Ionic liquid crystal as a hole transport layer of dye-sensitized solar cells

Article abstract of DOI:10.1039/b417610c

Use of a new ionic liquid crystal, 1-dodecyl-3-methylimidazolium iodide, and iodine as an electrolyte of dye-sensitized solar cells leads to a high short circuit photocurrent density and a high light-to-electricity conversion efficiency, due to a self-assembled structure of the imidazolium cations, resulting in high conductivity of the electrolyte. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b417610c

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Ionic liquid crystal as a hole transport layer of dye-sensitized solar cells
Noriyo Yamanaka,a Ryuji Kawano,b Wataru Kubo,a Takayuki Kitamura,a Yuji Wada,a
Masayoshi Watanabeb and Shozo Yanagida*c
Received (in Cambridge, UK) 19th November 2004, Accepted 20th December 2004
First published as an Advance Article on the web 12th January 2005
DOI: 10.1039/b417610c
A few examples of the ILC with the SA phase, such as imi-
dazolium salts consisting of cations with alkyl chains of C12–C18
and anions of hexafluorophosphate or bromide, have been
reported.7,8 Iodide is indispensable in a DSSC electrolyte, because
Use of a new ionic liquid crystal, 1-dodecyl-3-methylimidazo-
lium iodide, and iodine as an electrolyte of dye-sensitized solar
cells leads to a high short circuit photocurrent density and a
high light-to-electricity conversion efficiency, due to a self-
assembled structure of the imidazolium cations, resulting in
high conductivity of the electrolyte.
the I2/I3 redox couple functions as a hole transport agent.
2
However, an imidazolium salt with iodide as the counter anion has
not been reported to be an ILC with a SA phase. Here, we show
for the first time that imidazolium iodides with alkyl chains longer
than C12 exhibit a SA phase and that the liquid crystalline nature is
preferable in terms of the hole transport layer in DSSC.
Imidazolium iodides with long alkyl chains were synthesized
by the quaternization reaction of 1-methylimidazole with an
equimolar amount of the corresponding alkyl iodide for 72 h
under N2 atmosphere at room temperature. The products were
washed with n-hexane to remove the remaining starting
materials and dried under vacuum at 40 uC for 4 h, and finally
identified by 1H NMR in CDCl3 and differential scanning
calorimetry (DSC). Imidazolium iodides with alkyl chains longer
than C12 showed a liquid crystalline phase. C12MImI showed the
lowest melting point and viscosity among them and these
properties were suitable for the hole transport layer in DSSC. In
this study, we selected C12MImI and applied it with 0.65 M iodine
(C12MImI/I2) as the hole transport layer in DSSC. We have
compared the properties of C12MImI/I2 with an ionic liquid
electrolyte; 1-undecyl-3-methylimidazolium iodide with 0.65 M
iodine (C11MImI/I2).
Dye-sensitized solar cells (DSSC), constructed by using dye
molecules, nanocrystalline metal oxides and liquid electrolytes,
have attractive features in terms of the high light-to-electricity
conversion efficiency, and the low production cost and energy.1
2
The electrolytes, usually composed of an I2/I3 redox couple in
organic solvents, are sealed between two electrodes. These organic
solvents cause a serious problem of low durability due to
evaporation.2 It has been reported that DSSC using ionic liquids
as a non-volatile solvent achieves high temperature stability.3,4
However, the conversion efficiency of the cells using ionic liquids is
lower than that using organic solvents, because the high viscosity
of the ionic liquids retards the physical diffusion of I2 and I3
2
.
Many attempts to reduce the viscosity have not yet been
successful.5 For enhancing the conductivity of ionic liquids leading
to the high light-to-electricity conversion efficiency of DSSC, it
seems to be necessary to arrange a pathway for fast charge
transport.
