Fabrication and analysis of dye-sensitized solar cells (DSSCs) using porphyrin dyes with catechol anchoring groups

Article abstract of DOI:10.1039/c5ra23584g

Herein we report the preparation and application of 4 different zinc(ii) tetrakis(dihydroxyphenyl) porphyrins (ZnTDHPP) as the sensitizing dyes in dye-sensitized solar cells (DSSCs). The experimental results include solution and solid state UV-Vis data, steady state current-voltage characteristics, and our theoretical analysis comprised of density functional theory (DFT) and Langmuir isotherm adsorption formalism. The results show that the Zn tetrakis(2,3-dihydroxyphenyl) porphyrin (Zn2,3TDHPP) and Zn tetrakis(3,4-dihydroxyphenyl) porphyrin (Zn3,4TDHPP) with adjacent hydroxyl groups attaches to a TiO₂ surface much more strongly than carboxylate. The catechol anchoring group showed high stability of the dye on the TiO₂ surface. The cells prepared from these porphyrins showed no significant desorption of dye from the TiO₂ surface after several days. The DSSCs based on Zn2,3TDHPP showed the best photovoltaic performance under AM 1.5 irradiation comparable to the conventional Zn tetrakis(p-carboxyphenyl) porphyrin (ZnTCPP), despite lower dye loading on the TiO₂ surface. However, non-cooperative OH bonding to TiO₂ for Zn2,4TDHPP and Zn2,5TDHPP shows weak attachment to the TiO₂ surface and lower efficiencies. DFT calculations showed that the Zn2,3TDHPP structure is more non-planar than the others, which may suppress dye aggregation. The adopted adsorption modeling is fitted to the experimental data to study the kinetics of dye loading. The study can herald development of a new class of porphyrin sensitizer for DSSC applications.

Full text of DOI:10.1039/c5ra23584g

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Fabrication and analysis of dye-sensitized solar cells (DSSCs) using porphyrin dyes with
catechol anchoring group
Maryam Adineha, Pooya Tahaya, Mohsen Amerib, , Nasser Safaria*,Ezeddin Mohajeranic
a Department of Chemistry, Shahid Beheshti University, Evin, 1983963113 Tehran, Iran
bDepartment of Physics, Bu- Ali Sina University, Hamedan, Iran
c Laser and Plasma Research Institute, Shahid Beheshti University, Tehran,
1983969411, Iran
Eꢀmail: nꢀsafari@sbu.ac.ir
Abstract
Herein, we report the preparation and application of
4
different Zinc(II)
tetrakis(dihydroxyphenyl) porphyrin (ZnTDHPP) as the sensitizing dyes in dye sensitized solar
cells (DSSCs). The experiment includes solution and solid state UVꢀVis data, steady state
currentꢀvoltage characteristics, and our theoretical analysis is comprised of density functional
theory (DFT) and langmuir isotherm adsorption formalism. The results show that the Zn
tetrakis(2, 3dihydroxyphenyl) porphyrin (Zn2, 3TDHPP) and Zn tetrakis(3, 4dihydroxyphenyl)
porphyrin (Zn3, 4TDHPP) with adjacent hydroxyl groups attached to TiO2 surface much
stronger than carboxylate The catechol anchoring group showed high stability of the dye on the
TiO2 surface. The cells prepared by these porphyrins showed no significant desorption of dye
from the TiO2 surface after days. The DSSCs based on Zn2, 3TDHPP showed the best
photovoltaic performance under AM 1.5 irradiation comparable to the conventional Zn
tetrakis(pꢀcarboxyphenyl) porphyrin(ZnTCPP), despite lower dye loading on TiO2 surface.
1

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However, nonꢀcooperative OH bonding on TiO2 for Zn2,4TDHPP and Zn2, 5TDHPP showed
weak attachment to TiO2 surface and lower efficiencies. DFT calculations showed that
Zn2,3TDHPP structure is more nonplanar than the others that may suppress dye aggregation.
The adopted adsorption modeling is fitted to the experimental data to study the kinetics of dye
loading. The study can be herald of developing new class of porphyrin sensitizers for DSSC
applications.
.Keywords: dye sensitized solar cells, dihydroxyphenyl, anchoring group, catechol porphyrin
1 Introduction
With population increase, resource depletion and the rising cost of fossil energy sources,
researchers have tried to develop clean and renewable energy such as solar energy. Solar energy
is an abundant, renewable and clean energy sources that could supply energy for societies. Dye
sensitized solar cells (DSSCs) are the third generation of solar cells which were first developed
by O’Regan and Grätzel in 19911. DSSC is generally comprised of a photoanode which is made
of a semiconductor, sensitizer (dye molecules) on transparent conducting oxide (TCO) substrate
and hole conducting material as electrolyte. Photo excitation of dye molecules leads to electron
injection into the conduction band of the semiconductor metal oxide (generally titanium dioxide)
which is in contact with an electrolyte solution, generating oxidized dye. The process is followed
by electron transfer from the electrolyte to sensitizer and dye regeneration. 2ꢀ4
Inspired by the important role of porphyrins in the photosynthesis, many researchers have used
porphyrin derivatives as sensitizer in DSSCs 5ꢀ14. Thus far, the highest DSSCs efficiency has
been reported for a porphyrin dye12. Porphyrins are macrocyclic compounds with highly πꢀ
2

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conjugated systems exhibiting intense absorptions in the visible region. Some functional groups
can extend porphyrin and increase their ability to absorb light, therefore making them good
choices for use in solar energy conversion. Catechol is such functional group which could extend
15
porphyrin πꢀsystem . Catechol porphyrins have potential application in solar cells 16, 17. Ortho
dihydroxy phenyl functional groups in catechol porphyrins are directly at the meso positions.
