Utilization of carboxylated 1,3-indandione as an electron acceptor in dye-sensitized solar cells

Article abstract of DOI:10.1246/bcsj.20120208

The performance of dyes containing carboxylated 1,3- indandiones as electron acceptors was examined. The compounds that included a monocarboxylic group on the 1,3- indandione moiety showed good overall power conversion efficiencies (ca. 2.38%), the values of which were comparable to those of cyanoacrylate derivatives with the same donor linker structure.

Full text of DOI:10.1246/bcsj.20120208

Short Articles
Bull. Chem. Soc. Jpn. Vol. 85, No. 12, 1329-1331 (2012) 1329
NPh2
NPh2
O
Utilization of Carboxylated 1,3-
Indandione as an Electron Acceptor
in Dye-Sensitized Solar Cells
O
HOOC
R
S
HOOC
R
O
2
O
1
(R = H or COOH)
Shoji Matsumoto,*1 Tatsuro Aoki,1
Tasutaka Suzuki,1 Motohiro Akazome,1
Atsushi Betto,2 and Yasumasa Suda2
Chart 1.
O
O
O
O
HOOC
O
O
1Department of Applied Chemistry and Biotechnology,
Graduate School of Engineering, Chiba University,
1-33 Yayoicho, Inage-ku, Chiba 263-8522
O
O
O
MeCOCH2CO2Et
(1 equiv)
MeCOCH2CO2Me
(2 equiv)
NEt3 (6 equiv)
NEt3 (6 equiv)
2TOYO INK SC HOLDINGS, CO., LTD.,
Ac2O,
Ac2O, 100 °C, 1.5 h
50%
70 °C
27 Wadai, Tsukuba, Ibaraki 300-4247
then -5 °C, 1 d
36%
Received August 3, 2012
E-mail: smatsumo@faculty.chiba-u.jp
O
O
O
OEt
OOC
OMe
O
O
O
O
O
HNEt3
O
2HNEt3
The performance of dyes containing carboxylated 1,3-
indandiones as electron acceptors was examined. The com-
pounds that included a monocarboxylic group on the 1,3-
indandione moiety showed good overall power conversion
efficiencies (ca. 2.38%), the values of which were compa-
rable to those of cyanoacrylate derivatives with the same
donor-³-linker structure.
H2SO4
AcONH4
MeOH, reflux, 3 h
89%
O
H2O, 0 °C
93%
O
HOOC
HOOC
HOOC
OMe
O
OEt
O
OH
3a
OH
3b
Dye-sensitized solar cells (DSSCs) have attracted consid-
erable attention since Grätzel’s report1 in 1991 on their easy
production compared to conventional semiconductor-type solar
cells. Sensitizers containing ruthenium, such as N3,2 N719,3
and black dye,4 showed efficiencies of up to ca. 11% under
AM 1.5 irradiation.5 There is also increasing interest in metal-
free organic sensitizers because of the expense and difficulty
associated with the purification of ruthenium-metal complexes.
Many organic sensitizers have been examined,6 and most of
the efficient organic dyes contain donor-³-acceptor (D-³-A)
structures. Wang and co-workers achieved a high conversion
efficiency (ca. 10%) using thienothiophene derivatives con-
nected with a diphenylamino moiety as the donor and cyano-
acrylic acid as the acceptor.7 Various donating groups have
been investigated, and aryl amine groups such as diphenyl-
amine, (difluorenyl)phenylamine, and indole are among the
best donors for DSSCs.8 On the other hand, the variation on the
acceptor side is rather limited; the cyanoacrylic acid group is
one of the most useful acceptors. To achieve more efficient
dyes, the developments of electron-accepting groups must be
necessitated.
Scheme 1. Synthesis of 1,3-indandione derivatives 3.
no report on the utilization of the 1,3-indandione structure as
an organic sensitizer for DSSCs. Herein, we report a study
on the photovoltaic ability of D-³-A-type chromophores 1
and 2 containing 1,3-indandiones (Chart 1). We selected the
diphenylamino group as the donor part, and phenylene and
thienylene as ³-linkers because of the synthetic facility and the
easy comparison with another dyes. Mono- and dicarboxylated
derivatives of 1,3-indandione were examined because the
number of carboxylic groups would affect both the electronic
properties and the interaction with the TiO2 surface. For
instance, perylene dyes were used as sensitizers through the
formation of two carboxylic groups by opening the carboxylic
anhydride ring onto the TiO2 surface.10
The synthesis of carboxylated 1,3-indandiones was reported
by Rotbergs (for monocarboxylate derivative)11 and Krief (for
dicarboxylate derivative).12 The monocarboxylated derivative
3a could be prepared in 45% yield (two steps) according
to the reported procedure (Scheme 1). Introduction of active
methylene to pyromellitic dianhydride proceeded in 36% yield
by warming the mixture to 70 °C to dissolve the pyromellitic
dianhydride in the acetic anhydride prior to the addition
of other reagents, whereas 18% yield was obtained by the
mentioned procedure.12 The protonation of the monoconden-
sation product proceeded easily to give dicarboxylated deriv-
ative 3b in 93% yield. In addition, 1 and 2 were synthesized
1,3-Indandione has a planar structure with a benzene ring
connected to two carbonyl groups, and contains an active
methylene moiety to facilitate the elongation of ³-conjugation
by condensation with aldehydes. In addition, because of its
electron-withdrawing strength, 1,3-indandione is employed
as an acceptor to produce nonlinear optical (NLO) chromo-
phores.9 However, to the best of our knowledge, there has been
Published on the web December 1, 2012; doi:10.1246/bcsj.20120208

