Triarylamine-substituted imidazole- and quinoxaline-fused push-pull porphyrins for dye-sensitized solar cells

Article abstract of DOI:10.1002/cssc.201200869

We have prepared a push-pull porphyrin with an electron-donating triarylamino group at the β,β-edge through a fused imidazole group and an electron-withdrawing carboxyquinoxalino anchoring group at the opposite β,β-edge (ZnPQI) and evaluated the effects of the push-pull structure of ZnPQI on optical, electrochemical, and photovoltaic properties. ZnPQI showed red-shifted Soret and Q bands relative to a reference porphyrin with only an electron-withdrawing group (ZnPQ), thus demonstrating the improved light-harvesting property of ZnPQI. The optical HOMO-LUMO gap was consistent with that estimated by DFT calculations. The ZnPQI-sensitized solar cell exhibited a relatively high power conversion efficiency (η) of 6.8 %, which is larger than that of the ZnPQ-sensitized solar cell (η=6.3 %) under optimized conditions. The short-circuit current and fill factor of the ZnPQI-sensitized solar cell are larger than those of the ZnPQ-sensitized solar cell, whereas the open circuit potential of the ZnPQI-sensitized cell is smaller than that of the ZnPQ-sensitized cell, leading to an overall improved cell performance of ZnPQI. Such fundamental information provides a new tool for the rational molecular design of highly efficient dye-sensitized solar cells based on push-pull porphyrins. Copyright

Full text of DOI:10.1002/cssc.201200869

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DOI: 10.1002/cssc.201200869
Triarylamine-Substituted Imidazole- and Quinoxaline-
Fused Push–Pull Porphyrins for Dye-Sensitized Solar Cells
Hironobu Hayashi,[a] Abeda Sultana Touchy,[b] Yuriko Kinjo,[a] Kei Kurotobi,[b] Yuuki Toude,[a]
Seigo Ito,[c] Hanna Saarenpꢀꢀ,[d] Nikolai V. Tkachenko,[d] Helge Lemmetyinen,[d] and
Hiroshi Imahori*[a, b]
We have prepared a push–pull porphyrin with an electron-do-
nating triarylamino group at the b,b’-edge through a fused
imidazole group and an electron-withdrawing carboxyquinoxa-
lino anchoring group at the opposite b,b’-edge (ZnPQI) and
evaluated the effects of the push–pull structure of ZnPQI on
optical, electrochemical, and photovoltaic properties. ZnPQI
showed red-shifted Soret and Q bands relative to a reference
porphyrin with only an electron-withdrawing group (ZnPQ),
thus demonstrating the improved light-harvesting property of
ZnPQI. The optical HOMO–LUMO gap was consistent with that
estimated by DFT calculations. The ZnPQI-sensitized solar cell
exhibited a relatively high power conversion efficiency (h) of
6.8%, which is larger than that of the ZnPQ-sensitized solar
cell (h=6.3%) under optimized conditions. The short-circuit
current and fill factor of the ZnPQI-sensitized solar cell are
larger than those of the ZnPQ-sensitized solar cell, whereas the
open circuit potential of the ZnPQI-sensitized cell is smaller
than that of the ZnPQ-sensitized cell, leading to an overall im-
proved cell performance of ZnPQI. Such fundamental informa-
tion provides a new tool for the rational molecular design of
highly efficient dye-sensitized solar cells based on push–pull
porphyrins.
Introduction
We have been reliant on fossil fuels to produce an enormous
amount of energy. However, there is a limit of fossil-fuel re-
serves, and they surely will be exhausted sooner or later.
Therefore, there is a need to develop sustainable and clean-
energy-producing systems.[1] In this regard, solar cells have at-
tracted much attention due to their direct conversion of solar
energy into electricity. In particular, a great deal of attention
has been devoted to dye-sensitized solar cells (DSSCs) made
from mesoporous TiO2 electrodes owing to the possibility of
low-cost production and high power conversion efficiency
(h).[2]
Until recently, ruthenium(II) bipyridyl complexes have
proven to be the most efficient TiO2 sensitizers.[3] However, in
view of the high cost and scarcity of ruthenium, organic dyes
without metals or inexpensive metal complexes are desirable
for highly efficient DSSCs.[4,5] To attain highly efficient solar
energy conversion, such organic dyes should fulfill the follow-
ing requirements: 1) broad light-absorption capability that
allows us to collect solar light efficiently in visible and near-in-
frared regions, 2) fast electron injection from the excited dyes
to a conduction band (CB) of the TiO2 electrode, 3) slow
charge recombination between the injected electrons and re-
[a] Dr. H. Hayashi, Y. Kinjo, Y. Toude, Prof. H. Imahori
Department of Molecular Engineering
Graduate School of Engineering
ꢀ
sulting dye cations or I3 in the electrolyte. So far, organic dyes
composed of p-conjugative molecules, such as coumarin,[6] in-
doline,[7] polyene,[8] thiophene,[9] cyanine,[10] hemicyanine,[11]
squaraine,[12] phthalocyanine,[13] perylenes,[14] and tetrathiafulva-
lene[15] derivatives have been explored as potential sensitizers
for DSSCs.
