Feasibility of cubic and trigonal KGeBr3 perovskite material in photovoltaic applications: Structural, optical, electronic, and mechanical properties via a DFT study

This study investigates potassium germanium bromide (KGeBr3) as a lead-free perovskite for photovoltaic applications using Density Functional Theory (DFT) with Quantum ESPRESSO. The cubic phase of KGeBr3 features a direct band gap of 0.57 eV, ideal for infrared absorption in multi-junction solar cells, while the trigonal phase shows a 0.97 eV band gap suitable for single-junction solar cells in the visible spectrum. The cubic phase demonstrates superior mechanical properties, with higher elastic constants and moduli, making it more resistant to stress. Additionally, it has a higher absorption coefficient (similar to 4.5 x 10(5) cm(-1)) in the visible range. Both phases meet the Born stability criteria, confirming mechanical stability. The cubic phase's higher Debye temperature (182.75 K) compared to the trigonal phase (153.59 K) suggests stronger atomic bonds and greater thermal stability, highlighting its potential in photovoltaic technologies.