A new class of cyclometalated ruthenium sensitizers of the type CNN for efficient dye-sensitized solar cells

Article abstract of DOI:10.1021/ic200872a

A new class of cyclometalated ruthenium sensitizers incorporating a CNN ligand and conjugated 2,2′-bipyridine in the ancillary ligand have been designed and synthesized. The photovoltaic performance of JK-206 using an electrolyte containing 0.6 M 1,2-dime

Full text of DOI:10.1021/ic200872a

ARTICLE
pubs.acs.org/IC
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A New Class of Cyclometalated Ruthenium Sensitizers of the Type CNN
for Efficient Dye-Sensitized Solar Cells
Jeum-Jong Kim,† Hyunbong Choi,† Sanghyun Paek,† Chulwoo Kim,† Kimin Lim,† Myung-Jong Ju,†
Hong Seok Kang,‡ Moon-Sung Kang,§ and Jaejung Ko*,†
†Department of New Material Chemistry, Korea University, Jochiwon, Chungnam 339-700, Korea
‡Department of Nano and Advanced Materials, College of Engineering, Jeonju University, Jeonju, Korea
§Department of Environmental Engineering, College of Engineering, Sangmyung University, 300 Anseo-dong, Dongnam-gu, Cheonan,
Chungnam Province 330-720, Republic of Korea
S Supporting Information
b
ABSTRACT:
0
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A new class of cyclometalated ruthenium sensitizers incorporating a CNN ligand and conjugated 2,2 -bipyridine in the ancillary
ligand have been designed and synthesized. The photovoltaic performance of JK-206 using an electrolyte containing 0.6 M 1,2-
dimethyl-3-propylimidazolium iodide, 0.05 M I2, 0.1 M LiI, and 0.5 M tert-butylpyridine in CH3CN gave a short-circuit
photocurrent density of 19.63 mA cmꢀ2, an open-circuit voltage of 0.74 V, and a fill factor of 0.72, affording an overall conversion
efficiency of 10.39%. The efficiency is the highest one reported for dye-sensitized solar cells based on the cyclometalated ruthenium
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sensitizer of the type CNN. Moreover, the same device using a polymer gel electrolyte exhibited a remarkable stability under 1000 h
of light soaking at 60 °C, retaining 91% of the initial efficiency of 7.14%.
’ INTRODUCTION
orbital (LUMO) levels of a dye by changing the substituent is an
attractive route to the panchromatic sensitization of TiO2 elec-
trodes.6 The second approach for broadening the absorption
band and increasing the molar absorption coefficient is to increase
the conjugation length of the bipyridyl ligand.7 Such an increase
can be achieved by replacing one of the 4,40-dicarboxy-2,20-bi-
pyridine (dcbpy) anchoring ligands in N3 with a highly con-
jugated ancillary group such as thiophene8 and alkoxybenzene
moieties.9 In the third approach, the replacement of a neutral
RuꢀN bond with an anionic RuꢀC bond in a multidentate
ligand is an effective strategy for shifting the MLCT band to the
red region.10 The broad and red-shifted absorption properties of
the cyclometalated complexes of ruthenium(II) render these as a
new class of promising sensitizers because they are shown to
dramatically change the electronic properties by raising the energy
of their HOMO. Recently, Nazeeruddin et al.,11 Ghadarr et al.,12
van Koten et al.,12 and Chi et al.13 reported thiocyanate-free
Dye-sensitized solar cells (DSSCs) based on mesoporous
TiO2 electrodes have attracted widespread attention because of
their low-cost fabrication and high power conversion efficiency.1
The photovoltaic performance of DSSCs has been rapidly
improved by molecular engineering of the sensitizer,2 minimiza-
tion of the interfacial charge recombination,3 and the develop-
ment of some additives.4 In these cells, the sensitizer is one of the
key components in achieving a high conversion efficiency. Several
poly(pyridylruthenium) complexes5 have achieved power con-
version efficiencies of over 11% in standard air mass 1.5 sunlight.
In spite of their high photovoltaic performances, the efficiency of
DSSCs still needs to be improved in order to become compe-
titive with conventional photovoltaic cells. The design of an op-
timal sensitizer with a red-shifted absorption band and increasing
molar absorption coefficient presents a challenging task. Our
strategy is to find ways to systematically shift the metal-to-ligand
charge-transfer (MLCT) band toward the red region of the dye.
In the first approach, the judicious tuning of the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
Received: April 26, 2011
Published: October 17, 2011
r
2011 American Chemical Society
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Figure 1. Molecular structures of JK-206 and JK-207.
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cyclometalated ruthenium sensitizers of the type CN, CNN, NN,
10 mM 3a,7a-dihydroxy-5b-cholic acid (Cheno)] and kept at room
temperature overnight. The FTO plate for counter electrodes was
cleaned with an ultrasonic bath in H2O, acetone, and 0.1 M HCl(aq),
subsequently. Counter electrodes were prepared by coating with a drop
of H2PtCl6 solution (2 mg of platinum in 1 mL of ethanol) on an FTO
plate and heating at 400 °C for 15 min. The dye-adsorbed TiO2 elec-
trode and platinum counter electrode were assembled into a sealed
sandwich-type cell by heating at 80 °C with a hot-melt ionomer film
(Surlyn) as a spacer between the electrodes. A drop of an electrolyte
solution was placed on the drilled hole in the counter electrode of the
assembled cell and was driven into the cell via vacuum backfilling. Two
electrolytes were used for device evaluation. Electrolyte A: 0.6 M 1,2-
dimethyl-3-propylimidazolium iodide (DMPII), 0.05 M iodine, 0.1 M
LiI, and 0.5 M tert-butylpyridine (TBP) in acetonitrile. Electrolyte B:
0.1 M iodine, 0.5 M NMBI, and 5 wt % poly(vinylidene fluoride-co-
hexafluoropropylene) (PVDF-HFP) in 3-methoxypropionitrile (MPN).
