Electrospun Poly (Ionic Liquid) Membrane Based Solar Cell For Sustainable Clean Energy

Solar energy

The yearly raise in global energy consumption will effect in increase of demands towards natural resources such as coal, petroleum and natural gas. These natural resources will take thousands of years to form and it cannot be replaced as fast as they are being consumed. These traditional fossil based fuel usages caused environmental problems such as carbon dioxide emission and irreversible climate changes (Luis et al., 2019). Among the energy sources existing, solar energy can fulfill the world’s ever growing energy needs (Muthuraaman et al., 2017). Photovoltaic cells directly convert solar energy to electricity.

Dye-sensitized solar cell (DSSC)

Among the photovoltaic cells, dye-sensitized solar cell (DSSC) has received widespread attention owing to their advantages such as simple fabrication, ecofriendly nature and remarkable energy conversion, which mimics some of the key processes in the natural photosynthesis during the operation (Ishanie et al., 2014). In general, DSSCs are composed of three parts: TiO2 photo anode, a counter electrode and an electrolyte.

Electrolyte

The electrolyte can be considered as central among the DSSC components, since it takes the main function as a medium for charge transport (Buraidah et al., 2017).

Basic requirements of the electrolytes used for DSSCs are:

• The electrolyte must transport the charge carriers between electrodes with high redox potential.

• The electrolyte must guarantee high ionic conductivity and make good contact with the electrodes.

• The electrolyte must contain very good long term stability.

Highly efficient DSSCs usually utilize liquid electrolyte, but liquid electrolytes are prone to leak and evaporate making them unstable for outdoor application. Thus, quasi-solid state (QSS) electrolytes have been favoured owing to their considerably excellent conductivity, strong interfacial contact and high penetrative ability into the semiconductor (Yi-Feng et al., 2016). Electrospun polymer nanofibrous membranes in the form of QSS electrolyte caught a lot of attention due to their high porosity and their capacity to entrap large amount of liquid electrolyte and possessed an extremely high specific surface area (Mini & Sheeja, 2019).

Poly (ionic liquid) (PIL)

Recently, imidazolium based poly (ionic liquid) (PIL) electrolytes in DSSCs have received a lot of attention; due to their ability to show multiple functions upon tuning their molecular structure and also provide excellent long-term stability and power conversion efficiency (Xiaojian et al., 2012). The non-volatile imidazolium cations stop the unfavorable recombination reactions by blocking the possible sites for electron leakage on the TiO2 surface and thereby enhance the performance of DSSCs (Hao-Wei et al., 2018). It also increases the ionic conductivity of polymer matrix through charge transport network formation via - stacking of imidazole segment (Silvia et al., 2017). The oxygen atom present in the methyl methacrylate (MM) forms a co-ordination bond with the Li atom and improves the ionic conductivity of the electrolyte. As the PIL could not be electrospun due to their lower molecular weight, the synthesized PIL was blended with a polymer possessing high molecular weight to obtain nanofibrous membrane. Poly (ethylene oxide) (PEO) was selected as the blending polymer due to its high chemical stability and better ion transport due to the consolidation of their cation-complexation capability (Yanyan et al., 2015; Zebardastan et al., 2017). Blending with PIL can solve the problem of PEO crystallization that occurs below 60°C.

Materials

Poly (ethylene oxide ) (PEO) (average Mw = 200,000g/mol), iodine (I2), lithium iodide (LiI), 4-tert-butylpyridine (TBP), Chloroform, Vinyl imidazole, Sodium tetrafluroborate, Azobisisobutyronitrile (AIBN) and absolute ethanol were purchased from Sigma-Aldrich Co., Ltd. Methyl methacrylate and dichloromethane (DCM) were purchased from SRL Chemicals Pvt. Ltd., India. Nanocrystalline TiO2 (nc-TiO2) paste coated on FTO conducting glass (15Ω/sq), cis-Diisothiocyanato-bis (2,20-bypyridyl-4,40-dicarboxylato) ruthenium(II) bis (tetrabutylammonium) (N719 dye) and Pt counter electrode were purchased from Greatcell Solar's DSL, Australia.

Preparation of electrospun polymer nanofibers

PEO solution (A) with a concentration of 11 w/v % was prepared in chloroform at room temperature. Solution (B) was prepared by adding 0.55 g of PIL in solution (A). The prepared polymer solution was loaded into a 5 mL syringe attached to a stainless steel needle with a diameter of 0.3 mm. The flow rate of the solution was adjusted to 3 mL/h with the help of a syringe pump. An aluminum foil was placed as a collector at a distance of 15 cm from the needle and a high voltage of 30 kV was applied at the end of the needle. The electrospinning parameters were set after optimization.

