Asymmetric phthalocyanine derivatives containing 4-carboxyphenyl substituents for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2014.09.018

The synthesis of two asymmetrical zinc (II) phthalocyanines (ZnPcs) containing 4-carboxyphenyl and 3-thienyl or 5′-hexyl-2,2′-bithiophene substituent was described. These Zinc Phthalocyanines were synthesized by a statistical condensation reaction between two different phthalonitriles. Each of the phthalonitrile precursors was accomplished by the Suzuki-Miyaura cross-coupling reactions with the aryliodide and corresponding boronic acids. The ZnPc dyes were characterized by MALDI-MS, FT-IR, ¹H NMR, UV-Vis, fluorescence and cyclic voltammetry methods. Compared with 3-thienyl substituted ZnPc-1 dye, the Q-band absorption of 5′-Hexyl-2,2′-bithiophene substituted ZnPc-2 was red-shifted by 13 nm because of the extension of π-system. The ZnPc dyes were used as sensitizers in dye-sensitized solar cells (DSSCs). ZnPc-2 sensitized solar cell devices using a 7 (transparent) + 5 (scattering) μm thin TiO₂ layer yielded a short-circuit photocurrent density of 3.81 mA/cm², an open-circuit voltage of 500 mV, and a fill factor of 0.59, corresponding to an overall conversion efficiency of 1.12% under standard AM 1.5 sun light.

Full text of DOI:10.1016/j.dyepig.2014.09.018

Dyes and Pigments 113 (2015) 474e480
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Asymmetric phthalocyanine derivatives containing 4-carboxyphenyl
substituents for dye-sensitized solar cells
Sadik Cogal e, Sule Erten-Ela b , Kasim Ocakoglu c d, Aysegul Uygun Oksuz e
a
,
,
**
,
, *
a
Mehmet Akif Ersoy University, Faculty of Arts and Science, Department of Chemistry, 15030 Burdur, Turkey
Ege University, Solar Energy Institute, Bornova, 35100 Izmir, Turkey
Advanced Technology Research & Application Center, Mersin University, Ciftlikkoy Campus, 33343 Yenisehir, Mersin, Turkey
Mersin University, Tarsus Faculty of Technology, Department of Energy Systems Engineering, 33480 Mersin, Turkey
Suleyman Demirel University, Faculty of Arts and Science, Department of Chemistry, 32260 Isparta, Turkey
b
c
_
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 June 2014
Received in revised form
The synthesis of two asymmetrical zinc (II) phthalocyanines (ZnPcs) containing 4-carboxyphenyl and 3-
thienyl or 5 -hexyl-2,2 -bithiophene substituent was described. These Zinc Phthalocyanines were syn-
thesized by a statistical condensation reaction between two different phthalonitriles. Each of the
0
0
15 August 2014
phthalonitrile precursors was accomplished by the Suzuki-Miyaura cross-coupling reactions with the
Accepted 9 September 2014
Available online 19 September 2014
1
aryliodide and corresponding boronic acids. The ZnPc dyes were characterized by MALDI-MS, FT-IR,
H
NMR, UVeVis, fluorescence and cyclic voltammetry methods. Compared with 3-thienyl substituted
0
0
ZnPc-1 dye, the Q-band absorption of 5 -Hexyl-2,2 -bithiophene substituted ZnPc-2 was red-shifted by
3 nm because of the extension of -system. The ZnPc dyes were used as sensitizers in dye-sensitized
solar cells (DSSCs). ZnPc-2 sensitized solar cell devices using a 7 (transparent) þ 5 (scattering)
Keywords:
Phthalocyanine
Asymmetric
1
p
m
m
2
4-Carboxyphenyl
2
thin TiO layer yielded a short-circuit photocurrent density of 3.81 mA/cm , an open-circuit voltage of
Dye-sensitized solar cell
Thiophene
Bithiophene
500 mV, and a fill factor of 0.59, corresponding to an overall conversion efficiency of 1.12% under
standard AM 1.5 sun light.
©
2014 Elsevier Ltd. All rights reserved.
1
. Introduction
Among the various synthetic dyes being investigated for appli-
cation in DSSCs, phthalocyanines (Pcs) have been considered to be
promising candidates because of their unique optical and electrical
properties [7]. These dyes are well known chromophores for their
strong absorption around 300 and 700 nm, as well as for their
excellent electrochemical, photochemical and thermal stability
[8,9]. Moreover, the redox properties of Pcs are suitable for the
Dye-sensitized solar cells (DSSCs) have received increasing
attention due to their high incident to photon efficiency, easy
fabrication and low production cost [1]. Tremendous research ef-
forts have been devoted to the development of new and efficient
2
sensitizers suitable for practical use. In TiO -based DSSCs, effi-
ciencies of up to 11.4% under simulated sunlight have been obtained
with rutheniumepolypyridyl complexes [2e4]. However, the main
drawback of ruthenium complexes is the lack of absorption in the
red region of the visible light and the high cost. For this reason, dyes
2
sensitization of wide band gap semiconductors such as TiO [10].
