Vinylene and benzo[c] [1,2,5]thiadiazole: Effect of the π-spacer unit on the properties of bis(2-oxoindolin-3-ylidene)-benzodifuran-dione containing polymers for n-channel organic field-effect transistors

Article abstract of DOI:10.1039/c8ra08890j

Two polymers based on (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-b′]difuran-2,6(3H,7H)-dione (BIBDF) coupled with (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) or dithienylbenzothiadiazole (TBT), namely PBIBDF-TVT and PBIBDF-TBT were synthesized via the Stille coupling reaction. The effect of benzothiadiazole or vinylene-π spacer of the copolymers on optical properties, energy levels, electronic device performance and microstructure were studied. It was found that PBIBDF-TBT based OFET devices, annealed at 180 °C, showed better performance with the highest electron mobility of 2.9 × 10^-2 cm² V s^-1 whereas PBIBDF-TVT polymer exhibited 5.0 × 10^-4 cm² V s^-1. The two orders of magnitude higher electron mobility of PBIBDF-TBT over PBIBDT-TVT is a clear indicator of the better charge transport ability of this polymer semiconductor arising from its higher crystallinity and better donor-acceptor interaction.

Full text of DOI:10.1039/c8ra08890j

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PAPER
Vinylene and benzo[c][1,2,5]thiadiazole: effect of
the p-spacer unit on the properties of bis(2-
oxoindolin-3-ylidene)-benzodifuran-dione
containing polymers for n-channel organic field-
effect transistors†
Cite this: RSC Adv., 2018, 8, 38919
a
a
b
Thu Trang Do,‡ Basanagouda B. Patil, ‡ Samarendra P. Singh,
a
cd
a
a
Soniya D. Yambem, Krishna Feron, Kostya (Ken) Ostrikov, John M. Bell
and Prashant Sonar
a
*
0
Two polymers based on (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-b ]difuran-2,6(3H,7H)-
dione (BIBDF) coupled with (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) or dithienylbenzothiadiazole
(TBT), namely PBIBDF-TVT and PBIBDF-TBT were synthesized via the Stille coupling reaction. The effect
of benzothiadiazole or vinylene-p spacer of the copolymers on optical properties, energy levels,
electronic device performance and microstructure were studied. It was found that PBIBDF-TBT based
ꢀ
OFET devices, annealed at 180 C, showed better performance with the highest electron mobility of 2.9
Received 29th October 2018
Accepted 5th November 2018
ꢂ2
2
ꢂ1
ꢂ4
2
ꢂ1
ꢁ
10 cm V s whereas PBIBDF-TVT polymer exhibited 5.0 ꢁ 10 cm V s . The two orders of
magnitude higher electron mobility of PBIBDF-TBT over PBIBDT-TVT is a clear indicator of the better
charge transport ability of this polymer semiconductor arising from its higher crystallinity and better
donor–acceptor interaction.
DOI: 10.1039/c8ra08890j
rsc.li/rsc-advances
9
and high thermal/electrochemical stabilities. Depending on
charge transport properties, polymer semiconductors are
Introduction
Organic eld-effect transistors (OFETs) based on donor– generally classied as p-type (holes are the majority charge
acceptor incorporated p-conjugated polymeric semiconductors carriers), n-type (electrons are the majority charge carriers) and
have attracted a great scientic interest in recent years due to ambipolar (electron and holes are the majority charge carrier)
their potential applications in low-cost, solution-processed and materials. Most polymer semiconductors are p-type that have
1
–4

exible organic electronics.
OFET based on such exible seen a signicant increase in performances with very high hole
2
ꢂ1 ꢂ1 10,11
prototypes have been successfully demonstrated for a wide mobility close to or greater than 10 cm V
range of applications including logic circuits, chemical contrast, n-type, electron transport polymer semiconductors
sensors, memory devices and light emitting transistors. In have largely lagged behind their p-type counterparts. So far
comparison with small molecules, polymers offer great advan- most efforts have been directed to the development of novel
tages such as desirable solution processability, higher viscosity, building blocks, particularly fused ring acceptor building
s
.
In sharp
5
6
7
8
1
2–15
better exibility, superior elasticity, great mechanical strength blocks such as diketopyrrolopyrrole (DPP),
naphthalene
16–18
19,20
diimide (NDI)
and perylene diimide (PDI)
for high
mobility polymers for OFETs. Among the most interesting
electron-depleted cores used for n-channel polymer building
blocks, (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-
a
School of Chemistry, Physics and Mechanical Engineering, Queensland University of
Technology (QUT), 2 George Street, Brisbane, QLD-4001, Australia. E-mail: sonar.
prashant@qut.edu.au
0
b
b ]difuran-2,6(3H,7H)-dione (BIBDF in Scheme 1) has demon-
Department of Physics, School of Natural Sciences, Shiv Nadar University (SNU),
3,21–24
strated the greatest potential.
BIBDF possesses a large
Gautam Buddha Nagar, Uttar Pradesh, India-201307
c
CSIRO Energy Centre, NSW-2304, Australia
fused aromatic ring with a symmetric and planar structure,
d
Centre for Organic Electronics, University of Newcastle, Callaghan, NSW 2308, which would enhance the p–p overlap and intermolecular
Australia
25–27
interaction.
Additionally, the electron withdrawing lactone
1
13
†
Electronic supplementary information (ESI) available: H-NMR and C-NMR
groups in the central core combined with the lactam groups
spectra of all synthesized compounds, HRMS spectrum of BIBDF, GPC of
26,27
make the BIBDF unit strongly electron-decient.
On the
ꢀ
polymers, UV-vis of thin lms at annealing temperature of 180 C, XRD of
other hand, a donor–acceptor (D–A) type conjugated polymer is
one of the most successful and effective strategies to enhance
polymer powders and XPS spectra of polymers. See DOI: 10.1039/c8ra08890j
‡
Both authors contributed equally to this work.