Previously, we reported that the charge transport rate at high
2
concentration of an I2/I3 redox couple in ionic liquids can be
attributed to the exchange reaction of I2 + I32 A I32 + I2.4,6 For
enhancing the short circuit photocurrent density (JSC) of DSSC
C12MImI showed a phase transition from a liquid to a liquid
crystal at 80 uC, although C11MImI did not show a liquid
crystalline phase. The ionic liquid crystalline phase of C12MImI
was confirmed by polarized optical microscopy (POM). The
characteristic focal conic domains observed during the cooling
process suggested that the liquid crystalline phase of C12MImI was
a SA phase.7
using ionic liquids with a high concentration of an I2/I3 redox
2
couple, the exchange reaction in the ionic liquids needs to be
promoted. In this study, we report a new strategy for enhancing
the conductivity of ionic liquid electrolytes; employing an ionic
liquid crystal (ILC) as a constituent of an electrolyte, which forms
a self-assembled structure and promotes the exchange reaction by
the locally increased concentrations of I2 and I32. We have
selected 1-dodecyl-3-methylimidazolium iodide (C12MImI) as an
ILC. This provides a self-assembled structure of the imidazolium
cations like a solid, while maintaining the molecular dynamics like
a liquid. The ILC with the smectic A phase (SA) has a bilayer
structure of interdigitated alkyl chains of the imidazolium cations,
Photoelectrochemical cells were fabricated as previously
described.9 C11MImI/I2 and C12MImI/I2 were used as the hole
transport layer, respectively. C12MImI/I2 maintained the SA phase
ranging from 27 to 45 uC on heating. On the other hand,
C11MImI/I2 showed a liquid phase above 37 uC.
The light-to-electricity conversion efficiencies of DSSC using
C12MImI/I2 and C11MImI/I2 were evaluated at 40 uC under AM
1.5 irradiation from a solar simulator, adaptable for amorphous
silicon solar cells according to the Japanese Industrial Standard.10
Each value for cell performance was taken as an average of at least
3 samples.
and I2 and I3 would be localized between the SA layers. The
2
locally high concentration would promote the exchange reaction.
So, the ILC with the SA phase would be suitable for the electrolyte
of DSSC when aiming at the high light-to-electricity conversion
efficiency.
Fig. 1 shows photocurrent–voltage curves of the DSSC using
C12MImI/I2 and C11MImI/I2. JSC of the DSSC using C12MImI/I2
was higher than that using C11MImI/I2, while the open circuit
voltage (VOC) and fill factor (FF) were the almost same values as
*yanagida@mls.eng.osaka-u.ac.jp
740 | Chem. Commun., 2005, 740–742
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Fig. 1 Photocurrent–voltage curves of the cells using C12MImI/I2 (bold
solid curve) and C11MImI/I2 (solid curve) under AM 1.5 irradiation.
for C11MImI/I2. This result suggests that the higher conductivity of
11,12
C12MImI/I2 than C11MImI/I2 could lead to a high JSC.
To examine the conductivities of C12MImI/I2 and C11MImI/I2,
the diffusion-limited currents (Ilim) corresponding to the reaction
of I3 + 2e2 A 3I2 were measured. The measurements were
2
Fig. 2 Ionic conductivities for the samples of C12MImI/I2 and C11MImI/
I2 (insert) along with the direction parallel siI (filled circles) and
perpendicular si) (open triangles) to the SA layer plane of the
homeotropically aligned ionic liquid crystal.
carried out at 40 uC by using a microelectrode and the diffusion
2
coefficients (D) of I3 were calculated using the values of Ilim as
previously described.6 According to this study, the D value
calculated from Ilim intrinsically includes not only the simple
physical diffusion coefficient but also the diffusion coefficient
based on the exchange reaction.
were observed. The ionic conductivities parallel (siI) to the SA
layer of C12MImI/I2 decreased and those perpendicular (si))
increased at 45 uC on heating. When the liquid crystalline phase-
order disappeared above the phase transition temperature, the
ionic conductivities measured in both cell A and cell B were on the
same line within the limits of error. Taking into account that
C11MImI/I2 did not show any discontinuous changes in siI and
si) over the whole temperature range, the discontinuous changes
of C12MImI/I2 should be attributed to the conductive pathways
formed between the SA layers. The observed siI values of
C12MImI/I2 below 45 uC, which showed ionic conductivities along
the direction of the conductive pathways, were enhanced because
the exchange reaction would be promoted at the conductive
pathways.
