Synthesis and characterization of a thiadiazole/benzoimidazole-based copolymer for solar cell applications

Article abstract of DOI:10.1002/pola.24235

In this study, we synthesized a new polymer, PCTDBI, containing alternating carbazole and thiadiazole-benzoimidazole (TDBI) units. This polymer (number-average molecular weight = 25,600 g mol^-1), which features a planar imidazole structure into the polymeric main chain, possesses reasonably good thermal properties (T_g = 105 °C; T_d = 396 °C) and an optical band gap of 1.75 eV that matches the maximum photon flux of sunlight. Electrochemical measurements revealed an appropriate energy band offset between the polymer's lowest unoccupied molecular orbital and that of PCBM, thereby allowing efficient electron transfer between the two species. A solar cell device incorporating PCTDBI and PCBM at a blend ratio of 1:2 (w/w) exhibited a power conversion efficiency of 1.20%; the corresponding device incorporating PCTDBI and PC₇₁BM (1:2, w/w) exhibited a PCE of 1.84%.

Full text of DOI:10.1002/pola.24235

Synthesis and Characterization of a Thiadiazole/Benzoimidazole-Based
Copolymer for Solar Cell Applications
GUAN-YU CHEN,1 SHANG-CHE LAN,1 PO-YU LIN,1 CHIH-WEI CHU,2,3 KUNG-HWA WEI1
1Department of Material Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
2Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan
3Department of Photonics, National Chiao Tung University, Hsinchu 300, Taiwan
Received 26 May 2010; accepted 9 July 2010
DOI: 10.1002/pola.24235
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: In this study, we synthesized
a
new polymer,
thereby allowing efficient electron transfer between the two
species. A solar cell device incorporating PCTDBI and PCBM at
a blend ratio of 1:2 (w/w) exhibited a power conversion effi-
ciency of 1.20%; the corresponding device incorporating
PCTDBI and PC71BM (1:2, w/w) exhibited a PCE of 1.84%.
PCTDBI, containing alternating carbazole and thiadiazole-ben-
zoimidazole (TDBI) units. This polymer (number-average mo-
lecular weight ¼ 25,600 g molꢀ1), which features a planar
imidazole structure into the polymeric main chain, possesses
reasonably good thermal properties (Tg ¼ 105 ꢁC; Td ¼ 396 ꢁC)
and an optical band gap of 1.75 eV that matches the maximum
photon flux of sunlight. Electrochemical measurements
revealed an appropriate energy band offset between the poly-
mer’s lowest unoccupied molecular orbital and that of PCBM,
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2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
48: 4456–4464, 2010
KEYWORDS: carbazole; conjugated polymer; copolymerization;
heteroatom-containing polymers; imidazole; solar cell
INTRODUCTION Research into conjugated polymers possess-
ing extended arrays of delocalized p-electrons has advanced
dramatically because of their potential application in organic
optoelectronic devices, especially for the development of or-
ganic solar cells based on bulk heterojunctions (BHJs) of
conjugated polymers.1–5 Solution-processed BHJ solar cells
are simple-to-prepare devices featuring an electron-donating
conjugated polymer and an electron-accepting fullerene as
the active layer. Significant progress has been achieved
recently on improving the power conversion efficiency (PCE)
of polymer BHJ solar cells. For example, BHJ polymer solar
cells based on regioregular poly(3-hexylthiophene)/[6,6]-
phenyl-C61-butyric acid methyl ester (rr-P3HT/PC61BM)
composites can exhibit PCEs of up to 6%.6–10 To increase the
PCEs of these devices further, it will be necessary to improve
the amount of light absorbed by the active polymer. For
example, the absorption band of rr-P3HT covers wavelengths
only of less than 650 nm, whereas the maximum photon flux
of sunlight is located at 700 nm (1.77 eV).11 Thus, it is criti-
cal that the band gap of the polymer be narrowed so that it
can absorb more photons (relative to rr-P3HT) from sunlight
and, thereby, increase the photocurrent and the PCE of the
corresponding devices.
thereby resulting in partial charge separation along the poly-
mer backbone and a lower band gap. Moreover, increasing
the degree of delocalization of the p-electrons by increasing
the coplanarity of the polymeric structure can also lead to a
lower band gap.18–21
Carbazole derivatives are electron-donating materials that ex-
hibit good hole-transporting characteristics22–24; copolymers
based on carbazole derivatives display good performance
in solar cell applications.25–30 For example, poly[N-90-hepta-
decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzo-
thiadiazole)] (PCDTBT) possesses a band gap of 1.88 eV and
exhibits a high PCE of 6.1%.31 Conjugated polymers contain-
ing imidazole-moieties as acceptor units have also been
applied in solar cells exhibiting good performance (PCE ¼
ꢂ4.1%).31–34 The planar structure of the imidazole moiety
linked covalently to the polymeric side chain extends the
effective length of conjugation and lowers the band gap.
