Effect of dimethylaminophenyl and thienyl donor groups on Zn-Porphyrin for dye sensitized solar cell (DSSC) applications

Article abstract of DOI:10.1016/j.ica.2017.08.020

We have designed and synthesized two Zn-Porphyrin derivatives Zn[5,15-dimethylaminophenyl-10,20-(4-carboxyphenyl)Porphyrin] (SKPor-1) and Zn[5,15-thiophene-10,20-(4-carboxyphenyl)Porphyrin] (SKPor-2) which possess donor modified structures as sensitizer for dye sensitized solar cells (DSSCs). These molecules contain the porphyrin unit as π-bridge, dimethylaminophenyl and thienyl groups as electron donor group and carboxylic acid group as anchoring group (electron acceptor unit). The SKPor-1 dye has large red-shift of the absorption maxima due to introduction of the dimethylaminophenyl moiety at the meso position of the porphyrin ring. But the absorption maxima of SKPor-2 is a little red-shifted due to the thienyl group attached with the porphyrin unit. The highly conjugated dimethylaminophenyl group is efficiently donating the electrons and electronic interaction between the porphyrin and dimethylaminophenyl unit is better compared to thienyl unit. The DSSC was made using commercial P25 TiO₂ material as photoanode, Zn-Porphyrin derivatives as sensitizer, I^?/I^3? as electrolyte and platinum (Pt) as counter electrode. The highest power conversion efficiency of the two Zn-Porphyrin derivatives (SKPor-1) based on DSSC reached 3.2 % with open circuit voltage (V_oc) of 0.68 V, short circuit photocurrent density (J_sc) of 9.62 mA/cm² and fill factor (ff) of 0.50, SKPor-2 based DSSC reached 2.5% open circuit voltage (V_oc) of 0.68 V, short circuit photocurrent density (J_sc) of 7.2 mA/cm² and fill factor (ff) of 0.51 under AM 1.5 G irradiation.

Full text of DOI:10.1016/j.ica.2017.08.020

Accepted Manuscript
Research paper
Effect of Dimethylaminophenyl and Thienyl donor groups on Zn-Porphyrin for
Dye Sensitized Solar Cell (DSSC) Applications
S. Kotteswaran, V. Mohankumar, M. Senthil Pandian, P. Ramasamy
PII:
S0020-1693(17)30920-9
http://dx.doi.org/10.1016/j.ica.2017.08.020
ICA 17814
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
10 June 2017
9 August 2017
10 August 2017
Please cite this article as: S. Kotteswaran, V. Mohankumar, M.S. Pandian, P. Ramasamy, Effect of
Dimethylaminophenyl and Thienyl donor groups on Zn-Porphyrin for Dye Sensitized Solar Cell (DSSC)
Applications, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.08.020
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Effect of Dimethylaminophenyl and Thienyl donor groups on Zn-Porphyrin for Dye
Sensitized Solar Cell (DSSC) Applications
S. Kotteswaran, V. Mohankumar, M. Senthil Pandian, P. Ramasamy
SSN Research Centre SSN College of Engineering, Kalavakkam, Chennai-603110,
Tamilnadu.
Email of the corresponding author – kotties555@gmail.com
Abstract
We have designed and synthesized two Zn-Porphyrin derivatives Zn[5,15-
dimethylaminophenyl-10,20- (4-carboxyphenyl)Porphyrin] (SKPor-1) and Zn[5,15-
thiophene- 10,20-(4-carboxyphenyl)Porphyrin] (SKPor-2) which possess donor modified
structures as sensitizer for Dye Sensitized Solar Cells (DSSCs). These molecules contain the
porphyrin unit as π-bridge, dimethylaminophenyl and thienyl groups as electron donor group
and carboxylic acid group as anchoring group (electron acceptor unit). The SKPor-1 dye has
large red-shift of the absorption maxima due to introduction of the dimethylaminophenyl
moiety at the meso position of the porphyrin ring. But the absorption maxima of SKPor-2 is
a little red-shifted due to the thienyl group attached with the porphyrin unit. The highly
conjugated dimethylaminophenyl gorup is efficiently donating the electrons and electronic
interaction between the porphyrin and dimethylaminophenyl unit is better compared to
thienly unit. The DSSC was made using commercial P25 TiO2 material as photoanode, Zn-
Porphyrin derivatives as sensitizer, I-/I3- as electrolyte and Platinum (Pt) as counter electrode.
The highest power conversion efficiency of the two Zn-Porphyrin derivatives (SKPor-1)
based on DSSC reached 3.2 % with open circuit voltage (Voc) of 0.68 V, short circuit
photocurrent density (Jsc) of 9.62 mA/cm2 and fill factor (ff) of 0.50, SKPor-2 based DSSC
1

