Novel D-π-A system based on zinc porphyrin dyes for dye-sensitized solar cells: Synthesis, electrochemical, and photovoltaic properties

Article abstract of DOI:10.1016/j.dyepig.2011.12.006

We have designed and synthesized novel zinc porphyrin dyes which have a D-π-A system based on porphyrin derivatives containing a triphenylamine (TPA) electron-donating group and a phenyl carboxyl anchoring group substituted at the meso position of the por

Full text of DOI:10.1016/j.dyepig.2011.12.006

Dyes and Pigments 94 (2012) 143e149
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel D-
p
-A system based on zinc porphyrin dyes for dye-sensitized solar cells:
Synthesis, electrochemical, and photovoltaic properties
,
Kang Deuk Seo a, Myung Jun Lee a, Hae Min Song a, Hong Seok Kang b, Hwan Kyu Kim a
*
a Department of Advanced Materials Chemistry & Centre for Advanced Photovoltaic Materials (ITRC), Korea University, ChungNam 339-700, Republic of Korea
b Department of Nano & Advanced Materials, College of Engineering, Jeonju University, Hyoja-dong, Chonju, Chonbuk 560-759, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We have designed and synthesized novel zinc porphyrin dyes which have a D-p-A system based on
Received 17 November 2011
Received in revised form
13 December 2011
Accepted 15 December 2011
Available online 27 December 2011
porphyrin derivatives containing a triphenylamine (TPA) electron-donating group and a phenyl carboxyl
anchoring group substituted at the meso position of the porphyrin ring, yielding the push-pull
porphyrins as the most efficient green dye for dye-sensitized solar cell (DSSC) applications. The
synthesis and characterization of a novel D-p-A system based on zinc-porphyrin derivatives have been
investigated through their photophysical and photoelectrochemical studies. A large red-shift of the
absorption maxima due to introduction of the TPA moiety at the meso position of the porphyrin ring was
Keywords:
Porphyrin
expected in the D-p-A porphyrins, but the absorption maxima of HKK-Por dyes were a little red-shifted
in contrast to Zn[5,-10,15-triphenyl-20-(4-carboxylphenyl)-porphyrin], due to the tilted structure
between TPA and the porphyrin unit. Under the photovoltaic performance measurement, the maximum
incident photon-to-current conversion efficiency (IPCE) value of the DSSC based on HKK-Por 5 was
slightly higher than the efficiencies of the DSSCs based on other HKK-Por dyes due to the introduction of
the alkoxy group into the TPA moiety at the meso position of the porphyrin ring. A maximum photon-to-
electron conversion efficiency of 3.36% was achieved with the DSSC based on HKK-Por 5 dye
(JSC ¼ 9.04 mA/cm2, VOC ¼ 0.57 V, FF ¼ 0.66) under AM1.5 irradiation (100 m Wcmꢀ2).
Ó 2011 Elsevier Ltd. All rights reserved.
Triphenylamine electron-donating group
D-p-A system
Dye-sensitized solar cell
Charge recombination
Short-circuit photocurrent density
1. Introduction
by the natural chlorophylls, their rigid molecular structures with
large absorption coefficients in the visible region, and their many
The ever increasing consumption of fossil fuels, causing global
warming and environmental pollution, has led to a greater focus on
renewable energy sources and sustainable development. Dye-
sensitized solar cells (DSSCs) have attracted considerable atten-
tion in recent years [1]. DSSCs based on mesoporous nanocrystal-
line TiO2 films have attracted significant attention due to their low
cost and high sunlight-to-electric power conversion efficiencies of
10e12% [2e10]. In recent years, interest in metal-free organic dyes
as an alternative to noble metal complexes has increased due to
their many advantages, such as diversity of molecular structures,
high molar extinction coefficient, and simple synthesis as well as
low cost and environmental benefits [11,12].
reaction sites, that is, four meso and eight beta positions, available
for functionalization. Fine tuning of their optical, physical, and
electrochemical properties thus becomes feasible [21e28]. In
addition, a high
h value of porphyrin dye of up to 10.2% in full
sunlight has been achieved by Eric Diau and co-workers [29]. To
further design and develop the more efficient porphyrin dyes for
DSSCs, the predominant light-harvesting abilities of dyes in visible
and near-IR light are important to get a larger photocurrent
response.
In our previous report [30], porphyrin with a carbazole-
containing triphenylamine (TPA) moiety and a mesityl group
introduced at the 5, 20 position was designed and synthesized.
Here, we have designed and synthesized novel zinc porphyrin dyes
Porphyrins are one of the most widely studied sensitizers for
DSSCs because of their strong Soret (400e450 nm) and moderate Q
bands (550e600 nm) [13e20]. The porphyrin-based dyes have
several intrinsic advantages, such as the good photostability offered
that have a D-p-A system based on porphyrin derivatives con-
taining a TPA electron-donating group and a meso-substituted
phenyl carboxyl anchoring group attached at the meso position of
the porphyrin ring. To increase the electron-donating ability of the
D-p-A system based on porphyrin derivatives, to decrease the dye
aggregation, a hexyloxy substituent was introduced into the TPA
moiety. Furthermore to decrease the rotation of the phenyl unit,
* Corresponding author. Tel.: þ82 41 860 1493; fax: þ82 41 867 5396.
