Synthesis of porphyrin sensitizers with a thiazole group as an efficient π-spacer: Potential application in dye-sensitized solar cells

Article abstract of DOI:10.1039/c6ra00353b

Herein, we report porphyrin sensitizers for DSSCs, coded CNU-OC8 and CNU-TBU, which were synthesized using a donor-π-bridge-acceptor approach. The porphyrin sensitizers were subjected to electrochemical experiments to study their electron distribution, in

Full text of DOI:10.1039/c6ra00353b

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Synthesis of porphyrin sensitizers with a thiazole group as an
efficient π-spacer: Potential application in dye-sensitized solar
cells
Received 00th January 20xx,
Accepted 00th January 20xx
Rangaraju Satish Kumara, Hansol Jeongb,c, Jaemyeng Jeonga, Ramesh Kumar Chitumallae, Min Jae
DOI: 10.1039/x0xx00000x
Kob,d, Kempahanumakkagaari Suresh Kumara, Joonkyung Jange and Young-A Son a,*
www.rsc.org/
Herein, we report porphyrin sensitizers for DSSCs, coded CNU-OC8 and CNU-TBU, which were synthesized using a donor-π-
bridge-acceptor approach. The porphyrin sensitizers were subjected to electrochemical experiments to study their
electron distribution, intramolecular charge transfer and HOMO-LUMO levels. The optical and photovoltaic properties of
these synthesized porphyrins were measured and compared with those of the YD2-OC8 benchmark dye. To further
characterize, we simulated the electrochemical and optical properties of the dyes, which are perfectly in agreement with
the experimental data. The new CNU-OC8 and CNU-TBU porphyrin sensitizers provided power conversion efficiencies of
6.49% and 3.19%, respectively, compared to a conversion efficiency of 6.10% for YD2-OC8 under similar conditions. These
results indicate that CNU-OC8 exhibits better photovoltaic performance than the benchmark YD2-OC8 sensitizer in a liquid
I−/I3− redox electrolyte.
mA·cm-2 for every 100 nm, which represents a maximum
cumulated JSC value of
∼
35 mA·cm−2. Therefore, an ideal
Introduction
production of extreme photocurrent should be possible with
dye that can absorb maximum sunlight in the range of 400-900
nm.
Dye-sensitized solar cells (DSSCs) are fascinating and of
extensive interest for the conversion of sunlight into electricity
because of their low cost, high conversion efficiency, colorful
nature, ease of fabrication and potential usefulness in solving
environmental problems.1-7 Dye-sensitized solar cells have
garnered considerable attention since the first report of DSSCs
by Grätzel and O’Regan in 1991.8 A highly efficient metal-
organic dye based on a ruthenium (Ru) sensitizer has produced
solar-energy-to-electricity conversion efficiencies (η) greater
than 11% under illumination with standard AM 1.5 sunlight.9-11
Although Ru complexes are suitable as photosensitizers,
because of their low molar extinction coefficient for incident
light with wavelengths greater than 600 nm, the low natural
abundance of Ru and related environmental issues limit their
extensive application. On the other hand, porphyrin dyes
shown high molar extinction coefficients and promising
properties with easy synthetic conversion ability to be a good
sensitizers for DSSCs.12-24
The first porphyrin used for the sensitization of
nanocrystalline TiO2 was [tetrakis(4-carboxyphenyl) porphyrin-
ato] zinc(II), with an overall conversion efficiency of 3.5%.25
Then, zinc porphyrin YD2-OC8 is co-sensitized with an Y123
using
a cobalt-based electrolyte that reached a power
conversion efficiency of 12.3%,14 SM315 has been
demonstrated to exhibit a conversion efficiency of 13%.16
Recently, thiophene or furan,26,27 tropolone,28 and pyridine-
based compounds29 have been used as acceptor anchoring
groups with good conversion efficiency properties. Plater and
coworkers synthesized the thiazole π-spacer porphyrin
molecules,30 Jin Yong Lee and Karthikeyan given the
theoretical calculations for thiazole spacer containing
porphyrins.31 Thiazole is an electrondeficiency unit with
coplanarity and a significant electron-withdrawing ability. It
has been widely applied in many fields, thiazole electron-
deficiency is because it contains one electron-withdrawing
nitrogen of the imine (–C=N) in place of the carbon atom at the
3-position of thiophene.32-35 Here in with the introduction of
thiazole into the dyes as π-spacer, we planned the
collaborative electron-withdrawing of thiazole and carboxylic
acid may increase spectral response through lengthening π-
conjugation and increasing the withdrawing nature, while
maintaining good charge injection.36
In general, the absorption spectra of porphyrin dyes show
a Soret band (B-band) with strong absorption at 400-450 nm
and low-intensity Q-bands at 500-650 nm; notably, the 500-
800 nm region is the most intense, photon-rich wavelength
region of sunlight. According to AM1.5G solar simulations, the
expected current density between 400 and 900 nm is
∼7
Finally based on the above consideration, we have
designed and synthesized two new efficient porphyrin
sensitizers (CNU-OC8 and CNU-TBU shown in Fig. 1, with
diarylamine as the donor, thiazole as a π-spacer, and a
carboxylic acid group acting as the anchoring group. Here, in
the present manuscript, we further extend this strategy to
describe the DSSC properties CNU-OC8 and CNU-TBU, which
feature a thiazole carboxylic acid that acts as a good acceptor
and anchoring group; additionally, we compare the
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performance of these compounds with that of the YD2-OC8
porphyrin dye (Fig. 1). We also performed theoretical
TIPS compound
4
(0.2 g, 0.128 mmol) was dissolved in 20
mL dry THF, and the resulting solution TBAF was added (0.16
mL, 1 M THF) at 0°C. The reaction mixture was stirred for 15
min under inert atmosphere and subsequently quenched with
20 mL water and extracted with DCM (2 × 50 mL). The
combined organic layers were dried over anhydrous Na2SO4.
