Synthesis of a novel alkylimidazolium iodide containing an amide group for electrolyte of dye-sensitized solar cells

Article abstract of DOI:10.1016/j.electacta.2010.04.103

A novel alkylimidazolium iodide containing an amide group, 1-(2-hexanamidoethyl)-3-methylimidazol-3-ium iodide (amido-ImI), was synthesized to act as the quasi-solid-state electrolyte of dye-sensitized solar cells (DSSCs). The DSSC with the amido-ImI electrolyte exhibited short-circuit photocurrent density (J_sc) and overall energy conversion efficiency (η) that were improved by 7.2% and 10.2%, respectively, compared to those obtained with the cell containing 1-hexyl-2,3-dimethylimidazolium iodide, a commonly used liquid electrolyte, at 100mWcm^-2. Furthermore, the stability of the DSSC was enhanced by the presence of amido-ImI.

Full text of DOI:10.1016/j.electacta.2010.04.103

Electrochimica Acta 55 (2010) 5652–5658
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Electrochimica Acta
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Synthesis of a novel alkylimidazolium iodide containing an amide group for
electrolyte of dye-sensitized solar cells
∗
Seung-Hwan Jeon, A.R. Sathiya Priya, Eun-Ji Kang, Kang-Jin Kim
Department of Chemistry, Korea University, 5-1, Anam Dong, Seoul 136-713, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel alkylimidazolium iodide containing an amide group, 1-(2-hexanamidoethyl)-3-methylimidazol-
Received 30 January 2010
Received in revised form 27 April 2010
Accepted 27 April 2010
3
-ium iodide (amido-ImI), was synthesized to act as the quasi-solid-state electrolyte of dye-sensitized
solar cells (DSSCs). The DSSC with the amido-ImI electrolyte exhibited short-circuit photocurrent den-
sity (Jsc) and overall energy conversion efficiency (ꢀ) that were improved by 7.2% and 10.2%, respectively,
compared to those obtained with the cell containing 1-hexyl-2,3-dimethylimidazolium iodide, a com-
Available online 5 May 2010
−2
monly used liquid electrolyte, at 100 mW cm . Furthermore, the stability of the DSSC was enhanced by
the presence of amido-ImI.
Keywords:
Amide-containing imidazolium iodide
Dye-sensitized solar cell
Short-circuit photocurrent
Overall energy conversion efficiency
Long-term stability
©
2010 Elsevier Ltd. All rights reserved.
1
. Introduction
[27], imidazole polymer [28], organic molecular [29], and inor-
ganic particles [30]. Several molten salt-type polymers have been
prepared by radical polymerization of vinyl derivatives containing
imidazolium salts [31,32]. Kubo et al. [29] reported a highly stable
DSSC (stable at 5% for almost 1000 h) using a gelated ionic liquid
(IL) electrolyte based on 1-alkyl-3-methylimidazolium iodide.
Recently, growing attention has been paid to room-temperature
ILs (RTILs), especially those containing imidazolium salts and
pyridinium salts, due to their favorable properties such as ther-
mal stability, non-flammability, high ionic conductivity, negligible
vapor pressure, and possibly wide electrochemical window [33].
New electrolytic materials such as RTIL, gels and polymers are
expected to be introduced for quasi-solid DSSCs in the future [34].
The unique properties offered by ILs such as non-volatility, non-
flammability [35,36], high ionic conductivity [37] and gel-forming
properties with polymers [38] have been reported to be benefi-
cial in their application as DSSC electrolytes in order to achieve
high temperature stability [39]. However, the conversion efficiency
of the cells using ILs is lower than that of the cells using organic
solvents, because the high viscosity of the ILs retards the physical
Dye-sensitized solar cells (DSSCs) convert sunlight to electricity
with high conversion efficiency [1,2]. Although a light-to-electricity
conversion efficiency of more than 11% has been achieved in liq-
uid electrolyte-based DSSCs [3–5], the major drawback of these
solar cells is their poor long-term stability owing to the evaporation
of the liquid phase in the cells. To improve the stability of DSSCs,
investigations have focused on replacing the liquid electrolyte by a
solid-state medium, such as inorganic p-type semiconductors [6],
organic hole-transport materials [7], room-temperature molten
salts [8–10], polymer gel electrolytes [11,12], and solid polymer
redox electrolytes [13,14].
