Oxidovanadium(IV/V) complexes as new redox mediators in dye-sensitized solar cells: A combined experimental and theoretical study

Article abstract of DOI:10.1021/acs.inorgchem.5b00159

Corrosiveness is one of the main drawbacks of using the iodide/triiodide redox couple in dye-sensitized solar cells (DSSCs). Alternative redox couples including transition metal complexes have been investigated where surprisingly high efficiencies for the

Full text of DOI:10.1021/acs.inorgchem.5b00159

Article
pubs.acs.org/IC
Oxidovanadium(IV/V) Complexes as New Redox Mediators in Dye-
Sensitized Solar Cells: A Combined Experimental and Theoretical
Study
Andigoni Apostolopoulou,†,∥ Manolis Vlasiou,‡ Petros A. Tziouris,§ Constantinos Tsiafoulis,#
Athanassios C. Tsipis,*,§ Dieter Rehder,*,⊥ Themistoklis A. Kabanos,*,§ Anastasios D. Keramidas,*,‡
and Elias Stathatos*,†
†Nanotechnology and Advanced Materials Laboratory, Electrical Engineering Department, Technological-Educational Institute of
Western Greece, GR-26334 Patras, Greece
‡Department of Chemistry, University of Cyprus, Nicosia 1678, Cyprus
#
§Section of Inorganic and Analytical Chemistry, Department of Chemistry and NMR Center, University of Ioannina, Ioannina
45110, Greece
⊥Department of Chemistry, University of Hamburg, D-20146 Hamburg, Germany
∥Department of Physics, University of Patras, GR-26500 Patras, Greece
S
* Supporting Information
ABSTRACT: Corrosiveness is one of the main drawbacks of using the
iodide/triiodide redox couple in dye-sensitized solar cells (DSSCs).
Alternative redox couples including transition metal complexes have been
investigated where surprisingly high efficiencies for the conversion of solar to
electrical energy have been achieved. In this paper, we examined the
development of a DSSC using an electrolyte based on square pyramidal
oxidovanadium(IV/V) complexes. The oxidovanadium(IV) complex
(Ph4P)2[VIVO(hybeb)] was combined with its oxidized analogue (Ph4P)-
[VVO(hybeb)] {where hybeb4− is the tetradentate diamidodiphenolate ligand
[1-(2-hydroxybenzamido)-2-(2-pyridinecarboxamido)benzenato}and applied
as a redox couple in the electrolyte of DSSCs. The complexes exhibit large
electron exchange and transfer rates, which are evident from electron
paramagnetic resonance spectroscopy and electrochemistry, rendering the oxidovanadium(IV/V) compounds suitable for redox
mediators in DSSCs. The very large self-exchange rate constant offered an insight into the mechanism of the exchange reaction
most likely mediated through an outer-sphere exchange mechanism. The [VIVO(hybeb)]2−/[VVO(hybeb)]− redox potential and
the energy of highest occupied molecular orbital (HOMO) of the sensitizing dye N719 and the HOMO of [VIVO(hybeb)]2−
were calculated by means of density functional theory electronic structure calculation methods. The complexes were applied as a
new redox mediator in DSSCs, while the cell performance was studied in terms of the concentration of the reduced and oxidized
form of the complexes. These studies were performed with the commercial Ru-based sensitizer N719 absorbed on a TiO2
semiconducting film in the DSSC. Maximum energy conversion efficiencies of 2% at simulated solar light (AM 1.5; 1000 W m−2)
with an open circuit voltage of 660 mV, a short-circuit current of 5.2 mA cm−2, and a fill factor of 0.58 were recorded without the
presence of any additives in the electrolyte.
DSSCs to improve their performance. The components of a
typical DSSC have more or less been standardized, and they are a
TiO2 nanocrystalline film deposited on fluorine-doped tin
dioxide (SnO2:F) transparent conductive electrode (negative
electrode), a ruthenium bipyridyl derivative adsorbed and
chemically anchored on TiO2 nanocrystallites, an electrolyte
bearing I−/I3− redox mediator, and a platinized SnO2:F electrode
(positive electrode). A fundamental component of the DSSC is
INTRODUCTION
■
Vanadium compounds are of great interest due to their biological
importance,1−3 catalytic activity,4−8 and their use in alternative
electrochemical energy storage technologies (i.e., batteries).9
Moreover, oxidovanadium(IV/V) compounds, such as [VO-
(salen)]0/+ 10 which exhibit reversible redox behavior, might be
,
essential in various electrochemical devices, such as redox
mediators in dye-sensitized solar cells (DSSCs).11
the redox couple in the electrolyte. The I−/I3 redox couple is
−
Third-generation DSSCs have been recognized as a low-cost
viable alternative to conventional amorphous silicon photo-
voltaics.12,13 The unique design of DSSCs offers choices for
changing the materials of the different parts that constitute
Received: January 22, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b00159
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
considered as the best-performing and efficient electrolyte that
has been more or less established due to the very fast
regeneration of the dye combining with the redox couple I−/
Scheme 1. Chemical Structure of the Oxidovanadium(IV/V)
Complexes
−
I3 . However, a large volume of the recent works on DSSCs is
devoted to the substitution of this redox couple with alternative
materials.14−17 This is mainly due to some disadvantages that the
presence of I−/I3− provokes: (a) this couple is highly corrosive
and is thus destructive to metals (Ag, Au, Cu), which are used
mainly as counter electrodes and metal grids in modules; (b) it
absorbs visible light, reducing the photocurrent and conse-
quently the power efficiency; and (c) the redox potential of the
couple is not customized well; that is, the open circuit potential
Voc ranges to 0.7−0.8 V.18−20 There is therefore a need to
optimize the cells and to develop alternative charge-transport
and regeneration systems, to replace iodide with a more
transparent and less corrosive redox couple. Several attempts
have been made to find a suitable alternative to the I−/I3− system,
including transition metal complexes.21−24 However, these redox
couples should exhibit improved physical and chemical proper-
ties such as good solubility, substantially optically clear at
concentrations permitting good conductivity, high thermal
stability, and finally noncorrosive toward the components of
the solar cell. Besides, in the conventional solar cell based on the
complexes are referred to as alternative redox mediators in
DSSCs.11
EXPERIMENTAL SECTION
■
Materials. All chemicals were bought from Merck. Acetylsalicyloyl
chloride and tetraphenylphosphonium chloride were used as received.
