Ultralow bandgap molecular semiconductors for ambient-stable and solution-processable ambipolar organic field-effect transistors and inverters

Article abstract of DOI:10.1039/c6tc05079d

The design and development of novel ambipolar semiconductors is very crucial to advance various optoelectronic technologies including organic complementary (CMOS) integrated circuits. Although numerous high-performance ambipolar polymers have been realized to date, small molecules have been unable to provide high ambipolar performance in combination with ambient-stability and solution-processibility. In this study, by implementing highly π-electron deficient, ladder-type IFDK/IFDM acceptor cores with bithiophene donor units in D-A-D π-architectures, two novel small molecules, 2OD-TTIFDK and 2OD-TTIFDM, were designed, synthesized and characterized in order to achieve ultralow band-gap (1.21-1.65 eV) semiconductors with sufficiently balanced molecular energetics for ambipolarity. The HOMO/LUMO energies of the new semiconductors are found to be ?5.47/?3.61 and ?5.49/?4.23 eV, respectively. Bottom-gate/top-contact OFETs fabricated via solution-shearing of 2OD-TTIFDM yield perfectly ambient stable ambipolar devices with reasonably balanced electron and hole mobilities of 0.13 cm² V^?1 s^?1 and 0.01 cm² V^?1 s^?1, respectively with I_on/I_off ratios of ～10³-10⁴, and 2OD-TTIFDK-based OFETs exhibit ambipolarity under vacuum with highly balanced (μ_e/μ_h ～ 2) electron and hole mobilities of 0.02 cm² V^?1 s^?1 and 0.01 cm² V^?1 s^?1, respectively with I_on/I_off ratios of ～10⁵-10⁶. Furthermore, complementary-like inverter circuits were demonstrated with the current ambipolar semiconductors resulting in high voltage gains of up to 80. Our findings clearly indicate that ambient-stability of ambipolar semiconductors is a function of molecular orbital energetics without being directly related to a bulk π-backbone structure. To the best of our knowledge, considering the processing, charge-transport and inverter characteristics, the current semiconductors stand out among the best performing ambipolar small molecules in the OFET and CMOS-like circuit literature. Our results provide an efficient approach in designing ultralow band-gap ambipolar small molecules with good solution-processibility and ambient-stability for various optoelectronic technologies, including CMOS-like integrated circuits.

Full text of DOI:10.1039/c6tc05079d

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January 2016 Pages 1–224
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Page 1 of 30
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DOI: 10.1039/C6TC05079D
Design, Synthesis, and Characterization of Ultralow Bandgap
Molecular Semiconductors for AmbientꢀStable and Solutionꢀ
Processable Ambipolar Organic FieldꢀEffect Transistors
1#
2#
1
2
2
Resul Ozdemir, Donghee Choi, Mehmet Ozdemir, Guhyun Kwon, Hyekyoung Kim,
3
2
1
Unal Sen, Choongik Kim*, and Hakan Usta*
1
Department of Materials Science and Nanotechnology Engineering, Abdullah Gül
University, Kayseri, Turkey
2
Department of Chemical and Biomolecular Engineering, Sogang University, Mapoꢀgu,
Seoul, Korea
3
Department of Mechanical Engineering, Abdullah Gül University, Kayseri, Turkey
#
These authors contributed equally to this work.
*
Correspondence to: Prof. Hakan Usta (Eꢀmail:hakan.usta@agu.edu.tr), Prof. Choongik Kim (Eꢀmail:
choongik@sogang.ac.kr).
Abstract
1

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The design and development of novel ambipolar semiconductors is very crucial to advance
various optoelectronic technologies including organic complementary (CMOS) integrated
circuits. Although numerous highꢀperformance ambipolar polymers have been realized to
date, small molecules have been unable to provide high ambipolar performance in
combination with ambientꢀstability and solutionꢀprocessibility. In this study, by
implementing highly πꢀelectron deficient, ladderꢀtype IFDK/IFDM acceptor cores with
bithiophene donor units in DꢀAꢀD πꢀarchitectures, two novel small molecules, 2ODꢀ
TTIFDK and 2ODꢀTTIFDM, were designed, synthesized and characterized in order to
achieve ultralow bandꢀgap (1.21ꢀ1.65 eV) semiconductors with sufficiently balanced
molecular energetics for ambipolarity. The HOMO/LUMO energies of the new
semiconductors are found to be ꢀ5.47/ꢀ3.61 and ꢀ5.49/ꢀ4.23 eV, respectively. Bottomꢀ
gate/topꢀcontact OFETs fabricated via solutionꢀshearing of 2ODꢀTTIFDM yield perfectly
ambient stable ambipolar devices with reasonably balanced electron and hole mobilities of
2
2
3
4
0
.13 cm /V·s and 0.01 cm /V·s, respectively with Ion/Ioff ratios of ~10 ꢀ10 , and 2ODꢀ
TTIFDKꢀbased OFETs exhibit ambipolarity under vacuum with highly balanced (ꢁ /ꢁ ~ 2)
electron and hole mobilities of 0.02 cm /V·s and 0.01 cm /V·s, respectively with Ion/Ioff ratios
e
h
2
2
5
6
of ~10 ꢀ10 . Furthermore, complementaryꢀlike inverter circuits were demonstrated with the
current ambipolar semiconductors resulting in high voltage gains of up to 80. Our findings
clearly indicate that ambientꢀstability of ambipolar semiconductors is a function of molecular
orbital energetics without being directly related to bulk πꢀbackbone structure. To the best of
our knowledge, considering the processing, chargeꢀtransport and inverter characteristics, the
current semiconductors stand out among the best performing ambipolar small molecules in
the OFET and CMOSꢀlike circuit literature. Our results provide an efficient approach in
designing ultralow bandꢀgap ambipolar small molecules with good solutionꢀprocessibility
and ambientꢀstability for various optoelectronic technologies including CMOSꢀlike integrated
circuits.
Introduction
2

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Ambipolar organic semiconductors are attractive functional materials as potential
alternatives to conventional inorganic semiconductors (i.e., Si, Ge, GaAs, etc.) both for
fundamental research and for the fabrication of flexible, lowꢀcost, and lightꢀweight
optoelectronic devices such as singleꢀcomponent CMOS (complementary metalꢀoxide
[
1–8]
semiconductor)ꢀtype transistor circuits and lightꢀemitting transistors (OLETs).
Device
[9]
[10]
optimizations such as dielectric surface treatments, electrode surface modifications, and
[
11]
using nonꢀsymmetric workꢀfunction electrodes, or depositing bilayer/blend semiconductor
[12–14]
films
have been successfully employed to realize ambipolar chargeꢀtransport in organic
fieldꢀeffect transistors (OFETs). However, the most ideal approach would be to have an
[
1,15]
intrinsically ambipolar πꢀsystem, regardless of the device architecture.
The ability of an
intrinsic ambipolar semiconductor to transport both electrons and holes, as an initial
approximation, usually requires energetically close HOMO and LUMO frontier orbitals
(energy gap < 2.0 eV), which are symmetrically positioned around the metal electrode fermi
level, and also requires good solidꢀstate orbital overlaps between neighboring πꢀ
[
12,16,17]
systems.
In order to utilize ambipolar semiconductors in ideal optoelectronic
/µ ≈ 1) with good
applications, charge transport of electrons and holes should be balanced (µ
e
h
mobility and ambientꢀstability. This is particularly important to achieve maximum gains in
logic circuits and lightꢀemission efficiencies in OLETs. In addition, in order to enable lowꢀ
cost, facile device fabrications via spinꢀcoating, solutionꢀshearing, and printing, the
semiconductors should exhibit good solubilities in common organic solvents.
To date, ambientꢀstable and solutionꢀprocessable ambipolar semiconductors have been
mainly reported with πꢀconjugated polymers, and charge carrier mobilities exceeding those of
2
amorphous silicon (ꢁe,h > 0.5 cm /V·s) have been achieved in polymerꢀbased organic fieldꢀ
[18–20]
effect transistors (OFETs).
Most of these polymers have highly extended donorꢀacceptor
(DꢀA) type πꢀbackbones, which leads to πꢀelectronic structures with small HOMOꢀLUMO
3

