Materials Design of Solar Cell Absorbers Beyond Perovskites and Conventional Semiconductors via Combining Tetrahedral and Octahedral Coordination

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.