5,6-Difluorobenzothiadiazole and silafluorene based conjugated polymers for organic photovoltaic cells

Article abstract of DOI:10.1039/c4tc00340c

To achieve 5,6-difluorobenzothiadiazole and 2,7-linked silafluorene based soluble conjugated polymers, flexible side chains were attached at different positions of the conjugated polymers. Three soluble polymers PSiF-D(OT)DFBT, PSiF-TTDFBT, and PDOSiF-DTDFBT were prepared and used as donor materials for polymer solar cells. PSiF-D(OT)DFBT exhibits a band gap of 2.06 eV with a deep HOMO of -5.64 eV. PSiF-TTDFBT shows a band gap of 1.75 eV with the HOMO of -5.23 eV. PDOSiF-DTDFBT is of a band gap of 1.86 eV with the HOMO level of -5.37 eV. Among these three polymers, PDOSiF-DTDFBT shows the highest field effect transistor (FET) hole mobility up to 3.31 × 10^-2 cm² V^-1 s^-1, PDOSiF-DTDFBT:PC₇₁BM blend films show the highest SCLC mobility up to 5.10 × 10^-4 cm² V^-1 s^-1, and polymer solar cells (PSCs) with the blend of PDOSiF-DTDFBT:PC₇₁BM (1:1, by weight) as the active layer gave a power conversion efficiency (PCE) of 4.03% with an open circuit voltage (V _oc) of 0.73 V, a short circuit current (J_sc) of 8.55 mA cm^-2, and a fill factor (FF) of 0.65. Our studies also reveal the structure-property relationship of 2,7-linked silafluorene and 5,6-difluorobenzothiadiazole based conjugated polymers.

Full text of DOI:10.1039/c4tc00340c

Journal of
Materials Chemistry C
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PAPER
5,6-Difluorobenzothiadiazole and silafluorene
based conjugated polymers for organic
photovoltaic cells†
Cite this: J. Mater. Chem. C, 2014, 2,
5116
a
a
a
a
a
a
*
Guangwu Li, Chong Kang, Xue Gong, Jicheng Zhang, Weiwei Li, Cuihong Li,
b
b
a
*
Huanli Dong, Wenping Hu and Zhishan Bo
To achieve 5,6-difluorobenzothiadiazole and 2,7-linked silafluorene based soluble conjugated polymers,
flexible side chains were attached at different positions of the conjugated polymers. Three soluble
polymers PSiF-D(OT)DFBT, PSiF-TTDFBT, and PDOSiF-DTDFBT were prepared and used as donor
materials for polymer solar cells. PSiF-D(OT)DFBT exhibits a band gap of 2.06 eV with a deep HOMO of
ꢀ5.64 eV. PSiF-TTDFBT shows a band gap of 1.75 eV with the HOMO of ꢀ5.23 eV. PDOSiF-DTDFBT is of
a band gap of 1.86 eV with the HOMO level of ꢀ5.37 eV. Among these three polymers, PDOSiF-DTDFBT
shows the highest field effect transistor (FET) hole mobility up to 3.31 ꢁ 10ꢀ2 cm2 Vꢀ1
s
ꢀ1, PDOSiF-
ꢀ1, and
DTDFBT:PC71BM blend films show the highest SCLC mobility up to 5.10 ꢁ 10ꢀ4 cm2 Vꢀ1
s
polymer solar cells (PSCs) with the blend of PDOSiF-DTDFBT:PC71BM (1 : 1, by weight) as the active layer
gave a power conversion efficiency (PCE) of 4.03% with an open circuit voltage (Voc) of 0.73 V, a short
circuit current (Jsc) of 8.55 mA cmꢀ2, and a fill factor (FF) of 0.65. Our studies also reveal the structure–
property relationship of 2,7-linked silafluorene and 5,6-difluorobenzothiadiazole based conjugated
polymers.
