Novel azobenzene nickel(II) sensitizer for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2015.12.001

A novel 1-D chain complex {[Ni(azobenzene-4,4′-dicarboxylic acid)(ethylenediamine)₂](N,N-dimethylformamide)₂}(named as Ni1) was designed, synthesized, characterized, and demonstrated as an efficient sensitizer for dye-sensitized solar cells. The structure of Ni1 was characterized by single-crystal X-ray diffraction, infrared (IR) spectra, and elemental analysis. The optical, electrochemical and photovoltaic properties of Ni1 were investigated by UV-visible spectroscopy, cyclic voltammetry, and the photocurrent-photovoltage measurement. The results show that the incident photo-to-current conversion efficiency of cells based on Ni1 was obviously improved in blue light region of solar spectra. The overall conversion efficiency of the dye-sensitized solar cells using Ni1 was increased by 62% under AM 1.5 illumination (100 mW cm^-2).

Full text of DOI:10.1016/j.jphotochem.2015.12.001

Accepted Manuscript
Title: Novel Azobenzene Nickel(II) sensitizer for
Dye-Sensitized Solar Cells
Author: Lingyun Zhang Yanan Luo Ruokun Jia Xuhui Sun
Chunguang Liu Yingjie Zhang
PII:
S1010-6030(15)00427-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2015.12.001
JPC 10076
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
27-5-2015
25-11-2015
1-12-2015
Please cite this article as: Lingyun Zhang, Yanan Luo, Ruokun Jia, Xuhui Sun,
Chunguang Liu, Yingjie Zhang, Novel Azobenzene Nickel(II) sensitizer for Dye-
Sensitized Solar Cells, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2015.12.001
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Novel Azobenzene Nickel(II) sensitizer for Dye-Sensitized
Solar Cells
Lingyun Zhanga* zhanglingyun@vip.sina.com, Yanan Luob, Ruokun Jiaa, Xuhui Suna, Chunguang
Liua, Yingjie Zhanga
aSchool of Chemical Engineering, Northeast Dianli University, Jilin 132012, P. R. China. Fax:
+86-432 64806371, Tel.: +86-432 64807273.
bCollege of Chemical and Pharmaceutical Engineering, Jilin Institute of Chemical Technology,
Jilin City, Jilin, 132022, P. R. China
*Corresponding author.
1

Graphical Abstract
2

Highlights




A novel 1-D chain complex was synthesized.
The complex show photoelectric activity in blue light region.
The complex was used as a sensitizer for dye-sensitized solar cells.
The complex improved the efficiency of cells by co-sensitization with N719.
3

Abstract
novel
A
1-D
chain
complex
{[Ni(azobenzene-4,4'-dicarboxylic
acid)( ethylenediamine)2]( N,N-dimethylformamide)2}(named as Ni1) was designed,
synthesized, characterized, and demonstrated as an efficient sensitizer for
dye-sensitized solar cells. The structure of Ni1 was characterized by single-crystal
X-ray diffraction, infrared (IR) spectra, and elemental analysis. The optical,
electrochemical and photovoltaic properties of Ni1 were investigated by UV-visible
spectroscopy, cyclic voltammetry, and the photocurrent-photovoltage measurement.
The results show that the incident photo-to-current conversion efficiency of cells
based on Ni1 was obviously improved in blue light region of solar spectra. The
overall conversion efficiency of the dye-sensitized solar cells using Ni1 was increased
by 62% under AM 1.5 illumination (100 mW cm-2).
Keywords: Dye-sensitized solar cells; Organic sensitizer; Azobenzene;
Co-sensitization; Optical properties
4

1. Introduction
In the face of the limited fossil fuels and the disastrous environmental problem,
people realize the urgent need to develop renewable energy resources for the
increasing global energy demand. Solar energy is the most hopeful of supplying
sustainable energy because it is a kind of clean and renewable energy. The
dye-sensitized solar cells (DSSCs) have attracted much attention as a promising
technology for the performance/price ratio cells [1, 2]. The dye is a key component
which decides the efficiency of DSSCs as it absorbs sunlight, excites electrons and
injects electrons into the conductor band of the semiconductor [3, 4]. In the future,
cost-effective organic dyes will play a pivotal role in the large-scale production and
application of DSSCs.
To date, the highest conversion efficiency of DSSCs has been achieved by using Ru
dyes (N719, N3) [5, 6]. However, Ru dyes have several disadvantages: (1) relatively
low extinction coefficients; (2) rare and expensive metal Ru; (3) dye aggregates on the
semiconductor; (4) only absorb visible light. Low extinction coefficients make against
the improvement of the light harvesting for DSSCs. Using rare metal Ru raises the
cost of DSSCs and hampers practical applications of DSSCs. The aggregates of Ru
dyes on the semiconductor make against the carrier transport in DSSCs and increase
-
the recombination of excited electrons and injected electrons with I3 /I- in the
electrolyte. At present, the main drawback of Ru dyes is the lack of light-absorbing in
the blue-violet and red region of solar spectrum. The facts above prompt researchers
to develop new-type dyes for the improvement in the efficiency of DSSCs. So far
5

