Molecular design of triarylamine-based organic dyes for efficient dye-sensitized solar cells

Article abstract of DOI:10.1039/b815649b

Three donor-(π-spacer)-acceptor dyes, coded as DS-3, DS-5 and DS-6, were designed and synthesized for dye-sensitized solar cells (DSSCs). These dyes possess the same donor (triarylamine) and acceptor/anchoring group (2-cyanoacrylic acid), but the varied π

Full text of DOI:10.1039/b815649b

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Molecular design of triarylamine-based organic dyes for efficient
dye-sensitized solar cells
Gang Li,ab Ke-Jian Jiang,*a Peng Bao,a Ying-Feng Li,a Shao-Lu Lia
and Lian-Ming Yang*a
Received (in Gainesville, FL, USA) 8th September 2008, Accepted 17th December 2008
First published as an Advance Article on the web 3rd February 2009
DOI: 10.1039/b815649b
Three donor–(p-spacer)–acceptor dyes, coded as DS-3, DS-5 and DS-6, were designed and
synthesized for dye-sensitized solar cells (DSSCs). These dyes possess the same donor
(triarylamine) and acceptor/anchoring group (2-cyanoacrylic acid), but the varied p-spacers
consisting of a combination of thiophene and/or 3,4-ethylenedioxythiophene (EDOT) units. They
were evaluated by photophysical and electrochemical measurements as well as in nanocrystalline
TiO2-based DSSCs fabricated using them as light-harvesting sensitizers. The results showed that
incorporation of the EDOT moiety in the dye molecules strongly affected the molar extinction
coefficient and photocurrent response. Among the three dyes, the DS-5-based DSSCs afforded the
best photovoltaic performance: a maximum monochromatic incident photon-to-current
conversion efficiency (IPCE) of 80%, a short-circuit photocurrent density (Jsc) of 14.6 mA cmꢀ2
,
an open-circuit photovoltage (Voc) of 0.59 V and a fill factor (FF) of 0.70, corresponding to an
overall conversion efficiency of 6.03% under 100 mW cmꢀ2 irradiation, which reached 77% with
respect to that of an N3-based device fabricated and measured under similar conditions. Further,
a method of molecular orbital calculations was employed for the insight into the geometrical,
electronic and optical properties of these dyes.
conversion efficiencies of up to 10%, the Ru sources and
Introduction
the resulting environmental issues would greatly limit their
applications outdoors. In recent years, metal-free organic dyes
as an alternative to the noble metal complexes have been
attracting increasing attention because of the following
advantages: easy preparation; low costs and little environment
issues; high molar extinction coefficient; and a wider variety of
structures, etc. Generally, a good organic sensitizing dye must
meet some basic requirements: (1) the HOMO energy level
should be sufficiently lower than the corresponding redox
couple (electrolytes, e.g., Iꢀ/I3ꢀ) so that the dye can be quickly
regenerated from its oxidized state, and the LUMO energy
level should be sufficiently more negative than the conduction
band edge level of TiO2 to ensure electron injection efficiently;
(2) the absorption of the dye should overlap the largest
possible range of the solar spectrum, especially the region
from the visible to near-infrared light because the photon flux
in this region is largest in the solar irradiation spectrum; and
(3) the molecular structure of dye should be a D–p–A mode to
facilitate charge transfer and subsequent charge separation,
and to slow down the charge recombination between the
injected electron and the oxidized dye. To date, remarkable
advances have been made in organic dyes as photosensitizers
for DSSCs. Various organic dyes, such as perylene,6 cyanine,7
porphyrin,8 phthalocyanine,9 merocyanine,10 coumarin,11
hemicyanine,12 indoline13 and triphenylamine14 dyes, have
been employed. Particularly, a high Z value of 9% was
achieved in full sunlight by Ito et al. using indoline dye as
photosensitizer in DSSCs.15 These results have shown a
promising potential for organic dye-based DSSCs in practical
applications.
Since the seminal publication of Gratzel and co-workers in
¨
1991,1 dye-sensitized solar cells (DSSCs) have attracted
considerable interest because of their higher conversion efficiency
of sunlight-to-electricity and potential low costs of production
and materials as compared to conventional silicon-based solar
cells. Typically, DSSCs are composed of a photoactive TiO2
anode coated with photosensitizers (sensitizing dyes), redox
electrolytes and a passive cathode (n-type DSSCs).2 In such
devices, the sensitizing dye must be adsorbed chemically onto
the surface of nanocrystalline mesoporous TiO2. Upon light
illumination, the adsorbed dye becomes an excited state and
then the excited dye injects electrons into the conduction band
of TiO2. The electrons are brought back to the oxidized dye
through an external circuit, a passive cathode and a redox
electrolyte system (typically Iꢀ/I3ꢀ), regenerating the oxidized
sensitizing dye.
Among all the elements of DSSCs, the photosensitizer is
fundamental to the cell’s performance and stability. Sensitizing
dyes for DSSCs are divided generally into two types, inorganic
metal complexes and metal-free organic dyes. Although
inorganic dyes, such as Ru complexes (N3, N719 and black
dyes),3–5 can provide efficient solar energy-to-electricity
a Beijing National Laboratory for Molecular Sciences (BNLMS),
Laboratory of New Materials, State Key Laboratory for Structural
Chemistry of Unstable and Stable Species, Institute of Chemistry,
Chinese Academy of Sciences, Beijing, 100190, P. R. China.
