Synthesis and photovoltaic properties of functional dendritic oligothiophenes

Article abstract of DOI:10.1002/pola.24613

Novel p-type and low bandgap functional dendritic oligothiophenes bearing hole-transporting carbazole as peripheral substituents and an electron-withdrawing dicyanovinyl core group, namely, DCT(n)-DCN, where n = 1 or 2 for solution-processable photovoltaic (PV) applications have been synthesized. With electron-donating carbazole surface-functionalized moieties conjugated with dicyanovinyl core group, the optical bandgap of these functional dendritic oligothiophene thin-films greatly reduces to 1.74 eV with a strong spectral broadening and a high ionization potential at ～5.5 eV as determined by UV photoelectron spectroscopy. The bulk heterojunction PV cells fabricated from these dendrimers blended with PC₇₁BM as an acceptor showed a power conversion efficiency up to 1.64% with an open circuit voltage of (V _oc) = 0.93 V in the annealed device. We have demonstrated that the desirable molecular and PV properties of dendritic oligothiophenes can be obtained/tuned by the incorporation of functional group(s) onto peripheral of the dendron and into the core. In addition, these functional dendritic oligothiophenes show superior functional properties even at low dendritic generation as compared to the unsubstituted higher generation dendritic oligothiophenes as a p-type, low-bandgap semiconductor for solution-processable bulk heterojunction PV cells.

Full text of DOI:10.1002/pola.24613

Synthesis and Photovoltaic Properties of Functional
Dendritic Oligothiophenes
WEIFENG ZHANG,1,2 GING MENG NG,3 HOI LAM TAM,3 MAN SHING WONG,1,2 FURONG ZHU4
1Institute of Molecular Functional Materials, Areas of Excellence Scheme, University Grants Committee, Hong Kong
2Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University,
Kowloon Tong, Hong Kong SAR, China
3Institute of Materials Research and Engineering, 3 Research Link, Singapore
4Department of Physics and Centre for Advanced Luminescence Materials, Hong Kong Baptist University,
Kowloon Tong, Hong Kong SAR, China
Received 20 October 2010; accepted 29 January 2011
DOI: 10.1002/pola.24613
Published online 4 March 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Novel p-type and low bandgap functional dendritic oli-
gothiophenes bearing hole-transporting carbazole as peripheral
substituents and an electron-withdrawing dicyanovinyl core
group, namely, DCT(n)-DCN, where n ¼ 1 or 2 for solution-proc-
essable photovoltaic (PV) applications have been synthesized.
With electron-donating carbazole surface-functionalized moieties
conjugated with dicyanovinyl core group, the optical bandgap of
these functional dendritic oligothiophene thin-films greatly
reduces to 1.74 eV with a strong spectral broadening and a high
ionization potential at ꢀ5.5 eV as determined by UV photoelectron
spectroscopy. The bulk heterojunction PV cells fabricated from
these dendrimers blended with PC71BM as an acceptor showed a
power conversion efficiency up to 1.64% with an open circuit volt-
age of (Voc) ¼ 0.93 V in the annealed device. We have demon-
strated that the desirable molecular and PV properties of dendritic
oligothiophenes can be obtained/tuned by the incorporation of
functional group(s) onto peripheral of the dendron and into the
core. In addition, these functional dendritic oligothiophenes show
superior functional properties even at low dendritic generation as
compared to the unsubstituted higher generation dendritic oligo-
thiophenes as a p-type, low-bandgap semiconductor for solution-
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processable bulk heterojunction PV cells. 2011 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 49: 1865–1873, 2011
KEYWORDS: dendrimers; dicyanovinyl; donor material; func-
tional oligothiophenes; organic solar cell
INTRODUCTION Bulk heterojunction photovoltaic (PV) cells
have recently received considerable attention from both aca-
demia and industry as promising alternative renewable
energy sources due to their low-cost, lightweight, ease of
large-area fabrication by solution-process, and compatible to
flexible substrate.1 Through the improvement of the material
properties, fabrication techniques, and device structures, the
power conversion efficiencies (PCE) of the bulk heterojunc-
tion PV cells based on solution-processable p-type conju-
gated semiconducting materials blended with a soluble full-
erene derivative such as [6,6]-phenyl C61-butyric acid
methyl ester (PCBM) have recently reached higher than 7%.2
Meanwhile, solution-processable small molecule-based
organic thin-film solar cells have also advanced significantly
with efficiencies more than 4% recently.3 However, more effi-
cient and stable devices would still need further develop-
ment of superior materials as well as processing and device
optimization before commercialization can be realized.
Among the factors that limit the PCE of the heterojunction
PV cells, the poor sunlight harvesting efficiency and the low
charge carrier transport efficiency of organic semiconducting
materials are the most determining ones. Thus, the develop-
ments of new p-type organic/polymer semiconductors that
possess a broad absorption and narrow band gap with high
charge carrier mobility are prerequisite for achieving high
efficiency organic PV cells.4
Dendrimers are monodisperse, well-defined, highly branched
macromolecules. Because of their unique and controllable
dendritic architectures, various functional dendrimers have
been exploited as an active component for molecular elec-
tronics and optoelectronics such as organic light emitting
diodes,5 field-effect transistors,6 and PV devices7 in the past
decades. Dendritic molecules have drawn increasing atten-
tion as alternative solution-processable p-type donor materi-
als because of the ease to obtain in a high degree of purity
and the monodisperse nature which can aid the film mor-
phology and charge transport.8 The strong p-p interactions
of p-conjugated dendrimers can provide high degree of mo-
lecular order, which has been shown to be beneficial to the
charge transport and PV device efficiency. Furthermore, the
Correspondence to: M. S. Wong (E-mail: mswong@hkbu.edu.hk) or F. Zhu (Email: frzhu@hkbu.edu.hk)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 1865–1873 (2011) 2011 Wiley Periodicals, Inc.
