Design, synthesis and photovoltaic properties of a new D-π-A polymer with extended π-bridge units

Article abstract of DOI:10.1039/c2jm32931j

New low band gap copolymers PBDTT-DTTBT and PBDTT-DTBT using BDT-T and BT as donor and acceptor, with different π-bridge units thieno[3,2-b]thiophene and thiophene, respectively, were synthesized. The absorption spectra, electronic energy levels, and phot

Full text of DOI:10.1039/c2jm32931j

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PAPER
Design, synthesis and photovoltaic properties of a new D–p–A polymer with
extended p-bridge units†
a
a
a
Xia Guo,ab Maojie Zhang, Lijun Huo, Feng Xu,a Yue Wuab and Jianhui Hou
*
*
Received 9th May 2012, Accepted 19th August 2012
DOI: 10.1039/c2jm32931j
New low band gap copolymers PBDTT–DTTBT and PBDTT–DTBT using BDT-T and BT as donor
and acceptor, with different p-bridge units thieno[3,2-b]thiophene and thiophene, respectively, were
synthesized. The absorption spectra, electronic energy levels, and photovoltaic properties of the
polymers were characterized. PBDTT–DTTBT and PBDTT–DTBT films show broad absorption in
the visible range with an absorption edge at over 751 nm, and possess relatively low-lying HOMO
levels at ꢀ5.11 eV and 5.15 eV. A high hole mobility of 1.97 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1 was recorded for
PBDTT–DTTBT, that is two orders of magnitude higher than PBDTT–DTBT (1.58 ꢁ 10ꢀ5 cm2 Vꢀ1
s
ꢀ1), which is ascribed to the application of thieno[3,2-b]thiophene as p-bridge units. The PCE of the
PSC device based on PBDTT–DTTBT/PC70BM (1 : 1, w/w) reached 6.03% with a Jsc of 12.46 mA
cmꢀ2, a Voc of 0.78 V and a FF of 0.62, while PBDTT–DTBT/PC70BM (1 : 1, w/w) only has a PCE of
2.34% with a Voc of 0.82 V, a Jsc of 5.78 mA cmꢀ2 and a FF of 0.495 under the illumination of AM
1.5 G, 100 mW cmꢀ2. These results indicate that PBDTT–DTTBT is a promising photovoltaic
material. Furthermore, it can be concluded that extending the conjugation of the backbone units, as
demonstrated in this work, has little influence on the molecular energy levels, but can be seen as a
feasible strategy to improve the photovoltaic properties of D–p–A conjugated polymers by enhancing
intermolecular interactions.
The design and synthesis of low band gap (LBG) polymers
Introduction
have been proven to be one of the successful strategies to improve
the absorption of the conjugated polymers, because a broad
absorption band can be realized by molecular structure design.
Therefore, high short circuit density values (Jsc) can be obtained
in PSCs based on LBG polymers. One of the broadly adapted
strategies to design LBG polymers is to alternate the electron-
rich (donor) and the electron-deficient (acceptor) units on the
backbones of conjugated polymers.3 Conjugated polymers with
alternating D–p–A (wherein p represents a conjugated bridge)
architecture are particularly desirable due to the facile tunability
of their electronic structures. In order to get D–p–A polymers for
efficient PSCs, much effort has been devoted to the design and
synthesis of new donor and acceptor building blocks.4 According
to the reported work, thiophene or alkylthiophene units are
mainly used as p-bridges in D–p–A polymers with efficient
photovoltaic properties. Although the p-bridge unit has great
influence on the photovoltaic properties of D–p–A polymers,
there are only a few studies which focus on it.5,6 LBG polymers
based on benzo[1,2-b:4,5-b’]dithiophene (BDT) and bis(thio-
phen-2-yl)-benzothiadiazole (DTBT) have been designed,
synthesized and applied in PSCs. A PCE over 5% has been
achieved based on some of these PBDTDTBT-based poly-
mers.7–10 Different strategies have been employed to improve the
photovoltaic properties of this type of polymers. For example,
Bulk heterojunction (BHJ) polymer solar cells (PSCs) have been
extensively investigated in both scientific research and industrial
applications during the last decade, because of their potential
applications in making large area, light weight, flexible photo-
voltaic panels through low-cost solution-processable tech-
niques.1 Conjugated polymers and fullerene derivatives are
commonly used as electron donors and electron acceptor mate-
rials, respectively, in PSCs with a BHJ structure.2 As it is well
known, an ideal polymer electron donor material should possess
a broad absorption band with a high extinction coefficient to
absorb more sunlight, good mobility to facilitate charge trans-
port, suitable molecular energy levels matching those of the
fullerene acceptor, and appropriate compatibility with the
fullerene acceptor to form a nanoscale bicontinuous inter-
penetrating network.
