Dye molecules for simple co-sensitization process: Fabrication of mixed-dye-sensitized solar cells

Article abstract of DOI:10.1002/anie.201108610

Sensitive kind of dye: Co-sensitization of the TiO₂ electrode using PcS15 and the dye D131 results in a dramatic enhancement of the photocurrent response for the entire visible-light region. This method provides a simple design for accessing dye-sensitized solar cells. Copyright

Full text of DOI:10.1002/anie.201108610

Angewandte
Chemie
DOI: 10.1002/anie.201108610
Energy Conversion
Dye Molecules for Simple Co-Sensitization Process: Fabrication of
Mixed-Dye-Sensitized Solar Cells**
Mutsumi Kimura,* Hirotaka Nomoto, Naruhiko Masaki, and Shogo Mori*
Dye-sensitized solar cells (DSSCs) made from mesoporous
TiO2 electrodes have been intensely investigated as a promis-
ing candidate for low-cost, lightweight, and scalable solar
cells.[1] To date, the highest conversion efficiencies above 11%
have been achieved with panchromatic polypyridylruthenium
complex sensitizers.[2] The high efficiency of the ruthenium
complexes can be attributed to their wide absorption range
from the visible to near-infrared (IR) regions.[3] However, the
issue with the ruthenium complexes is the difficulty of further
improvement in the conversion efficiencies, because of their
low molar extinction coefficients (e.g., e < 10000mÀ1 cmÀ1 at
the metal to ligand charge-transfer (MLCT) band of black
dye) and the limited use of precious ruthenium metal for the
practical applications. To avoid these issues, much effort has
been devoted to the development of organic dye molecules
which possess no metal or utilize nonprecious metals.[4] So far,
the conversion efficiencies of DSSCs with organic dye
molecules are modest compared to those with the ruthenium
complexes, partially because of their insufficient light-har-
vesting ability in the near-IR region.
Metallophthalocyanines (MPcs) show potential for near-
IR sensitizers because of their intense Q bands (l = 600–
700 nm), high molar extinction coefficients (e >
100000mÀ1 cmÀ1), and good thermal, chemical, and photolytic
stabilities.[5] Nevertheless, MPc sensitizers displayed rather
low conversion efficiencies in DSSCs. Major factors for the
low efficiency of MPcs were the formation of aggregates on
the surface of the TiO2 crystal and the lack of electron-
transfer directionality in the excited state. In 2007, Nazeer-
uddin et al. and Torres et al. reported high conversion
efficiencies (h = 3.1 and 3.6%) for DSSCs using unsymmet-
rical the zinc phthalocyanines (ZnPcs) PCH001 and TT1,
which have three tert-butyl groups and one carboxylic acid
moiety.[6–8] Unsymmetrical substitution of ZnPcs provides
electron-transfer directionality, thus giving high incident
photon-to-current conversion efficiency (IPCE) in the near-
IR region, whereas little sensitization was observed in the
blue and green spectral regions.
To exploit such dyes, co-sensitization with different dyes
(cocktail-type) has been applied.[9] The combination of two
dyes, which have complementary absorption properties in the
visible region, could result in broad responses to the solar
spectrum, thus improving conversion efficiencies. However,
mixing two dyes usually resulted in a decrease of efficiency,
which is probably due to the decreased injection efficiency
caused by intermolecular interactions between the two dyes.
Thus, prevention of electronic interactions between the dyes
on the TiO2 surface is important for the development of
efficient cocktail-type DSSCs. A simple way is to separate the
adsorption sites on the TiO2; various methods have been
proposed, for example, sensitization using two separate
layers,[10,11] and sensitization using a double dye layer.[12]
However these methods require complex processes. Another
way is to use a co-adsorbant, and as in the case for the TT1
dye, which was co-sensitized with the organic red dye JK2 to
cover the blue and green regions.[8] The efficiency of the TT1/
JK2 cocktail-type DSSC was higher than that of the DSSCs
using each dye separately. While the co-adsorbant, cheno-
deoxycholoc acid (CDCA), was used to suppress the aggre-
gation of ZnPcs and to suppress interaction between TT1 and
JK2, complete suppression of aggregation by CDCAwas hard
to achieve.