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES, ZHANG ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
dendritic structure often renders the solubility of a material
for solution processing of large area thin films. Dendritic oli-
gothiophenes have recently shown promise as a p-type
donor material for PV cell applications in which the effi-
ciency increases with the dendritic generation.9 The use of
the intramolecular charge transfer from an electron-rich unit
crude product was purified by silica-gel column chromato-
graphy using petroleum ether as eluent affording the desired
product as a colorless liquid (4.3 g, 91%).
1H NMR (400 MHz, CDCl3, d) 7.08 (d, J ¼ 3.2 Hz, 1H), 6.98
(d, J ¼ 3.2 Hz, 1H), 0.29 (s, 9H). 13C NMR (100 MHz, CDCl3,
d) 143.3, 134.3, 131.1, 116.7, ꢁ0.26. MS (FAB): m/z 235.9
(Mþ).
to an electron-deficient unit has become
a promising
approach to obtain low bandgap molecular/polymer sys-
tems.10 In addition, the choice of the electron-rich and/or
electron-deficient moieties provide a means to proper con-
trol of the positions of the frontier molecular orbitals of the
donor and acceptor materials which are indispensable to
achieve effective exciton dissociation at the heterojunction
and a large open-circuit voltage (Voc) of a solar cell and also
directly related to the photo-oxidation stability of the semi-
conducting materials.
5,500-Bis(trimethylsilyl)-2,20:30,200-terthiophene (3)
To magnesium turnings (1.3 g, 56.0 mmol) in diethyl ether
(40 mL) was added dropwise a solution of 2-bromo-5-(tri-
methylsilyl)thiophene 2 (8.8 g, 37.4 mmol) in diethyl ether
(20 mL). After complete addition, the mixture was refluxed
for 2 h and then transferred into a round-bottom flask
containing
a solution of 2,3-dibromothiophene (3.3 g,
13.6 mmol) and Ni(dppp)Cl2 (100 mg) in 40-mL diethyl
ꢂ
ether at 0 C. The reaction mixture was refluxed for 4 h and
We report, herein, the synthesis and functional properties of
novel p-type and low bandgap functional dendritic oligothio-
phenes bearing hole-transporting carbazole as peripheral
substituents and an electron-withdrawing dicyanovinyl
group, namely, DCT(n)-DCN, where n ¼ 1 or 2 for solution-
processable PV applications. With carbazole surface-function-
alized moieties conjugated with dicyanovinyl core group, the
optical bandgap of these functional dendritic oligothiophene
thin-films greatly reduces to 1.74 eV with a strong spectral
broadening and a high ionization potential in the range of
5.47–5.61 eV as determined by UV photoemission spectros-
copy. The bulk heterojunction PV cells fabricated from these
dendrimers blended with PC71BM as an acceptor showed a
PCE up to 1.64% with Voc ¼ 0.93 V in the annealed devices.
Our findings suggest that the desirable molecular and PV
properties of dendritic oligothiophenes can be obtained/
tuned by the incorporation of functional group(s) onto pe-
ripheral of the dendron and into the core. In addition, these
functional dendritic oligothiophenes show superior func-
tional properties even at low dendritic generation when
compared with the unsubstituted higher generation dendritic
oligothiophenes as a p-type, low-bandgap semiconductor for
solution-processable bulk heterojunction PV cells.
then stirred at room temperature overnight under N2. The
resulting solution was poured into ice-water and extracted
with diethyl ether (3 ꢃ 50 mL). The combined organic
extract was washed with brine and water and then dried
over anhydrous Na2SO4. The solvent was removed by rotary
evaporation, and the residue was purified by silica gel
column chromatography using petroleum ether as eluent
affording the desired product 3 as a light yellow viscous oil
(4.4 g, 83%).
1H NMR (400 MHz, CD3COCD3, d) 7.55 (d, J ¼ 5.2 Hz, 1H),
7.27 (d, J ¼ 5.2 Hz, 1H), 7.25 (d, J ¼ 3.6 Hz, 1H), 7.21 (d,
J ¼ 2.4 Hz, 1H), 7.20 (d, J ¼ 2.4 Hz, 1H), 7.17 (d, J ¼ 3.6 Hz,
1H), 0.31 (s, 9H), 0.29 (s, 9H). 13C NMR (100 MHz,
CD3COCD3, d) 143.4, 142.8, 141.0, 140.8, 135.4, 135.3, 133.0,
131.7, 130.6, 130.1, 128.6, 126.2, ꢁ0.04, ꢁ0.09. MS (MALDI-
TOF): m/z 392.1 (Mþ þ 1).