E-mail address: hkk777@korea.ac.kr (H.K. Kim).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.12.006

144
K.D. Seo et al. / Dyes and Pigments 94 (2012) 143e149
a mesityl group was introduced at the 5, 20 position of the HKK-Por
4 and HKK-Por 5, yielding the push-pull porphyrins as the most
efficient green dye for DSSC applications.
the solution was stirred under argon at room temperature for an
additional hour and then quenched with triethylamine (0.4 ml).
The mixture was diluted with toluene (150 ml), washed with brine,
and then dried with MgSO4. The solvent was removed under
reduced pressure and then the unreacted pyrrole was removed by
vacuum distillation at room temperature. The residue was dis-
solved in CH2Cl2 and filtered through a short pad of silica using
CH2Cl2 as the eluent. Evaporation of the solvent under reduced
pressure resulted in a brown solid. This solid was washed with
cyclohexane and then with hexane, giving a pale yellow solid,
which was collected by filtration (yield: 70%). 1H NMR (CDCl3)
2. Experimental
2.1. Materials
Pyrrole, benzaldehyde, trifluoroacetic acid, 2,4,6-trimethyl-
benzaldehyde, TPA, copper iodide, 18-crown-6, 4-iodophenol, 3-
bromomethyl-heptane, aniline, potassium carbonate, triethyl-
amine, zinc acetate dihydrate, 1,2-dichlorobenzene, acetic acid,
methyl 4-formylbenzoate, sodium hydride, and potassium hydride
were used as received from SigmaeAldrich without further puri-
fication. Chloroform, toluene, dichloromethane, n-hexane, ethanol,
tetrahydrofuran, methanol, N,N-dimethylformamide, hydrochloric
acid, ether, and anhydrous potassium carbonate were purchased
from Samchun Co. 1,2-Dichloroethane was purchased from TCI Co.
d
(TMS, ppm): 7.95 (s, 2H), 6.87 (s, 2H), 6.67 (s, 2H), 6.18 (m, 2H),
6.02 (s, 2H), 5.93 (s, 1H), 2.27 (s, 3H), 2.06 (s, 6H).
2.4.3. 4-Formyl triphenylamine (3)
POCl3 (4.47 ml, 48.67 mmol) was added dropwise to 8.5 ml DMF
with stirring under dry conditions, and the temperature was kept
below 10 ꢁC. After adding a mixture of TPA (5.97 g, 24.34 mmol) and
2 ml DMF to the reaction vessel at this temperature, the reaction
mixture was maintained at 35 ꢁC for 24 h. Subsequently, the reac-
tion solution was poured into ice water and the precipitated
mixture was filtered and washed with water. The crude product
was purified by column chromatography (silica gel, dichloro-
methane) to afford a yellow solid (yield: 83%). 1H NMR (CDCl3)
2.2. Measurement
The 1H NMR spectra were recorded at room temperature with
Varian Oxford 300 spectrometers and chemical shifts were re-
ported in parts per million (ppm) with tetramethylsilane as an
internal standard. FT-IR spectra were measured on KBr pellets with
a Perkin Elmer Spectrometer. UVevisible absorption spectra were
obtained in THF on a Shimadzu UV-2401PC spectrophotometer.
Cyclic voltammetry was carried out with a Versa STAT3 (AMETEK).
A three-electrode system was used, consisted of a reference elec-
trode (Ag/AgCl), a working electrode, and a platinum wire elec-
trode. The redox potential of dyes on TiO2 was measured in CH3CN
d
(TMS, ppm): 9.81 (s, 1H), 7.68 (d, 2H), 7.36e7.31 (m, 4H), 7.19e7.14
(m, 6H), 7.02 (d, 2H).
2.4.4. 1-(2-Ethylhexyloxy)-4-iodobenzene (4)
4-Iodophenol (10 g, 10 mmol) was dissolved in 200 ml of DMF,
and then K2CO3 (9.42 g, 68.18 mmol) was added. The mixture was
stirred in a N2 atmosphere at 60 ꢁC. After 30 min, 2-ethyl-
hexylbromide (9.70 ml, 54.54 mmol) was dissolved in 50 ml of DMF
and added to the flask dropwise. The reaction was continued for
48 h. K2CO3 was filtered out, the solvent was evaporated, and then
the crude product was washed with water and dried with MgSO4.
The pure white liquid product was obtained by silica column
chromatography (eluent: hexane) (yield: 77%). 1H NMR (CDCl3)
with 0.1 M TBAPF6 with a scan rate of 50 mV sꢀ1
.
