Donor-acceptor oligomers and polymers composed of benzothiadiazole and 3-hexylthiophene: Effect of chain length and regioregularity

Article abstract of DOI:10.1002/cjoc.201300519

A series of donor-acceptor oligomer OBTTh_n (n=1-7) and polymer PBTTh₁ and PBTTh₂ composed of alternative 2,1,3-benzothiadiazole and 3-hexylthiophene have been designed and synthesized for the purpose of investigation on the effect of chain length and side-chain regioregularity on their basic properties and photovoltaic performance. In the OBTTh_n oligomers and PBTTh₁ polymer, all the hexyl side chains on thienyl units orient toward the same direction. Upon elongation of the chain length, the intramolecular charge transfer (ICT) absorption band in solution gradually redshifts from 398 nm for OBTTh₁ to 505 nm for OBTTh₇, then to 512 nm for PBTTh₁ polymer. Meanwhile, the HOMO energy level increases from -5.45 eV (OBTTh₁) to -5.08 eV (OBTTh₇) and -5.09 eV (PBTTh₁), and the LUMO energy level decreases from -3.11 eV (OBTTh₁) to -3.30 eV (OBTTh₇) and -3.33 eV (PBTTh₁), thus giving a smaller and smaller energy bandgap for higher oligomers and polymers. Theoretical calculation suggests straight line-like backbone geometry for this series of oligomers and polymer. On the other hand, polymer PBTTh₂ possesses a different side-chain regioregularity, in which every two neighbor hexyl side chains are arranged in different orienting direction. It is theoretically suggested to have curved line-like backbone geometry. In solution, it shows similar photophysical and electrochemical properties as PBTTh₁. However in film state, it displays a less redshift in the ICT band as refer to that in solution than PBTTh₁. In combination with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), these oligomers and polymers were used as donor material to fabricate organic bulk heterojunction solar cells. Again, chain length-dependent device photovoltaic performance was observed. The device based on OBTTh₄ showed a power conversion efficiency of 0.16%, while it increased to 0.36% and 0.49% for the devices based on OBTTh₆ and PBTTh₁, respectively. However, the side-chain regioregularity has less influence on the device photovoltaic output since the device based on PBTTh₂ displayed an efficiency of 0.52%, comparable to that of PBTTh₁. A series of donor-acceptor oligomer OBTTh_n (n=1-7) and polymer PBTTh₁ and PBTTh₂ composed of alternative 2,1,3-benzothiadiazole and 3-hexylthiophene have been designed and synthesized. The different orientation pattern of the side hexyl chains endows the polymers different regioregularity. It was found that the chain length of the oligomers and polymers has great impact on their photophysical, electrochemical and photovoltaic properties. Whereas, the side-chain regioregularity has less influence on their basic physical properties and photovoltaic performance. Copyright

Full text of DOI:10.1002/cjoc.201300519

FULL PAPER
DOI: 10.1002/cjoc.201300519
Donor-Acceptor Oligomers and Polymers Composed of
Benzothiadiazole and 3-Hexylthiophene: Effect of
Chain Length and Regioregularity
Jintu Wang,a Huaiying Ye,a Hongjiao Li,a Chongyu Mei,*,a Jun Ling,b
Weishi Li,*,a and Zhiquan Shenb
a Laboratory of Organic Functional Materials and Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, Shanghai 200032, China
b Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China
A series of donor-acceptor oligomer OBTThn (n＝1－7) and polymer PBTTh1 and PBTTh2 composed of al-
ternative 2,1,3-benzothiadiazole and 3-hexylthiophene have been designed and synthesized for the purpose of in-
vestigation on the effect of chain length and side-chain regioregularity on their basic properties and photovoltaic
performance. In the OBTThn oligomers and PBTTh1 polymer, all the hexyl side chains on thienyl units orient to-
ward the same direction. Upon elongation of the chain length, the intramolecular charge transfer (ICT) absorption
band in solution gradually redshifts from 398 nm for OBTTh1 to 505 nm for OBTTh7, then to 512 nm for PBTTh1
polymer. Meanwhile, the HOMO energy level increases from −5.45 eV (OBTTh1) to −5.08 eV (OBTTh7) and
−5.09 eV (PBTTh1), and the LUMO energy level decreases from −3.11 eV (OBTTh1) to −3.30 eV (OBTTh7) and
−3.33 eV (PBTTh1), thus giving a smaller and smaller energy bandgap for higher oligomers and polymers. Theo-
retical calculation suggests straight line-like backbone geometry for this series of oligomers and polymer. On the
other hand, polymer PBTTh2 possesses a different side-chain regioregularity, in which every two neighbor hexyl
side chains are arranged in different orienting direction. It is theoretically suggested to have curved line-like back-
bone geometry. In solution, it shows similar photophysical and electrochemical properties as PBTTh1. However in
film state, it displays a less redshift in the ICT band as refer to that in solution than PBTTh1. In combination with
[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), these oligomers and polymers were used as donor material to
fabricate organic bulk heterojunction solar cells. Again, chain length-dependent device photovoltaic performance
was observed. The device based on OBTTh4 showed a power conversion efficiency of 0.16%, while it increased to
0.36% and 0.49% for the devices based on OBTTh6 and PBTTh1, respectively. However, the side-chain regio-
regularity has less influence on the device photovoltaic output since the device based on PBTTh2 displayed an effi-
ciency of 0.52%, comparable to that of PBTTh1.
Keywords conjugated polymers, organic solar cells, regioregularity, donor-acceptor, oligomers
Introduction
mers, such as polythiophenes and poly(phenylene-
vinylene)s. In addition, the combination of D and A
units benefits to tune energy levels of highest occupied
molecular orbital (HOMO) and lowest unoccupied mo-
lecular orbital (LUMO). These advantages have raised
extensive investigations on D-A copolymers for organic
photovoltaics (OPVs) in the past decades. So far, a huge
number of D-A copolymers have been designed and
developed,[2] some of which show promising photo-
voltaic properties with a power conversion efficiency
(PCE) over 7%.[3] However, the most reported works
focused only on polymer systems, which are generally a
mixture of homologues and have issues of batch-
dependent average molecular weight and polydispersity
Conjugated polymers including donor-acceptor (D-A)
copolymers have attracted significant attention due to
their one-dimensional π-extended backbones, which
endow them outstanding photophysical and photo-
chemical properties and a wide range of applications in
opto- and electronic devices.[1] The backbone of D-A
copolymers is composed of alternative electron-rich and
electron-deficient π-conjugated moieties. This specific
structural feature generally narrows the polymer band-
gap, enables a wide light absorption spectrum extending
into far visible and even near infrared region, and thus
affords a better solar spectral coverage than homopoly-
*
E-mail: meichy@sioc.ac.cn (C.Y.M.), liws@mail.sioc.ac.cn (W.S.L.); Tel. & Fax: 0086-021-54925381
Received July 4, 2013; accepted August 13, 2013; published online XXXX, 2013.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201300519 or from the author.
Chin. J. Chem. 2013, XX, 1—13
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Wang et al.
FULL PAPER
(PDI). Therefore, the basic structure-property relation-
ship for most D-A copolymers remains unclear, which
hampers the fundamental understanding of the origina-
tion and influence factors for their photovoltaic per-
formance. Since oligomers have a definite chemical
structure and chain length, thus enabling an easy corre-
lation with their properties, oligomer approaching is a
good way to address the above issue. With this in mind,
we are motivated recently to explore D-A oligomers and
to systematically study their chain length-dependent
photovoltaic properties.
include those copolymerized with C-[11] or Si-bridged
dithiophenes,[12] or ladder-type electron-donating
units.[13] For oligothiophenes, dimer to octamer have
been reported to pair with BT for construction of D-A
copolymers.[14] An outstanding work was done by Chen
and coworkers, who synthesized BT-quaterthiophene
copolymers and achieved a PCE of 6.26%.[15] In our
work, we chose a very simple donor unit, 3-hexylthio-
phene, to cooperate with BT. Either polymer or oli-
gomer with such simple alternative BT and thiophene
structure has not yet been reported.