researchers have synthetized many kinds of dyes. These dyes have high molar
extinction coefficients, multiple optical absorption bands in ultraviolet, and visible
and near infrared region in spectrum. But the conversion efficiency of these dyes is
still behind that of Ru dyes. An approach to the high power efficiency of DSSCs is
using the optimal dye that can absorb all sunlight from ultraviolet to near infrared
region. Single dye cannot satisfy the demand above. On the basis of these strategies,
the combination of mixed dyes is used to solve the previous problem. The
light-absorbing response of DSSCs is extended and the aggregates on the
semiconductor of dyes are reduced by using two dyes in DSSCs. Although some
progresses in the co-sensitization of nanocrystalline TiO2 with two dyes have been
achieved, further investigating the co-sensitization properties of DSSCs with different
dyes is still needed. Most co-sensitization systems have supplemented the visible and
near infrared spectroscopy, the absorption of blue-violet light is still limited.
In this work, we synthesize a new dye (Ni1) using low cost and toxicity metal Ni
through a simple method for DSSCs. Ni1 and N719 are used to co-sensitize TiO2 film
and they present excellent photovoltaic performances. The combination of Ni1 and
N719 has successfully extended the light-absorbing response of DSSCs to the blue
region and reduced the aggregates on the semiconductor of dyes.
2. Experimental Section
2.1 Materials and reagents
All chemicals, LiI, I2, NiCl2, TiO2, 4-tertbutylpyridine, N,N-dimethylformamide,
ethylenediamine,
cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylate]
6

ruthenium(II) bis-tetrabutylammonium (N719), acetonitrile-propylene carbonate, were
analytical grade and were used without further purification.
Azobenzene-4,4'-dicarboxylate was synthesized according to a published procedure
[7,8].
2.2 Synthesis of Ni1
An aqueous solution (16 mL) containing azobenzene-4,4'-dicarboxylate (5 mg, 0.019
mmol), NiCl2 (30 mg, 0.32 mmol), H2O (1 mL), DMF (15 mL) and ethylenediamine
(0.03 mL) was stirred at room temperature for 30 min until a clear red solution formed,
finally transferred into a 25 mL glass bottle and heated to 120 °C for 72 h under
autogenous pressure. Afterwards, it was cooled to 25 °C at a rate of 4 °C·h–1. Red
sheet crystals were obtained after being washed with DMF and dried in air, in about
38 % yield (based on Ni). Anal. calcd. for C24H38N8NiO6 (593.33): C 48.54; H 6.60;
N 18.84; found: C 48.67; H 6.62; N 18.79. IR data (KBr pellet, cm-1, Figure S1): 3313
(s), 3294 (s), 2943 (w), 1614 (s), 1593 (s), 1535 (s), 1039 (s), 1027 (s), 796 (s), 706
(m), 619 (w), 513 (m).
2.3 Fabrication of DSSCs
A colloidal solution of TiO2 was obtained by being coated on the fluorine-doped tin
o
oxide (FTO) conducting glass by screen printing and then dried for 6 min at 125 C.
o
The TiO2 electrodes were gradually heated under an air flow at 325 C for 5 min, at
375 oC for 5 min, at 450 oC for 15 min, and at 500 oC for 15 min. The area of film was
0.25 cm2. The film electrodes were soaked in the Ni1 solution (concentration: 5 × 10–4
M, solvent: absolute ethanol for 5 h and N719 solution (concentration: 5 × 10–4 M,
7

