Boosting the Charge Transport Property of Indeno[1,2-b]fluorene-6,12-dione though Incorporation of Sulfur- or Nitrogen-Linked Side Chains

Article abstract of DOI:10.1002/adfm.201702318

Alkyl chains are basic units in the design of organic semiconductors for purposes of enhancing solubility, tuning electronic energy levels, and tailoring molecular packing. This work demonstrates that the carrier mobilities of indeno[1,2-b]fluorene-6,12-d

Full text of DOI:10.1002/adfm.201702318

FULL PAPER
Organic Semiconductors
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Boosting the Charge Transport Property of
Indeno[1,2-b]fluorene-6,12-dione though Incorporation
of Sulfur- or Nitrogen-Linked Side Chains
Zhi-Ping Fan, Xiang-Yang Li, Xiao-E. Luo, Xian Fei, Bing Sun, Li-Chuan Chen, Zi-Fa Shi,
Chun-Lin Sun, Xiangfeng Shao, and Hao-Li Zhang*
of large area and low-cost fabrication, and
good flexibility.[3] To design organic semi-
conductors for high-performance OFETs,
fine adjustment to their band gaps and
energy level positions are very important.
A large number of aromatic frameworks
with different band structures have been
developed to meet different carrier injec-
tion requirements.[4] In many cases, the
selected aromatic frameworks need to be
modified by electron-donating or -with-
drawing substitutions in order to fine tune
their energy levels to achieve ideal prop-
erties. For examples, Bao and co-workers
have reported that chlorination is an effi-
cient method to obtain high-performance
organic semiconductors.[5] Dou et al. have
reported that fluorination can effectively
decrease energy level of the lowest unoc-
cupied molecular orbital (LUMO) level to
achieve high electron mobility.[6] Besides,
Bunz and co-workers,[7] Miao and co-
workers,[8] and our group[9] have studied
how to modulate the charge carrier mobil-
ities of through fusing heteroatom, like
Alkyl chains are basic units in the design of organic semiconductors for
purposes of enhancing solubility, tuning electronic energy levels, and tai-
loring molecular packing. This work demonstrates that the carrier mobili-
ties of indeno[1,2-b]fluorene-6,12-dione (IFD)-based semiconductors can
be dramatically enhanced by the incorporation of sulfur- or nitrogen-linked
side chains. Three IFD derivatives possessing butyl, butylthio, and dibutyl-
amino substituents are synthesized, and their organic field-effect transistors
(OFET) are fabricated and characterized. The IFD possessing butyl substitu-
ents exhibits a very poor charge transport property with mobility lower than
10−7 cm2 V−1 s−1. In contrast, the hole mobility is dramatically increased to
1.03 cm2 V−1 s−1 by replacing the butyl units with dibutylamino groups (DBA-
IFD), while the butylthio-modified IFD (BT-IFD) derivative exhibits a high and
balanced ambipolar charge transport property with the maximum hole and
electron mobilities up to 0.71 and 0.65 cm2 V−1 s−1, respectively. Moreover,
the complementary metal–oxide–semiconductor-like inverters incorporated
with the ambipolar OFETs shows sharp inversions with a maximum gain
value up to 173. This work reveals that modification of the aromatic core with
heteroatom-linked side chains, such as alkylthio or dialkylamino, can be an
efficient strategy for the design of high-performance organic semiconductors.
nitrogen and sulfur, into pentacene frameworks.
