Unsymmetrical squaraines incorporating quinoline for near infrared responsive dye-sensitized solar cells

Article abstract of DOI:10.1021/ol302481k

Two new unsymmetrical squaraines (WCH-SQ10 and WCH-SQ11), wherein the electron-rich 3,4-ethylenedioxy-thiophene conjugated fragment was linked unconventionally to the squaraine core and triphenyl amine donor, and carboxylic acid substituted quinoline was

Full text of DOI:10.1021/ol302481k

ORGANIC
LETTERS
2012
Vol. 14, No. 21
5420–5423
Unsymmetrical Squaraines Incorporating
Quinoline for Near Infrared Responsive
Dye-Sensitized Solar Cells
Jheng-Ying Li,† Chia-Yuan Chen,† Wen-Chein Ho,‡ Szu-Han Chen,‡ and
Chun-Guey Wu*,†,‡
Department of Chemistry, National Central University, Jhong-Li, 32001, Taiwan, ROC
and Research Center for New Generation PhotoVoltaics (RCNPV), National Central
University, Jhong-Li, 32001, Taiwan, ROC
t610002@cc.ncu.edu.tw
Received September 9, 2012
ABSTRACT
Two new unsymmetrical squaraines (WCH-SQ10 and WCH-SQ11), wherein the electron-rich 3,4-ethylenedioxy-thiophene conjugated fragment
was linked unconventionally to the squaraine core and triphenyl amine donor, and carboxylic acid substituted quinoline was used as an acceptor,
were prepared. WCH-SQ10 and WCH-SQ11 dyes in ethanol have the λmax of 686 and 673 nm, respectively. The corresponding photovoltaic devices
exhibit an attractively panchromatic response over 1000 nm, suggesting that quinoline benefits the low energetic electron injection.
Dye-sensitized solar cell (DSSC) is one of the most
promising candidates for the next generation photovoltaic
in virtue of their low manufacturing cost and impressive
photovoltaic performance.1 In DSSCs, the dye molecule
(sensitizer) plays a vital role in the light-harvesting and
charge separation as well as the overall photon-to-current
conversion efficiency. DSSCs sensitized by polypyridyl
Ru(II) complexes have achieved remarkably high conver-
sion efficiencies exceeding 11%.2ꢀ6 Recently 12.3% effi-
ciency was achieved by using metal-prophyrin sensitizer
and CoII/CoIII electrolyte.7
Nevertheless, the spectral response of those devices in the
far-red (FR) and near-infrared (NIR) regions is still insuffi-
cient, which shall be overcome through a delicate molecular-
engineering of the photosensitizers.8ꢀ12 Squaraine is one of
† Research Center for New Generation PhotoVoltaics (RCNPV).
‡ Department of Chemistry, National Central University.
(6) Chen, C. Y.; Wang, M.; Li, J. Y.; Pootrakulchote, N.; Alibabaei,
€
L.; Ngoc-le, C. H.; Decoppet, J. D.; Tsai, J. H.; Gratzel, C.; Wu, C. G.;
€
(1) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737.
(2) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han,
€
Zakeeruddin, S. M.; Gratzel, M. ACS Nano 2009, 3, 3103.
(7) Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;
L. Jpn. J. Appl. Phys. 2006, 45, L638.
(3) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.;
€
Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gratzel, M. J. Am. Chem. Soc.
2005, 127, 16835.
(4) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.;
Nazeeruddin, M.; Diau, E. W. G.; Yeh, C. Y.; Zakeeruddin, S. M.;
€
Gratzel, M. Science 2011, 334, 629.
(8) Hao, Y.; Yang, X.; Cong, J.; Tian, H.; Hagfeldt, A.; Sun, L.
Chem. Commun. 2009, 4031.
€
Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gratzel, M.
J. Am. Chem. Soc. 2008, 130, 10720.
(5) Cao, Y.; Bai, Y.; Yu, Q.; Cheng, Y.; Liu, S.; Shi, D.; Gao, F.;
(9) Tian, H.; Yang, X.; Chen, R.; Hagfeldt, A.; Sun, L. Energy
Environ. Sci. 2009, 2, 674.
(10) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H.
Wang, P. J. Phys. Chem. C 2009, 113, 6290.
