First Principle Investigation of CsXBr3 (X=Si, Ge) for Solar Cells Applications

Abstract

Abstract Lead has been at the core of many recent studies on solar cells because it has unique properties that enhance light absorption and charge transport. Most of these studies are not practically employed due to the toxicity and instability of lead. This study focused on finding an alternative to lead that maintains or exceeds its performance in solar cells. The development of environmentally friendly, stable, and efficient perovskite materials for future solar cell applications has long-term practical significance and can eventually be commercialized. The current reported efficiency in MAPbBr3 is of the order of 22%, and this research aims to explore whether this efficiency could be improved beyond having lead in the crystal. It is for this reason that density functional theory, using the GGA-PBE and HSE06 correlation functionals, was employed on CsXBr3 (X=Si, Ge) to understand their optoelectronic properties and to calculate their efficiency as potential solar cell materials. The trends in electronic band structure in the cubic phase of CsXBr3 all inorganic perovskites for X= Cs, Ge were determined. The absorption spectrum was calculated using a simplified mathematical function that approximated the real spectrum. Here, the absorption spectra of CsXBr3 (X = Si, Ge) perovskites were calculated at the quantum-mechanical level of accuracy using the DFT method. This realistic absorption spectrum was employed in Quantum Espresso and Siesta, with post-processing using the Yambo package. The materials exhibited optical absorption of over 105 cm in the ultraviolet spectrum. Determination of elastic, electronic, structural, and optical properties in the cubic stable phase of CsXBr3 (X=Si, Ge) was done, and the solar conversion efficiency of CsXBr3 (X=Si, Ge) were 11.02 % and 11.65 % respectively, under standard solar illumination conditions. The band gap of 0.7eV and the Poisson’s ratio (ν) of 0.36 for CsGeBr3 were obtained. While for CsSiBr3, the band gap of 0.29 eV and Poisson’s ratio (ν) of 0.255 were obtained. Dielectric constants, which include the real (ε1) and imaginary part (ε2) of CsGeBr3 and CsSiBr3, were 2.0 eV and 2.7 eV, and the absorption begun from 2.5eV to match the band gap. The results obtained can be used in experimental research to develop CsGeBr3 and CsSiBr3 materials as potential high-performance solar cells with higher efficiencies and good performance, and to verify experimentally the use of CsGeBr3 and CsSiBr3 perovskites in solar cells. The results are significant in determining a suitable solar cell material for industrial applications. The study dealt only with theoretical findings on the structural, electronic, dielectric, and elastic properties of CsGeBr3 and CsSiBr3 at ground-state conditions. The structures were optimized using the Murnaghan equation of state in LUNA Quantum Espresso. It was concluded that the cubic phase of CsGeBr3 and CsSiBr3 are suitable materials for solar cell materials because of their suitable band gap. An extension of this work could be to use other exchange-correlation functionals.