Effect of oxygen-containing functional side chains on the electronic properties and photovoltaic performances in a thiophene-thiazolothiazole copolymer system

Article abstract of DOI:10.1002/hc.21196

We report the synthesis and characterization of new thiazolothiazole-based semiconducting polymers with alkoxy and/or ester groups as the side chain. An introduction of the alkoxy groups raised the HOMO energy level of the polymers, whereas the introduction of the ester groups lowered the LUMO energy level. It is found that, interestingly, in the monomers the electronic effect of the ester group was more significant than that of the alkoxy groups, whereas in the polymers the effect of the alkoxy group was more significant. PTzBT-oBOeHD with both the alkoxy and ester groups gave the most redshifted absorption spectrum compared to the polymers with the alkyl and alkoxy groups (PTzBT-oBOHD) and with the alkyl and ester groups (PTzBT-BOeHD). Solar cells using PTzBT-oBOeHD gave the highest power conversion efficiencies of up to 4.5% with the highest short-circuit current density (JSC) of 10.9 mA/cm² among those with the newly synthesized polymers.

Full text of DOI:10.1002/hc.21196

Heteroatom Chemistry
Volume 25, Number 6, 2014
Effect of Oxygen-Containing Functional Side
Chains on the Electronic Properties and
Photovoltaic Performances in a
Thiophene–Thiazolothiazole Copolymer
System
Masahiko Saito,1 Itaru Osaka,1,2 Tomoyuki Koganezawa,3
and Kazuo Takimiya1,2
1Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,
Higashi-Hiroshima, Hiroshima 739-8527, Japan
2Emergent Molecular Function Research Group, RIKEN Center for Emergent Matter Science,
Wako, Saitama 351-0198, Japan
3Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo 679-5198, Japan
Received 25 February 2014; revised 16 May 2014
riodicals, Inc. Heteroatom Chem. 25:556–564, 2014;
ABSTRACT: We report the synthesis and characteri-
View this article online at wileyonlinelibrary.com.
zation of new thiazolothiazole-based semiconducting
DOI 10.1002/hc.21196
polymers with alkoxy and/or ester groups as the side
chain. An introduction of the alkoxy groups raised the
HOMO energy level of the polymers, whereas the intro-
duction of the ester groups lowered the LUMO energy
level. It is found that, interestingly, in the monomers
the electronic effect of the ester group was more sig-
nificant than that of the alkoxy groups, whereas in
the polymers the effect of the alkoxy group was more
significant. PTzBT-oBOeHD with both the alkoxy and
ester groups gave the most redshifted absorption spec-
trum compared to the polymers with the alkyl and
alkoxy groups (PTzBT-oBOHD) and with the alkyl
INTRODUCTION
Bulk heterojunction (BHJ) solar cells using semicon-
ducting polymers as the p-type photoactive materials
together with fullerene derivatives as the n-type ma-
terials are of great interest for the development of
low-cost, flexible, and large area renewable energy
sources [1]. During the last decade, a number of new
semiconducting polymers have been reported, and
the power conversion efficiencies (PCEs) of polymer-
and ester groups (PTzBT-BOeHD). Solar cells using
PTzBT-oBOeHD gave the highest power conversion ef-
based BHJ solar cells have rapidly improved [2]. To
ficiencies of up to 4.5% with the highest short-circuit
current density (JSC) of 10.9 mA/cm2 among those
develop high-efficiency solar cells, semiconducting
polymers are required to have a wide absorption
range, namely a narrow bandgap, to absorb as much
sunlight as possible [3]. They also need to form a
highly ordered structure in thin films to ensure high
charge carrier transport to efficiently collect the gen-
erated charge carriers.
ꢀC
with the newly synthesized polymers. 2014 Wiley Pe-
Correspondence to: Itaru Osaka; e-mail: itaru.osaka@riken.jp.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
ꢀC
2014 Wiley Periodicals, Inc.
556

Effect of Oxygen-Containing Functional Side Chains on the Electronic Properties and Photovoltaic Performances 557
FIGURE 1 Chemical structure of thiophene-thiazolothiazole copolymers. PTzBT-BOHD: the previously reported polymer;
PTzBT-oBOHD, PTzBT-BOeHD, PTzBT-oBOeHD: newly synthesized in this work.
SCHEME 1 Synthesis of the monomers.
