Polyvinylidene fluoride based gel polymer electrolyte to be used in solar energy to electrical energy conversion

There is a great attention towards fulfilling energy requirement via renewable energy sources. Solar energy is one of the uprising renewable energy sources for the planet earth. Dye sensitized solar cells (DSSCs) have been identified as a promising technology to harvest solar energy and to convert that to electricity. In this study, it was attempted to optimize the composition of a gel polymer electrolyte consisting with polyvinylidene fluoride (PVdF), ethylene carbonate (EC), propylene carbonate (PC), tetrahexylammonium iodide (Hex4NI) and iodide (I2) to be utilized for DSSC application. The composition was fine tuned to get the highest conductivity by varying the salt concentration. The composition, 100 PVdF / 400 EC / 400 PC / 150 Hex4NI / 7.3 I2 exhibited the highest conductivity of 3.43 x 10 -3 Scm at room temperature. Sample was purely an anionic conductor. DSSCs were fabricated in the configuration, FTO / TiO2 / Dye / GPE / Pt / FTO. The open circuit voltage and short circuit current density were 799 mV and 1.06 mA respectively. The calculated fill factor was 0.62 and the efficiency was 2.08% .


INTRODUCTION
Due to the warning of doubling the energy requirement of the globe within next few years and dreadful climatic consequences of the green house effect by fossil fuel combustion, seeking alternative renewable energy sources has become an urgent requirement. Upon the first report made by M. Gratzel in 1991 about dye sensitized solar cells (DSSCs), an intense interest aroused to develop them all over the world owing to some of their fascinating properties like low cost, simple preparation procedure and benign effect on the environment. 1 DSSCs are fallen into the third generation of photovoltaic technology. Conventional solar cells are based on crystalline silicon and depend on P-N junction to separate photo generated charge carriers. In DSSCs, light absorption is separated from the charge carrier transport. 2 Even the solar power conversion efficiencies of DSSCs are lower than that of silicon based solar cells, there is a great potential to improve their performance. Since the work of Gratzel, many attempts have been put forward to develop DSSCs but most were consisting with a liquid electrolyte. The presence of a liquid electrolyte has given rise to numerous drawbacks such as leakage, volatilization, corrosion, photo degradation and low flexibility of device fabrication with respect to shape. 3 These have led to replace liquid electrolytes by solid electrolytes. Mainly, due to the lower conductivity of solid electrolytes, attention has been diverted towards polymer electrolytes which are known to be lying in between solid electrolytes and liquid electrolytes.
As polymer electrolytes are also suffering from poor ambient temperature were used as received from ALDRICH. Required amounts of the materials were weighed and magnetic stirring was done for about 1 hour until all are mixed thoroughly. The resulting homogeneous solution was heated at 80  C for 1 hour. Thereby, it was possible to get a viscous solution which was pressed in between two well cleaned glass plates. Upon cooling, a thin bubble free film could be obtained.

Optimizing the composition of GPE
Samples were prepared varying the salt concentration. Weight of I2 was kept as the molar ratio of the salt : I2 equals 10 : 1.

AC conductivity measurements
A sample of circular shape was cut from the electrolyte film and it was loaded inside a spring loaded sample holder in between two stainless steel (SS) electrodes. Impedance measurements were taken from room temperature to 60  C in the frequency range 37 kHz -0.01 Hz using a METROHM 101 frequency response analyser.

DC polarization test
A circular shape sample was loaded as explained in section 2.3 to perform the DC polarization test under blocking electrodes. Current variation with time was monitored applying a DC bias voltage of 1 V. A similar setup was used with iodide electrodes for the DC polarization test under non-blocking electrodes.

Preparation of photo anode
First, fluorine doped tin oxide (FTO) glass strips were cleaned well. Titanium dioxide (TiO2) paste was prepared by grinding TiO2 (Degussa P-25) for few minutes alone. Few drops of acetic acid and ethanol were mixed in during grinding process. Then, the resulting slurry was spread on an active area of 0.25 cm 2 on FTO glass strips using doctor blade method. The electrodes were heated at 450 C for 45 minutes. Those nano porous electrodes were allowed to cool down to room temperature. Thereafter, they were dipped in ethanolic Ruthenium dye solution for dye absorption overnight.

