C O M M U N I C A T I O N S
conjugated π-system of the dye and provides strong electronic
coupling to the conduction band of TiO . Second, the asymmetry
2
created by the octyl chain prevents surface aggregation and limits
self-quenching of the excited state. To the best of our knowledge,
these results represent a major breakthrough in the design and
development of squaraine-based sensitizers.
In order to see the impact of high molar extinction coefficient
of the squaraine sensitizer on photovoltaic properties, we have
fabricated solar cells using transparent TiO
thicknesses. The photocurrent increased with the thickness of the
TiO nanocrystalline layer. On the other hand, the photovoltage
2
membranes of various
2
decreased with increasing thickness. Despite the very narrow
absorption band of the squaraine sensitizer, devices using a 2.5
2
µm thin TiO layer yielded remarkably high photocurrent and IPCE
Figure 1. Top: isodensity plot of selected molecular orbitals of the
squaraine dye. Bottom: charge density difference between the excited (596
nm) and ground state; the yellow (white) color indicates an increase
2
of 8.20 mA/cm and 80%, respectively, which we have attributed
to the high molar extinction coefficient of the dye. Further
optimization was achieved by using 10 mM concentration of cheno
as coadsorbing additive (see Table S1 in Supporting Information).
In conclusion, we have demonstrated that selective functional-
ization of squaraine sensitizer yielded very high incident mono-
chromatic photon-to-current conversion and 4.50% power conver-
sion efficiency under one sun. Our data demonstrate that creating
directionality and inhibiting self-quenching of the excited state of
the sensitizer are the key for unprecedented efficiency of squaraines.
This should spark a broad spectrum of interest in the field of organic
sensitizers useful for photovoltaic windows that transmit part of
the visible light.
(decrease) of charge density in a given molecular region.
Acknowledgment. We acknowledge financial support of this
work by the Swiss Federal Office for Energy (OFEN), Swiss
Competence Center for Energy and Mobility CCEM-CH, and
Institute for Information Technology Advancement (Korea).
Figure 2. Photocurrent action spectrum (inset) and current-voltage
characteristics of the squaraine dye obtained with a nanocrystalline TiO2
film supported onto a conducting glass sheet and derivatized with a
monolayer of squaraine dyes in the presence of chenodeoxycholic acid. A
sandwich-type cell configuration was used to measure the spectrum.
Supporting Information Available: Experimental and synthetic
details of the sensitizer. This material is available free of charge via
the Internet at http://pubs.acs.org.
factor, together with the high dye molar extinction coefficient, is
probably responsible of the high photocurrent and overall photo-
voltaic efficiency measured experimentally; see below.
The screen-printed double layer TiO film consisting of a 10
2
µm transparent layer and a 4 µm scattering layer was prepared and
References
(
1) Special Issue: Michael Graetzel Festschrift, A tribute for this 60th
Birthday: Dye Sensitized Solar Cells; Nazeeruddin, M. K., Ed.; Elsevi-
er: Amsterdam, 2004; Vol. 248.
2) Park, N.-G.; Kang, M. G.; Kim, K. M.; Ryu, K. S.; Chang, S. H.; Kim,
D.-K.; Van de Lagemaat, J.; Benkstein, K. D.; Frank, A. J. Langmuir
2004, 20, 4246-4253.
(
treated with 0.05 M titanium tetrachloride solution using a pre-
viously reported procedure.4,13 The films were heated to 500 °C in
(3) Kamat, P. V.; Haria, M.; Hotchandani, S. J. Phys. Chem. B 2004, 108,
5166-5170.
air and sintered for 20 min before use. Dye solutions were prepared
(4) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi,
-
4
G.; Liska, P.; Ito, S.; Bessho, T.; Gr a¨ tzel, M. J. Am. Chem. Soc. 2005,
in the concentration range of 1 × 10 M solution in ethanol
127, 16835-16847.
containing 1, 10, and 60 mM 3a,7a-dihydroxy-5b-cholic acid
(
5) Lenznoff, C. C.; Lever, A. B. P. Phthalocyanines; VCH Publishers, Inc.:
New York, 1993; Vol. 3.
(cheno) as an additive. The electrodes were dipped into the dye
(6) Otsuka, A.; Funabiki, K.; Sugiyama, N.; Yoshida, T.; Minoura, M.; Matsui,
M. Chem. Lett. 2006, 35, 666-667.
solution for 4 h at 22 °C, and the dye-coated electrodes were rinsed
quickly with ethanol and used as such for photovoltaic measure-
ments. The fabrication procedure for solar cells, the testing condi-
tions, and the equipment used were reported in a previous work.4
Figure 2 shows the photocurrent action spectrum obtained with
a sandwich cell. The incident monochromatic photon-to-current
conversion efficiency (IPCE) plotted as a function of excitation
wavelength shows 85% efficiency. Under standard global AM 1.5
solar condition, the squaraine-sensitized cell gave a short circuit
(7) Li, C.; Wang, W.; Wang, X.; Zhang, B.; Cao, Y. Chem. Lett. 2005, 34,
554.
(8) Alex, S.; Santhosh, U.; Das, S. J. Photochem. Photobiol. A 2005, 172,
65.
(9) Burke, A.; Schmidt-Mende, L.; Ito, S.; Gr a¨ tzel, M. Chem. Commun. 2007,
234.
(10) Chen, Y.; Zeng, Z.; Li, C.; Wang, W.; Wang, X.; Zhang, B. New J. Chem.
2005, 29, 773-776.
(
11) Nazeeruddin, M. K.; Humphry-Baker, R.; Gr a¨ tzel, M.; W o¨ hrle, D.;
Schnurpfeil, G.; Schneider, G.; Hirth, A.; Trombach, N. J. Porphyrins
Phthalocyanines 1999, 3, 230.
(
12) Liu, G.; Jaegermann, W.; He, J.; Sundstr o¨ lm, V.; Sun, L. J. Phys. Chem.
B 2002, 106, 5814.
2
photocurrent density jsc of 10.50 ( 0.20 mA/cm , an open circuit
voltage of 603 ( 30 mV, and a fill factor (ff) of 0.71 ( 0.03,
corresponding to an overall conversion efficiency η of 4.5%, as
derived from the equation η ) jsc × Voc × ff (Figure 2). The high
efficiency of the squaraine sensitizer is attributed to the particular
molecular design. First, the carboxylic acid group is part of the
(13) Nazeeruddin, M. K.; Pe’chy, P.; Renouard, T.; Zakeeruddin, S. M.;
Humphry-Baker, R.; Comte, P.; Liska, P.; Le, C.; Costa, E.; Shklover,
V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gr a¨ tzel, M. J. Am. Chem.
Soc. 2001, 123, 1613.
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