Design of NIR-absorbing simple asymmetric squaraine dyes carrying indoline moieties for use in dye-sensitized solar cells with Pt-free electrodes

Article abstract of DOI:10.1021/ol300054a

Novel near-infrared (NIR)-sensitizing (up to 800 nm) simple asymmetric squaraine dyes (Sq 31 and Sq 33) carrying indoline moieties that did not require the introduction of any linker groups were developed. DSSCs fabricated with Sq 33 exhibited remarkable

Full text of DOI:10.1021/ol300054a

ORGANIC
LETTERS
2012
Vol. 14, No. 5
1246–1249
Design of NIR-Absorbing Simple
Asymmetric Squaraine Dyes
Carrying Indoline Moieties for
Use in Dye-Sensitized Solar Cells
with Pt-Free Electrodes
Kazumasa Funabiki,*,† Hiroyoshi Mase,† Yasuteru Saito,‡ Atsuhiro Otsuka,§
Atsuhiko Hibino,† Nagisa Tanaka,† Hidetoshi Miura, Yosuke Himori,^
Tsukasa Yoshida,^ Yasuhiro Kubota,† and Masaki Matsui†
Department of Materials Science and Technology Faculty of Engineering, Gifu
University, 1-1 Yanagido, Gifu, Japan, Dai-ichi Kogyo Seiyaku Co., Ltd., 5 Oogawara-cho,
Kisshoin, Minami-ku, Kyoto, 601-8391, Japan, Sekisuijushi Technical Research
Corporation, 731-1, Kagami, Ryuoh-chou, Gamou-gun, Shiga, 520-2596, Japan,
Tsukuba Research Center, Chemicrea Inc., D-14 2-1-6, Sengen, Tsukuba, Ibaraki,
305-0047, Japan, and Environmental and Renewable Energy System Division, Graduate
School of Engineering, 1-1, Yanagido, Gifu University, Gifu 501-1193, Japan
funabiki@gifu-u.ac.jp
Received January 11, 2012
ABSTRACT
Novel near-infrared (NIR)-sensitizing (up to 800 nm) simple asymmetric squaraine dyes (Sq 31 and Sq 33) carrying indoline moieties that did not
require the introduction of any linker groups were developed. DSSCs fabricated with Sq 33 exhibited remarkable characteristics in the long-
wavelength visible and NIR region (up to 800 nm), such as a conversion efficiency of 3.75% (AM 1.5G) with an incident photon-to-current conversion
efficiency of 63% (650 nm), a short-circuit photocurrent density of 13.64 mA, an open-circuit photovoltage of 0.48, and a fill factor of 0.57.
Dye-sensitized solar cells (DSSCs) that use metal-free
organic dyes continue to attract interest as promising
alternatives for practical photovoltaic devices because of
their potential low manufacturing cost, ease of purifica-
tion, high molar extinction, and structural diversity in the
field of photovoltaic energy conversion.1 Various types of
metal-free organic dyes have been developed, and some
have achieved power conversion efficiencies of greater
† Department of Materials Science and Technology, Faculty of Engineer-
ing, Gifu University.
‡ Dai-ichi Kogyo Seiyaku Co., Ltd.
(1) For a recent review, see: Hagfelt, A.; Boschloo, G.; Sun, L.; Kloo,
L.; Petterson, H. Chem. Rev. 2010, 110, 6595.
(2) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;
§ Sekisuijushi Technical Research Corporation.
Chemicrea Inc.
^ Environmental and Renewable Energy System Division, Graduate
School of Engineering, Gifu University.
Nazeerudine, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddine, S. M.;
€
Gratzel, M. Science 2011, 334, 629.
r
10.1021/ol300054a
Published on Web 02/09/2012
2012 American Chemical Society

than 12% in standard AM 1.5G sunlight.2 However, the
main drawback of these dyes is a lack of absorption in the
near-infrared (NIR) region.
We report here the details of the synthesis of novel NIR-
absorbing simple asymmetric Sq dyes carrying indoline
moieties for use in DSSCs with nanocrystalline TiO2.