Among the many polymer systems studied
so far, polymers with thiazlothiazole (TzTz)
is a promising system that provides highly ordered
structures and thus high charge carrier mobili-
ties [4]. In fact, several groups have reported on
the synthesis of TzTz-based polymers incorporat-
ing various donor units in the backbone, which
showed good PCEs of ꢀ5.8% [5]. Independently,
we have also reported a series of TzTz-based poly-
mers with alkylthiophenes as the counit and demon-
strated that one of the polymers (PTzBT-BOHD,
Fig. 1) showed quite high PCEs of 6−7.5% in con-
ventional single-junction cells depending on the n-
type material and the active layer thickness [6].
However, the relatively narrow absorption range
of ꢀ680 nm for PTzBT-BOHD limited the light
harvesting property, and thus broadening the ab-
sorption range, i.e., narrowing the bandgap, has
been an issue for the further improvement of the
performances.
In this work, we introduced the electron-
donating alkoxy and/or electron-withdrawing es-
ter groups on the thiophene moieties instead of
the alkyl groups introduced in the side chain of
PTzBT-BOHD while using the same backbone sys-
tem (PTzBT-oBOHD, PTzBT-BOeHD, and PTzBT-
oBOeHD; Fig. 1), since the backbone has been
proven to offer highly ordered structures [5]. It is ex-
pected that the combination of these two functional
groups with different electronic natures would al-
low us to have polymers with narrow bandgaps with
various HOMO and LUMO levels. We here describe
the synthesis, electronic structures, and the order-
ing structures of new thiophene-TzTz polymers with
alkoxy and/or ester groups as the side chain and their
solar cell performances.
Heteroatom Chemistry DOI 10.1002/hc

558 Saito et al.
SCHEME 2 Synthesis of PTzBT-oBOHD, PTzBT-BOeHD, and PTzBT-oBOeHD.
TABLE 1 Polymerization Resultsa
RESULTS AND DISCUSSION
Synthesis
Polymer
Mn (kDa)
Mw (kDa)
PDI
PTzBT-oBOHD
PTzBT-BOeHD
PTzBT-oBOeHD
PTzBT-BOHDb
32
25
29
35
132
115
235
110
4.0
4.8
8.2
3.1
The synthetic routes to the monomer with the alkoxy
groups [7] and the ester groups [8] are shown in
Scheme 1. The dithienyl-TzTz with the alkoxy groups
on the thiophene rings (2) was prepared by heating
the mixture of 1 and dithiooxiamide. Compound 2
was then dibrominated using N-bromosuccinimide
(NBS) to give 3, which was then stannylated via
the treatment of n-BuLi followed by the addition
of trimethyltin chloride, giving the alkoxy monomer
(4). As for the dithienyl-TzTz with the ester groups
on the thiophene rings (8), 5-bromothiophene-3-
carboxylic acid (5) was first formylated at the 2 po-
sition by the treatment of N,Nꢁ-dimethylformamide
(DMF) through lithiation with lithium diisopropy-
lamide (LDA) to yield 6. However, this reaction gave
the 4-formyl-3-thiophene-carbxylic acid as a byprod-
uct, which could not be removed by column chro-
matography. The crude of 6 was thus subjected to
esterification with 2-hexy-1-bromodecane to afford
7, which was successfully isolated by column chro-
matography. The ester monomer (8) was prepared
by the reaction of 7 and dithiooxamide in DMF.
All polymers were synthesized via the Stille
Abbreviation: PDI, polydispersity indice.
aDetermined by GPC using polystyrene standard and o-
dichlorobenzene as the eluent at 140°C.
bFrom [6b].
monomer (10) to give PTzBT-oBOeHD. All poly-
mers were soluble in warm chloroform (CF) and
chlorobenzene (CB), and the molecular weight of
polymers evaluated by gel permeation chromatog-
raphy (GPC) at 140°C were mostly above 30 kDa
(Mn) (Table 1). Polydispersity indices of the poly-
mers synthesized here were relatively large (>4), ow-
ing to the bimodally appeared signal in the chro-
matograph (Fig. S14 in the Suporting Information).
This is likely because the polymers with the alkoxy
and ester groups have higher tendency to aggregate,
resulting in the bimodal signals.
