First principle studies of thermo-physical and optoelectronic properties of LiBP (B = Zn, Mg) Half-Heusler alloys for photovoltaic application

Abstract

This study employs first-principles density functional theory (DFT) calculations to investigate the thermophysical and optoelectronic properties of LiBP (B = Zn, Mg) Half-Heusler (HH) alloys, with a focus on their potential photovoltaic and thermoelectric applications. The key findings are their semiconducting nature with direct band gaps of $E_g=1.34\mathrm{eV}$ (LiZnP) and $E_g=1.34\mathrm{eV}$ (LiMgP), calculated using the PBE-GGA functional. Lithium Zinc Phosphide (LiZnP) demonstrates a higher refractive index ($n=2.37$), and a larger dielectric constant, ${\varepsilon }_i=5.6$, and stronger optical absorption and light interaction, making it a promising candidate for high absorption optoelectronic applications. In contrast, Lithium Magnesium Phosphide (LiMgP) offers advantages in transparency and lower thermal conductivity. It exhibits a slightly lower refractive index ($n=2.29$), and a lower dielectric constant (${\varepsilon }_i=5.27$). Both materials exhibit mechanical stability, with LiZnP displaying a higher Bulk modulus, $B=637.408$, and Shear modulus, $G=507.533$, making it more incompressible. Both alloys are dynamically stable, as indicated by the absence of imaginary frequencies in the Brillouin zone. LiZnP is particularly advantageous for high-absorption applications, while LiMgP excels in transparency and lower thermal conductivity, with a Debye temperature ${\Theta }_D=489.7$ K and average sound velocity, $V_m=4129.696\mathrm{m/s}$. Both materials are brittle, with Pugh’s modulus ratio, $k$, less than 1.75, and negative Cauchy pressure, $C^{\prime }$. LiZnP exhibits greater hardness, with a Vickers hardness, $H_{\nu }$ of 97.27 compared to 88.46 for LiMgP. Dynamic stability is confirmed by phonon dispersion analysis. The thermoelectric analysis reveals high figures of merit $\mathrm{zT}$ reaching 2.05 for LiZnP and 2.38 for LiMgP at 1000 K. These results confirm LiZnP is more suited for photovoltaic and optoelectronic applications, while LiMgP does well in high-temperature thermoelectric energy conversion, hence giving clear guidance for their future experimental developments.