Kyoto University
Nishikyo-ku, Kyoto 615-8510 (Japan)
[b] Dr. A. S. Touchy, Prof. K. Kurotobi, Prof. H. Imahori
Institute for Integrated Cell-Material Sciences (WPI-iCeMS)
Kyoto University
Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2571
Porphyrins are among the most widely studied sensitizers
for DSSCs because of their strong Soret (400–450 nm) and
moderate Q bands (550–600 nm).[16–23] More importantly, their
optical, electrochemical, and photophysical properties can be
modulated by peripheral substitutions and inner metal com-
plexations. Nevertheless, until recently porphyrins as sensitizers
typically have shown poor cell performances relative to ruthe-
nium polypyridyl complexes. The major cause resulted from in-
sufficient light-harvesting abilities of typical porphyrins in opti-
mized cells, especially at around 500 nm and at wavelengths
beyond 600 nm.[5a,19d] To overcome this drawback, the pertur-
E-mail: imahori@scl.kyoto-u.ac.jp
[c] Prof. S. Ito
Department of Electrical Engineering and Computer Sciences
Graduate School of Engineering
University of Hyogo
2167 Shosha, Himeji, Hyogo 671-2280 (Japan)
[d] H. Saarenpꢀꢀ, Prof. N. V. Tkachenko, Prof. H. Lemmetyinen
Department of Chemistry and Bioengineering
Tampere University of Technology
P.O. Box 541, FIN-33101 Tampere (Finland)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200869.
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bation of the porphyrin p system is an effective methodology
to tailor the optical, photophysical, and electrochemical prop-
erties. For instance, a push–pull porphyrin with an electron-do-
nating diarylamino group at the meso position and an elec-
tron-withdrawing carboxyphenylethynyl anchoring group at
the opposite meso position led to a remarkably improved h
value of up to 12.3%, which was achieved by co-sensitization
with an organic dye under standard air mass (AM) 1.5 condi-
tions.[23g–j]
troduction of the electron-donating triarylamino group as
a result of enhanced charge-transfer (CT) character,[4–15] which
would have a significant impact on the photovoltaic proper-
ties. Overall, we can evaluate the substituent effects of the tri-
arylamino group through the fused imidazole moiety as an
electron-donating group on the optical, electrochemical, and
photovoltaic properties of the porphyrins.
We have also proposed that the unsymmetrical p elongation Results and Discussion
of porphyrins is a promising strategy to improve light-harvest-
ing properties.[5a,20] Especially quinoxalino[2,3-b]porphyrin acid
Synthesis and characterization
(ZnPQ, Figure 1) showed broadened, red-shifted, and amplified
The key features of the synthetic route to ZnPQI involve the
stepwise asymmetrical functionalization of symmetrical meso-
tetraarylporphyrin to build up the push–pull molecular struc-
ture of ZnPQI (Scheme 1). Therefore, ZnPQI was designed to
possess the fused imidazole group at one side of the porphy-
rin ring and the fused quinoxaline group at the opposite side.
The starting compounds 2-(4-bromophenyl)-5,5-dimethyl-1,3-
dioxane (1) and bis(4-hexylphenyl)amine (2) were prepared fol-
lowing literature procedures.[24,25] The coupling of 1 and 2
yielded the protected triarylamine 3. Formyl triarylamine (4)
was obtained by acidic hydrolysis of 3. Porphyrin dione 7 was
prepared by acetoxylation of meso-tetraarylporphyrin 5 and
subsequent oxidation of 6, which was employed for the next
oxidation reaction without isolation, according to established
methods.[26] Subsequent acetoxylation of 7 followed by oxida-
tion of 8 yielded porphyrin tetraone 9.[27] The coupling of 9
with 4 under mild acidic conditions led to a triarylamine-sub-
stituted imidazole-fused porphyrin dione (10). The condensa-
tion of 10 with methyl diaminobenzoate in pyridine afforded
the desired triarylamine-substituted imidazole and quinoxaline
fused porphyrin (11). Finally, treatment of 11 with zinc acetate
followed by hydrolysis under basic conditions yielded ZnPQI.
Initially, we attempted to synthesize 11 through a different
synthetic route: the synthesis of quinoxaline-fused porphyrin
dione from quinoxaline fused porphyrin, as in our previous re-
port,[20h] followed by condensation with 4 to yield 11. Unfortu-
nately, the quinoxaline fused porphyrin dione did not react
with 4, resulting in decomposition of the porphyrin. Structures
Figure 1. Molecular structures used in this study.
light-absorption properties relative to those of the correspond-
ing porphyrin reference, resulting in h values of 6.3%.[5a,20g,h]
Therefore, quinoxaline-fused porphyrins are highly attractive as
excellent sensitizers for DSSCs. On the other hand, introduction
of both electron-donating and electron-withdrawing substitu-
ents, including an anchoring group to the core of the p
system, is also appealing for the modulation of the light-har-
vesting properties of sensitizers, as demonstrated for other
sensitizers in DSSCs.[4–15] Considering that the fused quinoxaline
group of ZnPQ possesses an electron-withdrawing character,
the additional introduction of an electron-donating group into
the ZnPQ core is expected to enhance the light-harvesting
properties of the sensitizer.
1
of all new compounds were verified by performing H NMR, IR,
and high resolution MS (HRMS) analyses (see the Supporting
Information).
