Co-sensitized dye-sensitized solar cells based on d10 coordinate complexes towards their optoelectronic properties

Article abstract of DOI:10.1039/c0nj00361a

Three five-coordinate transition metal complexes [2,6-(PhNCMe) ₂C₅H₃NMCl₂·CH₃CN] (M = Zn, Cd, Hg) (named M1) have been assembled onto a nanocrystalline TiO ₂ film to prepare transition meta

Full text of DOI:10.1039/c0nj00361a

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www.rsc.org/njc | New Journal of Chemistry
Co-sensitized dye-sensitized solar cells based on d10 coordinate complexes
towards their optoelectronic propertiesw
Xin Wang, Yulin Yang,* Ruiqing Fan* and Zhaohua Jiang
Received (in Gainesville, FL, USA) 18th April 2010, Accepted 11th June 2010
DOI: 10.1039/c0nj00361a
Three five-coordinate transition metal complexes [2,6-(PhNQCMe)2C5H3NMCl2ÁCH3CN]
(M = Zn, Cd, Hg) (named M1) have been assembled onto a nanocrystalline TiO2 film to prepare
transition metal complex/N719 co-sensitized photoelectrodes for dye-sensitized solar cell
applications. The metal center is chelated in a tridentate manner by the ligand and further
coordinated by two chlorine atoms, resulting in distorted trigonal bipyramidal geometry. In the
tandem structure of the TiO2/M1/dye electrode, the M1 undergoes a re-organization of energy
levels due to its single-crystal structure, which is advantageous to electron injection and hole
recovery. The co-sensitized structure is proved to have a superior ability, when compared to a
single N719 dye’s influence on TiO2. Therefore, co-sensitized solar cells based on TiO2/M1/N719
electrodes yield a remarkably high photocurrent density (Jsc), open circuit voltage (Voc) and
energy conversion efficiency under standard global AM1.5 solar irradiation conditions, which are
relatively higher than those for DSSCs using single organic sensitizers.
sensitizer for DSSCs should have an inverted arrangement of
Introduction
the donor and acceptor moieties in the dye molecule for
Conversion of solar energy into electrical energy using photo-
efficient hole injection into the photocathode. A way to
voltaic cells (PVC) is arguably one of the most successful and
improve efficiency is by the co-adsorption of dyes with additives
environmentally friendly technologies for energy generation.1,2
and by structural modifications with bulky substituents that
can prevent p–p stacking or dye aggregation.11–13 The
In the original dye-sensitized solar cell (DSSC) developed in
1991 by O’Regan and Gratzel,3 TiO nanoparticle films were
sensitized with N3 (Ruthenium organic dye) of high cost.4
¨
2
co-sensitization approach is also very appealing for using
multiple dyes to obtain panchromatic absorption, but the
Recently, efforts for increasing solar cell efficiency while
fine-tuning seems to be quite difficult. Ehret et al. studied a
maintaining a low production cost have been the primary
nanocrystalline TiO2 solar cell sensitized by various
objective of solar energy conversion, such as the use of
dicarboxylated cyanines and found that the use of mixed
combining QD sensitization, polymers and other cheap
cyanine dyes could improve photoelectric conversion
efficiency.14 Zhang and co-workers examined the mechanism
metal–organic complexes as sensitizers in TiO2 nanoparticle
solar cells.5–7 Metal coordination organic dyes have been used
of co-sensitization with squarylium and N3 dyes by time-
resolved spectroscopy.15 Guo et al. studied the co-sensitization
and applied in DSSCs to assess the current status in research
and future developments in a broad and rapidly growing area
of cyanine dyes. A combination of the two dyes in a 1 : 3 ratio
of technology due to their low cost of production.8–10 Organic
was found to cover the entire visible spectrum, thereby
dyes offer infinite possibilities for improving a wide range of
generating a conversion yield of 3.4%, which is higher than
that of the TiO2 electrode sensitized with a single dye.16
properties, such as molecular structure and function, efficient
light-harvesting ability in different parts of the solar spectrum,
A combination of three dyes, such as 4-dimethylaminophenyl-
control over the molecular energy levels, charge generation
