Poly(ionic liquid)/ionic liquid/graphene oxide composite quasi solid-state electrolytes for dye sensitized solar cells

Article abstract of DOI:10.1039/c5ra10702d

Poly(ionic liquid)/ionic liquid/graphene oxide (poly(IL)/IL/GO) composite gel electrolytes containing poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide), 1-propyl-3-methylimidazolium iodide and graphene oxide are prepared for dye-sensitized solar cells (DSSCs), without any volatile organic solvent. The conductivity of the composite electrolyte is significantly increased by adding an appropriate amount of GO, and the DSSCs based on composite electrolytes containing GO show higher power conversion efficiency performance and better long-term stability compared to those without GO. The DSSC based on the composite electrolytes which contain 2 wt% of GO show an overall power conversion efficiency of 4.83% under simulated AM 1.5 solar spectrum irradiation. The superior long-term stability of the DSSCs indicates that this type of composite electrolyte could overcome the drawbacks of volatile liquid electrolytes, and offer a feasible method to fabricate DSSCs in future practical applications.

Full text of DOI:10.1039/c5ra10702d

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Poly(ionic liquid)/ionic liquid/graphene oxide composite
quasi solid-state electrolytes for dye sensitized solar cells
Bencai Lin a, b*, Tianying Feng a, Fuqiang Chu a, Shuai Zhang a, Ningyi Yuan a, b, Jianning Ding a, b, c *
Received 00th January 20xx,
Accepted 00th January 20xx
Poly(ionic liquid)/ionic liquid/graphene oxide (poly(IL)/IL/GO) composite gel electrolytes containing poly(1-butyl-3-
vinylimidazolium bis(trifluoromethanesulfonyl) imide), 1-propyl-3-methylimidazolium iodide and graphene oxide are
prepared for dye-sensitized solar cells (DSSCs), without any volatile organic solvent. The conductivity of the composite
electrolyte is significantly increased by adding a proper amount of GO, and the DSSCs based on composite electrolytes
containing GO show higher power conversion efficiency performance and better long-term stability compare to that
without GO. The DSSC based on the composite electrolytes which containing 2 wt% of GO show an overall power
conversion efficiency of 4.83% under simulated AM 1.5 solar spectrum irradiation. The superior long-term stability of the
DSSCs indicates that this type of composite electrolytes could overcome the drawbacks of volatile liquid electrolytes, and
offer a feasible method to fabricate DSSCs in future practical applications.
DOI: 10.1039/x0xx00000x
www.rsc.org/
stability, broad electrochemical potential window and high
ionic conductivity. These unique properties make IL very
appealing materials as stable and safe electrolyte media in
Introduction
Dyeꢀsensitized solar cell (DSSC), a kind of photovoltaic device
which could directly convert solar energy into electrical energy
without emissions, is attracting a great deal of interest because
of its low cost, ease of fabrication and high power conversion
efficiency.1ꢀ3 As one of the key components of DSSC, the
electrolyte which transfers charge from the counter electrode to
the dye has much effect on the energy conversion efficiency
and stability of the DSSCs. Recently, power conversion
efficiency up to 12.3% have been achieved for DSSC using a
liquid organic electrolyte.4 However, the presence of the
organic solvents substantially shorten the working life and
affect the practical application of DSSC because of the
evaporation and leakage of organic solvents. To overcome
lithium batteries,16 highꢀtemperature proton exchange
17,18
membrane fuel cells,
and alkaline anion exchange
membrane fuel cells.19 Recently, much work has been done to
develop DSSCs with ILꢀbased electrolytes.20ꢀ22 The evaporation
problem of the traditional organic solvent based electrolytes can
15
be solved by using IL as electrolyte. Recently, we prepared
the DSSCs with IL/ILꢀtethered graphene oxide composite
electrolytes which showed an overall power conversion
efficiency of 7.04% under simulated AM 1.5 solar spectrum
irradiation at 100 mW cm−2.23
As a new class of polymers, poly(ionic liquid) (poly(IL))
which combine both the properties of ionic liquid and polymers
have attracted much attention in recent years. Poly(IL) was
successfully used for quasiꢀsolidꢀstate DSSCs, and the devices
these problems of DSSC, pꢀtype inorganic semiconductor,5, 6
inorganic hole transportation materials
7ꢀ9
and polymerꢀbased
showed high power conversion efficiency and excellent longꢀ
10ꢀ14
gel electrolytes
were prepared to substitute for the organic
25
term stability.24,
It is reported that addition of inorganic
solventsꢀbased electrolytes. Among these technologies, the
polymersꢀbased quasiꢀsolidꢀstate electrolytes have been
attracting much attention due to their nonflammability,
negligible vapor pressure, and good contact with the
nanocrystalline TiO2 electrode and counter electrode.15
nanoparticles into the electrolytes is an effective way to
improve the DSSC performance.26 Lin et. al prepared DSSCs
with poly(ether urethane)/poly(ethylene oxide)ꢀmodified SiO2
based electrolytes, and the devices showed a power conversion
efficiency of 4.86%.27 The efficiency of DSSCs has been
significantly increased by addition of small amounts of
graphene sheets into ILꢀbased electrolytes reported by Brennan.
