Tuning the photovoltaic parameters of β-substituted porphyrin analogues: An experimental and theoretical approach

Article abstract of DOI:10.1016/j.jphotochem.2012.08.011

Spectroscopic and theoretical techniques were used to examine the effects of adsorption solvents and electrolyte additives on the photovoltaic performance of dye sensitized solar cells of two β-substituted zinc porphyrin analogues (A1-H and A1-M), where A

Full text of DOI:10.1016/j.jphotochem.2012.08.011

Journal of Photochemistry and Photobiology A: Chemistry 248 (2012) 63–72
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Tuning the photovoltaic parameters of ␤-substituted porphyrin analogues:
An experimental and theoretical approach
Mannix P. Balanay, Kang Hee Kim, Sang Hee Lee, Dong Hee Kim∗
Department of Chemistry, Kunsan National University, Gunsan, 573-701 Jeonbuk, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 June 2012
Received in revised form 31 July 2012
Accepted 23 August 2012
Available online 7 September 2012
Spectroscopic and theoretical techniques were used to examine the effects of adsorption solvents and
electrolyte additives on the photovoltaic performance of dye sensitized solar cells of two ␤-substituted
zinc porphyrin analogues (A1-H and A1-M), where A1-M has an added methyl group at the phenyl ring
attached to the meso-position of the porphyrin macrocycle. These results showed that the different sol-
vents used in the sensitization process affects how the dyes are aggregated and how much of the dyes
are adsorbed on the semiconductor surface. The highest efficiency of 4.2% (30 min soaking time) and
4.0% (1 h soaking time) was achieved for A1-H and A1-M analogues, respectively. The hypsochromic shift
observed in the UV/Vis spectra of the analogues when THF was used as a solvent was due to the partial
deprotonation of the hydrogen at the carboxylic moiety of the analogues induced by the interaction of
the THF molecule. Theoretical calculations at the B3LYP/6-31G(d) level of theory indicated that when THF
was used as an adsorption solvent, it tends to form six- or five-coordinate complexes causing the dyes
to be largely separated when attached to the semiconductor thus lowering the amount of dye. The dye
molecular plane of A1-M was 13◦ closer to the TiO2 surface than A1-H due to steric hindrance introduced
by the methyl group. This increases the surface area occupied by the A1-M dye, as indicated by the smaller
amount of dye adsorbed on the TiO2 surface. The addition of 4-tert-butylpyridine or the replacement of
lithium ions with guanidinium thiocyanate decreased the solar cell efficiency by 12 and 37%, respectively,
which was attributed to a decrease in the electron injection processes.
Keywords:
Dye sensitized solar cell
Density functional theory
Solvent effect
Multi-coordinate porphyrin complex
Tert-butylpyridine
Guanidinium thiocyanate
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
analogues, the substitution at the meso- or ␤-position of the macro-
cycle with electron-withdrawing or -donating groups leads to a
Porphyrins, which are considered to be natural light harvesters,
have long been studied for a range of material applications [1].
Currently, the focus is more on the applications of these macro-
cycles in photoelectric devices, particularly for solar cells [2–16].
Dye sensitized solar cells (DSSCs), which are also known as Grätzel
cells, offers a viable alternative to conventional all-inorganic solar
cells due to its lower production cost. There are three essential
parts of DSSCs: the photoelectrode, electrolyte, and counter elec-
trode. The photoelectrode is composed of fluorine-doped SnO2
(FTO) glass coated with a sensitizer that is anchored chemically to
the semiconducting TiO2 surface, whereas the counter electrode is
FTO coated with platinum [17]. Most studies focused on enhancing
the properties of the photoelectrode, specifically the dye, which is
responsible for most light absorption. Some of the important prop-
erties that need to be considered when designing sensitizers for
DSSCs are the geometric structures, molecular orbital (MO) energy,
absorption profiles, and aggregation states of the dye. For porphyrin
perturbation of the MO, which affects the photophysical properties.
Substitution at the ␤-position was reported to exert more steric
and electronic effects in the porphyrin macrocycle than those at
the meso-positions, which leads to a more nonplanar distortion of
the macrocycle often resulting in a larger red-shift in the absorption
spectra [18–24].
This paper reports the synthesis and application of two
␤-substituted Zn porphyrin analogues, one with
a known
photo-to-current conversion efficiency (cyano-3-(2ꢀ-5ꢀ,10ꢀ,15ꢀ,20ꢀ-
tetraphenylporphyrinato zinc(II)yl)acrylic acid (A1-H)) [3] and
the other with an added electron-donating moiety (cyano-3-(2ꢀ-
5ꢀ,10ꢀ,15ꢀ,20ꢀ-tetratolylporphyrinato zinc(II)yl)acrylic acid (A1-M))
as a sensitizer in dye sensitized solar cells (Scheme 1). The
photophysical properties and photovoltaic performance of these
analogues were assessed with different types of soaking solu-
tions using different spectroscopic and theoretical techniques.
Porphyrins containing electron-donor and -withdrawing groups or
substituents that contribute to ␲–␲ stacking of the analogues can
exhibit solvatochromic behavior [25–27]. The ability of the solvent
to chelate to the central metal of the porphyrin macrocycle can
affect the aggregation and orientation of the porphyrin monomers
∗
Corresponding author. Tel.: +82 63 469 4576; fax: +82 63 469 4571.
E-mail address: dhkim@kunsan.ac.kr (D.H. Kim).
