Ruthenium(II) sensitizers with heteroleptic tridentate chelates for dye-sensitized solar cells

Article abstract of DOI:10.1002/anie.201006629

Two new for Ru: Unprecedented Ru^II sensitizers are coordinated by two tridentate ligands, a tricarboxyterpyridine, and a dianionic bis(pyrazolyl)pyridine derivative with an extended π-conjugated auxochrome (see picture, Rumagenta, Nblue, Ored, Fgreen, Syellow, Cgray). This new generation of thiocyanate-free Ru^II sensitizers offers solar conversion efficiency in dye-sensitized solar cells up to I·=10.7 %.

Full text of DOI:10.1002/anie.201006629

Communications
DOI: 10.1002/anie.201006629
Dye-Sensitized Solar Cells
Ruthenium(II) Sensitizers with Heteroleptic Tridentate Chelates for
Dye-Sensitized Solar Cells**
Chun-Cheng Chou, Kuan-Lin Wu, Yun Chi,* Wei-Ping Hu, Shuchun Joyce Yu,
Gene-Hsiang Lee, Chia-Li Lin, and Pi-Tai Chou
Dye-sensitized solar cells (DSSCs) are a promising technol-
ogy with the potential to harvest sunlight at low cost.[1]
Specifically, DSSCs based on either organometallic dyes[2]
or organic push-and-pull dyes[3] adsorbed on nanocrystalline
TiO2 photoanodes have attracted intensive research interest
in the past two decades. The use of ruthenium(II)-based
sensitizers[4] is still the most attractive approach, because their
photophysical and electrochemical properties can be system-
atically fine-tuned to achieve optimal material characteristics.
Key examples of RuII sensitizers for DSSCs include the well-
established N3 and N719 dyes and their functionalized
derivatives.[5,6] These dyes are commonly assembled by
incorporation of at least one 4,4’-dicarboxy-2,2’-bipyridine
chelate (Scheme 1), on which the carboxy anchors allow
efficient electron injection into the TiO2 nanoparticles. The
accompanying thiocyanate ancillary ligands are pointed away
from the TiO2 surface, thus providing intimate contact to the
IÀ/I3 redox couple in the electrolyte and subsequently
À
triggering rapid regeneration of the oxidized sensitizer. The
lower-lying absorption bands are attributed to metal-to-
ligand charge transfer (MLCT) and display absorption
thresholds close to 800 nm, unmatched by the majority of
organic dyes known to date.
For optimizing RuII sensitizers, it has been reported that,
upon introduction of 4,4’,4’’-tricarboxy-2,2’:6,2’’-terpyridine
(H3tctpy), the lowest energy transition could be extended
toward the near-IR region, thus affording the panchromatic
sensitizer known as N749, or black dye [nBu4N]3[Ru(Htctpy)-
(NCS)3].[7] Although this design seems to show satisfactory
sensitization up to 900 nm, it still possesses two major
deficiencies. One is the inferior absorptivity in the visible
region, attributed to the lack of effective auxochrome. The
second is the presence of three thiocyanate ligands, which not
only reduce the synthetic yield owing to coordination isomer-
ism[7,8] but also deteriorate the lifespan of the as-fabricated
À
cell units, because the weak Ru NCS dative bonding causes
notable dye decomposition during operation.
In conjunction with current endeavors, we described one
panchromatic dye PRT4, which possesses a styrene-substi-
tuted pyridyl pyrazolate (pypz) auxochrome.[9] Its recorded
solar-cell performance supersedes our best reference cells
using N749, thus providing the first success for our research
initiative. Herein we present a breakthrough design employ-
ing both the H3tctpy anchor and a newly designed tridentate
ancillary ligand; the latter is also targeted for replacing the
last remaining thiocyanate in our PRT4 dye as well as all three
thiocyanates in N749 sensitizer. Thiophene derivatives or
other auxochrome units were tethered to the central pyridyl
group in an attempt to increase the light-harvesting capability.
