The optical absorption of Fe-doped ZnO thin films in the wavelength range of 300–800 nm was investigated, as depicted in Fig. 4. It shows that Fe-doped ZnO thin films have very low absorbance in the visible region of the spectrum and a strong absorbance for wavelengths below 350 nm. The observed dramatic increase in absorption for wavelength between 350 and 400 nm has been attributed to the fact that intrinsic absorption originated from direct transition of electrons [24]. Moreover, the absorbance increases with the increase in Fe-doping which might be due to the fact that lots of defects introduced by Fe dopant would enhance the absorption of films and this is especially conspicuous in the lower wavelengths below 400 nm. This shifting of the absorption edge towards higher wavelengths (red shift) is due to the decrease in band gap caused by defects and crystallite size. The band gap of these films can be gained by the following formula: