Bis-imidazolium based poly(ionic liquid) electrolytes for quasi-solid-state dye-sensitized solar cells

Article abstract of DOI:10.1039/c2jm33273f

Bis-imidazolium based poly(ionic liquid), poly(1-butyl-3-(1- vinylimidazolium-3-hexyl)-imidazolium bis(trifluoromethanesulfonyl)imide) (Poly[BVIm][HIm][TFSI]) and mono-imidazolium based poly(ionic liquid), poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide) (Poly[BVIm][TFSI]), were synthesized and dissolved in room temperature ionic liquids (ILs) to form quasi-solid-state electrolytes for dye-sensitized solar cells (DSSCs), without using any volatile organic solvent. Compared with mono-imidazolium based poly[BVIm][TFSI], bis-imidazolium based poly[BVIm][HIm][TFSI] electrolytes showed a higher thermal stability and conductivity due to the charge transport networks formed in the gel electrolytes via the π-π stacked imidazolium rings. The DSSCs based on the poly[BVIm][HIm][TFSI] gel electrolyte showed a superior long-term stability and yielded a power conversion efficiency of 5.92% under the simulated air mass 1.5 solar spectrum illumination at 100 mW cm^-2. This study offered a feasible method to fabricate quasi-solid-state DSSCs in future practical applications.

Full text of DOI:10.1039/c2jm33273f

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PAPER
Bis-imidazolium based poly(ionic liquid) electrolytes for quasi-solid-state
dye-sensitized solar cells†
Xiaojian Chen,‡a Jie Zhao,‡a Jinyu Zhang,‡a Lihua Qiu,a Dan Xu,a Haigang Zhang,a Xiaoyuan Han,b
b
Baoquan Sun, Gaohui Fu, Ye Zhang and Feng Yan
a
a
a
*
Received 22nd May 2012, Accepted 12th July 2012
DOI: 10.1039/c2jm33273f
Bis-imidazolium based poly(ionic liquid), poly(1-butyl-3-(1-vinylimidazolium-3-hexyl)-imidazolium
bis(trifluoromethanesulfonyl)imide) (Poly[BVIm][HIm][TFSI]) and mono-imidazolium based
poly(ionic liquid), poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide) (Poly[BVIm]
[TFSI]), were synthesized and dissolved in room temperature ionic liquids (ILs) to form quasi-solid-
state electrolytes for dye-sensitized solar cells (DSSCs), without using any volatile organic solvent.
Compared with mono-imidazolium based poly[BVIm][TFSI], bis-imidazolium based poly[BVIm][HIm]
[TFSI] electrolytes showed a higher thermal stability and conductivity due to the charge transport
networks formed in the gel electrolytes via the p–p stacked imidazolium rings. The DSSCs based on the
poly[BVIm][HIm][TFSI] gel electrolyte showed a superior long-term stability and yielded a power
conversion efficiency of 5.92% under the simulated air mass 1.5 solar spectrum illumination at 100 mW
cmꢀ2. This study offered a feasible method to fabricate quasi-solid-state DSSCs in future practical
applications.
liquid electrolyte by using solid or semi-solid electrolytes to
Introduction
overcome the drawbacks.
Dye-sensitized solar cells (DSSCs) have attracted both academic
and industrial interest due to their high-efficiency, potential low-
cost and easy-handling fabrication.1 Recently, a photoelectric
conversion efficiency over 12% has been achieved by incorpo-
Among the solid and semi-solid electrolytes studied, polymer
based quasi-solid-state electrolytes have been attracting a great
deal of interest because of their good contact with the nano-
crystalline TiO2 electrodes and counter electrodes, maintaining
good ionic conductivity, reducing the cell leakage problems and
their comparable efficiencies to cells using liquid electrolytes.10,18
Polymers or co-polymers, such as poly(ethylene oxide) (PEO),10
poly(styrene-co-acrylonitrile) (poly-(St-AN)),11 poly(ethylene
glycol) (PEG),18 poly(methylmethacrylate) (PMMA),19 poly-
(acrylonitrile-co-vinyl acetate) (PAN-VA)20 and poly(vinylidene
fluoride-co-hexafluoropropylene) (PVDF-HFP)9,21 have been
applied as the gel electrolytes. However, preparation of these
polymer-based quasi-solid-state electrolytes usually requires
conventional volatile organic solvents, such as acetonitrile and 3-
methoxypropionitrile,9–21 which gives rise to potential problems
such as flammability and high-temperature instability.
