Conjugated and nonconjugated substitution effect on photovoltaic properties of benzodifuran-based photovoltaic polymers

Article abstract of DOI:10.1021/ma301254x

In order to investigate the influence of two-dimensional (2D) conjugated structure on photovoltaic properties of benzo[1,2-b:4,5-b']difuran (BDF)-based polymers, two low band gap photovoltaic polymers, named PBDFTT-CF-O and PBDFTT-CF-T, were designed and synthesized. These two polymers have the same backbones and different side groups. Although these two polymers show similar optical band gaps (ca. 1.5 eV), the polymer with alkylthienyl side groups, PBDFTT-CF-T, exhibits stronger absorption in long wavelength direction than the polymer with alkoxyl side groups, PBDFTT-CF-O. Meanwhile, PBDFTT-CF-T exhibits a HOMO level of -5.21 eV, which is 0.23 eV lower than that of PBDFTT-CF-O due to weaker electron-donating ability of alkylthienyl side groups than that of aloxyl side groups. The hole mobility of the blend of PBDFTT-CF-T/PC₇₁BM (1:1.5, w/w) is 0.128 cm² V^-1 s^-1, which is 1 order of magnitude higher than that of the blend of PBDFTT-CF-O/PC ₇₁BM. Density functional theory (DFT) model shows thiophene pendants on dithienyl-BDF are more coplanar than it on dithienyl-BDT. These results indicate that the 2D-conjugated structure is helpful for molecular structure design of the BDF-based polymers in enhancing the intermolecular π-π stacking and improving charge transport property. Furthermore, the photovoltaic devices based on these two polymers show similar short circuit density and fill factor values, while the open circuit voltage of the PBDFTT-CF-T-based device is 0.78 V, which is 0.15 V higher than that of the PBDFTT-CF-O-based device. Therefore, the efficiencies of the devices based PBDFTT-CF-T/PC₇₁BM and PBDFTT-CF-O/PC₇₁BM are 6.26% and 5.22%, respectively. The results in this work demonstrate that the weak electron-donating ability of alkylthienyl side groups can be seen as an effective strategy to improve photovoltaic properties of the BDF-based polymers and the 2D-conjugated molecular structure is favorable to improve hole mobility.

Full text of DOI:10.1021/ma301254x

Article
pubs.acs.org/Macromolecules
Conjugated and Nonconjugated Substitution Effect on Photovoltaic
Properties of Benzodifuran-Based Photovoltaic Polymers
Lijun Huo, Long Ye, Yue Wu, Zhaojun Li, Xia Guo, Maojie Zhang, Shaoqing Zhang, and Jianhui Hou*
State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
*
S Supporting Information
ABSTRACT: In order to investigate the influence of two-
dimensional (2D) conjugated structure on photovoltaic
properties of benzo[1,2-b:4,5-b′]difuran (BDF)-based poly-
mers, two low band gap photovoltaic polymers, named
PBDFTT-CF-O and PBDFTT-CF-T, were designed and
synthesized. These two polymers have the same backbones
and different side groups. Although these two polymers show
similar optical band gaps (ca. 1.5 eV), the polymer with
alkylthienyl side groups, PBDFTT-CF-T, exhibits stronger
absorption in long wavelength direction than the polymer with
alkoxyl side groups, PBDFTT-CF-O. Meanwhile, PBDFTT-CF-T exhibits a HOMO level of −5.21 eV, which is 0.23 eV lower
than that of PBDFTT-CF-O due to weaker electron-donating ability of alkylthienyl side groups than that of aloxyl side groups.
2
−1 −1
The hole mobility of the blend of PBDFTT-CF-T/PC BM (1:1.5, w/w) is 0.128 cm V s , which is 1 order of magnitude
71
higher than that of the blend of PBDFTT-CF-O/PC BM. Density functional theory (DFT) model shows thiophene pendants
71
on dithienyl-BDF are more coplanar than it on dithienyl-BDT. These results indicate that the 2D-conjugated structure is helpful
for molecular structure design of the BDF-based polymers in enhancing the intermolecular π−π stacking and improving charge
transport property. Furthermore, the photovoltaic devices based on these two polymers show similar short circuit density and fill
factor values, while the open circuit voltage of the PBDFTT-CF-T-based device is 0.78 V, which is 0.15 V higher than that of the
PBDFTT-CF-O-based device. Therefore, the efficiencies of the devices based PBDFTT-CF-T/PC BM and PBDFTT-CF-O/
71
PC BM are 6.26% and 5.22%, respectively. The results in this work demonstrate that the weak electron-donating ability of
71
alkylthienyl side groups can be seen as an effective strategy to improve photovoltaic properties of the BDF-based polymers and
the 2D-conjugated molecular structure is favorable to improve hole mobility.
