Molecular design and synthesis of ruthenium(ii) sensitizers for highly efficient dye-sensitized solar cells

Article abstract of DOI:10.1039/c1jm11760b

We synthesized hydrophobic ruthenium(ii) sensitizers (SY-04 and SY-05) with high molar extinction coefficient by extending the π-conjugation of 3,4- or 3-alkylthiophene-substituted bipyridine ligands. Both dyes displayed a remarkably high molar extinction coefficient of 21.7 × 10³ M^-1 cm^-1 arising from red-shift of their metal-to-ligand charge transfer band when compared to a commonly used N3 sensitizer. The solar-to-electrical energy conversion efficiency (η) of the SY-04 based dye-sensitized solar cell (DSC) was 7.70%, which is 27% higher than that (6.05%) of the N3-based DSC under the same cell fabrication conditions. The increased η was attributed to the increase in life time and recombination half-life measured by electrochemical impedance and transition absorption spectroscopy, respectively. Density functional theory and time-dependent density functional theory calculations of two dyes in both gas phase and solution were performed. The calculated values of the highest occupied and the lowest unoccupied molecular orbitals and absorption spectra are in good agreement with the experimental results.

Full text of DOI:10.1039/c1jm11760b

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PAPER
Molecular design and synthesis of ruthenium(II) sensitizers for highly efficient
dye-sensitized solar cells†
A. Anthonysamy,a Y. Lee,a B. Karunagaran,b V. Ganapathy,b S.-W. Rhee,b S. Karthikeyan,c K. S. Kim,c
d
M. J. Ko, N.-G. Park, M.-J. Ju and J. K. Kim
e
f
a
*
Received 22nd April 2011, Accepted 8th June 2011
DOI: 10.1039/c1jm11760b
We synthesized hydrophobic ruthenium(II) sensitizers (SY-04 and SY-05) with high molar extinction
coefficient by extending the p-conjugation of 3,4- or 3-alkylthiophene-substituted bipyridine ligands.
Both dyes displayed a remarkably high molar extinction coefficient of 21.7 ꢀ 103 Mꢁ1 cmꢁ1 arising from
red-shift of their metal-to-ligand charge transfer band when compared to a commonly used N3
sensitizer. The solar-to-electrical energy conversion efficiency (h) of the SY-04 based dye-sensitized
solar cell (DSC) was 7.70%, which is 27% higher than that (6.05%) of the N3-based DSC under the same
cell fabrication conditions. The increased h was attributed to the increase in life time and recombination
half-life measured by electrochemical impedance and transition absorption spectroscopy, respectively.
Density functional theory and time-dependent density functional theory calculations of two dyes in
both gas phase and solution were performed. The calculated values of the highest occupied and the
lowest unoccupied molecular orbitals and absorption spectra are in good agreement with the
experimental results.
DSCs based on N3 and N719 sensitizers combined with a thick
Introduction
mesoscopic titania film and an acetonitrile-based electrolyte show
a solar-to-electrical energy conversion efficiency (h) of up to 11.1%
under the air mass 1.5 global (AM 1.5G) illumination,8–11 a further
improvement in the efficiency is required for the cost-effective
commercial production of DSCs. To achieve this objective, one
should increase the absorption at near-infrared wavelengths or the
molar extinction coefficient at visible wavelengths.
Increasing energy demandsand concerns over global warming have
led to a greater focus on renewable energy sources. Dye-sensitized
solar cells (DSCs) based on the ruthenium(II) dyes have been shown
to be very efficient candidates for photovoltaic technology due to
their high photon-to-current conversion efficiency, low
manufacturing cost, long-term stability, and tunable electro-
chemical properties covering wide absorption ranges from the
visible to the near-infrared (NIR).1–7 Although the currently used
To enhance both the molar extinction coefficient and red-shift
arising from the metal-to-ligand charge transfer (MLCT) band,
while maintaining good solubility, various ancillary ligands,
such as 2-hexylthiophene (HRS-1), furan, carbazole,
dialkylaminobenzene, dipyridylamine, hexylsulfanylphenyl
(TG6) and triazole derivatives, have been attached to the
bipyridine ligand in the ruthenium dyes.12–27 Among many
ancillary ligands, the thiophene moiety showed high efficiency.
Wang and coworkers14–17 synthesized a series of ruthenium dyes
(C101–C107) with extended p-conjugation length by using
a single thiophene or furan derivative. Ho and coworkers19–21
synthesized bithiophene (CYC-B1) and thiophene-attached
carbazole derivatives (CYC-B6S and CYC-B6L) as the ancillary
ligands that are attached to bipyridine ruthenium(II) sensitizers.
These dyes showed a high molar extinction coefficient and red-
shift of the MLCT. The h of a dye with thiophene-functionalized
ancillary ligands is known to be higher than that of another dye
without the ligands.14–27
aNational Creative Research Initiative Center for Block Copolymer
Self-Assembly, Department of Chemical Engineering, Pohang University
of Science and Technology, Pohang, Kyungbuk, 790-784, Republic of
Korea. E-mail: jkkim@postech.ac.kr
bSystem on Chip Chemical Process Research Center, Department of
Chemical Engineering, Pohang University of Science and Technology,
Pohang, Kyungbuk, 790-784, Republic of Korea
cCenter for Superfunctional Materials, Department of Chemistry, Pohang
University of Science and Technology, Pohang, Kyungbuk, 790-784,
Republic of Korea
dSolar Cell Research Center, National Agenda Research Division, Korea
Institute of Science and Technology, Seoul, 136-791, Republic of Korea
eSchool of Chemical Engineering and Department of Energy Science,
Sungkyunkwan University, Suwon, 440-746, Republic of Korea
fDepartment of Advanced Materials Chemistry & Center for Advanced
Photovoltaic Materials (CAPM), Korea University, ChungNam, 339-
700, Republic of Korea
† Electronic supplementary information (ESI) available: Synthesis,
characterization and optimized structure for ancillary ligands and
sensitizers, calculated molecular orbitals/energy-levels, and transition
assignments. See DOI: 10.1039/c1jm11760b
However, the ancillary ligands of the thiophene derivative in
ref. 14–21 were synthesized via the Stille coupling reaction of
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4,40-dibromo-2,20-bipyridine with corresponding stannane thio-
phene ligands in the presence of palladium as a catalyst.14–21 The
yield of the product was relatively low due to the poor solubility
of the tin compound in the solvent. Furthermore, since the
product might have both cis and trans isomers, multiple (at least
3 (or possibly 4) times) purifications by using column chroma-
tography should be required to obtain the desirable product
(only cis isomer). Thus, an efficient synthetic method is of the
utmost importance to produce high yields of ruthenium(II) dyes
with thiophene ancillary ligands without sacrificing high molar
extinction coefficients as well as the red-shift of the MLCT band.
