High efficiency co-sensitized solar cell based on luminescent lanthanide complexes with pyridine-2,6-dicarboxylic acid ligands

Article abstract of DOI:10.1039/c2dt30430a

Two lanthanide complexes, Ln(HPDA)₃·4EtOH (Ln = Tb, Dy) (H₂PDA = pyridine-2,6-dicarboxylic acid, EtOH = ethanol), have been successfully synthesized using hydrothermal or solvothermal methods, and their crystal structures were analyz

Full text of DOI:10.1039/c2dt30430a

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PAPER
High efficiency co-sensitized solar cell based on luminescent lanthanide
complexes with pyridine-2,6-dicarboxylic acid ligands†
Xin Wang,a,b Yu-Lin Yang,*a Ping Wang,a Liang Li,a Rui-Qing Fan,*a Wen-Wu Cao,c Bin Yang,c Hui Wangd
and Jing-Yao Liud
Received 22nd February 2012, Accepted 17th June 2012
DOI: 10.1039/c2dt30430a
Two lanthanide complexes, Ln(HPDA)3·4EtOH (Ln = Tb, Dy) (H2PDA = pyridine-2,6-dicarboxylic acid,
EtOH = ethanol), have been successfully synthesized using hydrothermal or solvothermal methods, and
their crystal structures were analyzed by single crystal XRD. Both crystals have orthorhombic symmetry
with space group Pbcn, exhibiting three-dimensional (3D) supramolecular architecture through hydrogen
bonding interactions. The metal center was coordinated to nine atoms by three pyridine-2,6-dicarboxylic
acid ligands. The nine-coordinated lanthanide metal complexes were assembled onto a nanocrystalline
TiO2 film to form co-sensitized photoelectrodes with N719 for dye-sensitized solar cells, and their
photoelectrochemical performance was studied. In the tandem structure of composite electrodes, the
energy levels of lanthanide metal complexes are reorganized in their single-crystal form, as verified by
ab initio calculations. The co-sensitized systems are far superior for electron-injection and hole-recovery
compared with single N719-sensitized systems. Luminescence properties were measured and
electrochemical analysis was also performed on these complexes.
different parts of the solar spectrum, controllable molecular
energy levels, charge generation and separation, and molecule-
Introduction
Since O’Regan and Grätzel pioneered the field of dye-sensitized
solar cells (DSSCs) in 1991,1 scientists have shown great enthu-
siasm in developing new sensitizers that can lead to higher inci-
dent photon-to-electron conversion efficiency (IPCE) over a
substantially wider band of wavelengths by molecular engineer-
ing. However, for most new sensitizers, optimization to give a
high IPCE value and a broad absorption band involves some
degree of trade-off. In order to improve the absorption ability of
a solar cell, co-sensitization by means of multiple sensitizers
would appear to be more achievable.2,3 However, unfavorable
interactions between different types of dye molecules often
decreases photovoltaic performance. Therefore, in recent years,
the arrangement of micro- and nanostructured building blocks
has gained popularity.4 Inorganic dyes offer a lot of possibilities
for improving a wide range of properties, such as molecular
structure and function, efficient light-harvesting ability in
to-molecule interactions. They can easily be designed to absorb
light more effectively, so that thinner films can be used to gener-
ate an optimal photovoltaic effect. Although advances have been
made with metal-coordination dyes as sensitizers in DSSCs,
there is still a need to optimize their chemical and physical prop-
erties to further improve the performance of solar cells. The co-
sensitization with multiple dyes is very appealing for obtaining
panchromatic absorption, but perfect tuning is quite difficult to
achieve. Ehret et al. studied the nanocrystalline TiO2 solar cell
sensitized by various dicarboxylated cyanines and found that the
use of mixed cyanine dyes could improve photoelectric conver-
sion efficiency.5 Zhang and coworkers examined the mechanism
of the co-sensitization with squarylium and N3 dyes by time-
resolved spectroscopy.6 Grätzel and coworkers combined
bisthiophene dye (JK2) with squarylium cyanine dye (SQ1) as
co-sensitizers and achieved an overall conversion efficiency as
high as 7.43%.7 This value is so far the highest value reported
for co-sensitized DSSCs with metal-coordination organic dyes.
