Spray Pyrloysed TiO2 Dye Sensitized Solar Cells

Thin film of Titanium dioxide (TiO₂) has generated a lot of interest because of its attractive properties such as wide band gap, high refractive index, high dielectric constant    and absence of non toxicity. TiO₂ thin films have been widely investigated for various optical applications and have attracted considerable attention for optical filter, gas sensor, ceramic membranes, photocatalysts and solar cells. Three crystallograpic modifications occur in TiO₂: Brookite, Anatase and Rutile. The anatase phase was formed when at growth temperature between 400°C – 700°C and then after 700°C the rutile was formed. The rutile phase has greater density (ρ=4.25 g/cm³) and refractive index (n=2.75 at λ=550 nm) than the anatase phase (ρ = 3.89 g/cm³ and n=2.54 at λ=550 nm).

The XRD profiles of TiO₂ films deposited on ITO substrates maintained at different substrate temperatures and annealed at 500ºC for 1h are polycrystalline in nature and oriented along (101), (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 intensity of the diffraction peaks gradually increases and full-width half maximum value increases with increase in substrate temperature and the observed peaks are in agreement with the reported value. The crystallite size is evaluated from the FWHM of the (101) plane using the Scherrer’s formula. The micro strain (ε) and dislocation density (δ) were also calculated. The effect of substrate temperature on the micro-structural parameters of TiO₂ indicates that the crystallite size of TiO₂ thin films can be tuned between 42 to 25 nm by varying the substrate temperature. It is observed that strain and dislocation density increases as there is decrease in the crystallite size.

Transmission spectra of the annealed TiO₂ films deposited at different substrate temperatures exhibit 30 to 50 % transmission which increases upto 70 %. The value of the absorption co-efficient(α) was calculated from the transmission spectra. Band gap of the films was calculated from the Tauc’s plot decreased from 3.45 eV to 3.20 eV with increase of substrate temperature. Similar behaviour was observed by earlier workers.

Gold ohmic contacts were evaporated on both the ends of the surface of the films. Linear I –V characteristics is observed. The room temperature resistivity of the films were 1x10⁷ ohm cm, 7x 10⁶ ohm cm, 4 x 10⁶ ohm cm and 1x 10⁶ ohm cm were obtained for the films deposited at 375°C, 400°C, 425°C, 450°C respectively. To reduce the resistivity of the films, they were annealed in vacuum at 450°C for 1 hour. The resistivity of the films decreased to 2 x 10³ ohm cm, 7 x 10² ohm cm, 3 x 10² ohm cm and 90 ohm cm for the films deposited at 375°C, 400°C, 425°C, 450°C respectively after vacuum annealing. The room temperature mobility and carrier concentration of the films after vacuum anneal varied in the range 10 cm²V^-1s^-1 to 3 cm²V^-1s^-1 and from 3.2 x 10¹⁵ cm^-3 to 2.31 x 10¹⁶ cm^-3 for the films deposited at 375°C, 400°C, 425°C, 450°C respectively.

Preliminary studies on Dye sensitized solar cells prepared using a combination of natural dyes (Strawberries, Hibiscus, Chlorophyll) in the ratio (1:1:1) as photo-sensitizer, showed the better photovoltaic performance compared with other single dyes. For the films deposited at 450°C and vacuum ennealed, V_oc of 0.46V, J_sc of 1.25 mA cm^-2, ff of 0.63, η of 0.36% were obtained for a cell area of 5 cm².