Synthesis and characterization of conjugated low band-gap terpolymers incorporating carbazole for photovoltaic application

Article abstract of DOI:10.1166/jnn.2012.5895

A series of photoactive conjugated low band-gap copolymer (CPSB) and terpolyemrs (TPSBCz-n, n = 1 to 4) based on N-alkyl carbazole, 4,4'-dialkyl dithienosilole, and bezothiadiazole were synthesized. The copolymer and terpolymers were built with the fraction of the carbazole unit varied for 0, 2.5, 5, 10 and 25 mol%. Among the mixtures, the composition of 25 wt% of terpolymer bearing 10 mol.% of the carbazole unit, TPSBCz-3, and 75 wt% of C₇₁-PCBM found a power conversion efficiency of 0.86% with a open-circuit voltage of 0.59 V, the short-circuit current of 4.85 mA and fill factor of 0.30 under AM 1.5 spectral illumination. Our findings suggest that terpolymer bearing low concentration of carbazole lead to a high power conversion efficiency with improved the short-circuit current due to hole mobility enhancement effect of carbazole unit. Copyright

Full text of DOI:10.1166/jnn.2012.5895

Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 4279–4283, 2012
Synthesis and Characterization of Conjugated Low
Band-Gap Terpolymers Incorporating Carbazole
for Photovoltaic Application
Ki-Won Kim, Hyungjun Na, Won-Wook So, Sung Cheol Yoon, and Won Suk Shin∗
Energy Materials Research Center, Korea Research Institute of Chemistry Technology,
Sinseongno 19, Yuseong-Gu, Daejeon 305-600, Korea
A series of photoactive conjugated low band-gap copolymer (CPSB) and terpolyemrs (TPSBCz-n,
n = 1 to 4) based on N-alkyl carbazole, 4,4ꢀ-dialkyl dithienosilole, and bezothiadiazole were synthe-
sized. The copolymer and terpolymers were built with the fraction of the carbazole unit varied for
0, 2.5, 5, 10 and 25 mol%. Among the mixtures, the composition of 25 wt% of terpolymer bearing
10 mol.% of the carbazole unit, TPSBCz-3, and 75 wt% of C71-PCBM found a power conversion
efficiency of 0.86% with a open-circuit voltage of 0.59 V, the short-circuit current of 4.85 mA and fill
factor of 0.30 under AM 1.5 spectral illumination. Our findings suggest that terpolymer bearing low
concentration of carbazole lead to a high power conversion efficiency with improved the short-circuit
current due to hole mobility enhancement effect of carbazole unit.
Keywords: Organic Solar Cell, Terpolymer, Carbazole, Photovoltaic.
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1. INTRODUCTION
this, we introduced the small portion of carbazole moiety
into the polymer backbone, by which the general polymer
property is retained with the polymer without carbazole,
but the hole transporting property have been expected to
be improved. In this investigation, we would like to report
the preparation and photovoltaic properties of terpolymers
incorporating carbazole moiety.
There has been a great deal of interests in organic
photovoltaic system since the first report by Tang1 and
Sariciftci2 in 1987 and 1992. Among many prerequisites
required for a compound to be useful in solar cell applica-
tion, high PCE (power conversion efficiency) with effec-
tive harvest of the solar spectrum is of utmost importance.
For many years, several groups of chemists proposed new
backbone structures for polymers for low band gap poly-
mers as internal charge transfer from electron-rich to elec-
tron deficient unit.3–7
2. EXPERIMENTAL DETAILS
2.1. Synthesis
The polycarbazole derivatives were widely investigated
and applied in the OLED and OTFT due to photocon-
ductive and high charge mobility of carbazole unit.8–11
Recently, Lelcerc group reported a new polycarbazole
derivative, PCDTBT, bearing a secondary alkyl side chain
on the nitrogen atom of the carbazole unit that showed
high solubility and some organization, resulting in a rel-
atively good PCE of 3.6%.12 This result appears that it
could be improved in the mobility of the charge carriers
if carbazole group is introduced in the main chain to con-
struct the conjugated backbones.