Therefore, Pcs are excellent alternative materials for DSSC appli-
cations and considerable research efforts have been made on the
synthesis of various Pcs and ZnPcs. Furthermore, to achieve higher
with large and stable
p-conjugated systems such as porphyrins and
efficiency, researchers have focused on extension of the p-system
phthalocyanines are important classes of potential sensitizers for
highly efficient DSSCs. Recently, the porphyrin dyes YD2-oC8 and
SM315 showed the highest power conversion efficiencies of 12.3%
and 13%, respectively, which broke the ruthenium based DSSCs
record [5,6].
and lowering the symmetry of the macro-cycle, which can result in
a broadened Soret band and a red-shifted Q band absorption
[11,12]. Recently, several groups have synthesized unsymmetrical
phthalocyanines to improve the efficiency of DSSC [13e15]. In order
to move the Q-band absorption of phthalocyanine to longer
wavelength, the p-electron conjugation system can be extended by
using various substituents [16e18]. In this study, we have designed
*
Corresponding author. Tel.: þ90 2462114082.
two new unsymmetrical phthalocyanines (Fig. 1) containing 4-
** Corresponding author. Tel.: þ90 2323111231.
0
0
carboxyphenyl, 3-thienyl and 5 -hexyl-2,2 -bithiophene substitu-
ent and investigated their performance in DSSC. 4-Carboxyphenyl
E-mail addresses: suleerten@yahoo.com (S. Erten-Ela), ayseguluygun@sdu.edu.tr
(
A.U. Oksuz).
http://dx.doi.org/10.1016/j.dyepig.2014.09.018
143-7208/© 2014 Elsevier Ltd. All rights reserved.
0

S. Cogal et al. / Dyes and Pigments 113 (2015) 474e480
475
group was selected as an anchoring moiety to the TiO
2
surface.
Substitution of 3-thienyl and 5 -hexyl-2,2 -bithiophene groups at
the -positions of the Pc ring could result in the extension of the
system. We reported the efficiencies of zinc phthalocyanines using
nanoporous TiO . ZnPc-2 sensitizer exhibited an efficiency of 1.12%
cool to room temperature. The reaction mixture was filtered and
solvent was evaporated. The obtained solid was subjected to silica
gel column chromatography and eluted with chloroform:hexane
0
0
b
p-
1
(3:1) mixture. Yield: 58%.
(ppm) ¼ 0.94 (m, 3H, CH
1.72 (m, 2H, CH ), 2.84 (m, 2H, CH
Ar-H), 7.29 (m, 1H, Ar-H), 7.79 (m, 1H, Ar-H), 7.80 (m, 1H, Ar-H), 7.86
H
NMR (CDCl
), 1.42 (m, 2H, CH
2
), 6.76 (m, 1H, Ar-H), 7.11 (m, 1H,
3
,
400 MHz):
2
d
3
), 1.36 (m, 2H, CH
2
2
),
using nanocrystalline 7 (transparent) þ 5 (scattering)
mm TiO
2
2
layers.
13
(
m, 1H, Ar-H), 7.97 (m, 1H, Ar-H). C NMR (CDCl
(ppm) ¼ 14.07, 22.57, 28.74, 30.24, 31.54, 112.74, 115.24, 115.52,
16.73, 124.33, 124.76, 125.22, 127.38, 128.71, 129.48, 132.44, 134.04,
3
, 400 MHz):
d
2
. Experimental
1
1
ꢁ
1
36.88,139.29,141.76,147.38. FTIR (KBr)
n
(cm ): 3106, 3067, 2954,
2.1. Materials
2920, 2841, 2229 (CN), 1719, 1593 (Ar C]C), 1554, 1495, 1446, 1439,
0
0
1411, 1369, 1314, 1280, 1219, 1202, 1175, 1104, 1065, 905, 879, 837,
4
-Aminophthalonitrile, 3-thienylboronic acid, 5 -hexyl-2,2 -
796, 777, 723, 521, 486, 430.
bithiophene-5-boronic acid pinacol ester, 4-methoxycarbonyl
phenylboronic acid, chloroform (CHCl ), tetrahydrofuran (THF),
hexane, tetrakis(triphenylphosphine)palladium(0) [Pd(PPh ], 1,2-
dimethoxyethane (DME), 2-dimethylaminoethanol (DMAE), 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU), zinc (II) acetate [Zn(OAc)
and sodium carbonate (Na CO ) were purchased from Aldrich;
3
2.4. Synthesis of 4-(4-methoxycarbonylphenyl)phthalonitrile (8)
3 4
)
4
-Methoxycarbonylphenylboronic acid (300 mg, 1.67 mmol), 4-
iodophthalonitrile (381 mg, 1.50 mmol) and Pd(PPh (173.34 mg,
.15 mmol) were dissolved in 20 mL of DME and stirred under
2
]
3 4
)
2
3
0
Silica Gel 60 (0.04e0.063) was purchased from Merck. 4-
Iodophthalonitrile (2) was synthesized as described in literature
argon for 10 min. Then, aqueous Na
2
CO
3
(318 mg, 3 mmol) was
ꢀ
added slowly in a few portions, and the mixture was heated at 80 C
for 12 h. After cooling to room temperature, the solvent was
removed under vacuum and the residue was dissolved in 100 mL
[
19].