18
Ortho dihydroxy phenyl can easily be oxidized to orthoquinone by electrochemical methods .
Porphyrins and catechol are both active in electrochemistry, and putting together and coupling
these two compounds could make new compounds with interesting properties 15ꢀ17, 19
.
Anchoring groups play an important role in DSSCs. In order to have a fast electron injection into
the TiO2 semiconductor, the existence of anchoring groups is crucial to attach the dye on a TiO2
surface. Carboxylic acid is an anchoring group which has been widely studied 20ꢀ25. The major
binding modes for the carboxylic acid anchoring groups are a bidentate chelating and/or a
bidentate bridging of the carboxylate on the TiO2 surface but an unidentate mode is also possible
(chart 1) 26.
Chart 1: unidentate(a) bidentate chelating (b) and bidentate bridging(c) mode of carboxilate on TiO2 surface
Similar to the carboxylate anchoring group, catechol can also bind to the TiO2 surface in
19
monodendate or bidentate modes. Li et al.
have shown that catechol can form two fullꢀ
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coverage Hꢀbonded structures, containing monodentate only or mixed monodentateꢀ bidentate
molecules on the TiO2 surface (Chart 2). Monodentate and bidentate structures can simply
convert into each other via proton exchange between catechol and the TiO2 surface. In the
bidentate mode, the formations of both bidentate mononuclear chelating (Chart 2b) and bidentate
binuclear bridging (Chart 2c) complexes have been reported 27ꢀ29
.
a
c
b
O
O
Ti
O
OH
O
O
Ti
Ti
Ti
Chart 2: unidentate(a) bidentate chelating (b) and bidentate bridging(c) mode of catechol on TiO2 surface
In this study, we used tetrakis(dihydroxyphenyl) Zinc(II) porphyrin (ZnꢀTDHPP) as sensitizer in
dye sensitized solar cells and compared the results with tetrakis(pꢀcarboxyphenyl) Zinc(II)
porphyrin(ZnTCPP) which has been previously studied 22, 30. To the best of our knowledge, this
is the first report of a porphyrin dye with a catechol moiety anchoring group so far.
Photosensitizers are characterized using NMR, FTꢀIR and UVꢀVis spectroscopy. To investigate
application of these sensitizers in DSSCs, the solution and solid state UVꢀVis spectroscopy are
used, and dye loading amount derived from these experiments. The currentꢀvoltage
characteristics are also provided for the DSSCs with the synthesized photosensitizers. To obtain
further evidences, density functional theory (DFT) calculations were carried out. Dye adsorption
modeling was also used to investigate the adsorption kinetics and estimate electron injection
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efficiencies from dye to TiO2 surface. General structures of metaloporphyrins used in this study
are shown in
scheme 1.
R3
R2
R4
R1
R5
R1
R2
R4
R5
N
N
N
N
R3
Zn
R3
R5
R4
R2
R1
R5
R4
R1
R2
R3
Zn2,3TDHPP: R1,R2=OH R3,R4,R5=H
Zn3,4TDHPP: R2,R3=OH R1R4,R5=H
Zn2,4TDHPP: R1,R3=OH R2,R4,R5=H
Zn2,5TDHPP: R1,R4=OH R2,R3,R5=H
ZnTCPP: R3=COOH R1,R2,R4,R5=H
Scheme1: General structures of sensitizers used in this study
2 Experimental
2.1 Materials and reagents
Tetrakis dihydroxyphenylporphyrins were obtained from Tetrakis dimethoxyphenylporphyrins
(ZnTDMPP). The Tetrakis (dimethoxyphenyl)porphyrins and Tetrakis (pꢀ
carboxyphenyl)porphyrin were prepared by the condensation of the appropriate
dimethoxybenzaldehyde with
pyrrole in refluxing propionic acid 27. The Tetrakis
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dihydroxyphenylporphyrins were prepared by boron tribromide demethylation of the
31
corresponding methyl ethers, according to the method presented by Milgrom . Solvents were
dried over appropriate drying agents. All other chemicals were reagent grade and used without
further purification.
2.2 Analytical instruments and measurements
Solution absorption spectra were recorded on an Analytilc Jena s600 spectrometer using a 1 cm
cuvette. Solutions were made at concentrations 5×10ꢀ5M in ethanol. Solid state absorption
spectra were recorded on a Shimadzo uvꢀ2100 spectrometer.
1H NMR and 13C NMR spectra were recorded in DMSOd6 using a 300 MHz
Bruker instrument. Photocurrentꢀvoltage (IꢀV) measurements were performed using a Sharif
solar (IꢀV) source measure unit and Prova 8300 sun simulator.