The observed D value of C12MImI/I2 (4.2 6 1028 cm2 s21) was
1.3 times as large as that of C11MImI/I2 (3.2 6 1028 cm2 s21),
while the viscosity of C12MImI/I2 was 2.5 times larger than that of
C11MImI/I2. The simple physical diffusion coefficient of C12MImI/
I2 should be smaller than C11MImI/I2. This result would lead to
the idea that the diffusion coefficient based on the exchange
reaction is increased in C12MImI/I2. It has been reported that
anisotropic long-range conductive pathways between the SA layers
are formed in liquid crystals with a SA phase.13 In C12MImI/I2,
2
these pathways should be formed, and I2 and I3 should be
located between the SA layers consisting of the imidazolium
cations. If a locally high concentration of I2 and I32 is achieved in
the pathways, the exchange reaction should be promoted in
C12MImI/I2, since the diffusion coefficient value based on the
exchange reaction is proportional to the concentrations of I2 and
I32. So, C12MImI/I2 could show a higher D value than C11MImI/
I2 in which the cations should exist at random.
In the DSSC using C12MImI/I2, although the long-range order
of ILC was not necessarily achieved in the mesoporous TiO2
electrode, the localization of I2 and I3 between the SA layers of
2
each domain should result in the promotion of the exchange
reaction in a hole transport layer and the enhancement in JSC
.
To prove that the exchange reaction was promoted in the
pathways between the SA layers of C12MImI/I2, we measured the
ionic conductivities along the direction parallel (siI) and
perpendicular (si)) to the SA layer plane13 at temperatures ranging
from 32.5 to 57.5 uC. A glass plate with comb-shaped platinum
electrodes (cell A) and a pair of indium tin oxide (ITO) electrodes
(cell B) were employed for the measurements of siI and si),
respectively. The conoscopic image at 40 uC for C12MImI/I2
revealed that the self-assemblies of the cations formed a home-
otropic alignment in the SA phase on the glass surface.
In conclusion, we have demonstrated a new strategy for
enhancing the conductivity of the ionic liquid electrolytes contain-
2
ing an I2/I3 redox couple. This strategy is based on the
2
promotion of the exchange reaction between I2 and I3 by the
locally increased concentrations of I2 and I32. As a means of
demonstrating this strategy,
C12MImI/I2 with a self-assembled structure of the imidazolium
cations was introduced. C12MImI/I2 showed high ionic
a new ionic liquid crystalline
a
conductivity in spite of its high viscosity. The ionic liquid
crystalline hole transport layer (C12MImI/I2) was applied to
DSSC, which showed a higher JSC and a higher light-to-electricity
conversion efficiency than that using the ionic liquid hole transport
layer (C11MImI/I2).
Fig. 2 shows anisotropic ionic conductivities of C12MImI/I2 and
isotropic ionic conductivities of C11MImI/I2 as a function of
temperature. It was reported that the ionic conductivities of
isotropic ionic liquids gradually increased with the increase in
temperature as shown in the insert of Fig. 2.14 In contrast, for
C12MImI/I2, the discontinuous changes of ionic conductivities
This research was supported in part by the New Energy and
Industrial Technology Development Organization (NEDO) under
the Ministry of Economy, Trade and Industry.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 740–742 | 741

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Noriyo Yamanaka,a Ryuji Kawano,b Wataru Kubo,a
Takayuki Kitamura,a Yuji Wada,a Masayoshi Watanabeb and
Shozo Yanagida*c
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aMaterial and Life Science, Graduate School of Engineering, Osaka
University, 2-1 Yamadaoka Suita, 565-0871, Japan.
E-mail: ywada@mls.eng.osaka-u.ac.jp
bDepartment of Chemistry and Biotechnology, Yokohama National
University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501,
Japan. E-mail: mwatanab@ynu.ac.jp
cCenter for Advance Science and Innovation Osaka University, Japan.
E-mail: yanagida@mls.eng.osaka-u.ac.jp
10 Japanese Industrial Standard Committee, Japanese Industrial Standard,
Solar simulators for amorphous solar cells and modules, JIS C 8933,
Japanese Standards Association, Tokyo, 1995.
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Notes and references
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742 | Chem. Commun., 2005, 740–742
This journal is ß The Royal Society of Chemistry 2005