Chem. Rev. 2010, 110, 6595.
r
10.1021/ol302481k
Published on Web 10/24/2012
2012 American Chemical Society

the photosensitizers with extraordinary high molar ab-
sorption coefficient and FR (or NIR) absorption.13ꢀ22 In
general, squaraine dye-sensitized DSSC can efficiently
convert the low-energy (∼1.4 eV) photons into electricity,
which makes the squaraine dyes not only the promising
cosensitizers for organic dyes with a complementary
absorption profile23,24 but also good candidates to be
combined with luminescent energy-relay dyes.25 How-
ever, there are only small amounts of squaraine-sensitized
DSSCs exhibiting a panchromatic or even FR/NIR re-
sponse.22 Recently we had reported26 two unsymmetrical
squaraine dyes (JYL-SQ5 and JYL-SQ6) where the elec-
tron-rich 3,4-ethylene-dioxythiophene or bithiophene
conjugated fragment was used to link unconventionally
the squaraine core and the hexyloxyphenyl amino group.
DSSCs based on those two sensitizers exhibited an attrac-
tively panchromatic response and also convert a portion
of the near-infrared photons into electricity. Neverthe-
less, their IPCE value is almost zero at the wavelength
over 900 nm.
red-shifting the absorption of squaraine dye. In this article,
we report the synthesis and photovoltaic performance of
two new unsymmetrical squaraine dyes, coded WCH-
SQ10 and WCH-SQ11, which are further optimized for
the FR and NIR sensitization of DSSCs.
The molecular structures of WCH-SQ10 and WCH-
SQ11 and their preparation procedure are depicted in
Scheme 1. The intermediates of the target dyes were char-
acterized by 1H NMR and the structure of the dyes were
further verified by 13C NMR, elemental analysis and mass
spectroscopy as given in the experimental section of the
Supporting Information (SI). The synthetic details and
NMR spectra of some intermediates are also provided in
the SI.
Scheme 1. Preparation Scheme for WCH-SQ10 and WCH-
SQ11
To obtain squaraine dyes with the absorption in the
infrared region, a more sophisticated molecular engineer-
ing is necessary. Recently, Guang et al. reported27 a sym-
metrical squaraine dye bearing carboxylic acid-substituted
quinoline acceptor with the λmax up to 754 nm, which is
much longer than that for the corresponding squaraine
bearing carboxylic acid substituted indoline (SQ1) devel-
oped by Graetzel et al.19 Quinoline may be a better unit for
ꢀ
(11) Altobello, S.; Argazzi, R.; Caramori, S.; Contado, C.; Da Fre,
S.; Rubino, P.; Chone, C.; Larramona, G.; Bignozzi, C. A. J. Am. Chem.
ꢀ
Soc. 2005, 127, 15342.
(12) Kinoshita, T.; Fujisawa, J. I.; Nakazaki, J.; Uchida, S.; Kubo,
T.; Segawa, H. J. Phys. Chem.Lett. 2012, 3, 394.
(13) Zhao, W.; Hou, Y.; Wang, X.; Zhang, B.; Cao, Y.; Yang, R.;
Wang, W.; Xiao, X. Sol. Energy Mater. Sol. Cells 1999, 58, 173.
(14) Li, C.; Wang, W.; Wang, X.; Zhang, B.; Cao, Y. Chem. Lett.
2005, 35, 554.
(15) Alex, S.; Santhosh, U.; Das, S. J. Photochem. Photobiol. A:
Chem. 2005, 172, 63.
(16) Otsuka, A.; Funabiki, K.; Sugiyama, N.; Yoshida, T. Chem.
Lett. 2006, 666.
(17) Tatay, S.; Haque, S. A.; O’Regan, B.; Durrant, J.; Verhees,
~
W. J. H.; Kroon, J. M.; Vidal-Ferran, A.; Gavina, P.; Palomares, E.
The electronic absorption spectra of WCH-SQ10 and
WCH-SQ11 measured in ethanol are shown in Figure 1.