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.08.011

64
M.P. Balanay et al. / Journal of Photochemistry and Photobiology A: Chemistry 248 (2012) 63–72
Scheme 1. Synthesis of Zn porphyrin analogues.
in solution, which affects the packing of the analogues at the surface
of the semiconductor during the sensitization process. In this study,
the effect of two solvents, tetrahydrofuran (THF) and toluene, with
different solvation characteristics was evaluated. THF is a common
adsorption solvent in DSSCs because it can dissolve a wide range
of non-polar and polar analogues. In Zn porphyrin analogues, THF
has a tendency to bind to the axial position of Zn forming five-
or six-coordinate structures [28–30], which can affect the pack-
ing of the analogue at the surface of the semiconductor. On the
other hand, toluene is a non-polar non-coordinating solvent. Theo-
retical calculations were used to better understand the effect of
the solvents in the zinc porphyrin analogues. There are 3 basic
ways of incorporating solvent effects into theoretical calculations.
The first is using an implicit solvent model, wherein the solvent
is modeled as a dielectric continuum of permittivity that forms
a cavity around the solute. The most common implicit solvation
frameworks are the polarizable continuum model (PCM) [31] and
its variant the conductor-like PCM (C-PCM) [32]. Of these two, the
C-PCM provides results that agree well with experimental data with
less computational time [20,32,33]. The second method employs
an explicit approach, where the number of solvent molecules is
included to allow an interaction with the solute molecules [34,35].
Lastly is the hybrid (implicit–explicit) solvation model, where a
small number of solvent molecules are included in the calculation
together with an implicit solvent model to account for the local
and long-range solvation effects [36]. Both experimental and the-
oretical approaches are needed to obtain insight into the effect of
the solvent in the photophysical state of the dye, which affects the
adsorbed amount and efficiency of the solar cell.
In an effort to further improve the solar-to-electric conver-
sion efficiency, the effect of the electrolyte composition on the
photovoltaic performance of porphyrin-based DSSCs was also
investigated. For most DSSCs, the electrolyte is composed mainly
of imidazolium iodide, iodine, lithium iodide, guanidinium thio-
cyanate (GuSCN), and 4-tert-butylpyridine (4-tBP). For the N719
dye, which achieved an efficiency of 11.18% at 1 sun, an A6141
electrolyte, which has a similar composition to the electrolyte men-
tioned above but without LiI, was used [37]. In studies involving
␤-substituted porphyrins, the researchers did not consider the use
of GuSCN and there is no clear correlation between the effect of
4-tBP on its photovoltaic performance [2–5]. However, in meso-
substituted porphyrins, researchers normally use 4-tBP as part of
the composition of the electrolyte [7,8]. Therefore, it is essential
to examine how the composition of the electrolyte can affect the
efficiency of the ␤-substituted porphyrin-based DSSCs.
In this study, the photovoltaic performance of Zn porphyrin ana-
logues was assessed based on the ability of the adsorption solvents
to coordinate with the axial position of the metal in the porphyrin
macrocycle. In addition, careful consideration is undertaken on the
soaking time, addition of the co-adsorbent, and the electrolyte com-
position to understand its effects on the solar cell efficiency.
2. Experimental method
2.1. Materials and synthesis
Analytical reagent grade solvents and reagents were used for
the synthesis of cyano-3-(2ꢀ-5ꢀ,10ꢀ,15ꢀ,20ꢀ-tetraarylporphyrinato
zinc(II)yl)acrylic acid analogues and distilled laboratory grade sol-
vents were used for chromatographic processes. A gravity or flash
chromatography using silica gel (0.040–0.063 mm) was used for
compound separation and purification processes. Thin-layer chro-
matography (TLC) was performed using precoated silica gel (TLC
silica gel 60 F254). All fractions or solutions containing a single spot
in TLC with the same Rf were combined and filtered, and the sol-
vent was removed using rotary evaporator and then dried overnight
under high vacuum.
The details of the synthetic procedure shown in Scheme 1 were
presented by Bonfantini et al. [38] and Officer et al. [3]. The syn-
thesis started with the typical porphyrin condensation reaction
followed by a series of Wittig reactions using the appropriate func-
tionalized aldehydes. The introduction of the substituents at the
␤-position was done via the Vilsmeier reaction. The final product
was obtained via a Knoevenagel condensation of the aldehyde with
cyanoacetic acid followed by the addition of phosphoric acid at pH
2 to give the desired carboxylic acid. The LC–ESI-MS data for A1-
M is 827.2 m/z. The 1H and 13C NMR spectra of A1-M are given
as supplementary data (Fig. S1 and S2), wherein it show typical
peaks at about 2.6 ppm for 1H and 21 ppm for 13C NMR spectra cor-
responding to methyl moiety on the phenyl ring attached to the
meso-position of the porphyrin macrocycle.
2.2. Analytical measurements
All UV–visible absorption spectra were measured on a Shimadzu
UV-2401PC Spectrometer. The concentration of solutions used in
the absorption experiment is set at 2 × 10−6 M. 1H and 13C NMR
spectra were obtained using a varian VNMRS 500 MHz NMR spec-
trometer. The compounds were dissolved in DMSO-d6 and chemical
shifts are relative to tetramethylsilane. LC-ESI-MS data were done

M.P. Balanay et al. / Journal of Photochemistry and Photobiology A: Chemistry 248 (2012) 63–72
65
using applied biosystems 3200 Q Trap at 200 ◦C and 19 psi with a
scan range of 500–1000 m/z.