solvent mixture: acetonitrile and absolute ethanol in volume ratio of 1:1) for 20 h at
room temperature, respectively. Then, the co-sensitized films were dried in air. The
counter electrodes were thermally platinized conducting glass (5 mM H2PtCl6 in dry
isopropanol, heated at 400 °C for 10 min). The electrolyte was consisted of 0.5 M LiI,
0.05 M I2, 0.1 M 4-tertbutylpyridine in 1:1 (volume ration) acetonitrile-propylene
carbonate.
2.4 Characterization
The elemental analyses were carried out on a Perkin-Elmer 240C elemental analyzer.
The IR spectra were recorded (400-4000 cm-1 region) on a Nicolet Impact 410 FT-IR
spectrometer using KBr pellets. The single-crystal X-ray diffraction data for Ni1 was
collected from a Bruker Apex Ⅱ CCD area-detector diffractometer (Mo Kɑ, 0.71073
Å), operating at 296 ± 2 K. An empirical absorption correction was applied to the date
using the SADABS program. The structure was solved by the direct method
(SHELXS-97) and refined by full matrix least squares. All non-hydrogen atoms were
refined anisotropically. All hydrogen atoms were located geometrically by the
program OLEX 2. The final formula was derived from crystallographic data
combined with elemental and thermogravimetric analyses data. The CCDC number of
Ni1 is 881780. A summary of the crystallographic data and structural determination
for Ni1 is listed in Table S2. Selected bond lengths and bond angles of Ni1 are listed
in Table S3. Hydrogen bond distances and angles for Ni1 are listed in Table S4.
UV-visible absorption spectra of the samples were measured at room temperature on
U-3010 spectrophotometer (Hitachi, Japan) with an integrating sphere (model
8

130-0632).
2.5 Photoelectrochemical measurements
The photocurrent-photovoltage (J-V) curves of the sealed cells were measured under
AM 1.5 illumination (100 mWcm-2) by using a solar simulator. Electrochemical
impedance measurements were carried out by electrochemical work-station (chi660d,
Chenhua, China) with three-electrode system in the dark. Frequency range was
0.05-105 Hz, and the applied potential was generally between 0～0.700 V. The IPCE
spectra were measured with a Model SR830 DSP Lock-In Amplifier and a Model
SR540 Optical Chopper (Stanford Research Corporation, USA), a 7IL/PX150 renon
lamp as light source, and a 7ISW301 Spectrometer. Cyclic voltammetry experiment
was performed with a three-electrode system, a platinum working electrode, a
platinum counter electrode, and an Ag/AgCl reference electrode. The potentials were
referenced to the ferrocene/ferrocenium (Fc+/Fc) couple.
3. Results and discussion
The synthetic route of Ni1 is depicted in Scheme 1. In Ni1, L2– ligand adopts
μ2-bidentate mode with the carboxyl group displaying monodentate fashion, as shown
in scheme 1. Single-crystal X-ray diffraction analysis reveals that Ni1 crystallizes in
the triclinic system, space group P-1, which exhibits an infinite 1D chain structure. As
shown in Figure 1, the asymmetrical unit of compound 1 contains one Ni(II) atom,
half a L2− ligand, one en molecule and one crystallization DMF molecule. Ni1 atom is
six coordinated by two oxygen atoms from two ligands and four nitrogen atoms from
two en molecules, forming a slightly distorted octahedral geometry. The distances of
9

Ni-O/N [2.076(2) Å-2.132(2) Å] are comparable with those found in other related
Ni(II) complexes.
L2– ligands link Ni(II) cations into an infinite 1D chain structure with Ni···Ni distance
of 17.76 Å, as shown in Figure 2(a). However, it's worth mentioning that the ligands
didn't generate any torsion by measuring the dihedral angle between the two benzene
rings planes. In the crystal building, each chain interacts with adjacent chains by
hydrogen bonds leading to two-dimensional (2D) supramolecular motif. The
coordinated en molecules play an important role in the formation of hydrogen bonds.
There exist NH···O [N3···O2iv = 3.094(4) Å, N3H3B···O2iv = 114°; N4···O2v =
3.230(4) Å, N4H4B···O2v = 134°] hydrogen bonds between adjacent chains, linking
the 1D chains into a 2D layer structure, as shown in Figure 2(b). In addition, N3
acting as a hydrogen bond donor bonds to O3vi [N3···O3vi = 3.163(6) Å,
N3−H3A···O3vi = 149°) to connect the DMF molecules with 2D layer structure, as
shown in Figure 2(c). The hydrogen bond interactions possibly play important roles
for constructing and stabilizing the 2D supramolecular structure of Ni1.
Figure 3 shows normalized absorption spectra of Ni1, mixed dyes of Ni1 and N719
(1:1 molar ratio) in absolute ethanol solution (10-5 mol/L) and mixed dyes of Ni1 and
N719 absorbed on TiO2 thin film. Ni1 shows absorbing response in the blue light
region (300nm-450nm) with maximum absorption peak at 400nm. The absorption
band of Ni1 is attributed to the π-π* transitions [9,10]. It is well known that absorption
band of N719 is in 450 nm-650nm. Therefore, Ni1 can effectively broaden the
absorption response of N719 by the combination of N719 and Ni1. The absorption
10

