The unprecedented increase in efficiency of perovskite solar cells in only a few years has not been followed with a correspondingly steep increase in the understanding of the fundamental properties of the devices. Besides their remarkable versatility, perovskite solar cells have puzzling properties such as hysteretic behaviour, that changes with the solar cell configuration or composition, the scan speed and even with temperature. It has been suggested that ionic movement inside the perovskite may explain not only the hysteretic behaviour, but also the low stability of the system, and it has been proposed through computational calculations, that the iodide ion is the most likely to move through the perovskite structure, with activation energies of the order of 0.6 eV. However, experimental measurements of ionic movement inside the perovskite layer can be difficult due to low stability of the perovskite cells and the fact that the results are usually open to interpretation. In spite of these difficulties, remarkable similar values of the activation energy for iodide diffusion in methylammonium lead iodide (MAPbI₃) perovskite solar cells have been obtained through experimental techniques such as electrochemical impedance spectroscopy (EIS), intensity modulated photovoltage spectroscopy (IMVS) and open-circuit photovoltage decay (OCVD) measurements. However, a profound understanding of the nature of the ionic movement inside the perovskite solar cells is still required in order to apply these characterization techniques to perovskite solar cells with a broader chemical composition.

In this work we started with the well-studied MAPbI₃ perovskite system and gradually change the halide concentration, replacing the iodide ion with the smaller bromide ion. Changes in the response of the system to small perturbations of potential (EIS) and light (IMVS) as a function of temperature and halide composition are reported . Our results show that experimental results usually associated with iodide movement are clearly influenced by bromide concentration in the perovskite. MAPbBr₃ Nyquist plots show that the semi-circle located at middle-frequencies is not necessarily related with iodide diffusion and that the activation energies for the ionic movement changes as the bromide concentration is increased. These results show the influence of the ionic environment and crystal structure on ion diffusion inside the perovskite layer and provide a broader picture of the nature of ionic movement along with the characterization techniques employed to obtain the activation energies of ionic movement in perovskite solar cells.