New D-π-A-conjugated organic sensitizers based on 4 H-pyran-4-ylidene donors for highly efficient dye-sensitized solar cells

Article abstract of DOI:10.1021/ol203298r

We have synthesized a series of four new promising D-π - A conjugated organic sensitizers with a proaromatic 4H-pyran-4-ylidene as a donor, a thiophene ring in the bridge, and 2-cyanoacrilic acid as acceptor. Comparison between different donor substituent

Full text of DOI:10.1021/ol203298r

ORGANIC
LETTERS
2012
Vol. 14, No. 3
752–755
New DꢀπꢀA-Conjugated Organic
Sensitizers Based on 4H-Pyran-4-ylidene
Donors for Highly Efficient Dye-Sensitized
Solar Cells
,†
Santiago Franco,* Javier Garın, Natalia Martınez de Baroja, Raquel Perez-Tejada,
†
†
†
ꢀ
´
´
Jesus Orduna, Youhai Yu, and Monica Lira-Cantu*
†
‡
,‡
ꢀ
ꢀ
ꢀ
´
ꢀ
Departamento de Quımica Organica, ICMA, Universidad de Zaragoza-CSIC, E50009
ꢀ
ꢁ
Zaragoza, Spain, Centre d’Investigacio en Nanociencia i Nanotecnologia
(CIN2, CSIC), Laboratory of Nanostructured Materials for Photovoltaic Energy,
Campus UAB E08193 Bellaterra, Barcelona, Spain
sfranco@unizar.es; monica.lira@cin2.es
Received December 9, 2011
ABSTRACT
We have synthesized a series of four new promising DꢀπꢀA conjugated organic sensitizers with a proaromatic 4H-pyran-4-ylidene as a donor, a
thiophene ring in the bridge, and 2-cyanoacrilic acid as acceptor. Comparison between different donor substituents and the modification of the
thiophene ring resulted in molar extinction coefficients as high as 36399 Mꢀ1 cmꢀ1 at 551 nm. The photovoltaic properties of the DSSCs
demonstrate power conversion efficiencies as high as 5.4%.
The conversion of solar energy to electricity appears as
one of the renewable energy sources that can replace fossil
fuels and directly concerns global warming. Recent pro-
gress on nanocrystalline TiO2-based dye-sensitized solar
cells (DSSCs) presents an important alternative to current
solar technology because of their low-cost, lightweight,
semitransparent characteristics, and potential applica-
tions.1 Inthesecells, the sensitizerisoneof the key elements
forhighpower conversionefficiency. Thesedevicesemploy
mostly ruthenium complexes as charge-transfer sensitizers.
Until now, only three prototype Ru complexes N3, N719
and the black dye have achieved power conversion effi-
ciencies over 10%. Although the ruthenium dyes exhibited
high efficiency and long-term stability, they contain a
limited precious metal and they are hard to purify. In this
context, organic dyes are considered to be alternative
photosensitizers in DSSCs owing to their ease in manu-
facturing and structural flexibility toward modification.
Organic dyes normally exhibit molar extinction coeffi-
cients surpassing those of ruthenium dyes and are expected
† Universidad de Zaragoza-CSIC.
‡
ꢀ
ꢁ
Centre d'Investigacio en Nanociencia i Nanotecnologia.
(1) (a) O’Regan, B.; Graetzel, M. Nature (London) 1991, 353, 737.
(b) Goetzberger, A.; Hebling, C.; Schock, H.-W. Mater. Sci. Eng. 2003,
R40, 1. (c) Robertson, N. Angew. Chem., Int. Ed. 2006, 45, 2338.
(d) Imahori, H.; Umeyama, T.; Ito, S. Acc. Chem. Res. 2009, 42, 1809.
(e) Yanagida, S.; Yu, Y. H.; Manseki, K. Acc. Chem. Res. 2009, 42, 1827.
(f) Boschloo, G.; Hagfeldt, A. Acc. Chem. Res. 2009, 42, 1819.
(g) O’Regan, B. C.; Durrant, J. R. Acc. Chem. Res. 2009, 42, 1799.