2.1.1. Represnetative Polymerization Procedure:
Poly[2,6-(4,4ꢀ-dihexyldithienosilole)-Co-
{4,7-(2,13-benzothiadiazole)}] (CPSB)
A mixture of compound 6 (0.40 g, 0.58 mmol) and
7 (0.17 g, 0.58 mmol), palladium(II) acetate (4.0 mg,
0.017 mmol) and tricyclohexylphosphine (15.0 mg,
0.052 mmol) were added to a degassed toluꢁene (2 mL).
The mixture was vigorously stirred at 120 C for 48 h
under a nitrogen atmosphere. After the mixture was cooled
to room temperature, it was poured into a mixture of
methanol and distilled water (10:1 (v/v)) while being
stirred. Dark red precipitate was collected on a filter. The
recovered polymer was washed with methanol followed by
Soxhlet extraction with methanol to remove oligomers and
In general, the amorphous polymers showed low hole
mobility to get low Jsc and PCE. In order to overcome
∗Author to whom correspondence should be addressed.
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doi:10.1166/jnn.2012.5895
4279

Synthesis and Characterization of Conjugated Low Band-Gap Terpolymers Incorporating Carbazole
Kim et al.
catalyst residue. The recovered yield was 70% (0.20 g).
1H NMR (300 MHz, CDCl3, ppm): ꢀ 8.23–7.31 (m, 4 H,
Ar–H), 1.69–1.33 (m, 16 H, −CH2–), 1.10–0.99 (m, 10 H,
Si–CH2– and CH3ꢁ.
Chlorobenzene as the solvent. The electrolyte the poly-
mer thin films were dip-coated on a platinum rod (2 mm in
diameter) using solution employed was 0.10 M tetrabuty-
lammonium tetrafluoroborate (Bu4NBF4ꢁ in acetonitrile.
The Ag/AgNO3 and Pt wire (600 ꢂm in diameter) elec-
trodes were utilized as reference and counter electrodes,
respectively. The scan rate was 50 mV/s.
2.1.2. TPSBCz-1
This polymer was prepared by the same manner as
described above for the preparation of CPSB. A mix-
ture of compound 6 (0.30 g, 0.44 mmol), 7 (0.12 g,
0.42 mmol) and 8 (0.011 g, 0.02 mmol), palladium(II)
acetate (2.9 mg, 0.013 mmol) and tricyclohexylphosphine
(10.8 mg, 0.039 mmol). The polymer obtained was a
dark red solid. The yield was 60% (0.13 g). 1H NMR
(300 MHz, CDCl3, ppm): ꢀ 8.58–7.59 (br, 1.03 H, Ar–H),
7.48–6.81 (br, 1 H, Ar–H), 3.71 (br, 0.014 H, N–CH2–),
1.66–1.31 (br, 8.02 H, –CH2–), 1.10–0.99 (m, 4.95, Si–
CH2– and CH3ꢁ.
2.3. Photovoltaic Device Fabrication
Composite solutions with polymers and C71-PCBM were
prepared using a 1,2-dichlorobenzene (DCB). The concen-
tration was controlled adequately in a range of 1.0–
2.0 wt%. The polymer photovoltaic devices were fabricated
with a typical sandwich structure of ITO/PEDOT: PSS/
active layer/LiF/Al. Current density–voltage (J–V) char-
acteristics of all polymer photovoltaic cells were mea-
sured under the illumination of simulated solar light with
100 mW/cm2 (AM 1.5 G) by Oriel 1000 W solar simulator.
Electric data were recorded using a Keithley 236 source-
measure unit and all characterizations were carried out in
an ambient environment. The illumination intensity used
was calibrated by a standard Si photodiode detector from
PV measurements Inc. which was calibrated at NREL. The
incident photon-to-current conversion efficiency (IPCE)
was measured as a function of wavelength from 360 to
800 nm (PV measurement Inc.) equipped with a halo-
gen lamp as a light source, and calibration was performed
2.1.3. TPSBCz-2
1
The yield was 68% (0.14 g). H NMR (300 MHz, CDCl3,
ppm): ꢀ 8.58–7.55 (br, 1.08 H, Ar–H), 7.42–6.85 (br,
1 H, Ar–H), 3.71 (br, 0.042 H, N–CH2–), 1.66–1.30 (br,
8.08 H, –CH2–), 1.10–0.99 (m, 4.85, Si–CH2– and CH3ꢁ.
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2.1.4. TPSBCz-3
using a silicon reference photodiode. Thickness of the thin
film was measured using KLA Tencor Alpha-step IQ sur-
face profilometer with an accuracy of 1 nm.
