Systematic evaluation of HOMO energy levels for efficient dye regeneration in dye-sensitized solar cells

Article abstract of DOI:10.1039/c4ta00613e

Thirty ruthenium complexes of the types [Ru(tctpy)(C^N)NCS] and [Ru(tctpy)(N^O)NCS] (C^N = cyclometalating ligand and N^O = pyridinecarboxylate and its derivatives) were synthesized and evaluated to identify the highest occupied molecular orbital (HOMO) energy level (E_HOMO) for efficient dye regeneration in dye-sensitized solar cells. E_HOMOof these complexes was systematically tuned by changing the electron-donating ability of the C^N and N^O ligands. For complexes with an E_HOMOin the potential range more negative than 0.5 V vs. a saturated calomel electrode (SCE), the incident photon-to-current conversion efficiency (IPCE) increased with a positive shift in E_HOMObut could not exceed 70%, suggesting that rapid dye regeneration is difficult. On the other hand, high IPCEs above 70% were often observed for complexes with an E_HOMOmore positive than 0.5 V vs. SCE. It is thus concluded that there is a threshold for efficient electron transfer near 0.5 V vs. SCE (ΔG₂≈ 0.3 eV) for the series of these sensitizers. This journal is

Full text of DOI:10.1039/c4ta00613e

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Materials Chemistry A
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Systematic evaluation of HOMO energy levels for
efficient dye regeneration in dye-sensitized solar
cells†
Cite this: J. Mater. Chem. A, 2014, 2,
15945
ab
*
Takashi Funaki,
Hiromi Otsuka,b Nobuko Onozawa-Komatsuzaki,ab
Kazuyuki Kasuga,b Kazuhiro Sayamaab and Hideki Sugihara
b
*
Thirty ruthenium complexes of the types [Ru(tctpy)(C^N)NCS] and [Ru(tctpy)(N^O)NCS] (C^N
¼
cyclometalating ligand and N^O ¼ pyridinecarboxylate and its derivatives) were synthesized and
evaluated to identify the highest occupied molecular orbital (HOMO) energy level (EHOMO) for efficient
dye regeneration in dye-sensitized solar cells. EHOMO of these complexes was systematically tuned by
changing the electron-donating ability of the C^N and N^O ligands. For complexes with an EHOMO in the
potential range more negative than 0.5 V vs. a saturated calomel electrode (SCE), the incident photon-
to-current conversion efficiency (IPCE) increased with a positive shift in EHOMO but could not exceed
70%, suggesting that rapid dye regeneration is difficult. On the other hand, high IPCEs above 70% were
often observed for complexes with an EHOMO more positive than 0.5 V vs. SCE. It is thus concluded that
there is a threshold for efficient electron transfer near 0.5 V vs. SCE (DG2 z 0.3 eV) for the series of
these sensitizers.
Received 5th February 2014
Accepted 22nd July 2014
DOI: 10.1039/c4ta00613e
www.rsc.org/MaterialsA
band can be extended to longer wavelengths by destabilizing
the metal t2g orbital using a strong donor ligand and/or intro-
Introduction
ducing a ligand with a low-lying p*-level molecular orbital. To
realize high performance solar cells, an enhanced spectral
response of the sensitizer in the lower energies is required while
maintaining sufficient thermodynamic driving forces for both
electron injection and dye regeneration. Systematic tuning of
the HOMO and LUMO energy levels (EHOMO and ELUMO) of
sensitizers is necessary to estimate the energy difference (DG)
for sufficient thermodynamic driving forces.
Katoh and co-workers reported that a sufficiently high energy
difference (DG1 > 0.2 eV) is required to attain efficient electron
injection for dye-sensitized ZnO lms.6 They further quantita-
tively evaluated ruthenium complexes and organic dyes in dye-
sensitized TiO2 lms.7 On the other hand, there are several
reports on the sufficient energy difference for efficient dye
regeneration (DG2). For the case of cobalt and ferrocene redox
couples, the dye regeneration kinetics for combination of
organic dyes and different redox couples were examined. By
varying the redox potential of cobalt complexes, Feldt et al.
showed that efficient dye regeneration can be achieved with a
DG2 of 0.25 eV when a highly concentrated electrolyte is used.8
Daeneke et al. reported that a DG2 of 0.20–0.25 eV for ferrocene
redox couples is sufficient to regenerate the dye quantitatively.9
For the case of the I3ꢀ/Iꢀredox couple with ruthenium
complexes, a minimum value of DG2 was reported to be about
0.75 eV.10 The redox potentials of I3ꢀ/Iꢀ and I2ꢀ_/Iꢀ in acetoni-
trile are reported to be 0.11 and 0.55 ꢁ 0.10 V vs. a saturated
calomel electrode (SCE), respectively.11 However, Schulze et al.