et al.28 In general, the uniformly dispersed nanomaterials in
electrolytes could bring beneficial contributions to the short
Ionic liquid (IL) had attracted much attention for its unique
properties such as negligible vapor pressure, excellent thermal
circuit current density (Jsc) and longꢀterm stability of the
26, 27
devices.24,
However, the aggregation of inorganic
nanomaterials in electrolyte was commonly observed.26 Proper
surface modification could enhance the compatibility of
inorganic nanomaterials and the ionic liquids in the preparation
of the corresponding electrolyte for DSSCs.25 Graphene oxide
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(GO) has much oxygenꢀcontaining functional groups such as for 3 days. The resultant viscous oil was washed with ethyl
1
hydroxyl, carboxylic acid, and can be easily chemical ether three times. HNMR: (400 MHz, CDCl3): 10.98 (s, 1H),
modification.29ꢀ31 In our previous work, the DSSCs based on 7.77 (s, 1H), 7.50 (dd, 1H), 7.46 (m, 1H), 5.95(m, 1H), 5.36 (m,
ionic liquidꢀtethered graphene oxide (ILꢀGO)/ionic liquid 1H), 4.38 (t, 3H), 1.91(m, 2H), 1.37(m, 2H), 0.96 (t, 3H).
electrolytes were fabricated.23 The introduced of ILꢀGO into Yield: 88%. Poly(1ꢀbutylꢀ3ꢀvinylimidazolium bromide was
ILꢀbased electrolyte is a successful method to improve the prepared via free radical polymerization of 1ꢀbutylꢀ3ꢀ
o
performance of DSSCs. However, the devices with liquid state vinylimidazolium bromide in ethanol at 60 C under a nitrogen
electrolyte were still suffering leakage problems in the practical atmosphere for 24 h. Poly(1ꢀbutylꢀ3ꢀvinylimidazolium
use, and the longꢀterm stability of the DSSCs should be further bis(trifluoromethanesulfꢀonyl) imide) was abtained by anionꢀ
enhanced. In addition, the preparation of ILꢀGO is not a simple exchange of [PBVIm][Br] with LiTFSI in aqueous solution.
process.
Here, we report the fabrication of high performance DSSCs
using poly(IL)/IL/GO composite gel electrolyte as
Preparation of graphene oxide (GO)
Graphene oxide (GO) was obtained from the graphite powder
a
33, 34
following previous work.
Typically, 1.00 g of graphite,
replacement for traditional organic solvent based gel
electrolytes. The influence of GO content on the DSSC
properties was systematically investigated. The GO could be
well dispersed in poly(IL)/IL to form quasiꢀsolidꢀstate gel
electrolytes, without using any volatile organic solvents. The
fabricated quasi solidꢀstate DSSCs with GO show better
performance and stability than that without GO gel electrolytes,
which indicating the poly(IL)/IL/GO composite gel electrolytes
are a promising candidate for DSSCs with good durability.
