Solid-state dye-sensitized solar cells using red and near-IR absorbing bodipy sensitizers

Article abstract of DOI:10.1021/ol1014762

Boron-dipyrrin dyes, through rational design, yield promising new materials. With strong electron-donor functionalities and anchoring groups for attachment to nanocrystalline TiO₂, these dyes proved useful as sensitizers in dye-sensitized solar cells. Their applicability in a solid-state electrolyte regime offers additional opportunities for practical applications.

Full text of DOI:10.1021/ol1014762

ORGANIC
LETTERS
2010
Vol. 12, No. 17
3812-3815
Solid-State Dye-Sensitized Solar Cells
Using Red and Near-IR Absorbing
Bodipy Sensitizers
Safacan Kolemen,† Yusuf Cakmak,‡ Sule Erten-Ela,§,| Yigit Altay,†
Johannes Brendel,| Mukundan Thelakkat,| and Engin U. Akkaya*,†,‡
Department of Chemistry, Bilkent UniVersity, Ankara 06800, Turkey, UNAM-Institute
of Materials Science and Nanotechnology, Bilkent UniVersity, Ankara 06800, Turkey,
Institute of Solar Energy, Ege UniVersity, BornoVa, Izmir 35100, Turkey, and
Macromolecular Chemistry I, Applied Functional Polymers, UniVersity of Bayreuth,
95440 Bayreuth, Germany
eua@fen.bilkent.edu.tr
Received June 27, 2010
ABSTRACT
Boron-dipyrrin dyes, through rational design, yield promising new materials. With strong electron-donor functionalities and anchoring groups
for attachment to nanocrystalline TiO2, these dyes proved useful as sensitizers in dye-sensitized solar cells. Their applicability in a solid-state
electrolyte regime offers additional opportunities for practical applications.
Dye-sensitized solar cells (DSSC) are successful alternatives
to more widely used traditional semiconductor-based designs.
The DSSC technology is being vigorously developed through
commercial enterprises.1 In fact, in the European Union
Photovoltaic Roadmap, it was suggested that by the year
2020, DSSCs are expected to be a significant contributor to
renewable electricity generation.2 However, most people
would agree that there is still room for improvement for a
few components of a typical DSSC.3 This is perhaps more
apparent for the electrolyte and the sensitizer dye component
itself. For use as redox mediator, I- and I2 (to generate iodide/
triiodide redox couple) is typically dissolved in organic
solvents (such as acetonitrile). However, the use of solvents
creates temperature stability problems, and because of the
volatility of the solvents, sealing of the cell is crucial. Most
plastics are not compatible with organic solvents, and thus
the use of liquid electrolytes effectively preclude integration
† Department of Chemistry, Bilkent University.
‡ UNAM-Institute of Materials Science and Nanotechnology, Bilkent
University.
§ Ege University.
| University of Bayreuth.
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10.1021/ol1014762  2010 American Chemical Society
Published on Web 08/12/2010

into flexible structures. Also, ruthenium dyes are expensive,
and their preparation includes lengthy purification steps.4
Accurate engineering of the sensitization wavelength would
also benefit from a replacement organic dye. Not surprisingly,
a large number of laboratories around the world are actively
pursuing potential candidates for sensitizers for DSSC
applications.5
Boron-dipyrrin or Bodipy dyes are interesting chro-
mophores with high quantum yields and absorptivity,6
typically with typical bright green fluorescence. We7 and
others8 have found ways to transform these dyes to absorb
essentially all colors of the rainbow and then some. A few
years ago, we published the first report9 of a rationally
functionalized Bodipy-based photosensitizer, taking advan-
tage of some of the superior characteristics of this class of
dyes. Others followed with equally promising Bodipy
derivatives.10 Calculations at various levels of the theory9,11
suggested that excitation of the Bodipy chromophore results
in significant reorganization of the electron distribution,
setting up the scene for efficient electron transfer to nanoc-
rystalline titania from the S1 state of the dye. Needless to
say, further optimization of the Bodipy derivatives may
provide better sensitizers for use in DSSCs.
In order to bypass the limitations imposed by liquid
electrolytes, one of the most common hole transport materials
(HTM) is 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-
9,9′-spirobifluorene (spiro-OMeTAD).5l,12 In this work, our
goal was to investigate the performance of rationally designed
boron-dipyrrin sensitizers in connection with spiro-OMeTAD
hole transport material.
In our previous work,9 we synthesized sensitizer 1 (Figure
1) and reported its efficiency in a standard DSSC setup using
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Figure 1. Sensitizers used in this study.
a iodide/triiodide redox couple in solution as electrolyte. In
this work, however, we targeted two more boron-dipyrrin
dyes, compounds 2 and 3, in an attempt to clarify relative
effects of various modifications on the efficiency. The
rationale behind the two new sensitizers was as follows. In
compound 1, the meso-phenyl substituent is orthogonal as a
result of the presence of methyl groups at the 3 and 5
positions of the Bodipy core. It is very likely that a new
sensitizer in which protruding methyls are not present (such
as sensitizer 2) could have the phenyl substituent with a
smaller dihedral angle, leading to extended conjugation and
facilitated charge transfer from the donor groups to the
electron-withdrawing (and anchoring) carboxylic acid ter-
minal. In addition, in sensitizer 2, we placed additional
electron-donor p-methoxy groups on the diphenylaminophe-
nyl charge donor moiety, again looking for a more efficient
excited state charge transfer. In the design of sensitizer 3,
we included two decyl chains on the meso-phenyl substituent
in order to minimize aggregation-induced losses in efficiency.
In addition, a cyanoacetic acid derived electron-withdrawing
anchor group was moved to position 2 of the Bodipy core.
It is apparent that in this design the cyano acetylidene group
will be in full conjugation with the Bodipy chromophore.
The syntheses of the novel sensitizers 2 and 3 were based
on versatile Bodipy chemistry. 8-Carboxyphenyl-Bodipy (4)
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3813

