Formamidinium (FA)-based hybrid lead halide perovskites (FAPbX3 , X=I or Br/I) have recently led to significant improvements in the performance of perovskite photovoltaics. The remaining major pitfall is the instability of α-FAPbI3 , causing the phase transition from the desired three-dimensional cubic perovskite phase to a non-perovskite one-dimensional hexagonal lattice. In this work, we report the facile, inexpensive, solution-phase growth of cm-scale single crystals (SCs) of variable composition Csx FA1-x PbI3-y Bry (x=0-0.1, y=0-0.6) which exhibit improved phase stability compared to the parent α-FAPbI3 compound. These SCs possess outstanding electronic quality, manifested by a high-carrier mobility-lifetime product of up to 1.2 × 10-1 cm2 V-1 and a low dark carrier density that, combined with the high absorptivity of high-energy photons by Pb and I, allows the sensitive detection of gamma radiation. With stable operation up to 30 V, these novel SCs have been used in a prototype of a gamma-counting dosimeter.