A first-principles investigation of lead-free novel direct band gap double perovskite oxides X2AlBiO6 (X = Mg, Ca, Ba) for implementation in optoelectronic and thermoelectric technologies

In this study, the structural, electronic, optical, and thermoelectric properties of novel double perovskite oxide materials X${}_2$AlBiO${}_6$ (X = Mg, Ca, Ba) are investigated using the widely used density functional theory. The tolerance factor, octahedral factor, and cohesive energy provide evidence of the structural and thermodynamic stability of these novel cubic materials. The calculated band structures using the Trans-Blaha modified Becke–Johnson potential show that the compounds exhibit a direct energy gap of 2.42 eV for Mg${}_2$AlBiO${}_6$, 2.15 eV for Ca${}_2$AlBiO${}_6$ and 1.3 eV for Ba${}_2$AlBiO${}_6$. In particular, the band gap of Ba${}_2$AlBiO${}_6$ is very close to the Shockley-Quiesser band gap value of a single-junction solar cell. Furthermore, optical characteristics show that remarkable absorption occurs within the visible region for Ba${}_2$AlBiO${}_6$ as well as the ultraviolet region for X${}_2$AlBiO${}_6$ (X= Mg, Ca). Finally, the compounds demonstrate notable thermoelectric properties, emphasizing the enhanced Seebeck coefficient at the golden range with a high figure of merit. Therefore, the calculated properties indicate that these materials are promising candidates for optoelectronic applications, such as photovoltaics, photocatalysts, and thermoelectric devices in the semiconductor industry.