PhD THESIS IN ENGINEERING

Development of
Small π-Conjugated Molecules with Long Alkyl Chainsfor Organic Field-Effect Transistors（長鎖アルキル基を導入した低分子トランジスタ材料の開発）

March, 2011

Iori Doi
Department of Applied Chemistry, Graduate School of Engineering
Hiroshima University

Contents
Abstract
Chapter 1

Ⅲ
General Introduction

1

Chapter 2
Development of Heteroaromatic-Fused TTFs as Organic Semiconductors for Solution Processible Field-Effect Transistors
2.1

Introduction for Organic Semiconductors

4

2.2

Bis(N-alkyl-pyrrolo[3,4-d])tetrathiafulvalenes (N-alkyl-PyTTFs)

10

2.3

Bis(N-acyl-pyrrolo[3,4-d])tetrathiafulvalenes (N-acyl-PyTTFs)

20

2.4

Bis(thieno[2,3-d])tetrathiafulvalenes (αDT-TTFs)

26

2.5

Correlation between Electronic Structure in Solid State and Field-Effect Mobility

35

2.6

Conclusion for Chapter 2

44

Chapter 3
Evaluation of C10-DNTT as a Organic Semiconductor for High
Performance Thin-Film Transistors
3.1

Introduction for Alkylated DNTT

46

3.2

Evaluation on Silicon Substrates

48

3.3

Fabrication of Polymer Dielectrics

53

3.4

Evaluation on Plastic Substrates

60

3.5

Conclusion for Chapter 3

64

Closing Remarks

65

References

66

Appendix
A1

OFET Operation

72

A2

Nomenclature of Tetrathiafulvalene Derivatives

74

A3

Crystallographic Data for X-ray Single-Crystal Structure Analyses

75

A4

Slideshow for Ph.D Defense

76

List of publications

86

Acknowledgment

IIIAbstract

Abstract
Chapter 1. General Introduction.
Organic field-effect transistors (OFET) have recently attracted much attention
because of such possibilities as flexible, low-cost, low processing temperature,and large-area fabrication. Performances of the OFETs reported so far are in fact
lower than that their potential applications required. Development of semiconductors has been needed.
The Ph.D thesis focused on a series of small π-conjugated molecules that havelong alkyl chains along the π-core. The alkylated molecules presented here were
investigated in terms of molecular design, syntheses, device characteristics,
solid-state structures, theoretical electronic properties, and application to flexible
devices. This work involves interdisciplinary fields of research.
Chapter 2. Development of Heteroaromatic-Fused TTFs as Organic Semiconductors for Solution Processible Field-Effect Transistors
A series of alkylated molecules based on a tetrathiafulvalene (TTF) framework
were designed as solution-processible p-channel organic semiconductors. The
TTF derivatives that have long alkyl or alkylthio groups showed high solubilities
and good FET responses, which can be attributed to well-ordered layer-by-layer
structures in the thin films as in the crystal structures. The alkyl chain plays animportant role for the self-organizing ability. Correlation between the solid-state
structures and the FET characteristics are clearly demonstrated by experimental
and theoretical analyses.
Chapter 3. Evaluation of C10-DNTT as a Organic Semiconductor for High
Performance Thin-Film Transistors
Although the development of semiconductors based on the TTF framework did
not afford superior materials, the molecular design strategy was applied to othersmall π-conjugated molecules to give several high performance materials. One of
the materials, alkylated dinaphto[2,3-b:2’3’-f]thieno[3,2-b]thiophene (DNTT), isa vacuum-processed semiconductor that shows excellent performances and airstability. The author contributed to accurate and practical evaluations of FET
characteristics of the alkylated DNTT by means of improving deviceconfigurations and changing substrates. High performance OFETs on plastic substrates demonstrated in the context can be regarded as a successful exemplification of prototypical OFET applications. Structural difference of the thin films depending on substrates are also examined.

IIIAbstract

要約
第一章：序論
有機電界効果トランジスタ（OFET）は、 高い柔軟性や大面積・低コスト・低温などでの素子作製の可能性を有するため、近年注目を集めている。しかしその特性は実用的な要求に答えられるものではなく、より一層力を入れた有機半導体材料の開発が望まれている。本論文では、π骨格に長鎖アルキル基を導入した一連の低分子材料に着目し、分子設計や合成、FET特性、固体構造、理論的電子特性、フレキシブル素子への応用などについて調査した。本研究は有機合成化学や有機構造化学、半導体物理などの学際的な研究分野を取り扱う。
第二章： ヘテロ芳香族縮環TTFを用いた溶液プロセス可能な有機トランジスタ半導体
材料の開発
テトラアチアフルバレン（TTF）を母骨格とした一連のアルキル基を置換した分子を溶液プロセスp型有機半導体として設計した。長鎖誘導体は高い溶解性と良好なFET特性を示した。構造解析からこれらの分子は薄膜中で秩序高い層状構造を形成しており、それがFET特性に寄与していると示唆される。アルキル基は溶解性だけでなく自己組織化能にも重要な役割を果たしていることが明らかになった。本文中では理論的な解析により、これらの構造と物性との相関を明確に示した。
第三章：高性能有機半導体アルキルDNTTの評価および応用第二章で述べたTTFを骨格とする化合物群自体は高性能材料とは成り得なかったが、これらの分子設計は他の低分子骨格に適用され、いくつかの高性能材料を与えた。その中の一つであるアルキルDNTTは、非常に高い移動度と大気安定性を示す。筆者は、素子構造の改善や基板を変更することによって、アルキルDNTTのFET特性の正
確かつ応用的な評価に取り組んだ。本文中で示されるプラスチック基板上の高性能トランジスタは、OFETの応用を鑑みた試験的な成功例と考えられる。また、基板による薄膜構造の変化についても調査し、わずかな結晶性や薄膜の表面形状などの違いが特性に影響していることが示唆された。

IV

Abstract

Zusammenfassung
Kapitel 1. Allgemeine Einführung
Organischen Feldeffekttransistoren (OFET) wird aufgrund ihrer vorteilhaften Eigenschaften seit einiger Zeit erhöhte Aufmerksamkeit gewidmet. Zu diesen Vorteilen zählt
z. B. ihre Flexibilität, sie sind kostengünstig, besitzen geringe Verarbeitungstemperaturen und sind zur Massenfertigung geeignet. Jedoch ist die Leistungsfähigkeit von
OFETs geringer als es ihre potentielle Einsatzzwecke erfordern würden, weshalb eine
Weiterentwicklung dieser Halbleiter notwendig ist.
Die vorliegende Doktorarbeit beschäftigt sich mit kleinen πkonjugierten Molekülen
mit langen Alkylketten in der πEbene. Die vorgestellten aklylierten Moleküle werden
hinsichtlich Moleküldesign, Synthese, Eigenschaften, Strukturen, theoretische elektronische Eigenschaften und Anwendungen für flexible Bauteile untersucht. Über dieshinaus schließt die vorliegende Arbeit interdisziplinäre Forschungsbereiche mit ein.
Kapitel 2. Entwicklung von heteroaromatisch kondensierten TTFs als organische
Halbleiter für lösungsverarbeitete Feldeffekttransistoren
Mehrere alkylierte Moleküle auf Basis von Tetrathiafulvalene (TTF) Systemen wurden konzipiert als lösungsverarbeitet organische pKanal Feldeffekttransistoren. Die TTF
Derivate langer Alkyl- oder AklylthioGruppen weisen hohe Löslichkeiten und gute FET
Eigenschaften auf, welche auf die geordneten Schichtenstrukturen der Dünnschichten
sowie der Kristallstrukturen zurückzuführen sind. Die Alkylketten nehmen hierbei eineentscheidende Rolle in der Fähigkeit zur Selbstorganisation ein. Zusammenhänge
zwischen Festkörperstrukturen und den FET Charakteristiken werden durch experimentelle sowie theoretische Analysen eindeutig demonstriert.
Kapitel 3. Bewertung von C10DNTT als organische Halbleiter für HochleistungsDünnschicht-Transistoren
Obwohl die Entwicklung von Halbleitern, welche auf TTF Anordnungen basieren,keine hochwertige Materialien bieten, wurde das molekulare Design auf andere kurze
πkonjugierte Moleküle angewendet, um verschiedene Hochleistungsmaterialien zu erhalten. Eines der Materialien, alkyliertes Dinaphto[2,3-b:2’3’-f]thieno[3,2-b]Thiophen(DNTT), ist ein vakuumverarbeiteter Halbleiter und weist exzellente Leistungsfähigkeit
und Luftbeständigkeit auf. Der Autor trägt zur präzisen und praktisch orientierten Bewertung von FET Charakteristiken alkylierter DNTTs bei. Hierbei wird Bezug auf Verbesserung der Bauelementstruktur sowie auf Wechsel zu anderen Trägermaterialien genommen. HochleistungOFETs auf Kunststoffträgermaterialien konnten in diesem Zusammenhang erfolgreich als prototypische OFET Anwendungen demonstrativ eingesetzt werden. Ferner wurden strukturbedingte Unterschiede der Dünnschichen in
Abhängigkeit von Trägermaterialien untersucht.
Translation: S. Maikowske, Germany

VAbstractVI1. General Introduction

Chapter 1.
General Introduction
Impact of Organic Field-Effect Transistor. Some people may not realize what a “transistor”
is in their daily life. However, it is significantly an important technology that dominate ourenvironment.1 For example, personal computers, displays, mobile phones, RFID tags, and otherintelligent electronic equipments. Most transistors around us are made of silicon, especiallysilicon-oxide-semiconductor field-effect transistor (MOS-FET).2 On the other hand, organic
field-effect transistors (OFETs) have been recently attracted much attention of many academic
and industrial research groups owing to the fascinate possibilities for being flexible, stretchable,
large-area manufacturable, low-cost, light-weight, and ecological.3 Although OFETs will not beable to displace silicon-based FETs, it will be indispensable technology which can produce plastic electronics by combining flexible polymer substrates (Figure 1.1.1).4,5
Current Position of OFETs among Thin-Film Transistors. Most OFETs can be classified
into the thin-film transistor (TFT) by its configuration or device structure.6 Organic thin-filmbased transistors (OTFTs) are often compared with hydrogenated amorphous silicon TFTs
(a-Si:H-TFTs), because their mobilities and the targeting applications are very close (Table
1.1.1).7 However, the rivals are competing in this field, e.g. carbon nanotube TFTs (CNT-TFTs)
and amorphous metal oxide TFTs (a-MO-TFTs) in addition to the conventional smaller Si-based
TFTs.8 Although the mobilities of OFETs are somewhat inferior to those of the other TFTs,
OTFTs have several advantages over the other TFTs, e.g., the bending radius, both p- and nchannel operations, and easy processability. In fact, demonstration of electronics applications of
OTFTs to rollable display, flexible complementary circuits for inverters (CMOS), and pressuresensors have been reported actively (Figure 1.1.2).9-11
NATURE MATERIALSARTICLES
DOI: 10.1038/NMAT2896

ab

Figure 1.1.2. (a) A rollable display demonstrated by Sony in 2009.
(b) An ultra flexible integrated circuit reported by Sekitani
et al. in
∞
2010.
VDD

2.0

Output voltage (V)Figure 1.1.1. Images of plastic electronics(Morph, published by Nokia in 2008).

p-channelTFT

Input

Outputpoly-Si (HTPS, LTPS)

10 - 600

10channelstability

processabilitypµsolubility

◎

n
◯

Vth

◎

×

◎

F

F

0.5

10

◎

×

×

×

(initial)

R = 300 µm

0

0.5

1.0

1.5

2.0

1.0
0.5
1.5
100
~ 350
Input voltage (V)2.0

Input voltage (V)
F

N

N

N

N

Cu

N

N

PECVD
N

F

1 mm

a-Si:HF

F

process
F

F

0
F

R=

0.5

n-channel TFT

N

40

F
F
F
F

F
F

F

×

CVD

×

PECVD, CVD

cost
30
Gain

Table 1.1.1. Comparison of thin-film transistors.
bending
materialµ / cm2V-1s-1 radius
/ mm

1.5

1.0

20

× 10
◎

0

temp / ºC
100 ~ 1000

0c

µ c-Si:H

1 - 500

10.00

◎

×

◯

◯

VDD

100 ~ 350◯
4

Metal Oxide

10 - 20

30

×

◎

△

◯

×

sputtering

<100

◯

R = ∞ (initial)

R = 300 µm

CNT

15

Organic(Small molecule)1

Organic (Polymer)

0.1

10
0.5
0.5

◎
◎
◎

×
◯
◯

NA
△
△

NA
△
△

Output voltage (V)3

×

PECVD, etc△

vacuumsolution◯solution

◎

◎

<100

△

2

<100
1

<100

VDD = 3 V
0

0
40
80
120
Abbreviations: HTPS (high temperature poly-Si). LTPS (low temperature poly-Si). µ c-Si:H (hydrogenated micro crystallineSi). Time
CNT(car-160(ms)
bon nanotube). CVD (chemical vapor deposition). PECVD (plasma enhanced chemical vapor deposition). vacuum (vacuum-deposition).
solution (solution process such as spin-coat, drop-casting, inkjet, and printing process)1 mm
Figure 4 | Ultraflexible organic circuits. a, Photograph of a polyimide substrate with functional organic TFTs and circuits wrapped around a cylinder with a
Characters: ◎ high performance; ◯ good; △ few examples; × almost impossible.
radius of 300 µm. b, Circuit diagram, photograph and electrical transfer characteristics of a complementary inverter (composed of a pentacene p-channel

TFT and a F16 CuPc n-channel TFT) operated with a supply voltage (VDD ) of 2 V. The TFTs have a channel length of 20 µm. The transfer characteristics were
measured in the flat state and while the inverter was bent into a radius of 300 µm. There is no discernible change in characteristics during bending.
c, Circuit diagram, photograph and output signals of a five-stage complementary ring oscillator operated with a supply voltage (VDD ) of 3 V in the flat state
and while bent into a radius of 300 µm. The circuit oscillates with a signal delay per stage of 4.5 ms, both in the flat state and during tight bending.
NATURE MATERIALS | VOL 9 | DECEMBER 2010 | www.nature.com/naturematerials

1

1019

1. General Introduction

poly-Si (HTPS)!TFT / cm2V-1s-1100

Large OLED display required• !: ~ 100 cm2/Vs• Vth shift: <0.3 V

poly-Si (LTPS)• !: ~ 20 cm2/Vs• Vth shift: <0.3 V

a-Metal Oxide*generation• !: ~ 10 cm2/Vs• Vth shift: <0.3 V

10! c-Si:H• !: <10 cm2/Vs

Organic SM (vac)• Vth shift: <0.3 V• !: ~ 1 cm2/Vs• Vth shift: !?

1

size / cm

473 " 92

5

110 " 13068150 " 180
216 " 240a-Si:H• !: <1 cm2/Vs• Vth shift: ~ 3 V

Organic SM (solution)

Organic (polymer)• !: ~ 1 cm2/Vs• Vth shift: !?

• !: 0.1 cm2/Vs• Vth shift: !?

~
~

0

~
~

*SM: small molecule4
68
Maximum Size of Substrate / generation*

Figure 1.1.3. Mobility of TFTs vs maximum size of substrate of several kinds of semiconductor materials. abbr.
vac = vacuum-deposition.

Issues for Developing OFETs. The features of OFETs described above are very fascinating.
The virtual performances organic semiconductors are, however, fairly insufficient for industrial
applications. For instance, solution-processible materials, which is capable of low-cost andlarge-area device fabrication, show much lower mobilities than those required for application tolarge OLED displays (Figure 1.1.3).12 Likewise, OFET has many issues to be improved: (i) mobility (ii) on-off ratio (iii) operating voltage (threshold voltage) (iv) air-stability (v) bias-stressstability (vi) processability (solubility) (vii) accessibility (material-cost) (viii) flexibility, and(ix) reproducibility. In this work, I have focused on two particular issues; one is development ofsolution-processible organic semiconductors, and the other is evaluation of high performanceorganic semiconductors.
Development of Semiconductors. Most of theabove issues can be solved by finding superior orS
Sganic semiconductors.13 In other words, design andS
S
TTF
synthesis of organic compounds is considered toS
be the most challenging and intrinsic subject for
S
S
S
N R
R N
S
S
developing OFETs. For this reason, I have mainly
S
S
S
S
1:
N-n-alkyl-PyTTF
DT-TTF
studied to develop organic semiconductors basedSSS
on a tetrathiafulvalene (TTF) framework, espe- O N S S N R RS
SR
S
S
S
S
S
O
cially heteroaromatic-fused TTFs as potential R
3: alkylthio-αDT-TTF
2: N-n-acyl-PyTTF
compounds for solution-processible and high mobility OFETs (Figure 1.1.4, described in Chapter 2).
Figure 1.1.4. Structures of heteroaromatic-fused
The benchmark material in the series of the TTFs.
heteroaromat-fused TTFs is dithiophenetetrathiafulvalene (DT-TTF).14 OFETs based on
single-crystals of DT-TTF showed mobilities up to 1.4 cm2/Vs. The other heteroaromatic-fused
TTFs are designed as an isoelectronic structure or a structural isomer of DT-TTF that can be
introduced alkyl chains along the long molecular axis; alkyl- or acyl-substituted pyrrolo[3,4d]fused TTFs (1 and 2) and alkylthio-substituted thieno[2,3-d] TTFs (3).15,16 The solubilizing

2

1. General Introduction

alkyl chains along the π-core are expected not to interfere the π-π and/or S-S intermolecularinteractions (section 2.2 - 2.4).
Since it is important to clarify the relationship between FET characteristics and the thin-film
structures for understanding molecular design strategies for superior organic semiconductors,
the molecular orientation in both bulk-single crystals and the thin film of the heteroaromaticfused TTFs were elucidated by X-ray diffractions (XRDs). In addition, theoretical studies werecarried out to evaluate electronic structures in the solid states more quantitatively (section 2.5).
These results were compared to each other in the present heteroaromatic-fused TTFs and thoseof high performance semiconductors in other class. An advantage or disadvantage of the TTF
framework are discussed.

Evaluation of Semiconductors.
Dinaphto[2,3-b:2’3’-f]thieno[3,2-b]thiophene (DNTT)R
S
is the vacuum-processed semiconductor that gives highperformance and highly air-stable organic thin-film tranSR
sistors (OTFTs) developed in Takimiya’s group (Figure
R = H : DNTT
1.1.5).17-21 Alkylated DNTT, having long alkyl chains
R = n-C10H21 : C10-DNTT
along the DNTT core, is the semiconductors developed
according to the same molecular design strategy as the Figure 1.1.5. Structures of DNTT derivatives
heteroaromatic-fused TTF. Although the alkylated DNTT
is insoluble, the vacuum-processed organic OFETs basedon the alkylated DNTT showed superior performance than the ones based on DNTT. The very
high performance of C10-DNTT required improvement of our conventional FET device structure
for evaluation of characteristics more accurately and more practically. In chapter 3, as the second part of the thesis, I made effort to develop our device structure to evaluate the alkylated
DNTT. I first improved the device geometry by defining the shadow mask for patterning of bothorganic layer and source-drain electrodes layer to evaluate alkylated DNTT as a highperformance new semiconductor (section 3.2).22 Second I tried to fabricate and evaluate ofpolymer dielectrics. The performances of dielectrics were examined by evaluation of characterPh.D defense：Development of !-Conjugated Molecules with Long Alkyl
istics of DNTT-based OTFTs on the dielectrics (section 3.3). Finally high performancealkylated
DNTT-based OFETs on plastic substrates
were demonstrated(section 3.4).23 It is alsoOF
important
CHAPTER
2 DEVELOPMENTSEMICONDUC
to clarify the correlation between FET characteristics and solid-state
structures, I examined
Designstructural difference between on the polymer dielectrics and on the Si/SiO2 dielectrics. These
Small !-Compounds
works were very challenging because designingdevice structures and fabrication and characSynthesis＋Long
Alkyl Chainsterization of polymer dielectrics were thefirst attempts
in Takimiya’sgroup.
9 stepsSolution-process

The two approaches. Even though the twoS chaptersseem to be different, each study gave
S
R first development of semiconductors
important result for developing OFETsR(FigureS 1.1.6).
TheS
FET characteristics
based on TTF derivatives afforded the superior molecular design strategy
with long alkyl chains: Pyrrole or Thiophene
both as solubility and self-assembling group. The high crystallinity of the series of TTF allowed
Heteroaromatic-fused
TTFs
systematic analysis of the crystal structures,
which enable
to elucidate of relationship between
Evaluation of Alkylated DNTTDevelopment of Semiconductors
Alkyl-substituted Heteroaromatic-fused TTFs
S
R

S
R

S

S

: Pyrrole or Thiophene

Chapter 2

OFET
SourceDrain

Organic Semiconductor

Dielectric
Gate
Substrate

S

S

S

S

N R

R N

10-4

Vd = -60 V
10-5
10-6

-Id / A

KOH + CS2

10-7
10-8
10-9
10-10
-60

-40
-20
Device Configuration
V /V
Structural/
theoreticalanalysis
Fabrication on Plastic Substratesg

S
C10H21

C10H21S

C10-DNTT

Chapter 3qrFigure 1.1.6. Schematic representation of this work.

3

p r
p

q

0

1. General Introduction

the structures and the mobilities clearly. In the second part of my thesis, I established the advanced evaluation system. which proved DNTT with long alkyl chains to be high performanceeven on the plastic substrates. Furthermore, structural analysis indicate the performance can still
be improved by control morphologies. The interdisciplinary study through the two approaches
should be essential for developing OFET materials.

42.1 Introduction for Organic Semiconductors

Chapter 2.
Development of Heteroaromatic-Fused TTFs as Organic Semiconductors
for Solution Processible Field-Effect Transistors
2.1

Introduction for Organic Semiconductors

History. Without exceptions, organic semiconductors consist of π-compounds because πorbitals (conjugated p- and/or d-orbitals) can electronically interact with neighboring moleculeswith van der Waals interaction. This character realizes both electronic transport and mechanicalelasticity simultaneously. A numbers of interesting π-compounds were already synthesized before 1980s such as phthalocyanines and polythiophenes.24,25 Nevertheless, examination of application to organic field-effect transistors (OFETs) had been very limited, and the field-effect mobilities (µFETs) of such FET characteristics were quite low (µFET <10-5 cm2/Vs) (Figure
2.1.1).26,27 This is because the characteristics of OFETs are very sensitive to the purity and themolecular arrangement of semiconductors on the substrates, and thus researchers had to wait
establishment of the standard OFET fabrication techniques which includes vacuum-deposition(= vapor-deposition in vacuum) techniques and ultra smooth dielectric surface, e.g. heavily
doped silicon with thermally grown silicon oxide (Si/SiO2) substrate (surface roughness < 0.1
nm).28
P3AT and Pentacene. High performance devices (µFET > 10-3 cm2/Vs) based on alkylpolythiophene (P3AT) and pentacene in the late of 1980s accelerated the investigations of
OFETs (Figure 2.1.2).29 P3AT is the most common polymer material, which had been attracting
many researchers due to the processability.30-38 The crystalline thin films of P3AT can be fabricated with solution-process such as drop-cast, spin-coating, dip-coat, and printing. These processes allows low-cost and large-area fabrication of devices. The µFET of P3AT was increased up
to ~0.1 cm2/Vs by improving regio-regularity of the polymers39 Recently developed thiophenecontaining co-polymer-based materials showed µFET ~1 cm2/Vs (Figure 2.1.3).40-46 The µFETs of
the polymer-based OFETs are, however, generally lower than those of vacuum-deposited smallR

103

S
S

µ

FET

2

/ cm V s

-1 -1

10

n

R
rrP3AT1

pentacene

Figure 2.1.2. Structures of pentacene andrrP3AT.

10-1

C10H21

-3

10

10-5

-7

polythiophenes
thiophene oligomers
phtalocyanines
tetrathiafulvalenes
pentacene
heteroacenes
single-crystal (rubrene)

101980 1985 1990 1995 2000 2005 2010 2015
Year

Figure 2.1.1. Evolution of OFET hole mobility for themost common p-type organic semiconductors. Examples ofcompounds of each category are described in Figure 2.1.3 2.1.5.

C8H17
O

N

O
S

R

S
S

n

S

S

n

S

O

N

R

PBTTT

O
C8H17
C10H21Nat. Mater. 2006, 5, 328.

Nature 2009, 457, 679.

R

R
S
S

S
PNDTBTn
R

JACS 2010, 132, 5000.

S

S

S

S
R

N

R
N

S

S

S

n

R
PTzQT
JACS 2009, 131, 2521.

Figure 2.1.3. Example of thiophene containing
co-polymers.
4

2. Development of Heteroaromatic-fused TTFs
R

Acenes

F

F

F

F

F

F
Appl.Phys.Lett. 2003, 83, 4345.

Adv. Mater. 2003, 15, 1090.

F

F

F
F

JACS 2006, 128, 9304.

R

F

F

F

F

JACS 2004, 126, 8138.

Si
R
R

S
S

R

R

S
S

R

Angew.Chem.Int.Ed. 2003, 42, 1159.

JACS 2005, 127, 2406.

JACS 2007, 19, 1864.
Science 2004, 303, 1644.

Si

Si

Si

Adv.Mater. 2003, 15, 2009.
S
F

Heteroacenes

S

S

JACS, 2008, 130, 2706.

X
X

S

S

X

S

S
Chem.Lett. 2008, 37, 284.

R

X

R

S

S

R

S

S

S
JACS 1998, 120, 2206.

S

S

S
S
J.Mater.Chem. 1999, 9, 2095.

S

S
S
S
S
J.Org.Chem. 2007, 72, 442.
O

S

S
S
Org.Lett. 2007, 9, 4187.

Ar

R
O

O

R N

R
N

R

O
R

N
R
R
Adv.Mater. 2005, 17, 849.

X = S or Se
J.Org.Chem. 2010, 75, 1228.

O

N R

O

O

Angew.Chem.Int.Ed. 2004, 43, 6363.

Nature 2000, 404, 478.

R

X

O

R N

N R

JACS, 2005, 127, 3065.
R

S

S

Org.Lett, 2005, 7, 5301.

XX
X = S or Se
Org.Lett. 2007, 9, 4499.

S
S

JACS 2005, 127, 13281.

X = S or Se

Ar
S

S

S
S
Adv.Mater. 2009, 21, 213.

S

X

X

S

R
R

S

S
S
J.Chem.Soc.PerkinTrans. 1992, 761

S

S
R

Adv.Mater. 1997, 9, 36.

S

Bull.Chem.Soc.Jpn. 2010, 83, 120.

S

S

S

R

S
JACS 2007, 129, 15732.

S
S

X

R

S

S
R

S
JACS 1998, 120, 664.

JACS, 2005, 127, 4986.

S

S

R

Si

S

Si
S

S
J.Mater.Chem. 2008, 18, 3442.

Chem.Mater. 2008, 19, 3018.

S

JACS 2007, 129, 2224.

S

S

S

F
S

N

N

N

CN

N

N

N

CN

Fullerenes

O
OMe

Angew.Chem.Int.Ed. 2004, 43, 6363.

Thiophene-containing oligomers
F

F

F

F

F

O

F

F

F
F

F

F
S

S
F

F3C

S

F

S

S

S

N

S

O

O
S

NC

S
S

F3C
R

N
S

CN

NC
NC

S

S
S

F3C
CN
CN

S

S

S

N

S
R

Chem.Comm. 2008, 4714.

Figure 2.1.4. Examples of organic semiconductors based on π-conjugated molecules.

N

N

Chem.Mater. 2007, 19, 1218.

N

M

N

N

S
N

N

N
N

N
CF3

N

M

N
S

R

R

JACS 2007, 129, 11684.

5

F
F
F
F
F
JACS 1996, 69, 3066.

N

N

F

F

CF3

N
N

OHex

S

N

N
JACS 2004, 127, 5336.

JACS 2010, 132, 10453.
OHex

S

S

N
N

Appl.Phys.Lett. 1996, 69, 3066.

O

N

M
N

F

J.Mater.Chem. 2003, 2004, 1787.
N

R

N

F

N

N
N

N

N

JACS 2005, 127, 14996.
O

F

N
M

N

S

N

CF3

F
N

N

N

F

F

F

F
N

N

S

F

R

n

Adv.Mater. 2003, 15, 917.

SSJ.Appl.Phys. 2005, 97, 083714.

Phthalocyanines
S

S

R

JACS 2006, 128, 5792.
N

O
JACS 2005, 127, 1348.

S

F

S

F

S
F

R

S
S

R

S

F

S

F

S

S

N
S

R

J.Mater.Chem. 2009, 19, 5913.

R2.1 Introduction for Organic Semiconductors

Tetrathiafulvalenes (TTFs)
A
C6H13

B

S

S

S

S

S

S

CN

Se

Se

Se

Se

S

S

S

S

S

S

CN

3.6 cm2/Vs (crystal)a
S

S

S

S

S

Appl. Surf. Sci. 1998, 130-132, 914.

S

S

S

S

S

S

S

C18H37 SSSS C18H37

C18H37 SSSS C18H37

S

S
S

S

S

S

S

S

S

S

S

S

BEDT-TTF
0.005 cm2/Vs (crystal)
CnH2n+1

S

S
S
S

3 × 10-6 cm2/Vs (vac)

S

S

S

S

S

S

N

N

S

S

N

N

S

S

N

N

S

S

N

R

Cl

N

S

S

N

S

Cl

N

S

S

N

S

S

S

S

S

R

S

N
S
S

N

S

N

F

N

S

S

N

F

0.2 cm2/Vs (Vac)

F

N

S

S

N

F

Chem. Lett. 2004, 33, 1632.

0.1 cm2/Vs (vac, n-type)S

JACS 2006,128, 9598.

S
S

S
C8H17

S

S

S

S

C8H17

0.11 cm2/Vs (spin-coat)Chem. Lett. 2009,38, 200.
O

d Chem. Mater. DOI:10.1021/cm1021867.

S

0.11 cm2/Vs (vac)Chem. Lett. 2010, 39, 538.

0.64 cm2/Vs (vac)Nanotech. 2007, 18, 424009.

b Appl. Phys. Lett. 2005, 86, 012110.

S

S

N

JACS 2004, 126, 8546.

a Org. Elect. 2008, 9, 1101.

S

2.3 cm2/Vs (crystal)
0.98 cm2/Vs (vac)c J. Mater. Chem. 2009, 19, 6548.

JACS 2005,127, 10142.

S

3 × 10-4 cm2/Vs (vac)N

0.2 cm2/Vs (vac)

S

S

S

3.6 cm2/Vs (vac)c
Chem. Mater. 2007, 19, 6382.

DiquinoxalinoTTF

0.003 cm2/Vs (crystal)S

10-4 cm2/Vs

S

S

S
S

S

0.05 cm2/Vs (crystal)

H2n+1CnSSS

BET-TTF

0.08 cm2/Vs (zone cast)
Chem. Mater. 2006, 18, 4724.

S

S

S

S

3.3 × 10-5 cm2/Vs (vac)

S

S

S

S

S

7 cm2/Vs (crystal)

DipyradinoTTF

0.12 cm2/Vs (crystal)

0.11 cm2/Vs (vac)Chem. Lett. 2005,134, 392

SS

HM-TTF0.42 cm2/Vs (vac)S
SS

DinaphtoTTF

0.37 cm2/Vs (crystal)S

S

0.06 cm2/Vs (vac)
1.0 cm2/Vs (crystal)b

1.4 cm2/Vs (crsytal)

7.5 × 10-9 cm2/Vs (vac)S

DB-TTF

DT-TTF0.02 cm2/Vs (vac)Synth. Met. 2005, 149, 219.

D

C
S

O
S

S

S

S

R N
O

8.1 × 10-2 cm2/Vs (vac)J. Mater. Chem. 2009, 19, 3648.
C6H13 S

S

S

S

S

S C6H13

C6H13 S

S

S

S

S

S C6H130.02 cm2/Vs (spin)
Chem. Comm. 2006, 2750.

N R

0.4 cm2/Vs (vac)0.33 cm2/Vs (cryst)dO

Adv. Mater. 2007,19, 3037.

S

S

S

S

0.27 cm2/Vs (vac)Chem. Comm. 2006, 2750.

Figure 2.1.5. Representative TTF derivatives for OFET materials reported so far. Described values are the highest
reported mobilities of the thin-film-based OFETs fabricated by vacuum-deposition (vac), spin-coating (spin), dipcoating (dip), drop-coat (drop), and single-crystal OFETs (crystal). They roughly are sorted with four groups, A is
the TTF with substituents, B is TTF annulated with sulfur-containing five or six membered rings, C is six membered aromatic-fused TTFs, and D is TTF annulated with saturated rings.

molecules-based OFETs. The drawbacks of polymer materials include dispersion of molecularweight, low purity, and synthetic limitation. In addition, difficulties to control properties andanalyze structures of polymer-based thin films are disadvantages as a research subject.
On the other hand, pentacene, is the most common organic semiconductors at present, gave
one of the highest performance OFETs which showed µFET of ~3 cm2/Vs.26,29,47,48 The performances of pentacene-based devices were improved in this decade by means of optimizing fabrication process of the thin films and chemical modification of the dielectric surface. The advantages of the small π-conjugated molecules are easy purification and possibility for fine tuning inproperties, such as solubility, ionization potential, energy gap of a highest occupied molecularorbital (HOMO) and a lowest unoccupied molecular orbital (LUMO) by modification of molecular structure (see Appendix).49,50 In fact, great numbers of extended π-compounds have beenreported to be promising organic semiconductors, which shows diversity of possible structures
for developing new superior organic semiconductors (Figure 2.1.4 and 2.1.5).51-55 Small molecules are also convenient for a research subject because their molecular arrangement can be analyzed in detail by means of X-ray diffractions (XRDs) of both thin films and bulk single
crystals.52
6

2. Development of Heteroaromatic-fused TTFs

(b)

(a)
O

(c)
Si

O

N
O S

S

n-CnH2n+1

S

S
S

n-CnH2n+1

6
4

Si

5

π-core

学位論文発表：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs π-core

溶解性置換基を導入した低分子半導体材料（１）ex. Anthra-dithiophene骨格

TIPS-pentaceneなど
R
R Si RS

Si/SiO2 substrateS

Figure 2.1.6. Molecular design strategy for solution-processible small π-conjugated molecules.
(Ron=substrates
Et)
Expected molecular2-Dordersare depicted in bottom.

R Si RR(a) π-stack(b) θ-type(c) herringbone

J. E. Anthony et al., J. Am. Chem. Soc. 2005, 127, 4986.

!FET = ~1.0 cm2/Vs

1-D (R = i-Pr)

溶解性

配向制御が困難 (1-D系に)適用可能な骨格が少ない

Figure 2.1.8. Explanation of packing fashion with view along molecu~ An
10-4identical
cm2/Vsπ-stacking. (b) Molecules are inclined for
FET =(a)Figure 2.1.7. Explanation of “molecu- lar long!axis;
lar long axis” and molecular short axis. some extent. This structure has no
13 particular name so I named the structure θ-type temporary.73 (c) A herringbone structure where molecules
can pack minimallyHigh potential of small π-compounds was evidenced by single-crystal-based OFETs.19,56-60
The single-crystal OFET is expected to provide idealized semiconductor channels of perfectly
ordered molecules. Takeya et al. reported the prominent µFETs of ~30 cm2/Vs in rubrene single
crystals.61-62
Soluble Small π-Conjugated Semiconductors. One drawback of high performance materials based on small π-compounds is necessity of vacuum-process. Although soluble precursors ofpentacene and phthalocyanine have been developed (4, Figure 2.1.6a), the performance of thedevices were lower than the parent vacuum-deposited ones.53,63-66 Approach to develop solution
processible small molecules by introducing solubilizing substituents into π-cores has been investigated by several research groups.67 Anthony et al. have reported that a series of soluble
acene-based semiconductors that have bulky solubilizing groups along the short-axis directionof an acene-core (Figure 2.1.6b).68-70,49 Bis(triethylsilylethynyl)-substituted anthradithiophene(5) gave high performance OFETs with µFETs up to 1 cm2/Vs.69 The molecules form thin films
that arranged with brick-wall-like π-stacking on a substrate, which could give two-dimensional
conducting layer. However, when the substituent or the π-core is alternated slightly, the ar7

2.1 Introduction for Organic Semiconductors

(a)

S

S

S

S

(b)

(c)

S

S

- e-

S

S

S

+ e-

S

6π

6π

S

- e-

S

S

S

+ e-

S 6π S

Figure 2.1.9. Introduction of tetrathiafulvalene (TTF): (a) Chemical structure (b) Distributionof HOMO coefficients (c) Fundamental electronic character of TTF as a donor.

rangement is drastically changed into 1D π-stacking that showed lower µFETs in most cases.
This fact indicates that it is difficult to control molecular orientation with this molecular design
strategy.
The other strategy to introduce soluble substituents is to use long aliphatic chains along a πcore long-axis direction (6, Figure 2.1.6c). The anthradithiophene core with alkyl chains has
been reported as a good example.71 Although the µFETs (~0.02 cm2/Vs bycasting) are lower thanthose of 5-based OFETs, it is reported that the molecule formed layer-by-layer structure on asubstrate to give conducting layers parallel to the substrate. I considered that the self-organizing
ability of this molecular design would be necessary for fabrication of thin films from solution.
Additionally, the resulting layer-by-layer structure allows aggregation of the π-cores with herringbone fashion, which is expected to give 2D intermolecular interaction. This thesis focusedon the latter molecular design for solution-processible semiconductors.
General explanation to understand the terms of the molecular short and long axis, π-stacking,herringbone, and their intermediate structure are illustrated in Figure 2.1.7 and 2.1.8.72,73
TTF derivatives. Because the long alkyl chains have small steric bulkiness, the molecules are
expected to be closely packed in the solid state. However, it is also expected to give not very
high solubility. Several similar π-compounds that have long alkyl chains have been reported as
vacuum-process materials.74-76,129 Thus choice of π-core will play an important role for not only
mobility but also solubility in this molecular design. Among the varieties of potential structures
for organic π-cores, a tetrathiafulvalene (TTF) framework is attractive for following reasons(Figure 2.1.9a). TTF is well-known skeleton as an excellent molecular donor which give conductors and superconductors (Figure 2.1.9c).77 With increasing interests in OFETs, TTF derivatives have been examined as promising candidates for p-type semiconductors (Figure
2.1.5).14,78-94 Impressive FET characteristics have been reported by using ring-fused TTFs suchas thiophene, benzene, pyridine, cyclopentane, and related nitrogen-containing aromatic moieties. The highest µFET among the TTF derivatives have been reported using hexamethylene-TTF(HM-TTF) both in single-crystal-based devices (~7 cm2/Vs)87 and in thin-film-based devices(~3 cm2/Vs).88 Although their characteristics show have small on-off ratios (~103), the high potential of the TTF framework have been shown obviously. The key feature of TTF is strong intermolecular interaction via four sulfur atoms where HOMO coefficients largely located (Figure
2.1.9b). Furthermore, TTF derivatives are somewhat soluble compared with other extended πcompounds because of the small π-core. Considering the benefits of the strong cohesive character of the TTF cores, I have identified TTF as a promising π-conjugated core for use as soluble
semiconducting materials for OFETs.
Heteroaromatic-fused TTFs for Solution-Processible Semiconductors. It is possible to design the TTF with two long alkyl chain in such a way that the resulting molecules can assume a8

2. Development of Heteroaromatic-fused TTFs
S

S

S

S

N R

R N

S

R

N
S

S

2: N-n-acyl-PyTTF

SDT-TTF
R

S

N

S

S

S

1: N-n-alkyl-PyTTF
O

S
S

O

S

S

S

S

S
SR

RS
S

3: alkylthio-αDT-TTF

Figure 2.1.10. Structures of heteroaromatic-fused
TTFs.

stretched structure along the TTF long axis direction, where introduced alkyl chains would nothinder intermolecular π-π or S-S interactions. When the mono-substitution on each 1,3-dithiole
ring is carried out, however, cis/trans isomerization caused by rotation of the central doublebond in the TTF core readily occurs.95,96 The formation of a mixture of two isomers having very
different molecular shapes are not optimal for the formation of highly ordered molecular arraysin the thin-film state.80 Another strategy for introduction of alkyl chains is tetra-substitution on a
TTF core.81,92 It also could result in large steric bulkiness which could disturb close packing insolid state and/or causes liquid crystalline character around room temperature. These may bringpoor thermal stability and device operation.94
The heteroaromatic-fused TTFs shown in Figure 2.1.10 are considered to be good skeletonsthat are amenable to di-substitution of alkyl chains along the molecular long-axis direction withavoiding the problems discussed above, i.e., steric hinderance of S-S interaction and problematic structural disorders caused by cis/trans isomerization will not occur. In this series,
dithiophene-tetrathiafulvalene (DT-TTF) has been reported as a high performancesemiconductor.14 The single-crystal OFETs based on DT-TTF showed µFETs ~1.4 cm2/Vs. Although the DT-TTF can not be substituted with alkyl groups along the π-cores, bis(pyrrolo[3,4d])tetrathiafulvalene (PyTTF, 1), which is isoelectronic structure of DT-TTF, can be modified
with alkyl chains easily on the N atoms. Bis(thieno[2,3-d])tetra-thiafulvalene (αDT-TTF) also
can be modified with alkyl chain at the α-positions of the fused thiophene rings with keeping thesame thiophene moieties as DT-TTF.97 Each molecular feature will be explained in each introduction of the following sections.
In this chapter, synthesis, properties, FET characteristics, and solid-state structures of theheteroaromatic-fused TTFs with various length of alkyl substituents are described. After description of the experimental results, theoretical analysis of the molecular structures and singlecrystal structures will be carried out to correlate between the FET characteristics and the structures and to discuss the potential of the present semiconductors.