1,2-phenylenediamine was purified by sublimation. [VIVO(acac)2],32
Na2[VO(hybeb)]·3CH3OH (1·3CH3OH),33 and Na[VO(hybeb)]·
CH3OH (2·CH3OH)33 were prepared by the literature procedures or
slight modifications of them (for the preparation of compounds 1·
3CH3OH and 2·CH3OH see Supporting Information). Dichloro-
methane and acetonitrile were dried and distilled over calcium hydride,
while tetrahydrofuran was dried and distilled over sodium wire. All the
solvents were distilled just prior to their use. Syntheses, distillations,
crystallization of the complexes, and physicochemical characterizations
were performed under high-purity argon using standard Schlenk
techniques. C, H, and N analyses were conducted by the University of
Hamburg microanalytical service; vanadium was determined gravi-
metrically as vanadium pentoxide or by atomic absorption.
I−/I3 system, optimized efficiency was hampered due to the
−
need of high driving force (∼500 mV) for the regeneration of
−
oxidized dye by I−/I3 couple. This loss represents a strong
limitation in the attainable open-circuit voltage of the cell and
hence the photovoltaic performance of the cell. For this reason,
the development of alternative redox mediators should further
improve the technology of DSSCs. Recently, the cobalt(II/III)
complexes [CoIII/II(ttcn)2]3+/2+ and [CoIII/II(bpy)3]3+/2+ with the
ligands trithiacyclononane (ttcn)25 and 2,2′-bipyrine (bpy),26
respectively, have been used in DSSCs as efficient redox couples.
DSSCs with 13% efficiency were achieved through the molecular
engineering of porphyrin sensitizers.27 It is worth noting that one
of the major advantages of using transition metal complexes as
redox couples in DSSCs is that their redox potentials can be
Chloroplatinic acid hexahydrate, [H2PtIVCl6]·6H2O, was purchased
from Sigma-Aldrich and used as received. Ruthenium(II) bis(tetrabutyl-
ammonium), N719, was purchased from Solaronix S.A, Switzerland.
SnO2:F transparent conductive electrodes (FTO, TEC A8) 8 Ohm/
square were purchased from the Pilkington NSG Group. Commercial
titanium isopropoxide (TTIP, 97%, Aldrich), Triton X-100 (poly-
ethylene glycol p-tert-octylphenyl ether) surfactant (99.8%, Fisher
Scientific), and glacial acetic acid (Aldrich) were used to make precursor
TiO2 sols. Titania powder P25 was provided by Degussa (Germany;
30% rutile and 70% anatase).
tuned by modifying the ligands, as distinguished from the I−/I3
−
system. Furthermore, the ligand properties can be tuned so as to
modulate the relative activation energies for electron-transfer
processes involving the metal complexes.28,29 In addition to the
use of cobalt(II/III) complexes as efficient redox mediators,
several other organometallic compounds were proposed as
1,2-Bis(2-acetoxybenzamido)benzene (H2hybebe). To a
stirred anhydrous tetrahydrofuran (60 mL) solution of 1,2-phenyl-
enediamine (2.722 g, 25.17 mmol) was added solid acetylsalicyloyl
chloride (10.000 g, 50.34 mmol) in one portion under high-purity argon.
The mixture was stirred for 18 h, and then water (300 mL) was added
dropwise to the stirred suspension. The suspension was cleared after the
addition of ∼30 mL of water, and then further addition of water (270
mL) resulted in the formation of a white precipitate. The resulting
precipitate was filtered off, washed with water (3 × 50 mL), and dried in
vacuum. Yield: 8.70 g (80%). Anal. Calcd for C24H20N2O6 (Mr =
432.19): C, 66.66; H, 4.66; N, 6.48. Found: C, 66.70; H, 4.58; N, 6.50%.
1,2-Bis(2-hydroxybenzamido)benzene (H4 hybeb·
0.6CH3NO2). H2hybebe (2.350 g, 5.44 mmol) was dissolved in acetone
(25 mL) at ambient temperature (∼20 °C), and an aqueous solution of
sodium hydroxide (12 mL, 6 M) was added dropwise (∼5 min) under
magnetic stirring. Then, the solution was cooled to ∼0 °C, and
concentrated (∼12 M) hydrochloric acid (11 mL, ∼48 mmol) was
added dropwise to the stirred solution under magnetic stirring. The final
pH value of the solution was ∼1. The resulting white precipitate was
filtered off and washed with water (2 × 50 mL). Yield: 1.88 g (99%). The
product was recrystallized from nitromethane34 (30 mL) to obtain off-
white crystals of H4hybeb·0.6CH3NO2. Yield after recrystallization:
1.450 g (77%). Anal. Calcd for C20.6H17.8N2.6O5.2 (Mr = 384.78): C,
64.25; H, 4.66; N, 9.46. Found: C, 64.30; H, 4.62; N, 9.51%. 1H NMR
(500 MHz; CD3CN, ppm): δ 7.80 (dd, J = 7.9, 1.2 Hz, 2H), 7.68 (dd, J =
6.2, 3.2 Hz, 2H), 7.46 (dt, J = 7.9, 2.3 Hz, 2H), 7.37 (dd, J = 6.2, 3.2 Hz,
−
alternatives to the I−/I3 electrolytes such as ferrocene/
f e r r o c e n i u m 3 0 ( F c / F c + ) , n i c k e l ( I I I / I V ) 3 1
[NiIII/IV(dicarbollide)2]0/−1, and very recently the
oxidovanadium(IV/V)11 couple ([VO(salen)]0/+ [salen =
N,N′-ethylenebis(salicylideneiminate)(2−)].
Herein, the syntheses and physicochemical characterization of
the square pyramidal oxidovanadium(IV/V) complexes
(Ph4P)2[VIVO(hybeb)] and Ph4P[VVO(hybeb)] are reported
(Scheme 1), where hybeb4− is a tetradentate diamidodipheno-
late(4−) ligand. The redox potential of the couple [VIVO-
(hybeb)]2−/[VVO(hybeb)]− and the energy of highest occupied
molecular orbital (HOMO) of the sensitizing dye and the
HOMO of [VIVO(hybeb)]2− were calculated by means of density
functional theory (DFT) electronic structure calculation
methods.
