Synthesis and photovoltaic properties of copolymers based on benzo[1,2- b:4,5- b ′]dithiophene and thiophene with different conjugated side groups

Article abstract of DOI:10.1021/ma202399n

Three novel copolymers (PT-ID, PT-DTBT, and PT-DTBTID) of benzo[1,2-b:4,5-b′]dithiophene and thiophene with different conjugated side groups (1,3-indanedione (ID), 4,7-dithien-5-yl-2,1,3-benzodiathiazole (DTBT), and DTBT-ID) were synthesized and developed for polymer solar cell applications. The effects of the different conjugated side groups on the thermal, photophysical, electrochemical and photovoltaic properties of these copolymers were investigated. As the length of the conjugated side groups increased, the absorption of the UV-vis region in solution was red-shifted. By changing the different side groups, the energy levels and band gaps of the resulted copolymers were effectively tuned. The three copolymers exhibit deep HOMO energy level and relatively high open-circuit voltage (V_oc). Bulk heterojunction solar cells with these copolymers as electron donors and (6,6)-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as an electron acceptor exhibit power conversion efficiencies of 2.48%, 4.18% and 1.16% for PT-ID, PT-DTBT, and PT-DTBTID, respectively.

Full text of DOI:10.1021/ma202399n

Article
pubs.acs.org/Macromolecules
Synthesis and Photovoltaic Properties of Copolymers Based on
Benzo[1,2-b:4,5-b′]dithiophene and Thiophene with Different
Conjugated Side Groups
Zhaojie Gu,† Peng Tang,† Bin Zhao,† Hao Luo,‡ Xia Guo,‡ Huajie Chen,‡ Gui Yu,‡ Xinping Liu,†
Ping Shen,† and Songting Tan*,†
†College of Chemistry, Key Laboratory of Polymeric Materials & Application Technology, and Key Laboratory of Advanced
Functional Polymeric Materials of the College of Hunan Province, Xiangtan University, Xiangtan 411105, P. R. China
‡Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
P. R. China
ABSTRACT: Three novel copolymers (PT-ID, PT-DTBT,
and PT-DTBTID) of benzo[1,2-b:4,5-b′]dithiophene and
thiophene with different conjugated side groups (1,3-inda-
nedione (ID), 4,7-dithien-5-yl-2,1,3-benzodiathiazole
(DTBT), and DTBT-ID) were synthesized and developed
for polymer solar cell applications. The effects of the different
conjugated side groups on the thermal, photophysical, elec-
trochemical and photovoltaic properties of these copolymers
were investigated. As the length of the conjugated side groups
increased, the absorption of the UV−vis region in solution was
red-shifted. By changing the different side groups, the energy levels and band gaps of the resulted copolymers were effectively
tuned. The three copolymers exhibit deep HOMO energy level and relatively high open-circuit voltage (Voc). Bulk heterojunction
solar cells with these copolymers as electron donors and (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron
acceptor exhibit power conversion efficiencies of 2.48%, 4.18% and 1.16% for PT-ID, PT-DTBT, and PT-DTBTID, respectively.
HOMO and LUMO energy levels of the conjugated
polymers.10,11 Among these main chain D−A copolymers, it
has been demonstrated that benzo(1,2-b:4,5-b′)dithiophene
(BDT) is the most potential and effective electron-rich donor
unit.12 In the past two years, a series of new D−A low band gap
polymers based on BDT have been reported. For example, the
solar cells based on a BDT copolymer PBDTTT have achieved
a series of high PCE values in the range of 5−7%.9,13 Although
main chain D−A copolymers have been witnessed great success
for achieving high performance PSCs, they may suffer from
lower hole mobility due to the influence of the acceptor units
on the polymer main chain.14
A new family of polymers with conjugated side groups or
side chains has been developed by several research groups,
including ours.8,15,16 This type of polymers features high hole
mobility benefiting from the overlapping of the conjugated side
chain interactions with the conjugated main chains, and broad
absorption spectra deriving from both the main chains and the
conjugated side chains, thus demonstrated prominent device
performances in PSCs.15 Here, we designed and synthesized
three copolymers PT-ID, PT-DTBT, PT-DTBTID (as shown
in Figure 1) based on BDT and thiophene with different
INTRODUCTION
■
Polymer solar cells (PSCs) have attracted considerable
attention in recent years due to their potential for low cost,
lightweight, and good compatibility with the roll-to-roll process
for making flexible large area devices.1−3 So far, the most
successful polymer solar cells are bulk heterojunction (BHJ)
devices,4−7 which use a phase separated blend of organic elec-
tron donor and acceptor components, where the conjugated
polymer is often used as the donor and the fullerene derivative
is used as the acceptor.8 It has been realized that an ideal
polymer donor in PSCs should exhibit broad absorption with
high absorption coefficient in the visible region, high hole mo-
bility, suitable energy level matching with the fullerene
acceptor, and appropriate compatibility with the fullerene
acceptor to form nanoscale bicontinuous interpenetrating
network.9 Optimizing the properties of the polymer donor
will offer higher short-circuit current density (Jsc), open-circuit
voltage (Voc), fill-factor (FF), and power conversion efficiency
(PCE) of the PSCs.
