An Indacenodithiophene-Quinoxaline Polymer Prepared by Direct Arylation Polymerization for Organic Photovoltaics

Article abstract of DOI:10.1021/acs.macromol.5b02324

Seeking preparation of high-performance donor-acceptor (D-A) polymers based on bare thiophene units in a more environmentally friendly and faster way, we have carried out a direct arylation polymerization (DAP) of two starting β-unprotected thiophene-cont

Full text of DOI:10.1021/acs.macromol.5b02324

Article
pubs.acs.org/Macromolecules
An Indacenodithiophene−Quinoxaline Polymer Prepared by Direct
Arylation Polymerization for Organic Photovoltaics
†
†
§
‡
,§
Shanshan Chen, Kyu Cheol Lee, Zhi-Guo Zhang, Dong Suk Kim, Yongfang Li,*
,†
and Changduk Yang*
†Department of Energy Engineering, School of Energy and Chemical Engineering, Low Dimensional Carbon Materials Center, Ulsan
National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea
§
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China
‡
KIER-UNIST Advanced Center for Energy, Korea Institute of Energy Research, Ulsan 689-798, South Korea
*
S Supporting Information
ABSTRACT: Seeking preparation of high-performance donor−acceptor (D−A)
polymers based on bare thiophene units in a more environmentally friendly and
faster way, we have carried out a direct arylation polymerization (DAP) of two
starting β-unprotected thiophene-containing monomers (indacenodithiophene
(
IDT) and thiophene−quinoxaline−thiophene (TQ)). Through modulating
DAP time and heating method, the resulting IDT−TQ polymer shows a relatively
well-defined structure with low content of structural defects, as demonstrated by
high temperature H NMR, MALDI-TOF-MS, and elemental analysis. Integrating
1
this polymer into bulk-heterojunction solar cells with PC BM can induce an
71
enhanced OPV performance compared to the other structural analogues that retain
a certain amount of unwanted structural defects. However, the film morphology
and crystallinity are negligibly influenced by the degree of the structural defects.
Through a combination of detailed electrical measurements using light intensity
dependence and net photocurrent, we are able to correlate the different
photovoltaic performances in structure−function relationships with the extent of the structural defects. Our study indicates
that DAP is a promising asset for environmental production of many valuable thiophene-containing polymers for electroactive
and photoactive applications.
INTRODUCTION
Despite its manifold advantages, DAP still suffers from side
reactions (i.e., branching, cross-linking, and homocouplings)
during the coupling process, which leads to additional structural
■
A rapid improvement in organic photovoltaic cells (OPVs) has
been achieved with power conversion efficiencies (PCEs)
31−33
1
−4
defects and alters polymers’ electronic properties.
For
exceeding 10%.
These great advances have been mainly
those reasons, many important D−A conjugated polymers have
not been sufficiently investigated by DAP. In particular, the
DAP of two β-unprotected thiophene-containing monomers,
enabling high selectivity of C−H bonds at α- and β-positions as
well as restraining homocoupling regiodefects, is a big
challenge.
To address this issue, we chose to work with indaceno-
dithiophene (IDT) as a coplanar ladder-type donor unit fused
by two outer thiophenes and thiophene−quinoxaline−
thiophene (TQ) as an acceptor counit for constructing a D−
A model polymer with the following in mind: (i) Because the
side chains introduced at both the cyclopentadienyl ring of the
IDT and quinoxaline of TQ can guarantee solubility of the
resulting polymer, it is unnecessary to have additional side
driven by the development of donor−acceptor (D−A) π-
conjugated polymers with increasingly complex structures.
Such polymers have been prepared by using a classical and
reliable transition-metal-catalyzed step-growth polycondensa-
tion method such as Stille- and Suzuki-type couplings.
5
−16
1
7,18
However, these methods generally involve time-consuming
preparation of pure organomentallic monomers and formation
17
of toxic byproducts. Given these financial and environmental
costs associated with the materials synthesis, a direct arylation
polymerization (DAP) as a C−H activation method is
emerging as a highly promising alternative to the above-
19−30
mentioned traditional cross-couplings.
In this reaction, the
preparation of organometallic intermediates is unnecessary; this
contributes not only to reducing the additional synthetic steps
and toxic wastes but also to minimizing the presence of
difficult-to-remove byproducts that can have a negative impact
on the devices.
Received: October 26, 2015
Revised: December 30, 2015
Published: January 14, 2016
©
2016 American Chemical Society
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Scheme 1. (a) Typical Design Motif Used by This Work, Where R = Aliphatic Side Chains; (b) Synthesis of IDT Monomers
sc
M1 and M2; (c) Synthesis of IDT−TQ Polymer with DAP under Either Conventional or Microwave Heating as Well as
Standard Stille Polymerization
chains on the outer thiophenes, which ensures the presence of
α- and β-proton activations of the flanked thiophenes toward
the central goal in this study. (ii) IDT−TQ-based D−A
polymers have been proven to be one of the high-performance
amount of structural defects arising from unselective C−H
activation and/or homocouplings within the polymer backbone.