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Scheme 1 The synthetic route to BIBDF.
ꢂ2
2
ꢂ1
the charge transporting capability of conjugated polymer. With mobility of 2.9 ꢁ 10 cm V s whereas PBIBDF-TVT polymer
ꢂ4
2
ꢂ1
this conguration, inter-/intra-molecular interactions of the exhibits 5.0 ꢁ 10 cm V s . Such higher mobility of PBIBDF-
polymer chains are easily tuned by varying the combination of TBT is due to the strong donor–acceptor interaction. The
different D and A units, and the strong inter chain D–A inter- comparative optoelectronic properties of both polymers and
action will result in ordered chain packing and shorten the p–p their OFET performance provide an important guideline for
stacking distance, facilitating the interchain charge trans- designing new polymer semiconductors.
28–30
port.
Following this approach, several BIBDF-based D–A
conjugated polymers have been reported to demonstrate high
charge-carrier mobilities with decent air stability. Recently,
Zhang and co-worker have reported D–A polymer based on
Results and discussions
Synthesis and characterization
BIBDF as an acceptor and (E)-2-(2-(thiophen-2-yl)vinyl) Scheme 1 shows the synthetic route to the electron-decient
30
thiophene (TVT) as a donor. The polymer PBIBDF-TVT fused aromatic ring (3E,7E)-3,7-bis(6-bromo-1-(4-octadecyldo-
0
showed close p–p distance and long-range ordered, lamellar cosyl)-2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-b ]difuran-
crystalline structure leading to the high charge carrier mobility. 2,6(3H,7H)-dione (BIBDF) as starting monomer. The synthesis
This research also exhibited that the donor unit plays important started from the bromination of commercially available 3-
roles in affecting the absorption bands, HOMO levels, lamellar octadecyl-henicosan-1-ol, giving compound (1) in a good yield.
packing and p–p stacking distances.
Compound 1 was then reacted with 6-bromo-1H-indole-2,3-
30
By taking inspiration from previous works, our investiga- dione (2) via N-alkylation to form its soluble derivative 6-
tion realizes the synthesis of a novel D–A conjugated polymer bromo-1-(4-octadecyldocosyl)indoline-2,3-dione (3). The long
based on BIBDF as A unit employing dithienylbenzothiadiazole branched alkyl chain 4-octadecyldocosyl was chosen to enhance
(
TBT) as donor–acceptor–donor (D–A–D) unit along with their the solubility of the nal BIBDF polymers. The compound 3,7-
0
detailed characterizations. The dithienylbenzothiadiazole dihydrobenzo[1,2-b:4,5-b ]difuran-2,6-dione (4) was synthesized
D–A–D is a weak electron-donating unit to reduce the energy from the dehydration reaction of 2,2 -(2,5-dihydroxy-1,4-
0
levels which may enhance air stability of the material. In order phenylene)diacetic acid. Finally the target compound BIBDF
to study the effect of stronger donor unit on the electron was prepared from reacting compound 3 with compound 4 in
transport properties of polymer, we also synthesized the previ- acetic acid at about reux temperature.
ously reported polymer PBIBDF-TVT for the comparison
The synthetic routes of PBIBDF-TVT and PBIBDF-TBT are
purpose. Benzothiadiazole-p spacer in TBT core is electron- illustrated in Scheme 2. (E)-1,2-bis(5-(trimethylstannyl)
decient whereas vinylene p-linker in TVT core is electron thiophen-2-yl)ethane (TVT), 4,7-bis(5-(trimethylstannyl)thio-
donor. Such arrangement can lead to lower HOMO and LUMO phen-2-yl)benzo[c][1,2,5]thiadiazole (TBT) comonomers and
energy levels of polymer PBIBDF-TBT compared to polymer BIBDF monomer were synthesized following the literature
21,31,32
PBIBDF-TVT. Vinyl linkage present in the backbone is electron procedures.
Two polymers were synthesized by Stille cross-
rich and can be susceptible for easy oxidation so incorporating coupling reaction using 1 : 1 monomer ratio in the presence of
electron withdrawing block can stabilise the system and mate- tris(dibenzylidene-acetone)dipalladium (Pd (dba) ) as a catalyst
2
3
rial can be more stable. HOMO/LUMO values of PBIBDF-TBT and tri(o-tolyl)phosphine (P(o-tol) ) as a ligand. The purication
3
and PBIBDF-TVT were ꢂ5.62/ꢂ4.28 eV and ꢂ5.51/ꢂ4.16 eV, of the crude polymers was performed by Soxhlet extraction with
respectively. Both synthesized copolymers were successfully methanol, hexane and acetone to remove the oligomers and
employed as active semiconductors in OFET devices and their other impurities present in the sample. At the last, chloroform
electrical and morphological properties have been studied in was used to extract the pure polymer product from the crude
detail. The PBIBDF-TBT polymer exhibits the highest electron sample. The nally extracted pure polymer solutions in
38920 | RSC Adv., 2018, 8, 38919–38928
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Scheme 2 The synthetic route to polymers PBIBDF-TVT and PBIBDF-TBT.
chloroform were kept under rotavapor to remove solvent and transition temperatures of polymers based on BIBDF with
3,23,30
the nal pure polymer was nally precipitated using methanol similar alkyl chain was observed,
which is possibly due to
as an anti-solvent. The puried polymers were investigated by X- the phase transition temperature caused by the melting of the
ray photoelectron spectroscopy (XPS) (Fig. S11†) to conrm that long alkyl chains. For the TGA measurement, both polymer
ꢀ
polymers are metal-free. The precipitated polymer sample was samples were heated from room temperature to 800 C using
ꢀ
then dried and used for further characterization and to study 10 C per minute heating rate. Both polymers exhibited good
their comparative behaviour. All the two polymers were dark thermal stabilities, with high decomposition temperature (at
ꢀ
black solids that gradually dissolved in chloroform. The which a compound loses 5% of its weight): PBIBDF-TVT: 365 C
ꢀ
number (M
n
) and the weight (M
w
) average molecular weights of and PBIBDF-TBT: 370 C. The thermally stable behaviour of
the polymers were determined by gel-permeation chromatog- polymers is benecial for performing higher temperature
ꢀ
raphy (GPC) with chloroform as an eluent at 30 C. The M
n
and annealing and thin lm processing application in optoelec-
M
w
were 173.5 kDa and 870.2 kDa, respectively for PBIBDF-TVT, tronic devices.