Although solid-state electrolytes solve some problems, they
show lower conversion efficiency than liquid electrolytes because
of poor contact between the solid-state charge transport material
and the dye-coated TiO surface. Polymer gel electrolytes offer the
2
advantages of low vapor pressure, good long-term stability, excel-
lent contacting and filling properties between the nanostructured
electrode and counter electrode, and higher ionic conductivity.
Therefore, polymer gel electrolytes have attracted intensive atten-
tion [15–23]. The various materials used as matrices for ionic gels
and ionic polymer electrolytes to fabricate quasi-solid-state DSSCs
have included ethylene oxide copolymer [24,25], polyvinyl pyri-
dine cross-linked copolymer [26], vinyliden–propylene copolymer
−
−
diffusion of I and I3 . An alkylimidazolium iodide which possesses
a hydrogen bonding group has been considered as an alternative to
overcome this drawback.
In this study, we synthesize a new alkylimidazolium iodide
−
(amido-ImI) with the aim of enhancing the conductivities of I and
−
I3 . The influence of amido-ImI on the performance and long-
term stability of the DSSC is investigated and compared to that of
the DSSC with 1-hexyl-2,3-dimethylimidazolium iodide (ref-ImI),
a commonly used liquid electrolyte, and the electrolyte contain-
∗ Corresponding author. Tel.: +82 2 3290 3127; fax: +82 2 3290 3121.
E-mail address: kjkim@korea.ac.kr (K.-J. Kim).
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.04.103

S.-H. Jeon et al. / Electrochimica Acta 55 (2010) 5652–5658
5653
Fig. 1. Chemical structures of the imidazolium iodides: amido-ImI, ref-ImI and decyl-ImI.
ing 1-decyl-3-methylimidazol-3-ium iodide (decyl-ImI) having no
amide group. The chemical structures of these imidazolium iodides
are shown in Fig. 1.
from 1-(3-aminopropyl)-imidazole was characterized using FT-
IR spectroscopy. The stretching vibrations of the NH peak
2
−
1
at 3200–3400 cm
were preserved in the spectrum of 1-(3-
aminopropyl)-imidazole, but the peak for the NH2 group of the
1-(3-aminopropyl)-imidazole disappeared, indicating the forma-
tion of N-hexamideimidazole.
2
. Experimental
1H NMR (300 MHz, CDCl3, ppm): 0.87–0.91 (3H, –CH3),
2.1. Materials
1
.28–1.33 (4H, –CH –CH –), 1.58–1.67 (2H, –CH –), 1.90–2.10 (2H,
2 2 2
Anhydrous LiI, I , 4-tert-butylpyridine, 1-(3-aminopropyl)-
–CH2–), 2.13–2.20 (2H, –CH2–), 3.25–3.30 (2H, –CH2–), 3.98–4.02
(2H, –CH2–), 6.96 (1H, –CH ), 7.08 (1H, –CH ), 7.54 (1H, –CH ).
Amido-ImI was then synthesized from N-hexamideimidazole as
the starting material. N-hexamideimidazole (8.3 mmol) was dis-
solved in 10 ml of trichloroethylene, to which 0.98 ml (10.0 mmol)
of iodomethane was added dropwise under an Ar atmosphere. The
resulting solution was heated to reflux for 3 h with constant stirring
and concentrated under vacuum. The residual salt was extracted
with water and trichloroethylene followed by evaporation. The
resulting product was dried under vacuum to obtain a white solid.
