Organic D-A-π-A solar cell sensitizers with improved stability and spectral response

Article abstract of DOI:10.1002/adfm.201001801

A novel concept "D-A-π-A" organic sensitizer instead of traditional D-π-A ones is proposed. Remarkably, the incorporated low bandgap, strong electron-withdrawing unit of benzothiadiazole shows several favorable characteristics in the areas of light-harves

Full text of DOI:10.1002/adfm.201001801

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Organic D-A-π-A Solar Cell Sensitizers with Improved
Stability and Spectral Response
Weihong Zhu, Yongzhen Wu, Shutao Wang, Wenqin Li, Xin Li, Jian Chen,
Zhong-sheng Wang,* and He Tian*
Dedicated to Professor Yong Cao on the occasion of his 70th birthday
triphenylamine[5] derivatives have been devel-
oped as promising candidates with the power
conversion efficiency closing to ruthenium
dyes. Currently, the efficiency of metal-free
organic DSSCs is limited mainly by three fac-
tors: i) non-optimized photocurrents arising
from the poor optical response of dyes in the
NIR, ii) non-optimized open-circuit voltages
arising from unmatchable energy levels as
well as unfavorable dark currents, and iii)
the instability and short lifetimes resulting
in the major problems of DSSCs with vul-
nerable organic dyes. As a consequence to
practical and commercial applications, there
is still much room for molecular engineering
to generate higher conversion efficiencies as
well as longer lifetimes for DSSC stability.
A novel concept “D-A-π-A” organic sensitizer instead of traditional D-π-A ones
is proposed. Remarkably, the incorporated low bandgap, strong electron-
withdrawing unit of benzothiadiazole shows several favorable characteristics
in the areas of light-harvesting and efficiency: i) optimized energy levels,
resulting in a large responsive range of wavelengths into NIR region; ii) a
very small blue-shift in the absorption peak on thin TiO2 films with respect to
that in solution; iii) an improvement in the electron distribution of the donor
unit to distinctly increase the photo-stability of synthetic intermediates and
final sensitizers. The stability and spectral response of indoline dye-based
DSSCs are improved by the strong electron-withdrawing benzothiadizole unit
in the conjugation bridge. The incident-photon-conversion efficiency of WS-2
reaches nearly 850 nm with a power conversion efficiency as high as 8.7% in
liquid electrolyte and 6.6% in ionic-liquid electrolyte.
As well known, extending π-conjugated
bonding bridges always brings a low sta-
1. Introduction
bility although it might efficiently red-shift the absorption
response.[3a] Tian et al. has reported a series of donor-donor-π
bridge-acceptor (D-D-π-A) structural organic dyes incorporating
several donor groups into the triphenylamine framework with
star-burst configuration, resulting in the red-shift in absorp-
tion and the suppression of charge recombination with elec-
trolyte.[6] However, considering their complicated synthesis and
the decrease in adsorbed amount arising from bulky molecules,
it seems rationally to find a more powerful electron-donating
unit as donor part instead of bulky multi-donor structure in the
molecular design. Recently, the indoline moiety has been found
to possess a stronger electron-donating ability than triphe-
nylamine unit in the series of isophorone bridged organic dyes,
thus giving a remarkable power conversion efficiency as high
as 7.41% under AM 1.5.[7] Moreover, a series of indoline dyes
(such as D-149 and D-205) containing rhodanine-3-acetic acid
as the acceptor and anchoring unit have shown very high per-
formance (η > 9%).[4c,4d] However, such DSSCs sensitized with
these indoline derivatives have been pointed out to maintain
short-term stability as a result of desorption.[2,5d,5e] With this in
mind, herein we report a series of novel cyanoacetic acid deriva-
tives with a novel specific donor-acceptor-π bridge-acceptor
(D-A-π-A) configuration to construct metal-free organic sen-
sitizers coded as WS-1 to WS-4 (Figure 1). The introduced
additional acceptor benzothiadiazole has been widely utilized
in the design of organic solar cells.[8] In the novel D-A-π-A
configuration dyes, the additional acceptor chromophore is
As a potential alternative to conventional inorganic photovoltaic
devices, dye-sensitized solar cells (DSSCs) have attracted ever-
increasing attention in the past decades due to their high per-
formance/price ratio.[1] The sensitizer is always a crucial element
in DSSCs, exerting a significant influence on the power conver-
sion efficiency (η) as well as the device stability. Metal-free organic
dyes,[2] commonly constructed with donor-π bridge-acceptor
(D-π-A) configuration, have become more promising due to their
facile structure modification, high extinction coefficients and low
cost with respect to classical ruthenium polypyridine complexes.
