Its lower bandgap makes formamidinium lead iodide (FAPbI(3)) a more suitable candidate for single-junction solar cells than pure methylammonium lead iodide (MAPbI(3)). However, its structural and thermodynamic stability is improved by introducing a significant amount of MA and bromide, both of which increase the bandgap and amplify trade-off between the photocurrent and photovoltage. Here, we simultaneously stabilized FAPbI(3 )into a cubic lattice and minimized the formation of photoinactive phases such as hexagonal FAPbI(3) and PbI2 by introducing 5% MAPbBr(3) , as revealed by synchrotron X-ray scattering. We were able to stabilize the composition (FA(0.95)MA(0.05)Cs(0.05))Pb(I-0.95 Br-0.05)(3), which exhibits a minimal trade-off between the photocurrent and photovoltage. This material shows low energetic disorder and improved charge-carrier dynamics as revealed by photothermal deflection spectroscopy (PDS) and transient absorption spectroscopy (TAS), respectively. This allowed the fabrication of operationally stable perovskite solar cells yielding reproducible efficiencies approaching 22%.