1H NMR spectra of ZnPQI showed a broad singlet resonance
at 8.6 ppm, which is assigned to the imidazole NH group. The
broad signal suggests that imidazole tautomerization occurs
1
on the H NMR timescale at room temperature as seen in pre-
vious reports.[27] Considering that regioisomers of dyes have an
impact on device performance[28] and ZnPQI involves the imi-
dazole moiety and a carboxylic acid group at the quinoxaline
ring, this slow tautomerization of the imidazole NH group may
affect device performance.
Herein, we report the synthesis and the optical, electro-
chemical, and photovoltaic properties of a push–pull porphyrin
with an electron-withdrawing carboxyquinoxalino anchoring
group at the b,b’-edge and an electron-donating triarylamino
group at the opposite b,b’-edge through a fused imidazole
moiety (ZnPQI, Figure 1). We expected that the absorption of
the porphyrin would be broadened and red-shifted by the in-
Spectroscopic and electrochemical studies
Figure 2 displays UV/Vis absorption spectra of ZnPQI and
ZnPQ[20g,h] in dichloromethane. Peak positions and molar ab-
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Scheme 1. Synthesis of ZnPQI: 1) tBu3PHBF4, [Pd2(dba)3] (dba= dibenzylideneacetone), NaOtBu, toluene, 26 h, 56%; 2) trifluoroacetic acid, aq. H2SO4 (10%),
2.5 h, 96%; 3) silver acetate, iodine, dichloromethane, 4 h; 4) a) K2CO3, methanol, dichloromethane, 8 h; b) Dess–Martin-Periodinane (DMP), dichloromethane,
4 h, 15% (two steps); 5) silver acetate, iodine, dichloromethane, 4 h; 6) a) K2CO3, methanol, dichloromethane, 8 h; b) DMP, dichloromethane, 4 h, 14% (two
steps); 7) 4, NH4OAc, chloroform/acetic acid (9:1), reflux, 10 h; 8) methyl 3,4-diaminobenzoate, pyridine, 1108C, 20 h, 20% (two steps). 9) a) Zn(OAc)2, metha-
nol, dichloromethane, 8 h; b) aq. KOH, THF/EtOH (1:1), reflux, 4 h, c) aq. NH4Cl, dichloromethane, 8 h, 96%.
The steady-state fluorescence spectrum of ZnPQI was mea-
sured in dichloromethane by exciting at the strongest peak
positions of the Soret bands (see the Supporting Information,
Figure S1). The wavelengths for emission maxima are listed in
Table 1. In accordance with the trend of the absorption maxi-
mum on the longest wavelength side, the emission maximum
of ZnPQI is red-shifted relative to that of ZnPQ.[20g,h] From the
intersection of normalized absorption and emission spectra,
the zero–zero excitation energy (E0–0) is determined to be
1.92 eV for ZnPQI, which is smaller than that of ZnPQ[20g,h]
(Table 1). The fluorescence lifetime (t) of ZnPQI was obtained
Figure 2. UV/Vis absorption spectra of ZnPQI (solid line) and ZnPQ[20g,h]
in dichloromethane by a time-correlated single-photon count-
(dashed line) measured in dichloromethane.
ing technique.[29] The excitation wavelength was 405 nm and
fluorescence was monitored at the strongest emission maxi-
sorption coefficients (e) of Soret and Q bands are listed in
Table 1. The Soret band of ZnPQI is split into two peaks and
becomes broader, and is shifted toward longer wavelengths
relative to that of ZnPQ. In addition, the Q bands are also red-
shifted relative to ZnPQ. The absorption properties of ZnPQI
are beneficial for harnessing solar energy in the visible region.
mum for ZnPQI. The decay curve of the fluorescence intensity
was single exponential and was fitted to give t=(1.25ꢁ
0.01) ns for ZnPQI, which is significantly longer than t=(0.99ꢁ
0.01) ns for ZnPQ.[20g,h] Because electron-injection processes
from the excited dyes to a conduction band (CB) of TiO2 take
place on a time scale of 0.1–100 ps,[30,31] the relatively slow
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DFT calculations
Table 1. Optical and electrochemical data for porphyrins and driving forces for electron transfer processes on
TiO2.
DFT calculations were employed
to gain insight into the equilib-
rium geometry and electronic
structures for the frontier orbi-
tals of the porphyrins. The cal-
culated structures do not show
negative frequencies, implying
that the optimized geometry is
in the global energy mini-
mum.[32] Notably, ZnPQI pos-
sesses a conjugated planar link-
age extending from the qui-
noxaline moiety to the phenyl-
ene ring attached to the imida-
zole moiety,[27b] whereas the
[a]
[b]
[c]
[c]
[e]
[f]
labs
e
lem
Eox
Ered
[V]
E0–0
[eV]
Eox*[d]
[V]
DGinj
[eV]
DGreg
[eV]
[nm]
[103 Mꢀ1 cmꢀ1
]
[nm]
[V]
ZnPQI
416
424
470
548
594
641
415
458
578
622
188
193
122
16.8
19.2
36.9
184
56.2
13.6
12.6
660
0.92
ꢀ1.11
ꢀ1.13
1.92
ꢀ1.00
ꢀ1.00
ꢀ0.50
ꢀ0.50
ꢀ0.42
ꢀ0.48
ZnPQ[g]
645
0.98
1.98
[a] Wavelengths for Soret and Q-band maxima in CH2Cl2. [b] Wavelengths for emission maxima in CH2Cl2 by ex-
citing at Soret wavelength. [c] First oxidation and reduction potentials (versus NHE). [d] Excited-state oxidation
potentials approximated from Eox and E0–0 (versus NHE). [e] Driving forces for electron injection from the por-
phyrin singlet excited state (Eox*) to the CB of TiO2 (ꢀ0.5 V versus NHE). [f] Driving forces for the regeneration
of porphyrin radical cation (Eox) by the Iꢀ/I3ꢀ redox couple (+0.5 V versus NHE). [g] From Ref. [20g].