cyanoacrylic acid, which absorb in the yellow (380 nm), red
and separation, and molecule-to-molecule interactions. The
(535 nm) and blue (642 nm) regions, respectively, were
major advantage of organic materials for solar cell applications is
employed as co-sensitizers in liquid DSSCs. Recently, Gratzel
¨
that they can function both as a hole transporter as well as a
sensitizer for such nanocrystalline solar cells. They are easy to
design with a very high light absorbing capacity. meaning that
thinner films can be used to generate optimal photovoltaic
performance. Although remarkable advances have been made
with metal coordination organic dyes as sensitizers in DSSCs,
there is still a need to optimize their chemical and physical
properties to further improve solar cells. An appropriate
and co-workers combined bisthiophene dye JK2 with
squarylium cyanine dye SQ1 as co-sensitizers and obtained
an overall conversion efficiency of 7.43%.17 This value is so far
the highest value reported for co-sensitized DSSCs with metal
coordination organic dyes. Herein, the research selects three
[bis(iminoalkyl)pyridine] ligand complexes we have previously
reported as co-sensitized photoelectrodes for a study of their
photoelectrochemical properties in DSSCs. It also discusses
the relationships between molecule structure and electrochemistry
properties. Worth mentioning here is that this research represents
the first example where 2,6-bis(imino)pyridyl-type inorganic
coordinate complexes are co-sensitized with N719 in TiO2,
forming composite photoelectrodes. Undoubtedly, they play
Department of Applied Chemistry, Harbin Institute of Technology,
Harbin, 150001, P. R. China. E-mail: ylyang@hit.edu.cn,
fanruiqing@hit.edu.cn
w Electronic supplementary information (ESI) available: Further
experimental data. See DOI: 10.1039/c0nj00361a
c
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an important role in optimization matching with dyes of
electricity conversion efficiencies in compounds.
Hg1 (C33H43N3HgCl2).
A
mixture of L2 (113 mg,
0.36 mmol) and HgCl2 (98 mg, 0.36 mmol) in CH3CN (20 mL)
was stirred under nitrogen at room temperature for 12 h.
Evaporation of the solvent gave the crude product as a
yellowish powder. Pure product Hg1 was obtained in 78%
yield (179 mg) by recrystallization from CH3CN–CH2Cl2
(2 : 1).
Experimental section
Synthesis of transition metal complexes
The pyridine-2,6-dicarboxylic acid was prepared by oxidation
reaction from 2,6-dimethylpyridine. 2,6-Dimethylpyridine
(20 mL, 0.17 mol) was added to 500 mL water, potassium
permanganate (54.4 g, 0.34 mol) slowly added and the mixture
then heating until the disappearance of the purple colour.
Another portion of potassium permanganate (54.4 g, 0.34 mol)
and water (300 mL) was then slowly added and the mixture
heated. After 2–3 h, the purple color disappeared again and
the reaction was cooled to room temperature. Filtration and
removal of the solvent was undertaken until the residual
volume was down to 200 mL. Then, sulfuric acid (70%, 35 mL)
was added slowly and the precipitate filtered to afford
pyridine-2,6-dicarboxylic acid as a white solid (20.62 g, yield:
71.8%).
Photoelectrochemical measurements
The sample was sandwiched between two FTO glass electro-
des. Optically transparent electrodes were made from an
F-doped SnO2-coated glass plate (FTO, 90% transmittance
in the visible, 15 OÀ1 per square) purchased from Geao
Equipment Company, Wu Han. The sensitizations were chosen:
N719 (cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylato)-
ruthenium(II) bis-tetrabutylammonium or RuL2(NCS)2 : 2
TBA (L = 2,2-bipyridyl-4,4-dicarboxylic acid, TBA = tetra-
butylammonium, Solaronix Company, Switzerland) at a
concentration of 3 Â 10À4 M in absolute ethanol solution,
and the co-sensitization dye of the three transition
metal complexes in the form of single crystals were prepared
in the same way. The electrolyte used in this work was
0.5 M LiI + 0.05 M I2 + 0.1 M tert-butyl pyridine in a 1 : 1
(volume ratio) of acetonitrile–propylene carbonate. The films
were stained by immersing them in 3 Â 10À4 M absolute
ethanol solution of M1 and N719 for 2 and 12 h, respectively.
