A new ruthenium polypyridyl dye, TG6, whose performance in dye-sensitized solar cells is surprisingly close to that of N719, the 'dye to beat' for 17 years

Article abstract of DOI:10.1039/b808255c

A new ruthenium polypyridine sensitizer for dye-sensitized solar cells (DSSCs) is proposed containing a hexasulfanyl-styryl-modified bipyridyl group as an ancillary ligand. The advantages of this dye are the much larger absorption coefficient and the small shift of the absorption envelope to the red. We compare this new dye, TG6 (cis-bis(thiocyanato)(2,2′-bipyridyl-4,4′- dicarboxylato){4,4′-bis[2-(4-hexylsulfanylphenyl)vinyl]-2, 2′-bipyridine}ruthenium(ii) mono(tetrabutylammonium) salt), to the current best-performing dye, N719 (cis-bis(thiocyanato)bis(2,2′-bipyridine-4, 4′-dicarboxylato)ruthenium(ii) bis(tetrabutylammonium) salt). We have applied a suite of evaluation tools including: varying the electrolyte, varying the TiO₂ film thickness, charge density and recombination rate constant measurements, fluorescence lifetime and magnitude, and transient absorption techniques. The combined results indicate that TG6, as presently constructed, can surpass the performance of N719 under some conditions, but is likely to need some modification before surpassing cells designed to give record energy efficiency using N719. The higher absorption coefficient may be relevant to mass-produced DSSCs on metal where thinner TiO₂ films are advantageous. The main disadvantage is the slight catalysis of the electron/electrolyte recombination, which is possibly due to the extended π-system. A factor that requires further optimization involves the complex interaction of the slightly lower LUMO position, the composition of the electrolyte, the band edge position of the TiO₂, and the electron injection rate. We show why the maximum output from TG6 cells will not occur using the exact electrolytes used to maximize N719 cells. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b808255c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
A new ruthenium polypyridyl dye, TG6, whose performance in dye-sensitized
solar cells is surprisingly close to that of N719, the ‘dye to beat’ for 17 years
a
Farah Matar, Tarek H. Ghaddar, Kate Walley, Tracy DosSantos, James R. Durrant and Brian O’Regan
a
b
b
b
b
*
*
Received 15th May 2008, Accepted 17th June 2008
First published as an Advance Article on the web 6th August 2008
DOI: 10.1039/b808255c
A new ruthenium polypyridine sensitizer for dye-sensitized solar cells (DSSCs) is proposed containing
a hexasulfanyl–styryl-modified bipyridyl group as an ancillary ligand. The advantages of this dye are
the much larger absorption coefficient and the small shift of the absorption envelope to the red. We
compare this new dye, TG6 (cis-bis(thiocyanato)(2,20-bipyridyl-4,40-dicarboxylato){4,40-bis[2-(4-
hexylsulfanylphenyl)vinyl]-2,20-bipyridine}ruthenium(II) mono(tetrabutylammonium) salt), to the
current best-performing dye, N719 (cis-bis(thiocyanato)bis(2,20-bipyridine-4,40-
dicarboxylato)ruthenium(II) bis(tetrabutylammonium) salt). We have applied a suite of evaluation
tools including: varying the electrolyte, varying the TiO2 film thickness, charge density and
recombination rate constant measurements, fluorescence lifetime and magnitude, and transient
absorption techniques. The combined results indicate that TG6, as presently constructed, can surpass
the performance of N719 under some conditions, but is likely to need some modification before
surpassing cells designed to give record energy efficiency using N719. The higher absorption coefficient
may be relevant to mass-produced DSSCs on metal where thinner TiO2 films are advantageous. The
main disadvantage is the slight catalysis of the electron/electrolyte recombination, which is possibly due
to the extended p-system. A factor that requires further optimization involves the complex interaction
of the slightly lower LUMO position, the composition of the electrolyte, the band edge position of the
TiO2, and the electron injection rate. We show why the maximum output from TG6 cells will not occur
using the exact electrolytes used to maximize N719 cells.