2,4,6-trimethylphenyl
groups
are almost perpendicular to the
fluorescence decay of ZnPQI may have little influence on elec-
porphyrin core to avoid the steric congestion around the
meso-positions[20d] (see the Supporting Information, Figure S3).
However, on the basis of cyclic voltammetry measurements,
there is a possibility that ZnPQI possesses the orthogonal dis-
position of the phenylene spacer against the fused imidazole
moiety and therefore the low conjugation between the elec-
trons of the N atom and the aromatic cloud of the porphyrin,
as seen in triarylamino moieties that are introduced at the
meso-position of porphyrins for DSSCs.[33] Rotation of the phen-
ylene spacer against the imidazole moiety would have little in-
fluence on an energy level of the optimized structure by DFT
calculations.
tron-injection efficiency (f ).
inj
To determine the first oxidation potential (Eox) and the first
reduction potential (Ered) of ZnPQI in solution, differential pulse
voltammetry (DPV) measurements were performed in dichloro-
methane containing Bu4NPF6 (0.1m) as a supporting electrolyte
(Table 1 and also see the Supporting Information, Figure S2).
The Eox value of ZnPQI was 0.92 V versus the normal hydrogen
electrode (NHE) and is shifted significantly to a negative direc-
tion relative to ZnPQ (0.98 V versus NHE),[20g] but the Ered value
of ZnPQI (ꢀ1.11 V versus NHE) is almost similar to that of
ZnPQ (ꢀ1.13 V versus NHE).[20g] This implies that the introduc-
tion of the electron-donating group, including the triarylamino
and fused imidazole moieties, affects the HOMO of the porphy-
rin rather than the LUMO. This trend leads to a decrease in the
electrochemical HOMO–LUMO gaps. However, the difference in
HOMO levels of ZnPQI and ZnPQ is moderate, despite the
presence of the triphenylamino and fused imidazole moieties
in ZnPQI. Fusion of imidazole to b-positions of a porphyrin
ring does not contribute to the extension of the porphyrin p
system significantly.[27] Although DFT calculations predict the
coplanarity between the porphyrin core and the phenylene
spacer attached to the diarylamino moiety, the energy barrier
around the phenylene spacer against the fused imidazole
moiety would be small, resulting in an unexpectedly subdued
effect of p conjugation in ZnPQI (vide infra). The electrochemi-
cal HOMO–LUMO gap of ZnPQI is determined to be 2.03 eV,
which is considerably smaller than that of ZnPQ (2.11 eV).[20g]
The trend largely agrees with that of the optical HOMO–LUMO
gaps (vide supra).
Porphyrins with D4h symmetry generally reveal two energeti-
cally degenerate LUMOs (LUMO+1, LUMO) and two nearly de-
generate HOMOs (HOMO, HOMOꢀ1).[20] The degenerate
energy levels of LUMO and LUMO+1 are split in both ZnPQI
and ZnPQ[20g] (Table 2). Although the degenerate energy levels
of HOMO and HOMOꢀ1 are split significantly in ZnPQI, the
near degeneracy of HOMO and HOMOꢀ1 is retained for ZnPQ.
The theoretical HOMO–LUMO gaps of these porphyrins are as
follows: ZnPQ (2.69 eV)[20g] > ZnPQI (2.26 eV). This trend is in
Table 2. Molecular orbital energy levels for porphyrins.[a]
Orbital
Energy level [ev]
ZnPQI
ZnPQ[b]
LUMO+1
LUMO
ꢀ2.08
ꢀ2.23
ꢀ2.55
ꢀ2.56
(ꢀ3.33)[c]
ꢀ4.81
(ꢀ3.31)[c]
ꢀ5.25
HOMO
From spectroscopic and electrochemical measurements,
driving forces for electron injection from the porphyrin excited
singlet state to the CB of the TiO2 (ꢀ0.50 V versus NHE; DGinj)
and the regeneration of the porphyrin radical cation by the Iꢀ/
(ꢀ5.36)[c]
ꢀ5.05
(ꢀ5.42)[c]
ꢀ5.28
HOMOꢀ1
[a] Some sets of molecular orbital energy levels for porphyrins were esti-
mated by DFT calculations with B3LYP/6-31G(d). The energies in eV are
quoted with respect to the vacuum (1 Hartree=27.2116 eV). [b] From
Ref. [20g]. [c] The energy levels of HOMO and LUMO were determined
electrochemically, given that the absolute energy level of NHE is
ꢀ4.44 eV under vacuum.
ꢀ
I3 redox couple (ꢀ0.42 V versus NHE; DGreg) for the ZnPQI-
sensitized solar cell are determined (Table 1).[20] Both processes
are thermodynamically feasible and sufficient for efficient elec-
tron transfer (<ꢀ0.3 eV; vide infra).