E-mail: yanglm@iccas.ac.cn. E-mail: kjjiang@iccas.ac.cn;
Fax: +86-10-62559373; Tel: +86-10-62565609
b Graduate School of Chinese Academy of Sciences, Beijing, 100049,
P. R. China
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Fig. 1 Molecular structures of as-synthesized organic dyes.
Organic dyes for DSSCs consist commonly of donor,
Photophysical properties
linker and acceptor (i.e., D–p–A structure), and hence their
various properties could be finely tuned by alternating
independently or matching different parts in a D–p–A dye.
Certainly, the p-spacers as bridging groups play an important
role in modifying properties and performances. We were
interested in introduction of 3,4-ethylenedioxythiophene
(EDOT) into p-linker groups in dye molecules because
EDOT is a conducting molecule and has presented some
specific properties as a building block in functionalized
p-conjugated systems or polymers.16 However, EDOT has
rarely been employed as the structural part of photosensitizers
in DSSCs, and only an extremely low efficiency was achieved
in one publication on EDOT-containing organic dye-based
DSSCs.17 Herein we wish to report our investigation into
three D–p–A chromophores coded as DS-3, DS-5 and DS-6
(Fig. 1) which contain the triarylamine group as donor
(D), 2-cyanoacrylic acid as acceptor (A) bridged by thiophene
and/or EDOT units.
The absorption spectra of the dyes in DMF solutions are
displayed in Fig. 2(a), and the corresponding data are
presented in Table 1. All the three dyes exhibited a p–p*
transition band in the range of 300–350 nm. Moreover, they
also give a strong absorption maximum (460–490 nm) in the
visible region, which may be assigned to an intramolecular
charge transfer (ICT) absorption because an efficient charge-
separated excited state could be produced between the
triarylamine and the cyanoacrylic acid moieties.19 As seen in
Table 1, the ICT absorption (lmax) appeared at 473 nm for
DS-3, 486 nm for DS-5, and 478 nm for DS-6, respectively;
molar exꢀti1nction coefficients at visible maxima (e) were
34 000 M cmꢀ1 for DS-3, 61 000 Mꢀ1 cmꢀ1 for DS-5, and
41 000 Mꢀ1 cmꢀ1 for DS-6, respectively, all of which are
twice as high as that of Ru(dcbpy)2(NCS)2 (N3 dye: ca. 1.6
ꢂ 104 Mꢀ1 cmꢀ1).20 The e values of DS-5 and DS-6
are superior to dye DS-3 due likely to a more extensive
conjugation of molecules containing the EDOT moiety. Also
as seen from Fig. 2(a), the maximum absorption of DS-5 is
slightly red-shifted in comparison with that of DS-6, which
should be attributed to a little more expansion of the DS-5
p-system by introduction of one additional methine unit into
its p-bridge. Further, the half-height absorption width of DS-5
is remarkably larger than those of both DS-6 and DS-3, and
thus it is predictable that dye DS-5 has an enhanced ability to
harvest solar light.
Results and discussion
Synthesis of materials
The photosensitive dyes in this study are depicted in Fig. 1.
Synthetic routes to the as-synthesized organic dyes are
illustrated in Scheme 1. (E)-1,2-Bis(5-formyl-2-thienyl)ethane
reacted with p-(di-p-tolylamino)benzyltriphenylphosphonium
bromide leading to compound 1 through a mono-Wittig
reaction.18 The thiophene moiety was coupled to the donor
to afford compound 2 by the Wittig–Horner reaction of
p-(di-p-tolylamino)benzyltriphenylphosphonium bromide and
Fig. 2(b) shows the normalized absorption and emission
spectra of DS-5 in air-equilibrated DMF solution (0.5 mM),
and the data were also collected in Table 1. The excitation
wavelength for emissions was 480 nm. The maximum
absorption and emission in DMF are at 486 and 633 nm,
respectively. On the other hand, no emission signal was
observed for all three dyes on the TiO2 colloidal electrodes,
suggesting that there existed an efficient electron injection
from the excited dye molecules into the conduction band
of TiO2.
thiophene-2-carbaldehyde. The intermediate
2 was then
converted to the corresponding aldehyde 3 in a moderate yield
by the Vilsmeier reaction in the presence of DMF and
POCl3 in refluxing anhydrous dichloromethane. A second
Wittig–Hornor reaction of 3 with 3,4-ethylenedioxy-2-thienyl-
methyl triphenylphosphonium bromide produced 4 which was
subjected to a Vilsmeier reaction to give 5. The intermediate 7
was also prepared by a process similar to the synthesis of
compound 3. Finally, target dyes were yielded by the
Knoevenagel reaction of the corresponding aldehydes with
cyanoacetic acid in refluxing acetonitrile in the presence of
piperidine. These target dyes were fully characterized by NMR
spectroscopy, mass spectrometry and elemental analysis,
and found to be consistent with the proposed structures.
The obtained dyes are purple or violet in the solid state, and
easily dissolved in common organic solvents, such as
dichloromethane, chloroform, THF, DMF, etc.
The absorption spectra of the dyes absorbed onto TiO2
nanocrystalline electrodes are shown in Fig. 3. The absorption
spectra of the dyes were distinctly broadened. Such
spectral broadening is favourable for harvesting visible light.
