Synthesis, photo-electrochemical properties and dye-sensitized solar cell performance of d-π-A structured porphyrins

Article abstract of DOI:10.1016/j.jphotochem.2013.08.010

Two new dye sensitizers 5-(N,N-4-(diphenylamino)phenyl)-10-(4-(tert-butyl- 2-cyanoacrylate)phenyl)porphyrin (TP) and 5-(N, N-4-(bis(4-butylphenyl) amino) phenyl)-10-(4-(tert-butyl-2-cyanoacrylate)phenyl)porphyrin (BUTP) with cyanoacrylic acid as electron acceptor were synthesized and researched. BUTP has strong absorption in a wide range of 300-726 nm. DSSCs sensitized by BUTP displayed a power conversion efficiency (η) 5.33%, J_SC 12.21 mA cm^-2, V_oc 0.68 V, FF 0.65, under the illumination of AM 1.5 G. Spectral properties, electrochemical properties, photovoltaic and electrochemical impedance properties of TP and BUTP were also investigated. Considering its simple synthesis, both TP and BUTP were qualified sensitizers for DSSCs.

Full text of DOI:10.1016/j.jphotochem.2013.08.010

Accepted Manuscript
Title: Synthesis, photo-electrochemical properties and
dye-sensitized solar cell performance of D-␲-A structured
porphyrins
Author: Hongxing Zhang Xiuwen Zheng Haiying Li Bin Jin
Chengyun Wang Yongjia Shen Jianli Hua
PII:
S1010-6030(13)00374-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2013.08.010
JPC 9496
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
17-4-2013
29-7-2013
8-8-2013
Please cite this article as: H. Zhang, X. Zheng, H. Li, B. Jin, C. Wang, Y. Shen, J. Hua,
Synthesis, photo-electrochemical properties and dye-sensitized solar cell performance
of D-␲-A structured porphyrins, Journal of Photochemistry and Photobiology A:
Chemistry (2013), http://dx.doi.org/10.1016/j.jphotochem.2013.08.010
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Synthesis, photo-electrochemical properties and dye-sensitized
solar cell performance of D-π-A structured porphyrins
Hongxing Zhang,a Xiuwen Zheng,b Haiying Li,b Bin Jin,a Chengyun Wang,a
Yongjia Shen,a Jianli Hua a
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
Technology, 130 Meilong Road, Shanghai 200237, China.
b Key Laboratory for Advanced Materials and Department of Chemistry, East China University of Science and Technology,
130 Meilong Road, Shanghai, 200237, China.
Abstract: Two new dye sensitizers 5-(N,N-4-(diphenylamino)phenyl)-10-(4-(tert-butyl-
2-cyanoacrylate)phenyl)porphyrin (TP) and 5-(N, N-4-(bis(4-butylphenyl) amino) phenyl)
-10-(4-(tert-butyl-2-cyanoacrylate)phenyl)porphyrin (BUTP) with cyanoacrylic acid as
electron acceptor were synthesized and researched. BUTP has strong absorption in a wide
range of 300-726 nm. DSSCs sensitized by BUTP displayed a power conversion efficiency
(η) 5.33 %, JSC 12.21 mA cm-2 , Voc 0.68V, FF 0.65, under the illumination of AM 1.5G.
Spectral properties, electrochemical properties, photovoltaic and electrochemical impedance
properties of TP and BUTP were also investigated. Considering its simple synthesis, both TP
and BUTP were qualified sensitizers for DSSCs.
Keyword: Porphyrin; Electronic structure; Absorption spectra; Power conversion efficiency;
Dye-sensitized solar cells
1 Introduction
The consumption of non-renewable resources such as coal and petrol are leading to a
potentially serious energy crisis.[1] Thus the devices which can use solar energy effectively are
of great significance. In 1991, professor Grätzel and his team developed a new low cost
solar cell called a nanocrystalline dye-sensitized solar cells (DSSCs), that features strong
power-conversion.[2] This new type of solar cell has drawn great attention of scientists and
researchers.