rating
a cobalt(II/III)-based tris(bipyridy) tetracyanoborate
complex as the redox mediator in conjunction with a donor-
p-bridge–acceptor zinc porphyrin dye in a liquid electrolyte.2
Cells with high conversion efficiency have been achieved by using
organic liquid electrolyte, however, the existence of the liquid
electrolyte in DSSCs suffers from several drawbacks such as the
leakage and volatilization of organic solvent, the possible
desorption of the surface-coated dyes, the corrosion of the
counter electrodes and the limited long-term performance which
restricts the widespread commercialization of DSSCs.3 Several
methods, including p-type semiconductors,4 organic and inor-
ganic hole conductors5,6 and gel and polymer electrolytes,7–17
have been proposed to reduce the evaporation and leakage of the
To overcome the evaporation problems of organic solvent
liquid electrolytes, room temperature ionic liquid (IL) based
electrolytes have been extensively studied and applied in DSSCs
due to their negligible vapor pressure, high ionic conductivity
and thermal stability, wide electrochemical window, the ability to
dissolve organic and inorganic solutes and tunable solvent
properties.22,23 However, the devices with room temperature IL-
based electrolytes are still suffering leakage problems in practical
use. More recently, growing attention has been paid to poly(ionic
liquids) (poly(ILs)) because they show specific functions such as
aJiangsu Key Laboratory of Advanced Functional Polymer Design and
Application, Department of Polymer Science and Engineering, College of
Chemistry, Chemical Engineering and Materials Science, Suzhou,
215123, China. E-mail: fyan@suda.edu.cn; Fax: +86-512-65880089; Tel:
+86-512-65880973
bJiangsu Key Laboratory of Carbon-Based Functional Materials
&
Devices, Institute of Functional Nano & Soft Materials (FUNSOM),
Soochow University, Suzhou, 215123, China
† Electronic supplementary information (ESI) available: 1H NMR
Spectra (PDF). See DOI: 10.1039/c2jm33273f
‡ These authors made an equal contribution.
18018 | J. Mater. Chem., 2012, 22, 18018–18024
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the transport of target ions, a specific polar environment and
mechanical strength, corresponding to their polymer structure.
Poly(ILs) have been successfully applied in quasi-solid-state
DSSCs with high power conversion efficiency and excellent
long-term stability of the devices.24–26 In our previous report,
mono-imidazolium based poly(ionic liquid), poly(1-butyl-3-
vinylimidazolium bis(trifluoromethanesulfonyl)imide) (poly
[BVIm][TFSI]) was synthesized and applied in quasi-solid-state
DSSCs. The superior long-term stability of the fabricated DSSCs
indicated that the poly(IL) based gel electrolyte could overcome
the drawbacks of the volatile solvents. However, the power
conversion efficiency of the devices fabricated with mono-imi-
dazolium type poly(IL)-based electrolyte is still low.
Scheme 1 General synthetic route for the bis- and mono-imidazolium
Compared with monocationic ILs, dicationic ILs show higher
thermal stability, lower volatility and more flexibility in tuning
the physicochemical properties.27–30 Here, bis-imidazolium-based
poly(IL), poly(1-butyl-3-(1-vinylimidazolium-3-hexyl) imidazo-
lium bis(trifluoromethane-sulfonyl)imide) (poly[BVIm][HIm]
[TFSI]), was synthesized and dissolved in the room temperature
ILs to form a gel electrolyte for high-performance quasi-solid-
state DSSCs. The fabricated bis-imidazolium based poly(IL) gel
electrolyte displayed a better power conversion efficiency than
mono-imidazolium based poly(IL) and is close to the efficiency
of ionic liquid electrolytes. In addition, the good long-term
stability of poly[BVIm][HIm][TFSI]/ionic liquid electrolytes
indicates that the bis-imidazolium based poly(IL) gel electrolyte
is a promising candidate for solar cells with good durability.
based poly(ILs).
1
temperature for 72 h. HNMR (400 MHz, CDCCl3): 10.98 (s,
1H), 7.77 (s, 1H), 7.50 (dd, 1H), 7.46 (m, 1H), 5.95 (m, 1H), 5.36
(m, 1H), 4.38 (t, 3H), 1.91 (m, 2H), 1.37 (m, 2H), 0.96 (t, 3H) (see
ESI, Fig. S1†).