aState Key Laboratory of Polymer Physics and Chemistry, Beijing
National Laboratory for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China. E-mail: hjhzlz@
iccas.ac.cn; huolijun@iccas.ac.cn
bGraduate University of Chinese Academy of Sciences, Beijing 100049,
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm32931j
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Hou et al. reported a PBDTDTBT polymer with a deeper
HOMO level;7 Huang et al. used a fused BT named naphtho[1,2-
c:5,6-c]bis[1,2,5] thiadiazole (NT) to replace the BT unit,8 You
et al. introduced fluorine atoms to BT units.9
The weight-average molecular weights (Mw) of PBDTT–DTTBT
estimated by the GPC method is 31 kDa with a polydispersity
index (PDI) of 2.1 (the temperature of the GPC test is room
temperature), while PBDTT–DTBT has a Mw of 56 kDa with a
PDI of 2.8. The thermal stability of the polymers was investi-
gated with TGA, as shown in Fig. 1. The TGA plot reveals that
the onset temperatures with 5% weight-loss (Td) of PBDTT–
DTTBT and PBDTT–DTBT are 440 ꢂC and 455 ꢂC, respectively.
This indicates that both of the two polymers’ thermal stabilities
are adequate for applications in optoelectronic devices. Through
differential scanning calorimetry (DSC) measurements, neither
endo- nor exothermic processes were observed in the range of 30–
In this work, we designed and synthesized a new D–p–A
polymer in which alkylthiophene-substituted benzo[1,2-b:4,5-b’]
dithiophene (BDT-T), benzothiadiazole (BT) and thieno[3,2-b]
thiophene (TT) are used as donor, acceptor and p-bridge units,
respectively. As shown in Scheme 1, the TT unit was used as the
p-bridge instead of thiophene. As it is known, TT has a rigid and
coplanar fused ring, which ensures a highly delocalized p-elec-
tron system and strong intermolecular p–p stacking, and thus it
has attracted much interest for the construction of high perfor-
mance organic/polymer semiconductors for applications in
optoelectronic devices.11–15 Some conjugated polymers with TT
units on their backbones exhibit interesting properties in opto-
electronic devices. For example, the field-effect transistor (FET)
ꢂ
280 C (as shown in Fig. S1†).
UV-visible absorption and X-ray diffraction
The ultraviolet-visible (UV-vis) absorption spectra provide
information on the electronic structures of the conjugated
polymers. Fig. 2 shows the UV-vis absorption spectra of the
polymers in chloroform solution and films casted from chloro-
form solution and o-DCB solution on quartz substrate. PBDTT–
DTTBT and PBDTT–DTBT exhibit similar optical band gaps,
which are 1.65 eV and 1.63 eV, respectively, indicating that the
introduction of the extended p-bridge, as shown in this work, has
little influence on the optical band gap. As shown in Fig. 2,
PBDTT–DTBT shows an absorption peak (lmax) at 555 nm in
chloroform solution and an absorption peak (lmax) at 619 nm in
solid film casted from chloroform, and the absorption spectra of
the films casted from chloroform and o-DCB solution are almost
identical. The absorption spectrum of PBDTT–DTTBT in
chloroform exhibits a peak at 607 nm, which is ꢃ60 nm red-
shifted compared to that of PBDTT–DTBT, indicating that in
solution state, the p-electrons of PBDTT–DTTBT can be delo-
calized better than in those of PBDTT–DTBT. Very interest-
ingly, for PBDTT–DTTBT, the film casted from chloroform
solution displays a different absorption spectrum compared to
the film casted from o-DCB solution; the absorption spectrum of
the film casted from o-DCB exhibits a strong shoulder peak at
680 nm. According to the reported work, this kind of shoulder
peak is observed in the conjugated polymers due to strong
intermolecular p–p stacking properties.17 Since o-DCB has a
much higher boiling point than chloroform, the slower drying
speed of the o-DCB solution gives longer time for the polymer to
form a more ordered structure. Compared to the PBDTT–DTBT
film processed by o-DCB, the absorption shoulder peak of the
PBDTT–DTTBT film indicates that stronger or/and ordered
intermolecular interactions might be formed in the PBDTT–
DTTBT film due to the introduction of the extended p-bridge
units.