Many researchers are interested in porphyrin, due to the properties of appropriate HOMO
and LUMO level, strong Soret band absorption in the range of 400-500nm and Q band in
500-700 nm.[3] Triphenylamine is widely used in DSSCs, owing to its non-planar structure,
 Corresponding author: Fax/Tel:+86-021-64252967; E-mail: cywang@ecust.edu.cn
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Hongxing Zhang et al./ Journal of Photochemistry and Photobiology A: Chemistry
strong electron-donating and aggregation-resisting capacity.[4-6] Recently reported, DSSCs
sensitized by dyes containing porphyrins and triphenylamine have shown to generally have
poor power conversion efficiency. Therefore, researchers have focused on improving the
photovoltaic performances of those devices. In 2009, Kim et al synthesized two dyes of this
type, in which triphenylamine and porphyrin were bridged by styrene.[7] DSSCs sensitized by
the two dyes reached a power conversion efficiency of 1.7 % and 3.1% respectively. In 2010,
Yeh et al reported a series of novel porphyrin derivates, most of which utilized alkoxy
substituted triphenylamine as electron donors.[8] Electron donors and acceptors were
connected by alkynyl and porphyrin units. The highest efficiency of DSSCs sensitized by
those dyes reached 5.13%. Tan et. al. reported dyes made of porphyrin derivatives
containing thiophene and triphenylamine units.[9] The efficiency of the devices containing
these derivatives ranged from 0.36% to 3.91%. Kim et. al synthesized several porphyrin
derivates for DSSCs, their devices efficiency ranged from 2.09% to 5.01%.[10-11] Xie et. al.
have ever studied the DSSCs’ efficiency influenced by triphenylamine derivatives’ number
and structure.[12] Liu and Deng et. al. have designed and synthesized a series of D-B-A
porphyrins, and gave a maximum power conversion efficiency of 2.38%. [13]
Some porphyrins derived by triphenylamine as electron donor, always have triphenylamine
or alkoxy-substituted triphenylamine located in the meso position, and carboxyl in the
para-position. The other two meso positions are occupied by phenyl or substituted phenyl.
The theoretical calculations indicate that phenyls in the meso position have little contribution
as electron donor. Therefore, a single donor group was introduced into the porphyrin core
rather than three groups. Also butyl was added to triphenylamine to increase the electron
donating capacity and reduce aggregation of dyes. Liu and Deng et. al. demonstrated that
broad absorption bands and effective intra-molecular charge transfer can be achieved by
introducing cyanoacrylic acid and porphyrin bridged phenylene.[13] These dyes improved
photovoltaic performances of DSSCs. Kim et. al. confirmed cyanoacetic acid unit occupied
the effect of electron-withdrawing .[14]
Based on above considerations, we designed and synthesized two novel porphyrins with
triphenylamine as electron donor and cyanoacrylic acid as electron acceptor (Scheme 1), and
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further studied their applications on DSSCs. Power conversion efficiency of DSSCs sensitized
by 5-(N,N-4-(diphenylamino)phenyl)-10-(4-(tert-butyl-2-cyanoacrylate)phenyl) porphyrin
(TP) and 5-(N,N-4-(bis(4-butyl-phenyl)amino)phenyl)-10-(4-(tert-butyl-2-cyanoacrylate)
phenyl)porphyrin (BUTP) reached 4.39% and 5.33% respectively. Considering the simple
structure and synthetic methodology of TP and BUTP, they may have promising prospects in
the field of DSSCs.
Scheme 1 Structures of TP and BUTP
2 Experimental section
2.1 Materials and measurements
All the reagents were purchased of AR or CP level. Methylene chloride was distilled using
standard methods. Other solvents were used directly without further processing except where
further noted. Fluorine-doped SnO2 conducting glass (FTO glass, transmission > 90% in the
visible, sheet resistance 15 Ω/square) was obtained from Geo Science and Educational Co.
Ltd. of China. The FTO conducting glass was washed with detergent, deionized water,
acetone and ethanol under ultrasonication before use.
1HNMR data were obtained from a Bruker Avance Ⅲ 400(in CDCl3 or DMSO-d6, TMS
as the internal standard). MS data were obtained using Micromass LCTTM (HREI-TOF). The
UV-vis spectra were recorded by a Varian Cary00 UV-vis Spectrometer (THF as solvent).
