Effect of functional groups on sensitization of dye-sensitized solar cells (DSSCs) using free base porphyrins

Article abstract of DOI:10.1142/S1088424617500390

Dye-sensitized solar cells (DSSCs) were fabricated with six meso-substituted A3B and A4 free base porphyrin dyes having different functional groups, as sensitizers. The two step synthesis and the effect of different functional groups and their positions on the photosensitization properties of these porphyrin dyes are reported. The highest power conversion efficiencies (η) of 3.26%, 2.94% and 2.84% were achieved for the DSSC fabricated using 5,10,15-tris(4′-pyridyl)-20-(4′-carboxyphenyl)porphyrin (H2TriPyMCPP), 5,10,15,20-tetrakis(4′-aminophenyl)porphyrin (H2TAPP) and 5-(4′-pyridyl)-10,15,20-tris(4′-carboxyphenyl)porphyrin (H2MPyTriCPP) dyes, respectively. The electron donating amino group is shown to enhance the power conversion efficiency while pyridyl appended porphyrin sensitizers are shown to be superior sensitizers as compared to carboxyphenylporphyrins. The investigation of effect of functional group and position of functional group of porphyrin dye on DSSC can serve as an important tool to guide further selection and synthesis of potential candidates as sensitizers.

Full text of DOI:10.1142/S1088424617500390

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2017; 21: 1–9
DOI: 10.1142/S1088424617500390
Published at http://www.worldscinet.com/jpp/
Effect of functional groups on sensitization of dye-sensitized
solar cells (DSSCs) using free base porphyrins
Nivedita Chaudhri,a Nipun Sawhney,b Bijjam Madhusudhan,a Anubhav Raghav,b
Muniappan Sankara*◊ and Soumitra Satapathib*
aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247667, India.
bDepartment of Physics, Indian Institute of Technology Roorkee, Roorkee-247667, India.
Received date: 8 February 2017
Accepted date: 27 April, 2017
ABSTRACT: Dye-sensitized solar cells (DSSCs) were fabricated with six meso-substituted A3B and A4
free base porphyrin dyes having different functional groups, as sensitizers. The two step synthesis and
the effect of different functional groups and their positions on the photosensitization properties of these
porphyrin dyes are reported. The highest power conversion efficiencies (h) of 3.26%, 2.94% and 2.84%
were achieved for the DSSC fabricated using 5,10,15-tris(4′-pyridyl)-20-(4′-carboxyphenyl)porphyrin
(H2TriPyMCPP), 5,10,15,20-tetrakis(4′-aminophenyl)porphyrin (H2TAPP) and 5-(4′-pyridyl)-10,15,20-
tris(4′-carboxyphenyl)porphyrin (H2MPyTriCPP) dyes, respectively. The electron donating amino group
is shown to enhance the power conversion efficiency while pyridyl appended porphyrin sensitizers are
shown to be superior sensitizers as compared to carboxyphenylporphyrins. The investigation of effect of
functional group and position of functional group of porphyrin dye on DSSC can serve as an important
tool to guide further selection and synthesis of potential candidates as sensitizers.
KEYWORDS: porphyrins, photosensitizers, dye-sensitized solar cell, power conversion efficiency.
increase the power conversion efficiency (PCE) of these
INTRODUCTION
solar cells [3]. Initially, Ru(II) based complexes had
Dye-sensitized solar cells (DSSCs) as discovered by
received particular interest as a photosensitizers because of
Grätzel in 1991, have attracted tremendous research interest
their long-term chemical stability, longer exciton lifetime
due to their low cost, ease of fabrication, short energy
and broad absorption spectra due to metal-to-ligand
payback time, low sensitivity to temperature variation,
charge transfer [4]. Polypyridyl Ru(II)-based dyes have
versatility, and environment friendly fabrication [1, 2].
been utilized to achieve the maximum PCE of 11.5% [5].