Mono-imidazolium based poly(IL), poly[BVIm][Br], was
prepared via free radical polymerization of 1-butyl-3-vinyl-
imidazolium bromide in ethanol at 60 ꢁC under a nitrogen
atmosphere for 24 h. Anion-exchange of poly[BVIm][Br] with
LiTFSI in aqueous solution yielded poly(1-butyl-3-vinyl-
imidazolium bis(trifluoromethanesulfonyl) imide) (poly[BVIm]
[TFSI]).32,33
3-(6-Bromohexyl)-1-butyl-imidazolium bromide ([BrHBIm]
[Br]) was synthesized via the reaction of 1-butylimidazole (3.10 g,
0.025 mol) and 1,6-dibromohexane (18.30 g, 0.075 mol) in
Experimental procedure
ꢁ
ethanol, and the solution was reacted at 60 C under a nitrogen
Materials
atmosphere for 48 h. The product was purified using a silica gel
column with acetone/methanol (v/v ¼ 3 : 1) as the eluent.
1HNMR: (400 MHz, CDCCl3): 10.66 (s, 1H), 7.35 (s, 1H), 7.31
(s, 1H), 4.37 (m, 4H), 3.40 (t, 2H), 1.93 (m, 6H), 1.50 (m, 2H),
1.40 (m, 4H), 0.97 (t, 3H) (see ESI, Fig. S2†).
Iodoethane, 1-iodopropane, 1-bromobutane, 1,6-dibromohex-
ane, 1-methylimidazole, 1-butylimidazole, 1-vinylimidazole, N-
butylbenzimidazole (NBB) and guanidine thiocyanate (GNCS)
were purchased from Alfa Aesar. Lithium bis(tri-
fluoromethanesulfonyl)imide (LiTFSI) was kindly provided by
Rhodia and used as received. 1-Ethyl-3-methylimidazolium
iodide (EMII), 1-propyl-3-methylimidazolium iodide (PMII) and
1-ethyl-3-methylimidazolium thiocyanate (EMISCN) were
synthesized according to the previous report.31 TiCl4 and H2PtCl6
were purchased from Aldrich. cis-Bis(isothiocyanato) bis(2,20-
bipyridyl-4,40-dicarboxylato) ruthenium(II) ([RuL2(NCS)2]) was
purchased from Solaronix SA (Switzerland) and used without
further purification. Fluorine-doped tin oxide overlayer (FTO)
glass electrodes (7 U Sqꢀ1) and slurries containing 20 nm-sized
mesoporous and 200 nm-diameter light-scattering colloidal TiO2
were purchased from Dalian Hepat Chroma Solar Tech. Co., Ltd
(China).
The bis-imidazolium ionic liquid, 1-butyl-3-(1-vinyl-
imidazolium-3-hexyl)-imidazolium bromide ([BVIm] [HIm][Br]),
was synthesized via the reaction of [BrHBIm][Br] (2.95 g, 0.008
mol) with 1-vinylimidazole (1.13 g, 0.012 mol) at room temper-
1
ature for 96 h. HNMR: (400 MHz, D2O): 8.77 (s, 1H), 7.76 (s,
1H), 7.55 (s, 1H), 7.47 (m, 2H), 7.10 (m, 1H), 5.78 (dd, 1H), 5.41
(m, 1H), 4.18 (m, 5H), 3.47 (t, 1H), 1.83 (m, 6H), 1.30 (m, 6H),
0.89 (t, 3H) (see ESI, Fig. S3†).
Bis-imidazolium based poly(IL), poly[BVIm][HIm][Br], was
prepared via fꢁree radical polymerization of [BVIm][HIm][Br] in
ethanol at 60 C under a nitrogen atmosphere for 24 h. Anion-
exchange of poly[BVIm][HIm][Br] with LiTFSI in aqueous
solution yielded poly[BVIm][HIm][TFSI].32
Synthesis of ILs and poly(ILs)
Poly(ILs),
methanesulfonyl)imide) (poly[BVIm][TFSI]) and poly(1-butyl-3-
(1-vinylimidazolium-3-hexyl)-imidazolium bis(trifluorometh-
poly(1-butyl-3-vinylimidazolium
bis(trifluoro-
Preparation of electrolytes
Room temperature IL electrolyte composed of 0.1 M I2, 0.5 M
NBB, 0.1 M GNCS, EMII/PMII/EMISCN (6 : 6 : 7, v/v/v) was
dried and stirred at room temperature before the application.
