Structural variation of donor-acceptor copolymers containing benzodithiophene with bithienyl substituents to achieve high open circuit voltage in bulk heterojunction solar cells

Article abstract of DOI:10.1039/c3ta13686h

Three new donor-acceptor copolymers P1, P2, and P3 were synthesized with benzodithiophene with bithienyl substituents as the donor and 5,6-difluorobenzo[c][1,2,5]thiadiazole, 4,7-di(thiophen-2-yl)benzo[c][1,2,5] thiadiazole, and 5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole as the acceptors, respectively. The insertion of thiophene spacer between the donor and the acceptor broadened the absorption of the polymers P2 and P3 and resulted in a red shift of ～30 nm as compared to that of the polymer P1. However, the inclusion of fluorine atoms on the polymer had detrimental effects on the photovoltaic properties of the polymers. The synthesized donor-acceptor polymers were tested in bulk heterojunction (BHJ) solar cells with [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) acceptor. Polymer P2 gave a PCE of 3.52% with PC₇₁BM in which the active layer was prepared in chloroform with 3% v/v 1,8-diiodooctane (DIO) additive. The effect of fluorine substitution and thiophene group insertion on the UV/Vis absorbance, photovoltaic performances, morphology, and charge carrier mobilities for the polymers are discussed.

Full text of DOI:10.1039/c3ta13686h

Journal of
Materials Chemistry A
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PAPER
Structural variation of donor–acceptor copolymers
containing benzodithiophene with bithienyl
substituents to achieve high open circuit voltage in bulk
heterojunction solar cells†
Cite this: J. Mater. Chem. A, 2013, 1,
15535
Ruvini S. Kularatne,a Ferdinand J. Taenzler,a Harsha D. Magurudeniya,a Jia Du,a
b
c
b
a
*
John W. Murphy, Elena E. Sheina, Bruce E. Gnade, Michael C. Biewer
a
*
and Mihaela C. Stefan
Three new donor–acceptor copolymers P1, P2, and P3 were synthesized with benzodithiophene with
bithienyl substituents as the donor and 5,6-difluorobenzo[c][1,2,5]thiadiazole, 4,7-di(thiophen-2-yl)-
benzo[c][1,2,5]thiadiazole, and 5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole as the
acceptors, respectively. The insertion of thiophene spacer between the donor and the acceptor
broadened the absorption of the polymers P2 and P3 and resulted in a red shift of ꢀ30 nm as
compared to that of the polymer P1. However, the inclusion of fluorine atoms on the polymer had
detrimental effects on the photovoltaic properties of the polymers. The synthesized donor–acceptor
polymers were tested in bulk heterojunction (BHJ) solar cells with [6,6]-phenyl C71 butyric acid methyl
ester (PC71BM) acceptor. Polymer P2 gave a PCE of 3.52% with PC71BM in which the active layer was
prepared in chloroform with 3% v/v 1,8-diiodooctane (DIO) additive. The effect of fluorine substitution
and thiophene group insertion on the UV/Vis absorbance, photovoltaic performances, morphology, and
charge carrier mobilities for the polymers are discussed.
Received 13th September 2013
Accepted 5th November 2013
DOI: 10.1039/c3ta13686h
www.rsc.org/MaterialsA
(BDT) donor–acceptor copolymers are in the forefront of the
eld of organic solar cells due to several advantages, such as a
1. Introduction
The low cost, light weight, exibility, and easy processability on
large area substrates make polymer solar cells promising
candidates for renewable energy resources. Power conversion
efficiencies (PCE) as high as 9.2% have been reported.1,2
Extensive research has been performed in the past decade to
improve the PCE of polymer solar cells in bulk heterojunction
(BHJ) device architecture.3–7 The active layer which is a blend of
donor semiconductor polymers and fullerene acceptor of a bulk
heterojunction solar cell (BHJ) plays a pivotal role. The short
planar backbone and exibility of attaching different substitu-
ents on the central benzene ring to ne tune the energy levels of
the polymer.4,8,9 Alkyl, alkoxy, thioalkyl, phenylethynyl, and
thienyl substituted BDT have been synthesized by various
research groups and PCEs as high as 7% have been achieved.8–17
Previously, our research group reported the synthesis of a
bis(bithienyl)-substituted BDT as a donor for organic solar cells
which gave a PCE of 2.52% with benzo[c][1,2,5]thiadiazole as
the acceptor unit (Table S5 and Fig. S29 in ESI†).18 The attach-
ment of six alkyl substituents per BDT repeat unit was highly
benecial due to several advantages: (1) highly soluble polymers
with very high molecular weights could be synthesized, (2) the
insertion of alkyl chains on the acceptor units was deemed
unnecessary, due to the solubilizing effect of the six alkyl
substituents attached to BDT unit, and (3) the conjugated
bithienyl substituents perpendicular to the polymer backbone
provided a broader absorption in the visible region.