Several organic sensitizers based on coumarin,[3] indoline[4] and
Prof. W. Zhu, Y. Wu, W. Li, X. Li, J. Chen, Prof. H. Tian
Key Laboratory for Advanced Materials and Institute of Fine
Chemicals
East China University of Science and Technology
Shanghai 200237, PR China
Fax: (+86) 21–6425-2758
E-mail: tianhe@ecust.edu.cn
S. Wang, Prof. Z.-S. Wang
Department of Chemistry & Laboratory of Advanced Materials
Fudan University
Shanghai 200433, PR China
E-mail: zs.wang@fudan.edu.cn
DOI: 10.1002/adfm.201001801
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with H NMR, 13C NMR and HRMS (shown
in Supporting Information). During the syn-
thetic process, the bromo-substituted indoline
(intermediate 1a, Figure S1, Supporting Infor-
mation) is extremely photosensitive. Reason-
ably, these nitrogen-containing electron donors
are very sensitive under irradiation due to their
low oxidation potential. Specifically for the
indoline unit, the saturated five-member ring
cannot disperse the lone pair electrons as the
phenylene in triphenylamine unit, resulting
in less photo-stability. In contrast, the inter-
mediate 2a (Figure S1, Supporting Informa-
tion) becomes completely photo-stable when
attaching a benzothiadiazole unit. Clearly, the
incorporated strong electron-withdrawing unit
of benzothiadiazole can distinctly increase
the photo-stability of 2a due to the favorable
electron distribution of donor section. Such a
stable intermediate is beneficial to the synthetic
process. Consequently, we expect to incorpo-
rate benzothiadiazole moiety as an additional
acceptor to construct D-A-π-A configuration
organic dyes for specifically improving their photostability, which
is always critical to the commercialization in solar cells.
1
S
S
N
N
N
N
NC
COOH
CN
COOH
S
N
N
WS-1
S
WS-2
S
N
N
N
N
NC
COOH
CN
COOH
S
N
N
WS-4
WS-3
COOH
S
N
CN
LS-1
Figure 1. Chemical structures of D-A-π-A sensitizers (WS-1 to WS-4) containing low bandgap
and strong electron-withdrawing unit of benzothiadiazole as additional acceptor, and reference
dye LS-1.
expected to function as an electron trap where charge separa-
tion occurs, and facilitating the electron migration direction to
cyanoacetic acetic unit. Remarkably, with incorporating adja-
cently to indoline or triphenylamine donors, the low bandgap
and strong electron-withdrawing unit of benzothiadiazole
shows several favorable characteristics in the beneficial light-
harvesting and efficiency: i) an optimization in energy levels
resulting in the long responsive wavelength to NIR region;
ii) exhibiting a very small blue-shift in absorption peak on thin
TiO2 fi lm with respect to that in solution; iii) an improvement in
the electron distribution of donor unit to distinctly increase the
photo-stability of synthetic intermediates and final sensitizers.
As demonstrated, using a liquid-electrolyte, DSSC based on
dye WS-2 exhibited broad action spectrum and high optimizing
efficiency (η = 8.7%) with a short circuit current density (Jsc) of
17.7 mA cm−2 , open circuit voltage (Voc) of 0.65 V, fill factor (FF)
of 0.76 under AM 1.5 illumination (100 mW cm−2 ). In the case
of ionic-liquid gel electrolyte, the cells based on dye WS-2 exhib-
ited high η = 6.6% and maintained 1000 h without desorption.
Consequently, WS-2 is an example of D-A-π-A configuration
dyes, providing a valuable principle in the design of efficient
sensitizers with a rational low regeneration and high injection
driving force for highly efficient dye-sensitized solar cells.
