Efficient sensitization of dye-sensitized solar cells by novel triazine-bridged porphyrin-porphyrin dyads

Article abstract of DOI:10.1021/ic400774p

Two novel porphyrin-porphyrin dyads, the symmetrical Zn[Porph]-Zn[Porph] (2) and unsymmetrical Zn[Porph]-H₂[Porph] (4), where Zn[Porph] and H₂[Porph] are the metalated and free-base forms of 5-(4-aminophenyl)-10,15,20-triphenylporphyrin, respectively, in which two porphyrin units are covalently bridged by 1,3,5-triazine, have been synthesized via the stepwise amination of cyanuric chloride. The dyads are also functionalized by a terminal carboxylic acid group of a glycine moiety attached to the triazine group. Photophysical measurements of 2 and 4 showed broaden and strengthened absorptions in their visible spectra, while electrochemistry experiments and density functional theory calculations revealed negligible interaction between the two porphyrin units in their ground states but appropriate frontier orbital energy levels for use in dye-sensitized solar cells (DSSCs). The 2- and 4-based solar cells have been fabricated and found to exhibit power conversion efficiencies (PCEs) of 3.61% and 4.46%, respectively (under an illumination intensity of 100 mW/cm² with TiO₂ films of 10 μm thickness). The higher PCE value of the 4-based DSSC, as revealed by photovoltaic measurements (J-V curves) and incident photon-to-current conversion efficiency (IPCE) spectra of the two cells, is attributed to its enhanced short-circuit current (J_sc) under illumination, high open-circuit voltage (V_oc), and fill factor (FF) values. Electrochemical impedance spectra demonstrated shorter electron-transport time (τ_d), longer electron lifetime (τ_e), and high charge recombination resistance for the 4-based cell, as well as larger dye loading onto TiO₂.

Full text of DOI:10.1021/ic400774p

Article
pubs.acs.org/IC
Efficient Sensitization of Dye-Sensitized Solar Cells by Novel Triazine-
Bridged Porphyrin−Porphyrin Dyads
Galateia E. Zervaki,† Mahesh S. Roy,‡ Manas K. Panda,† Panagiotis A. Angaridis,† Emmanouel Chrissos,†
Ganesh D. Sharma,*,§ and Athanassios G. Coutsolelos*,†
†Laboratory of Bioinorganic Chemistry, Department of Chemistry, University of Crete, Voutes Campus, P.O. Box 2208, 71003
Heraklion, Crete, Greece
§R&D Center for Engineering and Science, JEC Group of Colleges, Jaipur Engineering College, Kukas, Jaipur (Raj.) 303101, India
‡Defence Laboratory, Jodhpur (Raj.) 342011, India
S
* Supporting Information
ABSTRACT: Two novel porphyrin−porphyrin dyads, the sym-
metrical Zn[Porph]−Zn[Porph] (2) and unsymmetrical Zn-
[Porph]−H2[Porph] (4), where Zn[Porph] and H2[Porph] are
the metalated and free-base forms of 5-(4-aminophenyl)-10,15,20-
triphenylporphyrin, respectively, in which two porphyrin units are
covalently bridged by 1,3,5-triazine, have been synthesized via the
stepwise amination of cyanuric chloride. The dyads are also
functionalized by a terminal carboxylic acid group of a glycine
moiety attached to the triazine group. Photophysical measurements
of 2 and 4 showed broaden and strengthened absorptions in their
visible spectra, while electrochemistry experiments and density
functional theory calculations revealed negligible interaction
between the two porphyrin units in their ground states but
appropriate frontier orbital energy levels for use in dye-sensitized solar cells (DSSCs). The 2- and 4-based solar cells have been
fabricated and found to exhibit power conversion efficiencies (PCEs) of 3.61% and 4.46%, respectively (under an illumination
intensity of 100 mW/cm2 with TiO2 films of 10 μm thickness). The higher PCE value of the 4-based DSSC, as revealed by
photovoltaic measurements (J−V curves) and incident photon-to-current conversion efficiency (IPCE) spectra of the two cells, is
attributed to its enhanced short-circuit current (Jsc) under illumination, high open-circuit voltage (Voc), and fill factor (FF) values.
Electrochemical impedance spectra demonstrated shorter electron-transport time (τd), longer electron lifetime (τe), and high
charge recombination resistance for the 4-based cell, as well as larger dye loading onto TiO2.
Porphyrins, owing to their light-harvesting potential
(exemplified by their role in photosynthesis) and their
physicochemical properties, is a class of compounds that
stimulated significant research interest as artificial antennae in
solar cells.7 Porphyrin derivatives exhibit intense absorption in
the visible regime (a strong Soret band in the 450−500 nm
region and moderate Q bands in the 550−650 nm region).8 In
addition, their frontier orbital energy levels allow for efficient
electron injection into the TiO2 band and regeneration of the
oxidized dye by the electrolyte in a solar cell. Furthermore, by
appropriate functionalization of the porphyrin ring, either in the
meso or β positions, their spectral and redox properties can be
tuned, allowing control of the solar cell efficiency. Over the last
years, a large number of reports on porphyrin-based solar cells
with remarkable performances appeared in the literature.9
Campbell et al. were the first to report in 2007 a DSSC based
on a porphyrin with a conjugated, dicarboxylic acid anchoring
INTRODUCTION
■
Dye-sensitized solar cells (DSSCs) are currently attracting
considerable attention as alternatives to conventional amor-
phous silicon-based solar cells because of their low production
cost, simplicity of fabrication, and high power conversion
efficiency (PCE). In these devices, a thin layer of a sensitizer (a
chromophore), chemically bound to nanoporous TiO2, is
photoexcited and ultimately provides the photocurrent.1
Among the most efficient DSSC sensitizers are ruthenium
polypyridyl complexes, which exhibit intense and wide-range
absorption from the visible to near-IR regime2 and attain cell
efficiencies above 11%.3 However, because of their high cost
and environmental concerns, their wide application is limited.
Metal-free organic dyes have also been used as sensitizers,4 but
the most efficient ones result in cell efficiencies in the range of
9−10%.5 A lot of effort is currently devoted to the development
of new, efficient chromophores that exhibit strong and
panchromatic absorption and good light-harvesting efficiency
and are suitable for practical use.6
Received: March 28, 2013
© XXXX American Chemical Society
A
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Scheme 1
group that gave a cell PCE above 7%.10 In 2010, Bessho et al.,
respectively. Photovoltaic measurements as well as incident
photon-to-current conversion efficiency (IPCE) and electro-
chemical impedance spectroscopy (EIS) spectra of the two cells
revealed enhanced short-circuit current (Jsc) under illumination,
higher open-circuit voltage (Voc), and fill factor (FF) values for
the DSSC sensitized by the unsymmetrical dyad 4, which are
related to its shorter electron-transport time (τd), longer
electron lifetime (τe), and larger dye loading onto the TiO2
surface.
