Spirobifluorene-bridged donor/acceptor dye for organic dye-sensitized solar cells

Article abstract of DOI:10.1021/ol902314z

Chemical Equation Presented A new dye, SSD1, featuring two donor/acceptor chromophores aligned in a spiro configuration with two anchoring groups separated at a distance of 10.05 A (closely matching the distance between the adsorption sites of the anatase TiO₂ surface) undergoes efficient dye adherence on TiO₂ films. A dye-sensitized solar cell incorporating SSD1 exhibited a short-circuit current of 8.9 mA cm^-2, an open-circuit voltage of 0.63 V, a fill factor of 0.67, and a power conversion efficiency of 3.75%.

Full text of DOI:10.1021/ol902314z

ORGANIC
LETTERS
2010
Vol. 12, No. 1
12-15
Spirobifluorene-Bridged Donor/Acceptor
Dye for Organic Dye-Sensitized Solar
Cells
Daniel Heredia, Jose Natera, Miguel Gervaldo, Luis Otero, Fernando Fungo,*
Chi-Yen Lin, and Ken-Tsung Wong*
Departamento de Qu´ımica, UniVersidad Nacional de R´ıo Cuarto, Agencia Postal 3
(5800), R´ıo Cuarto, Argentina, and Department of Chemistry, National Taiwan
UniVersity, Taipei 106, Taiwan
ffungo@exa.unrc.edu.ar; kenwong@ntu.edu.tw
Received October 6, 2009
ABSTRACT
A new dye, SSD1, featuring two donor/acceptor chromophores aligned in a spiro configuration with two anchoring groups separated at a
distance of 10.05 Å (closely matching the distance between the adsorption sites of the anatase TiO2 surface) undergoes efficient dye adherence
on TiO2 films. A dye-sensitized solar cell incorporating SSD1 exhibited a short-circuit current of 8.9 mA cm-2, an open-circuit voltage of 0.63
V, a fill factor of 0.67, and a power conversion efficiency of 3.75%.
The growing worldwide demand for environment friendly
energy sources has led to greater interest in solar energy
conversion devices. Dye-sensitized solar cells (DSSCs) have
been the subject of much research1 since their report by
Regan and Gratzel et al. in 1991.2 The performance and
relatively low production costs render DSSCs as competitive
alternatives to conventional silicon-based photovoltaic de-
vices.3 A crucial issue in DSSC design is the choice of dye
used to capture the solar energy. Although many novel
structures have been developed and tested as light absorbers
in DSSCs, polypyridyl Ru(II) complexes remain dominant
in the most successful systems.4 However, Ru-based dyes
are expensive and hard to purify relative to organic sensitiz-
ers. Thus, the search for new, highly efficient metal-free dyes
remains an active aspect of DSSC development.3 Organic
dyes have many advantageous features, such as the huge
diversity of molecular structures available and the possibility
of obtaining materials at relatively low cost. Moreover,
because organic dyes normally possess higher molar extinc-
tion coefficients than Ru dyes (< 20 000 M-1 cm-1), they
allow thinner nanostructured oxide semiconductor films to
be prepared with comparable light-harvesting efficiencysa
key factor for practical solid state DSSCs. Another important
advantage of organic dyes is that they can be used to
construct semitransparent and/or multicolor solar cells, which
are potentially useful, for example, in power-producing
windows. DSSCs incorporating dyes that are transparent over
a region of the visible spectrum would allow part of the
visible light to enter a building while simultaneously
converting some of the solar power into electricity.
(1) Gunes, S.; Sariciftci, N. S. Inorg. Chim. Acta 2008, 361, 581.
(2) Regan, B. O.; Gra¨tzel, M. Nature 1991, 353, 737.
(3) Moreira Gonc¸alves, L.; de Zea Bermudez, V.; Aguilar Ribeiro, H.;
Magalha˜es Mendes, A. Energy EnViron. Sci. 2008, 1, 655.
(4) Gra¨tzel, M. J. Photochem. Photobiol. C 2003, 4, 145.
A common strategy in the design of highly efficient
organic dyes for DSSCs is the linking of electron donor/
10.1021/ol902314z  2010 American Chemical Society
Published on Web 12/04/2009

