Enhanced Charge Separation Efficiency in Pyridine-Anchored Phthalocyanine-Sensitized Solar Cells by Linker Elongation

Article abstract of DOI:10.1002/asia.201500756

A series of zinc phthalocyanine sensitizers (PcS22-24) having a pyridine anchoring group are designed and synthesized to investigate the structural dependence on performance in dye-sensitized solar cells. The pyridine-anchor zinc phthalocyanine sensitizer PcS23 shows 79% incident-photon to current-conversion efficiency (IPCE) and 6.1% energy conversion efficiency, which are comparable with similar phthalocyanine dyes having a carboxylic acid anchoring group. Based on DFT calculations, the high IPCE is attributed with the mixture of an excited-state molecular orbital of the sensitizer and the orbitals of TiO₂. Between pyridine and carboxylic acid anchor dyes, opposite trends are observed in the linker-length dependence of the IPCE. The red-absorbing PcS23 is applied for co-sensitization with a carboxyl-anchor organic dye D131 that has a complementary spectral response. The site-selective adsorption of PcS23 and D131 on the TiO₂ surface results in a panchromatic photocurrent response for the whole visible-light region of sun light. To dye for: Dye-sensitized solar cells using zinc phthalocyanine dyes exhibit good power conversion efficiency (PCE). For example, dye PcS23 (see picture) exhibited a PCE of 6.1% due to a good hybridization between sensitizer and TiO₂ molecular orbitals.

Full text of DOI:10.1002/asia.201500756

DOI: 10.1002/asia.201500756
Communication
Dye-Sensitized Solar Cells
Enhanced Charge Separation Efficiency in Pyridine-Anchored
Phthalocyanine-Sensitized Solar Cells by Linker Elongation
Takuro Ikeuchi,[a] Saurabh Agrawal,[a] Masayuki Ezoe,[b] Shogo Mori,*[a] and
Mutsumi Kimura*[a]
have achieved a high power conversion efficiency (PCE) above
Abstract: A series of zinc phthalocyanine sensitizers
11 % under one sun conditions.[3] While its absorption range
(PcS22–24) having a pyridine anchoring group are de-
extends close to 900 nm, the low absorption coefficient of the
signed and synthesized to investigate the structural de-
ruthenium complexes is the main drawback to further en-
pendence on performance in dye-sensitized solar cells.
hancement of the PCE.[4] Alternatively, p-conjugated macrocy-
The pyridine-anchor zinc phthalocyanine sensitizer PcS23
cles such as porphyrins and phthalocyanines (Pcs) have attract-
shows 79% incident-photon to current-conversion effi-
ed a special attention due to their very high extinction coeffi-
ciency (IPCE) and 6.1% energy conversion efficiency,
cients.[5] However, while having two intense absorption bands
which are comparable with similar phthalocyanine dyes
of the Soret and Q bands in visible light region, porphyrin and
having a carboxylic acid anchoring group. Based on DFT
phthalocyanine dyes lack absorption in the green spectral
calculations, the high IPCE is attributed with the mixture
region of solar light. Thus, co-sensitization by two dyes with
of an excited-state molecular orbital of the sensitizer and
complementary absorption spectra has been examined. Grätzel
the orbitals of TiO2. Between pyridine and carboxylic acid
et al. reported a PCE of over 12% by co-sensitization of D-p-A
anchor dyes, opposite trends are observed in the linker-
type porphyrin dye YD2-o-C8 with another organic dye Y123
length dependence of the IPCE. The red-absorbing PcS23
in combination with cobalt redox shuttle.[6] We also reported
is applied for co-sensitization with a carboxyl-anchor or-
the enhancement of PCE in the co-sensitized DSSCs using steri-
ganic dye D131 that has a complementary spectral re-
cally protected zinc phthalocyanine dye PcS15 with an organic
sponse. The site-selective adsorption of PcS23 and D131
dye by the prevention of intermolecular interactions among
on the TiO2 surface results in a panchromatic photocurrent
adsorbed dyes.[7]
response for the whole visible-light region of sun light.
