Mg2Si and Ca2Si semiconductors for photovoltaic applications: Calculations based on density-functional theory and the Bethe-Salpeter equation

We conduct a comprehensive assessment of the electronic and optical properties, as well as photovoltaic (PV) performance parameters, for low-cost, nontoxic Mg2Si and Ca2Si using methods based on densityfunctional theory and the Bethe-Salpeter equation (BSE). The band gap for Mg2Si (Ca2Si) is found to be in the range of 0.25-0.6 (0.57-0.96) eV when the Perdew-Burke-Ernzerhof (PBE), PBE for solids (PBEsol), and modified Becke-Johnson (mBJ) functionals are used. The effective masses at the last-filled valence and first-empty conduction bands are in the range of 0.14-0.17 (1.17-1.25) me and 0.27-0.29 (0.3-0.41) me, respectively. In the independent-particle approximation (IPA), the real and imaginary parts of the dielectric function show maximum values of <^> 50 (16.3) at <^> 2.6 (1.0) eV and <^> 61 (16.2) at <^> 3.24 (3.4) eV, respectively. Within the BSE, these respective values change to <^> 59 (17) at <^> 2.5 (0.86) eV and <^> 65 (16.6) at <^> 2.68 (3.1) eV. The excitonic effect is found to be crucial in understanding the experimental optical spectra of Mg2Si. However, this effect is relatively weaker in Ca2Si. In the active region of the solar spectrum, the highest absorption coefficient and lowest reflectivity change from <^> 1.5 (0.75) x 106 cm-1 and <^> 0.39 (0.3), respectively, in the IPA, to <^> 1.6 (0.88) x 106 cm-1 and <^> 0.41 (0.27) in the BSE. The present study highlights the importance of different levels of theoretical approximations for obtaining optical spectroscopy data of silicides with a high level of accuracy. Finally, we have evaluated the PV efficiency by using a spectroscopic limited maximum efficiency (SLME) calculation. On top of radiative recombination, we have also incorporated nonradiative carrier recombination at a defect trap state via the Shockley-Read-Hall (SRH) mechanism to evaluate the efficiency. Among the studied defects, interstitial Mg (Si) is identified as the most stable in Mg2Si (Ca2Si), and this provides an SRH lifetime of <^>2 mu s (11.3 ms). The estimated maximum SLME using the BSE absorption spectrum is <^> 1.3% (31.2%), which decreases to <^> 1.2% (28.5%) due to SRH recombination. The present study suggests that Ca2Si (Mg2Si) is a potential candidate for single-junction (bottom cell in multijunction) thin-film PV devices.