SYNTHESIS AND CHARACTERIZATION OF TIO 2 THIN FILM ELECTRODE BASED DYE SENSITIZED SOLAR CELL

Dye-Sensitized Solar Cells (DSSCs) are prominent alternative devices to conventional p-n junction silicon based solar cells because of their low fabrication cost and high-power conversion efficiency, good cost/efficiency ratio. In the present work, DSSC devices were made-up with fluorine doped tin oxide (FTO) glass substrate, a TiO 2 compact layer was deposited on FTO, Ruthenium (II) dye (N 719 ), an iodide - triiodide electrolyte and a platinum (Pt) counter electrode. Photo anode with thin film layers of TiO 2 and Pt counter electrode (photo-cathode) were prepared. Field emission electron microscope (FESEM) was employed to investigate the surface morphology of TiO 2 layers. The DSSC device efficiency was evaluated by J-V characteristics. Fabricated devices were exhibited high power conversion efficiencies. The electrochemical impedance characteristics were analyzed by fitting the experimental results to the corresponding electrical equivalent circuit simulated data.

Synthesis of Dye precursor solution. Dye solution was prepared by dissolving 36 mg N 719 dye in 100 ml absolute ethanol. Shake well until the solution dissolved properly.
Device Fabrication and characterization. A Systematic DSSCs consists of several majors parts like Photoanode, Dye, Electrolyte, and Photocathode which is mentioned in the diagrammatic structure of DSSC shown below as Figure 1. Firstly, for the synthesis of photoanode, FTO glass substrate was cleaned with DI water acetone and iso-propanol by ultrasonic bath process [15]. A scotch tape was paste on the conducting side of FTO then, TiO2 films were applied with the help of Doctor Blade technique. The deposited films were annealed at 470°C for 45 minutes in high temperature furnace. The TiO 2 films were deposited immersed in a dye solution for 24 hours in dark. The Pt also deposited on FTO substrate by using doctor blade method as a counter electrode and for 30 minutes annealed at 500°C. The TiO 2 films were formed as dye loaded photoanode and counter electrode as Pt deposited substrate [16,17]. Each sandwich cell was held in place with the help of two heavy duty clips on both opposites' sides of electrode. Liquid electrolyte was introduced between both [18]. Along with basic electron transfer processes in DSSC prospects of a state-of-art device based on dye adsorbed on TiO 2 and I -/I 3in the form of redox pairs electrolyte is shown in Figure 1(a). On illumination, an electron photo generated by dye molecules as like in photosynthesis [19]. By photon (hυ) absorption a dye molecule is excited whereas electron is excited from HOMO into LUMO subsequently, the free electron is injected into the TiO 2 conduction band and left the oxidized dye molecule. Then, the electron reach the Pt catalyst layer where redox reactions occur by the recombination with holes with in the electrolyte by reducing tri iodide (I 3-) to iodide ion (I -). However, the negative charge of I, in the final step, diffuses back to the dye molecules and it will react with the oxidized molecule. Thus, it completes the electrical cycle and repeats again. Besides this process some recombination happened like recombination of injected electron in the TiO 2 with either acceptors or oxidized dyes, which degrade the performance of the cell [20,21].

RESULTS AND DISCUSSION
The surface microstructure features of the film were characterized using Field Emission Scanning Electron Microscope (FESEM) as shown in the Figure 2. FESEM image of the TiO 2 film sample annealed at 470° C for 45 min. The image reveals a film that was crack-free, uniform and smooth on the surface. It also shows that the TiO 2 thin film possesses a nanocrystalline and nonporous structure which consists of nanoparticles.
The photovoltaic performance of the DSSCs based on Titanium Dioxide thin film photoanode were investigate under a simulated solar irradiation of 100mW cm -2 (AM 1.5G) [22,23]. Figure 3(a) display the measured J-V characteristics of solar cell based on TiO 2 films and 3(b) shows different efficiencies of DSSC based on wavelength. The corresponding photovoltaic parameters short circuit current (J sc ), fill factor (FF), open circuit voltage (V oc ), and conversion efficiency (η) for DSSCs for the TiO 2 films has been calculated. We have found the values of cell parameters such as; V oc , J sc , FF, I max , V max , P max and efficiency are of 0.60V, 3.337mA/cm 2 , 56.13%, 0.00289A, 0.4704V, 1.36 and 1.13%. The performance of cell is considerable at low cost DSSC technology available today [24]. On other hand we have found different values of efficiency at different range of wave length as mentioned in figure 3(b), and are continuously increase. From all the parameters we have measured overall highest efficiency of fabricated cell is 1.5% at 900-1100 nm range of wavelength. Photovoltaic performances are influenced by the dye adsorption solvent [25]. The adsorption behavior of dye is significant at this wavelength. Electrochemical impedance spectroscopy (EIS) is a beneficial technique to investigate the internal impedance of photovoltaic device [26], such as DSSC. Electrochemical interface of split in three parallel circuits, additionally the equivalent circuit of DSSC is related to the experimental result of the Nyquist plot as display in Figure 4  In Nyquist plot equivalent circuit in the horizontal axis represent the serial resistance (R 1 ) between wire and substrate, from the impedance spectra represent resistance (R 2 ) and (R 3 ) at interface between electrolyte and Pt counter electrode and as well as between electrolyte and TiO 2 film. On the other side at the interface C 1 shows the double layer capacitance between electrolyte and Pt counter electrode and as well as between electrolyte and TiO 2 film [27,28]. The values of all the parameters of Nyquist plots of DSSC based on TiO 2 photoanode with the equivalent circuit are summarized in Table.    In the bode plot of cell based on photoanode TiO 2 under the solar irradiation, the frequency was shifted in higher frequency region with the TiO 2 photoanode as shows in figure 4(b).According to curve recombination charge transfer processes at the TiO 2 /dye/electrolyte interface is low and enhance the electron transmission [29,30]. On the other hand illumination condition f max is inversely proportional to the electron transport time as τ s =1/ (2πf max ), increment in f max shows the increased rate of charge transport process in DSSC [24]. Hence, with the reduced value τ s of indicates that the electrons reaches at FTO electrode at a faster rate, so that we can improve the performance of DSSC by further modification.

SUMMARY AND CONCLUSIONS
We have successfully fabricated and characterized Dye sensitized solar cell based on TiO 2 photoactive layer. Which shows the PCE of 1.13%, J sc of 3.337mA/cm 2 , V oc of 0.60V and FF of 56.13%, and overall highest PCE is 1.5% on 900 to1100 nm wavelength ranges. The performance of cell is good at simple laboratory condition, but we can improve photovoltaic performance by using this methodology. The EIS analysis observes that the resistance between electrolyte and TiO 2 film increase, which indicates that a larger resistance at the interface between TiO 2 film and electrolyte is beneficial for suppression of charge recombination. Hence, we can achieve enhancement in performance of DSSC by modification in TiO 2 (like doping and annealing etc) and Dye and electrolyte (natural sources achieved by immediate environment).