Moreover, the oxidovanadium(IV) complex (Ph4P)2[VIVO-
(hybeb)] was combined with its oxidized analogue Ph4P[VVO-
(hybeb)] and applied as redox couple in the electrolyte of
DSSCs. Their output and electrical efficiency were tested. To the
best of our knowledge, it is only the second time that vanadium
B
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Article
2H), 6.97 (m, 2H) and 6.95 (m, 2H), 4.31 (s) for CH3NO2. 13C NMR
(500 MHz; CD3CN, ppm): δ 161.5 C(7), 169.4 C(9), 135.1 C(2), 131.9
C(10′), 131.4 C(10), 128.4 C(4), 127.4 C(6), 127.1 C(5), 120.0 C(3),
118.9 C(1), 115.9 C(8′), 115.6 C(8), 63.51 (s) for CH3NO2 (see
Scheme 1 for atomic numbering).
and a slightly platinized FTO counter electrode was pushed by hand on
the top. The platinized FTO glass was made by casting a few drops of a
solution of [H2PtIVCl6]·6H2O (5 mg/mL of ethanol) followed by
heating at 500 °C for 10 min.
Electrochemistry. Cyclic voltammetry (CV) and rotating disk
experiments were conducted on an EG&G Princeton Applied Research
273A potentiostat/galvanostat. Electrochemical procedures were
performed with a three-electrode configuration: a platinum disk
electrode (surface area of 0.13 mm2) was used as the working electrode,
and a platinum wire was used as the auxiliary electrode and as reference.
Excess of electrolyte (0.9 M tetrabutylammonium perchlorate (TBAP))
was added to minimize ohmic resistance. The potential was calibrated
using ferrocene as internal standard (0.63 V vs normal hydrogen
electrode (NHE)).38 The diffusion coefficient D was calculated by
Levich’s equation.39
Bis[tetraphenylphosphonium] [1-(2-hydroxybenzamido)-2-
(2-pyridinecarboxamido)benzenato]oxidovanadate(IV),
(Ph4P)2[VIVO(hybeb)]·0.8CH2Cl2 (3·0.8CH2Cl2). To a stirred suspen-
sion of Na2[VO(hybeb)]·3CH3OH (0.500 g, 0.94 mmol) in CH2Cl2
(20 mL) was added in one portion solid Ph4PCl (0.673 g, 1.88 mmol).
The mixture was stirred for 3.0 h, under rigorous exclusion of oxygen.
The complex dissolved, and a white solid (NaCl) separated. After
filtration and concentration of the solution to ∼3 mL, the complex was
precipitated by adding dropwise and with stirring 10 mL of diethyl ether.
The resulting pale green precipitate was filtered off, washed with diethyl
ether (2 × 10 mL), and dried in vacuo. Yield: 0.816 g (75%). High-
resolution electrospray ionization mass spectrometry (HR-ESI(−)-
MS): calcd for C20H12O5N2V ([M ]−2) 205.5145, 205.5198 found. Anal.
Calcd for C68.8H53.6Cl1.6N2P2O5V (Mr = 1157.32): C, 71.34; H, 4.67; N,
2.42; V, 4.40. Found: C, 70.95; H, 4.70; N, 2.36; V, 4.41%.
Tetraphenylphosphonium [1,2-bis(2-hydroxybenzamido)-
benzenato]-oxidovanadate(V), Ph4P[VVO(hybeb)]·0.3CH2Cl2 (4·
0.3CH2Cl2). The complex was prepared in a fashion similar to that used
for complex 3·0.8CH2Cl2 except that 1 equiv of Ph4PCl was used to get
4·0.3CH2Cl2 in 67% yield. HR-ESI(−)-MS: calcd for C20H12O5N2V
([M ]− ) 411.0180, 411.0147 found. Anal. Calcd for
C44.3H32.6Cl0.6N2O5PV (Mr = 775.70): C, 68.54; H, 4.24; N, 3.61, V,
6.57. Found: C, 68.31; H, 4.30; N, 3.70; V, 6.60%.
The heterogeneous electron-transfer rate constant k0 (cm s−1) for the
electrode reaction was determined by Nicholson’s equation, using the D
value calculated from Levich’s equation.39
Impedance measurements of the as-prepared solar cells based on the
aforementioned complexes were performed under solar light illumina-
tion provided from a Xe light source using a Solar Light Co. solar
simulator (model 16S-300) equipped with AM0 and AM 1.5 direct Air
Mass filters to simulate solar radiation at the surface of the Earth. In
particular, electrochemical impedance spectroscopy (EIS) measure-
ments were performed with Metrohm Autolab 3.v potentiostat
galvanostat (model PGSTAT 128N) through a frequency range from
100 kHz to 0.01 Hz, using a perturbation of 10 mV over the open-
circuit potential. Experimental data are presented by scattering symbols,
while lines represent the fitted plots obtained using Nova 1.10 software.
Nuclear Magnetic Resonance Measurements. Samples were
prepared by diluting a proper amount of compound in 0.600 mL MeCN-
d3 in a 5 mm NMR tube. NMR Spectra were recorded in a Bruker AV
500 MHz instrument. The 51V spectrum was recorded at 105.32 MHz
using a spectral width of 720 ppm with 16 000 data points, 90° pulse, a
relaxation delay of 0.5 s, and an acquisition time of 0.1 s. A 3 Hz line
broadening was applied prior to the Fourier transformation. The
chemical shifts are reported with respect to VVOCl3 at 0 ppm.
