New organic dye based on a 3,6-disubstituted carbazole donor for efficient dye-sensitized solar cells

Article abstract of DOI:10.1002/asia.201100661

We have synthesized and characterized four organic dyes (H1-H4) based on a 3,6-disubstituted carbazole donor as sensitizers in dye-sensitized solar cells. These dyes have high molar extinction coefficients and energy levels suitable for electron transfer from an electrolyte to nanocrystalline TiO₂ particles. Under standard air mass 1.5 global (AM 1.5 G) solar irradiation, a device using dye H4 exhibits a short-circuit current density (J_sc) of 13.7 mA cm^-2, an open-circuit voltage (V_oc) of 0.68 V, a fill factor (FF) of 0.70, and a calculated efficiency of 6.52 %. This performance is comparable to that of a reference cell based on N719 (7.30 %) under the same conditions. After 1000 hours of visible-light soaking at 60 °C, the overall efficiency remained at 95 % of the initial value. Metal-free sensitizer: Four organic dyes based on a 3,6-disubstituted carbazole donor have been synthesized for their potential use as sensitizers in dye-sensitized solar cells. These dyes have high molar extinction coefficients and energy levels suitable for electron transfer from an electrolyte to nanocrystalline TiO ₂ particles. Under standard global AM 1.5 solar irradiation, a device using one of the prepared dyes (see graphic) exhibited a calculated efficiency of 6.52 %.

Full text of DOI:10.1002/asia.201100661

DOI: 10.1002/asia.201100661
New Organic Dye Based on a 3,6-Disubstituted Carbazole Donor for
Efficient Dye-Sensitized Solar Cells
Woochul Lee,[a] Nara Cho,[b] Jongchul Kwon,[a] Jaejung Ko,*[b] and Jong-In Hong*[a]
Dedicated to Professor Eun Lee on the occasion of his retirement and 65th birthday
Abstract: We have synthesized and
characterized four organic dyes (H1–
H4) based on a 3,6-disubstituted carba-
zole donor as sensitizers in dye-sensi-
tized solar cells. These dyes have high
molar extinction coefficients and
energy levels suitable for electron
transfer from an electrolyte to nano-
crystalline TiO2 particles. Under stan-
dard air mass 1.5 global (AM 1.5G)
solar irradiation, a device using dye H4
exhibits a short-circuit current density
(Jsc) of 13.7 mAcmꢀ2, an open-circuit
voltage (Voc) of 0.68 V, a fill factor
(FF) of 0.70, and a calculated efficiency
of 6.52%. This performance is compa-
rable to that of a reference cell based
on N719 (7.30%) under the same con-
ditions. After 1000 hours of visible-
light soaking at 608C, the overall effi-
ciency remained at 95% of the initial
value.
Keywords: electrochemistry · elec-
tron transfer · sensitizers · solar
cells · substituted carbazole
Introduction
Certain fundamental requirements must be met when de-
signing highly efficient sensitizers. First, efficient sensitizers
should have good light-harvesting ability, which is related to
the molecular extinction coefficient and the overlap of the
dye absorption with the solar spectrum. To improve their
light-harvesting ability, dyes should have a high absorptivity
and a greater absorption in the visible region. Second, to
achieve efficient electron injection from the excited dye to
the conduction band of TiO2, the lowest unoccupied molecu-
lar orbital (LUMO) of the sensitizer must be more negative
than the conduction band of the nanoporous metal oxide.
On the other hand, the highest occupied molecular orbital
(HOMO) of the sensitizer must be more positive than the
Solar energy has infinite potential to address the challenges
of global warming and declining fossil fuel resources be-
cause it is renewable and offers unlimited access.[1] In recent
years, considerable effort has been devoted to converting
photo energy into electrically available energy using solar
cells. Among several types of photovoltaic devices, dye-sen-
sitized solar cells (DSCs) have attracted a great deal of at-
tention owing to their low production cost and relatively
high energy conversion efficiency.[2] Conventional DSCs typ-
ically consist of nanocrystalline titanium oxide, a sensitizer,
and an electrolyte.[3] Of these, the photosensitizer is crucial
for realizing highly efficient DSCs. Ruthenium-based sensi-
tizers, such as N3,[1,4,5] N719,[6] and black dye,[7] have ach-
ieved efficiencies of up to 11% under air mass 1.5 global
(AM 1.5G) irradiation. However, ruthenium-based sensitiz-
ers have some drawbacks, such as low molecular extinction
coefficients and costly synthesis and purification.[8] Metal-
free organic dyes are an attractive alternative because they
offer high absorption, diverse structures, low material cost,
and ease of purification.[5,9,10]
ꢀ
redox potential of the electrolyte chosen, such as Iꢀ/I3 , for
rapid electron injection into the oxidized dye.