(dialkylamino)benzene18 or alkoxybenzene moieties (such as
K19).12,19,20 However, despite the advantages shown by these dyes,
and the relatively high efficiencies reported, none of them have
displaced N719 for use in record cells. Given this situation, it
seems reasonable to explore further the parameter space of this
family of dyes, such as changing the alkoxybenzene moieties to the
more electron-rich sulfanyl groups. This led us to the strategy
of designing an amphiphilic ruthenium-based sensitizing dye,
TG6 cis-bis(thiocyanato)(2,20-bipyridyl-4,40-dicarboxylato){4,40-
bis[2-(4-hexylsulfanylphenyl)vinyl]-2,20-bipyridine}ruthenium(II)
mono(tetrabutylammonium) salt (Scheme 1). The TG6 dye
is engineered in a way to have a high absorption coefficient and
extended absorption in the visible region of the solar spectrum,
due to the extended p-conjugation of the ancillary ligand.
Such conjugation would drive the MLCT transition between
the metal center and the anchoring ligand to lower energies,
and thus the dye could have higher efficiency.21 In addition to
this, the presence of the alkyl groups acting as hydrophobic
moieties on the ancillary ligands can protect the complex from
water-induced desorption from the TiO2 surface, improving the
long-term stability of the DSSC.5 Alkyl chains may also slow
charge recombination processes.22 Furthermore, the presence
of the S-atom at the para-position of the phenyl ring has
greater radial extension in its bonding than the second-row
elements, such as oxygen, and would present a more electron-
rich moiety.
Introduction
Ruthenium polypyridyl based dyes have received great attention
due to their promising performance as sensitizers,1–6 especially
after the introduction of the practical dye-sensitized solar cell
(DSSC) in 1991.7 In a DSSC, a dye molecule is anchored to the
surface of nanocrystalline TiO2, and upon visible light excitation
an electron is injected from the excited dye to the TiO2 conduc-
tion band. To date, cis-bis(thiocyanato) bis(2,20-bipyridyl-4,40-
dicarboxylato)ruthenium(II) bis(tetrabutylammonium) (N719)
has been a paradigm in this field due to its outstanding perfor-
mance. In spite of this, one of the main drawbacks of this
sensitizer is the relatively low molar extinction coefficient in the
visible and near-IR.8 In order to overcome this problem,
different strategies have been employed, such as extending
the conjugation in the ancillary ligand,2,9–12 modifying the
ancillary ligand with electron-donating groups,5,13 the use of
multi-chromophoric dyes7,14,15 or both.16
Several publications have shown amphiphilic dyes with
extended conjugation and bearing either thiophene,2,5 pyrrole,17
aDepartment of Chemistry, American University of Beirut, Beirut, 11-0236
Lebanon. E-mail: tg02@aub.edu.lb; Fax: +961 1-365217; Tel:
+961-3-464568
bDepartment of Chemistry, Imperial College London, London, SW7 2AZ,
UK. E-mail: b.oregan@imperial.ac.uk; Fax: +44 (0) 207 594 5801; Tel:
+44 (0) 207 594 5031
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Scheme 1
Table 1 Photophysical and electrochemical properties of TG6 and N719
Results and discussion
Absorption l/nm
(3/105 Mꢀ1 cmꢀ1
Emission
l/nm
E1/2/V (RuII/III
vs. NHE
)
ELUMO/V
vs. NHE
Photophysical and electrochemical properties
a
)
The ruthenium-based dye TG6 was synthesized by a stepwise
protocol as shown in Scheme 1. The bipyridine ligand L was
synthesized by a one-step reaction starting from 4,40-dimethyl-
TG6
309 (0.54), 365
780a
755b
1.02a
1.09c
ꢀ0.86
ꢀ0.98d
(0.59), 547 (0.23)
306 (0.44), 379
(0.14), 525 (0.14)
N719
2,20-bipyridine. The TG6 dye was characterized by H and 13C
1
NMR, UV/Vis, FTIR and ESI mass spectrometry. The UV/Vis
and emission spectra of TG6 in DMF are shown in Fig. 1, in
addition to the N719 absorption spectrum as a reference. The
data is collected in Table 1. The band at 309 nm is due to
a bipyridine intra ligand (p–p*) charge transition, whereas the
maxima at 365 and 547 nm are due to the metal-to-ligand charge
transfer bands (MLCT). Compared to the N719 absorption
spectrum, the TG6 lowest energy MLCT is red-shifted by 22 nm
in DMF and the molar extinction coefficient is greatly higher (3 ¼
23 400 and 13 600 Mꢀ1 cmꢀ1 for TG6 and N719, respectively).
a
b
c
Measured in DMF. Measured in ethanol (ref. 8). Measured in
acetonitrile (ref. 8). d From ref. 24.
