Synthesis and electronic properties of semiconducting polymers containing benzodithiophene with alkyl phenylethynyl substituents

Article abstract of DOI:10.1021/ma101709h

Semiconducting polymers containing benzodithiophene with decyl phenylethynyl and hexadecyl phenylethynyl substituents have been synthesized by Stille coupling polymerization. The optoelectronic properties of the synthesized polymers have been investigated

Full text of DOI:10.1021/ma101709h

Macromolecules 2010, 43, 8063–8070 8063
DOI: 10.1021/ma101709h
Synthesis and Electronic Properties of Semiconducting Polymers
Containing Benzodithiophene with Alkyl Phenylethynyl Substituents
Prakash Sista,† Hien Nguyen,† John W. Murphy,† Jing Hao,† Daniel K. Dei,†
Kumaranand Palaniappan,† John Servello,† Ruvini S. Kularatne,† Bruce E. Gnade,† Bofei Xue,‡
Paul C. Dastoor,‡ Michael C. Biewer,† and Mihaela C. Stefan*,†
†Departments of Chemistry and Materials Science and Engineering, University of Texas at Dallas,
800 West Campbell Road, Richardson, Texas 75080, and ‡Centre for Organic Electronics, The University of
Newcastle, University Drive, Callaghan NSW 2308, Australia
Received July 28, 2010; Revised Manuscript Received September 7, 2010
ABSTRACT: Semiconducting polymers containing benzodithiophene with decyl phenylethynyl and
hexadecyl phenylethynyl substituents have been synthesized by Stille coupling polymerization. The opto-
electronic properties of the synthesized polymers have been investigated. The synthesized polymers were
tested in bulk heterojunction solar cells.
Introduction
alternating copolymers with fluorene and carbazole cores.29
Benzodithiophene monomers with phenylethynyl substituents
The field of organic electronics is based on the development of
organic semiconducting materials as a low-cost alternative to
traditional inorganic electronic materials.1 Conjugated organic
molecules, oligomers, and polymers are currently used as semi-
conducting materials in organic field-effect transistors (OFET),2
organic light-emitting diodes (OLED),3 solar cells,4-6 sensors,
and organic circuits for integration in low-cost large-area elec-
tronics.7 Currently, most studies of organic semiconductors focus
on solution processable semiconducting polymers due to their
advantages over conventional inorganic electronic semiconduc-
tors in terms of facile deposition, large coverage area, and
flexibility.1 While poly(3-hexylthiophene) is still at the forefront
of organic electronics research, the number of new semiconduct-
ing polymers with improved electronic properties is increasing
rapidly.8 Chemical modification of thiophene cores and synthesis
of fused-ring thiophene derivatives are two of the most common
synthetic strategies to generate novel semiconducting polymers
with improved optoelectronic properties.8 In order to maintain
the electronic and photonic benefits of regioregular poly-
(3-hexylthiophene), but still allow control of the molecular
orbital levels for the polymer, the fused benzodithiophene mono-
mer core has been studied.9-27 The fused central ring allows the
incorporation of substituents on the central benzene core, while
also maintaining planarity of the two thiophene units. In addi-
tion, the symmetric nature of the benzodithiophene core elim-
inates any potential regioirregularity during the polymerization
process.
have aromatic conjugation in two perpendicular directions. The
benzodithiophene core along one direction can be used to create a
semiconducting thiophene based polymer through typical poly-
merization techniques. The perpendicular direction, however, has
three aromatic units separated by conjugating alkyne units. The
bis(phenylethynyl)aromatic conjugation direction can be used as
a handle to control the supramolecular alignment of the aromatic
units. By having this control, a better conjugation of the π
orbitals of the aromatic units can be obtained, and thus better
optoelectronic properties can be achieved.30-32
In this paper, we present the synthesis and characterization of
novel polymers containing benzodithiophene with phenylethynyl
substituents containing more soluble longer decyl and hexadecyl
alkyl chains, and we demonstrate that these new polymers show
promise as active layer components in bulk heterojunction
organic photovoltaic devices.
Experimental Section
Materials. All commercial chemicals were purchased from
Aldrich Chemical Co., Inc., and were used without further
purification unless otherwise noted. All reactions were con-
ducted under purified nitrogen. The polymerization glassware
and syringes were dried at 120 °C for at least 24 h before use and
cooled under a nitrogen atmosphere. Tetrahydrofuran was
dried over sodium/benzophenone ketyl and freshly distilled
prior to use. 4,8-Dihydrobenzo[1,2-b:4,5-b0]dithiophen-4,8-dione
was prepared according to the literature.33
Previously groups have attached electron-donating alkyl and
alkoxy substituents on the central benzene ring.28 In order to
create polymers with a better molecular orbital overlap for
molecular electronic devices, however, we targeted the synthesis
of monomers with greater electron delocalization to generate
polymers with a lower band gap. The substitution of two
phenylethynyl groups on the central fused benzene ring allows
extended electron delocalization. We have recently published the
synthesis of benzodithiophene monomers with phenylethynyl
substituents and the polymerization of the monomer, along with
Synthesis of 4-Decanoyl-1-bromobenzene. Decanoyl chloride
(6.55 g, 0.0344 mol) was added dropwise to a solution of
bromobenzene (11.39 g, 0.0725 mol) and aluminum chloride
(5.17 g, 0.0389 mol). The reaction mixture was heated at 60 °C
for 1 h, cooled by pouring it into 300 mL of ice water, and
extracted with methylene chloride. The organic layer was
washed with 200 mL of 2 N HCl and 200 mL of brine, dried
with magnesium sulfate, and concentrated to yield a white solid
which was purified by recrystallization form hexane to obtain
6.11 g of solid (0.0196 mol, 57%). 1H NMR (CDCl3, 400 MHz)
δ: 0.86 (t, 3H), 1.25 (m, 12H), 1.72 (m, 2H), 2.9 (t, 2H), 7.58 (m,
1H), 7.81 (d, 2H).