In this synthetic protocol (Scheme 2), the functionalized
2,6-bis(5-pyrazolyl)pyridine chelate was first prepared using a
multistep process starting from chlorination of chelidamic
acid,[10] except for the synthesis of parent 2,6-bis(5-pyrazo-
lyl)pyridine, which employed the commercially available 2,6-
diacetylpyridine. Bis-tridentate RuII complexes were then
obtained from addition of the relevant 2,6-dipyrazolylpyr-
idine derivative with source complex [Ru(tectpy)Cl3] in
Scheme 1. RuII sensitizers N3, N719, N749, and PRT4.
[*] C.-C. Chou, K.-L. Wu, Prof. Y. Chi
Department of Chemistry, National Tsing Hua University
Hsinchu, Taiwan 30013 (Taiwan)
E-mail: ychi@mx.nthu.edu.tw
Prof. W.-P. Hu, Prof. S. J. Yu
Department of Chemistry and Biochemistry
National Chung Cheng University (Taiwan)
Dr. G.-H. Lee, C.-L. Lin, Prof. P.-T. Chou
Department of Chemistry
National Taiwan University (Taiwan)
ethanol
solution
(tectpy = 4,4’,4’’-triethoxycarbonyl-
2,2’:6’,2’’-terpyridine). The crude ethoxycarbonyl RuII prod-
ucts were purified on a silica gel column and eluted with a
mixture of hexane and ethyl acetate. To confirm their
molecular structure, single-crystal X-ray analysis of one
derivative (TF-2OEt; TF = thiocyanate-free) was carried
[**] This research was supported by the National Science Council
(NSC-98-3114-E-007-005)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006629.
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Angew. Chem. Int. Ed. 2011, 50, 2054 –2058

comparison. Their pertinent photophysical and electro-
chemical properties are summarized in Table 1. These TF
dyes show intense visible absorption bands down to
510 nm, which are assigned to metal-to-ligand charge
transfer (MLCT) to the H3tctpy chelate, together with
contributions from ligand-to-ligand charge transfer
(LLCT) originating from the pyrazolate groups. A
further difference from N749 is that all TF dyes exhibit
broad absorption at the longer-wavelength region, with
two weak peak maxima centered at about 650 and 720 nm
and with a shoulder extending to 800 nm; these features
are attributed to a mixed MLCTand LLCT transition (to
H3tctpy). Notably, the intense MLCT band at 606 nm in
N749 is substantially suppressed in all TF dyes, which
could be due to the lack of the three thiocyanate anions.
This spectral assignment is further supported by theoret-
ical calculations (see details in the Supporting Informa-
tion). Using TF-2 as an example, Figure 2 depicts selected
optically active electronic transitions (black bars)
employing time-dependent (TD) DFT PCM calculations
in DMF as media. According to the calculations, the
Scheme 2. Synthetic route to the RuII sensitizers TF-1–TF-4. Reagents and
conditions: a) PCl5, CHCl3, MeOH; b) EtOAc, NaOEt, reflux; c) HCl(aq)
reflux; d) Suzuki or Stille coupling, [Pd(PPh3)4]; e) CF3CO2Et, NaOEt, THF,
reflux; f) N2H4, EtOH, reflux; g) [Ru(tectpy)Cl3], EtOH, reflux; h) NaOH,
acetone/H2O, acidification.
,
out (see Figure S1 in the Supporting Information for the
ORTEP diagram and the associated metric parameters). Then
hydrolysis of ethoxycarbonyl groups was conducted in a
basified acetone/water mixture, and the RuII sensitizers were
precipitated from the solution by adjusting the pH value to 3.
To improve purity, these precipitates were then crystallized by
slow diffusion of diethyl ether into a saturated DMF solution
at room temperature. The identity of all sensitizers TF-1–TF-
4 was verified by routine mass spectrometry, 1H NMR
spectroscopy, and elemental analysis. Owing to the dual
tridentate nature of the complex, the sole product was
obtained free from 1) the interference of thiocyanate isomer-
ism and 2) the otherwise mandatory H+/Bu4N+ metathesis.