Because TG6 is a physically larger molecule than N719, there
was the possibility that the increased absorption coefficient could
be offset by a lower monolayer surface density of molecules on
the TiO2. Fig. 2 shows that this does not occur; the increased
light absorption on the TiO2 film matches exactly the increased
absorption in equimolar solutions.
The emission spectrum of TG6 obtained in air-equilibrated
DMF shows a maximum at 780 nm when excited at 530 nm
(Fig. 1). The E0ꢀ0 was calculated to be 1.87 eV from the inter-
section of the lowest energy MLCT band and the emission
band.23
In order to evaluate the oxidation potential (Eox*) of the
excited state of the TG6 dye, we performed cyclic voltammetry
measurements on TG6 in DMF with TBAPF6 as the supporting
electrolyte. The RuII/III redox couple of TG6 was measured to be
E1/2 ¼ 1.02 V vs. NHE. The Eox* of TG6 was approximated to be
about ꢀ0.85 V vs. NHE, whereas that of N719 is reported as
ꢀ0.98 V vs. NHE.8,24 In either case the Eox* is more negative than
the conduction band level of nanocrystalline TiO2 (ꢀ0.5 V) vs.
NHE.25 Such a negative value of the TG6 LUMO level is
a prerequisite for fast electron injection into TiO2.
Computational studies
We performed DFT calculations using the B3LYP/6-31G*
exchange correlation functional with the Los Alamos effective
core potential LanL2DZ26 as implemented in the Gaussian03
Fig. 1 Absorption (solid-blue) and emission (dotted-blue) spectra of
TG6 in DMF and absorption spectrum of N719 in DMF (dashed-red).
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program package,27 in order to gain insight on the electronic
structure of TG6 when compared to the protonated form of the
N719 dye (N3). We optimized the geometries of the two fully
protonated complexes (N3 and TG6) in water solution using the
C-PCM algorithm.25 The results are shown in Scheme 2. The
HOMO, HOMO ꢀ 1 and HOMO ꢀ 2 for both dyes have
a ruthenium t2g character in addition to a significant contribution
from the thiocyanate ligands (NCS). The HOMO ꢀ 3 and
HOMO ꢀ 4 of TG6 are combination of p-bonding orbitals of the
styryl-modified bipyridine ligand, the sulfur non-binding orbitals
and small contribution from the NCS ligands and the ruthenium
center. When compared to N3, the higher absorption extinction
coefficient measured for TG6 might be attributed to the existence
of these two additional occupied energy levels (HOMO ꢀ 3 and
HOMO ꢀ 4) in combination with the higher transitions’ oscil-
lator strength (Table 2). The HOMO ꢀ 3 of N3 and the HOMO
ꢀ 5 of TG6 are mainly non-bonding combinations localized on
the NCS ligands. The LUMO and LUMO + 1 of N3 are p*
orbitals delocalized on the dc-bpy ligands. However, the LUMO
and LUMO + 1 of TG6 are mainly p* combinations localized on
the dc-bpy and on the styryl-modified bipyridine ligand,
respectively. As can be seen from Scheme 2, the HOMO–LUMO
energy gap of TG6 (2.50 eV) is lower than that of N3 (2.60 eV) by
0.1 eV. This translates into a lower excitation energy of TG6,
Fig. 2 Absorption spectra of TG6 in DMF (solid-blue), TG6 on 4 mm
TiO2 film (dotted-blue), N719 in DMF (dashed-red) and N719 on 4 mm
TiO2 film (dashed-dotted-red). Film data has been normalized to solution
data without changing the ratio between dyes.
Scheme 2 Molecular orbital energy diagram of N3 and TG6. The HOMO–LUMO gaps are reported in eV, and lowest TD-DFT excitation energies in
nm (data in parantheses). The isodensity plots shown are for the HOMO ꢀ 3, HOMO ꢀ 1, HOMO, LUMO and LUMO + 1 of TG6.
