Structural design of benzo[1,2- b:4,5- b ′]dithiophene-based 2D conjugated polymers with bithienyl and terthienyl substituents toward photovoltaic applications

Article abstract of DOI:10.1021/ma401846n

In this contribution, six conjugated polymers consisting of benzo[1,2-b:4,5-b′]dithiophene-bithiophene (BDT-BT) and benzo[1,2-b:4,5-b′]dithiophene-benzothiadiazle (BDT-BTD) as building blocks in the main chain were synthesized by coupling polymerization and utilized for photovoltaic applications. By directly attaching three kinds of alkylthienyl side chains to the conjugated main chain, the resulted two-dimensional configuration revealed a broader absorption range due to the ground state electron transition of their corresponding alkylthienyl units and polymer backbone. Temperature-dependent absorbance, emission spectra, and thermal annealing further verify that the shoulder band(s) were originated from the aggregated (crystalline) species of polymers. The photovoltaic properties of the donor-acceptor polymers revealed well-defined side chain geometries, physical, and electronic structures and showed the highest power conversion efficiency of 4.25% among polymer solar cells based on two-dimensional (2-D) bithienyl- or terthienyl-substituted benzodithiophene.

Full text of DOI:10.1021/ma401846n

Article
pubs.acs.org/Macromolecules
Structural Design of Benzo[1,2‑b:4,5‑b′]dithiophene-Based 2D
Conjugated Polymers with Bithienyl and Terthienyl Substituents
toward Photovoltaic Applications
Cheng-Yu Kuo,†,∥ Wanyi Nie,‡ Hsinhan Tsai,† Hung-Ju Yen,† Adytia D. Mohite,‡ Gautam Gupta,‡
Andrew M. Dattelbaum,‡ Darrick J. William,‡ Kitty C. Cha,§ Yang Yang,§ Leeyih Wang,∥
and Hsing-Lin Wang†,*
†Physical Chemistry and Applied Spectroscopy (C-PCS), Chemistry Division, Los Alamos National Laboratory, Los Alamos, New
Mexico 87545, United States
‡Center of Integrated Nanotechnology, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos,
New Mexico 87545, United States
§Department of Materials Science and Engineering, University of California, Los Angeles, California 90095-1595, United States
∥Center for Condensed Matter Science, National Taiwan University, 1 Roosevelt Road, 4th Sec., Taipei 10617, Taiwan
S
* Supporting Information
ABSTRACT: In this contribution, six conjugated polymers consisting of
benzo[1,2-b:4,5-b′]dithiophene−bithiophene (BDT-BT) and benzo[1,2-b:4,5-
b′]dithiophene−benzothiadiazle (BDT-BTD) as building blocks in the main
chain were synthesized by coupling polymerization and utilized for
photovoltaic applications. By directly attaching three kinds of alkylthienyl
side chains to the conjugated main chain, the resulted two-dimensional
configuration revealed a broader absorption range due to the ground state
electron transition of their corresponding alkylthienyl units and polymer
backbone. Temperature-dependent absorbance, emission spectra, and thermal
annealing further verify that the shoulder band(s) were originated from the
aggregated (crystalline) species of polymers. The photovoltaic properties of
the donor−acceptor polymers revealed well-defined side chain geometries,
physical, and electronic structures and showed the highest power conversion
efficiency of 4.25% among polymer solar cells based on two-dimensional (2-
D) bithienyl- or terthienyl-substituted benzodithiophene.
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels allow the good exciton
dissociation and higher open circuit voltage (Voc) value,6 (3)
the crystalline nature of the polymer and good miscibility with
electron acceptor which lead to the increased π−π stacking and
charge mobility,7 and (4) good solubility to various organic
solvents, which would be beneficial for their fabrication of large-
area, thin-film optoelectronic devices by cost-effective solution
processing techniques. Therefore, to achieve a good combina-
tion of the above-mentioned criteria by synthetic approaches is
one of the crucial and ongoing investigations.8
INTRODUCTION
■
The conjugated polymers have gained much attention because
of their unique semiconducting properties in fabricating large
area, flexible, and low cost electronic devices, such as organic
photovoltaics (OPV),1 light emitting diodes,2 electrochromic
windows,3 and thin film transistors.4 Of particular interest are
the OPV as they have advantages over the traditional inorganic-
based material. The versatility and processability of the
polymers offer tunable electronic, optical, and crystalline
properties for optimizing the device performance. Increasingly,
power conversion efficiencies (PCE) of the organic solar cells
have been improved from 1% in the early 90′s to up to 9% until
very recently through modifying the polymer structures,
processing conditions and device structures.5 To achieve highly
efficient OPV devices, strategies of designing conjugated
polymer that have to meet the following prerequisites are
taken into consideration: (1) relatively broad coverage (from
UV to near-IR region) and high absorbance to the maximal
overlap with solar emission, (2) adequate highest occupied
Benzodithiophene- (BDT-) based conjugated polymers have
garnered considerable interest in the past few years due to their
good performance in both organic field-effect transistors and
OPVs.9 In addition to the planar backbone, BDT core offers the
Received: September 4, 2013
Revised: January 7, 2014
Published: January 21, 2014
© 2014 American Chemical Society
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Scheme 1. Synthetic Route to Monomers M1, M2, and M3
flexibility of attaching different substituents on the central
benzene core to fine-tune the energy levels of the resulted
polymers,9b which could be easily changed by adjusting
structure and affinity of the donor10 and/or acceptor.11
Moreover, the BDT-based two-dimensional conjugated poly-
mers prepared by attaching the alkylthienyl side chains could
effectively increases the thermal stability, gives broader
absorption spectra, lowers the HOMO energy levels, and
thus improves the photovoltaic properties of the device.12
Furthermore, the incorporation of thienyl substituents to the
BDT could result in an increased polymer solubility in common
organic solvents as it allows attaching a larger number of alkyl
substituents on the thiophene ring,13 which allows the emerged
thienyl-subsituted BDT as an excellent building block for
conjugated polymers. To this day, by using BDT unit as donor,
a polymer with low HOMO level (∼−5.50 eV) and high Voc up
to 0.91 could be achieved.14 Despite previous work using
conjugated side chains to increase the rigidity of the repeating
unit and show higher OPV performance, the effect of
substituents on the physical properties of two-dimensional
terthienyl/bithienyl BDT-based conjugated polymers has not
been fully investigated yet.
series of two-dimensional BDT-based conjugated polymers
(P1−P6) with terthienyl/bithienyl side chains by coupling
polymerizations. The electron-abundant bithiophene (BT) and
electron-deficient benzothiadiazole (BTD) units were chosen as
part of the polymer backbone due to the minimized curvature
of the resulted polymeric backbone.15 Moreover, the
incorporation of terthienyl and bithienyl side chains allows
the attachment of up to eight and six alkyl substituents per
BDT repeating unit, respectively, which in turn could produce
organosoluble conjugated polymers with high molecular weight
and provide a broader absorption covered from UV to near-IR
region. OPVs have been fabricated based on these 2-D
copolymers with well-defined structural configurations, side
chain geometries, electronic structures, and enhanced PCE.