Recently, Wang et al. reported that the enhanced performance
of chlorophyll-sensitized solar cells have been obtained by co-
sensitization of a-type Chl with b-, or c-type Chl.8
aDepartment of Applied Chemistry, Harbin Institute of Technology,
Harbin 150001, P.R. China. E-mail: fanruiqing@hit.edu.cn,
ylyang@hit.edu.cn; Fax: +86 451 86418270; Tel: +86 451 86413710
bDepartment of Food and Environmental Engineering, Heilongjiang
East University, Harbin 150086, P.R. China
On the other side, trivalent rare earth doped semiconductors
are technologically important materials in optoelectric devices
and have received a lot of attention,9 in which an efficient
energy transfer from semiconductor host to RE ions is essential
for most potential applications. Unfortunately, the doping of RE
ions into the crystal structure of semiconductors remains unsuc-
cessful.10 The difficulty is due to the inappropriate energy level
cCondensed Matter Science and Technology Institute, Harbin Institute of
Technology, Harbin 150001, P.R. China
dInstitute of Theoretical Chemistry, State Key Laboratory of Theoretical
and Computational Chemistry, Jilin University, Changchun 130023,
P.R. China
†Electronic supplementary information (ESI) available. CCDC 740890
and 818568. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt30430a
This journal is © The Royal Society of Chemistry 2012
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position of rare earth ions relative to the valence band and con-
duction band of the semiconductor host. To optimize the match-
ing of energy band structure with TiO2 in solar cells, we
designed a series of lanthanide complex sensitizers with 4f struc-
ture. In the research of coordination complexes constructed by
carboxylate ligands, the focus has been on lanthanide and tran-
sition metal coordination complexes constructed by multi-
carboxylate ligands, such as benzene-1,2,4,5-tetracarboxylate,
benzene-1,3,5-tricarboxylate, pyridine-2,3(5)-dicarboxylate and
pyridine-2,6-dicarboxylate.11–16 These multi-carboxylate ligands
possess interesting features that can facilitate the formation of
versatile coordination structures. Owing to steric hindrance, the
multi-carboxylate groups on the molecules may be completely or
partially deprotonated and the carboxylate groups may not lie in
the phenyl and pyridine ring plane upon coordination to metal
ions. As a result, the molecules may connect metal ions in differ-
ent directions, generating multidimensional architectures.17
In this work, we selected two lanthanide complexes, whose
crystal structures have not been reported, as co-sensitizers for the
study of photoelectrochemical properties in dye-sensitized solar
cells. The relationships among molecule structures, photolumi-
nescent and electrochemical properties have also been investi-
gated. This is the first time that lanthanide complexes based on
pyridine-2,6-dicarboxylic acid ligands have co-sensitized photo-
anodes of TiO2 to form composite photoelectrodes with N719.
According to the ab initio calculations, the complexes play an
important role in the optimization matching with dyes in the
compounds for increasing electrical conversion efficiency.
Scheme 1 Synthesis of lanthanide complexes.
30 min, and then sealed in a 15 mL Teflon-lined reactor, heated
at 160 °C for 5 days, and then slowly cooled to room tempera-
ture. Well-shaped, light, yellow single crystals of complex Tb1
suitable for X-ray four-circle diffraction analysis were obtained.
Yield: 45.23% (based on Tb). Elemental analysis for
C29H36N3O16Tb (Mr: 841.53): Calcd: C, 41.39; N, 4.99; H,
4.31%. Found: C, 41.64; N, 4.74; H, 4.46%. IR (KBr, cm−1):
3545(s), 2990(s), 2776(s), 2420(m), 1623(vs), 1463(m), 1429
(vs), 1388(vs), 1278(m), 1191(m), 1077(m), 1020(s), 917(m),
777(m), 734(s), 700(m), 665(m).