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Table 2 Calculated spectra of TG6 and N3 above 460 nm
Table 3 Photophysical properties of DSSCs with TG6 and N719 as
sensitizers
TDDFT excitation
energy/nm
Oscillator
strengtha
Assignmentb
Film
thickness/mm Electrolyteb Jsc/mA cmꢀ2 Voc/V ff
h (%)a
TG6
N3
682
585
550
522
476
655
550
0.037
0.077
0.212
0.158
0.065
0.039
0.140
219 / 220 (66%)
219 / 221 (66%)
217 / 220 (58%)
218 / 221 (55%)
219 / 222 (55%)
163 / 164 (68%)
161 / 164 (50%),
162 / 165 (45%)
161 / 165 (50%),
162 / 164 (34%)
163 / 166 (69%)
TG6
N719
TG6
N719
TG6 12
N719 12
4
4
4
4
EL04
EL04
EL01
EL01
EL01
EL01
11.4
9.0
7.9
8.1
14.0
14.2
0.62
0.65
0.78
0.81
0.75
0.74
0.43 3.0
0.44 2.6
0.47 2.9
0.48 3.1
0.55 5.8
0.52 5.5
a
The efficiency of these cells is limited by the fill factor (ff), which is low
due to the large geometry and low conductivity glass. We include it for
completeness but note that comparisons should be made at the Jsc and
527
466
0.021
0.091
b
Voc level. Electrolytes. EL04: methoxyproprionitrile (MPN), 0.6 M
a
Only bands with oscillator strength f $ 0.02 are listed. b The HOMO is
219 (TG6) and 163 (N3), and the LUMO is 220 (TG6) and 164 (N3).
propylmethylimidazolium iodide (PMMI), 0.1 M LiI, 0.1 M tert-
butylpyridine (TBP), 0.25 M I2. EL01: MPN, 0.8 M PMMI, 0.5 M
TBP, 0.1 M guanidine thiocyanate, 0.05 M I2. Measured under 100
mW cmꢀ2 simulated AM1.5 spectrum.
which is consistent with the experimental UV/Vis spectrum of
TG6 when compared to N3.
Solar cell properties
The greatest advantage in the increased absorption coefficient
will be seen in sensitizing relatively thin TiO2 films such as found
in solid state DSSCs.28 Fig. 3 shows the photocurrent vs. voltage
(IV) response of two cells made with 4 mm layers of TiO2. In this
case, TG6 shows a higher photocurrent (11.4 mA) than N719 (9.0
mA) (Table 3). The electrolyte used (EL04, see Table 3) was one
which gives a lower voltage but promotes high quantum effi-
ciency of electron injection for both dyes. Fig. 4 shows the IPCE
(spectral response) of these two cells, compared to the fraction of
light absorbed by the dye in that cell at each wavelength. The two
curves for each dye lie on top of each other, showing that the
APCE (absorbed photon to current efficiency) is near 100% for
both dyes in this electrolyte. A high APCE means that all steps
after light absorption have minimal losses.
Fig. 4 IPCE of TG6 (solid-blue) and N719 (dashed-red), and absorption
spectra in fractional units of TG6 (dotted-blue) and N719 (dotted-
dashed-red), for the cells described in Fig. 3.
When a different electrolyte is used which gives higher voltages
(EL01, Table 3), the current decreases for both dyes, but the
decrease is much larger for TG6, resulting in equal current from
both cells. This electrolyte achieves the higher voltage by moving
the TiO2 conduction band edge potential away from the iodine/
iodide couple and closer to the excited state potential of the dye.
This decreases the driving force for electron injection from the
excited dye, slowing injection for both dyes. In EL01 injection
becomes sufficiently slow for both dyes that luminescence and
thermal decay of the exited state begin to compete with injection.
Two features contribute to the larger loss of injection in TG6.
One is that TG6 has a slightly lower LUMO energy than N719
(Table 1), reducing the driving force for injection at any given
band edge potential. The second is that electrolyte EL01 results
in a higher band edge position in the TG6 cell, compared to the
N719 cell. Fig. 5 shows the charge density vs. voltage plot for
these cells.29 The rightward shift in the charge density curve can
Fig. 3 Photocurrent–voltage characteristics of DSSCs with 4 mm TiO2
films sensitized with N719 (dotted-red) and TG6 (solid-blue) using EL04
as the electrolyte.
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Fig. 6 Electron lifetimes for TG6 (triangles-blue) and N719 (circles-red)
cells plotted vs. charge density, electrolyte EL01.
Fig. 5 Charge density vs. voltage plots for TG6 (open circles-blue) and
N719 (closed circles-red), electrolyte EL01.
30ꢂ depending on the exact conditions of the comparison. To
further optimize the use of TG6 we hope to find electrolyte/TiO2
combinations where the acceleration of the dark current by TG6
is minimized.
be interpreted as an upward movement of the TiO2 energy levels
relative to the electrolyte. The higher band edge potential in the
TG6 cell will be caused in part by the smaller number of protons
on the TG6 molecule (1) relative to N719 (2). However, the shift
is larger than the ꢁ20 mV expected from this change. The
additional 40 mV shift is related to the bulkier TG6 dye holding
the large imidazolium cations farther from the surface, and to
any molecular dipole differences. The higher band edge potential
creates a larger barrier to injection from TG6, compared to
N719, specifically in this electrolyte. Fig. 5 thus shows that
identical electrolytes will not be appropriate for both dyes.
Losses at the injection step can be estimated by TCSPC.29
Preliminary TCSPC results show that the luminescence from
TG6 on TiO2 films in EL01 is not strongly quenched relative to
TG6 on ZrO2 films in EL01, where injection is thought not to
occur.
When the film thickness is increased, still using electrolyte
EL01, the current increases from both dyes (Table 3, and Fig. 7).
The proportional increase is the same for both dyes, showing the
losses during transport through the thick films, if any, are not
significantly larger for TG6 than for N719. Discounting the fill
factor difference, the two dyes give identical efficiencies. It is
interesting to note that in the case of thick films, the voltage is the
same for both dyes. In this case the increased band offset and
increased recombination still occur, but cancel each other.
Another point of loss could be at the regeneration step where
the oxidized dye is reduced by iodide. We have applied TAS
(transient absorption spectroscopy) to study the regeneration
rate and efficiency of TG6 relative to N719. Preliminary TAS
results indicate no change in the regeneration rate, consistent
with the small change in the oxidation potential between TG6
and N719.
Table 3 also shows that the voltage is lower for TG6 in both
electrolytes, despite the TiO2 band edge being higher in the TG6
cell. This is caused by the effect of dye structure on the
recombination of injected electrons with the iodine/iodide elec-
trolyte.30 In the case of TG6, the extended p-structure may
increase the binding of iodine to the dye, resulting in an
increased surface concentration of the acceptor.30 Fig. 6 shows
the electron lifetimes for the TG6 and N719 cells plotted vs.
charge density. At higher charge densities, corresponding to the
one sun Voc, the lifetime for TG6 cells is ꢁ8 times smaller than
for N719. An increased recombination rate is not unique to
TG6, but also occurs for most dyes, when compared to N719.30
The difference between TG6 and N719 varies between 8 and
Fig. 7 Photocurrent–voltage characteristics of 12 mm TiO2 electrodes
sensitized with dyes: N719 (dashed-red) and TG6 (solid-blue) using EL01
as the electrolyte.
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light source for which the intensity was measured with a silicon
photodiode traceable to NREL in two steps. Photocharge
measurements at Voc were measured using the charge extraction
technique.34,35 Recombination lifetimes at Voc were measured
using small perturbation photovoltage transients using a white
bias light and a red pulse diode.36,37 Time-correlated single
photon counting (TCSPC) was measured on TiO2 and ZrO2 films
using a Yobin Yvon spectrometer.38 TAS spectroscopy was
measured using a nitrogen dye laser pump and a monochromatic
probe as previously described.39
Experimental section
Materials and instrumentation
All organic chemicals were purchased from Aldrich and used as
supplied. Di-m-chloro bis[(h6-p-cymene)chlororuthenium(II)] was
purchased from Strem Chemicals Inc. (USA). Sephadex LH-20
was purchased from Pharmacia (USA). The N719 dye was
purchased from Dyesol (Australia). FTO glass ‘Tec15’ was
purchased from Pilkington (UK). 2,20-Bipyridine-4,40-dicarbox-
ylic acid was prepared according to a reported procedure in the
literature.31 Electrolyte components were purchased from Sigma-
Aldrich except pyropylmethylimidazolium iodide (PMMI),
which was purchased from Solvent Inovation. Methox-
yproprionitrile (MPN) was distilled prior to use, and all other
components were used as delivered. Mettler Toledo FP62
apparatus was used to determine melting points. The infrared
spectra were recorded as KBr pellets using a Nicolet AVATAR
360 FTIR ESP spectrometer. The NMR spectra (1H and 13C)
were measured on a Bruker AM 300 MHz spectrometer. UV-vis
spectra were recorded on a Jasco V-570 UV/vis/NIR. The ESI
mass spectroscopy measurements were done at Georgia Tech.
