Improved performance of solution-processed n-type organic field-effect transistors by regulating the intermolecular interactions and crystalline domains on macroscopic scale

Article abstract of DOI:10.1021/cm501780p

The development of four new n-channel naphthalene diimide (NDI) and perylene diimide (PDI) copolymers (NDI-Ph, NDI-BT, PDI-Ph, and PDI-BT) and their solution processed thin film transistor (TFT) devices are reported. Remarkable enhancements in the electron transport behavior for all the four copolymers were achieved on improving the intermolecular interactions in their thin film structures. These solution processable n-type copolymers having NDI and PDI backbone were synthesized in high yields (83-86%) by palladium catalyzed Suzuki coupling reactions, and their excellent solubility in several organic solvents allowed their deposition in organic thin film transistor (OTFT) devices from solution directly. Since these copolymers possess crystalline domains, annealing their films induced crystalline phases in the thin film structures with a very high degree of enhancement in crystallinity that was more prominent for PDI copolymers as compared to NDI derivatives. This resulted in significant enhancement in the intermolecular interactions in the thin film state on the macro scale, facilitating improved and higher charge carrier transport in annealed devices as compared to the as-spun devices that have lesser crystalline phases. The transport measurements performed for these four copolymers helped us to understand the difference in transport mechanism between D-A and A-A moiety and confirmed that tuning the thin film structures and the electronic properties by modifying the copolymer backbone structures as well as annealing them at appropriate temperature has profound implications on the level of improvement in electron transport behavior. The enhancement in μ_e values for all four copolymers is very large for any reported n-type copolymers. It is observed that the extended conjugation in the four copolymer structures, the efficient intermolecular interactions in the thin film state, and the formation of crystalline domains in the copolymers after annealing are, in principle, responsible for the enhanced device performance. These copolymers demonstrated electron mobility enhancement of several orders and are reported to be as high as 0.8 cm² V^-1 s^-1 and 0.2 cm ² V^-1 s^-1 with I_on/I_off ratios 10⁵ for NDI-Ph and NDI-BT, while those of PDI-Ph and PDI-BT are 0.04 cm² V^-1 s^-1 and 0.032 cm² V^-1 s^-1, respectively, with I_on/I_off ratios of 10³-10⁴.

Full text of DOI:10.1021/cm501780p

Article
pubs.acs.org/cm
Improved Performance of Solution-Processed n‑Type Organic Field-
Effect Transistors by Regulating the Intermolecular Interactions and
Crystalline Domains on Macroscopic Scale
Suresh Vasimalla,§ Satyaprasad P. Senanayak,‡ Meenakshi Sharma,§ K. S. Narayan,*,‡
and Parameswar Krishnan Iyer*,§,†
†
§Department of Chemistry and Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781039, Assam,
India
‡Jawaharlal Nehru Centre for Advanced Scientific Research Jakkur, Bangalore 560064, India
S
* Supporting Information
ABSTRACT: The development of four new n-channel
naphthalene diimide (NDI) and perylene diimide (PDI)
copolymers (NDI-Ph, NDI-BT, PDI-Ph, and PDI-BT) and
their solution processed thin film transistor (TFT) devices are
reported. Remarkable enhancements in the electron transport
behavior for all the four copolymers were achieved on
improving the intermolecular interactions in their thin film
structures. These solution processable n-type copolymers
having NDI and PDI backbone were synthesized in high
yields (83−86%) by palladium catalyzed Suzuki coupling
reactions, and their excellent solubility in several organic solvents allowed their deposition in organic thin film transistor (OTFT)
devices from solution directly. Since these copolymers possess crystalline domains, annealing their films induced crystalline
phases in the thin film structures with a very high degree of enhancement in crystallinity that was more prominent for PDI
copolymers as compared to NDI derivatives. This resulted in significant enhancement in the intermolecular interactions in the
thin film state on the macro scale, facilitating improved and higher charge carrier transport in annealed devices as compared to
the as-spun devices that have lesser crystalline phases. The transport measurements performed for these four copolymers helped
us to understand the difference in transport mechanism between D−A and A−A moiety and confirmed that tuning the thin film
structures and the electronic properties by modifying the copolymer backbone structures as well as annealing them at appropriate
temperature has profound implications on the level of improvement in electron transport behavior. The enhancement in μe
values for all four copolymers is very large for any reported n-type copolymers. It is observed that the extended conjugation in the
four copolymer structures, the efficient intermolecular interactions in the thin film state, and the formation of crystalline domains
in the copolymers after annealing are, in principle, responsible for the enhanced device performance. These copolymers
demonstrated electron mobility enhancement of several orders and are reported to be as high as 0.8 cm2 V−1 s−1 and 0.2 cm2 V−1
s−1 with Ion/Ioff ratios 105 for NDI-Ph and NDI-BT, while those of PDI-Ph and PDI-BT are 0.04 cm2 V−1 s−1 and 0.032 cm2 V−1
s−1, respectively, with Ion/Ioff ratios of 103−104.