The solvent was removed under reduced pressure. To the
obtained residual compound 2-bromothiazole-5-carboxylic
acid (0.266 g, 1.28 mmol) was added and dissolved in dry THF
(20 mL). NEt3 was then added to the reaction mixture (8 mL),
which was subsequently degassed with Ar for 15 min. To the
combined reaction mixture, Pd(dba)2 (0.044 g, 0.0768 mmol)
and AsPh3 (0.137 g, 0.45 mmol) were added and refluxed for
12 h under an Ar atmosphere. After the reaction was
completed, the solvent was removed under reduced pressure.
The resulting residue was purified by silica-gel column
chromatography using DCM:CH3OH (20:1) as the eluent to
obtain a solid, which was further washed with CH3OH to give
1
(0.112 g, 57.81%) as a green solid. H NMR (600
compound
1
MHz, CDCl3 + 1 drop pyridine-d5, ppm) δ 9.47 (d, J = 4.5 Hz,
2H), 8.94 (d, J = 4.7 Hz, 2H), 8.73 (d, J = 4.5 Hz, 2H), 8.51-8.48
(m, 2H), 6.98-6.83 (m, 10H), 6.78-6.66 (m, 5H), 3.73 (t, J = 4.8
Hz, 8H), 2.34 (t, J = 7.5 Hz, 4H), 1.51 (s, 4H), 1.46-0.29 (m, 78H).
13C NMR (125 MHz, CDCl3 + 1 drop pyridine-d5, ppm) δ 159.7,
152.1, 151.2, 150.2, 150.2, 134.7, 134.1, 132.1, 131.4, 130.2,
129.7, 129.5, 128.3, 123.3, 121.5, 114.3, 105.1, 100.1, 68.4,
45.2, 35.1, 32.0, 31.6, 31.3, 29.5, 29.2, 29.0, 28.5, 28.4, 25.0,
22.5, 22.4,22.2, 13.9, 13.7. UV-Vis [0.01 mM, λmax (nm) & ε
(mol-1cm-1)] 453 (2, 05, 842), 575 (14,495) and 650 (31.891).
FT-IR (KBr) 3344, 3093, 2952, 2184, 1724, 1584, 1296, 1094,
933, 791, 713. HR-MS (ESI-MS) m/z calcd. For
C94H119N6O6SZn[M+H]+: 1524.8133, found 1524.8135.
Fig. 1 Molecular structures of the CNU-OC8, YD2-OC8, and CNU-TBU porphyrin dyes.
calculations to verify the electrochemical and photophysical
properties of the two dyes, CNU-OC8 and CNU-TBU.
Experimental Section
Materials and Characterization
Synthesis of CNU-TBU (2)
All solvents and reagents (analytical and spectroscopic grades)
were commercially obtained and used as received unless
otherwise noted. An AVANCE III 600 spectrometer was
TIPS compound
5 (0.2 g, 0.158 mmol) was taken in 20 mL
THF and was added TBAF (0.16 mL, 1 M THF) at 0°C. The
reaction mixture was stirred for 15 min under inert
atmosphere and subsequently quenched with 20 mL of water
and extracted with DCM (2 × 50 mL). The combined organic
layers were dried over anhydrous Na2SO4. The solvent was
removed under reduced pressure. To the obtained residual
compound, 2-bromothiazole-5-carboxylic acid (0.332 g, 1.6
mmol) was added and dissolved in dry THF (20 mL). NEt3 was
then added to the reaction mixture (8 mL), which was
subsequently degassed with Ar for 15 min. To the combined
reaction mixture Pd(dba)2 (0.055 g, 0.095 mmol) and AsPh3
(0.169 g, 0.553 mmol) were added and refluxed for 12 h under
an Ar atmosphere. The solvent was removed under reduced
pressure after completion of the reaction. The resulting
residue was purified by silica-gel column chromatography
using DCM:CH3OH (20:1) as an eluent to obtain a solid, which
1
operated at 600 MHz for H NMR and 150 MHz for 13C NMR
(Akishima, Japan); the Alice 4.0 software and CDCl3 and
pyridine-d5 (Sigma–Aldrich) were used as solvents in both
cases. The chemical shifts (δ values) are reported in ppm as
downfield from an internal standard (Me4Si) (1H and 13C NMR).
