A panchromatic boradiazaindacene (BODIPY) sensitizer for dye-sensitized solar cells

Article abstract of DOI:10.1021/ol8010612

(Chemical Equation Presented) A novel distyryl-substituted boradiazaindacene (BODIPY) dye displays interesting properties as a sensitizer in DSSC systems, opening the way to further exploration of structure-efficiency correlation within this class of dyes.

Full text of DOI:10.1021/ol8010612

ORGANIC
LETTERS
2008
Vol. 10, No. 15
3299-3302
A Panchromatic Boradiazaindacene
(BODIPY) Sensitizer for Dye-Sensitized
Solar Cells
Sule Erten-Ela,† M. Deniz Yilmaz,‡ Burcak Icli,‡ Yavuz Dede,‡ Siddik Icli,† and
Engin U. Akkaya*,§
Solar Energy Institute, Ege UniVersity, Izmir, Turkey TR-35100, Department of
Chemistry, Middle East Technical UniVersity, Ankara, Turkey TR-06531, and
Department of Chemistry and UNAM-Institute of Materials Science and
Nanotechnology, Bilkent UniVersity, Ankara, Turkey TR-06800
eua@fen.bilkent.edu.tr
Received May 21, 2008
ABSTRACT
A novel distyryl-substituted boradiazaindacene (BODIPY) dye displays interesting properties as a sensitizer in DSSC systems, opening the
way to further exploration of structure-efficiency correlation within this class of dyes.
Among environment friendly and renewable sources of
energy, solar cells have always been high on the list of likely
candidates. Unfortunately, large-scale commercialization is
hampered by the production costs. Dye-sensitized solar cells1
(DSSCs) appear to be highly promising alternatives to more
expensive solar cell technologies. Considering the current
maximal level of overall conversion efficiency (η) under
simulated sunlight for DSSCs (11%),2 improvements in
efficiency and durability would certainly facilitate widespread
utilization of this technology.
It is clear that there are a number of factors determining
the efficiency of solar cells, but the structural and physical
properties of the sensitizer are clearly important ones. As a
result, many laboratories around the world have active
† Ege University.
‡ Middle East Technical University.
§ Bilkent University.
(2) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.;
Viscardi, G.; Liska, P.; Ito, S.; Bessho, T.; Gratzel, M. J. Am. Chem. Soc.
2005, 127, 16835–16847.
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10.1021/ol8010612 CCC: $40.75
Published on Web 06/28/2008
2008 American Chemical Society

research programs in optimizing the dye component of the
DSSCs. Ruthenium dyes, while holding the record for
conversion efficiencies, have relatively low extinction coef-
ficients, and they are also considered to be expensive and
hard to purify. Motivated by the possibility of finding a
replacement for metal-complex dyes, a number of chro-
mophores, including coumarins,3 indolines,4 oligoenes,5
merocyanines,6 hemicyanines,7 oligothiophenes,8 function-
alized thiophenes,9 squaraines,10 benzothiadiazoles,11 peryl-
enetetracarboxylic acid derivatives,12 diphenylaminosty-
renes,13 and phthalocyanines,14 have been studied, with
varying degrees of success. For the most efficient of these
sensitizers, the overall conversion efficiency is limited to
4-7.5% (Figure 2).
Boradiazaindacenes (the parent dye, 1), commonly known
as BODIPY dyes, have been recognized15 as useful fluo-
rescent labels for biomolecules for some time. Within the
past decade, there is much renewed interest16 in these dyes,
due to the development of new avenues for derivatization
and novel applications in a highly diverse field, including
chemosensors,17 logic gates,18 light harvesters,19 energy
transfer casettes,20,21 and photodynamic therapy.22 It is
known that photostability of the BODIPY dyes is signifi-
cantly better23 than many sensitizers proposed. One simple
derivative of BODIPY was in fact studied24 as a sensitizer
within the DSSC context, and charge separation was clearly
observed; however, very low overall conversion efficiencies
were obtained.
We now propose that, when judiciously designed, BO-
DIPY dyes have certain unique features that could make them
highly advantageous compared to most other organic dyes,
and thus, they are highly promising in this regard. (i)
BODIPYdyeshavehighextinctioncoefficients(70 000-80 000
M-1 cm-1) and can easily be modified with any desired
functionalities. (ii) Absorption peak can be moved to longer
wavelengths through simple modifications, keeping strong
absorption cross sections. (iii) BODIPY dyes have inherent
asymmetry in charge redistribution when they undergo
S0fS1 transition upon excitation, increasing the charge
density on the meso-carbon (C-8), while decreasing it in most
other positions in the boradiazaindacene system (Supporting
Information). This inherent directionality of charge redistri-
bution pinpoints C-8 as the optimal position of charge
injection. (iv) The directionality observed in excitation can
be further enhanced with strategically placed electron-
withdrawing and electron-donating groups, and on the basis
of earlier work,25 cyanoacrylic acid and 4-N,N′-diphenyl-
aminophenyl groups are to be of tremendous utility in this
regard.
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With these considerations, we designed compound 2
(Figure 1). The synthesis is convenient; the BODIPY core
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Figure 1. Structure and numbering of the parent boradiazaindacene
1 and the structure of the target sensitizer.
was synthesized using monoprotected terephthaldehyde under
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Org. Lett., Vol. 10, No. 15, 2008

