Fluorene Side-Chained Benzodithiophene Polymers for Low Energy Loss Solar Cells

Article abstract of DOI:10.1021/acs.macromol.7b00998

Here we design and synthesize one novel fluorene side-chained benzodithiophene (BDT) monomer for polymer solar cells (PSCs) donor. By copolymerizing this monomer with 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT) or 4,7-di(4-(2-ethylhexyl)-2-thienyl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT), two donor-acceptor (D-A) conjugated polymers PFBDT-DTBT and PFBDT-DTffBT are prepared. PSCs are prepared with these polymers as donor and PC₇₁BM as acceptor. The maximum power conversion efficiency (PCE) of the two polymers PFBDT-DTBT and PFBDT-DTffBT based PSCs is 7.13% (V_OC = 0.90 V, J_SC = 13.26 mA cm^-2, and FF = 0.598) and 7.33% (V_OC = 0.96 V, J_SC = 13.24 mA cm^-2, and FF = 0.577). The UV-vis absorption and electrochemical cyclic voltammetry test results show that F atoms in DTffBT unit present an obvious influence on intermolecular effect and molecular energy levels of polymers. Furthermore, the energy loss of two PSCs devices in this work is confirmed to be 0.78 and 0.71 eV, lower than most results based on BDT PSCs devices, which is critical to obtain high PCE PSCs devices with a decent trade-off between J_SC and V_OC.

Full text of DOI:10.1021/acs.macromol.7b00998

Article
pubs.acs.org/Macromolecules
Fluorene Side-Chained Benzodithiophene Polymers for Low Energy
Loss Solar Cells
Xin Song,† Yongchao Zhang,‡ Yonghai Li,‡ Feng Li,§ Xichang Bao,‡ Dakang Ding,† Mingliang Sun,*,†
and Renqiang Yang*,‡
†Institute of Material Science and Engineering, Ocean University of China, Qingdao 261000, People’s Republic of China
‡CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of
Sciences, Qingdao 266101, People’s Republic of China
§Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Engineering,
Qingdao University of Science & Technology, 53 Zhengzhou Road, Qingdao 266042, People’s Republic of China
S
* Supporting Information
ABSTRACT: Here we design and synthesize one novel fluorene
side-chained benzodithiophene (BDT) monomer for polymer solar
cells (PSCs) donor. By copolymerizing this monomer with 4,7-
di(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT) or 4,7-di(4-(2-
ethylhexyl)-2-thienyl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT),
two donor−acceptor (D−A) conjugated polymers PFBDT−DTBT
and PFBDT−DTff BT are prepared. PSCs are prepared with these
polymers as donor and PC71BM as acceptor. The maximum power
conversion efficiency (PCE) of the two polymers PFBDT−DTBT
and PFBDT−DTff BT based PSCs is 7.13% (VOC = 0.90 V, JSC
=
=
13.26 mA cm−2, and FF = 0.598) and 7.33% (VOC = 0.96 V, JSC
13.24 mA cm−2, and FF = 0.577). The UV−vis absorption and
electrochemical cyclic voltammetry test results show that F atoms in
DTff BT unit present an obvious influence on intermolecular effect and molecular energy levels of polymers. Furthermore, the
energy loss of two PSCs devices in this work is confirmed to be 0.78 and 0.71 eV, lower than most results based on BDT PSCs
devices, which is critical to obtain high PCE PSCs devices with a decent trade-off between JSC and VOC
.
achieve a good balance among all the photovoltaic parameters
including open circuit voltage (VOC), short circuit current
density (JSC), and fill factor (FF), which are directly
INTRODUCTION
■
Polymer solar cells (PSCs) have received significant attention
due to the potential for the fabrication of light-weight, large-
area, and flexible light harvesting devices through low-cost
solution processing.1−6 Among them, bulk heterojunction
(BHJ) solar cells based on the blend of donor and acceptor
materials have been proved to be one of the most efficient
structures, and the power conversion efficiency (PCE) with
fullerene acceptors has achieved over 10% in both single- and
multijunction cells.7−10 However, the classical fullerene accept-
ors (for example, PC71BM: [6,6]-phenyl-C71-butyric acid
methyl ester) still have intrinsic drawbacks including the
relatively narrow absorption region and low molar extinction
coefficients, which would cause inefficient harvesting of solar
energy. Therefore, the basic optoelectronic properties of donor
materials are important and mostly determine photocurrent
and photovoltaic efficiencies of polymer solar cells.