HRMS spectra were obtained using a micrOTOF-Q
Ⅱ mass
spectrometer. UV–Visible absorption spectra were recorded
using an Agilent 8453 spectrophotometer. The electrochemical
measurements, i.e., CV experiments, were conducted using a
VersaSTAT 3 instrument. The electrolyte used was anhydrous
methylene chloride along with 0.1 M TBAP as a supporting
electrolyte. The electrodes used included a platinum disc as
the working electrode, a Ag/Ag+ electrode as the reference
electrode and platinum wire as the counter electrode.
was further recrystallized from CH3OH to give compound
2
(0.124 g, 63.29%) as a green solid. 1H NMR (600 MHz, CDCl3 + 1
drop pyridine-d5, ppm) δ 9.71 (d, J = 4.6 Hz, 2H), 9.23 (d, J = 4.6
Hz, 2H), 8.92 (d, J = 4.5 Hz, 2H), 8.74 (d, J = 4.5 Hz, 2H), 7.97(s,
Synthesis
Synthesis of CNUOC8 (1)
2 | J. Name., 2012, 00, 1-3
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Fig. 2 Synthesis strategies for the preparation of the new porphyrin dyes 1 and 2.
4H),
7.76
(s,
2H),
7.22-7.16
(m,
5H), 6.94 (d, J = 8.4 Hz, 4H), 2.46 (t, J = 7.5 Hz, 4H), 1.50 (s, modeled as such without any modifications. We obtained the
36H), 1.33-1.18 (m, 16H), 0.83 (t, J = 7.1 Hz, 6H). 13C NMR (125 electron density distribution isosurfaces of the two dyes, by
MHz, CDCl3 + 1 drop pyridine-d5, ppm) δ 152.0, 151.8, 150.2, performing population analysis in dichloromethane solvent
149.9, 149.7, 148.1, 134.3, 133.0, 132.5, 130.4, 129.3, 128.4, medium. To characterize the photophysical properties of the
121.6, 120.4, 114.3, 105.1, 58.4, 45.2, 34.7, 34.5, 31.2, 31.0, two Zn-porphyrin dyes, we performed TDDFT simulations on
29.3, 28.6, 23.6, 22.1, 19.3, 13.6, 13.2. UV-Vis [0.01 mM, λmax the ground state optimized structures. The vertical excitation
(nm) & ε (mol-1cm-1)] 448 (2, 11, 388), 578 (12,711) and 655 energies of the two dyes for the first 25 vertical singlet
(34,775). FT-IR (KBr) 3359, 3026, 2954, 2185, 1736, 1696, excitations were calculated in tetrahydrofuran solvent
1590, 1433, 1285, 1207, 1066, 946, 790, 727. HR-MS (ESI-MS) medium. The solvent effects of dichloromethane and
m/z calcd. For C78H86N6O2SZn [M+H]+: 1235.5897, found tetrahydrofuran were modeled using the polarizable
1235.5819.
continuum model42,43 within the self-consistent reaction field
(SCRF) theory.
Computational details
Procedure for the preparation of the porphyrin-modified TiO2
electrode and for photovoltaic measurements
Density functional theory (DFT) and time dependent DFT
(TDDFT) calculations were performed on both the Zn-
porphyrin dyes CNU-OC8 and CNU-TBU. All the calculations Transparent conducting glass substrates were cleaned
reported in this paper were carried out with the Gaussian 09 sequentially with ethanol, Isopropanol and acetone with
ab initio quantum chemical program37. The geometry ultrasonication.
A TiO2 paste was prepared using ethyl
optimization of the two dyes has been carried out using M0638 cellulose (Aldrich), lauric acid (Fluka) and terpineol (Aldrich).
hybrid meta exchange correlation density functional at 6- The TiO2 particles used were ca. 20-30 nm in diameter. A
31G(d,p) level of theory. We used, quasi-relativistic pseudo- prepared TiO2 paste was doctor-bladed onto the pre-cleaned
potentials proposed by Hay and Wadt and a double zeta (
ξ)
glass substrates, followed by drying at 70 °C for 30 min and 30
quality basis set, LanL2DZ.539-41 for Zn throughout. During the min calcination at 500 °C. The calculated thickness is 8 μm. A
geometry optimization, we did not impose any symmetry scattering layer consisting of rutile TiO2 particles (250 nm in a
constraints. The frequency analysis was carried out on size) was deposited on the mesoporous TiO2 films and
optimized geometries to ensure that each configuration is thickness is was 14 μm. The sensitizers were dissolved in
indeed a minimum on the potential energy surface. To model Ethanol(4):THF(1) (0.3 mM) with 0.4mM chenodeoxycholic
the CNU-OC8 dye, we have replaced the -OC8H17 groups acid at room temperature and stirred for 24 h. The TiO2 layers
present on the two phenyl rings with simple -OCH3 for were immersed in the solutions for 24 h.