RESULTS AND DISCUSSION
■
3
4−39
OPVs materials.
Therefore, the optimized DAP for a class
Material Synthesis and Characterization. A typical
design motif used by our research group and synthetic routes
for the monomers are shown in Scheme 1. While TQ monomer
of such polymers should bring a valuable usability for the low-
cost, large-scale, and commercially viable preparation of high-
performance semiconducting materials.
4
0
was synthesized according to literature reports, IDT
monomer (M1) was synthesized from diethyl 2,5-dibromo-
terephthalate via several steps (Stille coupling, double
nucleophilic addition, and intramolecular Friedel−Crafts
cyclization). After lithiation of M1 followed by quenching
with trimethyltin chloride, the corresponding bis-stannylated
monomer M2 was obtained in a satisfying yield of 65%.
In addition to Stille polymerization of M2 with TQ for a
reference polymer, we carried out the various DAPs by using
M1 and TQ for the synthesis of IDT−TQ polymer. The
resulting polymers were purified by Soxhlet extraction with
Herein, we report an efficient DAP protocol based on simple
time control along with proper heating mode toward a well-
defined IDT−TQ polymer via two starting β-unprotected
thiophene-containing monomers. We then compared their
structural defects and properties, as well as OPV characteristics,
with the polymer obtained from standard Stille polymerization.
Compared to its Stille analogue, the OPVs based on P4
prepared by DAP under conventional heating exhibits an
improved PCE of up to 5.1%, thanks to the relatively small
5
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Table 1. Reaction Conditions and Characteristics of the Polymers Obtained from Stille Polymerization and DAP
a
b
reaction time
[h]
insoluble
material
Mn
yield
a
entry
catalyst system [mol %]
salt [equiv] solvent [mL]
heating/T [°C]
[kDa]
PDI
[%]
P1 Pd (dba) (2)/P(o-Tol) (10)
−
toluene (5)
toluene (5)
DMAc (5)
DMAc (5)
DMAc (5)
DMAc (5)
DMAc (5)
DMAc (5)
conventional/110
conventional/100
conventional/100
conventional/100
conventional/100
microwave/100
microwave/100
microwave/100
72
6
no
no
no
no
no
no
yes
yes
22.9
1.51
64
2
3
3
c
c
c
P2 Pd(OAc) (2)
K CO (2.5)
2
−
−
−
2
3
P3 Pd(OAc) (2)
K CO (2.5)
2
12
24
48
1
16.8
27.3
25.8
9.9
1.42
1.62
2.24
1.48
1.9
65
71
64
40
59
2
3
P4 Pd(OAc) (2)
K CO (2.5)
2 3
2
P5 Pd(OAc) (2)
K CO (2.5)
2 3
2
P6 Pd(OAc) (2)
K CO (2.5)
2 3
2
P7 Pd(OAc) (2)
K CO (2.5)
2
2
21.9
2
3
d
d
d
P8 Pd(OAc) (2)
K CO (2.5)
2
4
−
−
−
2
3
a
b
Estimated by permeation chromatography (GPC) using THF as a solvent and calibrated on polystyrene standard. Yield is based on the amount of
c
d
chloroform fractions. No polymerization occurred. Gelation occurred. DMAc = N,N-dimethylacetamide, M = number-average molecular weight,
n
PDI = polydispersity index.
1
Figure 1. H NMR spectra of P1, P4, and P7 in C D Cl at 70 °C. The solvent peak was marked as an asterisk.
2
2
4
methanol, acetone, hexanes, and chloroform. All entries of
chloroform-soluble fraction and molecular weight (M = 22.9
n
IDT−TQ made via DAP (P2−P8) and Stille polymerization
kDa) with polydispersity (PDI) of 1.51 (Table 1, entry P1).
On the basis of the literature data of DAP in terms of not
only the simplicity but also unnecessary removal of phosphine
(
P1) are compiled in Table 1. First, the Stille polymerization
was performed essentially following the reported standard
8
condition using Pd (dba) /P(o-Tol) as a catalyst in a refluxing
compounds, the investigation into time-control DAP began
2
3
3
toluene for 72 h, which afforded a yield of 64% for the
under phosphine-free conditions using Pd(OAc) (2 mol %) as
2
5
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Figure 2. UV−vis absorption spectra of the polymers in (a) chloroform solution and (b) thin films. (c) CV curves of polymer films at a scan rate of
−1
5
0 mV s . (d) Ultraviolet photoelectron spectroscopy (UPS) spectra of polymer films.
a catalyst and K CO (2.5 equiv) as a base in N,N-
dimethylacetamide (DMAc, 0.2 M) at a mild reaction
temperature of 100 °C. By employing the conventional heating,
different heating tools. Note that all the polymers (P1, P4, and
P7) have similar M and PDI values (M = 21.9−27.3 kDa and
2
3
n
n
PDI = 1.51−1.90), which can rule out the molecular-weight-
42−47
the DAP reaction for 12 h gave P2 with M = 16.8 kDa with a
dependent properties as a complicating variable.
n
6
5% yield. As the reaction time increased to 24 h, the obtained
To characterize the structural differences between the three
polymers, high temperature H NMR spectroscopy was
1
polymer (P4) showed higher M of 27.3 kDa, with an improved
n
yield of 74%. However, the M value did not further increase
performed in C D Cl at 70 °C and compared in Figure 1.