density shift from the π framework of the BODIPY core
and the styryl substituents to the pendant carboxyphenyl
moiety upon a vertical excitation of the π-π* type.
Compound 2 has a relatively broad absorption peak at
around 700 nm in chloroform solution, consistent with its
charge transfer (ICT) characteristics (Figure 4). The molar
Figure 2. Schematic drawing of a dye-sensitized solar cell: the
BODIPY dye is placed under the F-doped SnO2 layer and adsorbed
to nanocrystalline TiO2.
condensed with N,N′-diphenylaminobenzaldehyde (prepared
from triphenylamine via Vilsmeier formylation). Following
the deprotection of the aldehyde group, it was condensed
with cyanoacetic acid to place electron-withdrawing cyano
and the anchoring carboxylic acid groups in the desired
positions. An important part of the design in this dye is to
place the anchor and cyano group at the C-8 position, where
the charge moves on excitation; a perfect placement for
efficient charge injection.
Figure 4. Overlayed absorption spectrum of the dye 2 in chloroform
(red dashes) and the photocurrent action spectrum (black solid line)
obtained under simulated sunlight (AM 1.5). Nearly constant
photon-to-current conversion level between 400 and 800 nm is
impressive but, at the same time, an indication of aggregation on
the TiO2 surface.
extinction coefficient at the peak wavelength was found to
be 69 500 M-1 cm-1. Two additional transitions at shorter
wavelengths result in a panchromatic performance for this
dye. Cyclic voltammetry of the compound 2 (Supporting
Information) revealed a HOMO energy level of 5.088 eV
and a LUMO energy level of 3.517 eV, which is at
sufficiently high energy for thermodynamically favorable
injection of electrons to nanocrystalline TiO2-CB. The dye-
sensitized cell device was constructed in analogy to literature
(for experimental details, see the Supporting Information).
Characterization of the sandwich type cell (ESI) yielded
short-circuit current (JSC) of 4.03 mA/cm2, open-circuit
voltage (VOC) of 562 mV, and fill factor (FF) of 73.5%,
giving an η of 1.66%. IPCE data (Figure 5) reveal a near
constant photon-to-current conversion efficiency with a peak
DFT calculations for the dye 2, at the B3PW91/6-
31+G(d,p)//B3LYP/6-31G(d) level of the theory, support
directional movement of charge on excitation (Supporting
Information). The HOMO is primarily comprised of π
framework of the BODIPY and the styryl groups, with
significant contributions from π electrons of the dipheny-
lamino substituent (Figure 3); whereas the LUMO is very
Figure 3. Schematic representation of the frontier orbitals (left,
HOMO; right, LUMO) of compound 2 computed at the B3PW91/
6-31+G(d,p)//B3LYP/6-31G(d) level of theory. The surfaces are
generated with an isovalue at 0.02.
Figure 5. Current versus voltage in the constructed cell. Voc ) 562
clearly confined to the π system of the anchor group. (See
Supporting Information for the nearly identical orbital
pictures obtained with other methods.) This overall picture,
found at all levels considered, shows a very clear electron
mV, Jsc ) 4.03 mA/cm2, FF ) 0.735, η ) 1.66%. Cell’s active
surface area was 0.159 cm2, and the illumination intensity was 100
mW/cm2 at 1.5 AM conditions.
Org. Lett., Vol. 10, No. 15, 2008
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value of 22% at 750 nm. This peak value, to the best of our
knowledge, corresponds to the largest reported monochro-
matic incident photon-to-current conversion efficiency in the
near IR region (i.e., at 750 nm) for organic dyes, and second
to only a single tris(thiocyanato)terpyridyl-ruthenium com-
plex dye.26 In addition, we would like to point out that the
reported η value was obtained without any of the additives
typically used (e.g., cheno; 3a,4a-dihydroxy-5b-cholic acid)
for enhancing conversion efficiencies.
In conclusion, we have demonstrated that boradiaza-
indacenes or BODIPYs can be transformed into satisfactory
sensitizers especially in the longer wavelength region of the
visible and near-IR region of the solar spectrum. Further
structural optimization reducing the chances of aggregation
and enhancing the directionality of charge redistribution in
the excited state is very likely to yield more efficient
sensitizers. Our work to that end is in progress.
Acknowledgment. The authors gratefully acknowledge
valuable advice and guidance provided by Prof. Dr. M.
Gratzel and Dr. S. M. Zakeeruddin in EPFL, Switzerland,
and State Planning Organization (DPT) and Turkish Acad-
emy of Sciences (TUBA) for funding.
Supporting Information Available: Synthesis proce-
dures, computational details, molecular orbital plots, details
of cell construction and characterization, and CV data for
the dye. This material is available free of charge via the
Internet at http://pubs.acs.org.
(26) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.;
Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover,
V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gratzel, M. J. Am. Chem.
Soc. 2001, 123, 1613–1624.
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