2.2. Photophysical and Electrochemical Properties Upon
Incorporating Benzothiadiazole Moiety
We preliminary studied the photophysical and electrochem-
ical properties of these sensitizers (Table 1). In chloroform/
methanol mixed solution (v/v = 4/1), all these dyes exhibit a
broad absorption band in the visible region corresponding to
the intramolecular charge transfer (ICT) band (Figure 2a). In
contrast with the small difference in molar extinction coeffi-
cients (13 000–16 700 mol−1 cm−1 ), the absorption peaks (λmax
)
for WS-1, WS-2, WS-3, and WS-4 shift to some different extent,
located at 496, 533, 455 and 497 nm, respectively. Obviously, in
the series of sensitizers containing the same cyanoacetic acid
unit, the ICT band is dependent upon the donor units and elec-
tron transport channels. As listed in Table 1, the donor effect
on absorption peak is distinct. The bathochromic shift by 36 or
41 nm was observed when replacing triphenylamine unit (WS-3
and WS-4) with indoline unit (WS-1 and WS-2) in the donor
moiety, indicative of the more powerful electron-donating capa-
bility of the indoline unit than that of triphenylamine, which is
consistent with our previous work.[7] When compared to the ref-
erence dye LS-1 (Table 1), the introduction of benzothiadiazole
unit into the molecular frame distinctly decreases the bandgap
between the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO), thus shifting
the ICT absorption peak from 486 nm (LS-1) to 533 nm (WS-2)
(Table 1 and Figure S2, Supporting Information). Simultane-
ously, the absorption onset is extended by 100 nm from 630 nm
(LS-1) to 730 nm (WS-2) in a dilute solution. Consequently, the
benzothiadiazole unit can efficiently decrease the bandgap and
optimize energy levels, thus resulting in the long responsive
wavelength to NIR region and several favorable characteristics
beneficial to light-harvesting and efficiency.
2. Results and Discussion
2.1. Design and Synthesis
The synthetic route of organic dyes WS-1 to WS-4 is depicted in
Scheme S1 (Supporting Information). Two steps of Suzuki cou-
pling reactions on 4,7-dibromobenzothiadiazole result in the
corresponding aldehyde precursors, followed by treatment with
cyanoacetic acid under typical Knoevenagel condensation to obtain
the target dyes. Their chemical structures were fully characterized
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T a b le 1 . Photophysical and electrochemical properties of sensitizers.
Dyes
Absorption
ε [mol−1 cm−1 a)
HOMO [V]b)
E
0–0 [V]c)
LUMO [V]d)
λ
max [nm]a)
496
]
λ
max on TiO2 [nm]
WS-1
WS-2
WS-3
WS-4
LS-1
14400
16700
16100
13000
21000
490
528
445
481
420
0.67
0.67
0.95
0.95
0.69
2.06
1.90
2.27
2.11
2.27
−1.39
−1.23
−1.32
−1.16
−1.58
533
455
497
486
a) Absorption peaks (λmax) and molar extinction coefficients (ε) were measured in CHCl3/CH3OH = 4/1 (≈10−5 M). b) The formal oxidation potentials (versus NHE) in
CH2Cl2 were internally calibrated with ferrocene (0.4 V versus NHE)[9a] and taken as the HOMO. c) E0–0 was estimated from the intersection of the normalized absorption
and emission spectra in solution. d) The LUMO was calculated with the expression of LUMO = HOMO – E0–0
.
absorption peak of reference dye LS-1 on 3 μm film is severely
blue-shifted by 66 nm from 486 nm in a mixed chloroform/
methanol solution to 420 nm on TiO2 fi lm (Figure S3a, Sup-
porting Information). Since the aggregation tendency is not
obvious within such thin films of TiO2, here the shift in absorp-
tion peak might be predominated by deprotonation effect.
Unexpectedly, this sharp hypsochromic shift of absorption
peaks has not been observed in D-A-π-A configuration WS-type
dyes (Table 1). As a case illustrated with WS-2, the ICT absorp-
tion peak is slightly blue-shifted by only 5 nm from 533 nm
in solution to 528 nm on 3 μm TiO2 fi lm (Figure S3b, Sup-
porting Information). Undoubtedly, it is the strong electron-
withdrawing ability of benzothiadiazole unit in “WS”-type dyes
that countervails the deprotonation effect and contributes to
the observed small blue-shift in absorption peak (λmax) when
anchoring sensitizers onto nanocrystalline TiO2 surface.