circuit current density (JSC) and the open circuit voltage (VOC
)
depend on the electronic properties of the active layer. A deeper
highest occupied molecular orbital (HOMO) and a strong
absorption overlapping the solar spectrum are desirable prop-
erties of donor semiconducting polymers which are expected to
provide high VOC and JSC, respectively. The benzodithiophene
aDepartment of Chemistry, University of Texas at Dallas, 800 West Campbell Road,
Richardson, TX 75080, USA. E-mail: mihaela@utdallas.edu; biewerm@utdallas.edu
bDepartment of Materials Science and Engineering, University of Texas at Dallas, 800
West Campbell Road, Richardson, Texas 75080, USA
Benzo[c][1,2,5]thiadiazole has been widely used as a strong
electron withdrawing acceptor moiety in D–A copolymers.19–21
Recently, several groups have reported the insertion of thio-
phene spacer group in between the donor and acceptor moieties
for efficient device performances in BHJ solar cells.22–24 The
insertion of the thiophene spacer groups relieved the steric
cPlextronics. Inc, 2180 WilliamPitt Way, Pittsburgh, PA 15238, USA
† Electronic supplementary information (ESI) available: NMR Spectra, UV-vis
Spectra, Cyclic Voltammograms, TGA thermograms, and 2D TMAFM phase
images are included here. See DOI: 10.1039/c3ta13686h
This journal is ª The Royal Society of Chemistry 2013
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hindrance between the acceptor and the donor units and hence running conditions for SEC analysis were as follows: ow
the polymers had a planar polymer backbone.19 This enhanced rate ¼ 1.0 mL minꢂ1, injector volume ¼ 200 mL, detector
the conjugation along the polymer backbone, reduced the band temperature ¼ 80 ꢁC, column temperature ¼ 80 ꢁC. All the
gap, and improved the solid state packing and hence increased polymer samples were dissolved in chlorobenzene and the
the JSC of the BHJ devices. The VOC of a BHJ solar cell device is solutions were ltered through PTFE lters (0.45 mm) prior to
largely determined by the HOMO level of the donor–acceptor injection.
(D–A) copolymer as the VOC is proportional to the energy
difference between the HOMO level of the D–A copolymer and
the LUMO level of the acceptor fullerene.25 It has been
2.2. Optoelectronic properties of the synthesized polymers
demonstrated that the use of uorine atom on the acceptor in
UV-Vis measurements. The UV-vis spectra of polymer solu-
D–A copolymers decreased the HOMO level and hence tions in chloroform solvent were carried out in 1 cm cuvettes
increased the VOC of the BHJ solar cell devices.26–29 In addition, using an Agilent 8453 UV-vis spectrometer. Thin lms of poly-
the smaller size of the uorine atom as compared to other mer were obtained by evaporation of chloroform from polymer
electron withdrawing groups such as triuoromethyl groups solutions on glass microscope slides. In order to measure
and cyano groups has not imparted too much steric hindrance absorption coefficient of the polymers, 5 mg mLꢂ1 polymer
on the polymer backbone.15
solutions were spin-coated at 600 rpm, 800 rpm and at 1000 rpm
Here we report the synthesis of three donor–acceptor poly- on pre-cleaned glass slides and the absorption were measured
mers with bis(bithienyl)-substituted BDT as the donor unit and at the absorption maxima of each polymer. The lm thickness
5,6-diuorobenzo[c][1,2,5]thiadiazole,
4,7-di(thiophen-2-yl)- for each glass slide was measured using a Veeco Dektak VIII
benzo[c][1,2,5]thiadiazole, and 5,6-diuoro-4,7-di(thiophen- prolometer.
2-yl)benzo[c][1,2,5]thiadiazole acceptors by Stille coupling
Cyclic voltammetry. CV was obtained with a BAS CV-50 W
polymerization. The effect of uorine atom substitution and Voltammetric Analyzer (Bioanalytical Systems, Inc.). Electro-
thiophene insertion on the photovoltaic properties of the D–A chemical grade tetrabutylammonium perchlorate (TBAP) was
copolymers has been investigated.
used without further purication. Acetonitrile was distilled over
calcium hydride and collected over molecular sieves. The elec-
trochemical 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
2. Experimental section
2.1. Structural analysis
NMR measurements. 1H and 13C NMR spectra were recorded placed in a cell and purged with argon. A drop of the polymer
using Bruker Avance 500 MHz spectrometer at 25 C or other- solution was evaporated in ambient air. The lm was immersed
ꢁ
wise at indicated temperatures. The spectra were recorded in into the electrochemical cell containing the electrolyte and the
CDCl3 or in toluene-d8. The data are reported as follows: oxidation potential was observed and recorded. All electro-
chemical shis are reported in ppm on the d scale relative to chemical shis were standardized to the ferrocene redox couple
trimethylsilane (TMS) as the internal standard, multiplicity at 0.474 V.
(br ¼ broad, s ¼ singlet, d ¼ doublet, t ¼ triplet, q ¼ quartet,
Preparation of solar cell devices. Glass substrates coated
with ITO were purchased from Luminescence Technology Corp.
m ¼ multiplet).
GC–MS measurements. GC–MS was obtained on an Agilent (Taiwan) and were patterned using standard photolithography.
6890-5973 GC/MS workstation. The GC column was a Hewlett- The substrates were cleaned by sonication for 20 minutes in
Packard fused silica capillary column cross-linked with 5% acetone, methanol, toluene, and isopropyl alcohol. The
phenylmethylsiloxane. Helium was the carrier gas (1 mL substrates were subjected to UV/ozone treatment for 20 minutes
minꢂ1). The following conditions were used for all GC/MS prior to use. Aer the ozone treatment, poly(3,4-ethyl-
analysis: injector and detector temperature, 250 ꢁC; initial enedioxythiophene):poly(styrenesulfonate) PEDOT:PSS was
temperature, 70 ꢁꢁC; temperature ramp, 10 ꢁC minꢂ1; nal spin coated on the substrates (4000 rpm, 1740 rpm sꢂ1, 90 s).
temperature, 280 C.
The substrates were annealed at 150 ꢁC for 10 minutes under a
Solid–liquid chromatography. Analytical thin layer chroma- nitrogen atmosphere. Polymer/PCBM blends were prepared in
tography was performed on EM reagents 0.25 mm silica gel 60-F chloroform with a total blend concentration of 15 mg mLꢂ1. For
plates. Column chromatography was performed using the the solar cells with 1,8-diiodooctane (DIO) additive, the blends
indicated solvent system on select silica gel (SiO2) 230–400 were prepared by adding DIO dissolved in chloroform to a
mesh.
polymer/PCBM blend such that the total concentration was 15
Size exclusion chromatography. The average molecular mg mLꢂ1. These blends were spin coated (2000 rpm, 1740 rpm
weights of the synthesized polymers were measured by size sꢂ1, 60 s) on to the PEDOT:PSS treated substrate. Films of 20 nm
exclusion chromatography (SEC) analysis on a Varian PL-GPC Ca and 100 nm Al were theꢂrm1 ally evaporated on to the
˚
220 system equipped with Varian PL-Gel Mixed D column substrates at a rate of 2.75 A s through a shadow mask to
(300 mm ꢃ 7.5 mm, 5 mm) and Varian PL-Gel guard column obtain the solar cell devices.
