Effect of incorporated nitrogens on the planarity and photovoltaic performance of donor-acceptor copolymers

Article abstract of DOI:10.1021/ma301362t

Systematic control of the chemical structure of conjugated polymers is critically important to elucidate the relationship between the conjugated polymer structures and properties and to optimize their performance in bulk heterojunction (BHJ) polymer solar cell (PSC) devices. Herein, we synthesized three new copolymers, i.e., P0, P1, and P2; these copolymers contain the same benzodithiophene donor unit but have different acceptor units with different numbers of nitrogen atoms in the range of 0-2. The effects of the introduced nitrogen atoms on the structural, optical, electrical, and photovoltaic properties of the conjugated polymers were investigated; the structural properties of the polymers, in particular, were studied using both experimental (grazing-incidence X-ray scattering (GIXS) measurements) and computational methods (molecular simulation). As the number of introduced nitrogen atoms increased, the planarity of the main chain conformation increased in the order of P0 < P1 < P2. Additionally, the P0, P1, and P2 polymers showed increased interlayer domain spacings of 1.61, 1.72, and 1.78 nm, respectively, with increased intermolecular ordering. These results were in excellent agreement with the simulation results. In addition, the enhanced planarity resulted in a red-shifting at the onset of absorption in the polymer film from 544 to 585 nm, a downshift in the lowest unoccupied molecular orbital (LUMO) energy level from -3.02 to -3.26 eV, and an increase in the hole mobility from 2.33 × 10^-6 to 3.78 × 10^-5 cm²/(V s). As a result, we observed dramatically enhanced performance of the PSCs in the order of P0 < P1 < P2. For example, the P2:PC₆₁BM device exhibited a 3.5-fold improvement in power conversion efficiency (PCE) compared to that of P0:PC₆₁BM. The further optimization of P2 with PC ₇₁BM showed the PCE of 3.22%.

Full text of DOI:10.1021/ma301362t

Article
pubs.acs.org/Macromolecules
Effect of Incorporated Nitrogens on the Planarity and Photovoltaic
Performance of Donor−Acceptor Copolymers
Han-Hee Cho, Tae Eui Kang, Ki-Hyun Kim, Hyunbum Kang, Hyeong Jun Kim, and Bumjoon J. Kim*
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon
305-701, Korea
S
* Supporting Information
ABSTRACT: Systematic control of the chemical structure of
conjugated polymers is critically important to elucidate the
relationship between the conjugated polymer structures and
properties and to optimize their performance in bulk
heterojunction (BHJ) polymer solar cell (PSC) devices.
Herein, we synthesized three new copolymers, i.e., P0, P1,
and P2; these copolymers contain the same benzodithiophene
donor unit but have different acceptor units with different
numbers of nitrogen atoms in the range of 0−2. The effects of
the introduced nitrogen atoms on the structural, optical,
electrical, and photovoltaic properties of the conjugated polymers were investigated; the structural properties of the polymers, in
particular, were studied using both experimental (grazing-incidence X-ray scattering (GIXS) measurements) and computational
methods (molecular simulation). As the number of introduced nitrogen atoms increased, the planarity of the main chain
conformation increased in the order of P0 < P1 < P2. Additionally, the P0, P1, and P2 polymers showed increased interlayer
domain spacings of 1.61, 1.72, and 1.78 nm, respectively, with increased intermolecular ordering. These results were in excellent
agreement with the simulation results. In addition, the enhanced planarity resulted in a red-shifting at the onset of absorption in
the polymer film from 544 to 585 nm, a downshift in the lowest unoccupied molecular orbital (LUMO) energy level from −3.02
to −3.26 eV, and an increase in the hole mobility from 2.33 × 10−6 to 3.78 × 10−5 cm2/(V s). As a result, we observed
dramatically enhanced performance of the PSCs in the order of P0 < P1 < P2. For example, the P2:PC61BM device exhibited a
3.5-fold improvement in power conversion efficiency (PCE) compared to that of P0:PC61BM. The further optimization of P2
with PC71BM showed the PCE of 3.22%.
introduction of an additional atom into the polymer backbone
provides a promising pathway to fine-tune the electrochemical
and physical properties of conjugated polymers, and this
approach generally causes dramatic changes in the photovoltaic
performance of the polymers.34,35 For example, Yu and co-
workers have reported that the introduction of fluorine atoms
into a polymer backbone lowered both the highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) energy levels, which induced
changes in the polymer’s electrochemical properties and
significantly increased the PCE of PSCs from 2.3% to 7.2%.29
In addition to fluorination, other research groups have reported
the effects of introduced oxygen atoms on the properties and
the photovoltaic performance of polymers.36−39
INTRODUCTION
■
Polymer solar cells (PSCs) have attracted significant attention
as a promising candidate for renewable energy sources because
of their flexibility, solution processability, and potential for low-
cost fabrication.1−4 Over the past decade, research efforts have
been focused on the development of new conducting polymers
and fullerene acceptors to achieve high power conversion
efficiency (PCE).5−13 A facile route for achieving a low-
bandgap polymer is the combination of electron-rich donor
units with electron-deficient acceptor units, resulting in the
formation of donor−acceptor alternating structures. However,
challenges still remain in further improving the PCE of PSCs
with enhanced stability.
The ability to control the chemical structure of conjugated
polymers is critically important to determine their properties
and optimize the performance of PSCs. Even small
modifications can induce dramatic effects on the physical and
chemical properties of conjugated polymers, including their
solubility, crystallinity, interchain packing, light absorption, and
electrochemical properties.14−20 In this regard, the effects of the
structural modifications in the alkyl solubilizing groups and the
polymer backbone on the properties of conjugated polymers
have been widely investigated.21−33 In particular, the
The introduction of electronegative nitrogen atoms into the
conjugated polymers has advantages in that they can be
incorporated into the polymer backbone without interfering the
conjugation of the polymers due to their multiple binding sites.
Thus, it is important to examine the effects of structural
Received: July 2, 2012
Revised: July 31, 2012
Published: August 10, 2012
© 2012 American Chemical Society
6415
dx.doi.org/10.1021/ma301362t | Macromolecules 2012, 45, 6415−6423

Macromolecules
Article
Figure 1. Synthetic routes and chemical structures of three different polymers, P0, P1, and P2.
changes in the polymer backbone on the properties of
conjugated polymers through the introduction of nitrogen
atoms. Recently, both the You and Leclerc groups studied the
effects of the introduction of nitrogen atoms in conjugated
polymers by replacing the benzene in the benzothiadiazole
group with a pyridine moiety.38,40 Interestingly, they observed
change in the LUMO levels and in the photovoltaic properties
of the polymers. However, the variation of the structural
changes by the substitution of benzene with pyridine moiety is
limited to the addition of one nitrogen atom. In addition, the
effect of the incorporated nitrogen atoms on the structural
properties of the polymer and its correlation with changes in
the device performance has not been investigated.