360.6 kDa and 3064.9 kDa, respectively for PBIBDF-TBT.
Optical properties
Thermal properties
The optical properties of PBIBDF-TVT and PBIBDF-TBT were
studied by ultraviolet-visible (UV-vis) absorption spectra in
dilute chloroform solutions (around 10 M) and as thin lms
The thermal properties of PBIBDF-TVT and PBIBDF-TBT were
investigated by thermogravimetric analysis (TGA) (Fig. 1) and
differential scanning calorimetry (DSC) (Fig. 2) respectively. For
the DSC measurement, polymer sample was employed for
heating and cooling cycles using 10 C per minute heating rate.
As shown in Fig. 2, during DSC heating and cooling scans,
ꢂ4
on glass substrate casted from chloroform (Fig. 3). In chloro-
form solution, both polymers showed broad and strong
absorption at the range of 350 nm to 900 nm. The shorter
absorption band at 350–600 nm corresponds to p–p* transi-
tions, and longer wavelength band at 650–900 nm corresponds
to intramolecular charge transfer (ICT). The longest wavelength
ꢀ
PBIBDF-TVT and PBIBDF-TBT showed endothermic peaks at
ꢀ
ꢀ
1
2–24 C and exothermic peaks at 2–13 C. The similar
Fig. 1 TGA thermograms of PBIBDF-TVT and PBIBDF-TBT under Fig. 2 Differential Scanning Calorimetric (DSC) analysis of PBIBDF-
ꢀ
ꢂ1
ꢀ
ꢂ1
a nitrogen atmosphere at a heating rate of 10 C min
.
TVT and PBIBDF-TBT with a scanning rate of 10 C min .
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result from their unique H-aggregation-induced interchain
25,36
packing in the solid state.
bands for PBIBDF-TVT and PBIBDF-TBT were at 920 and
25 nm, respectively, corresponding to optical bandgaps of 1.35
The edges of the lms absorption
9
and 1.34 eV, respectively. The narrow bandgaps of polymers
could be ascribed to the extension of p-conjugation as well as
strong intramolecular and intermolecular interactions arising
from the donor and acceptor moieties along the polymer
26,30
backbone.
Electron energy level determination
Photoelectron spectroscopy in the air (PESA) was used to
determine HOMO energy levels of polymers. In our measure-
ments, we spin-coated polymer thin lms from chloroform
Fig. 3 UV-vis absorption spectra of PBIBDF-TVT and PBIBDF-TBT in solution on the glass substrates for PESA measurement (Fig. 4,
chloroform solutions and as thin films.
data in Table 1). From the onset energy level using PESA data,
we calculated HOMO values for PBIBDF-TVT and PBIBDF-TBT
polymers and they were found to be ꢂ5.51 and ꢂ5.62 eV,
respectively. It is clear from the observed energy levels that by
introducing a stronger electron affinity spacer, the polymer
PBIBDF-TBT possesses lower-lying HOMO levels. The LUMO
energy levels of PBIBDF-TVT and PBIBDF-TBT, calculated from
the optical band gap and HOMO value difference and they were
determined to be ꢂ4.16 and ꢂ4.28 eV, respectively. The ob-
tained LUMO values are appropriate and signicantly lower for
better air-stable electron transport.
absorption maxima (lmax) exhibited by PBIBDF-TVT and
PBIBDF-TBT were similar at 837 nm. The absorption spectrum
and absorption maximum of polymer PBIBDF-TVT in this study
are similar to the same one reported by Zhang et al. group. In
addition, the absorption spectrum of PBIBDF-TBT was quite
broader than that of PBIBDF-TVT in the long wavelength band.
Similar to some other polymers based on BIBDF, both
PBIBDF-TVT and PBIBDF-TBT exhibited insignicant spectral
shi upon going from the solution to the lm, suggesting that
polymers might form some pre-aggregates in solution due to
3
0
21
Furthermore, the cyclic voltammetry (CV) analysis with
a glassy carbon working electrode, platinum-wire counter elec-
trode and Ag/AgCl reference electrode using 0.1 M tetra(n-butyl)
33–35
their strong intermolecular interactions.
blue shi of absorption spectra and peaks of two polymers in
the thin lm state compared to those in the solution state may
Furthermore, the
ammonium
hexauorophosphate
(n-Bu NPF )
in
4
6
Fig. 4 Photoelectron spectroscopy in air (PESA) analysis of (a) PBIBDF-TVT and (b) PBIBDF-TBT in thin film.
Table 1 Optical and electrochemical data for the polymers PBIBDF-TVT and PBIBDF-TBT
l
max (nm)
opt
g
a
b
c
c
Polymer
Solution
Film
E
(eV)
HOMO (eV)
LUMO (eV)
HOMO (eV)
LUMO (eV)
PBIBDF-TVT
PBIBDF-TBT
837
837
834
830
1.35
1.34
ꢂ5.51
ꢂ5.62
ꢂ4.16
ꢂ4.28
ꢂ5.53
ꢂ5.67
ꢂ3.85
ꢂ3.90
a
b
PESA
opt c
The HOMO value determined by PESA.