OPV devices with highest PCE are prepared from P1 (4.26%)
and P5 (3.65%) with strongest polymer aggregates and
crystallinity, as manifested by the XRD, temperature-dependent
UV−vis, and PL spectra.
RESULTS AND DISCUSSION
■
Monomer Synthesis. The synthetic routes to the BDT
monomers are outlined in Scheme 1. Monomers M1, M2, and
M3 are all composed of head-to-tail alkylthienyl structure as
side chains, which would facilitate the chain packing of
prepared polymers in solid state. The oligthiophene on M1
This manuscript is a systematic work using a specific type of
conjugated side chain, which does not involve forming a “side
conjugated ring” on the repeating unit. We have synthesized a
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Scheme 2. Synthetic Procedure of 2-D Conjugated Polymers P1−P6
a
Table 1. Molecular Weight and Absorption of Conjugated Polymers
λabs,max (nm)
chlorobenzene (10 μM) solution
solid film
b
Mn
PDI
d
P1
P2
P3
P4
P5
P6
38 000
2.47
3.32
3.08
3.28
2.15
2.62
406 (388)
533
522
530
538
550
550
574
423
419
447
370
373
430
543
545
543
618
642
584
587
582
c
e
8 000
396 (388)
424 (396)
368 (388)
378 (388)
409 (396)
560
39 000
19 000
14 000
18 000
570
e
e
e
e
656, 712
646, 712
666, 712
658
e
718
e
631
683
a
b
Calibrated with polystyrene standards, using THF as the eluent at a constant flow rate of 1.0 mL/min at 40 °C. Polydispersity index = Mw/Mn.
c
d
e
Partially soluble in THF. Values in parentheses are data of corresponding monomers M1−M3. Shoulder.
and M3 were synthesized through a series of Grignard
metathesis and bromination reaction starting from 3-
bromothiophene while M2 monomer was started from
thiophene. Because the hydrogen on the 2-position of the
thiophene ring is more reactive, it could easily be reacted with a
strong base such as n-butyl lithium to form the strong
nucleophile 2-lithium thiophene and then substituted by the
bromide on the 1-bromohexane to yield 2-hexylthiophene.
After the bromination of 2-hexylthiophene with N-bromosucci-
nimide, the 2-bromo-5-hexylthiophene was obtained with high
yield and then coupled with (3-hexylthiophen-2-yl)magnesium
bromide under nickel catalysis to get the compound 10. In
addition, the BDT core was prepared according to the literature
with some modification.16 As shown in Scheme S1, Supporting
Information, by attaching the bromine group on the BDT core
from route 2 in advance, the as-prepared monomers after
attaching the oligothiophene groups would be ready to be
polymerized without further modification. Figures S1−S22
illustrate the NMR spectra of the synthesized compounds and
monomers, suggesting the successful preparation of the target
alkylthienyl-substituted BDT monomers.
weight-average molecular weights (Mn) and polydispersity
index (PDI) in the range of 8000−39000 Da and 2.15−3.32,
respectively, relative to polystyrene standards (Table 1). The
bithiophene (BT)-based polymers P1, P2, and P3 were
obtained as dark red solid while BTD-based polymers P4,
P5, and P6 powder are black in color. The structures of these
polymers were verified by NMR spectroscopy and typical set of
1H NMR spectra were showed in Figures S23−S28.
Considering the side chain architecture, P1 and P4 have
bithienyl moiety with six alkyl chains while there is only four
alkyl chains for bithienyl-substituted P2 and P5. In contrast, P3
and P6 with the terthienyl groups have eight alkyl chains
(Scheme 1). The comparative studies between these polymers
allow us to better understand the effect of molecular structural
on electronic, optical and crystalline properties of the as-
prepared polymers. Furthermore, the alternating electron-
deficient (acceptor) and electron-rich (donor) backbone
coupled with tunable side chain architecture allows optimizing
overall optical and crystalline properties.
Absorption and Photoluminescence. The conjugated
polymer chains have high affinity though π−π interaction,
which leads to the formation of aggregated domains
(crystallites) even in highly diluted solution. Realizing the
factors that affect the polymer aggregation during the early
stages in forming the solid films will provide insights in
designing polymer structures with enhanced crystallinity, hence
charge transport property. Herein, we measure the optical
Polymer Synthesis. Six conjugated polymers with two
building blocks as backbone and different conjugated side
chains are prepared via Pd-catalyzed stille coupling and Suzuki
coupling with distannyl compound and boronic monomer,
respectively (Scheme 2). All polymerization reactions pro-
ceeded smoothly. The obtained conjugated polymers had
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Figure 1. Normalized absorption spectra of polymers and the corresponding monomer M1 in chlorobenzene solution (10 μM).
Figure 2. Normalized photoluminescence (PL) spectra of polymers (a) P1, (b) P5, and their corresponding monomers (10 μM).
properties of the polymers by UV−vis absorption and
photoluminescence (PL) spectroscopy, and the absorption
data was summarized in Table 1. Figures 1 and S29 depict the
UV−vis absorption of these two-dimensional polymers. In the
case of chlorobenzene solutions (concentration: 10 μM), all the
polymers showed broad visible absorption with distinct bands
in the low and high-energy ranges. The high-energy absorption
could be attributed to the corresponding monomers (M1, M2,
and M3) while the low-energy region is resulting from the
polymer backbone. As shown in Figure 1, parts c and d, the
bathochromic shift of polymers P3/P6 in the high-energy
region compared with the rest of the polymers was ascribed to
the extension of conjugated length by introducing an extra
alkylthienyl units as side chain. On the other hand, the
absorbance in low-energy band resulting from donor−acceptor
characteristic does not change much for BT-based polymers P1,
P2, P3 (λonset ∼ 640 nm) and BTD-based polymers P4, P5, P6
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Figure 3. Normalized temperature-dependence absorption (upper) and PL (bottom) spectra of (a) (c) P1 and (b) (d) P5 in chlorobenzene
solution (10 μM). The shoulder band in P1 and P5 decreases profoundly (gray arrows) with increasing temperature. Illustation shows the heat
provides the needed energy to break the aggregates to form isolated chains.
(λonset ∼ 790 nm), respectively, because they share the same
polymer backbone structure.
The photoluminescence (PL) spectra of the polymers have
been used as a probe to better understand their optical and
electronic properties with respect to the polymer structure. PL
spectra of polymers in diluted chlorobenzene solutions (10
μM) are shown in Figure 2 and S30. Two different excited
wavelengths corresponding to the polymeric backbone
absorption and their side chain counterpart were used. Notably,
the PL spectra of the polymers show no trace of monomer
emission except P6, which reveals a weak intensity at around
530 nm, reminiscent the emission of the monomer. It was
evident that the energy-transfer from the side chain to main
chain occurs while the excited wavelength corresponding to the
absorption of the conjugated side chain was applied.