0.50 g of NaNO3 and 23 mL of H2SO4 (98%) was added into a
100 mL flask, and then the mixture was stirred for 15 min in an
ice bath. Then 3.00 g KMnO4 was added and the mixture was
stirred at 35oC for 30 min, followed 46 mL of deionized water
was added into the mixture and stirred at room temperature for
30 min. Then, 10 mL of 30% H2O2 was added into the mixture,
and finally cooled down to room temperature. The obtained
precipitated was washed with 5% HCl and H2O several times
before being dried under vacuum.
Preparation of poly(IL)/IL/GO electrolytes
The liquid electrolyte was composed of 1M PMII, 0.1M I2,
0.2M LITFSI, 0.5M TBP, 20 wt% of the poly(IL) (based on the
weight of the liquid electrolyte) and different amount of GO
was added into the liquid electrolyte and stirred at 60 oC for 6 h,
then the homogeneous poly(IL)/IL/GO electrolytes were
obtained.
Experimental
Materials
TiO2 nanoparticles (Titanium (IV) oxide, Tꢀ20nm), H2PtCl6 and
iodine were obtained from SigmaꢀAldrich. Flaky graphite
powder (45 ꢁm , Qingdao Huatai Lubricant Sealing S&T
China), sulfuric acid, potassium hydroxide, ethyl acetate, 1ꢀ
Fabrication of DSSCs
iodopropane,
1ꢀmethylimidazole,
3ꢀbromopropylamine
The DSSCs were fabricated by using a conventional process
according to the previous report:35,36 TiO2 working electrodes
were prepared on the cleaned FTO coated glass substrates, the
FTO glasses were sticky with adhesive tape on both parallel
edges to control the thickness of the TiO2 films, and were
annealed at 500 °C for 30 min to ensure good electrical contact
between the TiO2 films and the FTO substrates. Dye adsorption
was carried out by immersing the TiO2 working electrodes in
N719 dye solution (0.5 mM in ethanol) at room temperature for
hydrobromide and 4ꢀtertꢀbutylpyridine (TBP) were purchased
from Aladdin. Lithiumbis (trifluoromethanesulfonyl) imide
(LiTFSI) were purchased from Alfa Aesar and used as received.
N719 dye
，Surlyn (ionomer films of 25ꢀꢁm thick) and FTO
conducting glass (resistance of 25 ꢂ/square, transmittance of 85
%) were purchased from Dalian Rainbow Solar Technology
Development Co Ltd., Dalian, China. All reagents were of
analytical grade and were used as received unless otherwise
stated.
24 h
，then the TiO2 electrodes were rinsed with ethanol and
Synthesis of 1-propyl-3-methylimidazolium iodide (PMII)
32
dried under nitrogen flow. The Pt counter electrode was
prepared by dripping a drop of 10 mM H2PtCl6 ethanol solution
placed onto FTO glass substrate, followed by annealed at 450
°C for 30 min. DSSCs were fabricated by sandwiching
poly(IL)/IL/GO electrolytes between dyeꢀsensitized TiO2
working electrode and Pt counter electrode, which were using a
sheet of a thermoplastic frame (25ꢀꢁm thick, Surlyn) as a spacer
between the two electrodes. The typical active area of the cell
was 0.2025 cm2.
PMII was synthesized as follows:
8.5 g (0.05 mol) 1ꢀ
iodopropane was dissolved in 20 ml ethyl acetate, 4.1 g (0.05
mol)1ꢀmethylimidazole was then added and the mixture was
stirred for 2 h at 0 C. The reaction bath was heated to room
temperature and then stirred for another 48 h. The resultant
viscous oil was washed with ethyl ether three times, and then
dried in dynamic vacuum at 80 oC for 24 h. 1H NMR (400 MHz,
D2O): δ: 8.64 (S, 1H), 7.39 (S, 1H), 7.34 (S, 1H), 4.07 (t, 2H),
3.80 (S, 1H), 1.82 (m, 2H) , 0.83 (m, 3H).
o
Characterization techniques and instrumental
Synthesis of poly(1-butyl-3-vinylimidazolium
XRD analysis was carried out using a RADꢀ3X (Rigaku
Corporation, Tokyo, Japan) diffractometer with CuꢀKα
radiation. AFM images were recorded using Agilent AFM with
Pico plus molecular imaging system in the nonꢀcontact mode.
