## 3 Results and Discussion
### Optical Properties  
The Al:CuO films were optically investigated by recording their transmittance as presented in Figure 6. It can be observed that with the increase in Al doping content, the transmittance was improved significantly from 15% for undoped CuO up to 37% for 0.50% Al, then continued to increase gradually for higher doping (inset of Figure 6), reaching around 48% in the visible wavelength region for 1.00% Al. The spectra recorded in transmission mode revealed two regions: (i) one that is of high transmission within the infrared region \(\geq\) 900 nm (ii) and a second region with a strong absorption in the visible range. As one may observe, the as-grown Al:CuO films display poor spectral transparency in the visible range. Next, in order to determine the energy of the optical band gap (\(E_{g}\)), the second region was used by adopting the Tauc relation [23]:  
\[(\alpha h\theta)^{2}=A\left(h\theta-E_{g}\right) \tag{5}\]  
Figure 4: Raman spectra of pure and Al–doped CuO thin films grown at 350 \({}^{\circ}\)C via spray pyrolysis on glass substrates. where \(A\) is a constant that characterizes the degree of disorder in an amorphous structure, \(\alpha\) is the coefficient of absorption, and \(h\
u\) is the energy of the photon. The absorption coefficient, calculated by using the Beer Lambert relation [23], is thus:  
\[\alpha=\frac{1}{d}\mathrm{ln}\bigg{(}\frac{100}{T}\bigg{)} \tag{6}\]  
where \(T\) represents the transmission for a thickness \(d\) of CuO film. By plotting \((\alpha h\theta)^{2}\) vs. \(h\theta\), the value of \(E_{g}\) represents the intercept with \(h\
u\)-axis of the straight line. To appreciate the influence of doping and more specifically the doping concentration on sub-band energies and sub-band populations, as well as band bending, several aspects had to be taken into consideration. The variation of the \(E_{g}\) value for Al:CuO films is shown in the inset of Figure 7 as a function of Al doping concentration. One may note that the value of the energy gap gradually decreases with increasing Al doping concentration from 3.7 eV in pure CuO up to 3.4 eV in 0.50 Al:CuO, then suddenly drops drastically up to 2 eV in 1.00 Al:CuO. This can be explained by the following: additional free electrons, reduction of grain size resulting in higher grain boundaries, narrowing of \(Eg\) in nanostructured films is associated with quantum confinement. In literature, similar results are reported [24; 25; 26; 27; 28]. Figure 5: Variation of the resistivity of pure and Al–doped CuO films grown at 350 \({}^{\circ}\)C via spray pyrolysis as a function of the Al concentration. Figure 6: UV-visible transmittance spectra of pure and Al–doped CuO thin films grown at 350 \({}^{\circ}\)C via spray pyrolysis as a function of the Al concentration. Figure 7: Typical \((\Delta h\theta)^{2}\) vs. \(h\upsilon\) plot of pure and Al–doped CuO films grown at 350 \({}^{\circ}\)C via spray pyrolysis as a function of Al concentration. [PERSON] presents the variation of the energy band gap \(E_{g}\) as a function of Al concentration. In brief, the influence of annealing and Al doping upon the optical, electrical and the structural properties of CuO thin films are correlated, presented and discussed. Raman spectroscopy analysis confirms the presence of vibrational bands characteristic of the CuO phase, while X-ray diffraction (XRD) confirms the polycrystalline structure of the pure CuO thin films. Furthermore, XRD analysis of the Al:CuO films indicates that the existence of a monoclinic-type structure (CuO phase) becomes favoured by a preferred alignment alongside the (002) plane with a significant grain size reduction with increasing Al content. Therefore, such a simple synthesis technique, which is both eco-friendly and cost-effective, providing pure or doped CuO thin films, has proven to be suitable in various high-tech applications.