Most of sensitizing dyes for DSSCs use carboxylic acid as an-
choring groups for the formation of an ester linkage on TiO2
surface as well as electron accepter units to promote a smooth
electron injection. Pyridine,[8] 8-hydroxylquinoline,[9] and 2-(1,1-
dicyanomethylene)rhodanine[10] units have been investigated
as anchoring groups of sensitizing dyes for DSSCs. These het-
erocycle ligands can also form coordination bond with TiO2
surface. Ooyama et al. found highly efficient electron injection
through the coordination bond of pyridine anchoring group of
D-p-A organic dyes with TiO2 surface.[8] They also reported that
the dyes having the pyridine anchoring group and carboxylic
anchoring group tend to be adsorbed on different TiO2 surface
sites. Then, Arakawa et al. exploited the different adsorption
behaviors for co-sensitization and found the adsorption of the
dye using pyridine anchoring group without decreasing the
amount of adsorbed dye using carboxylic acid anchoring
group.[11] Thus, the total adsorption density was increased and
the photovoltaic performance of the co-sensitized DSSCs was
improved, showing employing dyes having two different an-
choring groups would be a strategy for co-sensitization.[11] On
the other hand, the amount of adsorbed dyes having pyridine
anchoring group depends on the structure of the dye frame-
work. Incident-photon to current-conversion efficiency (IPCE)
also seems to depend on the length of p-conjugated linker in
the sensitizers. However, these points have not been ad-
dressed explicitly.
Introduction
Considerable effort has been made to develop various types of
sensitizing dyes for dye-sensitized solar cells (DSSCs) using
wide-bandgap semiconductors such as titanium oxide
(TiO2).[1,2] Upon light irradiation, the dye adsorbed onto the sur-
face of TiO2 injects an electron into the conduction band of
TiO2, and the oxidized dye is regenerated to the ground state
by electron transfer from redox shuttle in the electrolyte.
DSSCs using panchromatic polypyridyl ruthenium complexes
[a] T. Ikeuchi, Dr. S. Agrawal, Prof. Dr. S. Mori, Prof. Dr. M. Kimura
Division of Chemistry and Materials
Faculty of Textile Science and Technology
Shinshu University
Ueda 386-8567 (Japan)
Fax: (+81)268-21-5499
E-mail: shogmori@shinshu-u.ac.jp
mkimura@shinshu-u.ac.jp
[b] Dr. M. Ezoe
Yamamoto Chemicals Inc.
Yao 581-0034 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500756.
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Results and Discussion
The performance of Pc-sensitized DSSCs has significantly im-
proved by electronic push-pull structure, steric suppression of
aggregation, and optimization of adsorption sites.[5c,d] Previous-
ly, we reported an overall PCE of 6.4% in DSSCs employing
asymmetrical PcS20 decorated with short alkoxyl chain.[12] In
this paper, we examined the applicability of pyridine anchoring
group to phthalocyanine dyes. We synthesized novel ZnPc
dyes PcS22–24 (Scheme 1) having a pyridine anchoring group
Figure 1. Absorption spectra of PcS23 in toluene (solid line) and adsorbed
on the TiO2 film (dotted line).
substituents.[14] The width of the split Q band for PcS23 is
almost the same as that of carboxyl-anchor PcS20 (675 and
695 nm),[13] indicating that the pyridine unit works as a strong
electron accepting unit in the asymmetrical ZnPcs. The Q
bands of PcS22 and PcS23 were slightly red-shifted as com-
pared with the PcS21 spectrum, which can be attributed to
the extension of p-conjugation by the attachment of 4-ethy-
nylpyridine unit with the phthalocyanine macrocycle. The high-
est occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels of PcS22–24 were de-
termined from the absorption spectral analyses and the first
oxidation potentials of differential pulse voltammetry measure-
ments (Table 1). The HOMO level of PcS24 having the acety-
lene linker was lower than that of PcS23, also indicating the
high electron accepting ability of 4-ethynylpyridine unit.
Scheme 1. Molecular structures of PcS22–24 sensitizers.
and different lengths of conjugated linker, and examined their
performance in DSSCs. We also applied PcS23 to the cocktail-
type DSSCs by the combination with organic dyes, D131,
having a carboxylic acid anchoring group.[7,13] DFT calculations
for Pcs having carboxylic acid and pyridine anchoring groups
on TiO2 surface were performed to understand charge injection
process. The site-selective adsorption of PcS23 and D131 on
the TiO2 surface resulted in an increased total dye adsorption
density and a panchromatic photocurrent response for the
whole visible light region of sun light.
