The role of conjugated side chains in high performance photovoltaic polymers

Article abstract of DOI:10.1039/c4ta05445h

Four new D-A type copolymers, namely, PBDT-DFQX-PP, PBDT-DFQX-TP, PBDT-DFQX-PT and PBDT-DFQX-TT, were designed and synthesized to investigate the influence of conjugated side chain pattern on photovoltaic properties of conjugated polymers. All the four copolymers have an identical conjugated backbone comprising benzo[1,2-b:4,5-b′]dithiophene (BDT) donor unit and quinoxaline (Qx) acceptor unit, but with varying conjugated side chains, p-alkoxyphenyl or 2-alkylthienyl, attached to the donor and acceptor units, respectively. As evidenced by UV/Vis absorption spectra, electrochemical cyclic voltammetry, density functional theory (DFT), grazing incidence X-ray scattering (GIXS), transmission electron microscope (TEM) and photovoltaic measurements, the difference in conjugated side chain modulation led to totally different physicochemical properties. Among the four copolymers, PBDT-DFQX-TT exhibits the broadest absorption spectrum, the most close-packed structure as well as a finest fibril structure when blended with PC₇₁BM. After systematic device optimization, the power conversion efficiencies (PCEs) of the bulk heterojunction (BHJ) photovoltaic devices based on the blends of PBDT-DFQX-PP, PBDT-DFQX-TP, PBDT-DFQX-PT and PBDT-DFQX-TT with PC₇₁BM achieved 3.96%, 6.08%, 6.54% and 7.68%, respectively. By systematic varying the side chains of the copolymers from all phenyl groups to all thienyl ones, PCEs was increased by 250% from 3.96% to 7.68%. To date, PBDT-DFQX-TT is one of a few Qx-based PSCs that exhibits PCE exceeding 7.5%, and the results suggest that simultaneously modulating the conjugated side chains on both donor and acceptor units of copolymers could be an effective strategy for constructing high performance photovoltaic copolymers.

Full text of DOI:10.1039/c4ta05445h

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Page 1 of 39
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DOI: 10.1039/C4TA05445H
The Role of Conjugated Side Chains in High
Performance Photovoltaic Polymers
Meng Wang 1,2, Di Ma 1, Keli Shi1,3, Shaowei Shi 1, Song Chen4, Changjiang Huang 1, Zi Qiao1,
ZhiꢀGuo Zhang3, Yongfang Li3*, Xiaoyu Li1*, and Haiqiao Wang1.2*
1.
State Key Laboratory of OrganicꢀInorganic Composite, Beijing University of Chemical
Technology, Beijing 100029, China
2.
Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education,
Beijing University of Chemical Technology, Beijing 100029, China
3.
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China
4.
China Textile Academy, Beijing, 100025, China
Corresponding author: eꢀmail: wanghaiqiao@mail.buct.edu.cn (H. Wang); liyf@iccas.ac.cn (Y.
Li); lixy@mail.buct.edu.cn (X. Li)
ABSTRACT: Four new DꢀA type copolymers, namely PBDTꢀDFQXꢀPP, PBDTꢀDFQXꢀTP,
PBDTꢀDFQXꢀPT and PBDTꢀDFQXꢀTT, were designed and synthesized to investigate the
influence of conjugated side chain pattern on photovoltaic properties of conjugated polymers. All
the four copolymers have an identical conjugated backbone comprising benzo[1,2ꢀb:4,5ꢀ
b']dithiophene (BDT) donor unit and quinoxaline (Qx) acceptor unit, but with varying
conjugated side chains, pꢀalkoxyphenyl or 2ꢀalkylthienyl, attached to the donor and acceptor
1

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units, respectively. As evidenced by UV/Vis absorption spectra, electrochemical cyclic
voltammetry, density functional theory (DFT), grazing incidence Xꢀray scattering (GIXS),
transmission electron microscope (TEM) and photovoltaic measurements, the difference in
conjugated side chain modulation led to totally different physicochemical properties. Among the
four copolymers, PBDTꢀDFQXꢀTT exhibits the broadest absorption spectrum, most closed
packed structure as well as a finest fibril structure when blended with PC71BM. After systematic
device optimization, the power conversion efficiencies (PCEs) of the bulk heterojunction (BHJ)
photovoltaic devices based on the blends of PBDTꢀDFQXꢀPP, PBDTꢀDFQXꢀTP, PBDTꢀDFQXꢀ
PT and PBDTꢀDFQXꢀTT with PC71BM achieved 3.96%, 6.08%, 6.54% and 7.68%, respectively.
By systematic varying the side chains of the copolymers from all phenyl groups to all thienyl
ones, PCEs was increased by 250% from 3.96% to 7.68%. To date 7.68% is one of a few Qxꢀ
based PSCs which exhibited PCE exceeding 7.5%, and the results suggest that modulating the
conjugated side chains on donor and acceptor units of copolymers simultaneously would be an
effective strategy for constructing high performance photovoltaic copolymers.
INTRODUCTION
Bulkꢀheterojunction (BHJ) polymer solar cells (PSCs) based on conjugated polymer donor and
fullerene derivative acceptor are promising devices for alternative energy sources because of
their potential applications in flexible, lightꢀweight, and lowꢀcost largeꢀarea solar cells through
rollꢀtoꢀroll printing.1 Stimulatingly, recent advances in the power conversion efficiencies (PCEs)
of solutionꢀprocessed PSCs have exceed 9% for singleꢀjunction devices,2 owing to the
development of new donor materials, engineered interfaces and enhanced control of polymeric
film microstructure. Nowadays, the majority of synthetic efforts and structureꢀproperty studies in
the field have been devoted to the new low bandꢀgap donorꢀacceptor (DꢀA) copolymers, which
2

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could be easily tuned to feature broad absorption spectra, high charge carrier mobilities and
suitable energy levels by selecting different donor or acceptor units. Low bandꢀgap DꢀA
copolymers have been successfully used in achieving both a high shortꢀcircuit current (Jsc) and a
high openꢀcircuit voltage (Voc) simultaneously, leading to enhanced PCEs.3
Generally a conjugated polymer can be arbitrarily divided into two constituting components:
the πꢀconjugated backbones and the side chains, where πꢀconjugated backbones mainly
determine the optoelectronic properties of the resulting polymers. Consequently hundreds of
different backbones have been reported so far.4 On the other hand, selecting the side chains is as
important as selecting the conjugated backbones when designing conjugated polymers. The side
chain engineering is indeed an essential and widely spread technique used to tune the physical
properties of a conjugated polymer, including absorption, emission, energy level, molecular
packing, and charge transport. Currently, the majority of reports on the side chain engineering
focused either on the donor units or on the acceptor units individually. For instance, Hou and coꢀ
workers2a recently reported the syntheses and study of 2Dꢀconjugated polymers base on BDT
and thienothiophene (TT) units, in which only the donor units have varying furan, thiophene or
selenophene side groups, while the acceptor units kept no change, and the resultant copolymers
exhibited totally different photovoltaic properties due to change of the conjugated side chains on
the donor units. In addition, some similar works were reported by Cao5a and KꢀH Wei5b (see
Scheme 1). However, upon further inspection of the followꢀup works, we found that very limited
study concentrated on those copolymers, in which the donor and acceptor units have conjugated
side chains simultaneously, and synergistic effect of the conjugated side chains is scarcely taken
into account to date.6 Whereas, it seems that adjusting the conjugated side chains on donor and
on acceptor units synchronously could be more effective method for tuning the film forming
3

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properties, steric hindrance, πꢀπ stacking and microstructure of the active layer in BHJ.
