Spray Pyrolyzed TiO2 / Sol Gel Dip Coated NiO Dye Sensitized Solar Cells

TiO₂ films were deposited on ITO coated glass substrates using spray pyrolysis technique. 0.1 M of Ti(acac) was dissolved in ethanol and sprayed onto indium tin oxide (ITO) substrates with dimensions of 75x25 mm² at different substrate temperatures (T=375, 400, 425 and 450ºC) and annealed at 500 ºC for 1h in air. The substrates were first cleaned with water, followed by dipping in con.HCl, acetone and ethanol successively. Finally the substrates were rinsed in deionised water and allowed to dry in a hot air oven. NiO films of different thickness were deposited by sol gel dip coating adopting the following procedure. 20 ml of  0.1 M of nickel nitrate was taken in a beaker, to this acrylamide, n,n, bis acrylamide solution was added. The pH was adjusted to 9 and a few drops of ammonium persulphate was added, just before gellation, ITO substrate coated with TiO₂ films were immersed in the above mixture and withdrawn at a slow rate. The coated film was dried for 15 min and then post heat vtreated in air at different temperatures ( T = 475,500 and 525°C) for 1 hour. For characterization of the films, individual films of TiO₂ and NiO films deposited under identical conditions and of the same thickness were used. With increase of heat treatment temperature, the thickness decreased and the crystallinity improved.

The XRD profiles of the annealed TiO₂ film is polycrystalline in nature and oriented along A (101) and A (200) planes, A (101) being the predominant peak. The films exhibited tetragonal crystal structure and the peak intensities are in agreement with the JCPDS data (No. 21-1272). The heat treatment NiO films indicated that the patterns show (1 1 1), (2 0 0) and (2 2 0) crystal planes of bulk NiO. For dense films obtained from Et/Bu, all peaks exhibit only cubic NiO structure from JCPDS 04-0835 with (1 1 1) preferential orientation.

The optical transmittance of NiO films increased with annealing temperature due to the increase in crystallite size and decrease in defects. At a wavelength of 550 nm. The E_g value reduced from 3.76 to 3.56 eV with increasing heat treatment temperature. Band gap of the TiO₂ films was calculated from the Tauc’s plot, the band gap decreased from 3.45 eV to 3.20 eV with increase of substrate temperature.

TiO₂ and TiO₂/NiO electrodes were immersed in an N719 solution for 24 h to adsorb sufficient dye. Pt was used as the cathode of the DSSC, and clamped together with the TiO₂/NiO anode for cell testing. The electrolyte solution was dropped between the electrodes. The I-V characteristics was measured at 100 mW cm^-2 illumination.  LiI and Iodine was used as electrolyte. Bare TiO₂ based DSSC cells exhibited a V_oc of 450 mV and J_sc of 3.2 mA cm^-2, these values increased to 690 mV and 7.0 mA cm^-2 for the TiO₂/NiO electrode post heat treated at 525°C. For the TiO₂/NiO electrodes post heat treated at lower temperatures the values decreased.