Finally, the hole was sealed using additional Surlyn and a cover glass
(0.1 mm thickness). The adsorbed quantity on TiO2 films is calculated by
the difference in the absorbance between 8-μm-thick TiO2 films each coated
with 5 ꢁ 10ꢀ4 M JK-206, JK-207, and N719 in N,N-dimethylformamide
(DMF) and desorbed JK-206, JK-207, and N719 from their corresponding
TiO2 films into aqueous 10ꢀ2 M KOH (2 mL) for 5 h.
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and NNN, which exhibit high power conversion efficiencies. Herein,
we report a new type of cyclometalated ruthenium complex as
efficient sensitizers, coded as JK-206 and JK-207, of the type
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CNN, NN, and NCS ligands. These novel ruthenium complexes
exhibit remarkable performances as sensitizers. The thiophene-
derived ancillary units were introduced for increasing the con-
jugation length to increase the light-harvesting ability and red-
shift the MLCT band of ruthenium sensitizers. The role of one
thiocyanato ligand is to easily regenerate the dye from the redox
system.14 The molecular structures of the two ruthenium sensi-
tizers are shown in Figure 1.
’ EXPERIMENTAL SECTION
General Methods. All reactions were carried out under an argon
atmosphere. Solvents were distilled from the appropriate reagents. All
reagents were purchased from Sigma-Aldrich. Dimethyl 2,20-bipyridine-
4,40-dicarboxylate (1),15 4,40-bis(5-hexylthiophen-2-yl)-2,20-bipyridine,16
and 4,40-bis(7-hexylethylenedioxythiophen-2-yl)-2,20-bipyridine17 were
synthesized using a modified procedure of previous references. 1H and
13C NMR spectra were recorded on a Varian Mercury 300 spectrometer.
Elemental analyses were performed with a Carlo Elba Instruments CHNS-O
EA 1108 analyzer. Mass spectra were recorded on a JEOL JMS-SX102A
instrument. The absorption and photoluminescence spectra were recorded
on a Perkin-Elmer Lambda 2S UVꢀvis spectrometer and a Perkin-Elmer
LS fluorescence spectrometer, respectively.
Time-Dependent Density Functional Theory (TD-DFT)
Calculations. Structure optimization was done with B3LYP/Lanl2dz.
On the one hand, TD-DFT calculations were done using B3LYP with a
mixed basis set, i.e., SDD ECP and 6-31+G(d,p) basis sets for Ru and
other atoms, respectively.18 All of the calculations were done using the
Gaussian03 program.19
Fabrication of DSSC. FTO glass plates (Pilkington TEC Glass-
TEC 8, Solar 2.3 mm thickness) were cleaned in a detergent solution
using an ultrasonic bath for 30 min and rinsed with water and ethanol.
The FTO glass plates were immersed in 40 mM TiCl4(aq) at 70 °C for
30 min and washed with water and ethanol. A transparent nanocrystal-
line layer on the FTO glass plate was prepared by doctor blade printing
TiO2 paste (Solaronix, Ti-Nanoxide D20, 20 nm) and then dried for 2 h
at 25 °C. The TiO2 electrodes were gradually heated under an air flow at
325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500 °C
for 15 min. The thickness of the transparent layer was measured by using
an Alpha-step 250 surface profilometer (Tencor Instruments, San Jose,
CA). A paste for the scattering layer containing 400-nm-sized anatase
particles (CCIC, PST-400C) was deposited by doctor blade printing
and then dried for 2 h at 25 °C. The TiO2 electrodes were gradually
heated under an air flow at 325 °C for 5 min, at 375 °C for 5 min, at
450 °C for 15 min, and at 500 °C for 15 min. The resulting film was
composed of a 10-μm-thick transparent layer and a 4-μm-thick scatter-
ing layer. The TiO2 electrodes were treated again by TiCl4 at 70 °C for
30 min and sintered at 500 °C for 30 min. The TiO2 electrodes were
immersed in JK-206, JK-207, and N719 [0.3 mM in EtOH containing
Cyclic Voltamogram. Cyclic voltammetry was carried out with a
BAS 100B (Bioanalytical Systems, Inc.) in combination with a conven-
tional three-electrode, one-compartment electrochemical cell. A plati-
num foil and Ag/AgCl/KClsat were used as the counter and reference
electrodes, respectively. A dye-coated nanocrystalline TiO2 electrode
was employed as the working electrode. The redox potential of dyes on
TiO2 was measured in CH3CN with 0.1 M (n-C4H9)4NPF6 at a scan rate
of 100 mV sꢀ1 [vs ferrocenium/ferrocene (Fc/Fc+)].