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energy gaps (< 2.0 eV) and low LUMO energies (<ꢀ4.0 eV) to facilitate charge
[21–23]
injection/transport of both electrons and holes under ambient conditions.
Nevertheless,
very few solutionꢀprocessed small molecules with ambipolar chargeꢀtransport are known to
[
24–26]
operate in ambient.
This is mainly due to the difficulty of achieving such low HOMOꢀ
LUMO energy gaps and sufficiently low LUMOs within a limited πꢀextension of a molecular
backbone. When compared with polymers, small molecular πꢀconjugated systems offer
advantageous properties including enhanced solubility, higher crystallinity, better synthetic
[27–31]
reproducibility, and higher degree of purity.
thiopheneꢀbased small molecules in OFETs,
Since the early studies on holeꢀtransporting
[32]
the design and development of novel
molecular πꢀarchitectures with solutionꢀprocessibility and ambientꢀstability have been quite
[
33–37]
important for the realization of printed electronic circuits which can operate in ambient.
2
To date, many high mobility (>1.0 cm /V·s) unipolar (
p
ꢀchannel or
nꢀchannel) molecular
[38,39]
semiconductors have been developed,
molecules exhibiting good solutionꢀprocessibility and ambient stability.
however, there are very few ambipolar small
[
40–42]
In addition, the
majority of these ambipolar small molecules are based on diketopyrrolopyrrole (DPP)
acceptor core, and they typically have electron/hole mobilities in the range of ~0.001ꢀ0.1
2
[43,44]
cm /V·s.
To the best of our knowledge, the highest ambientꢀstable electron mobility
2
reported to date was for an ambipolar DPPꢀbased small molecule with ꢁ
e
= 0.6ꢀ0.8 cm /V·s;
ꢀ3
2
however, this molecule exhibited much lower hole mobility of ~10 cm /V·s for the same
[45]
device.
Therefore, the continued research efforts to design and develop new ambipolar
small molecules are crucial to realize high and balanced charge carrier mobilities, to fully
reveal their technological potential and to elucidate the fundamentals of electron vs. hole
transport processes in novel molecular solids.
4

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Figure 1. The chemical structures of 2ODꢀTTIFDK and 2ODꢀTTIFDM showing the αꢀ
substituted bithiophene donor units in yellow, indeno[1,2ꢀb]fluoreneꢀ6,12ꢀdione (IFDK)
acceptor unit in green, and indeno[1,2ꢀb]fluoreneꢀ6,12ꢀdiylidene)dimalononitrile (IFDM)
acceptor unit in red.
To achieve reasonably balanced ambipolarity in a small molecular backbone, we
utilized the strategy of building very strong DꢀAꢀD πꢀarchitecture using ladderꢀtype
indeno[1,2ꢀb]fluoreneꢀ6,12ꢀdione
(IFDK)
and
indeno[1,2ꢀb]fluoreneꢀ6,12ꢀ
diylidene)dimalononitrile (IFDM) πꢀacceptors and αꢀsubstituted bithiophene terminal πꢀ
[46]
donor units.
This approach can yield very small HOMOꢀLUMO gaps since HOMO is
dominated by πꢀdonor unit while LUMO is dominated by πꢀacceptor unit. In addition,
structural and electronic properties of cyclopentaꢀfused scaffolds can be favorable to stabilize
[
47]
both positive and negative charges on these new molecular backbones. Specifically, IFDM
was used due to its great coplanarity and extremely strong electronꢀaccepting ability in a
[48]
proper sized πꢀcore. Swallowꢀtailed alkyl substitutents, 2ꢀoctyldodecyl (2ꢀOD), are placed
at molecular termini (α,ωꢀpositions) to ensure good solubility while minimizing interꢀring
[49]
twists. We report herein the design, synthesis and characterization of two new ladderꢀtype
small molecules, 2,8ꢀbis(5'ꢀ(2ꢀoctyldodecyl)ꢀ2,2'ꢀbithiophenꢀ5ꢀyl)indeno[1,2ꢀb]fluoreneꢀ6,12ꢀ
dione (2ODꢀTTIFDK) and 2,2'ꢀ(2,8ꢀbis(5'ꢀ(2ꢀoctyldodecyl)ꢀ2,2'ꢀbithiophenꢀ5ꢀyl)indeno[1,2ꢀ
b]fluoreneꢀ6,12ꢀdiylidene)dimalononitrile (2ODꢀTTIFDM) (Figure 1). The chemical
5

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structures, physicochemical and optoelectronic properties of the new compounds were
1
13
characterized by H/ C NMR, TGA, DSC, UVꢀVis and CV. Our design yields ultraꢀlow
solidꢀstate band gaps of 1.65 eV for 2ODꢀTTIFDK and 1.21 eV for 2ODꢀTTIFDM with
favorable HOMO/LUMO energy levels of ꢀ5.47/ꢀ3.61 eV and ꢀ5.49/ꢀ4.23 eV, respectively,
for balanced ambipolarity. Specifically, the sufficiently low LUMO of dicyanovinyleneꢀ
functionalized molecule, 2ODꢀTTIFDM, enables ambientꢀstable electronꢀtransport, while
properly aligned HOMO level facilitates hole injection/transport. Topꢀcontact/bottomꢀgate
OFETs fabricated via solutionꢀshearing of 2ODꢀTTIFDM yield perfectly ambientꢀstable
2
ambipolar devices with relatively balanced electron and hole mobilities of 0.13 cm /V·s and
2
3
4
0
.01 cm /V·s, respectively, and I /I ratios of 10 – 10 . On the other hand, the carbonylꢀ
on off
e h
functionalized molecule, 2ODꢀTTIFDK, exhibits highly balanced ambipolarity (ꢁ /ꢁ ~ 2)
2
2
under vacuum with electron and hole mobilities of 0.02 cm /V·s and 0.01 cm /V·s,
5
6
respectively and Ion/Ioff ratios of 10 – 10 . We have also successfully demonstrated the
application of the new ambipolar transistors to complementaryꢀlike inverters, which is the
building block of integrated circuits for data processing. The new inverters exhibit high
voltage gains of 30 in ambient for 2ODꢀTTIFDM and 223 in vacuum for 2ODꢀTTIFDK.
Our findings indicate that although DꢀAꢀD πꢀskeleton stays the same for both small
molecules, energetic stabilization of LUMO (ꢀ3.61 eV → ꢀ4.23 eV) as a result of functional
group modification is key to the ambient stability of the corresponding OFETs. The
theoretical calculations explain the differences in molecular energetics and provide insights
into the current functionalization strategies. Our results clearly indicate that IFDM πꢀcore is a
properꢀsized, favorable acceptor unit for building efficient, soluble ambipolar small
molecules to operate in ambient. Considering the processing, chargeꢀtransport and inverter
characteristics, the current molecular semiconductors are among the best performing
ambipolar small molecules in the OFET and CMOSꢀlike inverter literature.
6