hole mobility of a conjugated polymer containing a TT unit
reached as high as 1.95 cm2 Vꢀ1 sꢀ1
15
, and in an earlier report,
when the thiophene bridge was replaced by thieno[3,2-b]thio-
phene, the FET hole mobility can be improved by one or two
orders of magnitude,16 from 8.6 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1 to 3.6 ꢁ
10ꢀ3 cm2 Vꢀ1 sꢀ1 or from 2.2 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1 to 1.5 ꢁ 10ꢀ4
cm2 Vꢀ1 ꢀ1, and the D–A copolymer PSCs based on TT as
s
donor unit exhibited a high PCE over 5%.14,15 Based on the
reported work, we believe that the hole mobility and thus the
photovoltaic properties of the polymer will be improved after
replacing the thiophene unit by the TT unit. However, fused
thiophene oligomers or polymers usually show poor solubility at
room temperature. In order to overcome this disadvantage, n-
C11H23 groups were introduced onto the 3-positions of TT, and
more bulky groups, hexyldecyl, were employed as substituents on
the BDT units. Consequently, PBDTT–DTTBT, the new D–p–A
copolymer, was designed on account of the above points. As
comparison, we also synthesized another new polymer PBDTT–
DTBT (shown in Scheme 1), in which the p-bridge unit is thio-
phene. Consequently, compared with the analogous BT-based
polymer, the TTBT-based polymer exhibited higher mobility,
higher Jsc and FF, and a much improved PSC performance, with
a PCE of 6%.
Results and discussion
Thermal properties
The synthetic routes of the monomers and the polymer of
PBDTT–DTTBT were shown in Scheme 2. The polymers have
good solubility in chloroform and ortho-dichlorobenzene (o-
DCB) but poor solubility in toluene and tetrahydrofuran (THF).
X-Ray diffraction (XRD) measurements have been used to
investigate the formation of ordered structures within the thin
films. Fig. 3 shows the XRD patterns of the films of PBDTT–
DTTBT and PBDTT–DTBT casted from o-DCB. The films of
PBDTT–DTTBT and PBDTT–DTBT show peaks at 2q ¼ 3.96ꢂ
ꢂ
ꢀ
(corresponding to a d-spacing of 22.9 A) and 2q ¼ 3.66 (corre-
ꢀ
sponding to a d-spacing of 24.11 A), respectively, and it is clear
that the peak of PBDTT–DTTBT is much stronger than that of
PBDTT–DTBT. Furthermore, the film of PBDTT–DTTBT
shows pronounced (200) reflection at 7.89ꢂ, while only a (100)
Scheme 1 Molecular structure of PBDTT–DTTBT and PBDTT–
DTBT.
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Scheme 2 Synthetic routes for the monomers and polymer.
Fig. 3 X-Ray diffraction patterns of the pristine PBDTT–DTTBT and
PBDTT–DTBT films casted from o-DCB onto a Si substrate.
Fig. 1 TGA plots of PBDTT–DTTBT and PBDTT–DTBT with a
heating rate of 10 ꢂC minꢀ1 in the inert atmosphere.
Fig. 2 Normalized UV-vis absorption spectra of PBDTT–DTTBT and
PBDTT–DTBT in dilute chloroform solutions and thin films on quartz
substrate casted from chloroform and o-DCB.
Fig. 4 Cyclic voltammograms of PBDTT–DTTBT and PBDTT–DTBT
films on a platinum electrode measured in 0.1 mol Lꢀ1 Bu4NPF6 aceto-
nitrile solutions at a scan rate of 100 mV sꢀ1
.