Copyright: American Scientific Publishers
1
The yield was 74% (0.18 g). H NMR (300 MHz, CDCl3,
ppm): ꢀ 8.58–7.59 (br, 1.17 H, Ar–H), 7.48–6.81 (br,
1 H, Ar–H), 3.71 (br, 0.088 H, N–CH2–), 1.66–1.22 (br,
8.18 H, –CH2–), 1.10–0.99 (m, 4.69 H, Si–CH2– and
CH3ꢁ.
3. RESULTS AND DISCUSSION
3.1. General Properties of Polymers
2.1.5. TPSBCz-4
Synthetic routes to the preparation of monomer and poly-
mers can be found in Scheme 1. All polymers were pre-
pared by the Stille coupling reaction13ꢃ14 with 2,6-bis
(tributylstannanyl)-4,4-dihexyldithienosilole (6), 4,7-dibro-
mobenzo-thiadiazole (7) and 2,7-dibromo-N-dodecanyl
carbazole (8). They are soluble at room temperature in
common organic solvents such as chloroform, tetrahydro-
furan (THF), toluene, xylene, and dichlorobenzene. Table I
summarizes the polymerization results and general prop-
erties of the five polymers prepared in this investigation.
The actual contents of the carbazole moiety (8) incorpo-
rated into the terpolymers are lower than the feed ratio, also
molecular weights of terpolymers (TPSBCz-2 and 3) are
significantly reduced when compared with the molecular
weight of CPSB (Table I). Among the polymers, CPSB is
unique in that it is of alternating sequence of dithienosilole
and benzothiadiazole, whereas the other four terpolymers
are random sequence.
1
The yield was 55% (0.13 g). H NMR (300 MHz, CDCl3,
ppm): ꢀ 8.58–7.59 (br, 1.77 H, Ar–H), 7.48–6.81 (br, 1 H,
Ar–H), 3.71 (br, 0.37 H, N–CH2–), 1.66–1.22 (br, 8.91 H,
–CH2–), 1.10–0.99 (m, 5.02, Si–CH2– and CH3ꢁ.
2.2. Measurements
Molecular weights of polymers were determined by
gel permeation chromatography (GPC, Yong-Lin M930)
equipped with TDA 302 detector and PL-gel (Varian)
column using polystyrene as standard. THF was employed
as an eluent.
Thermal properties were studied under a nitrogen atmo-
sphere on a Mettler DSC 821 instrument. Thermogravi-
metric analysis (TGA) was performed under a nitrogen
ꢁ
atmosphere at the heating/cooling rate of 10 C/min on
a Mettler TGA 50 thermogravimetric analyzer. The redox
properties of polymers were examined by cyclic voltam-
metry (EG&G 1025).
Thermogravimetric analysis performed in a nitrogen
atmosphere shows that thermal stability of the present
4280
J. Nanosci. Nanotechnol. 12, 4279–4283, 2012

Kim et al.
Synthesis and Characterization of Conjugated Low Band-Gap Terpolymers Incorporating Carbazole
Br
S
n–BuLi
Ni(dppp)Cl2
r.t, 12 h
Mg
NBS
Br
S
S
Br
Et2O
–78ºC 1 h
S
S
Et2O
r.t 1 h
Br
AcOH, CHCl3
60ºC, 12 h
Br
(1)
(2)
Br
S
Si(C6H13)2Cl2
Et2O, 1 h
S
S
TMS–Cl
n–BuLi
TMS
TMS
TMS
–78 to R.T
12 h
TMS
S
Et2O, –78ºC
1 h
Si
Br
C6H13
C6H13
(3)
(4)
S
S
NBS
Br
S
S
Br
n–Butyl Li
THF, –78ºC, 1 h
SnMe3Cl
Sn
Sn
AcOH, CHCI3
60ºC, 12 h
–78 to r.t
12 h
Si
Si
C6H13
C6H13
C6H13
C6H13
(5)
(6)
S
N
N
S
S
S
Pd(OAc)2, P(C6H11 3
)
S
Sn
Sn
+
Br
Br
toluene, 120ºC, 48h
Si
Si
N
N
C6H13
C6H13
C6H13 C6H13
S
n
(7)
CPSB
S
N
N
S
S
Sn
Sn
+
+
Br
Br
Br
Br
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Si
C6H13
C6H13
12H25
Copyright: American Scientific Publishers
(8)
S
S
Pd(OAc)2, P(C6H11 3
)
toluene, 120ºC, 48 h
N
Si
N
N
C6H13
C6H13
C12H25
S
x
y
z
TPSBCz–n(n=1,2,3 and 4)
Scheme 1. Synthetic route to the monomer and CPSB and TPSBCz-n polymers.
polymers is fairly good. The decomposition temperature is
increased from 227 ꢁC for CPSB to 243 ꢁC for TPSBCz-4.