2
.2. Synthesis of 4-(3-thienyl)phthalonitrile (4)
CHCl
Na SO
evaporated in vacuum. The resulting solid was purified by column
chromatography on silica gel by using CH Cl as an eluent. Com-
pound 8 was obtained as a white solid (250 mg, 65.4%). H NMR
(CDCl , 400 MHz):
(ppm) ¼ 8.23-8.21 (1H, Ar-H), 8.08 (1H, Ar-H),
.01 (1H, Ar-H), 7.99 (1H, Ar-H), 7.96-7.94 (1H, Ar-H), 7.71 (1H, Ar-
3
, washed three times with distilled water and dried over
2
4
. The drying agent was removed by filtration and CHCl
3
4
-Iodophthalonitrile (127 mg, 0.5 mmol), 3-thienylboronic acid
70.05 mg, 0.55 mmol) and Pd(PPh (57.8 mg, 0.05 mmol) were
dissolved in 20 mL of DME. To this mixture, aqueous solution of
(
3 4
)
2
2
1
Na
2
CO
3
(26.5 mg, 0.25 mmol) was added. The resultant reaction
ꢀ
3
d
mixture was heated at 80 C under argon atmosphere for 16 h and
then the reaction mixture is allowed to cool to room temperature.
The reaction mixture was filtered and solvent was evaporated and
the solid obtained was subjected to silica gel column chromatog-
8
ꢁ
1
H), 7.69 (1H, Ar-H), 4.00 (m, 3H, CH
104, 3071 (H-Ar), 2956, 2849, 2389, 2291, 2230 (CN), 1714 (C]O),
600 (Ar C]C), 1484 (Ar C]C), 1426, 1384, 1295 (C-O), 1186, 1109,
3
). FTIR (KBr):
n
(cm ) ¼ 3422,
3
1
raphy and eluted with chloroform:hexane (3:1) mixture. Yield:
1
1015, 956, 918, 841, 765, 695, 519.
7
(
5%. H NMR (CDCl
3
, 400 MHz):
d
(ppm) ¼ 7.26 (m, 1H, Ar-H), 7.40
m, 1H, Ar-H), 7.50 (m, 1H, Ar-H), 7.67 (m, 1H, Ar-H), 7.84 (m, 1H, Ar-
13
2.5. Synthesis of 2-(4-carboxyphenyl)-9(10), 16(17), 23(24)-tris(3-
thienyl)phthalocyaninato zinc(II) (ZnPc-1)
H), 7.91e8.00 (m, 1H, Ar-H).
3
C NMR (CDCl , 400 MHz):
d
(ppm) ¼ 113.57, 115.39, 115.50, 116.66, 124.26, 125.52, 128.14,
ꢁ
1
1
2
8
30.36, 131.07, 134.08, 138.18, 140.78. FTIR (KBr) n (cm ): 3102,
4-(3-Thienyl)phthalonitrile (1.05 mmol, 220.76 mg), 4-(4-
230 (CN), 1654, 1594 (Ar C]C), 1523, 1423, 1353, 1264, 1216, 1089,
methoxycarbonylphenyl)phthalonitrile (0.35 mmol, 91.79 mg)
91, 846, 794, 702, 651, 584, 521.
and Zn(OAc)
2
(10 mmol, 183 mg) were dissolved in 10 mL DMAE.
ꢀ
The mixture was added by three drops of DBU and heated at 130 C
under argon atmosphere for 12 h. The reaction mixture was then
allowed to cool to room temperature and the solvent was removed
0
0
2
.3. Synthesis of 4-(5 -hexyl-2,2 -bithiophene)phthalonitrile (6)
4
-Iodophthalonitrile (254 mg, 1 mmol), 5’-hexyl-2,2’-bithio-
phene-5-boronic acid (414 mg, 1.1 mmol) and Pd(PPh
115.56 mg, 0.1 mmol) were dissolved in 20 mL of DME. To this
(53 mg, 0.5 mmol) was added.
The resultant reaction mixture was heated at 80 C under argon
atmosphere for 24 h and then the reaction mixture is allowed to
2
under vacuum. The residue was washed with a MeOH:H O (5:1)
3 4
)
mixture. The solid obtained was purified by column chromatog-
raphy on silica gel. A THF eluent removed most of the symmetric
tetrakis(3-theinyl)phthalocyanine and a THF:DMF (10:1) eluent to
(
mixture, aqueous solution of Na
2
CO
3
ꢀ
separate the ZnPc-1 methyl ester. This fraction was dissolved in
ꢀ
2
0 mL of THF:MeOH (3:1) mixture and was hydrolyzed at 60e70 C
for 1 h. The reaction mixture was then allowed to cool to room
temperature and the organic solvent was removed under vacuum.