by introducing donor−acceptor (or “push−pull”) porphyrins,
raised the cell efficiencies to 11%.11 In 2011, Gratzel and co-
̈
workers synthesized a “push−pull” zinc-metalated porphyrin
that, in combination with an organic cosensitizer and a cobalt-
based electrolyte, achieved the remarkable cell efficiency of
12.3%, the highest cell efficiency hitherto reported for DSSCs.12
Additionally, Wang et al. reported a porphyrin-based DSSC
that, without cosensitization, gave a cell PCE above 10%.13
One of the intrinsic limitations of porphyrins is their
relatively narrow absorbing range and weak light-harvesting
ability in the green and red regions of the solar spectrum,
affecting their overall performance in DSSCs. Nevertheless, by
incorporating covalently linked arrays of porphyrin units
absorbing in different spectral regions, panchromatic sensitiza-
tion and enhancement of the overall light-harvesting capacity
might be achieved. Indeed, examples from the literature reveal
that multiporphyrin arrays lead to DSSCs with increased light-
harvesting ability, but the cell efficiencies depend on the type of
linkage between the porphyrin units. For example, porphyrin
dyads (the simplest multiporphyrin arrays), with the porphyrin
units linked through their peripheral aryl groups by an ether
bridge, resulted in cells with low efficiency because of very poor
communication between the porphyrin units.14 However, when
the two porphyrin units are linked through their meso
positions, either directly15 or by a suitable π-conjugated
linker,16 broadening of spectral coverage and improved light-
harvesting properties have been observed. Furthermore, it has
been proposed that unsymmetrical porphyrin dyads can lead to
enhancement of the overall solar cell efficiency.17
EXPERIMENTAL SECTION
■
Materials and Techniques. All manipulations were carried out
using standard Schlenk techniques under a nitrogen atmosphere. 2,4,6-
Trichloro-1,3,5-triazine (cyanuric chloride), diisopropylethylamine
(DIPEA), glycine methyl ester hydrochloride, Zn(CH3COO)2·2H2O,
Na2SO4, KOH, and other chemicals and solvents were purchased from
the usual commercial sources and used as received, unless otherwise
stated. Tetrahydrofuran (THF) was freshly distilled from sodium/
benzophenone. 5-(4-Aminophenyl)-10,15,20-triphenylporphyrin
(H2[TPP-NH2]) and 5-(4-aminophenyl)-10,15,20-triphenylporphyri-
natozinc (Zn[TPP-NH2]) were prepared according to literature
procedures.18
Synthesis of 5-[4-[3-[5-(4-Aminophenyl)-10,15,20-triphenyl-
porphyrinato]-5-(glycine methyl ester)triazinyl]aminophenyl]-
10,15,20-triphenylporphyrin (1). To a THF solution (1 mL) of
cyanuric chloride (14.6 mg, 0.079 mmol) and DIPEA (17 μL, 0.095
mmol) was added a THF solution (1 mL) of H2[TPP-NH2] (50 mg,
0.079 mmol) under argon at 0 °C. The mixture was stirred at 0 °C for
15 min, and upon reaction completion [monitored by thin-layer
chromatography (TLC)], it was left to warm at room temperature.
Next, a THF solution (4 mL) of H2[TPP-NH2] (100 mg, 0.16 mmol)
and DIPEA (33 μL, 0.19 mmol) was added, and the mixture was
stirred at room temperature overnight. An excess of glycine methyl
ester hydrochloride (100 mg, 0.79 mmol) and DIPEA (0.3 mL, 1.9
mmol) were added, and the mixture was stirred at 65 °C for 24 h. The
volatiles were removed under reduced pressure, and after dilution in
CH2Cl2, the residue was purified by column chromatography (silica
gel, CH2Cl2/EtOH 5%). The product 1 was isolated as a purple solid.
In present work, we report the synthesis of two novel
porphyrin−porphyrin dyads, the symmetrical Zn[Porph]−
Zn[Porph] (2) and unsymmetrical Zn[Porph]−H2[Porph]
(4), where Zn[Porph] and H2[Porph] are the metalated and
free-base forms of 5-(4-aminophenyl)-10,15,20-triphenylpor-
phyrin, respectively. The porphyrin units are covalently linked
through peripheral arylamino groups by 1,3,5-triazine, which is
further functionalized by a terminal carboxylic acid group
(Scheme 1). 1,3,5-Triazine is a known mediator of intra-
molecular photoinduced electron transfer between two or three
chromophores. Furthermore, the broaden and strengthened
spectral absorptions of 2 and 4, together with their appropriate
frontier orbital energy levels, make them promising candidates
as efficient sensitizers in DSSCs. The PCEs of the 2- and 4-
based solar cells were found to be 3.61% and 4.46%,
1
Yield: 40 mg (36%). H NMR (300 MHz, CDCl3): δ 8.97 (s, 4H),
8.84 (s, 12H), 8.18 (m, 17H), 7.72 (m, 18H), 7.55 (s, 4H), 6.06 (s,
2H), 4.40 (d, J = 5.4 Hz, 2H), 3.82 (s, 3H), −2.75 (s, 4H). HRMS
(MALDI-TOF). Calcd for C94H67N14O2: m/z 1424.5493 ([M + H]+).
Found: m/z 1424.5504. UV−vis [CH2Cl2; λ, nm (ε, ×10−3 M−1
cm−1)]: 419 (540.4), 516 (26.3), 552 (11.2), 590 (8.5), 647 (6.4).
Anal. Calcd for C94H66N14O2: C, 79.31; H, 4.67; N, 13.77. Found: C,
79.22; H, 4.75; N, 13.65.
Synthesis of 5-[4-[3-[5-(4-Aminophenyl)-10,15,20-
triphenylporphyrinato]zinc-5-glycinetriazinyl]aminophenyl]-
10,15,20-triphenylporphyrin Zinc (2). Metalation. To a CH2Cl2
B
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solution (12 mL) of 1 (40 mg, 0.028 mmol) was added a saturated
MeOH solution (4 mL) of Zn(CH3COO)2·2H2O in MeOH (500
mg/20 mL), and the reaction mixture was stirred at room temperature
overnight. The organic phase was washed with H2O (3 × 10 mL) and
dried with Na2SO4, and the solvent was removed under reduced
pressure. The crude product 1-Zn2 was purified by column
chromatography (silica gel, CH2Cl2/EtOH 1%), resulting in 36 mg
of a purple solid (yield: 90%). 1H NMR (300 MHz, CDCl3): δ 9.04 (s,
5H), 8.93 (s, 11H), 8.17 (m, 16H), 7.88 (m, 5H), 7.70 (m, 17H), 7.02
(s, 1H), 4.01 (d, J = 5.4 Hz, 2H), 3.61 (s, 3H). The absence of the
hydrogen signals due to the amide groups has very low intensity, and
they are probably masked under stronger signals in the aromatic
region. HRMS (MALDI-TOF). Calcd for C94H63N14O2Zn2: m/z
1547.3763 ([M + H]+). Found: m/z 1547.3742. UV−vis [CH2Cl2; λ,
nm (ε, ×10−3 M−1 cm−1)]: 422 (783.2), 550 (33.8), 591 (10.5). Anal.
Calcd for C94H62N14O2Zn2: C, 72.82; H, 4.03; N, 12.65. Found: C,
72.77; H, 4.14; N, 12.61.
dropwise, until pH ∼5. The precipitate was filtered, washed with water,
extracted with CH2Cl2, and purified by column chromatography (silica
gel, CH2Cl2/EtOH 10%). The product 4 was isolated as a purple solid.
Yield: 25 mg (85%). 1H NMR (300 MHz, CDCl3/MeOD = 1/0.01): δ
8.76 (m, 16H), 7.90 (m, 28H), 6.56 (s, 2H), 4.15 (s, 2H). The absence
of any signal due to the amide groups as well as the pyrrolic hydrogen
atoms of the free-base porphyrin is attributed to H/D exchange caused
by MeOD. HRMS (MALDI-TOF). Calcd for C93H63N14O2Zn: m/z
1471.4472 ([M + H]+). Found: m/z 1471.4505. UV−vis [toluene; λ,
nm (ε, ×10−3 M−1 cm−1)]: 422 (442.5), 517 (17.9), 552 (20.7), 592
(8.6), 648 (4.6). Anal. Calcd for C93H62N14O2Zn: C, 75.83; H, 4.24;
N, 13.31. Found: C, 75.77; H, 4.16; N, 13.42.
