Donor-acceptor semiconducting polymers containing benzodithiophene with bithienyl substituents

Article abstract of DOI:10.1021/ma301624t

Synthesis and photovoltaic properties of two donor-acceptor polymers containing benzodithiophene with 3,3,5-trihexylbithienyl substituents are reported. Benzo[c][1,2,5]thiadiazole and 5-hexylthieno[3,4-c]pyrrole-4,6-dione were used as acceptor building bl

Full text of DOI:10.1021/ma301624t

Article
pubs.acs.org/Macromolecules
Donor−Acceptor Semiconducting Polymers Containing
Benzodithiophene with Bithienyl Substituents
Ruvini S. Kularatne, Prakash Sista, Hien Q. Nguyen, Mahesh P. Bhatt, Michael C. Biewer,
and Mihaela C. Stefan*
Department of Chemistry, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States
S
* Supporting Information
ABSTRACT: Synthesis and photovoltaic properties of two
donor−acceptor polymers containing benzodithiophene with
3,3′,5-trihexylbithienyl substituents are reported. Benzo[c]-
[1,2,5]thiadiazole and 5-hexylthieno[3,4-c]pyrrole-4,6-dione
were used as acceptor building blocks for the synthesis of
donor−acceptor polymers. The photovoltaic properties of the
synthesized donor−acceptor polymers were investigated in
bulk heterojunction solar cells with [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) acceptor.
level of the donor polymer will be a crucial factor in enhancing
the Voc.18 Moreover, a donor−acceptor copolymer with a
deeper HOMO level will enhance the oxidative stability of the
polymer. Hou and co-workers synthesized a donor−acceptor
polymer with 5-alkylthiophene-substituted BDT with a deep-
lying HOMO of −5.3 eV and obtained an air-stable donor
polymer that gave a Voc of 0.88 V in bulk heterojunction solar
cells.12
INTRODUCTION
■
Benzodithiophene semiconducting polymers have garnered
considerable interest in the past few years due to their good
performances in both organic field-effect transistors and
polymeric solar cells.1,2 In addition to having a planar
backbone, the benzodithiophene core offers the flexibility of
attaching different substituents on the central benzene core to
fine-tune the energy levels of the polymer.2−4
To generate highly soluble semiconducting polymers with
higher molecular weight and broader absorption in the visible
region, we targeted the synthesis of benzodithiophene with
bithienyl substituents. The bithienyl substituent allows the
attachment of up to six alkyl substituents per benzodithiophene
repeating unit, which in turn should allow the synthesis of
higher molecular weight polymers with good solubility in
organic solvents. The conjugated bithienyl substituents
perpendicular to the polymer backbone is also expected to
provide a broader absorption in the visible region. Additionally,
by incorporating this building block in donor−acceptor
structures, a deeper HOMO energy level can be obtained,
which is expected to generate higher Voc values in bulk
heterojunction solar cells.
Various research groups have exploited this property and
attached different substituents such as alkyl, alkoxy, and
thioalkyl to the benzene core of benzodithiophene.2,3,5−9 Our
group reported the synthesis and electronic properties of
benzodithiophene polymers with alkyl phenylethynyl substitu-
ents.10 More recently, the Yang group reported the synthesis of
donor−acceptor polymers containing benzodithiophene with a
conjugated thienyl substituent.11 Huo showed that replacing
the alkoxy with alkylthienyl substituents on the BDT core
increases the thermal stability, gives broader absorption spectra,
lowers the HOMO and LUMO energy levels, and improves the
photovoltaic properties of the polymers.12−14 Furthermore, the
attachment of thienyl substituents to the BDT can result in
increasing the solubility of the polymer as it provides the ability
to attach a larger number of alkyl substituents on the thiophene
ring.15 For example, Cao attached four alkyl chains on the BDT
building block which improved the solubility of the resulting
polymer.15 Recent reports have shown that benzodithiophene
semiconducting polymers displayed power conversion efficien-
cies (PCE) in excess of 7% in bulk heterojunction (BHJ) solar
cells.16,17
