Poly(4,8-bis(2-ethylhexyloxy)benzo1,2-b:4,5-b ldithiophene vinylene): Synthesis, Optical and photovoltaic properties

Article abstract of DOI:10.1002/pola.23952

A new benzodithiophene (BDT)-based polymer, poly(4,8-bis(2-ethyl hexyloxy )benzo[1,2-b:4,5-b']dithiophene vinylene) (PBDTV), was synthesized by Pd-catalyzed Stille-coupling method. The polymer is soluble in common organic solvents and possesses high thermal stability. PBDTV film shows a broad absorption band covering from 350 nm to 618 nm, strong photoluminescence peaked at 545 nm and high hole mobility of 4.84 χ 10 ³ cm ²/Vs. Photovoltaic properties of PBDTV were studied by fabricating the polymer solar cells based on PBDTV as donor and PC⁷⁰BM as acceptor. With the weight ratio of PBDTV: PC⁷⁰BM of 1:4 and the active layer thickness of 65 nm, the power conversion efficiency of the device reached 2.63% with V^oc = 0.71 V, Z_sc,. = 6.46 mA/cm ², and FF = 0.7 under the illumination of AM1.5, 100 mW/cm ².

Full text of DOI:10.1002/pola.23952

Poly(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene vinylene):
Synthesis, Optical and Photovoltaic Properties
YOUJUN HE,1,2 YI ZHOU,3 GUANGJIN ZHAO,1,2 JIE MIN,1 XIA GUO,1 BO ZHANG,1 MAOJIE ZHANG,1,2 JING ZHANG,1,2
3
YONGFANG LI,1 FENGLING ZHANG,3 OLLE INGANAS
¨
1Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
2Graduate University of Chinese Academy of Sciences, Beijing 100039, China
3Biomolecular and Organic Electronics, IFM, Linkoping University, Linkoping SE-581 83, Sweden
¨
¨
Received 10 December 2009; accepted 21 January 2010
DOI: 10.1002/pola.23952
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A new benzodithiophene (BDT)-based polymer,
poly(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene vi-
nylene) (PBDTV), was synthesized by Pd-catalyzed Stille-cou-
weight ratio of PBDTV: PC70BM of 1:4 and the active layer
thickness of 65 nm, the power conversion efficiency of the de-
vice reached 2.63% with Voc ¼ 0.71 V, Isc ¼ 6.46 mA/cm2, and
FF ¼ 0.57 under the illumination of AM1.5, 100 mW/cm2.
pling method. The polymer is soluble in common organic
C
V
solvents and possesses high thermal stability. PBDTV film
shows a broad absorption band covering from 350 nm to 618
nm, strong photoluminescence peaked at 545 nm and high
hole mobility of 4.84 ꢀ 10ꢁ3 cm2/Vs. Photovoltaic properties of
PBDTV were studied by fabricating the polymer solar cells
based on PBDTV as donor and PC70BM as acceptor. With the
2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
48: 1822–1829, 2010
KEYWORDS: conjugated polymers; PC70BM; poly(benzodithio-
phene vinylene); polymer solar cells; synthesis; UV-Vis
spectroscopy
INTRODUCTION Conjugated polymer photovoltaic materials
and polymer solar cells (PSCs) have attracted great attention
in recent years because of the promising application of PSCs
in solar energy conversion.1–4 PSCs are composed of a blend
film of conjugated polymer donor and soluble fullerene de-
rivative (typically PCBM) acceptor sandwiched between an
ITO positive electrode and a low workfunction metal nega-
tive electrode. At present, the power conversion efficiency
(PCE) of the PSCs is still low in comparison with the Si-
based solar cells. To improve the efficiency, most of the
efforts have been devoted to the design and synthesis of
new conjugated polymers with broad and strong absorption
band in visible region, higher hole mobility and suitable
LUMO and HOMO energy levels.5–19 For example, polythio-
phene derivatives with conjugated side chains were synthe-
sized for broadening their absorption band and improving
the hole mobility of the polymers.5 Crosslinked polythio-
phene derivatives with conjugated bridges were prepared for
enhancing their hole mobility.6 Various low bandgap D-A
copolymers were designed and synthesized for red-shifting
their absorption and dropping their HOMO energy levels.7–15
Poly(thienylenevinylene) derivatives also draw attention for
their simple molecular structure, low bandgap characteristic
and high hole mobility.16 Recently, Hou et al. synthesized a
new series of conjugated polymers containing benzodithio-
phene (BDT) unit, and the polymers show broad absorption,
low energy bandgap, tunable electronic energy levels and
promising photovoltaic properties.17 By now, the copolymers
composed of BDT unit and thieno[3,4-b]thiophene unit show
high photovoltaic efficiency of 5–7%.18,19 The results indicate
that BDT could be a useful unit in constructing new conju-
gated polymer donors for high performance PSCs.