Fluorescence spectra were recorded by a Varian Cary Eclipse Fluorescence Spectrometer
(THF as solvent). Cyclic voltammograms were performed with a Versastat II electrochemical
workstation. I-V curves were obtained with a Newport Oriel PV reference cell system (Model
91150V). Action spectra of the incident monochromatic photon-to-electron conversion
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efficiency (IPCE) were obtained with Newport-74125 system (Newport Instruments). The
electrochemical impedance spectroscopy (EIS) for the solar cells was measured with Zahner
IM6e Impedance Analyzer. The intensity of monochromatic light was measured with a Si
detector (Newport-71640). All the above tests were performed at room temperature.
2.2 Synthetic procedures and characterizations
4-Formyltriphenylamine[15] (1) Triphenylamine (4.0 g, 16.3 mmol) was dissolved in 8.0mL
DMF. POCl3 (7.6 mL, 81.5 mmol) was added drop-wise to the mixture at 0°C. As the
temperature rose to room temperature, the mixture changed to a clear bright red solution. The
reaction was heated to 50°C for an additional 5h. After being cooled, the mixture was poured
into an ice-bath with stirring and later neutralized with sodium carbonate. Pale yellow solids
1
were collected by filtering and recrystallizing in ethanol (3.43 g, 77% yield). H-NMR
(400MHz, CDCl3) δ: 7.01(d, J = 8.7 Hz, 2H), 7.15-7.18(m, 6H), 7.34(t, J = 8Hz, 4H), 7.68(d,
J = 8.8 Hz, 2H), 9.81(s, 1H).
Tert-Butyl isocyanoacetate[16] (2)
Cyanoacetic acid (1.705 g, 20 mmol) was dissolved in 20 mL acetonitrile. Tert-butyl alcohol
(2.7 mL, 28 mmol) was added dropwise to the mixture under stirring at 25°C. Then 24 mL of
1 M acetonitrile solution of DCC was added through a dropping funnel carefully. The reaction
mixture was stirred for an additional 5h and filtered through a sintered glass. Upon
evaporation of the solvents, the reaction mixture was purified by flash column
chromatography (silica gel, petroleum ether: ethyl acetate, 10:1) to afford the product (2.365
g, 84% yield) as colourless oil. 1H-NMR (400MHz, CDCl3) δ: 1.50(s, 9H), 3.38(s, 2H).
Tert-butyl-2-cyano-3-(4-formylphenyl)acrylate (3) 1,4-phthalaldehyde (2.682 g, 20 mmol)
was dissolved in 30 mL DMF. Basic alumina (6 g, 5.88 mmol) was added to the mixture
under stirring. Solution of (2) (3.205 g, 22.7 mmol) in DMF (30 mL) was added drop-wise to
the mixture at 25°C. The reaction mixture was stirred for another 3h at room temperature. The
resultant mixture was filtered through a sintered glass. Then the mixture solution was poured
into an ice-bath with stirring and the precipitate was filtered and recrystallized from
1
ethanol-water to get a light-yellow solid (2.48g, 48% yield). H-NMR (400MHz, CDCl3) δ:
1.60 (s, 9H), 8.00(d, J = 6.8 Hz, 2H), 8.11(d, J = 8.3 Hz, 2H), 8.21(s, 1H), 10.09(s, 1H).
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Dipyrromethane (4) Dipyrromethane was synthesized according to literature methods.[17]
A
solution of pyrrole (10 mL, 9.661 g, 0.144 mol) and paraformaldehyde (0.108g, 3.60mmol)
was treated with glacial acetic acid (15 mL) and methanol (5 mL) at room temperature under
an atmosphere of nitrogen. The mixture was stirred for 24 h in the dark. The reaction mixture
was then diluted with dichloromethane, washed with water, 0.1 mol L−1 NaOH aqueous
solution and water, and dried over anhydrous magnesium sulfate. The solvent was removed
under reduced pressure. The resultant slurry was purified by column chromatography (silica
gel, petroleum ether: dichloromethane, 1:1) to afford dipyrromethane (0.23 g, 43%) as a white
1
solid. H-NMR(400MHz, CDCl3) δ: 3.97(s, 2H), 6.03(m, 2H), 6.14-6.16(m, 2H),
6.64-6.66(m, 2H), 7.83(br, s, 2H).