Characterization of the Dye-Sensitized Solar Cells. For the
current density−voltage (J−V) curves, the samples were illuminated
with Xe light using a Solar Light Co. solar simulator (model 16S-300)
equipped with AM 0 and AM 1.5 direct Air Mass filters to simulate solar
radiation at the surface of the Earth. The light intensity was kept
constant at 1000 W/m2 measured with a CMP 3 Kipp & Zonen
pyranometer. Finally, the current−voltage (I−V) curves were recorded
by connecting the cells to a Keithley Source Meter (model 2601A),
which was controlled by Keithley computer software (LabTracer). The
cell active area was constant at 2 cm2, while mask of 0.3 cm2 was used in
all measurements. For each case, we made three devices, which were
tested under the same conditions to avoid any misleading estimation of
their efficiency. Cell performance parameters, including short-circuit
current (ISC), open-circuit voltage (VOC), maximum power (Pmax), fill
factor (FF), and overall cell conversion efficiency, were measured and
calculated from each J−V characteristic curve.
Computational Details. All calculations were performed using the
Gaussian 09 program suite.40 The geometries of the oxidovanadium(IV/
V) complexes were fully optimized, without symmetry constraints,
employing the 1997 hybrid functional of Perdew, Burke, and
Ernzerhof41 as implemented in the Gaussian09 program suite. This
functional uses 25% exchange and 75% correlation weighting and is
denoted as PBE0. For the geometry optimizations, we used the Def2-
TZVP basis for all atoms. Hereafter the method used in DFT
calculations is abbreviated as PBE0/Def2-TZVP. All stationary points
were identified as minima (number of imaginary frequencies Nimag = 0).
Acetonitrile solvent effects were taken into account with the polarizable
continuum model (PCM) using the integral equation formalism variant
(IEFPCM) being the default self-consistent reaction field (SCRF)
method.42 The energies of the frontier MOs of the N719 Ru dye were
obtained from a single-point energy calculation in CH3CN solution
(PCM model) at the PBE0/Def2-TZVP using the X-ray structure of the
dye.43
Electron Paramagnetic Resonance Spectroscopy. The con-
tinuous wave X-band electron paramagnetic resonance (EPR) spectra of
the compounds were acquired on an ELEXSYS E500 Bruker
spectrometer at a resonance frequency of ∼9.5 GHz and a modulation
frequency of 100 MHz. For the full EPR spectra (ΔB = 100 mT, 3000
points), 20 scans were accumulated. The optimization of the spin
Hamiltonian parameters and EPR data simulation were performed by
using the software package easy spin 4.5.5.35
Preparation of TiO2 Photoanodes Sensitized with N719 Dye. TiO2
thin films were first deposited on fluorine-doped tin oxide (FTO)
glasses as blocking layers, following a previously reported proce-
dure.36,37 Briefly, for 5.4 mL of solution, 0.72 g of Triton X-100 were
mixed with 4 mL of ethanol, followed by addition of 0.4 mL of glacial
acetic acid and 0.32 mL of titanium(IV) isopropoxide under vigorous
stirring. After this solution was stirred for a few minutes, FTO glasses
were dipped in the above solution and withdrawn at rate of 2 cm/sec.
The films were heated to 500 °C for 30 min using a 20 °C/min heating
ramp rate. The procedure was applied for the deposition of only one
TiO2 layer; the film thickness was ∼150 nm. Then, a layer of TiO2 paste
was deposited by doctor blading followed by heating to 500 °C. The
fabrication of TiO2 paste is as follows: 3 g of TiO2-Degussa P25 was
mixed with 0.5 mL of acetic acid in a mortar for ∼3 min. Thereafter, 2.5
mL of Millipore water and 17.5 mL of ethanol were alternately added to
break all TiO2 aggregates and form a homogeneous solution. The
solution was transferred to a crucible with 50 mL of ethanol and was
mixed with 10 g of terpineol and an amount of ethyl cellulose. The
solution was ultrasonicated for ∼2 min, and then the crucible was placed
in a rotary evaporator at 40−45 °C to remove the excessive solvent and
form the TiO2 paste. The TiO2 films have a total thickness of ∼5 μm and
were prepared by the previously described procedure and immersed into
a 5 × 10−4 M (ethanol/acetonitrile 50:50) solution of N719 dye
overnight to complete the photoanode preparation. However, note that
thicker TiO2 films might improve efficiency of the cells. Excessive dye
molecules that were not adsorbed on the TiO2 surface were removed by
rinsing with acetonitrile.
Vanadium Electrolyte Preparation. In the construction of the solar
cell, a liquid electrolyte based on vanadium redox couple was used. The
composition of the first electrolyte is as follows: 0.05 M of the
vanadium(IV) complex (Ph4P)2[VIVO(hybeb)] and 0.005 M of the
vanadium(V) complex Ph4P[VVO(hybeb)] were diluted in acetonitrile.
After this solution was stirred for 6 h, one drop of each electrolyte was
placed on the top of the titania electrode with adsorbed dye molecules,
C
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RESULTS AND DISCUSSION
■
Syntheses. The organic molecules H2hybebe and H4hybeb·
0.6 CH3NO2 were prepared by modifications of the reported
literature procedures.33 In particular, H4hybeb·0.6CH3NO2 was
synthesized with substantial modifications of the literature-
reported synthesis33 to increase the yield and to reduce the time
of the synthesis (5 min vs 12 h). The sodium ions of the
oxidovanadium(IV/V) compounds 1·3CH3OH and 2·CH3OH
were substituted for tetraphenylphosphonium counterions to
increase their solubility and their hydrolytic stability in
acetonitrile. High purity of [VIVO(acac)2]44 and H4hybeb·0.6
CH3NO2 was essential for the preparation of 1·3CH3OH.
Electron Paramagnetic Resonance Spectroscopy. The
EPR spectrum of the oxidovanadium(IV) compound 3·0.8
CH2Cl2 in acetonitrile solution gave the characteristic eight-line
isotropic spectrum (Figure 1) at room temperature (RT) and the
16-line anisotropic spectrum (Figure 2) at 120 K.
Figure 2. (A) X-band EPR spectrum of a frozen solution of the
compound 3·0.8CH2Cl2 in dried CH3CN (7.00 mM) at 120 K (black
line) and the simulated spectrum (red dashed line). (B) X-band EPR
spectrum of a frozen CH3CN solution (containing 1% H2O) of 3·
0.8CH2Cl2 (2.59 mM), which had been partially oxidized by bubbling air
for 1 h into it, at 120 K (black line) and the simulated spectrum (red
dashed line) obtained using g⊥= 1.980, g∥ = 1.954, A⊥= −50.8 × 10−4
cm−1 and A∥ = −158.6 × 10−4 cm−1 for [VIVO(hybeb)]2− (21%) and g⊥=
1.975, g∥ = 1.942, A⊥= −60.1 × 10−4 cm−1 and A∥ = −168.6 × 10−4 cm−1
for the new species (79%).