Research Corporation (Atlanta, GA). The electrochemical setup
consisted of a three-electrode cell, with a glassy carbon electrode
as the working electrode, a Pt wire ꢁ1 mm diameter as the
counter-electrode, and Ag/AgCl as the reference electrode. All
solution phase electrochemical measurements were done in 0.1 M
TBAPF6 in DMF.
Preparation of 4-(hexasulfanyl)benzaldehyde
To a stirred solution of hexane-1-thiol (2.7 g, 0.023 mol) and
anhydrous potassium carbonate (50.4 g, 0.37 mol) in dry DMF at
90 ꢃC, 4-chlorobenzaldehyde (3.2 g, 0.023 mol) was added
slowly. The resulting mixture was stirred at the same temperature
for 24 h. After cooling, the reaction mixture was quenched with
water and extracted with diethyl ether. The organic layer was
washed with brine and water, consecutively, and dried over
anhydrous magnesium sulfate. The solvent was then evaporated
under vacuum to obtain an oily product. Vacuum distillation of
the product at 200 ꢃC/0.8 mm Hg afforded the pure product (2.70
g, 54% yield). Mp 20–22 ꢃC. 1H NMR (CDCl3, 300 MHz),
d (ppm): 0.91 (t, 3H), 1.27–1.37 (m, 4H), 1.43–1.49 (m, 2H), 1.67–
1.77(m, 2H), 3.02 (t, 2H), 7.35 (d, J ¼ 6.0 Hz, 2H), 7.76 (d, J ¼
6.0 Hz, 2H), 9.93 (s, 1H). 13C NMR (CDCl3, 75 MHz), d (ppm):
14.02, 22.52, 22.74, 28.60, 31.32, 31.78, 126.25, 130.03, 133.07,
+
147.19, 191.28. ESI MS: m/z ¼ 222.4 [M + H] calculated for
Computational methods
C13H18OS, 222.1.
Calculations were carried out using Gaussian 03.27 Geometries
were optimized using the 6-31G* basis set with (B3LYP) together
with the Los Alamos effective core potential LanL2DZ26 in water
(C-PCM algorithm).
Preparation of 4,40-bis[2-(4-hexylsulfanylphenyl)vinyl]-2,20-
bipyridine
Potassium t-butoxide (1.0 g, 9.0 mmol) was slowly added to
a stirred mixture of 4-(hexasulfanyl)benzaldehyde (1.5 g, 6.7
mmol) and 4,40-dimethyl-2,20-bipyridine (0.6 g, 3.2 mmol) in
80 ml anhydrous DMF. The reaction was stirred at room
temperature for 24 h in the dark. The reaction was quenched with
water, and the precipitated product was filtered and air-dried.