(50 mm ꢃ 7.5 mm, 5 mm). A GPC solvent/sample module was
IV testing was carried out under a controlled nitrogen
used with chlorobenzene with 0.0125% BHT as the eluent and atmosphere using a Keithley 236, model 9160 interfaced with
the calibrations were based on polystyrene standards. The LabView soware. The solar simulator used was
a
15536 | J. Mater. Chem. A, 2013, 1, 15535–15543
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Journal of Materials Chemistry A
THERMOORIEL equipped with a 300 W Xenon lamp; the
intensity of the light was calibrated to 100 mW cmꢂ2 with a
NREL certied Hamamatsu silicon photodiode. The active area
of the devices was 0.1 cm2. The active layer lm thickness was
measured using a Veeco Dektak VIII prolometer.
Synthesis of 5,6-diuorobenzo[c][1,2,5]thiadiazole (2). 4,5-
Diuorobenzene-1,2-diamine (3.30 g, 0.0229 mol) and triethyl-
amine (12.87 mL, 0.0916 mol) were dissolved in chloroform
(300 mL) under a nitrogen atmosphere and the reaction mixture
was stirred until all the 4,5-diuorobenzene-1,2-diamine was
dissolved. Thionyl chloride (SO2Cl) (5.94 mL, 0.0503 mol) was
added drop-wise and the reaction mixture was reuxed for 3
days. The reaction mixture was cooled to room temperature and
the organic phase was extracted with methylene chloride (3 ꢃ
100 mL), washed with water (3 ꢃ 100 mL), and dried over
anhydrous MgSO4. Evaporation of the solvent generated 3.03 g
Space charge limited current measurements. ITO patterned
glass substrates were cleaned by sonication for 20 minutes in
acetone, methanol, toluene, and isopropyl alcohol. The
substrates were subjected to UV/ozone treament for 20 minutes
prior to use. Aer the ozone treatment, poly(3,4-ethyl-
enedioxythiophene):poly(styrenesulfonate) PEDOT:PSS was
spin coated on the substrates (4000 rpm, 1740 rpm sꢂ1, 90 s).
The substrates were annealed at 150 ꢁC for 10 minutes under a
nitrogen atmosphere. Polymer solutions (15 mg mLꢂ1 concen-
tration) were prepared in chloroform and were spin coated
(2000 rpm, 1740 rpm sꢂ1, 60 s) on to PEDOT:PSS treated
substrate. Aluminum (100 nm thickness) was thermally evapo-
1
of product as brown needle-like crystals (0.0176 mol, 77%). H
NMR (500 MHz, CDCl3): dH 7.75 (t, 2H).
Synthesis of 5,6-diuoro-4,7-bis(trimethylsilyl)benzo[c]-
[1,2,5]thiadiazole (3). Freshly distilled diisopropylamine
(6.06 mL, 0.0424 mol) was diluted with THF (40 mL) under a
nitrogen atmosphere and at ꢂ78 ꢁC and 2.5 M n-BuLi in
hexane (16.4 mL, 0.0409 mol) was added drop-wise to the
reaction mixture. The reaction mixture was stirred for 2 h at
ꢂ78 ꢁC at which time the synthesized lithium diisopropyl-
amine (LDA) was cannulated to a three neck ask containing
the 5,6-diuorobenzo[c][1,2,5]thiadiazole (3.2 g, 0.0186 mol)
in THF (80 mL) at ꢂ78 ꢁC. Aer the addition, the reaction was
rated on the substrates at a rate of 2.75 A sꢂ1 through a shadow
˚
mask to obtain the solar cell devices.
IV testing was carried out under a controlled nitrogen
atmosphere using a Keithley 236, model 9160 interfaced with
LabView soware. The active area of the devices was 0.1 cm2.
The active layer lm thickness was measured using a Veeco
Dektak VIII prolometer.
ꢁ
stirred for 30 min at ꢂ78 C, followed by the drop-wise addi-
Tapping Mode Atomic Force Microscopy (TMAFM). Tapping
Mode Atomic Force Microscopy (TMAFM) was carried out on the
active area of the solar cell devices and on the area in between the
channels of the OFET devices. AFM studies were carried out on a
VEECO-dimension 5000 scanning probe microscope with a hybrid
xyz head equipped with Nano-Scope Soware. AFM images were
obtained using silicon cantilevers with nominal spring constant
of 42 N mꢂ1 and nominal resonance frequency of 300 kHz
(OTESPa). Image analysis soware 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 con-
ducted 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 mm.
tion of trimethylsilylchloride (5.58 mL, 0.0436 mol). The
reaction mixture was stirred at ꢂ78 ꢁC for 3 h. A saturated
solution of NH4Cl (10 mL) was added to the reaction mixture
which was allowed to warm up to room temperature for 1 h.
An additional portion of saturated NH4Cl was added and the
organic phase was extracted with chloroform (3 ꢃ 100 mL),
washed with water (5 ꢃ 100 mL), and dried over anhydrous
MgSO4. The solvent was removed in vacuum to obtain a brown
oil, which was further puried by column chromatography
using hexane as the eluent to obtain 1.76 g of colorless oil
(0.00558 mol, 30%). 1H NMR (500 MHz, CDCl3): dH 0.52 (s,
18H). 13C NMR (125 MHz, CDCl3): dC 0.07, 117.0 (t), 154.6 (t),
(158.0, 156.0 (dd)).