*Corresponding author. E-mail: mci071000@utdallas.edu.
r 2010 American Chemical Society
Published on Web 09/16/2010
pubs.acs.org/Macromolecules

8064 Macromolecules, Vol. 43, No. 19, 2010
Sista et al.
Synthesis of 4-Decyl-1-bromobenzene. 4-Decanoyl-1-bromo-
benzene (14.20 g, 0.0457 mol), hydrazine monohydrate (12 mL,
0.160 mol), and potassium hydroxide (18.80 g, 0.336 mol) were
diluted in 14 mL of 1-octanol. Solution was heated at reflux for
3 h. After cooling to room temperature, the mixture was diluted
with ether (200 mL), washed with 2 N HCl (200 mL), washed
with brine (200 mL), dried with magnesium sulfate, filtered, and
concentrated to yield a yellow oil which was purified by column
chromatography (hexane eluent) to yield 8.607 g of clear oil
(0.0290 mol, 64%). 1H NMR (CDCl3, 400 MHz) δ: 0.87 (t, 3H),
1.25 (m, 14H), 1.56 (m, 2H), 2.54 (t, 2H), 7.03 (d, 2H), 7.37
(d, 2H).
Synthesis of 4-Decyl-1-trimethylsilylethynylbenzene. 4-Decyl-
1-bromobenzene (5.50 g, 0.0185 mol), 0.050 g of copper iodide,
and 0.605 g of bistriphenylphosphine dichloropalladium(II) was
diluted with 25 mL of triethylamine under nitrogen before
adding trimethylsilylacetylene (2.80 mL, 0.0201 mol) at 50 °C.
After addition, solution was heated at 80 °C for 24 h before
reaction was quenched with 2 N HCl solution (200 mL),
extracted with ether (200 mL), washed with brine (200 mL),
dried with magnesium sulfate, filtered, and concentrated to yield
a brown oil which was purified by column chromatography
(hexane eluent) to obtain 4.36 g of clear oil (0.0139 mol, 75%).
1H NMR (CDCl3, 400 MHz) δ: 0.24 (s, 9H), 0.88 (t, 2H), 1.26
(m, 14H), 1.58 (m, 2H), 2.58 (t, 2H), 7.09 (d, 2H), 7.37 (d, 2H).
Synthesis of 4-Decyl-1-ethynylbenzene. 4-Decyl-1-trimethyl-
silylethynylbenzene (1.99 g, 0.006 34 mol) was diluted with
30 mL of CH2Cl2, to which a solution of 2 g of potassium
hydroxide dissolved in 30 mL of methanol was added. The
mixture was stirred at room temperature for 1 h before being
diluted with water (200 mL) and extracted with CH2Cl2
(200 mL), dried with magnesium sulfate, filtered, and concen-
trated to yield a clear oil which was purified by column (hexane)
to obtain 1.53 g of oil (0.006 32 mol, 99%). 1H NMR (CDCl3,
400 MHz) δ: 0.87 (t, 3H), 1.26 (m, 14H), 1.57 (m, 2H), 2.59
(t, 2H), 3.02 (s, 1H), 7.12 (d, 2H), 7.39 (d, 2H).
water (2 ꢀ 200 mL), dried with magnesium sulfate, filtered, and
concentrated to obtain a thick oil, which was poured into
200 mL of methanol. The yellow solid that precipitated was
filtered to obtain 0.375 g of product (0.000 453 mol, 75%). 1H
NMR (CDCl3, 400 MHz) δ: 0.88 (t, 6H), 1.28 (m, 28H), 1.64
(m, 4H), 2.66 (t, 4H), 7.23 (d, 4H), 7.55 (d, 4H), 7.64 (s, 2H). 13
C
NMR (CDCl3, 100 MHz) δ: 14.1, 22.7, 29.28, 29.34, 29.5, 29.6,
29.63, 31.3, 31.9, 36.0, 84.1, 100.3, 110.2, 117.2, 119.5, 126.0,
128.7, 131.7, 137.3, 141.2, 144.5. Anal. calculated for
C
46H52Br2S2: C, 66.66%; H, 6.32%. Found: C, 65.93%; H,
6.13%.