spectrum of mixed dyes of Ni1 and N719 in solution exhibits two light absorption
band in 300nm-450nm and 450nm-650nm, respectively. As shown in Figure 3, light
absorption response is obviously broadened by the combination of Ni1 and N719. The
absorption spectrum of Ni1 and N719 absorbed on the TiO2 film exhibits there light
absorption bands in 350nm-400nm, 400nm-500nm, and 500nm-650nm, respectively.
The absorption bands of Ni1 (400nm-500nm) has narrowed relative to that in absolute
ethanol solution. It is known that the dyes have the strong attractive forces between
the molecules at the solid-liquid interface. The attractive forces can lead the shift of
absorption peak. The attractive forces have two forms: red-shift J-aggregation and
blue-shift H- aggregation. The maximum absorption peak of Ni1 attached to TiO2 film
has obviously red-shifted by 33nm from 400nm to 433nm compared to that in
absolute ethanol solution, owing to dye anchoring and J - aggregation on TiO2
semiconductor surface. The absorption bands of N719 (450nm-650nm) has broadened
relative to that in absolute ethanol solution. The maximum absorption peak of N719
has not obviously shift relative to that in ethanol solution. As a whole, the
combination of Ni1 and N719 broadens and intensifies the light absorption of DSSCs,
which should be profitable for light harvesting and photocurrent generation in DSSCs.
Figure 4 shows the incident photo-to-current conversion efficiency (IPCE) data of
DSSCs based on N719/TiO2 and Ni1/N719/TiO2 photoelectrodes. The IPCE of
DSSCs based on N719/TiO2 and Ni1/N719/TiO2 photoelectrodes match their
absorption spectra, indicating that Ni1 and N719 anchor on TiO2 semiconductor
surface and possess the ability for photo-induced injection and separation. For
11

N719/TiO2 DSSCs, the maximum IPCE value is 59% at about 540nm, which stays in
the range of 420nm-670nm. As shown in Figure 4, the IPCE of DSSCs based on
Ni1/N719/TiO2 photoelectrode exhibits two peaks, 350nm-450nm with top value of
70% at 400nm and 450nm-700nm with top value of 62% at 520nm, respectively.
Compared to that of N719/TiO2 DSSCs, the combination of Ni1 and N719 extends the
IPCE response to 350nm in the UV light region. In addition, IPCE value in
350nm-530nm increases by the combination of Ni1 and N719. The co-sensitization of
Ni1 and N719 on TiO2 film effectively increases the light harvesting of DSSCs, which
should improve the conversion efficiency of DSSCs. Moreover, the both ends of IPCE
peak for DSSCs based on N719 are low, which is different with the Nazeeruddin’s
report [11]. Many factors influence the shape of IPCE spectrum, such as the kind of
the photoanode, the thickness of the photoanode, the morphology of the photoanode,
and composition of the electrolyte [12,13]. The TiO2 photoanode and composition of
the electrolyte in this paper are not the same as that in Nazeeruddin’s report. Therefore,
the shape of IPCE spectrum shows the difference with that in Nazeeruddin’s report.
The insufficient driving force for injection of excited N719 molecule and regeneration
of the oxidized N719 molecule in our system led to a low electron injection efficiency
and light-harvesting efficiency in the UV and red part of the solar spectrum [13].
Cyclic voltammetry is used to study the electrochemical properties of Ni1, the
mechanism of electron injection from the Ni1 to conductor band of TiO2, the
oxidation and reduction regeneration of Ni1 in the electrolyte. Figure 5 shows the
cyclic voltammetry of Ni1 in CH2Cl2 solution. Ni1 exhibits a reversible oxidation-
12