One feasible approach toward broadening the absorption
spectrum and tuning the energy levels is to design alternating
donor−acceptor (D−A) copolymers, in which the hybrid-
ization of the highest occupied molecular orbital (HOMO)
located on the donor moiety with the lowest unoccupied
molecular orbital (LUMO) located on the acceptor moiety
provides a means of narrowing the band gap and tuning the
Received: December 8, 2011
Revised: February 20, 2012
Published: March 1, 2012
© 2012 American Chemical Society
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Figure 1. Molecular structures of PT-ID, PT-DTBT, and PT-DTBTID.
Scheme 1. Synthetic Routes of the Monomers and the Corresponding Copolymers
conjugated side groups. In order to broaden the absorption
spectra of the π-conjugated polymers, 1,3-indanedione (ID)
and 4,7-dithien-5-yl-2,1,3-benzodiathiazole (DTBT) as elec-
tron-deficient units are to be introduced into side chains of PT-
ID and PT-DTBT, respectively. Furthermore, a long side chain
(DTBT-ID) composed of DTBT and ID is introduced to form
the polymer PT-DTBTID. These conjugated side groups
attached to the main chains via vinylene groups. The vinylene
linkage serves to enhance coplanarity of the polymer backbone
and extend the π-conjugation by further eliminating torsional
strains between the main chain and electron-deficient side
groups, leading to a lower optical band gap and tunable frontier
molecular orbital energy levels. The effects of the conjugated
side groups on the thermal, photophysical, electrochemical, and
photovoltaic properties of the copolymers were investigated in
detail.
bromination of thiophene-3-carbaldehyde with NBS under
argon atmosphere. The important intermediate, monomer M2,
was synthesized from compounds 2 and 3 in a moderate yield
(65%) according to the Wittig−Horner reaction, and purified
using a silica gel column with an ether/dichloromethane (1/1,
v:v) eluent. The aldehyde-functionalized monomer M3 was
prepared by Vilsmeier−Haack formylation of M2 under a mild
condition.18 The precursor polymers PT-CHO and PT-
DTBTCHO were synthesized by the Stille coupling reaction.
Two target copolymers, PT-ID and PT-DTBTID, were ob-
tained by the Knoevenagel condensation between the aldehyde-
functionalized precursor polymers (PT-CHO and PT-
DTBTCHO) and 1,3-indanedione in yields of 49% and 46%,
respectively. The other target copolymer PT-DTBT was
directly synthesized by the Stille coupling reaction between
the monomer M2 and M4. The chemical structures of the
1
copolymers were verified by H NMR and elemental analysis.
RESULTS AND DISCUSSION
The complete disappearance of the aldehyde proton signals at
10.19 for PT-CHO and 10.10 ppm10,19 for PT-DTBTCHO,
and the appearance of olefinic proton signals at 8.75 for PT-ID
and 8.72 ppm for PT-DTBTID confirm the efficient
conversion reaction of the aldehyde and the successful
■
Synthesis and Chemical Characterization. Synthetic
routes of the monomers and copolymers are shown in
Scheme 1. The crucial monomer M4 was synthesized by the
reported method.17 The monomer M1 was obtained by
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preparation of target copolymers. PT-ID, PT-DTBT, and PT-
DTBTID exhibit good solubility in common organic solvents
such as CHCl3, THF, CH2Cl2 and chlorobenzene at room
temperature. Table 1 summarized the polymerization results
Table 1. Molecular Weights and Thermal Properties of
Copolymers
a
b
c
copolymers
yield (%) Mn (kg mol−1
)
PDI Tg (°C) Td (°C)
PT-ID
49
78
46
12.1
20.2
12.5
2.2
2.6
1.8
160
155
165
321
368
301
PT-DTBT
PT-DTBTID
a
b
c
Determined by GPC in THF based on polystyrene standards.