As the pyridine substituent lie on the same plane of ZnPc
macrocycles in PcS22–24, the coordination between pyridine
substituent and central Zn ion leads to the formation of phtha-
locyanine assemblies.[15] No significant changes in the absorp-
tion and fluorescence spectra of PcS22–24 were detected after
the addition of a large excess of pyridine. Furthermore, the
MALDI-TOF mass spectra of PcS22-PcS24 did not show any
molecular ion peaks corresponding to oligomeric phthalocya-
nine assemblies. These results suggest that the pyridine-
anchor PcS22–24 do not form the phthalocyanine assemblies
through the formation of intermolecular coordination bonds.
Palladium-catalyzed Stille or Sonogashira coupling reactions
with 4-iodophthalonitrile afforded two phthalonitriles having
a pyridine ring. The target dyes PcS22–24 were prepared by
the statistical condensation of two phthalonitriles in the pres-
ence of Zn(CH3COO)2, and were separated by column chroma-
tography as a second fraction.
While the pyridine ring is at-
tached directly to the phthalo-
Table 1. Optical and electrochemical data, HOMO and LUMO energy levels, and DSSC device performance pa-
rameters of PcS22–24.
cyanine macrocycle in PcS22,
PcS23 and PcS24 have an acety-
lene linker between the phthalo-
cyanine macrocycle and the pyri-
dine anchoring group. Figure 1a
shows the absorption spectrum
of PcS23 in toluene, and the
spectral data of PcS22–24 are
summarized in Table 1. The spec-
trum of PcS23 showed a split Q
band at 678 and 702 nm, typical
[b]
Dye
lmax [nm][a] Eox HOMO[c] LUMO[c] Adsorption amount[d] Thickness[e] [mm] Voc Jsc
FF PCE
[mV] [mAcmÀ2
] [%]
(log e
[V] [V]
[V]
”10À5 [molcmÀ3
]
[MÀ1 cmÀ1])
PcS22 695 (5.00) 0.24 0.87
PcS23 702 (4.95) 0.26 0.89
À0.90 1.1
À0.87 3.8
12.6+5.0
12.6+5.0
4.8+3.4
560 8.1
0.71 3.2
0.74 5.4
0.74 6.1
0.76 4.6
580 12.6
610 13.5
600 10.0
PcS24 702 (4.95) 0.24 0.87
À0.90 0.8
12.6+5.0
[a] In toluene. [b] Eox vs. ferrocene/ferrocenium (Fc/Fc+) were measured by differential pulse voltammetry using
dye-stained electrodes in acetonitrile containing 0.1m Bu4NPF6. [c] Vs. normal hydrogen electrode (NHE). [d] Ad-
sorption densities were determined by measuring the absorbance of dyes released from the TiO2 films by im-
mersing into THF containing acetic acid. [e] Film thicknesses of TiO2 transparent and scattering layers for
DSSCs.
of
asymmetrical “push-pull”
ZnPcs with donor and acceptor
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Steric crowding around a ZnPc macrocycle with twelve alkoxy
chains at the 2 and 6 positions of peripheral phenoxy units in
PcS22–24 can prevent the pyridine rings of the other dyes
from coming close to the central metal.
Porous TiO2 films on quartz substrate were immersed into
toluene solutions of PcS22–24 to obtain absorption spectra of
dye-stained films. The absorption spectrum of PcS23 adsorbed
on a TiO2 film showed sharp Q bands and the spectrum was in
fare agreement with that in toluene solution, revealing the
prevention of molecular aggregation among ZnPcs within the
adsorbed monolayer of PcS23 on TiO2 surface (Figure 1b). The
dye density adsorbed onto the TiO2 films were determined
from the absorbance at the Q band of ZnPc desorbed from
the dye-stained TiO2 film by the treatment with CH3COOH/THF
(Table 1). The adsorption density of PcS23 was about one-fifth
of that of carboxyl-anchor PcS20 (2.0”10À4 molcmÀ3),[12] indi-
cating large exposed TiO2 surface among the sensitizers. The
adsorption property of pyridine-anchor organic dyes on TiO2
surface was investigated by Harima et al.[16] They found a large
difference in adsorption equilibrium constant between two
dyes possessing pyridine and carboxyl anchors. The low ad-
sorption density of PcS23 also suggests that the binding of
pyridine unit in PcS23 with TiO2 surface is weaker than that of
carboxyl unit in PcS20. Moreover, lacking the acetylene spacer
in PcS22 decreased the adsorption density, which was consis-
tent with the previous report showing lower adsorption densi-
ty with the dyes having shorter p-conjugation linker.[11] Increas-
ing bulkiness around the ZnPc macrocycle in PcS24 also dimin-
ished the adsorption density.