We realize that the stereoselectivity of the Horner–Emmons–
lowest unoccupied molecular orbitals (HOMO and LUMO) and
absorption spectra are in good agreement with experiment
results. Fig. 1 shows the molecular structures of the two sensi-
tizers (SY-04 and SY-05) synthesized in this study and N3 for the
reference dye.
Experimental section
General procedures
All reactions were performed under a dried nitrogen atmosphere.
Solvents and liquid reagents were transferred by nitrogen flushed
syringes or cannula. Reaction mixtures were stirred with Teflon
coated magnetic stir bars. Reaction solvents used were HPLC
grade, dried further by passage through a column of appropriate
desiccants. Commercial reagents were used as such unless
otherwise noted. Thin layer chromatography was performed
using Merck 60 F524 pre-coated silica gel plates. Subsequent to
elution, the plates were visualized by ultra-violet light at 254 nm.
Column chromatography was performed using Merck silica gel
60 (230–400 mesh).
Wadsworth (HEW)28 reaction gives only the E-form alkene with
29,30
€
very high yield. Previously, Gratzel and coworkers
synthe-
sized K-series (K19, K77), ruthenium(II) sensitizers by using
alkyloxybenzaldehyde attached-bipyridine ligands via the HEW.
But the alkyloxybenzaldehyde moieties showed low molar
extinction coefficients compared with the thiophene attached
ruthenium(II) sensitizers.14–21
The amount of a dye sensitizer adsorbed on the TiO2 surface
has a significant influence on the open circuit voltage (Voc)
because the Fermi energy level of TiO2 is influenced by the
density of photo-injected electrons. Since the aggregation of
a dye on the TiO2 surface sometimes decreases Voc, the solubility
of a dye in the solvent should be increased to enhance the dye
adsorption.31 For an ancillary ligand with thiophene, the intro-
duction of alkyl chains increases the solubility. However, the
ruthenium sensitizers (C101–C107, and CYC-B1 and CYC-B6S)
reported in the literature have the alkyl chain attached to posi-
tion 2 (or 5) in the thiophene ring.14–21 It is noted that the
attachment of the alkyl (for instance, n-hexyl) group to position 3
(or 4) of the thiophene ring shows very high solubility compared
with the attachment of the alkyl group to position 2 (or 5) of the
thiophene ring.32,33 Thus, when the alkyl chain could be located
at position 3 (or 4) in the thiophene ring attached to the bipyr-
idine ligand (or its ruthenium complexes), the solubility is very
much enhanced in common organic solvents (chloroform,
tetrahydrofuran, ethanol, toluene, and acetonitrile) compared
with other ligands (or their ruthenium complexes) having the
alkyl chain attached to other positions (2 or 5) in the
thiophene.12–27
UV-Vis spectra were obtained with a Carry 5000 UV-Vis-near
infrared double beam spectrophotometer. Photoluminescence
(PL) spectra were collected with a laser excitation source
instrument. An 800 nm laser beam from a continuous wave
Ti/sapphire laser was used as an excitation source. A combina-
tion of the monochromator (Oriel 77200), InSb (Judson SP28) or
PIN detector (Thorlabs DET410), lock-in amplifier, and
mechanical chopper was used to record the photoluminescence
of dyes.
1
The H and 13C NMR spectra were measured with a Bruker
(1H 500 MHz and 13C, 125 MHz) spectrometer. Chemical shifts
are reported in parts per million using the residual solvent peaks
as internal reference. The C, H, N contents of the ligands and
complexes were carried out using Carlo Erba Elemental analyzer
Model 1106 and Haereus CHN rapid analyzer. High resolution
mass spectral analysis was performed by using 6224 TOF LC-MS
and the value is in the unit of mass to charge (m/z). The elec-
trospray ionization (ESI) source was selected for positive voltage
for ligands and negative voltage for complexes. Finally, melting
points of the ligands were measured on a capillary melting point
apparatus (Optimelt/SRS).
Based on the above argument, in this study, we synthesized
two heteroleptic ruthenium(II) dyes (SY-04 and SY-05) con-
taining the ancillary ligands of 3,4-dihexylthiophene and
3-hexylthiophene units which were synthesized based on the
HEW reaction. We found that the overall yield for the synthesis
of the ancillary ligands was very high (more than 85%). Also, the
solubility of the dyes in solvents is very good due to the existence
of the 3,4-dihexylthiophene or 3-hexylthiophene groups. Because
the ligands employed in this study extended the p-conjugation,
the dyes increased the absorption coefficients and red-shifted the
MLCT band. The solar-to-electrical energy conversion efficiency
(h) of the SY-04-based DSC was 7.70%, which is 27% higher than
that (6.05%) of the N3-based DSC under the same cell fabrica-
tion conditions. The effects of the alkyl thiophene ancillary
ligands on energy levels and electronic transitions of the sensi-
tizers were discussed by the density functional theory (DFT) and
the time-dependent density functional theory (TD-DFT) calcu-
lations. The calculated values of the highest occupied and the
The cyclic voltammograms of the sensitizers (10ꢁ3 M in DMF)
with 0.1 M (n-C4H9)4NPF6 as a supporting electrolyte were
obtained with a CHI600A electrochemical analyzer at a scan rate
of 100 mV sꢁ1. The measurements were carried out under oxygen-
free conditions using a three-electrode cell in which a carbon
electrode was used as a working electrode, a saturated Ag/AgCl
electrode as a reference electrode, and platinum wire was used as
an auxiliary electrode. The ferrocene/ferrocenium (+1) couple
Fig. 1 The molecular structures of SY-04, SY-05 and N3 dyes.