5,500-Diiodo-2,20:30,200-terthiophene (4)
To
a
solution of 5,500-bis(trimethylsilyl)-2,20:30,200-terthio-
phene 3 (4.2 g, 10.7 mmol) in 30-mL tetrahydrofuran (THF)
was added dropwise in a solution of iodine monochloride
ꢂ
(2.5 mL in 40-mL THF, 50 mmol) at ꢁ78 C. After complete
addition, the solution mixture was stirred at ꢁ78 ꢂC for 1h,
followed by quenching with 100 mL 1 M Na2SO3 solution.
The organic layer was separated and the aqueous layer was
extracted with diethyl ether (3 ꢃ 50 mL). The combined or-
ganic extract was washed with brine, water and then dried
over anhydrous Na2SO4. After removal of solvent, the residue
was purified by silica gel column chromatography using pe-
troleum ether as eluent affording the desired product as
light yellow viscous oil (5.2 g, 97%).
EXPERIMENTAL
Synthesis
2-Bromo-5-(trimethylsilyl)thiophene (2)
To a solution of diisopropylamine (2.83 g, 3.92 mL, and 28
mmol) in THF (40 mL) at ꢁ78 ꢂC was added dropwise
1.6 M n-butyllithium (15 mL, 24.0 mmol). The mixture was
warmed to 0 ꢂC for 5 min and then recooled to ꢁ78 ꢂC. 2-
Bromothiophene, 1 (1.94 mL, 20 mmol) was added drop-
wise, and then the solution was warmed to 0 ꢂC for 5 min.
After cooling to ꢁ78 ꢂC, trimethylsilyl chloride (3.95 mL,
24.0 mmol) was added in one portion, and the solution was
allowed to warm to room temperature for 30 min. The reac-
tion mixture was poured into water with 0.3 mL of 3 M
hydrochloric acid and the aqueous layer was extracted with
diethyl ether (3 ꢃ 50 mL). The organic extracts were washed
with Na2CO3 solution and brine. After drying over anhydrous
Na2SO4, the solvent was removed by rotary evaporation. The
1H NMR (400 MHz, CD3COCD3, d) 7.57 (d, J ¼ 5.2 Hz, 1H)
7.32 (d, J ¼ 3.6 Hz, 1H), 7.27 (d, J ¼ 3.6 Hz, 1H), 7.25 (d,
J ¼ 5.2 Hz, 1H), 6.93 (d, J ¼ 3.6 Hz, 1H), 6.91 (d, J ¼ 3.6 Hz,
1H). 13C NMR (CD3COCD3, 100MHz, d) 143.7, 140.9, 138.5,
138.2, 132.7, 131.1, 130.9, 130.1, 129.59, 127.1, 76.4, 74.9.
MS (MALDI-TOF): m/z 499.8 (Mþ þ 1).
5,500-Bis[(9-butylcarbazol)-3-yl]-2,20:30,200-terthiophene (9)
To a 100-mL two-neck round-bottom flask was added 9-
butylcarbazole-3-boronic acid, 8 (2.56 g, 9.6 mmol), 5,500-
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ARTICLE
diiodo-2,20:30,200-terthiophene 4 (2.0 g, 4.0 mmol), Pd(PPh3)4
(100 mg), THF (60 mL), and 2M K2CO3 (32 mL). The solu-
tion mixture was heated to 80 ꢂC overnight under N2. After
cooling to room temperature, the mixture was poured into
water and extracted with dichloromethane (3 ꢃ 30 mL). The
combined organic phase was dried over anhydrous Na2SO4
and evaporated to dryness. The crude product was purified
by silica gel column chromatography eluting with petroleum
ether/dichloromethane affording the desired product 9 as a
yellow solid (2.2 g, 84%).
reflux overnight under N2. After cooling to room tempera-
ture, the reaction mixture was poured into water, neutralized
with 0.1 M HCl, and extracted with CHCl3 (3 ꢃ 20 mL). The
combined organic phase was washed with water, dried over
anhydrous Na2SO4, filtered, and evaporated to dryness. The
residual solid was purified by silica gel column using petro-
leum ether/dichloromethane as eluent affording a black red
solid which was further purified by precipitation with
CHCl3/CH3OH, washed with CH3OH and dried in vacuo
affording the desired product as a black solid (204 mg,
93%).
1H NMR (400 MHz, CDCl3, d) 8.29 (s, 2H), 8.11 (d, J ¼ 7.6 Hz,
2H), 7.70 (dd, J ¼ 8.4 Hz , J ¼ 0.8 Hz, 2H), 7.47–7.43 (m, 2H),
7.39–7.35 (m, 4H), 7.30–7.14 (m, 8H), 4.29 (t, J ¼ 7.0 Hz, 4H),
1.87–1.79 (m, 4H), 1.42–1.33 (m, 4H), 0.94 (t, J ¼ 7.6 Hz, 6H).
13C NMR(100 MHz, CDCl3, d) 146.9, 145.6, 140.9, 140.1,
139.8, 135.7, 133.0, 131.9, 131.4, 129.9, 128.9, 127.5, 125.9,
125.8, 125.5, 125.2, 124.4, 124.0, 123.2, 123.1, 122.7, 122.2,
120.6, 120.5, 119.0, 118.9, 117.7, 117.6, 109.8, 108.9, 108.8,
42.9, 31.1, 20.5, 13.9. MS (MALDI-TOF): m/z 690.7 (Mþ þ 1).
