Iodide-free ionic liquid with dual redox couples for dye-sensitized solar cells with high open-circuit voltage

Article abstract of DOI:10.1002/cssc.201403204

A novel ionic-liquid mediator, 1-butyl-3-{2-oxo-2-[(2,2,6,6-tetramethylpiperidin-4-yl)amino]ethyl}-1H-imidazol-3-ium selenocyanate (ITSeCN), has been successfully synthesized for dye-sensitized solar cells (DSSCs). ITSeCN possesses dual redox channels, imidazolium-functionalized 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) and selenocyanate, which can serve as the cationic redox mediator and the anionic redox mediator, respectively. Therefore, ITSeCN has a favorable redox nature, which results in a more positive standard potential, larger diffusivity, and better kinetic heterogeneous rate constant than those of iodide. The DSSC with the ITSeCN electrolyte shows an efficiency of 8.38 % with a high open-current voltage (V_OC) of 854.3 mV, and this V_OC value is about 150 mV higher than that for the iodide-based DSSC. Moreover, different electrocatalytic materials were employed to trigger the redox reaction of ITSeCN. The ITSeCN-based DSSC with the CoSe counter electrode achieved the best performance of 9.01 %, which suggested that transition-metal compound-type materials would be suitable for our newly synthesized ITSeCN mediator.

Full text of DOI:10.1002/cssc.201403204

DOI: 10.1002/cssc.201403204
Full Papers
Iodide-Free Ionic Liquid with Dual Redox Couples for Dye-
Sensitized Solar Cells with High Open-Circuit Voltage
Chun-Ting Li,[a] Chuan-Pei Lee,[a] Chi-Ta Lee,[a] Sie-Rong Li,[b] Shih-Sheng Sun,*[b] and Kuo-
Chuan Ho*[a, c]
A novel ionic-liquid mediator, 1-butyl-3-{2-oxo-2-[(2,2,6,6-tetra-
methylpiperidin-4-yl)amino]ethyl}-1H-imidazol-3-ium selenocya-
nate (ITSeCN), has been successfully synthesized for dye-sensi-
tized solar cells (DSSCs). ITSeCN possesses dual redox channels,
imidazolium-functionalized 2,2,6,6-tetramethylpiperidine N-oxyl
(TEMPO) and selenocyanate, which can serve as the cationic
redox mediator and the anionic redox mediator, respectively.
Therefore, ITSeCN has a favorable redox nature, which results
in a more positive standard potential, larger diffusivity, and
better kinetic heterogeneous rate constant than those of
iodide. The DSSC with the ITSeCN electrolyte shows an efficien-
cy of 8.38% with a high open-current voltage (VOC) of
854.3 mV, and this VOC value is about 150 mV higher than that
for the iodide-based DSSC. Moreover, different electrocatalytic
materials were employed to trigger the redox reaction of
ITSeCN. The ITSeCN-based DSSC with the CoSe counter elec-
trode achieved the best performance of 9.01%, which suggest-
ed that transition-metal compound-type materials would be
suitable for our newly synthesized ITSeCN mediator.
Introduction
The increased consumption of fossil fuel calls for the urgency
of developing renewable and green energy resources. Dye-sen-
sitized solar cells (DSSCs) are one of the promising clean
power sources of solar-to-electrical energy conversion for the
next generation. The DSSCs are photoelectrochemical devices
that are beneficial for academic and industrial development
due to their high conversion efficiency, potential low-cost roll-
to-roll printing production possibility, and good performance
under weak light intensity, as well as under various light inci-
dent angles.[1] It is well known that a standard DSSC is com-
posed of three important parts: the photoanode, electrolyte,
and counter electrode (CE). The photoanode contains a meso-
porous titanium dioxide (TiO2) film adsorbed with a single
layer of dye molecules (sensitizer) and provides photoinduced
electrons. The electrolyte mostly contains iodine/triiodide (Iꢀ/
I3ꢀ) redox mediator and acts as a hole conductor for dye re-
generation. The CE usually contains a platinum catalytic thin
ꢀ
film for the reduction of Iꢀ/I3 redox mediator.[1]
In recent years, the performance of DSSCs was limited by
ꢀ
several inherent drawbacks of Iꢀ/I3 redox mediator:[2] 1) strong
light absorption prohibits photons from reaching the dye mol-
ecules; 2) high corrosion to the platinum CE and metal-based
electron collectors reduces the durability of DSSCs; 3) the
more negative redox potential leads to a lower open-circuit
voltage (VOC) of DSSCs and less excessive driving force for dye
regeneration; 4) the triiodide ion is strongly electron deficient
and causes charge recombination and great energy loss; and
5) the complex internal chemical reaction (involving two-elec-
tron transfer) delays dye regeneration. Therefore, many alterna-
tive redox mediators were investigated,[2] including halogen
redox mediators (i.e., bromide),[3] pseudohalogen redox media-
tors (i.e., cyanide, cyanate, thiocyanate, and selenocyanate),[4]
polysulfide redox mediators (Sn2ꢀ/Sn+12ꢀ),[5] metal-complex
redox mediators (i.e., Co,[6] Fe,[7] Cu,[8] and Ni complexes[9]), and
organic redox mediators (i.e., 2,2,6,6-tetramethylpiperidine N-
oxyl (TEMPO) free radicals,[10] disulfide/thiolate,[11] tetrame-
thylthiourea derivatives,[12] 2-mercapto-5-methyl-1,3,4-thiadia-
zole derivatives,[13] l-cysteine/l-cystine,[14] and quinone deriva-
tives[15]). The cobalt(II/III) tris(bipyridyl)-based redox complex
was reported to achieve a high DSSC efficiency of 13.0% in
conjunction with a custom-synthesized zinc porphyrin dye
(designated SM315).[6b]
[a] C.-T. Li,+ Dr. C.-P. Lee,+ C.-T. Lee, Prof. K.-C. Ho
Department of Chemical Engineering, National Taiwan University
No. 1, Sec. 4, Roosevelt Road, Taipei 10617 (Taiwan)
E-mail: kcho@ntu.edu.tw
[b] Dr. S.-R. Li, Dr. S.-S. Sun
Institute of Chemistry, Academia Sinica
No. 128, Sec. 2, Academia Road
Nankang District, Taipei 11529 (Taiwan)ca.edu.tw
E-mail: sssun@chem.sini
Of these mediators, selenocyanate (SeCNꢀ/(SeCN)3ꢀ) redox
mediator is a pseudohalogen redox mediator, which has at-
tracted much attention due to its interesting chemical proper-
ties and good mass-transport characteristics. The internal
double or triple bonds of pseudohalogen redox mediators do
not affect their redox reactions; this results in faster kinetic
[c] Prof. K.-C. Ho
Institute of Polymer Science and Engineering
National Taiwan University, No. 1, Sec. 4
Roosevelt Road, Taipei 10617 (Taiwan)
[+] These authors contributed equally to this work.
