Preparation of donor-acceptor type organic dyes bearing various electron-withdrawing groups for dye-sensitized solar cell application

Article abstract of DOI:10.1039/c1cc11130b

Novel donor-π-spacer-acceptor type organic dyes bearing various electron-withdrawing groups on their acceptor part were synthesized by Knoevenagel condensation with acetic acid silyl ester derivatives. Dye-sensitized solar cells with the new dyes present solar light-to-electricity conversion efficiency of 2.51-4.05%.

Full text of DOI:10.1039/c1cc11130b

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COMMUNICATION
Preparation of donor–acceptor type organic dyes bearing various
electron-withdrawing groups for dye-sensitized solar cell applicationw
Youhei Numata, Islam Ashraful, Yasuhiro Shirai and Liyuan Han*
Received 25th February 2011, Accepted 13th April 2011
DOI: 10.1039/c1cc11130b
Novel donor–p-spacer–acceptor type organic dyes bearing various
electron-withdrawing groups on their acceptor part were synthe-
sized by Knoevenagel condensation with acetic acid silyl ester
derivatives. Dye-sensitized solar cells with the new dyes present
solar light-to-electricity conversion efficiency of 2.51–4.05%.
the dyes determine the efficiency of electron injection into the
TiO2 electrode and dye regeneration. The HOMO–LUMO
band gap corresponds to the absorption wavelength. Because
the organic dyes absorb light strongly compared with
Ru–polypyridyl dyes, the bathochromic shift of the absorption
maxima is one of the most important challenges to improve
DSC performance. The HOMO level relates to the donor and
the p-spacer parts, and the LUMO level depends on the
acceptor parts. A wide variety of the donors and the p-spacers
have been explored.3–5,10 However, a knowledge of the
acceptors and the anchoring parts is still limited, even though
they also affect DSC performance. The major reason for minor
knowledge is less flexibility in their design. Most dye syntheses
start with the donors, then the p-spacers are connected to
elongate the p system, and finally the anchoring acceptors
are attached. Thus, it is essential to develop new synthetic
procedures that will allow us to efficiently explore their new design
and to optimize the HOMO and the LUMO energy levels at the
late stage of dye synthesis. We have developed a new method to
introduce various electron-withdrawing groups (EWGs) as the
acceptor part of D-p-A dyes for free exploration of the acceptor
designs. Here, we describe a new synthetic procedure for
the convenient introduction of EWGs, spectral properties and
photovoltaic performances of new three D-p-A dyes.
Dye-sensitized solar cells (DSCs) based on the TiO2 nano-
crystalline electrode have attracted great attention since
O’Regan and Gratzel achieved high solar light-to-electricity
¨
conversion efficiency using a Ru–polypyridyl complex as a
photosensitizer.1 Through great efforts of many researchers,
the best efficiency has been increased up to 11%.2 Recent
attentions have focused on metal-free organic dyes as DSC
sensitizers because of their strong molar absorption coefficient,
ease of structure modification, and low material cost.3 Most
organic dyes used for highly efficient DSCs follow a donor–
p-spacer–acceptor (D-p-A) architecture.3,4 Triarylamine moieties
are often used as the donor owing to their chemical stability,
stabilized photoexcited state by charge delocalization in the aryl
substituents, and prevention of direct charge recombination
À
between TiO2 and I3 by the bulky aryl group covering TiO2
surfaces.5 The acceptor part behaves as both an electron
acceptor in the charge transfer process from the donor and as
an anchoring group to adsorb onto TiO2 surfaces, which is
essential for the efficient electron injection from the dye into the
TiO2. Cyanoacrylic acid,6 rhodamine-,7 and thiazolidinorhod-
amine-N-carboxylic acid moieties8 are often used as the
acceptors to fulfil these requirements described above. The
donors and the acceptors are linked by p-conjugated spacers
such as polyene, oligophenylenevinylene, oligothiophene and
their derivatives.9 DSCs with D-p-A-type dyes have achieved up
to 9% efficiency under AM 1.5 G irradiation.6e,8c
We employed a triphenylamine- and bithiophene-based dye
as a common structure (Fig. 1), because this motif is easy to
prepare and give relatively high efficiencies of 5% to 6%.3,4
First, we attempted to synthesize the dyes by means of
Knoevenagel condensation of
a precursor aldehyde (2)
with acetic acid derivatives. We selected three acetic acid
derivatives: 3,3,3-trifluoropropanoic acid, p-nitrophenylacetic
In the design of dye molecules, the highest occupied
molecular orbital (HOMO), the lowest unoccupied molecular
orbital (LUMO), and the HOMO–LUMO band gap are
among the most important properties dominating the dye
performance in DSC. The HOMO and the LUMO levels of
Photovoltaic Materials Unit, National Institute for Materials Science,
Sengen 1-2-1, Tsukuba, Ibaraki, 305-0047, Japan.
E-mail: Han.Liyuan@nims.go.jp; Fax: +81 29-859-2301;
Tel: +81 29-859-2305
w Electronic supplementary information (ESI) available: Experimental
details, DFT calculation, and photovoltaic results. See DOI: 10.1039/
c1cc11130b
Fig. 1 Molecular structure of triphenylamine and bithiophene-based
new dyes with three EWGs.
c
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Chem. Commun., 2011, 47, 6159–6161 6159

