Lithium-coordinating ionic conductor for solid-state dye-sensitized solar cells

Article abstract of DOI:10.1039/c5ra09688j

A new solid-state ionic conductor is synthesized by linking an ether group to the nitrogen-atom of 1,2-dimethylimidazole with an iodide counter anion, and the single crystal structure is determined using X-ray crystallographic analysis. Replacement of the

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Lithium-Coordinating Ionic Conductor for Solid-State Dye-Sensitized
Solar Cells
Juan Li and Zhong-Sheng Wang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A new solidꢀstate ionic conductor is synthesized by linking an ether group to the nitrogenꢀatom of 1,2ꢀ
dimethylimidazole with an iodide counter anion, and the single crystal structure is determined with Xꢀray
crystallographic analysis. Replacement of the butyl group in 1ꢀbutylꢀ2,3ꢀdimethylimidazolium iodide
with an ether group induces significant improvement of conductivity. When the solid ionic conductor is
mixed with LiI alone, conductivity enhancement is more remarkable for the etherꢀcontaining ionic
conductor due to the lithium coordination to the ether oxygen, which is able to avoid the aggregation of
lithium cations with iodides and hence improves transport properties of Li+. Owing to the πꢀstacking of
imidazolium rings for the etherꢀcontaining ionic conductor, the increment of ionic conductivity is also
more significant upon further doping with I2. The etherꢀcontaining ionic conductor mixed with LiI alone
as the solid electrolyte can even make the solidꢀstate dyeꢀsensitized solar cells work. Further doping with
iodine achieves power conversion efficiency of 7.1%, which is much higher than that (5.3%) for the alkyl
analogue due to the positive shift of conduction band edge of titanium dioxide and suppression of charge
recombination caused by the ether group.
imidazolium ring,6,9 which results in significant improvement of
photovoltaic performance of ssDSSCs.
Introduction
Dyeꢀsensitized solar cells (DSSCs) employing nanoporous
TiO2 electrode1 have been attracting considerable attention in
industrial and academic fields because of their potential lowꢀcost
and high efficiency.2ꢀ5 Although high performance can be
achieved with volatile organic liquid electrolytes, solidꢀstate
DSSC (ssDSSC) devices mediated with solidꢀstate electrolytes
Figure 1. Chemical structures of (a) DOII and (b) DBII
have high application potential due to the stability and safety
concerns.6ꢀ10
In this study, we synthesized a new ionic conductor, 1,2ꢀ
dimethylꢀ3ꢀmethoxyethylimidazolium iodide (DOII, Figure 1a),
based on 1ꢀbutylꢀ2,3ꢀdimethylimidazolium iodide15 (DBII, Figure
1b) by replacing one CH2 unit in the butyl group with an oxygen
atom. The aim of introducing an ether group is to induce πꢀπ
stacking of imidazolium rings through the H…O hydrogen bonds,
which is favourable for efficient charge exchange and can thus
increase ionic conductivity. Furthermore, lithium coordination to
the ether oxygen can avoid strong aggregates of lithium cations
with iodides, thus improving their transport properties, which
may influence the conduction band level of titania and the
interfacial charge recombination. As compared to DBII, the
introduction of an ether group to the imidazolium improved the
ionic conductivity significantly for the pure ionic conductors and
their mixtures with LiI and with LiI and I2 as well. When DOII
mixed with appropriate amount of LiI and I2 was employed as the
solid electrolyte, the ssDSSC with a metalꢀfree organic dye16
(FNE29, Figure S1) achieved a power conversion efficiency of
7.1% under AM1.5G fullꢀsun illumination, which was much
higher than that (5.3%) with the DBII/LiI/I2 solidꢀstate
electrolyte. Impedance analyses reveal that the ether group
Ionic liquids (ILs) have attracted growing interests for use in
DSSCs due to their wonderful properties such as negligible vapor
pressure, excellent thermal stability, good ionic conductivity and
wide electrochemical window.11,12 1ꢀmethylꢀ3ꢀpropylimidazolium
iodide is often used to construct solventꢀfree DSSCs.13 To make
the DSSC device more stable, a series of solid imidazolium
iodides with various functional groups have been developed for
use in ssDSSCs.6,9
The acidic hydrogen at Cꢀ2 position in imidazolium ring
usually causes the IL unstable.14 Replacement of the H atom at Cꢀ
2 position can improve the stability and meanwhile enhance the
melting point significantly.14 The solid 1,2ꢀdimethylꢀ3ꢀ
propylimidazolium iodide is an often used additive in the organic
solvent based electrolyte for highꢀperformance organic liquid
electrolyte based DSSCs. However, the solid 1,2,3ꢀ
trialkylimidazolium iodide mixed with iodine and LiI without
adding organic solvent cannot make the solidꢀstate DSSCs
(ssDSSCs) work efficiently due to the pretty low ionic
conductivity.9 To improve the ionic conductivity of the solid IL,
different functional groups have been introduced to the
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induces a positive shift of conduction band edge of titanium
dioxide and suppression of charge recombination.