Synthesis of 4,7-dibromo-5,6-diuorobenzo[c][1,2,5]thiadia-
zole (4). 5,6-Diuoro-4,7-bis(trimethylsilyl)benzo[c][1,2,5]thia-
diazole (2.0 g, 0.00633 mol) was added to a three neck ask and
it was diluted with chloroform (30 mL). When the 5,6-diuoro-
4,7-bis(trimethylsilyl)benzo[c][1,2,5]thiadiazole was dissolved
completely, bromine (19.68 mL, 0.0127 mol) was added to the
reaction mixture and the reaction was stirred for 2 days at 60 ꢁC
in dark. The reaction mixture was then cooled to room
temperature and it was quenched in 4 M NaOH solution. The
organic phase was extracted with chloroform and was washed
with brine (3 ꢃ 100 mL), water (3 ꢃ 100 mL), and dried over
anhydrous MgSO4. The solvent was evaporated to obtain a white
solid which was further puried by column chromatography
using hexanes as the eluent to obtain 0.627 g of white needle-
like crystals (0.001899 mol, 30%).13C NMR (125 MHz, CDCl3): dC
99.40 (t), 148.8 (t), (150.6, 152.8 (dd)).
2.3. Synthesis of the monomers and polymers
The monomers 1 to 6 were synthesized according to previously
reported procedures with slight modications in each step.30,31
Synthesis of 4,5-diuorobenzene-1,2-diamine (1). 4,5-
Diuoro-2-nitroaniline (5.00 g, 0.0288 mol) was dissolved in
concentrated HCl (150 mL). At 0 ꢁC tin (16.56 g, 0.140 mol) was
added in small portions over 15 minutes. The reaction mixture
was stirred at room temperature until all the tin was dissolved,
and was further stirred for another one hour aer which it was
refrigerated overnight. The reaction mixture was poured into
deionized water and a solution of NaOH (4 M) was added
gradually until the solution reached pH 10. The organic phase
was extracted with ethyl acetate (4 ꢃ 100 mL), washed with
water (4 ꢃ 100 mL), and dried over anhydrous MgSO4. The
solution was concentrated in vacuum to obtain 2.98 g of pink
crystals. (0.0207 mol, 72%). 1H NMR (500 MHz, CDCl3): dH 6.50
(t, 2H), 3.24 (s, 4H).
Synthesis of 5,6-diuoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-
thiadiazole (5). 4,7-Dibromo-5,6-diuorobenzo[c][1,2,5]thiadia-
zole (0.360 g, 0.00109 mol), 2-(tributylstannyl) thiophene
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Synthesis of poly{4-(4,8-bis(3,30,50-trihexyl-2,20-bithiophen-5-yl)-
benzo[1,2-b:4,5-b']dithiophen-2-yl)-alt-4,7-di(thiophen-2-yl)benzo-
[c][1,2,5]thiadiazole} (P2). Using a procedure similar to that
described above for the polymer P1, a mixture of 4,7-bis(5-bro-
mothiophen-2-yl)benzo[c][1,2,5]thiadiazole (103.2 mg, 2.18 ꢃ
10ꢂ4 mol),30 (4,8-bis(3,30,50-trihexyl-2,20-bithiophen-5-yl)benzo-
[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(trimethylstannane) (304
mg, 2.18 ꢃ 10ꢂ4 mol) and tetrakis(triphenylphosphine)palla-
dium (0) (7.50 mg, 7.266 ꢃ 10ꢂ6 mol) in dry toluene was poly-
(0.720 mL, 0.00226 mol) and tetrakis(triphenylphosphine)-
palladium(0) (43.5 mg, 4.35 ꢃ 10ꢂ5 mol) were added to a three
neck ask under a nitrogen atmosphere and was dissolved in a
mixture of toluene (20 mL) and DMF (0.5 mL). The reaction
mixture was reuxed for 12 h at which time a sample was taken
to determine the monomer conversion. When there was no
starting material le in the reaction mixture the reaction was
cooled and potassium uoride (6 g) in water (10 mL) was added
to the reaction and it was stirred for 2 h. The reaction mixture
was quenched in ice cold water (100 mL) and the organic phase
was extracted with methylene chloride (3 ꢃ 100 mL). The
organic phase was washed with 10% HCl (3 ꢃ 100 mL), satu-
rated NaHCO3 (3 ꢃ 100 mL), and dried over anhydrous MgSO4.
The solvent was evaporated in vacuum to obtain a yellow solid,
which was further puried by column chromatography using
99 : 1 hexanes : ethyl acetate as the eluent to obtain 0.146 g of
bright yellow needle like crystals. (0.000436 mol, 40%). 1H NMR
1
merized to generate polymer P2. (100 mg, 35% yield). H NMR
(500 MHz, CDCl3): dH 0.92 (br, 18H), 1.53 (br, 56H), 1.81 (br,
24H) 2.70 (t, 8H), 2.90 (t, 4H), 6.79 (s, 2H), 7.53 (s, 2H), 7.70 (s,
2H), 7.86 (s, 2H).
Synthesis of poly{4-(4,8-bis(3,30,50-trihexyl-2,20-bithiophen-5-yl)-
benzo[1,2-b:4,5-b']dithiophen-2-yl)-alt-4,7-di(thiophen-2-yl)-5,6-
di(uoro)benzo[c][1,2,5]thiadiazole} (P3). Using
a procedure
similar to that described above for the polymer P1, a mixture of
4,7-bis(5-bromothiophen-2-yl)-5,6-diuorobenzo[c][1,2,5]thia-
diazole (122 mg, 2.48 ꢃ 10ꢂ4 mol), (4,8-bis(3,30,50-trihexyl-2,20-
bithiophen-5-yl)benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl)bis(trim-
ethylstannane) (335 mg, 2.48 ꢃ 10ꢂ4 mol) and tetrakis-
(triphenylphosphine)palladium(0) (8.39 mg, 8.266 ꢃ 10ꢂ6 mol)
in dry toluene was polymerized to generate polymer P3.