9

2.2 Bis(N-alkyl-pyrrolo[3,4-d])tetrathiafulvalenes (N-alkyl-PyTTFs)

2.2

Bis(N-alkyl-pyrrolo[3,4-d])tetrathiafulvalenes (N-alkyl-PyTTFs)

Introduction. Bis(pyrrolo[3,4-d])TTF (PyTTF, 1, Figure 2.2.1) is
first focused as a solution-processible skeleton among theSS
N Rheteroaromatic-fused TTFs. As mentioned in section 2.1, PyTTF R NSSseems to be an identical system that confirm the strategy for soluble
1a: R = H
OFET semiconductors based on the TTF core structure. Unsubsti1b: R = CH3
1c: R = n-C4H9
tuted (1a), methyl (1b), and n-butyl (1c) PyTTF derivatives aselectron donors for conducting materials were already synthesized Figure 2.2.1. Structure of
PyTTF.
via efficient synthetic route in 2000.98,99
Synthesis and Characterization. The parent PyTTF (1a) was synthesized via the reportedroute (Scheme 2.2.1, Route A). The synthesis using tosyl protecting group on the intermediate(11) gave PyTTF reproducibly. However, there are three problematic steps in this route; first,
dimethyl acetylenedicarbonate (reaction of 7 to 8) is expensive for use as the starting material.
Second, reduction of 8 to 9 is not reproducible, and the third, toxic Hg(OAc)2 is required for thesynthesis of 12 from 11. As a result of search for another synthetic route to the key intermediateof 12, I successfully found a new route by combining several efficient reactions (Route B).100-102
Route B can afford 12 in an improved total yield (22 % to 31 %) with fewer steps (6 steps to 5
steps) and easier purification procedures.103 From the key intermediate (12), the parent PyTTF(1a) was synthesized with gram scale in one batch successfully. N-alkylated PyTTFs with various chain lengths were then prepared by reaction of 1a with alkyl halides (Scheme 2.2.2). All
alkyl derivatives were isolated as stable yellow powders or crystals and fully characterized byspectroscopic and elemental analysis.
To evaluate the solubilizing effect of the introduced alkyl groups, the solubilities of 1b-g were
determined semi-quantitatively (see Experimental Procedures). Although the methyl derivative(1b) was virtually insoluble in toluene, elongation of the alkyl groups enhanced the solubility, as
expected. However, in compounds with very long alkyl groups (i.e., 1f with n-cetyl groups and1g with n-icosyl groups), the solubility was decreased (Table 2.2.1). These results indicate thatelongation of the alkyl groups enhances the affinity to solvents by enhancing solubility but thatvery long alkyl groups, such as the n-icosyl group, facilitate strong intermolecular hydrophobic
Scheme 2.2.1. Synthesis of parent PyTTF (1a).
Route A: 6 step 22 %CO2CH3Br

CS2, Bu4NBr
NaOH aq

Br

73%

S

CO2CH3

S

76%

S

S

7

NaBH4-LiBr

80%

S
S
S

OH

PBr3

OH

78%

CO2CH3S
S

CO2CH3
8

S

Br

S

Br

S

TsNHNaS

Hg(OAc)2

S

10

9

N Ts

S

80%

80%

11

S

N Ts

O
S

Route B: 5 step 31 %KOH + CS2

12

~ 80%

KS

S

3-chloro-2-butanone

S

iPrOH

OiPr

NHTsNa

OiPr

S

95 %

O

76 %

14

13
H2SO4

95 %

S

NBS

S

57 %

O

15

O

S

Br

S

Br
16

10

2. Development of Heteroaromatic-fused TTFs

DDQ

12

P(OMe)3

S

N Ts

O

82%

S

97%

Ts N

17

S

S

S

S

NaOMe
N Ts

quant.

HN

S

S

S

S

18

NH

1a

Scheme 2.2.2. Synthesis of N-alkyl-PyTTF.
R-Br or R-I
NaH

1a

S

S

S

S

N R

R N

N-alkyl-PyTTF

Table 2.2.1. Synthetic yields and properties of N-alkyl-PyTTFs
solubility
sublimation
Ryield/ %mp / ºC/ g L-1
at ~10-3 Pa
1b CH379252< 0.001✓
1c n-C4H971
198
10
1d n-C8H17
75
164
29
1e n-C12H25
61
127
50
1f
n-C16H33
79
122
4
1g n-C20H41
53
121
0.4
in toluene at r.t.
(a)

(b)

(c)

(d)

Figure 2.2.2. Molecular structures of 1c (a), 1d (b), 1e (c), and 1f (d).

interactions between the groups, consequently reducing the solubility of the molecules. The increased hydrocarbon-like character in the molecules are also seen in the melting points which
dramatically decrease with increment of alkyl chain lengths.
Among the PyTTFs investigated in this study, unsubstituted (1a), n-butyl (1c), n-octyl (1d), ndodecyl (1e), and n-cetyl (1f) derivatives afforded single crystals of high quality that could befully characterized by single-crystal XRD. (The crystallographic data are listed in Table A1 in
Appendix). Figure 2.2.2 shows the molecular structures of 1c-f, where the PyTTF cores are almost flat in all compounds but the alkyl groups have different conformations. The n-butyl
112.2 Bis(N-alkyl-pyrrolo[3,4-d])tetrathiafulvalenes (N-alkyl-PyTTFs)

S

C10H21S(Au)

D (Au)

iconductor (600 nm)
mide Dielectric(640 nm)
G (IZO)S (Au)

D (Au)

Organic semiconductor
SiO2
n+-Si (Gate electrode)

olycarbonate Film

Figure 2.2.3. Schematic representation of
OFET devices.
Figure 2.2.4. FET characteristics of 1g (n-icosyl) (a) output and(b) transfer characteristics at Vd = -60 V.

Table 2.2.2 FET characteristics of devices based on 1a-g on the Si/SiO2
substrate.
OFETb
R
processa
Vth / Vµ /cm2V-1s-1
Ion/Ioff
1aH
vac
no field-effect
1b
CH3
vac
40
5×10-51cn-C4H9spin
30
2×10-5101
1dn-C8H17spin
no field-effect1en-C12H25
spin20
6×10-3102-3
1f
n-C16H33
spin
10
8×10
104
-2
1g
n-C20H41
spin
8
1×10
104
a spin = spin-coat, vac = vacuum-deposition. b Typical value. Devices are
top-contact configurations with Au electrodes. W and L are 1500 and 50 µm,
respectively.

groups in 1c, taking an all-anti conformation, lie on the same plane as that of the PyTTF core,
whereas the n-octyl groups in 1d take a partial gauche conformation, and at the second methylene carbon from the nitrogen atoms in the pyrrole rings, the CH2 moiety is normal to the PyTTF
plane, while the rest of the n-octyl groups lie on almost the same plane as that of the PyTTF
core. These molecular structures most likely minimize the space required, enabling dense packing in bulk crystals (vide infra). In contrast to the fairly flat molecular structures of 1c and 1d,1e and 1f form stretched and doubly bent molecular structures in which the n-dodecyl groups in
1e and the n-cetyl groups in 1f take a staggered, all-anti conformation that minimizes the stericenergy in the aliphatic chains.
Deposition of Thin Films by Spin-Coating and Evaluation of FET Devices. As previously
mentioned, PyTTF derivatives with alkyl chains longer than n-butyl (except 1g) were solubleenough to be spun from various solvents (e.g., dichloromethane, chloroform, THF, toluene, andchlorobenzene). Among these solvents, toluene enabled the reproducible formation of homogeneous thin films of 1c-f by spin-coating on the Si/SiO2 substrate. In contrast, homogeneous thinfilms of 1g, which has n-icosyl groups and showed a low solubility in toluene at room temperature, were obtained only when a warm solution of 1g in chlorobenzene (ca. 80 °C) was used. On
the top of the thin-films, gold source and drain electrodes are deposited to give top-contact
OFETs (Figure 2.2.3).
Evaluation of the devices was carried out under ambient conditions without any precautions
taken to eliminate air and moisture. Table 2.2.2 summarizes the FET characteristics of the devices evaluated immediately after fabrication. * The data were obtained by averaging several de* The devices were degraded gradually under ambient conditions, owing to the low oxidation potentials (i.e., high HOMO levels) of

1c-1g. See section 2.3.

12

2. Development of Heteroaromatic-fused TTFs

1c (n-butyl)d = 10.2, 8.5 Å

1d (n-octyl)
d = 12.6 Å
Table 2.2.3. Interlayer spacing (d-spacing) of thin-film of 1a-g
d-spacinga / Åδd b / Åc-axisc / Å

1a H
1b CH3no peakno peak-

9.7-

1c n-C4H9
1d n-C8H17
1e n-C12H25

10.2 , 8.5
12.6
21.0

4.0
8.5

8.1
12.6
23.8

1f n-C16H33

26.8

5.8

26.9

R
1e (n-dodecyl)d = 21 Å

1g n-C20H41
32.3
5.5
a d = layer distance, determined on the basis of the (001) peaks in the
XRD patterns. b Increment of d-spacing (δd ) by elongation of alkyl
groups with the addition of four CH2 units; δd = d0 - dn-1. c In the singlecrystal unit cell. 1b and 1g did not form single-crystals sufficient foranalysis.

1f (n-cetyl)d = 27 Å

1g (n-icosyl)
d = 33 Å

0

5

10

15 20
2θ/θ

25

30

Figure 2.2.5. XRD patterns of thin-films of 1c-g.

vices independently fabricated with each compound. OFETs using spin-coated thin-film of 1cwith short alkyl groups showed poor responses with very small field-effect modulation of draincurrent; thus, field-effect mobility (µFET) and on-off ratio (Ion/Ioff) are very low. OFETs based on
1d showed no current modulation on gate bias application, and thus, the FET parameters of thedevices were not determined. Similarly, OFETs based on 1a and 1b fabricated by vacuumdeposition showed no and poor field-effect responses, respectively.
In sharp contrast, 1e-based devices showed a typical p-channel FET response with µFET = 6.8
× 10-3 cm2/Vs and Ion/Ioff = 2 × 102. Spin-coated thin films of 1f and 1g with long alkyl groups
also acted as semiconducting channels in the devices, and the µFETs of these devices were in theorder of 10-2 cm2/Vs with Ion/Ioff 104 (Figure 2.2.4 and Table 2.2.2). These FET characteristics
are fairly good for solution-processible OFET devices and comparable to OFETs based onpoly(3-hexylthiophene) tested in our laboratory.
Evaluation of Molecular Ordering by XRD Studies of Thin Films and Bulk Single Crystals. Figure 2.2.5 shows the XRD patterns of thin films of 1c-g deposited on an Si/SiO2 substrate by spin-coating. Since all the spin-coated films (except 1c) showed a series of peaks as13

2.2 Bis(N-alkyl-pyrrolo[3,4-d])tetrathiafulvalenes (N-alkyl-PyTTFs)

signable to (00l) reflections, it is clear that well-oriented crystalline films were formed in the
spin-coated film. In contrast, the vacuum-deposited film (1a-b) showed no peaks, indicating
amorphous structure. Calculated d-spacings based on the first-layer line are listed in Table
2.2.3. For the thin film of 1c, there exist two intense peaks at 2θ = 8.7º and 10.4º, which correspond to d-spacings of 10.2 and 8.5 Å, respectively, indicating the existence of two crystalline
phases or two different orientations normal to the substrate surface.
The calculated d-spacings were increased with the elongation of the alkyl groups, which can bequalitatively understood by considering that longer molecules form thicker layers. However,focusing on the increment of the d-spacing (δd) by the addition of four CH2 units, a large increase of d-spacing from 1d to 1e is noticeable, which may correspond to a change of molecularorientation. Interestingly, the trend of FET characteristics is also drastically changed at thispoint, and thus, we speculate that the change of molecular orientation in the thin film and the
FET characteristics can be correlated.
To gain further insight into the relationship between molecular orientation and FET characteristics, we examined the structures of 1c-f in bulk single crystals. By comparing the d-spacings
with the cell parameters of the bulk single crystals (Table 2.2.3), I noticed a similarity between
d-spacings and crystallographic c-axes in the bulk single crystals, indicating that the preferredorientation normal to the substrate in the thin films is the c-axis for all the compounds.
Molecular Orientation in Bulk Single Crystals. Regardless of the length of alkyl groups,
the crystal structures of 1c-f are similar to one another, that is, a layer-by-layer structure consisting of alternately stacked aliphatic layers and PyTTF layers (Figures 2.2.6 and Figure 2.2.7).
However, detailed examination of the molecular packing revealed that intermolecular interactions, and consequently intermolecular overlaps, are very much dependent on the compounds
and dominate the carrier transport properties of FET devices. Interestingly, these four crystal
structures can be classified into two groups from the viewpoint of molecular arrangement: one is
1c and 1d with a monoclinic space group (Figure 2.2.6), and the other is 1e and 1f with a triclinic space group (Figure 2.2.7).
In the PyTTF layers of the former two crystal structures with relatively short alkyl groups (1cand 1d), the molecules tilt markedly from the c-axis (Figure 2.2.6, left), resulting in the formation of very thin active semiconducting layers where no stacking of the TTF cores exists owing
to the slipping of molecules along the molecular long-axis direction (Figure 2.2.6, right). There(a)

(b)

c

a

Figure 2.2.6. Molecular arrangement in single crystal of 1c (a) and 1d (b): side view of molecular layers (b-axis
projection, left) and projections of one layer normal to the PyTTF plane (right). Side-by-side S-S short distancesindicated by dashed lines are 3.63 Å for 1c and 3.36 Å for 1d.

14

2. Development of Heteroaromatic-fused TTFs

(a)

(b)

Figure 2.2.7. Molecular arrangement in single crystal of 1e (a) and 1f (b): side view of molecular layers (left) and projections of one layer normal to the PyTTF plane (right). Side-byside S-S short distances indicated by dashed lines are 3.43 Å for 1e and 3.40 Å for 1f.

fore, molecular overlap along the a-axis direction is very small, although there are intermolecular side-by-side interactions through S−S contacts in the b-axis direction (3.63 Å for 1c and 3.36
Å for 1d).
In contrast, in the PyTTF layers of the latter group (1e and 1f), both well-overlapped πstacking between PyTTF moieties in the a-axis direction and transverse intermolecular interaction through S−S contacts (3.43 Å for 1e and 3.40 Å for 1f) was observed (Figure 2.2.7). This
indicates that these two compounds have typical two-dimensional (2-D) TTF conducting layers,
as observed in many highly conducting charge-transfer TTF salts. The existence of 2-D conducting layers on the substrate is considered to be one of the critical prerequisites to realizing
high-performance OFET devices.104,105 From these structural features, it appears that the FET
characteristics of the present PyTTF-based OFETs are well-explained by the electronic nature of
the semiconducting layer.
For inspection of poor crystallinity of thin films of unsubstituted PyTTF (1a) and methyl derivatives (1b), molecular orientation in crystal of 1a was examined (Figure 2.2.8). The arrangement of 1a consist of two different layers which stacked alternately along the a-axis, neither ofwhich has efficient overlap between the π-core. It is very differed from those of the alkylated
PyTTFs where repeated layer-by-layer structures were formed. The structure and the poor FET

15

2.2 Bis(N-alkyl-pyrrolo[3,4-d])tetrathiafulvalenes (N-alkyl-PyTTFs)

(c)

(a)

(b)

(d)

Figure 2.2.8. Single-crystal structure of parent PyTTF (1a): (a) a molecule and (b) view along b-axis. The
crystal consists of two layers of (c) and (d) that stacked alternately along the a-axis.

characteristics of 1a and 1b indicate that the alkyl groups bring the PyTTF the self-assembling
character to form 2-D layer-by-layer structure on the substrate from solution.
Summary. The present study on N-alkyl-substituted PyTTFs (1c-g) gave a good demonstration in which PyTTFs as are potential compounds for OFET applications. The structural analyses clarified the effect of N-alkyl groups in PyTTF system, which enhance not only solubility
but also self-organizing ability in thin film that suitable for FET operation. Molecules 1e-g,
which have long alkyl groups, tend to take 2-D molecular ordering both in the bulk single crystals and in the thin films, whereas 1c and 1d with relatively short alkyl groups also oriented
layer-by-layer structure but have no efficient overlap of π-core. 1a and 1b did not form oriented
structure. The present performance are not best among recently reported high performancesolution-process OFETs (up to µFET = 0.01 cm2/Vs on the 1g). However, an important conclusion that can be drawn from the present study is that an appropriate combination of a πconjugated core based on a small molecule with long alkyl groups can provide organic semiconductors capable of affording high-quality thin films with a well-organized 2-D moleculararray.
Experimental Procedures
General. All chemicals and solvents were of reagent grade unless otherwise indicated. DMF was distilled from CaH2 under reduced pressure. THF was distilled before use. Acetonitrile and hexane were distilled from CaH2 under N2 gas-flow. Trimethylphosphite was treated with sodium for 24 h, then distilled
in an inert atmosphere onto activated molecular sieves 4 Å. o-Chloranil was recrystallized from toluene.
NMR spectra were obtained in deuterated solvents with a JEOL Lambda 400 spectrometer operating at
400 MHz for 1H and at 100 MHz for 13C with tetramethylsilane as internal reference; chemical shifts (δ)
were reported in parts per million. EI-MS spectra were obtained on a Shimadzu QP-5050A spectrometerusing an electron impact ionization procedure (70 eV). MALDI-TOF-MS spectra were obtained on a
Shimadzu KRATOS KOMPACT MALDI spectrometer. The molecular ion peaks of the sulfur-containing
compounds showed a typical isotopic pattern, and all mass peaks are reported based on 32S.
Synthesis of N-alkyl-PyTTF:
Bis(N-n-octylpyrrolo[3,4-d])tetrathiafulvalene (1d). Bis(pyrrolo[3,4-d])tetrathiafulvalene (1a) (434
mg, 1.54 mmol) was dissolved in dry DMF (50 mL), cooled to 0 °C, and degassed by an argon stream for
15 min. 1-Bromooctane (2.19 g, 15.4 mmol) was added to the mixture, and then sodium hydride (0.485 g,
11.1 mmol) washed with dry hexane was added in one portion. The mixture was stirred for 1.5 h at 0 °C.
Brine (200 mL) was added, and the resulting yellow precipitate was collected by filtration, washed withwater, and dried in vacuo. The yellow solid was collected and purified by column chromatography (silica
gel, CH2Cl2) to give 1d as a yellow solid (Rf = 0.7). Recrystallization from hexane gave analytically pure
1d as yellow needles (380 mg, 75%): mp 165.5- 166.5 °C; 1H NMR (CDCl3) δ = 0.87 (t, J = 7.0 Hz, 6H),
16

2. Development of Heteroaromatic-fused TTFs

1.26 (m, 20H), 1.71 (m, 4H), 3.79 (brs, 4H), 6.43 (s, 4H); 13C NMR (DMSO-d6) δ = 13.9, 22.0, 25.9,
28.5, 30.8, 31.1, 49.8, 113.2, 116.5, 118.9; MS (EI, 70 eV) m/z = 506 (M+); Anal. calcd for C26H38N2S4:
C, 61.61; H, 7.56; N, 5.53. Found: C, 61.60; H, 7.54; N, 5.54. CV: Eox1/2 = +0.33 V, +0.75 V vs Ag/AgCl.
Bis(N-n-dodecylpyrrolo[3,4-d])tetrathiafulvalene (1e). The title compound was synthesized in a similar manner to 1d to give yellow needles (hexane) in 61% yield: mp 126.5-127.5 °C; 1H NMR (CDCl3) δ= 0.88 (t, J = 7.0 Hz, 6H), 1.25 (m, 36H), 1.71 (m, 4H), 3.79 (brs, 4H), 6.43 (s, 4H); 13C NMR (C6D6) δ =14.4, 23.2, 26.7, 29.5, 29.9 (×2), 30.08, 30.14 (×2), 31.5, 32.4, 50.3, 112.3, 119.3, 120.8; MS (EI, 70 eV)
m/z = 618 (M+); Anal. calcd for C34H54N2S4: C, 65.96; H, 8.79; N, 4.53. Found: C, 65.94; H, 9.08; N,
4.53. CV: Eox1/2 = +0.34 V, +0.75 V.
Bis(N-n-cetylpyrrolo[3,4-d])tetrathiafulvalene (1f). A yellow powder from hexane; 79% yield; mp120.0-121.0 °C; 1H NMR (CDCl3) δ = 0.88 (t, J = 6.8 Hz, 6H), 1.25 (m, 52H), 1.57 (m, 4H), 3.79 (brs,
4H), 6.49 (s, 4H); 13C NMR (C6D6) δ = 14.4, 23.1, 26.7, 29.5, 29.9 (×2), 30.08, 30.16 (×2), 30.22 (×4),
31.5, 32.4, 50.4, 112.3, 119.4, 120.8; MS (EI, 70 eV) m/z = 730 (M+); Anal. calcd for C42H70N2S4: C,
68.98; H, 9.65; N, 3.83. Found: C, 68.97; H, 9.69; N, 3.86. CV: Eox1/2 = +0.33 V, +0.75 V.
Bis(N-n-icosylpyrrolo[3,4-d])tetrathiafulvalene (1g). A yellow powder from hexane; 53% yield; mp
120.7-121.7 °C; 1H NMR (CDCl3) δ = 0.88 (t, J = 6.8 Hz, 6H), 1.25 (m, 68H), 1.71 (m, 4H), 3.79 (t, J )6.8 Hz, 4H), 6.43 (s, 4H); 13C NMR (100 MHz, C6D6) δ = 14.3, 23.1, 26.8, 29.5, 29.82, 29.84, 30.05,
30.13 (× 2), 30.21 (× 8), 50.4, 112.3, 119.6, 120.9; MS (MALDI-TOF) m/z = 842.51 (M+); Anal. calcd for
C50H86N2S4: C, 71.20; H, 10.28; N, 3.32. Found: C, 71.06; H, 10.25; N, 3.21. CV: Eox1/2 = +0.34 V, +0.76
V vs Ag/AgCl.
Synthesis forward to PyTTF:
Potassium O-isopropyl dithiocarbonate (13). KOH grains (22.4 g, 400 mmol) was placed into a 1,000mL three-neck flask and dissolved in iPrOH (600 mL) with heating ca. 60 °C for 30 min. After the grains
were dissolved completely, the solution was cooled down below 0 °C using ice-NaCl bath, and CS2 wasadded slowly over 10 min. Then the ice-bath are removed, the reaction mixture was stirred with a mechanical stirrer overnight (ca. 12 h). The resulting light-yellow solids were collected by filtration by icecooled iPrOH to give 13 (ca. 55 g, >80%). 1H NMR (400 MHz, DMSO-d6) δ = 1.156 (d, J = 6.4 Hz, 6H),
5.450 (sept, J = 6.4 Hz, 1H).
2-Oxo-1-methylpropyl O-isopropyl dithiocarbonate (14). EtOH (500 mL) was pour into a 1,000 mL
three-neck flask, and degassed by reducing pressure using water aspirator for 10 min. After an additionof 13 (24.2 g, 0.139 mol), the suspension was cooled down to 0 °C. 3-chloro-2-butanone (14.77 g, 14 mL,
0.139 mol) was added and allowed to stir overnight (ca. 16 h) at room temperature. The resulting mixture
was filtrated (× 2) to remove the KOH salt. The filtrated solution was evaporated in vacuo and then redissolved in CH2Cl2 (300 mL), which was washed with water (300 mL × 3), saturate NaClaq (300 mL ×1), and dried with MgSO4. Evaporation in vacuo gave 14 as orange oil (27.3 g, 95%).1H NMR (400 MHz,CDCl3) δ = 1.394 (d, J = 6.3 Hz, 3H), 1.398 (d, J = 6.3 Hz, 3H), 1.470 (d, J = 7.3 Hz, 3H), 2.316 (s, 3H),
4.427 (q, 7.3 Hz, 1H), 5.729 (sept, J = 6.3 Hz, 1H).
3,4-Dimethyl-1,3-dithiole-2-one (15). Under N2 atmosphere, 14 (27.3 g, 0.132 mol) was added into
conc. H2SO4 (150 mL) at 0 °C before stirred for ca. 1.5 h at 75 °C. At the conclusion of the reaction, themixture was cooled to room temperature and poured into ice (500 mL) and stirred for a while. The resulting black solution was extracted with CH2Cl2 (200 mL × 1, 100 mL × 2), combined organic phase waswashed with water (200 mL × 3), saturated NaCl aq (200 mL), then dried with MgSO4. After evaporation
distillation under reduced pressure (135 °C, 12 mmHg) afforded 15 as colorless crystals (18.1 g, 94 %).
1H NMR (60 MHz) δ = 2.12 (s).
4,5-Bis(bromomethyl)-1,3-dithilole-2-one (16). Under N2 atmosphere, 15 (14.6 g, 100 mmol) andNBS (35.6 g, 200 mmol) was placed in a 100 mL three-neck flask with CCl4 (50 mL), was irradiated withhalogen lump for ca. 1 h. The resulting mixture was evaporated to remove the CCl4 and dissolved in
CH2Cl2. The solution was washed with saturated NaHCO3 aq (400 mL × 1), water (300 mL ×3) and sat.
NaClaq (300 mL × 2), it was dried with MgSO4. Recrystallization from CHCl3/hexane gave 16 as whitesolids (1st crystallization.: 14.5 g, 48 %, 3rd and 4th crystallization.: 2.91 g, 10 %, total: 17.4 g , 57 %).
1H NMR (400 MHz) δ = 4.39 (s).
4,6-Dihydro-N-tosyl-(1,3)-dithiolo[4,5-c]pyrrole-2-one (12). Under nitrogen atmosphere, 16 (9.12 g,
30 mmol) was dissolved in DMF (600 mL) and deaerated by argon stream for 10 min. Finely ground sodium tosylaminde (12.2 g, 63 mmol) was added into the solution at 0 °C: the resulting solution immediately turned yellow in color. After 10 min, ice-bath was removed, and the solution was stirred for additional 50 min at room temperature. Then the solution was poured into saturated NaCl aqueous solution (1L), whereupon a precipitated solid was collected by filtration and washed with water. The solid dried invacuo was dissolved into CH2Cl2, and the solution was filtrated to remove impurity and then purified bycolumn chromatography (silica-gel, CH2Cl2, Φ 6 cm × 6 cm, Rf = 0.4) to give a practically pure sample aswhite solid (7.10 g, 76 %). Mp 175.6–176.2 °C (174.0–174.5 °CS2) ; 1H NMR (400 MHz, CDCl3) δ =17

2.2 Bis(N-alkyl-pyrrolo[3,4-d])tetrathiafulvalenes (N-alkyl-PyTTFs)

2.45 (s, 3H), 4.49 (s, 4H), 7.37 (d, J = 8.2 Hz, 2H), 7.75 (d, J = 8.2 Hz, 2H); MS (EI, 70 eV) m/z = 313(M+); IR (KBr) ν = 1694 cm–1 (C=O).
N-tosyl-1,3-dithiolo[4,5-c]pyrrolo-2-one (17). Synthesis of 17 with DDQ was carried out according topaper with 97 %.99 Here synthesis of 17 with cheaper o-chloranil reagent are described. Under nitrogen
atmosphere, a mixture of 12 (5.33 g, 17 mmol) and o-chloranil (20.9 g, 85 mmol) in o-dichlorobenzene(40 mL) was stirred at 195 °C until all the starting material was consumed (ca. 1.5 h). The reaction mixture was concentrated in vacuo, and the resulting solid was dissolved in CH2Cl2 (100 mL) and filtrated
through a Celite pad (1.5 cm). The filtrate was washed consecutively with 10 % sodium hydroxide aqueous solution (200 mL × 2) and saturated NaCl aqueous solution (200 mL × 3), and then dried overMgSO4. 17 was purified by column chromatography (silica-gel, CH2Cl2, Φ 6 cm × 5 cm, Rf = 0.5) andrecrystallization from toluene/hexane as a white solid (4.02g, 76 % up to the second crops). Mp 179.3–
179.8 °C (178.5–179.0 °CS2); 1H NMR (400 MHz , CDCl3) δ = 2.44 (s, 3H), 7.23 (s, 2H), 7.34 (d, J = 8.4
Hz, 2H), 7.79 (d, J = 8.4 Hz, 2H); MS (EI, 70 eV) m/z = 311 (M+); IR (KBr) ν = 1716 cm–1 (C=O).
OFET Device Fabrication. Fabrication of OFET devices was carried out in ambient conditions. Topcontact OFET devices were fabricated on a heavily doped n+-Si (100) wafer, with a 200 nm thermallygrown SiO2 (Ci ) 17.3 nF cm-2) as dielectric layer. A semiconductor layer was deposited on the substrateby spin-coating for soluble compounds (1c - 1g) and by vacuum-deposition (~3 × 10-3 Pa) for insolublecompounds (1a - 1b). Toluene was used as the spin-coating solvent except for n-icosyl PyTTF (1g) which
was dissolved in hot chlorobenzene (~80 ºC) due to relatively low solubility. On top of the organic thinfilms gold (80 nm thick) was deposited through a shadow mask. Characteristics of the OFET devices
were measured at room temperature in air, and µFET was extracted from a drain current (Id) of transfer
characteristics in saturation regime using the equation2
Id = (WCi/2L) µFET (Vg - V
th)Commercialpoly(3-hexylthiophene)(purchasedfrom Aldrich) was used for device fabrication.S3 0.5 wt%
where
C
is
the
capacitance
of
the
and
Vg and Vth are the gate and threshold voltages, rei
2 insulator,
solution of P3HT in chlorobenzene was spun on SiO
the OTS-treated
Si/SiO2 substrates (1000 rpm, 30 sec.),
spectively.
andon top of the thin films, gold source and drain electrode were vacuum deposited through the shadow
For devices based on P3HT, commercial poly(3-hexylthiophene) (purchased from Aldrich) was used formask.
device0.5 wt%solutionof P3HTin chlorobenzenewasspun
From FET fabrication.
characteristics shownin FigureS3, field-effectmobility(!FET) was determined
to be
0.02 oncm2the OTS-treated Si/SiO2 sub32 The FET characteristics are shown in Fig–1 –1
strates
(1000
rpm,
30
sec.),
followed
by
the
same
procedure.
V s for typical devices.
ure 2.2.9.
(a)

(b)

-5

-4 10

!"#$#
%&'#!

0.0001
Vd = -60 V

-5

-3 10

%)'#!

-5

10

-Id / A

-5

-2 10

d

I /A

%('#!

-6

10

%*'#!

-5

-1 10

%+'#!
0

%,'#!
%'#!
0

-10

-20

-30
V /V
d

-40

-50

-60

-7

10

20

0

-20

-40

-60

Vg / V

FigureS6.2.2.9.
FET characteristicsof P3HT-based
OFET: output(left)
Figure
FET characteristics
of spin-coatedrrP3HT
onand transfer (light) characteristics.

bare Si/SiO2 substrates. Typical extracted µFET is ca. 0.02 cm2/
Vs.

Solubility Determination. To a precisely weighed sample (ca. 5 or 10 mg), toluene was added in inReferences
crements
of 0.1mL,
andR.;theBoudreaux,resultingshakenroom temperature (20-25 °C).
S1. (a) Brédas,J.-L.;
Silbey,D. mixture
S.; Chance,was
R. R.
J. Am. andChem.sonicatedSoc. 1983, at
105,
The 6555–6559.
total amountof tolueneJ.;required
the entire
solid sample(b) Pommerehne,
Vestweber, toH.; dissolve
Guss, W.; Mark,R. F.; Bässler,
H.; Porsch,wasM.; converted into solubility (g/L) Daub, J. Adv. Mater. 1995, 7, 551–554.
Measurements.
measurementsof thin films fabricated on the Si/SiO2
S2.X-rayOng, B.Diffraction
S.; Wu, Y.; Liu,(XRD)
P.; Gardner
S. J. Am. Chem. Soc.XRD
2004, 126,
3378–3379.
substrate
obtainedwith a A.
RigakuRINT
2200
with a Cu KR source (λ = 1.541 Å) in
S3. Bao, Z.;wereDodabalapur,
A.; Lovinger,
J. Appl. Phys.
Lett.
1996, diffractometer
69, 4108–4110.
air. The d-spacings were calculated by the Bragg equation: λ = 2dsinθ. Single-crystal X-ray structural
analyses were conducted on a Rigaku Rapid-S Imaging Plate (Mo Kα radiation, λ = 0.71069 Å, graphite
monochromator, T = 200 K, and 2θmax = 55.0°) for 1c-f, and on a Rigaku MSC Mercury CCD (Mo Kα
radiation, λ = 0.71075 Å, graphite monochromator, T = 294 K, 2θmax = 55.0°) for 1a. Detail crystallographic data are shown in Table A1 (in Appendix). The structure was solved by direct methods(SIR92)106. Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were included in thecalculations but not refined.107 All calculations were performed using the crystallographic software package TeXsan.108
Cyclic voltammetry (CVs). CVs were recorded on a Hokuto Denko HA-301 potentionstat and a Hokuto Dneko HB-104 function generator in benzonitrile containing tetrabutylammonium hexafluorophosphite (TBAPF6, 0.1 M) as the supporting electrolyte, at a scan rate of 100 mV/s. Counter and working

5.

S5

18

2. Development of Heteroaromatic-fused TTFs

electrodes were made of Pt, and the reference electrode was Ag/AgCl. Cyclic voltammograms of 1c-g areshown in Figure 2.2.10. Oxidation potentials of 1c-g were identical with in experimental error (Figure
2.2.10 and Table 2.2.4). 1a showed slightly lower oxidation potential than those of 1c-g.

Figure 2.2.10. Cyclic voltammograms of 1c-g.
Table 2.2.4. Oxidation potentials
oxidation potentiala / V
E11/2
E21/2
onset
1a
-0.15
1c-g
-0.18
a vs. Fc/Fc+.

19

-0.06
-0.11

0.39
0.31

2.3 Bis(N-acyl-pyrrole[3,4-d])tetrathiafulvalenes (N-acyl-PyTTFs)

2.3

Bis(N-acyl-pyrrole[3,4-d])tetrathiafulvalenes (N-acyl-PyTTFs)
0.0001

-5

10

(b)
O

S
N R

S

S

1: N-n-alkyl-PyTTF

R

S

S

-6

10

N

N

1a: R = H
1b: R = CH3
1c: R = n-C4H9
1d: R = n-C8H17
1e: R = n-C12H25
1f: R = n-C16H33
1g: R = n-C20H41

S

R

S

O

fresh
2h later
4h later
19h later

d

S
R N

I /A

(a)

-7

10

2: N-n-acyl-PyTTF

-8

10

-9

10

Figure 2.3.1. Structures of (a) N-alkyl- and (b) N-acyl-PyTTFs with thecalculated HOMOs by DFT method (B3LYP/6-31G(d)).111 The calculated
HOMO energy levels are ca. -4.2 eV for 1 and -4.9 eV for 2.

-60

-40

-20

0

20

V /V
g

Figure 2.3.2. FET characteristics of N-n-icosyl-substituted
PyTTF (1g), R = n-C20H41) depending on the duration of exposure to ambient conditions.

Introduction. N-alkyl-substituted TTF were successfully developed as solution-processible
semiconductors, as described in section 2.2 (1, Figure 2.3.1a). The fabricated FET devices withthe spin-coated thin-films of 1g showed µFETs as high as 0.01 cm2/Vs in air. Although their FET
performances were relatively good for solution-processed OFETs without intensive optimizationin device fabrication, their sensitivity to ambient air was a severe drawback: shortly after exposure to air, off-current of the OFETs rose significantly (Figure 2.3.2). The poor stability in air islikely due to their low oxidation potentials (E1/2 = -0.11 V vs. Fc/Fc+), in other words, thehigher-lying energy level of the highest molecular occupied orbital (HOMO) makes 1 susceptible to air oxidation.109,110
In order to stabilize 1-based organic semiconductors, I intended to introduce electronwithdrawing groups on the nitrogen atom instead of rather electron-donating N-alkyl groups.
Molecular orbital calculations predicted that N-acyl-substituted PyTTFs (2) would have a lower
HOMO energy level than that of 1with keeping the large HOMO coefficients on the sulfur atoms (2.3.1b).111 In this section, I describe the synthesis of N-acyl-PyTTFs and the characteristics of FET devices using their vapor-deposited thin-films.
Synthesis and Characterization. The N-alkylation of the parent PyTTF (1a) was easily doneby the reaction of 1a with alkyl halide in the presence of sodium hydride.15 Analogously, introduction of acyl substituents on 1a was first attempted under a similar condition using acyl chloScheme 2.3.1. Synthesis of 2. The yields are described in Table 2.3.1.
O
R

HN

S

S

S

S

1a

NH

NaH or Et3N

Cl

O
R

O
O

R

NaH

R

S

S

S

S

O
N

N
O

R

2a-c
Table 2.3.1. Properties of synthesized 2a-c.
solubilityRyield / %/ g L-1
2a CH32b n-C5H112c n-C12H25

methylhexanoyl
dodecanoyl

60
56
74

<10-4
0.010
0.002

sublimation
at ~10-3 Pa✓✓
in toluene at r.t.

20

2. Development of Heteroaromatic-fused TTFs

Figure 2.3.3. Molecular structure of 2b in the single crystal. Two crystallographically independent molecules
exist.