Accordingly, the yield of isolated product was increased to
67% starting from [Ru(tectpy)Cl3] (cf. 46% for N749).[7]
The absorption spectra of our TF dyes in DMF solution
are depicted in Figure 1 together with that of N749 for
Figure 2. Experimental absorption spectrum of TF-2 sensitizer in DMF
and the associated calculated spectrum. Black bars represent the
computed vertical electronic excitations intensities (f) by TD-DFT
method. The electron–hole density plots are shown for the selected
electronic transitions at 412, 476, 493, and 748 nm, for which hole and
excited electron densities are represented in pink and cyan, respec-
tively.
absorption bands in the higher-energy region of 410–450 nm
are ascribed to S11–S15 excitation, for which the frontier orbital
analyses indicate that electron density, in part, is distributed
around the thiophene moiety (see electron–hole density at
412 nm, Figure 2).
Cyclic voltammetry was conducted to ensure that the
highest occupied molecular orbitals (HOMOs) of all TF
sensitizers match the redox potential of electrolyte and to
verify whether their lowest unoccupied molecular orbitals
(LUMOs) are higher than the conduction band of TiO2. As
shown in Table 1, the onset potential for RuII metal oxidation
Figure 1. Absorption spectra of TF-1–TF-4 and N749 in DMF.
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Communications
Table 1: Photophysical and electrochemical data of TF-1 to TF-4 measured in DMF. Also included are the respective photovoltaic parameters under
global AM 1.5 irradiation for DSSCs adsorbed on nanocrystalline TiO2 anodes.
Dye
labs [nm] (e [LmolÀ1 cmÀ1])[a]
lem [nm][a]
E8ox [V][b] E0À0 [V][c] ELUMO [V][d] VOC [V] JSC [mAcmÀ2
]
FF
h [%][e]
TF-1
318 (24906), 383 (9123), 417 (8507),
510 (9067), 640 (1623), 721 (1735)
329 (47898), 421 (18396), 509 (19170), 770, 827 (sh) 0.95
654 (2541), 719 (2430)
322 (51599), 387 (18763), 426 (21956), 760, 825 (sh) 0.97
513 (21890), 653 (2671), 723 (2606)
333(39037), 404(32366), 447 (29334),
516 (30926), 662 (2349), 716 (2350)
772, 830 (sh) 0.94
1.70
1.68
1.72
1.69
–
À0.76
À0.73
À0.75
À0.75
–
740
790
760
770
720
18.22
20.00
21.39
20.27
19.49
0.676 9.11
0.665 10.5
0.660 10.7
0.675 10.5
0.657 9.22
TF-2
TF-3
TF-4
770, 826 (sh) 0.94
N749 400 (8219), 606 (7003)
–
–
[a] Photophysical data were measured in DMF; sh=shoulder. [b] Oxidation potential of dyes was measured in DMF with 0.1m (nBu)4NPF6 and with a
scan rate of 50 mVsÀ1 (vs. the normal hydrogen electrode, NHE). It was calibrated with ferrocene/ferrocenium as internal reference and converted to
NHE by addition of 0.63 V. [c] E0À0 (the difference in zero-point energy between the S0 and T1 states) was determined from the average of absorption
(lowest-lying peak) and emission peak in terms of energy (eV) in DMF. [d] ELUMO was calculated according to the equation EoxÀE0À0. [e] All devices were
fabricated using a 15 mm layer of TiO2 nanoparticles (20 nm) and a 5 mm layer of 400 nm scattering particles (TiO2), while performance characteristics
were measured with a 0.144 cm2 working area.
appeared in the range 0.94–0.97 V (vs. NHE), showing that
the IÀ/I3 redox couple (ca. 0.4 V vs. NHE) is more negative
À
than the respective HOMOs and is able to regenerate the dye
efficiently. The LUMO energy levels, estimated from the
difference of the HOMO and the onset of the optical energy
gap (see Table 1), are also sufficiently more negative than the
conduction band edge of the TiO2 electrode (ca. À0.5 V vs.
