Phthaloylchitosan-Based Gel Polymer Electrolytes for Efficient Dye-Sensitized Solar Cells

Phthaloylchitosan-based gel polymer electrolytes were prepared with tetrapropylammonium iodide, Pr 4 NI, as the salt and optimized for conductivity. The electrolyte with the composition of 15.7 wt.% phthaloylchitosan, 31.7 wt.% ethylene carbonate (EC), 3.17 wt.% propylene carbonate (PC), 19.0 wt.% of Pr 4 NI, and 1.9 wt.% iodine exhibits the highest room temperature ionic conductivity of 5.27 × 10 S cm. The dye-sensitized solar cell (DSSC) fabricated with this electrolyte exhibits an efficiency of 3.5% with SC of 7.38mA cm , OC of 0.72V, and fill factor of 0.66. When various amounts of lithium iodide (LiI) were added to the optimized gel electrolyte, the overall conductivity is observed to decrease. However, the efficiency of the DSSC increases to a maximum value of 3.71% when salt ratio of Pr 4 NI : LiI is 2 : 1. This cell has JSC, OC and fill factor of 7.25mA cm , 0.77V and 0.67, respectively.


Introduction
A solar cell is a device that directly converts light energy into electrical energy through the photovoltaic process.The solar cells currently produced are generally divided into several types according to their material composition and operating mechanism, such as photoelectrochemical solar cells and thin film junction devices.Photoelectrochemical solar cells have become attractive devices since Gratzel and O' Regan first reported the prototype dye-sensitized solar cell (DSSC) using wide band gap TiO 2 semiconductor in 1991 [1].The DSSC has gained a great deal of attention due to its relatively high energy conversion efficiency and low production cost [2].DSSCs are also semitransparent and can be produced in various colours, making it possible for them to be applied in building integrated photovoltaic (BIPV) applications [3].A typical DSSC consists of a nanocrystalline semiconductor coated photoactive electrode, an iodide based liquid electrolyte, and a counter electrode (normally platinum or carbon-based coating on a conducting glass substrate).When the DSSC is irradiated with light, photoexcited electrons of the dye are transferred to the conduction band of TiO 2 , and the oxidized dye cations are regenerated by the I − in the electrolyte.The I 3 − formed in this process is reduced at the counter electrode, thus completing the cycle and generating the photocurrent [4].I − /I 3 − redox mediator shuttles between the two electrodes.
In the determination of high efficiency and long time stability of the DSSCs, electrolytes play an important role [5].A gel polymer electrolyte has been used in this work in order to replace liquid electrolytes that may cause some practical limitations of physical and chemical stability at higher temperatures [5,6].The conductivity of gel type electrolytes is usually high compared to that of solid polymer electrolytes and it can be improved to be almost comparable to that of the conventional electrolyte solutions in case of high solution fraction [7].Since the solvent is entrapped in the polymer network structure in gel electrolytes, leakage problems can be avoided.The solvents entrapped in the polymer matrix not only improve the conductivity of electrolytes but also improve the contacts between interfaces of the electrolyte and the dye covered nanocrystalline TiO 2 surface and electrolyte-counter electrode leading to improved efficiency of DSSCs [5,7].
Many host polymers have been used to prepare electrolytes for use in DSSCs [8][9][10][11].In this work, chitosan has been chosen as the host polymer for the electrolyte since this polysaccharide is nontoxic and biocompatible.Chitosan is also being explored intensively for applications in pharmaceutical, cosmetics, biomedical, biotechnological, agricultural, food, and nonfood industries [12][13][14][15].Chitosan is the deacetylated form of the chitin polysaccharide and contains the -(1-4) linked 2-amino-2-deoxy-D-glucopyranose repeating structure [16,17].Chitosan is generally insoluble under normal conditions due to the presence of -1, 4 glycosidic linkages that gives chitosan its structural rigidity and crystalline structure besides promoting the formation of intramolecular hydrogen bonds.This limits its usage and further investigation for the biological activities and development has been restricted [18].Since chitosan and its derivatives are favourable for many applications, chitosan should be effectively modified to fully explore its ability.The chemical modification of chitosan can provide functional groups as primary amine as well as secondary hydroxyl groups in its monomers [12].
In order to improve the solubility of chitosan in general organic solvents, N-phthaloylation is found to be one of the convenient methods.Kurita et al. have applied Nphthaloylchitosan as an organosoluble key intermediate for a series of controlled modification reactions [19][20][21].Chemical introduction of some functional groups of hydrophobic nature will result in disruption of the formation of specific hydrogen bond interactions between its amino and hydroxyl groups with the solvents [22].N-Phthaloylchitosan exhibits high affinity for organic solvents, namely, dimethylsulfoxide (DMSO), dimethylacetamide (DMAc), dimethylformamide (DMF), and pyridine [23].In this work, we aim at developing a best performing DSSC using a N-phthaloylchitosan-based gel polymer electrolyte.Therefore, gel polymer electrolytes were prepared with various amounts of tetrapropylammonium iodide, Pr 4 NI, as the salt and investigated in order to find the best composition for high conductivity.Pr 4 NI has been chosen as the primary iodide salt for the electrolyte as it has been shown to produce good efficiencies in DSSCs by enhancing the iodide conductivity in the electrolyte [24,25].The performances of the electrolytes were tested in DSSCs based on mesoporous TiO 2 sensitized with N719 dye as the sensitizer.The performance of the best DSSC was improved further with the addition of a second iodide salt, LiI.

