Unsymmetrical squaraines incorporating the thiophene unit for panchromatic dye-sensitized solar cells

Article abstract of DOI:10.1021/ol102127x

Two unsymmetrical squaraines, where the electron-rich 3,4- ethylenedioxythiophene or bithiophene conjugated fragment was used to link unconventionally the squaraine core and the hexyloxyphenyl amino group, were applied for DSCs. The corresponding photovol

Full text of DOI:10.1021/ol102127x

ORGANIC
LETTERS
2010
Vol. 12, No. 23
5454-5457
Unsymmetrical Squaraines Incorporating
the Thiophene Unit for Panchromatic
Dye-Sensitized Solar Cells
Jheng-Ying Li,† Chia-Yuan Chen,† Chuan-Pei Lee,‡ Szu-Chien Chen,†
Tsu-Han Lin,† Hui-Hsu Tsai,† Kuo-Chuan Ho,‡ and Chun-Guey Wu*,†
Department of Chemistry, National Central UniVersity, Jhong-Li, 32001, Taiwan, ROC,
and Department of Chemical Engineering, National Taiwan UniVersity,
Taipei, 10617, Taiwan, ROC
t610002@cc.ncu.edu.tw
Received September 24, 2010
ABSTRACT
Two unsymmetrical squaraines, where the electron-rich 3,4-ethylenedioxythiophene or bithiophene conjugated fragment was used to link
unconventionally the squaraine core and the hexyloxyphenyl amino group, were applied for DSCs. The corresponding photovoltaic devices
exhibit an attractively panchromatic response and also convert a portion of the near-infrared photons into electricity.
Dye-sensitized solar cells (DSCs) have been developed as
one of the promising alternatives for practical photovoltaic
application in virtue of their low manufacturing cost and
impressive photovoltaic performance.1 In DSCs, the dye
molecule (sensitizer) self-assembles onto the surface of a
wide band gap semiconductor (e.g., TiO2) and plays a vital
role in the light-harvesting, charge separation as well as the
overall photon-to-current conversion efficiency of the cor-
responding device. At present, DSCs sensitized by polypy-
ridyl Ru(II) complexes have achieved remarkably high
conversion efficiencies exceeding 11%.2-6 Nevertheless, the
spectral response of those devices in the far-red and near-
infrared regions is still insufficient, which shall be overcome
through a delicate molecular engineering of the photosen-
sitizers.7-9 Recently, squaraines with attractive intrinsic far-
red absorption and extraordinary high molar absorption
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Sci. 2009, 2, 674
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† National Central University.
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‡ National Taiwan University.
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10.1021/ol102127x  2010 American Chemical Society
Published on Web 11/03/2010

