Improved electrochemical and photovoltaic performance of dye sensitized solar cells based on PEO/PVDF–HFP/silane modified TiO₂ electrolytes and MWCNT/Nafion^® counter electrode

Abstract

A polymer blend electrolyte membrane was prepared for dye sensitized solar cells (DSSCs) based on poly ethylene oxide (PEO) and poly (vinylidene fluoride-co-hexaflouropropylene) (PVDF–HFP) filled with surface modified titanium dioxide (M-TiO₂) nanofillers. The surface of TiO₂ was modified using aminopropyltrimethoxysilane. The electrochemical studies indicated that the addition of surface modified nanoparticles increased the ionic conductivity up to 7.21 × 10⁻⁴ S cm⁻¹ for 7 wt%, whereas the ionic conductivity was about 8.14 × 10⁻⁵ S cm⁻¹ with the addition of an unmodified counterpart as the filler into the PEO/PVDF–HFP blend system. In addition to this ionic mobility, charge carrier concentration and ion diffusion coefficient were also increased with the addition of surface modified TiO₂ nanoparticles. Wide angle X-ray diffraction (WAXD) results showed the reduction in the crystalline phase of PEO/PVDF–HFP blend electrolyte with the addition of M-TiO₂. The influence of TiO₂ surface functionality on the degree of crystallinity of the polymer matrix was analyzed using differential scanning calorimetry (DSC). Thermo mechanical behavior of the composite membranes was studied by dynamic mechanical analysis (DMA). The thermo gravimetric analysis (TGA) investigations of membranes indicated that the thermal degradation temperatures of hybrid nanocomposites were enhanced upon the addition of nanosized inorganic fillers. The morphological characterizations were carried out by atomic force microscopy (AFM). The solid state dye sensitized solar cell has been fabricated by using a silane modified TiO₂/PEO/PVDF–HFP polymer nanocomposites electrolyte, and multiwalled carbon nanotube (MWCNT)/Nafion^® as the counter electrode. The photovoltaic characteristics of constructed cells showed an enhancement of open circuit voltage (V_oc) from 0.62 to 0.71 V and the best efficiency achieved was about 2.84%. The enhancement of the DSSC was further confirmed by electrochemical impedance spectroscopy (EIS) studies for lowest Warburg resistance (R_diff).

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1. Introduction

Since 1991, the great discovery of dye sensitized solar cells (DSSCs) by Gratzel has been attracting much attention because of their simple structure and low cost.¹ The typical structure of a DSSC consists of two sandwiched conducting glass plates; one is called a photo anode and is coated with a wide band gap and mesoporous semiconducting layer of nanoparticles with a self-assembled monolayer of chemisorbed dye molecules filled with an electrolyte for dye regeneration, and another glass plate coated with Pt, carbon, etc., is used as a counter electrode.² Remarkable improvements have been achieved in the performance and the maximum efficiency of DSSCs; about 11% has been reported by using liquid electrolyte and ruthenium dyes.³ However, the major drawbacks of these solar cells are leakage of solvent, high temperature instability, dye desorption, and electrode corrosion which causes the sealing and performance degradation of DSSCs.⁴ To overcome these problems for commercial exploitation of DSSCs, many efforts have been focusing on long term stability and replacing the liquid electrolyte medium by using solid alternatives such as p-type inorganic materials, organic hole conductors, ionic liquids and gel/solid polymer electrolytes.^5–8 Among these alternatives, solid polymer electrolytes (SPEs) have received considerable attention. Because of their outstanding performance in solid-state batteries, electrochromic devices, fuel cells and photoelectrochemical cells, they have been extensively studied.^9–11 Among the polymers used in electrochemical device applications, PEO and PVDF–HFP are the most searched polymers due to their unique characteristics. The smallest ionic radius and high electronegativity of fluorine (F) in PVDF–HFP is expected to improve the ionic conductivity and reduce the recombination rate at the semiconducting photo anode–polymer electrolyte interface in DSSCs.¹² PVDF–HFP is also known to be photochemically stable even in the presence of photo catalysts such as TiO₂ and Pt, which makes it a suitable material in DSSCs for long term stability outdoor applications.¹³ It has a high dielectric constant of ε = 8.4 which can help achieve better dissociation of ionic salts.¹⁴ The crystalline phase vinylidene fluoride (VDF) units provide excellent chemical stability and mechanical strength and amorphous hexafluoropropylene (HFP) units increase its plasticity and enhance ionic conductivity. Even in the presence of HFP, the ionic conductivity of PVDF–HFP is about 10⁻⁸ to 10⁻¹⁰ S cm⁻¹. Utilization of PEO based electrolytes has improved the performance and stability, which originated from the existence of a greater number of channels between electrodes to maintain the liquid-like nature for fast ion transport within electrolyte. The CF₂ group in PVDF–HFP can give a better compatibility while blending with PEO, due to the strong interaction of C–O–C.¹⁵

The additional incorporation of nanoparticles into the polymer matrix can reduce the crystallinity of the polymer, reducing recombination rate at the electrode/electrolyte interface,¹⁶ giving a three dimensional, mechanically stable and porous network structure. The anions in the present structure (iodide/tri-iodide) can easily move into the porous structures. In addition, highly charged surfaces of the nanoparticles provides the conductive path for ion transport.¹⁷

A variety of nanoparticles such as SiO₂, ZnO, SnO₂, TiO₂, ZrO₂, etc. have been employed as fillers in polymer nanocomposite electrolytes for dye sensitized solar cell applications.^18–24 Among them SiO₂ and TiO₂ nanoparticles were investigated more as fillers for solid state DSSC electrolyte applications. The interfacial resistance of each interface (photo anode/electrolyte/counter electrode) in DSSCs plays the main role in the photo conversion efficiency. When TiO₂ is added into PEO/PVDF–HFP electrolytes, the compatibility between polymer electrolytes and the photo anode based on TiO₂ nanoparticles is increased.²⁵ Hence, photo-current and efficiency will be enhanced. Recently, comparative studies of TiO₂ and SiO₂ within the polymer electrolyte have been reported by N. Tiautit et al. It is reported that the charge transfer resistance and diffusion resistance of TiO₂ incorporated electrolyte is much less than SiO₂.²⁰

