Novel zinc porphyrin sensitizers for dye-sensitized solar cells: Synthesis and spectral, electrochemical, and photovoltaic properties

Article abstract of DOI:10.1016/j.molstruc.2012.12.003

Two donor-π-spacer-acceptor porphyrin dyes were synthesized for use in dye-sensitized solar cells. The dyes comprised the same donor (porphyrin derivative) consisting of 3,4,5-trimethoxybenzaldehyde and acceptor/anchoring group (carboxyl group) but varying π-spacer consisting of a Schiff base structure. Each of the dyes displayed different adsorption behavior and coverage on the TiO₂ surface. The porphyrin dyes P_ZnBIACOOH studied in this work exhibit red-shifted and broadened electronic spectra respect to the reference P_ZnCOOH as expected. By the introduction of Schiff base unit at the meso positions, the energy level of E_ox (excited-state oxidation potentials) is significantly shifted to the positive compared with the reference P_ZnCOOH, indicating a decreased HOMO-LUMO gap. The highest power conversion efficiency of the two dyes based on DSSCs reached 1.75% under AM 1.5 G irradiation.

Full text of DOI:10.1016/j.molstruc.2012.12.003

Journal of Molecular Structure 1035 (2013) 400–406
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Novel zinc porphyrin sensitizers for dye-sensitized solar cells: Synthesis
and spectral, electrochemical, and photovoltaic properties
⇑
Qinglong Tan, Xuejun Zhang , Lijun Mao, Guanqiong Xin, Shuanfen Zhang
School of Science, North University of China, Taiyuan, Shanxi 030051, PR China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Molecular structure of PZnACOOH and PZnABIAACOOH.
"
Synthesis of two novel
unsymmetrical zinc porphyrin
sensitizers.
"
"
Characterization of prepared
compounds.
Properties of novel zinc porphyrin
sensitizers.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2012
Received in revised form 1 December 2012
Accepted 3 December 2012
Available online 10 December 2012
Two donor–
dyes comprised the same donor (porphyrin derivative) consisting of 3,4,5-trimethoxybenzaldehyde and
acceptor/anchoring group (carboxyl group) but varying -spacer consisting of a Schiff base structure.
Each of the dyes displayed different adsorption behavior and coverage on the TiO2 surface. The porphyrin
dyes PZnABIAACOOH studied in this work exhibit red-shifted and broadened electronic spectra respect to
the reference PZnACOOH as expected. By the introduction of Schiff base unit at the meso positions, the
energy level of Eox (excited-state oxidation potentials) is significantly shifted to the positive compared
with the reference PZnACOOH, indicating a decreased HOMO–LUMO gap. The highest power conversion
efficiency of the two dyes based on DSSCs reached 1.75% under AM 1.5 G irradiation.
p-spacer–acceptor porphyrin dyes were synthesized for use in dye-sensitized solar cells. The
p
Keywords:
Porphyrin derivative
Schiff base unit
Donor–acceptor systems
Power conversion efficiency
Solar cells
Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
Photovoltaic performances
1. Introduction
ing more than 11% efficiencies because of their intense and wide-
range absorption of visible light [2,3]. However, the ruthenium
Dye-sensitized solar cells (DSSCs) are currently attracting
considerable attention because of their high light-to-electricity
conversion efficiencies, ease of fabrication, and low production
costs [1]. Ruthenium sensitizers have been distinguished by attain-
dyes that are facing the problem of costs and environmental issues
which will limit the large-scale application of DSSCs. In recent
years, the complexes derived from common metals or metal-free
organic dyes have been attracting increasing attention, owing to
their modest cost, large molar absorption coefficients, ease of syn-
thesis and modification and satisfactory stability [4]. Among the
alternative dyes, porphyrins show strong absorption and emission
in the visible region as well as tunable redox potentials. These
⇑
Corresponding author. Address: North University of China, Xueyuan Road No. 3,
Taiyuan, Shanxi 030051, PR China. Tel./fax: +86 351 3922152.
E-mail address: zhangxuejun@nuc.edu.cn (X. Zhang).
