High transparency is one of the most important factors for the application of ZnO thin films as TCOs. Fig. 7 presents the optical transmittance spectra with wavelengths from 200 to 1100 nm of the NiAl:ZnO films. All films showed a high average transmittance more than 80% and the transmittance decreased in the visible region with increasing annealing temperature from 450 to 600 °C. As SEM images revealed that the roughness of the NiAl:ZnO films increased with increasing the annealing temperature, the decrease in transmittance of the films at higher annealing temperature may due to the scattering effect from the rough surfaces. Tneh et al.  reported that surface roughness strongly affects the transmittance of ZnO based thin films. Sengupta et al.  showed the same trend for the decreasing transmittance of ZnO films with increasing annealing temperature from 400 to 700 °C. In addition, all NiAl:ZnO films exhibited interference fringes of the spectra with shallow valleys in the visible and near infrared regions due to an interference phenomenon between the top and bottom surfaces of the film . Besides, the SB-505124 edge was blue-shifted as the annealing temperature was increased. To explain this phenomenon, the band gap of the films was investigated. Fig. 8 shows the band gap estimated from the absorption edges of the samples annealed at different temperatures. The absorption edge for a direct interband transition can be estimated from the following equation:equation(4)αhv=C(hv−Eg)1/2αhv=Chv−Eg1/2where C is a constant for a direct transition, and α is the optical absorption coefficient . The optical energy gap, Eg, can then be obtained from the intercept of (ahv)2 vs. hv for a direct transition. The energy bang gaps were obtained by extrapolating the linear absorption edge part of the curve using Eq. (4), as shown in Fig. 8. The band gaps of the samples increased with increasing annealing temperature from 450 to 600 °C, reaching a maximum of 3.55 eV at 600 °C. The band gaps of the NiAl:ZnO films are determined as 3.30, 3.31, 3.42, and 3.55 eV for 450, 500, 550 and 600 °C annealed samples, respectively. Dutta et al.  reported the observation of an increase in band gaps with an increase of ZnO grain size due to the less band bending at grain boundaries. In a similar study, Vinodkumar et al.  reported the enlargement of band gap with increase in grain sizes in Cd-doped ZnO films with increasing annealing temperature. In nanocrystalline, smaller grain sizes have a higher surface to volume ratios, which cause more band bending effect at the grain boundaries and the band edge becomes flatter than that of larger grains. Whereas in bigger grains, the band edge becomes sharper and band gap can be wider than that of smaller grains  and . Therefore, the blue shift of band gaps of the NiAl:ZnO films can be attributed to the decrease in band bending effect at the grain boundaries, owing to the larger grain sizes with increasing annealing temperature (Table 1 and Fig. 2).