Received 19th February 2014
Accepted 16th April 2014
DOI: 10.1039/c4tc00340c
www.rsc.org/MaterialsC
polymer main chain usually brings about the DFBT containing
polymers with poor solubility.3
Introduction
Silauorene has been used as the donor unit for the
construction of D–A alternating conjugated polymers for high
efficiency PSCs.12,17–19 The conjugated polymer PSiF-DBT, as
shown in Chart 1, with BT as the acceptor unit, silauorene as
the donor unit, and thiophene as the spacer has been reported
as the donor material with good device performance.12,20,21 In
our previous paper, we have reported the synthesis of 5,6-
bis(octyloxy)benzothiadiazole and silauorene based conju-
gated polymer P2, as shown in Chart 1, for high efficiency
PSCs.21 Due to the good solubility, high molecular weight P2 has
The synthesis of new donor–acceptor (D–A) alternating conju-
gated polymers with high power conversion efficiency (PCE) is
still an important topic in the research eld of polymer solar
cells (PSCs).1–7 Benzothiadiazole (BT) is a widely used acceptor
unit, and many BT containing D–A alternating conjugated
polymers have afforded high device performance when blended
with PC71BM as the active layer for PSCs.8–12 The introduction of
two uoro atoms in the 5,6-positions of the benzothiadiazole
unit not only lowers the HOMO and LUMO energy levels of the
resulted conjugated polymers to achieve higher open circuit
voltage (Voc), but also increases the intramolecular polymer
chain interactions to afford higher hole mobility.13 Compared
with the corresponding BT based conjugated polymers, 5,6-
diuorobenzothiazole (DFBT) based conjugated polymers
usually gave better device performance.14–16 Except the above
mentioned advantages, the introduction of DFBT into the
aBeijing Key Laboratory of Energy Conversion and Storage Materials, College of
Chemistry, Beijing Normal University, Beijing 100875, China
bBeijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail:
licuihong@bnu.edu.cn; zsbo@bnu.edu.cn
† Electronic supplementary information (ESI) available: Experimental part
includes 1H and 13C NMR spectra. See DOI: 10.1039/c4tc00340c
Chart 1
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been synthesized and PSCs with P2:PC71BM as the active layer
gave a PCE of 6.05%.
Due to the fascinating properties of both DFBT and sila-
uorene, herein we would like to report the rational design and
synthesis of DFBT and silauorene based conjugated polymers.
To achieve polymers with good photovoltaic performance, four
D–A conjugated polymers PSiF-DTDFBT, PSiF-D(OT)DFBT, PSi-
TTDFBT, and PDOSiF-DTDFBT, as shown in Chart 1, have been
designed and synthesized. We rst designed and synthesized
PSiF-DTDFBT, which is insoluble even at elevated temperature
in commonly used organic solvents such as chloroform, chlo-
robenzene (CB), 1,2-dichlorobenzene (DCB), and 1,2,4-tri-
chlorobenzene (TCB), and cannot be used for the fabrication of
PSCs. To obtain soluble polymers, we have attached an octyl
chain on the 4-position of the thiophene unit, and synthesized
polymer PSiF-D(OT)DFBT. The disadvantage of such a design is
that the octyl chain on the thiophene unit will give rise to a large
dihedral angle between the thiophene unit and silauorene and
result in poorer conjugation. To enhance the polymer chain
conjugation, we incorporated two additional thiophene units
into the polymer main chain and synthesized polymer PSiF-
TTDFBT. It is worth noting that similar uorene based D–A
polymers PFO-M1-3 with four thiophene units in each repeating
unit have been reported by Wang and Cao et al. and PFO-M-3
exhibited improved photovoltaic performance when compared
with PFO-DTBT.22 Alternatively, we have designed and synthe-
sized PDOSiF-DTDFBT by attaching four exible chains onto the
silauorene unit to enhance the solubility of polymers. The
structures of three soluble polymers were fully characterized,
and their optical, electrochemical, and photovoltaic properties
were investigated in detail to explore their structure and prop-
erty relationships.
