Improved charge transport in fused-ring bridged hemi-isoindigo-based small molecules by incorporating a thiophene unit for solution-processed organic field-effect transistors

Article abstract of DOI:10.1039/c9tc05374c

Two acceptor-donor-acceptor small molecules based on a fused ring indacenodithieno[3,2-b]thiophene (IDTT) as a donor, and hemi-isoindigo units, methyleneoxindole (IDTT-MI) and thienylmethyleneoxindole (IDTT-T-MI) as acceptors were synthesized and characte

Full text of DOI:10.1039/c9tc05374c

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Improved charge transport in fused-ring bridged
hemi-isoindigo-based small molecules by
incorporating a thiophene unit for solution-
processed organic field-effect transistors†
Cite this:DOI: 10.1039/c9tc05374c
ab
ab
ab
c
ab
Guobing Zhang,
Hongbo Lu,
*
Ruikun Chen, Yue Sun, Boseok Kang, Mingxiang Sun,
ab
ab
d
b
Longzhen Qiu,
*
Kilwon Cho
and Yunsheng Ding
Two acceptor–donor–acceptor small molecules based on a fused ring indacenodithieno[3,2-b]-
thiophene (IDTT) as a donor, and hemi-isoindigo units, methyleneoxindole (IDTT-MI) and thienyl-
methyleneoxindole (IDTT-T-MI) as acceptors were synthesized and characterized for application in
solution-processed organic field-effect transistors. The incorporation of a thiophene bridge which
extended the conjugation of the backbone maintained highly co-planar structures, and also endowed
the small molecule with ordered crystalline structures and interconnected morphology. Consequently,
Received 30th September 2019,
Accepted 22nd November 2019
2
À1 À1
IDTT-T-MI-based organic field-effect transistors exhibited a highest hole mobility of 0.80 cm
V
s ,
DOI: 10.1039/c9tc05374c
which was the highest mobility for solution-processed hemi-isoindigo-based organic semiconductor
materials.
rsc.li/materials-c
6–8
OFET semiconductors to date. The ID unit which also possesses
two lactam groups, has received particular attention in high-
1
. Introduction
Organic semiconductor materials which contain polymers, performance semiconductor materials for OFET and organic
oligomers, and small molecules have received tremendous solar cell applications since Reynolds et al. first reported
9
,10
attention in organic field-effect transistors (OFETs) due to their ID-based organic semiconductors.
ID has a symmetrical
potential application in low-cost, light-weight, and flexible structure containing two hemi-isoindigo units, deep lowest
1
–3
electronics. Significant progress in OFETs has been achieved unoccupied molecular orbital (LUMO)/highest occupied mole-
during the past years, with the hole and electron mobilities cular orbital (HOMO) energy levels, large local dipole, excellent
2
À1 À1
both surpassing 10 cm V
synthesis of new constructing units for developing their new high yields,
conjugated polymers and small molecules with excellent device and polymeric ID-based materials with superior photo-
s , which has also promoted the solubility via alkyl-substitution, and scalable synthesis with
1
1,12
all which endow a large quantity of molecular
4
,5
13,14
performances.
Within the constructing block library for electronic properties to develop rapidly.
For example, Chen
small molecules and conjugated polymers, imide- and lactam- reported a conjugated polymer based on ID as the acceptor and
containing dyes such as diketopyrrolopyrrole (DPP), isoindigo (ID), dithiophene as the donor, which exhibited a high field-effect
2
À1 À1
15
naphthalene diimide (NDI), and so on have been the most impor- mobility up to 8 cm
V
s
with excellent air stability.
tant electron-deficient units for constructing high-performance Therefore, ID has proven to be an effective electron-acceptor
unit for the design and synthesis of semiconductors for organic
electronic applications. It is well known that the ID unit is a
twisted conjugated structure, with two hemi-isoindigo units
rotating at a torsion angle of about 201 along the C–C double
bond because of the steric interaction between the hydrogen
a
Special Display and Imaging Technology Innovation Center of Anhui Province,
State Key Laboratory of Advanced Display Technology, Academy of Opto-Electronic
Technology, Hefei University of Technology, Hefei, 230009, P. R. China.
E-mail: gbzhang@hfut.edu.cn, lzhqiu@hfut.edu.cn
b
atoms of the one hemi-isoindigo and the carbonyl oxygens on
School of Chemistry and Chemical Engineering, Hefei University of Technology,
Key Laboratory of Advance Functional Materials and Devices of Anhui Province,
Hefei, 23009, P. R. China
1
6
the other hemi-isoindigo unit. The backbone structures with
excellent coplanarity are considered to be beneficial for the
c
Department of Nano Engineering and SKKU Advanced Institute of Nanotechnology
17
charge transport.