Phthaloylation of Chitosan.
The synthesis procedure used in this work is similar to that reported by Nishimura et al. [22].Chitosan (Aldrich) and phthalic anhydride (Aldrich) were refluxed between 100 ∘ C and 120 ∘ C under nitrogen atmosphere for 6 h in dimethylformamide (DMF) purchased from R & M Chemicals.The temperature was then reduced to 60 ∘ C, and the mixture was left overnight.The product of reaction between chitosan and phthalic anhydride dissolved in DMF is a yellowish solution.This solution was poured into ice water to precipitate the product, N-phthaloylchitosan.The precipitate was collected and washed with distilled ethanol (from J. T. Baker) in a soxhlet extractor for 8 h.The final product was dried in vacuum at 60 ∘ C for about 6 h.

Preparation of Gel Polymer Electrolyte (GPE).
Fixed amounts of ethylene carbonate (0.2 g) and propylene carbonate (0.2 g) from Aldrich were stirred well in a closed glass container with the relevant weights of Pr 4 NI (see Table 1).0.1 g of phthaloylchitosan was then added to each salt solution.The mixture was heated to 150 ∘ C and stirred until the mixture becomes a homogeneous gel.Then heater was stopped and the solution was allowed to cool down to room temperature.Iodine (10 wt.% of salt) was then added to the gel polymer electrolyte and stirred until the solution becomes homogeneous.The impedance of the resulting gel polymer electrolyte was then measured.
To the most conducting gel polymer electrolyte (Sample 6), lithium iodide, LiI, was added in such a way that the total weight of the iodide salts remains the same (0.12 g) in all the electrolytes.The salt compositions used in the electrolytes are shown in Table 2.

