Suppressing Charged Cation Antisites via Se Vapor Annealing Enables p‐Type Dopability in AgBiSe₂–SnSe Thermoelectrics

Abstract

figure-of-merit (zT) of a TE material, which is given by zT = S 2 σT/κ, where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. [3] Nevertheless, the competing interplay between electrical and thermal properties has stagnated the rise of zT for decades. [4] Ternary ABX 2 -type (A = group I, B = group V, X = group VI elements) compounds have gained considerable attention as promising candidates for TE applications. [5] Lone-pair electrons of B-site cations contribute to strong phonon-phonon interactions and lower the lattice thermal conductivity. [6] Therefore, several ABX 2 compounds show temperature-independent thermal conductivities unlike those of conventional TE materials. [7] This intriguing property provides a venue for modulating electronic properties with more degrees of freedom because ABX 2 compounds are relatively free from the complicated trade-off relationship between electrical and thermal properties. However, utilizing ABX 2 compounds has been limited by serious problems: an absence of rational design rules, complex phase stability, and poor reproducibility. [8,9] For example, an undoped hexagonal AgBiSe 2 shows a wide range of Seebeck coefficients from −450 to 250 µV K −1 depending on the synthesis condition (Figure 1a,b). In addition, the B-site cation determines the sign of the Seebeck coefficient, where most Sb-based ABX 2 compounds are intrinsically p-type and Bi-based Cation disordering is commonly found in multinary cubic compounds, but its effect on electronic properties has been neglected because of difficulties in determining the ordered structure and defect energetics. An absence of rational understanding of the point defects present has led to poor reproducibility and uncontrolled conduction type. AgBiSe 2 is a representative compound that suffers from poor reproducibility of thermoelectric properties, while the origins of its intrinsic n-type conductivity remain speculative. Here, it is demonstrated that cation disordering is facilitated by Bi Ag charged antisite defects in cubic AgBiSe 2 which also act as a principal donor defect that greatly controls the electronic properties. Using density functional theory calculations and in situ Raman spectroscopy, how saturation annealing with selenium vapor can stabilize p-type conductivity in cubic AgBiSe 2 alloyed with SnSe at high temperatures is elucidated. With stable and controlled hole concentration, a peak is observed in the weighted mobility and the density-ofstates effective mass in AgBiSnSe 3 , implying an increased valley degeneracy in this system. These findings corroborate the importance of considering the defect energetics for exploring the dopability of ternary thermoelectric chalcogenides and engineering electronic bands by controlling self-doping.

Reduced Thermal Conductivity in Nanostructured AgSbTe2 Thermoelectric Material, Obtained by Arc-Melting

“…The minimum value of the lattice thermal conductivity, observed at 623 K, followed by a conspicuous increase ( Figure 5 ) can be explained, based on the effect of the atomic ordering happening at that temperature. It has been reported that this atomic ordering effect can be inferred from the unconventional temperature dependence of the transport properties [ 30 ], such as in the lattice thermal conductivity, as well as in the resistivity, the Seebeck coefficient and the weighted mobility.…”

“…Therefore, the precise determination of the Ag/Sb occupancy in this telluride has a significant relevance in terms of transport properties. This cation disordering is often found in other ternary cubic chalcogenide compounds, with the associated point defects usually leading to a poor reproducibility of the properties of these materials [ 30 ]. Recently, Roychowdhury et al have proven that the atomic ordering can be achieved in AgSbTe 2 with cadmium (Cd) doping [ 31 ].…”

Stepwise Alloying in Liquid-like Solid Solutions to Achieve Crystallographic Distortion for Regulating Thermoelectric Transport Behavior

“…In parallel, considering the positive role of Sn, Ag, and Bi in reducing the lattice thermal conductivity value of Cu 2 Se, as well as the potential advantages of stepwise alloying engineering, (SnSe) 1– x (AgBiSe 2 ) x was selected as the dopant for alloying. Furthermore, the stable existence of high-pressure cubic rock salt phase SnSe can be promoted by adjusting the x content in (SnSe) 1– x (AgBiSe 2 ) x . , The large degenerate number caused by the highly symmetric structure in cubic (SnSe) 0.75 (AgBiSe 2 ) 0.25 is beneficial to the improvement of Seebeck coefficient. Inherently low thermal conductivity (SnSe) 0.75 (AgBiSe 2 ) 0.25 was used as an additive to combine these effects to optimize the thermoelectric properties of Cu 1.9 Se.…”

“…Furthermore, the stable existence of high-pressure cubic rock salt phase SnSe can be promoted by adjusting the x content in (SnSe) 1−x (AgBiSe 2 ) x . 23,24 The large degenerate number caused by the highly symmetric structure in cubic (SnSe) 0.75 (AgBiSe 2 ) 0.25 is beneficial to the improvement of Seebeck coefficient. Inherently low thermal conductivity (SnSe) 0.75 (AgBiSe 2 ) 0.25 was used as an additive to combine these effects to optimize the thermoelectric properties of Cu 1.9 Se.…”

VTAnDeM: A Python Toolkit for Simultaneously Visualizing Phase Stability, Defect Energetics, and Carrier Concentrations of Materials

“…Native defects often regulate electron-hole recombination mechanisms in, e.g., photovoltaics, [1][2][3][4][5] whereas electrical and thermal transport properties can be tuned by controlling point defects and the resulting carrier concentrations in, e.g., thermoelectrics. [6][7][8][9][10][11][12][13][14] The phase stability, defect formation energies, and carrier concentrations are interconnected properties of a semiconductor that are influenced by the thermodynamic environment. 15,16 In particular, a thermodynamic state is defined by a fixed set of elemental chemical potentials that are determined by the impurity phases with which a material is in equilibrium.…”