Scheme 1 Synthesis of monomers, reagents and conditions: (i) aniline,
SOCl2, toluene, 100 ꢂC; (ii) NIS, AcOH, H2SO4, rt; (iii) Pd2(dba)3, P(o-
tol)3, thiophene-2-boronic acid pinacol ester, NaHCO3, THF/H2O,
reflux; (iv) Br2, CH2Cl2, rt; (v) Pd2(dba)3, P(o-tol)3, 4-octylthiophene-2-
boronic acid pinacol ester, NaHCO3, THF/H2O, reflux; (vi) Br2, CH2Cl2,
rt; (vii) Pd2(dba)3, P(o-tol)3, thiophene-2-boronic acid pinacol ester,
NaHCO3, THF/H2O, reflux; (viii) NBS, CHCl3/DMF, rt; (ix) n-BuLi,
dichlorodioctylsilane, ꢀ100 ꢂC; (x) 4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane, PdCl2(PPh3)2, Et3N, 1,2-dichloroethane, 80 ꢂC.
Results and discussion
Material synthesis and characterization
The syntheses of monomers M-1, M-2, M-3, and M-4 are out-
lined in Scheme 1. M-1 was synthesized using commercially
available 4,5-diuorobenzene-1,2-diamine as the starting
material according to the literature procedures.23 Suzuki cross-
coupling of 3 with 4-octylthiophene-2-boronic acid pinacol ester
was followed by bromination of the cross-coupling product with
Br2 in methylene chloride at room temperature to afford M-2,
which bears two octyl chains on the two thiophene units.
Coupling of M-2 and thiophene-2-boronic acid pinacol ester
using the Suzuki–Miyaura cross-coupling reaction furnished
compound 6 in a yield of 75%, which was converted to M-2 by
bromination with NBS in a solvent mixture of chloroform and
DMF at room temperature. Compound 8 was synthesized
according to literature procedures.19 Reaction between 8 and
4,4,5,5-tetramethyl-1,3,2-dioxaborolane under Miyaura condi-
tions afforded M-4 in a yield of 59%. The synthesis of polymers
PSiF-DTDFBT, PSiF-D(OT)DFBT, PSiF-TTDFBT, and PDOSiF-
DTDFBT are outlined in Scheme 2. Suzuki polycondensation of
M-1 with M-5,21 M-2 with M-5, M-3 with M-5, and M-1 with M-4
afforded PSiF-DTDFBT, PSiF-D(OT)DFBT, PSi-TTDFBT, and
PDOSiF-DTDFBT, respectively. Except PSiF-DTDFBT, all the
Scheme 2 Synthesis of polymers.
other polymers are fully soluble in CB, DCB, and TCB at elevated
temperature. Molecular weights and molecular weight distri-
butions of these three soluble polymers were measured by gel
permeation chromatography (GPC) using TCB as an eluent at
150 ꢂC with narrowly distributed polystyrenes as the calibration
standards, and the results are summarized in Table 1. PSiF-
D(OT)DFBT showed a number average molecular weight (Mn) of
69.6 kg molꢀ1, a weight average molecular weight (Mw) of 218.3
kg molꢀ1, and a polydispersity index (PDI) of 3.14. PSiF-TTDFBT
showed a Mn of 31.0 kg molꢀ1, a Mw of 100.6 kg molꢀ1, and a PDI
of 3.25. PDOSiF-DTDFBT showed a Mn of 40.0 kg molꢀ1, a Mw of
119.8 kg molꢀ1, and a PDI of 3.00. For many polymers, the
photovoltaic performance of devices is highly dependent on
their molecular weight. Only high molecular materials can
furnish good device performance. It is worth noting that we
have obtained soluble and high molecular weight PSiF-D(OT)
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Table 1 Physical properties of the polymers
Mna (kg molꢀ1
)
Mwa (kg molꢀ1
)
PDI
b
Td (ꢂC)
Polymer
PSiF-D(OT)DFBT 69.6
PSiF-TTDFBT 31.0
PDOSiF-DTDFBT 40.0
218.3
100.6
119.8
3.14 447
3.25 440
3.00 397
a
Mn, Mw, and PDI of polymers were determined by GPC at 150 ꢂC using
b
polystyrene standards with TCB as an eluent.