Device Assembly. Fluorine tin oxide (FTO) glasses (from
Solaronix SA), which were used as the substrates, were rinsed with distilled water and ethanol before use.For preparing TiO 2 layer on the substrate, TiO 2 colloidal suspension was prepared by grinding 0.5 g powder of TiO 2 with 2 mL of HNO 3 , 0.1 g of carbowax, and few drops of surfactant octylphenol ethylene oxide condensate (Triton X-100) in an agate mortar.The TiO 2 colloidal suspension was spread on the clean (FTO) glass by applying the doctor blade technique.The TiO 2 electrode was sintered in a furnace at 450 ∘ C for 45 minutes.The TiO 2 electrode was cooled down to about 60 ∘ C and then dipped in a warm ethanol solution of Ruthenium dye (N719) for 24 hours for dye adsorption.The photovoltaic performance of DSSCs was investigated using a LOT-Oriel solar simulator (1.5 AM, 1000 W m −2 ) and a computer controlled eDAQ Potentiostat.From the - (current density-cell potential) characteristic curves, the short-circuit current ( SC ) and open circuit voltage ( OC ) were obtained and the fill factor (FF) and energy conversion efficiency () were calculated from the following equations: where  max is the current density at maximum power output,  max is the voltage at maximum power output, and  in is the power of the incident light.This behavior of the conductivity can be attributed to the change in the amount of mobile ions in the gel electrolytes [26].The increase in number of mobile ions due to the increased salt dissociation with the increasing amount of salt in the electrolyte can result in the conductivity increase.The Pr 4 NI salt molecules are expected to dissociate into Pr 4 N + cations and I − anions in the GPE.As the Pr 4 N + cations are bulky they are expected to have sluggish movement or get trapped in the polymer matrix and therefore ionic conductivity mainly comes from the I − ions.Initially as the salt concentration increases, since dissociated ions are less, both I − ions and Pr 4 N + ions can be expected to be mobile and contribute to ionic conduction and the conductivity rises sharply.On further increase of salt content, the number of both Pr 4 N + ions and I − ions increases but the mobility of Pr 4 N + ions is hindered due to entanglement in the polymer matrix and I − ions may be main contributor to the conductivity.Therefore, the conductivity increases only slightly with further increase in salt content up to about 16.4 wt.% of salt.Beyond this concentration of the salt the conductivity again rises sharply due to the increase in iodide ions coming from increased salt dissociation and increased added iodine amount, which is 10 wt.% of iodide salt, giving the maximum conductivity at 19 wt.% of salt content.At 21.4 wt.% and 23.7 wt.% of Pr 4 NI, ion association may have taken place in the system as the conductivity begins to decrease.In addition, conductivity can drop due to blocking effects imposed by the undissociated salt or due to higher ion aggregates present in the electrolyte at high salt concentrations.

