Modification Methods
Over the past two decades, extensive efforts [6, 8, 9, 16] have been devoted to the modification of interfaces in organic field‐effect transistors (OFETs), including physical modification methods, such as encapsulation or insertion on the surface, and chemical modification methods, such as covalently bonded self‐assembled monolayer (SAM) modification.
2.1 Noncovalent Modification Methods
2.1.1 Charge Insertion Layer at the Electrode Surface
Because the injection barrier originates generally from the gap between the Fermi level of metal electrodes and the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels of organic semiconductors (OSCs) according to the energetic alignment, a reasonable solution is to insert a suitable additional layer to modify the electrode and function as a springboard for charge carriers, thus possibly overcoming the poor injection [70]. Initially, this method of introducing a thin buffer layer of several nanometers at metal OSC/electrode interfaces, referred to as a charge insertion layer (CIL) [71, 72], had already been applied to increase the efficiency of organic light‐emitting diodes by improving the contact.
A preliminary study [73] was carried out by Hajlaoui et al., who inserted a thin film of tetracyanoquinodimethane (TCNQ) (1), working as molecular dopant, between Au and OSC active layers in OFETs to achieve an increased hole mobility. By using an ultrathin LiF interlayer as CIL between Al electrodes and F16CuPc active layers [33], enhanced n‐channel performance was achieved. Since then, CIL modification has attracted more and more attention in terms of enhancing the hole/electron injection efficiency. Various materials, such as inorganic compounds (metal oxides and inorganic salts) and organic compounds (small molecules and polymers), have been used as CILs, as shown in Figure 2.1.
Figure 2.1 Chemical structures of the charge insertion layer (CIL). (a) Metal oxides, (b) metal salts, (c) small molecules, (d) polymers, and (e) other materials.
Metal oxides [74] (Figure 2.1a) provide a variety of candidates for CILs because of their wide range of work functions. Transition metal oxides in particular [75, 76], whose work functions range from very low (ϕ ≈ 3 eV for ZrO2) to extremely high (ϕ ≈ 7 eV for V2O5), are suitable for modifying different metals with matched energy levels. After introducing the CIL, the thickness of the depletion layer between the metal and metal oxide decreases to a few nanometers, producing a reachable distance for tunneling injection, and the contact resistance could be significantly reduced once the tunneling injection is dominant. The CIL layer can also act as a protection layer against the diffusion [77] of metals from electrodes or the permeation [78] of O2 or H2O into the active layer, thus improving the device stability.
Metal salts are another big family of inorganic materials working as CILs (Figure 2.1b) [79, 80]. They experience a similar mechanism as metal oxides. One of the benefits of using metal salts as CILs is their compatibility with the solution‐process method. For example, CuSCN [81, 82] can be spin‐casted on Au electrodes as a novel CIL with an intrinsic electron‐blocking property. However, there are some drawbacks of metal salts when applying them as CILs. First, the thickness of the CIL affects electron transport. A thicker layer of CsF [83] (more than 0.6 nm) decreased the mobility owing to its insulating nature. Second, when depositing a CIL into ambipolar OFETs by using a single component, the outcome performance of p‐ and n‐channels cannot be enhanced simultaneously. This problem could be solved by mixing two different materials to form a hybrid CIL [84]. The injection barrier for different types of carriers was tuned finely according to the molecular ratio of the two compounds.
When it comes to organic small molecules as CILs (Figure 2.1c), i.e. charge‐transfer (CT) complex (2) [85], metal complex (3–5) [86, 87], metal tetraphenylporphyrin and phthalocyanine (6–10), [88] and N‐heterocyclic compounds (11–15) [89], direct CT occurs at the interface between the organic CIL and the semiconductor, which helps to reduce the contact resistance by the additional interface dipole. However, a problem that still exists is the possibility of matrix–dopant hybridization [90, 91]. The organic polymers with dipolar groups [23, 92, 93] (16 and 17) (Figure 2.1d) can also act as electrode surface modifiers to achieve electrodes with a low work function. Polyelectrolytes (18–22), which comprise the groups capable of being charged with cations or anions, were proven recently to be a promising CIL to achieve ohmic contacts with a wide range of work functions (3.0–5.8 eV) [23].
Other materials (Figure 2.1e), such as DNA [94] and graphene oxides (GOs) [95–97], have also been used as CILs. The presence of phosphate groups in the DNA leads to a dipole layer, which could change the orientation in response to applied gate voltages and enhance the charge injection of both types of carriers. When GOs were applied as CILs in OFETs, the improvement in device performance could be attributed to the good contact nature of carbon‐based materials with semiconductors, which will be discussed in Section 3.3.2.
2.1.2 Dielectric Surface Passivation Methods
Polymer encapsulation is a main choice for modifying the inorganic dielectric surface. Most of the polymer modifier, for example poly(imide‐siloxane) [98], poly(4‐vinylpyridine) (PVPyr) [99], and polystyrene (PS) [100], are used to change the surface energy of the dielectric and thus influence the grain size of OSCs growing whereon. There is still a debate on how the surface energy of polymer modifiers influence performance of OSC thin films; but as a general rule, the closer the surface energy of the dielectric layer is to that of the OSC, the higher the carrier mobility [101]. More fundamentally, it is a competition between the aggregation force within the OSC and the adsorption of OSC on the substrate.