A DSSC is composed of a nanocrystalline porous layer of
However, scarcity of noble metals, lower molar extinction
titanium dioxide (TiO2) coated photoanode, a monolayer
coefficient, highcostandenvironmentalissueofRu(II)dyes
of dye molecules that absorbs sunlight, an electrolyte for
limit their large scale commercialization [6]. In this regard,
dye regeneration and a cathode. The anode and cathode
a large number of steps have been taken to develop efficient
form a sandwich structure, while the photosensitizer (dye)
sensitizers that exhibit high molar absorption coefficients
plays a crucial role by absorbing photons from sunlight.
with high suitability for large scale applications. As
A significant amount of research has been undertaken to
modify the structure and composition of dye molecules
feasible alternatives, metal free organic dyes with D-p-A
or push–pull architecture have been used to improve short
in order to enhance the absorption of sunlight in order to
circuit current density JSC [7]. To overcome the high cost
of ruthenium complexes, metal free organic dyes have been
◊ SPP member in good standing.
extensively explored in the recent years.
Organic dyes including porphyrins, coumarins,
perylenes,phthalocyanines,carbazoles,triarylamine,pery-
lene, cyanine, fluorene, merocyanine and hemicyanine
*Correspondence to: Muniappan Sankar, email: sankafcy@iitr.
ac.in, tel: +91 1332-284753,, Saumitra Satapathi, email: ssphf.
fph@iitr.ac.in
Copyright © 2017 World Scientific Publishing Company

2
N. CHAUDHRI ET AL.
derivatives etc. have been utilized extensively [8, 9].
Among these, porphyrins have attracted considerable
attention as light-harvesting antennae for DSSCs due to
their unique photophysical and electrochemical redox
properties, thermal and chemical stability, and high
efficiency in absorbing solar energy in the visible region
duetotheirhighmolarextinctioncoefficientsmakingthem
suitable candidates for the development of panchromatic
sensitizers [10, 11]. Metalloporphyrins, especially Zn(II)
porphyrins have been commonly used in DSSCs to attain
the high PCE value and have gained tremendous research
interest in recent years [11a, 12, 13]. Despite their high
efficiencies in DSSCs, the effect of different functional
groups on light harvesting properties of porphyrin dyes
remains relatively unexplored [14]. Herein, we have
synthesized highly symmetrical A4 and unsymmetrical
A3B type free base porphyrins (Chart 1). Further, we have
examined photophysical and photovoltaic properties of
these porphyrin dyes to provide a detailed explanation
of the effect of functional groups on their sensitization
properties in DSSCs.
Dyesol, Australia. Titanium(IV) isopropoxide, spectro-
scopic grade ethanol and tetrahydrofuran were
purchased from HiMedia, India and used without further
purification.
Instrumentation and Methods
UV-visible absorption spectra were recorded with a
pair of quartz cells of 3 ml volume and 10 mm path length
in distilled CH2Cl2 using an Agilent Cary 100 spectro-
photometer. The fluorescence emission spectra were
recorded on a Hitachi F-4600 spectrofluorometer using
a quartz cell of 10 mm path length and 3 ml volume.
1H NMR spectra were recorded on JEOL ECX 400 MHz
spectrometers using CDCl3 as a solvent containing
0.03% TMS (v/v). MALDI-TOF-MS spectra were
measured using a BrukerUltrafleXtreme-TN MALDI-
TOF/TOF spectrometer using HABA as a matrix and
DFT studies were carried out using B3LYP functional
with 6-31 G basis set with the Gaussian 03 software
package. The HOMO and LUMO levels were obtained
from ground state redox potentials and the difference
of the potentials for the NHE (Uredox) an electron in
vacuum (E, Fredox).
Scanning Electron Microscopy images of electrodes
were taken after sputtering the surface with gold using
Scanning Electron Microscope (JEOL JEM2010) operated
at 15 kV and magnification of 2000X. All cells corres-
ponding were tested under 350 W/m2 illuminations using
artificial sunlight using Zahner’s CIMPS-QE/IPCE system
and Newport CIMPS-QE/IPCE system respectively.