point (m.p.) was derived from Differential Scanning Calorimetry
(DSC) performed on a Shimadzu DSCꢀ60A at a heating rate of 5
°C minꢀ1. The film thickness of TiO2 was measured with a surface
profiler (Veeco Dektak 150, USA). The current densityꢀvoltage
(JꢀV) characteristics of DSSCs were measured by recording the Jꢀ
V curves with a Keithley 2420 source meter under illumination of
simulated AM1.5G solar light coming from a solar simulator
(Newport, 94023A equipped with a 450 W Xe lamp and an
AM1.5G filter). The incident light intensity was calibrated using
a standard Si solar cell (Newport 91150). Action spectra of the
incident monochromatic photonꢀtoꢀelectron conversion efficiency
(IPCE) for the solar cells were obtained with an Orielꢀ74125
system (Oriel Instruments). The intensity of monochromatic light
Experimental
Materials and Reagents. LiI and I2 were obtained from
Acros.
2ꢀmethoxyethanol,
1,2ꢀdimethylimidazole,
pꢀ
toluenesulfonyl chloride, and 1,1,1ꢀtrichloroethane were obtained
from Aldrich. Organic solvents were dried according to the
standard methods. Transparent conductive glass (Fꢀdoped SnO2,
FTO, 14 ꢁ per square, transmittance of 85%, Nippon Sheet Glass
Co., Japan) was used as the substrate for the fabrication of TiO2
thin film electrodes. The organic dye FNE29 (Figure S1) was
prepared according to the reported method.16
was measured with
a
Si detector (Oriel71640). An
Fabrication of ssDSSCs. TiO2 films (12 ꢀm) composed of a
7 ꢀm nanoparticle (25 nm) layer in direct contact with the FTO
substrate and a 5 ꢀm light scattering particle (80% 25 nm TiO2 +
20% 100 nm TiO2) layer17 were fabricated with a screenꢀprinting
technique. The films were sintered at 500 °C with a rising rate of
10 °C minꢀ1 and treated with 0.05 M TiCl4 aqueous solution for
30 min at 70 °C followed by heating at 450 °C for 30 min. Then
the films were immersed in the dye solution (0.3 mM in toluene)
when the film was about 120 °C for 16 h. The dyeꢀsensitized
TiO2 and the Ptꢀcoated FTO as a counter electrode were separated
by a hotꢀmelt Surlyn film (30 ꢀm) and sealed together by
pressing them under heat. The methanol solution of the solid
electrolyte was injected into the internal space of the cell from the
two holes predrilled on the back of the counter electrode and
dried on a hot plate with the temperature of 80 °C until the TiO2
porous film was filled with the solid electrolyte. The devices
were further dried in a vacuum oven at 50 °C for 24 h to remove
the residue solvent. Finally, the back holes were sealed with a
Surlyn film covered with a thin glass slide under heat.