proportional to power conversion efficiency (PCE = JSC
×
VOC × FF/Pin), where Pin is the input light power. Benzo[1,2-
b:4,5-b′]dithiophene (BDT) is one of the most ideal electron-
rich units for efficient D−A polymers due to its rigid
conjugated planarity and suitable electron-donating ability.13−16
Yu’s group first reported efficiency polymer PTB7 based on
BDT unit as donor material. The PSCs devices based on PTB7
and PC71BM exhibited excellent photovoltaic property with
PCE up to 7.4%.17 Furthermore, Li and Hou developed two-
dimensional (2D) conjugated polymers with π-conjugated
groups as side chains of BDT unit instead of alkoxy chains to
modify the absorption spectra and molecular energy levels,
which successfully enhanced the JSC and VOC simultane-
ously.18−20 Various π-conjugated substitutions with different
electronic properties and steric effect have been investigated as
BDT side chains, and some efficient D−A polymers (for
D−A conjugated polymers including electron-rich and
-deficient units along the polymer backbones have been
intensively investigated for the convenience of tuning band
gap and energy levels through the intramolecular charge
transfer (ICT) effect.11,12 However, the design of efficient
donor materials is still a challenging task because it should
Received: May 16, 2017
Revised: August 2, 2017
© XXXX American Chemical Society
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Scheme 1. Synthetic Routes of PFBDT−DTBT and PFBDT−DTffBT Polymers
example, PTB7-Th and PBDB-T) were obtained with PCEs
exceeding 10% in fullerene and fullerene-free based solar
cells.21−25 The results reveal that the varieties of π-conjugated
substitutions in BDT side chains present a significant influence
in optoelectronic properties of the resulted D−A polymers and
photovoltaic efficiencies of PSCs.
In this work, we design and synthesized two D−A conjugated
polymers based on novel electron-donating 2D BDT unit with
fluorene as the conjugated side chains and electron-with-
drawing units 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole
(DTBT) and 4,7-di(4-(2-ethylhexyl)-2-thienyl)-5,6-difluoro-
2,1,3-benzothiadiazole (DTff BT). The results show that F
atoms in DTff BT unit present an obvious influence on
intermolecular effect and molecular energy levels of polymers.
BHJ-PSCs were fabricated with PC71BM as the acceptor
material. PSCs devices based on PFBDT−DTBT show the best
PCE up to 7.13%, with VOC = 0.90 V, JSC = 13.26 mA cm−2, and
FF = 0.598. Meanwhile, devices based on PFBDT−DTff BT
show largely enhanced VOC (0.96 V) and the best PCE up to
7.33% without any additives or annealing process. Molecular
conformations, photophysical/electrochemical properties, and
film morphology of blend films were systematically investigated
to reveal the structure−property relationships. This work
suggests that fluorene moiety is a promising π-conjugated side
chain group attached in BDT for the construction of efficient
D−A conjugated photovoltaic polymers.
As we all know, the fluorene unit has usually been used as
electron-donating moiety to design D−A conjugated polymers
due to its rigidity, high hole mobility, and highly conjugated
structures.26,27 Andersson’s group introduced the fluorene unit
to design donor materials of PSCs.28 The resulting polymer
PFDTBT exhibited good efficiency with PCE of 2.2%. Next,
Andersson reported a series of D−A materials based on
fluorene with different alkyl and alkoxy side chains with PCE up
to 3.5%.29 In order to improve the solubility and aggregation
properties of polymers, Hou synthesized the D−A copolymer
PFDTBT and BisDMO-PFDTBT with the PCE of 4.5% and
3.5%, respectively.30 Woo also brought the fluorene unit into
the backbone of highly crystallized polymers to disrupt partially
the interchain organizations of excessive aggregation and
improved the PCE from 2.6% to 6.6%.31 Besides, the
derivatives of fluorene (including PFN and PFN-Br) were
wildly used as interface materials in PSCs field.32−34 However,
to the best of our knowledge, the fluorene unit is generally
widely used as the electron-rich unit to construct the polymer
backbone, and the photovoltaic polymers with fluorene unit as
side chains have not been reported. Considering the excellent
property of fluorene unit in polymer backbones, we are
attracted by its potential application when it is used as π-
conjugated side chains of D−A polymers.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The structure and
synthetic routes of D−A polymers are outlined in Scheme 1.