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good agreement with those of the optical and electrochemical
HOMO–LUMO gaps (vide infra).
Figure 3 illustrates the electron-density distributions of
ZnPQI and ZnPQ in their respective HOMOs and LUMOs. The
electron-density distribution of LUMOs around an anchoring
Figure 4. Plots of porphyrin surface density (G) as a function of immersion
time for ZnPQI (solid line with closed circles) and ZnPQ[20g,h] (dashed line
with open circles), which adsorb on TiO2 films without the scattering layer.
1.1ꢂ10ꢀ10 molcmꢀ2. Taking into account the good agreement
between calculated and experimental G values and the saturat-
ed adsorption behavior of ZnPQI, ZnPQI forms nearly densely
packed monolayers on TiO2.
Attenuated total reflectance Fourier transform infrared (ATR-
FTIR) spectroscopy is a useful tool for gaining information on
the binding mode of the molecules adsorbed on TiO2 sub-
strates.[19,20] The ATR-FTIR spectrum of ZnPQI obtained from
a solid sample reveals the characteristic band of n˜(C=O) of the
carboxylic acid group at around 1700 cmꢀ1 (see the Supporting
Information, Figure S4a),[19,20] which disappears in spectra of
TiO2/ZnPQI (see the Supporting Information, Figure S4b).
ATR-FTIR spectra of TiO2/ZnPQI exhibit a marked increase in
the symmetric carboxylate band, n˜(COOsꢀ), at around
1400 cmꢀ1.[19,20] The disappearance of n˜(C=O) and the increased
intensities of n˜(COOsꢀ) corroborate the assumption that
a proton is detached from the carboxylic acid group during ad-
sorption of the porphyrin on the TiO2 surface, leading to the
bidentate binding of the carboxylate group to the TiO2 surface.
This is consistent with the previous assignment that a carboxyl-
ic acid of analogous porphyrins is bound to a TiO2 surface
through a bridging bidentate mode.[19,20,22a]
Figure 3. Sets of molecular orbital diagrams for (a) ZnPQI and (b) ZnPQ[20d]
obtained by DFT calculations with B3LYP/6-31G(d).
group influences the electronic coupling between the excited
adsorbed dye and the 3d orbital of TiO2.[34] The quinoxaline
group, including the anchoring moiety, possesses slightly
smaller electron densities in the LUMO of ZnPQI (29.8%) rela-
tive to those in the LUMO of ZnPQ (32.8%). Accordingly, we
can anticipate the efficient f value of ZnPQI from the por-
inj
phyrin excited singlet state to the CB of TiO2, as in the case of
ZnPQ[20g,h] (vide infra). On the other hand, the electron density
of the HOMO is mainly distributed in the triarylamine-substitut-
ed imidazole moiety, which is at the opposite side of the fused
quinoxaline moiety. These results are consistent with the ex-
pected push–pull electronic structure of ZnPQI.
Porphyrin adsorption on TiO2
Photovoltaic properties of porphyrin-sensitized TiO2 cells
The TiO2 electrodes were immersed into solutions of ZnPQI in
ethanol (0.2 mm) to yield the porphyrin-stained TiO2 elec-
trodes. Total amounts of the porphyrins adsorbed on TiO2 films
were determined by measuring the difference in absorbance
of the TiO2 films without the scattering layer before and after
immersion of the TiO2 film into the porphyrin solution in etha-
nol. After immersion, ZnPQI in EtOH reached almost saturated
surface coverage (G) on the TiO2 films in 4 h (Figure 4). The
trend is similar to our previous report on the G value of ZnPQ
on TiO2 as a function of immersion time,[20d,f,g] but the adsorp-
tion speed of ZnPQI is slower than that of ZnPQ, which agrees
with the small electron density of the quinoxaline group in-
cluding the anchoring moiety in the LUMO of ZnPQI relative to
ZnPQ.[20] The saturated G value of ZnPQI on the TiO2 film is de-
termined to be about 1.0ꢂ10ꢀ10 molcmꢀ2, which is compara-
ble to that of ZnPQ (1.1ꢂ10ꢀ10 molcmꢀ2). Assuming that the
porphyrin monolayer is densely packed and vertically orientat-
ed to the TiO2 surface, the G value of ZnPQI is calculated to be
We evaluated the performance of TiO2/ZnPQI and TiO2/ZnPQ
solar cells and drew comparisons between them. The TiO2 elec-
trode was sensitized with ZnPQI by immersing the TiO2 elec-
trode into the porphyrin ethanol solution for 1–8 h. The h
value is derived from the equation: h=JSC ꢂVOC ꢂff, where JSC is
the short circuit current, VOC is the open circuit potential, and ff
is the fill factor. The h value of the TiO2/ZnPQI solar cell reveals
the immersion-time dependency (Figure 5), as observed in the
immersion-time dependence of h values of TiO2 solar cells sen-
sitized with ZnPQ.[20g,h] After reaching the maximum h value in
2 h, the h value is decreased slightly with an increase in im-
mersion time, which can be rationalized by an increase in por-
phyrin aggregation with increasing immersion time.[19,20,31b] The
difference in immersion time for maximal h and G values can
be explained as a trade-off between the effect of the extent of
porphyrin adsorption and that of porphyrin aggregation,
which would adversely affect h values. Notably, the h value of
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than that of the ZnPQ-sensitized cell, leading to the overall im-
proved cell performance of ZnPQI.
