Annealing effects on structural, optical and electrical properties of TiO2/FTO heterojunction

Nanostructure titanium dioxide (TiO2) thin films were grown by atomic layer deposition on florin-doped tin oxide (FTO) substrates at 200 °C of thickness 100 nm. The TiO2 films were annealed at 300–500 °C in air for a period of 1 h. The prepared thin films have been studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV–Vis spectroscopy, photoluminescence (PL) spectroscopy and current–voltage (I–V) properties. From X-ray diffraction patterns, as-deposited TiO2 thin films have a mixed anatase and rutile phases with FTO peaks. When the films were annealed the phase transformed to single anatase phase of the TiO2 films. The optical transmittance of as-deposited thin films is more than 58% and decreased with the increase in annealing temperatures. The energy band gap value was found to change with annealing temperature. The room-temperature PL spectra of TiO2 thin films show the band gap and defect peak at 409 nm and 420 nm with excitation at 325 nm. An enhancement of luminescence intensity is found due to change in the crystalline environment and reduction in oxygen amount with temperature. The value of ideality factor of prepared heterojunction is more than one, while the calculated values of saturation current, series resistance and barrier height altered with annealing because of defect-induced carrier capturing on interface between TiO2 and FTO.