1H NMR (400 MHz, CDCl3, d) 8.33 (dd, J ¼ 7.2 Hz, J ¼ 1.6
Hz, 2H), 8.12 (d, J ¼ 4.0 Hz, 2H), 7.74–7.70 (m, 2H), 7.69 (s,
1H), 7.52 (s, 1H), 7.49–7.45 (m, 2H), 7.41–7.38 (m, 4H),
7.31–7.29 (m, 3H), 7.25–7.20 (m, 4H), 7.07 (s, 1H), 4.33–
4.29 (m, 4H), 2.89–2.81 (m, 6H), 1.90–1.82 (m, 4H), 1.79–
1.66 (m, 6H), 1.45–1.35 (m, 24H), 0.96–0.92 (m, 13H). 13C
NMR (100 MHz, CDCl3, d) 149.9, 147.1, 145.9, 144.1, 141.9,
140.9, 140.62, 140.60, 140.58, 140.2, 140.1, 135.2, 134.4,
133.9, 133.1, 132.6, 131.99, 131.91, 131.7, 131.6, 131.5,
131.1, 129.4, 128.8, 128.6, 127.9, 125.99, 125.96, 125.4,
125.1, 124.1, 123.2, 122.74, 122.72, 122.3, 122.2, 120.56,
120.55, 119.08, 119.03, 117.72, 117.66, 114.5, 113.6, 109.02,
109.00, 108.9, 75.6, 42.9, 31.72, 31.65, 31.59, 31.1, 30.6,
30.5, 29.9, 29.5, 29.3, 29.2, 22.69, 22.66, 22.60, 20.6, 14.17,
14.14, 14.11, 13.88. HRMS (MALDI-TOF): calcd: For
11
To a 100-mL two-neck round-bottom flask was added 5,500-
bis[(9-butylcarbazol)-3-yl]-2,20:30,200-terthiophene-50-boronic acid
and 10 (295 mg, 0.4 mmol), which was prepared by 5,500-
bis[(9-butylcarbazol)-4-yl]-2,20:30,200-terthiophene, 9 (1.04 g, 1.5
mmol), 1.6 M n-BuLi (3.0 mL, 4.5 mmol), and trimethyl borate
(0.5 mL, 5 mmol) using the procedure of 9-butylcarbazole-3-
boronic acid, 8), 5-bromo-3,40,400-terthexyl-[4,20,200]terthiophene-
500-carboxaldehyde, 15 (364 mg, 0.5 mmol), Pd(PPh3)4 (50 mg),
THF (20 mL), and 2 M K2CO3 (1.0 mL). The solution mixture
C
C
78H80N4S6: 1264.4707; Found: 1264.4694. Anal calc. for
78H80N4S6: C 74.01, H 6.37, N 4.43; found: C 74.39, H 6.32,
N 4.33.
ꢂ
was heated to 80 C overnight under N2. After cooling to room
5000,500000-Bis{5,500-bis[(9-butylcarbazol)-3-yl]-2,20:30,200-
terthiophen-50-yl}-2000, 20000:30000,200000-terthiophene (12)
The synthetic procedure for 9 was followed using boronic
acid 10 (1.03 g, 1.4 mmol), 5,500-diiodo-2,20:30,200-terthio-
phene 4 (290 mg, 0.58 mmol), Pd(PPh3)4 (60 mg), 2 M
K2CO3 (6 mL), THF (60 mL) at 80 ꢂC. The crude product
was purified by silica gel column chromatography using
petroleum ether/dichloromethane as eluent affording a red
solid (780 mg, 82%).
temperature, the reaction mixture was poured into water
and extracted with dichloromethane (3 ꢃ 30 mL). The com-
bined organic phase was dried over anhydrous Na2SO4 and
evaporated to dryness. The crude product was purified by
silica gel column chromatography using petroleum ether/
dichloromethane as eluent affording the desired product 11
as a yellow solid (389 mg, 80%).
1H NMR (400 MHz, CDCl3, d) 9.81 (s, 1H), 8.32 (dd, J ¼ 8.0
Hz, J ¼ 1.4 Hz, 2H), 8.30–8.09 (m, 2H),7.72–7.69 (m, 2H),
7.57 (s, 1H), 7.48–7.35 (m, 6H), 7.29–7.19 (m, 7H), 7.13 (s,
1H), 7.04 (s, 1H), 4.30–4.26 (m, 4H), 2.88–2.79 (m, 6H),
1.88–1.80 (m, 4H), 1.76–1.66 (m, 6H), 1.44–1.32 (m, 24H),
0.95–0.89 (m, 13H). 13C NMR (100 MHz, CDCl3, d) 182.6,
147.0, 145.9, 141.3, 140.9, 140.5, 140.3, 140.1, 140.0, 139.9,
139.2, 135.2, 134.1, 133.4, 132.9, 132.7, 132.5, 131.9, 131.5,
130.6, 130.5, 129.1, 128.8, 128.4, 127.9, 125.96, 125.93,
125.4, 125.1, 124.0, 123.2, 122.73, 122.71, 122.24, 122.19,
120.5, 119.1, 119.0, 117.7, 117.6, 109.00, 108.98, 108.88,
42.93, 31.72, 31.68, 31.63, 31.1, 30.6, 30.5, 30.2, 29.7, 29.44,
29.41, 29.3, 29.2, 29.16, 22.69, 22.65, 22.62, 20.5, 14.17,
14.15, 14.12, 13.9. MS (MALDI-TOF): m/z 1216.4 (Mþ þ 1).