Dye-sensitized solar cells (DSSCs) have been extensively inves-
tigated because of their ease of fabrication and low production
costs.1 Extensive effort has been devoted to the development of
new, highly efficient sensitizers, as they play a critical role in cell
performance. Of these various sensitizers, [Ru(Hdcbpy)2-
(NCS)2](TBA)2 (dcbpy ¼ 4,40-dicarboxy-2,20-bipyridine and TBA
¼ tetrabutylammonium) (N719)2 and [Ru(Htctpy)(NCS)3](TBA)3
(tctpy ¼ 4,40,400-tricarboxy-2,20:60,200-terpyridine) (N749)3 are two
of the most effective complexes, and DSSCs based on these dyes
have achieved a solar-to-electricity conversion efficiency of
11%.2,4
To improve the conversion efficiency of DSSCs, sensitizers
exhibiting absorption over a wide range of the solar spectrum
and a high molecular extinction coefficient have been investi-
gated. The absorption properties of ruthenium(II) complexes,
for example, can be tuned by careful consideration of the
highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) energy levels.5 The absorption
aResearch 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. E-mail: takasi-funaki@aist.go.jp; Fax: +81-29-
861-4641; Tel: +81-29-861-4892
bEnergy 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. E-mail: sugihara-hideki@aist.go.jp; Tel: +81-29-861-6273
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta00613e
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suggested that approximately 0.66 V vs. SCE is required for of 1 ꢂ 10ꢀ3 M. Ferrocene was added to each sample solution at
efficient dye regeneration,12 and Islam et al. suggested that the end of the experiment, and a ferrocenium/ferrocene redox
sensitizers should possess an EHOMO greater than 0.53 V vs. couple (0.39 V vs. SCE) was used as an internal potential
SCE.13 We reported that there seems to be a threshold for rapid reference.
dye regeneration near 0.5 V vs. SCE.14 It is essential to know the
required difference between the EHOMO of a sensitizer and the
iodine redox potential to design highly efficient sensitizers.
Herein we investigate the inuence of EHOMO on the dye
regeneration process in detail. Thirty ruthenium complexes of
Photovoltaic performance
Nanocrystalline TiO2 lms (area: 0.25 cm2; thickness: 38 mm)
were prepared via screen printing on conductive glass (Nippon
Sheet Glass Co., Japan, F-doped SnO2 layer) as described
the types [Ru(tctpy)(C^N)NCS] and [Ru(tctpy)(N^O)NCS] (C^N ¼
previously.15 The TiO2 lms were heated to 525 ꢃC for 1 h in air
cyclometalating ligand and N^O ¼ pyridinecarboxylate and its
and then allowed to cool to 120 ꢃC before being dipped in 0.1 M
derivatives) were synthesized. The introduction of C^N and N^O
ꢃ
HCl at 80 C for 2 h. Aer rinsing with water and ethanol, the
ligands made it possible to tune the EHOMO of the resulting
TiO2 photoelectrodes were immersed in a dye solution for 22 h
complexes while maintaining an ELUMO for efficient electron
ꢃ
at 25 C. We tried to nd conditions giving the highest short-
injection into the conduction band of a TiO2 electrode. By
circuit current density. Therefore, the concentration was
changing the electron-donating ability of the C^N and N^O
different between 1–23 and 24–30. Complexes 1–23 were dis-
ligands, the EHOMO of each of the resulting complexes was
solved in ethanol (2 ꢂ 10ꢀ4 M) containing 4 ꢂ 10ꢀ4 M TBA(OH)