1. Introduction
To achieve high-performance solution-processable organic
semiconductors, various alkyl chains are widely utilized as
side groups to the aromatic cores. Through varying the struc-
tures of alkyl chains (such as the length, shape, and branching
structures), a variety of physical properties related directly to
the device performance can be tailored, including solubility,
molecular packing in solid, and thin-film morphology.[10] For
instance, Pei and co-workers reported that the branching point
of alkyl chain has a significant effect on the charge transport
properties of isoindigo-based conjugated polymers.[10c] With
variation of side-chain branching position, Zhang et al. further
utilized the branched alkyl chains to boost the mobility of NDI-
DTYM2-based semiconductors.[11] We have reported that the
variation of alkyl side-chain length can tune the carrier polarity
of dithiophene-4,9-dione-based ambipolar semiconductors.[12]
Although playing a crucial role in the rational design of high-
performance organic semiconductors, alkyl chain is considered
to be an inefficient substitution for energy level adjustment,
due to its limited electron-donating ability. However, recent
researches in organic solar cell materials have shed new light
on the role of alkyl chain substitutions. Li and co-workers and
Organic filed-effect transistors (OFETs)[1] are key elements to
the organic electronic devices and have received significant
attention in recent years due to potential applications in various
electronic devices such as display and electronic skin,[2] by virtue
Z.-P. Fan, X.-Y. Li, X.-E. Luo, X. Fei, B. Sun, L.-C. Chen, Prof. Z.-F. Shi,
Dr. C.-L. Sun, Prof. X. Shao, Prof. H.-L. Zhang
State Key Laboratory of Applied Organic Chemistry (SKLAOC)
Key Laboratory of Special Function Materials
and Structure Design (MOE)
College of Chemistry and Chemical Engineering
Lanzhou University
Lanzhou 730000, P. R. China
E-mail: haoli.zhang@lzu.edu.cn
Prof. H.-L. Zhang
Tianjin Key Laboratory of Molecular Optoelectronic Sciences, and
Collaborative Innovation Center of Chemical Science and Engineering
Tianjin 300072, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201702318.
DOI: 10.1002/adfm.201702318
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ambipolar carrier transport properties, which have been suc-
cessfully used in the fabrication of high-performance comple-
mentary metal–oxide–semiconductor (CMOS)-like inverters.
2. Result and Discussion
2.1. Design and Synthesis
The structures of the three IFD derivatives are shown in
Scheme 1. B-IFD contains two butyl chains on the 2- and
8-positions of IFD framework and was designed as a refer-
ence. BT-IFD and DBA-IFD possess butylthio and dibutyl-
amino units, respectively, at the 2- and 8-positions of the IFD
framework. The synthetic route of the three IFD derivatives is
depicted in Scheme 2, and the detailed synthetic procedures are
described in Scheme S1 of the Supporting Information. Briefly,
compound 1 was treated with n-BuLi and trimethylborate to
give the intermediate boronic acid, which was used for the coup-
ling reaction with compound 2 to produce compound 3. Com-
pound 3 was subsequently hydrolyzed to the corresponding
dicarboxylic acid, and then the dicarboxylic acid was reacted in
polyphosphoric acid to obtain the desired IFD derivative. These
newly synthesized IFD molecules were carefully characterized
by NMR (1H, 13C, and distortionless enhancement by polariza-
tion transfer (DEPT); Figures S1–S6, Supporting Information),
mass spectra, and melting point measurement, as shown in the
Supporting Information.
Scheme 1. Chemical structures of the IFD derivatives.
Hou and co-workers have reported the use of alkylthio side
chains to improve the power conversion efficiency of polymer-
based photovoltaics.[13] In their works, the alkylthio side chains
play two roles simultaneously, i.e., tailoring the thin-film mor-
phology and modulating the frontier orbital energies. However,
whether such strategy is applicable to design new semicon-
ductors for OFETs is unknown. To address this issue, we have
designed indeno[1,2-b]fluorene-6,12-dione (IFD)-based mole-
cules modified by different side chains, and investigated the
impact of different heteroatom linkage in the side chains on
their OFET performance.
IFD is selected as the core mainly because of its simple struc-
ture and easy synthesis. Unmodified IFD is commercially avail-
able, making it an ideal candidate as starting materials for low-cost
organic electronics. However, unmodified IFD and IFD modified
by simple alkyl substituents show barely observable filed-effect
phenomenon in OFETs, which may be attributed to their low-
lying highest occupied molecular orbital (HOMO) energy levels
and relatively large band gaps.[14] Yamashita and co-workers and
Park et al. have designed fluorinated IFD derivatives to decrease
the LUMO energy level and obtained appreciable electron mobility
up to 0.17 cm2 V−1 s−1.[14b,15] The indeno[2,1-b]fluorene derivative,
a regioisomer of the IFD, has also been found efficient in elec-
tron transport.[16] Besides, the IFD unit was utilized to construct
donor–acceptor (D–A) type of small molecules and polymers, and
achieved ambipolar charge transport with maximum hole and
electron mobilities of 0.02 and 0.12 cm2 V−1 s−1,
2.2. Optical and Electrochemical Properties
The UV–vis absorption spectra of the three IFD derivatives
were measured in both solutions and thin films (Figure 1a,b).