EXPERIMENTAL SECTION
Chemicals and Materials
Pyrrole, KOH and Na2SO4 were purchased from
HiMedia, India and used as received. Propionic
acid was purchased from SDFCL and used without
further purification. All aromatic aldehydes used in
the presenting work were purchased from Alfa Aesar
and used as received. All the solvents employed in the
electronic spectral and electrochemical studies were of
analytical grade and distilled or dried before use. TBAPF6
for cyclic voltammetric studies was purchased from Alfa
Aesar and twice recrystallized from ethanol and dried
for two days under vacuum at 60°C. FTO coated Glass,
Fabrication of DSSCs
FTO slides were successively cleaned in distilled
water, ethanol and acetone for cycles of 30 min each
respectively. Synthesis of nano-crystalline TiO2 was done
through hydrolysis of titanium(IV) isopropoxide [15].
Titanium(IV) isopropoxide was mixed with ethanol,
and the mixture was stirred at 800 rpm for 30 min on a
-
I-/I3 electrolytes and Pt Paste were purchased from
Chart 1. Molecular structures of synthesized free base porphyrins
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EFFECT OF FUNCTIONAL GROUPS ON SENSITIZATION OF DYE-SENSITIZED SOLAR CELLS (DSSCS)
3
magnetic stirrer. This was followed by ultrasonicated for
information of these dyes has been explained using DFT
calculations.
30 min and then 24 mL distilled water was added at pace
of nearly 0.5 mL/min for 48 min using a micropipette.
The mixture was then heated to 353 K under reduced
pressure to remove the solvent. It was then further dried
in an oven at 393 K for a few hours to obtain dry TiO2
powder. TiO2 powder was then calcined in a muffle
furnace at 673 K to produce anatase TiO2 nanoparticles.
TiO2 slurry was then obtained by slowly adding acetic
acid solution (pH = 3). The slurry was ultrasonicated for
30 min, and was placed on a magnetic stirrer at 1200 rpm
for 30 min. Ultrasonication and stirring was repeated four
times to get consistent viscous slurry of TiO2.
Synthetic Procedures
5-(4′-carbomethoxyphenyl)-10,15,20-tris(4′-pyridyl)
porphyrin (H2TriPyMCMPP) and 5-(4′-carboxyphenyl)-
10,15,20-tris(4′-pyridyl)porphyrin (H2TriPyMCPP) were
synthesized according to the reported procedure and
the spectroscopic data exactly matches with reported
procedure [17].
The prepared TiO2 slurry was then doctor bladed
on FTO glass slide using doctor tape (30 mm) on one
side and sintered at an optimized temperature of 823 K
to prepare mesoporous nanocrystalline TiO2 photo
anodes. The photo anodes were dipped in the respective
dye solutions for 12 h for complete dye loading. A pt
counter electrode was fabricated through drop casting
and sintering [16]. The electrodes and counter electrodes
were used to make sandwiched devices with active cell
area of 0.15 cm2.
Synthesis of 5-(4′-pyridyl)-10,15,20-tri(4′-
carboxyphenyl) porphyrin (H2MPyTriCPP)
(H2MPyTriCPP) was synthesized by two step pro-
cedure as reported in literature [18]. First step is simple
condensation of a 3:1 mixture of pyridine-4-carbaldehyde
and 4-methoxy carbonylbenzaldehyde with pyrrole in
propionic acid using Adler’s Method which gives 5-(4′-
pyridyl)10,15,20-tris(4′-methoxycarbonylphenyl)
porphyrin (H2MPyTriCMPP). The second step is alkaline
hydrolysis of H2MPyTriCMPP, where 0.1 g (0.126 mmol)
of H2MPyTriCMPP was dissolved in 25 ml of THF. To this,
0.35g (6.2 mmol) of KOH in 0.4 ml of water was added
and heated at 75°C for 16 h. The course of the reaction was
monitored by TLC and the crude porphyrin was treated
with a 2N HCl solution, which yielded a green-colored
precipitate that was filtered and washed with water several
times and dried under vacuum. The protonated porphyrin
was neutralized by triethylamine and solvent was removed
by rotatory evaporator. The yield of the product was found
to be 72% (68 mg, 0.091 mmol). Spectroscopic data are in
accordance with the reported literature.