(h) Mishra, A.; Fischer, M. K. R.; Bauerle, P. Angew. Chem., Int. Ed.
2009, 48, 2474. (i) Hagfeldt, A.; Boschloo, G.; Sun, L. C.; Kloo, L.;
Pettersson, H. Chem. Rev. 2010, 110, 6595.
r
10.1021/ol203298r
Published on Web 01/20/2012
2012 American Chemical Society

to achieve maximum power conversion efficiencies of
20.25% with absorption onset at 940 nm.2 The latter make
these dyes ideal molecular systems for small-molecule
organic photovoltaics (OPVs).3 Organic dyes with high
extinction coefficients are also of high interest in solid-
state dye-sensitized solar cells (ss-DSSCs) due to the req-
uirement for thin TiO2 electrodes, viscous electrolytes, or
solid hole conductors.
An important factor that causes low conversion effi-
ciency of many organic dyes in the DSSCs is the formation
of aggregates on the TiO2 surface. This aggregation needs
to be avoided, for instance, by adding deoxycholic acid
(DCA) as a coadsorbate4 or by appropiate structural
modifications, usually the incorporation of long alkyl
chains in the spacer. It has been described that the number
and position of alkyl chains linked to the thiophene
improved the open circuit photovoltage (Voc) due to an
effective reduction of charge recombination processes.5
Recently, Tian and co-workers have reported that the
aggregation between molecules can also be suppressed
incorporating non planar (starburst) triarylamines in the
donor unit.6
4H-pyran-4-ylidene-unit, RPT-9 and SFO-346 dyes, re-
spectively. The thiophene groups have been modified by
the addition of two bulky hexyl groups, the NAT-440 and
NAT-622 dyes, respectively (Figure 1). The dyes can be
easily synthesized in moderate yields by well-known or-
ganic reactions including WittigꢀHorner, Knoevenagel,
and formylation reactions. The synthetic protocols and
characterization of the dyes are provided in the Supporting
Information.
Based on these ideas, in this communication we have
designed and synthesized four new promising D-π-A con-
jugated organic sensitizers (RPT-9, NAT-440, NAT-622,
and SFO-346) with a 4H-pyran-4-ylidene as a donor, a
thiophene ring in the bridge and 2-cyanoacrilic acid as
acceptor. The choice of this donor moiety relies on its
proaromatic character,7 that is expected to improve the
charge transfer process through the gain in aromaticity
experienced by the donor fragment. The deliberate use of
proaromatic donors for DSSCs has not yet been explored,
although the photovoltaic properties of some benzothia-
zolylidene merocyanines8 and other pyran derivatives9
have been recently reported.
The aim of this work is the study of the effect of the side-
chain modification of these dyes on the photovoltaic pro-
perties of dye-sensitized solar cells. Thus, we havemodified
the donor and the thiophene groups in these dyes. The
donor group has been exchanged between a phenyl and a
tert-butyl group, anchored to the positions 2 and 6 of the
Figure 1. Molecular structures of the 4H-pyran-4-ylidene-based
dyes: (a) RPT-9, (b) NAT-440, (c) NAT-622, and (d) SFO-346.
Table 1 shows the absorption and electrochemical prop-
erties of the synthesized dyes. The UVꢀvis spectra of the
dyes in CH2Cl2 (Figure 2) show an intense absorption
band which can be attributed to the intramolecular charge
transfer from the proaromatic donor to cyanoacrylic acid.
The high molar extinction coefficient absorption bands are
in favor of light-harvesting and hence photocurrent gen-
eration in DSSCs.
The redox potentials of the sensitizers were measured by
differential pulse voltammetry in CH2Cl2. As shown in
Table 1, the excited-state reduction potential (calculated
from Eox ꢀ E0ꢀ0) of the four dyes is more negative than the
conduction band edge (ꢀ0.5 V vs NHE) and the ground-
state oxidation potential is in all cases higher than the
redox potential of the iodide/triiodide couple (þ0.4 V vs
NHE). These values are adequate for an effective electron
injection into the semiconductor and ensure the regenera-
tion of the oxidized form of the dyes in a DSSC.