performances have appeared.12,13 For the electronics industries
that rely on organic materials, development of low-cost solution
processable n-type semiconducting materials with high
performance remains a challenge. One of the major drawbacks
INTRODUCTION
■
Polymer field effect transistors (PFETs) have attracted
significant interest during the past several years because they
have contributed significantly in the fabrication of low cost
organic optoelectronic devices such as p−n junctions,1,2
with n-type materials is their instability at ambient conditions.
photovoltaic cells,3,4 complementary circuits,5 and driving
In this regard, we demonstrate n-type PFETs with electron
mobilities ∼1 cm2 V−1 s−1 which are relatively stable under
ambient conditions and could be aligned to improve the
transport properties by several orders.
organic light emitting diodes (OLED) for flexible display.6,7
All these applications require both hole transporting (p-type)
and electron transporting (n-type) materials with suitable
physical, chemical, electrical, and photochemical properties.
The n-type materials development has lagged behind the p-type
materials over the years since the majority of studies reported
so far have focused on p-type semiconductor materials and
devices.8,9 In recent years, few reports on the development of
ambipolar10,11and n-type semiconductors with encouraging
There are numerous building blocks being developed to
fabricate air stable n-channel semiconducting materials, while
Received: May 17, 2014
Revised: June 13, 2014
Published: June 18, 2014
© 2014 American Chemical Society
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rylene based polymers, especially naphthalene,14,15 peryle-
ne,16,17and pyromellitic tetracarboxylic diimides18,19are the
most promising. Naphthalene (NDI) and perylene diimides
(PDI) possess aromatic planar conjugated structure and are
favorable for good charge transport with performances
comparable to p-type semiconducting materials. These classes
of materials have an excellent combination of thermal, optical,
redox, and electrical properties as well as very high moisture
and oxygen resistance.20 The functionalization at the diimide
position with aliphatic alkyl chain, either branched or long
chain, provides good solubility in common organic solvents,
enhancing their processability.21 Additionally, the possibility to
obtain interdigitated structures limits the percolation of
moisture to the conjugated core, thereby making it stable for
ambient performance.22 Many NDI and PDI based small
molecules23,24 and oligomers25,26 have been reported with self-
assembling properties and good charge transporting ability. Few
newly developed materials showing high electron mobility
include: ladder type naphthalene,27 perylene diimide,28 and D−
A type copolymers of NDI, PDI with thiophene,29,30
bithiophene,31 dithienothiophene,32 phenothiazine,33 dithieno-
pyrrole,34and phenylene.35High electron mobility of up to
0.45−0.85 cm2 V−1 s−1 was reported with NDI/PDI derivative
and bithiophene copolymers36 and NDI−vinylene−thiophene
copolymers which exhibited mobilities of up to 1.8 cm2 V−1
s−1.37 In PDI based copolymer, electron mobilities of up to
0.075 cm2 V−1 s−1 have been possible with the dithienothio-
phene unit,38 and with liquid crystalline copolymers.39,40
Despite these efforts it remains unclear why D−A based NDI
and PDI copolymers have better μFET. In addition, the role of
anisotropic properties, improving the structural order in
solution-processed n-type polymers, and the enhanced film
morphology on the macroscopic scale are also very crucial for
obtaining high-performance OFETs. In this manuscript, we
have synthesized four (thiophene-free) copolymers based on
phenyl and/or benzothiadiazole as copolymerizing units with
linear C18 substituted NDI and PDI derivatives in order to gain
a relatively higher understanding on the roles and features of
different D−A and A−A structures and carefully study the
transport behavior in these polymers. We also demonstrate that
enhancing the polymer anisotropic properties by annealing at
appropriate temperature significantly improves the intermolec-
ular interactions as well as the formation of crystalline domains
on the macroscopic scale, facilitating large enhancement in
charge carrier transport. Since conjugated polymers inherently
possess anisotropic characteristics, they can be realized in
device applications only when these conjugated chains are
aligned in such a manner that the crystallites do not remain
entrenched within the amorphous polymer phases. Tuning the
thin film structures of the polymers by annealing facilitates the
conjugated chains alignment over larger macrodomains and
enhances the structure−property relationships by demonstrat-
ing highly improved charged transport behavior here. Hence,
developing processing strategies in these materials to overcome
the amorphous domains and recognizing the importance of
conjugated backbone macrostructure chain alignment are, in
principle, extremely crucial for device improvements.