The effects of the introduced nitrogen atoms on the structural
properties of the polymers were investigated using X-ray
analysis, and the results were compared with those obtained
from molecular simulations. The introduction of nitrogen
atoms into the main backbone of the polymers caused an
increase in the interlayer domain spacing, a decrease in the π−π
stacking distance, and significant improvement in the planarity
of the polymer. In addition, the increased planarity resulted in a
red-shifted absorption onset of the polymer film, a downshifted
LUMO level, and an increase in hole mobility by more than an
order of magnitude. As a result, dramatic enhancement in the
performance of the PSCs was observed in the order of P0 < P1
< P2.
Herein, we systematically investigated the effect of nitrogen
atoms introduced into the backbone of conjugated polymers
with respect to the polymers’ structural properties and focused
on their correlation with the polymers’ photovoltaic perform-
ance. A series of three different polymers, P0, P1, and P2, with
0−2 nitrogen atoms were carefully designed and synthesized
(Figure 1). To control the number of nitrogen atoms in the
polymer backbone, 1,4-bis(4-hexylthiophen-2-yl)benzene
(HTB), 2,5-bis(4-hexylthiophen-2-yl)pyridine (HTP), and
3,6-bis(4-hexylthiophen-2-yl)pyridazine (HTPz) were used as
the acceptor unit in the donor−acceptor (D−A) alternating
conjugated polymer. Benzo[1,2-b:4,5-b′]dithiophene (BDT)
was used as the donor unit in the D−A conjugated polymers
because it has a planar conjugated structure that can enhance
electron delocalization and easily form π−π stacking.10,25,41,42
RESULTS AND DISCUSSION
■
Synthesis of Monomers and Polymers. To investigate
the effect of introduced nitrogen atoms on the properties of the
D−A conjugated polymers, three polymers were designed with
different numbers of nitrogen atoms. The new copolymers, P0,
P1, and P2, were composed of the same BDT donor unit but
contained different acceptor units, i.e., HTB, HTP, and HTPz,
respectively. These acceptor units contain benzene, pyridine,
and pyridazine groups with the number of nitrogen atoms
ranging from 0 to 2, respectively. We replaced the benzene in
the HTB unit with pyridine and pyridazine, which led to the
new acceptor units of HTP and HTPz. The chemical structures
and synthetic routes for the new series of three different
monomers are outlined in Figure 1. The monomer 2-(tri-n-
6416
dx.doi.org/10.1021/ma301362t | Macromolecules 2012, 45, 6415−6423

Macromolecules
Article
Figure 2. (a) 2D-GIXS images of films of P0, P1, and P2; (b) out-of-plane line cuts of GIXS. Inset: schematic illustration of the edge-on orientation
of the polymers with the backbone perpendicular to the substrate. The interchain lamellar spacing and the π−π stacking distance are labeled as d1
and d2, respectively.
butylstannyl)-4-hexylthiophene (1) was synthesized via the
method described in the literature,43 and the key acceptor units,
HTB, HTP, and HTPz, were obtained via the Pd-catalyzed
Stille coupling reaction of 1 with the corresponding monomers,
2, 3, and 4, under the same conditions. The bromination of
each monomer was then performed with N-bromosuccinimide
(NBS) to produce compounds 8, 9, and 10, which were
polymerization conditions, including the reaction time, were
carefully controlled to produce similar Mn and PDI values for
P0, P1, and P2. The Mn of all the polymers ranged from 10 to
15 kg/mol, with similar PDI values of 1.5−1.7. The thermal
properties of the three different polymers were measured by
thermal gravimetric analysis (TGA). The onset points of weight
loss of P0, P1, and P2 occurred at temperatures greater than
300 °C, which demonstrated the high thermal stability of the
polymers (Figure S5).
Structural Properties: X-ray Analysis. The effect of the
incorporated nitrogen atoms on the structural properties of the
polymers was investigated by examination of P0, P1, and P2 via
GIXS measurements. A thin layer (40−50 nm) of PEDOT:PSS
was spin-coated onto silicon substrates, and the polymer thin
films were subsequently spin-coated on top of the PEDOT:PSS
layer. The samples were annealed for 1 h at 180 °C and then
slowly cooled. Figure 2 shows the GIXS images of P0, P1, and
1
prepared successfully and identified by H NMR spectroscopy.
The BDT donor unit 11 was synthesized according to the
method described in the literature.10 Then, P0, P1, and P2
were synthesized via the Stille coupling reaction between
compounds 8−10 and 11, respectively.
The number-average molecular weight (Mn) and the
polydispersity index (PDI) were measured using high-temper-
ature gel-permeation chromatography (GPC) with o-dichlor-
obenzene (DCB) as the eluent; the instrument was calibrated
with polystyrene standards using UV and RI detectors. The
6417
dx.doi.org/10.1021/ma301362t | Macromolecules 2012, 45, 6415−6423

Macromolecules
Article
P2, in which each of the 2D GIXS pattern images is divided
into a component in the plane of the substrate (qxy) and a
component perpendicular to the substrate (qz). For all samples,
the diffraction peaks were strongest in the out-of-plane
direction, which indicates that the polymer films had a well-
organized structure with lamellar conjugated polymer stacks
oriented along the perpendicular axis of the substrate. The
domain spacings of the polymers were extracted from the
scattering pattern in the in-plane direction. The (100) peak for
the P0 film in Figure 2 was observed at qxy = 0.39 Å−1,
indicating the corresponding interlayer domain spacing (d1)
(inset, Figure 2b) was 1.61 nm. Similarly, the d1 values of P1
and P2 were found to be 1.72 and 1.78 nm, respectively.
