# Effect of Methylammonium Iodide (MACI) on MAPbI\({}_{3}\)-Based Perovskite UV-C Photodetectors
## 3 Results and Discussion  
### Crystallinity and Surface Morphological Properties of the Samples  
The thin films produced in this method are labeled with FABr 0, 5, 10, 15, 20, and 25 (mg) depending on the Br\({}^{-}\) mixture. Figure 2 shows the XRD patterns. This pattern indicated the formation of crystals in the thin film. Figure 2 shows the XRD patterns of bare MAPbl\({}_{3}\) coated at 4000 rpm, MAPbl\({}_{3}\) with the antisolvent process, and the MAPbl\({}_{3}\) thin film with 0.10 M MACI. As shown in Figure 2, sharp diffraction peaks are observed at 20 = 14.12\({}^{\circ}\) and 28.46\({}^{\circ}\), which are assigned to the planes (110) and (220), respectively. For the bare coating at 4000 rpm, the 12.5\({}^{\circ}\) (001) peak of PbI\({}_{2}\) was very large. This can be attributed to the formation of PbI\({}_{2}\) crystals owing to the thermal decomposition of CH\({}_{3}\)NH\({}_{3}\)I [26; 27; 28]. In the case of MAPbl\({}_{3}\), with the addition of an antisolvent process, the PbI\({}_{2}\) peak decreased rapidly compared with the bare MAPbl\({}_{3}\), and when 0.10 M of MACI was added, the PbI\({}_{2}\) peak became negligibly small. The sizes of the MAPbl\({}_{3}\)-coated crystal for each variable calculated using the Debye-Scherrer equation were approximately 38.02, 36.37, and 34.85 nm. The XRD peak attributed to the (110) crystal plane of the MAPbl\({}_{3}\) film with the addition of 0.10 M MACI shifted to a larger diffraction angle of 14.18\({}^{\circ}\). The lattice constant calculated using Bragg's law was 3.156 A at a diffraction angle of 14.12\({}^{\circ}\), and decreased to 3.1432 A at 14.18\({}^{\circ}\). The lattice constant is believed to decrease because of the addition of Cl, which has a smaller atomic radius. This suggests that, in this study, a UV PD with improved performance was fabricated by adding MACI to the precursor solution. Figure 2 illustrates that the crystallinity of the film steadily enhances with increasing FABr content in the MAPbl\({}_{3}\) precursor solution with the addition of 0.10 M MACI. The reduced peak intensity of FABr 25 indicates that the high density of FABr causes particle shrinkage, which negatively affects the crystalline film formation. The optimal crystallinity is observed for FABr 20. The optical properties are improved through the anion addition process, which means that the performance of PD is improved [29].  
Figure 1: (**a**) Schematic and (**b**) manufacturing process of MAPbl3-based deep-UV PD.  
Figure 3 shows the results of observing the surface morphology of each perovskite film using SEM. By adding MACI to the precursor solution, overall, a more stable surface formation was observed compared with that of the MAPbI3 thin film without the addition of MACI. As the post-treatment solution was added, the surface of the film was stabilized and particle formation was improved. This indicates improved photoelectric behavior, characterized by extended carrier lifetime and enhanced light absorption [30,31]. Figure 3a shows that in the case of the bare MAPbI3 film, the particle size is not uniform, pinholes are observed, and surface formation is unstable. Additionally, PbI2 residues are observed, which may be due to the dewetting phenomenon [32]. Therefore, the thickness of the thin film in the vertically structured device becomes unstable and deteriorates in quality. The thin film of MAPbI3 with the added anti-solvent process in Figure 3b shows a more uniform surface than Figure 3a. The MAPbI3 thin film with 0.10 M MACI added in Figure 3c forms large perovskite particles due to its high surface coverage, facilitating the movement of electrons and improving device performance. Consequently, Figure 3 shows that adding an anti-solvent process or adding Cl anions to MAPbI3 affects the structural change in the perovskite, resulting in a change in its properties [33].