Figure 1. X-Band EPR spectrum of compound 3·0.8CH2Cl2 in dried
CH3CN solution (7.00 mM) at RT (black line) and its simulated
spectrum (red dashed line).
oxidovanadium(V) analogue [VVO(hybeb)]−, which is sensitive
to hydrolysis, and it is hydrolyzed releasing protons to the
solution; these protons protonate the phenolic oxygen atom of
[VIVO(hybeb)]2− species (Scheme 2).
The g- and A-EPR parameters of the anisotropic spectrum of 3·
0.8 CH2Cl2 (Table 1) were calculated by simulation of its
anisotropic experimental spectrum (Figure 2). These parameters
are close to those reported33 for the Na+ salt of [VIVO(hybeb)]2−
(g⊥= 1.978, g∥ = 1.960, A⊥= −53.6 × 10−4 cm−1 and A∥ = −156.2
× 10−4 cm−1 and A0 = 87.8 × 10−4 cm−1).
This is also supported by the HR-ESI(−)-MS (in methyl
alcohol not dried; Figure 3) and 51V NMR (in CD3CN not dried)
spectra of Ph4P[VVO(hybeb)]·0.3 CH2Cl2.
However, partial aerial oxidation (i.e., in the solution coexist
VIV and VV species) of the compound 3·0.8CH2Cl2 in CH3CN
solution (containing 1% H2O) gave peaks from an additional
species (Table 1). The simulation of this spectrum (Figure 2B)
resulted in larger hyperfine splitting and a shift to lower g values
for the new species in comparison to those of [VIVO(hybeb)]2−.
The new species was assigned to the oxidovanadium(IV)
compound with a protonated phenolic oxygen (Scheme 2).
The protonation of the phenolic oxygen, according to the
additivity rule,45 increases the A∥ to ∼10 × 10−4 cm−1, which is
the same as the observed increment by the experiment. The
formation of either VIVO····VVO or OVIV−O−VVO
dinuclear adduct should be excluded because such complexes are
expected to exhibit higher g values and smaller hyperfine
splitting.46 In addition, any ligation to the sixth axial empty
position of [VIVO(hybeb)]2− is excluded because it would result
in a smaller orbital splitting, depopulation of the dxy orbital and
thus smaller hyperfine splitting.45d,47 Electrochemistry and UV−
vis experiments have shown that the species [VIVO(hybeb)]2−
retains the square pyramidal geometry in strong donating
solvents.28
In Figure 4 are shown the EPR spectra of (Ph4P)2[VIVO-
(hybeb)]·0.8CH2Cl2 and of various mixtures of this species with
its oxidized counterpart Ph4P[VVO(hybeb)]·0.3CH2Cl2 in dried
CH3CN at room temperature.
In dried CH3CN, the hydrolysis of the complex anion
[VVO(hybeb)]− does not take place, and thus, the complex
anion [VIVO(Hhybeb)]− is undetectable by EPR spectroscopy
(vide supra). Clearly, in dried CH3CN and rigorous exclusion of
oxygen both oxidovanadium(IV/V) complexes are stable. The
observed rate constant of the electron transfer kobs between the
oxidovanadium(IV/V) complexes [VIVO(hybeb)]2− and [VVO-
(hybeb)]− was determined from the EPR line-width broadening
of the eight peaks of the oxidovanadium(IV) signal, at room
temperature, according to eq 1.
kobs = 31/2πγ (ΔB − ΔB0)/{(1 − p)([VVO(hybeb)−]}
e
i
(1)
where γe is the gyromagnetic ratio of the electron (γe = 2.803 ×
107 mT−1 s−1), pi is the statistical population factor of the EPR
line i, and ΔB0 and ΔB are the line widths calculated from the
simulation of the spectra (Figure 5) in the absence and presence
of the self-exchange broadening, respectively.11,45e Using pi =
Although the complex anion [VIVO(hybeb)]2− is stable in
solution toward hydrolysis, it can be easily oxidized to its
D
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Table 1. EPR Parameters of the Oxidovanadium(IV) Complexes [VIVO(hybeb)]2− and Its Protonated Analogue [VIVO(Hhybeb)]−
complex
g⊥
g∥
A⊥ × 10−4, cm−1
A∥ × 10−4, cm−1
g0
A0 × 10−4, cm−1
[VIVO(hybeb)]2−
[VIVO(Hhybeb)]−
1.980
1.975
1.954
1.942
−50.8
−60.1
−158.6
−168.6
1.975
1.967
87.3
96.8
Scheme 2. Possible Mechanism of the Protonation of the
Complex Anion [VIVO(hybeb)]2− Leading to
[VIVO(Hhybeb)]− through Hydrolysis of [VVO(hybeb)]−
Figure 5. Plot of changes in the line width for the EPR signals found
from the simulation of the experimental EPR spectra in mT vs the
concentration of [VVO(hybeb)]−.
1/kex = 1/kobs − 2/kdiff
(2)
where kdiff = (8RT/3η), the exchange rate for the [VIVO-
(hybeb)]2−/[VVO(hybeb)]− couple was found to be kex = 3.9 ×
109 M−1 s−1, which is less than the rate constant observed for the
[VIVO(salen)]0/[VVO(salen)]+ couple (kex = 7.4 × 109 M−1 s−1)
but much higher than that reported for the nitroxide radicals such
as TEMPO0/+ (2.7 × 107 M−1 s−1),48 which typically has been
attributed to an outer-sphere exchange mechanism. At this point,
it is worth noting that the kex of the nonoxidovanadium(IV/V)
species49a [VIV/V(hidpa)2]2−/− is ∼1 × 105 M−1 s−1, while the kex
of the redox couple49b cis-[VIVO(OH)(bpy)2]+/[VVO2(bpy)2]+
is (1.0 0.3) × 10−1 M−1 s−1.