Compared to the absorption spectra in solutions, the film
absorptions of DS-3 and DS-6 show a large red-shift
(DS-3: 34 nm; and DS-6: 30 nm) due to J-aggregation, while
dye DS-5 was slightly blue-shifted (from 486 to 480 nm) which
could be ascribed to slight H-aggregation on the TiO2
surface. Such aggregation phenomena have been observed in
several organic dyes.21,22 Thus, chenodeoxychlolic acid is
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Scheme 1 Synthesis of the chromophores. Reagents and conditions: (a) DMF/18-crown-6, K2CO3, rt; (b) THF/t-BuOK, rt; (c) DMF/POCl3,
CH2Cl2, reflux; (d) CNCH2COOH, piperidine, MeCN, reflux.
usually used as a co-adsorbent to suppress dye aggregation in
DSSC assembly since dye aggregation tends to impair electron
injection from the excited dye to the TiO2.
where E0–0 was estimated from the intersection of the
absorption and emission spectra. As listed in Table 1, the
excited state potentials of the dyes (DS-3: ꢀ1.23 V vs. NHE;
DS-5: ꢀ1.21 V vs. NHE; DS-6: ꢀ1.36 V vs. NHE) are much
more negative than the conduction band energy level of TiO2
(ca. ꢀ0.5 V vs. NHE), clarifying that the electron injection
should be possible thermodynamically. Furthermore, the
positive shift of the excited state oxidation potentials for
DS-3 and DS-5, compared to dye DS-6, can be explained by
the presence of two EDOT units in DS-6 which enrich the
electron density and make the molecule more easily oxidized.
Electrochemical properties
The electrochemical behavior of the dyes was investigated by
cyclic voltammetry (CV), and the data are listed in Table 1. The
oxidation potentials of the as-synthesized dyes are determined
from the oxidation peak potential and reduction peak potential
for the oxidation process. As seen in Fig. 4, the dyes can be
reversibly oxidized at a moderately high oxidation potential
under low concentration conditions. The oxidation potential
vs. NHE (Eox) corresponds to the highest occupied molecular
orbital (HOMO). As listed in Table 1, the first oxidation
potentials of the dyes (0.96–1.04 V vs. NHE) are more positive
than the Iꢀ/I3ꢀ redox couple (B0.4 V vs. NHE). The sufficiently
low HOMO energy level of the dye can ensure more effective dye
regeneration and restrain the recapture of the injected electrons
by the dye cation radical or the Iꢀ/I3ꢀ redox couple. Moreover,
the oxidation potential of dye DS-5 (1.03 V) is slightly negative
than that of DS-3 (1.04 V) likely because the former has a strong
electron-donating EDOT moiety. Interestingly, dye DS-6
(0.96 V) bearing two EDOT units attains only a relatively low
oxidation potential compared to DS-5.
Photovoltaic devices
DSSCs were fabricated using the dyes DS-3, DS-5 and DS-6,
as photosensitizers with an effective working area of 0.2 cm2,
nanocrystalline anatase TiO2 on FTO, the electrolyte composed
of 0.6 M 1-propyl-3-methylimidazolium iodide, 0.1 M LiI,
0.05 M I2 and 0.75 M 4-tert-butylpyridine in a co-solvent of
acetonitrile–valeronitrile (1 : 1, v/v). The action spectra of the
incident photon-to-current conversion efficiency (IPCE) as a
function of wavelength, which is defined as the number of
electrons generated by light in the external circuit divided by
the number of incident photons, is calculated using eqn (2),
and the data are plotted in Fig. 5.
The excited state oxidation potential (Eox*) of the sensitizer,
corresponding to the lowest unoccupied molecular orbital
(LUMO), is determined from the first oxidation potential
(Eox) of the ground state and the zero–zero transition energy
(E0–0) according to eqn (1):
1240 ðeV nmÞ Jsc ðmA cmꢀ2
Þ
IPCEðlÞ ¼
ꢂ
ð2Þ
j ðmW cmꢀ2
Þ
l ðnmÞ
In eqn (2), Jsc is the short-circuit photocurrent generated by
monochromatic light, l the wavelength of incident mono-
chromatic light, and j the incident light intensity.
E*ox = Eox ꢀ E0–0
(1)
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Fig. 3 Absorption spectra of the three dyes absorbed onto TiO2 film
electrodes.
Fig. 2 (a) UV-Vis absorption spectra of organic dyes in DMF.
(b) Normalized absorption (solid line) and emission (dashed line)
spectra for DS-5 in DMF.
Fig. 4 Cyclic voltammetry of the dye DS-5. Scanning rate is 100 mV sꢀ1
;
working electrode and counter electrode: Pt wires; reference electrode:
Hg/Hg2Cl2.
For DS-5, a high IPCE (above 70%) was obtained in the
range from 400 to 560 nm with a maximum value of 80% at
480 nm; the onset of IPCE was extended to 800 nm.
