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Materials Science Forum Vols. 610-613 (2009) pp 347-352
© (2009) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/MSF.610-613.347
Online: 2009-01-02
Study on Properties of Quasi Solid Polymer Electrolyte based on
PVdF-PMMA Blend for Dye-sensitized Solar Cells
Yan YANG 1, a, Jie TAO 2,b and Li MA 3,c
1,2,3
College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics,
Nanjing 210016, China
a
b
c
[email protected], [email protected], [email protected] mail
Keywords: Dye-sensitized Solar Cell; PVdF–PMMA Blend; Polymer Gel Electrolyte; Porous
Membrane; Ionic conductivity; Photovoltaic Performance.
Abstract. Poly(vinylidene fluoride)(PVDF) is photochemically stable even in the presence of TiO2
and Pt nanoparticles, and poly(methacrylate)(PMMA) has good solvent retention. The quasi-solid
electrolytes based on PVDF-PMMA blend polymer were prepared in this work by soaking a porous
membrane in an organic electrolyte solution containing the I−/I3− redox couple. The as-prepared
electrolytes were characterized by means of Fourier Transform Infrared Spectroscopy, Scanning
Electron Microscope respectively. Moreover, the conductivity and the voltage-current curves of the
electrolytes were measured by electrochemical workstation. The results indicated that the optimum
blend proportion of PVDF and PMMA was 6:4. The porous structure prepared with the addition of
propanetriol was beneficial to ion diffusion and thus enhanced the conductivity of the electrolytes.
The gel polymer electrolyte had a conductivity of 0.14 mS·cm-1 under the ambient atmosphere.
Furthermore, electrolytes were assembled to fabricate DSSCs and the performance of the cells was
tested. The good properties with the open-circuit voltage of 0.60V and the short-circuit current of
1.1mAcm-2 were achieved upon illumination with visible light.
1. Introduction
In recent years, dye-sensitized solar cells (DSSCs) have received much attention because of their high
efficiency and low cost. Although liquid electrolyte dye-sensitized solar cells reach power conversion
efficiencies of over 11%[1], the main problem is that the liquid electrolytes limit device stability
because the liquid may evaporate when the cell is imperfectly sealed, and more generally, the
permeation of water or oxygen molecules and their reaction with the electrolytes may worsen cell
performance. Recently, many attempts have been made to solve the above problems by the
replacement of liquid electrolyte with solid and quasi-solid electrolyte [2-6]. New functional
materials, such as room temperature ionic liquids, organic and inorganic hole-transport materials,
solid and quasi-solid state polymer electrolytes and the sol-gel method developed gel electrolyte
[7-13], were developed as alternatives to the liquid electrolyte.
Polymer gels have been actively developed as highly conductive electrolyte material for lithium
secondary batteries and fuel cells. By definition, a polymeric gel is defined as a system that consists of
a polymer network swollen with a solvent [14]. Owing to their unique hybrid network structure, gels
have both the cohesive properties of solids and the diffusive transport properties of liquids.
Conventional methods used in DSSCs are solution casting and direct dissolution of the polymer in the
electrolyte solution. The softening of the polymer induced by the impregnation of liquid electrolyte
into a polar polymer makes the cell assembly difficult and increases the incidence of short-circuiting
between the electrodes. Thus an activation process in which a porous polymer membrane is soaked in
an electrolyte solution has been investigated.
The first attempt was made by Dong-Won Kim et al. [15]. In their research, the co-polymer used in
preparing the porous membrane was acrylonitrile–methyl methacrylate co-polymer with phase
inversion method. The gel polymer electrolyte was prepared by immersing the porous membrane in
organic liquid electrolyte. The DSSCs employing the gel polymer electrolyte yields a conversion
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Materials Research
efficiency of 2.4%. In 2006, Jihuai Wu et al. reported a polymer gel electrolyte by soaking
poly(acrylic acid)–poly(ethylene glycol) hybrid in the conventional organic liquid electrolyte[16]. X.
Zhang, et al. prepared PVDF-HFP porous polymer membrane applying water as
plasticizer[17].Quasi-solid-state solar cells fabricated with this gel electrolyte displayed energy
conversion efficiency of 6.0%.
