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

Abstract

In thermoelectric (TE) semiconductors, there are three physical parameters that govern the TE performance (i.e. Seebeck coefficient (), electrical conductivity (), and thermal conductivity ()); 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 (Eg = 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 MCu(Ag)2+ and 2VCu(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 0.25, = c/2a 1 (here and are the anion position displacement parameters, and a and c are the lattice parameters), and the cationanion distances are not equal (dCuTedInTe). 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 Cu1-xInMnxTe2 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 MnIn- 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 MnCu+ and the acceptor defect MnIn-. 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 (|| = |-0.25| and ||= |-1.0|) reduce, this only yields a subtle change in the difference between mean cation-anion distance (RInTe-RCuTe), indicating a small distortion tendency in lattice structure as Mn content increases. Because of this, there is a limited enhancement in lattice thermal conductivity (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.