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201403204.
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electron transfer than Iꢀ/I3ꢀ. The pseudohalogen redox media-
tors can provide more positive redox potentials than that of
Iꢀ/I3ꢀ, and thereby offer a larger energy difference between
their redox potentials and the Fermi level of the TiO2 photoa-
node than that of Iꢀ/I3ꢀ. Therefore, the theoretical higher VOC
values and lower energy losses for dye regeneration can be ex-
pected by using pseudohalogen redox mediators. In 2001, the
selenocyanate-based redox mediator was first introduced into
liquid-electrolyte-based DSSCs by Oskam et al.[4b] However, its
DSSC performance and the corresponding incident photon-to-
current conversion efficiency (IPCE) were poor due to ineffi-
cient dye regeneration of N3 dye by the SeCNꢀ ion. In 2004,
Wang et al. made a breakthrough by synthesizing the seleno-
cyanated-based ionic-liquid electrolyte 1-ethyl-3-methylimida-
zolium selenocyanate (EMISeCN), and the outstanding efficien-
cy of 8.3% demonstrates that the selenocyanate is a potential
bility.[10d] Min et al. mixed three kinds of benzimidazole (BI) de-
rivatives, namely, benzimidazole (BI), 1-methylbenzimidazole
(1MBI), and 5-methylbenzimidazole (5MBI), into the TEMPO-
based electrolytes separately, and the addition of those BIs
largely increased the VOC values from 690 mV to 860–924 mV
by shifting the band edge upwards.[10e] Chen et al. synthesized
a
novel imidazolium-functionalized TEMPO derivative, 1-
methyl-3-[2-oxo-2-(2,2,6,6-tetramethyl-1-oxyl-4-piperidoxyl)bu-
tyl]imidazolium bis(trifluoromethanesulfonyl)imide ([MeIm-
TEMPO][TFSI]), to prepare an imidazolium-functionalized
TEMPO/iodide hybrid redox mediator, and the corresponding
DSSC showed a high efficiency of 8.20%.[10f] Chu et al. synthe-
sized a novel ionic-liquid mediator, 1-butyl-3-{2-oxo-2-[(2,2,6,6-
tetramethylpiperidin-4-yl)amino]ethyl}-1H-imidazol-3-ium
iodide (JC-IL), which contained dual redox channels of imidazo-
lium-functionalized TEMPO and iodide.[10g] With respect to the
DSSC with the iodide mediator (6.90%), the DSSC with the JC-
IL mediator performed with a higher efficiency of 8.12% and
a high VOC of 858 mV, a short-circuit current density (JSC) of
13.70 mAcmꢀ2, and a FF of 0.69.
ꢀ
alternative for the Iꢀ/I3 redox mediator.[4c] Bergeron et al. re-
ported that the SnO2-based DSSC achieved comparable IPCE
ꢀ
and VOC values in the SeCNꢀ/(SeCN)3 electrolyte to those in
ꢀ
ꢀ
the Iꢀ/I3 electrolyte.[4d] Song et al. applied a mixture of Iꢀ/I3
ꢀ
and SeCNꢀ/(SeCN)3 mediators to reduce visible-light absorp-
tion and increase the VOC in the DSSC. A good cell efficiency of
7.6% with high VOC and fill factor (FF) values was achieved.[4f]
Bella et al. incorporated selenocyanate mediators into the
acrylic/methacrylic polymer membranes as the polymer elec-
trolyte for quasi-solid DSSCs.[4g] The electrochemical behavior
of the selenocyanate-based polymer electrolyte revealed fast
charge-transfer kinetics and outstanding long-term stability;
therefore, its DSSC showed a comparable efficiency to that of
the DSSC with the selenocyanate-based liquid electrolyte.
To summarize the research mentioned above, the mediator
containing the dual redox channels prepared by either physical
mixing or chemical synthesis seems to be a promising ap-
proach for further improving the performance of DSSCs in the
future. Because both selenocyanate and TEMPO mediators are
promising substitutions for Iꢀ/I3ꢀ, in this study, we synthesized
a novel ionic liquid, 1-butyl-3-{2-oxo-2-[(2,2,6,6-tetramethylpi-
peridin-4-yl)amino]ethyl}-1H-imidazol-3-ium
selenocyanate
(ITSeCN), through a simple, easy to scale up, and highly repro-
ducible three-step process with a good overall yield of about
60%. ITSeCN is designed to contain dual redox channels of
imidazolium-functionalized TEMPO and selenocyanate. Imida-
zolium-functionalized TEMPO serves as the cationic redox me-
diator, whereas selenocyanate serves as the anionic mediator.
Thus, ITSeCN has favorable redox features, which result in
a more positive standard potential, larger diffusivity, and
a better kinetic heterogeneous rate constant than those of
iodide.
On the other hand, the TEMPO organic radical, which con-
+
C
tains nitroxide as the redox couple (RNꢀO /RN=O ), also serves
as a desirable mediator in DSSCs due to its several attractive
features, including rapid and reversible one-electron redox ki-
netics, a more positive redox potential, a large diffusion coeffi-
cient, great heterogeneous electron-transfer rate constant,
rapid electron self-exchange reaction, and negligible visible-
light absorption.[16] Zhang et al. first introduced TEMPO /
TEMPO+ as an effective mediator with a high VOC value of
C
830 mV and reached a higher cell efficiency of 5.4% than that
ꢀ
of the DSSC with the Iꢀ/I3 redox mediator.[10a] However,
TEMPO may reach its limits for applications in DSSCs because Results and Discussion
unfavorable side reactions (i.e., irreversible ring cleavage)
Synthesis of ITSeCN
occur under oxidation. Fortunately, these side reactions can be
avoided by modifying the basic ring structure of TEMPO.[10b]
Therefore, research into TEMPO derivatives becomes impor-
tant. Kato et al. synthesized three substituted TEMPO redox
mediators (OH-TEMPO, NHCOCH3-TEMPO, and CN-TEMPO) for
DSSCs, and extremely high VOC values of around 860 mV could
be observed due to a better suppression ability for charge re-
combination in the DSSCs.[10c] They further synthesized an
isomer of TEMPO, namely, 2-azaadamantane N-oxyl (AZA). AZA
contains nitroxide radicals as the redox couple similar to
TEMPO, but AZA possesses a rigid, symmetric adamantane
framework, which provides steric protection around the un-
paired electrons. Eventually, the AZA-based DSSC reached
a high VOC value of 850 mV with good electrochemical reversi-
In accordance with our previous report,[10g] ITSeCN, was synthe-
sized by a three-step synthetic pathway (Scheme 1). First, inter-
mediate A of ITSeCN, 2-chloro-N-(2,2,6,6-tetramethylpiperidin-
4-yl)acetamide (ITSeCN-A), is prepared by an aminolysis reac-
tion from 4-amino-2,2,6,6-tetramethylpiperidine and chloroace-
tyl chloride. Second, ITSeCN-A is used to enable a quaterniza-
tion reaction with 1-butylimidazole, and thus, intermediate B
of ITSeCN, 1-butyl-3-{2-oxo-2-[(2,2,6,6-tetramethylpiperidin-4-
yl)amino]ethyl}-1H-imidazol-3-ium chloride (ITSeCN-B), is ob-
tained. Third, the NꢀH site on ITSeCN-B is converted into the
C
NꢀO radical group through an oxidation reaction. The final
product, ITSeCN, was obtained through an anion-exchange re-
action with potassium selenocyanate.