In this study, we prepared a new planar imidazole unit and
incorporated it into the polymeric main chain to extend the
effective conjugation length of the system and to increase
intramolecular charge transfer. Scheme
1 displays our
route toward the synthesis of the thiadiazole/benzoimidazole
(TDBI) unit M1. We expected the presence of the TDBI moi-
eties in the polymer backbone to lower the band gap and that
this polymer would possess a suitable lowest unoccupied
Several low-band gap conjugated polymers12–17 have been
synthesized that feature a main chain donor/acceptor struc-
ture to increase the degree of intrachain electron transfer,
Correspondence to: K.-H. Wei (E-mail: khwei@mail.nctu.edu.tw)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 4456–4464 (2010) 2010 Wiley Periodicals, Inc.
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ARTICLE
SCHEME 1 Reagents and condi-
tions: (i) n-octylmagnesium bro-
mide, ꢀ78ꢁC; then rt, overnight;
(ii) Et3N, Me3NꢃHCl, p-TsCl, rt, 2 h;
(iii) 4-bromophenol, K2CO3, DMF,
110 ꢁC, overnight; (iv) n-BuLi, ꢀ78
ꢁC, 1 h; then dry DMF, rt, over-
night; (v) Br2, HBr(aq), 100 ꢁC, 3 h;
(vi) TfOH, HNO3, H2SO4, rt, over-
night; (vii) 2-tributylstannylthio-
phene, PdCl2(PPh3)2, THF, 80 ꢁC,
20 h; (viii) Fe powder, AcOH,
80 ꢁC, 5 h; (ix) 4, H2O2, HCl, DMF,
rt, 1 h; (x) NBS, 0 ꢁC, 1 h.
molecular orbital (LUMO) and sufficiently large LUMO offset
for efficient electron transfer to PCBM. The LUMO energy
level of the polymer must be positioned above that of PCBM
sured through gel permeation chromatography (GPC) using a
Waters chromatography unit interfaced with a Waters 2414
different refractometer. Three 5-lm Waters styragel columns
were connected in series in decreasing order of pore size
(104, 103, and 102 Å); polystyrene was the standard and
THF was the eluant. UV-Vis absorption spectra were mea-
sured using an HP Agilent-8453 diode array spectrophotome-
ter. Cyclic voltammetry (CV) was performed using a BAS 100
electrochemical analyzer operated at a scan rate of 100 mV
by at least 0.3 eV to overcome the exciton binding energy35,36
;
if the LUMO energy level of the polymer is very close to that
of PCBM, there will be a loss of photocurrent.37 Since the elec-
tron-rich 2,7-carbazole possess good hole transport proper-
ties, incorporating 2,7-carbazole units into polymeric main
chains provide not only a high degree of conjugation but also
good intramolecular charge transfers.38,39 Scheme 2 displays
the copolymerization of the TDBI (M1) and carbazole (M2)
units, performed using a versatile Suzuki cross-coupling
reaction.
s
ꢀ1; the solvent was anhydrous MeCN containing 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as the
supporting electrolyte. A disk glassy carbon electrode coated
with a thin film of polymer was used as the working elec-
trode; a Pt wire was the counter electrode; Ag/Agþ (0.01 M
AgNO3) was the reference electrode; ferrocene/ferrocenium
(Fc/Fcþ) was used as the internal standard. Topographic
images of the copolymer/PCBM films were obtained using a
Digital Instruments Nanoscope III atomic force microscope
operated in the tapping mode under ambient conditions.
EXPERIMENTAL
Materials
2,1,3-Benzothiadiazole40 and M227,41 were prepared accord-
ing to reported procedures. Dimethylformamide (DMF) was
dried over MgSO4; tetrahydrofuran (THF) and diethyl ether
were dried over Na/benzophenone ketyl. All other reagents
were used as received from commercial sources, without
further purification.
Measurements and Characterization
1H and 13C NMR spectra were recorded using a Varian
Unity-300 NMR spectrometer. Differential scanning calorime-
try (DSC) was performed using a Perkin–Elmer Pyꢀri1s DSC1
ꢁ
instrument operated at a heating rate of 10 C min under
N2 purge. Thermogravimetric analysis (TGA) was performed
using a Du Pont TꢀG1A 2950 instrument operated at a heating
rate of 10 ꢁC min under a N2 purge. The number-average
(Mn) and weight-average (Mw) molecular weights were mea-
SCHEME 2 Reagents and conditions: (i) tetraethylammonium
hydroxide, Pd2dba3, THF, 80 ꢁC, 8 h.
THIADIAZOLE/BENZOIMIDAZOLE-BASED COPOLYMER, CHEN ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Solar Cells Device Fabrication and Characterization
reduced pressure. The crude product was purified through
An indium tin oxide (ITO, 10 X) coated glass substrate was
cleaned sequentially with detergent, DI water, acetone, and
isopropyl alcohol and then dried in the oven. Before use, the
substrate was treated with oxygen plasma. The active layers
were prepared by dissolving PCTDBI and PCBM at different
weight ratios but with a fixed total concentration (1.5 wt %)
in 1,2-dichlorobenzene, and then the solutions were spin-
coating at a speed of 1500 rpm onto the ITO/poly(3,4-ethyl-
enedioxthiophene):poly(styrene sulfonate) (PETDOT:PSS) (30
nm) substrates for 60 s (The model of spin coater is Laurell
WS-400B-6NPP/LITE). The films were dried at room temper-
ature. The thickness of each active layer was ꢂ90–100 nm.