The glass transition temperatures (Tgꢁ of the polymers
obtained by differential scanning calorimetry (DSC) are
74 ꢁC for CPSB, 75, 77, 78, 83 ꢁC for TPSBCz-1, 2, 3 and
4. The relatively lower Tg values than expected for these
polyaromatic polymers can be ascribed to the relatively
low Mw and the presence of long alkyl groups in the
Table I. General properties of copolymer (CPSB) and terpolymers (TPSBCz).
Transition and
Repeating unit
ratio (x/y/z)
Average molecular
weights
decomposition
temperature (ꢁC)
Polymers
Feed ratio
Actual ratio (by NMR)
Mw
PDI
Tg
Td
CPSB
50/50/0
50/47.5/2.5
50/45/5
50/40/10
50/25/25
50/50/0
8ꢃ500
9ꢃ800
6ꢃ700
6ꢃ600
11ꢃ200
3.1
3.7
2.8
3.8
2.1
74
75
77
78
83
227
226
234
232
243
TPSBCz-1
TPSBCz-2
TPSBCz-3
TPSBCz-4
50/49.3/0.7
50/47.9/2.1
50/45.6/4.4
50/31.3/18.7
Notes: All GPC measurements were performed by using THF as an eluent with polystyrene as the calibration standard. The actual fractions of TPSBCz-n were determined
by 1H NMR. DSC and TGA was performed under a nitrogen atmosphere at the heating/cooling rate of 10 ꢁC/min.
J. Nanosci. Nanotechnol. 12, 4279–4283, 2012
4281

Synthesis and Characterization of Conjugated Low Band-Gap Terpolymers Incorporating Carbazole
Kim et al.
1.0
3.3. Photovoltaic Properties
CPSB
TPSBCz-1
The device configuration was glass/ITO/PEDOT:PSS
(50 nm)/polymer: C71-PCBM (∼70 nm)/LiF (0.6 nm)/Al
(120 nm). The active layer was spin-coated onto the
PEDOT:PSS layer using the polymer/C71-PCBM solu-
tion in dichlorobenzene. The optimal performances of
polymers were obtained from the conformation of
polymers/C71-PCBM ratio of 25:75 (w/w%). The I–V char-
acteristics and the power conversion efficiencies of repre-
sentative cells prepared from CPSB or TPSBCz blend with
C71-PCBM are shown in Figure 2(b) and Table III. Among
the mixtures, the TPSBCz-3 cell has highest power conver-
sion efficiency (PCE) of 0.86%, with a short circuit current
density (Jsc) of 4.85 mA/cm2, an open circuit voltage (Vocꢁ
of 0.59 V, and a fill factor (FF) of 0.30 under AM 1.5
spectral illumination.
0.8
0.6
0.4
0.2
0.0
TPSBCz-2
TPSBCz-3
TPSBCz-4
300
400
500
600
700
800
900
Wavelength(nm)
Fig. 1. UV-vis absorption spectra of the CPSB and TPSBCz-n in film.
dithienosilole and carbazole units. The presence of the car-
bazole units in the main chain of terpolymers appears to
increase the glass transition temperature.
The IPCE for solar cells with both active materials has
been measured, as shown in Figure 2(a). The cell with a
TPSBCz-3/C71-PCBM active layer exhibits a higher max-
imum IPCE (36% at 520 nm) and a wider coverage of
the visible spectral range, as expected from the absorption
spectrum of the polymers.