Fabrication and characterization of the DSSC
The GPE that had shown the highest conductivity was used to fabricate the DSSC. It

RESULTS AND DISCUSSION
Impedance data analysis was done with the NOVA programme using corresponding equivalent circuits. The conductivity was calculated using the equation, where Rb is the bulk electrolyte resistance, t and A are thickness and area of the electrolyte film respectively.
Room temperature (28 C) conductivity (σRT) variation with salt concentration is shown in Figure 1. It is seen that with increasing salt concentration, conductivity increases.
After the salt concentration of 150 (by weight), conductivity decreases. This hints that there should be some competitive factors to govern the variation of conductivity with salt concentration. It is a well-accepted fact that conductivity depends on the charge carrier concentration as well as their mobility. When increasing the salt concentration, it is natural that charge carrier concentration increases giving rise to higher conductivities. Further increment of salt concentration may increase the viscosity of the medium disturbing the charge mobility. In addition, more and more charge aggregates can form at high salt concentrations which do not assist for conductivity. 9 These factors may reduce conductivity.    Figure 3.
The ionic transference number (ti) was calculated using the following equation. ti = (It-Is) / It (2) Here, It is the total current, Is is the saturated current.
The initial current (It) which is due to ions and electrons has dropped quickly and has reached a stable value (Is) due to the electrons. The abrupt drop of current is due to the blocking nature of SS electrodes towards ions. Soon after polarization of ions, the current due to electrons is existing. The value of ti is 0.9 which clearly indicates that the electrolyte sample is predominantly an ionic conductor. 12 Figure 3: DC polarization test results obtained with blocking electrodes Figure 4 shows the variation of current with time when using the non-blocking electrodes.
The anion transference number (t-) was calculated using the following equation 13 .
It is the total current due to cations and anions and Is is the saturated current due to anions.
In this test, initial current has a very small drop followed by a stable state. The initial current is due to the cations and anions in the system whereas the saturated current is due to anions as the electrodes are blocking cations. The small drop of initial current hints the fact that cations do not provide a considerable contribution. Therefore, it can be inferred that the system under study is a good anion or iodide conductor. This further confirms the fact that when the cation of the salt is large, anions show a higher mobility. This is one of the most essential requirements for the electrolyte to be used for DSSCs.
In iodide/triiodide based DSSCs, there is a possibility for triiodide to get recombined with electrons ejected to TiO2. This is negatively affecting the performance of DSSCs by lowering Fermi level of TiO2 and in turn Voc. When the cation of the salt is large in size, triiodide diffusion is low and hence, there is no probability for the recombination. So, Fermi level of TiO2 increases and as a result, Voc is also increasing. 14  where Jmax. and Vmax are respective current density and voltage at the maximum power point, Jsc and Voc are short circuit current density and open circuit voltage, Pin is the incident power. 15,16 The calculated value of the FF was 0.62 whereas the efficiency was 2.08. The system we have reported with PVdF, EC, PC and the salt 1methyl 3propyl Immidazolium Iodide has resulted a FF of 0.58 which is slightly lower even the conductivity value is higher than the present system 10 . The performance characteristics are somewhat attractive and need further studies to improve them. PVdF has been largely used for batteries. 17,18 And also, the use of PVdF with Hex4NI has not been considered for DSSCs as per the literature survey.
PAN and PEO have been used for fabrication of DSSCs by Bandara et al. with this salt. 8 In terms of conductivity as well as cell performance, the present study shows better values.
Hence, this study may boost the interest on the selected combination of PVdF, EC, PC, and Hex4NI to be employed for DSSCs.

CONCLUSION
By using PVdF, EC, PC, Hex4NI and I2, thin and mechanically stable electrolyte films can be prepared. By varying the salt concentration, the optimum composition that results the highest conductivity of 3.43 x 10 -3 Scm -1 was determined as 100 PVdF / 400 EC / 400 PC / 150 Hex4NI / 7.3 I2. Even upon increasing temperature, this salt concentration yields the highest conductivity irrespective of the fact that conductivity increases with increasing temperature. The resulting sample is a good ionic conductor having more contribution from anions. The electrolyte under investigation is an ideal candidate for DSSCs.