Published on Web 10/18/2006
Alkyl-Functionalized Organic Dyes for Efficient Molecular Photovoltaics
Nagatoshi Koumura,*,† Zhong-Sheng Wang,† Shogo Mori,‡ Masanori Miyashita,‡ Eiji Suzuki,‡ and
Kohjiro Hara*,†
National Institute of AdVanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba,
Ibaraki 305-8565, Japan, and Department of Fine Materials Engineering, Shinshu UniVersity, 3-15-1 Tokida,
Ueda, Nagano 386-8567, Japan
Received July 11, 2006; E-mail: n-koumura@aist.go.jp; and k-hara@aist.go.jp
Recently there has been a considerable increase in interest in
organic devices such as light-emitting diodes, field effect transistors,
and photovoltaic cells.1 The performances of these devices depend
mainly on the properties of functionalized organic molecules. In a
dye-sensitized nanocrystalline TiO2 solar cell (DSSC),2 which is a
promising type of molecular photovoltaics, the properties of photo-
sensitizer such as absorption spectrum and electron-transfer rate
are important factors determining the photovoltaic performance.
For example, Ru-polypyridyl-complex dyes, such as cis-dithiocy-
anato bis(4,4′-dicarboxy-2,2′-bipyridine)ruthenium(II) (called N3
dye), developed by Gra¨tzel and co-workers, satisfy the criteria, and
the DSSCs based on the Ru dyes can produce solar energy-to-
electricity conversion efficiency (η) of up to 11% under AM 1.5 G
irradiation.2
Besides Ru dyes, metal-free organic dyes have been also utilized
in DSSCs because of their wide variety of the structures, facile
modification, and high molar absorption coefficient. Various organic
dyes for DSSCs have been developed, such as coumarin dyes,3
merocyanine derivatives,4 and polyene dyes,5 and high η values of
up to 9% under AM 1.5 G irradiation have been attained so far.
The performance of DSSCs based on organic dyes, however, have
not exceeded those on Ru dyes yet. According to our recent report,
it was observed that the electron lifetimes (τ) for the DSSCs with
coumarin dyes were shorter than with N719, a Ru dye. The shorter
values of τ are probably the cause of the observed lower open-
circuit voltage (Voc) from the DSSCs using the coumarin dyes.6
The dye aggregation, which causes intermolecular energy transfer,
is also considered to be one of the factors decreasing photovoltaic
performance of DSSCs based on organic dyes.3d
the steric hindrance due to the long alkyl chains. MK-3 dye, which
has no alkyl groups at the oligothiophene part, was also synthesized
in order to compare with MK-1. (Syntheses of dyes MK-1, 2, and
3, in detail, are described in Supporting Information.)
The inset in Figure 2 shows the absorption spectra of new dyes
MK-1, 2, and 3 in chloroform. Absorption maxima (λmax) were
observed at 463 nm for MK-1, 473 nm for MK-2, and 443 nm for
MK-3, respectively, and the molar absorption coefficient (ꢀ) at λmax
was 41 400 M-1 cm-1 for MK-1, 35 800 M-1 cm-1 for MK-2,
and 42 100 M-1 cm-1 for MK-3, respectively. These results indicate
that these dyes could be suitable for DSSCs because of the wide
wavelength range of absorption and the relatively high molar
absorption coefficient compared with N3 dye (ca. 16 000 M-1
cm-1).2b The LUMO levels of the dyes adsorbed on a nanocrys-
talline TiO2 film, which were estimated from the oxidation potentials
and the maximum wavelengths of the UV-visible absorption
spectra, were more negative than -0.84 V (vs NHE), suggesting
that efficient electron transfer from the dyes to TiO2 should occur.
Action spectrum of incident photon-to-current conversion ef-
ficiency (IPCE) for a DSSC based on MK-2 is shown in Figure 2.
The onset wavelength of the IPCE spectrum was 800 nm. This
value is almost equal to that for a DSSC based on N3 dye.2e IPCE
values around 70% were observed in the range of 400 to 650 nm
with a maximum value of 72% at 500 nm for the DSSC based on
MK-2, showing the highly efficient performance of the solar cells.
The photovoltaic performance of the DSSCs based on N719 dye
and MK dyes are summarized in Table S1 (Supporting Information).
It is noteworthy that the Voc (0.71-0.72 V) for DSSCs based on
MK-1 and MK-2 dyes, which have long alkyl chains, were higher
than that for MK-3 (0.63-0.64 V) in the same condition. The Voc,
however, was lower with MK-1 and MK-2 than with N719 dye
(0.79 V). The maximum η value of 7.7% (short-circuit current den-
sity Jsc ) 14.0 mA cm-2, Voc ) 0.74 V, and fill factor FF ) 0.74)
was obtained with a DSSC using MK-2 under AM 1.5 G irradiation
(100 mW cm-2) with an aperture mask but no antireflection film.
To understand the high Voc observed with MK-1 and MK-2, we
measured electron lifetimes in the conduction band of TiO2. As
shown in Figure 3, it is remarkable that the values of τ for the
DSSCs with MK-1 and MK-2 were larger than with coumarin dyes,
NKX-2587 and NKX-2697, which have oligothiophene moiety
without alkyl chains.3f,6 In addition, the τ for the DSSC with MK-
3, having no alkyl chain, was much shorter than those with MK-1
and MK-2. These results obviously indicate that alkyl chains are
markedly effective in increasing the electron lifetimes. The longer
electron lifetime may be realized by two possible mechanisms: one
Taking these points into account, further strategical molecular
design of organic dyes is required to achieve higher η values. Our
concept toward the high photovoltaic performance of organic-dye-
sensitized solar cells is based on the interface engineering of a dye-
adsorbed TiO2 surface, which can be controlled by the structural
modification of the dye molecule to diminish the charge recombina-
tion between the electrons and acceptors, that is, dye cation and
-
I3 ions, and the aggregation of dye molecules. In addition, since
the synthetic schemes for dye molecules should be simplified to
facilitate developing dyes for DSSCs, we designed new organic
dyes having three separated functional parts: donor, linkage, and
acceptor groups shown in Figure 1. The design allows us to examine
different molecules systematically for the three parts. The most
important feature of the new dye structure (MK-1 and MK-2) is
the existence of n-hexyl groups substituted at the oligothiophene
linkage. With the long alkyl chains, an increase of the electron
lifetimes (τ) can be expected by preventing the approach of
acceptors (i.e., I3- ion) to the TiO2 surface and/or by reducing the
reorganization energy of the dye, resulting in the desired situation
for the kinetic competition for the reduction of the dye cation.7
The aggregation of dye molecules would be also suppressed by
-
is that the alkyl chains block the I3 or cations approaching the
-
TiO2 surface, decreasing the I3 concentrations at the vicinity of
the TiO2, and the other is that the alkyl chains reduce the reorgan-
ization energy of the dyes, increasing the rate of dye-cation re-
duction. The latter implies that shorter values of τ for MK-3 could
be due to the recombination between the conduction band electrons
and dye cations, because of the slower electron-transfer process
† National Institute of Advanced Industrial Science and Technology.
‡ Shinshu University.
9
14256
J. AM. CHEM. SOC. 2006, 128, 14256-14257
10.1021/ja0645640 CCC: $33.50 © 2006 American Chemical Society