In solution, B-IFD shows a strong peak at 294 nm and a broad
absorption band at 511 nm. In comparison, the unmodified
IFD and IFD derivative with dodecyl chains substituted on
the central phenyl ring were reported to show a broad absorp-
tion band at about 484 nm.[17a,18] However, the broad absorp-
tion band was not observed for the difluoro-, dichloro-, or
respectively.[17]
To study the effects of heteroatom linkage,
we have designed and synthesized three
IFD molecules containing butyl, butylthio,
and dibutylamino side chains (B-IFD, BT-
IFT, and DBA-IFD) as shown in Scheme 1.
The lengths of the carbon chains are kept
identical in order to unveil the effects of the
heteroatom linkages. Thin-film transistors
of the three molecules were fabricated and
their electron and hole transport properties
were carefully compared. We found that
the butylthio and dibutylamino side chains
can significantly change the energy levels
and molecular packing, resulting in seven
orders of enhancements to the hole and
electron mobilities. The butylthio-modified
IFD molecule exhibited high and balanced
Scheme 2. Synthetic route of the IFD derivatives.
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Figure 1. The optical and electrochemical characteristics of the IFD derivatives. a) Solution absorption spectra, b) film absorption spectra, c) cyclic
voltammograms (CVs), and d) the calculated molecular orbitals and the diagram of energy levels for the IFD derivatives.
dibromo-substituted IFD derivatives.[14b,17a] For BT-IFD and
DBA-IFD, the broad absorption bands are redshifted to 554 and
721 nm, respectively, along with increased absorption strength,
especially for DBA-IFD. The UV–vis spectra indicate that sulfur
and nitrogen linkages can significantly lower the band gap
of the IFD framework. In thin film, all the three compounds
show enhanced absorption strength in the visible region, indi-
cating strong intermolecular π–π interaction in the thin films.
The electrochemical properties of the three compounds were
investigated by cyclic voltammetry (Figure 1c), which enables
a rough estimation of the energy levels of the HOMO and the
LUMO. The molecular orbitals for the three IFD derivatives
were calculated by the density functional theory (DFT) using
Gaussian 09 package with basis set of B3LYP 6−311G++(2d,2p)
(Figure 1d). Table 1 shows that the theoretical energy levels are
in good agreement with that obtained from the experiment.
B-IFD shows a LUMO energy of −3.6 eV and a HOMO energy
of −6.0 eV. When the butyl chains are replaced by the butylthio
groups, the LUMO energy of BT-IFD is lowered to −3.7 eV and
the HOMO energy is raised to 5.6 eV. The effect of the sulfur
linkage can be understood as that the sulfur group is a weak
electron-donating unit. Meanwhile, the sulfur atom has some
π-acceptor capability due to the formation of pπ(C)–dπ(S) orbital
overlap.[19] Therefore, the sulfur linkage lowers the LUMO
and raises the HOMO energy levels of BT-IFD. For DBA-IFD,
the introduction of dibutylamino groups results in a rise of
the HOMO energies to −5.0 eV, along with a small rise of the
LUMO energy to about −3.4 eV (Table 1). The dramatic rise
to the HOMO level can be attributed to the strong electron-
donating effect of the nitrogen atoms.
Neutral bond orbital (NBO) analysis is a useful tool to ana-
lyze the formation of molecular orbitals from bond orbitals.
As shown above, the sulfur and nitrogen linkages can dramat-
ically change the molecular orbitals of the IFD derivatives. To
gain further insights into effects of sulfur and nitrogen link-
ages on the frontier molecular orbitals’ (FMOs) energy levels,
the NBOs and their contributions to FMOs were analyzed by
the DFT using Gaussian 09 package, as shown in Figure S7
and Tables S1–S2 in the Supporting Information. Tables S1
and S2 (Supporting Information) break down the FMOs of the
three IFD derivatives into contributions of bonding (BD)-type,
antibonding (BD*)-type, and nonbonding (LP)-type NBOs,
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Table 1. Optical, electrochemical, and theoretical properties of the IFD derivatives.
a)
b)
c)
c)
d)
Compound
B-IFD
Abs.Sol.