computational simplicity. The other dye CNU-TBU has been
Pt counter electrodes were prepared by thermal reduction
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of the films dip-coated in H2PtCl6 (7 x 10-3 M) in 2-propanol at
400 °C for 20 min. The dye-adsorbed TiO2 and Pt counter
electrodes were sandwiched between a 60μm-thick Surlyn
(Solaronix) layer, which was used as a bonding agent and
given in Table
1. The spectra of the dyes exhibit a
characteristic intense Soret band in the range of 400-500 nm;
this band is assigned to the second excited state. They also
exhibit less intense Q-bands in the range from 550 to 700 nm;
these bands are assigned to the first excited states that belong
to π-π* electron transitions. The Soret bands of the CNU-OC8
and CNU-TBU are red-shifted by 5-10 nm from that of the YD2-
OC8 dye. The dyes containing thiazole group as π-spacers
exhibited red-shifts of their absorption bands; this result
confirms that the thiazole group attached to the porphyrin
cycles improve the UV-Visible absorption characteristics of the
porphyrin dyes. This enhancement is likely due to the greater
electron-withdrawing nature of the thiazole group compared
to that of a benzene group.
-
spacer. A liquid electrolyte (I-/I3 redox couple) which was
composed of 0.6M BMII, 0.03M I2, 0.5M TBP, 0.05M LiI, 0.05M
GuSCN in Acetonitrile was then introduced through a pre-
punched hole on the Pt counter electrode that was finally
sealed. The active area of the dye-adsorbed TiO2 films (0.24
cm2) was estimated using a digital microscope camera with
image-analysis-software (Moticam 1000). Electrical Impedance
Spectra were obtained using an impedance analyzer (Solartron
1287) at frequencies ranging from 0.01Hz to 1MHz, the
magnitude of the signal was 10mV. The photocurrent-voltage
measurement was performed using a Keithley model 2400
Source Meter and a YAMASHITA solar simulator system
(equipped with a 1 kW xenon arc lamp, Oriel). A Si solar cell
calibrated by the National Renewable Energy Laboratory(NREL)
was used to adjust light intensity to the AM 1.5G 1 sun
condition(100mW cm-2) with KG-3 filter. A black aperture mask
was attached on the cells during the measurements. Incident
photon-to-current conversion efficiency (IPCE) was measured
as a function of wavelength ranging from 300 to 900 nm using
a K3100 EQX Spectral IPCE measurement system for DSSCs (PV
measurements, Inc.). A 75 W Xenon lamp was used as a light
source for generating monochromatic beam. An ellipsoidal
reflector collects light from the lamp and focuses on the
monochromatic entrance slit via a mechanical chopper to
create a small modulated signal. While the modulated,
monochromatic light was applied to the test devices, a
continuous bias light (ca. 1 sun) was also applied.
The emission spectra of porphyrin sensitizers were
measured at room temperature in THF solvent; representative
spectra of the compounds are given in Fig. 4, and the
corresponding emission maxima are reported in Table 1. The
fluorescence emission wavelengths of CNU-OC8, CNU-TBU,
and YD2-OC8 are 675, 700, and 663 nm, respectively. The red
shift of the emission values of the present dyes compared to
the emission of YD2-OC8 can be explained by the fact that the
2.4
2.2
2.0
CNU-OC8
YD2-OC8
CNU-TBU
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Results and Discussion
Synthesis of dyes 1 and 2
We synthesized new porphyrin sensitizers based on the donor-
π-acceptor concept. The synthetic protocol for the preparation
350
400
450
500
550
600
650
700
750
800
of the target dyes CNU-TBU and CNU-OC8 is shown in Fig. 2
Both compounds and were synthesized by adopting the
literature-reported procedure.14 Compounds
and 5 were first Fig. 3 UV-Vis absorption spectra of CNU-OC8, YD2-OC8 and CNU-TBU in THF (0.01mM).
.