The polymer modifiers, however, suffer from high roughness and trap density, which hamper charge transport on the surface. The same problem exists when directly using insulating polymers as gate dielectrics [102–105]. These applications call for the efficient surface modification methods of polymer films. However, the lack of functional groups makes it very hard to modify polymer dielectrics. Several strategies have been developed recently to solve this problem:
(i) By directly depositing a bilayer dielectric consisted of a bulk dielectric layer and an ultrathin interface layer (
Figure 2.2
a,b);
(ii) By using blends of two components to induce a phase‐separated bilayer structure (
Figure 2.2
c,d);
(iii) By utilizing a monolayer capable of self‐assembly on the polymer surface (
Figure 2.2
e–g).
Figure 2.2 Modification of polymer dielectrics. (a) Schematic representation showing the bilayer dielectric made from an initiated chemical vapor deposition process. (b) Molecular structures of the interfacial layer and the bulk dielectric layer. (c) A schematic structure of the vertical phase separation of the blended dielectric. (d) Schematic representation showing the effect of the polaronic disorder and carrier concentration in OFETs with three dielectrics. (e) A few‐layer triptycene film featuring 2D nested hexagonal packing and one‐dimensional (1D) layer stacking on a parylene dielectric. (f) Molecular structure of the triptycene molecule. (g) Atomic force microscopy (AFM) height images of evaporated OSC films on parylene (top) and triptycene‐coated parylene (bottom) gate dielectrics.
Source: (a, b) Adapted from Pak et al. [34]. (c, d) Adapted from Khim et al. [106]. (e–g) Reproduced from Yokota et al. [107], © 2018 Springer Nature.
For the first strategy, a bilayer polymer dielectric (Figure 2.2a) was employed [34], which consisted of a bulk layer of electropositive copolymer (26) for inducing electrostatic polarization along the channel and an ultrathin interface layer of 23 on top to form a good interface with OSCs. The polymer dielectric was prepared via a one‐step, low‐temperature, solvent‐free chemical vapor deposition process with different ratios of two monomers 24 and 25 (Figure 2.2b) [35].
The second strategy can be realized by blending a fluorinated high‐k polymer (27) with poly(methyl methacrylate) (PMMA) (28) as a dielectric [106]. Generally, the pure high‐k dielectric layer of 27 can enhance the hole mobility but suppress the electron mobility because of the interfacial C–F dipole. In the blends, however, both p‐ and n‐channel behaviors were increased simultaneously in ambipolar polymers because of the existence of vertical phase separation that induces a PMMA‐rich region adjacent to the semiconductor layer (Figure 2.2c), leading to reduced interfacial dipolar disorder (Figure 2.2d). These discoveries provided new strategies for designing dielectric materials by choosing multiple components with proper chemical and morphological properties.
Last but not least, the technique of using SAMs via covalent bonding in inorganic dielectrics could not work well on polymer substrates, because of the lack of particular anchoring sites on the polymer surface whose chains are randomly oriented. To overcome this problem, Yokota et al. [108] applied a particular paraffinic triptycene (29) (Figure 2.2f), which can form completely oriented thin films with a high structural order featuring 2D nested hexagonal packing and 1D layer stacking for the surface functionalization of a polymer substrate. The triptycene formed noncovalently on polymer substrates, where the paraffinic side chains were arranged vertically to the substrate, thus equalizing the surface properties of these substrates (Figure 2.2e) [107]. With a few‐layer triptycene film, the surface energies of different gate dielectrics were almost identical (∼22.2 mJ m−2), thus ensuring the high crystallinity and enhanced structural integrity of OSCs (Figure 2.2g). This is a more general strategy by depositing a few‐layer film of triptycene on polymer substrates, shedding new light on interface engineering of the OSC/polymer dielectric interface.
2.2 Covalent Modification Methods
2.2.1 SAM Modification of Electrodes
The metal electrode surface, which is capable of absorbing adventitious organic materials, offers a good platform for studying SAMs formed by chemisorption. SAM modification is not only a universal method to modify the surface properties [109,36,110–112], but also a method for providing more functionalities [37, 113]. Herein, we focus on SAM modification methods applied to the metal electrodes in OFETs (Figure 2.3).
Figure 2.3 Chemical structures of SAMs used to modify metal electrodes.
Commonly used SAM molecules are thiol derivatives (30–53), which have a sulfhydryl group for docking to the surfaces of metals such as Au. This forms metal–sulfur interactions through the sulfur long‐pair electrons, called the thiolate—gold bond (Au—S), whose strength is close to that of the Au—Au bond [114, 38]. Organic compounds with disulfide (S–S) (54–56) or sulfide acetyl (–SAc) (57–59) bonds can also form SAMs on Au or Ag electrode surfaces [115].
Thioketone (60), which is structurally similar to pentacene, was used as a SAM to modify Au electrodes prior to the deposition of pentacene [116], which induced a complementary surface for the growth of pentacene and reduced the contact resistance. In addition, dithiocarbamate (DTC)‐based molecules (61–69) have been proven to be stably adsorbed on metals under various types of environmental stresses [117], and could reduce the work function of noble metals (Au and Ag) to an ultralow value (such as ∼3.1 eV for Au) [118]. Recently, N‐heterocyclic carbenes (NHCs) (70) emerged as a versatile surface modification of gold surfaces [119, 120], exhibiting higher stability under harsh conditions in contrast to the most commonly used linear organosulfur ligands.
Despite the progress, great challenges remain in the SAM modification of metal electrodes. The relatively poor mechanical performance and scratch resistance hamper their practical application and commercialization. Carbon nanomaterials, i.e. carbon nanotubes (CNTs) and graphene (Gr), have emerged as potential electrodes with high conductivity, stability, and producibility. However, their chemical inertness makes them hard to functionalize without breaking the well‐defined structures and scarifying their performances. Moreover, the mass production, purification, and integration of carbon nanomaterials as electrodes is a huge systematic project. There is still a long way to go.
