Dye Sensitized Solar Cells with PAN-MgI2 Plasticized Electrolyte
T.M.W.J. Bandara1,2,3, B.-E. Mellander2 and M.A.K.L. Dissanayake1
1
Department of Physics and Postgraduate Institute of Science, University of Peradeniya,
Peradeniya, Sri Lanka.
2
Department of Applied Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden.
3
Department of Physical Sciences, University of Rajarata, Mihintale, Sri Lanka.
Abstract
In the recent past, the use of solid polymer electrolytes in electrochemical devices has
increased rapidly. However research work done on solid state solar cells with polymer
electrolyte is very limited even though polymer electrolytes can be used favourably in
Photo-electrochemical (PEC) solar cells. In this work, required conductivity enhancement
for Polyacrylonitrile (PAN) and MgI2 electrolyte was obtained by blending with
plasticizers such as ethylene carbonate and propylene carbonate. The conductivity
measurements showed Arrhenius behavior and this polymer blend electrolyte showed
optimum ionic conductivity of 1.9×10-3 S cm−1 at 30 °C. DC polarization test revealed
dominant ionic contribution to the conductivity. The minimum glass transition
temperature, -103.03 °C, was shown for the best electrolyte. Overall energy conversion
efficiency of 0.84 % was achieved for the solar cell fabricated. The short circuit current,
open circuit voltage and fill factor of the cell are 2.04 mA , 691.8 mV and 59.3%
respectively.
Keywords: Solar cells; Polymer electrolyte; Ionic conductivity; Plasticizers; Dye
sensitized:
1. Introduction
The dye sensitized solar cell is a promising device to
supplement future energy
requirements, due to low fabrication cost, environmentally friendly operation and
recently reported relatively high efficiency [1,2,3,4,5]. Nano-porous TiO2 Photo-electrodes
sensitized with Ruthenium dyes are attractive materials for dye sensitized solar cells [2,
1
4]. The dominance of the photovoltaic field by inorganic solid state junction devices is
now being challenged by the third generation of solar cells based on dye sensitized nanocrystaline photo-electrodes and solid polymer electrolytes. [2,4,6,7].
A dye sensitized photo-electrochemical (PEC) solar cell mainly comprises of a photoelectrode, an electrolyte membrane and a counter electrode. The salient part of the solar
cell is the photo-electrode and it has been reported that ruthenium polypyridyl dye
complexes gives better energy conversion efficiencies for TiO2 nano-crystalline photoelectrodes [7,8]. Even though wet type electrolytes has been reported high energy
conversion efficiencies their practical use has been limited due to many drawbacks side
reaction and leakage problems which limits the stability and lifetime of the cell [9,10].
Fabrication of low cost and mutually compatible, non-liquid electrolyte system with good
ionic conductivity for TiO2 solar cells will facilitate large scale fabrication of high
efficient and viable TiO2 solid state solar cell. Thus, investigation for suitable solid or gel
electrolytes to be used in PEC solar cells is very important. In addition, the usage of
polymers as host materials in such electrolytes is advantages due to their chemical
inertness towards electrodes and mechanical and geometrical flexibility. Polyacrylonitrile
(PAN) is one of the host polymers widely used for polymer electrolytes in
electrochemical applications and its chemical and physical properties are suitable to be
used as host polymer matrix in gel electrolyte based PEC cells.
In the recent past, the use of solid or gel polymer electrolytes in prototype
electrochemical devices like secondary batteries, fuel cells and electrochromic devices
increased rapidly. However, research reported on solid state solar cells using polymer
electrolyte has been very limited, even though polymer electrolytes are very favorable for
PEC solar cells [2,4,6,7]. We are not aware of any literature on PEC solar cells using
polymer electrolytes containing divalent metal salts. This may thus be the first report on
such PEC solar cells using a polymer electrolyte complexed with a divalent metal salt.
The advantage of using a divalent salt like MgI2 is the possibility of make the cation
immobilized via cross linking and to provide efficient charge transfer mechanism at the
interfaces. I-/I3- is the widely used and mutually compatible redox couple for the
2
electrolytes used in TiO2 PEC solar cells which shuttle in between the counter electrode
and photo-electrode giving efficient charge transfer mechanism at the interfaces enabling
electrons to travel
in the external circuit[6,7,8]. However, ambient temperature
conductivities of PAN based electrolytes are in general very low.
Therefore, a
conductivity enhancement is needed for solar cell applications. This is attained by
incorporating plasticizers such as ethylene carbonate (EC) and propylene carbonate (PC)
which have relatively high dielectric constants giving rise to improved ionic mobility
and salt salvation[11,12].
In this study, conductivity measurements were carried out in order to determine the
optimum salt concentration and this optimum conductivity sample was selected to
fabricate the PEC solar cell. Further the electrolyte is characterized using deferential
scanning colorimetry and DC polarization tests. cis-diisothiocyanato-N,N”-bis(2,2/bipyridyl–4,4/-dicarboxylicacid)-Ruthenium(II) dihydrate [RuL2(NCS)2.2H2O] sensitizer
was used to make the wide band gap TiO2 electrode photosensitive. I-V characteristic
curves were used to estimate energy conversion efficiency, fill factor and short circuit
current and open circuit voltage.