In this study, the quasi-solid polymer electrolytes (QSPE) based on poly(vinylidene
fluoride)-poly(methyl methacrylate) blend polymer(PVDF-PMMA) were prepared by soaking a
porous membrane in an organic electrolyte solution containing the I−/I3− redox couple. Both of PVDF
and PMMA have been used as gel polymer electrolyte in DSSC, but to our knowledge, PVDF-PMMA
blend polymer is seldom reported. An amount of propanetriol was added to produce porous polymer
membrane for improving the structure of the membrane.
2. Experimental
2.1 Preparation of porous polymer membrane and gel electrolyte. The dry porous polymer
membrane composed of poly(vinylidene fluoride) (PVDF, SOLEF 1015) and poly(methyl
methacrylate) (PMMA, Mw =200,000) was synthesized by the following processes. The dried
polymer powders of PVDF and PMMA were dissolved in N, N-dimethylformamide (DMF).
Propanetriol was then added under continuous stirring at 60℃ to form homogeneous hybrid. The
resulting viscous mixture spread on a glass substrate was put it into a hot air stream for 5–10 min to
remove solvent. When the surface of the membrane was dry, the solvent was further removed at 80
℃ under vacuum for 24 h.
The porous polymer membrane once peeled off glass substrate was soaked in liquid electrolyte.
The liquid electrolyte used was 0.5M KI, 0.05M I2 in the binary organic solvents mixture acetonitrile
and glycol with 4:1 (v/v).
2.2 Preparation of electrodes. TiO2 colloidal paste that consisted of TiO2 powder (Degussa P25),
ethylene glycol and Triton X-100 was cast on to a previously cleaned, fluorine-doped tin oxide (FTO)
glass substrate by means of a glass rod. After drying in air for 15 min at room temperature, the
electrode was heated to 450℃ at a rate of 5℃ min−1, and then left at 450 ℃ for 30 min. The TiO2 film
formed on the FTO was 20µm thick and 1cm×1cm in size.
2.3 Fabrication of dye-sensitized solar cells. The TiO2 film was immerged in a 0.5mMOL
ethanol solution of cis-[(dcbH2)2Ru(SCN)2] (N719; Solalonix) for 24 hours to absorb the dye
adequately, the other impurities were washed up with anhydrous ethanol and dried in moisture-free air.
After that, a dye-sensitized TiO2 film was prepared.
A quasi-solid-state dye-sensitized solar cell was fabricated by sandwiching a slice of quasi-solid
polymer electrolyte between a dye sensitized TiO2 electrode and a platinum counter electrode. The
two electrodes were clipped together with clamps. The active area of the cell is 1cm2.
2.4 Measurements. The morphology of dry porous polymer membrane was characterized by
scanning electron micrograph.
The liquid electrolyte absorbency (Qle) of hybrid is calculated as:
Qle =(W−Wo) / Wo
(1)
where W is the weight of swollen hybrid and Wo is the original weight of dry hybrid.
The symmetric cell was built to characterize electrochemical properties of electrolyte with a
similar design as DSSC by sandwiching the QSPE sample between two Pt electrodes (TCO-Pt
/electrolyte / TCO-Pt).
The electrolyte resistance Rb was measured using CHI660 electrochemical workstation. The ionic
conductivity of the QSPE is calculated with the equation:
(2)
σ= d / RbS
where d is the thickness of electrolyte and S is the area of electrolyte.
Materials Science Forum Vols. 610-613
349
To investigate the diffusion coefficients of I3− in the electrolyte, steady-state voltammograms of
QSPE was performed. The apparent diffusion coefficients (D) of triiodide can be calculated according
to the equation as follows:
I lim d
(3)
D
= 2nFC
where I lim is limiting current density , n is the electron number per molecule, d is the thickness of gel
electrolyte, F is the Faraday constant and C is the bulk concentration of electroactive species.
Photocurrent-voltage measurements were performed upon illumination with visible light. The fill
factor (ff) of the cell is calculated by the following equations:
P max Im p × Vmp
ff =
=
(4)
Isc × Voc Isc × Voc
where Isc is the short-circuit current density(mA cm-2),Voc the open-circuit voltage(V), Imp (mA cm-2)
and Vmp(V) are the current density and viltage in the I-V curves, respectively, at the point of maximum
power output.