Then 25 wt% of poly[BVIm][TFSI] and poly[BVIm][HIm][TFSI]
was added into the liquid electrolyte and stirred at 70 ꢁC to form
a quasi-solid-state gel electrolyte.
anesulfonyl)imide) (poly[BVIm][HIm][TFSI]) were synthesized as
shown in Scheme 1.
Mono-imidazolium based ionic liquid, 1-butyl-3-vinyl-
imidazolium bromide ([BVIm][Br]), was firstly synthesized via
the reaction of 1-bromobutane with 1-vinylimidazole at room
This journal is ª The Royal Society of Chemistry 2012
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Fabrication of quasi-solid-state DSSCs34,35
5 mV. The photocurrent density–voltage (J–V) curves of the
assembled DSSCs shielded by an aluminum foil mask with an
aperture area of 0.1 cm2 were measured with a digital source
meter (Keithley, model 2612) under simulated air mass (AM) 1.5
The cleaned FTO glass was covered with perforated adhesive
tape to control the thickness and area of the mesoporous TiO2
film, the area of TiO2 film was about 0.283 cm2. Two layers of
TiO2 particles were deposited onto cleaned FTO glass and used
as photoelectrodes. A 7 mm-thick film of 20 nm-sized TiO2
particles was deposited onto the FTO glass electrode by the
doctor-blade technique. The film was dried at 125 ꢁC for 5 min.
Then, a second 5 mm-thick layer of 200 nm light-scattering
anatase particles were coated on the top of the first TiO2 layer.
The resulting TiO2 films were annealed at 500 ꢁC for 30 min.
After cooling to 80 ꢁC, the obtained TiO2 electrode was
immersed in 0.5 mM [RuL2(NCS)2] ethanol solution at room
temperature for 24 h. Afterwards, the dye-coated TiO2 electrode
was washed with anhydrous ethanol and dried with nitrogen
stream. To prepare the Pt counter electrode, two drops of 2 mM
H2PtCl6 in ethanol were placed onto the cleaned FTO glass
substrate, followed by drying and annealing at 400 ꢁC for 15 min.
DSSCs were fabricated by sandwiching gel electrolytes
between a dye-sensitized TiO2 electrode and a Pt counter elec-
trode, which were separated by a 25 mm thick hot melt ring
(Surlyn, Dupont) and sealed by heating. The gel electrolytes were
solar spectrum illumination at 15, 50, and 100 mW cmꢀ2
,
respectively. Incident photo-to-current conversion efficiency
(IPCE) plotted as a function of excitation wavelength was
recorded on a Keithley 1612 source meter under the irradiation
of a Xenon lamp with a monochromater (Oriel Cornerstoneꢀ
260 1/4). Hermetically sealed cells were used for long-term
stability tests. For thermal stress the cells were covered with a UV
cut-off filter, put on the magnetic heater at 60 ꢁC and were
irradiated at open circuit under one sun visible-light soaking. The
photo-electrochemical parameters, such as the fill factor (FF)
and light-to-electricity conversion efficiency (h) were calculated
according to the previous reports.37
Results and discussion
Scheme 1 shows the facile synthetic strategy for the synthesis of
bis- and mono-imidazolium based poly(ILs). The chemical
structure of the produced poly(ILs) was confirmed by FT-IR
spectra (Fig. 1). The adsorption peaks centred at 3150 cmꢀ1 and
2940 cmꢀ1 are attributed to the C–H stretching vibration mode
of the imidazole ring and the alkyl chains. An absorption peak at
1138 cmꢀ1 can be assigned as the stretching and asymmetric
stretching vibrations of the C–N of the imidazole rings. The C]
N stretching vibration belonging to the imidazole ring is
observed at about 1628 cmꢀ1. The absorption peaks corre-
ꢁ
heated to 70 C under stirring until the gels completely melted.
The internal space of the cell was filled with electrolytes via
vacuum backfilling. The electrolyte injection hole on the ther-
mally platinized FTO counter electrode was finally sealed with a
Surlyn sheet and a thin glass by heating.