reached 2.5% open circuit voltage (Voc) of 0.68 V, short circuit photocurrent density (Jsc) of
7.2 mA/cm2 and fill factor (ff) of 0.51 under AM 1.5 G irradiation.
Keywords: Zn-Porphyrin derivatives, DSSC, UV-Vis NIR spectrum, cyclic voltammetry,
power conversion efficiency.
Corresponding author (S. Kotteswaran)
1. Introduction:
To maintain global economic growth and to reduce global warming and environmental
pollution, the exploration of renewable energy resources is of great significance. Sunlight is
the most generous resource that can bring clean and efficient energy to meet the increasing
demand worldwide. Since past few years there has been more attention in photo-active
molecules to convert solar energy into electrical power. Dye-sensitized solar cells (DSSCs)
have been paid much more attention due to the relatively high solar energy conversion and
potentially low cost [1-5]. Dye sensitized solar cells (DSSCs), based on electrochemical
devices, are very good alternatives to conventional silicon based photovoltaic technologies
for solar energy conversion owing to their high power conversion efficiency (PCE) and low
cost of fabrication [6-7]. In dye sensitized solar cells, light is absorbed by the dye and charge
separation takes place owing to photo induced electron injection from the dye into the
conduction band of semiconductors, the separated charges then move towards respective
electrode thereby yielding a photocurrent in an external circuit. To develop the most efficient
dyes in DSSC the essential design requirements are: 1) The sensitizing dyes must strongly be
adsorbed into photocatalyst (semiconductors) surface to ensure efficient electron injection
into conduction band of semiconductors, 2) The lowest unoccupied molecular orbital
(LUMO) of the dye must be sufficiently higher than the conduction band of semiconductors
2

for efficient charge injection, and the highest occupied molecular orbital (HOMO) of the dye
must be lower than the redox potential for the efficient regeneration of the oxidized dye, 3)
The dye must absorb in the UV, visible and IR regions of the solar spectrum. The research on
dye sensitized solar cells (DSSCs) has intensified in recent years. Nowadays, DSSCs based
on the ruthenium sensitizers have achieved power conversion efficiencies (η) of 12.4%.
However, ruthenium dyes are expensive due to the cost of ruthenium and the typically
lengthy purification steps involved in their preparation. Alternative low cost and readily
accessible ruthenium dye replacements are under active investigation [8-16].
Porphyrins are one of the alternative dyes used as sensitizers with many advantages as
follows. 1) Nature has chosen porphyrin-related pigments in the light-harvesting antennae of
photosynthetic organisms that power biological systems. The chromophores in the
photosynthetic reaction centre capture sunlight efficiently and convert solar energy into
usable chemical energy. The porphyrins derived from chlorophylls which are the key
components of natural photosynthetic systems in green plants and absorb strongly in the
range of 400 - 700 nm with high molar absorbance coefficient. 2) Porphyrin dyes have been
demonstrated to possess charge-transfer kinetics indistinguishable from those of ruthenium
polypyridyl complexes. 3) Metal porphyrins undergo facile reduction and oxidation, so their
optical, photophysical, and electrochemical properties can be systematically tailored by the
substitutions and inner metal complexations, by taking advantage of their multiple reactive
sites, including four meso and eight β -positions. Elongation of strong electron-donating and
electron-withdrawing groups can shift and broaden the absorption range of porphyrins
making it possible to increase the light-harvesting property and the resulting DSSC
efficiency. The optical, photophysical, and electrochemical properties of the porphyrin can
be tuned through functional groups or substitution at their meso and β position. The most
efficient porphyrin sensitizers consist of electron donor (D) and electron acceptor (A) groups,
3

connected via p-conjugated spacers, with the highest achieved power conversion efficiency in
DSSCs (13%). Most of these D–p–A structured porphyrin sensitizers contain dialkylamine or
diarylamine units as donor groups and ethynyl benzoic acid moieties as acceptors, which are
covalently linked to the macrocyclic core. In recent years, perovskite solar cells (PSCs) have
become an attractive research area because of their high cell performance. The drawbacks of
the high cost of hole-transporting materials, instability of devices and environmentally
dangerous lead based materials limit the application of perovskite photovoltaics in the very
good future. In this regard, dye-sensitized solar cells based on porphyrin photosensitizers and
less harmful iodide– triiodide electrolytes offer a compromising solution to energy
conversion in ambient environments. Zn Porphyrin appears to be an essential requirement to
develop relatively simple and cost-effective dyes for practical encapsulation of DSSCs into
end products for the community. That is, the molecular design of sensitizers should be a
trade-off between large scale synthesis and device efficiency. Among the alternative dyes,
porphyrins have attracted a great deal of attention because of their strong Soret (400–450 nm)
and moderate Q-bands (550–600 nm) absorption properties as well as their natural role in
photosynthesis [17]. Furthermore, the photo-chemical and electrochemical stabilities, easy
synthetic process and handy control of redox potential by metallisation also make them as
promising candidates for light harvesting dyes for DSSC [18-28]. Porphyrins showed highest
performance (13%) compared to other dyes in DSSC [29]. The practical application of
commercially available (Ruthenium-based) dyes is obstructed by limited resources and high
cost of ruthenium metal. Ruthenium based dyes contain lengthy purification process,
environmental issues and lower molar extinction coefficients but Zn-Porphyrin based dyes
are abundant, easily tuneable structure, the Q band and Moderate band reveals the strong
absorption spectrum in the Uv-Vis NIR spectral regions, higher molar extinction coefficients
4

