## 1 Introduction  
Human evolution and society are intimately related to environmental and energy concerns. Presently, the excessive use of fossil fuels has triggered an energy crisis and climate change, resulting in concerns for the future progression of human civilization. Consequently, the utilization of renewable energy sources that reduce CO\({}_{2}\) emissions, such as energy from natural sources (wind, solar, and hydroelectricity), has become a common global objective. Based on its environmentally friendly functioning principle and high accessibility, solar energy is an outstanding option. The photovoltaic (PV) technique, which instantly transforms solar power into electricity, is employed in many engineering applications. Currently, academia and industry are investing resources into researching ways to enhance solar-energy-harvesting efficiency at a reasonable cost. However, less than 1% of the global energy demand is currently met by solar energy. A key determinant is the high cost per kWh, which is primarily attributable to the complicated manufacturing procedure, high intake of material, and low productivity of the current solar cells that are based on crystalline. Thus, crystalline silicon-based solar cells account for 91% of the global consumption. Second-generation thin-film PVs constitute only 9% of the market because they are primarily constrained by the high costs incurred in the use of rare and hazardous materials [1; 2; 3]. Organic PVs, quantum dot solar cells, and dye-sensitized solar cells, which are third-generation thin-film PVs, can be inexpensive and lightweight. However, after more than 10 years of development, these PVs still exhibit a low power conversion efficiency (PCE; approximately 23%), which is most likely the result of a trade-off between light absorption and charge collection [4]. During the past decade, the primary achievement in PV technology has been related to metal halide perovskite materials because they have excellent optoelectronic features, such as very high absorption coefficients, long carrier diffusion lengths, exciton binding energy, and carrier charge balance [5; 6; 7; 8; 9; 10; 11; 12; 13]. The PCE of PV perovskites has rapidly improved from the initial 3.5% to 26.1% over a short research period [14; 15; 16; 17; 18; 19; 20; 21]. The crystal ABX\({}_{3}\) framework is the common structure of metal halide perovskites, as shown in Figure 1, where the A site represents monovalent cations, such as cesium (Cs\({}^{+}\)), methyl ammonium (MA\({}^{+}\)), or formamidinium (FA\({}^{+}\)), whereas the B site is bonded to a divalent cation, such as Sn\({}^{2+}\) or Pb\({}^{2+}\). The X site is a halide anion (Cl\({}^{-}\), I\({}^{-}\), and Br\({}^{-}\)) [22; 23]. More importantly, these materials can be employed in low-cost manufacturing methods, such as roll-to-roll printing, because they have excellent solution-handling properties [24; 25; 26; 27]. Owing to these qualities, metal halide perovskites can provide effective and affordable light-absorbing layers for PV applications. Organometal halide perovskite solar cells (PSCs) display a nanostructured morphology that inhibits the charge separation of photocarriers composed of diverse materials. Long diffusion lengths are generated for efficient charge collection via the swift injection of positive charges from the absorber to the electron and hole conveyance materials; this precludes the recombination of electron-hole pairs [29]. Typically, a hole transport layer (HTL) and an electron transport layer (ETL) are sandwiched around the photocative perovskite layer [30]. As one of the most vital parts of PV systems, the ETL plays a significant role in collecting and conveying photogenerated electron carriers and acting as a hole-blocking film that limits charge recombination [31]. The performance of a PV system relies on the physical features of the ETL, including energy level alignment, charge mobility, morphology, defect states, and associated interphase peculiarities [32].