acceptor (DA) systems through conjugated spacers, with TiO2
surface anchoring groups (e.g., carboxylate) integrated onto
the acceptor moiety.5 Irradiation of these bipolar molecules
generates photoinduced intramolecular charge transfer states,
which can inject electrons to the TiO2 conduction band. The
preferential orientation of the dye on the surface not only
improves charge injection but also maintains the photo-
oxidized donor at a distance from the photoinjected electrons,
diminishing the impact of deleterious back electron transfer
processes. Using this strategy, many bipolar metal-free dyes
have been incorporated as efficient sensitizers in DSSCs in
recent years.6-10 Diphenylamine (DPA) and cyanoacrylic
acid moieties are among the most commonly employed
subunits for the electron donor and electron acceptor/
anchoring groups, respectively, in the design of organic dyes
for highly efficient DSSCs.9-14 Variation of the conjugated
spacer connecting the donor and acceptor has resulted in dyes
reaching remarkable incident photon-to-current efficiencies
(IPCEs) and energy conversion efficiencies.15 The nature of
the π-conjugated bridge in the dye influences not only the
region of light absorbed by the DSSCs but also the degree
of electron injection from the dye’s excited state to the TiO2
surface. A common strategy for efficient harvesting of solar
energy is the use of long π-conjugated bridges that red-shift
the absorption spectra.5 The presence of lengthy rodlike
structures, however, can lead to aggregation and, therefore,
self-quenching and inefficient electron injection into the
TiO2.13,16 These deleterious processes can be avoided, or
suppressed, when the dye molecules possess bulky structural
features that impede aggregation.
than their single components. Spiro linkages are particularly
beneficial for fluorescent emitters, suppressing the excimer
formation that is frequently encountered in the solid state.
Molecules possessing a spirobifluorene core and tailor-made
optical and redox properties are used widely in organic
optoelectronics.17 For example, 2,2′,7,7′-tetrakis(N,N-di-p-
methoxyphenylamino)-9,9′-spirobifluorene (spiro-MeOTAD)
is one of the most efficient organic hole transporters used in
the development of solid-state DSSC.18 Nevertheless, it is
rare for DSSC dyes to be arranged in a spiro configuration,
even though fluorene-containing dyes have been incorporated
into DSSCs.14,19-21 In this study, we synthesized the
spirobifluorene-based bipolar dye SSD1 (Scheme 1) featuring
Scheme 1. Synthesis of SSD1
a diphenylamino (DPA) group as the donor and a 2-cy-
anoacrylic acid unit as the acceptor, with a fluorene bridge
ensuring efficient DA interactions. This novel dye possesses
several unique features that are potential advantages for
application in DSSCs. For example, SSD1 is composed of
two DA branches in a rigid cross-shaped molecular structure,
which not only doubles the light absorption efficiency but
also minimizes dye aggregation. In addition, two anchoring
carboxylate groups are presentsone at each electron acceptor
sitesto improve dye adsorption on basic TiO2 surfaces and
to direct the photoinduced electron injection.15
Scheme 1 outlines our synthesis of SSD1. 2,2′-Diformyl-
7,7′-bisdiphenyamino-9,9′-spirobifluorene (1)22 was reacted
with cyanoacetic acid and ammonium acetate in glacial acetic
acid at 135 °C for 24 h. The product was purified through
reprecipitation from EtOAc and hexane to afford SSD1 as a
red solid in 95% yield.
The bridging of two chromophores perpendicularly via an
sp3-hybridized atom into a spiro configuration allows the
constituted π-systems to retain their individual electronic
properties (e.g., absorption and emission characteristics). In
addition, the high steric demand resulting from the rigid
structure efficiently suppresses intermolecular interactions,
thereby diminishing the tendency to form aggregates;17 as a
corollary, spiro-configured compounds have higher solubility
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Figure 1a presents the absorption and emission spectra of
SSD1 in MeCN. We observed two distinct absorption bands:
one in the UV region (300 nm), corresponding to the π-π*
electronic transition of the spirobifluorene core, and the other
in the visible region (392 nm; ε392 ) 4.7 × 104 cm-1 mol-1),
corresponding to the strong DA intramolecular charge
.
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Org. Lett., Vol. 12, No. 1, 2010
13

