Syntheses and solar cell applications of conjugated copolymers containing tetrafluorophenylene units

Article abstract of DOI:10.1016/j.polymer.2015.06.052

Novel conjugated copolymers containing tetrafluorophenylene unit have been synthesized and evaluated in bulk heterojunction solar cell. The tetrafluorophenylene unit, as the strong electron deficient moeity, has been applied for the syntheses of donor-acceptor type copolymers with a narrow-band-gap for bulk heterojunction solar cells. DTBT, tetrafluorophenylene and four types of BDT derivatives as the electron rich units were incorporated using Stille polymerization to generate PE-BDTF, PO-BDTF, PE-BDTTF and PO-BDTTF. The introduction of even 1% of tetrafluorophenylene unit substituting DTBT of BDTDTBT type of polymers results in significant decrease of the band gap of the polymers. The device with PO-BDTF:PC₇₁BM (1:1) showed an open-circuit voltage (V_OC) of 0.75 V, a short circuit current (J_SC) of 11.80 mA/cm², and a fill factor (FF) of 0.59, which yields PCE of 5.22%.

Full text of DOI:10.1016/j.polymer.2015.06.052

Polymer 71 (2015) 113e121
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Syntheses and solar cell applications of conjugated copolymers
containing tetrafluorophenylene units
a
c
a
a
a
b
Jinwoo Kim , Taehyo Kim , Nam Hee Kim , Juae Kim , Joo Young Shim , Il Kim ,
d
c
e
e
e, *
Ho Hwan Chun , Jin Young Kim , Jong Sung Jin , Jong Pil Kim , Euh Duck Jeong
Hongsuk Suh
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea
The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of Polymer Science and Engineering, Pusan National University, Busan
09-735, Republic of Korea
Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea
Department of Naval Architecture and Ocean Engineering, Pusan National University, Busan 609-735, Republic of Korea
Division of High-Technology Materials Research, Busan Center, Korea Basic Science Institute, Busan 618-230, Republic of Korea
,
a, *
a
b
6
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 April 2015
Received in revised form
Novel conjugated copolymers containing tetrafluorophenylene unit have been synthesized and evaluated
in bulk heterojunction solar cell. The tetrafluorophenylene unit, as the strong electron deficient moeity,
has been applied for the syntheses of donor-acceptor type copolymers with a narrow-band-gap for bulk
heterojunction solar cells. DTBT, tetrafluorophenylene and four types of BDT derivatives as the electron
rich units were incorporated using Stille polymerization to generate PE-BDTF, PO-BDTF, PE-BDTTF and
PO-BDTTF. The introduction of even 1% of tetrafluorophenylene unit substituting DTBT of BDTDTBT type
of polymers results in significant decrease of the band gap of the polymers. The device with PO-
18 June 2015
Accepted 25 June 2015
Available online 2 July 2015
Keywords:
BDTF:PC71BM (1:1) showed an open-circuit voltage (VOC) of 0.75 V, a short circuit current (JSC) of
Tetrafluorophenylene
Photovoltaic
Polymer
2
1
1.80 mA/cm , and a fill factor (FF) of 0.59, which yields PCE of 5.22%.
© 2015 Elsevier Ltd. All rights reserved.
0
1. Introduction
deficient units have been reported including benzo[1,2-b:4,5-b ]
dithiophene (BDT) and 2,1,3-benzothiadiazole (BT) unit, respec-
tively [12e20].
Caused by the rapid progress of the development of new con-
jugated polymers as the electron donors, the power conversion
efficiency (PCE) of bulk heterojunction (BHJ) solar cells with full-
enrene derivatives, as the electron acceptors, has now reached
around 10% [1]. The most common type of polymer solar cell is
employing BHJ active layer which is composed of a blend of
electron-donating conjugated polymer, and fullerene-derived
acceptor such as (6,6)-phenyl C61-butyric acid methyl ester
Fluorinated conjugated polymers for BHJ solar cells have raised
much interest, originated from the high electronegativity and small
size of the fluorine atom. The mono and difluorinated conjugated
moiety has been incorporated extensively to provide strong elec-
tron deficiency, planar structure, good solubility, and improved
morphology [21e24].
Here, we report conjugated polymers with tetrafluorophenylene
moiety [25] to provide lower band gaps and deeper HOMO energy
levels for the improvement of the coverage of the solar spectrum
and higher open-circuit voltage, respectively. The copolymers of
two units, BDT-DTBT and BDT-tetrafluorophenylene, are synthe-
sized and utilized for the OPVs [26]. Dialkoxy or dithiphene
substituted BDTs are chosen here as the electron rich co-unit to
make use of its solubilizing characteristics and effective tuning of
the band gap. The ratios of two units, BDT-DTBT and BDT-
tetrafluorophenylene, should be optimized to provide the proper
processability, hole-mobility and band gaps. The properties of the
(
[
PC61BM) or (6,6)-phenyl C71-butyric acid methyl ester (PC71BM)
2e7]. Narrow-band-gaps (1.2 < Eg < 2.0 eV), to cover the long
wave-length region for the improvement of total photovoltaic
current, have been provided by many donoreacceptor (DeA) types
of electron donating conjugated polymers, using electron rich unit
and electron deficient unit [8e11]. Many types of electron rich and
*
Corresponding authors.
E-mail address: hssuh@pusan.ac.kr (H. Suh).
http://dx.doi.org/10.1016/j.polymer.2015.06.052
032-3861/© 2015 Elsevier Ltd. All rights reserved.