Characterization of Solar Cell Devices. JꢀV measurements
were characterized under simulated 100 mW cmꢀ2 AM 1.5G irradiation
from a 1000 W xenon arc lamp (Oriel 91193). The light intensity was
adjusted with a silicon solar cell that was calibrated with an NREL-calibrated
silicon solar cell (PV Measurement Inc.). The applied potential and
measured cell current were measured using a Keithley model 2400
digital source meter. The incident photon-to-current conversion effi-
ciency (IPCE) spectra for the cells were measured on an IPCE mea-
suring system (PV Measurements Inc.). The light and thermal tests were
performed with Suntest CPS/CPS+ equipment (AB Nexo Co).
Electron Transport Measurements. The electron diffusion
coefficients (De) and lifetimes (τe) in the TiO2 photoelectrode were
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measured by the stepped light-induced transient measurements of the
photocurrent and voltage.20 The transients were induced by a stepwise
change in the laser intensity. A diode laser (λ = 635 nm) as a light source
was modulated using a function generator. The initial laser intensity was
a constant 90 mW cmꢀ2 and was attenuated up to approximately
10 mW cmꢀ2 using a neutral density (ND) filter, which was positioned
at the front side of the fabricated samples (TiO2 film thickness = ca. 10 μm;
active area = 0.04 cm2). The photocurrent and photovoltage transients
were monitored using a digital oscilloscope through an amplifier. The De
value was obtained by a time constant (τc) determined by fitting a decay
of the photocurrent transient with exp(ꢀt/τc) and the TiO2 film thick-
ness (ω) using the equation De = ω2/(2.77τc).20a The τe value was also
determined by fitting a decay of the photovoltage transient with exp(ꢀt/τe).20a
All experiments were conducted at room temperature.
4,40-Bis(methoxycarbonyl)-2,20-bipyridine-1-oxide (2). To
a CHCl3 solution of 4,40-bis(methoxycarbonyl)-2,20-bipyridine (1.0 g,
3.67 mmol) was added slowly at room temperature 3-chloroperoxyben-
zoic acid (MCPBA; 821 mg, 3.67 mmol) as a solid. The reaction was
continued for 10 h. The reaction solvent was removed under reduced
pressure. The pure product 2 was obtained by column chromatography
on silica gel (CH2Cl2:MeOH = 30:1, Rf = 0.42). Yield: 78%. 1H NMR
(CDCl3): δ 9.30 (dd, 1H, J = 1.4 Hz), 8.87 (dd, 1H, J = 4.9 Hz), 8.75 (d,
1H, J = 2.4 Hz), 8.32 (d, 1H, J = 6.7 Hz), 7.91 (dd, 1H, J = 4.9 Hz), 7.85
(d, 1H, J = 6.8 Hz), 3.95 (s, 3H), 3.93 (s, 3H). 13C NMR (CDCl3): δ
165.2, 163.9, 150.2, 149.9, 146.6, 141.0, 137.8, 128.3, 126.4, 125.3, 124.3,
123.8, 52.8, 52.7. Anal. Calcd for C14H12N2O5: C, 58.33; H, 4.20.
Found: C, 58.12; H, 4.14.
Anal. Calcd for C18H10F2N2O4: C, 60.68; H, 2.83. Found: C, 60.41;
H, 2.90.
Complex JK-206. A mixture 4,40-bis(5-hexylthiophen-2-yl)-2,20-
bipyridine16 (137 mg, 0.28 mmol) and a dichloro(p-cymene)ruthenium-
(II) dimer (87 mg, 0.14 mmol) in argon-degassed DMF (15 mL) was
stirred at 80 °C for 4 h under reduced light. Subsequently, 6-(2,
4-Difluorophenyl)-2,20-bipyridine-4,40-dicarboxylic acid (100 mg,
0.28 mmol) was added into the flask, and the reaction mixture was stirred at
160 °C for 4 h. An excess of NH4NCS (126 mg, 1.42 mmol) was added
to the resulting dark solution, and the reaction continued for another 8 h
at 140 °C. Then the reaction mixture was cooled to room temperature,
and the solvent was removed under vacuum. Water was added to induce
precipitation. The resulting solid was filtered, washed with water, and
dried under vacuum. The resulting solid was dissolved in methanol
containing 2.2 equiv of tetrabutylammonium hydroxide to confer
solubility by deprotonating the carboxylic group and purified on a
Sephadex LH-20 column with methanol as the eluent. The collected
main band was concentrated, and the solution pH was lowered to 5.1
using 0.02 M nitric acid. The precipitate was collected on a sintered glass
crucible by suction filtration and dried in air. Yield: 60%. MS (FAB,
101Ru). Calcd for C49H45F2N5O4RuS3: m/z 1003.21. Found: m/z
1
1002.64 (M+). H NMR (CD3OD): δ 9.59 (d, 1H, J = 6.0 Hz), 9.12
(s, 1H), 8.90 (s, 1H), 8.32 (d, 1H, J = 3.3 Hz), 8.29 (d, 1H, J = 3.3 Hz),
8.12 (s, 1H), 8.07 (s, 1H), 7.84 (m, 1H), 7.73 (s, 1H), 7.43 (s, 1H), 7.34
(m, 1H), 6.96 (m, 3H), 6.87 (m, 2H), 6.22 (m, 1H), 2.97 (t, 2H, J = 4.2
Hz), 2.90 (t, 2H, J = 4.2 Hz), 1.66 (m, 4H), 1.40 (m, 12H), 1.01 (m, 6H).