Therefore, as more nitrogen atoms were introduced into the
polymer backbone, the d1 values of P0, P1, and P2 increased
from 1.61 to 1.72 to 1.78 nm, respectively. In addition, as
shown in Figure 2b, the P0 film exhibited the only strong first-
order diffraction of a (100) peak in the out-of-plane direction,
whereas both P1 and P2 films showed strong (100), (200), and
(300) peaks with more pronounced reflections; this result
indicates that the polymer stacks in the P1 and P2 films
exhibited a higher structural organization. The introduced
nitrogen atoms also affected the π−π stacking distance between
the conjugated polymers. The (010) peaks that correspond to
the π−π stacking distance (d2) (inset, Figure 2b) were the
strongest in the in-plane direction for all three polymers.
However, the (010) peaks of the P0, P1, and P2 films were
observed at different qxy values of 1.51, 1.72, and 1.75 Å−1,
respectively, which indicates that the d2 values of the P0, P1,
and P2 were 0.42, 0.37, and 0.36 nm, respectively.
Furthermore, as shown in Figure 2a, the intensity of the
(010) peak of P0 was significantly weaker than those of P1 and
P2. Because π−π stacking is critical for both charge transport
and light absorption in the conjugated polymers,44−46 the
change in the number of nitrogen atoms can induce different
intermolecular interactions that lead to different optical and
electrical properties. Therefore, the GIXS results indicated that
the planarity of the main chains and their intermolecular
ordering increased in the order of P0 < P1 < P2.
Structural Properties: Theoretical Calculation. To gain
insight into the effect of the incorporated nitrogen atoms on
the planarity of the polymer main chain, molecular simulations
were performed using density functional theory (DFT) at the
B3LYP/6-31G(d,p) level. The characteristic change in the
packing structure of the polymers can be explained by the
calculated results of the oligomers’ optimized conformation that
corresponds to the three different polymers. The dihedral
angles between adjacent rings in the repeating unit are denoted
as D1, D2, and D3, as shown in Table 1. The values of the
dihedral angles, D1, D2, and D3, can be controlled by tuning the
number of nitrogen atoms in the acceptor unit of the polymer
main chain. The dihedral angles D1, D2, and D3 of P0 were
estimated to be 27.3°, 23.5°, and 24.7°, respectively. Similarly,
those for P1 were 18.6°, 4.5°, and 24.7°, respectively, and those
for P2 were 21.0°, 1.9°, and 1.6°, respectively. No significant
change was found for the dihedral angles between the BDT unit
and thiophene (D1). In contrast, the dihedral angle D2 of P0
(23.5°) was significantly larger than that of P1 (D2 = 4.5°) or
P2 (D2 = 1.9°). Additionally, P2 exhibited a dramatically lower
D3 value (1.6°) relative to that of P0 (D3 = 24.7°) or P1 (D3 =
24.7°). The reduced dihedral angle was likely due to the
interaction between the thiophene and the nitrogen atom in the
pyridine or pyridazine, which stabilizes the conformation of the
Table 1. Key Dihedral Angles (deg) of the Three Different
Polymers
D1
D2
D3
P0
P1
P2
27.3
18.6
21.0
23.5
4.5
24.7
24.7
1.6
1.9
polymers with less-twisted structures.47,48 As a result, in the
case of P1, the nitrogen atom oriented toward the thiophene on
the left-hand side induced the decrease in the dihedral angle D2.
Furthermore, in the case of P2, D2 and D3 decreased
simultaneously because the nitrogen atoms were located on
both sides of the pyridazine group, which promoted the
interaction of the pyridazine group with the adjacent
thiophenes. Therefore, the planarity was increased significantly
in the order of P0 < P1 < P2 when the nitrogen atoms were
added, and the most planar conformation of the main backbone
of P2 led to the strongest interchain interaction, which is
consistent with the GIXS results.
Deeper insight into the effect of the nitrogen atoms on the
structural properties of the polymers can be obtained from
calculations of the interlayer domain spacings (d1) of the
polymers P0, P1, and P2. The d1 values of the polymers can be
estimated by taking the previously calculated dihedral angles of
D2 and D3 into account. Since all three polymers have the same
electron-rich moiety of BDT even though they showed
different d1 values, the d1 spacing between the polymer
backbones is assumed to be dependent predominantly on the
acceptor unit of each polymer. The center-to-end distance of 3-
hexylthiophene in the electron-deficient unit (0.87 nm) was
used in the calculation. As shown in Figure 3 and Table 2, the
calculated d1 spacings of P0, P1, and P2 were 1.59, 1.69, and
1.75 nm, respectively; these values show excellent agreement
with the d1 spacings of P0 (1.61 nm), P1 (1.72 nm), and P2
(1.78 nm) determined from the GIXS patterns. The agreement
is remarkable and shows that the measured values of the
Figure 3. Calculated d1 values for P0, P1, and P2 based on the
dihedral angles D2 and D3.
6418
dx.doi.org/10.1021/ma301362t | Macromolecules 2012, 45, 6415−6423

Macromolecules
Article
the strongest red-shifted λmax of 507 nm and λonset of 585 nm.
The optical bandgaps of P0, P1, and P2 were estimated to be
2.28, 2.16, and 2.12 eV, respectively, based on the onset of the
light-absorption spectra. Therefore, the polymers displayed
stronger red-shifts in the absorption onsets and maxima and,
thus, lower bandgaps as the number of incorporated nitrogen
atoms in the polymer backbone was increased. This
phenomenon can be accurately predicted from the changes in
the structural properties of the polymers. The GIXS results
clearly revealed that the planarity of the main chains and their
intermolecular ordering increased in the order of P0 < P1 < P2,
which resulted in the red-shift absorption feature and, thus,
better light-absorption ability.35
Table 2. Comparison of the Measured d1 Spacings (GIXS)
with the Spacings Calculated from the Simulations
a
a
domain spacing
calcd domain spacing π−π stacking distance
(nm) d1
(nm) d1
(nm) d2
P0
P1
P2
1.61
1.72
1.78
1.59
1.69
1.75
0.42
0.37
0.36
a
Obtained from GIXS measurements.
differences between the d1 spacings of P0 and P1 (0.11 nm)
and between the d1 spacings of P1 and P2 (0.06 nm) are
identical to the calculated values of the differences. Therefore,
the nitrogen atoms resulted in reduced dihedral angles and,
thus, the increased planarity. In addition, the trend of the
change in the planarity clearly explains the decreasing trend in
the π−π stacking distance via the incorporation of nitrogen
atoms (Table 2). Similar trends have been observed in previous
studies:49−52 poly(3,3′-dioctyl-2,2′-bithiophene) showed a
reduced interlayer domain spacing relative to that of poly(3-
octylthiophene) because of the 40° twisted structure that was
induced by head-to-head and tail-to-tail coupling.