Synthesis of 2,6-(Trimethyltin)-4,8-bis(4-decylphenylethynyl)-
benzo[1,2-b:4,5-b0]dithiophene. 4,8-Bis(4-decylphenylethynyl)-
benzo[1,2-b:4,5-b0]dithiophene (0.272 g, 0.000 406 mol) was
dissolved in 100 mL of dry THF. Under nitrogen at -78 °C,
0.60 mL of 1.6 M n-BuLi in hexane was added (0.000 96 mol,
2.3 equiv). The solution turned to a fluorescent green color
upon addition. Reaction was stirred at -78 °C for 1 h before
0.90 mL of 1 M trimethyltin chloride was added. Reaction was
stirred at -78 °C for 30 min before allowing to warm to room
temperature. Solvent was evaporated, and the yellow solid was
dissolved in chloroform and solution was washed with water
(2 ꢀ 200 mL), dried with magnesium sulfate, filtered, and
concentrated. The solid was dissolved in 10 mL of THF and
added to 200 mL of methanol. The yellow solid was filtered to
obtain 0.346 g of product (0.000 348 mol, 86%). 1H NMR
(CDCl3, 400 MHz) δ: 0.47 (s, 18 H) 0.89 (t, 6H), 1.27 (m, 28H),
1.62 (m, 4H), 2.66 (t, 4H), 7.23 (d, 4H), 7.61 (d, 4H), 7.73 (s,
2H). 13C NMR (CDCl3, 100 MHz) δ: -8.3, 14.1, 22.7, 29.27,
29.34, 29.5, 29.6, 29.7, 31.3, 31.9, 36.0, 85.7, 98.8, 110.2, 120.3,
128.5, 130.9, 131.7, 138.9, 143.4, 143.9, 144.6. Anal. calculated
for C52H70S2Sn2: C, 62.66%; H, 7.08%. Found: C, 62.33%; H,
7.08%.
Synthesis of Poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-
b:4,5-b0]dithiophene} (P1). To a three-neck round-bottom flask
were added 2,6-dibromo-4,8-bis(4-decylphenylethynyl)benzo-
[1,2-b:4,5-b0]dithiophene (1.17 g, 1.413 mmol), 2,6-(trimethyltin)-
4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene (1.49 g,
1.5 mmol), and tetrakis(triphenylphosphine)palladium(0) (0.08 g,
0.07 mmol) under a nitrogen atmosphere. Toluene (60 mL) and
DMF (2 mL) were added to dissolve the monomers. The solution
was heated at reflux for 2 h, at which time 20 mL of toluene was
added to the reaction mixture. After an additional 3 h at reflux,
40 mL of toluene was added to the reaction mixture. The polymer-
ization was stopped after 48 h by precipitating the polymer in
methanol. The polymer was filtered, and the polymer was purified
by Soxhlet extractions with methanol, diethyl ether, hexane, dichlor-
omethane, and chloroform. The polymer was obtained from the
chloroform fraction upon evaporation of solvent. The polymer was
obtained as a dark red solid (1.3 g, 70% yield). 1H NMR (toluene-d8,
400 MHz) δ: 0.42 (s, 18H), 0.95 (t, 6H), 1.25 (br, 56H), 2.7 (br, 4H),
7.2 (br, 8H), 7.55 (br, 2H).
4-Hexadecanoyl-1-bromobenzene was synthesized as 4-decan-
oyl-1-bromobenzene using hexadecanoyl chloride. 1H NMR
(CDCl3, 400 MHz) δ: 0.86 (t, 3H), 1.24 (m, 26H), 1.7 (m, 2H),
2.9 (t, 2H), 7.6 (d, 2H), 7.8 (d, 2H).
4-Hexadecyl-1-bromobenzene was synthesized as 4-decyl-1-
bromobenzene. 1H NMR (CDCl3, 400 MHz) δ: 0.87 (t, 3H), 1.26
(m, 26H), 1.57 (m, 2H), 2.55 (t, 2H), 7.04 (d, 2H), 7.37 (d, 2H).
4-Hexadecyl-1-trimethylsilylethynylbenzene was synthesized
as 4-decyl-1-trimethylsilylethynylbenzene. 1H NMR (CDCl3,
400 MHz) δ: 0.22 (s, 9H), 0.87 (t, 2H), 1.27 (m, 26H), 1.54 (m,
2H), 2.56 (t, 2H), 7.08 (d, 2H), 7.35 (d, 2H).
Synthesis of 4,8-Bis(4-decylphenylethynyl)benzo[1,2-b:4,5-
b0]dithiophene. 4-Decyl-1-ethynylbenzene (1.50 g, 0.006 20 mol)
was dissolved in 30 mL of dry THF. At 0 °C, 2.5 mL of 2.5 M
n-BuLi in hexane (0.006 25 mol) was added dropwise to the
solution of 4-decyl-1-ethynylbenzene. The reaction mixture was
stirred for 30 min at 0 °C followed by the addition of 4,8-
dihydrobenzo[1,2-b:4,5-b0]dithiophen-4,8-dione (0.650 g, 0.002 95
mol) dissolved in 50 mL of THF. After addition, the reaction
mixture was heated at reflux for 1 h. At 0 °C, 3.85 g of tin
chloride dihydrate dissolved in 50 mL of 20% HCl solution was
added, and then the reaction was heated at reflux for 2 h. The
reaction was diluted with ether (200 mL), and the organic layer
was washed with water (2 ꢀ 200 mL), dried with magnesium
sulfate, filtered, and concentrated to 10 mL of reddish solution
which was added to 200 mL of methanol, upon which a yellow
solid precipitated out of solution. The yellow solid was filtered
1
to obtain 1.033 g of product (0.001 54 mol, 52%). H NMR
(CDCl3, 400 MHz) δ: 0.88 (t, 6H), 1.27 (m, 28H), 1.64 (m, 4H),
2.66 (t, 4H), 7.22 (d, 4H), 7.57 (d, 2H), 7.59 (d, 2H), 7.7 (d, 2H).