The product was purified on a silica column using CH2Cl2 as the
eluting solvent. The fraction containing the product was evapo-
rated under reduced pressure, and the product was dried under
vacuum for 24 h to afford the pure product (0.5 g, 26% yield). Mp
144–145 ꢃC 1H NMR (CDCl3, 300 MHz), d (ppm): 0.82 (t, 6H),
1.21–1.25 (m, 8H), 1.30–1.37 (m, 4H), 1.40–1.64 (m, 4H), 2.89 (t,
4H), 7.05 (d, J ¼ 16.2 Hz, 2H), 7.22 (d, J ¼ 8.0 Hz, 4H), 7.30 (s,
2H), 7.31 (d, J ¼ 8.0 Hz, 4H), 7.42 (d, J ¼ 16.2 Hz, 2H), 8.46 (s,
2H), 8.60 (d, J ¼ 6.0 Hz, 2H). 13C NMR (CDCl3, 75 MHz),
d (ppm): 14.05, 22.56, 28.58, 29.02, 31.37, 33.09, 118.17, 121.06,
Solar cell fabrication
Dye-sensitized solar cells were fabricated using standard proce-
dures. TiO2 colloids were fabricated following reported proce-
dures in the literature.32,33 The TiO2 films were made from
colloidal solutions using the doctor blading method and then
heated to 450 ꢃC for 30 min. A TiCl4 post-treatment was applied
to the films following reported procedures in the literature34 or
using a similar route with 40 mM TiCl4–THF complex in
aqueous solution. These films were then reheated at 450 ꢃC.
Dyeing was done using 0.5 mM solutions in t-butyl alcohol for
N719 and methylene chloride for TG6. The counter electrodes
were fabricated by applying 2–3 ml cmꢀ2 of 5 mM H2PtCl6 in
isopropanol to the FTO glass, followed by heating in an oven at
ꢃ
400 C for 20 min. Cell assemblies were formed by sealing the
1125.53, 127.43, 128.33, 132.77, 133.48, 138.33, 145.78, 149.56,
+
counter-electrodes to the TiO2 electrode with heat-setting glue
(Surlyn, Dupont) at ꢁ100 ꢃC for 1 min. The corresponding
electrolyte was introduced through two small holes, previously
drilled through the counter-electrode, which were then sealed
with surlyn. Photocurrent vs. voltage characteristics were
measured on a solar simulator illuminated by a Xenon arc lamp
through an AM1.5 simulation filter (ScienceTech). Photocurrent
values were cross-checked with other laboratories (EPFL).
Spectral response (IPCE) was measured using a monochromatic
156.48. ESI MS: m/z ¼ 592.6 [M + H]
calculated for
C38H44N2S2, 592.3.
Preparation of cis-bis(thiocyanato)(2,20-bipyridyl-4,40-
dicarboxylato){4,40-bis[2-(4-hexylsulfanylphenyl)vinyl]-2,20-
bipyridine}ruthenium(II) mono(tetrabutylammonium) salt, TG6
In an argon-degassed anhydrous DMF, a mixture of 4,40-bis-[2-
(4-hexylsulfanylphenyl)vinyl]-2,20-bipyridine (400 mg, 0.67
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mmol) and di-m-chloro-bis[(h6-p-cymene)chlororuthenium(II)]
References
ꢃ
(206 mg, 0.33 mmol) were heated at 100 C for 4 h. Then, 4,40-
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1
Mp >300 ꢃC. H NMR (DMSO, 300 MHz), d (ppm): 9.36 (d,
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1H), 8.13 (s, 1H), 7.81 (d, 1H), 7.07–7.74 (m, 16H), 3.02–3.35 (m,
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In summary, we have synthesized an efficient dye, TG6, char-
acterized by its extended conjugation with alkylsulfanyl
substituents as electron-donating groups. The photovoltaic
performance of the solar cell sensitized with TG6 was compared
with that of the N719-sensitized cell. A solar-to-electricity
conversion efficiency of h ¼ 5.8% for TG6 is higher than h ¼
5.5% for the N719-sensitized solar cell with 12 mm thick TiO2
film and EL01 as the supporting electrolyte, while maintaining
a slightly higher cell voltage (Voc ¼ 0.75 and 0.74 V for TG6 and
N719, respectively). However, the TG6-sensitized cell gave
higher current than N719 with 4 mm thick TiO2 film and EL04
as the supporting electrolyte. The efficient performance of TG6
as a sensitizing dye is attributed to its high absorption extinction
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solar spectrum. Further optimization of the electrolyte will be
required to complete the comparison of N719 and TG6. Addi-
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TG6-based DSSCs. Finally, in order to further improve TG6
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Acknowledgements
This work was supported by the University Research Board
(URB) at the American University of Beirut (AUB) and the
Lebanese National Council for Scientific Research (LNCSR), the
UK EPSRC (Grant EP/E035175/1) and the EU project
‘RobustDSC’.
4252 | J. Mater. Chem., 2008, 18, 4246–4253
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