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 conducted 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. THF
was distilled from sodium/benzophenone ketyl. 2-Bromo-3-hexylth-
Donor−acceptor polymers with lower band gaps are highly
desirable for organic solar cell applications. Additionally, the
ability to improve the open-circuit voltage (Voc) has attracted
much attention. Since the Voc of a BHJ solar cell is directly
proportional to the difference between the HOMO of the
donor and the LUMO of the acceptor, lowering the HOMO
Received: August 1, 2012
Revised: September 5, 2012
© XXXX American Chemical Society
A
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iophene and 4,8-dihydrobenzo[1,2-b:4,5-b′]dithiophene-4,8-dione
were prepared according to the literature.19,20
nitrogen atmosphere at −78 °C, 0.639 mL of 2.5 M n-BuLi in hexane
(1.47 × 10−4 mol) was added dropwise. Upon the addition the
solution turned to a bright green color. The reaction was stirred at
−78 °C for 2 h, at which 1.50 mL of 1 M trimethyltin chloride in
hexane (1.50 × 10−4 mol) was added to the reaction mixture. The
reaction was stirred at −78 °C for 15 min and was allowed to warm to
room temperature. The solvent was evaporated, the obtained yellow
oil was diluted with chloroform (200 mL), and the solution was
washed with water (5 × 200 mL), dried with anhydrous magnesium
sulfate, filtered, and concentrated to obtain the product (0.76 g, 5.63 ×
Synthesis of 3,3′-Dihexyl-2,2′-bithiophene. Magnesium turn-
ings (0.218 g, 0.009 mol) were added to a three-neck round-bottomed
flask and were kept under vacuum for 2 h. The vacuum was canceled
with nitrogen followed by the addition of 2-bromo-3-hexylthiophene
(2.0 g, 0.0080 mol) and anhydrous ether (60 mL). The reaction was
initiated by heating the flask and by adding several drops of 1,2-
dibromoethane. After 30 min the reaction mixture turned to a gray
color, indicating the formation of 2-magnesiobromo-3-hexylthiophene
(flask A). In another round-bottomed flask [1,3-bis-
(diphenylphosphino)propane]dichloronickel(II) (0.1626 g, 0.0003
mol), 2-bromo-3-hexylthiophene (2.0 g, 0.0080 mol), and anhydrous
diethyl ether (60 mL) were added under a nitrogen atmosphere (flask
B). The 2-magnesiobromo-3-hexylthiophene from flask A was
cannulated to flask B. The reaction mixture was stirred overnight at
40 °C. The reaction mixture was extracted into ethyl acetate, and the
organic phase was washed with water, brine, dried over anhydrous
magnesium sulfate, filtered, and concentrated to obtain a brown oil
which was further purified by column chromatography (eluent hexane)
to obtain a clear oil of 3,3′-dihexyl-2,2′-bithiophene (2.10 g, 0.0063
1
10−4 mol, 82%). H NMR (CDCl3, 270 MHz) δ: 0.40 (s, 18H), 0.87
(m, 9H), 1.33 (m, 18H), 1.68 (m, 6H), 2.58 (m, 4H), 2.79 (t, 2H),
6.66 (s, 1H), 7.36 (s, 1H), 7.77 (s, 1H). 13C NMR (CDCl3, 270 MHz)
δ: −8.25, 14.23, 22.70, 29.01, 29.28, 29.32, 30.37, 30.85, 31.54, 31.68,
31.74, 122.31, 125.71, 130.39, 137.27, 139.56, 142.07, 142.07, 142.25,
142.56, 143.26, 145.85.
Synthesis of Polymer P1. Benzodithiophene monomer 3 (404
mg, 2.99 × 10−4 mol), 4,7-dibromobenzo[c][1,2,5]thiadiazole (88 mg,
2.99 × 10−4 mol), and tetrakis(triphenylphosphene)palladium(0) (9.9
mg, 9.86 × 10−6 mol) were added to a three neck round-bottomed
flask under a nitrogen atmosphere. Toluene (20 mL) and DMF (0.5
mL) were added to dissolve the monomers and the catalyst. The
reaction mixture was heated at reflux for 3 h, at which time another 20
mL of toluene was added to the reaction mixture. The polymerization
was stopped after 30 h by precipitating the polymer in methanol. The
polymer was filtered and was purified by Soxhlet extractions with
methanol, ether, hexane, and dichloromethane with successive drying
after each extraction. The polymer was obtained from the dichloro-
methane fraction as a dark blue solid upon the evaporation of the
solvent (268 mg, 77% yield). 1H NMR (CDCl3, 270 MHz) δ: 0.72 (br,
6H), 0.82 (br, 12H), 1.28 (m, 24H), 1.36 (m, 12H), 1.67 (m, 12H),
2.63 (m, 8H), 2.75 (m, 4H), 6.65 (m, 2H) 7.44 (br, 2H), 7.50 (br,
1H), 7.90 (br, 1H), 9.00 (br, 2H). SEC: Mn = 40 800 g/mol; PDI =
4.4.