Synthesis of Dy(HPDA)3·4EtOH (2). The procedure was the
same as that for Tb1 except that Tb(NO3)3·5H2O was replaced
by Dy(NO3)3·5H2O (43.8 mg, 0.1 mmol). Yield: 68% (based on
Dy). Elemental analysis for Dy1: C29H36N3O16Dy (Mr: 845.11):
Calcd: C, 34.03; N, 5.67; H, 1.56%. Found: C, 34.08; N, 5.71;
H, 1.59%. IR (KBr, cm−1): 3566(s), 2988(s), 2776(s), 2420(m),
1635(vs), 1462(m), 1426(vs), 1387(vs), 1278(m), 1191(m),
1020(s), 917(m), 777(s), 735(s), 679(m), 666(m).
Experimental section
Characterization
Synthesis of lanthanide complexes
Infrared spectra were obtained from KBr pellets on a Nicolet
Avatar-360 Infrared spectrometer in the 4000–400 cm−1 region.
Elemental analyses were performed on a Perkin-Elmer 240c
element analyzer. The single-crystal X-ray diffraction data for
complexes Tb1 and Dy1 were collected on a Rigaku R-AXIS
RAPID IP or a Siemens SMART 1000 CCD diffractometer
equipped with graphite-monochromated Mo Kα radiation (λ =
0.71073 Å), operating at 293 2 K. The structures were solved
by direct methods and refined by full-matrix least-squares based
on F2 using the SHELXTL 5.1 software package. All non-hydro-
gen atoms were refined anisotropically. Hydrogen atoms were
fixed at calculated positions and refined by using a riding mode.
Crystal data and details of the data collection and the structure
refinement are given in Table 1. Selected bond lengths and
angles of complexes Tb1 and Dy1 are listed in Table S1 and S2,
respectively.†
All reagents were commercially available and used without
further purification. The pyridine-2,6-dicarboxylic acid was pre-
pared by oxidation reaction from 2,6-dimethylpyridine. 2,6-
Dimethylpyridine (20 mL, 0.17 mol) was added into 500 mL
water and potassium permanganate (54.4 g, 0.34 mol) was
slowly added to the solution Then the solution was heated to
25 °C and maintained for 2 h until the disappearance of the
purple color. Further potassium permanganate (54.4 g, 0.34 mol)
and water (300 mL) was slowly added and the mixture was again
heated and maintained at 25 °C. After 2–3 h, the purple color
disappeared again. Afterwards, the reaction solution was cooled
to room temperature. Repeat filtration and removal of solvent
was carried out until the residual volume was reduced to
∼200 mL. Then sulfuric acid (70%, 35 mL) was added slowly
and the precipitate was filtered to yield pyridine-2,6-dicarboxylic
acid as
a white solid (20.62 g, yield: 71.8%). Mp:
220.1–221.3 °C. IR (KBr, cm−1): 3456(m), 3070(m), 1704(vs),
1575(m), 1460(m), 1416(m), 1331(m), 1300(m), 1267(s), 1163(w),
1082(w), 997(w), 927(w), 753(w), 702(m).
Photoelectrochemical measurements. The sample was sand-
wiched between two FTO glass electrodes. Optically transparent
electrodes were made from an F-doped SnO2-coated glass plate
(FTO, 90% transmittance in the visible, 15 Ω−1 per square) pur-
chased from Geao Equipment Company, Wuhan, China. The
sensitizations were chosen N719 (cis-bis(isothiocyanato)bis(2,2-
bipyridyl-4,4-dicarboxylato)-ruthenium(II) bis-tetrabutylammo-
nium, or RuL2(NCS)2:2TBA (L = 2,2-bipyridyl-4,4-dicarboxylic
acid TBA = tetrabutylammonium, Solaronix Company, Switzer-
land) at a concentration of 3 × 10−4 M in pure ethanol solution,
The syntheses of complexes Tb1 and Dy1 are summarized in
Scheme 1.