Synthesis and Characterization
The above mentioned A3B and A4 type porphyrins
(Chart 1) have been synthesized in two steps using
modified literature methods [17–19] via simple
condensation reaction between pyrrole and various
substituted benzaldehydes in propionic acid. The detailed
syntheticproceduresaregivenintheexperimentalsection.
Ester porphyrins were subjected to alkaline hydrolysis
to obtain the corresponding carboxyphenylporphyrins.
Scheme 1 shows the synthetic route for the preparation
of 2a and 2b whereas, Scheme S1 represents the synthetic
route for the preparation of 1a and 1b. The nitroporphyrins
were subjected to reduction using SnCl2/HCl as shown in
Scheme S2 in supporting information.
1H NMR (DMSO-d6), d (ppm) : 13.30 (s, 3H, -COOH),
9.07 (d, J = 4 Hz, 2H, Py-meso-m-phenyl), 8.91 (s, 6H,
b-pyrrole), 8.80 (s, 2H, b-pyrrole), 8.36–8.28 (m, 12H,
meso-m, p-phenyl), 8.24 (d, J = 4 Hz, 2H, Py-meso-o-
phenyl), -2.97 (s, 2H). UV/Vis in CH2Cl2: lmax [nm]
416, 513, 547, 589, 648. MALDI-TOF-MS (m/z): found
748.06, calculated 747.77 for C46H29N5O6.
The synthesized porphyrins were characterized by
various spectroscopic techniques viz. UV-Vis, fluores-
1
cence, H NMR and mass spectrometry. The structural
Scheme 1. Synthetic route for the preparation of H2MPyTriCMPP (2a) and H2MPyTriCPP (2b).
Copyright © 2017 World Scientific Publishing Company
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N. CHAUDHRI ET AL.
Synthesis of 5,10,15,20-tetrakis(4′-aminophenyl)
7.0 (d, 2H, meso-o-phenyl NH2), 4.0 (s, 2H, -NH2), -2.8
(s, 2H, NH). UV/Vis in CH2Cl2: lꢀmax [nm] 420, 516,
553, 592, 646.
porphyrin (H2TAPP)
Synthesis of H2TAPP was started with the prepar-
ation of 5,10,15,20-tetrakis(4′-nitrophenyl)porphyrin,
H2TNPP. H2TNPP was synthesized by condensing
pyrrole with 4-nitrobenzaldehyde in propionic acid as
reported in the literature [19]. H2TAPP was prepared
using modified reported procedure [19]. A 100 ml two
neck RB was charged with H2TNPP (0.1 g, 1.26 mmol)
in 15 ml of conc. HCl and then purged with argon for
RESULTS AND DISCUSSION
Optical Characterization
The absorption and emission spectra of the free base
porphyrinic dyes were recorded in CH2Cl2 at 298 K
(Fig. 1). These dyes exhibit a strong absorption band
(Soret band) in the range of 400–440 nm followed by
four Q bands in the visible region of 510–655 nm, which
can be attributed to p–p* transition of the porphyrin
core, which is the characteristic feature of free base
porphyrins. Table S1 lists the UV-Vis and fluorescence
spectral data of porphyrinic dyes in CH2Cl2 at 298 K.