E
LUMO ¼ EHOMO + E
g
.
DFT-estimated data.
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Fig. 5 The calculated molecular orbitals for PBIBDF-TVT and PBIBDF-TBT.
ꢀ
dichloromethane (DCM) solution as an electrolyte at a potential to 2q ¼ 3.18 , increasing the d-spacing slightly to 2.88 nm.
ꢂ1
scan rate of 100 mV s was also studied. Unfortunately, poly- However, in case of PBIBDF-TBT, the rst peak was shied from
ꢀ
ꢀ
ꢀ
mers are not completely soluble in DCM, then the mixture of 3.29 to 3.18 upon annealing at 180 C, leading to a d-spacing
chlorobenzene and DCM (7 : 3 v/v) was used, however polymers shi from 2.69 nm to 2.78 nm. Simultaneously, the FWHM was
are aggregated in the solution (Fig. S12†), therefore good CV changed from 1.07 to 0.37 upon annealing. While comparing
curves could not be obtained.
XRD, we clearly notice that an annealed thin lm of PBIBDF-
In addition, the structural and electric parameters of these TBT exhibits higher crystallinity and lower d-spacing than that
two polymers were also investigated by density functional of the PBIBDF-TVT due to the presence of the strong electron
theory (DFT) calculations using the Gaussian 09 at the B3LYP/6- withdrawing benzothiadiazole spacer moiety.
31g(d) level to study the electron density. As shown in Fig. 5, the
HOMO electron densities of PBIBDF-TVT and PBIBDF-TBT are
mainly located on the TVT and TBT moieties whereas their
LUMO electron densities are mainly located on the BIBDF units,
respectively. The calculated HOMO/LUMO energy levels of
PBIBDF-TVT and PBIBDF-TBT are ꢂ5.53/ꢂ3.85 eV and ꢂ5.67/
ꢂ3.90, respectively. The obtained theoretical HOMO and LUMO
energy levels are in a good agreement with PESA measurements.
OFET device characteristics and performance
The schematics of OFET devices of PBIBDF-TVT and PBIBDF-
TBT polymer semiconductors are shown in Fig. 7a and b,
respectively. The output and transfer characteristics of PBIBDF-
X-ray diffraction (XRD) analysis
X-ray diffraction measurement was performed to examine the
ordering and crystallinity of two polymers with the difference of
p-space unit. The thin lm XRD, for both polymers, was
ꢀ
measured before- and aer-annealing at 180 C (this is the best
optimized thin lm annealing temperature condition which
exhibits better charge transport-see next OFET device section)
using exact device geometry (Si/SiO substrates) shown in Fig. 6.
2
In case of PBIBDF-TVT, variation in the d-spacing and full width
half maximum (FWHM) of annealed and non-annealed thin

lms were almost identical. We noticed the rst diffraction
ꢀ
peak for the non-annealed thin lm at 2q ¼ 3.08 , corre-
ꢀ
sponding to a d-spacing of 2.87 nm, with an FWHM of 0.58 . For
Fig. 6 X-ray diffraction (XRD) Patterns of Thin Film XRD at RT and after
ꢀ
ꢀ
the thin lms annealed at 180 C, the diffraction peak changed annealing at 180 C for PBIBDF-TVT and PBIBBDF-TBT polymers.
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Fig. 7 Bottom-gate-top-contact OFET device structure using (a) PBIBDF-TVT and (b) PBIBDF-TBT based polymer semiconductors.
ꢂ3
2
ꢂ1
TVT and PBIBDF-TBT polymer semiconductors based OFETs polymer based OFETs, were 1.3 ꢁ 10 cm V s (linear) and
ꢂ2
2
ꢂ1
were measured under an inert condition inside a glove box. The 2.9 ꢁ 10 cm V s (saturation), respectively with an Ion/Ioff
2
OFET devices of non-annealed thin lms of both polymers did ratio of 10 and Vth of 12 volts. The two order of magnitude
not exhibit any OFET characteristics. The devices with an higher electron mobility of PBIBDF-TBT over PBIBDT-TVT in the
ꢀ
annealed (at 180 C) polymer thin lm as an active layer, saturation regime is a clear signature of better charge transport
showed n-type behaviour. The output and transfer characteris- ability in this polymer. The high crystallinity of PBIBDF-TBT
ꢀ
tics of PBIBDF-TVT and PBIBDF-TBT based OFETs are shown in polymer thin lms, annealed at 180 C, can be attributed for
Fig. 8 and 9, respectively. The electron mobilities, Ion/Ioff ratio high charge transport.
and Vth for both polymers are summarised in Table 2. For
PBIBDF-TVT based OFET, the electron mobility determined
AFM thin lm morphology
from the linear and saturation region, was found to be ꢃ4.2 ꢁ
ꢂ4
2
ꢂ1
ꢂ4
2
ꢂ1
In order to further investigate the reasons for better charge
transport in PBIBDF-TBT based OFETs as compared to the same
of PBIBDF-TVT, we further studied the surface morphology and
10
cm V s and ꢃ5.0 ꢁ 10 cm V s , respectively with an
2
Ion/Ioff ratio of 10 , and V of 16 volts. The electron mobility,
th
determined from the transfer characteristics of PBIBDF-TBT
Fig. 8 Output (a) and transfer (b) characteristics of PBIBDF-TVT Fig. 9 Output (a) and transfer (b) characteristics of PBIBDF-TBT
ꢀ
ꢀ
polymer based OFET devices after annealing at 180 C.
polymer based OFET devices after annealing at 180 C.
