# Effect of Methylammonium Iodide (MACI) on MAPbI\({}_{3}\)-Based Perovskite UV-C Photodetectors
## 1 Introduction  
Recently, ultraviolet-light (UV) or deep ultraviolet-light (DUV) photodetectors (PDs) have garnered significant interest from many scientists or experts due to their ob-jective in various optoelectronics devices, for example, solar UV monitoring, medical im-age analysis, covert communication (CC), lithography aligners, photodetector, secure space-to-space communications, and detection of missiles [1; 2; 3]. Generally, UV-light rays are divided into three parts depending on the wavelength range: UV-A (\(\sim\)400-320 nm), UV-B (\(\sim\)320-280 nm), and UV-C (\(\sim\)280-100 nm) [1]. From the Sun, UV radiation is directly irradiated to the Earth at all wavelengths but a little UV light is allowed to the Earth's surface due to the ozone layer surrounding the Earth. Before most UV-B and UV-C rays reach the surface of Earth, the ozone layer absorbs all of them. However, some UV-A reaches the surface of Earth despite the ozone layer and other protective layers [4]. Sometimes UV-C can be observed from the corona discharge sur-rounding wire or insulator in the electric power poles, lightning strikes, or artificial light sources (lamp or laser) [5]. When short-wavelength ultraviolet rays of 300 nm or less reach human skin, they penetrate into the epidermis and have adverse effects on skin diseases, eye diseases, and the immune system, emerging as a hazardous substance to the human body [6; 7]. Therefore, studies on PDs have detection functions for the UV-C region and need conversion process that changes from the irradiated UV-C light to an electrical signal. In previous studies, methods using semiconductor materials with a high bandgap (>3.0 eV) including AlGaN, MgZnO, Ga\({}_{2}\)O\({}_{3}\), ZnGa\({}_{2}\)O\({}_{4}\), and MoS\({}_{2}\) were attempted. However, these semiconductor materials should be synthesized at high temperatures in sophisti-cated equipment [8; 9; 10]. As an alternative, silicon (Si) photodiode that areequipped with a UV filter have been studied for UV-C sensor due its narrow bandgap, but excellent per-formance cannot be expected in UVC detection of Si diodes because light-generated carri-ers do not reach the depletion layer [11; 12]. In contrast, low-temperature solution pro-cessing is increasingly applied using perovskite materials compared with other semi-conductors [13; 14]. Perovskite has a notable absorption coefficient of nearly \(10^{5}\) cm\({}^{-1}\) in the UVC spectral range and exhibits high mobility. This suggests that perovskite can exhibit higher sensitivity and faster response as a UV-C sensor than other sensors [15; 16]. In recent research, the organic-inorganic hybrid perovskite used as a light absorber due to its excellent optical properties has a crystal structure of ABX\({}_{3}\). Hybrid perovskite consists of organic and inorganic cations. Organic cations such as methylammonium (MA\({}^{+}\), CH\({}_{3}\)NH\({}_{3}\)) and formamidinium (FA\({}^{+}\), CH(NH\({}_{2}\))\({}_{2}\)) are placed in A, and metal cations such as lead and tin are placed in B. At X, halogen anions such as I\({}^{-}\), Br\({}^{-}\), and Cl\({}^{-}\) are arranged to form a structure [17; 18; 19]. Organic-inorganic hybrid perovskites can control the band gap through halogen and anion substitution, and show its characteristics such as high absorption coefficient of light, long diffusion distance, and high electron mobility research on this has been conducted to determine the optimal perovskite composition. There has been a shift toward engineering [20; 21]. MAPbX\({}_{3}\), a representative organic-inorganic perovskite material, changes its valence band level by the 3p, 4p, and 5p orbitals of Cl\({}^{-}\), Br\({}^{-}\), and I\({}^{-}\) halogen ions respectively. When I\({}^{-}\) ions are arranged at the X position, it exhibits a band gap of approximately 1.5 eV, and when substituted with Br\({}^{-}\) ions, it increases to about 2.2 eV, when substituted with Cl\({}^{-}\) ions, it further increases to around 3.1 eV [22; 23].