In this work, we are focusing on a family of D-A oli-
gomers (OBTThn, n＝1－7) composed of 2,1,3-benzo-
thiadiazole (BT) and 3-hexylthiophene (Th), as shown
in Figure 1. BT is a famous electron-deficient unit and
has been applied in the synthesis of a variety of D-A
copolymers.[2c,4] For example, the first D-A copolymer
for OPVs reported by Janssen et al. in 2001 is com-
posed of pyrrole, thiophene and BT.[5] In 2003, Ander-
son and coworkers developed PFDTBT copolymer
based on fluorene, thiophene and BT, and demonstrated
an impressing large open-circuit voltage (VOC) around 1
V.[6] Getting clues from PFDTBT, several analogue
D-A copolymers have been designed by using other
similar units, like carbazole,[7] dibenzosilole,[8] germa-
fluorene,[9] and dithienopyrrole,[10] to replace fluorene
unit, some of which have shown outstanding photo-
voltaic properties. In addition to PFDTBT and its ana-
logues, other famous BT-containing D-A copolymers
In addition to π-conjugated skeleton, we also pay
special attention to the orientation of the side chain of
the thiophene units in this work. Since just one hexyl
chain is attached to the 3-position of the thiophene ring,
the different orientation of these alkyl tails in the poly-
mer/oligomer backbone will produce different regio-
regularity. It was reported by Kim et al. in 2006 that the
side-chain regioregularity has great impact on photo-
voltaic performance of poly(3-hexylthiophene)
(P3HT).[16] They found the devices based on P3HT
having regioregularity of 95.2% exhibited a PCE of
2.4%, which was further increased to 4.4% after opti-
mization. In contrast, the devices based on P3HT hav-
ing a slightly smaller regioregularity (93%) showed a
PCE of 1.8%. The PCE further decreased to 0.7% when
the regioregularity was 90.7%. They believed that
P3HT with high regioregularity can easily form regular
and tight π-π stacking structure in film, which could
S
S
N
N
N
N
H
n
S
S
S
H
H
H
n
N
N
S
OBTThn (n = 1 7)
PBTTh1
PBTTh2
e.g.
S
N
N
S
N
S
N
S
N
S
S
N
S
N
S
S
N
N
N
S
S
S
N
N
S
N
N
S
OBTTh7
Figure 1 Chemical structures of OBTThn (n＝1－7) D-A oligomers and PBTTh1 and PBTTh2 polymers. OBTTh7 is shown as an
example for regioregular oligomers.
2
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Chin. J. Chem. 2013, XX, 1—13

Donor-Acceptor Oligomers and Polymers Composed of Benzothiadiazole and 3-Hexylthiophene
afford good hole mobility.[17] However, Fréchet et al.
proved later that the devices based on three P3HTs
having regioregularity of 96%, 90% and 86% respec-
tively, outputted similar PCE values (3.8%－3.9%) after
device optimization individually.[18] They further dem-
onstrated that the devices using lower regioregular
P3HT had better thermal stability. These conflict results
indicate that the real impact of side-chain regioregularity
on OPV performance is still ambiguous even for well-
studied P3HT. As for D-A copolymers, to the best of
our knowledge, there is no precedent example to study
the effect of side-chain regioregularity.
able-wavelength UV-vis detector. UV-vis absorption
spectroscopy was performed on a Hitachi U-3310 spec-
trophotometer at room temperature. Cyclic voltammetric
(CV) measurements were performed in CH2Cl2 with a
sample concentration of 0.1 mmol•L−1 on a CHI 660C
instrument using a three-electrode cell with a glassy
carbon as working electrode, a platinum wire as counter
electrode, and Hg/Hg2Cl2 (SCE) as reference electrode.
0.1 mol•L−1 Bu4NPF6 was used as electrolyte and the
scan rate was 100 mV•min−1. Atomic force microscopy
(AFM) was performed on a Veeco Nanoscope IIIa Mul-
timode apparatus by tapping mode with a silicon tip.
In combination of the above two aspects, we report
here the synthesis, characterization and properties of
D-A oligomers (OBTThn, n＝1－7) and two polymers
(PBTTh1 and PBTTh2) having different side-chain re-
gioregularity (Figure 1). In OBTThn and PBTTh1, all
neighboring BT-Th units are linked by a head-to-tail
style, thus making all the hexyl side chains orienting
toward the same direction. While, PBTTh2 is another
regioregular polymer but having completely different
linkage patterns between the neighboring BT-Th units
(head-to-head or tail-to-tail styles). Thus, in PBTTh2,
every two neighbor hexyl side chains are arranged in
different orientation direction. Studies revealed that their
optical, electrochemical, and photovoltaic properties are
highly dependent on their chain length, but not on the
side-chain regioregularity.
Synthesis of 4-bromo-7-(4-hexylthiophen-2-yl)-2,1,3-
benzothiadiazole (Br-BTTh)
A mixture of sodium 4-hexyl-2-thienylboronate
(1.00 g, 4.0 mmol), 4,7-dibromo-2,1,3-benzothiadiazole
(4.50 g, 15.3 mmol), Na2CO3 aqueous solution (2 mol•
－
1
L , 6 mL), and THF (75 mL) was subjected to thor-
oughly degassing by freeze-pump-thaw cycle three
times, and then added with Pd(PPh3)4 (0.136 g, 0.12
mmol). After degas again by freeze-pump-thaw cycle
three times, the reaction mixture was stirred and re-
fluxed overnight, followed by extraction with CH2Cl2
several times. The organic phases were combined,
washed with water, and dried over anhydrous Na2SO4.
After filtration, the filtrate was concentrated under re-
duced pressure and the residue was subjected to silica
columnar chromatography using CH2Cl2/petroleum
ether (1/3, V/V) as an eluent, affording 0.582 g
Experimental
1
Br-BTTh as yellow solid in a yield of 38%. H NMR
Tetrahydrofuran (THF) and ethyl ether were distilled
over sodium/benzophenone. N,N-Dimethylformamide
(DMF) was distilled over CaH2 under reduced pressure.
Other reagents were used as received without further
purification unless stated otherwise. Sodium 4-hexyl-2-
thienylboronate,[19] 4-bromo-2,1,3-benzothiadiazole,[20]
4,7-dibromo-2,1,3-benzothiadiazole,[20] and 4,7-bis-
(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-2,1,3-
benzothiadiazole (B-BT-B)[21] were prepared via litera-
ture methods.
(300 MHz, CDCl3) δ: 7.96 (s, 1H), 7.84 (d, J＝7.8 Hz,
1H), 7.70 (d, J＝7.8 Hz, 1H), 7.07 (s, 1H), 2.69 (t, J＝
7.8 Hz, 2H), 1.75－1.63 (m, 2H), 1.45－1.28 (m, 6H),
0.90 (t, J＝6.9 Hz, 3H).
Synthesis of 4-(4-hexylthiophen-2-yl)-7-(boronic
acid)-2,1,3-benzothiadiazole (B-BTTh)
A mixture of Br-BTTh (0.525 g, 1.38 mmol),
bis(pinacolato) diboron (0.412 g, 1.62 mmol),
PdCl2(dppf)•CH2Cl2 (0.118 g, 0.138 mmol), and KOAc
(0.387 g, 3.94 mmol) in 1,4-dioxane (15 mL) was thor-
oughly degased by freeze-pump-thaw cycle several
times. After stirring at 80 ℃ for 2 h, the reaction mix-
ture was extracted with CH2Cl2 several times. The com-
bined organic phases were washed with water and dried
over anhydrous MgSO4. After filtration, the filtrate was
concentrated under reduced pressure and the residue was
subjected to silica columnar chromatography using
CH2Cl2/petroleum ether (1/3, V/V) as an eluent, afford-
ing 0.366 g product as a form of B-BTTh in a yield of
77% due to the hydrolysis by the acidic silica gel during
the column chromatography. 1H NMR (300 MHz,
CDCl3) δ: 8.22 (d, J＝6.0 Hz, 1H), 8.06 (s, 1H), 7.90 (d,
J＝6.0 Hz, 1H), 7.10 (s, 1H), 6.35 (s, 2H), 2.75－2.64
(m, 2H), 1.71－1.67 (m, 2H), 1.40－1.30 (m, 6H), 0.90
(t, J＝6.0 Hz, 3H).