Synthesis of Tb(HPDA)3·4EtOH(1). A mixture of Tb-
(NO3)3·5H2O (43.5 mg, 0.1 mmol), pyridine-2,6-dicarboxylic
acid (34.0 mg, 0.2 mmol), distilled water (2 mL), N,N-dimethyl-
formamide (3 mL) and ethanol (1 mL) (pH 2–3) was stirred for
10620 | Dalton Trans., 2012, 41, 10619–10625
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Table 1 Crystal data and structure refinement for complexes Tb1 and
Dy1
Method of calculation
In this work, the three parameter hybrid method proposed by
Becke19 was used with the gradient corrected functional of Lee
et al. The central metal atoms were treated and the SDD basis set
was used to determine the geometry. The non-metallic atoms,
oxygen, carbon and hydrogen, were treated by the 3-21G**
basis set of double zeta quality plus p, polarization function, in
hydrogen atoms and d functions in oxygen and carbon atoms.
The calculation method used as well as the basis sets are
included in the GAUSSIAN 03 package. The orbital are labeled
by the principal and the orbital quantum numbers of the atom. A
valence basis set consists of all valence atomic orbitals, occupied
or unoccupied, up to the shell of the principal quantum number
of highest occupied valence electrons.
Data
Dy1
Tb1
Formula
Fw
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C29H36N3O16Dy
845.11
Orthorhombic
Pbcn
16.947(3)
10.654(2)
18.659(4)
90.00
90.00
90.00
3368.9(12)
4
1.666
C29H36N3O16Tb
841.53
Orthorhombic
Pbcn
16.967 (1)
10.672 (1)
18.648(1)
90
90
90
3376.4(4)
4
1.655
β (°)
γ (°)
Volume (Å3)
Z
Dcalcd, mg m−3
μ, mm−1
2.296
2.172
F(000)
θ range for data collection
Limiting indices
1700
1696
3.14° to 27.46°
−21 ≤ h ≤ 21
−13 ≤ k ≤ 13
−23 ≤ l ≤ 24
2.18° to 25.72°
−20 ≤ h ≤ 17
−13 ≤ k ≤ 12
−16 ≤ l ≤ 22
Results and discussion
Crystal structures
Absorption correction
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
The single-crystal XRD analyses revealed that compounds Tb1
and Dy1 have the same structure. The local coordination
environment around Ln3+ in compound Tb1 is depicted in
Fig. 1. Both coordination complexes Tb1 and Dy1 crystallized
into orthorhombic symmetry with space group Pbcn. The asym-
metric molecular unit of complex Tb1 contains one distinct Tb3+
ion, three HPDA ligands, and four EtOH, as shown in Fig. 1.
Tb3+ ion is nine-coordinated by six carboxylate oxygen atoms
(OCOO−) and three pyridine ring N atoms from three neighboring
HPDA ligands. The HPDA ligand adopted the coordination type
as shown in Fig. 1, in which the metal ion is a Tb3+ cation.
Moreover, the adjacent units are connected through hydrogen
bonding and π–π stacking interaction to form a 3D supramole-
cule, which makes the framework of complex Tb1 very stable.
3839/1/227
0.962
3208/0/223
0.967
a
R1
0.0351
0.0887
0.0482
0.1211
b
wR2
R indices (all data)
a
R1
wR2
0.0475
0.0962
0.0538
0.1248
b
a R1 = ∑kFo| − |Fck/∑|Fo|. b wR2 = [∑[w(Fo2 − Fc ) ]/∑[w(Fo ) ]]1/2
.
2 2
2 2
and prepared a co-sensitization dye of three lanthanide com-
plexes with the form of single crystals in the same way. The elec-
trolyte used in this work was 0.5 M LiI + 0.05 M I2 + 0.1 M
tert-butyl pyridine in 1 : 1 (volume ratio) acetonitrile–propylene
carbonate. The films were immersed in 3 × 10−4 M pure ethanol
Photoelectrochemical analysis
solutions of Tb1 or Dy1 for 2 h, then immersed in 3 × 10−4
M
The lanthanide complexes were assembled onto a nanocrystalline
TiO2 film to prepare a complex/N719 co-sensitized photoelec-
trode for dye-sensitized solar cell application. Fig. 2 is the
surface photovoltage spectrum (SPS) of the photoanodes with a
single sensitizer and co-sensitizer. There are two mechanisms
emerging in the enhancement of the solar cells. The first process
involves the energy levels of N719 and Ln1, in which photo-
induced electrons of excited complexes inject into the conductive
band of TiO2 easily. The second one is the down-conversation
process, in which complexes produce emission under
pure ethanol solutions of N719 for 12 h. Photovoltaic perform-
ance was measured by using a mask with an aperture area of
0.2 cm2 and the irradiance of sunlight was 100 mW cm−2
.