Sq 31 and 33 were synthesized as shown in Scheme 1.
Indoles 2 were prepared in 76% to quantitative yields
via the BuchwaldꢀHartwig cross-coupling reaction of
1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole (1) with the
corresponding bromoaromatics, such as 2-bromo-9,9-
dimethyl-9H-fluorene and (2-(4-bromophenyl)ethene-1,
1-diyl)dibenzene, in the presence of palladium acetate
(8 mol %), tri-tert-butylphosphine (7 mol %), and potassium
tert-butoxide (1.7 equiv) in toluene at 80 ꢀC. The reaction
of the indoles 2 with 3,4-dichlorocyclobut-3-ene-1,2-dione
(3) in toluene at 80 ꢀC gave3-substituted 3-chlorocyclobut-
3-ene-1,2-diones 4 in yields of 50ꢀ84%, and subsequent
hydrolysis in a mixed solvent composed of acetic acid and
water (v/v = 4/1) at reflux temperature for 4ꢀ29 h gave
3-substituted 4-hydroxycyclobut-3-ene-1,2-diones 5 in
yields of 46ꢀ64%.
Recently, squaraines (Sq) have attracted attention be-
cause of their strong absorption and sensitizing abilities in
the long-wavelength visible region of solar light. Conse-
quently, the longest edge of incident photon-to-current
conversion efficiencies (IPCEs) inDSSCs withSq dyes that
have been reported to date are ca. 750 nm.3 By analogy to
the molecular design of visible light-absorbing organic
dyes, the introduction of not only linker groups, such as
phenyl, alkenyl, thienyl, and pyrrole groups, between
donor and acceptor groups in Sq dyes, but also another
Sq moiety has been shown to lead to a red shift in the
absorption maximum and sensitization in the NIR region
(up to ca. 900 nm).3bꢀd
During the development of long-wavelength visible
light- and NIR-absorbing organic dyes, such as asym-
metric Sq4 and heptamethincyanine (KFH)5 dyes for use
in DSSCs, we synthesized for the first time (1) two novel
NIR-sensitizing simple asymmetric Sq dyes by changing a
di-n-octylaminophenyl group to an indoline moiety.
Furthermore, (2) the absorption maxima (λmax) of these
Sq dyes showed a red shift of 11 nm compared with that of
dialkylaminophenylated Sq. Finally, (3) the simple asym-
metric Sq dyes that do not require the introduction of linker
groups, such as phenyl, thienyl, or pyrrole groups, are quite
efficiently sensitized on titanium oxide (TiO2) with the
long-wavelength visible and NIR region (up to 800 nm) of
the spectrum and show a remarkable conversion efficiency
(η) of 3.75% (AM 1.5G) with an IPCE of 63% (650 nm), a
short-circuit photocurrent density (Jsc) of 13.64 mA, an
open-circuitphotovoltage (Voc) of 0.48, and a fill factor (ff)
of 0.57.
Scheme 1. Synthesis of Sq 31 and Sq 33
(3) For a recent review, see: (a) Beverina, L.; Salice, P. Eur. J. Org.
Chem. 2010, 1207. For selected examples, see: (b) Paek, S.; Choi, H.;
Kim, C.; Cho, N.; So, S.; Song, K.; Nazeeruddin, M. K.; Ko, J. Chem.
Commun. 2011, 47, 2874. (c) Maeda, T.; Hamamura, Y.; Miyanaga, K.;
Shima, N.; Yagi, S.; Nakazumi, H. Org. Lett. 2011, 13, 5994. (d) Li, J. -Y.;
Chen, C.-Y.; Lee, C.-P.; Chen, S.-C.; Lin, T.-H.; Tsai, H.-H.; Ho, K.-C.;