To combine the advantage of the BDT unit and the simple vi-
nylene unit in the polymer main chain, in this paper, we syn-
thesized a new BDT-containing polymer, poly(4,8-bis(2-ethyl-
hexyloxy)benzo[1,2-b:4,5-b0]dithiophene vinylene) (PBDTV),
as shown in Scheme 1. We used two 2-ethylhexyloxy groups
as the substituents on the BDT unit for improving the solu-
bility of the polymer. PBDTV shows good solubility in com-
mon organic solvents, high thermal stability with 5% weight
loss at the temperature higher than 300 ꢂC, and high hole
mobility of about 5 ꢀ 10ꢁ3 cm2/Vs. The power conversion
efficiency (PCE) of the PSC based on PBDTV as donor and
PC70BM as acceptor reached 2.63% with Voc ¼ 0.71 V, Isc
¼
6.46 mA/cm2 and FF ¼ 0.57 under the illumination of
AM1.5, 100 mW/cm2.
Correspondence to: Y. Li (E-mail: liyf@iccas.ac.cn) or Y. Zhou (E-mail: yzhou@ifm.liu.se)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1822–1829 (2010) 2010 Wiley Periodicals, Inc.
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ARTICLE
Fabrication of Photovoltaic Devices
Polymer solar cells were fabricated with ITO glass as a posi-
tive electrode, LiF/Al as a negative electrode and the blend
film of the polymer/PC70BM between them as a photosensi-
tive layer. The ITO glass was precleaned and modified by a
thin layer of PEDOT: PSS (Baytron AI 4083 from H. C. Starck)
which was spin-cast from a PEDOT: PSS aqueous solution on
the ITO substrate, and the thickness of the PEDOT: PSS layer
is about 45 nm. The photosensitive layer was prepared by
spin-coating a blend solution of the polymer and PC70BM
with a weight ratio of 1:2, 1:3, or 1:4 in o-dichlorobenzene
at different spin speed on the ITO/PEDOT: PSS electrode.
Then the LiF/Al cathode was deposited on the polymer layer
by vacuum evaporation under 6 ꢀ 10ꢁ6 Pa. The thickness of
the photosensitive layer is 45–125 nm (depending on the
weight ratio and spin speed), determined by a Dektak 6M
surface profilometer. The effective area of one cell is between
4 mm2 and 6 mm2 which is calculated from the measured
width and length of the crossover area of the ITO and metal
electrodes of the device. The current-voltage (I-V) measure-
ment of the devices was recorded using a Keithley 2400
Source Meter under illumination of AM 1.5 with intensity of
100 mW cmꢁ2 from a solar simulator (Model SS-50A, photo
Emission Tech).
SCHEME 1 Molecular structure of PBDTV.
EXPERIMENTAL
Materials
Bromine, n-Butyllithium (2.5 mol/L in hexane, heptanoyl
chloride), NaOH and tetrabutylammonium bromide were
obtained from Acros organics. (E)-1,2-bis(tributylstannyl)
ethane was purchased from Alfa Company. Tetrahydrofuran
(THF) was dried over Na/benzophenone ketyl and freshly
distilled before use. Compound 1 was synthesized with the
method reported in the literature.17 The other materials
were common commercial level and used as received.
Synthesis of the Monomers
4,8-Di(2-ethylhexyloxy)benzo[1,2-b;3,4-b]dithiophene, 2
Compound 1 (8.8 g, 40 mmol), zinc powder (5.46 g, 84
mmol), and 100 mL of water were put into a 250 mL flask;
then 24 g of NaOH was added into the mixture. The mixture
was well stirred and heated to reflux for 2 h. Then, 3-(bro-
momethyl)heptane (23 g, 120 mmol) and a catalytic amount
of tetrabutylammonium bromide were added into the flask.
After being refluxed for 6 h, the reactant was poured into
cold water and extracted by diethyl ether for several times.
The ether layer was dried over anhydrous MgSO4. After
removing solvent, the crude product was re-adsorbed in
silica gel and purified by silica gel chromatography using
hexane as eluent. 12.5 g of compound 2 (28 mmol, yield
70%) was obtained as light yellow oil. 1H NMR (CDCl3, 400
MHz), d (ppm): 7.65 (d, 2H), 7.45 (d, 2H), 4.18 (t, 4H), 1.78
(m, 2H), 1.53–1.17 (m, 16H), 0.88 (t, 12H). Elemental analy-
sis: Calculated for C26H38O2S2: C, 69.96; H, 8.52. Found: C,
69.56; H, 8.47.