Anal. Calcd for C49H45F2N5 O4RuS3: C, 58.67; H, 4.52; N, 6.98. Found:
C, 58.31; H, 4.42; N, 6.71.
Dimethyl-6-chloro-2,20-bipyridine-4,40-dicarboxylate (3).
A mixture of 4,40-bis(methoxycarbonyl)-2,20-bipyridine-1-oxide (200
mg, 0.69 mmol) in POCl3 (0.4 mL) was refluxed for 1.5 h. H2O (50 mL)
and CH2Cl2 (50 mL) were added. The organic layer was separated and
dried with MgSO4. The solvent was removed under reduced pressure.
The pure product 3 was obtained by column chromatography on silica
Complex JK-207. A mixture 4,40-bis(7-hexylethylenedioxythio-
phen-2-yl)-2,20-bipyridine15 (270 mg, 0.44 mmol) and a dichloro(p-
cymene)ruthenium(II) dimer (141 mg, 0.22 mmol) in argon-degassed
DMF (15 mL) was stirred at 80 °C for 4 h under reduced light. Sub-
sequently, 6-(2,4-difluorophenyl)-2,20-bipyridine-4,40-dicarboxylic acid
(159 mg, 0.44 mmol) was added into the flask, and the reaction mixture
was stirred at 160 °C for 4 h. An excess of NH4NCS (195 mg, 2.20 mmol)
was added to the resulting dark solution and the reaction continued for
another 8 h at 140 °C. Then, the reaction mixture was cooled to room
temperature, and the solvent was removed under vacuum. Water was
added to induce precipitation. The resulting solid was filtered, washed
with water, and dried under vacuum. The resulting solid was dissolved in
methanol containing 2.2 equiv of tetrabutylammonium hydroxide to
confer solubility by deprotonating the carboxylic group and purified on a
Sephadex LH-20 column with methanol as the eluent. The collected
main band was concentrated, and the solution pH was lowered to 5.1
using 0.02 M nitric acid. The precipitate was collected on a sintered glass
crucible by suction filtration and dried in air. Yield: 45%. MS (FAB,
101Ru). Calcd for C53H49F2N5O8RuS3: m/z 1119.28. Found: m/z
1
gel using CH2Cl2 as the eluent (Rf = 0.54). Yield: 80%. H NMR
(CDCl3): δ 8.87 (m, 3H), 7.91 (m, 2H), 4.00 (s, 3H), 3.99 (s, 3H). 13
C
NMR (CDCl3): δ 165.5, 164.4, 157.0, 155.1, 151.9, 150.2, 141.3, 138.8,
124.2, 123.8, 120.8, 119.2, 53.1, 52.9. Anal. Calcd for C14H11ClN2O4: C,
54.83; H, 3.62. Found: C, 54.51; H, 3.50.
Dimethyl-6-(2,4-difluorophenyl)-2,20-bipyridine-4,40-di-
carboxylate (4). A mixture of 3 (260 mg, 0.85 mmol), 2,4-difluor-
ophenylboronic acid pinacol ester (223 mg, 0.93 mmol), Pd(PPh3)4
(54 mg), and Na2CO3 (99 mg) was dissolved in tetrahydrofuran (THF;
50 mL)/H2O (10 mL), and the mixture was refluxed for 12 h. After
evaporation of the solvent under reduced pressure, H2O (50 mL) and
CH2Cl2 (50 mL) were added. The organic layer was separated and dried
with MgSO4. The solvent was removed under reduced pressure. The
pure product 4 was obtained by column chromatography on silica gel
using CH2Cl2 as the eluent (Rf = 0.63). Yield: 68%. 1H NMR (CDCl3):
δ 8.98 (s, 1H), 8.89 (d, 1H, J = 1.8 Hz), 8.85 (d, 1H, J = 5.1 Hz), 8.34
(d, 1H, J = 1.5 Hz), 8.12 (s, 1H), 7.88 (dd, 1H, J = 1.8 and 1.5 Hz), 7.06
(m, 1H), 6.96 (m, 1H), 4.92 (s, 3H), 4.90 (s, 3H). 13C NMR (CDCl3):
δ 165.7, 165.6, 156.4, 156.2, 152.9, 150.1, 139.3, 138.5, 132.3, 123.7,
123.6, 123.2, 120.5, 119.2, 112.2, 104.9, 104.5, 104.2, 52.8, 51.7. Anal.
Calcd for C20H14F2N2O4: C, 62.50; H, 3.67. Found: C, 62.38; H, 3.54.
6-(2,4-Difluorophenyl)-2,20-bipyridine-4,40-dicarboxylic
Acid (5). A mixture of 4 (1.88 g, 4.89 mmol) and KOH (1.1 g, 4eq) was
dissolved in MeOH (50 mL)/H2O (10 mL), and the mixture was
refluxed for 12 h. The reaction mixture was neutralized with a 2 M HCl
solution, and the precipitate was filtered and washed with water and gave
1.6 g as a white solid. Yield: 92%. 1H NMR (DMSO-d6): δ 8.88 (d, 1H,
J = 5.7 Hz), 8.79 (s, 1H), 8.75 (d, 1H, J = 1.5 Hz), 8.18 (s, 1H), 8.13 (s,
1H), 7.89 (d, 1H, J = 1.5 Hz), 7.43 (d, 1H, J = 5.7 Hz), 7.31 (m, 1H). 13C
NMR (DMSO-d6): δ 166.4, 166.2, 156.0, 155.6, 152.6, 150.9, 140.9,
140.0, 132.7, 124.0, 123.5, 119.9, 119.1, 113.0, 112.7, 105.6, 105.3, 104.9.
