Synthesis and photovoltaic properties of polythiophene incorporating with 3,4-difluorothiophene units

Article abstract of DOI:10.1002/cjoc.201300505

Two polythiophene derivatives using fluorine atoms and hexyl or hexyloxy group as electron-withdrawing and donating substituents have been synthesized. The introduction of fluorine atoms to the polythiophene backbones simultaneously lowers the HOMO and narrows the bandgap, and the stronger electron-donating ability of hexyloxy side chain further reduces the bandgap. As a result, poly[3-hexylthiophene-2,5-diyl-alt-3,4-difluorothiophene] (PHTDFT) shows HOMO and bandgap of -5.31/1.83 eV and poly[3,4-dihexyloxythiophene-2,5-diyl-alt-3,4- difluorothiophene] (PDHOTDFT) shows HOMO and bandgap of -5.14/1.68 eV, both are lower than -4.76/2.02 eV of P3HT. Benefiting from the lower HOMO, PHTDFT:PC ₆₁BM (1:1) polymer solar cells obtain a power conversion efficiency of 1.11% and an impressed open-circuit voltage of 0.79 V under solar illumination AM1.5 (100 mW/cm²). Two new polythiophene derivatives incorporating with 3,4-difluorothiphene units, PHTDFT and PDHOTDFT, show lower HOMO and narrower bandgap than that of P3HT. Benefiting from the lower HOMO, PHTDFT:PC₆₁BM (1:1) polymer solar cells obtain a power conversion efficiency of 1.11% and an impressed open-circuit voltage of 0.79 V under solar illumination AM1.5 (100 mW/cm²). Copyright

Full text of DOI:10.1002/cjoc.201300505

FULL PAPER
DOI: 10.1002/cjoc.201300505
Synthesis and Photovoltaic Properties of Polythiophene
Incorporating with 3,4-Difluorothiophene Units
Linquan Huang,a,b Dong Yang,a,b Qiang Gao,a,b Yan Liu,*,a Shengmei Lu,a
Jian Zhang,*,a and Can Li*,a
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian National Laboratory for Clean Energy, Dalian, Liaoning 116023, China
b Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Two polythiophene derivatives using fluorine atoms and hexyl or hexyloxy group as electron-withdrawing and
donating substituents have been synthesized. The introduction of fluorine atoms to the polythiophene backbones
simultaneously lowers the HOMO and narrows the bandgap, and the stronger electron-donating ability of hexyloxy
side chain further reduces the bandgap. As a result, poly[3-hexylthiophene-2,5-diyl-alt-3,4-difluorothiophene]
(PHTDFT) shows HOMO and bandgap of －5.31/1.83 eV and poly[3,4-dihexyloxythiophene-2,5-diyl-alt-3,4-di-
fluorothiophene] (PDHOTDFT) shows HOMO and bandgap of －5.14/1.68 eV, both are lower than －4.76/2.02
eV of P3HT. Benefiting from the lower HOMO, PHTDFT:PC61BM (1∶1) polymer solar cells obtain a power
conversion efficiency of 1.11% and an impressed open-circuit voltage of 0.79 V under solar illumination AM1.5
(100 mW/cm2).
