Structurally simple dipolar organic dyes featuring 1,3-cyclohexadiene conjugated unit for dye-sensitized solar cells

Article abstract of DOI:10.1021/ol802630x

(Chemical Equation Presented) A series of structurally simple dipolar light-harvesting organic dyes featuring 1,3-cyclohexadiene in the aromatic π framework for dye-sensitized solar cells has been synthesized and characterized. The highest conversion effi

Full text of DOI:10.1021/ol802630x

ORGANIC
LETTERS
2009
Vol. 11, No. 2
377-380
Structurally Simple Dipolar Organic
Dyes Featuring 1,3-Cyclohexadiene
Conjugated Unit for Dye-Sensitized
Solar Cells
Kuan-Fu Chen,†,‡ Ying-Chan Hsu,† Qiongyou Wu,† Ming-Chang P. Yeh,*,‡ and
Shih-Sheng Sun*,†
Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan, Republic of China
and Department of Chemisrty, National Taiwan Normal UniVersity,
Taipei 11677, Taiwan, Republic of China
sssun@chem.sinica.edu.tw; cheyeh@scc.ntnu.edu.tw
Received November 13, 2008
ABSTRACT
A series of structurally simple dipolar light-harvesting organic dyes featuring 1,3-cyclohexadiene in the aromatic π framework for dye-sensitized solar cells
has been synthesized and characterized. The highest conversion efficiency of the DSSCs based on these dyes can reach up to 4.4%.
The effective harvesting solar energy and its conversion to
electrical power has been a contemporary research topic for
searching an energy alternative for the fossil fuel due to the
increasing energy demands worldwide as well as the concerns
of the global warming issue related to CO2 emission from
burning fossil fuel.1 Dye-sensitized solar cells (DSSCs) based
on organic and coordination complexes have received great
attention among many research groups since the seminal
report by Gra¨tzel and co-workers.2 Ru(II)-based complexes
such as N3, N719, and the black dye have shown the best
photoconversion efficiency up to 11% under AM 1.5
irradiation thus far.3 Although the efficiency of DSSCs based
on organic dyes are generally lower than the cells based on
Ru(II) dyes, the highest photoconversion efficiency of organic
dye-sensitized solar cells has exceeded 9%.4 The advantages
of organic dyes over the Ru(II)-based metal complexes in
DSSCs include the typically high molar absorption coef-
ficients of organic dyes, a variety of specific functional
groups of organic dyes available for tuning the absorption
spectra coverage, the relative low costs of organic dye
compared with those of ruthenium metal complexes, and
essentially no limitation of resources.
Many kinds of organic dyes, such as coumarin-,5 cyanine-,6
hemicyanine-,7 triarylamine-,8 thiophene-,9 oligoene-,10 por-
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Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc.
1993, 115, 6382–6390. (b) Nazeeruddin, M. K.; Pe´chy, P.; Renouard, T.;
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Photobiol. A 2004, 164, 3–14.
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Charvet, R.; Comte, P.; Nazeeruddin, M. K.; Pe´chy, P.; Takata, M.; Miura,
H.; Uchida, S.; Gra¨tzel, M. AdV. Mater. 2006, 18, 1202–1205. (b) Horiuchi,
T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Soc. Chem. 2004, 126, 12218–
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M.-H.; Lee, W.; Park, J.; Kim, K.; Park, N.-G.; Kim, C. Chem. Commun.
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† Academia Sinica.
‡ National Taiwan Normal University.
(1) (a) Balzani, V.; Credi, A.; Venturi, M. ChemSusChem 2008, 1, 26–
58. (b) Gra¨tzel, M. Nature 2001, 414, 338–344. (c) Hagfeldt, A.; Gra¨tzel,
M. Acc. Chem. Res. 2000, 33, 269–277. (d) Robertson, N. Angew. Chem.,
Int. Ed. 2006, 45, 2338–2345.
(2) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737–740.
10.1021/ol802630x CCC: $40.75
Published on Web 12/09/2008
2009 American Chemical Society