0

114
J. Kim et al. / Polymer 71 (2015) 113e121
ꢂ2
series could be compared with the reported BDT-DTBT, with PCE
value of 3.93%, which has exactly same structure as compared to
our PO-BDTF except BDT-tetrafluorophenylene unit [19].
simulator with an irradiation intensity of 1000 W/m . An aperture
2
(12.7 mm ) was used on top of the cell to eliminate extrinsic effects
such as cross-talk, waveguiding, shadow effects, etc. The spectral
mismatch factor was calculated by comparison of solar simulator
spectrum with AM 1.5 spectrum at room temperature.
All reagents were purchased from Aldrich or TCI, and used
without further purification. Solvents were purified by normal
procedure and handled under moisture-free atmosphere. 4,7-
Bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (2) [12], 2,6-
For the evaluation in bulk heterojunction solar cell, new con-
jugated copolymers in relation to contents of BDT-DTBT and BDT-
tetrafluorophenylene have been synthesized using Stille polymer-
ization to generate poly[2,1,3-benzothiadiazole-4,7-diyl-2,5-thio-
0
phenediyl[4,8-bis[2-ethyhexyloxy]benzo[1,2-b:4,5-b ]dithio-
phene-2,6-diyl]-2,5-thiophenediyl-co-2,6-ditetrafluorophenyl[4,8-
0
0
bis[2-ethyhexyloxy]benzo[1,2-b:4,5-b ]dithiophene-2,6-diyl] (PE-
bis(trimethyltin)-4,8-bis(2-ethyhexyloxy)benzo[1,2-b:3,4-b ]
BDTF),
poly[2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl
dithiophene (4) [27], and 2,6-bis(trimethyltin)-4,8-bis(5-(2-
ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene (8) [28]
were synthesized using similar methods reported.
0
[4,8-bis[2-octyldodecyloxy]benzo[1,2-b:4,5-b ]dithiophene-2,6-
diyl]-2,5-thiophenediyl-co-2,6-ditetrafluorophenyl[4,8-bis[2-
octyldodecyloxy]benzo[1,2-b:4,5-b ]dithiophene-2,6-diyl]
0
(PO-
BDTF),
poly[2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl
2.1.1. Synthesis of 4,8-bis(2-octyldodecyloxy)benzo[1,2-b:3,4-b’]
dithiophene (3)
0
[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b ]dithio-
0
phene-2,6-diyl]-2,5-thiophenediyl-co-2,6-ditetrafluorophenyl[4,8-
bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b ]dithiophene-
To the solution of benzo[1,2-b:4,5-b ]dithiophene-4,8-dion (1)
0
(4.0 g, 18.0 mmol), zinc powder (2.6 g, 40.0 mmol) in EtOH (16 ml)
and DMF (16 ml) was added 5 N NaOH aqueous solution (25 ml) and
the mixture was stirred under reflux for 3 h. After adding 1-iode-2-
octyldodecane (22.3 g, 54.0 mmol) and tetrabutylammonium-
bromide (0.9 g, 3.6 mmol), the reaction mixture was further
refluxed for 12 h. The reaction mixture was then extracted with
ethyl acetate, and the combined organic layer was washed with
2
,6-diyl] (PE-BDTTF) and poly[2,1,3-benzothiadiazole-4,7-diyl-2,5-
thiophenediyl[4,8-bis[5-(2-octyldodecyloxy)-2-thienyl]benzo[1,2-
b:4,5-b ]dithiophene-2,6-diyl]-2,5-thiophenediyl-co-2,6-ditetra-
fluorophenyl[4,8-bis[5-(2-octyldodecyloxy)-2-thienyl]benzo[1,2-
b:4,5-b ]dithiophene-2,6-diyl] (PO-BDTTF).
0
0
2
. Experimental section
4
water and dried over anhydrous MgSO . After removing the solvent
under reduced pressure, the residue was purified by flash chro-
1
2.1. Materials and instruments
matography to give 4.9 g (35%) of compound 3 as a colorless oil; H
NMR (300 MHz, CDCl
3
)
d
(ppm) 7.48 (d, 2H, J ¼ 5.5 Hz), 7.36 (d, 2H,
1H and 13C NMR spectra were recorded with a Varian Gemini-
00 (300 MHz) spectrometer and chemical shifts were recorded
in ppm units with TMS as the internal standard. Flash column
chromatography was performed with Merck silica gel 60 (particle
size 230e400 mesh ASTM) with ethyl acetate/hexane or methanol/
methylene chloride gradients unless otherwise indicated. Analyt-
ical thin layer chromatography (TLC) was conducted using Merck
J ¼ 5.5 Hz), 4.17 (d, 4H, J ¼ 4.7 Hz), 1.92e1.80 (m, 2H), 1.72e1.60 (m,
13
3
4H), 1.53e1.20 (m, 60H), 0.90e0.87 (t, 12H, J ¼ 6.9 Hz); C NMR
(75 MHz, CDCl (ppm) 144.9, 131.7, 130.2, 126.1, 120.5, 39.4, 32.2,
31.6, 30.3, 30.1, 30.0, 29.9, 29.6, 27.2, 22.9, 14.4; HRMS (FABþ, m/z)
calcd for C50 783.6148, found 783.6158.
3
) d
86 2 2
H O S
2.1.2. Synthesis of 2,6-bis(trimethyltin)-4,8-bis(2-octyldodecyloxy)
0
0
.25 mm silica gel 60F pre-coated aluminum plates with fluores-
benzo[1,2-b:3,4-b ]dithiophene (5)
cent indicator UV254. High resolution mass spectra (HRMS) were
recorded on a JEOL JMS-700 mass spectrometer under electron
impact (EI) or fast atom bombardment (FAB) conditions in the
Korea Basic Science Institute (Daegu). Molecular weights and
polydispersities of the polymers were determined by gel perme-
ation chromatography (GPC) analysis with a polystyrene standard
calibration. The UVevis absorption spectra were recorded by a
Varian 5E UV/VIS/NIR spectrophotometer, while the Oriel InstaSpec
IV CCD detection system with xenon lamp was used for the pho-
toluminescence and electroluminescence spectra measurements.