Decomposition
temperatures were determined by TGA under N2 based on 5% weight
loss.
DFBT, PSiF-TTDFBT, and PDOSiF-DTDFBT. These three poly-
mers are of good thermal stability as indicated by thermogra-
vimetric analysis (TGA) measurements. The 5% decomposition
temperatures of PSiF-D(OT)DFBT, PSꢂiF-TTDFBT, and PDOSiF-
ꢂ
DTDFBT are 447 C, 440 ꢂC, and 397 C, respectively. Differen-
tial scanning calorimetry (DSC) measurements from 20 to
300 ꢂC under a nitrogen atmosphere showed that there is no
obvious glass transition for these polymers. Powdery X-ray
diffraction (XRD) measurements were carried out to investigate
the packing of polymer chains in the solid state. As shown in
Fig. 1, XRD diffraction patterns of PSiF-D(OT)DFBT, PSiF-
TTDFBT, and PDOSiF-DTDFBT exhibited two diffraction peaks.
The rst peak in a small angle region, which reects the
distance of polymer backbones separated by the exible side
chains, is located at 2q of 4.36ꢂ for PSiF-D(OT)DFBT, 5.69ꢂ for
PSiF-TTDFBT, and 4.57ꢂ for PDOSiF-DTDFBT, corresponding to
˚
a distance of 20.27, 15.53, and 19.69 A, respectively. The three
diffraction peaks in the wide angle region reect the p–p
stacking distances between polymer backbones, which are
located at 2q of 20.94ꢂ for PSiF-D(OT)DFBT, 25.43ꢂ for PSiF-
TTDFBT, and 24.16ꢂ for PDOSiF-DTDFBT, corresponding to
Fig. 2 UV-vis absorption spectra of PSiF-D(OT)DFBT (a), PSiF-TTDFBT
(b) and PDOSiF-DTDFBT (c) in DCB solutions at 100 ꢂC and as thin
films.
˚
p–p stacking distances of 4.24, 3.50, 3.68 A, respectively.
Optical properties
summarized in Table 2. In comparison with their solution
spectra, those of the lms are all red-shied and broadened due
to the aggregation of polymer chains in the solid state. For PSiF-
D(OT)DFBT, due to the steric hindrance of the octyl chain on
the thiophene unit, the absorption spectrum of PSiF-D(OT)
DFBT is much narrower, and its lm absorption onset is
603 nm, corresponding to an optical band gap of 2.06 eV. For
PSiF-TTDFBT, in comparison with PSiF-D(OT)DFBT, the
UV-vis absorption spectra of polymers PSiF-D(OT)DFBT, PSiF-
TTDFBT, and PDOSiF-DTDFBT in DCB solutions at 100 C and
ꢂ
as thin lms are shown in Fig. 2. To eliminate the inuence of
aggregation of polymer chains in DCB solutions, UV-vis
absorption spectra of polymer solutions were measured at
100 ꢂC. All polymers exhibit two absorption peaks both in
solutions and as thin lms, and the peak locations are
Table 2 Electrochemical and optical properties of polymers
lmaxa [nm]
solution
Eg,opt
(eV)
HOMO
(eV)
LUMOc
(eV)
b
Polymer
PSiF-D(OT)DFBT
PSiF-TTDFBT
PDOSiF-DTDFBT
366 501
411 538
397 548
2.06
1.75
1.86
ꢀ5.64
ꢀ5.23
ꢀ5.37
ꢀ3.58
ꢀ3.48
ꢀ3.51
a
Measured at 100 ꢂC. b Calculated from the absorption band edge of the
c
copolymer lm, Eg,opt ¼ 1240/ledge
.