The incorporated nitrogen atoms in the polymer backbone
and the resulting change in the planarity of the polymers could
affect the electron distribution and consequently alter their
HOMO and LUMO levels.53 The electrochemical properties of
P0, P1, and P2 were measured using cyclic voltammetry (CV)
(Figure S6). The CV curves were recorded with an Ag quasi-
reference electrode, which was calibrated with the absolute
energy level of Fc/Fc+ (4.8 eV under vacuum) as an external
standard. The electrochemical properties of these materials are
summarized in Table 3. On the basis of the onset oxidation
potentials, the HOMO energy levels of P0, P1, and P2 were
determined to be −5.30, −5.35, and −5.38 eV, respectively; the
LUMO levels of P0, P1, and P2 were estimated to be −3.02,
−3.19, and −3.26 eV, respectively, based on the difference
between the HOMO level and the optical bandgap of each
polymer. The results showed that the incorporation of the
nitrogen atoms into the polymer backbone reduced both the
HOMO and the LUMO energy levels; however, the trend in
the change of LUMO energy levels was significantly more
pronounced than that for the HOMO energy levels. This result
was probably due to the change in the strength of the electron-
deficient acceptor unit in the polymers. The nitrogen atoms
acted as electron-withdrawing substituents because of their high
electronegativity;54,55 the strength of the acceptor unit
consequently increased, which resulted in the decrease in the
LUMO levels and the bandgap of the polymers.56 In addition,
because the π electrons tend to delocalize in the planar
structure and to localize in the twisted structure, the bandgaps
of the polymers decreased in the order of P0 > P1 > P2.
Photovoltaic Properties. To elucidate the relationship
between the molecular structure of the conjugated polymers
and the performance of the solar cell devices, BHJ-type PSCs
(ITO/PEDOT:PSS/active layer/LiF/Al) were fabricated based
on blends of P0, P1, and P2 with PC61BM, and their
performance was measured. Figure 5 shows the current density
versus voltage (J−V) curves for the devices under AM 1.5
illumination at 100 mW/cm2. The representative characteristics
of the PSC devices are summarized in Table 4. Each of the
devices was prepared under optimized conditions. The active
layers were spin-coated with DCB solution of the polymers and
PC61BM. The thicknesses of the films of P0, P1, and P2 with
PC61BM showed similar values of ∼100 nm, as measured using
an alpha-step profiler. The PCEs of the devices increased in
order of P0 < P1 < P2, which indicates a linear dependence of
the device performance on the number of the nitrogen atoms in
the polymer backbone. Among the P0-, P1-, and P2-based BHJ
PSCs, the device that consisted of P2:PC61BM (1:0.7, w/w)
exhibited the highest PCE of 2.78% with Voc = 0.73 V, Jsc = 6.78
mA/cm2, and FF = 0.56, which represents a 3.5-fold increase
compared with the PCE of P0:PC61BM (1:0.6, w/w) (PCE =
Optical and Electrochemical Properties. The absorption
spectra of the polymers were strongly dependent on their
planarity and packing structure. The UV−vis absorption spectra
of the three polymers as thin films were measured (Figure 4).
Figure 4. UV−vis absorption spectra of the polymer films.
Table 3. Optical and Electrochemical Properties of the
Polymers Used in This Study
polymer λmax (nm) λonset (nm) Eg (eV) HOMO (eV) LUMO (eV)
P0
P1
P2
470
487
507
544
573
585
2.28
2.16
2.12
−5.30
−5.35
−5.38
−3.02
−3.19
−3.26
The results are summarized in Table 3. The absorption spectra
of P0, P1, and P2 exhibited two maxima in the range from 450
to 600 nm; however, significant differences were observed in
the locations of the peaks. The spectrum of P0 showed an
absorption maximum (λmax) at 470 nm with an absorption
onset (λonset) at 544 nm. The λmax and λonset values of P1 were
487 and 573 nm, respectively; these values were considerably
red-shifted compared to those of P0. In addition, P2 showed
6419
dx.doi.org/10.1021/ma301362t | Macromolecules 2012, 45, 6415−6423

Macromolecules
Article
Figure 5. (a) Current density−voltage (J−V) characteristics of the PSCs: (i) P0:PC61BM (1:0.6), (ii) P1:PC61BM (1:0.7), (iii) P2:PC61BM (1:0.7),
and (iv) P2:PC71BM (1:0.7)-based BHJ devices under AM 1.5 illumination at 100 mW/cm2. (b) Measured space-charge-limited J−V characteristics
of the films of P0, P1, and P2 under dark conditions.
pronounced improvements in the structural and electronic
properties and in the PSC performance.
Table 4. Characteristics of the PSC Devices Composed of
Three Different Polymers as the Electron Donor and
Fullerene as the Electron Acceptor under AM 1.5G-
Simulated Solar Illumination (100 mW/cm2)
CONCLUSIONS
■
In summary, we successfully synthesized the conjugated
polymers P0, P1, and P2, which contain 0−2 nitrogen atoms
in their repeating unit, and investigated the effects of the
introduced nitrogen atoms on the structural and photovoltaic
properties of the polymers. As the number of the introduced
nitrogen was increased, the planarity of the polymer was
dramatically enhanced, with increased d1 spacings, decreased d2
spacings, and better interchain packing between the polymers,
as demonstrated by GIXS measurements. The changes in the
polymer structure can be explained quantitatively using
molecular simulations, which clearly revealed the reduction in
the dihedral angles of the polymer chain as a result of the
introduction of the nitrogen atoms. In addition, the red-shifted
absorption onset and the increased hole mobility of the
polymers were achieved due to the increase in planarity and
resulted in an increase in the Jsc and FF in the PSCs. Thus, an
increased PCE of 3.22% was finally obtained from the
P2:PC71BM-based PSC, which is ∼4-fold increase in the PCE
compared to that of P0:PC61BM. Therefore, our study provides
a model system to investigate the effect of the addition of
electronegative nitrogen atoms on the properties of conjugated
polymers.
Voc
(V)
Jsc
PCE
(%)
μh
a
active layer (w/w)
(mA/cm2)
FF
(cm2/(V s))
P0:PC61BM
0.52
0.59
0.73
0.72
4.19
4.89
6.78
7.97
0.37
0.81
1.13
2.78
3.22
2.33 × 10−6
8.63 × 10−6
3.78 × 10−5
(1:0.6)
P1:PC61BM
(1:0.7)
0.39
0.56
0.56
P2:PC61BM
(1:0.7)
P2:PC71BM
(1:0.7)
a
Hole mobility of the pristine polymer films of P0, P1, and P2
0.81% with Voc = 0.52 V, Jsc = 4.19 mA/cm2, and FF = 0.37).