Fabrication of DSSC devices

The TiO2 photo electrode was fabricated by immersing the annealed screen printed nc-TiO2 into 0.5mM of N719 dye in absolute ethanol for 24 h at room temperature under dark conditions followed by rinsing with anhydrous ethanol and air dried. A gasket frame (MSOO4610-10) with an optimal thickness of 60 m was inserted between the dye-adsorbed nc-TiO2 electrode and Pt counter electrode and the cell was pressed together using clips. The gasket thickness was sufficient to allow for the addition of a sufficient amount of electrolyte. Quasi-solid state DSSCs was assembled using electrospun nanofibrous membrane of definite dimension being sandwiched between the dye imbibed TiO2 working electrode and the Pt counter electrode (Marwa et al., 2016; Tan et al., 2019) and clipped tightly. A drop of the prepared electrolyte solution containing 0.05 M iodine, 0.1 M lithium iodide, 0.6 M of PMII, and 0.5 M of TBP in 1 mL of a mixture of valeronitrile (15%) and acetonitrile (85%) were then introduced into the clamped electrodes. The active area of the DSSC was found to be 0.49 cm2. For comparative studies, another DSSC was fabricated using liquid electrolyte without electrospun nanofibers.

Synthesis of poly (ionic liquid)

Poly (ionic liquid) (PIL), 3-decyl-vinyl imidazolium methyl methacrylate tetrafluoroborate was synthesized via a simple procedure, as shown in Figure 01 . The ionic liquid (3-decyl-vinyl imidazolium bromide) was prepared by taking 1-vinyl imidazole (5 mL), 1-bromodecane (5 mL) in a round bottomed flask and immersed in an oil bath for 2 h at 100oC. It was cooled for 15 min and again heated for 25 min in an oil bath to extract the ionic liquid by adding the solvent dichloromethane (Deyab et al., 2017). An aqueous solution of NaBF4- (2 g) was then added and the solution was stirred continuously until phase separation occurred. The compound obtained was purified and dried in vacuum. The poly (ionic liquid) was prepared by free radical polymerization process. The monomers 3-decyl-vinyl imidazolium bromide and methyl methacrylate was taken and AIBN were used as an initiator. These three solutions were mixed well and kept in an oil bath under N2 atmosphere and refluxed for 5 h. After 5 h, the polymer was precipitated out using water and evaporated in vacuum and the product poly (3-decyl-vinyl imidazolium methyl methacrylate tetrafluoroborate) was obtained as a pale yellow solid. The structure of the product was confirmed by 1H NMR studies as follows: 1H NMR (500 MHz) and solvent used was CDCl3; 0.83 (3H, –CH3, t), 1.03-1.92 (2H, -CH2, m), 7.40 (1H, –CH, d), 7.59 (1H, -CH, d), 8.01 (1H, -CH, d), 0.96 (3H, -CH3, t), 1.77(2H, -CH2, m), 2.45(1H, -CH, s),2.59(2H, -CH2, m),1.96 (3H, -CH3, s), 4.10(3H , -CH3, s),3.68 (3H, -CH3, s).

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Study of the morphology

Figure 02 (A1) exhibited uniform; beadless, smooth pure PEO nanofibers with mean diameter of 181.3 nm as shown in Figure 02 (A2). The highly porous interconnected nanofibrous membrane imbibed large amount of liquid electrolyte into the pores. Figure 02 (B2) shows the SEM micrograph of PIL/PEO membrane which showed an increased fiber diameter and rougher fiber surface. Increased fiber diameter of 191.12 nm could be attributed to the increased viscosity of the solution upon the addition of PIL and rougher fiber surface may be due to the change in surface tension of the precursor solution (Chang et al., 2018). The rougher fiber surface increased the surface area thereby enabling higher uptake of electrolyte. The EDX spectrum in Figure 02 (B1) shows the presence of oxygen, nitrogen and boron apart from carbon. These results confirm the presence of imidazolium IL and methyl methacrylate. The presence of boron indicates the presence of boron trifluorate on the nanofibers.

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FTIR study

Figure 03 displays the FTIR spectra of synthesized poly (3-decyl-vinyl imidazolium methyl methacrylate tetrafluoroborate) (PIL), PEO nanofibers, PIL/PEO nanofibrous membrane in the 500-4000cm-1 wave number range. In Figure 03 (A), the peak at 1250 cm-1 shows the C-N stretching. The peak at 1160 cm-1 indicates the presence of ester C-O-C stretching vibration. The band appearing at 1640 cm-1 displayed C=C stretching vibration.