Recently, we reported the photovoltaic properties of the
unsymmetrical ZnPc PcS6, which has one benzoic acid on one
side of the macrocycle and six bulky 2,6-diphenylphenol
groups on the other positions.[13] Three-dimensional (3D)
enlargement of the molecular structure prevented the cofacial
aggregation of ZnPcs adsorbed onto the TiO2 surface and
PcS6 gave a high efficiency of 4.6% without the addition of
a coadsorbent. Such non-interacting features of the dye would
be desirable for cocktail-type DSSCs. The introduction of
electron-donating phenoxy groups onto a ZnPc core led to
significant improvement of the IPCE for the entire absorption
range. The IPCE could be further improved by adding greater
directional electron-transfer features (pushing ability).
Herein, we report the novel ZnPc dye PcS15 having
a 5.3% efficiency, which is achieved by enhancing the
unsymmetrical and 3D ZnPc structure. The introduction of
electron-donating methoxy groups to the peripheral 2,6-
diphenylphenol units increased the photoresponse in the l =
400–500 nm range and improved the solubility. We then
applied PcS15 to the cocktail-type DSSCs through combina-
tion with red or orange organic dyes, D102 and D131,
respectively. The co-sensitization of the TiO2 electrode by
PcS15 and D131 showed little interaction between the dyes,
thus resulting in a dramatic enhancement of the photocurrent
response throughout the entire visible-light region.
[*] Prof. M. Kimura, H. Nomoto, Dr. N. Masaki, Prof. S. Mori
Division of Chemistry and Materials
Faculty of Textile Science and Technology
Shinshu University, Ueda 386-8567 (Japan)
E-mail: mkimura@shinshu-u.ac.jp
shogmori@shinshu-u.ac.jp
[**] This work has been supported by Grants-in-Aid for Scientific
Research (B) (No. 22350086) from the Japan Society for the
Promotion of Science (JSPS).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108610.
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Scheme 1 illustrates the synthesis of the ZnPc PcS15. The
phthalocyanine precursor 3 was synthesized from 4-methoxy-
phenol (R1 = OMe) in five steps. Reaction of 4-methoxyphe-
nol with tetrabutylammonium tribromide in a CH2Cl2/meth-
anol readily gave 2,6-dibromo-4-methoxyphenol.[14] After the
(see Figure S4 in the Supporting Information). The preven-
tion of intermolecular aggregation leads to high solubility and
single-site isolation in the respective physical environments.
Whereas PcS6, possessing 2,6-diphenylphenoxy units, did not
dissolve in toluene, the introduction of methyl and methoxy
groups to the 2,6-diphenylphenoxy units improved the
solubility in toluene. The absorption spectrum of PcS15 in
toluene showed a sharp and strong peak at lmax = 686 nm
(loge = 4.97) and a relatively weak peak at lmax = 360 nm as
the Q band and the Soret band of ZnPcs.[16] The synthesized
ZnPc exhibited a broad absorption band around l = 450 nm,
which was attributed to the intramolecular charge transfer
between the ZnPc core and the electron-donating phenoxy
units.[16] When PcS15 was excited at the Soret band of the
ZnPc core in a degassed toluene solution, it exhibited a strong
fluorescence at l = 702 nm. The Q bands, Soret bands, and the
emission maxima of the ZnPcs remained unaltered for PcS13,
PcS14, and PcS15, each of which possess different peripheral
units, thus suggesting that the electronic conditions in all
ZnPcs were similar to each other.
The porous TiO2 electrodes were immersed in toluene
solutions of PcS13–15 to obtain the phthalocyanine-stained
TiO2 electrode. The absorption spectrum of PcS15 adsorbed
on a 4 mm TiO2 film shows a sharp Q band at l = 699 nm and
the intensity ratio between the peaks at l = 699 and 624 nm
was in fare agreement with that in toluene solution, thus
revealing the prevention of molecular aggregation among
ZnPc cores within the adsorbed PcS15 monolayer on the TiO2
surface (see Figure S5 in the Supporting Information).[16] The
fluorescence of adsorbed PcS15 was completely quenched,
thereby implying an efficient electron transfer from the
excited PcS15 to TiO2. The molecular enclosure of a ZnPc
core with six bulky 2,6-diphenylphenoxy units in PcS13–15
enables steric isolation of the photoactive ZnPcs within the
adsorbed monolayer on the TiO2 surface.
