## 1 Introduction  
Organic-inorganic perovskites have recently attracted considerable attention as absorbers in solar cells; they have a chemical formula of ABX3 and have a three-dimensional structure with organic cations, such as CH\({}_{3}\)NH\({}_{3}\), CH(NH2)\({}_{2}\), Cs, and Rb at A, cations such as Pb, Sn, and Ge at B, and halogen anions such as I, Br, Cl, and F at X [1, 2]. Perovskites exhibit different performances and properties in solar cells depending on their constituent elements [3]. Since the first study on CH\({}_{3}\)NH\({}_{3}\)PbX\({}_{3}\) perovskites in 2009, their efficiency has considerably improved to 25.7% or more, resulting in the maximum power conversion efficiency (_PCE_) [4, 5, 6, 7, 8]. Representative perovskites include CH\({}_{3}\)NHPbI\({}_{3}\) (MAPbI\({}_{3}\)) and CH(NH\({}_{2}\))\({}_{2}\)PbI\({}_{3}\) (FAPbI\({}_{3}\)). The advantages of perovskites include the possibility of a low-cost production through a low-temperature solution process and a high efficiency because of a high light-absorption coefficient [9]. However, some issues need to be solved; in particular, Pb is harmful to the environment; thus, Pb-free materials should be developed [10]. In addition, existing structures using organic materials for the common layers, Electron Transport Layer (ETL) and Hole Transport Layer (HTL), are sensitive to moisture [11, 12]. Therefore, in order to overcome these disadvantages, research on using inorganic materials instead of organic materials for common layers such as ETL and HTL is being actively conducted [13]. Among the widely studied inorganic Electron Transport Layers (ETL) and Hole Transport Layers (HTL) in the device structure of perovskite solar cells (PSCs), looking first at ETL materials, substances such as SnO\({}_{2}\), ZnO, and TiO\({}_{2}\), which exhibit n-type semiconductor characteristics, are commonly utilized [14, 15]. SnO\({}_{2}\) is one of the most widely researched transparent conducting oxides (TCOs) as a transparent electrode since the early days of TCOs research, due to its transparent properties and high conductivity [16]. Furthermore, by controlling oxygen deficiency, its electrical characteristics can be adjusted from conducting to semiconductor, enabling extensive application research on thin film transistors as channel layers [17]. In particular, significant research has been conducted to control the electrical properties and improve reliability by doping SnO\({}_{2}\) with other materials, such as Al, In, Ta, etc. [18; 19; 20]. Additionally, ZnO is one of the most abundant materials on earth and is currently being evaluated in various multi-layer device structures as an Electron Transport Layer (ETL) material in perovskite solar cells. ZnO has a wide bandgap energy (Eg = 3.26 eV) and excellent mobility [21]. With its wide bandgap and significant exciton binding energy, ZnO is considered one of the most interesting ETL materials [22]. In particular, there has been active research in doping with various other materials to create p-type ZnO [23; 24]. However, a reliable p-type ZnO has not yet been reported [25]. Nonetheless, in the case of IGZO (a mixture of In\({}_{2}\)O\({}_{3}\), Ga\({}_{2}\)O\({}_{3}\), and ZnO), although it exhibits n-type characteristics, it is commercially utilized as the channel layer of the thin film transistor (TFT) in large organic light-emitting devices, due to its reliability under operating voltage and current conditions [26; 27]. TiO\({}_{2}\) is the most researched material for the electron transport layer in the multi-layer structure of perovskite solar cells. This preference is due to TiO\({}_{2}\)'s high chemical and optical stability, non-toxicity, and low cost, making it well-suited for applications in the optical field [28; 29]. Among TiO\({}_{2}\) structures, anatase is preferred in the photovoltaic field due to its higher conduction band edge energy and lower electron-hole recombination rate [30]. These various excellent properties led to the selection of TiO\({}_{2}\) as the ETL material in this study.