Measurement and Characterization
General Methods
All new compounds were characterized by 1H NMR. Nuclear
magnetic resonance (NMR) spectra were taken on a Bruker
DMX-400 spectrometer. Chemical shifts of 1H NMR were
reported in ppm relative to the singlet of CDCl3 at 7.26 ppm.
Splitting patterns were designated as s (singlet), d (doublet),
t (triplet), m (multiplet), and br (broaden). Absorption spec-
tra were taken on a Hitachi U-3010 UV-Vis spectrophotome-
ter. Photoluminescence spectra were measured using a Hita-
chi F-4500 spectrophotometer. Molecular weight of the
polymers was measured by GPC using THF as solvent and
polystyrene as a standard. TGA measurement was performed
on a Perkin–Elmer TGA-7. The electrochemical cyclic voltam-
metry was conducted on a Zahner IM6e Electrochemical
Workstation with Pt disk, Pt plate, and Ag/Agþ electrode as
working electrode, counter electrode, and reference electrode
respectively in a 0.1 mol/L tetrabutylammonium hexafluoro-
phosphate (Bu4NPF6) acetonitrile solution. Polymer thin
films were formed by drop-casting 1.0 mm3 of polymer solu-
tions in THF (analytical reagent, 1 mg /mL) onto the work-
ing electrode, and then dried in the air. Atomic force micros-
copy (AFM) measurements were performed under ambient
conditions using a Digital Instrument Multimode Nanoscope
IIIA operating in the tapping mode.
2,6-Dibromo-4,8-di(2-ethylhexyloxy)benzo[1,2-b;3,4-
b]dithiophene, 3
Compound 2 (8.92 g, 20 mmol) was dissolved into 300 mL
of methylene chloride in a 500 mL flask. Bromine (6.4 g, 40
mmol) was dissolved into 60 mL of methylene chloride in a
funnel and slowly dropped into the flask under an ice-water
bath, and then the reactant was stirred for 12 h at ambient
temperature. After removing solvent, the crude product was
re-adsorbed in silica gel and purified by silica gel chromatog-
raphy using hexane as eluent. 10.87 g of compound 3 (18
mmol, yield 90%) was obtained as a light yellow oil. 1H
NMR (CDCl3, 400 MHz), d (ppm): 7.43 (s, 2H), 4.17 (t, 4H),
1.76(m, 2H), 1.54–1.20 (m, 16H), 0.89 (t, 12H). Elemental
OPTICAL AND PHOTOVOLTAIC PROPERTIES OF PBDTV, HE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
1.83–1.30 (m, 22H), 1.00 (t, 12H). Elemental analysis for
(C28H38S2O2)n Calculated: C, 71.49; H, 8.09; S, 13.62. Found:
C, 70.58; H, 7.72; S, 12.76.
RESULTS AND DISCUSSION
Synthesis of the Monomer and Polymer
The synthetic routes of monomer and polymer PBDTV are
outlined in Scheme 2 and 3, respectively. PBDTV was pre-
pared by Stille-coupling reaction.18–20 Molecular structures
of the monomer and the polymer PBDTV were confirmed by
1H NMR spectra (see Figs. 1 and 2) and elemental analysis.
The molecular weight (Mw) of PBDTV is 9.4 kg/mol.
SCHEME 2 Synthetic route of the monomer. (i) Zinc powder,
NaOH, H2O, reflux 1 h; then, 2-ethy-1-bromo-hexane, tetrabuty-
lammonium bromide, reflux 2 h; (ii) Bromine, methylene, room
temperature, 24 h.
1
Figure 2 shows the H NMR spectrum of PBDTV. The hydro-
gen atoms in the benzo[1,2-b:4,5-b0]dithiophene ring and
carbon-carbon double bond peak at 7.60–6.80 ppm, the
hydrogen close to oxygen atoms in alkyl chain of PBDTV
peaks at 4.17 ppm. The peak at about 0.1 ppm could be
ascribed to the hydrogen atoms in TMS in the solvent used
in the NMR measurement. The positions of other hydrogen
atoms are shown in Figure 2. PBDTV with the lower Mw is
soluble in common organic solvents, such as chloroform,
toluene, and THF at room temperature. But the solubility
will become poorer when the molecular weight higher than
10 kg/mol.
analysis: Calculated for C26H36Br2O2S2: C, 51.66; H, 5.96.
Found: C, 51.55; H, 5.89.
Synthesis of the Polymer PBDTV
The synthesis of PBDTV was carried out using palladium-cat-
alyzed Stille-coupling between monomer 3 and (E)-1,2-bis
(tributylstannyl)ethane, as shown in Scheme 3. All starting
materials, reagents, and solvents were carefully purified,
and all procedures were performed under an air-free
environment.
Thermal Property
Under the protection of Argon atmosphere, monomer 3 (1
mmol) was dissolved in 10 mL dried toluene, (E)-1, 2-bis
(tributylstannyl) ethane (1 mmol) was added to the mixture.