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was used as an internal standard. The half potential (E1/2
)
Synthesis of ligand (7)
between the cathodic peak and the anode peak for the ferrocene/
ferrocenium couple is 0.47 V in dimethylformamide (DMF)
medium.
Ligand 7 was synthesized similarly to ligand 6 by using 5 instead
of 2. The crude product was purified by column chromatography
(silica gel, acetone) to give as a white solid (0.56 g). Yield: 85%;
mp: 127–128 ꢂC; H NMR (500 MHz, CDCl3): d 8.67 (d, J ¼
1
5.0 Hz, 2H), 8.50 (s, 2H), 7.60 (d, J ¼ 15.5 Hz, 2H), 7.39 (dd, J ¼
5.0 Hz, 2H), 7.21 (d, J ¼ 5.0 Hz, 2H), 6.94 (d, J ¼ 16.0 Hz, 2H),
6.90 (s, 2H), 2.77 (t, J ¼ 8.0 Hz, 4H), 1.66 (m, 6H), 1.42–1.34 (m,
10H), 0.93 (t, J ¼ 5.0 Hz, 6H). 13C NMR (125 MHz, CDCl3):
156.70; 149.56; 145.96; 143.32; 135.53; 130.08; 125.16; 124.75;
124.70; 120.90;120.72; 31.81; 31.10; 29.17; 28.67; 22.72; 14.19.
LC-MS (ESI, positive mode): Calcd for C34H40N2S2: [M+]:
540.26. Found. [MH+]: 541.2709. Anal. Calcd for C34H40N2S2:
C: 75.51; H:7.45; N: 5.18. Found. C: 75.57; H: 7.52: N: 5.23%.
Materials
3,4-Dihexylthiophene (1) and 2-bromo-3-hexylthiophene (4)
were synthesized according to the literature.34,35 Two ancillary
ligands (6 and 7) were first synthesized from the HEW reaction28
between 3,4-dihexylthiophene carbaldehyde (2) or 3-hexylth-
iophene carbaldehyde (5) and 4,40-bis(diethylphosphonomethyl)-
2,20bipyridine36 (3). Then, a one-pot synthetic procedure devel-
oped for heteroleptic polypyridyl ruthenium(II) sensitizer36 was
used for the synthesis of SY-04 and SY-05, as shown in Scheme 1.
(A detailed synthesis of 1–5 is given in the supporting
information.†)
Synthesis of ruthenium(II) sensitizer (SY-04)
In a nitrogen-flushed three-neck flask, 6 (0.135 g, 0.190 mmol)
and
dichloro(p-cymene)ruthenium(II)
dimer (0.059 g,
ꢂ
Synthesis of ligand (6)
0.096 mmol) in DMF were heated at 70 C for 4 h in the dark.
Subsequently, 4,40-dicarboxylic acid-2,20-bipyridine (8) (0.046 g,
0.190 mmol; Sigma Aldrich Co.) was added and the reaction
mixture was heated to 150 ꢂC and reacted at this temperature for
4 h. The solution became dark green. Solid NH4NCS (0.376 g,
4.94 mmol) was added to ꢂthis solution and the reaction was
carried out for 4 h at 140 C. DMF was removed on a rotary
evaporator under vacuum, and water (200 mL) was added to
cause precipitation. The purple solid was filtered off, washed
with water and dried under vacuum. The crude compound was
dissolved in basic methanol with tetrabutylammonium
hydroxide and then purified using the Sephadex LH-20 resin in
column chromatography with methanol as eluent. The main
band was collected and solution was slowly titrated with acidic
methanol (HNO3) to become pH 5.6. The precipitate was
collected on a sintered glass crucible by suction filtration and
A solution of 2 (0.5 g, 1.78 mmol) in dry THF (20 mL) was added
dropwise into the solution of 3 (0.4 g, 0.87 mmol) and t-BuOK
(0.2 g, 1.78 mmol) in dry THF (30 mL) under dry argon and
stirred for 5 h at room temperature. The solvent was evaporated
under a vacuum. Aqueous HCl (0.05 M, 20 mL) was added, and
the aqueous phase was extracted three times with CH2Cl2
(30 mL). The combined organic layers were washed with water
and dried over MgSO4. The solvent was evaporated under
a vacuum. The crude product was purified by column chroma-
tography (silica gel with 60–120 mesh size, acetone) to give a light
yellow solid (0.54 g). Yield: 87%; mp: 108–109 ꢂC; 1H NMR (500
MHz, CDCl3): d 8.66 (d, J ¼ 5.1 Hz, 2H), 8.46 (s, 2H), 7.59 (d,
J ¼ 15.5 Hz, 2H) 7.39 (dd, J ¼ 5.0 Hz, 2H), 6.92 (d, J ¼ 16.0 Hz,
2H) 6.84 (s, 2H), 2.70 (t, J ¼ 8.0 Hz, 4H), 2.52 (t, J ¼ 8.0 Hz, 4H),
1.65 (m, 4H), 1.53 (m, 4H), 1.40–1.35 (m, 24H), 0.93 (t, J ¼ 5.0
Hz, 12H). 13C NMR (125 MHz, CDCl3): 156.58; 149.43; 145.93;
143.57; 142.41; 135.88; 125.14; 124.57; 120.45; 119.99; 118.27;
31.76; 31.66; 31.21; 29.67; 29.40; 28.99; 27.06; 22.66; 14.13.
LC-MS (ESI, positive mode): Calcd for C46H64N2S2: [M+]:
708.45. Found. [MH+]: 709.4586. Anal. Calcd for C46H64N2S2:
C: 77.91; H: 9.10; N: 3.95. Found. C: 77.98; H: 9.15; N: 3.91%.