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iodide, we focus on the second
redox pair (a2 and c2) because it
represents the major redox reac-
tion (Iꢀ/I3ꢀ) in the DSSCs.
2 I3 $ 3 I2 þ 2 eꢀ
3 Iꢀ $ I3ꢀ þ 2 eꢀ
ð1Þ
ð2Þ
ꢀ
In Figure 1b, TEMPO has one
redox pair (redox pair 3) at
0.44 V versus Ag/Ag+ (0.93 V vs.
NHE), which corresponds to
Equation (3) (the reaction of the
+
C
TEMPO radical, NꢀO /N=O ) and
is represented by a3 and c3.
þ
ꢀ
C
TEMPO $ TEMPO þ e
ð3Þ
Scheme 1. Synthetic pathway to ITSeCN. DCM=dichloromethane, ACN=acetonitrile.
Selenocyanate has one redox
pair (redox pair 4) at 0.12 V
versus Ag/Ag+ (0.61 V vs. NHE),
which corresponds to the reac-
UV/Vis spectroscopy
UV/Vis absorption spectra of various electrolytes, including
iodide, TEMPO, selenocyanate, a mixture of TEMPO and seleno-
cyanate (referred to hereafter as binary), and ITSeCN, are
shown in Figure S1 in the Supporting Information. For
a proper demonstration of the light absorption effect of an
electrolyte in its DSSC, each electrolyte contained a tenth of
the concentration of that used for the DSSC. The standard
iodide electrolyte shows a strong light absorption in the wave-
length range from l=400 to 500 nm, which causes a loss of
visible-light radiation for dye excitation. The TEMPO electrolyte
shows a negligible absorption band in the visible wavelength
range from l=400 to 500 nm. The selenocyanate, binary, and
ITSeCN electrolytes show similar absorption bands in the wave-
length range from l=300 to 350 nm. Among these three sele-
nocyanate-based electrolytes, ITSeCN shows the lowest visible-
light absorption; thus it can be confirmed that ITSeCN is more
beneficial for dye excitation in its DSSC.
tion of Equation (4) and is represented by a4 and c4.
3 ðSeCNÞꢀ $ ðSeCNÞ3ꢀ þ 2 eꢀ
ð4Þ
According to Equations (3) and (4), the binary electrolyte
possesses two redox pairs. The redox pair at a more positive
potential of 0.54 V versus Ag/Ag+ (1.03 V vs. NHE) corresponds
to TEMPO [Eq. (3)]; the redox pair at a more negative potential
of 0.12 V versus Ag/Ag+ (0.61 V vs. NHE) corresponds to sele-
nocyanate [Eq. (4)]. Similarly, the synthesized ITSeCN electro-
lyte (the chemical combination of both TEMPO and selenocya-
nate) possesses two redox pairs. The redox pair at a more posi-
tive potential of 0.56 V versus Ag/Ag+ (1.05 V vs. NHE) corre-
sponds to TEMPO [Eq. (3)]; the redox pair at more negative po-
tential of 0.15 V versus Ag/Ag+ (0.64 V vs. NHE) corresponds to
selenocyanate [Eq. (4)]. Both binary and ITSeCN electrolytes
perfectly match the corresponding redox pairs of both TEMPO
and selenocyanate electrolytes. However, a higher current den-
sity can be observed for ITSeCN due to the stronger interaction
between TEMPO and selenocyanate. ITSeCN has promising
Cyclic voltammetry
Cyclic voltammograms of various mediators, including iodide,
TEMPO, selenocyanate, binary, and ITSeCN, were measured in
ACN containing 0.1m LiClO4 and 10.0 mm of different redox
mediators separately. The corresponding parameters are sum-
marized in Table 1. An imidazolium-functionalized iodide spe-
cies, 1,2-dimethyl-3-propylimidazolium iodide (DMPII), was
used as the iodide redox mediators for comparison with newly
synthesized ITSeCN (also containing the imidazolium-function-
alized cation). In Figure 1a, iodide possesses two redox pairs.
Redox pair 1 at a more positive potential of 0.48 V versus Ag/
Ag+ (0.97 V vs. NHE), represented by a1 and c1, corresponds
to the reaction shown in Equation (1); redox pair 2 at a more
negative potential of 0.03 V versus Ag/Ag+ (0.52 V vs. NHE),
represented by a2 and c2, corresponds to Equation (2). As for
Table 1. Electrochemical properties of various electrolytes.
Electrolyte
Redox
pair
DE
[V]
E0’[a]
[V vs. NHE]
D0 ꢁ10ꢀ6
[cm2 sꢀ1
k0 ꢁ10ꢀ3
]
[cmsꢀ1
]
a1&c1
a2&c2
a3&c3
a4&c4
a3&c3
a4&c4
a3&c3
a4&c4
0.29
0.51
0.47
0.40
0.16
0.42
0.33
0.36
0.97
0.52
0.93
0.61
1.03
0.61
1.05
0.64
6.27
7.00
75.58
7.71
76.85
7.94
101.47
8.07
1.06
1.18
11.85
1.45
11.98
1.48
14.33
1.49
iodide
TEMPO
selenocyanate
binary
ITSeCN
[a] NHE=normal hydrogen electrode.
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Figure 2. Energy level diagram for various mediators: a) TiO2, b) CR147 dye,
c) iodide, d) TEMPO, e) selenocyanate, f) binary, and g) ITSeCn.
Rotating disk electrode (RDE) analysis
A rotating disk electrode (RDE) system was used to obtain the
diffusion coefficients (D0) and heterogeneous electron-transfer
rate constants (k0) of various redox pairs [Eqs. (1)–(4)]. By oper-
ating at different rotating speeds (50, 100, 200, 400, 600, 800,
and 1000 rpm), RDE analysis was performed under a linear
sweep voltage (LSV) scan from ꢀ0.4 to 0.8 V at a scan rate of
1.0 mVsꢀ1. The corresponding reciprocal currents (iꢀ1) were ob-
tained from the LSV curves (not shown) at the formal poten-
tials (E0’) of the redox pair. Figure 3a and b shows the plots of
iꢀ1 versus wꢀ1/2, and the values of D0 and k0 were calculated
from the slopes and intercepts of the fitting lines of the plots,
Figure 1. Cyclic voltammograms of a) iodide and b) the other electrolytes,
including TEMPO, selenocyanate, binary, and ITSeCN, measured at a scan
rate of 100 mVsꢀ1
.
redox abilities, which are consistent with predictions from its
structure (see Scheme 1).