2.2.2 SAM Modification of Dielectrics
The SAM modification strategy is worth being highlighted at the OSC/dielectric interface. Based on different dielectric materials, we summarize three aspects to discuss the topic in detail: (i) SAM/SiO2 dielectrics, (ii) SAM/high‐k dielectrics, and (iii) self‐assembled monolayer field‐effect transistors (SAMFETs). In terms of different types of molecules that have been used [121], two categories of the most‐commonly‐used SAM molecules are discussed:
(i) Silane‐based SAM molecules
[122]
, which prefer to react with the hydroxyl group of –SiOH on the silicon‐based dielectrics (SiO
2
and Si
3
N
4
) to form SAMs via covalent linkage of Si—O—Si bonds.
(ii) The
phosphonic acid
(
PA
) and its phosphonate ester derivative‐based SAM molecules
[111]
, which can form PA–SAMs on a large range of inorganic oxide dielectrics, including high‐
k
materials, by multidentate binding of strong covalent P–O–M anchoring (M stands for metal atom). Under normal ambient conditions, phosphonic acid molecules do not react chemically on the surface of SiO
2
[123]
.
2.2.2.1 SAM/SiO2 Dielectrics
Surface modification with organosilanes (71–102) (Figure 2.4) has been a common method for a long time to prepare SAMs on a wide range of surfaces bearing –OH groups [124], such as silica, aluminum oxide, and zinc oxides. The modification can be done by reactions in solution or in the vapor phase. The challenge is that uniform silane monolayers are still difficult to obtain [121], thus limiting the device performance to some extent.
Figure 2.4 Molecular structures of SAMs used to modify the SiO2 surface. Surface modification with organosilanes (71–102) is the most common method. Phosphonic acid (103–106) can also be used in some cases.
The processing conditions have a great impact on the quality of the resulting SAM. The SAM coverage was found to be very sensitive to the pretreatment conditions. In general, vapor‐deposited SAMs are likely to result in high but not full coverage, different from the solution‐processing procedure. Theoretically, full surface passivation and zero‐onset voltage are only possible at near 100% SAM coverage, and if the coverage is incomplete or uncontrolled, high variability in the onset voltage occurs. A partially covered gate‐dielectric SAM would cause two impacts on the transport of an OSC [125]. The first impact is the surface‐induced electronic disorder in the adjacent semiconductor, which broadened the distribution of charge‐transport states. The second impact is the rigid shift of the transport levels of the semiconductor owing to electrostatic interactions between the SAM and the semiconductor, inducing either hole or electron accumulation accordingly. To get a comprehensive understanding of the relationship between SAM materials, preparation methods, and the field‐effect transistor (FET) performances, we have listed below recent works of SAM‐modified dielectric surfaces, which are shown in Table 2.1.
Table 2.1 The device performance of the devices with SAM‐modified SiO2 dielectrics.
Type
OSC material
OSC film deposition
Silane SAM
SAM preparation
Electrode
Average mobility (highest)
On/off ratio
References
p
Pentacene
OMBD
a)
ODTS
Dipping
TC Au
1.2
10
6
[126]
p
Pentacene
Single crystals
ODTES
PDMS stamp printing
BC Au
∼0.3 (0.36)
10
5
[127]
p
Rubrene
Single crystals
ODTES
PDMS stamp printing
BC Au
0.6 ± 0.5 (2.4)
10
7
[127]
n
C60
Single crystals
ODTES
PDMS stamp printing
BC Au
0.03
10
2
[127]
n
TCNQ
Single crystals
ODTES
PDMS stamp printing
BC Au
10
−4
10
1
[127]
p
MEH‐PPV
Spin‐coating
ODTS
Vapor
TC Au
1.2 × 10
−4
10
2
[128]
p
MEH‐PPV,
Spin‐coating
HMDS
Vapor
TC Au
1.3 × 10
−3
10
2
[128]
p
OC1C10‐PPV
Spin‐coating
ODTS
Vapor
TC Au
1.6 × 10
−4
10
2
[128]
p
OC1C10‐PPV