No considerable shift was observed in the absorption
.ꢀ
1 h at 0°C. To this, 24 equivalents of SnCl2 2H2O in 10 ml
of concentrated HCl was added under Argon atmosphere
and then the reaction mixture was heated for 20 h at
65°C. At the end of this period, the reaction mixture was
cooled to room temperature and neutralized using 25%
NH4OH solution. The porphyrin precipitate was filtered
using G-4 crucible and washed with 20% NaOH solution
followed by extraction with CHCl3. The yield of the
product was found to be 74% (63 mg, 0.093 mmol). The
crude porphyrin was purified by column chromatography
on basic alumina using CHCl3/acetone as eluent.
1H NMR in CDCl3, d (ppm) 8.8 (s, 8H, b-pyrrole-H),
7.9 (d, 8H, meso-o-phenyl H), 7.0 (d, 8H, meso-m-
phenyl-H), 4.02 (s, 8H, NH2), -2.9 (s, 2H, NH). UV/Vis
in CH2Cl2: l max [nm] 427, 522, 564, 596, 654. ESI-MS
(m/z): 675.3002 [M]+ (calcd., 675.300) for C44H34N8.
Synthesis of 5,10,15-triphenyl-20-(4′-aminophenyl)
porphyrin (H2TriPMAPP)
The free base 5,10,15-triphenyl-20-(4′-aminophenyl)
porphyrin (H2TriPMAPP) was synthesized by a two-step
procedure.Thefirststepisthesynthesisof5,10,15-triphenyl-
20-(4′-nitrophenyl)porphyrin (H2TriPMNPP) by Adler’s
method. This involves the condensation of 3:1 mixture
of benzaldehyde and 4-nitrobenzaldehyde with pyrrole
in propionic acid. The desired porphyrin was obtained by
column chromatography using CHCl3/acetone mixture
as second fraction with 12% yield. Thus, the isolated
5,10,15-triphenyl-20-(4′-nitrophenyl)porphyrin was con-
verted into amine porphyrin by reduction with tin chloride
dihydrate. H2TriPMNPP (0.4 g, 0.606 mmol) was dissolved
in con. HCl and then purged with argon for 45 min by
.
keeping it in an ice bath. To this, 16 eq. of SnCl2 2H2O in
20 ml of HCl was added and then the reaction mixture was
refluxed for 20 h. The reaction mixture was then cooled to
room temperature and neutralized with ammonia solution
by keeping it at ice bath. The precipitated solid was
filtered with a G4 sintered crucible. The solid porphyrin
was dissolved in CHCl3 and extracted with 20% NaOH
solution. The solvent was evaporated and the crude
porphyrin was purified by column chromatography with
85% yield (324 mg, 0.515 mmol).
1H NMR in CDCl3 d (ppm): 8.9 (d, 2H, b-pyrrole), 8.8
(s, 6H, b-pyrrole), 8.2 (d, 6H, meso-o-phenyl), 7.9 (d, 2H,
meso-m-phenyl NH2), 7.7 (m, 9H, meso-m, p-phenyl),
Fig. 1. (a) The comparative absorption, (b) Emission Spectra of
H2MPyTriCPP, H2MPyTriCMPP, H2TriPMAPP and H2TAPP in
CH2Cl2 at 298K
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EFFECT OF FUNCTIONAL GROUPS ON SENSITIZATION OF DYE-SENSITIZED SOLAR CELLS (DSSCS)
5
wavelength of the porphyrin dyes having carboxylic acid
Structural Characterization
and pyridyl groups, whereas tetra-aminophenylporphyrin
(H2TAPP) exhibited 7–11 nm red shift in the Soret band
and 8 nm in Qx(0,0) band as compared to other dyes due
to the presence of four electron donating amino groups
which raise the HOMO level resulting in a decrement of
the HOMO–LUMO gap.
Density Functional Theory (DFT) Calculations
The B and Q bands both arise from p–p* transitions
and can be explained by a four-molecular orbital model.