The fluorene-substituted BDT monomer was designed as
electron-donating moiety (D), and DTBT and DTff BT are
selected as two electron-withdrawing moieties (A). These two
polymers PFBDT−DTBT and PFBDT−DTff BT were pre-
pared by Stille polymerization in the presence of Pd2(dba)3 and
P(o-tolyl)3 as catalyst and ligand, respectively. The thermal
B
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Table 1. Decomposition Temperature, Molecular Weight, Optical Properties, and Frontier Energy Levels of Polymers
λmax (nm)
a
b
c
c
c
polymers
Td (°C)
Mn (kDa)
PDI
solution
film
Egopt (eV)
HOMOcv (eV)
LUMOcv (eV)
Egcv (eV)
PFBDT− DTBT
PFBDT−DTff BT
371
392
45.1
57.1
1.59
1.09
607
588
636
645
602
648
1.71
1.79
−5.41
−5.82
−3.60
−3.70
1.81
2.12
a
b
c
Measured under N2 atmosphere. Calculated based on the onset of thin films absorption spectra. Calculated from cyclic voltammograms.
Figure 1. (a) UV−vis absorption spectra of PFBDT−DTBT and PFBDT−DTff BT in dilute o-DCB solutions and thin films. (b) Temperature-
dependent absorption spectra of PFBDT−DTff BT in dilute o-DCB solutions.
Figure 2. (a) Cyclic voltammograms of polymers on glassy carbon electrodes in 0.1 M Bu4NPF6−CH3CN at a scan rate of 100 mV s−1. (b) Energy
level diagrams of the materials in this work.
stabilities of the polymers were determined by thermogravi-
metric analysis (TGA) in a nitrogen atmosphere at a heating
rate of 10 °C min−1. The TGA curves are shown in Figure S1,
and data were collected in Table S1. TGA plots reveal that
polymers PFBDT−DTBT and PFBDT−DTffBT exhibit good
thermal stability with decomposition temperatures (Td) 371
and 392 °C at 5% weight loss under nitrogen, respectively,
which indicates that the thermal stability of the polymers is
good enough for PSCs applications. The number-average
molecular weight (Mn) and polydispersity index (PDI = Mw/
Mn) were measured by gel permeation chromatography (GPC)
using tetrahydrofuran (THF) as the eluent and polystyrenes as
the internal standards, and the data are collected in Table 1. As
Table 1 indicates, polymer PFBDT−DTff BT shows higher Mn
and more favorable polydispersity index (1.09), suggesting
more orderly polymer backbones which should benefit the
charge transport of PSCs devices.
in Figure 1a, and the relevant data are summarized in Table
1.The two polymers show similar spectra profiles with broad
absorption from 350 to 730 nm covering most of visible
spectrum, with two distinct absorption bands at 380−480 and
480−730 nm in solutions and films. The peaks at 380−480 nm
are probably corresponding to the π−π* transition, and the
major peaks at 480−730 nm should be attributed to the strong
intermolecular charge transfer (ICT) effect between the donor
and acceptor moieties. Moreover, the UV−vis spectra in thin
films are red-shifted compared to those of solutions, which is
attributed to the enhanced intermolecular interactions in the
solid state. Nevertheless, there are still some differences
between polymer PFBDT−DTBT and PFBDT−DTff BT.
The absorption spectra of PFBDT−DTffBT in solution and
thin film exhibit obvious hypochromatic shift compared to
those of PFBDT−DTBT. Analogous results were observed in
other fluorine-substituted oligomers and polymers.35 Different
from PFBDT−DTBT, the absorption spectra of PFBDT−
DTff BT exhibit strong shoulder peaks at 636 nm in solution
Photophysical Properties. The optical absorption spectra
of polymers in dilute o-DCB solutions and thin films are shown
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Figure 3. Theoretically calculated molecular conformations, corresponding torsion angles and distributions of frontier energy levels of two polymers.
