Syntheses of pyrimidine-based polymers containing electron-withdrawing substituent with high open circuit voltage and applications for polymer solar cells

Article abstract of DOI:10.1002/pola.27910

Polymers using new electron-deficient units, 2-pyriminecarbonitrile and 2-fluoropyrimidine, were synthesized and utilized for the photovoltaics. Donor-acceptor (D-A) types of conjugated polymers (PBDTCN, PBDTTCN, PBDTF, and PBDTTF) containing 4,8-bis(2-octyldodecyloxy)benzo[1,2-b;3,4-b′]dithiophene (BDT) or 4,8-bis(5-(2-octyldodecyloxy)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTT) as electron rich unit and 2-pyriminecarbonitrile or 2-fluoropyrimidine as electron deficient unit were synthesized. We designed pyrimidine derivatives in which strong electron-withdrawing group (C-N or fluorine) was introduced to the C2 position for the generation of strong electron-deficient property. By the combination with the electron-rich unit, the pyrimidines will provide low band gap polymers with low highest occupied molecular orbital (HOMO) energy levels for higher open-circuit voltages (V_OC). For the syntheses of the polymers, the electron-rich and the electron-deficient units were combined by Stille coupling reaction with Pd(0)-catalyst. Absorption spectra of the thin films of PBDTTCN and PBDTTF with BDTT unit show shift to a longer wavelength region than PBDTCN and PBDTF with BDT unit. Four synthesized polymers provided low electrochemical bandgaps of 1.56 to 1.96 eV and deep HOMO energy levels between -5.67 and -5.14 eV.

Full text of DOI:10.1002/pola.27910

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Syntheses of Pyrimidine-Based Polymers Containing Electron-
Withdrawing Substituent with High Open Circuit Voltage and
Applications for Polymer Solar Cells
1
1
2
2
3
1
Juae Kim, Joo Young Shim, Jihoon Lee, Dal Yong Lee, Sangmin Chae, Jinwoo Kim,
4
3
2
1
Il Kim, Hyo Jung Kim, Sung Heum Park, Hongsuk Suh
1
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Korea
Department of Physics, Pukyong National University, Busan 608-737, Korea
2
3
4
Department of Organic Material Science and Engineering, Pusan National University, Busan 609-735, Korea
Department of Polymer Science and Engineering, Pusan National University, The WCU Center for Synthetic Polymer
Bioconjugate Hybrid Materials, Busan 609-735, Korea
Correspondence to: H. Suh (E-mail: hssuh@pusan.ac.kr)
Received 25 June 2015; accepted 28 August 2015; published online 10 October 2015
DOI: 10.1002/pola.27910
ABSTRACT: Polymers using new electron-deficient units, 2-
pyriminecarbonitrile and 2-fluoropyrimidine, were synthesized
and utilized for the photovoltaics. Donor-acceptor (D-A) types
of conjugated polymers (PBDTCN, PBDTTCN, PBDTF, and
PBDTTF) containing 4,8-bis(2-octyldodecyloxy)benzo[1,2-b;3,4-
energy levels for higher open-circuit voltages (VOC). For the
syntheses of the polymers, the electron-rich and the electron-
deficient units were combined by Stille coupling reaction with
Pd(0)-catalyst. Absorption spectra of the thin films of PBDTTCN
and PBDTTF with BDTT unit show shift to a longer wavelength
region than PBDTCN and PBDTF with BDT unit. Four synthe-
sized polymers provided low electrochemical bandgaps of 1.56
to 1.96 eV and deep HOMO energy levels between 25.67 and
0
b ]dithiophene (BDT) or 4,8-bis(5-(2-octyldodecyloxy)thiophen-
0
2
-yl)benzo[1,2-b:4,5-b ]dithiophene (BDTT) as electron rich unit
and 2-pyriminecarbonitrile or 2-fluoropyrimidine as electron
deficient unit were synthesized. We designed pyrimidine deriv-
atives in which strong electron-withdrawing group (CBN or flu-
orine) was introduced to the C2 position for the generation of
strong electron-deficient property. By the combination with the
electron-rich unit, the pyrimidines will provide low band gap
polymers with low highest occupied molecular orbital (HOMO)
2
5.14 eV. V
2015 The Authors. Journal of Polymer Science Part
C
A: Polymer Chemistry Published by Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2016, 54, 771–784
KEYWORDS: conducting polymers; high performance polymers;
structure-property relations
INTRODUCTION For the polymer solar cells (PSCs), the bulk
heterojunction structure has been examined intensively over
the past decades especially caused by their features such as
low cost, light weight, flexibility, and the possibility of large
extended p-conjugated system and (ii) the introduction of suita-
ble side chain for better solubility. BDT-based copolymers have
3–6
been reported to provide PSCs with PCE values up to 9%.
1
,2
area fabrication by loll-to loll process.
Many types of electron-deficient units, including 2,1,3-benzo-
7
8
thiadiazole,
diketopyrrolo[3,4-c]-pyrrole-1,4-dione,
and
D-A type of polymers are advantageous for the construction
of efficient low band gap polymers with extended long wave-
length absorption and reduced HOMO energy level for the
9
thieno[3,4-c]pyrrole-4,6-dione, have been reported for the
1
0
D-A type of low band-gap polymers. Among them, com-
pounds containing pyrimidine unit have been studied as
promising materials to present a compact stacking for high
mobility owing to coplanar conformation. However, there
have not been many reports on copolymers with pyrimidine
3
enhancement of open-circuit voltage (VOC).
0
Benzo[1,2-b:4,5-b ]dithiophene (BDT) is one of the best electron-
rich sub-structure, due to the rigid and coplanar fused ring sys-
tem, for (i) the enhancement of p-stacking, hole mobility and
1
1–13
for organic solar cells (OSCs).
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which per-
mits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifica-
tions or adaptations are made.
V
C
2015 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc.
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In this article, we report D-A types of conjugated polymers,
PBDTCN, PBDTTCN, PBDTF, and PBDTTF, containing BDT or
BDTT as donor (D) unit and 2-pyriminecarbonitrile or 2-
1
4,15
fluoropyrimidine as acceptor (A) unit (Schemes 1 and 2).
By using the Stille coupling condition, BDT/BDTT and 2-pyrimi-
necarbonitrile/2-fluoropyrimidine fragments were incorpo-
rated to synthesize poly(4,6-bis(4-hexyl-2-thienyl)-2-pyrimi
dinecarbonitrile-alt-4,8-di(2-octyldodecyloxy)benzo[1,2-b;3,4-
FIGURE 1 Approach towards new electron-deficient moiety.