13C NMR (CDCl3, 100 MHz) δ: 14.1, 22.7, 29.28, 29.34, 29.5,
29.6, 29.63, 31.3, 31.9, 36.0, 85.1, 99.5, 112.09, 120.0, 123.3,
128.0, 128.6, 131.7, 138.2, 140.3, 144.8. Anal. calculated for
C46H54S2: C, 82.33%; H, 8.11%. Found: C, 81.4%; H, 8.21%.
Synthesis of 2,6-Dibromo-4,8-bis(4-decylphenylethynyl)benzo-
[1,2-b:4,5-b0]dithiophene. 4,8-Bis(4-decylphenylethynyl)benzo-
[1,2-b:4,5-b0]dithiophene (0.404 g, 0.000 603 mol) was dissolved
in 100 mL of dry THF and cooled to -78 °C. 0.85 mL of 1.6 M
n-BuLi in hexane (0.001 36 mol, 2.2 equiv) was added, and the
solution was stirred for 1 h at -78 °C. After n-BuLi addition
the solution turned to a fluorescent green color. Carbon
tetrabromide (0.477 g) dissolved in 20 mL of THF was added.
After stirring for 15 min at -78 °C, the solution was allowed to
warm to room temperature for 1 h. Solvent was removed, and
the brown solid was dissolved in chloroform and washed with
4-Hexadecyl-1-ethynylbenzene was synthesized as 4-decyl-1-
1
ethynylnenzene. H NMR (CDCl3, 400 MHz) δ: 0.86 (t, 3H),
1.24 (m, 26H), 1.56 (m, 2H), 2.58 (t, 2H), 3. (s, 1H), 7.09 (d, 2H),
7.39 (d, 2H).
4,8-Bis(4-hexadecylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene
was synthesized as 4,8-bis(4-decylphenylethynyl)benzo[1,2-
b:4,5-b0]dithiophene. 1H NMR (CDCl3, 400 MHz) δ: 0.88

Article
Macromolecules, Vol. 43, No. 19, 2010 8065
(t, 6H), 1.27 (m, 52H), 1.64 (m, 4H), 2.65 (t, 4H), 7.22 (d, 4H), 7.57
(d, 2H), 7.6 (d,2H), 7.71 (d, 2H). 13C NMR (CDCl3, 100 MHz) δ:
14.0, 22.7, 29.28, 29.38, 29.51, 29.6, 29.68, 29.71, 31.3, 31.9, 36.0,
85.1, 99.5, 112.09, 120.0, 123.3, 128.0, 128.6, 131.7, 138.2, 140.3,
144.2. Anal. calculated for C58H78S2: C, 82.99%; H, 9.37%. Found:
C, 82.2%; H, 8.96%.
2,6-Dibromo-4,8-bis(4-hexadecylphenylethynyl)benzo[1,2-b:4,5-b0]-
dithiophene was synthesized as 2,6-dibromo-4,8-bis(4-decyl-
phenylethynyl)benzo[1,2-b:4,5-b0]dithiophene. 1H NMR (CDCl3,
400 MHz) δ: 0.88 (t, 6H), 1.27 (m, 52H), 1.65 (m, 4H), 2.65 (t, 4H),
7.21 (d, 4H), 7.50 (d, 4H), 7.64 (s, 2H). 13C NMR (CDCl3, 100
MHz) δ: 14.1, 22.7, 29.3, 29.4, 29.5, 29.6, 29.71, 29.72, 29.74, 29.75,
31.2, 31.9, 36.1, 84.2, 100.6, 110.5, 117.4, 119.7, 126.2, 128.7, 131.9,
137.6, 141.5, 144.7. Anal. calculated for C58H76Br2S2: C, 69.86%;
H, 7.68%. Found: C, 69.64%; H, 7.38%.
2,6-(Trimethyltin)-4,8-bis(4-hexadedecylphenylethynyl)benzo-
[1,2-b:4,5-b0]dithiophene was synthesized as 2,6-(trimethyltin)-
4,8-bis(4-dedecylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene.
1H NMR (CDCl3, 400 MHz) δ: 0.48 (s, 18 H), 0.89 (t, 6H), 1.29
(m, 52H), 1.6 (m, 4H), 2.65 (t, 4H), 7.22 (d, 4H), 7.61 (d, 4H),
7.73 (s, 2H). 13C NMR (CDCl3, 100 MHz) δ: -8.25, 14.1, 22.7,
29.28, 29.37, 29.52, 29.6, 29.7, 31.26, 31.94, 36.01, 85.7, 98.77,
110.2, 120.3, 128.57, 130.9, 131.7, 138.99, 143.35, 143.9, 144.6.
Anal. calculated for C64H94S2Sn2: C, 65.98%; H, 8.13%.
Found: C, 67.0%; H, 8.08%.
solution in chloroform was placed on tip of the platinum elec-
trode. The solvent was evaporated in air. The film was immersed
in the electrochemical cell containing the electrolyte prior to
measurements. All electrochemical shifts were standardized to
the ferrocene redox couple at 0.471 V.
