Ladder-type nonacyclic structure consisting of alternate thiophene and benzene units for efficient conventional and inverted organic photovoltaics

Article abstract of DOI:10.1021/cm202612t

A ladder-type nonacyclic thienyl-phenylene-thienylene-phenylene-thienyl TPTPT unit, consisting of alternate interfused thiophene and benzene units, is designed and synthesized. This multifused distannyl-TPTPT monomer was polymerized with two electron-defi

Full text of DOI:10.1021/cm202612t

Article
pubs.acs.org/cm
Ladder-Type Nonacyclic Structure Consisting of Alternate Thiophene
and Benzene Units for Efficient Conventional and Inverted Organic
Photovoltaics
Yen-Ju Cheng,* Chiu-Hsiang Chen, Yu-Shun Lin, Chih-Yu Chang, and Chain-Shu Hsu*
Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsin-Chu, 30010 Taiwan
S
* Supporting Information
ABSTRACT: A ladder-type nonacyclic thienyl-phenylene-thienylene-phenylene-thienyl
TPTPT unit, consisting of alternate interfused thiophene and benzene units, is designed
and synthesized. This multifused distannyl-TPTPT monomer was polymerized with two
electron-deficient acceptors, 4,7-dibromo-2,1,3-benzothiadiazole BT and 5,8-dibromo-2,3-
diphenylquinoxaline QX monomers, by Stille coupling reaction to afford two alternating
donor−acceptor copolymers, PTPTPTBT and PTPTPTQX, respectively. Because of the
covalent planarization of the conjugated framework, PTPTPTBT simultaneously possess
excellent solubilities for solution-processability, low bandgaps with suitable position of
HOMO/LUMO energy levels, and high hole mobilities. The devices based on the
PTPTPTBT/PC71BM blend not only showed a promising PCE of 5.3% with conventional
configuration but also achieved a high PCE of 5.9% with inverted configuration. This value is
among the highest performance from the inverted solar cells incorporating a donor−acceptor
low bandgap polymer.
KEYWORDS: ladder-type structure, organic photovoltaics, alternating copolymer
charge mobility.6 On the basis of the aforementioned
consideration, integration of alternate thiophene and benzene
INTRODUCTION
■
Research on excitonic solar cells using organic p-type (donor)
units into a coplanar entity with forced rigidification becomes a
and n-type (acceptor) semiconductors has attracted tremen-
novel molecular design. Pentacyclic fused indacenodithiophene
dous scientific and industrial interest in recent years.1 The most
(IDT) unit exemplifies a successful system in this category. The
critical challenge at molecular level is to translate excellent
devices utilizing the IDT-based D−A polymers have shown
microscopic properties of photoactive materials into optimal
superior photovoltaic performance.7 The other pentacyclic
macroscopic device characteristics.2 Donor−acceptor (D−A)
arrangement system is the fused diindenothiophene (DIDT)
with highly planar structure. We recently reported a DIDT-
based alternating copolymer poly(DIDT-alt-dithienylbenzo-
thiadiazole) (PDIDTDTBT) and its corresponding device
polymers incorporating tricyclic 2,7-fluorene unit have shown
to possess deep-lying HOMO energy levels that are an
important prerequisite to guarantee greater open-circuit
voltages (Voc).3 Nevertheless, because of high aromatic
has shown a moderate power conversion efficiency (PCE) of
stabilization energy of the benzene rings, fluorene-based
1.65% (Figure 1).8 If the 3,7-position of the DIDT units in
polymers exhibit relatively large optical band gaps (>2 eV)
that restrict their absorption ability and, thus, result in
insufficient photocurrents. 4H-cyclopentadithiophene
(CPDT) appears to be another attractive thiophene-based
PDIDTDTBT are covalently rigidified with the 3-position of
two adjacent thiophene rings by a carbon bridge, an alternate
thienyl-phenylene-thienylene-phenylene-thienyl (TPTPT)
nonacylic building block with forced coplanarity will be
emerged (Figure 1). Compared to DIDT, TPTPT unit may
exhibit improved optical and electronic properties due to the
tricyclic analogue. CPDT-based D−A polymers have shown
narrower optical bandgaps and higher hole mobilities, yielding
very high short-circuit currents (Jsc).4 Unfortunately, the
higher thiophene content and extended coplanarity of the
conjugated backbone. In this research, we report the synthesis
of the distannyl-TPTPT monomer which is copolymerized
with 4,7-dibromo-2,1,3-benzothiadiazole BT and 5,8-dibromo-
2,3-diphenylquinoxaline QX acceptor monomers to afford two
alternating D−A copolymers poly(TPTPT-alt-benzothiadia-
resulting devices deliver only moderate Voc values (<0.65 V).