Therefore, it is of interest to investigate the synergistic effect of conjugated side chains on both
donor and acceptor units on polymer photovoltaic properties in an effort to further understand the
structureꢀproperty relationships.
As wellꢀknown, amongst the various acceptor monomers utilized, quinoxaline (Qx) is a strong
electron acceptor unit widely used as building block for optoelectronic applications, since it can
be easily modified and side substituted to finely tune the optoelectronic properties of the
resulting polymer.7 On the other hand, benzodithiophene (BDT) is one of the most promising
electron donor units for photovoltaic applications thanks to its desirable peculiarities such as
structural rigidity and planarity and favorable interꢀchain πꢀπ stacking, other than the presence of
additional substitution sites for the incorporation of side chains.8 The feature of easy
modification of BDT and Qx provides a selectivity to construct polymers through dual side chain
modulation. It has been recently demonstrated that use of a vertical πꢀconjugation extension on
the Qxꢀbased polymers can dramatically increase the PCE.9 Thus, how to select conjugated side
chain combination to achieve high efficiency of BDTꢀDFQXꢀbased PSCs is an important and
challenging issue.
Prompted by the above considerations, herein, we designed and synthesized four new pꢀtype
conjugated DꢀA copolymers, PBDTꢀDFQXꢀPP, PBDTꢀDFQXꢀTP, PBDTꢀDFQXꢀPT and PBDTꢀ
DFQXꢀTT (whose molecular structures are shown in Scheme 2) based on benzodithiophene
(BDT) donor unit and fluorinated quinoxaline (DFQX) acceptor unit. The conjugated side
chains both on donor and on acceptor units of these copolymers were varied from pꢀ
alkoxyphenyl to 2ꢀalkylthienyl in turn, in order to investigate the effect of location and kind of
the conjugated side chains on morphology, charge transport and photovoltaic performance of the
4

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copolymers. Owing to the difference in electronꢀdonating ability, aromaticity, delocalization and
planarity, pꢀalkoxyphenyl (P) or 2ꢀalkylthienyl (T) side group was chosen to tune the solubility,
energy level and the chain packing of the polymers. In addition, electronꢀwithdrawing fluorine
substitutions were employed to further decrease HOMO energy levels and enhance the interchain
interaction of the BDTꢀDFQX copolymers. As evidenced by UV/Vis absorption spectra, Xꢀray
diffraction (XRD), grazing incidence Xꢀray scattering (GIXS) and transmission electron
microscope (TEM), PBDTꢀDFQXꢀTT with all thienyl side chains possesses the highest ordered
structure with the best planarity and tightest molecular packing, therefore, the PSC based on its
blend with PC71BM achieved the promising PCE of 7.68% with an exceptional fill factor of
about 70%. Whereas the device based on PBDTꢀDFQXꢀPP with all phenyl side chains just
exhibited a moderate PCE of 3.96%. These results demonstrate that modulating the conjugated
side chains both on donor and on acceptor units of copolymers simultaneously is an effective
way for constructing high performance photovoltaic copolymers.
Scheme 1. Chemical structures of copolymers with varied side groups reported in literature.
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Scheme 2. Molecular structures of designed BDTꢀDFQXꢀbased copolymers with different
conjugated side chains.
C4H9
C2H5
O
C2H5
C4H9
Ard
S
O
Ard
S
S
S
S
(Me)3Sn
Sn(Me)3
(Me)3Sn
Sn(Me)3
or
S
S
S
S
ii
i
O
1
S
Ard
2
O
M1
C4H9
M2
C2H5
C2H5
C4H9
C2H5
C4H9
C2H5
C4H9
O
O
C2H5
C2H5
C4H9
Ara
Ara
Ara
O
Ara
C4H9
S
S
S
Sn(Bu)3
O2N
NO2
Br
O
N
F
N
N
F
N
F
N
F
N
Br
S
S
S
S
Br
Br
or
iv
iii
F
F
F
F
5a
3
4
5b
C2H5
C4H9
C2H5
C4H9
O
O
C2H5
C2H5
C4H9
C4H9
S
S
N
F
N
N
F
N
S
S
S
S
Br
Br
Br
or
Br
v
F
F
M3
M4
Ara
Ara
Ara
=
=
OEH
M3
M3
Ard
=
=
OEH
EH
M1
M2
M1
M2
PBDT-DFQX-PP
PBDT-DFQX-TP
+
Ard
S
N
N
F
Ard
Ara
=
OEH
EH
+
+
+
S
S
S
S
S
vi
S
PBDT-DFQX-PT
PBDT-DFQX-TT
OEH
EH
n
Ard=
Ara
M4
M4
F
S
Ard
Ard
=
EH
Ara
=
6

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Scheme 3. Synthetic routes of the monomers and the corresponding copolymers. (i) 0°C, 15
min and then 50°C, 30 min; SnCl2/HCl/H2O, 50°C, 1 h. (ii) nꢀButyllithium, ꢀ78°C, 30 min;
trimethyltin chloride, ambient temperature, 30 min. (iii) Fe, CH3COOH, reflux, 12 h. (iv)
PdCl2(PPh3)2, 120°C, 24 h. (v) NBS, THF, ambient temperature,12h. (vi) Pd2(dba)3/P(oꢀtol)3,
130°C, 48h.
RESULTS AND DISCUSSION
Synthesis and Characterization.