Preparation of Solar Cell Devices. OLED-grade glass slides
were purchased with prepatterned ITO electrodes from Lumi-
nescense Technology Corp. (Taiwan.) The coupons were
cleaned with deionized water, acetone, and isopropanol succes-
sively by sonication for 5 min each and then washed for 10 min in
an oxygen plasma prior to use. Immediately following plasma
treatment, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) was spin-coated (3000 min-1, 60 s) followed by
annealing at 180 °C for 5 min under nitrogen, resulting in a film
thickness of 40 nm. The PCBM/polymer blend was prepared in
dichlorobenzene at a 1:1 weight ratio and total concentration of
35 mg/mL. This blend was then spin-cast (600 min-1, 200 min-1 s-1
,
60 s) onto the PEDOT-PSS/substrate, after which it was immedi-
ately placed into a closed Petri dish to finish drying. For the slow
evaporation samples a small Petri dish (diameter = 90 mm, height =
13 mm) was used, and drying took ∼60 min; for the fast evaporation
a larger Petri dish (diameter = 137 mm, height = 20 mm) was used,
and the drying time was about 15 min. Cathodes consisting
of aluminum (100 nm) were thermally evaporated at a rate of
˚
∼2.5 A/s through a shadow mask to define solar cell active areas.
Synthesis of Poly{4,8-bis(4-hexadecylphenylethynyl)benzo-
[1,2-b:4,5-b0]dithiophene} (P2) was synthesized as polymer P1.
1H NMR (toluene-d8, 400 MHz) δ: 0.5 (s, 18H), 0.96 (t, 6H), 1.30
(br, 56H), 2.6 (br, 4H), 7.2 (br, 8H), 7.55 (br, 2H).
IV testing was carried out under a controlled N2 atmosphere
using a Keithley 236, model 9160 interfaced with LabView
software. The solar simulator used was an THERMOORIEL
equipped with a 300 W xenon lamp; the intensity of the light was
calibrated to 100 mW cm-2 with a NREL certified Hamamatsu
silicon photodiode. The active area of the devices was 0.1 cm2.
The active layer film thickness was measured using a Veeco
Dektak VIII profilometer.
Structural Analysis. 1H NMR spectra of the synthesized
monomers and polymers were recorded on a Bruker 400 MHz
spectrometer at 25 °C. 1H NMR data are reported in parts per
million as chemical shift relative to tetramethylsilane (TMS) as
the internal standard. Spectra of monomers were recorded in
CDCl3. 1H NMR spectra of the polymers were recorded in
toluene-d8 at 55 °C. GC/MS was performed on an Agilent 6890-
5973 GC-MS workstation. The GC column was a Hewlett-
Packard fused silica capillary column cross-linked with 5%
phenylmethylsiloxane. Helium was the carrier gas (1 mL/min).
The following conditions were used for all GC/MS analyses:
injector and detector temperature, 250 °C; initial temperature,
70 °C; temperature ramp, 10 °C/min; final temperature, 280 °C.
The UV-vis spectra of polymer solutions in chloroform solvent
were carried out in 1 cm cuvettes using an Agilent 8453 UV-vis
spectrometer. Thin films of polymer were obtained by evapora-
tion of chloroform solvent on glass microscope slides. Fluores-
cence spectra were recorded on a Perkin-Elmer LS 50 BL
luminescence spectrometer. Molecular weights of the synthe-
sized polymers were measured by size exclusion chromatogra-
phy (SEC) analysis on a Viscotek VE 3580 system equipped with
ViscoGEL columns (GMHHR-M), connected to a refractive
index (RI) detectors. GPC solvent/sample module (GPCmax)
was used with HPLC grade THF as the eluent, and calibration
was based on polystyrene standards. Running conditions for
SEC analysis were flow rate=1.0 mL/min, injector volume=
100 μL, detector temperature = 30 °C, and column temperature
= 35 °C. All the polymers samples were dissolved in THF, and
the solutions were filtered through PTFE filters (0.45 μm) prior
to injection.
TMAFM (Tappping Mode Atomic Force Microscopy). AFM
studies were carried out on a VEECO-dimension 5000 scanning
probe microscope with a hybrid xyz head equipped with Nano-
Scope Software. AFM images were obtained using silicon
cantilevers with nominal spring constant of 42 N/m and nominal
resonance frequency of 300 kHz (OTESPa). Image analysis
software Nanoscope 7.30 was used for surface imaging and
image analysis. All AFM measurements were conducted under
ambient conditions. All cantilever oscillation amplitude was
equal to ca. 375 mV, and all images were acquired at 1 Hz scan
frequency. Sample scan area varied from 1 to 5 μm. Samples
were prepared from either chloroform or trichlorobenzene
solutions (1 mg/mL) via drop-casting onto a mica substrate.
The chloroform was slowly evaporated in a cover Petri dish
filled with chloroform. The trichlorobenzene was evaporated by
heating the mica substrate for 1 h at 60 °C. Samples were
annealed at 120 °C for 30 min. For the solar cell devices the
AFM images were recorded for spin-cast blends of polymer/
PCBM in dichlorobenzene.