Photovoltaic performance was measured by using a mask with
an aperture area of 0.2 cm2; the irradiance of sunlight was set
2,6-Bis[1-(2,6-dimethylphenylimino)ethyl]pyridine (L2) was
synthesized according to modified published procedures in
good yield by condensation of 2,6-diacetylpyridine with the
corresponding aniline in refluxing absolute methanol in the
presence of a catalytic amount of formic acid (Scheme 1).
Zn1 (C33H43N3ZnCl2). Zinc complexes were synthesized by
reactions of the corresponding bis(imino)pyridyl ligands with
ZnCl2 in CH2Cl2 at room temperature (Zn1). A mixture of
ZnCl2 (53 mg, 0.39 mmol), L2 (187 mg, 0.39 mmol), CH2Cl2
(35 mL), pyridine-2,6-dicarboxylic acid (42.3 mg, 0.1 mmol)
and distilled water (2 mL) was prepared. After the mixture had
been stirred for 12 h, it was slowly cooled to room temperature
and recrystallized twice with VCH CN:CH Cl = 2 : 1. Well-
at 100 mW cmÀ2
.
Based on a J–V curve, the fill factor (FF) is defined as:
FF = (Jmax  Vmax)/(Jsc  Voc), where Jmax and Vmax are
the photocurrent density and photovoltage for maximum
power output (Jsc and Voc are the short-circuit photocurrent
density and open-circuit photovoltage, respectively), Z =
(FF  Jsc  Voc)/Pin the overall energy conversion efficiency
(Z is defined as where Pin is the power of the incident light).
The EIS of cell measurements were taken at a working
electrode potential of 550 mV over a frequency range of
0.1–100 000 Hz. Potential values are referred to with respect
to a saturated calomel reference electrode (SCE). Glassy
carbon and platinum electrodes were used as the working
3
2
2
shaped, light-yellow single crystals of complex Zn1 (183 mg)
suitable for X-ray four-circle diffraction analysis were obtained
(yield ca. 76%, based on Zn).
Cd1 (C33H43N3CdCl2). CdCl2Á6H2O (90 mg, 0.39 mmol)
and L2 ligand (124 mg, 0.39 mmol) were stirred for 12 h in a
reaction with a stoichiometric amount of the corresponding
bis(imino)pyridyl ligands in 40 mL dichloromethane solution
at room temperature. Cd1 was obtained as yellowish crystals
in good yield (yield ca. 76%, 177 mg).
electrodes and
a platinum wire was employed as the
counterelectrode. A solution of 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6, Fluka, electrochemical grade)
in CH2Cl2 (Aldrich, anhydrous, 99.8%) were used as the
supporting electrolyte. A mixed solvent system was employed
to prevent adsorption on the electrode. Analyte concentrations
of typically 1 mM were used.
Results and discussion
Photoelectrochemical measurements
Fig. 1 presents the performance of the DSSCs in terms of Jsc
and Voc in the transition metal complexes. The transition
metal complex was assembled onto a nanocrystalline TiO2
film to prepare a complex/N719 co-sensitized photoelectrode
for DSSC applications. The results in Table 1 indicate that the
Scheme 1 Synthesis process of transition metal complexes.
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Cyclic voltammetric measurements
In order to explore the electrochemical behaviors of the title
compounds, we carried out cyclic voltammetric measurements.
Fig. 2 displays the cyclic voltammogram for 1.0 mM M1 in
CH2Cl2 containing 0.1 M TBAPF6 solutions with scan rate of
100 mV sÀ1 at room temperature.
Fig. 2 shows that all the isopolyanions underwent a one-step
two-electron transition metal reduction process (eqn (1))
according to the formula E1/2 = |Epa + Epc|/2, E1/2 = 1.52
(2.303 RT/nF) (mV)18 [n denotes the electrons that
transferred].