Development of a new donor moiety that can extract the
advantages of benzene-based fluorene and thiophene-based
CPDT units may overcome the trade-off between Jsc and Voc.
Chemical planarization by covalently fastening adjacent
aromatic units in the polymeric backbone can facilitate π-
electron delocalization and increase effective conjugation
length.5 Moreover, such rigidification also suppresses the
rotational disorder around interannular single bonds and lowers
the reorganization energy, which in turn enhances the intrinsic
Received: September 1, 2011
Revised: October 3, 2011
Published: October 19, 2011
© 2011 American Chemical Society
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the formation of benzylic alcohols in 3 which was subjected to
intramolecular Friedel−Crafts cyclization under acidic con-
dition to furnish the nonacyclic arene TPTPT in a high yield of
81%. It is noteworthy that the reaction sites occur regio-
selectively at 3,7-position of DIDT unit. TPTPT can be
efficiently lithiated by t-butyllithium followed by quenching
with trimethyltin chloride to afford the distannyl Sn-TPTPT in
a moderate yield of 62%. 2,7-dibromo-2,1,3-benzothiadiazole
(BT) and 5,8-dibromo-2,3-diphenylquinoxaline (QX) were
copolymerized with the donor Sn-TPTPT by Stille coupling to
afford PTPTPTBT (Mn = 30 kDa, PDI = 1.69) and
PTPTPTQX (Mn = 34 kDa, PDI = 1.60), respectively.
These copolymers purified by successive reprecipitation and
Soxhlet extraction showed narrow molecular weight distribu-
tions with polydispersity index below 1.7. The resulting
copolymers flanked with eight side chains on TPTPT showed
excellent solubilities in common organic solvents, such as
chloroform, toluene, chlorobenzene, and 1,2-dichlorobenzene.
Thermal Properties. Thermal stability of PTPTPTBT
and PTPTPTQX was analyzed by thermal gravimetric analysis
(TGA). The decomposition temperatures (Td) of PTPTPTBT
and PTPTPTQX are located at 389 and 409 °C (Figure 2),
indicating sufficient thermal stabilities for PSCs applications.
From the DSC measurement, both of the polymers showed
neither glass transition temperature nor melting point,
suggesting that these copolymers tend to be amorphous
(Table 1).
Figure 1. Chemical structure of DIDT and TPTPT units, fused
PTPTPTBT, PTPTPTQX copolymers, and nonfused PDIDTDTBT.
zole) (PTPTPTBT) and poly(TPTPT-alt-quinoxaline)
(PTPTPTQX), respectively (Figure 1). Bulk-heterojunction
(BHJ) solar cells incorporating these polymers have shown
promising photovoltaic performance in both conventional and
inverted architectures.
RESULTS AND DISCUSSION
■
Synthesis. The synthetic route toward Sn-TPTPT mono-
mer is depicted in Scheme 1. Suzuki coupling of 2, 8-diboronic
ester DIDT (1) with ethyl 2-bromothiophene-3-carboxylate
yielded compound 2. Double nucleophilic addition of the ester
groups in 2 by (4-octyloxy)phenyl magnesium bromide led to
Optical Properties. The optical properties of the polymers
are shown in Table 2. Both of PTPTPTBT and PTPTPTQX
Scheme 1. Synthetic Route of the Sn-TPTPT Monomer Leading to the Targeted PTPTPTBT and PTPTPTQX Copolymers
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Figure 2. Thermogravimetric analysis (TGA) of PTPTPTBT and PTPTPTQX.
should be emphasized that PTPTPTBT showed significantly
red-shifted absorption maximum in comparison with its
corresponding nonfused PDIDTDTBT analogue (444 vs 415
nm for the localized transition and 628 vs 544 nm for the ICT
transition in chloroform solution), demonstrating that the
effective conjugated length of the coplanar donor is increased
and electron coupling between the rigidified donor and the
acceptor units is substantially enhanced. Note that the
intensities of the shorter wavelength bands of the polymer in
the solid state are apparently stronger than those in the solution
state, which also suggests that the rigid and coplanar nonacyclic
units can enhance their light absorption ability in the solid state.
Theoretical Calculations. To further understand the
effect of covalent planarization on the molecular structures
and properties, we performed theoretical calculations by density
functional theory at the level of B3LYP/6-31G(d). One
repeating units TPTPTBT and DIDTDTBT were used as
simplified models for simulation of PTPTPTBT and
PDIDTDTBT copolymers, respectively. All the side-chain
substituents were replaced with methyl groups for simplicity.