Considering reflection and absorption losses in the transparent
conducting oxide substrate, the net photon-to-current conversion
efficiency exceeds 95%, indicating a highly efficient performance
of the DS-5-based solar cell. In the cases of DS-3 and DS-6,
relatively low IPCE values were observed, with maximum
values of 67% at 483 nm (for DS-3) and 50% at 548 nm
(for DS-6). The higher IPCE values for DS-5 should stem
from its broader absorption and enhanced molar extinction
coefficient in the visible region. Relative to the dye DS-5,
relatively low IPCE values of DS-6 could be caused by a
serious self-quenching of the electronically excited state
because of cis–trans isomerization and the disordered alignment
of chromophores in J-like aggregation.21b
Fig. 6 presents photocurrent density–voltage (J–V) curves
of the cells measured under simulated solar irradiation
(1 sun, 100 mW cmꢀ2, 1.5 air mass global). The open-circuit
photovoltage (Voc), short-circuit photocurrent density (Jsc),
fill factor (FF), and solar-to-electrical energy conversion
efficiencies (Z) are listed in Table 1. The conversion efficiency
(Z) of the DSSCs is calculated from the short-circuit
photocurrent density (Jsc), the open-circuit photovoltage
Table 1 Optical properties, redox potentials and DSSC performance of the dyes
l
ab/nma
(e/Mꢀ1 cmꢀ1 b
Eoxd/V vs.
NHE
Eox* f/V vs.
NHE
Dye
)
lexc/nm
E0–0e/eV
Jsc/mA cmꢀ2
Voc/V
FF
Zg (%)
DS-3
DS-5
DS-6
N3
473 (34 000)
486 (61000)
478 (41000)
641
633
586
1.04
1.03
0.96
2.27
2.24
2.32
ꢀ1.23
ꢀ1.21
ꢀ1.36
11.1
14.6
9.46
16.87
0.630
0.590
0.585
0.680
0.73
0.70
0.65
0.68
5.12
6.03
3.59
7.82
a
b
Absorption spectra were measured in DMF solution at room temperature. The molar absorbance coefficient of the absorption spectra at
c
d
The maximum emission of dyes in DMF. The oxidation potential of the dyes were measured in DMF with 0.1 M (n-C4H9)4NPF6 as
lmax
.
electrolyte. Scan rate: 100 mV sꢀ1. Working electrode and counter electrode: Pt wires; reference electrode: Hg/Hg2Cl2. E0-0 was estimated
e
f
from the intersection between the absorption and emission spectra. The reduction potential of the dyes was calculated from Eox ꢀ E0–0
.
g
Performances of the DSSCs were measured with 0.2 cm2 working area.
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Fig. 5 Action spectra of monochromatic incident photo-to-current
conversion efficiency (IPCE) of N3 and the three dye-sensitized solar
cells. The electrolyte used was 0.6 M 1-propyl-3-methylimidazolium
iodide, 0.1 M LiI, 0.05 M I2 and 0.75 M 4-tert-butylpyridine in a
co-solvent of acetonitrile–valeronitrile (1 : 1, v/v).
Fig. 7 Current–voltage curves obtained with the DSSCs based on
DS-3 and DS-5 dyes under dark conditions.
response due to the strong aggregation of DS-6 on TiO2 films.
Here, the DS-6-sensitized cell gave Jsc of 9.46 mA cmꢀ2, Voc
of 0.585 V and FF of 0.65, corresponding to only 3.59%
of overall conversion efficiencies (Z). For comparison, the
device with standard N3 dye has Jsc = 16.87 mA cmꢀ2
,
Voc = 680 mV, FF = 0.68 and Z = 7.82% under the similar
conditions. The results suggest a promising prospect of the
as-synthesized organic dyes applied in DSSCs, particularly DS-5.
It is necessary to make a further comparison between DS-3
and DS-5 since they have a similar structure but display
distinctly different photocurrent values. The principal factor
should be that DS-5 has better light harvesting ability (broader
absorption and larger molar extinction coefficient). The dark
current–voltage curves obtained from the DS-3 and DS-5
based DSSCs are shown in Fig. 7. DS-5 showed a larger dark
current and thus a higher degree of charge recombination
mainly with the electrolyte species.14a So the introduction of
the EDOT moiety may change the arrangement and orientation
manner of dye molecules and further influence the access
of electrons in TiO2 to oxidized electrolyte. The charge
recombination at the interface would directly result in low
Voc values.24 Consistent with the above-mentioned dark
current analyses, the open-circuit voltage (Voc) value is in
the order of DS-5 o DS-3. On the other hand, DS-5 can
furnish a relatively higher short-circuit photocurrent density in
contrast to DS-3. Finally the DS-5 based photocell produced a
higher conversion efficiency than that of the DS-3. So a
suitable p-spacer should be adopted to the photosensitizer
structure for the balance between Voc and Jsc in pursuit of a
superior conversion efficiency. A further study on dynamics of
electron transport and recombination of DSSCs based on the
two dyes are underway.
Fig. 6 Photocurrent–voltage characteristics of the three dye-sensitized
solar cells under illumination of simulated solar light (AM 1.5,
100 mW cmꢀ2). The electrode used was the same as that for Fig. 5.
(Voc), the fill factor (FF), and the intensity of the incident light
(Pin),23 by eqn (3):
Jsc ðmA cmꢀ2Þ ꢂ Voc ðVÞ ꢂ FF
Z ¼
ð3Þ
Pin ðmW cmꢀ2
Þ
Under solar-simulated light irradiation, the DS-5-sensitized
solar cell shows a short-circuit photocurrent density (Jsc) of
14.6 mA cmꢀ2, open-circuit photovoltage (Voc) of 0.59 V and
fill factor (FF) of 0.70, corresponding to an overall energy
conversion efficiency (Z) of 6.03%; the DS-3-sensitized cell
gave Jsc of 11.1 mA cmꢀ2, Voc of 0.63 V and FF of 0.73,
corresponding to an overall conversion efficiency Z of 5.12%.