further purification. The dielectric material hydroxyl free divinylte-
tramethylsiloxanebis-benzocyclobutene (BCB) is obtained from Dow
Chemicals. 2,6-Dibromo naphthalene tetracarboxylic dianhydride
(NTCDA-Br241and 1,7-dibromo-3,4:9,10-perylene teracarboxylic dia-
nhydride (PTCDA-Br2), N,N′-bis(octadecyl)-1,7-dibromo-3,4:9,10-
perylenetetracarboxylicdianhydride, and N,N′-bis(octadecyl)-2,6-di-
bromonaphthalene-1,4,5,8-tetracarboxylic dianhydride were synthe-
sized following the published literature procedure (Scheme 1).31 All
a
Scheme 1. Synthesis of Monomers and Copolymers
a
(i) o-Xylene, propionic acid, n-octadecylamine, 140 °C, 6 h; (ii)
Pd(PPh3)4, K2CO3, 2:1 mixture of THF/H2O, aliquat, 80 °C, 36 h,
1,4-benzene diboronic acid, or 2,1,3-benzothiadiazole-4,7-bis(boronic
acid pinacol ester).
1
the polymers were synthesized by Suzuki coupling reaction. H NMR
and 13C NMR were recorded on Varian AS 400 MHz and Bruker 600
MHz NMR spectrometers. MALDI-TOF-MS experiments were done
on AB SCIEX, U.S.A., instrument with a4800 plus MALDI TOF/TOF
Analyzer. Elemental analysis was done on CHNS Analyzer,
PerkinElmer, 2400 (U.S.A.). UV−vis spectra were recorded on a
PerkinElmer Model Lambda-25 spectrophotometer. Photolumines-
cence spectra were recorded on Varian Cary Eclipse spectropho-
tometer. Electrochemical measurements were carried out under argon
on a deoxygenated solution of tetra n-butyl ammonium perchlorate
(0.1M) in acetonitrile using a CH instruments Model 700D series
Electrochemical workstation. A glassy carbon as working electrode,
platinum wire as counter electrode, and Ag/Ag+ as reference
electrodes were used. Thermogravimetric analysis (TGA) was
performed on Mettler Toledo, model TG/SDTA 851 e, thermogravi-
metric analyzer under a nitrogen flow at a heating rate of 10 °C/min.
Differential scanning calorimetry was done on a Mettler Toledo,
Model DSC 1, Stare system. The gel permeation chromatography
measurements were performed on a waters 515 chromatograph
connected to waters 2414 refractive index detector using THF as
eluent and polystyrene standards as calibrants. AFM images were taken
by an Agilent 5500-STM instrument, and FESEM images were
recorded in a Sigma Carl ZEISS scanning electron microscope. XRD
analyses were done by Bruker D8 Discover Diffractometer. A Leica
polarizable optical microscope was used to study the anisotropic
morphology of the copolymer films.
Preparation of Films for AFM, FESEM, XRD, and POM. The
copolymers were spin coated on glass substrates at 500 rpm for 2 min
and then heated for 30 min at 50 °C to evaporate the solvent. FESEM
images were recorded with these films. Similarly, AFM images of the
films were recorded for as-spun and annealed (30 min at 150 °C)
films. For XRD measurement the films were prepared under the same
conditions as followed for the device fabrication. For POM studies, the
copolymers were dissolved in chloroform and drop-casted on a glass
EXPERIMENTAL SECTION
■
Materials and Characterization. Perylene tetracarboxylic dia-
nhydride, naphthalene tetracarboxylic dianhydride, 1,4-benzene
diboronic acid, and 2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol
ester) were purchased from Sigma-Aldrich and were used without
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Table 1. GPC Characterization and % Yields of Polymers, Electrochemical and Optical Band Gaps, and Thermal Data
a
a
a
b
b
c
d
polymer
Mn (Da)
Mw (Da)
PDI (yield %)
HOMO (eV)
LUMO (eV)
Eg (eV)
Eg (eV)
Td (°C)
NDI-Ph
NDI-BT
PDI-Ph
PDI-BT
6811
5171
13579
6627
1.99 (84%)
1.74 (85%)
1.53 (83%)
1.16 (86%)
−5.173
−5.14
−5.89
−5.49
−3.15
−3.03
−3.49
−3.08
2.02
2.10
2.40
2.41
1.55
1.72
1.57
341
350
363
385
16082
30619
24657
35739
1.59
b
a
Mn, Mw, number-average and weight-average molecular weights, and PDI, polydispersity index, are obtained from GPC. HOMO and LUMO
estimated from the onset oxidation and reduction potentials, respectively, assuming the absolute energy level of ferrocene/ferrocenium couple to be
c
d
4.8 eV below vacuum. HOMO−LUMO gap estimated by electrochemistry. Optical band gap calculated from absorption. Td: degradation
temperature by TGA.