(a)(b)

Figure 2.3.5. AFM images of evaporated thin filmsof (a) 2a on HMDS-treated substrate and (b) 2b on
ODTS-treated substrate (Tsub = rt).
(a)

(b)

Figure 2.3.4. Cyclic voltammograms of (blue line)
N-alkylated-PyTTF (1c-g) and (red line) N-hexanylPyTTF (2b).

Figure 2.3.6. XRD patterns of evaporated thin films of
2a on HMDS-treated substrate (a) and 2b on ODTStreated substrate (b) (Tsub = 100 °C).

rides. The reaction, however, gave a complex mixture, and the acylated 2 was not isolated. Concurrent hydrochloric acid may react with the TTF part, inducing decomposition of the TTF
framework. Instead of acyl chloride, the use of acid anhydrides turned out to give 2 successfully
in reasonable isolated yields (Scheme 2.3.1). Thus, N-acetyl- (2a), n-hexanoyl (2b), and ndodecanoyl-PyTTF (2c) were obtained. All new derivatives were fully characterized by spectroscopic and combustion analyses.
Unambiguous structural determination of N-n-hexanoyl derivative 2b was achieved bysingle-crystal X-ray analysis (see Experimental Procedures for detail). In the crystal structure,
two crystallographically independent molecules exist (Figure 2.3.3). Although the conformations of n-C5H11 chains in two molecules are different, the central core parts of the molecules
are similar to each other: the carbonyl moieties are in the same plane that the PyTTF part defines, indicating that the conjugation effectively extends to the N-acyl moieties.
Oxidation Potential. Cyclic voltammograms of 2 showed two reversible oxidation couples
typical for the TTF derivatives (Figure 2.3.4). As expected, the oxidation peaks of 2a-c (E11/2 =+0.25 V vs. Fc/Fc+) were shifted positively by more than 0.35 V compared with those of N-

21

2.3 Bis(N-acyl-pyrrole[3,4-d])tetrathiafulvalenes (N-acyl-PyTTFs)
(b)

(a)

(d)

(c)

extend Hükel
p = -40 meV
q = -45 meVcr = -47 meV
s = -45 meV
total = 135 meVDFT
p = -33 meV
q = -25 meV
r = 45 meV
s = 43 meV
total = 117 meVFigure 2.3.7. Molecular arrangement of 2b in crystal. (a) b-Axis projection to show layer-by-layer structure along
c-axis. (b) a-axis projection. Stacking columnar structure along the a-axis. Black dash lines show inter columnarhydrogen bond like C=0•••H-C contacts (2.62 Å). Green dash line show S-S inter columnar contacts shorter than thesum of Van der Waals radii (3.40 ~ 3.57 Å). (c) Molecular long-axis projection to show many S-S contacts (d) View
along normal to the π-plane. Overlap of π-core appears small.

alkyl-PyTTFs (1c-g), indicating a significant decrease of the energy level of the HOMO(EHOMO). The EHOMOs of 2a-c were estimated from the onset of first oxidation waves (Eonset =+0.15 V) to give EHOMO = -4.95 eV below the vacuum level, which is almost comparable withthat of DT-TTF (EHOMO = -4.98 eV).112,113
Solubility and Fabrication of Thin-Films, and their Morphologies. In contrast to thehighly soluble N-alkyl-PyTTFs (1c-g), the solubilities of N-acyl-PyTTFs (2a-c) even with thelong alkyl chains were very poor: the solubilities in toluene at room temperature were 0.01 g/L
for 2b and 0.002 g/L for 2c, which were lower than those of 1c-g by more than two orders ofmagnitude. Although the reason for extremely low solubility of 2a-c are not clear, I speculatethat incorporation of the electron-negative carbonyl groups in the conjugation of PyTTF core
and thus induced planer and rigid molecular structure confirmed by X-ray structural analysis.
Poor solubility of 2a-c in organic solvents prevented deposition of thin-films by solutionprocess. Instead, vacuum-deposition of 2a-c gave homogeneous thin-films with metallic luster
on Si/SiO2 substrates except for 2c, which thermally decomposed during the deposition. The
thin-films consist of well-ordered polycrystalline grains as confirmed by the atomic force mi22

2. Development of Heteroaromatic-fused TTFs

croscopy (AFM) images (Figure 2.3.5). Such morphology is generally a prerequisite for highperformance OFETs.114-116
The XRD measurements of the thin-films of 2a and 2b indicate that the films have crystalline
order in the direction of the substrate normal (Figure 2.3.6). The calculated interlayer distances(d-spacing) are 17.0 Å for 2a and 22.6 Å for 2b. The d-spacing of 2b is almost the same as thelength of the crystallographic c-axis in the bulk single crystal, which suggests that moleculararrangements in the thin-film and in the bulk single-crystal are similar to each other.
Molecular Arrangement in the Bulk Single-Crystal. The molecular arrangement of Nhexanoyl-PyTTF (2b) consists of layer-by-layer structure along crystallographic c-axis as seenin the structures of the long alkyl-substituted PyTTFs (Figure 2.3.7a). The overlap of π-cores isrelatively small (Figure 2.3.7d). However, all four sulfur atoms in the TTF core contact withthose of the neighboring molecules, indicating strong intermolecular interactions via S-S contacts (Figure 2.3.7b and c). Although all hydrogen atoms are not refined analytically, the hydrogen bond like intermolecular interaction between the pyrrole-H and the acyl-C=O are present,
which may contribute the dense packing between the TTF cores.

(a)

(b)

-5 10-5

-5

-4 10

10-4
Vg = -60 V

10

10-6
-I / Add-I / A-3 10-5 -50 V-2 10-5 -40 V

10-7
10-8

-1 10-5 -30 V-20 V
0 0 V,10V-60 -50

Vd = -60 V

-5

-40

-30
Vd / V

-20

-10

0

10-9
-60

fresh
7h
20h
30h
-40

-20

0

20

Vg / V

Figure 2.3.8. FET characteristics of 2b-based OFETs. (a) Output characteristic. (b) Transfer characteristics with the dependence of duration of exposure to the ambient conditions.
Table 2.3.2. FET characteristics of 2a- and 2b-based devicesa.
compd
substrateb
Tsub / ºCµFET / cm2 V–1 s–1
Ion/Ioff
Vth / V
2aHMDS
rt
2.5 × 10–2
104
–25
60
6.4 × 10–4
5 × 103
1.0
100
5.2 × 10–2
103
–11
OTS
rt
5.1 × 10–4
104
–8.0
–3
60
5.5 × 10
104
–8.0
100
2.1 × 10–3
103
3.0
ODTS
rt
1.8 × 10–4
103
1.0
60
6.9 × 10–2
5 × 104–14100
3.7 × 10–3
5 × 102
2.0
2b
HMDS
rt
2.2 × 10–2
5 × 103–1560
2.2 × 10–2
104
–7.0
100
6.8 × 10–2
104
–8.0
OTS
rt
3.3 × 10–2
104
–10
60
7.6 × 10–2
105
–3.0
100
6.2 × 10–2
5 × 104
–2.0
ODTS
rt
1.5 × 10–2
104
–11
60
4.4 × 10–2
5 × 105–9.0
100
8.1 × 10–2
5 × 105–4.0
a Top-contact configuration with W and L were 1500 µm and 50 µm respectively. b SiO
2
surface were treated with alkylsilyl reagents. See Experimental Procedures.

23

2.3 Bis(N-acyl-pyrrole[3,4-d])tetrathiafulvalenes (N-acyl-PyTTFs)

FET characteristics. On the top of the evaporated thin-films, gold source and drain electrodes were thermally deposited through a shadow mask to define the channel of the transistors
with a length (L) of 50 or 100 µm and width (W) of 1.5 mm, respectively. The OFET devices
were evaluated under ambient conditions, and the 2a- and 2b-based devices showed normallyoff FET characteristics (Figure 2.3.8). Extracted FET mobilities from the saturation regime are
up to 0.05 cm2/Vs for 2a-based devices and 0.08 cm2/Vs for 2b-based devices, respectively (Table 2.3.2).
As already mentioned, the 1g-based OFETs suffered from severe air-oxidation: although the
fresh device showed typical normally-off FET characteristics, off-current increased significantly
after exposure to air for 2 h by more than two orders of magnitude (Figure 2.3.2). In contrast,
the present 2a- and 2b-based devices show improved air-stability. As shown in Figure 2.3.8b,
the devices showed typical normally-off FET responses even after exposure to ambient lab conditions for more than 30 h. This stabilization can be accounted for the lowered HOMO energy
level of 2 caused by introduction of the electron-withdrawing acyl groups on the PyTTF core.
Summary. I successfully synthesized N-acyl-PyTTFs (2) and evaluated the FET devices
based on their vapor-deposited thin-films. The electron-withdrawing acyl groups introduced
brought about lower HOMO energy level than those of N-alkyl-PyTTFs (1c-g), which contributes to stabilization of 2a and 2b-based FET devices under ambient conditions without degrading the FET characteristics. These results indicate that the tuning of the energy level of HOMO
is important in developing air-stable p-channel organic semiconductors.
Experimental Procedures
General: All chemicals and solvents are of reagent grade unless otherwise indicated. DMF was distilledfrom CaH2 under reduced pressure. Melting points were measured with Yanaco MR approximately. Nuclear magnetic resonance spectra were obtained in deuterated chloroform with a JEOL Lambda 400 spectrometer operating at 400 MHz for 1H and 100 MHz for 13C or a JEOL EX 270 spectrometer operating at
270 MHz for 1H and 68 MHz for 13C with TMS as internal reference; chemical shifts (δ) are reported inparts per million. EI-MS spectra were obtained on a Shimadzu QP-5050A spectrometer using an electron
impact ionization procedure (70 eV). The molecular ion peaks of the sulfur-containing compounds
showed a typical isotopic pattern, and all the mass peaks are reported based on 32S.
Synthesis of N-acyl-PyTTFs:
Bis(N-methylpyrrolo[3,4-d]tetrathiafulvalene (2a). Under N2 atomosphere, anhyd DMF (30 mL)
solution of 1a (1 mmol) in a 100 mL round-bottom flask was degassed by N2 stream for 15 min. Acetic
anhydride (7.7 mL, 7 mmol) was added, subsequently sodium hydride (168 mg, 7 mmol, washed withhexane) was added in one portion at 0 °C. The yellow forming solution was allowed to continue stirring
for approximately 18 h. The resulting mixture was poured into brine (100 mL) and filtrated with water,then dried in a decicator. The solid was washed with MeOH to give a analytically pure compound of 2a(298 mg, 81 %). Sample for device fabrication was purified by sublimation under reduced pressure: coldfinger sublimation gave 98 mg from 238 mg (41 %), and subsequent temperature-gradient vacuum sublimation to give yellow crystals (40 mg from 72 mg (56 %). Mp > 300 °C; 1H NMR (270 MHz, DMSO-d6,
35 °C) δ = 1.358 (s), 7.479 (s); MS (EI, 70 eV) m/z = 366 (M+), 324 (M+ - COCH2), 282 (M+ (COCH2)2); IR (C=O) 1708 cm-1; Anal. calcd for C14H10N2O2S4: C,45.88, H, 2.75, N, 7.64%. Found forC, 45.86; H, 2.75; N, 7.64%.
Bis(N-n-hexanoylpyrrolo[3,4-d])tetrathiafulvalene (2b). Under N2 atmosphere, 1a (0.5 mmol) in a 50
mL round-bottom flask was dissolved in anhyd DMF (30 mL), cooled to 0 °C, and degassed by argon
stream for 15 min. Hexanoic anhydride (1.07 g, 1.16 mL, 5 mmol) was added, and then sodium hydride(120 mg, 5 mmol, washed with hexane) was added in one portion. The foaming, yellow mixture wasstirred for 1 h at 0 °C and then allowed to warm up to room temperature. The mixture was poured intobrine (150 mL). The formed sticky and olive-color precipitates were collected by filtration, and washedwith water, and then dried in a decicator. The olive-color solids were dissolved in heated CHCl3 and filtrated using a celite (1 cm) in hot. Recrystallization from CHCl3 gave 16 as olive-color solids (133 mg, 56%). Higher purified sample was obtained by temperature-gradient vacuum sublimation (source temperature: 220 °C under ~10-3Pa) as yellow solids (83 mg from 127 mg, yield of sublimation: 65 %). Mp. 166.7- 168.2 (with decomposition); 1H NMR (400 MHz, CDCl3 at 50°C) δ = 0.917 (t, J = 6.8 Hz, 6H),

24

2. Development of Heteroaromatic-fused TTFs

1.3568/1.3653/1.3745/1.3812/1.3928 (m, 7H), 1.772 (t, J = 7.4 Hz, 4H), 2.715 (t, J = 7.4 Hz, 4H), 7.109(bs, 4H); 13C NMR (400 MHz, CDCl3 at 50°C) δ = 14.0, 22.5, 24.5, 34.3, 52.0, 109.7, 121.3, 126.7; MS(EI, 70 eV) m/z = 478 (M+), 380 (M+ - COC5H11), 282 (M+ - 2×COC5H11): Anal. calcd for C22H26N2O2S4:
C, 55.20; H, 5.47; N, 5.85; S, 26.79%. Found: C, 55.02; H, 5.58; N, 6.12; S, 26.67%. IR 1699 cm-1.
Bis(N-n-dodecanoyl[3,4-d])tetrathiafulvalene (2c). Under N2 atmosphere, anhyd DMF (50 mL)
solution of a 1a (0.5 mmol) in a 100 mL round-bottom flask was degassed by argon stream for 15 min.
Lauric anhydride (1.34 g, 3.5 mmol) was added and the vessel was warmed up (~40 °C) to dissolve thereagent, then cooled to room temperature before sodium hydride (84 mg, 3.5 mmol, washed with hexane)
was added in one portion. The foaming and yellow mixture was stirred for 3 h at room temperature. After
addition of sodium hydride (~80 mg) it was allowed to stir overnight (ca. 16 h) at room temperature. The
resulting forming and yellow reaction mixture was poured into brine (100 mL) and filtrated with water,
and then the solids were dried in a decicator. The yellow solids were reprecipitated from PhCl / hexane,
and pass through flash column-chromatography in hot (selite - silica-gel – selite three layers, CHCl3) followed by recrystallization from PhCl to give yellow crystals (238 mg, 74%). Mp. 147.7 - 149.6 °C; 1H
NMR (270 MHz, CDCl3, at 50 °C) δ = 0.880 (t, J = 6.7 Hz, 6H), 1.269 (m), 1.763 (t, J = 6.9 Hz, 3H),
2.712 (t, J = 7.1 Hz, 3H), 7.107 (s, 3H); MS (EI, 70eV) m/z = 646 (M+); IR (KBr) 1697 cm-1; Anal. calcd
for C34H50N2O2S4: C, 63.11; H, 7.79, N, 4.33. Found for C, 63.11, H, 7.62, N, 4.31.
Lauric anhydride (Dodecanoic anhydride) (CH3(CH2)10CO)2O). A mixture of lauric anhydride (5.01
g, 0.25 mmol) and acetic anhydride (5.01 g, 50 mmol, 4.7 mL) was refluxed over 2 h. Then the leaving
acetic anhydride and forming acetic acid were distilled out under atmospheric pressure and then reducepressure at ~170 °C. A white solid, obtained by cooling down the remaining solution to room temperature, was recrystallized from Et2O (room temperature → ~50 °C). Lauric anhydride was collected by filtration using a Büchner funnel and washing with cooled Et2O as colorless plates (4.31 g, 90 %). 1H NMR(270 MHz, CDCl3) δ = 0.879 (t, J = 6.4 Hz, 6H), 1.258 (m, 32 H), 1.655 (quint, J = 7.2 Hz, 4.2H), 2.441(t, J = 7.4 Hz, 4H); 13C NMR (400 MHz, CDCl3) δ = 14.2, 22.8, 24.3, 29.0, 29.4, 29.5 29.68, 29.70, 32.0,
35.4; mp 42.0 - 42.5 °C; MS (EI, 70eV) m/z = 382 (M+, < 1 %), 183 (C11H23CO+, 100 %).
X-ray diffraction (XRD) measurement of thin-films and single-crystal. XRD patterns of spin-coatedthin films deposited on Si/SiO2 substrates were obtained with a Rigaku RiNT-2200 diffractometer with a
Cu Kα source (λ = 1.541 Å) in air. The d-spacings were calculated by the bragg equation: λ = 2dsinθ. Single crystals of N-n-hexanoyl-PyTTF (2b) suitable for structural analysis were obtained by vapor
sublimation. The single-crystal analysis was made on Rigaku MSC Mercury CCD. The structure wassolved by direct methods (SIR92).106 Non-hydrogen atoms were refined anisotropically, and hydrogenatoms were included in the calculations but not refined.107 All calculations were performed using the crystallographic software package TeXsan.108 Detailed condition and crystallographic data are listed in Table
A1 (in Appendix).
Cyclic voltammograms (CVs). CVs were recorded on a Hokuto Denko HA-301 potentiostat and a
Hokuto Denko HB-104 function generator in dichloromethane containing tetrabutylammonium
hexafluorophosphate (Bu4NPF6, 0.1 M) as supporting electrolyte at a scan rate of 100 mV/s. Counter andworking electrodes were made of Pt, and the reference electrode was Ag/AgCl. All the potentials were
calibrated with the standard ferrocene/ferrocenium redox couple (E1/2 = +0.46 V measured under identicalconditions). Oxidation potentials of 2a–2c were identical with in experimental error.
Atomic Force Microscopy (AFM). The AFM images were measured with a Molecular Imaging PicoPlus microscope in air.
Device fabrications and evaluations. OFETs were fabricated in a top-contact configuration on a heavily doped n+-Si (100) wafer with 200-nm-thick thermally grown SiO2 (Ci = 17.3 nF cm–2). The surface of
the SiO2 dielectrics was treated with silane-based self-assembled monolayer (SAM) reagents with following methods; the substrates with an octyltrichlorosilane (OTS)- or octadecyltrichlorosilane (ODTS)- wasobtained by immersing the Si/SiO2 substrate in 0.1 M OTS or ODTS in toluene at 60 °C for 20 min.
HMDS (hexamethyldisilazane)-treatments were carried out by exposing the SiO2 to HMDS vapor at room
temperature in a box under nitrogen for 12 h. A thin film (50 nm thick) of N-acetyl- (2a) or N-n-hexanoylPyTTF (2b) as the active layer was vacuum-deposited on the Si/SiO2 substrates under a pressure of ~2 ×
10–3 Pa. Substrate temperature (Tsub) was varied at rt, 60 ºC, and 100 ºC. Deposition rate was ~1.0 Å s–1.
On top of the organic thin film, gold (80 nm) as drain and source electrodes was deposited through ashadow mask. For a typical device, the drain-source channel length (L) and width (W) are 50 or 100 µm
and ca. 1.5 mm, respectively. The characteristics of the OFET devices were measured at room temperature in air with a Keithley 4200 semiconductor parameter analyzer. Field-effect mobility (µFET) was calculated in the saturation regime (Vd = –60 V) of the Id using the following equation: Id = (W Ci/2L) µFET (Vg– Vth)2, where Ci is the capacitance, and Vg and Vth are the gate and threshold voltages, respectively. Current on/off ratio (Ion/Ioff) was determined from the Id at Vg = 0 V (Ioff) and Vg = –60 V (Ion). The µFET datareported in Table 2.3.2 are values from more than 10 different devices.

25

2.4 Bis(thieno[2,3-d])tetrathiafulvalenes (αDT-TTFs)

2.4

Bis(thieno[2,3-d])tetrathiafulvalenes (αDT-TTFs)

Introduction. Pyrrolo[3,4-d]tetrathiafulvalenes (PyTTFs, 1 and 2, Figure 2.4.1) described in
section 2.2 and 2.3 were developed as solution processible p-type semiconductors. N-alkylPyTTF with long alkyl chains (1g) is highly soluble and form a crystalline thin-film that arranged with layer-by-layer structure.15 N-hexanoyl-PyTTF (2b) that showed improved airstability and mobilities in air, however, were low solubility.16 The field-effect mobilities (µFETs)
up to ~0.08 cm2/Vs of PyTTFs-based OFETs are lower than that of DT-TTF. That is regarded as
the prototypical compound of the heteroaromatic-fused TTF (µFET = ~1.4 cm2/Vs in singlecrystal OFETs).14 In fact, OFETs based on vacuum-deposited thin-films of DT-TTF showed performances with µFETs up to 0.3 cm2/Vs (see Experimental Procedures, Figure 2.4.9). The superior performances of DT-TTF to those of the PyTTFs impelled me to investigate thieno[2,3d]TTF (αDT-TTF, 3a), an isomer of DT-TTF, which could be platform for introducing alkyl
chains at the α-positions (C(α)) of the thiophene (Figure 2.4.2).
3a has been reported as a donor material as an unseparable mixture of the cis/trans isomers
depending on the direction of the thiophene because of the easy rotation of the central doublebond of the TTF core promoted by a protonation of the bond to give the isomer.97,118-120 For thisreason, αDT-TTF should contains two cis/trans isomers rings in the solid state. Nevertheless,relative positions of the α-carbon atoms in the two isomers should not be changed so much
(Figure 2.4.3a). This is because the α-carbons are on the apex of the five-membered ring, as wasreported for 5-hexylthiophene-fused porphyrazines (C6H2Pc) by Miyazaki et al. (Figure
2.4.3).117 Indeed C6H2Pc has well-ordered direction of the hexyl groups substituted at the αcarbons, in spite of the completely disordered thiophene moieties (Figure 2.4.3b and c). In addition, strong intermolecular interaction can be expected regardless of the direction of the thiophene rings, because the HOMO coefficients of 3a are mostly located on the TTF core (Figure
2.4.2, right).

S

S
n-C20H41 NSS

N n-C20H41

S

S

N

N
S

S

n-C5H11

S

S

S

S

N-n-cetyl-PyTTF (1g)O

SS

DT-TTFC(β)

n-C5H11

C(α)

O

S

S

S

S

S

αDT-TTF (3a)N-n-hexanyl-PyTTF (2b)Figure 2.4.2. Structures of thiophene-fused TTFs
(left) and their HOMOs calculated by DFT (B3LYP/631G(d))111 (right).

Figure 2.4.1. Structures of pyrrole-fused TTFs
(a)

(b)

(c)
R

R

S
N

R

S

S

S

S

S

S

S

S

S

NH

N

N

HN

N

N

R
S

S
R

N
S

C6H2PcR

Figure 2.4.3. Explanation of disordered model. (a) Illustration of alkylated 3 with superimposed cis and transisomers. (b) Structure of 5-hexylthiophene-fused porphyrazine and (c) the 5-hexylthiophene moiety in crystal,
where two thiophene ring with different directions are superimposed.
26

2. Development of Heteroaromatic-fused TTFs

Scheme 2.4.1. Synthesis of the unsubstituted αDT-TTF (3a).
1) n-BuLi
2) S
3) CS23) S
4) MeSCN

R

R

S

NaI

S
R = OTHP
R = OHR = OTsO

R = OTHP

SMe

94 %

S

61 %

HClaq 94 %TsCl 82 %P(OMe)3

S
S

S

78 %
R = OH

DDQ

S

S

S

Hg(OAc)2

S

60 %

3a

20 %

19

74 %

S

S

DDQ

O
S

S

S

P(OMe)3

O
S

64 %

S

3a

40 %

20

Scheme 2.4.2. Attempt to synthesize of methyl-substituted αDT-TTF (3b) from 3a.

3a

LDA
MeI

S

S

S

S

S

Me

Me
S

3b

Scheme 2.4.3. Synthesize of methyl-substituted αDT-TTF (3b)
19

LDA
MeI

Hg(OAc)2

S

78 %

S

P(OMe)3

S

Me

S
S

S

87 %

21

3b

Me

O
S

22

30 %

Scheme 2.4.4. Synthesize of octylthio-substituted αDT-TTF (3c)19

LDA
C8H17I

S

S
S

few %

S

S

S

23
1) LDA
2) S8
3) C8H17I

19

24
56 %

S

S

C8H17

S

SC8H17

S
S

95 %

S
O
S

S

S

SC8H17P(OMe)3S

36 %Table 2.4.1. Solubility of 3a-c
solubilityaR/ g L-1
1
<0.1
25

S S

26

S
C8H17S

S

S

S

S

S

SC8H17

3c

27

3a H
3b Me
3c S-C8H17
a in toluene at r.t.

S

25

24

Hg(OAc)2S

S

sublimation
at ~10-3 Pa✓✓-

In this section, I describe syntheses, solid-state structures, and FET characteristics of αDTTTFs substituted with alkyl chains.
Synthesis. According to the reported procedure,97 3a was synthesized (Scheme 2.4.1). I first
tried to synthesize methyl-substituted αDT-TTF from 3a as a model alkylating reaction. Methylation of 3a, however, did not proceeded via a lithiation reaction followed by a reaction withmethyl iodide (Scheme 2.4.2). Since chemical instability of the TTF skeleton was thought tocause the failure, I methylated intermediate 19 or 20. Although the lithiation reaction of 20 did
not gave methylated compound, the reaction of 19 gave the desired methylated compound 21
27

2.4 Bis(thieno[2,3-d])tetrathiafulvalenes (αDT-TTFs)

(a)

(b)

(c)

Figure 2.4.4. Molecular structures of (a) 3a, (b) 3b, and (c) 3c illustrated with thermal ellipsoids at 50 % probability. The structures of different direction of thiophenerings are included in the trans form structure for 3a and 3b, and superimposed on theeach thiophene site for 3c.

(Scheme 2.4.3). Subsequent conversion to the ketone (22) and the following coupling reaction
successfully gave methyl-substituted αDT-TTF (3b). The similar reaction was then carried out
using octyl iodide to synthesize the octyl derivative (Figure 2.4.4). The reaction, however, afforded a mixture of octyl-substituted compound (23) and the compound that seems to be noctylthio-substituted compound (24) as a byproduct together with a large amount of unidentifiable polymeric product. This result suggests that the reaction involves unintended decomposition such as a ring-opening reaction and an attack to the substrates to give intermediate like 25
and 26, which reacts with the octyl iodide reagent to give 24. This result indicates that the lithiated 19 is not so reactive and tends to decompose via the ring-opening reaction to form elemental sulfur. At the same time, the thiolate intermediate 25 may be stable and reactive enough toafford 24. Therefore, I added elemental sulfur into this reaction mixture, which resulted in a
successful synthesis of 24 with an acceptable yield (56 %). The following reactions from 24
gave n-octylthio-substituted αDT-TTF (3c). 3c is highly soluble in contrast to virtually insoluble
3b (Table 2.4.1).
Disorder in Crystal Structures. 3a-c afforded single crystals that could be fully characterizedby X-ray analysis (Figure 2.4.4). All molecular structures have disordered thiophene moieties,
as seen in the other thiophene-fused crystals.14b,121 The two thiophenes with different direction
are drawn with two methods: the structures of 3a and 3b are depicted as the trans isomers whereoccupancies of the S and C(β) are mixed up with each other, and the structure of 3c is shownwith superimposing the two thiophene rings. Although the crystal analysis do not informwhether the cis isomers are contained or not, this trans-form-based disordered model gave de-28

2. Development of Heteroaromatic-fused TTFs
(a)

(b)

(c)

c

Figure 2.4.5. (a) Molecular arrangement of 3a with c-axis projection (left) and depiction of one trimer-unit (right).
(b) Arrangement of 3b with view along the molecular short axis (left) and projection of one layer normal to the πplane (right). (c) Arrangement of 3c with b-axis projection (left), the molecular-long axis projection (middle, alkyl
moieties are omitted), and normal to the π-plane (right). Green dash lines show S-S short contacts of 3.47 Å for 3b
and 3.46 Å for 3c.

cent result for crystallographic analysis (see Table A1 in Appendix). Probabilities of the different directions of thiophene ring are 30%, 50%, and 33% for 3a, 3b, and 3c, respectively. In spiteof these disorders of the thiophene rings, 3a-c are highly crystalline as evidenced by the ease of
crystallization from solution. The n-octyl groups of 3c is directed to anti-conformation withsmall disorders, which allows the well-ordered packing structure in the solid state (vide infra).
Molecular Arrangement in the Bulk Single-Crystals. In the single crystal of 3a, moleculartrimer units are packed in a herringbone like manner (Figure 2.4.5a). The molecules of 3b are
packed in a so-called “slipped herringbone-like packing” with completely no overlapped π-core,
although short S-S contacts are observed (Figure 2.4.5b). As a result, neither arrangement of 3anor 3b has an effective conducting layer. In contrast, the molecules of 3c in the crystal are regu-29

2.4 Bis(thieno[2,3-d])tetrathiafulvalenes (αDT-TTFs)

larly packed to form a layer-by-layer (alkyl - π-core - alkyl) structure (Figure 2.4.5c). Thechange in the molecular arrangements in the single crystals of 3a-c are similar to those of
PyTTFs (1), where the long alkyl groups play an important role in constructing a desirable
structure for carrier transport as discussed in the previous section 2.2. The π-cores in the crystalof 3c are interacted with each other in all neighboring molecules via side-by-side short S-S contacts (3.46 Å) (Figure 2.4.5, middle). Since 3c takes the herringbone packing, the direct overlapof π-core is relatively small, the motif of which is similar to that of 2b showing moderately highmobility (Figure 2.3.7).
Thin-Film Fabrication. A thin film of 3b could not be fabricated by spin-coating method because of the low solubility (Table 2.4.1). Thin films of 3b were fabricated by vacuum-deposition
on Si/SiO2 substrates to give lustrous films of 3b. Although 3a has relatively good solubility (~1g/L in toluene at room temperature), fabrication of thin films with spin-coating method was notpossible. Thin films of 3a were fabricated by vacuum-deposition, which gave clouded films,
which implies that, however, homogeneous thin films were not obtained. In contrast, highly
soluble 3c can be spin-coated at room temperature to give lustrous thin films of 3c on the bare
Si/SiO2 substrate from 0.6 wt % toluene or benzene solution.
Orientations and Morphologies of Thin Films. The thin-films of 3a-c showed sharp XRD
peaks assignable to crystallographic (00l) reflections, indicating formation of crystalline thinfilms (Figure 2.4.6). The peak positions of (001) and calculated d-spacings are listed in Table
2.4.2. The d-spacing of spin-coated 3c film is 34 Å, which corresponds to the crystallographic c001
002

004

Intensity / a.u.

(a)
001

(b)
001

(c)

002

004

0

5

10

006

15
20
25
30
2θ/θ
Figure 2.4.6. XRD pattern of (a) 3a, (b) 3b, and (c) 3c.
(a)

(b)

(c)

Figure 2.4.7. AFM images of (a) 3a, (b) 3b, and (c) 3c. Difference of height on thegreen lines are as high as 400 nm for 3a, 180 nm for 3b, and 40 nm for 3c.

30

2. Development of Heteroaromatic-fused TTFs

Table 2.4.2. Interlayer distance of XRD pattern and FET characteristics of 3a-c and DT-TTF.
XRD patterns
OFET characteristicse
Rprocessa
2θd / ºd-spacing / Å
Vth / Vµ / cm2V-1s-1
3a H

-

-

Ion/Ioff

vac

6.8

13.0

-

3b Me

vac

6.9

12.8

-

-

-

3c S-C8H17

spinb

2.6

34

57

0.19

102

vacc

6.7

0.3
105
2 surface was treated with
HMDS. d (001) peak in XRD pattern. e Typical value on top-contact devices with W = 50 µm and L= 1500 µm.
DT-TTF

13.1

3

a vac = vacuum-deposition, spin = spin-coat. b from 0.4 wt % toluene solution. c SiO(a)-5 10-4(b)
V = -80 V

10-3

0.016

g

V = -60Vd-4 10-4

0.014
0.012

-Id / A

-40 V

0.008
10

0.006

-5

0.004

-4

-1 10

/ A1/2

Id / A0.01

1/2-2 10-4

10-4

-60V

(-Id)

-3 10-4

-20 V
0V
0
-80
-60

0.002

-40
Vd / V

-20

0

10-6
-40

-20

0

20

40

60

0
80

Vg / V

Figure 2.4.8. FET characteristics of 3c on bare SiO2 substrate: (a) output(b) transfer characteristics.

axis of the bulk crystal (34.06 Å). This correspondence indicates formation of the layer-by-layer
structure in the thin film as in the bulk crystal. On the other hand, the d-spacings of the thinfilms of 3a and 3b are different from any of the lengths of crystallographic axes, indicating thatthe structures of 3a and 3b in thin films are different from those of the bulk crystals. The dspacings are almost equal to the molecular lengths, which implies that molecules stand uprightto the substrate to form crystalline films with different arrangement from those in the bulk crystals.
Surface morphologies of the thin-film were studied by an atomic force microscopy (AFM) to
examine quality of the films (Figure 2.4.7). The AFM image of thin film of 3a deposited on thebare Si/SiO2 substrate shows large crystalline grains with very large height differences (ca. 400nm), indicating that non-uniform crystalline grains form. This is certainly due to too high crystallinity of 3a. The AFM image of deposited thin film of 3b shows crystalline grains with lower
height differences (ca. 180 nm) than that of 3a. This indicates that the uniformity of thin-film is
improved, although the surface is fairly rough. In contrast, the AFM image of spin-coated 3bfilm shows a relatively smooth surface with ca. 40 nm in step height. In general, uniform crystalline film is essential for a FET channel to achieve better performances.122
FET Characteristics. OFET devices were fabricated by deposition of gold source-drain electrodes on top of the thin films to give top-contact devices (see Experimental Procedures). The
OFETs based on 3a and 3b showed no field-effect responses. Non-uniformity of the thin films
should be responsible for the former, whereas the lack of π-overlap in the molecular orientation
as discussed above is supposed for the latter. On the other hand, 3c-based OFETs showed typical p-channel FET characteristics under ambient conditions (Table 2.4.2, Figure 2.4.8). The extracted µFETs are up to 0.2 cm2/Vs, which are higher than those of PyTTF derivatives (1g and2b) and are almost comparable to those of devices based on DT-TTF fabricated by vacuumdeposition. The 3c-based FETs are, however, suffered from heavily normally-on behavior (Vth =~50 V), probably owing to its relatively low oxidation potential (vide infra).

31

2.4 Bis(thieno[2,3-d])tetrathiafulvalenes (αDT-TTFs)

Table 2.4.3. Oxidation potentiala and the estimated HOMO levels.
R
onset
E11/2
E21/2
∆E
EHOMOb / eV
3a H
-0.05
0.03
0.38
0.35
-4.75
3b Me
-0.12
-0.04
0.32
0.36
-4.68
3c S-C8H17
-0.04
0.03
0.34
0.31
-4.77
DT-TTF
0.18
0.30
0.63
0.33
-4.98
a Vs Fc/Fc+. b E+
1/2 (Fc/Fc ) = -4.8 eV under vacuum level as a criteria.