Many organic dyes have recently been designed and syn-
thesized, including coumarin,[11] indoline,[9,12] porphyrin,[13]
merocyanine,[14] squaraine,[15] and oligothiophene, which usu-
ally contain triphenylamine (TPA) or difluorenylamine as
an electron donor.[16] Among these various molecular struc-
tures, a maximum efficiency of about 9.5% was achieved
with an indoline dye.[12a] Further, a strikingly high efficiency
of 10% was reported when electron-rich 3,4-ethylenedioxy-
thiophene was combined with dihexyl-substituted dithienosi-
lole as a linker between the donor and the anchoring unit.[17]
TPA moieties, which have good electron-donating ability as
well as hole-transporting properties, have been frequently
used as the donor unit.[18]
In this study, we synthesized and characterized four or-
ganic dyes consisting of 3,6-bis(triphenylamine)-substituted
carbazole, dithiophene, and cyanocarboxylic acid as an an-
choring group to bind to TiO2 nanoparticles. The photophys-
ical and electrochemical properties and device performance
of the sensitizers were systematically investigated. The ab-
[a] W. Lee, J. Kwon, Prof. Dr. J.-I. Hong
Department of Chemistry
Seoul National University
Seoul 151-747 (Korea)
Fax : (+82)02-889-1568
E-mail: jihong@snu.ac.kr
[b] N. Cho, Prof. Dr. J. Ko
Department of New Material Chemistry
Korea University
Jochiwon, Chungnam 339-700 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100661.
Chem. Asian J. 2012, 7, 343 – 350
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
343

FULL PAPERS
sorption peaks were red-shifted and better matched to the
solar spectrum by extending the conjugation length between
the donor and bridge units, which contributed to an increase
in the short-circuit current (Jsc) . The molar extinction coef-
ficients of the four dyes varied with the conjugation length.
Energy levels suitable for efficient electron transfer from an
electrolyte to nanocrystalline TiO2 particles were also ob-
tained by controlling the conjugation length and donor
group. The best performance obtained in this study was
6.52% for the dye with the longest conjugation length be-
tween the donor and the bridge group, H4. This dye also
showed good long-term stability.
Figure 1. Absorption and photoluminescence spectra of sensitizers H1–
H4 in a DMF solution.
Results and Discussion
The molecular structures of the dyes and their syntheses are
shown in Schemes 1 and 2, respectively. Compounds 1, 3, 8,
and 10 were synthesized by a palladium-catalyzed Suzuki re-
when compared with H1, owing to the better electron-do-
nating ability of TPA. The difference in the molar extinction
coefficients of H1 and H2 at 300–375 nm can be attributed
to the p–p* transition of the conjugated system, which de-
pends on the absorbance of the electron donor unit. Under
the same conditions, a TPA moiety is more suitable as a
donor group in terms of its light harvesting ability. The ab-
sorption bands of H3 and H4 are more intense and broader
than those of H1 and H2. Extending the conjugated system
by the addition of a phenylvinylene unit at the carbazole
core leads to a shift in absorption onset to longer wave-
length. In addition, sensitizer H4, which has phenyldithio-
phene as a bridging group, shows the highest molecular ex-
tinction coefficient at its maximum absorption values (lmax
=
390 nm, e>100000mꢀ1 cmꢀ1) and the longest wavelength ab-
sorption band reaching up to 500 nm. In addition, the H4
dye also shows stronger and broadened UV/Vis absorption
bands when adsorbed on a TiO2 film (Figure S1 in the Sup-
porting Information). These results imply that the electron-
donating substituents at the 3- and 6-positions of the carba-
zole ring contribute to its light-harvesting ability. Thus, H4
exhibits the best molar absorptivity and absorption range
for visible light. The electrochemical properties of these sen-
sitizers were determined by using cyclic voltammetry in a
N,N-dimethylformamide (DMF) solution containing tetrabu-
tylammonium hexafluorophosphate (TBAPF6) as a support-
ing electrolyte (Table 1). Dyes H2–H4 show similar electro-
chemical profiles with quasi-reversible waves; their recorded
oxidation potentials are 1.11, 0.97, and 1.01 V, respectively,
versus the normal hydrogen electrode (NHE) (Figure S2 in
the Supporting Information). However, H1 exhibits an irre-
versible oxidation potential. Slight changes in the oxidation
potentials of about 0.1 V appear between H2 and H3 (or
H4) owing to the increase in p-conjugation length in the
dyes. For H1, it is more difficult to regenerate the oxidized
sensitizer (compared to the redox potential of the electro-
lyte) when compared to the other H dyes because of the in-
corporation of a phenylcarbazole moiety into the 3- and 6-
positions of the carbazole core, which lowers the oxidation
Scheme 1. Molecular structures of the organic dyes.
action, and compound 5 was synthesized by the Heck C–C
coupling reaction of 10. Compounds 2, 4, 6, and 7 were pre-
pared from compound 8 by the copper-catalyzed Ullmann
coupling reaction, and compound 9 was prepared by the
Vilsmeier–Haack formylation of compound 8. Finally, all
the sensitizers were obtained as a mixture of E/Z isomers by
the Knoevenagel condensation reaction between cyanoace-
tic acid and formylated intermediates. A representative ex-
perimental protocol is described in the Experimental Sec-
tion.