´
In comparison with various electrolytes, ITSeCN possesses
the highest redox potential by its imidazolium-functionalized
TEMPO cation, which provides a minimized energy loss for dye
regeneration, as shown in Figure 2. On the other hand, ITSeCN
possesses a higher redox potential for its selenocyanate anion
than that of Iꢀ/I3ꢀ; therefore, ITSeCN provides a larger voltage
difference to the Fermi level of TiO2, and thus, a higher VOC
value for the cell is achieved. Moreover, ITSeCN provides DE
values of 0.33 (from TEMPO) and 0.36 V (from selenocyanate),
which are smaller than the values of bare TEMPO and bare se-
lenocyanate electrolytes, respectively. Thus, it can be conclud-
ed that ITSeCN can provide better reversibility for the desired
redox reaction than those of bare TEMPO and bare selenocya-
nate.
respectively, by using the simplified Koutecky–Levich equation
[Eq. (5)].[17]
1
i
1
1
¼
þ
ð5Þ
0:62nFACD20=3uꢀ1=6w1=2
nFAk0C
in which i is the current obtained from a LSV curve at E0’, n is
the number of electrons transferred, F is the Faraday constant,
A is the active surface area of the RDE, C is the bulk concentra-
tion of each mediator (10.0 mm), n is the kinematic viscosity of
ACN, and w is the angular velocity converted from the rotating
speed.
In Table 1, all D0 values represent the diffusion coefficients
for the reduced species of each redox mediator. The extremely
high D0 values of redox pairs 3 in TEMPO, binary, and ITSeCN
mediators can be observed; this indicates a large diffusivity
To further investigate the reversibility of our newly synthe-
sized ITSeCN, a long-term cyclic voltammetry (CV) test was per-
formed for 300 scan cycles (see Figure S2 in the Supporting In-
formation). With increasing scan cycles from 1 to 300, the
values of the anodic and cathodic peak current densities, cor-
responding to TEMPO (a3&c3) and selenocyanate (a4&c4)
redox pairs, all slightly decreased, but they all maintained up
to 97% of their initial values after 300 scan cycles. Thus, it can
be verified that ITSeCN possesses both favorable electrochemi-
cal reversibility and stability.
C
and small resistance of TEMPO . Generally, a larger molecular
structure has smaller diffusion coefficient. Surprisingly,
a
TEMPO, containing the NꢀO radical, possesses a larger molecu-
lar structure, but has much higher D0 values than those of
more compact iodide and selenocyanate. Among all mediators
containing TEMPO, synthesized ITSeCN gives the highest D0
C
values of TEMPO due to the beneficial cationic design by com-
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100 mWcmꢀ2 light illumination, the standard DSSC with iodide
electrolyte reached a conversion efficiency of 7.44% with
a normal VOC of 699.6 mV, JSC of 16.88 mAcmꢀ2, and FF of 0.63.
In Figure 4a, the DSSC with TEMPO electrolyte shows a promis-
ing high VOC value of 826.6 mV and FF of 0.66 due to its high
redox potential, large diffusivity, and fast charge-transfer ability.
The DSSC with selenocyanate electrolyte shows a higher VOC of
713.5 mV than that of iodide due to its more positive redox
potential, and provides a good JSC value of 14.25 mAcmꢀ2 due
to its fast electron-transfer kinetics. The DSSC with the binary
electrolyte combines the advantages of both TEMPO and sele-
nocyanate electrolytes. A slight enhancement of the cell per-
formance can be observed by increased VOC and FF values
compared with those values of the DSSC with selenocyanate
electrolyte. The DSSC with the newly synthesized iodide-free
ITSeCN electrolyte reaches the highest performance of 8.38%
with an extremely high VOC value of 854.3 mV, JSC of
Figure 3. Plots of iꢀ1 versus wꢀ0.5 for a) the redox pairs of DMPII, TEMPO, and
selenocyanate; b) the redox pairs of binary and ITSeCN.
bining imidazolium and TEMPO. On the other hand, the higher
D0 values of redox pairs 4 in selenocyanate, binary, and ITSeCN
mediators indicate better diffusivity and smaller resistance of
SeCNꢀ than those of iodide species. Based on different media-
tors, the k0 value for each redox pair can be individually evalu-
ated, as shown in Table 1. The extremely high k0 values of
redox pairs 3 in TEMPO, binary, and ITSeCN mediators can be
observed; this indicates their very fast charge-transfer ability
and high heterogeneous reactivity. Among these three media-
tors containing TEMPO, synthesized ITSeCN achieves the high-
est k0 values of redox pairs 3 due to the enhanced resonance
phenomenon at its imidazole ring, which makes the NꢀO radi-
cal highly reactive. Moreover, the k0 values of redox pairs 4 in
selenocyanate, binary, and ITSeCN mediators are all higher
than the k0 values of iodide species. Thus, the preferable kinet-
ic heterogeneous reactivity of selenocyanate can be clearly ob-
served. In brief, the combination of TEMPO and selenocyanate
results in both the highest D0 and k0 values for ITSeCN and
brings the largest diffusivity, least resistance, and best kinetic
heterogeneous reactivity compared with other medi-
Figure 4. Current density–voltage curves of the DSSCs with various electro-
lytes measured under a) 100 mWcmꢀ2 (AM 1.5G) light intensity and b) dark
conditions.
ators.
Table 2. Photovoltaic parameters of the DSSCs with various electrolytes.