The solution was flushed with argon for 10 min, and then
10 mg of Pd (PPh3)4 were added. After another flushing
with argon for 20 min, the reactant was heated to reflux for
10 h. The reaction solution was cooled to room temperature,
the reaction mixture was added dropwise to 200 mL metha-
nol, and then filtered through a Soxhlet thimble, which was
then subjected to Soxhlet extraction with methanol, hexane,
and chloroform. Polymer was recovered from the chloroform
fraction by rotary evaporation as solid. The polymer was
purified with bio-beads S-1 column chromatography eluted
with THF, the solvent was removed in vacuum, and the solid
was dried under vacuum for 1 day to get PBDTV. The yield
of the polymerization reaction was about 30–50%. GPC: Mw
¼ 9.4 kg/mol, Mn ¼ 5.1 kg/mol, Mw/Mn ¼ 1.84. 1H NMR
(CDCl3, 400 MHz) d (ppm) 7.60–6.80 (br, 4H), 4.19 (m, 4H),
Thermal stability of PBDTV was investigated with thermogra-
vimetric analysis (TGA), as shown in Figure 3. PBDTV shows
onset decomposition temperature with 5% weight loss at
ꢂ
307 C. The thermal stability is enough for the application of
PBDTV in polymer optoelectronic devices.
Optical Property
Figure 4(a) shows the absorption spectra of PBDTV solution
in chloroform and film on quartz plate. There are two
absorption peaks, located at 515 nm and 553 nm, respec-
tively, in the visible absorption band of PBDTV solution. The
two absorption peaks agree with that reported in the litera-
ture for the similar polymer,17 which should be ascribed to
p-p* transition absorption of the polymer main chains. The
visible absorption band of PBDTV film is broadened but the
two peak positions are almost the same in comparison with
that of the PBDTV solution, which indicates that the intermo-
lecular interaction of PBDTV film is weak probably due to
SCHEME 3 Synthetic route of
PBDTV. (i) Pd (PPh3)4, toluene,
Argon, reflux, 10 h.
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ARTICLE
FIGURE 1 1H NMR spectrum of com-
pound 3.
the two big alkoxy side chains of the polymer. The absorp-
tion edge of the PBDTV film is at about 617 nm, which is
corresponding to a bandgap of 2.01 eV. The bandgap of
PBDTV is smaller than that (2.13 eV) of the homopolymer
PBDT,17,21 which implies that the vinylene unit in PBDTV can
reduce the bandgap of the polymer for some extent. But it is
larger than that (ca. 1.5ꢃ1.6 eV) of the copolymers of BDT
unit and thieno[3,4-b]thiophene unit,19b indicating that the
thieno[3,4-b]thiophene unit is very important for reducing
the bandgap of the copolymers. In addition, PBDTV solution
shows strong photoluminescence in the wavelength range
from 550 to 900 nm peaked at about 664 nm, as shown in
Figure 4(b).
reduction potentials from the electrochemical measure-
ments.22 Therefore, we studied the electrochemical proper-
ties of PBDTV by cyclic voltammetry.
Figure 5 shows the cyclic voltammogram of the polymer film
on Pt electrode. It can be seen that there are p-doping/
dedoping (oxidation/re-reduction) processes in positive
potential range and n-doping/dedoping (reduction/re-oxida-
tion) processes in negative potential range. The HOMO and
LUMO energy levels of the polymer were calculated from its
onset oxidation potential (uox) and its onset reduction poten-
tial (ured) according to the following equations:23
EHOMO ¼ ꢁeðuox þ 4:71ÞðeVÞ;
ELUMO ¼ ꢁeðured þ 4:71ÞðeVÞ;
Where the units of uox and ured are V vs Ag/Agþ. The value
of uox and ured are 0.45 V and ꢁ1.89 V vs. Ag/Agþ,
Electrochemical Property
The electrochemical property is one of the most important
properties of conjugated polymers and the electronic energy
levels can be estimated from the onset oxidation and onset
FIGURE
2
1H NMR spectrum of
PBDTV.
OPTICAL AND PHOTOVOLTAIC PROPERTIES OF PBDTV, HE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 TGA plot of the polymer with a heating rate of 10
ꢂC/min under inert atmosphere.