1
dried under vacuum. Yield: 65%. H NMR (500 MHz, DMSO-
d6): d 9.35 (d, J ¼ 5.5 Hz, 1H); 9.15 (d, J ¼ 6.0 Hz, 1H); 9.07 (s,
1H); 8.91 (s, 1H); 8.87 (s, 1H); 8.71 (s, 1H); 8.21 (t, J ¼ 6.5 Hz,
2H); 8.06 (d, J ¼ 16.0 Hz, 1H); 7.78 (s,1H); 7.74 (d, J ¼ 5.5 Hz
1H); 7.58 (dd, J ¼ 6.0 Hz 1H); 7.43 (d, J ¼ 6.0 Hz, 1H); 7.31 (d,
J ¼ 6.0 Hz, 1H); 7.26 (s, 1H); 7.18 (s, 1H); 7.07 (d, J ¼ 15.5 Hz,
1H); 6.81 (d, J ¼ 15.5 Hz, 1H); 2.83 (t, J ¼ 7.4 Hz, 2H); 2.81 (t,
J ¼ 7.4 Hz, 2H); 1.78 (t, J ¼ 7.4 Hz, 2H), 1.63 (t, 2H, 7.4); 1.56 (t,
J ¼ 7.4 Hz, 2H); 1.43–1.26 (m, 10H); 0.91 (t, J ¼ 6.0 Hz, 3H);
0.83 (t, J ¼ 7.0 Hz, 3H). 13C NMR (125 MHz, DMSO): 165.57;
165.16; 158.62; 158.21; 157.32; 152.39; 151.45; 144.77; 144.06;
143.46; 143.33; 143.25; 135.40; 135.18; 133.89; 133.69; 127.40;
127.18; 125.81; 122.71; 122.40; 122.17; 121.76; 121.69; 119.90;
aliphatic carbon: 30.99; 30.95; 30.84; 30.68; 29.20; 19.13; 28.89;
28.49; 28.44; 28.37; 28.16; 28.09; 22.99; 22.01; 21.96; 21.93; 19.11;
13.81; 13.80; 13.79; 13.71; 13.34. LC-MS (ESI, negative mode):
Calcd for C60H72N6O4S4: [M+]: 1170.35. Found. [MHꢁ]:
1169.4049. Anal. Calcd for C60H72N6O4S4: C: 61.56; H: 6.20; N:
7.18. Found. C: 61.63; H: 6.28; N: 7.23%.
Synthesis of ruthenium(II) sensitizer (SY-05)
The synthesis of SY-05 is similar to that of SY-04 by using 7
instead of 6. The crude product was purified on the Sephadex
Scheme 1 The synthesis of SY-04 and SY-05.
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LH-20 resin in column chromatography with methanol as eluent.
The main band was collected and solution was slowly titrated
with acidic methanol (HNO3) to become pH 5.6. Yield: 65%. 1H
NMR (500 MHz, DMSO- d6): d 9.70 (d, J ¼ 6.0 Hz, 1H); 9.28 (d,
J ¼ 6.0 Hz, 1H); 9.12 (s, 1H); 8.96 (s, 1H); 8.51 (s, 1H); 8.43 (s,
1H); 8.13 (d, J ¼ 6.0 Hz 2H); 8.06 (s, 1H); 7.83 (d, J ¼ 15.5 Hz
2H); 7.71 (dd, J ¼ 6.0 Hz 1H); 7.56 (d, J ¼ 5.0 Hz 1H); 7.49 (d,
J ¼ 5.0 Hz 1H); 7.35 (d, J ¼ 5.0 Hz, 1H); 7.20 (d, J ¼ 6.5 Hz 1H);
7.15 (d, J ¼ 5.5 Hz 1H); 7.09 (d, J ¼ 5.0 Hz, 1H); 7.01 (d, J ¼ 15.5
Hz 1H); 6.82 (d, J ¼ 15.5 Hz, 1H); 2.83 (t, J ¼ 7.4 Hz, 2H); 2.81
(t, J ¼ 7.4 Hz, 2H); 1.78 (t, J ¼ 7.4 Hz, 2H); 1.63 (t, 2H, 7.4); 1.56
(t, J ¼ 7.4 Hz, 2H); 1.43–1.26 (m, 10H); 0.91 (t, J ¼ 6.0 Hz 3H);
0.83 (t, J ¼ 7.0 Hz 3H). 13C NMR (125 MHz, DMSO): 165.58;
165.19; 158.58; 158.23; 157.30; 157.00; 152.31; 151.57; 152.47;
150.81; 144.68; 144.49; 144.43; 144.32; 143.97; 143.81; 140.75;
140.57; 134.91; 134.69; 133.83; 133.67; 130.32; 130.22; 126.83;
126.61; 126.47; 126.39; 125.81; 125.18; 123.18; 123.06; 122.94;
122.65; 122.39; 122.12; 120.11; 119.78; aliphatic carbon: 31.02;
31.00; 30.95; 30.91; 30.88; 29.72; 29.58; 27.53; 22.98; 22.00; 21.93;
19.11; 13.96; 13.90; 13.89; 13.61; 13.32. LC-MS (ESI): Calcd for
C48H48N6O4S4: [M+]: 1002.17. Found. [M–Hꢁ]: 1001.2114. Anal.
Calcd for C48H48N6O4S4: C, 57.52; H, 4.83; N, 8.39. Found. C,
57.58; H, 4.86; N, 8.45%.
the solvent (ethanol) medium using the polarized continuum
model (PCM).43
Device fabrication
Transparent TiO2 electrodes were prepared by screen printing
a paste of TiO2 nanoparticles (T20/SP; Solaronix) on the FTO-
coated glass (8 U cmꢁ2) and annealed at 500 ꢂC in air for 30 min.
Then, a reflecting scattering layer containing ꢃ400 nm sized TiO2
anatase particles (Ti-Nanoxide R/SP; Solaronix) was also
ꢂ
prepared by screen printing and sintering at 500 C for 30 min.
The thickness of the TiO2 electrode was approximately 17 mm.
ꢂ
After cooling to 80 C the sintered TiO2 electrode was immedi-
ately immersed into the ethanol solution with dyes (SY-04 and
SY-05) (0.3 mM) synthesized in this work and kept at room
temperature for 24 h. For the comparison, the same TiO2 elec-
trode was also immersed into ethanol solution with N3 (0.3 mM).
