5-alkyloxy-6-fluorobenzo[c][1,2,5]thiadiazole- and silafluorene-based D-A alternating conjugated polymers: Synthesis and application in polymer photovoltaic cells

Article abstract of DOI:10.1021/ma500417r

Three donor-acceptor (D-A) alternating conjugated polymers with silafluorene as the donor unit, 5-alkyloxy-6-fluorobenzothiadiazole as the acceptor unit, and thiophene as the spacer has been synthesized and used as donor materials for polymer solar cells (PSCs). The introduction of a fluorine atom on the benzothiadiazole unit can lower the HOMO and LUMO energy level of the resulted polymers to afford higher open circuit voltage (V_oc); whereas the introduction of a flexible alkyloxy chain on benzothiadiazole unit can increase the solubility of the resulted polymers without interfering the close packing of polymer chains in the solid state. High molecular weight polymers P-1a, P-1b, and P-1c, which are fully soluble in 1,2-dichorobenzene (DCB) at elevated temperature, have been prepared by Suzuki polycondensation. Among these polymers, P-1c exhibited the highest hole mobility up to 1.36 × 10^-2 cm² V^-1 s^-1. PSCs based P-1b:PC₇₁BM demonstrated the highest V_oc up to 0.98 V. P-1a:PC₇₁BM based PSCs gave the highest power conversion efficiency (PCE) of 6.41%, which is the highest value among solar cells with benzothiadiazole- and silafluorene-containing polymers as the donor material.

Full text of DOI:10.1021/ma500417r

Article
pubs.acs.org/Macromolecules
5‑Alkyloxy-6-fluorobenzo[c][1,2,5]thiadiazole- and Silafluorene-
Based D−A Alternating Conjugated Polymers: Synthesis and
Application in Polymer Photovoltaic Cells
Guangwu Li,† Chong Kang,† Xue Gong,† Jicheng Zhang,† Cuihong Li,*,† Youchun Chen,‡ Huanli Dong,§
Wenping Hu,§ Fenghong Li,‡ and Zhishan Bo*,†
†Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing
100875, China
‡State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130023, China
§Key Laboratory of Organic Solids, Institute of Chemistry CAS, Beijing 100190, China
S
* Supporting Information
ABSTRACT: Three donor−acceptor (D−A) alternating conju-
gated polymers with silafluorene as the donor unit, 5-alkyloxy-6-
fluorobenzothiadiazole as the acceptor unit, and thiophene as the
spacer has been synthesized and used as donor materials for
polymer solar cells (PSCs). The introduction of a fluorine atom on
the benzothiadiazole unit can lower the HOMO and LUMO energy
level of the resulted polymers to afford higher open circuit voltage
(Voc); whereas the introduction of a flexible alkyloxy chain on
benzothiadiazole unit can increase the solubility of the resulted
polymers without interfering the close packing of polymer chains in the solid state. High molecular weight polymers P-1a, P-1b,
and P-1c, which are fully soluble in 1,2-dichorobenzene (DCB) at elevated temperature, have been prepared by Suzuki
polycondensation. Among these polymers, P-1c exhibited the highest hole mobility up to 1.36 × 10−2 cm2 V−1 s−1. PSCs based
P-1b:PC71BM demonstrated the highest Voc up to 0.98 V. P-1a:PC71BM based PSCs gave the highest power conversion
efficiency (PCE) of 6.41%, which is the highest value among solar cells with benzothiadiazole- and silafluorene-containing
polymers as the donor material.
(DFBT)24−27 have been used for the synthesis of donor−
INTRODUCTION
■
acceptor (D−A) alternating conjugated polymers for PSCs.
