用于高效太阳电池的离子型尖晶石半导体的结构稳定性和缺陷容限
Structural Stability and Defect-Tolerance of Ionic Spinel Semiconductor for High-Efficiency Solar Cells
摘要：
缺陷容限与结构稳定性之间的不相容性是阻碍高效太阳能电池广泛应用的严重问题。通常，以Si/CdTe晶体为原型的共价/极性半导体由于其复杂的四面体结构单元而表现出很好的结构稳定性，但由于组成元素的相似电负性而表现出极差的缺陷容限。相反，离子半导体，如钙钛矿系列，总是表现出良性的电子性质的本征缺陷，由于阴离子和阳离子之间的电负性的巨大差距，但结构不稳定，因为稀疏组成的八面体构建块由大阳离子支撑。结合共价半导体的稳定骨架和离子化合物的良性缺陷，我们发现HgX_2S_4（X=In，Sc，Y）尖晶石半导体兼具这两种优点。四面体和八面体紧密结合保证了结构的稳定性，而Hg-6s和S-3 p轨道分别主导导带最低（CBM）和价带最高（VBM）的带边离子特征使HgX 2S 4具有缺陷容忍性. Hg-6s球轨道的空间扩展引起的CBM的显著向下弯曲不仅诱导了合适的光学带隙（在离子化合物中通常过大），而且促进了n型载流子的形成和输运。研究表明，Hg基硫属尖晶石是一种很有前途的高效太阳能电池材料，在尖晶石晶体骨架下引入离域轨道的阳离子是合成稳定、耐缺陷光伏材料的另一种途径。
英文摘要：
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.
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