21026 | J. Mater. Chem., 2012, 22, 21024–21031
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Photovoltaic properties
Fig. 5 shows the current density–voltage (J–V) characteristics of
the devices with different D/A (polymer/PC70BM, w/w) ratios of
PBDTT–DTTBT and PBDTT–DTBT under the illumination of
AM 1.5 G, 100 mW cmꢀ2. Table 1 lists the photovoltaic
performances of the PSCs fabricated under different conditions.
The best power conversion efficiency (PCE) of PBDTT–DTTBT
was obtained from the PSC device with a D/A ratio of 1 : 1, with
a Voc of 0.78 V, a Jsc of 12.46 mA cmꢀ2 and a FF of 0.620,
leading to a PCE of 6.03%, and the PCEs are reproducible. For
example, in more than 20 devices, the average PCE of the PSCs is
over 5.9%, and 8 devices are listed (see Table S1 and Fig. S2 in
ESI†). Under the same conditions, the PBDTT–DTBT based
device gives a PCE of 2.34% with a Voc of 0.82 V, a Jsc of 5.78 mA
cmꢀ2 and a FF of 0.495, which is quite similar with the reported
work.8
Fig. 5 J–V curves of the polymer solar cells based on PBDTT–DTTBT
and PBDTT–DTBT with PC70BM as acceptor under the illumination of
The external quantum efficiencies (EQEs) and the absorption
spectra of the PSC devices under different conditions are shown
in Fig. 6a and b. Since the absorption at long wavelength range is
mainly from the polymer, the PSC device with a D/A ratio of
1 : 1.5 shows the weakest absorbance at long wavelengths and
also the lowest quantum efficiency (QE). Since the absorption in
the range from 550 nm to 700 nm is mainly PC70BM, the PSC
device with a D/A ratio of 1.5 : 1 shows weaker absorbance and
thus a lower QE in this range; the PSC device with a D/A ratio of
1 : 1 exhibits more efficient QE in the whole response range, and
therefore the highest Jsc was recorded under these conditions. It
is worth pointing out that the discrepancy between the current
density values calculated from the EQE and the J–V measure-
ments is within 3%, indicating that the photovoltaic results are
reliable in this work. In comparison with the reported work and
the results obtained from this work, PBDTT–DTTBT exhibits
much better photovoltaic performance than the PSCs based on
PBDTT–DTBT, indicating that the extended conjugation of the
backbone units, as demonstrated in this work, can be seen as a
feasible method to improve the photovoltaic properties of
conjugated polymers.
AM 1.5 G, 100 mW cmꢀ2
.
peak can be observed in the film of PBDTT–DTBT. The
comparison between the XRD patterns of the two films clearly
indicates that a more ordered structure can be formed in the film
of PBDTT–DTTBT, which may be ascribed to the introduction
of the extended p-bridge units. The results obtained from XRD
measurements are in agreement with the phenomena observed in
UV-vis absorption measurements.
Electrochemical properties
Cyclic voltammetry (CV) measurements were used to measure
the oxidation and reduction potentials of the polymers required
for determining the molecular energy levels of the conjugated
polymers.18 As shown in Fig. 4, PBDTT–DTTBT presents a
reversible p-doping (Eox ¼ 0.37 V vs. ferrocene/ferrocenium (Fc/
Fc+)) and an irreversible n-doping (Ered ¼ ꢀ1.41 V vs. Fc/Fc+)
process. The highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) levels of the
polymers were calculated from the electrochemical onset oxida-
tion potential (Eox) and the onset reduction potential (Ered),
respectively, where HOMO ¼ ꢀe(Eox + 4.80) (eV) and LUMO ¼
ꢀe(Ered + 4.80) (eV).19 The HOMO and LUMO levels, as well as
the electrochemical band gap (EEg C), of the polymer are ꢀ5.11 eV,
Morphology
Tapping mode atomic force microscopy (AFM) measurements
were carried out to demonstrate the morphology of the blend
films of PBDTT–DTTBT/PC70BM (w/w) with different ratios
and PBDTT–DTBT/PC70BM (w/w, 1 : 1). The AFM images
under D/A ratios of 1 : 1, 1.5 : 1 and 1 : 1.5 are shown in Fig. 7.