2. Experimental
PAN, MgI2, Iodine chips [I2], EC, PC, all with purity greater than 98% purchased from
Aldrich were used as starting materials. All these chemicals, except I2 and PC were
vacuum dried for 24 hours in a “GALLENKAMP” vacuum oven prior to use. Ruthenium
dye was purchased from solaronix sa.
Electrolyte Preparation
For preparing the electrolyte samples the weights of
PAN (0.100 g), I2 (0.008 g), EC
(0.400 g) and PC (0.400 g) were kept unchanged and the x weight % of MgI2 was varied
according to the weight ratio PAN : MgI2 = 100: x. All the samples were prepared by
using hot press method. i.e. the selected compositions of chemicals were mixed in glass
bottle and magnetically stirred at 80 0C for 2 minutes, until a homogeneous viscous slurry
3
was obtained. The resulting slurry after stirring was casted on to a glass plate and pressed
by another glass plate to fabricate a thin polymer electrolyte film.
Photo-electrode Preparation
Nano-porous TiO2 thin films with thickness 5-10 microns were prepared on fluorinedoped tin oxide (FTO) glass plates using an already published procedure [ 13]. The TiO2
coated electrode was immersed in the ethonolic solution of cis-diisothiocyanato-N,N”-bis
(2,2/ -bipyridyl–4, 4/- dicarboxylicacid)-Ruthenium(II)
dihydrate [RuL2(NCS)2.2H2O]
while both were hot (~600C). After 24 h absorption, the electrode was withdrawn from
the dye solution and then washed thoroughly with acetone to remove unabsorbed dyes
and loosely bound TiO2 particles from the dye-coated plate.
Characterizing Electrolyte
Complex impedance measurements were taken using a HP 4291 A RF impedance
analyzer in the frequency range of 1 MHz- 1GHz and in the temperature range of 20 °C to
60 °C. Disc shaped electrolyte films of 5 mm diameter and 0.1-0.4 mm thickness were
sandwiched between two polished stainless steel blocking electrodes and ac-impedance
measurements were performed in order to obtain dielectric data. A flow of nitrogen gas
was maintained over the sample in order to prevent contact with atmospheric moisture
DC polarization tests were carried out at room temperature by sandwiching disc shaped
electrolyte samples sandwiched in between two stainless steel blocking electrodes.
Thermal properties were studied using a Mettler Toledo DSC 30 differential scanning
calorimeter using a heating rate of 10 °C min-1 between -140 °C and 120 °C in the heating
and cooling cycles. The samples were first cooled from room temperature to -140 °C and
subsequently heated to120 °C (1st heating). Then they were again cooled to -120 °C and
subsequently heated to 120 °C in the 2nd cooling and 2nd heating runs.
Characterizing solar cells
4
Solid state PEC solar cell was fabricated by sandwiching polymer electrolyte films of
thickness from 0.1 mm to 0.4 mm between the dye sensitised TiO2 film and a previously
prepared
platinized
conducting
glass
plate
Glass/FTO/TiO2/Dye/Electrolyte/Pt/FTO/Glass.
Solar
with
cell
the
configuration
was
characterised
of,
by
measuring I-V curves using computer controlled Potentiostat/Galvanostat HA-301
instrument with 1000 W m-2 Xenon lamp.
3. Results and Discussion
Fig. 1 shows the temperature dependence of the conductivity of plasticized electrolyte
samples with different PAN: MgI2=100: x weight percentages. The highest conductivity is
given by the electrolyte sample contaminating 60 wt% MgI2 for all the temperatures
measured. This sample shows ionic conductivity of 1.9×10-3 S cm−1 at 30 °C and 2.3×103
S cm−1 at 50 °C. A conductivity enhancement of one order of magnitude has been
observed merely due to change of salt concentration. However it can be observed that the
conductivity variation shows Arrhenius behaviour within the measured temperature
range. Activation energy, Ea, was calculated fitting the data (not shown) to the Arrhenius
equation.
 E
σT = B exp − a
 k BT



(1)
where, B is an exponential factor, Ea the activation energy, kB the Boltzmann constant,
and T the absolute temperature. The Ea values are shown in the Table 1.
5
Fig. 1. The conductivity versus 1000/T for plasticized PAN based electrolytes with wt % of MgI2 (x values).
Table 1. Activation energy, Ea, values of plasticized PAN electrolyte for different MgI2 contents.
