## 4 Interface Design Rules for Efficient SnO2 ETL  
### Charge Carrier Transport  
A detailed diagram of the PSC charge dynamics is shown in Figure 6. Sunlight exposes the devices to photons, which the perovskite absorbs and converts into excitons. Owing to the low exciton binding energy, the excitons split into holes and electrons at ambient temperature. The charge-carrier lifetime had a duration of 10-100 ns [89]. Both the holes and electrons were moved to the perovskite-HTL and perovskite-ETL interfaces under the effect of the electric fields and carrier concentration gradients, respectively. The interface area experiences a continuous potential drop caused by the offsets in the valence and conduction bands, which ensures that charge transport is moved back to the HTL or ETL before their annihilation, typically within a timeframe of 100 ps [89]. Finally, the corresponding electrodes gather the electrons and holes. The internal quantum efficiency (IQE) should be extremely close to 100% if each absorbed photon produces a different hole-electron pair and all charge transport is gathered by the electrodes [90; 91]. As a result of the carrier recombination occurring in the bulk perovskite or at its grain boundaries, at the interface of perovskite-HTL, or perovskite-ETL and HTL-ETL interfaces when perovskite is missing in some places, the usual IQE of PSCs is approximately 90% [92; 93]. Figure 5: Cross-sectional SEM images of (**a**) indium-doped tin oxide (ITO)/SnO2 (10%), (**b**) ITO/SnO2 (15%), (**c**) ITO/SnO2 (20%), and (**d**) ITO/SnO2 (30%). Top-view SEM images of (**e-h**) the prepared ITO/SnO2(x) films at \(\times\)50,000 magnification and (**i-l**) films at \(\times\)200,000 magnification. Reprinted with permission from [PERSON] et al. [84]. The mechanism behind charge-carrier transport in planar PSCs is typically clarified by measuring the transient photovoltage (TPV) and transient photocurrent (TPC) [31, 95, 96]. A bi-exponential function was used to fit the photovoltage (\(V_{ph}\)) decay traces. Accordingly, the following equation was used to determine the apparent recombination time constant (\(\tau_{r}\)) and transport time constant (\(\tau_{t}\)):  
\[\tau=\frac{A_{1}\tau_{1}+A_{2}\tau_{2}}{A_{1}+A_{2}}\,\]  
where \(A_{2}\) and \(A_{1}\) are constants that define the impact on decay with lifetimes \(\tau_{1}\) and \(\tau_{2}\), respectively. With an increase in \(V_{ph}\), the \(\tau_{r}\) value shifted from milliseconds to microseconds. Notably, the connection between \(\tau_{r}\) and \(V_{ph}\) for planar PSCs on the semi-logarithmic axis exhibits a roughly linear correlation, suggesting that the charge-carrier recombination process should proceed in a manner similar to that of trap-limited recombination [97]. To determine the amount of charge (\(q\)) in a cell, current transients were unified with respect to time. The following equation was used to determine the overall charge density (\(n\)) [98]:  
\[n(Vph)=q(Vph)/edA\]  
where \(d\) is the thickness of the photoactive layer in the device, \(e\) is the elementary charge, and \(A\) is the device area. Thus, the important elements in regulating the accumulation of carriers and minimizing the recombination of defects, for instance, ETL layers and their interaction with the perovskite film, should be considered to increase the effectiveness of the devices used under such settings. Creating a smooth surface and thin layer of SnO\({}_{2}\) has been proven as an efficient method to increase the quality of an SnO\({}_{2}\) ETL, contributing to minimizing the formation of defects on the surface of the perovskite layer and improving PSC performance [99, 100]. Figure 6: Charge-transfer mechanisms in perovskite devices occurring at various time scales are schematically illustrated by arrows with various colors.