As shown, very uniform films can be formed under three different
D/A ratios, and the roughness of the films is below 1.4 nm. As
shown in Fig. S3,† the roughness of the PBDTT–DTBT/PC70BM
ꢀ3.33 eV and 1.78 eV, respectively, calculated from 4ox and 4red
.
PBDTT–DTBT presents reversible p-doping (Eox ¼ 0.40 V vs.
ferrocene/ferrocenium (Fc/Fc+)) and n-doping (Ered ¼ ꢀ1.48 V
vs. Fc/Fc+) processes, and the HOMO and LUMO levels are
ꢀ5.14 eV and ꢀ3.22 eV, and EEg C is 1.88 eV.
Table 1 Photovoltaic properties of the PSCs based on polymers/PC70BM (different ratio, w/w) under the illumination of AM 1.5 G, 100 mW cmꢀ2
Jsc
(mA cmꢀ2
D : A ratio
Thickness (nm)
Voc (V)
)
FF (%)
PCE (%)
PBDTT–DTTBT : PC70BM 1.5 : 1
PBDTT–DTTBT : PC70BM 1 : 1
PBDTT–DTTBT : PC70BM 1 : 1.5
PBDTT–DTBT : PC70BM1.5 : 1
PBDTT–DTBT : PC70BM 1 : 1
PBDTT–DTBT : PC70BM 1 : 1.5
82
85
87
80
86
85
0.80
0.78
0.77
0.82
0.82
0.79
11.35
12.46
9.00
5.01
5.78
4.23
51.5
62.0
57.4
46.6
49.5
49.7
4.68
6.03
3.99
1.91
2.34
1.66
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Fig. 6 (a) EQEs of the PSCs; (b) absorption spectra of the active layers based on PBDTT–DTTBT/PC70BM (w/w).
film is only 0.85 nm, indicating that a uniform film can also be
formed in the PBDTT–DTBT/PC70BM (w/w, 1 : 1) blend. Based
on all the phase contrast images, the films show similar phase
separation properties.
Hole mobility
Hole mobility is an important parameter for conjugated polymer
donor photovoltaic materials. Herein, the hole mobility of the
PBDTT–DTTBT/PC70BM blend film and the PBDTT–DTBT/
PC70BM blend film with a D/A ratio (w/w) of 1 : 1 were measured
by the space charge limited current (SCLC) method by a hole-only
device with a structure of ITO/PEDOT:PSS/polymer:PC70BM
(w/w, 1 : 1)/Au and estimated through the Mott–Gurney equa-
tion.20 Fig. 8 shows the ln(JL3/V2) versus (V/L)0.5 plot of the blend
films. The hole mobilities of the PBDTT–DTTBT/PC70BM blend
film and the PBDTT–DTBT/PC70BM blend film are 1.97 ꢁ 10ꢀ3
Fig. 8 ln(JL3/V2) versus (V/L)0.5 plot of PBDTT–DTTBT/PC70BM and
PBDTT–DTBT/PC70BM (1 : 1, w/w) blend films for the measurement of
hole mobility by the SCLC method.
cm2 Vꢀ1 sꢀ1 and 1.58 ꢁ 10ꢀ5 cm2 Vꢀ1
s
ꢀ1, respectively. As
reported, the hole mobilities of the two analogous polymers using
thiophene as a bridge unit between the BT and BDT units are 1.6
ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1 to 3.8 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1
8,9
,
which is
Fig. 7 AFM topography images (a, c and e) and phase contrast images (b, d and f) of PBDTT–DTTBT/PC70BM (with different ratio) films: 1.5 : 1 (a
and b), 1 : 1 (c and d), 1 : 1.5 (e and f).