Based on the 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 and Jsc and Voc are the short-circuit photocurrent density
and open-circuit photovoltage, respectively. η = (FF × Jsc × Voc)/
Pin for the overall energy conversion efficiency (Pin is the power
of incident light).
The electrochemical impedance spectroscopy (EIS) of cell
measurements were set at a working electrode potential of
550 mV, over a frequency range of 0.1–100 000 Hz. Potential
values are referenced with respect to the saturated calomel refer-
ence electrode (SCE). Glassy carbon and platinum electrodes
were used as the working electrodes and a platinum wire was
employed as the counter electrode. A solution of 0.1 M tetra-
butylammonium hexafluorophosphate (TBAPF6, Fluka, electro-
chemical grade) in CH2Cl2 (Aldrich, anhydrous, 99.8%) was
used as the supporting electrolyte. A mixed solvent system was
employed to prevent adsorption to the electrode. Analyte con-
centrations of typically 1 mM were used.
Fig. 1 (a) Molecular structural unit of Tb1 and (b) perspective view of
the packing seen along [010] of Tb1.
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Fig. 2 The relational SPS of composite photoanodes.
Fig. 3 J–V (current density–voltage) curves for the sunlight-illumi-
nated dye-sensitized solar cells with different lanthanide ligand
complexes.
illumination, so that the lower energy emission light would be
absorbed by N719 to extend the absorption region of the photo-
anode. With the SPS method, both processes coexist and
cooperate. The curves in Fig. 2 indicated that co-sensitizing
could enhance the photoelectric signal between 400 and 600 nm.
The photovoltage signal increased with Dy1, Tb1, TiO2, which
indicated that the separate efficiency of electrons and holes corre-
late strongly with Dy1, Tb1, TiO2. On the other hand, visible
light response range broadened from 400 to 600 nm with Dy1,
Tb1, TiO2. The results show that the composite TiO2 is ben-
eficial for extending the absorption region after co-sensitization
in DSSC photoelectrodes. The improvement of composite photo-
anode co-sensitized by Dy1 is more evident than Tb1.
Table 2 Comparison of J–V performance between TiO2/Ln1/N719
and raw TiO2 photoelectrodes
Photoelectrode
Voc/mV
Jsc/mA cm−2
FF
η/%
Raw TiO2
Tb1/TiO2
Dy1/TiO2
0.698
0.707
0.756
5.27
7.75
8.75
0.74
0.75
0.78
2.86
4.13
4.91
Fig. 3 presents the performance of the DSSCs in terms of Jsc
and Voc of co-sensitized solar cells. The result indicates that the
photochemical performance of co-sensitized photoelectrodes has
been enhanced. Our earlier work indicated that d10 metal coordi-
nation complexes could affect the photochemical performance of
DSSCs due to proton number.18 In the present work, the co-sen-
sitized photoelectrode has more protons in the central metal of
the complex, so that the performance of DSSCs improved. Dy
possesses a higher proton number than Tb, and the results
coincide with the conclusion reported before18 (Table 2). The
photochemical performance results indicated that the co-sensi-
tized solar cells based on TiO2/Ln1/N719 electrode yielded a
remarkably high photocurrent density (Jsc), open circuit voltage
(Voc) and energy conversion efficiency under standard global
AM1.5 solar irradiation conditions. The conversion of the TiO2/
Tb1/N719 DSSC system increases by 44% compared with the
ones using single organic N719 sensitizers. The TiO2/Dy1/N719
system even shows a greater efficiency, which is 72% higher
than that of single-DSSC.