To gain insight into the performance of the PSCs, the space-
charge-limited current (SCLC) for the polymers was measured
(Figure 5). The SCLC mobilities of the polymers are
summarized in Table 4. Hole-only devices were constructed
with the structure of ITO/PEDOT:PSS/polymer/Au, and hole
mobilities were calculated using the Mott−Gurney equation.
For films of pristine P0, P1, and P2 polymers, the SCLC
mobilities were 2.33 × 10−6, 8.63 × 10−6, and 3.78 × 10−5 cm2/
(V s), respectively. The SCLC results can be explained by the
structural changes in the polymers in that the nitrogen atoms
induced better planarity in the conjugated polymer chain, a
decrease in the π−π stacking distance, and a highly ordered
polymer packing, which are critical factors that determine the
charge mobility. Thus, P2, having a shorter π−π stacking
distance relative to those of P0 and P1, showed the highest
hole mobility. Therefore, the trend observed for SCLC mobility
clearly explained the increase in the Jsc and FF of the PSCs in
the order of P0 < P1 < P2. Further optimization to enhance the
PCE was performed with P2:PC71BM. When PC71BM was
used as an electron acceptor, the resulting solar cell could
exhibit an increased Jsc due to the absorption of PC71BM itself,
whereas other parameters, including Voc and FF, remained
almost unchanged compared to those in the P2:PC61BM
device. Finally, an even higher PCE (3.22%) was achieved with
Voc = 0.72 V, Jsc = 7.97 mA/cm2, and FF = 0.56. Therefore, the
small modification in the structure of the conjugated polymer
induced by the introduction of nitrogen atoms led to
EXPERIMENTAL SECTION
■
Synthesis of Monomers and Polymers. All commercially
available reagents were used without further purification. The organic
solvents were used as anhydrous solvents. Analytical thin-layer
chromatography (TLC) was performed using Merck silica gel 60
F254 precoated plates (0.25 mm) with a fluorescent indicator and
visualized with UV light S2 (254 and 365 nm). Silica gel
chromatography was performed on Merck silica gel 60 (230−400
mesh).
2-(Tri-n-butylstannyl)-4-hexylthiophene (1). Under an argon
atmosphere, 3-hexylthiophene (18.24 g, 108 mmol) and 500 mL of
freshly distilled dry THF were placed in a 1000 mL one-necked flask.
n-Butyllithium (47.69 mL, 2.5 M in hexane, 119 mmol) was added
dropwise at −78 °C, and the solution was stirred at −78 °C for 1 h.
Then, tributyltin chloride (38.74 g, 119 mmol) was added, and the
mixture was allowed to reach room temperature slowly; the mixture
was subsequently stirred for another 24 h. Finally, the mixture was
poured into 200 mL of cooled water and extracted with hexane. The
organic layer was dried over anhydrous MgSO4. Removal of the
6420
dx.doi.org/10.1021/ma301362t | Macromolecules 2012, 45, 6415−6423

Macromolecules
Article
solvent by rotary evaporation yielded a brown liquid (48.00 g, 97%).
The crude product was used in the next step without further
purification. 1H NMR (CDCl3, ppm): 7.19 (s, 1H), 6.98 (s, 1H), 2.69
(t, 2H), 1.50−1.67 (m, 14H), 1.30−1.50 (m, 12H), and 0.92 (t, 12H).
1,4-Bis(4-hexylthiophen-2-yl)benzene (5). A mixture of 2-(trin-
butylstannyl)-4-methylthiophene (4.57 g, 10.0 mmol), 1,4-diiodoben-
zene (1.5 g, 4.5 mmol), and tetrakis(triphenylphosphine)palladium(0)
(0.053 g, 0.05 mmol) in toluene (100 mL) was degassed twice with
N2. The reaction mixture was then heated at 80 °C for 6 h; after
cooling, the mixture was poured into water (100 mL) and extracted
with diethyl ether. The organic layer was dried over anhydrous
MgSO4. The crude compound was purified by silica gel chromatog-
raphy, and white crystals were obtained (0.95 g, 51%). 1H NMR
(CDCl3, ppm): 7.62 (d, 4H), 7.19 (s, 2H), 6.90 (s, 2H), 2.66 (t, 4H),
1.55−1.65 (m, 4H) 1.20−1.40 (m, 12H), and 0.88 (t, 6H).
then concentrated by evaporation and precipitated into methanol (200
mL).
Poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]-
dithiophene-alt-1,4-bis(4-hexylthiophen-2-yl)benzene]
(PBDTHTB, P0). 2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo-
[1,2-b:4,5-b′] dithiophene (0.380 g, 0.492 mmol) and compound 8
(0.28 g, 0.492 mmol) were used. The reddish-brown solid was
obtained (183 mg). Mn = 12.5 kg/mol; Mw = 21.3 kg/mol; PDI = 1.7.
Poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]-
dithiophene-alt-2,5-bis(4-hexylthiophen-2-yl)pyridine]
(PBDTHTP, P1). 2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo-
[1,2-b:4,5-b′] dithiophene (0.271 g 0.351 mmol) and compound 9
(0.200 g, 0.351 mmol) were used. The reddish-brown solid was
obtained (80 mg). Mn = 10.1 kg/mol; Mw = 15.2 kg/mol; PDI = 1.5.
Poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]-
dithiophene-alt-3,6-bis(4-hexylthiophen-2-yl)pyridazine]
(PBDTHTPz, P2). 2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)-
benzo[1,2-b:4,5-b′] dithiophene (0.271 g 0.351 mmol) and compound
10 (0.200 g, 0.351 mmol) were used. The brown solid was obtained
(83 mg). Mn = 15.2 kg/mol; Mw = 22.8 kg/mol; PDI = 1.5.
2,5-Bis(4-hexylthiophen-2-yl)pyridine (6). The synthetic pro-
cedure for 6 was similar to that for 5, except that 2,5-dibromopyridine
(1 g, 4.2 mmol) was used as the reactant instead of 1,4-diiodobenzene.
The crude product of yellow oil (1.05 g, 60%) was obtained and used
in the next step without further purification.