In addition to the distinct 12 peaks arising from the backbone
repeating unit in the aromatic regions and two peaks in the
n
2
2
4
when the reaction time was extended (48 h, P5), though a
lower yield (chloroform-soluble fraction) was obtained due to
the high content of insoluble fractions caused by the occurrence
of cross-linking side reaction. Although toluene is a widely used
solvent for the synthesis of most conjugated polymers, no
polymeric solid (P2) was obtained from replacing DMAc with
toluene, probably because of poor solubility of potassium
4.01−3.83 ppm due to the OCH protons of the alkoxy side
2
chains on each IDT and TQ, extra peaks a and b with low
intensity were observed in all cases. To determine such extra
1
peaks in the H NMR spectra, PIDT and PTQ homopolymers
were prepared by DAP under conventional heating, respec-
32,41
1
carbonate in the less polar solvent.
tively, and the corresponding H NMR spectra are provided in
In expansion of our studies, microwave-assisted DAP
reaction was also investigated under the same conditions
except for the reaction time (P6−P8). Under microwave
heating for 4 h, a solvent-swollen gelation was observed,
resulting in only insoluble product. By decreasing the reaction
the Supporting Information (Figures S1 and S2). Referring to
the H NMR spectra of the monomers (TQ and Br-TQ-Br)
1
(Figure S2), the peaks b at around 7.54 and 7.24 ppm can be
speculated to different end groups. It is noteworthy that P7
exhibits the broad, split resonances in the 8.25−8.15 ppm
range, implying a certain amount of structural defects.
A closer look at the NMR spectra of the homopolymers
(PIDT and PTQ) and IDT−TQ polymers (P1, P4, and P7)
reveals that the peaks a at 7.65 and 4.07 ppm, related to the
backbone signals of the homopolymer PTQ (see Figure S2),
are visible in both P1 and P7, but fail for P4, which indicates
that P4 is a relatively well-defined structure with low content of
the structural defects.
time to 2 h, P7 with M of 21.9 kDa (chloroform-soluble
n
fraction) was obtained, though it still contained a lot of
insoluble fractions in the Soxhlet thimble even after additional
extraction with hot dichlorobenzene. For a shorter reaction
time (1 h), P6 with a low M of 9.9 kDa was isolated without
n
the insoluble fractions after chloroform extraction. These
results indicate that the microwave-assisted DAP can indeed
accelerate the reaction rate, yet simultaneously stimulating the
β-H activation in the unsubstituted thiophene monomers,
which causes the formation of cross-linking ill-defined
structures.
Next, in order to compare the degree of the structural defects
and optoelectronic properties, as well as OPVs performance,
with the same IDT−TQ (P1) synthesized via Stille polymer-
ization, we chose each P4 and P7 sample made by DAP under
To further validate their structural features, the polymers
were also analyzed via MALDI-TOF-MS spectrum and
elemental analysis (Figures S3−S5). In the measurable
1
molecular weight range, linking to our findings from
H
NMR spectra, MALDI-TOF-MS of P4 showed a series of
IDT−TQ alternating peaks except for an lesser extent
containing TQ−TQ sequences, while the peaks corresponding
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to TQ−TQ segments can be clearly detected in P1 and P7. In
particular, P7 exhibited large quantities of TQ−TQ sequences,
suggesting the presence of severe homocoupling or even
branching defects, as a result of a negative side effect from the
enhanced reactivity system induced by microwave heating.
Surprisingly, other nonalternating IDT−IDT fragments that are
expected to form accompanying with the occurrence of TQ−
TQ homocouplings have not been detected yet. A possible
reason for the result is that the directly positioned bulky side
chains close to the IDT unit render large steric hindrance,
suppressing these nonalternating IDT−IDT structures. Com-
pared to those of both P1 and P7, the elemental analysis for the
composition of C, H, N, and S for P4 matched well with the
values calculated from the composition of the repeating unit
(
Table S1). This also supports a relatively higher purity of P4.
Figure 3. (a) Optimized frontier orbitals and (b) top and side views of
a single repeating unit of IDT−TQ at the level of the B3LYP/6-31 G*
basis set.
On the basis of all the data above, one can conclude that in the
case of DAP between two unprotected thiophene-containing
monomers P4 made by the carefully time-controlled DAP
under conventional heating has less homocoupling compared
to P1 and P7.