2.3 Synthesis of dyes
2.3.1 Tetrakis dimethoxyphenylporphyrins
Propionic acid (150 ml) was brought to a boil and then, the appropriate dimethoxybenzaldehyde
(20 mmol) was added, followed by pyrrole (20 mmol) being added dropwise. After refluxing for
1.5h, the solution was cooled to room temperature and then crystallized over 3 days. The product
was filtrated and washed with hot water and methanol. In the case of Tetrakis(3,4ꢀ
Dimethoxyphenyl) Porphyrin and Tetrakis(2,5ꢀDimethoxyphenyl) Porphyrin no precipitate was
achieved so the propionic acid was removed from crude. The solid black residue was purified
using column chromatography with dichloromethane and methanol (50:1) as eluents.
1
Tetrakis(3,4ꢀDimethoxyphenyl) Porphyrin: Yield: 350 mg 4%, H NMR (CDCl3, 300 MHz), δ,
ppm: 8.9 (s, 8H) , 7.8 (s, 4H), 7.7 (d, 4H) , 7.2 (d, 4H), 4.1 (s, 12H); 3.9 (s, 12H), ꢀ2.70 (s, 2H).
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13C NMR (CDCl3, 100 MHz), δ, ppm: 148.8, 147.0, 134.7, 131.1, 130.0, 127.34, 126.7, 119.8,
118.2, 110.2, 109.4, 108.7, 56.0, 55.9; UV– vis, (CHCl3 λmax): 421 (sorret); 518.; 556; 592;
649.
1
Tetrakis(2,3ꢀDimethoxyphenyl) Porphyrin: Yield: 680mg 8%, H NMR (300 MHz, CDCl3,
25℃): δ, ppm, 8.7(s, 8H), 7.2(m, 4H), 7.1(d, 4H), 7.0(m, 4H), 3.9 (s, 12H), 1.8 (s, 12H) ꢀ2.8(s,
2H).
1
Tetrakis(2,4ꢀDimethoxyphenyl) Porphyrin: Yield: 682mg 8%, H NMR (CDCl3, 300 MHz), δ,
ppm, 8.7(s, 8H), 7.7(s, 4H), 7.0(s, 8H), 4.1 (s, 12H), 1.8 (s, 12H) , ꢀ2.7(s, 2H).
Tetrakis(2,5ꢀDimethoxyphenyl) Porphyrin: Yield: 500mg 6%,1H NMR (CDCl3, 300 MHz), δ,
ppm, 9 (s, 8H), 7.4 (d, 4H), 7.1 (s, 8H) ꢀ2.8(s, 2H), 3.9 (s, 12H), 1.9 (s, 12H) .
2.3.2 Tetrakis (p-carboxyphenyl)porphyrin (TCPP)
Propionic acid (150 mL) was brought to a boil and then the pꢀphthalaldehydic acid (20 mmol)
was added followed by pyrrole (20 mmol) being added dropwise. After refluxing for 1.5 h, the
solution was cooled to room temperature and then crystallized overnight. The product was
filtrated and washed with hot water and a small amount of methanol.
Yield: 10%, 1H NMR (300 MHz, DMSO d6, 25ᵒC) δ (ppm): 8.7 (s, 8H), 8.3 (d, 8H), 8.1 (d, 8H),
ꢀ2.9(s, 2H).
2.3.3 Zinc tetrakis (p-carboxyphenyl)porphyrin
A mixture of TCPP and 1.2eq Zn(OAc)2.4H2O in methanol were stirred at room temperature.
The metalation process was followed by Uvꢀvis spectroscopy. After the completion of the
reaction, the solvent was evaporated with rotaevaporator at low temperature. The crude product
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was washed several times with water to remove the excess zinc salt and then dried at low
pressure using a vacuum pump.
1
Zn TCPP: Yield: 94%, H NMR (300 MHz, DMSO d6, 25ᵒC) δ (ppm): 8.9 (s, 8H), 8.4 (d, 8H),
8.3 (d, 8H). UVꢀvis (EtOH ,λmax, nm ):425 (soret), 558 , 601.
Tetrakis (dihydroxyphenyl)porphyrins(TDHPP)
The appropriate tetrakis (dimethoxyphenyl) porphyrin (0.5 mmol) was dissolved in the minimum
amount of dry dichloromethane. The solution was cooled in a methanolꢀliquid nitrogen bath (to ꢀ
98 °C). Boron tribromide (6 mmol) was slowly added to this solution. The mixture was stirred
for 1 h at ꢀ98 °C, and then allowed to come to room temperature, and was then stirred overnight
at room temperature. Water (20 mL) was added to quench the iceꢀcold reaction mixture.
Neutralization with triethylamine produced a brownꢀpurple precipitate, which was collected by
filtration and washed with water and dichloromethane. Characterization for 3,4TDHPP: Yield:
1
75%, H NMR (300 MHz, DMSOd6, 25ᵒC): δ, ppm 9.4 (s, 8H), 8.9 (s, 8H), 7.7 (s, 4H), 7.4 (s,
4H), 7.1 (s, 4H), ꢀ2.9 (s, 2H). 13C NMR (75 MHz, DMSOd6, 25 °C): δ, ppm 145.0, 144.1,
132.8, 131.6, 126.7, 122.9, 120.4, 115.9, 114.7. IR (KBr): ν, cmꢀ1 3386, 1653, 1597, 1508,
1433, 1346, 1260, 1201, 1108, 928, 871, 801, 711. UVꢀvis (EtOH ,λmax, nm ): 428 (soret), 520 ,
559 , 592 , 653.