WCH-SQ10 owns an intense absorption band centered
at 686 nm with a high molar absorption coefficient (ε)
of 9.24 ꢁ 104 Mꢀ1 cmꢀ1. The fwhm (full width at half-
maximum), λmax and ε value of the absorption band for
WCH-SQ10 are broader, slightly blue-shifted and lower
compared to those for JYL-SQ5 (for its structure, see SI)
that we reported previously.26 Nevertheless, WCH-SQ10
can efficiently adsorb the photons at a much lower energy
compared to other unsymmetrical squaraines reported in
literature.13ꢀ22 As expected, the λmax of the absorption
spectrum for WCH-SQ11 blue-shifted 13 nm compared to
that for WCH-SQ10. The ε value (8.72 ꢁ 104 Mꢀ1 cmꢀ1) is
also lower than that of WCH-SQ10 although the fwhm
of the absorption bands for both dyes are similar: 122 and
120 nm for WCH-SQ10 and WCH-SQ11, respectively.
To understand how the electron density redistributed
after photoexcitations, the density functional theory (DFT)
J. Mater. Chem. 2007, 17, 3037.
(18) Burke, A.; Schmidt-Mende, L.; Ito, S.; Gratzel, M. Chem.
€
Commun. 2007, 234.
(19) Yum, J. H.; Walter, P.; Huber, S.; Rentsch, D.; Geiger, T.;
€
€
Nuesch, F.; Angelis, F. D.; Gratzel, M.; Nazeeruddin, M. K. J. Am.
Chem. Soc. 2007, 129, 10320.
(20) Geiger, T.; Kuster, S.; Yum, J. H.; Moon, S. J.; Nazeeruddin,
€
€
M. K.; Gratzel, M.; Nuesch, F. Adv. Funct. Mater. 2009, 19, 2720.
(21) Beverina, L.; Ruffo, R.; Mari, C. M.; Pagani, G. A.; Sassi, M.;
€
De Angelis, F.; Fantacci, S.; Yum, J. H.; Gratzel, M.; Nazeeruddin,
M. K. ChemSusChem 2009, 2, 621.
(22) Choi, H.; Kim, J. J.; Song, K.; Ko, J.; Nazeeruddin, M. K.;
€
Gratzel, M. J. Mater. Chem. 2010, 20, 3280.
(23) Chen, Y.; Zeng, Z.; Li, C.; Wang, W.; Wang, X.; Zhang, B. New
J. Chem. 2005, 29, 773.
(24) Choi, H.; Kim, S.; Kang, S. O.; Ko, J.; Kang, M. S.; Clifford,
€
J. N.; Forneli, A.; Palomares, E.; Nazeeruddin, M. K.; Gratzel, M.
Angew. Chem., Int. Ed. 2008, 47, 8259.
(25) 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.
(26) 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.
(27) Yan, Z.; Guang, S.; Su, X.; Xu, H. J. Phys. Chem. C 2012, 116,
8894.
Org. Lett., Vol. 14, No. 21, 2012
5421

density to the anchoring ligand, revealing the advantages
of EDOT unit in ameliorating the spectral response of
squaraine dye. Furthermore, replacing indoline with
qoinoline unit in the unsymmetrical squaraines does
not affect significantly the electron distribution in the
frontier orbitals. Nevertheless, the difference between the
calculated and experimental λmax for both WCH-SQ10
and WCH-SQ11 is smaller than that for JYL-SQ5. This
phenomenon may due to the difference in the solvation
effect between indoline and quinoline or between butyl
and methyl in JYL-SQ5 and WCH-SQ11, respectively.
However, the C-PCM solvent model does not consider
the soluteꢀsolvent interaction explicitly. Therefore, one
must be very carefully when using the calculation data to
predict the real absorption profile of a dye molecule.
Figure 1. Electronic absorption spectra of WCH-SQ10, WCH-
SQ11 and JYL-SQ5 in ethanol.