1330 Bull. Chem. Soc. Jpn. Vol. 85, No. 12 (2012)
© 2012 The Chemical Society of Japan
Table 1. Physical Properties and DSSC Performance Parameters of 1a, 1b, 2a, and 2b
-
max/nm
EHOMO
/eVb)
Edge of
absorption/nm
ELUMO
/eVc)
JSC
/mA cm
VOC
/Vd)
©
/%d)
Compound
ff d)
¹1 a)
¹2 d)
(¾max/M¹1 cm
)
1a
2a
1b
2b
492 (49400)
525 (66500)
503 (40400)
547 (67500)
®
¹5.73
¹5.78
¹5.70
¹5.62
®
546
566
564
596
¹3.46
¹3.59
¹3.50
¹3.54
®
5.46
5.87
3.32
4.00
12.68
0.66
0.60
0.57
0.55
0.77
0.66
0.62
0.67
0.64
0.54
2.38
2.20
1.26
1.40
5.29
N719
a) Measured in THF (3.0 © 10¹5 M). b) Measured with photoemission yield spectroscopy on RIKEN KEIKI AC-1.
c) Estimated from EHOMO and the edge of absorption (-) by the equation of ELUMO (eV) = EHOMO (eV) + 1240/-
(nm). d) DSSC performances were measured under AM 1.5 irradiation.
NPh2
(a)
1a
1b
2a
2b
40
20
0
O
O
OHC
NPh
2 HOOC
3a or 3b
3a or 3b
AcOH
R
100 °C
1a (R=H), 68%
1b (R=COOH), 82%
300
500
700
NPh2
S
Wavelength / nm
O
OHC
NPh
2HOOC
S
(b)
6
1a
1b
2a
2b
AcOH
R
4
2
0
O
2a (R=H), 47%
70 °C
100 °C
2b (R=COOH), 65%
Scheme 2. Formation of D-³-A chromophores 1 and 2.
0
0.2
0.4
0.6
0.8
Voltage / V
1a
1b
2a
Figure 2. (a) J-V characteristics and (b) IPCE spectra of
DSSCs based on 1a, 1b, 2a, and 2b.
40000
2b
conduction band (CB) energy level of TiO2 (¹4.16 eV),13
implying that electron injection into the CB of TiO2 is feasible.
The HOMO levels were also able to accept electrons from the
iodine/iodide redox system (¹4.85 eV AVS (absolute vacuum
scale))14 after electron injection into the CB of TiO2. The
photocurrent density-voltage (J-V) curves and incident photon-
to-current conversion efficiency (IPCE) spectra are shown in
Figure 2, and the DSSC performance results are summarized
in Table 1. We found that the 1,3-indandione structure with a
carboxylate moiety worked as the electron-accepting part for
the DSSCs. The maximum IPCE value of 1a was 39% at
439 nm, and the wavelength range of the photocurrent active
zone was from 390 to 650 nm. The cell with 1a showed a
short-circuit current density (JSC) of 5.46 mA cm¹2, an open-
circuit voltage (VOC) of 0.66 V, and a fill factor ( ff ) of 0.66,
leading to an overall power conversion efficiency (©) of 2.38%
based on the device performance of © = 5.29% for N719. 3-
[4-(Diphenylamino)phenyl]-2-cyanoacrylic acid, which comes
from the conversion of the 1,3-indandione moiety of 1a into
cyanoacrylic acid, showed resembling properties on JSC and
VOC (Table S1). But its ff gave a slightly smaller value
than 1a, which suggests that the efficient conversion can be
achieved in 1a. And an overall power conversion efficiency of
3-[4-(diphenylamino)phenyl]-2-cyanoacrylic acid is 1.2-2.79%
(compared to 3.1-8.04% for N719).15 Therefore, the 1,3-
indandione acceptor has a comparable ability to that of cyano-
acrylate for use in DSSCs. The wavelength range of the
0
200
400
600
Wavelength / nm
Figure 1. Absorption spectra of 1a, 1b, 2a, and 2b in THF
(3.0 © 10¹5 M).
in moderate to good yields by the condensation reaction of
3 with the corresponding aromatic aldehydes (Scheme 2).
The absorption spectra of 1 and 2 were measured in THF
(Table 1 and Figure 1). The maximum absorption (-max) of the
monocarboxylated compound with a phenylene linker (1a) was
observed at 492 nm with a high molecular absorption coef-
ficient (¾max). Upon introduction of another carboxylic group at
the benzene ring of the 1,3-indandione moiety (1b), -max was
shifted by about 11 nm to a longer wavelength. This is because
the additional carboxylate has electron-withdrawing character.
A further bathochromic shift was achieved by changing the
³-linker from phenylene to thienylene, which resulted in
absorption peaks at 526 and 547 nm in 2a and 2b, respectively.
Increases in ¾
were also observed. The highest occupied
max
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy-level composition lies on the proper
level for the TiO2-I /I3 system (Table 1). The LUMO levels
of all the compounds were sufficiently more positive than the
¹
¹

