First-principles calculations and device characterizations of formamidinium-cesium lead triiodide perovskite crystals stabilized by germanium or copper

To avoid formation of the photo-inactive d-phase of formamidinium-cesium lead triiodide, copper or germanium was added to the perovskite compounds to stabilize the photoactive a-phase. It was found that the substitution of lead by germanium (Ge) or copper (Cu) provided the stabilization of the a-phase in the present work. The first-principles molecular dynamics calculations indicated that displacements of formamidinium molecules were suppressed by the Ge doping. X-ray diffraction results indicated that the Ge or Cu doping of the perovskite compounds could be effective for suppression the phase transition from a- to d-phase.