New class of NCS-free cyclometalated ruthenium(II) complexes with 6-phenylpyridine-2-carboxylate for use as near-infrared sensitizers in dye-sensitized solar cells

Article abstract of DOI:10.1016/j.inoche.2014.05.022

Three examples, FT102, FT90, and FT117 of a new class of NCS-free cyclometalated ruthenium(II) complexes, Ru(tctpy)(O^N^C) (where O^N^C is a tridentate 6-phenylpyridine-2-carboxylate), were synthesized for use as near-infrared (IR) sensitizers in dye-sensitized solar cells (DSSCs). A tridentate donor ligand, 6-phenylpyridine-2-carboxylate was introduced in order to enhance the light harvesting efficiency in the longer wavelength region for the first time. Modifying the ligand improved the photovoltaic performance, and DSSCs sensitized with FT117 exhibited efficient panchromatic sensitization over the entire visible wavelength, extending into the near-IR region. The highest incident photon-to-current conversion efficiency (68%) was found at 600 nm, and the action spectrum onset was near 920 nm.

Full text of DOI:10.1016/j.inoche.2014.05.022

Inorganic Chemistry Communications 46 (2014) 137–139
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
New class of NCS-free cyclometalated ruthenium(II)
complexes with 6-phenylpyridine-2-carboxylate for use
as near-infrared sensitizers in dye-sensitized solar cells
b
a,b
Takashi Funaki a,b, , Hiromi Otsuka , Nobuko Onozawa-Komatsuzaki
,
⁎
Kazuyuki Kasuga b, Kazuhiro Sayama a,b, Hideki Sugihara
Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
b,⁎⁎
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Three examples, FT102, FT90, and FT117 of a new class of NCS-free cyclometalated ruthenium(II) complexes,
Ru(tctpy)(O^N^C) (where O^N^C is a tridentate 6-phenylpyridine-2-carboxylate), were synthesized for use as
near-infrared (IR) sensitizers in dye-sensitized solar cells (DSSCs). A tridentate donor ligand, 6-phenylpyridine-
2-carboxylate was introduced in order to enhance the light harvesting efficiency in the longer wavelength region
for the first time. Modifying the ligand improved the photovoltaic performance, and DSSCs sensitized with FT117
exhibited efficient panchromatic sensitization over the entire visible wavelength, extending into the near-IR
region. The highest incident photon-to-current conversion efficiency (68%) was found at 600 nm, and the action
spectrum onset was near 920 nm.
Received 3 February 2014
Accepted 19 May 2014
Available online 23 May 2014
Keywords:
Cyclometalated ruthenium(II) complex
6-Phenylpyridine-2-carboxylate
Dye-sensitized solar cell
Near-IR sensitizer
© 2014 Elsevier B.V. All rights reserved.
The use of ruthenium(II) polypyridyl complexes as sensitizers
in dye-sensitized solar cells (DSSCs) has received much attention
since Grätzel and co-workers reported the successful use of
Ru(dcbpy)2(NCS)2 (dcbpy = 4,4′-dicarboxy-2,2′-bipyridine; N3) [1,2]
and [Ru(tctpy)(NCS)3](TBA)3 (tctpy = 4,4′,4″-tricarboxy-2,2′:6′,2″-
terpyridine and TBA = tetra-n-butylammonium; N749) [3]. Modified
polypyridine ligands have been extensively investigated with the aim
of improving DSSC performance [4]. In these ruthenium(II) complexes,
the isothiocyanate (NCS) ligands tune the spectral and redox properties
of the complexes by destabilizing the metal t2g orbital. However, the
NCS ligand can undergo photosubstitution or photodegradation reac-
tions, which will decrease both the long-term stability of the complexes
and the photovoltaic DSSC performance [5–9]. The occurrence of these
undesirable processes can be decreased by replacing the NCS ligands
with a multidentate donor ligand.
the longer-wavelength region is important because there is a strong
photon flux in the near-infrared (IR) region above 800 nm in the solar
spectrum. Few NCS-free ruthenium(II) complexes that exhibit good
light-harvesting efficiencies above 800 nm have been reported [13–18].