In parts a and b of Figure 1, the red-shifted absorbance onset
by ∼10 nm in solid film state relative to those of the solutions
could be attributed to the enhanced interchain packing in the
solid state. Similar phenomenon is also found in other polymers
(Figure S29). Moreover, the smaller optical band gap of the
BTD-based polymers P4, P5, P6 (∼1.60 eV) as oppose to BT-
based polymers P1, P2, P3 (∼1.95 eV) is mainly due to the
electron-deficient acceptor BTD unit, which coupled with
electron-rich BDT core and resulted in a charge transfer
characteristic accompanied by increased persistent length of the
polymer backbone and lowered energy band gap.
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Figure 4. Normalized absorption spectra and XRD pattern of the polymer P1 and P5 thin films before and after annealed at 120 °C for 30 min.
Illustration shows interchain distance in P1 and π−π stacking in P5.
According to the above results, the effective conjugation
length and bulkiness of the side chain both impact the physical,
optical, and electronic properties of the polymers. Moreover,
the shoulders of absorption and PL suggesting a stronger
interchain interaction are well manifested in the temperature-
dependent spectra.
Temperature-Dependent Optical Properties. System-
atic spectroscopic studies were carried out to probe the
presence of polymer aggregates in chlorobenzene by raising the
solution temperature to dissolve the crystallites. According to
the paper by Chu et al,17 higher temperatures would provide
the energy needed to increase molecular motion that results in
breaking the aggregates to form isolated chains. In view of this
work, P1 and P5 were used as our standard due to the high
pronounced aggregated peaks. The temperature-dependent
absorption spectra are shown in Figure 3a and 3b. As the
temperature increased from 20 to 100 °C for P1, the
absorbance peak at 530 nm originated from isolated chains
was hypsochromic shifted and the shoulder at 570 nm, which
was attributed to the crystalline peak18 gradually disappeared,
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indicating the formation of isolated polymer chain from
aggregated state. Similar phenomenon was also observed in
P5 as the intensity of shoulder from 610 to 800 nm diminished
with a concomitant increase of the absorption band at 560 nm
(isolated chain). Moreover, the red shift of the absorption
maximum is typical for the formation of J-aggregates formed at
low temperature.19
The temperature-dependent PL spectra were also measured
and the results are summarized in Figure 3c and 3d. The PL
peaks at 625 and 760 nm for P1 and P5 gradually decreased as
the temperature increases, respectively; meanwhile, the
concomitant decrease in the intensity of the shoulder band in
absorption spectrum suggest that the peaks are originated from
the polymer aggregates and the remaining peaks at 560 and 650
nm are originated from the single chain species of P1 and P5,
respectively. It is important to note that lacking of these
shoulder bands in the absorption and PL spectra for the rest of
the polymers (P2, P3, P4, and P6) suggests the presence of
aggregates (crystallites) in P1 and P5, but not in the other
polymers. (UV−vis and PL spectra of P2, P3, P4, and P6 are
shown in Figure S29 and S30, respectively)
Table 2. XRD Results of Conjugated Polymers
lamella (Å)
π−π (Å)
P1
22.5
23.7
21.7
24.5
21.1
23.3
22.6
23.9
23.2
24.5
3.65
3.65
3.67
3.70
3.96
3.96
3.71
3.60
4.00
4.00
P1 annealed
P2
P3
P4
P4 annealed
P5
P5 annealed
P6
P6 annealed
consistent with the absorbance change in P1 via thermal
annealing. In contrast, the π−π stacking distance of P5 exhibits
shortened stacking distance from 3.71 to 3.60 Å after thermal
annealing, suggesting that the more planar chain conformation
is formed with lesser alkyl chain density, thus results in an
ordered packing structure and improved absorption properties.
XRD results before and after annealing are consistent with the
UV−vis absorbance shoulder bands of P1 and P5 in Figure 4,
parts a and b, respectively. P1 shoulder band is slightly
decreased after annealing as oppose to the increase in the
shoulder band for P5 after annealing. Moreover, conceptual
illustration of P3 and P6 assembles are shown in Figure 5,
revealing a stronger π−π stacking interaction ability and
coplanarilty of BT-based P3 (3.70 Å) between polymer chains
facilitate charge transport as oppose to BTD-based P6 (4.0 Å).
The thermal properties of these polymers were examined by
DSC and are summarized in Figure S33−S38, P1, P4, and P5
revealed clear glass transition or crystallization temperature.
Furthermore, the smaller π−π stacking distance for P1 and P5
suggesting that the more planar chain conformation is formed
thus would result in an ordered packing structure and higher
OPV properties.
Electrochemistry. It is crucial to obtain the electronic
structure of the conjugated polymers as they have implications
toward electronic and photovoltaic devices. The electro-
chemical properties of these polymers were investigated by
cyclic voltammetry (CV) and the results are shown in Figure 6.
All the polymers exhibit clear oxidation and reduction couples
at the onset potential (Eonset) of around 0.96 to 1.05 V and
−0.82 to −1.23 V, respectively. The redox potentials of the
polymers and their respective HOMO and LUMO (versus
vacuum) are calculated and summarized in Table 3. The similar
HOMO levels for all polymers implying that the HOMO is
basically localized on the donor BDT cores. In addition, the
LUMO levels of the BTD-based polymers P4−P6 are laying
around −3.85 eV, a 0.4 eV drop in comparison with the BT-
based polymers P1−P3. The above results suggest that the
electron affinity of the polymers was increased by the
incorporation of the electron-deficient unit, leading to a lower
LUMO level. Besides, the smaller optical band gaps of BTD-
based polymers P4−P6 could be attributed to the donor−
acceptor ability originated from the intrachain charge transfer
character and the straightened backbone structure. On the
other hand, the oxidation Eonset of terthienyl-substituted
polymers (P3 or P6) is ∼1.0 V lower than bithiophene-
substituted polymers (P1 or P4) presumably due to their
extended conjugation.
In view of the above results, the shoulder band whether in
absorption or emission spectra are directly relates to the
polymer aggregates with strong interchain π−π stacking.
Temperature-dependent UV−vis and PL spectra further
validate the presence of these aggregates. The above results
suggest that the side chain geometry, number of alkyl chains,
and the polymer backbone structures have a huge impact on
the interchain stacking and lamellae packing.
Thermal Annealing. Previous study by Hollinger et al
insinuates vibronic shoulder band of P3HT becomes more
pronounced as interchain π−π interaction is enhanced by heat
treatment.18 We perform thermal annealing in our polymers
following the same procedure and the results are shown in
Figure 4, S31, and S32. It is particularly interesting to find that
annealing of BTD-based polymer P5 for 30 min results in
increase in the vibronic shoulder band, implying that the
annealing process provide the energy for polymer chain to
reorganize into a more ordered structure due to enhanced
interchain stacking. In contrast, the structural order of BT-
based polymer P1 seems to be sabotaged by the thermal
annealing as the decreased absorbance intensity at 584 nm.