Figure 1. (a) UV−vis spectra in THF (10−5 M), (b) UV−vis spectra in thin films (∼50−60 nm), (c) PL spectra in THF (10−5 M) of polymers. (d)
TGA chromatograms carried out at a rate of 5 °C/min. (e) DSC chromatograms of the second cycle of the polymers carried out at a rate 5 °C/min
under nitrogen atmosphere. (f) Cyclic voltammograms of the drop casted polymer films (∼100 nm) on carbon working electrode performed at a
scan rate of 0.5 mV/s in 0.1 M TBAP in acetonitrile under argon atmosphere.
slide and heated for 2 h at 50 °C to remove residual solvent. During
the POM annealing experiments the films were heated from 50 to 180
°C at a rate of 5 °C/min and then cooled to 50 °C at the same rate,
and the images were recorded at 50× resolution.
performed using standard Keithley 4200 semiconductor character-
ization systems and cross-checked with measurement using Keithley
2400 Source meters and high-impedance Keithley 6514 electrometer.
Typical FET performance is estimated from average characteristics of
five to seven devices.
Device Fabrication and Characterization. Detailed transport
study of the NDI and PDI copolymers was performed using bottom
gate top contact FET geometry. For device fabrication, Al metal is
coated as the gate electrode by physical vapor deposition (10−6 mbar,
40 nm thick) on RCA cleaned glass substrate. Dielectric layer hydroxyl
free divinyltetramethylsiloxane bis-benzocyclobutene (BCB) is then
coated at 1000 rpm for 1 min and annealed in N2 filled glovebox
atmosphere at 290 °C. The resultant dielectric layer had a Ci ∼ 4 nF/
cm2 measured directly using the C−V meter inbuilt in Keithley 4200
semiconductor parameter analyzer. Typical dielectric thickness is
estimated to be around 0.5−0.6 μm as measured using Filmetrics
(F20EXR) thickness measurement setup. The surface of the
dielectric was coated (1500 rpm for 30 s) with a thin monolayer of
hexamethyldisilazane (HMDS) and annealed at 110 °C for 2 h. The
copolymer layer is then introduced from the solution phase (15 mg/
mL in THF) by spin coating at 1000 rpm for 1 min and annealed at
different temperatures (110−150) °C for 30 min to optimize the
performance. In order to facilitate electron injection Al source-drain
electrodes were then coated (10−6 mbar, 40 nm thick) to complete the
device fabrication. We observe an injection limited response with
lower μFET for Au S-D electrodes. The device characterizations were
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The syntheses of four
new conjugated copolymers were achieved in 83−86% yields
(Scheme 1). The synthesis details of the monomers and
copolymers are reported in the Supporting Information. The
Suzuki coupling reaction of dibrominated NDI and PDI
derivatives with benzene-1,4-diboronic acid and 2,1,3-benzo-
thiadiazole bisboronic ester resulted in the formation of NDI-
Ph, NDI-BT, PDI-Ph, and PDI-BT copolymers. All the
monomers and polymers were well-characterized by NMR
(Supporting Information Figures S1−S2, S4−S9, and S11−
S14), MALDI (Supporting Information Figure S3 and S10)/gel
permeation chromatography (GPC), elemental analysis, DSC,
TGA, PL, UV−vis, and cyclic voltammetry and their
morphologies studied by AFM, FESEM, POM, and XRD
techniques (Figure 2), (Supporting Information Figure S15).