The peaks at 2850 cm-1 and 1390 cm-1 indicate the presence of C-H stretching and C-H bending vibrations respectively. The FT-IR spectrum of PEO nanofibers in Figure 03 (B) showed an intense peak at 2892 cm-1 due to -CH2 asymmetric stretching and at 839 cm-1 due to -CH2 asymmetric bending vibrations respectively. The peaks at 1478 cm-1 and 1278 cm-1 represent the -CH2 scissoring and -CH2 symmetric twisting vibrations respectively. The band at 1097 cm-1 shows the presence of symmetric and asymmetric –C–O–C– stretching vibrations (Elashmawi & Gaabour, 2015). In the case of PIL/PEO nanofibers shown in Figure 03 (C), the peak nearer to 3000 cm-1 shows the asymmetric C–H stretching of -CH2 and at 1700 cm-1 shows the C=O stretching vibration. The peaks at 1250 cm-1 shows the C-N stretching and the peak at 2000 cm-1 shows the C=N stretching vibration. All the functional groups of PEO and PIL were present in the blended fibers which revealed the interaction between PEO and PIL.

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XRD analysis

Figure 04 shows the XRD pattern of PEO nanofibers and PIL/PEO blended nanofibers. The maximum intensity peaks were observed at 19.28^o and 23.6^o respectively for the PEO nanofibrous membrane. The sharp peaks indicate the crystalline nature of PEO, which originate from the ordering of PEO side chains due to strong intermolecular hydrogen bonding. In the case of PIL/PEO blend nanofibrous membrane, the crystalline peaks of PEO were not observed and an amorphous phase was dominant. The reason could be due to the disruption of the rigid bonding in PEO due to the addition of PIL which enabled the ion migration (Azli et al., 2015). The relative percentage of crystallinity (X_c%) was calculated by using following equation (1) where, A_c is the crystalline area and A_a is the amorphous area and summarized in Table 1 .

$x_c\%=[A_c/\left(A_c+A_a\right)]\times 100$ (1)

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Differential scanning calorimetric studies

Figure 05 shows the DSC thermograms of PEO nanofibers, PIL and PIL/PEO nanofibers in the temperature range of -70C to 200C. The melting peaks of PEO appeared at 64.55C. The addition of PIL into PEO broadened the melting peak of PEO and the melting enthalpy decreased from 142 J/g to 137.12 J/g. This suggest that there was a reduction in the crystallization of PEO and thereby an enhancement in amorphous phase occurred which facilitated an increase in ionic conductivity. DSC thermogram of PIL showed an endothermic broad peak at 150C due to the melting of PIL. In the case of PIL/PEO nanofibrous membranes, two endothermic peaks at 65C and 150C were observed corresponding to the melting of PEO and PIL respectively. Although the result showed the immiscibility of the polymer blend solution, the enhancement of ionic conductivity in the blended solution was found to be beneficial for the DSSC application.

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Electrolyte uptake (U), porosity (P) and Ionic Conductivity (σ)

The porosity (P) of the nanofibrous membrane was determined by immersing the electrospun membrane into n-butanol for 1 h. The weights of the nanofibrous membranes before (m_i) and after (m_a) immersion were measured. The densities of n-butanol and the sample were represented as ‘_b’ and‘_s’ respectively. Porosity was evaluated by using the equation (2) (Mini & Sheeja, 2019).

$\rho \left(\%\right)=\frac{\frac{m_a}{{\rho }_b}}{\frac{m_a}{{\rho }_b}+\frac{m_i}{{\rho }_s}}\times 100$ (2)

The equation (3) (Mini & Sheeja, 2019) was used to measure liquid electrolyte uptake in the electrospun membranes, where ‘m_o’ and ‘m’ are the weight of the nanofibrous membrane before and after imbibing the liquid electrolyte.

$U=\frac{m-m_0}{m_0}\times 100$ (3)

The ionic conductivity () of the membranes was obtained using equation (4) (Karuppanan et al., 2015).

Where, ‘ l’ is the thickness of the membrane, ‘A’ is the area of the membrane and ‘R’ is the bulk resistance of the electrolyte which was gained from the interception of an inclined straight line with the real axis in Nyquist plot.