The UV/Vis absorption spectra of sensitizers H1–H4 are
shown in Figure 1, and their photophysical data are listed in
Table 1. As shown in Figure 1, H2 exhibits an improved red
shift of its absorption spectrum in the range of 375–475 nm
344
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 343 – 350

Substituted Carbazole Dyes as Sensitizers
higher than the conduction
band of TiO2 (ꢀ0.5 V vs.
NHE). Thus, these dyes show
optimum energy levels for effi-
cient electron transfer from an
electrolyte to a nanocrystalline
electrode.
To better understand the cor-
relation between the molecular
structure and the photophysical
properties, we performed mo-
lecular
orbital
calculations
using density functional theory.
The ground state geometries of
the H dyes were optimized in
vacuum using the B3LYP/6-
31G* level in the Gaussian 03
package. The optimized struc-
tures and isodensity plots of the
frontier molecular orbitals are
shown in Figure 2. The HOMO
levels of the synthesized dyes
are delocalized on the donor
group consisting of TPA and
the phenylcarbazole, extending
to the carbazole core, and the
LUMO is located mainly over
the cyanoacrylic moiety and the
dithiophene or phenyldithio-
phene. The well-separated orbi-
tal contributions in the electron
density distribution of the
HOMO and LUMO indicate
efficient charge separation.
The
photocurrent
action
spectra of H1–H4 are shown in
Figure 3. The DSCs were fabri-
cated using a liquid electrolyte
(0.6m 1,2-dimethyl-3-n-propyli-
midazolium
iodide,
0.05m
iodide, 0.1m LiI, and 0.5m 4-
tert-butylpyridine in acetoni-
trile) and 12 mm-thick TiO2
nanoparticles. We estimated the
photovoltaic performance of
the dyes with a black tape mask
under standard global AM 1.5G
solar radiation. The power con-
version efficiencies for all the
devices, except that using H1,
exceeded 5%. Under the stan-
dard conditions, the device
based on H4 exhibited a short-
Scheme 2. Synthetic routes for H1–H4.
circuit current density (Jsc) of
potential to 1.51 V versus NHE. The excited state oxidation
potentials (E0ꢀ0*) of the dyes are calculated from the equa-
tion E0ꢀ0*=Eox.ꢀE0ꢀ0. The calculated potentials are much
13.7 mAcmꢀ2, an open-circuit voltage (Voc) of 0.68 V, a fill
factor (FF) of 0.70, and a calculated efficiency of 6.52%.
These results compare favorably with those obtained for a
Chem. Asian J. 2012, 7, 343 – 350
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345

J. Ko, J.-I. Hong et al.
FULL PAPERS
Table 1. Photophysical and electrochemical properties and photovoltaic performance[a] of the dyes H1–H4 and
N719.
sitions of the carbazole core
and a phenyl unit at the bridge
group in H4. The light absorp-
tion abilities of H1 and H2 are
similar (see Figure 1), but H1
has a more positive oxidation
potential than H2; this inhibits
efficient regeneration of the
oxidized sensitizer. Moreover,
TPA has a three-dimensional
structure, in contrast to the
planar carbazole moiety. This
structure inhibits aggregation of
the sensitizer on the TiO2 parti-
cles, thereby resulting in a sig-
nificantly high photocurrent.
Voc increases from H1 to H4,
which is related to charge trans-
port from the dye to TiO2 and
the electron life time within the
TiO2 nanoparticles. To under-
stand the correlation between
Voc and the donor unit, we mea-
sured electrochemical impe-
dance spectra.[19] This method is
generally used to investigate
electron life times and photoin-
duced electron injection proper-
ties under open circuit and illu-
minated conditions. The second
semicircle of the Nyquist plot
(Figure S3 in the Supporting In-
formation) indicates charge
generation and transport. As
shown in Figure S3, the order
of the second semicircle radius
decreases from H1 (10.6 W) to
H4 (7.96 W), thus indicating
that H1 inhibits photoinduced
electron injection to TiO2 nano-
particles when compared with
H4. In the Bode phase plot
(Figure S3 in the Supporting In-
Dye labs
(eꢁ104 Mꢀ1
cmꢀ1 [b] [nm]
lmax[b] [nm] Eox[b] [V] E0ꢀ0[c] [V] E0ꢀ0 [d] [V] Jsc [mAcmꢀ2
* ] Voc [V] FF [%] h [%]
)
H1
309
340
393
337
393
304
386
303
390
441
–
N
535
1.51
2.74
ꢀ1.23
6.32
0.53
0.72
2.41
H2
H3
H4
511
479
519
1.11
0.97
1.01
2.71
2.77
2.65
ꢀ1.60
ꢀ1.80
ꢀ1.64
12.2
13.0
13.7
0.60
0.63
0.68
0.77
0.75
0.70
5.64
6.14
6.52
N719
–
–
–
–
14.7
0.68
0.73
7.30
[a] Measurements were based on a 0.11–0.13 cm2 working area with a black mask on the fabricated cells.
[b] Measured in DMF. Oxidation potentials were determined from the onset of cyclic voltammograms with a
scan rate of 200 mVsꢀ1 Potentials were calibrated with the internal ferrocene standard (E1/2 =+0.454 mV vs.