The pH of the reaction mixture was set to pH 3-4 by adding 1 M HCl
solution, and the precipitate was formed. The precipitate was
2
collected and washed several times with MeOH:H O mixture and
dried to give ZnPc-1 as a green solid (11% overall yield for two
1
steps). H NMR (CDCl
3
, 400 MHz):
d
(ppm) ¼ 9.42-9.11 (broad, m,
1
2H, Ar-H), 8.41-7.97 (4H Ar-H), 6.89-6.53 (12H, thienyl, Ar-H). FTIR
ꢁ1
(KBr):
n
(cm ) ¼ 3630, 3059 (Ar-H), 2953, 2912, 2866 (aliphatic
CeH), 1691 (C]O), 1653, 1607, 1526, 1486, 1435, 1411, 1390, 1363,
320, 1276, 1229, 1168, 1081, 1059, 888, 859, 832, 778, 759, 742, 688,
1
6
M
67, 623, 520, 461, 430. UVeVis (THF):
l
max
,
nm (log
3
,
ꢁ1.cm ) ¼ 358 (4.92), 618 (4.62) and 684 (5.30). MALDI TOF MS:
ꢁ1
þ
Fig. 1. Molecular structures of ZnPc-1 and ZnPc-2.
calcd for C51
H
26
N
8
O
2
S
3
Zn ([MþH] ) 944.41; found 944.07.

476
S. Cogal et al. / Dyes and Pigments 113 (2015) 474e480
0
2
.6. Synthesis of 2-(4-carboxyphenyl)-9(10), 16(17), 23(24)-tris(5 -
3. Results and discussion
0
hexyl-2,2 -bithiophene)phthalocyaninato zinc(II) (ZnPc-2)
Asymmetrically substituted phthalocyanines show interesting
properties due to the presence of different groups in the same
macrocycle. In the literature, three methods have been commonly
used for the synthesis of unsymmetrical phthalocyanines: a sta-
tistical condensation [21], a polymer support [22] or using sub-
phthalocyanines [23]. Among these methods, the statistical
cyclotetramerization was used to prepare the phthalocyanines in
this study. According to this method, two phthalonitriles (A and B)
mixture was used, for which one used in excess (A) to obtain in
good of the desired A B structure. Scheme 1 shows the synthetic
3
route of the phthalocyanines.
Fig. 2 shows absorption spectra of phthalocyanines recorded in
THF solution. The spectrum of ZnPc-1 shows the Q-band at 684 nm,
0
0
4
-(5 -hexyl-2,2 -bithiophene)phthalonitrile
03 mg), 4-(4-methoxycarbonylphenyl)phthalonitrile (0.18 mmol,
7.2 mg) and Zn(OAc) (10 mmol, 183 mg) were dissolved in 10 mL
(0.54
mmol,
2
4
2
DMAE. The mixture was added by three drops of DBU and heated at
30 C under argon atmosphere for 12 h. The reaction mixture was
ꢀ
1
then allowed to cool to room temperature and the solvent was
removed under vacuum. The residue was washed with
MeOH:H O (5:1) mixture. The solid obtained was purified by col-
umn chromatography on silica gel. A THF eluent removed most of
the symmetric tetrakis(5 -hexyl-2,2 -bithiophen)phthalocyanine
and a THF:DMF (10:1) eluent to separate the ZnPc-2 methyl ester.
This fraction was dissolved in 20 mL of THF:MeOH (3:1) mixture
a
2
0
0
ꢀ
and hydrolyzed at 60e70 C for 1 h. The reaction mixture was then
whereas ZnPc-2 shows a maximum absorbance at 700 nm. The Q-
*
allowed to cool to room temperature and the organic solvent was
removed under vacuum. The pH of the reaction mixture was set to
pH 3-4 by adding 1 M HCl solution, and the precipitate was formed.
The precipitate was collected and washed several times with
MeOH:H
band observed in phthalocyanines was attributed to
pe
p
transi-
tions from the highest occupied molecular orbital (HOMO) and to
lowest unoccupied molecular orbital (LUMO) of the conjugated
macrocycle. Both compounds, ZnPc-2 and ZnPc-3, present a Soret
band absorption around at 358 and 359 nm, respectively. The Soret
1
2
O mixture and dried to give ZnPc-2 as a green solid. H
NMR (CDCl
2
2
3
, 400 MHz):
.11-0.85 (several m, 39H, alkyl). FTIR (KBr): 3063 (aromatic C-H),
923-2850 (aliphatic C-H), 1686, 1607, 1489, 1433, 1398, 1329, 1283,
253, 1172, 1091, 1052, 946, 894, 824, 791, 761, 740, 693, 653, 583,
d
(ppm) ¼ 9.25-6.80 (several m, 28H, Ar-H),
band was observed due to the transitions from the deeper
to the LUMO. ZnPc-2 additionally presents an absorption band
between 400 and 550 nm, which is related to the peripheral,
conjugated, bisthiophene substituent [24]. The fluorescence
spectra of phthalocyanines were presented in Fig. 3. The fluores-
cence maxima of ZnPc-1 and ZnPc-2 were observed at 690 and
710 nm, respectively.
p levels
p
-
1
4
6
C
ꢁ1
ꢁ1
86, 430. UVeVis (THF):
32 (4.54) and 700 (5.23). MALDI TOF MS: calcd for
lmax, nm (log
3 , M .cm ) ¼ 359 (4.93),
þ
H
81 68
N
8
O
2
S
6
Zn ([MþH] ): 1443.26; found 1441.26.
Zeroezero excitation energy (E0-0) was obtained from the
intersection of absorption and emission spectra (lint):
2.7. DSSC preparation
E0 ¼ 1
240
TiO
2
electrodes consist of an adsorbent mesoporous layer with
particle size in 7 mm thicknesses and a second
particle size of
mm thicknesses. Preparation and characterization of double-layer
ꢁ0
2
0 nm anatase TiO
light scattering layer with 400 nm anatase TiO
2
l
int
2
The E0-0 energies of ZnPcs were given in Table 1.