However, when carefully comparing the absorption onset of
“WS”-type between the solution and the transparent thin film
onto 3 μm TiO2, a significant red-shift of absorption onset was
interestingly observed (Figure 2b). For example, WS-2 showed
the absorption onset at >800 nm (the transparent thin film onto
3 μm TiO2) versus 730 nm (in dilute solution). A more distinct
red-shift of absorption onset even to 850 nm has been observed
on a 16 μm thick film as shown in Figure S3c (Supporting Infor-
mation). Accordingly, such a red-shift in the absorption onset can
maintain a high absorption in the visible region (450–650 nm)
and also extend to the NIR region (800 nm) under solar spec-
trum, which is beneficial to high light utilization.
Cyclic voltammetry (CV) was employed to evaluate the redox-
stability and oxidation potential of the WS-type dyes. These dyes
can be reversibly oxidized at moderately high oxidation poten-
tials, which are more positive than ferrocene/ferrocenium and
Figure 2. a) Absorption spectra of WS-1, WS-2, WS-3 and WS-4 in chlo-
roform/methanol mixed solution (v/v = 4/1), b) Absorption spectra of
WS-1, WS-2, WS-3 and WS-4 on 3 μm TiO2 transparent films.
−
I−/I3 redox couples, suggesting sufficient driving force for dye
regeneration (Figure S4, Supporting Information). Notably,
under scanning for fifteen times, the voltammograms of WS-2
still maintained a well symmetrical and reversible behavior,
indicative of these dyes with high stability (Figure S5, Sup-
porting Information). The HOMO values of WS-1 and WS-2 are
almost identical, about 0.67 V. While for WS-3 and WS-4, it is
shifted to 0.95 V as the donor is changed from indoline unit to
triphenylamine unit. Observed from these electrochemical data,
it seems that the HOMO energy level is exclusively determined
by donors. Generally, the effect of π-bridge on the HOMO
energy levels is remarkable in traditional D-π-A structure
In a first set of experiments, the thin transparent TiO2 fi lms
(3 μm in thickness) loaded with a monolayer of these dyes were
employed in order to accurately measure the spectral response.
Generally, when sensitizers anchor onto nanocrystalline TiO2
surface, the deprotonation and aggregation of dye molecules
affect UV–vis absorption profiles. Typically, the molecular
aggregation contains H- and J-aggregates. Deprotonation[4f]
and H-aggregates always result in blue-shift of absorption peak,
while J-aggregates mainly lead to the red-shift of absorption
peak. Like most D-π-A conjugated organic sensitizers, the ICT
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dyes.[5c] However, in our D-A-π-A configuration dyes, this influ-
ence (including the phenylene and thiophene) may be blocked
from the electron-withdrawing unit in the bridge. It means we
can conveniently tune the energy levels without consideration
the effect of π-bridge in such D-A-π-A configuration dyes. Esti-
mated from E0–0, the LUMO energy levels of WS-1, WS-2, WS-3
and WS-4 are −1.39, −1.23, −1.32, and −1.16 V, respectively,
indicative of thermodynamic feasibility for electron injection to
conduction band (Ecb) of TiO2 (-0.5 V versus NHE).[9b]
The ground state structures of WS-1, WS-2, WS-3 and WS-4
were further optimized by using the hybrid B3LYP functional
in combination with the standard 6–31G∗ basis set.[10] Actu-
ally as shown in Figure 3, the electron of each LUMO orbital
is evenly delocalized across the entire A-π-A system indirectly
suggests that in the LUMO orbitals the well inductive or with-
drawing electron tendency from donor unit to the cyanoacetic
acid unit. It can be seen that the HOMOs in WS-2 and WS-4
are more delocalized than those in WS-1 and WS-3, due to
the better coplanarity between the benzothiadiazole unit and
the thiophene ring (Figure S6, Supporting Information). The
dihedral angles between benzothiadiazaole and neighboring
benzene ring or thiophene ring are calculated to be 32.9° for
WS-1, 2.7° for WS-2, 34.3° for WS-3, and 0.4° for WS-4. The
relative small torsion angle for thiophene-bridged WS-2 and
WS-4 ensures a good molecular coplanarity to keep high elec-
tronic communication with respect to phenylene-bridged WS-1
and WS-3, resulting in the above-mentioned larger red-shift in
absorption band by 37 and 42 nm in solution.