The TGA analysis reveals that the copolymers are stable up to
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350 °C. These polymers were fabricated and tested for solution
processable polymer field effect transistors (PFETs). It was
observed that these NDI and PDI derivatives exhibit field effect
electron mobilities (μFET) that are as high as 0.8 cm2 V−1
s−1(NDI-Ph), 0.2 cm2 V−1 s−1(NDI-BT), 0.04 cm2 V−1 s−1
(PDI-Ph), and 0.032 cm2 V−1 s−1 (PDI-BT) with a current
modulation in the range of 104−105. High μFET can be
attributed to the enhanced π−π stacking originating from the
intermolecular overlap of the D−A orbitals. Table 1 represents
their molecular weight and polydispersity index (PDI) as
obtained from GPC in THF (polystyrene standard). Due to the
presence of alkyl chains all four copolymers were readily soluble
in common organic solvents such as toluene, THF, chloroform,
dichloromethane, chlorobenzene, and xylenes and could be
solution processed onto desired substrates for achieving
smooth films.
nitrogen flow. All the polymers were very stable, and their onset
degrading temperatures were found to be in the range of 341−
385 °C (Figure 1d). The onset thermal degradation temper-
atures of all the polymers are shown in Table 1. In DSC, the
polymers were heated from 25 to 325 °C and then again cooled
to 25 °C with a rate of 5 °C/min for three cycles under
nitrogen flow. The chromatograms of the second cycle are
shown in Figure 1e. NDI-Ph copolymer shows an endothermic
peak at 145 °C in the second heating cycle and an exothermic
peak at 132 °C in the second cooling cycle which indicates the
melting and crystalline temperatures of the polymer,
respectively. Similarly, NDI-BT copolymer shows an endother-
mic peak at 142 °C and an exothermic peak at 125 °C in the
second heating and cooling cycles corresponding to the melting
and crystalline temperatures respectively showing that both
polymers are crystalline in nature.
Photophysical Properties. To understand the aggregation
mechanism, the UV−visible spectra of the copolymers were
studied in THF at a concentration of 10−5 M. NDI-BT shows
absorption peaks at 360, 371, and 449 nm, which are assigned
to basic naphthalene diimide core transitions while the peak at
538 nm is due to general polymer aggregation. Another peak
observed at 438 nm is generally observed for the
benzothiadiazole moiety. NDI-Ph shows peaks at 360 nm and
370 nm which are assigned to the basic naphthalene core
transitions and a peak of polymer aggregation at 540 nm. The
PDI-BT gives absorption bands at 550 nm, 525 nm, and 489
nm and those of PDI-Ph are at 548 nm, 513 nm, and 485 nm
which are assigned to the general PDI based (So−So, So−S1,
So−S2) transitions (Figure 1a).42The absorption spectra of the
spin coated polymer films on glass (Figure 1b) show a red shift
in the peaks for all the copolymers as compared to the solution
state absorption peaks. For NDI-BT copolymer the 538 nm
peak was significantly red-shifted to around 625 nm with similar
red shift of up to 371, 391, and 449 nm from the initial
positions of 360, 371, and 438 nm, respectively. Similarly, the
540 nm peak of the NDI-Ph copolymer was shifted to 600 nm
caused by the strong polymer interchain interaction. The other
peaks observed at 360 nm and 370 nm for NDI-Ph are also
shifted to 370 nm and 390 nm in thin films as compared to the
solution spectra. The shoulders at 675 nm in PDI-BT and 660
nm in PDI-Ph which are assigned to π−π polymer interactions
are observed to be shifted to 739 and 721 nm, respectively, in
thin films indicating the general polymer aggregation in solid
state. All other peaks of PDI-BT are also found to be red-
shifted. The optical band gaps calculated from the absorption
edge for NDI-Ph and NDI-BT were found to be 1.55 eV and
1.72 eV and those of PDI-BT, PDI-Ph copolymers are 1.57 and
1.59 eV, respectively (Table 1). PL spectroscopy measurements
of polymers were performed in dilute 10−5 M THF solutions.
The NDI-BT copolymer shows an emission peak at 480 nm
which corresponds to the main absorption peak at 371 nm of
the naphthalene core transition. NDI-Ph polymer exhibits an
emission peak at around 472 nm corresponding to that of 370
nm in the absorption spectra. The PDI-BT polymer shows
emission peaks at 537 nm and 577 nm corresponding to 525
nm and 550 nm peaks in UV (Figure 1c). PDI-Ph polymer
gives emission peaks at 538 and 575 nm corresponding to 513
and 548 nm absorption peaks in THF.