The optimized structures for TPTPTBT and DIDTDTBT are
shown in Figure 4. The strategy to sew-up C64/C65 and C61/
C62 of DIDTDTBT effectively planarizes the whole molecule
(TPTPTBT), in which the dihedral angels φ(C42−C43−
C44−C45) and φ(C55−C54−C60−C61) are zero. For
comparison, the corresponding angles in DIDTDTBT are
φ(C64−C25−C24−C65) = 26.6° and φ(C62−C34−C39−
C61) = 23.6°. Furthermore, such ring-fusion also makes impact
on the coplanarity between the BT and the neighboring
thiophene rings: the dihedral angles between these two
Table 1. Molecular Weights and Thermal Properties of
Polymers
a
copolymer
Mw
Mn
PDI
Tg (°C)
Td (°C)
PTPTPTBT
PTPTPTQX
50700
54400
30000
34000
1.69
1.60
389
409
b
Decomposition temperature (5% weight loss) measured by TGA.
Table 2. Optical Properties of the Copolymers
chloroform solution
thin film
λmax
(nm)
λonset
(nm)
Egopt
(eV)
λmax
(nm)
λonset
(nm)
Egopt
(eV)
polymer
PTPTPTBT
444,
628
713
1.74
452,
639
715
1.73
PTPTPTQX
449,
593
674
1.84
452,
593
688
1.80
exhibited two distinct bands in the absorption spectra (Figure
3). One band at shorter wavelengths region is due to localized
π−π* transitions and the other band at longer wavelengths is
attributed to intramolecular charge transfer (ICT). Compared
to PTPTPTQX showing the absorption maxima at 452 and
593 nm in the thin film, PTPTPTBT exhibited a similar
absorption maximum at 452 nm but a bathochromic shift of the
opt
ICT band at 639 nm. In addition, the optical band gaps (Eg
)
deduced from the absorption edges of thin film spectra are
determined to be 1.73 eV (715 nm) for PTPTPTBT and 1.80
eV (688 nm) for PTPTPTQX. These results indicate that
accepting strength of BT is stronger than that of QX unit. It
Figure 3. Normalized absorption spectra of (a) PTPTPTBT and (b) PTPTPTQX in the chloroform solution and the solid state. Absorption
spectrum of PDIDTDTBT in chloroform is included in (a) for comparison.
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Figure 4. Optimized conformations for the structure of DIDTDTBT (top) and TPTPTBT (bottom), respectively, at the level of B3LYP/6-31G(d).
Yellow: sulfur; blue: nitrogen; and gray: carbon; hydrogens are omitted for clarity.
Figure 5. Wave functions of the HOMO (left) and LUMO (right) orbitals of the TPTPTBT calculated at the level of B3LYP/6-31G (d,p).
structural motifs are φ(C40−C41−C49−C50) = 7.32° for
DIDTDTBT and φ(C62−C63−C71−C72) = 0.07° for
TPTPTBT.
The frontier orbitals of the model compound TPTPTBT
were also shown in Figure 5. The electron density of the
HOMO for TPTPTBT is homogeneously distributed along the
nonacyclic donor unit, whereas the electron density of the
LUMO is redistributed to the BT acceptor unit. Such an
electronic redistribution shows a pronounced intramolecular
charge separation between donor and acceptor after excitation.
The energy of HOMO−LUMO transition for TPTPTBT is
calculated to be 631 nm, which is in good agreement with its
experimental absorption maximum (628 nm in chloroform).
Electrochemical Properties. Cyclic voltammetry (CV)
was employed to examine the electrochemical properties and
evaluate the HOMO and LUMO levels of the polymer (Figure
6 and Table 3). Both polymers showed a stable and reversible
p-doing and n-doping processes, which are important
prerequisites for p-type semiconductor materials. The
HOMO energy levels being estimated to be −5.24 eV for
PTPTPTBT and −5.21 eV for PTPTPTQX are in an ideal
range to ensure better air-stability and greater attainable Voc in
the final device. The LUMO energy levels are approximately
located at −3.38 eV for PTPTPTBT and −3.30 eV for
PTPTPTQX, which are higher than the LUMO level of the
PC71BM acceptor (−3.8 eV) to ensure energetically favorable
electron transfer. This can be unambiguously evidenced by the
complete photoluminescence quenching in the film of the
Figure 6. Cyclic voltammograms of PTPTPTBT and PTPTPTQX in
the thin film at a scan rate of 80 mV s−1
.
Table 3. Electrochemical Properties of the Polymers
HOMO
(eV)
LUMOel
(eV)
Egel
(eV)
copolymer
Eoxonset(V) Eredonset(V)
PTPTPTBT
PTPTPTQX
0.44
0.41
1.42
1.50
5.24
5.21
3.38
3.30
1.86
1.91
PTPTPTBT/PC71BM (1:4, w/w) and PTPTPTQX/PC71BM
(1:2, w/w) blends (Figure 7).