5
The electrochemical behavior of the phthalocyanines was
investigated using cyclic voltammetry (CV). The experiments were
carried out in a typical three-electrode cell in which a glass sheet
with deposited indiumetin-oxide (ITO) was used as the working
electrode, a platinum wire was used as the counter electrode, an
Ag/AgCl electrode was used as the reference electrode and THF
TiO
2
electrode were previously described by Wang et al. [20].
ꢀ
TiO
2
coated FTO substrates were sintered at 450 C and
immersed in a 0.5 mM solution of the dye solutions in THF:t-
butanol (1:1) at room temperature overnight, and dried under a
flow of nitrogen. The active solar cell area was 0.16 cm . Pt catalyst
was deposited on the FTO glasses by coating with a drop of platinic
acid solution with the heat treatment at 450 C for 15 min. The cell
2
containing 0.1
M tetrabuthylammonium hexaflourophosphate
ꢀ
(
TBAPF6) was used as electrolyte. Fig. 4 shows the cyclic voltam-
was sealed using a Surlyn (60
mm, Solaronix) and the 0.6 M N-
mograms of ZnPc-1 and ZnPc-2. The oxidation and reduction be-
haviors of metallophthalocyanine derivatives are due to the
interaction between the phthalocyanine ring and the central metal
25]. The LUMO of phthalocyanines should have a higher energy
than the conduction band edge of TiO and the HOMO energy level
methyl-N-butyl-imidazolium iodide (BMII) þ 0.1 M LiI þ 0.05 M
I
2
þ 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile as redox elec-
trolyte solution was introduced through pre-drilled holes in the
counter electrode. The filling holes were sealed using Surlyn and a
microscope cover glass. Currentevoltage (JeV) curves were ob-
tained using Keithley measurement unit and the light source con-
sisted of an Oriel Xe-lamp.
[
2
of phthalocyanines should have a lower energy than the redox
potential of the electrolyte for dye regeneration. The HOMO energy
level can be calculated from the onset oxidation potential. The
electrochemically HOMO energy levels of ZnPc-1 and ZnPc-2 were
calculated as ꢁ4.81 and ꢁ4.61 eV, respectively. The lowest unoc-
cupied molecular orbital (LUMO) levels were estimated from the
differences between the HOMO energy levels and E0-0 [26]. The
LUMO levels of the phthalocyanines were estimated to be ꢁ3.01
and ꢁ2.87 eV, respectively. The LUMO levels are lower than the
2
.8. Characterization
FTIR spectra were measured between 400 and 4000 cm-1 from
KBr pellets on a Perkin Elmer Spectrum BX FTIR system (Beacons-
field, Buckinghamshire, HP91QA, England). The NMR (400 MHz)
spectra were recorded using a Bruker Ultrashield Plus Biospin
conduction band energy of TiO
enough thermodynamic driving force for electron injection from
the excited state of the dyes into the conduction band of TiO
Furthermore, the HOMO energy levels of the dyes were also lower
than the energy level of the redox couple I /I in the electrolyte,
3
indicating more efficient dye regeneration by electron transfer from
2
, suggesting that there should be
3
GmbH spectrometer in CDCl . The absorption spectra were recor-
ded using Perkin Elmer Lambda 20 UVeVis spectrometer. The
fluorescence spectra were measured on Varian Cary Eclipse fluo-
rescence spectrophotometer. Mass spectra were measured using
Bruker Microflex LT MALDI-TOF mass spectrometer. The electro-
chemical measurement was recorded using a Gamry PCI4/300
model the potentiostat.
2
.
ꢁ
-
ꢁ
I
[27]. The HOMO-LUMO gap decreased upon increasing the
conjugation length [28]. Muto et al. reported the similar

S. Cogal et al. / Dyes and Pigments 113 (2015) 474e480
477
ꢀ
ꢀ
ꢀ
Scheme 1. i) H
2
4
SO , NaNO
2
, KI; ii) Pd(PPh
3
)
4
, Na
2
CO
3
, DME, 80 C, reflux; iii) Zn(OAc)
2
, DBU, DMAE, 130 C, reflux; iv) THF:MeOH (3:1), NaOH, 70 C, reflux.
0
observation for 2-thienyl and [2-2 -bithiophene]-5-yl substituted
1
phthalocyanines showed the second oxidation peak at 1.10 and
0.88 V, respectively.
8
,4,8,11,15,18,22,25-octabutoxyphthalocyanines ((OBu) Pcs) [17].
The results of CV experiments of ZnPc-1 and ZnPc-2 were sum-
The photovoltaic characterizations of solar cells based on the
0
marized in Table 1. Both the 3-thienyl substituted ZnPc-1 and 5 -
ZnPc-1 and ZnPc-2 sensitizers using TiO
sandwich cell using electrolyte of 0.6 M N-methyl-N-butyl-imida-
zolium iodide (BMII) þ 0.1 M LiI þ 0.05 M I þ 0.5 M 4-tert-
2
layers obtained with a
0
hexyl-2,2 -bithiophene substituted ZnPc-2 showed reversible
oxidation peaks at 0.61 and 0.43 V, respectively. In addition, both of
2

478
S. Cogal et al. / Dyes and Pigments 113 (2015) 474e480
Fig. 2. UVeVis absorpsiton spectra of ZnPc-3 and ZnPc-4 in THF (1.0 ꢂ 10ꢁ6 M).