Long alkyl chains were replaced by methyl groups.
Table 2. Photovoltaic Performance of Devices Based on PFBDT−DTBT and PFBDT−DTffBT
a
polymer/acceptor
w/w
VOC (V)
JSC (mA cm−2
)
FF
PCE (%)
Eloss (eV)
PFBDT−DTBT/PC71BM
PFBDT−DTBT/PC71BM
PFBDT−DTBT/PC71BM
PFBDT−DTBT/PC71BM
PFBDT−DTff BT/PC71BM
PFBDT−DTff BT/PC71BM
PFBDT−DTff BT/PC71BM
1:1
0.91
0.88
0.90
0.90
0.98
0.96
0.97
10.63
13.69
13.26
11.85
11.69
13.24
11.51
0.444
0.566
0.598
0.617
0.478
0.577
0.521
4.29 (3.86)
6.82 (6.22)
7.13 (7.07)
6.58 (6.14)
5.47 (5.37)
7.33 (7.15)
5.81 (5.54)
0.76
0.79
0.78
0.77
0.69
0.71
0.70
1:2
1:2.5
1:3
1:1
1:1.5
1:2
a
The data were based on more than 10 devices and were provided in “highest (average)” format.
(648 nm in film state) which are directly correlated with
polymer aggregation in solution and film state, indicating an
improved intermolecular interaction from C−F dipole effect in
the DTff BT moiety.22,36,37 The temperature-dependent
absorption spectra arranging from 0 to 100 °C of PFBDT−
DTff BT in dilute solution were investigated to further
understand the formation of polymer aggregates. As Figure
1b shows, when the solution is heated from 0 to 100 °C, the
shapes of the aggregation peak at 648 nm decreased step by
step during the heating process. More importantly, the shoulder
peak can be detected even at 100 °C, which indicates that the
aggregation of PFBDT−DTff BT was strong even at high
temperature (100 °C). Furthermore, the optical band gap
(Egopt) of polymers PFBDT−DTBT and PFBDT−DTff BT are
determined to be 1.71 and 1.79 eV from the onset of the
and PFBDT−DTff BT are calculated to be −5.41 eV/−3.60
eV, and −5.82 eV/−3.70 eV, respectively, and the relevant data
are summarized in Table 1. The deep HOMO energy level of
PFBDT−DTff BT would help to obtain a high VOC in polymer
solar cells. Moreover, as shown in Figure 2b, the gap of HOMO
and LUMO levels (ΔHOMO and ΔLUMO) between
polymers and PC71BM are both larger than 0.3 eV, which is
essential to yield a sufficient driving force for exciton separation
in BHJ solar cells.
Molecular Conformation Study. To further understand
the influence of fluorene unit in BDT and F atoms in DTff BT
moiety on the polymer backbone conformations, quantum-
chemical calculations (based on density functional theory,
B3LYP/6-31G) were performed for the repeat unit of two
polymers. The 2-ethylhexyl alkyl chains in BDT units were
replaced by a methyl group to reduce the calculation time.
From Figure 3 we can see that the bulky fluorene units in BDT
produce a large dihedral angle over 58° for two polymers. This
highly twisting side groups are beneficial to improve the
solubility of polymers and may induce decreased π-orbital
overlap between side groups and polymer backbones. Never-
theless, it is clearly observed that both of two polymers exhibit
favorable planarity along polymer backbone, which will benefit
the charge transport in PSCs active layer. Moreover, the
planarity of PFBDT−DTff BT is better than PFBDT−DTBT
for the construction of weak noncovalent interactions between
S and F atoms in the DTff BT moiety. As a result, polymer
PFBDT−DTff BT will possess enhanced intermolecular
interactions, which would probably be one reason for the
raised aggregation peaks in absorption spectra. From the
absorption spectra in the thin film states according to Egopt
1240/λonset
=
.