(2) Ito, S.; Zakeeruddin, S. M.; Humphry-Baker, R.; Liska, P.;
Charvet, R.; Comte, P.; Nazeeruddin, M. K.; Pechy, P.; Takata, M.;
Miura, H.; Uchida, S.; Gratzel, M. Adv. Mater. 2006, 18, 1202.
(3) Buerckstuemmer, H.; Tulyakova, E. V.; Deppisch, M.; Lenze,
M. R.; Kronenberg, N. M.; Gsaenger, M.; Stolte, M.; Meerholz, K.;
Wuerthner, F. Angew. Chem., Int. Ed. 2011, 50, 11628.
(4) Hara, K.; Dan-oh, Y.; Kasada, C.; Ohga, Y.; Shinpo, A.; Suga, S.;
Sayama, K.; Arakawa, H. Langmuir 2004, 20, 4205.
(5) (a) Wang, Z. S.; Koumura, N.; Cui, Y.; Takahashi, M.; Sekiguchi,
H.; Mori, A.; Kubo, T.; Furube, A.; Hara, K. Chem. Mater. 2008
20, 3993. (b) Choi, H.; Baik, C.; Kang, S. O.; Ko, J.; Kang, M. S.;
Nazeeruddin, M. K.; Gratzel, M. Angew. Chem., Int. Ed. 2008, 47, 327.
(6) (a) Ning, Z. J.; Zhang, Q.; Wu, W. J.; Pei, H. C.; Liu, B.; Tian, H.
J. Org. Chem. 2008, 73, 3791. (b) Ning, Z. J.; Tian, H. Chem. Commun.
2009, 5483.
(7) Andreu, R.; Carrasquer, L.; Franco, S.; Garı
Martınez, de B. N.; Alicante, R.; Villacampa, B.; Allain, M. J. Org.
Chem. 2009, 74, 6647.
´
n, J.; Orduna, J.;
´
(8) Sayama, K.; Tsukagoshi, S.; Hara, K.; Ohga, Y.; Shinpou, A.;
Abe, Y.; Suga, S.; Arakawa, H. J. Phys. Chem. B 2002, 106, 1363.
(9) (a) Awuah, S. G.; Polreis, J.; Prakash, J.; Qiao, Q.; You, Y.
J. Photochem. Photobiol., A 2011, 224, 116. (b) Bolag, A.; Nishida, J.;
Hara, K.; Yamashita, Y. Chem. Lett. 2011, 40, 510.
The molecular geometries of these new organic dyes in
CH2Cl2 have been optimized using the B3LYP hybrid
Org. Lett., Vol. 14, No. 3, 2012
753

Table 1. Absorption, Fluorescence, and Electrochemical Properties of the Dyes
dye
λabsa (nm)
λema (nm)
ε (Mꢀ1 cmꢀ1
)
Eoxb (V) (vs NHE)
E0ꢀ0c (eV)
Eredd (V) (vs NHE)
a
b
c
RPT-9
556
580
584
551
652
667
629
627
33899
28840
23700
36399
þ0.81
þ0.83
þ0.85
þ0.86
2.00
1.94
2.03
2.05
ꢀ1.19
ꢀ1.11
ꢀ1.18
ꢀ1.19
NAT-440
NAT-622
SFO-346
d
a Absorption and emission spectra were measured in CH2Cl2 solutions (10ꢀ5 M) at room temperature. b The oxidation potentials, measured in CH2Cl2
with 0.1 M TBAPF6 as electrolyte and Ag/AgCl as reference electrode, were converted to normal electrode (NHE) by addition of 0.199 V. c E0ꢀ0 was
estimated fromthe intersectionbetween the absorptionandemissionspectra. d The estimated reductionpotential ofthe dye was calculatedfromEox ꢀ E0ꢀ0
.