[
Color figure can be viewed in the online issue, which is avail-
0
b ]dithiophene) (PBDTCN), poly(4,6-bis(4-hexyl-2-thienyl)-2-
able at wileyonlinelibrary.com.]
pyrimidinecarbonitrile-alt-4,8-bis(5-(2-octyldodecyloxy)thio-
0
phen-2-yl)benzo[1,2-b:4,5-b ]dithiophene) (PBDTTCN), poly(4,6-
To craft efficient electron-deficient moiety, we designed pyrimi-
dines in which strong electron-withdrawing group (CBN or fluo-
rine) was introduced to its C2 position, which will lead to low
bis(4-hexyl-2-thienyl)-2-fluoropyrimidine-alt-4,8-di(2-octyldode-
cyloxy)benzo [1,2-b;3,4-b ]dithiophene) (PBDTF), and poly(4,6-
0
b i s (4-hexyl-2-thienyl)-2-fluoropyrimidine-alt-4,8-bis(5-(2-octyl-
3,11
0
band gap polymers for PSCs as shown in Figure 1.
In case of
dodecyloxy)thiophen-2-yl)benzo[1,2-b:4,5-b ]dithiophene)
pyrimidines in which CBN or fluorine was introduced, the cyano
(PBDTTF). Novel conjugated polymers showed good solubility
at room temperature in various organic solvents. The photo-
voltaic properties of the polymers were investigated by
group and fluorine groups are strong acceptor units to provide
6
,11
properties of higher conjugation, coplanarity, and rigidity.
SCHEME 1 Synthetic route for the syntheses of the monomers.
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SCHEME 2 Synthetic route for the syntheses of the polymers.
fabrication of the polymer solar cells with the configuration of
ITO/PEDOT:PSS/polymer:PC BM/Al.
Merck silica gel 60 (particle size 230–400 mesh ASTM) with
ethyl acetate/hexane or methanol/methylene chloride gra-
dients unless otherwise indicated. Analytical thin layer chro-
matography (TLC) was conducted using Merck 0.25 mm
silica gel 60F precoated aluminum plates with fluorescent
indicator UV254. High resolution mass spectra (HRMS) were
recorded on a JEOL JMS-700 mass spectrometer under elec-
tron impact (EI) conditions in the Korea Basic Science Insti-
tute (Daegu). Molecular weight and polydispersity of the
polymer were determined by gel permeation chromatogra-
phy (GPC) analysis with a polystyrene standard calibration
and carried out at 35 8C. The differential scanning calorime-
try (DSC) analysis was performed on a DSC Q200 at heating
7
1
EXPERIMENTAL
Instrumental Characterization
All reagents were purchased from Aldrich or TCI, and used
without further purification. Solvents were purified by nor-
mal procedure and handled under moisture-free atmosphere.
1
13
H and
C NMR spectra were recorded with a Varian
Gemini-300 (300 MHz) spectrometer and chemical shifts
were recorded in ppm units with TMS as the internal stand-
ard. Flash column chromatography was performed with
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TABLE 1 Polymerization Results and Thermal Properties of
drain electrodes was 50 nm. Channel length (L) and channel
width (W) were 50 lm and 3.0 mm, respectively. Electrical
characterization was performed using a Keithley semiconduc-
tor parametric analyzer (Keithley 4200). All fabrication and
measurement processes were carried out in the glove box
Polymers
a
a
w
a
b
Polymer
M
n
M
PDI
DSC (T
g
)
d
TGA (T )
PBDTCN
PBDTTCN
PBDTF
20,000
38,000
10,000
52,000
60,000
220,000
26,000
260,000
3.00
5.70
2.54
4.93
101
99
324
410
329
429
filled with N . The mobility (l) was determined using the
2
following equation in the saturation regime by
102
101
2
PBDTTF
Isat5ðlWCi=2LÞ ꢀ ðVGS2VTÞ
a
Molecular weight (M
w
) and polydispersity (PDI) of the polymers were
-2
where C
VT is the threshold voltage.
i
is the capacitance per unit area of 15 nF cm and
3
determined by gel permeation chromatography (GPC) in CHCl using
polystyrene standards.
b
Onset decomposition temperature (5% weight loss) measured by TGA
under N
2
.
Solar cells were fabricated on an indium tin oxide (ITO)-
coated glass substrate with the following structure; ITO-
coated glass substrate/poly(3,4-ethylenedioxythiophene)
polystyrene sulfonate (PEDOT:PSS)/polymer:PC BM/Al. The
ITO-coated glass substrate was first cleaned with detergent,
ultrasonicated in acetone and isopropyl alcohol, and subse-
quently dried overnight in an oven. PEDOT:PSS (Baytron PH)
was spin-cast from aqueous solution to form a film of 40 nm
thickness. The substrate was dried for 10 min at 140 8C in
rates of 10 8C/min. The thermal gravimetric analysis (TGA)
was performed with a SDT Q600 under nitrogen atmosphere
at a heating rate of 10 8C/min to 800 8C in the Korea Basic
Science Institute (Busan). The UV-Vis absorption spectra
were recorded by a Varian 5E UV/VIS/NIR spectrophotome-
ter. Cyclic voltammogram of the polymers was obtained
using an Wona-WPG100 at room temperature in a three-
electrode cell under a Ar atmosphere at a scan rate of 100
mV/s, with Pt wire as the counter electrode, an Ag/AgNO3
reference electrode, and 0.1 M tetraethylammonium tetra-
7
1
fluoroborate (TBABF ) in acetonitrile as the electrolyte. The
4
AFM instrumentation consisted of a Veeco NanoScope AFM
and standard silicon cantilever (Veeco; tip radius, 8 nm; nor-
mal spring constant, 21–78 N/m; scan rate, 2.0 Hz) at ambi-
ent conditions (in air, 20 8C). GIWAXS were measured at the
5
Å beamline in the Pohang Accelerator Laboratory (PAL,
South Korea). 2D GIWAXS patterns were recorded using CCD
detector positioned at the end of a vacuum guide tube in
which the X-ray pass through the thin films under vacuum,
where operation conditions were set to a X-ray wavelength
of 1.07153 Å and a sample-to-detector distance (SDD) of
482.08 mm. The incidence angle (0.1) was carefully chosen
to allow for complete X-ray penetration of the film. The scat-
tering spectra were collected as a two-dimensional image
map oriented along the plane of the substrate (q ) and the
y
plane perpendicular to the substrate (qz).