Optical and Electrochemical Properties. The optical
properties of the three polymers (P1, P4, and P7) were
investigated by UV−vis spectroscopy in dilute chloroform
solution and solid state (Figure 2a,b). In chloroform solution,
the absorption spectra of P1 and P4 were almost identical, each
exhibiting absorption maxima at 570 nm (low-energy band)
and 440 nm (high-energy band). In contrast, for P7 solution,
the main backbone took place. Therefore, we propose that the
IDT-based polymer investigated here remains largely disor-
dered in the solid state (see the following GIWAXS data
below).
OPV Performance. For evaluating the impact of the
synthetic methods on the characteristics of the OPV materials,
bulk-heterojunction (BHJ) solar cells were fabricated using a
conventional structure of ITO/PEDOT:PSS/active layer/Al
−
2
and measured under 100 mW cm , AM 1.5G solar
illumination. It was found that unannealed as-spun films from
o-dichlorobenzene (o-DCB) with a 1:3 polymer:PC BM ratio
the low-energy absorption maximum was blue-shifted (λmax
5
=
51 nm), and a broad high-energy band with indefinable feature
7
1
was exhibited. Transitioning from solution to solid state, all
absorption spectra of P1, P4, and P7 were found to be slightly
red-shifted compared to the corresponding ones observed from
the solution, and the optical bandgaps were determined to be
gave the best performance. Figure 4a shows the representative
current density−voltage (J−V) curves of the three polymers,
and the detailed photovoltaic parameters are summarized in
Table 2. The OPV cells with an active layer of P1:PC BM
7
1
1.84, 1.84, and 1.81 eV from the onsets in their films,
afforded performance with an open-circuit voltage (VOC) of
−2
respectively. The different optical features of P7 are presumably
due to the above-mentioned high content of nonalternating
and/or branching structures.
0.89 V, a short-circuit current density (J ) of 10.42 mA cm ,
SC
Using cyclic voltammetry (CV) of polymer films (Figure 2c),
the HOMO/LUMO values of P1, P4, and P7 were estimated to
be −5.21/−3.35 eV, −5.20/−3.35 eV, and −5.25/−3.45 eV,
respectively. These results indicate that the energy levels are
somewhat sensitive to the extent of structural defects in even
the identical repeating backbone. The bandgaps determined
from CV are in perfect agreement with the measured optical
bandgaps above. Ultraviolet photoelectron spectroscopy (UPS)
was also used to measure the ionization potential (IP) levels of
the three polymers (Figure 2d). Despite the slight discrepancy
between the IP and HOMO values, the IPs (5.04, 5.02, and
5.11 eV for P1, P4, and P7, respectively) showed the same
trends as the HOMO values obtained from CV.
Theoretical Calculation. To demonstrate the optimal
molecular geometry and electronic properties, theoretical
calculations were performed by using density functional theory
(
DFT) B3LYP with 6-31G* basis set. All of the alkoxy side
chains were replaced by methoxy groups to simplify the
calculation. As shown in Figure 3a, the HOMO wave functions
are distributed entirely over the planar conjugated backbone,
which improved gain of high hole mobility, while its LUMO
wave functions are mainly localized on the acceptor moiety
(
TQ). We found that the alkoxy phenyl side chains on IDT
units extended mostly perpendicular to the conjugated plane of
the polymer backbone, while a rotation of the phenyl side
chains on TQ by only about 0.03° with respect to the plane of
Figure 4. (a) J−V curves of IDT-TQ-based OPVs under 100 mW
−
2
cm , AM1.5G solar illumination. (b) The corresponding EQE
spectra.
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Table 2. OPVs Performance of the Polymers
JSC [mA cm−2]
VOC [V]
FF [%]
PCE [%] max/av
a
EQE
μh [cm V s−1
b
2
−1
]
polymer
blend ratio [polymer:PC BM]
7
1
−
−
−
3
3
4
P1
P4
P7
1:3
1:3
1:3
10.42
10.75
9.23
0.89
0.89
0.84
52.10
53.40
43.10
4.82/4.77
5.10/5.05
3.35/3.28
9.35
9.40
8.41
1.50 × 10
4.09 × 10
1.23 × 10
a
b
Average values at least six runs. Space-charge-limited current (SCLC) measured hole mobilities.
Figure 5. AFM images of blend films of polymer/PC BM; the size of the image is 5 μm × 5 μm.
71
Figure 6. GIWAXD images of (a−c) neat P1, P4 and P7 polymer films and (d−f) as-cast blend films of P1:PC BM, P4:PC BM, and P7:PC BM.