Keywords polythiophene derivatives, 3,4-difluorothiophene, polymer solar cell, open-circuit voltage
Introduction
fullerene derivative.[9,10] In recent years, several new
fullerene derivatives with higher-lying LUMO energy
levels than PC61BM were synthesized, which signifi-
cantly increased the Voc of the PSCs based on P3HT and
new fullerene derivatives.[11-13]
Another method to improve the Voc is to lower the
HOMO energy level of polythiophene by incorporating
the electron-withdrawing substituents. By incorporating
an ester group, Chen et al. reported PCTDT polymer
with deep HOMO of −5.27 eV, with a high Voc of 0.68
V.[14] Krebs et al. obtained an increased Voc of 0.73 V
with P3MHOCT.[15] Similarly, by introducing a sub-
stituent carbonitrile group to an alternated copolymer of
polythiophene, Guillerez and coworkers reported poly-
(3-carbonitrile-3'-hexyl-2,2'-bithiophene) (PD1) with a
large Voc of 0.797 V.[16]
In the last few years, fluorine atoms have been used
as the most attractive electron-withdrawing group in
conjugated polymers for high-efficiency PSCs.[17-19] The
fluorine atom with small size could be introduced into
the polymer backbone without any deleterious steric
effects. Density functional theory calculations predicted
a 0.11 eV decreases in the HOMO energy level by add-
ing two fluorine atoms to the benzotriazole unit.[19b]
Herein, we report the synthesis of polythiophene deriva-
tives incorporating with 3,4-difluorothiphene unit and
Polymer solar cells (PSCs) based on solution proc-
essed bulk-heterojunctions (BHJs) of conjugated poly-
mers have been a world-wide growing field of interest
in green energy, due to light weight, flexibility, and
low-cost manufacturing, as well as large-area feasibil-
ity.[1-6] Among the materials used in PSCs, regioregular
poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-
C61-butyric acid methyl ester (PC61BM) are the most
prominent material system, and the power conversion
efficiency (PCE) of the P3HT:PC61BM PSCs can reach
around 4%－5% by thermal treatment, solvent anneal-
ing, or mixture solvent treatment.[7] In addition, the de-
vice performance of the P3HT:PC61BM PSCs is not
sensitive to the thickness of active layers in the range of
ca. 100－300 nm, which is crucial for the production of
the PSCs by solution-processing in large area at low
cost.[8]
The PCE of P3HT:PC61BM PSCs is limited by the
low open-circuit voltage (Voc) values due to the higher-
lying highest occupied molecular orbital (HOMO) en-
ergy level of P3HT and the relatively lower-lying lowest
unoccupied molecular orbital (LUMO) energy level of
PC61BM, because the Voc of the PSCs is proportional to
the difference between the HOMO energy level of the
donor polymer and the LUMO energy level of the
*
E-mail: yanliu503@dicp.ac.cn, jianzhang@dicp.ac.cn, canli@dicp.ac.cn; Tel.: 0086-0411-84379008; Fax: 0086-0411-84694447
Received June 28, 2013; accepted August 20, 2013; published online September 16, 2013.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201300505 or from the author.
Chin. J. Chem. 2013, 31, 1385—1390
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Huang et al.
FULL PAPER
mm2 defined by metal masks. All devices were charac-
terized with a Keithley 2400 source measure unit under
ambient conditions, and the typical illumination inten-
sity was 100 mW/cm2 (AM 1.5G Oriel solar simulator).
their photovoltaic performance in bulk heterojunction
PSCs. Two polythiophene derivatives using fluorine
atoms and hexyl or hexyloxy group as electron-with-
drawing and donating substituents have been synthe-
sized. The relationships between molecular structure
and photovoltaic property of these polythiophene de-
rivatives are investigated. We find that the introduction
of fluorine atoms to the polythiophene backbones si-
multaneously lowers the HOMO and narrows the band-
gap, and the stronger electron-donating ability of hexy-
loxy side chain further reduces the bandgap. And the
deep HOMO energy levels of the polythiophene deriva-
tives lead to an enhanced Voc.
Scheme 1 Synthetic routes for the monomers and polymers
Br
C6H13
C6H13
C6H13MgBr, Ether
Ni(dppp)Cl2
NBS, THF
Br
Br
S
S
S
1
C6H13
NaHSO4, Hexanol
O
OC6H13
H3CO
OCH3
NBS, THF
Toluene
S
2
S
Experimental
C6H13
O
OC6H13
Br
Materials and instrumental measurements
Br
S
All the solvents were obtained from Sigma-Aldrich
and further purified by standard techniques. The re-
agents were bought from Sigma-Aldrich and used as
received. The synthetic routes and molecular structures
of the monomers and polymers were shown in Scheme
1. 3-Hexylthiophene and 2,5-dibromo-3,4-difluorothio-
phene were prepared according to the previously re-
ported procedures.[20,21]
3
F
F
F
F
n-BuLi, THF
Br
Br
Me3Sn
SnMe3
S
S
4
Me3SnCl
C6H13
Br
F
F
Pd(PPh3)4
DMF, Toluene
+
SnMe3
Br
Me3Sn
1H NMR and 13C NMR spectra were recorded on
Bruker AVANCE 400 MHz at ambient temperature us-
ing TMS as reference. Molecular weights and distribu-
tions were determined by gel permeation chromatogra-
phy (GPC) on a Waters 1500 HPLC Pump equipped
with UV-vis detector (model Waters2489) working at
270 nm, and a set of Waters Styragel columns (HT3,
and HT4, 7.8×300 mm). GPC measurements were car-
ried out at 35 ℃ using THF as eluent at a flow rate of
1.0 mL/min, and polystyrene as the standard. The
UV-vis spectra were evaluated on a Varian Cary 5000
UV-vis spectrometer at wavelength from 300 to 800 nm.