.
Ag/AgNO3). [c] The band gap E0ꢀ0 was derived from the onset of the absorption spectra. [d] E0ꢀ0* was calcu-
lated as Eox.ꢀE0ꢀ0
.
Figure 2. Computed isosurface plots of the frontier orbitals and energy states of sensitizers H1–H4.
reference cell based on JK-2 (7.22%)[16 g] and the commer-
cially available N719 dye (7.30%) under the same condi-
tions. All the DSC devices fabricated using the H dyes have
different Jsc and Voc values. Jsc decreases in the following
formation), the middle-frequency peak (in the 10–100 Hz
range) is indicative of the charge-transfer process of injected
electrons in TiO2. Thise finding is related to the electron life
time and is ultimately correlated with Voc. The electron life
time is obtained from the middle-frequency peak (in the 10–
100 Hz range) in the Bode phase plot using Equation (1), in
which f is the frequency of the peak.
order:
H4
(13.7 mAcmꢀ2)>H3
(13.0 mAcmꢀ2)>H2
(12.2 mAcmꢀ2)>H1 (6.32 mAcmꢀ2), which is explained by
the trends observed in molar absorption coefficients and in
the photoresponse regions of the dyes. As shown in
Figure 3, the incident photon-to-electron conversion effi-
ciency (IPCE) of H4 shows a plateau around 400–550 nm
and higher values than those of H1–H3. In addition, H3 and
H4 exhibit a red-shifted IPCE spectrum up to 650 nm. This
results from the extension of the p-conjugated system by in-
corporation of a vinylene-phenyl moiety at the 3- and 6-po-
t ¼ 1=2pf
ð1Þ
The injected electron lifetime of the H dyes increases
from H1 (1.38 ms) to H4 (5.66 ms). This result is correlated
with the fact that H2–H4, which have a TPA moiety incor-
porated as a donor, possess higher Voc values (0.60–0.68 V)
346
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Chem. Asian J. 2012, 7, 343 – 350

Substituted Carbazole Dyes as Sensitizers
Figure 4. Variation of the photovoltaic parameters of H4 under light
soaking (AM 1.5G, 100 mWcmꢀ2) at 608C.
Conclusions
In summary, four organic dyes based on 3,6-disubstituted
carbazole donors were synthesized and characterized re-
garding their efficiency in dye-sensitized solar cells. These
dyes exhibited high molar extinction coefficients and energy
levels suitable for electron transfer from an electrolyte to
nanocrystalline TiO2 particles. The device based on H4 ex-
hibited the highest conversion efficiency of 6.52%, which
corresponds to approximately 89% of the conversion effi-
ciency of cells based on the commercially available dye
N719 (7.30%) under similar conditions. After 1000 hours of
visible-light soaking at 608C, the overall efficiency remained
at 95% of the initial value. The results showed that H4,
which featured the largest conjugation length, had the best
light harvesting ability and long-term stability among the
dyes investigated. Moreover, the triphenylamine unit as the
donor was found more suitable for enhancing the efficiency
than the carbazole moiety.
Figure 3. a) I–V curves and b) IPCE spectra of DSCs fabricated using
H1–H4.
than H1 (0.53 V). The device based on H4 showed the lon-
gest electron life time, which was four times longer than
that of the H1-based device. Thus, the presence of a TPA
unit in the H dyes as a donor increases not only charge gen-
eration and injection but also the electron life time, which
improves Voc and Jsc, and ultimately the photoconversion ef-
ficiency.
Because the long-term stability of devices is an important
issue to evaluate commercial applications, we measured
device performance using the H4 dye with light soaking
(AM 1.5G, 100 mWcmꢀ2) at 608C, as shown in Figure 4.
After 1000 hours of light soaking, the Voc slightly decreases
by 40 mV and the FF declines from 0.70 to 0.68, but the Jsc
increases by 0.47 mAcmꢀ2. This enhancement of short-cir-
cuit current is likely the result of an improved percolation
of the liquid electrolyte into cavities in the dye-adsorbed
nanoporous TiO2 layer that could not be accessed initially.
Thus, the overall efficiency remains at 95% (6.15%) of the
initial value (6.52%). These results suggest that the dye H4
has potential for commercial applications.
Experimental Section
General Information
All organic chemicals were purchased from Aldrich and TCI. Column
chromatography was conducted on Merck silica gel 60 (70–230 mesh).
All solvents and reagents were commercially available and used without
further purification unless otherwise noted. 1H and 13C NMR spectra
were recorded using an Avance 300- or 500 MHz Bruker spectrometer.