INTRODUCTION
To find a method to simultaneously obtain narrower band
gap, a deeper HOMO level as well as higher mobility in one
conjugated polymer is crucial to the study of photovoltaic
polymers. Recently, our groups introduced two-dimensional
■
Polymer solar cells (PSCs) have made great progress in recent
1
years. Up to now, several groups have developed some highly
efficient photovoltaic polymers, which showed promising
power conversion efficiencies (PCEs) of 7−9% in bulk
heterojunction cells devices. The optimization of molecular
(
2D) conjugated structure to benzodithiophene (BDT)-based
2a
polymers, and this kind of polymer shows similar band gaps,
slightly deeper HOMO levels, higher mobilities, and hence
better photovoltaic properties than the analogues without the
2
structure of photovoltaic polymers is of great importance to
realize high photovoltaic performance. It has been well-
recognized that band gap, molecular energy level, and mobility
are three key parameters for photovoltaic polymers. Different
strategies have been developed to optimize those three
parameters by modifying molecular structures of photovoltaic
polymers. For example, band gaps of conjugated polymers can
be reduced effectively by introducing D−A structure or quinoid
structure; molecular energy levels of conjugated polymers can
be tuned by introducing electron-rich/deficient building blocks
or substituents; mobilities of conjugated polymers can be3
improved by increasing regularity of the backbone units.
Beside those strategies listed as above, there are still a lot of
successful examples for molecular structure optimization of
2
D-conjugated structure. (see Scheme 1) Although the
introduction of the 2D-conjugated structure has proven to be
helpful in improving photovoltaic properties of BDT-based
polymers, how this concept functions in other polymer systems
is still unknown. Therefore, it is still necessary to do further
investigation on the influence of 2D-conjugated structure on
photovoltaic properties of different polymer systems.
In the field of organic semiconductors, furan and its
derivatives attracted much less attention compared to
thiophene-based heteroaromatic units. However, since furan-
Received: June 20, 2012
Revised: August 8, 2012
4
photovoltaic polymers.
©
XXXX American Chemical Society
A
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Scheme 1. Molecular Structures of the BDT-Based and BDF-Based Polymers
a
Scheme 2. Synthetic Routes of PBDFTT-CF-O and PBDFTT-CF-T
a
Reagents and conditions: (i) oxalyl chloride, room temperature, 12 h; (ii) diethylamine, methylene chloride, room temperature, 1 h; (iii) 0 °C,
THF, n-butyllithium, then room temperature,18 h, then HCl; (iv) 0 °C, ethanol, sodium borohydride, 3 h, HCl; then 2-ethylhexyl bromide, 150 °C,
5 h; (v) THF, n-butyllithium, room temperature, 1 h, then chlorotrimethylstannane, 30 min; (vi) n-butyllithium, THF, 0 °C, then 50 °C, 2 h;
1
benzo[1,2-b:4,5-b′]difuran-4,8-dione, 50 °C, 1 h, then SnCl , HCl; (vii) 0 °C, THF, n-butyllithium, then room temperature, 2 h, then
2
chlorotrimethylstannane, 1 h; (viii) toluene/DMF, Pd(PPh ) , 110 °C, 16 h.
3
4
based conjugated polymers exhibit some interesting properties,
EXPERIMENTAL SECTION
■
5
6
like planar molecular configuration, high solubility, and
Materials. 4,8-Dehydrobenzo[l,2-b:4,5-b′]dithiophene-4,8-dione
(3), 4,8-bis(2-ethylhexyloxy)benzo[1,2-b;3,4-b]difuran (4), and 2,6-
bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dfuran
5) were prepared according to our modified method. 1-(4,6-
7
biodegradable and biorenewable properties, several conjugated
polymers were developed and showed promising properties in
9
(
8
optoelectronic devices in recent years. Our group previously
Dibromo-3-fluorothieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one
(TT-CF) and 2-(2-ethylhexyl)thiophene were purchased from Solar-
mer Materials Inc.; Pd(PPh ) was purchased from Frontiers Scientific
designed and synthesized a conjugated polymer based on
alkyoxy-substitutd benzo[1,2-b:4,5-b′]difuran (BDF) and 4,7-
dithienyl-2,1,3-benzothiadiazole (DTBT), named PBDF-
3
4
Inc. All of these chemicals were used as received. Tetrahydrofuran
THF) was dried over Na/benzophenone ketyl and freshly distilled
(
9
DTBT, which showed promising photovoltaic properties. In
prior to use. The other materials were commercial level and used as
received.
this work, in order to develop new photovoltaic polymers based
on BDF unit and investigate the influence of 2D-conjugated
structure on photovoltaic properties of the BDF polymer, we
tried to build a new polymer backbone based on BDF and
thieno[3,4-b]thiophene (TT) and then apply the 2D-
conjugated structure to this backbone, and hereby a new
polymer, named PBDFTT-CF-T in Scheme 1, was designed. As
a reference material, another new polymer based on alkoxy-
substituted BDF, named PBDFTT-CF-O in Scheme 1, was also
prepared and characterized in parallel with PBDFTT-CF-T.