The electrochemical cyclic voltammetry was conducted
on a CHI 410B Electrochemical Workstation with Pt
disk coated with the polymer film, Pt plate, and Ag/Ag＋
electrode as working electrode, counter electrode, and
reference electrode respectively in a 0.1 mol/L tetrabu-
tylammonium hexafluorophosphate (Bu4NPF6) acetoni-
trile solution. The electrochemical potential was cali-
brated against Fc/Fc＋.
S
S
4
1
C6H13
S
*
n
S
F
F
PHTDFT
F
F
C6H13
O
OC6H13
Br
Pd(PPh3)4
+
Br
Me3Sn
SnMe3
DMF, Toluene
S
3
S
4
C6H13
O
OC6H13
S
*
S
n
F
F
PDHOTDFT
Synthesis of the monomers and polymers
2,5-Dibromo-3-hexylthiophene (1) To an ice-
cooled solution of 3-hexylthiophene (1.68 g, 10.0 mmol)
in anhydrous N,N-dimethylformamide (DMF) (20.0 mL)
was added dropwise a solution of N-bromosuccinimide
(NBS, 3.56 g, 20.0 mmol) in anhydrous DMF (20.0 mL).
After stirred overnight at room temperature, the mixture
was then quenched with water, extracted with ether (20
mL×3). The organic phase was combined and washed
with water and brine, dried over anhydrous magnesium
sulfate, concentrated and purified by column chroma-
tography using n-hexane as eluent to give a colorless oil
(2.6 g, 80% yield). 1H NMR (400 MHz, CDCl3) δ: 6.78
(s, 1H), 2.51 (t, J＝7.5 Hz, 2H), 1.62－1.47 (m, 2H),
1.39－1.30 (m, 6H), 0.89 (t, J＝6.6 Hz, 3H); 13C NMR
Fabrication of polymer solar cells
Polymer solar cells with the structure of ITO/
PEDOT:PSS/polymer:PC61BM/Ca (20 nm)/Al (150 nm)
were fabricated under conditions as follows: After spin-
coating a 50 nm layer of poly(3,4-ethylenedioxythio-
phene):poly(styrenesulfonate) (PEDOT:PSS) onto a pre-
cleaned indium-tin oxide (ITO)-coated glass substrates,
the polymer (10 mg/mL)/PC61BM blend solution in di-
chlorobenzene was spin-coated. The thickness of the
active layer was about 60 nm. After thermal annealed
for 15 min under argon, the devices were completed by
evaporating Ca/Al metal electrodes with an area of 3.14
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Synthesis and Photovoltaic Properties of Polythiophene
(100 MHz, CDCl3) δ: 143.15, 131.11, 110.45, 108.07,
31.71, 29.69, 29.63, 28.93, 22.71, 14.21.
After washing with methanol and n-hexane using a
Soxhlet apparatus, the product was recovered with
chloroform. The product was finally dried under vac-
uum for 1 d.
Poly[3-hexylthiophene-2,5-diyl-alt-3,4-difluoro-
thiophene] (PHTDFT) Following the general polym-
erization procedure, 326 mg (1.0 mmol) of 2,5-dibromo-
3-hexylthiophene (1) was reacted for 48 h. The polymer
was recovered with chloroform as a red dark powder
(40 mg, 14 % yield).