systematically tuned. The molecular structures of the prepared
and 2 ꢂ 10ꢀ2 M deoxycholic acid (DCA), whereas complexes 24–
ruthenium complexes with EHOMO values ranging from 0.33 to
30 were dissolved in ethanol (1 ꢂ 10ꢀ4 M) containing 2 ꢂ 10ꢀ4
0.77 V vs. SCE are shown in Fig. 1.
M TBA(OH) and 1 ꢂ 10ꢀ2 M DCA. Although the concentrations
were different, the molar ratio of 1–30 to DCA is 1 : 100. N749
was dissolved only in ethanol (2 ꢂ 10ꢀ4 M), and N719 was dis-
solved in a mixed solution of acetonitrile and tert-butyl alcohol
(1 : 1, v/v; 3 ꢂ 10ꢀ4 M). Photoelectrochemical measurements
were performed using a two-electrode solar cell consisting of the
dye-coated TiO2 electrode, a platinum lm counter electrode, a
polypropylene lm spacer (thickness: 60 mm), and an electrolyte
solution consisting of 0.6 M 1,2-dimethyl-3-propylimidazolium
iodide, 0.05 M iodine, 0.1 M lithium iodide, and 0 or 0.5 M tert-
butylpyridine (TBP) in acetonitrile.
Experimental
General
Commercially available chemicals and solvents were used as
received. Microwave irradiation reactions were carried out using
a CEM Discover microwave synthesizer and reaction tempera-
tures were monitored using an equipped IR sensor. Nuclear
magnetic resonance (NMR) spectra were obtained on a Varian
INOVA 400 spectrometer, a Bruker Avance 400 spectrometer, a
Bruker Avance 500 spectrometer, and a JEOL JNM-LA 600
spectrometer. Chemical shis are given in ppm relative to
standard. Electrospray ionization mass spectra (ESI-MS) were
recorded on a Waters ZQ mass spectrometer. Cyclic voltam-
mograms and differential pulse voltammograms were collected
Results and discussion
The synthesis of complexes 1–23 of the type [Ru(tctpy)(C^N)NCS]
is shown in Scheme 1. For complexes 1–18, the cyclometalation
step was conducted in EtOH–water (3 : 1, v/v), whereas for
complexes 19–23, both the cyclometalation step and the
hydrolysis of the ester group occurred in DMF–water (3 : 1, v/v)
at an elevated temperature. Complexes 1–23 should exist as
mixtures of stereoisomers; however, it was possible to obtain
the single isomer depicted in Scheme 1 using a microwave
synthetic technique. The structures of the resulting complexes
were identied via NMR and ESI-MS analysis. The most upeld
signal in the aromatic region of the NMR spectra, which is
assigned to the proton at the ortho position to the organome-
tallic bond, is useful spectroscopic evidence for cyclo-
metalation.16 The further upeld shi of this proton in the NMR
using
a BAS-100 electrochemical analyzer (Bioanalytical
System). The photovoltaic measurements were performed using
a Xe lamp light source simulating the AM 1.5 spectrum (Wacom,
WXS-80C-2, 100 mW cmꢀ1). The incident photon-to-current
conversion efficiency (IPCE) was measured using an Eko Seiki
SPM-005B equipped with a Xe lamp and a halogen lamp.
Synthesis
Details on the synthesis of C^N ligands and ruthenium
complexes are provided in the ESI.†
Electrochemical studies
Cyclic voltammograms and differential pulse voltammograms spectra of the obtained single isomers is due to the ring current
of the ruthenium complexes were measured in a solution con- effect of the 2,20:60,200-terpyridine ligand.17
taining 0.1 M lithium perchlorate using a three-electrode
The synthesis of complexes 24–28 of the type [Ru(tctpy)(N^O)NCS]
apparatus comprising a platinum wire counter electrode, a is shown in Scheme 2. Complexes 24–28 were prepared as
platinum working electrode, and a Ag/AgCl (saturated aqueous mixtures of stereoisomers, but it was possible to obtain the
KCl) reference electrode in contact with a NaCl salt bridge. single isomer depicted in Scheme 2 as a pure compound using
Complexes 1–23, N719, and N749 were dissolved in N,N-dimethyl- column chromatography. Complexes 29 and 30 were also
formamide at
a
concentration of
1
ꢂ
10ꢀ3 M, and synthesized using the procedure shown in Scheme 2. However,
complexes 24–30 were dissolved in methanol at a concentration they exist as stereoisomers with the relative disposition of the
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Fig. 1 Molecular structures of ruthenium complexes.