Synthesis of Polymer P2. Benzodithiophene monomer 3 (252
mg, 1.867 × 10−4 mol), 1,3-dibromo-5-hexylthieno[3,4-c]pyrrole-4,6-
dione (74 mg, 1.867 × 10−4 mol), and tetrakis(triphenylphosphene)-
palladium(0) (6.29 mg, 6.2 × 10−6 mol) were added to a three neck
round-bottom flask under a nitrogen atmosphere. Toluene (20 mL)
and DMF (0.5 mL) were added to dissolve the monomers and the
catalyst. The reaction mixture was heated at reflux for 3 h, at which
time another 20 mL of toluene was added to the reaction mixture. The
polymerization was stopped after 36 h by precipitating in methanol.
The polymer was filtered and was purified by Soxhlet extractions with
methanol, ether, hexane, and chloroform with successive drying after
each extraction. A dark red solid polymer was obtained after
evaporation of the chloroform extract (176 mg, 75% yield). 1H
NMR (CDCl3, 270 MHz) δ: 0.70 (br, 3H), 0.80 (br, 18H), 1.25 (m,
42H), 1.54 (m, 16H), 2.59 (m, 4H), 2.65 (m, 4H), 2.74 (m, 4H) 3.61
(m, 2H), 6.63 (m, 2H), 7.47 (m, 2H), 8.85 (m, 2H). SEC: Mn = 29
000 g/mol; PDI = 2.5.
1
mol, 78%). H NMR (CDCl3, 270 MHz) δ: 0.78 (t, 6H), 1.16 (m,
12H), 1.47 (m, 4H), 2.42 (t, 4H), 6.89 (d, 2H), 7.21 (d, 2H). 13C
NMR (CDCl3, 270 MHz) δ: 14.14, 22.65, 28.87, 29.18, 30.78, 31.71,
125.29, 128.60, 128.79, 142.42. GC-MS: m/z = 334.3.
Synthesis of 3,3′,5-Trihexyl-2,2′-bithiophene (2). 3,3′-Dihex-
yl-2,2′-bithiophene (1.80 g, 0.005 38 mol) was diluted in 125 mL of
THF followed by the dropwise addition of n-BuLi in hexane (2.5 M)
(0.005 28 mol, 2.29 mL) under a nitrogen atmosphere at 0 °C. The
reaction mixture was stirred at 0 °C for 30 min, followed by the
addition of 1-bromohexane (1.132 mL, 0.008 07 mol), and was heated
at reflux for 2 h. The reaction was stopped and was diluted with diethyl
ether (200 mL). The organic phase was washed with water (4 × 200
mL) and brine (200 mL) and dried with anhydrous magnesium sulfate
and concentrated to obtain a oil, which was further purified by column
chromatography (eluent hexane) to obtain a yellow color oil (0.89 g,
0.002 mol, 40%). 1H NMR (CDCl3, 270 MHz) δ: 0.87 (m, 9H), 1.26
(m,12H), 1.33 (m, 6H), 1.52 (m,4H), 1.68 (m, 2H), 2.42 (t, 2H), 2.51
(t, 2H), 2.76 (t, 2H), 6.63 (s, 1H), 6.93 (s, 1H), 7.24 (d, 1H).
13CNMR (CDCl3, 270 MHz) δ: 14.16, 22.67, 28.89, 28.97, 29.04,
29.20, 29.24, 30.32, 30.79, 31.54, 31.68, 31.74, 124.95, 125.84, 128.53,
129.5, 142.03, 142.05, 145.67.
Synthesis of 4,8-Bis(3,3′,5′-trihexyl-[2,2′-bithiophen]-5-yl)-
benzo[1,2-b:4,5-b′]dithiophene (2′). 3,3′,5-Trihexyl-2,2′-bithio-
phene (0.80 g, 0.0019 mol) was dissolved in 60 mL of dry THF
followed by the dropwise addition of n-BuLi in hexane (2.5 M) (0.002
20 mol, 0.95 mL) under a nitrogen atmosphere at 0 °C. The reaction
mixture was stirred at 0 °C for 30 min, followed by the addition of 4,8-
dihydrobenzo[1,2-b:4,5-b′]dithiophen-4,8-dione (0.190 g, 0.000 864
mol). After the addition the reaction mixture was heated at reflux for 1
h. The reaction mixture was cooled to room temperature, and tin
chloride dihydrate (0.985 g, 0.004 37 mol) dissolved in 20 mL of 20%
HCl was added. The reaction mixture was heated at reflux for 2 h. The
solvent was evaporated to obtain a yellow oil, which was dissolved in
diethyl ether (200 mL), and the solution was washed with water (4 ×
200 mL), dried with anhydrous magnesium sulfate, filtered, and
concentrated to obtain a yellow oil, which was further purified by
column chromatography (eluent hexane) to obtain 0.78 g of the
product (7.6 × 10−4 mol, 80%). 1H NMR (CDCl3, 270 MHz) δ: 0.87
(m, 9H), 1.33 (m, 18H), 1.68 (m, 6H), 2.58 (m, 4H), 2.79 (t, 2H),
6.66 (s, 1H), 7.34 (s, 1H), 7.47 (d, 2H), 7.72 (d, 2H). 13CNMR
(CDCl3, 270 MHz) δ: 14.19, 22.71, 22.68, 28.98, 29.09, 29.21, 29.25,
29.31, 30.36, 30.82, 30.91, 31.55, 31.69, 31.78, 123.97, 125.44, 130.73,
136.48, 138.85, 138.99, 142.23, 142.35, 145.99. Anal. Calcd for
C62H86S6: C,72.80%; H, 8.41%; S, 18.79%. Found: C, 73.12%; H,
8.46%; S, 18.42%.
