The wavelength dependences of the optical transmittance spectra of the AZO films subjected to the three different substrate temperatures are plotted in Fig.6.All of the AZO films show an average transmittance of 82.3–88.8% in the visible wavelength range from 400 to 800 nm and a sharp fundamental absorption edge in the UV region, which is suitable for display devices and solar cells as transparent electrodes [28]. It seemed that the transmittance decreased and a red shift of the absorption edge occurred with increasing the substrate temperature. The transmittance may be reduced by increasing the surface bonding, which absorbs visible photons [29]. The absorption edge is related to the optical band gap, which can be calculated from the relationship between the optical absorption coefficient(a) and the transmittance (T). The optical band gap of a direct band gap semiconductor can be determined by using Tauc model [30]: