## 5 Surface Engineering of SnO\({}_{2}\) ETL
### Interfacial Physical Interaction
#### 5.1.1 Simplistic Synthesis Route
Figure 10: (**a**) Transmittance spectra of SnO\({}_{2}\) films that were sputtered and spin coated and then placed on FTO glass. (**b**) UV–visible spectrum of SnO\({}_{2}\) films that have been spin coated and vapor-deposited with MAPbJ\({}_{3}\). (**c**) Photoluminescence spectrum of MAPbJ\({}_{3}\) that was vapor-deposited and placed on spin-coated and sputtered SnO\({}_{2}\) films. Reprinted with permission from [PERSON] et al. [128]  
Contact angle of SnO\({}_{2}\) thin films at various UVO exposure times: (**d**) 0 min, (**e**) 10 min, (**f**) 20 min, and (**g**) 30 min. Perovskite films that were deposited on a layer of SnO\({}_{2}\) are shown in the top-view SEM images at (**h**) 0 min, (**i**) 10 min, (**j**) 20 min, and (**k**) 30 min. Reprinted with permission from [PERSON] et al. [130]. The UVO treatment of the SnO\({}_{2}\) ETLs altered the performance of the planar PSCs at various time intervals [130]. A 30 min treatment period was found to be ideal. The characterization results revealed that the SnO\({}_{2}\) film became more hydrophilic with a contact angle of 24.5\({}^{\circ}\) by extending the UVO exposure period (30 min), which supported the elimination of organic residues associated with the precursor reagents at the film surface (as shown in Figure 10d-g). It is possible to use UVO treatment to improve the perovskite layer coverage and ETL-perovskite interface, which facilitates the electron transfer from the perovskite film to the ETL. Interestingly, the cells based on SnO\({}_{2}\) did not significantly decrease the efficiency of solar cells when exposed to UVO for more than 30 min. Furthermore, as seen in Figure 10h,i, the continuity of the SnO\({}_{2}\) films treated with UVO for 0 and 10 min was subpar, with evident voids and non-uniform coverage. The ETL and HTL layer separation were caused by the non-uniformity break, creating recombination centers and impairing the PSC performance. It was discovered that thin films without pinholes and cracks were formed as the UVO treatment period increased (Figure 10j,k). The ETL-perovskite interface was improved by increasing the wettability of the SnO\({}_{2}\) layer, which also decreased recombination and increased electron injection. This was explained by the fact that the oxygen atoms chemisorbed during UVO treatment contributed to the formation of hydroxyl groups on the SnO\({}_{2}\) surface. Therefore, the oxygen vacancies were reduced, film wettability was enhanced, and the ETL-SnO2 interface was improved [131; 132]. The effects of UVO treatment include the following: (1) increasing the wettability of the perovskite film and (2) passivating the ETL surface by lowering the recombination centers and increasing J\({}_{\text{Sc}}\). Contemporary approaches to altering the surface morphology are also highlighted. The increasing need for additive patterning of functional multilayers and device components can be achieved using inkjet technology [133]. The process is easily scalable to the meter format and delivers direct, cost-effective, and mask-free patterning. Compared with the commonly used spin coating process, the inkjet printing approach has attracted considerable attention regarding the up-scaling manufacturing of PSCs, together with lower material consumption and negligible waste. In this study, the inkjet printing techniques for TiO\({}_{2}\), SrTiO\({}_{3}\), and SnO\({}_{2}\) ETLs were assessed. The drying qualities of the cosolvent are advantageous for uniformly producing SnO\({}_{2}\) films by optimizing the PSC performance based on the printed ETLs. The low-cost printability of the method, particularly beneficial for flexible PSCs, is one of its encouraging features. However, it is currently difficult to print a vast and uniform ETL on rough and brittle plastic substrates without hysteresis. High-quality SnO\({}_{2}\) films for flexible PSCs with high efficiency can be created using a slot-die method.