Cation compositional engineering has revealed a powerful design tool to manipulate the perovskite structural and optoelectronic characteristics with a tremendous impact on device performances. Tuning the bandgap by cation and anion compositional mixing, for instance, is paramount to target different optoelectronic segments, from light emitting applications to tandem solar cells. However, structural and photo instabilities, and phase segregation come along, imposing a severe control on the material composition and structure. Here we develop highly uniform alloy of mixed cation FA(1−x)CsxPbBr3 perovskite thin films pushing for the first time the Cs content up to 30%. In contrast to what has been reported so far, this composition leads to a high quality crystalline film, maintaining a single cubic phase arrangement. In addition, a remarkably high robustness against moisture and phase purity is observed. The experimental finding is also supported by density functional theory simulations, demonstrating at the atomistic level Cs segregation starting from Cs concentration around 37.5%. Beyond that, phase segregation happens, leading to formation of an unstable pure Cs-rich region. Low temperature photoluminescence (PL) measurements reveal that the addition of Cs eliminates the non-radiative channel into mid-gap traps, as evident by the lack of the broad emission band, often associated with recombination of self-trapped exciton, present for 0% Cs. This, in turn, reduces the non-radiative recombination losses which manifests as high performance solar cells. Indeed, when embodied in solar devices, Cs incorporation leads to enhanced device performances, with an open circuit voltage beyond 1.33 V.