According to the Gouterman four-orbital model, the
absorption band in the porphyrin system arises from
transitions between both HOMOs and LUMOs, and it is
the identities of the metal center and the substituent on the
ring that affect the relative energies of the transitions. For
a perfect porphyrin system these orbitals are degenerate
due to symmetry of the system which gives two Q states
[20]. The substituents attached to the porphyrin core
may distort the symmetry while also leading to extended
conjugation which strongly influences the frontier
orbitals of the porphyrin moiety.
The DFT calculations of these dyes have been carried
out using Gaussian 09 with the basis set of 6-31G. The
functional (B3LYP) is used due to the reasonable accuracy
with affordable computational cost [21]. In all cases,
a planar conformation of the porphyrin core has been
observed, as shown in Fig. S1. Figure 2 and Figs. S2–S6
show the frontier molecular orbitals of H2TriPyMCPP
and other five porphyrinic dyes used in this study.
To elucidate the role of different donor functional
groups on the emission profile of the porphyrins, the
fluorescence spectra of all synthesized porphyrins were
recorded. Figure 1b shows the comparative electronic
emission spectra of representative porphyrin dyes in
CH2Cl2 at 298 K. Porphyrins were excited at B band (in the
range of 416–427 nm), and fluorescence was monitored
between 575 to 800 nm. The emission spectral data of
these porphyrins are listed in Table S1. No significant
shift in the emission band was observed for these dyes
except in the case of H2TriPyMCPP and H2TAPP. These
dyes exhibit a marginal red shift of 8–10 nm as compared
to other dyes due to the presence of electron-donating
groups such as amine and electron-withdrawing pyridyl
groups. The electronic absorption and emission spectral
data are in good agreement with the reported literature
[17–19].
Fig. 2. Frontier molecular orbitals (FMOs) of H2TriPyMCPP using Gaussian 09 with B3LYP/6-31 G basis sets
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N. CHAUDHRI ET AL.
of the -NH2 group causes a higher shift in the HOMO
energy level of H2TAPP as compared to H2TriPMAPP
as shown in Table 1. The distribution of HOMO–LUMO
energy levels in porphyrinic dyes is largely localized in
donor and acceptor groups [23]. The conduction band of
TiO2 is located at -4.0 eV, while the redox potential of the
electrolyte is located at -4.9 eV as shown in Fig. 3 [1, 4a].
These porphyrins are sufficient to ascertain the injection
and regeneration reaction from a thermodynamic point of
view. Figure 3 clearly indicates the feasibility of electron
transfer from porphyrin to the CB of TiO2.
SEM images of a bare, sintered TiO2 layer and the
porphyrin dye loaded TiO2 layer are shown in Figs 4a
and 4b, respectively. Highly porous electrode layers
with large surface areas are readily available to dyes for
rapid adsorption, thus enhancing the short circuit current
density. As a result, mesoporous electrode materials show
enhanced electrochemical performance compared with
their bulk counterparts [24]. The presence of holes and
extensive bridging as depicted in Fig. 4a is an indicator
of highly conducting mesoporous and crystalline TiO2
layers whereas Fig. 4b reveals extensive adsorption of
the porphyrin dye on the TiO2 nanoparticles.
Fig. 3. Energy level diagram of free base porphyrin sensitizers
compared with conduction band of nanocrystalline TiO2 and
I-/I3- redox couple
DFT calculations reveal the significant variation of the
HOMO and LUMO energy levels in mono/tetra-amino
porphyrins as compared to pyridylporphyrins as indicated
by the presence of electron density on the phenyl ring
of the amino group(s) (Figs. S2 and S3). HOMO–1 of
H2TAPP represents the electronic contribution of amino
donor moiety which clearly indicates the possibility
of charge transfer from HOMO-1 to LUMO. DFT
studies indicate that the major transitions are HOMO to
LUMO+1 and HOMO–1 to LUMO.
