## 5 Surface Engineering of SnO\({}_{2}\) ETL
### Interfacial Physical Interaction
#### 5.1.3 Bilayer  
During the formation of the perovskite layer, oleic acid ligands can passivate the trap sites [141]. Controlling perovskite layer defects can be extremely important for perovskite grain boundaries and ETL-perovskite heterointerfaces, resulting in PSCs with improved solar performance. Enhancing the perovskite crystalline quality can reduce the internal defects of the active layer and current loss caused by the internal non-radiative recombination of the device (Figure 12a). As a result, the PV performance is synergistically improved via the NaYF\({}_{4}\):Ce and Tb@NaYF\({}_{4}\) nanocrystals acting as light scattering centers, UV down-converters, and defect passivators. A cross-sectional image of the PSCs captured using SEM is displayed in Figure 12b. SnO\({}_{2}\) was coated on the FTO conductive glass with a thickness of 20 nm, and spin coating was used to manufacture NaYF\({}_{4}\):Ce, Tb@NaYF\({}_{4}\) nanocrystals on SnO\({}_{2}\) (50 nm). The perovskite active layer, FA\({}_{0.85}\)MA\({}_{0.15}\)Pb(I\({}_{0.85}\)Br\({}_{0.15}\))\({}_{3}\) (400 nm), was created by using a one-step spin coating technique. Before being added to the HTL, spiro-OMeTAD was mixed with Cu-In\({}_{0.7}\)-Ga\({}_{0.3}\)-Se\({}_{2}\) (CIGSe) quantum dots and Au nanorods (NRs) (205 nm). A silver electrode (117 nm) was placed on top of the device. The addition of NaYF\({}_{4}\):Ce and Tb@NaYF\({}_{4}\) phosphor nanocrystals improved the crystalline quality of the perovskite, reduced the recombination of grain boundaries, and boosted the utilization of UV light by light down-conversion. The natural deterioration of perovskites is a possible explanation for this phenomenon. The main reason for this phenomenon was the natural degradation of the perovskite. The NaYF\({}_{4}\):Ce,Tb@NaYF\({}_{4}\) affected the long-term stability of the perovskite layer because it enhanced the crystallinity of the perovskite films. The increased grain size of the perovskite films reduced the grain boundary density, which enhanced the resistance of the perovskite films to the water and oxygen in the environment. In addition, the main reason for the moderately enhanced device stability of NaYF\({}_{4}\):Ce, Tb@NaYF\({}_{4}\) nanocrystals was the presence of thinner oily ligands on their surfaces after the synthesis process. The hydrophobic nature of oily ligands enabled the perovskite film to improve its stability under humid conditions. The NaYF\({}_{4}\):Ce, Tb@NaYF\({}_{4}\) enhancement increased the crystallinity of the perovskite films, which was the key factor affecting their long-term stability. The larger grain size of perovskite films results in a decrease in the grain boundary density, which improves their resistance to oxygen and water in natural environments (Figure 12c) [142]. The increased hydrophobicity of the improved film helps to increase the interfacial contact, less the upper perovskite grain boundary defects, and prevent heterogeneous nucleation. The grain boundary movement was hampered because the wetting surface's grain boundaries were tethered by imperfections on the ETL surface. As a result, the enhanced hydrophobia suggested that the flaws at the perovskite grain boundaries were being reduced, improving the perovskite active layer's quality. This could be attributed to the ETL surface's smoothness and lack of wetness, which inhibit heterogeneous nucleation and promote grain boundary migration. This finding demonstrated that perovskite could grow its grain sizes due to its hydrophobic surface. Less grain boundary defects were present in the larger grain sizes, which reduced charge non-radiative recombination at the perovskite-ETL interface. Figure 12d displays the contact angle measurement results. After the addition of tetrabutylammonium fluoride (TBAF), the contact angle increased from 7.0\({}^{\circ}\) to 29.9\({}^{\circ}\). Furthermore, the RMS roughness of the SnO\({}_{2}\)/TBAF film (0.98 nm) was lower than that of the SnO\({}_{2}\) film (2.12 nm) (Figure 12e,f); the former film was smoother and more homogeneous.