Instruments. 1H spectra were measured on a Bruker arx-400
spectrometer in CDCl at 293 K. Absorption spectra were taken on a
3
Hitachi U-3100 UV−vis spectrophotometer. The molecular weights of
the polymers were measured by the GPC method, wherein
polystyrene was used as a standard by using chloroform as eluent.
TGA measurements were performed on a TA Instruments, Inc., TGA-
2
050. The electrochemical cyclic voltammetry (CV) was conducted on
a CHI650D electrochemical workstation with Pt disk, Pt plate, and
+
Ag/Ag electrode as working electrode, counter electrode, and
reference electrode, respectively, in a 0.1 M tetrabutylammonium
hexafluorophosphate (Bu NPF )−acetonitrile solution. Atomic force
4
6
microscopy (AFM) measurement of the surface morphology of
B
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samples was conducted on a Nanoscope III (Vecco) in tapping mode
with a 2 μm scanner. Transmission electron microscopy (TEM) was
performed using a JEOL 2200FS instrument at 160 kV accelerating
voltage. The samples for the TEM measurements were prepared as
follows: the thin active-layer (about 100 nm) films were spin-cast on
ITO/PEDOT:PSS substrates, and the ITO glass with the active layers
was submerged in deionized water to make the active layers floated
onto the air/water interface. Then the floated films were picked up on
unsupported 200 mesh copper grids for the TEM measurement. The
external quantum efficiency (EQE) was measured by an Enli
Technology solar cell spectral response measurement system (QE-
R3011). The light intensity at each wavelength was calibrated with a
standard single-crystal Si photovoltaic cell.
stannane (1.0 M in hexane, 2 mL) was added, and the mixture was
stirred for 1 h at ambient temperature. Then the mixture was extracted
by ethyl ether, and the combined organic phase was concentrated to
obtain compound 7. Further purification was carried out by
recrystallization using ethanol as solvent to obtain pure compound 7
1
as light yellow solid (0.27 g, yield 77%). H NMR (CDCl , 400 MHz),
3
δ (ppm): δ 7.72(d, 2H), 7.50 (s, 2H), 6.90 (d, 2H), 2.88 (d, 4H), 1.74
1
3
(m, 2H), 1.54−1.33 (br, 16H), 0.95 (m, 12H), 0.47 (s, 18H).
C
NMR (CDCl , 100 MHz), δ (ppm): 166.26, 152.37, 144.64, 135.52,
3
127.19, 125.68, 123.51, 117.97, 108.64, 41.64, 34.30, 32.65, 29.14,
25.80, 23.23, 14.36, 11.10, −8.79. Elements analysis: calcd: C, 55.07;
H, 6.70; found: C, 55.11; H, 6.71.
General Method of Polymerization by Stille Coupling
Reaction for the Polymers. Compound 5 or compound 7 (0.5
mmol) and TT-CF (0.5 mmol) were mixed in 10 mL of toluene and 2
mL of DMF. After being purged by argon for 5 min, 30 mg of
Hole Mobility Measurement. We used a device structure of
ITO/PEDOT:PSS/polymers:PC BM/Au for the hole mobility
7
1
measurement, based on the space-charge-limited current (SCLC)
3
2
model. According to the following equation: ln(JL /V ) ≃ 0.89(1/
Pd(PPh ) was added as catalyst, and then the mixture was purged by
3 4
0
.5
0.5
E ) (V/L) + ln(9εε μ /8), where μ is the zero-field mobility, E is
argon for 25 min. The reactant was stirred and heated to reflux for 16
h. Then the reactant was cooled to room temperature, and the
polymer was precipitated by addition of 50 mL of methanol, and then
filtered through a Soxhlet thimble, which was then subjected to
Soxhlet extraction with methanol, hexane, and chloroform. The
polymer was recovered as solid from the chloroform fraction by
precipitation from methanol. The solid powders were dried under
vacuum. The yields and molecular weight results of the polymers are
as follows: PBDFTT-CF-O: yield: 56%. Elements analysis: calcd: C,
69,13; H, 7.40; found: C, 69.22; H, 7.33. Mw = 14K, PDI = 3.0.
PBDFTT-CF-T: yield: 47%. Elements analysis: calcd: C, 69.69; H,
0
0
0
0
0
the characteristic field, J is the current density, ε is the dielectric
constant of the polymer, ε is the permittivity of the vacuum, L is the
0
thickness of the polymer layer, V = Vappl − V , V is the applied
bi
appl
potential, and V is the built-in potential (in this device structure, V =
bi
bi
0
.2 V). According to the equation, hole mobility can be calculated.
Fabrication of Polymer Solar Cells. Polymer solar cell devices
were fabricated under conditions as follows: After spin-coating a 35
nm layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(
PEDOT:PSS) onto a precleaned indium−tin oxide (ITO)-coated
glass substrates, the polymer/PC BM blend solution was spin-coated.
71
The concentration of the polymer:PC BM blend solutions used in
6.70; found: C, 69.48; H, 6.78. M = 17K, PDI = 3.6.