1H NMR (400 MHz, CDCl3, d) 8.21 (dd, J ¼ 9.6 Hz, J ¼ 1.6 Hz,
4H), 8.01 (d, J ¼ 8.0 Hz, 4H), 7.62–7.57 (m, 4H), 7.38–7.33
(m, 4H), 7.30–7.07 (m, 26H), 7.04 (d, J ¼ 2.0 Hz, 1H), 6.99 (d,
J ¼ 3.6 Hz, 1H) 4.19 (q, J ¼ 6.8 Hz, 8H), 1.78–1.69 (m, 8H),
1.33–1.22 (m, 8H), 0.85–0.82 (m, 12H). 13C NMR (100 MHz,
CDCl3, d) 146.93, 146.86, 145.92, 145.88, 140.82, 140.06,
139.97, 137.95, 136.71, 136.60, 135.30, 135.13, 135.05,
134.98, 133.95, 132.78, 132.71, 132.25, 132.21, 131.79,
131.30, 130.84, 130.59, 129.86, 128.84, 128.77, 128.72,
128.01, 127.98, 126.54, 126.63, 126.39, 125.89, 125.83,
125.37, 125.14, 125.12, 125.06, 124.22, 124.17, 124.04,
123.16, 122.72, 122.70, 122.20, 122.12, 120.56, 119.01,
118.95, 117.65, 117.59, 108.94, 108.82, 108.79, 42.88, 31.08,
20.51, 13.85. MS (MALDI-TOF): m/z 1626.7 (Mþ þ 1).
DCT(1)-DCN
To a 100-mL round-bottom flask containing 11 (210 mg,
0.20 mmol) and chloroform (20 mL) was added malononi-
trile (0.40 g, 6.1 mmol) and pyridine (0.8 mL) at room tem-
perature. The solution mixture was stirred and heated to
14
The synthetic procedure of 11 was followed using 5000,500000
-
bis{5,500-bis[(9-butylcarbazol)-3-yl]-2,20:30,200-terthiophen-50-yl}-
2000,20000:30000,200000-terthiophene-50000-boronic acid, 13 (500 mg, 0.3
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES, ZHANG ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of dendritic oligothiophenes, DCT(n)-DCN, where n ¼ 1 and 2.
mmol) [which was prepared by 12 (970 mg, 0.6 mmol), 1.6 M
n-BuLi (1.2 mL, 1.8 mmol), and trimethyl borate (0.3 mL, 3
mmol) according to the synthetic procedure of 8], 5-bromo-
3,40,400-terthexyl-[4,20,200]terthiophene-500-carboxaldehyde, 15 (364
mg, 0.5 mmol), Pd(PPh3)4 (50 mg), THF (20 mL), and 2 M
K2CO3 (1.0 mL). The crude product was purified by silica gel
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ARTICLE
SCHEME 1 (continued)
column chromatography using petroleum ether/dichloro-
methane as eluent affording the desired product as a red
solid (220 mg, 34%).
was purified by silica gel column using petroleum ether/
dichloromethane as eluent affording a black red solid which
was further purified by precipitation with CHCl3/CH3OH,
washed with methanol, and dried in vacuum affording the
desired product as a black solid (148 mg, 73%).
1H NMR (400 MHz, CDCl3, d) 9.81 (s, 1H), 8.29 (d, J ¼ 10.0
Hz, 4H), 8.09 (d, J ¼ 7.6 Hz, 4H), 7.69–7.64 (m, 4H), 7.56 (s,
1H), 7.44–7.12 (m, 33H), 7.03 (s, 1H), 4.25–4.23 (m, 8H),
2.84–2.81 (m, 6H), 1.84–1.81 (m, 8H), 1.73–1.68 (m, 6H),
1.43–1.25 (m, 24H), 0.93–0.86 (m, 21H). 13C NMR (100 MHz,
CDCl3, d) 182.5, 146.99, 146.93, 145.99, 145.94, 141.2,
140.9, 140.7, 140.35, 140.29, 140.1, 140.0, 139.1, 137.9,
137.1, 136.1, 135.2, 135.1, 134.9, 134.8, 133.7, 132.78,
132.76, 132.73, 132.6, 132.30, 132.28, 131.8, 131.3, 131.0,
130.8, 130.5, 130.2, 129.1, 128.79, 128.76, 128.6, 128.3,
128.1, 128.0, 126.7, 126.5, 125.92, 125.86, 125.4, 125.2,
125.0, 124.3, 124.1, 123.2, 122.76, 122.74, 122.22, 122.15,
120.6, 119.04, 118.99, 117.67, 117.62, 108.9, 108.84, 108.81,
42.9, 32.2, 31.9, 31.72, 31.68, 31.63, 31.1, 30.56, 30.50,
30.44, 30.2, 29.96, 29.70, 29.48, 29.45, 29.42, 29.37, 29.29,
29.24, 29.16, 26.4, 23.4, 22.69, 22.65, 22.62, 20.5, 14.18,
14.14, 14.12, 13.9. MS (MALDI-TOF): m/z 2152.6 (Mþ þ 1).