Figure 2. (a) Geometric optimization (AM1) of the structure of
SSD1 and (b) HOMO and LUMO frontier molecular orbitals of
SSD1 obtained from AM1 semiempirical calculations.
Figure 1. Normalized absorption (black) and emission (red) spectra
of SSD1 in MeCN.
transfer (ICT) band,23 as confirmed by the strong reverse
solvatochromism [λmax: 446 nm (toluene), 427 nm (THF),
Figure S-1, Supporting Information]. SSD1 features an
emission band centered at 535 nm in MeCN. The absorption
spectrum of SSD1 adsorbed onto fluorine-doped tin oxide
(FTO)/TiO2 nanostructured films; the signals are broader and
red-shifted (ca. 30 nm) relative to those in solution (MeCN)
(Figure S-2, Supporting Information), presumably because
of the interaction between the dye and the nanostructured
TiO2. In addition, the dye exhibits strong absorption in the
blue region, giving a film that was light yellow in color. More
importantly, a high amount of SSD1 remained on the
electrode surface after several solvent rinses (electrode
absorbance was as high as 3.2 after 72 h soaking, and longer
soaking time does not change the amount of uploaded dye).
We ascribe the high anchoring power of SSD1 to its two
carboxylic groups, which enable the formation of mono- or
bidentate binding states with TiO2.24,25 The FT-IR spectra
(Figure S-3, Supporting Information) reveal that a broadband
at 1685 cm-1 assigned to the carboxylic acid groups of free
SSD1 completely disappears as SSD1 absorbed on the TiO2
surface, providing clear evidence for the double-anchoring
behavior of SSD1. This result agrees with the observation
recently reported by Abbotto et al.26
of SSD1 is localized mostly on the diphenylamino groups,
whereas the lowest unoccupied molecular orbital (LUMO)
is localized on the cyanoacrylic acid units. Thus, we
anticipated that efficient photoinduced charge separation and
the relatively stronger anchoring power would enhance the
degree of electron injection to the TiO2 electrode.
We used cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) with Pt as the working electrode to probe
the redox properties of SSD1. When we scanned the potential
between -0.50 and +0.75 V, a reversible redox wave
appeared at a value of E1/2 of 0.55 V (vs ferrocene/
ferrocenium redox couple; equal to 1.10 V vs Normal
Hydrogen Electrode (NHE)). We obtained a similar oxidation
potential when using DPV (Figure S-4, Supporting Informa-
tion). Taking into account the optical band gap (2.75 eV) of
SSD1, we calculated the LUMO energy level to be -1.65
V (vs NHE). The energy levels of TiO2, SSD1, and the I-/
I3- redox couple reveal that both electron injection into TiO2
from the dye’s excited states and dye reduction by the I-/
I3- couple are exothermic, making DSSC operation feasible
(Figure S-5, Supporting Information).
Figure 3 presents the short circuit photocurrent action
spectra in terms of the IPCEs28 of SSD1-DSSCs with
AM1 semiempirical optimization revealed that the distance
between the two anchoring acid groups was ca. 10.05 Å
(Figure 2a), close to that between adsorption sites on the
anatase TiO2 surface (10.23 Å).24,25 One important feature
of the popular Ru-based dye N3 is its strong adherence to
TiO2 surfaces through its carboxylic groups, which also allow
electronic coupling for electron injection.15,25 Interestingly,
the anchoring groups in N3 are separated by 10.0 Å.24
Therefore, it is likely that SSD1 possesses a suitable
structural configuration for efficient surface adherence and
subsequent electron transfer. Moreover, the frontier molecular
orbitals we obtained through AM1 calculations27 (Figure 2b)
revealed that the highest occupied molecular orbital (HOMO)
Figure 3. Short circuit photocurrent action spectra (IPCE%) of
SSD1-containing DSSCs obtained after various soaking times (from
a few minutes to 72 h). Inset: change in IPCE% at a wavelength of
405 nm with the electrode absorbance at the same wavelength.
(24) Shklover, V.; Ovchinnikov, Y. E.; Braginsky, L. S.; Zakeeruddin,
S. M.; Gra¨tzel, M. Chem. Mater. 1998, 10, 2533
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(25) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269
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(26) Abbotto, A.; Manfredi, N.; Marinzi, C.; De Angelis, F.; Mosconi,
E.; Yum, J.-H.; Xianxi, Z.; Nazeeruddin, M. K.; Gra¨tzel, M. Energy EnViron.
Sci. 2009, 2, 1094.
different absorbances.29 The IPCE of the resulting photoac-
tive layer shows that SSD1 adsorbed on TiO2 successfully
extended the photocurrent response into the visible region.
(27) Balanay, M. P.; Dipaling, C. V. P.; Lee, S. H.; Kim, D. H.; Lee,
K. H. Sol. Energy Mater. Sol. Cells 2007, 91, 1775.
14
Org. Lett., Vol. 12, No. 1, 2010