Results and Discussion
We have previously reported the synthesis and characteriza-
tion of polymers containing benzodithiophene with pentylphe-
nylethynyl substituents. Poly{4,8-bis(4-pentylphenylethynyl)benzo-
[1,2-b:4,5-b0]dithiophene} was synthesized by Stille coupling and
was found to have relatively low solubility in organic solvents, such
as tetrahydrofuran and chloroform.29 Because of the low solubility,
the electronic properties of this polymer could not be investigated.
Attachment of longer alkyl substituents to the phenyl ring was
expected to increase the solubility of the polymers. In contrast to the
commercially available 4-pentylphenylacetylene, the larger alkyl
chains needed to be synthesized starting with bromobenzene and
attaching an acyl group through a Friedel-Crafts acylation. The
decanoyl (C10) and hexadecanoyl (C16) groups were attached in
this manner. The carbonyl was reduced by a Wolf-Kishner
reaction to generate the para-substituted bromoalkyl derivatives.
Optoelectronic Properties of the SynthesizedPolymers. Cyclic
Voltammetry. Electrochemical grade tetrabutylammonium per-
chlorate (TBAP) was used as the electrolyte without further
purification. Acetonitrile (low water 99.9% grade) was distilled
over calcium hydride (CaH2). Electrochemical experiments
were performed using a BAS CV-50W voltammetric analyzer
(Bioanalytical Systems, Inc.). The electrochemical cell was com-
prised of a platinum electrode, a platinum wire auxiliary elec-
trode, and an Ag/AgCl reference electrode. Acetonitrile solutions
containing 0.1 M of tetrabutylammonium perchlorate were
placed in a cell and purged with N2. A drop of the polymer

8066 Macromolecules, Vol. 43, No. 19, 2010
Scheme 1. Synthesis of 4,8-Bis(4-alkylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene Monomers
Sista et al.
Scheme 2. Synthesis of Poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene} (P1) (n = 8) and
Poly{4,8-bis(4-hexadecylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene} (P2) (n = 14)
The alkyne group was attached via a Sonogashira coupling with
trimethylsilylacetylene (TMS), and the TMS protecting group was
subsequently removed in basic conditions to generate the decyl- and
hexadecyl-substituted phenylacetylenes, as shown in Scheme 1.
These alkynes were reacted with 4,8-dihydrobenzene[1,2-b:4,5-
b0]dithiophen-4,8-dione to generate the benzodithiophene mono-
mers, which were subsequently reacted to form the dibromo and
distannylated derivatives, which were used in Stille polymerization
to generate poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]-
dithiophene} (P1) and poly{4,8-bis(4-hexadecylphenylethynyl)-
benzo[1,2-b:4,5-b0]dithiophene} (P2), as shown in Scheme 2.
The number-average molecular weights for both P1 (23400
g/mol) and P2 (27420 g/mol) are appreciably larger than the
previously synthesized poly{4,8-bis(4-pentylphenylethynyl)benzo-
[1,2-b:4,5-b0]dithiophene}(P3) (1700 g/mol)29 due to the higher
solubility during polymerization reactions (Table 1). Polymers P1
and P2 are soluble in chloroform, tetrahydrofuran, and chloro-
benzenes. 1H NMR analysis of polymers in CDCl3 proved to be
difficult as the polymers aggregate in solution, resulting in broad
of polymers were recorded in deuterated toluene at 60 °C
(Supporting Information).
The HOMO and LUMO energy levels of polymers P1 and P2
were estimated by cyclic voltammetry. From the value of onset
oxidation potential and onset reduction potential of polymers P1
and P2, the HOMO, LUMO, and the band gap were calculated
(Table 1 and Supporting Information). Both P1 and P2 polymers
have reversible oxidation and reduction peaks. The HOMO and
LUMO energy levels for polymers P1 and P2 are very similar, as
expected based on their structural similarity. The length of the
alkyl substituent does not significantly influence the HOMO and
LUMO energy levels, as seen from the results presented in
Table 1. The HOMO energy level of polymers P1 and P2
(-5.62 and -5.61 eV) is below the air oxidation threshold (ca.
-5.27 eV), indicating a good air stability of the synthesized
polymers. The band gaps of polymers P1 and P2 are lower than
our previously reported benzodithiophene polymer with pentyl
phenylethynyl substituents (P3), which is due to the larger
molecular weights, which in turn render a longer conjugation
length.
1
features. In order to avoid this drawback, the H NMR spectra

Article
Macromolecules, Vol. 43, No. 19, 2010 8067
Table 1. Molecular Weights as Well as Optical and Electronic Energy Levels of Poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-
b0]dithiophene} (P1), Poly{4,8-bis(4-hexadecylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene} (P2), and Poly{4,8-bis-
(4-pentylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene} (P3)29
polymer
Mn (g/mol)
PDI
λmaxL (nm)
λmaxS (nm)
Eox (V)
Ered (V)
HOMOa (eV)
LUMOb (eV)
Eg (eV)
P1
P2
P3
23 400
27 420
1 700
3.0
2.0
1.9
520
520
517
525
525
506
0.91
0.90
0.71
-0.81
-0.84
-1.33
-5.62
-5.61
-5.42
-3.90
-3.87
-3.38
1.72
1.74
2.04
a Estimated from onset oxidation wave. b Estimated from onset reduction wave.