Device Fabrication and Measurements
The FETs were fabricated on heavily doped n-type silicon
(
Si) wafers each covered with a thermally grown silicon
dioxide (SiO ) layer with thickness of 200 nm. Whole Si sub-
2
strates were cleaned with ultrasonic bath in acetone and iso-
propyl alcohol successively, dried over 2 h in the dry oven at
1
00 8C, and then subjected to UV-ozone treatment for 30
min. The active layer was deposited by spin coating at
000 rpm. Before active layer deposition, SiO surfaces were
2
2
treated with octadecyltrichlorosilane (OTS) to make surface
hydrophobic. All solutions were of 10 mg/mL concentration
in chlorobenzene. The thickness of the deposited films was
about 60 nm. Before deposition of source drain electrodes,
the films were dried at room temperature for 30 min. Source
and drain electrodes using Au were deposited by thermal
evaporation using shadow mask. The thickness of source and
FIGURE 2 Thermogravimetric analysis of polymers under N2.
Inner: differential scanning calorimetry of polymers under N2.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
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FIGURE 4 Cyclic voltammetry curves of the polymers in 0.1 M
TBAF in acetonitrile solution at a scan rate of 100 mV/s at
room temperature (vs. an Ag quasi-reference electrode). [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
22
intensity of 1000 W m . The spectral mismatch factor was
calculated by comparison of solar simulator spectrum with
AM 1.5 spectrum at room temperature.
Synthesis of Monomers and Polymers
Synthesis of 4,8-Bis(2-Octyldodecyloxy)Benzo[1,2-B:3,4-
0
B ]Dithiophene (2)
To the solution of benzo[1,2-b:4,5-b ]dithiophene-4,8-dion (1)
0
(
4.0 g, 18.0 mmol) and zinc powder (2.6 g, 40.0 mmol) in
EtOH (16 mL) and DMF (16 mL), was added 5 N NaOH aque-
ous solution (25 mL) and the mixture was stirred under
reflux for 3 h. After 1-iode-2-octyldodecane (22.3 g, 54.0
mmol) and tetrabutylammoniumbromide (0.9 g, 3.6 mmol)
were added to the mixture at room temperature, the reaction
mixture was refluxed for 12 h. The reaction mixture was then
extracted with ethyl acetate, and the organic layer was
FIGURE 3 UV-Visible absorption spectra of polymers in chloro-
form solution and the solid state. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.
com.]
air and then transferred into a glove box to spin-cast the
active layer. A solution containing a mixture of poly-
mer:PC BM in chlorobenzene solvent with concentration of
4
washed with water and dried over anhydrous MgSO . After
removing the solvent under reduced pressure, the residue
was purified by flash to give 4.9 g (35%) of compound 2 as a
colorless oil.
7
1
20 mg/mL was then spin-cast on top of the PEDOT:PSS layer.
The film was dried for 30 min at room temperature in the
glove box. Then, an aluminum (Al, 100 nm) electrode was
deposited by thermal evaporation in a vacuum of about 3 3
1
H NMR (300 MHz, CDCl ) d (ppm) 7.48 (d, 2H, J5 5.5 Hz),
3
26
7.36 (d, 2H, J 5 5.5 Hz), 4.17 (d, 4H, J 5 4.7 Hz), 1.92–1.80
10
Torr. Current density-voltage (J–V) characteristics of
(
m, 2H), 1.72–1.60 (m, 4H), 1.53-1.20 (m, 60H), 0.90–0.87 (t,
2H, J 5 6.9 Hz); C NMR (75 MHz, CDCl ) d (ppm) 144.9,
3
the devices were measured using a Keithley 2400 Source
Measure Unit. Solar cell performance utilized an Air Mass
1
3
1
1
2
31.7, 130.2, 126.1, 120.5, 39.4, 32.2, 31.6, 30.3, 30.1, 30.0,
1.5 Global (AM 1.5 G) solar simulator with an irradiation
1
9.9, 29.6, 27.2, 22.9, 14.4; HRMS (FAB , m/z) calcd. for
C H O S 783.6148, found 783.6158.
5
0 86 2 2
TABLE 2 Characteristics of the UV-Vis Absorption Spectra
Synthesis of 2,6-Bis(Trimethyltin)-4,8-Bis(2-
0
Octyldodecyloxy)Benzo[1,2-B:3,4-B ]Dithiophene (3)
Polymer
In solution
In thin film
To a solution of compound 2 (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 278 8C under argon
atmosphere. The mixture was stirred at 278 8C for 30 min and
then at room temperature for 30 min. After the mixture was
PBDTCN
PBDTTCN
PBDTF
427, 520, 554
437, 521, 561
336, 453, 535, 569
405, 500, 548
337, 450, 536, 575
437, 513, 555
PBDTTF
336, 451, 526, 559
337, 445, 525, 565
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TABLE 3 Electrochemical Potentials and Energy Levels of the Polymers
a
b
c
d
d
Polymers
Optical band gap (eV)
HOMO (eV)
LUMO (eV)
E
ox (V)
Ered (V)
PBDTCN
PBDTTCN
PBDTF
2.04
2.01
2.07
2.06
25.39
25.67
25.14
25.58
23.35
23.66
23.07
23.52
0.59
0.87
0.34
0.78
21.34
21.15
21.22
21.18
PBDTTF
a
c
Estimated by HOMO 1 E go pt
Onset oxidation and reduction potential measured by cyclic
Optical energy band gap was estimated from the onset wavelength of
the optical absorption.
Calculated from the oxidation potentials.
.
d
b
voltammetry.
1
3
cooled to 278 8C again, trimethyltin chloride (1 M in THF)
(m, 60H), 0.94–0.54 (t, 12H, J 5 7.1 Hz), 0.45 (s, 18H);
C
(
25.5 g, 25.5 mmol) was added. The mixture was warmed to
NMR (75 MHz, CDCl3) d (ppm) 143.5, 140.6, 134.1, 133.1,
room temperature and stirred for 12 h. After quenching the
reaction with water, the volatile species were evaporated under
reduced pressure. The residue was extracted with diethyl ether,
and the organic layer was washed with water, dried over anhy-
drous MgSO4, and concentrated. Recrystallization afforded
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, -8.1; HRMS (FAB , m/z) calcd. for
C56H102O2S2Sn2 1110.5374, found 1110.5370.
1
Synthesis of 4,8-Bis(5-octyldodecylthiophen-2-
0
Yl)Benzo[1,2-B;4,5-B ]Dithiophene (4)
5.0 g (88%) of compound 3 as a colorless crystals; mp 38.5 8C.