Photocurrent action spectra of TiO2/ZnPQI+CDCA and TiO2/
ZnPQ+CDCA cells were also compared (Figure 7). Note that
the maximal incident photon-to-current efficiency (IPCE) values
Figure 5. Plots of h as a function of immersion time for ZnPQI- (solid line
with closed circles) and ZnPQ-sensitized[20g,h] (dashed line with open circles)
TiO2 cells. The porphyrins were adsorbed on the TiO2 electrodes by immers-
ing them into a solution of ZnPQI or ZnPQ (0.2 mm) in EtOH without co-ad-
sorbent.
the ZnPQI solar cell (5.5%) is comparable to that of the ZnPQ
cell (5.4%), but the degree of decrease in the h value of the
ZnPQI solar cell as a function of immersion time is smaller than
that of the ZnPQ solar cell, probably due to the larger steric
hindrance of the ZnPQI molecule.[20g,h]
Figure 7. Photocurrent action spectra of the TiO2/ZnPQI+CDCA cell (solid
line) and TiO2/ZnPQ+CDCA cell[20g,h] (dashed line).
at Soret bands are 83% for the TiO2/ZnPQI+CDCA cell and
75% for the TiO2/ZnPQ+CDCA cell.[20g,h] The photocurrent
action spectra follow the absorption features of the corre-
sponding porphyrin adsorbed on the electrodes (Figure 8),
To further optimize cell performance, ZnPQI (0.2 mm) was
adsorbed onto the TiO2 surface for 2 h in EtOH, in which che-
nodeoxycholic acid (CDCA, 0.6 mm) was added as co-adsorb-
ent to reduce porphyrin aggregation on TiO2. Figure 6 depicts
Figure 8. Light-harvesting efficiencies of the TiO2/ZnPQI+CDCA (solid line)
and TiO2/ZnPQ+CDCA[20g,h] (dashed line) electrodes. The scattering TiO2
layers were not applied to TiO2 electrodes to measure absorbance
accurately.
Figure 6. Photocurrent–voltage characteristics of the TiO2/ZnPQI+CDCA cell
(solid line; h=6.8%, JSC =13.9 mAcmꢀ2, VOC =0.68 V, ff=0.71), the TiO2/
ZnPQI cell (dashed line; h=5.5%, JSC =11.2 mAcmꢀ2, VOC =0.67 V, ff=0.74),
and the TiO2/ZnPQ+CDCA cell[20g,h] (dotted line; h=6.3%,
J
SC =13.2 mAcmꢀ2, VOC =0.71 V, ff=0.67). Electrolyte: 1,3-dimethylimidazoli-
um iodide (1.0m), I2 (0.03m), LiI (0.05m), guanidinium thiocyanate (0.1m),
and 4-tert-butylpyridine (0.50m) in acetonitrile/valeronitrile (85:15); input
power: AM 1.5 under simulated solar light (100 mWcmꢀ2).
demonstrating that the porphyrin is the main source for pho-
tocurrent generation. IPCE is divided into three components
according to Equation (1):
the photocurrent–voltage characteristics of TiO2/ZnPQI and
TiO2/ZnPQI+CDCA solar cells under the respective maximal h
conditions. Co-adsorption of CDCA had a large impact on the
performance of the TiO2/ZnPQI+CDCA cell. Under the opti-
mized conditions the TiO2/ZnPQI+CDCA cell exhibits: h=6.8%
with JSC =13.9 mAcmꢀ2, VOC =0.68 V, and ff=0.71. It should be
noted that this value is larger than that of the TiO2/
ZnPQ+CDCA cell[20g,h] under optimized conditions (h=6.3%,
JSC =13.2 mAcmꢀ2, VOC =0.71 V, ff=0.67). The short-circuit cur-
rent and fill factor of the ZnPQI-sensitized solar cell are larger
than those of the ZnPQ-sensitized solar cell, whereas the
open-circuit potential of the ZnPQI-sensitized cell is smaller
IPCE ¼ LHE ꢂ ꢀinj ꢂ hcol
ð1Þ
where LHE (light-harvesting efficiency) corresponds to the
number of absorbed photons per number of incident photons,
f
inj
is the quantum yield for electron injection from the por-
phyrin excited singlet state to the CB of the TiO2 electrode,
and hcol is the efficiency of charge collection. It should be
noted that the TiO2/ZnPQI+CDCA cell had superior IPCE values
at wavelengths of 650–750 nm (Figure 7). Considering the
effect of the light-scattering TiO2 layer, the LHE value of the
TiO2/ZnPQI+CDCA cell is larger than that of the TiO2/
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ZnPQ+CDCA cell, especially at 650–750 nm (Figure 8), leading
to the improvement in IPCE values at 650–750 nm. Although
the IPCE values of the TiO2/ZnPQI+CDCA cell at 500–650 nm
are lower than those of the TiO2/ZnPQ+CDCA cell, the higher
IPCE values of the TiO2/ZnPQI+CDCA cell at 380–500 and 650–
750 nm result in overall high JSC values of the TiO2/ZnPQI+CD-
CA cell relative to the TiO2/ZnPQ+CDCA cell.
