Ladder-type heteroacene polymers bearing carbazole and thiophene ring units and their use in field-effect transistors and photovoltaic cells

Article abstract of DOI:10.1039/c0jm01897j

A family of ladder-type π-excessive conjugated monomer (dicyclopentathienocarbazole (DCPTCz)) integrating the structural components of carbazole and thiophene into a single molecular entity is synthesized and polymerized by oxidative coupling to yield pol

Full text of DOI:10.1039/c0jm01897j

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Ladder-type heteroacene polymers bearing carbazole and thiophene ring units
and their use in field-effect transistors and photovoltaic cells
Ravi Kumar Cheedarala,a Gi-Hwan Kim,a Shinuk Cho,b Junghoon Lee,a Jonggi Kim,a Hyun-Kon Song,a
a
Jin Young Kim and Changduk Yang
a
*
Received 15th June 2010, Accepted 8th October 2010
DOI: 10.1039/c0jm01897j
A family of ladder-type p-excessive conjugated monomer (dicyclopentathienocarbazole (DCPTCz))
integrating the structural components of carbazole and thiophene into a single molecular entity is
synthesized and polymerized by oxidative coupling to yield poly(dicyclopentathienocarbazole)
(PDCPTCz). Moreover, through the careful selection of 2,1,3-benzothiadiazole unit as a p-deficient
building block, the dicyclopentathienocarbazole-based donor–acceptor copolymer
(poly(dicyclopentathienocarbazole-alt-2,1,3-benzothiadiazole) (PDCPTCz-BT)) is prepared by Suzuki
polycondensation. The optical, electrochemical, and field-effect charge transport properties of the
resulting polymers (PDCPTCz and PDCPTCz-BT) are not only characterized in detail but also their
bulk-heterojunction (BHJ) solar cell in combination with PC71BM are evaluated. The values of
field-effect mobility (m) for PDCPTCz and PDCPTCz-BT are 8.7 ꢀ 10ꢁ6 cm2 Vꢁ1 sꢁ1 and 2.7 ꢀ
10ꢁ4 cm2 Vꢁ1 sꢁ1, respectively. A power conversion efficiency (PCE) of 1.57% is achieved on the
PDCPTCz-BT/PC71BM device, implying that the push–pull copolymers based on ladder-type
dicyclopentathienocarbazole as an electron-donating moiety are promising for organic electronic
devices.
deficient moieties have revealed excellent PCE values (up to
Introduction
3.6%).21 Among these classes of molecules, the poly[N-90-hep-
tadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-ben-
zothiadiazole)] (PCDTBT) showed particularly interesting
achievement of the optimized solar cell with PCE of 6% in BHJ
composites with fullerene derivative22 as well as remarkable
stability at higher temperature for extended periods of time.23
To ensure effective charge carrier transport to the electrodes
and reduce the photocurrent loss in solar cells, high carrier
mobility is also needed for the improved polymer solar cells.12,13
Therefore, another important synthetic strategy toward better
performance of the solar cells is the utilization of ring-fused
aromatic architectures since their rigid and flat p-conjugated
skeletons are beneficial to attaining a high charge carrier mobility
by the formation of densely packed structures owing to the
strong p–p interactions in the solid state.24–27
Development of efficient organic photovoltaics (OPVs), specifi-
cally bulk-heterojunction (BHJ) polymer based solar cells, has
become an active area of research in the recent years because of
their potential for use in low-cost, lightweight, solution-
processable, and flexible large area panels.1–10 Significant
progresses have been made in the BHJ solar cells using poly-
(3-hexylthiophene) (P3HT) as an electron-donor material and
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an elec-
tron-acceptor, leading to power conversion efficiency up to ca.
4–5%.11,12 Although approaches to improving the efficiency of
P3HT/PCBM cells are still being disclosed, a congestion in
conversion efficiency seems to be reached with P3HT since it
harvests photons with wavelengths below ꢂ650 nm, which covers
a small portion of the whole solar spectrum.13 From a funda-
mental electronic band gap point of view, semiconducting
polymers with low band gaps can powerfully harvest solar energy
over a broader spectrum, achieving further improvement of the
solar cell performance.14
We thus direct our attention toward the synthesis of ladder-
type heterocyclic carbazole-cored polymer incorporating thio-
phene subunits of which the fundamental strategies are based on
the following: (i) the planarization by methylene bridges between
carbazole and thiophene units can contribute to reduce the band
gap as well as to enhance the intrinsic charge mobility.
(ii) Furthermore, upon ICT structural design, the ladder-type
heterocycle monomer containing carbazole and thiophene rings
as electron-rich segments is capable of copolymerization with
suitable electron-deficient moieties such as 2,1,3-benzothiadia-
zole. As a ‘win–win’ strategy, this can potentially produce
a copolymer having further low band gap and better charge-
carrier mobility, which most likely induces its outstanding
features in electronic applications.
Conjugated polymers incorporating alternating p-excessive
and p-deficient aromatic moieties depict significant decrease in
their optical band gaps due to the introduction of intramolecular
charge transfer (ICT) structures,15–20 which affords the necessary
motivation for further work toward the preparation of novel
donor-acceptor materials. Recently, donor-acceptor copolymers
based on poly(2,7-carbazole) derivatives with different electron-
aInterdisciplinary School of Green Energy, Ulsan National Institute of
Science and Technology (UNIST), Ulsan, 689-798, South Korea.