reduction behavior. The ground-state oxidation potential (Eox) corresponding to the
high occupied molecular orbital (HOMO) level is determined by the peak potential of
CV [14]. Eox of Ni1 is 1.71V (vs. NHE), which is more positive than the redox
-
potential of I3 /I- couple (0.4V vs NHE) [15]. It suggests that the oxidized Ni1 formed
after electron injection into the conductor band of TiO2 should be thermodynamically
-
reduced by I3 /I- in the electrolyte. The absorption threshold of Ni1 is 450nm, which
corresponds to the band gap energy of 2.76 eV according to Eq. (1) [16].
Eg(eV)=1240/λ(nm)
(1)
The excited-state redox potential (Ered) corresponding to the low unoccupied
molecular orbital (LUMO) level is calculated to be -1.05V according to Eq. (2) [17].
Ered = Eox + Eg
(2)
Ered of Ni1 is more negative than the bottom of conductor band of TiO2 (-0.5V vs
NHE), which confirms that the electron injection from excited state Ni1 is
thermodynamically favorable. Figure 6 shows the schematic energy levels of Ni1
based on the absorption and the electrochemical data. The exciting, electron injecting
and redox regenerating of Ni1 are exhibited in Figure 6, indicating Ni1 can efficiently
sensitize TiO2 photoanode of DSSCs.
Electrochemical impedance spectroscopy (EIS) is used to correlate device structure
with a suitable model for the study of kinetics of electron transfer and
photoelectrochemical processes which occurred at the interface of DSSCs. Figure 7
shows electrochemical impedance spectroscopy of Ni1/N719/TiO2 and
N719/TiO2-based DSSCs in the dark. In Figure 7a, experimental data are represented
13

by symbols while the solid lines correspond to the fit obtained by Zsimpwin software
using the equivalent circuit Rs(C1(R1O1))(R2Q2) [18,19]. The spectra show one large
semicircle at medium frequency region and a small semicircle at high frequency
region [20,21]. In equivalent circuit, the symbols R and C describe resistance and
capacitance, respectively；O, which depends on the parameters Yo,1 and B, accounts
for a finite-length Warburg diffusion, and Q is the symbol for the constant phase
element, CPE (its parameters are Yo,2 and n)[20]. Yo,1 and Yo,2 are Warburg
coefficients at the photoanode/dye/electrolyte interface and the Pt/electrolyte interface,
respectively, corresponding to the module 1 and 2 in the equivalent circuit. Parameters
obtained by fitting the impedance spectra of composite solar cells using the equivalent
circuit are listed in table 1. Rs is the series resistance of DSSCs, accounting for the
resistance of polymer electrolyte, the resistance within the photoelectrode film and the
FTO electrode, as well as contacts. No obvious difference for the series resistance (Rs)
is observed between DSSCs based on Ni1/N719/TiO2 (4.09 Ω cm2) and DSSCs based
on N719/TiO2 (3.43Ω cm2). R2 deceases from 16.22 to 8.66 Ω.cm2 by Ni1 and N719
co-sensitization. R2 at high frequency is associated to the resistance at Pt/electrolyte
-
interface. R2 is the diffusion impedance of I3 at the counter electrode. The decrease of
-
R2 is attributed to the increased activation between I3 in the electrolyte and Pt counter
-
electrode. The decrement of diffusion impedance favors the reduction of I3 at the
-
counter electrode. When C1 increase and R2 decrease, much more I3 is reduced at Pt
electrode surface [19]. This is propitious to the increase of FF of the DSSCs. C1 and
R1 at the medium frequency are attributed to the capacitance and charge transfer
14

resistance of electron transfer process at the TiO2/dye/electrolyte interface. C1
increases from 11.96×10-4 to 20.31×10-4F.cm-2. The increasing film capacitance means
more electrons at the interface of TiO2/dye/electrolyte as the light absorption is
enhanced because of Ni1 and N719 co-sensitization. R1 slightly deceases from 14.37
to 11.86 Ω.cm-2 by Ni1 and N719 co-sensitization. The decreasing value of R1 means
that the recombination resistance is low at the interface of TiO2/Ni1/N719/electrolyte.
The low recombination resistance indicates that the recombination of electrons at the
interface of TiO2/Ni1/N719/electrolyte slightly increased. This slight increasing of
electron recombination may be caused by the increasing of the injected and excited
electrons at the interface of TiO2/Ni1/N719/electrolyte. Meanwhile, the recombination
characteristic peak of DSSCs based on Ni1/N719/TiO2 photoanode shifts to a lower
frequency (5.52 Hz) compared with that of N719/TiO2 photoanode (8.16Hz) in Figure
7b. This indicates a longer electron lifetime for DSSCs based on Ni1/N719/TiO2 [22].
The high film capacitance and low recombination characteristic frequency makes the
highest η possible [23].
Figure 8 shows the current density versus voltage curves of DSSCs based on the films
of Ni1/N719/TiO2 and N719/TiO2 under 100 mW/cm2 illumination. The photovoltaic
performances of N719/TiO2-based DSSCs, e.g. the short photocurrent density (Jsc),
the open circuit voltage (Voc), the fill factor (FF), and overall energy conversion
efficiency (η) are 8.78mA/cm2, 0.76V, 0.62, 4.17%, respectively. Compared with that
of N719/TiO2-based DSSCs, the photovoltaic performances of Ni1/N719/TiO2-based
DSSCs have obviously improved as shown in Figure 8. Jsc of Ni1/N719/TiO2-based
15

