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

Article abstract of DOI:10.1002/chem.200801572

Novel meso- or β-derivatized porphyrins with a carboxyl group have been designed and synthesized for use as sensitizers in dye-sensitized solar cells (DSSCs). The position and nature of a bridge connecting the porphyrin ring and carboxylic acid group show

Full text of DOI:10.1002/chem.200801572

FULL PAPER
DOI: 10.1002/chem.200801572
Novel Zinc Porphyrin Sensitizers for Dye-Sensitized Solar Cells: Synthesis
and Spectral, Electrochemical, and Photovoltaic Properties
Cheng-Wei Lee,[a] Hsueh-Pei Lu,[b] Chi-Ming Lan,[b] Yi-Lin Huang,[a] You-Ren Liang,[a]
Wei-Nan Yen,[a] Yen-Chun Liu,[a] You-Shiang Lin,[a] Eric Wei-Guang Diau,*[b] and
Chen-Yu Yeh*[a]
Abstract: Novel meso- or b-derivatized
porphyrins with a carboxyl group have
been designed and synthesized for use
as sensitizers in dye-sensitized solar
cells (DSSCs). The position and nature
of a bridge connecting the porphyrin
ring and carboxylic acid group show
significant influences on the spectral,
conjugated electron-donating group at
the meso position opposite the anchor-
ing group. Upon introduction of an
delocalization of charge between the
porphyrin ring and the amino group in
the first oxidative state of diarylamino-
substituted porphyrin 5, which exhibits
the best photovoltaic performance
among all the porphyrins under investi-
gation. From a comparison of the cell
performance based on the same TiO2
films, the devices made of porphyrin 5
coadsorbed with chenodeoxycholic acid
ethynylACTHNUGTRNEUNGene group at the meso position,
the potential at the first oxidation
alters only slightly whereas that for the
first reduction is significantly shifted to
the positive, thus indicating
a de-
electrochemical,
properties of these sensitizers. Absorp-
tion spectra of porphyrins with
phenylethynyl bridge show that both
and
photovoltaic
creased HOMO–LUMO gap. Quan-
tum-chemical (DFT) results support
the spectroelectrochemical data for a
a
(CDCA) on TiO2 in ratios [5]/ACHTUNGTRENNUNG[CD-
G
CA]=1:1 and 1:2 have efficiencies of
power conversion similar to that of an
N3-based DSSC, which makes this
green dye a promising candidate for
colorful DSSC applications.
Soret and Q bands are red-shifted with
respect to those of porphyrin 6. This
phenomenon is more pronounced for
porphyrins 3 and 4, which have a p-
Keywords: cross-coupling · donor–
acceptor systems · electrochemistry ·
porphyrinoids · solar cells
Introduction
ciencies of conversion of light to electric power of up to
11% have been obtained with polypyridyl ruthenium com-
plexes.[2] The advantages of using such ruthenium complexes
are that they exhibit broad absorption in the near-UV and
visible regions and appropriate excited-state oxidation po-
tentials for electron injection into the conduction band of
TiO2.[3] The cost, rarity, and environmental issues of rutheni-
um complexes limit their wide application and encourage
exploration of cheaper and safer sensitizers.
Dye-sensitized solar cells (DSSCs) have attracted much at-
tention because they present a highly promising alternative
to conventional photovoltaic devices based on silicon.[1] In
nanocrystalline TiO2 solar cells sensitized with a dye, effi-
[a] C.-W. Lee, Y.-L. Huang, Y.-R. Liang, W.-N. Yen, Y.-C. Liu, Y.-S. Lin,
Prof. C.-Y. Yeh
Department of Chemistry
National Chung Hsing University
Taichung 402 (Taiwan)
Fax : (+886)4-2286-2547
E-mail: cyyeh@dragon.nchu.edu.tw
Considerable effort has been devoted to the development
of new and efficient sensitizers suitable for practical use.
Among these, organic sensitizers have drawn great interest
because of their modest cost, ease of synthesis and modifica-
tion, large molar absorption coefficients, and satisfactory
stability. Organic dyes, such as coumarin,[4] indoline,[5] oli-
goene,[6] thiophene,[7] triarylamine,[8] perylene,[9] cyanine,[10]
and hemicyanine[11] derivatives, have been investigated as
sensitizers in DSSCs. Some organic dyes with conversion ef-
ficiencies in a range of 5–9% have been prepared.[5,12]
[b] H.-P. Lu, C.-M. Lan, Prof. E. W.-G. Diau
Department of Applied Chemistry
National Chiao Tung University
Hsinchu 300 (Taiwan)
Fax : (+886)3-572-3764
E-mail: diau@mail.nctu.edu.tw
Supporting information for this article, which contains the experimen-
tal details, is available on the WWW under http://dx.doi.org/10.1002/
chem.200801572.