Physicochemical Properties of the Polymers
coupling reaction using
a microwave reactor
(Scheme 2). The alkoxy monomer (4) was copoly-
merized with the dibrominated alkyl monomer (9)
and the ester monomer (8) to give PTzBT-oBOHD
and PTzBT-BOeHD, respectively. Compound 8 was
also copolymerized with the distannylated alkyl
To confirm the electronic effect of the side chains,
cyclic voltammetry (Fig. 2a) and UV–vis absorp-
tion spectroscopy (Fig. 2b) of dithienyl-TzTzs with
the alkyl (TzBT-HD, 9), alkoxy (TzBT-oBO, 3), and
ester groups (TzBT-eHD, 8) were carried out in
Heteroatom Chemistry DOI 10.1002/hc

Effect of Oxygen-Containing Functional Side Chains on the Electronic Properties and Photovoltaic Performances 559
(b)
(a)
1.4
TzBT-HD (9)
TzBT-oBO (3)
TzBT-eHD (8)
TzBT-HD (9)
TzBT-oBO (3)
TzBT-eHD (8)
1.2
1
0.8
0.6
0.4
0.2
0
300
350
400
Wavelength (nm)
450
500
0
0.5
1
1.5
2
Potential (V vs. Ag/AgCl)
FIGURE 2 Cyclic voltammograms (a) and UV–vis absorption spectra (b) of the monomers in the solution.
TABLE 2 Electronic Properties of the Monomers and Polymers
opt
Compound
λmax (nm)
λedge (nm) / Eg (eV)a
EHOMO (eV)b
ELUMO (eV)c
TzBT-HD (9)
403
415, 440
432
573, 624
600, 653
578, 633
621, 675
448/2.77
457/2.71
481/2.58
685/1.81
709/1.75
700/1.77
730/1.70
−5.59
−5.29
−5.80
−5.23
−5.02
−5.28
−5.08
−2.82
−2.58
−3.22
−3.42
−3.27
−3.51
−3.38
TzBT-oBO (3)
TzBT-eHD (8)
PTzBT-BOHD
PTzBT-oBOHD
PTzBT-BOeHD
PTzBT-oBOeHD
aλedge: absorption edge, Egopt: bandgaps calculated with λedge
.
bHOMO energy levels; cyclic voltammetry and photoelectron spectroscopy in air (PESA) were used to evaluate them for the monomers and
polymers, respectively.
cLUMO energy levels estimated by adding the band gap (Egopt) to EHOMO
.
the dichloromethane solution. EHOMO of TzBT-BO,
TzBT-oBO, and TzBT-eHD were estimated from the
onset oxidation potentials. EHOMO of TzBT-oBO was
higher by 0.3 eV (−5.29 eV) and that of TzBT-eHD
was lower by 0.21 eV (−5.80 eV) than that of TzBT-
BO (−5.59 eV) (Table 2). These results are consistent
with what is expected from the electron-donating
and electron-withdrawing nature of the alkoxy and
the ester groups. ELUMOs were estimated by the addi-
tion of Eg determined from the absorption onset. As
is the case in EHOMO, ELUMO of TzBT-oBO (−2.58 eV)
was higher by 0.24 eV and that of TzBT-eHD
(−3.22 eV) was lower by 0.40 eV than that of TzBT-
BO (−2.82 eV) (Table 2). These results indicate that
the alkoxy group affects EHOMO and the ester group
affects ELUMO more strongly. The absorption max-
ima (λmax) of both TzBT-oBO (415 nm) and TzBT-
eHD (430 nm) were redshifted as compared to that
of TzBT-BO (403 nm). The more significant redshift
in TzBT-eHD than in TzBT-oBO might be due to the
resonance effect of the ester groups.
and PTzBT-oBOeHD with the alkoxy groups were
−5.02 and −5.08 eV, respectively, which were about
0.15−0.2 eV higher than that of PTzBT-BOHD
(−5.23 eV) (Table 2), owing to the electron-donating
nature of the alkoxy group. Slightly lower EHOMO
of PTzBT-oBOeHD compared to PTzBT-oBOHD is
likely due to the electron-withdrawing nature of the
ester group. EHOMO of PTzBT-BOeHD (−5.28 eV)
was slightly lower than that of PTzBT-BOHD
(Table 2), which is also attributed to the electron-
withdrawing nature of the ester groups. It should be
noted that the downward shifts of EHOMO by the in-
troduction of the ester group, i.e., shifts from PTzBT-
BOHD to PTzBT-BOeHD and from PTzBT-oBOHD
to PTzBT-oBOeHD, were 0.05−0.06 eV, which was
far less than the upward shift by the alkoxy group.
ELUMO of the polymers were estimated by adding
the band gap (Egopt), which was determined from
the absorption onset in the film, to EHOMO (Table 2).