UV/Vis spectra were recorded on a Beckman DU 650 spectrophotome-
ter. Mass spectra were obtained using a gas chromatography–mass spec-
trometry instrument from JEOL (JMS-AX505WA, HP 5890 Series II)
and a matrix-assisted laser desorption/ionization time-of-flight (MALDI-
TOF) mass spectrometer from Bruker. Fluorescence spectra were record-
ed on a Jasco FP-7500 spectrophotometer. Cyclic voltammetry (CV)
measurements were performed at room temperature on a CHI 660 Elec-
trochemical Analyzer (CH Instruments, Inc., Texas, USA) using a solu-
tion of tetra-n-butylammonium hexafluorophosphate (Bu4NPF6, 0.1m) in
DMF as electrolyte solution. CV data were recorded using a reference
electrode of Ag/Ag+ (0.1m AgNO3 in acetonitrile) with a platinum wire
Chem. Asian J. 2012, 7, 343 – 350
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347

J. Ko, J.-I. Hong et al.
FULL PAPERS
counter electrode at a scan rate of 200 mVsꢀ1. The absolute potential
was calibrated to the NHE using an internal ferrocene/ferrocenium stan-
dard. The photovoltaic performances of the DSCs were measured using a
1000 W xenon light source, whose power as an AM 1.5 Oriel solar simu-
lator was calibrated using a KG5-filtered Si reference solar cell. The inci-
dent photon-to-current conversion efficiency (IPCE) spectra for the cells
were measured on an IPCE measuring system (PV Measurements). AC
was purified by silica-gel column chromatography and then recrystallized
from dichloromethane and methanol to produce a green oily product
(yield: 280 mg, 72%). Rf =0.310 (CH2Cl2/n-hexane=2:1); 1H NMR
(300 MHz, CDCl3): d=9.95 (s, 1H), 8.54 (s, 2H), 8.20 (d, 9 Hz, 4H), 7.99
(d, 9 Hz, 4H), 7.88 (d, 9 Hz, 2H), 7.77–7.71 (m, 6H), 7.56–7.44 (m, 10H),
7.38–7.31 ppm (m, 6H); 13C NMR (125 MHz, CDCl3): d=182.5, 146.3,
142.2, 141.6, 140.9, 140.6, 139.6, 137.3, 136.5, 134.2, 133.9, 128.6, 127.4,
126.2, 126.0, 125.7, 125.0, 124.5, 123.4, 120.3, 120.0, 119.0, 110.9,
109.8 ppm; MS (MALDI-TOF): m/z calcd for C57H35N3OS2: 841.222
[M+H]+; found: 841.313.
impedance measurements were performed under
1 sun illumination
(100 mWcmꢀ2) using an impedance analyzer (PARSTAT 2273, Princeton
Applied Research USA).
Synthesis of 1, 3, 8, and 10
5’-[3,6-Bis-(4-diphenylamino-phenyl)-carbazol-9-yl]-[2,2’]bithiophenyl-5-
carbaldehyde (4)
These compounds were prepared following the same protocol. Thus, only
the synthesis of 1 is described. Spectral data for the other compounds are
provided.
1
Rf =0.310 (CH2Cl2/n-hexane=2:1); H NMR (300 MHz, CDCl3): d=9.92
(s, 1H), 8.34 (s, 2H), 7.72 (d, 9 Hz, 4H), 7.64–7.58 (d, 9 Hz, 6H), 7.47 (d,
3 Hz, 2H), 7.37–7.17 (m, 20H), 7.06 ppm (t, 7 Hz, 6H); 13C NMR
(125 MHz, CDCl3): d=182.5, 147.7, 146.8, 142.0, 141.0, 140.0, 137.3,
135.6, 134.3, 133.8, 129.3, 127.9, 125.7, 125.3, 125.0, 124.4, 124.3, 122.8,
118.4, 110.5 ppm; MS (MALDI-TOF): m/z calcd for C57H39N3OS2:
845.254 [M+H]+; found: 845.408.
3,6-Bis(4-carbazol-9-yl-phenyl)-9H-carbazole (1)
A mixture of 3,6-dibromo-9H-carbazole (0.5 g, 1.57 mmol), 4-(9H-carba-
zol-9-yl)phenylboronic acid (1.00 g, 3.46 mmol), potassium carbonate (2m
aq., 50 mL), and tetrakis(triphenylphosphine)palladium (0.08 g,
0.06 mmol) in toluene was refluxed for 24 h. After cooling to room tem-
perature, the reaction mixture was extracted with dichloromethane, and
the organic phase was washed with H2O and dried over Na2SO4. The sol-
vent was evaporated to give the product, which was purified by silica-gel
column chromatography and then recrystallized from dichloromethane
and methanol to produce a green solid (yield: 417 mg, 41%). Rf =0.357
(CH2Cl2/n-hexane=1:1); 1H NMR (300 MHz, CDCl3): d=8.52 (s, 2H),
8.25 (s, 1H), 8.20 (d, 6 Hz, 4H), 7.99 (d, 6 Hz, 4H), 7.84 (d, 6 Hz, 2H),
7.71 (d, 6 Hz, 4H), 7.62 (d, 6 Hz, 2H), 7.54 (d, 6 Hz, 4H), 7.47 (t, 6 Hz,
4H), 7.33 ppm (t, 6 Hz, 4H); 13C NMR (125 MHz, CDCl3): d=141.1,
140.9, 139.7, 136.2, 132.3, 128.6, 127.4, 126.0, 125.7, 124.1, 123.4, 120.4,
119.9, 119.0, 111.3, 109.9 ppm; MS (MALDI-TOF): m/z calcd for
C48H31N3: 649.252 [M+H]+; found: 649.443.