Spin‐coating
HMDS
Vapor
TC Au
1.3 × 10
−3
10
2
[128]
p
Pentacene
OMBD
ordered ODTS
Dipping
TC Au
∼0.6
10
6
[129]
p
Pentacene
OMBD
disordered ODTS
Dipping
TC Au
∼0.3
10
6
[129]
p
Pentacene
Vacuum sublimation
HDMS
Vapor
BC Au
0.1–0.6
10
5
[130]
p
Pentacene
Vacuum
β‐PhTS
Immersion
TC Cu
0.4
—
[131]
p
8QT8
Drop‐casting
β‐PhTS
Coating
TC Au
0.014
10
6
[132]
p
8QT8
Drop‐casting
β‐PhTS
Coating
TC Au
0.17
—
[132]
p
pBTCT
Spin‐coating
ODTS
Immersion
Au
0.021
10
7
[133]
p
pBTCT
Spin‐coating
OTS
Immersion
Au
0.018
10
6
[133]
p
pBTCT
Spin‐coating
HMDS
Refluxing
Au
0.005
10
5
[133]
p
pBTCT
Spin‐coating
PTS
Immersion
Au
1.6 × 10
−3
10
5
[133]
p
pBTCT
Spin‐coating
APTES
Immersion
Au
5 × 10
−5
10
6
[133]
p
PB16TTT
Spin‐coating
β‐PhTS
Immersion
BC Au
0.24 (0.25)
10
7
[134]
p
PB16TTT
Spin‐coating
HMDS
Immersion
BC Au
0.28 (0.30)
10
7
[134]
p
PB16TTT
Spin‐coating
OTS
Immersion
BC Au
0.48 (0.51)
10
7
[134]
p
PB16TTT
Spin‐coating
FOTS
Immersion
BC Au
0.56 (0.74)
10
7
[134]
p
PB16TTT
Spin‐coating
β‐PhTS
Immersion
TC Au
0.35 (0.38)
10
6
[134]
p
PB16TTT
Spin‐coating
HMDS
Immersion
TC Au
0.41 (0.46)
10
7
[134]
p
PB16TTT
Spin‐coating
FOTS
Immersion
TC Au
0.59 (0.65)
10
7
[134]
p
PB16TTT
Spin‐coating
β‐PhTS
Immersion
TC Au
0.84 (1.0)
10
6
[134]
p
Pentacene
Thermal evaporation
ODTS
Immersion
TC Au
0.2
10
5
[135]
p
Pentacene
Thermal evaporation
ODTMS
Vapor
TC Au
0.6 (0.9)
10
6
[136]
p
Pentacene
Thermal evaporation
ODTMS
LB technique
TC Au
2.1 (2.3)
10
6
[136]
n
C60
Thermal evaporation
ODTMS
Vapor
TC Au
0.2 (0.27)
10
6
[136]
n
C60
Thermal evaporation
ODTMS
LB technique
TC Au
4.1 (5.3)
10
7
[136]
p
PTAA
Spin‐coating
FPPTS
Immersion
BC Au
5.2 (±1.3) × 10
−4
10
4
[137]
p
PTAA
Spin‐coating
MPTMS
Vacuum
BC Au
5.2 (±1.5) × 10
−4
10
5
[137]
p
PTAA
Spin‐coating
APTES
Vacuum
BC Au
1.4 (±0.7) × 10
−4
10
4
[137]
p
PTAA
Spin‐coating
TAATS
Immersion
BC Au
2.0 (±0.1) × 10
−4
10
5
[137]
p
PTAA
Spin‐coating
ODTS
Immersion
BC Au
—
—
[137]
p
Pentacene
Thermal evaporation
ODTMS
Spin‐casting
TC Au
2.8 ± 0.2
10
6
[138]
p
Pentacene
Thermal evaporation
ODTS
Vapor
TC Au
0.52 ± 0.04
10
5
[138]
n
C60
Thermal evaporation
ODTMS
Spin‐casting
TC Au
4.7 ± 0.41
10
7
[138]
n
C60
Thermal evaporation
ODTS
Vapor
TC Au
0.27 ± 0.15
10
5
[138]
n
PTCDI‐C4F7
Thermal evaporation
ODTMS
Spin‐casting
TC Au
1.4
10
6
[138]
n
PTCDI‐C4F7
Thermal evaporation
ODTS
Vapor
TC Au
0.72
—
[138]
p
Pentacene
Thermal evaporation
9‐Ant‐PA
T‐BAG treatment
TC Au
0.8 (0.9)
10
7
[139]
p
Pentacene
Thermal evaporation
2‐Ant‐PA
T‐BAG treatment
TC Au
1.6 (2.4)
10
7
[139]
p
Pentacene
Thermal evaporation
DiPh‐2‐Ant‐PA
T‐BAG treatment
TC Au
1.8 (3.6)
10
7
[139]
p
Pentacene
Thermal evaporation
DiNaph‐2‐Ant‐PA
T‐BAG treatment
TC Au
2.5 (4.7)
10
7
[139]
p
P‐BTDT
Thermal evaporation
High humidity (HH)‐ODTS
Immersion
TC Au
0.22 (0.250)
10
8
[125]
p
P‐BTDT
Thermal evaporation
anhydrous (AA)‐ODTS
Immersion
TC Au
0.085 (0.090)
10
8
[125]
p
Pentacene
Thermal evaporation
ODTS
Vapor
TC Au
0.517 ± 0.015
—
[140]
p
Pentacene
Thermal evaporation
FDTS
Vapor
TC Au
0.449 ± 0.002
—
[140]
p
C10–DNF–VV
Edge‐cast
DTES
Vapor
TC Au
1.3
—
[141]
p
C10–DNF–VW
Edge‐cast
DTES
Vapor
TC Au
1
—
[141]
p
C10–DNF–VW
Edge‐cast
β‐PhTS
Vapor
TC Au
1.1
—
[141]
p
Pentacene
OMBD
ODTS
Dipping at −30 °C
TC Au
0.46 (0.61)
—
[142]
p
Pentacene
OMBD
ODTS
Dipping at −5 °C
TC Au
0.31
—
[142]
p
Pentacene
OMBD
ODTS
Dipping at 20 °C
TC Au
0.19
—
[142]
p
P3HT
Spin‐coating
b)
OTS
Casting
BC Au
0.156 ± 0.010
10
3
[39]
Ambipolar
PNDTI‐BT‐DT
Spin‐coated
ODTS
Immersion
TC Au
0.2 (p)/0.28 (n)
10
3
(p)/10
3
(n)
[143]
Ambipolar
PNDTI‐BT‐DT
Spin‐coated
FDTES
Vapor
TC Au
0.43 (p)
10
6
[143]
Ambipolar
PNDTI‐BT‐DT
Spin‐coated
MAPTES
Immersion
TC Au
0.24 (n)
10
6
[143]
a) OMBD: organic molecular‐beam deposition.
b) Spin‐coating from ultrasonicated solution for four minutes.