In polymer nanocomposite electrolytes the high surface energy of nanoparticles restricts the uniform dispersion of the fillers within matrix. This will restrict to achieve the desired ionic transport properties for DSSC applications. Surface modification of nanoparticles has been identified as a feasible method to reduce the surface energy of nanoparticles while improving its dispersion to achieve the required performance characteristics. Many results have been reported on surface modifications of nanoparticles employing organic molecules, including silanes, phosphonic acid and ethylene diamine mostly for dielectric applications. Among the modifiers, silane coupling agents have the structure of (XO)₃SiY, where XO is a hydrolysable alkoxy group which can be ethoxy (OC₂H₅) or methoxy (OCH₃) and Y is an organo functional group. The formation of silane functional groups is based on the condensation reactions between silanols and the hydroxyl group on the metal oxide surface.²⁶ Recently, our group has investigated the effect of APS silane modified TiO₂ on the electrical properties of PVDF–HFP composites. It was found that reduction in crystallinity and glass transition temperature as well as uniform distribution of TiO₂ was achieved,²⁷ because the amine (–NH₂) group of APS is expected to react with C–F groups of PVDF through hydrogen bonds. Hence, this can improve the compatibility between nanofillers and polymers. Even the strategy of surface modified TiO₂ nanoparticles to reinforce PEO/PVDF–HFP nanocomposites has seldom been studied for its effects on electrochemical properties. In addition, the surface modification of TiO₂ by aminopropyltrimethoxy silane (APS) is a widely accepted technique for poly urethane based coating applications, because this forms hydrated oxides and captures the formation of hydroxyl radicals. Hence it will help to minimize the photo catalytic activity of pure TiO₂ nanoparticles.²⁸

Furthermore, the counter electrode plays an important role in highly efficient dye sensitized solar cells. Generally, a thin layer of Pt-coated transparent conducting oxide substrate is widely used as the counter electrode. But it is well known that Pt is expensive and issues of corrosion in tri-iodide solution restrict the practical viability. To consider commercialization of DSSCs, Pt should be replaced with other cheaper materials like conducting polymers (CPs)/carbon based nano materials.

In this work, the aforesaid key issues of DSSCs have been taken into account to develop solid state dye sensitized solar cells. At first, we mainly studied the effect of surface modified TiO₂ nanoparticles on PEO/PVDF–HFP on structural, thermal, morphological, optical, thermo mechanical and electrical properties. Then, MWCNT/Nafion^® composite was explored as a counter electrode and photovoltaic performance of solid state dye sensitized solar cells was studied.

2. Experimental procedure

2.1. Materials

Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP, Kynar flex 2801) was supplied by Arkema (India), poly ethylene oxide (PEO) (M_w 5 × 10⁶), aminopropyltrimethoxysilane (APS), Nafion^® 117 solution, Triton X-100 and chloroplatinic acid hexahydrate (H₂PtCl₆) were purchased from M/s. Sigma Aldrich, multiwalled carbon nanotubes (MWCNTs) of >98% purity and diameter of 80–100 nm were purchased from M/s. Nanoshel, Intelligent Materials Pvt. Ltd, India, bis(tetrabutylammonium)dihydrogen bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) (N719) dye was purchased from Solaronix, Switzerland. Anhydrous dimethyl formamide (DMF), ethanol, anhydrous lithium iodide (LiI), iodine (I₂), lithium perchlorate (LiClO₄), acetylacetone, n-butanol, and isopropyl alcohol were purchased from Himedia, India, and FTO glass plates (R_sh < 10 ohm cm⁻²) were supplied by M/s. Shilpa Enterprises, India. TiO₂ nanoparticles (P25), 25–30 nm, Degussa, AG, Germany, were dried at 80 °C for 10 h in a vacuum oven prior to use.

2.2. Preparation of polymer blend nanocomposite electrolyte

The surface of TiO₂ nanoparticles was modified by using APS as reported in our previous literature.²⁷ The hybrid nanocomposite polymer electrolyte membranes were prepared using a solvent-casting technique. 0.5 g of optimized composition of (6 : 4 wt%) of PVDF–HFP and PEO is dissolved in DMF solvent under continuous stirring at 80 °C. The pre-dried silane modified TiO₂ was slowly added with different weight percent (1–10 wt%). The mixture solution was thoroughly mixed under continuous stirring for about 8 h until the nanoparticles dispersed. Then, the obtained viscous polymer solutions were poured into glass Petri dishes and the solvent was allowed to evaporate. Finally, the dimensionally stable polymer nanocomposite electrolyte membranes were obtained. The formulation of surface modified TiO₂ was optimized by ionic conductivity as 7 wt%. For comparison, 7 wt% of unmodified TiO₂ nanoparticles were also separately incorporated into PEO/PVDF–HFP electrolyte. Hereafter, APS modified TiO₂ and unmodified TiO₂ nanoparticles incorporated blends are named as PEO/PVDF–HFP/M-TiO₂ and PEO/PVDF–HFP/U-TiO₂ respectively in the forthcoming sections.

2.3. Fourier transform infrared (FTIR) spectroscopy

FTIR spectra of the PEO/PVDF–HFP electrolyte samples were performed on a NICOLET 6700, USA FTIR spectrometer from 4000 to 400 cm⁻¹ with a scan rate of 5 cm⁻¹ s⁻¹.

2.4. Wide angle X-ray diffraction (WAXD)

Wide angle X-ray diffraction analysis was conducted in Shimadzu X-ray diffractometer, Japan, using CuK_α (1.514 Å) at a scanning rate of 5° min⁻¹ over a 2θ interval from 2° to 80°.

2.5. Thermal characterization

Differential scanning calorimetry (DSC) of the polymer composite electrolyte was carried out using Q20, M/s, TA Instruments (USA) equipment. Samples of ≤7 mg were heated from −60° to 200 °C, at the heating rate 5 °C min⁻¹ under nitrogen atmosphere.

Thermal stability of the polymer composites was studied by thermo gravimetric analyzer (TGA, Q50, TA Instruments, USA). Samples of about 7 mg were heated from room temperature to 800 °C at a linear heating rate of 10 °C min⁻¹ under N₂ flow (20 ml min⁻¹).

2.6. Dynamic mechanical analysis (DMA)

Dynamic mechanical properties were measured using a dynamic mechanical analyzer (M/s. TA Instruments, USA), operating in the tension film mode at an oscillation frequency of 1.0 Hz. The samples were heated from −60 °C to 120 °C at a heating rate of 10 °C min⁻¹.

2.7. UV-vis spectra studies

The tri-iodide and iodide (I⁻/I₃⁻) absorption spectra of PEO/PVDF–HFP and its composite samples were performed using a UV-vis spectrophotometer (UV-2450, M/s. Shimadzu, Japan) in the range of 200 nm to 600 nm.

2.8. Surface characterization

Surface characterization for the PEO/PVDF–HFP blend nanocomposite electrolytes was carried out in PARK XE-100, South Korea, using the non-contact mode.

2.9. Electrochemical impedance spectroscopy (EIS)

The ionic conductivity measurements were carried out using the EIS technique by using a WonATech-ZiveSP2 (Korea) electrochemical workstation. A perturbation of the sinusoidal voltage signal of 10 mV was applied over a frequency range of 1 mHz–1 MHz at room temperature. The impedance plots were fitted using Zview2-smart manager software. The samples were sandwiched between mirror polished circular stainless steel electrodes with a diameter of 10 mm. The thickness of polymer composite electrolyte membranes was varied from 40 to 200 microns measured by a thickness gauge.