71
71
71
and a fill factor (FF) of 0.52, resulting in a PCE of 4.8%. Very
interestingly, without the solvent additives or postprocessing
techniques, P4:PC BM-based devices exhibited an enhanced
light. P1 and P4 devices showed a similar photoresponse from
300 to 800 nm with a maximum EQE value of 67.8% at 410
nm, while the device based on P7 exhibited a decreased
photoresponse intensity in the whole wavelength range, with a
maximum EQE value of 59.7% at 410 nm. The results are
concordant with the lower JSC of P7 device compared to those
of P1 and P4 devices, revealing the limited photocurrent
generation when a large amount of structural defects was
7
1
−2
PCE of 5.1% with a JSC of 10.75 mA cm , FF of 0.53, and an
identical VOC of 0.89 V. On the other hand, with even the same
conditions, a decent PCE of 3.35% with the decreased every
−2
parameter (JSC = 9.23 mA cm , FF = 0.43, V = 0.83 V) was
OC
obtained from the devices based on P7 having the higher
content of the nonalternating structures. Recently, other groups
have also reported a similar reduction in OPVs when the
remained. The current density (J ) values calculated from
SC
integration of the EQEs of P1, P4, and P7 devices are 9.35,
4
8−50
−2
abundance of the homocouplings was present.
Con-
9.40, and 8.41 mA cm , respectively, which agree well with the
sequently, we attribute the improved performance of P4 to the
increased alternating D−A structures with higher regioregu-
larity.
JSC values obtained from J−V measurements (within 10%
mismatch).
To investigate charge transport characteristics, space-charge-
limited current (SCLC) models were used to determine the
hole mobilities of each optimized polymer film (Table 2). The
detailed fabrication and analysis are described in the
As shown in Figure 4b, the external quantum efficiency
(
EQE) as a function of wavelength of the OPVs based on the
polymers is measured under illumination of monochromatic
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Figure 7. (a) Light intensity dependence of J . (b) Light intensity dependence of VOC. (c) J−V curves of IDT-TQ-based OPVs in the dark. (d)
SC
Photocurrent density versus effective voltage characteristics.
Experimental Section. The hole mobilities of P1, P4, and P7 are
face-on orientation relative to the substrate surface. In addition,
the calculated coherence lengths of the three polymers were
also nearly equal at around 9 Å. Even though the lamellar
packing distances are slightly shortened from P7 to P1 to P4, at
this point, the impact on molecular packing of the neat films
remained unclear as a function of the structure defects content
in the polymer backbone.
−3
−3
−4
2
−1 −1
1.50 × 10 , 4.09 × 10 , and 1.23 × 10 cm V s ,
respectively. There does seem to be strong correlation between
the mobilities and relative abundance of structural defects.
Moreover, the observed higher mobility of P4 compared to the
other samples is one important reason that can contribute to
the enhanced JSC and FF values, though this cannot be solely
the result of the changes seen in PCEs.
When each polymer was then blended with PC BM, the
7
1
Morphological Properties. The morphology of the
−1
emergence of a new isotropic ring at 1.3−1.4 Å was assigned
polymer:PC BM blend films with the optimized weight ratio
71
to PC BM. However, both the lamellar packing and π−π
71
(
1:3) was investigated by tapping-mode atomic force
stacking peaks in the films were faded, suggesting a lack of the
ordered structures induced by π-stacking, which coincides with
the observations from the AFM images. When the findings
from AFM and GIWAXD were considered, we were unable to
clearly point out the exact reason why P4-based devices showed
better performance since there was no obvious variation in
morphology and microstructural order of the three polymers.
This means there must be some other cause for the difference
in OPVs performance (vide infra).
microscopy (AFM) (Figure 5). All the blend films show fairly
homogeneous surfaces in size and feature. No distinct
difference was observed in the three AFM images with a
similar root-mean-square (rms) roughness of 0.411−0.482 nm
for the blend films of P1, P4, or P7 with PC BM. We were
7
1
unable to see clear polymer−PC BM domains, and there was
7
1
no evidence of ordered nanostructures in the films, presumably
due to the rather amorphous nature of the IDT-based
polymers. These results seem to be directly linked to the
above-observed independency of PCEs on the application of
various processing techniques, such as addition of additives,
solvent annealing, and thermal annealing.
Charge Recombination and Transport Properties. To
assess the effects of the structure defects on the charge
recombination properties in OPVs, the light-intensity-depend-
ent J−V characteristics of the three polymers were measured.
For further investigation into the relevant structural features,
grazing-incidence wide-angle X-ray diffraction (GIWAXD)
measurements of neat polymers and blends with PC BM
Figure 7a shows a log−log plot of J as a function of the light
SC
intensity (I). This study could give a clue for answering the
71
above question. A power-law relationship is found, given by J
were performed, and detailed crystallographic parameters are
summarized in Table S2. As shown in Figure 6, the neat films of
all three polymers showed very similar scattering patterns in
SC
α
∝
I , where α would be unity (α = 1), when the bimolecular
recombination can be negligible under short circuit condition.