In the photosynthetic cores of bacteria and plants, solar
energy is collected by chromophores based on porphyrin,[13]
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and the captured radiant energy is converted efficiently to
chemical energy. Various artificial photosynthetic model sys-
tems have been designed and synthesized to elucidate the
factors that control the photoinduced electron-transfer reac-
tion.[14] Valuable knowledge has been acquired from these
artificial systems. Inspired by the efficient energy transfer in
naturally occurring photosynthetic reaction centers, numer-
ous porphyrin[15] and phthalocyanine[16] dyes have been syn-
thesized and used for photovoltaic solar cells. The best-per-
forming porphyrin dyes have been reported to have conver-
sion efficiencies in DSSCs in a range of 5–7%.[17] The effi-
ciency of power conversion depends on how the sensitizer is
attached to the surface of a semiconductor.[18]
To investigate how the structure of porphyrins affects the
cell performance of devices, we have designed and synthe-
sized novel carboxylated porphyrin-based sensitizers 1–5
and 7–12, in which the porphyrin ring and the carboxyl an-
choring group are connected with a vinyl, phenyl, or phenyl-
ethynyl (PE) bridging unit at the meso or b position.
Herein, we report the synthesis and the spectral, electro-
chemical, and photovoltaic properties of these porphyrin-
based sensitizers. For comparison, porphyrin sensitizer 6 was
prepared according to a literature method.[19] Our results in-
dicate that the cell performance of the device using porphy-
rin 5 as sensitizer outperforms the best-reported porphyrin
dye and has a performance similar to that of an N3-based
DSSC. Therefore, 5 might be a promising green dye for
future colorful DSSC applications.
Results and Discussion
Design and synthesis: The major factor in decreasing the ef-
ficiency of sensitized photocurrent generation is the forma-
tion of dye aggregates on the semiconductor surface.[20] To
decrease that aggregation, 3,5-di-tert-butylphenyl groups
were introduced at the meso positions of the porphyrin ring.
Our approach to the enhancement of absorption by porphy-
rin dyes in the visible region is to expand the p-conjugation
system, which causes a red shift and broadening of both
Soret and Q bands. In research on porphyrin arrays and por-
phyrin-based push–pull chromophores, bridges of the ethyne
type have been shown to allow efficient conjugation and
strong electronic interaction between chromophores, and to
provide a well-defined and rigid structural arrangement.[21]
We thus designed porphyrins with an ethyne bridging unit,
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Porphyrin Sensitizers for Solar Cells
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which might facilitate electron transfer from the excited dye
to the TiO2 surface, and which are expected to show an im-
proved efficiency of energy conversion. The synthetic ap-
proach to these ethynyl-bridged porphyrins has been de-
signed on the basis of Sonogashira coupling reactions. As an
example, porphyrin 1 was obtained by Sonogoshira coupling
of zinc 5-iodo-10,15,20-tris(3,5-di-tert-butylphenyl)porphyrin
with 4-carboxyphenylethyne in satisfactory yield.[22]
with a phenyl or PE bridge at the b position were synthe-
sized. Porphyrins 9 and 10 were obtained by Suzuki coupling
of
2-bromo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)por-
phyrin (16) to the corresponding phenylboronic acids.[27,28]
Porphyrins 11 and 12 were prepared from reactions between
16 and the corresponding carboxylphenylethynes with Sono-
gashira coupling.
Most highly efficient dyes used in DSSCs have a push–
pull structure.[23] To increase the electron-donating ability of
the porphyrin ring, compound 3 with a triarylamine moiety
was designed. The synthesis of 3 involved stepwise Sonoga-
shira coupling reactions. Porphyrin 5 with a diarylamine
group at the meso position was synthesized according to the
route in Scheme 1. Bromination[24] of porphyrin 13 with
Spectral and electrochemical properties: The UV/Vis spec-
tra of these compounds in solution exhibit the features typi-
cal of a porphyrin ring, with an intense Soret band in the
range 400–500 nm and less intense Q bands in a range from
550 to 750 nm. As expected, the electronic absorption bands
of porphyrins are sensitive to substituents on the periphery
of the porphyrin ring. Examples of absorption spectra for
porphyrins 1, 3, 5, 6, and 9 appear in Figure 1; UV/Vis data
Figure 1. UV/Vis absorption spectra of 1, 3, 5, 6, and 9 in CH2Cl2/pyridine
(100:1).