In a dried three-neck 250 mL argon purged flask, n-butyl-
lithium (2.5 M in hexane) (10.9 mL, 27.2 mmol) was added
dropwise (30 min) to a mixture of 2-octyldodecylthiophene
1
H NMR (300 MHz, CDCl ) d (ppm) 7.51 (s, 2H), 4.17 (d, 4H,
3
J 5 4.9 Hz), 1.91–1.80 (m, 2H), 1.70–1.60 (m, 4H), 1.52–1.26
FIGURE 5 Optimized geometries and molecular orbital surfaces of the HOMO and LUMO of polymers at the B3LYP/6-3111G level
of theory. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 4 OFET Characteristics of Polymers
2
21 21
s
Polymers
Mobility (cm V
)
Ion/Ioff
Vth (V)
26
24
26
25
3
3
PBDTCN
PBDTTCN
PBDTF
1.64 3 10
4.06 3 10
1.07 3 10
6.60 3 10
10 (2.8 3 10 )
210
222
28.4
25.5
4
3
10 (7.3 3 10 )
4
3
10 (9.6 3 10 )
4
3
PBDTTF
10 (6.2 3 10 )
The OFETs are in bottom-gate, top-contact configuration with channel
width (W) of 3 mm, and channel length (L) of 50 lm.
(
6.0 g, 18.2 mmol) in 40 mL THF at 0 8C. The mixture was
then warmed to 50 8C and stirred for 2 h, after that 4,8-
0
dehydrobenzo[l,2-b:4,5-b ]dithiophene-4,8-dione (1) (2.0 g,
9.1 mmol) was subsequently added to the reaction mixture
followed by stirring for 1.5 h at 50 8C. After cooling the reac-
:
2
tion mixture to ambient temperature, SnCl 2H O (15.4 g,
2
68.1 mmol) in 30 mL HCl (10%) was added and the mixture
was stirred for additional 2 h, after which it was subse-
quently poured into ice water and extracted with diethyl
ether. The combined organic layer was dried with anhydrous
MgSO4. After removing the solvent under reduced pressure,
the residue was purified by flash to give 5.0 g (30%) of com-
pound 4 as a yellow oil.
1
H NMR (300 MHz, CDCl ) d (ppm) 7.65 (d, 2H, J 5 5.9 Hz),
3
7.45 (d, 2H, J 5 5.9 Hz), 7.29 (d, 2H, J 5 3.6 Hz), 6.89 (d, 2H,
J 5 3.6 Hz), 2.86 (d, 4H, J 5 6.5 Hz), 1.78–1.72 (m, 2H), 1.36–
1
3
1
.22 (m, 64H), 0.89 (t, 12H, J 5 7.0 Hz); C NMR (75 MHz,
CDCl ) d (ppm) 146.0, 139.3, 137.5, 136.8, 127.9, 127.6,
3
1
2
9
25.6, 124.3, 123.7, 40.3, 34.9, 33.6, 32.2, 30.2, 29.9, 29.6,
1
6.9, 22.9, 14.3; HRMS (FAB , m/z) calcd. for C H S
5
8
91
4
15.6004, found 915.6007.
Synthesis of 2,6-Bis(Trimethyltin)-4,8-Bis(5-
0
octyldodecylthiophen-2-Yl)Benzo[1,2-B:4,5-B ]Dithiophene
5)
In a dry two-neck 50 mL argon purged flask, compound 4
1.5 g, 1.4 mmol) was dissolved in 20 mL anhydrous THF.
(
(
The solution was cooled to 0 8C and a solution of n-butyl-
lithium (2.50 M in hexane) (1.5 mL, 3.8 mmol) was added
dropwise with stirring. The reaction mixture was then
stirred for 2 h at room temperature. Next, the reaction mix-
3
ture was cooled to 0 8C and a Me SnCl solution (1 M in
THF) (5.5 mL, 5.5 mmol) was added in one portion. The
reaction mixture was stirred at 0 8C for 30 min and then
warmed to room temperature. After stirring for 2 h, the
reaction mixture was quenched by the addition of 10 mL
distilled water, and extracted by diethyl ether. The combined
organic phase was dried with anhydrous MgSO4 and concen-
trated to obtain yellow viscous crude product. Recrystalliza-
tion afforded 1.0 g (74%) of compound 5 as light-yellow oil.
1
H NMR (300 MHz, CDCl ) d (ppm) 7.69 (s, 2H), 7.32 (d, 2H,
3
FIGURE 6 The OFET of polymers. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.
com.]
J 5 3.5 Hz), 6.90 (d, 2H, J 5 3.6 Hz), 2.87 (d, 4H, J 5 6.5 Hz),
1.80–1.70 (m, 2H), 1.40–1.22 (m, 64H), 0.88 (t, 12H, J 5 7.0
1
3
Hz), 0.40 (s, 18H); C NMR (75 MHz, CDCl ) d (ppm) 145.6,
3
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1
H NMR (300 MHz, CDCl ) d (ppm) 7.73 (s, 2H), 7.57 (s,
3
1
H), 7.18 (s, 2H), 2.65 (t, 4H, J 5 7.6 Hz), 1.67 (qi, 4H, J 5 7.6
1
3
Hz), 1.45–1.33 (m, 12H), 0.90 (t, 6H, J 5 6.5 Hz); C NMR
75 MHz, CDCl ) d (ppm) 161.8, 145.0, 140.1, 129.9, 126.1,
(
1
3
1
20.3, 107.1, 31.7, 30.5, 30.4, 28.9, 22.6, 14.1; HRMS(EI , m/
z) calcd for C24 31ClN 446.1617, found 446.1617.
H
2 2
S
Synthesis of 4,6-Bis(4-hexyl-2-Thienyl)-2-
Pyrimidinecarbonitrile (8)
To a stirred solution of 2-chloro-4,6-bis(4-hexyl-2-thienyl)-
pyrimidine (7) (1.6 g, 3.6 mmol) in 20 mL of THF at RT
under argon was added dropwise NaCN (0.8 g, 16.8 mmol)
solution in TMA/H O (1:3). After stirring for 24 h, the reac-
2
tion mixture was treated with 200 mL of water and 200 mL
of diethyl ether. The aqueous phase was extracted with
diethyl ether and combined organic layer was dried over
MgSO . After removing the solvent under reduced pressure,
4
the residue was purified by flash to give 1 g (60%) of com-
pound 8 as a yellow oil.
1
H NMR (300 MHz, CDCl ) d (ppm) 7.75 (s, 2H), 7.73 (s,
3
1
H), 7.23 (s, 2H), 2.66 (t, 4H, J 5 6.3 Hz), 1.67 (qi, 4H, J 5 6.4
1
3
Hz), 1.46-1.34 (m, 12H), 0.91 (t, 6H, J 5 6.5 Hz); C NMR
75 MHz, CDCl ) d (ppm) 160.3, 155.7, 145.3, 139.6, 130.2,
(
3
1
26.9, 115.8, 110.1, 31.6, 30.5, 30.4, 28.9, 22.6, 14.1;
1
HRMS(EI , m/z) calcd. for C25
31 3 2
H N S 437.1959, found
437.1962.