pyridyl group on the N^O ligand and the central pyridyl group vs. SCE (Fig. S1†). Therefore, the DG2 values of 1–30, N719, and
on the 2,20:60,200-terpyridine ligand differing for each isomer. N749 are also listed in Table 1. The redox potentials for
1
Each isomer was identied via H NMR analysis. For complex complexes 1–30 were strongly affected by the introduction of the
29, the most downeld signal (d 9.37 ppm) is assigned to the C^N and N^O ligands. The negative shi in the EHOMO of
proton in the 6-position of the pyridyl group, which is close to complexes 1–23 relative to that of N749 (0.66 V vs. SCE) was
the NCS group.18 On the other hand, the signal of the proton in attributed to the introduction of the C^N ligand, because
the 6-position of the pyridyl group of 30 (d 6.74 ppm) is upeld cyclometalating ligands have a strong electron-donating ability.
shied, probably due to the ring current effect of the 2,20:60,200- On the other hand, because the electron-donating ability of the
terpyridine ligand.
single 2-pyridinecarboxylate ligand is inferior to that of two
Cyclic voltammetry and differential pulse voltammetry (DPV) isothiocyanate ligands,5a the EHOMO of complexes 24–30 was
were then performed to determine the EHOMO of the ruthenium positively shied. Moreover, structural modication of the C^N
complexes. A quasi-reversible wave for the Ru3+/2+ couple was and N^O ligands had an effect on the electrochemical proper-
obtained, and the peak potentials of the differential pulse vol- ties of the resulting complexes. For example, a positive shi in
tammograms for 1–30, N719, and N749 are listed in Table 1. the EHOMO of 8 relative to that of 3 was attributed to the intro-
Typical DPV spectra of ruthenium complexes and the electrolyte duction of the electron-withdrawing substituent on the C^N
(I3ꢀ/Iꢀ) are shown in Fig. S1.† Under the experimental condi- ligand, which decreased its electron-donating ability. In
tions employed, the peak potential for DPV of I3ꢀ/Iꢀ was 0.23 V contrast, a negative shi in the EHOMO was observed when
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Scheme 1 Synthesis of 1–23: (a) 2-phenylpyridine or a derivative, Et3N, EtOH, H2O. (b) 1. NH4SCN, DMF, H2O, microwave. 2. Et3N. (c) Derivative
of 2-phenylpyrimidine, Et3N, DMF, H2O, microwave. (d) NH4SCN, DMF, H2O, microwave.
electron-donating substituents were introduced. Thus, the
EHOMO was systematically tuned by changing the substituents of conversion efficiency (h) of the cells based on complexes 1–23
the ligands. increased as the DG2 increased. These ndings suggest that the
As shown in Fig. 2, Jsc, open-circuit voltage (Voc), and overall
The photovoltaic performance of the DSSCs under standard EHOMO of ruthenium complexes strongly affects the photovol-
air mass 1.5 irradiation (100 mW cmꢀ2) conditions is shown in taic performance of DSSCs when the EHOMO is less than 0.5 V vs.
Table 1. We tried to nd conditions giving the highest short- SCE. When the DG2 is small, rapid dye regeneration is difficult
circuit current density (Jsc). Therefore, an electrolyte without because of an insufficient driving force. The Jsc, Voc, and h values
TBP was used for the cells based on complexes 24–30 because decreased as the energy difference decreased, most likely
the ELUMO of each of these complexes was relatively close to the because the ratio of charge recombination increased. The
TiO2 conduction band edge.19 The open-circuit voltage (Voc) energy matching between the HOMO and the iodine redox
values for the DSSCs based on complexes 24–30 were lower than mediator is crucial for attaining high-performance DSSCs. At
those for N719 and N749 due to a shi of the TiO2 conduction this point, it remains unclear whether other parameters, such
band edge (Fig. 2(b)). It has been reported that the addition of as the molecular structure and interactions of the dye with the
TBP or a similar compound to the electrolyte increases the Voc of electrolyte solution components, affect the efficiency of dye
DSSCs by shiing the conduction band of TiO2 to more negative regeneration. However, given the similar structures of
potentials.20 Furthermore, it should be noted that the efficiency complexes 1–23, the low Jsc, Voc, and h values for the DSSCs
of complexes 28–30 was slightly lower than that reported based on 1–12 (0.33–0.42 V vs. SCE) were mainly attributed to an
previously,19 for which the photovoltaic performance data were insufficient driving force for rapid dye regeneration.