2.10. Preparation of TiO₂ photo anode

0.1 g of TiO₂ nanoparticles was taken in a mortar and pestle and ground for 10 min with the sequential addition of 0.2 ml of distilled water and 0.02 ml of acetyl acetone to achieve a viscous colloidal form. Then, 0.34 ml of distilled water and 20 μl of triton-X 100 were slowly added into the colloidal mixure to a uniform paste. In order to remove the organic impurities and other contaminants, the FTO glasses (1.5 cm × 1.5 cm dimensions; R_sh < 10 ohm cm⁻²) were introduced into ultrasonication and thoroughly cleaned in acetone, ethanol and distilled water, for 15 min in each step. The colloidal paste was uniformly spread over the area of 0.5 cm² on the FTO coated glass substrate (R_sh < 10 ohm cm⁻²) by the doctor blade technique. The slide was kept for 5 min in atmospheric conditions for drying. Then it was calcined at 450 °C for 30 min and cooled to 80 °C in an oven. Subsequently, the films were immersed in 0.3 mM N-719 dye in acetonitrile and n-butanol (1 : 1 volume ratio) solution for 24 h.

2.11. Preparation of counter electrode

FTO glasses (1.5 cm × 1.5 cm dimensions) were cleaned with the same procedure mentioned in the above section. Different amounts (0.5 wt%, 1 wt%, 1.5% and 2 wt%) of MWCNTs are dispersed in ethanol/Nafion^® solution with the volume percentage of 90 : 10%. The mixture was kept under ultrasonication for 3 h at room temperature. The viscous solution of MWCNT/Nafion^® composite was coated on a FTO glass plate by drop casting. The MWCNT/Nafion^® coated FTO sample was kept at 50 °C for over 5 h. For comparison purposes, a Pt-coated counter electrode has been prepared by spreading a drop of 5 mM chloroplatinic acid hexahydrate (H₂PtCl₆) in isopropyl alcohol on a separate FTO substrate and calcinated at 450 °C for 15 min under ambient air.

2.12. Cyclic voltammetry studies

Cyclic voltammetry (CV) was performed to investigate electrocatalytic properties of the counter electrodes. The CV was carried out with a three-electrode electrochemical system, by using an electrode of MWCNT/Nafion^® composite as the working electrode, a Pt foil as the counter electrode, and saturated calomel electrode as a reference electrode in an electrolyte solution (10.0 mM I, 1.0 mM I₂, and 0.1 M LiClO₄ in acetonitrile). For comparison purposes, an as prepared Pt counter electrode was also employed as a working electrode. All the samples were investigated with a 10 mV s⁻¹ scan rate.

2.13. Construction of solid state dye sensitized solar cells (SSDSSCs)

An optimal composition of PEO/PVDF–HFP composite electrolyte solutions were casted onto the dye adsorbed TiO₂ photo anode by drop casting. Then the MWCNT/Nafion^® coated FTO glass plate was compressed on the photo anode without any short circuit. Finally, the sandwich type SSDSSC configuration was clamped with a binder clip and kept in the oven at 50 °C for 12 h to remove the residual solvent completely. The schematic diagram of the DSSC is shown in Scheme 1.

Scheme 1 Solid state sensitized solar cells.

2.14. Characterization of SSDSSC

The photovoltaic performance of the DSSC is studied using an Oriel Class-A Simulator (M-91900 A, Newport) with a xenon lamp as a light source having an intensity of 100 mW cm⁻². A computer controlled Auto lab PGSTAT302N electrochemical workstation was used for J–V measurements. The active area of the cell was 0.5 cm². To analyze the internal resistance of the DSSC, EIS measurements were carried out using a WonATech-ZiveSP2 (Korea) electrochemical workstation with a signal frequency range from 1 mHz–1 MHz under the illumination of 100 mW cm⁻².

3. Results and discussion

3.1. FTIR studies

FTIR spectra of the PEO/PVDF–HFP, PEO/PVDF–HFP/U-TiO₂ and PEO/PVDF–HFP/M-TiO₂ blend electrolyte membranes are shown in Fig. 1(a–c). The IR spectra of the electrolyte membranes were characterized in the range of 4000–400 cm⁻¹ at room temperature. It is well-known that lithium iodide (LiI) forms complexes with C–O–C in PEO and C–F in PVDF–HFP which results in decreasing stretching vibration frequency of these groups in FTIR spectra.²⁹

Fig. 1 FTIR spectra of electrolytes (a) PEO/PVDF–HFP, (b) PVDF–HFP U-TiO₂ and (c) PEO/PVDF–HFP M-TiO₂.

The stretching and bending modes of CH₂ in PEO are observed around 1470 and 1350 cm⁻¹.³⁰ The deformed vibration of CH₂ groups appears at 1403 cm⁻¹.³¹ The characteristic peaks corresponding to 1071 cm⁻¹, 759 cm⁻¹ and 607 cm⁻¹ are primarily due to the crystalline form of PVDF–HFP, whereas the bands at 872 and 841 cm⁻¹ are assigned to the amorphous phase of PVDF–HFP. The peaks observed between the frequencies 1280–1050 cm⁻¹ are denoted as –CF and –CF₂ stretching vibrations.³² Other important characteristic CH₂ symmetric and –CH₂ asymmetric peaks were observed at 1280 cm⁻¹ and 3018 cm⁻¹ respectively. The typical deformed vibrational frequency of the CH₂ group was observed at 1400 cm⁻¹. The vibrational peaks around 504 cm⁻¹ and 420 cm⁻¹ are assigned to bending and wagging frequencies of –CF₂. The scissoring vibration of vinylidene was observed at 1404 cm⁻¹. The peak at 880 cm⁻¹ corresponds to the vinylidene group of PVDF.³³ The strong absorption band around 1095 cm⁻¹, assigned to be the main characteristic peak of a non-resolved anti-symmetric and symmetric stretching of C–O–C in polymer back bone chains of pure polycrystalline PEO, is shifted from 1119 cm⁻¹. This red shift is observed due to the formation of a transient cross-linking complex between the cations of the electrolyte salt and the ether oxygen of the PEO, so that the generated transition state will weaken the C–O–C stretching vibration and the crystallization of PEO will be decreased.³⁴

The intensity of characteristic peak of PEO around 1095 cm⁻¹ and crystalline peak of PVDF–HFP about 1070 cm⁻¹ were weakened in PEO/PVDF–HFP blend electrolyte system, which indicates the PVDF–HFP is well blended with PEO matrix. The peaks were broadened and intensity decreased with the addition of U-TiO₂ and M-TiO₂. This may be due the reinforcement effect of nanoparticles on the polymer matrix, which reduces the recrystallization. It suggests that the nanoparticles significantly affected the crystallinity of high molecular weight semicrystalline PEO/PVDF–HFP. The possible interactions can be explained in the forthcoming sections.