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EXPERIMENTAL
Materials and Methods
The reagents were purchased from commercial sources and used as received unless
otherwise noted. Conventional vacuum/nitrogen manifold system was used, and all the
1
13
reactions were carried out under N . The H/ C NMR spectra were recorded on a Bruker 400
2
1
13
spectrometer ( H, 400 MHz; C, 100 MHz), and elemental analyses were performed on a
Leco Truspec Micro model instrument. Thermogravimmetric analysis (TGA) and differential
scanning calorimetry (DSC) measurements were performed on Perkin Elmer Diamond model
instruments under nitrogen at a heating rate of 10 °C/min. Cyclic voltammetry measurements
were performed on a C3 Cell Stand equipped with the Epsilon potentiostat/galvanostat
(Bioanalytical Systems, Inc., Lafayette, IN). UVꢀvis absorption measurements were
performed on a UVꢀvis spectrophotometer (Shimadzu, UVꢀ1800). MALDIꢀTOF was
performed on a BrukerMicroflex LT MALDIꢀTOFꢀMS instrument. The total energy
minimizations and the optimization of the molecular geometries were carried out using
[
50]
density functional theory (DFT) at the B3LYP/6ꢀ31G** level with Gaussian 09.
The
synthesis of 2ꢀoctyldodecylbromide and 2,8ꢀdibromoindeno[1,2ꢀb]fluoreneꢀ6,12ꢀdione
(IFDKꢀBr2) were conducted in accordance with the reported procedures (See Supporting
[49]
Information).
Synthesis and Characterization
Synthesis of 5ꢀ(2ꢀoctyldodecyl)ꢀ2,2'ꢀbithiophene (1). To a solution of 2,2’ꢀbithiophene
(
1.353 g, 8.14 mmol) in THF (25 mL) at −78 °C was added 3.41 mL (8.52 mmol) of
nꢀ
butyllithium (2.5 M in ꢀhexane) dropwise under nitrogen. The resulting mixture was stirred
n
at −78 °C for 30 min and at room temperature for an additional 1 h. Then, 2ꢀ
octyldodecylbromide (3.11 g, 8.95 mmol) was added to this mixture slowly at −78 °C. The
o
resulting reaction mixture was stirred at room temperature for 1 h and then heated to 85 C
7

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for 12 h. The reaction was quenched with water, and the product was extracted with
chloroform. The organic phase was washed with water, dried over Na SO , filtered, and
2
4
evaporated to dryness to yield a crude product, which was purified by column
chromatography on silica gel using hexane as the eluent to give the pure product (1.415 g,
1
3
8.8% yield). H NMR (400 MHz, CDCl ): δ 0.89 (m, 6H), 1.27ꢀ1.35 (m, 33H), 2.72 (d, 2H,
3
J = 6.8 Hz), 6.66 (d, 1H, J = 3.2 Hz), 6.99 (m, 2H), 7.11 (d, 1H, J = 3.2 Hz), 7.17 (d, 1H, J =
.2 Hz).
5
Synthesis of trimethyl(5'ꢀ(2ꢀoctyldodecyl)ꢀ[2,2'ꢀbithiophen]ꢀ5ꢀyl)stannane (2). To a
solution of 5ꢀ(2ꢀoctyldodecyl)ꢀ2,2'ꢀbithiophene (1) (1.415 g, 3.17 mmol) in THF (30 mL) at
−
78 °C was added 1.33 mL (3.33 mmol) of nꢀbutyllithium (2.5 M in nꢀhexane) under
nitrogen. The mixture was stirred at −78 °C for 30 min and then at room temperature for 1 h.
Then, trimethyltin chloride (0.695 g, 3.49 mmol) was added slowly at −78 °C, and the
resulting reaction mixture was allowed to warm to room temperature and stirred at room
temperature overnight. The reaction was quenched with water, and the product was extracted
with hexane. The organic phase was washed with water, dried over Na SO , filtered, and
2
4
1
evaporated to dryness to give the pure product (1.889 g, 97.7% yield). H NMR (CDCl
MHz): δ 0.39 (s, 9H), 0.89 (m, 6H), 1.27ꢀ1.35 (m, 33H), 2.72 (d, 2H, J = 6.4 Hz), 6.65 (d,
H, J = 3.2 Hz), 6.98 (d, 1H, J = 3.2 Hz), 7.07 (d, 1H, J = 3.6 Hz), 7.22 (d, 1H, J = 3.6 Hz).
3
, 400
1
Synthesis of 2,8ꢀbis(5'ꢀ(2ꢀoctyldodecyl)ꢀ2,2'ꢀbithiophenꢀ5ꢀyl)indeno[1,2ꢀb]fluoreneꢀ6,12ꢀ
dione (2ODꢀTTIFDK). The reagents 2,8ꢀdibromoꢀindeno[1,2ꢀb]fluoreneꢀ6,12ꢀdione(IFDKꢀ
Br
2
) (0.282 g, 0.641 mmol), trimethyl(5'ꢀ(2ꢀoctyldodecyl)ꢀ[2,2'ꢀbithiophen]ꢀ5ꢀyl)stannane (2)
Cl (45.0 mg, 0.0641 mmol) in anhydrous DMF (40 mL)
(0.860 g, 1.41 mmol), and Pd(PPh
3
)
2
2
were heated at 125 °C under nitrogen for 2 days. Then, the mixture was then cooled to room
temperature and quenched with water. The reaction mixture was extracted with chloroform,
and the organic phase was washed with water, dried over Na SO , filtered, and evaporated to
2
4
8