NMR Spectra. 1H and 13C NMR spectra were recorded on a
Bruker DPX-300 MHz spectrometer as solutions in deuterated
solvents using the solvent peak as the internal standard.
Mass Spectra. High-resolution mass spectrometry (HRMS)
spectra were recorded on a Bruker UltrafleXtreme MALDI-TOF/
TOF spectrometer.
FTIR Spectra. FTIR were recorded on a Perkin-Elmer 16PC FTIR
spectrometer.
Ester Hydrolysis. To a THF solution (15 mL) of the product
isolated from the metalation reaction (36 mg, 0.023 mmol) were
added 4.8 mL of MeOH, 6 mL of H2O, and KOH (450 mg, 0.008
mol). The reaction mixture was stirred at room temperature overnight.
The organic solvents were removed under reduced pressure, and then
a solution of HCl (1.5 M) was added dropwise, until pH ∼5. The
precipitate was filtered, washed with H2O, extracted with CH2Cl2, and
purified by column chromatography (silica gel, CH2Cl2/EtOH 5%).
Photophysical Measurements. UV−vis absorption spectra were
measured on a Shimadzu UV-1700 spectrophotometer using 10-mm-
path-length cuvettes. Emission spectra were measured on a JASCO
FP-6500 fluorescence spectrophotometer equipped with a red-
sensitive WRE-343 photomultiplier tube (wavelength range 200−
850 nm). Emission lifetimes were determined by the time-correlated
single-photon-counting technique using an Edinburgh Instruments
mini-τ lifetime spectrophotometer equipped with an EPL 405 pulsed
diode laser at 406.0 nm with a pulse width of 71.52 ps and pulse
periods of 200 and 100 ns and a high-speed red-sensitive
photomultiplier tube (H5773-04) as the detector.
Electrochemistry. Cyclic and square-wave voltammetry experi-
ments were carried out at room temperature using an AutoLab
PGSTAT20 potentiostat and appropriate routines available in the
operating software (GPES, version 4.9). Measurements were carried
out for 2 and 4 in freshly distilled and deoxygenated THF, with a scan
rate of 20 mV/s, and for 1-Zn2 and 3 in freshly distilled and
deoxygenated CH2Cl2, using a scan rate of 100 mV/s, with a solute
concentration of 1.0 mM in the presence of tetrabutylammonium
hexafluorophosphate (0.1 M) as the supporting electrolyte. A three-
electrode cell setup was used with platinum as the working electrode, a
saturated calomel (SCE) reference electrode, and a platinum wire as
the counter electrode.
Computational Methods. Density functional theory (DFT)
calculations19 were performed using the Gaussian 03 program
suite.20 Gas-phase geometry optimizations were carried out by
employing Becke three-parameter exchange in conjunction with the
Lee−Yang−Parr correlation functional (B3LYP).21,22 For geometry
optimizations, the LANL2DZ basis set was used for zinc atoms and the
6-31G(d) basis set for lighter atoms. The input geometries were
modeled using ChemCraft software.23 The optimized minimum-energy
structures were verified as stationary points on the potential energy
surface by vibrational frequency analysis calculation. Time-dependent
DFT (TDDFT) calculations were performed using the gas-phase-
optimized structures in CH2Cl2 with standard dielectric constant ε =
8.93. Solvent effects were evaluated using the polarizable continuum
model implemented in Gaussian 03, in which the cavity is generated
via overlapping spheres, previously developed by Tomasi et al.24,25 For
the TDDFT calculations, the LANL2DZ basis set was used for zinc
atoms and the 6-311G* basis set for lighter atoms. A total of 20
lowest-energy, spin-allowed, singlet transitions were used to simulate
the absorption spectra. Computed structures and molecular orbitals
were visualized and analyzed by ChemCraft software.
1
The product 2 was isolated as a purple solid. Yield: 30 mg (85%). H
NMR (300 MHz, CDCl3, MeOD): δ 8.86 (s, 5H), 8.74 (s, 11H), 8.05
(m, 17H), 7.57 (m, 22H), 6.50 (s, 2H), 4.08 (s, 2H). The absence of
any signal due to the amide groups as well as the pyrrolic hydrogen
atoms of the free-base porphyrin is attributed to H/D exchange caused
by MeOD. 13C NMR (75 MHz, CDCl3, MeOD): δ 150.2, 149.9,
144.0, 143.3, 138.0, 134.9, 134.4, 131.5, 127.1, 126.2, 120.5, 120.3,
118.4, 29.7, 29.3. HRMS (MALDI-TOF). Calcd for C93H62N14O2Zn2:
m/z 1534.3607 ([M + 2H]+). Found: m/z 1534.3576. UV−vis
[CH2Cl2; λ, nm (ε, ×10−3 M−1 cm−1)]: 425 (627.9), 556 (29.5), 596
(12.0). Anal. Calcd for C93H60N14O2Zn2: C, 72.71; H, 3.94; N, 12.76.
Found: C, 72.79; H, 4.02; N, 12.68.
Synthesis of 5-[4-[3-[5-(4-Aminophenyl)-10,15,20-triphenyl-
porphyrinato]-5-(glycine methyl ester)triazinyl]aminophenyl)-
10,15,20-triphenylporphyrin Zinc (3). A THF solution (1 mL) of
Zn[TPP-NH2] (50 mg, 0.072 mmol) was added to a THF solution (1
mL) of cyanuric chloride (13.3 mg, 0.072 mmol) and DIPEA (15 μL,
0.086 mmol) under argon at 0 °C. The mixture was stirred at 0 °C for
15 min, and upon reaction completion (monitored by TLC), it was left
to warm at room temperature. Then, a THF solution (4 mL) of
H2[TPP-NH2] (90.7 mg, 0.144 mmol) and DIPEA (30 μL, 0.173
mmol) was added, and the mixture was stirred at room temperature
overnight. Glycine methyl ester hydrochloride (90.4 mg, 0.72 mmol)
and DIPEA (0.3 mL, 1.9 mmol) were added, and the mixture was
stirred at 65 °C for 24 h. The volatiles were removed under reduced
pressure, and after dilution in CH2Cl2, the residue was purified by
column chromatography (silica gel, CH2Cl2/EtOH 5%). The product
1
3 was isolated as a purple solid. Yield: 38 mg (34%). H NMR (300
MHz, CDCl3): δ 9.05 (d, J = 4.5 Hz, 2H), 8.93 (s, 7H), 8.82 (s, 7H),
8.20 (m, 15H), 8.02 (d, J = 8.1 Hz, 4H), 7.71 (m, 19H), 5.68 (s, 2H),
4.28 (d, J = 5.1 Hz, 2H), 3.77 (s, 3H), −2.75 (s, 2H). The absence of
the hydrogen signals due to the amide group has very low intensity,
and they are probably masked under stronger signals in the aromatic
region. HRMS (MALDI-TOF). Calcd for C94H65N14O2Zn: m/z
1485.4628 ([M + H]+). Found: m/z 1485.4642. UV−vis [CH2Cl2; λ,
nm (ε, ×10−3 M−1 cm−1)]: 421 (696.0), 515 (21.3), 551 (25.0), 591
(10.6), 646 (5.3). Anal. Calcd for C94H64N14O2Zn: C, 75.93; H, 4.34;
N, 13.19. Found: C, 75.82; H, 4.43; N, 13.27.