The conductivity of the electrolytes was characterized in an
bis(trifluoromethanesulfonyl) imide) ([PBVIm][TFSI], poly(IL)
Poly(IL) was synthesized according to the previous report: 24 1ꢀ
butylꢀ3ꢀvinylimidazolium bromide ([BVIm][Br]) was firstly
synthesized from 1ꢀbromobutane and 1ꢀvinylimidazole under a
nitrogen atmosphere using a magnet stirred in an ice water bath
2 | J. Name., 2012, 00, 1-3
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ordinary cell composed of 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 0.1 Hzꢀ1M Hz.24
Steadyꢀstate voltammetry was tested in a conventional photo
electrochemical cell equipped with a radius of 5.0 ꢁm platinum
ultramicroelectrode (CHI107) as the working electrode, and a
platinum foil as a counter electrode and reference electrode,
respectively.10 The currentꢀvoltage (
JꢀV) characteristics were
examined by a Keithley model 2400 source meter (Keithley
Instruments, Inc. Cleveland, USA) under AM 1.5 solar
spectrum irradiation of 100mW/cmꢀ2. Electrochemical
impedance spectra (EIS) were conducted using a Zahner IM6ex
electrochemical workstation. A perturbation of 10 mV was
applied, and the frequency ranged from 0.1 Hz to 1M Hz.
Cyclic voltammetry (CV) tests were carried out using a
Fig. 2 The AFM image of GO.
computerꢀcontrolled potentiostat (Autolab
Type
III)
by a polished platinum electrode vs a saturated KCl calomel
reference electrode from –0.7 to 1.2 V at an ambient
atmosphere, and the scan rates is 75 mV·s−1). Intensityꢀ
modulated photocurrent spectroscopy (IMPS) and photovoltage
spectroscopy (IMVS) were measured on a controlled intensity
modulated photo spectroscopy (Zahner Co., Kansas City, MO,
USA) in ambient conditions under illumination through the
FTO glass side, using a blue lightꢀemitting diode as the light
source (BLL01, λmax = 470 nm, spectral halfꢀwidth = 25 nm;
Zahner Co.) driven by a frequency response analyzer, and the
light intensity (incident photon flux) of the DC component was
controlled at 2.5 × 1016 cm−2s−1. During the IMVS and IMPS
measurements, the cell was illuminated with sinusoidally
modulated light having a small AC component (10 % or less of
the DC component).
It has been demonstrated that it is difficult to disperse inorganic
nanomaterials into common organic solvent due to the lack of
surfaceꢀfunctional groups.26 In this work, graphene oxide (GO)
was prepared and applied as the additive for the preparation of
ILꢀbased quasi solidꢀstate electrolyte for DSSCs. In the present
work, GO was prepared following the Hummers method.33,34
Poly(IL) and IL were obtained according to the previous
24
reports.15,
The purity and chemical structure of the
synthesized IL was confirmed by 1H NMR measurements (see
Experimental Section). XꢀRay diffraction (XRD) measurements
were used to investigate the structure of GO. The XRD spectra
of graphite and GO are shown in Fig. 1. It can be clearly seen
that an intense (002) diffraction peak of graphite appears at
26.4°, and the interlay space is 0.34 nm, while the (001)
diffraction peak of GO appears at 12.2° and the interlay space is
0.73 nm. The increase of interlayer distance of GO was caused
by the intercalation oxygenated functional groups on the
surface of graphite nanoꢀsheet. The atomic force microscope
(AFM) measurements were used to confirm the morphology of
GO, and the results indicating GO nanoꢀsheets were obtained
(Fig. 2).