Finally, Ca (40 nm) was thermally evaporated through a
shadow mask and then Al (100 nm) was evaporated to form
the top electrode, and the device area of 0.12 cm2. All fabri-
cations were performed in nitrogen-filled glovebox. The devi-
column chromatography (SiO2, EtOAc/hexane 1:9, Rf : 0.76)
to obtain 2 as a colorless oil (8.64 g, 90%, mp: 50–54 C).
ꢁ
1H NMR (300 MHz, CDCl3, ppm): d 7.79 (d, J ¼ 8.1 Hz, 2H),
7.32 (d, J ¼ 8.1 Hz, 2H), 4.56–4.52 (m, 1H), 2.44 (s, 3H),
1.56–1.54 (m, 4H), 1.27–1.17 (m, 24H), 0.88 (t, J ¼ 6.9 Hz,
6H). 13C NMR (75 MHz, CDCl3, ppm): d 144.9, 135.1, 129.8,
128.0, 84.9, 34.4, 32.1, 29.6, 29.5, 29.4, 24.9, 22.9, 21.8, 14.3.
MS (m/z): [M]þ calcd. for C24H42O3S, 410.3; found 410.
1-Bromo-4-(heptadecan-9-yloxy)benzene (3)
A mixture of 2 (2.00 g, 4.87 mmol), 4-bromophenol (1.01 g,
5.84 mmol), K2CO3 (3.36 g, 24.4 mmol), and DMF (60 mL)
ꢁ
was stirred overnight at 110 C in a three-necked flask. After
cooling to room temperature, the reaction mixture was
poured into water and extracted with EtOAc (3 ꢄ 100 mL).
The organic phase was then washed with water and brine.
After evaporating the solvent, the crude product was purified
through column chromatography (SiO2, hexane, Rf: 0.84) to
obtain 3 as a colorless oil (1.79 g, 90%).
1H NMR (300 MHz, CDCl3, ppm): d 7.34 (d, J ¼ 8.7 Hz, 2H),
7.75 (d, J ¼ 8.7 Hz, 2H), 4.18–4.11 (m, 1H), 1.64–1.61 (m,
4H), 1.40–1.25 (m, 24H), 0.85 (t, J ¼ 7.2 Hz, 6H). 13C NMR
(75 MHz, CDCl3, ppm): d, 157.9, 132.2, 117.6, 112.3, 78.5,
33.8, 31.8, 29.7, 29.5, 29.2, 25.3, 22.7, 14.1. MS (m/z): [M]þ
calcd. for C23H39BrO, 410.22; found 410.
ces were measured under AM 1.5 radiation (100 mW cmꢀ2
)
using an Agilent 4156 semiconductor parameter analyzer.
The spectral mismatch factor was calculated by comparison
of the solar simulator spectrum and the AM 1.5 spectrum at
room temperature. The external quantum efficiency (EQE)
was measured using a system established by Optosolar, Inc.
Monochromatic light was created from 500-W Xe lamp
source passing through a monochromator. The photocurrent
of the device was detected using a lock-in amplifier under
short-circuit conditions by illuminating the monochromatic
incident beam. A calibrated mono silicon diode exhibiting a
response at 300–800 nm was used as a reference.
4-(Heptadecan-9-yloxy)benzaldehyde (4)
n-BuLi (2.5 M in hexane, 1.17 mL, 2.94 mmol) was added
dropwise to a solution of 3 (1.00 g, 2.44 mmol) in dry THF
Synthetic Procedures
Heptadecan-9-ol (1)
ꢁ
1-Bromooctane (22.3 mL, 152 mmol) in dry ether (30 mL)
was added dropwise to a suspension of Mg (3.24 g, 135
mmol) in dry ether (40 mL) at room temperature. After stir-
ring at room temperature for 1 h, the resulting solution was
added slowly to a solution of ethylꢁ formate (4.00 g, 54.0
mmol) in dry ether (40 mL) at ꢀ78 C and then warmed to
room temperature. After stirring at room temperature over-
night, the mixture was poured into water and extracted with
CH2Cl2 (3 ꢄ 100 mL). The organic layer was dried (MgSO4)
and the solvent evaporated under reduced pressure. The
crude product was purified through recrystallization (MeCN)
(10 mL) in a 50-mL three-necked flask at ꢀ78 C. After stir-
ꢁ
ring for 1 h at ꢀ78 C, dry DMF (0.280 mL, 3.62 mmol) was
added to the reaction mixture, which was then stirred at
room temperature for 8 h. The reaction mixture was poured
into water and extracted with EtOAc (3 ꢄ 100 mL). The or-
ganic phase was dried (MgSO4) and the solvent evaporated
under reduced pressure. The crude product was purified
through column chromatography (SiO2, EtOAc/hexane 1:4,
Rf: 0.87) to obtain 4 as a viscous oil (0.61 g, 70%).
