A novel organic sensitizer combined with a cobalt complex redox shuttle for dye-sensitized solar cells

Article abstract of DOI:10.1021/ol300807x

A novel donor-π-acceptor organic dye (HIQ7) was used in dye-sensitized solar cells with a cobalt redox shuttle. The cells showed broad incident monochromatic photon-to-current conversion efficiency spectra covering the entire visible range and extending into the near-infrared region.

Full text of DOI:10.1021/ol300807x

ORGANIC
LETTERS
2012
Vol. 14, No. 10
2532–2535
A Novel Organic Sensitizer Combined with
a Cobalt Complex Redox Shuttle for
Dye-Sensitized Solar Cells
Chuanjiang Qin, Wenqin Peng, Kun Zhang, Ashraful Islam, and Liyuan Han*
Photovoltaic Materials Unit and NIMS Saint-Gobain Center of Excellence for Advanced
Materials, National Institute for Materials Science, Sengen 1-2-1, Tsukuba,
Ibaraki 305-0047, Japan
Han.Liyuan@nims.go.jp
Received April 2, 2012
ABSTRACT
A novel donor-π-acceptor organic dye (HIQ7) was used in dye-sensitized solar cells with a cobalt redox shuttle. The cells showed broad incident
monochromatic photon-to-current conversion efficiency spectra covering the entire visible range and extending into the near-infrared region.
Dye-sensitized solar cells (DSCs) have been the subject
of much academic and industrial research, owing to their
potential use for low-cost production of renewable energy
and in large-area, flexible, colorful, lightweight devices.1
Recently, a certified energy conversion efficiency (η) of
11.4% was achieved under standard AM 1.5 conditions
with DSCs based on ruthenium sensitizers.2 Because of the
limited availability of ruthenium, ruthenium-free organic
dyes based on the donor-π-bridge-acceptor (D-π-A) con-
figuration have attracted attention over the past decade.3
Almost all reported high-efficiency DSCs (η > 10%)
have used a iodide/triiodide redox shuttle to regenerate the
oxidized dye.4 However, the cells generate only a limited
open-circuit voltage (Voc) of 0.7ꢀ0.8 V because the redox
potential of iodide ions is too low for most sensitizing dyes,
even though the dyes have a relatively large energy gap.
Energy loss during the regeneration process is large. More-
over, the iodide/triiodide system is limited by the corro-
siveness of iodide/triiodide and by competitive absorption
of light by triiodide. To overcome these limitations and
increase conversion efficiency, researchers have recently
explored new redox species possessing higher redox poten-
tials than iodide ions, such as iron-, cobalt-, and copper-
based complexes.5 Among them, divalent/trivalent cobalt
bipyridine complexes exhibit impressive performance in
€
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Phys., Part 2 2006, 45, L638. (c) Bessho, T.; Zakeeruddin, S. M.; Yeh, C.-Y.;
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r
10.1021/ol300807x
Published on Web 05/04/2012
2012 American Chemical Society