5’-(3,6-Bis-{4-[2-(4-diphenylamino-phenyl)-vinyl]-phenyl}-carbazol-9-yl)-
[2,2’]bithiophenyl-5-carbaldehyde (6)
1
Rf =0.310 (CH2Cl2/n-hexane=2:1); H NMR (300 MHz, CDCl3): d=9.93
(s, 1H), 8.40 (s, 2H), 7.78–7.71 (m, 8H), 7.64 (d, 8 Hz, 5H), 7.56 (d, 8 Hz,
2H), 7.48 (d, 4 Hz, 2H), 7.45 (d, 8 Hz, 3H), 7.33–7.25 (m, 12H), 7.21–
7.04 (m, 18H), 6.81 (q, 11 Hz, 6 Hz, 1H), 5.84 (d, 18 Hz, 1H), 5.31 ppm
(d, 9 Hz, 1H); 13C NMR (125 MHz, CDCl3): d=182.5, 147.5, 147.3,
146.4, 142.1, 141.3, 140.2, 139.8, 137.3, 136.3, 134.3, 133.9, 131.5, 129.3,
128.0, 127.4, 127.3, 126.8, 126.5, 125.9, 125.4, 125.0, 124.5, 123.6, 123.0,
118.6, 110.6 ppm; MS (MALDI-TOF): m/z calcd for C73H51N3OS2:
1049.347 [M+H]+; found: 1049.209.
5’-[4-(3,6-Bis-{4-[2-(4-diphenylamino-phenyl)-vinyl]-phenyl}-carbazol-9-
yl)-phenyl]-[2,2’]bithiophenyl-5-carbaldehyde (7)
3,6-Bis-(4-diphenylamino-phenyl)-9H-carbazole (3)
This intermediate was prepared as described in a previous report.[20]
5-(4-Bromophenyl)-[2,2’]bithiophene (8)
1
Rf =0.345 (CH2Cl2/n-hexane=2:1); H NMR (300 MHz, CDCl3): d=9.88
(s, 1H), 8.40 (s, 2H), 7.87–7.62 (m, 12H), 7.55–7.36 (m, 7H), 7.32–7.20
(m, 9H), 7.19–7.02 (m, 19H), 6.81 (q, 11 Hz, 6 Hz, 1H), 5.85 (d, 18 Hz,
1H), 5.30 ppm (d, 9 Hz, 1H); 13C NMR (125 MHz, [D6]DMSO): d=
183.1, 145.0, 144.8, 143.3, 141.2, 141.1, 138.3, 134.6, 134.1, 132.6, 131.8,
131.7, 128.8, 127.9, 127.7, 127.6, 127.1, 125.5, 125.2, 124.9, 124.8, 124.7,
121.0 ppm; MS (MALDI-TOF): m/z calcd for C79H55N3OS2: 1125.379
[M+H]+; found: 1126.237.
Rf =0.567 (CH2Cl2/n-hexane=1:4); 1H NMR (300 MHz, [D6]acetone):
d=7.65–7.62 (m, 4H), 7.50–7.46 (m, 2H), 7.34 (d, 1 Hz, 1H), 7.30 (d,
4 Hz, 1H), 7.12 ppm (t, 4 Hz, 1H); 13C NMR (125 MHz, CDCl3): d=
141.6, 137.2, 133.0, 132.0, 127.9, 127.0, 124.7, 124.6, 124.3, 124.1, 123.8,
121.3 ppm; MS (MALDI-TOF): m/z calcd for C14H9BrS2: 319.933
[M+H]+; found: 320.149. This intermediate was reported in literature.[21]
5’-(4-Bromophenyl)-[2,2’]bithiophenyl-5-carbaldehyde (9)
3,6-Bis-(4-vinyl-phenyl)-9H-carbazole (10)
POCl3 (0.87 mL, 6.23 mmol) was added dropwise to DMF (2 mL) with
stirring under dry conditions, and the temperature was kept below 108C.
Then compound 8 (1.00 g, 3.11 mmol) in dichloromethane (20 mL) was
added to the reagent obtained from POCl3 and DMF, the mixture was
stirred overnight at room temperature. The reaction mixture was extract-
ed with dichloromethane, and the organic phase was washed with H2O
and dried over Na2SO4. The solvent was removed to give the product,
which was purified by silica-gel column chromatography to produce an
orange solid (yield: 716 mg, 49%). Rf =0.345 (CH2Cl2/n-hexane=2:1);
1H NMR (300 MHz, [D6]acetone): d=9.95 (s, 1H), 7.96 (d, 3 Hz, 1H),
7.72–7.64 (m, 4H), 7.58–7.53 ppm (m, 3H); 13C NMR (125 MHz,
[D6]DMSO): d=184.3, 145.5, 143.9, 141.8, 139.6, 135.2, 132.5, 128.7,
127.8, 126.4, 125.8 ppm; MS (MALDI-TOF): m/z calcd for C15H9BrSO2:
347.928 [M+H]+; found: 348.156.