0022-2860/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.12.003

Q. Tan et al. / Journal of Molecular Structure 1035 (2013) 400–406
401
properties make them as promising candidates for light harvesting
2.2. Analytical measurements
dyes for DSSCs. Although many advantages have been put for-
warded, porphyrins showed inferior performances compared to
those high performance dyes for DSSCs [5–7]. One way to tackle
1H NMR spectra were recorded with a Varian Inova 400 MHz
NMR system. MALDI-TOF mass spectrometric measurements were
performed on Bruker Bifiex III MALDI-TOF. UV–vis spectra of the
dyes were measured on a Techcomp 2300 spectrophotometer.
The PL spectra were obtained using Hitachi F-2500 luminescence
spectrometer. Cyclic and differential pulse voltammetry measure-
ments were performed on an electrochemical workstation
(CHILK2005A, Tianjin Lanlike Chemistry electronic high-tech Co.,
Ltd., PR China).
those problems is to elongate the p-system of porphyrin by mod-
ifying a b position with olefinic linkage, quinoxaline moiety or
naphthyl group [8–10]. Until now, the most efficient porphyrin
dye for DSSCs was reported by Grätzel et al. [11] and the porphyrin
dye gave the
g up to 12.3% for YD2-o-C8+Y123 under one sun and
the cobalt-based electrolyte. The breakthrough of the efficiency of
porphyrin dyes indicate that porphyrins have the potential to be
efficient as higher as the rutheniums were. Another way to im-
prove the performance of porphyrin based DSSCs is to modify por-
phyrin molecules at meso-position by incorporation of aryl
2.3. General procedure for preparation and test of solar cells
derivatives to extend the
been recognized as the most promising dyes for such application
because of their photochemical and electrochemical stability,
strong absorbing ability in the visible region due to
p
-system. Recently, porphyrins have
TiO2 photoelectrode was prepared by screen-printing methods.
TiO2 photoelectrode (area: ca. 0.8 cm ꢁ 1.2 cm) was prepared by a
similar method reported in the literature [21–23]. Nanocrystalline
p
–
pꢀ transi-
TiO2 films of 10–15 lm thickness were deposited onto transparent
tions of the conjugated macrocycle and high molar extinction coef-
ficient [12–16]. However, the narrow absorption spectra of
porphyrins, which poorly matched solar light distribution, limits
the performance of porphyrin-sensitized solar cells. In order to
solve this problem, many model compounds based on porphyrin
have been synthesized for application in DSSCs [17–19].
To investigate how the structure of Schiff base affects the cell
performance of devices, we have designed and synthesized two
meso-position modified porphyrin sensitizers with porphyrin moi-
eties as donors, aryl derivatives consisting of Schiff base unit as
conducting glass (which has been coated with a fluorine-doped
stannic oxide layer, thickness of 5 mm, sheet resistance of 19–
23
X). These films were dried at 150 °C for 30 min and then were
gradually sintered at 450 °C for 30 min. The sensitizer was dis-
solved in DMF at a concentration of 1 ꢁ 10ꢂ4 mol Lꢂ1. The photo-
electrode was dipped into the dye solution immediately after the
high-temperature annealing and it was still hot (70 °C) and then
kept at room temperature for 24 h so that the dye was adsorbed
onto the TiO2 films. After completion of the dye adsorption, the
photoelectrode was withdrawn from the solution and washed
thoroughly with ethanol to remove non-adsorbed dye under a
stream of dry air or nitrogen.
p-conjugated linkers, and carboxyl group as acceptors or anchoring
groups (Fig. 1). The compounds were expected to have broader and
intenser absorption than the corresponding porphyrin without
Schiff base unit. Herein, we report the synthesis and the spectral,
electrochemical, and photovoltaic properties of these porphyrin
based sensitizers. For comparison, porphyrin sensitizer PZnACOOH
was prepared according to a literature method [20]. Our results
indicate that the cell performance of the device using porphyrin
A sandwich cell was assembled by using the dye anchored TiO2
films as the working electrode and conducting glass coated with
carbon as the counter electrode. The two electrodes were placed
on top of each other using a thin polyethylene film (50 lm) thick
as a spacer to form the space for electrolyte. The empty cell was
tightly held, and the edges were heated to 65 °C to seal the two
electrodes together. The active surface area of TiO2 film electrode
was ca. 0.96 cm2. The electrolyte was introduced into cell through
pre-drilled whole of the counter electrode and later covered by
cover glass to avoid the leakage of the electrolyte solution. The
composition of electrolyte is 0.05 mol Lꢂ1 iodine, 0.5 mol Lꢂ1 LiI
in acetonitrile. Photoelectrochemical data were obtained using a
450 W xenon light source focused to give 100 mW cmꢂ2, the equiv-
alent of one sun at AM 1.5 (the luminance of the lamp has been
normalized and corrected by the supplier), at the surface of the test
PZnABIAACOOH as sensitizer outperforms the better-reported por-
phyrin dye. Therefore, PZnABIAACOOH might be a promising green
dye for future colorful DSSC applications.