(500 MHz, CDCl3): dH 7.27 (t, 2H), 7.61(d, 2H), 8.29 (d, 2H). 13
C
NMR (125 MHz, CDCl3): dC 111.0 (t), 127.5, 129.0, 131.0, 131.6,
148.8, (149.0, 151.0 (dd)).
Synthesis of 4,7-bis(5-bromothiophen-2-yl)-5,6-diuorobenzo-
[c][1,2,5]thiadiazole (6). 5,6-Diuoro-4,7-di(thiophen-2-yl)benzo[c]-
[1,2,5]thiadiazole (0.180 g, 5.85 ꢃ 10-4 mol) was dissolved in a
1
ꢁ
mixture of chloroform (20 mL) and acetic acid (20 mL). At 0 C,
(100 mg, 30% yield). H NMR (500 MHz, CDCl3): dH 0.92 (br,
NBS (0.219 g, 0.00123 mol) was added slowly to the reaction
mixture. Aer 1 hour an orange color compound precipitated.
And the reaction was quenched in water (100 mL) and the organic
phase was extracted into chloroform (3 ꢃ 100 mL). The organic
phase was washed with water (3 ꢃ 100 mL) dried over anhydrous
MgSO4. The solvent was evaporated in vacuum to obtain 0.231 g of
orange crystals (4.66 ꢃ 10ꢂ4 mol, 80%). 1H NMR (500 MHz,
Toluene-d8): dH 6.83 (d, 2H), 7.78(d, 2H). 13C NMR (125 MHz,
Toluene-d8): dC 111.5 (t), 117.6, 130.5, 131.46, 134.0, 148.6,
(149.0, 150.1 (dd)).
18H), 1.53 (br, 56H), 1.81 (br, 24H) 2.70 (t, 8H), 2.90 (t, 4H), 6.70
(s, 2H), 7.70 (s, 2H), 7.91 (s, 2H), 8.20 (s, 2H).
3. Results and discussion
3.1. Synthesis
Three alternating donor–acceptor polymers have been synthe-
sized by Stille coupling polymerization using (4,8-bis(3,30,50-
trihexyl-2,20-bithiophen-5-yl)benzo[1,2-b:4,5-b0]dithiophene-2,6-
diyl)bis(trimethylstannane)as the donor monomer. Equimolar
quantities of (4,8-bis(3,30,50-trihexyl-2,20-bithiophen-5-yl)benzo-
[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(trimethylstannane) were
reacted with 4,7-dibromo-5,6-diuorobenzo[c][1,2,5]thiadia-
zole, 4,7-bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole,
and 4,7-bis(5-bromothiophen-2-yl)-5,6-diuorobenzo[c][1,2,5]-
thiadiazole (Scheme 1 and S1 in the ESI†) in the presence
of tetrakis(triphenylphosphene)palladium(0) catalyst. The
synthesis of polymers P1, P2 and P3 is shown in Scheme 2. The
General procedures for the synthesis of polymers P1, P2 and
P3 through Stille coupling polymerization
Synthesis of poly{4-(4,8-bis(3,30,50-trihexyl-2,20-bithiophen-5-yl)-
benzo[1,2-b:4,5-b']dithiophen-2-yl)-alt-5,6-diuorobenzo[c][1,2,5]-
thiadiazole} (P1). 4,7-Dibromo-5,6-diuorobenzo[c][1,2,5]thia-
diazole (98.2 mg, 2.964 ꢃ 10ꢂ4 mol), (4,8-bis(3,30,50-trihexyl-2,20-
bithiophen-5-yl)benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl)bis(tri-
methylstannane) (403 mg, 2.986 ꢃ 10ꢂ4 mol) and tetrakis-
(triphenylphosphine)palladium(0) (9.97 mg, 9.80 ꢃ 10ꢂ6 mol)
were added to a three neck ask under a nitrogen atmosphere.
The compounds were dissolved in a mixture of toluene (20 mL)
and DMF (0.5 mL) and were reuxed at 110 ꢁC for 12 hours,ꢁat
which time the reaction temperature was decreased to 100 C
and was diluted with toluene (40 mL). The polymerization was
stopped aer 24 h by precipitating the polymer in a mixture of
methanol (300 mL) and HCl (30 mL). The polymer was puried
by Soxhlet extractions with methanol, hexane, ether and chlo-
roform with successive drying between each extraction. A dark
blue polymer was obtained by the evaporation of the chloroform
extract. (0.120 g, 35% yield). 1H NMR (500 MHz, CDCl3): dH 0.88
(br, 6H), 0.99 (br, 12H), 1.26 (br, 56H), 1.43, (br (12H), 1.55
(br, 12H), 2.72 (t, 8H), 2.84 (t, 4H), 6.74 (s, 2H), 8.50 (s, 2H), 9.52
(s, 2H).
Scheme
1
Synthesis of 4,7-bis(5-bromothiophen-2-yl)-5,6-difluorobenzo[c]-
[1,2,5]thiadiazole
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show any red-shi in the absorption bands of the polymer thin
lms as compared to that of the solution. The absence of a red-
shi was attributed to the fact that the polymer P1 has a rigid
rod conformation both in solution and thin lms.33,34 The red-
shi in absorption maxima in thin lms compared to that of the
solution, for the polymer P2 and P3 indicated more structural
organization and ordered packing which enhanced the aggre-
gation of the polymer in thin lms compared to that of the
solution. The incorporation of thiophene spacer in between the
donor and the acceptor in the polymers P2 and P3, may result in
a less rigid and planar structure in solution due to the twisting
of the polymer chain and this may be prevented in solid state
resulting in ordered packing, enhanced conjugation and
p-delocalization hence a red shi in thin lms.35 The onset of
absorption for the polymer P1 was 725 nm, for polymer P2 was
737 nm and for the polymer P3 was 750 nm corresponding to
optical band gaps of 1.71 eV, 1.69 eV, and 1.65 eV, respectively.