The synthesis routes for copolymers are depicted in Scheme 3. For donor moiety, starting from
benzo[1,2ꢀb:4,5ꢀb']dithiopheneꢀ4,8ꢀdione 1, a Grignard reagent synthesis afforded compound 2a
or 2b. Compound 2a or 2b was converted into the corresponding distannylated monomer 1 or
monomer 2, which was purified by recrystallization with methanol. For the other coꢀmonomer 3
or 4, Stille coupling reaction between compound 4 and tributyl(thiophenꢀ2ꢀyl)stannane yielded
compound 5a or 5b. Bromination of compound 5 with Nꢀbromosuccinimide (NBS) in THF gave
the M3 or M4. M3 is a bright yellow solid, while M2 is an orange liquid. All the compounds 1-
5a (5b) and M1-4 were satisfactorily characterized by1H NMR, 13C NMR and elemental
analysis. PBDTꢀDFQXꢀPP, PBDTꢀDFQXꢀTP, PBDTꢀDFQXꢀPT and PBDTꢀDFQXꢀTT were
prepared by Stille coupling polymerization reaction between the corresponding monomers
(Scheme 3) catalyzed by Pd2(dba)3/P(oꢀtol)3. The synthesized polymers were purified by
continuous extractions with methanol, hexanes and chloroform, and the chloroform fractions
were recovered. The thiophene πꢀbridge in the main chain of the polymers was employed to
overcome the steric hindrance between the adjacent donor and acceptor units, and four branched
alkyl chain groups (see Scheme 2) were introduced to ensure the good solubility of the resulting
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polymers. As expected, all polymers exhibit good solubility in common organic solvents, such as
chloroform, toluene, chlorobenzene, and oꢀdichlorobenzene (DCB). Table 1 summarized the
polymerization results and thermal properties of the copolymers. The numberꢀaverage molecular
weight (Mn), weightꢀaverage molecular weight (Mw) and the polydispersity index (PDI) of the
copolymers, measured by GPC in THF at 30 ℃using polystyrene standards, showed that the Mn
values of PBDTꢀDFQXꢀPP, PBDTꢀDFQXꢀTP, PBDTꢀDFQXꢀPT and PBDTꢀDFQXꢀTT are 20.8,
40.1, 24.7, and 44.5 kDa, respectively with PDIs of 2.08, 1.59, 1.63 and 1.82 respectively (Table
1). We note that the molecular weights of the polymers with thienyl side chains are higher
compared with their phenyl side chain analogues. In particular the polymer of PBDTꢀDFQXꢀTT
containing four thienyl side chains, exhibits the highest molecular weight, whereas PBDTꢀ
DFQXꢀPP, which has four phenyl groups, shows the lowest molecular weight and broadest
polydispersity. This can be explained by the fact that poor solubility of the growing chain can
make polymers precipitate earlier in the reaction preventing further chain growth.11 These results
indicate that the number and nature of the conjugated sideꢀchains have significant influences on
the molecular weight of the polymers formed. The thermal transition and stability of polymers
were measured by TGA and DSC. DSC scans of the four polymers revealed that there were no
obvious phase transitions up to 250℃ (Supporting Information, Figure S1). As shown in Figure
1 and Table 1, TGA thermograms showed that PBDTꢀDFQXꢀPP, PBDTꢀDFQXꢀTP, PBDTꢀ
DFQXꢀPT and PBDTꢀDFQXꢀTT are thermally stable with onset decomposition temperatures
corresponding to 5% weight loss at 386, 335, 361, and 402℃, respectively. The results suggest
that all the polymers are stable enough for application in PSCs and can be potentially annealed at
elevated temperatures.
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Figure 1. TGA plots of the polymers with a heating rate of 10 ℃/min under an N2 atmosphere.
Table 1. Molecular weights and thermal properties of the polymers
yield
Td (℃)b
Polymer
Mn (kDa)a
MW(kDa)a
PDI
(%)
55.4
20.8K
40.1K
24.7K
44.5K
43.2K
63.9K
40.4K
81.3K
2.08
1.59
1.63
1.82
386
335
361
402
PBDT-DFQX-PP
PBDT-DFQX-TP
PBDT-DFQX-PT
PBDT-DFQX-TT
53.2
87.6
83.4
aMn, MW and PDI of the polymers were estimated by GPC using polystyrene as
standards in THF . b The 5% weightꢀloss temperatures in nitrogen.
Optical Properties.
Figure 2 shows the normalized optical absorption spectra of the BDTꢀDFQX based polymers
in dilute chloroform and as thin films deposited on quartz glass. The optical data, including the
absorption peak wavelengths (λmax), absorption edge wavelengths (λonset), and the optical band
gap (Egopt) are summarized in Table 2. Both solution and thin film spectra showed three
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characteristic absorption bands. The absorption in the wavelength range of 300ꢀ500 nm
originated from the localized πꢀπ* transition, whereas a relatively broad and intense absorption in
the range of 500ꢀ700 nm was attributed to the intramolecular charge transfer (ICT) between the
strong electronꢀaccepting fluorinated quinoxaline units (DFQXꢀP, DFQXꢀT) and the electronꢀ
donating 2DꢀBDT units (BDTꢀP,BDTꢀT).12 In the solution spectra, the UV absorption maximum
and the visible ICT peak of the DFQXꢀT based polymers are red shifted by ca.25nm compared to
the DFQXꢀP based polymers (Table 1). This could be due to another stronger ICT absorption,
which caused by the more electronꢀrich thienyl groups pendant onto the central quinoxaline
chromophore.13 Similar to the solution spectra, the thin film absorption spectra of the polymers
were identical in shape but obvious redꢀshifts of the absorption maxima (by ca. 30 nm) were
observed, indicating that aggregation of the polymer main chains and πꢀπ intermolecular
interactions occurred in the solid state.14 However, larger redꢀshift in λmax from solution to film
for PBDTꢀDFQXꢀTT (ca.60 nm) can be observed in comparison with that of the other three
polymers (ca.30 nm). This difference can be ascribed to that, with four conjugated thienyl groups,
PBDTꢀDFQXꢀTT is more favorable for the πꢀπ coꢀfacial overlapping perpendicular to polymer
chains.15 In addition, all the polymers, except PBDTꢀDFQXꢀPP, displayed vibronic features at
lowꢀ energy absorption band in thin film, implying the presence of ordered πꢀπ stacks, and these
ordered structures favor the charge mobility. The Egopt of PBDTꢀDFQXꢀPP, PBDTꢀDFQXꢀTP,
PBDꢀTDFQXꢀPT and PBDTꢀDFQXꢀTT are 1.83, 1.78, 1.75 and 1.72 eV, respectively,
calculated from the λonset of solid state films (Table 2). It is worth noting that PBDTꢀDFQXꢀTT
displayed the narrowest bandgap, whereas PBDꢀDFQXꢀPP displayed the largest bandgap. This
phenomenon could be attributed to the less steric effect, more aggregated conformation and more
electronꢀrich capability of thienyl side groups in PBDTꢀDFQXꢀTT, when contrasted with phenyl
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groups in PBDTꢀDFQXꢀPP.16 These results demonstrate convincingly that PBDTꢀDTQXꢀTT has
a better ability of lightꢀharvesting, therefore it could achieve a higher Jsc than the others when it
is employed to fabricate solar cells as donor materials. Clearly, to construct the BDTꢀDFQX
based polymers with a more effective chainꢀchain stacking and a narrower bandgap, it is better to
choose thienyl group as the conjugated side chains both for donor and acceptor units.
