LAUSR

Tetrahedral coordination structures, e.g. crystalline Si, GaAs, CdTe, and octahedral coordination structures, e.g. perovskites, represent two classes of successful crystal structures hitherto for solar cell absorbers. Here, via first-principles calculations and crystal symmetry analysis, the two classes of semiconductors are shown exhibiting complementary properties in terms of bond covalency/ionicity, optical property, defect tolerance, and stability, which are correlated with their respective coordination number. Therefore, a spinel structure is proposed, which combines tetrahedral and octahedral coordination into a single crystal structure, as an alternative to perovskite and conventional semiconductors for potential photovoltaic applications. The case studies of a class of 105 spinel AB2 X4 systems identify five spinel compounds HgAl2 Se4 , HgIn2 S4 , CdIn2 Se4 , HgSc2 S4 , and HgY2 S4 as promising solar cell absorbers. In particular, HgAl2 Se4 has suitable bandgap (1.36 eV by GW0 calculation), small direct-indirect bandgap difference (24 meV), appropriate carrier effective mass (me = 0.08 m0 , and mh = 0.69 m0 ), strong optical absorption, and high dynamic stability. This study suggests that crystal systems with mixed tetrahedral and octahedral coordination may open a viable route for emerging solar cell absorbers.