Calculated by the equation
ELUMO ¼ EHOMO + Eg,opt
.
Fig. 1 Powdery XRD patterns of polymers.
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introduction of two additional thiophene units in the polymer mobility is very crucial for achieving high performance polymer
main chain can release the steric hindrance of the octyl chain solar cells. Transport properties of polymers were therefore
and result in a small dihedral angle between the two adjacent investigated by fabrication of eld effect transistors (FETs).
thiophene units. Therefore, the lm absorption spectrum of Bottom-contact devices were fabricated on Si/SiO2 substrates
PSiF-TTDFBT is much broad, and the lm absorption onset is with the low resistance Si as gate and SiO2 (500 nm) as gate
708 nm, which gave an optical band gap of 1.75 eV. For PDOSiF- insulator. Polymer thin lms were spin-coated on the OTS-
DTDFBT, due to the supramolecular S–O interaction, the two modied Si/SiO2 substrates from DCB solutions, and Au elec-
octyloxy substituents on the silauorene can planarize the trodes with a thickness of 25 nm were vacuum deposited onto
polymer main chain to some extent.24,25 As a result, PDOSiF- polymer thin lms. The transfer and output characteristic
DTDFBT as a lm exhibited broader absorption with an curves of FET devices are shown in Fig. 4. Hole mobility (m) was
absorption onset at 668 nm, giving an optical band gap of 1.86 deduced from the derivative plots in the saturated regime
eV. The absorption onsets and optical band gaps (Eg,opt) are also through the equation IDS ¼ m(W/2L)Ci(VG ꢀ VT)2 where IDS is the
summarized in Table 2.
drain current, W is the channel width, L is the channel length, Ci
is the capacitance per unit area of the gate dielectric layer (SiO2,
300 nm, Ci ¼ 11 nF cmꢀ2), VG and VT are the gain voltage and
threshold voltage, respectively. Hole mobilities of PSiF-D(OT)
Electrochemical properties
Cyclic voltammetry (CV) was used to study the electrochemical
properties of polymers and the CV curves are shown in Fig. 3.
The onset oxidation potentials of PSiF-D(OT)DFBT, PSiF-
TTDFBT, and PDOSiF-DTDFBT determined by CV are 0.93 V,
0.52 V, and 0.66 V, respectively. According to the equation
EHOMO ¼ ꢀe(Eox + 4.71), HOMO levels were calculated to be
ꢀ5.64 eV for PSiF-D(OT)DFBT, ꢀ5.23 eV for PSiF-TTDFBT, and
ꢀ5.37 eV for PDOSiF-DTDFBT. Among these three polymers,
PSiF-D(OT)DFBT is of the lowest HOMO level; compared with
PSiF-D(OT)DFBT, the introduction of two additional thiophene
units in the main chain has increased the HOMO level of PSiF-
TTDFBT from ꢀ5.64 eV to ꢀ5.23 eV. Compared with PSiF-D(OT)
DFBT, the introduction of two electron donating octyloxy
substituents on the silauorene has raised the HOMO level of
PDOSiF-DTDFBT from ꢀ5.64 eV to ꢀ5.37 eV. LUMO levels were
calculated according to the equation ELUMO ¼ EHOMO + Eg,opt to
be ꢀ3.58 eV for PSiF-D(OT)DFBT, ꢀ3.48 eV for PSiF-TTDFBT,
ꢀ3.51 eV for PDOSiF-DTDFBT. The data are also summarized in
Table 2. The low-lying HOMO level of PSiF-D(OT)DFBT is
favorable for achieving higher open circuit voltage (Voc), as
conrmed by device results (vide infra).