Rf =0.464 (CH2Cl2/n-hexane=1:1); 1H NMR (300 MHz, [D6]DMSO): d=
11.4 (s, 1H), 8.63 (s, 2H), 7.82–7.71 (m, 6H), 7.60–7.56 (m, 6H), 6.85–
6.75 (m, 2H), 5.89 (d, 12 Hz, 2H), 5.28 ppm (d, 12 Hz, 2H); 13C NMR
(125 MHz, CDCl3): d=141.4, 139.4, 136.5, 135.8, 132.6, 127.3, 126.7,
125.4, 124.0, 118.7, 113.5, 111.0 ppm; MS (MALDI-TOF): m/z calcd for
C28H21N: 371.167 [M+H]+; found: 371.231.
Synthesis of 2, 4, 6, and 7
The preparation of these compounds followed similar protocols. Thus,
only the synthesis of 2 is described. Spectral data for the other com-
pounds are provided.
5’-[3,6-Bis-(4-carbazol-9-yl-phenyl)-carbazol-9-yl]-[2,2’]bithiophenyl-5-
carbaldehyde (2)
3,6-Bis-{4-[2-(4-diphenylamino-phenyl)-vinyl]-phenyl}-9H-carbazol (5)
ortho-Dichlorobenzene was added to a mixture of 3,6-bis(4-carbazol-9-yl-
phenyl)-9H-carbazole (1) (0.3 g, 0.46 mmol), 5’-bromo-[2,2’]-bithiophen-
yl-5-carbaldehyde (0.14 g, 0.51 mmol), copper powder (0.02 g,
0.32 mmol), potassium carbonate (0.25 g, 1.84 mmol), and [18]crown-6
(0.04 g, 0.14 mmol). After the reaction mixture was refluxed for 48 h, it
was cooled to room temperature. The reaction mixture was extracted
with dichloromethane, and the organic phase was washed with H2O and
dried over Na2SO4. The solvent was removed to give the product, which
A mixture of compound 10 (300 mg, 0.81 mmol), 4-bromotriphenylamine
(654 mg, 2.02 mmol), palladium acetate (11.0 mg, 0.04 mmol), tri-o-tolyl-
phosphine (50 mg, 0.16 mmol), and triethylamine in DMF (10 mL) was
refluxed overnight. After cooling to room temperature, the solvent was
evaporated under high vacuum, and the reaction mixture was extracted
with dichloromethane. The organic phase was washed with water and
dried over Na2SO4. The solvent was evaporated to give a crude mixture,
348
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Chem. Asian J. 2012, 7, 343 – 350

Substituted Carbazole Dyes as Sensitizers
which was purified by silica-gel column chromatography and recrystal-
lized from dichloromethane and hexane to produce a green solid (yield:
460 mg, 80%). Rf =0.393 (CH2Cl2/n-hexane=1:1); 1H NMR (300 MHz,
CDCl3): d=8.34 (s, 2H), 8.11 (s, 1H), 7.76–7.71 (m, 6H), 7.63 (d, 6 Hz,
4H), 7.54–7.44 (m, 8H), 7.32–7.27 (m, 6H), 7.17–7.04 (m, 17H), 6.88–
6.77 (m, 1H), 5.85 (d, 18 Hz, 1H), 5.31 ppm (d, 9 Hz, 1H); 13C NMR
(125 MHz, CDCl3): d=147.5, 147.3, 140.8, 139.4, 135.9, 132.6, 131.6,
129.3, 127.8, 127.4, 127.3, 126.8, 126.7, 125.4, 124.5, 124.0, 123.6, 123.0,
118.6, 111.0 ppm; MS (MALDI-TOF): m/z calcd for C64H47N3: 857.377
[M+H]+; found: 857.349.
127.1, 127.0, 126.9, 126.2, 125.9, 125.5, 125.0, 124.9, 124.5, 124.3, 123.9,
123.6, 123.4, 119.9, 119.1, 111.4, 110.7 ppm. MS (HRMS): m/z calcd for
C82H56N4O2S2: 1193.3947 [M+H]+; found: 1193.3923.
Device fabrication of dye-sensitized solar cells
Fluorine-doped tin oxide (FTO) glass plates (Pilkington, 8 W sqꢀ1
,
2.3 mm thick) were cleaned in a detergent solution using an ultrasonic
bath for 15 min and then rinsed with water and ethanol. The washed
FTO glass plates were immersed in an aqueous 40 mm TiCl4 solution for
30 min at 708C. A transparent nanocrystalline layer was deposited on the
FTO glass by the doctor blade printing method using TiO2 paste (Ti-
Nanoxide T/SP, Solaronix) and a scattering layer paste (CCIC, PST-
400C). The TiO2 layer was gradually sintered according to a programmed
procedure. The sintered layer consisted of a 10 mm-thick transparent
layer and 4 mm-thick scattering layer. The TiO2 electrodes were treated
again with TiCl4 at 708C for 30 min and sintered at 5008C for 30 min.
