## 1 Introduction  
Hybrid organic/inorganic halide perovskite (ABX\({}_{3}\), A = methyl ammonium, formamidinium, or metallic Cs\({}^{+}\); B = Pb\({}^{2+}\), Sn\({}^{2+}\); X = I\({}^{-}\), Br\({}^{-}\), Cl\({}^{-}\)) materials have attracted substantial attention for use as next-generation solar cells [1, 2]. Perovskite materials have many advantages, such as an excellent absorption coefficient, tunable bandgap, low exciton binding energies, high carrier mobility, long charge diffusion length, ambipolar charge transport, etc. [3, 4, 5, 6, 7, 8, 9]. Based on these characteristics, organic-inorganic halide perovskite solar cells (PSCs) have attracted attention, as their power conversion efficiency (PCE) has increased remarkably from 3.8% in 2009 to 25.7% in recent years [10]. Despite these advantages, many efforts are needed to achieve high efficiency and high stability in PSCs due to the loss of the incomplete interface between the perovskite and the interfacial layer [11]. In particular, PSC performance is highly dependent on the interfacial charge transport layer (CTL) as well as the quality of the perovskite active layer. By strategically selecting the CTL of the PSC, it is possible to control the high-quality perovskite active layer and inhibit carrier recombination in the device, thereby promoting carrier injection into the electrode [12, 13, 14]. This charge transport layer is an important factor in the performance and stability of PSCs [15, 16, 17]. Planar PSCs can be divided into two well-known categories, which include n-i-p and p-i-n structures, both of which are beneficial for high-performance solar cells [18, 19]. Recently, many studies have examined p-i-n devices due to their advantages, such as low hysteresis effect, low process temperature, and potential tandem solar cells [20]. A material commonly used as a hole transporting layer (HTL) in the p-i-n structure is poly(3,4-ethylenedioxythiophene)polystyrene-sulfonate (PEDOT:PSS), which has a high work function, simple process, optical transparency and high conductivity [21]. However, PEDOT:PSS-based p-i-n PSCs exhibit low open circuit voltage (V\({}_{\text{OC}}\)) and insufficient stability due to their acidic and hygroscopic properties as well as a large energy barrier between PEDOT:PSS and perovskite [22; 23; 24]. Therefore, researchers are exploring various materials that could replace PEDOT:PSS. Recently, nickel oxide (NiO) has been widely used as a promising candidate for HTL in p-i-n structures because of its high light transmittance through a large band gap (3.6-4.0 eV), deep valence band compatible with perovskite layer, and thermal and chemical stability [25; 26; 27]. However, there are still some obstacles to achieving the high performance and high stability of NiO-based PSCs: low intrinsic conductivity, energy level at NiO/perovskite interface, and deprotonation reactions of under-coordinated Ni ions [28]. Therefore, this represents one of the technological challenges in improving charge transport and minimizing charge recombination at the interface of perovskite/HTL. Recently, there have been many studies to strategically improve the performance and stability of PSCs through surface engineering, such as improving charge extraction by reducing recombination by controlling interface defects and traps, energy alignment, and improving the quality of perovskite films [29; 30; 31; 32]. For surface engineering, various materials such as organic materials, self-assembled monolayer (SAM), and passivation materials have been reported. It has been reported that hole extraction is improved by tailoring the energy level of the HTL through addition of F6 TCNNQ and F2 HCNQ molecules and insertion of polymers such as insulating polystyrene (PS) and poly(methyl methacrylate) (PMMA), thereby suppressing energy loss and improving V\({}_{\text{OC}}\)[33; 34; 35].