First-principles study on structural, mechanical and optoelectronic properties of lead-free mixed Ge-Sn hybrid organic-inorganic perovskites

Lead-based perovskite materials are commercially restricted as ideal candidates for solar cell applications due to their toxic nature. Here, we have performed a first-principles investigation of structural, mechanical and optoelectronic properties of the orthorhombic perovskite structures of MAGe(1-x)Sn(x)I(3) (MA = CH3NH3) by using density functional theory (DFT). In this research work, mixed Ge/Sn in different proportion have been considered that allows additional opportunities to find better and improve the practical performance of perovskites for solar cell application. The calculations have revealed that the investigated materials are direct bandgap semiconducting materials and the obtained bandgaps range between 0.569 eV and 1.615 eV. The optical properties have shown that MAGeI(3), MAGe(0.875)Sn(0.125)I(3), MAGe(0.375)Sn(0.625)I(3) and MASnI(3) have an excellent absorption coefficient for visible light that makes them useful as photovoltaic materials. Further mixed materials exhibit higher dielectric performance resulting in higher dispersion of light. These materials also behave as superluminescent semiconducting material at high photon energy. Thus the mixed pemvskite structures improve the performance of the solar cells.