In order to further realize the annealing induced structural
change, wide-angle X-ray diffraction was used to investigate the
crystal structure before and after thermal treatment. The XRD
spectra of polymer thin films were measured by drop-casting
their corresponding solution on quartz glasses. Parts c and d of
Figure 4 show the XRD spectra of P1 and P5, respectively. The
d-spacing are obtained by Bragg’s equation, nλ = 2dhkl sin θ, in
which λ is the wavelength of radiation, dhkl is the specific lattice
spacing and θ is the diffraction angle. The X-ray diffraction
reveals both the π−π stacking distance in higher angle region
and the interchain lamellae packing spacing in the low angle
region, respectively, which were summarized in Table 2. As can
be seen, d-spacing in the XRD, the lamellae spacing and π−π
stacking distance of bithiophene-substituted polymers are
smaller than terthienyl-substituted polymers, suggesting that
the shorter oligothiophene side chain with lesser alkyl chains
facilitate a ordered packing structure. The lamellae spacing at
22.5 Å of P1 film increased to 23.7 Å upon thermal treatment,
while no change in π−π stacking distance, indicating thermal
annealing resulted in negligible impact on the interchain
packing but increased the lattice spacing. The above result was
Solar Cell Performance. All the polymers were used to
fabricate OPV devices by blending with PC71BM as an active
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Figure 5. Conceptual illustration of (a) P3 and (b) P6 assemble via π−π stacking.
separation (Figure S41).21 The effect of additive and thermal
annealing using P5 as representative have been studied and
results are summarized in Figure S42 and Table S1, revealing
that thermal annealing and additive do not enhance the OPV
performance. OPV devices using all polymers:PC71BM were
fabricated using the optimized weight ratio of 1:1.5. The J−V
curves and EQE for all polymers are shown in Figure 7. Devices
fabricated by polymers with lower band gap (P4−P6) showed
an extended absorption up to 800 nm as opposed to polymers
(P1-P3) with wider band gap that absorb from 300 to 650 nm.
The EQE for P1 shows a higher quantum efficiency of 72% at
480 nm significantly better than P2 (48%) and P3 (44%) with
similar band gaps. EQE for P4, P5, and P6 is 34%, 47%, and
41%, respectively. The highest EQE for P1 is attributed to the
most densely packed structure, as manifested by the small
lamellae (22.5 Å) and π−π stacking (3.65 Å) d-spacings, see
Table 2.
Moreover, all the polymers show a good Voc around 0.8−0.9
V, due to the deep HOMO level of the donor materials
determined by CV except for P2. This is most likely due to the
poor solubility in common organic solvent, the low quality of
the P2 film results in some defects, pinholes, and subsequently
lower the Voc. Comparing the BT-based P1−P3 with different
conjugated side chains varying from bithiophene to terthio-
phene, the device fabricated with P1 showed a highest PCE,
EQE, short circuit current (Jsc), and fill factor (FF) compared
to P2 and P3, which is consistent with the stronger interchain
π−π stacking interaction observed by UV−vis absorption
shoulder at the band edge at 650 nm and PL spectra. Similar
phenomena were also observed for P5 in BTD-based polymers.
The primary contribution for the enhanced device performance
of P5 is attributed to the higher degree of crystallinity as
manifested by the sharp shoulder in UV−vis absorption and
smaller π−π stacking distance in XRD spectra.22 The inferior
photovoltaic properties for P3 and P6 are believed to be
resulting from the less ordered polymer interchain packing that
was interrupted by the longer conjugation side chains.
Figure 6. Cyclic voltammetric diagrams of polymers P1−P6 in 0.1 M
TBAP at a scan rate of 100 mV/s. The inset figures show the enlarged
reduction diagrams for P4−P6.
layer; the light J−V characteristics are summarized in Table 4.
1,2-Dichlorobenzene (o-DCB) was chosen as the carrier solvent
for all polymers as its relatively high boiling point allows slow
drying of the polymer film to render OPV devices with a high
solar cell efficiency (Figure S39).20 The optimized donor
(polymer) acceptor (PC71BM) ratio was initially determined to
be 1:1.5 by varying the acceptor weight percentage when
polymer concentration was kept constant. As shown in Figure
S40, the device light current density−voltage (J-V curve) and
external quantum efficiency (EQE) for P5:PC71BM (1:x, x =
0.5−2.0) increased from 5.48 to 6.77 mA/cm2 and 35% to 57%,
respectively, with increasing PC71BM weight ratio up to 50%
and then dropped for higher loadings due to the D/A phase
Hole mobility are measured by space-charge-limited-current
method using the following equation:23a,b
9
8
JL3 = εrε0μ (V − V )2
app
bi
h
Here J is the current density; Vapp is applied bias, Vbi is the build
in voltage obtained from the electrode work function difference.
L is the film thickness; ε0 is the vacuum permittivity; εr is
estimated to be 3 for all conjugated polymers. We measure the
hole current under dark and plotted the (JL3)0.5 as a function of
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Table 3. Redox Potentials and Energy Levels of Conjugated Polymers
a
redox potentials (V)
energy levels (eV)
b
c
d
e
f
code
λonset
Eox,onset
Ered,onset
Egopt
Egec
HOMO
LUMOopt
LUMOec
P1
P2
P3
P4
P5
P6
650
631
638
780
790
786
1.04
0.97
0.96
1.05
0.98
0.99
−1.23
−1.14
−1.17
−0.82
−1.14
1.91
1.97
1.94
1.59
1.57
1.58
2.27
2.11
2.13
1.87
2.12
1.95
−5.47
−5.40
−5.39
−5.48
−5.41
−5.42
−3.56
−3.43
−3.45
−3.89
−3.84
−3.84
3.20
3.29
3.26
3.61
3.29
3.47
−0.96
a
b
From cyclic votammograms vs Ag/AgCl in CH3CN. The data were calculated from polymer films by the equation: Egopt = 1240/λonset (energy gap
c
d
between HOMO and LUMO). Egec, electrochemical band gap is derived from the difference between Eox,onset and Ered,onset. The HOMO energy
e
levels were calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV ; onset = 0.37 V). The LUMO energy levels were
f
calculated from HOMO − Egopt. The LUMO energy levels were calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV ; onset
= 0.37 V).
a
Table 4. Photovoltaic Properties of Solar Cells under AM 1.5 Illumination and Mobility
polymer
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
peak PCE (%)
hole mobility (cm2 × V−1 × S−1
)
P1
P2
P3
P4
P5
P6
0.822
0.683
0.832
0.896
0.804
0.850
9.12
4.88
6.15
6.61
6.77
5.47
54.61
40.53
49.29
41.18
60.40
35.62
4.10
1.33
2.53
2.44
3.30
1.66
4.25
1.46
2.68
2.45
3.68
1.72
1.08 × 10−4
3.60 × 10−5
1.30 × 10−5
3.40 × 10−4
5.85 × 10−4
2.40 × 10−4
a
Devices were fabricated by polymers: PC71BM at weight ratio of 1:1.5 using o-DCB as solvent.