Characterization and photovoltaic measurements
sponding to the TFSI anion are observed at 1357, 1198 and 1048
cmꢀ1
1H NMR spectra were carried out on a Varian 400 MHz spec-
trometer using chloroform-d (CDCl3) or deuterated water (D2O)
as a solvent (Fig. S1–S3†). Fourier-transform infrared (FT-IR)
spectra were recorded on a ProStar LC240 FTIR spectrometer in
the range of 4000–400 cmꢀ1. Thermal analysis of polymers was
evaluated using a Perkin Elmer TGA 4000 thermogravimetric
analyzer under a nitrogen atmosphere with a heating rate of
10 ꢁC minꢀ1. The conductivity of the electrolytes was charac-
terized in an ordinary cell composed of a Teflon tube and two
identical stainless steel electrodes (diameter of 1 cm) on a
CHI660c electrochemical workstation at room temperature,
using the AC impedance method over the frequency range of
1 Hz to 1 MHz. The conductivity was calculated using the
following equation:
25,32,38,39
.
Compared with poly[BVIm][TFSI], no new peaks
were observed in poly[BVIm][HIm][TFSI]. However, the char-
acteristic absorption peaks belonging to the alkyl chains, TFSI
anions and imidazole rings were highly intensified (Fig. 1).
The thermostability of the synthesized poly[BVIm][TFSI] and
poly[BVIm][HIm][TFSI] were studied by thermogravimetric
analysis (TGA). As shown in Fig. 2 the poly[BVIm][HIm][TFSI]
showed a thermal stability up to about 350 ꢁC (Fig. 2A), which is
higher than that of poly[BVIm][TFSI] (ꢂ322 ꢁC, Fig. 2B).
The higher thermal stability of poly[BVIm][HIm][TFSI] than
poly[BVIm][TFSI] might be due to the higher charge density in
the dicationic poly(ILs) than the monocationic poly(ILs). The
l
s ¼
(1)
RS
where s is the conductivity in S cmꢀ1, R is the ohmic resistance of
the electrolyte, l is the distance between the two electrodes and S
is the area of the electrodes.36
Steady-state voltammetry was tested in a conventional photo
electrochemical cell equipped with a radius of 5.0 mm platinum
ultramicroelectrode (CHI107) as the working electrode, and a
platinum foil as a counter electrode and reference electrode. The
electrochemical impedance spectra (EIS) of the devices were
tested using a CHI660c electrochemical workstation using the
AC impedance method, the forward bias voltage for the
impedance measurement was ꢀ0.70 V and the frequency ranged
from 0.01–105 Hz under dark condition and the amplitude was
Fig. 1 FTIR spectra of poly(ILs) (A) poly[BVIm][TFSI], (B) poly
[BVIm][HIm][TFSI].
18020 | J. Mater. Chem., 2012, 22, 18018–18024
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It should be noted that the values of both the ion conductivity
and diffusion coefficient of the poly[BVIm][HIm][TFSI] gel
electrolyte are only a little lower than those of the ionic liquid
based liquid electrolyte, however, they are much higher than
those of poly[BVIm][TFSI]. It has been demonstrated that the
p–p stacked imidazolium rings of the PILs provide the charge
transport from the counter electrode to the photoanode.24,44
However, the higher charge density of the dicationic poly(ILs)
ꢀ
would lead to a higher ion conductivity, in which therefore I3
and Iꢀ can move more freely than in monocationic poly(ILs).25
In the case of dicationic poly(IL) units, two imidazolium cations
were covalently attached to the polymer main chain. The weak
interaction force between the alkyl chain and the imidazolium
cation and the flexibility of the imidazolium cation enables the
electron to be easily transferred in the electrolyte.24 Thus, the
Fig. 2 TGA curves of poly(ILs) (A) poly[BVIm][HIm][TFSI] and (B)
poly[BVIm][TFꢀSI1] under a nitrogen gas flow (100 mL minꢀ1). Heating
ꢀ
rate: 10 ꢁC min
.
ionic conductivity and diffusion coefficient of I3 in the poly
[BVIm][HIm][TFSI]-based gel electrolyte still reached reason-
able values.
small displacement of the ion’s vibrations could suppress the
decompositions of the ions.28 Such a high thermal stability of
PILs will be beneficial for durable high temperature DSSCs.
