The stability of CsGeX₃ (X = I, Br, Cl) selectively tuned by the crystal structures and halide ions as inferred from the calculated phonon spectrum

In spite of the considerable advancements achieved in enhancing the power conversion efficiency (PCE) of lead-based all-inorganic perovskite solar cells, there persists a need for materials that are both more stable and environmentally friendly. This investigation systematically explores the structural and thermodynamic stability, and electronic properties of Ge-based all-inorganic perovskite CsGeX₃ (X = Cl, Br, I) in two space groups, <i>Pm</i>3̄<i>m</i> and <i>R</i>3<i>m</i>, utilizing first-principles calculations. Introducing the novel concept of the "imaginary frequency coefficient" alongside the tolerance factor and stabilizing the chemical potential window, we collectively characterize the stability of CsGeX₃ based on the phonon spectrum and phonon density of states calculations. The findings reveal that the stable phase of the Ge-based perovskite differs from that of lead-based systems, with the <i>R</i>3<i>m</i> structure of CsGeX₃ being the most stable in the rhombohedral phase. Moreover, the stability of <i>R</i>3<i>m</i>-CsGeX₃ can be manipulated by adjusting the halide composition with a gradual increase in stability observed as halogen atoms shift from I to Cl. This comprehensive approach, integrating the phonon spectrum, innovative measurement indicators, and tolerance factor, presents an effective strategy for designing materials that are both non-toxic and stable.