and better efficiency so the scientific community is looking forward to Zn-Porphyrin based
research.
In this study we have synthesized two D-π-A Zn- porphyrin (SKPor-1 and SKPor-2)
sensitizers containing same electron withdrawing moiety (carboxylic acid), same metal centre
(Zn) and we have modified donor groups such as dimethylaminophenyl, thiophene and we
compare the effect of dimethylaminophenyl, thiophene donor groups on the UV-Vis NIR
absorption spectrum, Cyclic voltammetry and Photovoltaic performance of the corresponding
DSSCs.
2. Experimental Section
2.1. Materials and Reagents
All starting materials were purchased from Sigma Aldrich and Merck. Methyl 4-
formylbenzoate, pyrrole, InCl3, 4-(dimethylamino)benzaldehyde, p-chloranil, Et3N, Zn
(OAc)2. 2H2O, KOH, HCl, thiophenealdehyde, methanol, hexane, chloroform,
imethylformamide, all chemicals and solvents used in this work were of analytical grade and
used without further purification. All chromatographic separations were carried out on silica
gel (120–160 mesh).
2.2. Analytical Methods
1H NMR and 13C NMR spectra of all the synthesized compounds were recorded in
DMSO-d6 on a Bruker AV III 500 MHz FT NMR spectrometer. The electronic spectra of the
dyes with solvent are recorded on a Perkin-Elmer Lambda 35 UV-Vis NIR
spectrophotometer. Cyclic voltammetry was carried out with a Versa STAT3 (AMETEK). A
three-electrode system consisting of a reference electrode (Ag/AgCl), a working electrode
(GC), and a counter electrode (platinum) was used. The redox potential of dyes was measured
in DMF with 0.1 M TBAPF6 with a scan rate of 100 mV s-1.
5

2.3. Solar Cell Fabrication
Fluorine doped SnO2 conducting glass (FTO 15Ω/cm2) was washed with isopropanol,
ethanol and deionized water using ultrasonic bath for 30 minutes. The FTO glass was treated
with 40 mM TiCl4 aqueous solution at 70 ◦C for 40 minutes and then the substrates were
sintered at 450 ◦C for 30 minutes in air. The TiO2 nanoparticles (Degussa P25 Solaronix) was
ground for appropriate hours with the Triton X 100, acetyl acetone, ethanol and water to
make colloid blend. The 10 µm thick film of TiO2 nanoparticles was deposited onto the side
of conducting layer of FTO by doctor blade technique. TiO2 substrates were sintered at 450
◦C for 30 minutes in air and then immersed in 40 mM TiCl4 aqueous solution at 70 ◦C for 40
minutes and then the substrates were sintered at 450 ◦C for 30 minutes in air. After cooling to
80 ◦C the TiO2 electrodes were immersed into 0.5 mM dye solution in ethyl acetate and left
for 24 h at room temperature. After dye adsorption, the dye coated TiO2 electrodes were
copiously rinsed with ethanol. Pt (Platisol solaronix) was deposited onto the side of
conducting layer of another FTO by doctor blading as the counter electrode and then the
substrates were sintered at 450 ◦C for 30 min in air. The photovoltaic measurements were
performed in a sandwich cell consisting of the porphyrin dyes sensitized TiO2 electrode as the
working electrode and Pt as the counter electrode. The electrolyte consists of I- /I3- (Iodolyte
Solaronix) redox potential and the efficient irradiated area of cell was 0.5 cm x 0.5 cm. The
photocurrent – voltage (J–V) characteristics were measured with Oriel Class-AAA Simulator
(Newport) with a xenon lamp as a light source having an intensity of 100 mW/cm2. Prior to
experiment the light intensity was adjusted with a standard PV reference cell (2 cm x 2 cm
monocrystalline silicon solar cell, calibrated at NREL, Colorado, USA) with a Keithley 2400
source-measure unit. All photovoltaic properties such as open circuit voltage (Voc), short
6

