In this study, experimental photovoltaic performance and their numerical SCAPS-1D simulations are compared for methylammonium-free perovskite solar devices based on the formamidinium organic cation. Experimentally, it is established that the incorporation of small amounts of cesium (Cs+) and bromide (Br-) into HC (NH2)(2)Pbl(3)(FAPI) stabilizes the optically active alpha - FAPbI(3) black phase, and boost the power conversion efficiency (PCE) of associated devices from 4 to 15% under standard illumination. The effect of series and shunt resistances (R-series & R-shunt) were theoretically evaluated and discussed by modeling the electrical characteristics of the cell as a function of active layer composition, using the SCAPS-1D software. Ideal devices built without these parasitic resistances do not match the experimental trends, although they reflect the influence of bandgap edge on the photocurrent generation. Feeding the experimental R-series and R-shunt values to SCAPS allows us to interpret the main limitations to the device current-voltage characteristics. The instability of the pure FAH phase is responsible for the drastic deterioration in R-series and R-shunt ultimately influencing the fill factor. Our data clearly confirm the beneficial effect of mixed cation and mixed halides on device operation.