Wavelength (nm)
4
5
4
silyl deprotected with TBAF in THF to obtain a common
intermediate compound that was Sonogashira coupled with
Pd(dba)2, AsPh3, Et3N and THF. For the synthesis of compound
1 and 2 we initially tried the reaction with 0.3 equiv. of
100
80
60
40
20
0
CNU-TBU
YD2-OC8
CNU-OC8
Pd2(dba)3 and
methods,14,26 but reaction yield was very low. We
subsequently obtained good yield upon changing the
2 equiv. of AsPh3 according to previous
a
reaction conditions to 0.6 equiv. of Pd(dba)2 and 3.5 equiv. of
AsPh3. The YD2-OC8 was obtained using a previously reported
synthesis procedure.14 Finally synthesized porphyrin sensitizers
were fully characterized by 1H NMR, 13C NMR, IR, emission,
and UV-Visible absorption spectroscopies as well as by high
resolution mass spectrometry and electrochemical methods.
Absorption and emission studies
600
650
700
750
800
Wavelength (nm)
The UV-Visible absorption spectra of porphyrin dyes CNU-OC8,
CNU-TBU and YD2-OC8 in THF solution are displayed in Fig. 3.
The corresponding wavelengths of maximum absorption and
the molar extinction coefficients of all of the compounds are
Fig. 4 Emission spectra of CNU-OC8 (λex = 570), YD2-OC8 (λex = 590) and CNU-TBU (λex =
580) in THF.
4 | J. Name., 2012, 00, 1-3
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Table 1 Absorption, fluorescence, and electrochemical data for porphyrin sensitizers.
Dye
Absorption (neutral form) a
λmax (cm-1), (log ε)
Emissionb λmax
nm(ф)
Potential V vs. SCEc
HOMO
(eV)
LUMO
EgEc (eV)
(eV)
Oxidation
Reduction
-1.280
CNU-OC8
CNU-TBU
YD2-OC8
453(205),575(14),649(32)
448(211), 578(13),650(34)
446(212),580(12),643(31)
675
700
663
+0.571,+1.169
+0.591,+1.158
+0.620,+1.189
-5.514
-5.474
-5.483
-3.511
-3.602
-3.592
2.003
1.872
1.891
-1.231
-1.351
aAbsorption and bemission data were measured in THF at 25°C (0.01 mM); error limits: λmax, ± 1 nm, ε ± 10%. cSolvent: CH2Cl2, 0.1 M TBAP, scan rate: 50 mV s-1, Pt
working electrode, Ag/Ag+ as reference electrode and Pt wire as counter electrode.
phenyl group of YD2-OC8 was replaced with a thiazole group in
CNU-OC8 and CNU-TBU.
energy gap values have an important role in DSSCs.47 Here the
electrochemical energy gap values of CNU-TBU, CNU-OC8 and
YD2-OC8 are 1.872, 2.003, and 1.891 eV respectively.
Electrochemical impedance spectroscopy
Electrochemical Characteristics
Cyclic voltammetry studies
Electrochemical impedance spectroscopy (EIS) was performed
using an electronic-chemical analyzer (Solartron 1287), in
order to investigate and elucidate electrochemical
The electrochemical properties of the CNU-OC8, YD2-OC8 and
CNU-TBU dyes were investigated by cyclic voltammetry
experiments in dichloromethane in the presence of 0.1 M characteristics and to get more information about interfacial
charge transfer at the interfaces of DSSC with the CNU-OC8,
YD2-OC8 and CNU-TBU dyes absorbed on TiO2. The Nyquist
plots of the above dyes were shown in Fig. 6 and fitting values
was shown in Table 2. The first semicircle in the above figure
obtained is assigned to resistance (RPt) and capacitance (CPt)
tetrabutylammonium perchlorate as a supporting electrolyte
at 18°C. All three dyes have quasi-reversible or reversible
redox voltammograms (Fig. 5). The corresponding oxidation
and reduction potentials are given in Table 1. All the oxidation
peaks correspond to the abstraction of electrons from the
amino functional groups and porphyrin ring.44,45 Similarly, all
the reduction peaks correspond to the reduction of acceptors
Table 2 Resistance extracted from the fitted results of the EIS spectra
Rs (Ω)
RPt (Ω)
Rct(Ω)
CPt
Cct
(μF cm-2)
136.83
87.41
(μF cm-2)
1013.8
1520.1
1734.6
YD2-OC8
CNU-OC8
CNU-TBU
1.428
1.363
1.316
3.23
2.117
1.949
446.2
13.45
13.62
88.93
-10
YD2-OC8
CNU-OC8
CNU-TBU
-5
Fig.
5 Cyclic voltammograms of YD2OC8, CNU-OC8 and CNU-TBU dyes. CV
0
0
measurements were carried out in DCM containing 0.1
perchlorate. Scan rate: 50 mV s-1 at 18°C.