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dryness to give the crude product. The crude was then purified by column chromatography on
silica gel using chloroform as the eluent to afford the pure product as a dark green solid
1
(
0.230 g, 30.6% yield). m.p. 217ꢀ218 °C. H NMR (400 MHz, CDCl
3
): δ 0.90 (m, 6H), 1.28ꢀ
1.32 (m, 33H), 2.72 (d, 2H, J = 6.4 Hz), 6.64 (d, 1H, J = 3.6 Hz), 6.98 (m, 2H), 7.18 (d, 1H, J
1
3
=
3.6 Hz), 7.41 (d, 1H, J = 7.6 Hz ), 7.62 (d, 1H, J = 8.5 Hz), 7.64 (s, 1H), 7.73 (s, 1H). C
): δ 14.1, 22.7, 26.6, 29.4, 29.5, 29.6, 29.7, 30.0, 31.9, 33.2, 34.6,
0.0, 115.6, 120.5, 120.8, 123.5, 123.7, 124.4, 125.9, 130.9, 134.4, 134.6, 135.3, 138.2,
NMR (100 MHz, CDCl
3
4
1
+
39.2, 140.2, 141.3, 144.4, 145.2, 192.1. MS (MALDIꢀTOF) m/z (M ): calcd. for
+
C H O S : 1170.64, found: 1171.74 [M+H] . Anal. calcd. for C H O S : C, 77.90; H,
76
98
2
4
76 98
2 4
8.43, found: C, 77.98; H, 8.64.
Synthesis of 2,2'ꢀ(2,8ꢀbis(5'ꢀ(2ꢀoctyldodecyl)ꢀ2,2'ꢀbithiophenꢀ5ꢀyl)indeno[1,2ꢀb]fluoreneꢀ
6,12ꢀdiylidene)dimalononitrile (2ODꢀTTIFDM). The reagents 2,8ꢀbis(5'ꢀ(2ꢀoctyldodecyl)ꢀ
2,2'ꢀbithiophenꢀ5ꢀyl)indeno[1,2ꢀb]fluoreneꢀ6,12ꢀdione (2ODꢀTTIFDKTT) (0.100 g, 0.085
mmol) and dicyanomethane (79 mg, 1.19 mmol) were dissolved in 12 mL of dry
chlorobenzene under nitrogen. Then, pyridine (0.131 mL, 1.62 mmol) and TiCl (0.094 mL,
4
0.853 mmol) were added and the resulting reaction mixture was heated at 110 °C for 5 h.
Then, the reaction was cooled to room temperature and quenched with water. The reaction
mixture was extracted with chloroform, and the organic phase was washed with water, dried
over Na SO , filtered, and evaporated to dryness to give the crude product. The crude was
2
4
then purified by column chromatography on silica gel using chloroform as the eluent to give
1
the pure product as a dark solid (63 mg, 58.8% yield). m.p. 320ꢀ321 °C. H NMR (400 MHz,
CDCl
3
): δ 0.90 (m, 6H), 1.27ꢀ1.35 (m, 33H), 2.57 (d, 2H, J = 6.0 Hz), 6.50 (d, 1H, J = 3.6
Hz), 6.81 (m, 2H), 7.01 (d, 1H, J = 3.6 Hz), 7.24 (d, 1H, J = 8.0 Hz ), 7.42 (d, 1H, J = 8.0
1
3
Hz), 8.02 (s, 1H), 8.04 (s, 1H). C NMR (100 MHz, CDCl ): δ 14.2, 22.7, 26.5, 29.3, 29.4,
3
29.6, 29.7, 29.8, 30.0, 32.0, 33.1, 34.5, 39.9, 112.6, 112.7, 117.4, 121.1, 122.3, 123.6, 123.9,
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124.8, 126.0, 129.9, 130.2, 134.0, 134.4, 135.4, 138.4, 138.5, 138.9, 139.1, 142.4, 145.0,
+
1
58.5. MS (MALDIꢀTOF) m/z (M ): calcd. for C82
H
98
N
4
S
4
: 1266.67, found: 1267.19
+
[
M+H] . Anal. calcd. for C82
H
98
N
4
S
4
: C, 77.68; H, 7.79; N, 4.42, found: C, 77.96; H, 7.95; N,
4.56.
OFET Device Fabrication and Characterization
All OFETs were fabricated on highly
00 nm SiO
n
ꢀdoped silicon wafers having thermally oxidized
2
3
2
dielectric (capacitance per unit area C
i
=11.4 nF/cm ) by adopting the topꢀ
contact/bottomꢀgate (TC/BG) device architecture. The substrates were cleaned via sonication
in acetone sonication for 10 min and followed by oxygen plasma cleaning for 5 min (Harrick
plasma, PDCꢀ32G, 18 W). The PS (polystyrene)ꢀbrush treatment was performed in
accordance with the reported procedures (M
w
= 1.7 – 10 kg/mol) to achieve favorable
[51–53]
dielectricꢀsemiconductor interfaces.
The organic semiconductor layers (2ODꢀTTIFDK
and 2ODꢀTTIFDM) were deposited via solutionꢀshearing method on PSꢀbrushꢀtreated
substrates. The solutionꢀshearing method was performed based on the reported procedure,
and the conditions were optimized with respect to solvent, concentration of solution,
[54]
temperature of substrate, shearing speed, and thermal annealing temperature. The solutionꢀ
sheared substrates were kept in a vacuum oven at 150 °C for 30 min, and then placed in a
desiccator for 24 h to remove the residual solvent. The profilometer (DEKTAKꢀXT, Brucker)
was used to measure the OSC film thicknesses (30 – 60 nm). The top electrodes were
thermally evaporated (deposition rate = 0.2 Å/s) as Au layers (50 nm) with various channel
widths (W; 1000 and 500 ꢁm) and lengths (L; 100 and 50 ꢁm). Keithley 4200ꢀSCS was used
to characterize the electronic performances of OFETs in ambient or in a vacuum probe station
ꢀ2
(
< 10 Torr) at room temperature. The electronic performance in the saturation region such as
charge carrier mobilities (ꢁ) and threshold voltages (V ) were extracted from the equation:
T
2
ꢁsat= (2I L)/[WC (V –V ) ]
DS
i
G
T
1
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where IDS is the drain current, L and W are the channel length and width, respectively, C
i
is
the areal capacitance of the gate dielectric, V is the gate voltage, and V is the threshold
G
T
voltage. The reported OFET characteristics are the average of 10 different devices with
standard deviations of less than 5ꢀ10%.The surface morphology and microstructure of thinꢀ
films were measured by atomic force microscopy (AFM, NX10, Park systems) and Xꢀray
diffraction (XRD, Smartlab, Rigaku), respectively.
Fabrication and Characterization of Complementaryꢀlike Inverters
+
+
The complementaryꢀlike inverters were fabricated on the PSꢀbrush treated n ꢀ
Si/SiO gateꢀdielectric substrate using two identical ambipolar OFETs based on 2ODꢀ
2
TTIFDK or 2ODꢀTTIFDM with a common gate as the input voltage (V ). The top
IN
electrodes with various channel widths (W; 2000, 1000, and 500 ꢁm) and lengths (L; 100 and
50 ꢁm) were deposited through shadow mask to define the electrode of supply bias (VDD), the
common electrode of output voltage (VOUT), and the ground electrode. The channel
dimensions for the two ambipolar OFETs were identical. The complementaryꢀlike inverters
were characterized with the Keithley 4200ꢀSCS instrument in ambient or under vacuum (<
ꢀ2
1
0 Torr).
RESULTS AND DISCUSSIONS
Computational Modeling, Synthesis and Characterizations
DensityꢀFunctional Theory (DFT) study was performed on the model compounds M1 and
M2 prior to the synthesis to gain initial insights into the molecular and electronic structures
of the new molecules (Figure 2). In these model compounds, shorter chains (isobutyl) were
employed instead of longer alkyl chains (2ꢀoctyldodecyl) to ease the calculations by reducing
degrees of freedom. Based on the energyꢀminimized geometries, IFDK and IFDM πꢀcores
are found to adopt high coꢀplanarity while some degree of acceptable interꢀring twists exist
between these cores and outer thiophene units (θ < 22˚). For both compounds, HOMO is
1
1