2. Experimental
2.1. Materials and reagents
cell. The solar-to-electricity conversion efficiency (g) of the DSSCs
All starting materials were purchased from Aladdin Company.
pyrrole were dried and distilled from standard distillation before
use. All other solvents and chemicals used in this work were ana-
lytical grade and used without further purification. All chromato-
graphic separations were carried out on silica gel (200–300 mesh).
was calculated from the short-circuit photocurrent density (Jsc),
the open-circuit photovoltage (Voc), the fill factor (ff) and the inten-
sity of the incident light (Pin) according to the following equation:
Fig. 1. Molecular structures of porphyrin derivatives used in this study.

402
Q. Tan et al. / Journal of Molecular Structure 1035 (2013) 400–406
À
Á
Jsc mA cmꢂ2 ꢁ Voc ðVÞ ꢁ ff
phenyl), 1458 (mC@H pyrrole), 1003 (mNAZn pyrrole). MALDI-TOF MS
calcd. for C53H47N5O9Zn 961.35; found 961.42.
g
¼
Pin ðmW cmꢂ2
Þ
The IPCE values were plotted as a function of the excited wave-
length and defined according to the following equation:
2.4.4. 5-[p-(4-Carboxyl benzyl idene amino)] phenyl-10,15,20-tris
(3,4,5-trimethoxy-lphenyl)porphyrin zine (PZnABIAACOOH)
À
Á
Jsc mA cmꢂ2
1240
A 50 ml dichloromethane solution of 3 (1.06 g, 1.1 mmol) and 4-
formylbenzoic acid (0.25 g, 1.7 mmol) was refluxed at 38 °C in the
presence of 0.2 ml acetic acid for 3 h under a nitrogen atmosphere.
After cooling to room temperature, the reaction mixture was re-
moved by rotary evaporation and recrystallied from the mixture of
CH2Cl2 and CH3OH (1:4, v/v), a purple solid of compound PZn-
ABIAACOOH was obtained (1.19 g, 98%) 1H NMR (CDCl3, 400 MHz,
d ppm): 7.95 (d, 4 H, ArAH), 8.22 (d, 12 H, ArAH), 8.35 (d, 4 H, ArAH),
8.63 (m, 8 H, pyrrole-H), 8.86 (s, 1 H, AC@NH), 3.96 (s, 27 H, AOCH3).
IPCE ¼
ꢁ
kðnmÞ /ðmW cmꢂ2
Þ
where Jsc is the short-circuit photocurrent density generated by
monochromatic light, k is the wavelength of incident monochro-
matic light, and / is the incident light intensity.
2.4. Synthesis
The synthetic routes to the two porphyrin dyes are shown in
Scheme 1. The detailed synthetic procedures were as follows:
IR (KBr pellet, cmꢂ1): 1236 (
C@C phenyl), 1657 ( C@O phenyl), 1458 (m
m
@CAOAC), 1657 (
mC@N Schiff base), 1562
(m
m
C@H pyrrole), 1007 (mNAZn
pyrrole); MALDI-TOF MS calcd. for C61H51N5O11Zn 1095.48; found
1095.45. Elem. Anal. For C61H51N5O11Zn Calc: C, 66.88; H, 4.69; N,
6.39. Found: C, 66.92; H, 4.71; N, 6.35.
2.4.1. 5-(4-Nitro) phenyl-10,15,20-tris (3,4,5-trimethoxylphenyl)
porphyrin (1)
In a 250 ml three-necked ask, 3,4,5-trimethoxybenzaldehyde
(1.96 g, 10 mmol) and 4-nitrobenzaldehyde (0.38 g, 2.5 mmol) were
dissolved in 45 ml propionic acid and 10 ml nitrobenzene. The solu-
tion was heated to reflux at 125 °C. Pyrrole (0.69 ml, 10 mmol) was
then added dropwise and the mixture stirred for another 45 min.
