Mn掺杂后三元黄铜矿结构半导体CuInTe2的缺陷特征与热电性能

三元黄铜矿结构(也称类金刚石结构)半导体是一类具有热电转换潜力的新型热电材料. 本次工作中采用电负性更小的Mn元素替换CuInTe₂黄铜矿结构半导体中的Cu元素, 设计制备贫Cu化合物Cu_1-xInMn_xTe₂. 研究表明, 当Mn含量较低时, Mn优先占位在In 位置产生受主缺陷Mn_In^-. 因此随着Mn含量的增大, 载流子浓度和电导率均得到改善. 但当Mn含量进一步增大后, Mn可同时占位在In位置和Cu位置, 除产生受主缺陷Mn_In^-外, 还能产生施主缺陷Mn_Cu⁺. 由于两类极性相反的缺陷之间的湮灭现象, 使得缺陷浓度及载流子浓度开始降低, 晶格结构畸变有变小趋势, 因此在高温下晶格热导率仅略有提高. 研究结果表明, 在某一特定的Mn含量(x=0.05)时, 材料具有最优的热电性能(ZT=0.84@810.0 K), 这一性能约是未掺杂CuInTe₂的2倍.

Defects and thermoelectric performance of ternary chalcopyrite CuInTe2-based semiconductors doped with Mn

In thermoelectric (TE) semiconductors, there are three physical parameters that govern the TE performance (i.e. Seebeck coefficient (α), electrical conductivity (σ), and thermal conductivity (κ)); they are interrelated, hence it is hard to optimize them simultaneously. In order to improve the TE performance, we need to further explore new materials. Ternary chalcopyrite (diamond-like) I-III-VI₂ semiconductors (E_g = 1:02 eV) are new materials of the TE family, which have potential in conversion between heat and electricity. Since in the ternary chalcopyrite structure, such as Cu(Ag) MTe₂, there is an inherent Coulomb attraction between charged defects M_Cu(Ag)²⁺ and 2V_Cu(Ag)^- (a native defect pair, i.e., metal M-on-Cu or Ag antisites and two Cu or Ag vacancies), hence the electronic and structural properties can easily be tailored if these two defects, along with the creation of other defects, are modified through the introduciton of foreign elements. Besides, the ternary I-III-VI₂ compounds often show tetragonal distortion because μ≠0.25, η= c/2a≠ 1 (here μ and η are the anion position displacement parameters, and a and c are the lattice parameters), and the cation–anion distances are not equal (d_Cu–Te≠d_In–Te). Any occupation by foreign elements in the cation sites of I-III-VI₂ will cause the redistribution of bond charges between I-VI and III-VI, thus leading to a tiny adjustment of the crystal structure and altering the phonon scattering behavior. In this work, we substitute Mn for Cu in the chalcopyrite CuInTe₂ and prepare the Cu-poor Cu_1-xInMn_xTe₂ semiconductors. Investigations of Z-ray patterns after Rietveld refinement reveal that Mn prefers In to Cu lattice sites for low Mn content (x < 0.1), thus creating Mn_In^- as an active acceptor, and improving the carrier concentration (n) and electrical conductivity as Mn content increases. However, Mn can either occupy In or Cu sites simultaneously when x ≥ 0.1, and generate both the donor defect Mn_Cu⁺ and the acceptor defect Mn_In^-. In this case, annihilation may occur between these two defects, allowing the reduction in both the defect and carrier concentrations. Because of the annihilation between the two defects, two values (|Δμ| = |μ-0.25| and |Δη|= |η-1.0|) reduce, this only yields a subtle change in the difference between mean cation-anion distance (R_In–Te-R_Cu–Te), indicating a small distortion tendency in lattice structure as Mn content increases. Because of this, there is a limited enhancement in lattice thermal conductivity (κ_L) at high temperatures. As a consequence, we attain an optimal TE performance at a certain Mn content (x = 0.05) with the dimensionless figure of merit (ZT) ZT = 0.84 at 810.0 K, which is about twice as much as that of Mn-free CuInTe₂.