2.3. Cell fabrication
A paste consisting of 20 nm sized TiO2 (CICC, PST-18NR) was
doctor-bladed on a fluorine-doped SnO2 (FTO, Pilkington TEC-8
glass, 6–9 Ohms/sq with 2.3 mm thickness) conducting glass and
then air-dried for 2 h. The electrode was gradually heated at 325 ◦C
for 5 min, at 375 ◦C for 5 min, at 450 ◦C for 15 min, and at 500 ◦C for
15 min. Subsequently, a second scattering layer made up of a paste
containing 400 nm anatase TiO2 particles (CICC, PST-400 C) was fur-
ther coated onto the first layer to form a light-scattering layer.
The electrode was air-dried and then sintered in the same way
as the first layer. The TiO2-coated FTO glass plate was immersed
in 40 mM aqueous TiCl4 solution at 70 ◦C for 30 min and subse-
quently washed with water and ethanol. The electrode was then
heated at 500 ◦C for 30 min and allowed to cool to 80 ◦C before dip-
ping into 0.2 mM dye solutions. After soaking, the electrodes are
kept in the dye solutions for 2 h. The dye-coated electrodes were
rinsed with ethanol. To prepare the counter electrode, a small hole
was drilled in an FTO glass and the Pt catalyst was deposited on the
FTO glass by spin-coating drops of H2PtCl6 solution (2 mg Pt in 1 mL
of ethanol) on the FTO plate and then sintering at 400 ◦C for 15 min.
A sandwich cell was prepared by using the dye-anchored TiO2
film as the working electrode and the counter Pt-electrode which
were put together with a thin Surlyn polymer transparent film in
between the electrodes (SX 1170-25, 25 ␮m). The sandwich cells
were lightly compressed at 110 ◦C to seal the two electrodes. A thin
layer of electrolyte was introduced into the inter electrode space
from the counter electrode side through pre-drilled holes using vac-
uum backfilling method. The drilled holes were sealed with Surlyn
and microscope cover slides to avoid leakage of the electrolyte solu-
tion. Three different kinds of electrolyte were used in the cells,
Electrolyte A (0.6 M butylmethylimidazolium iodide (BMII), 0.05 M
I2, 0.10 M LiI in 1:1 acetonitrile/valeronitrile (CH3CN/C4H9CN) sol-
vent); Electrolyte B (0.6 M BMII, 0.05 M I2, 0.10 M GuSCN in 1:1
CH3CN/C4H9CN); and Electrolyte C (0.6 M BMII, 0.05 M I2, 0.10 M
LiI, 0.5 M 4-tBP 1:1 CH3CN/C4H9CN).
Scheme 2. Equivalent circuit proposed to fit the EIS data. The (A and B) labels in the
circuit correspond to the counter electrode/electrolyte and TiO2/electrolyte inter-
faces, respectively. R corresponds to resistance, while CPE is the constant phase
element.
2.6. Photovoltaic measurements
The current–voltage characteristics of the devices were carried
under simulated AM 1.5G irradiation using Oriel sol 3A class AAA
Solar simulator equipped with a 450 W xenon lamp attached to a
Keithley 2400 source meter. The light intensity was calibrated with
a crystalline silicon reference cell (VLSI standard, SRC-1000-TC-K-
KG5-N). The solar cell efficiency (ꢁ) is obtained with these relations,
ꢁ = (JSC · VOC · FF)/Pin in which JSC (mA/cm2) is the current density
measured at short circuit, VOC (V) is the voltage measured at open
circuit, FF is the fill factor and Pin is the input radiation power (for 1
sun illumination (AM 1.5G), Pin = 100 mW cm−2). A black mask was
applied to the area surrounding the TiO2 with an illuminated active
area of 0.159 cm2 for all measurements. The incident photon-to-
current conversion efficiency (IPCE) spectra were measured using
the Oriel QE-PV-SI QE/IPCE measurement kit with a silicon detector.
2.7. Electrochemical impedance measurement
The electrochemical impedance spectroscopy (EIS) for DSSCs
with a forward bias of −0.5 V under dark condition was measured
with Princeton Applied Research VersaStat MC. The spectra were
scanned in a frequency range of 10−1–105 Hz at room temperature.
The alternate current amplitude was set at 10 mV. The device was
connected in a three-electrode configuration: the sensitized porous
electrode was connected as working electrode and the counter and
reference electrodes were connected at the Pt counter electrode
making it a pseudo-reference electrode. The resulting curves were
fitted to an appropriate equivalent circuit comprising a series of two
Randles-type circuits with series resistance shown in Scheme 2.
Each circuit represent one electrode interface: circuit A is the redox
charge transfer response at the counter electrode/electrolyte inter-
face and circuit B is electron transfer and recombination kinetics at
the TiO2/electrolyte interface. RS is the series resistance accounting
for the transport resistance of the FTO glass and contact resistance
of the cell.
2.4. Measurement of absorption spectra in TiO2
The preparation of TiO2 films were done using a microscope
slide and was doctor-bladed with 20 nm sized TiO2 with a cell area
of 1 cm2. The glass slide coated with TiO2 was air-dried and then
sintered in the same way as the FTO coated glass. The TiO2-coated
glass plate was immersed in 40 mM aqueous TiCl4 solution at 70 ◦C
for 30 min and subsequently washed with water and ethanol. The
electrode was then heated at 500 ◦C for 30 min and allowed to cool
to 80 ◦C before dipping into 0.2 mM Zn porphyrin dye solutions and
were kept in the dark for 2 h. The dye-coated electrodes were rinsed
with ethanol and the absorption spectra of the dyes in TiO2 were
taken.
