## 5 Surface Engineering of SnO\({}_{2}\) ETL
### Interfacial Chemical Interaction
#### 5.2.1 Functional Groups
The SnO\({}_{2}\) crystal structure is stable because there is no diminishment in strength peaks when an increase in phosphoric acid concentration causes no shift. The P\({}^{3+}\) and P\({}^{5+}\) ions have distinct radii. SnO\({}_{2}\)'s crystal structure is unaltered from that of Sn\({}^{4+}\). Phosphorus was introduced, demonstrating that the SnO\({}_{2}\) lattice does not have any lateral doping of phosphorus. A 3-mercaptoethanolpropyltrimethoxysilane (MPTMS) SAM interlayer was used. This interlayer has three functional methoxy groups acting as anchoring groups and a sulfhydryl group acting as the terminal group (Figure 14a,b) [153]. First, the sulfhydryl groups of the MPTMS SAM might combine with PbI\({}_{2}\), decreasing crystal formation and widening the perovskite film crystal size. Additionally, the SnO\({}_{2}\) ETL surface might be smoothened by MPTMS SAM modification, which would benefit the film quality of the perovskite absorber. Second, by anchoring the Pb atoms with the help of S atoms in the MPTMS SAM, the photogenerated electron extraction may be improved. Finally, by reacting with SnO\({}_{2}\), the hydrolyzed methoxy groups on MPTMS could enhance the perovskite structure. As a result, the average PCE for our fully air-processed PSCs with MPTMS SAM modification was 18.75%, with a highest PCE of 20.03%. Additionally, in an ambient air environment, the proposed MPTMS-SAM-modified PSCs showed improved illumination and long-term stability. RbF modifications of the SnO\({}_{2}\) ETL and mesoporous structures are effective approaches to improving PSC performance [154]. A mesoporous structure can increase the efficiency of electron collection because it has a larger specific surface area than a normal planar structure; however, it also introduces a higher interfacial trap density. RbF modification can increase the conductivity of the SnO\({}_{2}\) ETL and passivate the interfacial trap by creating F-Sn bonds [155]. Rb\({}^{+}\) ions can diffuse into the perovskite layer simultaneously, serving as a passivation agent and limiting ion migration. Here, the synergy between the mesoporous structure and RbF modification related to the SnO\({}_{2}\) ETL was successfully achieved. Not only was the disadvantage of the mesoporous structure in interfacial traps reduced by the RbF modification, but the carrier collection efficiency was also further boosted. Because of the chemical interaction at the SnO\({}_{2}\)-perovskite junction, charge transport was improved. An inorganic sulfur functionalization of the SnO\({}_{2}\) ETL was introduced through xanthate decomposition at low temperatures to modify the SnO\({}_{2}\)-perovskite interface and anchor Pb\({}^{2+}\) in both perovskite and SnO2. By utilizing inorganic sulfur atoms at the interface of the perovskite active materials, effective interfacial passivation was studied. Although rapid charge transfer at the interface and suppression of hysteresis have been accomplished using complex organic molecules, interfacial trap passivation may be constrained by potential weaknesses in the interconnection of organic molecules, which hinders the performance or stability of PV devices under ambient conditions. Through utilizing inorganic sulfur atoms at the interface between the perovskite active layer and SnO\({}_{2}\) ETL, effective interfacial passivation was examined (Figure 14c) [155]. Through xanthate breakdown at low temperatures, the inorganic sulfur functionalization on the SnO\({}_{2}\) ETL was introduced to modify the SnO\({}_{2}\)-perovskite interface and simultaneously anchor Pb\({}^{2+}\) in both SnO\({}_{2}\) and perovskite. The result of the three xanthate treatments yielded the best solar cell (Figure 14d). The J-V characteristics of a perovskite device built on SnO\({}_{2}\) and SnO\({}_{2}\)/S substrates are shown in Figure 14e for both forward and backward scan orientations. Notably, sulfur functionalization clearly reduced the hysteresis.