Determined by DSC at scan rate of 20 °C/min under nitrogen.
Decomposition temperature, determined by TGA in nitrogen, based
on 5% weight loss.
Figure 3. DSC curves of copolymers collected at a scan rate of
20 °C/min.
and thermal properties of the copolymers. The number-average
molecular weight (Mn) of PT-CHO and PT-DTBTCHO are
11.6 and 11.4 kg mol−1 with polydispersity index (PDI) of 1.66
and 2.00, respectively. The Mn of the target copolymers PT-ID
(12.1 kg mol−1) and PT-DTBTID (12.5 kg mol−1) show a
slight increase than those of PT-CHO and PT-DTBTCHO due
to the reaction between of 1,3-indanedione and aldehyde
group.
Thermal Properties. The thermal properties of the target
copolymers were obtained by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) measure-
ments. As shown in Figure 2 and Table 1, the TGA curves
Photophysical Properties. The photophysical properties
of the copolymers were investigated by UV−vis and
fluorescence spectroscopy. The correlative data of the
copolymers are summarized in Table 2. Figure 4 gives the
UV−vis absorption spectra of PT-ID, PT-DTBT, and PT-
DTBTID in diluted CHCl3 solution. All copolymers show two
distinct absorption bands. The first absorption bands with the
absorption maxima (λs,max) at 423 nm for PT-ID, 390 nm for
PT-DTBT and 387 nm for PT-DTBTID can be identified with
a delocalized excitonic π−π* transition and the second
absorption bands of them with λs,max at 565, 504, and 526 nm,
respectively, can be attributed to a localized transition between
donor−acceptor charge transfer states.21,22 PT-DTBTID
exhibits the broader absorption which derived from the ICT
interaction between the main chain and the conjugated side
chains.23 Obviously, the ICT interactions of the PT-DTBTID
are progressively enhanced due to the increase of the electron
affinity of the DTBTID group.10 In comparison with the
absorption spectra of the polymer solutions, the absorption spectra
of the films are broadened and the peaks are red-shifted. This
phenomenon results from the enhanced interchain interaction in
the solid films, which is probably related to the increased extent of
π−π stacking of the backbones and increased polarizability of the
film, or both.24−26 The optical band gaps (Egopt) of PT-ID, PT-
DTBT and PT-DTBTID are 1.75, 1.92, and 1.66 eV, respectively,
calculated from the onset of the film absorptions.
The photoluminescence (PL) spectra of copolymers in the
diluted CHCl3 solution are shown in Figure 5. The copolymers are
excited at the wavelength corresponding to the two λs,max. For
example, PT-DTBT exhibits the same PL spectra when it is
excited at 390 and 504 nm, respectively, which indicates that there
is a thorough intramolecular energy transfer of the excitons from
the conjugated side chains to the main chains. This phenomenon
ensures that all photons absorbed by the copoly-
mers are useful for the photovoltaic conversion.27 In CHCl3 solu-
tion, PT-ID exhibits a maximum emission peak (λf,max) at 663 nm.
With DTBT and DTBTID as the conjugated side groups, the λf,max
of PT-DTBT (682 nm) and PT-DTBTID (705 nm) are red-
shifted by 19 and 42 nm, respectively, compared with that of PT-
ID. All the results clearly indicate that the photophysical properties
of these copolymers could be tuned by changing conjugated side
groups.
Figure 2. TGA curves of copolymers at scan rate of 20 °C/min under
nitrogen atmosphere.
reveal that the degradation temperatures (Td) of 5% weight
loss of PT-ID, PT-DTBT, and PT-DTBTID, are 321, 368, and
301 °C, respectively. And the glass transition temperatures (Tg)
are 160, 155, and 165 °C for PT-ID, PT-DTBT, and PT-
DTBTID, respectively (Figure 3). We have also measured the
DSC curves at different scan rates, and no crystallization or
melting peaks are observed upon further heating beyond the Tg.