The formation of the amido-ImI was finally identified by 1H NMR
2
imidazole, imidazole, 1-bromodecane and trichloroethylene were
purchased from Aldrich, sodium hydroxide, MgSO , CH OH
4
3
and CH Cl from Dae Jung, Korea, and hexanoyl chloride
2
2
and iodomethane from Tokyo Chemical Industry. 1-Hexyl-
,3-dimethylimidazolium iodide was obtained from C-TRI, 3-
methoxypropionitrile, used as received, from Fluka, and N719 dye
from Solaronix SA, Switzerland.
2
2
1
.2. Synthesis of
-(2-hexanamidoethyl)-3-methylimidazol-3-ium iodide
(
amido-ImI)
(Fig. 2) and mass spectroscopy.
1
H NMR (300 MHz, D O, ppm): 0.65–0.69 (3H, –CH ), 1.08–1.09
2
3
First, N-hexamideimidazole was synthesized by stirring the
(4H, –CH2–CH2–), 1.35–1.40 (2H, –CH2–), 1.88–1.91 (2H, –CH2–),
2.01–2.06 (2H, –CH2–), 3.01–3.05 (2H, –CH2–), 3.71 (3H, –CH3),
4.02–4.06 (2H, –CH2–), 7.27 (1H, –CH ), 7.30 (1H, –CH ), 8.55 (1H,
–CH ).
required quantity of 1-(3-aminopropyl)-imidazole with 1 N NaOH
and CH Cl . Hexanoyl chloride was added dropwise to the stirred
2
2
solution, which was allowed to stir overnight and again treated
with excess of NaOH and CH Cl . The formed organic layer
2
2
was washed with water and dried over MgSO . Finally, it was
2.3. Synthesis of 1-decyl-3-methylimidazol-3-ium iodide
(decyl-ImI)
4
extracted with CH Cl2 and evaporated to obtain the product,
2
which was separated by silica column chromatography using
CH OH or CH Cl as the eluent. The obtained N-hexamideimidazole
First, 1-decyl-1H-imidazole was synthesized from imidazole
and 1-bromodecane. A tetrahydrofuran (THF) solution of imidazole
(86 mmol) was added to a suspension of oil-free NaOH (95 mmol)
3
2
2
1
was confirmed by a Varian H NMR (Oxford AS 400), mass
spectroscopy and Fourier transformed infrared (FT-IR) spec-
trometers (Bomen MB-04). N-hexamideimidazole synthesized
◦
and allowed to stir for 1 h at 60 C. To the stirred solution, bro-
Fig. 2. 1H NMR spectrum of amido-ImI.

5
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S.-H. Jeon et al. / Electrochimica Acta 55 (2010) 5652–5658
Table 1
Photovoltaic parameters and charge transfer resistance (Rct) of the DSSCs fabricated using electrolytes with the imidazolium iodides (ImI) and viscosity of the electrolytes. .
a
Jsc (mA cm−2)
Voc (V)
FF
ꢀ (%)
−2
Rct (ꢁ cm )
ImI (0.7 M)
Viscosity (mPa s)
Ref-ImI
Amido-ImI
Decyl-ImI
1.48
1.91
2.37
14.43
15.47
12.40
0.71
0.74
0.70
0.52
0.52
0.53
5.37
5.92
4.62
6.45
5.20
–
a
Electrolyte consisted of 0.7 M imidazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile.
◦
modecane (90 mmol) was added dropwise and stirred overnight at
thickness of the film. The film was dried for 10 min at 70 C, fol-
lowed by removing the tape and coating the film with 2% TiCl4
◦
6
0 C. Then the solvent was removed in a rotary evaporator and H O
2
◦
(
250 ml) was added to the residue. The resulting product, obtained
solution by the spin coating method and annealing it at 450 C for
by extraction with CH Cl2 and evaporation, was separated by sil-
30 min. A porous TiO2 film with a thickness of about 10 ␮m was
thus produced. The annealed film was sensitized with N719 dye by
immersing it for 24 h in an ethanol solution of N719 dye (0.3 mM
2
ica column chromatography using CH OH or CH Cl as the eluent.