The absorption band of ruthenium(II) complexes can be extended to
longer wavelengths by introducing a ligand with a low-lying π*-level
molecular orbital and/or by destabilizing the metal t2g orbital using a
strong donor ligand. Therefore, the absorption properties can be tuned
by careful manipulation of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) energy
levels [19–21]. Ruthenium(II) terpyridyl complexes have been reported
to exhibit better photoresponses at longer wavelengths than
ruthenium(II) bipyridyl complexes [3]. Therefore, ruthenium(II)
terpyridyl complexes with strong donor ligands may exhibit absorption
bands extending to longer wavelengths [14]. An advantage of using a
multidentate donor ligand is that its electron-donating ability can be
changed by introducing a substituent [22,23].
Although initial attempts gave only limited success, several NCS-
free ruthenium(II) complexes have been reported to exhibit conver-
sion efficiencies of up to 10% [10–13]. However, most NCS-free
ruthenium(II) complexes exhibit insufficient light-harvesting effi-
ciencies above 800 nm. Improving the light-harvesting efficiency in
Here, we report the synthesis of
a new class of NCS-free
cyclometalated ruthenium(II) complexes, Ru(tctpy)(O^N^C) (where
O^N^C is a tridentate 6-phenylpyridine-2-carboxylate). These are the
first examples of ruthenium(II) complex sensitizers that contain 6-
phenylpyridine-2-carboxylate as a donor ligand. Three NCS-free
cyclometalated ruthenium(II) complexes, FT102, FT90, and FT117,
were synthesized using the stepwise protocol illustrated in Scheme 1.
The O^N^C ligands were prepared using the Suzuki-coupling reaction
of ethyl-6-bromopicolinate and phenylboronic acid or its derivatives,
⁎
Correspondence to: T. Funaki, Research Center for Photovoltaic Technologies, National
Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-
1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. Tel.: +81 29 861 4892; fax: +81 29 861 4641.
⁎⁎ Corresponding author. Tel.: +81 29 861 6273; fax: +81 29 861 4641.
E-mail addresses: takasi-funaki@aist.go.jp (T. Funaki), sugihara-hideki@aist.go.jp
(H. Sugihara).
http://dx.doi.org/10.1016/j.inoche.2014.05.022
1387-7003/© 2014 Elsevier B.V. All rights reserved.

138
T. Funaki et al. / Inorganic Chemistry Communications 46 (2014) 137–139
Table 1
Photovoltaic performances of DSSCs containing FT102, FT90, FT117, and N749.
Dye
TBP/M
Jsc/mA cm−2
Voc/V
ff
η/%
IPCEmax/%
FT102
FT90
FT117
N749
0.1
0.1
0.1
0.5
9.1
16.2
18.3
19.0
0.53
0.62
0.62
0.71
0.66
0.64
0.63
0.71
3.2
6.5
7.2
9.6
47
63
68
70
aqueous KCl) reference electrode with a NaCl bridge. Ferrocene was
added to each sample solution at the end of the experiments, and
the ferrocenium/ferrocene redox couple was used as an internal
potential reference. A quasi-reversible wave caused by Ru3+/2+ redox
reactions was found between 0.4 and 0.9 V vs. a saturated calomel elec-
trode (SCE). The peak potentials for the differential pulse voltammo-
grams of FT102, FT90, and FT117 were 0.46, 0.52, and 0.56 V vs. SCE,
respectively. The redox potentials (Eox) of FT102, FT90, and FT117
were shifted by 0.10–0.20 V compared to the Eox for N749 (0.66 V vs.
SCE), and this was attributed to the presence of the O^N^C ligand in
the FT dyes. We previously proposed that rapid regeneration of a dye
is difficult if its HOMO energy level is below 0.49 V vs. SCE [23]. The en-
ergy difference between the Eox for FT102 and the iodide/triiodide cou-
ple (0.2 V vs. SCE) was too small to allow the efficient regeneration of
the oxidized complexes with a thermodynamic driving force. Structural
modification of the O^N^C ligand should affect the electrochemical
properties of the FT dyes. The electron withdrawing substituents intro-
duced into the O^N^C ligands in FT90 and FT117 made their Eox values
more positive than the value for FT102. The energy differences between
the Eox values and the iodide/triiodide couple could be large enough to
allow FT90 and FT117 to regenerate efficiently. The excitation energies
for FT102 (1.51 eV), FT90 (1.51 eV), and FT117 (1.55 eV) were estimated
using absorption thresholds [25,26], and the excited-state redox poten-
tials (Eox*) of the complexes were calculated to be −1.05, −0.99, and
−0.99 V vs. SCE, respectively. Therefore, a thermodynamic driving
force for electron injection might be provided, due to the Eox* of FT dyes
are more negative than the TiO2-conduction band edge (−0.7 V vs. SCE).