2.2.2.2 SAM/High‐k Dielectrics
High‐k materials [144] include different kinds of materials, such as the most abundant inorganic oxides [145] and polymers [146, 147], as well as hybrid dielectrics [148]. We focus here on the widely used inorganic oxide high‐k materials to achieve low‐voltage OFETs. They are usually prepared by solution processing and vapor deposition, such as the sol–gel and atomic layer deposition (ALD) methods. Owing to some distinct characteristics of high‐k metal oxides, i.e. (i) the unavoidable charge‐carrier trap sites on the surface (usually associated with –OH groups), (ii) the induced ionic polarization owing to the interaction between charge carriers and the high‐k lattice, and (iii) the surface roughness, they can adversely affect the device performance. In order to mitigate the problems, surface modification is needed for controlling the surface properties of high‐k dielectrics.
PA–SAMs (107–134) (Figure 2.5) are prepared easily in air and are able to form well‐ordered, strongly covalently bonded films on high‐k oxide surfaces [149]. PA–SAMs show better stability to moisture and less tendency to homocondensation between phosphonic acids. Once the monolayer is formed, the SAM [150, 151] shows more resistance of phosphonate to hydrolysis than siloxane. PA–SAMs [152, 153] can also be used to modify transparent electrodes based on metal oxides, such as indium tin oxide (ITO), with almost no impact on the transparency. To get a comprehensive understanding of the relationship between SAM materials, preparation methods, and FET performances, we have listed below recent works of SAM‐modified high‐k dielectrics, which are shown in Table 2.2.
Figure 2.5 Chemical structures of SAMs used to modify high‐k inorganic dielectrics.
Table 2.2 The device performance of the devices with SAM‐modified high‐k dielectrics.
Material
High‐
k
preparation
PA‐SAM
OSC material
OSC type
OSC deposition
V
th
Average mobility (highest mobility)
On/off ratio
References
CeO
2
–SiO
2
Electron‐beam deposition
HMDS
Pentacene
p
Vacuum
—
0.32
10
3
[154]
Al
2
O
3
–SiO
2
Magnetron sputtering
COOH‐anthracene
Pentacene
p
Thermal evaporation
−18
0.17
—
[155]
Al
2
O
3
–SiO
2
Magnetron sputtering
Ba‐C10
Pentacene
p
Thermal evaporation
−9
0.35
—
[155]
HfO
2
–SiO
2
Sol–gel
ODPA
Pentacene
p
Thermal evaporation
−0.53
0.15
10
5
[156]
HfO
2
–SiO
2
Sol–gel
π‐σ‐PA1
Pentacene
p
Thermal evaporation
−0.41
0.22
10
5
[156]
HfO
2
–SiO
2
Sol–gel
π‐σ‐PA2
Pentacene
p
Thermal evaporation
−0.41
0.15
10
5
[156]
Al
2
O
3
O
2
plasma treated Al
ODPA
Pentacene
p
Thermal evaporation
—
0.4
—
[157]
Al
2
O
3
O
2
plasma treated Al
ODPA
F16CuPc
n
Thermal evaporation
—
0.01
—
[157]
HfO
2
Sol–gel
ODPA
PTzQT‐14
p
Spin‐coating
−0.32
0.10 ± 0.01 (0.11)
10
5
[158]
HfO
2
flexible
Sol–gel
ODPA
PTzQT‐14
p
Spin‐coating
−0.85
0.061 ± 0.0045 (0.068)
10
5
[158]
HfO
2
Sol–gel
π‐σ‐PA
Pentacene
p
Thermal evaporation
−0.5
0.32
10
5
[159]
HfO
2
Sol–gel
ODPA
Pentacene
p
Thermal evaporation
−0.6
0.3
10
5
[159]
HfO
2
Sol–gel
π‐σ‐PA
TIPS‐PEN
p
Drop cast
−0.5
0.38
10
5
[159]
HfO
2
Sol–gel
ODPA
TIPS‐PEN
p
Drop cast
—
—
—
[159]
HfO
2
–SiO
2
Sol–gel
C18‐PA
Pentacene
p
Thermal evaporation
−1.06
0.25 ± 0.08
10
6
[160]
HfO
2
–SiO
2
Sol–gel
C16‐PA
Pentacene
p
Thermal evaporation
−0.88
0.30 ± 0.03
10
6
[160]
HfO
2
–SiO
2
Sol–gel
C14‐PA
Pentacene
p
Thermal evaporation
−0.86
0.82 ± 0.03
10
6
[160]
HfO
2
–SiO
2
Sol–gel
C12‐PA
Pentacene
p
Thermal evaporation
−0.69
1.09 ± 0.02
10
6
[160]
HfO
2
–SiO
2
Sol–gel
C10‐PA
Pentacene
p
Thermal evaporation
−0.59
0.84 ± 0.13
10
6
[160]
HfO
2
–SiO
2
Sol–gel
C8‐PA
Pentacene
p
Thermal evaporation
−0.58
0.74 ± 0.03
10
6
[160]
HfO
2
–SiO
2
Sol–gel
C6‐PA
Pentacene
p
Thermal evaporation
−0.56