Figure 7. (a) J−V curves and (b) EQE of OPV devices fabricated using polymers P1−P6 and PC71BM at weight ratio of 1:1.5 under 100 mW/cm2
illumination with AM 1.5 filter.
applied bias (Vapp) subtracted by the build in voltage (Vbi). The
obtained curve can be fitted linearly and the slope the prefactor
(9εrε0μh/8) that can be used for hole mobility calculation. The
calculated hole mobility are comparable to literature reported
values.23a,c Comparing the hole mobility for all the polymers,
P4−P6 generally show higher mobility than P1−P3 group,
which is due to a more planar backbone structure. In addition,
polymers P1 and P5 both exhibit highest mobility among the
BT-based and BTD-based polymers, respectively, consistent
with their higher PCE. Overall, the high Jsc (9.12 mA/cm2), FF
(54.61%) for P1 and Jsc (6.77 mA/cm2), FF (60.40%) for P5
are attributed to the proper band gaps and high degree of
crystallinity due to favorable π−π stacking between polymer
chains, both factors lead to the highest PCE of 4.25% among
the bithienyl-substituted benzodithiophene-based OPV devices.
CONCLUSIONS
■
In summary, six conjugated polymers consist of benzo[1,2-
b:4,5-b′]dithiophene−bithiophene as building blocks in the
main chain were synthesized by coupling polymerization. By
directly attaching three kinds of alkylthienyl side chains to the
conjugated main chain, the resulted two-dimensional polymers
revealed a broader absorption range from 300 to 800 nm.
Moreover, the enhanced conjugation of the side chain, from
bithiophene to terthiophene, leads to the interchain disorder
and reduced π−π stacking. Temperature-dependent absorb-
ance, emission spectra, and thermal annealing further verify that
the shoulder band(s) were originated from the aggregated
(crystalline) species of polymers. The as-synthesized donor−
acceptor polymers, from P1 to P6, reveal strong correlation
between the side chain geometries, backbone structures and the
crystallinity (carrier mobility). Our study also showed OPV
device performance consistent with what was predicted based
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extracted with 150 mL of diethyl ether. The collected organic phase
was washed several times with brine to remove the DMF, dried with
MgSO4, and evaporated under reduced pressure to get the crude
product as yellowish oil. The crude product was purified by silica gel
column chromatography with hexane as eluent to yield 3.46 g of
on the chain packing and crystallinity; bithienyl-substituted
benzodithiophene-based OPVs, P1, has a highest peak PCE of
4.25% under AM 1.5 100 mW/cm2 illumination. Our results
offer insights in designing two-dimensional polymers with
donor−acceptor backbone to enhance absorption overlaps with
solar emission, and side chain architecture allowing improved
solubility and crystallinity to promote PCE in OPV devices.
1
yellow sticky oil. (95% in yield). H NMR (CDCl3, δ, ppm): 6.75 (s,
1H), 2.65 (t, 2H), 2.44 (t, 2H), 1.57 (m, 2H), 1.52 (m, 2H), 1.30 (m,
12H), 0.89 (t, 6H). 13C NMR (CDCl3, δ, ppm): 140.22, 138.65,
131.07, 31.84, 31.74, 30.80, 29.21, 29.04, 28.30, 28.00, 22.76, 22.72,
14.22.
EXPERIMENTAL SECTION
■
3′,4′,5′-Trihexyl-2,2′-bithiophene (5). First, 1.92 g (7.77 mmol) of
2-bromo-3-hexylthiophene and 0.38 g (0.015 mol) of magnesium were
put into a two-necked rounded flask equipped with condenser. After
flushing with N2 for several minutes, 60 mL of dry ether was added,
followed by stirring at 45 °C for 2 h. The as-prepared Grignard reagent
was injected into a solution of 2.51 g (7.58 mmol) 2-bromo-4,5-
dihexylthiophene and 0.041 g (0.07 mmol) of Ni(dppp)Cl2 in 50 mL
of diethyl ether. After being stirred for 12 h, the reaction was quenched
by saturated ammonium chloride aqueous solution and extracted with
100 mL of ether. The organic phase was washed by brine, dried with
MgSO4, and evaporated under reduced pressure to get the crude
product. Finally, the yellow oil was obtained by being purified though
silica gel column chromatography with hexane as eluent. (1.90 g, 60%
in yield). 1H NMR (CDCl3, δ, ppm): 7.11 (d, 1H), 6.90 (d, 1H), 6.83
(s, 1H), 2.73 (m, 4H), 2.50 (t, 2H), 1.54−1.67 (m, 6H), 1.28−1.37
(m, 18H), 0.85−0.97 (m, 9H). 13C NMR (CDCl3, δ, ppm): 139.31,
138.88, 138.29, 131.72, 131.54, 130.00, 127.74, 122.98, 31.95, 31.89,
31.82, 31.78, 30.90, 30.83, 29.37, 29.33, 29.19, 28.38, 27.98, 22.77,
14.23.
Materials. 5,5′-Bis(trimethylstannyl)-2,2′-bithiophene, 2,3-dihex-
ylthiophene (3), thiophene-3-carbonyl chloride (11),16 and N,N-
diethylthiophene carboxamide (12)16 were synthesized according to
the literatures. 4,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
benzo[c][1,2,5]thiadiazole was directly purchased from TCI America.
All other reactants, catalysts, and solvents were used as received from
commercial sources.
3-Hexylthiophene (1). A 5.96 g (0.25 mol) sample of magnesium
was placed into a 250 mL 2 neck-rounded flask equipped with a
condenser and after purging with N2 gas for 10 min, diethyl ether (50
mL) was added. Then, 20.2g (0.12 mol) of 1-bromohexane was
injected into the solution at 0 °C. After the vigorous reaction, the
temperature was increased to 50 °C and stirred for another 1 h. The
as-prepared Grignard reagent was then transferred into a well-stirred
60 mL diethyl ether solution containing 10 g (0.06 mol) of 3-
bromothiophene and 0.3324 g (0.6 mmol) of dichloro(1,3-bis-
(diphenylphosphino)propane)nickel [Ni(dppp)Cl2] catalyst at 0 °C
under N2 protection. After 30 min, the ice bath was removed and the
solution was kept stirring overnight. When the reaction was finished, it
was then quenched by 100 mL of NH4Cl(aq) and extracted with 100
mL of ether. The organic phase was collected and washed several times
with brine. After dried over MgSO4, the solvent was evaporated under
reduced pressure. Finally, the crude product was purified through SiO2
column chromatography by using hexane as eluent. (8.26 g, 83% in
5-Bromo-3′,4′,5′-trihexyl-2,2′-bithiophene (6). To a solution of
2.60 g (6.2 mmol) 3′,4′,5′-trihexyl-2,2′-bithiophene in 30 mL DMF
was added 1.10 g (6.2 mmol) of N-bromosuccinimide, followed by
stirring at room temperature for 5 h. The solution was poured into 100
mL of brine and extracted with 150 mL of diethyl ether. The organic
phase was washed several times with brine to remove the DMF, dried
with MgSO4, and evaporated under reduced pressure to get the crude
product as yellowish oil. The crude product was purified by silica gel
column chromatography with hexane as eluent to yield 2.86 g of
1
yield). H NMR (CDCl3, δ, ppm): 7.21 (m, 1H), 6.90 (m, 2H), 2.61
(t, 2H), 1.61 (m, 2H), 1.29 (m, 6H), 0.87 (t, 3H). 13C NMR (CDCl3,
δ, ppm): 143.2, 128.2, 125.0, 119.7, 31.7, 30.6, 30.3, 29.0, 22.6, 14.1.