Characterizations. The chemical structures of materials were
characterized with 1HꢀNMR (Varian 400 MHz NMR
spectrometer) and 13CꢀNMR (Varian 500 MHz NMR
spectrometer). Highꢀresolution mass spectra (HRMS) were
recorded on a MicroTOF II Mass Spectrometer (Bruker Daltonics
instrument) with ionization method of electrospray ionization
(ESI). The Xꢀray diffraction data of the DOII single crystal were
collected on a Bruker SMART APEX (II)ꢀCCD area detector
diffractometer using graphite monochromatic Mo Kα radiation (k
= 0.71073 Å) with the ω scan mode 2.59 to 27.90°. Empirical
absorption correction was made with multiꢀscan based on
symmetryꢀrelated measurements. The structure was solved by a
direct method (SHELXLTꢀ97) and refined with full matrix leastꢀ
squares techniques based on F2. All nonꢀhydrogen atoms were
refined with anisotropic thermal parameters. All computations
were carried out using the SHELXLꢀ97 program package. Fourier
transition infrared (FTꢀIR) measurements were performed on
Shimadzu IRAffinityꢀ1 FTꢀIR spectrometer. The ionic
conductivity of the solid electrolytes, which were sandwiched
between two identical Pt electrodes, was determined with an ac
impedance technique on an electrochemical workstation (Zahner
CIMPSꢀ1, Germany) with applied bias and ac amplitude were set
at 0 V and 10 mV, respectively. The decomposition temperature
was determined with Thermo gravimetric (TG) analysis
performed on a TGꢀDTA 2000S system (Mac Sciences Co. Ltd.,
Yokohama, Japan) at a heating rate of 10 °C min−1. The melting
electrochemical workstation (ZAHNER ZENNIUM CIMPSꢀ1,
Germany) was used to perform intensity modulated photovoltage
spectroscopy (IMVS) and charge extraction for ssDSSCs under
illumination of a green light emitting diode (LED, 532 nm). The
intensityꢀmodulated spectra were measured at room temperature
with light intensity ranging from 5 to 45 W mꢀ2 in modulation
frequency ranging from 0.1 Hz to 10 kHz with modulation
amplitude less than 5% of the light intensity.
The synthetic route of DOII was illustrated in Scheme S1 in
the Supporting Information (SI). DBII was also synthesized to
highlight the effect of ether group on the ionic conductivity and
the corresponding photovoltaic performance.
Synthesis of 1-chloro-2-methoxyethane. 20.8 g (173 mmol)
2ꢀmethoxyethanol and 10.0 g (250 mmol) NaOH dissolved in 50
mL water were charged in a 1 L threeꢀnecked flask. Then 36.2 g
(190 mmol) 4ꢀmethylꢀbenzenesulfonyl chloride in 50 mL THF
o
was added to the above solution at ꢀ5 C under the protection of
nitrogen. The mixture was stirred at 0 oC for another 3 h. The
resultant mixture was mixed with 200 mL ice water and extracted
with CH2Cl2 (3×100 mL). The collected organic phase was
washed with hydrochloric acid (1 M, 2×250 mL) and brine
followed by drying over MgSO4. After the solvent was removed
under reduced pressure, the product was obtained as yellowish
oil. Yield: 25 g (68%).
Synthesis of 1-iodo-2-methoxyethane. 11.5 g (50 mmol) 1ꢀ
chloroꢀ2ꢀmethoxyethane and 15.0 g NaI (100 mmol) were
dissolved in acetone (200 mL) under protection of nitrogen in the
dark. The mixture was stirred for 48 h at 25 oC. After acetone was
removed under reduced pressure, CH2Cl2 (60 mL) and distilled
water (40 mL) were added to extract the organic phase, which
was washed with 5% Na2S2O3 aqueous solution to remove iodine.
Then the organic phase was further washed with saturated
NaHCO3 solution (3×50 mL) and distilled water (3×50 mL)
followed by drying over Na2SO4. After filtration, the product, 1ꢀ
iodoꢀ2ꢀmethoxyethane, was obtained by removing CH2Cl2 in the
filtrate with rotary evaporator at 50 oC. 1HꢀNMR (500 MHz,
DMSOꢀd6): δ 2.83~2.79 (t, 2H), 2.55~2.53 (d, 3H), 2.39~2.42 (t,
2H).