Scheme 1. Synthesis of 5. i) NBS, CHCl3. ii) Bis(4-tert-butylphenyl)amine,
NaH, Pd
acid, [Pd CHTUNGTRENNUNG
G
for each compound are summarized in Table 1. Comparison
of the spectra of 9 and 10 with that of 6 reveals that substi-
tution of carboxyphenyl at the b position causes no signifi-
cant red shift of either Soret or Q bands because of an or-
thogonal orientation between the carboxyphenyl and por-
phyrin rings. For 11 and 12, in which ethyne serves as the
bridge between the porphyrin ring and anchoring group, the
Soret and Q bands are more red-shifted than those of 9 and
10. Perturbation of the energy of the HOMO and LUMO of
the porphyrin ring is pronounced when meso- or b-ethenyl
is employed as the linker, as indicated by significant broad-
ening and a red shift of the absorption bands of porphyrins
7 and 8 relative to 9–12. Compounds 1 and 2 show a similar
spectral feature and substitution of carboxyphenyl at their
meso positions by an ethyne linker that results in red shifts
of the absorption bands which are larger than those for b-
substituted analogues 11 and 12. Compound 5 shows broad-
ening of the Soret band and a red shift of the Q band rela-
tive to the spectrum of 1 due to electronic interaction be-
tween the diarylamino group and the porphyrin ring of 5.
Comparison of the absorption spectra of porphyrins 3 and 4
with that of 1 shows that red shifts and broadening increase
DPEphos: bis(2-diphenylphosphinophenyl)ether, TBAF: tetrabutylam-
monium fluoride, dba: dibenzylideneacetone.
NBS gave 14 in satisfactory yield; subsequent amination af-
forded 15.[25] Deprotection of 15 with TBAF followed by So-
nogashira coupling to 4-iodobenzoic acid produced porphy-
rin 5.
An alternative approach to increase the domain of ab-
sorption in the visible region, and thus to increase the con-
version efficiency, is to use a unit of ethenyl type as the
linker.[26] The best porphyrin-based sensitizers, which exhibit
efficiencies of energy conversion of 5%, are those with a b-
substituted cyanoacetic or malonic acid, reported by Grꢁtzel
and co-workers.[17] We thus undertook the synthesis of por-
phyrins 7 and 8, which have a cyanoacetic acid substituted
at the meso and b positions, respectively, by employing a
procedure similar to that reported by Grꢁtzel et al.[17b]
The position and nature of the link between the porphyrin
and the carboxyl group are expected to influence significant-
ly the efficiency of energy conversion. Compounds 9–12
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E. W.-G. Diau, C.-Y. Yeh et al.
Table 1. Electronic absorption and emission data for porphyrins 1–12.[a]
Porphyrin
Absorption
Emission
lmax [nm]
lmax [nm] (e [103 mꢀ1 cmꢀ1])
1
2
3
4
5
6
7
8
445 (282), 579 (9.5), 636 (24.8)
445 (231), 582 (8.2), 632 (19.4)
451 (117), 680 (30.6)
653[b]
651[b]
707[c]
454 (283), 668 (51.0)
687[c]
448 (194), 601 (8.3), 654 (29.7)
430 (616), 565 (20.7), 605 (14.7)
455 (106), 571 (7.1), 636 (8.4)
451 (129), 564 (11.7), 613 (11.1)
434 (326), 567 (12.9), 607 (7.7)
433 (409), 566 (16.3), 609 (10.7)
442 (348), 574 (21.0), 618 (13.0)
444 (375), 575 (25.1), 618 (16.3)
687[c]
617, 660[b]
659[d]
666[d]
631[b]
630[b]
635[b]
635[b]
9
10
11
12
[a] Absorption and emission data were measured for (CH2Cl2/pyridine=
100:1) solutions at 298 K. The excitation wavelengths were [b] 550 nm,
[c] 650 nm, and [d] 600 nm.
Figure 2. Cyclic voltammograms of a) 1, b) 4, and c) 6 in THF containing
0.1m TBAPF6.
The electron-withdrawing property of the carboxyl an-
choring group through a PE linker in 1 has no significant in-
fluence on its oxidation potential. The first oxidation is shift-
ed only slightly by 40 mV to the positive relative to porphy-
rin 6; we attribute this property to the strong electronic in-
teraction between the carboxyl group and the porphyrin
macrocycle through the PE linker, which allows positive
charge to delocalize over the PE unit upon oxidation. The
effect of the electron-withdrawing ability of the carboxyl PE
linker on the oxidation potential is thus partially compensat-
ed by the charge delocalization. In contrast, the significant
shift of the first reduction of 1 versus 6 by 170 mV to the
positive indicates that both electron withdrawal and elec-
tronic delocalization influence the reduction potential in the
same direction. A similar trend of the reduction potentials
of 4 was also observed. As shown in Table 2, the half-wave
potentials for the first abstraction of an electron from the
porphyrin ring, except for 3 and 5, are in a small range
+0.96 to +1.05 V, whereas those for the first reduction span
a large range of ꢀ1.06 to ꢀ1.41 V. The electrochemical
HOMO–LUMO energy gap decreases as the p conjugation
is extended, consistent with red shifts of both Soret and
Q bands in the electronic absorption spectra. The potential
for the first oxidation of porphyrins corresponds to the
HOMO level. The LUMO energy level is predictable from
the HOMO and the absorption threshold. In our new por-
phyrins, the LUMO levels are more negative than the con-
duction-band edge (ꢀ0.5 V vs. normal hydrogen electrode)
and the HOMO levels are more positive than the oxidation
systematically with increasing p conjugation.[29] A similar
trend was observed in the fluorescence spectra.