showed improved light-harvesting properties relative to the
reference porphyrin. This push–pull porphyrin-sensitized solar
cell exhibited a relatively high power conversion efficiency of
6.8%, which is larger than that of the reference porphyrin-sen-
sitized solar cell (6.3%). Further improvement may be possible
by introducing other electron-donating groups at the opposite
side of the carboxyquinoxalino anchoring group in an effective
p-conjugated manner and/or utilizing CoII/III tris(bipyridyl)-
based redox electrolytes, which are known to increase VOC rela-
The VOC value of the TiO2/ZnPQI+CDCA cell is slightly lower
than that of the TiO2/ZnPQ+CDCA cell. To elucidate the reason
for this, we measured current–voltage characteristics under
dark conditions (Figure 9). The onset for the TiO2/ZnPQI+CDCA
ꢀ
tive to conventional Iꢀ/I3 redox electrolytes.[23h] Such funda-
mental information on electron-donating groups in porphyrin
DSSCs will be useful for the rational molecular design of highly
efficient DSSCs based on push–pull type porphyrins.
Experimental Section
Synthesis
ZnPQI was synthesized following the protocol developed by Cross-
ley et al.[26] and by our group.[20g,h] Detailed synthetic routes to the
target compound and the intermediates are described in the Sup-
porting Information.
Figure 9. Current–voltage characteristics of the TiO2/ZnPQI+CDCA (solid
line) and the TiO2/ZnPQ+CDCA[20g,h] (dashed line) cells under dark condi-
tions.
Spectroscopy
UV/Vis absorption spectra of the porphyrins in dichloromethane
and the porphyrin monolayers on TiO2 electrodes without a light-
scattering layer (vide infra) were recorded using a Perkin–Elmer
Lambda 900 UV/VIS/NIR spectrometer. Steady-state fluorescence
spectra were acquired by using a SPEX Fluoromax-3 spectrofluor-
ometer. A time-correlated single photon counting (TCSPC) method
was used for the time-resolved fluorescence measurements in the
nanosecond and sub-nanosecond time-scale and time resolution
was approximately 60–70 ps (full width at half maximum).[29] The
excitation wavelength was 405 nm and the emission decay was
monitored at 658 nm. ATR-FTIR spectra were recorded with the
golden gate diamond anvil ATR accessory (NICOLET 6700, Thermo
Scientific), using typically 256 scans at a resolution of 2 cmꢀ1. All
samples were placed in contact with the diamond window using
the same mechanical force.
cell appears at a less positive value than that of the TiO2/
ZnPQ+CDCA cell. This indicates that the degree of charge re-
combination between the injected electrons in the CB of TiO2
ꢀ
and I3 for the TiO2/ZnPQI+CDCA cell is higher than that for
the TiO2/ZnPQ+CDCA cell, although the electron densities in
the HOMO of ZnPQI are anticipated to be far from the TiO2 sur-
face relative to ZnPQ obtained from DFT calculations.[35] This
result can be explained plausibly by the more tilted geometry
of ZnPQI relative to ZnQP on the TiO2 surface, as we have pro-
posed previously.[5d,31b] It should be also pointed out that
longer dyes tend to be tilted on TiO2 surfaces when they are
adsorbed.[31b] Given that the hcol primarily depends on the rela-
tive rates of charge transport against charge recombination,
the higher charge recombination rate for the TiO2/ZnPQI+CD-
CA cell relative to the TiO2/ZnPQ+CDCA cell may also account
for the feasible lower charge-collection efficiency, resulting in
the slightly lower VOC value.[36] At present, it is unclear why the
ff value of ZnPQI-sensitized solar cells is slightly larger than
that of ZnPQ-sensitized solar cells and this may stem from the
relatively small cell serial resistances including electron-trans-
port resistances in the TiO2 film.
Electrochemistry
Electrochemical measurements were performed using an ALS 630a
electrochemical analyzer. Redox potentials of the porphyrins were
determined by DPV analysis in dichloromethane containing
Bu4NPF6 (0.1m) as a supporting electrolyte. A glassy carbon work-
ing electrode (3 mm in diameter), an Ag/AgNO3 (0.01m in acetoni-
trile) reference electrode, and a Pt wire counter electrode were em-
ployed. Ferrocene/ferrocenium (+0.642 V versus NHE) was used as
an internal standard for all measurements. All of the measured po-
tentials were converted to the NHE scale.
Conclusions
We have synthesized a push–pull porphyrin with an electron-
donating triarylamino group at the b,b’-edge through the
fused imidazole group and an electron-withdrawing fused car-
boxyquinoxalino anchoring group at the opposite b,b’-edge to
address the substitution effects of the electron-donating group
on the photovoltaic properties. This push–pull porphyrin
DFT calculations
Geometry optimization and electronic structure calculations of the
porphyrins were performed using the B3LYP functional and the 6-
31G(d) basis set implemented in the Gaussian 03 program pack-
age.[37] Molecular orbitals were visualized by using the Molstu-
dio 3.0 software.
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with sandpaper. A solder was applied on each edge of the FTO
electrodes. The electrolyte solution used consisted of 1,3-dimethyl-
imidazolium iodide (1.0m), I2 (0.03m), LiI (0.05m), guanidinium thi-
ocyanate (0.1m), and 4-tert-butylpyridine (0.50m) in acetonitrile/va-
leronitrile (85:15).