Cell Fabrication. The DSSCs were fabricated with electrodes
based on fluorine-doped tin oxide (FTO)-coated glass substrates,
which were precleaned with deionized water, acetone, and ethanol and
then dried in air. For the working electrode, TiO2 paste, prepared as
reported earlier,26 was deposited on the FTO substrate by the doctor
blade technique. TiO2 films of different thicknesses were prepared (4,
6, 8, 10, and 12 μm) in order to optimize the photovoltaic
Synthesis of 5-[4-[3-[5-(4-Aminophenyl)-10,15,20-triphenyl-
porphyrinato]-5-glycinetriazinyl]aminophenyl]-10,15,20-tri-
phenylporphyrin Zinc (4). To a THF solution (15 mL) of 3 (30
mg, 0.020 mmol) were added 4.8 mL of MeOH, 6 mL of H2O, and
KOH (450 mg, 0.008 mol). The reaction mixture was stirred at room
temperature overnight. The organic solvents were removed under
reduced pressure, and then a solution of HCl (1.5 M) was added
C
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Scheme 2
Scheme 3
D
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performance of DSSCs. In all cases, the electrode was sintered at 450
°C for 30 min. After cooling to room temperature, it was immersed in
a 20 mM TiCl4 aqueous solution for 30 min at 80 °C, washed with
deionized water and ethanol, sintered again at 450 °C for 30 min, and
cooled at 60 °C. Finally, it was immersed in a porphyrin solution (200
μM in CHCl3/EtOH = 1/1, containing 20 mM chenodeoxycholic acid
as an aggregation inhibitor) for 12 h to give the porphyrin-sensitized
TiO2 working electrode. The platinum wire counter electrode was
prepared by spin-coating drops of a H2PtCl6 solution onto a FTO-
coated glass substrate and heating at 350 °C for 15 min. The DSSCs
were assembled by separating the working electrode from the counter
electrode by a 20-μm-thick Surlyn hot-melt gasket. In the space
between the working and counter electrodes, the electrolyte
[consisting of 0.05 M I2, 0.5 M LiI, 0.6 M (dimethylpropyl)-
benzimidiazole iodide, 0.5 M 4-tert-butylpyridine in an acetonitrile
solution] was introduced through a hole and sealed with Surlyn.
The current−voltage (J−V) characteristics of the DSSCs under
illumination were measured by a Keithley sourcemeter and a solar
simulator coupled with a 150 W xenon lamp and an AM optical filter
to give 100 mW/cm2 illumination at the DSSC surface. The EIS
spectra, in the dark and under illumination, were recorded using an
electrochemical workstation (Autolab PGSTAT) with s frequency
response analyzer. The frequency range was from 10 mHz to 100 kHz,
and an alternating-current potential of 10 mV was used. A direct-
current bias equivalent to the open-circuit voltage of DSSC was
applied. The impedance data were analyzed using Z-View software with
an appropriate equivalent circuit. The IPCE was measured as a
function of the wavelength with a xenon lamp, a monochromator, and
a Keithley sourcemeter at 1 mW/cm2. The intensity calibration for the
IPCE measurement was performed using a standard silicon photo-
diode.
third chlorine atom of cyanuric chloride by a glycine methyl
ester moiety took place at 65 °C in the same pot and gave the
1
symmetrical, free-base porphyrin dyad 1, as confirmed by H
NMR spectroscopy and MALDI-TOF spectrometry. It should
be noted that, in the 1H NMR spectrum of 1, the signals for the
aromatic hydrogen atoms ortho to the H2[TPP-NH2] amino
groups were downfield displaced after attachment to the
triazine ring. Its UV−vis absorption spectrum in CH2Cl2,
except from the Soret band at 419 nm, exhibits four Q bands at
516, 552, 590, and 647 nm, which is characteristic of free-base
porphyrins.
The reaction of 1 with an excess of Zn(CH3COO)2·2H2O in
MeOH/CH2Cl2 gave 1-Zn2, while basic hydrolysis of the
methyl ester group of 1-Zn2, followed by aqueous workup and
chromatographic separation, resulted in almost quantitative
conversion to the symmetrical zinc-metalated dyad 2, as
confirmed by 1H and 13C NMR and UV−vis absorption
spectroscopy and MALDI-TOF spectrometry. The most
1
noticeable feature in the H NMR spectrum of 2, with respect
to the analogous free-base dyad 1, is the disappearance of the
pyrrolic hydrogen atom’s signal at −2.75 ppm. Also, after ester
hydrolysis, the absence of the signal at 3.82 ppm of the methyl
ester protons is obvious. The UV−vis absorption spectrum of 2
in CH2Cl2 shows only two Q bands at 556 and 596 nm, which
is characteristic of zinc-metalated porphyrins.
Synthesis of the unsymmetrical dyad 4 was carried out in a
similar fashion, except that in the initial step cyanuric chloride
was left to react with Zn[TPP-NH2] at 0 °C to give the
metalated monoporphyrin triazine adduct. The free-base
porphyrin was introduced in the following substitution reaction
with H2[TPP-NH2], which was carried out at room temper-
ature. Glycine methyl ester substitution at 65 °C gave
compound 3, which after basic hydrolysis resulted in the
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The syntheses of the
symmetrical dyad Zn[Porph]−Zn[Porph] (2) and unsym-
metrical dyad Zn[Porph]−H2[Porph] (4) are presented in
Schemes 2 and 3, respectively. In both dyads, the porphyrin
units are covalently linked through their peripheral arylamino
groups by a 1,3,5-triazine group. The third position of the
triazine group is functionalized by a carboxylic acid of a glycine
moiety (as an anchoring group for attachment on the TiO2
surface of DSSC electrodes).
1
formation of dyad 4. H NMR, UV−vis absorption spectros-
copy, and MALDI-TOF spectrometry confirmed the purity and
identity of 4. In the 1H NMR spectrum of 4, it is worth noting
the absence of any signal due to the pyrrolic hydrogen atoms of
the free-base porphyrin, which is attributed to H/D exchange,
because a CDCl3/MeOD = 1/0.01 mixture was used in order
to increase the solubility.
The precursor of 1,3,5-triazine, cyanuric chloride, is a
commercially available and inexpensive reagent that has been
extensively used in a variety of chemical transformations27 and
as a template for the synthesis of macrocycles,28 dendrimers,29
and multiporphyrin arrays.30,31 Its reactivity is based on the
temperature-dependent stepwise substitution of its three
chlorine atoms and the sequential introduction of different
nucleophiles. As a result, it allows the use of one-pot protocol
reactions, providing a modular synthetic route to the desired
multiporphyrin arrays in good yields and avoiding extensive and
expensive preparative and purification procedures. Further-
more, it offers a simple route to the challenging synthesis of
unsymmetrical multiporphyrin arrays bearing different chro-
mophores (in our case, zinc-metalated and free-base porphyr-
ins), which are difficult to synthesize with respect to
symmetrical dyads.
Symmetrical dyad 2 was prepared in a one-pot reaction,
starting from cyanuric chloride and H2[TPP-NH2] in the
presence of the base DIPEA at 0 °C in THF. Monitoring of the
reaction by TLC indicated the disappearance of the reactants
and the formation of the free-base monoporphyrin triazine. The
latter was neither isolated nor characterized and further reacted
at room temperature with 1 equiv of H2[TPP-NH2], resulting
in the symmetrical free-base porphyrin dyad. Substitution of the
Photophysical Properties. The UV−vis absorption
spectra of 2 and 4 in solution (0.3 mM in CHCl3/EtOH =
1/1) are shown in black in parts a and b of Figure 1a,
respectively. Both compounds exhibit typical porphyrin
absorption bands, with an intense Soret band in the 400−450
nm range and moderate Q bands in the 540−600 nm range.