Synthesis of 1,2-dimethyl-3-(2-methoxyethyl)imidazolium
iodide (DOII, Figure 1a). 96 mg (1 mmol) 1,2ꢀ
dimethylimidazole and 2.80
g
(1.5 mmol) 1ꢀiodoꢀ2ꢀ
methoxyethane were added to 1,1,1ꢀtrichloroethane (10 mL)
under protection of nitrogen. The mixture was stirred for 3 h at 40
oC. After the reaction, the 1,1,1ꢀtrichloroethane was decanted and
2
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the raw product was washed with ether for several times to get
DOII as a white powder. Yield: 2.5 g (95%). 1HꢀNMR (500 MHZ,
DMSOꢀd6): δ 7.58~7.59 (t, J = 3.3 Hz, 2H), 4.29~4.31 (m, 2H),
3.78~3.79 (d, J = 2.7 Hz, 3H), 3.60~3.63 (t, J = 4.9 Hz, 2H), 3.22
(s, 3H), 2.55~2.56 (d, J = 3.0 Hz, 3H). 13CꢀNMR (500MHZ,
DMSOꢀd6): 121.545, 120.574, 101.311, 69.191, 57.546, 46.791,
between the two adjacent molecules. The crystal structure of
DOII is expanded via linking the dimers with H8c…O (2.957 Å,
Figure 2) hydrogen bonds, forming a lamellar structure. The
centroidꢀcentroid distance is 4.4 Å for DOII and 5.9 Å for DBII.
As πꢀπ interaction exists when the centroidꢀcentroid distance is
smaller than 4.6 Å,18 it is inferred that the imidazolium rings are
πꢀstacked for DOII while they are not for DBII.
o
34.134, 8.770. The decomposition temperature: 310 C; m.p. 83
+
oC. HRMS (ESI. m/z): calcd for C8H15ON2 , 155.1184; found
155.1208.
Synthesis of 1,2-dimethyl-3-butylimidazolium iodide
(DBII, Figure 1b). DBII was prepared according to the reported
method.15 96 mg (1 mmol) 1,2ꢀdimethyl imidazole and 2.76 g
(1.5 mmol) 1ꢀbutyliodide were charged in 1,1,1ꢀtrichloroꢀethane
(10 mL) under protection of nitrogen. The mixture was stirred for
3 h at 40 oC. After the reaction, the 1,1,1ꢀtrichloroethane was
decanted and the raw product was washed with ether for several
times to get a white powder as the final product. Yield: 2.5 g
(95%). 1HꢀNMR (500 MHZ, DMSOꢀd6): δ7.62~7.61 (d, 2H),
7.46~7.45 (d, 3H), 4.2~4.18 (t, 2H), 4.01 (s, 3H), 2.84 (s, 3H),
1.82~1.81 (m, 2H), 1.42~1.41 (m, 2H), 0.98~0.97 (t, 3H). m.p.
105 oC, the decomposition temperature 298 oC.
Figure 3. Packing structure of DOII viewed along the b axis
R
esults and Discussion
The different packing structure caused by different functional
groups in the imidazolium ring is expected to influence the ionic
conductivity. The ionic conductivities at room temperature of the
pure ionic conductors and their mixtures doped with LiI (molar
ratio of ionic conductor/LiI = 2/1) and further doped with iodine
(molar ratio of ionic conductor/LiI/I2 = 2/1/0.2) were measured
with EIS using dummy cells, in which the solid electrolytes were
sandwiched between two identical Pt electrodes. Figure S4
illustrates the EIS Nyquist plots of the above solid electrolytes.
The left semicircle is attributed to the charge transfer resistance
(Rct) at the electrode/electrolyte interface while the right
semicircle is attributed to the resistance of solid electrolyte
(REL).19 Fitting the impedance spectra with the equivalent circuit
A
colourless lamellar crystal was obtained by slow
evaporation of a solution in methanol at room temperature.
Singleꢀcrystal Xꢀray analysis reveals that the DOII salt
crystallizes in monoclinic space group (CCDC 948215) with
crystallographic details listed in Table S1. To emphasize the
function of ether group, the crystal structure of DBII (Figure S2)
was downloaded from the Cambridge Crystallographic Data
Centre (CCDC 634384) for comparison. Figure 2 shows the
singleꢀcrystal structure of DOII. In the single crystal structure,
there is one cation for DOII (Figure 2) while there are two cations
for DBII (Figure S2). The ether oxygen does not affect the space
group but reduces the cell lengths. The cell volume is reduced by
more than 2ꢀfold from butyl to ether substitution. In the DBII
crystal, there is one H…I hydrogen bond with a distance of 2.915
Å. For DOII, there are three H…O hydrogen bonds while the
H…I hydrogen bonding is absent. This indicates that the ether
oxygen competes with the iodide for the hydrogen bonding.