circuit current density (Jsc), fill factor (FF) obtained from the J-V curve were used in the
following equation to obtain power conversion efficiency.
Jsc (mAcm-2) x Voc (V) x FF
η = ---------------------------------------
P in (mW cm-2)
2.4. Synthesis
The synthesis routes of the two porphyrins are shown in Fig. 1. The detailed synthesis
procedure is as follows.
2.4.1. Synthesis of 5-4(Carbomethoxyphenyl) dipyrromethane
The reaction was made by methyl 4-formylbenzoate (1 mmol) and pyrrole (2 mmol)
in the presence of InCl3 (0.1 mmol) taken in round bottom flask and stirred 3 hr at room
temperature. Purple colour solid was formed which was purified by column chromatography
Methanol: Hexane (10:90) as the eluent. 1H NMR (300 MHz, DMSO-d6) δ (ppm): 3.01 (s,
3H, OCH3), 3.44 (s, 1H, Meso-H), 3.99 (m, 2H, Pyrrolic- 2C3-H), 4.03 (d, J= 3.6 Hz, 2H,
Pyrrolic- 2C4H), 4.83 (m, 2H, Pyrrolic- 2C5-H), 7.81, (d, J= 7.4 Hz, 2H, aromatic- H), 7.47,
(d, J= 7.8 Hz, 2H, aromatic-H), 8.10, (s, 2H, 2N-H). 13C NMR (400 MHz, DMSO-d6); δ
(ppm) 40.12, 45.09, 76.84, 77.15, 77.48, 111.07, 115.42, 118.34, 162.20, 162.35. GC-MS:
m/z calculated for [C17 H16 N2 O2 + H] + 280.3211. Found: 280.2127 [M+H] +.
2.4.2. Formation of Porphyrin Ring
Solution of 4-(dimethylamino)benzaldehyde (2 mmol) and 5-(4-
carbomethoxyphenyl)dipyrromethane (2 mmol) in 250 mL of chloroform was passed with
argon gas for 40 minutes. Trifluoroacetic acid (2 mmol) was added and the obtained solution
was stirred in the dark at nitrogen atmosphere for 18 hr. Finally the para-chloranil (6 mmol)
was added and stirred for 3 hr. Triethylnitrate (2 mL) was added to complete the reaction.
The solvent was evaporated and the mixture was passed through TLC silica to remove the
7

non-porphyrin products and then the purification was done by column chromatography using
dichloromethane: ethyl acetate (95: 5) as an eluent to give the porphyrin as a pure product. 1H
NMR (300 MHz, CDCl3) δ (ppm): 8.90 (d, J=5.4 Hz, 4H, Aromatic H), 8.77 (d, J=7.5 Hz,
4H, Aromatic H), 8.48 (d, J=7.5 Hz, 4H, Pyrrolic H), 8.36 (d, J=7.5 Hz, 4H, Pyrrolic H), 8 (d,
J=9.2 Hz, 4H, Aromatic H), 6.7 (d, J=9.2 Hz, 4H, Aromatic H), 4.28 (s, 6H, COOMe), 4.12
(s, 12H, NMe2), 1.07 (s, 2H, Porphyrin NH). 13C NMR (300 MHz, CDCl3); δ (ppm) 153.2,
151.6, 151.2, 150.7, 145.8, 145.3, 143.6, 142.1, 140.8, 139.9, 133.7, 133.4, 131.8, 131.4,
131.1, 127.5, 127.2, 126.3, 113.9, 112.7, 29.8. GC-MS: m/z calculated for [C44 H30 N2 O4 S2
+H] + 742.8634. Found: 742.8623 [M+H] +.
2.4.3. Metallisation of Zn-Porphyrin (SKPor-1)
The solution of 2.4.2. (1 mmol) in 15 mL of CHCl3, a methanol solution of zinc acetate
dehydrate (1.5 mmol) was added and the solution was refluxed under nitrogen atmosphere for
8 hr. The solvent was evaporated, the residue was washed with distilled water (20 mL). The
solution (0.5 mmol) with 25 mL of tetrahydrofuran– methanol (1: 1) mixture was added to 10
mL of aqueous KOH (1 mmol) solution and refluxed for 46 hr. Then the solvent was
evoparated and the resulting residue was added to 10 mL of distilled water. HCl solution was
added to the precipitation of the porphyrin, which was filtered and washed with water and
then dried under vacuum to give the final product as a purple SKPor-1. 1H NMR (400 MHz,
DMSO-d6) δ (ppm): 11.9 (s, 2H, COOH), 9.79 (s, 4H, Porphyrin Pyrrolic H), 7.89 (s, 4H,
Porphyrin Pyrrolic H), 7.66 (d, J=8.8 Hz, 8H, Aromatic H), 7.31 (d, J=8 Hz, 4H, Aromatic
H), 7.07 (d, J= 8 Hz, 4H, Aromatic H), 2.76 (s, 12H, NMe2). 13C NMR (400 MHz, DMSO-
d6); δ (ppm) 190.44, 153.40, 147.90, 146.21, 131.33, 129.77, 129.50, 129.23, 129.17, 128.94,
126.35, 125.15, 124.20, 122.81, 122.70, 119.40, 35.46. GC-MS: m/z calculated for [C42 H24
N4 O4 S2 Zn+ H] + 778.1743. Found: 778.1758 [M+H] +.
2.4.4. Formation of Porphyrin Ring
8