1H NMR (300 MHz, CDCl3, ppm): d 9.84 (s, 1H), 7.79 (d, J ¼
8.7 Hz, 2H), 6.79 (d, J ¼ 8.7 Hz, 2H), 4.34–4.31 (m, 1H),
1.66–1.60 (m, 4H), 1.40–1.23 (m, 24H), 0.85 (t, J ¼ 7.2 Hz,
6H). 13C NMR (75 MHz, CDCl3, ppm): d 190.7, 164.0, 132.1,
129.4, 115.6, 78.4, 33.8, 31.8, 29.6, 29.5, 29.2, 25.3, 22.6,
14.1. MS (m/z): [M]þ calcd. for C24H40O2, 360.3; found 361.
ꢁ
to obtain 1 as a white solid (11.3 g, 82%, mp: 58–62 C).
1H NMR (300 MHz, CDCl3, ppm): d 3.59–3.58 (m, 1H), 1.43–
1.38 (m, 8H), 1.36–1.27 (m, 21H), 0.88 (t, J ¼ 6.9 Hz, 6H).
13C NMR (75 MHz, CDCl3, ppm): d 72.3, 37.7, 32.1, 30.0,
29.9, 29.5, 25.9, 22.9, 14.3. MS (m/z): [M]þ calcd. for
C17H36O, 456.3; found 455.
4,7-Dibromo-2,1,3-benzothiadiazole (5)
9-Heptadecane p-Toluenesulfonate (2)
Br2 (3.36 mL, 65.5 mmol) was added slowly to a mixture of
2,1,3-benzotꢁhiadiazole (3.00 g, 22.0 mmol) and HBr(aq) (6.6
mL) at 100 C, followed by another charge of HBr(aq) (5 mL).
After stirring at 100 ꢁC for 3 h, the reaction mixture was
A solution of p-TsCl (4.09 g, 25.8 mmol) in CH2Cl2 (20 mL)
was added dropwise to a solution of 1 (6.00 g, 23.4 mmol),
Et3N (8.14 mL, 58.51 mmol), and Me3NꢃHCl (2.23 g, 24.4
mmol) in CH2Cl2 (30 mL) in a 100-mL flask at room temper-
ature. After stirring for 2 h, the mixture was poured into
water and extracted with CH2Cl2 (3 ꢄ 200 mL). The organic
phase was dried (MgSO4) and the solvent evaporated under
cooled to room temperature, poured into
a saturated
Na2S2O3 solution, and then filtered. The crude product was
purified through recrystallization (EtOH) to obtain 5 as a
ꢁ
white solid (5.1 g, 80%, mp: 184–188 C).
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ARTICLE
TABLE 1 Molecular Weights and Thermal Properties of PCTDBI
(15 mL). After stirring at room temperature for 1 h, the
reaction mixture was poured into water and extracted with
EtOAc (3 ꢄ 100 mL). The organic layer was dried (MgSO4)
and the solvent evaporated under reduced pressure. The
crude product was purified through column chromatography
(SiO2, EtOAc/hexane 1:9, Rf: 0.58) to yield 9 as a red solid
Mn
Mw
PDI
Tg (ꢁC)
Td (ꢁC)a
PCTDBI
25,600
32,560
1.27
105
396
a
Temperature at which 5% loss of the initial weight occurred.
ꢁ
(0.41 g, 50%, mp: 140–145 C).
1H NMR (300 MHz, CDCl3, ppm): d 9.63 (s, 1H), 9.07 (dd, J
¼ 1.2, 3.9 Hz, 1H), 8.12 (d, J ¼ 8.7 Hz, 2H), 8.01 (dd, J ¼ 1.2,
3.9 Hz, 1H), 7.56 (td, J ¼ 0.9, 5.7 Hz, 2H), 7.35–7.29 (m, 2H),
7.04 (d, J ¼ 8.7 Hz, 2H), 4.37–4.33 (m, 1H), 1.69-1.55 (m,
4H), 1.50–1.24 (m, 24H), 0.86 (t, J ¼ 7.2 Hz, 6H). 13C NMR
(75 MHz, CDCl3, ppm): d 162.3, 156.8, 150.6, 150.1, 144.6,
137.1, 136.8, 136.6, 130.9, 128.9, 128.3, 128.1, 127.3, 125.9,
120.2, 114.8, 113.8, 104.4, 98.8, 78.2, 33.8, 31.8, 29.7, 29.5,
29.3, 25.4, 22.6, 14.1. MS (m/z): [M]þ calcd. for
C38H46N4OS3, 670.3; found 671.