4,4’-Dibutyltriphenylamine (5) Compound 5 was prepared according to the literature.[18]
A
solution of 1-bromo-4-butylbenzene (7.033 g, 33 mmol) was added to
tris(dibenzylideneacetone) dipalladium (Pd2(dba)3) (0.302 g, 0.33 mmol) and
bis(diphenylphosphino)ferrocene (DPPF) (0.274 g, 0.495 mmol) in toluene (70 mL) under
nitrogen atmosphere at room temperature, and the resultant mixture was stirred at that
temperature for 10min. Afterwards, sodium tert-butoxide (3.618 g, 37.65 mmol) and aniline
(1.397 g, 15 mmol) was added to the solution and stirred at 100°C for 24 h. Following the
standard workup procedure, the reaction mixture was purified by flash column
chromatography (silica gel, petroleum ether: dichloromethane, 10:1) to attain the product
1
(4.885g, 91.4% yield) as yellowish oil. H-NMR (CDCl3, 400 MHz) δ: 0.93(t, J = 7.3 Hz,
6H), 1.32-1.41(m, 4H), 1.55-1.62(m, 4H), 2.55(t, J = 7.8 Hz, 4H), 6.92(t, J = 7.3 Hz, 1H),
6.98(d, J = 8.5 Hz, 4H), 7.03-7.05(m, 6H), 7.16-7.21(m, 2H).
4-(bis(4-butylphenyl)amino)benzaldehyde (6) The title compound was prepared according
to the literature procedure.[19] Compound (5) (1.072 g, 3 mmol) was dissolved in 60 mL
1,2-dichloroethane and DMF(1.16 mL, 15 mmol) was added through the syringe. POCl3 (1.4
mL, 15 mmol) was added drop-wise to the mixture at 0°C. The reaction solution was stirred
for 20min at room temperature. Then the reaction was heated to 85°C for an additional 7 h.
After being cooled, the reaction mixture was poured slowly into 40mL saturated sodium
acetate aqueous solution. Then the reaction mixture was stirred for 20min at room
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Hongxing Zhang et al./ Journal of Photochemistry and Photobiology A: Chemistry
temperature. The organic layer was diluted with ether, washed with saturated salt solution and
water, and dried over anhydrous Na2SO4 overnight. Upon evaporation of the solvents, the
reaction mixture was purified by flash column chromatography (silica gel, petroleum ether:
dichloromethane, 3:1) to afford the product (1.090g, 94% yield) as yellowish oil.
1H-NMR(400MHz,CDCl3) δ: 0.94(t, J = 7.3 Hz, 6H), 1.33-1.42(m, 4H), 1.57-1.63(m, 4H),
2.60(t, J = 7.8 Hz, 4H), 6.94(d, J = 8.8 Hz, 2H), 7.08(d, J = 8.4 Hz, 4H), 7.15(d, J = 8.4 Hz,
4H), 7.64(d, J = 8.8 Hz, 2H), 9.77(s, 1H).
5-(N, N-4-(diphenylamino)phenyl)-10-(4-(tert-butyl-2-cyanoacrylate)phenyl) porphyrin
(7) Compound 1 (0.405 g, 1.483 mmol), 3 (0.381 g, 1.482 mmol) and 4 (0.430 g, 2.94 mmol)
were dissolved in 300 mL dichloromethane. Argon was bubbled into the solution under
stirring for 0.5 h. The contents were protected from light and trifluoroacetic acid (TFA) was
added (37µL, 0.5mmol), and the mixture was stirred for 16 h. 2, 3-dichloro-5,
6-dicyanobenzoquinone (1.271 g, 5.595 mmol) was then added, and the mixture was stirred
for 3 h. Triethylamine was added (0.3 mL), and the mixture was stirred for an additional 5
min. The porphyrin was purified by flash column chromatography (silica gel, petroleum ether:
dichloromethane, 1:2→1:4, gradient) to afford the product (0.356g, 31% yield) as a purple
solid. 1H-NMR(400MHz, CDCl3) δ: -3.05(br, s, 2H), 1.70(s, 9H), 7.15-7.19(m, 2H), 7.44(d, J
= 4.1 Hz, 8H), 7.51(d, J = 8.3 Hz, 2H), 8.13(d, J = 8.3 Hz, 2H), 8.43(s, 4H), 8.54(s, 1H), 9.05
(d, J = 4.6 Hz, 2H), 9.23(d, J = 4.6 Hz, 2H), 9.42(d, J = 4.5 Hz, 4H), 10.33(s, 2H).