In PTzBT-oBOHD, ELUMO was elevated to −3.28 eV
from that in PTzBT-BOHD (−3.42 eV), being af-
fected by the alkoxy group. On the other hand, ELUMO
of PTzBT-BOeHD was lower (−3.51 eV) than that
of PTzBT-BOHD owing to the electron-withdrawing
EHOMOs of the polymers were evaluated by
photoelectron spectroscopy in air using the poly-
mer thin films (Fig. 3a). EHOMOs of PTzBT-oBOHD
Heteroatom Chemistry DOI 10.1002/hc

560 Saito et al.
(b)
(a)
250
200
150
100
50
1.4
1.2
1
PTzBT-BOHD
PTzBT-oBOHD
PTzBT-BOeHD
PTzBT-oBOeHD
PTzBT-BOHD
PTzBT-BOeHD
PTzBT-oBOHD
PTzBT-oBOeHD
0.8
0.6
0.4
0.2
0
0
4.5
300
400
500
600
700
800
5.0
5.5
6.0
Photon energy (eV)
Wavelength (nm)
FIGURE 3 Photoelectron spectra (a) and UV–vis absorption spectra (b) of the polymer thin films.
nature of the ester group. In PTzBT-oBOeHD, de-
spite having the ester groups, ELUMO was −3.38 eV
that is almost the same as that in PTzBT-BOHD.
This implies that the electronic effect of the ester
groups on the ELUMO is compensated by the effect
of the alkoxy groups. These perturbations of EHOMO
and ELUMO suggest that the electronic effect of the
alkoxy group was more significant in this system.
UV–vis absorption spectra of the polymers in
the film are shown in Fig. 3b. As expected, all the
polymers synthesized here exhibited redshifted spec-
tra compared to that of PTzBT-BOHD with λmax
of 624 nm. PTzBT-oBOeHD with both the alkoxy
and ester groups had the most redshifted absorp-
tion spectra with λmax of 675 nm and the smallest
optical bandgap (Egopt) of 1.70, which was reduced
by ca. 0.1 eV compared to that of PTzBT-BOHD
(Table 2). It was interesting that λmax of PTzBT-
BOeHD (633 nm) was located at the shorter wave-
length region than λmax of PTzBT-oBOHD (653 nm)
(Table 2), which sharply contrast to the result in the
monomer system. This suggests that the electronic
effect of the alkoxy group is more significant than
the ester group in the polymers, as also observed in
perturbation of EHOMO and ELUMO. These phenom-
ena could be explained as follows. In the monomers,
the π-electron system of the molecule is limited to
three aromatic heterocyles and thus the electronic
state is strongly reflected by the resonance effect of
the ester groups. However, in the polymers where the
π-electron system is well developed along the back-
bone, the resonance effect of the ester groups might
be weakened.
methyl ester (PC61BM) with the weight ratio of
1:2 in CB onto the PEDOT:PSS spin-coated indium
tin oxide (ITO) glass, followed by vacuum depo-
sition of Ca/Al as the cathode. The current den-
sity (J)–voltage (V) curves and the external quan-
tum efficiency (EQE) spectra of the cells under 1
Sun of simulated AM 1.5G solar irradiation (100
mW/cm2) are displayed in Figs. 4a and 4b, respec-
tively, and photovoltaic parameters are summarized
in Table 3. Although only the PTzBT-oBOeHD-based
cell gave higher short-circuit current density (JSC
)
of 10.9 mA/cm2 than the PTzBT-BOHD-based cell
(10.1 mA/cm2), all the cells using the new poly-
mers (PTzBT-oBOHD, PTzBT-BOeHD, and PTzBT-
oBOeHD) showed photoresponse in wider spectral
ranges compared to that with PTzBT-BOHD, as ex-
pected from the absorption spectra. Owing to the
lower lying EHOMO of PTzBT-BOeHD, its cell showed
slightly but higher open-circuit voltage (VOC) of
0.92 V than PTzBT-BOHD-based cell (0.91 V). In the
cells with PTzBT-oBOHD and PTzBT-oBOeHD, VOC
were lower by more than 0.1 V than the cell with
PTzBT-BOHD, reflecting their higher lying EHOMO
s
.
All the cells with the new polymers showed lower
fill factors of below 0.60 than that of the cell with
PTzBT-BOHD (0.68). This is likely due to the lower
hole mobility of the new polymers. As revealed by the
hole-only device characterization, the hole mobili-
ties for PTzBT-oBOHD, PTzBT-BOeHD, and PTzBT-
oBOeHD (5–9 × 10−5 cm2/Vs) calculated as the space
charge limited current model were about one order
of magnitude lower than that for PTzBT-BOHD (4.8
× 10−4 cm2/Vs) (Fig. S16 in the Suporting Informa-
tion). Although the PCEs of the new polymer-based
cells (3.8–4.5%) were lower than that of PTzBT-
BOHD (6.1%), the cell using PTzBT-oBOeHD with
the most redshifted absorption showed the highest
PCE of 4.5% among the cells using new polymers.