After the TiO2 electrodes were immersed in 0.3 mm dye solution in DMF
containing 10 mm 3a,7a-dihydroxy-5b-cholic acid, they were kept at
room temperature for 18 h in the dark. The FTO plate for the counter
electrode was cleaned in an ultrasonic bath containing H2O, acetone, and
0.1m HCl aqueous solution. Subsequently, counter electrodes were pre-
pared by coating with a drop of H2PtCl6 solution (2 mg of Pt in 1 mL of
ethanol) on an FTO plate followed by heating at 4008C for 15 min. The
dye-adsorbed TiO2 electrode was then attached to the counter electrode
by heating with a hot-melt polymer film (Surlyn 1702, 100 mm thick,
DuPont). A drop of electrolyte solution (0.6m 1,2-dimethyl-3-n-propyl-
imidazolium iodide, 0.05m iodine, 0.1m LiI, and 0.5m 4-tert-butylpyridine
in acetonitrile) was permeated through a hole in the counter electrode,
which was driven into the cell by vacuum backfilling. Finally, the hole
was sealed using a hot-melt polymer film with a cover glass (0.1 mm
thick). N719 was purchased from Solaronix.
Synthesis of H1–H4
The preparation of these compounds followed similar protocols. Thus,
only the synthesis of H1 is described. Spectral data for the other com-
pounds are provided.
3-{5’-[3,6-Bis-(4-carbazol-9-yl-phenyl)-carbazol-9-yl]-[2,2’]bithiophenyl-5-
yl}-2-cyano-acrylic acid (H1)
A mixture of compound 2 (150 mg, 0.18 mmol) and cyanoacetic acid
(23 mg, 0.27 mmol) was dissolved in 50 mL of MeCN and 50 mL of THF
containing piperidine (46 mg, 0.54 mmol). The reaction mixture was re-
fluxed overnight. After cooling, 1n HCl was added to the reaction mix-
ture for neutralization, and the reaction mixture was extracted with di-
chloromethane. The organic phase was washed with water and dried over
Na2SO4. The solvent was evaporated to give the crude mixture, which
was purified by silica-gel column chromatography to give a reddish-
brown solid product (yield: 120 mg, 73%). Rf =0.309 (CH2Cl2/MeOH/
AcOH=10:1:0.1); 1H NMR (300 MHz, [D6]DMSO): d=8.94 (s, 2H),
8.28 (d, 8 Hz, 4H), 8.15 (d, 8 Hz, 4H), 8.09 (s, 1H), 8.00 (d, 9 Hz, 2H),
7.80–7.76 (m, 6H), 7.72 (d, 4 Hz, 1H), 7.68 (d, 4 Hz, 1H), 7.58 (d, 3 Hz,
2H), 7.51–7.44 (m, 8H), 7.32 ppm (t, 6 Hz, 4H). 13C NMR (500 MHz,
[D6]DMSO): d=143.4, 142.0, 141.8, 141.0, 140.5, 138.4, 137.6, 136.6,
136.3, 134.7, 134.5, 134.4, 134.2, 133.6, 129.2, 127.9, 127.1, 126.7, 125.8,
125.6, 125.0, 123.7, 121.4, 120.9, 120.2, 118.9, 111.7, 110.6 ppm; MS
(HRMS): m/z calcd for C60H36N4O2S2: 909.2355 [M+H]+; found:
909.2358.
Acknowledgements
3-{5’-[3,6-Bis-(4-diphenylamino-phenyl)-carbazol-9-yl]-[2,2’]bithiophenyl-
5-yl}-2-cyano-acrylic acid (H2)
This work was supported by the Basic Science Research Program
through a National Research Foundation of Korea (NRF) grant funded
from the Ministry of Education, Science, and Technology (MEST) of
Korea to the Center for Next Generation Dye-sensitized Solar Cells
(Grant No. 2011-0001055). We also acknowledge the support of BK21
fellowship grants (to W.L.) and thank Prof. Soo-Young Park for his
advice.
Rf =0.309 (CH2Cl2/MeOH/AcOH=10:1:0.1); 1H NMR (300 MHz,
[D6]DMSO): d=8.63 (s, 2H), 8.12 (s, 1H), 7.75–7.69 (m, 7H), 7.60 (d,
7 Hz, 3H), 7.53 (d, 4 Hz, 1H), 7.45 (d, 4 Hz, 1H), 7.31 (t, 8 Hz, 9H),
7.09–7.04 ppm (m, 16H). 13C NMR (500 MHz, [D6]DMSO): d=147.6,
146.6, 141.1, 140.7, 138.1, 136.6, 136.5, 135.2, 133.8, 133.6, 130.5, 129.9,
128.2, 128.1, 126.3, 125.8, 125.4, 125.2, 124.9, 124.4, 124.3, 123.4, 119.6,
119.0, 111.9, 110.9 ppm; MS (HRMS): m/z calcd for C60H40N4O2S2:
913.2670 [M+H]+; found: 913.2671.
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350
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 343 – 350