Oxidation Potentials. In order to estimate molecular electronic structures, oxidation potentials of the thiophene-fused TTFs were measured with cyclic voltammetry (Table 2.4.3). First
oxidation potential of 3a (E11/2 = +0.03 V vs Fc/Fc+) is lower by 0.27 V than that of DT-TTF(E11/2 = 0.30 V). This large difference indicates that the electronic structure of 3a markedly differs from that of DT-TTF. The lower oxidation potential of 3a may related to the resonancestructure where TTF moiety can obtain 6π-aromaticity in the cationic state, in contrast to that of
DT-TTF that may become 10π-aromaticity or break aromaticity of thiophene moiety in the cationic state.
The methyl groups increase the oxidation potential of 3a slightly by 0.07 V (E11/2 of 3b =-0.04 V). On the other hand, the oxidation potential of 3c (E11/2 = 0.03 V) is almost equal to thatof the 3a, indicating electronically neutral character of alkylthio group in this case, as a consequence of compensation of the inductive effect by the resonance effect. The energy level of the
HOMO (EHOMO) of 3c estimated from the oxidation onsets are -4.77 eV. This value is, unfortunately, higher than that of DT-TTF (EHOMO = -4.98 eV) and N-acyl-substituted PyTTF (2b,
EHOMO = -4.95 eV) which showed good air-stability in their devices. This higher EHOMO (lowoxidation potential) is presumably responsible for the normally-on characteristics of the OFET
devices. The low oxidation potential makes the molecule susceptible to the air-oxidation similar
to that observed for the N-alkyl-PyTTF (1g).
Summary. The methyl (3b)- and n-octylthio (3c)-substituted αDT-TTF were successfully synthesized. Single-crystal X-ray analysis revealed small positional disorders in the α-positions of
the thiophene rings and the n-octylthio groups of 3c, which promotes well-ordered layer-bylayer molecular arrangement in the solid state. This can be understood by considering hydrophobic interaction between the alkyl chains. In the structure, the TTF core of 3c are interactedwith all neighboring ones via the short S-S contacts, which is similar to that of DT-TTF. The
solution-processed OFETs based on 3c showed high µFETs up to 0.2 cm2/Vs, which is comparable to those of vacuum-processed DT-TTF-based OFETs. The heavily normally-on behavior of
3c-based devices can be attributed to its lower oxidation potential caused by the different contribution of the fused thiophene ring to the electronic structure of the whole molecules. Further
improvement of molecular design strategy is required to keep the HOMO energy level lower.
Experimental Procedures
Synthesis:
General: All chemicals and solvents are of reagent grade unless otherwise indicated. DMF was distilledfrom CaH2 under reduced pressure. THF was distilled from Na before use. Trimethylphosphite wastreated with sodium for 24 h, then distilled in an inert atmosphere and stored with activated molecularsieves 4 Å. Melting points were not collected. Nuclear magnetic resonance spectra were obtained in deuterated chloroform with a JEOL EX 270 spectrometer operating at 270 MHz for 1H with TMS as internalreference; chemical shifts (δ) are reported in parts per million. EI-MS spectra were obtained on a Shimadzu QP-5050A spectrometer using an electron impact ionization procedure (70 eV). The molecular ion
peaks of the sulfur-containing compounds showed a typical isotopic pattern, and all the mass peaks arereported based on 32S.
32

2. Development of Heteroaromatic-fused TTFs

5-Metylthieno[2,3-d]-1,3-dithiole-2-thione (21). Under N2 atmosphere, thieno[2,3-d]-1,3-dithiole-2thione (19) (952 mg, 5 mmol) was dissolved in THF (35 mL). The mixture was then cooled down to -80ºC before addition of 0.5 M lithium diisopropyl amide (LDA) in THF (12 mL, 6 mmol). Iodomethane(1.87 mL, 30 mmol) was added and stirred for 30 min, the mixture was kept stirring for 2 h at 60 ºC.
After quench the reaction by addition of water, the resulting mixture was extracted by CH2Cl2 (50 mL ×
4). The organic layer was washed with sat. NaClaq (75 mL) and dried with anhydrous MgSO4. Yellow
solids, obtained by evaporation of the solvent, were purified by column-chromatography (SiO2, CH2Cl2/
hexane=1:1, Rf = 0.79) to give yellow crystals of 21 (797 mg, 78 %). 1H-NMR (270 MHz, CDCl3, TMS)δ 6.76 (q, J = 1.16 Hz, 1 H), 2.58 (d, J = 1.15 Hz, 3 H).
5-Metylthieno[2,3-d]-1,3-dithiole-2-one (22). Mercury (Ⅱ) acetate (3.73 g, 11.7 mmol) was added to asolution of compound 21 (797 mg, 3.9 mmol) in CHCl3 (150 mL), and the mixture was stirred for 6 h at
rt. The resulting mixture was filtered though celite, and the filtrate was evaporated. The resulting yellow
solids was purified by column-chromatography (SiO2, CH2Cl2/Hexane = 1:1, Rf = 0.79) to give 22 aswhite crystals (636 mg). 1H-NMR (270 MHz, CDCl3, TMS) δ 6.79 (q, J = 1.20 Hz, 1 H), 2.56 (d, J = 1.23
Hz, 3 H).
(Z)/(E)-Bis(5,5’-dimethylthieno)[3,4-d]tetrathiafulvalene (3b). Under N2 atmosphere, 22 (636 mg,
3.38 mmol) was added into trimethylphoshite (5 mL). The coupling reaction was completed by refluxing
mixture at 140 ºC for 2.5 h. After cooling down the mixture, the trimethylphoshite was removed byevaporation in vacuum. The resulting red solids were purified by column-chromatography (SiO2, CS2, Rf= 0.85) followed by recrystallization from toluene/hexane. 3b was obtained as red crystals (175 mg, 30%). 1H-NMR (270 MHz, CDCl3, TMS) δ 1.26 (s), 6.55 (bs). DI-MS (70 eV, EI) m/z = 344 (M+).
5-octylthiothieno[2,3-d]-1,3-dithole-2-thione (24). Under N2 atmosphere, 0.5 M LDA in THF (5.5mL, 2.75 mmol) was added to a solution of 19 (480 mg, 2.5 mmol) in THF (23 mL) at 0 ºC. Subsequently
elemental sulfur (88 mg, 2.75 mmol) was added to the mixture and stirred for 10 min at 0 ºC, which isfollowed by addition of iodooctane (2.7 mL, 15 mmol). The reaction mixture was warmed up ~60 ºC andthen kept stirring for 2 h. After that, the reaction was quenched by addition of water. The mixture was
extracted with CH2Cl2 (50 mL × 4). The resulting organic phase was washed with NaClaq (75 mL) anddried with anhydrous MgSO4. Evaporation of the solvent gave red solids, which was purified by columnchromatography (SiO2, CH2Cl2 / hexane = 1:4, Rf = 0.65). 3c was obtained as red crystals (477 mg, 57 %).
1H-NMR (270 MHz, CDCl , TMS) δ 7.07 (s, 1H), 2.84 (t, 2H) 1.54~1.65 (m, 7H) 0.88 (t, 3H)
35-Octylthiothieno[2,3-d]-1,3-dithiole-2-one (27). Mercury (Ⅱ) Acetate (1.36 g, 4.3 mmol) was added
to a solution of 24 (477 mg, 1.43 mmol) in CHCl3 (160 mL), and the solution was stirred for 2 h at rt. The
resulting mixture was filtered though celite, and subsequent evaporation gave yellowy solids. The crude
product was purified by column-chromatography (SiO2, CH2Cl2 = 1:1, Rf = 0.65) to give compound 27 asbrown oil (454 mg, 95 %). 1H-NMR (270 MHz, CDCl3, TMS) δ 7.07 (s, 1H), 2.84 (t, 2H) 1.54 - 1.65 (m,7H) 0.88 (t, 3H).
(Z)/(E)-Bis(5,5’-dioctylthiothieno[2,3-d])tetrathiafulvalene (3c). Under N2 atmosphere, 27 (430 mg,
1.35 mmol) was added to trimethylphoshite (3 mL). After reflux of the mixture at 140 ºC for 2.5 h, thesolvent was evaporated to give red solids. The solids were purified by column-chromatography (SiO2,
CS2, Rf = 0.85) followed by recrystallization from cool toluene/hexane. 3c was obtained as orange
crystals (146 mg, 36 %) For device fabrication, the product was recrystallized twice. MS (70 eV, EI) m/z= 604 (M+). 1H-NMR (270 MHz, CDCl3, TMS) δ 7.07 (s, 1H), 2.84 (t, 2H) 1.54 - 1.65 (m, 7H) 0.88 (t,3H).
X-ray diffraction (XRD) measurements of thin-films and single-crystals. XRD patterns of spincoated thin films deposited on Si/SiO2 substrates were obtained with a Rigaku RiNT-2200 diffractometerwith a Cu Kα source (λ = 1.541 Å) in air. The d-spacings were calculated by the Bragg equation: λ =2dsinθ. The single crystal XRDs were measured on a Rigaku Rapid-S Imaging Plate with graphite monochromated (2θmax = 55º) Mo Kα source (λ = 0.71069 Å) for 3a and 3b. The X-ray analysis of crystal of 3c
was made on Rigaku DSC imaging plate system by using Si-monochrommated synchrotron (λ = 1.00000
Å) at beam line BL8B of Photon Factory (PF) in High Energy Accelerator Research Organization (KEK).
Detailed crystallographic data are listed in Table A1 (p62). The structure was solved by direct methods(SIR92).106 Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were included in thecalculations but not refined.107 Calculations were performed using the crystallographic software package
TeXsan108 and Yadocari software
package for 3a-b and 3c, respecTable 2.4.4. FET characteristics of 3c.
tively.
solventwt %rpm
Vth / V µ /cm2V-1s-1
Ion/Ioff
Atomic Force Microscopy
toluene
0.6
2000
57
0.19
102
(AFM). The AFM images were
2.0
4000
55
0.15
101
measured on a Molecular Imaging
benzene
0.6
2000
66
0.27
101
PicoPlus microscope in air.
1
2.0

33

4000

77

0.14

10

2.4 Bis(thieno[2,3-d])tetrathiafulvalenes (αDT-TTFs)

Device Fabrications and Evaluations. OFETs were fabricated in a top-contact configuration on aheavily doped n+-Si (100) wafer with 200-nm-thick thermally grown SiO2 (Ci = 17.3 nF cm–2). The 50
nm in thickness of thin films of 3a, 3b, and DT-TTF were fabricated on the substrates by vacuumdeposition with room substrate temperature and deposition rate of ~1.0 Å/s. The thin films of 3c was fabricated by spin-coating from the 0.6 and 2.0 wt % of benzene or toluene solution with the spin-coat rate of2000 and 4000 rpm, respectively (Table 2.4.4). The OFET devices was fabricated by deposition of gold
source-drain electrodes on the thin films. The channel width (W) and length (L) are 50 µm and 1500 µm,
respectively. The characteristics of the OFET devices were measured in air with a Keithley 4200 semiconductor parameter analyzer. Field-effect mobility (µFET) was calculated in the saturation regime (Vd = –60 V) of the Id using the following equation: Id =(WCi/2L) µFET (Vg – Vth)2, where Ci is the capaci- (a)(b)
tance, and Vg and Vth are the gate and thresholdvoltages, respectively. The FET characteristics of
3c-based devices fabricated with different spin-coatconditions are described in Table 2.4.4. The characteristics are averaged values of more than three derives. The FET characteristics of DT-TTF wasshown Figure 2.4.9. The extracted µFETs are up to
0.3 cm2/Vs on the HMDS treated substrates.
Molecular orbital (MO) calculation. Geometry Figure 2.4.9. FET characteristics of thin-film-based DT-TTF
optimization and normal mode MO calculation of devices on the HMDS treated SiO2 substrate: (a) output (b)3a-c were carried our by the DFT method at the transfer characteristics.
B3LYP/6-31G(d) level using the Gaussian03
program.111 The HOMO of 3c (alkyl moiety is converted to methyl group) is depicted in Figure 2.4.10.
Cyclic voltammetry (CV). CVs were recorded on a
Vd = -60V-4

10-2.5 10-4

10

-5

0.02

0.015

10-6

-2 10-4-Id / A

-40V

-1.5 10-4

d

d

10-3

Vg = -60V-3 10-4(-I )1/2 / A1/2

I /A

-3.5 10-4

-7

10

0.01

10-8

-4

-1 10

-5 10-5

10-9

-20V

0.005

-10

10

0V
-50

0
-60

-40

-30

-20

-10

0

10-11
-60

-40

V /V
g

Hokuto Denko HA-301 potentionstat and a Hokuto Dneko HB104 function generator in 10-3 M benzonitrile solution containing 0.1 M Bu4N+PF6- as a supporting electrolysis with scan rate
is 100mV/s, by using Pt work/counter electrodes and Ag/AgCl
reference electrode. All the potentials were calibrated with thestandard ferrocene/ferrocenium (Fc/Fc+) redox couple (E1/2 =+0.43 V measured under identical conditions). Cyclic voltammogram of 3a-c and DT-TTF are shown in Figure
2.4.11.

20

0

Figure 2.4.10. HOMO of methylthiosubstituted αDT-TTF estimated by single point
calculation based on the crystal structure of 3cwith DFT (B3LYP/6-31G*) method.

0

UV-Vis absorption spectrum. UV-Vis spectra of 3ac in dichloromethane solution are recorded on Shimazu
UV-3100 spectator. The spectra are shown in Figure
2.4.12.

-20
0
Vg / VE / V (vs. Fc/Fc+)
0.5

1

3a

3b

30000

DT-TTFn-octylthio-αDT-TTF (3c)ε/ L cm-1 mol-13cmethyl-αDT-TTF (3b)20000

αDT-TTF (3a)DT-TTF

10000

TTF

0
250

300

0
350

400

450

500

550

600

λ/ nm

Figure 2.4.12. UV-Vis spectra of 3a-c, DT-TTF, and TTF.

0.5
1
E / V (vs. Ag / AgCl)

1.5

Figure 2.4.11. Cyclic voltammograms of 3a-c and DTTTF.

34

2. Development of Heteroaromatic-fused TTFs

2.5

Correlation between Electronic Structure in Solid State and Field-Effect Mobility

Introduction.
The synthesis, properties, and structures of the heteroaromatic-fused TTFs substituted withlong alkyl groups were described in section 2.2 - 2.4. The correlation between the FET characteristics and the structures of the TTF derivatives clearly demonstrated that such molecular design as the TTF π-cores with long alkyl group successfully give solution processible organic
semiconductors.
The two-dimensional (2-D) electronic structures via π-π and/or S-S interactions between the
TTF-cores may be realized in the solid states for the present TTF derivatives, which results inrelatively good FET characteristics, e.g., N-n-cetyl-PyTTFs (1f), N-n-hexanoyl-PyTTF (2b) andn-octylthio-αDT-TTF (3c) (Figure 2.5.1). Nevertheless, the µFETs of the derivatives are lower
than those of recently developed superior semiconductors such as pentacene,47,123
dinaphtho[2,3-b:2´,3´-f]thieno[3,2-b]thiophene (DNTT),17 2,7-didodecyl[1]dibenzothieno[3,2b][1]benzothiophene (C12-BTBT),124,125 and hexamethylenetetrathiafulvalene (HM-TTF)88 (Figure 2.5.2). The FET characteristics of these materials are summarized in Table 2.5.1.
Among these superior µFET semiconductors, the solution-processible C12-BTBT are developed
along with the same molecular design strategy as the present heteroaromatic-fused TTFs. ThereS
C16H33 N

S

S
S

O

N C16H33

S

S

N

N
C5H11

N-n-cetyl-PyTTF (1f)

S

S

C5H11
C8H17SO

N-n-hexanyl-PyTTF (2b)

S

S

S

S

S

S
SC8H17

αDT-TTF (3c)
c

b
a

b
a
b

Figure 2.5.1. Structures of the 1f, 2b, and 3c (top) with their molecular arrangement viewed along themolecular-long axis with space-filling model (bottom, alkyl(thio) moieties are omitted for clarity).
S

S
C12H25

C12H25S

pentacene

DNTT

S

C12-BTBT

S

S

S

S

HM-TTF

Figure 2.5.2. Structure of high performance organic semiconductors recently reported.
Table 2.5.1. Summary of FET characteristics of the heteroaromatic-fused TTFs
and high performance materials reported recently.
TFT characteristicsb
processa
Vth / Vµ / cm2V-1s-1
Ion/Ioff
1d N-n-octyl-PyTTF
spin
no field effect 1f N-n-cetyl-PyTTF
spin
10
0.008
104
2b N-hexanoyl-PyTTF
vac
-4
0.08
105
3b methyl-αDT-TTF
vac
no field effect 3c n-octylthio-αDT-TTF
spin
50
0.2
102
DT-TTF
vac
3
0.3
105
pentacene
vac
-11
0.30
108
DNTT
vac
-10
2.9
107
C12-BTBT
spin
-20
1.7
108
c
HM-TTF
vac
27
3.6
103
a vac = vacuum-deposition, spin = spin-coat. b All top-contact configuration with Auelectrodes (W = 1500 µm, L = 50 µm) measured in air. c Reported value.88
35

2.5 Correlation between Electronic Structure in Solid State and Field-Effect Mobility

DT-TTFε/ L cm-1 mol-1

20000

N-alkyl-PyTTF (1b-g)
αDT-TTF (3a)N-acyl-PyTTF (2)

10000

TTF
PyTTF (1a)

0
250

300

350

400

450

500

550

600

λ/ nm
Figure 2.5.3. UV-Vis spectra of the TTF derivatives in CH2Cl2 solution. The spectra of
3b and 3c is shown in Figure 2.4.12, which is similar to that of 3a.

Table 2.5.2. HOMO and LUMO levels estimated experimentally from CV and UV-Vis spectra.
oxidation potentiala / V
abs / nmEHOMObΔEgc
ELUMOd
E11/2
E21/2
/ eV
/ eV
/ eV
onset
λmax
λedge
1f
N-n-cetyl-PyTTFe
-0.18
-0.11
0.31
-4.62
284
481
2.58
-2.04
2b
N-n-hexanoyl-PyTTF
0.15
0.22
0.59
-4.95
337
449
2.76
-2.19
3b
methyl-αDT-TTF
-0.12
-0.04
0.32
-4.68
306
550
2.25
-2.43
3c
n-octylthio-αDT-TTF
-0.04
0.03
0.34
-4.76
300
520
2.38
-2.39
DT-TTF
0.18
0.30
0.63
-4.98
312
521
2.38
-2.60
TTF
-0.18
-0.11
0.34
-4.62
311
500
2.48
-2.14
pentacene
-0.17
-4.97
600
2.06
-2.91
DNTTf
0.64
0.75
-5.44
401
420
2.95
-2.49
C12-BTBTg
0.7
0.89
NA
-5.5
NA
NA
3.5
-2.0
HM-TTFh
NA
-0.19
0.29
-4.5
NA
NA
NA
NA
a vs. Fc/Fc+. b E
cde
HOMO = -4.8 - onset voltage (eV). Eg = 1240 / λedge. ELUMO = EHOMO + Eg. The same as that of 1c.
f -h referred from the reports17,124,88.

fore, it should be very interesting to see the difference in the packing structure in qualitatively.
In order to clarify the difference in the electronic structures of π-cores at the molecular level andthen correlate the crystal structures and transport properties in OFETs, molecular orbitals(MOs), reorganization energy (λ), and transfer integrals (t) in bulk single crystal structure arecalculated for the heteroaromatic-fused TTFs together with the high performance OFET
materials.126-128 The theoretically determined electronic structures are correlated to experimentally determined electrochemical and optical properties to confirm the suitability of the calculations. Comparisons of the electronic structures will give insight into understanding theperformance of the TTF-based semiconductors and will give a new design strategy for better
materials.
Oxidation Potential and UV-Vis Spectra. Figure 2.5.3, Table 2.5.2 summarized the electronic properties of the heteroaromatic-fused TTFs experimentally by measuring their cyclic
voltammetries and UV-Vis absorption spectra. As already described in the previous sections,oxidation potential of the 1f and 3b are low, which makes their devices susceptible to airoxidation. 2b-based OFETs showed improved air-stability because of the higher oxidation potential. The HOMO energy level (EHOMO) of 2b estimated from first oxidation onset is ca. -4.95
eV.112 Nevertheless, the EHOMO of 2b is still higher by more than 0.4 eV than those of DNTT and

36

2. Development of Heteroaromatic-fused TTFs

C12-DNTT. DNTT-based devices do not degrade at least six or more months under ambient conditions. This fact indicates that the introduction of electron-withdrawing carbonyl group to the
TTF framework is insufficient for lowering the EHOMO to fully air-stable level. Further development of methods for introducing strongly electron-deficient moieties into the TTF π-coreshould be required83,75
UV-Vis spectra of the TTF derivatives are shown in Figure 2.5.3. Most of the TTF derivatives
have weak absorptions on their shoulders, which is ascribed to forbidden HOMO-LUMO excitations as seen for the parent TTF.129 Optical HOMO-LUMO energy gaps (∆Eg) were estimatedfrom the edge of the weak absorption, and then the energy level of the LUMOs (ELUMOs) are
estimated by using EHOMOs and ∆Egs.113 Among the present TTF derivatives, 2b shows an extraordinary spectrum, which has no shoulder. Because the same spectra were obtained in the
different solvents or concentrations for 2b, the HOMO-LUMO excitation of 2b is thought to beallowed, and has become the main peak (λmax = 337 nm). Thus the ELUMO of 2b was estimatedfrom the absorption edge and the EHOMO to be -2.19 eV, which is lower than that of 1f reasonably. Since no further information for the electronic structure related to the FET characteristics
was obtained form the experimental analyses, theoretical calculations were carried out as follows.
Molecular Electronic Structures: MO Calculation. Molecular orbital (MO) calculation
were carried out to examine shapes andenergy levels of frontier orbitals, which Table 2.5.3. Calculated frontier orbitalsa.
related to the electron transport properEHOMO / eV ELUMO / eV
ties. It should be noted that molecular
1f
N-n-cetyl-PyTTF
-4.22
-0.56
structure experimentally determined by
2bN-n-hexanoyl-PyTTF
-4.92
-1.24
X-ray analysis were used for initial mo3aαDT-TTF
-4.66
-2.32
3b
methyl-αDT-TTF
-4.51
-1.01
lecular coordinates except for 2b. This3cn-octylthio-αDT-TTFb
-4.63
-1.27
approach is very important for TTF deDT-TTF
-4.94
-1.19
rivatives, because the geometrical optimiTTF
-4.52
-0.95
zation of TTF molecules by DFT calculapentacene
-4.60
-0.94
tion often results in bended molecular
DNTT
-5.18
-1.81
shapes, which is far away from real moC12-BTBT
-5.40
-1.14
lecular in the solid states.130
HM-TTF-4.32-0.72
The calculated EHOMOs are in good a Based on DFT with B3LYP/6-31G(d) basis set. The alkyl
agreements with the experimental results, groups are inverted to methyl groups for ease of calculations.
molecules were kept in planar geometry while calculation.
indicating the calculations give the decent Allb single-point calculation.
results (Table 2.5.3). The tendency of thecalculated ELUMOs is also consistent with
(b)

(a)

(c)

(d)

(e)

(f)

Figure 2.5.4. Coefficients of LUMO (top) and HOMO (bottom) of (a) 1f, (b) 2b, (c) 3b, (d) 3c, (e) DT-TTF, andTTF (f).
(a)

(b)

(c)

(d)

Figure 2.5.5. Coefficients of HOMO of (a) pentacene (b) DNTT, (c) BTBT, and (d) HM-TTF.

37

2.5 Correlation between Electronic Structure in Solid State and Field-Effect Mobility

experimental results, although the absolute values are different from experimental results because the DFT calculation does not consider a configured interaction in an excited state.131
The MO calculation together with time-dependent DFT calculation supports the allowed
HOMO-LUMO excitation of 2b (HOMO → LUMO: oscillator strength = 0.51 at 377 nm)131,
which consistent with the distinct absorption spectra for 2b, probably reflecting the LUMO
shape of 2b which fairly differed from those of the other TTF derivatives (Figure 2.5.4).
On the other hand, the calculated HOMOs of the TTF derivatives are similar to each other,where the coefficients are largely on the sulfur atoms. All the derivatives thus could stronglyinteract intermolecularly via S-S contacts in solid states. Note that the large HOMO coefficientson the sulfur atoms and/or on the periphery of the molecules are also found in the high µFET materials (Figure 2.5.5).132
The overview of molecular electronic structures indicates capability of the heteroaromaticfused TTF to realize high µFET comparable to those of the state-of-the-art materials, whereas the
air-stability should be no good.
Parameters in Charge-Transport in OFETs. Understanding the theory of charge-transport in
OFETs is necessary for further evaluation of the electronic properties in the solid state.133 It isrationally believed that carrier transport in the majority of organic semiconductors can be classified into the hopping model.134 The hopping µFET can be expressed by135µ FET =

qd 2
kET …(1)k BT

kET = t 2

⎛ ( ΔE − λ )2 ⎞π…(2)exp ⎜ −
k BT λ4 λ k BT ⎟⎠⎝where q is the number of charge, d is intermolecular distance, ħ is Plank constant, kB is Boltzmann constant, T is temperature, kET is electron-transfer rate, t is transfer integral (intermolecular electronic coupling), λ is reorganization energy, and ∆E is site-energy difference of initialand final state (which also depend on t).136 The eq (2) is so-call the Marcus equation,137 which
expresses that µFET depends on λ and t.
Hopping process can be depicted as a model picture where an electron (or hole) is transferredfrom one molecule to the neighboring one. Figure 2.5.6 explains the transfer reaction when the
carrier is hole (p-type). After the charge transfer, relaxation of the structure of each molecule(+1 charged and neutral) allows the molecule to take each optimal geometry of each charged
state. The reorganization energy for hole (λh) correspond to the sum of the two structural relaxation energies (λh = λrel(1) + λrel(2)).138,139 These two portions are typical nearly identical (λh ≈ 2λrel). The λ in fact gives a measure of energy loss (or hopping efficiency). More strictly, λ includes intermolecular reorganization energy, that is polarization energy and molecular vibrationof surrounding molecules.136 However, the intermolecular λ can be neglect because the value is
expected to be much smaller than the intramolecular λ. That is why I consider only the intramolecular λ. Here I estimated intramolecular λh values on a isolated molecule based on the same
DFT method (B3LYP/6-31G(d). The λh values of the compounds are compared with each otherin order to evaluate the hole transport ability.
On the other hand, transfer integral (t) is electronic coupling between two molecules. The t isan energy gain when a charge is delocalized (or transferred) between two molecular orbitals.
The meaning of t is similar to a resonance integral (β) when two atomic orbitals give molecularorbital on the basis of linear combination of atomic orbitals (LCAO) approximation. Here I consider hole mobility, thus the t related to hole transport is important. That value can be estimatedby considering interaction of HOMO of neutral dimer (tHOMO, Figure 2.5.7).
The calculation of tHOMOs in dimers in the crystals are based on DFT with the PW91 functional and Slater-type triple-ζ plus polarization (TZP) basis set by using a Amsterdam Density38

2. Development of Heteroaromatic-fused TTFs

for Hole ! h

+1

e-

e-

S

S

S

S

S

S

S

S

S

S

S
S

(2)
!rel

S

E

S

S

S

S

S

S

+1 optimized

S

l

!

S

Ground(1)relS

Figure 2.5.6. Explanation of hopping model and reorganization energy.
Reorganization energy is defined as sum of relaxation energies.
(a)

S
S

S

S S

S

S S

S
S

e-

Ne
u

tra

S

S

S
S

S

S
S

(b)

S

S

S

S

S
S

S

S
S

S

S

S

S

S

S

S

S

S

S

S
S

S

S

S
S

Energy loss of charge transport
Calculated !h of an isolated
t of HOMO molecule in vacuum by DFT method.

!

Atomic Orbital " Molecular Obital.
Figure 2.5.7. Explanationof calculation of a transfer
integral.
(a) A dimer is picked up from
PROGRAM ADF (Amsterdam density functional)t
Molecular Obitalof" HOMOs
Band in crystal
a crystal structureMETHOD(b) Schematic
imageof the interaction
in a crystal structure.
Slater-type orbital
approximation
Triple-! polarization (TZP) basis set

LEVEL

GGA:PW91

40

Functional (ADF) program
package.138-141 The ADF program gaveplausible results as compared to theprogram based on the extend Hückel
method developed by Mori et al.142,143
The extend Hückel program tends tooverestimate tHOMO in the case of
sulfur-containing compounds (seeSupporting Information). It should be
mentioned that the tHOMOs in this report are absolute values, |tHOMO|. The
phase (+ or -) are neglected in order todiscuss the strength of electronic coupling simply.

Table 2.5.4. λh on isolated moleculesa.
λrel(1) / meV λrel(2)/ meV λh / meV
1f N-n-cetyl-PyTTF
137
143
280
2b N-n-hexanoyl-PyTTF
180
170
350
3b methyl-αDT-TTF
130
125
255
3c n-octylthio-αDT-TTF-b
DT-TTF
125
127
252
TTF
142
137
279
pentacene
48
49
97
DNTT
67
63
130
C12-BTBT
120
119
239
HM-TTF137
147
284
a calculated based on DFT with B3LYP/6-31G(d) basis set. b could
not be calculated a appropriate value owing to take boat conformation during the calculation.

Reorganization energy. The calculated λh of the TTF derivatives (> 250 meV) are higher thanthose of pentacene and DNTT (< 130 meV) (Table 2.5.4). This is responsible for the generalelectronic character of TTF core, that is, the TTF has non-aromatic structure in the neutral stateand gain aromaticity in the cation state. Thus, the structural difference between the neutral andcationic states should be large. In addition, 2b has larger λh (350 meV) than those of the other
TTF derivatives, which is attributable to its large structural freedom caused by single bond in
the conjugation. The high λhs of the TTF compounds could cause low µFETs of theheteroaromatic-fused TTFs. HM-TTF and C12-BTBT, nevertheless, also have high λhs ~250

39

37

Transfer Integrals (Pyrrole-fused TTFs)
S

S
C8H17 NN C8H17SS

C16H33 N

S

S

S

S

O

N C16H33

S

C5H11

S
N

N
S

C5H11

S

O

1f Electronic Structure in Solid State2b
2.5 Correlation between
and Field-Effect Mobility

1d!FET = ~0.01 cm2/Vs (spin)!FET = ~0.08 cm2/Vs (vac)(a) N-n-octyl-PyTTF(1d)(b) N-n-cetyl-PyTTF (1f)(c) N-n-hexanyl-PyTTF (2b)qq

no field-effect (spin)

p

q

q

p

r

s
q

r

q

p

q

r

r

p

s

q

p

c

p

b

b

a

Transfer Integrals (Thiophene-fused TTFs)
tHOMO / meV
p
14
q
17a

a

a S-S 3.37 Å

S
H3C

S

S
S

p
q
r
s
S

a !-! 3.57
S Å. b S-S 3.40 Å.

S

S
CH3

S

tHOMO / meV
87a
3b
1

p
q
r

C8H17 SSSS

S C8H17S

tHOMO / meV
33a
25a
45b
S 43b

a !-! 3.49 Å. b S-S 3.40
S - 3.57 Å.
SSS

Figure 2.5.8.3b
Molecular arrangement viewed along3a
long-molecular axis together with DT-TTF
transfer integrals of (a) 1d,
2/Vs (spin)(b) 1f,and(c)2b(alkylmoietiesareomittedforclarity).
!
=
~0.3
cm
FET
no field-effect (vac)!FET = ~0.3 cm2/Vs (vac)(a) methyl-αDT-TTF (3b)

q

p

r

(b) n-octylthio-αDT-TTF (3c)

q
r

q

r
p

p

p

p

r

q

(c) DT-TTF

q

q

r
r

p

c

tHOMO / meV
p
2
q
12
r
10

p
q
r

tHOMO / meV
14a
54b
53b

a !-! 3.53 Å. b S-S 3.46 Å.

a
b

p
q
r

tHOMO / meV
55a
34b
34b

c
b

a !-! 3.58 Å. b S-S 3.55 Å.

Figure 2.5.9. Molecular arrangement viewed along long-molecular axis together with transfer integrals of (a)3b, (b) 3c, and (c) DT-TTF (alkyl moieties are omitted for clarity).

meV. These fact indicate relationally small contribution of λh to µFET, which may related to small
λh in actual solid state, because molecules are embedded into surrounding molecular lattice.130
Transfer Integrals in Crystal Structures. The calculated tHOMO values are shown in figuresof crystal structures viewed along the molecular-long axis direction (Figure 2.5.8 - 2.5.9). Intermolecular interactions related to FET characteristics can be expressed with interactions between the surrounding six molecules for most cases. In addition, the six interactions frequently
include the identical pair(s) owing to the crystallographic symmetry. Thus, the interaction canbe explained, in general, with two transverse (S-S) directions and one perpendicular (π-staking)direction.
For 1d that gives no FET response with low Id current, very small tHOMOs (< 22 meV) arefound in all directions (Figure 2.5.8a). In contrast, the large tHOMOs (87 meV) is found along the
π-staking direction (designated as p) for 1f that give moderate µFETs (Figure 2.5.8b). These results are reasonable to consider the relation between the structure and properties of the FET
characteristics. This result also indicates very weak electronic interactions at the transverse direction, in which S-S short contacts are present (q = 17 meV for 1d, q = 3 meV for 1f). The
small interaction is probably explained for the slipping π-cores along the molecular long-axis,
and thus HOMOs on the sulfur atoms do not spread over the molecules in such direction. The
calculation asserts that not all S-S short contacts give electronic interaction.

40

Transfer Integrals (Heteroacenes) Transfer Integrals (OSS

C12H25

C12H25

2. Development of Heteroaromatic-fused TTFs

S

S

!FET = ~3 cm2/Vs (vac)(a) pentacenerp!FET = ~3 cm2/Vs (vac)(b) DNTTqp
q
rtHOMO / meV
48
61a
67a

a CH-! 2.7 - 2.8 Å

p
q

p
q
r

tHOMO / meV
71
91a
14b

a CH-! 2.78 Å.
b CH-! 2.73 Å(d) HM-TTF

q

q

q

p

p

29

q
r

q

q

(c) C12-BTBT

p

r
r

28

!FET = ~2 cm2/Vs (spin)

p

p
q

q

tHOMO / meV
62a
46b

r

a

p
q
r

p

r

p
r

p

tHOMO / meV
40a
105b
28

a !-! 3.6 Å. b S-S 3.55 Å.

a S-S 3.54 Å. b S-! 3.22 Å.

XFigure 2.5.10. Molecular arrangement viewed along long-molecular axis together with transfer integralsof (a)
pentacene, (b) DNTT, (c) C12-BTBT (alkyl moieties are omitted for clarity), and (d) HM-TTF.

In the structure of 2b, moderate tHOMOs (25 - 45 meV) in all direction are calculated in the
herringbone-like structure (Figure 2.5.8c). The isotropic electronic coupling within the π-corelayer ought to form 2-D conducting layer parallel to the substrate. This is contrasted to the structure of 1f which has 1D intermolecular interaction. With considering the better µFETs of 2b thanthose of 1f, 2-D electronic structure is suitable for thin-film OFETs.14b This is in good agreement with the 2-D electronic structure in pentacene film (Figure 2.5.10a).
Calculation of transfer integrals of thiophene-fused TTFs also show good agreement with the
FET characteristics (Figure 2.5.9). Very small tHOMOs (< 12 meV) were calculated for 3b thatgives no FET characteristics. In contrast, moderately large tHOMOs (q and r = 54 meV) in 3cwere calculated along the transverse directions. Similar tHOMOs were calculated in the DT-TTF
packing structure along the π-π interaction (p = 55 meV), in addition to moderate transverse
interaction (q and r = 34 meV). The 2-D electronic interaction with moderately large tHOMOs for3c and DT-TTF is consistent with their good µFETs ~0.3 cm2/Vs.
On the other hand, the high performance materials such as pentacene, DNTT, C12-BTBT, and
HM-TTF has generally larger tHOMOs than those in the present heteroaromatic-fused TTF (Figure 2.5.10). Pentacene and C12-BTBT have the moderately large tHOMOs (ca. 50 - 65 meV) in all
directions in the solid states. DNTT has large tHOMOs along the two directions (p = 71m eV andq = 91 meV). Pentacene, C12-BTBT, and DNTT are estimated to form 2-D conducting layerswith strong electronic coupling. For HM-TTF, a very large tHOMO (q = 105 meV) along thetransverse direction and a moderate tHOMO (p = 40 meV) along the π-staking direction are calculated, indicating the formation of a 1-D-like quasi 2-D electronic structure (vide infra).
Regardless of the kind of material, the present calculations of tHOMOs indicate that tHOMOs in
the solid state is well-correlated with µFETs of OFETs, having polycrystalline thin-film channel
with grain boundaries. First, a 2-D semi-conducting layer is necessary for high µFETs, which isachieved by tHOMOs more than 20 meV along more than two directions. In other words, fieldeffect response in the thin-film-based OFETs seems to require tHOMO at least ~20 meV, whichmight be boarder region of whether carriers hop or not between molecules beyond the energy
barrier such as λh and or other carrier traps. Second, stronger electronic coupling give higherµFET, in which a ca. 60 meV of tHOMO in one direction appears to correspond to µFETs 1 cm2/Vs.
Discussion for Developing Semiconductors for OFETs. The experimental and theoreticalevaluations suggest that the lower mobility of the OFETs based on the heteroaromatic-fused

41

q

q

p
q
r

r
q

p

tHOMO
63 952 -a S-! 3.45

Transfer Integrals (Other TTF materials)
2.5 Correlation between Electronic Structure in Solid State and Field-Effect Mobility

(a)

28

29
S

S

S

S

28

q
r

p
q
r

p

r

p
q

q

r

p

tHOMO / meV
40a
105b
28

a !-! 3.6 Å. b S-S 3.55 Å.

p
q
r

S

(b)
N

N

S
S N
S
S S N
N
N S

N

S

S

N

29

p

r

q

tHOMO / meV
63 - 66a
9 - 1352 - 53a

S

S

q

r

p

S

p

a

p
q
r

r
q

c

tHOMO / meV
46a
26b
71

a !-! 3.42 Å. b S-S 3.59 Å.

a S-! 3.45 Å.

Figure 2.5.11. Structures and molecular arrangements viewed along the molecular-long-axis withcalculated tHOMO values: (a) Naphthalene-fused TTF (28). (b) Quinoxaline-fused TTF (29).

TTFs than that based on the high performance semiconductors is ascribed to the smaller tHOMO
values in the solid states. The tHOMOs are much smaller than those expected from the existenceof short S-S contacts for 2b, 3c, and DT-TTF. I suggest that their packing fashion (can be classified into θ-type, Figure 2.1.8) is disadvantageous to take electronic coupling between TTF
cores. On the basis of these results and consideration, I shall suggest a strategies for molecular
design of TTF-based materials as follows.
DNTT type: herringbone packing. High performance acene and heteroacene-based semiconductors are packed in herringbone fashion. The densely packed herringbone structure driven by
the CH-π pseudo-hydrogen bonds should be one of the most promising strategy for large tHOMOs
with neighboring molecules to give 2-D semi-conducting layers. In general, a molecule having
benzene ring tend to take the herringbone structure. For instance, naphthalene-fused TTF (28) ispacked in the herringbone fashion in the solid state (Figure 2.5.11a).91 The calculated tHOMOs,
nevertheless, are moderate with ~50 meV in all directions. In this case, fused benzene rings
probably perturb the S-S interactions sterically. Accordingly, the µFETs of OFETs based on 28
are up to 0.4 cm2/Vs, which indicate that the strategy for herringbone structure seems to be unsuitable for TTF derivatives.
HM-TTF type: π-stacking. It is remarkable that HM-TTF has the large tHOMOs (105 meV). The
large tHOMO is found in the S-S contact direction, in spite of the longer S-S distance (3.55 Å)
than those in 2b or 3c. This fact indicates the importance of relative position of the π-cores (videsupra for 1d and 1f). In addition, the highly conductive charge-transfer salts based on TTF derivative so far reported are often packed with π-stacking structure (so-call β-sheet structure)
rather than the herringbone-like θ-type structure.73,143 Therefore, the π-stacking structure seems
to be suitable for TTF derivatives to realize high µFET. For the formation of π-stacking structure,
introducing bulky substituents into a molecule is one of the potential strategies (see Figure
2.1.6).49 On the other hand, the other strategy includes annulation of electron-deficient moiety
such as nitrogen-containing ring. For instance, quinoxaline-fused TTF (29) formed π-staking
structure (Figure 2.5.11b). This molecular design strategy is in agreement with decrease in
EHOMO for improving air-stability. However, polarization in molecules often gives low solubility75 and unpredictable structures in the solid state owing to local interaction between electrondonating and accepting moieties.72,144 In fact the OFETs based on 29 showed the moderate µFETs

42

2. Development of Heteroaromatic-fused TTFs

(0.2 cm2/Vs), relating to the packing slipped along to the molecular long axis. A molecular
structure should be designed carefully.
Summary. The electronic structure of the heteroaromatic-fused TTFs are determined experimentally and theoretically. Evaluations on molecular characteristics relating to hole transport interms of coefficients and energy levels of HOMO and reorganization energy suggest that theenergy level should be decreased to obtain air-stable operation, and relatively high λh can be attributed to low µFET. However, the molecular electronic structures are not correlated with µFET
strongly. On the other hand, calculated electronic structures in the solid state, transfer integrals
for HOMO (tHOMO), indicated that the µFETs of thin-film-based OFETs can be well-correlated tothe tHOMOs. The tHOMOs larger than ~25 meV in more than two direction seem to be a criticalfactor for high performance materials as indicated by 2b, 3c, and DT-TTF. The tHOMOs in 2b, 3c,and DT-TTF are, however, lower than those of the state-of-the-art high performance materials.
This result means that the intermolecular interaction between TTF cores is rather smaller thanthose expected from the packing structure, where S-S short contacts and/or π-stacking are present. It indicates that the present heteroaromatic-fused TTF core is not suitable for the present
molecular design. Another molecular design strategy is necessary for developing further better
TTF-based semiconductors.

Supporting Information
Transfer Integrals calculated based on an Extend Hückel method. Transfer integrals in 1f, 2b, andDT-TTF were calculated by using a program developed by Mori et al., which are shown in Table 2.5.4.
The program calculates overlap integral for HOMO, and then transfer integrals (tHOMOs) are calculated by
the following equation: tHOMO = -10 × SHOMO (eV). Parameters in the extend Hückel calculation is shownin Table 2.5.5.
The calculated tHOMOs show similar tendency with those calculated by DFT method. The value of tHOMOs, however, tends to have large values in the direction of side-by-side S-S directions. For instance, 1f
was calculated to be relatively large interaction (53 meV) in the direction of S-S contact (designated byq), in contrast to quite few value (3 meV) calculated by the DFT method.
Table 2.5.4 Transfer integrals calculated byextend Huckel programmolecule

contactat (meV)

1f

p
q
r
p
q
r
s
p
q
r

-135
53
0.08
-40
-45
-47
-45
30
47
50

2b

DT-TTFa Contact directions are equal to 2.5.8 and 2.5.9.

43

Table 2.5.5. Parameters in extend Huckl transfer integral calculations
atomorbitalSlater exponent ζ
ionization potential
S
3s
2.122
-1.47
3p
1.827
-0.808
3d
1.5
-0.4
O
2s
2.275
-2.38
2p
2.275
-1.09
N
2s
1.95
-0.1912p
1.95-0.990C
2s
1.625-1.5732p
1.625-0.838
H
1s
1.0
-1.0

2.6. Conclusion for Chapter 2

2.6.

Conclusion for Chapter 2
S

S

S

S

N R

R N

S

R

R

S
N

N
S

S

2: N-n-acyl-PyTTFµFET = 0.08 cm2/Vs

S

S

S

S

S

S

S

S

DT-TTFµFET = 0.3 cm2/Vs

1: N-n-alkyl-PyTTFµFET = 0.01 cm2/Vs
O

S
S

O

S
SR

RS
S

3: n-alkylthio-αDT-TTFµFET = 0.2 cm2/Vs

Figure 2.6.1. Heteroaromatic-fused TTFs with their µFETs as high as obtained.