It is well known that the efficiency of the DSSC is criticallꢀy
dependent on the ion conductivity and diffusion coefficient of I3
of the electrolyte applied in the cell.40,41 Fig. 3 shows the steady-
state voltammograms of the ionic liquid and poly(IL) based
electrolytes. The apparent diffusion coefficient of triiodide
(Dapp(I3ꢀ)) was calculated from the anodic and cathodic steady-
state Iss by using the following equation:
Fig. 4a shows the J–V curves of the DSSCs fabricated with
ionic liquid, poly[BVIm][TFSI] and poly[BVIm][HIm][TFSI]
based electrolytes under simulated AM 1.5 solar spectrum illu-
mination at 100 mW cmꢀ2, respectively. The photovoltaic
parameters including open circuit voltage (Voc), short circuit
current density (Jsc), fill factor (FF) and the photoelectric
conversion efficiency (h) of the DSSCs are summarized in Table 1
as well.
As it can be seen the electrochemical parameters of the devices
containing poly(ILs) electrolytes decreased if compared with the
ionic liquid electrolyte due to the limited transport property of
the gel electrolytes. The increase in Jsc of the poly[BVIm][HIm]
[TFSI]-based gel electrolyte is related to the improved diffusion
coefficients of I3ꢀ, that the dye molecules could be regenerated
more easily and the p–p stacked imidazolium rings of poly
[BVIm][HIm][TFSI] could provide the charge transport from the
counter electrode to the photoanode.45,46 The enhanced Voc is
assigned from the suppression of the dark current at the TiO2/
electrolyte interface. The dark current originates from the
Iss ¼ 4nrFCDapp
(2)
where n is the number of electrons per molecule, F is the Faraday
constant, r is the radius of the ultramicroelectrode and C is the
bulk concentration of the electroactive species.42,43
The calculated ionic conductivity and diffusion coefficient
ꢀ
values of I3 are summarized in Table 1. It can be clearly seen
that both the ionic conductivity and diffusion coefficient values
of the poly[BVIm][HIm][TFSI]-based gel electrolyte are higher
than those of the poly[BVIm][TFSI]-based gel electrolyte and
close to those of the ionic liquid based electrolyte. The ionic
conductivities were 6.76, 2.92 and 5.83 ꢃ 10ꢀ3 S cmꢀ1 and the
diffusion coefficients of I3ꢀ were 4.05, 2.23 and 3.93 ꢃ 10ꢀ6 cm2
ꢀ
reduction of I3 by conduction band electrons from TiO2.37 As
shown in Fig. 4b the onset of the dark current is shifted by
approx. 31 mV from the poly[BVIm][HIm][TFSI] to the poly
[BVIm][TFSI] based gel electrolyte. The lower dark current onset
value indicates the lower I3ꢀ reduction rate (ket). The decrease of
ket(I3ꢀ) should lead to an increase of Voc according to the
following equation:25
s
ꢀ1 for the ionic liquid, poly[BVIm][TFSI] and poly[BVIm][HIm]
[TFSI] based electrolytes, respectively.
!
kT
Iinj
ꢀ
ꢁ
Voc
¼
ln
(3)
ꢀ
e
ncbket I3
where Iinj is the flux of charge resulting from sensitized injection
related to the electron back transfer rate, ncb is the concentratioꢀn
of electrons at the TiO2 surface and ket is the rate constant for I3
reduction. It is not surprising that the poly[BVIm][TFSI] based
cell yielded the lowest Voc due to its highest dark current. At the
interface of the dye-coated TiO2 photoanode and electrolyte, a
higher diffusion coefficient of I3ꢀ to the Pt counter electrode for
the Iꢀ form could also lead to a lower I3 reduction (ket) rate,
ꢀ
restraining the dark current and obtaining a higher Voc, as the
described explanation. Furthermore, the diffusion of Iꢀ to the
dye-coated TiO2 photoanode is in favor of the regeneration of
Fig. 3 Steady-state voltammograms of different electrolytes with a
platinum ultramicro-electrode at a scan rate of 10 mV sꢀ1 at 25 ꢁC.