Fig. 4. Cyclic voltammograms of ZnPc-1 and ZnPc-2 recorded in 0.1 M TBAPF6/DMF at
a scan rate of 25 mV/s.
an overall conversion efficiency h, derived from the equation (h) Jsc
Voc ff/light intensity, of 1.12% (see Table 2).
ZnPc-2 was modified over ZnPc-1 with the thiophene bridged
bulky alkyl chains. Introduction of soluble thiophene alkyl chain
moieties to phthalocyanine allowed for an efficient sensitizer for
dye sensitized solar cells. In dye sensitized solar cell devices, it was
found that ZnPc-1 exhibited low photovoltaic performance that
could be rationalized by lack of thiophene bridged alkyl chains that
prevents back electron reactions as a spacer. The ZnPc-2 dye,
however, exhibited high photocurrent and high photoresponse
than ZnPc-1 for efficient dye sensitized solar cell. The introduction
of thiophene bridged alkyl chain groups in ZnPc-2 dye reduced
molecular aggregation as well as increased solubility. ZnPc-1
showed higher photocurrent than ZnPc-2 because of a little bit
large amount of absorbed photons in absorption spectra.
The incident photon to current conversion efficiency (IPCE) of
ZnPc-1 and ZnPc-2 was displayed in Fig. 6. The ZnPc-2 based DSSC
device exhibited a broad IPCE between 550 and 800 nm, but was
limited to low efficiency. The absorption of ZnPc-2 was red shifted
compared to ZnPc-1 sensitizers while maintaining the energy
Fig. 3. Fluorescence spectra of ZnPc-3 and ZnPc-4 in THF (1.0 ꢂ 10ꢁ6 M).
butylpyridine (TBP) in acetonitrile as redox electrolyte were pre-
sented in Fig. 5. And all photovoltaic data were summarized in
Table 2. ZnPc-1 and ZnPc-2 sensitized DSSCs were fabricated using
substrates with 7
mm of standard 20 nm particle titania paste, along
with a 5 m thick scattering layer. Under standard global AM 1.5
m
solar conditions, the ZnPc-2 sensitized cell devices gave a short-
2
circuit photocurrent density (Jsc) of 3.81 mA/cm , an open-circuit
voltage (Voc) of 500, and a fill factor (ff) of 0.59, corresponding to
Table 1
Electrochemical and optical properties of ZnPc-1 and ZnPc-2.
on
(V)a
int (nm)b
E
0-0 (eV)c
HOMO (eV)d
LUMO (eV)e
E
l
ox
ZnPc-1
ZnPc-2
0.61
0.43
1.10
0.88
687
705
1.80
1.76
ꢁ4.81
ꢁ4.63
ꢁ3.01
ꢁ2.87
a
Oxidation onset potential vs. Ag/AgCl reference electrode.
b
Measured from the intersection of the normalized absorption and emission
spectra.
c
int
Estimated from the l .
HOMO ¼ ꢁ[(E ꢁ Eferrocene)þ4.8]eV, where 0.6 V is the value for ferrocene vs.
d
on
ox
Ag/AgCl and 4.8 eV is the energy level of ferrocene below the vacuum.
e
LUMO ¼ HOMO þ E0-0
.
Fig. 5. JeV curves of dye sensitized solar cells.

S. Cogal et al. / Dyes and Pigments 113 (2015) 474e480
479
Table 2
Photovoltaic performance of TiO2 based dye sensitized solar cells.
4. Conclusions
J
sc (mAcmꢁ2)
V
oc (mV)
FF
h
(%)
In conclusion, we have successfully synthesized two unsym-
0
metrical phthalocyanine dyes containing 3-thienyl and 5 -hexyl-
ZnPc-1
ZnPc-2
1.08
3.81
500
500
0.47
0.59
0.25
1.12
0
2,2 -bithiophene groups at peripheral position of the macrocycle.
Photophysical properties (absorption, emission and electro-
chemical) of phthalocyanines were investigated. It was found that
0
5
-‘hexyl-2,2 -bithiophene substituent shifted the absorption max-
levels necessary for efficient electron injection and hole regenera-
tion in dye sensitized solar cells. In addition, the selection of thio-
phene bridged bulky alkyl chains supplied for additional
functionality such as minimizing recombination, increasing solu-
bility, and preventing dye aggregation. The molecular engineering
approach demonstrated herein was an example of the possibility to
prepare new types of phthalocyanines. Further structural optimi-
zation reducing the chances of aggregation and enhancing the
directionality of charge redistribution in the excited state is very
likely to yield more efficient sensitizers. Our work will continue in
fine-tuning phthalocyanine sensitizer for more efficient dye for dye
sensitized solar cells.
ima to red region and decreased the HOMO-LUMO gap upon
increasing conjugation length. We have also demonstrated phtha-
locyanine sensitizers in the visible and near-IR region of the solar
spectrum. We have successfully fabricated dye-sensitized solar
cells using Zinc phthalocyanines. We reported the efficiency ob-
tained with 7 (transparent) þ 5 (scattering)
2
mm TiO transparent
layers under standard conditions for ZnPc-2 dye that achieved a
2
short circuit current (Jsc) of 3.81 mA/cm , an open circuit voltage
Voc) of 500 mV, and a fill factor (ff) of 0.59 for a total efficiency of
(
1.12%.
