Copper (Cu) or germanium (Ge) was added to the perovskite compounds to stabilize the photoactive α-phase of formamidinium-cesium lead triiodide. The strain for the PbI₆ octahedra was relaxed by the Cu doping, which increased the displacement of formamidinium (FA) molecules, resulting in increasing the kinetic energy. The 3d orbitals of Cu were localized near the conduction band minimum and valence band maximum, which suppressed carrier diffusion resulting in lower efficiencies of the cells. The structural distortion for PbI₆ octahedra was caused by the Ge doping, which decreased the displacement of FA molecules, resulting in a decrease in the kinetic energy. Suppression of formation of photo-inactive δ-phase resulted in formation of α-phase. Electron density distribution showed the charge transfer from Ge to iodine (I) ions, which promoted carrier diffusion from I 5p to Ge 4p orbitals. The cells doped with 12.5% Ge also provided photovoltaic properties.