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Table 1 The performance of the DSSCs based on different electrolytes under simulated AM 1.5 solar spectrum illumination at 100 mW cmꢀ2
Conductivity/10ꢀ3
S cmꢀ1
Jsc/mA
cmꢀ2
Electrolytea
Iss(I3ꢀ)/10ꢀ7
A
Dapp(I3ꢀ)/10ꢀ6 cm2 sꢀ1
Voc/V
FF
h
Ionic liquid electrolyte
poly[BVIm][TFSI]
poly[BVIm][HIm][TFSI]
6.76
2.92
5.83
1.56
0.86
1.52
4.05
2.23
3.93
0.695
0.645
0.676
12.93
10.72
12.92
0.702
0.690
0.678
6.31
4.77
5.92
a
Ionic liquid electrolyte contains 0.1 M I2, 0.5 M NBB, and 0.1 M GNCS in a mixture of EMII/PMII/EMISCN (6 : 6 : 7, v/v/v). poly[BVIm][TFSI] and
poly[BVIm][HIm][TFSI]-based gel electrolytes were prepared by dissolving the corresponding polymer (25 wt%) in the ionic liquid electrolyte.
consistent with the photoelectric conversion efficiency (h). The
photovoltaic performance parameters of the devices under
different light intensity illuminations are also summarized in
Table 2. The device containing the poly[BVIm][HIm][TFSI] gel
electrolyte displays a maximum h of 6.47% under the simulated
air mass 1.5 solar illumination with an intensity of 50 mW cmꢀ2
.
Electrochemical impedance spectroscopy (EIS) technique was
used to reveal the appreciable difference in the charge transfer
resistance at the conducting layer/TiO2, Pt/electrolyte, dyꢀe
coated TiO2/electrolyte interface and diffusion of the Iꢀ/I3
redox electrolyte, especially for understanding the recombination
dynamics at the dye coated TiO2/electrolyte.47 Fig. 6a shows the
Nyquist plot of the ionic liquid electrolyte, poly[BVIm][TFSI]
and poly[BVIm][HIm][TFSI] gel electrolyte based cells measured
at ꢀ0.7 V bias in a dark environment and the values of resistance
are listed in Table 3. From high to low frequency, three semi-
circles are observed in the Nyquist plots. R1, R2 and R3 represent
the charge-transfer resistance at the counter electrode, the
resistance of the TiO2/FTO, TiO2/electrolyte interface and the
Nernst diffusion in electrolyte, respectively. The dark reaction
impedance of the intermediate frequency semicircle in the
Nyquist plot revealed the electron recombination at the dye
coated TiO2 electrode/electrolyte interface which was caused by
the conduction band electrons of the mesoscopic TiO2 electrode
Fig. 4 The J–V curves of the DSSCs assembled with different electro-
lytes (a) under simulated AM 1.5 solar spectrum irradiation at 100 mW
cmꢀ2; (b) in the dark. Cells were tested using a mask with an aperture area
of 0.1 cm2.
ꢀ
captured by the I3 ions. The bigger the diameter of the middle
frequency semicircle is, the slighter the charge recombination at
the dye-coated TiO2 photoanode/electrolyte interface is.21,36 The
charge-transfer resistance (R2) values of the poly[BVIm][TFSI]
and poly[BVIm][HIm][TFSI] based gel electrolytes were calcu-
lated to be 65.3 and 74.8 U, respectively, indicating that the
charge-transfer resistance (R2) at the dye-coated TiO2 photo-
anode/electrolyte interface of the poly[BVIm][HIm][TFSI] gel
electrolyte cell increased, reducing the charge recombination at
the dye-coated TiO2 photoanode/electrolyte interꢀface due to the
enhanced charge transport properties of an Iꢀ/I3 redox couple
in the gel electrolyte.
dye for sustaining the operation. Therefore, the poly[BVIm]
[HIm][TFSI] based cell shows a higher Voc (0.676 V) and a higher
h (5.92%) in the polymer gel devices.
The incident photon-to-current conversion efficiency (IPCE)
curves of these DSSCs are shown in Fig. 5. The maximum
IPCE value at 530 nm is 78.41%, 55.08%, 67.72% for ionic liquid,
poly[BVIm][TFSI] and poly[BVIm][HIm][TFSI] based electro-
lytes, respectively. The IPCE values of these devices are
Table 2 The PCE values of DSSCs based on different electrolytes under
simulated AM 1.5 solar irradiation
PCE under different incident light
intensities irradiation
Electrolyte
1.0 sun
0.5 sun
0.15 sun
Ionic liquid electrolyte
poly[BVIm][TFSI]
poly[BVIm][HIm][TFSI]
6.31%
4.77%
5.92%
6.87%
5.14%
6.47%
6.33%
4.86%
6.03%
Fig. 5 The IPCE curves of DSSCs based on different electrolytes.
