An Ab Initio Insight into the Structural, Mechanical, Electronic and Optical Properties of Cesium Based Double Perovskites Cs 2 Ge 2 X 6 (X=Cl, Br, I)

The Cubic Cs 2 Ge 2 X 6 (X=Cl, Br, I) double perovskite's structural, electronic, elastic, mechanical stability, phonons and optical properties are examined within the framework of Density Functional Theory (DFT). The calculated formation energy ∆H F and Goldschmidt tolerance factor ( t ) confirm the stability of the cubic phase in Cs 2 Ge 2 X 6 (X = Cl, Br, I) compounds. Direct band gap (E g ) values for Cs 2 Ge 2 Cl 6 , Cs 2 Ge 2 Br 6 , and Cs 2 Ge 2 I 6 are 1.02 eV, 0.98 eV, and 0.89 eV, correspondingly. Excellent optical absorption qualities were revealed by the computed absorption spectra of Cs 2 Ge 2 Cl 6 , Cs 2 Ge 2 Br 6 , and Cs 2 Ge 2 I 6 indicating their applicability for photovoltaic applications. Based on the calculated optical spectra of the investigated compounds, the noted maximum absorption coefficients were 1.8 × 10 5 cm -1 for Cs 2 Ge 2 Cl 6 , 3.3 × 10 5 cm -1 for Cs 2 Ge 2 Br 6 and 8.8 × 10 4 cm -1 for Cs 2 Ge 2 I 6 . Among the investigated compounds, Cs 2 Ge 2 B 6 exhibited the highest absorption coefficient, indicating high light harvesting capability and making it the most suitable candidate for thin film solar cells. Although Cs 2 Ge 2 Cl 6 displays a slightly lower absorption coefficient, it possesses excellent optical absorption in the visible and ultraviolet regions. The high absorption coefficient demonstrate that these materials are perfect for solar cells.