Photovoltaic and Hole-Mobility Characteristics. De-
spite the amorphous nature in thin films, PTPTPTBT and
PTPTPTQX showed good hole transporting properties
because of their rigid and coplanar structures. Hole-only
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small phase separation domains, suggesting its good morpho-
logical properties for efficient charge separation and transport.
BHJ devices with inverted architecture are known to possess
much improved long-term stability compared to conventional
structure. However, PCEs of P3HT/PCBM-based inverted
PSCs are mostly in the range of ca. 2−4%, which is inferior to
that of conventional PSCs.9 Therefore, recent research of
inverted solar cells is mainly devoted to device engineering and
interfacial modification. The utilization of low bandgap (LBG)
conjugated polymers other than P3HT as p-type photoactive
materials is only sporadically explored.10 It is highly desirable to
evaluate the new copolymers in the application of inverted solar
cells. The inverted devices were fabricated based on the
configuration of ITO/ZnO/C-PCBSD/copolymer:PC71BM/
PEDOT:PSS/Ag where a cross-linked PC61BM derivative,
[6,6]-phenyl-C61-butyric styryl dendron ester (C-PCBSD) was
used as an interlayer.11 Encouragingly, the inverted device using
PTPTPTQX/PC71BM (1:2, w/w) blend delivered an
enhanced PCE of 4.7%. Similarly, the device incorporating
PTPTPTBT/PC71BM (1:4, w/w) blend further achieved a
high PCE of 5.9% with Jsc = 12.10 mA/cm2, Voc = 0.76 V and
FF = 64%. This performance outperforms not only the P3HT/
PC61BM-based device under the identical device configuration
(PCE = 4.4%)11 but also an inverted tandem cell (PCE = 5.1%)
combining P3HT and poly[bis(2-ethylhexyl)dithienosilole)-alt-
(2,1,3-benzothiadiazole)] (PSBTBT) as the photoactive
materials.12 To confirm the accuracy of the measurements of
the devices, the corresponding external quantum efficiency
(EQE) for both the conventional and inverted solar cells was
measured under illumination of monochromatic light (Figure
8). The Jsc calculated from integration of the EQE with an AM
1.5 G reference spectrum agrees well with the Jsc obtained from
the J−V measurements.
Figure 7. Emission spectra of PTPTPTBT, PTPTPTBT/PC71BM at
excitation wavelength of 628 nm, PTPTPTQX and PTPTPTQX/
PC71BM (1/2, w/w) at excitation wavelength of 593 nm in the thin
film.
devices(ITO/PEDOT:PSS/polymer/Au) were fabricated to
estimate the intrinsic hole mobilities of these polymers by
means of the space-charge limit current (SCLC) theory.
PTPTPTBT and PTPTPTQX exhibited higher hole mobilities
of 5.1 × 10−4 cm2/(V s) and 3.7 × 10−4 cm2/(V s), respectively,
compared to PDITIDTBT with 1 × 10−4 cm2/(V s). On the
basis of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al configu-
ration, bulk heterojunction solar cells were fabricated and
characterized under simulated 100 mW cm−2 AM 1.5 G
illumination. The current density−voltage characteristics of the
devices are shown in Figure 8 and the device parameters are
summarized in Table 4. The preliminary device based on
PTPTPTQX/PC71BM (1:2, w/w) blend exhibited a Voc of 0.74
V, a Jsc of 9.0 mA/cm2, a FF of 61% and a high PCE of 4.1%.
More encouragingly, the device using PTPTPTBT/PC71BM
(1:4, w/w) blend delivered superior performance with a Jsc of
11.4 mA/cm2, a Voc of 0.76 V, a FF of 61%, and an exceptional
PCE of 5.3%, which dramatically outperforms the device based
on the nonfused PDIDTDTBT/PC71BM (1:2, w/w) blend by
3-fold. (Voc = 0.7 V, Jsc = 5.30 mA/cm2, FF = 44%, and a PCE
of 1.65%).7 This result reveals that the nonacyclic TPTPT unit
having rigid and extended coplanar conjugated system indeed
improves all the device characteristics.
CONCLUSIONS
■
In summary, we have successfully synthesized a nonacylic
distannyl-TPTPT unit that consists of alternate thiophene and
benzene units fused by four embedded cyclopentadienyl rings.
