## 3 Results and Discussion
### Optical, Electrochemical and Electrical Properties
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample** & **Type** & \(\boldsymbol{V_{T}}\) **(V)** & \(\boldsymbol{I_{F}}\) **(A)** & \(\boldsymbol{I_{R}}\) **(A)** & \(\boldsymbol{n}\) & \(\boldsymbol{I_{0}}\) **(A)** \\ \hline \multirow{3}{*}{S1a} & Dark & 0.858 & 0.24 \(\times\)\(10^{-3}\) & 0.14 \(\times\)\(10^{-3}\) & 3.95 & 1.66 \(\times\)\(10^{-9}\) \\ \cline{2-7}  & UV & 0.513 & 0.41 \(\times\)\(10^{-3}\) & 0.22 \(\times\)\(10^{-3}\) & - & 1.82 \(\times\)\(10^{-9}\) \\ \hline \multirow{3}{*}{S2a} & Dark & 0.980 & 10 \(\times\)\(10^{-3}\) & 3 \(\times\)\(10^{-3}\) & 3.40 & 0.013 \(\times\)\(10^{-9}\) \\ \cline{2-7}  & UV & 0.791 & 13 \(\times\)\(10^{-3}\) & 4 \(\times\)\(10^{-3}\) & - & 0.025 \(\times\)\(10^{-9}\) \\ \hline \multirow{3}{*}{S3a} & Dark & 1.310 & 0.11 \(\times\)\(10^{-3}\) & 0.16 \(\times\)\(10^{-3}\) & 6.48 & 0.55 \(\times\)\(10^{-9}\) \\ \cline{2-7}  & UV & 0.812 & 0.24 \(\times\)\(10^{-3}\) & 0.14 \(\times\)\(10^{-3}\) & - & 3.93 \(\times\)\(10^{-9}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Electrical parameters of the \(n\)-TiO\({}_{2}\)/\(p\)-CuMnO\({}_{2}\) heterojunction. factors greater than 2 that were found in our case [50]. Table 3 shows the obtained ideality factor that depends on corrosion time. A small difference regarding the ideality factor between the S1a and S2a sensor types, 3.95 versus 3.40, and a significant increase in the value for the sensor type S3a to an ideality factor equal to 6.48 were seen. This means that a high increase in the corrosion time can move the junction away from an ideal one. The high value of \(n\) suggests that the transport mechanism of the \(n\)-TiO\({}_{2}\)/p-CuMnO\({}_{2}\) heterojunction is not determined by the thermionic emission from the Sah-Noyce-Shockley theory, but that it results from several factors: (i) defect-assisted tunnelling; (ii) the accelerated recombination of electrons and holes in the depletion region; (iii) the existence of high-density surface states, or combinations of these mechanisms [39; 51]. Figure 9 shows the photo-detective properties under UV and Vis light exposure of the heterojunction device at a bias of 2 V. The switching of the photocurrent from the \"ON\" to the \"OFF\" state could be achieved reproducibly by turning the UV illumination. The performance of the sensors was measured under UV (0.1 mW/cm\({}^{2}\)) in the atmospheric environment. The response time of the heterojunction sensor upon UV lighting depends on the corrosion time, and it was about 6.29 s (S1a), 1.56 s (S2a) and 0.58 s (S3a), but only S1a required a recovery time of about 4.89 s. This improvement of the response and recovery speed that depend on the parameters of the production process resulted from the increase in surface area favoring light trapping. The photocurrent measurement in the dark was about 0.057 mA (S1a), 2.52 mA (S2a), and 0.095 mA (S3a), and under UV illumination for 20 s, the current increases to 0.262 mA (S1a), 12.58 mA (S2a), and 0.22 mA (S3a). The sensitivity of the sensor was calculated as a ratio between photocurrent to darkness (\(I_{UV}\)/\(I_{DARK}\)), which was about 4.89 for S1a, 4.99 for S2a, 2.31 for S3a. The responsivity values were calculated using Equation (2) [48]:  
\[R=(I_{UV}-I_{DARK})/P_{opt}.S \tag{2}\]  
where \(I_{UV}\) is the photocurrent at UV lighting, \(I_{DARK}\) is the current in the dark, \(S\) is the effective area (1 cm\({}^{2}\)) and \(P_{opt}\) is the optical power of the UV source. From this equation, knowing that the light power is 0.1 mW/cm\({}^{2}\), the responsivity values of the as-prepared \(n\)-TiO\({}_{2}\)/p-CuMnO\({}_{2}\) heterojunction sensors are 2.05 A W\({}^{-1}\) cm\({}^{2}\) (S1a), 100.6 A W\({}^{-1}\) cm\({}^{2}\) (S2a) and 1.25 A W\({}^{-1}\) cm\({}^{2}\) (S3a). The rejection ratio between UV and visible light was calculated by the division of the maximum responsivity in the UV range to the responsivity at visible light [52].