We investigated the electrochemical properties of these
porphyrins with cyclic voltammetry. Because the solubility
of some of these porphyrins in CH2Cl2 was poor, we dis-
solved them in THF. In general, two oxidations and two re-
ductions are expected for a zinc porphyrin under our elec-
trochemical conditions. For some porphyrins, the oxidation
waves were irreversible under ambient conditions; the elec-
trochemical reactions of these porphyrins were therefore in-
vestigated at low temperatures. The measured oxidation and
reduction potentials of these porphyrins are listed in
Table 2. Figure 2 shows examples of cyclic voltammograms
Table 2. Electrochemical data for porphyrins 1–12.[a]
Porphyrin
Oxidation E1/2 [V]
Reduction E1/2 [V]
1
2
3
4
5
6
7
8
+1.00
ꢀ1.19
+1.05[b]
ꢀ1.24
+0.86, +1.02
ꢀ1.08
+0.98
ꢀ1.06
+0.87[a] (+0.79, +1.05, +1.73)[c]
ꢀ1.11[a] (ꢀ1.02)[c]
ꢀ1.36
+0.96
+1.03
+0.98
+1.04[b]
+0.95
+1.00
+0.99
ꢀ1.21[b]
ꢀ1.36[b]
ꢀ1.41
9
10
11
12
ꢀ1.35
ꢀ1.36[b]
ꢀ1.40[b]
[a] Electrochemical measurements were performed at ꢀ208C in THF
containing TBAPF6 (0.1m) as supporting electrolyte. Potentials [V] are
reported versus Ag/AgCl and reference to the ferrocene/ferrocenium
(Fc/Fc+) couple (THF, ꢀ208C, +0.57 V). [b] Irreversible process Epa or
Epc. [c] Electrochemical measurements were performed at ꢀ208C in
CH2Cl2 containing TBAPF6 (0.1m) as supporting electrolyte.
ꢀ
potential for Iꢀ/I3 , which meets the requirement for effec-
tive electron injection and dye regeneration in a DSSC
system.[1–3]
The cyclic voltammogram of 3 shows that the first and
second 1ꢀeꢀ oxidations, E
1/2ACHTUNGTERNNUG(ox1)=+0.86 and E1/2ACHTUTGNREN(NUGN ox2)=+
1.02 V, overlap, which one can resolve by using differential
pulse voltammetry (Figure 3). The first reduction of the por-
phyrin ring was observed at ꢀ1.08 V. The oxidation centers
are identified by comparison with those of the correspond-
ing triarylamine and porphyrin components, with reduction
at E1/2 =+0.91 and +0.99 V, respectively. The first oxidation
for porphyrins 1, 4, and 6 in THF containing tetrabutylam-
monium hexafluorophosphate (TBAPF6; 0.1m) at ꢀ208C.
For porphyrin 1, the first oxidation at E1/2ACTHUNRTGNEUNG(ox1)=+1.00 V
and the first reduction at E
N
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Porphyrin Sensitizers for Solar Cells
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only the porphyrin ring and the PE linker at the LUMO;
LUMO+1 shows the localization of electron density solely
on the porphyrin ring. Both diarylamino and porphyrin
units are hence responsible for the first and second oxida-
tions at E1/2 =+0.79 and +1.05 V. In contrast, the reduction
at E1/2 =ꢀ1.02 V involves charge delocalization only on the
porphyrin ring and the PE linker, thus facilitating electron
transfer from the dye to TiO2 through the linker.