(
SAINT), Sungkyunkwan University (SKKU), Suwon, 16419, Korea
d
Based on the planar backbone requirements mentioned
above, effective strategies were used to optimize the backbone
Department of Chemical Engineering, Pohang University of Science and
Technology, Pohang, 37673, Korea
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tc05374c coplanarity by designing ID derivatives. On the one hand,
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thieno-ID, aza-ID, and halogen-substituted ID units were synthe- large-plane conjugated center, which are in favor of the p-orbital
sized by replacing the outer benzene ring with thiophene, overlap and charge transport mobility. Solution-processed OFET
1
8–20
pyridine, and halogen-based rings,
respectively. The corres- devices based on small molecules as the semiconducting layers,
ponding conjugated semiconductors based on the ID derivatives which were characterized under ambient conditions, exhibited
exhibited high mobility due to the improved backbone hole transport characteristics. IDTT-T-MI had a highest mobility
2
À1 À1
2
À1 À1
coplanarity. For example, Yu et al. reported aza-ID and its of 0.80 cm V
s
and an average mobility of 0.73 cm V
s
5
conjugated polymers by nitrogen substitution. The aza-ID with an Ion/Ioff ratio of over 10 . The results indicate that the hemi-
possessed a highly planar structure and exhibited dihedral isoindigo small molecules based on a fused-ring are promising
angles between a hemi-isoindigo unit of about 01. Conse- semiconductors for solution-processed OFET applications.
2
À1 À1
quently, a high hole mobility up to 7 cm V
s
was obtained
2
1
for OFET devices. On the other hand, dithiophene, thienyl-
ethylene, and their derivatives were used as bridges instead of
a double bond to connect the hemi-isoindigo unit.
corresponding ID analogues not only possessed the extended
conjugated core but also possessed a planar conjugated back- The synthetic route to two small molecules is outlined in
2
. Results and discussion
2
2,23
The
Synthesis and characterization
2
2
bone because of the noncovalent interactions between SÁ Á ÁO.
Scheme 1. 1-Hexyl-indoline-2,3-dione (1) was obtained by
The small molecules and polymers based on these extended attaching the hexyl side chain to the lactam nitrogen atoms
units have been extensively investigated for application in in the commercially available indoline-2,3-dione. The com-
2
4,25
OFETs.
Nevertheless, the device performances were rela- pound was reduced by hydrazine hydrate to form 1-hexyl-
tively low and the highest mobility was no more than 0.5 cm
2
indoline-2-one (2). Then monomer 3 was synthesized using
the Knoevenagel condensation reaction of 1-hexyl-indoline-2-
À1 À1
26
V
s
for both small molecules and polymers to date.
Therefore, further improving the charge transport mobility by one and 5-bromothiophene-2-carbaldehyde in the presence of
designing new semiconductors based on the extension unit and weak basic conditions. Finally, two small molecules were
investigating the relationship between their structures and synthesized via the Knoevenagel condensation reaction and
properties in detail are still significant for OFET applications.
the Stille reaction for IDTT-MI and IDTT-T-MI, respectively.
In this paper, a ladder-type arene, indacenodithieno[3,2-b]- The chemical structures of the small molecules were confirmed
thiophene (IDTT), was introduced between two hemi-isoindigo by nuclear magnetic resonance (NMR) and matrix-assisted
units, and then two new small molecules, named IDTT-MI and laser desorption ionization time-of-flight mass spectrometry
1
IDTT-T-MI (Scheme 1), were synthesized based on IDTT as the (MALDI-TOF-MS). The H spectrum of IDTT-T-MI was measured
13
donor and methyleneoxindole (MI) and thienylmethyleneoxin- under high-temperature conditions (90 1C), and both C spectra
dole (T-MI) as acceptors for solution-processed OFET applications. were not obtained due to the limited solubility. The thermal
IDTT has a heptacyclic ring with a planar structure and a properties of the two small molecules were investigated by
Scheme 1 Synthetic route to two small molecules.