PTPTPTBT and PTPTPTQX copolymers incorporating this
rigidified and coplanar TPTPT units simultaneously possess
excellent solubilities for solution-processability, low bandgaps
with suitable position of HOMO/LUMO energy levels, and
high hole mobilities, leading to promising PCEs of 4.1 and
5.3%, respectively. Most significantly, the PTPTPTBT/
PC71BM-based device with inverted architecture achieved an
impressively high PCE of 5.9%. This value is among the highest
The morphology of thin film was evaluated by atomic force
microscopy (Figure 9). The image of the PTPTPTBT/
PC71BM (1:4, w/w) blend showed a very smooth surface and
Figure 8. Current density−voltage characteristics (left) and EQE spectra (right) of the conventional devices (ITO/PEDOT:PSS/polymer:PC71BM/
Ca/Al) and inverted devices (ITO/ZnO/C-PCBSD/polymer: PC71BM/PEDOT:PSS/Ag) devices under illumination of AM 1.5 G, 100 mW cm−2
.
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Table 4. Hole Mobilities and Photovoltaic Characteristics
copolymer
polymer:PC71BM wt% ratio
mobility (cm2/V s)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
a
b
c
PTPTPTBT
PTPTPTBT
1:4
1:4
1:2
1:2
5.1 × 10−4
0.76
0.76
0.74
11.4 (11.2)
61
64
61
62
5.3
5.9
4.1
4.7
c
12.1 (11.6)
a
b
c
PTPTPTQX
PTPTPTQX
3.7 × 10−4
9.0 (9.1)
c
0.74
10.2 (9.6)
a
b
Conventional device structure: ITO/PEDOT:PSS/copolymer:PC71BM/Ca/Al. Inverted device structure: ITO/ZnO/C-PCBSD/copoly-
c
mer:PC71BM/PEDOT:PSS/Ag. Calculated from the EQE spectrum.
Figure 9. AFM height (left) and phase images (right) of the surface of PTPTPTBT/PC71BM (1:4, w/w) blend (10.0 μm × 10.0 μm). The surface
roughness is estimated to be 0.4 nm.
dichlorobenzene (ODCB) (0.47 wt.%) and PC71BM (purchased from
Nano-C) was added to reach the desired ratio. The solution was then
heated at 110 °C and stirred overnight. Prior to deposition, the
solution was filtrated through a 0.45 μm filter and the substrates were
transferred in a glovebox. The photoactive layer was then spin coated
at different spin coating speed in order to tune its thickness. After
drying, the samples were annealed during 15 min. The detailed
processing parameters (spin coating speed; annealing temperature) are
shown as follows: PTPTPTBT/PC71BM (700 rpm; 150 °C) and
PTPTPTQX/PC71BM (700 rpm; 150 °C). The cathode made of
calcium (35 nm thick) and aluminum (100 nm thick) was evaporated
through a shadow mask under high vacuum (<1 × 10−6 Torr). Each
device is constituted of 4 pixels defined by an active area of 0.04 cm2.
Finally, the devices were encapsulated and I−V curves were measured
in air.
For the inverted architecture, a ZnO precursor solution, consisting
of 157 mg mL−1 zinc acetate dehydrate in 96% 2-methoxy ethanol and
4% ethanolamine, was spin coated onto ITO-coated glass, followed by
thermal annealing in air at 300 °C for 10 min to crystallize the film
(thickness = ca. 50 nm). Subsequently, PCBSD solution (5 mg mL−1
in ortho-dichlorobenzene) was spin coated onto the ZnO layer and the
as-cast film was heated at 180 °C for 10 min for thermal cross-linking
(thickness = 10 nm). The same process for the active layer in the
conventional architecture was used for the inverted devices. The
PEDOT:PSS solution diluted with equal volume of isopropyl alcohol
and 0.2 wt % of Triton X-100 nonionic surfactant, was then spin
coated on top of the active layer, followed by thermal annealing at 120
°C for 10 min (thickness = 60 nm). A silver top electrode (thickness =
ca. 150 nm) was then thermally evaporated to complete the inverted
device.
efficiency ever reported for the inverted solar cells incorporat-
ing a D−A type LBG polymer. We envisage that further
improvement of device performance is achievable by optimizing
the processing conditions.
EXPERIMENTAL SECTION
■
General Measurement and Characterization. All chemicals
are purchased from Aldrich or Acros and used as received unless
1
otherwise specified. H and 13C NMR spectra were measured using
Varian 300 MHz instrument spectrometer. The molecular weight of
polymers was measured by the GPC method (Viscotek VE2001GPC),
and polystyrene was used as the standard (THF as the eluent).
