Optical Band Energy, Urbach Energy and Associated Band Tails of Nano Crystalline TiO<sub>2</sub> Films at Different Annealing Rates

Abstract

Increase in world population has led to more energy demand. Therefore, there is need for utilization of green and renewable energy. Dye sensitized solar cells (DSSCs) based on TiO₂ have attracted a lot of attention as an alternative source as compared to current silicon technology. In this study, TiO₂ thin films were deposited on doped fluorine tin oxide layer (FTO) glass substrates using sol-gel doctor blading technique. The films were annealed at different rates (1step, 2°C/min and 1°C/min) up to a temperature of 450°C followed by sintering at this temperature for 30 minutes. UV-VIS spectrophotometry was employed to probe the absorbance and reflectance of the films. It was found that, the optical parameters, such as the reflectance, the real (ε₁) and imaginary (ε₂) parts of dielectric constant, skin depth, Urbach energy and the energy gap; all depend on the annealing rate. The skin depth for the samples in visible region were found to increase from 6.319 x 10^-5 to 11.968 x 10^-5 cm^-1 due to annealing. The Optical band energy (Eg) decreased from 5.04eV for as deposited film to 4.35eV at annealing rate of 1°C/min for direct allowed and from 2.76 to 2.56 eV for indirect transitions. Urbach tails in weak absorption region decreased with annealing rate. Urbach energies (Eu) were in the range of 432-505 meV for as deposited and annealed films. This was used to account for the disorder of the films. An inverse relation between Urbach energy and optical band energy as result of annealing rate was reported.