The DSSC performance of PcS13–15 was examined using
double-layered TiO2 electrodes. The TiO2 electrodes were
immersed in toluene solutions of PcS13–15 for 48 hours at
258C. Figure 1a shows the photocurrent density-voltage
curve of the DSSCs using PcS15-adsorbed TiO2 electrodes
under a standard global AM 1.5 solar condition and Table 1
shows the short-circuit current density (Jsc), open-circuit
voltage (Voc), fill factor (FF), and overall power conversion
efficient (PCE) of PcS6 and PcS13–15. The PcS15 cell showed
the highest PCE value of 5.3% among the ZnPcs. This
efficiency is the highest for the DSSCs using ZnPcs as a light-
harvesting dye. Whereas the Voc values of the PcS6 and PcS15
cells were the same, the Jsc value for the PcS15 cell was higher
than that of PcS6 cell.
Scheme 1. Synthesis of PcS13–15. Reagents and conditions:
a) nBu4NBr3, CH2Cl2/MeOH (5:2); b) (CH3CO)2O, pyridine; c) Suzuki
coupling, [Pd(PPh3)4], phenyl boronic acid, toluene/THF/2m Na2CO3
(aq.); d) aq NaOH; e) 4,5-dichlorophthalonitirile, K2CO3, DMF; f) 4,
Zn(CH3COO)2, DMAE, reflux; g) aq. NaOH. See the Supporting
Information for details.
acetylation of the phenol group, a palladium-catalyzed
coupling reaction between the 2,6-dibromophenol derivatives
and phenylboronic acid gave 2,6-diphenyl-4-methoxyphenol
(R1 = OMe, R2 = H), which was was reacted with 4,5-
dichlorophthalonitrile in the presence of K2CO3 to afford
4,5-di(2’,6’-diphenyl-4’-methoxyphenoxy)phthalonitirile
(3).[15] The unsymmetrical ZnPc PcS15 was prepared through
a mixed cyclo-tetramerization reaction of 3 and 4 in a 3:1 ratio
by heating to reflux in 2-(dimethylamino)ethanol (DMAE) in
the presence of Zn(OAc)2, and subsequent hydrolysis with an
aqueous alkaline solution. The other ZnPcs, PcS13 and PcS14,
were synthesized from 1 and 2 by the same procedure of
PcS15.
The IPCE spectrum of the PcS15 cell was similar to the
absorption feature of PcS15 adsorbed onto the TiO2 electrode
(Figure 1b). The onset of the IPCE spectrum was at l =
800 nm and the maximum IPCE was 72% at l = 600–
720 nm, which correspond to the Q band of ZnPc. The
IPCE was higher than those values obtained with the
previously reported PcS6 in all visible-light regions (see
Figure S7 in the Supporting Information). Noticeably, the
IPCE values for the PcS15 cell in the region between l = 400
and 520 nm was significantly increased by the introduction of
The peripheral 2,6-diphenyl-4-methoxyphenoxy units in
PcS15, which are perpendicular to a planar phthalocyanine
ring, introduces severe steric crowding adjacent to the
phthalocyanine core, thus avoiding cofacial self-association
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Chemie
Figure 1. a) Photocurrent-voltage curve obtained with the DSSC
derived from PcS15 under a standard global AM 1.5 solar conditions
(solid line) and dark current (dotted line). b) IPCE spectrum for DSSC
based on PcS15.
Figure 2. a) Optical images of PcS15, PcS15/D131, and PcS15/D102
cells. b) Photocurrent–voltage curve obtained with DSSCs based on
PcS15/D131 (orange line: PCE=6.2%, Jsc =15.3 mAcmÀ2
Table 1: Photovoltaic performance of DSSCs based on PcS6 and
PcS13-15, compared to N719 dye.[a]
,
Dyes
Voc [mV]
Jsc [mAcmÀ1
]
FF
PCE [%]
V
oc =630 mV, FF=0.64) and PcS15/D102 (red line: PCE=5.6%,
Jsc =13.9 mAcmÀ2, Voc =625 mV, FF=0.64) under a standard global
AM 1.5 solar conditions and dark currents (dotted lines). b) ICPE
spectrum for DSSCs based on PcS15/D131 (orange line) and PcS15/
D102 (red line).
PcS6
610
610
600
610
640
11.0
10.9
11.5
12.8
15.3
0.70
0.70
0.70
0.68
0.68
4.7
4.7
4.8
5.3
6.7
PcS13
PcS14
PcS15
N719
[a] See the Supporting Information for details.
darkened as shown in Figure 2a (middle and left). The DSSC
performance of the cocktail-type PcS15/D102 and PcS15/
D131 cells were significantly enhanced in comparison to that
of the PcS15 cell. Figure 2b shows the photocurrent density-
voltage curve of the cocktail-type DSSCs adsorbed TiO2
electrodes. The cocktail-type PcS15/D102 and PcS15/D131
cells showed 17 and 28% increase, respectively, in the
Jsc values, compared to that of the PcS15 cell. Furthermore,
the Voc values of cocktail-type DSSCs were slightly increased.