Electrolyte
h
VOC
[mV]
JSC
[mAcmꢀ2
FF
JSC-IPCE
[mAcmꢀ2
Rct2
[W]
Rrec
[W]
Photovoltaic performance
[%]
]
]
The current density–voltage characteristics (J–V
curves) of the DSSCs with various electrolytes, includ-
ing iodide, TEMPO, selenocyanate, binary, and
ITSeCN, are shown in Figure 4. The corresponding
photovoltaic parameters are listed in Table 2. Under
iodide
TEMPO
selenocyanate
binary
ITSeCN
7.44
3.01
6.37
6.96
8.38
699.6
826.6
713.5
735.3
854.3
16.88
5.54
14.25
14.24
14.70
0.63
0.66
0.63
0.67
0.67
15.86
4.56
13.23
13.12
13.94
12.25
31.43
16.40
15.96
15.78
11.86
23.21
18.97
19.08
31.92
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14.70 mAcmꢀ2, and FF of 0.67. The highest VOC can be ascribed
to the fact that ITSeCN provides the most positive redox po-
tential. The good FF and JSC values are attributed to the attrac-
tive properties of ITSeCN, including its negligible absorption in
the visible region, most positive redox potential, highest diffu-
sivity, and best kinetic heterogeneous reactivity; these proper-
ties can reduce the energy loss for dye regeneration, charge
transfer, and charge recombination.[18]
The higher IPCE value indicates a higher photocurrent densi-
ty under short-circuit conditions. In Figure S3 in the Supporting
Information, the IPCE values were carefully calibrated by elimi-
nating the effect of background transmittance limitation of the
fluorine-doped tin oxide (FTO) conducting substrate. The maxi-
mum IPCE value of DSSCs with various electrolytes, including
iodide (Iꢀ/I3ꢀ), TEMPO, selenocyanate (SeCNꢀ/(SeCN)3ꢀ), binary,
and ITSeCN, were 95.46, 48.15, 90.89, 90.89, and 95.67%, re-
spectively. The area under each IPCE curve can be further inte-
grated to calculate the theoretical short-circuit current density
(JSC-IPCE) of each DSSC. The calculated JSC-IPCE values were 15.86,
4.56, 13.23, 13.12, and 13.94 mAcmꢀ2, corresponding to the
DSSCs with iodide, TEMPO, selenocyanate, binary, and ITSeCN,
respectively. Thus, it can be concluded that the JSC-IPCE values
showed great consistency with the JSC values obtained from
the J–V curves.
To further investigate the high VOC phenomenon, the dark
current density–voltage characteristics of DSSCs with various
electrolytes are used to estimate the corresponding onset bias
values. The values of onset bias can be obtained from the in-
tersection points of the two fitting lines: one is the tangent to
the curve, starting from the voltage axis, and the other is the
zero-current line. In Figure 4b, the onset bias values of DSSCs
with iodide, TEMPO, selenocyanate, binary, and ITSeCN electro-
lytes occur at 594.6, 728.3, 624.3, 683.8, and 777.9 mV, respec-
tively. These values show good consistency with the trend of
the VOC values of their DSSCs. The DSSC with the ITSeCN elec-
trolyte shows the largest onset bias for the dark current, and it
represents the most efficient suppression for charge recombi-
nation; thus leading to the highest VOC value. On the other
hand, the highest VOC of the ITSeCN-based DSSC can be ex-
plained by the redox potentials of ITSeCN. Generally, the theo-
retical VOC of a DSSC is determined by the difference between
the redox potential of a mediator and the Fermi level of a TiO2
photoanode. However, a DSSC would have inevitable energy
loss during the charge-transfer process (i.e., recombination or
heterogeneous electron-transfer reaction); thus, the practical
VOC often differs from its theoretical value. According to the
energy diagram mentioned above (Figure 2) and the photovol-
taic parameters of our DSSCs, TEMPO-based DSSC would have
an energy loss of around 0.6 V, whereas the selenocyanate-
based DSSC would have an energy loss of around 0.4 V (see
Scheme S1 in the Supporting Information). If selenocyanate
dominates the redox reaction in the ITSeCN-based DSSC, the
VOC value is predicted to be about 0.74 V. Considering that the
real VOC of ITSeCN-based DSSC is 0.85 V, we infer that it may be
mainly determined by the selenocyanate mediator; with the
presence of TEMPO, the energy loss of the ITSeCN-based DSSC
could be further reduced, and thus, the real VOC value (0.85 V)
is larger than that predicted (0.74 V). On the other hand, if
TEMPO dominates the redox reaction in the ITSeCN-based
DSSC, the real VOC value (0.85 V) is much smaller than that pre-
dicted (0.95 V); therefore, we infer that the final VOC of the
ITSeCN-based DSSC may not be dominated by TEMPO.
Electrochemical impedance spectroscopy (EIS)
The above-mentioned results obtained from the J–V curves
and IPCE spectra can be further verified by the interfacial resis-
tances obtained from electrochemical impedance spectroscopy
(EIS). The EIS results were analyzed by means of the equivalent
circuit model, as shown in the inset of Figure 5a, based on the
DSSC configuration of FTO/photoanode/electrolyte/CE/FTO.
The EIS spectrum of a DSSC generally shows three semicircles
in the frequency range of 10 mHz to 65 kHz. The ohmic series
resistance (Rs) is determined in the high-frequency region
where the phase is zero. The first and second semicircles in the
medium-frequency range represent the heterogeneous elec-
tron-transfer resistance at the CE/electrolyte interface (Rct1) and
TiO2/dye/electrolyte interface (Rct2), respectively.[19] The third
semicircle refers to Warburg diffusion process in the electrolyte
(Rdiff), which is often overlapped with Rct2 due to the short dif-
fusion length controlled by the thin spacer (25 mm thick), and
owing to the low viscosity of the ACN solvent used in our elec-
trolyte.[19] Based on the same photoanode and platinum CE,
we focused on investigating the charge-transfer phenomenon
introduced by different electrolytes at the photoanode/electro-
lyte interface by comparing the Rct2 values measured under
100 mWcmꢀ2 light illumination. The Rct2 value of each DSSC
refers to the diameter of the second semicircle (Figure 5a). The
larger Rct2 value implies less charge transfer through the photo-
anode/electrolyte interface, and thus, results in a lower JSC
value for the DSSC. In Table 2, the Rct2 values show an order of
TEMPO>selenocyanate>binary>ITSeCN>iodide, which re-
veals good consistency with the JSC values for the DSSCs. The
lowest Rct2 of the iodide-based DSSC is attributed to the faster
charge-transfer nature supported by Li+ from the LiI compo-
nent in the iodide electrolyte; therefore, the highest JSC can be
observed. However, lithium ions are highly reactive and cause
severe charge recombination, so that the VOC value of the
iodide-based DSSC is lowest. Among the non-iodide-based
DSSCs, the ITSeCN-based DSSC provides the lowest Rct2. The
lowest Rct2 is due to the largest diffusivity (D0) and best kinetic
heterogeneous reactivity (k0); thus resulting in a high JSC value
for the DSSC.
The IPCE is defined as the percentage of the incident pho-
tons transferred to the electrons at a given wavelength, which
can be measured under short-circuit conditions and calculated
by using Equation (6).
1240JSCðlÞ
ð6Þ
IPCEðlÞ ¼
lf
in which l is the wavelength, JSC (l) is the short-circuit photo-
current density obtained at a specific wavelength (mAcmꢀ2),
and f is the incident radiation flux (mWcmꢀ2).
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Figure 5. EIS results for DSSCs with various electrolytes measured a) under
100 mWcmꢀ2 (AM 1.5G) light illumination and b) under dark conditions with
a ꢀ0.90 V bias.
Figure 6. a) Photocurrent density–voltage curves of the DSSCs coupled with
various electrocatalytic CEs based on the newly synthesized ITSeCN electro-
lyte measured under 100 mWcmꢀ2 (AM 1.5G) light intensity. b) Cyclic vol-
tammograms of various CEs measured in the ITSeCN electrolyte at a scan
rate of 100 mVsꢀ1
.