Figure 2. UV/Vis absorption spectra of polymers in CHCl3 solutions and in thin films.
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Table 2. Optical and electrochemical properties of the polymers
λ
max(nm)
λonset(nm) Egopt(e HOMO( LUMO
ec
Polymer
solution
film
Eg
film
V)
eV)
(eV)
327,413, 346,419,
554 584
PBDTꢀDFQXꢀPP
PBDTꢀDFQXꢀTP
PBDTꢀDFQXꢀPT
PBDTꢀDFQXꢀTT
678
1.83
ꢀ5.35
ꢀ3.53
1.82
1.73
1.71
1.70
347,418, 359,431,
559 583
695
707
717
1.78
ꢀ5.25
ꢀ5.29
ꢀ5.20
ꢀ3.52
ꢀ3.58
ꢀ3.50
350,428, 347,431,
578 595
1.75
359,435, 361,456,
586 642
1.72
Electrochemical Properties.
The electronic structures of the four copolymers, viz. highest occupied molecular orbital
(HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels, were estimated by cyclic
voltammetry.17 The measured cyclic voltammograms are illustrated in Figure S2 in the
Supporting Information and the results are summarized in Table 2. The onset potentials for
oxidation (Eox) were observed to be 0.64, 0.58, 0.54 and 0.49 V vs. Ag/Ag+ for PBDTꢀDFQXꢀPP,
PBDTꢀDFQXꢀPT, PBDTꢀDFQXꢀTP and PBDTꢀDFQXꢀTT, respectively. The onset potentials
for reduction (Ered) of them were found to be ꢀ1.18, ꢀ1.13, ꢀ1.18 and ꢀ1.21 V vs. Ag/Ag+,
respectively. The HOMO and LUMO energy levels of the copolymers were calculated from the
onset oxidation potential and the onset reduction potential according to the following equations
(2) and (3):18
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Figure 3. Energy level diagrams for the BDTꢀDFQXꢀbased polymers.
As shown in Figure 3, the LUMO energy levels of PBDTꢀDFQXꢀPP, PBDTꢀDFQXꢀTP,
PBDTꢀDFQXꢀPT and PBDTꢀDFQXꢀTT, are ꢀ3.53, ꢀ3.52, ꢀ3.58, and ꢀ3.50 eV, respectively.
Obviously, the LUMO energy levels of the conjugated polymers do not vary appreciably with
variation of the conjugated side chains. Meanwhile, all the LUMO energy levels are higher than
that of the nꢀtype material PC71BM (ꢀ3.91eV),19 ensuring energetically favorable electron
transfer. The HOMO energy levels of PBDTꢀDFQXꢀPP, PBDTꢀDFQXꢀTP, PBDTꢀDFQXꢀPT
and PBDTꢀDFQXꢀTT are estimated to be ꢀ5.35, ꢀ5.25, ꢀ5.29 and ꢀ5.20 eV, respectively. Thus
one can found that the four copolymers all possess rather lowꢀlying HOMO levels, due to the
incorporation of electronꢀwithdrawing fluorine atoms. The lowꢀlying HOMO level is expected
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for higher openꢀcircuit voltage (Voc) of the PSCs with the polymers as donor materials because
the Voc is usually proportional to the difference between the HOMO level of the donor and the
LUMO level of the acceptor. On the other hand, it is worth noting that the HOMO energy levels
of the polymers rise gradually as the phenyl conjugated side chains, either on donor or on
acceptor units, were replaced by thienyl step by step, and the difference in the HOMO energy
level between PBDTꢀDFQXꢀTT with all thienyl substituents and PBDTꢀDFQXꢀPP with all
phenyl substituents is 0.15 eV. The elevation of the HOMO energy level of thienyl substituted
copolymers contrasted to phenyl substituted analogues is mainly due to the enhanced electronꢀ
donating strength of thiophene compared to benzene.16 The phenomenon indicates that the
conjugated side chains significantly affect the HOMO levels of the DꢀA copolymers. Also, it can
be anticipated that PBDTꢀ DFQXꢀPPꢀbased PSCs would achieve a higher Voc.
Theoretical Calculations.
To evaluate the impact of the different conjugated side chains on molecular architecture and
consequently on the optoelectronic properties of the copolymers, density functional theory (DFT)
calculations were performed to verify stationary points as stable states for the optimized
conformations and single point energies, with a molecular main chain length n= 1, at b3lyp/6ꢀ
31g(d,p) level of theory in vacuum using the Gaussian program package. The final energies were
calculated as the sum of single point and zero point energies. In particular, the HOMO and
LUMO level positions and related electron distributions were calculated. Moreover, all the alkyl
chains were replaced by methyl groups in the calculation to avoid excessive computation
demand. Optimized geometries of the five copolymers in ground state are shown in Figure S3
(Supporting Information). Calculated dihedral angles and corresponding HOMO, LUMO levels
and bandgaps (Egcal) are summarized in Table 3. The general molecule structure sketch used for
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the computational data is shown in Figure 4, where θ1 is dihedral angle between BDT unit and
the side chain groups, θ2 is dihedral angles between Qx unit and the side chain groups.
Figure 4. Molecular geometry sketch used for the computational data.
Table 3. Calculated dihedral angles, bond angle and corresponding HOMO, LUMO levels
and bandgaps (Egcal) of the Polymers.