2,3TDHPP: Yield: 80%, 1H NMR (300 MHz, DMSOꢀd6, 25℃): δ, ppm 9.7 (s, 8H), 8.7(s, 8H),
13
7.2(m, 4H), 7.1(d, 4H), 7.0(m, 4H), ꢀ2.8(s, 2H). C NMR (75MHz, DMSOꢀd6, 25℃): δ, ppm,
146.5, 145.5, 129.1, 128.6, 127.8, 126.8, 118.1, 116.8, 116.1. IR (KBr): ν, cmꢀ1, 3423, 2923,
2853, 1615, 1586, 1466, 1346, 1204, 1145, 927, 844, 804, 768, 730. UVꢀVis (EtOH, λmax, nm):
420 (soret), 516,550, 591, 647.
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1
2,4TDHPP: Yield: 78%, H NMR (300 MHz, DMSOꢀd6, 25℃): δ, ppm 9.1 (s, 8H), 8.8(s, 8H),
13
7.7(s, 4H), 7.0(s, 8H), ꢀ2.7(s, 2H). C NMR (75MHz, DMSOꢀd6, 25℃): δ, ppm, 146.5, 145.5,
129.1, 128.6, 127.8, 126.8, 118.1, 116.8, 116.1
2.3.4 Zn- Tetrakis (dihydroxyphenyl)porphyrins
A mixture of TDHPP and 1.2eq Zn(OAc)2.4H2O in methanol were stirred at room temperature.
The metalation process was followed by uvꢀvis spectroscopy. After the completion of the
reaction, the solvent was evaporated with a rotaevaporator at low temperature. The crude product
was washed several times with water to remove the excess zinc salt and then dried at low
pressure using a vacuum pump.
1
Zn2,3TDHPP: Yield:95%, H NMR (300 MHz, DMSOꢀd6, 25℃): δ, ppm 9.7 (s, 8H), 8.8(s,
8H), 7.4(m, 4H), 7.2(d, 4H), 7.0(m, 4H).13C NMR (75MHz, DMSOꢀd6, 25℃): δ, ppm, 146.6,
145.5, 129.2, 128.8, 128.0, 126.8, 118.3, 116.9, 115.9. IR (KBr): ν, cmꢀ1, 3426, 2926, 2851,
1620, 1586, 1466, 1346, 1210, 1145, 927, 844, 809, 768, 735,547 ,469. UVꢀvis (EtOH ,λmax,
nm ): 425 (soret), 556 , 592. Elemental Analysis Calc. for C44H28N4O8Zn: C, 65.56; H, 3.50;
N, 6.95; Found C, 65.58; H, 3.45; N, 6.98%.
Zn 3,4TDHPP: Yield:95%, 1H NMR (300 MHz, DMSOd6, 25ᵒC): δ, ppm 9.4 (s, 8H), 8.8 (s, 8H),
7.7 (s, 4H), 7.4 (s, 4H), 7.1 (s, 4H), 13C NMR (75 MHz, DMSOd6, 25 °C): δ, ppm 145.0, 144.1,
132.8, 131.6, 126.7, 122.9, 120.4, 115.9, 114.7. UVꢀvis (EtOH ,λmax, nm ): 428 (soret), 559 ,
602.
1
Zn 2,4TDHPP: Yield:96%, H NMR (300 MHz, DMSOd6, 25ᵒC): δ, ppm 9.4 (s, 8H), 8.8 (s,
8H), 7.7 (s, 4H), 6.9 (s, 8H). UVꢀvis (EtOH ,λmax, nm ): 425 (soret), 556 , 597.
1
Zn 2,5TDHPP: Yield:95%, H NMR (300 MHz, DMSOd6, 25ᵒC): δ, ppm 9.7 (s, 8H), 9 (s, 8H),
7.4 (d, 4H), 7.1 (s, 8H). UVꢀvis (EtOH ,λmax, nm ): 424 (soret), 556 , 591.
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2.4 Fabrication
measurements
of
dye-sensitized
solar
cells
and
photovoltaic
2.4.1 Preparation of nanoporous TiO2 paste
Two kinds of TiO2 paste containing nanocrystalline TiO2 (20 nm, paste A) and submicroparticle
TiO2 (350 nm, paste B) were prepared by a previously reported procedure 32, 33. 3 g of TiO2
nanopowder was added to mortar and grinded with gradual addition of 0.5 ml of acetic acid, 2.5
ml of deionized water and 15 ml of ethanol. The TiO2 dispersion was transferred with 50 ml of
ethanol to a round bottom flask and homogenized by an ultrasonic bath for 30 min. 10 g of
terpineol was added to the dispersion and sonication was resumed for another 30 min.
Alternating stirring and sonication were used three consecutive times after adding 1.5 g of ethyl
cellulose (10% solution in ethanol) to the TiO2 dispersion. Finally ethanol was removed by a
rotary evaporator. The resulting screenꢀprinting paste corresponds to 21 wt.% TiO2, 10 wt.%
ethyl cellulose and 69 wt.% terpineol (paste A).
Paste B which was used in the lightꢀscattering layers was prepared with the same method using
350 nm TiO2 nanoparticles.