Results and discussion
Characterization of electrolytes
ILꢀbased quasiꢀsolidꢀstate electrolytes containing various
content of GO were prepared and coded as electrolyte AꢀF: (A)
0 wt% GO, (B) 1.0 wt% GO, (C) 1.5 wt% GO, (D) 2.0 wt% GO,
(E) 2.5 wt% GO, (F) 2.5 wt% GO. The DSSCs based on
electrolyte AꢀF are denoted as cell AꢀF, respectively. All the
electrolytes are quasiꢀsolidꢀstate gels at room temperature. The
Graphite
GO
performance of
a DSSC is usually influenced by the
conductivity of electrolyte. The conductivities of electrolytes
containing different content of GO were measured with a
CHI660c electrochemical workstation at room temperature, and
the results listed in Table 1. It can be clearly seen that the
conductivities of the composite electrolytes increases with the
incorporation of proper amount of GO. For example, the
conductivity of electrolyte A is 5.54 × 10−4 S cm−1, and it
reaches to the maximum value of 7.82 × 10−4 S cm−1 by the
electrolyte D which contains 2.0 wt% of GO. The addition of
GO favors the formation of ion transportation network in the
quasi solidꢀstate electrolyte, which improved the movement of
10
15
20
25
30
35
40
ο
(
2θ
)
Fig. 1 XRD spectra of graphite and GO.
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Table 1 Photovoltaic performance parameters of the DSSCs based on
electrolytes AꢀF measured under AM 1.5 solar spectrum irradiation of
100mW cmꢀ2.
10
8
A
B
C
D
E
F
GO
(wt.%)
0
Conductivity
( 10-4 S cm-1)
5.54
Jsc
Voc
η
Cell
FF
(mA cm-2)
(Mv)
0.60
(%)
1.46
6
A
B
C
D
E
3.34
0.72
0.75
0.75
0.76
0.75
0.73
1.0
1.5
2.0
2.5
3.0
6.72
7.45
7.82
6.54
6.06
4.07
7.42
8.84
5.59
3.47
0.70
0.71
0.72
0.69
0.68
2.16
3.85
4.83
2.90
1.73
4
2
F
0
0.0
0.2
0.4
0.6
0.8
Voltage(V)
free ions in a regular direction.37, 38 However, the excess GO
result in the decrease of the conductivity of the quasi solidꢀstate
electrolyte, indicating potential saturation of ion transport
channels formed by the incorporation of GO, and the further
addition of GO could block the formed ion transport channels.31
Therefore, it is not surprising that excess GO resulted in low
ionic conductivity.
Fig. 4 The J-V curves of DSSCs assembled with electrolytes A-F under
simulated AM 1.5 solar spectrum irradiation at 100 mW cm−2. The devices
are tested with an aluminum foil mask with an aperture area of 0.1 cm2.
Cyclic voltammetry (CV) was tested to investigate the effect
of GO content on the redox activity of the ILꢀbased electrolytes.
Characterization of DSSCs
Fig. 4 shows the photocurrent densityꢀvoltage characteristics
of DSSCs based on electrolytes AꢀF containing different
content of GO, which were measured under AM 1.5 solar
spectrum irradiation of 100 mW cmꢀ2. The data of the open
–
The CV curves of the I–/I3 system with different content GO
were measured by a polished platinum electrode vs a saturated
KCl calomel reference electrode from –0.7 to 1.2 V, and the
results were showed in Fig. 3. There are two redox waves can
be seen in Fig. 3, the left one which is located at lower potential
circuit voltage (
FF) and the photoelectric conversion efficiency (
summarized and listed in Table 1. It can be seen that the DSSC
based on electrolyte A without GO shows a
sc of 3.34 mA cmꢀ
of 1.46%, respectively. By
V
oc), short circuit current density (
J
sc), fill factor
(
η) are also
–
corresponds to the redox reaction of I3 +2e– ↔3I– , and the
–
other one corresponds to the redox reaction of 3I2+2e–↔2I3 . In
J
general, the higher peak current density of the CV curves
means that the reaction kinetics is more active.39 The values of
peak current density increase with increasing the content of GO
from 0 to 2.0 wt%. These results indicating that the GO is
acting as a catalyst for the redox reaction in the gel electrolytes
and allows for an increase in current density. However, the
values of peak current density decreases with a further addition
of GO. As mentioned before, the further addition of GO could
block the ion transport channels formed in the gel electrolytes
which resulted in low current density.