Under dark conditions, the EIS spectra measured at ꢀ0.90 V
can provide the charge recombination resistance (Rrec) at the
photoanode/electrolyte interface. The Rrec value of each DSSC
can be estimated by fitting the diameter of the second semicir-
cle[19] (Figure 5b). The Rrec values show a tendency of ITSeCN>
TEMPO>binary>selenocyanate>iodide. The higher Rrec value
implies that less charge recombination occurs at the photoa-
node/electrolyte interface. The ITSeCN-based DSSC shows the
highest Rrec value, which contributes to the great retardation
ability for charge recombination provided by the large imida-
zolium cation of ITSeCN. Therefore, the highest VOC value for
the ITSeCN-based DSSC can be explained.
9.01%, respectively, than that of the DSSC with Pt CE (8.38%).
The higher efficiencies come from the clear increase in the VOC
and FF values, as summarized in Table S1 in the Supporting In-
formation. The increased VOC and FF values are attributed to
the fact that the PEDOT and CoSe CEs provide greater electro-
catalytic kinetics for ITSeCN regeneration; thus preventing
charge recombination and energy loss at the photoanode/elec-
trolyte interface. Overall, the DSSC with a CoSe CE achieves the
best performance of 9.01% with an extremely high VOC value
of 862.3 mV, a JSC of 14.86 mAcmꢀ2, and FF of 0.70. Therefore,
among all electrocatalytic materials, transition-metal com-
pound-type materials would be suitable for our newly synthe-
sized ITSeCN mediator.
Photovoltaic performance of ITSeCN-based DSSCs contain-
ing various CEs
To further investigate a suitable electrocatalytic material for
triggering redox of the ITSeCN mediator, photocurrent densi-
ty–voltage curves of the DSSCs coupled with various electroca-
talytic films as the CEs were measured against the newly syn-
thesized ITSeCN electrolyte under 100 mWcmꢀ2 light illumina-
tion. Several materials were used for pertinent comparisons:
for instance, platinum (metal type), poly(3,4-ethylenedioxythio-
phene) (PEDOT; conducting polymer type), CoSe (transition-
metal compound type), and carbon black (carbon type) films
were chosen to represent four types of electrocatalytic materi-
als in DSSCs. As shown in Figure 6a, the DSSCs with PEDOT
and CoSe CEs achieved better performances of 8.67 and
Surface morphology and CV of various CEs
The performance of ITSeCN-based DSSCs with different CEs
can be further explained by the surface morphology and CV of
the CEs. In Figure S4 in the Supporting Information, the field-
emission (FE) SEM image of platinum shows a noncontinuous
thin film covering on the FTO substrate. This indicates unfavor-
able charge transfer at the interface between the Pt CE and
ITSeCN electrolyte. In Figure S3b and c in the Supporting Infor-
mation, the PEDOT film shows a highly porous structure with
some puny aggregations, and the CoSe film shows a coral-like
morphology. Both PEDOT and CoSe films can provide a large
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surface area for electrocatalyzing redox of the ITSeCN media-
tor; therefore, the high JSC values can be obtained for their
DSSCs. In Figure S3d in the Supporting Information, the
carbon black film shows a relatively dense structure, which
suggests a smaller electrocatalytic surface area, so that the
DSSC delivered a lower JSC value and lower cell efficiency.
On the other hand, for various CEs in the ITSeCN electrolyte,
the major reaction at the CE/electrolyte interface can be repre-
sented by Equation (7).
electrolyte had an h of 7.44% with the same CR147 dye. The
VOC value of the cell with ITSeCN was far higher than that of
the cell with iodide, and this could be attributed to the fact
that ITSeCN could provide a much higher redox potential than
that of the iodide. The good FF and JSC values of the cells with
ITSeCN could be explained by its negligible absorption in the
visible region. The highest redox potential, highest D0, and
best k0 of ITSeCN could reduce the energy loss for dye regen-
eration, charge transfer, and charge recombination processes.
Moreover, different electrocatalytic materials were employed to
trigger the redox reaction of the ITSeCN mediator. The ITSeCN-
based DSSC with the CoSe CE achieved the best performance
ðSeCNÞ3ꢀ þ 2 eꢀ ! 3 ðSeCNÞꢀ
ð7Þ
Figure 6b shows the CV characteristics in the ITSeCN electro-
lyte under various electrocatalytic films. Two key factors,
namely, the cathodic peak current density (Jcp) and peak sepa-
ration (DEp), can be used to evaluate the overall electrocatalyt-
ic ability and kinetic reduction capability for ITSeCN, respec-
tively. A larger value of Jcp results in a better overall electroca-
talytic ability. On the other hand, based on the same ITSeCN
electrolyte, a smaller value of DEp results in a faster electrocata-
lytic reduction capability of the films.[20] As shown in Table S1
in the Supporting Information, both PEDOT and CoSe films
possess higher Jcp and lower DEp values than those of the Pt
film; thus, preferable electrocatalytic ability and better redox
kinetics for ITSeCN can be confirmed. Among all electrocatalyt-
ic materials, CoSe provides the highest Jcp value of
1.95 mAcmꢀ2 and lowest DEp value of 0.43 V; this indicates the
best electrocatalytic ability toward ITSeCN. Additionally, CoSe
of 9.01% with a high VOC value of 862.3 mV, JSC of
14.86 mAcmꢀ2, and FF of 0.70. Thus, the transition-metal com-
pound-type materials were suitable for our newly synthesized
ITSeCN mediator.
Experimental Section
Materials
ACN (99.99%), HNO3 (ca. 65% solution in water), DCM (99.99%),
and acetone (99%) were obtained from J. T. Baker. 4-Amino-2,2,6,6-
tetramethylpiperidine (>97%), chloroacetyl chloride (99%), and
TEMPO (99%) were purchased from Fluka. 1-Butylimidazole (98%),
H2O2 (ca. 35% solution in water), LiClO4 (ꢁ98.0%), NOBF4 (99%), ti-
tanium(IV) tetraisopropoxide (TTIP,>98%), EtOH (99.5%), isopropa-
nol (IPA, 99.5%), MeOH (99.5%), HCl (ca. 37% solution in water), 2-
methoxyethanol (ꢁ99.5%), KSeCN (99%), 3,4-ethylenedioxythio-
phene (EDOT, 99%), Nafion solution, KOH (99%), and H2WO4 (99%)
were bought from Sigma Aldrich. Selenourea (99%) was obtained
from Alfa Aesar. CoCl2·6H2O (98%) was purchased from Showa
Chemical Co. Ltd. Carbon black nanoparticles (CB-NPs, black pearls
2000) were purchased from Cabot Corporation, Alpharetta, USA. LiI
(analytical grade), I2 (analytical grade), and poly(ethylene glycol)
(PEG; MWꢂ20000) were obtained from Merck. DMPII (99%) was
purchased from Tokyo Chemical Industry Co. Ltd. Br2 (99.99%) was
obtained from Kanto Chemical Co., Inc. The 25 mm-thick Surlyn
(SX1170–25) was obtained from Solaronix (S.A., Aubonne, Switzer-
land). FTO (TEC-7, 7 Wsqꢀ1) conducting glasses were purchased
from NSG America, Inc., NJ, USA. Commercial light-scattering TiO2
particles, ST-41 at 200 nm, were obtained from Ishihara Sangyo
Ltd.