Photovoltaic studies
Figure 3 represents the HOMO–LUMO energy
levels of free base porphyrin dyes calculated from
DFT studies in comparison with the conduction band
of TiO2 and I-/I3- redox couple indicates the viability of
electron transfer. The calculated HOMO energy levels
lie in the range of -4.592 to -5.732 eV, while the LUMO
energy levels are in the range of -2.041 to -3.017 eV
as listed in Table 1. The calculated energy levels are in
agreement with transformed CV values [22] as indicated
by the data obtained from redox potential and optical
energies. The HOMO–LUMO energy gap is lower for
amino porphyrins (2.55–2.60 eV from DFT calculation
and 2.17–2.20 V from CV studies) as compared to
pyridylporphyrins (2.68–2.71 eV from DFT calculations
and 2.35–2.37 eV from CV studies) due to the strong
electron-donating nature of amino groups. The HOMO
and LUMOs are similar in pyridyl porphyrins, whereas
the difference in amino porphyrins can be clearly seen
in Figure 3. It appears that the electron-donating nature
The DSSCs were sensitized using synthesized free
base porphyrin dyes on mesoporous nanocrystalline TiO2
photoanodes assembled into standard sealed sandwiched
cells with Pt sheet counter electrode and iodide/triiodide
electrolyte. Figure 5 shows the I–V characteristics of the
DSSC using various free base porphyrins. Table 2 lists
the photovoltaic parameters of free base porphyrinic
dyes. We have obtained the photon-to-current conversion
efficiency (PCE%) in the range of 0.0039% to 3.26%.
The DSSCs sensitized with H2TriPyMCPP have shown
the highest conversion efficiency of 3.26%, possibly due
to the presence of a higher number of pyridyl groups
acting as electron injecting anchoring groups. It is
reported that the pyridyl ring present in the dye enables
good electronic communication between the dye and
TiO2 due to the formation of a coordinate bond between
the pyridine ring of the dye and the Lewis acid site of the
TiO2 surface [25].
Table 1. Calculated energy levels and HOMO–LUMO gap in (eV) obtained from DFT calculations
Porphyrins HOMO-1 (ev) HOMO (ev) LUMO (ev) LUMO+1 (ev) HOMO–LUMOgap (ev)
H2TriPMAPP
H2TAPP
-5.425
-5.106
-5.913
-6.003
-6.025
-6.056
-4.999
-4.592
-5.563
-5.658
-5.690
-5.723
-2.367
-2.041
-2.875
-2.969
-2.984
-3.017
-2.331
-2.004
-2.857
-2.915
-2.963
-2.996
2.60
2.55
2.68
2.69
2.70
2.71
H2MPyTriCMPP
H2MPyTriCPP
H2TriPyMCMPP
H2TriPyMCPP
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EFFECT OF FUNCTIONAL GROUPS ON SENSITIZATION OF DYE-SENSITIZED SOLAR CELLS (DSSCS)
7
Fig. 4. SEM images of (a) bare TiO2 layer and (b) porphyrin dye loaded TiO2 layer
anchoring groups. In agreement with this H2TAPP with
four anchoring -NH2 groups shows approximately four
times higher efficiency as compared to H2TriPMAPP
having one anchoring -NH2 group. The dyes with ester
groups have shown negligible PCE% values as compared
to their counterpart dyes having acid groups, as they
cannot coordinate with TiO2 effectively, unlike -COOH.
The power conversion efficiency (h) of the above
reportedfreebasedyesfollowstheorder:H2TriPyMCMPP
(h = 0.0083%) < H2MPyTriCMPP (h = 0.0093%) <
H2TriPMAPP (h = 0.7%) < H2MPyTriCPP (h = 2.84%) <
H2TAPP (h = 2.94%) H2TriPyMCPP (h = 3.26%).