2.4.2 Fabrication of porous-TiO2 electrodes
The FTO glass was first cleaned with detergent solution, distilled water and methanol using an
ultrasonic bath (10 min for each solution), and then rinsed with methanol. After treatment in 40
mM aqueous TiCl4 solution at 70 °C for 30min, the FTO glass was again washed with water and
methanol. Paste A was screen printed (manual screen printer, 90T, Estal Mono, Schweiz.
Seidengazefabrik, AG, Thal) on the FTO glass and kept in a clean box for 10 min to let the paste
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relax and reduce the surface irregularity. After that, FTO glasses were dried for 10 min at 125
°C. This screen printing procedure with paste A (coating, storing and drying) was repeated to get
an appropriate thickness of 10–12 ꢁm for the working electrode. After drying the (paste A) films
at 125 °C, one layer of paste B was deposited by the screenꢀprinting method, resulting in a lightꢀ
scattering TiO2 film containing 350 nm anatase TiO2 particles. The electrodes coated with the
TiO2 pastes were gradually heated under airflow at 325 °C for 5 min, at 375 °C for 5 min, and at
450 °C for 15min, and finally, at 500 °C for 15 min.
The TiO2 films were once again treated with 40 mM TiCl4 solution, as previously described, then
rinsed with water and methanol and heated at 450 °C for 15 min and 500 °C for 15 min. After
cooling to 80 °C, the TiO2 electrodes were immersed into 0.1 mM dye solution with 35 mM 4ꢀ
tertꢀButylpyridine as additive in a mixture of ethanol and THF (volume ratio, 4:1) and kept at
room temperature for 2 h to uptake the dye.
2.4.3 Preparation of counter Pt-electrodes and DSSC assemblage
FTO glasses were washed with the same method as used for the working electrode. 0.05 M
H2PtCl6 solution was spinꢀcoated on the FTO glass substrate at 2000 rpm for 15 s. The formed
film was annealed at 450 °C for 1 hour in atmosphere.
Clamps assembled the dyeꢀadsorbed TiO2 electrode and counter electrode into a sandwichꢀtype
cell. Finally a drop of electrolyte solution (0.10
M
lithium iodide, 0.60
M
butylmethylimidazolium iodide, 0.03 I2, and 0.05
M
M 4ꢀtertꢀbutylpyridine in
acetonitrile/valeronitrile 75:25) was introduced into the clamped electrodes.
2.5 Desorption measurement
The
amounts
of
dyeꢀloading
of
each
dye
determined by dye desorption in a basic solution. The dye loaded anodes were soaked in 10%
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solution of tetrabuthylammonium hydroxide in water/EtOH. The solutions absorbances were
measured during the time till the dyes completely desorbed from electrode.
3 RESULTS AND DISCUSSION
We have studied four types of porphyrin dyes, Zn2,3TDHPP, Zn2,4TDHPP, Zn2,5TDHPP,
Zn3,4TDHPP and compared them with previously studied dye ZnTCPP (scheme1). Each of
these dyes was chosen because they have two hydroxyl groups on each porphyrin meso phenyls
as anchoring groups. In the case of Znꢀ2,3TDHPP and Znꢀ3,4TDHPP these two hydroxyl groups
were adjacent and cooperatively bond to TiO2 surface but for Znꢀ2,4TDHPP and Znꢀ2,5TDHPP
they were nonꢀadjacent.
3.1 Photophysical properties of the dyes in solution and on TiO2
film
3.1.1 Photophysical properties of the dyes in solution
Uvꢀvis spectra of metaloporphyrins are shown in figure 1. Absorption spectra were measured
between 400 and 750 nm in ethanolic solutions. All studied metalloporphyrins have strong soret
bands at around 425 – 428 nm and two Q bands at around 500 ꢀ 700 nm which are uvꢀvis spectra
characteristics of metalloporphyrins and makes them potential candidates for light harvesting in
DSSCs.
Zinc is a closed shell metal and has very little effect on the porphyrin π to π* energy gap in
porphyrin electronic spectra. The soret band for ZnꢀTDHPPs was 5 ꢀ8 nm red shifted relative to
zinc tetraphenylporphyrin which indicates the conjugation of the loneꢀpair electrons of the eight
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oxygen atoms that are directly attached to the phenyl rings, with π electrons of the macrocycle
rings 18, 34. Extinction coefficients for ZnꢀTDHPPs were lower than ZnTPP due to both electronic
and steric effects 35, 36. ZnꢀTDHPPs with adjacent hydroxyl groups have relatively higher
extinction coefficients than the porphyrins with nonꢀadjacent hydroxyl groups. The lower
extinction coefficient in such porphyrins i.e. Zn2,5TDHPP, could be due to the resonance forms
37
shown in chart3 . The π system extension, results in a red shift in the porphyrin uvꢀvis spectra
and the electronegativity of oxygen, attracts electron density from the porphyrin ring and
lowered the extinction coefficient 36.