The results indicate that these copolymers are amorphous. It is
clear that the introduction of 1,3-indanedione group in the side
chains of the conjugated polymers improves the glass transition
temperature. The good thermal stability of the copolymers
retards the deformation of the polymer morphology and the
degradation of the active layer at elevated temperatures, which
are desirable for polymers in PSCs applications.20
Electrochemical Properties. The highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
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Table 2. Optical and Electrochemical Properties of the Three Copolymers
a
solution λs,max (nm)
abs (nm)
b
copolymers
abs
PL
film λf,max(nm)
film λedge(nm)
Egopt (eV)
HOMO (eV)
LUMO (eV)
Egec (eV)
PT-ID
423
504
526
663
682
443
517
534
707
644
748
1.75
1.92
1.66
−5.22
−5.44
−5.25
−3.40
−3.44
−3.54
1.82
2.00
1.71
PT-DTBT
PT-DTBTID
705
b
a
Measured in chloroform solution. Band gap estimated from the optical absorption band edge of the films.
Figure 4. UV−vis absorption spectra of the copolymers in chloroform
solutions and in the film states.
Figure 6. Cyclic voltammograms of the polymer films on platinum
electrode in 0.1 mol/L Bu4NPF6 acetonitrile solution.
PT-DTBTID, respectively. On the other hand, the onset potentials
for reduction (Ered) of them were found to be −0.89, −0.85,
and −0.75 V, respectively. Accordingly, the HOMO energy
levels of PT-ID, PT-DTBT, and PT-DTBTID, are −5.22, −5.44,
zand −5.25 eV, respectively. Regarding the threshold
HOMO energy level for air stable conjugated polymers being
estimated to be −5.2 eV,29,30 the deep HOMO levels of the
three copolymers with acceptor groups (DTBT, ID, and
DTBTID) should be beneficial to their chemical stability in
ambient conditions. In addition, the deep HOMO levels of the
copolymers are desirable for higher open-circuit voltage (Voc) of
the PSCs with copolymers as donor materials31 because the Voc
is usually proportional to the difference between the HOMO
energy level of the donor and the LUMO energy level of the
acceptor. The HOMO energy level of PT-DTBT is ∼0.2 eV
lower than those of PT-ID and PT-DTBTID, predicting that
PT-DTBT would achieve the highest Voc value. On the other
hand, a donor polymer intended for use with a soluble fullerene
acceptor (e.g., PC61BM) should have a LUMO offset of
approximately 0.3−0.4 eV relative to PC61BM (−4.2 eV)32,33
for the effective charge transfer. The LUMO levels of PT-ID,
PT-DTBT, and PT-DTBTID are −3.40, −3.44, and −3.54 eV,
respectively, which imply an effective charge transfer could
occur from the copolymers to PC61BM.34 The electrochemi-
cally estimated band gaps of the copolymers are 1.71−2.00 eV
and they are different from corresponding optical band gaps
(Egopt), but within the range of error (0.2−0.5 eV).35
Furthermore, compared with PT-ID, the lower LUMO energy
levels of polymers PT-DTBT and PT-DTBTID should be a
result of the extension of π-electron delocalization in the
polymer backbone.36 In a word, with the increase the length of
conjugated side chains, the LUMO energy levels of the three
copolymers are lowered gradually.
Figure 5. Photoluminescence spectra of copolymers in the CHCl3 solution.
orbital (LUMO) energy levels of the conjugated polymers are
important parameters in the design of optoelectronic devices,
and they can be estimated from the onset oxidation and
reduction potentials of cyclic voltammogram (CV).28 Figure 6
shows the CV curves of the three copolymers and that of
ferrocene for potential calibration. The redox potential of
ferrocene is 0.51 V vs SCE. On the basis of 4.8 eV below
vacuum for the energy level of Fc/Fc+, the HOMO and LUMO
energy levels of as well as the electrochemical energy gaps
(Egec) of copolymers were calculated according to the following
equations:
HOMO = −e(E + 4.29) (eV)
ox
LUMO = −e(E + 4.29) (eV)
red
ec
E
= e(E − E ) (eV)
ox red
g
The onset potentials for oxidation (Eox) were observed to
be 0.93, 1.15, and 0.96 V for PT-ID, PT-DTBT, and
Photovoltaic Properties. In order to investigate whether
the conjugated side groups of the three copolymers make a
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contribution to the photoelectric conversion in the PSCs, the
bulk heterojunction PSC devices based on the blend of the
copolymers and PC61BM have been fabricated. The structure of
fabricated devices is ITO/PEDOT:PSS/polymer:PC61BM-
(1:4,w/w)/Ca/Al. Figure 7 shows the current−voltage (J−V)
Figure 8. IPCE of the polymer solar cells based on PT-ID, PT-DTBT,
and PT-DTBTID.