DFBT, PSiF-TTDFBT, and PDOSiF-DTDFBT are 6.94 ꢁ 10ꢀ3
,
3.09 ꢁ 10ꢀ2 and 3.31 ꢁ 10ꢀ2 cm2 Vꢀ1 sꢀ1, respectively, which
are close to the electron mobility of PC71BM. Hole mobilities,
VT, and on/off ratios are summarized in Table 3.
Photovoltaic properties
Photovoltaic properties of polymers were evaluated by using the
device conguration of ITO/PEDOT:PSS/active layer/LiF/Al. The
performance of devices was optimized by variation of the weight
ratio of polymer to PC71BM, the concentration of blend solu-
tion, the spin-coating speed, and the volume of 1,8-diio-
dooctane (DIO) or 1-chloronaphthalene (1-CN) as additives. The
J–V curves of these polymer solar cells are illustrated in Fig. 5a.
The use of DIO as the additive only showed a positive effect for
PSiF-D(OT)DFBT based PSCs, but a negative effect for PSiF-
TTDFBT and PDOSiF-DTDFBT based PSCs. The use of 1-CN as
the additive has a positive effect for polymers PSiF-TTDFBT and
PDOSiF-DTDFBT and a negative effect for PSiF-D(OT)DFBT. For
PSiF-D(OT)DFBT, the active layer spin-coated from poly-
mer:PC71BM in a weight ratio of 1 : 3 in DCB solutions gave the
best device performance with a PCE of 0.96%, a Voc of 0.97 V, a
Jsc of 2.63 mA cmꢀ2, and an FF of 0.38. The high Voc of devices
Field-effect transistor fabrication and characterization
Hole mobility up to 10ꢀ3 cm2 Vꢀ1 sꢀ1 is usually required for agrees well with the wide band gap and the deep HOMO level of
polymer donors in order to obtain balanced electron and hole PSiF-D(OT)DFBT (vide supra). The use of DIO (2%, by volume) as
mobility for the polymer and PCBM blends, and balanced the additive, the PCE was increased to 2.19% with a Voc of
0.96 V, a Jsc of 3.75 mA cmꢀ2, and an FF of 0.61. For PSiF-
TTDFBT based PSCs, the active layer spin-coated from poly-
mer:PC71BM in a weight ratio of 1 : 3 using DCB as the solvent
gave a PCE of 1.61% with a Voc of 0.70 V, a Jsc of 5.09 mA cmꢀ2
,
and an FF of 0.45. By the use of 1-CN (2%, by volume) as the
additive, the PCE was increased to 2.39% with a Voc of 0.70 V, a
Jsc of 5.34 mA cmꢀ2, and an FF of 0.65. For PDOSiF-DTDFBT,
devices fabricated from DCB solutions with a polymer to
PC71BM weight ratio of 1 : 1 gave the highest PCE of 3.61% with
a Voc of 0.72 V, a Jsc of 8.23 mA cmꢀ2, and an FF of 0.61. By the
use of 1-CN (2%, by volume) as the additive, the PCE was
increased to 4.03% with a Voc of 0.73, a Jsc of 8.55 mA cmꢀ2, and
an FF of 0.65. The data are summarized in Table 4. Comparing
PSiF-TTDFBT with PDOSiF-DTDFBT, we should mention here
that the solubility of PSiF-TTDFBT was poorer than
Fig. 3 Cyclic voltammograms of the polymers.
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Fig. 4 Transfer and output characteristics of PSiF-D(OT)DFBT (a and b), PSiF-TTDFBT (c and d), and PDOSiF-DTDFBT (e and f) devices,
respectively.