Chart3. The resonance forms of 2,5TDHPP
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Figure1. Uvꢀvis spectra of metalloporphyrins at 0.05mM concentration
3.1.2 Photophysical properties of the dyes adsorbed on the TiO2 film
The solid state uvꢀvis spectra of zinc tetrakis(dihydroxyphenyl) porphyrin and zinc tetrakis(pꢀ
carboxyphenyl) porphyrin dyes adsorbed on the TiO2 film have been shown in fig 2. The
broadened absorption bands of the dyes on TiO2 might be a sign of intermolecular interactions of
38
the molecules aggregated on the TiO2 surfaces . All zinc tetrakis(dihydroxyphenyl) porphyrin
dyes have sharper soret band to that of ZnTCPP indicating lower aggregation in former
compounds. Dihydroxophenylporphyrins also show red shift upon binding to the TiO2 film
which could be ascribed to the stabilization of the LUMO level in TiO2ꢀPorphyrin complexes39.
In the case of ZnTCPP the existence of blue shoulders for the soret bands may have been derived
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from the deprotonation of the carboxylic acid upon binding to the TiO2 surface 40 and/or Hꢀtype
aggregation of the dye molecules41, 42
.
Figure 2. The solid state uvꢀvis spectra of ZnTDHPP dyes adsorbed on the TiO2 film
3.1.3 Desorption of dyes from TiO2 film and adsorption model
In order to measure dye loading amount and also the time for complete desorption of dye from
TiO2 surface we carried an experiment to desorb of dye from TiO2 surface. Figure 3 shows
desorption of dyes from the TiO2 surface versus time in TBAOH solution.
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Figure 3. Desorption of dyes from TiO2 surface during the 30 hrs (Oꢀexperimental ●ꢀmodel).
ZnTCPP and porphyrins with nonꢀadjacent hydroxyl groups desorbed from TiO2 surface very
quickly; in comparison Zn2,3TDHPP and Zn3,4TDHPP desorbed more slowly from the TiO2
surface. Zn2,4TDHPP and Zn2,5TDHPP almost completely desorbed from the TiO2 surface after
30 minute and ZnTCPP after two hours yet this process took six hours for Zn2,3TDHPP and
Zn3,4TDHPP. There was no significant desorption in acidic and neutral media after days.
Therefore Zn2,3TDHPP and Zn3,4TDHPP showed almost three times more stability on TiO2
surface. This significant increase in stability of catechol anchoring group can arise from
cooperative bonding of dihydroxy phenyl via stable five member ring (form b in chart2).
Table 1 shows data concerning spectral properties and dye loading amount of all the synthesized
porphyrin derivatives on the TiO2 surface calculated from desorption and calibration data.
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For nonꢀadjacent hydroxyl group dyes, the loading amount and also desorption time were very
low, which indicates that in such porphyrins, there was no effective attachment of dye on the
TiO2 surface. Although all of the electrodes were soaked in dye solutions of the same
concentration for the same period of time, the amount of adsorbed dye forZn2,3TDHPP was
lower than Zn3,4TDHPP. This observation suggests that the flat orientation of adsorbed
porphyrin with respect to the TiO2 surface of Zn2,3TDHPP(25.3 nmol.cmꢀ2) results in less dye
20, 43
intake compared to Zn3,4TDHPP (36.7 nmol.cmꢀ2) and ZnTCPP ( 33.5 nmol.cmꢀ2)
(Figure4).
Figure 4 . Orientation of adsorbed porphyrin with respect to the TiO2 surface
Here, we implement a model, developed in our previous work 44, for investigation of time
evolution of dye loading to extract the dye adsorption. The adsorption from dye solution by solid
nanoparticle is studied assuming kinetic form of the time dependent Langmuir formalism
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framework. In the present microscopic model it is assumed that the bulk density of dye
molecules are adsorbed only by nanoparticles network by a constant adsorption and desorption
rates, also the interconnection of nanoparticles is influenced by particle necking which leads to
partial aggregation of nanoparticles.
Using kꢀ and kꢁ as the rate constants for adsorption and desorption, respectively, we can write
the net rate of adsorption for solution phase species i as the difference between the rate of
adsorption and the rate of desorption:
Rꢂ,ꢀꢁꢃ,ꢄꢅꢆ = kꢀꢂCꢂꢃꢇꢈꢉCꢂꢊꢃꢂꢆꢅ,ꢆꢇꢆꢀꢈ − Cꢂꢊꢃꢂꢆꢅꢃ,ꢇꢋꢋꢌꢍꢂꢅꢁ ꢎꢏ ꢂꢐ − kꢁ,ꢃCꢂꢊꢃꢂꢆꢅꢃ,ꢇꢋꢋꢌꢍꢂꢅꢁ ꢎꢏ ꢂ
(1)
which means that the rate of adsorption should be proportional to the concentration of dye
molecules in the solution phase (Cꢂꢃꢇꢈ) and to the number of sites available on the surface of
particles. If the dye molecules in the solution around the adsorbent solid particles occupy some
fraction ∈ of a volume V, and if this volume contains an adsorbing dye molecule i, then the rate
of adsorption of the dye molecules onto the adsorbent and the rate of depletion of that species
from the solution phase are coupled batch processes. The component mass balances for the
solution and solid phases are as follows
Solution phase
ꢁꢑ
∈ ꢖ
ꢒ,ꢓꢔꢕ
ꢗ
ꢘ
= − 1−∈ Rꢂ,ꢀꢁꢃ,ꢄꢅꢆAꢃρꢃV
(2)
(3)
ꢁꢆ
Solid phase
ꢁꢑ ꢗꢙꢊ∈ꢘꢚ
ꢛꢔꢛ
ꢒ,ꢓ
ꢗ
ꢘ
= 1−∈ Rꢂ,ꢀꢁꢃ,ꢄꢅꢆAꢃρꢃV
ꢁꢆ
where Aꢃ is effective surface area of nanoparticle matrix per unit of mass and ρꢃ, solid
nanoparticle density. The further derivation of equations can be found in literature44. The
obtained adsorption amount on the semiconductor matrix can be assumed to calculate the
injection efficiency by relation 4.