2.8. Theoretical calculations
The ground state geometries of the analogues using B3LYP
[39,40] exchange correlation (xc) functional with 6-31G(d) basis
set were recovered from reference [41]. Solvation effect was done
using the C-PCM framework [32]. The calculation of singlet state
excitation energies based on time-dependent density functional
theory was done with CAM-B3LYP [42]. xc functional, since B3LYP
normally fails to describe valence, Rydberg, and charge transfer
state transitions [43–46].
2.5. Determination of the amount of dye
To determine the amount of dye absorbed on the TiO2 films
(using the TiO2-coated glass used in absorption measurements on
TiO2), the dye was desorbed with 5 mL of 0.1 M KOH in CH3OH
overnight and a corresponding aliquot was further diluted with
the desorption solvent. Calibration curves for A1-H and A1-M in
0.1 M KOH in CH3OH at their corresponding ꢀmax were obtained.
The amount of dye (nmol cm−2) was calculated according to the
Beer’s law based on the measured absorbance.
The supermolecular method was used to calculate the binding
energies (adsorption energies) of the system based on the following
relation:
ꢀ
Eads = Ecomplex
−
(Eanalogue + nEsolvent)The Ecomplex is the energy
of the complex, Eanalogue and Esolvent are the energies of the iso-
lated Zn porphyrin analogue and the THF molecule, respectively,

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M.P. Balanay et al. / Journal of Photochemistry and Photobiology A: Chemistry 248 (2012) 63–72
Table 1
Absorption spectral data of A1-H and A1-M in different solvents.
Analogue
A1-H
Condition
Solvent/soaking solution
Absorption peaks, nm (ε × 104, M−1 cm−1
)
B-band
Q2-band
Q1-band
THF
Toluene
THF
436(17.4)
454(10.4)
460
567 (1.37)
568 (0.81)
570
612(0.75)
616(0.75)
620
In solution
In film
Toluene
461
570
621
A1-M
THF
Toluene
THF
438(14.8)
456(8.6)
460
568 (1.32)
569 (0.70)
572
616(0.85)
617(0.63)
622
In solution
In film
Toluene
461
572
523
and n is the number of solvent molecules explicitly included in the
formation of the complex.
All theoretical calculations were performed using the Gauss-
ian 09 software package [47]. Natural bond orbital (NBO) analysis
was performed with NBO version 3.1 incorporated in the Gaussian
package using ground state structures.
All of the analogues exhibit negative solvatochromism, espe-
cially at the Soret band region, wherein both analogues are
blue-shifted by 18 nm when THF (ε = 7.43) was used as a solvent as
compared to toluene (ε = 2.38). The blue-shifting was also observed
in free-base porphyrin compounds which suggest that the metal-
center of the porphyrin macrocycle has a minor influence on the
solvatochromatic character of the analogues [48,49]. Moreover, this
negative solvatochromatic behavior was even seen in phthalocya-
nines [50,51] and fluorene-based organic dyes [52].
3. Results and discussion
To properly identify the cause of the spectral shifts, an implicit
method was first used to calculate the excited state energies using
the C-PCM framework at different solvent conditions with the TD-
CAM-B3LYP/6-31G(d)//B3LYP/6-31G(d) theoretical method. This
method represents the solvent as a continuous medium and nor-
mally accounts for the electrostatic nature of the solvent. As shown
in Table 2, all of the excitation energies were all underestimated
compared to the experimental data. A slight blue-shift at the
B-band region was observed going from toluene to THF, which
was observed in the experimental data. Contrary to experimental
results, all the excitation energies at the Q-bands are red-shifted.
One theory of the observed blue-shift of the absorption spectra
of the analogues is probably due to the hydrogen bond interac-
tion of the oxygen of the tetrahydrofuran and the hydrogen of the
carboxylic acid of the analogue. In order to prove this theory, we
explicitly modeled A1-H analogue and THF to interact through H-
bonding. As shown in Fig. 2a, the O H bond undergoes elongation
of 2.63 pm when interacted with THF. An analysis of the second
3.1. Absorption profile of Zn porphyrin analogues
The absorption spectra of the zinc porphyrin analogues (A1-H
and A1-M) were recorded in two solvents: (a) tetrahydrofuran, a
polar coordinating solvent, which has a capability to coordinating
with the axial position of the metal in the porphyrin macrocycle and
also undergoes hydrogen bonding interaction with the carboxylic
acid moiety of the analogue that may induce partial deprotonation;
and (b) toluene, a non-polar non-coordinating solvent. The relative
intensities and wavelengths of the Soret or B-band and Q-bands
for the analogues are presented in Table 1 and their corresponding
spectra are shown in Fig. 1.
Between the analogues, A1-M exhibited a more red-shifted
spectra as compared to A1-H in both solvents indicating the addi-
tion of methyl groups to the meso-substituted phenyl rings in A1-M,
contributes to the increased electron-donating capacity of the por-
phyrin macrocycle, which increases the electron charge-transfer
from the donor to the acceptor group.
Fig. 1. Absorption spectra of A1-H (a and c) and A1-M (b and d) in the THF (solid lines) and toluene (dashed lines) solvents. The insets show an enlarged version of the
absorption spectra at the Q-band regions.

M.P. Balanay et al. / Journal of Photochemistry and Photobiology A: Chemistry 248 (2012) 63–72
67
Table 2
The four lowest excitation energies (nm) with its corresponding oscillator strengths (f) of A1-H and A1-M.