51
The calculated α values for the devices of P1, P4, and P7 were
.88, 0.90, and 0.85, respectively, suggesting that the
GIWAXD analysis; along the out-of-plane (q ) axis, all films
z
−1
0
exhibited (100) lamellar packing reflections at 0.271−0.277 Å
corresponding to d-spacing distances of 23.1, 22.7, and 23.1 Å,
for P1, P4, and P7, respectively) and almost identical (010)
(
bimolecular recombination can function as the content of
structural defects within the identical repeating backbone. As a
result, P4:PC71BM-based devices having the highest α value
could lead to relatively better performance via the reduced
bimolecular recombination.
−1
π−π stacking reflections at ∼1.37 Å (corresponding to d-
spacings of ∼4.6 Å), while the reflections along in-plane (q )
xy
axis did not appear, suggesting preferential self-assembly into a
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Besides, the trap-assisted recombination process was also
investigated via measuring the dependence of VOC on the light
intensity. Normally, when bimolecular recombination domi-
nates in OPVs, the slope of VOC versus light intensity should
equal kT/q, while when trap-assisted recombination predom-
EXPERIMENTAL SECTION
■
1
General Measurement and Characterization. H NMR were
recorded on a Varian VNRS 400 MHz (Varian, USA) spectropho-
tometer using CDCl as solvent and tetramethylsilane (TMS) as a
3
reference as well as Varian VNRS 600 MHz (Varian, USA)
spectrophotometer using C D Cl as solvent. UV−vis absorption
inates the loss of charge carriers, a stronger dependence of V
2
2
4
OC
52
spectra were recorded on Cary 5000 (Varian USA) spectropho-
on light intensity with a slope larger than kT/q is observed. As
shown in Figure 7b, compared to slopes of 1.40 kT/q and 1.50
kT/q for each device made by P1-and P7-based blends, the
calculated slope of 1.32 kT/q for P4:PC BM indicated the
tometer. The number-average (M ), weight-average (M ) molecular
n
w
weights, and polydispersity index (PDI) of the polymers were
determined by GPC with Agilent 1200 HPLC and miniDAWN
TREOS using polystyrene as a standard in THF (HPLC grade). Cyclic
voltammetry (CV) were measured on Solartron electrochemical
station (METEK, Versa STAT3) equipped with a three-electrode
cell in tetra-n-butylammonium hexafluorophosphate solution in
acetonitrile (0.1 M) at a scan rate of 50 mV s under an argon
atmosphere at room temperature. Ag/Ag electrode, a platinum wire,
and a glass carbon disk were used as the reference electrode, counter
electrode, and working electrode, respectively. The HOMO energy
7
1
weakest dependence of VOC on the light intensity. These results
demonstrated that increasing structural defects leads to more
trapping centers during charge transport, consequently
reducing the FF and JSC parameters in OPVs.
−
1
+
To further probe the charge dissociation and collection
process, the photocurrent density (J ) as a function of effective
ph
onset
applied voltage (V ) were examined for the three polymer
levels were obtained from the equation HOMO (eV) = −(E
−
eff
(ox)
onset
devices. The J is defined as J − Jdark, where Jlight and Jdark are
E
(ferrocene)
+ 4.8) (S1). The LUMO levels of polymers were
ph
light
onset
obtained from the equation LOMO (eV) = −(E
−
the current densities under illumination and in the dark,
(red)
onset
E
+ 4.8) (S2). MALDI-TOF-MS were conducted from
(
ferrocene)
respectively, and the Veff is defined as V − V, where V is the
0
0
52,53
Ultraflex (Bruker, Germany). Elemental analyses were performed with
Flash 2000 (Thermo Scientific, Netherlands).
compensation voltage at Jph = 0 and V is the applied bias.
As shown in Figure 7d, Jph reaches saturation (J ) at a
sat
Materials. All starting materials were purchased from Acros or
Aldrich and used without further purification. 4-Bromo-1-(2-ethyl-
hexyl)oxy)-2-fluorobenzene and 5,8-bis(5-bromothiophen-2-yl)-2,3-
bis(3-(octyloxy)phenyl)quinoxaline (TQ) were prepared according
sufficiently large reverse voltage (i.e., Veff > 2.0 V). Thus, the
charge dissociation efficiency can be evaluated by using J /J
ph sat
ratio. At the short-circuit condition, assuming Jsat at 2.5 V , P4-
eff
4
0
to previous reports.
based blends exhibited a higher J /J value of 81% than those
ph sat
Synthesis of Compound 1. To a solution of 2-(tributylstannyl)-
thiophene (9.0 g, 24 mmol) and diethyl 2,5-dibromoterephthalate
3.70 g, 10 mmol) in anhydrous toluene (50 mL), Pd(PPh ) (0.58g,
.5 mmol) was added under nitrogen. The resulting mixture was kept
at 110 °C for 24 h and then quenched with water and extracted with
diethyl ether. The diethyl ether solution was dried over MgSO . After
removing the solvent, the crude compound 1 was purified by silica gel
chromatography using a mixture of dichloromethane and hexane (1:1)
of the other samples (78% and 77% for P1 and P7,
respectively), indicating a higher charge-dissociation efficiency.