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ꢊꢑ
ꢝ
ꢗ
ꢘ
φꢂꢄꢜ = φ 1 − 10
(4)
The highest rate of injection efficiency belongs to ZnTCPP. The estimated injection quantum
yield together with 100% collection efficiency in DSSCs,44 strongly suggest that the limiting
performance factor for DSSCs can be the inefficient injection efficiency of the cells (Table1).
Table 1. Absorption wavelength, extinction coefficient and loading amount of dyes on TiO2
dye
λmax
(nm)
ε×10ꢀ5
λmax (nm)
on TiO2
surface
Dye loading
(nmol.cmꢀ2)
Injection
Adsorption
coefficient(ꢢꢣ)
nmol/cmꢀ1
Desorption
coefficientꢀ
(ꢢꢤ)
(Mꢀ1cmꢀ1)
efficiency
ethanol
%(ꢞꢟꢠꢡ
)
nmol/cmꢀ1
2.85*10ꢊꢥ.ꢧ
3.5*10ꢊꢥ.ꢧ
2.85*10ꢊꢥ.ꢧ
3.8*10ꢊꢥ.ꢨꢩ
2.5*10ꢊꢥ.ꢨꢩ
2.63
2.6
2.5
0.9
0.5
100*10ꢊꢥ.ꢦ
100*10ꢊꢥ.ꢙ
100*10ꢊꢥ.ꢦꢙ
57*10ꢊꢥ.ꢦꢙ
55*10ꢊꢥ.ꢦꢙ
ZnTCPP
425
425
428
424
425
3.466
4.538
3.098
2.84
425
430
435
429
432
33.5
25.3
36.7
15.1
10.4
Zn2,3TDHPP
Zn3,4TDHPP
Zn2,5TDHPP
Zn2,4TDHPP
2.24
3.2 Photovoltaic properties
To consider the potential of ZnꢀTDHPP as photosensitizers for DSSC, their DSSCs
performances were tested. Fig 5 shows JꢀV curves for the studied dyes. Detailed photovoltaic
parameters such as open circuit voltage (VOC), short circuit current (JSC), filling factor (FF) and
power conversion efficiency (ƞ) for different dyes under AM 1.5 solar illumination have been
listed in Table 2. power conversion efficiency is derived from the equation: ƞ= JSC × VOC × ff,
where JSC is the short circuit current, VOC is the open circuit potential, and ff is the fill factor.
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Figure 5. Currentꢀvoltage curves for DSSCs based on Zn2,3TDHPP, Zn3,4TDHPP, Zn2,4TDHPP and Zn2,5TDHPP
under illumination of AM 1.5 simulated sunlight
The VOC values of all the dye cells ranked: ZnTCPP = Zn2,3TDHPP > Zn3,4TDHPP >
Zn2,5TDHPP > Zn2,4TDHPP. Despite the lower dye loading amount for Zn2,3TDHPP
compared to ZnTCPP and Zn3,4TDHPP; the Voc value for this porphyrin is similar to ZnTCPP.
This could be because of the flat orientation of Zn2,3TDHPP on the TiO2 surface, meaning it
could cover the surface more than the other dyes and prevent direct contact of electrolyte with
TiO2 as well as decrease the recombination.
Similar to power conversion efficiencies, the Jsc values of all the dye cells ranked: ZnTCPP >
Zn2,3TDHPP > Zn3,4TDHPP > Zn2,4TDHPP > Zn2,5TDHPP. The order is the same as
extinction coefficient for ZnTDHPPs indicating that short circuit current increases with increase
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in light absorption ability. However, in the case of Znꢀ2,4ꢀTDHPP and Znꢀ2,5ꢀTDHPP
photovoltaic results were poor because of weak binding on TiO2 surface and in some extent due
to lower extinction coefficients of the corresponding dye.
Table 2. Photovoltaic parameters of porphyrinꢀbased dyeꢀsensitized solar cells under AM 1.5 illumination
Dye
Voc(mV)
449
Jsc (mAcmꢀ2)
Fill factor(%)
Efficiency(%)
ZnTCPP
2.7
2.1
67
62
51
34
38
0.8
0.6
Zn2,3TDHPP
Zn3,4TDHPP
Zn2,4TDHPP
Zn2,5TDHPP
449
410
1.11
0.28
0.12
0.23
0.03
0.02
312
390
3.3 Density functional theory (DFT) calculations:
To obtain further evidences for the above results, DFT calculations of two porphyrins with
adjacent hydroxyl group and ZnTCPP dyes were carried out, using the B3LYP/3ꢀ21G level. The
calculated structures do not show negative frequencies, inferring that the optimized geometries
are in the global energy minima. Figure 6 illustrates the electron density distributions of dyes in
their respective HOMO and LUMO levels. The energy levels of frontier orbitals and the HOMO
ꢀ LUMO energy gaps for these three dyes are listed in table3.