Abs.Film
HOMOCal./CV
LUMOCal./CV
Eg Cal./CV/Abs.
294 337 511
313 349 554
326 394 721
271 341 517
298 364 568
564 714 776
2.6/2.5/2.2
2.2/2.1/1.9
1.9/1.6/1.4
−6.0/−6.0
−5.7/−5.6
−4.8/−5.0
−3.4/−3.5
−3.5/−3.7
−2.9/−3.4
BT-IFD
DBA-IFD
a)Solution absorption measured in chloroform with concentration of 1 × 10−5 m; b)Film absorption measured with the sample prepared by using the physical vapor-
deposited method; c)The HOMO and LUMO energy levels were calculated by the density functional theory (DFT) using Gaussian 09 package with basis set of B3LYP
6−311G++(2d,2p), while that of the experimental values were determined by CV measured in 1 × 10−3 m chloroform solution. Potential values are reported in voltage versus
saturated calomel electrode (SCE) using the Fc+/Fc couple as an internal standard. HOMO and LUMO energy levels were approximated using SCE = −4.68 eV versus
reduction
vacuum[27] and the half-wave potential (E1/2) values, where EHOMO = −4.68 − E1/2oxidation and ELUMO = −4.68 − E1/2
;
d)Band gap for calculation or CV was calculated with
Eg = HOMO − LUMO, while the absorption band gap was estimated with Eg = 1240/λonset
.
showing how contributions of each type are generally found
mixed into FMOs, whether occupied or virtual. The NBO
analysis reveals that the HOMO of B-IFD mainly comes from
the CC π-bonds, while the LUMO is composed primarily of
the carbonyl (CO) groups (38.6%) and the centered benzene
ring (20.45%). For BT-IFD, the presence of the sulfur atoms
changed the contributions of NBOs to HOMO. As shown in
Table S1 (Supporting Information), the LP (2) S21 and LP (2)
S22 of BT-IFD contribute about 35% of HOMO, which occur
at much higher energy than the CC π-bonds and therefore ele-
vate the HOMO energy level. Meanwhile, the linkage of sulfur
atoms changes a little on the distributions of LUMO, but
lowers the energy levels of the CO π*-orbitals with main con-
tributions, as well as the other NBOs with substantial contri-
butions (Table S2, Supporting Information). For the DBA-IFD
molecule, the LPs of the nitrogen atom compose about 21%
of the HOMO orbital and obviously raise the energy levels
of the CC π-bonds (Table S1, Supporting Information). Con-
sequently, the effect of the nitrogen linkage in DBA-IFD on
the promotion of HOMO energy level is stronger than that of
the sulfur atom in BT-IFD. However, the linkage of nitrogen
atoms raises the energy levels of the NBOs that have substan-
tial contributions to LUMO of DBA-IFD, which explains the
higher LUMO energy level of DBA-IFD than that of B-IFD
and BT-IFD.
The energy levels of organic semiconductors play a key role
in the charge carrier injection at the electrode/organic semicon-
ductor interface and greatly influence device performance.[9c,20]
To the three IFD derivatives, their relative hole and electron
injection barriers can be estimated from Figure 1d. It is known
that the work function of clean gold electrode is −5.1 eV. How-
ever, most literatures reported that the work functions of gold
electrode in OFET devices were found in the range of −4.3 to
−4.7 eV, due to either contamination or interface dipole.[21]
Therefore, it is expected that either hole and electron injec-
tion to B-IFD would be difficult due to large injection barriers
(around 1.2 eV), which explains the poor charge transport
properties of most IFD analogs in literature reports.[14] As for
BT-IFD, the energy barriers for either hole or electron injection
are estimated to be around 1 eV, which is likely to show ambi-
polar charge transport according to our previous research. Fur-
thermore, it can be seen that the HOMO of DBA-IFD matches
well with the gold work function, implying small hole injection
barrier.