1H NMR spectra were recorded on a Varian Mercury
spectrometer operated at 300 MHz using deuterated
solvents and tetramethyl silane (TMS) as an internal
reference. Matrix-assisted laser deionization time-of-
flight (MALDI-TOF) mass spectroscopy was carried out
on a Shimadzu Biotech Axima Performance mass spec-
trometer using dithranol or α-cyano-4-hydroxycinnamic
acid (CHCA) as a matrix. Gel permeation chromatogra-
phy (GPC) was carried out on a Waters 1515 HPLC
instrument equipped with a Waters 2489 UV detector,
using THF as an eluent. The molecular weight and PDI
were calculated based on polystyrene standards. Recy-
cling preparative size-exclusion chromatography (SEC)
was performed with CHCl3 as an eluent using JAIGEL
1H and 2.5H on a JAI model LC-9201 recycling HPLC
system equipped with a JASCO model MD-2010 vari-
Chin. J. Chem. 2013, XX, 1—13
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Wang et al.
FULL PAPER
Synthesis of 4-(4-hexylthiophen-2-yl)-2,1,3-benzo-
thiadiazole (OBTTh1)
1
of 76%. H NMR (300 MHz, CDCl3) δ: 8.13 (s, 1H),
8.04 (s, 1H), 7.95－7.91 (m, 3H), 7.71 (d, J＝9 Hz, 1H),
7.64－7.61 (m, 1H), 7.08 (s, 1H), 2.75－2.67 (m, 4H),
1.76－1.69 (m, 4H), 1.35－1.23 (m, 12H), 0.91 (t, J＝6
Hz, 3H), 0.82 (t, J＝6 Hz, 3H); MS (MALDI-TOF) m/z
(%): 602.0 (M＋, 100). Anal. calcd for C32H34N4S4: C
63.75, H 5.68, N 9.29, S 21.27; found C 64.40, H 6.36,
N 7.98, S 18.10.
A mixture of 4-bromo-2,1,3-benzothiadiazole (1.09 g,
5.07 mmol), sodium 4-hexyl-2-thienylboronate (2.31 g,
9.16 mmol), Pd(PPh3)4 (0.184 g, 0.15 mmol), aqueous
Na2CO3 solution (2 mol•L−1, 7.7 mL, 15.4 mmol) and
THF (25 mL) was degased thoroughly by freeze-pump-
thaw cycle several times. After refluxing overnight, the
reaction mixture was extracted with CH2Cl2 several
times. The combined organic phases were washed with
water and dried over anhydrous MgSO4. After filtration,
the filtrate was concentrated under reduced pressure and
the residue was subjected to silica columnar chromato-
graphy using CH2Cl2/petroleum ether (1/5, V/V) as an
eluent, affording 1.16 g OBTTh1 as oil in a yield of
Synthesis of OBTTh2-Br
OBTTh2-Br was prepared with OBTTh2 (1.08 g,
1.79 mmol), NBS (0.33 g, 1.83 mmol) in anhydrous
THF (25 mL) following the general procedure to afford
1.21 g in a yield of 99%. 1H NMR (300 MHz, CDCl3) δ:
8.12 (s, 1H), 7.97－7.81 (m, 4H), 7.74－7.59 (m, 2H),
2.73 (d, J＝7.5 Hz, 2H), 2.66 (d, J＝7.5 Hz, 2H), 1.71
－1.69 (m, 4H), 1.43－1.29 (m, 12H), 0.91 (t, J＝7.5
Hz, 3H), 0.85 (t, J＝7.5 Hz, 3H).
1
76%. H NMR (300 MHz, CDCl3) δ: 7.97 (s, 1H), 7.89
(d, J＝9.0 Hz, 1H), 7.80 (d, J＝6.9 Hz, 1H), 7.61－7.56
(m, 1H), 7.04 (s, 1H), 2.69 (t, J＝7.7 Hz, 2H), 1.72－
1.64 (m, 2H), 1.48－1.27 (m, 6H), 0.90 (t, J＝6.4 Hz,
3H).
Synthesis of OBTTh3
OBTTh3 was prepared with OBTTh2-Br (1.21 g,
1.77 mmol), B-BTTh (0.84 g, 2.43 mmol), Pd(PPh3)4
(0.10 g, 0.086 mmol), Na2CO3 aqueous solution (2
mol•L−1, 2.7 mL, 5.40 mmol) and THF (11 mL) follow-
ing the general procedure to afford 1.22 g in a yield of
General procedure for synthesis of OBTThn−1-Br
(n≥2)
The THF solution of N-bromosuccinimide (NBS)
was dropwise added into a solution of OBTThn−1 (n≥2)
in anhydrous THF under ice/water bath within 10 min.
After stirring overnight, the reaction mixture was con-
centrated under reduced pressure. The residue was sub-
jected to silica columnar chromatography to separate the
corresponding OBTThn-1-Br (n≥2) product.
1
79%. H NMR (300 MHz, CDCl3) δ: 8.18 (s, 1H), 8.14
(s, 1H), 8.05 (s, 1H), 8.00 (d, J＝7.2 Hz, 1H), 7.95－
7.92 (m, 3H), 7.73 (dd, J＝7.5, 2.2 Hz, 2H), 7.67－7.62
(m, 1H), 7.09 (s, 1H), 2.80－2.71 (m, 6H), 1.73－1.60
(m, 6H), 1.40－1.18 (m, 18H), 0.92－0.83 (m, 9H); MS
(MALDI-TOF) m/z (%): 902.1 (M＋, 100). Anal. calcd
for C48H50N6S6: C 63.82, H 5.58, N 9.30, S 21.30; found
C 64.15, H 5.76, N 9.05, S 20.92.
General procedure for synthesis of OBTThn (n≥2)
A mixture of OBTThn−1-Br (n ≥ 2), B-BTTh,
Pd(PPh3)4, aqueous Na2CO3 solution and THF was de-
gased thoroughly by freeze-pump-thaw cycle several
times. After refluxing overnight, the reaction mixture
was extracted with CH2Cl2 several times. The combined
organic phases were washed with water and dried over
anhydrous MgSO4. After filtration, the filtrate was con-
centrated under reduced pressure and the residue was
subjected to silica columnar chromatography to separate
the corresponding OBTThn (n≥2) product.
Synthesis of OBTTh3-Br
OBTTh3-Br was prepared with OBTTh3 (1.08 g,
1.20 mmol), NBS (0.23 g, 1.28 mmol) in anhydrous
THF (25 mL) following the general procedure to afford
1.03 g in a yield of 88%. 1H NMR (300 MHz, CDCl3) δ:
8.18 (s, 1H), 8.14 (s, 1H), 8.01 (d, J＝7.5 Hz, 1H), 7.96
－7.90 (m, 2H), 7.89－7.83 (m, 2H), 7.73 (dd, J＝7.6,
3.5 Hz, 2H), 7.67－7.62 (m, 1H), 2.76 (t, J＝7.5 Hz,
4H), 2.68 (t, J＝7.5 Hz, 2H), 1.76－1.67 (m, 6H), 1.41
－1.19 (m, 18H), 0.94－0.81(m, 9H).
Synthesis of OBTTh1-Br
OBTTh1-Br was prepared with OBTTh1 (0.53 g,
1.76 mmol), NBS (0.31 g, 1.72 mmol) in anhydrous
THF (25 mL) following the general procedure to afford
0.59 g in a yield of 88%. 1H NMR (300 MHz, CDCl3) δ:
7.96 (s, 1H), 7.84 (d, J＝7.7 Hz, 1H), 7.69 (d, J＝7.7 Hz,
1H), 7.07 (s, 1H), 2.69 (t, J＝7.5 Hz, 2H), 1.73－1.68
(m, 2H), 1.45－1.28 (m, 6H), 0.90 (t, J＝6.9 Hz, 3H).
Synthesis of OBTTh4
OBTTh4 was prepared with OBTTh3-Br (1.25 g,
1.27 mmol), B-BTTh (0.60 g, 1.73 mmol), Pd(PPh3)4
(0.068 g, 0.059 mmol), aqueous Na2CO3 solution (2
－
1
mol•L , 2.0 mL, 4.0 mmol) and THF (8 mL) following
the general procedure to afford 1.16 g in a yield of 76%.