Figure 1. UV-vis absorption spectra of poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene} (P1) and poly{4,8-bis(4-hexadecylphenylethynyl)-
benzo[1,2-b:4,5-b0]dithiophene}(P2) in chloroform solutions and as thin films drop-cast from chloroform solution.
Table 2. Fluorescence Data for Poly{4,8-bis-
(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene} (P1), Poly-
solution and films are shown in Figure 1. P1 and P2 polymers
The UV-vis spectra of the polymers P1 and P2 in chloroform
{4,8-bis(4-hexadecylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene}
(P2), and Poly{4,8-bis(4-pentylphenylethynyl)benzo-
have similar electronic absorption spectrain chloroform solution,
[1,2-b:4,5-b0]dithiophene} (P3)29
displaying two absorption maxima, at ∼363 and ∼520 nm. The
absorption band in the visible region (∼520 nm) is due to the
polymera
excitation (nm)
emission (nm)
quantum yield (%)
π-π* transition of the benzodithiophene backbone, while the
band in the UV region (∼363 nm) originates from the 1,4-bis-
(phenylethynyl)benzene substituents.34,35 The intensity of both
benzodithiophene and (phenylethynyl)benzene bands correlates
with their relative ratio in the repeating unit. The UV-vis
absorption spectra in chloroform show a vibronic peak at
570 nm for P1 and 566 nm for P2. A particular feature of the
absorption spectra of polymers P1 and P2 is the relatively small
red shift observed for the UV-vis spectra of films vs chloroform
solutions. The absorption bands shifted from 363 nm (in
solution) to 366 nm (in film) and from 520 nm (in solution) to
525 nm (in film) for polymer P1, while polymer P2 displayed a
shift from 363 nm (in solution) to 366 nm (in film) and from
520 nm (in solution) to 525 nm (in film). Both the presence of
the vibronic peak in the solution spectra and the small shift of the
absorption maxima from solution to films indicate that the
polymers have a rigid rod conformation in the solution and solid
state.25,36
Fluorescence spectra of polymers P1 and P2 have been
recorded (Table 2 and Supporting Information). Polymer P1
displays emission at 526 nm, while polymer P2 displays emission
at 519 nm, upon excitation at 355 nm. The fluorescence quantum
yields for polymers P1 and P2 have been calculated by using
rhodamine B as standard. The quantum yields of the three
polymers P1, P2, and P3 are 18.8%, 14.5%, and 48%, respec-
tively.
P1
P2
P3
355
355
400
526
519
505
18.8
14.5
48.0
a Fluorescence was measured in chloroform solution.
deposited from trichlorobenzene followed by the evaporation
of solvent at 60 °C displays different morphological features as
compared to films deposited from chloroform. Figure 2 shows
the TMAFM phase and height images for thin films of polymer
P1 deposited from both chloroform and trichlorobenzene solu-
tions. The 3D AFM images displayed in Figure 3 show that the
polymer film deposited from trichlorobenzene solution forms
rounded grains, which result in a nonuniform film surface. A
possible explanation for the presence of the rounded grains could
be the development of a columnar morphology, which cannot be
proved by using TMAFM imaging. Further insight into the
morphology of these polymers is currently under investigation by
using cross-section TEM measurements on films deposited from
various high boiling point solvents.
The photovoltaic properties of polymers P1 and P2 were
investigated in bulk heterojunction solar cells with [6,6]-phenyl-
C61-butyric acid methyl ester (PCBM). The solar cells were
fabricated using a conventional device structure: ITO/PEDOT:
PSS/P:PCBM/Al. The PCBM/polymer blend was prepared in
dichlorobenzene at 1:1 weight ratio. This blend was spin-cast
onto the substrate, after which it was placed into a closed Petri
dish to finish drying. Slow and fast evaporation samples were
obtained. The approximate drying time for the fast evaporation
was 15 min, while for the slow evaporation the drying time was
60 min. The I-V curves (Figure 4) for both polymers P1 and P2
are shown in Figure 4, and Table 3 lists the photovoltaic proper-
ties obtained from the I-V curves. As listed in Table 3, the power
The morphology of polymers P1 and P2 was investigated by
TMAFM. Films were deposited by drop-casting from chloro-
form and trichlorobenzene solutions onto mica substrates. Films
deposited from chloroform solution for polymers P1 and P2
display granular morphology as shown in Figure 2 and the
Supporting Information. However, the film of polymer P1

8068 Macromolecules, Vol. 43, No. 19, 2010
Sista et al.
conversion efficiencies (PCE) of devices prepared with polymer
P2 are 50% lower as compared to polymer P1.
