First-Principles Investigation of the Structural Stability and Physical Properties of Lead-Free Ge-Based Halide Perovskites

Inorganic lead halide perovskite semiconductor materials exhibit great potential in the optoelectronic field due to their excellent optical and electrical properties. However, lead toxicity and limited material stability hinder their commercial applications. Consequently, the pursuit of non-toxic, stable alternatives is imperative for the sustainable development of halide-perovskite semiconductors. Non-toxic germanium-based halide perovskites, as promising candidates, have attracted considerable attention. Here, we present a systematic first-principles investigation of the structural, electronic, elastic, and optical properties of cost-effective germanium-based halide perovskites NaGeX3 (X = Cl, Br, I). Energy and phonon-spectrum calculations demonstrate that NaGeX3 with the R3c space group exhibits the highest structural stability, rather than the commonly assumed cubic phase. Hybrid functional calculations reveal that the band gaps of R3c NaGeX3 decrease monotonically with increasing halogen radius, that is, 4.75 eV (NaGeCl3) → 3.76 eV (NaGeBr3) → 2.69 eV (NaGeI3), accompanied by a reduction in carrier effective masses. Additionally, mechanically stable R3c NaGeX3 exhibits lower hardness and ductility than that of the cubic phase. Optical properties indicate that NaGeX3 materials have strong absorption coefficients (>106 cm−1) and low loss in the photon energy range of 9–11 eV, suggesting that such cost-effective germanium-based halide perovskites can be used in various optoelectronic devices in the ultraviolet region.