In recent years, the research on polymer solar cells (PSCs) has
experienced a rapid development since a large number of new
donor−acceptor (D−A) alternating conjugated polymers have
The introduction of two flexible alkoxy chains on the BT unit
can increase the solubility of polymers without interfering the
ordered packing of polymer chains.18,28 The introduction of
been synthesized and used as donor component of the active
layer.1−8 Donor−acceptor design is a very appealing design
two fluorine atoms in the 5,6-positions of BT unit can lower the
HOMO and LUMO energy levels of the resulted conjugated
style; one can easily tune the absorption and energy level of
polymers by changing the donor or/and acceptor unit.9,10
Benzo[c][1,2,5]thiadiazole (BT) is a commonly used strong
acceptor unit, as shown in Chart 1, including the parent BT five
polymers to achieve higher open circuit voltage (Voc) as well as
increase the intramolecular polymer chain interaction to afford
higher hole mobility and PSC performance.24−27,29 However,
kinds of derivatives, namely, BT,11−16 5-alkoxybenzo[c][1,2,5]-
compared with nonfluorinated conjugated polymers, the
thiadiazole (ABT),17 5,6-di(alkoxy)benzo[c][1,2,5]-thiadiazole
corresponding fluorinated conjugated polymers usually ex-
hibited much poorer solubility.30 To achieve high performance
polymer donor materials, careful side chain design is usually
required to balance the solubility and the chain packing of
polymers. Herein, we focus on the design and synthesis of
novel silafluorene (SiF) and BT based soluble conjugated
polymers for high efficiency PSCs. As shown in Chart 2, the
first SiF and BT based conjugated polymer PBSDTBT with a
number-average molecular weight (Mn) of 15 kDa was
(DABT),18 , 19 5-fluorobenzo[c][1,2,5]-thiadiazole
(FBT),17,20−23 and 5,6-difluorobenzo[c][1,2,5]thiadiazole
Chart 1. Chemical Structures of Benzothiadiazole
Derivatives Used for the Synthesis of Conjugated Polymer
Donor Materials
Received: February 25, 2014
Revised: June 23, 2014
© XXXX American Chemical Society
A
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Chart 2. Chemical Structures of SiF- and BT-Based Conjugated Polymers Used for PSCs
a
Scheme 1. Synthesis of Monomers and Copolymer
a
Key: (i) aniline, SOCl2, toluene, 100 °C; (ii) NIS, AcOH, H2SO4, room temperature; (iii) Pd2(dba)3, P(o-tol)3, 4,4,5,5-tetramethyl-2-(thiophen-2-
yl)-1,3,2-dioxaborolane, NaHCO3, THF/H2O, reflux; (iv) KOBut, alkanol, THF, 80 °C; (v) NBS, CHCl3, room temperature; (vi) Pd(PPh3)4,
NaHCO3, NBu4Br, toluene/H2O, reflux.
prepared by Leclerc et al. and a power conversion efficiency
(PCE) of 1.6% was achieved.31 Later, Cao et al. reported the
PCE of high molecular weight PSiF-DBT based PSCs could
reach 5.4%.32 We previously reported the synthesis of SiF and
5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole based conjugated
polymer P2, and P2 based PSCs gave a PCE of 6.05%.33 You
et al. have recently shown that the introduction of fluorine
atom onto the polymer backbone can lower the HOMO level
of polymers to acquire high Voc and suppress the charge
recombination to enhance short circuit current (Jsc) and fill
factor (FF).20,23 At the beginning of this research, we first
designed and synthesized DFBT and SiF based polymer PSiF-
DTDFBT, whose structure is shown in Chart 2. However,
PSiF-DTDFBT is unfortunately insoluble in any organic
solvent even at elevated temperature. As we known, conjugated
polymers are of rod-liked structures, in order to endow them
with good solubility, flexible side chains are required to act as
the inherent solvent.34−36 To achieve soluble polymers, we for
the first time designed and synthesized 5-alkoxy-6-fluorobenzo-
[c][1,2,5]thiadiazole (AFBT) and SiF based conjugated
polymer PSiF-DTAFBT. Because of the intramolecular S−O
supramolecular interaction, the replacement of one fluorine
atom on DFBT unit with alkoxy substituent can markedly
enhance the solubility of polymers without interfering the close
packing of polymer chains in the solid state.18,28,37 To study the
influence of side chains on the performance of PSCs, we have
synthesized three polymers with different alkoxy substituent on
the BT unit. These polymers are of high molecular weight and
fully soluble in organic solvents such as chlorobenzene (CB)
and dichlorobenzene (DCB). Pure polymer films exhibited field
effect transistor (FET) hole mobilities ranging from 1.51 ×
10−3 to 1.36 × 10−2 cm2 V−1 s−1, which match the electron
mobility of PC71BM. Bulk heterojunction PSCs based on these
polymers and PC71BM exhibited high Voc up to 0.98 V. A high
PCE up to 6.41% has been obtained. To the best of our
knowledge, a PCE of 6.41% is the best result for PSCs based on
silafluorene and benzothiadiazole containing polymers, indicat-
ing these polymers are promising donor materials for high
efficiency PSCs.