1H NMR (300 MHz, CDCl3) δ: 8.20 (s, 2H), 8.14 (s,
1H), 8.05 (s, 1H), 8.03 (s, 1H), 8.01 (s, 1H), 7.94－7.92
(m, 3H), 7.77－7.73 (m, 3H), 7.76－7.62 (m, 1H), 7.09
(s, 1H), 2.80－2.72 (m, 8H), 1.73－1.69 (m, 8H), 1.35
－1.23 (m, 24H), 0.92 (t, J＝6 Hz, 3H), 0.86－0.82 (m,
9H); MS (MALDI-TOF) m/z (%): 1202.3 (M＋, 100).
Anal. calcd for C64H66N8S8: C 63.86, H 5.53, N 9.31, S
Synthesis of OBTTh2
OBTTh2 was prepared with OBTTh1-Br (1.02 g,
2.69 mmol), B-BTTh (1.22 g, 3.52 mmol), Pd(PPh3)4
(0.157 g, 0.134 mmol), aqueous Na2CO3 solution (2
－
1
mol•L , 4.1 mL, 8.20 mmol) and THF (20 mL) fol-
lowing the general procedure to afford 1.24 g in a yield
4
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Chin. J. Chem. 2013, XX, 1—13

Donor-Acceptor Oligomers and Polymers Composed of Benzothiadiazole and 3-Hexylthiophene
21.31; found C 63.97, H 5.49, N 9.29, S 21.90.
8.25－8.17 (m, 4H), 8.24 (s, 1H), 8.03－8.00 (m, 4H),
7.95－7.91 (m, 2H), 7.88－7.84 (m, 2H), 7.77－7.71 (m,
5H), 7.67－7.61 (m, 1H), 2.81－2.74 (m, 10H), 2.67 (t,
J＝7.5 Hz, 2H), 1.77－1.68 (m, 12H), 1.40－1.15 (m,
36H), 0.90－0.82 (m, 18H).
Synthesis of OBTTh4-Br
OBTTh4-Br was prepared with OBTTh4 (1.08 g,
0.90 mmol), NBS (0.18 g, 1.00 mmol) in anhydrous
THF (25 mL) following the general procedure to afford
1.03 g in a yield of 89%. 1H NMR (300 MHz, CDCl3) δ:
8.19 (s, 2H), 8.14 (s, 1H), 8.04 (s, 1H), 8.02 (s, 1H),
7.95－7.84 (m, 4H), 7.77－7.71 (m, 3H), 7.64－7.62 (m,
1H), 2.80－2.75 (m, 6H), 2.67 (t, J＝7.2 Hz, 2H), 1.76
－1.68 (m, 8H), 1.35－1.24 (m, 24H), 0.92 (t, J＝6.9
Hz, 3H), 0.86－0.82 (m, 9H).
Synthesis of OBTTh7
OBTTh7 was prepared with OBTTh6-Br (0.66 g,
0.35 mmol), B-BTTh (0.18 g, 0.52 mmol), Pd(PPh3)4
(0.047 g, 0.041 mmol), aqueous Na2CO3 solution (2
mol•L−1, 0.4 mL, 0.8 mmol) and THF (6 mL) following
the general procedure to afford 0.68 g in a yield of 92%.
1H NMR (300 MHz, CDCl3) δ: 8.21－8.20 (m, 5H),
8.14 (s, 1H), 8.08－7.98 (m, 6H), 7.95－7.91 (m, 3H),
7.82－7.70 (m, 6H), 7.67－7.62 (m, 1H), 7.09 (s, 1H),
2.82－2.70 (m, 14H), 1.78－1.69 (m, 14H), 1.38－1.17
(m, 42H), 0.96－0.75 (m, 21H); MS (MALDI-TOF) m/z
(%): 2105.3 (M＋, 100). Anal. calcd for C112H114N14S14:
C 63.90, H 5.46, N 9.32, S 21.32; found C 63.40, H 5.50,
N 8.81, S 20.13.
Synthesis of OBTTh5
OBTTh5 was prepared with OBTTh4-Br (1.03 g,
0.80 mmol), B-BTTh (0.36 g, 1.04 mmol), Pd(PPh3)4
(0.047 g, 0.041 mmol), aqueous Na2CO3 solution (2
－
1
mol•L , 1.2 mL, 2.4 mmol) and THF (8 mL) following
the general procedure to afford 0.99 g in a yield of 82%.
1H NMR (300 MHz, CDCl3) δ: 8.20 (s, 3H), 8.14 (s,
1H), 8.07－7.98 (m, 4H), 7.97－7.88 (m, 3H), 7.78－
7.72 (m, 4H), 7.67－7.62 (m, 1H), 7.09 (s, 1H), 2.87－
2.70 (m, 10H), 1.78－1.68 (m, 10H), 1.37－1.19 (m,
30H), 0.90－0.80 (m, 15H); MS (MALDI-TOF) m/z
(%): 1503.6 (M＋, 100). Anal. calcd for C80H82N10S10: C
63.88, H 5.49, N 9.31, S 21.32; found C 64.03, H 5.49,
N 9.18, S 21.20.
Synthesis of 4-(4-hexylthiophen-5-bromo-2-yl)-7-
(boronic acid)-2,1,3-benzothiadiazole (B-BTTh-Br)
To the solution of B-BTTh (0.32 g, 0.92 mmol) in
THF (20 mL) was added the solution of NBS (0.17 g,
0.94 mmol) in THF (10 mL) under ice/water bath. After
stirring at room temperature overnight in darkness, the
reaction mixture was concentrated by evaporating off
solvents under reduced pressure. The residue was sub-
jected to silica columnar chromatography using CH2Cl2/
petroleum ether (1/4, V/V) then CH2Cl2/methanol (9/1,
V/V) as eluents, affording 0.37 g B-BTTh-Br in a yield
Synthesis of OBTTh5-Br
OBTTh5-Br was prepared with OBTTh5 (0.99 g,
0.66 mmol), NBS (0.13 g, 0.72 mmol) in anhydrous
THF (15 mL) following the general procedure to afford
0.96 g in a yield of 92%. 1H NMR (300 MHz, CDCl3) δ:
8.19 (s, 3H), 8.14 (s, 1H), 8.03－8.00 (m, 3H), 7.95－
7.91 (m, 2H), 7.88－7.84 (m, 2H), 7.77－7.71 (m, 4H),
7.67－7.62 (m, 1H), 2.81－2.74 (m, 8H), 2.67 (t, J＝7.5
Hz, 2H), 1.77－1.67 (m, 10H), 1.38－1.18 (m, 30H),
0.92－0.81 (m, 15H).
1
of 96%. H NMR (300 MHz, CDCl3) δ: 8.22 (d, J＝9.0
Hz, 1H), 7.86 (s, 1H), 7.83 (d, J＝9.0 Hz, 1H), 6.28 (s,
2H), 2.65 (t, J＝9.0 Hz, 2H), 1.73－1.63 (m, 2H), 1.46
－1.30 (m, 6H), 0.90 (t, J＝7.5 Hz, 3H).
Synthesis of polymer PBTTh1
Monomer B-BTTh-Br (374 mg, 0.88 mmol) and
Pd(PPh3)4 (55 mg, 0.048 mmol) were added into a mix-
ture of toluene (20 mL) and 20% aqueous Et4NOH (6
mL). The reaction mixture was subjected to thoroughly
degassing by freeze-pump-thaw cycles and then re-
fluxed under vigorous stirring for 72 h under Ar. After-
ward, the reaction mixture was added with B-BTTh
(0.0427 g, 0.12 mmol) and 6 h later with OBTTh1-Br
(0.0641 g, 0.17 mmol) as monofunctional end-capping
reagents. After further refluxing for 6 h, the reaction
mixture was cooled down to room temperature and ex-
tracted with chloroform for three times. The combined
organic layers were washed with water, dried over an-
hydrous Na2SO4, filtered, and evaporated off solvents to
dryness. The solid residue was subjected to Soxhlet ex-
traction with methanol, acetone, hexane, and chloroform.