(inset of Figure S4) using the Zꢀview software gives Rct and ZEL
.
The ionic conductivity (σ) is calculated with σ = d/( ZEL×S),
where d is the thickness and S is the area of the solid electrolyte.
Interestingly, replacement of one CH2 unit in the butyl group of
DBII with an oxygen atom can increase the conductivity by more
than 3ꢀfold from 0.017 to 0.055 mS cmꢀ1 (Figure 4). Structural
difference is likely the main reason for the different ionic
conductivities. The ether bond diminishes the van der Waals
interactions among the side chains,20 making the organic cations
move more freely and thus resulting in higher ionic conductivity
for DOII. As the ionic conductivity is low, the Rct is high: 34 ꢀ
for DOII and 110 ꢀ for DBII.
Figure 2. Singleꢀcrystal structure of DOII along with the hydrogen bonds
The packing structures of DOII and DBII are shown in Figure
3 and Figure S3, respectively. DOII and DBII exhibit different
packing structures because of their different hydrogen bonds
between neighbouring molecules. Dimers of DOII are formed by
means of two H5B…O (2.880 Å, Figure 2) hydrogen bonds
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thus increases the number of charged species, resulting in
enhanced conductivity.20
Upon further doping with iodine (ionic conductor/LiI/I2
=
2/1/0.2), the conductivity increases to 2.6 mS cmꢀ1 and 1.4 mS
cmꢀ1 for DOII and DBII, respectively, as shown in Figure 4. The
remarkable increase in ionic conductivity stems from the
ꢀ
formation of polyiodides such as I3 ions by means of reaction
between iodide and iodine. The formation of polyiodides
facilitates the charge transfer along the polyiodides chain via the
Grotthusꢀtype exchange mechanism,22ꢀ24 which is responsible for
the remarkable enhancement of conductivity. The increment of
ionic conductivity arising from the Grotthus charge exchange for
DOII is larger than that for DBII upon iodine doping. This is
attributed to the πꢀπ stacking in DOII, which is advantageous to
efficient Grotthus bond exchange due to the reduced distance
between the iodide/polyiodide species. In this case, the Rct further
decreased to 2 ꢀ for DOII and 3 ꢀ for DBII.
It is seen from Figure 4 that DOII exhibits higher conductivity
than DBII. The ionic conductivity increases from pure ionic
conductor to LiI doping and then to LiI + I2 doping for both
compounds. For each doping, the increment of conductivity for
DOII is larger than that for DBII. Evidently, the ether oxygen
contributes positively to the increased ionic conductivity. As a
consequence, DOII based solid electrolytes show higher
conductivity than DBII based electrolytes in either pure phase or
mixture.
Figure 4. Ionic conductivities for various solid electrolytes
When LiI is doped into the solid ionic conductors (ionic
conductor/LiI = 2/1), the ionic conductivity for both DOII and
DBII increased due to the increased number of charged species.
However, the increment of conductivity is quite different for
DOII and DBII. Upon doping of LiI into the solid ionic conductor,
the conductivity of DBII increased slightly from 0.017 to 0.024
mS cmꢀ1 while the conductivity of DOII increased significantly
from 0.055 to 0.32 mS cmꢀ1 (Figure 4). The Rct decreased to 5 ꢀ
for DOII and 35 ꢀ for DBII due to the enhanced conductivity
upon LiI doping. The increase in conductivity for DOII is more
pronounced than that for DBII upon LiI doping. As reported in
the literature,20 the strong interaction of Li+ with iodide can
diminish the transport properties of Li+ ions, and therefore, the
increase in ionic conductivity is not notable. As the ether oxygen
in DOII can, however, compete with iodide anions for Li+, the
mobility of lithium ions is expected to increase, resulting in
significant improvement of conductivity.20 It is thus likely that
the Li+ ion dominates the contribution to increase the
conductivity for the mixture of ionic conductor and LiI.