FIGURE 5 Cyclic voltammogram of PBDTV film on Pt electrode
in 0.1 mol/L Bu4NPF6, CH3CN solution at a scan rate of 100 mV/s.
respectively. Accordingly, the HOMO and LUMO energy levels
of PBDTV were calculated to be ꢁ5.16 eV and ꢁ2.82 eV,
respectively. In comparison with the high efficiency copoly-
mers of benzo[1,2-b:4,5-b0]dithiophene and thieno[3,4-b]thio-
phene, PBDTTT-E, PBDTTT-C and PBDTTT-CF,19b the HOMO
energy level of PBDTV (ꢁ5.16 eV) is a little lower than that
of PBDTTT-E (ꢁ5.01 eV) and PBDTTT-C (ꢁ5.12 eV), and a
little higher than that of PBDTTT-CF (ꢁ5.22 eV).19b The rela-
tive lower HOMO energy level of PBDTV is desirable for get-
ting higher open-circuit voltage when it is used as donor
blended with PCBM in PSCs. The electrochemical bandgap of
PBDTV calculated from the difference of the LUMO and
HOMO energy levels is 2.34 eV, which is a little higher than
that (2.01 eV) of the optical bandgap aforementioned. The
higher electrochemical bandgap than optical bandgap is a
common phenomenon for the conjugated polymers,5,24
FIGURE 6 Current-voltage data from the device of ITO/
PEDOT:PSS/PBDTV/Au, plotted in the format ln(Jd3/V2) vs
(V/d)0.5, where J is the current density and d is the thickness of
the polymer layer. The line is the fit to the experimental data
points.
FIGURE 4 (a) Normalized absorption spectra of PBDTV in chlo-
roform solution and film spin-coated on quartz plate; (b) Photo-
luminescence spectrum of PBDTV in chloroform solution
(concentration 10ꢁ5 M) excited at 480 nm.
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ARTICLE
TABLE 1 Photovoltaic Performance of the PSC Devices Based on PBDTV: PC70BM Under the Illumination of AM1.5, 100 mW/cm2
PBDTV: PC70BM (w/w)
1:2
Spin Speed (rpm)
Thickness (nm)
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
2,000
2,500
3,000
1,000
2,000
2,500
3,000
2,000
2,500
3,000
4,000
60
50
40
125
70
55
40
80
75
65
60
5.47
4.71
4.27
4.49
6.30
6.00
4.73
6.97
6.81
6.46
6.26
0.71
0.70
0.70
0.64
0.70
0.70
0.71
0.68
0.69
0.71
0.70
0.62
0.63
0.63
0.39
0.57
0.58
0.61
0.48
0.53
0.57
0.58
2.43
2.07
1.86
1.11
2.50
2.45
2.04
2.30
2.51
2.63
2.54
1:3
1:4
because of the energy barrier of the charge transfer at elec-
trodes during the electrochemical measurement.
factor, because of limited charge transport property. Too thin
active layer (thinner than 60 nm) leads to lower short circuit
current (Jsc) due to the limited absorption of the thin active
layer. Furthermore, when comparing the device performance
among different weight ratio of PBDTV: PC70BM, the
decrease of FF as a function of PC70BM loading can be
observed, which may be caused by the decrease of hole mo-
bility. Moreover, the increase of Jsc as a function of PC70BM
loading can be ascribed to the increasing absorption contri-
bution from PC70BM part. By balancing the absorption and
carrier transport properties of the photosensitive layer, the
optimized device with the PBDTV: PC70BM weight ratio of
1:4 and an active layer thickness of 65 nm shows a PCE of
2.63% with Voc ¼ 0.71 V, Isc ¼ 6.46 mA/cm2 and FF ¼ 0.57
under the illumination of AM1.5, 100 mW/cm2. Actually, the
LUMO level (ꢁ2.82 eV) of PBDTV is too high when it is used
as donor blended with PCBM as acceptor, which results in
big energy loss during the charge separation of excitons. The
PCE could be further improved by decreasing the LUMO and
HOMO energy levels of the polymer according to the design
rule for the polymer donor in high performance PSCs.26
Hole Mobility of PBDTV
Hole mobility is an important parameter of the conjugated
polymers for the photovoltaic application. Here, we investi-
gated the hole mobility of PBDTV with the space-charge-lim-
ited current (SCLC) model25 with a device structure of ITO/
PEDOT: PSS/PBDTV (135 nm)/Au (70 nm). The results are
plotted as ln (Jd3/V2) vs (V/d)0.5, as shown in Figure 6.
Herein, J stands for current density, d the thickness of the
device, V ¼ VapplꢁVbi, Vappl is the applied potential, and Vbi is
the built-in potential. Hole mobility of PBDTV is 4.84 ꢀ10ꢁ3
cm2/Vs calculated from the intercept of the corresponding
lines on the axis of ln (Jd3/V2)25(e) according to the following
equations.
"
#
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
9
8
ðV ꢁ VbiÞ
V ꢁ Vbi
jSCLC
¼
ꢀ0ꢀrl0
exp 0:89c
d3
d
The hole mobility of about 5 ꢀ 10ꢁ3 cm2/Vs is a high value
for the solution processed conjugated polymer films.