The solution of 2-thophenecarboxyaldehyde (2 mmol) and 5-(4-
carbomethoxyphenyl)dipyrromethane (2 mmol) in 250 mL of chloroform was passed with
argon gas for 40 minutes. Trifluoroacetic acid (2 mmol) was added and the obtained solution
was stirred in the dark at nitrogen atmosphere for 18 hr. Finally the para-chloranil (6 mmol)
was added and stirred for 3 hr. Triethylnitrate (2 mL) was added to complete the reaction.
The solvent was evaporated and the mixture was passed through TLC silica to remove the
non-porphyrin products and then the purification was done by column chromatography using
dichloromethane: ethyl acetate (95: 5) as an eluent to give the porphyrin as a pure product. 1H
NMR (300 MHz, CDCl3) δ (ppm): 8.78 (d, J=9.2 Hz, 4H, Porphyrin Pyrrolic H), 8.46 (d,
J=9.2 Hz, 4H, Porphyrin Pyrrolic H), 8.32 (d, J=8.4 Hz, 4H, Aromatic H), 8.18 (d, J=8.4 Hz,
4H, Aromatic H), 7.87 (d, J=9.4 Hz, 2H, Thienyl H), 7.42 (s, 4H, Thienyl H), 5.32 (s, 6H,
COOMe), 3.89 (s, 12H, NMe2), 0.80 (s, 2H, Porphyrin NH). 13C NMR (300 MHz, CDCl3); δ
(ppm) 158.8, 149.8, 148.3, 141.6, 132.3, 129.9, 128.4, 128.1, 127.8, 127.5, 126.3, 125.7,
124.6, 121.9, 121.4, 117.8, 36.2. GC-MS: m/z calculated for [C52 H44 N6 O4 + H] + 817.3502.
Found: 817.3289 [M+H] +.
2.4.5. Metallisation of Zn-Porphyrin (SKPor-2)
The solution of 2.4.4. (1 mmol) in 15 mL of CHCl3, a methanol solution of zinc
acetate dehydrate (1.5 mmol) was added and the solution was refluxed under nitrogen
atmosphere for 8 hr. The solvent was evaporated, the residue was washed with distilled water
(20 mL). The solution (0.5 mmol) with 25 mL of tetrahydrofuran– methanol (1: 1) mixture
was added to 10 mL of aqueous KOH (1 mmol) solution and refluxed for 46 hr. Then the
solvent was evoparated and the resulting residue was added to 10 mL of distilled water. HCl
solution was added to the precipitation of the porphyrin, which was filtered and washed with
1
water and then dried under vacuum to give the final product as a purple SKPor-2. H NMR
9

(400 MHz, DMSO-d6) δ (ppm): 12.7 (s, 2H, COOH), 7.92 (s, 4H, Porphyrin Pyrrolic H), 7.90
(s, 4H, Porphyrin Pyrrolic H), 7.88 (d, J=8.4 Hz, 4H, Aromatic H), 7.18 (d, J=8.8 Hz, 4H,
Aromatic H), 6.67 (d, J= 6.8 Hz, 4H, Thienyl H), 6.10 (s, 4H, Thienyl H). 13C NMR (400
MHz, DMSO-d6); δ (ppm) 178.6, 167.1, 147.6, 132, 131.8, 129.9, 128.7, 128.4, 118.5, 117.6,
108.4, 107.7, 106.1. GC-MS: m/z calculated for [C50 H38 N6 O4 Zn+ H] + 851.2324. Found:
851.2345 [M+H] +.
3. Results and Discussions:
3.1 Absorption Spectrum
The Zn-porphyrin dye exhibits the typical features of a porphyrin ring that is an
intense Soret band in the range 350–450 nm and a less intense Q band in the range 450–700
nm. The UV–vis NIR absorption spectra of the SKPor-1 and SKPor-2 dyes in DMF solution
are shown in Fig. 2. The peak positions and molar absorption coefficients (ε) of Soret and Q-
bands of SKPor-1 and SKPor-2 dyes are listed in Table.1. The UV–vis NIR absorption
spectra of SKPor-1 dye exhibits a strong Soret band at 381 nm and two moderate Q-bands at
483 and 650 nm and SKPor-2 dye exhibits a strong Soret band at 374 nm and two moderate
Q-bands at 476 and 619 nm. The strong intense Soret band of both dyes between 350 nm to
435 nm are due to intermolecular π-π* electronic transition and the porphyrin species consist
of a strong transition to the second excited state (S0-S2). The broad less intense second Q-
bands of both dyes between 450 nm to 675 nm are due the intermolecular n-π* electronic
transition and this involves weak transition to the first excited state (S0-S1). However, the
strong Soret band and moderate Q-bands of SKPor-1 are broader and red- shifted than that of
SKPor-2. The absorption spectrum of SKPor-1 the Soret band and the first Q-bands are red-
shifted around 10 nm and the second Q-band is red shifted around 30 nm than SKPor-2.
This is because SKPor-1 contains dimethylaminophenyl group at meso-position which have
10