3
2
2
1
The obtained 1-decyl-1H-imidazole was confirmed by a Varian H
NMR (Oxford AS 400), mass spectroscopy and FT-IR spectrometers
ꢀ
ꢀ
of (Ru(II)L (NCS) :2TBA, where L = 2,2 -bipyridyl-4,4 -dicarboxylic
2
2
(
Bomen MB-04).
acid, Solaronix SA). A 2-electrode sandwiched DSSC was fabricated
according to the procedure described elsewhere [41]. The DSSC had
an active area of 0.4 cm × 0.4 cm. The electrolyte consisted of 0.05 M
Decyl-ImI was then synthesized from 1-decyl-1H-imidazole as
the starting material. 1-Decyl-1H-imidazole (0.058 mol) was dis-
solved in 100 ml of trichloroethylene, to which 6.8 ml (0.069 mmol)
of iodomethane was added dropwise under an Ar atmosphere. The
resulting solution was heated to reflux for 3 h with constant stirring
and concentrated under vacuum. The residual salt was extracted
with water and trichloroethylene followed by evaporation, after
which the product was dried under vacuum to obtain a white solid.
The formation of decyl-ImI was finally identified by 1H NMR and
I , 0.1 M LiI, 0.7 M amido-ImI, and 0.5 M 4-tert-butylpyridine in 3-
2
methoxypropionitrile. Reference DSSCs were also prepared under
identical conditions with 0.7 M ref-ImI.
2.6. Photovoltaic performance
The photocurrent density–voltage (J–V) curves of the DSSCs
mass spectroscopy.
fabricated with the amido-ImI, decyl-ImI and ref-ImI electrolytes
were obtained using a Keithley M236 source measure unit. A
300 W Xe arc lamp (Oriel) with an AM 1.5 solar simulating fil-
ter for spectral correction served as the light source, and its light
1
H NMR (300 MHz, CDCl , ppm): 0.86–0.89 (3H, –CH ),
3
3
1
4
–
.10–1.35 (18H, –CH –CH –CH –CH –CH –CH –CH –CH –CH –),
2 2 2 2 2 2 2 2 2
.13 (3H, –CH ), 7.34 (1H, –CH ), 7.44 (1H, –CH ), 10.18 (1H,
3
CH ).
−2
intensity was adjusted to 100 mW cm using a Si solar cell. Ther-
mally sealed cells were used to test the long-term stability of the
DSSCs. The sealed cells were stored in a desiccator and subjected to
electrochemical measurements every 24 h in order to study their
long-term stability.
2
.4. Characterizations of the electrolyte
Three different types of electrolyte were prepared for the DSSCs:
amido-ImI, decyl-ImI and ref-ImI electrolytes. The amido-ImI elec-
trolyte consisted of amido-ImI, 0.1 M LiI, 0.05 M I , and 0.5 M
3. Results and discussion
2
4
-tert-butylpyridine in 3-methoxypropionitrile. The decyl-ImI and
ref-ImI electrolytes were prepared with decyl-ImI and ref-ImI
instead of amido-ImI, respectively. The viscosity of the electrolytes
was measured under an Ar atmosphere using an SV-10 viscometer.
The electrical conductance was then measured using the complex
impedance technique [40] at 25 C. The cells consisted of two iden-
tical, Pt-sputtered, fluorine-doped tin oxide (FTO) electrodes. The
electrolyte resistance was measured using a Solartron instrument
3.1. Characterization of the amido-ImI
The recorded viscosities of the three electrolyte solutions are
given in Table 1. The viscosity of the electrolyte solution in which
ref-ImI was replaced with amido-ImI was increased. The electrolyte
containing decyl-ImI showed the highest viscosity of 2.37 mPa s.