Solar Cell Characteristics
Solar cells were fabricated by spin coating the solu-
tions of polymer and [6,6]-phenyl-C61-butyric acid
Heteroatom Chemistry DOI 10.1002/hc

Effect of Oxygen-Containing Functional Side Chains on the Electronic Properties and Photovoltaic Performances 561
(a)
(b)
5
0
100
80
60
40
20
0
PTzBT-BOHD
PTzBT-oBOHD
PTzBT-BOeHD
PTzBT-oBOeHD
PTzBT-BOHD
PTzBT-oBOHD
PTzBT-BOeHD
PTzBT-oBOeHD
–5
–10
–15
–0.2
0
0.2
0.4
0.6
0.8
1
300
400
500
600
700
800
Voltage (V)
Wavelength (nm)
FIGURE 4 J–V curves (a) and EQE spectra (b) of the solar cells based on PTzBTs.
TABLE 3 Photovoltaic Properties of the Solar Cells Based
tures in the thin film were investigated by
X-ray diffraction studies. Two-dimensional graz-
ing incidence X-ray diffraction (2D-GIXD) images
of the polymer/PC61BM blend films on the PE-
DOT:PSS/ITO substrate are shown in Fig. 5a. In
all cases, diffractions assignable to the π−π stack-
on PTzBTs
JSC
VOC
(V)
PCEmax
Polymer
(mA/cm2)
FF
[PCEave] (%)a
PTzBT-BOHD
PTzBT-oBOHD
PTzBT-BOeHD
PTzBT-oBOeHD
10.1
9.4
8.1
0.91 0.68
0.71 0.57
0.92 0.57
0.78 0.53
6.1 [5.8]
3.8 [3.5]
4.2 [4.0]
4.5 [4.2]
ing structures (q ࣈ 1.7 A−1) appeared along the qz
˚
axes (out of plane), indicating that the polymers
formed face-on orientation on the substrate surface
[9], which would facilitate the out-of-plane charge
transport and thus are ideal for the solar cells. π–π
10.9
Abbreviations: PCEmax, maximum power conversion efficiencies;
PCEave, average power conversion efficiencies; FF, fill factor.
˚
Stacking distances were estimated to be ca. 3.5 A for
Thin Film Structure and Morphology
all the polymers, suggesting that the alkoxy and es-
ter groups are not detrimental to the π–π stacking
order.
To further understand the photovoltaic perfor-
mances of the cells, polymer ordering struc-
FIGURE 5 2D-GIXD patterns (a) and AFM images (b) of the polymer/PC61BM blend films.
Heteroatom Chemistry DOI 10.1002/hc

562 Saito et al.
We also investigated the morphology of the
crowave reactor (Biotage Initiator, Uppsala, Swe-
den). Molecular weights were determined by GPC
using a TOSOH HLC-8121GPC/HT at 140°C using
o-dichlorobenzene as a solvent and calibrated with
polystyrene standards.
polymer/PC61BM blend films by the atomic force
microscopy (AFM) (Fig. 5b). The polymers with the
ester groups (PTzBT-BOeHD and PTzBT-oBOeHD)
showed needle-like textures that are slightly different
from the nodule-like textures observed in other poly-
mers. Nevertheless, all the films seem to have formed
well phase separated morphologies, which would
ensure the charge generation. On the other hand,
the surface roughness (rms) of the blend of PTzBT-
oBOHD, PTzBT-BOeHD, and PTzBT-oBOeHD is
larger than that of PTzBT-BOHD, which means that
the blend films of PTzBT-oBOHD, PTzBT-BOeHD,
and PTzBT-oBOeHD are less uniform compared to
that of PTzBT-BOHD, which could be a reason for
the lower hole mobility and the lower solar cell per-
formances of the new polymers. We believe that the
solar cell performances of the new polymers could
be improved by optimizing the film morphology.