more phenyl-conjugated double bonds and more electron withdrawing nature compared to
thienyl group of SKPor-2 [30-31]. The molar extinction coefficients (ε) of SKPor-1 Soret
band 2.08 x 105 mol L-1 cm-1, first Q-band is 0.42 x 105 mol L-1 cm-1, second Q-band is 0.45 x
105 mol L-1 cm-1. For SKPor-2 dye Soret band 1.68 x 105 mol L-1 cm-1, first Q-band is 0.29 x
105 mol L-1 cm-1, second Q-band is 0.21 x 105 mol L-1 cm-1 which are lower than that of
SKPor-1. As a consequence of an extended π-conjugation, both Soret and Q bands of
SKPor-1 exhibit a significant red shift and a Soret band broader than that of SKPor-2,
because the electronic interaction with the dimethylaminophenyl group is more pronounced
than for the thienyl group. The results clearly show that more π -conjugation with a highly
conjugated phenyl bridge unit can improve the light-harvesting capability of Zn-porphyrin
dyes. These results indicate that the SKPor-1 has more light harvesting ability compared to
SKPor-2.
3.2. Cyclic Voltammetry
The electrochemical studies are performed on the SKPor-1 and SKPor-2 dyes to
elucidate the oxidation and reduction potentials corresponding to the HOMO energy level and
LUMO energy level of the dyes via cyclic voltammetry. The cyclic voltammetry
measurements for SKPor-1 and SKPor-2 in DMF were conducted using
tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M) as supporting electrolyte at scan
rate of 100 mV/s with three electrode system. The potentials are listed in table.2. All
potentials are quoted with respect to a non-aqueous silver/silver chloride (Ag/AgCl) reference
electrode, this electrode was externally calibrated with the Fc/Fc+ redox couple in the
dimethylformamide solution and has a potential of 0.552 V versus NHE. SKPor-1 contains
three successive quasi-reversible redox couples with the first anodic and cathodic peak
potentials -0.72 V, -0.85 V; the second anodic and cathodic peak potentials 0.17 V, 0.11 V;
11

and the third anodic and cathodic peak potentials 1.14 V, 1.00 V respectively. The HOMO
value of the SKPor-1 is -5.79 eV which was calculated using the oxidation potential 1.14 V.
The LUMO value is -3.03 eV which was estimated from the HOMO energy value with the
intersection point of absorption spectrum. SKPor-2 contains two successive quasi-reversible
redox couples with the first anodic and cathodic peak potentials 1.24 V, -0.32 V and the
second anodic and cathodic peak potentials 1.49 V, -0.88 V respectively which is shown in
Fig.3. The HOMO value of the SKPor-2 is -6.09 eV which was calculated using the
oxidation potential 1.49 V. The LUMO value is -3.20 eV which was estimated from the
HOMO energy value with the intersection point of absorption spectrum. In addition, with the
introduction of the stronger donor group, the energy levels of HOMO and LUMO are largely
changed. To ensure efficient electron injection from the LUMO of the sensitizer into the
conduction band of TiO2, the LUMO level must be higher than the conduction band edge of
TiO2. The HOMO level of the sensitizer must be lower than the redox potential of I-/I3-
electrolyte for efficient regeneration of the sensitizer cations after the photo-induced electron
injection into the conduction band of TiO2 [32-35]. The resulted HOMO value of the SKPor-
1 is -5.79 eV which is lower than the redox potential of I-/I3- value (-4.94 eV) and LUMO
value is -3.03 eV which is higher than the conduction band value of TiO2 (-4.04 eV) so this
provides enough driving force to inject electrons from the excited SKPor-1 singlet state to
the conduction band of TiO2.
SKPor-1 and SKPor-2 dyes have been fully optimized with B3LYP/6-311g (d,p)
level of theory without any symmetry constraints. The vibrational frequency calculations
were performed at the same level of theory to ensure that the SKPor-1 and SKPor-2
structures are true minima (no imaginary frequencies) on the potential energy surface. Fig. 4
shows the optimized molecular structure of SKPor-1 and SKPor-2 dye molecules. All
calculations were performed using Gaussian 09 W programme package [36]. The ground-
12