The Soret bands are broadened and split into doublets, which
suggests intermolecular exciton coupling between porphyrin
units of different dyads in their excited S2 states.32 The split
Soret bands may also be caused by lowered molecular
symmetry. The Q bands show large molar absorption
coefficients (nearly double those of the monomeric porphyrin
units) and no other additional spectral features, which may be
attributed to the negligible interaction of the porphyrin units in
their S1 states.16,33 The stronger Q bands should be related to
lessened configuration interactions caused by lowered molec-
ular symmetry.8 The additional band observed in the case of the
unsymmetrical dyad 4 at 552 nm may be assigned to the higher
vibrational Q1,0 band of the free-base porphyrin.
The UV−vis absorption spectra of 2 and 4 adsorbed onto
TiO2 films of 8 μm thickness are shown in parts a and b of
Figure 1 (red color), respectively. Both dyads exhibit the usual
Soret- and Q-band absorptions, but compared to their solution
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Figure 2. Fluorescence spectra of (a) 2 and (b) 4 in isoabsorbing
CH2Cl2 solutions (black) and after their adsorption on TiO2 (red).
Figure 1. Normalized UV−vis absorption spectra of (a) 2 and (b) 4 in
a CHCl3/EtOH solution (black) and adsorbed onto a TiO2 films
(red).
transfer process from the zinc-metalated porphyrin units
(which act as electron donors) to the TiO2 surface.35
spectra, these are much broader and shifted. Generally, when
porphyrins are anchored onto the TiO2 surface, they form
either H aggregates, which result in broader Soret bands shifted
to shorter wavelengths (blue shift) relative to their solution
spectra, or J aggregates, which give sharp Soret bands shifted to
longer wavelengths (red shift). The broader and blue-shifted
Soret bands of 2 and 4 provide an indication of H-type
aggregation for the porphyrin dimers on the TiO2 surfaces.34
The broadness and red shift of the Q bands is a sign of
enhanced light-harvesting capacity to longer wavelengths of the
solar spectrum of the two dyads.
Electrochemical Studies. The electrochemical properties
of dyads 2 and 4 were investigated by cyclic voltammetry
measurements. The cyclic voltammograms of both compounds
in THF are depicted in Figures S1 and S2 in the Supporting
Information). Both compounds exhibit two oxidation waves at
1
2
Eox = +1.16 V (quasi-reversible) and Eox = +1.50 V vs NHE
for 2 and Eox1 = +1.16 V (quasi-reversible) and Eox2 = +1.49 V
vs NHE for 4 (Table S1 in the Supporting Information).
1
Furthermore, dyad 2 exhibits one reduction process at Ered
=
−1.13 V vs NHE (quasi-reversible), while for 4, there are two
1
On the basis of the onset absorption edge λonset of the Q
bands of 2 and 4 on thin TiO2 films and using the expression
Eogpt = 1240/λonset, the optical band gaps of 2 and 4 were
calculated as 2.08 and 2.02 eV, respectively.
reduction processes at Ered = −0.89 V (quasi-reversible) and
2
Ered = −1.13 V vs NHE. These data give an indication of
negligible electronic interaction between the two porphyrin
moieties in the ground states of the dyads.
The steady-state fluorescence spectra of 2 and 4 in CH2Cl2
solutions and after their adsorption on TiO2 are depicted in
parts a and b of Figure 2, respectively. In solution, excitation of
the two dyads at 422 nm (Soret band) results in photo-
luminescence, with two weak peaks of equal intensity at 603
and 650 nm for 2 and a strong peak at 655 nm and a moderate
peak at 720 nm for 4 (black). After adsorption of 2 and 4 onto
TiO2, the fluorescence intensities for both dyes are reduced,
providing significant quenching (red). This fluorescence
quenching can be attributed to a photoinduced electron-
The electrochemical properties of the methyl ester analogues
of the carboxylic acid dyads 2 and 4, compounds 1-Zn2 and 3,
were also investigated. The cyclic voltammogram of the
symmetrical dyad 1-Zn2 in CH2Cl2 exhibits two oxidation
and two reduction waves, while that of 3 shows four oxidation
and three reduction waves (Figures S3−S5 and Table S2 in the
Supporting Information). These are due to the usual stepwise
oxidations and reductions by two electrons of the π-ring system
of the porphyrin units of the dyads and not due to electronic
interaction between the two porphyrin moieties of the dyads.
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The additional irreversible oxidation and reduction processes
observed in the voltammogram of 3 might be attributed to
triazine moiety chemical side reactions.
In both 2 and 4, the two porphyrin moieties, bridged by a
triazine group, are nearly perpendicular to each other. In
addition, 2 consists of two zinc-metalated porphyrin units,
while 4 consists of a zinc-metalated and a free-base porphyrin
unit. This structural difference may cause the difference in the
HOMO and LUMO energy levels for each porphyrin moiety,
favoring energy transfer to take place between the two moieties
and toward the carboxylic acid group,38 when dyad is excited
with solar radiation.
DFT Calculations. To gain further insight into the
electronic properties of 2 and 4, DFT calculations at the
B3LYP/6-31G level of theory were performed. The gas-phase
geometry-optimized structures of the dyads are shown in Figure
4, while their optimized coordinates are provided in Tables S3
For a sensitizer used for DSSC, suitable highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels are required to enable efficient
dye regeneration and electron injection processes. The HOMO
and LUMO energies correspond to the first oxidation potential
Eox and first reduction potential Ered of the sensitizer,
respectively. In order to ensure efficient electron injection
from the photoexcited sensitizer into the TiO2 conduction
band (CB), the LUMO energy level of the sensitizer (Ered)
should be higher than the TiO2 CB edge (−0.5 V vs NHE). In
addition, for efficient electron regeneration of the sensitizer
radical cation (after photoinduced electron injection), the
HOMO energy level of the sensitizer (Eox) should be lower
than the potential of the electrolyte redox I−/I3 couple (+0.4
−
V vs NHE).36 As shown in Figure 3, in which the
Figure 3. Electrochemical potential diagram showing the electron flow
in 2- and 4-based DSSCs.
Figure 4. Geometry-optimized structures of 2 and 4 in the gas phase.
electrochemical potential data of 2, 4, the TiO2 CB edge, and
the I−/I3− couple are depicted, the reduction potentials Ered of 2
and 4 are more negative than that of the TiO2 CB edge and
hence electron injection from the photoexcited dyads into the
TiO2 CB is thermodynamically favored (Table 1; ΔGinj < 0). In
addition, their oxidation potentials (Eox) are more positive than
and S4 (Supporting Information), respectively. Both com-
pounds exhibit a “butterfly-like” structure, in which the
porphyrin units are attached in a perpendicular fashion to the
triazine frame and they are slightly twisted against each other.