Synthesis of 4,6-Bis(5-bromo-4-hexyl-2-Thienyl)-2-
Pyrimidinecarbonitrile (9)
To a stirred solution of 4,6-bis(4-hexyl-2-thienyl)-2-pyrimidi-
necarbonitrile (8) (0.5 g, 1.2 mmol) in DMF at 80 8C was
added N-bromosuccinimide (NBS) (1.2 g, 6.9 mmol). After
stirring over 6 h at 80 8C, 100 mL of water, and 100 mL of
FIGURE 7 Current density-potential characteristics of the poly-
mers solar cells under the illumination of AM 1.5, 100 mW/
2
cm . A) Current density–voltage (J–V) of polymer:PC71BM
2 2
CH Cl were added. The organic phase was washed with 3
blends without DIO, B) current density–voltage (J–V) of poly-
mer:PC71BM blends with 3% DIO. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.
com.]
3 100 mL of water. The organic phase was concentrated
under reduced pressure and purified by flash column chro-
matography to give 0.5 g (73%) of compound 9 as yellow
solid; mp 150 8C.
1
H NMR (300 MHz, CDCl ) d (ppm) 7.61 (s, 2H), 2.61 (t, 4H,
1
43.5, 142.4, 138.2, 138.0, 131.4, 127.8, 125.6, 122.7, 40.3,
3
J 5 7.6 Hz), 1.65 (qi, 4H, J 5 7.6 Hz), 1.47–1.35 (m, 12H),
34.9, 33.7, 32.2, 30.3, 30.0, 29.6, 27.0, 23.0, 14.4, 28.1;
1
3
1
0.91 (t, 6H, J 5 6.5 Hz); C NMR (75 MHz, CDCl ) d (ppm)
HRMS (FAB , m/z) calcd. for C64
H
106
S
4
Sn
2
1242.5221, found
3
1242.5226.
TABLE 5 Photovoltaic Properties of Polymers
2
Donor
3% DIO
V
OC (V)
J
SC (mA/cm )
FF
PCE (%)
Synthesis of 2-Chloro-4,6-Bis(4-hexyl-2-Thienyl)
Pyrimidine (7)
To a stirred solution of 2,4,6-trichloropyrimidine (10.0 g, 54.5
mmol) and (4-hexyl-2-thienyl)(tributyl)stannane (6) (38.4 g,
PBDTCN
Without 0.52
With 0.74
PBDTTCN Without 0.87
With 0.86
Without 0.76
With 0.64
Without 0.87
With 0.82
0.70
2.37
0.68
3.14
0.61
2.25
1.08
3.28
0.30 0.11
0.36 0.63
0.44 0.26
0.40 1.09
0.38 0.17
0.35 0.50
0.50 0.47
0.39 1.05
1
14.5 mmol) in 30 mL of THF at room temperature was
added dichlorobis(triphenylphosphine)palladium(II) (2 mol
). The reaction mixture was stirred for 24 h at 80 8C. After
PBDTF
%
cooling room temperature, the reaction mixture was concen-
trated under reduced pressure, and purified by flash column
chromatography to give 10.0 g (20%) of compound 7 as yel-
low oil.
PBDTTF
Polymer:acceptor(1:1.5) 20 mg/mL in CB.
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FIGURE 8 The GIWAX data of PBDTCN, PBDTTCN, PBDTF, or PBDTTF/PC71BM blend films. A) 2D patterns of PBDTCN, B) 2D pat-
terns of PBDTTCN, C) 2D patterns of PBDTF, D) 2D patterns of PBDTTF. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
1
59.4, 145.1, 144.4, 139.1, 129.6, 117.5, 115.5, 108.9, 31.6,
6 h at 80 8C, water and CH Cl2 were added. The organic
2
1
29.7, 29.6, 28.9, 22.6, 14.1; HRMS(FAB , m/z) calcd. for
phase was washed with 3 3 100 mL of water. The organic
phase was concentrated under reduced pressure and puri-
fied by flash column chromatography to give 0.2 g (20%) of
compound 11 as yellow solid, m.p. 97 8C.
C H N Br S 594.0248, found 594.0246.
2
5
30
3
2 2
Synthesis of 2-fluoro-4,6-Bis(4-hexyl-2-Thienyl)
Pyrimidine (10)
A solution of 2-chloro-4,6-bis(4-hexyl-2-thienyl)pyrimidine
1
H NMR (300 MHz, CDCl ): d (ppm) 7.11 (s, 2H), 2.73 (t, 4H,
3
(
6
1
7) (3.0 g, 6.7 mmol), KF (2.0 g, 33.6 mmol), DCH-18-crown-
(0.15 g, 0.4 mmol), and tetraglyme (14 mL) was heated at
50 8C overnight. After cooling to room temperature, the
J 5 7.6 Hz), 1.76 (qi, 4H, J 5 7.6 Hz), 1.45–1.30 (m, 12H),
1
3
0.90 (t, 6H, J 5 7.0 Hz); C NMR (75 MHz, CDCl ) d (ppm)
3
2
1
207.1, 164. 5, 162.4 (d,
J
C-F 5 12.7 Hz), 141.9 (d,
J
C-
reaction mixture was treated with water and CH Cl . The
aqueous phase was extracted with ethyl acetate and com-
F
5 338.6 Hz), 129.4, 116.6, 105.5, 31.6, 29.7, 29.6, 28.9,
22.6, 14.1; HRMS(EI , m/z) calcd. for C H29Br FN S
24 2 2 2
2
2
1
bined organic layer was dried with MgSO . After concentra-
586.0123, found 586.0120.
4
tion of the organic phase under reduced pressure, the
residue was purified by column chromatography to give
Synthesis of Poly(4,6-Bis(4-hexyl-2-Thienyl)-2-
pyrimidinecarbonitrile-alt-4,8-Di(2-
0.5 g (28%) of compound 10 as yellow oil.
0
Octyldodecyloxy)Benzo[1,2-B;3,4-B ]Dithiophene)
1
(PBDTCN)
H NMR (300 MHz, CDCl ) d (ppm) 7.75 (s, 2H), 7.60 (d, 1H,
3
Carefully purified 2,6-bis(trimethyltin)-4,8-bis(2-octyldodecy-
loxy)benzo[1,2-b:3,4-b ]dithiophene (3) (1 eq), 4,6-bis(5-
bromo-4-hexyl-2-thienyl)-2-pyrimidinecarbonitrile (9) (1 eq),
3 2 3
P(o-tolyl) (0.4 eq) and Pd (dba) (3 mol %) were dissolved
in chlorobenzene. The reaction mixture was refluxed with
vigorous stirring for 2 days under argon atmosphere. After
cooling to room temperature, the reaction mixture was
poured into methanol and 1 M HCl. The precipitated material
was recovered by filtration. The resulting solid material was
reprecipitated using 100 mL of chloroform/1.0 L of methanol
several times to remove residual amount of the catalyst. The
solid was washed for 2 days in a Soxhlet extractor using an
J 5 4.2 Hz), 7.19 (s, 2H), 2.66 (t, 4H, J 5 7.7 Hz), 1.67 (qi, 4H,
1
3
0
J 5 7.6 Hz), 1.48–1.34 (m, 12H), 0.91 (t, 6H, J 5 6.5 Hz);
C
NMR (75 MHz, CDCl ) d (ppm) 163.4, 145.0, 140.2, 130.0,
3
1
26.2, 106.6, 106.5, 31.6, 30.5, 30.4, 28.9, 22.6, 14.1;
1
HRMS(EI , m/z) calcd. for C24
2 2
H31FN S 430.1913, found
430.1916.
