## 1 Introduction
There are various synthesis methods to achieve nanoscale size; the difference between them lies in controlling the shape, size and other properties of the prepared nanostructured material. Several methods of preparation and/or deposition are well known, in particular sol gel, thermal evaporation, hydrothermal methods, plasma torch processing and chemical spray pyrolysis (CSP) [13; 14; 15]. In addition to the classical methods, other techniques exist, such as chemical vapour deposition (CVD), pulsed laser deposition (PLD), sputtering by magnetron (classic or pulsed) and electrochemistry. In our previous studies, we investigated the influence of the annealing process on the physico-chemical properties of CuO thin films grown via CSP. Several types of precursors (0.1 M) have been studied, such as copper chloride or copper acetate dissolved in an appropriate volume of water [14; 15]. A decrease of the band gap energy was observed, shown to be related to increasing crystallite size and finally explained by the quantum confinement effect. Here, we further advance by assessing the influence of doping effects, in particular by using aluminium. Doping is an often-used method in which one or more elements or compounds are doped into the substrate to generate specific electrical and/or optical properties in order to change material properties. Dopants are very often introduced to modify grain boundary energy. Furthermore, grain boundary mobility is slowed by segregated dopant ions, while the concentration of maximum dopant increases with surface-to-volume ratio of the grain and is therefore higher for nanosized grains. Above a given concentration, a new (dopant-rich) phase precipitates; however, this occurs at lower dopant concentrations as grain size increases [16]. Dopant diffusion has been studied extensively because of its importance in device fabrications. The importance of our research subject is therefore perfectly justified and is essentially based on the fact that the potential fields of application are wide. The question \"why\" has a very simple answer: studies in the literature and in particular those presenting calculations based on density functional theory, reveal that an impurity level is created within the energy gap of the structure due to Al doping, in agreement with our experimental results. The occurrence of Al ions within the CuO lattice has also been found to critically influence optical properties due to the formation of defects and due to host and dopant ions' electro-negativity difference. The bandgap energy of CuO, which is associated with resulting particle size and oxygen vacancies, decreases with the level of Al doping. Accordingly, we present and discuss the performance results of CuO thin films, both pure and doped (Al-doped, up to 1%), grown on cost-efficient, optical grade, borosilicate-crown (BK7) glass substrates via chemical spray pyrolysis. The thickness of CuO films grown at 350 \({}^{\circ}\)C and various annealing intervals reveals that it is proportional to annealing time. Previous studies have shown that the electrical, optical and morphological properties of CuO, as well as the roughness, crystallite size and phase purity of the synthesized films, are all influenced when the annealing takes place at temperatures between 300 and 500 \({}^{\circ}\)C. However, annealing of CuO films at higher temperatures (e.g., above 500 \({}^{\circ}\)C) is not recommended, since surface roughness increases due to growing crystallite size. Consequently, the influence of annealing time and Al doping on the electrical, optical and structural properties are highlighted. Analysis via Raman spectroscopy confirms the presence of vibrational bands characteristic of the CuO phase, while X-ray diffraction (XRD) confirms the polycrystalline nature of the thin layers of pure CuO.