Photovoltaic Property
The motivation of design and synthesis of the polymer
PBDTV is to look for novel conjugated polymer photovoltaic
materials. We fabricated the PSCs with the structure of ITO/
PEDOT-PSS/PBDTV: PC70BM/LiF(6Å)/Al(80 nm), and opti-
mized photovoltaic performance of the devices by changing
the PBDTV: PC70BM weight ratio, spin speed, and thickness
of the active layer.
Table 1 shows the photovoltaic performance of the devices
based on the blend of PBDTV and PC70BM at different device
fabrication conditions under the illumination of AM1.5, 100
mW/cm2. Herein, the thickness of the active layer is an im-
portant parameter for the photovoltaic performance. It can
be seen from Table 1 that, within the same weight ratio of
PBDTV: PC70BM, the PSCs with the active layer thickness in
the range of 60–75 nm exhibit higher PCEs than others. Too
thick active layer (thicker than 80 nm) results in lower fill
FIGURE 7 External quantum efficiency of the device with the
PBDTV: PC70BM weight ratio of 1:4 and an active layer thick-
ness of 65 nm.
OPTICAL AND PHOTOVOLTAIC PROPERTIES OF PBDTV, HE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 8 AFM images of the devices based on PBDTV and PC70BM with different weight ratio.
Figure 7 shows the external quantum efficiency (EQE) of the
optimized PSC device with the PBDTV: PC70BM weight ratio
of 1:4 and an active layer thickness of 65 nm. Comparing the
EQE of device and the absorption of PBDTV, pronounced
contribution from PC70BM to the photo current make the
EQE much more broadened, and the EQE of device covers
most of the visible wavelength range from 350 nm to 700
nm. The highest EQE value reached about 55% at around
500 nm corresponding to the absorption peak of PBDTV.
(KJCX2-YW-M11) and the Ministry of Science and Technology of
China (863 project, 2006AA03Z220).
REFERENCES AND NOTES
1 (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J
Sci 1995, 270, 1789–1791; (b) Peet, J.; Kim, J. Y.; Coates, N. E.;
Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Nat Mater
2007, 6, 497–500; (c) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.;
Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger,
A. J. Nat Photonics 2009, 3, 297–303.
Morphology of the Active Layer
AFM images of the blend films of PBDTV and PC70BM with
different weight ratios of 1:2, 1:3, and 1:4 are shown in
Figure 8. There is not much difference among the height
images of the three films. The surfaces of the three films are
quite smooth, their root mean square (RMS) roughness
values are 1.25 nm, 1.14 nm, and 1.24 nm for the 1:2, 1:3
and 1:4 films, respectively. The results indicate that PBDTV
possesses good film-forming properties when blended with
PC70BM in dichlorobenzene solvent. The uniform morphol-
ogy of the blend films is coincident with the good photovol-
taic performance of the PSC devices.
2 (a) Li, G.; Shrotriva, V.; Huang, J. S.; Yao, Y; Moriarty, T.; Em-
ery, K.; Yang, Y. Nat Mater 2005, 4, 864–868; (b) Ma, W. L.;
Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv Funct Mater
2005, 15, 1617–1622.
3 (a) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv Mater
2009, 21, 1323–1338; (b) Li, Y. F.; Zou, Y. P. Adv Mater 2008,
20, 2952–2958.
4 Chen, J. W.; Cao, Y. Acc Chem Res 2009, 42, 1709–1718.
5 (a) Hou, J. H.; Tan, Z. A.; Yan, Y.; He, Y. J.; Yang, C. H.; Li, Y.
F. J Am Chem Soc 2006, 128, 4911–4916; (b) Hou, J. H.; Huo, L.
J.; He, C.; Yang, C. H.; Li, Y. F. Macromolecules 2006, 39,
594–603; (c) Zhou, E. J.; Tan, Z. A.; Huo, L. J.; He, Y. J.; Yang,
C. H.; Li, Y. F. J Phys Chem B 2006, 110, 26062–26067; (d) Zou,
Y. P.; Wu, W. P.; Sang, G. Y.; Yang, Y.; Liu, Y. Q.; Li, Y. F. Mac-
romolecules 2007, 40, 7231–7237.
CONCLUSIONS
Poly(4,8-bis(2-ethylhexyloxy)benzo[1,2-b4,5-b0]dithiophene
vinylene) (PBDTV) was synthesized by the Pd-catalyzed
Stille-coupling method. PBDTV shows good solubility in com-
mon organic solvents, high thermal stability, broader absorption
band covering from 350 nm to 618 nm, higher hole mobility of
about 5 ꢀ 10ꢁ3 cm2/Vs and deeper HOMO energy level of
ꢁ5.16 eV. The higher hole mobility and deeper HOMO level are
beneficial to the photovoltaic application. The PSCs were fabri-
cated with PBDTV as donor and PC70BM as acceptor, and the
device fabrication conditions such as weight ratio of
PBDTV:PC70BM and the active layer thickness were optimized.