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thermogravimetric analysis (TGA) under a nitrogen atmosphere
The electrochemical properties of the two small molecules
(Fig. S1, ESI†). High decomposition temperatures (T
d
) with 5% were investigated using cyclic voltammetry (CV). As shown in
weight loss of both over 400 1C were obtained, indicating Fig. 2a, the reduction and oxidation peaks of IDTT-MI and
excellent thermal stability for the two small molecules.
IDTT-T-MI were located at À1.23/0.63 and À1.15/0.59 eV,
respectively. According to the equation (Table 1), the LUMO
and HOMO energy levels were calculated to be À3.52/À5.38 eV
Optical and electrochemical properties
The optical properties of two small molecules were investigated and À3.60/À5.34 eV for IDTT-MI and IDTT-T-MI, respectively,
by UV-vis absorption spectroscopy. The normalized absorption indicating that the two small molecules might be potential
spectra characterized in chloroform solutions, on drop-cast p-type transport materials in OFET devices when a suitable
thin films, and annealed films are shown in Fig. 1. In solution, metal is used as the electrode. The incorporation of a thiophene
IDTT-T-MI exhibited an absorption maximum at B558 nm bridge and extension of the effective-conjugation-length
(Table 1), which was red-shifted by ca. 5 nm compared with resulted in a slight decrease in the LUMO energy level and an
IDTT-MI (553 nm). Moreover, IDTT-T-MI showed a significant increase in the HOMO energy level. Consequently, IDTT-T-MI
increase of the 0–0 vibrational peak. As expected, the two exhibited a narrower bandgap (1.74 eV) than IDTT-MI (1.86 eV).
small molecules showed apparent red-shifts and broadened
Density functional theory (DFT) calculations at the B3LYP/
absorption spectra in thin films. For example, the absorption 6-31G(d) level were also used to investigate the electronic
maxima at 558 nm in solution was red-shifted to 600 nm with structures of IDTT-MI and IDTT-T-MI. All the hexyl side chains
significant shoulders emerging at 520 and 558 nm for the thin in small molecules were replaced with methyl groups, and the
film of IDTT-T-MI, suggesting strong intermolecular inter- corresponding calculation results are shown in Fig. 2b and
1
7
actions in the solid state. Relative to the absorption of the Fig. S2 (ESI†). IDTT-MI exhibited a small dihedral angle of 0.11
IDTT-MI film, IDTT-T-MI displayed distinctly red-shifted between the fused ring and the hemi-isoindigo unit, implying a
absorption peaks with a red shift of ca. 20 nm. The significant coplanar backbone for IDTT-MI probably because of the intra-
bathochromic-shift for IDTT-T-MI may be attributed to the molecular interactions. Upon incorporating the thiophene
extended effective-conjugation-length by incorporating the bridge between the hemi-isoindigo and thiophene units, the
thiophene bridge between the hemi-isoindigo and fused ring dihedral angle dropped to 0.051, indicating minimal torsional
units. Annealing the films led to further red-shifts of about effects. Moreover, the other dihedral angle for IDTT-T-MI
5
nm for the absorption maxima of IDTT-T-MI, albeit slightly, between the fused ring and the thiophene unit was calculated
indicating the greater planarization and the improvement of to be 0.51. These results indicate that the incorporation of the
2
7
the molecule ordering. The optical band gaps of IDTT-MI thiophene bridge did not exert a detrimental effect and the
and IDTT-T-MI are 1.97 and 1.94 eV, respectively, which were extended conjugation also possessed a planar backbone, which
calculated from the thin film absorption onset (628 and is beneficial for the ordered aggregation, thus enhancing the
648 nm).
charge-carrier transport. The SÁ Á ÁO distance was calculated to
Fig. 1 UV-vis absorption of (a) IDTT-MI and (b) IDTT-T-MI in chloroform solution and thin films with and without annealing.
Table 1 Decomposition temperatures, optical properties, and energy levels of two small molecules
lmax [nm]
a
d
opt b
g
c
c
cv
Molecule
T
[1C]
Solution
Film
lonset [nm]
E
[eV]
E
HOMO [eV]
E
LUMO [eV]
g
E [eV]
IDTT-MI
IDTT-T-MI
406
414
522/553
532/558
538/584
558/600
628
640
1.97
1.94
À5.38
À5.34
À3.52
À3.60
1.86
1.74
a
b
opt
c
ox
red
+
The 5 wt% loss temperature.
E
g
= 1240/lonset
.
HOMO = À(4.75 + Eonset) and LUMO = À(4.75 + Eonset); the redox Fc/Fc was located at 0.05 V
+
related Ag/Ag .