The broad band absorptions of –OH stretching should be negligible in high average molecular weight PEO, but they are observed in the frequency range 3600–3200 cm⁻¹. This might be due to the fact that a large number of intermolecular and intramolecular hydrogen bonds are formed by the interactions of solvent and PEO/PVDF–HFP. In U-TiO₂ incorporated electrolyte membrane, water molecules on the surface of nanoparticles might be a reason for absorption in these regions. The intensity of –OH stretching was decreased in M-TiO₂ incorporated electrolyte membrane, which indicates the hydrolysis condensation reaction of APS molecules present on the TiO₂ nanoparticles.

In PEO/PVDF–HFP/7% M-TiO₂ membrane, the NH₂ group was observed at 1600 cm⁻¹ which is shifted to the lower wave number around 1580 cm⁻¹. This indicates the fact that a greater number of ions coordinate with –NH₂. The new interaction of salt–TiO₂ and TiO₂–polymer in the FTIR spectra of PEO/PVDF–HFP blend nanocomposite electrolyte membranes can be expected to improve the ionic conductivity of the system. The possible interactions of other characteristic peaks of APS such as Si–O–Si and C–N are not distinguishable as these peaks overlapped with the major peaks of PEO/PVDF–HFP blend matrix. Hence, it could not be resolved from the major peaks of PEO/PVDF–HFP matrix and it can be explained in the forthcoming sections.

3.2. Wide angle X-ray diffraction

Fig. 2(a–e) shows the WAXD patterns of PEO, PVDF–HFP, PEO/PVDF–HFP blend and its nanocomposite electrolyte membranes. The diffraction peaks at 19.36° and 23.52°, and 18.4°, 20.3° and 26.9° were observed as the characteristic peaks of crystalline phases of PEO and PVDF–HFP respectively in Fig. 2(a & b).^35,36 The peaks of pristine PEO and PVDF–HFP decreased in PEO/PVDF–HFP blend electrolyte, which indicates that PEO is properly blended with PVDF–HFP and the crystallization of PEO is controlled by PVDF–HFP in the blends. The diffraction peaks blend nano composites are slightly shifted towards the higher angle side and broadened with less intensity. In all the polymer electrolytes, there are no additional peaks observed for lithium iodide and I₂, suggesting that the electrolyte salts were in the polymer blend system.

Fig. 2 X-ray diffraction study of (a) PEO, (b) PVDF–HFP, (c) PEO/PVDF–HFP, (d) PEO/PVDF–HFP/U-TiO₂ and (e) PEO/PVDF–HFP/M-TiO₂ composite electrolyte membranes.

In the case of PEO/PVDF–HFP blend nanocomposite electrolytes in Fig. 2(d–e), TiO₂ nanoparticles disturb the crystalline region and decrease the crystalline phase of PEO/PVDF–HFP blend more than pure blend (Fig. 2(c)). This behavior is due to the large surface area of TiO₂ nanoparticles, which inhibits the recrystallization of the PEO/PVDF–HFP host. The addition of TiO₂ nanoparticles could lead to a decrease in the intermolecular interaction between polymer chains, hence the increased amorphicity.³⁷ These observations suggest that the M-TiO₂ incorporated PEO/PVDF–HFP undergoes more structural reorganization. This amorphous nature may lead to higher ionic conductivity PEO/PVDF–HFP nanocomposite electrolytes which is generally observed in amorphous polymer electrolytes with flexible backbones.

3.3. Surface morphology of the electrolyte membranes using AFM

Three-dimensional topographical images of PEO/PVDF–HFP blend nanocomposite electrolyte membranes are presented in Fig. 3. The PEO/PVDF–HFP/U-TiO₂ electrolyte shows a rough surface morphology and is composed of spherical grains with the size of 200–300 nm. This is mainly due to the high surface energy of unmodified TiO₂ nanoparticles causing agglomeration via particle–particle interaction among the TiO₂ nanoparticles. But, in the case of surface modified TiO₂ (M-TiO₂) incorporated blend electrolytes, a comparatively smoother surface is shown, which indicates that M-TiO₂ particles are uniformly distributed within the matrix. Also the smoother surface pointed out that one is more amorphous in nature than the other. This behavior can be expected to improve ion transport properties in polymer electrolytes. The possibility of surface modified TiO₂ nanoparticle dispersion and complexation in the polymer matrix is shown Scheme 2.

Fig. 3 Surface morphology of (a) PEO/PVDF–HFP/U-TiO₂ and (b) PEO/PVDF–HFP/M-TiO₂ electrolytes.

Scheme 2 Dispersion of nanoparticles in PEO/PVDF–HFP blends.

3.4. Differential scanning calorimetry

DSC thermograms of pure PEO, PVDF–HFP, PEO/PVDF–HFP and PEO/PVDF–HFP nanocomposite electrolyte membranes in a wide range of temperatures from −60 °C to 200 °C were shown in Fig. S1(a–e).† From the DSC thermograms, the glass transition temperature (T_g), the melting temperatures of PEO (T_m1) and PVDF–HFP (T_m2) and melting enthalpy of PEO (ΔH_m1) and PVDF–HFP (ΔH_m2) were determined. Here, T_m was obtained as the peak of the melting endotherm, and T_g as the inflection point. The glass transition temperature is an important parameter of the amorphous phase for the flexibility of polymers at room temperature. The T_g of pure PEO and PVDF–HFP electrolytes are about −41.8 °C and −21.5 °C respectively is shown in Fig. S1(a and b).†

A single glass transition temperature, which indicates that all polymer composites are miscible with homogenous phase in Fig. S1(c–e).† Similar results have been observed by J. Zhang et al.³⁸ The decreasing of T_g of about −23.3 °C for PEO/PVDF–HFP electrolytes than pristine PVDF–HFP electrolyte is maybe due to the addition of the PEO, which has softened the rigid segment of the main matrix (PVDF–HFP). The further reduction in T_g was observed with the addition of U-TiO₂ and M-TiO₂ nanoparticles. The incorporated nanoparticles are acting as solid plasticizers and significantly they reduce the cohesive forces between the long range polymer chains of high molecular weight polymers. Hence, it is expected to increase the ionic conductivity. In Fig. S1(b)† there are few small peaks observed in between the glass transition temperature and melting temperature which is probably due to the crystal phase transition of ferroelectric to paraelectric phase in PVDF copolymer known as Curie temperature.³⁹ The endothermic sharp peak with melting temperature of PEO and PVDF–HFP was observed around 60 °C and 147 °C respectively.