1
1118.67 (M+). H NMR (CD3OD): δ 9.47 (d, 1H, J = 3.3 Hz), 9.08
(s, 1H), 8.90 (d, 1H, J = 1.2 Hz), 8.37 (m, 3H), 8.14 (d, 1H, J = 4.2 Hz),
7.48 (s, 1H), 7.37 (m, 1H), 7.23 (d, 1H, J = 2.7 Hz), 7.14 (d, 1H, J = 3.6
Hz), 7.04 (m, 1H), 6.36 (m, 1H), 4.50ꢀ4.25 (m, 8H), 2.76 (t, 2H, J = 4.5
Hz), 2.66 (t, 2H, J = 4.2 Hz), 1.63 (m, 4H), 1.31 (m, 12H), 0.89 (m, 6H).
Anal. Calcd for C53H49F2N5O8RuS3: C, 56.87; H, 4.41; N, 6.26. Found:
C, 56.61; H, 4.35; N, 6.08.
’ RESULTS AND DISCUSSION
The two cyclometalated ruthenium sensitizers JK-206 and JK-
207 were synthesized by the stepwise synthetic protocol illu-
strated in Scheme 1. The dimethyl-6-chloro-2,20-bipyridine-4,40-
dicarboxylate 3 was prepared by the oxidation reaction of 4,40-
bis(methoxycarbonyl)-2,20-bipyridine with MCPBA, followed by
chlorination of 2 with POCl3. The Suzuki coupling reaction21 of
3 with 2,4-difluorophenylboronic acid yielded 4. The hydrolysis
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Scheme 1. Schematic Diagram for the Synthesis of Ruthenium Sensitizers JK-206 and JK-207a
a Reagents: (a) MCPBA, CH2Cl2; (b) POCl3; (c) 2,4-difluorophenylboronic acid pinacol ester, Pd(PPh3)4, Na2CO3, THF, 80 °C; (d) KOH, aqueous
MeOH; (e) for JK-206, 4,40-bis(5-hexylthiophen-2-yl)-2,20-bipyridine; for JK-207, 4,40-bis(7-hexylethylenedioxythiphen-2-yl)-2,20-bipyridine
[Ru(Cl)2(p-cymene)]2, DMF, 80 °C; (f) 6-(2,4-difluorophenyl)-2,20-bipyridine-4,40-dicarboxylic acid, DMF,160 °C; (g) NH4NCS, DMF, 140 °C.
^ ^
reaction of 4 with aqueous KOH was converted to the NNC
ligand 5. JK-206 and JK-207 were synthesized in a one-pot
reaction from the sequential reaction of [Ru(p-cymene)Cl2]2
with 4,40-bis(5-hexylthiophen-2-yl)-2,20-bipyridine or 4,40-bis(7-
hexylethylenedioxythiophen-2-yl)-2,20-bipyridine, followed by
the reaction of the resulting ruthenium intermediates with 5. The
chlororuthenium complexes reacted with an excess of ammonium
thiocyanate to give the ruthenium sensitizers JK-206 and JK-207.
Figure 2 shows the UV/vis spectra of JK-206 and JK-207 in
EtOH as well as that of N719 for reference and also shows
normalized emission spectra. The absorption spectrum of JK-
206 exhibits two peaks at 391 and 527 nm, which are attributable
to the MLCT transitions. The molar extinction coefficients of
these two peaks are 19.8 ꢁ 103 and 18.0 ꢁ 103 Mꢀ1 cmꢀ1, re-
spectively (Table 1). The molar extinctions of the lower energy
absorption bands of JK-206 and JK-207 increase 31% and 21%
relative to that of N719. The low-energy MLCT band of JK-207
is red-shifted about 3 and 10 nm compared with those of JK-206
and N719. The red-shifted absorption is attributable to the electron-
rich π-conjugation unit in an ancillary ligand. The cyclometalated
complexes resulted in a broadening and red-shifted for the absorp-
tion band.10a Such broadening and red shifting are desirable for
harvesting the solar spectrum and lead to a large photocurrent.
The absorption spectra of JK-206 on the TiO2 film are sig-
nificantly red-shifted and much broadened because of the J aggrega-
tion and interaction of the anchoring group with the surface
titanium ions, ensuring a good light-harvesting efficiency. Excitation
of the low-energy MLCT bands of JK-206 and JK-207 resulted in
strong emissions centered at 800 and 795 nm, respectively.
We performed the molecular orbital calculation of JK-206 and
JK-207 to gain insight into the photophysical properties using
DFT and TD-DFT. Figure 3 presents the isodensity plots of
frontier molecular orbitals of JK-206 and JK-207.