Power conversion efficiency of the PSC with the PBDTV:PC70BM
weight ratio of 1:4 and the active layer thickness of 65 nm
reached 2.63% with Voc ¼ 0.71 V, Isc ¼ 6.46 mA/cm2 and FF ¼
0.57 under the illumination of AM1.5, 100 mW/cm2. The results
indicate that PBDTV with the simple structure is a promising
photovoltaic material.
6 (a) Zhou, E. J.; Tan, Z. A.; Yang, C. H.; Li, Y. F. Macromol
Rapid Commun 2006, 27, 793–798; (b) Zhou, E. J.; Tan, Z. A.;
Yang, Y.; Huo, L. J.; Zou, Y. P.; Yang, C. H.; Li, Y. F. Macromo-
lecules 2007, 40, 1831–1837.
7 (a) Svensson, M.; Zhang, F. L.; Veenstra, S. C.; Verhees, W. J.
H.; Hummelen, J. C.; Kroon, J. M.; Inganas, O.; Andersson, M.
¨
R. Adv Mater 2003, 15, 988–991; (b) Zhang, F. L.; Mammo, W.;
Andersson, L. M.; Admassie, S.; Andersson, M. R.; Inganas, O.
¨
Adv Mater 2006, 18, 2169–2173; (c) Zhang, F. L.; Bijleveld, J.;
Perzon, E.; Tvingstedt, K.; Barrau, S.; Inganas, O. Andersson,
¨
M. R. J Mater Chem 2008, 18, 5468–5474; (d) Lindgren, L. J.;
Zhang, F. L.; Andersson, M.; Barrau, S.; Hellstrom, S.; Mammo,
W.; Perzon, E.; Inganas, O.; Mats R. Andersson, M. R. Chem
¨
Mater 2009, 21, 3491–3502; (e) Qin, R. P.; Li, W. W.; Li, C. H.;
Du, C.; Veit, C.; Schleiermacher, H.-F.; Andersson, M.; Bo, Z. S.;
This work was supported by NSFC (No. 50633050, 20721061,
20874106 and 50933003), Chinese Academy of Sciences
Liu, Z. P.; Inganas, O.; Wuerfel, U.; Zhang, F. L. J Am Chem
¨
Soc 2009, 131, 14612–14613.
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INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
8 (a) Yang, R. Q.; Tian, R. Y.; Yan, J.; Zhang, Y.; Yang, J.; Hou,
Q.; Yang, W.; Zhang, C.; Cao, Y. Macromolecules 2005, 38,
244–253; (b) Wang, F.; Luo, J.; Yang, K. X.; Chen, J. W.; Huang,
F.; Cao, Y. Macromolecules 2005, 38, 2253–2260; (c) Wang, E.
G.; Wang, L.; Lan, L. F.; Luo, C.; Zhuang, W. L.; Peng, J. B.;
Cao, Y. Appl Phys Lett 2008, 92, 033307; (d) Wang, E. G.;
Wang, M.; Wang, L.; Duan, C. H.; Zhang, J.; Cai, W. Z.; He, C.;
Wu, H. B.; Cao, Y. Macromolecules 2009, 42, 4410–4415.
16 (a) Hou, J. H.; Tan, Z. A.; He, Y. J.; Yang, C. H.; Li, Y. F. Mac-
romolecules 2006, 39, 4657–4662; (b) He, Y. J.; Wu, W. P.;
Zhao, G. J.; Liu, Y. Q.; Li, Y. F. Macromolecules 2008, 41,
9760–9766; (c) Huo, L. J.; Chen, T. L.; Zhou, Y.; Hou, J. H.;
Yang, Y.; Li, Y. F. Macromolecules 2009, 42, 4377–4380.
17 Hou, J. H.; Park, M. H.; Zhang, S. Q.; Yao, Y.; Chen, L. M.;
Li, J. H.; Yang, Y. Macromolecules 2008, 41, 6012–6018.
18 (a) Liang, Y. Y.; Wu, Y.; Feng, D. Q.; Tsai, S. T.; Son, H. J.;
Li, G.; Yu, L. P. J Am Chem Soc 2009, 131, 56–57; (b) Liang, Y.
Y.; Feng, D. Q.; Wu, Y.; Tsai, S-T; Li, G.; Ray, C.; Yu, L. P. J Am
Chem Soc 2009, 131, 7792–7799.