2-Bromo-3-hexylthiophene (2). After 8.04 g (0.048 mol) of 3-
hexylthiophene was dissolved in 50 mL of N,N-dimethylmethanamide
(DMF), 8.50 g (0.048 mol) of N-bromosuccinimide was added and
the reaction was kept for 3 h. After that, 100 mL of water was added
and the mixture was extracted with 100 mL of diethyl ether. The
organic phase was collected and washed several times with brine. After
dried over MgSO4, the solvent was evaporated under reduced pressure.
Finally, the crude product was purified through SiO2 column
1
yellow sticky oil. (93% in yield). H NMR (CDCl3, δ, ppm): 6.86 (s,
1H), 6.76 (s, 1H), 2.64−2.73 (m, 4H), 2.50 (t, 2H), 1.52−1.66 (m,
6H), 1.27−1.41 (m, 18H), 0.91 (m, 9H).
3,4′,4″,5″-Tetrahexyl-2,2′:5′,2″-terthiophene (7). First, 1.26 g (5.1
mmol) of 2-bromo-3-hexylthiophene and 0.25 g (0.01 mol) of
magnesium were put into a two-necked rounded flask equipped with a
condenser, after flushing with N2 for several minutes, 60 mL of dry
ether was added, followed by stirring at 45 °C for 2 h. The as-prepared
Grignard reagent was injected into a solution of 2.48 g (4.98 mmol) of
5-bromo-3′,4′,5′-trihexyl-2,2′-bithiophene and 0.027 g (0.05 mmol) of
Ni(dppp)Cl2 in 50 mL of diethyl ether. After being stirred for 12 h, the
reaction was quenched by slowly adding saturated ammonium chloride
solution and extracted with 100 mL ether. The organic phase was
washed by brine, dried with MgSO4, and evaporated under reduced
pressure to get the crude product. Finally, the yellow oil was obtained
by purified though silica gel column chromatography with hexane as
1
chromatography using hexane as eluent. (8.90 g, 95% in yield). H
NMR (CDCl3, δ, ppm): 7.19 (d, 1H), 6.80 (d, 1H), 2.57 (t, 2H), 1.56
(m, 2H), 1.34 (m, 6H), 0.90 (t, 3H).
2,3-Dihexylthiophene (3). First, 1.60 g (0.066 mol) of magnesium
was placed into a 250 mL two-necked rounded flask equipped with a
condenser, after flushing with N2 for few minutes, 50 mL of dry ether
and 5.48 g (0.033 mol) of 1-bromohexane was added, followed by
stirring for 1 h at 45 °C. To a stirred solution 4.06 g (0.016 mol) of 2-
bromo-3-hexylthiophene and 0.086 g (0.16 mmol) of Ni(dppp)Cl2 in
50 mL of ether was added to the as-prepared Grignard reagent in one
portion under ice/water bath. The reaction was quenched by saturated
ammonium chloride solution and extracted with 100 mL ether. The
organic layer then was washed several times with brine, dried with
anhydrous MgSO4, and evaporated under reduced pressure to get
crude product as brown oil. The crude product was purified by silica
gel column chromatography with hexane as eluent to obtain 2.97 g of
colorless sticky oil. (65% in yield). 1H NMR (CDCl3, δ, ppm): 7.03 (d,
1H), 6.82 (d, 1H), 2.73 (t, 2H), 2.52 (t, 2H), 1.62 (m, 2H), 1.56 (m,
2H) 1.35 (m, 12H), 0.90 (t, 6H). 13C NMR (CDCl3, δ, ppm): 138.95,
137.85, 128.83, 121.02, 32.08, 31.90, 31.79, 30.99, 29.36, 29.19, 28.37,
27.93, 22.79, 22.75, 14.23.
1
eluent. (1.58 g, 60% in yield). H NMR (CDCl3, δ, ppm): 7.14 (d,
1H), 6.93 (d, 1H), 6.91 (s, 1H), 6.85 (s, 1H), 2.71−2.80 (m, 6H), 2.51
(t, 2H), 1.54−1.68 (m, 8H), 1.28−1.35 (m, 24H), 0.89−0.91 (m,
12H). 13C NMR (CDCl3, δ, ppm): 139.52, 139.46, 139.01, 138.38,
133.43, 131.51, 131.45, 130.93, 130.18, 128.79, 127.61, 123.48, 31.95,
31.90, 31.83, 31.79, 30.91, 30.81, 30.71, 29.42, 29.38, 29.34, 29.19,
28.40, 28.02, 22.79, 22.75, 14.24.
2-Hexylthiophene (8). First, 74 mL of 1.6 M n-BuLi in hexane was
added dropwise into a stirring solution of 10 g (0.12 mol) of 2-
bromothiophene in tetrahydrofuran (THF) at 0 °C for 1 h. Then,
20.32 g (0.12 mol) of 1-bromohexane was injected via syringe in one
portion, and the reaction was then allowed warm to room temperature
and kept stirring overnight. The reaction was quenched by pouring
into 100 mL of brine and extracted with 100 mL of diethyl ether. The
collected organic layer was washed several times with brine, dried over
MgSO4, and evaporated under reduced pressure. The crude product
2-Bromo-4,5-dihexylthiophene (4). To a solution of 2.78 g (0.011
mol) of 2,3-dihexylthiophene in 30 mL of DMF, 1.95 g (0.11 mol) of
N-bromosuccinimide was added slowly, followed by stirring for 5 h at
room temperature. The solution was pouring into 100 mL brine and
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1
was purified by distillation under vacuum. (16.16g, 80% in yield). H
NMR (CDCl3, δ, ppm): 7.11 (m, 1H), 6.92−6.94 (m, 1H), 6.79−6.80
(m, 1H), 2.84 (t, 2H), 1.66−1.73 (m, 2H), 1.28−1.40 (m, 6H), 0.91−
0.92 (m, 3H).
then lowered the temperature to −78 °C, 17.6 mL of n-BuLi was then
added dropwisely into the solution. The mixture was allowed warm to
room temperature, and kept stirring for another 2 h. The reaction was
then quenched with saturated ammonium chloride solution, and the
yellow precipitate was obtained. Collect the filtrate and wash several
times with water, dry under vacuum. The final product was obtained
by recrystallization from THF. (3.18 g, 30% in yield). 1H NMR
(CDCl3, δ, ppm): 7.57 (s, 1H). 13C NMR (CDCl3, δ, ppm): 172.37,
144.91, 142.55, 129.32, 123.94.
