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K. Neuthe et al. / Polyhedron 81 (2014) 583–587
In this paper, we show for the first time symmetric nickel and
3. Results and discussion
palladium bis-iminosemiquinonate and bis-diiminosemiquinonate
complexes with a 1,2-arrangement of nitrogen or oxygen coordi-
nating atoms that are applied in DSSCs. To identify dyes that are
applicable in dye-sensitized solar cells, we introduced different
electron-withdrawing ligands in order to alter the redox behavior
of the complexes and evaluated their electrochemistry. Based on
the electrochemical studies, the most promising dyes were then
tested in prototypical dye-sensitized solar cells.
3.1. Complex design
Fig. 1 shows the NIR-absorbing metal complexes synthesized in
this study. Generally, the structures can be divided into two
groups: (a) nickel complexes with diiminosemiquinonate (N,N)
ligands and (b) palladium complexes with iminosemiquinonate
(N,O) ligands.
We selected the different substituents on the aryl moiety of the
ligands in the nickel complexes Ni–H, Ni–CF
Ni–Prop to investigate the effect of these substituents on the elec-
trochemical behavior of the dyes. To be applicable in TiO -based
DSSCs, the dye has to fulfill two requirements. First, the LUMO
lowest unoccupied molecular orbital) of the dye has to be energet-
ically higher than the conducting band edge of the TiO to facilitate
3
, Ni–Cl, Ni–EE, and
2
. Experimental
2
2.1. Materials and reagents
(
Details in the synthesis and characterization of ligands and
2
metal complexes can be found in the supporting information.
Electrolyte C, which was used in the photovoltaic application tests,
was provided by Merck KGaA.
electron injection from the dye into the conducting band of the
semiconductor. Second, the HOMO (highest occupied molecular
orbital) has to be energetically lower than the reduction potential
of the redox mediator used in the electrolyte of the DSSC device.
We introduced the electron-withdrawing group (EWG) to lower
the HOMO as well as the LUMO of the dyes with respect to the
unsubstituted nickel complex Ni–H. We expected to see a direct
correlation of the energy levels of the HOMO and LUMO, as mea-
sured electrochemically, and the strength of the EWG, i.e. the
stronger the electron-withdrawing group the lower the HOMO
and LUMO of the particular dye. In addition to the Ni diiminosemi-
quinonate complexes, tert-butyl-functionalized iminosemiquino-
nate palladium complexes were prepared. Complexes with this
general ligand structure are known to have HOMO/LUMO features
that are close to the desired levels [18]. Introduction of additional
functional groups in the ligands may further improve the afore-
mentioned features.
Another requirement of a dye sensitizer for solar cells is the
availability of anchoring groups. These anchors enable a firm
attachment of the dye to the semiconductor surface. While
different moieties, such as carboxylic, phosphonic, sulfonic acids,
or nitro groups, may be used, carboxylic acids are most commonly
applied as anchors. Hence, dyes Ni–EE, Ni–Prop, Pd–H, and
Pd–Tolyl were equipped with carboxylic acid or ester moieties.
2.2. Analytical and physical measurements and instrumentation
Absorption spectrometry in solution was performed on a Scinco
S-3100 spectrometer for UV–Vis wavelengths or a Perkin Elmer
Lambda 950 spectrometer for the measurement of near IR wave-
lengths. UV–Vis absorption spectra of sensitized films were
recorded with a Varian UV–Vis spectrophotometer Cary 4000.
The spectra cover the wavelength range from 300 to 1200 nm.
The cyclic voltammetry data were recorded on an Autolab
PGSTAT302N potentiostat. Cyclovoltammetric measurements were
3
performed in acetonitrile (MeCN), CHCl , dichloromethane (DCM),
or ethanol containing 0.1 M tetra-n-butylammonium hexafluoro-
phosphate (tetra-n-butylammonium perchlorate for ethanol) as
conducting salt in a 3-electrode compartment at a scan rate of
5
0 mV/s. The working electrode was a platinum disc electrode with
a diameter of 0.2 mm, the counter electrode was a platinum wire
electrode, and the reference electrode was a Ag/AgCl electrode
with 1 M LiCl in ethanol as inner electrolyte. Before all the mea-
surements, the electrolyte was purged with nitrogen for 20 min.
The reference electrode was calibrated against 1 mM ferrocene.
The potentiostat and all electrodes were purchased from Metrohm.
The cells used were provided by Fraunhofer Institute für Solare
Energiesysteme (Fh ISE). The cells were imprinted on so-called
3.2. Photophysical and electrochemical characterization
master plates with 5 cells on each master plate. The individual cell
The optical and electrochemical qualities of the dyes were
assessed by UV–Vis spectroscopy and cyclic voltammetry.
2
size was 2.0 cm . The porous nano-crystalline TiO
2
layer was
2
paste DSL
applied in a single screen-printing step using the TiO
8NRT from Dyesol. After sintering, the resulting TiO
ness amounted to 3 m. The dyes were adsorbed onto the TiO
1
2
layer thick-
l
2
surface using a filling station. More details about the cell prepara-
tion have been described in a previous publication [17].
The I–V characteristics of the assembled DSSCs were measured
under simulated sunlight with an irradiance equivalent to approx-
À2
imately 1000 W m . A sulfur lamp from Solaronix served as the
light source. An electrochemical workstation from Zahner was
used to record the J/V curves. The reported current densities and
2
conversion efficiencies refer to the photoactive area of 2.0 cm of
the solar cells.
Three electrolytes A–C were used for the applications tests in
the photovoltaic devices. Electrolyte A was a standard acetonitrile
based mixture, containing 0.100 M guanidinium thiocyanate
(
GSCN), 0.600 M 1-methyl-3-propylimidazolium iodide (PMII),
and 0.030 M iodine. Electrolyte B was composed of 0.030 M iodine
and 0.300 M lithium iodide in acetonitrile. Electrolyte C was com-
posed of 0.050 M iodine, 0.011 M 1-butyl-2,3-dimethylimidazoli-
um iodide (BMMIM I) and 0.087 M 1-ethyl-3-methylimidazolium
tetracyanoborat (EMIM TCB).
Fig. 1. Structures of NIR dyes: (a) general structure of Ni dyes and (b) structure of
Pd–N, O dyes.