State
Excitation energies/nm (f)
Assignment
A1-H
A1-M
Gas
Gas
Toluene
THF
Toluene
THF
561.0
(0.0400)
556.8
(0.0182)
386.8
(1.6702)
378.2
569.0
(0.0841)
562.5
(0.0229)
412.9
(2.0878)
399.8
571.8
(0.0851)
563.4
(0.0289)
411.9
(2.0049)
398.8
564.0
(0.0523)
557.9
(0.0144)
389.5
(1.7286)
380.9
572.3
(0.1013)
563.3
(0.0240)
415.3
(2.1477)
401.4
575.0
(0.1020)
564.1
(0.0223)
414.0
(2.0700)
400.3
Q1
Q2
B1
B2
1
2
3
4
(1.0010)
(1.3175)
(1.1840)
(0.9956)
(1.3176)
(1.1878)
order perturbation theory of the Fock matrix based in NBO basis
half maximum (FWHM) at the Soret band (Fig. 1c and d) indicating
that toluene has a wider FWHM compared to THF (A1-H: toluene
(FWHM = 2088 cm−1) > THF (FWHM = 1828 cm−1); A1-M: toluene
(FWHM = 2145 cm−1) > THF (FWHM = 2144 cm−1)). This shows that
toluene is more aggregated in film as compared to THF.
indicated that the stabilization energy at the nO1 → nH interaction
before and after the THF association decreased by 70.56 kcal mol−1
.
A nO2 → nH interaction with a 34.87 kcal mol−1 stabilization energy
was observed when THF was explicitly added to A1-H which corre-
sponds to the H-bond formation. These resulted in the elongation
of the O H bond supporting the partial deprotonation theory.
A hybrid solvation model was used to calculate the excited state
energies of A1-H with an H-bonded THF moiety, while only the
implicit model was applied when toluene was used as a solvent.
The schematic spectrum of the corresponding solvents of A1-H is
shown in Fig. 2b. The transition energies at both bands (B- and Q-
bands) were consistent with the blue-shift in the absorption spectra
in solution when THF was used as a solvent as compared to toluene.
This confirms the theory of partial deprotonation when THF was
used as a solvent.
To determine why THF has a smaller amount of dye adsorbed
in the semiconductor, different interactions of THF in A1-H were
explicitly modeled to account for the solute–solvent interactions
in solution. THF has a tendency to chelate to the central metal
atom of the porphyrin macrocycle, acting as a donor solvent,
and at the same time has the ability to bind to carboxylic acid
through H-bonding. Fig. 3 shows the optimized geometry of the
A1-H(THF)n(+1) complex calculated at B3LYP/6-31G(d), where the
+1 in the subscript indicates THF bonding to the carboxylic acid.
The most favored conformation is A1-H binding with three THF
molecules with a binding energy of −36.14 kcal mol−1, where
two THF molecules form a six-coordinate Zn complex and the
other THF interacts with the carboxylic acid of the acceptor group
through H-bonding (Fig. 3a). The mean Zn O bond distance of
3.2. Influence of the adsorption solvent on DSSCs performance
˚
this complex was 2.392 A, which was only 1.2 pm larger than the
The soaking solvent plays a vital role in the efficiency of the
solar cells which influences the coverage of the dye on the TiO2
surface and also plays an active role in the formation of dye-
solvent–TiO2 complex [53]. The solar cell performances of the zinc
porphyrin analogues using THF and toluene as adsorption solvents
are presented in Table 3. It was observed that all the photovoltaic
parameters, such as the short circuit photocurrent density (JSC),
open circuit voltage (VOC) and fill factor (FF), were affected by the
soaking solvent. Toluene produced a lower photovoltaic values as
compared to THF under standard AM 1.5 solar conditions. The TiO2
surface concentration of A1-H sensitized in THF and toluene were
determined to be 37 and 41 nmol cm−2, respectively, while 23 and
27 nmol cm−2 for THF and toluene, respectively, was observed for
A1-M. For both analogues the increase in the absorbed amount
when toluene was used as a solvent resulted in lower photovoltaic
efficiencies which could be due to the aggregation of the dye at the
TiO2 surface.
experimentally determined crystal structure of ZnTPP(THF)2 [43].
The five-coordinate Zn porphyrin structures with another THF H-
bonded to the carboxylic acid (Fig. 3b and c) has about 6 kcal mol−1
higher binding energy than the six-coordinate counterpart. A1-
H(THF)1+1 undergoes out-of-plane distortion of the macrocycle
forming a square pyramidal configuration as shown in Fig. 3b and
c. The conformation of the five-coordinate Zn porphyrin structure
is similar to that observed with high-spin iron(II) porphyrins [54].
The presence of either six- or five-coordinate complexes in solution
makes the dye far apart from each other hence lowers the amount
of dye adsorbed in the semiconductor. This multi-coordinated por-
phyrin complex could also be the reason for the increase of VOC
when THF was used as a solvent (Table 3) as compared to toluene
due to the minimization of the interaction between the redox
species and the TiO2 surface.
A comparison of the two analogues using the same adsorp-
tion solvent showed that the A1-M analogue produced a lower
solar-to-electric conversion efficiency than A1-H. Assuming that
The aggregation of the dye when toluene was used as a sol-
vent was even supported with the analysis of the full width at
Fig. 2. (a) A truncated view of A1-H analogue showing the H-bond interaction with THF together with the selected bond lengths (bond length before interaction with THF is
enclosed in parenthesis). (b) Comparative schematic spectrum of A1-H in THF (solid line, hybrid solvation model) and toluene (dashed line, explicit solvation model).