71
w
this study for spin-coating was 10 mg/mL (polymer/o-dichloroben-
zene), and o-dichlorobenezene was used as the solvent. The additive,
RESULTS AND DISCUSSION
1
,8-diiodooctane (DIO) with 3% volume ratio was added prior to the
■
spin-coating process. The thickness of the active layer was controlled
by changing the spin speed during the spin-coating process and
measured on an Ambios Tech. XP-2 profilometer. The devices were
completed by evaporating Ca/Al metal electrodes with an effective
Synthesis and Structural Characterization. The syn-
thetic routes of PBDFTT-CF-O and PBDFTT-CF-T are shown
in Scheme 2. Pure 4,8-dehydrobenzo[1,2-b:4,5-b′]dithiophene-
4
,8-dione (3) can be obtained from several recrystallizations in
2
area of 4 mm as defined by masks. The current−voltage curves were
−2
acetic acid. 4,8-Bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b′]difuran
4) and 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo-
measured under 100 mW cm standard AM 1.5 G spectrum using a
XES-70S1 (San-EI Electric Co., Ltd.) solar simulator (AAA grade, 70
mm × 70 mm photobeam size). Two cm × 2 cm monocrystalline
silicon reference cell (SRC-1000-TC-QZ) was purchased from VLSI
Standards Inc.
(
[
1,2-b:4,5-b′]dfuran (5) were prepared according to modified
9
our previous work. The 2D-conjugated BDF monomer was
synthesized easily through two steps. Under 0 °C, n-
butyllithium was added to 2-ethylhexylthiophene to form (5-
Synthesis. The synthetic routes of the monomers and polymers
are shown in Scheme 2. The detailed synthetic processes are as
follows.
(
2-ethylhexyl)thiophen-2-yl)lithium, and then benzo[1,2-b:4,5-
b′]difuran-4,8-dione was added at 50 °C and the reactant was
stirred under 50 °C for 2 h, and then a solution of SnCl in
hydrochloric acid was added. Compound 6 can be purified by
silica gel column using petroleum ether as eluent and obtained
in a yield of 44%. Subsequently, compound 7 was prepared
with a yield of 77% through the same method as used in the
synthesis of 2D-conjugated BDT monomer. These two
polymers were prepared by Stille coupling reactions, and the
yields are 56% and 47% for PBDFTT-CF-O and PBDFTT-CF-
T, respectively. Both of these two polymers show excellent
solubility in tetrahydrofuran, chloroform, toluene, dichloroben-
zene, etc. Although different methods were tried to increase the
polymerization degree, the molecular weights of the polymers
are still lower than the majority of the conjugated polymers
used in photovoltaic applications. The molecular weights of
these two polymers were estimated by the gel permeation
chromatography (GPC) method using chloroform as eluent
and monodispersed polystyrene as standard. For PBDFTT-CF-
O and PBDFTT-CF-T, the weight-average molecular weights
4
,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]-
difuran (6). In a 100 mL argon purged flask, n-butyllithium (2.5M,
.52 mL) was added into a solution of 2-(2-ethylhexyl)thiophene
1.57 g, 8 mmol) in THF (30 mL) dropwise at 0 °C; then the mixture
2
3
(
was warmed up to 50 °C and stirred for 1 h. Subsequently, 4,8-
dehydrobenzo[l,2-b:4,5-b′]difuran-4,8-dione (0.38 g, 2 mmol) was
added, and the mixture was stirred for 1 h at 50 °C. After cooling
2a
down to ambient temperature, a mixture of SnCl ·2H O (4.5 g, 20
2
2
mmol) in 10% HCl (8 mL) was added, and the mixture was stirred for
an additional 1.5 h and then poured into ice water. The mixture was
extracted by ethyl ether twice, and the combined organic phase was
concentrated to obtain the raw compound 6. Further purification was
carried out by a silica gel column using petroleum ether as eluent to
obtain pure compound 6 as a pale yellow sticky liquid (0.48 g, yield
1
4
2
1
(
1
4%). H NMR (CDCl , 400 MHz), δ (ppm): δ 7.77(d, 2H), 7.68 (d,
3
H), 7.37 (d, 2H), 6.89 (d, 2H), 2.85 (d, 4H), 1.70 (m, 2H), 1.463−
13
.25 (br, 16H), 0.95−0.91 (m, 12H). C NMR (CDCl , 100 MHz), δ
3
ppm): 148.87, 145.72, 145.28, 134.36, 127.78, 125.89, 123,43, 110.34,
07.36, 41.71, 34.38, 32.68, 29.14, 25.83, 23.26, 14.38, 11.09. Elements
analysis: calcd: C, 74.68; H, 7.74; found: C, 74.65; H, 7.73.