ꢀ
has a large electrocatalytic surface area for (SeCN)3 reduction
(Figure S4 in the Supporting Information). CoSe facilitates the
heterogeneous charge-transfer rate, and thus, reduces the
energy loss in an ITSeCN-based DSSC, even better than plati-
num. Thus, the JSC, FF, and efficiency of the cell with CoSe CE
are higher than those of the cell with Pt.
Conclusions
In this study, a new ionic-liquid mediator, ITSeCN, was synthe-
sized for use in DSSCs. The cationic part of ITSeCN gave the
redox channel of imidazolium-functionalized TEMPO contain-
+
C
ing the nitroxide (RNꢀO /RN=O ) as the redox center. The
anionic part of ITSeCN gave the redox channel of selenocya-
nate (SeCNꢀ/(SeCN)3ꢀ). Therefore, the ITSeCN possessed dual
redox channels of imidazolium-functionalized TEMPO and sele-
nocyanate, and both exhibited distinct redox potentials and at-
tractive electrochemical characteristics. After characterizing
each redox couple by CV and RDE analyses, ITSeCN showed an
outstanding redox nature, including the most positive redox
Synthesis of ITSeCN through a three-step synthetic pathway
The new ionic liquid ITSeCN, containing the imidazolium functional
group, TEMPO free radical, and selenocyanate anion, was synthe-
sized as previously reported.[10e] All reactions were carried out
under a nitrogen atmosphere by using standard Schlenk tech-
niques. All organic solvents used were purified by standard proce-
dures or purged with nitrogen before use, and all chemicals used
herein were of the purest quality. The synthetic procedure for
ITSeCN can be split into the following three steps, as shown in
Scheme 1.
’
potential (E0 ), the largest diffusivity (D0), and the largest kinetic
heterogeneous rate constant (k0), compared with the other
mediators. The UV/Vis absorption spectrum of ITSeCN showed
a negligible absorption in the visible range of l=400–800 nm;
this was beneficial for dye excitation in the DSSC. Five types of
electrolytes, containing iodide, TEMPO, selenocyanate, binary,
and ITSeCN mediators, were used in DSSCs. The DSSCs with
the ITSeCN electrolyte had a power conversion, h, of 8.38%
with CR147 dye, whereas the DSSC with a conventional iodide
Step 1: Preparation of ITSeCN-A
In the first step, intermediate A of ITSeCN (ITSeCN-A) was prepared
by aminolysis of the starting materials. A mixture of 0.50m 4-
amino-2,2,6,6-tetramethylpiperidine and 0.60m chloroacetyl chlo-
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ride in DCM was stirred at 608C for 8 h. The reaction was
quenched by the addition of an aqueous solution of HCl, and the
mixture was extracted with aqueous solution of HCl. A solution of
KOH was added, the solution was filtered, the precipitate was
dried under vacuum to give 2-chloro-N-(2,2,6,6-tetramethylpiperi-
din-4-yl)acetamide (ITSeCN-A) as a white solid (80%).
and reference electrodes, respectively. The PEDOT thin films were
synthesized in an ACN-based electrolyte containing 10 mm EDOT
monomer and 0.1m LiClO4 under a constant current of 4 mA to
reach a charge capacity of 40 mCcmꢀ2 [24]
The CoSe thin films were
.
electrodeposited in a three-electrode electrochemical system by
using a potentiostat/galvanostat. The 1 cm2 cleaned FTO substrate
was used as the working electrode, and the Pt foil and Ag/AgCl
electrode were used as the counter and reference electrodes, re-
spectively. The CoSe thin films were synthesized in an aqueous
deposition bath containing 5.0 mm CoCl2·6H2O and 0.75m sele-
nourea under a pulse potential deposition procedure for 10 cycles,
which consisted of a ꢀ0.9 V cathodic bias for 6 s and a 0.1 V
anodic bias for 24 s.[25] The carbon black thin films were prepared
by drop coating the slurry, containing 10.0 wt% CB-NPs and
2.0 vol% of Nafion (2.00% with respect to EtOH-based aqua solu-
tion), on the cleaned FTO at 608C.[26]
Step 2: Preparation of ITSeCN-B
In the second step, intermediate B of ITSeCN (ITSeCN-B) was pre-
pared by quaternization of ITSeCN-A with 1-butylimidazole.[21] The
mixture of 0.20m ITSeCN-A and 0.25m 1-butylimidazole in ACN
was stirred at 608C for 20 h in a nitrogen atmosphere. After cool-
ing to room temperature, the solvent was removed, and the pre-
cipitate was poured into diethyl ether to remove excess reactants.
The precipitate was filtered and dried under vacuum to give 1-
butyl-3-{2-oxo-2-[(2,2,6,6-tetramethylpiperidin-4-yl)amino]ethyl}-1H-
imidazol-3-ium chloride (ITSeCN-B) as a white solid (90%).
Assembly of DSSCs
A TiO2 colloid was synthesized by adding 0.5m TTIP to a 0.1m
aqueous solution of nitric acid with constant stirring and heating
at 888C for 8 h. To further grow the TiO2 nanoparticles uniformly
(ca. 20 nm), the colloid was transferred to an autoclave (PARR
4540, USA) and heated at 2408C for 12 h. Then, the TiO2 colloid
was concentrated to contain 8 wt% TiO2 nanoparticles (the con-
centrated TiO2 colloid). Subsequently, the transparent layer (TL)
paste was prepared by adding 25 wt% PEG to the concentrated
TiO2 colloid. The scattering layer (SL) paste was prepared by
adding 25 wt% PEG and 100 wt% of ST-41 (with respect to the
weight of TiO2 nanoparticles) to the concentrated TiO2 colloid to
prevent the aggregation of TiO2 nanoparticles and reduce the loss
of light, respectively. In the next stage, FTO conducting glasses
were first cleaned with a neutral cleaner and then washed with de-
ionized water (DIW), acetone, and isopropanol sequentially. A good
mechanical contact layer was first formed on the cleaned FTO con-
ducting surface through treatment with a mixture of TTIP and 2-
methoxyethanol (weight ratio of 1:3). The 6 mm porous TiO2 film
was coated on the treated FTO by means of the doctor blade tech-
nique, and consisted of a 3 mm TL and a 3 mm SL by using the TL
and SL pastes mentioned above, respectively. After a sintering pro-
cess at 5008C for 30 min in air, the TiO2 photoanode (total 6 mm)
was controlled at 0.16 cm2 as the active area and immersed in
a 3ꢁ10ꢀ4 m DCM-based solution of CR147 organic dye (Scheme S2
in the Supporting Information)[27] at room temperature for 24 h.