S. Matsumoto et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 12 (2012) 1331
M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker,
E. Müller, P. Liska, N. Vlachopoulos, M. Grätzel, J. Am. Chem.
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3
thienylene derivative 2a was shifted to longer wavelengths
than that of 1a. This is rationalized by the bathochromic shift
of -max observed between 1a and 2a. The JSC value of 2a
increased to 5.87 mA cm¹2, whereas VOC decreased by 0.60 V.
As a result, © of 2a (2.20%) was smaller than that of 1a.
Low efficiencies were obtained for 1b and 2b. A decrease
of IPCE was observed with decreasing conversion efficiency,
4
M. K. Nazeeruddin, P. Péchy, T. Renouard, S. M.
Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey,
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even though the ¾
of 1b and 2b showed the same value
max
on that of 1a and 2a, respectively. To find the reason for
the inefficient performances, the effect of the addition of DCA
(deoxycholic acid) was examined to suppress dye aggregation
on TiO2 surface by coadsorption.16 However, the same perfor-
mances in 1b were observed either with or without DCA.
Therefore, the aggregation of the dyes was not influenced. We
carried out a study using density functional theory (DFT)
calculations at the B3LYP/6-31+G* level (Figure S1). The
geometry of the double bond in 1a was not affected on
the orbital shapes and energy levels of the HOMO and the
LUMO. Both the HOMOs of 1a and 1b lie on the donating
diphenylamino group and the phenylene linker, and the
LUMOs spread to the carboxylic group attached to the 1,3-
indandione moiety, which allows facile electron transfer from
the excited dye to TiO2. However, only one carboxylic moiety
is involved in the LUMO of 1b, although there are two
carboxylic groups. The energy levels of LUMO+1 of 1b,
which lies in both carboxylic moieties, is higher than that of
the LUMO. In the case of 1b, both carboxylic groups can be
applied as the anchor to connect to TiO2, but, because of the
problems of dianion formation and/or the proper distance on
TiO2 for connecting both anchors, 1b may be connected to
TiO2 using only one carboxylic groups which is not selective
whether the LUMO is possessed or not. Therefore, the only
compound connected with carboxylate possessing the LUMO
orbital will be utilized to transfer the electron to TiO2 effi-
ciently because of orbital correlation restrictions. It is the
reason for the low efficiencies observed for 1b and 2b.
In summary, we synthesized dyes 1a, 1b, 2a, and 2b
containing a 1,3-indandione structure with a carboxylic moiety
as an acceptor for DSSCs. All the materials showed photo-
voltaic sensitizing abilities with © µ 2.38%. It is shown that
the 1,3-indandione derivatives possess a comparable ability to
the well-examined cyanoacrylic acid group. We examined the
effect of the additional carboxylic group on 1,3-indandione,
and found that a decrease in © was obtained in spite of
the bathochromic shift of the absorption spectra compared to
those of the monocarboxylated materials. Further investiga-
tions to improve the performance of DSSCs based on the 1,3-
indandione structure are underway.
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Supporting Information
Experimental procedure of 1, 2, fabrication and compari-
son of DSSCs, and results of DFT calculation. This material
is available free of charge on the web at http://www.csj.jp/
journals/bcsj/.
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