To further understand the cause of variation in solar cell
performance, the hole mobility of these copolymers was
measured using the OTFT technique under the same
conditions. The mobility values of the copolymers are listed
in Table 3. PT-DTBT with the DTBT side chain demonstrated
a relatively high hole mobility of 3.2 × 10−3 cm2 V−1 s−1 which is
two order higher than those of PT-ID (6 × 10−5 cm2 V−1 s−1)
and PT-DTBTID (3 × 10−5 cm2 V−1 s−1) (Table 3). Fur-
thermore, the hole mobility of PT-ID is double of PT-DTBTID.
Therefore, the high hole mobility of the PT-DTBT further clearly
explain why the photovoltaic properties of the PT-DTBT are
better than those of PT-ID and PT-DTBTID.
Figure 7. J−V curves of the photovoltaic cells based on PT-ID,
PT-DTBT, and PT-DTBTID under the illumination of AM 1.5,
100 mW/cm2.
curves of the PSCs based on the three copolymers. The
photovoltaic parameters of the PSCs are summarized in
Table 3. Under standard global AM 1.5 solar condition, the
Table 3. Photovoltaic Properties and Mobility of PT-ID, PT-
DTBT, and PT-DTBTID
CONCLUSION
copolymers/PC61BM
(1:4, w/w)
Voc
Jsc
PCE
(%)
mobility
■
(V) (mA/cm2)
FF
(cm2/V S)
In summary, three novel alternating copolymers based on
benzo 1,2-b:4,5-b′ dithiophen and thiophene with different
conjugated side groups have been successfully synthesized and
characterized. The three copolymers show good solubility and
film-forming ability. Through manipulating the length of the
acceptor groups attached at the conjugated side chains of the
copolymers, the photophysical and electrochemical properties
of the copolymers are effectively tuned. The best power
conversion efficiency of the PSCs based on PT-DTBT/
PC61BM (1:4, w/w) reached to 4.18% under the illumination
of AM1.5, 100 mW cm−2. The broad absorption spectrum,
deep HOMO level and the noticeably high hole mobility of
PT-DTBT contribute to the outstanding Jsc, Voc as well as the
high device efficiency.
PT-ID
0.71
0.81
0.62
7.19
12.65
4.67
0.49
0.41
0.40
2.48
4.18
1.16
6 × 10−5
3.2 × 10−3
3 × 10−5
PT-DTBT
PT-DTBTID
photovoltaic cells based on PT-DTBT exhibit better photo-
voltaic performance (Jsc = 12.65 mA/cm2, Voc = 0.81 V, FF =
0.41, and PCE = 4.18%) than PT-ID based device (Jsc = 7.19
mA/cm2, Voc = 0.71 V, FF = 0.49, and PCE = 2.48%) and PT-
DTBTID based device (Jsc = 4.67 mA/cm2, Voc = 0.62 V, FF =
0.40, and PCE = 1.16%). In comparison with PT-ID, the better
overlap of the absorption spectrum of PT-DTBT with the solar
spectrum can account for the increase in Jsc as revealed by incident-
photon-to-current efficiency (IPCE) spectra (Figure 8). On the
other hand, the PCE based on PT-DTBTID is considerably lower
than those of PT-ID and PT-DTBT, despite the fact that the
absorption spectrum of PT-DTBTID better match the solar
spectrum in comparison with the other two. That is because the
electron deficient nature of the DTBTID group provides a wider
photo responsive range (Figure 8), but it can also act as a trapping
site, inhibiting electron transfer from the main chain to the
acceptor and as a result to produce low IPCE values and poor
photovoltaic performance for PT-DTBTID.37 In addition, the
bulky side chain (DTBTID) of the PT-DTBTID leads to a
significant decrease in Jsc, consequently, a decrease in the PCE of
the solar cell. PT-DTBT with DTBT as the side chains shows the
highest PCE due to the highest Jsc and Voc. Therefore, the side
group DTBT seems to be better than ID, and ID seems to be
better than DTBTID.
EXPERIMENTAL SECTION
■
Materials and Characterization. All the chemicals were
purchased from Alfa Aesar or Shanghai Medical Company (China).