The absorption coefficients of the polymers were determined
by spin coating polymer solutions on glass substrates at
different spin rates (1000 rpm, 800 rpm and 600 rpm) to obtain
different lm thicknesses. The absorption coefficients of these
polymer lms were calculated at the maximum absorption of
the polymers. Table S1 in ESI† summarizes the absorption
coefficients obtained for the polymers at different polymer lm
thicknesses. The absorbance coefficient for all the polymers
decreased with the increasing the thickness of the polymer lm.
Scheme 2 Synthesis of the donor–acceptor polymers P1, P2 and P3.
SEC analysis was used to determine the molecular weights of
the polymers. The estimated molecular weights had values
ranging from 20 000 g molꢂ1 to 28 000 g molꢂ1 (Table 1).
The synthesized polymers had relatively broad polydispersity
index (PDI) which are due to the non-living nature of the Stille
coupling polymerization. Polymers P1, P2, and P3 had good
solubility in common organic solvents such as chloroform,
toluene, and chlorobenzene. The polymers were characterized
1
1
by H NMR and the H NMR spectra of the reported polymers
are shown in (ESI†).
3.2. UV-Vis spectroscopy
3.3. Cyclic voltammetry
The UV-vis absorption spectra were recorded for the polymers
P1–P3 in chloroform solution and in thin lms obtained by the The HOMO and LUMO energy levels of the polymers were
drop-casting the chloroform solutions onto glass substrates determined by cyclic voltammetry using Ag/Ag+ non-aqueous
(Table 1, Fig. S19 in ESI†). Polymers P2 and P3 showed two electrode as the reference electrode as well as using the photo-
absorption maxima, one at 350 nm and the second one in the electron spectroscopy. For the cyclic voltammetric measure-
visible region (> 550 nm), whereas polymer P1 showed several ments the HOMO and LUMO energy levels for the polymers
absorption maxima bands, among which the most prominent were calculated by using the following equations: HOMO ¼ ꢂ
bands with very high intensity are at ꢀ350 nm and ꢀ650 nm. [Eox + 4.71] eV; LUMO ¼ ꢂ [Ered + 4.71] eV, where the Eox and Ered
The strong absorption peaks in the visible region (ꢀ653 nm for are the onset of oxidation and reduction potentials respectively
polymer P1, ꢀ621 nm for polymer P2 and ꢀ615 nm for polymer (Fig. S20, ESI†). The HOMO levels of the polymers measured
P3) are due to the p–p* transition along the conjugated polymer from cyclic voltammetry and that from photoelectron spec-
backbone.17,32 The absorption maxima observed in the UV troscopy showed similar trends (Table 1).
region (ꢀ343 nm for polymer P1, ꢀ374 nm for polymer P2 and
Polymer P1 had a HOMO level of ꢂ5.64 eV while polymer P3
ꢀ368 nm for polymer P3) is due to the conjugation along the had a HOMO energy level of ꢂ5.52 eV. By contrast, polymer P2
side chain of the benzodithiophene moiety. Both polymers P2 had a higher HOMO energy level of ꢂ5.47 eV. This was attrib-
and P3 showed a red-shi (ꢀ28 nm for polymer P2 and ꢀ13 nm uted to the incorporation of electron withdrawing uorine
for polymer P3) in absorption bands of polymer thin lms substituents to the conjugated polymer backbone which
compared to that of the solutions. However, polymer P1 did not decreased the HOMO level of polymers P1 and P3.15 Both
Table 1 Molecular weight, optical and electronic energy levels of P1, P2, P3
Mn (gmolꢂ1
)
PDI
HOMOb (eV)
LUMOc (eV)
Egd (eV)
HOMOe (eV)
lmax (nm)
lmaxg (nm)
a
f
Polymer
P1
P2
P3
22 600
28 097
20 100
1.8
2.9
2.9
ꢂ5.64
ꢂ5.47
ꢂ5.52
ꢂ3.39
ꢂ3.13
ꢂ3.15
2.25
2.34
2.37
ꢂ5.57
ꢂ5.29
ꢂ5.40
343, 494, 593, 653
374, 590, 621
368, 579, 615
343, 494, 595, 656
393, 606, 649
377, 598, 628
a
b
c
Determined by SEC (chlorobenzene eluent). Estimated from the onset of oxidation wave of cyclic voltammogram. Estimated from the onset
of reduction wave of cyclic voltammograms. Eg¼(LUMOc–HOMOb). Estimated from the AC2 measurements. Absorption maxima of
d
e
f
chloroform solution. g Absorption maxima of thin lms.
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polymers P2 and P3 have a 0.2 eV difference in their HOMO and with increasing the PC71BM weight ratio and a decrease in the
LUMO values compared to that of the HOMO and LUMO energy open circuit voltage (VOC) was also observed. The polymer P3
levels of the polymer P1 which was attributed to the more with 5,6-diuoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole
extended conjugation and the p-electron delocalization arising as the acceptor moiety gave a PCE of 2.67% (VOC ¼ 0.90 V, FF ¼
from the incorporation of thiophene spacer in between the 0.37 and JSC ¼ 8.03 mA cmꢂ2) for the polymer : PC71BM 1 : 1
donor and the acceptor moieties. All reported polymers have weight ratio. The short circuit current density (JSC) and the PCE
deep lying HOMO energy levels which are lower than the air of the polymer P3 decreased with the increase in PC71BM weight
oxidation threshold (ca. ꢂ5.27 eV) indicating good air ratio. The polymer P1 gave a lower PCE of 1.19% for the weight
stability.36,37 Moreover, due to the lower HOMO energy level of ratio of polymer: PC71BM 1 : 1 (VOC ¼ 0.96 V, FF ¼ 0.39 and JSC
¼
these polymers a higher VOC for BHJ could be potentially 3.20 mA cmꢂ2). A similar trend was observed for polymer P1 as
obtained.25
compared to polymers P2 and P3 upon increasing the PC71BM
weight ratio. All polymers gave exceptionally high VOC which
was attributed to the low lying HOMO levels of the polymers
(HOMO < ꢂ5.2 eV) as the VOC is related to the energy difference
between the HOMO the polymer and the LUMO of the
acceptor.25 The insertion of uorine atom on the acceptor
decreased the HOMO and the LUMO levels of the polymers P1
and P3 as compared to the polymer P2.