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RESULTS AND DISCUSSION
■
Material Synthesis and Characterization. The syntheses
of monomers M-1a, M-1b, and M-1c and polymers P-1a, P-1b,
and P-1c are outlined in Scheme 1. The syntheses of
monomers M-1a, M-1b, and M-1c were rather straightforward.
Starting from commercially available 4,5-difluorobenzene-1,2-
diamine (1), 5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-
thiadiazole (4) was synthesized according to the literature
procedures.38 The treatment of compound 4 with alkyl alcohols
(octan-1-ol, 2-ethylhexan-1-ol, and 2-hexyldecan-1-ol) and
potassium tert-butoxide in THF at reflux temperature afforded
mono fluoro and mono alkoxy substituted compounds 5a, 5b,
and 5c, respectively, in yields of 56−72%. Bromination of
compounds 5a, 5b, and 5c with NBS in chloroform containing
a few drops of acetic acid as the catalyst afforded monomers M-
1a, M-1b, and M-1c, respective, in yields of 91−99%. Suzuki
polycondensations of M-1a, M-1b, and M-1c with 9,9-dioctyl-
9H-silafluorene-2,7-bis(boronic acid pinacol ester) were carried
out with Pd(PPh3)4 as the catalyst precursor and tetrabuty-
lammonium bromide as the phase transfer catalyst in a solvent
mixture of aqueous NaHCO3 and toluene to afford polymers P-
1a, P-1b, and P-1c, respectively, in yields of 82−90%. Since the
introduction a flexible alkoxy chain on the benzothiadiazole
ring, all the as-synthesized polymers are fully soluble in CB,
DCB, and 1,2,4-trichlorobenzene (TCB) at elevated temper-
ature. Molecular weights and molecular weight distributions,
which were measured by gel permeation chromatography
(GPC) at 150 °C with TCB as an eluent and narrowly
distributed polystyrenes as the calibration standards, are
summarized in Table 1. P-1a with an octyloxy chain on the
Figure 1. Powdery XRD curves of polymers.
are located at 2θ of 22.45, 22.11, and 20.51° for P-1a, P-1b, and
P-1c, respectively, corresponding to π−π stacking distances of
3.96, 4.01, and 4.31 Å.
Optical Properties. UV−vis absorption spectra of polymers
P-1a, P-1b, and P-1c in DCB solutions at 100 °C and as thin
films are shown in Figure 2. To eliminate the influence of
polymer chain aggregation in solution at room temperature, the
UV−vis absorption spectra of polymer solutions were therefore
measured at 100 °C. Both in solutions and as thin films, all the
Table 1. Physical Properties of the Novel Polymers
polymer
Mn (kg/mol)
Mw (kg/mol)
PDI
Tg (°C)
P-1a
P-1b
P-1c
34.8
97.8
80.6
360.2
247.1
2.31
3.68
2.15
357
338
338
114.9
benzothiadiazole unit showed a Mn of 34.8 kg/mol and a
polydispersity index (PDI) of 2.31. Of the three polymers, P-1b
and P-1c with branched alkoxy chain on benzothiadiazole unit
were better soluble in hot toluene, which was the organic
solvent used for Suzuki polycondensation, allowing us to
achieve high molecular weight polymers. P-1b showed a Mn of
97.8 kg/mol and a PDI of 3.68, and P-1c showed a Mn of 114.9
kg/mol and a PDI of 2.15. Thermo gravimetric analysis (TGA)
and differential scanning calorimetry (DSC) were used to
investigate the thermal properties of these polymers. Under
nitrogen atmosphere at a heating rate of 10 °C/Min, all the
three polymers exhibited good thermal stability with the 5%
decomposition temperature up to 357 °C for P-1a, 338 °C for
P-1b, and 338 °C for P-1c. DSC measurements under nitrogen
atmosphere at a heating rate of 20 °C/Min in the range of 20 to
300 °C also showed that there was no obvious glass transition
for all the three polymers. Powdery XRD measurement (Figure
1) was used to investigate the packing of polymer chains in the
solid state. All these polymers showed one diffraction peak at
small angle region and one broad diffraction peak at wide angle
region. The first diffraction peak at small angle region is located
at 2θ = 3.95° for P-1a, 2θ = 4.44° for P-1b, and 2θ = 3.70° for
P-1c, revealing the distance of polymer chains separated by the
flexible chains is 22.35 Å for P-1a, 19.89 Å for P-1b, and 23.86
Å for P-1c. The broad diffraction peaks at the wide angle region
Figure 2. UV−vis absorption spectra of P-1a (a), P-1b (b), and P-1c
(c) in DCB solutions at 100 °C and as thin films.