THF and toluene were refluxed over sodium and benzophenone, then
distilled prior to use. DMF was dried and distilled under reduced
pressure. All other commercially available materials were used as
received unless noted otherwise. Column chromatography was carried
out on silica gel (merck, 200−300 mesh)
1H NMR and 13C NMR spectra were measured with Bruker
AVANCE 400 spectrometer. FT-IR spectra were obtained on Perkin-
Elmer Spectra One. UV−vis spectra and photoluminescence (PL)
spectra of the copolymers were measured on Perkin-Elmer Lamada 25
spectrometer and Perkin-Elmer LS-50 luminescence spectrometer,
respectively. Molecular mass was determined by matrix assisted laser
desorption-ionization time-of-flight mass spectrometry (MALDI−
TOF MS) using a Bruker Aupoflex-III mass spectrometer. The ele-
mental analysis of all monomers and copolymers were performed with
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an Elementar Vario EL III element analyzer for C, H, and S determi-
nation. Thermogravimetric analyses (TGA) were performed under
nitrogen at a heating rate of 20 °C/min with Netzsch TG 209 analyzer,
and differential scanning calorimetric measurement (DSC) was recorded
with TA DSCQ10 instrument at a heating rate of 20 °C/min. The
average molecular weight and polydispersity index (PDI) of the copoly-
mers were determined using Waters 1515 gel permeation chromatog-
raphy (GPC) analysis with THF as eluent and polystyrene as standard.
Cyclic voltammetry (CV) was conducted on an electrochemistry work-
station (CHI660A, Chenhua Shanghai) with the polymer film on Pt plate
as the working electrode, Pt slice as the counter electrode, and saturated
calomel electrode (SCE) as a reference electrode in a 0.1 M tetra-
n-butylammonium hexafluorophosphate acetonitrile solution at a scan rate
of 50 mV/s.
Fabrication and Characterization of Polymer Solar Cells. The
photovoltaic cells were constructed in the traditional sandwich
structure through several steps. The ITO coated glass substrates
were cleaned by successive ultrasonic treatment in acetone and
isopropyl alcohol, then treated with a nitrogen−oxygen plasma oven
for 5 min. Poly(3,4-ethylene dioxythiophene)−poly(styrenesulfonate)
(PEDOT:PSS, from Bayer AG) was spin-coated from an aqueous
solution on a cleaned indium tin oxide (ITO) glass substrate giving a
thickness of about 40 nm as measured by Ambios Technology XP-2
surface profilometer, then it was dried at 150 °C for 15 min.
Subsequently, the photoactive layer was prepared by spin-coating the
dichlorobenzene solution of polymer:PC60BM (1:4, w/w) with the
polymer concentration of 6 mg/mL on the top of the PEDOT:PSS
layer and the thickness of the active layers of ca. 80 nm, and then
annealed at 150 °C for 5 min in a nitrogen-filled glovebox. Finally, the
substrates were transferred into an evaporator and pumped down to
5 × 10−4 Pa. to deposit 10 nm of Ca and 100 nm of aluminum
cathodes, producing an active area of 4 mm2 for each cell. Current
density−voltage (J−V) characteristics were measured by a Keithley
2602 Source Meter under 100 mW cm−2 irradiation using a 500W Xe
lamp equipped with a global AM 1.5 filter for solar spectrum
simulation, and the incident light intensity was calibrated using a
standard Si solar cell. The measurement of monochromatic incident
photon-to-current conversion efficiency (IPCE) was performed using
a Zolix DCS300PA Data acquisition system.
(15 mL) in the dark. The resulting solution was stirred at room
temperature under argon overnight. Then, the organic material was
extracted with dichloromethane and washed with water. After the
solvent was removed, the crude product was purified on silica gel
chromatography using a petroleum ether/dichloromethane mixture
(3:1 by volume) as eluent. A light yellow solid was obtained. Yield:
1
2.82 g (70%). H NMR (400 MHz, CDCl3, δ/ppm): 9.82 (s, 1H),
7.36 (s, 1H). 13C NMR (100 MHz, CDCl3, δ/ppm): 183.04, 139.37,
128.70, 124.03, 113.34.
3-Hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[1,2,5]thiadiazol-
4-yl)thiophene-2-carbaldehyde (2). The compound 2 was
synthesized by the Vilsmeier reaction. To the mixture of compound
1 (2.4 g, 5.1 mmol) in dichloroethane (25 mL) and anhydrous DMF
(0.4 mL, 5.1 mmol) was added dropwise under nitrogen atmosphere,
then POCl3 (0.48 mL, 5.1 mmol) was added slowly. The mixture was
stirred at 85 °C for 12 h. Then the mixture was cooled to room
temperature, 30 mL of water was added to quench the reaction. The
mixture was extracted with CH2Cl2. After the solvent was removed, the
crude product was purified on silica gel chromatography using a
petroleum ether/dichloromethane mixture (1:1 by volume) as eluent.