18022 | J. Mater. Chem., 2012, 22, 18018–18024
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recombination in the TiO2 photoelectrode can be related to the
inverse of the characteristic frequency and is estimated by the
following equation:48
1
1
se ¼
¼
(4)
umax 2pfmax
where fmax is the maximum frequency of the low-frequency peak.
The maximum frequency of the middle-frequency peak (fmax
)
and the lifetime of electrons are also summarized in Table 3. The
electron lifetime for recombination (se) for an ionic liquid elec-
trolyte-based cell gives the highest value of 29.01 ms, while the
poly[BVIm][TFSI]-based cell shows the lowest value of 16.30 ms,
and the poly[BVIm][HIm][TFSI]-based cell gives a value of
23.96 ms. Compared with the poly[BVIm][TFSI]-based cell, the
poly[BVIm][HIm][TFSI]-based cell yields a longer electron
recombination lifetime, which favors the electron transport
through a longer distance with less diffusive hindrance and
finally leading to enhanced photoconversion efficiency.7
The long-term stability of the fabricated DSSCs was investi-
gated via an accelerating aging test of sealed devices. The cells
were covered with a UV cutoff filter, put on a magnetic heater at
ꢁ
60 C and were irradiated at open circuit under one sun visible-
light soaking. The total efficiencies were normalized to the values
measured on the first day, as shown in Fig. 7. During the first
several days, the efficiency of all the cells was enhanced due to the
increased regeneration rate of the dye.9 It should be noted that
both the poly(IL) electrolytes exhibit superior long-term stability
than the ionic liquid electrolyte because the gel network hindered
the leakage of the liquid electrolyte effectively. The poly[BVIm]
[HIm][TFSI] based cell retains about 96% of the initial efficiency
even after a test of about 1200 h, which is slightly higher than the
poly[BVIm][TFSI] based cells (ꢂ92% of the initial efficiency).
This is probably because poly[BVIm][HIm][TFSI] is more
thermal stable than poly[BVIm][TFSI]. However, the ionic liquid
electrolyte based cells only retain ꢂ73% of their initial efficiency,
which might be due to the possible electrolyte leakage. These
results indicate that the quasi-solid-state DSSCs based on poly
[BVIm][HIm][TFSI] gel electrolyte have an excellent long-term
stability.
Fig. 6 The EIS Nyquist plots (a) and Bode phase plots (b) in the dark for
DSSCs based on different electrolytes.
Table 3 The parameters obtained by fitting the EIS of the DSSCs
fabricated with the different electrolytes
Device
R1/U
R2/U
R3/U
fmax/Hz
se/ms
Ioinc liquid electrolyte
poly[BVIm][TFSI]
poly[BVIm][HIm][TFSI]
4.2
3.8
3.7
100.7
65.3
74.8
14.3
23
18.5
5.486
9.766
6.643
29.01
16.30
23.96
Fig. 6b shows the Bode phase plots of EIS spectra display the
characteristic frequency peaks of the charge transfer process for
all the cells. The effective lifetime of the electrons (se) before
Conclusions
In summary, bis-imidazolium based dicationic poly[BVIm][HIm]
[TFSI] electrolytes have been prepared and employed for the
fabrication of organic solvent free quasi-solid-state DSSCs. The
resultant device shows an efficiency of 5.92% under the simulated
air mass 1.5 solar spectrum illumination at 100 mW cmꢀ2, which
is higher than that of the monocationic poly[BVIm][TFSI]-based
gel electrolyte. Furthermore, the poly[BVIm][HIm][TFSI] gel
electrolyte exhibits a superior long-term stability than ionic
liquid-based liquid electrolyte and monocationic poly(IL) gel
electrolyte. These results offer us a feasible method to quasi-
solid-state DSSCs in future practical applications.
Acknowledgements
This work was supported by the Natural Science Foundation of
China (nos 20974072, 21174102), the Natural Science Founda-
tion of Jiangsu Province (BK2011274), Research Fund for the
Fig. 7 Time-course variation of the normalized efficiency for the DSSCs
during accelerated aging tests at 60 ꢁC.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 18018–18024 | 18023

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