Compared to the dye DS-3, the DS-5-based cell exhibits
larger energy conversion efficiency due mainly to its larger
short-circuit photocurrent density (Jsc). The reason may be
that DS-5 has both a broader absorption and a larger molar
extinction coefficient in the visible region and hence could
more efficiently capture the solar radiation energy. On the
other hand, the DS-6-sensitized cell exhibited much lower
photovoltaic performance compared to the DS-3 and DS-5
based cells, which could be attributed to its lower spectral
Molecular orbital calculations
To gain insight into the geometrical, electronic and optical
properties of dyes, we mimicked the optimized geometries of
the three dyes by the hybrid density functional theory
(B3LYP) with 6-31G(D) basis set as implemented in
the Gaussian 03 program.28 The electron distributions
of the HOMO and LUMO of the dyes are shown in Fig. 8.
At the ground state for the dyes, electrons are homogeneously
distributed in both the electron donor and p-bridge, and upon
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Fig. 8 Frontier orbitals of the three dyes optimized at the B3LYP/6-31G(D) level.
the light illumination, electrons move from the HOMO to the
Experimental
LUMO by an intramolecular charge transfer, and are finally
located in anchoring groups through the p-bridge. Fig. 8
reveals that the cyanoacrylic acid group is essentially coplanar
with respect to the thiophene and/or EDOT fragments in their
energy minimised structures. Assuming that the dye molecule
has a similar molecular orbital geometry when anchored to
TiO2, the position of the LUMO close to the anchoring group
would enhance the orbital overlap with the titanium 3d orbital
and thus favor electron injection from dye to TiO2.
General
All reactions and manipulations were carried out under
nitrogen atmosphere unless otherwise stated. THF was
distilled from sodium/benzophenone. DMF, dichloromethane
1
and POCl3 were distilled from CaH2. H NMR spectra were
obtained on a Bruker Avance 400 (400 MHz) or Bruker DMX
300 (300 MHz) spectrometer with tetramethylsilane as internal
standard. 13C NMR spectra were recorded on a Bruker
Avance 400 (100 MHz). MALDI-TOF mass spectrometric
measurements were performed on the Bruker Biflex III
MALDI-TOF. Elemental analyses were performed on a
FLASH EA1112 elemental analyzer. EI were recorded on
the APEXII FT-ICR spectrometer. UV–Vis and fluorescence
spectra of the dyes in DMF solutions were recorded in a
quartz cell with 1 cm path length on an HP-8453 spectrometer
and F-4500 fluorescence spectrophotometer, respectively.
Potassium tert-butoxide, thiophene-2-carbaldehyde, tetra-
butylammonium perchlorate, chenodeoxycholic acid (DCA),
Triton X-100, 4-tert-butylpyridine (TBP), TiCl4, LiI and I2
from Aldrich, N3 from Solaronix Company, and cyanoacetic
acid from TCI were used as received without further purifica-
tion. 3,4-Ethylenedioxy-2-formylthiophene and 5-formyl-2,20-
bi(3,4-ethyleneoxythiophene) were synthesized according to
the literature procedures.25 All other solvents and chemicals
used in this work were analytical grade and used without
further purification. Chromatographic separations were
carried out on silica gel (200–300 mesh). Transient conducting
oxide (TCO, F–doped SnO2, 10 O &ꢀ1, Nippon Sheet Glass)
Conclusions
We have investigated three metal-free organic dyes (DS-3, DS-5
and DS-6) that feature triarylamine as electron-donating
moiety and cyanoacrylic acid as electron-accepting moiety
bridged by thiophene and/or EDOT moieties. Introduction
of the EDOT moiety could strongly affect the photocurrent
response of the dyes by controlling photophysical and
photovoltaic properties. Under the photovoltaic performance
measurement,
a maximum photo-to-electron conversion
efficiency of 6.03% was achieved with the DSSC based on
dye DS-5 under AM 1.5 irradiation (100 mW cmꢀ2), which
reached 77% with respect to that of an N3-based device
fabricated and measured under similar conditions. Our results
demonstrated the necessity and importance of designing or
modifying p-spacers in D–p–A dyes for DSSCs. Further
structural modification of DS-5 for broadening the spectral
absorption and increasing the molar extinction coefficient is
anticipated to give some new organic dyes with even better
performances.
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was washed with a basic solution, ethanol, and acetone
successively under supersonication for 15 min before use.
removal by rotary evaporation, followed by column chromato-
graphy over silica gel with a mixture of ethyl acetate and
petroleum (1:6) as eluent, yielded a yellow solid (0.39 g). The
crude product was used directly in the next step without
further purification. MS-EI: m/z 547 (M+).
Synthesis
5-{2-[5-[p-Di(p-tolyl)aminostyryl]thiophene-2-yl]vinyl}thiophene-
2-carbaldehyde (1). The synthetic procedure and characterizations
were presented in an earlier publication.18
5-2-[5-[p-Di(p-tolyl)aminostyryl]thiophene-2-yl]vinyl-3,4-
ethylenedioxythiophene-2-carbaldehyde (5). POCl3 (1 mL,
11 mmol) was added dropwise with stirring to a solution of
anhydrous CH2Cl2 (20 mL) and anhydrous DMF (5 mL) at
0–5 1C. After stirring for 30 min, a solution of compound 4
(0.39 g, 7.12 mmol) in CH2Cl2 (10 mL) was added slowly, and
then the solution turned brown–red. The solution was allowed
to warm to room temperature, and then refluxed for 12 h.