Figure 5. IR spectra of DOII and DOII/LiI
Figure 5 shows the FTIR spectra of DOII and DOII/LiI
mixture. The peak at 1282 cmꢀ1 attributed to the asymmetric
stretching vibration of CꢀOꢀC bond21 is clearly observed for DOII,
but it diminishes for the DOII/LiI mixture. This is due to the
coordination of ether oxygen to Li+ ions. Therefore, the ether
oxygen atom can avoid aggregation of Li+ with iodide anions and
Figure 6. (a) J-V curves and (b) IPCE spectra for ssDSSCs with DOII or
DBII solid electrolytes (DOII/LiI/I2 = 2/1/0.2, DBII/LiI/I2 = 2/1/0.2)
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Next, we apply these solid ionic conductors and their
mixtures as solid electrolytes for ssDSSCs. It is noted that the
used electrolytes are solid state under the operating conditions of
solar cells. Since the pure ionic conductors have pretty low
conductivity and high Rct values, they cannot make the ssDSSC
work without any doping. Upon doping with LiI, the ssDSSC
with DOII works with a power conversion efficiency (η) of 3.4%
because of the increased conductivity and decreased Rct. However,
the ssDSSC with DBII+LI does not work because the
conductivity is still low and the Rct is still high. These results
indicate that the ssDSSC can work only when the ionic
conductivity is larger than a threshold value. Upon further doping
with iodine, it is expected to observe improved performance due
to the enhanced conductivity. Figure 6a displays the JꢀV curves of
ssDSSCs recorded under simulated AM1.5G solar light (100 mW
cmꢀ2). The ssDSSC based on DBII/I2/LiI produced a short circuit
photocurrent (Jsc) of 14.26 mA cm−2, an openꢀcircuit
photovoltage (Voc) of 622 mV, and a fill factor (FF) of 0.60,
corresponding to η of 5.3%. The ssDSSC based on DOII/I2/LiI
produced a Jsc of 15.73 mA cm−2, Voc of 642 mV, and FF of 0.70,
corresponding to a PCE of 7.1%. The replacement of an oxygen
atom in the butyl group increases the Jsc and FF significantly,
resulting in much improved PCE from 5.3% to 7.1%. The higher
FF for DOII is the result of its higher conductivity and lower Rct
(left arc in Figure S4c). The higher Jsc for DOII is attributed to its
greater conductivity and higher electron injection yield to be
discussed below. The IPCE action spectra are shown in Figure 6b.
The maximum IPCE value is higher than 80% for DOII while it is
about 65% for DBII. The difference of IPCE values accounts for
the different Jsc values.
As the conduction band (CB) edge of TiO2 may shift with the
electrolyte,25 the relative position of CB, which can influence Jsc,
Voc, and PCE, has been investigated. Figure 7 shows the plot of
charge density (Q) at open circuit against openꢀcircuit
photovoltage under LED light with various intensities. For both
electrolytes, the Voc increases with the logarithm of charge
density, and the slope is almost the same (120 mV). As compared
to the DBII/LiI/I2 electrolyte, the DOII/LiI/I2 electrolyte shows
lower CB edge by about 50 mV, as seen from Figure 7. The
interaction of ether oxygen with Li+ ions diminishes the strong
coulombic interaction of Li+ ions with iodides, which improves
the transport properties of Li+ ions.20 It is thus reasonable that
more Li+ ions for the DOII case can be adsorbed on the TiO2
surface than for the DBII case, which accounts for the relatively
lower CB edge for DOII. As compared to DBII, the positive shift
of CB for DOII is beneficial for higher electron injection yield,
which in combination with the higher ionic conductivity is
responsible for the observed higher Jsc and IPCE for DOII.
However, the positive shift of CB should lead to a decrease in
Voc,25 contrasting to the observation that the device with DOII
shows slightly higher Voc than that with DBII. This suggests that
charge recombination rate in the two devices should be different.