The dependence of the electrical conductance (ꢂ) of the electrolyte
solutions on the amido-ImI concentration, given in Table 2, showed
that ꢂ increased with increasing amido-ImI concentration, despite
the high viscosity of the corresponding solutions.
◦
(
Model 1287) with a 1260 frequency response analyzer controlled
by a computer. The frequency limits were typically set between
−
2
6
1
0
Hz and 10 Hz. The AC oscillation was 10 mV. The data were
analyzed by Z-plot software. Chronoamperometric curves were
obtained from the acetonitrile solutions containing 0.7 M imida-
3.2. Photovoltaic performance
zolium iodide, 0.05 M I , 0.1 M LiI, and 0.5 M 4-tert-butylpyridine
2
using an EG&G PARC 263A potentiostat, with an electrochemical
cell consisting of a Pt electrode, a Pt-wire auxiliary electrode and
an Ag/AgCl reference electrode.
Fig. 3 presents the J–V curves of the DSSCs with the electrolytes
based on different amido-ImI concentrations at AM 1.5 illumi-
nation. The results are summarized in Table 2. The short-circuit
photocurrent density (Jsc) increased with increasing concentration
from 0.4 M to 0.7 M. The performance was optimized for the elec-
trolyte containing 0.7 M amido-ImI, with Jsc, open-circuit voltage
(Voc), fill factor (FF), and overall energy conversion efficiency (ꢀ)
2
.5. Preparation of TiO electrodes
2
Dye-coated TiO films were prepared as the working electrodes
2
−
2
for the DSSCs as follows [41]. In each case, a thin buffer layer of
values of 15.47 mA cm , 0.74 V, 0.52, and 5.92%, respectively. How-
ever, the Jsc and ꢀ values decreased with further increase in the
amido-ImI concentration above 0.7 M.
non-porous TiO was deposited on a cleaned FTO conducting glass,
2
purchased from Libbey-Owens-Ford (TEC 8, 75% transmittance in
the visible region), from 5% titanium(IV) butoxide in ethanol by
spin coating at 3000 rpm. The thin layer coated FTO glass was
Fig. 4 compares the J–V curves of the DSSCs based on the amido-
ImI, decyl-ImI and ref-ImI electrolytes at 0.7 M concentration and
the results are included in Table 1. As it can be seen in the table, the
DSSCs constructed with the amido-ImI electrolyte exhibited Jsc, Voc
and consequently ꢀ values that were improved by 7.2%, 4.2% and
◦
cleaned and annealed at 450 C. Dyesol titania paste, purchased
from Dyesol Ltd., was deposited on the above-pretreated FTO glass
by the doctor blade technique, using two scotch tapes to limit the

S.-H. Jeon et al. / Electrochimica Acta 55 (2010) 5652–5658
5655
Table 2
Photovoltaic parameters and Rct of the DSSCs fabricated using electrolytes with different amido-ImI concentrations and electrical conductance (ꢂ) of the electrolytes. .
a
Jsc (mA cm 2
−
)
Voc (V)
FF
ꢀ (%)
Rct (ꢁ cm
−2
)
−1
ꢂ (mS cm )
Amido-ImI (M)
0
0
0
0
0
.4
.5
.6
.7
.8
11.31
13.07
14.36
15.47
13.62
0.72
0.74
0.76
0.74
0.73
0.58
0.50
0.52
0.52
0.48
4.71
4.80
5.71
5.92
4.71
8.08
6.80
5.76
5.20
5.68
0.74
0.88
1.04
1.15
1.06
a
The same electrolyte as in Table 1.
ited by the presence of 0.7 M amido-ImI up to the incident light
−
2
intensity of 100 mW cm
.