2.3. Density functional theory (DFT)/time-dependent DFT (TD-DFT)
calculations
Structural optimization of HKK-Por dyes was done with a PBE
exchange-correlation function using the Vienna ab initio simula-
tion package (VASP) [31,32]. We used 400 eV as the cut-off energy,
and the conjugate gradient method was employed to optimize
the geometry until the force exerted on an atom was less than
0.03 eV/Å. In order to calculate the absorption spectra, we per-
formed the TD-DFT calculations in the gas phase using the 6-31G(d)
basis set in GAUSSIAN03 program [33]. We focused on transitions
occurring in the range of 350e800 nm, specifically those whose
oscillation strengths were greater than 0.1.
d
(TMS, ppm): 7.54 (d, 2H), 6.69 (d, 2H), 3.81 (d, 2H), 1.17e1.75 (m,
1H), 1.36e1.55 (m, 8H), 0.95 (t, 6H).
2.4.5. Bis-[4-(2-2ethylhexyloxy)-phenyl]-phenylamine (5)
Aniline (1.78 g, 19.5 mmol), 1-(2-ethylhexyloxy)-4-iodobenzene
(14.4 g, 43.3 mmol) (5 g, 15.05 mmol), copper(I) iodide (4.83 g,
76.0 mmol), 1,10-phenanthroline (0.05 g, 0.3 mmol), and potassium
hydroxide flakes (2.96 g, 52.7 mmol) were dissolved in 1,2-
dichlorobenzene (50 ml). The reaction mixture was rapidly heated
over a period of 30 min to 125 ꢁC and maintained at this temper-
ature for 24 h. After cooling, the mixture was diluted with
dichloromethane and washed with 1 N HCl and brine before being
dried over MgSO4. The crude product was purified by column
chromatography to give the product (yield: 80%) as colourless oil.
2.4. Synthesis
2.4.1. 5-Phenyl dipyrromethane (1)
A solution of benzaldehyde (1 mmol) in 3 ml pyrrole (43 mmol)
with a catalytic amount of trifluoroacetic acid (0.1 mmol) was
vigorously stirred at room temperature for 15 min. The crude
product is obtained by dilution with CH2C12, washing with dilute
NaOH, and concentration of the organic layer. The excess pyrrole is
recovered by vacuum distillation at room temperature. Column
chromatography on silica of the resulting brownish solid affords
the white 5-phenyl dipyrromethane in 29% yield. 1H NMR (CDCl3)
1H NMR (CDCl3)
d (TMS, ppm): 7.46 (d, 2H), 7.17 (m, 1H), 7.03 (d,
4H), 6.90 (d, 2H), 6.85 (d, 4H), 3.81 (d, 4H), 1.17e1.75 (m, 2H),
1.36e1.55 (m, 16H), 0.95 (t, 12H).
2.4.6. 4-{Bis-[4-(2-ethylhexyloxy)-phenyl]-amino}-benzaldehyde (6)
POCl3 (0.73 ml, 7.8 mmol) was slowly added to a stirred solution
of
bis-[4-(2-ethylhexyloxy)-phenyl]-phenylamine
(3.78
g,
d
(TMS, ppm): 7.21e7.34 (m, 5H), 6.70 (q, 2H), 6.17 (q, 2H), 5.93
7.67 mmol) and dimethylformamide (0.6 ml, 7.8 mmol) in 50 ml of
1,2-dichloroethane. Subsequently the solution was refluxed for 2 h,
cooled, and poured into water. The resulting mixture was extracted
with dichloromethane. After drying with MgSO4, the solvent
together with the 1,2-dichloroethane and DMF was removed by
evaporation under reduced pressure. The crude product was puri-
fied by silica gel column chromatography to give a product (yield:
(s, 2H), 5.48 (s, 1H).
2.4.2. 5-Mesityl dipyrromethane (2)
A solution of 2,4,6-trimethylbenzaldehyde (2.65 ml, 18 mmol)
and pyrrole (50 ml, 720 mmol) was degassed by argon bubbling for
15 min. Trifluoroacetic acid (0.14 ml, 1.8 mmol) was then added and

K.D. Seo et al. / Dyes and Pigments 94 (2012) 143e149
145
78%) as colourless oil. 1H NMR (CDCl3)
7.57 (d, 2H), 7.09 (d, 4H), 6.85 (d, 2H), 6.79 (d, 4H), 3.81 (d, 4H),
1.71e1.75 (m, 2H), 1.36e1.55 (m, 16H), 0.95 (t, 12H).
d
(TMS, ppm): 9.92 (s, 1H),
then purged with N2 and slowly heated to 80 ꢁC. It was then
brought to reflux under N2 until there was no free base left as
revealed by TLC (24 h). The mixture was then cooled to room
temperature and the solvent was removed by vacuum distillation.