NHE).
The DSSC performance of these sensitizers was then
examined using a 15 mm thick layer of 20 nm TiO2 nano-
particles plus a 5 mm thick layer of 400 nm scattering particles
(TiO2) with an acetonitrile electrolyte containing 0.6m 1,2-
dimethyl-3-propylimidazolium iodide, 0.1m iodine, 0.5m tert-
butylpyridine, and 0.1m lithium iodide. The incident photon-
to-current conversion efficiencies (IPCEs) of these DSSC
dyes are shown in Figure 3a. The onsets of the IPCE spectra
are all close to 820 nm, and excellent IPCE performance was
observed from 420 to 700 nm. It is notable that, except for the
parent TF-1, which exhibits a maximum IPCE of 85% at
515 nm, all other sensitizers (TF-2–TF-4) show a much better
IPCE value of 92% at 515 nm and maintain a high value of
approximately 75% in the longer wavelength region up to
685 nm. For comparison, DSSC fabricated using N749 showed
inferior IPCE below 560 nm and in the region between 640
and 710 nm and increased IPCE in the regions between 560
and 640 nm and beyond 730 nm.
Figure 3. a) Photon-to-current action spectra and b) current–voltage
characteristics of solar cells sensitized with various TF dyes and N749.
Figure 3b shows the photocurrent density–voltage curve
of the DSSC devices recorded under AM 1.5 G simulated
sunlight at a light intensity of 100 mWcmÀ2. The N749 cells
showed short-circuit current density (JSC), open-circuit volt-
age (VOC), fill factor (FF), and overall conversion efficiency
(h) of 19.49 mAcmÀ2, 720 mV, 0.657, and 9.22%, respectively.
We note, however, that this conversion efficiency is lower
than that reported by Wang et al. (10.5%) and by Han et al.
(11.1%).[11,12] Remarkably, most of our TF-sensitized solar
cells were endowed with much better, double-digit efficien-
the case, N749 would be expected to have an even higher VOC ,
because it carries only one proton, while the investigated TF
sensitizers carry three protons. For comparison, a reduction of
the number of protons in either N3 or N719 was found to
induce an increase of VOC as large as 90 mV.[13] Alternatively,
we propose that the replacement of the three thiocyanate
ligands with the extremely bulky 2,6-bis(5-pyrazolyl)pyridine
ligand may allow better packing upon adsorption on the TiO2
surface, which could prevent interfacial charge recombina-
tion,[14] thus giving the first advantage. Moreover, in compar-
cies. For instance, TF-3 gives JSC = 21.39 mAcmÀ2, VOC
=
760 mV, and FF = 0.66, corresponding to an overall conver-
sion efficiency h = 10.7%. The higher VOC seems unrelated to
the neutral characteristics of the TF sensitizers. If this were
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Angew. Chem. Int. Ed. 2011, 50, 2054 –2058

ison to the prevailing negative contribution of the thiocyanate
ligands that reside at the similar site of N749, the neutral
pyridyl group flanked by two 2,6-bis(5-pyrazolyl) termini may
be intrinsically more capable of exerting a dipole moment
pointing toward the TiO2 surface.[15] In other words, in TF
sensitizers the negative pole of the dipole moment is expected
to localize much closer to the TiO2 surface, thus giving better
negative dipole of sensitizers to reside closer to the TiO2
surface, which is the prerequisite for raising the TiO2
conduction-band energy level to achieve high open-circuit
voltage (VOC) and decent overall efficiency (h).
Received: October 22, 2010
Published online: January 26, 2011
lifting of conduction-band energy level and consequently
[16]
Keywords: N ligands · pyrazole · ruthenium · sensitizers ·
thiophene
affording higher VOC
.
Finally, all functionalized sensitizers
.
(TF-2 to TF-4) show higher current density of 20.00–
21.39 mAcmÀ2 versus that of 19.49 mAcmÀ2 for N749 and
18.22 mAcmÀ2 for the parent TF-1, manifesting the increase
of molar absorptivity upon extending the p conjugation of the
auxochrome.