Regarding electron transfer in DSSCs, absorption of a
photon promotes an electron from the ground state of the
dye into an excited state. The electron is then transferred
from the excited dye to the semiconductor on an ultrarapid
timescale. The interfacial electron transfer between the ex-
cited dye and the TiO2 semiconductor can be investigated
by detection of the temporally resolved absorption signals
of dye cations through spectrometric techniques. To gain in-
sight into the electronic absorption properties of the oxi-
dized species of the sensitizers, we studied electrochemical
reactions of compounds 1, 3, and 5 with a spectroelectro-
chemical method.[30] Thin-layer UV/Vis absorption spectra
of porphyrin 1 recorded during electrochemical oxidation at
an applied potential of +1.26 V in THF containing TBAPF6
(0.1m) at ꢀ208C are displayed in Figure 5. Scans of UV/Vis
Figure 3. Cyclic voltammogram (c) and differential pulse voltammo-
gram (a) of 3 (top) in THF and cyclic voltammogram of 5 (bottom) in
CH2Cl2 containing TBAPF6 (0.1m) at ꢀ208C.
of 3 at E
being abstracted from the triarylamine unit; the second ab-
straction at E1/2A(ox2)=+1.02 V corresponds to oxidation of
1/2ACHTUNGTRENNUNG(ox1)=+0.86 thus corresponds to one electron
CHTUNGTRENNUNG
the porphyrin ring. The assignments of these oxidation cen-
ters for compound 3 are confirmed with thin-layer spectro-
A
ACHTUNGTRENNUNGchemistry, as discussed below.
+0.87 and ꢀ1.11 V in THF at ꢀ208C for the first oxidation
and first reduction, respectively, but in CH2Cl2 porphyrin 5
exhibits three reversible couples at potentials E1/2 =+0.79,
+1.05, and +1.73 V, which correspond to oxidations from
the amino functional group and the porphyrin ring; the first
reduction of the porphyrin ring was observed at a potential
of ꢀ1.02 V. To identify the redox centers, we performed
quantum-chemical calculations on 5 by density functional
theory (DFT) at the B3LYP/6-31G(d) level (Spartan ꢂ06
package). As shown in Figure 4, the electron density of 5 is
Figure 5. Spectral changes of 1 in THF containing TBAPF6 (0.1m) at an
applied potential of +1.26 V at ꢀ208C.
spectra at +1.26 V show that the intensities of the Soret and
Q bands decrease significantly; furthermore, the Soret band
broadens and a new and broad band appears at about
660 nm, which is characteristic of the formation of the por-
phyrin cation radical. The electrochemical oxidation is re-
versible because the porphyrin cation generated at +1.26 V
can be reconverted to its neutral form in a ratio >90% at
an applied potential +0.80 V.
The oxidative electrolysis of porphyrin 3 was also per-
formed at low temperature because the oxidized species was
unstable under electrolysis at ambient temperature. Figure 6
shows the spectral changes of porphyrin 3 at applied poten-
tials of +0.95 and +1.24 V in THF containing TBAPF6
(0.1m) at ꢀ208C. In the first oxidation, the signal at 332 nm
Figure 4. Surfaces of frontier molecular orbitals of 5 predicted with DFT
calculations. To simplify the computations, tert-butyl groups at the para
and meta positions were replaced with methyl groups and hydrogen
atoms, respectively.
significantly distributed to the p system of the porphyrin
ring, the diphenylamino moiety, and the PE linker at the
HOMO and HOMO-1, but the p conjugation is extended to
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E. W.-G. Diau, C.-Y. Yeh et al.
oxidation (Eappl. =+0.94 V), the absorption bands at 441 and
647 nm corresponding to the porphyrin ring and the band at
303 nm corresponding to the amino unit decrease, whereas
those at 807 and 1384 nm increase with several clear isosbes-
tic points (Figure 7). As mentioned for compound 3, the
Figure 6. Spectral changes of 3 in THF containing TBAPF6 (0.1m) at ap-
plied potentials of +0.95 V (top) and +1.24 V (bottom) at ꢀ208C.
corresponding to the triarylamine unit decreases while that
at about 566 nm, attributed to formation of the triarylamine
cation, increases. The intensity of the Q band at 677 nm de-
creases and a new band at about 920 nm arises with clear
isosbestic points. One-electron oxidation of a porphyrin ring
generally results in a significantly decreased intensity of the
Soret band, but in this case we observed sharpening and a
slightly increased intensity of the Soret band upon 1ꢀeꢀ oxi-
dation. The first oxidation is hence assigned to a triaryl-
Figure 7. Spectral changes of 5 in CH2Cl2 containing TBAPF6 (0.1m) at
applied potentials of +0.94 V (top) and +1.20 V (bottom) at ꢀ208C.
first electron abstraction occurs essentially from the triaryl-
AHCTUNGTREGaNNNU mine unit for which a significantly decreased Soret band
A
was not observed. In contrast, both the Soret band and the
band at 303 nm of compound 5 show a significant decrease
upon abstraction of the first electron, indicating that the
positive charge was delocalized over both the porphyrin ring
and the amino unit. Further oxidation at an applied poten-
tial Eappl. =+1.20 V resulted in decreased signals at 438 and
1384 nm and an increased band at 486 nm. These spectroe-
lectrochemical results are consistent with those obtained
from the quantum-chemical calculations; that is, the elec-
tronic densities of HOMO and HOMOꢀ1 are distributed to
both the porphyrin and diarylamine units discussed earlier.