Electrochemical Properties. The oxidation and reduction
potentials of the polymers were determined by cyclic
voltammetry (CV) with the polymer coated on glassy carbon
electrodes.38 In order to obtain the oxidation and reduction
potentials of the polymer, the reference electrode was calibrated
using ferrocene/ferrocenium (Fc/Fc+), which had a redox
potential with an absolute energy level of 0.40 eV in a vacuum.
CV curves of polymers are shown in Figure 2a. The HOMO
and LUMO energy levels were calculated from their onset
oxidation potential (φox) and onset reduction potential (φred)
according to the empirical equation HOMO = −(φox + 4.80 −
0.40) (eV) and LUMO = −(φred + 4.80 − 0.40) (eV). The
HOMO/LUMO energy levels of polymers PFBDT−DTBT
D
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Figure 4. (a) J−V curves of polymer/PC71BM solar cells under optimal weight ratio. (b) EQE curves of the corresponding solar cells.
distributions of electron density, both polymers show a
delocalized HOMO which is equally distributed over both
the donor and acceptor parts of D−A polymer. However, the
LUMO energy levels are almost localized on the acceptor units.
Moreover, we can see from Figure 3 that polymer PFBDT−
DTff BT exhibits deeper frontier energy levels, which is
consistent with the results form CV measurement. And we
also have calculated the structure of PFBDT−DTBT and
PFBDT−DTff BT with the orientation of two thiophenes in
different sides as shown in Figure S2 (Supporting Information),
which show similar results as thiophenes in the same sides
(Figure 3).
Heterojunction Morphology Studies. Atomic force
microscopy (AFM) and transmission electron microscopy
(TEM) (Figure 5 and Figure S6) were applied to investigate
Photovoltaic Properties. Single BHJ-PSCs were fabricated
with a configuration of indium tin oxide (ITO)/poly(3,4-
ethylenedioxythiophene)polystyrenesulfonate (PEDOT:PSS)/
polymers:PC71BM/Ca/Al. The donor−acceptor weight ratio
was screened from 1:1 to 1:3. The current density−voltage (J−
V) curves at different D/A ratios under AM 1.5G (100 mW
cm−2) illumination are shown in Figure S3, and the device
parameters are summarized in Table 2. As Table 2 shows, the
optimal donor−acceptor weight ratio of PFBDT−DTBT/
PC71BM is 1:2.5, with PCE up to 7.13% and VOC = 0.90 V, JSC
= 13.26 mA cm−2, and FF = 0.598. For the devices based on
polymer PFBDT−DTff BT, the optimal donor−acceptor
weight ratio is proved to be 1:1.5, with PCE up to 7.33%
and VOC = 0.96 V, JSC = 13.24 mA cm−2, and FF = 0.577. The
J−V curves under optimal weight ratio are displayed in Figure
4a. Notably, two PSCs devices exhibit close short-circuit
current densities and fill factors, while devices based on
PFBDT−DTff BT exhibit significantly enhanced open-circuit
voltage near 1.0 V. This should be attributed to its deep
HOMO energy levels. To further optimize the solar cells
performances, 1,8-diiodooctane (DIO) additive and ITIC
acceptor based devices were also systematically investigated.
The detailed device performance is summarized in Tables S2
and S3, and the J−V curves are shown in Figures S4 and S5.
Unfortunately, they do not show positive improvement.
The corresponding external quantum efficiencies (EQEs) of
the PSCs were measured and are shown in Figure 4b. Two
devices demonstrate broad photo response in 300−750 nm
with the maximum value up to 80% at around 500 nm.
Obviously, devices based on PFBDT−DTff BT reveal im-
proved EQE response at 500−670 nm, owing to the strong
aggregation peak in absorption spectra. Furthermore, the
calculated JSC for PFBDT−DTBT and PFBDT−DTff BT
based devices shows consistent well with the results from J−
V measurement.