Fig. 4 Nyquist plots of the lanthanide complex/N719 co-sensitized
DSSC in standard global AM1.5 solar irradiation.
Electrochemical impedance spectroscopy (EIS)
As shown in Table 2, the performances of TiO2/Ln1/N719
composite photoelectrodes were relatively better than that for
DSSCs with single organic sensitizers. Table 2 indicated that the
cell performance of DSSCs co-sensitized by a mixture of lantha-
nide complex and N719 were improved. And the co-sensitized
DSSCs with Dy1 showed a better performance than that with
Tb1. As the proton number of the central metal atom in the
lanthanide complexes increased, the efficiency of the DSSCs co-
sensitized by Ln1/N719 increased.
Fig. 4 presented the Nyquist plots of the lanthanide complex/
N719 co-sensitized DSSC under standard global AM1.5 solar
irradiation. The results revealed charge carrier dynamics in the
interior and interfacial regions of solid–liquid layers, and the
effects of mixed sensitizers in composite photoelectrodes. Each
curve contains three arcs that are representative for the resistance
states of different interfaces. The first arc in the high frequency
range (>1000 Hz) of the Nyquist plot was assigned to the
10622 | Dalton Trans., 2012, 41, 10619–10625
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electron transfer process at the electrolytes/counter electrode
(CE) interface. This feature arose from an increase in the electron
transport resistance of the TiO2 film due to the increase of elec-
tron concentration. The electron transport process and charge
recombination dominated the impedance spectra in the inter-
mediate frequency range (from 100 to 1000 Hz). The increase of
the semicircle radius in the Nyquist plot indicates a reduction in
the interfacial charge recombination rate due to the decrease in
the conduction band electron concentration. The Warburg diffu-
sion impedance for holes transport in electrolytes could be
observed in the low frequency range (less than 1 Hz). In accord-
ance with this observation, the impedance of composite photo-
electrode decreased with the increase of protons in atoms, which
is beneficial for the photochemical performance of DSSC.
A physical model has been proposed to understand the
complex charge-transfer processes in DSSCs.20,21 The equivalent
circuit used to model this system was Rs(Q1R1)(Q2(R2Zw)), repre-
senting interfaces in composite solar cells. As shown in Fig. 5,
Rs represents series resistance consisted of Ohmic components.
The R1 and R2 were resistance components forming a parallel
circuit with constant elements Q1 and Q2.
The complex impedance plot (Nyquist plot) and fitting data
using the equivalent circuit for DSSCs are listed in Table 3. As
shown in the data, R2 estimated from semicircle 2 decreased
remarkably with the introduction of Dy1 or Tb1. The series
resistance R2 represents electron transport process and charge
recombination impedance in the interfacial regions of TiO2/Ln1
composite photoelectrode layer. With increasing protons in
lanthanide atoms, the energy of relevant frontier molecular
orbital increases, which is beneficial for photoelectron transfer
between HOMO and LUMO. Therefore, the photochemical per-
formance of co-sensitization of composite TiO2 photoelectrodes
increased compared with single phase TiO2. With the increase of
the semicircle radius in the Nyquist plot, a reduction in the inter-
facial charge recombination rate emerged due to the decrease of
electron concentration in the conduction band. Hence, we con-
cluded that the intermediate frequency semicircle 2 originated
from TiO2/Ln1 composite photoelectrodes, which behaved like
an n-type inorganic semiconductor. The results also
demonstrated that co-sensitization in TiO2 could utilize more
UV-visible light than DSSCs containing individual dyes
(Table 2).
Relationship between structures and electrochemical properties
The geometric and electronic structures of the sensitizer are the
key factors for solar cells, which determine related electrochemi-
cal properties. Crystal structural analyses revealed that com-
pounds Tb1 and Dy1 are isostructural (see ESI†). Their unit cell
dimensions, volumes, related bond distances and bond angles
differ only slightly. In the coordination polyhedron around Ln,
the N1, N2 and N3 atoms came from three phenyl rings. This is
one of the common coordination patterns for complexes with a
coordination number of nine. As shown in Fig. 1, there are three
independent planes of phenyl rings, which orient approximately
orthogonally to the plane of the three coordinated nitrogen
atoms. The dihedral angles between the two phenyl rings
increase with the proton number, and the distance from Ln atom
to the coordination plane also increases. This induced more elec-
trons transferring across the same plane due to the enlarged
space.
