## 3 Results and Discussion
### Degradation Characterization  
In order to characterize cell degradation, we monitored the \(J\)-\(V\) curves of cells under one sun illumination until the efficiency stabilized. Figure 5 shows the time evolution of the normalized parameters of one typical cell for the three different thicknesses. We observe a fast and abrupt decay of the cell efficiency during the first minutes of degradation. This dramatic drop in the efficiency is commonly found in organic [47] and perovskite [48] solar cells, and it is known as the \"burn-in\" effect. The causes of this burn-in degradation have been previously attributed to a worsening of the current extraction, energy transfer, or enhancing of non-radiative recombination, and/or the formation of trap states in the bandgap [49]. As we can observe in Figure 5, the time evolution of the efficiency for the three active layer thicknesses shows similar trends but different degradation parameters, \(T_{80}\) and stabilization times. Thin samples stabilize faster losing only 20% of their initial value. For medium samples, the stabilization takes almost the same time, around 5 min, but the decrease in the efficiency is almost 40% of the initial value. Finally, the worst scenario is found in the thickest samples, where the efficiency does not stabilize within the first 40 min of operation, reaching values lower than 30% of the initial efficiency. On the other hand, the efficiency decay is dominated in all samples by a dramatic drop in \(J_{\text{SC}}\), which decreases by a similar percentage, while \(V_{\text{OC}}\) and FF barely change (Figure 5). Impedance measurements were carried out in similar samples during the degradation experiments, starting after the burn-in process. Results of the IS characterization can be found in Figure 6, where Nyquist plots for the 350 nm (a) and 650 nm (b) samples are  
Figure 5: Normalized characteristic parameters, (**a**) PCE, (**b**) \(J_{\text{SC}}\), (**c**) FF and (**d**) \(V_{\text{OC}}\), of the different samples as function of the illumination time. Missing points are discontinuities found when disconnecting the samples for IS characterization. represented at different degradation stages. After 45 min of degradation, the thinner sample does not show any significant change in the Cole-Cole diagram. However, for the thickest sample, a significant increase in both the high frequency and the medium-low frequency arcs, is observed. Cole-Cole diagrams at different degradation stages for the 350 nm and 650 nm were fitted with the equivalent circuit described above. Figures 7 and 8 present the evolution of the circuital capacitances and resistances with the irradiation time for the thinnest (a) and thickest (b) sample, respectively. In the case of 350 nm, low and high frequency parameters do not vary significantly. Regarding medium frequency parameters, \(R_{\text{MF}}\) increases while \(C_{\text{MF}}\) decreases, which yields an increment in the medium frequency impedance. As has been previously mentioned, the authors believe that this feature is associated with the impedance of a low-conductivity intermediate layer. The reduction in \(C_{\text{MF}}\) is clearly more pronounced in the 65 nm device, falling from 5.8 \(\times\) 10\({}^{-3}\) F down to 5 \(\times\) 10\({}^{-6}\) F. This huge drop, together with the increase in \(R_{\text{MF}}\) in an order of magnitude, from 3 up to 27 \(\Omega\), yields to a dramatic increase in medium frequency impedance. This remarkable rise supports our hypothesis of the appearance of a low-conductivity intermediate layer for the thickest devices. We believe that this intermediate layer is directly related to an inefficient carrier extraction caused by the accumulation of mobile ions at one or both active layer interfaces, as will be discussed and supported by numerical simulations in the next section. Figure 6: Nyquist plots during degradation time of (**a**) 350 nm and (**b**) 650 nm samples. Figure 7: Evolution of resistances with irradiation time for (**a**) 350 nm and (**b**) 650 nm.