In line with these statements, we reported the efficiency under
Acknowledgments
standard conditions obtained for ZnPc-2 sensitizer using
7
(
transparent) þ 5 (scattering)
m
m TiO layers that showed 500 mV
2
This work was supported financially by the grant of Suleyman
Demirel University, Scientific Research Projects (2950-D-11).
2
open circuit voltage, 3.81 mA/cm short-circuit current, 0.59 fill
factor and 1.12% overall conversion efficiency.
The new dyes including ZnPc-1 and ZnPc-2 show an extended
Appendix A. Supplementary data
p-conjugation system, which produces a redshift in the maximum
absorption, in comparison with three tert-butyl groups substituted
TT1 dye [29]. TT1 dye shows an Q-band absorption at 679 nm,
whereas ZnPc-1 and ZnPc-2 dyes show a maximum absorbtion at
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.09.018.
6
84 and 700 nm, respectively. The use of tert-butyl groups en-
References
hances the solubility, minimizes the aggregation, and tunes the
LUMO level of the phthalocyanine [29,30]. In ZnPc-2 dye, three 5 -
hexyl-2,2 -bithiophene groups decrease the aggregation and in-
0
[1] O'Regan B, Gr €a tzel M. A low-cost, high-efficiency solar cell based on dye-
0
2
sensitized colloidal TiO films. Nature 1991;353(6346):737e40.
[
[
2] Han L, Islam A, Chen H, Malapaka C, Chiranjeevi B, Zhang S, et al. High-effi-
ciency dye-sensitized solar cell with a novel co-adsorbent. Energy Environ Sci
2012;5(3):6057e60.
crease the conversion efficiency. However, this efficiency is gener-
ally lower those of TT1 and other tert-butyl substituted
phthalocyanine dyes [29e31]. Similar results were reported by Ince
et al. for ZnPc dye 2 with 5’-hexyl-2,2’-bithiophene groups [24]. It
was indicated that the efficiency was increased to 2.70% by
changing the electrolyte and by using coadsorbent chenodeox-
ycholic acid (CDCA). As a result, the aggregation phenomena play
an important role on the overall efficiency and explain the low
efficiency obtained by ZnPc-2 dye.
3] Abbotto A, Sauvage F, Barolo C, De Angelis F, Fantacci F, Gr a€ tzel M, et al.
Panchromatic ruthenium sensitizer based on electron-rich heteroarylvinylene
p-conjugated quaterpyridine for dye-sensitized solar cells. Dalton Trans
2011;40(1):234e42.
[4] Giribabu L, Bessho T, Srinivasu M, Vijaykumar C, Soujanya Y, Reddy VG, et al.
A new family of heteroleptic ruthenium(II) polypyridyl complexes for sensi-
tization of nanocrystalline TiO films. Dalton Trans 2011;40(17):4497e504.
2
[5] Mathew S, Yella A, Gao P, Humphry-Baker R, Curchod BFE, Ashari-Astani N,
et al. Dye-sensitized solar cells with 13% efficiency achieved through the
molecular engineering of porphyrin sensitizers. Nat Chem 2014;6(3):242e7.
[
6] Yella A, Lee H-W, Tsao HN, Yi C, Chandiran AK, Nazeeruddin MK, et al.
Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte
exceed 12 percent efficiency. Science 2011;334(6056):629e34.
[7] Ragoussi M-E, Ince M, Torres T. Recent advances in phthalocyanine-based
sensitizers for dye-sensitized solar cells. Eur J Org Chem 2013;2013(29):
6475e89.
[
[
8] Mack J, Kobayashi N. Low symmetry phthalocyanines and their analogues.
Chem Rev 2011;111(2):281e321.
9] De la Torre G, Claessens CG, Torres T. Phthalocyanines: old dyes, new mate-
rials. Putting color in nanotechnology. Chem Commun 2007;20:2000e15.
[
[
[
[
[
[
10] Giribabu L, Vijay Kumar C, Reddy VG, Reddy PY, Rao CS, Jang S-R, et al. Un-
symmetrical alkoxy zinc phthalocyanine for sensitization of nanocrystalline
TiO films. Sol Energy Mater Sol Cells 2007;91(17):1611e7.
2
11] Yu L, Lin L, Zhang X, Li R, Peng T, Li X. Highly asymmetric phthalocyanine-
sensitized solar cells: the effect of coadsorbent and adsorption temperature
of phthalocyanine. Electrochim Acta 2013;111:344e50.
12] Haas M, Liu S-X, Neels A, Decurtins S. A synthetic approach to asymmetric
phthalocyanines with peripheral metal-binding sites. Eur
006;2006(24):5467e78.
J Org Chem
2
13] Yang L, Guo L, Chen Q, Sun H, Liu J, Zhang X, et al. Theoretical design and
screening of panchromatic phthalocyanine sensitizers derived from TT1 for
dye-sensitized solar cells. J Mol Graph Model 2012;34:1e9.