[1] a) A. Hagfeldt, G. Boschloo, H. Lindstroem, E. Figgemeier, A.
Holmberg, V. Aranyos, E. Magnusson, L. Malmqvist, Coord.
Chem. Rev. 2004, 248, 1501; b) L. M. Gonꢀalves, V. de Zea Ber-
mudez, H. A. Ribeiro, A. M. Mendes, Energy Environ. Sci. 2008,
1, 655; c) S. Yanagida, Y. Yu, K. Manseki, Acc. Chem. Res. 2009,
42, 1827; d) M. Grꢁtzel, Acc. Chem. Res. 2009, 42, 1788; e) S.
Ardo, G. J. Meyer, Chem. Soc. Rev. 2009, 38, 115; f) Y. Luo, D.
Li, Q. Meng, Adv. Mater. 2009, 21, 4647; g) Z. Ning, Y. Fu, H.
Tian, Energy Environ. Sci. 2010, 3, 1170; h) A. Hagfeldt, G.
Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110,
6595.
To gain more insight into the cell characteristics, alternat-
ing current (AC) electrochemical impedance spectroscopy
was performed to analyze the effects on charge generation,
transport, and collection. Figure S2a in the Supporting
Information depicts the impedance spectra of DSSCs fabri-
cated with all TF cells in the dark under forward bias
(À0.74 V). As a result, the radius of these semicircles reveals a
descending order of TF-2(148.15 W) > TF-4(117.55 W) > TF-
3(101.39 W) > TF-1(74.63 W) > N749(65.8 W), thus indicat-
ing that the recombination process from the as-prepared TF
dye-sensitized solar cells is slower than with N749, leading to
the higher photovoltage. Concomitantly, the AC electro-
chemical impedance of the cells was also measured under
solar illumination at 1 sun (100 mWcmÀ2). As can be seen in
Figure S2b in the Supporting Information, the radius of the
semicircle in the intermediate frequency regime, according to
the Nyquist plot, follows the opposite order (cf. Figure S2a
measured in the dark) of TF-2 < TF-4 < TF-3 < TF-1 < N749.
These results clearly indicate that the charge transport is in
the order of TF-2 > TF-4 > TF-3 > TF-1 > N749, which coin-
cides with the overall device performances shown in Table 1.
Finally, the lifespan of solar cells fabricated with TF-2 on a
(8 + 5) mm thick TiO2 photoanode was examined using a long-
term accelerated aging experiment under accelerated visible-
light soaking at 608C. After 1000 h illumination, the device
efficiency changed only slightly for all the parameters: JSC
15.73 to 15.95 mAcmÀ2, VOC 760 to 690 mV, FF 0.673 to 0.688,
and h 8.07 to 7.57% (see Figure S3 in the Supporting
Information), thus supporting the advantage of thiocyanate-
free, charge-neutral RuII sensitizers suited for dye-sensitized
solar cells.
In summary, promising device performance data have
been obtained with the new design of panchromatic RuII
sensitizers that incorporate both 4,4’,4’’-tricarboxy-2,2’:6,2’’-
terpyridine and 2,6-bis(5-pyrazolyl)pyridine ligands; the best
cell showed power conversion efficiencies as high as 10.7%.
This result completely rules out the need for thiocyanate
ancillary ligands. From a synthetic point of view, the dual
tridentate structure avoids problematic isomerization, thus
simplifying the purification process and rendering good
product yield. Chemically, the multidentate coordination, in
theory, should improve the long-term stability, as evidenced
by the great lifespan of solar cells fabricated with TF-2.
Further development is versatile. Particularly, our strategy
even simplifies the structural design that may allow the
[2] a) M. K. Nazeeruddin, C. Klein, P. Liska, M. Grꢁtzel, Coord.
Chem. Rev. 2005, 249, 1460; b) N. Robertson, Angew. Chem.
2006, 118, 2398; Angew. Chem. Int. Ed. 2006, 45, 2338; c) H.