Figure 5. AFM height (a) (5 μm × 5 μm) and phase (b) (5 μm × 5
μm) images of the PFBDT−DTBT:PC71BM (1:2.5, w/w) blend films;
AFM height (c) (5 μm × 5 μm) and phase (d) (5 μm × 5 μm) images
of the PFBDT−DTff BT:PC71BM (1:1.5, w/w) blend films.
the morphology of blend films. From Figure 5, we can see that
both films of PFBDT−DTBT/PC71BM and PFBDT−
DTff BT/PC71BM display uniform and fine nanoscale
structures. As shown in Figure 5, the root-mean-square
(RMS) roughness of PFBDT−DTBT:PC71BM and PFBDT−
DTff BT:PC71BM are 0.35 and 0.98 nm, respectively. The
difference of RMS is probably attributed to the stronger
molecular interaction and aggregation ability. However, in the
TEM of blend films (Figure S6), we do not observe this
aggregation. We propose it may be related to the miscibility
between donors and acceptors as well as the film forming
mechanism of BHJ structure. Furthermore, some small holes
can be observed in two blend films, which is preferable neither
for efficient exciton dissociation nor for balanced charge
transport. And the defects in blend films should be blamed
partially for the low fill factors of two PSCs devices (see Table
2).
E
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Table 3. Comparison of Photovoltaic Properties and Energy Loss of Polymers in Analogues
polymers/acceptor
VOC (V)
JSC (mA cm−2
)
FF
PCE (%)
Eloss (eV)
ref
PFBDT−DTBT/PC71BM
0.90
0.96
0.85
0.78
0.72
0.68
0.68
0.76
13.26
13.24
12.72
8.83
0.598
0.577
0.621
0.520
0.620
0.565
0.552
0.619
7.13 (7.07)
7.33 (7.15)
6.71 (6.58)
3.57 (3.51)
4.98 (4.90)
3.24 (3.10)
4.46 (4.35)
6.20 (6.03)
0.78
0.71
0.82
0.89
0.95
0.77
0.99
0.91
this work
PFBDT−DTff BT/PC71BM
PBDTB−DTff BT/PC71BM
P3TBDT−DTBT/PC71BM
PBDTODO−DTBT/PC71BM
PBDTTHO−DTBT/PC71BM
PBDTTEH−DHff BT/PC71BM
PBDTTTEH−DTEHff BT/PC71BM
this work
21
22
40
41
42
42
11.16
8.45
11.87
13.17
1.5 Hz, 1H), 7.44 (dd, J = 8.0, 1.5 Hz, 1H), 7.37−7.33 (m, 1H), 7.31
(d, J = 7.3 Hz, 1H), 7.28 (d, J = 7.2 Hz, 1H), 2.00−1.91 (m, 4H),
0.92−0.67 (m, 22H), 0.51 (m, 6H). 13C NMR (151 MHz, CDCl3): δ
151.75, 148.96, 139.25, 128.76, 126.30, 125.90, 125.84, 123.07, 123.04,
123.00, 119.86, 118.64, 54.08, 43.33, 33.56, 32.60, 26.97, 26.03, 21.71,
13.03, 9.33.
Energy Loss Analysis. The high VOC of PFBDT−DTff BT
based device would afford a reduced energy loss (Eloss), which is
defined as Eloss = Eg − eVOC, where Eg is the lowest optical band
gap between donor and acceptor materials.39 In this work, Eg is
defined to be 1.67 eV from the onset absorption of PC71BM
film at 743 nm (see Figure S7). The Eloss values of PFBDT−
DTffBT based device are calculated to be 0.69−0.71 eV,
smaller than most of the reported fullerene based PSCs and
approaching the empirically low threshold of 0.6 eV (see Tables
2 and 3). The decreased energy loss is critical to afford an ideal
trade-off between VOC and JSC and strengthen these two
parameters simultaneously for high photovoltaic performance.