obtained for cells using an electrolyte solution with TBP (0.2 or
Fig. 3 shows the relationship between the maximum IPCE
0.1 M). The efficiency of 11, 18, and 21 was also lower than that values (IPCEmax) of the DSSCs and the EHOMO and DG2 values of
reported previously because these data were obtained using the ruthenium complexes for estimation of the appropriate
sealed cells with an antireection lm and a black metal EHOMO for rapid dye regeneration. The IPCE spectra of the
mask.14 The efficiency of 11, 15, and 18–23 was similar to that DSSCs based on the ruthenium complexes are shown in
reported previously,21 for which the data were obtained using Fig. S3.† As depicted in Fig. S2,† molecular extinction coeffi-
cells with different TiO2 lms (thickness: 31 mm).
cients (3) of ruthenium complexes are slightly different. To
compensate for the low 3 of the ruthenium complexes, very
Scheme 2 Synthesis of 24–28: (a) derivative of 2-pyridinecarboxylic acid, LiCl, Et3N, EtOH. (b) 1. NH4SCN, DMF, H2O. 2. Et3N.
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Table 1 Electrochemical data and photovoltaic performance of DSSCs
Complexes
EHOMO/V vs. SCE
DG2/eV
IPCEmax/%
TBP/M
Jsca/mA cmꢀ2
Vocb/mV
ffc
hd/%
1
2
3
4
5
6
7
8
0.33
0.33
0.37
0.37
0.38
0.39
0.39
0.40
0.40
0.42
0.42
0.42
0.43
0.43
0.48
0.48
0.48
0.49
0.49
0.51
0.52
0.55
0.56
0.61
0.65
0.68
0.69
0.71
0.73
0.77
0.85
0.66
0.10
0.10
0.14
0.14
0.15
0.16
0.16
0.17
0.17
0.19
0.19
0.19
0.20
0.20
0.25
0.25
0.25
0.26
0.26
0.28
0.29
0.32
0.33
0.38
0.42
0.45
0.46
0.48
0.50
0.54
0.62
0.43
28
43
32
29
41
45
32
45
32
45
40
40
59
32
61
57
49
65
69
71
74
73
75
62
76
65
80
73
73
62
77
71
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0
4.4
9.1
6.1
4.6
9.1
9.6
6.3
10.8
5.6
11.0
7.7
0.51
0.57
0.53
0.52
0.58
0.56
0.54
0.57
0.53
0.56
0.54
0.55
0.62
0.55
0.61
0.63
0.59
0.62
0.65
0.64
0.69
0.68
0.70
0.53
0.53
0.55
0.55
0.52
0.55
0.54
0.73
0.71
0.69
0.70
0.69
0.71
0.70
0.70
0.73
0.70
0.72
0.69
0.72
0.70
0.68
0.72
0.68
0.69
0.70
0.68
0.66
0.65
0.68
0.68
0.67
0.71
0.68
0.66
0.67
0.66
0.69
0.71
0.74
0.69
1.5
3.6
2.2
1.7
3.7
3.8
2.5
4.3
2.1
4.3
3.0
3.2
6.9
2.7
6.4
6.8
5.2
7.7
8.6
8.9
9.9
9.4
9.3
5.9
7.3
6.9
8.0
6.8
7.1
5.3
8.9
9.6
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
N719
N749
8.3
16.0
6.7
15.4
15.8
12.4
18.3
20.2
21.5
21.3
20.3
20.0
15.7
20.3
19.0
21.5
19.9
18.8
13.8
16.6
19.6
0
0
0
0
0
0
0.5
0.5
a
Short-circuit current density. b Open-circuit voltage. c Fill factor. d Overall solar light-to-electrical conversion efficiency.
thick TiO2 lms (38 mm) were used in this study. Furthermore, IPCEmax of 71%, although the EHOMO of 20 was 0.34 and 0.15 eV
to obtain a sufficient thermodynamic driving force for electron more negative than that of N719 and N749, respectively.