5-(N, N-4-(bis(4-butylphenyl)amino)-10-(4-(tert-butyl-2-cyanoacrylate)phenyl)phenyl)
porphyrin (8) Compound 3 (0.366g, 1.425mmol), 4(0.416 g, 2.85 mmol) and 6 (0.604 g,
1.568 mmol) was dissolved in 300 mL dichloromethane. Argon was bubbled into the solution
with stirring for 0.5 h. The contents were protected from light and TFA was added (0.5 mmol)
and the mixture was stirred for 16h. 2,3-dichloro-5,6-dicyanobenzoquinone (1.23 g, 5.415
mmol) was then added, and the mixture was stirred for 3 h. Triethylamine was added (0.3
mL) and the mixture was stirred for additional 5 min. The porphyrin was purified by flash
column chromatography (silica gel, petroleum ether: dichloromethane, 2.5:1→1:3, gradient)
to afford the product (0.216 g, 17% yield) as a purple solid. 1H-NMR(400MHz,CDCl3) δ:
-3.04(br, s, 2H), 0.98(t, J = 7.3 Hz, 6H), 1.28-1.37(m, 4H), 1.41-1.46(m, 4H), 1.70(s, 9H),
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2.67(t, J = 7.6Hz, 4H), 7.24 (d, J = 8.4 Hz, 4H), 7.35(d, J = 8.5 Hz, 4H), 7.46(d, J = 8.5 Hz,
2H), 8.09(d, J = 8.4Hz, 2H), 8.42(s, 4H), 8.54(s, 1H), 9.05(d, J = 4.6 Hz, 2H), 9.24(d, J = 4.6
Hz, 2H), 9.41(d, J = 4.6 Hz, 4H), 10.32(s, 2H).
5-(N, N-4-(diphenylamino) phenyl)-10-(4-(tert-butyl-2-cyanoacrylate) phenyl) porphyrin
(TP) Compound 7 (0.136 g, 0.174 mmol) was dissolved in 8 mL of TFA. A solid
precipitated after stirring at room temperature for 1 h. TFA was evaporated in vacuo, and the
solid was triturated with water, filtered, and dried to give the product (0.116 g, 92% yield) as
1
a purple solid. H-NMR (400MHz, DMSO-d6) δ: -3.19(br, s, 2H), 7.30-7.50(m, 12H),
8.10-8.16(m, 2H), 8.53(dd, J = 22.0, 8.3 Hz,4H), 8.75(s, 1H), 9.06(d, J = 4.6 Hz, 2H), 9.18(d,
J = 4.5 Hz, 2H), 9.67(d, J = 4.4 Hz, 4H), 10.64(s, 2H). HRMS (ESI, m/z): [M+H]+ calcd for
C48H33N6O2, 725.2665; found, 725.2661.
5-(N,N-4-(bis(4-butylphenyl)amino)phenyl)-10-(4-(tert-butyl-2-cyanoacrylate)phenyl)
porphyrin (BUTP) Compound 8 (0.195 g, 0.219 mmol) was dissolved in 8 mL of TFA. A
solid precipitated after stirring at room temperature for 1 h. TFA was evaporated in vacuo, the
solid was triturated with water, filtered, and dried to give the product (0.173 g, 95% yield) as
a purple solid. 1H-NMR(400MHz, DMSO-d6) δ: -3.19(br, s, 2H), 0.93(t, J = 7.3 Hz, 6H),
1.34-1.39(m, 4H), 1.59-1.62(m, 4H), 2.60(t, J = 7.5 Hz, 4H), 7.28-7.35(m, 10H), 8.12(d, J =
8.1 Hz, 2H), 8.52(dd, J = 17.4, 8.0 Hz, 4H), 8.68(s, 1H), 9.07(d, J = 4.5 Hz, 2H), 9.19(d, J =
4.3 Hz, 2H), 9.67(d, J = 4.1 Hz, 4H), 10.64(s, 2H). HRMS (ESI, m/z): [M+H]+ calcd for
C56H49N6O2, 837.3917; found, 837.3923.