Synthesis of 3. Under the protection of argon, n-butyllithium (9.38
mL,15 mmol) was added to 2 (7.042 g,15 mmol) in dry THF (30 mL)
at −78 °C and stirred for 2 h. Subsequently, dihydrobenzo[1,2-b:4,5-
b′]dithiophene-4,8-dione (1.1 g, 5 mmol) was added at −78 °C and
stirred for 1 h.Then the mixture solution was warmed to room
temperature and stirred overnight. SnCl2 (11.28 g, 50 mmol) in 15%
HCl (65 mL) was added, and the mixture was heated to 70 °C and
stirred further for 6 h. 10 mL of ice water was added, and the mixture
as extracted by petroleum ether. The extract was washed with water for
three times and dried over anhydrous sodium sulfate. After removing
the solvent by the rotary evaporator, the residue was purified by
column chromatography on silica gel with petroleum ether as the
eluent to afford 3 as light yellow solids (yield: 50.37%). 1H NMR (600
MHz, CDCl3): δ 7.89 (d, J = 7.7 Hz, 2H), 7.78 (d, J = 7.4 Hz, 2H),
7.74−7.63 (m, 4H), 7.42 (d, J = 7.3 Hz, 2H), 7.39−7.35 (m, 4H),
7.34−7.26 (m, 4H), 2.11−1.97 (m, 8H), 1.03−0.69 (m, 44H), 0.69−
0.52 (m, 24H). 13C NMR (101 MHz, CDCl3): δ 140.44, 140.21,
137.60, 135.71, 135.37, 130.32, 126.26, 124.84, 124.50, 122.22, 121.65,
121.59, 119.56, 119.40, 119.33, 118.00, 109.01, 108.08, 47.03, 37.11,
31.09, 30.82, 30.76, 29.30, 28.96, 28.65, 28.48, 28.22, 25.89, 25.68,
25.66, 21.62, 21.57, 13.08.
CONCLUSIONS
■
In this work, a novel fluorene side-chained BDT unit is design
and synthesized. By copolymerizing with DTBT or DTff BT
unit, high open circuit voltage and low energy loss PSCs donor
materials are obtained. Polymer PFBDT−DTff BT shows
enhanced PCE (7.33%) and VOC (0.96 V) than polymer
PFBDT−DTBT PCE (7.13%) and VOC (0.90 V) due to the F
induced low HOMO energy level. Furthermore, the fluorene
side-chained polymers show low energy loss (0.78 and 0.71 eV)
compared with analogues in the literature which contributes to
the overall improved photovoltaic performances. The results in
this work exhibit that fluorene moiety is a promising π-
conjugated side chain group attached in BDT for the
construction of efficient D−A conjugated photovoltaic
polymers and low energy loss PSCs devices.
Synthesis of 4. Under the protection of argon, n-butyllithium (5.18
mL, 3.23 mmol) was added to 3 (1.16 g, 1.19 mmol) in dry THF (40.0
mL) at −78 °C for 1 h. Then the mixture solution was warmed to
room temperature and stirred for 1 h. Then SnMe3Cl (4.20 mL, 4.20
mmol) was added at −78 °C. The mixture was stirred overnight. 10
mL of ice water was added, and the mixture was extracted by
petroleum ether. After the solvent was removed, the raw product was
purified by recrystallization from acetone, and a light yellow solid 4
EXPERIMENTAL SECTION
■
Materials and Characterization. Acceptor materials PC71BM
and monomers (2BrDTBT and 2BrDTff BT) are purchased from
Solarmer Materials Inc. All solvents and reagents were purchased from
Alfa Aesar, Aladdir, and TCI Inc. and utilized directly unless stated
otherwise. Toluene or tetrahydrofuran (THF) was distilled from
sodium with benzophenone as an indicator, and N,N-dimethyl-
formamide (DMF) was distilled from CaH2 under an argon
atmosphere before use. The characterization techniques used in this
work are collected in the Supporting Information.
1
could be received (yield: 45.79%). H NMR (600 MHz, CDCl3): δ
7.92−7.89 (m, 2H), 7.79 (d, J = 7.6 Hz, 4H), 7.63 (m, 2H), 7.42 (d, J
= 6.4 Hz, 2H), 7.36 (d, J = 7.4 Hz, 4H), 7.32 (d, J = 7.3 Hz, 2H),
2.12−1.97 (m, 8H), 1.13−0.70 (m, 44H), 0.62 (m, 24H), 0.38−0.26
(m, 18H). 13C NMR (151 MHz, CDCl3): δ 151.56, 151.30, 151.03,
141.24, 141.21, 141.15, 141.05, 138.56, 130.83, 129.65, 128.47, 126.94,
126.75, 125.34, 124.18, 120.05, 119.86, 55.26, 55.17, 55.09, 45.41,
45.31, 34.83, 34.10, 28.72, 27.50, 22.91, 22.86, 14.18, 10.87, −8.31,
−8.32.