polymer
θ1(deg)
ꢀꢀ
θ2(deg) HOMO(eV)
LUMO (eV)
ꢀ2.35
Egcal (eV)
2.47
PBDTꢀDFQX
ꢀꢀ
ꢀ4.82
ꢀ4.89
ꢀ4.87
ꢀ4.84
ꢀ4.71
PBDTꢀDFQXꢀPP
PBDTꢀDFQXꢀTP
PBDTꢀDFQXꢀPT
PBDTꢀDFQXꢀTT
56.9
48.7
62.5
54.7
45.2
42.1
43.5
40.7
ꢀ2.32
2.57
ꢀ2.41
2.46
ꢀ2.29
2.55
ꢀ2.27
2.44
Different structures of conjugated side chain groups significantly affect the molecular
architecture, as shown in Table 3. The dihedral angles between the conjugated side chain groups
and the main chains of polymers are all 40°∼60°, indicating twisted conformation of
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copolymers. Note that θ1 and θ2 in thiophene substituted polymers is smaller than that in benzene
substituted polymers, owing to the relatively small bulk of thiophene.16 The smaller dihedral
angles of θ1 and θ2 is beneficial to the πꢀπ stacking of polymer main chains. On the other hand,
with the side chains varied from PP to TP, to PT then to TT, conformations of the relevant
polymers become more planar, which can be beneficial for charge transport. We speculated that
this thiophene side groups can be better to form coꢀplanar structure with BDTꢀDFQX, which is
consistent with previous reports.5a In this cases, the πꢀorbital overlap between neighboring
polymer main chains are increased, leading to changes in the frontier orbital energy levels and,
more specifically, to decrease in bandꢀgaps of thiophene substituted polymers in turn. The wave
functions of the frontier molecular orbital are depicted in Figure S4 (Supporting Information). As
can be seen in Table 3, although there are some discrepancies between the calculation and
experimental results, the calculated HOMO energy levels of the four model compounds also
follow the trends of variation: PBDTꢀDFQXꢀPP (ꢀ4.89 eV) <PBDTꢀDFQXꢀTP (ꢀ4.87 eV)
<PBDTꢀDFQXꢀPT (ꢀ4.84 eV) <PBDTꢀDFQXꢀTT (ꢀ4.71 eV). Furthermore, as demonstrated in
Figure S4, for all model compounds, the LUMO electron density is mainly located on the
acceptors (DFQXꢀP. DFQXꢀT), while the HOMO is delocalized along the whole πꢀconjugated
backbone. Similar to CV, the DFT results revealed that benzene substituted polymers have lower
HOMO levels and larger electrochemical bandgaps than thiophene substituted polymers.
Photovoltaic Properties.
To investigate the effect of the conjugated side chain pattern on the photovoltaic properties of
the copolymers, the devices were fabricated with the ITO/PEDOT: PSS/polymer: PC71BM/Ca/Al
structure. The polymer and PC71BM was dissolved into oꢀdichlorobenzene to make the DꢀA
mixed solution for spinꢀcoating. The device fabrication process is described in detail in the
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Experimental Section. Different weight ratios of polymer: PC71BM and thicknesses of the active
layers were carefully optimized. The measurements of photovoltaic performances were carried
out under illumination of AM1.5G simulated solar light at 100 mW·cmꢀ2. The corresponding
openꢀcircuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and power conversion
efficiency (PCE) of the devices are summarized in Table 4, while Figure 5 shows the
representative current densityꢀvoltage (JꢀV) curves of the best devices. The detailed photovoltaic
data of the optimization process for the PSCs are listed in Table S1 in the Supporting
Information.
Figure 5. Current densityꢀvoltage characteristics of the PSCs based on polymer:PC71BM blends
under illumination of AM1.5G, 100 mW·cmꢀ2.
Figure 6. Plots of ln(JL3/V2) versus (V/L)0.5for the measurement of the hole mobility in
polymer/PC71BM devices by the SCLC method.
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Table 4. Photovoltaic performances of the PSCs based on polymer/PC71BM under the
illumination of AM1.5G, 100 mW·cmꢀ2
Mobility
Jsc(
FF
(%)
Voc(V)
Ratioa
PCEc(%)
mA/cm2)
Polymer
(cm2V−1s−1)d
PBDTꢀDFQXꢀPP
PBDTꢀDFQXꢀPP
PBDTꢀDFQXꢀTP
PBDTꢀDFQXꢀPT
PBDTꢀDFQXꢀTT
1:2
1:2b
1:1
0.80
0.89
0.82
0.85
0.85
8.11
8.66
45.60
51.38
60.12
63.46
68.51
69.93
2.96
3.96
6.08
6.54
7.29
7.68
1.10×10ꢀ4
8.85×10ꢀ4
1.33×10ꢀ3
2.94×10ꢀ3
4.09×10ꢀ3
6.28×10ꢀ3
12.34
12.13
12.53
12.77
1:1
1:1.2
PBDTꢀDFQXꢀTT 1:1.2b 0.86
[a]Polymer/PC71BM weight ratio. [b]Treatment with methanol.[c] Optimized data.
[d] Measured by using the SCLC method.
As shown in Table 4, under the optimal conditions without any treatment and solvent additive,
the highest PCE of the devices were 2.96%, 6.08%, 6.54% and 7.29% for PBDTꢀDFQXꢀPP,
PBDTꢀDFQXꢀTP, PBDTꢀDFQXꢀPT and PBDTꢀ DFQXꢀ TT, respectively. As shown in Figure 5
and Table 4, PBDTꢀDFQXꢀPP exhibited a lower Jsc of 8.11mA·cmꢀ2, most likely due to a
relatively broad band gap and blueꢀshifted absorption, whereas the Jscs of the other three
polymers were relatively higher, in the range of 12.13∼12.53 mA·cmꢀ2. On the other hand, it can
be observed in Table S1, the Voc of devices decreased from 0.89 V to 0.85 V, owing to the
upshift of the HOMO levels from PBDTꢀDFQXꢀPP to PBDTꢀDFQXꢀTT. An increase in the
PC71BM content also resulted in a slight decrease in the Voc and these results are in accordance
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with previously reported ones.20Although the introduction of phenyl side chains successfully
enhances the Voc of PBDTꢀDFQXꢀPPꢀbased device, this is not sufficient to offset the reduction in
Jsc and FF, especially for an unsatisfactory FF of 45%, leading to a disappointing PCE of 2.96%.
It is interesting to note that thienyl substituted polymers exhibit superior photovoltaic properties
achieving PCE of >6.50% to phenyl substituted ones, especially, the polymers with thienyl side
chains on acceptor units possess better performance than those polymers with thienyl side chains
on donor units, and lowering the ratio of thienyl in the conjugated side chains leads to lower
performance: as the proportion of thienyl side chains decreased from 100% to 50% to 0, the
maximum PCE dropped from 7.29% to 6.54% to 6.08%, then to 2.96%. Thus, the PBDTꢀ
DFQXꢀTTꢀbased device exhibits the highest PCE of 7.29% at the optimized film thickness of
110 nm, which is 2.5 times higher than that of the PBDTꢀ DFQXꢀPPꢀbased device. This
phenomenon indicates that thienyl side chains are more beneficial to achieve high PCEs of
relevant polymers, which, on the one hand, can be attributed to the higher hole mobilities of
thienyl substituted polymers (the hole mobilities were shown in table 4 and Figure 6), on the
other hand, can be ascribed to the excellent selfꢀaggregation and proper phase separation of
thiophene side chains substituted polymers when blended with PC71BM.5a It is wellꢀknown that
processing additives, such as1,8ꢀdiiodooctane (DIO), 1ꢀchloronaphthalene (CN) can dramatically
enhance device performance. Thus we also checked the effect of DIO or CN additive on the four
copolymers. Disappointingly, when an amount of DIO or CN(3%, v/v) was added as the
processing additive, the results showed the additive of DIO or CN has no positive effect on the
device performance of the four polymers. PCE of the relevant devices exhibited a slight
reduction due to the decrease of Voc or FF (Table S1).