was applied to study the molecular geometries, molecular
orbitals and UVꢀvis spectra of WCH-SQ10 and WCH-
SQ11. The ground-state molecular geometry was opti-
mized by the Becke, three-parameter, LeeꢀYangꢀParr
(B3LYP) function and 6-31G (d,p) basis set, as imple-
mented in the Gaussian 09 program.28 The Conductor-
like Polarizable Continuum Model (C-PCM)29 was used
to account for the salvation effect (in ethanol). The time-
dependent DFT (TDDFT) calculations were performed
to calculate the UVꢀvis spectra. To account for the charge
transfer excitations, the coulomb-attenuating method
(CAM)30 was applied (at TD-CAM- B3LYP/6-31G (d,p)
level) to calculate the UVꢀvis spectra of these two sensiti-
zers based on their corresponding optimized geometries
calculated at the B3LYP/6-31G (d,p) level. The calculated
absorption band of WCH-SQ10 is centered at 651 nm (see
SI Figure S6), which mainly arises from the highest occupied
molecular orbital (HOMO) to the lowest unoccupied mo-
lecular orbital (LUMO) vertical excitation (see SI Table S1
for details). For WCH-SQ11, the calculated absorption
band is centered at 645 nm. Similarly, this transition mainly
arises from the HOMO to LUMO vertical excitation.
The graphical representation of the frontier orbitals is
displayed in Figure 2. The HOMO and HOMO-1 for both
WCH-SQ10 and WCH-SQ11 are π-orbitals delocalizing
throughout the backbone and the terminal amino group.
These results indicate that the incorporation of EDOT
effectively elongates the conjugation length of the squar-
aines and induces the electronic coupling between the
squaraine core and the terminal amino group. The LUMO
and LUMOþ1 for both dyes are mainly located at the
anchoring motif including the carboxylic group. These
results suggest that the main transitions of WCH-SQ10
and WCH-SQ11 can effectively convert the electron
Figure 2. Isodensity surface plots for the frontier orbitals of
WCH-SQ10 and WCH-SQ11 (isodensity value = 0.02).
The absorption spectra (see SI, Figure S7) of these two
squaraine dyes immobilized on the surface of the porous
TiO2 film (ca. 8.0 μm thick) are both broader and blue-
shifted compared to those of dyes dissolved in ethanol.
Furthermore, the absorption profiles of these two squar-
aine dyes on TiO2 film extend to 1000 nm. These results
imply that the interaction between dye molecules and the
electronic coupling betweenthe dyeand TiO2 increase. The
dye molecules anchored on TiO2 films are predominantly
H-aggregated but also partially J-aggregated.31,32 This
phenomenon may be associated with the dihedral angle
between squaraine core and quinoline (ca. 5.8° and 6.4° in
WCH-SQ10 and WCH-SQ11, respectively) is larger than
that for the indoline-SQ system such as their predecessor,
JYL-SQ5 (ca. 2.5°), as deduced from the DFT calculation.
The energy level of the ground state (or HOMO) for
WCH-SQ10 and WCH-SQ11 determined with the square-
wave voltammograms (see SI Figure S8) is 0.73 and 0.71 V
(versus NHE), respectively. The HOMO energy level for
both squaraines is more positive than the redox potential
of the iodide/triiodide couple employed as an electrolyte in
DSSCs. The optical transition energy, E0ꢀ0, of WCH-
SQ10 and WCH-SQ11 determined from the intersection
of absorption and emission spectra measured in ethanol
(28) Frisch, M. J. et al. Gaussian 09, Revision A.02; Gaussian, Inc.:
Wallingford, CT, 2009 (complete reference is provided in the Supporting
Information).
(29) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem.
2003, 24, 669.
ꢀ
(31) de Miguel, G. S..; Ziozek, M.; Zitnan, M.; Organero, J. A.;
Pandey, S. S.; Hayase, S.; Douhal, A. J. Phys. Chem. C 2012, 116, 9379.
(32) Sayama, K.; Hara, K.; Mori, N.; Satsuki, M.; Suga, S.;
Tsukagoshi, S.; Abe, Y.; Sugihara, H.; Arakawa, H. Chem. Commun.
2000, 1173.
(30) Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004,
393, 51.
5422
Org. Lett., Vol. 14, No. 21, 2012

(see SI Figure S9) is 1.65 and 1.68 eV, yielding the
potentials of the LUMO for these two dyes to be ꢀ0.92
V and ꢀ0.97 V (vs NHE), respectively. The frontier orbital
energy levels of the two dyes certify the facile electron
injection from the photoexcited sensitizers to TiO2 con-
duction band and regenerating the oxidized dyes by the
iodide/triiodide redox couple. Moreover, the frontier
orbitals energy level of WCH-SQ10, WCH-SQ11, and
JYL-SQ5revealsthatintroductionofanelectron-donating
quinoline (instead of indoline) nearby the anchoring unit
can destabilize the potential of both the ground and excited
states of dye, which may increase the driving force for the
electron injection from the photoexcited sensitizers to TiO2
conduction band. High injection driving force ishelpful for
converting low energy photo into current.