Synthesis of Polymers PFBDT−DTBT and PFBDT−DTff BT. In a 25
mL flask, 4 (192.97 mg, 0.15 mmol) and 2BrDTBT (68.73 mg, 0.15
mmol) or 2BrDTff BT (74.13 mg, 0.15 mmol), Pd2(dba)3 (2.06 mg,
0.0022 mmol), and tri(o-tolyl)phosphine (4.1 mg, 0.0134 mmol) were
added under argon. After the addition of dry toluene (8.0 mL), the
mixture was heated to 110 °C for 24 h. After cooling to room
temperature, the mixture was poured into methanol. The precipitate
was collected and filtered into a Soxhlet funnel successively with
methanol, acetone, and hexane. The residue was collected and dried
overnight under vacuum to afford polymers.
Synthesis. Synthesis of 2. The synthetic procedures start with the
commercially available 1. 1 (10.0 g, 41 mmol) in 50% aqueous NaOH
solution (50 mL) and 3-(bromomethyl)heptane (44.42 g, 230 mmol)
dissolved in DMSO (85 mL) were stirred at room temperature. After
tetrabutylammonium bromide (1.0 g) was added to the mixture, the
mixture was stirred vigorously at 60 °C for 5 h. After the reaction
mixture was allowed to cool to room temperature, it was extracted
with petroleum ether. The organic phase was washed with water for
three times and dried over anhydrous sodium sulfate. After removing
the solvent by rotary evaporator, a yellow transparent liquid was
collected by distillation under reduced pressure. It was purified by
column chromatography on silica gel with petroleum ether as the
eluent to afford 2 as oil (yield: 71.75%). 1H NMR (600 MHz, CDCl3):
δ 7.65 (d, J = 7.3 Hz, 1H), 7.54 (d, J = 8.0 Hz, 1H), 7.50 (dd, J = 6.4,
Polymer PFBDT−DTBT was obtained as a black solid (yield:
57%). Mn (GPC, THF, 40 °C) = 45.1 kDa, PDI = 1.59.
Decomposition temperature (N2, 5% weight loss): 371 °C. 1H
F
DOI: 10.1021/acs.macromol.7b00998
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Macromolecules
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NMR (400 MHz, 1,2-dichlorobenzene-d4, 100 °C): δ 8.16 (m), 7.91
(br), 7.45 (br), 7.32 (br), 3.34−3.26 (m), 2.11 (br), 1.59 (m), 1.26−
0.92 (br), 0.73−0.66 (m).
Polymer PFBDT−DTff BT was obtained as black solid (yield:
61%). Mn (GPC, THF, 40 °C) = 57.1 kDa, PDI = 1.09.
Decomposition temperature (N2, 5% weight loss): 392 °C. 1H
NMR (400 MHz, 1,2-dichlorobenzene-d4, 100 °C): δ 7.97 (br), 7.77
(br), 7.61 (br), 7.46 (br), 7.31 (br), 3.34−3.26 (m), 2.14 (br), 1.26
(m), 0.98 (br), 0.73−0.66 (br).
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.7b00998.
(14) Pan, H.; Wu, Y.; Li, Y.; Liu, P.; Ong, B. S.; Zhu, S.; Xu, G.
Benzodithiophene CopolymerA Low-Temperature, Solution-Pro-
cessed High-Performance Semiconductor for Thin-Film Transistors.
Adv. Funct. Mater. 2007, 17, 3574.
General information, TGA analysis, calculated molecular
orbitals, details of the PSCs fabrication and performance
1
data, H NMR and 13C NMR spectra (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: mlsun@ouc.edu.cn (M.S.).
*E-mail: yangrq@qibebt.ac.cn (R.Y.).
ORCID
Xichang Bao: 0000-0001-7325-7550
Mingliang Sun: 0000-0002-6245-3844
Renqiang Yang: 0000-0001-6794-7416
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from the
National Natural Science Foundation of China (51573205,
21502205, and 21274134).
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