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Fig. 2 (a) Cyclic voltammogram and (b) the optimized structures according to the DFT calculation at the B3LYP/6-31G(d) level.
be 2.79 Å (Fig. S2, ESI†), shorter than the combined van der
Waals radii of ca. 3.32 Å, indicative of the existence of intra-
2
8
molecular SÁ Á ÁO interactions.
Field-effect properties
The field-effect properties of the IDTT-MI and IDTT-T-MI were
investigated using a bottom-gate/top-contact (BG/TC) configu-
ration. The OFET devices were fabricated in a glovebox and
characterized under ambient conditions. The corresponding
device performances are summarized in Table S1 (ESI†). First,
the semiconductor films were spin-coated from a conventional
À1
halogenated solution (6 mg mL ). No field-effect perfor-
mances were observed for both as-prepared OFETs, probably
because of the amorphous structures for the non-annealing
(N/A) films. The two small molecules displayed typical hole
transport characteristics after annealing at different tempera-
tures ranging from 180 to 260 1C (Fig. 3 and Fig. S3, ESI†). In
the case of IDTT-MI, the device was optimized via thermal
annealing and exhibited a maximum hole mobility (mmax)
2
À1
À1
of 0.036 cm
V
s
and an average mobility (mavg) of
2
À1 À1
0
.023 cm V
s , with an current ON/OFF ratio (ION/IOFF) of
5
over 10 and a threshold voltage (Vth) of À8.8 V. With the Fig. 3 The transfer and output characteristics of OFETs for the two small
molecules. The films were spin-coated from chloroform solution.
incorporation of the thiophene bridge into the backbone, IDTT-
T-MI exhibited much-improved device performances at least
one order of magnitude compared to IDTT-MI without thio-
phene bridge. After annealing at 180 1C, the hole mobility that IDTT-T-MI should be a high-performance semiconductor
2
À1 À1
reached 0.26 cm V
s , and when the annealing temperature which exhibited much higher hole mobilities compared to
was further increased to 210 1C, the hole mobility continued to those of IDTT-MI under the same conditions. The chemical
2
À1 À1
reach 0.37 cm
V
s
(Fig. S3, ESI†). The OFET devices structures of the two small molecules are different in the
exhibited optimized performances when annealed at 240 1C, thiophene bridge, which not only extends the conjugation
2
À1 À1
29–31
and IDTT-T-MI showed mmax as high as 0.80 cm V
s
with a length but also enhances the intramolecular interactions,
5
high ION/IOFF ratio (over 10 ) and a low Vth (À4.8 V, Fig. 3b). which both probably result in different crystal structures and
Further increase in the annealing temperature above 240 1C thin-film morphology.
resulted in lower device performances. For instance, the m
It is well known that the chloroform used above is not an
max
2
À1 À1
decreased to 0.30 cm V
s
for the 260 1C-annealing devices. environmentally-friendly solvent. To date, most OFET devices
IDTT-T-MI based devices displayed very low V , which is are fabricated using a halogenated solvent, which may result in
th
beneficial for fabricating low-power-consumption devices. serious environmental issues. Herein, the nonhalogenated
Moreover, all the output curves showed negligible hysteresis solvent, tetrahydrofuran (THF), was used for fabricating
even at a high gate voltage (VGS = 100 V). These results indicated IDTT-T-MI-based devices. The device results are summarized
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Fig. 4 The transfer and output characteristics of OFETs for the IDTT-T-MI. The films were spin-coated from tetrahydrofuran (THF) solution.
À1
in Table S1 (ESI†), and the transfer/output curves are shown in second-order diffraction peak at 0.38 Å
corresponding to
Fig. 4. IDTT-T-MI-based OFET devices also exhibited excellent (002) along the out-of-plane (Fig. S5, ESI†). Upon incorporating
performances even if the nonhalogenated solvent was used. the thiophene bridge, IDTT-T-MI exhibited an obvious
The devices fabricated from the THF solvent showed a similar enhanced diffraction peak (002) as well as a strong peak
level of performance compared to those from the chloroform (001). These results indicated that the incorporation of the
2
À1 À1
solvent (Table S1, ESI†). A highest hole mobility of 0.32 cm V
s
thiophene bridge into the backbone endowed IDTT-T-MI with a
2
À1 À1
and an average mobility of 0.27 cm V
s
were obtained, with a more ordered lamellar crystalline structure. The corresponding
5
high ION/IOFF ratio of over 10 and a low Vth of below À6 V after
annealing at 210 1C.