2,5-Bis(3-((butyloctyl)oxy)thiophen-2-yl)
thiazolo[5,4-d] thiazole (2)
Dithiooxamide (1.1 g, 9.2 mmol) and 3-((2-
hexyldecyl)oxy)thiophene-2-carbaldehyde (1) (6.0 g,
20.2 mmol) were combined in a round-bottom flask
and heated at temperature 200°C for 12 h. The crude
product was purified by column chromatography
on silica gel. Recrystallization from hexane gave
pure product as yellow solids (1.8 g, 29%). 1H NMR
(400 MHz, CDCl3) δ 7.32 (d, J = 5.5 Hz, 2H), 6.89 (d,
J = 5.5 Hz, 2H), 4.14 (d, J = 4.8 Hz, 4H), 1.85–1.82
(m, 2H), 1.62–1.49 (m, 8H), 1.40–1.20 (m, 24H), 0.86
(t, J = 7.0 Hz, 12H). 13C NMR (100 MHz, CDCl3) δ
159.8, 156.3, 149.9, 126.8, 116.4, 115.5, 74.5, 38.6,
32.0, 31.4, 31.1, 29.8, 29.3, 27.1, 23.2, 22.8, 14.3,
14.3. EI-MS (70 eV) m/z = 674 (M+); anal. calcd
for C36H54N2O2S4: C, 64.05%; H, 8.06%; N, 4.15%.
Found: C, 64.19%; H, 8.36%; N, 4.02%.
Summary
New thiazolothiazole-based semiconducting poly-
mers with the alkoxy and/or ester groups as the side
chain have been synthesized and investigated. As ex-
pected, the introduction of the alkoxy groups raised
the HOMO energy levels of the polymers, and the
introduction of the ester groups lowered the LUMO
energy levels. Interestingly, the electronic effect of
the alkoxy group was more significant compared to
the ester group in the polymers, which was in sharp
contrast to what was observed in the monomers.
This is likely because the electronic effect of the ester
group is dominated by the resonance effect, which
is weakened in the polymers with well-developed
π-conjugation. Having both the alkoxy and ester
groups, PTzBT-oBOeHD gave the most redshifted
absorption spectrum among the polymers studied
here. Although the overall efficiencies were lower
for the cells using the new polymers compared to
the polymer with all alkyl groups, all the cells using
new polymers showed photoresponse in wider spec-
tral regions. Further device optimization might lead
to the improvement of the efficiency.
2,5-Bis(5-bromo-3-((2-butyloctyl)oxy)thiophen-
2-yl)thiazolo[5,4-d]thiazole (3)
To a solution of 2,5-bis(3-((2-octyldodecyl)oxy)
thiophen-2-yl)thiazolo[5,4-d]thiazole (2) (1.5 g,
2.2 mmol) in CHCl3 (50 mL) and acetic acid (25
mL) solutions, NBS (870 mg, 4.9 mmol) solution in
CHCl3/AcOH (20/10 mL) was added dropwise and
stirred at 0°C for 2 h. Then, the reaction solution
was stirred at room temperature overnight. The mix-
ture was washed with water and brine, and the or-
ganic layer was concentrated. Purifications with col-
umn chromatography on silica gel and recrystal-
lization from hexane gave pure product as yellow
1
solids (1.57 g, 85%). H NMR (400 MHz, CDCl3) δ
6.90 (s, 2H), 4.09 (d, J = 4.9 Hz, 4H), 1.84–1.82 (m,
2H), 1.60–1.49 (m, 8H), 1.35–1.29 (m, 24H), 0.93–
0.86 (m, J = 7.0 Hz, 12H). 13C NMR (100 MHz,
CDCl3) δ 158.7, 154.8, 145.0, 119.8, 116.9, 115.5,
74.9, 38.5, 32.0, 31.4, 31.1, 29.8, 29.3, 27.0, 23.2, 22.8,
14.3. EI-MS (70 eV) m/z = 830 (M+); anal. calcd for
C36H52Br2N2O2S4: C, 51.91%; H, 6.29%; N, 3.36%.
Found: C,51.70%; H, 6.19%; N, 3.46%.
EXPERIMENTAL
Synthesis
3-((2-Hexyldecyl)oxy)thiophene-2-carbaldehyde (1)
[7] and 5-bromothiophene-3-carboxylate (5) [8]
were synthesized according to the reported proce-
dure. All chemicals were of reagent grade unless
otherwise indicated. All solvents were distilled prior
to use. Polymerization was carried out using a mi-
2,5-Bis(5-trimethystannyl-3-((2-butyloctyl)oxy)
thiophen-2-yl)thiazolo[5,4-d]thiazole (4)
To a solution of 2,5-bis(5-bromo-3-((2-butyloctyl)
oxy)thiophen-2-yl)thiazolo[5,4-d]thiazole (3) (1.2 g,
Heteroatom Chemistry DOI 10.1002/hc

Effect of Oxygen-Containing Functional Side Chains on the Electronic Properties and Photovoltaic Performances 563
1.44 mmol) in 50 mL of tetrahydrofuran (THF),
2,2ꢁ-(Thiazolo[5,4-d]thiazole-2,5-diyl)bis(5-bro-
mothiophene-3-carboxylate). A mixture of 7 (2.02 g,
4.4 mmol) and dithiooxamide (240 mg, 2 mmol) was
heated to 140°C for 24 h, and then cooled to room
temperature. The reaction mixture was poured into
water and extracted with dichloromethane. The
combined organic layer was dried over magnesium
sulfate and concentrated under reduced pressure.