phyrin-,11 and perylene-based12 dyes have been exploited as
photosensitizers for DSSCs. The structural composition of
commonly seen organic dyes emploited in the DSSCs typically
consists of an electron donor, such as para-disubstituted aniline,
and an electron acceptor, most notably cyanoacrylic acid, aiming
to facilitate vectorial charge transfer upon light absorption, a
π-conjugated spacer between the donor and acceptor to tune
the spectral coverage and light harvesting efficiency, and an
anchoring group to the TiO2 surface integrated into the acceptor
end.
integration of extended π-framework such as ethenyl,
thiophene, and bithiophene unit would expect to further
increase the spectrum coverage and better light harvesting
efficiency. We have found that an appreciable photoconver-
sion efficiency up to 4.4% can be achieved even with such
structurally simple dyes.
The synthesis of dyes 4a-c is summarized in Scheme 1,
which followed the similar procedures. Lithiation with
n-butyl lithium to 4-bromo-N,N-dimethylbenzenamine (4-
In this study, we design a new class of organic sensitizers
with a 1,3-cyclohexadiene conjugated unit as part of the
spacer to link the electron donor and electron acceptor. The
structures of these dyes are shown in Figure 1. A model
Scheme 1. Synthesis of Dipolar Dyes 4a-c
Figure 1. Structures of organic dyes.
compound 16 was also synthesized for assessing the role of
1,3-cyclohexadiene played in the photovoltaic efficiency. One
obvious advantage of employing 1,3-cyclohexadiene unit in
the framework of light-harvesting dyes is the essentially
planar conformation in the structural skeleton which yields
more dense packing of the dyes adsorbed on TiO2 surface,
not necessarily multilayer aggregation, and, therefore, in-
creases the amounts of dye loading on the surface. The
bromo-N,N-diphenylbenzenamine or 4-bromo-N-carbazoyl-
benzenamine)followedbythenucleophilicaddition-elimination
reaction with 3-ethoxycyclohex-2-enone provided 1a-c.
Acylation of 1a-c with ethyl chloroformate provided 2a-c.
The ester compounds 3a-c were obtained by consecutive
steps of reduction of cyclohexenone, methanesulfonylation,
and elimination. Finally, the hydrolysis of the ester groups
in compounds 3a-c in aqueous KOH afforded the target
dyes 4a-c.
Scheme S1 (Supporting Information) describes the syn-
thetic routes for dye 9, which followed similar procedures
to dyes 4a-c. The key intermediate, compound 6, was
prepared by conventional Wittig reaction of 4-(diphenylami-
no)benzaldehyde with diethyl (3-oxocyclohex-1-enyl)meth-
ylphosphonate. Dyes 14a and 14b were also synthesized in
a similar manner by first synthesizing compounds 10a and
10b followed by palladium-catalyzed Suzuki coupling reac-
tions to afford 11a and 11b. Subsequent acylation of 11a
and 11b followed by reduction of cyclohexenone, methane-
sulfonylation, elimination, and hydrolysis afforded desired
dyes 14a and 14b (Scheme S2, Supporting Information).
Compound 16 was synthesized by palladium-catalyzed
suzuki coupling reaction of 4-(diphenylamino)phenylboronic
acid with ethyl 4-bromobenzoate followed by hydrolysis in
aqueous KOH (Scheme S3, Supporting Information). All
dyes with 1,3-cyclohexadiene in the structure framework
exhibit fairly good thermal stability. Except for 4c, which
starts to decompose at ∼150 °C, the TGA data of other dyes
showed no decomposition up to 200 °C. The photochemical
stability of dyes 4b and 14a adsorbed on TiO2 were also
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378
Org. Lett., Vol. 11, No. 2, 2009

