## 4 Conclusions  
In this work, we have measured _J-V_ and IS in perovskite solar cells based on CsFAP-blBr with different active layer thicknesses, from 350 nm to 650 nm. The cell performance in a pristine state is not affected significantly by changes in the active layer thickness. Similar ideality factors, \(n=\)1.2-1.5, have been obtained from the dependence of the \(R_{\text{LF}}\), \(C_{\text{LF}}\) and \(R_{\text{HF}}\) on \(V_{\text{OC}}\), indicating that SRH is the main recombination mechanism. Interestingly, the degradation pattern varies significantly depending on the perovskite thickness. The thickest samples, 650 nm, show a more pronounced burn-in process and no stabilization of the cell efficiency when compared to 350 and 500 nm samples. Moreover, IS measurements at different degradation stages show three distinctive features that have been fitted with a three RC/CPE elements equivalent circuit. \(C_{\text{LF}}\) rises with degradation time in the 650 nm cell, indicating an increase in the ionic mobile species. Moreover, the medium frequency impedance increases dramatically as a result of a significant decay of \(C_{\text{MF}}\) and \(R_{\text{MF}}\) increasing an order of magnitude. This medium frequency behavior is attributed to the appearance of a low-conductivity intermediate layer for the thickest devices. Finally, numerical simulations reveal that this higher ion population partially screens the electric field in the perovskite, which is ultimately responsible for the reduction in the carrier  
Figure 11: Electric field profile and electron concentration along the active layer at short circuit conditions. Active layer is 350 nm (black) and 650 nm (red) thick, and ion concentrations are set to 0 (line) and \(10^{17}\) cm\({}^{-3}\) (dashed).  
extraction and, therefore, in the cell efficiency. Therefore, we can conclude that for this configuration, thickness is not a determinant for pristine device performance but results in a key parameter when searching for better long-term stability.  
All authors contributed to the manuscript and were involved in the discussion of results. Device manufacture, Y.G. and M.N.; characterization and circuital modeling, M.C.L.-G., L.M.-D., J.C.P.-M., G.d.P. and E.H.-B.; data processing, M.C.L.-G., L.M.-D. and J.C.P.-M.; physical simulations, D.M.-M. and G.d.P.; writing-review and editing, B.A., B.R. and M.C.L.-G.; funding acquisition, B.A., B.R. and Y.G. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Comunidad de Madrid under the SINFOTON2-CM Research Program (S2018/NMT-4326-SINFOTON2-CM), and by Universidad Rey Juan Carlos with research projects \"Materials nanoresamblados para sensado y manipulation de luz en ample rango spectral\", reference M2417, and by the Community of Madrid in the framework of the Multinannual Agreement with the Rey Juan Carlos University in line of action 1, \"Encouragement of Young Phd students investigation\" Project Ref. M2180, \"SolGenALEE\", and \"Grupo DELFO de alto rendimiento\", reference M2363, under research program \"Programa de fomento y desarrollo de la investigacion\". Y.G. acknowledges the Ministry of Science and Technology of Taiwan (MOST) for the financial support provided by grant with project number 110-2222-E-002-001-MY3, and by National Taiwan University with grant number 110L104042.  
The authors declare no conflict of interest.