Theoretical optoelectronic analysis of MgIn2S4 and CdIn2S4 thiospinels: Effect of transition-metal substitution in intermediate-band formation

We analyzed and compared two families of intermediate-band materials derived from indium thiospinels: MgIn2S4, having an inverse spinel structure, and CdIn2S4, a direct spinel. First-principles studies of the electronic structures of these two parent semiconductors were carried out to understand the nature of their band gaps. Optical properties were also analyzed and we found good agreement with experiments. As derivatives of these semiconductors, alloys where transition metals (M = Ti and V) substitute for In atoms at octahedral sites are presented as a class of intermediate-band materials. First, the effect of the substitution on structural parameters is assessed. Then, electronic structures are determined for Mg2MIn3S8 and Cd2MIn3S8 to show that the t(2g) d states of the transition metal form a partially filled localized band within the band gap of the host semiconductor. The suitability of these compounds as photovoltaic high-efficiency absorbers is discussed. An increase in absorption is assessed by studying the contribution of the transition-metal band toward their optical properties, in the range of higher solar emission, and comparing them with those of the host semiconductors. An analysis of transmittance spectra is carried out to predict the range of optimum thicknesses for samples of this type of thin film absorber. We compare, by means of structural, electronic, and optical behavior, Ti and V as substituents, to evaluate the resulting alloys for potential photovoltaic applications.