## 3 Results and Discussion
### SnO\({}_{2}\) vs. TiO\({}_{2}\) for CsFAMAPIBr Based Devices
Using V\({}_{\text{OC}}\) stabilization, it is possible to track the influence of ionic mobility over carrier recombination, as this technique correlates a slow V\({}_{\text{OC}}\) stabilization with a slow ionic redistribution that leads to an increase interfacial recombination, and, thus, reduced V\({}_{\text{OC}}\)[26]. Another important feature is that SnO\({}_{2}\)-based devices do not show any change in the J\({}_{\text{SC}}\), while we observed a reduction in TiO\({}_{2}\)-based devices. This is related to TiO\({}_{2}\) causing photo-instability in the perovskite as it absorbs in the UV region [27; 28], producing a slight degradation in the perovskite layer. Therefore, both from performance and from stability, we can confirm that SnO\({}_{2}\) works better than TiO\({}_{2}\) as ESC in n-i-p perovskite solar cells. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**ESC/Perovskite** & **Aging Days** & **Sweep** & **J\({}_{\text{SC}}\) [mA/cm\({}^{2}\)]** & **V\({}_{\text{OC}}\) [V]** & **FF** &
\begin{tabular}{c} **PCE [\%]** \\ **(Champion Cell)** \\ \end{tabular} \\ \hline \multirow{4}{*}{TiO\({}_{2}\)/CsFAMAPbIBr} & \multirow{2}{*}{1} & Forw & 21.7 \(\pm\) 1.1 & 0.930 \(\pm\) 0.011 & 0.53 \(\pm\) 0.01 & 10.7 \(\pm\) 0.4 \\  & & Rev & 22.1 \(\pm\) 1.1 & 1.054 \(\pm\) 0.029 & 0.71 \(\pm\) 0.01 & **(11.5)** \\  & & & Rev & 22.1 \(\pm\) 1.1 & 1.054 \(\pm\) 0.029 & 0.71 \(\pm\) 0.01 & 16.6 \(\pm\) 0.9 \\  & & & & & & **(18.4)** \\  & & & & & & 15.3 \(\pm\) 1.7 \\  & & & & & & **(16.8)** \\  & & & & & & 16.3 \(\pm\) 0.6 \\  & & & & & & **(17.0)** \\ \hline \multirow{4}{*}{SnO\({}_{2}\)/CsFAMAPbIBr} & \multirow{2}{*}{1} & Forw & 22.0 \(\pm\) 0.9 & 1.062 \(\pm\) 0.013 & 0.50 \(\pm\) 0.02 & 11.8 \(\pm\) 0.5 \\  & & & & & & **(2.5)** \\ \cline{1-1}  & & & & & 15.6 \(\pm\) 1.0 \\ \cline{1-1}  & & & & & & **(17.9)** \\ \cline{1-1}  & & & & & 17.5 \(\pm\) 1.0 \\ \cline{1-1}  & & & & & & **(19.6)** \\ \cline{1-1}  & & & & & & 17.5 \(\pm\) 0.6 \\ \cline{1-1}  & & & & & & **(18.6)** \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the photovoltaic parameters for CsFAMAPbIBr on either TiO\({}_{2}\) or SnO\({}_{2}\) layers measured in fresh (1 day) and aged (28 days) conditions. Then, we moved onto the study of the dependence of the ESC choice on the carrier recombination. For doing this, we employed a set of optoelectronic transient techniques, transient photovoltage (TPV) and transient photocurrent (TPC), which we have widely used in previous studies on perovskite solar cells, as well as on organic solar cells or dye sensitised solar cells [11; 15; 29]. Both techniques are small-perturbation techniques based on the application of a laser pulse that generates a small perturbation in voltage while using different background illumination intensities, which allow registering the experiment at different voltages. With TPV, we study the time that it takes for the voltage to return to its steady state, which depends on the background bias, and it is correlated with the recombination in the solar cell. TPC allows us to determine the number of charges that the laser pulse generates in the device. By combining both techniques, we can determine the charge density in the device with a technique termed as differential capacitance (DiffCap). We study the recombination scenario with the small perturbation lifetimes obtained from the TPV decays at stable photovoltages. As it has been previously highlighted, at low-light intensities (low values of voltages), the lifetimes obtained from TPV are affected by the discharge of the ionic layer accumulated at the perovskite interfaces, which is the ionic redistribution once the device is illuminated [10; 17]. Therefore, we should compare the lifetimes at high voltage values (Figure S2), which we can identify as the region in which carriers accumulate in the bulk of the perovskite (Figure S3). For this reason, we compare the recombination kinetics at different carrier densities (Figure 2A), obtained from the differential capacitance method (Figure 2B and Figure S3).