MgI2 (wt%)
10
20
40
60
80
100
Ea /eV
0.26
0.14
0.13
0.11
0.11
0.10
Fig. 2 shows the differential scanning calorimetric (DSC) thermograms of plasticized
polymer electrolyte samples with different PAN : MgI2=100: x weight percentages for 2nd
heating run. There were no significant differences between DSC thermograms obtained
during the 1st and the 2nd heating runs. A clear glass transition can be observed for all the
samples. The glass transition temperatures, Tg, obtained in this work are shown also in
6
Fig. 3. These values closely agree with the values reported in literature for other PAN
based other plasticized electrolyte systems [14]. However no significant first order phase
transition could be observed in the DSC in the temperature range -140 to 120 °C. This
amorphous sample shows good thermal stability in the temperatures range that solar cells
are operated. Tg values obtained are also included in the Fig. 2.
Fig. 2. DSC thermograms of plasticized PAN based polymer electrolytes with different MgI 2 contents (x
values).
Fig. 3 shows variation of Tg and ionic conductivity at temperature 20 °C and 30 °C with
different salt contents. As the glass transition temperature (Tg) is related to the segmental
flexibility of the host polymer and the disordered structure [15,16], the Tg result can be
related to a possible change in the segmental flexibility of polymeric chains of the
electrolyte due to incorporation of the salt. Minimum Tg , -103.03 °C was observed for
the electrolyte containing 60 wt% MgI2 salt with respect to weight of PAN and which in
general relates to the flexibly of material and finally to the mobility of charge carriers[17].
7
As expected this salt concentration is given the maximum conductivity. For low salt
concentrations Tg is higher and with increase of salt concentration it decreases may be
due to structural disorder caused by Mg++ and I- ionic species. When the salt
concentration is increased further, the Tg shows an increase. This Tg increase could
possibly be due to the “geometrical constrictions” imposed by non-ionized MgI2 grains,
making the long PAN chains “immobilized”. In addition the Mg++ ions can cross linked
long polymer chains reducing their segmental flexibility. The maximum conductivity is
obtained for the composition with the minimum Tg, exhibiting the contribution to the
ionic conductivity enhancement due to structural modifications caused by the salt. The
conductivity increase shown in the Fig. 3 may comes from increasing amount of mobile
ions as well as segmental flexibility facilitated by increasing amount of ionic species as
indicated by decrees of Tg. Then the conductivity drop given for higher salt concentration
seems to be due to “blocking effect” of the non-ionized MgI2 grains and reduction of the
segmental flexibility ensured by increase of Tg.
8
Fig. 3 Relationship between Tg and conductivity as a function of different salt contents for Plasticized PAN
based electrolyte.
The results of dc polarisation measurements on electrolyte sample with 60% MgI2 taken
using stainless steel blocking electrodes and under +2.0 V dc bias oltages at 30 °C are
shown in Fig. 4. The predominantly ionic nature of the electrolyte is seen from the DC
polarisation data. The ionic and electronic transference numbers estimated from these
data are ion, 0.957 and e, 0.043, respectiely [18,19].
Fig. 4.
The photocurrent vs. photovoltage curves for the cell fabricated from PAN electrolyte
with composition 60 wt% MgI2 at 30 °C shown in Fig. 4. The short-circuit photocurrent
(ISC) and open-circuit voltage (VOC) for a solar light intensity of 1000W m -2 for PAN
-
electrolyte are found to be 2.04 mA cm 2 and 0.69 V, respectively. The fill factor
9
calculated using equation (1) for fabricated quasi-solid state dye-sensitized solar cell is
59.3%.
ff =
J opt Vopt
J SC VOC
(1)
where, Jopt= Current density at maximum power output and
Vopt= Voltage at maximum power output
The power conversion efficiency, η, of the PEC cell calculated using equation (2) is
0.84%.
η =
J SC VOC ff
Total incident power density
(2)
10
Fig. 5. Photocurrent versus voltage curve for the solar cell with plasticized PAN electrolyte containing 60
wt% MgI2 with respect to PAN under irradiation of a 1000 W m-2 Xenon lamp. The dashed line shows the
power output against voltage on loading. The calculated parameters are also shown.
The output power density of the solar cell against voltage under loading also shown in
Fig. 1 The photocurrent and voltage at maximum power point is 1.62 mA and 515.3 mV.
Maximum power output of the cell is 8.4 W m-2.
4. Conclusion
A gel polymer electrolyte, based on PAN has been prepared by incorporating the salt
MgI2 and plasticizers EC and PC. The optimum conductivity of the best electrolyte
membrane is 1.9×10-3 S cm−1 at 30 °C. The predominantly ionic nature of the electrolyte
is seen from the DC polarisation data. The glass transition temperature of the best
electrolyte is -103.03 °C. PEC solar cell fabricated using this electrolyte has shown an
open circuit voltage of 691.8 mV and short circuit current 2.04 mA cm -2 for an incident
light intensity of 1000 W m-2. The overall efficiency of the cell is 0.84 %. It should be
possible to further improve the efficiency by optimising the cathode fabrication and
electrolyte preparation. The research is in progress to investigate enhancing the properties
of photo-electrode and electrolyte to increase the energy conversion efficiency.
Acknowledgements
Research support from IRQUE project Faculty of Applied Sciences, Rajarata University
of Sri Lanka and IPPS, VR/SIDA Sweden are gratefully acknowledged.
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