I designed, synthesized, and characterized a series of heteroaromatic-fused TTFs as semiconductors for OFETs (Figure 2.6.1, with the highest µFETs obtained). This study successfully demonstrated the promising molecular design for solution-processible semiconductors (1 and 3).
The small and cohesive TTF-cores with long alkyl chains along the π-core are highly soluble,
and formed layer-by-layer structures on the substrates with spin-coat process. The longer alkyl
derivatives showed the better FET characteristics than those of short or unsubstituted derivatives, which completely agreed with the well-ordered layer-by-layer structure of the latter suitable for OFET operations. In the solid-state structures, 2-D intermolecular interactions between
the TTF-cores via π-π and/or S-S interactions can be observed. In addition, the hydrophobic interaction between the long alkyl chains effected the large overlaps between the TTF-cores.
On the other hand, the drawbacks of the heteroaromatic-fused TTFs are mainly two fold; incompatibility of solubility with air-stability (2) and the relatively low µFETs of the devices ascompared to those of the state-of-the-art high performance semiconductors. The elucidation of
the electronic structures indicates that the weaker electronic couplings between the TTF-cores inthe solid states than expected from the 2-D packing structures is most likely responsible. However, the strategy described here is utilized in other π-cores, and gave the high performancesemiconductors for both solution-process124 and vacuum-process22 (Chapter 3). Although 1-3
were not used in the following study owing to the inferior performances, TTF derivatives arestill excellent candidates for high performance semiconductors and very interesting as researchsubjects.

44

2. Development of Heteroaromatic-fused TTFs

45

3.1 Introduction for Alkylated DNTT

Chapter 3
Evaluation of C10-DNTT as a Organic Semiconductorfor High Performance Thin-Film Transistors
3.1

Introduction for Alkylated DNTT

Organic thin-film transistors (OTFTs) have attracted interest because of the possibility for realizing flexible electronic devices.5 Among the many organic semiconductors reported so far, pentacene is the most common organic semiconductors as it shows the highest field effect mobility(µFET > 3.0 cm2/Vs).48,145-147 On the other hand, Takimiya’s group has developed several newheteroacene-based organic semiconductors that show high µFET in the TFT configuration: 2-7diphenyl[1]benzothieno[3,2-b]benzothiophene (DPh-BTBT, 2.0 cm2/Vs),148 dinaphto[2,3-b:
2’3’-f]thieno[3,2-b]thiophene (DNTT)17, and alkylated BTBTs (Cn-BTBT, 3.9 cm/Vs)124 (Figure
3.1.1).
DNTT has been reported as the high performance and air-stable vacuum-processed organic
semiconductor. Flexible transistors and integrated circuits based on DNTT have been reported.18
Some people call DNTT as post-pentacene due to the reproducible and the stable highperformance. On the other hand, Cn-BTBT was developed along with the same molecular
design-strategy as the heteroaromatic-fused TTFs (comparison of the characteristics was discussed in section 2.5), which has long alkyl chains along the π-core. The alkyl chains can act as
the driving force for molecular ordering in solid state owing to the van der Waals intermolecularinteraction between the alkyl groups, or the so-called molecular fastener effect, that renders thesemiconducting core to pack tightly, thereby enhancing carrier mobility (Figure 3.1.2).125 The
application of the molecular design strategy to the DNTT framework is considered to be a
promising approach for much higher performances and solution-processible organic semiconductors.
M. J. Kang in Takimiya’s group synthesized di-n-alkyl-substituted DNTT (Cn-DNTT) andinvestigated their basic properties and characteristics. Although Cn-DNTT has low solubility,
vacuum-processed Cn-DNTT-based TFT devices showed higher performances than those basedon DNTT. In particular, di-n-decyl-DNTT (C10-DNTT) gives the highest mobility over 5 cm2/
Vs at least. However, our conventional device configuration is somewhat inappropriate inevaluating such high mobility organic semiconductors as C10-DNTT, owing to over estimationof drain-current (Id) thereby field-effect mobility (µFET). Furthermore, TFT characteristics of
C10-DNTT-based devices on plastic substrates should be interesting. Although many of organic
semiconductors that show high performances on the silicon substrates have been reported so far,demonstrating high performance OTFTs with µFETs over 1 cm2/Vs on flexible substrates is verySS

DPh-BTBTS
CnH2n+1

H2n+1CnS

Cn-BTBTSS

DNTTS
C10H21

C10H21S

C10-DNTT

Figure 3.1.1. Structure of theheteroacene-based semiconductors.

Figure 3.1.2. Schematic representation of the molecularorientation on Si/SiO2 substrates (a) for DNTT (b) for alkylated DNTT.
46

3. Evaluation of C10-DNTT

limited; pentacene is in fact almost only the semiconductor that well evaluation on plastic substrates.
In this chapter, evaluation of a high performance C10-DNTT semiconductor is described: first,
I improved the device structure by designing new shadow masks, and determined the µFETs on
the silicon substrate. Second, I established fabrication and evaluation system of polymer dielectrics on the plastic substrates. Finally, the high performance C10-DNTT-based OTFTs on plastic
substrates are demonstrated.

47

3.2 Evaluation on Silicon Substrates

3.2

Evaluation on Silicon Substrates

Designing Shadow Mask. Our conventional top-contact/bottom-gate device with a parallel S/
D electrode configuration is fabricated by deposition of gold source-drain electrode through ashadow mask on the organic thin film coated over the substrate (Figure 3.2.1). The conventional
device system is easy to fabricate many devices in one deposition (12 arrays, 3 devices of eacharray). However, it has some problems. First, fringe current may give overestimation of µFET
(Figure 3.2.2).149 Second, large areas of electrodes (~1.5 mm2 for each) often brigs large offcurrent and frequently give rise breakdown when voltage is applied to the device.150 Third, thechannel is defined with tungsten wires, which gives inaccurate channel width (W) and (L). The
rough shadow mask also prohibits fabrication of multi-layer electrodes, e.g., a MoO3 injectionlayer beneath the Au electrode.151
I designed a new device structure shown in Figure 3.2.3 to address these issues. This device
geometry needs two masks; for the organic layer and for the S-D electrodes. The organic mask
defines the organic layers in a square shape. The width of organic layer regulates the width ofchannel region W of a device with this comb-type electrode structure. Therefore, overestimationof µFETs caused by the fringe current can not occur with this device geometry. The separatedorganic layer also contributes lowering the off-current (Ioff) because it can exclude leakage currents generated in the other organic region. The area of S-D electrode of this device (~0.6 mm2
for each) is smaller than that of our conventional one, which contributes to reduce Ioff significantly and prevent dielectric breakdown. In particular, the Ioff can be minimized when gateelectrodes are patterned, which is quite important for fabrication of devices on polymer dielectrics (see section 3.3 and 3.4). On one substrate, 10 devices can be fabricated by using the maskwith various L (20, 40, 90, 140, and 190 µm, measured) and W (1500 µm) dimensions, whichallows evaluation of the contact and channel resistance of the devices separately. Eight squares(2 mm × 2 mm dimension) on the side of the pattern are metal-insulator-metal (MIM) devices
for capacitance-measurement and metal-insulator-semiconductor-metal (MIS) devices for I-V
characteristics of semiconductors, respectively. The shadow mask is made of stainless (SUS304H). Its moderately magnetic character allows the mask to stick to a substrate that placed on apiece of strong neodymium magnets. This is important for ease of the fabrication of devices andpreventing a damage to the mask during the peeling.
C10H21(a)(b)
S-D electrodesL
D (Au)WDrain

MIM structure(for capacitance)

n+-Si (Gate electrode)ate Film

Figure 3.2.1. Schematic representation of the conventional OFET device (a) structure (b) top-view of sourcedrain electrodes. Channel width (W) and length (L) are
1500 µm and 50 µm, respectively.

1.5 cmr (600 nm)
ric(640 nm)
O)

S (Au)

Organic semiconductor
SiO2

SourceD (Au)MIS structure(for I-V)

For alignment

1.0 cmFigure 3.2.3. New designed device. Shadow area represents organic layer.
Figure 3.2.2. Schematic image fringe current.

48

40-4 ~ -7
1.9 ~ 2.1
1090
-7
1.3 ~ 1.9
109
140
-7
1.3 ~ 1.4
108
190
-5 ~ -7
1.4 ~ 1.5
109
ODTS
20
+6 ~ +80.2
10640+1 ~ +4
0.1
106
90-13 ~ +1
0.1 ~ 0.2106140
-6 ~ +1
0.1 ~ 0.2
107
3. Evaluation of C10-DNTT
190+1 ~ +3
0.1
106
HMDS
20
-6 ~ -5
0.9 ~ 1.1
108
40
-8 ~ -6
0.6 ~ 0.7107
90
-7 ~ -9
0.6 ~ 0.7
108
(a)
140
-6 ~ -7
0.6 ~ 0.7
109
Table 3.2.1 Characteristics of DNTTa-based OFET devices fabri190-4 ~ -6
0.6 ~ 0.7
109
cated by the conventional mask (W = 1500 µm, L
=
50
µm).
a DNTT was sublimated twice after the Soxhlet extraction. b T
sub = 60 ºC, except
b
2
-1
-1
substrate
L / µmVth / Vµ / cmforVc atsr.t. Rdep = 1.5
Ion~/I2.0
off Å/s.

bare Si/SiO2 c
50-10 ~ +60.015 ~ 0.57
103
OTS
50
-16 ~ -4
1.2 (a)
~ 2.2
107
(b)
(b)
ODTS
50
-18 ~ 00.48 - 1.1
106
HMDS
50
-32 ~ -10
0.59 ~ 1.0
106Organic
β-PTS
50+5 ~ +26
0.034 ~ 0.011
105
D(Au)
S(Au)
PTS
50
+2 ~ +110.029 ~ 0.055
105
LW
a Purified by sublimated twice after the Soxhlet extraction. Deposition(1500 !m) (40 ~ 190 !m)crate (Rdep) was 1.5 ~ 2.0 Å/s. b Tsub = 60 ºC, except for
r.t..(a)See
Ex-representation of the device (b) a top view of the TFTs.
Figureat
3.2.4.
Schematic
perimental Procedures for treated reagents.
Figure 3.2.4. (a) Schematic representation of the device (b) a top view of
the TFTs.
20 !mTable 3.2.2. Characteristics of DNTTa-based OFET devices fabri40 !m 0.1-1.6 10
V = -60V
90 !mcated by the new mask (W = 1500 µm).
10
140 !m190 !m 0.08
-1.2 10
b
2
-1
-1
10
substrate
L / µmVth / Vµ / cm V s
Ion/IVoff= -60 V
0.06
bare Si/SiO2 c
20-6 ~ -8
0.6 ~ 0.810 ! = 7.1 cm /Vs 108-8 10 -40 V
20 µm
8
0.04
40
-7 ~ -9
0.4 ~ 0.510 2.1
1040 µm 0.1
-4 10
7
1.3
90 µm
90
-7 ~ -8
0.3 ~ 0.410
10
0.02
10-3
1.4
140 µm-20V
7140-7 ~ -8
0.4 ~ 0.510 1.4
10
0V
0
0
190 µm 0.08
-60 -50 -40 -30 -20 -10 0-60 -40 -20
0 7 2040-510
190-7 ~ -8
0.4 ~ 0.5
V /V
V 10
/V
V
=
-60 V
9
2
d
OTS
20
-11 ~ -14
4.4 ~Figure
7.1 3.2.5. Characteristics
10 of DNTT-based OTFT
µ =on7.1
cm /Vs
OTS-treated
Si/SiO2 substrates 0.06
10-7
9fabricated
by
the
new
designedmask.
(a)Transfer(b)
Outputcharacteristics.
40-4 ~ -7
1.9 ~ 2.1
10
0.04
90
-7
1.3 ~ 1.9
109
10-9 2.1140-7
1.3 ~ 1.4
108
0.02
10-11 1.3
190
-5 ~ -7
1.4 ~ 1.5
109
1.4
1.4
6
-13
ODTS
20
+6 ~ +80.2
10
100-60 -40 -20
0
20
40
40
+1 ~ +4
0.1
106
V /V
g
90
-13 ~ +1
0.1 ~ 0.2
106
-3
140
-6 ~ +1
0.1 ~ 0.2
107
-1.6 10
V = -60V
190+1 ~ +3
0.1
106
g
HMDS
20
-6 ~ -5
0.9 ~ 1.1
108
-3
-1.2 1040-8 ~ -6
0.6 ~ 0.7107
90
-7 ~ -9
0.6 ~ 0.7
108
-8 10-4 -40 V
140-6 ~ -7
0.6 ~ 0.7
109
190
-4 ~ -6
0.6 ~ 0.7
109
-4 10-4
a DNTT was sublimated twice after the Soxhlet extraction. R
dep = 1.5 ~bc
2.0 Å/s. Tsub = 60 ºC, except for at r.t.
-20V
-3

-3

g

-3

d

-9

-I d / A2d-7(-I )1/2 / A1/2

-I d / A

-5

-4

-4

-11

-13

1/2

-I / A

(-Id)

d

g

d

/A

-Id / A

1/2

0 0V
-60 -50 -40 -30 -20 -10
Vd / V

0

Figure 3.2.5. Characteristics of
DNTT-based OTFT on OTS-treated
Si/SiO2 substrates fabricated by the
new designed mask. (a) Transfer (b)Output characteristics.

Characteristics of DNTT-based OTFTs. In order to examine the new shadow mask, I fabricated the OFET devices with DNTT using both conventional mask (old ones) and newly designed mask (new ones, Figure 3.2.4). The old ones showed typical p-type performances with
µFETs up to 2.2 cm2/Vs on the octyltrichlorosilane (OTS)-treated Si/SiO2 substrates (Table 3.2.1,
see Experimental Procedures, Figure 3.2.8). On the other hand, the new ones L = 40 µm fabricated by the same conditions gave µFETs up to 2.1 cm2/Vs and on-off ratio (Ion/Ioff) of ca. 109(Table 3.2.2, in bold). Although the calculated µFETs of both devices are similar to each other,
the device-to-device deviation regarding to the µFETs and the threshold voltages (Vth) of the newones are smaller, and Ion/Ioff of the new ones are higher by ~102 than those of the old ones (Table
3.2.2). Additionally, the drain-current (Id) of the new one increased rapidly in the transfer char49

3.2 Evaluation on Silicon Substrates

acteristics (Figure 3.2.5), which is better than those of the old ones. The characteristics of thedevices with L = 20 µm, however, lack reproducibility. The short L devices frequently gave highIoff (~108) and short circuits (the S-D electrodes are connected). These fact indicates that such asmall gap by 20 µm is difficult to achieve reproducibly with the technique using the shadow
mask, although the extracted µFETs up to 7 cm2/Vs in the devices with L = 20 is very interesting.
I concluded that the newly designed mask could give reliable devices, except for the ones with L= 20 µm.
Characteristics of C10-DNTT-based OTFTs. C10-DNTT-based OFET devices were fabricated for several times under the conditions that Kang optimized; substrate temperature (Tsub) of
60 ºC and deposition rate (Rdep) of ca. 0.1 Å/s. Table 3.2.3 and Figure 3.2.6 summarize the characteristics of the devices. The C10-DNTT-based devices showed higher performances than thoseof DNTTs with µFETs more than 2 cm2/Vs regardless of the dielectric surfaces; bare, OTS, octa-µFET / cm2V-1s-1

Table 3.2.3. Characteristics of C10-DNTT-based OTFT devices fabricated by
the new mask (W = 1500 µm)a.
substrate
L / µmVthb / VµFETb / cm2V-1s-1
Ion/Ioffbare Si/SiO2
40-20 ~ -13 [-16]2.3 ~ 2.7 [2.9]109
90-17 ~ -7 [-13]2.0 ~ 3.7 [2.8]108140-15 ~ -10 [-13]
2.3 ~ 3.4 [2.8]109
190-16 ~ -7 [-12]2.2 ~ 3.3 [2.8]
108
OTS
40
-17 ~ -3 [-9]2.0 ~ 3.9 [3.0]10990-15 ~ -3 [-9]2.8 ~ 3.2 [3.0]109
140-14 ~ -3 [7]2.5 ~ 3.4 [3.0]108190-12 ~ 3 [-6]2.5 ~ 3.7 [3.4]
109
ODTS
40
-24 ~ -6 [-14]3.4 ~ 4.9 [4.0]10990-19 ~ -3 [-13]
4.2 ~ 6.5 [5.7]1010
140-21 ~ +1 [-12]4.9 ~ 7.6 [6.9]1010
190-15 ~ +1 [-5]6.6 ~ 7.8 [7.3]
109
HMDS
40
-20 ~ 0 [-13]
2.3 ~ 3.5 [2.9]10990-12 ~ -1 [-8]1.8 ~ 2.8 [2.3]
109
140-9 ~ -1 [-6]1.8 ~ 3.1 [2.5]109
190-7 ~ +1 [-4]2.3 ~ 3.1 [2.6]109aT
sub = 60 ºC, Rdep = 1.5 ~ 2.0 Å/s. C10-DNTT was sublimed twice before use.
bAverage values are in the parentheses.

bare Si/SiO2

10
9
8
7
6
5

OTS-treated

ODTS-treated

HMDS-treated

4
3

2

1

40

90

140

190

40

90

140

190

40

90

140

190

40

90

140

190

L / µmFigure 3.2.6. Distribution map of the extracted µFETs of C10-DNTT-based OTFTs.

50

3. Evaluation of C10-DNTT
(b)
10-3
10-5

-4 10-4

10-9

0.02

-2 10-4
-40 V

10-13
-60

0.01

-40

-20
Vg / V0020

Vd = -5 V

0
-60

10-13

-20 V
-10 V

-50

10-9
10-11

-1 10-4

10-11

Vd = -60 V

10-7
Id / A

0.03

10-5-3 10-4

0.04

10-7

10-3

Vg = -60 V(-Id)1/2 / A1/2

-Id / A

(c)

0.06

40 µm
90 µm
140 µm 0.05
190 µmVd = -60 V-Id / A

(a)

-40

-30
Vd / V

-20

-10

0

10-15
-60 -50 -40 -30 -20 -10
Vg / V0

10 20

Figure 3.2.7. Characteristics of C10-DNTT-based devices. (a) Transfer characteristics in saturation regime withvarious L. The extracted µFET from the transfer are 2.8, 6.4, 7.6, 7.8 cm2/Vs at L are 40, 90, 140, 190 µm, respectively. (b) Output characteristic of device with L = 190 µm. (c) Hysteresis of transfer characteristics in the linear (Vd= -60 V, red line with filled circle) and saturate (Vd = -5 V, green line with open circle) region. The µFETs is calculated to be 5.1 cm2/Vs in linear region.

decyltrichlorosilane (ODTS), and hexamethylenedisilazane (HMDS)-treated Si/SiO2. It is interesting to note that the devices on ODTS-treated substrate showed pronounced increase of µFET
with increasing L, the µFET reaches more than 7.0 cm2/Vs, and the highest observed µFET is 7.8
cm2/Vs with quite small hysteresis in the saturate region (Figure 3.2.7). This very high mobility
difference can be ascribed to better molecular ordering on the ODTS substrate, effectively reducing the channel resistance between the S and D electrodes. In case of the short L devices on
the ODTS-treated substrates, it is expected that relatively large contact-resistance limits the Id of
the on-state, resulting in the lower µFETs calculated from the device characteristics.152 These
unique characteristics are in good agreements with those of devices fabricated by Kang, whichshowed µFETs up to 7.9 cm2/Vs. With the good reproducibility we concluded that C10-DNTTbased OFET devices show very high µFETs close to 8 cm2/Vs, which are among the highest reported for organic thin-film-based transistors.153,154
Summary. I designed the shadow masks for patterning S-D electrodes and organic layers. The
DNTT-based OTFTs fabricated with the new mask showed better performances reproducibly
than those fabricated with the conventional mask. The OTFTs based on C10-DNTT were fabricated with the new mask, and the performances of the devices with µFETs up to 8 cm2/Vs were
definitely characterized, which is among the highest mobilities of thin-film-based OFETs. The
present results indicate that the combination of a π-extended heteroacene core with long alkyl
groups is promising approach to develop superior organic semiconductors.

Experimental Procedures
Fabrication of OTFT devices: OTFTs were fabricated in a top-contact configuration on a heavily
doped n+-Si (100) wafer with 200-nm-thick thermally grown SiO2 (Ci = 17.3 nF cm–2). The surface of the
SiO2 dielectrics was treated with silane-based self-assembled monolayer (SAM) reagents with following
methods: the Si/SiO2 substrate was washed in acetone and isopropanol with sonication for ca. 5 min andthen exposed in UV/O3 for 15 min. Subsequently the substrate was immersed in a toluene solution of 0.1
M octyltrichlorosilane (OTS) or octadecyltrichlorosilane (ODTS) at 60 °C for 20 min.155 HMDS
(hexamethyldisilazane)-treatment was carried out by exposing the substrate to HMDS vapor at room temperature in a box under nitrogen for 12 h. A thin film of DNTT or C10-DNTT (ca. 50 nm in thick, countedwith a quartz oscillator) as the active layer was vacuum-deposited on the Si/SiO2 substrates under a pressure of ~2 × 10–3 Pa, substrate temperature (Tsub) at 60 ºC, and deposition rate (Rdep) of 1.5 ~ 2.0 Å/s for
DNTT and 0.1 Å/s for 10-DNTT. On top of the organic thin film, gold (80 nm) was deposited through theshadow mask to give S-D electrodes. The characteristics of the OTFT devices were measured at room
temperature in air with a Keithley 4200 semiconductor parameter analyzer. Field-effect mobility (µFET)
was calculated in the saturation (Vd = −60 V) or linear regime (Vd = −5 V) of the Id using the following
equation,51

速度：1.0 - 1.2 Å/s
洗浄法：単蒸留アセトン(10min) - iPrOH (10min) - 120°C bakeの目玉：(UV) ＝SAM 処理の際、UV-オゾン処理を取り入れた処理基板。
-3

10

5 10
02-1 OTS-8 transfer

-5

-4

10

-5

10

-1 10

-4

10

-4

-5

5 10

02-2 OTS-8

-5

-5 10

-6

3.2 Evaluation on Silicon Substrates

02-2 OTS-8 output

0 10
-5 10

0

-5

d

-4

d

-9

-4

-1 0

-4

-1 1

g

1/2

-8

-4

d

d

-8

-4

1/2

-7

-7

-I (A)-6-I (A)

210-1 10
Saturation
regime: Id = (WCi /2L)µ
FET (Vg – Vth)-1.5 1010
10-1.5 10
Linearregime:
I
=
V
(WC
/L)µ
(V
–
V
)
d
d
i
FET
g
th
-2 10
10
10
where
Vg and Vth are the gate -2andthreshold voltages, respectively.
10-2.5 10Ci is the capacitance, and10
Typicalcharacteristics of DNTT-based
OTFT devices
10
-3 10
10
-2.5 10 with the conventional device structure is shown
20 10 0 -10 -20 -30 -40 -50 -60
0-10 -20 -30 -40 -50 -60
20 10 0 -10 -20 -30 -40 -50 -60
0-10 -20 -30 -40 -50 -60
V (V)
V (V)
V (V)
in Figure 3.2.8.V (V)-I (A)d-I (A)

10-3

10

02-1 OTS-8

0

d

10

-5

0 10

Square Root of -I (A )Square Root of -I (A )

-4

-4

-4

d

g

d

-9

1/2

-1 0

10

10

-6

-2 10

-4

-4 10

-4

-8 10

-3

-1 10

20 10 0 -10 -20 -30 -40 -50 -60
V (V)

0

-10

-20

-30 -40
V (V)

-50

-9

-1 0

10

20 10 0 -10 -20 -30 -40 -50 -60
V (V)

-4
-4

-2 10

-4

-2.5 10

-4

-3 10

-4

-3.5 10

-4

-4

-5 10

Vg = -60 V

-4

-6 10

-4

-7 10

20 10 0 -10 -20 -30 -40 -50 -60
V (V)

0

-10

-20

-30 -40
V (V)

-50

-60

d

10

-4

10

-6

10

-8

1 10

10-3 HMDS

-1 0

10

0

-10

-20

g

-30 -40
V (V)

-50

-60

10-3 HDMS
0

-4

-1 10

-4

-2 10

-4

-3 10

-4

-4 10

-1 2

10

-4

0 10

1/2

10

-1 10
-1.5 10

-50 V

-4

-4 10

d

10

-8

1/2

-7

d

10

-5

d

-6

09-2 HMDS

0

-5 10

-4

-3 10

Figure 3.2.8. Transfer (a) and output (b) characteristic of a
DNTT-based OTFT.

-5

0 10

-40 V

-4

-2 10

g

-I (A)

10

-9

10

-60

d

-5

5 10

09-1 HMDS-I (A)d-I (A)

10

10

Square Root of -I (A )-4Square Root of -I (A )

10

-8

d

S
10

-7

10

-1 0

g

-3

10

-4

-6 10

06-1 UV-OTS-8 output

0

-4

-1 10

d

10

-5

-4

0 10

-I (A)

-8

-4

10

1 10

06-1 UV-OTS-8 transfer

Vd = -40 V

d

-7

10

-4

-I (A)

10-3

10

05-1 UV-OTS-8 output

d

-6

10

0

-I (A)

-5

10

d

10

-I (A)

d

-4

-4

0 10

1/2

10

2 10

05-1 UV-OTS-18 transferd-3

Square Root of -I (A )

-2

10

d

10

Square Root of -I (A )-I (A)

(UV)

-4

-5 10

20 10 0 -10 -20 -30 -40 -50 -60
V (V)
g

d

0

-10 -20 -30 -40 -50 -60
V (V)d

S (UV)-1.5 10

-3

-2 10

-3

10

-4

10

-5

d

-3

-3

10

-6

10

-7

-9

0 -10 -20 -30 -40 -50 -60
V (V)
g

0

-10

-20

-30 -40
V (V)
d

-50

-60

10

1/2

1/2

-8

10

1 10

14-1 UV-HMDS

d

10

20 10

-1 10

10

d

-7

-4

-I (A)

-6

10

-5 10
d

d

-5

10

0 10

0

Square Root of -I (A )10

-I (A)

13-2 UV-HMDS

Square Root of -I (A )

-4

10

-4

-8

-4

0 10

14-1 UV-HMDS

0

-4

-1 10

-4

-2 10

-4

-3 10
d

5 10
13-2 UV-HMDS-I (A)-3

10-I (A)

-2

10

-4

-4 10

-4

-5 10

-4

-6 10

-4

20 10 0 -10 -20 -30 -40 -50 -60
V (V)
g

-7 10

0

-10

-20

-30 -40
V (V)

-50

-60

d

・ SAM 作製時に UV オゾン洗浄を取り入れた処理基板を用いて作製した FET 素子は、そうでないものと比較してに安定して高い特性を示すような気がする。
・ OTS-8 ではトランスファー特性における電流の立ち上がりが悪い素子が多かった。
・ HMDS(UV)で瞬間最大風速 6 が観測された。off は高めだが、特性図はきれい。

52

3. Evaluation of C10-DNTT

3.3

Fabrication of Polymer Dielectric films

Introduction.
of gate-dielectrics.
The gate dielectric material plays a crucial Table 3.3.1. Classificationmaterial
general features
part in the OFET operation, because chargeInorganic
High-k (high polar)
Low-k (non-polar)
carriers move in a few layer close to the in- Gate dielectrics Organic (polymers)
SAM
High-k, low-voltage
terface between the gate dielectric and theIonic liquid
Very high-k, liquid
Multi-component
114-116
semiconductor.
In addition, the molecular arrangement in the first few layers are
Table 3.3.2. Dielectric constant (k) of various
much influenced by the structure and surface en- gate insulators.
compoundkergy of the dielectric.156,157 The gate dielectric
Inorganic
requires such characteristics as (i) high dielectric
SiO2
3.9
constant (k) (= relative permittivity, εr) (ii) low
Si3N4
7
leakage current, (iii) smooth surface, (iv) low
Al2O3
9
ZrO
23
2
surface energy, (v) absence of carrier-trap (vi)
HfO2
24
stability, and (vii) mechanical flexibility.158 Over
Ta2O5
25
the past decade, a wide range of dielectric mateTiO2
58
rials have been developed (Table 3.3.1 andOrganic
3.3.2).28,159
polyvinylalcohol (PVA)
3.3
The commercially available Si/SiO2 is the mostpolyvinylphenol (PVP)
3.5
reliable test bed for organic semiconductors as itpoly(methyl methacrylate) (PMMA)
3.5
polyimide
3.5
was readily available from conventional Si techpalylene N
3.1
nology. The SiO2 has an excellent smooth surface
palylene C
2.7(roughness < 0.1 nm), stability, low leakage curCytopTM
2.1
rent, and high-k. The Si/SiO2 gate-dielectric is,
however, much differed from possible applications of OFETs because of its less flexibility.
Flexible dielectrics have been developed based on both inorganic and organic materials. The
surface-modified alumina on aluminum (Al/AlOx) with self-assembled monolayers (SAM) is
one of the superior inorganic flexible dielectric.48 DNTT-based OTFT devices on the Al/AlOx
showed 0.3 cm2/Vs with low-operating voltage according to Klauk et al.18 The fabrication of thedielectrics, however, requires vacuum-process and special techniques similar to the other inorganic dielectrics.
On the other hand, organic dielectrics are expected as inexpensive and flexible gate-dielectrics
because a variety of solution-processible polymer insulators have been demonstrated. In general, OTFT devices on the polymer dielectrics show higher µFETs compared to those of inorganic ones.160 Also, polymer dielectrics are compatible with general plastic substrates.
In this section, I examined several polymer dielectrics to OTFTs; Cytop, polyimide, andparylene N (Figure 3.3.1). The fabrication of the polymer dielectrics followed by evaluations of

Cl

Cl
Cl
Di-p-xylene(dimer)

Parylene N

Cl
Parylene CCl

ClParylene D


n

CF2

F2C

FC

CF

Au

C
F2

n

Cytop

Poly-p-xylene

Figure 3.3.1. Structure of Parylenes and Cytop. The structure of thepolyimide in this work is unknown.

53

Au
DNTT

CF2

O

polymer dielectric

ITO
glass

Figure 3.3.2. Device structure.

3.3 Fabrication of Polymer Dielectric films

(a)

(b)

-1

Electric Field / MV cm

2

10

-5

10-7

J

10

10-6

-9

10-11
-80

-40

0

40

Applied Voltage / V80-7

6 10-4V = -50 V

5 10

d

-4

10-8

4 10-410-9

3 10-410-10

2 10-410-11

1 10-4d-2/ A cm1

10-12-60 -40 -20 0
Vg / V

20

d

leak

0

(-I )1/2 / A1/2

10

-1

-I / A

10

-3-2

0

Figure 3.3.3. (a) Plot of leakage current density vs both applied voltage of Parylene N
dielectric. (b) A FET characteristics of DNTT-based OTFT on parylene N dielectric.

the dielectrics by means of measuring the insulating characteristics, capacitance, and TFT characteristics of DNTT-based devices on the dielectrics are described (Figure 3.3.2).
Parylene N. Parylenes (di-p-xylenes) are commercially available coating precursors that has inthree forms: parylene N, C, and D.161-163 The monomer pyrolyzes in high temperature (~ 650ºC)
and then polymerizes at room temperature. under ~10 Pa to give a transparent film on a substrate, which has low air- and water-permeability. Parylenes have frequently been used as dielectrics in top- or bottom-gate transistors or as protecting layer in devices.164.165
I built up an apparatus, and ca. 400 nm in thick of transparent parylene films on indium-tinoxides (ITO) glass substrates were fabricated. The parylene films showed Ci of ~10 nF/cm2 (k =~4.7). The DNTT-based OTFTs with the parylene films, however, showed poor characteristics
owing to the large gate-leakage current density (Jleak) over 10-7 A/cm2 at -40 V (Figure 3.3.3a).
One device showed an appreciable characteristic with µFET of 2 × 10-3 cm2/Vs and Ion/Ioff of~103 (Figure 3.3.3b). The low performances are responsible for low quality of the parylene
films. Fabrication procedure should be improved.

54

3. Evaluation of C10-DNTT-2-7

2 10-32

0.1 cm /Vs

1/2

10

510 nm-3

3 10-32

0.07 cm /Vs

d

leak

10-8

4 10

2

0.6 cm /Vs

1640 nm

10

10-9
10-11
-200

-100

0

100

Applied Voltage / V

Figure 3.3.4. Leakage current of Cytop
films with different thickness.

200

-10

10-12
-60

2

0.7 cm /Vs

-40

-20

0
20
V /V

/A

J

110 nm

310 nm

-6

40 µm
90 µm
140 µm
190 µm

1/2

-5

10

d

d

10

V = -40V

(-I )

10-3

L=

1640 nm
510 nm
310 nm
110 nm-1/ A cm10

1-I / A10

1 10-3

40

0
60

g

Figure 3.3.5. Transfer characteristics of DNTT-based
OTFTs on Cytop films of 1460 nm in thickness.

Table 3.3.3. OFET characteristics of DNTTa on Cytop films fabricated on
ITO glass substrate.
conc.bd / nmLc / µm
Vth / VµFET / cm2V-1s-1
Ion/Ioff
9
1460
40
-25
0.05 - 0.07
105
90
-19
0.12 - 0.13
106
140
-20
0.59
106
190
-8
0.09 - 0.69
106
6
510
40
-30
0.004 - 0.02
106
90
-20
0.67 - 0.82
106
140
-29
0.82 - 0.83
108
190
-24
0.85 - 0.91
107
5
310
140
-16
0.06
105
3
110
no field-effect response
a DNTT layer was deposited at T
sub = 60 ºC, Rdep = 1.0 Å/s, thickness ca. 500 Å.
b Dissolved in fluoro CTL-180v. c W = 1500 µm.

Cytop. Cytop™ is the fluoropolymer developed by Asahi Glass Chemical. Since Cytop provides highly hydrophobic surface (low surface energy), transparent, and low-k dielectric film,
several researchers have reported high performance OTFTs on Cytop films.166-174 The OTFTs on
Cytop include DNTT-based bottom-gate devices that showed µFETs up to 0.2 cm2/Vs.175
Cytop films were fabricated by spin-coating followed by annealing process (see Experimental
Procedures). Various thickness of Cytop films were prepared by changing concentration (9 % 3 %). All films showed typical dielectric constant of k = ~ 2.0 nF/cm2. The Cytop dielectrics
with relatively larger thickness (1460 and 510 nm) showed good insulating properties with Jleak< 10-7 A/cm2 at 100 V, although thinner ones (310 and 110 nm) showed poor characteristics withlarge Jleaks up to 10-3 A/cm2 at Vg < 100 V (Figure 3.3.4).
The OTFTs were fabricated by deposition of DNTT on the Cytop layer followed by depositionof gold S-D electrodes. The OTFTs on 1460 nm- thick of Cytop dielectrics showed good µFETs
up to 0.7 cm2/Vs and IonIoff ~106 (Table 3.3.3). The OTFT devices on the 510 nm of Cytop/ITOglass also showed good performances with µFETs ~0.9 cm2/Vs. The characteristics on the Cytop
dielectrics, however, have positively higher turn-on voltage (> +30 V) and large sub-thresholdswing (> 10 V/dec) than those on the Si/SiO2 (Figure 3.3.5). The OTFTs on the thinner Cytop
films (310 nm and 110 nm) showed large Id in the subthreshold regions. The devices on 110 nm
of Cytop were no field-effect response at all. This is attributable to low quality (many pin-holes,etc) of the Cytop films. Realizing TFTs characteristics on polymer dielectrics as good as thoseon the SiO2 dielectric is more difficult than expected.

55

3.3 Fabrication of Polymer Dielectric films-1Electric Field / MV cm
-2
0
2

-4

10

-9

I

leak

2

J

leak

/A
10-11

10-9

10

break
down

break
down
10-13
200

-11

-200

-100
0
100
Applied Voltage / V

Figure 3.3.6. Plot of leakage current
density (leakage current) vs applied voltage (electric field) of 500 nm in thickness of the polyimide film.
(a)V = -60 V
d

10

0.02

3 10-4

10-6

-40

-20 0
V /V
g

20

d

d

-60

0
40

-4

2 10

-60V

1/2

10-13
-80

0.005

/A

10

4 10

0.01

-11

-4

0.025

0.015

1 10-4-20 V, 0 V

-80

-60

-40

-20

0

V /V
d

Figure 3.3.7. Transfer (a) and output characteristics (b) of DNTT-based TFT
on the polyimide dielectric. The extracted µFETs in saturate region are 0.67 cm2/
Vs, 0.55 cm2/Vs, 0.43 cm2/Vs, 0.40 cm2/Vs, and 0.49 cm2/Vs in devices with Lof 20 µm, 40 µm, 90 µm, 140 µm and 190 µm, respectively.

5d

10-8

10-10

-40V
0

V = -60V
d

d

1/2

10

-9

10-2

04-2 (20µm) output
V = -80V

d

10

-7

5 10-4-4(-I )-I / A

10-5

(b)
0.03

-I / A

10-3

04-1(20µm)
04-5(90µm)
04-3(40µm)
04-7(140µm)
04-9(190µm)d/ A cm4

sample 1

3

Table 3.3.4. Characteristics of DNTT-based OTFTs on
polyimide dielectrics.ad / nm
L b / µm
Vth / VµFET / cm2V-1s-1 Ion/Ioff
870 - 880
40
-2 - +170.40 - 0.53
107
90
+7 - +18
0.44 - 0.56
107
140
+4 - +18
0.38 - 0.67
107
190
0 - +220.27 - 0.63
107
490 - 500
40
0 - +60.45 - 0.55
10890
+5 - +10
0.41 - 0.50
107140
+9 - +110.43 -0.44
107
190+3 - +12
0.08 - 0.50
107405 - 41040
+2 - +6
0.38 - 0.52
107
90
+3 - +50.30 - 0.55
107
140
+4 - +80.24 - 0.53
108
190
+4 - +80.45 - 0.50
107
a top-contact configuration. DNTT was deposited with T
sub = 60ºC and Rdep = ca. 1.0 Å/s. b W = 1500 µm,

10-7-I / A

10

-7

-2

10

-5

4

10-12
-80

fresh
5day later
100ºC anneal
-60

-40

-20

0

20

40

V /V
g

Figure 3.3.8. Transfer characteristics of DNTT devices on thepolyimide being stored for 5 days,
followed by annealing in N2 atmosphere at 100 ºC.

Polyimide. Polyimide is one of the popular insulating materials widely used in electronics,
because of their high chemical resistance and mechanical properties.176-179 Although high curing
temperature (~300 ºC) prevented application to TFTs for the decades, recent progress in polymer technology makes it possible to commercialize a class of polyimide precursors, which canbe cross-linked at temperature as low as 180 ºC. Someya et al have reported high performancepentacene-based OTFTs on polyimide gate-dielectrics with mobilities up to 1 cm2/Vs.180 The
OTFTs are integrated into complex electronics devices.165
Polyimide films were fabricated on ITO-glass substrates by spin-coating polyimide precausor
followed by annealing procedure ~180 ºC. Thickness of the polymide films are measured to be
880 nm to 410 nm depending on the spin rates (see Experimental Procedures, Table 3.3.11), allof which showed typical k (ca. 3.1) and high insulating properties with Jleak < 10-7 A/cm2 at Vg <100 V (Figure 3.3.6).
Independent of the channel length (L) and dielectric thickness, DNTT-based OTFTs on thepolyimide dielectrics showed high performances with µFETs of ca. 0.5 cm2/Vs, IonIoff of ~107
(Table 3.3.4, Figure 3.3.7). Large turn-on voltage (Von) up to +30 V were observed in all characteristics, which were shifted positively after allowing the device to stand in ambient air (Figure
3.3.8). The shift was recovered by annealing the device in N2 atmosphere at ~100ºC. The resulting transfer characteristics of annealed devices were better than those of fresh devices. The Von
shift can be explained by absorption of impurities such as H2O and O2.160

56

3. Evaluation of C10-DNTTIntensity / a. u.

Cytop

polyimide

OTS-treated SiO2

0

5

10

15

20 25
2θ/θ

30

35

40

Figure 3.3.9. XRD patterns of thin films of DNTT on Cytop, polyimide, and OTS-treated Si/
SiO2. d-Spacings calculated from first peak are 16.3 Å.
(a)

(b)

(c)

Figure 3.3.10. AFM images of thin films of DNTT on (a) Cytop(b) OTS-treated SiO2 (c) polyimide.