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comparable with the mobility of PBDTT–DTBT. In comparison
with these analogues, the hole mobility of PBDTT–DTTBT is
about two orders of magnitude higher, which may benefit from the
long range ordered structure of the PBDTT–DTTBT film due to
the extended conjugation of the backbone units. The higher hole
mobility of PBDTT–DTTBT is desirable for its application as a
photovoltaic donor material in PSCs.
working electrode, counter electrode and reference electrode,
respectively, in
a
0.1
M
tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) acetonitrile solution. The potential
of the Ag/Ag+ reference electrode was internally calibrated by
using the ferrocene/ferrocenium redox couple (Fc/Fc+), which has
a known energy level at ꢀ4.8 eV. The thickness of the
PEDOT : PSS and active layer were determined by an Ambios
Tech. XP-2 profilometer. The current density–voltage (J–V)
characteristics were recorded with a Agilent B2912A Precision
Source/Measure unit. The power conversion efficiencies of the
resulting polymer solar cells were measured under 1 sun, AM
1.5 G (air mass 1.5 global) spectrum from a solar simulator (100
mW cmꢀ2) using a XES-70S1 (SAN-EI ELECTRIC CO., LTD.)
solar simulator (AAA grade, 70 mm ꢁ 70 mm photo-beam size).
A 2 ꢁ 2 cm monocrystalline silicon reference cell (SRC-1000-TC-
QZ) was purchased from VLSI Standards Inc. The external
quantum efficiency (EQE) was measured by a solar cell spectral
response measurement system QE-R3011 (Enli Technology Co.,
Ltd.). The light intensity at each wavelength was calibrated with a
standard single-crystal Si photovoltaic cell.
Conclusion
In conclusion, we designed and synthesized the new copolymers
PBDTT–DTTBT and PBDTT–DTBT using BDT-T and BT as
donor and acceptor, with different p-bridge units TT and thio-
phene, respectively. The absorption spectra, electronic energy
levels, and photovoltaic properties of the polymers were char-
acterized. PBDTT–DTTBT and PBDTT–DTBT films show
broad absorption in the visible range with an absorption edge at
over 751 nm, and possess relatively low-lying HOMO levels at
ꢀ5.11 eV and 5.15 eV. A high hole mobility of 1.97 ꢁ 10ꢀ3 cm2
V
ꢀ1 sꢀ1 was recorded for PBDTT–DTTBT, that is two orders of
magnitude higher than PBDTT–DTBT (1.58 ꢁ 10ꢀ5 cm2 Vꢀ1
s
ꢀ1), which is ascribed to the application of TT as p-bridge units.
Fabrication of polymer solar cells
The PCE of the PSC device based on PBDTT–DTTBT/PC70BM
(1 : 1, w/w) reached 6.03% with a Jsc of 12.46 mA cmꢀ2, a Voc of
0.78 V and a FF of 0.62, while PBDTT–DTBT/PC70BM (1 : 1, w/
w) only has a PCE of 2.34% with a Voc of 0.82 V, a Jsc of 5.78 mA
cmꢀ2 and a FF of 0.495 under the illumination of AM 1.5 G, 100
mW cmꢀ2. These results indicate that PBDTT–DTTBT is a
promising photovoltaic material. Furthermore, it can be
concluded that extending conjugation of the backbone units, as
demonstrated in this work, has little influence on the molecular
energy levels, but can be seen as a feasible strategy to improve the
photovoltaic properties of D–p–A conjugated polymers by
enhancing intermolecular interactions.
Polymer solar cell devices with the structure of ITO/PEDOT-PSS/
Polymers:PC70BM/Ca(10 nm)/Al(80 nm) were fabricated under
the conditions as follows: patterned indium tin oxide (ITO)-
coated glass with a sheet resistance of 10–15 Ohm per square was
cleaned with a surfactant scrub and then underwent a wet-clean-
ing process inside an ultrasonic bath, beginning with deionized
water, followed by acetone and isopropanol. After oxygen plasma
cleaning for 10 min,
a 40 nm thick poly(3,4-ethylene-
dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Bayer
Baytron 4083) anode buffer layer was spin-cast onto the ITO
substrate and then dried by baking in an oven at 150 ꢂC for 15 min.
The active layer, with a thickness in the range of 80–90 nm, was
then deposited on top of the PEDOT:PSS layer by casting from a
10 mg mlꢀ1 o-DCB solution. Finally, 10 nm Ca and 80 nm Al
layers were successively deposited in vacuum onto the active layer
at a pressure of ca. 4 ꢁ 10ꢀ4 Pa. The overlapping area between the
cathode and anode defined a pixel size of 4 mm2. Except for the
deposition of the PEDOT:PSS layers, all the fabrication processes
were carried out inside a controlled atmosphere of a nitrogen
drybox containing less than 10 ppm oxygen and moisture.