Differential scanning calorimeter (DSC) was measured on TA Q200
Instrument and thermal gravimetric analysis (TGA) was recorded on
Perkin-Elmer Pyris under a nitrogen atmosphere at a heating rate of 10
°C/min. Absorption spectra were taken on a HP8453 UV−vis
spectrophotometer. The electrochemical cyclic voltammetry was
conducted on a CH instruments electrochemical analyzer. A carbon
glass coated with a thin polymer film was used as the working
electrode and an Ag/AgCl as the reference electrode, whereas 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile
was the electrolyte. CV curves were calibrated using ferrocence as the
standard, whose HOMO is set at 4.8 eV with respect to zero
vacuum level. The HOMO energy levels were obtained from the
onset
onset
equation HOMO = −(Eox
− E(ferrocene)
+4 .8) eV. The LUMO
levels of polymer were obtained from the equation LUMO =
−(Eredonset − E(ferrocene)onset + 4.8) eV. Surface images were measured by
Veeco di-Innova Atomic Force Microscope.
Fabrication and Characterization of BHJ Devices. ITO/Glass
substrates were ultrasonically cleaned sequentially in detergent, water,
acetone and isopropyl alcohol. Then, the substrates were covered by a
30 nm thick layer of PEDOT:PSS (Clevios P provided by H. C. Stark)
by spin coating. After annealing in air at 150 °C during 30 min, the
samples were cooled down to rt. Polymers were dissolved in ortho-
Hole-Only Devices. To investigate the respective hole mobility of
the different copolymer films, we have prepared unipolar devices
following the same procedure except that the active layer is made of
pure polymer and the Ca/Al cathode is replaced by evaporated gold
(40 nm). The hole mobilities were calculated according to space
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charge limited current theory (SCLC). The J−V curves were fitted
mL, 3.73 mmol) was introduced dropwise by syringe to the solution.
The mixture solution was quenched with water and extracted with
hexane (300 mL × 3) and water (100 mL). The combined organic
layer was dried over MgSO4. After removal of the solvent under
reduced pressure, Sn-TPTPT (1.18 g, 62%) was obtained as an orange
oil and used without further purification; 1H NMR (CDCl3, 300
MHz): δ 7.29 (s, 2 H), 7.27 (s, 2H), 7.21 (d, J = 8.7 Hz, 8 H), 7.06 (s,
2 H), 6.81 (d, J = 8.7 Hz, 8 H), 3.94 (t, J = 6.3 Hz, 8 H), 2.20−2.00
(m, 8 H), 1.85−1.70 (m, 8 H), 1.50−0.60 (m, 112 H), 0.40 (s, 18 H).
13C NMR (CDCl3, 75 MHz): 157.8, 153.5, 152.1, 148.8, 147.3, 144.5,
140.5, 137.2, 136.9, 134.1, 130.5, 129.1, 129.0, 115.8, 114.0, 113.2,
67.8, 61.3, 55.0, 40.4, 35.7, 31.8, 31.7, 30.3, 29.6, 29.4, 29.3, 29.2, 26.0,
24.2, 22.6, 14.06, 14.03, −8.0. Elemental anal. Calcd (%) for
according to the following equation
Where ε is the dielectric permittivity of the polymer, μ is the hole
mobility, and L is the film thickness (distance between the two
electrodes)
Synthesis of Compound 2. To a 50 mL round-bottom flask was
introduced 1 (2.20 g, 2.29 mmol), ethyl 2-bromothiophene-3-
carboxylate (1.24 g, 5.27 mmol), Pd(PPh3)4 (0.265 g, 0.23 mmol),
K2CO3 (1.90 g, 13.75 mmol), and Aliquant 336 (0.23 g, 0.57 mmol) in
a solution of degassed toluene (17 mL) and degassed H2O (3.5 mL).
The mixture was heated to 90 °C under nitrogen for 72 h. The
reaction solution was extracted with ethyl acetate (300 mL × 3) and
water (150 mL). The combined organic layer was dried over MgSO4.
After removal of the solvent under reduced pressure, the residue was
purified by column chromatography on silica gel (hexane/ethyl
acetate, v/v, 20/1) and then recrystallized from hexane to give a light
C
122H176O4S3Sn2: C, 71.82; H, 8.69. Found: C, 71.67; H, 8.78.
Synthesis of PTPTPTBT. To a 15 mL round-bottom flask was
introduced Sn-TPTPT (145.4 mg, 0.07 mmol), 4,7-dibromo-2,1,3-
benzothiadiazole BT (20.6 mg, 0.07 mmol), Pd2(dba)3 (3.2 mg,
0.0035 mmol), tri(o-tolyl)phosphine (6.8 mg, 0.022 mmol), and dry
chlorobenzene (3 mL). The mixture was then degassed by bubbling
nitrogen for 10 min at room temperature. The round-bottom flask was
placed into the microwave reactor and reacted for 45 min under 270
W. To end-cap the resulting polymer, we added tributyl(thiophen-2-
yl)stannane (13.1 mg, 0.035 mmol) to the mixture solution and
reacted it for 10 min under 270 W. Finally, 2-bromothiophene (6.2
mg, 0.038 mmol) was added to the mixture solution and reacted for 10
min under 270 W. The solution was added into methanol dropwise.