Solar cells were fabricated on an indium tin oxide (ITO)-coated
glass substrate with the following structure; ITO-coated glass sub-
strate/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate)
To a solution of compound 3 (4.0 g, 5.1 mmol) in anhydrous
THF (35 ml) was added dropwise n-butyllithium (2.5 M in hex-
ꢀ
ane) (8.2 ml, 20.4 mmol) via syringe at ꢂ78 C under argon at-
ꢀ
mosphere. The mixture was stirred at ꢂ78 C for 30 min and then
at room temperature for 30 min. After the mixture was cooled
ꢀ
to ꢂ78 C again, trimethyltin chloride (1 M in THF) (25.5 g,
25.5 mmol) was added. The mixture was warmed to room tem-
perature and stirred for 12 h. After quenching the reaction with
water, the volatile species were evaporated under reduced pres-
sure. The residue was extracted with diethyl ether, and the
combined organic layer was washed with water, dried over
anhydrous MgSO
4
, and concentrated under reduced pressure.
Recrystallization affords 5.0 g (88%) of compound 5 as a colorless
ꢀ
1
(
PEDOT:PSS) (4 nm)/polymer:PC
X
BM (~100 nm)/Al (100 nm). The
crystals; mp 38.5 C; H NMR (300 MHz, CDCl ) d (ppm) 7.51 (s,
3
ITO-coated glass substrate was first cleaned with detergent, ultra-
sonicated in acetone and isopropyl alcohol, and subsequently dried
overnight in an oven. PEDOT:PSS (Baytron PH) was spin-casted
from aqueous solution to form a film of 40 nm thickness. The
substrate was dried for 10 min at 140 C in air and then transferred
into a glove box to spin-cast the charge separation layer. A solution
2H), 4.17 (d, 4H, J ¼ 4.9 Hz), 1.91e1.80 (m, 2H), 1.70e1.60 (m, 4H),
1.52e1.26 (m, 60H), 0.94e0.54 (t, 12H, J ¼ 7.1 Hz), 0.45 (s, 18H);
13
3
C NMR (75 MHz, CDCl ) d (ppm) 143.5, 140.6, 134.1, 133.1, 128.2,
39.4, 32.2, 31.6, 30.4, 30.1, 30.0, 29.9, 29.7, 29.6, 27.3, 22.9,
ꢀ
14.4, ꢂ8.1; HRMS (FABþ, m/z) calcd for C56
102 2 2 2
H O S Sn 1110.5374,
found 1110.5370.
X
containing a mixture of polymer:PC BM in ODCB solvent with
concentration of 7wt/ml % was then spin-casted on top of the
2.1.3. Synthesis of 4,8-bis(5-octyldodecylthiophen-2-yl)benzo[1,2-
b; 4,5-b’]dithiophene (7)
In a dried three-neck 250 ml argon purged flask, n-butyllithium
(2.5 M in hexane) (10.9 ml, 27.2 mmol) was added dropwise
ꢀ
PEDOT/PSS layer. The film was dried for 60 min at 70 C in the glove
ꢀ
box. The sample was heated at 80 C for 10 min in air. Then, an
aluminum (Al, 100 nm) electrode was deposited by thermal evap-
ꢂ7
oration in a vacuum of about 5 ꢁ 10 Torr. Current densityevolt-
(30 min) to a solution of 2-octyldodecylthiophene (6.0 g,
ꢀ
ꢀ
age (JeV) characteristics of the devices were measured using a
Keithley 236 Source Measure Unit. Solar cell performance was
measured by using an Air Mass 1.5 Global (AM 1.5 G) solar
18.2 mmol) in 40 ml THF at 0 C. After warming to 50 C and stirring
for 2 h, the reaction mixture was treated with 4,8-dehydrobenzo
0
[l,2-b:4,5-b ]dithiophene-4,8-dione (1) (2.0 g, 9.1 mmol) and

J. Kim et al. / Polymer 71 (2015) 113e121
115
ꢀ
0
stirred for 1.5 h at 50 C. After cooling the reaction mixture to
ambient temperature, SnCl $2H O (15.4 g, 68.1 mmol) in 30 ml HCl
10%) was added and the mixture was stirred for additional 2 h,
bis[2-octyldodecyloxy]benzo[1,2-b:4,5-b ]dithiophene-2,6-diyl]
(PO-BDTF) and PO-BDTF5.
2
2
(
This dark purple copolymer was prepared by a procedure similar
after which it was subsequently poured into ice water and extrac-
ted with diethyl ether. The combined extracts were dried with
to that for PE-BDTF, using 2,6-bis(trimethyltin)-4,8-bis(2-
0
octyldodecyloxy)benzo[1,2-b:3,4-b ]dithiophene
(5),
and
7-bis(5-
tetra-
anhydrous MgSO
4
and then concentrated under reduced pressure.
bromo-2-thienyl)-2,1,3-benzothiadiazole
(10)
The residue was purified by flash chromatography to give 5.0 g
fluorodibromobenzene (11) as the monomers. For the synthesis of
PO-BDTF5, BDT-DTBT and BDT-tetrafluorophenylene units were
used with a ratio of 95:5 following exactly same procedure.