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Table 5. Characteristics of PBDTꢀDFQXꢀTTꢀbased PSCs under different processing conditions
Ratioa
1:1
1:1.2
1:1.5
1:2
1:3
1:1b
1:1.2b
1:1.5b
3.15×10ꢀ 4.09×10ꢀ 3.73×10ꢀ 3.89×10ꢀ 2.53×10ꢀ 1.66×10ꢀ 6.28×10ꢀ 7.08×10ꢀ
Mobility(cm2V−1s−1)c
3
3
3
3
3
3
3
3
FF (%)
69.15
7.16
68.51
7.29
67.90
6.99
69.92
6.92
69.07
5.62
64.49
7.36
69.93
7.68
70.79
7.41
PCE (%)d
[a] Polymer/PC71BM weight ratio. [b] Treatment with methanol.
[c] Measured by using the SCLC method. [d] Optimized data.
Inspired by recent work on postꢀsolvent treatment,21 we tried to employ methanol to further
optimize the performance of PSCs with a modified procedure:(i) spinꢀcoated the active layer and
dried under vacuum; (ii) 60µl methanol was added atop the active layer and 30 seconds was
allowed; (iii) solvent was removed by spin coating at high speed (3000 rpm); (iv)the Ca/Al was
evaporated (as shown in experimental section). From Table 4 and Table S1 in the Supporting
Information, one can observe that different polymers showed different performance for the
methanol treatment. Cheerfully, for PBDTꢀDFQXꢀPP and PBDTꢀDFQXꢀTT, photovoltaic
performance of the two polymers had been enhanced to some extent, achieving PCEmax of 3.96%
and 7.68% respectively. To date 7.68% is one of a few Qxꢀbased PSCs which exhibited PCE
exceeding 7.5%. It is worthwhile to note that the FF of PBDTꢀDFQXꢀPP was enhanced from
45% to 51% after methanol treatment, as well as the hole mobility was promoted to 8.85×10ꢀ4
from 1.10×10ꢀ4 cm2V−1s−1. These might be originated from the relatively low surface resistance
of PBDTꢀDFQXꢀPP blend films,22 which caused by methanol. Figure S5 (Supporting
Information) shows the hole mobilitiy, FF and PCE of the PBDTꢀDFQXꢀTTꢀbased PSCs under
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different processing conditions and the results are summarized in Table 5. As demonstrated by
Figure S5 and Table 5, it appears that PBDTꢀDFQXꢀTT is a star polymer, its PSC achieved high
fill factor of > 62.0% and promising PCE of > 5.6% at a relative broad weight ratio of
polymer/PC71BM. Meanwhile, the hole mobility of all relevant devices based on PBDTꢀDFQXꢀ
TT keep the value of 10ꢀ3 cm2V−1s−1, and the high hole mobility partially account for stable and
high Jsc and FF values. So with four thienyl side chains, the PBDTꢀDFQXꢀTT demonstrated the
best photovoltaic performance with an excellent PCE of 7.68%. However for those BDTꢀDFQXꢀ
based polymers reported in literature, in which the optimized conjugated side chains were
attached either on donor or acceptor units, the relative PSCs just exhibited the highest PCE of
6.1%.23 Based on the above obtained experimental results, we can conclude that optimizing the
conjugated side chains both on donor and on acceptor units synchronously would be a more
effective strategy to improve the photovoltaic properties of BDTꢀDFQXꢀcontaining copolymers.
Figure 7. EQE spectra of the PSCs based on polymer/PC71BM blends.
The external quantum efficiency (EQE) curves of the devices fabricated under the optimal
conditions are shown in Figure 7. It can be noted that the EQE responses of all the polymers in
the visible region can be attributed to both the intrinsic absorption of the polymers and the
response of PC71BM. Among them, the device based on PBDTꢀDFQXꢀPP/PC71BM (1:2, w/w)
exhibits a response range from 350 nm to 650 nm and a relatively low EQE value, with two
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maximum peak values of 49% at∼384nm and of 51% at ∼520nm. After methanol treatment,
EQE of PBDTꢀDFQXꢀPP is slightly improved both in long wavelength region and in short
wavelength one, which should be ascribed to the change of microstructure of blend film.
However, compared with the PBDTꢀDFQXꢀPPꢀbased device, the other devices show a
significant improvement of EQE in the wavelength range 400ꢀ600 nm, with the peak values
exceeding 70%. For the convenience of analysis, the absorption spectra of the blend films
prepared by the optimal conditions were shown in Figure S6 (Supporting Information). One can
see that the shapes of the EQE spectra of the devices are similar to the absorption spectra of their
active layer used. The blend films showed remarkably broad absorption in the range of 300ꢀ450
nm mainly due to the PC71BM absorption in this region. The absorption ability of PBDTꢀDFQXꢀ
TP and PBDTꢀDFQXꢀTT based devices were stronger than that of others from 500 to 700 nm.
The high EQE and enhanced optical absorption of the active layer means more efficient
photoelectron conversion in this range, hence benefits for the improvement of Jsc. The difference
in EQE and absorption spectra curves under the optimal conditions result in the different Jsc
values of the devices. These results were consistent with the data in JꢀV measurements, and the
difference between the Jsc and the integral of the EQE by the solar irradiation spectrum, AM
1.5G, was∼4%, which provided the solid evidence for the reliability of the photovoltaic results.
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Microstructure
Figure 8. Topographic AFM images (5×5ꢁm2) of the blend films of PBDTꢀDFQXꢀPP/PC71BM
1:2 w/w (a, without methanol treatment; b, with methanol treatment);PBDTꢀDFQXꢀTP/PC71BM
1:1 w/w (c); PBDTꢀDFQXꢀPT/PC71BM 1:1 w/w (d); PBDTꢀDFQXꢀTT/PC71BM 1:1.2 w/w (e,
without methanol treatment; f, with methanol treatment).
To investigate the effect of microstructure of polymer/PC71BM films on device performance,
AFM, TEM and GIXS measurements were performed. The surface morphology recorded by
tapping mode AFM is shown in Figure 8 (topography images). Under the conventional
conditions, all polymers display uniform and smooth films, with a small rootꢀmeanꢀsquared
(RMS) of 0.61∼1.53 nm. In contrast, the treatment of methanol for the PBDTꢀDFQXꢀPP
increased the RMS of blend film from 0.61 nm to 1.29 nm (see Figure 8 a, b). Similar to PBDTꢀ
DFQXꢀPP, the surface of the active layer became clearly rougher with increasing RMS of 1.23 to
3.61nm for PBDTꢀ DFQXꢀTT after the methanol treatment (see Figure 8 e, f). The larger RMS
probably resulted from the enhanced aggregation of the donor polymers and acceptor fullerene in
the active layer. Although polar solvent treatment did not provide an adequate driving force for
reconstructing the morphologies of the polymer films, some of the polar solvent could penetrate
the active layer.24 After wetting for 30 seconds, PC71BM particles could be redistributed to
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reduce the contact areas with methanol and led to sufficient aggregations with donor polymers.