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three polymers exhibited two absorption peaks, and their
locations are summarized in Table 2. Compared with their
Polymer thin films were spin-coated from DCB solutions onto
the OTS-modified Si/SiO2 substrates, and Au electrodes with a
thickness of 25 nm were vacuum deposited atop polymer thin
films. The transfer and output characteristic curves of FET
devices are shown in Figure 4. Hole mobilities of P-1a, P-1b,
and P-1c were calculated to be 1.51 × 10−3, 8.0 × 10−3, and
1.36 × 10−2 cm2 V−1 s−1, respectively, which are close to the
electron mobility of PC71BM (2 × 10−2 cm2 V−1 s−1).28 The
hole mobilities, VT and the on/off ratio are summarized in
Table 3. The hole mobility of polymer films is usually related to
two main factors, namely, the π−π stacking distance and
molecular weight of polymers. Usually shorter π−π stacking
distance and higher molecular weight are prone to endow
polymer films with higher hole mobility.39 The π−π stacking
distance of P1-a is shorter than that of P1-c, but the molecular
weight of P1-a is much smaller than that of P1-c. Therefore, it
is reasonable that the mobility of P1-c is larger that of P-1a.
Photovoltaic Properties. Photovoltaic properties were
investigated in a device configuration of ITO/PEDOT:PSS/
active layer/interfacial layer/Al. The thickness of active layer
was about 100 nm. Two different interfacial layers, LiF and FN-
C60, were tested. As shown in Chart 3, FN-C60 is an alcohol-
soluble cationic fullerene derivative, which can be spin-coated
atop the active layer without damaging the underlayer. We have
screened the weight ratio of polymer to PC71BM, the
concentration of blend solutions, the spin-coating speed, and
the volume of 1,8-diiodooctane (DIO) additive to achieve the
better device performance. The preliminary results are
summarized in Table 4. For all the three polymers, a polymer
to PC71BM weight ratio of 1:3 gave the best photovoltaic
performances. For P-1a, devices fabricated from DCB solutions
using LiF as the interfacial layer for cathode gave a PCE of
4.56% with a Voc of 0.91 V, a Jsc of 9.46 mA/cm2, and an FF of
0.53. When DIO (2.0%, by volume) was used as the processing
additive, the PCE was slightly enhanced to 5.80%. The
increasing of PCE was mainly attributed to the increasing of
Jsc and FF. When FN-C60 was used as the interfacial layer
instead of LiF, the PCE was further increased to 6.41%,
accompanied by the simultaneously increasing of the Voc, Jsc,
and FF. It is believed that the use of FN-C60 as the interfacial
layer for cathode can enhance the interfacial contact, electron
transportation, and collection. For P-1b, PSC devices with
active layers spin-coated from DCB solutions containing DIO
(2.0%, by volume) also gave better photovoltaic performance
than that fabricated from DCB solutions. The best device
performance with a PCE of 4.85%, a Voc of 0.98 V, a Jsc of 8.5
mA/cm2, and an FF of 0.59 was achieved when FN-C60 was
used instead of LiF as the interfacial layer for cathode. For P-1c,
devices fabricated from DCB solutions containing DIO (2.0%,
by volume) also gave the best device performance with a PCE
of 5.06%, a Voc of 0.96 V, a Jsc of 8.54 mA/cm2, and an FF of
0.62. The trend of Jsc for the devices is P-1a > P-1c > P-1b,
which is inconsistent with the trend of hole mobility of the FET
devices based on these polymers. The inconsistency is probably
due to that the FET mobility is measured for the pure polymer
films in FET devices. Namely, the FET mobility reflects the
transport property along the thin polymer film. The short
circuit current (Jsc) of solar cells is related to the transport
property of polymer:PC71BM blend instead of pure polymer
films. The Jsc is mainly related to the nanostructures of
polymer:PC71BM blend films. The above preliminary device
results indicated that the substituent on the benzothiadiazole
unit had significant influence on the performance of PSCs.