work showed that the introduction of a triarylamine unit
onto a porphyrin ring through an ethyne linker results in
broadening and a red shift of the absorption bands because
of strong electronic communication between the porphyrin
and triACHTUNGTRENNUNGarylACHTUNGTRENNUNG
amine units.[30] Upon oxidation of the triarylamine
moiety, the sharpening and increased intensity of the Soret
band might be ascribed to the decreased interchromophoric
interaction between the porphyrin ring and the triarylamine
cation. Further oxidation of porphyrin 3 at an applied po-
tential of +1.24 V leads to a significantly decreased Soret
band and extinction of the Q band, which are characteristics
of a porphyrin-centered oxidation, and the presence of an
intense near-IR band in the range 800–1100 nm. We report-
ed previously the electrochemical and spectroelectrochemi-
cal properties of porphyrin–triarylamine conjugates, and a
similar near-IR band has been observed upon oxidation of
the conjugates.[29] These electrochemical oxidations are es-
sentially reversible as more than 90% of the oxidized spe-
cies generated at +1.24 V can be reconverted to the neutral
form of porphyrin 3 according to the intensity of the
Q band.
Cell fabrication and photovoltaic characteristics: The por-
phyrins were sensitized onto TiO2 nanoparticulate films to
serve as working electrodes in DSSC devices. The TiO2
nanoparticles (size ꢁ20 nm), prepared with a sol–gel meth-
od,[2b,31] were screen-printed onto the F-doped SnO2 (FTO,
30 Wcmꢀ2, Sinonar, Taiwan) glass substrate.[32] Crystalliza-
tion of TiO2 films (thickness ꢁ9 mm and active area
0.16 cm2) was performed by annealing in two stages: heating
at 4508C for 5 min followed by heating at 5008C for 30 min.
The TiO2 film was then immersed in an aqueous solution of
TiCl4 (50 mm, 708C, 30 min) followed by the same two-stage
thermal treatment for final annealing of the electrode. The
electrode was then immersed in a porphyrin/ethanol solu-
We performed spectroelectrochemical measurements on
compound 5 in CH2Cl2 because the 2ꢀeꢀ oxidized species
(52+) were unstable in THF even at ꢀ208C. In the first 1ꢀeꢀ
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Porphyrin Sensitizers for Solar Cells
FULL PAPER
tion (0.2 mm, 258C, 2 h) con-
taining chenodeoxycholic acid
(CDCA; 0.2 mm unless other-
wise specified) for dye loading
onto the TiO2 film. The Pt
counter electrodes were pre-
pared by spin-coating drops of
H2PtCl6 solution onto FTO
glass and heating at 4008C for
15 min. To prevent a short cir-
cuit, the two electrodes were
assembled into a cell of sand-
wich type and sealed with a
hot-melt film (SX1170, Solaro-
nix, thickness 25 mm). The elec-
trolyte solution containing LiI
(0.1m), I2 (0.05m), 1-butyl-3-
methylimidazolium
iodide
(0.6m), and 4-tert-butylpyridine
(0.5m) in a mixture of acetoni-
trile and valeronitrile (volume
ratio 1:1)[17b] was introduced
into the space between the two
electrodes, so completing the
fabrication of these DSSC devices.
Figure 8. a)–c) Visible absorption spectra of porphyrin-sensitized TiO2 with sensitizers 1–11; d)–f) I–V curves
of porphyrin-sensitized solar cells made of the corresponding sensitizers as in (a)–(c) under AM 1.5 illumina-
tion (power 100 mWcmꢀ1).
the meta positions. The absorption spectra of the porphyrin/
TiO2 films and the corresponding I–V curves of the solar
Through measurement of an I–V curve with an air mass
(AM) 1.5 solar simulator (Newport–Oriel 91160), we as-
sessed the performance of a DSSC device. The solar simula-
tor uses filters and other optical components to mimic solar
radiation with an AM 1.5 spectrum; the output intensity is
evenly distributed for illumination of a large area. When the
device is irradiated with the solar simulator, the source
meter (Keithley 2400, computer-controlled) sends a voltage
(V) to the device, and the photocurrent (I) is read at each
step controlled by a computer through a general-purpose in-
terface bus. The solar simulator was calibrated with a Si-
based reference cell (S1133, Hamamatsu) and an IR filter
(KG5) to correct the spectral mismatch of the lamp.[33] The
efficiency (h) of conversion of light to electricity is obtained
with the relations [Eq. (1)]:
cells made of the same sensitized films are shown in Fig-
ure 8a–c and 8d–f, respectively; the corresponding photo-
voltaic parameters are summarized in Table 3.