tested under AM 1.5 irradiation for 3 h and showed no
notable changes in the absorption spectra.
The absorption spectra of the dipolar dyes in acetonitrile
solution are shown in Figure 2. The related photophysical
the two phenyl rings in model 16, which suggests a more
planar conformation in 4b. The emission data in acetonitrile
is also summarized in Table 1. The emission spectra exhibit
significant solvatochromic shift in polar solvents indicative
of an eminent ICT character.
The absorption spectra of the six dyes adsorbed on a
transparent TiO2 film are also shown in Figure 2. Compared
to the corresponding spectra in CH3CN solution, the spectral
coverage become broader upon adsorption on TiO2 but the
absorption maxima shifted to higher energy when compared
to that of in solution. Such blue shifts of the absorption
spectra is attributed to either the formation of H-type
aggregate13 or deprotonation of the carboxylic acid.9a It
should be noted here that the high absorption before 370
nm primarily belongs to TiO2 and transparent FTO glass.
The amount of dyes adsorbed on the TiO2 surface was
estimated from the difference in concentration of each
solution before and after TiO2 film immersion followed by
rinsing with acetonitrile to ensure minimum physically
adsorbed dyes left on the films. The data is also included in
Table 1. It is pleased to see that the amount of dye adsorbed
on TiO2 film is exteremely large and almost one to two orders
higher than most of literature examples.14 The dye adsorption
is directly related to the molecular size of dyes. The high
dye-loading on the TiO2 film indicates these structurally
simple dyes with small molecular sizes formed a well
organized monolayer on the TiO2 surface, which is beneficial
for the light harvesting efficiency.15
Figure 2. (Left) Normalized UV-vis absorption spectra of dyes
in acetonitrile solution. (Right) Normalized absorption spectra of
different dyes on 1.5 µm TiO2 films.
data is summarized in Table 1. The general features of the
absorption spectra consist of one broad band in the visible
region and a less intense absorption band in the near UV
Table 1. Absorption and Fluorescence Propertiesa
λmax, nm
(ꢀ × 10-4, M-1cm-1
dye loading,
10-6mol/cm2
The oxidation potentials E(S+/S) of the dyes were mea-
sured by cyclic voltammetry and the results are collected in
Table 2. The oxidation potentials ranging from 0.87 to 1.50
dye
)
λem, nm
4a
4b
4c
391 (1.4)
481
536
447
528
527
542
501
1.0
1.4
1.0
1.5
0.43
0.38
0.97
299 (2.3), 387 (1.5)
298 (1.9), 379 (1.9)
300 (2.1), 404 (2.7)
284 (1.6), 416 (2.3)
279 (1.3), 417 (1.1)
350 (1.9)
9
14a
14b
16
Table 2. Electrochemical Data
dye
E0s0, eV
E(S+/S), Va
E(S+/S*), Va
Egap, Vb
a Absorption and fluorescence spectra were measured in acetonitrile
solution at 293 K. The fluorescence spectra were measured by excitation at
the absorption maximum of the lowest energy band.
4a
4b
4c
2.84
2.81
2.81
2.71
2.77
2.66
3.01
0.87
1.04
1.50
0.96
0.99
1.05
1.22
-1.97
-1.77
-1.31
-1.75
-1.78
-1.61
-1.79
1.47
1.27
0.81
1.25
1.28
1.11
1.29
9
14a
14b
16
region, which correspond to an intramolecular charge transfer
(ICT) transition and a locally excited π-π* transition,
respectively. The broad absorption bands in these compounds
suggest a good electronic delocalization throughout the whole
molecular system. A systematic bathochromic shift in the
absorption spectra with increasing conjugation was observed.
Meanwhile, the spectra of 14a and 14b with thiophene
moiety integrated into the conjugated structures are broader
when compared to other four dyes due to the enhanced
conjugation in these two molecules. DFT calculations at the
B3LYP/6-31G* level of the theory support the directional
charge transfer on excitation. The HOMO is primarily located
on π-framework of triphenyl amine; whereas the electron
density of LUMO is delocalized over the 1,3-cyclohexadienyl
carboxylic acid moiety (Supporting Information). DFT
calculations also indicated a smaller dihedral angle (28.0 and
33.5°) between the 1,3-cyclohexadienyl and phenyl moieties
in 4b when compared to the dihedral angle (34.6°) between
a Potentials are vs NHE. b Energy gap between the excited-state oxidation
potential and the TiO2 conduction band edge.
V vs NHE are more positive than the redox the efficienct
thermodynamic driving force (0.37-1.10 V) for dye regen-
eration.
The excited-state oxidation potential E(S+/S*) of these
dyes can be obtained from the difference between the ground-
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N.; Cui, Y.; Takahashi, M.; Sekiguchi, H.; Mori, A.; Kubo, T.; Furube, A.;
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Org. Lett., Vol. 11, No. 2, 2009
379