More interestingly, under the maximum power output
(
0
3 4
condition, P4 devices again show a slightly higher J /J
ph sat
value of 79%, compared to both P1 and P7 devices (74% and
4
7
6%), implying that alternating structures with relatively higher
purity can improve the charge extraction and collection as well.
Not only do all the results observed above demonstrate that the
charge recombination and transport characteristics are very
sensitive to relative amount of the structural defects within
polymer distribution, but they are also consistent with the
trends in performance of the tested polymer-based devices (vide
supra).
1
as the eluent to afford a colorless solid (3.3 g, 82%). H NMR (400
MHz, CDCl , δ): 7.80 (s, 2H), 7.39 (dd, 2H), 7.11 (m, 4H), 4.21 (d,
3
4
H), 1.15 (t, 6H).
Synthesis of 4-Bromo-1-(2-ethylhexyl)oxy)-2-fluorobenzene. To a
solution of 4-bromo-2-fluorophenol (10.0 g, 52.5 mmol) and K CO
(8.5 g, 62.8 mmol) in DMF (50 mL), 1-bromo-2-ethylhexane (10.0 g,
2.4 mmol) was added, and the stirred mixture was kept at 130 °C for
2 h, then cooled to room temperature, and extracted with ethyl
2
3
5
1
acetate. The combined organic phase was dried over MgSO . After
removing the solvent, the resulting crude compound was purified by
4
CONCLUSIONS
■
silica gel chromatography using hexane as the eluent to afford a
In summary, we have demonstrated a well-defined IDT−TQ
model polymer with satisfied and controllable molar mass
synthesized by DAP of two β-unprotected thiophene-
containing monomers. The resulting polymer (P4) made by
DAP under conventional heating is a well-definded structure
1
colorless oil (13.0 g, 81%). H NMR (400 MHz, CDCl , δ): 7.24 (dd,
3
1
1
H), 7.16 (d, 1H), 6.88 (t, 1H), 3.92 (d, 2H), 1.78 (m, 1H), 1.59−
.32 (m, 8H), 0.92−0.86 (m, 6H).
Synthesis of Compound M1. To a solution of 4-bromo-1-(2-
ethylhexyl)oxy)-2-fluorobenzene (3.0 g, 12.5 mmol) in THF (25 mL)
at −78 °C was added n-BuLi (5.5 mL, 2.5 M in hexane, 13.75 mmol);
the mixture was kept at −78 °C for 1 h, and then a solution of
compound 1 (0.8 g, 2.1 mmol) in THF (15 mL) was added slowly.
After the addition, the mixture was stirred at room temperature
overnight and then quenched with water and extracted with ethyl
1
with low content of structural defects, as proven by H NMR,
MALDI-TOF-MS, and elemental analysis data. Even though
there are no obvious changes in their morphology and packing
orientation as the amount of the embedded structural defects
into the backbone, P4-based OPVs show slightly higher PCEs
acetate. The combined organic phase was dried over MgSO . After
4
(
up to 5.1%) than the Stille analogue-based ones (4.7%). By
removing the solvent, the crude product was directly dissolved in
acetic acid (100 mL) and concentrated H SO (2 mL) was added. The
using experimental photocurrent measurement in combination
with light intensity dependence, the performance enhancement
is clarified as the corresponding charge recombination and
transport characteristics affected by the structural defects
content in the polymer backbone. The presented study brings
a simple and efficient synthetic method, which paves the way
for the preparation of various bare thiophene-containing D−A
polymers via DAP for organic photovolatics.
2
4
mixture was refluxed for 4 h and cooled to room temperature. After
pouring into water, the mixture was extracted with ethyl acetate and
dried over MgSO . The resulting crude compound was purified by
4
silica gel chromatography using a mixture of hexane/dichloromethane
1
(
5:1) as the eluent to afford a yellow solid (1.6 g, 68%). H NMR (400
MHz, CDCl , δ): 7.36 (s, 2H), 7.29−7.28 (d, 2H), 6.97−6.94 (m,
3
6
H), 6.93−6.89 (d, 4H), 6.88−6.79 (m, 4H), 3.86−3.84 (m, 8H),
13
1.75−1.52 (m, 4H), 1.46−1.27 (m, 34H), 0.92−0.86 (m, 24H).
C
5
34
DOI: 10.1021/acs.macromol.5b02324
Macromolecules 2016, 49, 527−536

Macromolecules
Article
NMR (100 MHz, CDCl , δ): 155.1, 153.5, 153.1, 151.1, 146.4, 141.2,
be 0.13°−0.135° for IDT-TQ polymer films and blended films.
GIWAXS patterns were recorded with a 2D CCD detector (Rayonix
SX165), and X-ray irradiation time was 6−9 s, dependent on the
saturation level of the detector. Diffraction angles were calibrated using
a sucrose standard (monoclinic, P21, a = 10.8631 Å, b = 8.7044 Å, c =
7.7624 Å, β = 102.938°), and the sample-to-detector distance was
∼231 mm.