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Table 2 OFET device characteristics of PBIBDF-TVT and PBIBDF-TBT based polymers
ꢀ
2
ꢂ1
2
ꢂ1
Polymers
Anneal. temp. ( C)
Vth (V)
Ion/Ioff
m
e
(Lin.) (cm V s
)
m
e
(Sat.) (cm V s
)
2
ꢂ4
ꢂ3
ꢂ4
ꢂ2
PBIBDF-TVT
PBIBDF-TBT
180
180
16
12
10
10
4.2 ꢁ 10
1.3 ꢁ 10
5.0 ꢁ 10
2.9 ꢁ 10
2
ꢀ
Fig. 10 AFM Images (2 mm by 2 mm) of (a) PBIBDF-TVT at room temperature and (b) at 180 C, (c) PBIBDF-TBT at room temperature and (d) at
ꢀ
1
80 C.
microstructures of the polymeric thin lms by atomic force polymer thin lms are known to facilitate charge transport in
37
microscopy (AFM) of the thin lms on the actual devices from OFETs leading to high charge carrier mobility.
which the characteristics were obtained. An AFM (area: 2 mmꢁ2
mm) examination for both PIBDF-TVT and PIBDF-TBT thin lms
exhibits uniformly arranged grains, as shown in Fig. 10. It is
clearly noticed that, upon annealing of both polymer thin lms,
Conclusion
the large domains with clear boundary areas emerge. It is also Two conjugated polymers PBIBDF-TVT and PBIBDF-TBT
observed that the annealed thin lms of both polymer exhibit composed of BIBDF and (E)-2-(2-(thiophen-2-yl)vinyl)thiophene
large polycrystalline grains. This observation is consistent with (TVT) or dithienylbenzothiadiazole (TBT) groups were prepared
previously reported PBIBDF-TVT based polymer thin lm to investigate the inuence of p-spacer on eld-effect charge
30
morphology in the literature. In annealed thin lms, the extent transport in OFETs, and microstructures in thin lms. The
of grain formation and their size is considerably higher in case of HOMO energy levels, for PBIBDF-TVT and PBIBDF-TBT poly-
PIBDF-TBT than that of the PIBDF-TVT. This observation is mers, were found to be ꢂ5.51 and ꢂ5.62 eV, respectively. These
further substantiated by estimating an average roughness of thin results indicate that a stronger electron affinity spacer benzo-
lm surfaces. A non-annealed thin lm of PBIBDF-TVT and thiadiazole (compared to vinylene) lowers the HOMO energy
PBIBDF-TBT polymer showed a surface roughness of 1.17 nm level. Furthermore, the investigation of the two polymers for n-
ꢀ
and 1.21 nm, respectively. Whereas, in case of 180 C annealed channel organic eld-effect transistors was also performed. The
polymer thin lms, the surface roughness is enhanced from PBIBDF-TBT polymer exhibits the highest electron mobility of
ꢂ2
2
ꢂ1
1
.56 nm (for PBIBDF-TVT) to 2.37 nm (for PBIBDF-TBT). The AFM 2.9 ꢁ 10 cm V s whereas PBIBDF-TVT polymer exhibits
ꢂ4 2 ꢂ1
image, Fig. 10d, shows evolution of higher interconnecting grains electron mobility of 5.0 ꢁ 10 cm V s . The high crystallinity
in annealed thin lm of PBIBDF-TBT. Such microstructures in and interconnecting microstructures observed in a thin lm of
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ꢀ
ꢀ
ꢀ
PBIBDF-TBT polymer, annealed at 180 C, led to an improved and 300 C. PBIBDF-TVT polymer thin lms were annealed at 62
electron mobility.
and 180 C. On top of PBIBDF-TVT or PBIBDF-TBT polymer thin

lm, an Au lm of 100 nm was deposited as source and drain
electrode by thermal evaporation. The Au electrodes were
patterned via shadow masking method. The channel width (W)
and length (L) of OFETs were 2 mm and 80 mm, respectively.
Experimental section
Materials and instruments
All starting materials were purchased commercially as analyt-
ical reagents and used directly without any further purication.
Synthesis
4-Octadecyl-henicosan-1-ol was purchased from Lymtech (Bei-
Synthesis of 19-(2-bromo-ethyl)-heptatriacontane (1). Tri-
phenylphosphine (4.65 g, 17.70 mmol) was dissolved in DCM
jing) Technology Ltd, purity 95%. (3E,7E)-3,7-bis(2-oxoindolin-
0
3-ylidene)benzo[1,2-b:4,5-b ]difuran-2,6(3H,7H)-dione (BIBDF)
(
100 mL) under argon for 15 minutes. Then the reaction mixture
was prepared according to the literature. High resolution mass
spectra were recorded on an Orbitrap Elite mass spectrometer
ꢀ
was cooled to 0 C and followed by the addition of bromine (1.28
mL, 24.78 mmol). Aer 3-octadecyl-henicosan-1-ol (10.0 g, 17.70
mmol) in 100 mL of DCM was added dropwise via an additional
funnel over 30 minutes, the reaction solution was stirred at
room temperature for overnight under argon. The organic
solvent was removed under the reduced pressure, and the
residue was extracted with excess amount of hexane. The
hexane was evaporated and the resulting crude yellow solid was
puried by silica gel column eluting with hexane. The title
(Thermo Fisher Scientic) equipped with an atmospheric
pressure chemical ionisation (APCI) source, operating in the
positive ion mode at a resolution of 120 000 (at m/z 400). Gel
Permeation Chromatography (GPC) against polystyrene stan-
ꢀ
dards was performed in chloroform at 30 C and a ow rate of 1
ꢂ1
mL min on a Waters GPC assembly equipped with a Waters
1515 isocratic HPLC pump, Waters 2707 autosampler with
a 100 mL injection loop, and a Waters 2487 dual wavelength
1
compound was isolated as white-off solid (10 g, 90%). H NMR
absorbance detector analysed at 254 nm in series with a Waters
ꢀ
(600 MHz, CDCl
.57 (m, 1H), 1.28 (m, 68H), 0.90 (t, J ¼ 9.6 Hz, 6H).
