Fig. 1. PCE development and stability comparison of n-i-p structured single-junction PSCs. (a) Evolution of record PCE of PSCs with SnO₂ ETL^[10,18-23]; (b) normalized average efficiency for devices with the structure ETL/FAMACs/EH44/MoO_x/Al where ETL is either TiO₂ or SnO₂^[24]

Fig. 2. Stability comparison of perovskite films deposited on SnO₂ and/or TiO₂ being the ETL^[28]. (a) Cross-sectional SEM images of perovskite films deposited on TiO₂ ETL and SnO₂ ETL before and after light soaking^[28]; (b) photoluminescence and time-resolved photoluminescence decays of perovskite films on TiO₂ ETL and SnO₂ ETL before and after light soaking^[28]; (c) XRD patterns (left panel) before and after 50 hours light soaking, UV-vis absorbance spectra (right panel) before and after 500 hours light soaking of perovskite films fabricated on TiO₂, SnO₂ and TiO₂/SnO₂ composite ETL^[28]

Fig. 3. Various synthetic approaches of SnO₂ ETL^[43-45]

Fig. 4. Schematic illustration of various defects present at the surface and in the bulk of SnO₂ ETL (upper left is the crystal structure of SnO₂)^[64]

Fig. 5. Several methods of passivation of SnO₂/perovskite interface by improved processing. (a) Schematic diagram of UV surface treatment^[80]; (b) UV-vis spectra of glass/ITO/SnO₂/perovskite thin films before and after UV treatment for 500 hours^[73]; (c) UV-vis spectra of glass/perovskite films before and after UV treatment of 500 h and storage in glovebox without UV for 500 h^[73]; (d) schematic illustration of the modified two-step sequential deposition method for preparing perovskite films^[82]; (e) energy level of the device^[82]

Fig. 6. Preparation strategies of large area PSCs based on SnO₂ ETL. (a) Schematic of slot-die coating of SnO₂ films^[83]; (b) structure of the 6 sections series connected large-area flexible PSCMs^[83]; (c) AFM images of the SnO₂ films without hot-air assistance and with hot-air blowing^[83]; (d) schematic illustration of PSCM architecture and SnO₂ films fabricated by CBD^[84]; (e) light stability of 5 cm×5 cm PSCs based on pristine SnO₂ and SnO₂/K ETL with encapsulation (inset is photographs of 5 cm×5 cm and 10 cm×10 cm PSCs)^[84]

Fig. 7. Schematic diagram of chemisorbed SAM on substrates^[91]

Fig. 8. Stability testing results of perovskite films and devices prepared on SnO₂ and Zw-SnO₂^[77]

Fig. 9. Effects of interfacial residual stresses on the device stability of PSCs. (a) Residual stresses of perovskite films prepared on SnO₂ with and without KPF₆ modification^[108]; (b) humidity, thermal and optical stability of SnO₂ unpackaged devices modified with KPF₆^[108]; (c) schematic diagram of NH₄F modification mechanism on ETL/perovskite interface^[62]

Fig. 10. Effects of additive doping in SnO₂ on the device stability of PSCs. (a) PCE evolution of the unencapsulated devices storing in N₂-filled glovebox^[65];^{(b) GIXRD patterns that present the residual stress and microstrain of perovskite films with and without HCOONH₄ dopant[39];(c) UV stability of the devices based on the SnO₂ ETL with and without CNDs in ambient atmosphere (20 °C, 20%‒30% humidity) [119]}

Fig. 11. Comparison of the stability of PSCs based on single and double ETL. (a) Energy level of PSCs with double ETL^[119]; (b) air stability of unencapsulated PSCs^[127]; (c) air-thermal-stability of unencapsulated PSCs^[127]; (d) energy level of PSCs based on In₂O₃/SnO₂ double ETL^[130]; (e) long-term stability without encapsulation under 1 sun continuous light illumination^[130]; (f) moisture stability without encapsulation under RH of 75%^[130]; (g) long-term stability of N-doped bilayer ETL devices with different mass concentrations^[131]; (h) stability of perovskite devices under continuous light illumination (AM 1.5 G) at room temperature^[131]

Table 1. Various approaches used to enhance device stability of PSCs by ETL doping