Analysis. 1H and 13C NMR spectra were recorded at room
temperature using either a 270 MHz JEOL or a 500 MHz Bruker
spectrometer, as indicated, and were referenced to residual protio
solvent (CHCl3: δ 7.27 ppm). The data are reported as follows:
chemical shifts are reported in ppm on the δ scale, multiplicity (br =
broad, s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet).
GC-MS was obtained on an Agilent 6890-5973 GC/MS work-
station. 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. Analytical thin layer chromatography was performed on EM
reagents 0.25 mm silica gel 60-F plates. Liquid chromatography was
performed using flash chromatography of the indicated solvent system
on Select silical gel (SiO2) 230−400 mesh.
Synthesis of 2,6-(Trimethyltin)-4,8-bis(3,3′,5′-trihexyl-[2,2′-
bithiophen]-5-yl)benzo[1,2-b:4,5-b′]dithiophene. 4,8-Bis-
(3,3′,5′-trihexyl-[2,2′-bithiophen]-5-yl)benzo[1,2-b:4,5-b′]dithiophene
(0.7 g, 6.83 × 10−4 mol) was dissolved in 50 mL of dry THF. Under a
Molecular weights of the synthesized polymers were measured by
size exclusion chromatography (SEC) analysis on a Viscotek VE 3580
system equipped with ViscoGEL columns (GMHHR-M), connected
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Scheme 1. Synthesis of Benzodithiophene with the 3,3′,5-Trihexylbithienyl Substituents
to a refractive index (RI) detector. A 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 as follows: flow rate = 1.0 mL/min, injector
volume = 100 μL, detector temperature = 30 °C, column temperature
= 35 °C. All the polymer samples were dissolved in THF, and the
solutions were filtered through PTFE filters (0.45 μm) prior to
injection.
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 evaporation of
chloroform from polymer solutions on glass microscope slides. The
films for the determination of absorption coefficients were deposited
by spin-casting solutions of 5 mg/mL of polymer in chloroform.
CV was obtained with a BAS CV-50W voltammetric analyzer
(Bioanalytical Systems, Inc.). Electrochemical grade tetrabutylammo-
nium perchlorate (TBAP) was used without further purification.
Acetonitrile was distilled over calcium hydride and collected over
molecular sieves. The electrochemical cell was comprised of a platinum
electrode, a platinum wire auxiliary electrode, and an Ag/AgCl
reference electrode. Acetonitrile solutions containing 0.1 M TBAP
were placed in a cell and purged with argon. A drop of the polymer
solution was evaporated in ambient air. The film was immersed into
the electrochemical cell containing the electrolyte, and the oxidation
potential was observed and recorded. All electrochemical shifts were
standardized to the ferrocene redox couple at 0.474 V.
Preparation of Solar Cell Devices. Glass substrates coated with
ITO were purchased from Luminescence Technology Corp. (Taiwan)
and were patterned using standard photolithography. The substrates
were cleaned by sonication for 20 min in acetone, methanol, toluene,
and isopropyl alcohol. The substrates were subjected to UV/ozone
treatment for 20 min prior to use. After the ozone treatment, poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) PEDOT−PSS was
spin-coated on the substrates (4000 rpm, 1740 rpm/s, 90 s). The
substrates were annealed at 120 °C for 10 min under a nitrogen
atmosphere. Polymer/PCBM blends were prepared in chloroform
with a total blend concentration of 12 mg/mL. For the solar cells with
1,8-diiodooctane (DIO) additive the blends were prepared by adding
DIO dissolved in chloroform to a polymer/PCBM blend such that the
total concentration was 12 mg/mL. These blends were spin-coated
(2000 rpm, 60 s) on the PEDOT−PSS treated substrate. Films of 10
nm Ca and 100 nm Al were thermally evaporated on the substrates at
a rate of 2.5 Å/s through a shadow mask to obtain the solar cell
devices.