The calculation illustrates that the band of the UV spectrum at
527 nm in JK-206 is mainly characterized by a mixture of
HOMO f LUMO+2 (47%) and HOMOꢀ1 f LUMO+2
(19%). The HOMO is mainly populated over ruthenium t2g
character and thiocyanate with a significant contribution on the
far-end S atom, while HOMOꢀ1 is delocalized over the ruthe-
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nium center, ꢀNCS, NNC unit, and bipyridyl ligand. The LUMO
of JK-206 is predominantly delocalized over the dcbpy portion of
^ ^
the CNN ligand, ꢀNCS, and the ruthenium center. On the other
hand, the LUMO+1 is delocalized over the bpy of the ancillary
ligand. Examination of the HOMO and LUMO of both sensiti-
zers indicates that HOMOꢀLUMO excitation moves the elec-
tron distribution from the Ru-NCS unit to the dcbpy moiety.
Accordingly, the photoinduced electron transfer from the dye to
the TiO2 electrode can occur efficiently at the HOMO f LUMO
excitation.
The electrochemical properties of the two ruthenium sensiti-
zers JK-206 and JK-207 were evaluated by cyclic voltammetry in
CH3CN with 0.1 M TBAPF6. TiO2 films stained with the sensitizers
were used as working electrodes. Ferrocene was added to the
solution, and the Fc/Fc+ redox couple was used as an internal
potential reference. The potentials vs NHE were calculated by
the addition of 630 mV to the potential vs Fc/Fc+. The oxidation
potentials of the JK-206 and JK-207 sensitizers adsorbed on TiO2
films show a quasi-reversible couple at 0.99 and 0.96 V vs NHE,
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which is assigned to the RuII/III couple, whereas the ex-
perimentally determined oxidation potential of N719 using
the same experimental conditions gave 1.13 V vs NHE. The 0.14ꢀ
0.17 V cathodic shift of the JK-206 and JK-207 oxidation
potentials relative to that of N719 is attributed to the influence
of the electron-donor nature of the ancillary ligand in both sen-
sitizers. The oxidation potentials of both sensitizers are favorable
for iodide oxidation.22 The excited-state reduction potentials of
JK-206 and JK-207 calculated from the oxidation potential and
E0ꢀ0 are summarized in Table 1. The excited-state oxidation
potentials (E*ox) of the sensitizers (JK-206, ꢀ1.00 V; JK-207,
ꢀ0.91 V vs NHE) were much more negative with respect to the
conduction band edge of TiO2, providing the thermodynamic
driving force for electron injection.23
The inset of Figure 4 shows the photocurrent action spectrum
of the JK-206, JK-207, and N719 sensitized cells using an
acetonitrile-based electrolyte. The cell was fabricated using a
double layer of 9-μm-thick TiO2 (20 nm) and 4-μm-thick scat-
tering TiO2 (400 nm). The band on the IPCE spectrum of JK-
206 tails off toward 830 nm and shows a performance of over
80% from 460 to 650 nm, peaking at 88% at 545 nm.
For reference, N719 dye affords a maximum of 84% at 610 nm
under the same conditions. The IPCE spectrum of JK-206 is
more enhanced in the 400ꢀ600 nm range compared to that of
N719 as a result of cyclometalation and extended π conjugation,
which is consistent with the absorption spectrum of JK-206.
From the overlap integral of the IPCE of JK-206 with the
standard global AM 1.5 solar emission spectrum, a short-circuit
photocurrent density of 19.08 mA cmꢀ2 is calculated, which is in
good agreement with the measured photocurrent. The JꢀV
curve for the devices based on JK-206 and JK-207 is shown
and compared with that of N719 in Figure 4. Under standard
global AM 1.5 solar conditions, the JK-206 and JK-207 sensi-
tized cells gave short-circuit photocurrent densities (Jsc) of 19.63
and 13.86 mA cmꢀ2, open-circuit voltages (Voc) of 0.74 and
0.69 V, and fill factors (ff) of 0.72 and 0.66, corresponding to
overall conversion efficiencies (η) of 10.39% and 6.32%, respec-
tively (Table 1). Under the same conditions, the N719 sensitized
cell gave a Jsc of 18.79 mA cmꢀ2, a Voc of 0.75 V, and a ff of 0.70,
corresponding to η of 9.80%. The significant decrease in the η
value for JK-207 compared to a JK-206-based device demonstrates
some role of the electron-donor ethylenedioxythiophen-2-yl
Figure 2. Absorption and emission spectra (a) of JK-206 (red solid
line), JK-207 (blue solid line), and N719 (black solid line) in EtOH and
absorption spectra (b) of JK-206 (red solid line), JK-207 (blue solid
line), and N719 (black solid line) adsorbed on the TiO2 film.
Figure 3. Isodensity surface plots of the HOMO, HOMOꢀ1, LUMO,
LUMO+1, and LUMO+2 of JK-206 and JK-207.