After cooling to room temperature, the solution was poured
onto 50 mL of iced water, and stirred for 10 min, followed by
neutralization with aqueous NaOH (10%, w/w). The mixture
was extracted with CH2Cl2, and the organic phase was washed
with water and dried over Na2SO4 and filtered. Solvent
removal by rotary evaporation and column chromatography
over silica gel with a mixture of dichloromethane and
petroleum ether (2 : 1, v/v) as eluent yielded 5 as a brown–red
solid (0.3 g, 73.3%). 1H NMR (300 MHz, CDCl3): d 2.32
(s, 6H), 4.35–4.39 (m, 4H), 6.84 (d, 1H, J = 16 Hz), 6.85–6.99
(m, 4H), 6.97 (d, 2H, J = 9 Hz), 7.00 (d, 4H, J = 8.4 Hz), 7.08
(d, 4H, J = 8.4 Hz), 7.20 (d, 1H, J = 16 Hz), 7.27 (d, 2H,
J = 9 Hz), 9.87 (s, 1H). MS-EI: m/z 575 (M+).
5-{2-[5-[p-Di(p-tolyl)aminostyryl]thiophene-2-yl]vinyl}thiophene-
2-cyanoacrylic acid (DS-3). The synthetic procedure and
characterizations were presented in an earlier publication.18
5-{2-[p-Di(p-tolyl)aminophenyl]vinyl}thiophene
(2).
A
mixture of 4-(di-p-tolylamino)benzyl triphenylphosphonium
bromide (3.77 g, 6 mmol) and thiophene-2-carbaldehyde
(0.467 ml, 5 mmol) were dispersed in anhydrous THF
(30 mL) and stirred at ambient temperature under nitrogen
atmosphere for 30 min. t-BuOK (0.785 g, 7 mmol) was
dissolved in anhydrous THF and added dropwise to the
solution, the mixture was stirred overnight at ambient
temperature. Then a molar excess of water was added and
the mixture was extracted with CH2Cl2 (50 mL ꢂ 3). The
combined extract was dried over anhydrous Na2SO4 and
filtered. Solvent removal by rotary evaporation, followed by
column chromatography over silica gel with a mixture of
CH2Cl2 and petroleum (1 : 8, v/v) as eluent, yielded a yellow
solid (1.42 g). This crude product was used directly in the next
step without further purification. MS-EI: m/z 381 (M+).
5-2-[5-[p-Di(p-tolyl)aminostyryl]thiophene-2-yl]vinyl-3,4-ethylene-
dioxythiophene-2-cyanoacrylic acid (DS-5). The compound 5
(0.22 g, 0.38 mmol), cyanoacetic acid (0.1 g, 1.17 mmol),
piperidine (0.2 mL) and acetonitrile (30 mL) was charged
sequentially in a three-necked flask and heated to reflux under
N2 atmosphere for 8 h. After cooling to room temperature,
solvent was removed by rotary evaporation, the solid was
absorbed on silica gel and purified by column chromatography
using CH2Cl2–acetic acid (150 : 1, v/v) as eluent, yielding the
product DS-5 as a purple solid (0.23 g, 93.6%); mp 268 1C.
1H NMR (400 MHz, d6-DMSO): d 2.3 (s, 6H), 4.43–4.49
(m, 4H), 6.84 (d, 2H, J = 8.6 Hz), 6.90 (d, 1H, J = 10.3 Hz),
6.93–6.95 (m, 5H), 7.11–7.14 (m, 5H), 7.25 (d, 1H. J = 16 Hz),
7.30 (d, 1H, J = 3.8 Hz), 7.38 (d, 1H, J = 16 Hz), 7.42 (d, 2H,
J = 8.7 Hz), 8.16 (s, 1H). 13C NMR (400 MHz, d6-DMSO):
d 20.88, 65.27, 66.40, 109.65, 116.17, 117.51, 119.90, 121.57,
125.21, 125.68, 127.93, 128.04, 129.41, 129.82, 130.58, 131.02,
133.24, 139.44, 140.13, 140.19, 144.66, 144.77, 147.98, 149.38,
164.56. MS (MALDI-TOF): m/z 642.0 (M+). Anal. Calc.
for C38H30N2O4S2: C, 71.00; H, 4.70; N, 4.36. Found:
C, 71.05; H, 5.02; N, 4.36%.
5-[p-Di(p-tolyl)aminostyryl]thiophene-2-carbaldehyde
(3).
POCl3 (0.867 mL, 9.55 mmol) was added dropwise with
stirring to a solution of anhydrous CH2Cl2 (20 mL) and
anhydrous DMF (8 mL) at 0–5 1C under nitrogen atmosphere.
After stirring for 30 min, a solution of compound 2 (1.42 g,
3.47 mmol) in CH2Cl2 (10 mL) was added slowly. The solution
was allowed to warm to room temperature and then refluxed
for 12 h. After cooling to room temperature, the solution was
added to 50 mL of iced water and stirred for 10 min, followed
by neutralization with aqueous NaOH (10%). The mixture
was extracted with CH2Cl2, and the organic phase was washed
with water, dried over Na2SO4, and filtered. Solvent removal
by rotary evaporation, followed by column chromatography
over silica gel with a mixture of ethyl acetate and petroleum
ether (1 : 10, v/v) as eluent, yielded 3 as a red solid (1.0 g,
65%). 1H NMR (400 MHz, CDCl3): d 2.32 (s, 6H), 6.96
(d, 2H, J = 8.67 Hz), 7.01 (d, 4H, J = 8.38 Hz), 7.05–7.08
(m, 7H), 7.31 (d, 2H, J = 8.70 Hz), 7.63 (d, 1H, J = 3.90 Hz),
9.82 (s, 1H). MS-EI: m/z 409 (M+).