After cooling to room temperature, half of the propionic acid was
evaporated and C2H5OH was added. The mixture was cooled over
night and filtered under vacuum. The crude product was purified
column chromatography (dichloromethane/petroleumether = 2:1
as an eluent). After recrystallization from the mixture of CHCl3 and
CH3OH (1:4, v/v), a desired purple solid of compound 1 was obtained
(0.89 g, 10%). 1H NMR(CDCl3, 400 MHz, ppm): 7.07 (d, 2 H, ArAH),
7.73–7.79 (d, 8 H, ArAH), 8.05 (d, 2 H, ArAH), 8.35 (d, 4 H, ArAH),
8.63 (m, 8 H, pyrrole-H), ꢂ2.76 (s, 2 H, NAH), 3.96 (s, 27 H, AOCH3).
2.4.5. 5-(4-Carboxyl) phenyl-10,15,20-tris (3,4,5-trimethoxylphenyl)
porphyrin (5)
The synthetic procedure for 5 was similar to that for 1, except
that 4-formyl benzoic acid (0.38 g, 2.5 mmol) was used instead of
4-nitrobenzaldehyde. The crude product was purified using silica
chromatograph with dichloromethane/cyclohexane mixture (1:1)
to give 5 as a purple solid (0.35 g, 15%) 1H NMR (CDCl3, 400 MHz,
ppm): 7.47 (d, 4 H, ArAH), 7.54–7.79 (d, 12 H, ArAH), 8.24 (m, 8
H, pyrrole-H), 3.95 (s, 27 H, AOCH3). IR (KBr pellet, cmꢂ1): 1236
(m@CAOAC), 3450 (
mNAH), 1562 (mC@C phenyl), 1458 (mC@H pyrrole),
1008 mNAH pyrrole). MALDI-TOF MS calcd. for C54H48N4O11
(
928.99; found 929.01.
IR (KBr pellet, cmꢂ1): 1236 (
nyl), 1610 ( C@O phenyl), 1458 (
m
@CAOAC), 1566 (
m
NO2), 1562 (
mC@C phe-
m
mC@H pyrrole), 1001 (m
NAH pyrrole).
2.4.6. 5-(4-Carboxyl) phenyl-10,15,20-tris (3,4,5-
trimethoxylphenyl)porphyrin (PZnACOOH)
MALDI-TOF MS calcd. for C53H47N5O11 929.98; found 930.19.
The synthetic procedure for PZnACOOH was similar to that de-
scribed for 3, except that 5 (0.02 g, 0.03 mmol) was used instead
of 2. A purple-blue solid of compound PZnACOOH was obtained
(0.21 g, 96%) 1H NMR (CDCl3, 400 MHz, ppm): 7.47 (d, 4 H, ArAH),
7.54–7.79 (d, 12 H, ArAH), 8.24 (d, 8 H, pyrrole-H), 3.95 (s, 27 H,
2.4.2. 5-(4-Amidogen) phenyl-10,15,20-tris (3,4,5-trimethoxylphenyl)
porphyrin (2)
Under a nitrogen atmosphere, 2 was obtained through restoring
the compound 1 (1.43 g, 1.6 mmol) in 36% HCl, and the solvent was
heated at 67 °C for 2.5 h. The reductant was SnCl2ꢃ2H2O (0.35 g,
1.6 mmol). When the reaction finished, stronger ammonia water
was used to adjust the pH of solution to 7–8. Then the crude product
was extracted by dichloromethane and purified using column chro-
matography (chloroform/cyclohexane = 1:1 as an eluent). A purple
solid of compound 2 was obtained (1.18 g, 85%) 1H NMR(CDCl3,
400 MHz, ppm): ꢂ2.76 (s, 2 H, NAH), 4.02 (s, 2 H, NH2), 7.07 (d, 2
H, ArAH), 7.73–7.79 (d, 8 H, ArAH), 8.05 (d, 2 H, ArAH), 8.35 (d, 4
H, ArAH), 8.63 (m, 8 H, pyrrole-H), 3.96 (s, 27 H, AOCH3). IR (KBr pel-
AOCH3). IR (KBr pellet, cmꢂ1): 1236 (
m@CAOAC), 3450 (mNAH),
1562 (mC@C phenyl), 1458 (mC@H pyrrole), 1007 (mNAZn pyrrole).