It is necessary to study the crystallinity of PEO, even though the amount (4 wt%) of PEO is less than the main matrix PVDF–HFP (6 wt%) in PEO/PVDF–HFP blend and its composite system in the present work. Because the crystallization temperature of PEO is near room temperature (50–60 °C), the ionic movement can be restricted. The addition of PVDF–HFP and nanoparticles with PEO results in broadening of the melting peak, which suggests that there is an interruption in the crystallization of PEO and an increase in the free volume fraction. The relative percentage of crystallinity (χ%) was calculated from the following eqn (1)

χ% = ΔH_m/ΔH⁰_m × 100% (1)

where, ΔH⁰_m is the heat of fusion of pure PEO, which is equal to 213.7 J g⁻¹ when the material is assumed to be 100% crystalline,⁴⁰ and ΔH_m is heat of fusion of polymer blend electrolyte. The melting enthalpy of the main matrix PVDF–HFP also reduced when it is blended with PEO. The changes in thermal parameters for M-TiO₂/PEO/PVDF–HFP blend electrolyte is higher than U-TiO₂/PEO/PVDF–HFP, which may be due to organic functional groups like Si–O–Si, NH₂ and CH₂, which may destroy the polymer chain conformation and coordinate the salt systems. The new interactions formed by APS molecules are not only interrupt structural conformation of polymer, but also provide a conduction path for easy ion migration (I⁻/I₃⁻). The relative percentage of crystallinity is comparatively decreased with the addition of M-TiO₂ than U-TiO₂ and it is corroborated with WAXD and FTIR in Fig. 1(c) and 2(e). The other thermal parameters and percentage of crystallinity derived from DSC were depicted in Table 1.

Table 1 Thermal parameters derived from DSC

Polymer electrolyte	(ΔHm1) (J g^−1)	(ΔHm2) (J g^−1)	Glass transition temperature (°C)	Crystallinity (%) (PEO)
PEO	109.1	—	−40.12	51.05
PVDF–HFP	—	24.70	−21.5	—
PEO/PVDF–HFP	49.32	22.96	−23.3	23.07
PEO/PVDF–HFP 7% U-TiO2	27.29	22.3	−23.8	12.70
PEO/PVDF–HFP 7% M-TiO2	26.53	10.33	−24.0	12.10

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3.5. Dynamic mechanical analysis

It is important to obtain membranes with high mechanical stability under dynamic conditions while developing polymer blend electrolytes for dye sensitized solar cells. DMA helps to study thermo mechanical properties and polymer/polymer miscibility in polymer blend systems, and also measures the glass transition temperature of polymers. Fig. 4 shows the variations of the storage modulus (G′), the loss modulus (G′′) and tan δ with temperature of PEO/PVDF–HFP blend and its composite electrolytes. It is clear from Fig. 4(a) that storage modulus decreases with the increase of temperature for all blend electrolyte samples; however the decrease is not linear. The storage modulus curves of all samples exhibit three distinct regions: a glassy high modulus region at low temperature where the segmental motion is restricted, a transition region where a substantial decrease takes place in G′ with an increase in the temperature, and a rubbery region where a drastic decay occurs in modulus values above the glass transition temperature. The storage modulus is increased in both glassy and rubbery regions with the addition of U-TiO₂ and M-TiO₂, moreover, it is a higher value in the glassy region than in the rubbery region as compared to PEO/PVDF–HFP electrolyte membrane.

Fig. 4 DMA curves of PEO/PVDF–HFP blend and its blend nanocomposite electrolyte membranes. (a) Storage modulus, (b) loss modulus and tan delta (insert figure).

The high storage modulus of PEO/PVDF–HFP blend nanocomposites at low temperatures confirms the reinforcement effect of nanoparticles. It can be attributed to the restriction of the molecular motion of the PEO/PVDF–HFP blend macromolecules due to the fine dispersion of the nanofillers, as shown in AFM analysis Fig. 3(b), which leads to increased interactions with the polymer matrix.⁴¹ The gradual decreasing trend of G′ has been observed between −40 °C to 0 °C, which is ascribed to the glass transition temperature of PEO/PVDF–HFP blends.⁴² It can be seen that the addition of nanoparticles has an influence on the glass transition temperature of PEO/PVDF–HFP blend electrolytes.

A further abrupt decay in G′ has been observed around 60 °C with an increase in the temperature. Undoubtedly it might be attributed to the melting characteristics of the semicrystalline PEO component in the blend electrolytes. Even though the modulus does not completely vanish near the melting temperature of PEO, it is mainly contributed by the crystalline portion of the PVDF in the PVDF–HFP component.

Fig. 4(b) shows the plots of the loss modulus and loss tangent (tan δ) (insert) as a function of temperature for PEO/PVDF–HFP blend and its nanocomposite electrolyte membranes. Generally, the glass transition temperature of polymeric materials is determined from the peak tan δ curves. But in the present case it is difficult to distinguish T_g from its other relaxation peaks due to the movement of molecular chains at the amorphous–crystal interface of the semicrystalline polymers like PEO and PVDF–HFP.⁴³ It is not easy to find sharp glass transition temperatures from DMA curves due the presence of more heterogeneities in the PEO/PVDF–HFP composite electrolytes resulting in a broader tan δ curve. Also the method of measurement plays a very important role in determining the T_g. The rate of heating of samples was kept at 10 °C min⁻¹, which may not be able to measure the micro-Brownian movement of molecular chains. Hence it is advisable to give a low heating rate for the determination of the glass transition temperature in DMA analysis.

The glass transition temperature can be explained in another way from the peak of the loss modulus curve of polymer systems. It is clear that the loss modulus curve is showing a single peak which corresponds to the glass transition temperature, even though the peak is observed around 0 °C for PEO/PVDF–HFP blend. But in the case of nanoparticle incorporated blend electrolytes, a peak at lower temperature region is exhibited and it is slightly decreased for M-TiO₂. Hence, it is undoubtedly understood that incorporated nanofillers might be perturbed by the crystalline domain and increase the free volume as well as flexibility of the long range chain. Similar behavior of crystalline phase reduction is also observed in WAXD and DSC.

3.6. UV-vis studies

The UV-vis absorption spectra of pure PEO/PVDF–HFP and U-TiO₂, M-TiO₂ incorporated PEO/PVDF–HFP electrolyte membranes are shown in Fig. 5. The peaks around 230 nm and 290 nm observed for all composite electrolytes, which correspond to I⁻ and I₃⁻ ions respectively. It is found that intensity of peaks increased with the addition of U-TiO₂ and M-TiO₂ nanoparticles. The incorporated nanoparticles within polymer electrolyte play the role of Lewis acid–base interaction centres with the ionic species in the electrolyte. The surface hydroxyl groups (–OH) of the nanoparticles are expected to interact with the salt anions through the hydrogen bonds; hence the ion concentration has increased. But, in PEO/PVDF–HFP/M-TiO₂ electrolyte membrane showed high absorption, shown in Fig. 5(c), indicates an increased free anion concentration (I⁻ and I₃⁻). The peak point is slightly shifted to a higher wavelength, which might be due to the interaction of silane molecules on the TiO₂ surface. The similar interaction of free anions (I⁻/I₃⁻) with the amine group (–NH) present in APS have been observed from FTIR analysis by J. Zhang et al.⁴⁴ More free anions in the PVDF–HFP/M-TiO₂ electrolyte membranes are expected to enhance the electrochemical and photovoltaic properties of DSSCs.