Wu, C.-G. Org. Lett. 2010, 12, 5454. (e) Pandey, S. S.; Inoue, T.; Fujikawa,
N.; Yamaguchi, Y.; Hayase, S. Thin Solid Films 2010, 519, 1066. (f) Pandey,
S. S.; Inoue, T.; Fujikawa, N.; Yamaguchi, Y.; Hayase, S. J. Photochem.
Photobiol., A 2010, 214, 269. (g) Holliman, P. J.; Davies, M. L.; Connell, A.;
Velasco, B. V.; Watson, T. M. Chem. Commun. 2010, 46, 7256. (h) Choi, H.;
Finally, 1-carboxyethyl-3,3-dibutyl-2-methylindolenium
iodide (6)5a reacted with 4-hydroxybut-3-ene-1,2-diones 5
in a mixed solvent of toluene and butanol (v/v = 1/1) at
reflux temperature for 5ꢀ24 h to produce Sq 31 and 33 in
yields of 9ꢀ29%.
€
Kim, J.-J.; Song, K.; Ko, J.; Nazeeruddin, M. K.; Gratzel, M. J. Mater.
Chem. 2010, 20, 3280. (i) Kim, S.; Mor, G. K.; Paulose, M.; Varghese,
O. K.; Baik, C.; Grimes, C. A. Langmuir 2010, 26, 13486. (j) Yum, J.-H.;
€
Baranoff, E.; Hardin, B. E.; Hoke, E. T.; McGehee, M. D.; Nuesch, F.;
€
Gratzel, M.; Nazeeruddin, M. K. Energy Environ. Sci. 2010, 3, 434. (k)
The UVꢀvis absorption in DMSO solution and on
nanoporous TiO2, along with the emission and photo-
chemical properties of Sq 31 and Sq 33, are listed in Table 1
and shown in Figure 1. The absorption spectra of Sq 31
and Sq 33 show peaks in the long-wavelength visible region
in DMSO solution, and the λmax values of Sq 31 and Sq 33
are the same (643 nm). These values are red-shifted (by 11 nm)
compared to that of Sq 3 (λmax = 632 nm, ε = 125 000
Mꢀ1 cmꢀ1), which has a di-n-octylaminophenyl group
instead of an indoline group,4 due to the potent electron-
donating properties of the indoline moiety. The molar
extinction coefficients (ε) of Sq 31 and Sq 33 were
€
Yum, J.-H.; Hardin, B. E.; Moon, S.-J.; Baranoff, E.; Nuesch, F.;
McGehee, M. D.; Gratzel, M.; Nazeeruddin, M. K. Angew. Chem.,
€
Int. Ed. 2009, 48, 9277. (l) Geiger, T.; Kuster, S.; Yum, J.-H.; Moon, S.-J.;
€
€
Nazeeruddin, M. K.; Gratzel, M.; Nuesch, F. Adv. Funct. Mater. 2009, 19,
2720. (m) Yum, J. -H.; Walter, P.; Huber, S.; Rentsch, D.; Geiger, T.;
€
€
Nuesch, F.; De Angelis, F.; Gratzel, M.; Nazeeruddin, M. K. J. Am.
Chem. Soc. 2007, 129, 10320. (n) Burke, A.; Schmidt-Mende, L.; Ito, S.;
€
Gratzel, M. Chem. Commun. 2007, 234.
(4) Otsuka, A.; Funabiki, K.; Sugiyama, N.; Yoshida, T.; Minoura,
H.; Matsui, M. Chem. Lett. 2006, 35, 666.
(5) (a) Funabiki, K.; Mase, H.; Hibino, A.; Tanaka, N.; Mizuhata,
N.; Sakuragi, Y.; Nakashima, A.; Yoshida, T.; Kubota, Y.; Matsui, M.
Energy Environ. Sci. 2011, 4, 2186. (b) Otsuka, A.; Funabiki, K.;
Sugiyama, N.; Mase, H.; Yoshida, T.; Minoura, H.; Matsui, M. Chem.