Information). During the photo-excitation, the electrons on
the indoline subunit of WS-2 are transferred to the benzothia-
diazole and cyanoacetic acid subunits, while in LS-1 the elec-
trons on the indoline subunit are directly transferred to the
cyanoacetic acid subunit. This is consistent with our expected
electron-trap role of the incorporated benzothiadiazole sub-
unit. That is, upon anchoring onto nanocrystalline TiO2
surface, the electrons could be successively transferred from
the donor to the benzothiadiazole subunit, then transferred to
the cyanoacetic acid subunit, and finally to TiO2.
2.3. Photovoltaic Device Performance
Figure 4 shows the incident photon-to-electron conversion effi-
ciency (IPCE) as a function of incident wavelength for DSSCs
based on these dyes, and Table S2 (Supporting Information)
summarizes the solar-cell performance parameters. In good
agreement with absorption spectra, the IPCE action spectrum
for DSSC based on WS-2 is broader than those of WS-1, WS-3
and WS-4. It is worth noting that the IPCE of WS-2 keeps a high
plateau at visible region until 720 nm, and extending the onset
to the NIR region at about 850 nm. Although the absorbance
around 800 nm is a little low, the IPCE values around 800 nm
are distinct owing to the scattering effect of the large particles
in the film and in combination with the reflective effect of the
Pt mirror.[11] Actually, the IPCE onset is in good agreement with
absorption onset of the dye-loaded film for WS-2.
Because unity IPCE should be 80–90%, taking the absorp-
tion and reflection of the conductive glass into account, the
dyes except for WS-3 produce maximum IPCE lower than
unity. Considering that all the four dyes have LUMO energy
levels sufficiently higher than the CB of TiO2, dye aggrega-
tion, which is unfavorable for photocurrent generation, would
occur for the dyes except for WS-3. As discussed in the calcu-
lation part, the thiophene-bridged WS-2 and WS-4 have better
planarity than the phenylene-bridged WS-1
To compare the charge transfer processes during the excita-
tion in WS-2 and LS-1, the Mulliken population analysis is uti-
lized to model the partial charges on each subunit. The Mulliken
charges for the ground state (S0) and the excited state (S1) are
calculated at the HF (Hartree-Fock) and the CIS (configuration
interaction with single substitution) level of theory, respectively,
in combination with the 6–31G∗ basis set. The charges are
simulated for the subunits and listed in Table S1 (Supporting
and WS-3, respectively. It is reasonable that
intermolecular interactions in WS-2 and
WS-4 are stronger than those in WS-1 and
WS-3, which leads to lower maximum IPCE
values for the former two dyes than those for
the latter two dyes, respectively. WS-2 gen-
erates the highest Jsc (Table S2) due to the
broad characteristics of IPCE spectrum.
Co-adsorption of deoxycholic acid (DCA)
with dye molecules, which suffer from aggrega-
tion, has been proven to improve solar cell per-
formance significantly.[12] Since WS-2 exhibits
the broadest absorption range among the four
dyes studied in this work, we focus on the
optimization of WS-2 based DSSC using co-
adsorption strategy. Upon co-adsorption with
DCA, the plateau of IPCE curve was lifted from
around 0.6 to greater than 0.8 (Figure 5a), and
the Jsc increased from 12.9 to 17.7 mA cm−2
(Figure 5b). The remarkable enhancement of
IPCE is attributed to the break-up of dye aggre-
Figure 3. Calculated frontier molecular orbitals of HOMO and LUMO and experimental energy
level diagram of WS-1, WS-2, WS-3 and WS-4.
gates upon co-adsorption. The increased Jsc is
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Compared with WS-1, WS-3 and WS-4 (Table 1), the prom-
ising enhanced performance of sensitizer WS-2 can be attrib-
uted to the high energy level match and broad action spectrum.