2
,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-
(
3
M ) are 14K and 17K, and the polydispersity index (PDI) are
benzo[1,2-b:4,5-b′]difuran (7). In a 50 mL argon-purged flask, n-
w
.0 and 3.6, respectively. Compared to other conjugated
butyllithium (2.5 M, 0.48 mL) was added into a solution of compound
6
(0.22 g, 0.4 mmol) in THF (20 mL) at 0 °C; then the mixture was
polymers, these two new BDF-based polymers show lower
molecular weight values, and higher molecular weight products
stirred for 2 h at ambient temperature. Subsequently, chlorotrimethyl-
C
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can be expected if microwave condition can be used in the Stille
10
polycondensation reaction.
Optical Properties. The photophysical characteristics of
the polymers were investigated by ultraviolet−visible (UV−vis)
absorption spectroscopy in dilute chloroform solutions and as
spin-coated films on quartz substrates. The absorption spectra
of PBDFTT-CF-O and PBDFTT-CF-T in solution and as solid
films are shown in Figure 1. According to the absorption
Figure 2. Cyclic voltammograms of PBDFTT-CF-O and PBDFTT-
CF-T films on a platinum electrode in acetonitrile solution containing
0
.1 M Bu NPF at a scan rate of 20 mV/s.
4 6
according to the following equations: ELUMO = −e(E + 4.8)
red
(
eV) and EHOMO = −e(E + 4.8) (eV), where the units of E
ox
ox
+
and Ered are in V vs Fc/Fc . The HOMO and LUMO levels of
PBDFTT-CF-O and PBDFTT-CF-T are −4.98 eV/−3.18 eV
and −5.21 eV/−3.20 eV, respectively. It is obvious that the
HOMO level of PBDFTT-CF-T is 0.23 eV lower than that of
PBDFTT-CF-O, implying that the weak electron-donating
ability of alkylthienyl side groups is favorable to lowering the
HOMO level of the polymer, and hence the PSC device based
Figure 1. Absorption spectra of PBDFTT-CF-O and PBDFTT-CF-T
in chloroform solutions and in solid films on quartz.
spectra of the films, the optical band gaps (Egopt) of PBDFTT-
CF-O and PBDFTT-CF-T are 1.51 and 1.49 eV, respectively.
Although the optical band gaps of these two polymers are quite
similar, PBDFTT-CF-T shows much narrower absorption peak
than PBDFTT-CF-O. In the visible range of 400−700 nm,
PBDFTT-CF-T shows the weaker absorption intensity
compared to that of PBDFTT-CF-O, indicating the π−π*
transition of PBDFTT-CF-T are located at the low-energy
region mainly, and thus the π-electrons of PBDFTT-CF-T
might be delocalized better than the π-electrons of PBDFTT-
CF-T. The solution and the film of PBDFTT-CF-T show two
absorption peaks at 660 and 706 nm. The absorption peak
located at 660 nm is ascribed to the intramolecular π−π*
transition of the backbones, indicating that the two polymers
should have similar effective conjugated length, and therefore
the replacement of alkoxy groups by alkylthienyl groups has
little influence on the conformation of the backbone units in
these two polymers; i.e., the dihedral angels between the
adjacent backbone units in these two polymers should be much
similar. The absorption peak at 706 nm is ascribed to the
intermolecular π−π* transition. Compared to PBDFTT-CF-O,
PBDFTT-CF-T shows much stronger absorption in long
wavelength direction, implying that the intermolecular π−π*
transition in PBDFTT-CF-T is much stronger than that in
PBDFTT-CF-O, and thus the introduction of 2D-conjugated
structure might be helpful to enhance intermolecular π−π
stacking of the polymer.
on PBDFTT-CF-T may possess higher open-circuit voltage
2a
(
Voc) than that of PBDFTT-CF-O-based PSC device. As
reported in our previous work in Table 1, in the polymer
Table 1. Electrochemical Data, Molecular Energy Levels and
Optical Band Gaps Comparisons among PBDFTT-CF-O,
PBDFTT-CF-T, PBDTTT-C, and PBDTTT-C-T
ox
red
opt
a
ec
b
E
E
HOMO LUMO
E
E
g
onset
(
onset
g
V)
(V)
(eV)
(eV)
(eV)
(eV)
PBDFTT-CF-O
PBDFTT-CF-T
0.18
0.41
0.27
0.31
−1.62
−1.60
−1.59
−1.55
−4.98
−5.21
−5.07
−5.11
−3.18
−3.20
−3.21
−3.25
1.51
1.49
1.60
1.58
1.80
2.01
1.86
1.86
c
PBDTTT-C
c
PBDTTT-C-T
a
Egopt = λabs,onset/1240. Calculated from E = e(Eonsetox − Eonsetred).
Reference 2a.
b
ec
g
c
system based on BDT and TT, HOMO levels of the polymers
can be slightly reduced by replacing alkyoxy groups with
alkylthienyl groups; i.e., the HOMO level of PBDTTT-C-T is
0.04 eV lower than that of PBDTTT-C-O. It is obviously that
the alkylthienyl side groups exhibit stronger effect on lowering
HOMO level of the BDF-based polymer than on BDT-based
polymer.