Synthesis of Poly[2,1,3-benzothiadiazole-4,7-diyl-2,5-thio-
phenediyl[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b ]
dithiophene-2,6-diyl]-2,5-thiophenediyl-co-2,6-ditetra-
1
(
30%) of compound 7 as a yellow oil; H NMR (300 MHz, CDCl
3
)
d
(ppm) 7.65 (d, 2H, J ¼ 5.9 Hz), 7.45 (d, 2H, J ¼ 5.9 Hz), 7.29 (d, 2H,
J ¼ 3.6 Hz), 6.89 (d, 2H, J ¼ 3.6 Hz), 2.86 (d, 4H, J ¼ 6.5 Hz), 1.78e1.72
13
0
(
(
1
1
m, 2H), 1.36e1.22 (m, 64H), 0.89 (t, 12H, J ¼ 7.0 Hz); C NMR
75 MHz, CDCl (ppm) 146.0, 139.3, 137.5, 136.8, 127.9, 127.6,
25.6, 124.3, 123.7, 40.3, 34.9, 33.6, 32.2, 30.2, 29.9, 29.6, 26.9, 22.9,
4.3; HRMS (FABþ, m/z) calcd for C58 915.6004, found
3
)
d
fluorophenyl[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-
0
H
91
S
4
b ]dithiophene-2,6-diyl] (PE-BDTTF) and PE-BDTTF5.
9
15.6007.
This dark purple copolymer was prepared by a procedure similar
to that for PE-BDTF, using 2,6-bis(trimethyltin)-4,8-bis(5-(2-
0
ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b ]dithiophene (8), 7-
2
.1.4. Synthesis of 2,6-bis(trimethyltin)-4,8-bis(5-
bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (10) and tetra-
fluorodibromobenzene (11) as the monomers. For the synthesis of
PE-BDTTF5, BDT-DTBT and BDT-tetrafluorophenylene units were
used with a ratio of 95:5 following exactly same procedure.
Synthesis of Poly[2,1,3-benzothiadiazole-4,7-diyl-2,5-thio-
phenediyl[4,8-bis[5-(2-octyldodecyloxy)-2-thienyl]benzo[1,2-
0
octyldodecylthiophen-2-yl)benzo[1,2-b:4,5-b ]dithiophene (9)
In a dry two-neck 50 ml argon purged flask, compound 7 (1.5 g,
1.4 mmol) was dissolved in 20 ml anhydrous THF. After cooling to
ꢀ
0
C, the reaction mixture was treated with a solution n-butyl-
lithium (2.50 M in hexane) (1.5 ml, 3.8 mmol). The reaction mixture
was then stirred for 2 h at room temperature. After cooling to 0 C,
ꢀ
0
b:4,5-b ]dithiophene-2,6-diyl]-2,5-thiophenediyl-co-2,6-ditetra-
the reaction mixture was treated with SnMe
3
Cl solution (1 M in
fluorophenyl[4,8-bis[5-(2-octyldodecyloxy)-2-thienyl]benzo[1,2-
b:4,5-b ]dithiophene-2,6-diyl] (PO-BDTTF), PO-BDTTF5.
0
THF) (5.5 ml, 5.5 mmol) in one portion. The reaction mixture was
ꢀ
stirred at 0 C for 30 min and then warmed to room temperature
This dark purple copolymer was prepared by a procedure similar
and stirred for 2 h. Subsequently, the reaction mixture was
quenched by the addition of 10 ml distilled water, and extracted by
diethyl ether. Finally, the combined organic phase was dried with
to that for PE-BDTF, using 2,6-bis(trimethyltin)-4,8-bis(5-(2-
octyldodecyl)thiophen-2-yl)benzo[1,2-b:4,5-b ]dithiophene (9), 7-
bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (10) and tetra-
fluorodibromobenzene (11) as the monomers. For the synthesis of
PO-BDTTF5, BDT-DTBT and BDT-tetrafluorophenylene units were
used with a ratio of 95:5 following exactly same procedure.
0
4
anhydrous MgSO and concentrated to obtain yellow viscous crude
product. Recrystallization afforded 1.0 g (74%) of compound 9 as
1
light-yellow oil; H NMR (300 MHz, CDCl
d, 2H, J ¼ 3.5 Hz), 6.90 (d, 2H, J ¼ 3.6 Hz), 2.87 (d, 4H, J ¼ 6.5 Hz),
.80e1.70 (m, 2H), 1.40e1.22 (m, 64H), 0.88 (t, 12H, J ¼ 7.0 Hz), 0.40
s,18H); 13C NMR (75 MHz, CDCl
(ppm) 145.6,143.5,142.4,138.2,
38.0,131.4,127.8,125.6,122.7, 40.3, 34.9, 33.7, 32.2, 30.3, 30.0, 29.6,
3
) d (ppm) 7.69 (s, 2H), 7.32
(
1
(
1
2
3. Results and discussion
3
) d
3.1. Synthesis and characterization of the polymers
7.0, 23.0, 14.4, ꢂ8.1; HRMS (FABþ, m/z) calcd for C64
106 4 2
H S Sn
1
242.5221, found 1242.5226.
The general synthetic routes toward polymers are outlined in
Scheme 1. 2,6-Bis(trimethyltin)-4,8-bis(2-ethyhexyloxy)benzo[1,2-
Synthesis of Poly[2,1,3-benzothiadiazole-4,7-diyl-2,5-thio-
0
0
phenediyl[4,8-bis[2-ethyhexyloxy]benzo[1,2-b:4,5-b ]dithio-
b:3,4-b ]dithiophene dithiophene (4), 4,7-bis(5-bromo-2-thienyl)-
phene-2,6-diyl]-2,5-thiophenediyl-co-2,6-ditetrafluorophenyl[4,8-
bis[2-ethyhexyloxy]benzo[1,2-b:4,5-b ]dithiophene-2,6-diyl] (PE-
BDTF) and PE-BDTF5.