The chloroform fraction was evaporated to dryness and
further dried in vacuum for 1 d, affording 0.21 g
PBTTh1 product as red-brown solid with a yield of
80%.
Synthesis of OBTTh6
OBTTh6 was prepared with OBTTh5-Br (0.96 g,
0.61 mmol), B-BTTh (0.37 g, 1.07 mmol), Pd(PPh3)4
(0.047 g, 0.041 mmol), aqueous Na2CO3 solution (2
－
1
mol•L , 0.8 mL, 1.6 mmol) and THF (8 mL) following
the general procedure to afford 0.94 g in a yield of 86%.
1H NMR (300 MHz, CDCl3) δ: 8.21 (s, 2H), 8.20 (s,
2H), 8.14 (s, 1H), 8.08－8.00 (m, 5H), 7.98－7.89 (m,
3H), 7.77－7.73 (m, 5H), 7.67－7.62 (m, 1H), 7.09 (s,
1H), 2.81－2.71 (m, 12H), 1.78－1.68 (m, 12H), 1.39－
1.23 (m, 36H), 0.92－0.82 (m, 18H); MS (MALDI-TOF)
m/z (%): 1804.0 (M＋, 100). Anal. calcd for C96H98-
N12S12: C 63.89, H 5.47, N 9.31, S 21.32; found C 63.83,
H 5.44, N 9.28, S 20.70.
Synthesis of OBTTh6-Br
OBTTh6-Br was prepared with OBTTh6 (0.67 g,
0.37 mmol), NBS (0.07 g, 0.39 mmol) in anhydrous
THF (10 mL) following the general procedure to afford
0.66 g in a yield of 94%. 1H NMR (300 MHz, CDCl3) δ:
Chin. J. Chem. 2013, XX, 1—13
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
5

Wang et al.
FULL PAPER
Synthesis of 4,7-bis-(3-hexyl-thiophen-2-yl)-2,1,3-
benzothiadiazole (ThBTTh)
Fabrication and characterization of organic photo-
voltaic devices
The mixture of 2-bromo-3-hexylthiophene (0.87g,
3.52 mmol), B-BT-B (0.70 g, 1.80 mmol), Pd(PPh3)4
(65 mg, 0.056 mmol), aqueous Na2CO3 solution (2
mol•L−1, 2.6 mL, 5.2 mmol), and THF (20 mL) was de-
gased thoroughly by freeze-pump-thaw cycles and re-
fluxed overnight under Ar. After cooled to room tem-
perature, the mixture was extracted with CH2Cl2 for
three times. The combined organic phases were washed
with water, dried over anhydrous Na2SO4, filtered and
concentrated. The residue was subjected to silica co-
lumnar chromatography using dichloromethane/
petroleum ether (1/9, V/V) as an eluent. The separated
crude product was further purified by preparative SEC,
The solar cell devices were fabricated with a struc-
ture of ITO/PEDOT:PSS/active layer/Ca/Al. A thin
layer of PEDOT:PSS (Heraeus Clevios P VP. Al 4083)
was spin-coated on top of well cleaned ITO glass at
4000 r/min and baked at 120 ℃ for 15 min, affording a
thickness of 30 nm. After transfer into a N2-filled glove
box, the active layer was spin-coated from a chloroben-
zene solution of a mixture of the test oligomer or poly-
mer and PC61BM (Lumitec LT-8905) at 2000 r/min.
Finally, 10 nm-thick Ca layer and a 100 nm-thick Al
layer were subsequently thermally deposited on the top
of the active layer under high vacuum (＜10−5 mbar)
through a shadow mask. The effective cell area is 7 mm2.
Layer thickness was measured on a Veeco Dektak 150
profilometer. Current density-voltage (J-V) curves were
recorded with a Keithley 2420 source meter. Photocur-
rent was acquired upon irradiation using an AAA solar
simulator (Oriel 94043A, 450 W) with AM 1.5G filter.
The intensity was adjusted to be 100 mW•cm−2 under
the calibration with a NREL-certified standard silicon
cell (Orial reference cell 91150). External quantum effi-
ciency (EQE) was detected with a 75 W Xe lamp, Oriel
monochromator 74125, optical chopper, lock-in ampli-
fier and a NREL-calibrated crystalline silicon cell.
1
affording 0.39 g of ThBTTh in a yield of 32%. H
NMR (300 MHz, CDCl3) δ: 7.65 (s, 2H), 7.44 (d, J＝5.2
Hz, 2H), 7.11 (d, J＝5.2 Hz, 2H), 2.66 (t, J＝7.5 Hz,
4H), 1.70－1.58 (m, 4H), 1.31－1.10 (m, 12H), 0.93－
0.70 (m, 6H).
Synthesis of 4,7-bis-(3-hexyl-thiophen-5-bromo-2-
yl)-2,1,3-benzothiadiazole (Br-ThBTTh-Br)
The solution of NBS (0.39 g, 2.17 mmol) in THF (10
mL) was dropwise added into a solution of ThBTTh
(0.39 g, 0.83 mmol) in anhydrous THF (20 mL) in
darkness under ice/water bath within 10 min. After stir-
ring overnight, the solvent was removed under reduced
pressure. The residue was subjected to silica columnar
chromatography using dichloromethane/petroleum ether
(1/9, V/V) as an eluent affording 0.47 g of desired prod-
Results and Discussion
Synthesis and characterization
1
The synthesis of regioregular OBTThn oligomers
followed an iterative growing approach, as shown in
Scheme 1. During this approach, compound B-BTTh
having a boron acid functionality at 7-position of BT
serves as a growing monomer and plays an important
role. For the synthesis of this growing monomer,
Br-BTTh is a key precursor and was prepared in a yield
of 38% by the Suzuki coupling between sodium
4-hexyl-2-thienylboronate and 4 molar equivalent of
4,7-dibromo-2,1,3-benzo-thiadiazole. After reacting
with bis(pinacolato)diboron in the presence of
Pd(dppf)Cl2 in 1,4-dioxane, Br-BTTh was converted
into a pinacol boronate compound, which was further
hydrolyzed during the silica column chromatography
into B-BTTh in a yield of 77% based on Br-BTTh.
Meanwhile, the first oligomer (OBTTh1) was obtained
though Suzuki coupling of 4-bromo-2,1,3-benzothia-
diazole with sodium 4-hexyl-2-thienylboronate. With
the growing monomer (B-BTTh) and the first oligomer
(OBTTh1) on hand, the iterative process was started for
synthesis of longer oligomers. Firstly, OBTTh1 was
brominated with NBS, affording an intermediate com-
pound OBTTh1-Br. Subsequently, the Suzuki coupling
was carried out between OBTTh1-Br and the growing
monomer B-BTTh, producing OBTTh2. The yield of
OBTTh2 for the above two steps was 67%. Following
the same way, higher oligomers OBTThn (n＝3－7)
uct in a yield of 90%. H NMR (300 MHz, CDCl3) δ:
7.60 (s, 2H), 7.06 (s, 2H), 2.61 (t, J＝7.5 Hz, 4H), 2.68
－2.55 (m, 4H), 1.31－1.10 (m, 12H), 0.82 (t, J＝7.5
Hz, 6H).
Synthesis of polymer PBTTh2
Monomer Br-ThBTTh-Br (63.4 mg, 0.10 mmol),
monomer B-BT-B (40.2 mg, 0.10 mmol), and Pd(PPh3)4
(5.8 mg, 0.005 mmol) were added into a mixture of
toluene (5 mL) and 20% aqueous Et4NOH solution (0.6
mL). The reaction mixture was subjected to thoroughly
degassing by freeze-pump-thaw cycles and then re-
fluxed under vigorous stirring for 7 d under Ar. After-
ward, the reaction mixture was added with B-BTTh (6.4
mg, 0.018 mmol) and 6 h later with 2-bromo-3-hexyl-
thiophene (3.6 μL, 0.018 mmol) as monofunctional
end-capping reagents. After further refluxing for 6 h, the
reaction mixture was cooled down to room temperature
and extracted with chloroform for three times. The
combined organic layers were washed with water, dried
over anhydrous Na2SO4, filtered, and evaporated off
solvents to dryness. The solid residue was subjected to
Soxhlet extraction with methanol, acetone, hexane, and
chloroform. The chloroform fraction was evaporated to
dryness and further dried in vacuum for 1 d, affording
34.3 mg PBTTh2 product as red-brown solid with a
yield of 57%.
