###### Abstract  
Perovskite solar cells (PSCs) have become very popular due to the high efficiencies achieved. Nevertheless, one of the main challenges for their commercialization is to solve their instability issues. A thorough understanding of the processes taking place in the device is key for the development of this technology. Herein, _J-V_ measurements have been performed to characterize PSCs with different active layer thicknesses. The solar cells' parameters in pristine devices show no significant dependence on the active layer thickness. However, the evolution of the solar cells' efficiency under ISOS-L1 protocol reveals a dramatic burn-in degradation, more pronounced for thicker devices. Samples were also characterized using impedance spectroscopy (IS) at different degradation stages, and data were fitted to a three RC/RPCE circuit. The low frequency capacitance in the thickest samples suffers a strong increase with time, which suggests a significant growth in the mobile ion population. This increase in the ion density partially screens the electric field, which yields a reduction in the extracted current and, consequently, the efficiency. This paper has been validated with two-dimensional numerical simulations that corroborate (i) the decrease in the internal electric field in dark conditions in 650 nm devices, and (ii) the consequent reduction in the carrier drift and, therefore, of the effective current extraction and efficiency. 1 2021 11 11668 116  
However, before entering the market, there are still some issues to be addressed, such as hysteresis [4; 5] and, particularly, device instability [6]. The instability of PSCs mainly originates from the degradation of the perovskite active layer as well as the degradation of the interfaces and electrodes. Degradation causes can be extrinsic, such as moisture [7; 8], oxygen [9; 10], UV light [11; 12], radiation [13] and temperature [14; 15], or intrinsic, such as chemical instability of the perovskite active layer [16] and instability associated with the mixed electronic/ionic properties of the device. The latter includes ion migration [17] that can reduce interlayer conductivity and, therefore, current extraction and device efficiency. Understanding the electronic and ionic processes that take place in the bulk and contact layers is a key issue to control the device performance and ultimately boost the development of this technology. Methylammonium lead iodide (CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) or MAPI) or formamidinium lead iodide (CH(NH\({}_{2}\))\({}_{2}\)PbI\({}_{3}\) or FAPI) have been widely used as PSCs' active layers. However, in the last years, mixed-cation and mixed-halide perovskites have become very popular due to their high efficiencies and improved stability [18; 19]. Indeed, many research groups are devoted to material engineering of mixed-cation and mixed-halide perovskite structures for stable and efficient devices. [PERSON] et al. showed that replacing 10% of formamidinium (FA) by cesium (C\({}_{5}\)) resulted in an improvement in the photo and moisture stability of the perovskite films [20]. This was attributed to an enhancement in the interaction between FA and iodide due to the reduction in the cube-octahedral volume. Regarding the anion, it was demonstrated that increasing the bromine (Br) amount raises the conduction band of perovskite and lowers the valence band. This results in a shift in the absorption edge towards shorter wavelengths, which in turn increases the energy gap and leads to higher open-circuit voltages (\(V_{\text{OC}}\)) without compromising the short circuit current (\(J_{\text{SC}}\)) [21; 22]. On the other hand, the active layer thickness is one of the most important parameters to optimize the device layer structure. Indeed, there is a trade-off between light absorption, which increases with perovskite thickness, and charge extraction, which can be limited in thicker active layers due to inefficient carrier transport or reduced charge mobility.