Figure 7. Openꢀcircuit photovoltage as a function of charge density
Figure 8 shows the relationship between electron lifetime and
charge density obtained from charge extraction. Electron lifetime
is enhanced by about 4ꢀfold from DBII to DOII at the same
charge density. This indicates that charge recombination between
electrons and triiodide is retarded remarkably when the
electrolyte is changed from DBII to DOII. The slower charge
recombination for the DOII case is caused by more Li+ ions
adsorbed on the TiO2 surface.26 The increase in electron lifetime
brings about more electrons stored in TiO2, resulting in
improvement of Voc from DBII to DOII. The charge densities,
which depend on the photoꢀinjected electron density and electron
lifetime at openꢀcircuit, are measured of 8 and 14 ꢂC cmꢀ2 under
the same LED light (30 W mꢀ2, 532 nm) for DBII and DOII,
respectively. It is derived from Figure 7 that Voc gain solely
arising from the increased charge density is about 67 mV.
Therefore, the collective effects of the positively shifted CB edge
(50 mV) and the increased charge density contribute to an
increase in Voc by 17 mV, which is in good agreement with the
observed increase in Voc. The Voc gain originating from the
increased electron lifetime compensates for the Voc loss arising
from the positive shift of conduction band edge, resulting in
slightly higher photovoltage.27
Figure 8. Electron lifetime as a function of charge density at open circuit
The stability for the ssDSSC based on the DOII based solid
electrolyte was recorded over a period of 1000 h under oneꢀsun
soaking. As shown in Figure 9, at the beginning stage below 200
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h, Jsc increases, Voc and FF decreases with time due to the aging
effect, resulting an increase in PCE with time up to 200 h. After
device aging for 200 h, the device performance becomes steady;
all parameters remain almost constant with time. These data
indicates that the ssDSSC with DOII based solid electrolyte and
an organic dye is longꢀterm stable under oneꢀsun soaking.
Research Program (No. 2011CB933302) of China and STCSM
(12JC1401500).
Notes and references
Department
of
Chemistry,
iChEM
(Collaborative
Innovation Centre of Chemistry for Energy Materials), Lab of
Advanced Materials, Fudan University, 2205 Songhu Road,
Shanghai 200438, P. R. China
* Corresponding author. Eꢀmail: zs.wang@fudan.edu.cn
§ Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/b000000x/
1. M. Grätzel, Nature 2001, 414, 338ꢀ334.
2. Y. Bai, Y. Cao, J. Zhang, M. Wang, R. Li, P. Wang, S. M. Zakeeruddin,
M. Grätzel, Nat. Mater. 2008, 7, 626ꢀ630.
3. H. N. Tian, X. A. Jiang, Z. Yu, L. Kloo, A. Hagfeldt, L. C. Sun, Angew.
Chem., Int. Ed. 2010, 49, 7328ꢀ7331.
4. A. Hagfeldt, G. Boschloo, L. C. Sun, L. Kloo, H. Pettersson, Chem.
Rev. 2010, 110, 6595ꢀ6663.
5. F. Gong, H. Wang, X. Xu, G. Zhou, Z. S. Wang, J. Am. Chem. Soc.
2012, 134, 10953ꢀ10958.
6. H. Wang, X. Zhang, F. Gong, G. Zhou, Z.ꢀS. Wang, Adv. Mater. 2011,
24, 121ꢀ124.
7. I. Chung, B. Lee, J. He, R. P. H. Chang, M. G. Kanatzidis, Nature,
2012, 485, 486ꢀ489.
8. J. Wu, S. Hao, Z. Lan, J. Lin, M. Huang, Y. Huang, P. Li, S. Yin, T.
Sato, J. Am. Chem. Soc. 2008, 130, 11568ꢀ11569.
9. H. Wang, H. Li, B. Xue, Z. Wang, Q. Meng, L. Chen, J. Am. Chem.
Soc. 2005, 127, 6394ꢀ6401.
10. N. Yamanaka, R. Kawano, W. Kubo, T. Kitamura, Y. Wada, M.
Watanabe, S. Yanagida, Chem. Commun. 2005, 6, 740ꢀ742.
11. J. Le Bideau, L. Viau, A. Vioux, Chem. Soc. Rev. 2011, 40, 907ꢀ925.
12. F. Mazille, Z. Fei, D. Kuang, D. Zhao, S. M. Zakeeruddin, M. Grätzel,
P. J. Dyson, Inorg. Chem. 2006, 45, 1585ꢀ1590.
Figure 9. The longꢀterm stability of ssDSSC with DOII based solid
electrolyte
Conclusions
13. D. Kuang, P. Wang, S. Ito, S. M. Zakeeruddin, M. Grätzel, J. Am.
Chem. Soc. 2006, 128, 7732ꢀ7733.