C33H43N3M(II)Cl2 + 2eÀ " C33H43N3M(0)Cl2
(1)
Compound 1 presented a similar electrochemical behavior to
that of complexes of 2 and 3 in aqueous solution, implying
that the polymeric structures of the compounds are nearly
identical. However, the redox peak potential separations
further increase and the peak positions are shifted to the
positive potential direction with decreasing protons, implying
that the reduction of isopolyanions is accompanied by
protonation to maintain charge neutrality. Table 2 lists the
peak potentials of cyclic voltammograms for the transition
metal complexes.
Fig.
1
J–V (current density–voltage) curves for the sunlight-
illuminated DSSCs with different organometallic ligand complexes.
cell performance in co-sensitizer DSSCs with a mixture of
transition metal complex and N719 are improved with fewer
protons in elements. As shown in Table 1, the TiO2/Zn1/N719
composite photoelectrode is selected as an example. The
photochemical performance results indicate that the co-sensitized
solar cell based on a TiO2/Zn1/N719 electrode yielded a
remarkably high photocurrent density (Jsc) of 5.47 mA cmÀ2
under standard global AM 1.5 solar irradiation conditions.
The cell showed an open circuit voltage (Voc) of 576 mV and
an energy conversion efficiency of 4.55%, which were
relatively higher than those for DSSCs using single organic
sensitizers. In this way, the cell efficiency was improved by
more than 2%.
Electrochemical impedance spectroscopy (EIS)
EIS is a powerful method of characterizing many of the
electrical properties of materials and their interfaces with
electronically conducting electrodes. It may be used to
investigate the charge carrier dynamics in the bulk or inter-
facial regions of any kind of solid or liquid material, including
ionic, semiconducting, mixed electronic–ionic conductors and
even insulators (dielectrics).19,20
Table 1 indicates that the photochemical performance of the
co-sensitized photoelectrode has been increased with fewer
protons in the transition metal complexes. The results also
suggest that co-sensitized TiO2 can be constructed with a
higher UV-visible light activity over DSSCs containing an
individual dye (see Table 1 for single dyes). This result suggests
that the arrangement of the dyes on the TiO2 film by, for
example, aggregation and ordering of the chromophores,
clearly influences the performance of the solar cell. The use
of dyes with efficient light-harvesting properties at low energies
and negligible intermolecular interactions is important for the
development of efficient multidye solar cells. With increasing
protons in atoms, the photoelectrons are extinguished, which
is of no benefit for the movement of photoelectrons in TiO2;
therefore, the photochemical performance of the composite
photoelectrode decreases. In a word, the photochemical
performance of the composite co-sensitized TiO2 photo-
electrodes was increased compared to single raw TiO2.
The first arc in the high frequency range of the Nyquist plot
is assigned to the electron transfer process at the electrolyte/
counterelectrode (CE) interface. This feature arises from an
increase in the electron transport resistance of the mesoscopic
TiO2 film due to a decrease of the electron concentration.
The electron transport process and charge recombination
dominate the impedance spectra in the middle frequency range
(from 100 to 1000 Hz). The increase of the radius of the
semicircle in the Nyquist plot indicates a reduction in the
Table 1 Contrast of J–V performance between TiO2/M1/N719 and
raw TiO2 photoelectrodes
Photoelectrode
Voc/mV
Jsc/mA cmÀ2
FF
Z (%)
Raw TiO2
Zn1/TiO2
Cd1/TiO2
Hg1/TiO2
544
576
567
531
2.66
5.47
3.69
3.26
0.76
0.72
0.75
0.72
2.21
4.55
3.14
2.51
Fig. 2 Cyclic voltammograms of 1.0 mM M1 and a blank sample in
CH2Cl2 containing 0.1 M TBAPF6 solutions with scan rate of 100 mV sÀ1
.
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Table 2 Peak potentials of cyclic voltammograms for Zn1, Cd1 and
Hg1
Complex
Epc/V
Epa/V
DEp/V
Zn1
Cd1
Hg1
À1.41
À1.09
À0.75
À0.85
À0.70
À0.50
0.56
0.39
0.25
interfacial charge recombination rate due to a decrease in the
conduction band electron concentration. The Warburg diffusion
impedance for hole transport in electrolytes can be observed at
low frequencies (less than 1 Hz). In accordance with Fig. 3, the
impedance of the composite photoelectrode is increased with
increasing protons in atoms, which is of no benefit for the
photochemical performance of DSSC.