The crude product was purified by column chro-
matography on silica gel and recrystallized from
hexane to give 8 as yellow solids (220 mg, 11%).
1H NMR (400 MHz, CDCl3) δ 7.44 (s, 1H), 4.26 (d,
J = 4.9 Hz, 4H), 1.78 (m, 2H), 1.60–1.56 (m, 8H),
1.35–1.27 (m, 40H), 0.90–0.86 (m, 12H). 13C NMR
(100 MHz, CDCl3) δ 162.3, 159.6, 153.8, 132.7, 128.8,
115.9, 68.6, 37.5, 32.1, 32.0, 31.5, 30.1, 29.8, 29.7,
29.5, 26.9, 22.9, 22.8, 14.3. EI-MS (70 eV) m/z = 998
(M+); anal. calcd for C46H68N2O4S4: C, 55.19%; H,
6.85%; N, 2.80%. Found: C, 54.98%; H, 6.61%; N,
2.91%.
1.6 M solution of n-butyllithium in hexane (2.25 mL,
3.6 mmol) was added dropwise at −78°C. The
solution was stirred at −78°C for 1 h and trimethyltin
chloride (800 mg, 4.0 mmol) was added. After the
solution was warmed to room temperature, 50 mL
of water and 50 mL of ethyl acetate were added.
The organic layer was washed twice with 50 mL of
water and dried over magnesium sulfate. After re-
moving the solvent, recrystallization from hexane
gave pure product as yellow solids (1.0 g, 70%).
1H NMR (400 MHz, CDCl3) δ 6.91 (s, 2H), 4.15 (d,
J = 4.9 Hz, 4H), 1.84–1.82 (m, 2H), 1.61–1.50 (m,
8H), 1.28–1.24 (m, 24H), 0.87–0.84 (m, J = 7.0 Hz,
12H), 0.41 (s, 18H). 13C NMR (100 MHz, CDCl3) δ
159.5, 157.5, 149.9, 141.2, 123.2, 120.7, 74.3, 38.6,
32.0, 31.4, 31.1, 29.8, 29.8, 29.5, 27.1, 22.8, 14.3.
EI-MS (70 eV) m/z = 1002 (M+); anal. calcd for
C42H70N2O2S4Sn2: C, 50.41%; H, 7.05%; N, 2.80%.
Found: C, 50.65%; H, 6.88%; N, 2.98%.
2-Hexyldecyl-5-bromo-2-formylthiophene-3-car-
General Procedure for Polymerization
boxylate (7)
To a reaction tube equipped with a stirring
bar, stannylated monomer (0.10 mmol), bromi-
nated monomer (0.10 mmol), Pd2(dba)3 (1.8 mg,
0.002 mmol), P(o-Tol)3 (2.4 mg, 0.008 mmol), and
CB (5 mL) were added. Then, the tube was purged
with argon and sealed. The reaction tube was set
into a microwave reactor and heated to 200 ºC for
10 min. After cooling to room temperature, the reac-
tion solution was poured into 200 mL of methanol
containing 5 mL of hydrochloric acid and stirred for
5 h. Then, the precipitated solid was subjected to
the sequential Soxhlet extraction with methanol and
hexane to remove low molecular weight fractions.
The residue was then extracted with CF and repre-
cipitated in 200 mL of methanol to yield dark purple
or dark blue solids (yield = 72–91%).