E-mail: yang@unist.ac.kr; Fax: +82-52-217-2909; Tel: +82-52-217-2920
bDepartment of Physics and EHSRC, University of Ulsan, Ulsan, 680-749,
South Korea
Herein, we present a straightforward synthesis toward ladder-
type p-conjugated polymeric skeleton containing carbazole and
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Results and discussion
The 2,7-dibromo-N-(2-decyltetradecyl)carbazole (1) was
synthesized according to literature method.28 AlCl3 promoted
Friedel–Crafts acylation of 1 with 4-octylbenzoyl chloride gave
2,7-dibromo-N-(2-decyltetradecyl)-3,6-bis(4-octylbenzoyl)car-
bazole (2), which was coupled under standard Stille-type
conditions with 2-tributylstannylthiophene in the presence of
Pd(PPh3)4 as catalyst to generate the dithiophene carbazole 3
in 88% yield. Treatment of 3 with 6 equivalents of MeLi in
anhydrous THF produced the corresponding dialcohol 4 and
the desired dicyclopentathienocarbazole 5 was obtained by
intramolecular annulation using boron trifluoride etherate
(80%). Subsequently, bromination of 5 by NBS gave the
monomer 6 (88%).
Fig.
1 Structure of dicyclopentathienocarbazole-based polymers
(PDCPTCz and PDCPTCz-BT).
thiophene
(PDCPTCz)) as well as a push–pull copolymer (poly(dicyclo-
pentathienocarbazole-alt-2,1,3-benzothiadiazole) PDCPTCz-
rings
(poly(dicyclopentathienocarbazole)
The poly(dicyclopentathienocarbazole) (PDCPTCz) was
synthesized by oxidative coupling of 5 using FeCl3 in anhydrous
chloroform. To construct a push–pull system, the copolymer
(poly(dicyclopentathienocarbazole-alt-2,1,3-benzothiadiazole)
(PDCPTCz-BT)) containing the combination of the dicyclo-
pentathienocarbazole as electron-donating units and 2,1,3-
benzothiadiazole as electron-accepting units was designed and
synthesized via Suzuki polycondensation (Scheme 1).
BT)) in which the electron-donating units are the dicyclopenta-
thienocarbazole groups and electron-withdrawing moieties are
2,1,3-benzothiadiazole groups (Fig. 1). In the course of our study
to assess a profile along electronic applications of these polymers
(PDCPTCz and PDCPTCz-BT), the characteristics of organic
field-effect transistors (OFETs) as well as BHJ solar cells in
combination with fullerene, in preliminary form, have been
investigated, respectively.
Scheme 1 Synthesis of ladder-type heteroacene polymers (PDCPTCz and PDCPTCz-BT). Reagents and conditions: (i) AlCl3, dichloromethane,
4-octylbenzoyl chloride, 50 ꢃC, 5 h; (ii) 2-tributylstannylthiophene, anhydrous DMF, Pd(PPh3)4, 80 ꢃC, 18 h; (iii) MeLi, anhydrous THF, ꢁ78 ꢃC to RT,
12 h; (iv) anhydrous dichloromethane, BF3$OEt2, RT, 4 h; (v) FeCl3, anhydrous CHCl3, RT, 48 h; (vi) NBS, CHCl3, RT, 4 h; (vii) 4,7-diboronicester-
2,1,3-benzothiadiazole, Pd(PPh3)4, 120 ꢃC, toluene, 2 M aqueous K2CO3, 48 h.
844 | J. Mater. Chem., 2011, 21, 843–850
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Fig. 3 Cyclic voltammograms of PDCPTCz and PDCPTCz-BT thin
films on the GC electrode in 0.1 M n-Bu4NPF6 acetonitrile solution at
room temperature.
Fig. 2 UV-Vis absorption spectra of PDCPTCz and PDCPTCz-BT in
chloroform solution (dashed line) and the solid state (solid line).
Electrochemical properties
The electrochemical properties of PDCPTCz and PDCPTCz-BT
were studied by cyclic voltammetry (CV) at a room temperature
in acetonitrile. The experiments were carried out using tetra-n-
butylammonium hexafluorophosphate (n-Bu4NPF6) (0.1 M) as
the supporting electrolyte, a platinum working electrode coated
with polymer films, a platinum-wire auxiliary electrode, a Ag
wire pseudo-reference electrode, and Fc/Fc+ as the external
standard (Fig. 3). The onset oxidation and reduction potentials
occur at 0.62 and ꢁ1.28 eV versus Fc/Fc+ for PDCPTCz and 0.59
and ꢁ1.10 eV for PDCPTCz-BT, respectively. According to the
Gel-permeation chromatography (GPC) analysis against PS
standard exhibits a number-averaged molecular mass (Mn) of
3.41 ꢀ 103 and 1.02 ꢀ 104 g molꢁ1 for PDCPTCz and
PDCPTCz-BT respectively with a polydispersity (PDI) of 1.87
and 2.77. Both the polymers (PDCPTCz and PDCPTCz-BT)
are readily soluble in organic solvents (THF, toluene,
chloroform, etc.).