similar to the well-studied P3HT/PCBM system, these features
are associated withthe phasesegregation of PCBMislands during
fast evaporation process.37,38 Interestingly, this structure-
function relationship is opposite to that observed P3HT/PCBM
solar cells, where slowly evaporated films produce rougher
surfaces and consequently better performing solar cell devices.39,40
Moreover, this phase segregation is more marked in P2/PCBM
blends compared with the P1/PCBM blend surfaces. It seems
reasonable to hypothesize, therefore, that the lower solar cell
performance of the P2/PCBM devices relative to the P1/PCBM
devices (together with the observed dependence of PCE upon
Thus, the solar cell device performance appears to depend
upon the ratio between the insulating alkyl substituents and the
semiconducting benzodithiophene backbone, with polymer P2
with hexadecyl substituents having lower performance as com-
pared to polymer P1 with decyl substituents. In addition, the
field-effect mobility measured in bottom-gate bottom-contact
transistors of polymer P2 (μ = 1 ꢀ 10-6 cm2 V-1 s-1) is 2 orders
of magnitude lower as compared to polymer P1 (μ = 1.6 ꢀ 10-4
cm2 V-1 s-1). The results presented in Figure 4 and Table 3 also
show that devices fabricated by slow evaporation have higher
PCE than those fabricated by fast evaporation. For example,
polymer P1 has a PCE of ∼0.5% in devices obtained by fast
evaporation, while devices obtained by slow evaporation have a
PCE of ∼1.05%. This trend is consistent for polymer P2, which
showed a power conversion efficiency of ∼0.25% in the fast
evaporation device, while a PCE of ∼0.47% was obtained for the
slow evaporation device. A comparison of the TMAFM images
of the fast and slow evaporated P1/PCBM and P2/PCBM binary
blend devices reveals that, for both systems, the morphology of
the fast evaporated surfaces are rougher than the slow evaporated
surfaces and show more evidence of phase-segregated features
(Figure 5 and Supporting Information). We speculate that,
Figure 4. I-V plots for the polymer solar cells based on P1 (top) and
P2 (bottom). [P1]:[PCBM] = 1:1 (w/w); [P2]:[PCBM] = 1:1 (w/w); the
blue curves represent devices obtained by fast evaporation, and the red
curves represent devices obtained by slow evaporation. The error bars
represent the standard deviation from the average for the working
devices on a coupon.
Figure 2. TMAFM phase (right) and height (left) images of poly{4,8-
bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene} (P1): (a, b)
films deposited by drop-casting from chloroform solution; (c, d) films
deposited by drop-casting from trichlorobenzene solution. Scan size:
5 ꢀ 5 μm.
Figure 3. 3D height TMAFM images of thin films of polymer P1: (a) film deposited by drop-casting from chloroform solution; (b) film deposited by
drop-casting from trichlorobenzene.

Article
Macromolecules, Vol. 43, No. 19, 2010 8069
Table 3. Photovoltaic Properties of Poly{4,8-bis(4-decylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene} (P1) and Poly{4,8-bis(4-
hexadecylphenylethynyl)benzo[1,2-b:4,5-b0]dithiophene} (P2)a
polymer
evaporation
active layer thickness (nm)
VOC (V)
ISC (mA/cm2)
FF
η (%)
P1
P1
P2
P2
slow
fast
slow
fast
280
250
170
190
0.75
0.73
0.85
0.80
4.0
2.0
3.3
0.87
0.34
0.30
0.33
0.35
1.05
0.50
0.47
0.25
a Spin-coated films of polymer/PCBM (1:1 w:w).
Figure 5. 3D TMAFM phase images of solar cell devices: (a) P1/PCBM active layer (slow evaporation); (b) P1/PCBM active layer (fast evaporation);
(c) P2/PCBM active layer (slow evaporation); (d) P2/PCBM active layer (fast evaporation).
Figure 6. P1, P2 donors and PCBM acceptor energy levels.
evaporation rate) arises from the observed differences in mor-
phology resulting in lower Jsc currents.
All the solar cell devices have relatively high open circuit
voltage (VOC) which is due to the relatively low HOMO energy
levels of polymers P1 and P2 (-5.62 and -5.61 eV). However, the
measured VOC (0.7-0.85 V) are lower than the theoretical
predicted values (1.12 and 1.11 eV),41 which is most likely
attributed to charge carrier recombination losses at the poly-
mer/fullerene interfaces and at the electrodes (Figure 6). Further
work on optimization of device structure of polymer P1 is

8070 Macromolecules, Vol. 43, No. 19, 2010
Sista et al.
expected to result in improved photovoltaic performance. Ad-
ditionally, further investigation regarding the morphological
features of pristine polymer P1 and its corresponding blends
with PCBM are currently under investigation. If polymer P1
proves to form columnar morphology by casting from high
boiling points solvents, a bilayer solar cell structure is expected
to outperform the bulk heterojunction solar cells presented in this
paper.
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with phenylethynyl substituents containing more soluble longer
decyl and hexadecyl alkyl chains have been synthesized. The
HOMO and LUMO energy levels of the synthesized polymers
with decyl and hexadecyl substituents were estimated from cyclic
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were investigated in bulk heterojunction solar cells with [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM). The benzodithio-
phene polymer with decyl phenylethynyl substituents showed
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Acknowledgment. Mihaela C. Stefan gratefully acknowl-
edges financial support from UTD (start-up funds) and NSF
(Career DMR-0956116). We gratefully acknowledge Dr. Anvar
Zakhidov for giving us access to his solar cell fabrication lab.
Supporting Information Available: 1H NMR spectra of
monomers and polymers, phase and height TMAFM images
of polymers, liquid and solid state UV-vis spectra of copoly-
mers, fluorescence spectra, cyclic voltammograms of polymers.
This material is available free of charge via the Internet at http://
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