9 (a) Blouim, N.; Michaud, A.; Leclerc, M. Adv Mater 2007, 19,
2295–2300; (b) Blouin, N.; Michaud, A.; Gendron, D.; Wakim,
S.; Blair, E.; Plesu, R. N.; Belleteˆte, M.; Durocher, G.; Tao, Y.;
Leclerc, M. J Am Chem Soc 2008, 130, 732–742; (c) Zou, Y. P.;
19 a) Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Chen, R. I.; Yang,
Y.; Wu, Y.; Li, G. J Am Chem Soc 2009, 131, 15586–15587; (b)
Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.;
Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat Photonics 2009, 3,
649–653.
Gendron, D.; Badrou-Aıch, R.; Najari, A.; Tao, Y.; Leclerc, M.
¨
Macromolecules 2009, 42, 2891–2894; (d) Zou, Y. P.; Gendron,
D.; Neagu-Plesu, R.; Leclerc, M. Macromolecules 2009, 42,
6361–6365.
10 (a) Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Li, G.; Yang, Y. J Am
Chem Soc 2008, 130, 16144–16145; (b) Huo, L. J.; Hou, J. H.;
Chen, H. Y.; Zhang, S. Q.; Jiang, Y.; Chen, T. L.; Yang, Y. Macro-
molecules 2009, 42, 6564–6571; (c) Huo, L. J.; Chen, H. Y.; Hou, J.
H.; Chen, T. L.; Yang, Y. Chem Commun 2009, 5570–5572.
20 (a) Bao, Z. N.; Chan, W. K.; Yu, L. P. Chem Mater 1993, 5,
2–3; (b) Bao, Z. N.; Wai Kin Chan, W. K.; Yu, L. P. J Am Chem
Soc 1995, 117, 12426–12435.
21 Shiraishi, K.; Yamamoto, T. Synth Met 2002, 130, 139–147.
11 (a) Zhou, E. J.; Nakamura, M.; Nishizawa, T.; Zhang, Y.;
Wei, Q. S.; Tajima, K.; Yang, C. H.; Hashimoto, K. Macromole-
cules 2008, 41, 8302–8305; (b) Zhou, E. J.; Yamakawa, S.;
Tajima, K.; Yang, C. H.; Hashimoto, K. Chem Mater 2009, 21,
4055–4061.
22 Li, Y. F.; Cao, Y.; Gao, J.; Wang, D. L.; Yu, G.; Heeger, A. J.
Synth Met 1999, 99, 243–248.
23 Sun, Q. J.; Wang, H. Q.; Yang, C. H.; Li, Y. F. J Mater Chem
2003, 13, 800–806.
24 (a) Johansson, T.; Mammo, W.; Svensson, M.; Andersson,
12 Liang, Y. Y.; Feng, D. Q.; Guo, J. C.; Szarko, J. M.; Ray, C.;
M. R., Inganas, O. J Mater Chem, 2003, 13, 1316–1323; (b)
˙
Chen, L. X.; Yu, L. P. Macromolecules 2009, 42, 1091–1098.
Chen, W. C.; Liu, C. L.; Yen, C. T.; Tsai, F. C.; Tonzola, C. J.;
Olson, N.; Jenekhe, S. A. Macromolecules, 2004, 37,
5959–5964.
13 (a) Wong, W. Y.; Wang, X. Z.; He, Z.; Djurii, A. B.; Yip, C.-T.;
Cheung, K.-Y.; Wang, H.; Mak, C. S. K.; Chan, W.-K. Nat Mater 2007,
6, 521–527; (b) Wong, W. Y.; Wang, X. Z.; He, Z.; Chan, K. K.; Djuri-
sic, A. B.; Cheung, K. Y.; Yip, C. T.; Ng, A. M.; Xi, Y. Y.; Mak, C. S. K.;
Chan, W. K. J Am Chem Soc 2007, 129, 14372–14380.
25 (a) Malliaras, G. G.; Salem, J. R.; Brock, P. J.; Scott, C. Phys
Rev B 1998, 58, 13411–13414; (b) Lampert, M. A.; Mark, P. Cur-
rent Injection in Solids; Academic Press: New York, 1970; (c)
Chu, T. Y.; Song, O. K. Appl Phys Lett 2007, 90, 203512; (d)
Murgatroyd, P. N.; J Phys D 1970, 3, 151–156; (e) Natali, D.;
Sampietro, M. J Appl Phys 2002, 92, 5310–5318.
14 Chen, C. P.; Chan, S. H.; Chao, T. C.; Ting, C.; Ko, B. T. J
Am Chem Soc 2008, 130, 12828–12833.
15 (a) Li, Y. W.; Xue, L. L.; Li, H.; Li, Z. F.; Xu, B.; Wen, S. P.;
Tian, W. J. Macromolecules 2009, 42, 4491–4499; (b) He, Y. J.;
Wang, X.; Zhang, J.; Li, Y. F. Macromol Rapid Commun 2009,
30, 45–51.
26 Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.;
¨
Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv Mater 2006, 18,
789–794.
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