1H NMR (300 MHz, CDCl3, ppm): d 7.71 (s, 2H). 13C NMR
(75 MHz, CDCl3, ppm): d 153.3, 132.7, 114.2. MS (m/z):
[M]þ calcd. for C6H2Br2N2S, 293.8; found 293.
4,7-Dibromo-5,6-dinitro-2,1,3-benzothiadiazole (6)
Trifluoromethanesulfonic acid (7.53 mL, 85.0 mmol) and
HNO3 (1.8 mL) were added dropwise to H2SO4 (10 mL) at
ꢁ
ꢁ
0 C. 5 (2.00 g, 6.85 mmol) was added to the acid mixture at
0 C; the system kept at room temperature overnight before
being poured onto ice-water and then filtered. The filter
cake was washed with water and dried. The crude product
was purified through column chromatography (SiO2, EtOAc/
hexane 1:9, Rf : 0.3ꢁ4) to obtain 6 as a white solid (1.28 g,
48%, mp: 190–195 C).
13C NMR (75 MHz, CDCl3, ppm): d 151.3, 127.5, 110.3. MS
(m/z): [M]þ calcd. for C6Br2N4O4S, 383.8; found 384.
5,6-Dinitro-4,7-dithien-2-yl-2,1,3-benzothiadiazole (7)
PdCl2(PPh3)2 (75 mg, 0.11 mmol) was added to a solution of
2-tributylstannylthiophene (3.50 mL, 11.0 mmol) and 6
(1.40 g, 3.67 mmol) in dry THF (20 mL) and then the reac-
tion mixture was heated at 80 ꢁC for 20 h. After cooling to
room temperature, the solvent was evaporated and the crude
product washed with hexane and dried to yield 7 as an
ꢁ
orange solid (1.2 g, 84.2%, mp: 240–245 C).
1H NMR (300 MHz, CDCl3, ppm): d 7.75 (dd, J ¼ 1.2, 5.1 Hz,
2H), 7.52 (dd, J ¼ 1.2, 3.6 Hz, 2H), 7.25–7.22 (m, 2H). 13C
NMR (75 MHz, CDCl3, ppm): d 152.2, 141.8, 131.4, 131.0,
129.5, 128.0, 121.5. MS (m/z): [M]þ calcd. for C14H6N4O4S3,
390.0; found 390.
5,6-Diamino-4,7-dithien-2-yl-2,1,3-benzothiadiazole (8)
AcOH (50 mL) was added to a mixture of 7 (1.00 g, 2.56
mmol) and Fe powder (1.70 g, 30.4 mmol). The mixture was
ꢁ
heated at 80 C for 5 h before being cooled to room temper-
ature, poured into the water, and extracted with ether (3 ꢄ
100 mL). The organic phase was washed sequentially with
5% NaOH(aq) and water and then dried (MgSO4). Evaporation
of the solvent under reduced pressure yielded 8 as an
ꢁ
orange solid (0.77 g, 91%, mp: 219–225 C).
1H NMR (300 MHz, CDCl3, ppm): d 7.56 (dd, J ¼ 1.2, 5.1 Hz,
2H), 7.36 (dd, J ¼ 1.2, 3.6 Hz, 2H), 7.25 (t, J ¼ 3.6 Hz, 2H),
4.39 (s, 4H). 13C NMR (75 MHz, CDCl3, ppm): d 151.2, 139.7,
135.6, 128.8, 127.8, 127.5, 107.4. MS (m/z): [M]þ calcd. for
C
14H10N4S3, 330.0; found 330.
FIGURE 1 (a) Normalized UV-Vis absorption spectra of PCTDBI
and M1. (The concentrations of solution were 1 ꢄ 10ꢀ5 M; film
(93 nm) was spin-coated on quartz glass). (b) Absoprtion coeffi-
cient of PCTDBI film (90 nm). The film was spin-coated on
quartz glass).
4,10-Bis(thiophene-2-yl)-7-[4-(heptadecan-9-yloxy)-
phenyl]-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole (9)
H2O2 (0.87 mL) and HCl (0.34 mL) were added to a solution
of 8 (0.400 g, 1.21 mmol) and 4 (0.52 g, 1.45 mmol) in DMF
THIADIAZOLE/BENZOIMIDAZOLE-BASED COPOLYMER, CHEN ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Optical and Electrochemical Properties of PCTDBI
Eogpt (eV)a
E
(V)
E
(V)
onset
HOMO (eV)
LUMO (eV)
Eegc (eV)b
ox
onset
red
kmax (nm) sol
kmax (nm) film
PCTDBI
394,596
408,620
1.75
0.33
ꢀ1.83
Eegc ¼ HOMO – LUMO.
ꢀ5.13
ꢀ2.97
2.16
a
b
Calculated from the edge of the absorption spectrum of the film.
4,10-Bis(5-bromothiophene-2-yl)-7-
[4-(heptadecan-9-yloxy)-phenyl]-6H-[1,2,5]
thiadiazole[3,4-g]benzoimidazole (M1)
NBS (0.270 g, 1.49 mmol) was added in five portions to a
solution of 9 (0.500 g, 0.910 mmol) in DMF (15 mL) at 0 C.