The precipitate was collected by filtration and washed by Soxhlet
extraction with acetone, hexane, and chloroform sequentially for one
week. The Pd-thiol gel (Silicycle Inc.) and Pd-TAAcOH were added to
above chloroform solution to remove the residual Pd catalyst and Sn
metal. After filtration and removal of the solvent, the polymer was
redissolved in chloroform again and added into methanol to
reprecipitate. The purified polymer was collected by filtration and
dried under vacuum for 1 day to give a dark-green fiber-like solid (215
1
yellow solid 2 (2.07 g, 62%); mp: 110 °C; H NMR (CDCl3, 300
MHz): δ 7.52 (d, J = 5.6 Hz, 2 H), 7.51−7.30 (m, 6 H), 3.17−3.58 (m,
4 H), 7.23 (d, J = 5.6 Hz, 2 H), 4.20 (q, J = 7.2 Hz, 4 H), 2.09 (t, J =
8.1 Hz, 8 H), 1.40−0.96 (m, 54 H), 0.82 (t, J = 13.4 Hz, 12 H); 13C
NMR (CDCl3, 75 MHz): δ 163.4, 152.0, 151.6, 149.9, 144.8, 139.3,
130.0, 128.8, 128.1, 123.6, 123.5, 117.3, 60.4, 55.6 40.2, 31.8, 30.3,
29.6, 29.5, 24.2, 22.6, 14.2, 14.0; MS (FAB, C64H88O4S3): calcd,
1017.58; found, 1017.
Synthesis of Compound 3. A Grignard reagent was prepared by
the following procedure. To a suspension of magnesium turnings (0.8
g, 33.3 mmol) and 3−4 drops of 1,2-dibromoethane in dry THF (20
mL) was slowly added 1-bromo-4-(octyloxy)benzene (8.56 g, 30.0
mmol) dropwise and stirred for 1 h. To a solution of 2 (0.80 g, 0.79
mmol) in dry THF (20 mL) under nitrogen was added freshly
prepared 4-(octyloxy)benzene 1-magnesium bromide (20 mL, 7.9
mmol) dropwise at room temperature. The resulting mixture was
heated at reflux for 16 h. The reaction solution was extracted with
ethyl acetate (150 mL × 3) and water (100 mL). The combined
organic layer was dried over MgSO4. After removal of the solvent
under reduced pressure, the residue was purified by column
chromatography on silica gel (hexane/ethyl acetate, v/v, 100/1) to
give a yellow oil 3 (1.17 g, 85%); 1H NMR (CDCl3, 300 MHz): δ 7.24
(d, J = 9.0 Hz, 2 H), 7.17 (d, J = 9.0 Hz, 2 H), 7.16 (d, J = 9.0 Hz, 8
H), 7.09 (d, J = 5.1 Hz, 2 H), 6.94 (s, 2 H), 6.84 (d, J = 9.0 Hz, 8 H),
6.41 (d, J = 5.6 Hz, 2 H), 3.97 (t, J = 6.5 Hz, 8 H), 3.07 (s, 2 H),
1.95−1.60 (m, 16 H), 1.55−0.40 (m, 112 H). 13C NMR (CDCl3, 75
MHz): δ 158.2, 152.5, 149.7, 144.5, 144.1, 140.7, 139.2, 138.8, 131.5,
131.0, 128.9, 123.1, 122.2, 117.9, 113.7, 80.4, 68.0, 55.4, 39.9, 31.9,
31.8, 30.2, 29.7, 29.5, 29.4, 29.3, 29.26, 26.1, 24.1, 22.7, 22.6, 14.1,
14.05. MS (FAB, C116H164O6S3): calcd, 1750.73; found, 1751.
Synthesis of TPTPT. To a solution of 3 (2.00 g, 1.14 mmol) in
boiling acetic acid (116 mL) was added conc. sulfuric acid (3.5 mL) in
one portion. The resulting solution was stirred for 18 h at 95 °C and
then was extracted with ethyl acetate (500 mL × 3) and water (250
mL). The combined organic layer was dried over MgSO4. After
removal of the solvent under reduced pressure, the residue was
purified by column chromatography on silica gel (hexane/ethyl
acetate, v/v, 100/1) to give an orange oil TPTPT (1.58 g, 81%);1H
NMR (d8-THF, 300 MHz): δ 7.45 (s, 2 H), 7.34 (d, J = 4.8 Hz, 2H),
7.33 (s, 2 H), 7.12 (d, J = 8.7 Hz, 8H), 6.99 (d, J = 4.8 Hz, 2H), 6.72
(d, J = 8.7 Hz, 8 H), 3.87 (t, J = 6.0 Hz, 8 H), 2.3−2.1 (m, 8 H),
1.50−0.95 (m, 96 H), 0.90−0.70 (m, 24 H). 13C NMR (CDCl3, 75
MHz): δ 157.9, 155.9, 153.2, 152.2, 148.9, 144.6, 141.3, 137.1, 137.0,
134.3, 129.0, 126.9, 123.0, 115.8, 114.1, 113.1, 67.9, 61.8, 55.1, 40.5,
31.9, 31.85, 31.8, 30.3, 29.7, 29.5, 29.3, 29.29, 29.2, 26.0, 24.2, 22.6,
14.1, 14.0. MS (FAB, C116H160O4S3): calcd, 1714.70; found, 1715.