Results and Discussion
The conductivity-temperature relationship of phthaloylchitosan-EC-PC electrolytes with different amounts of Pr 4 NI is exhibited in a plot of log  versus 1000/ shown in Figure 2. It is observed that the conductivity of each electrolyte increases as the temperature is increased from 25 ∘ C to 60 ∘ C. The variations of ionic conductivity with temperature for the polymer electrolytes obey Arrhenius law, suggesting that the conductivity is thermally activated.The Arrhenius model employed in describing the temperature dependence of conductivity is as follows: where   is the preexponential factor,   is the activation energy of ionic conduction,  is the Boltzmann constant, and  is temperature in Kelvin.The electrolytes with various amounts of Pr 4 NI were employed in DSSCs sensitized with N719 dye and the photocurrent density versus cell potential (-) characteristics were obtained under light irradiation of 100 mW cm −2 at room temperature.The - curves for the DSSCs are shown in Figure 3. Very poor solar cell performance is shown by the DSSC with the electrolyte containing 3.8 wt.% of Pr 4 NI.This can be due to the poor conductivity and the low density of redox mediators in the electrolyte.However with increasing amount of iodide salt together with increasing iodine content in the electrolyte, better solar cell performance is observed as can be seen from the - characteristic curves.The performance parameters for the DSSCs obtained from the - curves shown in Figure 3 are given in Table 3.The cell with electrolyte having 19.0 wt.% of Pr 4 NI shows both the highest values of  OC = 0.72 V and  SC = 7.38 mA cm −2 .Electrolyte with the lowest salt content of 3.8 wt.% shows the lowest values of  OC and  SC of about 0.19 V and 1.59 mA cm −2 , respectively.
In addition to  OC and  SC , the performance of the DSSCs is reflected by the parameters of fill factor (FF) and energy conversion efficiency ().The calculated values of FFand  are summarized in Table 3.The DSSC fabricated using the electrolyte containing 19.0 wt.% of Pr 4 NI with  of 3.50 exhibits fill factor of 0.66.The highest fill factor was obtained for the DSSC that had the electrolyte containing 19.0 wt.% of Pr 4 NI.This can be attributed to the combined effect of series resistance and shunt resistances as reported in [26].In a DSSC, low shunt resistance provides a substitute path for light generated current and hence lowers the power output which in turn decreases the fill factor.As series resistance in the DSSC is usually lower when compared to conventional solar cells, improved fill factor is expected.The relationships between Pr 4 NI content in the electrolyte with efficiency and short-circuit current density are shown in Figure 4. Efficiency of the cells is observed to increase with the Pr 4 NI content until 19.0 wt.% of Pr 4 NI and further increase of salt decreases the efficiency.Thus the cell with 19.0 wt.% of Pr 4 NI shows the highest efficiency value of 3.5%, the highest value obtained with the single iodide salt polymer electrolyte system used in this work.The efficiency variation shown in Figure 4 correlates with the conductivity variation shown in Figure 1. Figure 4 also shows that the short-circuit current density of DSSCs increases with increasing concentration of the salt up to 19.0 wt.% of Pr 4 NI.It can be seen that the variation of efficiency of the cells is parallel with variation of short-circuit current density as well as the conductivity of the electrolyte with the salt content.The short-circuit current of the cell is governed by the ionic conductivity dominated by the transport of I − /I 3 − redox couple as exhibited by a comparable PAN based electrolyte system using the same salt and plasticizers [27].As explained earlier, the iodide ions are the main contributors to the ionic conductivity in the GPE used.Since iodides (I − /I 3 − ) play a major role in determining the current in DSSCs, the  SC should follow the variation of free iodide ions content in the electrolyte and therefore must increase sharply as the salt content increases as more and more iodide ions are expected to be produced due to salt dissociation.According to the results in Figure 1, the number of iodide ions increases up to 19 wt.% of salt and the number decreases beyond this salt content due to possible ion association taking place.Hence the  SC is expected to follow this trend.But in Figure 4, as the salt content increases,  SC increases sharply at the beginning up to 7.4 wt.% of salt and beyond 13.6 wt.% up to 19 wt.% of salt content.The increase becomes less steep for the salt contents between 7.4 and 13.6 wt.%.It should be noted that the current in the DSSC is not only controlled by the iodide conduction in the electrolyte but also controlled by the electron injection rate from the dye molecules to TiO 2 and the recombination rate of injected electrons with the iodide ions in the electrolyte.The injection rate is also affected by the cations as they can stick to the TiO 2 surface and shift the Fermi level.The combined effect of all these processes results in the observed variation of  SC with salt content.Further experiments and analysis are needed to explain the exact correlation between ionic conductivity and the  SC .
To our best knowledge, the  SC and efficiency values obtained in this study for DSSC containing 19.0 wt.% of Pr 4 NI are the highest values reported so far for a quasisolid state DSSC having chitosan host polymer based electrolyte system with a single iodide salt.
The dependence of  OC and FF on the weight percentage of Pr 4 NI salt in the electrolyte is shown in Figure 5.The  OC and FF show similar variations but different to that shown by  SC in Figure 4.These results show that the efficiency variation of the DSSCs is determined mainly by the changes in  SC .It has been reported that the addition of small cations such as Li + , K + , and Mg 2+ to the gel polymer electrolytes so as to have two mixed cations in the electrolyte improves the solar cell performance [28][29][30].In particular these small cations adsorbed onto the TiO 2 surface alter the flat band potentials so as to improve the charge injection rates from excited dye molecules to the TiO 2 enhancing the photocurrent [31].Hence in order to improve the photocurrent density of the DSSC various amounts of lithium iodide (LiI) have been added to the most conducting polymer electrolyte, that is, phthaloylchitosan with 19.0 wt.% of Pr 4 NI. Figure 6 shows the room temperature ionic conductivity variation of the gel polymer electrolyte for different mass fractions of the LiI and Pr 4 NI.The conductivities of gel polymer electrolytes with various compositions of LiI : Pr 4 NI at different temperatures are shown in Figure 7.With the addition of LiI conductivity drops are observed at all temperatures in the measured temperature range.This conductivity drop can be attributed to the reduction of polymer flexibility due to inter-and intramolecular cross links by Li + due to its high charge density.The reduction of polymer flexibility can  increase the local viscosity of electrolyte medium hindering mobility of charge carriers.As can be seen from Figure 7, the temperature dependence of conductivity for all electrolytes approximately obeys the Arrhenius relationship as observed in other polymer systems [32].
As expected the incorporation of small amount (less than 0.04 g) of LiI into the electrolyte has improved the solar cell performance (see Figure 8) even though these electrolytes have shown a slight decrease in conductivity as shown in Figure 6.The DSSC parameters for the devices having the binary salt system are tabulated in Table 4.The efficiency reaches a maximum value of 3.71% for a combination of 0.08 g Pr 4 NI and 0.04 g LiI in the electrolyte.This increase in efficiency can be attributed to the mixed cation effect as observed with many binary salt electrolytes [29,30].Mixed cation effect is observed when binary iodide salts having smaller and larger cations are present in the gel electrolyte.The smaller cations get adsorbed to the TiO 2 and reduce the Fermi level in such a way to improve the electron injection and, at the same time, the number of iodide ions in the electrolyte increases due to the addition of LiI salt and both these effects increase the current in the DSSC.However, higher amounts of added LiI (more than 0.04 g) have reduced the solar cell efficiency due to the ionic conductivity drop as explained above.Variations of efficiency and photocurrent density with different ratio of Pr 4 NI : LiI are shown in Figure 9.The efficiency variation closely follows that of the current density variation.