2.3. Single Crystal and Thin-Film Structure Analyses
Single crystals of the three IFD derivatives were obtained in solu-
tions, and their crystal structures were determined by single-
crystal X-ray analysis, which are shown in Figure 2. It is found
that the different side chains induce significantly different molec-
ular packing in the crystals. B-IFD formed needle-like crystal,
which exhibits one dimensional (1D) column stacking with a π–π
distance of 3.447 Å between the IFD planes (Figure 2a). When
the butylthio substituents are used as side chains, the crystal
structure of BT-IFD is changed to two dimensional (2D) brick-
wall stacking with a shortest π–π distance of 3.217 Å between
the IFD planes (Figure 2b). In the crystal of DBA-IFD, the mole-
cules adopt 2D crossed stacking with a π–π distance of 3.387 Å
between the IFD planes (Figure 2c). As discussed above, the
sulfur and nitrogen linkages in the BT-IFD and DBA-IFD mole-
cules exhibit electron-donating effect, hence increase the local
polarity. Therefore, the BT-IFD and DBA-IFD molecules prefer
2D molecular packing motif, which can be stabilized by the
dipolar interaction between the adjacent molecules.[22]
The 2D grazing-incidence X-ray diffraction (2D-GIXD) anal-
ysis was utilized to investigate the crystallinity and packing
motif of the physical vapor-deposited (PVD) thin film.[23]
Polycrystalline films were fabricated in vacuum tube furnace
at 5 × 10−4 Pa with an evaporate temperature below 230 °C.
Thermal gravimetric analysis confirmed that the IFD deriva-
tives are stable at this temperature (Figure S8, Supporting
Information). Figure 2d–f shows the 2D-GIXD patterns of
the thin films; the corresponding out-of-plane (qz-axis) and
in-plane (qxy-axis) line-cut profiles are shown in Figures S9–S11
in the Supporting Information. The 2D-GIXD patterns of
the three IFD derivatives exhibit clear Bragg reflection points
along the qz- and qxy-directions, revealing the good crystallinity
of the thin films (Figure 2d–f). Generally, the reflections of
(00l) and {h,k} planes are contained along the qz and qxy-axes,
respectively. Therefore, the d-spacing can be calculated from
the reflection of (001) plane, while the unit cell parameters can
be determined by the {h,k} plane reflections.[23b] B-IFD clearly
shows two Bragg reflections along the qz-direction at 4.411 and
13.208 nm−1 (Figure 2d; Figure S9, Supporting Information),
corresponding to the reflections of the (001) and (003) planes.
The d-spacing was calculated from the reflection of (001) plane
to give the value of 14.24 Å. Along the qxy-direction, four col-
umns of reflections at 8.13, 13.26, 14.01, and 16.29 nm−1 were
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Figure 2. The single-crystal structures for a) B-IFD, b) DBA-IFD, and c) BT-IFD molecules; and 2D-GIXD patterns of vacuum deposited d) B-IFD,
e) BT-IFD, and f) DBA-IFD films.
observed, which were identified as {0,1}, {1,0}, {1,1}, and {0,2}
planes, respectively. Then from the reflections along the qxy-
direction, the crystal parameters of the B-IFD thin film were
deduced to give a unit cell with a = 4.85 Å, b = 7.91 Å, and
γ = 102.22°. BT-IFD has only (001) reflection along the qz-direc-
tion with a value of 3.13 nm−1, giving the d-spacing of 20.06
(Figure 2e; Figure S10, Supporting Information). Along the
qxy-direction, the reflections of {0,1}, {1,0}, and {1,1} planes
were observed at 11.10, 12.06, and 18.52 nm−1, from which the
crystal parameters were calculated with a = 5.23 Å, b = 5.60 Å,
and γ = 94.64°. As for DBA-IFD, the d-spacing was calculated
from the (001) reflection with a value of 17.13 Å (Figure 2f;
Figure S11, Supporting Information). Along the qxy-direction,
the reflections of {1,0}, {1,1}, {0,2}, {2,0}, and {2,1} planes were
found at 7.28, 9.52, 12.28, 14.56, and 15.79 nm−1. Therefore, the
crystal parameters were determined with the values of a = 8.63
Å, b = 10.23 Å, and γ = 90.04°. The calculated crystal parameters
of the films for the three IFD derivatives are summarized in
Table 2, which reveals that the molecular packing motif of the
three IFD derivatives in the thin film is same as that in their
single crystal structures.