14] Sarker AK, Kang MG, Hong J-D. A near-infrared dye for dye-sensitized solar
cell: catecholate-functionalized zinc phthalocyanine. Dye Pigments
2012;92(3):1160e5.
15] Ragoussi M-E, Cid J-J, Yum J-H, De la Torre G, Di Censo D, Gr €a tzel M, et al.
Carboxyethynyl anchoring ligands: a means to improving the efficiency of
phthalocyanine-sensitized solar cells. Angew Chem Int Ed Engl 2012;51(18):
4375e8.
Fig. 6. Incident photon to charge carrier efficiency of dye sensitized solar cells.

480
S. Cogal et al. / Dyes and Pigments 113 (2015) 474e480
[
16] Zhang X, Mao L, Zhang D, Zhang L. Synthesis, characterization and electro-
chemistry of novel unsymmetrical zinc phthalocyanines sensitizer with
extended conjugation. J Mol Struct 2012;1022:153e8.
[24] Ince M, Cardinali F, Yum J-H, Martinez-Diaz MV, Nazeeruddin MK, Gr a€ tzel M,
et al. Convergent synthesis of near-infrared absorbing, “push-pull”,
bisthiophene-substituted, zinc(II) phthalocyanines and their application in
dye-sensitized solar cells. Chem Eur J 2012;18(20):6343e8.
[25] Sakamoto K, Furuya N, Soga H, Yoshino S. Cyclic voltammetry of non-
peripheral thioaryl substituted phthalocyanines. Dye Pigment 2013;96(2):
430e4.
[17] Muto T, Temma T, Kimura M, Hanabusa K, Shirai H. Elongation of the pi-
system of phthalocyanines by introduction of thienyl substituents at the pe-
ripheral beta positions. Synthesis and characterization.
001;66(18):6109e15.
J Org Chem
2
[
[
[
18] Zhang D, Zhang X-J, Zhang L, Mao L-J. A new unsymmetrical zinc phthalo-
cyanine as photosensitizers for dye-sensitized solar cells. Bull Korean Chem
Soc 2012;33(4):1225e30.
[26] Zhu L, Yang H Bin, Zhong C, Li CM. Rational design of triphenylamine dyes for
highly efficient p-type dye sensitized solar cells. Dye Pigment 2014;105:
97e104.
19] Marcuccio SM, Svirskaya PI, Greenberg S, Lever ABP, Leznoff CC, Tomer KB.
Binuclear phthalocyanines covalently linked through two- and four-atom
bridges. Can J Chem 1985;63(11):3057e69.
[27] Balanay MP, Kim DH. DFT/TD-DFT molecular design of porphyrin analogues
for use in dye-sensitized solar cells. Phys Chem Chem Phys 2008;10(33):
5121e7.
20] Wang P, Zakeeruddin SM, Comte P, Charvet R, Baker RH, Gr €a tzel M. Enhance
[28] Maya EM, V ꢀa zquez P, Torres T, Gobbi L, Diederich F, Pyo S, et al. Synthesis and
the performance of dye-sensitized solar cells by co-grafting amphiphilic
electrochemical properties of homo- and heterodimetallic diethynylethene
bisphthalocyaninato complexes. J Org Chem 2000;65(3):823e30.
[29] Cid J-J, Yum J-H, Jang S-R, Nazeeruddin MK, Martínez-Ferrero E, Palomares E,
et al. Molecular cosensitization for efficient panchromatic dye-sensitized solar
cells. Angew Chem Int Ed 2007;46(44):8358e62.
sensitizer and hexadecylmalonic acid on TiO
003;107:14336e41.
2
nanocrystals. J Phys Chem B
2
[
[
[
21] Kimura T, Kanota N, Matsui K, Tanaka I, Tsuboi T, Takaguchi Y, et al. Prepa-
ration and electrochemical and optical properties of unsymmetrically
substituted phthalocyanines with one or two trithiole rings and related
symmetric derivatives. Inorg Chem 2008;47(9):3577e83.
[30] Reddy PY, Giribabu L, Lyness C, Snaith HJ, Vijaykumar C, Chandrasekharam M,
et al. Efficient sensitization of nanocrystalline TiO
2
films by a near-IR-
22] Leznoff CC, Svirskaya PI, Khouw B, Cerny RL, Seymour P, Lever ABP. Syntheses
of monometalated and unsymmetrically substituted binuclear phthalocya-
nines and a pentanuclear phthalocyanine by solution and polymer support
methods. J Org Chem 1991;56(1):82e90.
23] Weitemeyer A, Kliesch H, Wohrle D. Unsymmetrically substituted phthalo-
cyanine derivatives via a modified ring enlargement reaction of unsubstituted
subphthalocyanine. J Org Chem 1995;60:4900e4.
absorbing unsymmetrical zinc phthalocyanine. Angew Chem Int Ed
2007;46(3):373e6.
[31] Cid J-J, García-Iglesias M, Yum J-H, Forneli A, Albero J, Martínez-Ferrero E,
et al. Structure-function relationships in unsymmetrical zinc phthalocyanines
for dye-sensitized solar cells. Chem Eur J 2009;15(20):5130e7.