Synthesis of 4,6-Bis(5-bromo-4-hexyl-2-Thienyl)-2-
Fluoropyrimidine (11)
To a stirred solution of 2-fluoro-4,6-bis(4-hexyl-2-thienyl)-
pyrimidine (10) (0.5 g, 1.1 mmol) in DMF at 80 8C was
added N-bromosuccinimide (NBS) (1.2 g, 6.4 mmol). After
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FIGURE 9 AFM phase images (A, C, E, G, I, K, M, and O) and topography (B, D, F, H, J, L, N, and P) of the polymer/PCBM (1:1.5,
w/w) without DIO (A, E, I, and M) and with DIO (C, G, K, and O). (scan size: 1 lm 3 1 lm). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
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acetone/methanol mixture to remove oligomers and catalyst
residue. The solid was washed for two days in a Soxhlet
extractor using an acetone/methanol mixture to remove
oligomers and catalyst residue. The final product was
obtained after drying under vacuum at 60 8C. The resulting
(1 eq), P(o-tolyl)3 (0.4 eq), and Pd (dba)3 (3 mol %) were
2
dissolved in chlorobenzene. The reaction mixture was
refluxed with vigorous stirring for 2 days under argon
atmosphere. After cooling to room temperature, the reaction
mixture was poured into methanol and 1 M HCl. The precipi-
tated material was recovered by filtration. The resulting solid
material was reprecipitated using 100 mL of chloroform/
3
polymer was soluble in THF, CHCl , ODCB, and toluene.
Synthesis of Poly(4,6-Bis(4-hexyl-2-Thienyl)-2-
pyrimidinecarbonitrile-alt-4,8-Bis(5-(2-
1.0 L of methanol several times to remove residual amount
of the catalyst. The solid was washed for 2 days in a Soxhlet
extractor using an acetone/methanol mixture to remove
oligomers and catalyst residue. The solid was washed for 2
days in a Soxhlet extractor using an acetone/methanol mix-
ture to remove oligomers and catalyst residue. The final
product was obtained after drying under vacuum at 60 8C.
Octyldodecyloxy)Thiophen-2-Yl)Benzo[1,2-B:4,5-
0
B ]Dithiophene) (PBDTTCN)
Carefully purified 2,6-bis(trimethyltin)-4,8-bis(5-octyldodecylth-
iophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene (5) (1 eq), 4,6-bis(5-
bromo-4-hexyl-2-thienyl)-2-pyrimidinecarbonitrile (9) (1 eq),
P(o-tolyl) (0.4 eq), and Pd (dba) (3 mol %) were dissolved in
3
2
3
The resulting polymer was soluble in THF, CHCl , ODCB, and
3
chlorobenzene. The reaction mixture was refluxed with vigorous
stirring for 2 days under argon atmosphere. After cooling to
room temperature, the reaction mixture was poured into metha-
nol and 1 M HCl. The precipitated material was recovered by fil-
tration. The resulting solid material was reprecipitated using
toluene.
RESULTS AND DISCUSSION
Synthesis and Characterization
The general synthetic routes toward the intermediates and
polymers are outlined in Schemes 1 and 2. 2,6-Bis(trimethyl-
100 mL of chloroform/1.0 L of methanol several times to
remove residual amount of the catalyst. The solid was washed
for 2 days in a Soxhlet extractor using an acetone/methanol mix-
ture to remove oligomers and catalyst residue. The solid was
washed for 2 days in a Soxhlet extractor using an acetone/meth-
anol mixture to remove oligomers and catalyst residue. The final
product was obtained after drying under vacuum at 60 8C. The
0
tin)-4,8-bis(2-octyldodecyloxy)benzo[1,2-b:3,4-b ]dithiophene
(
3),
which
was
prepared
from
benzo[1,2-b:4,5-
0
b ]dithiophene-4,8-dione (1) over two steps, and 4,6-bis(5-
bromo-4-hexyl-2-thienyl)-2-pyrimidinecarbonitrile (9), which
was prepared from 2,4,6-trichloropyrimidine over three
steps, were coupled to provide PBDTCN through Stille reac-
resulting polymer was soluble in THF, CHCl , ODCB, and toluene.
3
tion with Pd(0)-catalyst. 2,6-Bis(trimethyltin)-4,8-bis(5-octyl-
Synthesis of Poly(4,6-Bis(4-hexyl-2-Thienyl)-2-
0
dodecylthiophen-2-yl)benzo[1,2-b:4,5-b ]dithiophene
(5),
fluoropyrimidine-alt-4,8-Di(2-Octyldodecyloxy)Benzo[1,2-
B;3,4-B ]Dithiophene) (PBDTF)
which was prepared from compound 1 over two steps, and
compound 9 were coupled to provide PBDTTCN through
Stille reaction with Pd(0)-catalyst. By using the same poly-
merization condition, compound 3 and 4,6-bis(5-bromo-4-
hexyl-2-thienyl)-2-fluoropyrimidine (11), which was pre-
pared from 2,4,6-trichloropyrimidine over three steps, were
coupled to provide PBDTF through Stille reaction with
Pd(0)-catalyst. Compound 5 and compound 11 were coupled
to provide PBDTTF through Stille reaction with Pd(0)-cata-
lyst. All of the polymers show good solubility at room tem-
perature in organic solvents such as chloroform, THF,
chlorobenzene, and o-dichlorobenzene (ODCB).
