## 2 The Role of Surface Modification  
An individual, thin coating of the surface modifier is applied to the SnO\({}_{2}\) layer for surface modification, acting as an interlayer between the SnO\({}_{2}\) and the perovskite layers. In this instance, the modifier influences the nucleation and development of perovskite, bonds the SnO\({}_{2}\)-ETL and the perovskite layer, and passivates interface defects (from both SnO\({}_{2}\) and perovskite bottom surfaces) [56]. Charge accumulation and charge dynamics are significantly impacted by the improvement of interfacial bonding and the elimination of charge traps, both of which are greatly influenced by the microstructure of the blocking layer [57; 58; 59]. For improving device performance, creating a non-pinhole SnO\({}_{2}\) layer at low temperatures is very important. The film produced using the wet chemical approach is not the desired PSC since it has several flaws and nonradiative centers [58; 59]. To create a compact layer without pinholes, surface modification is essential [60; 61]. Low-temperature SnO\({}_{2}\) compact layer surface modification results in a small improvement in crystal quality and a small shift in energy levels. These improve charge transportation with full coverage on conductive substrates and successfully induce the ordered alignment of energy levels close to the electrodes [59]. Although SnO\({}_{2}\) has been utilized as a substitute ETL in PSCs, electron mobility on its surface still poses challenges. Figure 2 shows the fundamental charge generation, collection, transport, and recombination in PSCs [62; 63]. Light absorption produces free charge carriers with electrons located in the holes in the valence band (VB) and conduction band (CB), owing to the extremely low effective exciton binding energy and high permittivity of perovskites [64]. Under a built-in electric field, these photogenerated electrons or free holes gravitate toward the perovskite-ETL or perovskite-HTL interfaces. The charge carriers undergo bulk recombination as they penetrate the perovskite layer. A portion of the charge carriers may be captured using bulk traps; subsequently, these charge carriers may be combined with other free charge carriers (first-order recombination dominates especially \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **TiO\({}_{2}\)** & **ZnO** & **SnO\({}_{2}\)** \\ \hline Crystal structure & Rutile, anatase, brookite & Rocksalt, zinc blende, wutzite & Rutile \\ Energy band gap (eV) & 3.0–3.2 & 3.2–3.3 & 3.50–4.0 \\ Surface work function (eV) & 4.5–5.0 & 4.45–5.30 & 4.71–5.33 \\ Electron mobility (cm\({}^{2}\) V s\({}^{-1}\)) & 0.1–4.0 & Bulk ZnO: 205–300; \\  & & Nanowire: 1000 & 240 \\  & & 2.0 & 2.0 \\  & & 0.26 & 0.3 \\  & & 8.5 & 9.6 \\  & & & \\  & & & \\  & & & \\ \end{tabular}
\end{table} 
Table 1: Comparison between the electrical properties of TiO\({}_{2}\), ZnO, and SnO\({}_{2}\)[54; 55]. at low charge carrier density). The energy barrier, ions in perovskite or charge transport layers, defects or charge/ion accumulation at perovskite/transport material interfaces, and charge mobility in charge transport layers also significantly affect the hysteresis and light soaking effect in heterojunction PSCs (HPSCs) because they possess the potential to either increase or screen the built-in potential (depending on the type of species) [65; 66], which in turn increases or decreases the open-system potential [67]. To avoid all the aforementioned problems, the interfacial materials should be appropriately engineered. Currently, interfacial engineering is recognized as a proven and promising solution to effectively impede the charge recombination and trap states between the ETL and perovskite active layer, which is conducive to enhancing the PV efficiency and reducing hysteresis. Here, we review the fundamental insights into the effect of surface variation on the performance of ETLs.