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highly delocalized along the molecular backbone and LUMO is more localized on the
acceptor cores. However, the proper size of the acceptor unit may still ensure efficient intraꢀ
/
interꢀmolecular πꢀdelocalization and therefore good electron transport. The theoretical
HOMOꢀLUMO energy gaps are estimated as 2.27 eV for M1 and 1.45 eV for M2. The large
reduction in the energy gap originates mainly from the LUMO level reduction (ꢀ2.90 eV → ꢀ
3.87 eV) as a result of functional group modification from carbonyl to dicyanovinylene. The
origin of this reduction is a result of favorable inductive and mesomeric effects of
dicyanovinylene vs. carbonyl ensuring very low LUMO level with minimal influences on
HOMO energy level. The low LUMO energy level of M2 is ideal for airꢀstable electron
transport, which, when combined with its still relatively high HOMO, should enable
ambipolarity in ambient. It is noteworthy that although ambipolarity may also be expected
from M1, based on its relatively high LUMO, the electronꢀtransport is not expected to be
stable in ambient.
Figure 2. Chemical structures of the model compounds M1 and M2. Optimized molecular
geometries showing interꢀring dihedral angles, computed HOMO and LUMO energy levels,
and topographical orbital representations (DFT, B3LYP/6ꢀ31G**).
Scheme 1 shows the synthesis of the stannylated πꢀdonor moiety, trimethyl(5'ꢀ(2ꢀ
octyldodecyl)ꢀ[2,2'ꢀbithiophen]ꢀ5ꢀyl)stannane (2) and the corresponding α,ωꢀdisubstituted
1
2

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molecules 2ODꢀTTIFDK and 2ODꢀTTIFDM. The synthesis of the ladderꢀtype acceptor
core IFDKꢀBr2 was carried out via Suzuki crossꢀcoupling and intramolecular FriedelꢀCrafts
acylation reactions in accordance with the established procedure by our group (Scheme S1).
Donor unit, 2, was prepared from bithiophene in two steps: in the first step, bithiophene was
monolithiated and capped with 2ꢀoctyldodecylbromide to yield 1 in 39% yield, and
subsequently,
2
was synthesized from
1
in 98% yield via
a
conventional
lithiation/stannylation reaction. The synthesis of the new carbonyl functionalized ladder type
compound 2ODꢀTTIFDK was carried out via conventional Stille crossꢀcoupling reaction by
reacting IFDKꢀBr2 and 2 in DMF using Pd(PPh ) Cl catalyst in 31% yield. Since the
3
2
2
solubility of IFDKꢀBr2 was very low in common organic solvents, the success of this
reaction relies on using highly polar solvent (DMF) at high temperature (125 °C). During the
course of this reaction, the coupling of the first πꢀdonor moiety having 2ꢀOD chain enhances
the solubility of the monoꢀcoupled adduct significantly so that the second coupling proceeds
more easily. Dicyanovinylene functionalized molecule, 2ODꢀTTIFDM, was synthesized via
Knoevenagel condensation between 2ODꢀTTIFDK and malononitrile in the presence of
pyridine and TiCl
substituents, the new DꢀAꢀD molecules are found to be very soluble in common organic
solvents (CHCl , THF, Toluene, etc.) and purifications were performed by flash column
4
in 59% yield. Thanks to the presence of swallowꢀtailed 2ꢀOD alkyl
3
chromatography using Silica gel and chloroform as the stationary phase and eluent,
respectively. The good solubilites of the new molecules also enable the fabrication of OFETs
via solutionꢀprocessing. The chemical structures and purities of the intermediate and final
1
13
products were confirmed by H/ C NMR (Figures S1, S2, S4, and S5), mass spectroscopy
(MALDIꢀTOF) (Figures S3 and S6), elemental analysis, and ATRꢀFTIR (Figure S7). Based
on thermogravimetric analysis (TGA), both compounds exhibited good thermal stabilities
with onset decomposition temperatures over 350 ˚C (Figure S8A). Differential scanning
1
3

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calorimetry (DSC) measurements of 2ODꢀTTIFDK and 2ODꢀTTIFDM exhibited highꢀ
temperature endotherms at 218 ˚C and 320 ˚C, respectively, which are in accord with the
conventional melting temperature measurements (Figure S8B). Surprisingly, higherꢀenergy
endothermic thermal transitions (179/208 ˚C for 2ODꢀTTIFDK and 209 ˚C for 2ODꢀ
TTIFDM) were observed for both compounds prior to the major melting process, which is
[
55]
typical characteristics of liquidꢀcrystalline materials. On cooling, the DSC traces of both
compounds displayed the same behavior of their heating cycle. Although the πꢀskeleton
remains the same for both compounds, the melting temperature significantly increases (ꢂT
m
=
+102 ˚C) when the functional group is changed from carbonyl (“C=O” in 2ODꢀTTIFDK)
2
to dicyanovinylene (“C=C(CN) ” in 2ODꢀTTIFDM). This indicates more effective cohesive
forces in the solidꢀstate packing for the latter molecule, which is likely a combined result of
larger local/molecular dipole moments and enhanced dipoleꢀdipole/πꢀπ interactions as a result
of stronger donorꢀacceptor characteristics. When compared with our previously reported
[
49]
compound, 2ODꢀTIFDKT (Figure S9),
the introduction of additional thiophenes at
molecular termini significantly increased T
enhanced πꢀcore rigidity.
m
by ~80 ˚C in 2ODꢀTTIFDK as a result of
Scheme 1. Synthesis of 2ODꢀTTIFDK and 2ODꢀTTIFDM.
As shown in Figure 3, when the concentrations of 2ODꢀTTIFDK and 2ODꢀTTIFDM
in CDCl increased from 1.0 mg/mL to 16.0 mg/mL, the chemical shifts of the aromatic and
3
1
4

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αꢀaliphatic protons moved upfield (ꢀꢂδ = 0.1–0.4 ppm), indicating the presence of shielding
effect. This shielding is due to enhanced electronꢀdensity on individual protons as a result of
molecular stacking in organic solvents via intermolecular πꢀπ stacking and van der Waals
interactions. The aromatic protons that show chemical shifts originated both from
IFDK/IFDM πꢀcores and bithiophene units, as well as the alphaꢀmethylene proton (ArꢀCH
2
ꢀ
CH(C H )C H ) adjacent to the thiophenes, indicating that all these protons are involved in
8
17
10 21
the intermolecular interactions in the molecular stacks. The aromatic peaks were broadened
with increasing concentration, which further supports the stacking model since intermolecular
attractions widen the local electronic profile around each proton. Similar stacking behaviors
[
56–58]
were observed for a number of πꢀconjugated small molecules in the literature.
This
indicates the existence of favorable intermolecular interactions for the present DꢀAꢀD πꢀcores
even in the solutionꢀphase, which may potentially render the formation of favorable
supramolecular architecture after solvent removal in the solidꢀstate (vide infra).
1
Figure 3. The concentration dependent H NMR spectra of 2ODꢀTTIFDK (A) and 2ODꢀ
3
TTIFDM (B) in CDCl . Inset is the schematic model of molecular stacking in solution as a
result of concentration. (The blue and green units represent πꢀbackbone and flexible
hydrocarbon chains, respectively.)
Optical and Electrochemical Properties
UVꢀvis absorption spectra (Figures 4A and 4B) of 2ODꢀTTIFDK and 2ODꢀ
TTIFDM in dichloromethane solutions exhibit similar higher energy maxima at 410 nm and
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400 nm, respectively, corresponding to πꢀπ* transitions of the bithiopheneꢀindeno[1,2ꢀ
b]fluorene core, which remains the same for both compounds although the functional groups
are different. However, lowerꢀenergy peaks appear at 568 nm and 740 nm for 2ODꢀTTIFDK
and 2ODꢀTTIFDM, respectively, reflecting the differences in the chemical nature of
carbonyl vs. dicyanovinylene functional groups. Therefore, these relatively weaker lowerꢀ
energy transitions are most likely to be governed by the functional groups and originates from
[59,60]
symmetry forbidden nꢀπ* transitions.
The optical band gaps in solution are calculated
from the lowꢀenergy absorption edges as 1.89 eV (2ODꢀTTIFDK) and 1.31 eV (2ODꢀ
TTIFDKM). Both compounds exhibit significant bathochromic shifts (∆λ = 16–216 nm) and
reductions in optical band gaps when going from dilute solution to solidꢀstate. This is
indicative of molecular planarization and enhanced πꢀπ stacking/donorꢀacceptor interactions
in the solidꢀstate. The solidꢀstate optical band gaps are estimated as 1.65 eV for 2ODꢀ
TTIFDK and 1.21 eV for 2ODꢀTTIFDM, which are, to the best of our knowledge, among
the lowest in the literature for πꢀconjugated small molecules. It is noteworthy that the optical
bandꢀgap and absorption maxima observed for the present compound, 2ODꢀTTIFDM, is
~
0.15 eV smaller and ~30 nm redꢀshifted, respectively, compared to those of structurally
[48]
related compound, βꢀC12ꢀTTIFDM (1.44 eV; λmax = 410, 711 nm).
This is due to
repositioning of the hydrocarbon chains from βꢀ to α,ωꢀpositions (Figure S9) in the new
semiconductor, which minimizes the interꢀring twists between donor and acceptor units (θPhꢀ
=
45ꢀ46° → 16ꢀ17°) and further extends the πꢀconjugation on the present compound. This
Th
is found to be very advantageous to the charge carrier mobilities in the corresponding OFET
devices (vide infra).
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6