3.2.1. Jsc increase
The increase in the Jsc value of the DSSC with amido-ImI com-
pared to that with ref-ImI (Table 1) was attributed to the structural
change of the electrolyte associated with the formation of hydro-
gen bonds among the amido-ImI molecules. Amido-ImI contains
an amide group through which the molecules are connected to one
another by hydrogen bonds. Given the increase in the electrical
conductivity with increasing amido-ImI concentration, the forma-
tion of hydrogen bonds apparently creates conducting channels for
−
−
the charge carriers, I3 and I , which facilitate the ionic transport
through the electrolyte, and, consequently, leads to an increase
in Jsc compared to that with the reference electrolyte containing
ref-ImI.
The Jsc increase was supported by the chronoamperometric
plots shown in Fig. 6. The plots were obtained with two electro-
chemical cells containing either amido-ImI or ref-ImI, with each
Fig. 3. J–V curves of the DSSCs vs. amido-ImI concentration. The light intensity was
−
2
1
00 mW cm
.
cell containing 1 mM I , 10 mM LiI, and 0.1 M LiClO in acetonitrile.
2
4
1
0.2%, respectively, compared to those obtained with the DSSC con-
The figure reveals that the diffusion-limited currents (i ), described
d
taining the ref-ImI electrolyte under AM 1.5 simulated Xe light of
1
−
−
in Eq. (1) [42], for both the oxidation of I and the reduction of I3
−2
00 mW cm . However, in the case of the DSSC with the decyl-ImI
were larger for the cell with amido-ImI than for that with ref-ImI.
This indicated that the diffusion of the charge carriers was more
facilitated in the amido-ImI electrolyte than in the ref-ImI elec-
electrolyte, the Jsc and ꢀ values were lower than those of the amido-
ImI and ref-ImI electrolytes, which were attributed to the higher
viscosity and lower electrical conductance of the decyl-ImI elec-
trolyte compared to those of the ref-ImI and amido-ImI electrolytes.
The FF remained the same, regardless of the electrolytes.
We also investigated the effect of incident light intensity on the
performance of the DSSC fabricated with 0.7 M amido-ImI. The J–V
curves as a function of the incident light intensity ranging from
−
trolyte. The smaller increase in id for the reduction of I3 relative
−
to that for the oxidation of I was attributed to the 10-fold lower
−
−
concentration of I3 than I in the electrolyte.
nFAD1/2
ꢃ1/2t1/2
C
id =
(1)
− −2
2
6
0 mW cm to 100 mW cm are shown in Fig. 5, which reveals
that the Jsc showed a linear response with Voc, FF remained essen-
tially constant over the light intensity range. These results indicated
that the light absorption and charge carrier diffusion were not lim-
where n is the electron transfer number per molecule, F the Fara-
day constant, A the surface area of the electrode, D the diffusion
coefficient, C the concentration, and t is the time.
Fig. 4. J–V curves of the DSSCs fabricated with amido-ImI, ref-ImI and decyl-ImI
−2
at 100 mW cm . The concentrations of the imidazolium iodides were 0.7 M. Inset
Fig. 5. J–V curves of the DSSCs fabricated with 0.7 M amido-ImI at various incident
shows the corresponding J–V curves obtained without using 4-tert-butylpyridine.
light intensities.

5
656
S.-H. Jeon et al. / Electrochimica Acta 55 (2010) 5652–5658
Fig. 7. Nyquist plots of the DSSCs fabricated with ref-ImI at 0.7 M and with amido-
ImI at various concentrations. The plots were recorded over a frequency range of
5
0
.05–10 Hz with an AC amplitude of 5 mV.
Fig. 6. Chronoamperometric curves of the electrochemical cells assembled with the
ref-ImI and amido-ImI electrolytes in acetonitrile.
Jsc for the pertinent DSSC, compared to the Jsc values of the cells
with other amido-ImI concentrations (Table 2) and with ref-ImI at
0
.7 M (Table 1).
The variation of Jsc according to the electrolyte composition was
further evidenced by electrochemical impedance spectroscopy.
