Thermal Annealing Effects on the Optical, Electrical, and Surface Characteristics of Electrodeposited Aluminium-Doped TiO₂ Thin Films

The thermal response of aluminium-doped titanium dioxide (Al:TiO₂) thin films is important for tailoring their optical and electrical properties for functional applications. In this study, Al:TiO₂ thin films were electrodeposited on indium tin oxide (ITO) glass substrates under constant precursor and deposition conditions and annealed at 200, 300, 400, 500, 600, and 700 °C for 60 min. The nominal Al concentration and deposition parameters were maintained constant to isolate the effect of annealing temperature. Optical properties were evaluated using UV–visible spectroscopy, electrical conductivity by the four-point-probe technique, and surface morphology by scanning electron microscopy (SEM). The films exhibited high visible-light transmittance, generally exceeding 80%, with values of approximately 91–93% between 450 and 800 nm. The apparent optical band gap decreased from 3.00 eV at 200 °C to 2.85 eV at 700 °C, accompanied by a red shift in the absorption edge. Electrical conductivity increased from 1.263 S cm⁻¹ at 200 °C to 3.609 S cm⁻¹ at 700 °C, while resistivity decreased from 0.792 to 0.277 Ω cm. SEM revealed temperature-dependent changes in particle development, interparticle contact, and surface organization. These results demonstrate that post-deposition annealing effectively regulates the optical, electrical, and morphological characteristics of electrodeposited Al:TiO₂ films. The combination of high optical transmission and enhanced conductivity supports their potential for transparent optoelectronic, sensing, photoelectrochemical, and photovoltaic-related structures. Complimentary structural, chemical, and device-level studies are required to clarify the mechanisms underlying these changes and establish practical device performance.