The electronic density distributions in the frontier orbitals of
2 and 4 are presented in Figure 5. In both dyads, no
delocalization of the electronic density is observed along the
two porphyrin units and the π-conjugated triazine linker, which
suggests a very weak electronic communication between the
two porphyrin units. This is an immediate consequence of the
structures of the two dyads, in which the two porphyrin units
are not coplanar with the triazine frame. HOMO and HOMO−
1 of the symmetrical dyad 2 (Figure 5a) have very similar
energies, and their electron densities are delocalized over both
zinc-metalated porphyrins but unequally distributed (mainly on
−
the redox potential of the I−/I3 couple, which means that
electron acceptance of the oxidized dyads from the electrolyte
−
I−/I3 couple is thermodynamically feasible (Table 1; ΔGreg
<
0). Consequently, there is sufficient driving force for
regeneration of the oxidized dyads 2 and 4 in DSSCs.
The HOMO−LUMO band gaps of 2 and 4 calculated from
these electrochemical data are found to be 2.29 and 2.05 eV,
respectively (Table 1). These are larger than the optical band
gaps estimated from the photophysical measurements, but this
is a common feature for organic dyes of this type, which can be
attributed to solvent effects.37
Table 1. Electrochemical Data and Calculated HOMO and LUMO Energy Levels, Band Gaps, and ΔG Values for Electron
Injection and Regeneration Processes for 2 and 4
a
a
b
b
b
c
c
compound
Eox /V
Ered /V
HOMO /eV
LUMO /eV
band gap /eV
ΔGinj /eV
ΔGreg /eV
2
4
1.16
1.16
−1.13
−0.89
−5.56
−5.56
−3.27
−3.51
2.29
2.05
−0.63
−0.39
−0.76
−0.76
a
b
Observed first oxidation and reduction potentials (vs NHE). HOMO and LUMO energy levels and band gaps calculated according to the
c
relationships EHOMO = −(Eox + 4.4) eV and ELUMO = −(Ered + 4.4) eV, measured with reference to the redox potential −4.4 eV. ΔGinj for electron
injection from the LUMO of the dyads to the TiO2 CB (−0.5 V vs NHE) and ΔGreg for regeneration of the dyad radical cations by the I−/I3− redox
couple (0.4 V vs NHE).
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This method has been extensively used for simulating the
absorption spectra, as well as for assigning the spectral
transitions, of porphyrins.39 In order to obtain deeper insight
into the electronic transitions of 2 and 4, TDDFT calculations
were carried out in CH2Cl2 as the solvent medium. The
calculation for the symmetrical dyad 2 with two zinc-metalated
porpyrin units revealed two closely spaced intense peaks at
395.7 nm (f = 1.73) and 395.6 nm (f = 1.95) (Figure 6a),
Figure 5. Frontier molecular orbitals of (a) 2 and (b) 4 and
corresponding energy levels from DFT calculations in CH2Cl2.
one of the zinc-metalated porphyrins), whereas LUMO and
LUMO+1 are degenerate and the electron density is localized
on one of the zinc-metalated porphyrins. For the unsym-
metrical dyad 4 (Figure 5b), the HOMO electron density is
localized only on the zinc-metalated porphyrin, while LUMO
and LUMO+1 are almost degenerate and the electron density is
localized only on the free-base porphyrin. Considering that a
zinc-metalated porphyrin acts as an electron donor (D) and a
free-base porphyrin unit as an electron acceptor (A), the dyads
2 and 4 can be described as D−π−D and D−π−A systems,
respectively, where π denotes the triazene−π system. It should
be noted that, for efficient application of a sensitizer in a DSSC,
in order to have a successful electron injection into the TiO2
CB, the LUMO electron density of the sensitizer should be
localized on (or close to) A, which is usually the anchoring
group. However, in both dyads, the LUMO is localized on one
of the porphyrin units and not on the anchoring carboxylic acid
group of the glycine moiety.
Figure 6. TDDFT-calculated vertical transitions of (a) 2 and (b) 4 in
CH2Cl2.
which involve contributions primarily from HOMO−3 →
LUMO, HOMO−2 → LUMO+2, HOMO−2 → LUMO+3,
and HOMO−3 → LUMO+1 and correspond to the π−π*
transitions of the zinc-metalated porphyrin Soret band. The
vertical transitions calculated at 557.2 nm (f = 0.1665) and
556.2 nm (f = 0.045) correspond to the Q-band transitions of
the zinc porphyrin units. These are π−π* transitions that have
primary contributions from HOMO → LUMO+3 and
HOMO−2 → LUMO+2 transitions.
In the case of 4, a larger number of peaks are observed than
those in 2 in the Soret-band as well as in the Q-band regions
(Figure 6b), presumably because of its asymmetrical structure
(which consists of a free-base porphyrin and a zinc-metalated
The HOMO−LUMO gap of 2 is estimated to be 2.71 eV,
which is close to the value of 2.29 eV, calculated from the
experimentally observed oxidation Eox and reduction potentials
Ered (Table 2). Similarly, the theoretically estimated HOMO−
LUMO gap of 4 in a CH2Cl2 solution is in good agreement
with the experimental value.
TDDFT calculations have been an important tool for the
prediction of electronic transitions for many types of molecules.
Table 2. Calculated HOMO and LUMO Levels and Band Gaps from DFT Calculations in the Solution Phase and
Electrochemical Data for 2 and 4
a
a
a
b
b
b
compound
HOMO /eV
LUMO /eV
band gap /eV
HOMO /eV
LUMO /eV
band gap /eV
2
4
−5.56
−5.56
−3.27
−3.51
2.29
2.05
−5.20
−5.21
−2.49
−2.64
2.71
2.57
a
b
Calculated from electrochemical data. Estimated from DFT calculations in the solution phase.
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Table 3. Photovoltaic Parameters of 2- and 4-Based DSSCs
a
b
c
d
e
f
g
compound
Jsc /(mA/cm2)
Voc /V
FF
PCE /%
dye loading/(mol/cm2)
τe /ms
τd /ms
ηcc
2
4
8.82
0.63
0.66
0.65
0.68
3.61
4.46
2.42 × 10−6
16
12
8
0.57
9.94
b
2.68 × 10−6
24
0.75
a
c
d
e
f
g
Short-circuit current. Open-circuit voltage. Fill factor. Photoconversion efficiency. Electron lifetime. Electron-transport time. Charge
collection efficiency calculated by ηcc = [1 + (τd/τe)]−1 (photoanode = TiO2 of 10 μm thickness).
porphyrin unit). The asymmetrical structure of 4 is also evident
by its calculated dipole moment of 2.1762 D, which is higher
than that of the symmetrical dyad 2 (1.8696 D). The observed
peaks at 404.3 nm (f = 1.11) and 397.8 nm (f = 1.41) have
major contributions from HOMO−3 → LUMO+1, HOMO−3
→ LUMO, and HOMO−1 → LUMO+1 and can be assigned
to π−π* transitions of the free-base porphyrin unit Soret band.
Another peak at 395.2 nm (f = 1.21), consisting of transitions
from HOMO → LUMO+2, HOMO−2 → LUMO+2, and
HOMO → LUMO+3, can be assigned to the Soret band of the
zinc porphyrin unit. The calculated Q-band peaks are observed
at 583.8, 556.4, 556.2, and 550.4 nm and can be attributed to
π−π* transitions of the two porphyrin moieties.
