## 3 Results and Discussion
### SnO\({}_{2}\) vs. TiO\({}_{2}\) for CsFAMAPIBr Based Devices
In this way, we compare the kinetics when the contacts are depleted with charges, and they start to accumulate exponentially in the bulk [15]. This part is easily distinguishable in the analysis of the charge density as a function of voltage (Figure S3), where two different regimes are distinguished. First, a linear increase with voltage of the charge at lower values of voltage. Then, after the contacts are depleted with charges, they accumulate in the bulk of the perovskite, which we can observe as an exponential increase with voltage [15]. In these samples, as expected, we observe slower lifetimes for the aged devices (Figure 2A), which confirm that the increase in V\({}_{\text{OC}}\) is associated with a decrease in the recombination scheme of the system. This is also supported by the recombination orders in the perovskite. The small perturbation lifetime follows a power law dependence with the charge density \(\tau_{\text{An}}\)= \(\tau_{\text{An}_{0}}\left(\frac{\text{Q}}{\text{Q}_{0}}\right)^{-\lambda}\), which is used to calculate the recombination order (\(\delta\)), (\(\delta=\lambda+1\)) [30]. All the devices are ruled by first and second order (\(\delta=1\)-\(2\)) corresponding to the trap-assisted and surface recombination and band-to-band free carriers [31]. However, a clear decrease in the recombination order of the samples is observed after the aging treatment. Again, we associated it with a reduced carrier recombination at the interfaces because of the reduced ionic activity and motion. Using the recombination orders, it is possible to estimate the total carrier lifetime, or a pseudo-first-order lifetime of total excess charge carries via \(\tau=\tau\Delta n\delta\) (Figure S4). We observe very similar trends independently of the ESC employed. Next, we study the ionic influence over carrier recombination. For this purpose, we employed a variation of TPV, known as transient of the transient (TROTR), previously used by our research group and others [14; 16; 18]. In this technique, TPV transients are registered at the early stages of illumination, prior to V\({}_{\text{OC}}\) stabilization, where the ionic population is not fully distributed in the perovskite layer [17; 18]. Figure 3 shows the V\({}_{\text{OC}}\) stability over time for fresh and aged devices. Here, we clearly observe a slower stabilization of voltage in illumination conditions for fresh devices mainly due to its higher hysteresis, which agrees well with results of previous studies [18]. This slow VOC stabilization is associated with the ionic redistribution in the perovskite layer, a slow process that affects carrier recombination, inducing slower VOC values until ions redistribute in the bulk of the perovskite [13; 16]. During this time, when TPV transient pulses are registered--at 3, 5, 10 and 30 --a negative spike appears. This negative transient, associated with an increased recombination at the interfaces is observable in every fresh device (see Figure 4A,C). Then, after the aging treatment (see Figure 4B,D), this spike is reduced for the case of TiO2, or it even disappears, as with SnO2-based devices and, as previously observed, after aging for 45 days [17]. These results suggest that the ionic accumulation at the interfaces enhances recombination, most likely, trap-assisted carrier recombination, thus yielding lower values of VOC. After storing the devices under dry air  
Figure 3: VOC stability over time for CsFAMAPbIBr on TiO2 and SnO2 for fresh and aged devices. Figure 2: (**A**) Carrier lifetime obtained from TPV decays as a function of the carrier density after the subtraction of the geometric capacitance. (**B**) Carrier density obtained from the integration of the capacitance and after the subtraction of the geometric capacitance. The orange circles represent the TiO2/CsFAMAPbIBr, and blue diamonds represent the SnO2/CsFAMAPbIBr. In both cases, the filled symbols represent the fresh samples and the empty symbols—the samples after the aging treatment.