Thin-film structures. The surface morphologies and the crystallinity were investigated by Xray diffraction (XRD) and atomic force microscopy (AFM), respectively. The XRD patterns of
DNTT films showed the same sharp reflections which assignable to crystallographic (00l) diffractions on ITO/Cytop, ITO/polyimide, and OTS-treated Si/SiO2 (Figure 3.3.9). The intensitiesof peaks are, however, in the order of OTS > polyimide > Cytop, indicating crystallinity of thethin films are slightly different with this order. On the other hand, the AFM images showed thatca. 100 nm size crystalline grains grew on the surfaces on all dielectrics (Figure 3.3.10). These
structures indicates the difference of the characteristics of devices depending on the dielectrics
may be attributed the the difference of the crystallinity of the films and subtle structural difference of dielectric and organic layer near the interface.
Summary. Three kind of polymer dielectrics, Parylene N, Cytop and polyimide, were fabricated on the ITO-glass substrates and evaluated their properties including TFT characteristics of
DNTT-based devices on the dielectrics. Fundamental techniques of fabrication/evaluation ofpolymer dielectrics were established.
Parylene N showed poor performances as the gate-dielectric and consequently poor TFT characteristics. In contrast, Cytop and polyimide showed good dielectric properties and TFT characteristics with high µFETs of up to 0.9 cm2/Vs and 0.5 cm2/Vs, respectively. The Cytop-based
OTFTs showed higher µFETs and air-stability, however, which also showed low-reproducibility
and large subthreshold swing, especially short channel devices. On the other hand, thepolyimide-based OTFT showed steady µFET and Ion/Ioff values regardless of the thickness of dielectric films or L of the devices. I thus decided to use polyimide as a dielectric material for fabrication of flexible C10-DNTT devices on plastic substrates in section 3.4, although relatively

573.3 Fabrication of Polymer Dielectric films

high curing temperature (~180 ºC) of polyimide could be a potential problem in the application
to a flexible substrate.

Experimental Procedures
Fabrication of dielectric films:
Parylene N: The parylene N was placed on a quartz glass tube, and reduced pressure under ~5 Pa. The
tube near the sample was heated slowly from 80 ºC to 170 ºC over ca. 1h while the tube far from the sample was heated to 660 ºC.
Cytop: Glass plates coated with indium-tin oxides (ITO-glass) were used for substrates. The ITO were
patterned as gate-electrodes (see Section 3.4). The Cytop dielectrics were simply fabricated on the ITOglass substrates by spin-coating of Cytop as purchased (CTL-809M, 9%) or diluted with the attachedfluorocarbon-based solvent for spin-coating (CTL-180v). Cytop films are also fabricated on Si/SiO2 substrates for comparison. The spin-coated Cytop were cured at 50 ºC for 1 h, 80 ºC 1h, and 120 ºC 2 h innitrogen atmosphere.
The Cytop films with some different thicknesses from 1460 nm to 110 nm were obtained from different
concentration of Cytop solvent from 9 % to 3 % (Table 3.3.10)
Table 3.3.10 Properties of Cytop dielectrics
conc.
spin rated / nm Ci / nF cm-2
9%
1460
1.3
6%
510
3.4
500 rpm 10s
1000 rpm 30 s
4.5%
310
5.5
3%
110
14.6

k
2.1
2.0
1.9
1.8

Polyimide. The polyimide precursor (KEMI-TITE CT4112, Kyocera Chemical) was spin-coated onto
the gate-patterned ITO-glass substrates with vairous spin rate: 1000 rpm (2 s) then 2000 rpm (58 s), 2000
rpm (2 s) then 4000 rpm (58 s), and 3000 rpm (2 s) then 6000 rpm (58 s) (Figure 3.3.11). The substrates
were cured at 80 ºC for 1 h, 120 ºC 1h, and 180 ºC 1h under nitrogen atmosphere to give the transparent
polyimide films.
Table 3.3.11 Properties of Polyimide Dielectrics
spin rated / nmCi / nF cm-2
k2000 rpm
870 - 880
2.9 - 3.2
2.8 - 3.2
4000 rpm
490 - 500
5.4 - 5.6
3.0 - 3.1
6000 rpm
405 - 410
3.1
3.1

Fabrication of OTFT devices. DNTT was deposited for 50 nm onto the dielectrics under the conditions of Tsub = 60 ºC and Rdep = ~2.0 Å/s. Au S-D electrodes were deposited with Rdep = ~2.0 Å/s throughthe shadow mask to give top-contact OTFT devices and MIM devices for measurement of characteristics
dielectrics.
Dielectric characteristics. A film capacitance (Ci) was measured on a ITO-dielectric-Au structure
which fabricated together with OTFT devices by using a Keithley 4200-SCS Semiconductor analyzer.
Standard measurement condition are DC bias 0 V, presoak 0 V, frequency 10 kHz, AC voltage 30 mV
RMS, sampling interval 0.5 s, sampling number 10 in voltage bias mode. The frequency (f) should approximately be equal to the sweep speed (S) of gate-voltage (Vg) of the transfer characteristics measurement. e.g., if Vg is swept from 40 V to -80 V in 2 sec, S would be 60 V/s. The 60 V/s corresponds to f = 1
kHz when AC = 30 mV RMS. Hint is (60 [V/s]) = (15 m[V]) / ((1 / 4f)[s]). Average of measured value(CDUT) is divided by the area of electrode (2.90 (±0.04) × 10-3 cm2) to give Ci. The dielectric constant isestimated by k = (Ci • d) / ε0, where ε0 = permittivity of vacuum (= 8.85 × 10-14 F/cm), d = film thickness(cm).
Ci of SiO2 (200 nm) were measured first for certification of the system. The resulting Ci = 16.1 nF/cm2
is most likely as a value of SiO2, although the value is different with 17.3 nF/cm2 that usually I employed.
Measurement of Insulating Properties. The measurement was carried out by connection of one sideof a MIM device with the prober that usually used as gate, and another side of the MIM device with theprober that used as source (ground). Electric field (E) in leakage characteristics is calculated by
E=

Vg
d

where Vg is the applied voltage [V], d is film thickness [cm].
Current density (J) is calculated by58

3. Evaluation of C10-DNTT

J=

Ig
S

where Ig is the gate current [V], S is area of electrode, 2.90×10-3 [cm2].
Thickness Measurement. Thickness of film (d) are measured with an AMBOS Stylus profilometer.
The general measurement conditions are speed 0.20 mm/sec, range 100 microns, stylus force 0.5 mg.
Atomic Force Microscopy (AFM). The AFM images were measured on a SII nanocute with cycliccontact dynamic force mode (DFM).
X-ray diffraction (XRD). Thin-film XRD were measured on a Rigaku Ultima Ⅵ X-ray diffractometerwith Cu Kα source (λ = 1.541 Å) in air. The d-spacings were calculated by the Bragg equation: λ =2dsinθ.

59

3.4 Evaluation on Plastic Substrates

3.4

Evaluation on Plastic Substrates

Introduction. The technology of the organic thin film transistors (OTFTs) has been attracted
much attention, because of their potential applications as a key device for the future plastic
electronics.3,8 One of the important features of OTFTs that can differentiate from ordinary
silicon- or metal oxide-based TFTs is lower processing temperature (<200 ºC) than those for the
inorganic-based TFTs (several hundreds degree), which allows OTFT to be fabricated on plastic
films. In fact, fabrication of OTFTs on the plastic substrates has been investigated actively, andtheir applications to various devices including the drivers for flexible displays,164,10b bendable
sensors,165,181 sheet-like scanners, rewritable braille sheets,179,182 and so on, have been
demonstrated.
Most of these plastic OTFTs have employed pentacene (Figure 3.4.1),145-147,180 the standard
vapor-processible organic semiconductor, as the active material that enables good performancewith mobility as high as ~ 3 cm2/Vs.48,8 However, the lack of air-stability and durability of
pentacene itself is an apparent drawback for actual plastic electronics applications.183 On theother hand, dinaphto[2,3-b:2’3’-f]thieno[3,2-b]thiophene (DNTT), developed as an air-stable
and high performance organic semiconductor with mobility of up to 3.0 cm2/Vs, is an attractive
alternative,17 and recently flexible, low-voltage OTFTs and circuits based on DNTT as theactive semiconducting material have been realized.18 In addition, very recently further material
improvement in terms of the mobility has been achieved based on the DNTT structure:alkylation of DNTT core turned out to be quite effective to enhance mobility,124,125 andoptimized di(n-dodecyl)-DNTT (C10-DNTT, Figure 3.4.1)-based devices on Si/SiO2 substrates
reaches up to 8 cm2/Vs.22
In the final section of this chapter, I describe C10-DNTT-based TFTs fabricated on thepolyimide gate dielectrics coated over polycarbonate (PC) substrates along with the parent
DNTT-based TFTs on the same substrate for comparison (Figure 3.4.2). The C10-DNTT-based
devices showed excellent performances with mobility as high as 2.4 cm2/Vs and Ion/Ioff grater
than 107 under ambient conditions.

SS

pentacene

DNTTS
C10H21

C10H21S
C10-DNTT

Figure 3.4.1. Chemical structure of organic
semiconductors.

Figure 3.4.2. Schematic device structure of TFTs
fabricated on PC substrates.

capton tapecapton tape (2 mm)

IZOetching

peel offPC

spin-coat polymer
dielectrics

protecting film
deposition
Au layer

deposition organic layer

curingpeel off

Figure 3.4.3. Device fabrication process.

60

3. Evaluation of C10-DNTTIntensity / a.u.

(001)

(a)

(002) (003)

Polyimide(b)

ODTS-treated Si/SiO2

4

8

12
2θ/degree

16

20

Figure 3.4.5. AFM images of C10-DNTT thinfilms: (a) on a PC/polyimide substrate and (b) a
Si/SiO2 substrates.

Figure 3.4.4. XRD patterns of C10-DNTT thin films: (a) on
a PC/polyimide substrate and (b) a Si/SiO2 substrates. The
calculated d-spacing = 3.8 nm.

Methodology. A PC film coated with indium-zinc oxide (IZO) layer was cut into pieces of 2
× 2 cm squares (Figure 3.4.3).184 The IZO layer was patterned for gate electrodes by etchingprocess: the substrates that protected by capton tape were immersed in diluted aqua regia (HCl :H2O : HNO3 = 1 : 4 : 0.08) for ~10 sec. After removal of the capton tape, the substrates were
washed with with water, acetone, and isopropanol. Polyimide precursor (KEMI-TITE CT4112,
Kyocera Chemical)180 was spin-coated on PC plastic substrates, edge of which had been protected by capton tape for gate-contact for measurement. The spin rate was 1000 rpm for 10 s
and 4000 rpm for 30 s. The spin-coated substrates were immediately placed on a hot plate for~10 min at ~80 ºC. The substrate was then annealed at 180 ºC for 1 h under nitrogen atmosphere, giving the transparent polyimide film (ca. 600 nm thickness).
C10-DNTT and DNTT were synthesized according to the reported procedures and purified by
vacuum gradient-sublimation.17,22 Vacuum deposition of the materials (60 nm thickness) on thesubstrate with 60 ºC of substrate temperature (Tsub) and 0.1 Å/s of deposition rate (Rdep) gave auniform semiconducting layer. On top of it, vapor deposition of gold (80 nm thickness) througha shadow mask defined the source and drain electrodes, giving OTFT devices with a topcontact, bottom-gate configuration having a constant channel width (W) of 1500 µm and various
channel lengths (L) of 40 ~ 190 µm. The TFT characteristics are evaluated with a Keithley4200-SCS semiconductor characterization system. Mobility was calculated from the transfer
curves in the saturate regime by using the following equation:µFET = (2 · Id · L) / [W· Ci · (Vg - Vth)2]
where µ is mobility, Id is source-drain current, Ci is capacitance of dielectric, and Vg is gatesource voltage. The thickness (d) and capacitance (Ci) of the dielectrics for each device were
measured with an AMBIOS Stylus Profilometer and Keithley 4200-CVU, respectively.
Experimentally decided Ci values showed good agreement with the value estimated from thethickness of the polyimide layer and reported εr value.
The atomic force microscopy (AFM) images were measured on a SII nanocute with cycliccontact dynamic force mode (DFM). Thin-film X-ray diffraction (XRD) were measured on a
Rigaku Ultima Ⅵ X-ray diffractometer with Cu Kα source (λ = 1.541 Å) in air.
Thin film morphology. Crystallinity and surface morphology of the C10-DNTT layer
deposited on the polyimide gate dielectric were investigated with X-ray diffraction (XRD) andatomic force microscopy (AFM), respectively. The XRD patterns (Figure 3.4.4) clearly testify
that a crystalline C10-DNTT thin film formed on the polyimide gate dielectric and the molecular61

3.4 Evaluation on Plastic Substrates

Figure 3.4.6. Transfer characteristics of C10-DNTT-TFTs: (a) fresh devices and (b) annealed devices at 100 ºC.

arrangement is basically the same with that on the Si/SiO2 substrate. Although the first XRD
peak, i.e., the 001 reflection is clearly observed on the polyimide layer, higher reflections arequite broad and weak (Figure 3.4.4a), whereas the thin film on the Si/SiO2 substrate shows clear
peaks up to 003 reflections (Figure 3.4.4b). This indicates that the crystallinity of the thin filmon the polyimide layer is lower than that on the Si/SiO2 substrate, which is presumably causedby the larger roughness of the former than the latter.
AFM images show that wire-like crystalline grains grew on the polyimide layer, whereas
relatively lager domains with rod-like crystals are observed for the thin film on the Si/SiO2(Figure 3.4.5). This morphological difference can be again ascribed to the surface roughness ofthe substrates. Rather flat and smooth surface of Si/SiO2 can induce the formation of larger
domains, whereas the polyimide surface with large roughness can facilitate rapid nucleation,
resulting in different surface morphologies.
TFT characteristics. The C10-DNTT-based TFTs on the polyimide gate dielectrics showed
decent p-channel FET responses under ambient conditions (Figure 3.4.6). Mobilities extracted
from the saturation regime at the drain (Vd) and gate voltage (Vg) of –60 V were higher than 0.5
cm2/Vs, regardless of the channel lengths (L) (Table 1). Noticeable channel length (L) dependence of the mobility was observed, as was the case for the devices on the Si/SiO2 substrates, andthe devices with longer L tend to give higher mobility. The highest mobility among the fresh
devices on the polyimide gate is ~1.5 cm2/Vs at L = 190 µm, which is about one fifth of the best
device fabricated on the Si/SiO2 substrate (7.9 cm2/Vs). It is highly likely that the reduced
mobility of the devices on the polyimide gate is ascribed to the reduced crystallinity of the thinfilm as described above, which may cause higher channel resistance owing to the increase thegrain boundary, in other words, the large numbers of carrier traps in the channel region. Similar
reduction of the mobility caused by changing the substrate and gate dielectric from the Si/SiO2
to PC/polyimide was also observed for the unsubstituted DNTT-based TFTs: the mobility ashigh as 3.1 cm2/Vs on the Si/SiO2, whereas 0.5 cm2/Vs on the present PC/polyimide substrate. It
is thus rational to consider that the surface roughness of the gate dielectric causing the limited
crystallinity of the semiconducting layer is one of the major reasons for reducing the mobility.
On the other hand, even on such less ideal substrate surface, the present C10-DNTT-TFTs
showed quite high mobility, clearly indicating the good potential of C10-DNTT molecule as anorganic semiconductor. It should be also noted that pentacene TFTs fabricated on the identical
PC/polyimide substrate showed mobilities up to 0.24 cm2 cm2/Vs, which is one order ofmagnitude lower than those of the C10-DNTT-TFTs.
Another noticeable differences of the TFT characteristics of the present devices comparedwith those on the Si/SiO2 substrate are positive turn-on voltage (~ +20 V) and sluggish increaseof Id in the subthreshold region. Although the actual reasons for these non ideal behaviors are62

3. Evaluation of C10-DNTT

not clear, it is generally believed that they are attributed to the residual dopant and carrier traps,
respectively, in the interface between the semicondcuting layer and the dielectric layer. Sincesuch non-ideal characteristics are often improved by thermal annealing after the device
fabrication,176 we tested a thermal treatment (~100 ºC) of the devices under nitrogen. Figure
3.4.5 shows the transfer characteristics of the annealed devices; although no significant
improvements of the Id increase in the subthreshold region was observed, increase of mobility
and reduction of off-current were observed.
I also checked the stability of the present C10-DNTT-TFTs. Different from thepentacene-based OTFTs, the C10-DNTT-TFTs showed good stability during storage underambient lab conditions: no apparent reduction was observed in the mobility and Ion/Ioff ratio
after 30 days without any encapsulations, although slight Vth shift was observed.
Summary. In summary, I have successfully fabricated C10-DNTT-TFTs on the PC/polyimide
plastic substrate. The devices showed good p-channel characteristics with the mobility over 2.4
cm2/Vs and Ion/off of 107 under ambient conditions. Although the present mobility is somewhat
reduced compared to those of the C10-DNTT-TFTs on the Si/SiO2 substrates, which may becaused by reduced crystallinity of the semiconducting layer on the polyimide gate dielectric asconfirmed by the AFM and XRD measurements, the mobility itself is quite high for so far
reported OTFTs fabricated on the plastic substrate. Further process optimizations as well as
applications of C10-DNTT to other flexible substrate/gate dielectrics are now underway in ourgroup.

63

3.5. Conclusion for Chapter 3

3.5.

Conclusion for Chapter 3

I successfully evaluated C10-DNTT as a high performancesemiconductor showing mobilities (µFETs) up to 8 cm2/Vs on
Si/SiO2 substrates and 2.4 cm2/Vs on PC/polyimide substrates (Figure 3.5.1).
The high µFETs were obtained reproducibly with the newly
designed device configuration. Channel length (L) dependence was observed in the TFT characteristics of the C10DNTT-based devices, which indicates contact resistance

Figure 3.5.1. Picture of the C10DNTT-based OTFT on the plastic
substrate.

limits the µFET significantly in the short L devices. In contrast, DNTT showed almost no L dependence, in spite of almost the same HOMO energy level of DNTT as that of C10-DNTT. The
structural difference in the solid state may affect the charge-injection characteristics.
The DNTT and 10-DNTT-based OTFT devices on the PC/polyimide substrates gave the µFETs
up to 0.6 cm2/Vs and 2.4 cm2/Vs, respectively. Apparently higher µFETs of C10-DNTT-based devices than those of DNTT-based devices indicate the important role of the long alkyl chains forachieving high µFETs, which enhances 2-D electronic interaction between the π-cores even on
the polymer dielectrics. Formation of the crystalline films was elucidated by the XRDs. However, the slightly increasing disorder in the films of DNTT and C10-DNTT on the polyimide canbe attributed to the lower µFETs as compared to those on the SiO2 dielectric.
The present performances and air-stability of the OTFTs based on C10-DNTT on bare Si/SiO2
and PC/polyimide substrates is comparable or superior to those of pentacene. However, the contact resistance limiting TFT characteristics and the low solubility limiting fabrication process
should be improved.185 Further active development of organic semiconductors will be expected.

64Closing Remarks

Closing Remarks

I have developed small π-conjugated molecules with long alkyl chains for thin-filmbased OFETs, in terms of the molecular design, synthesis, FET characteristics, structural analysis, theoretical analysis, and application to flexible devices. I believe this interdisciplinary studies contribute to development of organic semiconductors. The FET
characteristics were well-correlated with the electronic structures in the layer-by-layer
structures, which may give new ideas for designing organic semiconductors. The success of this study is come from the study of TTF derivatives that gave the good molecular concept followed by application of the concept to the high performance DNTT πcores. The result could not be achieved only by myself. The collaboration should be essential for the development of materials and devices.

65

References

References(1)

秋田純一, ゼロから学ぶ電子回路, 講談社, 2002.

(2)

大西一功, 藤田実, 絵ときでわかる半導体デバイス, オーム社, 2002年 livepage.apple.co.jp

(3)
(4)
(5)
(6)

Klauk, H. Organic Electronics: Materials, Manufacturing, and Applications; Wiley-VCH, 2006.
Nokia Coporate; http://www.nokia.com/press/media_resources/photos/corporate/showphotos?category=rd
NIKKEI MICRODEVICES, July 2009 vol. 288; NikkeiBP.
鵜飼育弘, 薄膜トランジスタ技術のすべて―構造、特性、製造プロセスから次世代TFTまで, 工業調査
会、2007.

(7)
(8)
(9)

Semiconductor FPD World, 2010, 29, 31-37.
Kagan, C. R.; Andry, P. Thin-Film Transistors; CRC Press, 2003.
“Extreme bending OTFT” Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Nat. Mater. 2010, 9, 10151022.
(10) (a) Sony Co Ltd; http://www.sony.co.jp/SonyInfo/technology/technology/theme/organictransistor_01.html. (b)Gelinck, G.; Heremans, P.; Nomoto, K.; Anthopoulos, T. D. Adv. Mater. 2010, 22, 3778-3798.
(11) (a) Roberts, M. E.; Sokolov, A. N.; Bao, Z. J. Mater. Chem. 2009, 19, 3351-3363.
(b) Semiconductor FPD World 2009, 28, 37-51.
(12) Nikkei Electronics 2009, 12-28, 73-82.
(13) Murphy, A. R.; Fréchet, J. M. J. Chem. Rev. 2007, 107, 1066-1096.
(14) (a) “Single-crystal DT-TTF-based OFETs” Mas-Torrent, M.; Durkut, M.; Hadley, P.; Ribas, X.; Rovira, C. J.
Am. Chem. Soc. 2004, 126, 984-985.
(b) “Correlation DT-TTF derivative” Mas-Torrent, M.; Hadley, P.; Bromley, S. T.; Ribas, X.; Tarrés, J.; Mas,
M.; Molins, E.; Veciana, J.; Rovira, C. J. Am. Chem. Soc. 2004, 126, 8546-8553.
(c) Recently higher performance of DT-TTF-based single-crystal OFET was reported: Leufgen, M.; Rost, O.;
Gould, C.; Schmidt, G.; Geurts, J.; Molenkamp, L.; Oxtoby, N.; Mas-Torrent, M.; Crivillers, N.; Veciana, J.;
Rovira, C. Org. Elect. 2008, 9, 1101-1106.
(d) “DT-TTF polymorph” Pfattner, R.; Mas-Torrent, M.; Bilotti, I.; Brillante, A.; Milita, S.; Liscio, F.; Biscarini,
F.; Marszalek, T.; Ulanski, J.; Nosal, A.; Gazicki-Lipman, M.; Leufgen, M.; Schmidt, G.; Molenkamp, L. W.;
Laukhin, V.; Veciana, J.; Rovira, C. Adv. Mater. 2010, 22, 4198-4203.
(15) “N-alkyl-PyTTFs” Doi, I.; Miyazaki, E.; Takimiya, K.; Kunugi, Y. Chem. Mater. 2007, 19, 5230-5237.
(16) “N-acyl-PyTTFs” Doi, I.; Miyazaki, E.; Takimiya, K. Chem. Lett. 2008, 37, 1088-1089.
(17) “DNTT” Yamamoto, T.; Takimiya, K. J. Am. Chem. Soc. 2007, 129, 2224-2225.
(18) “DNTT on AlOx/SAM” Zschieschang, U.; Ante, F.; Yamamoto, T.; Takimiya, K.; Kuwabara, H.; Ikeda, M.; Sekitani, T.; Someya, T.; Kern, K.; Klauk, H. Adv. Mater. 2010, 22, 982-985.
(19) “DNTT-based single-crystal OFETs” Haas, S.; Takahashi, Y.; Takimiya, K.; Hasegawa, T. Appl. Phys. Lett.
2009, 95, 022111.
(20) “3D-OFET” Uno, M.; Tominari, Y.; Yamagishi, M.; Doi, I.; Miyazaki, E.; Takimiya, K.; Takeya, J. Appl. Phys.
Lett. 2009, 94, 223308.
(21) “Anisotropy DNTT” Yamagishi, M.; Soeda, J.; Uemura, T.; Okada, Y.; Takatsuki, Y.; Nishikawa, T.; Nakazawa,Y.; Doi, I.; Takimiya, K.; Takeya, J. Phys. Rev. B 2010, 81, 161306.
(22) “Cn-DNTT” Kang, M. J.; Doi, I.; Mori, H.; Miyazaki, E.; Takimiya, K.; Ikeda, M.; Kuwabara, H. Adv. Mater. inpress (DOI: 10.1002/adma.201001283).
(23) “C10-DNTT on Plastic Substrates” Doi, I.; Myong-Jin, K.; Kazuo, T. Cur. Appl. Phys. submitted.
(24) Heilmeier, G. H.; Zanoni, L. A. J. Phys. Chem. Solids 1964, 25, 603-611.
(25) “Polythiophene OFETs” Tsumura, A.; Koezuka, H.; Ando, T. Appl. Phys. Lett. 1986, 49, 1210-1212.
(26) Dimitrakopoulos, C.; Malenfant, P. Adv. Mater. 2002, 14, 99-117.
(27) Dimitrakopoulos, C. D.; Furman, B. K.; Graham, T.; Hegde, S.; Purushothaman, S. Synth. Met. 1998, 92, 47-52.
(28) Veres, J.; Ogier, S.; Lloyd, G.; de Leeuw, D. Chem. Mater. 2004, 16, 4543-4555.
(29) “Review” Katz, H. E. Chem. Mater. 2004, 16, 4748-4756.
(30) Osaka, I.; McCullough, R. D. Advanced Functional Regioregular Polythiophenes in Leclerc, M.; Morin, J. Design and Synthesis of Conjugated Polymers; Wiley-VCH, 2010.
(31) Takimiya, K.; Sakamoto, K.; Otsubo, T.; Kunugi, Y. Chem. Lett. 2006, 35, 942-943.
(32) Bao, Z.; Dodabalapur, A.; Lovinger, A. J. Appl. Phys. Lett. 1996, 69, 4108-4110.
(33) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741-1744.
(34) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685-688.
(35) Wang, G.; Swensen, J.; Moses, D.; Heeger, A. J. J. Appl. Phys. 2003, 93, 6137-6141.

66

References(36) Chang, J.; Sun, B.; Breiby, D. W.; Nielsen, M. M.; Sölling, T. I.; Giles, M.; McCulloch, I.; Sirringhaus, H.
Chem. Mater. 2004, 16, 4772-4776.
(37) Kline, R.; McGehee, M.; Kadnikova, E.; Liu, J.; Fréchet, J. Adv. Mater. 2003, 15, 1519-1522.
(38) Joseph Kline, R.; McGehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 222-228.
(39) Osaka, I.; McCullough, R. D. Acc. Chem. Res. 2008, 41, 1202-1214.
(40) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.;
Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 328333.
(41) Umeda, T.; Kumaki, D.; Tokito, S. J. Appl. Phys. 2009, 105, 024516-5.
(42) (a) Li, Y.; Wu, Y.; Liu, P.; Birau, M.; Pan, H.; Ong, B. Adv. Mater. 2006, 18, 3029-3032. (b) Nakayama, K.;
Uno, M.; Nishikawa, T.; Nakazawa, Y.; Takeya, J. Org. Elect. 2010, 11, 1620-1623.
(43) Osaka, I.; Zhang, R.; Sauvé, G.; Smilgies, D.; Kowalewski, T.; McCullough, R. D. J. Am. Chem. Soc. 2009,
131, 2521-2529.
(44) Osaka, I.; Abe, T.; Shinamura, S.; Miyazaki, E.; Takimiya, K. J. Am. Chem. Soc. 2010, 132, 5000-5001.
(45) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler, M.; Facchetti, A. Nature 2009, 457,
679-686.
(46) Anthony, J. E.; Facchetti, A.; Heeney, M.; Marder, S. R.; Zhan, X. Adv. Mater. 2010, 22, 3876-3892.
(47) Kelley, T. High-performance Pentacene Transistors; Wiley-VCH Verlag GmbH & Co. KGaA, 2006.
(48) Kelley, T. W.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. P. J. Phys. Chem. B
2003, 107, 5877-5881.
(49) Anthony, J. E. Chem. Rev. 2006, 106, 5028-5048.
(50) Takimiya, K.; Kunugi, Y.; Otsubo, T. Chem. Lett. 2007, 36, 578-583.
(51) Bao, Z.; Locklin, J. Organic Field-Effect Transistors; CRC Press, 2007.
(52) 森 健彦, 長谷川達夫, 有機トランジスタ材料の評価と応用Ⅱ, 第１〜２章, p3 - 16, シーエムシー出版,
2008.
(53) 小野昇, 低分子有機半導体の高性能化, Science&Technology, 2009.
(54) Murphy, A. R.; Fréchet, J. M. J. Chem. Rev. 2007, 107, 1066-1096.
(55) Sun, Y.; Liu, Y.; Zhu, D. J. Mater. Chem. 2005, 15, 53-65.
(56) Takeya, J.; Kato, J.; Hara, K.; Yamagishi, M.; Hirahara, R.; Yamada, K.; Nakazawa, Y.; Ikehata, S.; Tsukagoshi,
K.; Aoyagi, Y.; Takenobu, T.; Iwasa, Y. Phys. Rev. Lett. 2007, 98, 196804.
(57) Braga, D.; Horowitz, G. Adv. Mater. 2009, 21, 1473-1486.
(58) Briseno, A. L.; Mannsfeld, S. C. B.; Ling, M. M.; Liu, S.; Tseng, R. J.; Reese, C.; Roberts, M. E.; Yang, Y.;
Wudl, F.; Bao, Z. Nature 2006, 444, 913-917.
(59) de Boer, R. W. I.; Klapwijk, T. M.; Morpurgo, A. F. Appl. Phys. Lett. 2003, 83, 4345-4347.
(60) Li, R.; Jiang, L.; Meng, Q.; Gao, J.; Li, H.; Tang, Q.; He, M.; Hu, W.; Liu, Y.; Zhu, D. Adv. Mater. 2009, 21,4492-4495.
(61) (a) Takeya, J.; Yamagishi, M.; Tominari, Y.; Hirahara, R.; Nakazawa, Y.; Nishikawa, T.; Kawase, T.; Shimoda,
T.; Ogawa, S. Appl. Phys. Lett. 2007, 90, 102120. (b) Uno, M.; Tominari, Y.; Takeya, J. Org. Elect.. 2008, 9,753-756.
(62) Hasegawa, T.; Takeya, J. Sci. Technol. Adv. Mater. 2009, 10, 024314.
(63) Afzali, A.; Dimitrakopoulos, C. D.; Breen, T. L. J. Am. Chem. Soc. 2002, 124, 8812-8813.
(64) Weidkamp, K. P.; Afzali, A.; Tromp, R. M.; Hamers, R. J. J. Am. Chem. Soc. 2004, 126, 12740-12741.
(65) Hirao, A.; Akiyama, T.; Okujima, T.; Yamada, H.; Uno, H.; Sakai, Y.; Aramaki, S.; Ono, N. Chem. Commun.
2008, 4714-4716.
(66) Murphy, A. R.; Fréchet, J. M. J.; Chang, P.; Lee, J.; Subramanian, V. J. Am. Chem. Soc. 2004, 126, 1596-1597.
(67) Sirringhaus, H. Adv. Mater. 2005, 17, 2411-2425.
(68) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4, 15-18.
(69) Payne, M. M.; Parkin, S. R.; Anthony, J. E.; Kuo, C.; Jackson, T. N. J. Am. Chem. Soc. 2005, 127, 4986-4987.
(70) Llorente, G. R.; Dufourg-Madec, M.; Crouch, D. J.; Pritchard, R. G.; Ogier, S.; Yeates, S. G. Chem. Commun.
2009, 3059-3061.
(71) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J. J. Am. Chem. Soc. 1998, 120, 664-672.
(72) Curtis, M. D.; Cao, J.; Kampf, J. W. J. Am. Chem. Soc. 2004, 126, 4318-4328.
(73) Kondo, R.; Higa, M.; Kagoshima, S.; Hoshino, H.; Mori, T.; Mori, H. J. Phys. Soc. Jpn. 2006, 75, 044716.
(74) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Ponomarenko, S.; Kirchmeyer, S.; Weber, W. Adv. Mater.
2003, 15, 917-922.
(75) Gao, X.; Wang, Y.; Yang, X.; Liu, Y.; Qiu, W.; Wu, W.; Zhang, H.; Qi, T.; Liu, Y.; Lu, K.; Du, C.; Shuai, Z.; Yu,
G.; Zhu, D. Adv. Mater. 2007, 19, 3037-3042.
(76) Meng, H.; Sun, F.; Goldfinger, M. B.; Jaycox, G. D.; Li, Z.; Marshall, W. J.; Blackman, G. S. J. Am. Chem. Soc.
2005, 127, 2406-2407.

67

References(77) (a) Yamada, J.; Sugimoto, T. TTF Chemistry: Fundamentals And Applications Of Tetrathiafulvalene; SpringerVerlag, 2004. (b) Otsubo, T.; Takimiya, K. Bull. Chem. Soc. J. 2004, 77, 43-58.
(78) Mas-Torrent, M.; Rovira, C. J. Mater. Chem. 2006, 16, 433-436.
(79) Iizuka, M.; Shiratori, Y.; Kuniyoshi, S.; Kudo, K.; Tanaka, K. Applied Surface Science 1998, 130-132, 914-918.
(80) Noda, B.; Katsuhara, M.; Aoyagi, I.; Mori, T.; Taguchi, T.; Kambayashi, T.; Ishikawa, K.; Takezoe, H. Chem.
Lett. 2005, 34, 392-393.
(81) Miskiewicz, P.; Mas-Torrent, M.; Jung, J.; Kotarba, S.; Glowacki, I.; Gomar-Nadal, E.; Amabilino, D. B.; Veciana, J.; Krause, B.; Carbone, D.; Rovira, C.; Ulanski, J. Chem. Mater. 2006, 18, 4724-4729.
(82)
Mori. T, J. Phys. Cond. Mater 2008, 20, 184010.
(83) Naraso; Nishida, J.; Ando, S.; Yamaguchi, J.; Itaka, K.; Koinuma, H.; Tada, H.; Tokito, S.; Yamashita, Y. J. Am.
Chem. Soc. 2005, 127, 10142-10143.
(84) Naraso; Nishida, J.; Kumaki, D.; Tokito, S.; Yamashita, Y. J. Am. Chem. Soc. 2006, 128, 9598-9599.
(85) Yoshino, T.; Shibata, K.; Wada, H.; Bando, Y.; Ishikawa, K.; Takezoe, H.; Mori, T. Chem. Lett. 2009, 38, 200201.
(86) Mas-Torrent, M.; Hadley, P.; Bromley, S. T.; Crivillers, N.; Veciana, J.; Rovira, C. Appl. Phys. Lett. 2005, 86,
012110.
(87) Takahashi, Y.; Hasegawa, T.; Horiuchi, S.; Kumai, R.; Tokura, Y.; Saito, G. Chem. Mater. 2007, 19, 6382-6384.
(88) Kanno, M.; Bando, Y.; Shirahata, T.; Inoue, J.; Wada, H.; Mori, T. J. Mater. Chem. 2009, 19, 6548-6555.
(89) Yamada, T.; Kumai, R.; Takahashi, Y.; Hasegawa, T. J. Mater. Chem. 2010, 20, 5810-5812.
(90) Inoue, J.; Kanno, M.; Ashizawa, M.; Seo, C.; Tanioka, A.; Mori, T. Chem. Lett. 2010, 39, 538-540.
(91) Morioka, Y.; Nishida, J.; Fujiwara, E.; Tada, H.; Yamashita, Y. Chem. Lett. 2004, 33, 1632-1633.
(92) Gao, X.; Wu, W.; Liu, Y.; Qiu, W.; Sun, X.; Yu, G.; Zhu, D. Chem. Commun. 2006, 2750-2752.
(93) Bando, Y.; Shirahata, T.; Shibata, K.; Wada, H.; Mori, T.; Imakubo, T. Chem. Mater. 2008, 20, 5119-5121.
(94) Katsuhara, M.; Aoyagi, I.; Nakajima, H.; Mori, T.; Kambayashi, T.; Ofuji, M.; Takanishi, Y.; Ishikawa, K.;
Takezoe, H.; Hosono, H. Synth. Met. 2005, 149, 219-223.
(95) Simonsen, K. B.; Svenstrup, N.; Lau, J.; Simonsen, O.; Mørk, P.; Kristensen, G. J.; Becher, J. Synthesis 1996,
1996, 407-418.
(96) Takimiya, K.; Oharuda, A.; Aso, Y.; Ogura, F.; Otsubo, T. Chem. Mater. 2000, 12, 2196-2204.
(97) Jigami, T.; Takimiya, K.; Otsubo, T.; Aso, Y. J. Org. Chem. 1998, 63, 8865-8872.
(98) Jeppesen, J. O.; Takimiya, K.; Thorup, N.; Becher, J. Synthesis 1999, 1999, 803,810.
(99) Jeppesen, J. O.; Takimiya, K.; Jensen, F.; Brimert, T.; Nielsen, K.; Thorup, N.; Becher, J. J. Org. Chem. 2000,
65, 5794-5805.
(100)Stueber, D.; Patterson, D.; Mayne, C. L.; Orendt, A. M.; Grant, D. M.; Parry, R. W. Inorganic Chemistry 2001,
40, 1902-1911.
(101)Ferraris, J. P.; Poehler, T. O.; Bloch, A. N.; Cowan, D. O. Tetrahedron Letters 1973, 14, 2553-2556.
(102)Renner, R. M.; Burns, G. R. Tetrahedron Letters 1994, 35, 269-272.
(103)Crivillers, N.; Oxtoby, N. S.; Mas-Torrent, M.; Veciana, J.; Rovira, C. Synthesis 2007, 2007, 1621,1623.
(104)Cornil, J.; Calbert, J. P.; Beljonne, D.; Silbey, R.; Brédas, J. Adv. Mater. 2000, 12, 978-983.
(105)Cornil, J.; Beljonne, D.; Calbert, J.; Brédas, J. Adv. Mater. 2001, 13, 1053-1067.
(106)Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. Journal ofApplied Crystallography 1994, 27, 435.
(107)Sheldrick, G. M. Program for the Refinement of Crystal Structures; University of Göttingen: Göttingen, Germany, 1997.
(108)teXsan: Single Crystal Structure Analysis Software, Version 1.19; Molecular Structure Corporation and Rigaku
Corporation, 2000.
(109)Takimiya, K.; Yamamoto, T.; Ebata, H.; Izawa, T. Sci. Technol. Adv. Mater. 2007, 8, 273-276.
(110)Winkler, M.; Houk, K. N. J. Am. Chem. Soc. 2007, 129, 1805-1815.
(111)Calculated by Gaussian 03 program. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J.
R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J.
Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M.
Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X.
Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B.
B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez,
J. A. Pople, Gaussian 03, Revision C.02, Gaussian, Inc., Wallingford CT, 2004.
(112)Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bässler, H.; Porsch, M.; Daub, J. Adv. Mater. 1995, 7,
551-554.
(113)Balaji, G.; Phua, D. I.; Shim, W. L.; Valiyaveettil, S. Org. Lett. 2010, 12, 232-235.
68