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 the AFM
measurements were taken of the active area of solar cell devices and
were conducted under ambient conditions. All cantilever oscillation
amplitude was equal to ca. 375 mV, and all images were acquired at 2
Hz scan frequency. Sample scan area was 5 μm.
Field-Effect Mobility Measurements. of the synthesized
polymers were performed on thin-film transistors with a common
bottom-gate, bottom-contact configuration. Highly doped, n-type
silicon wafers with a resistivity of 0.001−0.003 Ω cm were used as
substrates. 200 nm of thermal oxide (SiO2) was grown at 1000 °C.
Chromium metal (5 nm) followed by 100 nm of gold was deposited
by E-beam evaporation as source-drain metals. The source-drain pads
were formed by photolithographically patterning the metal layer. The
SiO2 on the backside of the wafer was etched with buffered oxide
etchant (7:1 BOE from J.T. Baker) to generate the common bottom
gate. The resulting transistors had a channel width of 475 μm and
channel length ranging from 2 to 80 μm. The measured capacitance
density of the SiO2 dielectric was 17 nF/cm2. After the SiO2 on the
backside was removed, the devices were cleaned under UV-ozone for 6
min using a Technics Series 85 RIE etcher and stored under vacuum.
This process removes any residual organics on the substrate. Prior to
the polymer deposition, the devices were treated with n-octyltri-
chlorosilane (OTS). For the surface treatment, the devices were rinsed
sequentially with deionized water, acetone, hexanes, and chloroform
and placed in a glass container in a solution of 8 × 10−3 M n-
octyltrichlorosilane silane in dried toluene. The sealed container was
placed in a glovebox at ambient temperature for 48 h. After 48 h, the
devices were taken out of the glovebox and rinsed with freshly distilled
toluene before baking them at 80 °C for 30 min in a vacuum oven.
The devices were allowed to cool under vacuum. The polymer films
were deposited in air by drop-casting 4−5 drops of 1 mg/mL of
polymer solution (in chloroform) filtered through a 0.2 μm PTFE
filter using a 25 μL syringe. The devices were allowed to dry in a Petri
dish saturated with chloroform. The devices were annealed under
vacuum for 30 min at 120 °C prior to measurements. The devices were
allowed to cool down to room temperature under vacuum after
annealing. A Keithley 4200-SCS semiconductor characterization
system was used to probe the devices. The probe station used for
electrical characterization was a Cascade Microtech Model Summit
Microchamber. When measuring current−voltage curves and transfer
curves, VGS was scanned from +20 to −100 V. All the measurements
were performed at room temperature in air.
IV testing was carried out under a controlled nitrogen atmosphere
using a Keithley 236, model 9160 interfaced with LabView software.
The solar simulator used was a 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.
Tapping Mode Atomic Force Microscopy (TMAFM). was
carried out on the active area of the solar cell devices. AFM studies
were carried out on a VEECO-dimension 5000 scanning probe
microscope with a hybrid xyz head equipped with Nano-Scope
RESULTS AND DISCUSSION
■
In this paper we describe the synthesis of a new
benzodithiophene building block containing 3,3′,5-trihexylbi-
thienyl substituents. This building block increases the
conjugation along the side chain to ensure a broader absorption
in the visible region and enhances the solubility of the
synthesized polymers due to the attachment of six alkyl
substituents per benzodithiophene repeating unit. We are also
C
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Scheme 2. Synthesis of the Donor−Acceptor Polymers
Table 1. Molecular Weight as Well as Optical and Electronic Energy Levels of Polymers
a
b
c
d
e
f
g
polymer
Mn (g/mol)
PDI
4.4
λmax (nm)
λmax (nm)
HOMO (eV)
LUMO (eV)
Eg (eV)
Eg (eV)
P1
P2
40 800
29 000
348, 589
366, 563
354, 593, 644
−5.53
−5.52
−3.57
−3.48
1.97
2.04
1.69
1.70
2.5
b
371, 561, 608
a
c
d
Determined by SEC (THF eluent). Absorption of chloroform solution. Absorption of films. Estimated from the onset of oxidation wave of cyclic
voltammogram. Estimated from the onset of reduction wave of cyclic voltammograms. Electrochemical band gap calculated from cyclic
voltammograms. Optical band gap calculated from the onset of the UV−vis spectra of the films.
e
f
g
Figure 1. UV−vis absorption spectra of (a) polymer P1 and (b) polymer P2: red, film; blue, solution (chloroform solvent).
reporting here the synthesis and photovoltaic properties of two
donor−acceptor polymers containing benzodithiophene with
3,3′,5-trihexylbithienyl substituents donor. Benzo[c][1,2,5]-
thiadiazole and 5-hexylthieno[3,4-c]pyrrole-4,6-dione were
used as acceptor building blocks for the synthesis of donor−
acceptor polymers. These strong acceptor units with the
conjugated bithienyl substituents on the BDT moiety were
expected to generate a donor−acceptor polymer with low-lying
HOMO energy level, which in turn should give relatively high
values for the Voc in BHJ solar cell devices.