The crude product left behind was then dried completely and
purified with a column to afford the compound Zn-por (yield: 86%).
2.4.7. 5,15-Bisphenyl-15-triphenylamino(4-methoxycarbonylphenyl)
porphyrin (7)
The mixture of methyl-4-formylbenzoate (0.67 g, 4.05 mmol), 4-
formyltriphenyl amine (1.1 g, 4.05 mmol), and 5-phenyldipyrro-
methane (1.8 g, 8.10 mmol) was condensed in dichloromethane
(2.41 L) with TFA (0.4 ml, 5.10 mmol) at room temperature for 1 h.
Then DDQ (2.35 g, 10.4 mmol) was added, after which the mixture
was stirred for 1 h at room temperature. Next TEA (3 ml) was
added, and the mixture was stirred for 1 h at room temperature.
The solvent was removed under reduced pressure. The residue was
then dissolved in CHCl3 and passed through a short silica gel
column. This mixture was purified with a second column to afford
1H NMR (CDCl3)
d (TMS, ppm): 9.01 (d, 2H), 8.85 (d, 2H), 8.88 (d,
2H), 8.78 (d, 2H), 8.44 (d, 2H), 8.31 (d, 2H), 8.22 (d, 4H), 8.07 (d, 2H),
7.77 (d, 6H), 7.40e7.46 (m, 10H), 7.14 (d, 2H), 4.11 (s, 3H); FT-IR (KBr
pellet, cmꢀ1): 1720 (ester, C]O).
2.4.9. 5,15-Bisphenyl-15-triphenylamino(4-carboxylphenyl)porphyri
natozinc (HKK-Por 3)
5,15-Bisphenyl-15-triphenylamino(4-methoxycarbonylphenyl)
porphyrinatozinc (0.1 g, 0.12 mmol) and KOH (0.08 g, 0.14 mmol)
were dissolved in THFeEtOHeH2O (1:1:0.1, 50 ml), and the solution
was refluxed for 12 h. The mixture was then cooled to room
temperature, acidified with concentrated HCl, and extracted with
CH2Cl2. The organic phase was washed with sodium bicarbonate
solution and water and dried with MgSO4. The crude product left
behind was then dried completely and purified with a column to
afford the compound Zn-por-COOH (yield: 68%). 1H NMR (CDCl3)
the compound FB-por (yield: 3%). 1H NMR (CDCl3)
d (TMS, ppm):
9.01 (d, 2H), 8.88 (d, 2H), 8.85 (d, 2H), 8.78 (d, 2H), 8.44 (d, 2H), 8.31
(d, 2H), 8.22 (d, 4H), 8.07 (d, 2H), 7.77 (d, 6H), 7.40e7.46 (m, 10H),
7.14 (d, 2H), 4.11 (s, 3H), ꢀ2.74 (s, 2H); FT-IR (KBr pellet, cmꢀ1): 1720
(ester, C]O), 3455 (amine, eNH).
2.4.8. 5,15-Bisphenyl-15-triphenylamino(4-methoxycarbonylphenyl)
porphyrinatozinc (8)
5,15-Bisphenyl-15-triphenylamino(4-methoxycarbonylphenyl)
porphyrin (0.15 g, 0.18 mmol) and zinc acetate dehydrate (0.2 g,
0.89 mmol) were suspended in dry THF (20 ml). The mixture was
d (TMS, ppm): 9.01 (d, 2H), 8.85 (d, 2H), 8.88 (d, 2H), 8.78 (d, 2H),
8.44 (d, 2H), 8.31 (d, 2H), 8.22 (d, 4H), 8.07 (d, 2H), 7.77 (d, 6H),
7.40e7.46 (m, 10H), 7.14 (d, 2H); lmax,abs in THF: 424.5, 556.5, 598.5.
lmax,em in THF: 608.9, 659.1; FT-IR (KBr pellet, cmꢀ1): 2400e3400
(carboxylic acid, eCOOH); FAB-MS (m/z): calcd. 887.2, found 888.0.
Scheme 1. Chemical structures and synthesis of HKK-Por dyes.

146
K.D. Seo et al. / Dyes and Pigments 94 (2012) 143e149
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.1
HKK-Por3
HKK-Por4
HKK-Por5
HKK-Por3
HKK-Por4
HKK-Por5
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
400
500
600
700
300
400
500
600
700
Wavelength(nm)
Wavelength(nm)
Fig. 1. Absorption spectra of HKK-Por dyes in THF solvent (1.0 ꢂ 10ꢀ4 M).
Fig. 2. Absorption spectra of HKK-Por dyes on TiO2 film.
It was then brought to reflux under N2 until there was no free base
left as revealed by TLC (24 h). The mixture was then cooled to room
temperature and the solvent was removed by vacuum distillation.
The crude product left behind was then dried completely and
purified with a column to afford the compound Zn-por (yield: 79%).