Photophysical properties
empirical equation EHOMO/LUMO ¼ [(Eox red
/
ꢁ EFC) + 4.8] eV,30
the HOMO and LUMO energy levels were estimated as
ꢁ5.42 and ꢁ3.52 eV for PDCPTCz and ꢁ5.39 and ꢁ3.70 for
PDCPTCz-BT, respectively. It is worth noting that the electro-
The UV-Vis absorption of the two ladder-type heteroacene
polymers (PDCPTCz and PDCPTCz-BT) is acquired in chlo-
roform solution and in the films as depicted in Fig. 2. The
absorption spectra of both the polymers in the solid state are
almost identical to those in solution, and almost same
featureless characters are observed, respectively. This suggests
that there is almost no change in the conformation of both the
polymers backbone when going from solution to the solid state.
PDCPTCz displays an absorption with a maximum at 535 nm
with well-resolved vibronic structure and the sharp edge,
arising from p–p* transitions in the polymer backbone, which
testifies to the rigidity of the ladder-type polymer. In the case of
PDCPTCz-BT, it presents two absorption bands at 410 nm and
617 nm (Fig. 2). The high-energy peak can be associated with
p–p* transition of the ladderised carbazole units, whereas the
broad band centered at 617 nm is clearly attributed to the
intramolecular charge transfer (ICT) between the dicyclo-
pentathienocarbazole electron-rich group and the 2,1,3-benzo-
thiadiazole electron-withdrawing segment. Notably, the
lower-energy absorption (lmax ¼ 617 nm) for PDCPTCz-BT is
substantially bathochromically shifted compared to the parent
non-fused copolymer, namely poly[N-90-heptadecanyl-2,7-
carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)]
(PCDTBT) (lmax ¼ 550 nm) by Leclerc et al.21,22,29 It has
been suggested the rigid and flat p-conjugated framework of
PDCPTCz-BT leads to the elongation of p-system along the
main backbone by the formation of densely packed struc-
tures, resulting in the downhill-energy shift. The optical
band gaps (Egopt) for both PDCPTCz and PDCPTCz-BT
determined from the absorption onset are 2.17 eV for
PDCPTCz and 1.67 eV for PDCPTCz-BT, respectively.
CV
CV
chemical band gaps (Eg ¼ 1.9 eV for PDCPTCz and Eg
¼
1.69 eV for PDCPTCz-BT) and optical band gaps estimated
from UV-Vis spectra are similar within experimental error. Both
the HOMO and LUMO energy levels of PDCPTCz-BT match
well with those of PCDTBT (HOMO ¼ ꢁ5.5 eV and LUMO ¼
ꢁ3.6 eV) as the acyclic parent polymer,21,29 which fits reasonably
well with the required electronic levels for BHJ solar cells
utilizing PCBM derivatives as the acceptor.31
Field effect transistors
All field-effect transistors (FETs) were fabricated in the top
contact geometry as described in the Experimental section
(Fig. 4a). Fig. 4b shows the transfer characteristics, |Ids| vs. Vgs
and |Ids|1/2 vs. Vgs (both at Vds ¼ ꢁ60 V), of FETs fabricated using
PDCPTCz and PDCPTCz-BT, respectively, as the active layer.
These Ids vs. Vgs curves obtained from both PDCPTCz and
PDCPTCz-BT are typical of p-type semiconductors, as might be
expected from the dicyclopentathienocarbazole building blocks
in the main backbone. However, forming push–pull copolymer
by introducing 2,1,3-benzothiadiazole electron-withdrawing
moiety induces significant enhancement of electronic properties.
The FET property of the PDCPTCz-BT is significantly higher
than that of PDCPTCz. The hole mobilities deduced from
1/2
the slopes of Ids vs. Vgs are estimated to be mPDCPTCz ¼ 8.7 ꢀ
10ꢁ6 cm2 Vꢁ1 sꢁ1 for the PDCPTCz and mPDCPTCz-BT ¼ 2.7 ꢀ
10ꢁ4 cm2 Vꢁ1 sꢁ1 for the PDCPTCz-BT. The relatively low hole
mobilities are observed in both PDCPTCz and PDCPTCz-BT
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Fig. 4 (a) Schematic diagram of p-type OFET structure (L ¼ 50 mm, W ¼ 1.5 mm) with PDCPTCz (black circles) and PDCPTCz-BT (red circles),
respectively (b) transfer curves in saturated regime; (c and d) the output curves at different gate voltages.
OFETs as compared to the acyclic carbazole-based alternating
copolymers, which can be largely attributed to the low molecular
weights of both polymers.32–34 However, both PDCPTCz and
PDCPTCz-BT provide excellent FET output characteristics with
clear saturation (Fig. 4c and d). There are many examples of the
application of microwave irradiation as heat source to reduce
reaction times and increase the yield and the molecular weight of
metal-mediated cross-coupling reactions.35–37 Therefore,
a microwave-facilitated synthesis of the PDCPTCz-based poly-
mers is underway to improve the molecular weights.