IPCE and photocurrent–voltage (J–V) performance were measured
by means of an action-spectrum-measurement setup (CEP-2000RR,
Bunkoukeiki) and a solar simulator (PEC-L10, Peccell Technologies)
with a simulated sunlight of AM 1.5 (100 mWcmꢀ2), respectively:
IPCE (%)=100ꢂ1240ꢂi/(Winꢂl), where i is the photocurrent densi-
ty [Acmꢀ2], Win is the incident light intensity [Wcmꢀ2], and l is the
excitation wavelength [nm]. During the photovoltaic measure-
ments, a black plastic mask was attached on the back of the TiO2
electrode, except for the TiO2 film region, to reduce scattering
light.
Preparation of the porphyrin-sensitized TiO2 electrode and
photovoltaic measurements
The preparation of TiO2 electrodes and the fabrication of sealed
cells for photovoltaic measurements were performed following
a previously reported method.[38] Nanocrystalline TiO2 particles (d=
20 nm, CCIC:PST18NR, JGC-CCIC, and d=30 nm, CCIC:PST30NR,
JGC-CCIC) were used as the transparent layer of the photoanode,
whereas
sub-microcrystalline
TiO2
particles
(d=400 nm,
CCIC:PST400C, JGC-CCIC) as the light-scattering layers of the pho-
toanode. The working electrode was prepared by cleaning a fluo-
rine-doped tin (FTO) glass (Solar, thickness of 4 mm, 10 W/ ,
&
Nippon Sheet Glass) with a detergent solution in an ultrasonic
bath for 10 min, rinsing with distilled water, ethanol, and air-
drying. The electrode was subjected to UV–O3 irradiation for
18 min, immersed into a solution of freshly prepared aqueous TiCl4
(40 mm) at 708C for 30 min, washed consecutively with distilled
water and ethanol, and dried. Nanocrystalline TiO2 paste was
coated onto the FTO glass by screen printing, followed by standing
in a clean box for a few minutes and dried at 1258C for 6 min, and
then repeating the process to attain a final thickness of 12 mm.
A layer of the sub-microcrystalline TiO2 paste (4 mm) was deposited
in the same fashion as the nanocrystalline layer. Finally, the elec-
trode was heated under an airflow at 3258C for 5 min, at 3758C for
5 min, at 4508C for 15 min, and at 5008C for 15 min. The thickness
of the films was determined using a surface profiler (Surfcom 130A,
Accretech). The size of the TiO2 film was 0.16 cm2 (4ꢂ4 mm). The
TiO2 electrode was then subjected to immersion into a solution of
freshly prepared aqueous TiCl4 (40 mm) at 708C for 25 min before
rinsing with distilled water and ethanol, and air-drying. The elec-
trode was sintered at 5008C for 30 min, cooled to 708C, and im-
mersed into the dye solution at 258C in the dark for the prescribed
times. The TiO2 electrode was immersed into a solution of porphy-
rin in ethanol (0.20 mm). The TiO2 electrode stained with ZnPQI is
denoted as TiO2/ZnPQI. To reduce dye aggregation on TiO2, the
TiO2 electrode was also immersed into a solution of porphyrin in
ethanol (0.20 mm) containing CDCA (0.60 mm) as co-adsorbent.
The TiO2 electrode stained with ZnPQI and CDCA is denoted as
TiO2/ZnPQI+CDCA. The porphyrin surface coverage adsorbed on
TiO2 films (molcmꢀ2) was determined by measuring the difference
in absorbance of the TiO2 films (TiO2 area of 1.0 cm2 with a thick-
ness of 12 mm) without the scattering layer before and after im-
mersion of the TiO2 film in the porphyrin solution, assuming that
the molar absorption coefficient in dichloromethane is identical to
that on TiO2.
Acknowledgements
This work was supported by the Grant-in-Aids (MEXT, Japan, No.
21350100 to H.I.), the Strategic Japanese-Finnish Cooperative Pro-
gram (JST), the Advanced Low Carbon Technology Research and
Development Program (ALCA, JST), and the WPI Initiative (MEXT,
Japan). H.H. would also wish to thank a JSPS for a fellowship for
young scientists. H.S., N.T., and H.L. would like to thank the Acad-
emy of Finland for financial support.
Keywords: dyes/pigments · electrochemistry · porphyrinoids ·
sustainable chemistry · synthesis design
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Received: November 14, 2012
Published online on && &&, 0000
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ChemSusChem 0000, 00, 1 – 11
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These are not the final page numbers!

FULL PAPERS
H. Hayashi, A. S. Touchy, Y. Kinjo,
K. Kurotobi, Y. Toude, S. Ito, H. Saarenpꢀꢀ,
N. V. Tkachenko, H. Lemmetyinen,
H. Imahori*
&& – &&
Triarylamine-Substituted Imidazole-
and Quinoxaline-Fused Push–Pull
Porphyrins for Dye-Sensitized Solar
Cells
Pushing and pulling: A push–pull por-
phyrin with an electron-donating and
an electron-withdrawing group at oppo-
site sites has been synthesized to evalu-
ate the effect of the push–pull structure
on optical, electrochemical, and photo-
voltaic properties. This system exhibited
a power conversion efficiency that is
higher than that of the dye without the
electron-donating group.
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 11
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These are not the final page numbers!