By replacing triphenylamine unit with indoline unit, the red-
shift in absorption peak of WS-2 was achieved with a decrease
in E0–0 (negative shift 270 mV in HOMO, and a smaller nega-
tive shift in LUMO) with respect to WS-4. As well known,
the driving force to dye regeneration and electron injection is
dependent upon the HOMO and LUMO energy levels of sen-
sitizers, respectively. Typically, the driving force of ≥0.20 eV
is required for efficient dye regeneration in most reported
DSSCs.[13a] Surprisingly, WS-2 exhibits good performance with
high HOMO, which is contrary to the fact that has previously
been reported.[13a] Grätzel and coworkers have proved using
time-resolved spectroscopy that the efficient regeneration of
sensitizers with driving force as little as 0.15 eV between the
oxidation potential of the sensitizer and the mediator is pos-
sible for some particular dyes.[13b] These findings imply that
each dye-TiO2-elctrolyte system might show different require-
ments in respect to the required driving force for efficient dye
regeneration. We think that the low driving force in our studied
system is also for the very particular system. In contrast, the
potential difference between the LUMO and the Ecb has a
remarkable effect on the electron injection. Previous research
has demonstrated that the difference value greater than 0.20 eV
was indispensable for efficient electron-injection.[14] Moreover,
when the difference is greater than 0.20 eV, Ecb can be nega-
tively shifted by adding additives such as 4-tert-butylpyridine or
guanidinium thiocyanate to obtain higher photovoltage and η
rather than waste energy in the electron injection driving force.
WS-2 is an example of D-A-π-A configuration dyes, which shows
exceptional performance and may help design new efficient
sensitizers with a rational low regeneration and high injection
driving force for high efficiency dye-sensitized solar cells.
Figure 4. IPCE action spectra for WS-1, WS-2, WS-3 and WS-4-based
DSSCs.
For future applications, ionic-liquid based electrolytes
are greater choice because of their high thermal stability,
non-flammability and negligible vapor pressure. We fabricated
WS-2 based DSSCs using ionic-liquid gel electrolytes. The photo-
voltaic parameters were Voc = 615 mV, Jsc = 14.48 mA cm−2 , FF =
0.74, and η = 6.62% under standard global AM 1.5 solar light
conditions (100 mW cm−2 ) shown in Figure 6. This result is
comparable to the record efficiency of well-known indoline dye
D149.[4c,4e] Moreover, the high stable DSSCs based on WS-2 with
ionic-liquid electrolyte have been successfully realized. As shown
in Figure 7, a WS-2 based DSSC in open-circuit mode was sub-
jected to visible-light soaking and showed good stability. Values
for Voc, Jsc, FF, and η were recorded over a period of 1000 h.
The overall efficiency remained at 94% of the initial value after
1000 h of visible-light soaking. This demonstrates that the
amount of dye on the TiO2 surface remained intact after long
time light soaking. To the best of our knowledge, this is the first
taste for stability test of DSSCs based on indoline dyes.
Figure 5. a) IPCE action spectra for untreated and DCA co-adsorbed
DSSCs based on dye WS-2. b) I–V curve for a DSSC with only WS-2 or
co-adsorbed with DCA using 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and
0.5 M TBP as redox electrolyte and the effect of coadsorption of DCA on
dark current.
due to the higher IPCE value caused by higher injection efficiency
resulting from relatively independent dye molecules arraying on
TiO2 surface. Here, the increased Voc upon co-adsorption is due
to the retarded charge recombination as evidenced by the dark
current onset potential shift from 0.48 to 0.55 V (Figure 5b).
Finally, all parameters (Jsc, Voc, FF) are optimized to the best with
17.7 mA cm−2 , 650 mV, and 0.76, respectively, yielding an overall
conversion efficiency of 8.7%, about 64% higher than that of the
cell using a film just sensitized with WS-2.
3. Conclusions
In summary, we incorporated a low bandgap, electron-
withdrawing benzothiadiazole unit in the conjugated bridge
to develop novel D-A-π-A organic sensitizers with different
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XE spectrometer. The absorption spectra of sensitizer dyes in solution
and adsorbed on TiO2 films were measured with a Varian Cary 500
spectrophotometer. Emission spectra of sensitized dyes in solution
were measured with Varian Cary Eclipse. The CVs were determined
with a Versastat II electrochemical workstation (Princeton Applied
Research) using a three-electrode cell with a Pt working electrode, a Pt
wire auxiliary electrode, and a saturated calomel reference electrode in
saturated KCl solution, 0.1 M tetrabutylammonium hexaflourophosphate
(TBAPF6) was used as the supporting electrolyte in CH2Cl2. Ferrocene
was added to each sample solution at the end of the experiments, and
the ferrocenium/ferrocene (Fc/Fc+) redox couple was used as an internal
potential reference.