PDCPTCz/PC71BM-based cell has a short circuit current density
(Jsc) of 1.92 mA cmꢁ2, an open circuit voltage (Voc) of 0.74 V,
a fill factor (FF) of 52%, generating a PCE (he) of 0.73%. In the
case of the solar cell made from PDCPTCz-BT/PC71BM, a Jsc of
4.59 mA cmꢁ2, a Voc of 0.81 V, respectively; combined with a FF
of 42%, it gives a PCE of 1.57%, a substantial improvement when
compared to that of PDCPTCz/PC71BM, because of its
increased photocurrent. This result is in good agreement with the
OFET measurements. The hole mobility in pure PDCPTCz-BT
is about two orders of magnitude higher than that obtained with
PDCPTCz. Therefore, this higher hole mobility in PDCPTCz-
BT, presumably related to a better organization, can explain the
higher value of the Jsc with PDCPTCz-BT. In addition, we
cannot count out that lower band gap for PDCPTCz-BT can
contribute to higher Jsc than that of PDCPTCz/PC71BM-based
cell. As shown in Fig. 5b, the difference in photocurrent between
two devices is confirmed by measurement of the incident photon-
to-current efficiency (IPCE) spectra that reveal the photon–
current response of devices with respect to the wavelength.
The Voc values of both PDCPTCz/PC71BM and PDCPTCz-
BT/PC71BM are improved in comparison with P3HT/PC71BM.38
This is not unexpected due to the carbazole building units in the
main chain since the polycarbazole derivatives lead to the rela-
tively deep-lying HOMO energy levels that can potentially
increase Voc.21,29,39,40 This is, indeed, supported by our CV results.
In the case of both PDCPTCz/PC71BM and PDCPTCz-BT/
PC71BM, the significant lower Jsc values, relative to the BHJ
solar cell-based polymers/fullerenes, lead to an overall decrease
in PCE (he). This may be attributed to one of the following two
factors that an undesirable morphology in the BHJ, due to
Polymer solar cells
The polymer solar cells had a layered structure of ITO/
PEDOT:PSS/polymers:PC71BM/Ca/Al. The active layer was
P3HT/PC71BM at 1 : 2 weight ratio. It was spin-coated from
a solution containing the polymer and PC71BM in o-dichloro-
benzene onto an ITO/glass substrate covered by poly(3,4-ethyl-
enedioxythiophene)/poly(styrenesulfonate) (PEDOT–PSS). Thin
layers of Ca (20 nm) and Al (80 nm) were thermally deposited
under vacuum. All data were obtained under white light AM
1.5G illumination from a calibrated solar simulator with irradi-
ation intensity of 100 mW cmꢁ2. The device performance of
a solar cell is determined by the open-circuit voltage (Voc), short-
circuit current (Jsc), and fill factor (FF). The power conversion
efficiency (he) of a solar cell is given as he ¼ (Jsc ꢀ Voc ꢀ FF) ꢀ
100/Pinc, where Pinc is the intensity of incident light.
The current–voltage characteristics of the solar cells based on
the two blends PDCPTCz/PC71BM and PDCPTCz-BT/PC71BM
are shown in Fig. 5a. Under the white light illumination, the
846 | J. Mater. Chem., 2011, 21, 843–850
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Fig. 5 (a) J–V characteristics of polymer solar cells under illumination of AM 1.5G (100 mW cmꢁ2). (b) Incident photon-to-current efficiency (IPCE)
spectra of PDCPTCz/PC71BM and PDCPTCz-BT/PC71BM.
‘unsymmetrical’ branched solubilizers (decyltetradecyl side
chains) or the low molecular weights for the both copolymers
(PDCPTCz (Mn ¼ 3.41 ꢀ 103 g/mol) and PDCPTCz-BT (Mn ¼
1.02 ꢀ 104 g/mol)), causes a decrease in the Jsc and low efficiency
in the BHJ solar cells. Although the efficiencies of both
PDCPTCz/PC71BM and PDCPTCz-BT/PC71BM show lower
PCEs compared to the reported polycarbazole derivatives
without ladderization as parent frameworks.29 In the case of the
non-fused D–A copolymer based on carbazole, the significant
improved Jsc (up to 10.6 mA cmꢁ2) was achieved only in highly
optimized devices, while Voc and FFs remain comparable to our
devices made of dicyclopentathienocarbazole-based polymers,
such as PDCPTCz and PDCPTCz-BT. Thereby, there is still
plenty of room to further improve efficiencies of BHJ solar cells
prepared from our polymers. We are currently not only
preparing PDCPTCz-BT with improved molecular weight as
well as symmetric branched solubilizing groups through new
synthetic strategies but also are carrying out the extensive device
optimization by utilizing processing additives, different solvents,
and thermal annealing, which will most likely result in substan-
tial enhancement of the device performance.
donor–acceptor copolymer (poly(dicyclopentathienocarbazole-alt-
2,1,3-benzothiadiazole) (PDCPTCz-BT)) reveals more bath-
ocromically shifted absorption spectrum (lmax ¼ 617 nm) and
opt
lower band gap (Eg ¼ 1.67 eV) in comparison with its corre-
sponding parent non-fused polymer (poly[N-90-heptadecanyl-2,7-
carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)]
(PCDTBT)) due to the further strength of ICT between the
electron-rich and the electron-withdrawing segments in the
polymer backbone through its planarized molecular skeleton. p-
Type organic field-effect transistor (OFET) devices, PDCPTCz
and PDCPTCz-BT manufactured from solution, have been
characterized. In a top-contact geometry, electron mobilities in
the saturation regime were found to be mPDCPTCz ¼ 8.7 ꢀ 10ꢁ6
cm2 Vꢁ1 sꢁ1 for the PDCPTCz and mPDCPTCz-BT ¼ 2.7 ꢀ 10ꢁ4 cm2
Vꢁ1 sꢁ1, respectively. We have also characterized the solar cells
using the polymers (PDCPTCz and PDCPTCz-BT) respectively
in BHJ composites with PC71BM. In the case of the device of
PDCPTCz-BT/PC71BM, a power conversion efficiency (PCE) of
1.57% as a preliminary measurement is obtained. Further
improvements are anticipated through a rational push–pull
architectural design of new symmetric copolymers containing
electron-rich unit based on dicyplopentathienocarbazole.