The PDI-Ph copolymer did not undergo any change during
the heating or cooling process. The PDI-BT copolymer shows
an endothermic peak at 164 °C during heating which
corresponds to melting and exothermic peak of crystallization
at 110 °C during cooling process, which indicates the crystalline
behavior of the copolymer. Hence, the DSC reveals that NDI-
Ph, NDI-BT, and PDI-BT copolymers possess crystalline
domains within their structures.
Electrochemical Properties. To estimate the HOMO and
LUMO energy levels of the copolymers, cyclic voltammetry
(CV) was performed. The copolymers were drop casted on
carbon working electrode. Silver as the reference electrode and
platinum wire as the counter electrode were used in these
experiments. The HOMO and LUMO levels were estimated by
the onset oxidation and onset reduction peaks assuming the
absolute energy level of the ferrocene/ferrocenium couple to be
4.8 eV below vacuum. 0.1 M tetrabutylammonium perchlorate
(TBAP) as electrolyte in deoxygenated acetonitrile and
ferrocene as standard were used in argon atmosphere. The
HOMO/LUMO energies were calculated by substituting the
onset reduction and onset oxidation peak values in ELUMO
[(Ered − E1/2(ferrocene)) + 4.8] eV, EHOMO = [(Eox
=
−
E1/2(ferrocene)) + 4.8] eV. The LUMO levels of NDI-BT,
NDI-Ph, PDI-BT, and PDI-Ph are −3.03 eV, −3.15 eV, −3.49
eV, and −3.08 eV, respectively. Similarly HOMO levels of the
copolymers NDI-BT, NDI-Ph, PDI-BT, and PDI-Ph calculated
from CV are −5.14 eV, −5.173 eV, −5.89 eV, and −5.49 eV,
respectively (Figure 1f). The energy levels and band gaps
calculated from cyclic voltammetry of thin films are represented
in Table 1
Morphology Characterization. Polymer thin films were
characterized by AFM, FESEM, XRD, and polarized optical
microscopy (POM) to obtain further information on
crystallinity and morphology which is relevant for device
characterization. Out-of plane XRD measurements were
performed on thin films of the NDI and PDI copolymers
(Figure 2) to obtain information regarding the molecular
reorganization. As-spun polymeric films reveal π−π stacking in
the range of 4.02−4.18 Å and a broad primary diffraction peaks
at 2θ = 4.09−4.34° corresponding to the d-spacing values of
18.51−27.84 Å. Annealing of the thin films reduced the d-
spacing to a range of 18.26−24.65 Å and π−π stacking distance
to 4.0−4.16 Å. This indicates that denser interdigitated lamellar
packing is obtained by annealing the polymer films, which is an
inherent anisotropic characteristic of conjugated backbone
possessing polymers. It should be noted that these magnitudes
of d-spacing and π−π stacking obtained in these polymers are
Thermal Properties. The thermal properties of the
polymers were studied by thermogravimetric analysis (TGA)
and differential scanning calorimeter (DSC). TGA analysis was
done in the range 25−800 °C at a rate of 5 °C/min under
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relatively lower than other nonthiophene based NDI polymers
reported35 which gets reflected in the enhanced transport
properties. In addition, annealing the polymer films decreases
the fwhm of the peaks (Table S1 in Supporting Information)
corresponding to π−π stacking as well as the d-spacing pointing
to the enhanced morphological ordering with annealing.
AFM imaging was carried out on optical quality films coated
under similar condition as those used in device studies. AFM
images for as-spun films do not exhibit any crystallite formation
(Figure 3a,c,e,g). However, it was observed that annealing the
thin films at 150 °C introduces crystallites in the film (Figure
3b,d,f,h). This trend is consistent with DSC measurement
which shows crystallization at ∼150 °C. It should be noted that
the degree of enhancement of crystallinity is more prominent
for PDI films compared to NDI copolymer films. This was
substantiated from observations of a larger number of
crystallites obtained in case of PDI-Ph and PDI-BT compared
to NDI-Ph and NDI-BT. Furthermore, FE-SEM measurements
were also performed on these copolymers to examine the thin
film microstructure. FE-SEM images show interconnected
crystalline fibrillar network of the copolymers which supports
enhanced transport, weakly affected by defects.43,44 In addition,
POM studies performed on films of the these four copolymers
confirmed the appearance of macroscale crystalline domains
after annealing above 150 °C (Figure 4) due to their inherent
structural anisotropic. These features can be directly correlated
to large enhancement of μFET upon thermal annealing in PDI
derivatives as compared to the NDI derivatives.
