Electronic, structural, and optical properties of Cs2SnGeX6 (X = I, Br, Cl) and mixed halides for solar cell applications

The mixed metal-halide perovskites of the form ABX3, (where, A is the organic or inorganic cation, B is the metal cation, and X is the halide anion) have a wide range of tunable structural and electronic properties leading to intense research over the past decade. This study reports the density functional theory calculations on the sn-Ge based perovskite Cs2SnGeX6 (where, X = I, Br, Cl), for determining the most stable structural configuration and the corresponding electronic properties based on GGA (Generalized Gradient Approximation) functional. In addition, two mixed halide variants like Cs2SnGeI3Br3 and Cs2SnGeBr3Cl3 are also simulated. The structures show a broadly tunable bandgap of 0.46 to 1.45 eV by the substitution of various halides in the mixed sn-Ge-based perovskite. The halide perovskites show ambipolar transport behavior with long diffusion lengths. The charge density and electronegativity difference analysis show the predominant ionic nature between the Cesium and halogen atoms and the covalent nature of bonding between the Tin/Germanium and halogen atoms. In the mixed halides, an asymmetric charge distribution is visualized for the first time, which is resulting due to the disproportionate bond strength caused by the difference in the electronegativities of I-Br and Br-Cl. This helps in the manifestation of stereochemically-active lone-pair electrons in Sn and Ge which lead to large variations in the Sn(Ge)-halogen bond lengths and enable the high mobility of the charge carriers. The major optical absorption is due to the transition between the hybridized Sn/Ge(s)-X(p) states to Sn/Ge(p) states. The long charge carrier diffusion lengths and significant covalent bonding interactions can enable in improving the efficiency and structural stability of these materials, making them suitable for solar cell applications.