state geometries and electronic properties such as highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) energy densities were calculated
at B3LYP/6-311g (d,p) level basis set. The frontier molecular orbital (FMO) plays a vital
role in the charge separated sates of organic molecules. The HOMO and LUMO energies of
dye sensitizers have suitable energy values for matching the conduction band edge of
semiconductor and redox potential of electrolyte in DSSCs. In DSSC, HOMOs are
delocalized over the donor part and LUMOs are delocalized over the acceptor part and
anchoring group of dye molecules. Fig. 5 shows the FMOs of SKPor-1 and SKPor-2 dyes.
From Fig. 5 we see that the electron densities for SKPor-1 and SKPor-2 HOMO-2 are
delocalized over the donor part, HOMO-1 is delocalized on porphyrin core and HOMO is
delocalized on porphyrin unit and donor part of the molecule. While the LUMO is
delocalized in porphyrin core unit and smaller electron densities are delocalized in acceptor
part, LUMO+1 are delocalized on porphyrin core and LUMO+2 delocalized on acceptor part
and anchoring group of the molecule. The FMOs analysis gives the clue for charge density
transfer in the dye molecule.
3.3. Photovoltaic Performance of the SKPor-1 and SKPor-2.
Liyang Luo, et al. and Ram Ambre, et al. have reported thienyl donor group based Zn-
Porphyrins. They achieved solar cell efficiency of 1.8% and 0.8% [37-38]. In order to study
the influence of different donor units of Zn-Porphyrin (SKPor-1 and SKPor-2) on the DSSC
device performance, photovoltaic experiments were conducted. The devices were assembled
using TiO2 (Degussa P25 Solaronix) film (10µm) in conjugation with commercially available
N719 dye, synthesized dyes (SKPor-1 and SKPor-2), I-/I3- (Iodolyte Solaronix) based
electrolyte and pt as cathode. Fig. 5 shows the photocurrent-voltage (Jsc-V) characteristics of
DSSC devices sensitized by SKPor-1 and SKPor-2 under standard air mass 1.5 global
spectral solar photon flux. The short-circuit photocurrent density (Jsc), open-circuit voltage
13

(Voc) and fill factor (FF) of the two devices are summarized in Table. 4. The overall power
conversion efficiency (PCE) of DSSC devices stained with dyes SKPor-1 and SKPor-2 was
3.2 % and 2.5 %, respectively. This implies the enhancement of the photo-induced charge
collection of SKPor-1 sensitized devices over SKPor-2 devices in association with the
standard I- /I3- based redox electrolyte. It can be concluded from these preliminary results that
the DSSCs sensitized with SKPor-1 bearing the electron donating dimethylaminophenyl core
unit, achieved better photovoltaic performance than those sensitized with SKPor-2 engaging
the electron donating thienyl core unit. The higher power conversion efficiency of the
SKPor-1 based DSSCs compared with that of the SKPor-2 based DSSCs was mainly due to
high Jsc that is attributed to the excessive conjugated double bonds of the
dimethylaminophenyl core unit possibly enhancing the efficiency, regeneration of electrons
and also increasing electron injection from the SKPor-1 molecules to TiO2. The poor PCE of
the SKPor-2 DSSCs may be resulted from their unfavorable orientation due to the lower
conjugation of the molecules and the aggregation of the Zn-porphyrin macrocycles.
3.4. Dye Adsorption measurement of the SKPor-1, SKPor-2 and N719 Dyes.
In order to find out the amount of dye molecule adsorbed on the TiO2 surface we
prepared 0.5 mM of dye (SKPor-1, SKPor-2 and N719 dyes) solutions, then analyzed with
the Uv-Vis NIR spectrum. The results are shown in Fig.7. Using these absorption values the
molar extinction coefficient of the initial dye solution was calculated through Beer- Lamberts
law. TiO2 films were immersed into the same dye solutions for 24 hrs, after that TiO2 films
were taken out. Dye desorption was carried out by 0.5 mM of NaOH in a mixed solvent
(water: ethanol = 1:1, v: v for N719, water: DMF = 1:9, v: v for SKPor-1 and SKPor-2).
Then, UV-Vis absorption spectra of the resultant solutions were measured to estimate the
amount of adsorbed dye molecules [39-42]. The results are shown in Fig.8. SKPor-1,
14