N
N
I
CHO
1
CN
COOC(CH3)3
NC
NC
N
HN
N
N
HN
N
COOH
COOC(CH3)3
VIII
VII
N
N
II
III
NC COOC(CH3)3
NH
NC COOH
NH
CHO
3
7
TP
2
IV
(HCHO)n
HN
NH
NC
NC
4
N
HN
VII
N
HN
VIII
COOH
COOC(CH3)3
N
N
CHO
N
NH
N
NH
NH2
VI
V
+
N
BUTP
8
N
Br
5
6
Scheme 2 Synthesis of TP and BUTP. Reaction conditions: (I) DMF, POCl3; (II) acetonitrile, TBA; (III) 1,4-phthalaldehyde,
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Hongxing Zhang et al./ Journal of Photochemistry and Photobiology A: Chemistry
DMF, Al2O3; (IV) pyrrole, acetic acid, methanol, CH2Cl2 ; (V) Pd2(dba)3, DPPF, toluene; (VI) DMF, POCl3; (VII)
CH2Cl2, TFA; (VIII) TFA.
3 Results and discussion
3.1 Syntheses of the porphyrin dyes
Generally speaking, an obvious drawback of DSSCs is the aggregation of dyes on TiO2
surface. So 4, 4’-dibutyltriphenyl amine was introduced into the molecule to reduce the
aggregation. The classical D-π-A system was employed in the structure design of TP and
BUTP, in which triphenylamine and cyanoacrylic acid worked as electron donor and acceptor
respectively. The detailed synthetic routes are showed in Scheme 2. The final products were
characterized by 1H-NMR, HRMS and IR spectra.
Table 1 Optical and electrochemical properties of TP and BUTP
Absorption
λma xa /nm
(ε/104 M- 1 cm- 1
Absorption
λma xb/nm
Emission
λma xa /nm
EH O MO c /V
(vs. NHE)
ELU MO e /V
(vs. NHE)
Dye
E0 - 0 d /V
)
406(13.2), 505(1.1),
548(1.1), 638(0.4)
406(11.4), 505(1.0),
548(0.9), 638(0.3)
TP
442, 514, 559, 645
443, 514, 559, 645
646, 706
646, 706
0.99
0.98
2.05
2.06
-1.06
-1.08
BUTP
a Absorption and emission spectra were measured in THF solution. b Absorption spectra were obtained on TiO2 films. c
d
EHOMO level of the dyes (vs. NHE) were measured on TiO2 film E0-0 of the dyes were estimated from the intersection of
the absorption and emission spectra. e ELUMO level was estimated by add E0-0 to EHOMO
3.2 UV-visible absorption and fluorescence spectra
Fig. 1 is the absorption and emission spectrum of TP and BUTP. Fig. 2 is the
absorption spectra of TP and BUTP adsorbed on TiO2 film. Fig. 3 is emission spectra
of TP and BUTP in different concentrations.
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Fig. 1 Absorption and emission spectra of TP and BUTP in THF
Fig. 2 Absorption spectra of TP and BUTP on TiO2 films
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Fig. 3 Fluorescence spectra of TP and BUTP in THF at different concentrations
As seen in Fig.1, TP and BUTP both have typical absorption spectra of porphyrin.
The maximum absorptions of TP and BUTP are at similar wavelengths and show
slight differences in intensity. The absorption spectra of TP and BUTP display four
absorption maxima at 406nm, 505nm, 548nm and 638nm, owing to the π-π*
transitions of the conjugated system. Compared to solution, absorption maxima of TP
and BUTP adsorbed on TiO2 both showed obvious red-shift of 736 nm and a
broadened absorption spectra. This may be related to the interaction of dyes and
[20-21]
TiO2.
It is noteworthy that absorption spectrum of TP and BUTP anchored on
TiO2 differs from each other; BUTP has stronger absorption than TP. This difference
could be partly attributed to the incorporation of butyl, which decreased the
aggregation of dyes on the TiO2 surface.
According to Fig. 1, emission maxima of TP and BUTP are next to each other,
similar to the relative position of their absorption maxima. As seen in Fig.3, emission
intensity of TP decreases with its concentration increasing in the range of 5×10-6
mol/L-2×10-5 mol/L. It can be explained by the ACQ (aggregation-caused quenching)
caused by rich dyes. However, emission intensity of BUTP behaves differently. It
increases in proportion to the concentration during 5×10-6 mol/L-1×10-5 mol/L, and
decreases when concentration grows from 1×10-5 mol/L to 2×10-5mol/L .The emission
behaviours of BUTP indicate that AEE (aggregation-enhance emission) dominates
when the dye solution was dilute and ACQ dominates in the dye solution of higher
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11
concentration. This phenomenon also reflects that the incorporation of butyl reduces
the aggregation of dyes.