5-{2-[5-[p-Di(p-tolyl)aminostyryl]thiophene-2-yl]vinyl}-3,4-
ethylenedioxythiophene (4). A mixture of 3,4-ethylenedioxy-2-
thienylmethyl triphenylphosphonium bromide (1.5 g, 3 mmol)
and 2 (0.82 g, 2 mmol) were dispersed in anhydrous THF
(30 mL), and stirred at ambient temperature under nitrogen
atmosphere for 30 min. t-BuOK (0.6 g, 5.3 mmol) was
dissolved in anhydrous THF and added dropwise to the
solution. The mixture was stirred overnight at ambient
temperature, and then 20 mL water was added. The mixture
was extracted with CH2Cl2 (10 mL ꢂ 3), and the combined
extract was dried over anhydrous Na2SO4 and filtered. Solvent
2-5-[p-Di(p-tolyl)aminostyryl]-3,4-ethylenedioxythiophene-2-yl-
3,4-ethylenedioxythiophene (6). A mixture of [4-di(p-tolyl)-
aminobenzyl] triphenylphosphonium bromide (2.55 g, 4.06 mmol)
and 5-formyl-2,20-bi(3,4-ethyleneoxythiophene) (0.95 g,
3.06 mmol) were dispersed in anhydrous THF (50 mL) and
stirred at ambient temperature under nitrogen atmosphere for
10 min. t-BuOK (0.56 g, 5 mmol) was dissolved in anhydrous
THF and added dropwise to the solution. The mixture was
stirred overnight at ambient temperature. After a molar excess
of water was added, the mixture was extracted with CH2Cl2
ꢁc
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(50 mL ꢂ 3). The combined extracts were dried over anhydrous
Na2SO4 and filtered. Solvent removal by rotary evaporation,
followed by column chromatography over silica gel with a
mixture of CH2Cl2 and petroleum ether (1 : 1, v/v) as eluent,
yielded a yellow solid (1.6 g). The crude product was used
directly in the next step without further purification. MS-EI:
m/z 579 (M+).
was added to each sample solution at the end of the
experiments, and the ferrocene/ferrocenium (Fc/Fc+) redox
couple was used as an internal potential reference.
Fabrication of the dye-sensitized nanocrystalline TiO2
electrodes
For fabrication of the devices, two layers of TiO2, the main
layer and scattering layer, were prepared by screen-printing
TiO2 pastes on FTO glass substrate (10 O &ꢀ1, Nippon Sheet
Glass). The main layer (thickness: 14 mm; TiO2 particle size:
18 nm) and scattering layer (thickness: 8 mm; TiO2 particle
size: 400 nm) were prepared from these two different TiO2
2-5-[p-Di(p-tolyl)aminostyryl]-3,4-ethylenedioxythiophene-2-
yl-3,4-ethylenedioxythiophene-5-carbaldehyde
(7).
POCl3
(0.4 mL, 4.4 mmol) was added dropwise with stirring to a
solution of anhydrous CH2Cl2 (10 mL) and anhydrous DMF
(0.32 mL) at 0–5 1C under nitrogen atmosphere. After stirring
for 30 min, a solution of 6 (1.5 g, 2.58 mmol) in CH2Cl2
(10 mL) was added slowly. Then the solution was allowed to
warm to room temperature, and refluxed for 12 h. After
cooling to room temperature, the solution was poured onto
50 mL of ice–water and stirred for 10 min, followed by
neutralization with aqueous NaOH (10%, w/w). The mixture
was extracted with CH2Cl2, and the organic phase was washed
with water and dried over Na2SO4 and filtered. Solvent
removal by rotary evaporation and column chromatography
over silica gel with CH2Cl2 as eluent yielded 7 as a red solid
colloids (PST-18NR and PST-400C from Catalysts
&
Chemicals Ind. Co., Ltd, CCIC in Japan). After screen-printing,
the TiO2 film was heated at 500 1C for 30 min, and treated with
0.04 M TiCl4 aqueous solution as reported by Ito.26 The three
dyes and N3 solutions were prepared in DMF at a concentration
of 0.5 mM; 1 mM chenodeoxycholic acid in tert-butyl
alcohol–acetonitrile (1 : 1, v/v) was added as reported in order
to effectively prevent unfavorable dye aggregation on the TiO2
surface.27 The TiO2 films were left in solution at room
temperature for 24 h. A Pt-sputtered FTO glass was used as
a counter electrode. The electrolyte was composed of 0.6 M
1-propyl-3-methylimidazolium iodide, 0.1 M LiI, 0.05 M I2
and 0.75 M 4-tert-butylpyridine in a co-solvent of acetonitrile–
valeronitrile (1 : 1, v/v). The photovoltaic performance of the
devices was recorded under 100 mW cmꢀ2 simulated air mass
(AM) 1.5 solar light illumination.