6
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© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2013, XX, 1—13

Donor-Acceptor Oligomers and Polymers Composed of Benzothiadiazole and 3-Hexylthiophene
Scheme 1 Synthesis of regioregular OBTThn oligomers
O
O
O
O
B
B
1)
C6H13
,
S
S
N
N
C6H13
S
N
N
C6H13
PdCl2(dppf),
KOAc, 1,4-dioxane
N
N
B(OH)3Na
S
HO
HO
Br
B
S
Br
Br
S
Pd(PPh3)4,
aq. Na2CO3, THF
2) H+, silica gel
Br-BTTh
B-BTTh
C6H13
S
S
N
N
N
N
C6H13
B(OH)3Na
S
Br
Pd(PPh3)4,
aq. Na2CO3, THF
S
OBTTh1
S
S
S
N
N
N
N
C6H13
Br
N
N
C6H13
H
C6H13
B-BTTh
NBS
THF
H
H
H
H
n
Pd(PPh3)4,
aq. Na2CO3, THF
S
S
S
n
1
n
1
OBTThn
OBTThn 1-Br
OBTThn
1
were finally prepared from their one-repeating unit-less
oligomers in a moderate to good yield (65%－85%).
Here, the bromination of all OBTThn oligomers with
NBS only took place on the 5-position of the end thienyl
unit. This is another important point to achieve the suc-
cessful iterative synthesis of OBTThn oligomers. The
structure of all oligomers was unambiguously confirmed
by 1H NMR, MALDI-TOF mass spectroscopy, and
elemental analysis.
Scheme 2 outlines the synthetic routes for PBTTh1
and PBTTh2 polymers. Polymer PBTTh1 with all side
hexyl chains orienting the same direction was achieved
by the Suzuki-coupling polymerization of an AB-type
monomer, B-BTTh-Br. It was prepared in a yield of
96% by the bromination of B-BTTh with NBS. The
polymerization of this AB-type monomer ensures the
formation of the polymer with all neighboring BT-Th
units linked by a head-to-tail style. The number-average
molecular weight (Mn) and PDI of the prepared PBTTh1
were 89.0 kDa and 2.31, respectively, as determined by
GPC using polystyrenes as standards. The synthesis of
polymer PBTTh2 with alternative orienting side chains
was a little complicated. Firstly, a diboronate-functiona-
lized BT compound, B-BT-B, was subjected to Su-
zuki-coupling with 2-bromo-3-hexyl thiophene, afford-
ing ThBTTh. Then, it was brominated to give monomer
Br-ThBTTh-Br. The final polymer PBTTh2 was pro-
duced in a yield of 57% via Suzuki-coupling copoly-
merization between Br-ThBTTh-Br and B-BT-B
monomers. The number-average molecular weight (Mn)
and PDI were 7.5 kDa and 1.73 refer to polystyrene
standards for the so-produced PBTTh2, quite different
from those of PBTTh1. Since the molecular weight was
reported to have great impact on polymer photovoltaic
performance,[22] it is not applicable to directly use the
so-prepared polymers (PBTTh1 and PBTTh2) to study
Scheme 2 Synthesis of PBTTh1 and PBTTh2 polymers
S
S
S
N
N
C6H13
N
N
N
N
C6H13
Br
C6H13
Pd(PPh3)4
HO
HO
HO
HO
NBS
THF
H
H
B
B
aq. Et4NOH, toluene
S
S
S
n
PBTTh1
B-BTTh
B-BTTh-Br
C6H13
S
S
N
N
N
N
Br
O
O
O
S
S
S
NBS
THF
B
B
O
Pd(PPh3)4,
aq. Na2CO3, THF
C6H13 C6H13
B-BT-B
ThBTTh
S
S
S
N
N
N
N
N
N
B-BT-B
S
S
S
S
Br
Br
Pd(PPh3)4,
aq. NEt4OH, toluene
n
C6H13 C6H13
Br-ThBTTh-Br
C6H13 C6H13
PBTTh2
Chin. J. Chem. 2013, XX, 1—13
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
7

Wang et al.
FULL PAPER
the effect of side-chain regioregularity on their basic and
photovoltaic properties. To ensure a comparison basis,
the two polymers were subjected to preparative SEC and
the fractions with the same elution time were collected.
The final polymers used for property study and organic
solar cells have Mn and PDI of 5.05 kDa and 1.86 for
PBTTh1, while 4.66 kDa and 1.16 for PBTTh2.
and ICT peak around 449 nm. The position of the π-π*
transition peak shifts to 312 nm for OBTTh3, 314 nm
for OBTTh4, and 316 nm for OBTTh5, OBTTh6, and
OBTTh7. Meanwhile, the ICT peak changes to around
476 nm for OBTTh3, 489 nm for OBTTh4, 497 nm for
OBTTh5, 501 nm for OBTTh6, and 505 nm for
OBTTh7. These observations suggest that the effective
conjugated length becomes longer and longer when the
number of repeating unit increases. It is also valuable to
point out that chain length-dependent absorption redshift
for ICT bands are larger than those for π-π* transition
band. This implies that the ICT band is delocalized to a
more extent through the backbone than π-π* transition
band. For polymers, PBTTh1 and PBTTh2 display ab-
sorption bands at similar positions, around 320 nm for
π-π* transition band and 512 nm for ICT peak. Since
both polymers have similar molecular weight and
polydispersity, the above fact suggests that the
side-chain regioregularity does not interfere electron
delocalization in the backbone. However, compared
with those of the oligomers, these two peaks of the
polymers exhibit further redshift, indicating the poly-
Photophysical properties
The UV-vis spectroscopy of all oligomers and poly-
mers were carried out in both CHCl3 solution and film
state (Figure 2). The data were summarized in Table 1.
In CHCl3 solution, all oligomers except OBTTh1 and
polymers show two distinguished absorption bands in
the range of 300－350 and 400－600 nm. The former
band originates from the π-π* transition of their conju-
gated backbones, while the latter is assignable to in-
tramolecular charge transfer (ICT) absorption between
BT and 3-hexylthiophene units. The apex position for
the both peaks displays red-shift along with the elonga-
tion of the oligomer chain length. For example,
OBTTh2 displays π-π* transition peak around 308 nm
Figure 2 Normalized UV-vis absorption spectra of OBTThn (n＝1－7) oligomers and PBTTh1 and PBTTh2 polymers (a) in CHCl3
solution and (b) in film state.
Table 1 Summary of optical, electrochemical properties and theoretical calculation results of OBTThn (n＝1–7) oligomers and
PBTTh1 and PBTTh2 polymers
λ
max/nm
Film
—
HOMO/eV
CVa
calcd
LUMO/eV
CVb calcd
Eg/eV
opticalc CV
2.34
Oligomer/
Polymer
λ
onset/nm Eox, onset/V Ered, onset/V
Solution
398
calcd
3.15
2.51
2.26
2.11
2.03
1.97
1.95
1.95d
1.93e
OBTTh1
OBTTh2
OBTTh3
OBTTh4
OBTTh5
OBTTh6
OBTTh7
PBTTh1
—
1.05
0.95
0.82
0.75
0.72
0.70
0.68
0.69
0.67
−1.29
−1.24
−1.27
−1.24
−1.12
−1.12
−1.10
−1.07
−1.15
−5.45 −5.97 −3.11 −2.82
—
308, 449 316, 468
312, 476 318, 501
314, 489 320, 522
316, 497 322, 528
316, 501 322, 532
316, 505 324, 536
318, 512 328, 560
320, 512 326, 548
574
618
642
657
661
664
700
685
−5.35 −5.53 −3.16 −3.02 2.16
−5.22 −5.37 −3.13 −3.11 2.01
−5.15 −5.28 −3.16 −3.17 1.93
−5.12 −5.23 −3.28 −3.20 1.89
−5.10 −5.20 −3.28 −3.23 1.88
−5.08 −5.19 −3.30 −3.24 1.87
−5.09 −5.19 d −3.33 −3.24d 1.77
−5.07 −5.17 e −3.25 −3.24e 1.81
2.19
2.09
1.99
1.84
1.82
1.78
1.76
1.82
PBTTh2
a
Estimated from the onset of oxidation potential in the CV measurement. b Estimated from the onset of reduction potential in the CV
measurement. c Estimated from the onset of the absorption spectra in film state. d OBTTh7 was used to represent PBTTh1 for theoretical
calculation. e A BT-Th heptamer having the same side-chain regioregularity as PBTTh2 was used to represent PBTTh2 for theoretical
calculation.