The IPCE spectra of the PcS15/D102 and PcS15/D131
cells were a summation of the IPCE response for each of the
dyes and the valley of IPCE response for the PcS15 cell filled
with the IPCE responses of D102 and D131 (Figure 2c). The
IPCE values at the near-IR region were not decreased by the
co-sensitization, thus indicating that PcS15 did not interact
with the organic dyes on the TiO2 surface. More than 80%
IPCE for the PcS15/D102 cell also suggests that there was no
effect on the injection efficiency of the organic dyes by the co-
sensitization. The bulky moieties attached to the phthalocya-
nine ring were to prevent the aggregation among the Pc dyes
but the moieties were also effective in preventing the
interaction with other dyes. Probably the key point of the
design is that the nonconjugated bond between the Pc core
and the bulky outer moiety. This is basically the same idea for
reducing the intermolecular forces as a result of the induced
dipole in the molecules, that is, the insertion of the bulky
moiety increases the distance between the conjugated frame-
works of the two molecules.[13]
the methoxy groups to the periphery of the molecule. The
PCE value and the IPCE spectrum for the PcS13 cell were
almost the same as those of the PcS6 cell, thus revealing that
the introduction of methoxy groups onto the outer benzene
rings of the molecule was not effective in enhancing the DSSC
performance. The PcS14 cell exhibited a slight improvement
in the PCE value compared to that of the PcS6 cell. Judging
from the Hammett sigma constants of methyl and methoxy
groups (s
: À0.170 and s
: À0.268),[18] the electron-
ÀCH3
ÀOCH3
pushing ability is on the order of -OCH3 >-CH3 > H, which
coincides with the overall performance of the device.
The PcS15-adsorbed TiO2 electrode showed a vivid green
color (Figure 2a, left). The green coloration of the PcS15 cell
resulted in a lack of light harvesting in the l = 480–600 nm
region. The IPCE spectrum of the PcS15 cell showed a dip in
the IPCE response at around l = 530 nm, where the IPCE
value decreased to 30% (Figure 1b). To compensate for the
low IPCE around l = 500 nm, co-sensitization of PcS15 with
either the red or orange dyes D102 and D131, respectively
(Mitsubishi Paper Mill),[19] was attempted. The absorption
maxima of D102 and D131 in solution are l = 491 and 425 nm,
respectively. The surface of the TiO2 electrode was initially
covered with PcS15 by immersing the TiO2 electrode into
a solution of PcS15 for 48 hours, followed by immersion into
the solution of either D102 or D131 for 2 hours. The color of
the resulting cells covered with the mixed dyes became
Yella et al. reported the highest efficiency (above 12%)
for the porphyrin-sensitized DSSCs by using cobalt (II/III)
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redox electrolytes.[20] The PcS15 cell showed a low efficiency,
less than 1% with cobalt redox electrolytes, because of the
lack of a barrier to the approach of the redox species on the
TiO2 surface. We are now investigating the ZnPc-based
sensitizers having an effective barrier to obtain higher
Voc values in DSSCs. In addition, since the absorption range
of phthalocyanines can shift to near l = 900 nm by the
expansion of p conjugation of the phthalocyanine rings, the
expansion of spectral response up to l = 900 nm in the
cocktail-type DSSCs is underway by using highly efficient
naphthalocyanine derivatives.[21]
In conclusion, the photovoltaic performance of DSSCs
based on near-IR-absorbing ZnPcs is enhanced by tuning of
electron-pushing abilities of the peripheral substituents
around the ZnPc core and the sterically isolated ZnPc
PcS15 exhibited a 5.3% efficiency when used as a light-
harvesting dye on a TiO2 electrode under standard solar
conditions. The dye was then used for co-sensitization with
other organic dyes that have a complementary spectral
response. The sensitization process involved immersion in
one dye bath after another. Without the coadsorbant, the two
dyes on the TiO2 showed high IPCE, thus suggesting that the
bulky peripheral moiety attached to the framework of the
dyes through a nonconjugated bond was effective in prevent-
ing intermolecular interactions between the different dyes.
Such a molecular design provides a simple sensitization
process for co-sensitized solar cells.
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