The dye-adsorbed TiO2 electrode was washed in ethanol and
dried with nitrogen. A 60 mm thick Surlyn spacer (SX1170-60
from Solaronix) was sandwiched between the photo anode and
a platinized FTO counter electrode. Iodide-based redox elec-
trolyte was used in the present work (the details are given in the
supporting information†).
The photovoltaic performance of the cells was measured with
a solar simulator (Newport, USA) equipped with an AM 1.5G
filter and a 300 W Xenon light source. The one sunlight intensity
of 100 mW cmꢁ2 was adjusted with a standard calibrated Si solar
cell (PV Measurements Inc.). The current–voltage characteristics
were obtained by applying an external bias to the cell and
measuring the generated photocurrent with a Keithley model
2400 digital source meter. Incident photon to current conversion
efficiency (IPCE) was recorded by using a computerized setup
consisting of a xenon arc lamp and a monochromator (PV
Measurements Inc).
Electrochemical impedance spectra
Electrochemical impedance spectra (EIS) were obtained in the
dark using an SP-200 electrochemical workstation with
frequencies ranging from 50 mHz to 100 kHz and potentials from
ꢁ0.7 to ꢁ0.3 V. The obtained impedance spectra were fitted with
Z-view software.
Nanosecond transient absorption spectra (TAS)
TAS were obtained by using a nanosecond pulse laser. The
504 nm pulse generated by hydrogen Raman shifting of third
harmonic generation (THG, 355 nm) from Q-switched Nd:YAG
laser (Continuum, Surelite II, pulse width of 4.5 ns fwhm) was
used to excite the samples. A 150 W Xenon Arc lamp (Newport,
70525) was focused on the sample solution as the probe light for
the transient absorption measurement. Temporal profiles were
measured with a monochromator (DongWoo Optron, Monora
500i) equipped with a photomultiplier (Zolix Instruments Co.,
CR 131) and a digital oscilloscope (Tektronix, TDS-784D). The
whole system was controlled with a computer through a GP-IB
interface. Transient absorption spectra were measured by ICCD
(Ando Technology, iStar) with the gate time of 2 ns at various
time-delays. Reported signals were averages of 500 events.
Results and discussion
Synthesis and structural characterization
The molecular structures of the ligands (6 and 7) and the sensi-
tizers (SY-04 and SY-05) were confirmed by 1H, 13C NMR,
HRMS-ESI and elemental analysis. (see Figs. S1ꢃS4†). The
peaks at 7.59 (J ¼ 15.5 Hz) and 6.92 (d, J ¼ 16.0 Hz) ppm in the
1H NMR spectra (see Figs. S1a and S2a) suggest that the protons
in the vinyl groups of ligand 6 (or 7) are in a trans configuration.44
This is because the ancillary ligand 6 (or 7) was synthesized from
2 (or 5) and 3 based on the HEW reaction.15 The peaks at 8.06,
and 7.07 ppm with the coupling constant (J) of 16.0ꢃ15.5 Hz,
(see Figs. S3a and S4a) suggested that the protons in the vinyl
groups in the SY-04 and SY-05 dyes are downfield shifted when
compared to free ligand and also trans configuration to each
other.4 13C NMR spectra of SY-04 (Fig. S3b) and SY-05
(Fig. S4b) have resonance signals between 119 and 166 ppm
arising from chemically and magnetically non-equivalent carbon
atoms in the four pyridyl rings, two thienyl rings, vinyl groups,
and carboxylic acid groups. Two peaks displayed at 133.89 and
133.69 ppm are assigned to the carbons of the two NCS ligands
of both SY-04 and SY-05, indicating that the nitrogen atom in
the NCS ligands coordinated with Ru. This confirms that only
coordinated NCS ligands are present in the dye molecules.4
Computational study
The molecular structure and electronic properties of SY-04 and
SY-05 were investigated with DFT and TD-DFT calcula-
tions.37–42 The calculations were performed using the B3PW91
exchange correlation functional, as implemented in the
Gaussian03 program package.43 The 6-31G* basis set was used
for S, N, O, C, and H, and CRENBL ECP basis set for Ru. The
geometries, electronic structure and optical absorption spectra of
the ruthenium complexes were optimized in the gas phase and in
12392 | J. Mater. Chem., 2011, 21, 12389–12397
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The overall yield for 6 and 7 was very high (>85%). This is
a great advantage compared to other ancillary ligands synthe-
sized previously.14–21 We found that even the efficiency based on
synthesized dyes that were purified only one time using column
chromatography was essentially the same as that based on the
dye purified three (or four) times. This clearly demonstrates that
a single isomer (cis form) was formed for the synthesis of SY-04
and SY-05.
The electron transfer from the excited state of the dye to the
conduction band of the TiO2 electrode was checked by cyclic
voltammetry, and the results are given in Fig. S6.† The cyclic
voltammograms showed that both dyes are redox-active, namely,
metal-centered (oxidation) and ligand-centered (reduction)
processes. The values of E1/2 ¼ (Eox + Ered)/2, where Eox and Ered
are the oxidation and reduction peak potentials, respectively, are
0.87 V and 0.82 V for SY-04 and SY-05, respectively, while that
of N3 is 1.04 V. The value of DE ¼ (Eox ꢁ Ered) ꢀ 1000) for SY-
04 and SY-05 was 130 eV, which indicates that a one-electron
quasi-reversible oxidation took place.27 The difference in the
oxidation potential between SY-04 and N3 is attributed to the
electron donating of the thiophene groups in the ancillary
ligands.12 The experimentally determined LUMO values of the
two dyes (SY-04 and SY-05) are similar to that of N3. However,
the HOMO values of the two dyes are lower than that of N3; thus
the band gaps of the two dyes are ꢃ0.06 eV smaller than that of
N3. This is attributed to the existence of the alkylthiophene unit
attached to the bipyridine ligand, which causes an increase in the
peak intensity of the MLCT band.