M tetrabutylammonium
10
20
Z'(ꢀ)
and/or the formation of di-anions and zinc porphyrin anions.44
The HOMO and LUMO energy levels were calculated by using
Fig. 6 Electrochemical impedance spectroscopy results (Nyquist plots) for DSSCs with
CNU-OC8, YD2-OC8 and CNU-TBU dyes as sensitizers.
the following equations: HOMO = -[4.8 + Eoxonset] eV and LUMO
onset
=
-[4.8
+
Ered
]
eV, respectively (vs. Fc+/Fc).46 The
corresponding HOMO, LUMO and electrochemical energy gaps
(EgEC) of the three dyes are given in Table 1. The sensitizers
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CNU-OC8
CNU-TBU
Fig. 7 Optimized ground state (S0) geometries of CNU-OC8 and CNU-TBU obtained at M06/6-31G(d,p)/LanL2DZ level of theory. Hydrogen atoms are omitted here for clarity.
Fig. 8 Electron density distribution in HOMO-1, HOMO, LUMO, and LUMO+1 for CNU-OC8 and CNU-TBU obtained at M06/6-31G(d,p)/LanL2DZ level of theory. Hydrogen atoms are
omitted for clarity.
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pattern of both the dyes is almost similar. In the HOMO-1, the
mainly obtained for charge transport at the electrolyte/Pt electron density is mainly delocalized over porphyrin ring but
counter electrode interface. In the middle frequency range, in the HOMO, the majority of the electron density is
the large semicircle is assigned to the charge transfer delocalized over donor and porphyrin and also partly extended
resistance(Rct) and chemical capacitance(Cct) related to charge to the thiazole. In the case of LUMO, the electron density is
recombination at the working electrode/electrolyte regeneration depends on the energy levels of the frontier
interface.48,49 This semicircle result from interface of molecular orbitals, they are of great interest in the context of
electrolyte/dye absorbed TiO2 is in the order of YD2-OC8 > DSSC. The calculated molecular orbital energy levels are given
CNU-OC8 = CNU-TBU. These results clearly indicate that the
electron recombination resistance of these three dyes that
affects open circuit voltage determined by the potential
difference between the quasi-Fermi level of TiO2 affected by
the specific internal donors present and the redox potential of
the electrolyte. This results also suggests shift of TiO2
conduction band edge in the order of YD2-OC8 > CNU-OC8 >
CNU-TBU that affects electron injection related to current
density from driving force between TiO2 Conduction band edge
and LUMO of each dye. So CNU-OC8 has higher current density
and higher efficiency than YD2-OC8 due to lower conduction
band edge than YD2-OC8 although CNU-OC8 has lower open
circuit voltage than YD2-OC8. CNU-TBU has lowest conduction
band edge that has an effect on lowest voltage.50-55
Computational studies
Fig.
9 Simulated UV-Visible absorption spectra of CNU-OC8 and CNU-TBU in
To gain further insights into the geometry, electronic
structure, and optical properties of the two Zn-porphyrin dyes,
we have carried out thorough DFT and TDDFT calculations. The
ground sate optimized geometries of the dyes are shown in
Fig. 7. The two phenyl rings directly attached to the porphyrin
ring are perpendicular to the porphyrin ring, whereas the
other two phenyl rings present in the donor part, are with a
dihedral angle of ca. 30˚. The non-planar geometry of the dyes
helps in preventing the dye aggregation on TiO2.
tetrahydrofuran obtained at TD-M06/6-31G(d,p)/Lanl2DZ level of theory.
Table
3 Energies of the frontier molecular orbitals calculated at M06/6-
31G(d,p)/LanL2DZ level of theory, along with the optical band gap (E0−0).
Dye
EHOMO (eV)
-5.25
E0−0 (eV)a
1.87
ELUMO (eV)
-3.38
CNU-OC8
CNU-TBU
-5.29
1.84
-3.45
aE0−0 is the TDDFT transition energy (S0 → S1), and the LUMO = HOMO + E0−0
.
The isodensity surface plots (isodensity contour: 0.02 e Å−3) of
selected molecular orbitals are shown in Fig. 8. From the
figure, it can be seen that, the electron density distribution
Table 4 Simulated absorption wavelengths (λcal), oscillator strengths (f), and coefficient of configuration interaction (CI) with dominant contribution to each transition of the two
dyes.
a
λcal
(nm)
662
λexp
(nm)
649
Dominant
Contribution (%)b
H ꢀ L (90)
Dye
Transition
f
CI Coefficient
0.67036
S0ꢀS1
0.5482
S0ꢀS2
597
0.0062
0.51926
H-1 ꢀ L (54)
S0ꢀS3
S0ꢀS4
S0ꢀS5
S0ꢀS1
S0ꢀS2
S0ꢀS3
S0ꢀS4
S0ꢀS5
542
489
450
673
601
549
496
448
0.0291
0.2313
1.4180
0.5111
0.0023
0.0469
0.2416
1.4075
0.63328
0.50185
0.45611
0.67392
0.49824
0.63571
0.48306
0.46621
H-2 ꢀ L (80)
H ꢀ L+1 (50)
H-1 ꢀ L+1 (42)
H ꢀ L (91)
453
650
H-1 ꢀ L (50)
H-2 ꢀ L (81)
H ꢀ L+1 (47)
H-1 ꢀ L+1 (43)
448
aExperimental absorption wavelengths.
bH and L denote HOMO and LUMO, respectively.