Whether charge transport occurs through a band-like or
hopping mechanism, the transfer integral (t) plays a vital role
in determining the carrier mobility.[24] As shown in Table 2,
B-IFD shows a large transfer integral of −76.7 meV along the
π–π interaction direction, and therefore the poor performance
is mainly attributed to large injection barrier. For BT-IFD, the
2D brick-wall structure allows the strong 2D electron coupling
for both hole and electron transport. The transfer integrals for
hole transport are −48.4 and −21 meV, while that of electron
transport are 52.8 and 34.4 meV. The similar injection bar-
rier and transfer integrals for both hole and electron transport
indicate that the BT-IFD material may exhibit balanced ambi-
polar performance, according to our previous investigations.[9]
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Table 2. Crystallographic packing and transfer integral data for the IFD derivatives.
Compound
B-IFD
Crystallographic packing data
Transfer integral
t2[meV]
A [Å]
B [Å]
7.90
d-spacing [Å]
t1 [meV]
−76.7
−23.1
−48.7
52.8
t3[meV]
0.3
γ [°]
102.28
102.22
94.79
94.64
90
Crystal
Thin film
Crystal
4.84
4.85
5.23
5.23
8.60
8.63
14.26
14.24
20.08
20.06
17.11
17.13
Hole
Electron
Hole
0.1
7.91
−2.3
−21.0
34.3
−8.3
−1.3
21.2
0.1
BT-IFD
5.59
Thin film
Crystal
5.60
Electron
Hole
DBA-IFD
10.26
10.23
−4.4
−4.3
−48.7
Thin film
90.04
Electron
11.7
−48.9
DBA-IFD exhibits that 2D crossed stacking with the transfer
integrals for hole transport along the t1 and t2 directions is
about −4.4 and −4.3 meV, respectively (Table 2). According to
Bao and co-workers, the essentially equivalent transfer integral
in two directions of a 2D electron coupling structure may facili-
tate charge transport.[25] The HOMO energy level of DBA-IFD
exhibited similar transfer integral along the t1 and t2 directions,
suggesting good hole transport property.
were fabricated and examined in a glove box. The representa-
tive characteristics for the IFD derivatives are shown in Figure 4
and Figure S12 (Supporting Information), and OFET properties
obtained at different substrate temperatures are summarized
in Table 3. As shown in Figure S12 (Supporting Information),
the source–drain currents (ISD) of B-IFD devices are at only
detectable current level (<10−12 A), which can be considered
as no field effect (mobility lower than 10−7 cm2 V−1 s−1) and is
consistent with literature reports.[14] As expected, the butylthio
and dibutylamino substituents significantly increase the charge
transport mobility. BT-IFD exhibits high and balanced ambi-
polar charge transport with maximum hole and electron mobili-
ties of 0.71 and 0.65 cm2 V−1 s−1, as well as the balanced average
2.4. Film Morphology Analysis
The film morphology is one of the important factors that
determine charge transport property of
thin-film OFETs. The morphology of the
films was optimized by altering substrate
temperature, and then was investigated by
atomic force microscopy (AFM) technology.
Figure 3 shows the AFM images of the poly-
crystalline films for the three molecules
fabricated under substrate temperature of
room temperature (RT), 60, and 100 °C,
respectively. The B-IFD films show a min-
imum roughness of 4.655 nm at RT, while
the films have increased roughness of 10.70
and 15.67 nm at 60 and 100 °C, respectively
(Figure 3a–c). For the BT-IFD films, layer-by-
layer terrace-like grains were formed with
roughness of 3.635, 3.133, and 3.325 nm at
RT, 60, and 100 °C, respectively (Figure 3d–f).
As shown in Figure 3g–i, DBA-IFD formed
terrace-like grains at 60 °C with a roughness
of 3.353 nm, which are smaller than that of
the films deposited at RT and 100 °C (8.37
and 5.605 nm at RT and 100 °C, respectively).
The smaller roughness of DBA-IFD and
BT-IFD than B-IFD suggests that the incor-
poration of sulfur- or nitrogen-linked side
chains may help to increase carrier mobility.
