Halide perovskite derivatives present unprecedented physical phenomena among those materials suitable for photovoltaics, such as a fast ion diffusion coefficient. Here we report how to take benefit from this property during the growth of halide perovskite in order to control the morphological and optoelectronic properties of the final thin film. Using a large enough halide reservoir, the nature of the halides present in the final perovskite layer can be exchanged respect the initial salt used in the two step deposition method. In particular, we report the preparation of methylammonium lead bromide (MAPbBr 3 ) thin film using a two-step method based on the transformation of PbI 2 , PbBr 2 and PbCl 2 salts into MAPbBr 3 perovskite after dipping in a MABr solution. The films prepared from different salts present different properties in terms of morphology and optoelectronic properties, thus providing significantly different performance when they are used for the preparation of photovoltaic devices. Interestingly, the use of PbI 2 and PbCl 2 salts reduce the charge recombination and increases the obtained open circuit potential, especially in the former case. However, the highest photocurrent is obtained when PbBr 2 is used. While for PbI 2 and PbCl 2 salts no traces of the former salt are observed in the obtained MAPbBr 3 layer after 10 minutes of dipping time, the presence of PbBr 2 still been detected when this salt is employed as it has been determined by Xray diffraction.2

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