2.4.10. 5,15-Bis(2,4,6-trimethylphenyl)-15-triphenylamino(4-methox
ycarbonylphenyl) porphyrin (9)
Methyl-4-formylbenzoate (0.56 g), 4-formyl triphenylamine
(0.9 g), and 5-mesityl dipyrromethane (1.8 g) were condensed in
dichloromethane (2.4 L) with TFA (0.8 ml) at room temperature
for 1 h. DDQ (1.98 g) was added, and the mixture was stirred for
1 h at room temperature. Then TEA (3 ml) was added, after which
the mixture was stirred for 1 h at room temperature. The solvent
was removed under reduced pressure. The residue was then dis-
solved in CHCl3 and passed through a short silica gel column to
remove the non-porphyrinic components from the crude reaction
mixture. This mixture was purified with a second column to afford
1H NMR (CDCl3)
d (TMS, ppm): 8.95 (d, 2H), 8.69e8.73 (m, 6H), 8.43
(d, 2H), 8.30 (d, 2H), 8.07 (d, 2H), 7.39e7.45 (m, 10H), 7.13 (m, 2H),
4.10 (s, 3H), 2.64 (s, 6H), 1.84 (s, 12H); FT-IR (KBr pellet, cmꢀ1): 1720
(ester, C]O).
2.4.13. 5,15-Bis(2,4,6-trimethylphenyl)-15-{4-{Bis-[4-(2-ethylhexylo
xy)-phenyl]-amino}}-(4-methoxycarbonylpheny)porphyrinatozinc
(12)
the compound FB-por (yield: 3%). 1H NMR (CDCl3)
d (TMS, ppm):
Compound 12 was synthesized by the same method as
compound 10.
8.95 (d, 2H), 8.69e8.73 (m, 6H), 8.43 (d, 2H), 8.30 (d, 2H), 8.07 (d,
2H), 7.39e7.45 (m, 10H), 7.13 (m, 2H), 4.10 (s, 3H), 2.64 (s, 6H), 1.84
(s, 12H), ꢀ2.59 (s, 2H); FT-IR (KBr pellet, cmꢀ1): 1720 (ester, C]O),
3455 (amine, eNH).
1H NMR (CDCl3)
d (TMS, ppm): 8.96 (d, 2H), 8.67e8.72 (m, 6H),
8.42 (d, 2H), 8.30 (d, 2H), 7.98 (d, 2H), 7.28e7.35 (m, 6H), 6.95 (d,
4H), 4.10 (s, 3H), 3.88 (d, 4H), 1.77e1.81 (m, 2H), 1.20e1.54 (m, 16H),
0.86e0.98 (m, 12H); FT-IR (KBr pellet, cmꢀ1): 1750 (ester, C]O).
2.4.11. 5,15-Bis(2,4,6-trimethylphenyl)-15-{4-{Bis-[4-(2-ethylhexy
loxy)-phenyl]-amino}}-(4-methoxycarbonylphenyl) porphyrin (11)
Compound 11 was synthesized by the same method as
compound 9.
2.4.14. 5,15-Bis(2,4,6-trimethylphenyl)-15-triphenylamino(4-carboxy
lphenyl) porphyrinatozinc (HKK-Por 4)
1H NMR (CDCl3)
d (TMS, ppm): 8.96 (d, 2H), 8.67e8.72 (m, 6H),
5,15-Bis(2,4,6-trimethylphenyl)-15-triphenylamino(4-
methoxycarbonylphenyl) porphyrinatozinc (0.18 g, 0.18 mmol) and
KOH (0.08 g, 0.14 mmol) were dissolved in THF-EtOH-H2O (1:1:0.1,
50 ml), and the solution was refluxed for 12 h. The mixture was
then cooled to room temperature, acidified with concentrated HCl,
and extracted with CH2Cl2. The organic phase was washed with
sodium bicarbonate solution and water and dried with Na2SO4. The
crude product left behind was then dried completely and purified
with a column to afford the compound Zn-por-COOH (yield: 70%).
8.42 (d, 2H), 8.30 (d, 2H), 7.98 (d, 2H), 7.28e7.35 (m, 6H), 6.95 (d,
4H), 4.10 (s, 3H), 3.88 (d, 4H), 1.77e1.81 (m, 2H), 1.20e1.54 (m, 16H),
0.86e0.98 (m, 12H), ꢀ2.57 (s, 2H); FT-IR (KBr pellet, cmꢀ1): 1750
(ester, C]O), 3450 (amine, eNH).
2.4.12. 5,15-Bis(2,4,6-trimethylphenyl)-15-triphenylamino(4-methox
ycarbonylphenyl) porphyrinatozinc (10)
5,15-Bis(2,4,6-trimethylphenyl)-15-triphenylamino(4-methox-
ycarbonylphenyl) porphyrin (0.5 g, 0.54 mmol) and zinc acetate
dehydrate (0.59 g, 2.71 mmol) were suspended in dry THF (20 ml).