Lett. 2008, 37, 176. (c) Matsui, M.; Hashimoto, Y.; Funabiki, K.; Jin,
J. -Y.; Yoshida, T.; Minoura, H. Synth. Met. 2005, 148, 147.
Org. Lett., Vol. 14, No. 5, 2012
1247

determined to be 139 000 and 86 200 Mꢀ1 cmꢀ1 at the
respective absorption maxima. The absorption spectra of
Sq 31 and Sq 33 are broadened on a TiO2 electrode.
Table 1. Absorption, Emission, and Electrochemical Properties
of Sq 31 and 33
εa
(Mꢀ1
Fmax
Eox
(V)b
E0ꢀ0
(V)
Ered
(V)c
a
a
λmax
Dyes
(nm)
cmꢀ1
)
(nm)
Sq 31
Sq 33
643
643
139 000
86 200
731
738
0.72
0.76
1.82
1.81
ꢀ1.10
ꢀ1.05
a Measured in DMSO at a concentration of 5 ꢁ 10ꢀ6 M. b Deter-
mined by cyclic voltammetry. c Calculated on the basis of Eox and λint
.
Figure 2. Graphic representation of the frontier orbital and the
geometry of Sq 33 (N blue, O red, C gray, H white) by B3LYP/6-
31G (d,p).
Table 2 summarizes the photovoltaic properties of
DSSCs fabricated with Sq 31 and Sq 33 on TiO2 films with
a liquid electrolyte consisting of 0.1 M iodine (I2), 0.1 M
lithium iodide (LiI), and 1.0 M 1,2-dimethyl-3-propylimi-
dazolium iodide (DMPImI) in 3-methoxypropionitrile
(3-MPN), together with the results for a DSSC with
N719 as a standard. A TiO2 film was prepared by the
screen-printing method (a transparent layer of up to
9 μm using 18 nm particles þ a scattering layer of up to
5 μm using 400 nm particles). The TiO2 electrode was
stained by immersion in a mixed solution of acetoni-
trileꢀtert-BuOH (v/v = 1/1) containing 0.1 mM Sq
dyes and 0.5 mM deoxycholic acid (DCA) as a coad-
sorbent. A PEDOT-PTS coated film on FTO glass was
used as a counter electrode. The cells have an effective
area of 0.50 cm2, and their performance was evaluated
under AM 1.5G illumination (100 mW cmꢀ2). Conse-
quently, the Sq 33-sensitized solar cell gave a slightly
Figure 1. UVꢀvis absorption in DMSO (solid line) and ad-
sorbed on TiO2 (dash-dotted line) and fluorescence spectra
(dotted line) of Sq 31 and Sq 33.
To estimate the HOMO levels of Sq 31 and Sq 33, cyclic
voltammetry was performed using a three-electrode cell.
The first oxidation potential (Eox) was found to corre-
spond to the HOMO level and was determined to be 0.72 V
vs NHE for Sq 31 and 0.76 V vs NHE for Sq 33. Since the
Ered values of Sq 31 and Sq 33 were not observed, the
LUMO levels of Sq 31 and Sq 33 could be obtained by
Eox ꢀ E0ꢀ0. The values of E0ꢀ0 for Sq 31 and Sq 33 were
obtained based on the intersection (683 nm for Sq 31 and
684 nm for Sq 33) of the normalized absorption and
fluorescence spectra, respectively. The energy levels of
Eox (HOMO) for Sq 31 and Sq 33 are more positive than
the redox potential of the iodide/triiodide couple used as
an electrolyte. The potential of Ered (LUMO) for Sq 31 and
Sq 33 is sufficiently negative to inject electrons into the
conduction band of TiO2.
better η value of 3.75%, with a Jsc of 13.64 mA cmꢀ2
,
a Voc of 0.48 V, and an ff of 0.57, compared to that
(3.50%) of Sq 31. The use of electrolytes either without
LiI or with both N-methylimidazole (NMBI) and gua-
nidine thiocyanate (GuNCS) resulted in a significant
reduction of Jsc.
