## 4 Interface Design Rules for Efficient SnO2 ETL
### Non-Radiative Recombination  
Surface flaws, reverse charge carrier transfer, and mismatched energy levels are the main causes of interface-induced non-radiative recombination. Effective interfaces should block minority carriers and extract only the dominant carriers. Because perovskite and ETLs exhibit distinct bandgaps, a power cliff or spike (E\({}_{\text{C}}\)) must occur at the interfaces, such as that at the ETL-perovskite contact. The ETL carriers could readily go over the barrier to return to the surface in a scenario where there was a surface with a structural cliff. The forward bias condition would result in the annihilation of electrons and holes because of the many defects functioning as recombination foci for charge transport at the ETL-perovskite interface (Figure 7a). An increase in the barrier (E\({}_{\text{C}}\)) against the ETL electron reverse insertion into the interface with a forward bias can result from the spike in the conduction band (CB) at the interface (Figure 7b). The charge injection-accumulation-diffusion-model-based nanocrystal charge transfer dynamics are shown in Figure 7c, together with the recombination and charge transfer related to SnO\({}_{2}\) nanoparticles (Figure 7d). Conventional nanoparticles typically have several organic molecules bonded to their interfaces, which causes deep-level flaws. These  
Figure 7: Equilibrium energy band diagrams at the interface with (**a**) a cliff structure and (**b**) a spike structure in the ETL–perovskite heterojunction under forward bias. The blue and red circles represent electrons and holes, respectively. E\({}_{\text{C}}\) stands for the conduction band minimum energy, E\({}_{\text{V}}\) for the valence band maximum energy, E\({}_{\text{F}}\) for the Fermi level, and E\({}_{\text{c}}\) for the conduction band offset. (**c**) Schematic of a nanocrystalline particle, electron transfer, and recombination. (**d**) Impact of defect states on charge transport in nanocrystalline films is depicted by using the charge injection–accumulation–diffusion model. The direction of the arrows depicts the processes through which electrons are introduced, trapped, or diffused as well as how the energy level changed in various materials. Reprinted with permission from [PERSON] et al. [94] Copyrights 2021 by AIP Publishing. conventional nanoparticles are formed from the precursors of metal oxides disseminated in an organic solvent [101]. Additionally, owing to the low construction energy, the inherent deep-level flaws in ETLs include transition metal defects (Ti\({}^{3+}\) or Ti\({}^{4+}\)), oxygen vacancies with positive charges (V\({}_{\rm O}\)\({}^{+}\), V\({}_{\rm O}\)\({}^{++}\)), and intrinsic defects (Sn\({}_{\rm i}\)) [102]. These flaws may trap charge transfers and lead to charge recombination. In addition, a vital factor in preventing charge accumulation is the balanced transit of holes and electrons throughout the entire cell. Owing to the existence of shallow-level defects in ETLs, including vacancies of oxygen with no positive carriers called V\({}_{\rm O}\)\({}^{0}\), electrons transferred from the perovskite layer to ETL layers may be trapped before being released and returned to the CB upon thermal excitation. Nanoparticle metal oxides were used to create a crosslinked ETL network. The crystalline components were joined together or in amorphous equivalents. The carrier transfers in the ETL layers, particularly in TiO\({}_{2}\) and SnO\({}_{2}\), have been extensively explained by the fluctuation-induced tunneling conductivity (FITC) mechanism. The effective tunneling width, w, effective tunneling area, A, and zero-field barrier height, B, play significant roles in the carrier transit rate through the junction.