The mixture was then purged with N2 and slowly heated to 80 ꢁC.
1H NMR (CDCl3)
d (TMS, ppm): 8.95(d, 2H), 8.69e8.73 (m, 6H), 8.43
(d, 2H), 8.30 (d, 2H), 8.07 (d, 2H), 7.39e7.45 (m, 10H), 7.13 (m, 2H),
2.64 (s, 6H),1.84 (s,12H); lmax,abs in THF: 426.5, 557.5, 599.5. lmax,em
Table 1
Absorption/emission and electrochemical properties of HKK-Por dyes.
Dye
Absorption (lmax/nm)a (ε/Mꢀ1 cmꢀ1
)
Emission (lmax/nm)
Eoxb/V (vs NHE)
Eoeoc/V
ELUMOd/V (vs NHE)
HKK-Por 3
HKK-Por 4
HKK-Por 5
424.5(340000), 554.5(12300), 598.5(8418)
426.5(335000), 557.5(20493), 599.5(8524)
425.5(365000), 558.0(18526), 601.0(8968)
608.9, 659.1
609.1, 660.0
612.0, 660.1
1.03
1.05
1.37
2.11
2.10
2.02
ꢀ1.08
ꢀ1.05
ꢀ0.65
a
Absorption was measured in THF solution (1.0 ꢂ 10ꢀ6 M) at room temperature.
b
c
The oxidation potential of the dye on TiO2 was measured in acetonitrile with 0.1 M TBAPF6 with a scan rate of 50 mV sꢀ1 (vs NHE).
E0e0 was determined from the intersection of the absorption and emission spectra in THF.
d
LUMO was calculated by EoxeE0e0
.

K.D. Seo et al. / Dyes and Pigments 94 (2012) 143e149
147
0.05 M iodine, 0.1 M LiI, and 0.5 M tert-butylpyridine in acetonitrile,
was then introduced into the cell. It was introduced into the inter-
electrode space from the counter electrode side through predrilled
holes. The drilled holes were sealed with a microscope cover slide
and Surlyn to avoid leakage of the electrolyte solution.
2.6. Photoelectrochemical measurements of DSSCs
Photoelectrochemical data were measured using a 1000 W
xenon light source (Oriel, 91193) that was focused to give1000 W/
m2, the equivalent of one sun at Air Mass (AM) 1.5, at the surface of
the test cell. The light intensity was adjusted with a Si solar cell that
was double-checked with an NREL-calibrated Si solar cell (PV
Measurement Inc.). The applied potential and measured cell
current were measured using a Keithley model 2400 digital source
meter. The currentevoltage characteristics of the cell under these
conditions were determined by biasing the cell externally and
measuring the generated photocurrent. This process was fully
automated using Wavemetrics software.
Fig. 3. Optimized structure of HKK-Por 3 dye (f1:wꢀ37.7ꢁf2:wꢀ61.1ꢁ).
in THF: 609.1, 660.0; FT-IR (KBr pellet, cmꢀ1): 2400e3400
(carboxylic acid, eCOOH); FAB-MS (m/z): calcd. 973.3, found 973.0.
3. Results and discussion
2.4.15. 5,15-Bis(2,4,6-trimethylphenyl)-15-{4-{Bis-[4-(2-ethylhexylo
xy)-phenyl]-amino}}-(4-carboxylphenyl) porphyrinatozinc
(HKK-Por 5)
The novel zinc porphyrins were synthesized according to
Scheme 1. Compounds 1e6 were prepared by adapting published
procedures [30,34]. Compound 7 was synthesized according to the
literature [35]. Compound 8 was synthesized by an insertion
reaction of Zn(II) ions and then hydrolysed (HKK-Por 3). HKK-Por 4
and HKK-Por 5 dyes were synthesized by the same method.
Fig. 1 shows the absorption and emission spectra of HKK-Por
dyes measured in THF solution; their photophysical properties are
summarized in Table 1. The absorption spectrum of HKK-Por 3
displays three absorption maxima at 424.5 nm, 554.5 nm, and
HKK-Por 5 was synthesized by the same method as HKK-Por 4.
1H NMR (CDCl3)
d (TMS, ppm): 8.96 (d, 2H), 8.67e8.72 (m, 6H),
8.42 (d, 2H), 8.30 (d, 2H), 7.98 (d, 2H), 7.28e7.35 (m, 6H), 6.95 (d, 4H),
3.88 (d, 4H), 1.77e1.81 (m, 2H), 1.20e1.54 (m, 16H), 0.86e0.98 (m,
12H); lmax,abs inTHF: 425.5, 558.0, 601.0. lmax,em inTHF:612.0, 660.1;
FT-IR (KBr pellet, cmꢀ1): 2400e3500 (carboxylic acid, eCOOH); MS
(MALDI-TOF): 1229.2 (Mþ).