Table 1. Optical, Redox, and DSSC Performance Parameters of Dyes
dye
λabsa/nm (ε/Mꢀ1 cmꢀ1
)
Eredoxb/V
E0ꢀ0c/V
ELUMOd/V
Jsc (mA cmꢀ2
)
Voc (V)
FF
ηe (%)
JK-206
JK-207
N719
391 (19 800), 527 (18 000)
368 (38 700), 530 (16 800)
380 (13 100), 520 (1300)
0.99
0.96
1.13
1.99
1.87
2.29
ꢀ1.00
ꢀ0.91
ꢀ1.16
19.63
13.86
18.79
0.74
0.69
0.75
0.72
0.66
0.70
10.39
6.32
9.80
a Absorption spectra were measured in an EtOH solution. b Redox potentials of dyes on TiO2 were measured in CH3CN with 0.1 M (n-C4H9)4NPF6 at a
scan rate of 100 mV sꢀ1 (vs NHE). c E0ꢀ0 values were estimated from the onset of the absorption spectrum. d ELUMO was calculated by Eox ꢀ E0ꢀ0
.
e Performances of DSSCs were measured with a 0.18 cm2 working area. The aperture area is controlled by a black mask. The average of these PV
measurements is 2ꢀ3 times. The errors are in the range of 0.6ꢀ0.8%. Electrolyte: 0.6 M DMPII, 0.05 M I2, 0.5 M TBP, and 0.1 M LiI in acetonitrile.
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ARTICLE
(EDOT) unit in the ancillary ligand of the ruthenium sensitizer.
A slightly higher Voc of JK-206 relative to JK-207 can be cor-
related with a decrease of the dark current. The Jsc enhancement
of JK-206 compared to JK-207 can be related to the dense
packing of the JK-206 monolayer on the TiO2 electrode. To see
the effect of packing in both sensitizers, we measured the amount
of dyes adsorbed on the TiO2 film by desorbing the dyes from the
TiO2 surface with KOH. The amounts of three dyes adsorbed on
the TiO2 film were measured to be 3.84 ꢁ 10ꢀ7, 3.04 ꢁ 10ꢀ7
,
and 3.76 ꢁ 10ꢀ7 mmol cmꢀ2 for JK-206, JK-207, and N719,
respectively. The low adsorption of JK-207 can be due to the
presence of two bulky EDOT units, the low solubility, and the
hydrophilic nature of EDOT. Therefore, the electron recombi-
nation occurred more significantly in the photoelectrodes by the
ineffective packing of large dyes.
Because long-term stability is a vital element for sustained cell
operation, we replaced the liquid electrolyte with a quasi-solid-
state one consisting of 0.5 M N-methylbenzimidazole (NMBI), 5
wt % PVDF-HFP, 0.6 M DMPII, and 0.1 M I2 in MPN. The JK-
206 sensitized cell yielded a remarkably high conversion effi-
ciency of 7.14% (see Figure 5). After 1000 h of light soaking at
60 °C, the initial efficiency of 7.14% in JK-206 decreased to
6.51%. On the other hand, the efficiency of N719 decreased from
7.03% to 5.25% under the same conditions. After 1000 h of light
soaking, Voc of JK-206 decreased by 110 mV, but the loss was com-
pensated for by an increase in Jsc from 15.27 to 18.35 mA cmꢀ2. The
long-term stability of the device is unprecented because a few
ruthenium dyes passed such a severe light-soaking stress by a
DSSC with an efficiency of over 6.5%.24 The enhanced long-
term stability of JK-206 can be attributed to the intrinsic
Figure 4. JꢀV curve and IPCE spectra of JK-206 (red line), JK-207
(blue line), and N719 (black line). The dark currentꢀbias potential
relationship is shown as dotted curves.
Figure 5. Evolution of solar cell parameters with N719 (black line) and
JK-206 (red line) during visible-light soaking (AM 1.5G, 100 mW cmꢀ2) at
60 °C. A 420 nm cutoff filter was put on the cell surface during illumination.
Figure 7. Nyquist plots of EIS spectra measured under illumination of
simulated AM 1.5 G (100 mW cmꢀ2) at open-circuit voltage.
Figure 6. Electron diffusion coefficients (a) and lifetimes (b) of the DSSCs employing JK-206, JK-207, and N719, respectively.
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ARTICLE
Figure 8. (a) Nyquist plots and (b) Bode phase plots measured in dark conditions at a forward bias of ꢀ0.67 V. The inset is the equivalent circuit used to
fit the impedance spectra.
^ ^
stability of a terdentate CNN coordination and the introduction
of long alkyl chains to the thienyl ligand.
The larger semicircle at the middle-frequency range represents
the interfacial charge recombination resistance (Rct) at the dyed
TiO2/electrolyte interface. The fitted Rct increased in the order
of JK-207 (119 Ω) < JK-206 (214 Ω) < N719 (229 Ω), which is
consistent with the Voc values measured in the devices. The
smaller Rct value means that electron recombination from TiO2
to electron acceptors in an electrolyte is occurring more easily
and thus results in lower Voc. By fitting the EIS curves, another
important parameter for DSSCs, the electron lifetime (τ), could
be extracted from the Cμ and Rct using τ = CμRct.26,27 Generally, a
negative shift of the conduction band edge of TiO2 leads to
improvement of Voc of DSSCs. Cμ increased in the order of N719
(527 μF) > JK-207 (456 μF) > JK-206 (410 μF), and the
electron lifetime (τ) increased in the order of N719 (120 ms) >
JK-206 (88 ms) > JK-207 (54 ms). Longer electron lifetime
contributed to a higher Voc. These results mean that it is the
charge recombination resistance, rather than the position of the
conduction band edge, that governs Voc. These results were in
accordance with the trends of the Voc and τ values.