1H NMR (400 MHz, CDCl3, d) 8.27 (dd, J ¼ 12 Hz, J ¼ 1.6 Hz,
4H), 8.07 (d, J ¼ 7.6 Hz, 4H), 7.68–7.62 (m, 4H), 7.51 (s,
1H), 7.46–7.11 (m, 33H), 7.03 (s, 1H), 4.25 (q, J ¼ 2.8 Hz,
8H), 2.84–2.75 (m, 6H), 1.84–1.78 (m, 8H), 1.76–1.62 (m,
6H), 1.45–1.31 (m, 26H), 0.93–0.87 (m, 21H). 13C NMR (100
MHz, CDCl3, d) 149.7, 146.94, 146.89, 145.96, 145.91, 144.0,
141.8, 140.85, 140.77, 140.61, 140.53, 140.1, 140.0, 137.9,
137.1, 136.1, 135.2, 135.1, 135.0, 124.9, 134.7, 134.4, 133.6,
133.3, 132.8, 132.7, 132.2, 131.8, 131.7, 131.6, 131.44,
131.38, 130.9, 130.74, 130.69, 129.3, 128.75, 128.74,128.6,
128.3, 128.03, 128.00, 126.7, 126.5, 125.93, 125.87, 125.41,
125.39, 125.16, 125.14, 124.2, 124.1, 123.2, 122.75, 122.73,
122.2, 122.1, 120.6, 119.04, 118.99, 117.61, 117.57, 114.5,
113.6, 108.95, 108.85, 108.02, 75.4, 42.9, 31.73, 31.66,
31.59, 31.1, 30.5, 30.4, 29.9, 29.53, 29.48, 29.3, 29.2, 22.70,
22.66, 22.61, 20.52, 14.18, 14.15, 14.11, 13.85. HRMS
(MALDI-TOF): calcd: For C134H122N6S12: 2200.6404; Found:
2200.6457. Anal calc. for C134H122N6S12: C 73.12, H 5.59, N
3.82; found: C 73.10, H 5.52, N 3.46.
DCT(2)-DCN
The synthetic procedure of DCT(1)-DCN was followed using
14 (200 mg, 0.092 mmol), malononitrile (0.20 g, 3 mmol),
pyridine (0.4 mL), and CHCl3 (10 mL). The residual solid
TABLE 1 Summary of Physical Measurement of DCT(n)-DCN
a
abs
k
(nm)
max
(104 Mꢁ1
cmꢁ1
Optical
gapb (eV)
E
Erped
HOMOc
(eV)
LUMOc
(eV)
Energy
gapd (eV)
Tm
Tdec
(ꢂC)
c
c
e
f
oxd
1=2
)
(V)
(V)
(ꢂC)
DCT(1)-DCN
DCT(2)-DCN
a
495 (7.83)
438 (3.77)
1.74
1.74
0.77, 0.90, 1.18
0.73g
ꢁ1.09
ꢁ1.12g
d
ꢁ5.12
ꢁ5.06
ꢁ3.26
ꢁ3.21
1.86
1.85
205
99h
396
386
Measured in toluene.
Energy gap ¼ HOMO-LUMO.
b
c
e
Estimated from the absorption edge of the thin film.
Determined by CV method using platinum disc electrode as a working
Determined by differential scanning calorimeter from re-melt after
cooling with a heating rate of 10 ꢂC minꢁ1 under N2.
f
electrode, platinum wire as a counter electrode and SCE as a reference
electrode with an agar salt bridge connecting to the oligomer solution
and ferrocene was used as an external standard, E1/2 (Fc/Fcþ) ¼ 0.45 V
versus SCE and calculated with ferrocene (4.8 eV vs. vacuum).
Determined by thermal gravimetric analyzer with a heating rate of 10
ꢂC minꢁ1 under N2.
g
E1/2 (Fc/Fcþ) ¼ 0.47 versus SCE.
h
Tg.
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES, ZHANG ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
borate protocol. Palladium catalyzed Suzuki crosss-coupling of
dendron substituted boronic acid 10 with bromoterthiophene-
carboxaldehyde 15, which was prepared according to the
modified literature procedures, gave the first-generation den-
dritic oligothiophene-carboxaldehyde 11 in 80% yield. Conden-
sation of aldehydes 11 with excess malononitrile in the pres-
ence of pyridine in chloroform afforded the desired product
DCT(1)-DCN in an excellent yield. To synthesize the second-
generation carbazole surface-functionalized thiophene-based
dendritic wedge, double palladium catalyzed Suzuki cross-cou-
pling of the first-generation dendron-substituted boronic acid
10 and iodosubstituted branched terthiophene 4 was carried
out affording the product 12 in 82% yield. Following the same
reaction sequence, the dendron 12 was first converted into
the boronic acid 13 and then cross-coupled with bromoter-
thiophenecarboxaldehyde 15 followed by the condensation
with malononitrile affording the second-generation dicyano-
vinyl-dendritic oligothiophene, DCT(2)-DCN in a good yield.
All the newly synthesized dendritic oligothiophenes were fully
characterized with 1H NMR, 13C NMR, high resolution matrix-
FIGURE 1 Absorption spectra of DCT(n)-DCN in (a) toluene and
(b) thin film. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
RESULTS AND DISCUSSION
Synthesis and Characterization
The synthesis of novel functional dendritic oligothiophenes is
outlined in Scheme 1. Lithiation of 2-bromothiophene, 1 at
ꢁ78 ꢂC with LDA followed by reaction with TMS-Cl afforded
2-bromo-5-trimethylsilylthiophene, 2 in 91% yield. Kumada
cross-coupling of Grignard reagent of 2 and 2,3-dibromothio-
phene in the presence of Ni(dppp)Cl2 as a catalyst afforded
the branched terthiophene 3 in a good yield. Iododesilylation
of 3 with ICl at low temperature afforded iodosubstituted
branched terthiophene 4 in an excellent yield. Butylated carba-
zole, 6 was prepared from the phase-transfer conditions.
