## 3 Results and Discussion
### Optical, Electrochemical and Electrical Properties
Figure 8a-c presents the \(I\)-\(V\) measurements at different immersion times in the dark state, UV and Vis illumination. The dark state and UV light showed reduced turn-on voltages from 0.858 V to 0.513 V (for S1a), 0.980 V to 0.791 V (for S2a), and from 1.310 V to 0.812 V (for S3a). In addition, an increase in the current in the forward bias between dark and UV illumination, from 0.24 mA to 0.41 mA at S1a, from 10 mA to 13 mA at S2a, and from 0.11 mA to 0.24 mA for S3a, was noted. This can be attributed to the narrowed heterojunction interface that is generated by UV excitation [42; 43]. In addition, it can be seen that the turn-on voltages increase with an increase in corrosion time, which is likely a result of the growth of the surface area. The higher specific surface area of the \(n\)-TiO\({}_{2}\)/\(p\)-CuMnO\({}_{2}\) heterojunction can enhance light-trapping processes and increase the charge-carrier concentration in the depletion region, as well as the carrier lifetime [44; 45]. The slightly higher value of the dark current in the case of the S2a sensor is possibly due to the injection of free electrons from the reverse bias. Because the ideality factor \"\(n\)\" is greater than 2, our junction does not block the total current on the reverse bias, thereby generating free electrons and conducting current on the forward bias [46]. The decrease in the reverse saturation current \(I_{0}\) (Tabel 3) for this junction supports our theory. The \(I\)-\(V\) characteristics in the visible spectrum show the variation of turn-on voltages as 0.7 V (S1a), 1.09 V (S2a), and 1.131 V (S3a) (Figure 8a-c). A small increase in the current at visible illumination compared to the current in the dark state, which is due to the generated photoexcitation, can also be observed. The inset of Figure 8a-c shows that the \(I\)-\(V\) characteristics for the TiTiO\({}_{2}\) and CuMnO\({}_{2}\) interface present a linear behavior, which indicates the ohmic nature of the contacts [47]. Figure 8d-f show the semi-logarithmic forward and reverse biases for the S1a, S2a, and S3a UV sensors. An increase in asymmetry between the forward and reverse biases can be noted, suggesting that all heterojunctions work at a higher rectification upon photo-illumination [48]. The electrical parameters of the \(n\)-TiO\({}_{2}\)/\(p\)-CuMnO\({}_{2}\) heterojunction both in the dark and under UV illumination conditions are shown in Table 3, where \(V_{T}\) is the turn-on voltage, \(I_{F}\) is the current under forward bias, \(I_{R}\) is the current under reverse bias, \(n\) is the ideality factor, and \(I_{0}\) is the reverse saturation current. The low values of the turn-on voltage of the heterojunction show that the junction has a relatively low power consumption. In order to evaluate the ideality factor of the \(n\)-\(p\) junction in dark conditions, the thermionic emission current-voltage relation (ideal diode Equation (1)) was used,  
\[I=I_{0}\left[\textit{exp}\left(V/nkT\right)-1\right] \tag{1}\]  
where \(I_{0}\) (reverse saturation current), \(V\) (applied voltage), \(n\) (ideality factor), \(k\) (Boltzmann's constant) and \(T\) (operation temperature). The ideality factor (\(n\)) is calculated in the dark by plotting log(\(I\))-\(V\), which is the slope of the straight-line region of the forward bias that is equal to \(q\)/\(nkT\) (electronic charge \(q=1.6\times 10^{-19}\) C, and operating temperature \(T=298\) K). The reverse saturation current (\(I_{0}\)) is found from the intercept of the straight-line region of the forward bias of the logarithmic current against the voltage, which gives us log(\(I_{0}\)) [41]. These parameters are presented in Table 3.  
The Sah-Noyce-Shockley formula for a classical \(p\)-\(n\) junction give an ideality factor value between 1 and 2 [49]. However, the theoretical model cannot account for the ideality