Consequently, the formation of phase separation of the blend films occurred. The nanoscale
morphologies of polymer/PC71BM films were further studied using TEM. As seen in Figure 9 a,
largeꢀscale phase separation could be observed and the clear phase boundary of PBDTꢀDFQXꢀ
PP/PC71BM is chapped and discontinuous, which is harmful to exciton diffusion, exciton
separation and charge transport. Thus, PBDTꢀDFQXꢀPPꢀbased device achieved both a relative
low Jsc and FF. Unlike the film of PBDTꢀDFQXꢀPP/PC71BM, a nanoscale interconnected
network structure between fibrilꢀlike PBDTꢀDFQXꢀPP and PC71BM is well developed in 1 : 1.2
ratio film (see Figure 8 d). The well developed interconnected network structure affords not only
high Jsc but also excellent FF. However, it could be observed that the PBDTꢀDFQXꢀTP or
PBDTꢀDFQXꢀPT blend film exhibits moderately homogeneous and there were no evidence of
detrimental scale phase segregation, indicating that both of them possessed good miscibility with
PC71BM (see Figure 8 b, c). It also resulted in the satisfactory PCEs of corresponding devices.
Figure 9. TEM images of the blend films of PBDTꢀDFQXꢀPP/PC71BM (a); PBDTꢀDFQXꢀ
TP/PC71BM (b); PBDTꢀDFQXꢀPT/PC71BM and PBDTꢀDFQXꢀTT/PC71BM.
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Figure 10. GIXS of polymer/PC71BM blended thin film (a) inꢀplane, (b) Outꢀofꢀplane.
To gain deeper insight of the crystallinity and molecular organization of the polymer thin films,
Xꢀray diffraction analysis (XRD) of the pristine polymer films was performed. As shown in
Figure S7 (Supporting Information), for all polymers, a relatively weak diffraction peak can be
observed at about 2θ= 22.8ꢀ23.4°, which corresponds to the πꢀπ stacking distances (d πꢀꢀπ= 3.79ꢀ
3.91 Å) between coplanar conjugated polymers. Compared with PBDTꢀDFQXꢀPP (d πꢀꢀπ= 3.91
Å), PBDTꢀDFQXꢀTT (d πꢀꢀπ= 3.79 Å) displayed an enhanced intensity diffraction as well as a
shortening of the πꢀπ stacking distance. Interestingly, PBDTꢀDFQXꢀTT thin film exhibited a
clear diffraction peak at 2θ= 3.7°, corresponding to the lamellar distance of 23.80 Å. While no
relative diffraction peaks were detected in other three polymer films. The results reveal that the
film of PBDTꢀDFQXꢀTT has a relatively order structure and stronger intermolecular interactions,
25 which can be partly supported by the observation from the UV spectra, where PBDTꢀDFQXꢀ
TT shows the strongest vibrational peak at long wave direction. All these differences could be
due to the less steric hindrance between thienyl side groups and the neighboring groups of
another molecule.26 In order to further understand the microstructures of polymers, the
preferential orientation of the blend films was investigated by GIXS. Figure 10a and 10b shows
inꢀplane (IP) and the outꢀofꢀplane (OOP) GIXS profiles of the blend films, including PBDTꢀ
DFQXꢀPP/PC71BM blend (1:2, w/w, treated with methanol), PBDTꢀDFQXꢀTP/ PC71BM blend
(1:1, w/w), PBDTꢀDFQXꢀPT/PC71BM blend (1:1, w/w) and PBDTꢀDFQXꢀTT /PC71BM blend
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(1:1.2, w/w, treated with methanol) cast from oꢀDCB (10 mg/ml), respectively. As shown in
Figure 10, for all the blend films, both the OOP (100) and IP (100) reflections could be observed.
However, the stronger intensity of the (100) reflection in the OOP profile than in the IP profile
indicated that the lamellar packing BDTꢀDFQXꢀbased polymers preferentially stacked out of the
film plane (edgeꢀon rich orientation), though with some randomness. The results may be
attributed to the introduction of branched alkyl sideꢀchains in polymers resulting in the
misorientation of the polymer lamellae. 27 When compared with XRD patterns of the pristine
polymer films (Figure S7), it is noteworthy that a clear lamellar diffraction peak could be
observed in the GIXS profiles (Figure 10). Additionally, stacking peaks in GIXS profiles became
sharper and more intense than those in XRD patterns upon blending with PC71BM, suggesting
that structures of the blend films became more ordered. The result may arise from the strong
polymerꢀfullerene interactions that delay the fullerene selfꢀassembly in the final drying process.28
Consequently, fullerene could induce polymer selfꢀassembly towards the kinetics equilibration of
the blends' phase separation.29 The IP profile of the blend films showed a pronounced (100)
diffraction peak at 0.235, 0.237, 0.244, and 0.265Åꢀ1, for PBDTꢀDFQXꢀPP, PBDTꢀDFQXꢀTP,
PBDTꢀDFQXꢀPT and PBDTꢀDFQXꢀTT respectively, corresponding to the conjugated side chain
packing distance of 26.74, 26.51, 25.75, and 23.71 Å. Thus, we can conclude that the polymers
with thienyl side chains exhibit a shorter interchain distance than those with phenyl side chains.
Due to the strong polymerꢀfullerene interactions, the reflection of polymers corresponding to the
πꢀ π stacking peak overlapped with the PC71BM peak.30 In addition, we can observe that
intensities of the πꢀ π and lamellar stacking peaks increased gradually from PBDTꢀDFQXꢀPP to
PBDTꢀDFQXꢀTT (Figure 10). This phenomenon indicated that a relatively higher degree of
crystallinity formed for PBDTꢀDFQXꢀTT, which coincides with the images estimated from TEM
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(see Figure 8 d). Moreover, the smaller lamellar spacing of PBDTꢀDFQXꢀTT relative to the
phenyl sideꢀchain derivatives benefits charge transport across domains as well as πꢀelectron
hopping among the polymer chains. 31 Obviously, the tighter packing and higher degree of
crystallinity of PBDTꢀDFQXꢀTT correspond well with the enhanced value of hole mobility and
FF (Table 4 and Table 5). These results suggest that the presence of large amounts of thienyl side
chains contribute to highly ordered structure and effective charge transport, leading to PBDTꢀ
DFQXꢀTT with a promising PCE of 7.68%. Whereas phenyl side chains suppress the
aggregation of BDTꢀDFQX polymers in the solid state, which may hinder the charge carrier
transport of the blend film of polymer and PC71BM, resulting in PBDTꢀDFQXꢀPP with a
moderate PCE of 3.98%.