3.2. Optical and Electrochemical Properties
Figure 1 shows the absorption spectra of the five polymers
in thin films. According to the UV-vis absorption spectra
shown in Figure 1, co- and terpolymers display similar
absorption spectral features in chloroform solution and in
CPSB/ PC71BM
(a) 40
TPSBCz–1/ PC71BM
TPSBCz–2/ PC71BM
TPSBCz–3/ PC71BM
TPSBCz–4/ PC71BM
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the film state. The first two peaks in short wavelength
30
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region are originated from carbazole and dithienosilole
Copyright: American Scientific Publishers
unit, the second from the ꢄ–ꢄ∗ transition of the whole
20
10
0
conjugated systems.
The redox behaviors of the polymers were investigated
by cyclic voltammetry (CV).15 The LUMO levels then
were estimated from the HOMO levels and optical band
gaps (Eg) of polymers determined from the absorption
edges of their UV-vis absorption spectra. The work func-
tion of ITO was taken to be 4.8 eV. The results are sum-
marized in Table II. The HOMO level of the main chain of
the polymers steadily increased from −5ꢅ24 eV for CPSB
to −5ꢅ17 eV for TPSBCz-4 with increasing the fraction of
carbazole unit. The high value of HOMO can be attributed
to the electron-donating effect of the nitrogen atom in car-
bazole moiety.16ꢃ17
400
500
600
700
800
Wavelength (nm)
2
(b)
0
–2
–4
–6
Table II. Optical and electrochemical properties of copolymer (CPSB)
and terpolymers (TPSBCz).
CPSB/PC71BM
TPSBCz–1/ PC71BM
TPSBCz–2/ PC71BM
TPSBCz–3/ PC71BM
TPSBCz–4/ PC71BM
ꢆmax
ꢆmax
Egopt Eox HOMO LUMO
Polymers (in CHCl3ꢁ (in film) ꢆonset (eV) (V)
(eV)
(eV)
CPSB
543
531
518
542
541
534
547
545
560
560
714 1.74 0.495 5.24
704 1.76 0.453 5.20
723 1.72 0.448 5.19
728 1.70 0.429 5.17
731 1.69 0.434 5.17
3.50
3.44
3.47
3.47
3.48
TPSBCz-1
TPSBCz-2
TPSBCz-3
TPSBCz-4
–0.2
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Fig. 2. (a) IPCE (incident photon-to-current conversion efficiency) and
(b) current density–voltage characteristics of photovoltaic devices (CPSB
or TPSBCz-n/C71-PCBM) under AM 1.5 spectral illumination condition.
Notes: Eg stands for the band-gap energy estimated from the onset wavelength of
optical absorption. Eox is the onset potential of oxidation of polymers.
4282
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Kim et al.
Synthesis and Characterization of Conjugated Low Band-Gap Terpolymers Incorporating Carbazole
Table III. Characteristic current–voltage parameters from device testing
at standard AM 1.5 G condition.
funded by the Korea Research Council Industrial Sci-
ence and Technology, Republic of Korea and by the
Korea Energy Management Corporation (KEMCO) under
the New and Renewable Energy R&D Grant (2008-N-
PV08-02).
Polymers
Jsc
Voc
FF (fill factor)
PCE (%)
CPSB
1.74
3.19
2.02
4.85
2.49
0.43
0.44
0.54
0.59
0.64
0.28
0.30
0.28
0.30
0.29
0.21
0.41
0.31
0.86
0.47
TPSBCz-1
TPSBCz-2
TPSBCz-3
TPSBCz-4
References and Notes
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The Jsc value of the polymers steadily increases from
1.74 for CPSB to 4.85 for TPSBCz-3 with increasing the
fraction of carbazole unit, but the Jsc value of TPSBCz-
4 is dropped than that of TPSBCz-3. Generally, Jsc is
related to the hole mobility and absorption of polymers.
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in the terpolymer increases its solubility and mobility on
one hand, but on other hand reduces the amount of light-
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4. CONCLUSION
This paper describes the successful synthesis and appli-
cation of new terpolymers containing carbazole as donor
materials in organic bulk heterojunction solar cells. The
heterojunction solar cells based on the terpolymers as
donor materials and C71-PCBM as acceptor materials
showed PCEs from 0.21 to 0.86%. Although the PCE
value of synthesized polymers is not higher than previous
best results, this result could serve as a guideline for
improved organic bulk heterojunction solar cells.
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Acknowledgment: This work was supported by a grant
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Received: 10 May 2010. Accepted: 15 June 2011.
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