Table 3 FET properties of the pure polymer films
DTDFBT is of a deep HOMO level. Among these polymers, the
pure PDOSiF-DTDFBT lm showed the highest hole mobility
Polymers
On/off
m (cm2 Vꢀ1 sꢀ1
)
VT (V)
and the PDOSiF-DTDFBT:PC71BM blend lm gave the highest
space charge limited current (SCLC) mobility (vide infra). The
hole mobility of the PDOSiF-DTDFBT:PC71BM blend lm is
almost three times higher than that of the PSiF-
TTDFBT:PC71BM blend lm. As we described (vide supra), the
PSiF-D(OT)DFBT
PSiF-TTDFBT
PDOSiF-DTDFBT
8.4 ꢁ 105
2.0 ꢁ 106
4.7 ꢁ 106
6.94 ꢁ 10ꢀ3
3.09 ꢁ 10ꢀ2
3.31 ꢁ 10ꢀ2
ꢀ40
ꢀ37
ꢀ35
PDOSiF-DTDFBT, which makes it difficult to make a high best photovoltaic performance was achieved for PSiF-
concentration solution for spin-coating. The high Voc for PDO- TTDFBT:PC71BM in a polymer to PC71BM weight ratio of 1 : 3;
SiF-DTDFBT based PSCs is consistent with the fact that PDOSiF- whereas for PDOSiF-DTDFBT based PSCs, the best photovoltaic
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Fig. 5 (a) J–V curves of PSiF-D(OT)DFBT, PSiF-TTDFBT, and PDOSiF-DTDFBT solar cells; (b) external quantum efficiency curves for the solar cell
devices under the best conditions.
Table 4 Photovoltaic cell performance of three polymers
Active layer
Solvent
Voc (V)
Jsc (mA cmꢀ2
)
FF
PCE
Thickness
SCLC mobilities
PSiF-D(OT)DFBT:PC71BM (1 : 3)
PSiF-D(OT)DFBT:PC71BM (1 : 3)
PSiF-TTDFBT:PC71BM (1 : 3)
PSiF-TTDFBT:PC71BM (1 : 3)
PDOSiF-DTDFBT:PC71BM (1 : 1)
PDOSiF-DTDFBT:PC71BM (1 : 1)
DCB
DCBa
DCB
DCBb
DCB
DCBb
0.97
0.96
0.70
0.70
0.72
0.73
2.63
3.75
5.09
5.34
8.23
8.55
0.38
0.61
0.45
0.65
0.61
0.65
0.96%
2.19%
1.61%
2.39%
3.61%
4.03%
95
97
102
105
101
106
4.56 ꢁ 10ꢀ5
6.52 ꢁ 10ꢀ5
1.21 ꢁ 10ꢀ5
9.65 ꢁ 10ꢀ5
1.25 ꢁ 10ꢀ4
3.21 ꢁ 10ꢀ4
a
With 2% DIO (by volume). b With 2% 1-CN (by volume).
performance was achieved in a polymer to PC71BM weight ratio
To correct the Jsc results obtained from J–V measurements,
of 1 : 1. Therefore we think that the high Jsc of PDOSiF-DTDFBT external quantum efficiencies (EQEs) of devices used for the J–V
based PSCs is due to the following two reasons: (1) the high hole investigation were measured and are shown in Fig. 5b. In the
mobility of the blend lm, which is conrmed by the SCLC range of 360 to 600 nm, EQEs of PSiF-D(OT)DFBT and PSiF-
measurement; (2) the high polymer content blend lms can TTDFBT are around 35%; whereas the EQE of PDOSiF-DTDFBT
absorb more photons, which is proved by the high absorption is around 50%. For all devices, Jsc obtained from the integration
coefficient of PDOSiF-DTDFBT:PC71BM (1 : 1) blend lms. UV- of the EQE with an AM 1.5G reference spectrum matched well
visible absorption coefficient curves of PSiF-D(OT) with the Jsc obtained from the J–V measurements.