2.5. Thin-Film OFET Characteristics
To evaluate the charge transport characteris-
tics of the films for the three IFD derivatives,
the bottom-gate top-contact OFETs devices
Figure 3. Atomic force microscopy images of the films for the IFD derivatives: for B-IFD at
a) RT, b) 60 °C, c) and 100 °C, respectively; for BT-IFD at d) RT, e) 60 °C, and f) 100 °C, respec-
tively; for DBA-IFD at g) RT, h) 60 °C, and i) 100 °C, respectively.
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Figure 4. Transfer curves and output curves of PVD OFETs bases on IFD derivatives, respectively. a,b) for DBA-IFD; c–f) for BT-IFD.
threshold voltage of −48 V (p-type) and 41 V (n-type), respec-
tively. Moreover, DBA-IFD exhibits p-type charge transport with
a maximum hole mobility of 1.03 cm2 V−1 s−1 (Figure 4a,b).
The results of OFET performance of the three IFD deriva-
tives illustrate that both molecular packing and energy level
are essential for charge carrier transport. The extremely low
carrier mobilities of the thin-film OFET based on B-IFD are
attributed to large injection barrier and 1D molecular packing.
In contrast, BT-IFD has appropriate energy level due to the
sulfur linkage, which allows both electron and hole injection
from the gold Fermi level. In addition, the 2D stacking provides
favorable charge transport channels for both types of carriers.
As a result, BT-IFD exhibits high and balanced ambipolar trans-
port behavior. DBA-IFD possesses 2D stacking structures that
Table 3. OFET characteristics of the IFD derivatives.
Compound
VT [V] Min (Avg)
–
Ion/Ioff Max (Avg)
VT [V] Min (Avg)
Ion/Ioff Max (Avg)
T
sub [°C]
RT
μ
h [cm2 V−1 s−1] Max (Avg)
N.O.
μe [cm2 V−1 s−1] Max (Avg)
B-IFDa)
–
N.O.
N.O.
–
–
60
N.O.
–
–
–
–
100
RT
N.O.
–
–
N.O.
–
62(66)
41(53)
60(64)
–
–
BT-IFD
0.14 (0.09)
0.71 (0.38)
0.24(0.19)
0.19 (0.13)
1.03 (0.41)
0.22 (0.19)
106(105)
105(104)
104(103)
106(105)
105(104)
106(105)
0.05(0.03)
0.65 (0.32)
0.14 (0.12)
N.O.
104(103)
−36 (−31)
−48 (−32)
−32 (−22)
−19 (−12)
−27 (−11)
−14 (−6.7)
60
104(103)
100
RT
105(104)
DBA-IFD
–
–
–
60
N.O.
–
100
N.O.
–
a)The OFETs based on B-IFD exhibited only detectable filed-effect mobility (<10−7 cm2 V−1 s−1), which were considered with no mobility. The term N.O. stands for “not
observed”.
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are favorable for charge transport. The nitrogen linkages signif-
icantly raise the HOMO level to allow efficient hole injection,
but the LUMO level remains too high for electron injection.
Therefore, the transistors from DBA-IFD exhibits high hole
mobility but no electron transport property.
equivalent n and p-type transistors, the ideal inversion voltage
(VINS) is located at VDD/2.[26] The gain, which is defined as the
slope of the voltage transfer characteristics curve, is an impor-
tant parameter to evaluate the performance of an inverter. For
VDD = −100 V, the maximum gain is up to 80 with a VINS of
−49 V (Figure 5a); while under positive bias with VDD = 100 V,
the highest gain is up to 173 with a VINS of 40 V (Figure 5b).
These results also confirm that BT-IFD is indeed a balanced
ambipolar organic semiconductor.
2.6. Inverter Property
CMOS-like inverter is an important application for ambipolar
OFETs. The ambipolar OFETs can transport both hole and
electron carriers so that one active layer is enough to construct
inverters, which avoid patterning n- and p-type materials into
specific areas. The inverter circuits were constructed by the
BT-IFD-based ambipolar OFETs, which possess excellent ambi-
polar charge transport property and balanced threshold voltage.