was synthesized from appropriate precursors, and then double
Knoevenagel condensation reactions with the appropriate
diphenylaminophenylbenzaldehyde compound resulted in the
sensitizer 2, following rather routine purification procedures.
In the synthesis of sensitizer 3, we first prepared 3,5-
didecyloxyphenyl-substituted Bodipy 5.7b Formylation fol-
lowing the procedure in a recent report13 resulted in
compound 6. Cyanoacetic acid reacts with the formyl-bodipy
6 in toluene, resulting in compound 7. In the final step, a
double Knoevenagel condensation with the appropriate
aldehyde yields the target sensitizer 3. All new compounds
were analytically pure (Supporting Information).
(Table 1). It is clear that the LUMO energies of the
sensitizers are appropriate for efficient electron injection
to TiO2.
Table 1. Optical and Electrochemical Properties of Sensitizers
1-3
b
b
λmax (abs)a
Eox
Ered
HOMOb LUMOb
a
dye
(nm)
εmax
(mV) (mV)
(eV)
(eV)
1
2
3
699
746
695
69 500
66 000
79 000
680
560
720
-890
-870
-940
-5.09
-5.05
-5.21
-3.52
-3.62
-3.55
First the absorbance spectra in solution (CHCl3) (Figure
2) and as adsorbed on TiO2 (Figure 3) were obtained. The
a Absorption data were collected in CHCl3. b Electrochemical data were
collected in CH2Cl2. Potentials are quoted with reference to the internal
ferrocene standard.
The solid-state cells were prepared as in the previous
reports. For a brief procedure, please see Supporting
Information. Incident photon to current conversion plots were
obtained under standard conditions (AM 1.5G, 100 mW
cm-2). The results show that the sensitizer 1 has the highest
efficiency (Figure 4). Only beyond 800 nm, sensitizer 2 has
Figure 2. Normalized absorption spectra of the sensitizers in CHCl3.
Figure 4. Incident photon to current conversion efficiency as a
function of wavelength for the solid-state DSSCs prepared as
described in Supporting Information.
IPCE values higher than those of 1. For sensitizer 3, IPCE
values are below 1% in the 450-850 nm region. The data
can be interpreted as follows. As we suggested previously,
the meso position (8 position) is particularly important. In
the parent Bodipy and in other derivatives, theoretical
calculations suggest that on excitation there is significant
charge relocalization on the meso carbon.9 Sensitizers 1 and
2 take advantage of this natural Bodipy tendency for charge
relocalization onto meso-carbon, by placing electron-accep-
tor/anchor groups on that position. The sensitizer 3 has the
anchor group on a different position, forcing electron flow
to an alternate position, apparently reducing the efficiency
of charge injection. The data also suggest that the cyanoacetic
acid derived anchor is better than a simple carboxylic acid
group in their dual role of charge-withdrawing and anchoring
Figure 3. Normalized absorption spectra of the sensitizers adsorbed
on nanocrystalline TiO2.
chromophores in solution have strong absorption peaks in
the red and near-IR regions of the visible spectrum. As
expected, on absorption over titania, peaks are significantly
broadened, suggesting aggregation of the sensitizers in the
adsorbed film.
The sensitizers 1-3 were further characterized by cyclic
voltammetry and absorption spectroscopy in solution
(13) Jiao, L.; Yu, C.; Li, J.; Wang, Z.; Wu, M.; Hao, E. J. Org. Chem.
2009, 74, 7525.
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Org. Lett., Vol. 12, No. 17, 2010

Table 2. DSSC Performance Parameters of BODIPY Dyesa
Scheme 2. Synthesis of Sensitizer 3
dye
Voc (V)
Jsc (mA cm-2
)
f
η (%)
1
2
3
0.80
0.64
0.59
2.27
1.61
1.49
0.37
0.28
0.38
0.68
0.28
0.33
a Voc is the open-circuit potential, Jsc is the short curcuit current, f is the
fill factor, and η is the overall efficiency of the cell under standard
conditions.
Scheme 1. Synthesis of Photosensitizer 2
properties only loosely translate into properties on titania.
However, Bodipy-based sensitizers still hold significant
promise as they can be derivatized as desired and the
absorption peaks can be moved along the visible and near-
IR region. It is interesting to note that one of the most
efficient dyes in terms of monochromatic incident photon to
current conversion is indeed a Bodipy dye. It looks like
Bodipy dyes, which are known for their bright fluorescence,
are likely to find novel applications as photosensitizers. We
will continue in fine-tuning the Bodipy structure toward ever
more efficient dyes for dye-sensitized solar cells through
rational design.
group. As expected, the sensitizers have very low fluores-
cence emissions due to strong charge transfer characteristics
of the diphenylaminophenyl substituent.
What is also remarkable is the near flat response of these
sensitizers in the visible wavelengths. Actually, the response
is one that could be expected from a black dye.
Table 2 lists some cell parameters for the solid-state
DSSCs prepared using these sensitizers. The overall conver-
sion efficiency is highest for the sensitizer 1 (η ) 0.68%),
which could be considered a respectable value for an organic
dye with a solid-state redox mediator. It appears that solution
Supporting Information Available: Experimental pro-
cedures, structural proofs, and additional spectral data for
all new compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL1014762
Org. Lett., Vol. 12, No. 17, 2010
3815