3.4. Thermogravimetric analysis
The thermal stability of the polymers was investigated using
thermogravimetric analysis (TGA) under a nitrogen atmosphere
with a heating rate of 10 ꢁC minꢂ1 (Fig. S21 in ESI†). The onset
of decomposition temperature for all the three polymers P1, P2
ꢁ
and P3 is above 400 C indicating good thermal stability.
Although the deeper HOMO levels are benecial for the VOC
,
due to the large band gap the excitons diffusion may be
hindered and may also increase the charge recombination
which in turn will decrease the short circuit current JSC of the
device. This may be the reason for the lower JSC values and lower
PCE measured for the polymer P1. It can be observed that the
insertion of the thiophene spacer had a signicant inuence on
solar cell performance. A two fold increase in JSC was observed
for the polymers P2 and P3 as compared to polymer P1. This was
attributed to the increased conjugation and enhanced absorp-
tion due to the thiophene spacer. However, the presence of
uorine had a negative effect on the device performance as
polymer P3 had lower PCE as compared to P2.
We attempted to further optimize the BHJ performance by
using 1,8-diiodooctane (DIO) as a processing additive for the
polymer–PC71BM blend (Table 3, Fig. 1, S22, S23 and S24 in
ESI†). This high boiling point solvent (DIO) can selectively
dissolve fullerene components in the polymer blend and
thereby can affect the morphology of the active layer.38 However,
the effect of DIO as an additives on the solar cell device
performance depends on the structure of the polymer and can
3.5. Bulk heterojunction (BHJ) solar cell studies
The photovoltaic properties of the polymers were investigated
for the polymers P1, P2 and P3 in bulk heterojunction solar cell
using PC71BM as the acceptor. The solar cells were fabricated
with a device structure of ITO/PEDOT:PSS/Polymer: PC71BM/Ca/
Al with a device area of 0.1 cm2 for the polymers P1, P2 and P3
and were tested under AM 1.5 AMG illuminations.
In order to maintain a constant active layer thickness for the
solar cell devices of the polymers a total blend concentration of
15 mg mLꢂ1 in chloroform was maintained and the polymer
blend was spin-casted at a spin rate of 2000 rpm on ITO
substrates that have been treated with PEDOT: PSS. The effect of
PC71BM on the photovoltaic properties of the polymers were
investigated by varying the blend ratio of polymer: PC71BM from
1 : 1 to 1 : 4 (Table 2).
Polymer P2 with 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadia-
zole as the acceptor unit gave the highest PCE of 2.96% (VOC
¼
0.91 V, FF ¼ 0.42 and JSC ¼ 7.90 mA cmꢂ2) for the polymer:
PC71BM 1 : 1 weight ratio. The PCE of polymer P2 decreased
Table 2 Photovoltaic properties of the P1, P2 and P3 in solar cell devicesa
Polymer
[P]:PC71BM
VOC (V)
JSC (mA cmꢂ2
)
FF
PCE (%)
Thickness (nm)
P1
1 : 1
1 : 2
1 : 3
1 : 4
1 : 1
1 : 2
1 : 3
1 : 4
1 : 1
1 : 2
1 : 3
1 : 4
0.96, (0.92)
0.94, (0.90)
0.78, (0.70)
0.29, (0.23)
0.91, (0.85)
0.84, (0.79)
0.73, (0.75)
0.67, (0.66)
0.90, (0.91)
0.89, (0.87)
0.89, (0.89)
0.89, (0.89)
3.20, (3.14)
2.97, (2.98)
2.80, (2.82)
2.60, (2.61)
7.90, (8.33)
7.78, (7.41)
7.54, (7.01)
7.66, (6.51)
8.03, (7.81)
6.16, (6.14)
5.49, (5.48)
5.62, (5.45)
0.39, (0.36)
0.33, (0.32)
0.30, (0.30)
0.29, (0.29)
0.42, (0.40)
0.36, (0.32)
0.36, (0.32)
0.38, (0.37)
0.37, (0.37)
0.47, (0.43)
0.50, (0.48)
0.47, (0.46)
1.19, (1.06)
0.92, (0.88)
0.64, (0.59)
0.22, (0.17)
2.96, (2.84)
2.34, (1.94)
1.99, (1.73)
1.94, (1.64)
2.67, (2.61)
2.60, (2.37)
2.48, (2.36)
2.38, (2.23)
81.44
77.13
83.91
80.52
87.89
80.71
84.89
77.07
79.11
79.71
83.57
75.65
P2
P3
a
Data in the parenthesis represents the average measured PCE values.
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Table 3 Photovoltaic properties P1, P2 and P3 with 3% DIO as an additive in solar cell devicesa
Polymer
VOC (V)
JSC (mA cmꢂ2
)
FF
PCE (%)
Thickness (nm)
P1
P2
P3
0.55, (0.44)
1.03, (0.85)
0.86, (0.86)
3.19, (3.22)
7.05, (8.22)
7.72, (7.57)
0.29, (0.29)
0.48, (0.49)
0.43, (0.43)
0.51, (0.41)
3.52, (3.43)
2.93, (2.82)
79.97
78.37
85.05
a
Data in the parenthesis represents the average measured PCE values.
were investigated using tapping mode atomic force microscopy
(TMAFM). It has been observed that smoother surface and a
good phase separation are critical for good device performances
as a smoother surface induces better contact with the electrode
and the bicontinuous phase separation inuence better charge
transport and excitons dissociation.18,44 The polymers with
thiophene spacers (P2 and P3) showed improved morphology
with less surface roughness and better phase separation
compared to the polymer P1, for which the PCBM and the
polymer phase segregated and formed large domains.
