Optimization of Benzoguanamine doped PVDF/KI/I2 Solid Polymer Electrolytes for Dye-Sensitized Solar Cell Applications

Benzoguanamine doped 0%, 10%, 20%, 30%, 40% and 50% PVDF/KI/I 2 polymer electrolytes were prepared by solution casting technique. The crystallinity, surface morphology, ionic conductivity and photovoltaic performance of polymer electrolytes were analyzed. The PXRD studies have conrmed the decreased and increased crystallinity of benzoguanamine doped polymer electrolytes. The surface morphology of polymer electrolytes is discussed using SEM analysis. From the AC-impedance analysis, ionic conductivity of benzoguanamine doped 0%, 10%, 20%, 30%, 40% and 50% PVDF/KI/I 2 polymer electrolytes were calculated as 5.57 × 10 − 6 Scm − 1 , 1.05 × 10 − 5 Scm − 1 , 5.95 × 10 − 5 Scm − 1 , 3.09 × 10 − 5 Scm − 1 , 1.56 × 10 − 5 Scm − 1 and 1.48 × 10 − 5 Scm − 1 , respectively. The photovoltaic performance of benzoguanamine doped 0%, 10%, 20%, 30%, 40% and 50% PVDF/KI/I 2 polymer electrolytes based DSSCs have achieved 1.5%, 1.9%, 2.8%, 2.5%, 2.3% and 2.1% power conversion eciency, respectively.


Introduction
Ion diffusion and ionic conductivity of the polymer electrolytes are lower than the liquid electrolytes. However, the research is going on to develop a novel polymer electrolyte with higher ion diffusion and ionic conductivity with good stability. Higher ionic conductivity occurs by the creation of an amorphous phase or suppression of the crystallinity of polymer electrolytes. The many efforts are taken to reducing the crystallinity or creating an amorphous phase. This can be achieved by various techniques. They are polymer blending, co-polymerization, addition of nano llers and the incorporation of organic plasticizers [1][2][3][4][5][6]. The addition of organic plasticizers decreases both the crystallinity and polymer-polymer chain interactions in polymer electrolytes. It will enhance the segmental motion of the polymer backbone and generate free volume. The ions can easily migrate via the free volume; it will improve ionic conductivity of polymer electrolytes and the performance of DSSCs.
The researchers have fabricated DSSCs with the organic plasticizer doped polymer electrolytes and achieved considerable power conversion e ciency [7][8].
In this present work, one of the organic plasticizer benzoguanamine compound is chosen to prepare the polymer electrolytes with PVDF/KI/I 2 . The addition of the benzoguanamine compound in the polymer matrix is one of the routes to create the amorphous phase or decreased crystallinity of polymer electrolytes. This will increase the ionic conductivity of polymer electrolytes. The benzoguanamine compound consists of nitrogen atoms, which will form complex with I 2 . This complex formation avoids the sublimation of iodine and it will increase the lifetime of the DSSC [9]. The benzoguanamine compound is of low cost and it is also a new material for the electrolyte preparation in DSSC.

Preparation of Benzoguanamine-doped PVDF/KI/I 2 Electrolyte Films
The benzoguanamine-doped PVDF/KI/I 2 electrolytes were prepared by dissolving PVDF (0.3 g), KI (0.03 g), I 2 (0.006 g) and benzoguanamine (different weight percentages of 0%, 10%, 20%, 30%, 40% and 50% with respect to KI) in DMF (20 mL) under continuous stirring at 80°C until the homogeneous polymer electrolyte solutions were obtained. The obtained homogeneous polymer electrolyte solutions were poured into glass petri dishes. The DMF solvent was removed by using a vacuum oven at 60°C for 12 hrs.

Fabrication of Dye Sensitized Solar Cell
The solvents (acetone, ethanol and Millipore water) were used to clean the uorine doped tin oxide (FTO) substrates. The cleaned FTO substrates were treated on TiCl 4 treatment (FTO immersed with ice (50 ml), Millipore water (50 ml) and TiCl 4 (2 ml) at 70°C for 30 min) after that washed by ethanol and then sintered at 500°C for 30 min. TiO 2 paste (TiO 2 paste was made by adding binder materials such as 2propanol and triton X-100 with TiO 2 nanoparticles) was deposited on the FTO and then sintered at 500°C for 1 h. TiCl 4 treatment is done, followed by sintering at 500°C for 30 min. The photoanode is immersed in N719 dye solution (0.05 mM concentration, absolute ethanol is used as solvent) for 24 h. After that, working electrodes were obtained. The platisol was deposited (doctor blading technique) on FTO substrate and then sintered at 400°C for 30 min. After that, counter electrode was obtained. The semisolid benzoguanamine doped PVDF/KI/I 2 polymer electrolyte was dropped onto working electrode. It was kept for a few minutes for solvent evaporation and the counter electrode was placed over it. The sandwiched cell was put together with binding clips. The active area of DSSC is 0.5 cm × 0.5 cm. The photovoltaic (J-V) measurements are monitored at air mass (AM) 1.5 illumination using Keithley source meter with a light source of 150 W Xenon arc lamp (Science Tech, Canada) at 100 mW/cm 2 .

Characterization Techniques
The powder X-ray diffraction pattern was recorded using Bruker advance D8 powder X-ray diffractometer with CuKα radiation (λ = 1.54 Ǻ). The AC-impedance analysis was taken using an electrochemical workstation (Ametek, V3-500). The scanning electron microscopy images were taken using JEOL 6390 model. The photovoltaic measurements were recorded at air mass (AM) 1.5 illumination using Keithley source meter with a light source of 150 W Xenon arc lamp (Science Tech, Canada) at 100 mW cm − 2 .
The DSSC with 20% benzoguanamine doped PVDF/KI/I 2 electrolyte has the highest power conversion e ciency of 2.8%.