2,1,3-benzothiadiazole (10) and tetrafluorodibromobenzene (11)
were copolymerized through Stille coupling reaction with Pd(0)-
catalyst to yield PE-BDTF and PE-BDTF5. 2,6-Bis(trimethyltin)-
0
0
The copolymers were synthesized by using BDT-DTBT and BDT-
tetrafluorophenylene units with ratio of 99:1. To a stirred solution
of 2,6-bis(trimethyltin)-4,8-bis(2-ethyhexyloxy)benzo[1,2-b:3,4-
4,8-bis(2-octyldodecyloxy)benzo[1,2-b:3,4-b ]dithiophene (5), 4,7-
bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (10) and tetra-
fluorodibromobenzene (11) were copolymerized through Stille
coupling reaction with Pd(0)-catalyst to yield PO-BDTF and PO-
BDTF5. 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophen-2-
0
b ]dithiophene (4) in 30 mL of chlorobenzene, were added 4,7-
bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole
(10),
tetra-
fluorodibromobenzene (11), 40 mol% of P(o-tolyl)
3
, and 5 mol% of
yl)benzo[1,2-b:4,5-b’]
thienyl)-2,1,3-benzothiadiazole
dithiophene
(8),
(10)
4,7-bis(5-bromo-2-
and tetra-
ꢀ
Pd
2
(dba)
3
. After being stirred at 80 C for 2 days, the reaction
mixture was added 1 ml (2.99 mmol) of phenyltributylstannane,
and further heated at 80 C for 3 h. Then the reaction mixture was
fluorodibromobenzene (11) were copolymerized through Stille
coupling reaction with Pd(0)-catalyst to yield PE-BDTTF and PE-
ꢀ
treated with 1 ml (9.55 mmol) of bromobenzene, and refluxed for
BDTTF5. 2,6-Bis(trimethyltin)-4,8-bis(5-(2-octyldodecyl)thiophen-
0
6
h. After cooling to room temperature, the reaction mixture was
2-yl)benzo[1,2-b:4,5-b ]dithiophene
thienyl)-2,1,3-benzothiadiazole
(9),
(10)
4,7-bis(5-bromo-2-
and tetra-
poured into 100 mL of methanol. The precipitated solid was filtered
and dissolved with minimum amount of chloroform, and poured
into the 200 mL of acetone. After filtration by glass filter, the pre-
cipitate was purified by Soxhlet extraction with methanol and dried
in vacuum for 48 h to generate PE-BDTF. For the synthesis of PE-
BDTF5, BDT-DTBT and BDT-tetrafluorophenylene units were used
with a ratio of 95:5 following exactly same procedure.
fluorodibromobenzene (11) were copolymerized through Stille
coupling reaction with Pd(0)-catalyst to yield PO-BDTTF and PO-
BDTTF5.
The molar feed ratios of monomer 4,7-bis(5-bromo-2-thienyl)-
2,1,3-benzothiadiazole (10) to tetrafluorodibromobenzene (11)
were 99:1 for PE-BDTF, PO-BDTF, PE-BDTTF and PO-BDTTF and
Synthesis of Poly[2,1,3-benzothiadiazole-4,7-diyl-2,5-thio-
phenediyl[4,8-bis[2-octyldodecyloxy]benzo[1,2-b:4,5-b ]dithio-
phene-2,6-diyl]-2,5-thiophenediyl-co-2,6-ditetrafluorophenyl[4,8-
95:5 for PE-BDTF5, PO-BDTF5, PE-BDTTF5 and PO-BDTTF5. The
0
1
structures and purities of the monomers were confirmed by
H
13
NMR, C NMR, and HRMS.

116
J. Kim et al. / Polymer 71 (2015) 113e121
Scheme 1. Synthetic route for the synthesis of the polymers.
Table 1 summarizes the polymerization results including mo-
lecular weight, polydispersity index (PDI) and thermal stability of
the polymers. As shown in Table 1, the number-average molecular
Table 1
Polymerization results and thermal properties of polymers.
weight (M
copolymers were in the range of 1000e37,000 (M
100e49,000 (M ), respectively, with PDI (polydispersity indices,
/M ) values in the range of 1.1e1.6.
The thermal properties of the polymers were characterized by
n
) and the weight-average molecular weight (M
w
) of the
a
a
PDIa
)b
Polymer
M
n
(g/mol)
M
w
(g/mol)
TGA (T
d
DSC (T
g
)
n
) and
1
w
PE-BDTF
PO-BDTF
5000
5700
1.1
1.3
1.3
1.1
1.3
1.1
1.5
1.6
337
315
348
391
310
315
350
362
101
109
103
111
97
98
98
89
M
w
n
37,000
10,000
34,000
35,000
1000
49,000
14,000
39,000
45,000
1100
PE-BDTTF
PO-BDTTF
PE-BDTF5
PO-BDTF5
PE-BDTTF5
PO-BDTTF5
both differential scanning calorimetry (DSC) and thermal gravi-
metric analysis (TGA). The DSC and TGA of the polymers are sum-
marized in Table 1. The DSC analysis was performed under a
nitrogen atmosphere (50 mL/min) on a annealing. The copolymers
5600
2200
8400
3500
a
showed good thermal stability with a glass transition temperature
w
Molecular weight (M ) and polydispersity (PDI) of the polymers were deter-
ꢀ
(
T
g
) of the range of 89e111 C measured by DSC 2920 at heating
mined by gel permeation chromatography (GPC) in THF using polystyrene
ꢀ ꢀ
rates of 10 C/min over the temperature range of 30e250 C. The
standards.
b
2
Onset decomposition temperature (5% weight loss) measured by TGA under N .

J. Kim et al. / Polymer 71 (2015) 113e121
117
Fig. 1. The normalized UVevisible absorption spectra of the polymer: PE-BDTF, PO-BDTF, PE-BDTTF and PO-BDTTF in chloroform solution (a) and in thin solid state (b), PE-BDTF5,
PO-BDTF5, PE-BDTTF5 and PO-BDTTF5 in chloroform solution (c) and in thin solid state (d).