1
3,6-Bis(4-hexylthiophen-2-yl)pyridazine (7). The synthetic
procedure for 7 was similar to that for 5, except that 3,6-
diiodopyridazine (2 g, 6 mmol) was used as the reactant instead of
1,4-diiodobenzene. After silica gel chromatography, the product of
Characterization Methods. All H NMR spectra were recorded
at 400 MHz using CDCl3 as the solvent, unless otherwise stated. The
1
chemical shifts of all H NMR spectra were referenced to the residual
signal of CDCl3 (δ 7.26 ppm) on a Bruker 400-MHz NMR
instrument. UV−vis absorption spectra were obtained with a JASCO
V-570 spectrophotometer. CV curves were measured using a CHI
600C electrochemical analyzer at room temperature with a conven-
tional three-electrode system (Pt-disk working electrode, Pt-wire
counter-electrode and Ag-wire quasi-reference electrode). The
oxidation potentials of the polymer solutions against the Ag quasi-
reference electrode were measured and calibrated against a ferrocene/
ferrocenium (Fc/Fc+) redox couple, where the absolute energy level of
Fc/Fc+ was assumed to be −4.80 eV. GIXS measurements were
performed on beamline 9A at the Pohang Accelerator Laboratory
(South Korea). X-rays with a wavelength of 1.1010 Å were used. The
incidence angle (∼0.15°) was chosen to allow for complete
penetration of the X-rays into the polymer film.
1
yellow oil (1.64 g, 66%) was obtained. H NMR (CDCl3, ppm): 7.73
(d, 2H), 7.54 (s, 2H), 7.10 (s, 2H), 2.73 (t, 4H), 1.55−1.73 (m, 4H),
1.27−1.42 (m, 12H), and 0.77 (t, 6H).
1,4-Bis(5-bromo-4-hexylthiophen-2-yl)benzene (8). To a
solution of compound 5 (0.95 g, 2.3 mmol) in chloroform (20 mL)
and acetic acid (30 mL), NBS (0.91 g, 5.1 mmol) was added at room
temperature. The mixture was stirred at room temperature under
darkness for 6 h. The solution was then poured into distilled water
(200 mL) and extracted with chloroform (200 mL). The organic layer
was dried over anhydrous MgSO4. The solvent was removed using a
rotary evaporator, and the residue was purified by silica gel
chromatography to yield the product (0.48 g, 37%). 1H NMR
(CDCl3, ppm): 7.52 (d, 4H), 7.06 (s, 2H), 2.61 (t, 4H), 1.55−1.66
(m, 4H) 1.20−1.41 (m, 12H), and 0.97 (t, 6H).
Device Fabrication and Measurement. BHJ PSC cells were
fabricated using an ITO/PEDOT:PSS/active layer/LiF/Al structure.
P0, P1, and P2 were used as electron donors, and either PC61BM or
PC71BM was used as the electron acceptor. ITO-coated glass
substrates were ultrasonicated in acetone and 2% Helmanex soap in
water; the substrates were then extensively rinsed with deionized water
and then ultrasonicated in deionized water and isopropyl alcohol. The
substrates were dried for 1 h in an oven at 80 °C. The ITO substrates
were treated with UV-ozone prior to PEDOT:PSS deposition. A
filtered dispersion of PEDOT:PSS in water (PH 500) was applied by
spin-coating at 3000 rpm for 40 s and baking for 30 min at 140 °C in
air. After application of the PEDOT:PSS layer, all subsequent
procedures were performed in a glovebox under a N2 atmosphere.
Separate solutions of P0, P1, P2, PC61BM, and PC71BM in DCB (30
mg/mL) were prepared and stirred at 120 °C for at least 24 h before
being filtered through a 0.45 μm PTFE syringe filter. Blend solutions
composed of P0, P1, and P2 mixed with PC61BM or PC71BM were
prepared to a final polymer concentration of 14 mg/mL. Each solution
was spin-cast onto an ITO/PEDOT:PSS substrate at 1200 rpm for 90
s. The substrates were then placed in an evaporation chamber and held
under high vacuum (less than 10−6 Torr) for at least 1 h before
deposition of ∼0.7 nm of LiF and 100 nm of Al with a shadow mask
that produced four independent devices on each substrate. The
photovoltaic performances of the devices were characterized with an
AM 1.5G filter. The intensity of the solar simulator was carefully
calibrated using an AIST-certified silicon photodiode. Current−voltage
behavior was measured using a Keithley 2400 SMU. The active area of
the fabricated devices was 0.102 cm2. The hole mobilities of the
synthesized polymer were measured via the SCLC method using an
ITO/PEDOT:PSS/polymer/Au device structure. The SCLC is
described by
2,5-Bis(5-bromo-4-hexylthiophen-2-yl)pyridine (9). The syn-
thetic procedure for 9 was similar to that for 8, except that compound
6 (1.05 g, 2.55 mmol) was used as the reactant instead of compound 5.
The product was purified by silica gel chromatography and then
recrystallized in hexane to yield a yellowish-white solid (0.32 g, 22%).
1H NMR (CDCl3, ppm): 8.74 (s, 1H), 7.80 (d, 1H), 7.59 (d, 1H),
7.36 (s, 1H), 7.09 (s, 1H), 2.65 (t, 4H), 1.45−1.68 (m, 4H), 1.25−
1.41 (m, 12H), and 0.89 (t, 6H).
3,6-Bis(5-bromo-4-hexylthiophen-2-yl)pyridazine (10). The
synthetic procedure for 10 was similar to that for 8, except that
compound 7 (1.64 g, 3.97 mmol) was used as the reactant instead of
compound 5. The product was purified by silica gel chromatography
and then recrystallized in hexane to yield a yellow solid (0.32 g, 15%).
1H NMR (CDCl3, ppm): 7.65 (d, 2H), 7.34 (s, 2H), 2.61 (t, 4H),
1.59−1.70 (m, 4H), 1.25−1.41 (m, 12H), and 0.85 (t, 6H).
2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-
b:4,5-b′]dithiophene. This compound was synthesized according to
the reported procedure.10 1H NMR (CDCl3, ppm): 7.50 (s, 2H), 4.18
(d, 4H), 1.54−1.83 (m, 8H), 1.32−1.50 (m, 10H), 1.01 (t, 6H), 0.93
(t, 6H), and 0.42 (s, 18H).