$\sigma =\frac{l\left(cm\right)}{R\left(\Omega \right)\times A\left(c{m}^2\right)}$ (4)

Ionic conductivity of PIL/PEO nanofibrous membrane based electrolyte was observed to have a higher value than that of the unblended PEO nanofibers. The reason could be due to the amorphous nature of the blend that could imbibe more liquid electrolyte into the pores. The charge carrier mobility enhanced due to an increase in amorphous nature. All the prepared polymer membranes exhibited good porosity due to their three dimensional network architecture (as shown in Table 01 ) that was appropriate for taking high amount of electrolyte which caused for the improvement in ionic conductivity.

Photovoltaic Performances of DSSC

The electron transfer mechanism (Malaisamy et al., 2014) taking place in DSSCs containing PIL/PEO electrolyte are shown below:

N719 dye (D) absorbs photons and produces excited electron (D*)

D/TiO₂ +hD*/ TiO₂

The excited electron in the dye goes to the conduction band of TiO_2, to form the oxidized dye (D⁺), and the electron diffuses through TiO₂ coated anode and moves through the external load and reaches the Pt counter electrode.

D*/ TiO₂D⁺/ TiO₂ + e^- / TiO₂

The oxidized dye (D⁺) was regenerated by accepting electrons from reduced state I^- to produce the oxidized state I^3- of the electrolyte

D⁺/ TiO₂ + I^-(re)D/TiO₂ + I^3- (ox)

Oxidized form of I^3- migrates to cathode through polymer electrolyte, and gets reduced to I^- by accepting electron from the Pt counter electrode (Pt CE).

I^3- (ox) + e^-/Pt CE I^- (re)

Thus, DSSCs produce current from solar light without any chemical transformation. Iodine forms charge-transfer complex with the lone pair of electrons of nitrogen (N) atom in the imidazolium based PIL which prevents the recombination of triiodide present in the electrolyte and thereby inject electrons which increases the iodide concentration of the polymer electrolyte. Interconnected PIL/PEO electrospun nanofibrous membrane helps to entrap large amount of liquid electrolyte and thereby increase the stability of DSSCs.

Photovoltaic performance of the DSSCs were measured using LOT Technologies solar simulator equipped with a 150 W xenon under air mass 1.5 and 100 mW cm^-2 of light intensity.

Photovoltaic parameters, light-to-current conversion efficiency (η) and Fill Factor (FF) were calculated by using equation (5) and (6) as shown below (Khushboo et al., 2018).

$ɳ\left(\%\right)=\frac{V_{oc}{J}_{sc}FF}{p_{in}}\times 100$ (5)

$FF=\frac{V_{max}{J}_{max}}{V_{oc}{J}_{sc}}$ (6)

Where, ‘ J _sc ’ is the short-circuit current density, ‘ V _oc ’, the open circuit voltage and ‘ p _in ’ is the incident light power and J _max and V _max are the current density and voltage in the J-V curve at the point of maximum power on DSSC devices prepared using the electrospun nanofibers. The best results of V _oc , J _sc , FF, and η of the DSSC devices using the electrospun PIL/PEO nanofibrous membrane were found to be 0.65 V, 13.9 mA/cm², 0.59 % and 5.3 % under AM_1.5, respectively. J–V curves are shown in Figure 06 . All the data of photovoltaic parameters of DSSCs under standard conditions are given in Table 02 .

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The overall power conversion efficiency of electrospun PEO was found to be lower than that of the PIL/PEO blend nanofibers as shown in Figure 06 . This high efficiency value of PIL/PEO membrane was attributed to (1) the porosity of PIL/PEO blend that was higher than PEO (2) the fact that it can imbibe large amount of electrolyte (3) the good ionic conductivity (4) suppression of recombination reaction of electrons on the TiO2 working electrode by the imidazolium part in the PIL (5) methyl methacrylate group in the PIL that helped to reduce the crystalline nature of PEO.

Stability measurement

In Figure 07 A, the stability of the QSS-DSSC using liquid electrolyte and PIL/PEO membrane was measured. In Figure 07 B, the total efficiencies were normalized to the values measured on the first day. The efficiency of the liquid cell decreased to 37% of their initial value after 5 days. The evaporation and leakage of organic solvent in liquid electrolyte caused the low stability of DSSC contained liquid electrolyte (Roger et al., 2014; Venkateswararao et al., 2014). Even if the power conversion efficiency of PIL/PEO membrane was initially 5.34%, it could be preserved up to 5.1% over the same test period of study. From the normalized power conversion efficiency graph it was clear that the efficiency of PIL/PEO membrane was maintained at 94% of their initial value after 5 days study period. It should be noted that PIL/PEO membrane exhibited better stability than liquid electrolyte cell because the polymer network prevented the leakage of electrolyte effectively.

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