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Figure 4. For compounds 2ODꢀTTIFDK and 2ODꢀTTIFDM, (A) Optical absorption in
dichloromethane solution, (B) Optical absorption as thinꢀfilms, and (C) Cyclic
+
ꢀ
ꢀ1
4 6
voltammograms in dichloromethane (0.1M Bu N PF , scan rate = 50 mV·s ).
As shown in Figure 4C, cyclic voltammetry (CV) measurements of 2ODꢀTTIFDK
and 2ODꢀTTIFDM in dichloromethane solutions exhibit reversible oxidation and reduction
peaks with the first halfꢀwave potentials located at 1.07 V/ꢀ0.79 V (vs. Ag/AgCl) and 1.09
V/ꢀ0.17 V (vs. Ag/AgCl), respectively. The existence of both oxidative and reductive
electrochemical processes with good reversibility indicates the intrinsic redoxꢀstable
ambipolar nature of the present compounds. Due to the stronger electron accepting nature of
dicyanovinylene in 2ODꢀTTIFDM compared to carbonyl in 2ODꢀTTIFDK, reduction
potential shows significant anodic shift (ꢀ0.79 V → ꢀ0.17 V) with minimal change on the
oxidation potentials. The corresponding HOMO/LUMO energy levels are estimated as ꢀ5.47
eV/ꢀ3.61 eV for 2ODꢀTTIFDK and ꢀ5.49 eV/ꢀ4.23 eV for 2ODꢀTTIFDM, using the vacuum
level energy of Ag/AgCl as ꢀ4.40 eV. These observations are consistent with our theoretical
findings (vide supra) that for both compounds, HOMOs are delocalized along the same
molecular backbone and LUMOs are localized on the acceptor core functionalities. On the
basis of the oxidation and reduction potentials, the electrochemical band gaps for 2ODꢀ
TTIFDK and 2ODꢀTTIFDM are estimated as 1.86 eV and 1.26 eV, respectively, which are
in perfect agreement with the optical band gaps. Based on the molecular orbital energetics
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and low band gaps, the HOMO (ꢀ5.47ꢀ ꢀ5.49 eV) and LUMO (ꢀ3.61ꢀ ꢀ4.23 eV) energies of
the current compounds should favor concurrent electron and hole injection/transport
(ambipolarity) in the corresponding thinꢀfilms. Specifically, the LUMO level of 2ODꢀ
TTIFDM is highly favorable for ambientꢀstable electronꢀtransport.
FieldꢀEffect Transistor and Complementaryꢀlike Inverter Fabrication and
Characterization
Topꢀcontact/bottomꢀgate (TC/BG) OFETs were fabricated by solutionꢀshearing 30ꢀ60 nm
++
2
thick films of 2ODꢀTTIFDK and 2ODꢀTTIFDM on PSꢀbrush treated n ꢀSi/SiO (300 nm)
substrates to achieve favorable semiconductorꢀdielectric interfaces with higher semiconductor
film quality and better control on the microstructure/morphology, as compared to
[
61,62]
conventional solutionꢀbased methods such as drop casting and spin coating.
For both
semiconductors, the most ideal solutionꢀshearing condition was identified as applying 1.0
mg/mL (in chlorobenzene) semiconductor formulations on substrates maintained at 50ꢀ60 ˚C
at a shearing speed of ~1.0 mm/min. OFET devices were characterized under positive and
negative gate biases in ambient and under vacuum to explore the ambipolarity, device
performance, and ambient stability. Typical transfer and output curves are shown in Figures 5
and S10ꢀS13, and the corresponding OFET data are summarized in Table 1.
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Figure 5. (A, B) Ambipolar (Pꢀchannel (in red) and NꢀChannel (in blue)) transfer curves of
the OFET devices based on solutionꢀsheared thinꢀfilms of 2ODꢀTTIFDK and 2ODꢀ
TTIFDM, and (C) Topꢀcontact/bottomꢀgate OFET device structure and the band lineups of
the molecular orbital energy levels with respect to O
the airꢀthin film interface.
2 2
/H O electrochemical redox couple at
As expected from the optical/electrochemical and theoretical characterizations (vide
supra), all OFET devices fabricated with the current semiconductors, 2ODꢀTTIFDM and
2ODꢀTTIFDK, were found to exhibit typical ambipolar characteristics. This indicates
intrinsic ambipolar semiconducting nature of the present compounds and can be attributed to
their low solidꢀstate band gaps (1.21ꢀ1.65 eV), which render both HOMO and LUMO levels
energetically accessible for hole and electron injection/transport, respectively. Consistent
with its highly stabilized LUMO energy level of ꢀ4.23 eV, 2ODꢀTTIFDM is found to be
perfectly ambientꢀstable as semiconductor thinꢀfilm in OFETs with electron and hole
2
2
3
4
mobilities of 0.13 cm /V·s and 0.01 cm /V·s, respectively, and Ion/Ioff ratios of ~10 –10 in
ambient conditions (Figure 5). These devices are found to exhibit negligible variations in
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transistor characteristics after few months of storage in ambient, showing prolonged stability
of the corresponding semiconductor thinꢀfilms (Figure S12). When the semiconductor
performance of 2ODꢀTTIFDM is compared with that of structurally related βꢀC12ꢀ
[
48]
TTIFDM
(Figure S9), which includes additional linear alkyl chains at thiophenes’ βꢀ
ꢀ3
2
positions, two orders of magnitude (×100) higher electron (ꢁ = 1×10 cm /V·s → 0.13
e
2
ꢀ4
2
2
cm /V·s) and hole (ꢁ
h
= 1×10 cm /V·s → 0.01 cm /V·s) mobilities were achieved with the
current semiconductor, 2ODꢀTTIFDM. The substantial mobility improvements observed for
the new semiconductor probably reflects a combination of DꢀAꢀD πꢀcore planarization as a
result of reducing interꢀring DꢀA twists (vide supra) and improved thinꢀfilm
microstructure/morphology due to the solutionꢀshearing process. From a structural design
standpoint, in the development of ambipolar small molecules employing donorꢀacceptor
linkages, we can suggest that it’s important to minimize the interꢀring twists between donor
and acceptor units. On the other hand, due to its relatively higher LUMO energy level of ꢀ
3.61 eV, OFETs fabricated with 2ODꢀTTIFDK were only active under vacuum; they exhibit
2
highly balanced (ꢁ /ꢁ ~ 2) ambipolar behavior with mobilities of 0.02 cm /V·s (for nꢀ
e
h
2
5
6
channel) and 0.01 cm /V·s (for pꢀchannel) with Ion/off ratios of ~10 –10 , respectively. When
2ODꢀTTIFDKꢀbased OFETs were measured in ambient conditions, although pꢀchannel
performance remained at the same level, nꢀchannel performance showed significant initial
drop (×100) and eventually became inactive (Figure S13). The transition of electron transport
from being unstable to ambientꢀstable was clearly observed for the present semiconductors
when the functional group was changed from carbonyl to dicyanovinylene, even though the
πꢀbackbone (bithiopheneꢀindeno[1,2ꢀb]fluoreneꢀbithiophene) remains the same. This
indicates the importance of molecular orbital energetics stabilization for ambientꢀstable
electronꢀtransport in semiconductor thinꢀfilms and an electrochemical threshold exists
between ꢀ3.6 eV and ꢀ4.2 eV. This is consistent with earlier reports, and corresponds to a
2
0