coefficient have been developed for DSCs.10-19 In general,
the squaraine dye sensitized DSCs can efficiently convert
the low-energy (∼1.9 eV) photons into electricity, which
makes the squaraine dyes not only promising cosensitizers
for organic dyes with a complementary absorption profile20,21
but also good candidates to be combined with luminescent
energy-relay dyes.22 However, there are only rare squaraine-
sensitized DSCs exhibiting a panchromatic response.19 Here
we report two novel unsymmetrical squaraine dyes, coded
JYL-SQ5 and JYL-SQ6, which are optimized for panchro-
matic sensitization of DSCs. The electron-rich 3,4-ethylene-
dioxythiophene (EDOT) and bithiophene (BT) fragments
with their benefits for improving the light-harvesting capacity
of the ruthenium sensitizers23,24 were incorporated, respec-
tively, to link unconventionally the squaraine core and the
hexyloxyphenyl amino donor moiety.
Figure 1. Electronic absorption spectra of JYL-SQ5 and JYL-
SQ6 in ethanol and immobilized on TiO2 thin films (ca. 8.0 µm).
The molecular structures of JYL-SQ5 and JYL-SQ6 and
their synthetic scheme are depicted in Scheme 1. The
absorption coefficient (ε) of 10.9 × 104 M-1 cm-1; therefore,
it can efficiently adsorb the photons with a much lower
energy compared to the previously reported squaraines10-19
designed for DSC application. These features make JYL-
SQ5 promising for harvesting more far-red and near-infrared
photons. Surprisingly, the λmax of JYL-SQ6 is 41 nm blue-
shifted compared to that of JYL-SQ5, and its ε value (7.5
× 104 M-1 cm-1) is also significantly lower than that of JYL-
SQ5 although its absorption band is broader (the full width
at half-maximum (fwhm) is 132.4 nm for JYL-SQ6 vs 107.7
nm for JYL-SQ5).
Scheme 1. Preparation for JYL-SQ5 and JYL-SQ6
To understand how the electron density redistributed after
photoexcitations, the density functional theory (DFT) was
applied to study the molecular geometries, molecular orbitals,
and UV-vis spectra of JYL-SQ5 and JYL-SQ6. The
ground-state molecular geometry was optimized by the
Becke, three-parameter, Lee-Yang-Parr (B3LYP) func-
tional and 6-31G(d,p) basis set, as implemented in the
Gaussian 09 program.25 The Conductor-like Polarizable
Continuum Model (C-PCM)26 was used to account for the
solvation effect (ethanol). The time-dependent DFT (TD-
DFT) calculations were performed to calculate the UV-vis
spectra. To account for the charge transfer excitations, the
coulomb-attenuating method (CAM)27 was applied (at TD-
CAM-B3LYP/6-31G(d,p) level) to calculate the UV-vis
spectra of the two sensitizers based on their corresponding
optimized geometries calculated at the B3LYP/6-31G(d,p)
synthetic details and structure characterization are provided
in the Supporting Information (SI). The electronic absorption
spectra of JYL-SQ5 and JYL-SQ6 measured in ethanol are
shown in Figure 1. Encouragingly, JYL-SQ5 owns an
intense absorption band centered at 691 nm with a high molar
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Org. Lett., Vol. 12, No. 23, 2010
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level. The calculated absorption band of JYL-SQ5 is
centered at 638 nm, which mainly arose from the HOMO to
LUMO vertical excitation (see SI for details, Table S1). For
JYL-SQ6, the calculated absorption band is centered at 635
nm. Similarly, this transition mainly arose from the HOMO
to LUMO vertical excitation. The graphical representation
of the frontier orbitals is displayed in Figure 2. The highest
µm thick) are also provided in Figure 1. The absorption bands
of JYL-SQ5 and JYL-SQ6 adsorbed on TiO2 film are both
broader than those dissolved in ethanol, implying the
increasing interaction between dye molecules and the
electronic coupling between the dye and TiO2. The λmax of
JYL-SQ6 anchored on TiO2 is red-shifted compared to that
in ethanol, indicating that JYL-SQ6 molecules anchored on
the TiO2 thin film are J-aggregated,28 which is probably due
to its planar molecular geometry. The DFT calculations
showed that JYL-SQ6 is nearly planar except for its terminal
amino group. Furthermore, the absorption profiles of the two
squaraine dyes on TiO2 thin film suggested that the device
sensitized by JYL-SQ5 may convert more longer-wavelength
photons into electricity than that based on JYL-SQ6 if the
potential of the ground state and excited state for these two
dyes satisfies the requirements for DSC application.29
The energy level of the ground state (or HOMO) for JYL-
SQ5 and JYL-SQ6 determined with the square-wave vol-
tammetry is 0.88 and 0.96 V (versus NHE, see the SI for
details, Figure S6), respectively, and both are more positive
than the redox potential of the iodide/triiodide couple
employed as an electrolyte in DSCs. The optical transition
energy, E0-0, of JYL-SQ5 and JYL-SQ6 determined from
the intersection of absorption and emission spectra measured
in ethanol (see Figure S7 in the SI) is 1.67 and 1.74 eV,
yielding the potentials of the LUMO for these two dyes to
be -0.79 and -0.78 V, respectively. The frontier orbital
energy levels of the two dyes not only certify the facile
electron injection from photoexcited sensitizers to TiO2
conduction band and the regeneration of oxidized dyes by
the iodide/triiodide redox couple but also reveal that EDOT
is a superior candidate to reduce the electronic transition
energy through destabilizing the potential of HOMO.
The preliminary evaluation of the incident monochromatic
photon-to-current conversion efficiency (IPCE) of the cells
based on JYL-SQ5 or JYL-SQ6 is displayed in Figure 3.
Figure 2. Isodensity surface plots for the frontier orbitals of JYL-
SQ5 and JYL-SQ6 (isodensity value ) 0.02).
occupied molecular orbital (HOMO) and HOMO-1 for both
JYL-SQ5 and JYL-SQ6 are π-orbitals delocalizing through-
out the backbone and the terminal amino group. These results
indicate that the incorporation of EDOT or BT effectively
elongates the conjugation length of the squaraines and
induces the electronic coupling between the squaraine core
and the terminal amino group. Nevertheless, the electron
density of HOMO is less or not populated at the anchoring
carboxylic group. The lowest unoccupied molecular orbital
(LUMO) and LUMO+1 for JYL-SQ5 and JYL-SQ6 are
also π-orbitals; their electron density is mainly located at
the anchoring motif including the carboxylic group. These
results indicate that the main transitions of JYL-SQ5 and
JYL-SQ6 can effectively convert the electron density to the
anchoring ligand, revealing the advantages of EDOT and
bithiophene units in ameliorating the spectral response of
the squaraine dye. Nevertheless, the blue-shifted absorption
of JYL-SQ6 with respect to JYL-SQ5 was not observed in
the calculations. It is expected that the EDOT unit has a
stronger solvation effect than that of the bithiophene unit;
in particular, the EDOT unit will form hydrogen bonds with
the ethanol which may lead to a red-shifted absorption.
However, the C-PCM solvent model does not consider the
solute-solvent hydrogen bond explicitly. Further computa-
tional study including solvent molecules explicitly to account
for the solute-solvent hydrogen bond will be helpful to
answer this question.
Figure 3. Incident monochromatic photon-to-current conversion
efficiency (IPCE) spectra of the devices sensitized by JYL-SQ5
or JYL-SQ6 and a blank device (without dye).
Encouragingly, both devices not only exhibit the panchro-
matic response (covers the window of the whole visible-
(28) Sayama, K.; Hara, K.; Mori, N.; Satsuki, M.; Suga, S.; Tsukagoshi,
S.; Abe, Y.; Sugihara, H.; Arakawa, H. Chem. Commun. 2000, 1173.
(29) Gra¨tzel, M. Inorg. Chem. 2005, 44, 6841.
The absorption spectra of these two squaraine dyes
immobilized on the surface of the porous TiO2 film (ca. 8.0
5456
Org. Lett., Vol. 12, No. 23, 2010