d001 spacings of two small molecules were calculated to be
17.4 Å for IDTT-MI and 16.4 Å for IDTT-T-MI, suggesting that
there may be at least in IDTT-T-MI interdigitation of the alkyl
chains along the lamella packing direction. Moreover, by using
Crystallinity and morphology
Grazing-incidence X-ray diffraction (GIXD) and atomic force the 1D intensity profiles of the 2D-GIXD pattern, the d100 and
microscopy (AFM) were employed in this work to investigate the
010 spacings were found to be 14.5/8.2 Å and 17.0/8.0 Å for
d
crystal structure and microstructure of the two small molecule IDTT-MI and IDTT-T-MI (Fig. S6, ESI†), respectively. According to
thin-films. The 2D-GIXD patterns of the annealing films are the verification from the DFT-simulated structure, the molecular
shown in Fig. 5. IDTT-MI showed strong diffraction peaks at lengths of IDTT-MI and IDTT-T-MI, which along the direction
À1
0
.36 Å corresponding to (001) and also showed a very weak of the molecular long axis, were calculated to be approximately
Fig. 5 GIXD patterns of two small molecules after thermal annealing at optimized temperatures.
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Fig. 6 AFM height images of two small molecule films spin-coated from different solutions (CF and THF) with and without annealing.
25.0 and 35.5 Å (Fig. S7, ESI†) respectively, indicating that there processed from chloroform and THF solution both exhibited
should be an overlap along the molecular long axis for the two continuous and nanofibrillar crystal structures over the entire
small molecules. The solid-state structures of IDTT-MI and area. The well-interconnected network could form a highly effi-
IDTT-T-MI were obtained from GIXD and simulated calculations, cient pathway for charge transport, which may be beneficial for
in which the estimated p–p stacking distance is half of the high field-effect mobility obtained for the annealing devices.
d010 distances, approximately 4.1 and 4.0 Å for IDTT-MI and In short, according to the results mentioned above, IDTT-MI
IDTT-T-MI, respectively. IDTT-T-MI had smaller p–p distances as exhibited low field-effect performances, which are mainly ascribed
well as more ordered lamellar crystalline structures compared to to the poor film-forming properties, such as the dewetting film
IDTT-MI, which may both contribute to the higher hole mobilities and too rough surface. The high hole mobility for IDTT-T-MI may
for IDTT-T-MI based devices.
be attributed to the extended conjugation, excellent crystal struc-
The AFM height images of the as-spun and thermally ture, close p–p stacking, and well-interconnected morphology.
annealed thin-films are shown in Fig. 6. The as-spun films
processed from chloroform solution both displayed amorphous-
like morphology and the whole AFM images exhibited a smooth
surface with no obvious crystalline region (Fig. 6). In contrast,
3
. Conclusions
the IDTT-T-MI film spin-coated from THF solution exhibited a Two new A–D–A small molecules (IDTT-MI and IDTT-T-MI)
fine crystal structure throughout the thin-film. However, the based on a fused ring and a hemi-isoindigo unit were synthe-
IDTT-T-MI crystallites are of small size and separate from each sized for application in solution-processed OFETs. The incor-
other. As a result, the discontinuous morphology may be difficult poration of a thiophene bridge into the backbone not only
to form an efficient pathway for charge carrier transport. These extended the conjugation length but also maintained the
results from the as-spun films well explained why the OFET highly co-planar structure. IDTT-T-MI showed more ordered
devices without annealing did not exhibit any field-effect. After crystalline structure, closer p–p stacking, and much better-
annealing, relatively large crystals formed in the IDTT-MI film interconnected morphology compared to IDTT-MI without a
while obvious dewetting was also observed, and this may be thiophene bridge. Consequently, OFET devices based on IDTT-
2
À1 À1
responsible for the low mobility. Moreover, the IDTT-MI film T-MI exhibited a high hole mobility of over 0.80 cm V
s
showed a seriously rough surface with a too large root mean with a high ION/IOFF ratio and a low V . Moreover, the film spin-
th
square roughness (RMS) of over 45 nm, which may also result in coated from a nonhalogenated solvent (THF) also displayed
poor interfacial contact between the thin-film and the electrode, excellent field-effect performances with a mobility as high
2
À1 À1
and is thus also unfavorable for charge transport. IDTT-T-MI films as 0.32 cm
V
s . These results indicated that the fused
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Foundation of China (NSFC, Grants 51573036 and 51703047),
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