After 1 h, the mixture was poured into water and extracted
with EtOAc (3 ꢄ 100 mL). The organic phase was dried
(MgSO4) and the solvent evaporated under reduced pressure.
The crude product was purified through column chromato-
graphy (SiO2, EtOAc/hexane 1:9, Rf :ꢁ 0.74) to yield M1 as
a red solid (0.35 g, 57%, mp: 95–100 C).
8 h. Phenylboronic acid (40.8 mg, 0.250 mmol) was then
added and the mixture stirred for 1 h at 80 ꢁC. Bromoben-
zene (30.0 lL, 0.250 mmol) was then added and the mixture
stirred for another 1 h at 80 ꢁC. The resulting mixture was
poured into MeOH (50 mL) and filtered. The precipitated
material was washed with acetone for 72 h in a Soxhlet
apparatus and then dried (75 mg, 58%).
ꢁ
1H NMR (300 MHz, CDCl3, ppm): d 9.04 (br, 2H), 7.97 (br,
4H), 7.59 (br, 5H), 7.05 (br, 2H), 6.63 (br, 1H), 4.71 (br, 1H),
4.34 (br, 1H), 2.37 (br, 2H), 1.66 (br, 8H), 1.32–1.15 (m,
1H NMR (300 MHz, CDCl3, ppm): d 9.27 (s, 1H), 8.73 (d, J ¼
4.2 Hz, 1H), 7.92 (d, J ¼ 8.7 Hz, 2H), 7.61 (d, J ¼ 3.9 Hz,
1H), 7.26–7.25 (m, 1H), 7.19 (d, J ¼ 3.9 Hz, 1H), 6.95 (d, J ¼
8.7 Hz, 2H), 4.38–4.34 (m, 1H), 1.68–1.58 (m, 4H), 1.46–1.25
(m, 24H), 0.86 (t, J ¼ 7.2 Hz, 6H). 13C NMR (75 MHz, CDCl3,
ppm): d 162.6, 156.9, 151.6, 149.1, 138.5, 138.3, 133.2,
133.1, 131.6, 130.2, 129.0, 128.7, 119.8, 119.4, 117.2, 115.5,
114.9, 114.7 110.0, 78.3, 33.8, 31.9, 29.8, 29.5, 29.3, 25.3,
22.6, 14.1. Anal. Calcd. for C38H44Br2N4OS3: C, 55.07; H, 5.35;
N, 6.76. Found: C, 55.35; H, 5.62; N, 6.80%. MS (m/z): [M]þ
calcd. For C38H44Br2N4OS3, 828.8; found 829.
PCTDBI
A mixture of M1 (100 mg, 0.120 mmol), M2 (79.6 mg, 0.120
mmol), and tetraethylammonium hydroxide (0.46 mL) in
ꢁ
THF (1.60 mL) was degassed at 50 C for 10 min. Tri(diben-
zlideneacetone)dipalladium(0) (2.20 mg, 2.40 lmol) and tri-
phenylphosphate (2.50 mg, 9.60 lmol) were added and then
the reaction mixture was heated under reflux at 80 ꢁC for
FIGURE 3 Current density-voltage characteristics of illuminated
(AM 1.5G, 100 mW cmꢀ2) polymer solar cells incorporating
active layers of (a) PCTDBI/PCBM and (b) PCTDBI/PC71BM.
FIGURE 2 CV trace of PCTDBI.
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ARTICLE
TABLE 3 Photovoltaic of Bulk Heterojunction Solar Cellsa
Heptadecan-9-ol (1), synthesized from n-octylmagnesium
bromide and ethyl formate, was reacted with p-TsCl to yield
9-heptadecane p-toluenesulfonate (2), which we then treated
with 4-bromophenol to obtain 1-bromo-4-(heptadecan-9-
yloxy)benzene (3). 4-(Heptadecan-9-yloxy)benzaldehyde (4)
was synthesized through carbonylation of compound 3. 4,7-
Dibromo-5,6-dinitro-2,1,3-benzothiadiazole (6) was obtained
through bromination and nitration of 2,1,3-benzothiadiazole;
it was then treated with 2-tributylstannylthiophene to
yield 5,6-dinitro-4,7-dithien-2-yl-2,1,3-benzothiadiazole (7)
via Stille coupling; reduction of the nitro groups provided
5,6-diamino-4,7-dithien-2-yl-2,1,3-benzothiadiazole (8). We
obtained 5,6-diamino-4,7-dithien-2-yl-2,1,3-benzothiadiazole
(9) from the condensation of 4 and 8. Subsequently, we pre-
pared M1 through the bromination of 9. We synthesized
PCTDBI through Suzuki cross-coupling copolymerization of
M1 and M2. The structures of the monomers and polymer
Weight Ratio of
Active Layer
Voc
Jsc
(mA cmꢀ2
PCE
(%)
(V)
)
FF
PCTDBI/PCBM ¼ 1:1
PCTDBI/PCBM ¼ 1:2
PCTDBI/PCBM ¼ 1:3
PCTDBI/PCBM ¼ 1:4
PCTDBI/PC71BM ¼ 1:1
PCTDBI/PC71BM ¼ 1:2
PCTDBI/PC71BM ¼ 1:3
PCTDBI/PC71BM ¼ 1:4
a
0.69
0.64
0.67
0.64
0.74
0.75
0.70
0.72
4.57
5.89
4.18
2.69
5.52
6.46
5.27
4.37
0.28
0.32
0.31
0.34
0.33
0.38
0.30
0.29
0.88
1.20
0.86
0.58
1.35
1.84
1.10
0.91
Device structure: ITO/PEDOT:PSS/active layer/Ca/Al.