Table 2. SMD TD-B3LYP/6-31G** Calculation Results in
CH2Cl2
HOMOꢀ
λmax
(nm)
HOMO LUMO
LUMO gap
(eV)
dye
f
(eV)
(eV)
a
b
c
RPT-9
561
571
534
523
1.23
1.11
1.02
1.04
ꢀ4.95
ꢀ4.85
ꢀ4.81
ꢀ4.92
ꢀ2.56
ꢀ2.51
ꢀ2.36
ꢀ2.43
2.39
2.34
2.45
2.49
NAT-440
NAT-622
SFO-346
d
(TGA) under nitrogen, with a heating rate of 10 °C/min.
The decomposition temperatures (Td) were estimated as
the temperature that is the intercept of the leading edge of
the weight loss by the baseline of the TGA scans. The
results show that all the dyes described in this communica-
tion are thermally stable with decomposition temperatures
higher than 135 °C (Td (°C): RPT-9, 205; NAT-440, 189;
NAT-622, 135; SFO-346, 185), in principle adequate for a
photovoltaic device.
The photovoltaic performance of the sensitizers, i.e., the
incident photon-to-current conversion efficiency (IPCE)
and the photocurrentꢀvoltage (JꢀV) curve, was carried
out in test devices using standard mesoporous 4 μm thick
TiO2 films with the iodolyte AN-50 (Solarionix) as the
electrolyte. The dye adsorption on TiO2 was carried out in
dichloromethane. The IPCE graphs in Figure 3 show a
IPCE peak at 440 nm for the RPT-9 and SFO-346 with
values of 53% and 62%, respectively. In the case of
the sensitizers NAT-622 and NAT-440 the maximum
IPCE peak is found at 540 nm with values very similar
around 53%. In all cases, the IPCE graphs are in accor-
dance with the shape and magnitude of the respective
extinction coefficients and absorption spectra shown in
Figure 2 and Table 1. The JꢀV curves of devices under
simulated full sunlight (AM 1.5G, 100 mW/cm2) are
shown in Figure 3, and the corresponding photovoltaic
parameters, i.e., the conversion efficiency (η), short-circuit
photocurrent density (Jsc), open-circuit photovoltage
(Voc), and the fill factor (ff), are given in Table 3 (average
of four samples).
Figure 2. Absorption and emission spectra measured in CH2Cl2
solutions (10ꢀ5 M) at room temperature for (a) RPT-9, (b)
NAT-440, (c) NAT-622, and (d) SFO-346.
functional10 and the SMD solvation model.11 These calcu-
lations result in a planar conjugatedπ-system in every case.
We have also calculated the lowest energy transitions using
a TD-DFT approach (Table 2). Calculations predict an
increase in both HOMO and LUMO energies whenadding
hexyl substituents or changing a phenyl by a tert-butyl
group. Comparing to reference dye RPT-9, hexyl substi-
tuents on the thiophene ring (NAT-440) cause a bath-
ochromic shift, while tert-butyl groups on the donor
moiety (SFO-346) give rise to an hypsochromic shift, in
agreement with the experimental results (see Table 1). It is
also noteworthy that hexyl groups attached to the thio-
phene ring cause a decreased oscillator strength that
correlates well with the decreased extinction coefficients of
NAT-440 and NAT-622 as compared to RPT-9 and SFO-
346, respectively. The calculated molecular geometries,
energies, and the molecular orbital contour plots are
included in the Supporting Information.
Another important issue for a sensitizer is the photo-
chemical and thermal stability. The thermal stabilities of
the dyes were studied by thermogravimetric analysis
Taking the RPT-9 sensitizer as the basic molecular
formula for this series of compounds (Figure 1), the
modification of the thiophene group of the RPT-9 dye
(10) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(11) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B
2009, 113, 6378.