0.56 ± 0.04
10
6
[160]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
PhO‐19‐PA
TIPS‐PEN
p
Spin‐cast
−0.7 to −0.9
0.02–0.11
10
6
[161]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
PhO‐19‐PA
PC61BM
n
Spin‐cast
0.4–0.6
0.02–0.06
10
6
[161]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
PhO‐19‐PA
Pentacene
p
Thermal evaporation
−0.8
0.9–0.11
10
6
[161]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
C10‐PA
DH4T
p
Thermal evaporation
−1.5
0.024
10
6
[162]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
PHDA
DH4T
p
Thermal evaporation
−1.3
0.018
10
6
[162]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
F15C18‐PA
DH4T
p
Thermal evaporation
−0.4
0.024
10
6
[162]
AlO
x
/banknote
O
2
plasma treated Al
ODPA
DNTT
p
Thermal evaporation
−1.0 to −1.4
—
10
4
(10
5
)
[163]
AlO
x
/banknote
O
2
plasma treated Al
ODPA
F16CuPc
n
Thermal evaporation
—
0.005
—
[163]
AlO
x
O
2
plasma treated Al
Phenyl PA
DH6T
p
Thermal evaporation
−1.100 ± 0.24
0.011 ± 0.003
—
[164]
AlO
x
O
2
plasma treated Al
Phenyl PA
Pentacene
p
Thermal evaporation
−1.780 ± 0.07
0.046 ± 0.03
—
[164]
AlO
x
O
2
plasma treated Al
Phenyl PA
F16CuPc
n
Thermal evaporation
0.31 ± 0.01
0.061 ± 0.005
—
[164]
AlO
x
O
2
plasma treated Al
C14‐PA
DH6T
p
Thermal evaporation
−0.860 ± 0.08
0.034 ± 0.007
—
[164]
AlO
x
O
2
plasma treated Al
C14‐PA
Pentacene
p
Thermal evaporation
−1.610 ± 0.05
0.833 ± 0.106
—
[164]
AlO
x
O
2
plasma treated Al
C14‐PA
F16CuPc
n
Thermal evaporation
0.330 ± 0.180
0.164 ± 0.056
—
[164]
AlO
x
O
2
plasma treated Al
4T‐C12‐PA
DH6T
p
Thermal evaporation
−0.80 ± 0.14
0.014 ± 0.005
—
[164]
AlO
x
O
2
plasma treated Al
4T‐C12‐PA
Pentacene
p
Thermal evaporation
−1.37 ± 0.07
0.001 ± 0.0001
—
[164]
AlO
x
O
2
plasma treated Al
4T‐C12‐PA
F16CuPc
n
Thermal evaporation
0.81 ± 0.070
0.014 ± 0.008
—
[164]
AlO
x
O
2
plasma treated Al
BTBT‐C12‐PA
DH6T
p
Thermal evaporation
−0.75 ± 0.02
0.015 ± 0.002
—
[164]
AlO
x
O
2
plasma treated Al
BTBT‐C12‐PA
Pentacene
p
Thermal evaporation
−1.23 ± 0.240
0.029 ± 0.022
—
[164]
AlO
x
O
2
plasma treated Al
BTBT‐C12‐PA
F16CuPc
n
Thermal evaporation
0.66 ± 0.04
0.031 ± 0.025
—
[164]
AlO
x
O
2
plasma treated Al
C60C18‐PA
DH6T
p
Thermal evaporation
0.450 ± 0.04
0.016 ± 0.001
—
[164]
AlO
x
O
2
plasma treated Al
C60C18‐PA
Pentacene
p
Thermal evaporation
−0.51 ± 0.07
0.155 ± 0.076
—
[164]
AlO
x
O
2
plasma treated Al
C60C18‐PA
F16CuPc
n
Thermal evaporation
0.83 ± 0.09
0.004 ± 0.002
—
[164]
AlO
x
O
2
plasma treated Al
F15C18‐PA
DH6T
p
Thermal evaporation
0.640 ± 0.01
0.022 ± 0.006
—
[164]
AlO
x
O
2
plasma treated Al
F15C18‐PA
Pentacene
p
Thermal evaporation
−0.60 ± 0.04
0.097 ± 0.067
—
[164]
AlO
x
O
2
plasma treated Al
F15C18‐PA
F16CuPc
n
Thermal evaporation
1.890 ± 0.120
0.043 ± 0.014
—
[164]
AlO
y
/TiO
x
Spin‐cast
ODPA
TIPS‐TAP
n
Thermal evaporation
0.83 ± 0.08
1.3–1.8
—
[40]
AlO
y
/TiO
x
Spin‐cast
PA1
TIPS‐TAP
n
Drop cast
1.1 ± 0.22
1.2 (2.5)
10
5
[40]
AlO
y
/TiO
x
Spin‐cast
PA2
TIPS‐TAP
n
Drop cast
0.9 ± 0.25
0.88
—
[40]
AlO
y
/TiO
x
Spin‐cast
PA3
TIPS‐TAP
n
Drop cast
—
—
—
[40]
AlO
y
/TiO
x
Spin‐cast
PA4
TIPS‐TAP
n
Drop cast
1.2 ± 0.25
0.94
—
[40]
AlO
y
/TiO
x
Spin‐cast
PA5
TIPS‐TAP
n
Drop cast
1.2 ± 0.24
0.97
—
[40]
AlO
y
/TiO
x
Spin‐cast
PA1
TIPS‐TAP
p
Drop‐cast
—
1.13
—
[40]
AlO
y
/TiOx
Spin‐cast
PA4
TIPS‐TAP
p
Drop‐cast
—
0.95
—
[40]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
BA‐11‐PA
Pentacene
p
Thermal evaporation
26.0 ± 12.3
3.5 ± 0.28
10
6
[165]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
12‐PD‐PA
Pentacene
p
Thermal evaporation
14.0 ± 7.3
3.15 ± 0.31
10
6
[165]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
DDPA
Pentacene
p
Thermal evaporation
18.6 ± 3.8
3.51 ± 0.39
10
6
[165]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