As shown in Figure 5, each inverter circuit is composed of
two adjacent OFETs with a common gate as the input voltage
(VIN) and a common drain as the output voltage (VOUT). In
Figure 5a, a typical input/output characteristic curve is shown
for an inverter based on two adjacent transistors with channel
length L = 150 mm and channel width W = 3000 mm. In con-
trast to the conventional inverters based on unipolar materials,
the inverters consisting of ambipolar transistors can work in
both negative and positive supply biases (VDD). The ambipolar
OFETs based on BT-IFD exhibited an averaged threshold voltage
of −48 V for the p-channel and 41 V for the n-channel. A VDD of
100 V or −100 V was selected to ensure that both the n-channel
and p-channel OFETs were operated in the saturation region
(Figure 5a,b). For an ideal CMOS-like inverter consisting of
3. Conclusion
We have demonstrated that by replacing the butyl chains with
butylthio or dibutylamino substituents, the carrier mobili-
ties of a classic organic semiconductor IFD can be boosted for
several orders of magnitudes. The sulfur and nitrogen link-
ages have significant impact on the electronic energy levels of
the IFD core, hence, could optimize the charge carrier injec-
tion. The OFETs of the butyl group terminated B-IFD exhib-
ited a very poor charge transport property with a mobility lower
than 10−7 cm2 V−1 s−1. Interestingly, the butylthio-modified
BT-IFD derivative exhibited a high and balanced ambipolar
charge transport property with maximum hole and electron
mobilities up to 0.71 and 0.65 cm2 V−1 s−1, respectively. High-
performance CMOS-like inverters incorporated with the
BT-IFD-based ambipolar OFETs showed sharp inversions with
a maximum gain value up to 173. The dibutylamino groups
containing DBA-IFD significantly boosted hole mobility up
to 1.03 cm2 V−1 s−1, with no observable electron mobility. This
Figure 5. Devices structure and input/output characteristic for the CMOS-like inverters work at a) negative and b) positive supply biases.
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work reveals that incorporation of sulfur or nitrogen link-
ages between the alkyl chains and the aromatic cores could
be a concise and efficient strategy for the design of high-
performance organic semiconductors.
Acknowledgements
This work was supported by the Ministry of Science and Technology
of China (2017YFA0204903), National Natural Science Foundation
of China (NSFC: 51733004, 51525303, 21233001, 21572086, and
21702085), Fundamental Research Funds for the Central Universities
(223000/862653), and the 111 Project. The authors thank beam line
BL14B1 (Shanghai Synchrotron Radiation Facility) for providing the
beam time.
4. Experimental Section
Synthesis: The detailed synthetic procedures of the IFD derivatives
were described in the Supporting Information.
Characterization: Chemical structures were fully identified by 1H
NMR and 13C NMR (Bruker, AVANCE-III-400; INOVA) and GC-Mass
(LTQ-Obitrap-ETD). UV–vis spectra were recorded on a T6 UV–Vis
spectrometer. HOMO levels of organic materials were obtained from the
cyclic voltammetry measurements. Cyclic voltammetry measurements
Conflict of Interest
The authors declare no conflict of interest.
were performed using
a CHI660C electrochemistry station (CHI,
USA) with one-compartment platinum working electrode, a platinum
wire counterelectrode, and a quasi Ag+/Ag electrode as the reference.
Measurements were performed in a 1 × 10−3 m dichloromethane solution
Keywords
ambipolar materials, inverters, organic field-effect transistors, side-chain
engineering
−
with 0.1 m N(Bu)4+BF4 as the supporting electrolyte, at a scan rate of
100 mV s−1. Potential values are reported in voltage versus saturated
calomel electrode (SCE) using the Fc+/Fc couple as an internal standard.
HOMO and LUMO energy levels were approximated using SCE = −4.68 eV
versus vacuum[27] and the half-wave potential (E1/2) values. Single-crystal
structure analysis was carried out with a Bruker X8 APEX diffractometer
with graphite monochromated Mo Kα radiation. The 2D-GIXD experiments
were performed on various semiconductor films at beam line BL14B1 of
the Shanghai Synchrotron Radiation Facility. Film surface topologies
were obtained by an Agilent 5500 with tapping mode. DFT calculations
were performed using Gaussian 09 package with the basis set of B3LYP,
6−311G ++(2d, 2p). The transfer and output characteristics of all OFET
devices were measured using a Keithley 4200 SCS.
Received: May 1, 2017
Revised: August 21, 2017
Published online:
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