## 3 Results and Discussion
### Characterization of Triple-Cation Perovskite Thin Films
#### 3.1.1 Structural Analysis of Triple-Cation Perovskite Thin Films
The effective ionic radius of the A-site cation (\(r_{\rm A}\)), a mixture of Cs\({}^{+}\), FA\({}^{+}\), and MA\({}^{+}\), was determined based on XRD analysis. Similarly, the B-site cation (\(r_{\rm B}\)) and halide anion (\(r_{\rm x}\)) radii were derived from the XRD data. Generally, perovskites are considered structurally stable in the cubic or slightly distorted cubic phase when 0.9 \(\leq t\leq\) 1.0 [24]. The calculated tolerance factors for the bare and Cs-doped samples (Table 1) indicate that all the samples remained within the stable perovskite structure region. Even with an increase in the Cs content, the tolerance factor remained within a stable range. As confirmed by XRD analysis, despite Cs\({}^{+}\) having a smaller ionic radius than FA\({}^{+}\) and MA\({}^{+}\), the incorporation of Cs does not compromise the stability of the structure. The crystal structure remained stable even with an increase in the Cs content. The octahedral factor is defined as follows:  
\[\mu=\frac{r_{B}}{r_{X}}\]  
The values of \(r_{B}\) and \(r_{X}\) for the Pb-I system were derived from XRD analysis, and the octahedral factor (\(\mu\)) was calculated, showing similar values of 0.54 across all the Cs-doping conditions (Table 1). This suggests that A-site cation substitution does not significantly affect the structural stability of the PbI\({}_{6}\) octahedra [24]. These results demonstrate that adjusting the tolerance factor through Cs doping can optimize the perovskite crystal structure while maintaining the octahedral factor within a stable range. Therefore, the structural stability of the material was preserved, confirming the feasibility of the proposed Cs-doping strategy for fabricating perovskite thin films with structural properties suitable for UVC PD applications. Scanning electron microscopy (SEM) analysis was used to further investigate the structural characteristics of the mixed-cation perovskite films, as shown in Figure 3. The SEM images revealed the morphological evolution of the perovskite films with increasing Cs content. The average grain size increased with an increase in the Cs content, reaching a maximum value at Cs = 0.05. Compared with the pristine FAMAPbI\({}_{3}\) film, which exhibited a smaller grain size, the Cs-incorporated films showed a noticeable increase in the grain size, particularly at Cs = 0.05, as can be clearly observed from the SEM images shown in Figure 3.  
However, when the Cs content exceeded 0.05 (Cs = 0.07 and 0.10), the grain size decreased. This reduction in the grain size at higher Cs concentrations can be attributed to excessive lattice contraction and an increase in the number of nucleation sites. The smaller ionic radius of Cs\({}^{+}\) (1.81 A) compared with that of FA\({}^{+}\) (2.53 A) induces lattice shrinkage, leading to internal strain that hinders further crystal growth. Additionally, excessive Cs incorporation can increase the number of nucleation sites, limiting continued grain growth [25; 26]. The SEM images indicated that, while Cs incorporation promoted grain growth up to a certain level, an excessive Cs content resulted in reduced grain size. Thus, careful  
Figure 3: SEM images of mixed-cation perovskite with varying Cs concentrations: (a) bare (FAMAPbI\({}_{3}\)), (b) Cs0.03, (c) Cs0.05, (d) Cs0.10. optimization of the Cs concentration was essential to achieve the desired balance between structural stability and grain morphology for optimal device performance. In particular, the Cs0.05 composition represents an optimal balance between lattice stabilization and lattice distortion. Excessive Cs incorporation increases lattice strain, which can introduce defects and non-uniformities at the grain boundaries, ultimately leading to performance degradation. This interpretation is supported by previous studies showing that Cs incorporation positively affects band alignment, charge transport, and defect passivation [27].