The effect of annealing and different molar concentrations on the structural and optical properties of zinc selenite thin films \((\text{ZnSe})\) were studied, as they were prepared using (ZnCl2 & Na2SeO3) compounds. It was deposited by spray pyrolysis technique on glass substrates with different molar concentrations ((0.1 − 0.075 − 0.05) \(\text{mol}/\text{L}\) ). Under a base temperature of (200 °C), it was annealed at (450 °C). The results of the X-ray diffraction pattern (XRD) showed the emergence of the structure of the cubic phase and that the preferred and dominant phase in all thin films is (200), and the increase in the crystal size of the thin films ZnSe after annealing, while the results of the diagnosis of (FE-SEM) to the presence of particle sizes within (55–83 nm) before annealing and these sizes decrease after annealing, where it approximates about (38–68 nm), while the analysis of (EDX) showed an increase in the proportion of (Zn & Se) with an increase in molar concentration After annealing, an increase in the elements was demonstrated with a decrease in selenium (Se) due to its conversion to (SeO2). The results of the optical properties of the different molar concentrations before annealing were that the absorbance increases with increasing concentrations and that the greatest amount of absorbance is at the wavelength (320 nm). Still, after annealing, we also see an increase in the absorbency of the molar concentrations. It was found that the edge of optical absorption varies with increasing film thickness. The band gap increased due to the variation of particle size in the thin film regions after annealing.
