Structural, electronic, mechanical, and optical properties of the lead-free halide perovskites XGeCl3(X = Cs, K, and Rb) for the photovoltaic and optoelectronic applications

Abstract

The present investigations into the chloroperovskites CsGeCl3, KGeCl3, and RbGeCl3 are conducted by using the density functional theory-based CASTEP tool with the GGA-PBE method. The space group Pm-3m (221) is used. The computed values of lattice parameters are CsGeCl3: 5.34 Å, KGeCl3: 5.26 Å, and RbGeCl3: 5.29 Å, and also an inverse relationship between volume and density is observed. Total density of states (DOS) and partial density of states (PDOS) calculations are performed to assess the semiconducting properties of the materials. The calculated energy bandgaps, indicative of their direct semiconducting nature are as follows: CsGeCl3: 1.076 eV, KGeCl3: 0.874 eV, and RbGeCl3: 0.952 eV. In the context of mechanical properties, the various mechanical parameters are determined using single-elastic constants, revealing that XGeCl3 (X=Cs, K, Rb) exhibits mechanical stability and anisotropic characteristics. Furthermore, all materials are ductile and hard because Pugh's ratio is greater than 1.75 and Vicker's hardness is less than 40 GPa, respectively. All materials contain the same value of compressibility. Regarding optical characteristics, the loss function, refractive index, dielectric function, optical conductivity, absorption, and reflectivity are comprehensively studied. It is found that the strong peaks of absorption, conductivity (Re & Im), plasmon peak, dielectric functions (Im), refractive indices (Im), and reflectivity fall in the UV region (10 nm-400 nm), while the largest peaks of dielectric functions (Re) and refractive indices (Re) parts belong to the visible region (400 nm- 700 nm). These compounds hold significant promise for applications in photovoltaics and optoelectronics.