Conclusions
Experimental section
From an electronic structural point of view, we have designed
and synthesized a new ladder-type p-conjugated heteroacene
with inclusion of both a carbazole core and two outer thiophene
subunits, namely dicyclopentathienocarbazole (DCPTCz). The
Materials and instruments
All starting materials were purchased either from Aldrich or
Acros and used without further purification. THF was distilled
over sodium/benzophenone. 1H NMR and 13C NMR spectra
were recorded on a Varian VNRS 600 MHz (Varian USA)
spectrophotometer using CDCl3 as solvent and tetramethylsilane
(TMS) as the internal standard and MALDI MS spectra were
obtained from Ultraflex III (Bruker, Germany). UV-Vis-NIR
spectra were taken on Cary 5000 (Varian USA) spectropho-
tometer. Number-average (Mn) and weight average (Mw)
molecular weights, and polydispersity index (PDI) of the poly-
mer products were determined by gel permeation chromatog-
raphy (GPC) with Agilent 1200 HPLC Chemstation using
ring-fused
DCPTCz
monomer
is
polymerized
by
oxidative coupling using FeCl3 and copolymerized with the
chosen electron-deficient 2,1,3-benzothiadiazole comonomer by
Suzuki
polymer
condensation
to
afford
and
poly(di-
(poly(di-
cyclopentathienocarbazole)
(PDCPTCz)
cyclopentathienocarbazole-alt-2,1,3-benzothiadiazole) PDCPTCz-
BT), respectively. The absorption spectra of PDCPTCz both in the
solution and the thin film show a clear vibronic fine structural band
with a maximum at 535 nm which gives evidence of its rigid and
coplanar motif. On the other hand, the profile of the
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a series of mono-disperse polystyrene as standards in THF
(HPLC grade) at 308 K. Cyclic voltammetry (CV) measurements
were performed on Solartron SI 1287 with a three-electrode cell
in a 0.1 M tetra-n-butylammonium hexafluorophosphate (n-
Bu4NPF6) solution in acetonitrile at a scan rate of 50 mV sꢁ1 at
room temperature under argon. A silver wire, a platinum wire
and a glass-carbon disk were used as the reference electrode,
counter electrode and working electrode respectively. The Ag/
Ag+ reference electrode was calibrated using a ferrocene/ferro-
cenium redox couple as an external standard, whose oxidation
potential is set at ꢁ4.8 eV with respect to zero vacuum level. The
HOMO energy levels were obtained from the equation HOMO ¼
through a 0.45 mm PTFE filter, spin-coated at 1500 rpm for 60 s then
moved to glove box with filled nitrogen. Those samples were
brought into a vacuum system (about 10ꢁ7 torr) and a Ca (20 nm)
and Al electrode (80 nm) was deposited on the top of the BHJ layer.
The active area is 13.5 mm2.
Synthesis of 2,7-dibromo-N-(2-decyltetradecyl)-3,6-bis(4-
octylbenzoyl)carbazole (2)
To a mixture of compound 1 (3 g, 4.5 mmol) and AlCl3 (1.42 g,
10.6 mmol) in 30 mL of anhydrous dichloromethane (CH2Cl2),
4-octylbenzoyl chloride (2.5 g, 9.9 mmol) was added slowly at
room temperature. The mixture was stirred for 5 h at 50 ꢃC, and
then quenched with ice. The inorganic precipitate was dissolved
in 2 M NaOH and the product was extracted with CH2Cl2. The
separated organic layer was dried over MgSO4 and removed
under reduced pressure. The residue was purified by column
chromatography (silica gel, 5% ethyl acetate in hexane) to afford
onset
onset
ꢁ(Eox
ꢁ E(ferrocene)
+ 4.8) eV. The LUMO levels of poly-
onset
mers were obtained from the equation LUMO ¼ ꢁ(Ered
ꢁ
E(ferrocene)onset + 4.8) eV.
OFET device preparation and measurement
All p-type OFETs were fabricated on heavily doped n-type
silicon (Si) wafers each covered with a thermally grown silicon
dioxide (SiO2) layer with thickness of 200 nm. The doped Si
wafer acts as a gate electrode, and the SiO2 layer functions as the
gate insulator. The active layer was deposited by spin-coating at
2500 rpm. All solutions were prepared at 0.5 wt% concentration
in chlorobenzene. The thickness of the deposited films was about
60 nm. Prior to vapor-deposition of source-drain electrodes, the
films were dried on a hot plate stabilized at 80 ꢃC for 30 minutes.