1
An orange solid was obtained. Yield: 1.78 g (70%). H NMR (400
MHz, CDCl3, δ/ppm): 10.12 (s, 1H), 8.08 (s, 1H), 8.06 (s, 1H), 7.99
(d, 1H, J = 7.64 Hz), 7.89 (d, 1H, J = 7.60 Hz), 7.11 (s, 1H), 3.06 (t,
2H, J = 7.66 Hz), 2.72 (t, 2H, J = 7.62 Hz), 1.72−1.83 (m, 4H), 1.44−
1.36 (m, 12H), 0.96 (m, 6H).
(E)-4-(5-(2-(2,5-dibromothiophen-3-yl)vinyl)-4-hexylthio-
phen-2-yl)-7-(4-hexylthiophen-2-yl)benzo[1,2,5]thiadiazole
(M2). Compound 2 (0.497 g, 1 mmol) and 3 (0.39 g, 1 mmol) were
dissolved in THF (20 mL) and the solution was stirred at room
temperature for 30 min under nitrogen atmosphere. Then potassium
tertbutoxide (0.13 g, 1.15 mmol) was dissolved in THF (20 mL) and
added dropwise to the solution. The reaction mixture was stirred for
5 h at room temperature, and then heated to 50 °C for 12 h. After
cooling to room temperature, the reaction mixture was extracted with
dichloromethane and washed with dilute aqueous HCl solution. The
organic phase was dried over anhydrous MgSO4, and then the solvent
was removed by rotary evaporation. The crude product was purified
on silica gel chromatography using a petroleum ether/dichloro-
methane mixture (1:1 by volume) as eluent. A red solid was obtained.
OTFTs Device Fabrication. OTFTs fabricated on OTS-modified
SiO2/Si substrates. OTFT devices were fabricated in a top-contact
configuration. Before the deposition of organic semiconductors,
octyltrichlorosilane (OTS) treatment was performed on the gate
dielectrics which were placed in a vacuum oven with OTS at a
temperature of 120 °C to form an OTS self-assembled monolayer.
Then the polymer thin films were spin-coated on the OTS modified
SiO2/Si substrates from the chloroform solutions, followed by Au
deposition through a shadow mask to define the source and drain
electrodes. The OTFTs characteristics of the devices were determined
at room temperature in air by using a Keithley 4200 SCS. The mobility
of the devices was calculated in the saturation regime. The equation is
listed as follows:
1
Yield: 0.48 g (65%). FT-IR (KBr, cm−1): 938 (trans-vinylene). H
NMR (400 MHz, CDCl3, δ/ppm): 8.01 (s, 1H), 7.98 (s, 1H), 7.86 (d,
2H, J = 1.44 Hz), 7.22 (s, 1H), 7.12 (d, 1H, J = 15.96 Hz), 7.07 (s,
1H), 6.90 (d, 1H, J = 15.96 Hz), 2.70 (m, 4H), 1.72−1.71 (m, 4H),
1.42−1.36 (m, 12H), 0.93−0.92 (m, 6H). 13C NMR (100 MHz,
CDCl3, δ/ppm): 152.63, 143.39, 143.37, 139.03, 137.39, 136.92,
130.55, 129.15, 127.31, 126.32, 125.47, 122.57, 121.69, 119.50, 111.91,
109.72, 31.68, 30.64, 29.01, 22.59, 14.02. Anal. Calcd for
C32H34Br2N2S4: C, 52.31; H, 4.66; N, 3.81; S, 17.46. Found: C,
53.26; H, 5.19; N, 3.80; S, 17.31. MALDI−TOF MS (C32H34Br2N2S4),
m/z: calcd, 734.000; found, 733.905.