0
Carefully purified 2,6-bis(trimethyltin)-4,8-bis(2-octyldodecy-
0
loxy)benzo[1,2-b:3,4-b ]dithiophene (3) (1 eq), 4,6-bis(5-
bromo-4-hexyl-2-thienyl)-2-fluoropyrimidine (11) (1 eq),
P(o-tolyl) (0.4 eq), and Pd (dba) (3 mol %) were dissolved
3 2 3
in chlorobenzene. The reaction mixture was refluxed with
vigorous stirring for 2 days under argon atmosphere. After
cooling to room temperature, the reaction mixture was
poured into methanol and 1 M HCl. The precipitated material
was recovered by filtration. The resulting solid material was
reprecipitated using 100 mL of chloroform/1.0 L of methanol
several times to remove residual amount of the catalyst. The
solid was washed for 2 days in a Soxhlet extractor using an
acetone/methanol mixture to remove oligomers and catalyst
residue. The solid was washed for 2 days in a Soxhlet extrac-
tor using an acetone/methanol mixture to remove oligomers
and catalyst residue. The final product was obtained after
drying under vacuum at 60 8C. The resulting polymer was
Table 1 summarizes the polymerization results including
molecular weights, PDI, and thermal stability of the copoly-
mers. The number-average molecular weights (M
20,000, 38,000, 10,000, and 52,000 and weight-average
molecular weights (M ) of 60,000, 220,000, 26,000, and
260,000 with PDI (poly dispersity index, M /M ) values of
) of
n
w
w
n
soluble in THF, CHCl
3
, ODCB, and toluene.
3.0, 5.7, 2.5, and 4.5 were determined by gel permission
chromatography (GPC) with a polystyrene standard in a
chloroform eluent for the PBDTCN, PBDTTCN, PBDTF, and
PBDTTF, respectively.
Synthesis of Poly(4,6-bis(4-hexyl-2-thienyl)-2-
fluoropyrimidine-alt-4,8-bis(5-(2-
octyldodecyloxy)thiophen-2-Yl)benzo[1,2-B:4,5-
0
B ]dithiophene) (PBDTTF)
The thermal properties of the polymers were characterized
by both DSC and TGA as shown in Figure 2 and summarized
in Table 1. The polymers show good thermal stability with
Carefully purified 2,6-bis(trimethyltin)-4,8-bis(5-octyldode-
cylthiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene (5) (1 eq),
4,6-bis(5-bromo-4-hexyl-2-thienyl)-2-fluoropyrimidine (11)
onset decomposition temperatures (T , 5% weight loss) of
d
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324, 410, 329, and 429 8C, respectively. All polymers have
ized in Table 3. The energy levels of HOMO and lowest unoccu-
glass transition temperature (T ) values above 100 8C, thus
pied molecular orbital (LUMO) were calculated according to
g
ox
they show thermal stability. PBDTTCN and PBDTTF with
BDTT unit show especially higher thermal stability than the
case of alkoxy-substituted BDT unit, which is attributed to
the empirical formula of (EHOMO 5 2([Eonset] 1 4.8) eV) and
opt
(ELUMO 5 2 (EHOMO 1 Eg ) eV), respectively.
1
6
The polymers, PBDTCN, PBDTTCN, PBDTF, and PBDTTF,
exhibited the absorption onset wavelengths of 608, 617, 599,
and 602 nm in solid thin film, which corresponds to band
gaps of 2.04, 2.01, 2.07, and 2.06 eV, respectively. The poly-
mers exhibit irreversible processes in an oxidation scan. The
oxidation onsets of the polymers, PBDTCN, PBDTTCN,
PBDTF, and PBDTTF, were estimated to be 0.59, 0.87, 0.34,
and 0.78 V, which correspond to HOMO energy levels of
25.39, 25.67, 25.14, and 25.58 eV, respectively. When the
alkoxy side chain was substituted with the alkylthiophene
conjugated side chain, the HOMO and LUMO levels of the
polymers are moved to lower energy values. The HOMO
energy levels of PBDTTCN and PBDTTF are lower than
the BDTT fragment with the more rigid structure. The high
thermal stability of the resulting polymers prevents the
deformation of the morphology and is important for OPV
device applications.
Optical Properties of the Polymers
The optical properties of solutions and films of the polymers
were investigated by UV-Vis absorption spectroscopy. The
solution was prepared using ODCB as a solvent and the thin
film by spin-coating on quartz plates from the solution in
ODCB. The UV-Vis absorption spectra of the polymers as sol-
utions and thin films are shown in Figure 3 and summarized
in Table 2. The solution of PBDTCN presents absorption
bands with maximum peaks at 427, 520 and 554 nm. The
PBDTCN thin film shows absorption band with peaks at
1
9
those of PBDTCN and PBDTF. Gadisa et al. have studied a
relationship between the oxidation potential of polythiopene
2
0
derivative and VOC in BHJ type PSCs. The VOC depends on
437, 521, and 561 nm and the absorption onset at 608 nm,
2
1
the oxidation potential of p-conjugated polymer as well.
corresponding to band gap of 2.04 eV. The maximum absorp-
The lower HOMO energy levels of the two-dimensional (2D)
conjugated polymers, as in the case of BDTT series, could
have higher VOC values for the improvement of photovoltaic
property and additional merit of improving oxidative
tion peaks of PBDTTCN appear at about 336, 453, 535, and
569 nm in ODCB solution. The spectra of the PBDTTCN thin
film shows absorption bands with maximum peaks at about
3
6
37, 450, 536, and 575 nm and the absorption onsets at
17 nm, corresponding to band gap of 2.01 eV. The maxi-
2
2,23
stability.
mum absorption peaks of PBDTF and PBDTTF appear at
05, 500, 548 nm and 336, 451, 526, 559 nm in ODCB solu-
tion, respectively. The maximum absorption peaks in solid
thin films were at around 437, 513, 555 nm and 337, 445,
The optimized geometry and the HOMO and LUMO energy
4
levels were calculated at the B3 LYP/6-311G (d,p) level of
2
4
theory using the Gaussian 03 package, as shown in Figure
. The HOMO and LUMO surfaces were plotted using the
5
525, 565 nm, and the absorption onsets at 599, 602 nm, cor-
GaussView version 4.1. To simplify the calculation, the alkyl
chains were replaced by methyl groups. For PBDTCN and
PBDTTCN, the electron densities of the HOMOs is mainly
distributed on the BDT unit (the electron rich unit), while
those of the LUMOs on the pyrimidine unit (the electronic
deficient unit), suggesting that the polymers are typical D–A
types. For PBDTF and PBDTTF, the electron densities of the
HOMOs show similar distribution mainly on the BDT unit
responding to band gap of 2.07, 2.06 eV, respectively. The
short-wavelength absorption peaks have been ascribed to a
delocalized excitonic p–p* transition in the polymer chains
and the long-wavelength absorption peaks attributed to the
intramolecular charge transfer (ICT) between the substituted
BDT/BDTT as electron rich unit and pyrimidine as electron
1
7
deficient unit. As shown in Figure 3, the absorption spectra
of the PBDTTCN and PBDTTF films show red shifts of the
long wavelength absorption peaks compared with the cases
of PBDTCN and PBDTF due to the extended p-conjugation
(
the electron rich unit), while those of the LUMOs is delocal-
25,26
ized on the polymer backbone.
1
6,18
of the BDTT unit.