3,4-Dihexyloxythiophene (2) A solution of 3,4-di-
methoxythiophene (1.44 g, 10.0 mmol), hexanol (4.08 g,
40.0 mmol) and NaHSO4 (0.12 g, 1.0 mmol) in anhy-
drous toluene (10 mL) was refluxed overnight at 110 ℃.
After the mixture was cooled to room temperature, 20
mL dichloromethane (DCM) was added. The resulting
mixture was washed with an aqueous solution of sodium
hydrogencarbonate and brine, dried over anhydrous
magnesium sulfate, concentrated and purified by col-
umn chromatography using n-hexane/ DCM (10∶1) as
Poly[3,4-dihexyloxythiophene-2,5-diyl-alt-3,4-
difluorothiophene] (PDHOTDFT)
Following the
1
eluent to give a colorless oil (1.70 g, 60% yield). H
general polymerization procedure, 442 mg (1.0 mmol)
of 2,5-dibromo-3,4-dihexyloxythiophene (5) was re-
acted for 48 h. The polymer was recovered with chloro-
form as a red dark powder (75 mg, 19% yield).
NMR (400 MHz, CDCl3) δ: 6.16 (s, 2H), 3.98 (t, J＝6.7
Hz, 4H), 1.88－1.75 (m, 4H), 1.46－1.33 (m, 12H),
0.91 (t, J＝6.2 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ:
147.76, 97.08, 70.72, 31.71, 29.15, 25.79, 22.71, 14.12.
2,5-Dibromo-3,4-dihexyloxythiophene (3) In the
absence of light, N-bromosuccinimide (NBS, 1.78 g,
10.0 mmol) was added in portions to a solution of
3,4-dihexyloxythiophene 2 (1.42 g, 5.0 mmol) in
anhydrous DMF (10 mL). The mixture was stirred at
room temperature for 3 h, then poured into water, ex-
tracted with DCM (10 mL×3). The organic phase was
washed with water and brine, dried over anhydrous
magnesium sulfate, concentrated and purified by col-
umn chromatography using n-hexane/DCM (10∶1) as
Results and Discussion
Characterization of polymers
The two polythiophene derivatives are soluble in
common organic solvents such as THF and dichloro-
benzene. Calibration against the polystyrene standard,
PHTDFT gives a number-average molecular weight (Mn)
of 2691 and a weight-average molecular weight (Mw) of
2929 with a polydispersity (PDI) of 1.09. Introducing
hexyloxy chains into the polymer main chain could in-
crease the solubility. Thus the PDHOTDFT has a Mn of
3861 and a Mw of 5439 with a PDI of 1.41. The mo-
lecular weights of these two polymers are similar to
their derivatives.[16,22] The low nucleophilicity of 3,4-
difluoro-2,5-bis-trimethylstannanylthiophene caused by
two strong electro-negative fluorine substituent group
could result in low reactivity toward nucleophilic attack
in the transmetallation reaction step in the Stille cross-
coupling reactions.[23]
1
eluent to give a colorless oil (1.33 g, 60% yield). H
NMR (400 MHz, CDCl3) δ: 4.05 (t, J＝5.6 Hz, 4H),
1.79－1.66 (m, 4H), 1.48－1.33 (m, 12H), 0.89 (t, J＝
6.3 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ: 147.81,
95.35, 74.11, 31.71, 30.06, 25.68, 22.75, 14.16.