68
M.P. Balanay et al. / Journal of Photochemistry and Photobiology A: Chemistry 248 (2012) 63–72
Table 3
Photovoltaic performances of Zn porphyrin analogues at different soaking solutions.a
Analogue
A1-H
Adsorption solvent
VOC (mV)
JSC (mA/cm2)
FF (%)
ꢁ (%)
THF
Toluene
452
424
12.9
13.6
64.1
62.7
3.73
3.62
A1-M
THF
Toluene
448
433
12.2
11.3
66.6
63.8
3.65
3.12
a
Photovoltaic performance were measured at AM 1.5 irradiation with 0.159 cm2 working area. Soaking time was 2 h. Electrolyte A: 0.6 M BMII, 0.05 M I2, 0.10 M LiI in 1:1
CH3CN:C4H9CN.
Fig. 3. Structures of the different interactions of THF with A1-H: (a) A1-H(THF)2+1, (b) A1-H(THF)1a+1 and (c) A1-H(THF)1b+1, calculated at B3LYP/6-31G(d). Also shown are
the adsorption energies and the Zn O and H-bond distances.
both analogues bind to the semiconductor through the carboxylic
group of the cyanoacrylic acid in bidentate mode, the lower
efficiency of A1-M might be due to the coupling orientation of
the acceptor group to the surface of the TiO2. A previous study
already established the geometry of A1-H [41] in that the acceptor
group is slightly co-planar with the porphyrin macrocycle and
should assume a bent configuration to obtain bidentate bridging
to the surface of the semiconductor. The Ti3O10H12 structure for
TiO2 was used to analyze the bending configuration of the dyes
in relation to the TiO2 surface, wherein only the atoms that are
responsible for attaching the dye to the TiO2 were optimized,
while the remaining atoms remain fixed. The additional methyl
group in A1-M introduces steric hindrance when it is attached to
TiO2, which bends the dye molecular plane to approximately 13◦
more than A1-H (Fig. 4). This also increases the adsorption energy
of A1-M to 22 kcal mol−1, which makes the binding of A1-H to the
semiconductor thermodynamically stable. The larger bending of
A1-M increases the surface area of the dye thus adsorbing only a
smaller amount, hence the lower solar cell efficiency. Aside from
the dye coverage, there are also a number of other possible reasons
which makes A1-M lower in efficiency compared to A1-H. Due
to the close proximity of the porphyrin macrocycle with the TiO2
surface the electron injection process could involve a transition
from the porphyrin macrocycle to the TiO2 surface without going
through the ␲-spacer group; it could be a result of the piling of
multiple porphyrin analogues resulting in higher dye–dye inter-
actions; or the lower dye coverage could result in the penetration
of the electrolyte species in to the TiO2 surface [53,55].
The incident photon-to-charge carrier efficiency action spectra
for DSSCs based on A1-H and A1-M in THF is shown in Fig. 5 (Elec-
trolyte A: 0.6 M BMII, 0.05 M I2, 0.10 M LiI in 1:1 CH3CN:C4H9CN).
The IPCE at the maximum absorption of the Soret band reaches 63%
Fig. 4. Possible adsorption configurations of (a) A1-H and (b) A1-M. The inset shows the bidentate adsorption of the dye in Ti3O10H12. C1 C2 O3 determines the dye
molecular angle against the TiO2.

M.P. Balanay et al. / Journal of Photochemistry and Photobiology A: Chemistry 248 (2012) 63–72
69
Table 4
Photovoltaic performances of Zn porphyrin analogues at different soaking time.a
Analogue
A1-H
Soaking time (h)
VOC (mV)
JSC (mA/cm2)
FF (%)
ꢁ (%)
0.5
1.0
2.0
3.0
478
461
452
450
13.0
12.7
12.9
12.9
67.5
66.6
64.1
63.5
4.19
3.91
3.73
3.68
A1-M
0.5
1.0
2.0
3.0
439
465
448
439
13.2
12.8
12.2
11.9
65.3
67.5
66.6
66.4
3.77
4.03
3.65
3.46
a
Photovoltaic performance were measured at AM 1.5 irradiation with 0.159 cm2 working area. Concentration of dye = 0.2 mM in THF. Electrolyte A: 0.6 M BMII, 0.05 M I2,
0.10 M LiI in 1:1 CH3CN:C4H9CN.
to attain the highest photovoltaic performance. We also performed
different soaking time of A1-M analogue in toluene. As shown
in Table S1 (supporting data), the solar-to-electric cell efficiency
trend in soaking time is the same as with the data when THF
70
60
50
40
30
20
10
0
was used as a soaking solvent. This is probably due to the small
difference between the amount of dye between solvents. Based on
these findings, we could also expect that A1-H in toluene would
have an optimal soaking condition nearing 30 min.
Further increasing immersing time leads to the decrease in
the solar cell efficiency which was attributed to increased dye
aggregation thus increasing the recombination effect or exciton
annihilation [56], as observed with the decreasing JSC values. It was
also observed with other porphyrin analogues that the longer sen-
sitization time could lead to quenching of the absorbed light energy
through radiationless process such as intramolecular relaxation
leading to the decrease in efficiency [53].
300
400
500
600
700
Fig. 5. Spectra of monochromatic incident photon-to-current conversion efficiency
(IPCE) for DSSCs based on A1-H (solid line) and A1-M (dotted line) with electrolyte
A.
To know the effect of CDCA on the photovoltaic efficiency,
different concentration of CDCA was added to the dye (0.2 mM con-
centration) as presented in Table 5. As shown in Table 5, the value
of the short circuit current density (JSC) decreased as the concen-
tration of the CDCA increased. Since CDCA undergoes competitive
adsorption with the dye, the decrease in the photocurrent with
increasing amount of CDCA is due to the decreased amount of dye at
the semiconductor surface thus limiting the photoinduced collec-
tion. The addition of CDCA improved the VOC values as compared to
the dye without the addition of the co-adsorbent. At 1:1 dye/CDCA
ratio, it achieved the highest efficiency of 3.79%, due to the 58 mV
increase of the VOC. The improvement of the open-circuit voltage
with CDCA could be due to the prevention of the electron recom-
bination between the redox species and the injected electrons on
the TiO2 surfaces [57].
at 460 nm with A1-H analogue, while for A1-M, 56% was observed
at the same wavelength as A1-H. This is also consistent with the
observed photovoltaic parameters wherein A1-H has a higher JSC
values than A1-M.