The red shift absorption demonstrates
Field-Effect Transistor and Photovoltaic Properties of
the Polymers
The field-effect carrier mobilities of the polymers were
measured by fabricating thin film field-effect transistors
(FETs) using the top-contact geometry. Figure 6 shows the
FET transfer characteristics of the polymer devices of the
that the p–p stacking or interchain interaction of the poly-
mers containing BDTT unit is better than those of the poly-
mers with BDT unit. Similar trends were also reported for
1
6
other BDT- and BDTT-based polymers.
Electrochemical Properties of the Polymers
OTS-modified SiO substrate. Hole mobilities of the polymers
2
The CV was performed with a solution of tetrabutylammonium
tetrafluoroborate (Bu NBF ) (0.10 M) in acetonitrile at a scan
were calculated from the transfer characteristics of the
OFETs and summarized in Table 4. The field-effect hole
mobilities of PBDTCN, PBDTTCN, PBDTF and PBDTTF are
4
4
rate of 100 mV/s at room temperature under argon atmos-
phere. A platinum electrode, Pt wire, and Ag/AgNO electrode
26
24
26
25
2
3
1
.6 3 10 , 4.1 3 10 , 1.1 3 10 , and 6.6 3 10 cm /
were used as the working, counter and reference electrode,
Vs, respectively. In case of the polymers with BDTT, the
mobilities were increased caused by its better crystallinity.
respectively. The energy level of the Ag/AgNO reference elec-
3
1
trode (calibrated by the FC/FC redox system) was 4.8 eV
below the vacuum level. The cyclic voltammetry curves are
shown in Figure 4 and the electrochemical data are summar-
The photovoltaic properties of the polymers were investi-
gated by fabricating the OPVs with ITO as positive electrode,
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the blend with polymers (PBDTCN, PBDTTCN, PBDTF, and
PBDTTF) and PC BM as active layer, and Al as negative
electrode. Figure 7 shows the current-voltage (I–V) curves of
4.39 nm, 8.70 nm, and 5.25 nm. In comparison, PBDTCN,
PBDTTCN, PBDTF and PBDTTF with 3% DIO exhibit
smaller RMS values of 2.61 nm, 1.85 nm, 1.95 nm, and
2.09 nm than the cases without DIO. The smoother surfaces,
without large domains caused by the addition of the DIO
additive, improves the interpenetrating network for optimal
charge generation and transport. The higher hole mobility in
addition to the improved morphology with 3% DIO is in
7
1
the OPVs with the configuration of ITO/PEDOT:PSS/poly-
2
mer:PC BM/Al under AM 1.5G irradiation (100 mW/cm )
7
1
which are summarized in Table 5. The BHJ active layers
were obtained by spin-coating of 2% (w/v) chlorobenzene
solutions comprising a blend of polymers and PC BM with
7
1
6
or without diiodooctane (DIO) additive. The device with
good accordance with their higher values of JSC and FF.
PBDTTCN and PC BM with 3% DIO show a V of 0.86 V, a
7
1
2
OC
JSC of 3.14 mA/cm , and a fill factor (FF) of 0.40, giving a
power conversion efficiency of 1.09%. The device comprising
PBDTTF and PC BM with 3% DIO show a V of 0.82 V, a
CONCLUSIONS
PBDTCN, PBDTTCN, PBDTF, and PBDTTF with substituted-
BDT/BDTT units as the electron rich units and pyrimidines,
with cyano or fluoro groups, as the electron deficient units,
were synthesized by Stille coupling reaction with Pd(0)-cata-
lyst to provide good solubility in common organic solvents.
The absorption spectra of PBDTTCN and PBDTTF thin films
show shift to a longer wavelength region than PBDTCN and
PBDTF. Solution-processed OPVs with the polymers as donor
and PC BM as the acceptor were fabricated, and the experi-
7
1
OC
2
short circuit current density (J ) of 3.28 mA/cm , and a FF
SC
of 0.39, giving a power conversion efficiency of 1.05%. The
PBDTCN and PBDTF device provided poor performance
because of its more disordered structure. The enhanced effi-
ciency of PBDTTCN and PBDTTF results from the good crys-
tallinity with the extended p-conjugation caused by the
BDTT unit. The better performance of PBDTTF and
PBDTTCN is in good accordance with their higher hole-
7
1
2
4
mental conditions with different blending ratios and DIO
additive were optimized. The device comprising PBDTTCN
and PC71BM (1:1.5) with 3% DIO showed a VOC of 0.86 V, a
mobilities around 10 order as compared with the case of
PBDTF and PBDTCN.
2
Two-Dimensional Grazing-Incidence Wide-Angle X-Ray
Scattering (2D-GIWAXS) and Structural Properties of the
Polymer
J
SC of 3.14 mA/cm , and a FF of 0.40, giving a power conver-
sion efficiency of 1.09%. The device comprising PBDTTF and
PC BM (1:1.5) with 3% DIO showed a V of 0.82 V, a JSC of
7
1
OC
2
2D-GIWAX was measured to determine the extent of cofacial
3.28 mA/cm , and a fill factor (FF) of 0.39, giving a power
p-stacking, crystallinity, and the polymer-packing orientation
relative to the substrate. Figure 8 shows the 2D-GIWAX
images of PBDTCN, PBDTTCN, PBDTF, or PBDTTF:PC BM
conversion efficiency of 1.05%. PBDTTCN and PBDTTF
provide the high V values and the hole mobility of around
OC
24
7
1
10 order and addition of DIO additive improved the mor-
phology significantly, which is in good accordance with the
device performances of the series. The 2D-GIWAX measure-
ment revealed that PBDTTCN and PBDTTF:PC BM blends
blends with 3% DIO. In Figure 8(A,C), 2D-GIWAX image of
PBDTCN or PBDTF: PC BM blends with BDT unit shows
7
1
2
1
weak diffraction peaks at q 5 0.27 and 0.28 Å , which cor-
z
7
1
respond to the (100) reflection of the polymer crystal with a
lamellar domain spacing (lattice parameter) of 2.37 and
with DIO have high structural organization.
2
.29 nm. In Figure 8(B,D) of PBDTTCN or PBDTTF:PC BM
71
blends, very strong (100) peaks at around 0.26 and 0.27
21
ACKNOWLEDGMENT
Å
were observed, which correspond to a lattice parameter
of 2.78 and 2.35 nm. Furthermore, the film with BDTT
showed (100), (200), and (300) peaks with more pro-
nounced reflections, indicating that the polymer stacks in the
film have higher structural organization than the case with
BDT. In both cases, a (010) p2p stacking peaks at around
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea Government (MEST)
(NRF-2014R1A1A2055318).
2
1
1.71 to 1.73 Å
were observed, which corresponds to a
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