3,4-Difluoro-2,5-bis-trimethylstannanyl-thio-
－
1
phene (4) n-BuLi in hexane (2.4 mol•L , 9.2 mL,
22.0 mmol) was added dropwise in a solution of
2,5-dibromo-3,4-difluoro-thiophene (2.78 g, 10.0 mmol)
in THF (50 mL) at －78 ℃ under argon, and the
mixture was stirred at － 78 ℃ for 30 min. The
Optical properties
－
1
Me3SnCl in THF (1.0 mol•L , 21.0 mL, 21.0 mmol)
was added dropwise. The resulting mixture was stirred
at －78 ℃ for 1 h, then allowed to warm to room tem-
perature and poured into water, extracted with ether (20
mL×3). The organic phase was washed with water and
brine, dried over anhydrous magnesium sulfate, concen-
trated and purified by column chromatography using
n-hexane as eluent to afford product as a white solid
The UV-vis absorption spectra of PHTDFT and
PDHOTDFT solutions in chloroform are shown in Fig-
ure 1. In the visible region, one absorption peak is ob-
served, which could be attributed to the π-π* transition
of the conjugated polymer main chains. In solutions, the
peak of absorption spectrum of PHTDFT is at about 430
nm, while the PDHOTDFT appeared at 515 nm that
shows a red shift of about 85 nm, which is due to the
stronger electron-donating ability of hexyloxy side
chain of PDHOTDFT than hexyl side chain of
PHTDFT.[24]
Figure 1 also shows the UV-vis absorption spectra of
the polymer films on quartz plate. The optical data in-
cluding the absorption edge wavelength (λonset, abs) and
the optical band gap ( Egopt ) of films are summarized in
Table 1. In comparison with the absorption spectra of
the polymer solutions in chloroform, the visible absorp-
tion of the films shows a red shift.[25,26] The optical band
gaps ( Egopt ) are 1.77 and 1.90 eV for PDHOTDFT and
PHTDFT, respectively.
1
(3.56 g, 80% yield). H NMR (400 MHz, CDCl3) δ:
0.39 (s, 18H).
General polymerization procedure In a 100 mL
Schlenk flask, 1.0 mmol of dibromothiophene monomer,
0.41 g (1.0 mmol) of 3,4-difluoro-2,5-bis-trimethyl-
stannanyl-thiophene (6) and 50 mg (0.044 mmol) of
tetrakis(triphenylphosphine)palladium(0) were added.
The flask was purged three times with argon. 16.0 mL
of degassed toluene and 4.0 mL of degassed DMF were
added, and the mixture was heated to 120 ℃ for 48 h.
Then the reactant mixture was cooled to room tempera-
ture. The crude polymer was precipitated in methanol.
Chin. J. Chem. 2013, 31, 1385—1390
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Huang et al.
FULL PAPER
Table 1 Optical and electrochemical properties of polymers
λmax, sol./nm λonset, film/nm Egopt /eV φox/V (vs. Ag/Ag＋) red/V (vs. Ag/Ag＋)
HOMO/eV
Polymer
E
φ
E
LUMO/eV
Egec /eV
1.83
PHTDFT
430
515
650
700
1.90
1.77
0.87
0.70
−5.31
−5.14
−4.76
−0.96
−0.98
−3.48
−3.46
−2.74
PDHOTDFT
P3HTa
a Data from ref 25.
1.68
2.02
Bandgaps of the two polymers are a little smaller
than that of P3HT, although both the HOMO and
LUMO dropped by some extent (Table 1). This phe-
nomenon indicated that the HOMO energy level can be
efficiently reduced by introducing fluorine atoms to the
polymer backbones. It was observed that PDHOTDFT
has a higher HOMO value (－5.14 eV) than that of
PHTDFT (－5.31 eV), which is in accordance with the
previous study on polythiophenes that conjugated poly-
mer with alkoxy group as a substituent usually exhibited
a higher HOMO level than alkyl-substituted polymer.[28]
Photovoltaic properties
To study the photovoltaic properties of PHTDFT and
PDHOTDFT, PSCs are fabricated with a device struc-
ture of ITO/PEDOT:PSS (50 nm)/polymer:PC61BM/Ca
(20 nm)/Al (150 nm). In the photovoltaic devices, the
polymer is used as electron donor while PC61BM is used
as electron acceptor.
Figure 1 UV-vis absorption spectra of PDHOTDFT and
PHTDFT in chloroform solutions and thin film on fused quartz.