Nanocrystalline TiO2 photoelectrodes (each with an area of 0.25 cm2
and a thickness of 31 μm) were produced using a screen-printing meth-
od. The TiO2 pastes that were used were obtained from the Sumitomo
Osaka Cement Co., Ltd. Each TiO2 film produced was immersed in a
solution containing a single FT dye for 22 h at 25 °C. The 2 × 10−4 M
FT102, FT90, and FT117 solutions were prepared in a 7:3 (v/v) mixture
Scheme 1. Synthesis of FT102, FT90, and FT117. a) Phenylboronic acid or its derivatives,
Pd(PPh3)4, Na2CO3, H2O, THF; b) NaOH, H2O, EtOH; c) Ru(Me3tctpy)Cl3, LiCl, Et3N, H2O, DMF.
followed by hydrolysis of the ester group. FT102, FT90, and FT117
were produced using a microwave synthesis method. FT117 exists
as stereoisomer because there are two possible sites for the formation
of organometallic bond. The 1H NMR spectrum of FT117 suggested
that only one isomer was obtained in this work (Fig. S3). This is
probably due to a steric hindrance between 2,2′:6′,2″-terpyridine
and trifluoromethyl group. Recently, a similar observation has
been reported by Chi and Chou [24].
The absorption spectra of FT102, FT90, and FT117 in ethanol are
compared with the absorption spectrum of N749 in Fig. 1 Two equivalents
of tetra-n-butylammonium hydroxide (TBA(OH)) was added to the solu-
tions to improve the solubilities of the complexes in ethanol. The com-
plexes exhibited intense and broad absorption bands over the entire
visible region, and this is a desirable attribute that will allow a large pho-
tocurrent to be produced. The lowest energy bands for FT102, FT90, and
FT117 had maxima at 540 nm (ε = 12,900 M−1 cm−1), 535 nm (ε =
16,900 M−1 cm−1), and 525 nm (ε = 13,900 M−1 cm−1), respectively.
The most notable spectral feature of this class of FT dyes was a distinct
shoulder in the lower energy region, at around 700 nm, which is probably
caused by the strong σ-donating cyclometalating ligand.
of ethanol and DMF containing 4 × 10−4 M TBA(OH) and 2 × 10−2
M
Cyclic voltammograms of FT102, FT90, and FT117 were measured in
DMF solution containing 0.1 M tetra-n-butylammonium perchlorate
using a three-electrode apparatus comprising a platinum wire counter-
electrode, a platinum working electrode, and an Ag/AgCl (saturated
deoxycholic acid, and a 2 × 10−4 M N749 solution was prepared in
ethanol. Photoelectrochemical measurements were performed using a
Fig. 2. Photocurrent–voltage curves for DSSCs containing FT102 (dotted line), FT90
(dashed line), FT117 (solid line), and N749 (solid gray line). The onsets of the dark current
densities of the cells are also shown.
Fig. 1. Absorption spectra of FT102 (dotted line), FT90 (dashed line), FT117 (solid line), and
N749 (solid gray line) in ethanol.

T. Funaki et al. / Inorganic Chemistry Communications 46 (2014) 137–139
139
In summary, we synthesized a new class of NCS-free ruthenium(II)
complexes (FT102, FT90, and FT117) for use as near-IR sensitizers in
DSSCs. These are the first examples of ruthenium(II) complex sensi-
tizers that contain 6-phenylpyridine-2-carboxylate as a donor ligand.
