Organic inorganic (hybrid) perovskite solar cells are considered one of the most promising technologies for future photovoltaics, due to the fast rise of the achievable power conversion efficiency (PCE). Besides the thorough understanding of the effect of morphology and crystallinity on the optical and electronic behavior of this class of semiconductors, the recent breakthrough in terms of device efficiency have been achieved through a rational chemical design of the perovskite absorbers. Apart from the substitution of the monovalent cation A and the halide X in the general perovskite formula AMX₃, the impact of the partial exchange of the divalent metal M on the optoelectronic properties of the semiconductor has been investigated on a much more limited basis. In this work we describe the partial replacement of Pb²⁺ by Sr²⁺ in MAPbI₃ and its effect on the morphological, optical and electronic behavior of the material. We found the addition of Sr²⁺ in MAPbI₃ to strongly enhance the charge carrier collection efficiency of the cells leading to increased current densities and very high fill factors (FF), going from 78% for the pure perovskite, up to 85% in the presence of 2% Sr²⁺. To gain insight on the marked improvements in the FF and current collection, the charge carrier dynamics of the MAPbI₃:Sr²⁺ films were investigated using time-resolved microwave conductivity (TRMC) measurements. These measurements indicate that, at low charge carrier concentrations (~10¹⁴ cm^-3), the charge carrier lifetime of Sr²⁺ containing perovskites is in excess of 40 μs. Such carrier lifetimes are among the longest observed for this family of perovskites, and even longer than those reported for perovskite single crystals. We also examine the surface electronic properties of the perovskite absorber layer, by means of photoemission spectroscopy to further detail the origin of the high charge carrier lifetimes as well as the enhancement in fill factor. We observe a small change in the perovskite work function as well as a significant Sr²⁺ enrichment at the thin-film surface, which might explain, in part, the altered photovoltaic performance. The observations of perovskite solar cells with fill factors as high as 85%, obtained merely by the addition of very small amounts of a different cation, is remarkable and can be easily extended to other hybrid lead halide perovskites, potentially boosting further the already high power efficiencies.