3,3′,5-Trihexyl-2,2′-bithiophene precursor (2, Scheme 1) was
synthesized by Kumada cross-coupling of 2-bromo-3-hexylth-
iophene (1, Scheme 1) and 2-magnesiobromo-3-hexylthio-
phene followed by lithiation and reaction with 1-bromohex-
ane.19,21 The reaction of 4,8-dihydrobenzo[1,2-b:4,5-b′]-
dithiophen-4,8-dione with lithiated 3,3′,5-trihexyl-2,2′-
bithiophene followed by the aromatization using tin chloride
generated the benzodithiophene precursor which was con-
verted to its distannylated derivative (3, Scheme 1).
The donor−acceptor polymers were synthesized by the Stille
coupling polymerization of distannylated monomer 3 with 4,7-
dibromobenzo[c][1,2,5]thiadiazole (4, Scheme 2) and 1,3-
dibromo-5-hexylthieno[3,4-c]pyrrole-4,6-dione (5, Scheme 2)
to generate polymers P1 and P2, respectively (Scheme 2).
Polymer P1 had a molecular weight of 40 800 g/mol, while
polymer P2 had a molecular weight of 29 000 g/mol (Table 1).
Both synthesized polymers had good solubility in common
organic solvents such as chloroform, toluene, and THF.
The UV−vis absorption spectra of polymers P1 and P2 in
chloroform and film are shown in Figure 1. Both polymers
display similar absorption spectra. The UV−vis spectra of both
polymers P1 and P2 display two absorption maxima at ∼350
and ∼560 nm and a shoulder peak at ∼600 nm. The absorption
maxima at ∼350 nm is due to the 3,3′,5-trihexyl-2,2′-bithienyl
D
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Figure 2. I−V curve for the best bulk heterojunction solar cell device of polymer: (a) P1 (P1:PCBM = 1:3); (b) P2 (P2:PCBM = 1:4).
Table 2. Photovoltaic Properties of Polymers P1 and P2
a
b
polymer
[P]:PCBM weight ratio
Voc (V)
Isc (mA/cm2)
FF
PCE (%)
thickness (nm)
P1
1:1
1:2
1:3
1:4
1:1
1:2
1:3
1:4
1.04
0.98
0.99
0.97
0.93
0.99
0.89
0.91
2.00
4.96
5.36
4.91
1.06
3.21
3.26
3.22
0.24
0.30
0.47
0.43
0.23
0.31
0.34
0.45
0.50 (0.42)
1.46 (1.24)
2.54 (2.38)
2.08 (1.89)
0.23 (0.19)
1.00 (0.88)
1.00 (0.90)
1.33 (1.18)
59.5
79.5
85.8
77.4
80.2
65.1
69.8
83.9
P2
a
The data outside of the parentheses represent the maximum measured PCE values, and the data inside the parentheses represent the average
b
measured PCE values. Thickness of the polymer/PCBM active layer blend.
substituents while the absorption band in the visible region is
due to the conjugation along the polymer backbone. A
significant feature of the UV/vis spectra of the films vs the
chloroform solutions is the broadening of the absorption bands
and the increase in the intensity of the shoulder peaks. A
relatively low red-shift was observed for the polymer films as
compared to the solution (Table 1), indicating that the
polymers have a rigid-rod confirmation in both solution and
solid state.22,23 The estimated absorption coefficients of
polymers P1 and P2 are 3.84 × 104 and 3.80 × 104 cm−1,
respectively (Supporting Information).
The HOMO and LUMO energy levels of polymers P1 and
P2 were estimated by cyclic voltammetry from the onset of the
oxidation and the reduction curves, respectively. The HOMO
and the LUMO energy levels of the polymer P1 were
determined as −5.53 and −3.57 eV and that of the polymer
P2 as −5.52 and −3.48 eV, respectively (Table 1). Both
polymers P1 and P2 have a band gap of ∼2.0 eV. The HOMO
energy levels of polymers P1 and P2 are lower than the air
oxidation threshold (ca. −5.27 eV), indicating good air
stability.24,25 Moreover, due to the lower HOMO energy
level of these polymers, a higher Voc for BHJ could be
potentially obtained.18
decomposition temperature for both polymers is around 400
°C, which indicates a good thermal stability.