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in Table 3. The simulated HOMO energies of CNU-OC8 and
CNU-TBU are -5.25 and -5.29 eV, respectively. The TDDFT first
transition energy (S0 → S1) is considered as the optical
bandgap of the dyes, which are found to be 1.87 and 1.84 eV,
respectively for CNU-OC8 and CNU-TBU. In the present study,
we calculated the LUMO energy by taking the sum of HOMO
energy and the TDDFT transition energy, rather than from the
unreliable Kohn–Sham LUMO eigenvalue.56 The obtained
LUMO energies of CNU-OC8 and CNU-TBU are -3.38 and -3.45
eV, respectively. The calculated energy levels of the frontier
molecular orbitals and band gaps are in excellent agreement
with the experimental CV data reported in Table 1. The
simulated HOMO energies of the dyes are well below the
redox potential (-5.20 eV) of the redox couple ꢀꢁ/ꢀꢂꢁ, which
facilitates the effective dye regeneration. The LUMO energies
of the two dyes are well above the conduction band of the
TiO2 (-4.20 eV), which fulfils the requirement for effective
electron injection from the dye excited state. The UV−Visible
spectra of CNU-OC8 and CNU-TBU were simulated in
tetrahydrofuran solvent and are depicted in Fig. 9. From the
figure, it can be seen that the TDDFT simulations reproduced
the main bands observed in the experimental UV−Visible
spectrum. Calculated first five singlet vertical excitation
energies along with their oscillator strengths are given in Table
70
60
50
40
30
20
10
0
YD2-OC8
CNU-OC8
CNU-TBU
300
400
500
600
700
800
Wavelength(nm)
Fig. 10 IPCE [%] spectrum of the devices made with CNU-OC8, YD2-OC8 and CNU-TBU
dyes.
of the three dyes. For CNU-OC8, higher dye-loading value
obtained compared to CNU-TBU and YD2-OC8, it may be an
important reason for the higher IPCE value, which implies that
this sensitizer would result in a relatively large photocurrent in
DSSCs, also shows strong -COOH anchoring on TiO2 Surface
improved by thiazole π-spacer.57 By contrast, CNU-TBU
exhibits a lower IPCE because of its weaker light-conversion
ability although it has higher dye-loading value than YD2-OC8
(Table 5).
4. The calculated absorption maxima (λmax) in the low energy
region of the dyes CNU-OC8 and CNU-TBU are located at 662
and 673 nm, respectively, are in well agreement with the
experimental results. These low energy absorptions mainly
occur from the transition of HOMO to LUMO for both the dyes.
The other intense band around 450 nm is due to the transition
from HOMO-1 to LUMO+1 with oscillator strength of more
than 1.4. The excitation energies and oscillator strengths were
interpolated by a Gaussian convolution with an FWHM of 2500
cm-1.
The current density-voltage (I-V) characteristic of the DSSCs
sensitized by CNU-OC8, CNU-TBU and YD2-OC8 is displayed in
Fig. 11, and the corresponding detailed photovoltaic
performance parameters recorded in Table 5. Under standard
global air mass 1.5 solar conditions, the CNU-TBU sensitized
cell provided a short circuit photocurrent density (JSC) value of
6.73 mA cm-2, an open circuit voltage (Voc) of 0.644 V, and a fill
factor (FF) of 73.61, corresponding to overall conversion
efficiency (η) of 3.19%. Under the similar conditions, the DSSCs
Photovoltaic characteristics
The incident photo-to-current conversion efficiencies
(IPCE) diagrams for the DSSCs of the three porphyrin dyes are
shown in Fig. 10. The dyes all respond in the broad range of
300-750 nm and show maximum IPCE values at 450 nm and
650 nm. This is consistent with UV-Visible absorption spectra
16
YD2-OC8
CNU-OC8
CNU-TBU
14
12
Table 5 Photovoltaic characteristics of the DSSC devices fabricated using dyes 1 and 2
compared to those of the device fabricated using the YD2-OC8 standard dye. The
approximate electrode areas were 0.24 cm2, and measurements were conducted at
approximately 100 mW cm-2
10
8
6
Dye
Jsc
(mA cm-2)
13.99
Voc
Ff (%)
η (%)
DLa
mmol/cm2
264.3
4
(V)
2
CNU-OC8
CNU-TBU
YD2-OC8
0.717
0.644
0.753
64.71
73.61
62.63
6.49
3.19
6.10
6.73
173.2
0
12.92
138.3
0.0
0.2
0.4
0.6
0.8
aThe amounts of dye loading, indicated as YD2-OC8, CNU-OC8 and CNU-TBU
were determined from desorption of dye molecules on immersion of the
transparent 8 µm TiO2 electrodes in a basic solution of 0.1 M sodium hydroxide
in THF and the calibrated absorption.