The bond distances can be divided into two different types
presented in the coordination sphere of the RE metal. The Ln–O
bond distances are 2.425(5) Å for Tb1 and 2.343(7) Å for Dy1,
and the Ln–N bond distances are 2.515(5) Å (Tb1) and 2.495(5)
Å (Dy1), respectively. The O–Ln–O bond angles range from
127.7° (Tb1) to 128.2° (Dy1), and the O–Ln–N bond angles
range from 120.2° (Tb1) to 120.7° (Dy1). The data indicated
that the Ln–N and Ln–O bonds in Dy1 are more stable compared
to Tb1. These bond distances and bond angels are comparable to
the values in other lanthanide complexes. Interestingly, the
N–Ln–N bond angles are nearly 120°, and the Ln atoms are located
in the center of the N3 equilateral triangles. Adjacent HPDA
groups in three neighboring layers of coordination complexes are
parallel and the distance between two neighboring parallel layers
is around 3.257 Å, which indicates π–π stacking interactions.
Fig. 6 shows the packing diagram of Ln1 along the c-axis.
Hydrogen bonds are indicated by dashed lines. Some observed
hydrogen bond interactions in coordination complexes are given
in Table 4. The data showed that the hydrogen bond lengths are
in the following order: DyO⋯H > TbO⋯H. Therefore, a 3D supra-
molecular structure is formed through π–π stacking interaction
and hydrogen bonding interactions between the carboxyl groups.
There is a large void between the layers in the complex.
From their electrochemical properties (data in Table 2) it can
be concluded that the electron-donating capability is better in a
stable single molecule structure. Generally speaking, the
decrease of the bond length and the deviation distance are
Fig. 5 Equivalent circuit in composite solar cells composed of FTO|
TiO2/Ln1-dye|I3−/I−||Pt|FTO. (Component explanation: Rs
= series
resistance, R1, R2 = the charge-transfer resistance, Q1, Q2 = the symbol
for the constant phase element, and Zw = Warburg impedance).
Table 3 Parameters obtained by fitting the impedance spectra of composite solar cells using the equivalent circuit
Sample
Rs/Ω
CPE1(YO2/Fsn−1
)
n1
R1/Ω
Zw(YO1/S)
R2/Ω
CPE2(YO2/Fsn−1
)
n2
TiO2
Tb1
Dy1
19.45
17.46
18.36
1.532 × 10–3
9.154 × 10–4
6.739 × 10–4
0.542
0.914
0.934
6.756
19.39
18.14
0.2057
0.2346
0.201
17.94
4.251
3.143
6.77 × 10–4
5.509 × 10–4
1.663 × 10–3
0.943
0.800
0.810
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advantageous for electrochemical reactions, while the enlarge-
ment of the space between the layers leads to superior cell
performance.
Titania to visible light through electron transfer from lanthanide
complex to anatase nanocrystals. Specifically, the EHL values of
lanthanide complex were very close to that of bulk TiO2.
Additional photoluminescent and electrochemical properties
could prove the same results as Tb(III) < Dy(III). The calculated
energy levels for complex Tb(III) and Dy(III) agreed well to the
experimental results. The EHL of complex Dy(III) was closer to
TiO2 than that of complex Tb(III). And the gap between ELUMO
of complex Dy(III) and ECB of TiO2 was smaller, which could
mean that it is easier to inject electrons into the conduction band
of TiO2 during the transfer process. Furthermore, the gap
between electrolyte and the EHOMO of Dy(III) is smaller than that
of Tb(III), leading to easier transfer from electrolyte to Dy(III).