Figure 2. Out of plane X-ray diffraction patterns of (a) as-spun and
(b) annealed (150 °C) copolymer films.
Figure 3. AFM topography image (5 μm × 5 μm) for the copolymer films: NDI-Ph (a) as-spun and (b) annealed; NDI-BT (c) as-spun and (d)
annealed; PDI-Ph (e) as-spun and (f) annealed; PDI-BT (g) as-spun and (h) annealed. Typical FE-SEM images (10 μm × 10 μm) of (i) NDI-Ph, (j)
NDI-BT, (k) PDI-Ph, and (l) PDI-BT films.
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Figure 4. Polarizable optical microscope images of (a) NDI-Ph before annealing and (b) after annealing, (c) NDI-BT before annealing and (d) after
annealing, (e) PDI-Ph before annealing and (f) after annealing, and (g) PDI-BT before annealing and (h) after annealing.
FET Measurements. Top contact bottom gated FETs were
fabricated with all the copolymers in the procedure as described
in the Experimental Section. The devices demonstrated well-
defined linear and saturation behavior with electron only
mobility and ON/OFF ratio exceeding 104 (Figure 5,
feature can be attributed to weaker electron donating ability of
the phenylene group. NDI-Ph showed the highest electron μFET
∼ 0.8 cm2 V−1 s−1 whereas FETs fabricated with NDI-BT had
μFET ∼ 0.2 cm2 V−1 s−1 after annealing at 150 °C. The PDI
derivatives, PDI-Ph and PDI-BT, also demonstrated μFET
∼
0.04 cm2 V−1 s−1 and 0.032 cm2 V−1 s−1 respectively, upon
annealing up to 150 °C. Annealing increases the μFET by several
orders, which are among the highest enhancement values for
NDI and PDI based copolymers (Table 2). The observed
Table 2. Performance Parameters of OFETs Obtained under
Different Annealing Conditions
annealing
temperature
average μFET
maximum μFET
polymers
NDI-Ph
(°C)
(cm2 V−1 s−1
)
(cm2 V−1 s−1
0.8
)
Ion/Ioff
∼ 105
as-spun
110
0.1
0.4
150
0.6
NDI-BT
PDI-Ph
PDI-BT
as-spun
110
0.007
0.05
0.08
0.005
0.008
0.015
0.001
0.004
0.01
0.2
>104
>104
>104
150
as-spun
110
0.04
0.032
150
as-spun
110
150
Figure 5. Typical output and transconductance characteristics for
FETs fabricated under vacuum with (a and b) NDI-Ph and (c and d)
PDI-Ph active layer and BCB as the dielectric (Ci ∼ 2−4 nF/cm2).
Typical device dimensions are W ≈ 1 mm and L ≈ 60−100 μm. Inset
(top) shows the schematic of the device.
enhancement of μFET is consistent with the occurrence of
crystallites with annealing in the microscopic images and a
crystalline transition temperature in DSC. In addition, XRD
measurements on these thin films also indicate a decrease in d-
spacing and π−π stacking distance with annealing for all the
polymers. Specifically, it was also observed that with annealing
the decrease in d-spacing and π−π stacking distance is higher
for benzothiadiazole based molecules compared to phenylene
substituted copolymers which corroborates with a larger change
in the μFET.
Supporting Information Figure S16). A typical plot showing
FET characteristics measured under inert conditions is shown
in Figure 5. Similar FET characteristic with lower μFET is
obtained when the devices are operated under ambient
conditions (Supporting Information Figure S17). Although
molecules with phenylene substitution are of D−A type, hole
transport was not observed even with Au S-D electrodes. This
Upon comparing the NDI and PDI derivatives, it is evident
that the NDI derivatives exhibit μFET values which are at least
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an order of magnitude higher than the corresponding PDI
derivatives. This can be attributed to the enhanced
interdigitated lamellar packing and better π−π stacking in
NDI polymers. Annealed films of NDI polymers exhibit d-
spacing in the range of 18.26−19.82 Å compared to 21.00−
24.65 Å for PDI based polymers. Similarly, the π−π stacking
distance increases from 4.06 Å in NDI-Ph to 4.16 Å in PDI-Ph
polymer (Table 3).