3.3 Electrochemical properties and DFT calculations
Fig. 4 Cyclic voltammetry plots of the two porphyrin dyes on TiO2 films
Cyclic voltammetry was used to study the electrochemical property of dyes. Fig. 4 is
the cyclic voltammogram of TP and BUTP anchored on TiO2 film. Three-electrode
system was applied: TiO2 film, platinum electrode and saturated calomel electrode
worked as the working electrode, counter-electrode and reference electrode
respectively. Tetrabutylammonium hexafluorophosphate was used as electrolyte. As
shown in Table 1, HOMO levels of TP and BUTP are 0.99 V and 0.98 V (vs NHE)
-
respectively, higher than the redox potential of I-/ I3 (0.4V vs NHE), which makes the
regeneration of dyes possible. LUMO levels of TP and BUTP are -1.06V and -1.08V,
[22]
lower than the conduction band edge energy level of the TiO2.
LUMO level ensures the electron injection from excited dyes to TiO2.
The appropriate
The molecular geometries and electron distributions of TP and BUTP were
obtained by using density function theory (DFT) calculations with Gaussian 09
program.[23] The calculations were performed with the B3LYP exchange correlation
functional under 6-31G (d) basis and the results are displayed in Fig. 5. As shown in
Fig. 5, the HOMOs of both sensitizers mainly populated over triphenylamine block,
while the LUMOs delocalized through the porphyrin and cyanoacrylic acid fragments.
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This suggests that the HOMO-LUMO excitation can disperse the electron density
distribution between triphenylamine and cyanoacrylic acid, favorable for the
photoinduced electron injection from excited dyes to TiO2.
Fig. 5 Frontier molecular orbital profiles of TP and TTP calculated by DFT
3.4 Photovoltaic performance of DSSCs
DSSCs were prepared with dye-coated TiO2 films which was used as working
electrode, platinized FTO glass as the counter electrode and 0.1M LiI, 0.05M I2, 0.5M
4-tert-butylpyridine (TBP), 0.6M 1,2-dimethyl-3-propylimidazolium iodide (DMPII)
in acetonitrile solution as the redox electrolyte. In all experiments, TiO2 films with
certain thickness were immersed in dye solution (3×10-4 mol/L) in ethanol and
dichloromethane mixed solution (volume ratio=3:7) at room temperature for 12 h.
The final cells were tested under AM 1.5G conditions. The obtained photovoltaic
parameters were listed in Table 2.
Table 2 Photovoltaic performance of DSSCs based on the porphyrin dyesa
Dye
TP
Jsc(mA/cm2)
9.60
Voc(V)
0.67
FF
η(%)
4.39
5.33
0.69
0.65
BUTP
12.21
0.68
a Under AM 1.5 illumination (power 100 mW cm-2) with an active area of 0.25 cm2.
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Hongxing Zhang et al./ Journal of Photochemistry and Photobiology A: Chemistry
13
Fig. 6 The IPCE spectra of DSSCs based on TP and BUTP
Fig. 7 The J-V curves of the DSSCs based on TP and BUTP
The incident photo-to-current conversion efficiency (IPCE) plots for DSSCs based
on the two porphyrin dyes is shown in Fig.6. IPCE spectrum of TP ranges from
300-725nm, and reaches its maximum value of 38% at the wavelength of 440 nm.
BUTP has the similar IPCE spectrum range of 300-726nm and obtains its largest
IPCE value of 48% at 448nm. Although the maximum IPCE of TP and BUTP are not
high enough, both of the two dyes have middle IPCE in a broad range of 300-567nm
and relatively lower IPCE from 567nm-659nm. That is to say TP and BUTP can
make use of most of the visible light. By comparison, BUTP has a higher IPCE
spectrum than TP, owing to the incorporation of butyl which reduces the aggregation
of dyes.
