Transition-Metal-Substituted Indium Thiospinels as Novel Intermediate-Band Materials: Prediction and Understanding of Their Electronic Properties

Results of density-functional calculations for indium thiospinel semiconductors substituted at octahedral sites with isolated transition metals ($M=\mathrm{Ti},\mathrm{V}$) show an isolated partially filled narrow band containing three $t_{2g}$-type states per $M$ atom inside the usual semiconductor band gap. Thanks to this electronic structure feature, these materials will allow the absorption of photons with energy below the band gap, in addition to the normal light absorption of a semiconductor. To our knowledge, we demonstrate for the first time the formation of an isolated intermediate electronic band structure through $M$ substitution at octahedral sites in a semiconductor, leading to an enhancement of the absorption coefficient in both infrared and visible ranges of the solar spectrum. This electronic structure feature could be applied for developing a new third-generation photovoltaic cell.

Figure 1

DOS and PDOS obtained with GGA calculations for ${\mathrm{In}}_{14}{\mathrm{M}}_2{\mathrm{S}}_{24}$ alloy materials and the ${\mathrm{In}}_2{\mathrm{S}}_3$ host semiconductor. (a),(b) $M=V$, (c) ${\mathrm{In}}_2{\mathrm{S}}_3$, and (d),(e) $M=\mathrm{Ti}$. Plots (b) and (d) show the total DOS and the partial $M$ contribution to the DOS; plots (a) and (e) show the splitting of the $M$ $d$-orbitals manifold into $t_{2g}$- and $e_g$-type components. The Fermi level is set to $E=0$.

Figure 2

DOS and PDOS curves obtained with GGA calculations for ${\mathrm{Mg}}_2{\mathrm{In}}_3{\mathrm{MS}}_8$ alloy materials and ${\mathrm{MgIn}}_2{\mathrm{S}}_4$ host semiconductor. (a) ${\mathrm{MgIn}}_2{\mathrm{S}}_4$, (b) $M=V$ (left panel) and $M=\mathrm{Ti}$ (right panel). Plot (c) displays the spin-up band diagrams (including high symmetry $k$ points of the Brillouin zones labeled as corresponds to the semiconductor space groups) for the alloy system: $M=V$ (left panel) and $M=\mathrm{Ti}$ (right panel). The Fermi level is set to $E=0$.

Figure 3

$\omega$-weighted imaginary part ${ϵ}^{(2)}$ of the total dielectric function (solid line) and the partial contribution of the different direct transitions to it, computed for the ${\mathrm{Mg}}_2{\mathrm{In}}_3{\mathrm{VS}}_8$ systems. The inset shows the optical absorption computed from the dielectric function for ${\mathrm{Mg}}_2{\mathrm{In}}_3{\mathrm{VS}}_8$ (solid line) and the semiconductor spinel ${\mathrm{MgIn}}_2{\mathrm{S}}_4$ (dots).