44H), 0.87–0.77 (m, 12H). Anal. Calcd. for C67H85N5OS3: C,
75.02; H, 7.99; N, 6.53. Found: C, 72.44; H, 7.87; N, 6.09%.
1
were confirmed using H and 13C NMR spectroscopy and ele-
mental analysis. Table 1 lists the molecular weights, thermal
decomposition temperature, and glass transition temperature
of the polymer. The number-average weight (Mn) and poly-
dispersity index (PDI) of polymer were 25,600 g molꢀ1 and
1.27, respectively. This polymer exhibited good solubility in
1,2-dichlorobenzene and good thermal stability, as evidenced
RESULTS AND DISCUSSION
Synthesis and Characterization
Schemes 1 and 2 display the synthetic routes that we used
to prepare the monomer M1 and the copolymer PCTDBI.
FIGURE 4 Topographic images of PCTDBI/PCBM films deposited at blend ratios of (a) 1:1, (b) 1:2, (c) 1:3, and (d) 1:4.
THIADIAZOLE/BENZOIMIDAZOLE-BASED COPOLYMER, CHEN ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
by its 5% weight-loss temperatureꢁ (Td) of 396 ꢁC and glass
transition temperature (Tg) of 105 C.
Optical Properties
Figure 1(a) presents the absorption spectra of the polymer
in dilute CHCl3 solution (1 ꢄ 10ꢀ5 M) and as a solid film
(93 nm) on quartz glass; Table 2 summarizes the corre-
sponding optical data. In solution, PCTDBI exhibits two main
absorption bands at 320–480 and 490–720 nm, respectively.
Relative to the absorption spectrum of TDBI unit (M1), the
absorption peak of PCTDBI exhibited a red shift of 78 nm
that was caused by an increase in the conjugation length and
the presence of intramolecular charge transport between
TDBI and carbazole units. The longer-wavelength absorption
maximum of PCTDBI was located at 596 nm; this signal
appeared at a higher wavelength than that of the corre-
sponding carbazole-derivative polymer featuring benzothia-
diazole segments as the acceptor (545 nm),27 presumably
because the presence of the coplanar imidazole structure in
the polymer backbone increased the degree of coplanarity
which provided a longer effective conjugation length and
better intramolecular charge transfer.42,43 The absorption
spectrum of the polymer thin film was red-shifted related to
its solution spectrum, suggesting intermolecular interactions
and aggregation in the solid state. Figure 1(b) displays the
absorption coefficient (1.09 ꢄ 105 at kmax ¼ 620 nm) of
PCTDBI film (90 nm) on quartz glass. The optical band gap
(1.75 eV), calculated from the absorption edge (705 nm) of
the solid state film, is very close to the photon flux maxi-
mum of the solar spectrum (ꢂ1.77 eV), suggesting that
PCTDBI might be
a
useful material for solar cell
applications.
Electrochemical Properties
Cyclic voltammetry is used widely to investigate the redox
behavior of polymers and to estimate their highest occupied
molecular orbital (HOMO) and LUMO energy levels. We
recorded the CV traces of PCTDBI in MeCN containing 0.1 M
TBAPF6 at a potential scan rate of 100 mV sꢀ1. Figure 2 dis-
plays the electrochemical behavior of PCTDBI; Table 2 sum-
marizes the CV data. PCTDBI exhibited reversible oxidation
and reduction processes, the oxidation and reduction onset
was located at 0.33 V and ꢀ1.83 V that were attributed to
the oxidation and reduction capability of PCTDBI, respec-
tively. The HOMO and LUMO energy levels of PCTDBI were
ꢀ5.13 and ꢀ2.97 eV, respectively, according to the energy
level of the ferrocene reference (4.8 eV below vacuum
level).44,45 The higher HOMO level of PCTDBI than that of
other carbazole-containing polymers, is presumably caused
by the presence of electron-donating alkoxy groups in
PCTDBI’s side chain,46,47 and the higher PCTDBI’s HOMO in
turn results in a lower Voc for its heterojunction device. The
LUMO offset was 1.13 eV relative to the LUMO energy level
of PCBM (ꢀ4.1 eV),48 suggesting that electrons could be
transported efficiently to PCBM. The electrochemical band
gap (Eegc) was higher than the optical band gap (Eogpt), pre-
sumably because of the interface energy barrier between the
polymer film and the electrode surface.15,49
FIGURE 5 (a) Absorption spectra of active layer at various
PCTDBI/PCBM and PCTDBI/PC71BM blend ratios. (b) Spectra of
EQE spectra of active layers based on PCTDBI/PCBM and
PCTDBI/PC71BM (blend ratio, 1:2).