However, the differences observed between the IPCE and
absorption spectrum of TiO2/SSD1 indicate the presence of
various surface interaction modes, which can affect the
charge injection and recombination efficiencies. Interestingly,
the IPCE grew continuously as the absorbance increased
(inset to Figure 3). The IPCE is given by the equation IPCE
) LHEꢀinjηc, where LHE is the light-harvesting efficiency
(LHE ) 1 - T ) 1 - 10-A), ꢀinj is the quantum yield of
electron injection, and ηc is the efficiency of collecting the
injected electrons at the photoanode. If ꢀinj and ηc are
constant for a given system, the IPCE should reach a plateau
at absorbance values greater than one. In this study, however,
the relationship between IPCE and absorbance indicates that
ꢀinjηc is not constant. Long soaking times allow the TiO2-
coated electrode to accumulate large amounts of SSD1;
therefore, because of the high value of ε of the SSD1 dye,
the incident light was efficiently extinguished within a thin
layer close to the illuminated FTO. Thus, the electrons that
were injected from SSD1 into the nanoporous TiO2 film had
to cross only a short pathway to reach to the FTO, producing
a concomitant greater value of ηc.8 However, with the present
evidence, we can not exclude the changes in the injection
efficiency along with the amount of dye adsorbed on TiO2.
Nevertheless, the result indicates that even at a high dye
concentration, where a close packing of dye on the TiO2 film
exists, there is a low tendency for self-quenching of the dye’s
excited state through dye aggregation, consistent with steric
impediment originating from the spiro configuration.
To compare the performance of SSD1 relative to that of
widely used dye N3, we constructed a series of DSSCs
featuring thin TiO2 layers (ca. 1.5 µm) deposited through
spin-coating. For both dyes, the electrode absorbances were
less than one. We estimated the value of (ꢀinjηc)N3/(ꢀinjηc)SSD1
to be 1.12, taken at the maximum absorption wavelength of
the electrodes (520 and 418 nm for N3 and SSD1, respec-
tively). This value indicates that the photoinduced electron
injection efficiency and electron collection of the DSSCs
assembled with SSD1 were similar to those of devices based
on the dye N3, which exhibits its very good performance in
part because of its ability to keep its positive charge away
from the TiO2 surface, thereby avoiding deleterious back
electron transfer.8 In the case of SSD1, the photogenerated
positive charge is located on the diphenylamino units, which
are positioned at a significant distance from the TiO2 surface
(Figure 2b). In addition, the two carboxylate anchoring
groups of SSD1 not only align with the TiO2 coordination
sites to provide strong bivalent binding but also separate the
electrode surface from the photo-oxidized electron donor
moieties, diminishing electron/hole recombination. Figure 4
displays the current-voltage curve of the SSD1 DSSC (TiO2
film thickness: ca. 7 µm) under AM 1.5 G simulated sunlight.
Figure 4. Current-voltage curves for DSSCs incorporating SSD1
(red) and N3 (black) under AM 1.5 G simulated sunlight.
The cell exhibits a short-circuit current of 8.9 mA cm-2, an
open-circuit voltage of 0.63 V, a fill factor of 0.67, and a
power conversion efficiency of 3.75%; for the corresponding
N3 DSSC, these values were 16.5 mA cm-2, 0.61 V, 0.71,
and 7.0%, respectively. From the point of view of practical
applications, SSD1 is not panchromatic: only part of the solar
spectrum is absorbed. This situation could be rectified,
however, through the synthesis of spirobifluorene DA dyes
possessing extended π backbones. On the other hand, as the
photograph in Figure 4 indicates, the SSD1 DSSC is yellow,
suggesting that it might serve in a semitransparent power-
producing window.
In summary, we have synthesized and characterized a
spiro-configured DA sensitizer, SSD1, for use in DSSCs. The
two anchoring groups of SSD1 are arranged perpendicularly
at a distance of ca. 10.05 Å, comparable to that between the
two acids in the popular dye N3 and a close match to the
distance between the adsorption sites on the anatase TiO2
surface (10.23 Å). Strong binding to TiO2 and suppressed
aggregation, due to steric impediment originating from its
spiro configuration, make SSD1 a promising candidate for
use in efficient DSSCs. To the best of our knowledge, this
paper provides the first example of a DSSC dye based around
a DA spirobifluorene structure. We hope that this molecular
design triggers further interest in using spiro-configured
compounds in DSSCs.
Acknowledgment. We thank Consejo Nacional de Inves-
tigaciones Cient´ıficas y Te´cnicas (CONICET-Argentina),
Agencia Nacional de Promocio´n Cient´ıfica y Tecnolo´gica
(ANPCYT-Argentina), Secretar´ıa de Ciencia y Te´cnica
Universidad Nacional de R´ıo Cuarto (SECYT-UNRC), and
the National Science Council of Taiwan for financial support.
LO, MG, and FF are scientific members of CONICET.
Supporting Information Available: Detailed experimen-
tal procedures; spectroscopic characterization, solvent-de-
pendent absorption, cyclic voltammogram, and NMR spectra
of SSD1; and energy levels alignments. This material is
available free of charge via the Internet at http://pubs.acs.org.
(28) Vlachopoulos, P.; Liska, P.; Augustynski, J.; Gra¨tzel, M. J. Am.
Chem. Soc. 1988, 110, 1216.
(29) The electrodes were obtained by soaking the TiO2 films in dye
solution, from a few minutes to 72 h, providing surfaces presenting different
amounts of adsorbed dye.
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