Fe2O3 as an efficient filler in PVDF-HFP based polymeric electrolyte for dye sensitized solar cell application
Graphical abstract
Introduction
Dye sensitized solar cells has gained interest with their acceptable power conversion efficiency with use of low cost and lesser purity of materials. It has an advantage of simple fabrication technique over silicon based solar cells. DSSCs with higher power conversion efficiency over 10% came into existence in the year 1991 invented by O'regan and Gratzel [1]. The traditional DSSCs were made of dye absorbed TiO2 as a photo-electrode, liquid electrolyte and platinum counter electrode. Despite of higher power conversion efficiency, traditional cells lagged behind with certain issues like electrolyte leakage and solvent evaporation. Those trailed issues can be remedied by using polymer electrolyte, where the polymer electrolytes entrap redox ions in its chain. With the catalytic effect of counter electrode, ions tend to hops in between the chains of polymer electrolyte to reach photo-electrode [2]. Hence, use of polymer electrolyte paves the way towards promising alternative for liquid electrolyte based DSSC system.
In this regard, various approaches has been used in preparation of polymer membranes and some of the processing techniques are electro-spinning [3], solution casting [4], template synthesis [5] etc. Among them, solution casting is the simple technique used to obtain polymer membrane with good flexibility and desired thickness. Many host polymers like Poly(ε-caprolactone) (PCL), poly(ethylene oxide) (PEO), poly(vinylidiene fluoride) (PVDF), polyurethane (PU) etc. [6] has been used. Out of those polymers PVDF based copolymer, PVDF-HFP has gained interest as a potential candidate due to its properties; mechanical support provided by PVDF crystalline phase whereas higher ionic conduction is facilitated by HFP amorphous phase [7]. Furthermore, incorporation of inorganic metal oxide including silica (SiO2) [8], alumina (Al2O3) [9], zirconium dioxide (ZrO2) [10]. titanium dioxide (TiO2) [11] etc. has been extensively used to prepare composite polymer electrolytes. As a result, the stability of polymer electrolytes improved and crystallinity reduction has also been achieved, ultimately leading to the enhancement in conductivity of the host polymer. These mentioned reasons are significant in enhancing photovoltaic performance of DSSCs.
Fe2O3 has been used in many optical applications: solar energy conversion, photocatalysis [12], interference filters [13], electrochromism [14], and photo-oxidation of water [15]. This particular abundant semiconductor material has the narrow band gap (2.0–2.2 eV) and shows a tendency to collect 40% of the solar spectrum [16]. So, here we focused on utilizing Fe2O3 in plasticized PVDF-HFP by using simple solution casting method to prepare a composite polymer membrane and investigated its crystallinity behaviour. Electrochemical properties like diffusion co-efficient and ionic conductivity of the composite polymer electrolyte and the observation showed that the polymer electrolyte could be an effective alternative to liquid electrolyte of traditional DSSCs.
Section snippets
Materials and methods
Anhydrous lithium iodide (LiI), Iodine (I2) and Iso-propanol were procured from Thermo Fisher Scientific India. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) (Kynar flex 2801) with melt viscosity of 23–27 K poise was purchased from Arkema (India). Di-tetrabutylammoniumcis-bis(isothiocyano)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium (II) (N-719) dye was purchased from Greatcell Solar Australia Pty Ltd. 4-tert-Butylpyridine (purity > 96%) and Tetrabutylammonium Iodide (TBAI)
X-ray diffraction
X-ray powder diffraction pattern of synthesized Fe2O3 are shown in Fig. 2a. The synthesized sample matches with the JCPDS card no 33-0664 and peaks appearing at 24.2°, 33°, 35°, 40.5°, 49°, 54° and 57° corresponds to (012), (104), (110), (113), (024), (116) and (018) crystalline plane signifying pristine Fe2O3. In addition, sharp and narrow peak indicates that the synthesized product is highly crystalline and also implies purity of the sample [19]. The crystallite size was calculated from
Conclusion
A series of polymer membranes were developed by simple solution casting method using PVDF-HFP as the host polymer and incorporation of various wt% Fe2O3 (1, 2 and 3) in host polymer. The reduction in crystallinity was shown by XRD studies. FTIR spectra showed the crystalline and amorphous peaks in polymer composite. The ionic conductivity of 6.36 × 10−4 S/cm and diffusion co-efficient of 1.02 × 10−6 cm2/s was observed upon addition of 2 wt% Fe2O3 in PVDF-HFP. The membrane is used as redox
Acknowledgement
The authors show their appreciation to HPCL-Mittal Energy Limited (HMEL) for financial support in DSSC research.
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