PhO‐19‐PA
Pentacene
p
Thermal evaporation
22.5 ± 7.5
0.91 ± 0.04
10
6
[165]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
ODPA
Pentacene
p
Thermal evaporation
27.4 ± 6.8
0.98 ± 0.21
10
6
[165]
Al
2
O
3
–SiO
2
O
2
plasma treated Al on Si/SiO
2
Trip‐12‐PA
Pentacene
p
Thermal evaporation
42.1 ± 6.7
0.51 ± 0.15
10
5
[165]
AlO
x
/PEN foil
Potentiostatic anodization of Al
C14‐PA
DNTT
p
Thermal evaporation
−0.61 ± 0.13
0.88 ± 0.13 (1.6)
10
7
[166]
AlO
x
/SiO
2
Potentiostatic anodization of Al
C14‐ PA
DNTT
p
Thermal evaporation
2
−0.76
10
7
[166]
AlO
y
/TiO
x
Spin‐coating
CDPA
Pentacene
p
Thermal evaporation
—
3.86 ± 0.47 (5.7)
—
[167]
AlO
y
/TiO
x
Spin‐coating
CDPA
Pentacene
p
Thermal evaporation
—
2.21 ± 0.46 (2.9)
—
[167]
AlO
y
/TiO
x
Spin‐coating
CDPA
C60
n
Thermal evaporation
—
2.98 ± 0.83 (5.1)
—
[167]
AlO
y
/TiO
x
Spin‐coating
CDPA
C60
n
Thermal evaporation
—
0.66 ± 0.28 (1.1)
—
[167]
AlO
y
/TiO
x
Spin‐coating
CDPA
TIPS‐PEN
p
Drop‐cast
—
1.64 ± 0.55 (2.7)
—
[167]
AlO
y
/TiO
x
Spin‐coating
CDPA
TIPS‐TAP
n
Drop‐cast
—
0.78 ± 0.32 (1.44)
—
[167]
Al
2
O
3
Atomic layer deposition (ALD)
HC18−PA
DNTT
p
Thermal evaporation
−0.6
2
—
[168]
Al
2
O
3
Atomic layer deposition (ALD)
HC18−PA
DNTT
p
Thermal evaporation
−0.4
2.3
—
[168]
Al
2
O
3
Atomic layer deposition (ALD)
FC18−PA
DNTT
p
Thermal evaporation
1.2
1.8
—
[168]
Al
2
O
3
Atomic layer deposition (ALD)
FC18−PA
DNTT
p
Thermal evaporation
1.3
2.1
—
[168]
AlO
x
/PEN foil
O
2
plasma treated Al
HC14‐PA
DNTT
p
Thermal evaporation
—
2.9 ± 0.3
10
6
[169]
AlO
x
/PEN foil
O
2
plasma treated Al
FC18‐PA
DNTT
p
Thermal evaporation
—
2.2 ± 0.2
10
6
[169]
AlO
x
/PEN foil
O
2
plasma treated Al
HC14‐PA
DPh‐DNTT
p
Thermal evaporation
—
4.1 ± 0.1
10
6
[169]
AlO
x
/PEN foil
O
2
plasma treated Al
FC18‐PA
DPh‐DNTT
p
Thermal evaporation
—
1.9 ± 0.1
10
6
[169]
AlO
x
/PEN foil
O
2
plasma treated Al
HC14‐PA
C10‐DNTT
p
Thermal evaporation
—
3.6 ± 0.1
10
6
[169]
AlO
x
/PEN foil
O
2
plasma treated Al
FC18‐PA
C11‐DNTT
p
Thermal evaporation
—
1.1 ± 0.04
10
6
[169]
AlO
x
/PEN foil
O
2
plasma treated Al
HC14‐PA
N1100
n
Thermal evaporation
—
0.5 ± 0.04
10
6
[169]
AlO
x
/PEN foil
O
2
plasma treated Al
FC18‐PA
N1100
n
Thermal evaporation
—
0.8 ± 0.1
10
6
[169]
AlO
y
/TiO
x
Spin‐coating
CDPA
TIPS‐TAP
n
Drop‐cast
11–15
7.6 ± 1.6
10
6
[41]
AlO
y
/TiO
x
Spin‐coating
CDPA
TIPS‐TAP
n
Dip‐coated
11–15
7.6 ± 1.9
10
6
[41]
AlO
y
/TiO
x
Spin‐coating
CDPA
TIPS‐TAP
n
Thermal evaporation
18–22
5.1 ± 1.2
10
6
[41]
2.2.2.3 Self‐Assembled Monolayer Field‐Effect Transistors (SAMFETs)
The covalent attachment of SAMs containing semiconducting cores onto an inorganic high‐k dielectric surface provides a direct method for the development of miniaturized devices of SAMFETs, with low operation voltages and high electrical performance by reducing the dielectric thickness and improving the dielectric quality.
For OSC monolayer growing on rigid or flexible substrates to construct a SAMFET (Figure 2.6a), three golden rules (Figure 2.6b) should be taken into consideration:
(i) Anchor group was chosen based on the dielectric used;
(ii) Chained tail was chosen by considering interactions;
(iii) Head group was chosen to integrate the functionality.
Figure 2.6 Organic semiconducting molecular design for SAMFEFs. (a) Schematic illustration showing a SAMFET device configuration. (b) Molecular design strategy of a typical SAMFET semiconductor.
Source: (a, b) Adapted from Schmaltz et al. [42], © 2017 American Chemical Society.
We have summarized in Figure 2.7 and Table 2.3 some widely used OSCs and their performances in SAMFETs.
Figure 2.7 Molecular structures of OSCs for SAMFETs.
Table 2.3 Summary of the device performance of SAMFETs.