The photovoltaic properties of polymers P1 and P2 were
investigated in BHJ solar cells with [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) acceptor. The solar cells were
fabricated using a conventional device structure ITO/
PEDOT:PSS/polymer:PCBM/Ca/Al and tested under
AM1.5G illumination. The polymer/PCBM blends were
prepared in chloroform maintaining a total blend concentration
of 12 mg/mL. To determine the optimal blending ratio, the
polymers were tested in BHJ solar cell at [P]:[PCBM] weight
ratios ranging from 1:1 to 1:4 (Table 2). Upon the increase of
polymer/PCBM weight ratio from 1:1 to 1:3 the PCE of the
polymer P1 increased from 0.59% to 2.54%, and the short-
circuit current density (Jsc) increased from 2.00 to 5.36 mA/
cm2. A maximum PCE of 2.54% was measured for polymer P1
for a weight ratio [P1]:[PCBM] = 1:3. The measured Voc
varied between 0.97 and 1.04 V, which represent a relatively
good agreement with the theoretically predicted values
calculated from the HOMO of the donor polymer and
LUMO of PCBM acceptor.18 The polymer P1 gave exception-
ally large Voc values which is an important factor in achieving
large PCEs since the PCE is given by the product between Voc,
Jsc, and fill factor (FF).
The thermal stability of the polymers P1 and P2 were
investigated using thermogravimetric analysis (TGA) (thermo-
grams shown in Supporting Information). The onset of
A maximum PCE of 1.33% was measured for polymer P2 at a
weight ratio of 1:4. Similar to polymer P1, the Voc values ranged
from 0.89 to 0.99 V. While the Voc values obtained for polymer
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P2 are comparable to P1, the Jsc values are almost twice lower.
The larger decrease of Jsc observed for polymer P2 is most
likely due to the unfavorable morphology of the active layer
blend. Moreover, the higher PCEs measured for polymer P1
are most probably due to its higher molecular weight as
compared to polymer P2 (Table 1).
To attempt further optimization of the photovoltaic
performance of the polymers P1 and P2, 1,8-diiodooctane
(DIO) was used as an additive as previously reported by other
research groups.26−28 It has been shown that the high boiling
point DIO does not evaporate completely during spin-coating,
and it can interact with either the donor polymer or the PCBM
acceptor phase in the blend.27 We used the DIO additive for
the weight ratios that gave the best performing BHJ solar cells,
i.e., [P1]:[PCBM] = 1:3 and [P2]:[PCBM] = 1:4 (Table 3). In
3).12,20,22 The decrease of PCE upon addition of DIO could be
due to the preferential interaction of DIO with the insulating
alkyl substituents of the benzodithiophene repeat unit which
affects the morphology of the blends. Moreover, it has been
shown that the effect of DIO on the solar cell performance is
dependent on the solubility of the polymer and also on the
chain stacking ability of the donor polymer.23 These factors
may also contribute to a decrease of PCE.
The surface morphology of the active layer of BHJ solar cells
was investigated by tapping mode atomic force microscopy
(TMAFM). TMAFM analysis of the devices with the highest
PCEs was preformed. The 3D-TMAFM images of P1/PCBM
and P2/PCBM blends are shown in Figure 3 and display a
smoother surface morphology for P1/PCBM blend (rms =
0.368 nm) as compared to the P2/PCBM blend (rms = 0.855
nm). This result is consistent with our previous report in which
we observed that smoother blend films displayed better
performance in bulk heterojunction solar cells.11,21 TMAFM
analysis was also performed for the solar cell devices of both P1
and P2 with 1% DIO additive (Supporting Information and
Figure 3). A comparison of the TMAFM images of P1/PCBM
blend with 0% and 1% DIO indicates that the film obtained
from 1% DIO has a more rougher surface morphology (rms =
0.432 nm) as compared to those obtained without DIO (rms =
0.368 nm), thereby reducing the PCEs of the devices. The same
trend was observed for P2/PCBM blends where the film
obtained from 1% DIO had a rougher morphology (rms = 2.19
nm) as compared to blend without DIO (rms = 0.855 nm).