MALDI-TOF MS calcd. for C54H46N4O11Zn 992.36; found 992.34.
Elem. Anal. For C54H46N4O11Zn calcd.: C, 65.35; H, 4.67; N, 5.65.
Found: C, 65.32; H, 4.69; N, 5.63.
3. Results and discussion
let, cmꢂ1): 1236 (
mC@O phenyl), 1458 (mC@H pyrrole), 1002 (mNAH pyrrole). MALDI-
m@CAOAC), 3450 (mNAH), 1562 (mC@C phenyl), 1610
3.1. Synthesis and chemical characterization
(
TOF MS calcd. for C53H49N5O9 897.98; found 898.21.
Both the two dyes have been synthesized according to several
classical reactions. The synthetic strategy is shown in Scheme 1.
Taking PZnABIAACOOH as an example, the dye was synthesized as
following steps. First, the starting material, 5-(4-nitro) phenyl-
10,15,20-tris (3,4,5-trimethoxylphenyl) porphyrin 1, was obtained
from 3,4,5-trimethoxybenzaldehyde in the presence of 4-nitrobenz-
aldehyde and pyrrole. Subsequently, compound 1 was converted
into 5-(4-amidogen) phenyl-10,15,20-tris (3,4,5-trimethoxylphe-
nyl) porphyrin 2 by Tin(II) chloride dehydrate with 36% HCl. Finally,
compound 2 was treated with Zn(OAc)2, and then reacted with 4-
formyl benzoic acid by bonus-elimination reaction to give PZn-
ABIAACOOH. PZnACOOH were synthesized by the same procedure
as that for PZnABIAACOOH except 4-carboxyl benzyl group was used
instead of p-(4-carboxyl benzyl idene amino) phenyl group at the
2.4.3. 5-(4-Amidogen) phenyl-10,15,20-tris (3,4,5-trimethoxylphenyl)
porphyrin zine (3)
In a 250 ml three-necked round-bottomed ask, a mixture of 2
(1.17 g, 1.3 mmol) and Zn (OAc)2 (0.58 g, 2.6 mmol) in a solution
of CHCl3 (45 ml) and DMF (10 ml) was refluxed for 4.5 h. After cool-
ing to room temperature, the mixture was washed with water. The
organic layer was dried over anhydrous MgSO4 and concentrated.
A purple-red solid of compound 3 was obtained (1.07 g, 85%) 1H
NMR(CDCl3, 400 MHz, ppm): 4.02 (s, 2 H, NH2), 7.07 (d, 2 H, ArAH),
7.73–7.79 (d, 8 H, ArAH), 8.05 (d, 2 H, ArAH), 8.35 (d, 4 H, ArAH),
8.63 (d, 8 H, pyrrole-H), 3.96 (s, 27 H, AOCH3). IR (KBr pellet,
cmꢂ1): 1236 (
m@CAOAC), 3450 (mNAH), 1562 (mC@C phenyl), 1610 (mC@O

Q. Tan et al. / Journal of Molecular Structure 1035 (2013) 400–406
403
Scheme 1. Synthesis of porphyrin dyes.

404
Q. Tan et al. / Journal of Molecular Structure 1035 (2013) 400–406
Fig. 2. UV–vis absorption spectra of porphyrins in DMF solution.
Fig. 3. PL spectra of PZnABIAACOOH and PZnACOOH in DMF solution.
meso position. The structures of the two dyes were verified by 1H
NMR, IR, and MOLDI-TOF mass spectra.
3.2. Optical and electrochemical properties
The UV–vis absorption spectra of PZnABIAACOOH and PZn-
ACOOH in DMF solution are shown in Fig. 2, and the peak positions
and molar absorption coefficients (e) of Soret and Q bands of porphy-
rin dyes are listed in Table 1. The UV–vis absorption spectra of PZn-
ACOOH exhibits a strong Soret band at 423 nm and two moderate
Q-bands at 559 and 600 nm. However, PZnABIAACOOH showed
broad and moderate strong absorption peak at 431 nm and stronger
absorption peak at 564 nm, which are broader and red-shifted than
that of PZnACOOH. As seen on the UV–vis absorption spectrum, the
Soret band of PZnABIAACOOH at 431 nm are red-shifted about
8 nm compared to PZnACOOH. The molar extinction coefficients (e)
of Soret bands are 1.83 ꢁ 105 mol Lꢂ1 cmꢂ1 for PZnABIAACOOH
and 1.94 ꢁ 105 mol Lꢂ1 cmꢂ1 for PZnACOOH. These results indicate
that PZnABIAACOOH and PZnACOOH have relatively strong light
harvesting ability, respectively, both of which are much higher than
that of Ru(dcbpy)2 (NCS)2 (N3 dye: ca.1.6 ꢁ 104 mol Lꢂ1 cmꢂ1) [24].