Optical and electrochemical property
Fig. 2 shows UV-visible spectra of SY-04, SY-05 and N3 in
ethanol solution. The absorption spectrum of SY-04 displays
three bands at 546 nm, 386 nm, and 312 nm. The band at 312 nm
is assigned to the intra-ligand p–p* transitions of the bipyridine
ligand. The bands at 546 and 386 nm are the characteristic
MLCT absorptions bands which are observed in the visible
region for heteroleptic polypyridyl ruthenium complexes.45,46
The p–p* transition of dialkylthiophene ancillary ligand
attached to SY-04 has a much stronger absorption coefficient
compared with N3. The peak at 546 nm for SY-04 and SY-05 is
15 nm red-shifted compared to N3, which represents the low
energy MLCT transition absorption peak. The measured molar
extinction coefficient (3) of the MLCT band at a peak at 546 nm
of SY-04 and SY-05 is 21.7 ꢀ 103 Mꢁ1 cmꢁ1, which is higher than
that (14.0 ꢀ 103 Mꢁ1 cmꢁ1) of N3.45,46 The enhanced molar
extinction coefficient and the red shift are attributed to the
extension of p-conjugation in both SY-04 and SY-05 dyes. The
red shift of the MLCT band suggests that the band gap between
the HOMO and LUMO levels for SY-04 and SY-05 becomes
smaller than that of N3. The UV-vis spectrum of HRS-1 sensi-
tizer also showed two absorption peaks at 371 and 542 nm with
The location of HOMO–LUMO and the electronic properties
of SY-04 and SY-05 dyes were theoretically investigated by using
DFT and TD-DFT in both gas and solution (ethanol) phase.
Fig. 3 shows the isodensity surface contours (interval of 0.02) of
LUMO, HOMO, HOMO-1, HOMO-2 for SY-04 (these
contours for SY-05 are given in Fig. S7†). The HOMO of both
SY-04 and SY-05 were localized on the ruthenium metal over
NCS units, whereas the LUMOs of both dyes were localized over
the 4,40-dicarboxy-2,20-bipyridine and thiophene attached ancil-
lary ligands. The HOMO-1 and HOMO-2 for both dyes have
a ruthenium excited state (t2g) character in addition to a signifi-
cant positive charge from the NCS to the bipyridine ligands. A
small change of HOMO level for both dyes compared with N3 is
mainly due to the Ru 4d orbital, ancillary ligand, and NCS
orbital. But, the LUMO of both dyes is almost the same as that
of the N3 dye because the LUMO results mainly from the orbital
delocalized on the dcbpy ligand. The proper location of the
frontier orbitals also accounts the high absorption coefficient of
the MLCT band for SY-04 and SY-05 dyes. The energy levels of
the HOMO and LUMO of SY-04 and SY-05 match well with the
redox potential of Iꢁ/Iꢁ3 and LUMO of TiO2.19
an 3 value of 18.7 ꢀ 103 M cmꢁ1 12
. The extinction coefficient of
SY-04 and SY-05 is slightly higher than that of HRS-1. This is
because of the better solubility SY-04 and SY-05 arising from the
hexyl groups in the b (or g) position of the thiophene ring.
The HOMO and LUMO levels of the dyes are estimated from
the oxidation potentials and UV-vis absorption data of the dyes,
and are given in Table 1. The excitation transition energies (E0–0
)
of both dyes are estimated to be 1.83 eV (678 nm) by taking the
crossing point of the absorption and emission spectra (Fig. S5†).
The calculated energy level diagram (HOMO and LUMO) of
SY-04, SY-05 and TiO2 is shown in Fig. S8.† The band gap of
both dyes is 1.68 eV in the gas phase, while the band gaps of SY-
04 and SY-05 in ethanol are 2.42, and 2.45 eV, respectively.
Because both SY-04 and SY-05 have similar bandgaps, the two
dyes have very similar chemical structure. The HOMO–LUMO
energy gap of N3 in the solution phase was reported to be 2.60 eV
calculated by DFT.23 The absorption intensity and the energy
levels (HOMO and LUMO) of the frontier orbitals are almost
the same for both dyes (SY-04 and SY-05), but those are smaller
compared to reference N3. The relative intensity predicted by
theory is consistent with experiment (cf. Figs. S9 and S10† with
Fig. 2). The energy-minimized structures of SY-04 and SY-05
and ancillary ligands are given in Figs. S11ꢃS14.† The transi-
tions between the HOMO and LUMO, the excitation energies,
and the detailed origins of the transitions for both dyes are
listed in the supporting information (Figs. S15ꢃS16 and
Tables S1–S3).†
Fig. 2 Absorption spectra of SY-04 (solid), SY-05 (dotted) and N3
(dashed) dyes.
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Table 1 Optical, redox and energy level for SY-04, SY-05, and N3
Dyes
labsa/nm
E0–0b/eV
DEc/V
Eox/V
HOMOd/eV
LUMOe/eV
SY-04
SY-05
N3
312, 386, 546
312, 385, 545
311, 385, 530
1.83
1.83
1.89
130
130
240
0.88
0.92
1.16
ꢁ5.21
ꢁ5.25
ꢁ5.36
ꢁ3.38
ꢁ3.42
ꢁ3.47
Absorption spectra were measured in ethanol. b E0–0 was determined from intersection of absorption and emission spectra in ethanol. c DE ¼ (Eox
ꢁ
.
a
Ered) ꢀ 1000, where Eox and Ered are oxidation and reduction peak potentials, respectively. d HOMO ¼ ꢁ4.8 ꢁ (Eox ꢁ 0.47). e LUMO ¼ HOMO + E0–0
open-circuit voltage (Voc) and fill factor (ff) of the device based
on SY-04 are 14.7 mA cmꢁ2, 0.756 V, and 0.695, respectively;
thus, the h was 7.70%. This value is 27% higher than that (6.05%)
of the N3-sensitized solar cell under the same cell fabrication
conditions. When a mask was used for another solar cell with
a larger active area of 0.46 cm2 to avoid diffusive lights, h for SY-
04 was slightly decreased to 6.5%, but this is again larger than
that for N3 (5.1%) (see Table S4†). The increase in Jsc of the two
dyes compared with N3 is due to higher incident photon-to-
current conversion efficiency (IPCE), as shown in Fig. 5. The
IPCE of SY-04 exhibits a high plateau in a broad spectral range
from 450 to 650 nm with a maximum 65% at 548 nm. This
indicates that the MLCT transition of the sensitizer can be
enhanced by attaching thiophene ancillary ligands. Considering
the light absorption and scattering loss by the conducting glass,15
the maximum efficiency for absorbed photon-to-collected elec-
tron conversion efficiency (APCE) would be close to unity over
a broad spectral range.