2-Bromo-5-hexylthiophene (9). First, 4.29 g (0.025 mol) of 2-
hexylthiophene was dissolved in 50 mL of glacial acetic acid, and 4.54
g (0.025 mol) of N-bromosuccinimide was added slowly into the
solution. After 2 h of stirring, the solution was poured into saturated
sodium hydroxide solution under ice-cooling and extracted by ether
three times. The organic phase was washed by water several times until
pH = 7 and then dried over anhydrous MgSO4. After the solvent was
removed under vacuum, the crude product was purified by silica gel
column chromatography using hexane as eluent. (5.56 g, 90% in yield).
1H NMR (CDCl3, δ, ppm): 6.85 (d, 1H), 6.54 (d, 1H), 2.75 (t, 2H),
1.60−1.67 (m, 2H), 1.28−1.38 (m, 6H), 0.88−0.91 (m, 3H).
3,5′-Dihexyl-2,2′-bithiophene (10). First, 4.58 g (0.019 mol) of 2-
bromo-3-hexylthiophene and 0.90 g (0.037 mol) of magnesium
turnings were placed into a two-necked rounded flask equipped with
condenser. After flushing with N2 for several minutes, 50 mL dry
diethyl ether was added, and then the reaction was heated to reflux for
2 h. In the meanwhile, 4.12 g (0.017 mol) of 2-bromo-5-
hexylthiophene and 0.09 g (0.17 mmol) of catalyst Ni(dppp)Cl2 was
homogeneously mixed with 50 mL dry ether under N2 protection.
Then the as-prepared Grignard reagent was then purged into the
above solution at 0 °C. After being stirred overnight, the reactant was
quenched with 100 mL of saturated ammonium chloride solution and
extracted with 100 mL diethyl ether. The organic extraction was
washed several time with water and dried with MgSO4. After the
solvent was removed under vacuum, the light yellow oil was obtained
by purified with silica gel column chromatography using hexane as
Synthesis of M1, M2, and M3. Firts, 2.1 equiv of oligothiophenes
(compound 5, 7, or 10) was dissolved in 40 mL of dry THF, and then
2.1 equiv of n-BuLi was added dropwisely to the solution at 0 °C. After
addition, the mixture was heated to 60 °C for 2 h, followed by addition
of compound 14. The reaction was kept stirring at 60 °C. After 2 h,
the reaction was cooled to room temperature, and 4 equiv of tin
chloride in 30 mL of 10% HCl(aq) was added. Then, the mixture was
kept stirring overnight. After completion, the reactant was poured into
100 mL of water and extracted with 150 mL of diethyl ether. The
organic layer was collected and dried over anhydrous MgSO4. After
removing the solvent, the crude product was purified though SiO2
column chromatography by using hexane as eluent. Yellow sticky oil
was obtained.
M1: 1H NMR (CDCl3, δ, ppm): 7.71 (s, 1H), 7.25 (s, 1H), 6.96 (s,
1H), 2.86 (t, 2H), 2.77 (t, 2H), 2.55 (t, 2H), 1.59−1.77 (m, 6H),
1.37−1.47 (m, 18H), 0.85−0.98 (m, 9H). 13C NMR (CDCl3, δ, ppm):
140.27, 140.09, 139.16, 138.57, 135.97, 135.51, 133.43, 131.25, 130.99,
128.15, 126.25, 122.24, 117.11, 31.96, 31.91, 31.87, 31.80, 30.95,
30.74, 29.50, 29.40, 29.36, 29.20, 28.41, 28.05, 22.86, 22.81, 22.77,
14.27. (47% in yield).
M2: 1H NMR (CDCl3, δ, ppm): 7.74 (s, 1H), 7.27 (s, 1H), 7.05 (d,
1H), 6.79 (d, 1H), 2.88 (t, 4H), 1.73−1.81 (m, 4H), 1.37−1.49 (m,
12H), 0.85−0.99 (m, 6H). 13C NMR (CDCl3, δ, ppm): 146.78,
140.20, 139.34, 135.88, 135.78, 133.13, 132.92, 131.16, 126.17, 126.08,
124.59, 122.12, 117.15, 31.86, 31.72, 30.74, 30.30, 29.45, 29.39, 28.96,
22.85, 22.74, 14.29, 14.24. (43% in yield).
1
eluent. (3.53 g, 62% in yield). H NMR (CDCl3, δ, ppm): 7.13 (d,
1H), 6.90−6.93 (m, 2H), 6.73 (d, 1H), 2.74−2.83 (m, 4H), 1.61−1.73
(m, 4H), 1.28−1.41 (m, 12H), 0.88−0.93 (m, 6H).
Thiophene-3-carbonyl Chloride (11).16 A 10 g (0.078 mol) sample
of thiophene-3-carboxylic acid was suspended in 50 mL of methylene
chloride and cooled down to 0 °C. Then 19.81 g (0.156 mol) of oxalyl
chloride was injected into the solution in one portion. The reactant
was kept stirring overnight until the turbid mixture becomes clear.
After evaporating the solvent and unreacted oxalyl chloride under
reduced pressure, a colorless solid was gotten. And the product was
subjected to the next step without further purification.
M3: 1H NMR (CDCl3, δ, ppm): 7.74 (s, 1H), 7.29 (s, 1H), 7.07 (s,
1H), 6.93 (s, 1H), 2.92 (t, 2H), 2.77−2.84 (m, 4H), 2.56 (t, 2H),
1.63−1.82 (m, 8H), 1.44−1.53 (m, 24H), 0.85−1.01 (m, 12H). 13C
NMR (CDCl3, δ, ppm): 140.30, 139.82, 139.70, 139.18, 138.47,
135.99, 132.81, 132.54, 132.23, 131.41, 131.33, 129.21, 127.75, 126.21,
125.67, 122.17, 117.23, 31.95, 31.91, 31.88, 31.84, 31.80, 30.93, 30.72,
30.48, 29.86, 29.62, 29.48, 29.41, 29.36, 29.20, 28.41, 28.04, 22.86,
22.80, 22.76, 14.27. (53% in yield).
N,N-diethylthiophene Carboxamide (12).16 A 9.97 g (0.068 mol)
sample of thiophene-3-carbonyl chloride was dissolved in 100 mL
methylene chloride under ice cooling and the diethylamine was then
added dropwisely to the solution. After the addition, ice bath was
removed and kept stirring for another 1 h. Finally, the reaction was
quenched with 100 mL of water and extracted with 50 mL of
methylene chloride. The collected organic phase was washed several
times with brine and dried with MgSO4. After removing the solvent,
the crude product was purified by distillation under vacuum to yield
the colorless liquid. (9.97 g, 80% in yield). 1H NMR (CDCl3, δ, ppm):
7.45−7.48 (m, 1H), 7.28−7.31 (m, 1H), 7.18−7.20 (m, 1H), 3.27−
3.47 (m, 4H), 1.17−1.23 (m, 6H).
