Enhanced electronic, optical and mechanical properties of non-toxic KGeX3 (X=Br, I) trigonal-perovskites by Sn-doping: A DFT study with photovoltaic application

Density Functional Theory (DFT) is one of the key computational tools to gain insights into the effects of doping on the electronic and optical properties of perovskite photovoltaics. This article investigates the effect of Sn-doping on the structural, mechanical, electronic and optical properties of KGeX3 (X = Br, I) perovskites with a trigonal crystal structure by DFT analysis. All the Sn-doped perovskite compositions are confirmed to be thermodynamically stable and direct band gap semiconductors. It is observed that Sn-doping makes the formation energy more negative and decreases the band gap within the perovskite compounds. In the undoped state the band gaps of KGeBr3 and KGeI3 are 2.460 eV and 1.453 eV, respectively and with Sn doping, these values decrease to 0.793 eV and 0.808 eV. The optical absorbance, static dielectric constant and static refractive index also show enhancement due to Sn-doping. Notably, a significant amount of Sn-doping in iodide perovskite (KGeI3) improves the elastic modulus and ductility (B/G), whereas its effect on elastic modulus in bromide perovskite (KGeBr3) is minimal. Moreover, the perovskite compositions kGeI3, KSn0.67Ge0.33Br3, and KSn0.33Ge0.67I3 with direct band gap values 1.453 eV, 1.694 eV and 1.422 eV respectively exhibit great potential as a light-harvesting layer in perovskite solar cells (PSC).