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required stabilization of LUMO energy level against electrochemical trapping by O
2
ꢀH
2
O
[
48,63]
redox couple.
To the best our knowledge, the OFET characteristics of 2ODꢀTTIFDM
are among the best solutionꢀprocessed, small molecular ambipolar performances in terms of
reasonably balanced and ambientꢀstable charge carrier mobilities. It’s noteworthy that further
[
20]
device structure optimizations such as using ferroelectric dielectrics and adding graphene
[
64]
[65]
oxide (GO) or singleꢀwalled carbon nanotube (SWNT) interlayers can be employed to
further balance electrical characteristics of holeꢀ vs electronꢀtransport with the current
ambipolar semiconductors.
Table 1. Electrical Performance of OFETs based on 2ODꢀTTIFDK and 2ODꢀTTIFDM
a
developed in this study.
Nꢀchannel
(V)
Pꢀchannel
(V)
Material
2
2
ꢁ
e
(cm /V·s)
V
T
Ion/Ioff
μ
h
(cm /V·s)
V
T
Ion/Ioff
5
4
6
3
2
2
ODꢀTTIFDK 0.02±0.002
ODꢀTTIFDM 0.13±0.012
34±3
13±2
1.4×10
1.0×10
0.01±0.001
0.01±0.002
ꢀ55±4
ꢀ33±2
2.2×10
1.1×10
a
The OFETs based on 2ODꢀTTIFDK and 2ODꢀTTIFDM were measured under vacuum and in ambient,
respectively.
Complementaryꢀlike inverter is an essential building block of integrated circuits for
data processing, and it typically requires the fabrication of separate pꢀ and nꢀchannel
transistors. However, when an ambipolar semiconducting material is used, only a single layer
of organic semiconductor is required without the need for separate patterning of unipolar pꢀ
and nꢀchannel semiconductors. Therefore, given the promising highꢀperformance, wellꢀ
balanced ambipolar OFET characteristics of the new semiconductors, complementaryꢀlike
inverters were fabricated from 2ODꢀTTIFDK or 2ODꢀTTIFDM semiconductor solutions
and characterized on single substrates using two identical ambipolar transistors with the
shared gold electrode (Figure 6ꢀinset). As shown in Figures 6 and S14, complementaryꢀlike
inverters based on 2ODꢀTTIFDK and 2ODꢀTTIFDM operated well under vacuum and in
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ambient, respectively, in the first and third quadrants with sharp switchings of VOUT’s at
supplied voltages (VDD) of +100 and ꢀ100 V, respectively. The voltage gain is calculated
from |dVOUT/dVIN|, where VOUT and VIN are the output and input voltages, respectively. The
ability of the current inverters to operate in both quadrants is the specific advantage of having
[
66,67]
ambipolar OFETs.
For 2ODꢀTTIFDKꢀbased inverters, very high voltage gain of 80 V/V
was achieved at positive VIN along with a maximum gain of 28 V/V at negative VIN. To the
best of our knowledge, the high voltage gain in the first quadrant is among the highest
voltage gains achieved with
a
solutionꢀprocessed, singleꢀcomponent ambipolar
High voltage gain from sharp voltage inversion demonstrates the
reliability of the current inverter devices, and can be attributed to the highly balanced
[68–70]
semiconductor.
[71]
ambipolar chargeꢀtransport behavior of 2ODꢀTTIFDK (vide supra). On the other hand,
inverters based on ambientꢀstable semiconductor, 2ODꢀTTIFDM, were characterized in
ambient, and they exhibited good voltage gains of 30 V/V (Figure S14). Our results show that
the current solutionꢀprocessable ambipolar small molecules are ideal semiconductors not only
for ambipolar OFETs but also for singleꢀcomponent CMOSꢀlike organic circuits. However,
it’s noteworthy that the observed Zꢀshaped voltage transfer characteristics (VTC) for both
semiconductors are typical for unoptimized ambipolar transistorꢀbased inverters, since both
the pullꢀup (pꢀchannel) and pullꢀdown (nꢀchannel) transistors cannot be completely switched
[64]
off at low and high V ’s. These features will cause limited noise margin and large static
IN
power consumption, and further inverter device optimizations such as semiconductor film
doping or incorporating interlayers are needed to further match charge carrier mobilities (ꢁ)
[65]
T
and threshold voltages (V ) in the pꢀ and nꢀchannel regions.
2
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Figure 6. Voltage transfer characteristics (VTC) of a complementaryꢀlike inverter fabricated
by two identical 2ODꢀTTIFDKꢀbased ambipolar transistors in first (A) and third (B)
quadrants with supplied voltages of +100 V and ꢀ100 V. Inset shows the circuit diagram and
the plots of gains (ꢀdVOUT/dVIN) are given in red.
ThinꢀFilm Microstructure and Morphology
The microstructural and morphological characteristics of the solutionꢀsheared thinꢀ
films of the current semiconductors were investigated by outꢀofꢀplane θꢀ2θ Xꢀray diffraction
(XRD) technique and atomic force microscopy (AFM). As shown in Figure 7, both
semiconductor thinꢀfilms showed crystalline patterns with distinct (00l) diffraction peaks up
to the third order along with broad (0k0) peaks. The primary diffraction peaks (001) were
observed at 2θ = 3.05° (dꢀspacing = 29.0 Å) for 2ODꢀTTIFDK and at 2θ = 2.37° (dꢀspacing
=
37.2 Å) for 2ODꢀTTIFDM, indicative of the formation of wellꢀorganized lamellar
microstructure having molecular/alkylꢀchain edgeꢀon orientation relative to the dielectric
surface. Further weak diffraction peaks were observed at 2θ = 4.24° (001’) and 5.34° (001’’)
for 2ODꢀTTIFDKꢀbased thinꢀfilm, which do not belong to the primary diffraction family,
indicating the existence of additional minor crystalline phases.
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Figure 7. θꢀ2θ Xꢀray diffraction (XRD) scans and AFM topographic images (inset) of 2ODꢀ
TTIFDK (A) and 2ODꢀTTIFDM (B) thinꢀfilms fabricated by solution shearing method. The
scale bar denotes 1ꢀ2 ꢁm and the arrow shows the shearing direction.
The dꢀspacings for both compounds are shorter than the computed molecular lengths
(~57.0 Å) with fully extended (allꢀtrans conformation) alkyl chains. This indicates that
significant alkyl chain interdigitation and/or molecular tilting from the substrate normal are
likely to occur in thinꢀfilm phases. Considering that the alkyl chains remain the same for both
semiconductors, the larger dꢀspacing observed for 2ODꢀTTIFDM reflects more vertically
oriented molecular πꢀcores relative to the dielectric surface, probably as a result of stronger
intermolecular interactions between DꢀAꢀD πꢀcores (vide supra). 2ODꢀTTIFDM thinꢀfilm
exhibited much stronger (001) diffraction peak intensity (×7 for the same film thickness) with
smaller FWHM (full width at half maximum) (~0.20° vs. ~0.35°) when compared with that of
2ODꢀTTIFDK. This shows that 2ODꢀTTIFDM thinꢀfilms have higher crystallinity with
[72]
larger mean crystallite size.
Broad diffraction peaks observed at 2θ ~ 22˚ (010) were
ascribed to πꢀπ stacking interactions with distances of ~4.1 Å between molecular backbones,
which is somewhat larger than the typical πꢀπ stackings observed for oligoꢀ/polythiophenes
[48,73,74]
(3.4ꢀ3.9 Å).
This points to relatively weaker πꢀπ stacking interactions for the current
semiconductors and it can be attributed to the presence of sterically bulky, swallowꢀtailed 2ꢀ
2
4