Quantum chemistry calculation by the DFT (B3LYP/6-
31G** level) method was employed to demonstrate the
electronic structures of these polymers. To simplify the
calculations, long alkyl side chains were replaced by methyl.
From simulated models in Table 2, we found that the LUMO
orbits of these four polymers are localized at the TT units
mainly, while their HOMO orbits are distributed along the
whole molecules. Moreover, two phenomena should be noted.
First, for PBDFTT-CF-T and PBDTTT-C-T, the wave
functions of their HOMO levels can be delocalized onto the
alkylthienyl groups, indicating that the π-electrons can be
delocalized better by introducing the 2-D conjugated structure.
Electrochemical Properties. Electrochemical cyclic vol-
tammetry (CV) was employed to measure the polymers’
oxidation/reduction onset potentials. From Figure 2, it can be
seen that p-doping processes of both PBDFTT-CF-O and
PBDFTT-CF-T are reversible. The oxidation onset potentials
of PBDFTT-CF-O and PBDFTT-CF-T are 0.18 and 0.41 V,
respectively. In the reduction processes, both two polymers
exhibit similar reduction onset values at ca. −1.6 eV. The
HOMO/LUMO levels of the polymers can be calculated
D
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Table 2. Optimized Geometries and Molecular Orbital Surfaces of the HOMO and LUMO of the Model Compounds, Obtained
at the DFT/B3LYP/6-31G* Level
Second, since the diameter of oxygen atom is much smaller
than that of sulfur atom, compared to BDT unit, BDF unit
exhibits smaller steric hindrance to the substituent on its 4- and
8-positions, and thus the torsion angle between the alkylthienyl
group and the backbone unit in PBDFTT-CF-T is smaller than
that in PBDTTT-C-T. Consequently, the wave function of the
HOMO level of PBDFTT-CF-T can be delocalized to the
alkylthienyl side groups more effectively than that of PBDTTT-
CF-T.
Photovoltaic Properties. The PSC devices with a
structure of ITO/PEDOT:PSS/polymers:PC BM/Ca/Al
7
1
were fabricated to investigate photovoltaic properties of these
two polymers. Both PBDFTT-CF-O/PC BM and PBDFTT-
7
1
CF-T/PC BM blends were dissolved into o-dichlorobenzene
71
to make the solutions of the D/A blends for spin-coating, and
Figure 3. J−V curves of the PSCs based on PBDFTT-CF-O/PC BM
different weight ratios of polymer donor to PC BM were
71
7
1
and PBDFTT-CF-T/PC BM under the illumination of AM 1.5G, 100
71
scanned. The current density−voltage (J−V) curves of the
2
mW/cm .
PBDFTT-CF-O/PC BM-based and PBDFTT-CF-T/
7
1
PC BM-based PSC devices with optimum D/A ratios under
7
1
J . According to the J−V curves, it is quite clear that the devices
2
sc
the illumination of AM 1.5G (100 mW/cm ) are shown in
Figure 3; the basic photovoltaic data for D/A ratio scan are
listed in Table 3, and the photovoltaic results of the BDT
analogues are included for making clear comparisons. The Voc,
based on PBDFTT-CF-O and PBDFTT-CF-T fabricated
through the optimum conditions show much similar Jsc and
FF values, but Voc of the PBDFTT-CF-T/PC BM-based
71
devices is 0.15 V higher than that of the PBDFTT-CF-O/
PC BM-based devices. Therefore, from PBDFTT-CF-O/
the short-circuit current density (J ), and the fill factors (FF) of
sc
7
1
PBDFTT-CF-O/PC BM-based devices with different D/A
7
1
PC BM to PBDFTT-CF-T/PC BM, PCE of the devices
71
71
2
ratios are 0.60−0.63 V, 9.76−12.59 mA/cm , and 47.1−59.7%,
respectively. Correspondingly, the PCEs of the devices vary
from 2.85% to 5.22%. From the device based on PBDFTT-CF-
improved by 20%. Comparing the optimized photovoltaic
results of the BDT-based analogues with these two BDF-based
polymers (see the data in Table 3), the PCEs of the polymers
can be improved by using the 2-D conjugated structure.
Interestingly, from PBDTTT-C to PBDTTT-C-T, an improve-
O/PC BM with a optimum D/A ratio (polymer:PC BM =
7
1
71
2
1
:1.5), a PCE of 4.52% with V = 0.63 V, J = 12.51 mA/cm ,
oc sc
FF = 0.57 was recorded. In order to further optimize
photovoltaic performance, 3% (v/v) 1,8-diiodooctane (DIO)
ment of 40 mV of V was observed; however, from PBDFTT-
oc
CF-O to PBDFTT-CF-T, an improvement of 160 mV can be
realized. The change of the Voc values matches well with the
results obtained from HOMO level (see Table 1), indicating
that the 2-D conjugated structure has different influence on
conjugated polymers with different backbone structures. From
PBDFTT-CF-O to PBDFTT-CF-T, the distinguished improve-
ment in HOMO level, and thus Voc should be due to the
reduced electron-donating effect caused by the side groups
11
was used as additive in the solution of active layer materials.