2
, Voc of 0.60 mV, FF of 0.72, and η
increasing the content of GO to 2.0 wt%, the maximum values
of Jsc, Voc and η are obtained, but these values decrease with
additional GO. The DSSC based on electrolyte D which
containing 2.0 wt% GO shows the best performance, and shows
a maximum photoelectric conversion efficiency of 4.83%.
Unfortunately, with the excess addition of GO into the ILꢀbased
gel electrolyte, the performance of the devices decrease
probably due to the high viscosity of the gel electrolyte and the
aggregation of GO which blocked the charge transfer in the gel
electrolyte.
0.6
0.4
0.2
0.7
A
B
C
D
E
F
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0
A
B
C
D
E
F
-0.2
-0.4
-0.5
0.0
0.5
1.0
1.5
Voltage(V)
400
500
600
700
800
Wavelength(nm)
Fig. 3 CV curves of electrolyte A-F.
Fig. 5 The IPCE curves of DSSCs based on different gel electrolytes.
4 | J. Name., 2012, 00, 1-3
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transport at the TiO2/electrolyte interface, as well as the
enhancement of transport of I−/I3− ions in the electrolyte.43
Intensityꢀmodulated photocurrent spectroscopy (IMPS) and
photovoltage spectroscopy (IMVS) measurements were
conducted to further investigate the GO content effect on the
electron transport and charge recombination in the DSSCs, and
the IMPS (Fig. 7A) and IMVS (Fig. 7B) plots of Cell AꢀF are
20
CPE2
CPE1
R1
A
B
C
D
E
F
W
RS
R2
10
shown in Fig. 7. The transit time (τd) and electron lifetime (τn)
can be calculated using the follow equations:
τ
τ
d=(2π min
n =(2π min
min(IMPS) and fmin(IMVS) are the frequencies at the
f
f
(IMPS) )ꢀ1
(IMVS) )ꢀ1
0
10
where
f
20
30
40
50
minimum imaginary component in the IMPS and IMVS plots.23
Table 3 lists the electron lifetime and transit time of cells AꢀF.
Cell D shows the longest electron lifetime of 96.89 ms and the
shortest transit time of 1.89 ms among the devices. A long τn
favors the collection of electrons before they recombine in the
cells,44 and indicating more electrons surviving from the back
reaction which results in high photocurrent.23,45 The τd is much
Z′(Ohm)
Fig. 6 EIS Nyquist plots of cell A-F.
Table 2 Electrochemical impedance spectroscopy results of Cell AꢀF.
Cell
A
R1/Ω
15.20
10.03
7.47
R2/Ω
14.24
6.02
3.61
3.52
4.45
8.93
Rdiff /Ω
39.13
15.41
10.10
5.04
shorter than
τ
n, and this is important for DSSCs to collect the
n and d are in
charge injected by dye.46 Both the tendencies of
τ
τ
B
good agreement with the fact that the performance of DSSCs is
improved by increasing the content of GO until it reaches 2
wt%. Compared with Cell A, the electron lifetime of DSSCs
based on composite gel electrolytes containing GO are enough
to favors electron transport through a longer distance with less
diffusive hindrance, and finally leads to enhanced photoelectric
conversion efficiency.47
C
D
E
5.97
8.82
11.07
30.92
F
10.68
The incident photoꢀtoꢀcurrent conversion efficiency (IPCE)
curves of cell AꢀF are shown in Fig. 5. The maximum IPCE
value as high as 64.3% at 530 nm was obtained for Cell D,
which is higher than that of Cell A (18.2%), Cell B (31.1%),
Cell C (44.6%), Cell E (33.3%), and Cell F (20.1%),
respectively. The IPCE values are consistent with the
photoelectric conversion efficiency of the DSSCs.
The kinetics of electrochemical and photoꢀelectrochemical
processes of DSSCs, such as the electronic and ionic processes
occurring in DSSCs were investigated by the electrochemical
impedance spectroscopy (EIS) technique. Fig. 6 shows the EIS
Nyquist plot of cells AꢀF which measured at ꢀ0.7V bias under
dark environment. Generally, three semicircles can be seen in
the EIS spectra of DSSCs. As shown in Fig. 6, from high to low
frequency, the three semicircles correspond to the charge
0.0
A
-0.2
Cell A
Cell B
Cell C
Cell D
Cell E
-0.4
Cell F
-0.6
0.0
0.5
1.0
1.5
2.0
Re
(
ꢀ
I
)(mA
)
0
-200
-400
-600
-800
-1000
B
transfer resistance at the counter electrode (
of TiO2/FTO, TiO2/electrolyte interface ( 2), and the Warburg
of I−/I3 in the electrolyte,
R1), the resistance
R
−
diffusion process
(Rdiff)
respectively.40ꢀ42 The overall series resistance was coded as Rs
as shown in Fig. 6. The corresponding fitting values of the
resistance showed in Fig. 6 are listed in Table 2. By increasing
the content of GO to 2.0 wt%, the maximum values of R1 was
obtained, but the value decrease with additional GO. This
phenomenon is in good coordination with the reversed order
Cell A
Cell B
Cell C
Cell D
Cell E
Cell F
0
400
800
1200
1600
2000
2400
Re
(
ꢀ
V
) (mV
)
observed for the
η of the DSSCs. The trends of R2 and Rdiff are
similar with 1, and cell D with 2 wt% GO shows the minimum
R
resistance values of 3.52 and 5.04 ꢂ. These results supports that
the addition of proper content of GO could enhances the charge
Fig. 7 IMPS (A) and IMVS (B) plots of cell A-F.
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Conclusions
Table 3 Detailed IMPS/IMVS parameters of cell AꢀF.
In summary, poly(IL)/IL/GO composite gel electrolyte have
been prepared for quasiꢀsolid state DSSCs. The conductivity of
the composite electrolytes was increased by incorporating of
proper content of GO. Compared with the DSSC based on
electrolyte without GO, the DSSCs based on composite
electrolyte with proper GO show higher open circuit voltage,
short circuit current density, photoelectric conversion
efficiency, and better longꢀterm stability. The DSSC based on
composite electrolyte with 2 wt% of GO showed maximum
efficiency of 4.83 % under AM 1.5 solar spectrum irradiation.
The composite ILꢀbased gel electrolytes prepared in the present
work could overcome the leakage problem of liquid electrolytes
based DSSCs and demonstrate a feasible approach for the
practical applications of quasiꢀsolidꢀstate DSSCs.
Cell
A
fmin(IMPS) (Hz)
17.1
fmin(IMVS)
24.56
7.58
τd (ms)
9.31
4.62
2.50
1.89
3.37
6.48
τn (ms)
6.48
B
34.44
21.01
55.37
96.89
33.35
13.76
C
63.71
2.88
D
E
84.48
1.64
47.28
4.78
F
24.56
11.57
The longꢀterm stability of DSSCs is still a challenge for their
practical applications. Here, the longꢀterm stability of DSSCs
was investigated via an accelerating aging test of the sealed
devices.48 The total efficiencies of Cell A, D and F are
normalized to the values measured on the first day, as shown in
Fig. 8. The efficiency of all the three devices was enhanced at
the early stage of longꢀterm stability testing due to the increased
of the dye regeneration rate which enhanced the Jsc of
DSSCs.47,49 As shown in Fig. 8, Cell D and Cell F retains 97%
and 91% of their initial efficiency after 1200 h, while Cell A
only maintained 85% of the initial efficiency under the same
conditions. The longꢀterm stability of DSSCs is greatly
improved by addition of GO probably due to the gel network
hindered the leakage of the ionic liquid of the electrolyte
effectively. In addition, the nanomaterial is effective in
preventing the release of ionic liquid component from the
composite electrolyte.18 Compared with that of Cell D, Cell F
showed a lower longꢀterm stability probably due to the
aggregation of GO in the electrolyte.24 All these results
demonstrating that DSSCs based on this type of gel electrolyte
containing proper content of GO have excellent practical
stability.
Acknowledgements
This work was supported by National Natural Science
Foundation of China (No. 51303017 and 21476031), the
Natural Science Foundation of the Jiangsu Higher Education
Institutions of China (Grant No. 13KJB150003), and a Project
Funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions.
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