such as deoxycholic acid nor optimizing the thickness of TiO2
films for enhancing conversion efficiency. The DSSC based
on model 16 with the 1,3-cyclohexadiene replaced by phenyl
group shows only ∼22% device efficiency of the one based
on 4b. With comparable Voc and fill factor, the primary
efficiency loss in 16 compared to 4b is from the photocurrent,
which is attributed to the more efficient charge transfer. The
more planar 1,3-cyclohexadiene-phenyl conformation in 4b
is expected to yield more effective charge transfer to the
anchoring carboxylate and perhaps better charge separation
when compared to the biphenyl analogue 16.
Figure 3. J-V curves (left) and IPCE plots (right) of the dyes.
DSSCs based on dyes 4b and 14a produced maximum
IPCE of 60-65% whereas those based on other dyes
produced maximum IPCE below 40%. DSSCs based on dye
9 produced comparable Voc and fill factor to dyes 4b and
14a but considerable lower photocurrent and, thus, IPCE
performance. The IPCE is derived from the product of light
harvesting efficiency (LHE) of the dye-loaded film, the
quantum yield of the electron injection (CIE), ad the
effeciency of collecting the injected electron (CCE).3a,14c
Since the absorbance is in the range of 0.6-1.3 for the dye-
loaded 1.5 µm TiO2 film, the LHE would be close to unity
for dye-loaded 16 µm TiO2 films. Electron injection yield
would be also comparable between dyes 4b, 14a, and 9
considering the similar driving force for electron injection.
Therefore, the reduced photocurrent in DSSC based on 9,
which exhibited highest dye loading on TiO2 surface, is
attributed to the serious charge recombination due to the
significant dye aggregates formed on the TiO2 surface which
may block the infusion of redox mediator into the nanoporous
TiO2.14c The same effect also influenced the resutling Voc
state oxidation potential E(S+/S) and zero-zero excitation
17
energy E0s0
,
which was estimated from the cross section
of the normalized absorption and emission spectra. As shown
in Table 2, the excited-state oxidation potentials of the six
dyes are more negative than the potential of TiO2 conduction
band edge (-0.5 V vs NHE), which provides sufficient
thermodynamic driving force (0.81-1.47 V) for electron
injection.18
The device performance of solar cells based on the dyes
with 1,3-cyclohexadiene linkage and the model 16 are
summarized in Table 3. Figure 3 illustrates the J-V curves
Table 3. Photovoltaic Performance Parameters of the Dyesa
dye
VOC, mV
JSC, mA/cm2
ff
η, %b
4a
4b
4c
9
14a
14b
16
577
660
626
660
682
585
660
695
7.89
9.40
7.92
7.39
9.92
9.42
2.06
11.9
0.59
0.65
0.66
0.64
0.65
0.60
0.66
0.71
2.69 ( 0.20
4.03 ( 0.01
3.27 ( 0.33
3.33 ( 1.01
4.40 ( 0.28
3.31 ( 0.35
0.90 ( 0.01
5.87 ( 0.02
-
values. The thicker dye aggregates would block the I3
approaching the TiO2 surface and reduced the probablity of
back electron transfer from injected electron to reduce the
-
I3 . As shown in Tables 1 and 3, the amount of dye adsorbed
on TiO2 surface qualitatively correlated with the Voc with
the lowest Voc generated from DSSC based on least adsorbed
14b.
N719
a Experiments were performed using TiO2 photoelectrodes with ap-
proximately 16 µm thickness and 0.25 cm2 working area on the FTO (15
Ω/sq.) substrates with electrolyte composed of 0.05 M I2, 0.5 M LiI, and
0.5 M tert-butylpyridine in acetonitrile solution under AM 1.5 illumination.
b Average of three measurements.
In summary, a series of structurally simple dipolar organic
dyes with 1,3-cyclohexadiene integrated in the π-conjugated
backbone has been synthesized in high yields. DSSCs based
on these dyes have shown appreciable phototo-electrical
energy conversion efficiency with the highest one up to 4.4%.
Further structural optimization with broad spectral coverage
and enhancing directionality of charge transfer in the excited-
state is expected to produce more efficient photosensitizers.
Our work toward these directions are underway.
and action spectrum of incident photon-to-current conversion
efficiency (IPCE) plots for the DSSCs based on the dyes
studied in this work. The performance of the devices based
on dyes 4b and 14a reach to 70% of the standard N719 dye.
It should be pointed out here the reported η values in Table
3 were obtained without any of the additives typically used
Acknowledgment. We thank the National Science Council
in Taiwan, Academia Sinica, and National Taiwan Normal
University for support of this research.
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Supporting Information Available: Synthesis, experi-
mental procedures, characterization data, molecular orbital
plots, and TGA plots. This material is available free of charge
via the Internet at http://pubs.acs.org.
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A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B
2003, 107, 597–606. (b) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95,
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