The incident photon to current efficiency (IPCE)
spectra were measured and was performed as a function
of incident wavelength to evaluate the photovoltaic
performance. The IPCE spectra of fabricated solar cells
are shown in Fig. S7 in the Supporting information. In
all cases, the solar cells fabricated using H2TriPyMCPP
exhibited good IPCE response in Soret band region,
which is consistent with a higher Jsc value (Table 2)
as compared to other dyes. The amino porphyrin dyes
Fig. 5. The typical I–V curve under 350 W/m2 solar illumination
for the DSSC using synthesized free base porphyrin dyes
H2TAPP achieved the second highest performance in
power conversion efficiency among the dyes tested, due
to the presence of more number of electron donating
Table 2 Photovoltaic parameters of free base porphyrins under 350 W/m2 solar
illumination with an active area of 0.15 cm2
Porphyrin
V (V)
Jsc (A/m2)
30.00
0.38
Fill factor
0.610
Efficiency (h)
2.84%
oc
H2MPyTriCPP
H2MPyTriCMPP
H2TAPP
0.555
0.233
0.547
0.575
0.233
0.502
0.370
0.00934%
2.94%
31.67
31.82
0.15
0.595
H2TriPyMCPP
H2TriPyMCMPP
H2TriPMAPP
0.642
3.26%
0.378
0.00385%
0.7%
23.44
0.545
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J. Porphyrins Phthalocyanines 2017; 21: 7–9

8
N. CHAUDHRI ET AL.
demonstrated moderate response while the ester group
containing porpyrinic dyes exhibited very poor response
towards IPCE, which is consistent with the Jsc and Voc
values.
H2TriPyMCPP showed Voc, Jsc and fill factor values
of 575 mV, 31.82 A/m2 and 0.642, respectively with the
PCE value of (h) 3.26% (Table 2).
Grätzel M. J. Am. Chem. Soc. 2005; 127: 16835–
16847. (b) Yu Y, Wang Z, Yi N, Zu J, Zhang M and
Wang P. ACS Nano 2010; 4: 6032–6038. (c) Spettel
KE and Damrauer NH. J. Phys. Chem. C 2016; 120:
10815–10829.
5. Chen CY, Wang M, Li JY, Pootrakulchote N, Aliba-
baei L, Ngoc-Le CH, Decoppet JD, Tsai JH, Grätzel
G, Wu CG, Zakeeruddin SM and Grätzel M. ACS
Nano 2009; 3: 3103–3109.
6. Giribabu L and Kanaparthi RK. Curr. Sci. 2013;
104: 847–855.
CONCLUSIONS
In summary, we have synthesized meso-substituted
A3B/A4 free base porphyrins having pyridyl, carboxy
phenyl and amino groups in good yields via a simplified
two-step process, and we have investigated the effect
of these functional groups on their photosensitization
properties. The highest power conversion efficiencies
(h) of 3.26%, 2.94% and 2.84% were achieved for the
DSSC fabricated using H2TriPyMCPP, H2TAPP and
H2MPyTriCPP dyes, respectively. The introduction of
more number of electron donating groups such as the
–NH2 group enhances the PCE% as depicted by H2TAPP
which shows a fourfold higher efficiency (2.84%) as
compared to H2TriPMAPP (0.7%). Tripyridyl mono-
carboxyphenylporphyrin has shown higher PCE value as
compared to monopyridyltricarboxyphenylporphyrin due
to efficient coordination and feasible electron injection
from pyridyl group(s) to TiO2. Porphyrins having more
numbers of electron-donating/electron-injecting groups
as well as anchoring amino/pyridyl groups show higher
power conversion efficiency as compared to porphyrins
with lower number of pyridyl/amino groups.
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(b) Ball JM, Davis NKS, Wilkinson JD, Kirkpat-
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Acknowledgments
We sincerely thank Science and Engineering Research
Board (SB/FT/CS-015/2012) and Board of Research
in Nuclear Science (2012/37C/61/BRNS) for financial
support. NC thanks CSIR, India for senior research
fellowship.
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