Synthesis of 6-bromo-1-(4-octadecyldocosyl)indoline-2,3-
3
, ppm): d 3.41 (t, J ¼ 10.2 Hz, 2H), 1.85 (m, 2H),
2
414 refractive index detector at 30 C. Three consecutive
1
Waters Styragel columns (HR5, HR4, and HR1, all 7.8 ꢁ 300
mm, 5 mm particle size), preceded by a Waters Styragel guard
column (WAT054405, 4.6 ꢁ 30 mm, 20 mm particle size) were
used during analysis. A typical concentration of 1 mg polymer
dissolved in 1 mL chloroform was used to run GPC samples.
Thermal analysis was performed using a Pegasus Q500TGA
thermogravimetric analyzer under a nitrogen atmosphere at
dione (3). 19-(2-Bromo-ethyl)-heptatriacontane (5.0 g, 7.96
mmol) was added under nitrogen atmosphere to a suspension
of 6-bromo-1H-indole-2,3-dione (1.8 g, 7.96 mmol), potassium
carbonate (2.2 g, 15.9 mmol) in a mixture of tetrahydrofuran (40
mL) and N,N-dimethylformamide (40 mL). The mixture was
ꢀ
ꢀ
ꢂ1
stirred at room temperature for 1 hour, and then stirred at 50 C
a heating rate of 10 C min . Differential scanning calorimeter
DSC) was conducted under nitrogen using a Chimaera instru-
for overnight. Aer the reaction mixture was cooled to room
temperature and extracted with DCM and water. The organic
layer was washed with water, brine and dried over anhydrous
Na SO . The solvent was removed under reduced pressure and
2 4
the residue was puried by column chromatography on silica
(
ꢀ
ꢂ1
ment Q100 DSC. The sample was heated at 10 C min from
ꢀ
ꢀ
ꢂ20 C to 300 C. Absorption spectra were recorded on Carry50
UV-vis Spectrophotometer. Photoelectron spectroscopies in air
(PESA) measurements were conducted using on an AC-2
gel using hexane/DCM (3 : 1, v/v) as eluent to give orange col-
photoelectron spectrometer (Riken-Keiki Co.). The AFM
images were taken on the sample on which the OFET devices
where characterized. We used the Cypher AFM from (Oxford
instrument, Asylum Research, Santa Barbara, CA). It was carried
1
oured oil (4.7 g, 75%). H NMR (600 MHz, CDCl , ppm): d 7.46
3
(
2
d, J ¼ 12 Hz, 1H), 7.28 (s, 1H), 7.06 (s, 1H), 3.67 (t, J ¼ 11.4 Hz,
H), 1.65 (m, 1H), 1.25 (m, 24H), 0.88 (m, J ¼ 9.6 Hz, 6H).
0
Synthesis of 3,7-dihydrobenzo[1,2-b:4,5-b ]difuran-2,6-dione
out by tapping mode in air using Etalon NT-MDT cantilever with
0
ꢂ1
(4). In the mixture of 2,2 -(2,5-dihydroxy-1,4-phenylene)
diacetic acid (2.0 g, 8.8 mmol) and anhydrous toluene (50
mL), acetic anhydride (20 mL) was added. The mixture was
force constant 3.5 N m
.
Device fabrication
ꢀ
stirred at 100 C for 5 h and then the solvent was removed under
A heavily p-doped silicon wafer with a layer of ꢃ200 nm SiO on pressure. The residue was puried by silica gel chromatography
2
the surface was used as the substrate for bottom-gate, top- with eluent (CHCl ), and recrystallized from toluene to give 3,7-
3
0
contact device geometry with channel length 80 mm and dihydrobenzo[1,2-b:4,5-b ]difuran-2,6-dione as a white crystal
1
channel width 2 mm. The SiO
and the doped Si as the gate. The active polymer thin lm was 3.87 (s, 4H).