DSSCs containing FT102, FT90, and FT117 exhibited efficiencies of 3.2%,
6.5%, and 7.2%, respectively, and modifying the ligand could improve
the photovoltaic performance. The cells sensitized with FT117 exhibited
efficient panchromatic sensitization over the entire visible wavelength,
extending into the near-IR region. A maximum IPCE value of 68% was
found at 600 nm, and the action spectrum onset was near 920 nm.
Structural modification of the O^N^C ligand could further improve the
light-harvesting properties in the near-IR region. Thus, these types of
ruthenium(II) complexes are promising new sensitizers for DSSCs. We
are currently optimizing the ligand structure and the cell fabrication
procedure of the complexes.
Fig. 3. Photocurrent action spectra of DSSCs sensitized with FT102 (dotted line), FT90
(dashed line), FT117 (solid line), and N749 (solid gray line).
Appendix A. Supplementary data
two-electrode solar cell comprising a dye-coated TiO2 electrode; a
platinum film counter-electrode; a polypropylene film spacer (thickness:
60 μm); and an electrolyte solution comprising 0.6 M 1,2-dimethyl-3-
propylimidazolium iodide, 0.05 M iodine, 0.1 M lithium iodide, and
0.1–0.5 M 4-tert-butylpyridine (TBP) in acetonitrile.
Details about the syntheses and 1H NMR spectra of FT dyes are
provided in a supplementary file, together with the description of the
photovoltaic studies. The supplementary data associated with this arti-
cle can be found in the online version at http://dx.doi.org/10.1016/j.
inoche.2014.05.022. These data include MOL files and InChiKeys of the
most important compounds described in this article.
The photovoltaic performances of the DSSCs sensitized with
FT102, FT90, and FT117 under standard air mass 1.5 irradiation
(100 mW cm−2) are shown in Table 1, in which Jsc is the short-circuit
photocurrent density, Voc is the open-circuit photovoltage, ff is the fill
factor, and η is the solar energy to current conversion efficiency. Data
were also obtained for a DSSC sensitized with N749 (Table 1) using
the same cell structure and measuring procedures. The best performance
of the DSSCs sensitized with the FT dyes was obtained with 0.1 M
TBP. Table S1 shows the influence of TBP concentration on the
efficiency of DSSCs sensitized with FT117. In contrast, the DSSC sensitized
with N749 exhibited its highest conversion efficiency with 0.5 M TBP.
The addition of TBP to the electrolyte has been reported to shift the
conduction band edge of TiO2 to more negative potentials [27–29].
These results suggest that the Eox* values of the FT dyes were relatively
close to the TiO2-conduction band edge. Thus, the shift of TiO2-
conduction band edge had a significant influence on the electron
injection and varied the Jsc values of DSSCs sensitized with FT dye.
The photocurrent–voltage curves for the DSSCs produced are shown
in Fig. 2. Although the DSSC containing FT102 had an efficiency of 3.2%,
the cells containing FT90 and FT117 had relatively high efficiencies of
6.5% and 7.2%, respectively. The cells containing FT90 and FT117 were
two times more efficient than the cells containing FT102 primarily
because they had higher Jsc and Voc values. These results indicate that
the thermodynamic driving force is insufficient for the rapid regenera-
tion of FT102 because of its relatively high HOMO energy level. The Jsc,
Voc, and η values were lower for FT102 than for the other FT dyes, prob-
ably because of the relatively large degree of charge recombination.
Fig. 3 shows the photocurrent action spectra for the DSSCs containing
the FT dyes and N749. The incident photon-to-current conversion
efficiency (IPCE) for each DSSC is plotted as a function of the wave-
length. As anticipated from the photovoltaic experiment results, slightly
lower IPCE values were obtained for the cell containing FT102 than for
the cells containing FT90 and FT117. The higher IPCE values can be
explained by the HOMO energy levels of the FT90 and FT117 dyes, be-
cause these FT dyes have similar structures and absorption properties.
Notably, the cell containing FT117 exhibited efficient panchromatic sen-
sitization over the entire visible wavelength range, extending into the
near-IR region, and the action spectrum onset was near 920 nm. There-
fore, structural modifications of the O^N^C ligand can improve the prop-
erties of the resulting complexes and provide better performing DSSCs.
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