We found that after adding DIO, the J of the device can be
sc
2
improved slightly, from 12.51 to 13.87 mA/cm , and thus a
PCE of 5.22% can be obtained. The optimum D/A ratio of
PBDFTT-CF-T/PC BM system is 1:1.5, and after adding DIO
7
1
as additive, photovoltaic performance of the device can be
improved, from 4.54% to 6.26%, benefiting from the increase of
E
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Table 3. Photovoltaic Results of the PSCs Based on PBDFTT-CF-O/PC BM and PBDFTT-CF-T/PC BM under the
7
1
71
2
Illumination of AM 1.5G 100 mW/cm
polymers: PC BM
D/A ratio
1:1
film thickness (nm)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
71
PBDFTT-CF-O
100
110
114
105
108
105
110
95
0.62
0.63
0.63
0.60
0.8
9.76
12.51
13.87
12.59
8.62
47.14
57.34
59.70
55.77
48.98
58.76
61.55
59.8
2.85
4.52
5.22
4.21
3.38
4.54
6.26
4.20
6.43
7.59
1:1.5
1:1.5
1:2
a
PBDFTT-CF-T
1:1
1
1
1
:1.5
0.79
0.78
0.78
0.70
0.74
9.79
a
:1.5
: 2
13.04
9.00
b
a
a
PBDTTT-C
1:1.5
1:1.5
120
140
15.51
17.48
59.2
b
PBDTTT-C-T
58.7
a
b
3
% v/v DIO was added. Optimized results from ref 2a.
because the alkoxy should have a stronger electron-donating
effect than the alkylthienyl group.
higher than that of the blend film of PBDFTT-CF-O/PC BM,
71
it can be concluded that the 2D conjugated structure is helpful
to facilate the charger transport of the polymer, benefiting from
the extended conjugated area and hence the enhanced
intermolecular π−π interaction. The above conclusion is also
coincident with the reported results from 2-D conjugated BDT
The external quantum efficiency (EQE) curves of the devices
based on PBDFTT-CF-O/PC BM and PBDFTT-CF-T/
7
1
PC BM (w/w, 1:1.5) prepared under optimum conditions
7
1
are shown in Figure 4. Since the two devices shows similar EQE
2a
system.
Surface morphology of the blend films of PBDFTT-CF-O/
PC BM and PBDFTT-CF-T/PC BM with and without DIO
71
71
were investigated by the atomic force microscope (AFM). As
shown in Figures 5a and 5b, without adding DIO, the root-
mean-square roughness (R ) of the PBDFTT-CF-O/PC BM
q
71
and PBDFTT-CF-T/PC BM films are 2.6 and 3.5 nm,
71
respectively. However, when DIO was used as additive during
the spin-coating process, R values of the blend films increased
q
to 8.9 and 6.0 nm for the films of PBDFTT-CF-O/PC BM
7
1
and PBDFTT-CF-T/PC BM, respectively. Transmission
7
1
electron microscopy (TEM) was employed to further
investigate the morphology of the blend films. The effects of
DIO treatment on morphology of the blend films are well
demonstrated as shown in Figure 5. As shown in Figures 5a and
5
b, PBDFTT-CF-O/PC BM and PBDFTT-CF-T/PC BM
71 71
exhibit very similar phase separation, wherein 100−350 nm
Figure 4. External quantum efficiency (EQE) curves of PBDFTT-CF-
O- and PBDFTT-CF-T-based PSCs with a D/A ratio of 1:1.5 treated
by using additive (DIO, 3% v/v).
dark domains are formed. Since PC BM has higher scattering
71
density than the polymer, these dark domains should be
1
2
ascribed to the aggregations of PC BM. These large
7
1
dimensions are much greater than the typical organic exciton
in the whole response region, the integral current density values
of the two devices deduced from the EQE curves and the
standard solar spectrum are quite similar, which is coincident
with we observed in J−V measurements. These two devices
exhibit broad response in the visible light region, from 400 to
13
diffusion length (ca. 10 nm as reported) and thereby limit
device performance. Upon addition of 3% DIO, a significant
reduction in phase separation is observed in Figures 5c and 5d.
On the basis of the observations in the AFM and TEM images,
it can be concluded that after adding DIO, the roughness of the
blend films based on both two polymers increased slightly,
while the phase separation of the blends reduced distinctly.
8
5
00 nm. The EQE peak values of these two devices reach over
0%. Considering that PBDFTT-CF-T/PC BM-based device
7
1
shows a very broad response range, from 400 to 800 nm, if the
EQE of the device can be improved to over 70%, the current
CONCLUSION
density of the device would be increased tremendously, and
■
2
theoretically, a J of ∼18 mA/cm can be expected.