Monobrominationꢂof 6 with NBS followed by lithium-bromide
exchange at ꢁ78 C and then quenched with trimethyl borate
gave carbazole-boronic acid 8.11 Palladium catalyzed Suzuki
cross-coupling of 8 and iodosubstituted branched terthiophene
4 afforded the first-generation carbazole surface-functionalized
thiophene-based dendritic wedge 9 in 84% yield. Transforma-
tion of 9 into the corresponding boronic acid 10 was carried
out using the lithiation followed by reaction with trimethyl
FIGURE 2 CV traces of DCT(n)-DCN.
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WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
fer character of these dendritic oligothiophenes. In addition,
these dendrimers show dramatic spectral broadening and
large red shifts of more than 100 nm of absorption cutoff in
solid-state thin films especially for DCT(2)-DCN, which is
attributed to the planarization of thiophene rings and the
presence of intermolecular interactions in the solid state,
which leads to narrowing of optical bandgap to 1.74 eV.
To investigate the electrochemical properties of these den-
drimers, cyclic voltammetry (CV) was carried out in a three-
electrode cell set-up with 0.1 M of Bu4NPF6 as a supporting
electrolyte in CH2Cl2. All the potentials reported are refer-
enced to Fc/Fcþ standard and the results are tabulated in
Table 1. Both of the dendritic oligothiophenes showed a dis-
oxd1
1=2
tinct first oxidation wave with E
at 0.73–0.77 V, corre-
sponding to the removal of an electron from the carbazole
surface-functionalized group. For DCT(1)-DCN, two addition
FIGURE 3 Typical ultraviolet photoemission spectroscopic
spectra of DCT(1)-DCN film and gold film (reference). [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
oxd2
oxidation waves were followed with E
at 0.90 V, and
1=2
oxd3
E
at 1.18 V, corresponding to the sequential removal of
1=2
electrons from dendritic oligothiophenes (Fig. 2). On the
other hand, no further oxidation was observed for DCT(2)-
DCN, which is attributed to the poor p-conjugation of the
second generation oligothiophene-dendritic wedge. They also
exhibit a reduction wave, Erped1 ¼ ꢁ1.09 to ꢁ1.12 V, corre-
sponding to the reductions of dicyanovinyl group. Hence, the
HOMO and LUMO levels of these dendritic oligothiophenes
were estimated to be ꢁ5.1 and ꢁ3.2 eV, respectively, based
on CV data. On the other hand, the ionization potentials of
these dendritic oligothiophene films are in the range of
5.47–5.61 eV as determined by UV photoelectron spectros-
copy (Fig. 3).11 Such a high first ionization energy is essen-
tial to achieve a large Voc of a PV device.
assisted laser desorption ionization time-of-flight (HR MALDI-
TOF) MS, and elemental analysis and found to be in good
agreement with their structures. These dendrimers are highly
soluble in common organic solvents and solution processable.
The thermal behavior and stability of these dendritic oligothio-
phenes were studied by differential scanning calorimetry
(DSC) and thermal gravimetric (TGA) analyses, respectively.
The results are summarized in Table 1. The first-generation
dendritic oligothiophene, DCT(1)-DCN exhibits no apparent
ꢂ
glass transition except a melting transition (Tm) of 205 C; on
the other hand, the second-generation dendrimer, DCT(2)-
DCN shows a distinct glass transition temperature (Tg) at 99
ꢂC suggesting that this dendrimer can form a morphologically
stable amorphous thin film. Both of the dendritic oligothio-
Device Characterization
To probe the potential of these dendritic oligothiophenes as
hole-transporting light-absorbing materials in solution-proc-
essable PV cells, the bulk heterojunction PV cells with device
structure of ITO/poly(3,4-ethylendioxythiophene:poly(styre-
nesulfonate) (PEDOT:PSS) (40 nm)/DCT(n)-DCN:PC71BM/Ca
ꢂ
phenes exhibit an excellent thermal stability with Td > 396 C.