All fabrication processes were carried out in a glove box filled
with N2. Source and drain electrodes using Au were deposited by
thermal evaporation using a shadow mask. The thickness of
source and drain electrodes was 50 nm. Channel length (L) and
channel width (W) was 50 mm and 1.5 mm, respectively. All
OFET devices were made in the top contact geometry as shown
in Fig. 4a. Electrical characterization was performed using
a Keithley semiconductor parametric analyzer (Keithley 4200)
under N2 atmosphere. The electron mobility (m) was determined
using the following equation in the saturation regime:
1
4.66 g (94%) of 2 as yellow gummy solid. H NMR (600 MHz,
CDCl3) d (ppm) 7.92 (s, 2H, Ar-H), 7.68–7.67 (d, 4H, Ar-H, J ¼
7.8 Hz), 7.60 (s, 2H, Ar-H), 7.18–7.17 (d, 4H, Ar-H, J ¼ 7.8 Hz),
4.1–4.09 (d, 2H, CH2, J ¼ 7.8 Hz), 2.6–2.58 (t, 4H, 2 ꢀ CH2-Ph,
J ¼ 7.8 Hz), 2.1 (m, 1H, CH), 1.57–1.53 (m, 4H, 2 ꢀ CH2, J ¼
7.2 Hz), 1.32–1.29 (m, 4H, 2 ꢀ CH2, J ¼ 6.0 Hz), 1.28–1.18 (m,
56H, 28 ꢀ CH2), 0.8–0.78 (t, 12H, 4 ꢀ CH3, J ¼ 7.8 Hz).13C
NMR (600 MHz, CDCl3) d (ppm) 195.6, 149.3, 142.66, 136.5,
134.7, 132.1, 130.5, 128.6, 120.9, 118, 115.5, 114.2, 40.6, 36.1,
31.9, 31.8, 31.6, 29.9, 29.6, 29.3, 26.4, 22.6, 14.1, 14.09. MALDI-
TOF-MS m/z: [M + Na]+c ¼ 1114.
Synthesis of 2,7-di(thiophen-2-yl)-N-(2-decyltetradecyl)-3,6-
bis(4-octylbenzoyl)carbazole (3)
2-Tributylstannylthiophene (2 g, 5.3 mmol) was added to
a solution of 2 (2.5 g, 2.3 mmol) in anhydrous DMF (25 mL) and
the resulting mixture was purged with argon for 30 min.
Pd(PPh3)4 (81 mg, 0.07 mmol) was then added, and the reaction
mixture was stirred at 80 ꢃC overnight. Excess DMF was
removed under high vacuum, and the residue was dissolved in
ethyl acetate and treated with 10% HCl solution. The mixture
was filtered through a pad of Celite. The separated organic layer
was dried over MgSO4, filtered, and removed under vacuum. The
crude product was purified by flash chromatography using 5%
ethyl acetate in hexane to afford 2.2 g (88%) of 3 as yellow gum.
1H NMR (600 MHz, CDCl3) d (ppm) 8.15 (s, 2H, Ar-H), 7.66–
7.65 (d, 4H, Ar-H, J ¼ 8.4 Hz), 7.55 (s, 2H, Ar-H), 7.19–7.18 (d,
2H, Th-H, J ¼ 1.2 Hz), 7.12–7.11 (d, 4H, Ar-H, J ¼ 8.4 Hz),
6.97–6.96 (d, 2H, Th-H, J ¼ 1.2 Hz), 6.88–6.86 (t, 2H, Th-H, J ¼
3.4 Hz), 4.28–4.26 (d, 2H, CH2, J ¼ 7.2 Hz), 2.6–2.58 (t, 4H, 2 ꢀ
CH2-Ph, J ¼ 7.8 Hz), 2.2 (m, 1H, CH), 1.58–1.56 (m, 4H, 2 ꢀ
CH2), 1.41–1.39 (m, 4H, 2 ꢀ CH2), 1.36–1.23 (m, 56H, 28 ꢀ
CH2), 0.88–0.85 (t, 12H, 4 ꢀ CH3, J ¼ 3.6 Hz).13C-NMR (600
MHz, CDCl3) d (ppm) 198.1, 148.4, 142.7, 135.7, 132.2, 131.6,
130.1, 128.2, 127.6, 127.5, 125.9, 122.0, 121.4, 35.9, 31.9, 31.8,
31.6, 31.0, 29.7, 29.6, 29.3, 26.5, 22.6, 14.1. MALDI-TOF-MS m/
z: [M + Na]+c ¼ 1123.
Ids ¼ (WCi/2L) ꢀ m ꢀ (Vgs ꢁ VT)2,
where Ci is the capacitance per unit area of the SiO2 dielectric
(Ci ¼ 15 nF cmꢁ2), VT is the threshold voltage.
Photovoltaic cells fabrication and testing
The organic photovoltaic cells, with the sandwiched structure of
Glass/ITO/PEDOT:PSS/polymer–PC71BM(1 : 2)/Ca (20 nm)/Al
(80 nm), were prepared on commercial Indium Tin Oxide (ITO)-
coated glass substrate (15 ꢀ 10 mm2) with a sheet resistance of ꢁ30
Ohm sqꢁ1 (M & M Tech, Korea). Prior to use, the substrates were
cleaned with detergent and deionized water. Then, they were
ultrasonicated for 10 minutes in deionized water, acetone and iso-
propanol respectively and dried in the oven at 100 ꢃC for overnight.