Synthesis of Sn-TPTPT. To a solution of TPTPT (1.60 g, 0.93
mmol) in dry THF (28 mL) was added 1.6 M solution of t-BuLi in
hexane (1.8 mL, 2.80 mmol) dropwise at −78 °C. After stirring at −78
°C for 1 h, 1.0 M solution of chlorotrimethylstannane in THF (3.7
1
mg, 62%, Mn = 30 000, PDI = 1.69). H NMR (CDCl3, 300 MHz): δ
8.20−8.00 (m, 2 H), 7.95−7.80 (m, 2 H), 7.50−7.30 (m, 4 H), 7.25−
7.15 (m, 8 H), 6.90−6.70 (m, 8 H), 4.00−3.80 (m, 8H), 1.85−1.60
(m, 16H), 1.50−1.00 (m, 88 H), 0.90−0.70 (m, 24 H
Synthesis of PTPTPTQX. To a 15 mL round-bottom flask was
introduced Sn-TPTPT (259.7 mg, 0.13 mmol), 5,8-dibromo-2,3-
diphenylquinoxaline QX (56 mg, 0.13 mmol), Pd2(dba)3 (4.7 mg,
0.0051 mmol), tri(o-tolyl)phosphine (12.4 mg, 0.041 mmol) and dry
chlorobenzene (5 mL). The mixture was then degassed by bubbling
nitrogen for 10 min at room temperature. The round-bottom flask was
placed into the microwave reactor and reacted for 45 min under 270
W. To end-cap the resulting polymer, tributyl(thiophen-2-yl)stannane
(23.8 mg, 0.064 mmol) was added to the mixture solution and reacted
for 10 min under 270 W. Finally, 2-bromothiophene (11.2 mg, 0.069
mmol) was added to the mixture solution and reacted for 10 min
under 270 W. The solution was added into methanol dropwise. The
precipitate was collected by filtration and washed by Soxhlet extraction
with acetone, hexane, and chloroform sequentially for one week. The
Pd-thiol gel (Silicycle Inc.) and Pd-TAAcOH were added to above
chloroform solution to remove the residual Pd catalyst and Sn metal.
After filtration and removal of the solvent, the polymer was redissolved
in chloroform again and added into methanol to reprecipitate. The
purified polymer was collected by filtration and dried under vacuum
for 1 day to give a dark-green fiberlike solid (155 mg, 61%, Mn = 34
1
000, PDI = 1.60). H NMR (CDCl3, 300 MHz): δ 8.10−8.00 (m,
2H), 7.95−7.85 (m, 2H), 7.80−7.70 (m, 4H), 7.55−7.30 (m, 10H),
7.20−7.00 (m, 8H), 6.90−6.70 (m, 8H), 4.00−3.80 (m, 8H), 1.85−
1.65 (m, 16H), 1.50−1.00 (m, 88 H), 0.90−0.70 (m, 24 H).
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
5074
dx.doi.org/10.1021/cm202612t|Chem. Mater. 2011, 23, 5068−5075

Chemistry of Materials
Article
(11) (a) Hsieh, C.-H.; Cheng, Y.-J.; Li, P.-J.; Chen, C.-H.; Dubosc,
M.; Liang, R.-M.; Hsu, C.-S. J. Am. Chem. Soc. 2010, 132, 4887.
(b) Cheng, Y.-J.; Hsieh, C.-H.; He, Y.; Hsu, C.-S.; Li, Y. J. Am. Chem.
Soc. 2010, 132, 17381.
(12) Chou, C.-H.; Kwan, W. L.; Hong, Z.; Chen, L.-M.; Yang, Y. Adv.
Mater. 2011, 23, 1282.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yjcheng@mail.nctu.edu.tw (Y.-J.C.); cshsu@mail.nctu.
edu.tw (C.-S.H.).
ACKNOWLEDGMENTS
■
This work was supported by the National Science Council and
“ATP Plan” of the National Chiao Tung University and
Ministry of Education, Taiwan. We thank Yi-Lin Wu for the
help of theoretical calculations.
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