HIQ7 was synthesized as follows (Scheme 1). The dye
precursor was obtained from two sequential asymmetrical
Suzuki coupling reactions8 of 2,6-dibromo-4,4-ethylene-di-
oxy-4H-cyclopenta[2,1-b:3,4-b0]dithiophen with 4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-N,N-bis(4-(ethylhexy-
loxy)phenyl)aniline and 5-formylthiophen-2-boronic acid,
respectively. The resulting precursor was subjected to a
Knoevenagel condensation9 with cyanoacetic acid to afford
1
HIQ7, which was characterized by H and 13C NMR,
elemental analysis, and HRMS. The obtained dye was a
black-red solid that dissolved readily in common organic
solvents, such as chloroform, DMF, and methanol.
Figure 1. Chemical structures of dye HIQ7 and cobalt complex.
DSCs sensitized with organic D-π-A dyes.6 A state-of-the
art DSC that combines a porphyrin dye and a cobalt
electrolyte and exhibits conversion efficiencies exceeding
12% was reported in late 2011.7 However, the absorption
spectra and the incident monochromatic photon-to-current
conversion efficiency (IPCE) spectra of the DSC did not
extend into the near-infrared (NIR) region. Thus, the
development of new sensitizers with broad absorption
spectra and energy levels matched with the cobalt complex
redox system is necessary for higher efficiency. Herein, we
report a novel D-π-A sensitizer (HIQ7, Figure 1) based on
4,4-ethylenedioxy-4H-cyclopenta[2,1-b:3,4-b0]dithiophen,
which exhibited broad absorption (extending into the NIR
region) on a TiO2 film and was employed successfully in
combination with a cobalt complex redox shuttle for DSCs.
Feldt and co-workers reported that the key to making
efficient cells is modifying the periphery of the sensitizing
dyes and the cobalt complexes with electrically inactive
groups with just enough bulk to slow down recombination
without blocking the necessary electron transfer proces-
ses.5b Thus, we designed a dye with the following features:
(1) the electron-rich 4,4-ethylenedioxy-4H-cyclopenta-
[2,1-b:3,4-b0]dithiophen group to tune the band gap for
harvesting a broad spectrum of the sun’s radiation, (2)
bulky groups on the donor moiety to suppress charge
recombination between the injected electron and the redox
couple and thus increase Voc, and (3) an electron-donating
alkyloxy group to elevate the energy level of the HOMO,
which facilitated dye regeneration by the redox shuttle and
reduced energy loss during the regeneration process.
Figure 2. Absorption spectra of HIQ7 in DMF (line), on TiO2
(dash dot line) and fluorescence emission spectrum (dash line).
In a DMF solution, HIQ7 absorbed strongly over
the entire visible region (Figure 2), with a peak at 488 nm
(ε = 4.7 ꢁ 104 Mꢀ1 cmꢀ1) due to intramolecular charge
transfer from the donor moiety to the acceptor moiety.
In solution, HIQ7 exhibited a strong red fluorescence
emission around 631 nm. When adsorbed on a transparent
TiO2 electrode, HIQ7 showed a broadened absorption
spectrum, with an absorption peak (at 510 nm) that was
strongly red-shifted compared to the corresponding peak
in the solution spectrum. The shift was attributed to
interaction of the anchoring group with surface titanium
anions and the formation of J-aggregates.10 Moreover, the
onset of absorption occurred at about 750 nm, making
HIQ7 a good candidate for an NIR sensitizer for DSCs.
To investigate the thermodynamics of electron injection
from the excited state of the dye to the conduction band of
the TiO2 electrode and the thermodynamics of dye regen-
eration via electron donation from cobalt complex redox
couples, we obtained a cyclic voltammogram in a typical
three-electrode electrochemical cell with dye-sensitized
Scheme 1. Synthesis Routes of HIQ7
(7) Yellla, A.; Lee, H.-W.; Tsao, H.; Yi, C.; Chandiran, A. K.;
Nazeerudin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.;
€
Gratzel, M. Science 2011, 334, 629.
(8) Suzuki, A. J. Organomet. Chem. 1999, 576, 147.
(9) Jones, G. Org. React 1942, 1, 210.
(10) Wang, Z.; Koumura, N.; Cui, Y.; Takahashi, M.; Sekiguchi, H.;
Mori, A.; Kubo, T.; Furube, A.; Hara, K. Chem. Mater. 2008, 20, 3993.
Org. Lett., Vol. 14, No. 10, 2012
2533