Figure 2. a) Photocurrent voltage curve obtained with DSSC based on PcS23
under a standard global AM 1.5 solar condition (solid line) and dark current
(dotted line). Thickness of TiO2 transparent layer is 4.8 mm. b) Incident
photon-to-current conversion efficiency spectrum for DSSC based on PcS23.
dye anchor and attached surface Ti5c atoms respectively (Fig-
ure S6). Calculated binding energy between PcS16 and PcS23/
24 dyes and TiO2 are À95.04 and À76.13 kJmolÀ1 respectively.
The better binding energy for PcS16 dye with carboxylate
anchor seems reasonable as it forms two covalent bonds with
two surface Ti atoms, whereas, pyridine anchors via one coor-
dinate bond with a surface Ti atom. Following optimization,
TD-DFT calculations are performed to estimate the optical
properties of PcS23/24-TiO2 and PcS16-TiO2 complexes. Com-
puted optical spectrum for PcS23/24-TiO2 shows Q-band peak
at 670 nm (1.85 eV) wavelengths (Figure S2). Looking into MOs
participated in CT-excitations, we see that HOMO contributes
in most of the high intensity photo-excitations and displays lo-
calization over phthalocyanine ring (Figure 3), which is charac-
teristically similar to the HOMO of dye (Figure S4). Among un-
occupied orbitals, mainly LUMO, LUMO+1 and LUMO+7 par-
ticipate in CT-excitations. MO plots (Figure 3) as well as MO
energy level diagram (Figure S8) suggest that LUMO+1 and
LUMO+7 of PcS23/24-TiO2 complex originate from respective
reallocation of LUMO and LUMO+1 of PcS23/24 dye. Consid-
erable localization of LUMO+1 over dye and TiO2 suggests
We fabricated DSSCs using TiO2 electrodes with electrolytes
containing 0.6m 1,2-dimethyl-3-propylimidazolium iodide
(DMPImI), 0.1m LiI, 0.05m I2, 0.5m t-butylpyridine (tBP) in ace-
tonitrile, and the solar cell performances of the PcS22–24 cells
were measured under global AM 1.5 simulated solar conditions
(Figure S9). Table 1 lists the short-circuit photocurrent density
(Jsc), open-circuit voltage (Voc), fill factor (FF), and PCE of
PcS22–24 cells. The PCEs of the PcS22 and PcS24 cells were
lower than that of the PcS23 cell, and the PCE value was in
the order of PcS22<PcS24<PcS23. This agrees with the
order of dye adsorption densities on TiO2 surface. When the
thickness of transparent TiO2 layer decreased from 13.5 to
4.8 mm, the Voc and Jsc values of PcS23 cell rose from 0.58 to
0.61 V and 12.6 to 13.5 mAcmÀ2 with keeping the same FF
values. The PcS23 cell with a 4.8 mm thickness of transparent
TiO2 layer yields a PCE of 6.1% (Figure 2a), and the incident-
photon to current conversion efficiency (IPCE) at the maximum
absorption of the Q band reaches 79% (Figure 2b). The opti-
mized thickness of TiO2 film for PcS23 was 4.8 mm.