3. Results and discussions
3.1 Effect of the blend proportion of PVDF and PMMA on ionic conductivity of the electrolytes.
The ionic conductivity of the QSPE with different blend proportion of PVDF and PMMA is listed in
Table 1. It can be seen that the ionic conductivity gradually increases with the increase of PVDF
content, while beyond the amount of PVDF content 60%, the conductivity decrease. The QSPE with
40wt.% PMMA and 60 wt.% PVDF has the highest ionic conductivity 3.4×10-5 S cm-1.
It is well known that fluorine, presented in PVDF, has the smallest ionic radius and the largest
electronegativity and thus is hopeful to improve the ionic transport. But PVDF is a rigid membrane
and has poor solvent retention ability. It has been reported that the solvent retention ability of polymer
gels decreases in the order of PMMA≥PAN≥P(VdF-HFP)≥PVdF, which is surely the relative order of
polymer affinity for the solvent[18]. Therefore the blend polymer with 40 wt.% PMMA and 60 wt.%
PVDF, taking advantage of both PVDF and PMMA, maintains reasonable electrochemical
performance. In the following experiment, the blend proportion of PVDF and PMMA is 6:4.
Table 1.Ionic conductivity of QSPE with different blend proportion of PVDF and PMMA
PVDF wt%
Ionic conductivity/Scm-1
0
8.7×10-1
20
1.6×10-
40
1.2×10-
60
3.4×10-
80
3.6×10-
0
7
5
5
6
100
7.7×10-7
3.2 Effect of propanetriol on ionic conductivity and diffusion coefficients of the electrolytes.
In order to improve the ionic conductivity of quasi-solid polymer electrolytes, propanetriol was added
as a porosity-increasing material. 600mg of PVDF and 400mg PMMA were dissolved in 10mlDMF.
Different weight propanetriol was added to the solution with continuous stirring. The surface of dry
polymer membranes with different weight of propanetriol was observed by SEM, (seeing Fig. 1).
(a)
(b)
(c)
Fig.1. SEM photos of dry polymer membranes.
(a) without propanetriol (b) mPropanetriol /mPolymer is 0.1 (c) mPropanetriol / mPolymer is 1
For comparison, the image for polymer membranes produced without propanetriol is also included
in this figure. The membrane produced without propanetriol displays compact structure with some
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Materials Research
cracks, while the membrane assisted by propanetriol shows porous structure. The size and amount of
the pores increase with the increase of the amount of propanetriol.
Figure 2 shows the Fourier transform infrared (FTIR) spectra of a) 60 wt% PVDF + 40 wt%
PMMA, b) 60 wt% PVDF + 40 wt% PMMA + Propanrtriol. Compare with Fig 2 (a) and (b), (b) show
a strong and broad absorption band at 3341.67 cm–1 attributed to O–H of the propanetriol. It can be
seen that the propanetriol exists in polymer network stably.
100
100
98
95
90
94
transmittance
transmittance
96
92
90
88
85
80
75
86
(a)
84
4000
70
3500
3000
2500
2000
1500
1000
500
(b)
3341.67
4000
3500
3000
2500
2000
1500
-1
Wavenumber (cm)
1000
500
-1
Wavenumber (cm)
Fig.2. Fourier transform infrared (FTIR) spectra of polymer membrane
a) 60 wt% PVDF + 40 wt% PMMA b) 60 wt% PVDF + 40 wt% PMMA + Propanrtriol.
Polymer membranes with different weight rate (mPropanetriol/mPolymer is 0.5, 1.0, 1.5) were
prepared respectively. Those kinds of polymer membranes were immersed into liquid electrolyte after
their weight was measured. The gel polymer membranes were weighed after 12hr. It encapsulated the
electrolyte solution well without solvent leakage and maintains good mechanical properties. The
liquid electrolyte absorbency (Q le) of hybrid, the ionic conductivity of the QSPE(σ)and the apparent
diffusion coefficient of triiodide (D) are summarized in Table 2. Q le and D increases with
propanetriol amount increasing. The gel polymer electrolyte shows the highest ionic conductivity
when mPropanetriol/mPolymer is 1.0 and then decreased. The ionic conductivity of QSPE depends on
both cation(K+) and anion. It can be assumed that proper content of propanetriol decreased conductive
resistance and increased both σ and D. Over dosage propanetriol will lead too much porous network
and then the complex format will deteriorate the K+ movement thus decreased the ionic conductivity
of the QSPE.
