## 3 Results and Discussion  
### SnO\({}_{2}\) vs. TiO\({}_{2}\) for CsFAMAPIBr Based Devices  
First, we studied the photovoltaic parameters of the solar cells with the different ESC (SnO\({}_{2}\) and TiO\({}_{2}\), n-type material), using the triple cation perovskite composition (CsFAMAPIBr) and spiro-OMeTAD as HSC (p-type material). The photovoltaic parameters, obtained from the measurements of the current voltage (JV) curves are shown in Figure 1 and listed in Table 1. For clarity, we just show the results from day 1 (fresh) and day 28 (aged). Figure S1 summarises the photovoltaic parameters of the different measurement days. During the whole duration of the experiment, the devices were kept in dark and under dry air conditions (RH < 10% H\({}_{2}\)O), obtained via a synthetic airflow. Figure 1: JV curves measured at 1 Sun conditions (100 mW/cm\({}^{2}\), AM 1.5 G) in forward (dashed line) and reverse (solid line) scan for CsFAMAPIBr on SnO\({}_{2}\) (**A**) and TiO\({}_{2}\) (**B**) in fresh and aged devices. For fresh devices, the J-V hysteresis is more accentuated for devices with TiO\({}_{2}\) as ESC. This might be attributed to the higher electron collection efficiency and better band alignment of energy levels of SnO\({}_{2}\), which helps to decrease and even eliminate hysteresis [19]. However, in both cases, a reduction in the JV hysteresis is observed after keeping the solar cells in dark and dry air conditions (Figure 1 and Table 1). In previous publications, we and others have reported this kind of behaviour [18; 19; 20; 21]. It has been suggested that O\({}_{2}\) might have two major effects over this perovskite configuration. First, spiro-OMeTAD becomes oxidised, which helps to increase its conductivity, so getting a better charge extraction, and also to lower the HOMO value, which results in better band alignment, reducing carrier losses due to band mismatching [22]. The better charge extraction properties and reduced losses produce better values of FF and V\({}_{\text{OC}}\) in the aged solar cells. This process happens in the very early days after the storage of the devices, being clearly observable in the results on day 4 (Figure S1). Despite this increase in the very early days, these devices still present a high degree of hysteresis, and we have observed that the storage in dry air conditions over long periods of time helps to reduce it [17; 18]. With longer aging times, device hysteresis reduces, obtaining higher values of V\({}_{\text{OC}}\) and FF. As well, the difference between the Voc values in the forward sweeps (from J\({}_{\text{SC}}\) to V\({}_{\text{OC}}\)) and reverse sweeps (from V\({}_{\text{OC}}\) to J\({}_{\text{SC}}\)) tends to be smaller (\(\Delta\)V\({}_{\text{OC}}\) = V\({}_{\text{OC}}\) (Rev) \(-\) V\({}_{\text{OC}}\) (fwd); \(\Delta\)V\({}_{\text{OC}}\)(fresh, TiO\({}_{2}\)) = 134 mV versus \(\Delta\)V\({}_{\text{OC}}\)(aged, TiO\({}_{2}\)) = 23 mV). We observe significant changes in V\({}_{\text{OC}}\), increasing 85 mV for TiO\({}_{2}\)-based devices and 70 mV for SnO\({}_{2}\)-based devices. This important improvement might be associated with an improved carrier recombination at the interfaces, which greatly affects V\({}_{\text{OC}}\). Metal halide perovskite are both ionic and electronic semiconductors, and the presence of mobile ions at the interfaces were appointed as the main cause for hysteresis in perovskite solar cells [13]. Thus, the reduction of hysteresis in our devices is related to a reduction in the influence of mobile ions over recombination. It has been previously suggested that the O\({}_{2}\) molecules might also have a passivation effect on the perovskite lattice [23; 24; 25]. They might occupy some of the halide defects in the perovskite lattice, improving both ionic mobility and non-radiative carrier recombination.