References(114)Jung, S.; Yao, Z. Appl. Phys. Lett. 2005, 86, 083505.
(115)Huang, J.; Sun, J.; Katz, H. E. Adv. Mater. 2008, 20, 2567-2572.
(116)Ruiz, R.; Papadimitratos, A.; Mayer, A.; Malliaras, G. Adv. Mater. 2005, 17, 1795-1798.
(117)Miyazaki, E.; Kaku, A.; Mori, H.; Iwatani, M.; Takimiya, K. J. Mater. Chem. 2009, 19, 5913-5915.
(118)Engler, E. M.; Patel, V. V.; Andersen, J. R.; Schumaker, R. R.; Fukushima, A. A. J. Am. Chem. Soc. 1978, 100,
3769-3776.
(119)Santalo, N.; Veciana, J.; Rovira, C.; Molins, E.; Miravitlles, C.; Claret, J. Synth. Met. 1991, 42, 2205-2209.
(120)Rovira, C.; Veciana, J.; Santalo, N.; Tarres, J.; Cirujeda, J.; Molins, E.; Llorca, J.; Espinosa, E. J. Org. Chem.
1994, 59, 3307-3313.
(121)Rovira, C. Chem. Rev. 2004, 104, 5289-5318.
(122)Kumaki, D.; Yahiro, M.; Inoue, Y.; Tokito, S. Appl. Phys. Lett. 2007, 90, 133511.
(123)Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Radlik, W.; Weber, W. J. Appl. Phys. 2002, 92, 5259-5263.
(124)“Cn-BTBT” Ebata, H.; Izawa, T.; Miyazaki, E.; Takimiya, K.; Ikeda, M.; Kuwabara, H.; Yui, T. J. Am. Chem.
Soc. 2007, 129, 15732-15733.
(125)Izawa, T.; Miyazaki, E.; Takimiya, K. Adv. Mater. 2008, 20, 3388-3392.
(126)Yamamoto, T.; Shinamura, S.; Miyazaki, E.; Takimiya, K. Bull. Chem. Soc. J. 2010, 83, 120-130.
(127)Sánchez-Carrera, R. S.; Atahan, S.; Schrier, J.; Aspuru-Guzik, A. J. Phys. Chem. C 2010, 114, 2334-2340.
(128)Valiyev, F.; Hu, W.; Chen, H.; Kuo, M.; Chao, I.; Tao, Y. Chem. Mater. 2007, 19, 3018-3026.
(129)Gao, X.; Qiu, W.; Liu, Y.; Yu, G.; Zhu, D. Pure Appl. Chem. 2008, 80, 2405-2423.
(130)(a) Bromley, S. T.; Mas-Torrent, M.; Hadley, P.; Rovira, C. J. Am. Chem. Soc. 2004, 126, 6544-6545.
(131)Casado, J.; Zgierski, M. Z.; Delgado, M. C. R.; Navarrete, J. T. L.; Mas-Torrent, M.; Rovira, C. J. Phys. Chem.
C 2007, 111, 10110-10118.
(132)Takimiya, K.; Kunugi, Y.; Konda, Y.; Ebata, H.; Toyoshima, Y.; Otsubo, T. J. Am. Chem. Soc. 2006, 128, 30443050.
(133)Cornil, J.; Calbert, J. P.; Brédas, J. L. J. Am. Chem. Soc. 2001, 123, 1250-1251.
(134)Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Brédas, J. Chem. Rev. 2007, 107, 926952.
(135)Kim, E.; Coropceanu, V.; Gruhn, N. E.; Sánchez-Carrera, R. S.; Snoeberger, R.; Matzger, A. J.; Brédas, J. J. Am.
Chem. Soc. 2007, 129, 13072-13081.
(136)Valeev, E. F.; Coropceanu, V.; da Silva Filho, D. A.; Salman, S.; Brédas, J. J. Am. Chem. Soc. 2006, 128, 98829886.
(137)Marcus, R. Rev. Mod. Phys. 1993, 65, 599-610.
(138)Delgado, M. C. R.; Pigg, K. R.; da Silva Filho, D. A.; Gruhn, N. E.; Sakamoto, Y.; Suzuki, T.; Osuna, R. M.;
Casado, J.; Hernández, V.; Navarrete, J. T. L.; Martinelli, N. G.; Cornil, J.; Sánchez-Carrera, R. S.; Coropceanu,
V.; Brédas, J. J. Am. Chem. Soc. 2009, 131, 1502-1512.
(139)ADF2008.01. Scientific Computing & Modeling (SCM), Theoretical Chemistry, Vrije Universiteit, Amsterdam,
The Netherlands, http://www.scm.com(140)Prins, P.; Senthilkumar, K.; Grozema, F. C.; Jonkheijm, P.; Schenning, A. P. H. J.; Meijer, E. W.; Siebbeles, L.
D. A. J. Phys. Chem. B 2005, 109, 18267-18274.
(141)Senthilkumar, K.; Grozema, F. C.; Bickelhaupt, F. M.; Siebbeles, L. D. A. J. Chem. Phys. 2003, 119, 9809-9817.
(142)Mori, T.; Kobayashi, A.; Sasaki, Y.; Kobayashi, H.; Saito, G.; Inokuchi, H. Bull. Chem. Soc. Jpn. 1984, 57, 627633.
(143)Mori, T. Bull. Chem. Soc. J. 1998, 71, 2509-2526.
(144)Appleton, A. L.; Miao, S.; Brombosz, S. M.; Berger, N. J.; Barlow, S.; Marder, S. R.; Lawrence, B. M.; Hardcastle, K. I.; Bunz, U. H. F. Org. Lett. 2009, 11, 5222-5225.
(145)Klauk, H.; Halik, M.; Zschieschang, U.; Eder, F.; Schmid, G.; Dehm, C. Appl. Phys. Lett. 2003, 82, 4175-4177.
(146)Kitamura, M.; Arakawa, Y. J. Phys.: Condens. Matter 2008, 20, 184011.
(147)Tan, H. S.; Mathews, N.; Cahyadi, T.; Zhu, F. R.; Mhaisalkar, S. G. Appl. Phys. Lett. 2009, 94, 263303.
(148)Takimiya, K.; Ebata, H.; Sakamoto, K.; Izawa, T.; Otsubo, T.; Kunugi, Y. J. Am. Chem. Soc. 2006, 128, 1260412605.
(149)Okamura, K.; Nikolova, D.; Mechau, N.; Hahn, H. Appl. Phys. Lett. 2009, 94, 183503.
(150)Walser, M. P.; Kalb, W. L.; Mathis, T.; Batlogg, B. Appl. Phys. Lett. 2009, 95, 233301.
(151)Kumaki, D.; Umeda, T.; Tokito, S. Appl. Phys. Lett. 2008, 92, 013301.
(152)Kano, M.; Minari, T.; Tsukagoshi, K. Appl. Phys. Lett. 2009, 94, 143304.
(153)(a) Anthopoulos, T. D.; Singh, B.; Marjanovic, N.; Sariciftci, N. S.; Ramil, A. M.; Sitter, H.; Colle, M.; deLeeuw, D. M. Appl. Phys. Lett. 2006, 89, 213504. (b) Itaka, K.; Yamashiro, M.; Yamaguchi, J.; Haemori, M.;
Yaginuma, S.; Matsumoto, Y.; Kondo, M.; Koinuma, H. Adv. Mater. 2006, 18, 1713-1716.
(154)Shukla, D.; Nelson, S. F.; Freeman, D. C.; Rajeswaran, M.; Ahearn, W. G.; Meyer, D. M.; Carey, J. T. Chem.
Mater. 2008, 20, 7486-7491.
(155)Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. J. Am. Chem. Soc. 2004, 126, 3378-3379.

69

References(156)Pernstich, K. P.; Haas, S.; Oberhoff, D.; Goldmann, C.; Gundlach, D. J.; Batlogg, B.; Rashid, A. N.; Schitter, G.
J. Appl. Phys. 2004, 96, 6431-6438.
(157)Suemori, K.; Uemura, S.; Yoshida, M.; Hoshino, S.; Takada, N.; Kodzasa, T.; Kamata, T. Appl. Phys. Lett. 2007,
91, 192112.
(158)森 健彦, 長谷川達夫, 有機トランジスタ材料の評価と応用Ⅱ, 第4章, p117-128, シーエムシー出版, 2008.
(159)Halik, M. Gate Dielectrics in Klauk, H Organic Electronics: Materials, Manufacturing, and Applications,.
Wiley-VCH Verlag GmbH & Co. KGaA, 2006.
(160)Kim, C.; Wang, Z.; Choi, H.; Ha, Y.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 6867-6878.
(161)Pokhodnya, K. I.; Bonner, M.; Miller, J. S. Chem. Mater. 2004, 16, 5114-5119.
(162)Podzorov, V.; Pudalov, V. M.; Gershenson, M. E. Appl. Phys. Lett. 2003, 82, 1739-1741.
(163)Podzorov, V.; Sysoev, S. E.; Loginova, E.; Pudalov, V. M.; Gershenson, M. E. Appl. Phys. Lett. 2003, 83, 35043506.
(164)Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T. Nat. Mater. 2009, 8, 494499.
(165)Someya, T.; Sekitani, T.; Iba, S.; Kato, Y.; Kawaguchi, H.; Sakurai, T. Proceedings of the National Academy of
Sciences of the United States of America 2004, 101, 9966 -9970.
(166)Umeda, T.; Kumaki, D.; Tokito, S. Org. Elect. 2008, 9, 545-549.
(167)Deshmukh, K. D.; Reuter, K.; Kempa, H.; West, J. E.; Katz, H. E. Appl. Phys. Lett. 2009, 95, 113307.
(168)Jang, J.; Kim, J. W.; Park, N.; Kim, J. Org. Elect. 2008, 9, 481-486.
(169)Kaji, Y.; Kawasaki, N.; Lee, X.; Okamoto, H.; Sugawara, Y.; Oikawa, S.; Ito, A.; Okazaki, H.; Yokoya, T.; Fujiwara, A.; Kubozono, Y. Appl. Phys. Lett. 2009, 95, 183302.
(170)Kalb, W. L.; Mathis, T.; Haas, S.; Stassen, A. F.; Batlogg, B. Appl. Phys. Lett. 2007, 90, 092104.
(171)Smith, J.; Hamilton, R.; McCulloch, I.; Heeney, M.; Anthony, J. E.; Bradley, D. D.; Anthopoulos, T. D. Synth.
Met. 2009, 159, 2365-2367.
(172)Walser, M. P.; Kalb, W. L.; Mathis, T.; Brenner, T. J.; Batlogg, B. Appl. Phys. Lett. 2009, 94, 053303.
(173)Walser, M. P.; Kalb, W. L.; Mathis, T.; Batlogg, B. Appl. Phys. Lett. 2009, 95, 233301-3.
(174)Uno, M.; Tominari, Y.; Yamagishi, M.; Doi, I.; Miyazaki, E.; Takimiya, K.; Takeya, J. Appl. Phys. Lett. 2009,
94, 223308.
(175)時任静士 他 “フルオロポリマーをゲート絶縁膜に用いたボトムコンタクト有機TFTアレイの試作”
2009年春期応用物理学関係連合講演会(176)Fukuda, K.; Sekitani, T.; Someya, T. Appl. Phys. Lett. 2009, 95, 023302-3.
(177)Kato, Y.; Sekitani, T.; Takamiya, M.; Doi, M.; Asaka, K.; Sakurai, T.; Someya, T. IEEE Trans. Electron Devices
2007, 54, 202-209.
(178)Ahn, T.; Choi, Y.; Jung, H. M.; Yi, M. Org. Elect. 2009, 10, 12-17.
(179)Sekitani, T.; Iba, S.; Kato, Y.; Noguchi, Y.; Someya, T.; Sakurai, T. Appl. Phys. Lett. 2005, 87, 073505.
(180)Kato, Y.; Iba, S.; Teramoto, R.; Sekitani, T.; Someya, T.; Kawaguchi, H.; Sakurai, T. Appl. Phys. Lett. 2004, 84,3789-3791.
(181)Roberts, M. E.; Mannsfeld, S. C.; Stoltenberg, R. M.; Bao, Z. Org. Elect. 2009, 10, 377-383.
(182)Kawaguchi, H.; Iba, S.; Kato, Y.; Semitani, T.; Someya, T.; Sakurai, T. IEEE Sensors Journal 2006, 6, 12091217.
(183)Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist, T. Chem. Mater. 2004, 16, 4980-4986.
(184)Hanada, T.; Negishi, T.; Shiroishi, I.; Shiro, T. Thin Solid Films 2010, 518, 3089-3092.
(185)Kano, M.; Minari, T.; Tsukagoshi, K. Appl. Phys. Express 2010, 3, 051601.

70

Appendix

Appendix
A1. OFET Operation

P- and n-channel operations. Energy levels of frontier orbitals, namely, a highest occupied
molecular orbital (HOMO) or a lowest unoccupied molecular orbital (LUMO) of an organic
semiconductor, determine the p- or n-channel FET operation. The frontier energy-levels also
affect the air-stability of the FET characteristics (Figure A1). The energy levels are determinedfrom the electronic potential of the compound in solutions. Work functions (Φ) (Fermi energy,εF) of metal electrodes and energy levels of HOMO or LUMO should be close for smooth carrier injection.

b)

Figure A1. Relationship between energy levels of frontier orbitals of organic semiconductors and stable/unstablep- or n-type operation of OFETs in air. Work functions (= εF) of metals are also depicted.

72

Appendix

1
1.00

Electronic Bonding Energy / eV

0

Evac
Ac

-1

-0.96
-1.63

-2
-3

IPc

IPg

Pc

ǻEc

-2.08

-2.39

ǻEg

-4
Ph

-5

-5.23

-4.86

-4.60

-5.79

-6

tetracene
pentacene-6.70
naphthaleneantracenebenzeneFigure A2. Energy band structure of the acene compounds. The calculation is based on DFT method at B3LYP/631(d) level.111

Relationship Between Molecular Structures and HOMO-LUMO energy levels. The
HOMO and LUMO levels of the π-compounds are changed linearly with increase in the extension of π-conjugation (Figure A2). The relationship can be confirmed by molecular orbital
calculations.111 It is noted that an energy level in a solid state increases somewhat (polarizationenergy, Pc or Ph) due to formation of a small band structure. Thus an ionization potential in agas state (IPg) is different from that in a solid state (IPc).

73

Appendix

A2. Nomenclature of tetrathiafulvalene derivatives

Common Name vs IUPAC Name. The compound and name of tetrathiafulvalene (TTF) iswidely known, however, the name of TTF is not admit by IUPAC (International Union of Pureand Applied Chemistry) nomenclature. Structure of TTF is named by IUPAC as “2,2’-bis(1,3dithiolylidene)”(Figure A3a), which derived from the name of “1,1’-bi(cyclopenta-2,4-diene-1ylidene)” (Figure A3b).
Other name of TTF is traditionally given by “∆2,2’-bi-1,3-dithiole”, where ∆ expresses that thesame structures are connected by double bond at the numbering positions. It is frequently used
when naming quinoidal compounds. The names of thiophene-fused TTFs are in Figure A4.
(a)
4'

5'

3'

S
1'

(b)

3

S

2' 2

S

4

3'

S

5

4'

2'

1' 1

5'

1

2

5

3
4

Figure A3. Numbering of (a) TTF and an (b) analogy.

3'

4'
5'S

S

2' 2

S

6'

4

3

S

b cd
a e

S

6

1

1'

S5

Nomenclature

Namecommon 1
common 2
traditional
IUPAC

dithiophene-tetrathiafulvalene (DT-TTF)
bis(thiophene[3,4-d])tetrathiafulvalene∆2,2’-bitheno[3,4-d]-1,3-dithiole
2-(thieno[3,4-d]-1,3-dithiole-2-ylidene)thieno[3,4-d]-1,3-dithiole
3

1'

6'
5'

2' 2

S

S

5

S

1

3'

4'

4

S

S

S

6

Nomenclature

Namecommon 1
common 2
traditional
IUPAC

α-dithiophene-tetrathiafulvalene (αDT-TTF)(Z)/(E)-bis(thieno[2,3-d])tetrathiafulvalene(Z)/(E)-∆2,2’-bithieno[2,3-d]-1,3-dithiole(Z)/(E)-2-(thieno[2,3-d]-1,3-dithiole-2-ylidene)thieno[2,3-d]-1,3-dithiole
4'

5'

H N
6'

3

3'

S

S

S 2'
1

2

4

5

N H
S

1

6

Nomenclature

Namecommontraditional
IUPAC

bis(pyrrolo[3,4-d])tetrathiafulvalene (PyTTF)∆2,2’-bipyrrolo[3,4-d]-1,3-dithiole
2-(pyrrolo[3,4-d]-1,3-dithiole-2-ylidene)pyrrolo[3,4-d]-1,3-dithiole
Figure A4. Nomenclatures of TTF derivatives.

74

75
c = 12.603(3) Å
β = 92.51(1) °V = 1356(9) Å 3

0.50 × 0.20 × 0.10mm
monoclinic

a = 10.237(4) Å

b = 11.170(4) Åc = 8.139(4) Å

β = 91.51(2)°V = 930(7) Å3yellow, platelet

0.25 × 0.20 × 0.10mm
monoclinic
a = 29.0(2) Å

b = 7.80(6) Åc = 9.72(7) Å

β = 93.26(3) °V = 2194(2) Å3

Crystal Color , Habit

Crystal Dimensions

Total: 9216
Unique: 2133
(Rint = 0.053)

1641

109

293 K

Total: 8736
Unique: 2512
(Rint = 0.038)

1471

145

5.2 % ; 8.0 %µ (MoKα)

Temperature

No. of Reflections
Measured

No. of Reflections(All, 2θ < 54.97°)

No. of Variables

Residuals: R; Rw

Diffractometor

Rigaku/MSC Mercury CCD

Rigaku Rapid-S
Imaging Plate

1.07

4.5 % ; 11.9 %200 K

8.33 cm-1

Goodness of Fit Indica1.02
tor

3.67 cm-1

5.13 cm-1

8

Rigaku Rapid-S
Imaging Plate

1.02

3.4 % ; 10.2 %

145

2628

Total: 12909
Unique: 3094
(Rint = 0.025)

200 K

2

2

C2/c (#15)

P21/a (#14)

b = 10.846(5) Å

a = 9.929(5) Å

0.25 × 0.50 × 0.40mm
monoclinic

yellow, blockchlorobenzene

Z value

P21/ a (#14)

tolueneSpace Group

Lattice Parameters

Crystal System

yellow, blockacetone506.84

Crystallization From

394.63282.41

C26H38N2S4Formula Weight

C18H22S4N2

C10H6N2S4

1c (n-butyl PyTTF) 1d (n-octyl-PyTTF)

Empirical Formula1a (H-PyTTF)

Table A1 Crystal Data for the heteroaromatic-fused TTFs

Rigaku Rapid-S
Imaging Plate

0.8

8.3 % ; 27.5 %

181

1599

Total: 8184
Unique: 3430
(Rint = 0.199)

200 K

3.10 cm-1

Rigaku Rapid-S
Imaging Plate

0.93

5.6 % ; 17.8 %

217

2751

Total: 7768
Unique: 3938
(Rint = 0.045)

200 K

2.56 cm-1

1

P1 (#2)1

V = 1053.8(9) Å 3

P1 (#2)

γ = 103.68(2)°V = 835(5) Å 3

β = 94.06(2)°γ = 85.96(3) °α = 92.34(2)°β = 83.92(3) °c = 26.92(2) Å

α = 80.75(3) °b = 7.502(3) Å

a = 5.394(3) Å

0.30 × 0.25 × 0.20mm
triclinic

yellow, platelet

chlorobenzene

731.27

C42S4N2H70

1f (n-cetyl-PyTTF)

c = 23.82(2) Å

b = 7.680(8) Å

a = 4.659(4) Å

0.30 × 0.20 × 0.05mm
triclinic

yellow , block

toluene619.05

C34H54N2S41e (n-dodecylPyTTF)

A3. Crystallographic Data for X-ray Single-Crystal Structure Analyses

Rigaku/MSC Mercury CCD

0.90

7.2 % ; 20.2 %

271

2266

Total: 8338
Unique: 4871
(Rint = 0.073)

293 K

4.36 cm-1

2

P1 (#2)V = 1145(1) Å 3

γ =90.84(1)°β = 91.54(1)°α = 96.18(1)°c = 22.87(2) Å

b = 10.260(8) Å

a = 4.914(4) Å

0.50 × 0.30 × 0.15mm
triclinic

Rigaku Rapid-S
Imaging Plate

1.09

4.2 % ; 13.1 %

217

2873

Total: 16990
Unique: 4038
(Rint = 0.026)

200 K

11.20 cm-1

6

P21/ n (#14)

V = 1772.0(2) Å3

β = 95.090(2) ºRigaku Rapid-S
Imaging Plate

1.08

6.5 % ; 28.6 %

163

1419

Total: 6900
Unique: 3155
(Rint = 0.031)

296 K

9.57 cm-1

2

P1 (#2)V = 696.5(4) Å3

γ = 105.033(8) ºβ = 103.041(9) ºα = 109.071(9) ºc = 10.785(3) Å

b = 9.742(3) Å

b = 15.455(1) Åc = 18.617(1) Å

2.00 × 1.60 × 0.10mm
triclinic
a = 7.742(3) Åred, platelet

toluene344.56

C12S6H8

3b (methyl-αDTTTF)

1.00 × 0.20 × 0.20mm
monoclinic
a = 6.1828(5) Åred, platelet

tolueneyellow, block316.50sublimation (~103Pa)

C10H4S6

3a (αDT-TTF)

478.70C22H26N2O2S4

2b (n-hexanoylPyTTF)

0.91

8.4 % ; 24.8 %

73

802

Total: 5952
Unique: 1445
(Rint = 0.063)

200 K

67.89 cm-1

2

P21/a (#14)

V = 603.0(3) Å3

β = 109.45(1) °c = 6.303(2) Å

b = 16.195(4) Å

a = 6.265(2) Å

0.50 × 0.20 × 0.01
mm
monoclinic

red, platelet

Rigaku DSC ImagRigaku Rapid-S
ing Plate (SynchroImaging Platetron in KEK)

2.5

8.6 % ; 27.6 %

159

1172

Total: 3803
Unique: 1711
(Rint = 0.055)

200 K

5.79 cm-1

2

P21/ a (#14)

V = 1476.7 Å3

β = 94.81 ºc = 34.056 Å

b = 4.214 Å

a = 10.326 Å

0.15 × 0.15 × 0.02
mm
monoclinic

orange, platelet

chlorobenzene

410.30605.03benzene

C10H4Se2S4

C26H36S8

3c (S-n-octyl-αDT(aDS-TTF)
TTF)

Appendix

Slideshow for Ph.D defense

A4. Slideshow for Ph.D Defense (February 7th, 2011)
Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

Presentation for Ph.D Defense

CHAPTER 1
General Introduction

Development of
Small !-Conjugated Moleculeswith Long Alkyl Chainsfor Organic Field-Effect Transistors

CHAPTER 2
Development of Heteroaromatic-Fused TTFs as Organic
Semiconductors for Solution Processible Field-Effect Transistors
I. Doi et al, Chem. Mater. 2007, 19, 5230.
I. Doi et al, Chem. Lett. 2008, 37, 1088.

（長鎖アルキル基を導入した低分子トランジスタ材料の開発）

D083606

CHAPTER 3
Evaluation of C10-DNTT as Organic Semiconductors for High
Performance Thin-Film Transistors

土井伊織

工学研究科
物質化学システム専攻
応用有機化学研究室

I. Doi et al, Cur. Appl. Phys. submitted.
Kang et al, Adv. Mater. in press.

1

2

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

OFF-state (insulating) 10-3
ON-state（conducting）
ON
Gate-electric field
-5

S

S

S

S

O

H
N

H2N

N
H

O
O
HO

O

HN
HN

H
N

N
H

O

H
N

O

O

O

How arrange neutral molecules regularly
on a substrateN
H

O!-Compounds＋carrier-dope give conductors
Planer Dielectric Self-organizing ability

Si/SiO2

S

S

S

S

S S

S

S

S

S

S S

S

S

S

S
S

S

S

S
S

S

Surface roughness< 0.1 nm

S
S

S

F

F

F

F

F

F

F

F

F

F

Si

F

F

F

Si

F

F
S

S
F

Si

F

F

N

Heteroacenes
S

S

Si

S

S

S

S

S

S

S

S
S

S

R
R

S

S

S

R

N
R

S

S
S

S

S

S

S

S

N

R

CN

S

CN

Se

Se

Se

R N

N R

R N

N R

O

O

S

S

Se

S

S

S

C18H37 SSSS C18H37

C18H37 SSSS C18H37

N
N

N
S

R

S

S

N

N

S

S

S

F

S

M

N

N

O

R

N

N

S

S

CF3

N
S

F3C

Sublimation

N

CF3

S

R

CN

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

OHex

OHex

S

S

10-3 Pa

CN

S

CN

S

S

S

S

O

NCDI

N

S

S

S

S

S

S

S
S

S

S

S

S

S

S

S

S C6H13

S

S

S

S

S C6H13

O

S

S

S

S

S

S

S

O

S

S

S

N R

S

O

O

Thiophene polymers

S

S

S

S

S

S

N

S

S

N

C8H17
O

N

O

N

O

S

R

R

S

S

S

N
N

n

S
R

PBTTT

S

N
N

S
S

S
S

N
N

O
C8H17

R
S

PNDTBTn
R

JACS 2010, 132, 5000.

S

S

S
S

n

Nature 2009, 457, 679.

R
S

S
R

rrP3HT

C10H21Nat. Mater. 2006, 5, 328.

S

S

n

S

S

S

S

S

S S

S

S
S

S

S

S
S

S

S

S
S

S

S
S

4

S
R

N

R
N

S

S

S

n

R
PTzQT
JACS 2009, 131, 2521.

Solution-process

101

ex. spin-coatinga-Si:H

573

in airOrganic Semiconductors(solution-process)Organic Semiconductors(Vacuum-process) *SM: small molecule

C10H21

S

S

S

R N

S
N

S

S S

INSULATOR

poly-Si(LTPS)
a-MetalOxide" c-Si:H

0

S

C6H13 S
N

S

S

S

S
N

S

S

S

S

S

S

z S

R

S

S

F

N

M

CN

F

F

S

N

N

N

F

F
R

N

N

N
F

N
N

N

O

PCDI

C6H13

F

N

M

N

F

N

N

F

N

N

N

N

N

N

N

O

F

F

F

F

N

M

CN

O

O

F
F

N

N

R

R

Phthalocyanines
N

N

R

R

R

N

S

S

R
N

R

N

S

S

R

S

S

CnH2n+1

S

S
Ar

R

S

S
N

Tetrathiafulvalenes

S

S

S

S

S

S

S
S

S

S

S

NC

S
S

S

N

N
S

H2n+1Cn

R

Ar

O

S

S

NC

S

S

S

S

O

O

O

NC

S
S

S

R

S

S

CF3

N

S

S

S

S

S

N

R
S

S
S

R

F3C

S

S
S

BTBT

S

S

S

S

F

S

S

R

Si

R

S
S

R

S

S

R

S

S
S

DNTT
R

F3C

Vacuum Process

N

S
S

S

F

S
S

F

S

S

S

Large OLED display required

poly-Si(HTPS)

100

C10H21

S

S

N
S

F

O

S

S

C10H21

F

F

S

S

S

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

S

S

F

F

S

R

rubrene
Si

F
S

F

S

Source

S

S

R

S

S

F

20

Gate voltage

3

S

S

S

R

S

S

F
R

S

O

R

S n

F

F
R

S

S

R

F

R

R

S

S

F
tetracene

0

Vg

Oligothiophenes

R
R

-20
V /V

METAL

METAL

R

-40

drain

Many compounds
have been developed

Inorganic
Insulating Polymers

INSULATOR

-60

g

S

(smaller than a molecule)

OFF

-13

~
~

S

S

S

S

~
~

S

S

S

!TFT / cm2V-1s-1S

Acenes

10-9
10-11

Id

Electrostatic potential
Dipoles
Hydrophobic/hydrophilic
Steric hindrance

Controlling the carrier by field-effect give transistors

d

10-7

10

I

V = -60V

10dOrganic compounds are insulator-I / AORGANIC ELECTRONIC MATERIALSn

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

ORGANIC FIELD-EFFECT TRANSISTOR（OFET）

GENERAL INTRODUCTION

Drain current

CHAPTER 1

180
200
Maximum Size of Substrate / cm

ADVANTAGES OF OFETs• Flexible• Low-process temperature• Low-cost• Large area fabrication

Need solution-process

Solution-process materials should be improved

676Appendix

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

CHAPTER 2 DEVELOPMENT OF SEMICONDUCTOR
DesignSMALL !-COMPOUNDS＋LONG ALKYL CHAINSSSSMALL !-COMPOUNDS＋LONG ALKYL CHAINS

9 steps

KOH + CS2

S

S

S

S

S

Vacuum-process

N R

R N

S
R

R

Device configuration"-coreSynthesis

Solution-process

Vd = -60 V
10-5

8 10-3-6

10-Id / A

C10H2110-7
4 10-3

10-8

Alkylated DNTT

Source

-9

10

10-10
-60

SourceDrain

Structural / theoretical analysis
-40

-20

0

C10H21S

Sq. Root of (-Id) / A1/2

: Pyrrole or Thiophene

Si/SiO2 substrate

S

10-4

FET characteristics

Heteroaromatic-fused TTFs

CHAPTER 3 EVALUATION ＆ APPLICATION

Drain

Plastic substrate

Organic Semiconductor

020

Organic Semiconductor

10-3

q

p r

r

0.02
40 !m
90 !m
140 !m190 !m 0.015

Vd = -60 V

10-5
10-7

-I d / A

Dielectric
Gate
Substrate

0.01

10-9

0.005

10-11
10-13
-60

q

(-Id)1/2 / A1/2

Dielectric
Gate
Substrate

Vg / V

-40

-20

p

0
Vg / V20

040

Chapter 3 is omitted in this presentation

7

8

APPLIED PHYSICS LETTERS 86, 083505 !2005"

Organic field-effect transistors with single and double pentacene layers
Suyong Jung and Zhen Yaoa!
Department of Physics, Texas Materials Institute, and Center for Nano- and Molecular Scienceand Technology, The University of Texas at Austin, Austin, Texas 78712!Received 17 August 2004; accepted 20 December 2004; published online 16 February 2005"
We report the characterization of field-effect transistors fabricated within individual grains of single
and double pentacene layers grown on silicon oxide. Field-effect mobilities are found to increase
with increasing gate voltage and exhibit a thermally activated form for the temperature dependence.
These characteristics can be explained by the multiple trapping and release model. The mobilities ofdouble-layer devices are one order of magnitude higher than those of single-layer devices. Possible
origins of the traps are discussed. The geometry of these ultrathin devices makes them suitable forsensing applications. © 2005 American Institute of Physics. #DOI: 10.1063/1.1865331$

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

CHAPTER 2 DEVELOPMENT OF SOLUTIONPROCESSIBLE ORGANIC SEMICONDUCTORSCHAPTER 1
General Introduction

CHAPTER 2
Development of Heteroaromatic-Fused TTFs as Organic Semiconductors for
Solution Processible Field-Effect Transistors
2.1 Introduction for Organic Semiconductors
2.2 Bis(N-alkyl-pyrrolo[3,4-d])tetrathiafulvalenes (N-alkyl-PyTTFs)
I. Doi et al, Chem. Mater. 2007, 19, 5230.
2.3 Bis(N-acyl-pyrrolo[3,4-d])tetrathiafulvalenes (N-acyl-PyTTFs)
I. Doi et al, Chem. Lett. 2008, 37, 1088.
2.4 Bis(thieno[2,3-d])tetrathiafulvalenes (!DT-TTFs)
2.5 Correlation between Electronic Structure in Solid State and Field-Effect Mobility
2.6 Conclusion for Chapter 2

Heteroaromatic-fused TTFs

OFETs with single grains of sexithiophene a few molecularlayers in thickness.4 Charge mobilities in these devices werea few orders of magnitude lower than bulk thin-film devices,which is attributed to the poor bottom contact which forms
the bottleneck to charge transport. Dinelli et al. have studied
the carrier mobility in ultrathin sexithienyl FETs as a function of film coverage and found that the mobility is dominated by the first two organic layers at the interface.5 However, their devices consist of many grains and the effect ofgrain boundaries was not separated out.
In this letter, we report on the fabrication and characterization of top-contact FETs based on individual grains of
single and double pentacene layers grown on silicon dioxideusing physical vapor deposition. The charge carrier mobility
at room temperature extracted from the FET measurement is0.01–0.04 and 0.10– 0.20 cm2 / V s for single- and double2 increases with inlayer devices, respectively. The mobility
FETcreasing gate voltage and shows a thermally activated form
as a function of temperature, consistent with the multiple
trapping and release model.6 These characteristics underline
the importance of the organic–insulator interface. These ultrathin
devicesare Phys.
remarkably
ambient
condiTanet al.,
Appl.
Lett.sensitive
2009, to94,
263303.
tions, which
thatPhys.
they may
be suitable
for sensing
Klauk
et al.,suggests
J. Appl.
2002,
92, 5259-5263.
applications.
Kelleyet al., J. Phys. Chem. B 2003, 107, 5877.
Ultrathin pentacene films are deposited on silicon dioxide !thickness %560 nm" thermally grown on highly doped

REPRESENTATIVE ORGANIC SEMICONDUCTOR

CHAPTER 3

Pentacene

Evaluation of C10-DNTT as Organic Semiconductors for High PerformanceThin-Film Transistors
Kang et al, Adv. Mater. in press.

3.3 Fabrication of Polymer Dielectrics
3.4 Evaluation on Plastic Substrates

Jung et al., Appl. Phys. Lett. 2005, 86, 083505-3.

Herringbone!
= ~ 3 cm /Vs(vacuum-process）

3.1 Introduction for Alkylated DNTT
3.2 Evaluation on Silicon Substrates

silicon wafers, which are employed as gates in FET measureImproving the performance of organic field-effect trandefense：Development
of !-Conjugated
Molecules with Long Alkyl Chains for OFETs
ments.Ph.D
Beforedeposition, the substrates
are ultrasonicallysistors !OFETs" has been a challenge to the development ofcleaned in nitric acid, acetone, and isopropanol and subsethe emerging organic electronics. Recent work has focusedquently annealed in a flow of Ar and H2 at 900 ° C foron OFETs based on single crystals.1 While such studies are
30 min. Thorough cleaning is necessary to reduce nucleation
useful to elucidate the fundamental transport mechanism andcenters promoting growth of large pentacene islands,
the ultimate performance limit in organic semiconductors,
whereas the H2 annealing is known to reduce trap density atpractical devices are mostly based on thin films where thethe Si/ SiO2 interface.7 The starting pentacene source matefilm morphology and the organic–insulator interface play anrial is purified multiple times by temperature gradient subliimportant role in device performance. It has been shown that
accumulated charge carriers in thin-film OFETs are restricted
Smation.SFigure 1!a" shows the schematic of our growth setup.
The physical vaporRdeposition is carried out in a 1-in.-diam
to a few layers of organic molecules at the interface R
with thegate insulator.2,3 Thus investigation of the charge transport
Sglass tube
S placed in a horizontal tube furnace using nitrogenas a carrier gas. Nitrogen is first flown for a few hours toprocess at the level of a few molecular layers and a few
dispense air and moisture from the growth tube. The source
crystalline grains represent a most direct way to understand: Pyrroleor Thiophenematerial is then heated up slowly, first to 200 ° C at
the effect of microstructures and the gate–insulator interface
2
°
C
/
min,
then to 230 ° C at 5 ° C / min. The sublimed molon the transport properties. Frisbie et al. have fabricated

I. Doi et al, Cur. Appl. Phys. submitted.

3.5 Conclusion for Chapter 3a"Electronic mail: yao@physics.utexas.edu

9Advances in Regioregular Polythiophenes Osaka and McCullough

FIG. 1. !a" Schematic of physical vapor deposition setup. !b" AFM image ofa pentaceneSCHEME
monolayer
sample anda line scan height profile. !c", !d" AFM
4. Synthesis
of Poly(3-hexylthiophene)-b-poly(3-dodecylthiophene)by Chain Extension through Sequential Monomer Addition
images of pentacene films with almost complete monolayer and doublelayer coverage. !e", !f" AFM images of typicalsingleTanet andal.,double-layer
Chem. Mater. 2007, 19, 3018.
devices.

2D semiconducting layer parallel to the substrates

100003-6951/2005/86"8!/083505/3/$22.50
86, 083505-1© 2005 American Institute of Physics
Downloaded 17 Jan 2011 to 133.41.91.74. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
nanofibrils, while high molecular weight rrP3HTs are likely toion, Ni(dppp)Cl2 is believed to act as an initiator rather than aform more closely packed nanofibrils indicating better intercatalyst, and therefore this limits polymerization to one end of
domain connectivity. Further investigation by our group has
the polymer chain. The prediction of the mechanism as livdemonstrated that the width of the nanofibrils as well as theing is supported by two experimental results: first, the degreecharge carrier mobility in FETs increase as the molecularof polymerization of rrP3ATs has been found to increase withweight increases; this correlates with the average contour
monomer conversion and can be predicted by the molar ratio
length of polymer chains. However, it is interesting to noteof the monomer to the nickel initiator; second, addition of varthat both the nanofibril width and the mobility saturate atious Grignard reagents (R!MgX) at the end of polymerization
higher molecular weight, which is believed to be due to theresults in end-capping of rrP3AT with an R! end group as“chain folding” of the rrP3HT backbones. These structural studdescribed below.27,28 Furthermore, we have successfully
achieved a chain extension of rrP3ATs. Sequential addition ofies, accomplished through careful control of molecular weightmonomer 6 with R ) dodecyl following the polymerization ofof the polymers, have allowed better understanding the cor6 with R ) hexyl gave poly(3-hexylthiophene)-b-poly(3-doderelation of the polymer structure with physical properties andcylthiophene) (15) as shown in Scheme 4.26 This result
device performance.
strongly indicates the living nature of the polymerization
HT Regioregular Polythiophenes withsystem.
Functionalized Side Chains
Supramolecular Structure in rrP3ATs. A fascinating
The initial purpose of introducing side chains on the PT backstructural feature of HT rrP3ATs that differs completely frombones was to improve solubility and hence processability.
regioirregular P3ATs is the supramolecular ordering driven by
Ph.D
of !-Conjugated
Molecules
LongitAlkyl
Chainsfor OFETs
However,
has beenestablishedthat the introduction of varthedefense：Development
self-assembly of planar polymer
backbones. X-ray
study withious kinds of functionalized side chains can also change thehas revealed a well-ordered lamellar structure with an intermaterials’ electrical and optical properties. A large number oflayer spacing of ∼16.0 Å for rrP3HT and very close π-πside chain functionalized HT regioregular poly(3-substituted
stacking of facing polymer backbones with a distance of∼3.8
thiophene)s (rrPTs) have been synthesized (Table 1).
Å.14 Atomic-force microscopy (AFM) study on thin films of
rrP3ATs show well-organized nanofibril morphologies of difHeteroatom-Containing Groups. Introducing electronfering dimensions (Figure 3).
donating groups, such as alkoxy or alkylthio groups, with the
Recently, several groups (including ours) have demonheteroatom directly connected to the ring in the 3-position ofstrated that rrP3HTs form various morphologies depending on
thiophene is expected to decrease the band gap by raising
molecular weight.29,30 AFM studies have shown that low
their HOMO level, which leads to low oxidation potentials andmolecular weight rrP3HTs are likely to form isolated
a highly stable conducting state. rrPTs with alkoxy substitu-

REPRESENTATIVE SOLUTION-PROCESSIBLE
ORGANIC SEMICONDUCTOR

Regio-regular Alkyl thiophene
R
S

S

S
R

S

S
S

S
S

S
S

n

!FET = 0.1 cm2/Vs

S
S

S
S

S
S

S
S

FIGURE 3. AFM phase images of rrP3HT thin films on FET devices: Mn ) 12000 (left); 21000 (right).