A physical model has been proposed to understand the
complex charge-transfer processes that take place in
DSSCs.21,22 As shown in Fig. 4, Rs represents series resistance
consisting of Ohmic components. R1 and R2 are resistance
components forming a parallel circuit with constant phase
elements (CPE). Furthermore, C is roughly equal to the
differential electric capacitances because there is almost no
depression of the semicircles. Under light irradiation conditions,
R1 estimated from semicircle 2 was remarkably larger with
decreasing protons. Hence, we conclude that lower frequency
semicircle 2 originated from TiO2/M1 composite photo-
electrodes as an n-type inorganic semiconductor.
Fig.
3
Nyquist plots of the transition metal complex/N719
co-sensitized DSSC in standard global AM 1.5 solar irradiation.
Fig. 4 Equivalent circuit used to model this system, representing inter-
À
faces in composite solar cells composed of FTO|TiO2/M1-dye|I3
IÀ||Pt|FTO (component explanation: Rs = series resistance, R1, R2
/
=
charge-transfer resistance, C = double layer capacitance, CPE = symbol
for the constant phase element and Zw = Warburg impedance).
Surface photovoltage spectrum analysis
role. When all these factors are taken into account, it should
be possible to design more sophisticated and appropriate dye
structures that satisfy the needs of DSSC technology.
The surface photovoltage spectrum (SPS) method is
commonly applied to TiO2 for DSSCs, which is a well-
established contactless technique for surface state distribution.23,24
As shown in Fig. 5, the SPS analysis is described for
samples with d10 transition metal co-sensitization of composite
TiO2 photoelectrodes. It shows one typical characteristic
response band close to 350 nm that originates from the
band–band electron transition of TiO2. However, for composite
TiO2 photoelectrodes, a new SPS signal from 500 to 600 nm is
observed, assigned to the band–band electron transition of the
d10 metal co-function with TiO2 in the composite photo-
electrode. The results also show that the composite TiO2 is
beneficial for visible light response range broadening from
400 to 500 nm and continuous after co-sensitization in DSSC
photoelectrodes.
The relationship between crystal structures and electrochemical
properties
From the synthetic chemists’ point of view, the geometric and
electronic structure of the sensitizer also play an important
Fig. 5 SPS of transition metal complex co-sensitized TiO2 and raw
TiO2.
Table 3 Parameters obtained by fitting the impedance spectra of composite solar cells using the equivalent circuit in Fig. 4
DSSC samples
Rs/O
C/F
R1/O
Zw (YO1/S)
R2/O
CPE (YO2/FsnÀ1
)
n
Raw TiO2
Zn1
Cd1
53.51
38.011
39.59
41.4
2.51E-5
54.97
46.28
51.16
78.61
2.119E-8
3.915E-2
1.462E-1
5.035E-2
25.85
25.62
27.36
35.21
1.854E-5
2.062E-4
5.121E-5
1.47E-4
0.803
0.664
0.704
0.554
6.339E-6
1.533E-4
8.861E-5
Hg1
c
2602 New J. Chem., 2010, 34, 2599–2604 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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Single-crystal structural analyses revealed that compounds
1, 2 and 3 are isostructural25,26 (see the ESIw). The complex
possesses a structure with approximate Cs symmetry about a
plane containing the cadmium atom, two chlorine atoms and
the pyridine nitrogen atom. For both molecules, the central
cadmium atom is coordinated to five atoms, and the geometry
about the cadmium/zinc/mercury atoms is distorted trigonal-
ꢀ
bipyramidal, crystallizing in the polar space group P1; the unit
cell dimensions, volumes, related bond distances and angles
are only slightly changed.