2.5. Fabrication and testing of DSSC
598.5 nm, which are assigned as the p-p* transitions of the
conjugated system. HKK-Por 4 and HKK-Por 5 also have similar
behaviour to HKK-Por 3. In contrast, the absorption spectra of
HKK-Por dyes adsorbed on the TiO2 surface are remarkably
broadened relative to the absorption spectra of the solution state
(Fig. 2). This might be due to dyeedye and/or dyeeTiO2 interactions
[36,37]. A large red-shift of the absorption maxima due to intro-
FTO glass plates (Pilkington) were cleaned in a detergent solu-
tion using an ultrasonic bath for 1 h and rinsed with water and
ethanol. The FTO glass plates were immersed in an aqueous solu-
tion of 40 mM TiCl4 at 70 ꢁC for 30 min and washed with water and
ethanol. The first TiO2 layer of 8 mm thickness was prepared by
screen printing TiO2 paste (Solaronix, 13 nm anatase), and the
second scattering layer containing 400 nm sized anatase particles
was deposited by screen printing. The TiO2 electrodes were
immersed into the dye solution [0.3 mM in ethanol/THF (1:1) with
DCA (40 mM)] and kept at room temperature overnight. Counter
electrodes were prepared by coating with a drop of H2PtCl6 solution
(2 mg of Pt in 1 mL of ethanol) on an FTO plate. The dye-adsorbed
TiO2 electrode and Pt-counter electrode were assembled into
a sealed sandwich-type cell. A drop of the electrolyte, which was
composed of 0.6 M 1,2-dimetyl-3-propyl imidazolium iodide,
duction of the TPA moiety was expected in the D-p-A porphyrins,
but HKK-Por dyes were a little red-shifted in contrast to Zn[5,-
10,15-triphenyl-20-(4-caboxylphenyl)-porphyrin], due to the tilted
structure
(
f1
:
wꢀ37.7ꢁ, f2wꢀ61.1ꢁ) between TPA and the
porphyrin unit, as shown in Fig. 3. In order to find out which
molecular transitions in our dyes are responsible for the electron
transport in our DSSC, we have made a detailed analysis of the TD-
DFT results. Table-S1 summarizes absorption peaks and major
contributions to each of them. Evidently, the most efficient electron
transport in the DSSCs will be contributed by the charge transfer
Fig. 4. Plots of the isodensity surfaces (PBEPBE/6-31G*) of HOMO-12 and LUMO of HKK-Por 5 dye.

148
K.D. Seo et al. / Dyes and Pigments 94 (2012) 143e149
70
HKK-Por3
HKK-Por4
HKK-Por5
60
50
40
30
20
10
0
300
400
500
600
700
Wavelength (nm)
Fig. 6. Typical action spectra of incident photon-to-current conversion efficiencies
(IPCEs) obtained for nanocrystalline TiO2 solar cells sensitized by HKK-Por dyes.
Fig. 5. Overview of the evaluated energy levels of HKK-Por dyes determined by cyclic
voltammetry.
values (ꢀ0.65 to ꢀ1.08 vs NHE) compared to the conduction band
edge energy level of the TiO2 electrode [38].
transition from the donor to the acceptor. In fact, the table shows
that the transition from HOMO-12 to LUMO exclusively corre-
sponds to the charge transfer transition among all transitions. The
transition makes a big contribution to the absorption of HKK-Por 5,
which peaked at 425.5 nm in Fig. 1, as indicated by the large
oscillation strengths of transitions at 400 and 414 nm in Table-S1.
Our analysis indicates that the HOMO-12 of HKK-Por 5 is raised by
0.35 eV with respect to the corresponding MO (¼ HOMO-11) of
HKK-Por 4, while LUMO levels of the two dyes are located almost at
the same energy level. Therefore, the HOMO-11 to LUMO transition
of HKK-Por 4, which corresponds to the UV region, is red-shifted to
400 and 414 nm by the electron-donating group of HKK-Por 5, as
shown in Table 1. The electrochemical properties were investigated
by cyclic voltammetry (CV) to obtain the HOMO and LUMO levels of
the HKK-Por dyes. The cyclic voltammogram curves were obtained
from a three-electrode cell in 0.1 M TBAPF6 in CH3CN at a scan rate
of 50 mV/s, using a dye-coated TiO2 electrode as a working elec-
trode, a Pt wire counter electrode, and an Ag/AgCl (saturated KCl)
reference electrode (þ0.197 V vs NHE) and calibrated with ferro-
cene. All of the measured potentials were converted to the NHE
scale. The LUMOs of the HKK-Por 3, HKK-Por 4, and HKK-Por 5
estimated from the oxidation potential and the energy at the
intersection point of absorption and emission spectra are 1.03 V,
1.05 V, and 1.37 V vs NHE, respectively. In addition, with the
introduction of the bulky stronger donor group, the energy levels of
HOMO and LUMO are largely changed. HKK-Por 5 dye has a HOMO
level of 1.37 V (vs NHE) and a LUMO level of ꢀ0.65 V (vs NHE)
(Fig. 4). HOMO values of HKK-Por dyes (1.03e1.37 V vs NHE) were
more positive than the Iꢀ/I3ꢀ redox couple (0.4 V vs NHE) so effective
dye regeneration could be possible (Fig. 5). The electron injection
from the excited sensitizers to the conduction band of TiO2 should
be energetically favourable because of the more negative LUMO
The photovoltaic properties of the solar cells constructed from
these organic dye-sensitized TiO2 electrodes were measured under
simulated AM1.5 irradiation (100 mW/cm2). The photovoltaic
performances of the HKK-Por-DSSCs are summarized in Table 2.