DSSCs (13.46 mA/cm2) has a remarkably increase relative to that of N719/TiO2-based
DSSCs (8.78mA/cm2), meaning much more injection electrons in CB of TiO2. This
increase of Jsc indicates that Ni1 and N719 effectively co-sensitized TiO2 photoanode
and resulted in a higher light harvesting. The Voc increases from 0.76 to 0.79 V by Ni1
and N719 co-sensitizing. Voc is the difference potential between the electrochemical
potential of the redox couple in the electrolyte and the conduction band edge of TiO2
photoanode [24]. The increase of Voc indicates that co-sensitization of Ni1 and N719
leads to an upward shift of the conduction band edge of TiO2 [25]. As shown in table 1,
co-sensitization of Ni1 and N719 decreases the resistance at Pt/electrolyte interface of
DSSCs, resulting in the activation or better contact between electrolyte and Pt catalyst.
Therefore, FF increases from 0.62 to 0.64. The overall solar conversion efficiency of
DSSCs based on Ni1/N719/TiO2 tremendously increases by 62% from 4.17% to
6.77% compared with DSSCs based on N719/TiO2.
4. Conclusions
In this paper, Ni1 was designed and synthesized as the dye to sensitize TiO2
photoanode with N719 for DSSCs application. The combination of Ni1 and N719 not
only extends the photoresponse of DSSCs to the UV region of the solar spectrum but
also intensify the optical spectrum absorption. The photovoltaic performances of
Ni1/N719/TiO2-based DSSCs obviously improved compared with to DSSCs based on
individual dye (N719).
16

Acknowledgments
This work was supported by the national science foundation of china (grant No.
21373043), Jilin Provincial Science & Technology Department (No.20130305017GX,
20140101090JC, 20130413046GH), scientific research fund of Jilin provincial
education department (No.2014105), Jilin city science and technology bureau
(201464036), and the Doctor Science Foundation of Northeast Dianli University (No.
BSJXM-201321).
17

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21

Figure Captions
Scheme 1. Synthetic route to Ni1
22

Fig. 1 The coordination environment of Ni1 (symmetry codes: i, 2-x, -y, -z; ii, 2-x, 1-y,
1-z; iii, x, 1+y, 1+z. All hydrogen atoms are omitted for clarity).
23

Fig.2 (a) 1D layer constructed from Ni(II) cations and L2- ligands; (b) 1D chain
structures are connected into a 2D layer structure; (c) DMF molecules are connected
with 2D layer structures via hydrogen bond interactions in Ni1 (symmetry codes: iv,
1-x, -y, -z; v, 1+x, y, z; vi, x, –1+y, z. All hydrogen atoms are omitted for clarity).
24

Fig. 3 Normalized absorption spectra of Ni1, mixed dyes of Ni1 and N719 in absolute
ethanol solution and mixed dyes of Ni1 and N719 absorbed on TiO2 thin film
25

Fig. 4 IPCE spectra of DSSCs based on N719/TiO2 and Ni1/N719/TiO2
photoelectrodes
26

Fig. 5 Cyclic voltammogram of Ni1 in CH2Cl2 solution at scan rate of 100 mV S-1
27

Fig. 6 LUMO and HOMO energy levels of Ni1 with respect to the conductor band
-
(CB) edge and the valence band (VB) edge energies of TiO2 and the I3 /I- redox
potential.
28

Fig. 7 (a) EIS spectra of DSSCs based on Ni1/N719/TiO2 and N719/TiO2 in the dark.
(b) Bode phase plots of DSSCs
29