Inverse design of stable spinel compounds with high optical absorption via materials genome engineering

Abstract

Conventional semiconductors, e.g., Si and GaAs with tetrahedral coordination (four-fold) structures, possess good stability but relatively low optical absorption compared with halide perovskites with octahedral coordination (six-fold) structures, which exhibit superior optical absorption but relatively poor stability. It is thus desirable to combine the complementary properties of four-fold and six-fold semiconductors to have a system with both good stability and optical absorption. To achieve such a goal, we investigate the spinel compounds AB₂X₄ (X = O, S and Se) as potential candidates because they possess both four-fold and six-fold coordination in one structure. Here, we built four basic structure units as material genes to design various types of spinel compounds with optimal optical absorption. The optical absorption of a semiconductor (α) is fundamentally determined by the dipole transition matrix element (|M|²) and joint density of states (J_cv). Our first-principles calculations show that in AB₂X₄ systems the type of cation at the B site determines the optical absorption. Two rules are identified through our study: (i) the s_v cations with the valence s orbital at the valence band maximum at the B site (type-I, type-II) will lead to allowed transitions, whereas the s_c cations with the valence s orbital at the conduction band minimum at the B site (type-III, type-IV) will lead to partially forbidden or fully forbidden transitions; (ii) when the s_v cation is located at the B site, the A site with the s_c cations (type-I) has higher |M|², while the A site with the s_v cations (type-II) has higher J_cv. Our study, therefore, provides guidelines to optimize the J_cv and |M|², and thus α, through cation engineering in AB₂X₄ compounds, which have higher stability compared to that of halide perovskites.