FTIR Spectra. The triazine group of the dyads 2 and 4 is
functionalized by a carboxylic acid of a glycine moiety, as the
anchoring group for attachment onto the TiO2 surface of
electrodes of solar cells. To get information about the
anchoring of 2 and 4 onto the TiO2 surface, FTIR powder
spectra of 2 and 4, in pure form and adsorbed onto TiO2 films,
were recorded (representative FTIR spectra of 2 and 2
absorbed onto TiO2 are shown in Figure S12 in the Supporting
Information). Both dyads in pure form exhibit very strong
absorption peaks at 1687 cm−1, which correspond to the
ν(CO) stretching vibrations of the carboxylic acid groups. In
the corresponding spectra of the dyad/TiO2 films, these peaks
are shifted to lower frequencies at 1645 and 1414 cm−1
[assigned as νasym(COO−) and νsym(COO−), respectively],
which is a sign of the strong binding and electronic coupling of
2 and 4 through their carboxylic acid groups on the TiO2
surface.40
Photovoltaic Performance. 2- and 4-based DSSCs have
been fabricated, and their photovoltaic cell parameters are
summarized in Table 3. Their current−voltage (J−V) curves
and the corresponding IPCE spectra are shown in parts a and b
of Figure 7, respectively. The 4-based solar cell yields a PCE
value of 4.46%, while the 2-based solar cell gives a lower value
of 3.61%. This difference can be primarily attributed to the
enhanced short-circuit current Jsc and second to the higher
open-circuit voltage Voc and fill factor FF values of the 4-based
cell.
Figure 7. (a) Current−voltage (J−V) curves under illumination and
(b) IPCE spectra of 2- and 4-based DSSCs.
The higher Jsc value for DSSC based on the 4 dye is
consistent with its larger IPCE value. The IPCE values at
wavelength 590 nm of 2- and 4-sensitized DSSCs were found
to be 43% and 51%, respectively (Figure 7b). IPCE is defined
as
Another critical factor in determining the PCE and IPCE
values of a DSSC is the dye loading on the TiO2 surface.41 To
obtain the dye loading value of the photoanode, the dye-
sensitized TiO2 layer was immersed into a mixture of CH2Cl2
and NaOH(aq). The total amounts of dye adsorbed onto the
TiO2 surface, i.e., dye loading, were obtained by measuring the
absorption spectra of dissolved porphyrin in the mixture
solvent. The dye loading values for 2 and 4 were found to be
2.42 × 10−6 and 2.68 × 10−6 mol/cm2, respectively. The IPCE
value is dominated by LHE = (1 − 10−abs), for which the
absorbance (abs) is linearly dependent on both the dye loading
on TiO2 and absorption coefficient. The IPCE values at both
Soret and Q bands are larger for 4 than 2. As shown in the
absorption spectra of porphyrin/TiO2 (Figure 1a,b) and dye
loading values, the LHE of the 4-based cell is larger than that
IPCE = LHE × η η
inj cc
where LHE is the light-harvesting efficiency (which depends on
the absorption of the sensitized sensitizer), ηinj is the electron-
injection efficiency (which depends on the electron-injection
rate from the sensitizer into the TiO2 CB), and ηcc is the charge
collection efficiency (which depends on the diffusion rate of
electrons in TiO2 and the recombination rate between the
electrons and redox couple of the electrolyte).
I
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for 2. We assume that, in addition to a larger amount of dye
loading to give larger LHE for 4, the possibility of a lower TiO2
CB edge upon dye uptake of the 4-based DSSC might lead to a
larger injection rate to give a higher electron yield for this
device. This may also be related to the formation of a blocking
layer caused by the higher dye loading.42
In general, the Voc value is determined by the difference
between the Fermi level of electrons in TiO2 and the redox
potential level of a redox couple in the electrolyte. Because the
potential levels of the redox couples in these devices are the
same, the value of Voc depends on the Fermi levels for electrons
after electron injection in both porphyrin systems. The Voc
value of the DSSC can be determined from the expression43
D−π−A, respectively, with their anchoring group located on
the π linker. As a result, although triazine mediates effectively
intramolecular electron-transfer and charge-separation phenom-
ena between the porphyrin units in the dyads and promotes
electron injection into the TiO2 electrode, the electron flow is
not as efficient as that in the “push−pull” systems.
The larger PCE value of the 4-based solar cell can be
attributed to its unsymmetrical structure with respect to the
symmetrical structure of 2, which consists of two identical zinc-
metalated porphyrin units. Asymmetry has been proposed as
the reason for increased efficiency of solar cells sensitized by
other porphyrin dyads17 as well as other non-“push−pull”
porphyrins.13,45 The completely symmetrical system of 2
provides no directionality for the electron-transfer process.
However, in the case of the unsymmetrical system of 4, the
presence of both zinc-metalated and free-base porphyrin units
causes a polarizing and cooperating effect, directing electron
transfer toward TiO2 and therefore leading to more efficient
electron-injection process.
In order to gain a better understanding of the observed
photovoltaic parameters of 2- and 4-based DSSC properties,
EIS was performed to analyze the effects on charge generation,
transport, and collection.46 Figure 8 depicts the EIS spectra of
2- and 4-based DSSCs, in the dark under a forward bias of 0.65
V. In general, the Nyquist plots of EIS spectra in the dark, in
the frequency range 0.1−10 kHz, show three semicircles. The
semicircle radius in the high-frequency range (left side of the
spectrum) represents the charge-transfer resistance at the
⎛
⎜
⎝
⎞
⎟
⎠
⎛
⎜
⎝
⎞
⎟
⎠
⎛
⎜
⎝
⎞
⎟
⎠
⎛
⎜
⎝
⎞
⎟
⎠
ECB
q
nc
Ered
q
kT
q
V =
+
ln
−
oc
N
CB
where ECB is the TiO2 CB edge, Ered is the electrolyte redox
potential, kT represents the thermal energy, and nc and NCB are
the number of electrons and density of states in the TiO2 CB,
respectively. After adsorption of the dye on the TiO2 surface, a
shift of ECB takes place and ΔECB is given by the expression43
−qμnormalγ
ΔECB
=
ε0ε
where μnormal represents the dipole moment of the adsorbed
dye vertical to the TiO2 surface, γ is the concentration of dye
molecules adsorbed on the TiO2 surface (dye loading), and ε0
and ε are the permittivity of the dye under vacuum and
dielectric constant on the dye monolayer, respectively. From
the above equation, it is apparent that a dye with large μnormal
and γ yields high Voc. Because the dipole moment of 4 is larger
than that of 2 and the 4-based solar cell exhibits higher dye
loading, then ΔECB is larger for the 2-based DSSC and
consequently the 4-based DSSC is expected to result in a
higher Voc value.
On the basis of the J−V characteristics obtained in the dark
(Figure S13 in the Supporting Information), the DSSC based
on 4 shows a lower current than the 2-sensitized DSSC.
Because of the fact that the dark current density is governed by
the impedance of the electron recombination between TiO2
and the electrolyte impedance of the electron recombination
between TiO2 and the electrolyte governs the dark current
density, the adsorbed porphyrin dye molecules on TiO2 might
play a role, by increasing the interfacial impedance depending
upon the degree of coverage of sensitizing dye on the surface of
TiO2. According to the results of dye loading, the larger dye
loading on the TiO2 surface in the 4-based DSSC behaves like a
blocking layer, which prevents charge recombination between
−
TiO2 and I3 of the electrolyte. However, for 2, which has a
−
lower surface coverage on the TiO2 surface, I3 in the
electrolyte can penetrate closely to the surface of TiO2 to
decrease the barrier for electrons to recombination with I3− and
to result in a higher dark current.
From the above, it is evident that both dyads 2 and 4 can act
as efficient sensitizers for DSSCs. However, the PCE values
reported herein are lower than those reported for DSSCs
sensitized by “push−pull” multiporphyrin arrays.44 The reason
for this difference lies on the different electron structures of the
sensitizers. The structure of a “push−pull” is a D−π−A system,
with the anchoring group residing on A, which results in very
effective electron flow into the TiO2 CB. The structures of
dyads 2 and 4 presented herein are described as D−π−D and
Figure 8. EIS spectra of (a) Nyquist and (b) Bode plots of 2- and 4-
based DSSCs, measured at a bias voltage of 0.65 V under dark
conditions.