Imahori, T. Umeyama, S. Ito, Acc. Chem. Res. 2009, 42, 1809.
[3] a) Y. Ooyama, Y. Harima, Eur. J. Org. Chem. 2009, 2903; b) Z.
Ning, H. Tian, Chem. Commun. 2009, 5483; c) A. Mishra,
M. K. R. Fischer, P. Bauerle, Angew. Chem. 2009, 121, 2510;
Angew. Chem. Int. Ed. 2009, 48, 2474; d) D.-Y. Chen, Y.-Y. Hsu,
H.-C. Hsu, B.-S. Chen, Y.-T. Lee, H. Fu, M.-W. Chung, S.-H. Liu,
H.-C. Chen, Y. Chi, P.-T. Chou, Chem. Commun. 2010, 46, 5256.
[4] a) K.-J. Jiang, N. Masaki, J.-B. Xia, S. Noda, S. Yanagida, Chem.
Commun. 2006, 2460; b) C.-Y. Chen, S.-J. Wu, J.-Y. Li, C.-G. Wu,
J.-G. Chen, K.-C. Ho, Adv. Mater. 2007, 19, 3888; c) K.-S. Chen,
W.-H. Liu, Y.-H. Wang, C.-H. Lai, P.-T. Chou, G.-H. Lee, K.
Chen, H.-Y. Chen, Y. Chi, F.-C. Tung, Adv. Funct. Mater. 2007,
17, 2964; d) C.-Y. Chen, J.-G. Chen, S.-J. Wu, J.-Y. Li, C.-G. Wu,
K.-C. Ho, Angew. Chem. 2008, 120, 7452; Angew. Chem. Int. Ed.
2008, 47, 7342; e) F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X.
Jing, R. Humphry-Baker, P. Wang, S. M. Zakeeruddin, M.
Grꢁtzel, J. Am. Chem. Soc. 2008, 130, 10720; f) F. Gao, Y.
Wang, J. Zhang, D. Shi, M. Wang, R. Humphry-Baker, P. Wang,
S. M. Zakeeruddin, M. Grꢁtzel, Chem. Commun. 2008, 2635;
g) C. Lee, J.-H. Yum, H. Choi, S. O. Kang, J. Ko, R. Humphry-
Baker, M. Grꢁtzel, M. K. Nazeeruddin, Inorg. Chem. 2008, 47,
2267; h) J.-H. Yum, I. Jung, C. Baik, J. Ko, M. K. Nazeeruddin,
M. Grꢁtzel, Energy Environ. Sci. 2009, 2, 100; i) Z. Jin, H.
Masuda, N. Yamanaka, M. Minami, T. Nakamura, Y. Nishikitani,
J. Phys. Chem. C 2009, 113, 2618; j) C.-Y. Chen, M. Wang, J.-Y.
Li, N. Pootrakulchote, L. Alibabaei, C.-H. Ngoc-Le, J.-D.
Decoppet, J.-H. Tsai, C. Grꢁtzel, C.-G. Wu, S. M. Zakeeruddin,
M. Gratzel, ACS Nano 2009, 3, 3103; k) Q. Yu, S. Liu, M. Zhang,
N. Cai, Y. Wang, P. Wang, J. Phys. Chem. C 2009, 113, 14559; l) K.
Chen, Y.-H. Hong, Y. Chi, W.-H. Liu, B.-S. Chen, P.-T. Chou,
J. Mater. Chem. 2009, 19, 5329.
[5] a) M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker,
E. Mueller, P. Liska, N. Vlachopoulos, M. Graetzel, J. Am.
Chem. Soc. 1993, 115, 6382; b) M. Grꢁtzel, J. Photochem.
Photobiol. A 2004, 164, 3.
[6] a) S. H. Wadman, J. M. Kroon, K. Bakker, R. W. A. Havenith,
G. P. M. van Klink, G. van Koten, Organometallics 2010, 29,
1569; b) Y. Sun, A. C. Onicha, M. Myahkostupov, F. N. Castel-
lano, ACS Appl. Mater. Int. 2010, 2, 2039; c) K.-L. Wu, H.-C.