Two new low band gap conjugated polymers based on BDF
were designed, synthesized, and applied in PSCs. In order to
investigate the influence of 2D conjugated structure on
photovoltaic properties of the BDF-based polymers, non-
conjugated alkoxy and conjugated alkylthienyl are used as side
groups in PBDFTT-CF-O and PBDFTT-CF-T, respectively.
Compared to PBDFTT-CF-O, PBDFTT-CF-T shows stronger
intermolecular π−π interaction in solution state and solid film,
deeper HOMO level, and higher hole mobility. Under
optimum device fabrication conditions, the PSC based on
PBDFTT-CF-T/PC BM exhibits similar J and FF but higher
sc
The hole mobility of the blend films of PBDFTT-CF-O/
PC BM and PBDFTT-CF-T/PC BM (w/w, 1:1.5) prepared
7
1
71
by the same conditions as used in fabricating the active layers of
the champion PSC devices were measured by the space-charge-
limited current (SCLC) method with a device structure of
ITO/PEDOT:PSS/polymers:PC BM/Au. Interestingly, both
7
1
two kinds of blends show rather high hole mobilities, 0.128 and
5
.76 × 10− cm V
2
2
−1 −1
s
for PBDFTT-CF-T/PC BM and
71
PBDFTT-CF-O/PC BM, respectively. Since the hole mobility
7
1
of PBDFTT-CF-T/PC BM is more than 1 order of magnitude
7
1
71
sc
F
dx.doi.org/10.1021/ma301254x | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
2
Figure 5. AFM height images (2 × 2 μm ) (a−d) and TEM images (e−h) of the blend films of PBDFTT-CF-O/PC BM and PBDFTT-CF-T/
71
PC BM (1:1.5 w/w) prepared by different processes.
7
1
Voc. Therefore, the efficiencies of the PSC devices based on
M.; Li, Z.; Ding, J.; Tao, Y. J. Am. Chem. Soc. 2011, 133, 4250.
e) Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.; Liu, S.; You, W.
Angew. Chem., Int. Ed. 2011, 50, 2995. (f) Huang, Y.; Guo, X.; Liu, F.;
(
PBDFTT-CF-T/PC BM and PBDFTT-CF-O/PC BM are
7
1
71
6.26% and 5.22%, respectively. The results in this work clearly
Huo, L.; Chen, Y.; Russell, T. P.; Han, C. C.; Li, Y.; Hou, J. Adv. Mater.
demonstrate that BDF-based conjugated polymers are promis-
ing photovoltaic materials, and the 2D-conjugated structure can
be seen as a simple but effective strategy to improve their
photovoltaic properties.
2
(
012, 24, 3383.
3) (a) Roncali, J. Chem. Rev. 1997, 97, 173. (b) Beaujuge, P. M.;
Frechet, J. M. J. J. Am. Chem. Soc. 2011, 133, 20009. (c) Zhou, H. X.;
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ASSOCIATED CONTENT
Supporting Information
Experimental details. This material is available free of charge via
the Internet at http://pubs.acs.org.
(4) (a) Chochos, C. L.; Choulis, S. A. Prog. Polym. Sci. 2011, 36,
■
*
S
1326. (b) Huo, L. J.; Hou, J. H. Polym. Chem. 2011, 2, 2453. (c) Huo,
L. J.; Hou, J. H.; Zhang, S. Q.; Chen, H.-Y.; Yang, Y. Angew. Chem., Int.
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(
(
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AUTHOR INFORMATION
Corresponding Author
E-mail: hjhzlz@iccas.ac.cn.
Notes
■
*
2
010, 132, 2148.
7) Gidron, O.; Dadvand, A.; Sheynin, Y.; Bendikov, M.; Perepichka,
D. F. Chem. Commun. 2011, 47, 1976.
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Frechet, J. M. J. J. Am. Chem. Soc. 2010, 132, 15547. (b) Bijleveld, J. C.;
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The authors declare no competing financial interest.
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Janssen, R. A. J. J. Mater. Chem. 2011, 21, 1600−1606. (c) Li, H.; Jiang,
P.; Yi, C.; Li, C.; Liu, S.-X.; Tan, S.; Zhao, B.; Braun, J. R.; Meier, W.;
Wandlowski, T.; Decurtins, S. Macromolecules 2010, 43, 8058.
(9) Huo, L.; Huang, Y.; Fan, B.; Guo, X.; Jing, Y.; Zhang, M.; Li, Y.;
Hou, J. Chem. Commun. 2012, 48, 3318.
ACKNOWLEDGMENTS
■
This work was supported by National High-tech R&D Program
of China (863 Program) (No. 2011AA050523) and National
Natural Science Foundation of China (NSFC) (No. 21104088,
5
1173189).
(10) Coffin, R. C.; Peet, J.; Rogers, J.; Bazan, G. C. Nat. Chem. 2009,
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dx.doi.org/10.1021/ma301254x | Macromolecules XXXX, XXX, XXX−XXX