Photocurrent density versus voltage (J-V) for DSSCs based on TP and BUTP are
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Hongxing Zhang et al./ Journal of Photochemistry and Photobiology A: Chemistry
shown in Fig. 7. As shown in Table 2, for TP-sensitized cells, the short-circuit
current density (JSC) 9.60 mAcm-2, the open-circuit voltage (VOC) 0.67V, and the
filled factor (FF) 0.69, corresponding to an overall conversion efficiency of 4.39%.
However BUTP-sensitized cells gave a JSC of 12.21 mAcm-2, VOC of 0.68 V, FF of
0.65, corresponding to an overall conversion efficiency of 5.33%. BUTP has a higher
short-circuit current than TP which is consistent with its better power conversion
efficiency. The J-V curve trend of TP and BUTTP are in accordance with their
IPCE-λ spectra.
3.5 Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) analysis was performed to further
explore the influence of alkyl chain in TP and BUTP on Voc in DSSCs. The EIS
analysis also provides information about interfacial charge transfer process. The
Nyquist plots of the DSSCs based on TP and BUTP under a forward bias of -0.70V
are shown in Fig. 8 (a). In the Nyquist plots, the high-frequency semicircle on the left
-
indicates the Pt/redox (I/I3 ) interface charge transfer and the middle-frequency
semicircle on the right corresponds to the electron transport process at the
TiO2/dye/electrolyte interface. According to Fig. 8 (a), BUTP has a larger resistance
in the charge transfer and is reflected in the improvement in Voc. The Bode phase
plots were shown in Fig. 8 (b). The peaks at low frequencies are related to the charge
recombination, and the peaks at high frequencies are due to the charge transfer at the
counter electrode. The reciprocal of frequency is regarded as the electron lifetime.
[24-25] The frequency of the peak maxima is in the order of BUTP < TP. Therefore, the
electron lifetime is in the order of TP < BUTP. BUTP has a longer electron lifetime
than TP, resulting in BUTP having a higher Voc due to reduced charge
recombination rate.
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Hongxing Zhang et al./ Journal of Photochemistry and Photobiology A: Chemistry
15
Fig. 8 Electrochemical impedance spectra of DSSCs based on TP, BUTP measured at -0.70V in
the dark. (a) Nyquist plots; (b) Bode phase plots.
4 Conclusion
In conclusion, two novel porphyrin dyes (TP and BUTP) with D-π-A structure
were synthesized. In the dye molecules, triphenylamine is applied as
electron-donating group and cyanoacrylic acid as electron-withdrawing group. The
introduction of triphenylamine breaks the planarity of the whole molecule, hindering
the stacking of dyes. Additionally, alkyl improved the electron-donating ability of
triphenylamine as well as the solubility of the whole molecule, and resists aggregation
of dyes. Combined with the strong electron-withdrawing effect of cyanoacrylic acid,
BUTP displayed satisfactory photovoltaic properties: JSC 12.21 mAcm-2, Voc 0.68 V,
and FF 0.65, power conversion efficiency (η) 5.33 % under AM 1.5G simulated light.
Moreover, structures of TP and BUTP are simple and the cost of preparation is low,
offering a promising prospect of its application in DSSCs.
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Hongxing Zhang et al./ Journal of Photochemistry and Photobiology A: Chemistry
Acknowledgements
The authors are grateful to financial support from the National Natural Science
Foundation of China (No.20872035, 21076078), and the East China University of
Science and Technology. The authors would also like to thank Dr. Emily Anglin
(Department of Chemistry and Biochemistry, University of California at San Diego)
for her help in the revising of this paper.
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Hongxing Zhang et al./ Journal of Photochemistry and Photobiology A: Chemistry
Graphical Abstract
Synthesis, photo-electrochemical properties and dye-sensitized solar
cell performance of D-π-A structured porphyrins
Hongxing Zhang,a Xiuwen Zheng,b Haiying Li,b Bin Jin,a Chengyun Wang,*a Yongjia Shen,a
Jianli Hua a
Two new D-π-A dyes based on triphenylamine and porphyrin were synthesized, power conversion
efficiency of DSSCs sensitized by them reached 4.39% and 5.33% respectively.
Page 18 of 19

Highlights
Two new D-π-A porphyrins TP and BUTP were synthesized for the first time.
Power conversion efficiency of DSSCs sensitized by TP reached 4.39%.
Power conversion efficiency of DSSCs sensitized by BUTP reached 5.33%.
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