  The incompatibility between defect-tolerance and structural stability is a
severe issue hindering the wide application of high-efficiency solar cells.
Usually, covalent/polar semiconductors with prototype of Si/CdTe crystals
exhibit great structural stability owing to their compactly composed
tetrahedral building blocks, but present extremely poor defect-tolerance due to
the similar electronegativity of component elements. On the contrary, ionic
semiconductors, such as perovskite series, always exhibit benign electronic
properties of intrinsic defects owing to the great disparity of
electronegativity between anions and cations, but are structurally unstable
because of the sparsely composed octahedral building blocks supported by large
cations. Combining the stable framework of covalent semiconductors and benign
defects of ionic compounds, we find that HgX2S4 (X=In, Sc and Y) spinel
semiconductors possess both the merits. The tightly combined tetrahedral and
octahedral blocks ensures the structural stability, and the band edge of ionic
characteristic, which is mainly dominated by Hg-6s and S-3p orbitals for
conduction band minimum (CBM) and valence band maximum (VBM), respectively,
makes HgX2S4 defect-tolerant. The prominent downward bending of CBM caused by
spatially spreading Hg-6s spherical orbital not only induces a suitable optical
band gap which is often too large in ionic compounds, but also promotes the
formation and transport of n-type carriers. This study presents that Hg-based
chalcogenide spinels are promising candidates for high-efficiency solar cells,
and suggests that adopting cations with delocalized orbitals under the
framework of spinel crystal is an alternative way for synthesizing the stable
and defect-tolerant photovoltaic materials.