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Fig. 2 UV-vis and emission spectra of the new dyes measured in
acetonitrile solutions.
The results are summarized in Table 1. The HOMO and the
LUMO levels of the three dyes are sufficient for efficient
electron injection into TiO2 and dye regeneration. All three
dyes showed weak absorption at around 300 nm and strong
absorption at around 420 nm. These absorptions were
assigned to the ICT between the donors and the acceptors
for the peaks at around 420 nm, and to the p–p* transition for
the peaks at around 300 nm. The lmax values of ICT were red-
shifted in the order of CF3 4 p-NO2Ph 4 o-NO2Ph. It was
assumed that the electron-withdrawing properties of o- and
p-NO2Ph groups are weaker than that of the CF3 group, being
presumably due to the large dihedral angle between phenyl
rings and the acrylic acids (Fig. S2, ESIw). Moreover, a larger
steric hindrance of the o-NO2 group increases the dihedral
angle between the phenyl ring and the CQC bond in
optimized structures.
Scheme 1 Preparation of triphenylamine and bithiophene-based new
dyes with three EWGs on the 1-position of acrylic acid.
acid, and o-nitrophenylacetic acid (Scheme 1) as alkyl, aryl,
and sterically-hindered EWG substituents, respectively. We
carried out Knoevenagel condensation between 2 and acetic
acid derivatives to obtain target dyes according to the popular
method to prepare 1-cyanoacrylic acid derivatives for DSC
chromophores.6
Unfortunately, the target compounds were obtained in trace
amounts at best. We suspected that the methylene protons of
these acid derivatives possess insufficient acidity to generate
reactive species. Consequently, we converted the carboxylic
acids into esters to increase the acidity of methylene protons.
We employed the tert-butyldimethylsilyl (t-BuMe2Si) ester as a
precursor due to facile preparation, bulkiness to obtain a
favorable isomer, stability under the reaction conditions,
sufficient reactivity for Knoevenagel condensation, and ease
of the O–Si bond cleavage in the presence of FÀ anions in
one-pot under mild reaction conditions. The t-BuMe2Si ester
derivatives were prepared easily by the reaction of corresponding
acetic acid derivatives with t-BuMe2SiCl in the presence
of pyridine and N,N-dimethylaminopyridine. Knoevenagel
condensation with the silyl ester was carried out under similar
conditions in the case of acetic acid derivatives. Tetra-n-
butylammonium fluoride, trifluoroacetic acid and water were
added before the reaction mixture was stirred overnight to give
target dyes in moderate yields (CF3: 30%, o-NO2Ph: 9%, and
p-NO2Ph: 38%). The low yield of o-NO2Ph may be ascribed to
the steric hindrance of the o-NO2 group.
Finally, we examined the photovoltaic properties of the
newly synthesized dyes. The DSCs were successfully fabricated
according to the literature method.11b The incident photon-to-
current conversion efficiency (IPCE) spectra of the DSCs with
the new dyes are shown in Fig. 3, and the corresponding
photovoltaic parameters are listed in Table 2. All measurements
were performed under AM 1.5 irradiation (100 mW cmÀ2) with
0.25 cm2 active surface area defined by a metal mask. Photo-
voltaic parameters were obtained by the measurements of two
different cells on each dye, and all measurements gave a
consistent result (Table S1, ESIw). The maximum value for
o-NO2Ph in IPCE reached ca. 80% at around 430 nm. The
short circuit current (JSC) and the open circuit voltage (VOC
)
of o-NO2Ph were 9.91 mA cmÀ2 and 0.58 V, respectively, with
the fill factor (FF) of 0.70. The overall conversion efficiency (Z)
was 4.05%, which was the highest value among the three dyes.
The overall conversion efficiencies of CF3 and p-NO2Ph were
Table 1 Photophysical properties of the new dyes
The optimized molecular structures and frontier orbitals of
the three dyes were calculated by using the density functional
theory computational method. The HOMOs were localized on
around the triphenylamine and the bithiophene moieties in all
three dyes (Fig. S1, ESIw). In contrast, the LUMOs were
distributed over the bithiophene, the acrylic acid, and EWGs.
Accordingly, directional intramolecular charge transfer (ICT)
from the donors to the acceptor parts along the bithiophene
p-spacer units occurred to inject an excited electron through
the anchoring carboxylic acids to the TiO2 conduction band.
Fig. 2 presents UV-visible absorption and emission spectra
of the new dyes measured in 0.1 M acetonitrile solutions.
HOMOb/ E0–0
/
LUMOd/
eV
c
Entry
lmaxa/nm (e/MÀ1 cmÀ1
)
eV
eV
CF3
301 (26800), 430 (54500) 5.95
o-NO2Ph 304 (14700), 416 (37100) 5.70
p-NO2Ph 302 (15000), 422 (29900) 5.54
2.40
2.52
2.45
3.55
3.18
3.09
a
Absorption spectra were measured in 0.1 M acetonitrile solutions.
The HOMO levels were estimated from work function measurements
c
with the dye adsorbed onto TiO2 film. E0–0 values were estimated
b
from the intersection between the absorption and emission spectra.
d
The LUMO levels were calculated from the HOMO levels and the
E0–0 values.
c
6160 Chem. Commun., 2011, 47, 6159–6161
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with strong extinction coefficients. The DSCs fabricated with
the new dyes exhibited the efficiencies of 2.51% to 4.05%. We
are now promoting this research in combination with compu-
tational methods to obtain the bathochromic dyes and to
produce high efficiency DSCs.
We would like to acknowledge a financial support from
Core Research for Evolutional Science and Technology
(CREST) of the Japan Science and Technology Agency.
Notes and references
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different EWGs on the acceptor part. Knoevenagel condensa-
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c
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Chem. Commun., 2011, 47, 6159–6161 6161