Electrochemistry. The CV study of 3·0.8CH2Cl2 in CH3CN
solution (1.00 mM) reveals one reversible [VIVO(hybeb)]2−/
[VVO(hybeb)]− redox wave at −0.047 V vs NHE. The CV study
of 4·0.3CH2Cl2 (the oxidized analogue of 3·0.8CH2Cl2) in
CH3CN solution provided results identical to those for 3·
0.8CH2Cl2. The cyclic voltammograms of compound 3·
0.8CH2Cl2 in CH3CN at various scan rates (Figure 6) show
one electron reversible electron transfer; ia/v1/2 and E1/2 are
independent of the scan rate (v) and ia/ic = 1, where ia and ic are
Figure 3. HR-ESI negative mass spectrum of Ph4P[VVO(hybeb)]·
0.3CH2Cl2 in methyl alcohol (not dried), showing isotopic distribution
envelopes of [VVO(hybeb)]− and [H3hybeb]− centered at m/z =
411.0147 and 347.1090, respectively.
Figure 4. X-Band EPR spectra of an acetonitrile solution, at RT, of (A)
(Ph4P)2[VIVO(hybeb)]·0.8CH2Cl2 (1.22 mM) (black line), (B)
(Ph4P)2[VIVO(hybeb)]·0.8CH2Cl2 (1.22 mM) and Ph4P[VVO-
(hybeb)]·0.3CH2Cl2 (0.593 mM) (brown line), and (C) (Ph4P)2[VIVO-
(hybeb)]·0.8CH2Cl2(1.22 mM) and Ph4P[VVO(hybeb)]·0.3CH2Cl2
(4.74 mM) (red line).
Figure 6. Cyclic voltammograms of compound 3·0.8 CH2Cl2 in dried
CH3CN solution (1.00 mM), containing 0.9 M TBAP, recorded at
various scan rates (v). The data were used for the calculation of the
heterogeneous electron transfer rate constant k0 = 7.7 × 10−3 cm s−1.
0.13 and correcting the exchange rate of the couple [VIVO-
(hybeb)]2−/[VVO(hybeb)]− using eq 2
E
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Figure 7. (A) Current−potential curves obtained with a rotated disk electrode of compound 3·0.8CH2Cl2 in CH3CN solution (1.00 mM) containing
0.9 M tBu4NClO4 as supporting electrolyte; at scan rate 10 mV s−1. (B) A plot of the diffusion limited currents vs the square root of the rotation rates of
the electrode obtained from Figure 7A used for the determination of the diffusion coefficient D = 1.0 × 10−5 cm2 s−1.
the anodic and cathodic peak currents, respectively, and ΔE = Ea
− Eb is close to 65 mV at low scan rates.
the Ph4P+ cation. The C(9)O [167.0 (169.4 for the free
ligand)] and the C(7)−O [164.6 (161.5 for the free ligand)] gave
the largest shifts in comparison with those of the free ligand
implying that the ligand hybeb4− is bonded to vanadium through
the two deprotonated amide nitrogen and two deprotonated
phenolic oxygen donor atoms (Scheme 1).
Solar Cell Performance. The energy levels of the main
components of the DSSC and the relative position of the redox
potential for the complexes (Ph4P)2[VIVO(hybeb)] and (Ph4P)-
[VVO(hybeb)] according to the electrochemistry data are
presented in Figure 8. Obviously, the relative position of this
The potential−current curves of compound 3·0.8CH2Cl2 in
CH3CN solution (1.00 mM) recorded at various rotation rates
with a rotating disc working electrode are shown in Figure 7A.
The diffusion coefficient D (1.0 × 10−5 cm2 s−1) for 3·0.8CH2Cl2
was calculated from the Levich plot (Figure 7B) This plot is
linear as a result of the electrochemical reversibility of the redox
couple [VIVO(hybeb)]2−/[VVO(hybeb)]−. The D value found
for 3·0.8CH2Cl2 is similar to the D value for the compound
[VIVO(salen)] (5.8 × 10−6 cm2 s−1).11
The heterogeneous electron transfer rate constant k0 was
estimated using the above D value and ΔE (= Ea − Eb) at various
scan rates at CVs. The value of k0 found for 3·0.8CH2Cl2 (7.7 ×
10−3 cm s−1) is almost one-third of the corresponding value for
the compound [VIVO(salen)] (2.7 × 10−2 cm s−1). The electron
transfer is rapid and reduces the electrode reaction-related cell
resistance.
51V Spectra of Compound 4·0.3 CH2Cl2. The 51V spectrum
of the oxidovanadium(V) compound [Ph4P]+[VVO(hybeb)]−
(4·0.3 CH2Cl2) in dry CD3CN solution shows one resonance
signal at −465 ppm with a half-height line width Δν1/2 of 293 Hz.
This chemical shift is the same as that observed for a CD2Cl2
solution of [(Ph3P)2N]+[VVO(hybeb)]− (5) {in which Ph4P+
was substituted for [Ph3PNPh3P]+} indicating that the
vanadium coordination environment in [VVO(hybeb)]− is the
same in both solvents.
On the basis of the heteroatom electronegativities tabulated by
Zang,50 we obtain ∑χ = 17.05 for the complex [VVO(hybeb)]−
[∑χ is the sum of the electronegativities of the three oxygen and
two nitrogen atoms coordinated to vanadium(V)]. Applying
these values to Rehder’s51 referencing scale, one predicts shift
values in the upfield region between −400 and −500 ppm for a
five-coordinate vanadium(V) compound. This is in agreement
with the observed value of −465 ppm for the complex
[VVO(hybeb)]−, and thus, the vanadium(V) atom most likely
retains its coordination number in the CH3CN solution.
13C NMR Spectra of Compound 4·0.3 CH2Cl2. The carbon
peaks of the CH3CN solution of [VO(hybeb)]− at 132.43 C(8),
117.62 C(6), 127.1, 127.4, 128.4, and 120 ppm, obtained from
the two-dimensional (2D) {1H, 13C} HSQC and the 2D {1H,
13C} HMBC spectra were assigned to the coordinated ligand.