Polymer Synthesis. Synthesis of P1, P2, and P3 via Stille
Coupling Polymerization. One equivalent of BDT monomers (M1,
M2 and M3) and 5,5′-bis(trimethylstannyl)-2,2′-bithiophene were
dissolved in 50 mL of degassed dry toluene under nitrogen
atmosphere, then 0.5 mol % tetrakis(triphenylphosphine)palladium
[Pd(PPh3)4] was added. The reaction was heated to reflux and kept for
72 h. After reaction completed, the solution was poured into 100 mL
of methanol, and the precipitate was collected by filtration. The crude
polymer was washed with methanol and acetone in a Soxhlet apparatus
to remove the impurities. Finally, the polymer solid was washed with
chloroform. After removing the solvent, the polymer was obtained as
dark red solid.
2,5-Dibromo-N,N-diethylthiophene 3-Carboxamide (13). To a
solution of 9.75 g (0.053 mol) of N,N-diethylthiophene-3-carboxamide
in 30 mL of DMF, 19.88 g (0.112 mol) of N-bromosuccinimide was
added slowly, followed by stirring at room temperature for 5 h. The
solution was poured into 100 mL of brine and extracted with 150 mL
of diethyl ether. The collected organic phase was washed several times
with brine to remove the DMF, dried with MgSO4, and evaporated
under reduced pressure to get the crude product as yellowish oil. The
crude product was purified by silica gel column chromatography with
hexane: ethyl acetate = 4:1 as eluent to yield 17.05 g of yellow sticky
1
P1: H NMR (CDCl3, δ, ppm): 6.5−7.8 (m, br, 10H), 2.3−3 (m,
br, 12H), 1.2−1.9 (m, br, 48H), 0.7−1.2 (m, br, 18H). (64% in yield).
1
P2: H NMR (CDCl3, δ, ppm): 6.5−7.8 (m, br, 12H), 2.7−3 (br,
8H), 1.65−1.95 (br, 8H), 1.2−1.65 (m, br, 24H), 0.71−1.2 (m, br,
12H). (53% in yield).
1
P3: H NMR (CDCl3, δ, ppm): 6.5−7.8 (m, br, 12H), 2.4−3 (m,
br, 16H), 1.2−1.9 (m, br, 64H), 0.7−1.2 (br, 24H). (57% in yield).
Synthesis of P4, P5, and P6 by Suzuki Coupling Polymerization.
One equivalent of BDT monomer (M1, M2, and M3) and 4,7-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzo[c][1,2,5]-
thiadiazole was put into a two-necked rounded flask equipped with
condenser. The flask was purged with nitrogen for 10 min. At the same
time, 3 mol % palladium(II) acetate [Pd(OAc)2] and 6 mol %
tricyclohexylphosphine [P(Cy)3] were dissolved in 30 mL of degassed
toluene. The as-prepared solution was then purged into the two-
1
oil. (95% in yield). H NMR (CDCl3, δ, ppm): 6.89 (s, 1H), 3.54−
3.56 (m, 2H), 3.26−3.28 (m, 2H), 1.22−1.26 (m, 3H), 1.10−1.14 (m,
3H). 13C NMR (CDCl3, δ, ppm): 163.93, 138.85, 129.29, 112.73,
109.26, 43.10, 39.46, 14.55, 12.96.
2,6-Dibromobenzo[1,2-b;4,5-b′]dithiophene-4,8-dione (14). First,
9.59 g (0.028 mol) of compound 13 was put into a dried 250 mL flask.
After filling with inert gas, 50 mL of dry diethyl ether was injected and
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necked flask and then heated to reflux for 30 min, followed by addition
of 2.1 equiv of tetrabutylammonium hydroxide. The reaction was kept
stirring overnight. After the reaction was completed, the reactant was
poured into 100 mL methanol and the precipitate was collected by
filtration. The crude polymer was washed with methanol, hexane and
acetone in a Soxhlet apparatus to remove the impurities. Finally, the
polymer solid was washed by chloroform. After removing the solvent,
the polymer was obtained as a black solid.
ASSOCIATED CONTENT
* Supporting Information
NMR spectra of monomers and polymers, absorption spectra,
photoluminescence spectra, DSC traces, XRD patterns, J−V
curve, and EQE of polymers and a table of photovolataic
properties. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
1
P4: H NMR (CDCl3, δ, ppm): 6.5−9 (m, br, 8H), 2.4−3 (m, br,
12H), 1.2−2.1 (m, br, 48H), 0.7−1.2 (br, 18H). (50% in yield).
AUTHOR INFORMATION
Corresponding Author
*E-mail: hwang@lanl.gov (H.-L.W.).
Notes
■
1
P5: H NMR (CDCl3, δ, ppm): 6.5−9 (m, br, 10H), 2.7−3 (br,
8H), 1.2−2.1 (m, br, 32H), 0.7−1.2 (br, 12H). (62% in yield).
1
P6: H NMR (CDCl3, δ, ppm): 6.5−9 (m, br, 10H), 2.4−3.1 (m,
br, 16H), 1.2−2.1 (m, br, 64H), 0.7−1.2 (br, 24H). (49% in yield).
Fabrication of Solar Cell Devices. Patterned indium tin oxide
(ITO) covered glass slides were cleaned by distilled water, acetone,
and isopropyl alcohol in ultrasonication bath for 30 min, respectively,
and dried on a hot plate for 1 h under ambient condition. By treating
with oxygen plasma for 3 min after drying, the substrates were
transferred to a vacuum chamber that pumped down to 2 × 10−7 Torr
to deposite 5 nm molybdenum trioxide (MoO3; 99.98% trace metals
basis,) for hole transporting layer. The active layer was fabricated by
spin-coating a blend solution of 15.0 mg polymer (P1−P6) and 22.5
mg of PC71BM in 1.0 mL of o-dichlorobenzene (o-DCB) at 1500 rpm
onto MoO3 layer. Finally, the cathode of 20 nm calcium and 80 nm
aluminum were thermally deposited in the same vacuum system onto
the active layer through a shadow mask that formed an active area of 3
mm2.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge support of the Basic Energy Science (BES),
Materials Sciences and Engineering Division, Biomolecular
Materials Program, US Department of Energy and Los Alamos
National Laboratory (LANL) Directed Research and Develop-
ment Funds. Los Alamos National Laboratory is operated by
Los Alamos National Security, LLC, for the National Nuclear
Security Administration of the U.S. Department of Energy
under Contract DE-AC52-06NA25396. L.W. acknowledges
financial support from National Science Council of Taiwan
(NSC 102-2113-M-002-003-MY3).
Fabrication of Hole-Type Devices. ITO slides were pre coated
with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PE-
DOT:PSS) for high work function. The polymer solutions were
predissolved in o-DCB (20 mg/mL) and then spin-coated onto
PEDOT/ITO slides at 1500 rpm for 35 s. All the film thicknesses are
calibrated by profilometer for mobility calculation. Once the film dried,
all devices were transferred to a vacuum chamber for gold deposition
to complete device electrodes. The I−V curves are scanned by a
Keithley 2400.
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