copolymers. The thin film structures and electronic anisotropic
properties in conjugated copolymers can be regulated, which
results in efficient intermolecular interactions, polymer packing,
π−π stacking, and the formation of crystalline domains over
large area after annealing, thereby facilitating large enhance-
ment in charge carrier transport. The optical, thermal, and
redox characterization of the copolymers indicates good
polymeric aggregates, high thermal stability with the tendency
to form crystalline domains, and suitable energetics which
support efficient electron injection. This was further supported
by the morphology patterns observed in the AFM, FESEM, and
POM images. The transport studies performed under different
annealing conditions up to 150 °C demonstrated huge
enhancement in electron μFET with maximum values of 0.8
cm2 V−1 s−1 for the NDI-Ph and 0.04 cm2 V−1 s−1 for the PDI-
Ph polymers. The extended conjugation and efficient
intermolecular interactions observed in thin films, along with
the formation of crystalline domains as also confirmed by DSC
analysis after annealing are collectively responsible for the
enhanced device characteristics. This study conclusively
illustrates that D−A copolymers display better properties
compared to A−A copolymers and demonstrates a route to
develop high-performing, solution processable copolymers.
Table 3. Summary of Variation in π−π Stacking and d
Spacing of the Polymers
as-cast films
annealed films
d (00n) spacing
d (00n) spacing
polymers
NDI-Ph
(00n)
2θ
(Å)
2θ
(Å)
(001)
(002)
π−π
(001)
(002)
π−π
(001)
(002)
π−π
(001)
(002)
π−π
4.33
-
19.85
-
4.34
-
19.82
-
21.25
4.65
-
4.07
18.51
-
21.27
4.71
-
4.06
18.26
-
NDI-BT
PDI-Ph
PDI-BT
21.48
-
4.02
-
21.58
-
4.00
-
6.64
20.84
-
25.91
4.14
-
6.66
20.97
-
25.81
4.12
-
ASSOCIATED CONTENT
■
6.12
21.06
28.11
4.10
6.14
21.05
28.02
4.10
S
* Supporting Information
The synthesis and characterization of the monomers and four
copolymers and scanned NMR and MALDI mass spectra are
included. The typical output and transconductance character-
istics with BT copolymers are also mentioned. This material is
available free of charge via the Internet at http://pubs.acs.org.
Importantly, the observed μFET values are higher than those
of other nonthiophene based NDI polymers and are
comparable to NDIOD-T2.36,45A plot of FET characteristics
obtained under similar conditions at our laboratory with
NDIOD-T2 is shown in Supporting Information Figure S16c.
This comparable μFET can be attributed to a combination of
factors, ranging from the molecular level to an optimum
dielectric-semiconductor interface. The presence of linear alkyl
chains in NDI-Ph and NDI-BT polymer improves the polymer
packing and the π−π stacking. In comparison to the typical d-
spacing of 23−24 Å observed for nonthiophene based NDI the
d-spacings in these NDI polymer films were obtained as 18.26−
19.82 Å,35 indicating better lamellar structure and closely
packed interdigitated structure. In addition, the π−π stacking
distance of the NDI copolymers is in the range of 4.00−4.06 Å
which is comparable to NDIOD-T2 (3.93 Å)46 and better than
other nonthiophene based NDI molecules (4.51 Å).35 Hence,
these molecules have a higher propensity to form ordered
crystallites and interconnected aggregates on the macroscopic
scale which supports enhanced transport as observed for other
high mobility polymers reported recently.43,47 Another
interesting observation in the transport measurement is the
enhanced performance of the D−A type molecules (NDI-Ph or
PDI-Ph) compared to A−A type (NDI-BT or PDI-BT
molecule). This indicates the apparent importance of the
intermolecular D−A type interaction on the charge transport
compared to the geometry and microstructure of the molecule
which can be a general way to achieve high mobility in NDI or
PDI polymeric materials.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: pki@iitg.ernet.in (P.K.I.).
*E-mail: narayan@jncasr.ac.in (K.S.N.).
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
■
The authors thank the Department of Science and Technology
(DST), India (Nos. DST/TSG/PT/2009/11, DST/TSG/PT/
2009/23, DST/SB/S1/PC-020/2014), and IGSTC/MPG/
PG(PKI)/2011A/48 for financial support. The Central Instru-
ments Facility, IIT Guwahati, is acknowledged for microscopy
and spectroscopy facilities. We thank Prof. S. S. Ghosh, IIT
Guwahati, for MALDI analysis and Dr. N. H. Khan, CSMCRI,
India, for elemental analysis. S.P.S. acknowledges CSIR India
for a fellowship.
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