spin coated on top of the SiO surface using a PBIBDF-TVT or Synthesis of BIBDF. In the mixture of compound 3 (787 mg,
2
functions as the gate insulator (1.5 g, 90%). H NMR (600 MHz, CDCl
3
, ppm): d 7.06 (s, 2H),
2
PBIBDF-TBT polymer solution in chloroform and dichloroben- 1.0 mmol) and acetic acid (20 mL), compound 4 (70 mg, 0.417
ꢂ1
zene (DCB) (6 mg mL ) respectively at 1000 rpm for 60 seconds. mmol) and p-toluenesulfonic acid monohydrate (TsOH, 23 mg,
The average thickness of lms was around 50 nm. Annealing of 0.12 mmol) was add under nitrogen atmosphere. The mixture
ꢀ
polymer thin lms was at different temperatures done for was stirred at 115 C under nitrogen for 17 h. The reaction
20 min on a hot plate in a nitrogen atmosphere. Specically, mixture was then cooled to room temperature and ltered. The
PBIBDF-TBT polymer thin lms were annealed at 100, 180, 260 solid was washed with acetic acid and then methanol. The
38926 | RSC Adv., 2018, 8, 38919–38928
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residue was puried by silica gel chromatography with eluent
Acknowledgements
(
(
hexane : DCM ¼ 1 : 1 v/v) to give BIBDF as a black solid
1
430 mg, 60%). H NMR (600 MHz, CDCl , ppm): d 9.11 (s, 2H), T. T. Do is thankful to QUT for offering here QUTPRA scholar-
3
8
6
0
.93 (d, J ¼ 13.2 Hz, 2H), 7.21 (dd, J ¼ 13.2 Hz, J ¼ 2.4 Hz, 2H), ship to conduct her research work. B. B. Patil is thankful for
1
2
.95 (s, 2H), 3.72 (t, J ¼ 11.4 Hz, 4H), 1.67 (m, 4H), 1.22 (m, 24H), Supervisor Bridging HDR Scholarship provided by QUT. We are
1
3
.86 (m, J ¼ 9.6 Hz, 12H). C NMR (150 MHz, CDCl
3
, ppm): thankful to CRC for Polymers, Central Analytical Research
d 167.13, 166.97, 151.85, 149.85, 135.88, 131.61, 128.98, 126.93, Facility (CARF), Institute of Future Environments, Queensland
1
3
1
26.64, 125.72, 119.83, 111.85, 111.06, 40.73, 37.07, 33.52, University of Technology (QUT) for providing equipment
1.94, 30.75, 30.12, 29.73, 29.68, 29.38, 26.69, 24.40, 22.71, support. We are thankful to Advanced Nano Fabrication Facility
4.14. HRMS (EI, m/z) calcd: 1729.1512 (M ). Found: 1729.1589. (ANFF) located at University of Queensland (UQ) for providing
+
Synthesis of PIBDF-TVT. The following synthesis was based support during Dicing of Substrates and during AFM
30
on a modied literature procedure. To a 20 mL dry Schlenk Measurements. P. S. is thankful to QUT for the nancial support
ask was added 7 (150 mg, 0.0868 mmol), (E)-1,2-bis(5- from the Australian Research Council (ARC) for the Future
trimethylstannyl)thiophen-2-yl)ethene (45.0 mg, 0.0868 mmol) Fellowship (FT130101337) and QUT core funding (QUT/ 322120-
and tri(o-tolyl)phosphine (4.3 mg, 16 mol%). Aer degassing 0301/07).
and relling argon for times, solution of

(
3
a
tris(dibenzylideneacetone)-dipalladium (2.4 mg, 3 mol%) in dry
chlorobenzene (10 mL) was added under an argon atmosphere.
The ask was sealed and stirred for at 130 C for 48 h. Four
References
ꢀ
drops of bromobenzene was added and the reaction was kept at
1 O. Knopfmacher, M. L. Hammock, A. L. Appleton,
G. Schwartz, J. Mei, T. Lei, J. Pei and Z. Bao, Nat. Commun.,
2014, 5, 2954.
2 H. Sirringhaus, Adv. Mater., 2014, 26, 1319–1335.
3 Y. Q. Zheng, T. Lei, J. H. Dou, X. Xia, J. Y. Wang, C. J. Liu and
J. Pei, Adv. Mater., 2016, 28, 7213–7219.
4 Z. Zhao, Z. Yin, H. Chen, L. Zheng, C. Zhu, L. Zhang, S. Tan,
H. Wang, Y. Guo, Q. Tang and Y. Liu, Adv. Mater., 2017, 29,
1602410.
ꢀ
130 C for an additional 12 h. The reaction mixture was cooled
to room temperature and added dropwise into methanol (100
mL) and the mixture was stirred for 1 h. Then the precipitated
product was collected by ltration, and subjected to consecutive
Soxhlet extractions with methanol, acetone, and hexane. The
remaining solid was extracted with chloroform and precipitated
with methanol, and ltered off to afford a black solid. Yield:
120 mg (80%).
Synthesis of PIBDF-TBT. The following synthesis was based
5 M. Li, D. K. Mangalore, J. Zhao, J. H. Carpenter, H. Yan,
H. Ade, H. Yan, K. Mullen, P. W. M. Blom, W. Pisula,
D. M. de Leeuw and K. Asadi, Nat. Commun., 2018, 9, 451.
6 B. Wang, T. P. Huynh, W. Wu, N. Hayek, T. T. Do,
J. C. Cancilla, J. S. Torrecilla, M. M. Nahid, J. M. Colwell,
O. M. Gazit, S. R. Puniredd, C. R. McNeill, P. Sonar and
H. Haick, Adv. Mater., 2016, 28, 4012–4018.
on a modied literature procedure. To a 20 mL dry Schlenk ask
was added 7 (150 mg, 0.0868 mmol), 4,7-bis(5-(trimethyl-
stannyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (55 mg, 0.0868
mmol) and tri(o-tolyl)phosphine (4.3 mg, 16 mol%). Aer
degassing and relling argon for 3 times, a solution of
tris(dibenzylideneacetone)-dipalladium (2.4 mg, 3 mol%) in dry
chlorobenzene (10 mL) was added under an argon atmosphere.
7 W. Lv, H. Liu, W. Wang, E. Yang, H. Zhen and Q. Ling, RSC
Adv., 2017, 7, 18384–18391.
ꢀ
The ask was sealed and stirred for at 130 C for 48 h. Four
drops of bromobenzene was added and the reaction was kept at
8 C. Soldano, R. D'Alpaos and G. Generali, ACS Photonics, 2017,
4, 800–805.
ꢀ
130 C for an additional 12 h. The reaction mixture was cooled
to room temperature and added dropwise into methanol (100
mL) and the mixture was stirred for 1 h. Then the precipitated
9 S. E. Root, S. Savagatrup, A. D. Printz, D. Rodriquez and
D. J. Lipomi, Chem. Rev., 2017, 117, 6467–6499.
product was collected by ltration, and subjected to consecutive 10 I. Kang, H. J. Yun, D. S. Chung, S. K. Kwon and Y. H. Kim, J.
Soxhlet extractions with methanol, acetone, and hexane. The
Am. Chem. Soc., 2013, 135, 14896–14899.
remaining solid was extracted with chloroform and precipitated 11 Y. Ji, C. Xiao, Q. Wang, J. Zhang, C. Li, Y. Wu, Z. Wei, X. Zhan,
with methanol, and ltered off to afford a black solid. Yield:
30 mg (80%).