Table 3. Photovoltaic parameters of porphyrin-based DSSCs under
AM 1.5 illumination (power 100 mWcmꢀ2) and active area 0.16 cm2.[a]
Dye
JSC [mAcmꢀ2
]
VOC [mV]
FF
h [%]
1
2
3
4
5
6
7
8
5.79
2.63
3.76
3.17
13.60
5.24
2.94
0.60
0.34
0.17
6.61
617
613
546
544
701
624
543
505
242
141
638
0.667
0.719
0.672
0.680
0.629
0.685
0.582
0.468
0.335
0.250
0.642
2.4
1.2
1.4
1.2
6.0
2.2
0.93
0.14
0.03
<0.01
2.7
9
10
11
Pmp JmpVmp
JSCVOCFF
¼
Pin
ð1Þ
h ¼
¼
[a] The photovoltaic parameters were obtained by fitting the correspond-
ing current–voltage curves of Figure 8d–f according to Equation (1). For
each sensitizer, two to four different devices of equal quality were tested
and the uncertainties of h were within 10%.
Pin
Pin
in which Pin is the input radiation power (for one solar radi-
ation Pin =100 mWcmꢀ2) and Pmp is the maximum output
power (=Jmp ꢃVmp); the fill factor (FF) is defined as
[Eq. (2)]:
With compound 1 as our standard, we compared the cell
performance for each porphyrin sensitizer. Compound 8 was
designed to mimic the molecular structure of Zn-3,[17b] in
which the COOH group is attached to the b position
through a vinylene group, but incorporating the tert-butyl
substituents to avoid dye aggregation. As shown in Fig-
ure 8c, compound 8 with a short linker attached at the b po-
sition of the porphyrin ring exhibits poor adsorption on the
TiO2 film, which causes poor cell performance (Figure 8 f);
compounds 9 and 10 similarly showed small efficiencies of
JmpVmp
JSCVOC
ð2Þ
FF ¼
JSC (mAcmꢀ2) is the current density measured at short cir-
cuit, and VOC (V) is the voltage measured at open circuit.
We classified compounds 1–11 into three categories
(Figure 8); compound 12 was not measured because poor
cell performance is anticipated for the anchoring groups in
Chem. Eur. J. 2009, 15, 1403 – 1412
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1409

E. W.-G. Diau, C.-Y. Yeh et al.
power conversion for the same reason—that the steric hin-
drance of the bulky tert-butyl groups impedes adsorption of
the dye on TiO2.
For compound 11 with a longer linker at the b position
that promoted sufficient dye to become adsorbed on the
TiO2 film (Figure 8c), the efficiency of power conversion
became comparable to that of compound 1 (h=2.7 vs.
2.4%), thus indicating that the cell performance of a por-
phyrin-sensitized solar cell was insensitive to the position of
the PE linker attached at the b or meso position. This result,
although contrary to reported data,[17a] provides new evi-
dence that meso-substituted porphyrins might serve as po-
tential photosensitizers in DSSC applications. In what fol-
lows, we discuss the photovoltaic performance of devices
fabricated with various meso-substituted porphyrins.
First, comparison of 1 and 2 shows that the anchoring
group (COOH) in the para position performs better than
that in the meta position (h=2.4 vs. 1.2%), consistent with
literature results.[17a] Relative to compound 1, the structure
of 6 lacks a triple bond in the linker so that its p conjugation
is decreased. The Q band of 6 thus became blue-shifted and
the photocurrent density of 6 decreased slightly relative to
1. As a result, the power-conversion efficiency of 6 became
slightly less than that of 1 (2.2 vs. 2.4%). In contrast, the
Q band of 7 is red-shifted relative to 1 because the structure
of 7 has an additional cyano group. The poor performance
of 7 in both JSC and VOC leads to the power-conversion effi-
ciency being much less than that of 1. We expect that the su-
perior electron-pulling capability of the cyano group in 7
might first pull the electrons and then transfer them back
into the porphyrin core through space; this process is im-
practicable for 8 or Zn-3, in which the linker is attached to
the b position.
Figure 9. Comparison of the a) visible absorption spectra and b) I–V char-
acteristics of porphyrin 5-sensitized solar cells under different concentra-
tions of coadsorber, CDCA.[17b]
varied as 1:0, 1:1, 1:2, 1:4, and 1:10; the corresponding dye/
TiO2 absorption spectra are shown in Figure 9a. Figure 9b
compares the I–V characteristics of the devices made of por-
Table 4. Photovoltaic parameters of 5-sensitized solar cells with various
dye/CDCA ratios under AM 1.5 illumination (power 100 mWcmꢀ2) and
active area 0.16 cm2.