The absorption spectra of DCT(n)-DCN (n ¼ 1 or 2) meas-
ured in toluene as shown in Figure 1 are characterized with
very broad but discernible bands and structureless absorp-
tion maximum (kambasx) at about 438 and 495 nm, respectively,
corresponding to the intramolecular charge transfer transi-
tion of these donor–acceptor dendrimers together with sev-
eral absorption bands at shorter wavelengths corresponding
to other higher energy transitions including the p ! p* tran-
sition of the oligothiophene backbone at 365–360 nm and
the n ! p* transition of carbazole-substituted dendron at
301 nm.11,12 DCT(2)-DCN exhibits a blue shift of the k
in
abs
max
toluene despite an increase in molar absorptivity relative to
that of the lower homologues, DCT(1)-DCN, which is attrib-
uted to the poorer p-conjugation of the higher generation
dendritic wedges arising from the highly twisted of the thio-
phene units in the branched structure of the dendrons.9b
These dendrimers also exhibit strong solvatochromic effect
in which the absorption cutoff red shifts from 590 to
620 nm when changing from toluene to chloroform indicat-
ing the charge-transfer character of the dendrimers in the
FIGURE 4 J–V characteristics of DCT(2)-DCN: PC71BM-based PV
cells. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
ground state. There is no noticeable fluorescence emission
abs
max
on excitation at k
which is due to the strong charge-trans-
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES, ZHANG ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Summary of PV Device Performance with Device Structure of ITO/PEDOT:PSS (40 nm)/DCT(n)-DCN:
PC71BM/Ca (10 nm)/Ag (100 nm)
Electron Donors
Solvents
Annealing
Voc (V)
Jsc (mA cmꢁ2
)
FF
PCE (%)a
DCT(1)-DCN:PC71BM (1:1)
Chloroform
As fabricated
120 ꢂC, 10 min
As fabricated
120 ꢂC, 10 min
As fabricated
100 ꢂC, 10 min
As fabricated
100 ꢂC, 10 min
As fabricated
100 ꢂC, 10 min
As fabricated
100 ꢂC, 10 min
0.79
0.73
0.94
0.85
0.80
0.90
0.85
0.96
0.84
0.92
0.79
0.93
1.61
1.56
3.00
2.99
3.09
3.68
2.73
2.93
3.82
4.31
3.94
4.79
0.28
0.29
0.37
0.36
0.30
0.31
0.29
0.31
0.32
0.35
0.32
0.37
0.36
0.33
1.06
0.91
0.75
1.02
0.68
0.88
1.02
1.40
0.99
1.64
DCT(1)-DCN:PC71BM (1:1)
DCT(1)-DCN:PC71BM (1:4)
DCT(2)-DCN:PC71BM (1:1)
DCT(2)-DCN:PC71BM (1:2)
DCT(2)-DCN:PC71BM (1:4)
a
Toluene
Toluene
Toluene
Toluene
Toluene
Measured using a simulated AM 1.5G illumination of 100 mW cmꢁ2 under a nitrogen atmosphere.
(10 nm)/Ag (100 nm) were fabricated. The current density–
voltage (J–V) characteristics of these devices were measured
using a simulated AM 1.5G illumination of 100 mW/cm2
under a nitrogen atmosphere. (Fig. 4) The results of the de-
vice performance are summarized in Table 2. Solvent used
for thin-film casting has tremendous effect on the device
performance. The devices fabricated from toluene gave supe-
rior device performance. These dendrimer-based PV cells
afforded relatively large Voc in the range of 0.8–0.96 V, which
was attributed to the low-lying highest occupied molecular
orbital (HOMO) levels (ꢁ5.1 eV) of these dendrimers and,
hence, the larger differences to the lowest unoccupied molec-
ular orbital (LUMO) level of PCBM. In general, thermal
annealing of the blended active layer could significantly
improve the device performance of these PV cells by enhanc-
ing Voc, short circuit current density (Jsc), fill factor (FF), and
thus PCE. As evidenced from the atomic force microscopy
(AFM) phase images, thermal annealing improved the pack-
ing and phase separation of the blended film. (Fig. 5) The
carrier mobility of a blended film could be greatly affected
by the donor: acceptor ratio. For DCT(2)-DCN:PC71BM, there
were also progressive improvement in Voc, Jsc, FF as well as
PCE on an increase in the PC71BM acceptor content. With a
1:4 DCT(2)-DCN:PC71BM ratio, the Jsc increased up to 4.79
mA/cm2 with Voc ¼ 0.93 V and PCE up to 1.64% in the
annealed devices. Similar to the nonfunctional dendritic oli-
gothiophenes, these functional dendrimers also exhibited an
enhancement in the device efficiency on increasing the den-
dritic generation. Our findings highlight the potential of this
class of materials promising for further device optimization
in high-performance PV applications.
CONCLUSIONS
In summary, novel solution-processable, p-type, low-optical-
gap functional dendritic oligothiophenes bearing hole-trans-
porting carbazole as peripheral substituents and an electron-
withdrawing dicyanovinyl core group have been synthesized.
With incorporation of carbazole surface-substituents and
dicyanovinyl core group, the functional dendritic oligothio-
phene exhibits a strong spectral broadening and a high ioni-
zation potential at ꢀ5.5 eV as determined by UV photoelec-
tron spectroscopy with a low optical bandgap of 1.74 eV.
The bulk heterojunction PV cells fabricated from these den-
drimers blended with PC71BM as an acceptor showed a PCE
up to 1.64% with Voc ¼ 0.93 V in the annealed devices. We
have shown that the desirable molecular and PV properties
of dendritic oligothiophenes can be obtained/tuned by the
incorporation of functional group(s) onto peripheral of the
dendron and into the core. In addition, these carbazole
surface-functionalized and dicyanovinyl core-substituted
dendritic oligothiophenes are promising solution-processable
p-type low bandgap photoactive materials for application in
bulk heterojunction PV cells.
This work was supported by a grant from the University Grants
Committee, Areas of Excellence Scheme (AoE/P-03/08) and
Faculty Research Grant (FRG2/09-10/021). We thank Prof. K.
W. Cheah, Department of Physics for the sample preparation of
FIGURE 5 AFM phase images, data scale from 0 to 5ꢂ, taken
for blend of DCT(2)-DCN:PC71BM in 1:4 ratio (a) as-fabricated
and (b) after annealed.
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ARTICLE
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