Synthesis: All sensitizers were synthesized quite straightforward with
the traditional Knoevenagel reaction in relatively high yield. The synthetic
routes as well as detailed synthesis of intermediates are list in Scheme S1
(Supporting Information).
Synthesis of WS-1: A mixture of aldehyde 3a (100 mg, 0.20 mmol) with
cyanoacetic acid (17 mg, 0.20 mmol) in acetonitrile (10 mL) was refluxed
in the presence of piperidine (0.5 mL) for 7 h under Argon. After cooling,
the mixture was diluted with CH2Cl2, and washed with water and brine,
dried over Na2SO4, and evaporated under reduced pressure. The crude
product was purified by column chromatography (CH2Cl2/methanol =
20/1) on silica gel to yield the product as a red powder, WS-1 (81 mg,
Figure 6. J–V curve for a DSSC with WS-2 using ionic-liquid gel
electrolyte.
1
0.15 mmol, 75%). H NMR (400 MHz, DMSO-d6, δ): 8.16 (s, 1H), 8.05
(d, J = 8.0 Hz, 2H), 7.99 (d, J = 8.1 Hz, 2H), 7.83 (d, J = 7.1 Hz, 1H),
7.77 (s, 1H), 7.71 (d, J = 7.1 Hz, 1H), 7.67 (d, J = 8.2 Hz, 1H), 7.19 (d,
J = 8.3 Hz, 2H), 7.15 (d, J = 8.4 Hz, 2H), 6.90 (d, J = 8.4 Hz, 1H), 4.84 (m,
1H), 3.82 (m, 1H), 2.27 (s, 3H), 2.02 (m, 1H), 1.77 (m, 3H), 1.59 (m, 1H),
1.39 (m, 1H). 13C NMR (100 MHz, DMSO-d6, δ): 163.93, 153.25, 153.22,
147.97, 147.56, 139.47, 139.20, 134.92, 133.16, 132.40, 130.80, 129.72,
129.57, 129.16, 128.93, 128.76, 128.73, 126.44, 125.86, 125.31, 119.58,
118.84, 112.20, 106.74, 68.32, 44.56, 34.80, 33.09, 23.94, 20.36. HRMS
(ESI, m/z): [M + H]+ calcd for C34H25N4O2S, 555.1855; found, 555.1857.
WS-2 was obtained as purple powder in similar way with WS-1 (yield
60%): 1H NMR (400 MHz, DMSO-d6, δ): 8.19 (s, 1H), 8.13 (d, J = 3.4 Hz,
1H), 8.07 (s, J = 7.2 Hz, 1H), 7.79 (m, 2H), 7.72 (m, 2H), 7.22 (d,
J = 8.3 Hz, 2H), 7.17 (d, J = 8.2 Hz, 2H), 6.91 (d, J = 8.4 Hz, 1H), 4.89 (m,
1H), 3.91 (m, 1H), 2.29 (s, 3H), 2.04 (m, 1H), 1.79 (m, 3H), 1.61 (m, 1H),
1.41 (m, 1H). 13C NMR (100 MHz, DMSO-d6, δ): 164.10, 152.87, 151.86,
147.70, 144.02, 141.09, 139.38, 137.50, 135.16, 134.98, 133.15, 130.92,
129.74, 128.84, 127.19, 127.01, 126.20, 125.66, 125.28, 122.50, 119.69,
118.97, 108.48, 106.73, 68.35, 44.53, 34.83, 33.06, 23.94, 20.37. HRMS
(ESI, m/z): [M + H]+ calcd for C32H23N4O2S, 561.1419; found, 561.1419.
WS-3 was obtained as red powder in similar way with WS-1 (yield
Figure 7. Variations of photovoltaic parameters (Jsc, Voc, FF, and η) with
aging time for the DSSC device based on WS-2 and ionic-liquid electrolyte
under visible-light soaking.
1
70%): H NMR (400 MHz, DMSO-d6, δ): 8.16 (d, J = 8.4 Hz, 2 H), 8.08
(s, 1 H), 8.05 (d, J = 8.5 Hz, 2 H), 8.02 (d, J = 7.6 Hz, 1 H), 7.97 (d,
J = 8.8 Hz, 2 H), 7.93 (d, J = 7.6 Hz, 1 H), 7.37 (t, J = 7.6 Hz, 4 H),
7.09–7.13 (m, 8 H). 13C NMR (100 MHz, DMSO-d6, δ): 163.89, 153.20,
153.13, 147.97, 147.52, 146.76, 139.08, 132.59, 132.30, 130.12, 130.02,
129.63, 129.34, 128.70, 127.12, 124.55, 123.60, 121.97, 118.77, 112.27.
