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Published on the web April 20, 2011
Dye-sensitized Solar Cells Based on Novel Diphenylpyran Derivatives
Altan Bolag,1 Jun-ichi Nishida,1 Kohjiro Hara,2 and Yoshiro Yamashita*1
1Department of Electronic Chemistry, Tokyo Institute of Technology,
G1-8, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502
2Advanced Organic Materials Team, Research Center for Photovoltaics, Advanced Industrial Science and Technology,
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565
(Received March 8, 2011; CL-110195; E-mail: yoshiro@echem.titech.ac.jp)
R
R
R
R
We have prepared
a series of novel diphenylpyran
derivatives as photosensitizers of dye-sensitized solar cells
(DSSCs) for the first time. Nonsubstituted dye 3a shows an
energy conversion efficiency of 2.3% and iodine-substituted dye
3d shows a higher efficiency of 2.9% due to the tuned energy
levels and steric effect.
O
O
a)
−BF4
−BF4
PBu3
DPP1
1
a
R = H
b R = F
c R = Cl
d R = I
b)
Worldwide concerns about energy provision and develop-
ment of recyclable energy source has made dye-sensitized solar
cells (DSSCs)1,2 an important area of research in new energy
utilization. A dye-sensitized system was long ago reported by
Moser et al. in 1887, explaining a charge-transfer phenomenon
from dye to semiconductor through optical excitation.3 DSSCs
have made remarkable progress due to contributions by Grätzel
et al. since 1991.2 Although well-known Ru-based DSSC
materials researched by Grätzel et al. have already achieved
noteworthy power conversion efficiency up to 11-12%,4 use
of rare metal and environmental issues restrict their further
development for practical application. On the other hand,
development of metal-free organic materials for DSSCs have
attracted much attention due to their appealing advantages such
as low-cost production, flexibility, and light weight. Compared
with Ru-based materials, pure organic dyes have further
advantages such as larger molar extinction coefficients, easier
preparation, and more controllable energy levels. Various
organic dyes have recently been developed, which are antici-
pated to be applied for colorful transparent production.5 Up to
date, conversion efficiency as high as 10.1% has been achieved
on organic dyes based on ethylenedioxythiophene and dithie-
nosilole blocks.6 However, these performances are still not good
enough for real application when compared with traditional
silicon solar cells. Development of new organic dyes is still very
important for progress of this field to explore new materials as
well as to investigate the relationship between structure and
device performance.
R
R
R
R
O
O
O
c)
CN
COOH
2
3
Reagents: a) Tributylphosphine, MeCN. b) Terephthalaldehyde, t-BuOK, dry THF.
c) Cyanoacetic acid, piperidine, MeCN.
Scheme 1. Synthesis of dyes 3a-3d.
pyran moiety can be simply prepared and various ³-conjugated
parts can be inserted between the donor and acceptor parts to
extend the absorption region. We present herein the synthesis,
physical properties, and DSSC device performances of novel
diphenylpyran dye 3a and its halogen-substituted derivatives
3b-3d to investigate the influence of halogen groups introduced
at the termini of organic dyes.
The synthesis was accomplished in only three steps as
shown in Scheme 1. Addition of tributylphosphine to an
acetonitrile solution of 2,6-diphenylpyrylium tetrafluoroborate
(DPP1a)7 gave tributyl(2,6-diphenyl-4H-pyran-4-yl)phospho-
nium tetrafluoroborate (1a), which produced aldehyde 2a by a
Wittig reaction in good yield. The aldehyde 2a was condensed
with cyanoacetic acid to give the final product 3a. Other dyes
3b-3d were similarly prepared.
In general, broad UV-vis absorption extending to the near-
infrared region and well-matched energy levels with the
conduction band of TiO2 electrode and iodine/iodide redox
potentials are desirable for achieving excellent energy-conver-
sion efficiencies in DSSCs. The electron donor part of DSSC
organic dye is largely responsible for the absorption spectrum
and HOMO energy level of materials. Diphenylpyran is
electron-donating, but this unit has not been used in DSSC
organic dyes until now. According to our previous report,7
compounds containing the diphenylpyran part have intense
absorptions in the visible region as well as proper HOMO
energy levels adjustable by introducing substituents at the end of
the phenyl groups. Moreover, organic dyes with the diphenyl-
The UV-vis absorption spectra of dyes 3a-3d measured in
dichloromethane are shown in Figure S1.14 The absorption
edges of the dyes are extended to 594-566 nm due to intra-
molecular charge transfer. Compared with dye 3a, the absorption
maxima of halogen derivatives 3b-3d are a little blue-shifted
from 497 to 491, 488, and 487 nm, respectively, due to the
minutely changed oxidation potentials as shown in Table 1. The
molar extinction coefficients also decreased according to the size
of halogen atom at the end of the phenyl groups. The redox
potentials were measured by cyclic voltammetry in DMF. The
first oxidation potential of dye 3a was observed at 1.09 V, which
is more positive than the iodine/iodide redox potential of 0.4 V.
The excited state oxidation potential of 3a of ¹0.99 V is more
Chem. Lett. 2011, 40, 510-511
© 2011 The Chemical Society of Japan
Bolag, Altan