Fig. 5 UV-vis spectra of (a) PEO/PVDF–HFP, (b) PEO/PVDF–HFP/U-TiO₂ and (c) PEO/PVDF–HFP/M-TiO₂ composite electrolytes.

3.7. Electrochemical impedance studies

The samples were optimized based on ionic conductivity measurements. The typical Nyquist plots of PVDF–HFP and its composite electrolyte membranes at room temperature are illustrated in Fig. 6. It was observed that the complex impedance spectra of electrolytes show only the slanted lines which indicates the electrode polarization plays the major role in the present polymer blend system. In practice the equivalent circuit model can help for a better understanding of the real system. Equivalent circuit for fitting the electrochemical impedance data is shown in Fig. S2.† It consists of a serial combination of bulk electrolyte resistance (R_b) and constant phase element (CPE).

Fig. 6 Nyquist plots of (a) PEO/PVDF–HFP, (b) PEO/PVDF–HFP/U-TiO₂ and (c) PEO/PVDF–HFP/M-TiO₂ composite electrolytes.

The real and imaginary parts of impedance can be expressed as the following eqn (2) and (3).⁴⁵

Z′ and Z′′ are the real and imaginary parts respectively, R_b is the bulk resistance of the blend electrolyte sample and k⁻¹ corresponds to capacitance of constant phase element, ω is angular frequency which is equal to 2πf, where f is the frequency in Hz, and p is the right angle which the slanted line makes with the horizontal axis in Nyquist plot (Z′ vs. Z′′).

The complete linear behavior in the high frequency region of blend electrolytes is convincing evidence of the integrity of the system, which means the PEO/PVDF–HFP blend is miscible. If any phase separation and/or more crystallite domains existed, it would be exhibited in the semicircles in the lower frequency region or, more generally, by deviation from linearity in the high frequency region of the impedance spectra.⁴⁶ It is understood that in Fig. 6(a) there is a slight deviation of the linear line at the lower frequency region in EIS spectra, which indicates the high percentage of crystallinity of PEO/PVDF–HFP blend.

The electrical parameters such as diffusion coefficient (D), ionic mobility (μ) and charge carrier density were calculated by using the following eqn (4)–(6) with the help of fitting parameters and plotted as a function of M-TiO₂ nanoparticles.^45,47

where d = thickness of sample/2, δ = d/λ, λ = ε₀εA/k⁻¹ is the electrical double layer thickness, A is the area of electrolyte membrane, ε₀ is vacuum permittivity, ε is the dielectric constant of polymer blend electrolytes, τ₂ = 1/ω₂, ω₂ is the angular frequency at which the slanted line intersects at the real impedance axis. The intercept of the Nyquist plot (Z′ vs. Z′′) with the real axis at higher frequency region is taken as a bulk resistance of polymer electrolyte membranes. The ionic conductivity (σ) of the electrolytes was calculated by using eqn (7)

σ = t/R_bA (7)

where, t is the thickness of the membrane, R_b is bulk resistance, and A is the area of electrode in contact with electrolyte membrane.

Fig. 7 shows the variation of ionic conductivity and mobility of ions in polymer blend nanocomposite electrolytes with inclusion of different amounts of M-TiO₂. The ionic conductivity of PEO/PVDF–HFP increased with the addition of M-TiO₂ and reached to a maximum value of 7.21 × 10⁻⁴ S cm⁻¹ for 7 wt%. The nano level M-TiO₂ particles may influence the kinetics of the PEO/PVDF–HFP chain which increases the localized amorphous regions. Also, the addition of nanoparticles into the polymer matrix gives a three dimensional, mechanically stable and porous network structure. The anions (iodide/tri-iodide) can easily move into the porous structures within the electrolyte. For comparison purposes, conductivity of 7 wt% of untreated TiO₂ incorporated PEO/PVDF–HFP composite was also found to be about 8.14 × 10⁻⁵ S cm⁻¹ and it is lower than the same amount of silane modified TiO₂. Further addition of M-TiO₂ exceeds the optimum level and decreases the ionic conductivity of PEO/PVDF–HFP electrolytes due to the insulation of M-TiO₂ nanoparticles. Because, the high concentration of nanoparticles also leads to well-defined crystallite regions and the added fillers beyond the optimum level may catalyze aggregation of polymer chains. It leads to increase the rate of recrystallization processes.⁴⁷ The addition of more nanofillers may cause an increase in the viscosity of polymer electrolyte; it hinders the ion mobility and dilution effect. Similar behavior has been observed for unmodified nanoparticle incorporated polymer electrolytes.⁴⁸ These crystallite regions of polymer electrolytes restrict the movement of ions, thus ionic mobility also decreased beyond the optimum level of nanofillers. From the above observations, it is clearly understood that the enhancement in ion conducting property cannot be attributed to the intrinsic electronic properties of the semiconducting nanoparticles. This is because the interactions in polymer nanocomposite electrolytes are between salt–polymer, plasticizer–salt and the polymer–TiO₂ but not the salt–TiO₂, as investigated by Forsyth et al.⁴⁹

Fig. 7 Ionic conductivity and ionic mobility of PEO/PVDF–HFP electrolyte membrane with different amounts of silane treated TiO₂ (M-TiO₂) nanoparticles.

In the present study, the disappearance of peaks is mainly observed in WAXD studies (Fig. 2(e)), and the lowering of T_g in DSC and DMA analysis (Fig. S1(e)†) and 5) compared with the unmodified counterpart. It is hinted that there may be an additional interaction of Li⁺ in salt with nanoparticles in a surface modified TiO₂ incorporated PEO/PVDF–HFP blend electrolyte system and similar behavior reported by Chin-Yeh Chiang et al.⁵⁰ supports our results. Such additional interactions/coordination might be enhanced by the ionic conductivity. Similar behavior has been observed in silane modified PEO/PVDF blends by Y. Yang et al.⁴⁴ This is because the same amount of untreated nanoparticle incorporated PEO/PVDF–HFP electrolytes exhibits lower ionic conductivity. The possible interaction of Li⁺ in salt with TiO₂ in surface modified TiO₂/PVDF–HFP electrolyte system is sketched in Scheme 2.