8
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Chin. J. Chem. 2013, XX, 1—13

Donor-Acceptor Oligomers and Polymers Composed of Benzothiadiazole and 3-Hexylthiophene
mers have a longer effective conjugated length.
6-311＋G(d,p)//B3LYP/6-31G(d,p) level. All the calcu-
The onset of the film absorption spectrum can be
used for the estimation of the material optical bandgap
(Eg) following the equation: Eg＝1240/λonset. The results
were included in Table 1. As expected, the bandgap
gradually decreases upon increment of oligomer chain
length, 2.16 eV for OBTTh2, 2.01 eV for OBTTh3,
1.93 eV for OBTTh4, 1.89 eV for OBTTh5, 1.88 eV for
OBTTh6, and 1.87 eV for OBTTh7. The energy band-
gap further decreased to 1.77 and 1.81 eV for polymers
PBTTh1 and PBTTh2 respectively.
lations were performed using a Gaussian 03 program.[25]
This inexpensive calculation was successfully applied in
previous studies on conjugated polymers.[23,24]
Figure 3 displays the optimized HOMO and LUMO
structures and the geometrical dihedral angles of
OBTTh7 and PBTTh2-like BT-Th heptamer. It is clear
that the different side-chain orientation pattern greatly
impacts the geometry of the backbone. A straight line-
like geometry was obtained for OBTTh7, while a
curved line-like shape for PBTTh2-like heptamer. Due
to just one head-to-tail linkage among the BT-Th re-
peating units in OBTTh7 backbone, the local environ-
ment for all BT rings is same, with one adjacent thienyl
unit having a far away side chain and the other adjacent
thienyl unit having a nearby side chain. In optimized
geometry, these two thienyl units adopt a flipped ori-
enting direction (parallel or antiparallel) as refer to the
central BT ring. The thienyl unit having a far away side
chain is almost in planar with BT, since the estimated
dihedral angle for C-C-C-C is 174°. But for that having
a nearby side chain, the dihedral angle for C-C-C-C be-
tween this thienyl unit and BT ring is around 31°, indi-
cating the side chain has imposed a large steric hin-
drance. In contrast, there are two different kinds of BT
rings in the structure of PBTTh2-like Th-BT heptamer,
one neighboring to two thienyl units having two far
away side chains, the other neighboring to two thienyl
units having two nearby side chains. Because of less
steric hindrance coming from the far away side chains,
the former BT rings are almost in planar with the two
neighboring thienyl units. The two dihedral angles are
173° and 177°. But for the latter kind of BT rings, two
dihedral angles both around 30° are suggested. Fur-
thermore, the two neighboring thienyl units cannot
Electrochemical properties
The material redox properties were investigated by
cyclic voltammetry (CV) in CH2Cl2 solution at room
temperature with a conventional three electrode con-
figuration consisting of a glassy carbon working elec-
trode, a Pt wire counter electrode and an SCE
(Hg/Hg2Cl2) reference electrode. The measurements
were carried out with a sample concentration of 0.1
mmol•L−1 in the presence of 0.1 mol•L−1 Bu4NPF6 elec-
trolyte under a scan rate of 100 mV•min−1. The meas-
ured onset oxidation and reduction potentials of oli-
gomer OBTThn (n＝1－7) and polymer PBTTh1 and
PBTTh2 are listed in Table 1. From these data, the
HOMO and LUMO energy levels can be estimated fol-
lowing equations: HOMO＝−(4.4＋Eox,onset) and LUMO
＝−(4.4＋Ered, onset), since the energy level of SCE is 4.4
eV below vacuum. It is reasonable to observe that
HOMO level increases while the LUMO level decreases
with the number increment of the repeating units in the
oligomers, making the electrochemical bandgap gradu-
ally become smaller and smaller. For polymers, no sig-
nificant difference was observed for the oxidation po-
tential between PBTTh1 and PBTTh2, thus giving a
similar HOMO level for both polymers. But for reduc-
tion potential, PBTTh2 exhibited a 0.08 V-lower onset
potential, affording a slightly higher LUMO and larger
bandgap than PBTTh1. As compared with optical values,
all the electrochemical bandgaps for oligomers and
polymers are in good consistence.
Theoretical studies
In order to theoretically understand the effect of
chain length and the side-chain regioregularity on their
basic properties, quantum chemical computations were
performed on oligomer OBTThn (n＝1－7) and an
(BT-Th) heptamer with the same side-chain orientation
pattern as PBTTh2. Here, the pendant hexyl side chains
were simplified as methyl units to save computational
time due to the fact that the geometries and energies
negligibly depend on the pendant alkyl groups.[23,24]
Geometry optimization of oligomers was carried out by
a density functional theory (DFT) method at the
B3LYP/6-31G(d) level. Vibrational frequencies at the
B3LYP/6-31G(d)//B3LYP/6-31G(d) level were used to
characterize stationary points as minima, i.e. no imagi-
nary frequencies. Energy was evaluated at the B3LYP/
Figure 3 The optimized HOMO and LUMO structures of
OBTTh7 and a (BT-Th) heptamer having the same side-chain
regioregularity of PBTTh2. The bottom figures show the dihedral
angles of the oligomer backbones.
Chin. J. Chem. 2013, XX, 1—13
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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9

Wang et al.
FULL PAPER
adopt the reverse direction as refer to the central BT
rings, thus making the geometry of the backbone
curved.
vestigate the photovoltaic properties of OBTThn (n＝4
－7) oligomers and PBTTh1 and PBTTh2 polymers.
The active layer was a bulk heterojunction film com-
posed of the checked oligomer or polymer as donor and
PC61BM as acceptor. OBTTh5 was first chosen to op-
timize device fabrication conditions. After checking the
weight ratio of OBTTh5 to PC61BM, spin-coating rate,
and the thermal anneal treatment, it was concluded that
the film prepared by spin-coating a chlorobenzene solu-
tion containing a mixture of OBTTh5 and PC61BM in a
weight ratio of 1/2 with a total concentration of 20
We also computed the chain length-dependent
HOMO and LUMO energy level (Table 1). The chang-
ing tendency is consistent very well with that observed
in the electrochemical measurement. When the chain
length elongates, the HOMO energy level gradually in-
creases while the LUMO decreases, giving a smaller and
smaller energy bandgap for higher oligomers. It was
reported that the energy bandgap is semiempirically
proportional to the reciprocal of the number of repeating
units and such linear relationship can be used for predic-
tion of the effective conjugated length.[26] Figure 4 plots
all the energy bandgaps of OBTThn obtained from dif-
ferent methods against 1/n and confirms their good lin-
ear relationship. From their linear projection, the poly-
mer having infinite number of the repeating unit would
have an energy gap of 1.76 eV based on the theoretically
calculated Eg, or 1.73 eV based on optical Eg, or 1.77 eV
based on electrochemical Eg. We noted that there is no
substantial difference between the electrochemically
measured energy bandgaps of OBTTh7 and PBTTh1
and the estimated value for infinite polymer, indicating
OBTTh7 and polymer PBTTh1 have sufficient chain
length to achieve a good photovoltaic performance.
mg•mL－ at a rate of 2000 r/min was good for OSCs.
1
Under such conditions, the thickness of the active layer
was around 80 nm.