General Procedure for Polymerization of P0, P1, and P2. 2,6-
Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]-
dithiophene, electron-deficient monomers (8, 9, and 10), Pd2(dba)3 (4
mol %), and P(o-tol)3 (16 mol %) were added into 100 mL flask, and
the flask was purged three times with successive vacuum and argon
filling cycles. Then, 8 mL of dry chlorobenzene was added
subsequently. The mixture was stirred for 36 h at 110 °C. The
mixture was cooled to room temperature and poured slowly in
methanol (400 mL). The precipitate was filtered through a Soxhlet
thimble, purified via Soxhlet extraction for 12 h with methanol, 24 h
with hexanes, 12 h with dichloromethane, and 24 h with chloroform,
and finally collected with chlorobenzene. The chloroform solution was
6421
dx.doi.org/10.1021/ma301362t | Macromolecules 2012, 45, 6415−6423

Macromolecules
Article
(13) Kim, K.-H.; Kang, H.; Nam, S. Y.; Jung, J.; Kim, P. S.; Cho, C.-
H.; Lee, C.; Yoon, S. C.; Kim, B. J. Chem. Mater. 2011, 23, 5090.
(14) Zhang, L.; Tajima, K.; Hashimoto, K. Macromolecules 2011, 44,
4222.
9
8
V2
JSCLC
=
εε0μ
L3
where ε0 is the permittivity of free space, ε is the dielectric constant of
the polymer, μ is the charge-carrier mobility, V is the potential across
the device (V = Vapplied − Vbi − Vr), and L is the thickness of the
polymer layer. The series and contact resistances of the device (∼25
Ω) were measured using a blank device (ITO/PEDOT:PSS/Au), and
the voltage drop caused by this resistance (Vr) was subtracted from the
applied voltage.
Theoretical Calculations. The simulations were performed using
density functional theory (DFT) at the B3LYP/6-31G(d,p) level with
the Spartan 08 software package. The oligomers with a chain length of
n = 3 were chosen as simplified models, and all of the alkyl chains were
replaced with methyl chains to make computation possible. In
addition, we selected one isomer between two regiochemical isomers
of P1 resulting from the asymmetric structure of 1,4-diiodopyridazine.
(15) Lee, W.; Choi, H.; Hwang, S.; Kim, J. Y.; Woo, H. Y. Chem.
Eur. J. 2012, 18, 2551.
(16) Gendron, D.; Morin, P.-O.; Berrouard, P.; Allard, N.; Aïch, B.
R.; Garon, C. N.; Tao, Y.; Leclerc, M. Macromolecules 2011, 44, 7188.
(17) Hong, Y.-R.; Ng, J. Y.; Wong, H. K.; Moh, L. C. H.; Yip, Y. J.;
Chen, Z.-K.; Norsten, T. B. Sol. Energy Mater. Sol. Cells 2012, 102, 58.
(18) Ahmed, E.; Subramaniyan, S.; Kim, F. S.; Xin, H.; Jenekhe, S. A.
Macromolecules 2011, 44, 7207.
(19) Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. J. Am.
Chem. Soc. 2011, 133, 4625.
(20) Wang, M.; Hu, X.; Liu, P.; Li, W.; Gong, X.; Huang, F.; Cao, Y.
J. Am. Chem. Soc. 2011, 133, 9638.
(21) Huang, Y.; Guo, X.; Liu, F.; Huo, L.; Chen, Y.; Russell, T. P.;
Han, C. C.; Li, Y.; Hou, J. Adv. Mater. 2012, 24, 3383.
(22) Ajayaghosh, A. Chem. Soc. Rev. 2003, 32, 181.
(23) Havinga, E. E.; ten Hoeve, W.; Wynberg, H. Synth. Met. 1993,
55, 299.
ASSOCIATED CONTENT
* Supporting Information
TGA data, CV curves, and H NMR spectra of materials. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
1
(24) Najari, A.; Beaupre,
́
S.; Berrouard, P.; Zou, Y.; Pouliot, J.-R.;
Lepage-Perusse, C.; Leclerc, M. Adv. Funct. Mater. 2011, 21, 718.
́
(25) Hou, J.; Park, M.-H.; Zhang, S.; Yao, Y.; Chen, L.-M.; Li, J.-H.;
Yang, Y. Macromolecules 2008, 41, 6012.
AUTHOR INFORMATION
Corresponding Author
*E-mail: bumjoonkim@kaist.ac.kr.
■
(26) Shi, Q.; Fan, H.; Liu, Y.; Chen, J.; Ma, L.; Hu, W.; Shuai, Z.; Li,
Y.; Zhan, X. Macromolecules 2011, 44, 4230.
(27) Duan, R.; Ye, L.; Guo, X.; Huang, Y.; Wang, P.; Zhang, S.;
Zhang, J.; Huo, L.; Hou, J. Macromolecules 2012, 45, 3032.
(28) Biniek, L.; Fall, S.; Chochos, C. L.; Anokhin, D. V.; Ivanov, D.
A.; Leclerc, N.; Leveque, P.; Heiser, T. Macromolecules 2010, 43, 9779.
́ ̂
(29) Son, H. J.; Wang, W.; Xu, T.; Liang, Y.; Wu, Y.; Li, G.; Yu, L. J.
Am. Chem. Soc. 2011, 133, 1885.
(30) Cho, C.-H.; Kim, H. J.; Kang, H.; Shin, T. J.; Kim, B. J. J. Mater.
Chem. 2012, 22, 14236.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Korea Research
Foundation Grant funded by the Korean Government (2011-
0030387) and the Research Project of the KAIST EEWS
Initiative (EEWS-N01110441). This research was also
supported by the New & Renewable Energy KETEP Grant
(2011-T100100587). Experiments at PLS were supported in
part by MEST and POSTECH.
(31) Braunecker, W. A.; Owczarczyk, Z. R.; Garcia, A.; Kopidakis, N.;
Larsen, R. E.; Hammond, S. R.; Ginley, D. S.; Olson, D. C. Chem.
Mater. 2012, 24, 1346.
(32) Li, Y. Acc. Chem. Res. 2012, 45, 723.
(33) Cho, C.-H.; Kang, H.; Kang, T. E.; Cho, H.-H.; Yoon, S. C.;
Jeon, M.-K.; Kim, B. J. Chem. Commun. 2011, 47, 3577.
(34) Scharber, M. C.; Koppe, M.; Gao, J.; Cordella, F.; Loi, M. A.;
Denk, P.; Morana, M.; Egelhaaf, H.-J.; Forberich, K.; Dennler, G.;
Gaudiana, R.; Waller, D.; Zhu, Z.; Shi, X.; Brabec, C. J. Adv. Mater.