LDA (freshly prepared from diisopropylamine
(4.2 mL, 33 mmol) and 1.6 M n-butyllithium
(20.1 mL, 33 mmol) in THF (50 mL)) was
added dropwise to a THF (50 mL) solution of 5-
bromothiophene-3-carboxylate (5) (2.07 g, 10 mmol)
at −30°C. After stirring the mixture for 1 h, DMF
(2.8 mL, 36 mmol) was added to the mixture at
−30°C, and then the mixture was warmed to room
temperature. The mixture was further stirred for 1
h, and then water (100 mL) was added to the mix-
ture and extracted with ether three times. The or-
ganic layer was dried over magnesium sulfate, and
the solvent was evaporated under reduced pressure,
giving 6 as brown solids (2.2 g). Compound 6 was
subjected to the next reaction without further purifi-
cation. Compound 6 (2.2 g), 2-hexyl-1-bromodecane
(3.35 g, 33 mmol), and sodium carbonate (10.5 g,
99 mmol) in DMF (50 mL) were heated to 125°C for
5 h, and then cooled to room temperature. The re-
action mixture was poured into water and extracted
with dichloromethane. The crude product was puri-
fied by column chromatography on silica gel to give
Instrumentation
UV–vis absorption spectra were measured using a
Shimadzu UV-3600 spectrometer. Cyclic voltammo-
grams were recorded on an ALS Electrochemical An-
alyzer Model 612D in dichloromethane containing
tetrabutylammonium hexafluoride (Bu4NF, 0.1 M)
as supporting electrolyte at a scan rate of 100 mV/s.
A Pt counter and working electrodes and a Ag/AgCl
reference electrode were used. All the potentials were
calibrated with the standard ferrocene/ferrocenium
redox couple (Fc/Fc+: E1/2 = +0.43 V measured
under identical conditions). Photoelectron spectra
were measured by using a photoelectron spectrome-
ter, model AC-2, in air (Riken Keiki, Tokyo, Japan).
1
7 as yellow oil (2.4 g, 52% in two steps). H NMR
(400 MHz, CDCl3) δ 10.48 (s, 1H), 7.51 (s, 1H), 4.26
(d, J = 4.9 Hz, 2H), 1.88–1.78 (m, 1H), 1.62–1.44
(m, 4H), 1.34–1.26 (m, 20H), 0.90–0.86 (m, 6H). 13C
NMR (100 MHz, CDCl3) δ 183.7, 161.5, 148.7, 137.1,
133.5, 122.7, 68.9, 37.7, 37.5, 32.0, 31.9, 31.5, 31.2,
30.0, 29.7, 29.5, 26.8, 22.8, 14.3. EI-MS (70 eV) m/z
= 458 (M+); anal. calcd for C22H35O3S: C, 57.51%;
H, 7.68%. Found: C, 57.36%; H, 7.46%.
Heteroatom Chemistry DOI 10.1002/hc

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The dynamic force mode AFM study was carried out
on a Nanocute scanning probe microscope system
(SII Nanotechnology). 2D-GIXD experiments were
conducted at the SPring-8 on the beamline BL19B2.
The sample was irradiated at a fixed incident angle
on the order of 0.12° through a Huber diffractome-
˚
ter. The X-ray energy of 12.39 keV (λ = 1 A) of the
2D-GIXD patterns was recorded using a 2D image
detector (Pilatus 300K, Dectris, Baden, Switzerland).
Samples for the X-ray measurements were prepared
by spin casting the polymer and polymer/PC61BM
solution on the ITO substrate.
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Solar Cell Fabrication and Measurement
ITO substrates were first precleaned sequentially by
sonicating in a detergent bath, deionized water, ace-
tone, and isopropanol at room temperature, and in
a boiled isopropanol bath each for 10 min, and then
baked at 120°C for 10 min in air. The substrates were
then subjected to a UV/ozone treatment at room
temperature for 20 min. The precleaned ITO sub-
strates were coated with PEDOT:PSS (Clevios P VP
Al4083, Heraeus, Hanau, Germany) by spin coat-
ing (5000 rpm for 30 s, thickness: ꢀ50 nm). The
photoactive layer was deposited in a glove box by
spin coating a CB solution, containing 3–6 g/L of the
polymer sample with respective amount of PC61BM,
at 400 rpm for 20 sec and 1500 rpm for 5 sec, in
which the solution was kept heated at 100°C. The
thin films were transferred into a vacuum evapo-
rator connected to the glove box, and the Ca layer
(20 nm) and the Al layers (100 nm) were deposited se-
quentially. The active area of the cells was 0.16 cm2.
J–-V characteristics for the cells were measured us-
ing a Keithley 2400 source measure unit in nitrogen
atmosphere under 1 Sun (AM1.5G) conditions us-
ing a solar simulator (XES-40S1, San-Ei Electric,
Osaka, Japan). The light intensity for the J--V mea-
surements was calibrated using a reference PV cell,
AK-100 (Konica Minolta, Tokyo, Japan), which was
certified at National Institute of Advanced Industrial
Science and Technology, Japan. EQE spectra were
measured using a spectral response measuring sys-
tem (S 9241, Soma Optics, Tokyo, Japan).
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Heteroatom Chemistry DOI 10.1002/hc