Hsu, K. Chen, Y. Chi, M.-W. Chung, W.-H. Liu, P.-T. Chou,
Angew. Chem. Int. Ed. 2011, 50, 2054 –2058
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2057

Communications
Chem. Commun. 2010, 46, 5124; d) K. C. D. Robson, B. D.
[13] M. K. Nazeeruddin, R. Humphry-Baker, P. Liska, M. Grꢁtzel,
J. Phys. Chem. B 2003, 107, 8981.
Koivisto, T. J. Gordon, T. Baumgartner, C. P. Berlinguette, Inorg.
Chem. 2010, 49, 5335; e) H.-J. Park, K. H. Kim, S. Y. Choi, H.-M.
Kim, W. I. Lee, Y. K. Kang, Y. K. Chung, Inorg. Chem. 2010, 49,
7340; f) J.-J. Kim, K. Lim, H. Choi, S. Fan, M.-S. Kang, G. Gao,
H. S. Kang, J. Ko, Inorg. Chem. 2010, 49, 8351; g) X. Lv, F. Wang,
Y. Li, ACS Appl. Mater. Int. 2010, 2, 1980.
[14] a) M. Miyashita, K. Sunahara, T. Nishikawa, Y. Uemura, N.
Koumura, K. Hara, A. Mori, T. Abe, E. Suzuki, S. Mori, J. Am.
Chem. Soc. 2008, 130, 17874; b) T. Marinado, K. Nonomura, J.
Nissfolk, M. K. Karlsson, D. P. Hagberg, L. Sun, S. Mori, A.
Hagfeldt, Langmuir 2010, 26, 2592.
[7] M. K. Nazeeruddin, P. Pechy, T. Renouard, S. M. Zakeeruddin,
R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V.
Shklover, L. Spiccia, G. B. Deacon, C. A. Bignozzi, M. Grꢁtzel,
J. Am. Chem. Soc. 2001, 123, 1613.
[8] M. K. Nazeeruddin, M. Grꢁtzel, J. Photochem. Photobiol. A
2001, 145, 79.
[9] B.-S. Chen, K. Chen, Y.-H. Hong, W.-H. Liu, T.-H. Li, C.-H. Lai,
P.-T. Chou, Y. Chi, G.-H. Lee, Chem. Commun. 2009, 5844.
[10] J. B. Lamture, Z. H. Zhou, A. S. Kumar, T. G. Wensel, Inorg.
Chem. 1995, 34, 864.
[11] Z.-S. Wang, T. Yamaguchi, H. Sugihara, H. Arakawa, Langmuir
2005, 21, 4272.
[15] The dipole sign is defined to have a direction pointing from the
negative to the positive charge.
[16] a) S. S. Rꢂhle, M. Greenshtein, S.-G. Chen, A. Merson, H.
Pizem, C. S. Sukenik, D. Cahen, A. Zaban, J. Phys. Chem. B
2005, 109, 18907; b) Z. Zhang, N. Evans, S. M. Zakeeruddin, R.
Humphry-Baker, M. Grꢁtzel, J. Phys. Chem. C 2007, 111, 398;
c) W. H. Howie, F. Claeyssens, H. Miura, L. M. Peter, J. Am.
Chem. Soc. 2008, 130, 1367; d) P. Chen, J. H. Yum, F. De Angelis,
E. Mosconi, S. Fantacci, S.-J. Moon, R. H. Baker, J. Ko, M. K.
Nazeeruddin, M. Grꢁtzel, Nano Lett. 2009, 9, 2487; e) U. B.
Cappel, S. M. Feldt, J. Schoeneboom, A. Hagfeldt, G. Boschloo,
J. Am. Chem. Soc. 2010, 132, 9096; f) Y. Liang, B. Peng, J. Chen,
J. Phys. Chem. C 2010, 114, 10992.
[12] Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, L. Han,
Jpn. J. Appl. Phys. 2006, 45, L638.
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