(E)-5-(7-(5-(2-(2,5-Dibromothiophen-3-yl)vinyl)-4-hexylthio-
phen-2-yl)benzo[1,2,5]thiadiazol-4-yl)-3-hexylthiophene-2-
carbaldehyde (M3). By following a similar method, as for compound
2, monomer M3 was synthesized from anhydrous DMF (0.80 mL,
10.2 mmol), POCl3 (0.94 mL, 10.0 mmol) and monomer M2 (0.743 g,
1 mmol). The crude product was purified on silica gel chromatography
using a petroleum ether/dichloromethane mixture (3:1 by volume) as
eluent. A black red solid was obtained. Yield: 0.32 g (40%). FT-IR (KBr,
cm−1): 1657 (vCO), 929 (trans-vinylene). 1H NMR (400 MHz, CDCl3,
δ/ppm): 10.12 (s, 1H), 8.09 (s, 1H), 8.04 (s, 1H), 7.99 (d, 1H, J =
7.60 Hz), 7.90 (d, 1H, J = 7.56 Hz), 7.23 (s, 1H), 7.11 (d, 1H, J =
15.88 Hz), 6.91 (d, 1H, J = 15.76 Hz), 3.06 (t, 2H, J = 7. 48 Hz), 2.76
(t, 2H, J = 7.42 Hz), 1.81−1.69 (m, 4H), 1.56−1.27 (m, 12H), 0.930−
0.927 (m, 6H). 13C NMR (100 MHz, CDCl3, δ/ppm): 181.98, 153.27,
147.38, 143.48, 139.05, 138.00, 137.52, 136.81, 131.41, 130.57, 127.26,
124.98, 122.35, 119.99, 112.03, 31.47, 28.96, 22.55, 14.00. Anal. Calcd for
C33H34Br2N2OS4 C, 51.92; H, 4.46; N, 3.67; S, 16.78. Found: C, 52.42;
H, 4.72; N, 3.44; S, 15.74. MALDI−TOF MS (C33H34Br2N2OS4) m/z:
calcd, 762.700; found 761.951.
2
I
= (W/2L)C μ(V − V )
DS
i
GS
th
Here W/L is the channel width/length, Ci is the insulator capacitance
per unit area, and VGS and Vth are the gate voltage and threshold
voltage, respectively.
Synthesis of the Monomers and Copolymers. The synthetic
routes of the monomers and copolymers are shown in Scheme 1.
4,7-Bis(4-hexylthiophen-2-yl)benzo[1,2,5]thiadiazole (1),38 (2,5-di-
bromothiophen-3-ylmethyl) phosphonic acid diethyl ester (3),39 and
2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]-
dithiophene (M4),17 were synthesized according to already published
procedures. The detailed synthetic procedures of other compounds are
as follows.
2,5-Dibromothiophene-3-carbaldehyde (M1). N-Bromosucci-
nimide (NBS) (5.337 g, 30 mmol) was dissolved in 35 mL of
anhydrous DMF. The mixture was added dropwise to a mixture
of thiophene-3-carbaldehyde (1.68 g, 15 mmol) in anhydrous DMF
2364
dx.doi.org/10.1021/ma202399n | Macromolecules 2012, 45, 2359−2366

Macromolecules
Article
2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-
Notes
b:4,5-b′]dithiophene (M4). M4 were synthesized according to
The authors declare no competing financial interest.
already published procedures17 with a yield of 75%. H NMR (400
1
MHz, CDCl3, δ/ppm): 7.55 (s, 2 H), 4.23−4.22 (d, 4 H, J = 4.52 Hz),
1.85−1.71 (m, 2 H), 1.65−1.29 (m, 16 H), 1.08−0.98 (m, 12 H), 0.48
(s, 18 H). 13C NMR (100 MHz, CDCl3, δ/ppm): 143.3, 140.3, 133.8,
132.9, 128.0, 75.66, 40.72, 30.58, 29.26, 23.95, 23.14, 14.12, 11.32.
MALDI−TOF MS (m/z): calcd, 772.160; found, 772.130.
ACKNOWLEDGMENTS
■
This work was supported by the National Nature Science
Foundation of China (Nos. 50973092, 51003089, 21004050,
51173154), National Basic Research Program of China (No.
2011CBA00701), and Specialized Research Fund for the
Doctoral Program of Higher Education of China (Nos.
20094301120005 and 20104301110003).
Synthesis of the Precursor Polymer PT-CHO. M1 (67.25 mg,
0.25 mmol) and M4 (193 mg, 0.25 mmol) were dissolved in 20 mL of
toluene, and the solution was flushed with argon for 15 min; then,
30 mg of Pd(PPh3)4 was added into the solution. The mixture was
again flushed with argon for 30 min. The polymerization was carried
out at 110 °C for 3 days. Finally, the reaction mixture was cooled to
room temperature and slowly added to methanol (120 mL). The
precipitate was collected by filtration from methanol and further
purified by Soxhlet extraction with methanol, hexane, and chloroform
in sequence. The chloroform fraction was evaporated by rotary
evaporation. The final product was obtained as a green solid and dried
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1
72 mg (49%). H NMR (400 MHz, CDCl3, δ/ppm): 8.75 (br, 1H),
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tanst2008@163.com.
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