## 1 Introduction  
Photodetectors (PDs), also known as photosensors, are devices designed to detect specific types of light (such as ultraviolet (UV), visible, and infrared) or other forms of electromagnetic radiation. PDs function as optical-to-electrical (O/E) converters that convert light from specific spectral regions into electrical output signals, such as voltages or currents, similar to photovoltaic devices [1, 2, 3, 4]. They are widely used in various applications such as video imaging, biomedical image sensing [5, 6, 7], optical communication [8], military-related systems [9], aerospace technologies [10], and industrial processes [11].  
To date, PDs have primarily been fabricated using inorganic semiconductor-based materials (e.g., silicon, III-V semiconductors, oxide or metal complexes, and alloys). However, inorganic semiconductor systems are fabricated under vacuum conditions, which involve high costs and complex manufacturing processes. In contrast, organic-based device fabrication processes offer advantages, such as low temperature and simplicity, compared with commercial semiconductor fabrication processes.  
Moreover, ABX3 perovskites have emerged as potential candidate materials for PDs because of their good optical (tunable wide bandgap and high light absorption) and electrical (high carrier mobility and long diffusion length) properties [12].  
Recent studies have explored the potential of perovskite materials for UV and deep-ultraviolet (UVC) PD applications. One study demonstrated a self-powered UVC PD based on a mixed-halide perovskite ((FAPbI3)0.97(MAPbBr3)0.03), which exhibited high responsivity (52.68 mA/W) and high response speed under 254 nm UV light without requiring an external power supply. The device also maintained stable performance after being stored in air for three weeks.  
In addition, the tunability of the perovskite bandgap has been leveraged to develop sensors with enhanced sensitivity to short-wavelength UV light, enabling improved detection precision and environmental stability. One study employed a fluoropolymer-coated CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\)\(\_\)Cl\({}_{\rm X}\) perovskite structure that achieved high responsivity (7.85 A/W), sub-microsecond response time, and long-term durability, maintaining stable operation even after 100 days of air exposure. A clear photoresponse at 254 nm was confirmed, and further evaluations across the UVA, UVB, and visible wavelength regions are planned for future studies.  
In particular, double-cation perovskites (FAMAPbI\({}_{3}\)), which incorporate formamidinium (FA) and methylammonium (MA) cations, have been extensively studied for UVC PD applications owing to their excellent optoelectronic properties.  
Despite their advantages, double-cation perovskites suffer from inherent stability issues, particularly under thermal stress and in humid environments. Methylammonium-based perovskites are prone to decomposition because of the volatility of MA cations, which leads to performance degradation over time. Additionally, FA-based perovskites, while offering better stability than their MA-based counterparts, still face challenges in maintaining structural integrity under prolonged exposure to heat and moisture [13; 14; 15; 16].  
In previous studies, various approaches have been attempted to address the stability issues of double-cation perovskites (FAMAPbI\({}_{3}\)). However, fully resolving their thermal and moisture stability limitations remains challenging. In this study, we introduced a triple-cation (CsFAMAPbI\({}_{3}\)) structure [17] and applied a two-step post-treatment process (methylammonium iodide (MAI)-isopropyl alcohol (IPA) treatment) to enhance the crystallinity of the structure and minimize its surface defects, thereby significantly improving stability compared with conventional methods [18].  
This study aims to demonstrate that a triple-cation perovskite-based UVC PD offers superior optoelectronic performance, enhanced stability, and improved durability compared with conventional double-cation structures.