The diffusion coefficient and number of charge carriers within the blend electrolytes were derived and illustrated in Fig. S3.† Both parameters increased with the addition of M-TiO₂ nanoparticles within the PEO/PVDF–HFP blend electrolyte systems. The highest ionic diffusion coefficient achieved was 7% for incorporated M-TiO₂ electrolyte; beyond this level it decreased. The high filler content may be reduced by the miscibility between PEO/PVDF–HFP and TiO₂ nanoparticles; it leads to phase separation.⁴⁹ Also the crystallinity of electrolytes can be increased, hence, it is believed to be an unfavorable environment for ion transport. The number of charge carriers (n) also decreased beyond the optimum level. The optimum amount of M-TiO₂ can dissociate/coordinate the maximum number of Li⁺ and gives high ionic conductivity. The dielectric constant is calculated and the obtained maximum was about 1260 for 8% of M-TiO₂ incorporated PEO/PVDF–HFP blends at 100 Hz. The decreasing trend of dielectric constant may be associated with exceeding the percolation threshold value. The electrical parameters of unmodified TiO₂ and surface modified TiO₂ incorporated electrolyte were calculated and summarized in Table 2.

Table 2 Calculated electrical parameters for 7% untreated TiO₂ incorporated PEO/PVDF–HFP

PEO/PVDF–HFP with different amounts of TiO2	Ionic conductivity σ (S cm^−1) × 10^−4	Mobility μ (cm^2 V^−1 s) × 10^−8	No. of charge carriers n (cm^−3) × 10^22	Diffusion coefficient (cm^2 s^−1) × 10^−10	Dielectric constant
a Optimized compositions.
1% M-TiO2	0.25	1.24	1.24	3.18	130
2% M-TiO2	0.26	1.35	1.21	3.45	170
3% M-TiO2	0.80	2.39	2.11	6.14	350
4% M-TiO2	0.81	2.55	1.99	6.55	420
5% M-TiO2	0.84	3.25	1.62	8.35	490
6% M-TiO2	0.88	3.37	1.63	8.66	550
7% M-TiO2^a	7.24	7.43	4.41	19.11	1020
8% M-TiO2	4.29	6.69	4.01	17.20	1260
9% M-TiO2	1.93	3.83	3.14	9.85	987
10% M-TiO2	1.25	3.19	2.43	8.21	927
7% U-TiO2^a	0.81	1.35	3.76	3.45	172

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3.8. Thermo gravimetric analysis

TGA and DTA curves of PEO/PVDF–HFP and its nanocomposite electrolyte membranes were shown in Fig. S4.† It is observed that the stability of all sample blends moves slightly down at lower temperatures around 50 °C with a weight loss of 8–10%. The evaporation of moisture and physically absorbed silane molecule attached on the PEO and TiO₂ surface is well known. It is seen for PEO/PVDF–HFP blends that no significant weight loss was observed until 240 °C followed by weight loss in the DTA exothermic peak which clearly shows that PEO/PVDF–HFP blend system is stable up to 370 °C. The majority of weight loss started around 400 °C. This may be due to thermal degradation of random chain scission of C–O bonds in PEO.⁵¹ The main degradation products are ethyl alcohol, methyl alcohol, alkenes, non-cyclic ethers (ethoxymethane, ethoxyethane and methoxymethane), formaldehyde, acetic aldehyde, ethylene oxide, water, CO and CO₂.⁵² Beyond this temperature the thermal stability of the membranes is due to the repeating units –(CF₂–CF₂)– of PVDF–HFP. The chemical bond strength of C–F is approximately 485 kJ mol⁻¹, which is higher than C–H and C–C bond strengths at around 435 kJ mol⁻¹ and 410 kJ mol⁻¹ respectively.⁵³ Upon further increasing temperature, the stability of nanocomposite increased due to TiO₂ nanoparticles. In the present case, the thermal stability of the composite electrolyte at high operating temperature is not required for practical applications of DSSC.

3.9. Cyclic voltammetry analysis of MWCNT/Nafion^®

To investigate the electrocatalytic activity of MWCNT/Nafion^® composite for I₃⁻ reduction at the electrode, the cyclic voltammetry technique was employed. Fig. 8 shows the typical CV characteristic curves of MWCNT/Nafion^® composite electrodes with different amounts of MWCNT at a scan rate of 10 mV s⁻¹. The catalytic activity is increased with an increase in the amount of MWCNT and reaches a maximum for 2 wt% (Fig. 8(d)). For comparison purposes, a Pt electrode is taken as a reference electrode and performed at the same conditions, as depicted in Fig. S5.† The peak position and shape of the curve for MWCNT/Nafion^® are similar to the Pt electrode. The cathodic peak at −0.167 V with the peak current (I_c) of −2.21 mA cm⁻² assigned to the redox reaction of I₃⁻/I⁻ is given in eqn (8).

I₃⁻ + 2e⁻ ↔ 3I⁻ (8)

Fig. 8 Cyclic voltammetry studies of MWCNT/Nafion^® composite. MWCNT: (a) 0.5, (b) 1, (c) 1.5 and (d) 2 wt%.

The anodic peak at 0.13 V with the peak current of 2.67 mA cm⁻² is assigned to the redox reaction given in eqn (9).

3I⁻ ↔ I₃⁻ + 2e⁻ (9)

The peak current density of MWCNT (2 wt%) is quite comparable with the Pt coated electrodes (Fig. S5.†). It is mainly due to the uniform distribution of MWCNTs caused by Nafion^® in the coating solution, which inhibits the aggregation in coated films. Hence, the electrocatalytic activity of the high electrochemically active surface area of MWCNT/Nafion^® composite can be improved. This indicates the carbon based nanoparticles can be utilized as an alternative candidate to replace the precious metals as counter electrodes. Similar work had reported by Min-Hsin Yeh et al. with graphene/Nafion^® nanocomposites as a counter electrode for DSSC applications.⁵⁴

3.10. Photovoltaic performance of SSDSSC

J–V characteristic curves of PEO/PVDF–HFP and blend nanocomposite based DSSC are shown in Fig. 9.

Fig. 9 J–V characteristic curves of SSDSSC PEO/PVDF–HFP and its composite electrolytes.