Molecular orbital calculations demonstrated that the
HOMO was largely delocalized over the triphenylamino
unit and a portion of the 4,4-ethylenedioxy-4H-cyclo-penta-
[2,1-b:3,4-b0]dithiophen moiety. These results suggest that
delocalization of the HOMO over the donor moiety may
have facilitated reduction of the oxidized dye by reaction
with the redox shuttle, making the dye suitable for highly
efficient solar cells. The LUMO was located predominantly
on the cyanoacrylic unit and extended to the 4,4-ethylene-
dioxy-4H-cyclopenta[2,1-b:3,4-b0]dithiophen moiety, facili-
tating electron injection from the photoexcited sensitizer
to the TiO2 semiconductor. HOMOꢀLUMO excitation
moves the electron from the donor moiety to the cya-
noacrylic acid moiety via a π-space group. Such electron
density distributions are beneficial for efficient charge sep-
aration and electron injection.
Figure 3. Cyclic voltammograms of the HIQ7 absorbed on TiO2
film and Co(Bpy)3 2PF6 in acetone nitrile.
3
TiO2 films as the working electrodes and 0.1 M tetra-n-
butylammonium hexafluorophosphate as the supporting
electrolyte (Figure 3). The reference electrode was Ag/
AgCl calibrated with ferrocene as an internal reference.
HIQ7 on TiO2 exhibited two reversible oxidative waves.
The ground-state oxidation potential corresponding to the
HOMO of HIQ7 was measured to be 0.95 V (vs Normal
Hydrogen Electrode (NHE)), and this value was low
enough for efficient regeneration of the oxidized dyes
by means of reaction with the redox couples (0.4 V for
Iꢀ, 0.56 V for Co(Bpy)3 2PF6 vs NHE).11 The zeroꢀzero
3
transition energy, which is related to the band gap energy,
was determined from the intercept of the normalized
absorption and emission spectra (Figure 2). The calcu-
lated excited-state oxidation potential (referred to as the
LUMO level) was ꢀ1.26 V and was much more nega-
tive than the conduction band level of TiO2 (approxi-
mately ꢀ0.5 V vs NHE), ensuring efficient electron injection
from the excited state of the dye to the TiO2 conduction
band. Thus, our 4,4-ethylenedioxy-4H-cyclopenta[2,1-
b:3,4-b0]dithiophen-based sensitizer possessed ideal en-
ergy levels for use in a cobalt redox system.
Figure 5. JꢀV and IPCE curves of DSCs sensitized with HIQ7
with iodine and cobalt as a redox couple, respectively.
We performed preliminary photovoltaic experiments to
evaluate the potential of HIQ7 as a sensitizer for DSCs. A
double-layer TiO2 photoelectrode (thickness 10 μm; area
0.25 cm2) was used as a working electrode. A 5 μm main
transparent layer with titania particles (∼25 nm) and a
5 μm scattering layer with titania particles (∼400 nm) were
screen-printed on a fluorine-doped tin oxide conducting
glass substrate. The TiO2 film was stained with HIQ7 by
immersion in a 0.3 mM dye solution in tert-butanol/
acetonitrile (1:1 v/v) for 12 h. The photovoltaic perfor-
mance of DSCs sensitized with HIQ7 was studied with
two redox shuttles under standard AM 1.5 irradiation
(100 mW cmꢀ2). During the measurements, a black metal
mask and an edge attached to the active area of the cells
were used to avoid inflated photocurrents due to stray
light.12 We used the following electrolyte solutions: 0.6 M
dimethylpropylimidazolium iodide, 0.05 M I2, 0.1 M LiI,
and 0.5 M tert-butylpyridine (TBP) in acetonitrile; 0.22 M
Co(Bpy)3 2PF6, 0.05 M Co(Bpy)3 3PF6, 0.5 M TBP, and
To investigate the molecular orbitals and molecular
geometries of the dye, we carried out time-dependent
density functional theory calculations on HIQ7 (Figure 4).
3
3
0.1 M LiClO4 in acetonitrile.
The Voc of the HIQ7 cell with a cobalt redox shuttle was
131 mV higher than that of a cell with an iodine electrolyte
system (Table 1), and the increase was ascribed mainly to a
160 mV negative shift of the redox potential of the cobalt
complex.6a The short-circuit current density (Jsc) of the cell
with the cobalt electrolyte was 34% higher than that of the
Figure 4. Frontier molecular orbital profiles of HIQ7 at the
B3LYP/6-31G* level.
(11) (a) Oskam, G.; Bergeron, B. V.; Meyer, G. J.; Searson, P. C.
J. Chem. Chem. B 2001, 105, 6867. (b) Yonezawa, Y.; Kozaki, M.;
Okada., K. Synth. Met. 2001, 119, 99.
(12) (a) Koide, N.; Han, L. Rev. Sci. Instrum. 2004, 75, 2828. (b)
Koide, N.; Chiba, Y.; Han, L. Jpn. J. Appl. Phys. 2005, 44, 4176.
2534
Org. Lett., Vol. 14, No. 10, 2012