Then, the photoanodes were coupled with various CEs with a fixed
cell gap of 25 mm by using the heated 25 mm-thick Surlyn. Various
electrolytes mentioned above were injected into the gap between
these two electrodes by capillarity.
Step 3: Preparation of ITSeCN
In the last step, the NꢀH sites on ITSeCN-B (as shown in Scheme 1)
C
were converted into NꢀO , and thus, there was one free radical on
the NꢀO site obtained through an oxidation reaction.[22] A mixture
of 0.20m ITSeCN-B, 1.0m H2O2, and 1.0 mm H2WO4 in MeOH was
stirred at 608C for 24 h. After cooling to room temperature, MeOH
was evaporated to give an orange precipitate. The final product of
ITSeCN was obtained through an anion-exchange reaction.[23]
A
mixture of the orange precipitate and an equimolar amount of
KSeCN were dissolved in ACN. This solution was filtered and dried
under vacuum to give a preliminary product, which was further
purified and washed with DCM to give ITSeCN as the orange–red
final product (85%).
Preparation of electrolyte solution
The five kinds of redox couples, which were used to measure the
performance of the corresponding DSSCs were 1) the iodide elec-
trolyte containing 0.1m LiI, 0.1m DMPII, and 0.05m I2 in ACN;
2) the TEMPO electrolyte containing 0.2m TEMPO and 0.05m
NOBF4 in ACN; 3) the selenocyanate electrolyte containing 0.2m
KSeCN and 0.05m (SeCN)2 in ACN; 4) the binary electrolyte contain-
ing 0.2m TEMPO, 0.2m KSeCN, and 0.05m (SeCN)2 in ACN; and
5) the ITSeCN electrolyte containing 0.2m ITSeCN and 0.05m
(SeCN)2 in ACN. Here, 0.05m (SeCN)2 was prepared by adding 0.1m
KSeCN and 0.05m Br2 to ACN, and the precipitate KBr was eliminat-
ed by filtration. For UV/Vis/near-IR spectroscopy, each electrolyte
was one tenth of the concentration of that used for the DSSCs. For
CV and RDE analyses, the five kinds of redox couples were
1) 10.0 mm DMPII and 0.1m LiClO4 in ACN; 2) 10.0 mm TEMPO and
0.1m LiClO4 in ACN; 3) 10.0 mm KSeCN and 0.1m LiClO4 in ACN;
4) 10.0 mm TEMPO, 10.0 mm KSeCN, and 0.1m LiClO4 in ACN; and
5) 10.0 mm ITSeCN and 0.1m LiClO4 in ACN.
Instruments
Absorption spectra were obtained by means of a UV/Vis spectro-
photometer (V-670 Jasco). CV was recorded by using a potentio-
stat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, the Nether-
lands) in a three-electrode system, which was composed of a plati-
num foil with a specific area of 1 cm2 as the working electrode, an-
other platinum foil as the CE, and an Ag/Ag+ reference electrode.
Here, the Ag/Ag+ reference electrode worked in an ACN-based
buffer solution containing 0.01m AgNO3 and the calibrated poten-
tial of the Ag/Ag+ reference electrode was 0.492 V versus NHE.
The scan rate was set at 100 mVsꢀ1. RDE experiments were per-
formed by means of a potentiostat (model 900B, CH Instruments)
equipped with a modulated speed rotator (MSR, PINE Instrument
Fabrication of various CEs
A standard Pt CE was prepared by direct current sputtering of a Pt
thin film on a cleaned FTO substrate with a thickness of 50 nm.
PEDOT thin films were electropolymerized by using the potentio-
stat/galvanostat in a three-electrode electrochemical system. The
1 cm2 cleaned FTO substrate was used as the working electrode,
and the Pt foil and Ag/Ag+ electrode were used as the counter
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Company). A pristine Pt electrode (PINE Instrument Company) was
used as the working electrode, and a Pt wire and Ag/Ag+ elec-
trode were used as the counter and reference electrodes, respec-
tively. The rotating speeds were controlled at 50, 100, 200, 400,
600, 800, and 1000 rpm. The cell performance of the DSSCs was
measured by means of a potentiostat/galvanostat (PGSTAT 30, Au-
tolab, Eco-Chemie, the Netherlands) under 100 mWcmꢀ2 light illu-
mination by using a class A quality solar simulator (XES-301S,
AM1.5G, San-Ei Electric Co., Ltd., Osaka, Japan). The incident light
intensity was calibrated with a standard Si cell (PECSI01, Peccell
Technologies, Inc., Kanagawa, Japan). The IPCE of the DSSCs was
obtained by using the potentiostat/galvanostat and another class
A quality solar simulator (PEC-L11, AM1.5G, Peccell Technologies,
Inc., Kanagawa, Japan) equipped with a monochromator (model
74100, Oriel Instrument, California, USA). The incident radiation flux
(f) was obtained by using an optical detector (model 71580, Oriel
Instrument, California, USA) and a power meter (model 70310,
Oriel Instrument, California, USA). The electrochemical properties of
the DSSCs could be obtained by EIS, which was measured by using
the above-mentioned potentiostat/galvanostat equipped with an
FRA2 module under an explored frequency range from 10 mHz to
65 kHz. Surface morphologies of various CEs were observed by FE-
SEM (Nova NanoSEM 230, FEI, Oregon, USA).
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Keywords: dye-sensitized solar cells · electrochemistry · ionic
liquids · radicals · redox chemistry
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Published online on && &&, 0000
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ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
10
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

FULL PAPERS
Split personality: The ionic-liquid medi-
ator 1-butyl-3-{2-oxo-2-[(2,2,6,6-tetrame-
thylpiperidin-4-yl)amino]ethyl}-1H-imida-
zol-3-ium selenocyanate (ITSeCN), pos-
sesses dual redox channels of imidazoli-
um-functionalized 2,2,6,6-tetramethylpi-
peridine N-oxyl (TEMPO) and
C.-T. Li, C.-P. Lee, C.-T. Lee, S.-R. Li,
S.-S. Sun,* K.-C. Ho*
&& – &&
Iodide-Free Ionic Liquid with Dual
Redox Couples for Dye-Sensitized
Solar Cells with High Open-Circuit
Voltage
selenocyanate, in which TEMPO and se-
lenocyanate operate as the cationic and
anionic redox couples, respectively. A
dye-sensitized solar cell coupled with
ITSeCN electrolyte and CoSe counter
electrode reaches a high efficiency of
9.01 % with high open-circuit voltage.
ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
11
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