In summary, an etherꢀfunctionalized imidazolium iodide
conductor was designed and synthesized for use as a solidꢀstate
electrolyte in ssDSSCs. When one methane unit in the butyl
group of DBII was replaced with an oxygen atom, the packing
structure changes greatly due to the presence of H…O hydrogen
bonds, resulting in different ionic conductivity. Upon doping with
LiI, the ionic conductivity was improved more significantly for
DOII than for DBII, as the ether oxygen diminished the
interaction between Li+ and iodide, which raised the number of
charged species. Further doping with iodine, the increment of
conductivity was also more remarkable for DOII than for DBII
because of the formation of πꢀstacking in the former. The
presence of ether bond was found to shift the CB edge of TiO2
positively and to retard charge recombination. As compared to
DBII, the DOII produced higher Jsc, due to the higher ionic
conductivity and higher electron injection yield caused by the
lower CB edge, and comparable Voc, due to the opposite effects of
CB edge shift and charge recombination suppression. A power
conversion efficiency of 7.1% was obtained for an organic dye
based ssDSSC with DOII/I2/LiI as the solid electrolyte, which
was much higher than the efficiency of 5.3% for the DBII case at
the same conditions. In addition, a longꢀterm stability test
demonstrated that the ssDSSC device was stable under oneꢀsun
soaking.
14. E. I. Izgorodina, R. Maganti, V. Armel, P. M. Dean, J. M. Pringle, K.
R. Seddon, D. R. MacFarlane, J. Phys. Chem. B 2011, 115, 14688ꢀ14697.
15. J. Kutuniva, R. Oilunkaniemi, R. S. Laitinen, J. Asikkala, J.
Kärkkäinen, M. K. Z. Lajunen, Naturforsch. 2007, 62b, 868ꢀ870.
16. Y. Li, H. Wang, Q. Y. Feng, G. Zhou, Z.ꢀS. Wang, Energy Environ.
Sci. 2013, 6, 2156ꢀ2165.
17. Z.ꢀS. Wang, H. Kawauchi, T. Kashima, H. Arakawa, Coord. Chem.
Rev. 2004, 248, 1381ꢀ1389.
18. C. Janiak, J. Chem. Soc.-Dalton Trans. 2000, 21, 3885ꢀ3896.
19. A. Hanch, A. Georg, Electrochim. Acta 2001, 46, 3457ꢀ3466.
20. M. J. Monteiro, F. F. Camilo, M. C. C. Ribeiro, R. M. Torresi, J.
Phys. Chem. B 2010, 114, 12488ꢀ12494.
21. Z. Fei, W. H. Ang, D. Zhao, R. Scopelliti, E. E. Zvereva, S. A.
Katsyuba, P. J. Dyson, J. Phys. Chem. B. 2007, 111, 10095ꢀ10108.
22. F. C. Küpper, M. C. Feiters, B. Olofsson, T. Kaiho, S. Yanagida, M.
B. Zimmermann, L. J. Carpenter, G. W. Luther, Z. L. Lu, M. Jonsson, L.
Kloo, Angew. Chem., Int. Ed. 2011, 50, 11598ꢀ11620.
23. V. K. Thorsmølle, G. Rothenberger, D. Topgaard, J. C. Brauer, D. B.
Kuang, S. M. Zakeeruddin, B. Lindman, M. Grätzel, J. E. Moser,
ChemPhysChem 2011, 12, 145ꢀ149.
24. R. Kawano, M. Watanabe, Chem. Commun. 2003, 3, 330ꢀ331.
25. G. Schlichthörl, S. Y. Huang, J. Sprague, A. J. Frank, J. Phys. Chem.
B 1997, 101, 8141ꢀ8155.
26. N. Kopidakis, K. D. Benkstein, J. Lagemaat, A. J. Frank, J. Phys.
Chem. B 2003, 107, 11307ꢀ11315.
27. Z.ꢀS. Wang, G. Zhou, J. Phys. Chem. B 2009, 113, 15417ꢀ15421.
Acknowledgment
This work was financially supported by the National Basic
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