Electron distributions of all three dyes, in the frontier orbitals, are mostly over the porphyrin ring
which is likely due to the fact that the phenyl rings are perpendicular to the porphyrin ring and do
not contribute to electron distributions. This could block electron transfer to TiO2 and reduce
photovoltaic performance for studied dyes to those that have anchoring group participate in
electron conjugation. It is well known that the electron density distribution of LUMO around an
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anchoring group , affect the electronic coupling between the excited adsorbed dye and 3d orbital
45
of TiO2 . Consequently, we can expect the low φinj values of dyes (table1).
Figure 6. The electron density distributions and energy levels of ZnTCPP, Zn2,3TDHPP and Zn3,4TDHPP
For ZnTDHPPs, variations in energies for different positions of the hydroxyl groups for the
HOMO
level
(ꢀ4.95eV
for
Zn2,3TDHPP
and
ꢀ4.85
eV
for
Zn3,4TDHPP) are larger than those for the LUMO level (ꢀ1.91 eV for Zn2,3TDHPP and ꢀ1.95
eV for Zn3,4TDHPP); indicating that variations in the hydroxyl group position mainly affect the
occupied orbitals of ZnTDHPP dyes (table3). In addition, dihedral angle between phenyl unit
and the porphyrin macrocycle for Zn2,3TDHPP is 84.1ᵒ but the corresponding angle for
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Zn3,4TDHPP and ZnTCPP is 69.1ᵒ and 69.5ᵒ; indicating that Zn2,3TDHPP is more nonplanar
than the others.
It is well known that for nonplanar and distorted dyes, dye aggregation and charge
recombination can be efficiently suppressed, which is favorable for enhancing cell efficiency42.
Similarly lower Voc for Zn3,4TDHPP could be a consequence of the lower dihedral angle
between the phenyl unit and the porphyrin macrocycle and the more planar structure which
favors aggregation. Therefore, Zn2,3TDHPP, compared to Zn3,4TDHPP could be considered a
8,
better dye due to its different mode of attachment,Fig4, and its relatively nonplanar structure
42, 46
.
Table3. The energy levels of frontier orbitals and the HOMO ꢀ LUMO energy gaps for ZnTCPP,Zn2,3TDHPP and
Zn3,4TDHPP
Dye
HOMO(eV)
ꢀ5.34
LUMO(eV)
ꢀ2.40
Eg
ZnTCPP
2.94
3.03
2.9
Zn2,3TDHPP
Zn3,4TDHPP
ꢀ4.95
ꢀ1.91
ꢀ4.85
ꢀ1.95
In order to have fast electron transfer, the HOMO must be sufficiently more positive than the
−
redox potential of I−/I3 to accept electrons effectively; the difference between these two levels is
47
given by ∆E . HOMO energy levels are ꢀ5.34,ꢀ4,95 and ꢀ4.85 for ZnTCPP, Zn2,3TDHPP and
ꢀ
Zn3,4TDHPP respectively; considering that Iꢀ/I3 electrolyte potential is ꢀ4.85eV48 the
differences between electrolyte and HOMO energy level (ꢂE) are 0.5, 0.1 and 0 for ZnTCPP,
Zn2,3TDHPP and Zn3,4TDHPP. For Zn3,4TDHPP there is no driving force for electron
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injection from electrolyte to dye oxidized state that could be the reason of poor photocurrent in
this dye (figure 7) 47.
Figure 7. Illustration of energy levels and electron injection efficiencies of ZnTCPP, Zn2,3TDHPP and
Zn3,4TDHPP
4
Conclusion
ZnꢀTDHPP dyes were successfully synthesized and used as sensitizer in DSSCs and compared
with ZnTCPP as reference dye. This is the first report of a porphyrin dye with a catechol
anchoring group so far. The Jsc values of all the dye cells as well as power conversion
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efficiencies, were ranked: ZnTCPP > Zn2,3TDHPP > Zn3,4TDHPP > Zn2,4TDHPP >
Zn2,5TDHPP; The order was the same as extinction coefficient for ZnTDHPPs. Despite lower
dye loading on TiO2 surface for Zn2,3TDHPP it has comparable photovoltaic performance(0.6
vs. 0.9) and more stability to ZnTCPP. The stability of the dye on TiO2 for Zn2,3TDHPP and
Zn3,4TDHPP might arise from bidandate coordination on TiO2 surface via chelating and
formation of five member ring. The higher efficiency observed for Zn2,3TDHPP relative to
Zn3,4TDHPP may be ascribed to, mode of coordination to the TiO2 surface which result in more
efficient electron transfer, suppressed dye aggregation and high extinction coefficient for the
former compound. Injection efficiencies of the cells were also calculated; the results indicate that
the limiting performance factor for DSSCs can be the inefficient injection efficiency of the cells.
Future strategy is to get advantage of high stability of catechole anchoring group and extend π
system contibution and also synthesis of asymmetric porphyrins to make dipole moment by
modification of porphyrin core to enhance the efficiencies.
5 Acknowledgments:
This work was supported by the Iranian National Science Foundation (INSF) and Shahid
Beheshti University Research Affaires.
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Zinc (II) porphyrins with catechol moiety anchoring group were used in dye
sensitized solar cells for the first time.