The steady-state fluorescence spectra of PZnABIAACOOH and
Fig. 4. Cyclic voltammograms and differential pulse voltammerty for PZn-
PZnACOOH were measured in DMF by exciting at the peak position
ABIAACOOH and
PZnACOOH in DMF solution, measured at a scan rate of
100 mV sꢂ1
.
of the Soret band (Fig. 3) and the wavelengths for emission maxima
are listed in Table 1. From the intersection of the normalized
absorption and emission spectra, the zero-zero excitation energies
(E0–0) are determined to be 2.88 eV for PZnABIAACOOH and 2.93 eV
for PZnACOOH. The redox potentials of the porphyrin dyes were
obtained by cyclic voltammetry and differential pulse voltammerty
(Fig. 4), and the corresponding data are summarized in Table 2
[26]. The oxidation potentials of PZnABIAACOOH and PZnACOOH
(1.195 and 0.937) are both higher than the redox potential (0.2 V
vs. SCE) of the iodide/triiodide couple. This could lead to fast dye
regeneration, avoiding the geminate charge recombination be-
tween oxidized dye molecules and photoinjected electrons in the
nanocrystalline titania film. As shown in Table 2, the excited-state
oxidation potentials of two porphyrin dyes are more negative than
the equivalent potential of the TiO2 conduction band edge (ꢂ0.74 V
vs. SCE), and the Egap ranges from 0.71 to 0.76 eV. Assuming that
energy gap of 0.2 eV is necessary for efficient electron injection
these driving forces are sufficiently large for effective electron
injection [25]. It is noted that the measured electrochemical
HOMO/LUMO gaps are 2.19 and 2.52 eV for PZnABIAACOOH and
PZnACOOH, which are broader than the above mentioned optical
gaps. This difference is a common feature of organic optoelectronic
materials and may be related to the solvent effect.
Table 1
UV, PL spectral data of porphyrin dyes.
, 105 mol Lꢂ1 cmꢂ1
)
kem (nm)
kint (nm)
Cd (10ꢂ9 mol cmꢂ2
)
a
b
c
Dyes
kabs
(e
P
P
ZnABIAACOOH
ZnACOOH
431 (1.83), 564 (0.19), 606 (0.09)
423 (1.94), 559 (0.24), 600 (0.09)
609, 660
609, 659
548
546
1.83
1.26
a
b
c
Absorption spectra was measured in DMF solution.
Wavelengths for emission spectra in DMF solution by exciting at Soret wavelength.
Measured by the intercept of the normalized absorption and emission spectra.
Dye loading density was obtained on TiO2 film.
d

Q. Tan et al. / Journal of Molecular Structure 1035 (2013) 400–406
405
Table 2
Electrochemical data for porphyrin dyes and driving forces for electron-transfer processes on the TiO2.
a
b
c
ꢀ
e
f
d
Dyes
E0–0 (eV)
Eox (V)
Ered (V)
D
Ging (eV)
DGreg (eV)
Eox (V)
P
P
ZnABIAACOOH
ZnACOOH
2.88
2.93
1.195
0.937
ꢂ0.764
ꢂ0.841
ꢂ1.685
ꢂ1.993
ꢂ0.945
ꢂ1.253
ꢂ0.995
ꢂ0.737
a
b
c
d
e
f
Determined from the intercept of the normalized absorption and emission spectra.
First oxidation potentials (vs. SCE).
First reduction potentials (vs. SCE).
Excited-state oxidation potentials approximated from Eox and E0–0 vs. SCE).
Driving forces for electron injection from the porphyrin excited singlet state Eꢀox to the conduction band of TiO2 (ꢂ0.74 V vs. SCE).
Driving forces for regeneration of the porphyrin radical cation by Iꢂ=I3ꢂ redox couple (+0.2 V vs. SCE).
