## 3 Importance of Surface Modification of SnO\({}_{2}\) ETL
### Pinholes  
The perovskite active layer and SnO\({}_{2}\) ETL in PSC devices are frequently disposed of using low-temperature synthesis methods. Because of the greater carrier mobility and reduced internal recombination of organic-inorganic hybrid perovskite materials, surface recombination is a key factor in determining the performance of PSCs. Recombination in SnO\({}_{2}\) ETLs is primarily caused by inherent and bulk defects. Similar to other semiconductor  
Figure 2: Schematic of the processes taking place in a HTL–perovskite–ETL sandwich configuration and the light-induced generation of holes (white spheres) and electrons (black spheres) in the perovskite film (hv). In the pristine perovskite, electrons can either be trapped (trap density N\({}_{T}\), trapping rate k\({}_{T}\)), in which case they can recombine with holes via k\({}_{R}\), or they can undergo second-order band-to-band recombination (k\({}_{2}\)). Through k\({}_{e}\) and k\({}_{h}\), the electrons and holes can be injected into an organic transport layer, respectively. Reprinted with permission from [PERSON] et al. [3] Copyright 2019 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. supplies, the electrical and optical features of SnO\({}_{2}\) are affected by point defects. Owing to their low binding energies, tin interstitials and oxygen vacancies are the two most widespread intrinsic defects on the SnO\({}_{2}\) surface [55]. The majority of bulk defects are pinholes that are caused by the cracks in the SnO\({}_{2}\) surface, which generate current leakage and lower the performance of PSCs. The performance of PSCs must be improved by lowering the power loss due to current leakage and delaying the charge recombination at the surface. Consequently, the pinholes is an un-intrinsic defect that mostly results from the surface deformation of SnO\({}_{2}\) ETLs. Although a decrease in performance is typically anticipated owing to inadequate surface coverage or pinholes, the effects of pinholes on performance metrics have been inconsistent. Surprisingly, the surface coverage has a greater influence on V\({}_{\text{oc}}\) than the pinhole size distribution. Nonetheless, the amount of surface coverage and the size distribution of the pinholes have an impact on the \(\text{J}_{\text{sc}}\) of the devices [68]. In addition, charge recombination at the HTL-ETL interface (caused by the lack of perovskite between them) can result in intriguing effects, such as impeccable device performance with inadequate surface coverage (Figure 3a,b) [69]. For various surface coverages, Figure 3c shows the photon volumetric integration absorbed per wavelength vs. perovskite (the diameter of the cell was approximately 1 \(\upmu\)m). The results for the device with 100% surface coverage were used to normalize the obtained values. Here, the surface area affects the number of photons that the perovskite can absorb. Additionally, in the moderate-to-wide wavelength region (greater than 460 nm), this rise is nonlinear with surface coverage. The effect of light diffraction on gaps that are comparable to the light wavelength in dimension is discussed, where the relevant wavelength is in the 300-800 nm range and considerable diffraction occurs for gaps smaller than 300 nm (as shown in Figure 3d). Additionally, the gap functions as a circular light source if its size is smaller than the incident light wavelength [69]. Although the absorption rate varies nonlinearly with surface coverage for devices with small diameters (cell size of approximately 0.1 \(\upmu\)m), the voids are minor in size for devices with large diameters (cell size of approximately 10 \(\upmu\)m). Therefore, the photon absorption in the perovskite film varies linearly with surface coverage owing to the negligible light diffraction. Because of the higher recombination in the cell caused by the HTL-ETL interface, a subsequent drop in V\({}_{\text{oc}}\) is anticipated in devices with pinholes. Despite the reliance of V\({}_{\text{oc}}\) on the surface coverage, it has a relatively minor impact on the size of the voids.