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DOI: 10.1039/C6TC05079D
octyldodecyl substituents, which may preclude the πꢀcores from approaching each other
effectively. AFM characterizations provide more information on the morphological aspects of
the current films, and, therefore, allow us to better understand the correlations with the
electrical performances. Both films reveal highly homogeneous morphologies with surface
roughness below 4ꢀ5 nm for 10×10 ꢁm scan areas. Although 2ODꢀTTIFDK films exhibited
small highlyꢀinterconnected isotropic spherulites (~100ꢀ200 nm diameter sizes), 2ODꢀ
TTIFDM films revealed twoꢀdimensional micronꢀsize (~1ꢀ2 ꢁm) plateꢀlike grains of terraced
islands with stepꢀheights of ~3.9 nm, which corresponds to the dꢀspacing measured from the
XRD characterization (~3.7 nm). The observed highly crystalline and edgeꢀon πꢀoriented
microstructure, and layerꢀbyꢀlayer grown 2ꢀD morphology of 2ODꢀTTIFDM film favors
efficient
chargeꢀtransport
in
OFETs.
Furthermore,
when
these
favorable
microstructural/morphological features are compared with those of 2ODꢀTTIFDKꢀbased
thinꢀfilm, it seems to significantly enhance the fieldꢀeffect electron mobility (×6) with
minimal influence on the fieldꢀeffect hole mobility. This shows that thinꢀfilm morphological
and microstructural characteristics may have totally different effects on electron and hole
chargeꢀtransport characteristics of molecular solids.
Conclusions
Two new solutionꢀprocessable ambipolar small molecules, 2ODꢀTTIFDK and 2ODꢀ
TTIFDM, have been designed, synthesized and fully characterized. The new semiconductors
having strong DꢀAꢀD πꢀarchitectures employ ladderꢀtype πꢀacceptors of IFDK/IFDM and αꢀ
substituted bithiophene πꢀdonors, which results in ultralow band gaps of 1.21ꢀ1.65 eV. The
HOMO/LUMO frontier orbital energies of the new molecules (ꢀ5.47/ꢀ3.61 eV for 2ODꢀ
TTIFDK and ꢀ5.49/ꢀ4.23eV for 2ODꢀTTIFDM) are found to be highly favorable for
balanced ambipolar chargeꢀtransport and ambientꢀstability for 2ODꢀTTIFDM. The resulting
bottomꢀgate/topꢀcontact OFET devices fabricated by solutionꢀshearing 2ODꢀTTIFDM show
2
5

Journal of Materials Chemistry C
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DOI: 10.1039/C6TC05079D
clear ambipolar characteristics in ambient with reasonably balanced carrier mobilites of 0.13
2
2
3
4
cm /V·s for electron and 0.01 cm /V·s for hole with Ion/Ioff ratios of ~10 ꢀ10 . 2ODꢀ
TTIFDKꢀbased OFETs exhibit ambipolarity under vacuum with highly balanced (ꢁ /ꢁ ~ 2)
electron and hole mobilities of 0.02 cm /V·s and 0.01 cm /V·s, respectively with I /I ratios
e
h
2
2
on off
5
6
of ~10 ꢀ10 . Most importantly, complementaryꢀlike inverters were demonstrated based on the
new ambipolar small molecules, which showed sharp signal switching with very high gains
of up to 80. When the charge transport stability of the current semiconductors are compared,
our observations show that the ambientꢀstability of chargeꢀtransport in ambipolar
semiconductors is governed mainly by the stabilization of conducting electrons in LUMO
energy level. To the best of our knowledge, the current DꢀAꢀD πꢀstructures are among the
best performing ambipolar small molecules for OFET and complementaryꢀlike inverter
devices. Our findings clearly provide an efficient approach for the preparation of ultralow
bandꢀgap small molecules as ambientꢀstable and solutionꢀprocessable ambipolar
semiconductors for various organic optoelectronic technologies including CMOSꢀlike
integrated circuits.
Acknowledgement
H.U. and R.O. acknowledges support from AGUꢀBAP (FYLꢀ2016ꢀ65). H. U.
acknowledges support from Turkish Academy of Sciences, The Young Scientists Award
Program (TUBAꢀGEBĐP 2015) and The Science Academy, Young Scientist Award Program
(BAGEP 2014). C. K. acknowledges support from Basic Science Research Program through
the National Research Foundation of Korea (NRF) (NRFꢀ2014R1A1A1A05002158).
Supporting Information
SI includes synthetic procedures and characterizations for compounds 2ꢀoctyldodecyl
bromide and 2,8ꢀdibromoindeno[1,2ꢀb]fluoreneꢀ6,12ꢀdione (IFDKꢀBr2) shown in Scheme
1
13
S1, Figures S1ꢀS8 ( H/ C NMR, MALDIꢀTOF, and ATRꢀFTIR spectra, TGA/DSC curves of
ODꢀTTIFDK and 2ODꢀTTIFDM), Figures S9 (Chemical structures and optimized
2
geometries of some reference compounds), and Figures S10ꢀS14 (OFET and Inverter
electrical curves).
2
6

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DOI: 10.1039/C6TC05079D
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