Fig. 7 compares the Nyquist plots of the DSSCs with ref-ImI at
3.2.2. Voc enhancement
0
.7 M and with amido-ImI at its different concentrations. The first
The enhanced Voc value of the DSSC with amido-ImI relative
to that with ref-ImI and decyl-ImI (Table 1) was related to the
semicircle represents the impedance related to the charge transfer
process occurring at the Pt/electrolyte interface in the DSSC [43,44].
The smallest charge transfer resistance (Rct) value (Table 2) with
adsorption of amido-ImI to the hydrophilic surface of the TiO par-
2
ticles through the amide group. This adsorption enabled the long
hydrophobic alkyl chain in amido-ImI to suppress the back electron
transfer from the TiO2 conduction band and the FTO/TiO2 inter-
0.7 M amido-ImI was well correlated with the highest Jsc value of
the corresponding DSSC. This reduced charge transfer resistance
with amido-ImI implied an easier electron transfer [44] from the
counter electrode to the I3 ions, and thus confirms the enhanced
−
face to the I3 ions in the electrolyte by reducing the direct contact
−
between the TiO surface and the electrolytic solution [45], thereby
2
Fig. 8. Variation of the photovoltaic parameters of the DSSCs assembled using the ref-ImI (black squares) and amido-ImI (red dots) electrolytes over a period of 21 d. The
concentrations of the imidazolium iodides were 0.7 M. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this
article.)

S.-H. Jeon et al. / Electrochimica Acta 55 (2010) 5652–5658
5657
Table 3
formation of hydrogen bonds among the amido-ImI molecules
in the electrolyte, which increased the electrical conductivity of
Photovoltaic parameters of the DSSCs fabricated without using 4-tert-
butylpyridine. .
a
−
−
the charge carriers, I3 and I , and reduced the charge transfer
resistance of the DSSC. The increase in the open-circuit voltage
was attributed to the adsorption of amido-ImI on the surface of
the TiO2 electrode, thereby reducing the back electron transfer
from the conduction band of TiO2 and the FTO/TiO2 interface to
Jsc (mA cm 2
−
)
Voc (V)
FF
ꢀ (%)
ImI (0.7 M)
Ref-ImI
Amido-ImI
Decyl-ImI
15.87
14.72
11.63
0.63
0.67
0.64
0.44
0.54
0.47
4.4
5.3
3.5
a
−
Electrolyte consisted of 0.7 M imidazolium iodide, 0.1 M LiI, and 0.05 M I2 in
-methoxypropionitrile.
the I3 ions in the electrolyte arising from the presence of a long
3
hydrophobic chain of amido-ImI. The DSSC with the amido-ImI
electrolyte exhibited better stability than that with the ref-ImI elec-
trolyte.
increasing Voc according to Eq. (2) [46]:
ꢀ
ꢁ
ꢀ
ꢁ
kT
e
Jsc
Jo
Voc =
ln
(2)
Acknowledgment
where Jo is the exchange current density and is related to the rate of
This work was supported by the Joint Research Project under
the KOSEF-JSPS Cooperative Program in 2009.
the back electron transfer. Furthermore, the adsorption of amido-
ImI to the surface of the TiO particles can also increase Voc by the
2
negative shift of the flatband potential (VFB) of the dye-coated TiO2
electrode, similarly to 4-tert-butylpyridine [47]. Under Fermi level
pinning, these two parameters are linked by Eq. (3) [48]:
References
ꢂ
ꢂ
[
[
1] B. O’Reagen, M. Grätzel, Nature 353 (1991) 737.
2] M. Grätzel, Nature 414 (2001) 338.
ꢂ
ꢂ
Voc = VFB − V
(3)
red
−
3
−
[3] M. Grätzel, Inorg. Chem. 44 (2005) 6841.
4] M. Grätzel, J. Photochem. Photobiol. A: Chem. 164 (2004) 3.
where V
is the reduction potential of the I /I redox couple.
red
[
Assuming that V remains the same regardless of the presence of
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amido-ImI in the electrolyte, the negative shift of VFB can increase
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did indeed enhance Voc by adsorption as 4-tert-butylpyridine.
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