Conclusions
In this work, a gel polymer electrolyte has been successfully prepared using phthaloylchitosan as the polymer host and it has been used to produce high performance in a dyesensitized nanocrystalline solar cell.The most conducting polymer electrolyte containing 19.0 wt.% of Pr 4 NI exhibited the highest efficiency of 3.5% with  SC of 7.38 mA cm −2 ,  OC of 0.72 V, and FF of 0.66.The efficiency of the DSSC was further improved to 3.7% when part of the Pr 4 NI salt in the electrolyte is replaced with LiI salt (Pr 4 NI : LiI = 2 : 1) with the value of  SC ,  OC and fill factor of 7.25 mA cm −2 , 0.77 V and 0.67, respectively.The presence of the LiI in addition to the Pr 4 NI improves the charge injection rates and increases the iodide contribution to the total conductivity and both factors contribute to the increase in efficiency of the DSSC.

Figure 1 :
Figure 1: Effect of Pr 4 NI concentration on the conductivity at 25 ∘ C.

Figure 1
Figure 1 shows the variation of conductivity at room temperature for the phthaloylchitosan-based gel polymer electrolytes with various Pr 4 NI salt compositions.The conductivity of the electrolyte with the lowest Pr 4 NI concentration, 3.8 wt.%, is 1.81 × 10 −3 S cm −1 at 25 ∘ C. With increasing amount of Pr 4 NI salt up to 19.0 wt.% in gel polymer host, the ionic conductivity increases until a maximum conductivity of 5.27 × 10 −3 S cm −1 at 25 ∘ C. On increasing the Pr 4 NI concentration further, the conductivity is observed to decrease.This behavior of the conductivity can be attributed to the change in the amount of mobile ions in the gel electrolytes[26].The increase in number of mobile ions due to the increased salt dissociation with the increasing amount of salt in the electrolyte can result in the conductivity increase.The Pr 4 NI salt molecules are expected to dissociate into Pr 4 N + cations and I − anions in the GPE.As the Pr 4 N + cations are bulky they are expected to have sluggish movement or get trapped in the polymer matrix and therefore ionic

Figure 2 :
Figure 2: Temperature dependence of conductivity for phthaloylchitosan doped with different wt.% of Pr 4 NI salt.

− 2 )Figure 4 :
Figure 4: Effects of weight percentage of Pr 4 NI salt in the electrolyte on efficiency and short-circuit photocurrent density of DSSCs.

FFFigure 5 :
Figure 5: Effects of weight percentage of Pr 4 NI salt in the electrolyte on open circuit voltage and fill factor of DSSCs.

)Figure 8 :
Figure 8: Photocurrent-voltage curves of DSSCs with phthaloylchitosan with based electrolyte different ratio of LiI : Pr 4 NI binary salt.

Figure 9 :
Figure 9: Dependence of  SC and  of DSSCs with different gel polymer electrolytes.

Table 1 :
Pr 4 NI content of different electrolyte samples.

Table 2 :
Composition of the salts in binary salt electrolytes.

Table 3 :
Performance parameters of DSSCs with phthaloylchitosan-based electrolytes containing various salt contents.
) Figure 3: Photocurrent density variation with cell potential for dye-sensitized solar cells having phthaloylchitosan-based electrolyte with various Pr 4 NI/I 2 contents.

Table 4 :
The performance parameters of DSSCs with different gel polymer electrolytes.