Electrochemical properties
The electrochemical properties of PHTDFT and
PDHOTDFT are investigated by cyclic voltammetry
(CV). The cyclic voltammograms of the polymer film
on platinum plates are shown in Figure 2. All measure-
ments are calibrated using ferrocene (Fc) value of
＋0.36 eV as the standard. The HOMO energy levels
can be calculated by an equation of HOMO＝－(Eonset,ox
－EFc＋4.8) (eV), and the LUMO energy levels by an
equation of LUMO＝－(Eonset,red－EFc＋4.8) (eV).[27]
From the values of Eonset,ox and Eonset,red, the HOMO and
the LUMO as well as the electrochemical bandgaps
( Egec ) of the polymers can be estimated by the following
equations:
Firstly, PSCs based on PHTDFT and PC61BM blends
with different ratios are fabricated. Then the device
performances of PHTDFT and PC61BM blend with the
best ratio (1∶1) at different thermal annealing
temperatures are investigated. The highest efficiency is
obtained in PSCs with PHTDFT:PC61BM blend (1∶1)
and thermal annealing under 100 ℃. Further increasing
annealing temperature to 160 ℃, the deterioration of
morphology of active layers induced lower Voc and
lower efficiency.[29] For PDHOTDFT, photovoltaic
performances are also investigated with different blend
ratios and thermal annealing. However, we observe a
significant decrease in the PCE for the cells based on
PDHOTDFT:PC61BM blend with thermal annealing
(see supporting information Table 1S). The current den-
sity-voltage (J-V) curves of the optimized PSCs based
on PHTDFT and PDHOTDFT under the illumination of
AM1.5, 100 mW/cm2 are shown in Figure 3. The PCE
of the PSCs based on PHTDFT and PDHOTDFT are
1.11% and 0.78%, respectively. The corresponding Voc,
short-circuit current (Jsc), fill factor (FF), and PCE of the
devices based on the two polymers are listed in Table 2.
The Jsc of the device based on the PDHOTDFT is larger
than that of the device based on PHTDFT, benefiting
from the broad response range of PDHOTDFT in visible
range, as showed in Figure 1. The Voc of the devices
based on PHTDFT are 0.79 V. In comparison with the
device based on P3HT (Voc is ca. 0.6 V), the Voc of the
device based on PHTDFT increased by ca. 0.2 V. The
higher Voc is expected from the lower HOMO energy
level of PHTDFT, which originated from the strong
E
E
LUMO＝－(Eonset,red＋4.44) (eV)
HOMO＝－(Eonset,ox＋4.44) (eV)
Egec＝e (Eonset,ox－Eonset,red) (eV)
Figure
2
Cyclic voltammograms of PHTDFT (a) and
PDHOTDFT (b) film on platinum plates in an acetonitrile solu-
tion of 0.1 mol/L Bu4NPF6 at a sweep rate of 0.1 V/s.
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Chin. J. Chem. 2013, 31, 1385—1390

Synthesis and Photovoltaic Properties of Polythiophene
Table 2 The corresponding Voc, short-circuit current (Jsc), fill factor (FF), and PCE of PSCs based on PHTDFT and PDHOTDFT
Polymer
Annealing temperature/℃
Jsc/(mA•cm−2)
Voc/V
0.79
0.57
FF/%
60.8
42
PCE/%
1.11
PHTDFT:PC61BM (1∶1)
PDHOTDFT:PC61BM (1∶1)
100
2.31
－
3.27
0.78
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Figure 3 J-V curves of the PSC devices based on PHTDFT and
PDHOTDFT under illumination of AM 1.5G, 100 mW/cm2.
electro-negative fluorine substituent group on the poly-
mer main chain.
Conclusions
Two new polythiophene derivatives incorporating
with 3,4-difluorothiphene units, PHTDFT and
PDHOTDFT, are designed and synthesized, and used as
donor material in PSCs. PHTDFT and PDHOTDFT ex-
hibited lower HOMO and narrow bandgap than that of
P3HT. Benefiting from the lower HOMO, PHTDFT:
PC61BM (1∶1) polymer solar cells obtain a power
conversion efficiency of 1.11% and an impressed
open-circuit voltage of 0.79 V under solar illumination
AM1.5 (100 mW/cm2).
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Acknowledgement
This work was financially supported by the National
Natural Science Foundation of China (No. 20904057)
and Solar Energy Initiative of the Knowledge Innova-
tion Program of the Chinese Academy of Sciences (No.
KGCX2-YW-395-2). Prof. J. Zhang acknowledges fi-
nancial support by 100 Talents Program of the Chinese
Academy of Sciences.
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