light) but also extend the photovoltaic performance to the
near-infrared region. This phenomenon is extremely uncom-
mon for metal-free organic dyes designed for DSCs. The
IPCE for the JYL-SQ5-sensitized cell reaches the maximum
of 46.8% at 620 nm, significantly higher than that (39.3%
at 590 nm) of the device based on JYL-SQ6. The remarkable
difference between the two devices in converting photons
with the energy of 1.51-2.10 eV to electricity is conceivable
from the intrinsic distinction of the absorption profiles for
the two sensitizers dissolved in ethanol and anchored on TiO2
thin film as shown in Figure 1. The IPCE value at ca. 350
nm is mainly from the charge separation between TiO2 and
electrolyte, not from the dye molecules as can be seen from
the performance of the blank device (device without dye).
recombination.30,31 The results demonstrate that the electron-
rich EDOT or BT units incorporating squaraine dyes are
promising candidates for the panchromatic sensitizers used
in DSCs. In addition to developing new unsymmetrical
squaraine dyes, the device optimization of JYL-SQ5 is in
progress for fully exploring its potential for DSC application.
In conclusion, a new strategy for designing new unsym-
metrical squaraine dyes by incorporating the electron-
abundant thiophene derivatives, such as EDOT or BT units,
between the squaraine core and the terminal amino donor
group is demonstrated. Even with the preliminary test, the
devices based on these two newly developed squaraines
display a panchromatic response and also convert the near-
infrared photons into electricity. The results present a new
paragon for developing high efficiency and panchromatic
response squaraine sensitizers for DSC applications.
The characteristic current density-voltage (J-V) curve
measured under the illumination of AM (air mass) 1.5 global
simulated sunlight (100 mW/cm2) (as illustrated in Figure
S8 of SI) provides a short-circuit current density (Jsc) of 11.12
mA/cm2, an open-circuit voltage (Voc) of 422 mV, and a fill
factor (FF) of 0.557, yielding the overall conversion ef-
ficiency of 2.61% for the JYL- SQ5 sensitized device. Under
the same device fabrication and efficiency evaluation pro-
cedure, the photovoltaic parameters (Jsc, Voc, FF) of the cell
based on JYL-SQ6 are 9.40 mA/cm2, 432 mV, and 0.578,
respectively, providing the efficiency of 2.34%. The mea-
sured Jsc values of the devices are in good agreement with
the photocurrent densities (11.45 and 9.69 mA/cm2 for JYL-
SQ5 and JYL-SQ6 sensitized devices, respectively) calcu-
lated from the overlap integral of the IPCE spectra. Com-
pared to the device sensitized by an analogue with the same
terminal donor group but no thiophenes,19 JYL-SQ5- and
JYL-SQ6-based devices have a higher photocurrent, which
is attributed to that the EDOT or BT unit can reinforce the
light-harvesting ability in the red-light region. Moreover, the
high concentration of lithium ion we used in the electrolyte
may also facilitate a charge injection from photoexcited dyes
to the conduction band of TiO2 and a retardation of charge
Acknowledgment. We gratefully acknowledge the finan-
cial support from the National Science Council, Taiwan,
ROC. 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 the two
new unsymmetrical sqauraines, the instruments used for the
physicochemical studies, the square-wave voltammograms,
the absorption and emission spectra of the dyes measured
in ethanol, the theoretical calculation data, and the detailed
device fabrication as well as the photovoltaic characteriza-
tions. This material is available free of charge via the Internet
at http://pubs.acs.org.
OL102127X
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