Table 2. Optical and Electrochemical Properties of Polymers
λmax
[nm]
film
λedge
[nm]
film
λmax [nm]
E
HOMO
(eV)
LUMO
c
a
g,opbt
polymer solution
(eV)
(opt, eV)
P-1a
P-1b
P-1c
387, 533
387, 529
387, 525
393,
561
624
621
618
1.99
−5.54
−5.56
−5.59
−3.55
−3.57
−3.59
394,
564
1.99
2.00
392,
563
a
b
Measured at 100 °C. Calculated from the absorption onset the
c
polymer film, Eg,opt = 1240/λedge. Calculated by the equation ELUMO
EHOMO + Eg,opt
=
solution absorption spectra, the corresponding film ones were
red-shifted due to the aggregation of polymer chains in the
solid state. The film absorption onsets (λedge) of P-1a, P-1b,
and P-1c were 624, 621, and 618 nm, respective. The increasing
of the volume of substituent group on the benzothiadiazole
moiety could increase the π−π stacking distance of polymer
chains in films and resulted in a blue-shift of the absorption
peak. The optical band gaps (Eg,opt) of P-1a, P-1b, and P-1c
films were therefore calculated to be 1.99, 1.99, and 2.00 eV,
respectively, according to the equation: Eg,opt = 1240/
absorption onset.
Electrochemical Properties. Electrochemical properties of
P-1a, P-1b, and P-1c were investigated by cyclic voltammetry
(CV) using a standard three electrodes electrochemical cell. As
shown in Figure 3, these three polymers exhibited quasi
Figure 3. Cyclic voltammograms of the polymers.
reversible redox processes. The onset oxidation potentials of P-
1a, P-1b, and P-1c are 0.83, 0.85, and 0.88 V, respectively.
HOMO levels of P-1a, P-1b, and P-1c were determined, using
the equation EHOMO = -e(Eox + 4.71), to be −5.54, −5.56, and
−5.59 eV, respectively. LUMO levels of P-1a, P-1b, and P-1c
were therefore calculated according to the equation ELUMO
=
EHOMO + Eg,opt to be −3.55, −3.57, and −3.59 eV, respectively.
The data are also summarized in Table 2. The above results
indicated that the introduction of big volume alkoxy chain on
the mono fluoro benzothiadiazole unit can only slightly lower
the HOMO energy level of polymers. Their low-lying HOMO
level is favorable for achieving PSCs with higher Voc. The
changing of alkoxyl substituent group on the benzothiadiazole
unit has a subtle influence on their electrochemical properties.
Transport Properties. Hole transport properties of P-1a, P-
1b, and P-1c were investigated by fabrication of bottom-contact
field effect transistors (FETs) on Si/SiO2 substrates with the
low resistance Si as gate and SiO2 (300 nm) as gate insulator.
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Figure 4. Transfer characteristics and output characteristics of P-1a (a, b), P-1b (c, d), and P-1c (e, f).
