## 5 Surface Engineering of SnO\({}_{2}\) ETL
### Interfacial Chemical Interaction
#### 5.2.3 Hybridization  
Considerable efforts have been made to limit the occurrence of these grain boundaries in perovskite films because there is evidence that they may be responsible for charge recombination owing to containing charge-trapping recombination sites [158]. The postulate is that under electron-rich conditions, additional electron traps in SnO\({}_{2}\) might be utilized, increasing mobility and reducing the work function. This is because of the sp\({}^{2}\)-hybridization of the carbon and nitrogen atoms in crystalline polymeric carbon nitride (cPCN), which results in totally conjugated electronic structures. There was a considerable influence on the charge extraction, collection, and recombination in the PSC due to the decrease in the energy barrier at the SnO\({}_{2}\)-perovskite interface [159]. The efficiency can be substantially improved, along with decreased charge accumulation at the ETL-perovskite interface and the suppression of hysteresis in PSCs [160]. Owing to their high average transmittance in the visible spectrum and good optical properties, these samples guarantee that most of the light can pass through and be absorbed by the perovskite layer. Moreover, the response heat generated during the cluster growth can be expanded more effectively. This is advantageous for creating the nucleus because of the comparatively higher interaction intensity. Topographic SEM images of perovskite films were used to study the effects of SnO\({}_{2}\)-cPCN hybridization on the morphology and crystallization of the films (made with the same composition and technique), as shown in Figure 16a,b. Both films exhibit a uniformly dense morphology with randomly joined grains. Meanwhile, the solvent and precursor ions that are sufficiently close are drawn to the surface, and then they are caught. Diffusion is slowed down by the hydrophilic surface, resulting in a slower rate of crystal formation and smaller granularity. On the other hand, non-wetting (hydrophobic) substrates may offer a greater free-energy barrier with higher grain sizes, crystal formation at a faster rate, and fewer grain boundaries. Interestingly, the SnO\({}_{2}\)-cPCN-based devices show good repeatability. This contrasts with the low standard deviation and reliability of the SnO\({}_{2}\)-based devices that are unmodified. This proves that the perovskite developed on the SnO\({}_{2}\)-cPCN ETL has fewer deep-level flaws than those in the grain boundary because of the larger grain size, which helps reduce carrier recombination and significantly improves the performance of the PSC device. A sufficient optical transmittance is required to guarantee adequate light transmission into the perovskite absorber so that the perovskite's energy level supplies generate the anticipated open-circuit voltage (Voc). To remove carriers from the active layer, high electron mobility is required to successfully prevent charge recombination [124]. To prevent charge accumulation at the device, quick carrier extraction is desired to reduce the hysteresis at the contact caused by ion movement in the planar PSCs. Consequently, high-quality ETLs with a reasonable energy level and excellent electron mobility have been prioritized in devices with high PCE. The shapes of the perovskite films deposited on various ETLs are shown in Figure 16c-e. These images clearly show that continuous pinhole-free films with complete surface coverage were produced. For the perovskite coated on SnO\({}_{2}\), Figure 16f displays a distribution diagram with an average grain size of approximately 309 nm. The particle size of the ethylenediamine tetraacetic acid (EDTA) sample is increased to approximately 518 nm. Surprisingly, for the E-SnO\({}_{2}\) substrates, the average perovskite grain size is further increased to 828 nm (Figure 16c,d). EDTA can create a transition metal oxide because of its ability to supply its lone-pair electrons to the empty d-orbital of the transition metal atoms. The grain size, when perovskite is placed, increases the film thickness threefold.