With the above optimized conditions, we fabricated
the solar cells based on the blend of OBTThn (n＝4－7),
PBTTh1, or PBTTh2 with PC61BM and characterized
their current-voltage performance under the illustration
of AM 1.5 G with a light density of 100 mW•cm−2 (Fig-
ure 5a). Their device parameters, such as VOC
,
short-circuit current (JSC), fill factor (FF) and PCE were
summarized in Table 2. The data clearly show that both
VOC and JSC increased when the donor material was
changed from OBTTh4 to OBTTh5, and then to
OBTTh6. Together with their comparable fill factors,
these increments finally resulted in the improvement in
the device efficiency from 0.16% for the OBTTh4-
based cell to 0.36% for that based on OBTTh6. How-
Figure 4 Plot of energy bandgap versus 1/n of OBTThn.
Although the computation suggests big different
backbone geometry for PBTTh1 and PBTTh2, the cal-
culated HOMO and LUMO structures are similar and
have comparable energy level (Figure 3 and Table 1).
This may be understandable when considering the fact
that the twisting degree and the number of twisted dihe-
dral angles are almost the same between OBTTh7 and
PBTTh2-like BT-Th heptamer. However, the different
backbone geometry would influence the chain packing
structure in solid state, which is reflected by the less
ICT red-shifts observed for the film absorption spectrum
of PBTTh2, as compared with that of PBTTh1.
Photovoltaic properties
Figure 5 (a) J-V curves and (b) EQE spectra of the solar cells
based on the blend of OBTThn (n＝4－7), PBTTh1 or PBTTh2
with PC61BM.
Organic solar cells (OSCs) with a structure of
ITO/PEDOT:PSS/active layer/Ca/Al were used to in-
10
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Chin. J. Chem. 2013, XX, 1—13

Donor-Acceptor Oligomers and Polymers Composed of Benzothiadiazole and 3-Hexylthiophene
Table 2 Photovoltaic properties of the OSCs based on OBTThn
(n＝4－7) and PBTTh1 and PBTTh2 as donor and PC61BM as
acceptor under the illumination of AM1.5G, 100 mW•cm−2
b
PBTTh1. In EQE spectroscopy, the PBTTh2-based de-
vice displayed smaller photocurrent in the UV region,
but a slightly larger photocurrent in the visible light re-
gion, finally giving a comparable total value. For these
two polymers, they have comparable molecular weight
and polydispersity, and exhibit similar optical and elec-
trochemical properties in solution. But they possess
quite different backbone geometry, and show some dif-
ferences in the film absorption spectroscopy. However,
these differences are small and not able to modify the
final device photovoltaic performance.
JSC
/
JSC
/
Material
VOC/V
FF/% PCE a/%
(mA•cm−2)
0.93
(mA•cm−2)
OBTTh4 0.59
OBTTh5 0.78
OBTTh6 0.89
OBTTh7 0.85
PBTTh1 0.80
PBTTh2 0.79
28.7 0.16 (0.15) 0.78
31.0 0.31 (0.26) 1.14
27.8 0.36 (0.31) 1.35
29.4 0.33 (0.32) 1.19
31.8 0.49 (0.46) 1.81
34.0 0.52 (0.49) 1.79
1.27
1.45
1.33
1.94
1.95
a Data in parentheses are the average values. b Calculated from
EQE spectrum.
ever, when the donor material was further changed to
OBTTh7, no further improvement but comparable per-
formance was observed. The situation was changed
again when polymer PBTTh1 was used as donor mate-
rial. Its devices displayed a larger JSC while maintained
comparable VOC and FF, finally resulting in an improved
efficiency (0.49%).
In general, the open circuit voltage of OSCs is
semi-empirically proportional to the energy level dif-
ference between the HOMO of donor and the LUMO of
the acceptor materials.[27] However in this work, the
devices showed an increasing tendency on VOC along the
chain length of OBTThn (n＝4－6), obviously not
obeying the above general correlation. This implies that
some other factors besides the material energy levels
affect the device VOC. Actually, the origin of VOC is still
under intense debate in the field, and some works have
already pointed out that a couple of factors including the
size of side chains, interchain interactions, ground state
charge transfer interactions between donor and fullerene
acceptors, and the morphology of the active layer have
[28]
noticeable influence on VOC
.
In this work, the differ-
ent chain length would endow the oligomer a different
interchain interactions and the ground state charge trans-
fer interactions with PC61BM, thus affording an unusual
increasing tendency for VOC
.
On the other hand, the device photocurrent (JSC) en-
hanced along with the increment in the chain length,
from OBTTh4, to OBTTh6, then to PBTTh1 polymer-
based cells. The increasing tendency was further coin-
cided with external quantum efficiency (EQE) spectral
measurement. As shown in Figure 5b, all the checked
devices exhibited photocurrent response in the range of
300－700 nm. The photocurrent generation efficiency
was gradually improved from shorter oligomer
(OBTTh4) to longer one (OBTTh6), then to PBTTh1
polymer. All the above observations indicate that the
chain length of the material has great impact on its
photovoltaic performance.
Figure 6 AFM topographical height images (2×2 μm2) of the
blend films of (a) OBTTh4, (b) OBTTh5, (c) OBTTh6, (d)
OBTTh7, (e) PBTTh1, and (f) PBTTh2 with PC61BM in weight
ratio of 1∶2.
Atomic force microscopy (AFM) was applied to
study the effect of the chain length and the side-chain
regioregularity on the morphology of the bulk hetero-
junction active films. As shown in Figure 6, the blend
films based on OBTThn (n＝4－7) with PC61BM
showed AFM images with very smooth and uniform
appearance, suggesting that all the checked oligomers
are phase-compatible with PC61BM. However, in the
case of polymer blend films, obvious phase separation
was observed. The domain size grew bigger from
PBTTh1 to PBTTh2. These observations indicate the
Unlike the chain-length effect, the side-chain regio-
regularity was found to have less influence on the device
performance. The devices based on PBTTh2 outputted
similar VOC, JSC, and FF values as those based on
Chin. J. Chem. 2013, XX, 1—13
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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11

Wang et al.
FULL PAPER
Chu, T.-Y.; Lu, J.; Beaupré, S.; Zhang, Y.; Pouliot, J.-R.; Wakim, S.;
Zhou, J.; Leclerc, M.; Li, Z.; Ding, J.; Tao, Y. J. Am. Chem. Soc.
2011, 133, 4250; (d) Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.;
Liu, S.; You, W. Angew. Chem., Int. Ed. 2011, 50, 2995; (e) Amb, C.
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occurrence of phase separation in the blend films re-
quires a sufficient chain length of BT-Th backbone and
is also affected by the side-chain regioregularity. Al-
though it is not clear at present that the observed phase
separation scale would benefit their photovoltaic per-
formance, appropriate phase separation between donor
material and fullerene acceptor components in the bulk
heterojunction is one of the prerequisites for high per-
formance OSCs.[2] This may be one of the reasons for
the performance improvement for polymer-based OSC
devices as compared with the oligomer-based ones.
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Conclusions
A series of oligomer OBTThn (n＝1－7) and poly-
mer PBTTh1 and PBTTh2 composed of alternating
3-hexylthiophene and 2,1,3-benzothiadiazole units were
designed and synthesized. The structural feature of the
oligomers and PBTTh1 polymer is that all the hexyl
side chains orienting toward the same direction.
Whereas in polymer PBTTh2, every neighboring hexyl
side chain changes its orientation direction one another,
thus endows it a different side-chain regioregularity. It
was found that the chain length has great impact on the
photophysical, electrochemical, and photovoltaic prop-
erties of the oligomers and polymers. The elongation of
the chain length enables the material red-shifted absorp-
tion bands, an enhanced HOMO energy level, a de-
creased LUMO, a smaller energy bandgap, and finally a
better photovoltaic performance. The change of side
chain regioregularity makes polymer PBTTh2 adopt
different backbone geometry, and influences its packing
structure in solid state. However, it has been proved to
have less influence on the UV-vis absorption spectros-
copy and electrochemical properties in solution, and the
final device photovoltaic outputs.
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Acknowledgement
This work was supported by the National Natural
Science Foundation of China (Nos. 20974119,
90922019, and 21074147), Chinese Academy of Sci-
ences, and Shanghai Science and Technology Commis-
sion (No. 13JC1407000).
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