As shown in Fig. 6, the equatorial plane is defined by the
N(2)(pyridine), Cl(1) and Cl(2) atoms, with the N(1) and
N(3)(imino) atoms in the axial positions. The molecular
structure of the complex is shown below, and selected bond
lengths and angles are presented in Tables S2–S4.w There are
two independent planes of phenyl rings, oriented approximately
orthogonally to the plane of the three coordinated nitrogen
atoms. The dihedral angles between the two phenyl rings
decrease with the proton numbers; Zn1 is chosen for explanation.
The angles between two molecules in the equatorial plane
range around 701, and the axial Zn–N(imino) bonds subtend
an angle of 1391, which decreases from Zn to Hg.
Fig. 7 Packing diagram of the complex Zn1 along the c axis.
Hydrogen bonds are indicated by dashed lines.
In the coordination polyhedron around Zn, the N1, N2 and
N3 atoms form two phenyl rings. This is one of the common
coordination geometries for complexes with a coordination
number of five. The crystal lattice distances are divided into
two different types presented in the coordination sphere of the
transition metal (see Table 4). The data show bond lengths
Hg–N 4 Cd–N 4 Zn–N, Cd–Cl 4 Hg–Cl 4 Zn–Cl,
indicating that the Zn–N and Zn–Cl bonds are more stable
than the others. The Zn atom deviates by 0.56 A away from
the coordination plane in a single molecule, while the mean
deviation of the Zn atom from the equatorial plane in a single
molecule is only 0.013 A; they all increase with proton
Fig.
8
The 3-D structure of complex Zn1 with p–p stacking
interaction.
numbers. Fig. 7 shows the packing diagram of Zn1 along
the c axis. Hydrogen bonds are indicated by dashed lines.
From the data shown in Table 4, the hydrogen bond lengths
decreased with increasing proton number. Corresponding to
their electrochemical properties, it can be concluded that the
electron-donating ability is in favor of a stable single molecule
structure with copious space between molecules. Therefore, it
is common that the decrease of bond lengths and deviation
distances are advantageous to electrochemical properties,
while the enlargement of dihedral angles leads to superior cell
performance according to Fig. 8.
Conclusions
In this paper, we have presented three d10 metal coordination
complexes with pyridine-type ligands through systematic basic
research that represent mosaic pieces for future research and
Fig. 6 The asymmetric unit of Zn1.
Table 4 The contrast in bond lengths and angles of the molecular structures of the transition metal complexes
Bond
lengths of lengths of bond
Bond
Hydrogen Distance of M deviated
Deviation distance of M Dihedral an- Dihedral angles between phenyl
gles of rings and the coordination
N–M–N (1) plane (1)
away from the coordination from the equatorial
M–N/A
Zn1 2.313
M–Cl/A
lengths/A plane/A
plane/A
2.251
2.411
2.364
2.887
2.744
2.746
0.560
0.652
0.740
0.013
0.027
0.032
72.88(8)
68.29(17)
65.4(2)
139.88(8)
131.99(16)
126.6(2)
Cd1 2.480
Hg1 2.513
c
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development. DSSC technology undoubtedly has an exciting
future and there remain vast opportunities to ameliorate
performance. Therefore, the development of a conceptually
new design for constructing metal coordination dyes is an
important and urgent challenge. We also believe that the
versatility of the synthetic design concept will continue to
inspire research on this topic.
8 T. Funaki, M. Yanagida, N. Onozawa-Komatsuzaki, K. Kasuga,
Y. Kawanishi, M. Kurashige, K. Sayama and H. Sugihara,
Inorg. Chem. Commun., 2009, 12, 842.
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10 W. Lee, S. H. Kang, S. K. Min, Y. E. Sung and S. H. Han,
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11 W. Y. Wong, J. Organomet. Chem., 2009, 694, 2644.
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Acknowledgements
15 Y. S. Chen, Z. H. Zeng, C. Li, W. Wang, X. S. Wang and
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16 M. Guo, P. Diao, Y. J. Ren, F. S. Meng, H. Tian and S. M. Cai,
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This work was supported by the National Science Foundation
of China (Grant no. 20971031, 20671025 and 20771030) and
the China Postdoctoral Science Foundation funded project
(no. 65204).
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