Table 3
3.3. Photovoltaic properties of porphyrin-sensitized TiO2 solar cells
Photovoltaic performance of DSSCs for the two porphyrin dyes.
DSSCs were fabricated using two dyes as the sensitizers, with an
Dyes
Jsc (mA/cm2)
Voc (V)
ff
g%
effective area of 0.96 cm2, single-layer TiO2 film with 10–15
lm
P
P
ZnABIAACOOH
ZnACOOH
5.5
4.0
0.49
0.40
0.65
0.66
1.75
1.06
thickness on ITO, and the electrolyte composed of 0.5 M LiI,
0.05 M I2, and 0.5 M 4-tert-butyl-pyridine (TBP) in 3-methoxypro-
pionitrile in methanol solution. The photovoltaic performance of
the porphyrin-sensitized TiO2 cells was obtained under optimized
conditions which the dyes were taken out and illuminated for
30 min after immersing in DMF for 15 min. Fig. 5 shows the J–V
curves of DSSCs based on PZnABIAACOOH and PZnACOOH. The de-
values is excluded. However, the biggest difference between the
two dyes is their orientation on the TiO2 surface. PZnABIAACOOH
will be tilted due to the C@C bond. This will result in different
dye loading density and different electron injection rate. Therefore,
the enhanced light-harvesting ability and higher electron collec-
tailed parameters (Jsc, Voc, ff, and g) are summarized in Table 3. The
DSSC based on PZnABIAACOOH dye shows a better comprehensive
properties with an open-circuit voltage (Voc) of 0.49 V, a short-cir-
cuit photocurrent density (Jsc) of 5.5 mA cmꢂ2, and a fill factor (ff)
of 0.65, corresponding to a solar-to-electricity conversion effi-
tion efficiency of PZnABIAACOOH probably lead to the higher
lue than PZnACOOH.
g va-
4. Conclusions
ciency (
ZnACOOH sensitized one yields
g
) of 1.75% under AM 1.5 irradiation (100 mW cmꢂ2), while
P
g = 1.06% with Voc = 0.40 V,
In summary, we have successfully synthesized two porphyrin
sensitizers (PZnABIAACOOH and PZnACOOH) in which porphyrin
unit act as electron-donating moiety and carboxyl group as
Jsc = 4.0 mA cmꢂ2, and ff = 0.66. It is easy to see that the Voc of PZn-
ACOOH dye is lower than that of PZnABIAACOOH which may be
explained by dark current voltage curves of DSSCs.
electron-accepting moiety. By introducing the p-spacer consisting
To a large extent, the two porphyrin dyes display rather similar
of a Schiff base unit, the light absorption of PZnABIAACOOH has been
broadened and slightly red-shifted compared to that of PZnACOOH.
Therefore, PZnABIAACOOH exhibits the maximum power conver-
sion efficiency of 1.75% with Jsc = 5.5 mA cmꢂ2, Voc = 0.49 V and ff
= 0.65. Since electron injectionefficiencies of the two porphyrindyes
are more or less similar, the light-harvesting ability and electron col-
lection efficiencies of porphyrin sensitizers have strong influence on
the photovoltaic performances. Our results demonstrate the neces-
Voc and ff values. The different
g values of the two porphyrin-sen-
sitized primarily result from the difference of Jsc. Because the sur-
face density of the dye-sensitized TiO2 films is an important factor
to determine the photocurrent generation. From Tables 1 and 3, we
can see that the low Jsc of PZnACOOH sensitized TiO2 cell compared
to that of the PZnABIAACOOH Sensitized cells mainly results from
the low surface density of TiO2/PZnACOOH. It seems that the higher
electron collection efficiency of PZnABIAACOOH determined by the
moderate surface density of TiO2/Dyes is suitable for preparing
high efficient DSSCs. Given the similar driving forces of two por-
sity and importance of modifying p-spacers in D–p–A structure of
porphyrin dyes. Inspired by the natural light-harvesting ability of
porphyrin derivatives, further structural modification of meso-
substituted porphyrin for broadening the spectral absorption is
anticipated to give some new porphyrin dyes with even better
performances.
phyrin dyes, the influence of electron injection efficiency in the
g
Acknowledgements
The authors would like to acknowledge financial support from
the National Nature Science Foundation of China (Grant number:
20871108) and the Natural Science Foundation of Shanxi Province,
China (Grant number: 2011011022-4).
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