The field-effect mobilities of the polymers P1 and P2 were
measured in bottom-gate bottom-contact field-effect transistors
(OFET).5,29 The IDS−VDS curve family was recorded at
different gate voltages. The field-effect mobility (μ) was
Table 3. Photovoltaic Properties of Polymers P1 and P2 with
1,8-Diiodooctane Additive
c
e
DIO
Voc
Isc
thickness
(nm)
d
polymer
(%)
(V) (mA/cm2)
FF
PCE (%)
a
P1
0
1
3
5
0
1
3
5
1.00
0.98
0.78
0.79
0.91
0.85
0.92
0.55
5.36
5.32
4.88
4.78
3.22
2.85
2.48
3.90
0.47
0.41
0.31
0.34
0.45
0.35
0.40
0.27
2.54 (2.38)
2.17 (1.65)
1.21 (1.15)
1.28 (1.03)
1.33 (1.18)
0.86 (0.77)
0.91 (0.80)
85.8
67.5
69.5
91.3
83.9
86.9
80.9
b
P2
0.58 (0.43)
67.2
a
b
c
d
[P1]:[PCBM] = 1:3. [P2]:[PCBM] = 1:4. Weight % DIO. The
data in the parentheses represent the average measured PCE values.
e
Thickness of the polymer/PCBM blend.
1/2
obtained by plotting IDS vs VGS in the saturation regime
contrast to previous reports, the PCE of both polymers P1 and
P2 decreased upon the addition of DIO additive (Table
(Figure 4), using the equation
Figure 3. 3D TMAFM phase images of solar cell devices: (a) polymer P1/PCBM (1:3 w/w) active layer without additives; (b) polymer P1/PCBM
(1:3 w/w) active layer with 1,8-diiodooctane; (c) polymer P2/PCBM (1:4 w/w) active layer without additives; (d) polymer P2/PCBM (1:4 w/w)
active layer with 1,8-diiodooctane; scan size 5 × 5 μm (spin-cast from chloroform).
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Figure 4. Current−voltage characteristic of polymer P1: (a) output curves at different gate voltages; (b) transfer curves at VDS = −50 V (μ = 7.3 ×
10−4 cm2/(V s), VT = 2.1 V, on/off ratio = 102, W = 475 μm, L = 20 μm). Current−voltage characteristic of polymer P2: (c) output curves at
different gate voltages; (d) transfer curves at VDS = −50 V (μ = 9.5 × 10−5 cm2/(V s), VT = −6.5 V, on/off ratio = 10, W = 475 μm, L = 20 μm).
⎡
⎢
⎣
⎤
⎥
⎦
tuning to generate highly soluble semiconducting polymers for
use in solar cells.
IDS
2L
WC
μ =
(VGS − VT)2
i
ASSOCIATED CONTENT
■
where IDS is the source-drain current, W is the channel width, L
is the channel length, Ci is the capacitance of the dielectric, VGS
is the gate voltage, and VT is the threshold voltage. The
polymer P1 had mobility of 7.3 × 10−4 cm2/(V s), and the
polymer P2 had a mobility of 9.5 × 10−4 cm2/(V s).
S
* Supporting Information
1H and 13C NMR spectra of monomers and polymers, cyclic
voltammograms of polymers, thermograms of polymers,
TMAFM images of polymer blends. This material is available
free of charge via the Internet at http://pubs.acs.org.
CONCLUSION
■
AUTHOR INFORMATION
Corresponding Author
*E-mail mci071000@utdallas.edu.
Notes
■
A novel benzodithiophene monomer with bithienyl substitu-
ents has been synthesized. Stille coupling of the benzodithi-
phene monomer with 4,7-dibromobenzo[c][1,2,5]thiadiazole
and 1,3-dibromo-5-hexylthieno[3,4-c]pyrrole-4,6-dione gener-
ated two donor−acceptor polymers P1 and P2, respectively. Six
alkyl substituents were attached per each benzodithiophene
repeating unit generating relatively higher molecular weight
polymers with good solubility in organic solvents. Both
polymers showed relatively good photovoltaic properties: P1
with a PCE of 2.54% and P2 with a PCE of 1.33%. In addition,
the lower HOMO energy levels of polymers P1 and P2 resulted
in high Voc values up to 1.04 V. The surface morphology of the
polymer/PCBM blends in solar cell devices suggested that
smoother films gave a higher PCE. While the photovoltaic
performance of the reported polymers is modest, the novel
benzodithiophene with 3,3′,5-trihexylbithienyl substituents
building block is highly versatile, and it allows further structural
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support for this project from NSF (Career DMR-
0956116) and Welch Foundation (AT-1740) is gratefully
acknowledged. We gratefully acknowledge the NSF-MRI grant
(CHE-1126177) used to purchase the Bruker Advance III 500
NMR instrument.
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