3.3. Effect of soaking time and co-adsorbent on DSSCs
performance
As mentioned in the previous section, the analogues in consider-
ation tend to form aggregates on the surface of the semiconductor.
By varying the soaking or immersing time or the amount of 3␣,7␣-
dihydroxy-5␤-cholanic acid (chenodeoxycholic acid, CDCA), which
acts as a co-adsorbent in the dye solutions, can control the degree
of aggregation in porphyrin analogues. The photovoltaic perform-
ances of the zinc porphyrin analogues at different immersing time
using THF as solvent and at different concentration of CDCA are
presented in Tables 4 and 5.
3.4. Influence of electrolyte composition on DSSCs performance
It was shown that A1-H has
a larger amount of dye
Lithium ions (Li+), a common electrolyte additive, can improve
JSC but at the same time often results in a decrease in VOC. The
decrease in VOC was often attributed to the positive shift of the
CB of TiO2, which eventually leads in a decrease in photovoltaic
performance due to the irreversible surface intercalation of these
ions at the surface of the TiO2 films [58–60]. An alternative
compound is to replace inorganic ions with guanidinium thio-
cyanate, which improves the solar cell performance of the N3 dye
by slowing the rate of surface recombination thus reducing the
dark current [61]. To determine how Li+ affects the photovoltaic
parameters, the DSSCs using Li+ and guanidinium ions as additives
in the electrolytes were compared, as presented in Table 6. The
replacement of 0.1 M Li+ with 0.1 M of GuSCN increased the VOC by
22%, however decreased the JSC by one-half. This suggests that the
use of GuSCN negatively shifts the CB of the semiconductor giving
a much higher VOC. This is probably due to the possible adsorption
of thiocyanate ion through its nitrogen side on the unoccupied
positions of the semiconductor. The lower JSC could be due to
(37 nmol cm−2) than A1-M (23 nmol cm−2) in THF due to the
wider angle of the dye with respect to the semiconductor surface
(Fig. 4) thus A1-H is more aggregated at the surface than A1-M.
This premise is also supported by the data presented in Table 4,
wherein the most efficient soaking condition for A1-H is 30 min
which could control the amount of dye adsorbed at the TiO2
surface, while A1-M having a smaller adsorbed amount needed 1 h
Table 5
Photovoltaic performances of A1-H analogue at CDCA content.a
Dye:CDCA ratio
VOC (mV)
JSC (mA/cm2)
FF (%)
ꢁ (%)
1:0
1:1
1:10
452
510
515
12.9
10.8
6.9
64.1
69.1
69.9
3.73
3.79
2.49
Photovoltaic performance were measured at AM 1.5 irradiation with 0.159 cm2
working area. Concentration of dye = 0.2 mM in THF. Soaking time = 2 h. Electrolyte
A: 0.6 M BMII, 0.05 M I2, 0.10 M LiI in 1:1 CH3CN:C4H9CN.
a

70
M.P. Balanay et al. / Journal of Photochemistry and Photobiology A: Chemistry 248 (2012) 63–72
(a)
(b)
Fig. 6. (a) Nyquist and (b) Bode phase plots of A1-H using various electrolyte composition measured in the dark at forward bias of −0.5 V. (i) Electrolyte A: 0.6 M BMII, 0.05 M
I2, 0.10 M LiI in 1:1 CH3CN:C4H9CN; (ii) Electrolyte B: 0.6 M BMII, 0.05 M I2, 0.10 M GuSCN in 1:1 CH3CN:C4H9CN; (iii) Electrolyte C: 0.6 M BMII, 0.05 M I2, 0.10 M LiI, 0.5 M
4-tBP in 1:1 CH3CN:C4H9CN.
Table 6
Photovoltaic performances of A1-H analogue using different electrolyte.a
Electrolyte
VOC (mV)
JSC (mA/cm2)
FF (%)
ꢁ (%)
R1 (ꢂ)
CPE1 (×10−5 F)
R2 (ꢂ)
CPE2 (×10−5 F)
ꢃ2 (ms)
16.4
8.6
37.1
A
B
C
452
552
576
12.9
6.5
8.1
64.1
66.4
70.4
3.73
2.36
3.29
5.0
68.1
24.5
1.2
0.7
0.9
57.1
314.8
386.6
28.8
2.7
9.6
a
Photovoltaic performance were measured at AM 1.5 irradiation with 0.159 cm2 working area. Concentration of dye = 0.2 mM in THF. Soaking time = 2 h. Electrolyte A:
0.6 M BMII, 0.05 M I2, 0.10 M LiI in 1:1 CH3CN:C4H9CN. Electrolyte B: 0.6 M BMII, 0.05 M I2, 0.10 M GuSCN in 1:1 CH3CN:C4H9CN. Electrolyte C: 0.6 M BMII, 0.05 M I2, 0.10 M
LiI, 0.5 M 4-tBP in 1:1 CH3CN:C4H9CN.
the suppression of the electron injection process brought by the
bulkiness of the GuSCN and the possibility of the thiocyanate ion
to chelate with the Zn metal of the porphyrin macrocycle similar
to the THF chelation discussed in Section 3.2.