TGA was performed with TGA 2950 in a nitrogen atmosphere at a
heating rate of 10 C/min to 600 C. TGA showed that copolymers
(Fig. 1b) The chloroform solutions of PE-BDTF5, PO-BDTF5, PE-
BDTTF5 and PO-BDTTF5 showed absorption bands with maximum
peaks at around 424e440 and 634e681 nm. (Fig. 1c) The solid films
of PE-BDTF5, PO-BDTF5, PE-BDTTF5 and PO-BDTTF5 showed ab-
sorption bands with maximum peaks at around 423e442 and
610e664 nm. (Fig. 1d) The polymer solutions with 5% ratio of tet-
rafluorophenylene unit showed red shift as compared to the case of
1%. The polymer solutions of high molecular weight (PO-BDTF, PO-
BDTTF and PE-BDTF5) with the longer effective conjugation length
exhibited red-shifted absorption as compared to the corresponding
polymers only with different alkyl group (PE-BDTF, PE-BDTTF and
PO-BDTF5) which has lower molecular weights. The shoulder
peaks of all the copolymers at about 400 nm have been ascribed to
ꢀ
ꢀ
are thermally stable with only about 5% weight loss in nitrogen at
ꢀ
temperatures (T
d
) of the range of 315e391 C. The copolymers have
high decomposition temperatures. The high thermal stability of the
resulting copolymers prevents the deformation of the polymer
morphology and is important for organic photovoltaics application
[29].
3.2. Optical properties
The solution was prepared using ODCB as a solvent and the thin
film by spin-coating on quartz plates from the solution in ODCB at
room temperature. The UVevis absorption spectra of the co-
polymers in solution and as thin films are shown in Fig. 1 and
summarized in Table 2.
The chloroform solutions of PE-BDTF, PO-BDTF, PE-BDTTF and
PO-BDTTF showed absorption bands with maximum peaks at
around 346e426 and 546e598 nm. (Fig. 1a) The solid films of PE-
BDTF, PO-BDTF, PE-BDTTF and PO-BDTTF showed absorption
bands with maximum peaks at around 368e461 and 602e644 nm.
a delocalized excitonic p-p* transition in the conjugated chains and
the long-wavelength absorption peaks attributed to the intra-
molecular charge transfer (ICT) between the electron rich and
electron deficient units [30]. The absorption onsets of the polymers
in solution were found to be 727e805 nm, corresponding to optical
band gaps of 1.54e1.71 eV.
3.3. Electrochemical properties
The HOMO and LUMO energy levels were estimated from the
oxidation and reduction onset values of the cyclic voltammetry
Table 2
Characteristics of the UVevis absorption spectra.
(CV). The CV was performed with a solution of tetrabutylammo-
a
Copolymers
Solution
l
max (nm)
Film lmax (nm)
nium tetrafluoroborate (Bu NBF ) (0.10 M) in acetonitrile at a scan
4
4
PE-BDTF
PO-BDTF
415, 567
426, 598
346, 546
359, 582
440, 681
425, 634
435, 675
424, 590
423, 622
433, 630
461, 644
368, 602
434, 664
423, 635
442, 623
441, 610
rate of 100 mV/s at room temperature under argon atmosphere. A
platinum electrode (~0.05 cm ) coated with a thin polymer film was
2
PE-BDTTF
PO-BDTTF
PE-BDTF5
PO-BDTF5
PE-BDTTF5
PO-BDTTF5
used as the working electrode. Pt wire and Ag/AgNO
3
electrode
were used as the counter electrode and reference electrode,
respectively. The energy level of the Ag/AgNO
3
reference electrode
þ
(calibrated by the Fc/Fc redox system) was 4.8 eV below the vac-
uum level. The CV spectra are shown in Fig. 2, and the oxidation
potentials derived from the onsets of electrochemical p-doping are
a
Solution absorption in chloroform.

118
J. Kim et al. / Polymer 71 (2015) 113e121
Fig. 2. Cyclic voltammograms of the polymers recorded from thin films coated onto platinum wire electrodes in an electrolyte solution of Bu
4
NBF
4
(0.10 M) in acetonitrile with a
reference electrode of Ag/AgNO
PO-BDTTF5.
3
(0.10 M) at room temperature. Scan rate ¼ 80 mV/s (a) PE-BDTF, PO-BDTF, PE-BDTTF and PO-BDTTF, (b) PE-BDTF5, PO-BDTF5, PE-BDTTF5 and
summarized in Table 3. HOMO and LUMO levels were calculated
for this purpose. Typical JeV characteristics of devices with the
configuration of ITO/PEDOT:PSS (40 nm)/polymer:PC BM (1:1, 1:2
ox
according to the empirical formula (EHOMO ¼ e([Eonset
]
þ 4.8) eV)
X
red
and (ELUMO ¼ e([Eonset
]
þ 4.8) eV), respectively. The polymers
or 1:3) (100 nm)/Al (100 nm) under AM 1.5G irradiation (100 mW/
2
exhibit irreversible processes in an oxidation scan. The oxidation
and reduction onsets of PE-BDTF, PO-BDTF, PE-BDTTF and PO-
BDTTF were exhibited at (0.89, ꢂ0.56), (0.95, ꢂ0.51), (0.87, ꢂ0.48)
and (0.95, ꢂ0.42), respectively. The HOMO levels of the polymers
estimated from the oxidation onsets were ꢂ5.75 to ꢂ5.67 eV. The
LUMO energy levels of the polymers were determined to be ꢂ4.32
to ꢂ4.24 eV. The PE-BDTF, PO-BDTF, PE-BDTTF and PO-BDTTF
exhibited electrochemical band gaps of 1.35e1.47 eV, which are
about 0.4 eV lower than those of the corresponding polymers [9,10]
without tetrafluorophenylene unit.