The preliminary evaluation of the incident monochro-
matic photon-to-current conversion efficiency (IPCE) of
DSSC based on WCH-SQ10 or WCH-SQ11 is displayed
in Figure 3. Encouragingly, both devices not only ex-
hibit the panchromatic response but also extend the photo-
voltaic performance to the near-infrared region. This
phenomenon (although had been observed in Os-based
dyes)11,12 is extremely uncommon for metal-free organic
dyes and also bare observed in squaraine dyes designed for
DSSCs. The IPCE for WCH-SQ10 sensitized cell reaches
the maximum of 37% at 600 nm, slightly higher than that
(34% at 594 nm) of the device based on WCH-SQ11. The
small difference in Jsc is probably attributed to the differ-
ence in the potential of excited states (LUMO energy level)
for the two sensitizersas summarizedin Table S2 of SI. The
IPCE value at ca. 350 nm is mainly from the charge
separation between TiO2 and electrolyte, not from the
dye molecules as proved in the previous report.26
The characteristic current densityꢀvoltage (JꢀV) curve
measured under the illumination of AM (air mass) 1.5
global simulated sunlight (100 mW cmꢀ2) (as illustrated in
SI Figure S10) provides a short-circuit current density (Jsc)
of 9.25 mA cmꢀ2, an open-circuit voltage (Voc) of 374 mV
and a fillfactor(FF) of0.51, yielding the overall conversion
efficiency of 1.77% for WCH-SQ10 sensitized device.
Under the same device fabrication and efficiency evalua-
tion procedure, the photovoltaic parameters (Jsc, Voc, FF)
of the cell based on WCH-SQ11 are 9.06 mA cmꢀ2, 391
mV and 0.55, respectively, providing the efficiency of
1.96%.
Figure 3. Incident monochromatic photon-to-current conver-
sion efficiency (IPCE) curves for the DSSC devices sensitized by
WCH-SQ10 and WCH-SQ11.
DSSCs. The optimization of these new dye-based device is
in progress for fully exploring its potential for DSSCs
application.
In conclusion, a new strategy for designing new unsym-
metrical squaraine dyes by using quinoline instead of
commonly used indoline to bearing the carboxylic acid
group in the acceptor is demonstrated. With the prelimin-
ary tests, the devices based on these two newly developed
squaraines display a panchromatic response and also con-
vert even the near-infrared (λ > 1000 nm) photons into
electricity. The results present a new paragon for developing
new IR squaraine sensitizers for DSSC application.
Acknowledgment. We gratefully acknowledge the fi-
nancial support from the National Science Council, Taiwan,
ROC via grant number NSC-98-2113-M-008-009-MY3.
The computer facilities used are from the National Center
for High-Performance Computing and the Vger computer
cluster at the National Central University, Taiwan, ROC.
Supporting Information Available. The detailed syn-
thetic procedures and structure characterizations of these
two new unsymmetrical sqauraines, the instruments used
for the physicochemical studies, the square-wave voltam-
mograms, the absorption and emission spectra of the dyes
measured in ethanol, the theoretical calculation spectra
and data, the detailed device fabrication and the photo-
voltaic characterizations as well as IꢀV curves for DSSC
devices. This material is available free of charge via the
Internet at http://pubs.acs.org.
Comparing to the analogue JYL-SQ5 with indoline-
carboxylic acid as an acceptor,26 WCH-SQ10 and WCH-
SQ11 with quinoline-carboxylic acid motif can reinforce
the light-harvesting ability in the near-infrared region
as well as enhance the driving force for low-energetic
electron injection. Therefore, the IPCE response extends to
1050 nm for both WCH-SQ10 and WCH-SQ11 sensitized
DSSC devices. The results demonstrate that quinoline
incorporating squaraine dyes are the promising candidates
for near-infrared or even infrared sensitizers applying to
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 21, 2012
5423