3
1
2
36.8, 135.0, 128.1, 123.3, 122.6, 117.2, 116.0, 114.3, 71.8, 39.3, 30.3,
8.9, 23.7, 23.0, 14.0. Anal. Calcd for C H F O S : C 74.83, H 7.50,
72
86
4
4 2
O 5.54, S 5.55. Found: C 74.65, H 7.61, O 5.64, S 5.60.
Synthesis of Compound M2. To a solution of M1 (1.1 g, 1.0
mmol) in THF (20 mL) at −78 °C was added n-BuLi (1 mL, 2.5 M,
2
1
.5 mmol). After addition, the mixture was kept at −78 °C for another
h, and then trimethyltin chloride (1.0 M, 4.0 mmol) was added. The
resulting mixture was stirred overnight at room temperature overnight
and then quenched with water and extracted with diethyl ether. The
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge via the
Internet at The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.macromol.5b02324.
■
*
S
diethyl ether solution was dried over MgSO . Recrystallization from
4
1
methanol afford a yellow solid (0.9 g, 65%). H NMR (400 MHz,
CDCl , δ): 7.35 (s, 2H), 7.25−6.98 (m, 6H), 6.97−6.89 (d, 4H),
3
6
.84−6.80 (m, 4H), 3.89−3.83 (m, 8H), 1.80−1.56 (m, 4H), 1.45−
13
1
.23 (m, 38H), 0.99−0.88 (m, 27H), 0.44−0.30 (t, 18H). C NMR
(
100 MHz, CDCl , δ): 156.8, 153.5, 153.3, 151.0, 146.3, 142.2, 137.3,
3
Figures S1−S5; Tables S1 and S2 (PDF)
134.7, 129.9, 123.4, 117.5, 116.2, 114.3, 71.8, 39.3, 30.3, 29.7, 23.7,
2
3.0, 14.0, −8.0. Anal. Calcd for C H F O S Sn : C 63.25, H 6.94,
78
102
4
4
2
2
O 4.32, S 4.33. Found: C 64.69, H 7.46, O 4.24, S 4.11.
AUTHOR INFORMATION
Corresponding Authors
E-mail: liyf@iccas.ac.cn (Y.L.).
E-mail: yang@unist.ac.kr (C.Y.).
■
Stille-Coupling Polymerization. The compound M2 (150.0 mg,
0
.112 mmol), TQ (95.9 mg, 0.112 mmol), Pd (dba) (2.0 mg, 0.002
2 3
*
*
mmol), and P(o-Tol)3 (3.4 mg, 0.010 mmol) were put into a Schlenk
flask and purged with argon for 10 min. Then 5 mL of anhydrous
toluene was added, and the mixture was heated at 110 °C for 72 h.
After cooling to room temperature, the mixture was poured into
methanol. The crude product was collected by filtration and washed by
Soxhlet extraction in methanol, acetone, and hexane. Finally,
chloroform-soluble fraction was reprecipitated in methanol to afford
a dark purple solid.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government
(MSIP) (2015R1A2A1A10053397) and Development Program
of the Korea Institute of Energy Research (KIER) (B6-2431).
GIWAXD experiment at PLS-II 6D UNIST-PAL beamline was
supported in part by MEST, POSTECH, and UNIST-UCRF.
DAP. The compound M1 (100.0 mg, 0.069 mmol), TQ (74.5 mg,
0
0
.069 mmol), Pd(OAc) (0.3 mg, 0.002 mmol), and K CO (23.9 mg,
2
2
3
.173 mmol) were put into a microwave vessel and purged with argon
for 10 min. Then 5 mL of anhydrous DMAc was added, and the
mixture was heated at 100 °C by using either conventional or
microwave tool for each reaction time noticed in Table 1. After cooling
to room temperature, the mixture was poured into methanol. The
crude product was collected by filtration and washed by Soxhlet
extraction in methanol, acetone, and hexane. Finally, chloroform-
soluble fraction was reprecipitated in methanol to afford a dark purple
solid.
Fabrication and Characterization of Solar Cells. The structure
of the all the polymer solar cells was glass/ITO/PEDOT: PSS/active
layer/Al. PEDOT:PSS (Bayer Baytron 4083) was spin-coated at 4000
rpm onto ITO substrate, followed by annealing at 150 °C for 15 min
to remove water completely. The active layer was spin-coated from o-
dichlorobenzene solution of polymers and PC BM (10 mg mL )
onto the PEDOT:PSS layer. Finally, 80 nm aluminum was thermally
evaporated under vacuum (<5.0 × 10 Pa). The active area of each
sample was 5.0 mm . The current density−voltage (J−V) character-
istics were measured on a Keithley 2400 source under illumination of
an AM1.5G solar simulator with an intensity of 100 mW cm . EQE
measurements were conducted in ambient air using an EQE system
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DOI: 10.1021/acs.macromol.5b02324
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