2010, 22, 367.
(35) Roncali, J. Macromol. Rapid Commun. 2007, 28, 1761.
(36) Padhy, H.; Huang, J.-H.; Sahu, D.; Patra, D.; Kekuda, D.; Chu,
C.-W.; Lin, H.-C. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 4823.
(37) Bijleveld, J. C.; Shahid, M.; Gilot, J.; Wienk, M. M.; Janssen, R.
A. J. Adv. Funct. Mater. 2009, 19, 3262.
REFERENCES
■
(1) Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. J. Mater. Chem. 2009,
19, 5442.
(2) Arias, A. C.; MacKenzie, J. D.; McCulloch, I.; Rivnay, J.; Salleo, A.
Chem. Rev. 2010, 110, 3.
(3) Thompson, B. C.; Frec
́
het, J. M. J. Angew. Chem., Int. Ed. 2008,
47, 58.
(4) Hauch, J. A.; Schilinsky, P.; Choulis, S. A.; Childers, R.; Biele, M.;
Brabec, C. J. Sol. Energy Mater. Sol. Cells 2008, 92, 727.
(5) Zhao, G.; He, Y.; Li, Y. Adv. Mater. 2010, 22, 4355.
(6) Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.;
Beaujuge, P. M.; Frec
(7) Chu, T.-Y.; Lu, J.; Beaupre,
Wakim, S.; Zhou, J.; Leclerc, M.; Li, Z.; Ding, J.; Tao, Y. J. Am. Chem.
Soc. 2011, 133, 4250.
(8) Kim, K.-H.; Kang, H.; Kim, H. J.; Kim, P. S.; Yoon, S. C.; Kim, B.
J. Chem. Mater. 2012, 24, 2373.
(9) Ong, K.-H.; Lim, S.-L.; Tan, H.-S.; Wong, H.-K.; Li, J.; Ma, Z.;
Moh, L. C. H.; Lim, S.-H.; de Mello, J. C.; Chen, Z.-K. Adv. Mater.
2011, 23, 1409.
(10) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. J.
Am. Chem. Soc. 2009, 131, 7792.
(11) Lenes, M.; Shelton, S. W.; Sieval, A. B.; Kronholm, D. F.;
Hummelen, J. C.; Blom, P. W. M. Adv. Funct. Mater. 2009, 19, 3002.
(12) Voroshazi, E.; Vasseur, K.; Aernouts, T.; Heremans, P.;
Baumann, A.; Deibel, C.; Xue, X.; Herring, A. J.; Athans, A. J.; Lada,
T. A.; Richter, H.; Rand, B. P. J. Mater. Chem. 2011, 21, 17345.
(38) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
Neagu-Plesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc, M. J.
̂
Am. Chem. Soc. 2007, 130, 732.
́
het, J. M. J. J. Am. Chem. Soc. 2010, 132, 7595.
S.; Zhang, Y.; Pouliot, J.-R. m.;
(39) Woo, C. H.; Beaujuge, P. M.; Holcombe, T. W.; Lee, O. P.;
́
́
Frechet, J. M. J. J. Am. Chem. Soc. 2010, 132, 15547.
(40) Zhou, H.; Yang, L.; Price, S. C.; Knight, K. J.; You, W. Angew.
Chem., Int. Ed. 2010, 49, 7992.
(41) Hou, J.; Chen, H.-Y.; Zhang, S.; Chen, R. I.; Yang, Y.; Wu, Y.;
Li, G. J. Am. Chem. Soc. 2009, 131, 15586.
(42) Liu, Y.; Wan, X.; Wang, F.; Zhou, J.; Long, G.; Tian, J.; Chen, Y.
Adv. Mater. 2011, 23, 5387.
(43) Zhou, W.; Shen, P.; Zhao, B.; Jiang, P.; Deng, L.; Tan, S. J.
Polym. Sci., Part A: Polym. Chem. 2011, 49, 2685.
(44) Kim, H. J.; Han, A. R.; Cho, C.-H.; Kang, H.; Cho, H.-H.; Lee,
M. Y.; Frec
215.
́
het, J. M. J.; Oh, J. H.; Kim, B. J. Chem. Mater. 2011, 24,
(45) Zhou, E.; Tan, Z. a.; Yang, C.; Li, Y. Macromol. Rapid Commun.
2006, 27, 793.
6422
dx.doi.org/10.1021/ma301362t | Macromolecules 2012, 45, 6415−6423

Macromolecules
Article
(46) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.;
Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R.
A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401,
685.
(47) Yamamoto, T.; Zhou, Z.-h.; Kanbara, T.; Shimura, M.; Kizu, K.;
Maruyama, T.; Nakamura, Y.; Fukuda, T.; Lee, B.-L.; Ooba, N.;
Tomaru, S.; Kurihara, T.; Kaino, T.; Kubota, K.; Sasaki, S. J. Am. Chem.
Soc. 1996, 118, 10389.
(48) Tian, Y.-H.; Kertesz, M. Macromolecules 2009, 42, 6123.
(49) Fell, H. J.; Samuelsen, E. J.; Mardalen, J.; Bakken, E.; Carlsen, P.
̊
H. J. Synth. Met. 1995, 69, 301.
(50) Łuzny, W.; Trznadel, M.; Pron
́
̇
M.; Andersson, M. R.; Pettersson, L. A. A.; Inganas, O. Macromolecules
̈
2000, 33, 5481.
(52) Aasmundtveit, K. E.; Samuelsen, E. J.; Hoffmann, K.; Bakken, E.;
Carlsen, P. H. J. Synth. Met. 2000, 113, 7.
(53) Reed, J. L. J. Chem. Educ. 1992, 69, 785.
(54) Janiak, C.; Scharmann, T. G.; Green, J. C.; Parkin, R. P. G.;
Kolm, M. J.; Riedel, E.; Mickler, W.; Elguero, J.; Charamunt, R. M.;
Sanz, D. Chem.Eur. J. 1996, 2, 992.
(55) Huang, Y.; Huo, L.; Zhang, S.; Guo, X.; Han, C. C.; Li, Y.; Hou,
J. Chem. Commun. 2011, 47, 8904.
(56) Zhou, H.; Yang, L.; Stoneking, S.; You, W. ACS Appl. Mater.
Interfaces 2010, 2, 1377.
6423
dx.doi.org/10.1021/ma301362t | Macromolecules 2012, 45, 6415−6423