Figure 6 shows the electron diffusion coefficients (De) and
lifetimes (τc) of the DSSCs employing different dyes JK-206, JK-
207, and N719 displayed as a function of Jsc and Voc, respectively.
No significant differences among the De values were seen at
identical short-circuit current conditions, showing a trend similar
to those of coumarin dyes.25 This result demonstrates that the De
values are hardly affected by structural changes in the dye molecules.
On the other hand, the τc values show a significant gap among the
dyes, resulting in the increasing order of N719 > JK-206 > JK-
207. The different τc values might be caused by the different
molecular structures of the dyes. The low value of τc in JK-207
compared to those of JK-206 and N719 may be due to the defects of
the JK-207 monolayer on the TiO2 electrodes. The results of the
electron lifetime are well consistent with those of Voc.
Electrochemical impedance spectroscopy (EIS) was employed
to study the electron recombination and electrolyte reduction pro-
cess in DSSCs based on three dyes. Figure 7 shows the alternat-
ing-current impedance spectra of the DSSCs measured under
illumination. A smaller radius of the semicircle in this intermedi-
ate-frequency regime implies a higher rate of electron transport at
the TiO2/dye/electrolyte interface. Upon illumination under
open-circuit voltage conditions (100 mW cmꢀ2), the response of
the intermediate semicircle is attributable to the deposited dye on
the TiO2/electrolyte. A capacitance (1.1 mF cmꢀ2) of JK-206
derived from (Y0R)(1/n)/R, where Y0 [se^cn cmꢀ2] is the symbol
for the constant phase element, n is the frequency power, and R is
the charge-transport resistance, is higher than those of JK-207
(0.49 mF cmꢀ2) and N719 (0.82 mF cmꢀ2), and the resistance
of JK-206 (11.98 Ω) is low compared with those of JK-207
(16.52 Ω) and N719 (15.26 Ω). As a result, the photocurrent of
JK-206 (19.63 mA cmꢀ2), JK-207 (13.68 mA cmꢀ2), and N719
(18.79 mA cmꢀ2) is in good agreement with the trends observed
in the above data.
In conclusion, a new class of cyclometalated ruthenium sensiti-
zers JK-206 and JK-207 was meticulously designed and synthe-
sized. A solar-to-electricity conversion efficiency of 10.39% in JK-
206 is better than η of 9.80% for the N719 sensitized cell. The
efficiency is the highest one reported for DSSCs based on the
cyclometalated ruthenium sensitizer. In addition, the JK-206
sensitized solar cell with a polymer gel electrolyte yielded an
overall conversion efficiency of 7.14%. Moreover, the JK-206
device showed excellent stability under light soaking at 60 °C for
1000 h. The high efficiency and excellent stability may be attributed
to the intrinsic stability of the cyclometalated ruthenium complex
^ ^
with the CNN ligand and the broad and red-shifted absorption
property of the cyclometalated complex. We believe that the
development of highly efficient DSSC devices with stability is
possible through a variety of cyclometalated complexes, and
work on these is now in progress.
Figure 8 shows the typical EIS Nyquist and Bode phase plots
measured in dark conditions at a forward bias of ꢀ0.67 V. The
equivalent circuit inset in Figure 8b was used to fit the experi-
mental data of all of the samples. Rs is the series resistance ac-
counting for the transport resistance of the TCO and the electrolyte.
Cμ and Rct are the chemical capacitance and the charge recombi-
nation resistance at the TiO2/electrolyte interface, respectively.
CPt and RPt are the interface capacitance and charge-transport
resistance at the platinum/electrolyte interface, respectively.
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR spectra of
S
b
JK-206 and JK-207, calculated absorption spectra of JK-206,
cyclic voltammogram of JK-206, and XYZ cordinates. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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ARTICLE
’ AUTHOR INFORMATION
(13) (a) Wu, K.-L.; Hsu, H.-C.; Chen, K.; Chi, Y.; Chung, M.-W.;
Chou, P.-T. Chem. Commun. 2010, 46, 5124. (b) Chou, C.-C.; Wu, K.-L.;
Chi, Y.; Hu, W.-P.; Yu, S. J.; Lee, G.-H.; Lin, C.-L.; Chou, P.-T. Angew.
Chem., Int. Ed. 2011, 50, 2054. (c) Chen, K.-S.; Liu, W.-H.; Wang, Y.-H.;
Lai, C.-H.; Chou, P.-T.; Lee, G.-H.; Chen, K.; Chen, H.-Y.; Chi, Y.;
Tung, F.-C. Adv. Funct. Mater. 2007, 17, 2964.
Corresponding Author
*E-mail: jko@korea.ac.kr. Fax: 82 41 867 5396. Tel: 82 41 860 1337.
’ ACKNOWLEDGMENT
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This work was supported by the WCU (the Ministry of Educa-
tion and Science) program (Grant R31-2008-000-10035-0), the
ERC [the Korean government (MEST)] program (Grant R11-
2009-088-02001-0), and the New & Renewable Energy of the
Korea Institute of Energy Technology Evaluation and Planning
(KETEP) grant funded by the Korea government Ministry of
Knowledge Economy (Grant 2010T100100674), and computa-
tions were performed using a supercomputer at the Korea Institute
of Science and Technology Information (Grant KSC-2011-C1-11).
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