Table 3 shows the photovoltaic parameters of SSDSSC derived from the J–V curve. It is observed that on addition of U-TiO₂ and M-TiO₂ nanoparticles within the PEO/PVDF–HFP blend, photo-current density (J_max) has increased from 1.26 mA cm⁻², to 2.77 mA cm⁻² and 5.36 mA cm⁻² respectively. It is well known that the performance of DSSC is mainly determined by effective transport behaviour of I⁻ and I₃⁻ions. The high concentration of I⁻ and I₃ ions shown in UV-vis (Fig. 5(c)) spectra as well as more number charge carriers (Fig. S3†) may cause an increase in the ionic conductivity and favours achievement of the high short circuit current density (J_sc) and open circuit voltage (V_oc) for M-TiO₂ incorporated PEO/PVDF–HFP blend nanocomposites. The improvement in the efficiency and fill factor for PEO/PVDF–HFP/M-TiO₂ electrolyte is mainly due to the reduction in the crystallinity followed by increasing ionic conductivity to about 7.21 × 10⁻⁴ S cm⁻¹. The more amorphous phase of PEO/PVDF–HFP/M-TiO₂ electrolyte can allow easy migration of ions from electrolyte to electrode. This can help to regenerate the dye molecules from their oxidized states (D⁺) to normal states (D). These are the main factors for improved performances. The APS molecules on the TiO₂ surface in PEO/PVDF–HFP electrolyte not only increases the ionic conductivity but also inhibits the charge recombination of conduction band electrons of dye sensitized TiO₂ and I₃⁻. Hence, it is beneficial for the enhancement of V_oc.

Table 3 Photovoltaic parameters for PVDF–HFP and its blend nanocomposite electrolytes

Polymer electrolytes	Voc (V)	Jsc (mA cm^−2)	Vmax (V)	Jmax (mA cm^−2)	FF (%)	Efficiency (%)	References
PEO/PVDF–HFP	0.62	2.13	0.45	1.26	43	0.6	Present
PEO/PVDF–HFP/U-TiO2	0.67	3.85	0.50	2.77	54	1.4	Present
PEO/PVDF–HFP/M-TiO2	0.71	6.37	0.53	5.36	62	2.8	Present
TiO2 nanotube (TNT)/PEO electrolyte	0.37	0.58	0.29	0.43	58	0.12	63
PEO/PVDF	0.61	7.2	—	—	77	3.38	65

---

Generally, the high efficiency (η > 3%) DSSCs are employing liquid electrolytes, plasticizers, and other toxic materials like 4-tertbutyl pyridine and iso-cyanate compounds as additives.^55–59 However, the efficiency (η = 2.8%) in the present work is quite comparable with recently reported efficiencies (η = 2–5%).^60–62 In another way it is interesting to note that the efficiencies of ZnO photo anode based DSSCs are higher than TiO₂ as a photo anode material. This is because the thickness of the ZnO based photo anode is higher than TiO₂ photo anode sintered at 400 °C.⁶⁴ Hence, the amount of dye absorption by TiO₂ is much less and its high grain boundary resistance impedes the electrons emitted from the dye. These factors also affect the photovoltaic performance of DSSCs. Currently, we are optimizing the photo anode materials with combinations of TiO₂, ZnO and Fe₂O₃ by changing the composition, morphology and particle size which will be discussed in the near future.

However, our main concern here is whether the surface modified TiO₂ can improve the conversion efficiency of DSSCs as compared with the PEO/PVDF–HFP/U-TiO₂ electrolyte system. The above results exposed that surface modification of TiO₂ nanoparticles within polymer electrolytes is a more effective approach for solid state DSSC applications than other expensive organic additives.

3.11. Internal resistance of DSSC

The internal resistance of various interfaces in DSSCs was measured by EIS. Fig. 10 shows EIS spectra of DSSCs based on PEO/PVDF–HFP and its composite electrolytes. Three distinct semicircles are observed in the frequency range of 10⁻³ Hz to 10⁶ Hz. These semicircles are due to Nernst diffusion within the electrolyte, the electron transfer at the TiO₂/electrolyte interface and the redox reaction at the MWCNT/Nafion^® counter electrode. The experimental EIS data fitted with equivalent circuit and charge transfer resistance of each interface have been derived.

Fig. 10 EIS spectra of DSSC with different electrolytes; (a) PEO/PVDF–HFP, (b) PEO/PVDF–HFP/U-TiO₂ and (c) PEO/PVDF–HFP/M-TiO₂.

R_s is denoted as the series resistance of the electrolytes and electric contacts in the DSSCs. R_ct1, R_ct2 and R_diff correspond to the charge transfer processes occurring at the MWCNT/Nafion^® counter electrode/electrolyte (corresponding to the first arc), the FTO/TiO₂/electrolyte interface (corresponding to the second arc) and the Warburg element of the ionic diffusion for the redox-couple (I⁻/I₃⁻) ion diffusion in the electrolyte (third arc) respectively.^66–68 The EIS parameters were summarized in Table S1.†.

From Table S1,† DSSCs based on PEO/PVDF–HFP/M-TiO₂ electrolyte show smaller R_ct2 and R_diff than pure and U-TiO₂ incorporated nanocomposites. It is attributed to the uniform distribution of M-TiO₂ within the matrix and reduced crystallinity of PEO/PVDF–HFP, which can be facilitated by the ion diffusion coefficient (I⁻/I₃⁻). Hence, the photo-current and solar energy conversion efficiency of PEO/PVDF–HFP/M-TiO₂ based DSSCs were marginally increased.

4. Conclusions

In summary, PEO/PVDF–HFP blend electrolyte membranes reinforced with surface modified TiO₂ nanofillers were prepared for dye sensitized solar cell applications. The electrochemical studies revealed that the addition of surface modified nanoparticles increased the ionic conductivity up to 7.21 × 10⁻⁴ S cm⁻¹ for 7 wt%, which is almost 10 times higher than the unmodified counterpart within PEO/PVDF–HFP blend. The ionic mobility, charge carrier concentration and ion diffusion coefficient are also found to increase with the incorporation of surface modified TiO₂ nanoparticles. The influence of the TiO₂ nanoparticle surface functionality upon the degree of crystallinity of the polymer matrix was analyzed using DSC and WAXD. It is observed that amorphous phase is increased. Thermo mechanical behavior of the composite membranes also confirmed this reduction in crystallinity. The thermo gravimetric investigations of membranes indicated the thermal stability of the blend improved upon the addition of nanosized inorganic fillers. The surface morphology of electrolyte membranes was studied by AFM. The solid state dye sensitized solar cell was fabricated by using silane modified TiO₂/PEO/PVDF–HFP polymer nanocomposite electrolytes, MWCNT/Nafion^® as counter electrode and N-719 as a sensitizer. The maximum solar conversion efficiency of 2.84% was achieved with silane modified TiO₂ incorporated electrolyte. The addition of surface modified TiO₂ within the electrolytes also decreases the interfacial resistance. These results confirmed the active role of silane modification of TiO₂ which leads to an increase in transport properties and photo-current conversion efficiency.

Acknowledgements

One of the authors K. Prabakaran is thankful to Prof. P. Ramasamy and Dr M. Senthil Pandian, SSN Research Centre, Chennai, Tamilnadu, for their support in photo anode fabrications. This research received no specific grant from any funding agency.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01770j

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