The ITO substrates were then kept in UVO cleaner for 1500 s, and
were spin-coated (5000 rpm, 60 s) with a thin film (40 nm) of poly-
(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT–
PSS, Baytron P, H. C. Starck) and baked at 140 ꢃC for 10 min in air.
A blend of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)
(Nano-C, USA) and polymer (2 : 1, 4.0 mg mLꢁ1 of polymer in
o-DCB) was solubilized in chlorobenzene for overnight and filtered
848 | J. Mater. Chem., 2011, 21, 843–850
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Synthesis of 2,7-di(thiophen-2-yl)-N-(2-decyltetradecyl)-3,6-
bis(1-(4-octylphenyl)ethanolyl)carbazole (4)
acetate in hexane to provide 1.94 g (88%) pure product. 1H NMR
(600 MHz, CDCl3) d (ppm) 7.7 (s, 2H, Ar-H), 7.27–7.26 (d, 2H,
Ar-H, J ¼ 2.4 Hz), 7.16–7.12 (dd, 4H, Ar-H, J ¼ 8.4Hz),
7.04–7.02 (d, 4H, Ar-H, J ¼ 8.4 Hz), 6.92 (d, 2H, Th-H, J ¼
1.8 Hz), 4.19–4.18 (d, 2H, CH2, J ¼ 7.8 Hz), 2.53–2.50 (q, 4H,
2 ꢀ CH2, J ¼ 7.8 Hz), 2.24 (m, 1H, CH), 1.91 (m, 6H, 2 ꢀ CH3,
J ¼ 16.2 Hz), 1.55–1.53 (m, 4H, 2 ꢀ CH2), 1.30–1.23(m, 60H,
30 ꢀ CH2) 0.89–0.86 (t, 12H, 4 ꢀ CH3, J ¼ 2.4 Hz). 13C-NMR
(600 MHz, CDCl3) d (ppm) 157.4, 147.1, 141.5, 141.4, 141.0,
134.4, 128.4, 126.0, 125.9, 124.9, 121.2, 115.1, 113.3, 99.3, 52.7,
37.6, 35.5, 35.4, 31.9, 31.3, 30.0, 29.7, 29.6, 29.2, 26.4, 24.9, 22.7,
22.6, 14.1. MALDI-TOF-MS m/z: [M + Na]+c ¼ 1274.
A solution of dithiopheno diketone 3 (2 g, 1.8 mmol) in anhy-
drous THF (30 mL) in a 100 mL Schlenk flask was cooled to
ꢃ
ꢁ78 C in an acetone-dry ice bath. Subsequently, a solution of
1.6 M methyllithium in diethyl ether (8 mL) was added to reac-
tion mixture in argon atmosphere. After 1 h at ꢁ78 ꢃC, the
reaction mixture was allowed to reach room temperature and
stirred for 12 h and then water was added to quench the reaction.
The product was extracted with CH2Cl2 (100 mL), treated with
saturated NaCl solution, dried over MgSO4 and the solvent was
removed in vacuo. The crude product was purified by flash
chromatography 10% ethyl acetate in hexane to get pure dia-
lcohol 4, 1.85 g (90%) as pale yellow solid.1H NMR (600 MHz,
CDCl3) d (ppm) 8.59 (s, 2H, Th-H), 7.26–7.25 (d, 4H, Ar-H, J ¼
1.8 Hz), 7.21–7.20 (d, 2H, Th-H, J ¼ 4.8 Hz), 7.06–7.04 (d, 4H,
Ar-H), 7.02–7.01 (d, 4H, Ar-H), 6.79–6.77 (t, 2H, Th-H, J ¼ 1.2
Hz), 6.21–6.20 (t, 2H, 2 ꢀ OH, J ¼ 3.6 Hz), 4.06–4.05 (d, 2H,
CH2, J ¼ 3.6 Hz), 2.59–2.56 (t, 4H, 2 ꢀ CH2–Ph, J ¼ 7.8 Hz),
2.1 (m, 1H, CH), 2.04–2.03 (s, 6H, 2 ꢀ CH3, J ¼ 7.2 Hz), 1.61–
1.56 (m, 4H, 2 ꢀ CH2, J ¼ 7.2 Hz), 1.36–1.23 (m, 60H, 30 ꢀ
CH2), 0.89–0.86 (t, 12H, 4 ꢀ CH3, J ¼ 7.2 Hz).13C-NMR
(600 MHz, CDCl3) d (ppm) 148.1, 141.1, 140.0, 137.86, 131.07,
128.1, 126.1, 125.1, 122.0, 118.4, 114.9, 37.7, 35.4, 32.4, 31.9,
29.9, 29.6, 29.5, 26.1, 22.7, 22.6, 14.1. MALDI-TOF-MS
m/z: [M + Na]+c ¼ 1154.