The stronger peaks at 135.1, 130.16, and 118.9 ppm belong to
Figure 8. Redox potential for (Ph4P)2[VIVO(hybeb)] and (Ph4P)-
[VVO(hybeb)] mediators in relation to the energy levels of the dye/
TiO2 photoanode.
redox mediator is energetically lower than that referred to for the
basic state of the N719 dye, and thus, it promotes the dye’s
regeneration in a successful way (see Figure 8). At this point, it is
worth noting that the theory confirms this statement (vide infra),
but although the SOMO of [VIVO(hybeb)]2− is more positive
than the HOMO of the N719 dye (see Figure 11), the energy
difference is relatively small; thus, the driving force for dye
regeneration should not be optimal.
The J−V typical curves were performed for several
concentrations of (Ph4P)2[VIVO(hybeb)] and (Ph4P)[VVO-
F
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(hybeb)] as redox mediators dissolved in dried acetonitrile.
Initially, the two complexes were used separately as electrolytes
solely dissolved in acetonitrile, but very poor results were finally
obtained. Optimized results appear in Figure 9 where solar cells
constructed with an electrolyte consisting of 0.05 M
(Ph4P)2[VIVO(hybeb)] and 0.005 M (Ph4P)[VVO(hybeb)]
dissolved in acetonitrile were applied.
Figure 10. Impedance spectrum plot for optimized electrolyte
composition at open-circuit voltage configuration irradiated with 1
sun solar light. (inset) DSSC equivalent circuit R(RQ)(RQ) used to fit
the experimental data from the EIS measurements.
Cpseudo = Y0(1/N)R(1/N−1)
(3)
The total series resistance of the cell can be calculated using eq
4. Rs was finally measured to be 35.5 Ω, which is a relatively low
value for internal resistance of the cell based on liquid electrolyte.
Figure 9. Characteristic current density (J) vs voltage (V) curve of the
best-performing DSSC fabricated with an electrolyte based on the
complexes (Ph4P)2[VIVO(hybeb)] and (Ph4P)[VVO(hybeb)].
RS = Rh + Rpt
(4)
Computational Studies. The [VIVO(hybeb)]2−/[VVO-
(hybeb)]− redox process has been studied by means of DFT
electronic structure calculation methods.54 The calculation of the
redox potential for the following charge transfer reaction was
based on the thermodynamic cycle shown in Scheme 3.
The highest energy conversion efficiency of 2% was achieved
under simulated sunlight (AM 1.5, 1000 W/m2), while lower
quantities of the complexes showed lower values for Jsc and Voc.
However, higher amount of the complexes showed no
remarkable changes to the short-circuit current densities and
open-circuit voltages. It is noticeable that no electrolyte additives,
such as lithium perchlorate or tert-butylpyridine, were applied for
current or voltage enhancement. The obtained results refer to the
use of a commercial dye (N719) as the most applicable sensitizer
in DSSCs with standard potential for its reduced form. We
recognize that the obtained efficiency could be even higher in
case of the use of other pure organic dyes as recently better
results were monitored for MK2 and D107 commercial dye
products and cobalt-based redox mediators,52,53 but our work is
referred to the more stable and most-used dye in the literature.
The DSSCs were further characterized using electrochemical
impedance spectroscopy (EIS). Figure 10 shows the Nyquist plot
obtained from cells with optimized electrolyte composition. The
first semicircle corresponds to the Pt/electrolyte interface, Rpt.
The charge transfer resistance at the counter electrode (Rpt) is
represented as a semicircle in the impedance spectra. The
resistance element related to the response in the intermediate
frequency represents the charge transport at the TiO2/dye/
electrolyte interface (Rtr) and shows diode-like behavior.
[VVO(hybeb)]−(soln. ) + e− → [VIVO(hybeb)]2−(soln. )
(5)
The redox potential of reaction 5 in solution is related to the
total change in the Gibbs free energy according to eq 6.55
Eo = −ΔGosoln./nF
(6)
where n is the number of electrons transferred (n = 1 in this case),
F is the Faraday constant (equal to 23.061 kcal mol−1 V−1), while
ΔGosoln. is calculated from the thermodynamic cycle (Scheme 3)
according to the following equation
ΔGo
= ΔGo + ΔGo[VVO(hybeb)]−
soln.
gas
soln.
− ΔGo[VIVO(hybeb)]2−
(7)
soln.
where ΔGogas is the change of the standard Gibbs free energy of
reaction 5 in the gas phase, while ΔGo[VVO(hybeb)]−
and
soln.
ΔGo[VIVO(hybeb)]2−soln. are the standard free solvation energies
of the [VVO(hybeb)]− and [VIVO(hybeb)]2−species, respec-
tively.
The equivalent circuits used to fit the experimental data are
presented as inset in Figure 10. For electrodes having a rough
surface the capacitance element (inset Figure 10) is replaced by a
constant phase element (CPE, Q), which depends on the
parameters Y0 and N. It is possible to convert a CPE element,
which is in parallel with a resistance, to a pseudo capacitance
using eq 3. Finally, the intercept of the horizontal axis stands for
the resistance of the sheet resistance of the FTO substrate and
the contact resistance of the FTO/TiO2 (Rh). The fitted
parameters are summarized in Table 2.
Using ΔG values, calculated at the PBE0/Def2-TZVP level of
theory, for the Born−Haber thermodynamic cycle given in
Scheme 3 and eq 6, we estimated the theoretical redox potential
Eo of reaction 5 in solution to be equal to 4.43 V. On the basis of
experimental and theoretical studies56 the standard free energy
for the standard process of formation of hydrogen from protons
(eq 8) on the NHE is found to be equal to −4.36 V. Therefore,
the redox potential of the couple [VIVO(hybeb)]2−/[VVO-
(hybeb)]− versus NHE is −0.07 V in excellent agreement with
the experimental value of −0.047 V.
