In the search for novel photovoltaic (PV) solar materials, half-Heusler (hH) compounds have recently been regarded as especially promising due to their favorable electronic and optical properties. Using first-principles density-functional theory and many-body excited-state calculations, we study the quasiparticle band structure, as well as the optical and excitonic properties, of two representative hH compounds, namely, $\mathrm{Li}\mathrm{Zn}\mathrm{As}$ and $\mathrm{Sc}\mathrm{Ag}\mathrm{C}$, for PV applications. Our results reveal a direct-band-gap semiconducting behavior in $\mathrm{Li}\mathrm{Zn}\mathrm{As}$ ($\mathrm{Sc}\mathrm{Ag}\mathrm{C}$) with a value of approximately 1.5 (1.0) eV under the accurate $G_0{W}_0$ method. The highest value of the imaginary part of the dielectric function is found to be approximately 52 (87), 77 (87), and 88 (91) using the independent-quasiparticle approximation, local field effects in the random-phase approximation, and the electron-hole interaction within the Bethe-Salpeter equation, respectively. Both materials demonstrate a high refractive index, high absorption coefficients (approximately $1.2–1.6\times {10}^6{\mathrm{cm}}^{-1}$), and low reflectivity (less than 40%) within the active region of the solar energy spectrum. The triply degenerate bright excitons (exciton A) at the main absorption peak, as well as a considerable number of bright excitonic states in the visible region, are observed; however, the oscillator strengths of the excitons are comparatively weaker in $\mathrm{Sc}\mathrm{Ag}\mathrm{C}$ than in $\mathrm{Li}\mathrm{Zn}\mathrm{As}$. We further discuss the exciton character contributing to intense optical interband transitions and reveal that the direct optical band gap is associated with the loosely bound exciton A state with a binding energy of approximately 45 (56) meV in $\mathrm{Li}\mathrm{Zn}\mathrm{As}$ ($\mathrm{Sc}\mathrm{Ag}\mathrm{C}$). Exciton A is found to be highly localized (delocalized) in momentum (real) space, indicating the presence of Mott-Wannier–type excitons at the band gap. Finally, we assess the solar efficiencies using the spectroscopic limited maximum efficiency (SLME) model and find SLME values of approximately 32% for $\mathrm{Li}\mathrm{Zn}\mathrm{As}$ and approximately 31% for $\mathrm{Sc}\mathrm{Ag}\mathrm{C}$ at an approximately 0.4 $\text{μ}\mathrm{m}$ thin-film thickness. These findings highlight the significant role of excitons in the solar energy absorption process and also suggest that both are highly suitable candidates for next-generation single-junction thin-film PV solar devices.

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• Electronic structure
• Excitons
• Optoelectronics
• Half-Heusler alloy
• Photovoltaic absorbers
• Solar cells
• Ab initio calculations
• Many-body techniques