SAM OSC
Anchor
Spacer
Dielectric
Mobility (cm
2
V
−1
s
−1
)
On/off ratio
V
th
(V)
Work voltage (V)
References
135
Ortho‐hydroxyl
—
Al
2
O
3
—
—
—
−2.5
[43]
136
Ortho‐hydroxyl
—
SiO
2
1
10
4
—
−2
[170]
143
Monochlorosilane
C11
SiO
2
0.04
10
8
—
−2
[171]
143
Monochlorosilane
C11
SU8
0.02
10
6
—
−2
[172]
144
Phosphonic acid
C12
SiO
2
10
−5
10
3
−2.15
−4
[173]
148
Phosphonic acid
C6
SiO
2
10
−4
10
4
3
4
[173]
145
Phosphonic acid
C11
Al
2
O
3
8.0 × 10
−6
10
2
−32 to −53
—
[174]
146
Phosphonic acid
C12
SiO
2
1.2 × 10
−2
10
2
—
−3
[175]
146
Phosphonic acid
C12
PEN
1.7 × 10
−3
10
3
—
−3
[175]
152
Phosphonic acid
C11
Al
2
O
3
1.5 × 10
−3
10
5
0
20
[176]
151
Phosphonic acid
C11
Al
2
O
3
10
−3
10
6
10
2
[177]
138
Tetramethyldisiloxane
C10
SiO
2
3 × 10
−3
10
5
15
−20
[178]
142
Trichlorosilane
C12
SiO
2
1.7 × 10
−2
10
6
−20
−1
[179]
149
Phosphonic acid
C11
HfO
2
/SiO
2
3.56 × 10
−2
10
3
−65
−3
[180]
150
Phosphonic acid
C12
HfO
2
/SiO
2
7.37 × 10
−2
10
3
−65
−3
[180]
153
Phosphonic acid
—
Al
2
O
3
(3.6 ± 0.3) × 10
−5
10
4
—
−5
[181]
139
Monochlorosilane
C10
SiO
2
1.4 × 10
−2
10
4
−30
—
[182]
147
Phosphonic acid
C11
Al
2
O
3
3 × 10
−2
Variable
—
−5
[42]
140
Triethoxysilane
—
SiO
2
1.8 × 10
−3
10
4
−20
—
[183]
Early attempts to fabricate SAMFETs resulted in relatively poor performance at channel lengths of more than 1 μm because of the lack of long‐range order of the OSC films. These molecules were designed by attaching the anchoring group directly to the rigid semiconductor core with no flexible chained tails [43, 170] which misses the second golden rule. For example, the Nuckolls' group [43] used a tetracene derivative (135) that had one of its terminal rings functionalized with o‐hydroxyl as a catechol to form upright monolayers on the AlOx surface. The SAM of semiconductor showed p‐type operation. The current–voltage curves, however, did not allow quantitative extraction of the transistor parameters.
The flexile chained tails are necessary to tune the self‐assembly process of monolayers. As described by Mottaghi et al. [44], who used a molecule (137) comprising a short alkyl chain (C6) linked to an oligothiophene moiety and a –COOH terminal group as an anchor on AlOx, an estimated mobility of ∼0.0035 cm2 V−1 s−1 was achieved in a submicrometer channel. A remarkable breakthrough was achieved by Smits et al. [171] who used a liquid‐crystalline molecule (143) consisting of an oligothiophene core separated by a long aliphatic chain (C11) from a monofunctionalized anchor group. The SAMFET exhibited bulk‐like carrier mobility (∼0.04 cm2 V−1 s−1) for a 40‐μm channel with high reproducibility. Another good example of the use of long chained tail was demonstrated by Parry et al. [179] using monochlorosilane (141) or trichlorosilane (142) anchors.
N‐type SAMFETs were demonstrated by employing the perylene diimide (PDI) derivates (151 and 152) [176, 177]. Because of the formation of a nanocrystalline monolayer, highly reproducible transistors were demonstrated with channel lengths up to 100 μm, showing electron mobilities of ∼10−3 cm2 V−1 s−1 and on/off current ratios up to 105.
In summary, by holistic molecular design, SAMFETs can achieve the FET property as good as bulk OFET devices. Considering their monolayer thickness, low‐voltage operation, and high‐sensitive OSC/environment, SAMFETs may have promising applications in chemo‐/biosensors and flexible electronics. The operation stability and reproducibility are main concerns hindering their development. It calls for rational design of the molecules as well as the engineering of device architectures. A deep understanding of surface self‐assembly mechanism [184], including the complicated inter‐/intramolecular interactions as well as SAM–dielectric interactions, should be taken in the preparation of high‐quality semiconducting SAMs.
2.3 Efforts in Developing New Methods
In organic electronics, the functionalization of dielectric substrates with SAMs or polymers is regarded as an effective surface modification strategy that may significantly improve the resulting device performance. This method, however, is not suitable for some latest substrates typically used in flexible electronics, e.g. stretchable substrates, paper substrates, and degradable substrates. The pioneer work has been demonstrated by Yokota et al. [107] by using a paraffinic tripodal triptycene, which self‐assembles into a completely oriented 2D hexagonal triptycene array on polymer surfaces. Such few‐layer films are analogous to conventional SAMs on inorganic substrates in that they neutralize the polymer surface to provide better electrical performance.
The result, however, is still far from perfect. The development of interfacial modification methods lags far behind the progress of active materials, namely, OSCs, electrodes, and dielectrics. To name but a few of the drawbacks:
(i) The ultrahigh stretchability of OSCs, electrodes, and substrates calls for ultrahigh density of SAMs, which is impossible to achieve under thermodynamic self‐assembly process;
(ii) For portable use, the surface conditions of substrates are more serve than regular flat substrates, how can we expect a perfect interfacial property based on general methodologies?
There is still plenty of room to develop efficacious surface modification methodologies to optimize the surfaces on flexible substrates and thus improve the device performance.