Photovoltaic Properties
Figure 3 displays the photovoltaic properties of devices hav-
ing the structure ITO/PEDOT:PSS/PCTDBI:PCBM/Ca/Al
under AM 1.5G illumination (100 mW cmꢀ2). Table 3 sum-
marizes the performance of the devices. These devices exhib-
ited open circuit voltages (Voc) of 0.64–0.69 V, which were
related to the energy difference between the HOMO energy
level of the copolymer and the LUMO energy level of
PCBM.50,51 When we increased the amount of PCBM from a
blend ratio of 1:1 to 1:2, the short circuit current density
(Jsc) increased significantly; it decreased upon increasing the
loading weight ratio of PCBM to 1:3 and 1:4 because of the
unfavorable morphologies of the active layer. Figure 4 dis-
plays AFM images of the surface morphologies of blended
films of PCTDBI/PCBM, prepared from 1,2-dichlorobenzene
solutions using a procedure identical to that used to fabri-
cate the active layers of the devices. For PCTDBI/PCBM at
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ARTICLE
3 Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hum-
melen, J. C.; van Hal, P. A.; Janssen, R. A. J Angew Chem Int
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weight ratios were 1:3 and 1:4, the images reveal rougher
surfaces compared with that obtained at a blend ratio at 1:2.
These rougher surfaces presumably arose from poor misci-
bility between PCTDBI and PCBM, which leaded to aggrega-
tion of PCBM that might limit the degree of charge dissocia-
tion and reduced the photocurrent.52–54 The AFM images
suggested that devices based on high PCTDBI/PCBM weight
ratios (1:3 or 1:4) would exhibit inefficient electron trans-
port to the electrode and, thus, lower values of Jsc. Accord-
4 Zhang, F.; Svensson, M.; Andersson, M. R.; Maggini, M.; Bucella,
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¨
5 Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R.;
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6 Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.;
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Dante, M.; Heeger, A. J. Science 2007, 317, 222–225.
and PCE (1.20%) at a blend ratio of 1:2. We also fabricated
a solar cell device incorporating PC71BM in place of PCBM as
the electron acceptor. Figure 5(a) reveals the absorption
spectra of the active layer at various PCTDBI with PCBM or
PC71BM weight ratios. As compare with the absorption spec-
tra of PCTDBI/PCBM, the spectra of PCTDBI/PC71BM exhibit
a broad absorption peaks around 500 nm that was contrib-
uted from PC71BM. Figure 5(b) displays the external quan-
tum efficiency (EQE) curves of the two devices; we observe
higher quantum efficiency at wavelengths below 550 nm for
the PCTDBI/PC71BM device, which was presumably caused
by the absorption of PC71BM in the shorter-wavelength
range and thus increased photocurrent.3 The short circuit
currents decreased when we increased the blend weight
ratios of PCTDBI/PC71BM to 1:3 and 1:4, similar to the
trend we observed for PCTDBI/PCBM. The device based on
PCTDBI/PC71BM (1:2) exhibited the highest values of Jsc
(6.46 mA cmꢀ2) and PCE (1.84%).
7 Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery,
K.; Yang, Y. Nat Mater 2005, 4, 864–868.
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CONCLUSIONS
15 Wang, E.; Wang, M.; Wang, L.; Duan, C.; Zhang, J.; Cai, W.;
The polymer PCTDBI, prepared through Suzuki coupling and
He, C.; Wu, H.; Cao, Y. Macromolecules 2009, 42, 4410–4415.
containing alternating carbazole and TDBI units in its main
16 Helgesen, M.; Gevorgyan, S. A.; Krebs, F. C.; Janssen, R. A.
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J. Chem Mater 2009, 21, 4669–4675.
396 ꢁC) and decent number-average molecular weight
(25,600 g molꢀ1). The band gap of this polymer was 1.75 eV
as a result of incorporating planar thiadiazole/benzoimida-
zole units into the polymeric backbone. Moreover, this poly-
mer exhibited a suitable LUMO energy level to allow efficient
charge dissociation. A solar cell device fabricated with
PCTDBI/PCBM at a blend ratio of 1:2 (w/w) exhibited a
value of Jsc of 5.89 mA cmꢀ2 and a PCE of 1.20%; these val-
ues increased to 6.46 mA cmꢀ2 and 1.84%, respectively, for
the corresponding device incorporating PCTDBI/PC71BM at
a blend ratio of 1:2 (w/w).
17 Mishra, S. P.; Palai, A. K.; Srivastava, R.; Kamalasanan, M.
N.; Patri, M.
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