As shown in Figure 6, external quantum efficiencies (EQE)
were measured to examine the accuracy of Jsc of devices
fabricated under the optimized conditions for each polymer. All
polymers exhibited broad photo to current responses in the
range of 350 to 700 nm. In the range of 400 to 600 nm, the
EQE value of P-1a is higher than 60% with the maximum close
to 70%. For P-1b and P-1c, these two polymers exhibited
similar EQE curves, in the range of 400 to 600 nm EQE values
are around 50%. For all the three polymers, the Jsc calculated
from the integration of EQE curves matched well with that
obtained from J-V measurements. This good agreement could
confirm the accuracy of PCE obtained from J−V measure-
ments.
Table 3. FET Properties of the Pure Polymer Films
polymers
on/off
μ (cm2/(V s))
VT (V)
P-1a
P-1b
P-1c
2.3 × 105
9.4 × 105
4.3 × 105
1.51 × 10−3
8.00 × 10−3
1.36 × 10−2
−25
−10
−42
Chart 3. Chemical Structure of FN-C60
Film Morphology. DIO is a higher boiling point solvent than
the processing solvent DCB, exhibits better solubility for
PC71BM, and can be used as additive to affect the morphology
of polymer:PC71BM blend films.18,40 In previous studies, it has
been shown that polymer component can form connected
fibrous network when using DIO as the processing additive.40
Atomic force microscopy (AFM) and transmission electron
microscopy (TEM) investigations on blend films fabricated
without and with DIO as the processing additive were
performed to study if DIO acts a similar effect in our system.
TM-AFM was utilized to study the blend film morphology of
polymer and PC71BM, AFM images of P-1a:PC71BM, P-
1b:PC71BM, and P-1c:PC71BM (1:3 by weight) blend films
(2.0 μm × 2.0 μm) fabricated from DCB solutions without and
with DIO as additives are shown in Figure 7. All blend films
showed nanoscale phase separation. As shown in Figure 7, the
Devices based on the monooctyloxy- and monofluoro
substituted benzothiadiazole containing conjugated polymer
P-1a gave the best device performance, even where the
molecular weight of P-1a was the lowest one among the three
polymers. Further increasing the photovoltaic performance was
anticipated after intensive device optimization. J−V curves for
the solar cells are found in Figure 5.
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Table 4. Photovoltaic Parameters of Devices Based on LiF/Al Cathode and FN-C60/Al Cathode without and with DIO as the
Processing Additive
device structure
solvent
DCB
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
ITO/PEDOT:PSS/P-1a:PC71BM/LiF/Al
ITO/PEDOT:PSS/P-1a:PC71BM/LiF/Al
ITO/PEDOT:PSS/P-1a:PC71BM/FN-C60/Al
ITO/PEDOT:PSS/P-1b:P71BM/LiF/Al
ITO/PEDOT:PSS/P-1b:P71BM/LiF/Al
ITO/PEDOT:PSS/P-1b:PC71BM/FN-C60/Al
ITO/PEDOT:PSS/P-1c:PC71BM/LiF/Al
ITO/PEDOT:PSS/P-1c:PC71BM/LiF/Al
ITO/PEDOT:PSS/P-1c:PC71BM/FN-C60/Al
0.91
0.88
0.92
0.80
0.94
0.98
0.93
0.92
0.96
9.46
12.00
12.07
7.50
8.13
8.50
7.83
8.60
8.54
0.53
0.55
0.58
0.53
0.49
0.59
0.52
0.57
0.62
4.56
5.80
6.41
3.17
3.72
4.86
3.77
4.54
5.06
a
DCB
a
DCB
DCB
a
DCB
a
DCB
DCB
a
DCB
a
DCB
a
With 2% DIO (by volume).
Figure 5. J−V curves for the solar cells.
Figure 7. AFM images (2 μm × 2 μm) of P-1a:PC71BM (1:3), P-
1b:PC71BM (1:3), and P-1c:PC71BM (1:3) blend films.