To improve the charge separation in the sensitizer, we
modified the structure of compound 1 by adding an efficient
electron-pushing group at the meso position of the porphy-
rin ring, and designed porphyrins 3–5 accordingly. The addi-
tional triple bond in 3 and 4 extends the absorption to the
near-IR region (Figure 8b), which might facilitate harvesting
of sunlight toward a more efficient region, but 3 and 4 are
poorly soluble in ethanol, thus leading to molecular aggrega-
tion in solution. The Q band of compound 3 adsorbed on
TiO2 became broad, an indication of serious dye aggregation
on the surface of the TiO2 films. As a result, the cell per-
formances of both 3 and 4 were poorer than that of 1. In
contrast, porphyrin 5 with a diarylamino group attached di-
rectly at the meso position exhibited an excellent cell perfor-
mance with JSC =13.60 mAcmꢀ2, VOC =0.701 V, FF=0.629,
and overall h=6.0% obtained for a TiO2 film of thickness
ꢁ9 mm. The photocurrent density of 5 is more than twice
that of 1, indicating the superior capabilities of light harvest-
ing and charge separation of that dye. The open-circuit volt-
age of 5 is notably the largest among all porphyrins under
investigation.
[5]:
[CDCA]
JSC [mAcmꢀ2
]
VOC [mV]
FF
h [%]
1:0
1:1
1:2
1:4
1:10
N3 dye
12.22
14.33
13.22
12.05
11.23
12.08
686
710
708
710
701
756
0.645
0.594
0.640
0.662
0.668
0.666
5.4
6.0
6.0
5.7
5.3
6.1
phyrin 5 under various dye/CDCA ratios, and the corre-
sponding photovoltaic parameters are summarized in
Table 4.
Our results indicate that treating with the coadsorbate
CDCA together with loading the dye on TiO2 films helps to
impede dye aggregation, which increases both JSC and VOC
and thus also h values; the best cell performance of a
CDCA-treated device is for a cell with a dye/CDCA ratio
between 1:1 and 1:2. Specifically, we found that the CDCA
coadsorbates occupied effectively the available space on the
TiO2 surface to compete with 5, because the absorption
spectra showed a systematic decrease of absorbance from
the greatest value in a 1:0 film to the least value in a 1:10
film. The device made of a 1:0 film had, however, a smaller
We used CDCA as a coadsorbate for dye loading to pre-
vent aggregation of the dye on the TiO2 surface.[17b] The
ratios of the concentrations of compound 5 and CDCA were
1410
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 1403 – 1412

Porphyrin Sensitizers for Solar Cells
FULL PAPER
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Because our TiO2 films were not optimized to provide an
absolute h value for porphyrin 5, we report a comparison of
the cell performance of 5 with the representative dye, N3,
which is a ruthenium complex-based dye widely used as a
reference in DSSC applications (h=10%).[2] Figure 9a
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of the device made of 5 became comparable to that made of
N3 dye. Our compound 5 is thus perhaps the best porphyrin
dye reported in the literature, which makes this green dye a
promising candidate for colorful DSSC applications.
Conclusion
We have prepared novel porphyrin-based dyes for DSSC ap-
plications. Our strategy for the porphyrin design is summar-
ized by three major points: 1) the introduction of tert-butyl
groups onto phenyl rings tends to suppress the formation of
dye aggregates on the TiO2 surface; 2) the extension of
p conjugation of the porphyrin ring tends to broaden and to
red-shift the Soret and Q bands to improve the light-har-
vesting effect; and 3) the introduction of an electron-donat-
ing group in the meso position of the porphyrin ring tends
to enhance the charge-separation capability. Among these
dyes, a device made of diarylamino-substituted porphyrin 5
adsorbed on a 9 mm TiO2 film exhibits a short-circuit photo-
current density (JSC) of 13.6 mAcmꢀ2, an open-circuit volt-
age (VOC) of 0.70 V, and a fill factor (FF) of 0.63, corre-
sponding to an overall efficiency of power conversion of
6.0%. From our comparison of cell performance with the
same TiO2 films, porphyrin 5 outperformed the best report-
ed porphyrin sensitizer and has a performance comparable
to that of an N3-based DSSC. Work is in progress to modify
the structure of the porphyrin-based sensitizer with a more
strongly electron-pushing group to increase the efficiency of
power conversion, and with hydrophobic long-chain sub-
stituents to improve the long-term stability of the device.
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Received: August 1, 2008
Revised: October 9, 2008
Published online: December 18, 2008
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