HRMS (ESI, m/z): [M – H]− calcd for C34H21N4O2S, 549.1385; found,
549.1388.
WS-4 was obtained as deep red powder in similar way with WS-1
(yield 55%): 1H NMR (400 MHz, DMSO-d6, δ): 8.22–8.26 (m, 2 H), 8.14
(s, 1 H), 7.99 (d, J = 8.4 Hz, 2 H), 7.93 (d, J = 7.6 Hz, 1 H), 7.83 (d, J =
3.6 Hz, 1H), 7.35–7.39 (t, J = 7.6 Hz, 4H), 7.09–7.14 (m, 8H). 13C NMR
(100 MHz, DMSO-d6, δ): 162.79, 153.04, 151.91, 147.68, 146.76, 143.14,
140.00, 138.35, 135.22 132.35, 130.21, 129.92, 129.68, 127.60, 127.24,
127.06, 124.59, 123.93, 123.66, 122.07, 119.24, 110.16. HRMS (m/z):
[M-H]− calcd for (C34H19N4O2S2), 555.0949; found, 555.0948.
Reagents: All chemicals and solvents used for DSSCs fabrication were
of reagent-grade quality, LiI, I2, Deoxycholic acid (DCA), acetonitrile
(AN) and 4-tert-butylpyridine (TBP) were obtained from Acros, and used
without further purification unless otherwise noted. In this work, two
kinds of electrolytes were employed: liquid electrolyte, 0.1 M LiI, 0.05 M
I2, 0.6 M 1, 2-dimethyl-3-N- propylimidazolium iodide (Tomiyama Pure
Chemical Industries Ltd.), and 0.5 M TBP in dehydrated acetonitrile;
ionic liquid gel electrolyte, 0.1 M LiI, 0.4 M I2, 0.4 M TBP in 1-methyl-3-n-
electron donors and electron-transporting channels. The addi-
tional acceptor of the benzothiadiazole unit adjacent to donors
in a π-conjugation has several advantages, such as improving
the distribution of donor electrons to enhance the photo-stability
of indoline compounds, red-shifting absorption spectra into the
NIR region and weakening the deprotonation effect on TiO2
film, beneficial to light-harvesting, as well as optimizing the
match in energy levels to get the maximum photon-to-electricity
conversion. Specifically, its outstanding IPCE response, high
energy conversion efficiency, and long-term stability reveals
that the D-A-π-A configuration is very effectual for the mole-
cular engineering in the design of indoline dyes.
4. Experimental Section
Characterization: 1H NMR and 13C NMR spectra were recorded on
Brucker AM-400 MHz instruments with tetramethylsilane as internal
standard. MS and HRMS were performed using a Waters LCT Premier
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propylimidazolium iodide (MPImI). Transparent conducting oxide (TCO,
10 Ω square−1 , F-doped SnO2, Nippon Sheet Glass Co.) was used as the
substrate for the TiO2 thin-film electrode after several washing.
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Fabrication of Nanocrystalline TiO2 Solar Cells: 16 μm TiO2 films
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a 5 μm scattering layer (60% 20 nm nanoparticles and 40% 100 nm
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Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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Acknowledgements
This work was supported by NSFC/China, Major NSFC Program
(90922004), National Basic Research Program of China (No.
2011CB933302, 2011CB933300), Shanghai Shuguang Project (07SG34),
Shanghai Pujiang Program (090PJ1401300), the Program for Professor of
Special Appointment at Shanghai Institutions of Higher Learning, Shanghai
Non-Governmental Cooperation Project (10530705300), Shanghai Leading
Academic Discipline Project (B108), Specialized Research Fund for the
Doctoral Program of Higher Education (SRFDP 200802510011), State Key
Laboratory of Precision Spectroscopy (ECNU), and the National College
Students Innovation Experiment Program (091025127).
Received: August 31, 2010
Revised: October 8, 2010
Published online: December 10, 2010
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