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

Abstract

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 (m_e = 0.08 m₀ , and m_h = 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.