Since the dyes synthesized in this study exhibited high molar
extinction coefficients, DSCs made of thinner mesoscopic TiO2
films or lower concentrations of the dyes could maintain higher
h. The effect of the TiO2 film thickness on h of the DSCs based on
SY-04, SY-05 and N3 is summarized in Table S5† and the
absorption spectra of SY-04 on TiO2 with different film thick-
nesses are shown in Fig. S17.† In this case, we do not use
a reflecting-scattering layer. Even though the TiO2 film thickness
decreased from 12 to 6 mm, the h of the DSCs prepared by SY-04
and SY-05 was almost maintained, but h decreased at a thickness
of 4 mm. This indicates that the h of the DSCs prepared by SY-04
and SY-05 would be saturated above a TiO2 thickness of 6 mm.
Interestingly, the dye adsorption amount was steadily increasing
with increasing TiO2 thickness, as shown in Fig. S17.† These
results suggest that with increasing the dye-adsorbed TiO2 layer,
the charge-recombination sites could be increased, which
adversely influences the h.47 On the other hand, the h of the DSC
prepared by N3 decreased up 30% when the film thickness
decreased from 12 to 6 mm. We measured the amount of dyes
Fig. 3 Isodensity surface plot (isodensity contour ¼ 0.02) of the SY-04
molecular orbitals: 294(LUMO), 293(HOMO), 292(HOMO-1) and 291
(HOMO-2).
Photovoltaic data
The current–voltage characteristics of the SY-04 and SY-05
along with N3 under AM 1.5G one sunlight (100 mWcmꢁ2) are
shown in Fig. 4, and the photovoltaic parameters of the cells are
listed in Table 2. The short-circuit photocurrent density (Jsc),
Table 2 Photovoltaic performance of DSCs with different sensitizers
under AM 1.5 light illumination (100 mW cmꢁ2
)
Dyes
Jsc/mA cmꢁ2
Voc/V
ff
h (%)
Thickness/mm
SY-04
SY-05
N3
14.7
13.6
13.1
0.756
0.750
0.669
0.695
0.694
0.691
7.70
7.08
6.05
12 + 5a
12 + 5a
12 + 5a
a
400 nm sized TiO2 anatase scattering layer. The active areas of the cells
were 0.250 cm2 without a mask.
Fig. 4 J–V curves of DSCs based on the dyes SY-04 (solid), SY-05
(dotted) and N3 (dashed).
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Fig. 6 Impedance spectra of the (Bode phase diagram) measured under
the dark for SY-04 (C), SY-05 (-), and N3 (O).
Fig. 5 IPCE spectra of SY-04 (solid), SY-05 (dotted), and N3 (dashed).
adsorbed on the TiO2 electrode on the surface by stripping off the
dyes with 0.1 M KOH solution in water and ethanol (20 : 80 v/v).
From the absorption spectra of the desorbed dye solution with
the peak intensity at 546 nm and the Beer–Lambert law, the
amounts for SY-04, SY-05, and N3 dyes were 6.8 ꢀ 10ꢁ7, 6.5 ꢀ
10ꢁ7, and 2.4 ꢀ 10ꢁ7 mmol cmꢁ2, respectively. The increased
adsorption of two dyes compared with the N3 dye is due to
higher solubility arising from hexyl groups located at the
3,4-position of the thiophene, thus the dye agglomeration. The
maintenance of higher h even at a 6 mm TiO2 layer indicates that
the absorption amount of SY-04 and SY-05 on TiO2 is greatly
increased compared with N3, as verified by the higher molar
extinction coefficient.
peaks that were assigned in the order of increasing frequency to
ꢁ
diffusion impedance of I3 ions, electron transport and recom-
bination in TiO2 electrode, and electron transfer process at
counter electrode–electrolyte interface, respectively. The life time
(s) of dyes attached to TiO2 conduction band is given by f,
s ¼ 1/(2pf)
where f is the characteristic frequency corresponding to the
middle peak in Fig. 6.
The first peak observed between ca. 1 and 10 Hz was not
distinct due to a small contribution of the electrolyte diffusion to
the impedance. The second peak corresponds to the electron
transport and recombination in the TiO2 electrode. From the
peak positions (57.50, 35.06, and 32.12 Hz for N3, SY-05, and
SY-04, respectively), the life times of these three dyes are calcu-
lated to be 2.76, 4.54 and 4.95 ms, respectively. It is generally
accepted that a decrease in frequency at the second semicircle (f)
is correlated with an increase in the Voc of electrochemical
impedance spectroscopy.54 The increased Voc values for SY-04
and SY-05 compared with that of N3 are well consistent with
long life times.
Also, we found that the h of the DSCs prepared by SY-04 and
SY-05 was almost the same even though the dye concentration
decreased by three times (0.3 to 0.1 mM). But, under the same
conditions, the h of the N3-based DSC was decreased to 25% (see
Table S6†). These results clearly demonstrate the advantage of
the sensitizers with higher molar extinction coefficient arising
from their metal-to-ligand charge transfer band.
It is also noted in Table 2 that the Voc values of SY-04 and
SY-05 are similar, because the absorption profile and the energy
levels (HOMO and LUMO) of the frontier orbitals are almost
the same. On the other hand, the Voc of the two dyes is 87 mV
higher than that of N3. The Voc depends on many factors such as
the electron life-time, the number of protons in the carboxylic
To elaborate the EIS results, transient absorption spectra
(TAS) measurements were carried out to find the recombination
half-life time (t1/2) between the oxidized redox couple and photo-
injected electrons in mesoporous TiO2 having 12 mm thickness
with liquid electrolyte.
groups of the dye, the inherent dye structure, and the conduction
47–50
band shift in TiO2.
Among them we consider two main
Fig. 7 shows the decay profiles of the absorption of the dyes.