1c blend films, respectively, after the addition of DIO (2.0%, by
volume) as the additive. The AFM results are consistent with
the device results. Since AFM can only probe the surface
roughness of the polymer:PC71BM blend films, TEM experi-
ments were therefore performed to investigate the nanostruc-
tures of the blend films without and with DIO. TEM images of
polymer:PC71BM blends spin-coated from DCB solutions
without and with 2.0% DIO are shown in Figures 8 and S1
(Supporting Information). As shown in Figure 8a, TEM image
of P-1a:PCBM blends spin-coated from DCB solutions
exhibited apparent phase separation in nanoscale with
Figure 6. EQE of PSCs fabricated under optimized conditions.
morphology of all blend films became smoother when DIO
(2%, by volume) was used as the additive. The root-mean-
square (rms) roughness values were decreased from 1.36, 0.93,
and 3.91 nm to 0.43, 0.61, and 0.53 nm for P-1a, P-1b, and P-
F
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1,3,2-dioxaborolan-2-yl)-9,9-dioctyl-9H-silafluoren-7-yl)-1,3,2-
dioxaborolane and the newly developed 4,7-bis(5-bromothio-
phen-2-yl)-5-ethoxy-6-fluorobenzo[c][1,2,5]thiadiazole mono-
mer. The introduction of a fluorine atom on the benzothiadia-
zole unit can usually lower the HOMO and LUMO level of the
resulted polymers; whereas the introduction of an alkoxy chain
on the benzothiadiazole unit can enhance the solubility of
polymers without interfering the dense packing of polymers in
the solid state. The as-synthesized three polymers, which are of
high molecular weight and fully soluble in CB, DCB, and TCB
at elevated temperature, can be used to fabricate PSCs by
solution processing. FET hole mobilities of these polymers are
in the range of 1.51 × 10−3 to 1.36 × 10−2 cm2 V−1 s−1, which
are close to the electron mobility of PC71BM. Because of their
deep HOMO energy level, blending with PC71BM one can
obtain BHJ PSCs with the highest Voc up to 0.98 V. High PCE
of 6.41% indicated that 5-alkyloxy-6-fluorobenzo[c][1,2,5]-
thiadiazole could be a promising acceptor unit for the synthesis
of donor materials for PSC applications.
ASSOCIATED CONTENT
■
S
* Supporting Information
Syntheses and characterizations of monomers and polymers,
experimental details for the fabrication and characterization of
1
OFETs and PSCs, TEM images, solar cell results, and H and
13C NMR spectra. This material is available free of charge via
Internet at http://pubs.acs.org.
Figure 8. TEM images of P-1a:PC71BM blends (a), P-1a:PC71BM
blends with 2.0% DIO (b), P-1b:PC71BM blends (c), P-1b:PC71BM
blends with 2.0% DIO (d); P-1c:PC71BM blends (e), and P-
1c:PC71BM blends with 2.0% DIO (f). The scale bar is 200 nm.
AUTHOR INFORMATION
■
Corresponding Authors
*(Z.B.) E-mail: zsbo@bnu.edu.cn.
*(C.L.) E-mail: licuihong@bnu.edu.cn.
Notes
PC71BM rich isolated dark domains. After the use of 2.0% DIO
as the additive, the PC71BM rich domains blurred and
entangled nanofibers formed as shown in Figure 8b. As
shown in Figure 8c, TEM image of P-1b:PCBM blend films
spin-coated from DCB solutions also exhibited similar larger
scale phase with many isolated PC71BM rich dark domains. The
TEM image of P-1b:PC71BM spin-coated from DCB
containing 2% DIO is shown in Figure 8d. After the use of
DIO as the additive, the PC71BM rich domains disappeared and
nanofibrous structures are formed. TEM image of P-
1c:PC71BM blend films spin-coated from DCB solution is
shown in Figure 8e. It can be clearly seen that the blend films
are of homogeneous structure without apparent large scale
phase separation between polymer and PC71BM; whereas after
the use of 2.0% DIO as the processing additive, as shown in
Figure 8f, the blend films comprised a large number of
entangled nanofibers, which are probably formed by the
aggregation of the polymers. From TEM investigations, we
can conclude that the enhance of device performance is mainly
due to the better phase separation between PC71BM and
polymer components and the formation of connected fibrous
polymer networks after the addition of DIO as the processing
additive.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the financial support from the NSFC of China
(Grant Nos. 21161160443 and 91233205).
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