Structural Stability and Defect-Tolerance of Ionic Spinel Semiconductor for High-Efficiency Solar Cells

TL;DR

This paper introduces HgX2S4 spinel semiconductors that combine structural stability and defect-tolerance, making them promising for high-efficiency solar cells by leveraging their unique ionic and covalent features.

Contribution

It demonstrates that HgX2S4 spinels integrate the stability of covalent frameworks with the defect-tolerance of ionic compounds, offering a new class of photovoltaic materials.

Findings

HgX2S4 spinels have stable tetrahedral and octahedral blocks.

Their electronic structure promotes defect-tolerance and suitable optical band gaps.

They are promising candidates for high-efficiency solar cells.

Abstract

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.