## 5 Surface Engineering of SnO\({}_{2}\) ETL
### Interfacial Chemical Interaction
#### 5.2.2 Vacancy Defects  
Nitrogen-doped graphene oxides (NGO) were added to SnO\({}_{2}\) to boost the efficiency of PeSCs because the oxygen vacancy defects passivated the ETL [156]. By monitoring the shift from Sn\({}^{2+}\) to Sn\({}^{4+}\) in the oxidation state of Sn in SnO\({}_{2}\), it was possible to determine that NGO successfully passivated the oxygen vacancy defects in the SnO\({}_{2}\) layer. The dark-current investigation revealed that the SnO\({}_{2}\):NGO composite layer had better electrical characteristics than that of the SnO\({}_{2}\) layer. The SnO\({}_{2}\):NGO composite layer is an excellent ETL for high-performance PeSCs because it has a superior PCE of 16.5, almost no hysteresis, and a significantly improved V\({}_{\text{OC}}\) and FF. The higher J\({}_{\text{SC}}\), V\({}_{\text{OC}}\), and FF values of PeSCs based on SnO\({}_{2}\) are in accordance with the time-resolved photoluminescence (TRPL) and V\({}_{\text{OC}}\) dependence on light intensity measurements: effective charge extraction and reduced charge carrier biomolecular recombination from the perovskite to the electrode cause NGO composite layers. This is a useful doping technique that may be applied to construct high-performance PeSCs based on SnO\({}_{2}\) at a relatively low temperature. It is also expected that other perovskite optoelectronic devices, including nip-structured perovskite light-emitting diodes (PeLEDs), will perform better when the SnO\({}_{2}\):NGO composite layer is used as an ETL, creating a unique method for the surface modification of SnO\({}_{2}\) nanoparticles [157]. Tetrabutylammonium iodide (TBAI) was added to the colloidal SnO\({}_{2}\) nanoparticle solution to produce modified T-SnO\({}_{2}\) nanoparticles (Figure 15a). TBA\({}^{+}\) ions affect the dispersion of SnO\({}_{2}\) nanoparticles, whereas I ions can interact with Sn\({}^{2+}\) to passivate the oxygen vacancies of SnO\({}_{2}\). The Van der Waals interactions between particles lead to aggregation. TBA\({}^{+}\) is surface-active owing to the interactions between water and hydrophobic molecules. The weakening of these connections inhibited aggregation. Elemental doping can control the free-electron concentration, which is essential at the Fermi level. The addition of I ions may increase the carrier concentration, raising the Fermi level of SnO\({}_{2}\), and hence raising the CB. Additionally, through electrostatic interactions with the anions at the interface, the TBA\({}^{+}\) ions serve as a bridge. This structure offers a quick path for electron extraction and transfer, increasing the capacity of the solar cell to collect charge (as shown in Figure 15b). The lower PL intensity in the multilayer of ITO/T-SnO\({}_{2}\)/perovskite than that in the pristine sample indicates that there was more electron transport from the perovskite to ETL (Figure 15c). Figure 15d presents the TRPL spectra to further illustrate the electron transport and extraction. The T-SnO\({}_{2}\)-nanoparticle-modified surfaces are responsible for the enhanced energy alignment between the T-SnO\({}_{2}\) ETL and the perovskite layer. In Figure 15e, the J-V curves of the devices that excel are based on the T-SnO\({}_{2}\) and pure SnOETLs of the nanoparticles. Based on the T-SnO\({}_{2}\) ETL, the PSC exhibited a PCE of 21.71%, which was significantly higher than that of a perfect gadget (18.85%). This higher value is due to improved electron extraction and a decrease in the interface defects between the perovskite and T-SnO\({}_{2}\) ETLs. The histogram shown in Figure 15f is based on the PCE distribution observed from 22 slices of various ETLs. The average PCE of the devices built using the T-SnO\({}_{2}\) ETL was higher (19.8%) than that of the reference device. By filling the iodide vacancies at the ETL-perovskite interface, the \(\Gamma\) of the T-SnO\({}_{2}\) nanoparticles passivated the interface defects, indicating that the lowered trap density for the perovskite film depended on the particle layers.