Incident photon-to-current conversion efficiencies (IPCEs) and
current density-voltage (J-V) characteristics of devices are shown in
Figs. 6 and 7, respectively.
The onset wavelengths of the IPCE spectra for DSSCs based on
HKK-Por dyes were <700 nm. IPCE values higher than 70% were
observed in the range of 400e500 nm. The maximum IPCE value of
the DSSC based on HKK-Por 5 was slightly higher than the effi-
ciencies of the DSSCs based on other HKK-Por dyes due to the
introduction of the alkoxy group into the TPA moiety. Under stan-
dard global AM 1.5 solar conditions, the HKK-Por 5 sensitized cell
gave a JSC of 9.04 mA cmꢀ2, VOC of 0.57 V, and FF of 0.66, corre-
sponding to an overall conversion efficiency of 3.36%. The intro-
duction of a bulky alkoxy group into the TPA donor unit gives
a strong donating effect and reducing the aggregation of dyes.
10
HKK-Por3
HKK-Por4
HKK-Por5
8
6
4
2
0
Table 2
Dye-sensitized solar cell performance data of the HKK-Por dyes.
Dye
JSC (mA/cm2)
VOC (V)
FF
h (%)
HKK-Por 3
HKK-Por 4
HKK-Por 5
5.38
7.57
9.04
0.59
0.54
0.57
0.66
0.67
0.66
2.09
2.74
3.36
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Voltage (V)
TiO2 thickness 9,5 þ 4,sc,TiCl4; electrolyte condition 0.6 M DMPII, 0.5 M LiI, 0.05 M I2,
Fig. 7. Photocurrentevoltage characteristics of representative TiO2 electrodes sensi-
tized with HKK-Por dyes.
0.5 M TBP in an acetonitrile solution; dye was dissolved in THF.

K.D. Seo et al. / Dyes and Pigments 94 (2012) 143e149
149
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4. Conclusions
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We have successfully synthesized a novel D-p-A system based
on zinc porphyrin dyes and obtained materials with interesting
optical and electrochemical properties. Under standard global AM
1.5 solar conditions, the HKK-Por 5 sensitizer cell gave a JSC of
9.04 mA cmꢀ2, VOC of 0.57 V, and FF of 0.66, corresponding to an
overall conversion efficiency of 3.36%. Because of a bulky donor
effect, HKK-Por dyes with an alkoxy-containing TPA moiety showed
a higher conversion efficiency. Further improvement of the power
conversion efficiency of porphyrin derivative dyes can be investi-
gated by using them as dye sensitizers instead of expensive
ruthenium dye sensitizers in the near future.
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Acknowledgements
[22] Imahori H, Umeyama T, Ito S. Large
p-aromatic molecules as potential
This research was supported by MKE (the Ministry of Knowl-
edge Economy), Korea, under the ITRC support programme super-
vised by the IITA (Institute for Information Technology
Advancement) (IITA-2008-C1090-0804-0013), the WCU (the
Ministry of Education and Science) programme (R31-2008-000-
10035-0), and the CRC Program through the National Research
Foundation of Korea (NRF) (2010K000973). Computations were
performed using a supercomputer at the Korea Institute of Science
and Technology Information (KSC-2011-C1-11).
sensitizers for highly efficient dye-sensitized solar cells. Acc Chem Res 2009;
42:1809e18.
[23] Walter MG, Rudine AB, Wamser CC. Porphyrins and phthalocyanines in solar
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Appendix. Supplementary material
[27] Lee CW, Lu HP, Lan CM, Huang YL, Diau EWG, Yeh CY, et al. Novel zinc
porphyrin sensitizers for dye-sensitized solar cells: synthesis and spectral,
electrochemical, and photovoltaic properties. Chem Eur J 2009;15:1403e12.
[28] Mathew S, Iijima H, Toude Y, Umeyama T, Matano Y, Imahori H, et al. Optical,
electrochemical, and photovoltaic effects of an electron-withdrawing tetra-
fluorophenylene bridge in a push-pull porphyrin sensitizer used for dye-
sensitized solar cells. J Phys Chem C 2011;115:14415e24.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.dyepig.2011.12.006.
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