1
(0.95 g, 60%). H NMR (300 MHz, CDCl3): d 2.31 (s, 6H),
4.32–4.42 (m, 8H), 6.85 (d, 1H, J = 16 Hz), 6.95 (d, 2H,
J = 8.7 Hz), 7.00 (d, 4H, J = 8.3 Hz), 7.03 (d, 1H, J = 16 Hz),
7.06 (d, 4H, J = 8.3 Hz), 7.27 (d, 2H, J = 8.7 Hz), 9.88
(s, 1H). MS-EI: m/z 607 (M+).
2-2-[5-[p-Di(p-tolyl)aminostyryl]-3,4-ethylenedioxy-thiophene-2-
yl]-3,4-ethylenedioxythiophene-5-yl]cyanoacrylic acid (DS-6).
The compound 7 (0.13 g, 0.21 mmol), cyanoacetic acid
(0.1 g, 1.17 mmol), piperidine (0.2 mL) and ethanol (40 mL)
was charged sequentially in a three-necked flask and heated to
reflux under N2 atmosphere for 8 h. After cooling to room
temperature, solvent was removed by rotary evaporation, the
residue was absorbed on silica gel and purified by column
chromatography using CH2Cl2–acetic acid (50:1, v/v) as
eluent, yielding the product DS-6 as a dark-purple solid
Photovoltaic measurements
The action spectra of monochromatic incident photon-
to-current conversion efficiency (IPCE) for solar cells was
performed by using a commercial setup for IPCE measurement
(PV-25 DYE, JASCO) under 5 mW cmꢀ2 monochromic light
illumination. The irradiation source for the photocurrent–
voltage (J–V) measurement is an AM 1.5 solar simulator
(YSS-50A, Yamashita Denso Co. Ltd.). A 500-W Xe lamp
serves as the light source in combination with a band-pass
filter (400–800 nm) to remove the ultraviolet and infrared light
and to give a light power of 100 mW cmꢀ2. The current–
voltage curves were obtained by a linear sweep voltammetry
1
(0.08 g, 55%); mp 249 1C. H NMR (400 MHz, d6-DMSO):
d 2.3 (s, 6H), 4.39–4.49 (m, 8H), 6.81 (d, 2H, J = 8.5 Hz), 6.89
(d, 1H, J = 16 Hz), 6.94 (d, 4H, J = 8.2 Hz), 7.06 (d, 1H,
J = 16 Hz), 7.13 (d, 4H, J = 8.2 Hz), 7.42 (d, 2H, J = 8.5 Hz),
8.16 (s, 1H). 13C NMR (400 MHz, d6-DMSO): d 20.38, 64.58,
65.07, 65.38, 65.73, 105.76, 108.85, 114.94, 115.65, 117.44,
119.72, 120.87, 121.02, 124.76, 127.37, 127.94, 129.49, 130.10,
132.78, 136.19, 138.53, 139.65, 140.70, 144.28, 147.38, 148.26,
164.42. MS (MALDI-TOF): 674.1 (M+). Anal. Calc. for
C38H30N2O6S2: C, 67.64; H, 4.48; N, 4.15; Found: C, 67.15;
H, 4.55; N, 4.08%.
method using an electrochemical workstation using
a
PC-controlled voltage–current source meter (R6246, Advantest)
under a solar simulator illumination (Yamashita Denso,
Yss-80) of Air Mass 1.5 (100 mW cmꢀ2) conditions at 25 1C.
The active electrode area was 0.2 cm2.
Molecular orbital calculations
Electrochemical measurements
All calculations were performed on the Gaussian 03 program
package by using Density Functional Theory (DFT).28
B3LYP and the 6-31 G (D) basis set were used. Before
optimizing ground-state geometries by B3LYP/6-31G(D),
Electrochemical redox potentials were measured via cyclic
voltammetry using a three-electrode cell under room temperature
in DMF solutions. The working and counter electrodes were
Pt wires, and the reference electrode was Hg/Hg2Cl2. The
electrolyte was 0.1 M (n-C4H9)4NPF6 in DMF. The E1/2 values
were determined as (Epa + Epc)/2, where Epa and Epc are the
anodic and cathodic peak potentials, respectively. Ferrocene
the molecular structures were initially optimized in
a
HF/ST0-3G calculation method. All the geometries and
electronic properties were calculated by assuming that the
target molecules would be isolated in gas phase.
ꢁc
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New J. Chem., 2009, 33, 868–876 | 875

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W. Wu, H. Pei, B. Liu and H. Tian, J. Org. Chem., 2008, 73, 3791;
(f) H. Choi, C. Baik, S. O. Kang, J. Ko, M.-S. Kang,
Acknowledgements
M. K. Nazeeruddin and M. Gratzel, Angew. Chem., Int. Ed.,
¨
2008, 47, 327; (g) P. Bonhote, J.-E. Moser, R. Humphry-Baker,
The authors thank the National Natural Science Foundation
of China (Projects No. 20672116 and No. 20774103) for
financial support of this work.
N. Vlachopoulos, S. M. Zakeeruddin, L. Walder and M. Gratzel,
¨
J. Am. Chem. Soc., 1999, 121, 1324.
15 S. Ito, S. M. Zakeeruddin, R. Humphry-Baker, P. Liska,
R. Charvet, P. Comte, M. K. Nazeeruddin, P. Pe0chy,
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876 | New J. Chem., 2009, 33, 868–876