SKPor-2 and N719 dye molecules adsorbed onto the TiO2 surface are 8.1 x 10-8M Cm-2, 2.3 x
10-8M Cm-2 and 2.8 x 10-8M Cm-2, respectively, The results are shown in Table. 5.
4. Conclusion
In conclusion, SKPor-1 sensitizer cell gave JSC of 9.62 mA cm-2, VOC of 0.68 V, and
FF of 0.50, corresponding to an overall conversion efficiency of 3.2 % because of a bulky
donor effect. SKPor-2 sensitizer cell gave JSC of 7.2 mA cm-2, VOC of 0.68 V, and FF of 0.51,
corresponding to an overall conversion efficiency of 2.5 %. Zn-porphyrin with
dimethylaminophenyl moiety (SKPor-1) showed a higher conversion efficiency because it
contains strong and wide absorption spectrum in the Uv-Vis NIR spectral regions, so
corresponding molar extinction coefficients have increased (2.08 x 105 mol L-1 cm-1 )
compared to SKPor-2 dye (1.68 x 105 mol L-1 cm-1). Cyclic voltammetry results show the
HOMO and LUMO energy levels of the SKPor-1 dye as -5.79 eV and -3.03 eV,
respectively. The HOMO and LUMO energy levels of the SKPor-2 dye are -6.09 eV and
-3.20 eV, respectively. The HOMO value of the SKPor-1 is -5.79 eV which is lower than
the redox potential of I-/I3- value (-4.94 eV) and LUMO value is -3.03 eV which is higher
than the conduction band value of TiO2 (-4.04 eV) so this provides enough driving force
to inject electrons from the excited SKPor-1 singlet state to the conduction band of TiO2
compared to SKPor-2. The amount of SKPor-1 dye adsorbed on semiconductor surface
is 81 x 10-8M Cm-2 and the amount of SKPor-2 dye adsorbed on semiconductor surface
is 23 x 10-8M Cm-2. Compared to SKPor-2 dye, SKPor-1 dye is adsorbed more so
SKPor-1 based solar cell gives higher efficiency. Compared to SKPor-1 and SKPor-2
dyes, commercially available N719 dye is having wide absorption and appropriate
HOMO-LUMO values resulting in higher efficiency. Further improvement of the power
15

conversion efficiency of Zn-porphyrin derivative dyes can be investigated by using them as
dye sensitizers instead of expensive ruthenium dye sensitizers in the near future.
Acknowledgements
The authors thank SIF, VIT, Vellore for NMR analysis, SAIF, IIT, Madras for
providing Mass spectrometry analysis and we also thank DST (SERI)
DST/TM/SERI/2k12/40(G) & 04.06.2014 for financial support.
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Figures
Figure.1. Synthesis of SKPor-1 and SKPor-2.
Figure.2. UV-Vis NIR spectrum of SKPor-1 and SKPor-2.
Figure.3. Cyclic voltammetry of SKPor-1 and SKPor-2 scan rate at 100mV/Sec.
Figure.4. Optimized molecular structures of SKPor-1 and SKPor-2 (For clarity hydrogen atoms are
omitted)
Figure.5. Frontier molecular orbitals of SKPor-1 (top) and SKPor-2 (bottom)
Figure.6. Current-voltage characteristics of the SKPor-1, SKPor-2 and N719 dyes.
Figure.7. UV-Vis NIR absorption spectrum of 0.5 mM dye (SKPor-1, SKPor-2 and N719
dyes) solutions.
Figure.8.
UV-Vis NIR absorption spectrum of dye (SKPor-1, SKPor-2 and N719 dyes)
desorbed solutions.
22

R= Dimethylaminophenyl for SKPor-1
R= Thienyl for SKPor-2
.
Figure.1. Synthesis of SKPor-1 and SKPor-2.
3.0
SKPor-1
SKPor-2
23

0.6
0.5
0.4
0.3
0.2
0.1
0.0
SKPor-1
SKPor-2
500
600
700
Wavelength (nm)
Figure.2. UV-Vis NIR spectrum of SKPor-1 and SKPor-2.
24

0.0005
0.0004
0.0003
0.0002
0.0001
0.0000
-0.0001
-0.0002
SKPor-1
SKPor-2
-2
-1
0
1
2
Potential Vs Ag/AgCl / V
Figure.3. Cyclic voltammetry of SKPor-1 and SKPor-2 scan rate at 100 mV/Sec.
25

SKPor-2
SKPor-1
Figure. 4. Optimized molecular structures of SKPor-1 and SKPor-2 (For clarity hydrogen
atoms are omitted)
26

Figure. 5. Frontier molecular orbitals of SKPor-1 (top) and SKPor-2 (bottom)
27

10
8
6
4
N719
SKPor-1
SKPor-2
2
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage (V)
Figure.6. Current-voltage characteristics of the SKPor-1 and SKPor-2 dyes.
28

4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
N719
SKPor-1
SKPor-2
400
500
600
700
800
Wavelength (nm)
Figure.7. UV-Vis NIR absorption spectrum of 0.5 mM dye (SKPor-1, SKPor-2
N719 dyes) solutions.
and
29