From the profiles, t1/2 values for SY-04 and SY-05 were 74.4 and
73.2 ms, respectively. These values are higher than that (49.7 ms)
of N3 when measured under identical conditions. Therefore, the
increased values of Voc and Jsc for the SY-04 and SY-05 dyes
compared with N3 might be attributed to the longer recombi-
nation half-life time measured by TAS and the longer electron
life time measured by EIS.55,56
factors: (1) the number of protons in the carboxylic groups of the
dye, and (2) the electron life-time.
The increase in the Voc of SY-04 and SY-05 compared with N3
might be due to the fact that only two protons in the carboxylic
groups are existent in the present dyes compared with four
protons for N3. Therefore, the offset between the quasi-Fermi
level of the TiO2 and the redox level of the electrolytes for SY-04
and SY-05 increased.51,52 To investigate the electron life-time of
the sensitizers in the mesoporous titania, we measured the dark
electrochemical impedance spectra53 of DSC cells at various bias
potentials.
Conclusions
Fig. 6 shows the Bode phase diagram of the DSCs measured
under the dark at open circuit voltage conditions for the samples
made with SY-04, SY-05, and N3. It contains three characteristic
We synthesized two heteroleptic ruthenium-based sensitizers
(SY-04 and SY-05) in which 3,4- or 3-hexylthiophene moieties
were incorporated via a vinyl group to the bipyridine ligand. Due
This journal is ª The Royal Society of Chemistry 2011
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11 Y.-S. Chen, C. Li, Z.-H. Zeng, W.-B. Wang, X.-S. Wang and
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€
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16 Y. Cao, Y. Bai, Q. Yu, Y. Cheng, S. Liu, D. Shi, F. Gao and P. Wang,
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17 Q. Yu, S. Liu, M. Zang, N. Cai, Y. Wang and P. Wang, J. Phys.
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€
18 F. De Angelis, S. Fantacci, A. Selloni, M. Gratzel and
M. K. Zakeeruddin, Nano Lett., 2007, 7, 3189.
19 C.-Y. Chen, S.-J. Wu, C.-G. Wu, J.-G. Chen and K.-C. Ho, Angew.
Chem., 2006, 118, 5954.
20 C.-Y. Chen, J.-G. Chen, S.-J. Wu, J.-Y. Li, C.-G. Wu and K.-C. Ho,
Angew. Chem., Int. Ed., 2008, 47, 7342.
Fig. 7 Transient absorbance decay of the dyes obtained by nanosecond
laser flash photolysis.
21 C.-Y. Chen, S.-J. Wu, J.-Y. Li, C.-G. Wu, J.-G. Chen and K.-C. Ho,
Adv. Mater., 2007, 19, 3888.
22 C. Y. Chen, M. Wang, J.-Y. Li, N. Pootrakulchote, L. Alibabaei,
to the employment of the HEW reaction, the yield of ancillary
ligands was very high, and the purification of these two dyes was
very easy. The functionalization of position 3,4 in the thiophene
ancillary ligands increases the molar extinction coefficient
considerably. Furthermore, the electron life time adsorbed on
TiO2 and the adsorption amount of these two dyes are larger
than those of N3. With increases of both Voc and Jsc, the h of the
DSC prepared using SY-04 was 7.70%, which is 27% higher than
that (6.05%) prepared with N3 under the same cell fabrication
conditions. The increased h might be attributed to the increase in
electron life time (4.95 ms) measured by electrochemical imped-
ance as well as the increased recombination half-life (74.4 ms)
measured by TAS for SY-04 compared with those (2.76 ms and
49.7 ms) of N3.
€
C. Ngoc-le, J.-D. Decoppet, J.-H. Tsai, C. Gratzel, C.-G. Wu,
S. M. Zakeeruddin and M. Gratzel, ACS Nano, 2009, 3, 3103.
€
23 F. Matar, T. H. Ghaddar, K. Walley, T. DosSantos, J. R. Durrant
and B. O’Regan, J. Mater. Chem., 2008, 18, 4246.
€
24 A. Abbotto, C. Barolo, L. Bellotto, F. De Angelis, M. Gratzel,
N. Manfredi, C. Marinzi, S. Fantacci, J.-H. Yum and
M. K. Nazeeruddin, Chem. Commun., 2008, 5318.
25 J.-J. Kim, H. Choi, C. Kim, M.-S. Kang, H. S. Kang and J. Ko, Chem.
Mater., 2009, 21, 5719.
26 S. Paek, C. Baik, M.-S. Kang, H. Kang and J. Ko, J. Organomet.
Chem., 2010, 695, 821.
27 C. Lee, J. H. Yum, H. Choi, S. O. Kang, J. Ko, R. Humphry-Baker,
€
M. K. Zakeeruddin and M. Gratzel, Inorg. Chem., 2008, 47, 2267.
€
28 K. H. Dahm, B. M. Trost and H. Roller, J. Am. Chem. Soc., 1967, 89,
5292.
29 P. Wang, C. Klein, R. Humphry-Baker, S. M. Zakeeruddin and
€
M. Gratzel, J. Am. Chem. Soc., 2005, 127, 808.
30 D. Kuang, C. Klein, S. Ito, J.-E. Moser, R. Humphry-Baker,
€
N. Evans, F. Duriaux, C. Gratzel, S. M. Zakeeruddin and
M. Gratzel, Adv. Mater., 2007, 19, 1133.
Acknowledgements
€
€
31 J.-J. Lagref, M. K. Nazeeruddin and M. Gratzel, Synth. Met., 2003,
138, 333.
32 H. Sirringhaus, N. Tessler and R. H. Friend, Science, 1998, 280, 1741.
This work was supported by the National Creative Research
Initiative Program supported by the National Research Foun-
dation of Korea (NRF) and BK-21 program. We thank Taiho
Park for critical comments and helpful suggestions.
ꢀ
33 G. Sauve and R. D. McCullough, Adv. Mater., 2007, 19, 1822.
34 C. Ringenbach, A. De Nicola and R. Ziessel, J. Org. Chem., 2003, 68,
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35 F. Wang, R. D. Rauh and T. L. Rose, J. Am. Chem. Soc., 1997, 119,
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36 S. R. Jang, C. Lee, H. Choi, J. Ko, J. Lee, R. Vittal and K. Kim,
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This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 12389–12397 | 12397