Experimental section
Materials
3-Bromothiophene, ethyl 2-mercaptoacetate, Pd(PPh3)4 were
purchased from Sigma-Aldrich Chemical Co.; dodecanoyl
chloride, 4,7-dibromo[2,1,3]benzothiadiazole were purchased
from Acros Chemical Co., and they were used as received.
Hexane and THF were dried over Na/benzophenone ketyl and
freshly distilled prior to use. The other materials were common
commercial level and were used as received.
Synthesis
3-Undecylthieno[3,2-b]thiophene (4). Compound 4 was synthe-
sized according to ref. 21 and was further purified by a chro-
matographic column using petroleum ether (60–90 ꢂC) as eluent to
obtain pure compound 4 as a viscous liquid (29.28 g, yield 42%).
1H NMR (CDCl3, 400 MHz), d (ppm): d 7.39 (d, 1H), 7.28 (d,
1H), 7.03 (s, 1H), 2.79 (t, 2H), 1.79 (m, 2H), 1.44–1.21 (br, 16H),
0.95 (m, 3H). 13C NMR (CDCl3, 100 MHz), d (ppm): d 140.02,
138.77, 134.93, 126.60, 121.79, 119.96, 31.97, 29.98, 29.73, 29.70,
29.64, 29.49, 29.46, 29.39, 28.70, 22.74, 14.15. MS: m/z ¼ 294.
Instruments and measurements
1H nuclear magnetic resonance (1H NMR) and 13C NMR spectra
were obtained with a Bruker DMX-400 spectrometer. UV-visible
absorption spectroscopy measurements were carried out using a
Hitachi U-3100 UV-vis spectrophotometer. Atomic force
microscopy (AFM) was performed on a Nanoscope III A (Vecco)
AFM in tapping mode. The molecular weight of the polymers was
measured by the GPC method, and polystyrene was used as the
standard by using chloroform as the eluent. Thermogravimetric
analysis (TGA) was performed on a TA Instruments, Inc., TGA-
2050. The CVs were conducted on a CHI650D electrochemical
workstation with a Pt disk, Pt plate, and Ag/Ag+ electrode as the
4,7-Bis(6-undecylthieno[3,2-b]thiophen-2-yl)benzothiadiazole
(5). In a 250 ml argon purged round flask, n-BuLi (2.88 M, 18.89
ml) was injected slowly with a syringe into a solution of
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J. Mater. Chem., 2012, 22, 21024–21031 | 21029

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ꢂ
compound 4 (16.0 g, 54.4 mmol) in THF (100 ml) at 0 C, then
of Sciences and National Natural Science Foundation of China
(NSFC) (no. 51173189, 21104088).
the mixture was stirred at this temperature for 1.5 h. Subse-
quently, chlorotrimethylstannane (1.0 M, 54.4 ml) was added at
0 ꢂC and warmed up to room temperature for another 6 h, then
4,7-dibromobenzothiadiazole (6.76 g, 23 mmol) and Pd(PPh3)4
(2.65 g, 2.3 mmol) were added together into the solution and the
reaction was refluxed for 12 h. After the solution was cooled to
room temperature, water (100 ml) was added and extracted by
chloroform twice and the combined organic phase was concen-
trated to remove the solvent. The residue was purified by a
chromatographic column using petroleum ether : dichloro-
methane (10 : 1) as eluent to obtain compound 5 as a deep red
powder (7.79 g, yield 47%).
1H NMR (CDCl3, 400 MHz), d (ppm): d 8.39 (s, 2H), 7.78 (s,
2H), 7.04 (s, 2H), 2.75 (t, 4H), 1.78 (m, 4H), 1.51–1.14 (br, 32H),
0.88 (m, 6H). 13C NMR (CDCl3, 100 MHz), d (ppm): d 152.48,
140.50, 140.18, 139.68, 135.17, 126.35, 125.36, 122.95, 121.00,
31.95, 29.97, 29.70, 29.67, 29.63, 29.46, 29.44, 29.39, 28.67, 22.72,
14.15. MS: m/z ¼ 721.
Notes and references
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Acknowledgements
This work was supported by National High-tech R&D Program
of China (863 Program) (no. 2011AA050523), Chinese Academy
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