impressive, owing to the relatively weak IPCE response,
the use of this type of D-π-A configuration offers a simple,
feasible approach to the construction of NIR dyes for
cobalt-based systems. Further research on 4,4-ethylene-
dioxy-4H-cyclopenta[2,1-b:3,4-b0]dithiophen, including
optimization of the structure, tuning of the energy levels
and electron injection efficiency, and optimization of the
electrolyte solution and TiO2 paste, is likely to result in
more-efficient NIR dyes.
Table 1. CurrentꢀVoltage Characteristics of DSCs Sensitized
with HIQ7 in Different Electrolytes
Jsc
[mA cmꢀ2
Voc
η
[%]
]
[V]
FF
cobalt
iodine
8.38
6.24
0.747
0.616
0.648
0.636
4.04
2.44
In conclusion, we developed a novel 4,4-ethylenedioxy-
4H-cyclopenta[2,1-b:3,4-b0]dithiophen-based D-π-A or-
ganic dye and introduced it into a DSC with a cobalt-
based redox system. By means of systematic and rational
molecular design, we fabricated a DSC that exhibited a
broad IPCE response spectrum (extending to 820 nm).
Intensity-modulated photovoltage and photocurrent spec-
troscopy indicated that the combination of HIQ7 with a
cobalt complex efficiently suppressed electron recombina-
tion between the TiO2 surface and the redox oxide species.
Finally, an HIQ7-sensitized DSC with a cobalt electrolyte
showed an overall conversion efficiency of 4.04%, which
was 1.65 times that observed with an iodine electrolyte.
Our results demonstrate the great potential of compounds
containing 4,4-ethylenedioxy-4H-cyclopenta-[2,1-b:3,4-b0]-
dithiophen-based D-π-A structures as NIR sensitizers for
cobalt-based DSCs.
iodine-based DSC. The increase was ascribed to two
factors: (1) competitive light absorption by triiodide,
which lowered the IPCE of the iodine system relative to
that of the cobalt system (Figure 5); (2) faster recombinꢀa-
tion between injected electrons in the photoanode and I3
,
which is smaller than the cobalt electrolyte and thus
penetrates more deeply into the TiO2 film.13 Consequently,
changing from an iodine-based redox system to a cobalt-
based system increased η from 2.44% to 4.04%. It is
noteworthy that the IPCE response of the HIQ7-based
cell extended to 820 nm, which is one of the longest-
wavelength responses among all DSCs based on cobalt
redox systems reported to date.
We used intensity-modulated photovoltage spectro-
scopy and intensity-modulated photocurrent spectroscopy
to deepen our understanding of the differences between the
two systems.14 For identical cell structures, the diffusion
coefficient was lower for the cobalt complex than for
iodine, owing to higher mass transport (Figure S1). How-
ever, investigation of the relationship between the electron
lifetime and Voc for the DSCs with the iodine and cobalt
redox shuttles (Figure S2) indicated that the latter electro-
lyte afforded a much longer electron lifetime than did the
former, implying that the recombination reaction between
the electrons on the TiO2 surface and the trivalent cobalt
complex in the electrolyte was considerably suppressed.
This result confirms that our molecular design is ideal
for cobalt-based DSCs. Although the efficiency was not
Acknowledgment. This work was supported by the
NIMS Saint-Gobain Center of Excellence for Advanced
Materials and by Core Research for Evolutional Science
and Technology of the Japan Science and Technology
Agency.
Supporting Information Available. Experimental de-
tails, characterization data for all new compounds, details
of the DSC fabrication, intensity-modulated photovolt-
age, and photocurrent spectroscopy data characteriza-
tions. This material is available free of charge via the
Internet at http://pubs.acs.org.
(13) Ohta, M.; Koumura, N.; Hara, K.; Mori, S. Electrochem.
Commun. 2011, 13, 778.
(14) Schlichthoerl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J.
Phys. Chem. B 1997, 101, 8141.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 10, 2012
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