Synthesis of poly(dicyclopentathienocarbazole) (PDCPTCz)
In a 50 mL flame dried Schlenk flask, monomer 5 (0.3 g,
0.27 mmol) was dissolved in anhydrous CHCl3 (7 mL), to this
a solution of anhydrous iron chloride (0.2 g, 1.2 mmol) in anhy-
drous CHCl3 (7 mL) was added. The mixture was vigorously
stirred at room temperature for 2 days. Then the solution was
precipitated in a mixture of methanol and ammonia (3 : 1 v/v,
270mL). This was filtered off through 0.45 mm nylon filter, washed
on Soxhlet apparatus with methanol (1 d) and acetone (1 d). 0.2 g
(60%) of the polymer was recovered as a orange red powder (Mn ¼
1
3.41 ꢀ 103 g molꢁ1, PDI ¼ 1.87). H NMR (600 MHz, CDCl3)
d (ppm) 7.8 (br, 2H), 7.36 (br, 4H), 7.15–7.12 (br, 4H), 7.01–6.95
(br, 4H), 6.87 (br, 2H), 4.1(br, 2H), 2.45 (br, 4H), 1.8 (br, 6H), 1.49
(br, 4H), 1.18–1.03 (br, 60H), 0.89–0.74 (br, 12H).
Synthesis of dicyclopentathienocarbazole (5)
Synthesis of poly(dicyclopentathienocarbazole-alt-2,1,3-
benzothiadiazole) (PDCPTCz-BT)
To a solution of dialcohol 4 (1.6 g, 1.4 mmol) in anhydrous
CH2Cl2 (30 mL) in a two neck flask, BF3$etherate (1 mL, 8
mmol) was added at room temperature. After 4 h, the reaction
mixture was quenched with MeOH, diluted with excess CH2Cl2,
washed with water, treated with saturated NaCl solution, dried
over MgSO4 and concentrated to get crude compound. 1.24 g
(80%) of the pure product was obtained after silica gel column
chromatography using 5% ethyl acetate in hexane. 1H NMR
(600 MHz, CDCl3) d (ppm) 7.73 (s, 2H, Ar-H), 7.35–7.34 (d, 2H,
Ar-H, J ¼ 3.0 Hz), 7.27–7.26 (dd, 2H, Th-H, J ¼ 3.0 Hz),
7.20–7.16 (dd, 4H, Ar-H, J ¼ 8.4 Hz), 7.04–7.0 (dd, 4H, Ar-H,
J ¼ 8.4 Hz), 6.92–6.91 (dd, 2H, Th-H, J ¼ 3.0 Hz), 4.19–4.18 (d,
2H, CH2, J ¼ 7.2 Hz), 2.53–2.50 (q, 4H, 2 ꢀ CH2, J ¼ 7.8 Hz),
2.24 (m, 1H, CH), 1.91–1.89 (d, 6H, 2 ꢀ CH3, J ¼ 16.2 Hz),
1.55–1.53 (m, 4H, 2 ꢀ CH2), 1.30–1.23 (m, 60H, 30 ꢀ CH2),
0.89–0.86 (t, 12H, 4 ꢀ CH3, J ¼ 7.2 Hz). 13C NMR (600 MHz,
CDCl3) d (ppm) 158.1, 148.1, 141.2, 140.0, 135.1, 131.07, 128.1,
126.1, 125.1, 122.0, 118.4, 114.9, 53, 37.7, 35.4, 32.4, 31.9,
29.9, 29.2, 29.3, 26.4, 22.4, 22.1, 14.6. MALDI-TOF-MS
m/z: [M + Na]+c ¼ 1119.
Ina flame dried 50 mL flask, monomer 6 (0.25 g, 0.2 mmol) and 4,7-
diboronicester-2,1,3-benzothiadiazole (0.2 mmol) were dissolved in
the mixture of anhydrous toluene (15 mL), 2 M aq. K2CO3 (7 mL)
under argon. After 15 min stirring under argon at room tempera-
ture, Pd(PPh3)4 (12 mg, 0.01 mmol) was added to the reaction
ꢃ
mixture and stirred at 120 C for 2 days. Then the solution was
precipitated in a mixture of methanol and diluted HCl (2 : 1 v/v,
200 mL). The separated solidwas filtered offthrough 0.45mm nylon
filter and washed on Soxhlet apparatus with methanol and 0.1 g
(50%) of the polymer was recovered as a deep bluish green powder
1
(Mn ¼ 1.02 ꢀ 104 g molꢁ1, PDI ¼ 2.77). H NMR (600 MHz,
CDCl3) d (ppm) 7.8 (br, 2H), 7.36 (br, 4H), 7.15–7.118 (br, 4H),
7.01–6.95 (br, 4H), 6.87 (br, 2H), 4.1(br, 2H), 2.45 (br, 4H), 1.8 (br,
6H), 1.49 (br, 4H), 1.18–1.03 (br, 60H), 0.89–0.74 (br, 12H).
Acknowledgements
This work was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (2010-
0002494) and the National Research Foundation of Korea Grant
funded by the Korean Government (MEST) (2010-0019408), (2010-
0026163) and (MEST) (NRF-2009-C1AAA001-2009-0093020).
Synthesis of dibromo-dicyclopentathienocarbazole (6)
To a mixture of compound 5 (2 g, 1.8 mmol) in anhydrous CHCl3
(30 mL) was added NBS (0.7 g, 3.9 mmol) at room temperature
under argon conditions. After 4 h, water was added to quench
the reaction. Subsequently, diluted with excess CH2Cl2, washed
with water, treated with saturated NaCl solution, dried over
MgSO4 and concentrated to afford crude compound. The crude
product was purified by column chromatography using 5% ethyl
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