CONCLUSION
In this work, four BDTꢀDFQXꢀbased derivatives were synthesized and characterized to
investigate the effect of conjugated side chain modulation on photovoltaic properties. In these
four polymers, different side chain combinations including pꢀalkoxyphenyl and pꢀalkoxyphenyl
(PP), 2ꢀalkylthienyl and pꢀalkoxyphenyl (TP), pꢀalkoxyphenyl and 2ꢀalkylthienyl (PT), 2ꢀ
alkylthienyl and 2ꢀalkylthienyl (TT) were used to tune the photoelectronic properties. DFT
calculations reveal that the thiophene side groups show weaker steric hindrance and smaller
dihedral angle to the backbones than that of benzene side groups. The difference of the steric
bulk, electronꢀdonating ability and aromaticity between thiophene and benzene appeared to
greatly affect the morphology, charge transport and photovoltaic performance of devices. The
photovoltaic results indicate that PBDTꢀDFQXꢀTT and PBDTꢀDFQXꢀPP show totally different
photovoltaic characteristics in device, and PCEs of 7.68% and 3.96% were obtained,
respectively. Whereas devices incorporating PBDTꢀDFQXꢀTP or PBDTꢀDFQXꢀPT with
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PC71BM exhibited PCEs of 6.08% or 6.54% .This can be ascribed to the enhanced interchain πꢀπ
stacking effect and wellꢀdeveloped fibril structure of PBDTꢀDFQXꢀTTꢀbased device. This work
could provide some insight into side chain engineering to construct efficiently photovoltaic
polymers.
EXPERIMENTAL SECTION
Materials
Scheme 2 shows the synthetic routes to the copolymers. Monomers 2,6ꢀBis(trimethyltin)ꢀ4,8ꢀ
bis(4ꢀethylhexyloxyꢀ1ꢀphenyl)ꢀbenzo[1,2ꢀb:4,5ꢀb’]ꢀdithiophene (M1),10a 2,6ꢀBis(trimethyltin)ꢀ
4,8ꢀbis(5ꢀ(2ꢀethylhexyl)thiophenꢀ2ꢀyl)ꢀbenzo[1,2ꢀb:4,5ꢀb’]ꢀdithiophene (M2),10b 5,8ꢀBis(5ꢀ(2ꢀ
ethylhexyl)thiophenꢀ2ꢀyl)ꢀ6,7ꢀdifluoroꢀ2,3ꢀbis(4ꢀ(ethylhexyloxy)phenyl)quinoxaline (M3)10c and
5,8ꢀBis(5ꢀ(2ꢀethylhexyl)thiophenꢀ2ꢀyl)ꢀ6,7ꢀdifluoroꢀ2,3ꢀbis(5ꢀethylhexythiophenꢀ2ꢀyl)
quinoxaline (M4)9b were synthesized according to literature procedures. Tetrahydrofuran (THF)
was dried over Na/benzophenoneketyl and freshly distilled prior to use. Other reagents and
solvents were of commercial grade and used as received without further purification. All
reactions were performed under a nitrogen atmosphere.
Synthetic Procedures
Synthesis of poly{4,8ꢀbis(4ꢀethylhexyloxyꢀ1ꢀphenyl)ꢀbenzo[1,2ꢀb:4,5ꢀb’]ꢀ dithiopheneꢀaltꢀ5,8ꢀ
bis(5ꢀthiophenꢀ2ꢀyl)ꢀ6,7ꢀdifluoroꢀ2,3ꢀbis(4ꢀ (ethylhexyloxy) phenyl)quinoxaline} (PBDTꢀ
DFQXꢀPP)
A mixture of 0.1 mmol of M1 (92.45 mg) and an equal quantity of M3 (89.70 mg) were mixed
in 6 mL of degassed chlorobenzene. After being purged with nitrogen for 15 min, Pd2(dba)3
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Page 29 of 39
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C4TA05445H
(0.005 mol, 5 mg) and P(oꢀtol)3 (0.005 mol ,12.5 mg) were added and the mixture was then
purged with nitrogen for another 15 min. The reaction mixture was vigorously stirred at 130℃
for 48 h. When the reaction mixture was cooled to room temperature, the polymer was
precipitated by slow dropwise addition into methanol (200 mL), collected by filtration and
washed with methanol. Then the solid was subjected to Soxhlet extraction with methanol,
hexane, and chloroform. The polymer was recovered as solid from the chloroform fraction by
precipitation from methanol, filtered off, and dried to afford PBDTꢀDFQXꢀPP copolymer as a
bronze black solid (74 mg, 55.4%). Mn=20.8 kDa; MW=43.2 kDa; PDI=2.08; 1H NMR (600
MHz, CDCl3): δ(ppm) 7.94ꢀ7.28 (br, 14 H), 7.18ꢀ6.83 (br, 10 H), 3.99ꢀ3.89 (br, 8 H), 1.82ꢀ
0.64 (br, 60 H); Elemental analysis calcd (%) for (C82H92N2F2O4S4)n: C 73.73, H 6.94, N 2.10, S
9.60; found: C73.29, H7.32, N 2.27, S 9.17.
Synthesis of poly{4,8ꢀbis(5ꢀ(2ꢀethylhexyl)thiophenꢀ2ꢀyl)benzo[1,2ꢀb:4,5ꢀb’]dithiopheneꢀaltꢀ5,8ꢀ
bis(5ꢀthiophenꢀ2ꢀyl)ꢀ6,7ꢀdifluoroꢀ2,3ꢀbis(4ꢀ(ethylhexyloxy) phenyl)quinoxaline} (PBDTꢀDFQXꢀ
TP)
PBDTꢀDFQXꢀTP was synthesized and purified following the same procedure for PBDTꢀ
DFQXꢀPP, but monomer M2 (90.45 mg, 0.1 mmol) was used instead of M1. PBDTꢀDFQXꢀTP
was obtained as a black solid (70 mg, 53.2%). Mn=40.1 kDa; MW=63.9 kDa; PDI=1.59; 1H NMR
(600 MHz, CDCl3): δ(ppm) 7.80ꢀ7.43 (br, 10 H), 7.09ꢀ6.87 (br, 8 H), 3.92ꢀ3.64 (br, 8 H), 1.93ꢀ
0.69 (br, 62 H); Elemental analysis calcd (%) for (C78H88N2F2O4S6)n: C 71.19, H 6.74, N 2.13, S
14.62; found: C 72.03, H 6.32, N 2.07, 14.93.
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