Figure 3 shows the incident photon-to-current conver-
sion efficiency (IPCE) spectra of DSSCs based on Sq 31
and Sq 33, respectively. Despite not only the absence of
thienyl and phenyl groups as linker groups in the dye
structure but also the red shift of λmax by only 11 nm
compared to that of Sq 3 carrying a di-n-octylaminophenyl
group, they show broader IPCE spectra from 300 to 800 nm,
withmaximum values of 68% at650 nm for Sq 31 and 63%
at 650 nm for Sq 33, respectively. However, the effect of the
To determine the electronic and geometrical structures
of Sq 33, a molecular orbital calculation was performed
for Sq 33 with the density functional theory (DFT) at
the B3LYP/6-31G level with Gaussian 09, and the elec-
tron distributions for HOMOꢀ2, HOMOꢀ1, HOMO,
LUMO, LUMOþ1, and LUMOþ2 are shown in Figure 2.
HOMO is delocalized throughout the dye, and HOMOꢀ2
is entirely localized within the squaraine core. On the other
hand, while LUMO is delocalized throughout the dye,
LUMOþ2 is entirely localized in the indolenium moiety,
including the carboxylic substituent.
1248
Org. Lett., Vol. 14, No. 5, 2012

transport in DSSCs based on Sq 31 and Sq 33 are similar,
but slightly worse than those of N719.
Table 2. Performance Parameters of Sq 31- and Sq 33-Based
DSSCsa
In conclusion, two novel NIR-absorbing simple asym-
metric squaraine dyes (Sq 31 and Sq 33) carrying indoline
moieties as potent electron-donating groups were synthe-
sized for use in NIR-active DSSCs with porous TiO2.
Despite the absence of any linker groups in the dye, Sq
33 was quite efficiently sensitized on TiO2 with the long-
wavelength visible and NIR region (up to 800 nm) of the
spectrum and showed remarkable performance, such as
an η of 3.75% (AM 1.5) with an IPCE of 63% (650 nm), a
Jsc of 13.64 mA, a Voc of 0.48, and an ff of 0.57.
Jsc
(mA cmꢀ2
Voc
η
(%)
Dyes
)
(V)
ff
Sq 31
Sq 33
N719
Sq 33b
Sq 33c
13.39
13.64
16.77
6.83
0.473
0.480
0.488
0.614
0.644
0.53
0.57
0.57
0.67
0.76
3.50
3.75
4.30
2.83
1.96
4.01
a Irradiation light: 100 mW cmꢀ2 simulated AM 1.5G solar light;
working area: 0.5 cm2; electrolytes 0.1 M I2, 1.0 M DMPII, 0.1 M LiI in
3-MPN. b Without LiI. c With NMBI and GuNCS.
Figure 4. Nyquist plots for Sq 31-, Sq 33-, and N719-based
DSSCs.
Figure 3. Action spectra of Sq 31- and Sq 33-based DSSCs.
Acknowledgment. The present work was partially fi-
nancially supported by a grant from the New Energy and
Industrial Technology Development Organization of
Japan (NEDO) (No. 09B39011d) to K.F. K.F. is also grate-
ful to the Japan Science and Technology Agency (JST)
(No. 07-81-021) and the Japan Society for the Promotion
of Science (JSPS) (No. 22234567) for financial support.
indoline moieties for the broader IPCE spectra is not clear
at the present time.
Figure 4 shows Nyquist plots of DSSC cells with Sq 31,
Sq 33, and N719, measured under illuminated conditions
(100 mW cmꢀ2, Voc conditions). Two semicircles were
observed in the Nyquist plots. The radii of the smaller
semicircles on the right, which are attributed to the de-
fusion of electrolytes, do not vary greatly among the three
dyes. The radii of the larger semicircles on the left, which
contain R1 from charge transfer at the counter electrode
and R2 from electron transport at the TiO2/dye/electrolyte
interface, decreased in the order Sq 31 ≈ Sq 33 > N719.
This result indicates that the electron generation and
Supporting Information Available. Detailed procedures
1
and characterization of all of the new compounds, H and
13C NMR spectra for 2, 4, 5, and Sq dyes. This material is
available free of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 5, 2012
1249