DFBT:PC71BM (1 : 3), PSiF-TTDFBT:PC71BM (1 : 3), and PDOSiF-
DTDFBT:PC71BM (1 : 1) blend lms fabricated under optimized
conditions are shown in Fig. S1.† The absorption coefficient was
Film morphologies
calculated by lm absorbance/lm thickness (nm). We can
clearly see that the absorption coefficient of the PDOSiF-
DTDFBT:PC71BM (1 : 1) blend lm is markedly higher than that
of the PSiF-D(OT)DFBT:PC71BM (1 : 3) blend lm in the range of
The morphology of polymer:PC71BM blend lms, which has a
signicant inuence on exciton separation, charge carrier
mobility, and photovoltaic performance, was investigated by
atomic force microscopy (AFM) in tapping-mode. AFM images
350 to 700 nm. The poorer solubility of PSiF-TTDFBT:PC71BM in of PSiF-D(OT)DFBT:PC71BM (1 : 3), PSiF-TTDFBT:PC71BM
1,2-dichlorobenzene prohibited the fabrication of a device with (1 : 3), and PDOSiF-DTDFBT:PC71BM (1 : 1) blend lms fabri-
a high polymer concentration. Compared with uorene based
D–A polymers PFO-M1-3,22 the introduction of two uorine
atoms on the benzothiadiazole ring resulted in poorer solubility
of the nal polymers, and led to poorer performance of PSiF-
TTDFBT:PC71BM based PSCs. Hole mobilities of blend lms
were measured by the SCLC method, and the results are also
cated from DCB solutions without and with additives are shown
in Fig. 6. For PSiF-D(OT)DFBT, the use of DIO as an additive led
to a smaller domain size with the root-mean-square (rms) value
decreased from 6.16 nm to 1.55 nm; whereas for PSiF-TTDFBT
and PDOSiF-DTDFBT, the use of 1-CN as an additive also led to
a smaller domain size with the rms values decreased from
summarized in Table 4. PDOSiF-DTDFBT and PC71BM lms 5.61 nm to 0.793 nm for PSiF-TTDFBT and from 0.401 nm to
exhibit the highest hole mobilities among these blend lms, 0.364 nm for PDOSiF-DTDFBT. As an experiential rule, DIO
which is consistent with the highest short circuit current
obtained from PDOSiF-DTDFBT based PSCs.
prefers to dissolve PCBM; whereas CN prefers to dissolve the
conjugated polymers. Usually, the use of DIO can reduce the
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Paper
Fig. 6 AFM height images of polymer:PC71BM blend films without and with additive.
domain size of PC71BM;26 whereas the use of CN can usually and 2,7-linked silauorene. PSiF-DTDFBT is insoluble in
reduce the domain size of polymers.27,28 The change in common organic solvents and not suitable for fabrication of
morphology aer the addition of additive is consistent with the PSCs by solution processing. PSiF-D(OT)DFBT, PSiF-TTDFBT,
enhancement of the performance of PSCs.
and PDOSiF-DTDFBT are soluble in DCB and TCB and could be
used for the fabrication of PSCs. PSiF-D(OT)DFBT with an octyl
chain on the thiophene unit is of a band gap of 2.06 eV with a
deep HOMO of ꢀ5.64 eV. PSiF-TTDFBT with two additional
thiophenes at each repeating unit in the polymer main chain
exhibits a band gap of 1.75 eV with the HOMO of ꢀ5.23 eV.
Conclusions
We have designed and synthesized four D–A alternating
conjugated polymers based on 5,6-diuorobenzothiadiazole
5122 | J. Mater. Chem. C, 2014, 2, 5116–5123
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Journal of Materials Chemistry C
PDOSiF-DTDFBT with two additional octyloxy chains on the 11 R. Qin, W. Li, C. Li, C. Du, C. Veit, H.-F. Schleiermacher,
silauorene unit has a band gap of 1.86 eV with the HOMO level
of ꢀ5.37 eV. The hole mobilities of these polymers are in the
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Acknowledgements
Financial supports from the NSF of China (21161160443,
91233205, and 51003006), the 973 Program (2011CB935702),
the Beijing Natural Science Foundation (2132042), and the
Fundamental Research Funds for the Central Universities are
gratefully acknowledged.
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