The results in Figures 2 and 3 prove that the surface morphologies and crystalline structures of the bilayer NiO/TZO thin films are dominated by the TZO thin films. For that, the transmittance rate of the NiO/TZO heterojunction bilayer thin films is also dominated by the TZO thin films and will be higher than that of the NiO thin film. All of the NiO/TZO heterojunction JIB04 nmr diodes showed a sharp absorption edge, but they did not exhibit the blueshift phenomenon
as the deposition power of the TZO thin films increased. Compared with other research, the NiO/TZO heterojunction diodes obtained in this study have the highest transmittance, even higher than that of deposited NiO thin films. The corresponding find more optical bandgap (E g ) was determined by applying the Tauc model and the Davis and Mott model  using Equation 4: (4) where α is the optical absorption coefficient, c is the constant for direct transition, h is Planck’s constant, and υ is the frequency of the incident photon. Figure 8b shows (αhυ)2 vs. hυ for the NiO/TZO heterojunction diodes. Their E g values increased when the deposition power of the TZO thin films increased from 75 to 125 W. The variations in E g values roughly agree with those of the carrier concentrations shown in Figure 3. Figure 8 NiO/TZO heterojunction diodes. (a) Transmittance and (b) αhυ 2 vs. E g plots of NiO/TZO heterojunction diodes. Figure 9 shows the
I-V characteristics of the NiO/TZO heterojunction diodes. The nonlinear and rectifying I-V characteristics confirmed that a p-n junction diode DMXAA was successfully formed in the NiO/TZO heterojunction structure. In the forward bias condition, the turn-on voltages of the NiO/TZO heterojunction diodes were about 2.57, 1.83, and 2.05 V as the deposition powers of the TZO thin films were 100, 125, and 150 W, respectively. The turn-on voltage of the NiO/TZO heterojunction diodes decreased as the deposition power increased from 75 to 125 W; then, it increased with a 150-W deposition power. As the deposition power increased from 75 to 125 W, the resistivity
linearly decreased (Figure 3), causing the decrease in turn-on voltage. However, even though TZO thin films deposited at 150 W have lower resistivity, the increase PJ34 HCl in turn-on voltage is due to the greater roughness of the TZO thin film (Figure 2d) and the defects that exist between the p-n heterojunction interfaces of the NiO and TZO thin films. In addition, the forward currents of the NiO/TZO heterojunction diodes abruptly increase when the turn-on voltages are over 2.57 V (deposition power 100 W), 1.83 V (125 W), and 2.05 V (150 W), which demonstrates that the I-V curves are a characteristic of a typical p-n junction diode. For TZO thin films deposited at 75 W, the symmetrical I-V curve of the NiO/TZO heterojunction device is not a typical characteristic of a p-n junction diode.