Table 2. The liquid electrolyte absorbency (Q le), the ionic conductivity (σ) and
the apparent diffusion coefficient of triiodide (D) of QSPE
mPropanetriol / mPolymer
0
Q le / %
45
0.5
83
σ/ S cm
3.43×10
D / cm2 s-1
9.3×10-11
-1
1.0
-5
1.23×10
1.5
91
-4
3.46×10-8
96
-4
0.99×10-4
3.76×10-8
6.76×10-7
1.42×10
3.3 Photovoltaic performance. The photocurrent performances for dye sensitized solar cell with
different QSPE (A to E) were tested. The values of the open-circuit voltage (VOC), short-circuit
current density (ISC), fill factor (ff) and maximum power output (Pmax) are summarized in Table 3.
Table 3. Photovoltaic performance of dye sensitized solar cell
Cell A
Cell B
Cell C
Cell D
Cell E
Voc /V
Isc /mA cm-2
ff
Pmax /W
0.35
0.56
0.57
0.60
0.59
0.058
1.00
1.04
1.10
1.41
0.525
0.223
0.348
0.480
0.385
1.06489E-5
1.25023E-4
2.06558E-4
3.17245E-4
3.20092E-4
Materials Science Forum Vols. 610-613
351
The quasi-solid polymer electrolyte used in Cell A is PVDF/PMMA(6:4 m/m);
in Cell B is PVDF/PMMA/Propanetriol(mPropanetriol/mPolymer=0.5);
in Cell C is PVDF/PMMA/Propanetriol (mPropanetriol/mPolymer=1.0);
in Cell D is PVDF/PMMA/Propanetriol (mPropanetriol/mPolymer=1.5);
The liquid electrolyte is used in Cell E to be a reference.
The QSPE based on pure PVDF/PMMA polymer membrane shows poor photovoltaic
performance. The addition of propanetriol brings a significant improvement of the cell performance.
Isc, Voc, and Pmax increase with the increase of the amount of propanetriol. The good properties with the
open-circuit voltage of 0.60V and the short-circuit current of 1.1mA was achieved when
mPropanetriol / mPolymer is 1.5. The result accords with Q le and D variation law as indicated in
Chapter 3.2. It proves that the apparent diffusion coefficient of triiodide (D) plays a major role in
DSSC performance [19].
The DSSC assembled with the QSPE reports a lower ISC and higher VOC than the DSSC with a
liquid electrolyte. The lower value of ISC in the DSSC with gel polymer electrolyte may originate from
its lower ionic conductivity. A higher resistance to ion migration reduces the supply of I3− to the Pt
counter-electrode. This causes depletion of I3− and also retards the kinetics of dye regeneration, and
therefore, decreases the ISC. The slight increase of VOC for the DSSC with gel polymer electrolyte is
related to the reduction of the back electron-transfer reaction that decreases the VOC [20]. When a gel
polymer electrolyte is used in the DSSC, the polymer contacted on the surface of TiO2 suppresses the
back electron-transfer from the conduction band of TiO2 electrode to the I3−in the gel polymer
electrolyte, leading to high value of VOC.
4. Conclusions
A quasi-solid electrolytes based on PVDF-PMMA blend polymer were prepared by soaking a porous
membrane in an organic electrolyte solution containing the I−/I3− redox couple. It encapsulated the
electrolyte solution well without solvent leakage and maintains good mechanical properties that
allowed application in the dye-sensitized solar cell. The addition of propanetriol improved the
DSSCs performance. The porous structure prepared with the addition of propanetriol was beneficial
to ion diffusion and thus enhanced the conductivity of the electrolytes. The gel polymer electrolyte
had a conductivity of 0.14 mS·cm-1 under the ambient atmosphere. A dye-sensitized solar cell
employing quasi-solid electrolyte yield an open-circuit voltage of 0.6V and short-circuit current of
1.1mAcm−2 upon illumination with visible light.
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Materials Research
10.4028/www.scientific.net/MSF.610-613
Study on Properties of Quasi Solid Polymer Electrolyte Based on PVdF-PMMA Blend for DyeSensitized Solar Cells
10.4028/www.scientific.net/MSF.610-613.347
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