1206

Osaka et al., Acc. Chem. Res. 2008, 41, 1202.

ACCOUNTS OF CHEMICAL RESEARCH

1202-1214

September 2008

Alkyl／"-" intermolecular interactions

Vol. 41, No. 9

Lamella structure (2D conducting layer)

Drawbacks
1177• Low mobility• Distribution Mw• Synthetic limitation• Difficulty of structural analysis
12

Slideshow for Ph.D defense

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

SMALL MOLECULES WITH SOLUBLE SUBSTITUENTS (1)Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

SMALL MOLECULES WITH SOLUBLE SUBSTITUENTS (2)ex. Anthra-dithiophene core

ex. Anthra-dithiophene core

TIPS group in acene coreR
R Si RS

n-CnH2n+1S

alkyl
n-CnH2n+1!-core(E/Z isomers)

S!-coreS

Si/SiO2 substrate!-corealkyl

2-D (R = Et)

R Si RR!FET = ~0.01 cm2/Vs (solution-cast)~0.15 cm2/Vs (vacuum)

Si/SiO2 substrate

Layer-by-layer

H.T. Karz et al., J. Am. Chem. Soc. 1998, 120, 664.

J. E. Anthony et al., J. Am. Chem. Soc. 2005, 127, 4986.

Self-organizing ability!FET = ~1.0 cm2/Vs

Low solubility

1-D (R = i-Pr)

SolubleS

C10H21

Organizing ability (1-D)

S

S

S

S

C10H21

Adv. Mater. 2007, 19, 1864-1868.

Adv. Mater. 2003, 15, 917.

Variation of !-cores!FETµFET = 0.1 cm2/Vs (vacuum process)= ~ 10-4 cm2/Vs

C8H17

S

C8H17

µFET = 0.03 cm2/Vs (vacuum process)µFET = 10-4 cm2/Vs (solution process)14

13

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETsTETRATHIAFULVALENE (TTF)
S

S

S

Small !-core・aggregation (S atoms)

causes disorder in thin film

Attention
R

S

S

S

S

S
S

S

- e+ e-S

6!

S

View Online

S
S

S- e+ e-6!

S

S 6! S

Tetra-substitution perturbs S-S interactions •• LC
1D

Rovira et al., JACS, 2004, 126. 984.
S

S

R

S

DT-TTF!FET (crystal) = ~1.4 cm2/Vs

Yamashita et al., JACS. 2005,
127, 10142.

S

R

S

S

S

!FET = 0.4 cm2/Vs

SS

cis/trans equilibrium in solution

S

S

R

S

S
S

TTF-based Semiconductors
S

S!-coreHOMOSSDonor in organic conductors
Large coefficient on the sulfur atoms
strong intermolecular interaction

S

S

S
S

S-S & !-! contacts!FET = 0.11 cm2/Vs（Herringbone like）

Mori et al., Chem. Lett. 2005, 34, 392.

C18H37 SSSS C18H37!FET = ~0.08 cm2/Vs (solution, dip)

C18H37 SSSS C18H37

Chem. Mater. 2006, 18, 4724.

C6H13 S

S

S

S

S

S C6H13

C6H13 S

S

S

S

S

S C6H13

Downloaded on 17 January 2011
Published on http://pubs.rsc.org | doi:10.1039/B603632E

S

S!FET = ~0.02 cm2/Vs (solution)

15

Chem. Comm. 2006, 2750.

Fig. 1 Molecular structure (top) and stacking structure (bottom) of1, in
which the H atoms are omitted for clarity.

16

The solution electrochemical properties of1–3 were investigatedby cyclic voltammetry (CV). The results are summarized in Table 1.
Similar to type A compounds,7,8 1–3 exhibit four sequential quasireversible one-electron transfer steps, ultimately yielding the fully
oxidized tetravalent cation. Their HOMO levels obtained from the
CV data are about 24.9 eV. The results (see Table 2) ofthermogravimetric analysis (TGA) and differential scanningcalorimetry (DSC) indicate that 1–3 have good thermal stability,
after melting; the onset decomposition occurred at about 300 uC.
The longer alkyl chains resulted in a lower melting point (Tm, from

201 to 163 uC), but the onset decomposition temperature (Td),from 296 to 301 uC, was slightly enhanced. The HOMO–LUMO
gaps of 1–3 are near to 2.9 eV (Table 2), estimated from theabsorption edges in CH2Cl2, indicating good redox stability.13
As compared with most mono-TTFs5f used for OFETs, 1–3
have more extended p-conjugation, which can increase intermolecular p–p interactions. On the other hand, they bear four
terminal alkyl chains (carbon atoms, 4, 6 or 8), which not only
ensure the solubility of the molecules but also facilitate the stacking
of central C14S12 skeletons by van der Waals interactions of thealkyl chains.14 These structural characteristics of 1–3 are expected
to improve the mobilities of their FET devices. Thin films (30–50 nm) of 1–3 were deposited on SiO2/Si substrates by spin-coating
the solutions (1, 2 in CHCl3 and 3 in 1,2-dichlorobenzene for its
lower solubility in CHCl3, 0.6–0.8 wt%). An n-type Si wafer with a
SiO2 layer of 450 nm and a capacitance of 10 nF cm22 was used as
the gate; gold source and drain contacts (50 nm) were deposited on
the organic layer through a shadow mask, affording a bottom-gate
configuration. The channel length (L) and width (W) were 50mm
and 3 mm, respectively. The FET measurements were carried out
at room temperature in air using a semiconductor parameter
analyzer. To our knowledge, these devices are the first examples ofsolution processed thin film OFETs based on TTFs and ofoligomeric TTF-based OFETs.
The FET characteristics are summarized in Table 2. Fig. 2
shows the output characteristics of the device based on2. All threedevices showed p-type behavior and the field-effect mobilities were
estimated from the linear regime.15 The device based on 2
(Scheme 2, R 5 n-C6H13) afforded the highest mobility of
0.02 cm2/Vs, which is quite high among the solution processed thinfilm FET devices based on small organic molecules.16 The
dependence of field-effect mobility on alkyl chain length was
also found for poly(3-alkylthiophene)-based FET devices, in which
the poly(3-hexylthiophene)-based devices showed the highest
mobilities.17 The threshold voltage of these devices is relatively
high (39, 54 and 17 V for 1–3, respectively), indicating the existenceof rich carriers in the active layer under zero gate voltage, whichmight be one of the main reasons for the low on/off current ratios.
To improve the FET performance, optimization of fabrication
techniques and synthesis of new benzene-fused bis-TTF
compounds with higher function are now in progress.

Table 1 Half-wave potentials of 1–3 vs. Ag/AgCl in CH2Cl2 with
0.1 M n-Bu4NPF6 as supporting electrolyte (scan speed 5 50 mV/s)
Compound

E11/2
(V)

E21/2
(V)

E31/2
(V)

E41/2
(V)

HOMOa
(eV)

1
2
3b

0.52
0.52
0.56

0.71
0.72
0.80

1.07
1.07
1.09

1.20
1.19
1.24

24.87
24.86
24.91

a
Calculated based on HOMO 5 2(Eoxonset + 4.4) eV. b Measured in
CHCl3 due to the poor solubility of 3 in CH2Cl2.

Table 2 Physical properties of 1–3

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

1
2
3

HETEROAROMATIC-FUSED TTFs

Band

Mobility

2.88
2.87
2.86

5 6 1023
2 6 1022
6.7 6 1025

Compound Tm (uC) Td (uC) gap (eV) (cm /Vs)
Ion/Ioff
Ph.D defense：Development of !-Conjugated
Molecules with Long Alkyl Chains for OFETs

Cn-PyTTF： SYNTHESIS (1)

201
174
163

296
299
301

2

102
102
103

This journal is ! The Royal Society of Chemistry 2006

Previous 6 step 22 %!-coreChem. Commun., 2006, 2750–2752 | 2751

J. Becher et al., J. Org, Chem. 2000, 65, 5794.

isoelectronic structure!FET = 1.4 cm2/Vs
S

S

S

S

N R

R N

O

S

S

S

S

2 Cn-AcPyTTF

S

S

S

S

S

S

S

S

R
O

S
SR

RS
S

S

S

S

S

S

S

S

S

S

S

S

S

N R

R N

DT-TTF

N

N
R

S
S

1 Cn-PyTTF

R

R

Improved: 5 step 31 %3 SCn-"DT-TTFs
same thiophene ring
Inorg. Chem. 2001, 40, 1902.

Heteroaromatic-fused TTF + Long alkyl chains•Solubility•S-S & !-! intermolecular interactions•Small steric disorder by cis/trans isomerization

Tetrahedron Lett. 1973, 2553.
Tetrahedron Lett. 1989, 134.

Tetrahedron Lett. 1994, 269.

17

18

78

Fig. 2 Drain current versus drain voltage as a function of gate voltage
for FETs based on 2 at room temperature.

Appendix

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

SYNTHESIS (2)
S

S

NaOMe
THF-MeOH
reflux, 30min

H N

quant.

N Ts

O

S

S

S

S

N H

Ts N

82%

S

97%

R-Br or R-I , NaH
R N
53 - 79 %

S

S

S

S

S

S

S

S

N Ts

deposition Au

2000 rpm
30 secN R

Cn-PyTTF

PyTTF

-5

(001)

Intensity (a.u.)/ cm2Vs-1

n-C4H9n-C8H1710.2 ; 8.5
12.6

2"10-5

C12 n-C12H25

21.0

6"10-3

C16 n-C16H33

26.8

8"10-3

C20 n-C20H41

32.3

0.01

20

0

-20
V /V

-40

-20V

-6

-1 10

0
0

0
-60

0V
-10

-20

-30 -40
V /V

-50

-60

g

d

g

Longer alkyl chains
higher performances

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

800
600
400
(002)

(003)

200

-

Small "-" overlaps!FET = no field-effect

1000

C4
C8

-40V

-3 10-6-2 10-60.000510-9-4 10-6

20

C8-PyTTF

1200

!FET

0.001

MOLECULAR ARRANGEMENTS IN SINGLE-CRYSTAL

THIN-FILM XRDs

d-spacing / Å

0.0015-5 10-6

19

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

R

-7

10

10-8

From toluene solutions

in toluene at rt.

-6 10-6
0.002

-I /A

H
C1 CH3C4 n-C4H9
C8 n-C8H17
C12 n-C12H25
C16 n-C16H33
C20 n-C20H41

Vg = -60V

-6

-7 10

0.0025dHigh solubility• C16 ＆ C20: Low

solubility/g L-1
<0.01
10
29
50
4
0.4

-6

-8 10

Vd = -60V
10-6

Sq. Root of (-I )

yield
/%
79
71
75
61
79
53

R
CH3
n-C4H9n-C8H17n-C12H25
n-C16H33
n-C20H41

R

FET characteristics! /cm2V-1s-1
Ion/Ioff
no field-effect-5
5"10
2"10-5101
no field-effect
6"10-3
102
8"10-3
104
0.01
104

0.003

10

Solution-Processible (> n-C4H9)

C1
C4
C8
C12
C16
C20

Top-contact type
W=1500, L =50 !morganic thin film

spin-coat

d

N Ts

O

THIN-FILM FABRICATION & FET CHAR.

P(OMe)3
120 °C, 5 h

I /A

S

2.0 eq. DDQ
PhCl, reflux, 2 hPh.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

2

4

6

8
2!"!

10

12

Large "-" overlaps

C16-PyTTF!FET = 0.008 cm2/Vs

Crystalline thin filmscLarge interlayer distance (d-spacing) : High mobility
d-Spacing = Crystallographic c-axis !!!!!! similar structure
21

22

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

SUMMARY OF Cn-PyTTF
S

S
N R

R N
S

S
N
S

R

SSolution R
N
S
O

S

S

S

S

S

S

S

Cn-AcPyTTF

S

Cn-PyTTFs
O

S
S

S

b

• Air-stability

a

S
R

R
S

Low air-stability

E / eVeoxidation-4.6 HOMO

-5

10

-5

10

-6

10

-7

10

-8

10

-9

d

O2 in air

-4

-I / A

Low oxidation potential causes（E11/2(vs. Fc/Fc+) = -0.11 eV）

-4

S

N R

N
R
2h

S

S

S

S

Cn-AcPyTTF

Introduction of EDW
groups

MO calc. (DFT:B3LYP/6-31G*)
EHOMO

N-Acyl groupsO-4.2 eV（exp. -4.6 eV)R
NO-4.9 eVstored in air

estimated from CV
-60

-40

-20
V /V
g

79

S

S

Cn-PyTTF

Longer alkyl chains＝Higher mobility
Hydrophobic
interactions

R = C20: !FET = 0.01 cm2/Vs

10

S
R N

#-# & S-S

layer-by-layer structure

Oxidation potentials

0

20

23

24

Slideshow for Ph.D defense

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

SYNTHESISOXIDATION POTENTIALSC16H33 N

O

HN

S

S

S

S

NH

O
R

PyTTF

NaH or Et3N

Cl

S

S

S

O

N C16H33

S

S

S

S

N

N
C5H11

Cn-PyTTF

C5H11O

Cn-AcPyTTF

HOMO level
Std：EFc/Fc+ = -4.8 eV

O
O

R

NaH

R

S

S

S

S

EHOMO / eV

R

S

O
N

N
O

R

Cn-AcPyTTF
R
C1-AcPyTTF CH3
C5-AcPyTTF n-C5H11
C11-AcPyTTF n-C11H23

nameyield (%)

acetylhexanoyl
dodecanoyl

-4

-4.62
-4.95

-5

solubility (g L-1)

60
56
74

PyTTF

< 5!10-4
1!10-2
2!10-3

AcPyTTF

Insoluble
Oxidation potential increases +0.3 eV

25

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs
-4

10

-5

10

-6

10

-7

10

-8

10

-9

d

-I / AFET CHARACTERISTICS

10

W = 1500
L = 50
top-contact OFET

Au
vacuumdeposition

26

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

THIN-FILM XRDs
2hvacuum dep.

Si/SiO2

Thin film (~50 nm)

(001)

Alkyl

-60

-40

-20
V /V

0

20

!FET /cm2V-1s-1

Ion/Ioff

C1: CH3~ 0.05 a

104

C5: n-C5H11~ 0.08 b

105

C11 n-C11H23

no field effect~ 0.01

104

10

-4

10

-5

10

-6

d

cf: N-n-alkyl-PyTTF-I / AR

a HMDS-treated SiO , T
b ODTS-treated SiO T
22 sub=100ºC
sub=100ºC.

C5-AcPyTTF
Improve mobility and air-stability

Intensity / a.u.

g

Vd = -60V

10

-7

10

-8

10

-9

d-spacing / Å

C11-AcPyTTF(002) (003)

-40

-20
V /V

0

(007)

C5-AcPyTTF

C1-AcPyTTF

30h

Acyl
-60

(006)

23
17

20

Crystalline thin films for C1 & C5

g27

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

28

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

SUMMARY OF Cn-AcPyTTF

MOLECULAR ARRANGEMENT IN SINGLE-CRYSTAL
C5-AcPyTTF%-coreHeteroaromatic-fused TTFs
isoelectronic structure
S

S

S

S

N R

R N

O

S

S

S

S

Cn-AcPyTTF

Herringbone + S-S contacts

R

S

S

O

S

S

S

S

S
SR

RS
S

Cn-"DT-TTF!

same thiophene ring

22.9 ÅcS

SDT-TTFNN
R

S-S short contacts
3.30 - 3.57 ÅSS

Cn-PyTTF

Formula C22H26N2O2S4
System triclinic
Lattice a = 4.914(4) Å
b = 10.260(8) Åc = 22.87(2) Å" = 96.18(1)°# = 91.54(1)°$ =90.84(1)°R merge 7.2%

Layer-by-layer structure
2D conducting layer!FET ~0.08 cm2/Vs
29

Air-stability

Solubility

30

80Appendix

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

THIOPHENE-FUSED TTF
10-3"-core!-DT-TTF

10-5

0.015

-Id / A

-7

10

0.01

10-8

isoelectronic structure
S

S

S

S

N R

S

S

10

S

R

N

Hg(OAc)2

S
S

+ e-

6!

S

S

S

- e-

S

LDA
MeI

N

S

R

S 6! SS

Small disorders
in alkyl moieties

Miyazaki et al., J. Mater.
Chem. 2009, 19, 5913.

S

S

S

S

S

BuLi or LDA
C8H17Br or MeI

S

S

Me
S

87 %

S
S

S

S

S

S

S

S

Me
S

S

C8H17

S
S

S
S

SC8H17

S

S

S

S
C8H17

S

S

S

S
S

Containing a different direction of thiophene

S
S

S S

S

b = 4.214 Å
c = 34.056 Å! = 94.81 º
R-factor 8.6%

Hg(OAc)2

S C8H17S

S

S

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

SC8-!DT-TTF

Addition of Elemental Sulfur

S

Me
S

S

Formula C26H36S8
System monoclinic
Lattice a = 10.326 Å

Could not isolate octylated compound

S

S

S
S
trace

32

Me

30 %

M+ + 32 (sulfur)

1) LDA
2) S
3) C8H17I

S

MOLECULAR STRUCTURE IN SINGLE CRYSTAL

P(OMe)3

S
O

S

S

THF

S

S

31

C1-!DT-TTF

S

S

S

Methyl derivative

Me
S

S
Me

THF -80ºC!DT-TTF
C8H17

Cn-!DT-TTF can be expected

Hg(OAc)2

S
S

78 %

S

S

40 %

T. Otsubo et al., J. Org. Chem. 1998, 63, 8865.

SYNTHESIS (2)LDA
C8H17-IS

Cn-!DT-TTF

N

Alkylation of intermediate
S

S

S

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

S

S

O

64 %

!
-DT-TTF (40%)!DT-TTF

N

S
R

+ e-

S

S

6!

cis/trans isomerization

S

S

HClaq 94 %TsCl / Pyrridine 82 %

S

N

S

- e-

S

S

61 %

P(OMe)3

S

DDQ

O
74 %

R

N
M

N

Attention

S(Follows)

S
N

S

SMe

R = OTHP
R = OHR = OTs, Camphorsulfonic acidR

R

same thiophene ring

S

S
S

S

Cn-!DT-TTF

Cn-AcPyTTF

S

NaI

S
78 %

94 %

S

S

S

S

S

S

O

R

S

S, MeSCN

S

0

20

S

S

S

S

S

SR
S

-20
0
V / V
g

RS

O

-40

R = OH

OTHP

S
S

THF

R = OTHP(vacuum-process)

10-11
-60

S

S

R
N

N
R

S

S

DT-TTFS

SS
S

Cn-PyTTF

1) n-BuLi
2) S
3) CS2R

0.005

0.3 cm2/Vs

-10

R N

O

10-9

(-Id)1/2 / A1/2

10-6

Heteroaromatic-fused TTFs

SYNTHESIS (1)0.02

Vd = -60V

10-4

S
O

CHCl3

S

56 %

S

S C8H17

Small disorder in alkyl moiety

95 %P(OMe)3

S

S

S

S

S

C8H17 SS

S C8H1736 %SC8-!DT-TTF

Synthesized octylthio (SC8) derivative

C(!) and alkyl moieties are not disordered

33

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

MOLECULAR ARRANGEMENT IN SINGLE-CRYSTAL
S

S

S

S

S

THIN-FILM FABRICATION & XRD

S
S

S

S

Me

!DT-TTF

Me
S

34

S

spin-coatingS

C1-!DT-TTF

S

S

S

S

S

OctS

Benezene 0.6 wt %2000 rpm 30 sec

SOctS

SC8-!DT-TTF

Thin films fabricated by spin-coating001

intensity / a.u.

S

Herringbone likeS-S 3.47 Å

002

0Long alkylthio chains81

Layer-by-layer

c* = 33.9 Å
35

S

S

S

S

OctS

d!34 "

5

003

10

SOct
S

SC8-!DT-TTF

004

15
20
2θ/θ

25

30

Crystalline thin films formed.

36

Slideshow for Ph.D defense

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

AFM & FET CHAR.
!DT-TTF
S

S

S

S

S

S

Me
S

S

S

S

400 nm

S

E11/2

E21/2

"E

EHOMO / eV

-0.05

0.03

0.38

0.35

-4.75

3b Me

-0.12

-0.04

0.32

0.36

-4.68

3c SC8H17

-0.04

0.03

0.34

0.31

-4.77

DT-TTF

0.18

0.30

0.63

0.33

R

S

OctS

Me

S

onset

3a H

SC8-!DT-TTF

C1-!DT-TTF

S

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETsOXIDATION POTENTIALSOn Si/SiO2 Top-contact type
W=1500, L =50 #m

SOct

S

S

180 nm

S

S

40 nm

AFM
Step
height

S
S

S

S

-4.98

E1/2(Fc/Fc+) = -4.8 eV

S

HOMO

S

!DT-TTF-5 10-4

V = -80 V

10-3

-60V

10-4

g

S

S

S

S

Me

V = -60Vd-4 10-4

S

S

S

S

S

C8H17 S

-2 10-4-1 10-4!FET-I d / A

Id / A-3 10-4-4

Me
S

C1-!DT-TTF

-40 V

10-5

= ~0.2 cm2/Vs

Normally-on" DT-TTF-40
Vd / V-20

10-6-40 -20

0

S

S

S

S

-4.75

-4.70

-4.75

3a

3b

3c

(H)

(Me)

(octylthio)

-5.0

-5

S

S

-20 V

0V
0
-80
-60

S C8H17

S

SC8-!DT-TTF

EHOMO / eVno field effectno field effectFETSDT-TTF020 40
Vg / V

60

80

Low oxidation potential（High EHOMO)

37

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

S

S

S

S

S

S

S

S

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

S

R

R

S
S

R
S

S

$-coreR = Me or SC8H17

alkyl

Heteroaromatic-fused TTFs
SS

S

C8H17 S
S

S

S

S

S

Solution-process

S C8H17

S

O

S

S

S

S

S

S

R

Small disorder
S-S & $-$ 2D interaction

S

S

S

alkyl

S
SR

S

Si/SiO2 substrate

3 SC8-!DT-TTF!FET = 0.2 cm2/Vs!FET = 0.08 cm2/Vs

solubility & Self-organization

S
RS

O

2 C16-AcPyTTF

Layer-by-layer
StructureAir-stability & Mobility

Solubility & Layer-by-layer$-coreMolecular design!FET = ~ 0.2 cm2/Vs (> PyTTFs, " DT-TTF)
Low oxidation potential

40

39

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

CORRELATION OF AIR-STABILITYLONG ALKYL CHAINS + HETEROACENE "-CORE

HOMO energy levels
S$-core$-coreno Thin Films

O

S
N R

R N
S

S

S

N
S

R

R

S

N
S

S

S

O

S
S R

R S
S

S

S

S

Air-stable

S

S

S

S
S

S

R

R
S

1 Cn-PyTTF

S
S$-coreS

DT-TTF!FET = 0.3 cm2/Vs

N

N
R

S

S

1 C16-PyTTF!FET = 0.008 cm2/Vs

EHOMO = -4.75 eV

S

S

N R

R N

SC8-!DT-TTF

Long alkylthio chains

38

SUMMARY OF EXPERIMENTAL RESULTS

SUMMARY OF !DT-TTF
S

-4.95

Acyl
DT-TTF PyTTF

2 Cn-AcPyTTF

3 SCn-!DT-TTF

DT-TTF

Cn-BTBT2 cm2/Vs (solution-process)S

Ebata et al. JACS 2007, 129, 15732.

S

BTBT

S
S

DNTT

3 cm2/Vs

8 cm2/Vs (vacuum-process)(vacuum-process)Kang et al. Adv. Mater. in press.

Yamamoto et al. JACS
2007, 129, 2224.

2.4 cm2/Vs

Layer-by-layer structure

EHOMO / eV

DNTTon Polyimide/PC

-4
-4.5

-5

Doi et al. Cur. Appl. Phys., submitted.

Air-stability & high mobility
Difference from heteroaromatic-TTFs !?

-5.5

-4.62
-4.95

-4.75

-4.98
-5.5

-5.4

CHAPTER 3
41

Higher energy levels for TTF derivatives

4282Appendix

Ph.D defense：Development of "-Conjugated Molecules with Long Alkyl Chains for OFETs

Ph.D defense：Development of "-Conjugated Molecules with Long Alkyl Chains for OFETs

CORRELATION OF MOBILITY
Heteroaromatic-fused TTFs S-S

Packing of C12-BTBT (DNTT is similar)

S

S

S

S

!-!

To evaluate electronic structure / properties relating to hole transport quantitatively

N R

R N

C12H25

Mechanism in well-ordered organic semiconductors!

!

S
C12H25MECHANISM OF ELECTRON-TRANSFER

3.40 Å

S

2 cm2/Vs

O

S

S

N

N
S

R

S

O

e-

!
< 3.57 Å
S

S

S

S

S

S

S

kET = t 2

S

S

!

S

S

S

S

Two major parameters

S

integralt ：transferPortionfor energy gain

3.46 Åb% ( $E # " )2 (!
exp ' #!kBT "
4 " kBT *)
&

S R

R S

a

c

Electronic Transfer rate kET(Marcus Theory)+

Hopping Model

R

S

S

S

S

S

S

!

Typical Herringbone fashion
S-S 3.54 Å!

3.55 Å

S
S

S

S
S

S

S

energy!：reorganization
Portion for energy loss

A hole is considered to betransfered via overlap of orbitals.

S

How was the strong S-S interaction !?
It is necessary to evaluate electronic structure quantitatively.

Calculate t & ! in following slides

43

Ph.D defense：Development of "-Conjugated Molecules with Long Alkyl Chains for OFETs

Ph.D defense：Development of "-Conjugated Molecules with Long Alkyl Chains for OFETs

REORGANIZATION ENERGY (!)

REORGANIZATION ENERGY (!)
S+1S

for hole !

S

Compound

h

E

S

Ne
u

(2)
transfers to the!relneighboring one

C16-PyTTF

R N

2

C5-AcPyTTF

O

S

S

S

S

S

S

S

3 SC8-"DT-TTF+1 optimized

C12-BTBT(1)!rel

An isolated molecule in vacuum
Keep planer conformation

S

S

S

S

0.08

-a

0.2

253

0.3

239

2

130

3

O

S
S R

S

S

S

S
S

S
S

S
S
R

R

S

DNTT

S

a Alkylthio derivative has large value because of taking a boat form

Aromaticity(2)! h = ! (1)rel + ! rel

2

Electronic coupling between
neighboring molecules
Evaluation of strength of electronicbonding
Neglect phases (+ or -)ADF (Amsterdam density functional)
Slater-type orbital approximation
Triple-# polarization (TZP) basis set
GGA:PW91

SSS

N C8H17

C8-PyTTF

q

Overlap integral

p

q

S
S

46

Non-aromaticity

e1 = 〈$1|%|$1〉qq

C16H33 N

S

S

S

S

O

N C16H33

C16-PyTTF
0.008 cm2/Vs (solution)

S

S

S

S

C5H11

N

N
C5H11

O

C5-AcPyTTF
0.08 cm2/Vs (vacuum)

c
p

q

s

r
q

r

p

p

q

r
q

p

r

p

c

s

t12 = 〈$1|%|$2〉tHOMO (meV)
p
14
q
17a
a S-S 3.37 Å

Similar to a resonance integral ($)
when the AO forms MO
Analogy：H2分子
1#*
H1s

S

no field-effect (solution)

HOMO-1(dimer)

1
t12 - (e1 - e2 )S122
t12eff =1 - S122

S

Ph.D defense：Development of "-Conjugated Molecules with Long Alkyl Chains for OFETs

C8H17 N

2
HOMO
$2

2t

S

S
S

TRANSFER INTEGRAL tHOMOHOMO(dimer)
1
HOMO
$1

S

Acyl derivative : the largest value

B3LYP/6-31G(d), Gaussian 03 program 45

Ph.D defense：Development of "-Conjugated Molecules with Long Alkyl Chains for OFETs

1

S

TTFs have large values

TRANSFER INTEGRAL (t)2"

tHOMO (meV)
p
87a
q
3b
r
1
a !-! 3.57 Å. b S-S 3.40 Å.

1-D

H1s

tHOMO (meV)
p
33a
q
25a
r
45b
s
43b
b S-S 3.40 - 3.57 Å.

2-D

2D electronic interaction = higher mobility

1#Calculate t of dimers in single-crystal structure

83

350

S

DEFINITION：Sum of two relaxation energy

Level

0.008

R

R S
S

DT-TTF

GroundFunction"FET (cm2/Vs)

280

N

N
R

S

tra
l

Program!h (meV)
N R

Structure

1

S

Energy-loss when a hole

S

44

47

S-S short contact S-S contact

Electronic Interaction48

Slideshow for Ph.D defense

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

TRANSFER INTEGRAL tHOMOS

C8H17 S

S

S

S

S

S

S

S

q

S

C12-BTBTS

DNTT

2 cm2/Vs (solution)

3 cm2/Vs (vacuum)

0.3 cm2/Vs (vacuum)
p

p
q

q

p

r
p

tHOMO (meV)
p
55a
q
34b
r
34b

b S-S 3.46 Å.

2-DModerate t in two directions

atHOMO (meV)
p
71
q
91a
r
14b
a CH-! 2.78 Å. b CH-! 2.73 Å

2-D

2-D

2D conducting layert (50 ~ 90 meV) in two directions
Stronger interactions than those in TTF derivatives

49

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

compd.

structure

C16-PyTTF

1

2 C5-AcPyTTF
3

S

S

S

S

N R

R N

O

R

S

S

N

N
R

S

S

SC8-!DT-TTF R S

S

S

S

S

S

S

S

S

EX. OF EFFECTIVE S-S INTERACTION IN TTF

横1

S-S contact direction
perp

tHOMO (meV)side1 side2

50

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

縦横2

SUMMARY OF
TRANSFER INTEGRALSqpqa S-S 3.54 Å.

Values are larger than those in PyTTFs

r
q

tHOMO (meV)
p
62a
q
46

b S-S 3.55 Å.

2-D

q

q
p

tHOMO (meV)
p
14a
q
54b
r
53b

p

r

q

r

q

r
p

S

C12H25SS

DT-TTF0.2 cm2/Vs (solution)r

S

C12H25SS

S C8H17

TRANSFER INTEGRAL tHOMO!FET (cm2/Vs)

87

3

1

0.008

29a

45

43

0.08

14

54

53

0.2

55

34

34

0.3

62

46

46

2

71

91

14

3

S

S

S

S

HM-TTFpq

Thin films： 3 cm2/Vs (vacuum)r

Crystal： 8 cm2/Vs

r

q

p

Chem Mater. 2007, 19, 6382.
J. Mater. Chem. 2009, 19, 6548.

O

tHOMO (meV)S
S RS

DT-TTFS

C12-BTBT

R

S

S
R

p
q
r

40a
105b
28

Quasi 2-D

a !-! 3.6 Å. b S-S 3.55 Å.

3.46 Å (SC8-!DT-TTF).

S
S

DNTT

S

t ＝correlates with mobility（dimensionality & abs value）
Not all S-S contact give electronic interactions

S-S interaction• Distance
Both needed.
• Position"-Orbital expands the "-plane
normal (anisotropic)

51

52

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

SUMMARY OF CHAPTER 2

Ph.D defense：Development of !-Conjugated Molecules with Long Alkyl Chains for OFETs

Chapter 2

alkyl"-core

alkyl
S

S

S

S

Si/SiO2 substrate
S

Mobility

Layer-by-layer

Cn-PyTTF

◯

"

0.01

◯

2 Cn-AcPyTTF

"

◯

0.08

◯

3 SC8-!DT-TTF

◯

"

0.2

◯

C10-DNTT• Low oxidation potentials（air-stability）• Large "h & small t in the Herringbone like packing(mobility)! DNTT＆BTBT

EHOMO

t values/dimensionality"h values

Heteroacene core + Long alkyl chains (vacuum-proces)

Correlation: FET Char. & Structures

tHOMO = ~50 meV

Air-stability

Chapter 3• 2D electronic interaction in layer-by-layer structures

cm2/VsSolubility

Correlated electronic Structure

Good Molecular Design Strategy
S

Long alkyl chains + heteroaromatic-fused TTFs (solution-process)Compd.
1"-coreSUMMARY53

8 cm2/Vs on Si/SiO2
2.4 cm2/Vs on PC/polyimide

DNTT~2.1 cm2/Vs~0.4 cm2/Vs

Long alkyl chains along a small !-conjugated molecule promoteformation of 2D conducting layers which suitable for thin-film-based
OFETs owing to its hydrophobic interaction.
The resulting semiconductors showed high characteristics even on
plastic substrates.

84

54

Appendix

85

List of Publications

List of Publications

Contributed Paper 1 (Article): Section 2.2
Doi, I.; Miyazaki, E.; Takimiya, K.; Kunugi, Y. “Development of N-Alkyl-SubstitutedBis(pyrrolo[3,4-d])tetrathiafulvalenes as Organic Semiconductors for Solution Processible
Field-Effect Transistors” Chemistry of Materials. 2007, 19, 5230.
Contributed Paper 2 (Letter): Section 2.3
Doi, I.; Miyazaki, E.; Takimiya, K. “Synthesis and Characterization of N-Acyl-Substituted PyrroloTTF Derivatives and Improvement Air-Stability of PyrroloTTF-based OFETs” Chemistry
Letters. 2008, 37, 1088.
Contributed Paper 3 (Letter): Section 3.2
Kang, M. J.; Doi, I.; Mori, H.; Miyazaki, E.; Takimiya, K.; Ikeda, M.; Kuwabara, H. “Alkylated
Dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophenes (Cn-DNTTs)” Advanced Materials in press.
Contributed Paper 4 (Letter). Section 3.4
Doi, I.; Kang, M. J.; Takimiya, K. “High Mobility Organic Thin-Film Transistors on Plastic
Substrates” Current Applied Physics, submitted.
Related Paper 1
Uno, M.; Doi, I.; Takimiya, K.; Takeya, J. “Three-dimensional organic field-effect transistors
with high output current and high on-off ratio” Applied Physics Letters 2009, 94, 103307.
Related Paper 2
Uno, M.; Tominari, Y.; Yamagishi, M.; Doi, I.; Miyazaki, E.; Takimiya, K.; Takeya, J. “Moderately anisotropic field-effect mobility in dinaphtho[2,3-b; 2’3’-f]thiopheno[3,2-b]thiophenes
single-crystal transistors” Applied Physics Letters, 2009, 94, 223308.
Related Paper 3
Yamagishi, M.; Soeda, J.; Uemura, T.; Okada, Y.; Takatsuki, Y.; Nishikawa, T.; Nakazawa, Y.;
Doi, I.; Takimiya, K.; Takeya, J. “Free-electron-like Hall effect in high-mobility organic thinfilm transistors” Phys. Rev. B 2010, 81, 161306.

86

Acknowledgment
This work had been carried out in Applied Organic Chemistry Research Group since 2005 until 2011.
If it had not been for the support of following persons, I would not have been able to complete this study.
Stuff Members
Prof. Kazuo Takimiya
瀧宮和男教授Dr. Eigo Miyazaki
宮碕栄吾助教Dr. Itaru Osaka
尾坂格助教
Prof. emeritus Tetsuo Otubo
大坪徹夫名誉教授
岩谷さん
Secretary Hashimoto Keiko
橋本恵子秘書Dr. Tatsuya Yamamoto
山本達也博士Thank you for everything about the Ph.D work and thesis.
お忙しい中本当にお世話になりました。明日から４時に起きます。
Thank you for his great support of X-ray and other technical advices for ca. 5 years.
５年間に渡る技術的な指導、特にX線解析と多岐にわたる助言に感謝いたします。
I really admit his challengeable carrier and optimism.
チャレンジ精神溢れるキャリアと楽観主義を尊敬しています。
His advices were correct for me now.
一年間の短期間でしたが指導していただきありがとうございました。
元素分析でお世話になりました。
She supported everything during writing this thesis.
論文執筆中には一番お世話になりました。
I will work as a post-doctor like him
DNTTは偉大です。

Takeya Research Group
Prof. Junichi Takeya
竹谷純一教授Dr. Takafumi Uemura
植村隆文助教Dr. Mayumi Uno
宇野真由美博士
Dr. Masakazu Yamagishi
山岸正和博士He gave me great inspiration and motivation for study on organic transistor.
恐縮ながら共同研究者として活動できたことに本当に感謝しています。
Thank you for his advice for device fabrication. I respect your fascinating techniques.
デバイス研究に関する御助言ありがとうございました。高い技術を見習います。
It was nice work with you although short time.
お手伝いさせていただいた時の実験が成果に結びついたようで非常に光栄です。
He is one of the few graduates in same year. I hope our success in the future.
研究室は違いましたが数少ない僕の同期生ですね。これからもよろしく。

Excellent Collaborators
Mr. Kiyotaka Goshome
五升目清剛

The Chapter 2.5 is almost your work! Thank you for grateful research.
本論文の第２章４節はほとんど君の成果です。ありがとう。

Excellent Doctoral Candidates
Dr.C. Hiroki Mori
Thank you for his leadership in our group with great beard for organic devices.
森裕樹博士(見込)
本研究室のリーダーとしてまたデバイスに関して多大なる支援ありがとう。
Dr.C. Myeong Jin Kang
I was able to enjoy my doctoral life with her cheerfulness and with her C10-DNTT.
姜明辰 (캉명진)博士 (見込)
いつも元気なミョンジンでいてください。
Dr.C. Shoji☆Sinamura
品村祥司博士 (見込)Dr.C. Tomoya Kashiki
樫木友也博士(見込)Research of the excellent, by the excellent, for the excellent.
分子軌道計算に関する多大な助言を頂き感謝したします。
I have no advice for your doctoral research.
博士論文の健闘をお祈りしています。

Dr.C. Masahiro Nakano
中野正浩 博士

I hope you will enjoy your synthesis on my desk and equipment.
頑張って下さい。

Labmates (順不同）
香原くん
石根希望
龍ちゃん
工場長
八田くん
P
Great classmates
伊澤隆文
じいちゃん
Great Seniors
渡久地さん
江端さん
鳴滝さん
宇野さん

西村健
典子さん
まろ

斉藤慎彦
林秀輝博士
キング

半田さん

中瀬くん

香山さん
本田さん

蔵田さん

阿部達
田辺寛幸博士
竹田さん

秋田誠広
島脇雅史
岩谷くん

多田さん

田中さん

橋本雄人
柳井直幸
野村くん

瀬田梢
小松
加来さん

P山
新見くん

何よりここまで辛抱強く支援していただきました母・土井千恵美（＋妹・小夜）に感謝申し上げます。
Sincerely
Iori Doi 土井伊織