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counter electrode/electrolyte interface. The charge recombina-
tion resistance at TiO2/dye/electrolyte corresponds to the
semicircle in the middle-frequency range. In the low-frequency
range, the impedance is associated with the Warburg diffusion
process of I−/I3− in the electrolyte. As can be seen in Figure 8a,
the semicircle radius in the middle-frequency range is larger for
the DSSC based on 4 compared to 2, indicating that the charge
recombination rate is slower for the former device.
The values of the electron lifetime and electron-transport
time are listed in Table 3. The electron-transport time is a
measure of the average time taken for the collection of injected
electrons, and a faster electron-transport time is associated with
a higher photocurrent because it indicates that the electrons
hop across the TiO2 network and are collected at the
photoanode at a faster rate.49 The decrease in Rtran and the
increase in Rrec for the DSSC sensitized with 4 indicate that the
electron-transport time has decreased for the DSSC based on 4.
This means that the electrons are reaching the FTO substrate
in the DSSC sensitized with 4 faster than they are with 2
because the electron-transport rate is enhanced.
Furthermore, the photovoltaic performance was evidently
reflected by the charge collection efficiencies ηcc = [1 + (τd/
τe)]−1,50 the values of which were found to be 0.57 and 0.75 for
DSSCs sensitized with 2 and 4, respectively (Table 3). A higher
charge collection efficiency value leads to an improved PCE
value in the case of the 4-based DSSC.
The electron lifetime was calculated from the peak frequency
(fpeak) at the intermediate-frequency region in a Bode phase
plot (Figure 8b) according to τe = 1/2πfpeak. The electron
lifetimes for the DSSCs based on 2 and 4 are 16 and 24 ms,
respectively, indicating that the charge recombination is
suppressed for 4, compared to 2. The higher electron lifetime
for 4 relative to that of 2 could be attributed to the D−π−A
−
nature of 4, which disturbs the dispersion forces by trapping I3
near the porphyrin more effectively, preventing the approach of
−
I3 to the linker group and the surface region. In this case, the
distance from the surface of TiO2 to the trapped I3− is expected
−
to be longer than the distance between the surface and the I3
attached to the conjugated linker of 2, explaining the longer
electron lifetime of DSSC based on 4 compared to 2. In
contrast, 2, which behaves as a D−D porphyrin dye, could
increase the surface I3− concentration by attracting the acceptor
to its linker through dispersion forces, leading to a shorter
electron lifetime and increasing the back-recombination of
injected electrons in TiO2.47
To get the more information about the charge-transport
mechanism in the DSSCs, we have also performed EIS
measurements under illumination at the bias voltage of 0.65
V, and the Nyquist plots for DSSCs based on 2 and 4 are
shown in Figure 9. The semicircle in the intermediate-
CONCLUSIONS
■
In summary, we have synthesized two novel porphyrin−
porphyrin dyads, the symmetrical Zn[Porph]−Zn[Porph] (2)
and unsymmetrical Zn[Porph]−H2[Porph] (4), which are
covalently linked through arylamino groups at their peripheries
by 1,3,5-triazine, by taking advantage of the stepwise and
temperature-dependent reactivity of cyanuric chloride. The
dyads are also functionalized by a terminal carboxylic acid
group of a glycine moiety attached to the triazine group. The
study of the photophysical and electronic properties of the two
compounds revealed not significant electronic interaction in
their ground states but broadened spectral absorptions and
suitable frontier orbital energy levels for use in DSSCs. 2- and
4-based DSSCs were fabricated, demonstrating efficient
intramolecular electron-transfer and charge-separation phenom-
ena mediated by the triazine group and PCE values of 3.61%
and 4.46%, respectively. The higher PCE value of the 4-based
DSSC is attributed to its larger dye loading and higher Jsc, Voc,
and FF values. On the basis of the DFT results, the
unsymmetrical structure of 4 does not have localized frontier
molecular orbital patterns, but the direction of the HOMO/
LUMO pattern shifts is not aligned with that of electron
injection. The J−V curves of the two cells as well as their IPCE
and EI spectra demonstrated enhancement of the short-circuit
current (Jsc) under illumination for the 4-based DSSC, due to
shorter electron-transport time (τd), longer electron lifetime
(τe), and high charge recombination resistance, as well as larger
dye loading onto the TiO2 surface. Further improvement of the
PCE values of DSSCs reported herein may be achieved by
using a modified photoanode, and we are currently working in
this direction.
Figure 9. Nyquist plots of EIS spectra measured at a bias voltage of
0.65 V under illumination, for 2- and 4-based DSSCs.
frequency region corresponds to the charge-transport mecha-
nism at the TiO2/dye/electrolyte interface after photo-
excitation. The diameter of this semicircle is the measure of
the charge-transport resistance (Rtran). It can be seen that the
diameter of this semicircle is smaller for the DSSC sensitized
with 4 than that for 2, resulting in a low value of Rtran, indicating
fast transport of electrons in the TiO2 layer toward the FTO
electrode. The electron-transport time (τd) of an injected
electron in the TiO2 film is related with τe, Rrec, and Rtran as
follows:48
ASSOCIATED CONTENT
■
S
* Supporting Information
Complementary data of cyclic voltammograms, H and 13C
1
NMR spectra of compounds 1, 1-Zn2, 2, 3, and 4, FTIR spectra
of 2 and 2 absorbed on TiO2, dark current-voltage character-
istics of 2- and 4-based DSSCs, and optimized coordinates from
DFT calculations for 2 and 4. This material is available free of
charge via the Internet at http://pubs.acs.org.
τ
Rtranr
Rrec
d
=
τ
e
K
dx.doi.org/10.1021/ic400774p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Torre, G.; Torres, T. Chem. Commun. 2010, 46, 7090. (d) Ball, J. M.;
Davis, N. K. S.; Wilkinson, J. D.; Kirkpatrick, J.; Teuscher, J.; Gunning,
R.; Anderson, H. L.; Snaith, H. J. RSC Adv. 2012, 2, 6846.
(10) Campbell, W. M.; Jolley, K. W.; Wagner, P.; Wagner, K.; Walsh,
P. J.; Gordon, K. C.; Schmidt-Mende, L.; Nazeeruddin, M. K.; Wang,
AUTHOR INFORMATION
Corresponding Author
*E-mail: coutsole@chemistry.uoc.gr (A.G.C.), gdsharma273@
gmail.com (G.D.S.).
■
Notes
Q.; Gratzel, M.; Officer, D. L. J. Phys. Chem. C 2007, 111, 11760.
(11) Bessho, T.; Zakeeruddin, S. M.; Yeh, C.-Y.; Diau, E. W.-G.;
̈
The authors declare no competing financial interest.
Gratzel, M. Angew. Chem., Int. Ed. 2010, 49, 6646.
̈
(12) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;
Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.;
ACKNOWLEDGMENTS
■
Financial support from the European Commission (FP7-
REGPOT-2008-1, Project BIOSOLENUTI No. 229927) is
greatly acknowledged. This research has also been cofinanced
by the European Union (European Social Fund) and Greek
national funds (Heraklitos II) through the Operational
Program “Education and Lifelong Learning” of the National
Strategic Reference Framework Research Funding Program
(THALIS-UOA-MIS 377252).
Gratzel, M. Science 2011, 334, 629.
̈
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