cm ) are depicted in Fig. 3 and summarized in Table 4. The
photovoltaic parameters of all the polymers, including open circuit
voltage (VOC), short circuit current (JSC), fill factor (FF), and power
conversion efficiency (PCE) are summarized in Table 4. The devices
with PE-BDTF:PC61BM (1:3), PO-BDTF:PC61BM (1:1), PE-
BDTTF:PC61BM (1:3) and PO-BDTTF:PC61BM (1:1) layers showed
open-circuit voltages (VOC) of 0.60, 0.77, 0.51 and 0.73 V, short
2
circuit currents (JSC) of 2.58, 6.79, 1.01 and 2.38 mA/cm , and fill
factors (FF) of 0.43, 0.52, 0.36 and 0.38, giving power conversion
efficiencies of 0.66, 2.47, 0.18 and 0.66%, respectively. The device
with PO-BDTF:PC71BM (1:1) layers, using dichlorobenzene as the
solvent, showed an open-circuit voltage (VOC) of 0.75 V, a short
The oxidation and reduction onsets of PE-BDTF5, PO-BDTF5,
PE-BDTTF5 and PO-BDTTF5 were exhibited at (0.8, ꢂ0.43),
2
(
0.93, ꢂ0.7), (0.83, ꢂ0.4) and (0.76, ꢂ0.42), respectively. The HOMO
circuit current (JSC) of 11.80 mA/cm , and a fill factor (FF) of 0.59,
levels of the polymers estimated from the oxidation onsets
were ꢂ5.73 to ꢂ5.56 eV. The LUMO energy levels of the polymers
were determined to be ꢂ4.40 to ꢂ4.10 eV. The PE-BDTF5, PO-
BDTF5, PE-BDTTF5 and PO-BDTTF5 exhibited electrochemical
band gaps of 1.18e1.63 eV, which are lower than those of the cor-
responding polymers with 1% tetrafluorophenylene unit, except the
case of PO-BDTF.
giving a power conversion efficiency of 5.22%, higher value as
compared to the cases with chloroform and chlorobenzene as the
solvents for the spin coating. The effect of annealing and adding
various kind additives was investigated without much success as
shown in the supplement part (Table S1eS4 and Fig. S1). The series
with 5% ratio of tetrafluorophenylene unit, PE-BDTF5, PO-BDTF5,
PE-BDTTF5 and PO-BDTTF5, showed too low solubility to fabricate
meaningful devices.
The field-effect carrier mobilities of the polymers were
measured by fabricating thin film field-effect transistors (FETs)
using the top-contact geometry. Fig. S2eS5 show the FET transfer
characteristics of the polymer devices of the OTS-modified SiO .
2
Hole mobilities of the polymers were calculated from the transfer
3
.4. Photovoltaic properties
The OPVs were fabricated by spin-casting of chlorobenzene (CB)
solutions of PC61BM/polymers or PC71BM/polymers. All polymers
were applied as donors into a conventional BHJ type OPV device
with PC61BM or PC71BM as acceptor, which has been widely used
characteristics of the OFETs. The field-effect hole mobilities of PE-
Table 3
Electrochemical potentials and energy levels of the copolymers.
Optical band gapa (eV)
HOMOb (eV)
LUMOc (eV)
E
ox (V)
d
Ered (V)
d
Electrochemical band gape (eV)
Copolymers
PE-BDTF
PO-BDTF
1.56
1.71
1.54
1.69
1.48
1.62
1.57
1.62
ꢂ5.69
ꢂ5.75
ꢂ5.67
ꢂ5.75
ꢂ5.60
ꢂ5.73
ꢂ5.63
ꢂ5.56
ꢂ4.24
ꢂ4.29
ꢂ4.32
ꢂ4.28
ꢂ4.37
ꢂ4.10
ꢂ4.40
ꢂ4.38
0.89
0.95
0.87
0.95
0.8
0.93
0.83
0.76
ꢂ0.56
ꢂ0.51
ꢂ0.48
ꢂ0.52
ꢂ0.43
ꢂ0.7
1.45
1.46
1.35
1.47
1.23
1.63
1.23
1.18
PE-BDTTF
PO-BDTTF
PE-BDTF5
PO-BDTF5
PE-BDTTF5
PO-BDTTF5
ꢂ0.4
ꢂ0.42
a
Optical energy band gap was estimated from the onset wavelength of the optical absorption. E
Determined by UPS.
LUMO ¼ HOMO þ opt film.
Onset oxidation and reduction potential measured by cyclic voltammetry.
Calculated from the Eox and Ered
g
¼ 1240/
onset
l .
b
c
DE
d
e
.

J. Kim et al. / Polymer 71 (2015) 113e121
119
Fig. 3. Current density-potential characteristics of the polymers solar cells under the illumination of AM 1.5, 100 mW/cm2 (a) PE-BDTF:PC61BM, (b) PO-BDTF:PC61BM, (c) PE-
BDTTF:PC61BM, (d) PO-BDTTF:PC61BM, (e) PO-BDTF:PC71BM (1:1) blend films cast from chlorobenzene (aed) and three types of solutions (e).
Table 4
Photovoltaic properties of the copolymer solar cells.
