<p>Cupric oxide (CuO) thin films were obtained from cuprous oxide (Cu₂O) precursor films deposited by spray pyrolysis and subsequently transformed by rapid thermal annealing (RTA) under a controlled atmosphere. Cu₂O films with different thicknesses were deposited at a low substrate temperature (280&#xa0;°C) using diethanolamine as a reducing agent, and then subjected to RTA to induce the Cu₂O-to-CuO phase transformation. X-ray diffraction and Raman spectroscopy confirmed that single-phase CuO films were obtained over a wide thickness range under optimized RTA conditions, independently of film thickness. Structural and morphological analyses showed that as thickness increased, crystallinity and density improved. Optical examination revealed a direct band gap of approximately 1.58&#xa0;eV, a high absorption coefficient (10⁵ cm^− 1), and a progressive decrease in the Urbach energy, indicating diminished structural disorder. Electrical experiments confirmed p-type conductivity and a notable reduction in resistivity for thicker CuO films. The results show that the combined spray pyrolysis and RTA method enables rapid production of high-quality CuO thin films with strong potential for solar cell and optoelectronic devices applications. In addition, the functional response of the CuO films to acetone vapor exhibited a clear thickness dependence, with a maximum response at intermediate thicknesses.</p> Graphical Abstract <p></p>