to be 57.1, 314.8, and 386.6 ꢂ for electrolytes A, B, and C, respec-
tively, which are consistent with the increase in VOC. The chemical
capacitance is directly proportional to the distribution of electronic
states below the conduction band. A higher chemical capacitance
indicates a higher density of states of electrons at the semi-
conductor surface [67]. This agrees with the increasing trend of
the short circuit current density values (electrolyte B < electrolyte
C < electrolyte A). However, the increase of density of electronic
states below the CB of the TiO2 surface can also introduce an
increase in recombination rate as seen in Table 6 for electrolyte
A. The electron lifetime increases in the order of electrolyte
B < electrolyte A < electrolyte C. Although A1-H in electrolyte C pro-
duced the longest electron lifetime and lower recombination rate,
it does not contain enough density of electronic states at the semi-
conductor surface leading to the lower JSC value hence lower in
efficiency compared to A1-H in electrolyte A. The higher charge
transfer resistance at the counter electrode (R1 = 68.1 ꢂ) when
using GuSCN as compared to electrolyte A (R1 = 5.0 ꢂ) indicated
Another common electrolyte additive, 4-tert-butylpyridine, can
be adsorbed via a Ti N bond on the unoccupied positions on the
semiconductor. This bonding shifts the conduction band in the
negative direction thereby increasing the open circuit voltage of
the DSSC. This also decreases the recombination effect between
the TiO2 and redox couple [58,62–66]. Table 6 shows that the VOC
increases by ∼ 27%, however, JSC decreases by ∼ 37% when 4-tBP
was added to the electrolyte. The decrease in the JSC value offsets the
gains of VOC and FF resulting in a decrease in the overall conversion
efficiency from 3.73% to 3.29%. A similar trend was observed for the
phthalocyanine and Ru-based DSSCs dyes [58,64–66]. One possible
reason for the decrease in efficiency was credited to the suppres-
sion of electron injection of the dye due to decreasing the density
of acceptor states caused by the negative shift in the conduction
band [66].
−
that there is a longer regeneration of the I3 species, since GuSCN
To further analyze the differences in the photovoltaic per-
formances of the different electrolyte solution, an electrochemical
impedance spectroscopy was performed. Fig. 6 shows the elec-
trochemical impedance spectra of A1-H analogue in different
electrolyte solutions measured in the dark at a forward bias
of −0.5 V. All impedance spectra were fitted using the circuit
presented in Scheme 2. In Fig. 6a, Nyquist plots consisting of
two semicircles are shown. The smaller semicircle occurring at
higher frequencies represents the redox charge transfer response
at the counter electrode (CE)/electrolyte interface (R1 and CPE1,
Scheme 2). The larger circle at the intermediate frequencies rep-
resents the electron transfer and recombination kinetics at the
TiO2/electrolyte interface (R2 and CPE2, Scheme 2).
provides an insulating barrier that prevents the oxidation of I− as
evidenced by the decrease in the JSC value from 12.9 to 6.5 mA/cm2
[68]. However, the device with GuSCN in its electrolyte produced
the shortest electron lifetime which indicates the need for inor-
ganic cations such as Li+ in the electrolyte to assist in the injection
dynamics and facilitates the charge separation of the dye due to its
high density of ions at the semiconductor surface [66,69].
4. Conclusion
Two Zn porphyrin sensitizers, A1-H and A1-M, were synthe-
sized. Their photovoltaic performances were found to be influenced
by the dye bath condition in terms of the sensitization process and
composition of the electrolyte. The solar-to-electric conversion effi-
ciency was related directly to the amount of dye adsorbed to the
semiconductor. When THF was used as a solvent it produced 3 and
17% higher solar-to-electric conversion efficiency as compared to
Based on the EIS results, R2 is the charge recombination resis-
tance at the TiO2 surface that is related to the charge recombination
rate, such that a smaller R2 means a larger recombination rate.
As shown in Table 6 and Fig. 6a, the R2 values were estimated

M.P. Balanay et al. / Journal of Photochemistry and Photobiology A: Chemistry 248 (2012) 63–72
71
toluene for A1-H and A1-M analogues, respectively, based on the
results of 2 h soaking time without CDCA. The results from the-
oretical calculations indicated that the use of THF as a sensitizing
solution interacted with the porphyrin analogue producing a six- or
five-coordinate Zn complexes making it more dispersed when sen-
sitized on the semiconductor minimizing dye aggregation, hence
the higher efficiency. The trend in the absorption spectra in solution
was well depicted through theoretical calculations when the A1-
H(THF)0+1 complex was used to calculate the excited state energy
supporting the partial deprotonation theory. The difference in the
photovoltaic properties between A1-H and A1-M was caused by the
amount of tilting of the molecular dye plane toward the dye sur-
face. The A1-M molecular plane was closer to the TiO2 surface due
to the increased steric effect caused by the addition of a methyl
group. This increases the surface area of the dye adsorbed in the
TiO2, resulting a much lower dye concentration. The amount of
aggregation of the dye can be controlled by the soaking condition
and the use of co-adsorbents giving a much higher photovoltaic
performances. The use of 4-tBP or GuSCN as an additive in the elec-
trolyte composition does not improve the efficiency of the resulting
solar cell, which is due mainly to the suppression of the electron
injection process, as shown by the decreased JSC values. This study
will help guide researchers in optimizing DSSCs to achieve high
solar-to-electric efficiency by efficiently designing the proper sen-
sitizer and by proper assessment of the dye adsorption solvent and
electrolyte composition.
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This work was supported by the Basic Science Research Pro-
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