## 3 Results and Discussion
### Optical, Electrochemical and Electrical Properties
Therefore, the UV rejection ratio \(R_{UV}\)/\(R_{vis}\) of the heterojunction photodetector was about 41 (S1a), 91.45 (S2a) and 41.66 (S3a), indicating that the n-TiO\({}_{2}\)/p-CuMnO\({}_{2}\) heterojunction could  
Figure 9: UV and Vis time-dependent photo-detective properties of the sensors at light exposure (ON/OFF = 20 s). work properly as a solar-blind UV sensor. The responsivity at visible light was calculated based on Equation (2) by replacing I\({}_{\rm uv}\) with I\({}_{\rm vis}\) (the photocurrent at visible light), due to the fact that visible light power is 0.2 mW/cm\({}^{2}\), and the resulting values were about 0.05 A W\({}^{-1}\) cm\({}^{2}\) (S1a), 1.1 A W\({}^{-1}\) cm\({}^{2}\) (S2a) and 0.03 A W\({}^{-1}\) cm\({}^{2}\) (S3a). The increases in the response and recovery speed for S2a and S3a is probably due to the pair of electron-hole in the recombination center. Thus, the increase in the velocity saturation during UV illumination determines the decrease in the resistance to the freely flowing UV-generated electrons in the channel [53]. In addition, the increase in the carrier densities for _S2_ and _S3_ that were highlighted in the Mott-Schottky analysis, which led to the increase in the recombination speed for the junction, further explains the increases in response and recovery speed. From the comparison of the physical parameters of the junctions, it can be seen that there is a close correlation between the ideality factor (\(n\)) and the sensitivity of the UV photodetector. Additionally, the sensitivity of S3a decreases almost proportionally to the difference in the S2a ideality factor. This means that approaching an ideal junction can lead to an improvement in sensitivity [54]. The best performance of the S2a sensor (sensitivity, responsivity, response and recovery speed) compared to S1a and S3a, was demonstrated by several parameters: crystallite size, porosity, layer thickness, band-gap energy values and flat-band potential. The thickness of the layer is a parameter that has a direct effect on the optical band gap. From the UV-Vis optical measurements (see Figure 6) it was observed that the optical band-gap energy of the Ti-TiO\({}_{2}\)-CuMnO\({}_{2}\) heterostructures (see Figure 6b,d) decreases proportionally with the thickness of the layer. The increase in the thickness of the TiO\({}_{2}\) layers determines the addition of new energy levels, thereby creating new \"cross-bridges\" from the electrons that are transmitted between the valence band and conduction band, which increases the electron lifetime and decreases the recombination rate of the electrons [55]. In addition, the roughness can improve the response and recovery speeds because of the increasing of the contact surface between TiO\({}_{2}\) and CuMnO\({}_{2}\), leading to the generation of more pairs of electron-holes. Furthermore, the enhanced sensibility and responsivity of the S2a sensor may be due to both the increased specific area [56] and the balance that was achieved between the size of the crystallites and charge-carrier concentration in the heterojunction interface [57; 58].