To understand the charge transfer nature of photo-excited
dye upon adsorption on TiO2 nanoparticle, we performed DFT
calculations for PcS16- and PcS23-TiO2 complexes. To reduce
the computational cost, alkyl chains of PcS23 were omitted,
which makes the model exactly same as of PcS24 dye without
alkyl chains. Further, PcS16 (Figure S5) consists of carboxylic
acid anchor instead of pyridine anchor as in case of PcS23/
24.[17] Optimized geometries of PcS16- and PcS23/24-TiO2
complexes show (2.03, 2.11) and 2.31 Š bond lengths between
Figure 3. Plotted frontier molecular orbitals HOMO (H), LUMO (L), L+1 and
L+7 (isovalue=0.02 ea.u.À3) involved in charge-transfer excitation of the
PcS23/24–TiO2 complex.
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strong hybridization between dye-TiO2 orbitals, implying effec-
tive charge transfer from photo-excitation dye to TiO2. Contrary
to PcS23/24-TiO2 complex, photo-excited PcS16-TiO2 complex
(LUMO+3, Figure S7) shows only marginal charge localization
at dye anchor/TiO2 interface. This suggests that among the
studied systems, mixing of dye/TiO2 states with pyridine-
anchor is stronger than carboxylate-anchor. The coupling of
dye/TiO2 states can be a determining factor of charge injection
rate.[18] This seems consistent with the lower Jsc and eventually
IPCE of PcS16 sensitized solar cells.
In view of the linker length between the phthalocyanine
core and adsorption site, higher IPCE was obtained with the
dyes having longer linker (Figure S9). On the other hand,
phthalocyanine dyes having carboxylic acid anchoring group,
IPCE was decreased with the increase of linker length of the
dyes, PcS15 to PcS16.[17] The reason of the opposite trend
could be different kinetics of recombination between injected
electrons and dye cation. Since pyridine anchor dyes seems to
have better molecular orbital hybridization with TiO2 particle, it
would make it easy to not only injection but also recombina-
tion with dye cation. Thus, longer linker would be better to
reduce the mixing of dye-TiO2 orbitals to retard charge recom-
bination. On the other hand, for dyes with carboxylic acid an-
choring group, shorter linker would be required to have suffi-
cient molecular orbital penetrations for charge injection.
To enhance response of DSSCs using organic dyes in the
red-region of solar light, co-sensitization of carboxyl-anchor
yellow dye D131 with PcS23 was attempted through the site-
selective adsorption of two dyes on TiO2 surface. The surface
of TiO2 electrode was initially covered with D131 by immersing
the TiO2 electrode into a D131 solution for 24 hr, followed by
immersion into the PcS23 solution for 24 hr. The adsorption
amount of D131 did not decrease after the treatment with
PcS23, indicating the successful site-selective adsorption of
two dyes on the TiO2 surface. The DSSC performance of the
co-sensitized D131/PcS23 cell was significantly enhanced in
comparison to the D131 cell. The D131/PcS23 cell showed
almost a double Jsc value compared to that of the D131 cell
(Figure 4a). The IPCE spectra of D131/PcS23 cell displayed
sums of each dyes (Figure 4b), and the IPCE values corre-
sponding to D131 in the co-sensitized D131/PcS23 cell were
the same as that of D131 cell.
Figure 4. a) Photocurrent voltage curve obtained with DSSCs based on
D131 (dotted line: PCE=3.8%, Jsc =8.5 mAcmÀ2, Voc =650 mV, FF=0.71, Ad-
sorption amount of D131=22.0”10À5 molcmÀ3) and D131/PcS23 (solid
line: PCE=7.4%, Jsc =16.0 mAcmÀ2, Voc =650 mV, FF=0.71, Adsorption
amounts of D131 and PcS23=22.0”10À5 and 2.1”10À5 molcmÀ3) under
a standard global AM 1.5 solar condition and dark currents (dashed lines).
b) Incident photon-to-current conversion efficiency spectrum for DSSCs
based on D131 (dotted line) and D131/PcS23 (solid line).
Acknowledgements
This work has been partially supported by Grants-in-Aid for Sci-
entific Research (A) (No. 15H02172) and (B) (No. 22350086)
from the Japan Society for the Promotion of Science (JSPS) of
Japan.
Keywords: dyes/pigments
·
energy
conversion
·
phthalocyanines · solar cells · TiO2
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Manuscript received: July 21, 2015
Accepted Article published: July 28, 2015
Final Article published: August 11, 2015
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