Voltage(V)
Fig. 11 Current-voltage characteristics of the devices made with CNU-OC8, YD2-OC8
and CNU-TBU porphyrin dyes with an I-/I3--based redox electrolyte.
8 | J. Name., 2012, 00, 1-3
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ARTICLE
12 S. H. Kang, I. T. Choi, M. S. Kang, Y. K. Eom, M. J. Ju and J. Y.
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14 A. Yella, H-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran and M. K.
Nazeeruddin, Science, 2011, 334, 629-633.
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Chem. Soc., 2007, 129, 4655-4665.
16 S. Mathe, A. Yella, P. Gao, R. H. Baker, B. F. E. Curchod, N. A.
Astani, I. Tavernelli, U Rothlisberger, M. K. Nazeeruddin and
M. Grätzel, Nature Chemistry, 2014, 6, 242-247.
17 C. KC, K. Stranius, P. D’Souza, N. K. Subbaiyan, H.
Lemmetyinen and N. V. Tkachenko, J. Phys. Chem. C, 2013,
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for CNU-OC8 and benchmark YD2-OC8 showed Jsc of 13.99 and
12.92 mA cm-2, Voc of 0.717 and 0.753 V, and FF of 64.71 and
62.63, corresponding to η of 6.49% and 6.10%, respectively.
These results indicate that thiazole carboxylic acid is a good
acceptor anchoring group for 2,6-dioctyloxyphenyl porphyrin
sensitizer. The high Jsc of CNU-OC8 might be attributed the the
lengthy and judiciously wrapped alkoxy chains of CNU-OC8,
impede efficiently the π−π aggregation, which increases the
charge injection of the dye and additionally protect the
porphyrin core against the electrolyte, which on the other
hand delays the charge recombination process with the
oxidized species of the redox shuttle.20 The obtained lower Jsc
and efficiency values in the case of CNU-TBU is attributable to
low light harvesting ability (Figure 10). Finally here, we
achieved good conversion efficiency in the case of CNU-OC8,
compare to benchmark YD2-OC8 under similar conditions. It
shows that thiazole π-spacer play a key role to replace the
phenyl group.
21 Z. Yao, M. Zhang, H. Wu, L Yang, R. Li and P. Wang, J. Am.
Chem. Soc., 2015, 137, 3799-3802.
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Zhu, J. Am. Chem. Soc., 2015, 137, 14055-14058.
23 Y. Wang, B. Chen, W. Wu, X. Li, W. Zhu, H. Tian and Y. Xie,
Conclusion
Angew Chem. Int. Ed., 2014, 53, 10779-10783.
24 X. Sun, Y. Wang, X. Li, H. Ågren, W. Zhu, H. Tian and Y. Xie,
Chem. Commun., 2014, 50, 15609-15612.
25 S. Cherian and C. C. Wamser, J. Phys. Chem. B, 2000, 104,
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27 W. Li, Z. Liu, H . Wu, Y-B. Cheng, Z. Zhao and H. He, J. Phys.
Chem. C, 2015, 119, 5265-5273.
We designed and synthesized the D-π-A porphyrin sensitizers
CNU-OC8 and CNU-TBU with
a thiazole carboxylic acid
acceptor and compared their photovoltaic performance and
electrochemical properties with those of YD2-OC8. Upon
photosensitization of nanocrystalline TiO2, the sensitizer CNU-
OC8 exhibited a good conversion efficiency of 6.49%, which is
better than that of the standard YD2-OC8 sensitizer (6.09%)
under the same photovoltaic conditions.
28 T. Higashino, Y. Fujimori, K. Sugiura, Y. Tsuji, S. Ito and H.
Imahori, Angew Chem. Int. Ed., 2015, 127, 9180-9184.
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Acknowledgements
This study was supported by the National Research Foundation
of Korea (NRF) funded by the Ministry of Science, ICT and
Future Planning (Grant no. 2015063131).
32 T. Yamamoto, H. Suganuma, T. Maruyama, T. Inoue, Y.
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DOI: 10.1039/C6RA00353B
GRAPHICALABSTRACT
CNUꢀOC8 exhibits better photovoltaic performance than the benchmark YD2ꢀOC8 sensitizer
−
in a liquid I−/I3 redox electrolyte.
16
YD2-OC8
C8H17O
OC8H17
C H13
6
CNU-OC8
CNU-TBU
14
12
10
8
N
X =
X =
COOH
N
N
N
N
S
N
X
Zn
CNU-OC8
COOH
C H13
6
OC8H17
C8H17O
YD2-OC8
6
C6H13
4
N
N
Zn
N
N
N
N
CNU-TBU
COOH
2
S
0
0.0
C H13
6
0.2
0.4
0.6
0.8
Voltage(V)