754
Org. Lett., Vol. 14, No. 3, 2012

Table 3. Photovoltaic Parameters of Devices with a 4 μm
Thick TiO2 Film under Simulated AM 1.5G Illumination
(100 mW/cm2)a
dye
Voc (V)
Jsc (mA)
ff
η (%)
a
RPT-9
0.545 ( 0.005 9.35 ( 0.53 68.5 ( 0.6 3.49 ( 0.19
b NAT-440 0.595 ( 0.007 12.32 ( 0.10 70.8 ( 1.1 5.19 ( 0.02
NAT-622 0.645 ( 0.011 10.78 ( 0.22 71.5 ( 1.4 4.97 ( 0.13
d SFO-346 0.610 ( 0.011 12.10 ( 0.29 72.8 ( 1.1 5.37 ( 0.11
N719 0.712 ( 0.012 14.13 ( 0.1 68.1 ( 1.8 6.90 ( 0.30
c
a For comparison purposes the photovoltaic values of a DSSC
applying the commercial N719 dye. Average of four samples.
a 4H-pyran-4-ylidene as a donor, a thiophene ring in the
bridge and 2-cyanoacrylic acid as acceptor. We have
demonstrated photovoltaic response on these sensitizers,
obtaining between ∼3.5ꢀ5.4% conversion efficiency. We
haveobservedthatthe highest photovoltaic performanceis
obtained when the dye shows a 2-cyanoacrylic acid sub-
stituent as the acceptor and a 4H-pyran-4-ylidene with two
tert-butyl groups as the donor unit linked to the thiophene
ring. The highest Voc of the devices were observed for the
NAT-622 and the SFO-346 dyes with values around 0.6 V,
substantially higher than the 0.5 V observed for the basic
dye molecule, RPT-9. The latter has been attributed to the
presence of the phenyl group. Voltage values below the
standard 0.75 V usually obtained for a good DSSC are an
indication of charge recombination losses. The latter
indicates that the application of a phenyl-based donor
group results in higher recombination losses than the
tert-butyl group.
Figure 3. Photocurrentꢀvoltage (above) and IPCE (below)
curves of test devices with 4 μm thick TiO2 films, sensitized with
(a) RPT-9, (b) NAT-440, (c) NAT-622, and (d) SFO-346 under
full 100 mW cmꢀ2 simulated AM 1.5G sunlight.
with two bulky hexyl groups results in the NAT-440 dye.
Comparison of the absorption spectra of both sensitizers
proves a red shift of the maximum from 556 nm of the
RPT-9 dye, to 580 nm for the NAT-440. The latter is in
good agreement with the IPCE max peak shift observed
from 460 to 540 nm for the same dyes, respectively. This
substitution on the thiophene group allowed an impressive
increase in power conversion efficiency from 3.49% to
5.19%. The replacement of the phenyl group by tert-butyl
side group resulted in the SFO-346 dye. In this case, an
increase on the power conversion efficiency up to 5.37%
obtaining a maximum in the IPCE spectra of 62% at 460
nm was observed. Nevertheless, the dual modification of
the RPT-9 dye with both, the tert-butyl donor group in
the 4H-pyran-4-ylidene unit and the bulky hexyl acceptor
group in the thiophene group, dye NAT-622, did not result
in the enhancement of the power conversion efficiency.
The latter is probably due to a steric effect that these
modifications infuse to the final molecule.
Acknowledgment. We thank the Spanish Ministry of
Science and Innovation, MICINN-FEDER (projects
CTQ2008-02942, CTQ2011-22727 and ENE2008-04373),
ꢀ
the Gobierno de Aragon-Fondo Social Europeo (E39),
the Consolider NANOSELECT project CSD2007-00041,
ꢁ
the Xarxa de Referencia en Materials Avanc-ats per a
l0Energia, XaRMAEoftheCataloniaGovernmentforfinan-
ꢀ
cial support. N.M.de B. thanks the Departamento Educacion,
Gobierno de Navarra, for a predoctoral grant. Y.Y. thanks the
Spanish National Research Council (CSIC) for a JAE post-
doctoral contract. R.P.T. thanks the Spanish National Re-
search Council (CSIC) for a JAE predoctoral grant.
Supporting Information Available. General experimen-
tal methods, synthesis, characterization, devices, mea-
surements, calculated molecular geometries, energies,
and molecular orbital contour plots. This material is
available free of charge via the Internet at http://pubs.
acs.org
In summary, we have successfully synthesized a series
of new promising DꢀπꢀA conjugated organic sensitiz-
ers (RPT-9, NAT-440, NAT-622, and SFO-346) with
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 3, 2012
755