G
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Table 2. EIS Results of DSSC Fabricated with an Electrolyte Based on (Ph4P)2[VIVO(hybeb)] and (Ph4P)[VVO(hybeb)]
Complexes in Optimized Concentration
fitted parameters
Rh (Ω)
Rpt (Ω)
Cpt (× 10−4 F)
Rtr (Ω)
Ctr (× 10−3 F)
1.18 0.01
values
20.0 0.1
15.5 0.1
0.22 0.01
16.9 0.1
Scheme 3. Thermodynamic Cycle Used to Calculate Gibbs
Free Energies
Table 3. Characteristics of [VIV/VO(hybeb)]2−/−
,
[VIV/VO(salen)]0/+, and Iodide/Triiodide-Based Electrolytes
b
kex
a
c
d
electrolyte
E1/2
(× 109)
D
k0
[VIV/VO(hybeb)]2−/−
[VIV/VO(salen)]0/+
−0.047
0.44
3.9
7.4
23
1.0 × 10−5
5.8 × 10−6
7.7 × 10−3
2.7 × 10−2
5.3 × 10−3
e
I−/I3
0.58
7.6 × 10−6,
−
a
b
c
In volts vs NHE. Self-exchange rate constant in M−1 s−1. Diffusion
d
coefficient in cm2 s−1. The heterogeneous electron transfer rate
H+(aq) + e−(g) → 1/2H2(g)
(8)
e
constant in cm s−1. Reference 43.
One way to improve the efficiency of a DSSC57 is to tune the
regenerating electrolyte couple to the HOMO of the sensitizing
dye to ultimately obtain a higher open-circuit voltage, Voc. The
HOMO of the sensitizing dye should lie below the energy level of
the regenerating electrolyte couple or hole transporting material
(HTM) so that after its oxidation upon electron injection to the
conduction band of TiO2 to be effectively regenerated by
accepting electrons from the HTM.58,59 The HOMOs of N719
dye and of [VIVO(hybeb)]2−, calculated at the PBE0/Def2-
TZVP level augmented with the PCM solvation model for
CH3CN solution, are shown in Figure 11..
−
I3 respectively. The other properties of the three electrolytes are
very similar, and this means that the oxidovanadium(IV/V)
redox couples can be potentially used as redox shuttles.
CONCLUSIONS
■
In conclusion, the oxidovanadium(IV/V) complexes
(Ph4P)2[VIVO(hybeb)] and (Ph4P)[VVO(hybeb)] were synthe-
sized and physicochemically characterized. The ideal properties
of the complexes, including the large electron exchange and
transfer rates and the reversible redox process resulting from EPR
and electrochemistry, render the vanadium compounds suitable
as candidates for redox mediators in DSSCs. The very large self-
exchange rate constant offered an insight into the mechanism of
the exchange reaction most likely mediated through an outer-
sphere exchange mechanism. The redox couple [VIV/VO-
(hybeb)]2−/−, with a redox potential 0.63 V more negative
than the I−/I3 system, was successfully applied to iodide/iodine-
−
free DSSCs sensitized with the ruthenium N-719 dye and yielded
devices with efficiencies of 2% at 1 sun irradiation without the
presence of any organic additive in the electrolyte.
The theoretical calculations revealed that the HOMO of
[VIVO(hybeb)]2−, which is involved in the oxidation process
upon removal of an electron, is more positive than the HOMO of
the N719 dye, but this energy difference is relatively small; thus,
the driving force for dye regeneration should not be optimal. The
synthesis of suitable dye/[VIV/VO]2+/3+ redox mediators based
on the theoretical calculations is underway in our laboratory to
improve the solar cell efficiency.
Figure 11. Three-dimensional contour plots of the HOMOs of
[VIVO(hybeb)]2−and N719 dye in CH3CN solution, at the PBE0/
Def2-TZVP level.
Perusal of Figure 11 reveals that, although the HOMO of
HTM is more positive than the HOMO of the N719 dye, the
energy difference is relatively small, and thus, the driving force for
dye regeneration should not be optimal. Notice that the HOMO
of [VIVO(hybeb)]2−, which is involved in the oxidation process
upon removal of an electron, is mainly located on one of the
phenyls ring of the hybeb4− ligand as well as on the vanadium
metal center.
Comparison of the Thermodynamic and Kinetic
Parameters of the Oxidovanadium(IV/V) Complexes
[VIV/VO(hybeb)]2−/−, [VIV/VO(salen)]0/+, and Iodide/Triio-
dide-Based Electrolytes. Many factors can affect the incident
photon to charge carrier efficiency (IPCE) and J−V behavior of
DSSCs, making comparisons with literature results very difficult.
In Table 3 are summarized the main thermodynamic and kinetic
parameters of the oxidovanadium(IV/V) complexes [VIV/VO-
(hybeb)]2−/−, [VIV/VO(salen)]0/+, and iodide/triiodide-based
electrolytes. The main difference between them is the E1/2 of the
redox couple [VIV/VO(hybeb)]2−/−, which is 0.49 and 0.63 V less
positive than redox potential of [VIV/VO(salen)]0/+and I−/
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details for the synthesis of the compounds
Na2[VO(hybeb)]·3CH3OH (1·3CH3OH) and Na[VO-
(hybeb)]·CH3OH (2·CH3OH). This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: attsipis@uoi.gr. (A.C.T.)
■
*E-mail:rehder@chemie.uni-hamburg.de. (D.R.)
*E-mail: tkampano@cc.uoi.gr. (T.A.K.)
*E-mail: akeramid@ucy.ac.cy. (A.D.K.)
*E-mail: estathatos@teiwest.gr. (E.S.)
Notes
The authors declare no competing financial interest.
H
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Article
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ACKNOWLEDGMENTS
■
Prof. E. Stathatos acknowledges that his research has been
cofinanced by the European Union (European Social Fund
(ESF)) and Greek national funds through the Operational
Program “Education and Lifelong Learning” of the National
Strategic Reference Framework (NSRF) - Research Funding
Program: ARCHIMEDES III, Investing in knowledge society
through the ESF. We also thank the Research Promotional
Foundation of Cyprus and the European Structural Funds for the
purchase of EPR through ANABAΘMIΣH/ΠAΓIO/0308/32
and Dr. C. Drouza for the EPR measurements. We are grateful to
the Greek Community Support Framework III, Regional
Operational Program of Epirus 2000-2006 (MIS 91629), for
supporting the purchase of the LC-NMR cryoinstrument. The
authors would like to thank the Unit of Environmental, Organic,
and Biochemical high-resolution analysis-ORBITRAP-LC-MS of
the Univ. of Ioannina for providing access to the facilities.
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