W. Hu, Z. Wang, R. A. Janssen and W. Li, Adv. Mater., 2016,
28, 943–950.
1
1
1
1
2 Y. Li, P. Sonar, L. Murphya and W. Hong, Energy Environ. Sci.,
2
013, 6, 1684–1710.
3 B. Sun, W. Hong, H. Aziz and Y. Li, Polym. Chem., 2015, 6,
38–945.
Author contributions
9
The manuscript was written through contributions of all
authors. All authors have given approval to the nal version of
the manuscript.
4 J. Yang, H. Wang, J. Chen, J. Huang, Y. Jiang, J. Zhang, L. Shi,
Y. Sun, Z. Wei, G. Yu, Y. Guo, S. Wang and Y. Liu, Adv. Mater.,
2017, 29, 1606162.
1
5 K. Guo, J. Bai, Y. Jiang, Z. Wang, Y. Sui, Y. Deng, Y. Han,
H. Tian and Y. Geng, Adv. Funct. Mater., 2018, 28, 1801097.
6 B. Kang, R. Kim, S. B. Lee, S. K. Kwon, Y. H. Kim and K. Cho,
J. Am. Chem. Soc., 2016, 138, 3679–3686.
Conflicts of interest
1
There is no conicts to declare.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 38919–38928 | 38927

View Article Online
RSC Advances
Paper
1
7 M. J. Sung, A. Luzio, W.-T. Park, R. Kim, E. Gann, 26 Z. Yan, B. Sun and Y. Li, Chem. Commun., 2013, 49, 3790–
F. Maddalena, G. Pace, Y. Xu, D. Natali, C. de Falco, 3792.
L. Dang, C. R. McNeill, M. Caironi, Y.-Y. Noh and 27 G. Zhang, P. Li, L. Tang, J. Ma, X. Wang, H. Lu, B. Kang,
Y.-H. Kim, Adv. Funct. Mater., 2016, 26, 4984–4997. K. Cho and L. Qiu, Chem. Commun., 2014, 50, 3180–3183.
8 Z. Chen, W. Zhang, J. Huang, D. Gao, C. Wei, Z. Lin, L. Wang 28 X. Wang, H. H. Choi, G. Zhang, Y. Ding, H. Lu, K. Cho and
and G. Yu, Macromolecules, 2017, 50, 6098–6107. L. Qiu, J. Mater. Chem. C, 2016, 4, 6391–6400.
9 Z. Chen, Y. Zheng, H. Yan and A. Facchetti, J. Am. Chem. Soc., 29 Y. He, C. Guo, B. Sun, J. Quinn and Y. Li, Polym. Chem., 2015,
009, 131, 8–9. 6, 6689–6697.
0 W. Zhou, Y. Wen, L. Ma, Y. Liu and X. Zhan, Macromolecules, 30 G. Zhang, J. Guo, M. Zhu, P. Li, H. Lu, K. Cho and L. Qiu,
012, 45, 4115–4121. Polym. Chem., 2015, 6, 2531–2540.
1 J.-H. Dou, Y.-Q. Zheng, T. Lei, S.-D. Zhang, Z. Wang, 31 H. D. Pham, H. Hu, K. Feron, S. Manzhos, H. Wang,
1
1
2
2
2
2
W.-B. Zhang, J.-Y. Wang and J. Pei, Adv. Funct. Mater.,
014, 24, 6270–6278.
2 J. H. Dou, Y. Q. Zheng, Z. F. Yao, T. Lei, X. Shen, X. Y. Luo,
Z. A. Yu, S. D. Zhang, G. Han, Z. Wang, Y. Yi, J. Y. Wang
and J. Pei, Adv. Mater., 2015, 27, 8051–8055.
Y. M. Lam and P. Sonar, Sol. RRL, 2017, 1, 1700105.
32 J. Eggert Carl ´e , J. Wenzel Andreasen, M. Jørgensen and
F. Christian Krebs, Sol. Energy Mater. Sol. Cells, 2010, 94,
774–780.
2
2
33 J. Kim and T. Swager, Nature, 2001, 411, 1030–1034.
23 X. Zhou, N. Ai, Z.-H. Guo, F.-D. Zhuang, Y.-S. Jiang, 34 N. Zhou, X. Guo, R. P. Ortiz, S. Li, S. Zhang, R. P. Chang,
J.-Y. Wang and J. Pei, Chem. Mater., 2015, 27, 1815–1820.
A. Facchetti and T. J. Marks, Adv. Mater., 2012, 24, 2242–
24 J. H. Dou, Y. Q. Zheng, Z. F. Yao, Z. A. Yu, T. Lei, X. Shen,
2248.
X. Y. Luo, J. Sun, S. D. Zhang, Y. F. Ding, G. Han, Y. Yi, 35 J. Kim, A. R. Han, J. Hong, G. Kim, J. Lee, T. J. Shin, J. H. Oh
J. Y. Wang and J. Pei, J. Am. Chem. Soc., 2015, 137, 15947–
5956.
5 Y. Deng, B. Sun, Y. He, J. Quinn, C. Guo and Y. Li, Chem.
and C. Yang, Chem. Mater., 2014, 26, 4933–4942.
36 J. Shin, H. A. Um, D. H. Lee, T. W. Lee, M. J. Cho and
D. H. Choi, Polym. Chem., 2013, 4, 5688–5695.
1
2
Commun., 2015, 51, 13515–13518.
37 D. Gao, Z. Chen, J. Huang, W. Zhang, C. Wei, Z. Lin, D. Li
and G. Yu, J. Mater. Chem. C, 2017, 5, 3568–3578.
38928 | RSC Adv., 2018, 8, 38919–38928
This journal is © The Royal Society of Chemistry 2018