Presumably these larger domains may be PCBM (Fig. 2, S25 and
S26 in ESI†). The active layer of the polymer P2 showed a very
smooth surface with an rms of 1.15 nm and the polymer P3 had
an rms of 1.22 nm. However, the active layer of polymer P1 had a
much less smooth surface and had an rms of 1.46 nm. Thus, the
smoother morphology of the active layer of P2 and P3 seemed to
aid the charge transport resulting in a better device perfor-
mance. However, the higher surface roughness and the aggre-
gation observed in the active layer of the polymer P1 reected in
the poor device performance of the BHJ device.
TMAFM analysis was also performed for the solar cell devices
of P1, P2, and P3 with 3% DIO additive. A comparison of the
TMAFM images of P2/PC71BM blend with 0% and 3% DIO
indicated that upon the addition of DIO a very smooth active
layer morphology was obtained (polymer P2: rms ¼ 0.877 nm)
which improved the device performance. The surface roughness
of the polymer P3 remained almost the same upon the addition
of DIO which reected in a very slight increase of PCE. However,
for polymer P1 the addition of DIO increased the surface
Fig. 1 J–V curves for polymers P1, P2 and P3.
either enhance39–41 or decrease11,42,43 the efficiency of the BHJ
devices. For the polymers P1, P2 and P3 the DIO concentration
was progressively increased from 1 to 5% v/v. The BHJ devices
were fabricated with the same set of conditions used to obtain
the highest PCEs for each polymer. For the polymer P1, addition
of DIO had a detrimental effect on the JSC, ll factor as well as on
VOC
,
thereby reducing the overall device performance
(Table 3and S2 in ESI†). Previous work in our group has also
shown that upon the addition of DIO as an additive decreased
the device performance of the polymer(Table S5, S6 and Fig. S29
in ESI†).18 For the polymers P2 and P3 an optimal DIO
concentration of 3% v/v was used to obtain an increase in the
PCE of the BHJ devices. Polymer P2 gave the highest PCE of
3.52% with an impressive open circuit voltage of 1.03 V (FF ¼
0.48 and JSC ¼ 7.04 mA cmꢂ2) for 3% (v/v) DIO content (Table 3
and S3 in ESI†). A similar trend was observed for the polymer P3
in which the highest PCE of 2.93% was obtained with a VOC
¼
0.86 V, FF ¼ 0.43 and a JSC ¼ 7.72 mA cmꢂ2 for 3% (v/v) DIO
content (Table 3, S4 in ESI†). For polymers P2 and P3 the ll
factor increased with increasing the DIO concentration which
could be attributed to the fact that upon the addition of DIO the
phase separation was improved which in turn enhanced the
charge transport and decreased the recombination losses.
3.6. Tapping Mode Atomic Force Microscopy (TMAFM)
analysis
Fig. 2 3D TMAFM phase images of the solar cell devices without DIO: (a)
polymer P1 : PC71BM ¼ 1 : 1blend (rms ¼ 1.46 nm); (b) polymer P2 : PC71BM ¼
1 : 1 blend (rms ¼ 1.15 nm); and (c) polymer P3 : PC71BM 1 : 1 blend (rms ¼
To investigate the differences in the efficiencies of the reported
polymers the morphology of the active layer of the BHJ devices 1.22 nm); scan size (5 mm ꢃ 5 mm).
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with the solar cell data, in which the highest PCE was obtained
by the polymer P2 due to its larger short circuit current density.
4. Conclusion
Three new donor–acceptor copolymers P1, P2 and P3 were
synthesized by Stille coupling polymerization with bis(bi-
thienyl)- substituted BDT as the donor and 5,6-diuorobenzo[c]-
[1,2,5]thiadiazole, 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadia-
zole and 5,6-diuoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thia-
diazole as the acceptor units, respectively. The polymers had
comparable molecular weights (Mn ꢀ 20 000 g molꢂ1) and they
had good solubility in common organic solvents. The insertion
of the thiophene spacer between the donor and the acceptor
units broadened the absorption spectrum of the polymers P2
and P3 as compared to polymer P1. Moreover, polymers P2 and
P3 showed a larger red shi in the UV/vis spectrum indicating
better solid state packing and improved the conjugation which
was benecial for the charge transport. The attachment of
the uorine atoms on the acceptor had an adverse effect on the
photovoltaic properties of the polymers, most likely because the
electron withdrawing uorine deepened the HOMO level of
the polymer, which in turn increased the band gap of the
polymer and consequently resulted in lower JSC in the BHJ solar
cells. Polymer P2 had the highest charge carrier mobility of
8.71 ꢃ 10ꢂ5 cm2 Vꢂ1 sꢂ1 as determined by SCLC method and
gave the highest PCE of 3.52% in BHJ with PC71BM acceptor.
Fig. 3 3D TMAFM phase images of the solar cell devices with 3% DIO: (a)
polymer P1 : PC71BM ¼ 1 : 1blend (rms-2.08); (b) polymer P2 : PC71BM ¼ 1 : 1
blend (rms ¼ 0.88); and (c) polymer P3 : PC71BM ¼ 1 : 1 blend (rms ¼ 1.24 nm);
scan size (5 mm ꢃ 5 mm).
Acknowledgements
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 Avance III 500 NMR
instrument.
Fig. 4 Current density and voltage curves of polymer P1, P2, and P3.
roughness (rms ¼ 2.08 nm), which contributed to the poor
device performance (Fig. 3, S27 and S28 in ESI†).
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