injection, an electrolyte without TBP was used for the cells
At present, the reason for the relatively small IPCEmax values
based on complexes 24–30. The efficiency of dye regeneration is of complexes 24, 26, and 30 is not clear. It is possible that the
an important factor determining the electron collection effi- molecular structures and interactions of the dyes with the
ciency. Aer electron injection, a competition is established electrolyte solution affect the efficiency of dye regeneration.
between dye regeneration and charge recombination. If the Indeed, there are several sensitizers with similar EHOMO values
thermodynamic driving force for dye regeneration is insuffi- but different IPCEmax in complexes 1–23. Thus, deviation seems
cient, charge recombination dominates, resulting in a low to be similar for whole complexes. The origin of the deviation is
IPCEmax for the DSSC. The ratio of charge recombination not clear, but it could be related to the different substituent
increases when the EHOMO shis toward more negative groups on the C^N and N^O ligands. Complexes 1–23 have
potentials.
several types of electron-donating and electron-withdrawing
As can be seen in Fig. 3, the IPCEmax increased with a positive groups on the C^N ligands, and the inuence of these groups is
shi in the EHOMO for the complexes with an EHOMO in the shown in Fig. S4.† These results suggest that higher IPCEmax
potential range more negative than 0.5 V vs. SCE, but did not values were obtained for complexes with ligands bearing an
exceed 70%, suggesting that rapid dye regeneration is difficult. electron-withdrawing substituent. In addition, the tri-
The reduced IPCEmax is probably due to a decrease in the uoromethyl group seems to be a particularly effective electron-
driving force for dye regeneration. On the other hand, a high withdrawing group.
IPCEmax above 70% was oen observed for the complexes with
Although several factors may affect dye regeneration, the
an EHOMO more positive than 0.5 V vs. SCE. It can thus be energy difference between the HOMO and the iodine redox
concluded that there is a threshold for efficient electron transfer mediator is one of the most important factors. To realize high
near 0.5 V vs. SCE (DG2 z 0.3 eV) in the series of these sensi- performance solar cells, an enhanced spectral response of the
tizers. Notably, the cell based on complex 20 exhibited an sensitizer at lower energies is required while maintaining a
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Fig. 2 (a) Short-circuit current density, (b) open-circuit voltage, (c) fill factor, and (d) overall conversion efficiency of DSSCs as a function of the
HOMO and DG2 of ruthenium complexes.
E
ELUMO for efficient electron injection into the conduction band
of a TiO2 electrode. Structural modication of the ligands
greatly inuenced the EHOMO and enabled systematic tuning.
For complexes with an EHOMO in the potential range more
negative than 0.5 V vs. SCE, the IPCE increased with a positive
shi in EHOMO, but did not exceed 70%, suggesting that rapid
dye regeneration is difficult. On the other hand, high IPCEs
above 70% were oen observed for complexes with an EHOMO
more positive than 0.5 V vs. SCE. It is thus concluded that there
is a threshold for efficient electron transfer near 0.5 V vs. SCE
(DG2 z 0.3 eV) in the series of these sensitizers. The present
study provides a new criterion for sensitizer designs, because it
is important to minimize the energy difference between the
HOMO and the iodine redox mediator. We believe that these
ndings will thus enable the design of highly efficient
sensitizers.
Fig. 3 Maximum IPCE values for DSSCs as a function of the EHOMO and
DG2 of ruthenium complexes.
Acknowledgements
sufficient thermodynamic driving force for dye regeneration.
Because DPV is a general analytical method, it can be a useful
tool for the optimization of the EHOMO of newly synthesized
sensitizers under the same conditions.
This research was partly supported by the Japan Society for the
Promotion of Science (JSPS) through its “Funding Program for
World-Leading Innovative R&D on Science and Technology”
(FIRST Program). We are grateful to Prof. Dr S. Mori from
Shinshu University for valuable discussion.
Conclusion
We have synthesized thirty ruthenium complexes to elucidate
the EHOMO required for efficient dye regeneration in DSSCs. The
introduction of C^N and N^O ligands made it possible to tune
the EHOMO of the resulting complexes while maintaining an
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