Copolymers
Ratio copolymer:PC
1:2
X
BM
V
OC (V)
J
SC (mA/cm2)
FF
PCE (%)
PE-BDTF (PC61BM)
0.56
0.60
0.72
0.73
0.71
0.75
0.75
0.77
0.32
0.51
0.73
0.67
2.41
2.58
6.79
7.26
7.83
0.40
0.43
0.45
0.39
0.35
0.59
0.54
0.54
0.32
0.36
0.38
0.40
0.61
0.66
2.47
2.08
1.98
5.22
3.29
2.41
0.06
0.18
0.66
0.49
1:3
PO-BDTF (PC61BM)
PO-BDTF (PC71BM)
1:1
1
1
:2
:3
1:1 (DCB)
11.80
8.15
5.87
0.61
1.01
2.38
1.81
1
1
:1 (CB)
:1 (CF)
PE-BDTTF (PC61BM)
PO-BDTTF (PC61BM)
1:1
:3
1:1
:3
1
1
ꢀ
2
Fig. 4. IPCE curves of the polymer solar cells after annealing at 100 C under the illumination of AM 1.5, 100 mW/cm . IPCE curves of (a) PE-BDTF:PC61BM (1:3), (b) PO-
BDTF:PC61BM (1:1), (c) PE-BDTTF:PC61BM (1:3), (d) PO-BDTTF:PC61BM (1:1), (e) PO-BDTF:PC71BM (1:1) blend films cast from chlorobenzene (aed) and three types of solutions (e).

120
J. Kim et al. / Polymer 71 (2015) 113e121
Fig. 5. Atomic force microscopy images of (a) PE-BDTF:PC61BM (1:3), (b) PO-BDTF:PC61BM (1:1), (c) PE-BDTTF:PC61BM (1:3), (d) PO-BDTTF:PC61BM (1:1), (e) PO-BDTF:PC71BM
(
1:1), (f) PO-BDTF:PC71BM (1:1), and (g) PO-BDTF:PC71BM (1:1) blend films cast from chlorobenzene (aed and f), dichlorobenzene (e), and chloroform (g).
ꢂ6
BDTF, PO-BDTF, PE-BDTTF and PO-BDTTF are 1.39 ꢁ 10
,
tetrafluorophenylene unit should be optimized to afford the
appropriate processability, hole-mobility and band gaps. The best
device with PO-BDTF:PC71BM (1:1) showed an open-circuit voltage
ꢂ4
ꢂ6
ꢂ7
2
2
.4 ꢁ 10 , 1.07 ꢁ 10 , and 9.67 ꢁ 10 cm /V, respectively.
The incident photon to current efficiency (IPCE) spectra of the
2
photovoltaic devices from PE-BDTF:PC61BM (1:3), PO-
BDTF:PC61BM (1:1), PE-BDTTF:PC61BM (1:3), and PO-
BDTTF:PC61BM (1:1) blend films cast with chlorobenzene solution
and PO-BDTF:PC71BM (1:1) blend films cast from three types of
solutions are presented in Fig. 4. The IPCE peak of PO-BDTF:PC61BM
1:1) film shows about 40% IPCE value from 300 nm to 750 nm with
maximum value at about 341 nm, higher value as compared to the
other polymers of the series.
Especially, the IPCE value of PO-BDTF:PC71BM (1:1) blend films
by spin-casting using dichlorobenzene solution showed higher
value at the broad range as compared to films using the other so-
lutions as shown in Fig. 4e.
The morphology of the copolymers/PC61BM or copolymers/
PC71BM films cast from their solution in chlorobenzene, dichloro-
benzene, or chloroform were studied by atomic force microscopy
(VOC) of 0.75 V, a short circuit current (JSC) of 11.80 mA/cm , and a
fill factor (FF) of 0.59, which yield PCE of 5.22%. Further optimiza-
tion of the ratio of BDT-tetrafluorophenylene unit may improve the
morphology, hole-mobility and band gap leading to the better
performance of the device. As compared to the band gap of BDT-
DTBT (2.18 eV) [19], the introduction of small amount of tetra-
fluorophenylene unit substituting DTBT decreased the band gap
(1.23e1.63 eV) significantly. In case of PTBDTDTBT also, by applying
same modification, the band gap of PTBDTDTBT (1.62 eV) [20] was
decreased to 1.18e1.47 eV. Since the introduction of even 1% of
tetrafluorophenylene unit substituting DTBT of BDTDTBT type of
polymers results in significant decrease of the band gap of the
polymers, similar type of tactics could be applied for the generation
of lower band gap polymers especially for the tandem solar cell
structure. similar type of tactics could be applied for the generation
of lower band gap polymers especially for the tandem solar cell
structure.
(
(
AFM) as shown in Fig. 5, where the images were obtained in a
2
surface area of 2.5 ꢁ 2.5
mm by the tapping mode.
The morphologies of PE-BDTF:PC61BM (1:3), PO-BDTF:PC61BM
1:1), PE-BDTTF:PC61BM (1:3), PO-BDTTF:PC61BM (1:1) blend films
(
Acknowledgments
from chlorobenzene solutuion are 9.25, 3.16, 9.77, and 8.47 nm,
respectively. The morphologies of PO-BDTF:PC71BM (1:1) blend
films from chlorobenzene, dichlorobenzene, and chloroform solu-
tions are 2.48, 2.65 and 7.92 nm, respectively. The surface of the
active layer of PO-BDTF:PC71BM (1:1) with dichlorobenzene is
quite smooth, with a root-mean-square (rms) value of 2.65 nm,
which results in the highest PCE value out of the series.
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea Government (MEST)
(NRF-2014R1A1A2055318) and the Grant no. POF017 from Busan
Metropolitan City, Korea.
Appendix A. Supplementary data
4
. Conclusion
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2015.06.052.
Novel conjugated copolymers in relation to contents of BDT-
DTBT and BDT-tetrafluorophenylene have been synthesized via
Stille coupling reaction with Pd catalyst to generate PE-BDTF, PO-
BDTF, PE-BDTTF, PO-BDTTF, PE-BDTF5, PO-BDTF5, PE-BDTTF5
and PO-BDTTF5 for OPV devices. The introduction of
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