Molecular orbitals and band structure calculations of lanthanide
complex
In order to further understand the energy band structure in
lanthanide complex, we performed ab initio calculations for the
molecular orbitals based on the time-dependent density func-
tional theory (TD–DFT) with 3-21G** basis set. The geometric
parameters obtained from X-ray diffraction analyses were fully
optimized in conjunction with the solid model. The MO diagram
for Ln(HPDA)3·4EtOH), (Ln = Tb, Dy), is shown in Fig. 7,
which presented the typical pattern of frontier molecular orbital
in lanthanide complexes for f orbitals. The three ligand extremes
differ in the number of electrons: two adopted the neutral form,
while another was described as anionic according to the electron
count based on X-ray diffraction analyses on the single crystal
structure. These results included the highest occupied (HOMO)
and the lowest unoccupied (LUMO) molecular orbitals for each
cluster. The molecular skeletons were shown as ball-stick models
(see ESI†).
For each cluster, the occupied FMO levels were found to be
very close to each other within 0.13 eV, and the same for the
unoccupied FMO levels. The occupied and unoccupied FMOs
related to the valence and conduction bands in the case of bulk
complexes.
In Table 4, some useful electronic properties are listed for the
stable clusters, such as the HOMO energy level (EH) and LUMO
energy level (EL). The EHL value of bulk TiO2 is around 3.05 eV
according to the literature.22 As shown in Fig. 7, sensitization of
Conclusions
In conclusion, two lanthanide complexes with pyridine-2,6-
dicarboxylic acid ligands have been synthesized under hydro-
thermal or solvothermal conditions. Complexes have mono-
nuclear structures and self-assemble to form
a
3D
supramolecular structure through hydrogen bond and aromatic
π–π interactions between adjacent metal–organic complex
coordination chains. Both nine-coordinate lanthanide metal com-
plexes were attached to a nanocrystalline TiO2 film to assemble
co-sensitized photoelectrodes with N719 for dye-sensitized solar
cells, which yielded a remarkably high optoelectronic efficiency
of 4.91%. Moreover, the Voc and Isc increased after introducing
the cosensitizers. It is worth mentioning that this is the first time
that 2,6-bis(imino)pyridyl types of lanthanide inorganic coordi-
nate complexes co-sensitized with N719 in TiO2 have been used
Fig. 7 Schematic energy diagrams showing proposed electron and
energy transfer mechanisms in composite photoelectrode of titania-
based lanthanide complexes (VB = valence band; CB = conduction
band).
Fig. 6 The unit of complex Tb1 extends into a 3D supramolecular by
O–H⋯O hydrogen bonds along the [001] direction.
Table 4 Comparison of the bond lengths and angles in the molecular structure of the lanthanide complexes
Bond
lengths of lengths of bond
Ln–O/Å Ln–N/Å angles/°
Bond
O–Ln–O
N–Ln–N Distance of Ln deviated Dihedral angles
Hydrogen Electron
between pyridine bond structure
lengths/Å of Ln
bond
away from the
coordination plane/Å
angles/°
rings plane/°
EHOMO/eV ELUMO/eV
Tb1 2.425(5) 2.515(5) 127.7(2)° 120.2(2)° 0.0105
Dy1 2.343(7) 2.495(5) 128.2(14)° 120.7(3)° 0.0665
89.06
89.13
3.080
3.439
[Xe]4f86s2 2.4768
[Xe]4f96s2 2.5394
5.909
5.894
10624 | Dalton Trans., 2012, 41, 10619–10625
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to form composite photoelectrodes. On the other hand, it has
been shown that the optoelectronic efficiency can be strongly
increased through postsynthesis functionalization of titania using
visible light sensitizing species, for example, grafting of dye
molecules or depositing narrow band gap semiconductor nano-
crystals onto premade titania matrixes. Undoubtedly, they play
an important role in the optimization matching with dyes for
increasing electricity conversion efficiencies in compounds.
More importantly, it demonstrates a way to design more sophisti-
cated and appropriate dye structures to satisfy the needs of
DSSC technology.
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This work was supported by the National Science Foundation of
China (Grant No. 20971031, 21071035 and 21171044), the
China Postdoctoral Science Foundation Funded Project (No.
65204), and the key Natural Science Foundation of the Heilong-
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